**4. Application of electrospun nanofibers in electrochemical sensors**

**Materials**

CNF 200-500

CNF 200-400

CNF 400-600

Pd/CNF 200-500

Pd/CNF 200-500

**Fiber diameter (nm)**

**Analytes**

DA UA AA

L-tryptophan L-tyrosine L-cysteine

catechol hydroquinone

> H2O2 NADH

> > DA UA AA

Pd/CNF 300-500 oxalic acid 1.07

NiO-Ag 82.1± 13.8 glucose

width 580

±44, glucose

NiO-Au

**Detection potential (V)**

Electrospun Nanofibers: From Rational Design, Fabrication to Electrochemical Sensing Applications

0.376 0.475 0.200

> 0.9 0.8 0.75

> > ~ ~


0.402 0.550 0.158

> 0.1 0.6

> 0.2 0.6

CNF 200-400 xanthine 0.85 0.03-21.19 0.02 [211]

CNF 100±25 glucose 0.2 ~ ~ [213]

Pd/CNF 200-500 hydrazine -0.32 10-4000 2.9 [216]

Ni/CNF 200-400 glucose 0.6 2 -2500 1 [218] Ni/CNF 200-400 ethanol 0.55 250-87500 250 [219] Rh/CNF 300-500 hydrazine 0.4 0.5-175 0.3 [220] Pt/CNF 200-500 H2O2 0.0 1-800 0.6 [221] ZnO 195-350 glucose 0.8 250-19000 1 [222] Mn2O3-Ag ~ glucose -0.45 ~1100 1.73 [223] Au 990±490 fructose 0.2 100-3000 11.7 [224] Co3O4 105 ±10 glucose 0.59 ~2040 0.97 [225] CuO 90-240 glucose 0.4 6-2500 0.8 [226] CuO ~2 μm glucose 0.4 0.2 -600 0.0022 [227] NiO 10 μm glucose 0.5 1-270 0.033 [228] Pd-CuO 90-140 glucose 0.35 0.2-2500 0.019 [229]

**Linear Rang (μM)**

> 0.04-5.6 0.8-16.8 2-64

0.1-119 0.2-107 0.15-64

> 1-200 1-200

0.2-20000 0.2-716.6

0.5-160 2-200 50-4000

200-13000 13000-45000

> ~590 ~2630

> ~2790 ~4550

1.37 0.72

0.65 1.32 [230]

[231]

**Limit of Detection (μM)**

http://dx.doi.org/10.5772/57099

0.04 0.2 2

> 0.1 0.1 0.1

> 0.2 0.4

> 0.2 0.2

> 0.2 0.7 15

**Ref.**

51

[209]

[210]

[212]

[214]

[215]

200 [217]

Electrospun nanofibers are featured with small diameter, extremely long length, high surface area and complex pore structure. As mentioned above, electrospinning has been applied to fabricate nanofibers with various compositions and secondary structures. These electrospun nanofibers readily assemble into three-dimensional membranes, which characterized as high porosity, interconnectivity, and a large surface-to-volume ratio, makes electrospun nanoma‐ terials highly attractive to different applications, including sensors. Several reviews on the application of electrospun nanofibers in constructing sensors with different read-out mode and for different target were published in past several years [7-9, 194, 195]. Among various read-out modes, electrochemical read-out has attracted remarkable attentions in the ultrasen‐ sitive detection due to its high sensitivity and selectivity, inexpensive equipment and easy miniaturization. Various electrospun nanofibers, including polymer nanofibers, composite nanofibers, and metal or metal oxide nanofibers, have been used to prepare electrochemical sensors for a wide range of analytes. A summary of electrochemical sensors based on electro‐ spun nanomaterials is illustrated in Table 1.


Electrospun Nanofibers: From Rational Design, Fabrication to Electrochemical Sensing Applications http://dx.doi.org/10.5772/57099 51

**4. Application of electrospun nanofibers in electrochemical sensors**

spun nanomaterials is illustrated in Table 1.

**Fiber diameter (nm)**

> ~ ~

**Analytes**

4-CP 2,4-DCP 2,4,6-TCP

400-500 superoxide anion

(O2˙ˉ)

glucose glucose

PVA 70-250 glucose 0.65 1000-10000 50 [40]

nylon-6 95(RSD 27%) glucose ~ 1000-10000 6 [197] nylon-6 ~ glucose 0.70 1000-9000 2.5 [198] nylon-6 ~ glucose 0.50 1000-10000 6 [199] nylon-6 ~ pyrocatechol -0.2 ~100 0.05 [200]

Pt/PANi ~ urea -0.1 ~20000 10 [202] DNA/SWNT/PEO 50-300 glucose 0.5 ~20000 [203]

PVDF/PAPBA 150 glucose 0.04 100-1600 [205] nylon-6 140±15 cysteine 0.3 100-400 15 [206] P2W18/PVA ~500 nitrite -0.2 100-1500 0.96 [207] CNF 200-500 NADH 0.45 0.02-11.47 0.02 [208]

**Detection potential (V)**

> 0.0 -0.15 0.1

> > 0.8 0.8

**Linear Rang (μM)**

> 1-25 1-25 1-25

0.3 ~ 0.3 [201]

0-7000 0-7000

**Limit of Detection (μM)**

> 12.09 2.70 9.33

> > 557 668

**Ref.**

[196]

[204]

**Materials**

50 Advances in Nanofibers

PMMA/PANi-Aunano

PANCAA MWNT/PANCAA

PVA/F108/AuNPs/Lac ~

Electrospun nanofibers are featured with small diameter, extremely long length, high surface area and complex pore structure. As mentioned above, electrospinning has been applied to fabricate nanofibers with various compositions and secondary structures. These electrospun nanofibers readily assemble into three-dimensional membranes, which characterized as high porosity, interconnectivity, and a large surface-to-volume ratio, makes electrospun nanoma‐ terials highly attractive to different applications, including sensors. Several reviews on the application of electrospun nanofibers in constructing sensors with different read-out mode and for different target were published in past several years [7-9, 194, 195]. Among various read-out modes, electrochemical read-out has attracted remarkable attentions in the ultrasen‐ sitive detection due to its high sensitivity and selectivity, inexpensive equipment and easy miniaturization. Various electrospun nanofibers, including polymer nanofibers, composite nanofibers, and metal or metal oxide nanofibers, have been used to prepare electrochemical sensors for a wide range of analytes. A summary of electrochemical sensors based on electro‐



glucose biosensor by electrospinning a solution of glucose oxidase (GOx) and PVA, and directly collecting the fibers on a electrode [40]. Then GOx was immobilized by cross-linking the electrospun PVA/GOx composite membranes with glutaraldehyde. The immobilized GOx remained active inside the electrospun PVA fibrous membranes after the harsh process of electrospinning. The apparent Michaelis-Menten constant (KMapp) for this biosensor was determined to be 23.66 mM. Liu et al. developed laccase (Lac) biosensor for the determination of phenolic compounds by in situ electrospinning of a mixture of PVA, Lac, PEO-PPO-PEO (F108) and Au nanoparticles, where F108 was used as an enzyme stabilizing additive and Au NPs were used to enhance the conductivity of the biosensor [196]. Under the optimal condi‐ tions, the biosensor showed a sensitivity following the order of 2, 4-dichlorophenol (2, 4-DCP) > 2, 4, 6-trichlorophenol (2, 4, 6-TCP) > 4-chlorophenol (4-CP). The obtained KMapp values were 426.06, 9.41 and 73.36 µM for 4-CP, 2, 4-DCP and 2, 4, 6-TCP, respectively. The results indicated that Lac encapsulated into electrospun nanofibers retained its high catalytic activity. The sensing performance of this biosensor was attributed to the suitable electrochemical interface (e.g. biocompatibility, high surface area-to-volume ratio and superior mechanical properties)

Electrospun Nanofibers: From Rational Design, Fabrication to Electrochemical Sensing Applications

http://dx.doi.org/10.5772/57099

53

**Figure 10.** (A) Schematic picture of the nylon nanofibrous biosensing unit coupled with a glassy carbon electrode. Al‐ so shown a scanning electron microscopy detail of the nanofibrous structure. (B) Response current of nylon nanofiberbased glucose biosensors after the addition of glucose (1 mM each). Detection potential, + 0.5 V vs. Ag/AgCl. Supporting electrolyte, 0.1 M PBS (pH 6.5) containing 0.1 mM ferrocene methanol. Inset (bottom-right) shows the cor‐ responding calibration plot. Inset (top-left) shows the catalyzed electrochemical oxidation of glucose mediated by fer‐ rocene methanol, where curve a and b are the cyclic voltammograms (CVs) of the blank and of the ferrocene methanol (0.1 mM) in the absence of glucose. Other CVs are obtained upon addition of glucose from 5 to 100 mM. In

In addition to the direct incorporation of enzyme into polymer nanofibers, post-spun modifi‐ cation is a widely used method for constructing enzyme-based biosensors. Mannino's group developed glucose biosensors by using electrospun nylon-6 nanofibrous membranes (NFM) as the enzyme immobilization matrix [197-199]. A piece of NFM was placed over the electrode

of PVA/F108/Au NPs/Lac.

all the CVs, scan rate=0.02 V s-1; 0.1 M PBS (pH 6.5) [199].

**Table 1.** Electrospun nanofibers based electrochemical sensors.

Abbreviations in the table: PVA: poly(vinyl alcohol); F108: PEO-PPO-PEO; Lac: laccase; 4-CP: 4-chlorophenol; 2, 4-DCP: 2, 4-dichlorophenol; 2, 4, 6-TCP: 2, 4, 6-trichlorophenol; PMMA: Poly(methyl methacrylate); PANi: polyaniline; PEO: poly(ethylene oxide); PANCAA: poly(acrylonitrile-co-acrylic acid); SWNT: Single-walled carbon nanotube; MWNT: Multiwalled carbon nanotube; PVDF: poly(vinylidene fluoride); PAPBA: poly(aminophenyl boronic acid); P2W18: α-K6[P2W18O62] 14H2O; CNF: carbon nanofiber; Hb: hemoglobin; TCA: trichloro‐ acetic acid.

#### **4.1. Polymer nanofibers based electrochemical sensors**

Since the first enzyme-based electrochemical biosensor was proposed by Clark and Lyons [241], numerous efforts have been afforded in this direction because of the simplicity and high selectivity of enzyme electrodes. The immobilization of enzymes on a suitable matrix and their stability are important factors in the fabrication of biosensors. Several methods have been developed for immobilization of enzymes, including physical adsorption, cross-linking, selfassembly, as well as entrapment in polymers or sol-gels. Due to the merits of high specific surface area and porous structure, electrospun polymer fibers would be a promising biocom‐ patible material for enzyme immobilization [1]. For example, Ren and co-workers prepared a glucose biosensor by electrospinning a solution of glucose oxidase (GOx) and PVA, and directly collecting the fibers on a electrode [40]. Then GOx was immobilized by cross-linking the electrospun PVA/GOx composite membranes with glutaraldehyde. The immobilized GOx remained active inside the electrospun PVA fibrous membranes after the harsh process of electrospinning. The apparent Michaelis-Menten constant (KMapp) for this biosensor was determined to be 23.66 mM. Liu et al. developed laccase (Lac) biosensor for the determination of phenolic compounds by in situ electrospinning of a mixture of PVA, Lac, PEO-PPO-PEO (F108) and Au nanoparticles, where F108 was used as an enzyme stabilizing additive and Au NPs were used to enhance the conductivity of the biosensor [196]. Under the optimal condi‐ tions, the biosensor showed a sensitivity following the order of 2, 4-dichlorophenol (2, 4-DCP) > 2, 4, 6-trichlorophenol (2, 4, 6-TCP) > 4-chlorophenol (4-CP). The obtained KMapp values were 426.06, 9.41 and 73.36 µM for 4-CP, 2, 4-DCP and 2, 4, 6-TCP, respectively. The results indicated that Lac encapsulated into electrospun nanofibers retained its high catalytic activity. The sensing performance of this biosensor was attributed to the suitable electrochemical interface (e.g. biocompatibility, high surface area-to-volume ratio and superior mechanical properties) of PVA/F108/Au NPs/Lac.

**Materials**

52 Advances in Nanofibers

Hb

SWNTs-Hb

acetic acid.

**Fiber diameter (nm)**

thickness 60 ±21

> width ~2.5μm, thickness ~600 nm

> width ~2.5μm, thickness ~600 nm

**Table 1.** Electrospun nanofibers based electrochemical sensors.

**4.1. Polymer nanofibers based electrochemical sensors**

**Analytes**

nitrite H2O2

TCA nitrite H2O2

NiO-Pt 214±77 glucose 0.6 ~3670 0.313 [232] CuO-NiO 10 μm glucose 0.5 3-510 0.001 [233] NiO-CdO ~ glucose 0.6 ~6370 0.35 [234] Mn2O3 105 hydrazine 0.6 ~644 0.3 [235] CuO/Co3O4 150-350 fructose 0.3 10-6000 3 [236]

TiO2-Pt 72.61±15.04 hydrazine 0.5 ~1030 0.142 [239] SiO2@Au ~ H2O2 -0.4 5-1000 2 [240]

Abbreviations in the table: PVA: poly(vinyl alcohol); F108: PEO-PPO-PEO; Lac: laccase; 4-CP: 4-chlorophenol; 2, 4-DCP: 2, 4-dichlorophenol; 2, 4, 6-TCP: 2, 4, 6-trichlorophenol; PMMA: Poly(methyl methacrylate); PANi: polyaniline; PEO: poly(ethylene oxide); PANCAA: poly(acrylonitrile-co-acrylic acid); SWNT: Single-walled carbon nanotube; MWNT: Multiwalled carbon nanotube; PVDF: poly(vinylidene fluoride); PAPBA: poly(aminophenyl boronic acid); P2W18: α-K6[P2W18O62] 14H2O; CNF: carbon nanofiber; Hb: hemoglobin; TCA: trichloro‐

Since the first enzyme-based electrochemical biosensor was proposed by Clark and Lyons [241], numerous efforts have been afforded in this direction because of the simplicity and high selectivity of enzyme electrodes. The immobilization of enzymes on a suitable matrix and their stability are important factors in the fabrication of biosensors. Several methods have been developed for immobilization of enzymes, including physical adsorption, cross-linking, selfassembly, as well as entrapment in polymers or sol-gels. Due to the merits of high specific surface area and porous structure, electrospun polymer fibers would be a promising biocom‐ patible material for enzyme immobilization [1]. For example, Ren and co-workers prepared a

**Detection potential (V)**

> -0.65 -0.377

> -0.65 -0.65 -0.364

**Linear Rang (μM)**

> ~4500 ~27

12-108 ~207 ~27.3

**Limit of Detection (μM)**

> 0.47 0.61

> 2.41 0.30 0.22

**Ref.**

[237]

[238]

**Figure 10.** (A) Schematic picture of the nylon nanofibrous biosensing unit coupled with a glassy carbon electrode. Al‐ so shown a scanning electron microscopy detail of the nanofibrous structure. (B) Response current of nylon nanofiberbased glucose biosensors after the addition of glucose (1 mM each). Detection potential, + 0.5 V vs. Ag/AgCl. Supporting electrolyte, 0.1 M PBS (pH 6.5) containing 0.1 mM ferrocene methanol. Inset (bottom-right) shows the cor‐ responding calibration plot. Inset (top-left) shows the catalyzed electrochemical oxidation of glucose mediated by fer‐ rocene methanol, where curve a and b are the cyclic voltammograms (CVs) of the blank and of the ferrocene methanol (0.1 mM) in the absence of glucose. Other CVs are obtained upon addition of glucose from 5 to 100 mM. In all the CVs, scan rate=0.02 V s-1; 0.1 M PBS (pH 6.5) [199].

In addition to the direct incorporation of enzyme into polymer nanofibers, post-spun modifi‐ cation is a widely used method for constructing enzyme-based biosensors. Mannino's group developed glucose biosensors by using electrospun nylon-6 nanofibrous membranes (NFM) as the enzyme immobilization matrix [197-199]. A piece of NFM was placed over the electrode surface and secured with an o-ring (Fig. 10A). The highly porous morphology of the NFM allowed the analytes to diffuse toward the transducer, while the proteins might be retained by physical or chemical bonding with its large available surface. With the presence of mediator (ferrocene methanol) in the detection cell, a linear current response of this biosensor was obtained in the range of 1-10 mM, with detection limit of micromole-level (Fig. 10B). The KMapp value for the immobilized GOx was 17 mM, which was greater than that obtained for homo‐ geneous enzyme catalysis, but was comparable to that of GOx covalently bound to nylon [199]. These results indicated that this NFM provided favorable environment for the immobilization of GOx enzyme. Additionally, the nylon nanofibers membrane was also used to immobilize tyrosinase and construct an amperometric biosensor for the detection of phenolic compounds [200]. This biosensor showed excellent performance in respect to sensitivity, selectivity and reproducibility. Santhosh et al. developed an electrochemical sensor for the detection of superoxide anion (O2˙ˉ) based on Au nanoparticles loaded PMMA-polyaniline (PANi) coreshell electrospun nanofiber membrane [201]. This membrane provided high surface area and porous structure for effective immobilization of superoxide dismutase (SOD), as well as offered excellent biocompatible microenvironment for SOD. Direct electron transfer was achieved between SOD and the electrode with an electron transfer rate constant of 8.93 s-1. Jia et al. prepared a urea biosensor based on Pt nanoflower/PANi nanofibers [202]. PANi nano‐ fibers were prepared by in situ polymerization of aniline on an electrospun PAN nanofiber template in an acidic solution with ammonium persulfate as the oxidant. Pt nanoflowers were further electrodeposited onto the PANi nanofibers backbone by cyclic voltammetry. Then, urease was physically adsorbed on the Pt/PANi modified electrode, followed with Nafion entrapment. This biosensor was applied for the sensitive urea detection in a flow injection analysis (FIA) system.

delayed. The MWNT filling also increased the KMapp value, indicating that the secondary

Electrospun Nanofibers: From Rational Design, Fabrication to Electrochemical Sensing Applications

http://dx.doi.org/10.5772/57099

55

Although good selectivity and high sensitivity were obtained with these enzyme-based biosensors, inevitable drawbacks such as the chemical and thermal instabilities originated from the intrinsic nature of enzymes as well as the tedious fabrication procedures might limit their analytical applications. Therefore, it is desirable to develop sensitive and selective nonenzymatic sensors. Manesh et al. prepared a non-enzymatic glucose sensor based on the composite electrospun nanofibrous membrane of PVDF and poly(aminophenyl boronic acid) (PAPBA), which was collected on indium tin oxide (ITO) glass plate [205]. The smaller size of PVDF/PAPBA nanofibers provided a large number of active sites for sensing action and the boronic acid groups in PAPBA were the sources for the preferential selectivity and sensing of glucose. The sensor retained 90% of the original activity after 50 days repeated usage and

Scampicchio et al. studied the protective properties of nylon-6 nanofiber membrane against fouling and passivation of the carbon working electrode [243]. For example, the polyphenols oxidation usually results in the severe passivation of carbon electrode due to the adsorption of analytes or reaction intermediates. However, no voltammetric waves appeared at the nylon-6 nanofiber membrane coated electrode for the flavonoids (quercetin, myricetin and cathechin) oxidation. On the contrary, when phenol acids (caffeic, synapic, syringic, vanillic and gallic acid) were used, their typical oxidation waves emerged. Therefore, nylon-6 nano‐ fiber membrane could be used as a selective barrier to preserve the active surface of the electrode from passivation of flavonoids and to construct sensors with high selectivity. Furthermore, this protective nylon-6 nanofiber membrane was used to adsorb MWNTs and construct a sensor for the electrochemical detection of sulfhydryl compounds [206]. The membrane was easily peeled off, leaving the bare electrode surface back to its original electrochemical behaviour. Preliminary experiments indicated that the membrane coating protected the bare electrode from the passivation occurred during oxidation of cysteine. Cao and co-workers prepared a nitrite sensor based on polyoxometalate hybrid nanofibers, which was fabricated by electrospinning of a mixture of PVA and α-K6[P2W18O62] 14H2O (P2W18) onto the surface of an ITO electrode [207]. After thermal crosslinking at 135 ℃ for 24 hours, the P2W18 hybrid nanofibers were insoluble in aqueous solutions even after a period of 24 hours, which ensured the electrochemical stability of the hybrid nanofiber-modified ITO electrode. This P2W18 hybrid nanofiber modified electrode presented excellent electrocatalytic activity toward the reduction of nitrite, which could be attributed to the large electroactive surface area

Carbon nanofibers (CNFs), a unique 1D carbon nanomaterial, have attracted great interests due to their high mechanical strength and excellent electric properties similar to carbon nanotubes (CNTs), but larger surface-active groups-to-volume ratio than that of the glassylike surface of CNTs [244]. CNFs can be used as immobilization matrixes for biomolecules, while at the same time they can relay the electrochemical signal acting as transducers.

structure of immobilized GOx was disturbed.

storage at 4 ℃, indicating an excellent long-term stability.

of the P2W18 hybrid nanofibers.

**4.2. Carbon nanofiber based electrochemical sensors**

Carbon nanotubes (CNTs) have become the subject of intense investigation due to their remarkable electrical, chemical, mechanical and structural properties. Recent studies have demonstrated that CNT could greatly promote the electron-transfer reaction of proteins [242]. Therefore, CNT-filled electrospun nanofibers as matrix for the immobilization of enzyme are expected to further improve the analytical performance of enzyme electrode. Liu et al. prepared CNT-filled composite nanofibers by electrospinning DNA/SWNT/PEO blended suspension [203]. The noncovalent binding of DNA to the sidewalls of SWNTs was used to highly disperse SWNTs in the solution. The DNA/SWNT/PEO composite nanofibers were deposited on Pt-coated glass slides, and then directly used as substrate electrode for immobi‐ lization of GOx. This biosensor displayed the direct electrochemistry of GOx, suggesting that GOx immobilized on the nanofibers still maintained its electrochemical properties and the composite nanofibers promoted the electron transfer between the electrode and the redox center of enzyme. Nanofibrous membranes filled with multiwalled carbon nanotube (MWNT) were also electrospun from the mixture of poly(acrylonitrile-co-acrylic acid) (PANCAA) and MWNTs [204]. These nanofibrous membranes were directly deposited on Pt electrodes for the fabrication of glucose biosensors. Glucose oxidase (GOx) was covalently immobilized on the membranes through the activation of carboxyl groups on the PANCAA nanofiber surface. Compared with PANCAA nanofiber membrane, MWNT-filled PANCAA nanofiber mem‐ brane enhanced the maximum current response, while the electrode response time was delayed. The MWNT filling also increased the KMapp value, indicating that the secondary structure of immobilized GOx was disturbed.

surface and secured with an o-ring (Fig. 10A). The highly porous morphology of the NFM allowed the analytes to diffuse toward the transducer, while the proteins might be retained by physical or chemical bonding with its large available surface. With the presence of mediator (ferrocene methanol) in the detection cell, a linear current response of this biosensor was obtained in the range of 1-10 mM, with detection limit of micromole-level (Fig. 10B). The KMapp value for the immobilized GOx was 17 mM, which was greater than that obtained for homo‐ geneous enzyme catalysis, but was comparable to that of GOx covalently bound to nylon [199]. These results indicated that this NFM provided favorable environment for the immobilization of GOx enzyme. Additionally, the nylon nanofibers membrane was also used to immobilize tyrosinase and construct an amperometric biosensor for the detection of phenolic compounds [200]. This biosensor showed excellent performance in respect to sensitivity, selectivity and reproducibility. Santhosh et al. developed an electrochemical sensor for the detection of superoxide anion (O2˙ˉ) based on Au nanoparticles loaded PMMA-polyaniline (PANi) coreshell electrospun nanofiber membrane [201]. This membrane provided high surface area and porous structure for effective immobilization of superoxide dismutase (SOD), as well as offered excellent biocompatible microenvironment for SOD. Direct electron transfer was achieved between SOD and the electrode with an electron transfer rate constant of 8.93 s-1. Jia et al. prepared a urea biosensor based on Pt nanoflower/PANi nanofibers [202]. PANi nano‐ fibers were prepared by in situ polymerization of aniline on an electrospun PAN nanofiber template in an acidic solution with ammonium persulfate as the oxidant. Pt nanoflowers were further electrodeposited onto the PANi nanofibers backbone by cyclic voltammetry. Then, urease was physically adsorbed on the Pt/PANi modified electrode, followed with Nafion entrapment. This biosensor was applied for the sensitive urea detection in a flow injection

Carbon nanotubes (CNTs) have become the subject of intense investigation due to their remarkable electrical, chemical, mechanical and structural properties. Recent studies have demonstrated that CNT could greatly promote the electron-transfer reaction of proteins [242]. Therefore, CNT-filled electrospun nanofibers as matrix for the immobilization of enzyme are expected to further improve the analytical performance of enzyme electrode. Liu et al. prepared CNT-filled composite nanofibers by electrospinning DNA/SWNT/PEO blended suspension [203]. The noncovalent binding of DNA to the sidewalls of SWNTs was used to highly disperse SWNTs in the solution. The DNA/SWNT/PEO composite nanofibers were deposited on Pt-coated glass slides, and then directly used as substrate electrode for immobi‐ lization of GOx. This biosensor displayed the direct electrochemistry of GOx, suggesting that GOx immobilized on the nanofibers still maintained its electrochemical properties and the composite nanofibers promoted the electron transfer between the electrode and the redox center of enzyme. Nanofibrous membranes filled with multiwalled carbon nanotube (MWNT) were also electrospun from the mixture of poly(acrylonitrile-co-acrylic acid) (PANCAA) and MWNTs [204]. These nanofibrous membranes were directly deposited on Pt electrodes for the fabrication of glucose biosensors. Glucose oxidase (GOx) was covalently immobilized on the membranes through the activation of carboxyl groups on the PANCAA nanofiber surface. Compared with PANCAA nanofiber membrane, MWNT-filled PANCAA nanofiber mem‐ brane enhanced the maximum current response, while the electrode response time was

analysis (FIA) system.

54 Advances in Nanofibers

Although good selectivity and high sensitivity were obtained with these enzyme-based biosensors, inevitable drawbacks such as the chemical and thermal instabilities originated from the intrinsic nature of enzymes as well as the tedious fabrication procedures might limit their analytical applications. Therefore, it is desirable to develop sensitive and selective nonenzymatic sensors. Manesh et al. prepared a non-enzymatic glucose sensor based on the composite electrospun nanofibrous membrane of PVDF and poly(aminophenyl boronic acid) (PAPBA), which was collected on indium tin oxide (ITO) glass plate [205]. The smaller size of PVDF/PAPBA nanofibers provided a large number of active sites for sensing action and the boronic acid groups in PAPBA were the sources for the preferential selectivity and sensing of glucose. The sensor retained 90% of the original activity after 50 days repeated usage and storage at 4 ℃, indicating an excellent long-term stability.

Scampicchio et al. studied the protective properties of nylon-6 nanofiber membrane against fouling and passivation of the carbon working electrode [243]. For example, the polyphenols oxidation usually results in the severe passivation of carbon electrode due to the adsorption of analytes or reaction intermediates. However, no voltammetric waves appeared at the nylon-6 nanofiber membrane coated electrode for the flavonoids (quercetin, myricetin and cathechin) oxidation. On the contrary, when phenol acids (caffeic, synapic, syringic, vanillic and gallic acid) were used, their typical oxidation waves emerged. Therefore, nylon-6 nano‐ fiber membrane could be used as a selective barrier to preserve the active surface of the electrode from passivation of flavonoids and to construct sensors with high selectivity. Furthermore, this protective nylon-6 nanofiber membrane was used to adsorb MWNTs and construct a sensor for the electrochemical detection of sulfhydryl compounds [206]. The membrane was easily peeled off, leaving the bare electrode surface back to its original electrochemical behaviour. Preliminary experiments indicated that the membrane coating protected the bare electrode from the passivation occurred during oxidation of cysteine. Cao and co-workers prepared a nitrite sensor based on polyoxometalate hybrid nanofibers, which was fabricated by electrospinning of a mixture of PVA and α-K6[P2W18O62] 14H2O (P2W18) onto the surface of an ITO electrode [207]. After thermal crosslinking at 135 ℃ for 24 hours, the P2W18 hybrid nanofibers were insoluble in aqueous solutions even after a period of 24 hours, which ensured the electrochemical stability of the hybrid nanofiber-modified ITO electrode. This P2W18 hybrid nanofiber modified electrode presented excellent electrocatalytic activity toward the reduction of nitrite, which could be attributed to the large electroactive surface area of the P2W18 hybrid nanofibers.

#### **4.2. Carbon nanofiber based electrochemical sensors**

Carbon nanofibers (CNFs), a unique 1D carbon nanomaterial, have attracted great interests due to their high mechanical strength and excellent electric properties similar to carbon nanotubes (CNTs), but larger surface-active groups-to-volume ratio than that of the glassylike surface of CNTs [244]. CNFs can be used as immobilization matrixes for biomolecules, while at the same time they can relay the electrochemical signal acting as transducers. Therefore, a great number of CNF-based sensors or biosensors have been developed [245]. In combination with the carbonization process, electrospinning has been actively exploited as a valuable and versatile method for preparation of CNFs with the controllable structure and texture [62]. As a result, electrospun CNFs or their composite materials are expected to be a promising material for constructing ultrasensitive electrochemical sensors.

Our group has successfully prepared CNFs by electrospinning, followed by stabilization and carbonization processes. These electrospun CNFs were directly used to modify carbon paste electrode (CNF-CPE) and construct a sensor for mediatorless detection of NADH (Fig. 11) [208]. This electrochemical sensor showed low detection limit down to nM-level, wide linear range and good selectivity for determination of NADH in the presence of ascorbic acid (AA). CNF-CPE was also employed for the simultaneous determination of AA, dopamine (DA), and uric acid (UA) by using differential pulse voltammetry (DPV) method [209]. Three welldefined peaks with remarkably increased peak current could be achieved at the CNF-CPE. Low detection limits of 0.04 µM, 2 µM and 0.2 µM for DA, AA and UA were obtained. Some oxidizable amino acids such as L-tryptophane, L-tyrosine and L-cysteine play important roles in many biochemical processes. However, the determination of these amine acids usually suffers from high overpotential and poor reproducibility. We found that the electrospun CNF modified electrode displayed high electrocatalytic activity toward the oxidation of these amino acids with enhanced peak currents and low overpotentials [210]. This sensor showed excellent analytical performance for the detection of the three amino acids. In addition, electrospun CNF modified electrode was also used for non-enzymatic electrochemical detection of xanthine [211], and simultaneous determination of dihydroxybenzene isomers (catechol and hydroqui‐ none) [212]. These sensors exhibited high sensitivity, stability and selectivity, as well as good anti-fouling properties. The practical application of these sensors for determining the target analytes was evaluated, and satisfactory results were obtained. Recently, we fabricated a novel composite electrode by mixing the electrospun CNF with the ionic liquid 1-butyl-4-methyl‐ pyridinium hexaflurophosphate (PFP) [246]. This CNF/PFP electrode exhibited strong current response and low background noise at the studied composite ratio. When used as electro‐ chemical sensor, it showed high sensitivity and good selectivity for simultaneous detection of DA, AA and UA, guanine and adenine, as well as high signal-to-noise ratio (S/N) and good stability for amperometric detection of NADH under physiological conditions.

subsequent analytical performance [213]. Raman spectra indicated the crystallization and orientation of the carbon fibers was improved with the increase of carbonization temperature. The electrical conductivity was also improved after heat treatment at higher temperature. The sample treated at 2473 K showed the highest sensitivity for glucose detection among the tested samples, which was ascribed to the high porosity, crystallization and orientation of the carbon structure. Additionally, CNTs were used as an electrically conductive additive to prepare CNT-embedded carbon fibers [248]. Combined with physical activation and oxyfluorination treatment, the prepared glucose sensor showed improved sensitivity and rapid response time

**Figure 11.** (A) SEM image of electrospun CNFs. Inset shows TEM image of CNFs. (B) CVs of 0.1 M PBS (pH 7.0) a) plain and b) containing 1 mM NADH at the CNF-CPE; c) Corresponding CV of (b) with the CPE. Scan rate: 50 mV s-1 [208].

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**Figure 12.** (A) Typical TEM image of Pd/CNF nanocomposites; (B) Current-time responses of the Pd/CNF-CPE on suc‐ cessive injection of specific concentration of H2O2 into N2-saturated PBS (0.1 M, pH 7.0), inset shows the calibration

Metal nanoparticle/CNF nanocomposites have received great attention in catalysis, fuel cell, and chemical/biological sensing applications. In conventional synthesis method, CNFs usually suffer from harsh oxidation or modification with polymers in order to realize

as a result of more efficient GOx immobilization and electron transfer.

curve for H2O2 concentration between 0.2 μM and 20 mM [214].

Recently, Lee's group prepared porous carbon nanomaterials by electrospinning, thermal treatment and activation process, and then constructed GOx-based glucose biosensors [213, 247, 248]. Silica nanoparticles with average size of 16±2 nm were used as physical activation agent. It was found that micro- and mesopores were induced through the physical activation process, which increased the specific surface area by over 42-fold compared to the untreated materials [247]. These carbon nanomaterials were also treated by oxyfluorination at 1 bar for 5 min using a mixed gas of oxygen and fluorine to introduce hydrophilic functional groups. After the activation and oxyfluorination treatment, the GOx immobilization was maximized by enlarged sites of carbon electrode and improved interfacial affinity between the carbon surface and the GOx. Subsequently, high sensitivity was obtained for this glucose sensor. They also investigated the influence of carbonization temperature on the carbon structure, and

Electrospun Nanofibers: From Rational Design, Fabrication to Electrochemical Sensing Applications http://dx.doi.org/10.5772/57099 57

Therefore, a great number of CNF-based sensors or biosensors have been developed [245]. In combination with the carbonization process, electrospinning has been actively exploited as a valuable and versatile method for preparation of CNFs with the controllable structure and texture [62]. As a result, electrospun CNFs or their composite materials are expected to be a

Our group has successfully prepared CNFs by electrospinning, followed by stabilization and carbonization processes. These electrospun CNFs were directly used to modify carbon paste electrode (CNF-CPE) and construct a sensor for mediatorless detection of NADH (Fig. 11) [208]. This electrochemical sensor showed low detection limit down to nM-level, wide linear range and good selectivity for determination of NADH in the presence of ascorbic acid (AA). CNF-CPE was also employed for the simultaneous determination of AA, dopamine (DA), and uric acid (UA) by using differential pulse voltammetry (DPV) method [209]. Three welldefined peaks with remarkably increased peak current could be achieved at the CNF-CPE. Low detection limits of 0.04 µM, 2 µM and 0.2 µM for DA, AA and UA were obtained. Some oxidizable amino acids such as L-tryptophane, L-tyrosine and L-cysteine play important roles in many biochemical processes. However, the determination of these amine acids usually suffers from high overpotential and poor reproducibility. We found that the electrospun CNF modified electrode displayed high electrocatalytic activity toward the oxidation of these amino acids with enhanced peak currents and low overpotentials [210]. This sensor showed excellent analytical performance for the detection of the three amino acids. In addition, electrospun CNF modified electrode was also used for non-enzymatic electrochemical detection of xanthine [211], and simultaneous determination of dihydroxybenzene isomers (catechol and hydroqui‐ none) [212]. These sensors exhibited high sensitivity, stability and selectivity, as well as good anti-fouling properties. The practical application of these sensors for determining the target analytes was evaluated, and satisfactory results were obtained. Recently, we fabricated a novel composite electrode by mixing the electrospun CNF with the ionic liquid 1-butyl-4-methyl‐ pyridinium hexaflurophosphate (PFP) [246]. This CNF/PFP electrode exhibited strong current response and low background noise at the studied composite ratio. When used as electro‐ chemical sensor, it showed high sensitivity and good selectivity for simultaneous detection of DA, AA and UA, guanine and adenine, as well as high signal-to-noise ratio (S/N) and good

promising material for constructing ultrasensitive electrochemical sensors.

56 Advances in Nanofibers

stability for amperometric detection of NADH under physiological conditions.

Recently, Lee's group prepared porous carbon nanomaterials by electrospinning, thermal treatment and activation process, and then constructed GOx-based glucose biosensors [213, 247, 248]. Silica nanoparticles with average size of 16±2 nm were used as physical activation agent. It was found that micro- and mesopores were induced through the physical activation process, which increased the specific surface area by over 42-fold compared to the untreated materials [247]. These carbon nanomaterials were also treated by oxyfluorination at 1 bar for 5 min using a mixed gas of oxygen and fluorine to introduce hydrophilic functional groups. After the activation and oxyfluorination treatment, the GOx immobilization was maximized by enlarged sites of carbon electrode and improved interfacial affinity between the carbon surface and the GOx. Subsequently, high sensitivity was obtained for this glucose sensor. They also investigated the influence of carbonization temperature on the carbon structure, and

**Figure 11.** (A) SEM image of electrospun CNFs. Inset shows TEM image of CNFs. (B) CVs of 0.1 M PBS (pH 7.0) a) plain and b) containing 1 mM NADH at the CNF-CPE; c) Corresponding CV of (b) with the CPE. Scan rate: 50 mV s-1 [208].

subsequent analytical performance [213]. Raman spectra indicated the crystallization and orientation of the carbon fibers was improved with the increase of carbonization temperature. The electrical conductivity was also improved after heat treatment at higher temperature. The sample treated at 2473 K showed the highest sensitivity for glucose detection among the tested samples, which was ascribed to the high porosity, crystallization and orientation of the carbon structure. Additionally, CNTs were used as an electrically conductive additive to prepare CNT-embedded carbon fibers [248]. Combined with physical activation and oxyfluorination treatment, the prepared glucose sensor showed improved sensitivity and rapid response time as a result of more efficient GOx immobilization and electron transfer.

**Figure 12.** (A) Typical TEM image of Pd/CNF nanocomposites; (B) Current-time responses of the Pd/CNF-CPE on suc‐ cessive injection of specific concentration of H2O2 into N2-saturated PBS (0.1 M, pH 7.0), inset shows the calibration curve for H2O2 concentration between 0.2 μM and 20 mM [214].

Metal nanoparticle/CNF nanocomposites have received great attention in catalysis, fuel cell, and chemical/biological sensing applications. In conventional synthesis method, CNFs usually suffer from harsh oxidation or modification with polymers in order to realize selective deposition of metal nanoparticles on the surface of CNFs. These surface function‐ alization approaches provide efficient avenues for the deposition of metal nanoparticles, but tend to degrade the mechanical and electronic properties of CNFs because of the introduction of a large number of defects or polymer shell. Electrospinning provided a simple and efficient method to prepare metal nanoparticle/CNF nanocomposites with high quality and purity. Recently, palladium nanoparticle-loaded carbon nanofibers (Pd/CNFs) were synthesized by the combination of electrospinning, reduction and carbonization processes [214]. The metallic Pd nanoparticles were well-dispersed on the surface or completely embedded into CNFs (Fig. 12A), which rendered the Pd nanopatticles high stability and resistance to the aggregation and desquamation. Pd/CNF-modified electrode exhibited high electrocatalytic activities towards the reduction of H2O2 and the oxidation of NADH. For H2O2, the Pd/CNF-modified electrode displayed a wider linear range from 0.2 µM to 20 mM with a detection limit of 0.2 µM at -0.2 V (Fig. 12B), and the detection was free of interference from the coexisted AA and UA. In the case of NADH, the linear range at the Pd/CNF-modified electrode was from 0.2 µM to 716.6 µM with a detection limit of 0.2 µM at 0.5 V. The high sensitivity, wide linear range, good reproducibility, and the minimal surface fouling make this Pd/CNF-modified electrode a promising candidate for amperometric H2O2 or NADH sensor. Pd/CNFs modified electrode also displayed excellent electrocatalytic activities towards DA, UA and AA [215]. The oxidation overpoten‐ tials of DA, UA and AA were decreased significantly compared with those obtained at the bare electrode. Due to the different extent of the peak potential shift, these three com‐ pounds could be determined simultaneously by CV or DPV at the Pd/CNF modified electrode. The Pd/CNF composite materials were also applied for the detection of hydra‐ zine and oxalic acid with attractive analytical performances [216, 217]. Nickel nanoparticleloaded carbon nanofibers (NiCF) were also fabricated by using the similar method to that of Pd/CNF [218]. NiCF paste (NiCFP) electrode exhibited excellent electrocatalytic perform‐ ance for the oxidation of glucose. The amperometric responses of the NiCFP electrode to glucose showed a linear range from 2 µM to 2.5 mM with the detection limit of 1 µM at the applied potential of 0.6 V. The proposed electrode, featured with good resistance to surface fouling and high operational stability, could be used as a promising nonenzymat‐ ic glucose sensor. The NiCFP electrode also showed high electrocatalytic activity toward the ethanol oxidation, and was used as enzyme-free ethanol sensor [219]. The detection exhibited high response, good stability and acceptable reproducibility. Similarly, Hu et al. prepared rhodium nanoparticle-loaded carbon nanofibers by electrospinning [220]. Rh nanoparticles with the diameter of 30-70 nm were uniformly distributed on the CNF surface. This nanocomposite was used for determination of hydrazine with high sensitivity and selectivity. Very recently, a Pt nanoparticle-loaded electrospun carbon nanofiber electrode was prepared by a simple wet-chemical method [221]. CNF paste electrode was firstly prepared using electrospun CNFs, then it was immersed into H2PtCl6 solution to adsorb [PtCl6] 2−. After that, HCOOH was added to reduce the metal precursors. Large amounts of Pt nanoparticles could be well deposited on the surface of the electrospun CNF electrode without using any stabilizer or pretreatment procedure. In application to electrochemical

sensing platform, the Pt/CNF electrode exhibited high sensitivity and good selectivity for

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Electrospinning has been proved to be a simple method for large-scale producing metal or metal oxide nanofibers. One of the most important applications of these nanomaterials is to develop their potential in chemical sensing or biosensing, profiting from their small size, large surface-to-volume ratios and high aspect ratios. Reliable and fast determination of glucose is of considerable importance in biotechnology, clinical diagnostics and food industry. Up to now, numerous electrospun metal/metal oxide nanofiber based glucose sensors or biosensors have been reported. For example, Ahmad and co-workers prepared an amperometric glucose biosensor based on a single ZnO nanofiber which was pro‐ duced by electrospinning of PVP/zinc acetate mixture solution and subsequent hightemperature calcination [222]. A single ZnO nanofiber was transferred on Au electrode and functionalized with GOx via physical adsorption. The KMapp value was estimated to be 2.19 mM, indicated that the immobilized GOx possessed a high enzymatic activity. Huang et al. fabricated highly porous Mn2O3-Ag nanofibers by a two-step procedure (electrospin‐ ning and calcinations) [223]. The as-prepared Mn2O3-Ag nanofibers were employed as the immobilization matrix for GOx to construct oxygen-reduction based glucose biosensor. The Mn2O3-Ag nanofibers could effectively mediated the direct electron transfer between the electroactive center of GOx and the electrode. This biosensor displayed good analytical performance for glucose detection due to the merits of this porous nanofiber, such as high surface area for enzyme loading, and high electrocatalytic activity toward the reduction of oxygen. Recently, electrospun Au nanofiber based biosensor for the detection of fructose and glucose was also developed by Russell's group [224]. The gold fibers were prepared by electroless deposition of gold nanoparticles on an electrospun PAN-HAuCl4 fiber. Fructose dehydrogenase was covalently coupled to the Au fiber surface through glutaralde‐ hyde crosslink to a cystamine monolayer. The enzyme exhibited mediated electron transfer directly to the gold electrode, and catalytic currents characteristic of fructose oxidation in the presence of a ferrocene methanol mediator were observed. This fructose sensor could also be used to determine glucose by using glucose isomerase to convert glucose to fructose.

Compared with the enzyme-based glucose biosensors, nonenzymatic glucose sensors are preferential because they avoid the problem of enzyme denature and intricate enzyme immobilization process. The nonenzymatic electrochemical glucose sensors significantly depend on the properties of electrode materials, on which glucose is oxidized directly. Various electrospun metal oxide nanofibers have been used to construct nonenzymatic glucose sensors. For example, Ding et al. fabricated Co3O4 nanofibers by electrospinning and subsequent calcination [225]. The as-prepared Co3O4 nanofibers were applied to construct a non-enzymatic sensor for glucose detection in alkaline solution. The catalytic property of the as-prepared Co3O4 nanofibers towards glucose oxidation was related to CoOOH and CoO2. The negatively charged Co3O4 nanofibers surface could strongly repel

**4.3. Metal/metal oxide nanofiber based electrochemical sensors**

amperometric detection of H2O2.

sensing platform, the Pt/CNF electrode exhibited high sensitivity and good selectivity for amperometric detection of H2O2.

#### **4.3. Metal/metal oxide nanofiber based electrochemical sensors**

selective deposition of metal nanoparticles on the surface of CNFs. These surface function‐ alization approaches provide efficient avenues for the deposition of metal nanoparticles, but tend to degrade the mechanical and electronic properties of CNFs because of the introduction of a large number of defects or polymer shell. Electrospinning provided a simple and efficient method to prepare metal nanoparticle/CNF nanocomposites with high quality and purity. Recently, palladium nanoparticle-loaded carbon nanofibers (Pd/CNFs) were synthesized by the combination of electrospinning, reduction and carbonization processes [214]. The metallic Pd nanoparticles were well-dispersed on the surface or completely embedded into CNFs (Fig. 12A), which rendered the Pd nanopatticles high stability and resistance to the aggregation and desquamation. Pd/CNF-modified electrode exhibited high electrocatalytic activities towards the reduction of H2O2 and the oxidation of NADH. For H2O2, the Pd/CNF-modified electrode displayed a wider linear range from 0.2 µM to 20 mM with a detection limit of 0.2 µM at -0.2 V (Fig. 12B), and the detection was free of interference from the coexisted AA and UA. In the case of NADH, the linear range at the Pd/CNF-modified electrode was from 0.2 µM to 716.6 µM with a detection limit of 0.2 µM at 0.5 V. The high sensitivity, wide linear range, good reproducibility, and the minimal surface fouling make this Pd/CNF-modified electrode a promising candidate for amperometric H2O2 or NADH sensor. Pd/CNFs modified electrode also displayed excellent electrocatalytic activities towards DA, UA and AA [215]. The oxidation overpoten‐ tials of DA, UA and AA were decreased significantly compared with those obtained at the bare electrode. Due to the different extent of the peak potential shift, these three com‐ pounds could be determined simultaneously by CV or DPV at the Pd/CNF modified electrode. The Pd/CNF composite materials were also applied for the detection of hydra‐ zine and oxalic acid with attractive analytical performances [216, 217]. Nickel nanoparticleloaded carbon nanofibers (NiCF) were also fabricated by using the similar method to that of Pd/CNF [218]. NiCF paste (NiCFP) electrode exhibited excellent electrocatalytic perform‐ ance for the oxidation of glucose. The amperometric responses of the NiCFP electrode to glucose showed a linear range from 2 µM to 2.5 mM with the detection limit of 1 µM at the applied potential of 0.6 V. The proposed electrode, featured with good resistance to surface fouling and high operational stability, could be used as a promising nonenzymat‐ ic glucose sensor. The NiCFP electrode also showed high electrocatalytic activity toward the ethanol oxidation, and was used as enzyme-free ethanol sensor [219]. The detection exhibited high response, good stability and acceptable reproducibility. Similarly, Hu et al. prepared rhodium nanoparticle-loaded carbon nanofibers by electrospinning [220]. Rh nanoparticles with the diameter of 30-70 nm were uniformly distributed on the CNF surface. This nanocomposite was used for determination of hydrazine with high sensitivity and selectivity. Very recently, a Pt nanoparticle-loaded electrospun carbon nanofiber electrode was prepared by a simple wet-chemical method [221]. CNF paste electrode was firstly prepared using electrospun CNFs, then it was immersed into H2PtCl6 solution to adsorb

2−. After that, HCOOH was added to reduce the metal precursors. Large amounts of Pt nanoparticles could be well deposited on the surface of the electrospun CNF electrode without using any stabilizer or pretreatment procedure. In application to electrochemical

[PtCl6]

58 Advances in Nanofibers

Electrospinning has been proved to be a simple method for large-scale producing metal or metal oxide nanofibers. One of the most important applications of these nanomaterials is to develop their potential in chemical sensing or biosensing, profiting from their small size, large surface-to-volume ratios and high aspect ratios. Reliable and fast determination of glucose is of considerable importance in biotechnology, clinical diagnostics and food industry. Up to now, numerous electrospun metal/metal oxide nanofiber based glucose sensors or biosensors have been reported. For example, Ahmad and co-workers prepared an amperometric glucose biosensor based on a single ZnO nanofiber which was pro‐ duced by electrospinning of PVP/zinc acetate mixture solution and subsequent hightemperature calcination [222]. A single ZnO nanofiber was transferred on Au electrode and functionalized with GOx via physical adsorption. The KMapp value was estimated to be 2.19 mM, indicated that the immobilized GOx possessed a high enzymatic activity. Huang et al. fabricated highly porous Mn2O3-Ag nanofibers by a two-step procedure (electrospin‐ ning and calcinations) [223]. The as-prepared Mn2O3-Ag nanofibers were employed as the immobilization matrix for GOx to construct oxygen-reduction based glucose biosensor. The Mn2O3-Ag nanofibers could effectively mediated the direct electron transfer between the electroactive center of GOx and the electrode. This biosensor displayed good analytical performance for glucose detection due to the merits of this porous nanofiber, such as high surface area for enzyme loading, and high electrocatalytic activity toward the reduction of oxygen. Recently, electrospun Au nanofiber based biosensor for the detection of fructose and glucose was also developed by Russell's group [224]. The gold fibers were prepared by electroless deposition of gold nanoparticles on an electrospun PAN-HAuCl4 fiber. Fructose dehydrogenase was covalently coupled to the Au fiber surface through glutaralde‐ hyde crosslink to a cystamine monolayer. The enzyme exhibited mediated electron transfer directly to the gold electrode, and catalytic currents characteristic of fructose oxidation in the presence of a ferrocene methanol mediator were observed. This fructose sensor could also be used to determine glucose by using glucose isomerase to convert glucose to fructose.

Compared with the enzyme-based glucose biosensors, nonenzymatic glucose sensors are preferential because they avoid the problem of enzyme denature and intricate enzyme immobilization process. The nonenzymatic electrochemical glucose sensors significantly depend on the properties of electrode materials, on which glucose is oxidized directly. Various electrospun metal oxide nanofibers have been used to construct nonenzymatic glucose sensors. For example, Ding et al. fabricated Co3O4 nanofibers by electrospinning and subsequent calcination [225]. The as-prepared Co3O4 nanofibers were applied to construct a non-enzymatic sensor for glucose detection in alkaline solution. The catalytic property of the as-prepared Co3O4 nanofibers towards glucose oxidation was related to CoOOH and CoO2. The negatively charged Co3O4 nanofibers surface could strongly repel the negatively charged UA and AA molecules, thus resulting in good selectivity. Other metal oxide nanofibers, such as CuO [226, 227], and NiO [228] were also prepared by using the similar method and used for nonenzymatic detection of glucose. The direct glucose detection on these metal oxide nanofiber modified electrodes usually carried out in alkaline electrolyte and mediated by Ni(OH)2/NiO(OH) or Cu(OH)2/CuO(OH) redox couples. The study also demonstrated that the content of metal precursor in the electrospinning solution and the calcination temperature greatly influenced the morphology and catalytic activity of the produced nanomaterials [227, 228]. In contrast to the monometallic nanomaterials, bimetallic ones usually show enhanced electrocatalytic activity due to the synergistic effect. Wang et al. initially prepared electrospun palladium (IV)-doped CuO composite nanofib‐ er based non-enzymatic glucose sensors [229]. The as-prepared nanofibers had a rough surface and consisted of the agglomeration of oxide nanoparticles with average size of about 40 nm. This sensor exhibited high sensitivity for the determination of glucose with the detection limit of 19 nM. Following a facile two-step synthesis route of electrospinning and calcination, Ding and co-workers prepared NiO-Ag hybrid nanofibers, NiO nanofibers, and porous Ag [230]. The NiO-Ag hybrid nanofibers consisted of homogeneous distribution of NiO and irregular distribution of Ag nanoclusters. The as-prepared samples were then applied to construct non-enzymatic sensors for glucose detection. The NiO-Ag hybrid nanofiber modified electrode showed 55-fold higher sensitivity than that obtained on the porous Ag modified electrode at 0.1 V, and 5.2-fold higher sensitivity, lower detection limit and wider linear range than that of the NiO nanofiber modified electrode at 0.6 V (Fig. 13). The significant improvement obtained with NiO-Ag nanofiber were attributed to the use of abundant nanofibers which could provide numerous electron transfer tunnels, the highly porous structure which minimized the diffusion resistance of analytes, and the synergetic effect between NiO and Ag. This method have also been extended to prepare NiO-Au [231], and NiO-Pt [232] bimetallic nanofibers. The as-prepared hybrid nanofibers were em‐ ployed for the nonenzymatic glucose detection in alkaline electrolyte and showed im‐ proved analytical performance compared to the monometallic counterparts. Binary metal oxide nanofibers, including CuO-NiO [233], and NiO-CdO [234] have also exploited as the candidates for developing nonenzymatic glucose sensors. These binary metal oxide nanofibers showed good analytical properties for glucose detection due to the large amounts of reactive sites on the electrode surface and improved conductivity of NiO nanofibers by the incorporation of secondary metal oxide.

**Figure 13.** (A) Hydrodynamic voltammograms of 200 μM glucose at the porous Ag/GCE, NiO NFs/GCE and NiO-Ag NFs/GCE; (B) Amperometric response of porous Ag/GCE and NiO-Ag NFs/GCE to successive additions of glucose at an applied potential of 0.1 V; (C) Amperometric response of porous NiO NFs/GCE and NiO-Ag NFs/GCE to successive ad‐

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Ding et al. developed an amperometric biosensor by directly electrospinning deposition of hemoglobin (Hb) microbelts on the surface of glassy carbon electrode (Fig. 14A) [237]. This porous Hb microbelt coating closely contacted to the electrode surface and showed enhanced direct electrochemistry of Hb (Fig. 14B). The Hb microbelts based amperometric biosensor showed a fast response to the analytes and low detection limits of 0.61 µM for H2O2 and 0.47 µM for nitrite. The KMapp value of 0.093 mM was obtained for the electrocatalytic reduction of H2O2, reflecting the high affinity of Hb to the substrate H2O2. SWNT-Hb composite microbelts were also fabricated by the same group and employed as active material to prepare mediatorfree biosensors [238]. The direct electrochemistry of Hb at SWNT-Hb/GCE was more promi‐ nent than that obtained at the Hb microbelt/GCE because of the enhanced electron transfer by incorporated/embedded SWNTs and the porous 3D structure of Hb microbelt coating. Sensitive amperometric detection of trichloroacetic acid (TCA), nitrite, and H2O2 was obtained with the detection limits of 2.41 µM, 0.30 µM and 0.22 µM, respectively. TiO2-Pt nanofibers were fabricated by electrospinning PVP/ethanol solution containing platinum acetate and Ti(O*i*Pr)4, followed by calcination in air at 500 °C for 3 h [239]. The as-prepared TiO2-Pt hybrid nanofibers were used as the electrochemical catalyst for hydrazine detection. Au-coated SiO2

ditions of glucose at an applied potential of 0.6 V; (D) the corresponding calibration curves [230].

**4.4. Other electrospun nanofibers based electrochemical sensors**

In addition to the predominant glucose sensors, the applications of electrospun metal/metal oxide nanofibers in the preparation of sensors for other important analytes were also investi‐ gated. For example, Ding et al. constructed an amperometric sensor for hydrazine detection by using electrospun Mn2O3 nanofibers [235]. Wang and co-workers exploited electrospun CuO-Co3O4 nanofibers as active electrode materials for direct enzyme-free fructose detection [236]. These works demonstrated that electrospun metal/metal oxide nanometerial is one of the promising catalytic electrode materials for constructing ultrasensitive electrochemical sensors.

Electrospun Nanofibers: From Rational Design, Fabrication to Electrochemical Sensing Applications http://dx.doi.org/10.5772/57099 61

**Figure 13.** (A) Hydrodynamic voltammograms of 200 μM glucose at the porous Ag/GCE, NiO NFs/GCE and NiO-Ag NFs/GCE; (B) Amperometric response of porous Ag/GCE and NiO-Ag NFs/GCE to successive additions of glucose at an applied potential of 0.1 V; (C) Amperometric response of porous NiO NFs/GCE and NiO-Ag NFs/GCE to successive ad‐ ditions of glucose at an applied potential of 0.6 V; (D) the corresponding calibration curves [230].

#### **4.4. Other electrospun nanofibers based electrochemical sensors**

the negatively charged UA and AA molecules, thus resulting in good selectivity. Other metal oxide nanofibers, such as CuO [226, 227], and NiO [228] were also prepared by using the similar method and used for nonenzymatic detection of glucose. The direct glucose detection on these metal oxide nanofiber modified electrodes usually carried out in alkaline electrolyte and mediated by Ni(OH)2/NiO(OH) or Cu(OH)2/CuO(OH) redox couples. The study also demonstrated that the content of metal precursor in the electrospinning solution and the calcination temperature greatly influenced the morphology and catalytic activity of the produced nanomaterials [227, 228]. In contrast to the monometallic nanomaterials, bimetallic ones usually show enhanced electrocatalytic activity due to the synergistic effect. Wang et al. initially prepared electrospun palladium (IV)-doped CuO composite nanofib‐ er based non-enzymatic glucose sensors [229]. The as-prepared nanofibers had a rough surface and consisted of the agglomeration of oxide nanoparticles with average size of about 40 nm. This sensor exhibited high sensitivity for the determination of glucose with the detection limit of 19 nM. Following a facile two-step synthesis route of electrospinning and calcination, Ding and co-workers prepared NiO-Ag hybrid nanofibers, NiO nanofibers, and porous Ag [230]. The NiO-Ag hybrid nanofibers consisted of homogeneous distribution of NiO and irregular distribution of Ag nanoclusters. The as-prepared samples were then applied to construct non-enzymatic sensors for glucose detection. The NiO-Ag hybrid nanofiber modified electrode showed 55-fold higher sensitivity than that obtained on the porous Ag modified electrode at 0.1 V, and 5.2-fold higher sensitivity, lower detection limit and wider linear range than that of the NiO nanofiber modified electrode at 0.6 V (Fig. 13). The significant improvement obtained with NiO-Ag nanofiber were attributed to the use of abundant nanofibers which could provide numerous electron transfer tunnels, the highly porous structure which minimized the diffusion resistance of analytes, and the synergetic effect between NiO and Ag. This method have also been extended to prepare NiO-Au [231], and NiO-Pt [232] bimetallic nanofibers. The as-prepared hybrid nanofibers were em‐ ployed for the nonenzymatic glucose detection in alkaline electrolyte and showed im‐ proved analytical performance compared to the monometallic counterparts. Binary metal oxide nanofibers, including CuO-NiO [233], and NiO-CdO [234] have also exploited as the candidates for developing nonenzymatic glucose sensors. These binary metal oxide nanofibers showed good analytical properties for glucose detection due to the large amounts of reactive sites on the electrode surface and improved conductivity of NiO nanofibers by

In addition to the predominant glucose sensors, the applications of electrospun metal/metal oxide nanofibers in the preparation of sensors for other important analytes were also investi‐ gated. For example, Ding et al. constructed an amperometric sensor for hydrazine detection by using electrospun Mn2O3 nanofibers [235]. Wang and co-workers exploited electrospun CuO-Co3O4 nanofibers as active electrode materials for direct enzyme-free fructose detection [236]. These works demonstrated that electrospun metal/metal oxide nanometerial is one of the promising catalytic electrode materials for constructing ultrasensitive electrochemical

the incorporation of secondary metal oxide.

sensors.

60 Advances in Nanofibers

Ding et al. developed an amperometric biosensor by directly electrospinning deposition of hemoglobin (Hb) microbelts on the surface of glassy carbon electrode (Fig. 14A) [237]. This porous Hb microbelt coating closely contacted to the electrode surface and showed enhanced direct electrochemistry of Hb (Fig. 14B). The Hb microbelts based amperometric biosensor showed a fast response to the analytes and low detection limits of 0.61 µM for H2O2 and 0.47 µM for nitrite. The KMapp value of 0.093 mM was obtained for the electrocatalytic reduction of H2O2, reflecting the high affinity of Hb to the substrate H2O2. SWNT-Hb composite microbelts were also fabricated by the same group and employed as active material to prepare mediatorfree biosensors [238]. The direct electrochemistry of Hb at SWNT-Hb/GCE was more promi‐ nent than that obtained at the Hb microbelt/GCE because of the enhanced electron transfer by incorporated/embedded SWNTs and the porous 3D structure of Hb microbelt coating. Sensitive amperometric detection of trichloroacetic acid (TCA), nitrite, and H2O2 was obtained with the detection limits of 2.41 µM, 0.30 µM and 0.22 µM, respectively. TiO2-Pt nanofibers were fabricated by electrospinning PVP/ethanol solution containing platinum acetate and Ti(O*i*Pr)4, followed by calcination in air at 500 °C for 3 h [239]. The as-prepared TiO2-Pt hybrid nanofibers were used as the electrochemical catalyst for hydrazine detection. Au-coated SiO2 core-shell nanofibers were prepared by the seed-mediated growth Au shell on electrospun SiO2 nanofibers [240]. Then horseradish peroxidase (HRP) was immobilized on the SiO2@Au nanofibers modified electrode via physical adsorption to construct an amperometric H2O2 biosensor. This biosensor exhibited high biological affinity to H2O2 and the HRP enzyme on the gold shell kept its activity with a low-diffusion barrier.

nanofibers with core/sheath, hollow and porous structures have been directly generated by electrospinning or prepared through the combination of electrospinning with some post-spun treatments. Due to the small size, high surface area, and high porosity, electrospun nanoma‐ terials have been witnessed as a promising candidate for a wide range of applications. One of the important applications is the construction of electrochemical sensors or biosensors, where electrospun nanomaterials acted as matrix for the immobilization of enzyme or as the active electrocatalysts. Electrospun nanofiber-based electrochemical sensors or biosensors have

Electrospun Nanofibers: From Rational Design, Fabrication to Electrochemical Sensing Applications

http://dx.doi.org/10.5772/57099

63

In spite of the significant progress in the area of electrospinning, several challenges have to be resolved before large-scale fabrication and extensive applications of electrospun nanomateri‐ als. Most important is that more experimental studies and theoretical modeling are required in order to achieve a better control over the size and morphology of electrospun fibers. To date, it is still not easy to generate uniform nanofibers with diameters below 100 nm, in particular, on the scale of 10-30 nm. Additionally, it is still necessary to systematically investigate the correlation between the processing/solution parameters and the secondary structures of produced nanofibers. Frankly speaking, the application of electrospun nanomaterials in electrochemical sensors is still in its infancy stage, where the applied materials and analytical targets are limited. The majority of polymers have poor conductivity, which limited their direct applications in electrochemical sensors. In this case, it is desirable to develop conductive polymer nanofibers based electrochemical sensors. However, it is still rarely reported in the literatures. Electrospun carbon nanofiber is another good alternative, but the limited catalytic activity and larger diameters confined their analytical performances. Metal nanoparticle loaded carbon nanofibers showed great promise in the preparation of ultrasensitive electro‐ chemical sensors, while the diameter of nanoparticles is difficult to control by using the current one-step method. For the analytical targets, it is still focused on the small molecules at the present research, predominated by glucose. Therefore, there is a large scope to extend the analytes to other significant molecules, particularly the biomolecules such as DNA, proteins,

There is no doubt that electrospinning has become one of the most powerful techniques for fabricating 1D nanomaterials with broad range of functionalities. Electrospun nanofibers have emerged as a kind of great promising material for constructing ultrasensitive electrochemical sensors or biosensors. We can believe that with the extensive interdisciplinary research more and more electrspun nanofiber-based electrochemical sensors or biosensors with excellent properties will emerge in the near future and will be practically applied in environmental

This work was financially supported by the National Nature Science Foundation of China (NO.

monitoring, food analysis and clinical diagnostics.

**Acknowledgements**

21155002, 21105098, 21222505).

exhibited excellent analytical performances for a number of analytes.

and cells.

**Figure 14.** (A) Typical SEM images of Hb microbelts at low (scale bar=10 μm) and high (inset, scale bar=1 μm) magnifi‐ cation; (B) CVs of the bare GC electrode (a) and Hb microbelts modified GC electrode (b) in 0.1 M pH 7.0 phosphate buffer solution. Scan rate, 100 mV s-1 [237].

#### **5. Conclusions and remarks**

In past few years, numerous studies have demonstrated that elctrospinning is a simple and versatile method for fabricating nanofibers of organic or inorganic materials. Various func‐ tional components, such as nanoparticles, CNTs, proteins, DNA and so on, have been incor‐ porated into the electrospun nanofibers. These composite nanofibers exhibited excellent properties and extended the applications of electrospun nanomaterials. With the profound understanding the electrospinning process and the development of setup for electrospinning, nanofibers with core/sheath, hollow and porous structures have been directly generated by electrospinning or prepared through the combination of electrospinning with some post-spun treatments. Due to the small size, high surface area, and high porosity, electrospun nanoma‐ terials have been witnessed as a promising candidate for a wide range of applications. One of the important applications is the construction of electrochemical sensors or biosensors, where electrospun nanomaterials acted as matrix for the immobilization of enzyme or as the active electrocatalysts. Electrospun nanofiber-based electrochemical sensors or biosensors have exhibited excellent analytical performances for a number of analytes.

In spite of the significant progress in the area of electrospinning, several challenges have to be resolved before large-scale fabrication and extensive applications of electrospun nanomateri‐ als. Most important is that more experimental studies and theoretical modeling are required in order to achieve a better control over the size and morphology of electrospun fibers. To date, it is still not easy to generate uniform nanofibers with diameters below 100 nm, in particular, on the scale of 10-30 nm. Additionally, it is still necessary to systematically investigate the correlation between the processing/solution parameters and the secondary structures of produced nanofibers. Frankly speaking, the application of electrospun nanomaterials in electrochemical sensors is still in its infancy stage, where the applied materials and analytical targets are limited. The majority of polymers have poor conductivity, which limited their direct applications in electrochemical sensors. In this case, it is desirable to develop conductive polymer nanofibers based electrochemical sensors. However, it is still rarely reported in the literatures. Electrospun carbon nanofiber is another good alternative, but the limited catalytic activity and larger diameters confined their analytical performances. Metal nanoparticle loaded carbon nanofibers showed great promise in the preparation of ultrasensitive electro‐ chemical sensors, while the diameter of nanoparticles is difficult to control by using the current one-step method. For the analytical targets, it is still focused on the small molecules at the present research, predominated by glucose. Therefore, there is a large scope to extend the analytes to other significant molecules, particularly the biomolecules such as DNA, proteins, and cells.

There is no doubt that electrospinning has become one of the most powerful techniques for fabricating 1D nanomaterials with broad range of functionalities. Electrospun nanofibers have emerged as a kind of great promising material for constructing ultrasensitive electrochemical sensors or biosensors. We can believe that with the extensive interdisciplinary research more and more electrspun nanofiber-based electrochemical sensors or biosensors with excellent properties will emerge in the near future and will be practically applied in environmental monitoring, food analysis and clinical diagnostics.
