**2. Biosensors with a nanotechnological approach**

Nanotechnological developments have played an important role in the development of biosensors along with many other scientific research areas [15]. Nano-sized materials can exhibit unique properties compared with their bulk structures. This has also been advantageous in biosensor applications and has revolutionized them. It enables rapid analysis of multiple analytes at any time and place [20]. The selectivity, detection, non-toxicity, biocompatibility, reversibility, fast response, and sensitivity required for transducer materials can be met by nanomaterials [15, 21]. The fact that nanomaterials can be synthesized in different sizes, shapes, distributions, and compositions makes them unique for biosensor applications [14]. The nanotechnological approach is indispensable for biosensor design, as it ensures superior optoelectronic, electrical conductivity, catalytic activity, and biocompatibility properties as a result of a high surface-to-volume ratio [14, 17]. Due to these unique multi-functional properties, many researchers have recently focused on the use of various nanomaterials in biosensors. There are several reported techniques to fabricate electrodes to develop highly sensitive, selective, and rapid nanosensors for biosensing applications. Various

#### **Figure 1.**

*The schematic diagram of the classification and application areas of biosensors.*

strategies, such as using nanocomposite structures, indium tin oxide (ITO)-polymer, conductive polymer nanoparticles, screen-printing water-based carbon ink method, surface molecular imprinting method, inkjet printing method, recrystallization

#### *Nanostructures in Biosensors: Development and Applications DOI: http://dx.doi.org/10.5772/intechopen.108508*

method, injection molding method, and charge transfer method, have been used for the fabrication of high-performance biosensors [22]. It is known that nanomaterials have good detection sensitivity with a high specific surface area and homogeneous particle distribution. In particular, graphene, GO, and reduced graphene oxide (rGO) metal oxide nanocomposites are the preferred nanomaterials for the production of electrochemical sensors. In the last decade, graphene and graphene-based nanocomposites have been investigated to design biosensors with improved performance. In 2016, Eftekhari-Sis et al. developed a novel GO/5-carboxy fluorescein-labeled DNAbased nano-biosensor for monitoring of the mutation in exon 19 of the EGFR gene in lung cancer [23]. Bao et al. (2019) proposed an effective 3D graphene/copper oxide nano-flowers-based acetylcholinesterase electrochemical biosensor for the detection of malathion [24]. The experimental results showed that the proposed biosensor had a low detection limit of 0.31 ppt in concentration, ranging from 1 ppt to 15.555ppb due to the excellent conductivity and adsorption property of the CuO NFs nanocomposite electrode. In 2021, a novel non-enzymatic PAN: β-rhombohedral borophene-based non-enzymatic electrochemical biosensor has been prepared for the detection of glucose by Taşaltın [25]. The two-dimensional (2D) PAN: β-rhombohedral borophene combined non-enzymatic glucose biosensor was developed, and the proposed biosensor detected glucose with a low LOD of 0.099 mM in a concentration range from 1.5 to 12 mM and a rapid response time (30 s) due to the electrochemical oxidation of glucose. Up to date, 0D–2D nanomaterials-based nanobiosensors have been reported with high sensing performance in the form of polymer-based, metal-based, carbonbased, and composite-based nanosystems.

### **2.1 Polymer-based nanobiosensors**

Polymer-based nanostructures are used for purposes such as to improve cell permeability, to increase therapeutic application, to control dosage frequency and amounts due to their adjustable nanoscale properties in order to improve efficiency in the use of biosensors. Although there are stand-alone applications, polymer-based nanostructures are generally used in composite-based biosensor systems and are preferred in areas such as drug delivery systems, nanomedicines, catalysts, wastewater treatment, etc. Polymer-based nanostructures used in biosensors can be in forms such as polymeric dendrimers, micelles, nanogels, polymersomes, and polymer nanoparticles [26]. In literature, Vais et al. [27] developed a novel DNA biosensor for the detection of *Trichomonas vaginalis* via an electropolymerized poly(orthoaminophenol) (POAP) thin-film-based transducer. The POAP film was developed as an electrochemical nanotransducer and acted as a redox active indicator. In this pursuit, the POAP nanotranducer exhibited long-time stability, good reproducibility, high selectivity for *Trichomonas vaginalis*, and reusability. It has also been reported that POAP film has significant potential in the development of biosensors for DNA immobilization in biomedical applications. In another study, Singh et al. [28] studied a novel L-asparaginase (L-ASP) immobilization by obtaining a polymer-based carrier of gelatin alginate nanoparticles (GANp) synthesized by the ionic gelation method. The produced nanoparticles in the study were tested as a polymer nanoparticle-based fiber optic asparagine biosensor. The tested biosensor works on the basis of detecting the fluorescence intensity of Rhodamine 6G with L-ASP as a bioreceptor when there is an ammonia release occurring in the presence of asparagine. The authors stated that the polymer-based nanostructure of GANp was successful in L-ASP immobilization and offered a selective, sensitive, reusable, and reproducible solution

for the asparagine biosensor and that it could also be used in leukemia patients. In 2020, Zahed et al. [29] successfully developed a flexible poly (3, 4-ethylene dioxythiophene)-poly (styrene sulfonate) (PEDOT:PSS) anchored 3-dimensional (3D) porous laser-induced graphene-based electrochemical glucose and pH biosensor with a highly sensitivity by modifying the surface electrode with PEDOT:PSS/ graphene. In this report, various advantages of conductive PEDOT:PSS, such as high electrical conductivity, solution workability, chemical stability, and biocompatibility, have been brought to the fore in biosensor applications. Tran et al. [30] highlighted the development of conducting polymer-based electrochemical biosensors for the detection of proteins and nucleic acids as biomarkers for COVID-19. Different methods of SARS-CoV-2 by biosensor have been highlighted, such as detection of virus and antigen, viral RNA, and antibody in next-generation diagnostic technologies. In this study, different detecting mechanisms such as gene interaction, protein-protein interaction, protein-aptamer interaction, protein-antibody interaction, antigenspecific antibody response, receptor binding domain interaction, and SARS-CoV-2-angiotensin-converting enzyme 2 (ACE2) interaction for electrochemical virus biosensors were presented. However, the desired success in COVID-19 sensors has not been achieved due to some disadvantages such as weak interaction forces, uncontrollable reactions, large diffusional barriers, poor water solubility, and poor stability.
