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Sensors & Transducers Journal, Vol. 113, Issue 2, February 2010, pp. 1-17

Fig. 4. Schematic diagram of hybridization reaction [Source: Ref. 121].

A review article on protein based lab-on-a-chip sensors reported by Borini et al. [65], describes different approaches for fabrication of biochips with PS and their future perspectives. Hu Lingang et al. [73] in their paper, reported a new technique for fabrication of a biochip on porous silicon and its application for detection of small molecule–protein interactions with desorption/ionization on PS (DIOS) [74]. Other applications of PS for building DNA sensors have also been reported earlier [108].

One of the major challenges in nanobiosensor industry is to design efficient sensors and integrating them on a biochip. The information captured by the sensors must be converted into a readable form [32]. Several optical, electrical, chemical, and biological data from the nanosensors are to be transformed into signals for processing, analysis, and for deciding actions. Thus, integration of sensing and controlling probes with the fluidic and the electronic world remains the main challenge while designing a multi-function biochip.

5. Optical Detection Techniques

For functioning of biochips, one of the most important criteria to be fulfilled is to attach a powerful transduction or signal processing unit to the system, which can directly and accurately detect the biological event and convert it in to a human readable output. The biological events like, antibody/ DNA binding, oxidation/reduction, etc., need to be transduced into a format understandable by a computer (voltage, light, intensity, mass, etc.), for analyzing and processing the signal to produce the final output. Several optical techniques are commonly used for detecting and quantifying biomolecules [62-63] as one does not require electrical contacts with the system for capturing data with optical devices. Instead, one can use fluorescent tags either with the probe or with the analyte to detect any change in the system. The major advantages of the optical transduction methods are that the devices are small, lightweight and portable due to the integrability of all optical components.

Several optical transduction methods such as FRET (fluorescence resonance energy transfer), SERS (surface enhanced Raman spectroscopy), and fluorescence spectroscopy are used for detection of biological samples. Spectroscopic techniques are used, for detecting biological samples or events occurring in it because cells or tissues can absorb or emit light, thereby producing a signal or spectrum, which is a characteristic of that particular event. From this fingerprint spectrum, one can directly identify or quantify the sample or the event.

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