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Basics of Biosensors and Nanobiosensors

Pravin Bhattarai and Sadaf Hameed

Peking University, Department of Biomedical Engineering, Beijing, 100871, PR China

1.1 Introduction

The conventional analytical methods, both qualitative and quantitative, based on the measurements of species in complex matrices dominated the era of chemical sensing. These methods were based on the complete separation of sample components followed by the identification and quantitation of the target analytes. However, (i) expensive nature of the measurement techniques both financially and temporally, (ii) difficulty in the analysis of complex samples within a limited sample concentration, and (iii) the employment of separation methods limiting real-time analysis during in vivo applications subtly challenged its future development [1]. At present, an inexpensive and facile way of biosensor fabrication for the real-time detection and/or quantification of biologically relevant analytes provides an analytically powerful tool over conventional techniques [2]. These biosensors can surpass the major limitations of traditional sensors such as sensitivity, speed, and sensibility. Such biosensors typically function by combining a biomolecular recognition unit that is capable to sense the biochemical reaction and a transducer that can convert the concentration of the target analytes into a measurable signal. In 1977, Karl Camman first coined the term biosensor, but the IUPAC (International Union of Pure and Applied Chemistry) disagreement led to the conception of a new standard definition in 1997 [3]. A standard definition of biosensor now is as follows: "A biosensor is a self-contained integrated device, which is capable of providing specific quantitative or semi-quantitative analytical information using a biological recognition element (biochemical receptor), which is retained in direct spatial contact with a transduction element. Because of their ability to be repeatedly calibrated, we recommend that a biosensor should be clearly distinguished from a bioanalytical system, which requires additional processing steps, such as reagent addition. A device that is both disposable after one measurement, i.e., single use, and unable to monitor the analyte concentration continuously or after rapid and reproducible regeneration should be designated as a single-use biosensor." Since the earliest enzymatic electrode-based biosensors developed by Clark, there has been a rapid development/improvement in the design and application of these biosensors (Figure 1.1). Recently, biosensors (electrochemical, optical, electronic, and piezoelectric) comprising various biorecognition molecules such as enzymes [4], aptamers [5], whole cells [6], antibodies [7], and deoxyribonucleic acid (DNA) [8] are widely applied in health care, food quality management, forensics, pharmaceutical industries, and several other areas (Figure 1.2). Improvised methods in the fabrication of biosensors have greatly augmented the characteristics of a biosensor measured in terms of selectivity, reproducibility, stability, sensitivity, and linearity. Moreover, rapid advancement in the fabrication technology together with electronic components has ushered miniaturization of such devices resulting a huge surge in the biosensor market. Notably, the use of nano-sized materials (having at least one dimension <100?nm) in the fabrication of biosensors leading to nanobiosensors have gained high momentum lately. The unique properties (mechanical, chemical, structural, and electrical) of these nanomaterials used in nanobiosensors have not only helped to overcome challenges based on the sensitivity and detection limit of the devices but has also improved the interfacial reaction owing to the better immobilization of biorecognition molecules [9,10]. In addition, hybridization of nanomaterial-based strategies with a microscale system has allowed a new type of biomolecular analysis together with a high level of sensitivity that can leverage nanoscale binding events to detect circulating tumor cells (CTCs) or sense rare analytes [11]. In brief, this chapter comprehends all the basic information about biosensors and also provides in-depth knowledge of the design, components, characteristics, and applications of biosensors.

Figure 1.1 Recent publication trend in biosensors.

Figure 1.2 Five-year publication trends of various types of biosensors. (a) 2017-2013 and (b) 2012-2008.

1.2 Biosensor and Its Working Principle

A simple design of any biosensors basically comprises four major components: (i) a bioreceptor, (ii) a transducer, (iii) electronic components, and (iv) a readout/display unit (Figure 1.3). Briefly, a bioreceptor is an external component of a biosensor that comes in direct contact with the target analyte during operation. The major function of a bioreceptor is to capture the target analytes with high specificity and selectivity [12]. Some examples of bioreceptors commonly used in the construction of biosensors are enzymes [4], aptamers [5], whole cells [6], antibodies [7], and DNA [8]. Construction mechanism typically follows the adsorption/immobilization of a biorecognition element on the surface of a biosensor. Therefore, techniques deployed for the adherence of such biorecognition elements remain central to the sensitivity and selectivity of a biosensor.

A most common approach for the immobilization of biorecognition elements includes adsorption, microencapsulation, entrapment, covalent bonding, and cross-linking [13-15]. Immobilization serves one or more of the following purposes: (i) continuous monitoring of analytes in flowing samples such as environmental samples, biological fluids having less amount of target molecules or bioreactor fluids, (ii) the biosensor can be used repeatedly, (iii) enhances the performance of biosensors in terms of reproducibility and sensitivity owing to the advancement of the biorecognition unit, and (iv) simplicity and flexibility of the immobilization technique. Toward a closer look in the fabrication strategies, (nano)biosensors confer multivariate interfacial region ranging between 1 and 10?nm, especially for the recognition of target analytes [11]. The detection of various biological molecules including protein-protein interactions can occur in this region. However, complexity during immobilization of such nanoscale components may be a challenging task. The chemical reaction at the site of bioreceptor, also termed as biorecognition, results in the generation of various signals such as light, changes in pH, heat generation, or changes in mass, which can be perceived by the physical component, transducer. The transducer can be defined as a device that can convert one form of energy to another. Therefore, depending on the type of biochemical reactions, several types of transducers can be used during construction of a biosensor; for instance, if the biorecognition process yields output in the form of light, then an optical transducer (e.g. photodetector) can perceive the incoming light and convert into a measurable electrical form [16]. Notably, all of the conversion processes are directly proportional to the amount of analyte-bioreceptor interactions at the biorecognition unit. The signals generated by the transducer (usually electrical) are in analogous form and cannot be read directly. Therefore, a signal conditioning unit assimilating various high pass/low pass/notch filters, amplifiers, and analogs to digital converters usually quantifies the signal that can be displayed directly in a readable format [17]. Biosensors may consist of different types of display units such as liquid crystal display (LCD), computer, or directly to the printer that comprises a pictorial representation of the measured signal. Depending on the user's requirement, the format of output signals may vary, e.g. the final data can be either numeric, tabular, graphics, or an image.

Figure 1.3 Schematic of biosensor components.

1.3 Characteristics of a Biosensor

The design of a biosensor defines the intended purpose of the application; however, other key factors are still central to manipulate the performance of these biosensors (Figure 1.4) [18].

1.3.1 Selectivity

Selectivity is the ability of a biosensor to detect a specific target analyte from a pooled sample containing mixtures of unwanted contaminants. The best classical example to explain selectivity is the interaction between an immobilized antibody and an antigen that is highly specific in nature.

1.3.2 Reproducibility

Reproducibility, on the other hand, is the ability of a biosensor to yield identical end results regardless of the number of times experiment is repeated. This is mainly determined by the precision and accuracy of the transducer or electronic components in a biosensor. The reliability of biosensor output is highly dependent on the reproducibility of the biosensor devices.

Figure 1.4 Biosensor characteristics.

1.3.3 Stability

Although precision and accuracy regulate the ability of biosensors to yield highly reproducible results, nevertheless, stability is another...