Optical transduction techniques

Comparing the two optical transduction techniques (absorption or SPR) used in this work, we can conclude that SPR technique appears to be more suitable for gas sensing even if it presents some limitation regarding the suitable film thickness for SPR excitation. Moreover, the response and recovery times during the anal5fle/sensing layer interaction appears shortest in the case of optical absorption measurements. Further investigations are in... 

In general, a vast number of optical transduction techniques can be used for biosensor development. These may employ linear optical phenomenon (e.g. adsorption, fluorescence, phosphorescence, and polarization) or nonlinear phenomena (e.g. second harmonic generation). The choice of a particular optical method depends on the analyte and the sensitivity needed. Total internal reflection fluorescence (TIRF) has been used with planar and fibre-optic wave-guides as signal transducers in a number of biosensors. 

An optical transduction method that is often used with ultrathin hlms, such as LB hlms, is that of surface plasmon resonance [30, 31]. Surface plasma waves are collective oscillations of the free electrons at the boundary of a metal and a dielectric. These can be excited by means of evanescent electromagnetic waves. This excitation is associated with a minimum in the intensity of the radiation reflected from the thin him system, called surface plasmon resonance (SPR). The sensitivity of SPR is noteworthy, and changes in refractive index of 10 may be monitored thus the technique compares favorably with ellipsometry. The method has been used with LB hlms to provide both gas detectors [29] and sensors for metal ions in solution [32]. 

This chapter discusses different categories of LOC devices based on the transduction technique involved ie, optical, electrochemical, thermal, etc. The approach of this chapter is to discuss the types of on-chip devices from the point of view of the working principle, followed by some highlights of recent advances in the development of such devices. 

Optical detection is the most commonly used transduction technique for application in laboratories due to its high sensitivity and immunity from coupled electromagnetic... 

One of the most common transduction techniques used in microfluidic LOC systems is electrochemical detection. Advantages of electrochemical detection include easy fabrication, integration, and miniaturization of electrodes within microfluidic devices. Although electrochemical techniques may lag behind optical detection in terms of sensitivity, nevertheless its low cost, ability to work with turbid samples, inertness to ambient light, temperature, etc., prompted researchers to incorporate it in LOC devices for POC applications. 

Apart from the classification based on sensing mechanisms, biosensors are classified into three categories considering the transduction mechanism used. They are optical, electrochenfical, or electrical. The optical transduction mechanism includes fluorescence, chemilununescence, interferometry, and surface plasmon resonance. These techniques involve either the production of light from chemical reactions or the change in refractive index at the interface of biosensing materials. 

Cantilevers are usually microfabricated from silicon by using conventional pho-tomasking and etching techniques. Typical dimensions of a cantilever are 100 pm in length, 20 pm in width, and 1 pm in thickness. Silicon and silicon nitride cantilevers and cantilever arrays that utilize optical beam deflection for signal transduction are commercially available. Piezoresistive cantilever arrays are also commercially available. Piezoresistive cantilevers are 120 pm in length, 1 pm in thickness, and 40 pm in width. 

It appears that fluorescence techniques are poised to receive more serious consideration for accelerated development efforts. Key obstacles remaining include stability of receptors and fluorophores, challenges that will possibly be met partially by results of the intense efforts of molecular biology, polymer science, and nanotechnology. Advances in nanomaterials such as quantum dots will likely enable improvements in optical stability and choice of excitation/emission wavelengths for various transduction methods. Stabilization of natural and artificial enzymes and rendering immunogenic protein receptors stealthy may also aid the pursuit. 

Fiber-optic biosensors are analytical devices in which a fiber optic device serves as a transduction element. The usual aim of fiber-optic biosensors is to produce a signal proportional to the concentration of target analyte to which the biological element reacts. Fiber-optic biosensors are based on the transmission of light along silica glass fiber, or POF to the site of analysis. They can be used in combination with different types of spectroscopic technique, e.g. absorption, fluorescence, phosphorescence, or surface plasmon resonance (SPR) (14). 

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