A Review on Biosensors and Recent Development of Nanostructured Materials-Enabled Biosensors

1.1. Sensors Nowadays, we enjoy the results of science and technology for the smoothly running lives. We frequently rely on various types of appliances or gadgets, such as computers, copy machines, mobile phones, microwave ovens, refrigerators, air conditioning and television remotes, smoke detectors, infrared (IR) thermometers, turning on and off lamps and fans, which help us interact with the physical environment. Many of these applications perform with the help of sensors. A sensor is defined as a device or module that aids in detecting changes in physical quantities, such as pressure, heat, humidity, movement, force, and an electrical quantity like current, and thereby converts these to signals that can be detected and analyzed [ 1 , 2 ]. A transducer is defined as a device that can convert energy from one form to another. The sensor is the heart of a measurement system. An ideal sensor should possess certain characteristics, such as range, drift, calibration, sensitivity, selectivity, linearity, high resolution, reproducibility, repeatability, and response time [ 3 , 4 ]. The progress of sensor technology has become increasingly important, owing to various applications, such as environmental and food quality monitoring, medical diagnosis and health care, automotive and industrial manufacturing, as well as space, defense, and security.

2.1. Design and Principle A biosensor is a device or probe that integrates a biological element, such as an enzyme or antibody, with an electronic component to generate a measurable signal. The electronic component detects, records, and transmits information regarding a physiological change or the presence of various chemical or biological materials in the environment. Biosensors come in different sizes and shapes and can detect and measure even low concentrations of specific pathogens, or toxic chemicals, and pH levels. A typical biosensor comprises (a) an analyte, (b) bioreceptor, (c) transducer, (d) electronics, and (e) display ( Figure 2 ) [ 7 ]. (a) Analyte: A substance of interest whose constituents are being identified or detected (e.g., glucose, ammonia, alcohol, and lactose). (b) Bioreceptor: A biomolecule (molecule) or a biological element that can recognize the target substrate (i.e., an analyte) is known as bioreceptor (e.g., enzymes, cells, aptamers, deoxyribonucleic acid (DNA or RNA), and antibodies). The process of signal production (in the form of light, heat, pH, charge or mass change, plant or animal tissue, and microbial products) during the interaction between bioreceptor and analyte is called biorecognition. (c) Transducer: A device that transforms energy from one form to another. The transducer is a key element in a biosensor. It converts the biorecognition event into a measurable signal (electrical) that connects with the quantity or in the presence of a chemical or biological target. This process of energy conversion is known as signalization. Transducers generate either optical or electrical signals proportional to the number of analyte–bioreceptor interactions. According to the operating principle, transducers are broadly categorized as electrochemical, optical, thermal, electronic, and gravimetric transducers (d) Electronics: The transduced signal is processed and prepared for the display. The electrical signals obtained from the transducer are amplified and converted into digital form. The processed signals are quantified by the display unit. (e) Display: The display unit is composed of a user interpretation system, such as a computer or a printer that generates the output so that the corresponding response can be readable and understandable by the user. Depending on the end-user prerequisite, the output can be in the form of a numerical, graphical, or tabular value, or a figure.

To develop a highly effective and capable biosensor system, certain static and dynamic requirements are necessary. Based on these specifications, the performance of the biosensors can be optimized for commercial uses [ 3 , 4 , 35 ]. (a) Selectivity: Selectivity is a crucial feature to consider when selecting a bioreceptor for a biosensor. A bioreceptor can detect a particular target analyte molecule in a sample comprised of admixture spices and unwanted contaminants. (b) Sensitivity: The minimum amount of analyte that can be correctly detected/identified in a minimum number of steps and in low concentrations (ng/mL or fg/mL) to verify the existence of analyte traces in the sample. (c) Linearity: Linearity contributes to the accuracy of the measured results. The higher the linearity (straight line), the higher the substrate concentration detection. (d) Response time: The time is taken for obtaining 95% of the results. (e) Reproducibility: Reproducibility is characterized by precision (similar output when the sample is measured more than once) and accuracy (capability of a sensor to generate a mean value closer to the actual value when the sample is measured every time). It is the ability of the biosensor to produce identical results whenever the same sample is measured more than once. (f) Stability: Stability is one of the key characteristics in biosensor applications where continuous monitoring is required. Stability is the extent of vulnerability to environmental disturbances inside and outside the biosensing device. The factors that affect stability are the affinity of the bioreceptor (the extent of binding of the analyte to the bioreceptor) and the degradation of the bioreceptor over time.

According to the biorecognition principle, biosensors are classified as catalytic biosensors and affinity or non-catalytic biosensors [ 36 ]. In a catalytic biosensor, analyte–bioreceptor interaction results in the development of a new biochemical reaction product. This biosensor includes enzymes, microorganisms, tissues, and whole cells. In the case of affinity (non-catalytic) biosensor, the analyte is bound to the receptor irreversibly, and during the interaction no new biochemical reaction product is formed. This type of sensor comprises antibodies, cell receptors, and nucleic acids as the target for detection [ 37 ]. 2.5.1. Enzyme-Based Biosensors Enzymes are common biocatalysts, which are efficient at increasing the biological reaction rate. The working principle of an enzyme-based biosensor depends on the catalytic reaction and binding capabilities for the target analyte detection [ 38 ]. Various possible mechanisms are involved in the analyte recognition process: ( i ) The analyte is metabolized by the enzyme, so the enzyme concentration is estimated by measuring the catalytic transformation of the analyte by the enzyme, ( ii ) an enzyme inhibited or activated by analyte, so the analyte concentration is related to decreased enzymatic product formation, and ( iii ) tracking of the alteration of enzyme characteristics [ 39 , 40 , 41 , 42 ]. Owing to the long history of enzyme-based biosensors, various biosensors can be produced on the basis of enzyme specificity. However, the enzyme structure is extremely sensitive, which makes it expensive and complicated to improve its sensitivity, stability, and adaptability [ 42 ]. Electrochemical transducers are most commonly used for enzyme-based biosensors. The most common enzyme-based biosensors are glucose and urea biosensors. Cordeiro et al. developed and characterized W-Au based amperometric enzyme-based glucose biosensors for real-time monitoring of glucose in the brain in vitro. Their experiments revealed that developed W-Au-based sensor can monitor changes in brain glucose in response to relevant pharmacological challenges [ 43 ]. Uygun et al. developed a highly stable potentiometric urea biosensor using nanoparticles. The response time and detection limit of their developed sensor were 30 s and 0.77 µM, respectively [ 44 ]. Integrating enzymes with nanomaterials has resulted in increased use of enzymes as recognition elements in biosensors [ 45 ].

Aptamers are synthetic single-stranded nucleic acids (sequences of DNA or RNA) that bind to target molecules selectively and can be folded into two-dimensional (2D) and three-dimensional (3D) structures. In 2D or 3D structures, the targets have high-binding performance due to greater surface density and less spatial blocking [ 50 , 51 , 52 ]. Due to the nucleic acid character of aptamers, they are structurally and functionally stable over a wide range of temperatures and storage conditions. Unlike antibodies that require biological systems to be generated, aptamers can be chemically synthesized, are stable in a pH range of 2–12, and have certain thermal refolding capabilities. A further benefit of aptamers is that they can be chemically modified according to the detection criteria for the target molecule [ 53 ]. By an in vitro selection mechanism, SELEX (Systematic Evolution of Ligands by EXponential enrichment), aptamers can be isolated from oligonucleotides libraries [ 54 , 55 ]. Several SELEX variants have recently been established, including cell-SELEX, capillary electrophoresis-based SELEX, microfluidics-SELEX, FACS-based SELEX, microtiter plate-SELEX, magnetic bead SELEX, and in vivo SELEX [ 56 ]. Optical, electrochemical, and piezoelectric techniques are the most frequently used in biosensors. Depending on different transduction techniques, these biosensors are further categorized as labeled or label-free aptasensors. Surface plasmon resonance (SPR) is the most commonly used method for the label-free optical sensors, whereas fluorescent dyes (fluorescein) are used for label-based optical aptasensors [ 57 ]. For tracking biological systems in real-time, fluorescent NPs, such as QDs, provides many benefits over regular fluorescent dyes. To identify targets, such as cancer cells, bacterial spores, and proteins, aptamer-QD conjugates were used [ 58 , 59 ]. Starno et al. demonstrated aptamer capped NIR PbS QDs to detect thrombin protein, based on selective charge transfer, within 1 min and with a detection limit of ~1 nM [ 60 ]. Gold nanoparticles (GNPs) have demonstrated interesting absorption characteristics that vary depending upon their aggregation state. Furthermore, GNPs are more biocompatible, easier to bioconjugate, and less toxic than QDs [ 61 ]. Feng et al. fabricated an electrochemical impedance spectroscopy (EIS)-based biosensor for L-Arg detection, which exhibited acceptable performance in the linear range of 0.1 pM–0.1 mM with a detection limit of 0.01 pM [ 62 ].

Apart from the above class of bioreceptors, NMs have recently been considered as a new category of bioreceptors. With the advancement of nanotechnology and nanoscience, different NMs have been used as bioreceptors [ 68 ]. NPs deliver a more vast range of applications in biosensing technology. NMs can act as bioreceptors as well as transducers. For example, cerium oxide-based NMs exhibit biomimetic catalytic activity favorable for bioreceptors [ 69 ]. Due to their effective transduction capabilities, different inorganic materials, such as graphene and CNTs-based NMs, noble metal NPs, and QDs, have successfully been used as transducers.

This method deals with the formation of strong chemical bonds, like covalent binding or covalent linking, between biorecognition element functional groups and the transducer surface. Based on the chemical bonding, chemical immobilization methods are categorized as (a) direct covalent binding and (b) covalent cross-linking [ 61 , 70 , 71 , 72 , 73 ]. (a) Covalent binding Direct covalent binding is the most widely used enzyme immobilization technique, in which the biorecognition element is firmly bonded either to the electrode/transducer surface or to the inert matrix of the membrane. The immobilization process through the membrane matrix involves two steps: Synthesis of the functional polymer and covalent immobilization. The binding mechanism depends on the interaction between biorecognition element and functional protein groups (usually side chains of amino acids) and reactive groups of the transducer/membrane matrix surface. The advantages of direct covalent binding include strong resistance to environmental changes, little leakage of biorecognition element (enzyme), and strong bond formation between the biorecognition element (enzyme) and matrix. The main disadvantages of this method are the use of harsh chemicals and that the developed matrix cannot be regenerated once used [ 61 , 70 , 71 , 72 , 73 ]. (b) Cross-linking The mechanism of cross-linking occurs via the creation of intermolecular covalent cross-linkages between biorecognition elements (enzymes) or between biorecognition elements and functionally inert proteins (for example bovine serum albumin). This process is performed with the help of multi-functional reagents that act as a linker to connect enzyme molecules in 3D cross-linked aggregates to the transducer surface. The optimal conditions required for cross-linking are pH, temperature, and ionic strength. It allows shorter response time, stronger attachment, and higher catalytic activity of enzymes. The advantages are less leakage of enzymes, stronger chemical binding, and the possibility to adjust the environment for the biorecognition element using appropriate stabilizing agents, The disadvantages are the formation of covalent cross-links between protein molecules instead of the matrix and protein and that partial denaturation of protein structure limit the application of cross-linking immobilization [ 61 , 70 , 71 , 72 , 73 ].

Optical biosensors are analytical devices consisting of a biorecognition element integrated into an optical transducer system. The working principle of an optical biosensor is to generate signals, which are proportional to the concentration of the analyte and to provide label-free and real-time parallel detection. Optical biosensors utilize enzymes, antibodies, aptamers, whole cells, and tissues, as biorecognition elements. In optical biosensors, the transduction process induces a change in the absorption, transmission, reflection, refraction, phase, amplitude, frequency, and-/or light polarization, in response to physical or chemical changes created by the biorecognition elements. According to the principle, optical biosensors are categorized as label-free and label-based. In label-free sensing, the detected signal is produced by the interaction of the analyte with the transducer. On the contrary, in label-based sensing, the optical signal is generated by calorimetric, fluorescence, or luminescent methods. Optical biosensors can be designed based on various optical principles, such as SPR, evanescent wave (EW) fluorescence, optical waveguide interferometry, chemiluminescence, fluorescence, refractive index, and surfaced-enhanced Raman scattering. The most commonly used optical-based biosensors are (a) fluorescence-based optical biosensors, (b) chemiluminescence-based optical biosensors, (c) SPR-based optical biosensors, and (d) optical fiber-based optical biosensors [ 70 , 83 , 84 , 85 ]. In Figure 7 , schematic diagrams of (a) chemiluminescence-based optical biosensors, (b) surface plasmon resonance (SPR)-based optical biosensors, and (c) evanescent wave-based optical fiber biosensors are shown. (a) Fluorescence-based optical biosensors: Fluorescence is the optical phenomenon that includes labeling for the detecting analyte or molecules. This phenomenon has drawn much attention to the design of fluorescence-based optical biosensors. This type of biosensor is the most widely investigated for medical diagnosis, and environment and food quality monitoring applications because of its high selectivity, sensitivity, and short response time. Different kinds of fluorescent dyes, such as QDs, dyes, and fluorescent proteins, are used in this biosensor. Fluorescence-based biosensors include three approaches: (i) Fluorescent quenching (turn-off), (ii) fluorescent enhancement (turn–on), and (iii) fluorescence resonance energy transfer (FRET). Recently, fluorescence resonance energy transfer (FRET)-based optical biosensors have prevailed in the study if the intercellular process owing to their higher sensitivity. The FRET process involves nonradiative energy transfer from an excited donor molecule (D) to the acceptor molecule (A) at the ground state via long-range multipole interactions. As FRET-based optical sensors can detect changes in angstroms to nanometers, they are widely employed in clinical applications, such as cancer therapy and aptamers analysis [ 74 , 86 ]. Liu et al. constructed a carbon dots (CDs)/Au NR assembly-based FRET sensor for detection of lead ions. The linear range was obtained from 0 to 155μM with a detection limit of 0.05μM [ 87 ]. (b) Chemiluminescence-based optical biosensors: Chemiluminescence is the phenomenon in which light energy is released because of the chemical reaction. By virtue of its simplicity, low detection limit, wide calibration limit, and affordable instrumentation, chemiluminescence-based biosensors have received considerable interest. In recent times, chemiluminescence studies have been expanded to nanomaterials to improve intrinsic sensitivity and extended to the new applications of detection [ 88 ]. He et al. developed a chemiluminescence-based biosensor to detect DNA using graphene oxide, which exhibited high sensitivity and selectivity and the linear range is 0.1–3 nm with a limit of detection of 34 pM [ 89 ]. (c) SPR-based biosensors: SPR-based biosensors detect the change in the refractive index caused by molecular interaction at a metal surface through surface plasmon waves. This biosensor falls into the group of label-free biosensing technology and operates on the principle of SPR. According to the SPR phenomenon, when polarized light illuminates a metal surface at the interface between two media of different refractive indices, at a certain angle it produces electron charge density waves called plasmons. Based on the thickness of the layer at the metal surface, the SPR phenomenon results in the declined intensity of the reflected light relative to the incident light at a specific angle known as the resonance angle. The decrease in intensity is proportional to the mass on the surface [ 90 , 91 ]. Moreover, the SPR method relies on refractive index variations connected with the binding of the analyte to the biorecognition element on the transducer or SPR sensor. SPR phenomenon offers various applications in disease diagnosis and environmental and food quality monito

Thermal biosensor exploits the basic characteristics of the biological reactions (exothermic or endothermic), i.e., measurement of heat energy absorbed or released during the reaction. The total heat energy absorbed/evolved or the temperature change (Δ T ) measured by thermal biosensor is proportional to the enthalpy (Δ H ) and to the total number of product molecules ( n p ) created in the biochemical reaction and inversely proportional to heat capacity ( C p ) of the reaction, which can be shown as Δ T = −( n p Δ H )/ C p [ 105 , 106 , 107 , 108 , 109 ]. Calorimetric-based biosensors measure the change in heat, which is directly monitored to calculate the extent of reaction (for catalyst) or structural dynamics of biomolecules in the dissolved state [ 108 , 110 ]. Calorimetric devices are limited because of the relatively long experimental procedures and lack of specificity in the temperature measurement; there is no direct way to discriminate between specific and non-specific heat changes. However, the invention of the enzyme thermistor-based on flow injection analysis in combination with an immobilized biocatalyst and heat-sensing element circumvented several of these shortcomings [ 107 ]. Usually, thermal biosensors employ a flow injection analysis method utilizing an immobilized enzyme reactor, together with a differential temperature measurement across the enzyme reactor. The configuration involves a pair of thermal transducers, such as thermistors or thermopiles, positioned across the enzyme column packed with immobilized enzymes for the conversion of given substrate to product. This differential measurement system generates a high common-mode-rejection ratio that greatly reduces the effects of ambient temperature fluctuations, allowing the specific measurement of enzyme catalysis. The thermal signals generated during the catalytic reaction sensed by the thermistor are proportional to the concentration of the substrate. Generally, the enthalpy changes for enzymatic catalysis are around −10 to −200 kJ mol −1 , which is adequate to determine the substrate concentrations at clinically interesting levels for a range of metabolites [ 106 , 107 , 108 , 109 , 110 ]. Thermometric detection is advantageous when multiple reactions are involved since it is the sum of all enthalpies that determines the sensitivity of the assay. Thermistors or thermopiles are the two most commonly used temperature sensors. The thermistor is a sensitive temperature transducer, which depends on the changes in the electric resistance with temperature from which the absolute temperature can be determined but have limited sensitivity. Thermopiles measure the temperature difference between the two regions. Thermopiles are the set of thermocouple junctions in series fabricated from metals, semiconductors, and various substrate semiconductor components [ 108 , 109 ]. An enzyme thermistor-based biosensor is shown in Figure 9 a. In addition to thermistors, bimetallic strips, liquid gas expansion, and pyroelectric systems (according to the production of an electrical signal because of a change in the temperature), metal resistance and microelectromechanical systems (MEMS) are being used as thermal transducers. Thermocouples with high sensitivity are also an excellent alternative for detecting changes in temperature [ 61 ]. Recently, microelectromechanical (MEMS) thermal sensors are being used to monitor metabolic applications on the basis of temperature detection. Low-cost integration of miniaturized devices and low-cost batch fabrication are the advantages of MEMS technology. MEMS thermal sensors exhibited improved thermal isolation, low thermal mass, and sample volume of the MEMS thermal biosensor provides linear range and high sensitivity, low measurement time, and low power consumption. It is possible to measure multiple samples in parallel [ 111 ].

Acoustic biosensors operate based on the change in the physical properties of an acoustic wave in response that can be correlated to the amount of analyte absorbed [ 118 ]. Piezoelectric materials are commonly used for sensor transducers, because of their ability to produce and transmit acoustic waves in a frequency-dependent manner. For acoustic-wave propagation, the optimum resonant frequency is highly dependent on the physical dimensions and properties of the piezoelectric crystal. The changes in material mass on the surface of the crystal can induce measurable variations in the natural resonant frequency of crystal [ 100 , 101 ]. There are two classes of mass-balance acoustic transducers: Bulk-acoustic wave (BAW) and surface-acoustic wave (SAW) devices. BAW devices can transmit an acoustic wave from one crystal surface to another whereas SAW devices can transmit an acoustic wave along a single crystal face from one location to another [ 119 , 120 ]. The operation of these devices in the gas phase is well understood, but in liquid media, it is not that well understood. For many years the piezoelectric mechanism has been established and can be an alternative for the transduction process in affinity biosensors if the challenges of non-specific binding and poor sensitivity are solved [ 72 , 83 ]. The surface acoustic wave-based biosensor is shown in Figure 9 c.

With advances in nanotechnology, research and development in the field of biosensors have become open and multidisciplinary. Exploring NMs, such as NPs (metal- and oxide-based), NWs, NRs, CNTs, QDs, and nanocomposites (dendrimers), for different characteristics provides the possibility of improving the performance of biosensors and increase the power of detection through size and morphology control. Different types of NMs-based biosensors (nanobiosensors) are shown in Figure 11 . The basic working principle of nanobiosensors is along the same lines of conventional macro- and micro-counterparts, but they are constructed using nanoscale components for signal or data transformation [ 123 ]. Nanobiosensors have an edge over their conventional macro- and micro-counterparts because of their multidisciplinary applications due to dimensionality. Nanobiosensors are instrumental in the field of nanotechnology for (a) monitoring physical and chemical phenomena in regions difficult to reach, (b) detecting biochemicals in cellular organelles and medical diagnosis, (c) measuring nanoscopic particles in industrial areas and the environment, and (d) detecting ultra-low concentrations of potentially harmful substances [ 123 ]. Based on the classification of the NMs, their involvement in the enhancement of biosensing mechanisms has been broadly investigated. For instance, NPs-based biosensors include all sensors that employ metallic NPs as enhancers of biochemical signals. Similarly, nanotube-based biosensors, if they involve CNTs, are used as enhancers of reaction specificity and efficiency, while biosensors using NWs as charge transport and carriers are termed as NW biosensors. Likewise, QD-based sensors employ QDs as contrast agents for improving optical responses. 4.1. Nanoparticle-Based Biosensors NPs have been widely used in various biomedical applications, like in the development of biosensors for health diagnosis, imaging, drug delivery, and therapy, owing to their unique properties. Because of their small size and shape, their physical and chemical properties are strongly influenced by the binding of target biomolecules [ 124 ]. These properties of NPs enable them to be exploited for various bioanalytical applications. They are considered suitable for electrode modification in which they increase the sensitivity and specificity of electrochemical catalysis [ 70 ]. Moreover, catalytic active NMs, such as transition metal oxides, have been developed as nanoenzymes, which allow providing catalysis of biochemical reactions on biosensors. The NPs include metal and noble metal NPs, such as gold (Au), silver (Ag), platinum (Pt), palladium (Pd), cobalt (Co), iron (Fe), and copper (Cu), and metal oxide NPs (ZnO, TiO 2 , SnO 2 and MnO 2 ), which exhibit excellent optical, electronic, magnetic, chemical, mechanical, and catalytic properties. NP biosensing performance is tailored by coating with various matrices, such as metal oxides, silica network, polymers, graphene, fibers, and dendrimers [ 125 ]. Gold NPs, which fall into the class of noble metal NPs, are extensively investigated and used owing to their unique optical, electronic, and physicochemical properties. They are widely used in biomedical research because of the following advantages: Simple synthesis techniques, easier fabrication procedures, greater chemical stability, biocompatibility, vast electrochemical potential range, high catalytic activity, and their nanocomposite forms [ 126 , 127 ]. Wu et al. demonstrated gold NP-based electrochemical sensors for sensitive detection of uranyl in natural water. The developed sensor determined uranyl in the range of 2.4 to 480 µg L −1 , and a detection limit of 0.3 µg L −1 was obtained by anodic stripping voltammetry [ 128 ]. Luo et al. established a novel “turn–on” fluorescent sensor for detecting Pb 2+ , based on graphene quantum dots (GQDs) and gold nanoparticles (AuNPs). The designed sensor showed an extremely broad detection range of Pb 2+ from 50 nm to 4 µm, with a detection limit of 16.7 nm [ 129 ]. Ghasemi et al. demonstrated a novel non-enzymatic glucose sensor based on gold-nickel bimetallic NPs doped aluminosilicate framework prepared from agro-waste material that exhibited a wide linear range for glucose (1–1900 µM) and low limit of detection (0.063 µM) [ 130 ]. Silver nanoparticles (Ag NPs) have gained much research interest in biomedical applications due to excellent surface-enhanced Raman scattering (SERS), biocompatibility, high conductivity, amplified electrochemical signals, and catalytic activity [ 131 , 132 , 133 ]. Rivero et al. demonstrated an optical fiber sensor based on both localized surface plasmon resonance (LSPR) and lossy-mode resonance (LMR) using Ag NPs. The devices showed high sensitivity (0.943 nm per RH %), a large dynamic range (42.4 nm for RH changes between 25% and 70%), and a fast response time (476 ms and 447 ms for rise and fall, respectively). This device can be used to monitor hum

NWs provide favorable conditions for creating robust, sensitive, and selective electrical detectors of biological binding events. The NWs exhibit highly reproducible optical and electrical characteristics. Current flow in any 1D systems, such as NWs and NTs is extremely sensitive to minor perturbations because, in such systems, flow is extremely close to the surface [ 172 ]. The diameter of the NWs are comparable to the biological and chemical species that are being sensed. They offer excellent transduction generating signals, which ultimately interface to macroscopic instruments. The combination of tunable conducting properties of semiconducting NWs and the ability to bind analytes on their surface yields a direct, label-free electrical readout [ 173 ]. NWs-based sensors operate on the principle of ion-selective FETs and rely on the interaction of external charges with carriers in the nearby semiconductor, which results in enhanced sensitivity at low ionic strength. Park et al. fabricated fiber optic sensor using ZnO NWs and AuNPs for highly sensitive plasmonic biosensing [ 174 ]. Priolo et al. demonstrated label-free and PCR-free silicon NWs-based optical biosensor for direct genome detection. They exhibited a detection limit of 2 copies per reaction for the synthetic genome and 20 copies per reaction for the genome extracted from human blood [ 175 ]. Nuzaihan et al. prepared a silicon NW-based biosensor with novel molecular gate control for electrical detection of Dengue virus (DENV) DNA. The developed sensor had a low detection limit of 2.0 fM concentration with high sensitivity of 45.0 μA M −1 [ 176 ].

CNTs are exciting 1D NMs and are the most extensively investigated nanotubes class of materials in biosensors, diagnostics, tissue engineering, cell tracking and labeling, drug delivery, and biomolecules. They are hollow cylindrical tubes composed of one, two, or several concentric graphite layers capped by fullerenic hemispheres, which are referred to as single-, double-, and multi-walled CNTs, respectively. They have unique structures, excellent electrical and mechanical properties, high thermal conductivity, high chemical stability, remarkable electrocatalytic activity, minimal surface fouling, low over-voltage, and high aspect ratio (surface-to-volume) [ 181 , 182 , 183 ]. Because of their high surface-to-volume ratio and novel electron transport properties, the electronic conductance of theses nanostructures is strongly influenced by minor surface perturbations, such as those associated with the binding of macromolecules. CNT-based biosensors and diagnostics have been employed for the highly sensitive detection of analytes in healthcare, industries, environmental monitoring, and food quality analysis. They have been predominantly used in electrochemical sensing, for glucose monitoring, but also for detecting fructose, galactose, neurotransmitters, neurochemicals, amino acids, immunoglobulin, albumin, streptavidin, insulin, human chorionic gonadotropin, C-reactive protein, cancer biomarkers, cells, microorganisms, DNA, and other biomolecules. Multi-wall-carbon nanotubes (MWCNTs) are represented in all applications of nanotubes in biosensing. Such 1D nanomaterials provide real-time and sensitive label-free bioelectronic detection and massive redundancy in nanosensor arrays [ 165 ]. Cui et al. developed a wearable-based amperometric biosensor painted onto gloves as a new sensing platform used to determine lactate [ 184 ]. Janssen et al. demonstrated a CNT-based biosensor to sense a standard protein, bovine serum albumin (BSA), as a proof-of-concept. The developed sensor had a detection limit of 2.89 ng mL −1 [ 185 ]. Tang et al. fabricated a single-walled carbon nanotube (SWNT)-based DNA sensors and described the sensing mechanism. This work demonstrated clear experimental evidence on SWNT-DNA binding on DNA functionality, which paved a path for the future design of SWNT biocomplexes for applications in biotechnology and DNA-based nanotube manipulation techniques [ 186 ]. Hong et al. constructed metallic floating electrode-based DNA sensors with controllable responses. They showed the enhancement in the sensor response on increasing the number of floating electrodes [ 187 ]. Park et al. demonstrated a CNT-based biosensor system-on-a-chip for detecting a neurotransmitter. Here, CNT-based sensors were integrated with CMOS chips, which is useful in various biomedical applications, such as sensing components in LoC (lab-on-a-chip) systems for neuronal culture [ 188 ].

In this review article, we have discussed types and mechanisms of biosensors based on receptors (enzymes, antibodies, whole-cell, and aptamers), transducers (electrochemical, electronic, optical, gravimetric, and acoustic), and nanomaterials (gold NPs, Ag NPs, Pt NPs, Pd NPs, NWs, NRs, CNTs, QDs, and dendrimers). Biosensors offer versatile applications in the fields of engineering and technology, medicine and biomedical, toxicology and ecotoxicology, food safety monitoring, drug delivery, and disease progression. With the application of NMs in biosensors, we have witnessed rapid growth in biosensing technology is witnessed in the recent decade. This is because of the employment of new biorecognition elements and transducers, progress in miniaturization, design, and manufacture of nanostructured devices at the micro-level, and new synthesis techniques of NMs, all of which bring together the life and physical scientists and engineering and technology. The sensing technology has become more versatile, robust, and dynamic with the induction of nanomaterials. The transduction mechanism has been improved significantly (like greater sensitivity, faster detection, shorter response time, and reproducibility) by using different nanomaterials (such as NPs, NRs, NWs, CNTs, QDs, and dendrimers) that each has different characteristics within biosensors. Though there is considerable improvement in the use of nanostructured materials in biosensors applications, there are few limitations, which hinder these applications for the next level. For instance, lack of selectivity remains a setback for the CNT-based gas sensors, hampering its usage in CNT-based devices. However, this hurdle can be overcome by coupling CNTs with other materials. The other issues in these sensors include (i) the sustainability of nanostructures in sensor applications, which have been insufficiently investigated, (ii) the fabrication of nanostructures, and (iii) the toxicity, which changes according to the physical properties of the material type. These issues should be investigated and addressed while expanding new nanostructured materials for their use in biosensors. Most nanobiosensor devices used in biomedical applications require a large sample for detection, which may lead to false-positive or false-negative results. Very few biosensors have attained commercial success at the global level, apart from electrochemical glucose sensors and lateral flow pregnancy tests. There is also a need for making nanostructure-based biosensors at an affordable cost that give rapid results with accuracy and are user-friendly. For example, nanomaterials should be integrated with a tiny biochip (lab-on-chip) for sample handling and analysis for multiplexed clinical diagnosis. More research should be done in this area and we expect the ongoing academic research to be realized into commercially viable prototypes by industries in near future.