Nanostructures in Biosensors: Development and Applications

Abstract

In recent years, there has been significant interest in advanced nanobiosensor technologies with their exceptional properties for real-time monitoring, ultra-sensing, and rapid detection. With relevant experimental data, highly selective and hypersensitive detection of various analytes is possible using biosensors based on nanostructures. In particular, biosensors focus on vital issues such as disease early diagnosis and treatment, risk assessment of quality biomarkers, food-water quality control, and food safety. In the literature, there has been great attention to the preparation and sensing behavior of several nanomaterials-based sensors, such as polymer frameworks, metal-organic frameworks, one-dimensional (1D) nanomaterials, two-dimensional (2D) nanomaterials, and MXenes-based sensors. This chapter gives points to all aspects of fabrication, characterization, mechanisms, and applications of nanostructures-based biosensors. Finally, some smart advanced sensing systems for ultra-sensing nanoplatforms, as well as a comprehensive understanding of the sensor performances, current limitations, and future outlook of next-generation sensing materials, are highlighted.

1. Introduction

Today, as a result of rapid industrialization, global problems bring about pollution, diseases, and many other problems. Early detection, prevention, and elimination of these problems have become very important for the continuity of the ecological system. Due to increasing technological developments, the rapid, precise, high-sensitivity, and reusable detection of these situations is made possible with biosensors. Biosensors are innovative, effective, and independent analytical devices that respond selectively and reversibly depending on the concentration or activity of the analyte to be determined in the sample [1, 2, 3]. With a brief historical overview of biosensors from past to present, in 1962, the first electrochemical enzymatic biosensor was invented by Clark et al. to detect glucose in biological samples due to the oxygenation of blood samples by reducing oxygen on the surface of the platinum electrode [1]. With this discovery, Leland C. Clark is referred to as the “father of the biosensor.” In 1962, Clark determined the concentration of glucose by immobilization of the glucose oxidase on the surface of the amperometric enzyme electrode. In 1962, Montalvo and Guilbault reported the first potentiometric urease enzyme sensor for the detection of the ammonium ion activity due to the enzyme-catalyzed hydrolysis [4]. Another fascinating work was presented by Opitz and Lubbers with the development of the optical biosensor for the detection of alcohol in 1975, which made the field of sensors very remarkable and rapid developments in this area continued [5]. One year later, Clemens et al. developed an electrochemical glucose biosensor for a bedside type of artificial bedside pancreas as a prominent work in the field of biotechnology [6]. At the beginning of the twentieth century, researchers at the University of Cambridge found a pen-sized detector for monitoring blood sugar levels [5]. In the light of ongoing research, different biosensors have been developed by integrating nanostructures for the selective detection of specific analytes. When biosensors were applied to pesticide determination, Ivanov et al. [7] emphasized that an enzymatic layer makes it difficult to operate the biosensor in real samples, especially in field conditions, and reduces the sensitivity and reproducibility of the results obtained. With this approach, they experimentally demonstrated the importance of producing new low-cost disposable pesticide biosensors in which the ultrathin film of the enzyme is directly immobilized to the surface. As a result of rapidly developing technologies in the last 20 years, Chinnappan et al. [8] proposed as an alternative to existing allergen detection methods. It was shown that graphene oxide (GO)-based biosensors for the detection of major shrimp allergens (tropomyosin) with affinity (30 nM) and LOD (2 nM) values in the low nanomolar range were highly sensitive when compared with traditional sensors. For an example of work on the detection of bacterial pathogens originating from water, Yaghoobi et al. [9] demonstrated the successful usage of the green selective, sensitive, stable, repeatable, and reproducible electrochemical biosensor in Streptococcus Pneumoniae bacteria with a low limit of detection (0.0022 ng/ml ~ 622 bacteria) and a high sensitivity (3432.9 Ω (ng/ml)⁻¹). A glassy carbon electrode (GCE) was modified with DNA-lead nanoparticles (Pb NPs) for sensitive and selective detection of the bacteria. In a study on biosensors for early detection of cancer, Alves et al. (2022) developed a novel electrochemical biosensor for monitoring of breast cancer by immobilizing the biotin-C3 and biotin-H₂ peptides in the screen-printed electrodes/poly 3-(3-aminophenyl) propionic acid/avidin system [10]. As we advanced deeper into the pandemic, we saw the need for advanced sensor technologies as a solution in the public health response to COVID-19. As it is known, since December 2019, humanity has been going through a historical process related to the coronavirus [COVID-19]. The urgency of developing a fast, easily accessible, and highly sensitive biosensor to monitor COVID-19 in all countries has emerged. In particular, the importance of developing and researching biosensors that do not give false-negative results for viruses that are at risk of rapid transmission and have lethal effects, which is another problem for humanity, has been understood. Dai et al. reported the development of a novel COVID-19 biosensor highly-sensitive and rapid enzymatic detection of the COVID-19 spike antigen without sample labeling for the accurate and rapid diagnosis of SARS-CoV-2 infection [11]. In this study, the anti-SARS-CoV-2 spike monoclonal antibodies were immobilized onto the surface of biosensor for the specific recognition of the SARS-CoV-2 spike antigen. Thus, it has been proven by rapid development and successful experimental results that biosensors have a wide range of applications such as environmental applications, drug delivery, diagnostics, biomedicine, food quality and safety [12].

In particular, biosensors have a wide range of applications, such as environmental applications, drug delivery, diagnostics, biomedicine, food quality and safety, etc. [12]. The global biosensor market was valued at $15.5 billion in 2015, but it was expected to grow to $24.9 billion by 2021 [13]. In other words, it has shown an increase of 60.6% in 6 years, an indication that it has a widespread usage network day by day. A biosensor is a compact device containing a biological or biomimetic sensing element. It consists of bioreceptor, electronic system, and transducer component [14]. The target analytes are the structures to be detected in the sample. Bioreceptors are structures that produce a measurable signal as a result of physicochemical changes that occur through interaction with the analyte as a result of physical or chemical bonding. These can be enzymes, nucleic acids (DNA or RNA), antibodies, tissues, aptamers, organelles, etc. [2, 12, 15]. After analyte and bioreceptor interaction, a number of physicochemical changes may occur, such as pH change, mass change, electron transfer, heat transfer, etc. [16]. When a physicochemical signal such as specific temperature, sound, light, weight or pressure is produced by the interaction of the bioreceptor with the analyte, the transducer converts it into a readable or measurable electrical signal [17]. Generally, biosensors can be classified according to the type of biorecognition element and transducer [18, 19]. Based on the biorecognition element, biosensors can be classified as antibody, enzyme, antigen, or oligonucleotide-based biosensors. Based on the transducer, biosensors can be classified as optical, electrochemical, magnetic, amperometric, potentiometric, piezoelectric, acoustic, or thermometric-based biosensors. In Figure 1, the schematic diagram of the classification and application areas of biosensors was presented. With the unique sensing performance of nanostructure-based biosensors, a variety of chemical and biosensing nanoplatforms have been reported.

Figure 1.

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

In sensor applications needing continuous monitoring of many analytes, e.g., pesticides, drugs, heavy metals, and bacteria residues detection in real samples, volatile organic compounds (VCOs), biomarkers, specific allergens detection in blood, glucose have been investigated [16]. As known, biosensors are under the influence of more than one scientific area and require a multidisciplinary study. Therefore, it is a very suitable area for development. Recently, developments in nanotechnology and, accordingly, nanotechnological applications of biosensors have attracted attention in the scientific world. In general, the impact of nanotechnology and nanomaterials on the development of biosensors, recent innovations in this area, and future expectations are reviewed in this chapter.

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 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 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 DNA-based 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 15.555 ppb 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, carbon-based, 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(ortho-aminophenol) (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, antigen-specific 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.

2.2 Metal-based nanobiosensors

Metal-based structures at the nanoscale are used in many fields due to their functionability, catalytic and electrical properties, surface plasmon resonance behavior, charge conduction, and sensing properties, and these properties provide unique advantages, especially for biosensor applications. Core@shell nanomaterials have been successfully synthesized by different methods, such as the hydrothermal method [31], sonochemical method [32], microwave irradiation [33], laser ablation [34] etc. In 2022, Mallick et al. highlighted the requirement of metal@metal oxide (M@MO) core@shell nanoparticles (NPs) with chemical, physical properties, and unique morphologies at the nanoscale in high-performance biosensor applications [35]. Various characterization methods such as photoluminescence (PL), UV-visible spectrophotometer, dynamic light scattering (DLS), scanning electron microscopy (SEM), wet-scanning electron microscopy (wet-SEM), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectrometer, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) are used to characterize nanostructures. However, thanks to new studies demonstrating advancements in nanotechnology, it is proven that the sensing performance of chemosensors and biosensors improves depending on the size and shape of nanostructures, including TiO₂, SnO₂, CuO, ZnO, WO₃, and Fe₂O₃ nanoparticles with different morphologies.

Metals (Au, Ag, Pt, etc.) and metal oxides (ZnO, ZrO₂, CuO, Fe₂O₃, TiO₂, etc.) were investigated in metal-based biosensor studies. Among metal nanoparticles, noble metal nanoparticles (such as silver (Ag) and gold (Au) NPs) have been widely studied for use in biosensor applications [21, 36]. In particular, Ag NPs and Au NPs have been used for the detection of heavy metals such as cadmium, arsenic, mercury, etc. [21]. Santhosh et al. [37] synthesized novel plant-derived Ag NPs and tested them for the detection of heavy metals. In this study, waste Allium cepa L peels were prepared as a plant extract and then Ag NPs were synthesized in a greener way by chemical precipitation via using the plant extract.

The synthesized Ag NPs were tested in the solution of various heavy metal salts and a significant color change was observed in the HgCl₂ solution. This result paves the way for the detection of mercury in the liquid phase by observation, which is easy and does not require any tools. The authors stated that green-produced Ag NPs can be used as a simple and inexpensive sensing partner in biosensor applications. In light of the metal-based nanosystems with sensing performances, Hasan et al. [38] worked on developing a gold nanoparticle-based biosensor that provides observation in a colorimetric way for the early diagnosis of ovarian cancer, which is one of the most common cancer types in women. In their study, a novel aptamer-based fluorescence sensor was develpoed for the detection of platelet-derived growth factor (PDGF). It aimed to develop a biosensor based on the fact that Au NPs show significant color change after aggregation. The authors stated that after aptamers and platelets were added to the Au NPs solution, aggregations occurred and changed the particle size of the NPs. In this case, the color of the solution changed from a pinkish color to a purplish color. Also, they said that the aptamers and Au NPs-based structure have potential for such applications by providing the observability of such an important disease in a simple, fast, inexpensive, and effective way for early diagnosis. Another metal-based nanostructure widely used in biosensor applications is metal-oxide nanoparticles (MO NPs). Many different forms of synthesized nano-sized metal oxides have many uses as transducers in biosensors, and many studies are being conducted on them [17]. Karakuş et al. [39] synthesized novel green polyphenol matrix CuO nanoparticles with the help of matcha tea powder extract by a low-cost, easy, green, and fast sonochemical method and then studied the development of the biosensor for the smartphone-based digital colorimetric detection of ammonia. In the study, the prepared CuO NPs solution was coated onto the gold electrochemical transducer by drop-casting and then dried at 40°C, and CuO NPs-based electrochemical biosensors were prepared. It was clear that the prepared biosensor carried out with the detection of color changes by performing red-green-blue (RGB) analysis in the examinations with the help of a smartphone provides an easy and highly selective detection of ammonia at nano levels.

2.3 Carbon-based nanobiosensors

Carbon is one of the most special elements in the material science world. It can be used in many areas thanks to its different atomic array versions, such as graphene, graphite, fullerenes, and nanotubes. Two general approaches, such as top-down (laser ablation, chemical ablation, electrochemical, and sonication) and bottom-up (ultrasonication, solvothermal, electrochemical, hydrothermal, and microwave methods), have been used for synthesizing carbon dots, carbon nanotubes, carbon nanorods, and carbon fibers. Due to its ability to improve electrical, mechanical, physical, and chemical properties, it was used in various fields. In recent years, various studies have been carried out on carbon-based nanobiosensors in biosensor applications [40]. Sreekanth et al. [41] studied the detection of cadmium metal in water with multi-walled carbon nanotube enhanced nanobiosensors. Heavy metals, especially cadmium, are harmful to both nature and humans and pose a serious threat to human health. In this study, a nanobiosensor was developed to detect cadmium (Cd) metal with a DNA-assisted electrochemical technique. In the study, the glassy carbon electrode (GCE) was decorated with a multi-walled carbon nanotube, and dsDNA was immobilized on the carbon nanotube decorated GCE. Furthermore, heavy metal detection was examined by using differential pulse voltammetry (DPV) analysis. In the presence of Cd (II) ions, dsDNA interacts with Cd to form ssDNA. ssDNA binds with ethyl green (EG), and this provides a noticeable change in reduction peak current. Higher reduction peak currents are observed at increasing Cd concentrations. The developed nanobiosensor has demonstrated the potential of multi-walled carbon nanotubes in nanobiosensor applications with its ability to detect Cd at a limit of detection (2 nM) and sensitivity (5 nA nM⁻¹). In addition, Ballen et al. focused on the development of cantiveler biosensors to detect the presence of cadmium. In their studies, they developed urease, (GO), and urease/GO-based nanobiosensors and investigated the properties of nanobiosensors for the detection of Cd. The urease nanobiosensor has a detection limit of 0.03776 ppb, while the GO/urease nanobiosensor has a more advanced detection limit of 0.01831 ppb [42]. In another study, Taşaltın et al. studied a nanobiosensor enhanced with rGO synthesized by an ultrasonic microwave assisted method for propamocarb pesticide detection. It has been reported that the developed biosensor in the study has superior properties such as high selectivity (101.1 μAμM⁻¹ cm⁻²), rapid response (1 min), a wide linear range (1–5 μM), and a low detection limit (0.6 μM) of pesticide [43]. In another study, Elugoke et al. [44] fabricated a novel electrochemical biosensor based on a modified electrode with carbon quantum dots and CuO nanocomposite for the detection of dopamine using square wave voltammetry (SWV). The electrochemical results showed that the carbon quantum dots-CuO nanocomposite-based biosensor exhibited a low LOD of 25.4 μM in a wide linear range from 1 to180 μM. Furthermore, it was proposed that the sensing mechanism was based on the negative charge of the oxygen-containing functionalities on the modified electrode, which attracted the positively charged analyte. With a similar approach, Gaidukevic et al. [45] prepared a sensitive electrochemical rGO-based biosensor for the determination of dopamine in the presence of malonic acid and P₂O₅ additives. Experimental results showed that the proposed biosensor exhibited high sensitivity of 28.64 μA μM⁻¹ cm⁻² and a low LOD value of 0.11 μM for the detection of dopamine. Additionally, it was reported that the sensing mechanism of the redox reaction of analyte was changed due to the change from reversible to irreversible transition. The biosensing mechanism of the redox reaction of analyte was changed due to the change from a reversible to an irreversible transition. In addition, the electrochemical process was a phenomenon occurring in the surface adsorption-controlled reaction.

2.4 Composite nanobiosensors

Another most commonly used material type is composite materials, in nanobiosensor applications. These materials have been developed with the combination of the desired properties of two or more materials in their structure, and these materials have also come to the fore in applications thanks to their superior properties [46]. So far, sensor studies have been reported on many nanomaterials-based biosensors that focused on flexible, stretchable, and wearable sensors to determine target analytes such as heart rate, blood pressure, breathing rate, serum electrolyte, temperature, creatinine, albumin, urea, DNA, RNA, and glucose. Based on the use of composite nanomaterials, Ebrahimi et al. developed a microRNA-199a-5p targeted electrochemical nanobiosensor for Triple-Negative Breast Cancer. The composite nanobiosensor was prepared using gold nanorods, GO, and graphene electrode glass. Fetal bovine serum and human serum samples were studied for detection of microRNA-199a-5p in this study. It was stated that high selectivity and sensitivity were observed in both sample environments. It has been reported that the prepared composite nanobiosensor offers an important potential due to its detection even at low concentrations [47]. In another study, Karakuş et al. studied glucose detection by developing an electrochemical nanobiosensor prepared with polyacrylonitrile (PAN) and rGO. It has been stated that due to the redox mechanism provided by rGO, it allows glucose detection with high stability and sensitivity. It has been stated that compared with the PAN-based sensor, the nanobiosensor supplemented with rGO provides glucose detection with higher sensitivity [48]. This shows the importance of developing composite structures for nanobiosensor applications. In another study, Baytemir et al. studied the detection of glucose with an electrochemical nanobiosensor and developed a nickel phthalocyanine-borophene composite based nanobiosensor. It has been reported that the borophene-doped nanobiosensor exhibits high electrical conductivity and sensitivity compared with the NiPc nanobiosensor. It was also stated that the composite nanobiosensor exhibited a lower limit of detection value [49]. In another study, Samak et al. developed a novel nanobiosensor for H₂S detection that is coupled with a DNA/sulfide fluorophore (SF) and a hybrid composite (alumina nanorods and GO nanosheet). In the study, it was stated that composite structures that can be produced in a controllable way are important in nanobiosensor applications and that the prepared composite nanobiosensor for sensitive and selective H₂S detection in wastewater can be developed [50]. As seen from the literature, composite structures have an important potential for nanobiosensor applications and can be used in many different areas. In Table 1, comparison of developed nanobiosensor for the detection of specific analytes was presented.

Biosensor	Bioreceptor	Detection	Results		Ref.
Polymer-based nanobiosensor	Polypyrole	H2O2 and glucose	Detection range for H2O2	10 mM–10 mM	[51]
			Linear range	1–5 mM
	Polypyrole	Prostate cancer	LOD	2.0 pg. mL^−1	[52]
			Linear range	0.01–4.0 ng mL^−1
Metal-based nanobiosensor	DNA-CuO nanoparticles	Single nucleotide polymorphism anemia	LOD	0.64 nM	[53]
			Linear range	2 nM to 12 nM
	Gold nanoparticles	Thyroid disease diagnosis	LOD	0.001 μIU/mL	[54]
			Linear variation	100 μIU/mL
	Cu Nanocluster	Detection of VEGF165 biomarker	LOD	12 pM	[55]
			Linear range	10–800 pM
	Gold nanoparticles	Prolactin hormone	Linear range	1–40 ng ml^−1	[56]
			LOD	0.8 ng ml^−1
			Sensitivity	10 pg. ml^−1
Carbon-based nanobiosensor	GO/urease	Cadmium	LOD	0.01831 ppb to 0.03776 ppb	[42]
	Multi-walled carbon nanotube	Cadmium	LOD	2 nM in a linear range of 2 nM–10.0 nM.	[41]
	Graphene	IgE detection	LOD	47 pM in a linear range 50 pM to 250 nM	[57]
Composite nanobiosensor	Molecularly imprinted polymers/carbon quantum dots	Hemoglobin (Hb)	Linear range	0.77–7.7 nM	[58]
			LOD	0.77 nM
			Recovery in serum sample	86.8% to 93.9%
	l-Glutathione modified Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate/carboxylated multiwall carbon nanotubes	Cadmium (II) ions	Sensitivity	10.69 μA/ppm	[59]
			LOD	1 ppb
	rGO/gold nanocomposite	miRNA-122 biomarker	LOD	1.73 pM in a range from 10 μM to 10 pM	[60]
MXene nanobiosensor	NiO/Ti3C2Tx MXene	H2O2	LOD	0.34 μM in a range from 0.01–4.5 mM	[61]
MXene composite biosensor	MXene@Ag nanoclusters and amino-functionalized multi-walled carbon nanotubes	Carbendazim	LOD	0.1 nM in a range from 0.3 nM–10 μM	[62]

Table 1.

Comparison of developed nanobiosensor for the detection of specific analytes.

2.5 MXene nanobiosensors

Discovered in 2011 and attracting great attention in recent years, especially in advanced nanosensor technologies, MXene is a 2D transition metal carbide and nitride material with different functional groups (-O, -OH, and -F). MXene sensors have been great attention with their excellent electrical, biological, chemical, surface, and mechanical properties in various applications such as water treatment systems, energy storage materials, photothermal systems, and sensor applications.

Recently, various types of 2D nanomaterials have been produced in small quantities for other applications outside the sensor field, whereas scale of production for healthcare applications poses a major challenge. In addition, it has been proven that most of the 2D materials produced have properties such as hydrophobic and instability in the air environment. Therefore, 2D MXene materials have a strong influence in the field of modern science, with the development of issues with superior metallic conductivity, ease of processing, hydrophilic character, chemical stability, and layered morphology. Soomro et al. developed a novel photo-electrochemical NiWO₄- MXene sensor for the detection of the prostate-specific antigen [63]. According to the experimental results, it was found that the sensor had a wide detection range from 1.2 fg.mL⁻¹ to 0.18 mg.mL⁻¹ and a low detection limit of 0.15 fg.mL⁻¹ for the prostate-specific antigen. In another study, Qin et al. reported that MXene/V₂O₅/CuWO₄-based sensor had a highly selective against ammonia at room temperature in few seconds [64]. Ranjbar et al. studied the sensing performance of the novel wearable conductive polymer/MXene-based pressure sensor for the human detection and information transmission using cotton fabric [65]. In another study, Zhu et al. developed a novel acetone sensor using ZnO/Ti₃C₂T_x-MXene composite nanomaterials [66]. The few-layered ZnO/Ti3C2Tx-MXene composite nanomaterial was prepared by a hydrothermal method Furthermore, the proposed nanosensor exhibited a high-sensing response nearly six times higher than that of the ZnO in a concentration range of 14.4–100 ppm of acetone at 320°C due to the large specific surface area and layered structure. Finally, the sensing mechanism was proposed based on the large specific surface area, sufficient adsorption and reaction sites, a large number of oxygen functional groups (-O, -OH) of MXene nanostructures, and the surface of the modified electrode for sensing gas molecules.

3. Future trends of sensing nanomaterials

With these developed nanomaterial-based sensors, various sensors with excellent sensing performance values have been fabricated, and their sensing performances have been observed to detect the target analyte in a short time. Accordingly, previous results show that advanced biosensors are low-cost, easier to use, selective, rapid, and sensitive. In addition, the synthesis methods of large-scale nanostructure-based sensors are still a hot topic. Considering the nanostructures outlined in this review, it is clear that many nanosensors, each with their own unique superior properties and advantages, are suitable for incorporation into biosensing nanoplatforms. In particular, MXene and its composites appear to be a promising choice for use in forensic biosensors due to their ultra-sensitivity and superior electrochemical properties that provide a broad absorption spectrum. The use of MXene and MXene composites with various detection systems such as antibodies, genes, viruses, biomarkers, and enzymes allows the fabrication of rapid and ultra-sensitive biosensors with very low LOD values. As research in the field continues to expand, there is no doubt that portable and flexible biosensors for real-time mobile detection of illicit drugs, early detection of disease, environmental pollutants, and biological traces will become universal in sensor applications and facilitate quality of life. However, it has also been shown that nanosensors can rapidly detect the target molecule among mixed components at very low concentrations, and detection research in the health field is gaining momentum. It is seen that successful experimental data have been obtained in the field of development and sensing mechanisms of 2D materials-based nanobiosensors, which is an interesting subject for biomedical and environmental applications. As a result, there are not enough reports on the research of sensitive, selective, and effective nanostructure-based sensors with different morphologies. For this reason, nanostructure-based sensors with high sensing potential are being investigated and will guide future studies in a wide spectrum of science.