Mechanical Performance of Carbon-Fiber-Reinforced Composite Textile Laminates Integrated with Graphene Nanosheets

*Vishwas Jadhav and Ajit D. Kelkar*

#### **Abstract**

This chapter discusses the fabrication and mechanical characterization of nanoengineered composite laminates fabricated using variable-thickness graphene sheets incorporated in non-crimp carbon fiber prepregs. The effect of graphene sheet thickness on interlaminar strength (Mode I fracture toughness) of the carbon fiber composites was evaluated. The graphene lattice structure used in the present research had linear and square grids. Linear grids were arranged parallel and perpendicular to the 0° fibers in the composite laminates and labeled as vertical and horizontal grid patterns, respectively. Mechanical characterization involved the study of the effects of sheet thickness and grid pattern with and without nanoengineered enhanced laminates at the midplane. The composite laminates fabricated using a lattice graphene structure had better interlaminar strength than those fabricated with straight graphene sheets. Nanoengineered sheets with minimal thickness showed better interlaminar strength than the thicker sheets. The polymer used to manufacture the graphene sheet could not bond with the epoxy used in the composite laminate. In the literature, when the graphene nanoparticles are dispersed in the epoxy, the challenge is a uniform distribution of the nanoparticles. To overcome this dispersion problem, sheets made using nanomaterials can be used to enhance the mechanical properties of the composite laminates.

**Keywords:** carbon-fiber-reinforcement, composite textile laminates, doble cantilever beam, fabric characterization, graphene nanoplatelets, graphene sheet, non-crimp fibers

#### **1. Introduction**

Due to superior specific properties over traditional materials, the use of textile composites for aerospace, automotive, and marine applications has increased dramatically. Metal failure depends on the yield strength of the material, whereas the applied stresses influence the failure of composite material, the interaction between the two phases, the stacking sequence, and voids or defects in the composite laminate. Composite laminates are bonded together by a thin layer of resin between them. The function of this interface layer is to transfer the displacement and the force from one layer to another. As this matrix is a weaker element in the composites, the matrix fails at the beginning and then the fibers. When these layers get damaged or weakened, they form a crack between adjacent plies, separating the adjacent layers. This results in a failure of the lamination, which is known as delamination. The weak interlaminar region of each ply is the primary reason for the interlaminar-induced delamination of composite laminates. Researchers have developed various methods from different perspectives in diverse fields to overcome this challenge and enhance interlaminar strengths. The next-generation reinforcements, non-crimp fibers (NCF), are being explored for various structural applications. Non-crimp fabric provides excellent laminate strength, and the fabrication cost is lower than that of traditional composite manufacturing due to its drape ability and lack of fiber crimp in the woven fabric.

Various techniques such as three-dimensional weaving, stitching, Z-pinning, edge notching, braiding, fiber hybridization, and edge capping have been explored. These techniques suppress delamination but degrade in-plane laminate properties and incur additional manufacturing costs due to the process modifications. Degradation of in-plane properties and an increase in the weight of the composite laminate also lead to macroscale defects in the composite laminate, which may act as a crack initiation point for failure.

Tsai and coworkers studied the effect of delamination and tensile strength in the thick and thin ply composite laminate. To achieve a thickness of less than one-third of conventional plies (thin ply), a constant airflow through a sagged fiber filament technique was used to spread thick tows without damaging the fibers. Smaller ply thickness provides more choice in optimizing the laminate composite structure. The authors found that the tensile strength was higher for the thin-plies than the thickerplies composite laminates due to the less resin-rich area used, but manufacturing thinner plies is more challenging [1]. With the advanced technology and concept of Prof. Stephen W Tsai's theory, known as the CHOMARAT, which was founded in 1998, four industrial sites in France, Tunisia, the USA, and China, manufactures non-crimp fabric (NCF) [2].

Nanomaterials, such as carbon nanotubes (CNTs)/graphene nanoplatelets (GNPs) incorporated polymer composites, are becoming popular due to their excellent mechanical, thermal, and electrical properties. Adding a small portion of nanomaterials improves the mechanical properties of the composite laminates, such as their strength, stiffness, impact toughness, and electrical and thermal conductivity by several orders of magnitudes, and allows them to work with inner plies However, due to strong intermolecular interactions based on dipole interactions and van der Waals forces, non-functionalized nanomaterials form bundles or agglomerates. Agglomeration causes the collection of bundles of these nanomaterials together, distributing them unevenly in the matrix.

Nanomaterials can be aligned and dispersed in the polymer composite by applying external force. Various techniques, such as mechanical stretching, electrical fields, spinning processes, and magnetic fields, are used to overcome the agglomeration issue. Mechanical methods, such as ultrasonication (bath or probe), three-roll milling (calendaring) ball milling techniques, or a combination of these methods in series or parallel, are used to achieve dispersion. Each method has its pros and cons. High energy created during the sonication or milling process often damages

#### *Mechanical Performance of Carbon-Fiber-Reinforced Composite Textile Laminates Integrated… DOI: http://dx.doi.org/10.5772/intechopen.114200*

the nanomaterials. So, surface modification techniques became famous for dealing with agglomeration problems. In these methods, the nanomaterials are treated with chemicals such as nitric, phenolic, and carboxylic groups to improve the interaction with the solvent, which leads to uniform dispersion and avoid agglomeration.

Various tactics are used to integrate carbon nanotubes or graphene into fiberreinforced plastic structural composites to improve the mechanical properties of the composite laminate. Micro/nanoscale materials incorporation into the matrix resin to enhance the interlaminar properties have become popular compared to mechanical insertion due to both in-plane and out-of-plane enhancement of the properties per unit mass. Furthermore, even though mechanical insertion improves the interlaminar strength, it degrades in-plane properties due to cutting of the fibers during the insertion and adding mass to the composite laminate. To overcome the agglomeration problem, researchers have used nanoengineered interleaving sheets typically inserted at various locations through the thickness of the laminate [3].

In the present research work, graphene sheets of various thicknesses were incorporated into composite laminate and the interlaminar strength investigated by performing a double cantilever beam test. Graphene sheet-embedded composite laminates showed degraded interlaminar strengths, due to low bonding between the resin and graphene sheets. To overcome this problem, grids were formed on the graphene sheet, which helped to form the thin layer of resin at the midplane by enhancing the interlaminar fracture toughness of the composite laminates. After the test to check the bonding at midplane, the tested specimens were opened at midplane for microscopic analysis.

#### **2. Literature background**

Nanomaterials have superior specific (per unit mass) mechanical properties as compared to resins and fibers. A small amount of nanomaterial added to the composites increases the mechanical properties by a few orders of magnitudes. Carbon nanotubes (CNTs)/graphene nanoplatelets (GnPs)-incorporated polymer composites have become popular in composite industries. The carbon nanotubes (CNTs) improve mechanical properties such as strength, stiffness, impact toughness, and electrical and thermal conductivity of the composites.

Toughening of the matrix resin, interleaving with short fibers or micro/nanoscale particles, became popular in recent years to overcome delamination issues by avoiding mechanical inserts (pins). The addition of nanofillers leads to additional toughening mechanisms such as fiber pull-out, peeling/pull-out hackle formation, and stickslip mechanism, which are controlled by: (i) the method by which the laminate is manufactured, (ii) the even dispersion of the nanofiller, and (iii) the matrix mixing method. **Table 1** presents the techniques used to improve the interlaminar using different nanofillers with various results/laminar improvements reported by researchers in the last few decades.

A key method of incorporating nanomaterials in composite processing is based on the use of vacuum assisted resin transfer molding method (VARTM). However, the use of nanomaterials in composite processing using VARTM presents a significant challenge due to the narrow gaps between fibers as nanoparticles filter out during infusion as depicted in **Figure 1**. To overcome these challenge, researchers utilize advanced threeroll mills, centrifugal mixers, functionalization, sprayers, etc., for uniform dispersion of nanoparticles into the resin to produce nanomodified prepregs [19].


#### **Table 1.**

*Summary of the techniques used to fabricate nanoengineered composite laminates and their effect on the mechanical properties.*

*Mechanical Performance of Carbon-Fiber-Reinforced Composite Textile Laminates Integrated… DOI: http://dx.doi.org/10.5772/intechopen.114200*

**Figure 1.** *Filtering of larger particles by fiber array and infiltration of smaller particles between fibers [18].*

Furthermore, the use of nanofillers poses additional challenges of nonuniform dispersion resulting from nano-agglomeration, which degrades the properties of the laminate. The advanced method of inserting the carbon nanotube buckypaper was used by various researchers with promising results. Wang et al. [20], for example, studied glass fiber reinforced polymer composite with and without buckypaper. They reported 25% more shear strength due to incorporating carbon nanotubes (CNTs) buckypaper. Li et al. [21], on the other hand, studied the effect of curing pressure on glass fiber reinforced plastic (GFRPs) laminates fabricated by adding CNTs buckypaper to the midplane layer resulting in fracture toughness increments of 174.81% for initiation and 179.97% for propagation relative to baseline composite for 2 MPa curing pressure. Then, Chen et al. [22] investigated fracture toughness by incorporating buckypaper at the midplane of reinforced fiber and found the initiation interlaminar fracture toughness was increased by 29% compared to that of the one without the buckypaper composite laminate. These studies demonstrated that the interlaminar fracture toughness of fiber-reinforced plastics could be significantly enhanced by incorporating CNTs buckypaper interleaf. The researchers used nanomaterials with a small percentage of the composite and the nanoengineered micro thickness sheets were incorporated into composite laminates.

On the other hand, the use of graphene sheet interleaf is not found in the literature. So, in the present study, the graphene sheets with 50, 120, and 240 μm thickness were incorporated at midplane in order to study the initiation and propagation fracture toughness of quasi-isotropic layered non-crimp carbon fibers. Straight graphene sheets of 50, 120, and 240 μm thickness incorporated composite laminates degraded the initiation interlaminar fracture toughness by 75, 92, and 86% compared to the composite laminate without graphene sheet. To overcome this problem, uniform grids were formed on the graphene sheet, aligned parallel or perpendicular to 0o fibers at the midplane, and labeled as either vertical or horizontal grids in the present research work.

#### **3. Materials and the process**

#### **3.1 Procurement and characterization of graphene sheets**

Graphene sheets of three different thicknesses: 50, 120, and 240 μm (PN40003, PN40008, and PN 40009) are supplied by XG Sciences Inc, Lansing, MI, USA, were used as interleaving material. SEM images of the graphene sheet, as shown in **Figure 2**, depict the two-dimensional graphene nanoplatelets (GnPs).

**Figure 2.** *SEM images of the graphene sheet.*

#### **3.2 Panel fabrication from NCF-MTM45-1 Prepreg**

Almost all high-tech composite companies manufacturing aerospace and sporting goods use prepreg because of the high quality of fiber alignment and uniform fiber volume fraction. In the present study, NCF-MTM45-1 pre-impregnated prepregs supplied by SHD composites, NC, USA, were used to fabricate nanoengineered composite laminates. The graphene sheets were employed as interleaf at the midplane of the composite laminate to analyze the fracture toughness at the initiation and the propagation point. Other supporting materials, including glass mold, vacuum bag, release film, breather, and sealant tape, were assembled as shown in **Figure 3**.

The NCF composite laminates were fabricated using 48 layers (24 bilayers) of biaxial spread-tow carbon NCF fabric prepreg. Each NCF prepreg layer had [0/−45] orientation. These 24 bilayers were symmetrically stacked about midplane with the orientation of [0/−45/90/45] to achieve a balanced layup in the final cured laminate. Hand-cutting fibers produced the composite laminate with dimension 12″ × 12″, with layup [(0/−45/90/45)6/GS/(45/90/−45/0)6] [22, 23]. **Figure 4a** depicts the debulking, using vacuum pressure to remove the entrapped air in the prepregs. Then, the 12 layers of biaxial spread-tow carbon fibers were placed on the mold, and a Teflon sheet of 14″ × 3″ was inserted at one end such that the 14″ side is perpendicular to 90o fibers in the laminate, as shown in **Figure 4b** and **c**.

**Figure 3.**

*Fabrication set up for nanoengineered composite laminate.*

*Mechanical Performance of Carbon-Fiber-Reinforced Composite Textile Laminates Integrated… DOI: http://dx.doi.org/10.5772/intechopen.114200*

**Figure 4.**

*Fabrication—nanoengineered composite laminate. (a) Debulking of Prepregs. (b) NCF prepreg layer preparation. (c) NCF prepreg layer arrangement. (d) GS at midplane.*

The graphene sheet (either 50, 120, or 240 μm) was incorporated next to the Teflon sheet at the midplane to produce nanoengineered composite laminate. The fabrication process included composite laminate, with and without nanoengineered graphene as depicted in **Figure 4d** [2, 23, 24].

Weak bonding between the matrix and graphene resulted in degraded interlaminar resistance causing delamination and separation at the midplane when manual force was applied. This was then corrected by perforating the graphene sheets forming a lattice structure.

#### **3.3 The curing cycle of the process**

The curing cycle for the process is shown in **Figure 5**. After oven curing, the coupons were waterjet cut as shown in **Figure 6a**, and tested as per American Society for Testing and Materials (ASTM) D5528 standards [25]. The edges were made smooth and uniform using the grinding machine to achieve uniform width, as depicted in **Figure 6b**.

#### **3.4 Fracture toughness (GIC)**

Strain energy release rate G, expressed as Mode I interlaminar fracture toughness in ASTM D 5528 standards is given as:

$$\mathbf{G}\_{\rm IC} = \frac{\mathbf{3P}\boldsymbol{\delta}}{2b\left(a + \mathrm{l}\boldsymbol{\Delta l}\right)}\tag{1}$$

**Figure 5.**

*Cure and post cure time-temperature cycle (MTM 45-1).*

**Figure 6.** *(a–c) Preparation and DCB test. (a) Waterjet cutting, (b) edge grinding of coupons and (c) DCB Test.*

where *P* = load, *δ* = load point displacement, *b* = specimen width, *a* = delamination length and Δ is constant, determined experimentally by generating the least-square plot of the cube root of compliance, 1/3 *C* as a function of delamination length. Compliance *C* is the ratio of crack opening displacement to the applied load, *δ*/*P*. For more details, readers are referred to appropriate literature sources [25]. Instron electromechanical system is used to record the load and displacement at every 10 s after test analysis was done to calculate fracture toughness and coupons were separated about midplane to observe the midplane bonding using microscopic images.

The present work uses the modified beam method to calculate fracture toughness as per ASTM 5528 standards.

#### **4. Results and discussion**

Strain energy release rate (GIC) values for with- and without-graphene sheet reinforced-composite laminate were calculated using Eq. (1) and the results are shown in **Figure 7**. The initiation and propagation values of strain energy release rate *Mechanical Performance of Carbon-Fiber-Reinforced Composite Textile Laminates Integrated… DOI: http://dx.doi.org/10.5772/intechopen.114200*

#### **Figure 7.**

*Initiation and propagation of GIC with and without graphene sheet.*

(GIC) with brittle graphene-epoxy for graphene sheet reinforced-composite reduced initiation fracture toughness by 75, 92, and 86% for 50, 120, and 240 μm thick graphene sheets, and similar trends observed for propagation fracture toughness [26].

Weak bonding between the matrix and graphene resulted in degraded interlaminar resistance. Because of this, the specimens were easily delaminated and separated at the midplane when manual force was applied, as shown in **Figure 8**. To overcome this challenge, the graphene sheet was perforated by forming a lattice structure.

The lattice grids were developed with 5 mm spacing horizontally, vertically, and in square form, as shown in **Figure 9**. The gap between the graphene sheets helped to form the fiber-epoxy bonding.

**Figure 10** represents the delamination resistance curve (R curve) for the horizontal, vertical, and square grid embedded graphene sheet at midplane in the composite laminate. It then compares it with the composite laminate without a graphene sheet. Delamination resistance for composite laminate without nanoengineered graphene sheet was dominant as compared to the plain and lattice structure graphene sheetembedded composite laminates. The lattice structure formed on the graphene sheet helped to improve the delamination resistance of the composite laminate.

**Figure 11** compares the fracture toughness of the 24 bilayers, NCF carbon fiber with and without incorporated lattice structure graphene sheet at the midplane with bare composites. Irrespective of the thickness, the vertical grids (grids parallel to 90° fibers in the laminate) had better performance than horizontal and square grids.

**Figure 8.**

*Testing of coupon manually. (a) Waterjet coupon, (b) manually applied force and (c and d) separation of the laminate at the midplane.*

#### **Figure 9.**

*Lattice structure formed on a graphene sheet.*

**Figure 10.** *Delamination resistance curve (R curve) for all GS.*

**Figure 11.**

*Mechanical Performance of Carbon-Fiber-Reinforced Composite Textile Laminates Integrated… DOI: http://dx.doi.org/10.5772/intechopen.114200*

**Figure 12.** *(a and b) Images of the coupons after the DCB test. (a) 50 μm straight GS and (b) 50 μm horizontal grid.*

The graph for the horizontal grids showed a sudden change in values due to graphene strips, because of the alternating resin patch.

The fracture toughness of the vertical grid 50 μm graphene sheet was the highest and was almost the same as the baseline composite laminate. The fracture toughness of the square grids was observed to be less than that of the other grid patterns. The fracture toughness of a straight 120 μm-thick graphene sheet was the lowest compared to those of similar groups.

Sample images for the above configurations are shown in **Figure 12a** and **b**.

For the straight graphene sheet-incorporated panels, it was observed that there was very little binding between the graphene sheet and adjacent plies. This degraded its interlaminar fracture toughness compared to the other configurations. In straight graphene composite laminates, laminates fabricated using 50 μm-thick graphene sheet-embedded showed higher fracture toughness when compared to that of the 120 and 240 μm thick graphene sheet-embedded composite laminate. This could be due to the smaller thickness of the sheet. Also, the vertical lattice structure on 50 μm sheet showed a fracture toughness of 0.28 kJ/m2 as compared to 0.18 and 0.24 kJ/m2 when 120 and 240 μm vertical lattice structures graphene sheets were used, respectively, as the matrix formed a thin layer in between the lattice structure, which helped to improve the interlaminar resistance. As vertical grids were arranged parallel to 0° fibers in the composite laminates, which formed the thin alternate layer of epoxy along the length of the DCB test specimen, it helped to increase the interlaminar strength of the composite laminate. The grids are aligned horizontally perpendicular to the 0o fibers in the composite laminate, which forms alternate thin layers of the matrix between two strips of graphene sheet along the length of the DCB test specimen, resulted in instability that was observed for delamination resistance (in the delamination curve) of horizontal grid pattern of graphene sheet embedded composite laminates.

To maintain uniformity in the fabrication process, horizontal-grid-formed sheets were arranged in a manner that a graphene sheet follows the same gap after the Teflon at midplane. Experimentally, maximum fracture toughness was observed for vertical grids. Furthermore, all the vertical grid samples exhibited fiber pullout, which showed bonding between the top and bottom layers at midplane, and this was not observed in the specimens with other grid patterns. Khan and Kim [13] used buckypaper intervals and reported a 31% increase in the Mode II interlaminar shear strength while reporting doubled fracture toughness, the cost of making CNT buckypaper is way too expensive compared to the commercial graphene sheets used in the present study. Also, Kelkar et al. [6] reported an 80% increase in interlaminar strength as compared to neat composite using electrospun fibers.

#### **5. Conclusion**

Graphene sheets-embedded composite laminate was used to incorporate the nanoparticles and avoid the nanoparticles' uniform dispersion problem in the epoxy resin. Mode I fracture toughness (strain energy release rate) studies indicated that when graphene sheets were inserted in the midplane of the carbon fiber composite laminate, the results for fracture toughness were poorer as there was no bonding between the graphene sheet and adjacent prepregs. Therefore, the graphene sheets were converted into three different lattice configurations, including horizontal, vertical, and square structures, so as to alleviate this problem. Grids formed on the graphene sheet showed enhanced properties and vertical grids showed peak mechanical properties as compared to horizontal and square grids. The addition of grids helped to form a tiny epoxy layer and to improve the fracture toughness of the 50 μm lattice structure sheet, more than that of the composite without a graphene sheet. Vertical grids formed on 50, 120, and 240 μm thick graphene sheets showed improvement in the fracture toughness by 4.5, 10, and 7 times compared to the straight graphene sheet, but the values were always less compared to composite without graphene sheet. Overall use of two-dimensional nanoengineered graphene sheet showed degraded interlaminar strength as compared to the composite laminate without a graphene sheet.

*Mechanical Performance of Carbon-Fiber-Reinforced Composite Textile Laminates Integrated… DOI: http://dx.doi.org/10.5772/intechopen.114200*

#### **Author details**

Vishwas Jadhav1 \* and Ajit D. Kelkar2

1 Joint School of Nanoscience and Nanoengineering, North Carolina A&T State University, Greensboro, NC, USA

2 Department of Mechanical Engineering, North Carolina A&T State University, Greensboro, NC, USA

\*Address all correspondence to: vsjadhav@aggies.ncat.edu; vishwas.ncat@gmail.com

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 6**

## Advanced Graphene-Based Materials for Electrochemical Biomarkers and Protein Detection

*Carmen Ioana Fort, Liviu Cosmin Cotet, Lucian Cristian Pop, Monica Baia and Lucian Baia*

#### **Abstract**

In this chapter, recent advances in the field of graphene materials-based (bio)sensors that are used for biomarker and protein electrochemical detection are presented. Approaches related to the synthesis of electrode material for (bio)sensors construction as well as to their morphological and structural characterization, are highlighted, pointing out the advantages of using graphene-based materials for (bio)sensors applications. This chapter focuses on obtaining graphene-based electrodes, detecting biomarkers and proteins, and evaluating the performance of the sensors. Different methods for designing sensors for a large variety of biomolecules are described and comparatively discussed. In order to increase their electroanalytical performances, such as sensitivity, selectivity, detection limit, linear range, and stability, the research performed in the last years was focused on different types of graphene structures including graphene oxide, reduced graphene oxide, graphene nanofoams, graphene nanocomposites, different functionalized graphene, etc. The detection of analytes including neurotransmitters and neurochemicals (dopamine, ascorbic acid, uric acid, serotonin, epinephrine, etc.), hydrogen peroxide, and proteins, have been discussed. The studies related to electrochemical (bio)sensors are presented in three subchapters, and the key results—electroanalytical performances—of the sensors are summarized. The final chapter provides the conclusions derived from the comparative analyses of different approaches related to these types of (bio)sensors based on graphene materials.

**Keywords:** biosensors, electrochemical sensors, graphene materials, graphene oxide, reduced graphene oxide, graphene-based composites, graphene nanomaterials

#### **1. Introduction**

In the last decade, the necessity for rapid, simple hand-held testing devices in medicine has prompted the development of biosensors for clinical purposes. Biological sensors were developed as optical, electrical, piezoelectrical devices or systems, which consist of biological and electronic components and are able to discover and detect biological compounds like nucleic acids, proteins, enzymes, and genes. Nowadays, biosensors are known as devices that are employed for qualitative and quantitative detection of biological analytes [1]. The biomolecules, the biological structures, or the microorganisms can play the role of biological analytes. As it is well-known, an electrochemical (bio)sensor involves three components, which are responsible for (i) the recognition of the analyte and the signal formation, (ii) the transducer of the received signal, and (iii) the reader device [2]. Owing to an interdisciplinary combination of approaches from physics, chemistry, biology, nanotechnology, and medical science, the achievements in the biosensor field are impressive. Consequently, they are becoming essential devices developed for diagnosis of lifethreatening syndromes [3, 4].

Electrochemical sensors are of high interests in different applications because of their great sensitivity, selectivity, inexpensive and simplistic production, and facile miniaturization [5, 6]. They depend on the use of sensitive biological molecules immobilized on the surface of solid electrodes that are able to capture target molecules by specific recognition [2, 3]. This process, at the electrode surface, occurs with a reaction signal. The modified electrode transforms the produced chemical signal into a measurable electrical signal, such as current, voltage, conductivity, impedance, etc. This way, the technique enables both the qualitative and quantitative analysis of target species [7, 8].

Electrochemical biosensors have some advantages as compared to other biosensor categories. First of all, the theory behind it is well-developed, and it involves a facile design based on simple structures for easy measurements [5, 9]. As compared to other analytical methods including fluorescence [10, 11] colorimetric [12], and chemiluminescence methods [13, 14], electrochemical biosensors can be used in many important fields such as biomedicine, pharmaceutical industry, food industry, and environmental analysis [3, 4, 15]. In order to increase the sensor performances with respect to detection limits, sensitivity, selectivity, time stability, and linear detection range and to lessen the time of response, recent research was focused on developing a new preparation method for biosensor electrodes using different electrode materials.

Thus, due to their low cost, compared to other materials such as Pt and Au metals, and because of their good conductivity, a variety of modified electrodes incorporating carbonaceous materials [16], such as graphite [17, 18], carbon aerogel [19–22], glassy carbon [23–28], graphene [25, 29–33], carbon fiber [34], and screen-printed electrode [35], have been developed for sensing of numerous biological compounds. Also, various materials including metals nanoparticles Au [25, 36–39], Ag [40]; phosphates such as zirconium phosphates [28, 41] and titanium phosphates [27]; mediators [42], polymers (PANI [37, 43], chitosan [44–46]), enzymes (HRP) [17, 23, 47], plasmodium falciparum lactate dehydrogenase (PfLDH) [48], cellobiose dehydrogenase (CDH) [20]; oxides (α-Fe2O3 [49], Al2O3 [50], ZnO [36, 51], Cu2O [52]), and non-metals (N [53]) have been used widely to modify different carbonaceous electrode materials.

Among the carbonaceous modified electrodes, those based on graphene matrix, i.e., graphene (G), graphene oxide (GO), and reduced graphene oxide (rGO) [54, 55], exhibit important advantages over the conventional ones. This is because the latter classes of materials, present significant drawbacks that range from limited active surface areas easy inactivation and high cost of production. G structure is composed only of carbon atoms that are all sp2 -hybridized and organized in a single atomic layer. Every carbon atom belonging to G is bonded to three other neighboring carbon atoms using three of its valency electrons, and the fourth electron is delocalized, thus facilitating the electrical conductivity of G [15]. The use of G for sensors' development is

#### *Advanced Graphene-Based Materials for Electrochemical Biomarkers and Protein Detection DOI: http://dx.doi.org/10.5772/intechopen.114011*

attributed to its additional unique features that include its ease of production; good chemical, morpho-structural, and mechanical characteristics, joined with other atypical properties including the high surface area to volume ratio, exceptional optical characteristics, outstanding carrier mobility, and extraordinary electrical and thermal properties as compared to those exhibited by other carbon allotropes [56]. Besides its exceptional properties, G can be functionalized easily, and many of its biomedical applications can benefit from non-toxic, biocompatible, and water-dispersible G layers obtained by chemical functionalization with different ligands [57].

The grafting of functional groups on the G surface structure is extremely reliable for binding molecules so as to further examine their communications with welldefined targets. In this respect, oxidized G presents one of the best solutions to obtain surface charges by anchoring on its surface oxygen-containing substituents like hydroxyl, carbonyl, carboxyl, and epoxide groups, which make it easier to understand the future specific interactions that take place on the surface of the G structure. Thus, G and its oxidized derivatives (GO or rGO) combine well-known properties of the carbonaceous structures, i.e., the great electric and thermal conductivities, mechanical strength, and impermeability to gases, with those related to the enhancement of their surface response after interaction with specific molecules. Accordingly, G-based materials are desired because of their essential functional groups such as those found in GO or N-G, very beneficial and effective in aligning with biomedical and related fields [58]. This is the reason why, in the last years, investigations were focused on different types of G structures including GO, rGO, G nanofoams, G nanocomposites, and different functionalized G derivatives [59], and the biosensor applications are now focused on biomarkers and protein detection.

In this chapter, the latest developments in the domain of biosensors, based on G materials, are described. Details about the synthesis and their morpho-structural investigations are also provided. Different approaches for designing sensors for a great variety of biomolecules are described and discussed. The detected analytes contain neurotransmitters and neurochemicals (dopamine, ascorbic acid, uric acid, serotonin, epinephrine, etc.), hydrogen peroxide, proteins, DNA, glucose, and others. The aim was to highlight biosensor performances for glucose, hydrogen peroxide, neurotransmitters and neurochemicals, and cancer and disease biomarkers detection. A special importance was accorded to dopamine, ascorbic acid, uric acid, serotonin, and epinephrine detection. The conclusions resulting from the presented research on the biosensors are summarized in the last subchapter.

#### **2. Synthesis, structure, and morphology of graphene materials for sensing applications**

G materials have been widely promoted due to their suitable properties and versatility in biomedical applications as like biosensors, diagnosis-imaging, and drug delivery [58]. G consists of a one-atom-thick carbon structure that forms a 2D hexagonal honeycomb nanomaterial (**Figure 1a**), but besides the pristine G materials, the functionalized G structures (containing hydroxyl, carbonyl, carboxyl, and epoxide groups on the edge or plane of the nanosheets) such as GO (**Figure 1b**) and rGO (**Figure 1c**) are tremendously involved and tested in the same biomedical application fields. As a convention, the limits of 2D G are accepted to have the thickness of up to 10 atomic layers (i.e., a few nanometers) and the lateral size larger than 10 nm up to more than 20 μm [61]. Those having less than 10 atomic layers and a few thousand

#### **Figure 1.**

*Suggested structural models for monoatomic layered nanosheet of G (a), GO (b), and rGO (c); the blue square (b) depicted the electrical conductor oasis of GO. The red ovoid (c) depicted electrical insulator oasis of rGO. Reproduced from Ref. [60] with permission from the Royal Society of Chemistry.*

nanometers in size will be termed as nanosheets. The properties that make G materials suitable for various purposes can be adjusted by selecting the appropriate synthesis pathway. If the target is represented by the sensing applications, then the obtained structures are desired to recognize entities having similar physical properties and to provide information about them [58].

One very important parameter of G for electrochemical sensing applications is its semiconductor behavior with zero band gap energy, which shows that G has electrical conductivity much greater than that of copper, resulting from its extended π-π conjugation of sp2 hybridized carbon atoms. In the same way, rGO is considered an electrical conductor that contains electrically insulating oases (**Figure 1c**), whereas GO is an electrical insulator with electrically conductive oases (**Figure 1b**). The electrical insulation behavior of GO results from the presence of sp3 hybridized carbon atoms that are involved in anchoring oxygen-containing groups on the edges or planes of GO/rGO nanosheets. Among them, the carboxyl and epoxide groups are very commonly involved in biomolecular immobilization for biosensor construction [58].

Generally, the synthesis of G materials can be grouped into two main classes: bottom-up and top-down methods [62]. The representative pathway for the first class is carbon vapor deposition (CVD) that consists of a G growth on catalytic support (e.g., nickel or copper) using a carbon-source gas (e.g., CH4, CH4/H2 mixture, C2H4) at a high temperature (around 1000° C). Free-standing monoatomic layer G as a crystal sheet with the size of catalytic metallic support is released after support etching [62].

Among the top-down methods is the one proposed by Paton et al. [63], which enables the obtaining of G nanosheets in a large-scale production method by physical exfoliation based on shear mixing of graphite. Even though, the obtained G has a high yield, good conductivity, and strong mechanical properties, its poor dispersion in common solvents (e.g., water and ethanol) is the primary disadvantage [64]. The most efficient ways to overcome this drawback is to use a dispersing agent (e.g., amphiphilic polymers, alkylamines, and molecules with hydrophilic carboxyl groups) or to generate organic functional groups (e.g., hydroxyl, carbonyl, carboxyl, and epoxide) on the planes and the edges of G sheets. The most known of such methods are Hummer [65] and its derivates [66], which are based on chemical oxidative exfoliations of graphite using concentrated sulfuric acid (H2SO4) and sodium nitrate (NaNO3) as the reaction medium combined with potassium permanganate (KMnO4) as oxidant. But toxic and hazardous gases (e.g., NO2 and N2O4) are generated if the reaction temperature overcomes certain limits. Marcano et al. [67] proposed an improved method of Hummers that consists in an oxidative exfoliation, in which phosphoric acid (H3PO4) was involved. Better efficiency in GO production and no

#### *Advanced Graphene-Based Materials for Electrochemical Biomarkers and Protein Detection DOI: http://dx.doi.org/10.5772/intechopen.114011*

hazardous gas generation were claimed. By using sonication and a lower reaction temperature, a more controlled sono-oxidative-exfoliation process is obtained [60, 68]. A new GO preparation route is proposed by Peng et al. [69], who used the potassium ferrate (K2FeO4) as the oxidant and concentrated H2SO4, as acidic medium for the reaction. Besides the higher quantity of GO that can be obtained in comparison with the other reported pathways, the approach presented a simple way of preparation with the additional benefit brought about by the recycling of H2SO4. In another pathway version promoted by Yu et al. [70], an oxidant mixture of K2FeO4 and KMnO4 was employed with boric acid (H3BO3) as a stabilizer in H2SO4. By involving such liquid phase exfoliation methods, GO nanosheets that are proper to form stable aqueous suspensions can be obtained. For sensor construction, the obtained 2D GO nanosheets have to be capable to form stable individual 3D structures as nanobricks or to form (nano)composites/modified structures by combination with other components.

Generally, the 3D structures of G materials can be grouped into three main classes: films (i.e., supported membranes), membranes (i.e., unsupported films), and porous structures (e.g., foams, aerogels/xerogels, frameworks, etc.).

In the case of G obtained by using the CVD technique and flat metallic support, a large-area continuous 2D film or membrane can be obtained after etching the support by using acid solutions. But, if the template is a porous 3D metallic structure (e.g., nickel foam, copper powder, magnesium, or aluminum oxide nanoparticles), a 3D G, as a coating film of metallic framework support, stabile hollow or macroporous scaffolds without metallic part can be achieved after the metallic support is etched [71]. Due to the lack of promoter-link groups on the surface of G it is difficult to obtain stable 3D structures if non-functionalized G nanosheet/nanoflake suspensions are involved. Besides, although the individual G nanosheets exhibit a very high electrical conductivity, because of the poor connection between the resulted 3D materials, a lower conductivity is measured. To overcome this drawback, the G nanosheets should be functionalized. The most known and used functionalized G material is GO. In this case, the most common methods for obtaining films are vacuum filtration, dip or drop-casting, spraying, screen printing, and the Langmuir–Blodgett method [62]. Due to the strong connection between functional groups of neighboring GO nanosheets, such as hydroxyl, carboxyl, carbonyl, and epoxy (see **Figure 1b**), very stable 3D laminar structures are formed as 3D GO films on the support materials after their drying. By removing (generally by a simple exfoliation) the forming support, stable free-standing GO membranes (i.e., unsupported films) can be obtained [68].

Besides this, Cotet et al. [60] proposed an elegant method that enables the harvesting of a stable membrane formed as a self-assembled pellicle at air-liquid interface after performing an isopycnic separation of a certain GO fraction. A proposed schematic distribution of functional groups present in the obtained dried GO membrane is illustrated in **Figure 2**.

For a deep understanding of the interaction processes between GO nanosheets that build up such a GO membrane, an assessment of the zeta potential was performed for initial GO suspension [72]. A variation in time of the zeta-potential values with high negative values (i.e., −63 and −67 mV) at about 3 weeks from the GO synthesis, followed by a stabilization (at about −45 mV) after 4–6 weeks, was observed. These high values of negative charge indicated a high density of functional groups present both on the plans and edges of GO nanosheets [72]. The presence of high number of functional groups is beneficial for functionalization due to the potential of attaching

**Figure 2.**

*Suggested 3D laminar GO structure with intraplanar (ovoid) or interplanar (rhombic) interactions, hydrogen bonding involving H2O (square) or not (rectangular), and ester linking (circular). Reproduced from Ref. [60] with permission from the Royal Society of Chemistry.*

specific groups or structures including sensitive biological molecules that can be involved in targeted sensing activity.

Porous 3D structures from the class of aerogel/xerogel/cryogel are obtained by drying in supercritical conditions of liquid CO2, in ambient conditions or by freezing-sublimation of wet hydrogels. A simple way to obtain hydrogels with GO nanosheets in their structure was promoted by Worsley et al. [73]. A GO suspension was involved as reaction medium for the polycondensation reaction of resorcinol with formaldehyde catalyzed by sodium carbonate (Na2CO3). In another approach based on a hydrothermal route [71], the added carbohydrates (e.g., glucose, cyclodextrin, and chitosan) played both the role of morphology-oriented agents and of reduction agents, which define the structural, physical, and electrochemical properties of the obtained 3D interconnected-pore aerogel.

A cheaper method for 3D GO porous structure synthesis without the necessity of complex equipment and supporting substrates is the room-temperature vacuum centrifugal evaporation of a properly concentrated GO suspension, which is described in the literature [71].

During the research and testing of GO materials for sensing applications, a very important step is represented by the reduction process that partially restores the structure and electrical properties of G. In this way, a conversion from insulating GO into electrically conductive rGO occurred with the increase of the C:O molar ratio. These reduction processes can be thermal, chemical, electrochemical, irradiationinduced, etc. [62, 68]. Among these approaches, the thermal and chemical reduction methods are identified as the most used techniques, and we will therefore refer to these methods later in this chapter.

In the case of GO materials modified with bio-structures, particular sensitivity (e.g., pH and temperature) of this kind of class of substances have to be taken into account. Moreover, if the modification will be carried out with more sensitive bio-structures, the reduction process, which is harsh, should be performed prior to this process. Additionally, it is known that the higher electrochemical properties of G materials are due to their intrinsic favorable properties represented by the π-π stacking interactions. These can enhance electrochemical signals and the irreversible anchoring of catalytic sites onto G materials. Thus, it is important that the new 3D structures formed present these structural advantages [68]. A method used to improve these properties is the doping of G material with nitrogen. Shao et al. [74] obtained nitrogen-doped G by exposing G materials to nitrogen plasma. The

*Advanced Graphene-Based Materials for Electrochemical Biomarkers and Protein Detection DOI: http://dx.doi.org/10.5772/intechopen.114011*

#### **Figure 3.**

*TEM (transmission electron microscopy) images of Fe-doped carbon aerogels showing G-like nanosheets (a and b); carbon nanoribbons, which are isolated (c) or around the Fe nanoparticles (a and c); and carbon nanotubes (d). Reproduced from Ref. [21] with permission from the Elsevier.*

as-obtained N-doped G materials produce the best result in the detection of hydrogen peroxide (H2O2) due to nitrogen and oxygen-containing groups. Interesting results were obtained by Fort et al. [21], who observed that the presence of Fe in the carbon aerogel matrix led to the formation of G-like structure as single-layer nanosheets (**Figure 3**), which increased the conductivity of the electrode materials and the electroanalytical performance of the obtained modified electrode for H2O2 detection [21].

The graphitic nanostructure formation in the presence of Fe and Bi was observed by Rusu et al. [75]. The graphitic structure obtained (**Figure 4**) is relevant in charge transport and thermo-chemical processes.

del Pino et al. showed that the use of the ultraviolet laser irradiation of flexible free-standing GO membranes in the ammonia-rich atmosphere and liquid environments generates significant integration of nitrogen groups, especially amines, in a partially reduced GO structure [68]. This method allowed for the obtaining of films with controlled geometry of reduced areas up to hundreds of square centimeters of N-doped rGO materials with high potential for (bio)sensing applications. So, flexible electrodes prepared on flexible polymeric supports with 10 μm thickness and low resistivity (6 × 10−4 Ω m) were obtained by an innovative laser reduction protocol [76].

There are various chemical agents that are used in GO reduction: hydrazine (N2H4), alcohols, sodium borohydride (NaBH4), hydriodic acid in acetic acid (HI/ CH3COH), sodium/potassium hydroxide (NaOH, KOH), iron/aluminum powder (Fe/ Al), ammonia (NH3), hexylamine (CH3-(CH2)5-NH2), sulfur-containing compounds, hydroxylamine hydrochloride (HONH2·HCl), urea (CH4N2O), manganese(II) oxide

#### **Figure 4.**

*Graphitic nanostructures evidenced in carbon matrix that contains bismuth-iron nanoparticles: (a) TEM and HR-TEM (high resolution-Transmission Electron microscopy) images after heating at 900°C with two highlighted regions during the in situ TEM experiment, (b) electron tomography results on a grain representative for the graphitized sample. On the left, a 2D projection extracted from the tilt series was used to reconstruct the grain volume; on the middle and right, two orthoslices took at different depths within the reconstructed volume evidencing a tubular morphology of the graphitic structure having a length of roughly 120 nm [75]. Reproduced from Ref. [75] with permission from Cambridge University Press.*

(MnO), enzymes, vitamin C, bacteria, etc. [77]. However, the most used reducing agent is hydrazine, which allows for the obtaining of rGO materials with a C:O of about 12.5:1 and a conductivity of 99.6 S cm−1 [78]. But, because of its toxicity, green alternatives are encouraged to be developed. From these, reduction with alcohols [79], hydriodic acid [80], and vitamin C [81] are reported [82].

Thermal reduction of GO materials consists of organic (i.e., hydroxyl, epoxy, carbonyl, carboxyl, etc.) group removal as O2, CO, CO2, and H2O by the aid of high temperatures (>120° C) [83]. Because hazardous reagents are not involved, this reduction method is considered to be safer than certain chemical reduction approaches. The degree of reduction can be adjusted by controlling the reaction temperature and the process time. A high temperature (of about 400–1050° C) and an oven equipped with an inert atmosphere (i.e., Ar or N2) or in hydrogen flow are required to obtain rGO materials with high electrical conductivity [62]. Nonetheless, even by the use of these methods, the materials obtained show a low surface and a smaller number of insulated oases (**Figure 1c**) due to the remaining functional groups from rGO materials that could be further involved in applications related to the sensing electrode construction.

To the best of our knowledge there has not yet been established a direct relationship between the morphology-structure of a composite and its sensing properties. However, there are several studies reported in which particular correlations between the structure and sensing properties were made by keeping in mind the possibility of improving the sensing performances as a result of understanding the structural particularities.

#### *Advanced Graphene-Based Materials for Electrochemical Biomarkers and Protein Detection DOI: http://dx.doi.org/10.5772/intechopen.114011*

Besides the excellent electrical conductivity, another remarkable property of G, when used in sensing applications, consists of its high specific surface area (theoretical specific surface area ~2630 m2 /g), which promotes the attachment of (bio) molecules, polymers, or nanoparticles to the G surface by exposing all carbon atoms to sense biomolecules. These results indicate an increased G's sensitivity as biosensor [84]. After synthesizing cobalt oxide (Co3O4) nanowires using a hydrothermal method based on 3D G foam grown by CVD, Dong et al. used this composite (G/ Co3O4) as a monolithic free-standing electrode for free-enzyme electrochemical detection of glucose. The porous structure of the G foam proved to play the role of a scaffold for the sensors fabrication [85]. A double-layered membrane of rGO-andpolyaniline (PANI) nanocomposites (to enhance the sensitivity of sensor) and molecularly imprinted polymers decorated with gold nanoparticles was prepared by Xue's research team in order to obtain an electrochemical serotonin sensing interface. When the morphology of the material was studied by SEM (Scanning Electron Microscopy), a uniform distribution of rGO/PANI nanocomposites spheres with the diameter of the single sphere of approximately 93 nm was observed. A rough and foliated structure embedded with AuNPs was observed. This indicates the achievement of imprinted membrane embedded with AuNPs, which led to an increased conductivity and electrocatalytical activity of the membrane [37]. By employing electrophoretic deposition in a magnesium salt/GO electrolyte, Akhavan et al. prepared GO nanowalls with extremely sharp edges and preferred vertical orientation, and then they deposited them on a graphite electrode. The results revealed a uniformly deposited film of GO nanowalls on the electrode surface. Interesting petal-like G nanosheets with lateral sizes of ∼500 nm and very sharp edges (1–15 nm thickness) were observed. Because of the formation of a large fraction of graphitic edge-plane defects that can lead to the obtaining of a higher surface activity than the G nanosheets placed parallel to the substrate, the authors speculate that such vertical nanosheets may exhibit distinctive electrochemical properties [86]. By SEM investigation [60] of free-standing selfassembled GO membranes (**Figure 5**), a continuous wavy feature (**Figure 5a**) and a compact layer-by-layer stacking (**Figure 5b**) were observed at the surface and the cross-section of obtained dried GO membranes.

In the meantime, in the case of this preparation pathway, by changing the time used for self-assembled process from 15 to 120 min, GO membranes with about 2.85 μm and 11.40 μm thickness, respectively, were obtained. This kind of GO membrane type could be properly reduced (e.g., by protected laser irradiation, **Figure 5**, [76]) to

#### **Figure 5.**

*SEM images of the self-assembled GO membrane surface (a), and its 3D view (see the inset), the cross-section (b) obtained by dispersing a certain GO fraction on an open area framework support and harvesting it after 15 min, and the average membrane thickness evolution against the self-assembly time (c) [60]. Reproduced from Ref. [60] with permission from the Royal Society of Chemistry.*

obtain 3D rGO membranes or films with controlled geometry, reduction degree, and morpho-structural properties suited for biosensor construction.

#### **3. Biosensor applications**

As already reported, the attractive properties of G-based nanomaterials can be successfully used in biomedical applications as electrochemical-based biosensors with improved analytical performance with respect to low detection limits, selectivity, high sensitivity, low response time, reproducibility, and large linear detection range [54, 56, 59, 71]. In addition, a significant improvement in the sensors is mechanical properties, relating to flexibility. An important work focused on the importance of flexible sensors was reported by Giaretta et al. [87]. They compared the biosensors performances based on the obtained material substrates. It was concluded that the polymeric substrates are the cheapest, but their performances were inferior when compared to those of the other substrates obtained from carbonaceous materials. This is due to the poorer electrical conductivity, reduced permeability and lack of porosity, respectively.

A bioelectrochemical sensor transforms a biological modification occurrence into an electrical signal. Hence, the working principle is based on the transfer of a negative elementary charge (the electron) between the G interfaces and species that present electrical activity. The species can be either the molecule to be analyzed or a species whose electrochemical measured signal can be associated with the existence of the target analyte.

#### **3.1 Biomarkers and protein detection**

Real-time quantitative monitoring of biomarkers, which represent a particular class of biological compounds whose presence in serum and saliva shows a relationship with a certain disease, has become essential for early disease detection, leading to adapted treatments and investigation of treatment efficiency. Guo et al. synthesized Ni nanosheet/G composites, which have an exceptional electrocatalytic signal for the detection of L-alanine by using a direct current (DC) arc plasma jet CVD method [88].

The obtaining of 1 to 10-layered G nanosheets, G nanoribbons, and core-shell Ni/G nanosheets by the above-mentioned method was evidenced. These G-based composites were used as sensors to detect proteins. It was revealed that the higher value of the specific surface area of the G, the higher adsorption ability for L-alanine, and the good transfer of the electron between the Ni nanosheet/G composite and the surface of glassy carbon electrodes (GCE) had considerably improved the performances of the obtained sensor (**Table 1**). Furthermore, by means of the obtained sensor, without enzyme presence, the direct electrooxidation process of amino acids was achieved [88].

Zhang and co-workers developed functionalized graphene oxide (FGO), which exhibited high affinity to (His)-tagged acetylcholinesterase (AChE) for paraoxon manufacture, an acetylcholinesterase inhibitor, biosensors [89]. In order to estimate the functionalization of GO and their capacity to be involved as multipurpose enzyme immobilization nanomaterials for the bioelectrochemical sensor design, the AChE was nominated as being a perfect enzyme. The authors evidenced the existence of an optimum amount of Nα,Nα-bis (carboxymethyl)-L-lysine hydrate (NTA-NH2) and

*Advanced Graphene-Based Materials for Electrochemical Biomarkers and Protein Detection DOI: http://dx.doi.org/10.5772/intechopen.114011*


**Table 1.**

*Electrochemical parameters of some G-based biosensors for different analytes detection.*

that Ni2+ (Ni-NTA) needed to attach on the GO surface. This connection has succeeded to an increased enzyme loading, which lead to an enhanced electrocatalytic activity and sensitivity. This work proved excellent stability, for both short-term and long-term, being the effect of the stable binding between Ni-NTA and His-tagged AChE. The paraoxon concentrations also influence the inhibition response, and a low detection limit value was reached (**Table 1**). The authors showed that the obtained FGO composite can be changed to multipurpose biosensor construction [89].

#### *3.1.1 Glucose*

Glucose, a monosaccharide, serves as an energy source and metabolic fuel and is involved in the processes of photosynthesis and respiration in most organisms. The high blood-glucose concentration, recognized as hyperglycemia, or the reduced glucose presence, identified as hypoglycemia, is caused by the effect of insufficiency of insulin in the body, known as Diabetes mellitus, an incurable disease. By monitoring the glucose concentration in blood, as an illness marker, it is possible to extend life expectancy. Thus, people with diabetes, people who have problems with glucose concentration in blood, can manage episodes of hypo- or hyper-glycaemia, hence providing improved control over their conditions and avoiding some of the incapacitating side effects. Moreover, by monitoring the glucose, the patient's treatment strategies can be optimized, and also, the effect of medications, physical exercise, and nutrition on the patient can be controlled.

Xue and co-workers prepared a biocompatible AuNPs/PPy/rGO nanocomposite that demonstrated exceptional electro-catalytical activity toward O2 reduction [37]. By encapsulating glucose oxidase (GOD) *via* chitosan (N-deacetylated derivative of chitin) cross-linking in the obtained composite, AuNPs/PPy/rGO/GOD/chitosan modified electrode was fabricated. The developed electrochemical sensor revealed a linear range for glucose from 0.2 to 1.2 mM with a good sensitivity value of 123.8 mA M−1 cm−2 (**Table 2**). The obtained performances were correlated with (i) the high surface area and high electric conductivity of rGO, (ii) good shielding of PPy and fixing capacity on rGO surfaces, (iii) biocompatibility of the dispersed small AuNPs and their electron transfer promotion ability. The authors showed that the enzyme, GOD, has a good connection with AuNPs/PPy/rGO nanocomposite, which was proved by the fast electron transfer between the used enzyme and electrode. Also, between the electrode surface and the enzyme, there seems to be a rather low barrier. Furthermore, the achieved AuNPs/PPy/rGO ternary nanocomposites can be favorable for encapsulating some other biomolecules, indicating, thus, a potential substitute in constructing multipurpose bioelectrochemical sensors and additional devices [37]. Chitosan, due to its uncommon mixture of properties including the exceptional membrane-forming capability, good bond, high mechanical strength


*GCE—glassy carbon electrodes, CS—chitosan, His—histidine, GOD—encapsulated glucose oxidase, AuNPs gold nanoparticles, PPy—polypyrrole, GDH—glucose dehydrogenase, PTZ-O—phenothiazone, PAMAM poly(amidoamine), GNs—G nanosheets.*

#### **Table 2.**

*Electrochemical parameters of some G-based modified electrodes for glucose and H2O2 detection.*

and water permeability, and extraordinary biocompatibility, is one of the best-suited biopolymers for the surface-deposition of nanocomposite for the electrode fabrication [45, 46, 92, 93].

With the aim of encapsulating redox enzymes, rGO film with adsorbed phenothiazine represents another composite that was synthetized by Ravenna et al. [90]. The prepared composite was very efficient for the electron transfer process between flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase and the obtained modified sensor. The study showed that for the glucose oxidation process, the determined redox potential was lower than 0 V vs. Ag/AgCl reference electrode. Furthermore, the obtained rGO-based biosensor presented an increased value of sensitivity and a large linear range for glucose detection (**Table 2**). Also, the obtained biosensor achieved reasonable reproducibility and stability, a great selectivity for different interfering compounds. The results demonstrated a promising sensor for several bioelectrochemical applications [90].

Luo and co-workers showed that by combining the rGO with poly(amidoamine) and silver (rGO–PAMAM–Ag), the newly developed nanocomposite offers an exceptional microenvironment to assure the direct electron transfer process of GOD enzyme fixed on the modified surface of GCE [40]. Besides, it was confirmed that the developed arrangement can preserve a great electrocatalytic activities of the enzyme. By using GO self-assembled with PAMAM-G3.5, as a growth pattern, and microwave irradiation, rGO-PAMAM-Ag new nanocomposite electrode material was prepared. Then, based on this type of sensitive nanocomposite, a biosensor for glucose detection was manufactured. A high value of sensitivity, a low value of the detection limit, and a wide linear range, were the analytical performances of the obtained biosensor (**Table 2**). The authors also showed that the interference between the signals originating from uric acid (UC) and ascorbic acid (AA), which are regularly detected in blood fluids together with glucose, is insignificant compared to the signal attained by the glucose biosensor. The achieved characteristics recommended the obtained rGO-PAMAM-Ag nanocomposite as an innovative exceptional electrode material for the construction of glucose biosensors, involving a direct electron transfer process [40].

#### *Advanced Graphene-Based Materials for Electrochemical Biomarkers and Protein Detection DOI: http://dx.doi.org/10.5772/intechopen.114011*

An interesting study is related to the involvement of an easy synthesis path for 3D G/Co3O4 composite production, which was further used as a sensor for glucose [85]. Thus, the synthesis process was based on the (i) Co3O4 nanowires achievement by *in situ* hydrothermal synthesis, and (ii) CVD growth of G foam. Through this study, it was shown that Co3O4 nanowires possess high crystallinity and constant diameter and form a compact nanomesh covering the 3D G matrix, which can function as a freestanding electrode due to the higher mechanical strength of G. 3D G/Co3O4 obtained composite was employed as electrode material and the acquired results proved significant in performance as sensor for glucose and as a supercapacitor. These results were based on the synergistic incorporation of the two, G and Co3O4, nanomaterials and represent an important step for developing enzyme-free ultrasensitive sensors for glucose. Co3O4 nanowires proved excellent electrochemical and electrocatalytic properties (**Table 2**). The authors showed that the 3D multiplexed and extremely conductive matrix provided by the defect-free G foam depicted fast electron transfer and conduction, and offered a high available active surface area. In addition, the open pores of 3D G/Co3O4 composite were found to be advantageous to ion diffusion and transport kinetics. Thus, the G foams showed unique morphological properties, and these could serve as 3D supports for embedding great capacity to homogeneously bind metal oxides with well-defined properties (size, shapes, and crystallinity), as demonstrated by Dong et al. [85]. Agglomeration, which represents a regular fact of metallic oxide synthesis, is no longer a problem. The multifunctional character and the improved performance of composites based on G and metal oxides were achieved thanks to the synergies between components, representing a positive aspect for developing novel applications [85].

In their work, Dhara et al. presented an easy and low-cost fabrication process with no enzyme-limited screen-printed electrodes [35]. Thus, it was possible to synthesize Pt-CuO/rGO nanocomposite electrocatalyst by a one-step chemical reduction. The electronic microscopy measurements of the composite revealed the nanocubes structure for Pt, and nanoflowers structure for CuO. The obtained sensor for glucose oxidation process exhibited exceptional sensitivity (**Table 2**) and good selectivity. The acquired linear response for glucose and the detection limit value are presented in **Table 2**. When the newly developed sensor measured the glucose quantity in blood sample, the results could be considered as being satisfactory [35].

Another study that is worth mentioning was performed by Liu et al. and was focused on glucose detection. They demonstrated that by the use of a non-enzymatic sensor composed of G nanosheet-covered Cu2O nanocubes (Cu2O/GNs), good sensor's characteristics for glucose oxidation could be obtained (**Table 2**) [52]. They also showed that Cu2O/GNs hybrids can be produced by an easy technique at low temperatures, and additionally, an outstanding electrocatalytic activity for electroreduction process of H2O2 can be obtained (**Table 2**). Therefore, electroanalytical detection of glucose and H2O2 was achieved, and the developed sensor revealed low limits of detection, good selectivity, good linear response, and a large detection range due to the enlarged electroactive surface area and the enhanced capacity for electron transfer of the fabricated G-hybrid nanostructure. Furthermore, the authors affirmed that due to the surface covered by G nanosheets, the Cu2O/GNs composite revealed enhanced electrochemical stability related to that acquired for Cu2O nanocubes only. The innovative Cu2O/GNs nanocomposite led to new possible applications in different categories of biosensors, such as sensitive electrode materials, bioelectronic devices, and (electro)catalysts [52].

By a simple electrodeposition method without any template, Chu et al. synthesized *in situ* an originally designed Au nanocubes/G composite film [91]. The obtained composite film possessed exceptional performance for glucose detection under a low potential value of −0.4 V *vs.* Ag/AgCl (**Table 2**). A possible explanation for this behavior was the synergistic consequence of regular morphology and uniform isolated distribution. The authors stated that Au nanocubes/G composite film presents the possibility of being used in applications for trace analysis for new physiological activators by changing the corresponding proteins. The high catalytic activity and conductive capacities of the new composite material represent important properties that were demonstrated not only in biosensor construction but also offer the possibility of being successfully used in other studies closely related to electrocatalysis [91].

#### *3.1.2 Hydrogen peroxide*

The production of reactive oxygen species represents a process that can be associated with early signs of cancer or neurodegenerative diseases such as Alzheimer's, Parkinson's, and multiple sclerosis. Precisely, reactive oxygen species appear as a consequence of changed cellular metabolism rising from a given disease state. These species are extremely reactive and induce redox reactions of cell structures leading to activation of immune response and apoptosis. Between these, hydrogen peroxide (H2O2) was intensively investigated as analyte to describe disease conditions. Easier H2O2 detection is as well a challenge in various domains like medicine, manufacturing, food production, and pharmaceuticals. Among the variety of possible applied techniques, electrochemistry consists of the simplest way of getting rapid and accurate results while demanding only a simple device, and it is realized by analyte oxidation or reduction (**Figure 6**) [21, 94–96].

By electrochemical reduction of GO-horseradish peroxidase (GO-HRP) to rGOhorseradish peroxidase (rGO-HRP) a new biocomposite was obtained by Selvakumar et al. [97]. The enzyme immobilization process was facile, and the obtained composite material was employed for the preparation of rGO-HRP modified screen-printed carbon electrode (SPE) in order to detect H2O2. The obtained sensor characteristics were a wide linear range, high selectivity, and good stability, which recommended its extended practical applications. The sensitivity value toward H2O2 reduction is

#### **Figure 6.**

*I vs. [H2O2] calibration curve recorded at (Fe-CA)-CPE (a) and undoped CA-CPE (b). Inset: amperograms recorded at an applied potential of 0.3 V vs Ag/AgCl, KClsat, and a rotating speed of 500 rpm, in 0.1 M phosphate buffer (pH 7) at (Fe-CA)-CPE (a) and CA-CPE (b) for consecutive addition of 1 mM H2O2. Reproduced from Ref. [21] with permission from the Elsevier.*

presented in **Table 3**. The authors suggested that the fabrication process used for obtaining of rGO-HRP biocomposites can be utilized for the construction of G-based composite materials of a large range of electrochemically essential molecules with redox properties [97].

Elsewhere, an innovative biosensor based on GCE, which was modified with the AuNPs and hemin-G nanosheets (H-GNs), was successfully fabricated by Song et al. [98]. Their work demonstrated that the newly fabricated H-GNs/AuNPs/GCE bioelectrochemical sensor exhibited an improved electrocatalytic activity for H2O2 reduction process, when compared with the AuNPs/GCE, H-GNs/GCE, and bare GCE. The authors proved that the improved electrocatalytic signal was the result of two properties: (i) the improved specific surface area of the electrode surface, and (ii) the great loading of the H-GNs on the modified electrode surface. Therefore, an enhanced synergistic electrocatalytic influence was revealed concerning the AuNPs and H-GNs. Enhanced electroanalytical parameters of the developed biosensor (high value of sensitivity, wide linear response range, good stability and reproducibility, fast response time, and good analyte specificity) were the result of the new sensor material properties (**Table 3**) [98].

Another interesting research was done by Zhou et al., who achieved a new H2O2 biosensor based on the modification of GCE using G, chitosan CS, Au, and HRP [23]. They prepared individual G sheets by the insertion of ∙SO3∙ radicals. By EDS (Energy Dispersive Spectroscopy) and TEM measurements, it was proved that the reduction and sulfonation techniques used for sensor material preparation did not produce the destruction of G morphology. As previously observed, the unaltered G structure is significant for the maintenance of the exceptional properties of G. By cyclic voltammetry techniques, the authors proved the existence of a direct electron transfer process among the fixed enzyme (HRP) and the surface of the electrode. Thus, a usual electrocatalytic reduction process of H2O2 occurs on the electrode surface. This work showed that in identical experimental conditions, the recorded current response of HRP/CS/GCE, Au/HRP/CS/GCE, sulfonated and reduced G/HRP/ CS/GCE, and Au/sulfonated and reduced G/HRP/CS/GCE can significantly enhance


*GCE—glassy carbon electrodes, CS—chitosan, His—histidine, AChE—acetylcholinesterase, GOD—encapsulated glucose oxidase, AuNPs—gold nanoparticles, PPy—polypyrrole, GDH—glucose dehydrogenase, PTZ-O phenothiazone, PAMAM—poly(amidoamine), GNs—G nanosheets, HRP—horseradish peroxidase, SPCE—screenprinted carbon electrode, H—hemin, Hb—hemoglobin.*

#### **Table 3.**

*Electroanalytical parameters of some G-based electrodes for H2O2 detection.*

the sensitivity of the developed biosensor due to the G structures presence (**Table 3**). It was also stated that a wide linear range, low detection limit, and long-term stability represent other excellent characteristics of this type of sensor [23].

The easy, low-priced, and highly sensitive and selective amperometric assessments employed for H2O2 and glucose detection, based on hemin functionalized G nanosheets (H-GNs), were developed by Guo et al. [99]. The obtained H-GNs hybrid nanomaterial combines the G nanosheet properties of high electrical conductivity and high surface area value with hemin properties (i.e., exceptional electrocatalysis and synthetic enzyme simulation). The study showed that the H-GNs are able to compete with the natural enzymes. The advantages of the obtained H-GNs are their facile synthesis method, the strength of materials, and the stability of materials in irregular conditions. Moreover, due to their excellent recorded results (**Table 3**), the obtained biosensor presents favorable possible applications in different fields, including clinical diagnostics, biotechnology, and chemical or pharmaceutical industry [99].

Another electrochemical biosensor for H2O2 selective detection was established by Nandini et al. [17]. The preparation method of this new electrode was realized by a co-deposition process of palladium and HRP, on the functionalized G-modified graphite surface electrode. The fabricated biosensor revealed an increased electrocatalytic activity concerning the reduction process of H2O2 at 0.02 V *vs* SCE. It was found that Pd, by its presence, decreases the over-potential of H2O2 reduction and grows the active surface area of the modified electrode. Furthermore, the HRP and Pd co-deposition escapes the poisoning occurrence of the modified graphite-based electrode. The sensor performance exhibited a fast response within less than 2 s and exceptional linear concentration range under determinate optimal experimental conditions (**Table 3**). For the acquired properties of the biosensor (i.e., selectivity, repeatability, feasibility and stability), acceptable results were obtained. The presented technique could be applied to produce biosensor for a wide range of applications [17].

Based on the hemoglobin (Hb) immobilized on an rGO, flower-like ZnO, and AuNPs nanocomposite modified GCE (AuNPs/ZnO/rGO/GCE), a new amperometric H2O2 biosensor was proposed by Xie and co-workers [36]. Each biosensor component was found to have an important contribution to the H2O2 reduction process. Therefore, the ZnO flower-like nanoparticles exhibit good biocompatibility and conductivity. Then, the ZnO-aminopropyl triethylene silane (APS)-AuNPs composite was found to have good uniformity and be suitable for protein attachment. rGO possesses high specific surface area and can increase the ZnO-APS-AuNPs composite conductivity and its mechanical resistance. The authors showed that by combining the advantages of each component used for bioelectrical sensor preparation (i.e., nanosized ZnO, AuNPs, and rGO), the Hb/AuNPs/ZnO/rGO/GCE), the obtained modified electrode can lead to acceptable sensors performances of high sensitivity, satisfactory construction reproducibility, and good storage stability. Their study has shown that AuNPs/ZnO/rGO/GCE amperometric third-generation biosensor offers an advantageous application for nanoparticles-based electrode materials in order to be employed in the investigation of direct electron transfer of proteins and the improvement of biosensors (**Table 3**) [36].

It is also important to reveal that the study performed by Liu et al., which focused on glucose detection and H2O2 reduction, demonstrated that with the non-enzymatic electrochemical sensor fabricated from G nanosheet-wrapped Cu2O nanocubes (Cu2O/GNs), good sensor characteristics for glucose oxidation can be obtained [52]. Also, they proved that Cu2O/GNs hybrids can be produced by an easy process at low temperature and that they exhibit excellent electrocatalytic activity toward

#### *Advanced Graphene-Based Materials for Electrochemical Biomarkers and Protein Detection DOI: http://dx.doi.org/10.5772/intechopen.114011*

H2O2 reduction. Therefore, they tested the electrochemical detection capacity of glucose and H2O2 (**Table 3**). The developed sensor revealed good performances. An explanation for this behavior can be the enhanced electrocatalytic surface area of the electrode and the high conductivity of the G-hybrid nanostructure, reflected in the improved electron transfer ability through this matrix. Additionally, the authors stated that their synthesized Cu2O/GNs structure revealed enhanced electrochemical stability in contrast with the Cu2O nanocubes alone due to the G nanosheets that covered the oxide nanocubes. The advanced Cu2O/GNs nanocomposite material opens up new opportunities for applications in several varieties of biosensors, bioelectronic devices, and catalysts [52].

Some advances in *in vivo* electrochemical analysis of H2O2 were reported by Deng and co-workers. Their interest was directed toward the electrochemical redox process of H2O2 at the electrode surface and on the diversity of catalysts (synthetic electrocatalysts or biomolecular electrocatalysts) used for improved electrochemical analysis of H2O2. A higher selectivity and sensitivity of the proposed biosensors were followed. The new technique, the photoelectrochemical (PEC) method for H2O2 detection was also discussed. The developments in the high selectivity analysis of H2O2 at the cellular and *in vivo* levels was highlighted [30]. Another review focused on the G as electrode materials for (bio)sensors applications was written by Shao et al. [32], who demonstrated the performances for hydrogen peroxide, NADH, dopamine, DNA, or heavy metals detection.

An important work dedicated to the significance of flexible sensors on H2O2 detection was realized by Giaretta et al. [87]. They compared the electroanalytical parameters for H2O2 detection obtained on different biosensors based on various material substrates. It was concluded that the carbonaceous materials are better in comparison with the polymeric substrates, which are the cheapest. The carbon materials-based biosensors results were based on the corroborated effect of the increased electrical conductivity, increased permeability, and increased porosity, respectively.

Different types of materials (noble metals, metal oxides, polymers, carbon materials, and other two-dimensional materials) employed in sensor development for H2O2 detection were presented in the review work of Yu and co-workers [100]. Additionally, their work presents the challenges and future prospects in the biological applications of electrochemical sensors for H2O2 detection.

#### *3.1.3 Neurotransmitters and neurochemicals*

Neurotransmitters are endogenous compounds that permit the transmission of nerve impulses between two neurons or between neuron and effector, named 'target' cell. Thus, the nervous system is based on the role of chemical couriers of the neurotransmitters, which transfer the information across the synapses by excitation or inhibition of the subsequent neural or effector cell. Neurotransmitters are ordered into monoamines (histamine, adrenaline, dopamine (DA), noradrenaline, serotonin, and melatonin), amino acids (aspartate, D-serine, glutamate, gamma-aminobutyric acid, and aminoacetic acid (glycine)), peptides (somatostatin, cocaine, and opioid), and other (including acetylcholine, adenosine, anandamide, and nitrogen monoxide) [101].

#### *3.1.3.1 Dopamine, ascorbic acid, uric acid*

It is well-known that monoamine neurotransmitters have in their structure one amino group connected by a chain of two atoms of carbon (∙CH2∙CH2∙) with

an aromatic ring. These types of neurotransmitters have a significant importance in secreting and producing neurotrophins via astrocytic glial cell. It is an essential local cellular source of trophic support, present both in healthy and unhealthy brain. Growth of neutrophins stimulates the survival of neurons and is known as neurotrophic factors. Based on the neuron behavior, the neurotransmitters act antagonistically, namely neurotransmitters that play an inhibitory role to relax the brain and those that play an excitatory role in stimulating the brain [102]. Dopamine (DA) owns both excitatory and inhibitory classification, being thus a unique neurotransmitter. DA vital roles lie in adjustable attention, motor control, cognition, executive functions, pleasure, motivation, arousal, reinforcement, reward, and hormonal processes. Also, dopamine is extensively dispersed in the main systems of the human body, such as central nervous, renal, hormonal, and cardiovascular.

Neurological problems in the human body can be due to the anomaly in the amount of dopamine. Thus, the illness such as Parkinson's disease (degenerative disorder of the central nervous system that mainly affects the motor system), restless leg syndrome (like Parkinson's disease, this is another long-term disease that manifests itself through the uncontrolled movement of the legs), attention deficit hyperactivity disorder (ADHD) (characterized by lack of ability to concentrate, hyperactivity and impulsivity), schizophrenia (a disease in the psychiatric spectrum in which episodes of psychosis occur), and infection with human immunodeficiency viruses (HIV) (a virus that over time can cause AIDS—the gradual collapse of the immune system leading to the onset of opportunistic infections and cancers) are strongly associated with a low level of dopamine. Also dopamine is also greatly correlated with the reward mechanism in the brain. On the other hand, the consumption of prohibited drugs or substance abuse leads to an increase in dopamine levels. Thus, the forbidden substances such as heroin or cocaine, and not compulsory substances such as nicotine or alcohol, block the DA carrying that inhibits the reuptake of dopamine. As was shown before, dopamine, a neurotransmitter, which is vital for message transfer functions, produces, in this case, an amplified risk of depression and drug dependence. Thus, the discovery of a reliable analytical technique is significant and necessary in order to estimate the disease evolution.

A simple and "green" technique to produce G flowers that were exploited to modify carbon fiber electrode (CFE) in order to detect AA, DA, and UC was employed by Du et al. [34]. The G flowers, well deposited on the surface of CFE, detect separately electroactive compounds as AA, DA and UC. Besides, this study proved that the obtained G flowers based modified electrodes, can detect simultaneously AA, DA and UC, with distinct signals from each other. Thus, great electrocatalytic activities of the electrode material toward the oxidation process of electroactive AA, DA and UC, good selectivity and sensitivity, were demonstrated (**Table 4**). Excellent performance was obtained with the modified electrode when the detection of real samples was proposed. The obtained results offer appreciable evidence for the use of G as a sensitive modifier material electrode in order to detect electroactive biomolecules [34].

Using a solution-gated G transistor (SGGT) with a G gate electrode, a very sensitive dopamine electrochemical sensor was achieved by Zhang et al. [103]. The detecting mechanism of the obtained system was ascribed to the electrooxidation process of DA at the gate electrode. This electrode changed the potential distribution at the boundaries between the G gate electrode and the G channel. A perfluorinated membrane with ionic properties and excellent selectivity for dopamine was obtained after the addition of a thin layer of Nafion to G gate electrode. The obtained sensor showed a limit of detection for dopamine down to 1 nM. This represents a worthy result for

*Advanced Graphene-Based Materials for Electrochemical Biomarkers and Protein Detection DOI: http://dx.doi.org/10.5772/intechopen.114011*


*CS—chitosan, AA—ascorbic acid, DA—dopamine, UA—uric acid, GEF/CFE—G flowers/carbon fiber electrode, SGGT—solution-gated G transistor, NG—nitrogen-doped G, MIP—molecularly imprinted polymer, ZnO NWAs/3D-GF—nanowire arrays fabricated on 3D G foam.*

#### **Table 4.**

*Electroanalytical parameters of some G-based electrodes for ascorbic acid, uric acid, and dopamine detection.*

dopamine detection in medical uses. The dopamine interference measurements with AA and UA showed good selectivity of the device, proved by a recorded signal up to four orders of magnitude lower when compared with that obtained for dopamine. Since the channel and the gate of the device are both made of G, they have the advantage that they can be produced on different substrate materials (containing flexible, elastic, pliable, and stretchy ones) at low temperatures by suitable procedures. Centered on the equivalent mechanisms, various additional types of biosensors could be imagined and designed in the following years, and the whole-G SGGT represents a good solution for one-use, bendable, and very sensitive biosensors [103].

Li and co-workers demonstrated that nitrogen-doped rGO (N-rGO) with a very porous matrix and adjustable structure can be produced in three steps: first, the molecular functionalization; second, the fast thermal expansion–exfoliation, and third, the covalent binding [53]. The nitrogen-doped configurations, controlled by varying the temperature of expansion–exfoliation process, were used for the preparation of screen-printed electrodes. Judging by the potential value of the peak current recorded to the oxidation process of AA, DA, and UC among the nitrogen-doped sample, the pyrrolic-N revealed the maximum electrocatalytic activity. Nevertheless, the corresponding peak currents, of the oxidation of AA, DA, and UC, are related to the corroborated effect of the nitrogen-doped sample distribution, and structural properties (specific surface area and porosity) and the electroanalytical parameters presented in **Table 4**. The prepared SPEs exhibited high peak currents (for biomolecules' electrooxidation process), good selectivity (good peak separation), and sensibility for the detection of AA, DA, and UC from a blend [53]. A molecularly imprinted polymers (MIPs) based on Chi–G composite, as the functional matrix, was used by Liu et al. to develop a sensor for DA electrochemical detection [44]. Thus, the obtained MIPs-GR composite was used to modify GCE, in order to fabricate the sensor (MIPs-G/GCE). The improved sensitivity and low value of the detection limit

of MIPs-G/GCE sensor for DA oxidation (**Table 4**) can be explained based on the special characteristics of G. The selectivity, stability, and reproducibility remained just as for the MIPs sensor. It was stated that the achieved information could offer a possible rapid and reliable method for DA determination in biological samples [44].

Another significant G-based sensor for dopamine was produced by Yue et al. [51]. Thus, the detection of three biomolecules (UA, DA, and AA) was realized by employing 3D G foam containing ZnO nanowires, vertically arranged at the electrode surface. Differential pulse voltammetry (DPV) technique was used for UA, DA, and AA electrochemical detection. The new structural design combined (i) the large mesoporous surface area 3D G structures that facilitated easy diffusion of ions through the electrode material, with (ii) the increased conductivity of 3D G foam, which led to a good electron transfer process, and (iii) the active sites of ZnO nanowires, which assure a high selectivity. The present study proved that the UA, DA, and AA selectivity was the result of thermal annealing of ZnO surface.

A high selectivity and a low value of detection limit for UA and DA were obtained with the optimized ZnO nanowire/3D G foam electrochemical sensor (**Table 4**). The obtained results were clarified by the gap variance among the LUMO (lowest unoccupied molecular orbitals) and HOMO (highest occupied molecular orbitals) of a biological molecule for a set of specified electrodes. For the Parkinson's disease test, the UA level was 25% lower than in healthy individuals. It was concluded that the reported work can open new perspectives for UA application as a biomarker for Parkinson's disease, which can offer better medical diagnostic control in addition to the possibility of tracking the disease [51].

#### *3.1.3.2 Serotonin*

Several chemically different types of G nanosheets were synthesized by Kim et al. with the aim of using them as electrocatalysts for serotonin (5-hydroxytryptamine, 5-HT) [24]. In order to estimate the G nanosheets surface morphologies, X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FE-SEM) techniques were used. By electrochemical impedance spectroscopy (EIS) technique the electrocatalytic activity was investigated. In this study, the three different arrangements of rGO obtained did not reveal some significant changes in the obtained XPS spectra and FE-SEM images but showed dissimilar electrochemical performance. Thus, EIS recorded spectra exhibited a dissimilarity in the electron transfer resistance. This result is in good agreement with the reducing agent. The obtained parameters for 5-HT determination and the acquired EIS results were in agreement as well. The rGO-based GCE sensor obtained for 5-HT detection exhibited high sensitivity, good selectivity, and smaller electron transfer resistance compared with previously obtained sensors. Between the evaluated G-modified GCEs, the best electrochemical sensor properties (i.e., lowermost detection limit, uppermost sensitivity and selectivity, broadest linear range, fastest response time, alongside the greatest defined peak of 5-HT) were obtained for rGO reduced using hydrazine and ammonia solution (**Table 5**). This study proved an irreversible diffusion-controlled electrode process for a 5-HT electrooxidation reaction mechanism [24].

A two-layered membrane sensing interface for serotonin detection was constructed by Xue et al. [37]. The production of the obtained device was based on the use of nanosized rGO/polyaniline (PANI) composites and molecularly imprinted polymers (MIPs) surrounded with AuNPs (AuNPs@MIPs). With the aim of obtaining a good sensitivity and selectivity of the elaborated device, the rGO/PANI

#### *Advanced Graphene-Based Materials for Electrochemical Biomarkers and Protein Detection DOI: http://dx.doi.org/10.5772/intechopen.114011*

nanocomposites were produced by the electrodeposition method. First, the protonated anilines were anchored by electrostatic adsorption on the rGO sheets. Afterward, the rGO/PANI nanocomposite film was made by cyclic voltammetry process on the surface of bare GCE. Over the nanosized rGO/PANI composites membrane, the AuNPs@MIPs were deposited. Interestingly, the obtained material interface showed improved properties for (i) selectivity to 5-HT, (ii) electric conductivity, and (iii) electrocatalytic activity (**Table 5**). The new AuNPs/PPy/RGO/GOD/chitosanmodified electrode was efficaciously applied toward 5-HT detection in human serum specimens. In the meantime, the interferences produced from AA, DA, UA, and epinephrine (EP) did not affect the 5-HT detection. Therefore, the approach was recommended by this research team for a sensitive and selective detection of targeted biomolecules in real sample [37].

#### *3.1.3.3 Epinephrine*

Adrenaline or epinephrine, manufactured by the suprarenal glands and certain neurons, is a hormone neurotransmitter. It is also a medication and fulfills a pivotal function in the fight-or-flight reaction. Thus, under its action, the blood circulation to muscle tissues, cardiac output, pupil dilation, and blood sugar increase. This happens because of the adrenaline binding to alpha and beta receptors. Due to its importance, many research works were focused on this subject [18, 25].

In order to prepare a sensor for epinephrine (EP), Cui et al. started from a method based on chemical reduction of both Au (III) and GO, to prepare rGO/Au nanocomposites [25]. Then, the prepared rGO/Au nanocomposites, used as modifier-sensitive material for GCE surface (rGO/Au/GCE) were found to confirm an improved electrochemical activity toward EP (**Table 5**). The role of gold nanoparticles (nano-Au) included in rGO matrix was that of a spacer in order to avoid the rGO sheets aggregation. The achieved rGO/Au/GCE proved high sensitivity for the detection of EP. The recorded CV in the presence of ascorbic acid (AA), which show total peak separation between EP and AA, demonstrated good electrochemical sensor selectivity. Furthermore, rGO/Au/GCE revealed exceptional electron transfer capacity


*GCE—glassy carbon electrodes, GO—graphene oxide, rGO—reduced graphene oxide, AuNPs—gold nanoparticles, PANI—polyaniline, CEA—carcinoembryonic antigen, MBs—magnetic beads, Ab—antibody, AFP—α-fetoprotein, PB—Prussian Blue, Thi—thionine.*

#### **Table 5.**

*Electroanalytical parameters of some G-based electrodes for different types of analytes.*

and exceptional electrocatalytic ability to additional biomolecules, including DA, AA, NADH, and pyrocatechol, opening up, thus, new perspective for applications improvement of rGO/Au nanocomposites in (bio)sensors [25].

#### *3.1.4 Cancer and disease biomarkers*

Cancer biomarker detection represents one of the most important achievements in the field of biosensor production. An interesting review work was done by Alsharabi et al., who highlighted the capacity of G and its various derivatives to conjugate their unique chemical structure and characteristic optical, electronic, mechanical, and thermal properties. These special materials may represent an excellent sensing platform for cancer biomarkers detection, representing a possible response to the important challenge in the diagnosis of cancer at the initial stages [29]. The G-based electrochemical biosensors for cancer biomarker detection were fabricated by using G surface that was improved with magnetic beads and enzyme-marked antibody-AuNPs [39]. One should emphasize that the development of an electrochemical biosensor for cancer biomarker determination remains essential for early discovery and diagnosis. In addition, the electrochemical sensor presents the advantages of improved characteristics (i.e., rapid, precise, and sensitive) in comparison with other investigation techniques.

In order to prepare a new biosensor for cancer-related biomarker, Jin and co-workers used a G platform that was prepared by CVD. Then, the MBs and enzyme-marked antibody-AuNP bioconjugate were added [39]. The attachment of MBs, covered with capture antibodies (Ab1), to the G sheet surface, was realized by applying an external magnetic field, with the aim to avoid the reduced G matrix conductivity. With the aim of increasing the sensitivity of the multi-nanomaterialbuilt biosensor, the AuNPs were improved with HRP and detection antibody (Ab2), forming the conjugate Ab2–AuNPs–HRP. In this electrode arrangement, the fast electron transfer between the multi-nanomaterial present on the electrode surface and analyte target was realized, and a low value for carcinoembryonic antigen detection limit was reached (**Table 5**). The acquired results evidenced a fast response and recovery time, which was more improved compared to that achieved when the old-style approaches were used. The good sensors achieved properties (sensitivity, specificity, simplicity of construction technique, ease of use, fast analysis, and reusability) confirm that the developed biosensor can be used for cancer's medical diagnosis [39].

A label-free electrochemical multiplexed immunosensor based on G nanocomposites was fabricated for the recognition of both carcinoembryonic antigen (CEA) and α-fetoprotein (AFP) by Jia et al. [38]. The indium tin oxide (ITO) sheets were used to be modified with anti-AFP fixed on G nanocomposite matrix. The electrode fabrication and voltametric sensing technique were centered on the electron transfer delay determined by the engineered antibody-antigen immunocomplex present on the ITO electrode surface. The obtained multiplexed immunosensor facilitated the concomitant detection of both CEA and AFP, and the found linear ranges are presented in **Table 5**. The limit of detection value for CEA and for AFP, are depicted in **Table 5**. A few aspects were pointed out as follows: (i) the fabricated immunosensor escaped the marking of both antigens or antibodies, making it easier and preventing the cross-talk among diverse analytes; (ii) G nanocomposites used as supporting scaffold were synthesized by a facile route, and the shapes and quantity of the immobilized AuNPs, by this method, could be easily controlled; (iii) the

immunoassay having a worthy stability, large linear ranges received a good correlation with ELISA (enzyme-linked immunosorbent assay) and could be used in medical diagnosis. It was also stated that this simple approach could be adapted and combined for new biosensor applications [38].

#### **4. Conclusions**

Since its discovery, G has been tested in a large diversity of biosensing applications owing to its remarkable electrical, mechanical, and optical properties as well as its unique structure. If one compares G-based biosensors with conventional ones, clear benefits, such as high sensitivity and selectivity, low detection limit, reproducibility, stability, fast response, or easy miniaturization, make G-based biosensors a real candidate for a novel and efficient class of biosensors for medical applications.

Electrochemical biosensors offer an inexpensive, facile, fast, sensitive, and selective detection of biomolecules, and the use of G and its composites for biosensors development is due to its unique features combined with some peculiar properties. Moreover, the reusability of the biosensors is aimed. The present chapter summarized the G-based electrochemical sensors developed for sensing biomolecules and highlighted their significant advances. Thus, G and GO present a wide range of electrochemical potential and fast electron transfer rate. The drawback of using G in biosensors construction is its possible toxicity, as stated in the international nanotechnology guidelines. On the other hand, G can be easily functionalized/modified by electrodeposition, polymerization, electrochemical doping, or other methods, and even if G could be cytotoxic, biomedical applications can benefit from non-toxic, biocompatible and water-dispersible G layers obtained by chemical functionalization with different ligands. By biosensors incorporation into strong, transportable, and miniaturized devices, the detection of biomolecules and toxins for usability in clinical and diagnostic fields can be achieved.

Although there is a massive investment both in academia and industry, G-based biosensors are still at an incipient level, and commercial biosensors are yet to come. Nevertheless, there is a slow but promising translation into medical applications.

#### **Acknowledgements**

This work was supported by a grant from the Ministry of Research, Innovation, and Digitization, CNCS/CCCDI—UEFISCDI, project number PN-III-P2-2.1- PED-2021-3156, within PNCDI III.

#### **List of abbreviations**



*Advanced Graphene-Based Materials for Electrochemical Biomarkers and Protein Detection DOI: http://dx.doi.org/10.5772/intechopen.114011*


#### **Author details**

Carmen Ioana Fort1,2\*, Liviu Cosmin Cotet1,2, Lucian Cristian Pop1,2,3, Monica Baia2,3,4 and Lucian Baia2,3,4\*

1 Faculty of Chemistry and Chemical Engineering, "Babes-Bolyai" University, Cluj-Napoca, Romania

2 Institute for Research-Development-Innovation in Applied Natural Sciences, "Babes-Bolyai" University, Cluj-Napoca, Romania

3 Interdisciplinary Research Institute on Bio-Nano-Sciences, "Babes-Bolyai" University, Cluj-Napoca, Romania

4 Faculty of Physics, "Babes-Bolyai" University, Cluj-Napoca, Romania

\*Address all correspondence to: ioana.fort@ubbcluj.ro and lucian.baia@ubbcluj.ro

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 7**

## Application of Graphene in Lithium-Ion Batteries

*Chuanlei Qi, Jiaran Wang, Shengping Li, Yuting Cao, Yindong Liu and Luhai Wang*

#### **Abstract**

Graphene has excellent conductivity, large specific surface area, high thermal conductivity, and sp2 hybridized carbon atomic plane. Because of these properties, graphene has shown great potential as a material for use in lithium-ion batteries (LIBs). One of its main advantages is its excellent electrical conductivity; graphene can be used as a conductive agent of electrode materials to improve the rate and cycle performance of batteries. It has a high surface area-to-volume ratio, which can increase the battery's energy storage capacities as anode material, and it is highly flexible and can be used as a coating material on the electrodes of the battery to prevent the growth of lithium dendrites, which can cause short circuits and potentially lead to the battery catching fire or exploding. Furthermore, graphene oxide can be used as a binder material in the electrode to improve the mechanical stability and adhesion of the electrodes so as to increase the durability and lifespan of the battery. Overall, graphene has a lot of potential to improve the performance and safety of LIBs, making them a more reliable and efficient energy storage solution; the addition of graphene can greatly improve the performance of LIBs and enhance chemical stability, conductivity, capacity, and safety performance, and greatly enrich the application backgrounds of LIBs.

**Keywords:** conductivity, electrochemical processes, electrode materials, graphene oxide, graphene materials, lithium storage, lithium-ion batteries

#### **1. Introduction**

During the Industrial Revolution, the world rapidly developed and economically prospered with clean, affordable and reliable energy [1]. The consumption of traditional fossil energy has attracted widespread attention from various industries, and environmental protection has driven research on alternative energy supplies, especially renewable energy, and efforts are being made to develop new renewable energy types while providing an effective environmental protection approach [2, 3].

In order to meet the growing energy demand and reduce greenhouse gas emissions, many countries have recently conducted extensive research on low-cost and environmentally friendly renewable energy sources, such as solar, tidal, wind, biomass, and geothermal [4]. At the same time, it is necessary not only to develop energy but also to maximize energy storage. In addition, there has been a renewed interest in electric vehicles as substitutes for internal combustion engine vehicles, which account for 25% of greenhouse gas emissions [5], making energy storage a priority for the new global energy management system.

Electrochemical energy storage technology has obvious advantages among the various energy storage technologies. The battery is not limited by geographical location, convenient and efficient charging and discharging processes, and higher efficiency [6]. As the main force of energy storage technology, electrochemical energy storage has received widespread attention in market development and scientific research fields. At present, mainstream electrochemical energy storage technologies include LIBs [7], lead batteries [8], and flow batteries [9]. Among them, the LIBs have the characteristics of long cycle characteristics, fast response speed, and high system comprehensive efficiency and are widely used in portable charging equipment, transportation systems, aerospace, and other fields [10, 11].

However, the energy storage processes of the LIB electrode systems are different from each other. With the demand for reliable and durable energy storage devices in portable electronic products and power grids, improving the power density and cycle life of LIBs has become an important goal [12, 13]. It is crucial to design and manufacture efficient electrode materials that can provide high specific capacity and energy density to meet the growing demand for high-performance electrical equipment [10, 14].

Since the energy density of LIBs depends largely on electrode materials, the research direction is aimed at high-specific capacity electrode materials. It has been recognized that nanostructured electrode materials with special electrochemical properties will be necessary to achieve the purpose. The dimensionality reduction of nanomaterials can shorten the diffusion time of Li<sup>+</sup> [15].

#### **2. Overview of the graphene chemistry**

Graphene and carbon nanotubes [16] have played important roles in nanomaterials, which can be applied to portable communication equipment, electric vehicles, and large-scale energy storage systems. Many research results have shown that energy storage technology could achieve a qualitative leap by breaking through the technical difficulty of electrode nanomaterials named graphene, a novel two-dimensional carbon material that was discovered by mechanical exfoliation of graphite in 2004 [17]. As illustrated in **Figure 1**, the typical structure of graphene [18] is composed of a carbon material 1 million times thicker than the diameter of a single hair comprising a hexagonal two-dimensional honeycomb lattice of sp2 hybridized carbon atoms. The structure can be divided into single-layer, double-layer, and multilayer, with an ultrahigh specific surface area of about 2630 m2 g−1.

Graphene has many desirable properties, with very high electron mobility at room temperature and rapid heterogeneous electron transfer at the edges, with values exceeding 15,000 cm2 V−1S−1 [19]. Graphene also has a significantly high Young's modulus (1.0 TPa), high breaking strength, extraordinary mechanical strength, excellent thermodynamic, electrical conductivity (5–6.4×106 S m−1), optical transmittance activity (about 97.7%), material density (less than 1 g cm−3), stability, catalytic properties, and other graphene tunable properties [20, 21].

The continuous two-dimensional conductive network formed by graphene can effectively improve the electron and ion transport kinetics of electrode materials, and *Application of Graphene in Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.114286*

**Figure 1.** *(a) and (b) Typical structure of graphene [18].*

#### **Figure 2.**

*(a) Schematic diagram of top-down and bottom-up approaches, (b) schematic diagram of graphite structure [29].*

graphene is used to improve the rate performance and cycle stability of LIBs because of its high specific surface area, stable chemical properties, excellent electrical and thermal conductivity [18]. With its excellent characteristics, graphene has lots of applications in electronic devices, photonic devices, photocatalysis, and advanced composite materials applicable in the military, aerospace, and other fields.

Although graphene shows excellent properties in chemical and mechanical aspects, its application in electronics and energy storage devices needs to be continuously explored [22]. Researchers have developed many methods in graphene applications, such as combining with other materials to develop graphene-based composites [23] including graphene/polymers [24], graphene/metals [25], and graphene/carbon nanotubes [26] composites for energy storage devices.

In addition, recent scientific advancements have allowed the development of various low-cost and environmentally friendly methods for preparing graphene. This is particularly important for large-scale production and application. The following analysis analyzes the application of graphene and graphene-based nanocomposites as electrode materials in LIBs, and provides possible development paths in the future.

The main production methods for graphene include bottom-up and top-down methods, and graphene properties have great differences in structural integrity, sheet size, and cost with different methods [27, 28]. As illustrated in **Figure 2**, the top-down approach refers to the method of obtaining a product by crushing or peeling off a large amount of material. The bottom-up approach, on the other hand, refers to the

method of synthesizing the desired product from smaller materials, continuously growing graphene by breaking the chemical bonds of carbon-containing compounds and depositing carbon atoms on a suitable substrate [29–32]. Various forms of graphene nanomaterials have been prepared, including graphene spheres, graphene scrolls, graphene networks, graphene tubes, graphene cages, and other structures of graphene [21].

#### **3. Lithium-ion batteries, LIBs**

As shown in **Figure 3**, lithium-ion batteries (LIBs) consist of two electrodes and a separator impregnated with an electrolyte to provide the electrons and ions needed for electrochemically active nanomaterials [33].

The metal-foil anode is the negative electrode of the battery, and it is made of lithium-containing material. During discharge, lithium ions are released from the anode and travel through the electrolyte to the cathode, where they are intercalated (inserted) into the electrode material. The anode also serves as the current collector for the negative terminal of the battery. The cathode, or positive electrode, is made of an electron-rich material that can store the lithium ions after they are released from the anode. It is typically coated onto a metal foil current collector. During discharge, electrons flow through the external circuit to the cathode, where they combine with the lithium ions to form the discharge product. The separator is a thin film that separates the anode and cathode to prevent direct contact between them. It is made of a material that is permeable to lithium ions but impermeable to electrons, ensuring that the electrons flow through the external circuit during discharge and charge. The separator also helps to prevent short circuits within the battery. The electrolyte is a liquid or solid material that transports the lithium ions between the anode and cathode. It is typically a lithium salt dissolved in a non-aqueous solvent.

The LIBs are based on the movement of lithium ions between the anode and cathode during discharge and charge cycles. When the battery is discharged, electrons

**Figure 3.** *Schematic diagram of the structure of commercial LIBs [33].*

*Application of Graphene in Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.114286*

flow through the external circuit to the cathode, where they combine with lithium ions to form the discharge product. At the same time, lithium ions travel through the electrolyte.

At the moment, LIBs are regarded as one of the most promising energy storage technologies due to their high energy density, energy conversion efficiency, and output voltage [34]. With the development of information electronics, electric vehicles, and smart grids, there is a huge demand for LIBs with high energy density, long cycle life, and lower cost [35, 36]. However, the electrode materials of LIBs generally have poor conductivity, and the charge-discharge reaction cannot be completely carried out due to electrode polarization during the charging and discharging process. Moreover, the effective capacity of the electrode material cannot be fully exerted [37–39].

At present, the conductive agents on the market mainly include conductive carbon black, conductive graphite, and carbon nanotubes [40]. The excellent thermal conductivity of graphene can improve the thermal stability of LIBs. The "face" contact between graphene and electrode materials can improve the conductivity of electrodes [41]. Based on the special physical and chemical properties of graphene, and it has great potential as an electrode material for LIBs. LIBs are composed of four parts: cathode electrode material, anode electrode material, separator, and electrolyte, and the electrode material plays an important role in battery performance [42, 43]. According to application fields, the application of graphene mainly has three directions in LIBs: (1) graphene use as an active electrode material: graphene can be used as an anode material for LIBs to provide reversible storage space for Li<sup>+</sup> , improving specific capacity and rapid charge and discharge efficiency [44]. (2) Graphene can be combined with cathode or anode materials to improve the performance of electrodes: the combination of graphene with a cathode active material enhances electrode conductivity [45]. In this respect, graphene can be compounded with anode-active materials to construct a three-dimensional structure and provide space for volume expansion [46]. (3) Graphene is used as a conductive additive to provide a fast channel for electron and Li<sup>+</sup> transport and improve the conductivity of the electrode [47, 48].

#### **4. Graphene as LIBs electrode conductive agent**

At present, the development of energy storage technology has made higher requirements for LIBs in terms of energy density, ion transport rate, and cycle performance [49]. However, the poor conductivity of electrode materials greatly limits the performance of LIBs. Adding a conductive agent can enhance the electron transport efficiency and reduce the polarization of the electrode. It is important to utilize the effective capacity of the active material. Compared with conductive agents such as commercial conductive agent carbon black (CB), graphene has higher conductivity and specific surface area, and studies have shown that advanced carbon materials, including carbon (one-dimensional) [50], graphene (two-dimensional) [51, 52], and 3D graphene backbones (three-dimensional) [53], have been used to build continuous conductive networks for LIBs [54].

Graphene is a powerful planar conductive additive, which is considered to be one of the most promising conductive additives due to its unique physicochemical properties including high aspect ratio, chemical resistance, excellent conductivity, and low dosage of effective characteristics [55–57]. Compared with the "point-to-point"

contact mode constructed by the traditional graphite conductive agent, graphene can form a "point-to-surface" contact mode with the electrode material in the electrode, providing a long-range and fast conduction path for electrons and Li<sup>+</sup> , reducing the amount of conductive agent is equivalent to increasing the content of the active material of the electrode and increasing the capacity of the electrode [54, 58]. Therefore, using a small amount of graphene as a conductive additive can greatly improve the electronic conductivity of the electrode.

It has been proposed as a simple and effective method to prepare graphene conductive slurry as a conductive agent for LIBs by combining mechanical stirring, ultrasonic dispersion, and dispersant modification [48]. **Figure 4a** shows the schematic diagram of graphene dispersion. Graphene slurry also exhibits excellent battery performance as a conductive agent for LIBs. At 100 mAg−1 current density, the first charge and discharge capacity are 1273.8 and 1723.7 mAhg−1, respectively, and the coulombic efficiency is 73.9%. The capacity retention rate of the anode is 84% (1070.2 mAhg−1) after 100 cycles at 200 mAg−1. Another article reported that graphene nanosheets (GN) with different sizes as conductive additives can affect the electrochemical performance of LiFePO4 (LFP) [61]. Compared with conventional conductive additives, GNs and Super P Conductive Carbon Black (SP) can construct an effective electronic conductive network and significantly improve the electrochemical performance of LFP as conductive additives. It also shows that with the increase of GN size, the specific capacity and rate performance of nanoscale LFP tended to deteriorate, which is due to the "barrier effect" of GN extending the length of the ion transport path and greatly reducing the ion conductivity. The small-size GN can effectively balance the rapid diffusion of Li+ and the electron transport of nanoscale LFP. The effect of the amount of graphene used in different thicknesses electrode on the electrode performance was also studied [59]. The results showed that when the thickness of the electrode is thin, the cycle performance and rate performance of

#### **Figure 4.**

*(a) Schematic diagram of graphene dispersion, (b) schematic diagram of the evolution of electrode thickness from laboratory half-coin battery to commercial soft-packaged battery [59], and (c) schematic diagram of lithium-ion transport path in LiFePO4 cathode with GN or HG + SP as conductive additive [60].*

#### *Application of Graphene in Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.114286*

the electrode increase with the increase of graphene addition (**Figure 4b**). When the electrode thickness is higher, the rate performance of the electrode has a linear relationship with the amount of graphene. The Li+ diffusion path is greatly extended, and the spatial effect of GN is amplified, causing slower ion transport kinetics. Different research results indicate that the conductivity of the electrode material is significantly improved when the amount of graphene is appropriate. However, when the size or the amount of the graphene nanosheet (GN) is too large in the higher-thickness electrode, the Li+ diffusion path will be greatly extended. The steric hindrance effect of graphene on the diffusion of Li+ is amplified, the polarizability is higher at high rates, and the rate of performance of the electrode is reduced. So, researchers should pay attention to maintaining the balance between ion diffusion kinetics and electron conduction when using graphene as a conductive agent.

In order to improve the conductivity and the rate performance of LIBs, researchers have studied a lot on graphene composite conductive agents and graphene modification. Graphene nanoribbons (GNBs) have been used as a conductive agent for LFP [62]. The cathode with olivine structures is used to promote rapid redox reactions and achieve high-rate cell performance. The results have shown that the cathodes with 5 wt% graphene nanoribbons and 10 wt% conductive carbon nanoparticles exhibited a capacity of 163.25 mAhg−1 at 0.1 C and 130.60 mAhg−1 at 2 C, the capacity retention rate is 98.21% after 100 cycles at 2 C. Graphene nanoribbons play the role of bridges creating connected networks to facilitate electron transport. Porous graphene (HG) has been prepared by KOH activation [60]. HG, with a large number of pores and a large specific surface area, greatly improved the electronic conductivity of the LFP electrode, but it did not affect the efficient transport of ions. LFP cells with traditional graphene additives exhibit lower rate performance because graphene with a planar structure hinders the transport of Li<sup>+</sup> . Binary conductive additives containing only 1 wt% HG and 1 wt% carbon black (such as SP) can make LIBs obtain higher rate performance comparable to batteries containing 10 wt% SP, and the schematic diagram of the Li<sup>+</sup> transport path of the composite conductive agent is shown in **Figure 4c**. A small amount of SP complements the remote conductive network formed by HG, which can fully contact LFP so that the entire LFP electrode has excellent conductivity. The simultaneous use of HG and SP can achieve a balance of electron conductivity and ion diffusivity.

#### **5. Application of graphene as LIBs cathode materials**

As the cathode of LIBs, the electrode material should have the requirements of high reversible capacity, high stability potential, and lower manufacturing cost [63, 64]. At present, the cathode materials of LIBs are mostly LiFePO4, LiCoO2, LiMn2O4, Li3V2(PO4)3, and LiNixCoyM1-x-yO2, which have the characteristics of high specific capacity, non-toxicity, and low cost, but the conductivity is poor, and the mobility of lithium-ion is low [65–67].

When LiFePO4 material is compounded with graphene, theoretically, the rate performance can be improved as conductivity improves [68, 69]. The LPF-graphene composite, LFP@C/G, was successfully synthesized, as illustrated in **Figure 5**, using the high energy ball milling-assisted rheological phase method [70]. The multilayer graphene film is not stacked on the carbon-coated LiFePO4 nanospheres, thus forming rich mesopores forming unique 3D "ball-in-chip" and "ball-on-chip" conductive network structures. High conductivity and rich mesopores facilitate the transport of

#### **Figure 5.**

*Schematic diagram of the synthesis of LFP@C/G composites [70].*

#### **Figure 6.**

*(a) Preparation and electrochemical reaction protocols of LFP@TC and LFP/GN, (b) rates data of LFP@TC and FP/GN [66].*

electrons and ions. The results show that the mixed materials with graphene content of about 3 wt% show excellent rate performance, and the initial discharge capacities were 163.8 and 147.1 mAhg−1, respectively. In addition, the composites also showed excellent cycling stability, with a capacity decay of only 8% after 500 cycles at 10 C. However, there are also reports that graphene composite LiFePO4 exhibits unfavorable electrochemical performance when used in cathode materials of LIBs. The effect *Application of Graphene in Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.114286*

of carbon coating on the electrochemical performance of LiFePO4 has been discussed [71]. The schematic diagram of the synthesis of lFP@graphene nanosheets (LFP@ GN), sucrose-derived amorphous carbon-coated LFP (LFP@TC), and LFP/GN (partial wrapping) is shown in **Figure 6a**. Graphene partially wrapped LiFePO4 enhances the conductivity of LFP/GN material, but when the cathode material (LFP/GN) is fully wrapped in graphene, the ion transport efficiency decreases, leading to a decline in rate performance (**Figure 6b**). The results show that although graphene coating improved the conductivity of Li+ and electrons, the complete and dense coating of high graphitic carbon is not conducive to the transport of electrons due to the influence of steric hindrance effect on Li+ diffusion in the cathode material. Therefore, the ideal coating structure should maintain a balance between increased electron transport and rapid ion diffusion.

#### **6. Application of graphene as LIBs anode materials**

As the most widely used anode material for LIBs, graphite anode has the advantages of easy access to abundant raw materials and low costs, but its low specific capacity falls short of meeting the requirements of LIBs [72–74]. The lithium dendrite formed during the charging and discharging process of graphite anode makes it difficult for LIBs to achieve high rate and cycle life [75], in addition to the fact that they can cause short circuits and potentially lead to the battery catching fire or exploding. Therefore, many studies are devoted to the modification of graphite anodes and the development of new anode materials.

At present, the typical lithium-ion battery anode materials can be divided into three categories: intercalation reaction electrode materials, conversion reaction electrode materials, and alloy electrode materials. The intercalated electrode materials are mainly composed of carbon materials [76]. There are many studies on the application of carbon materials in LIBs, and the research on graphene application in anode materials is more extensive and in-depth than in cathode. Graphene can react with Li+ on both sides of the graphene nanosheet to form LiC3, and each C atom corresponds to about 0.33 Li+ , which is twice the amount of lithium intercalation in the traditional graphite electrode with a theoretical specific capacity of 744 mAhg−1 [77, 78]. Porous graphene foam (GF) has been prepared and applied as the anode of lithium-ion batteries. It comprises a loosely porous three-dimensional network structure with excellent electrical conductivity and chemical stability. GF has high performance in specific capacity and cycle stability. However, graphene as the anode material for LIBs may cause stacking between graphene sheets, reducing the specific surface area of the material, resulting in a decrease in lithium storage and failure to achieve higher capacity as a single anode material.

The graphene-metal composite material produced by the graphene coating method can, however, improve the electrochemical performance of LIBs [79]. The flexible characteristics of graphene can effectively inhibit the metal electrode volume expansion during the charging and discharging process, and the morphology of graphene can change with changes in the preparation method [52, 80, 81]. With excellent electrical conductivity, graphene can establish a conductive network between particles, and the high specific surface area can also increase the storage capacity of lithium. Numerous studies have shown that the graphene-metal composite materials applied as anode materials can greatly improve the performance of LIBs [52, 80–82]. The bilayer graphene (BGra) was synthesized by the thermal evaporation-deposition

#### **Figure 7.**

*(a) Schematic diagram of the preparation process of Si@BGra and Si@BGra/Ni (yellow represents silicon nanoparticles, orange is copper powder, gray is double-layer graphene, green is nickel foam), (b) SEM image of Si@BGra/Ni, (c) HRTEM image of Si@BGra/Ni, (d) Si@BGra/Ni magnification performance image at 1.0-50 Ag−1, (e) Si@BGra/Ni cycle performance at 20 Ag−1, and (f) cycling performance of Si@BGra/Ni at 3, 5, and 7 Ag−1 [83].*

assisted chemical vapor deposition (CVD) method by coating on Si nanoparticles [83], as shown in the process diagram in **Figure 7a**–**c**. The results showed that the electrode composed of this material, Si@BGra, can provide a capacity of 2500 mAhg−1 at the current density of 3 Ag−1. At the same time, the capacity retention rate was 85% after 1000 cycles, exhibiting excellent cyclic stability (**Figure 7d**–**f**). The graphene-MnO2 composite was also prepared as anode materials for LIBs [84]. The material also demonstrated excellent cycle stability at current rates of 2 C, 5 C, and 10 C after 500 charge-discharge tests.

The excellent cycle stability and higher rate performance of these composites can be attributed to the integration of graphene and mesoporous metallic material. Graphene, as an exceptional charge carrier, enhances the electronic conductivity of composite materials and enables complete reversible redox reactions in metallic materials. As in LIBs anode materials, graphene can also act as a buffer medium for large volume changes of the negative electrode material during the charging and discharging process and inhibit mechanical strain and the crushing of electrodes.

#### **7. Application of other graphene derivatives in LIBs**

Although graphene exhibits excellent electrochemical performance in electron and ion transport kinetics, its hydrophobic properties are challenging in electrode applications. As a strong and flexible carbon atom thin film, graphene offers a variety

#### *Application of Graphene in Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.114286*

of possibilities for the modification and functionalization of its carbon backbone, such as chemical modification of H and O functional groups, so that its hydrophilic version of oxygen/hydrogen functionalization has recently gained popularity [85–87]. At present, the doping of heteroatoms into graphene to produce more Li+ storage sites has been widely studied. Heteroatom doping produces a large number of defects on the graphene surface, which not only prevents the irreversible aggregation of the graphene layer but also provides a rich lithium reservoir [86]. At the same time, the surface of graphene derivatives such as graphene oxide (GO), nitrate graphene, and fluorographene, has a large number of functional groups, defects, and other active sites, which enhance the electronic and mechanical properties of graphene [88].

Graphene oxide is usually prepared by stripping of graphite oxide generated by chemical oxidation, and the use of strong oxidants to generate graphite oxide is the most common method for preparing graphene oxide [89]. At present, the production of GO and reduced graphene (r-GO) have been commercialized. They form the basic units for forming other 3D graphene complex material, which also has the problem of low relative conductivity [90, 91]. High-quality foamed graphene and vertical graphene prepared by chemical vapor deposition (CVD) and plasma-enhanced chemical vapor deposition (PECVD) have emerged in the latest report, but the preparation process involves high temperatures and complex techniques. It is inferred that highend 3D graphene derivatives with few defects and high electronic conductivity are the development direction in the future [92, 93].

Many researchers have devoted themselves to the study of graphene derivatives as lithium-ion cathode materials. A series of reduced porous graphene oxide (rhG-x) has been synthesized by the chemical oxidation and annealing reduction process of porous waste graphite (hSG) [94], and the schematic diagram of the process is shown in **Figure 8**. Because of the unique oxygen-containing groups and electronic conductivity, the rhG-x series with "worm-like" segments and porous structure diagrams shows excellent lithium storage performance as the cathode for LIBs.

Few-layer graphene (FLG) has been prepared by a simple method to modify FLG using nitrogen doping. With the doping of heteroatoms, the rate performance and cycle stability of graphene were significantly improved [95].

In addition, graphene-based compounds are also widely used in LIBs. Uniformly dispersed Cu-hexahydroxytriphenyl (HHTP)/graphene (G) composites were synthesized using an in-situ growth strategy, and electrochemical performance was studied

#### **Figure 8.**

*Schematic diagram of rhG-x synthesis process [94].*

for the first time as anode for LIBs [96]. It was observed that graphene formed a two-dimensional network of conductivity in the composites and effectively improved the energy storage of Cu-HHTP.

#### **8. Application of graphene in thermal management of LIBs**

Lithium-ion batteries have a wide range of applications in mobile communications, automobiles, and aerospace. With the rise of electric and hybrid electric vehicles (HEVs), there is another push for battery technology [97]. The battery and its management system are two of the three main technologies of electric vehicles, and the thermal management technology of the battery is an important part of the battery system. The operation life and efficiency of batteries are affected by high temperatures during working, thereby affecting the maintenance, life, and cost of electric vehicles [98–100]. Extending battery life and safe use of batteries requires controlling the operating temperature of LIBs within a safe range, and high or low temperatures will create adverse effects on LIBs. In addition, the electrolyte could solidify and fail to transport electrons at a lower temperature. Thus, the LIBs could experience thermal runaway or battery rupture or even explosion at high temperatures.

Many studies have shown that high, low, or uneven temperatures affect the charge-discharge efficiency and cycle stability of the power battery [101–103]. Therefore, it is necessary to design a reasonable battery thermal management system to effectively control and maintain a stable and uniform temperature of the battery in the battery pack. At present, the cooling methods of lithium-ion battery thermal management systems are mainly divided into three cooling methods: air cooling, liquid cooling, and phase change material (PCM) [104, 105].

A common method of thermal management of lithium-ion battery packs is based on the utilization of phase change materials (PCM) [106]. Phase change materials are a special class of functional materials that, in the phase change process, keep a small temperature change range or constant temperature and can absorb or release a large amount of latent heat [107, 108]. During the technological development of PCMs, researchers have studied many different kinds of materials, including inorganic systems (salts and salt hydrates), organic compounds (such as paraffins or fatty acids), and polymeric materials (such as polyethylene glycol) [109]. Paraffin wax is widely used as a phase change material in LIBs due to its excellent characteristics such as safety and non-toxicity, low price, small volume change during phase change, stable chemical properties, and low vapor pressure [110, 111]. However, there are some nonnegligible shortcomings in the use of paraffin, such as easy leakage and low thermal conductivity during use of paraffin. Some materials with high thermal conductivity are usually combined with paraffin wax to improve their thermal conductivity and performance as PCM [112–114], considering the defect of low thermal conductivity of paraffin.

Researchers have combined some carbon materials with paraffin wax while using it in the thermal management of LIBs, and graphene is widely used due to its excellent thermal conductivity [115]. Graphene-epcm hybrid composites have been prepared by dispersing liquid phase stripping (LPE) graphene and FLG solutions in paraffin [116], and mixing them with high shear with a magnetic stirrer on a hot plate at 70°C. It is proved that graphene and FLG as fillers in organic phase change materials can improve their thermal conductivity by more than two orders of magnitude while maintaining their latent heat storage capacity. Graphene-coated nickel foam was

*Application of Graphene in Lithium-Ion Batteries DOI: http://dx.doi.org/10.5772/intechopen.114286*

prepared using chemical vapor deposition technology, and paraffin wax was used as a phase change material to penetrate into the voids of graphene-coated nickel foam. The thermal characteristics of saturated paraffin graphene-coated nickel foam and its application in the thermal management of LIBs were studied [117]. The results showed that: (1) the thermal conductivity of graphene @Ni/saturated paraffin wax was increased by 23 times compared with pure paraffin; (2) the melting temperature and freezing temperature of graphene-coated nickel foam composites of nickel foam/saturated paraffin were higher than paraffin wax and lower than paraffin, respectively.

#### **9. Conclusions and perspectives**

In conclusion, the application of graphene in lithium-ion batteries has shown significant potential in improving battery performance. Graphene's exceptional electrical conductivity, high specific surface area, and excellent mechanical properties make it an ideal candidate for enhancing the capabilities of these batteries. The various approaches, graphene derivatives, and graphene-based electrodes have been successfully utilized to improve the capacity, rate capability, cycling stability, and thermal stability of lithium-ion batteries.

However, there are still challenges that need to be addressed for the widespread application of graphene in lithium-ion batteries. One of the main challenges is the production of high-quality graphene in a scalable manner. The development of efficient and cost-effective methods for the synthesis of graphene is crucial for its commercialization in battery applications. Additionally, understanding the mechanisms behind the electrochemical performance of graphene-based electrodes is crucial for optimizing their properties.

Future perspectives in this field include exploring new applications of graphene beyond electrodes, such as in separators, electrolytes, and other battery components. The combination of graphene with other materials, such as metal oxides, carbon nanotubes, or polymers, may lead to the development of novel electrode architectures with improved performance. Furthermore, the development of 3D-printed graphene composites for battery applications is a promising direction that could lead to the production of customized battery components with improved mechanical properties and conductivity.

Overall, the application of graphene in lithium-ion batteries holds great promise for the development of next-generation energy storage devices with higher energy density, longer cycle life, and better rate capability. Continuing research efforts in this field are expected to lead to further advancements in the field of energy storage and pave the way for a sustainable future.

#### **Author details**

Chuanlei Qi1 , Jiaran Wang2 , Shengping Li1 , Yuting Cao1 , Yindong Liu1 and Luhai Wang1 \*

1 PetroChina Company Limited, Petrochemical Research Institute, Changping, Beijing, P.R. China

2 State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, Changping, P.R. China

\*Address all correspondence to: wlh459@petrochina.com.cn

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Section 4
