**3. Transducers and transduction mechanisms in biosensors**

Integrated biosensors can be categorized in several ways, one of which is their transduction mechanism. Below is a brief description of some of the main transducers used in biosensor mechanisms:

#### **3.1 Transduction mechanisms**

*Electrochemical or Electroanalytical transducers:* These types of transducers exploit analyte capturing to change the electrochemical characteristics of electrodeelectrolyte systems. Hassibi, and Lee [29] described one of the first reports of a CMOS electrochemical biosensor array chip capable of performing impedance spectroscopy, amperometric analysis, and cyclic-voltammetry techniques. The Electrical performance of this CMOS multi-functional chip is comparable to stateof-the–art electrochemical measurement instruments currently used in molecular biology. Prior efforts in this area are limited only to ISFET [30] and conductionbased sensor arrays [31] in CMOS compatible processes.

*Mechanical transducers:* Mechanical transducers in biosensors are systems in which an electromechanical parameter of the system (e.g., mass of a cantilever) is changed by the additional mass of the captured analytes. Mechanical transducers cannot be fabricated using standard CMOS processes. This is largely because CMOS processes, unlike many MEMs processes, offer no component or device, which can move in response to mechanical motion [32, 33].

*Nanomaterial-Enhanced Receptor Technology for Silicon On-Chip Biosensing Application DOI: http://dx.doi.org/10.5772/intechopen.94249*

*Thermal transducers:* They are used to measure the temperature change during biological thermal reactions to detect the total number of molecules involved in the reaction [34].

*Acoustic transducers:* Acoustic transducers are based on the principle of using acoustic waves to develop biosensing devices. Acoustic waves can be used to analyze biochemical reactions in different fluid (liquid and gas) environments because they have the ability to travel through them easily. Different mechanisms can be employed to either generate or receive acoustic waves, including piezoelectric, magnetostrictive, optical and thermal techniques. The surface or the acoustic biosensor can be functionalized using standard methods, and the binding of the target analyte will induce a change in the acoustic wave. By measuring and subsequently analyzing this variation in the resonant frequency, it is possible to determine the correlation between the acoustic signal and the interaction of the molecule. One important advantage of acoustic transducers is the ability to operate using wireless excitation. This makes it extremely important for sensing in difficult-to-reach or hazardous environmental conditions. Additionally, the compatibility with integrated circuit technology permits a high efficiency in the fabrication process. The main drawback of this technique is the fact that any little vibration induced in the system can produce artifacts that will require complex algorithms to differentiate from the true biosensing signals [35].

*Optical transducers:* Optical biosensors can easily fulfill the requirement for high sensitivity, fast response and the potential for real-time measurements. They also lend themselves to optical measurements through different techniques like emission, absorption, fluorescence, refractometry, or polarimetry. Optical biosensors based on evanescent wave detection have unique properties such as extremely high sensitivity for direct, real-time measurement of biomolecular interactions in labelfree schemes. The advantages of optical sensing are significantly improved when this approach is used within an integrated optics context. Integrated optics technology allows the integration of passive and active optical components (including fibers, emitters, detectors, waveguides, and related devices) onto the same substrate, allowing the flexible development of miniaturized compact sensing devices, with the additional possibility to fabricate multiple sensors on a single chip. The integration offers additional advantages such as miniaturization, robustness, reliability, potential for mass production with consequent reduction of production costs, low energy consumption, and simplicity in the alignment of the individual optical elements [36].

**Figure 2.** *Categories of optical transducers used for biosensor applications.*

Independent of the transduction method used in integrated biosensors, one challenge is always to adequately immobilize capturing probes on the surface to create the capturing spot. Our design addresses this challenge by etching a receptor cavity through the silicon substrate, and by using AgNPs as immobilization layer in the receptor layer to enhance selective absorption of the target analyte.
