Biosensors evanescent wave sensor

Sol-gel coating technique for optical chemical sensors and biosensors is now in extensive research phase. For example, the side-coating of optical fibers or waveguides in evanescent-wave sensors it is particularly important to control precisely the sensitivity determining parameters, such as the coating thickness and length45. 

Loev, W. F., Button, L. J., Slovacek, R. E. (1991) Optical Characteristics of Fiberoptic Evanescent Wave Sensor Theory and Experiment, Biosensor with Fiberoptics, the Humana Press Inc., pp.l39... 

DeMarco DV, Lim DV (2001) Direct detection of escherichia coliol57 h7 in unpasterized apple juice with an evanescent wave sensor. J Rapid Meth Automation Micro 9 241-257 Diez A, Andres MV, Cruz JL (2001) In-line fiber-optic sensors based on the excitation of surface plasma modes in metal-coated tapered fibers. Sensors Actuators B Chem 73 95-99 Dostalek J, Ctyroky J, Homola J, Brynda E, Skalsky M, Nekvindova P, Spirkova J, Skvor J, Schrofel J (2001) Surface plasmon resonance biosensor based on integrated optical waveguide. Sensors Actuators B Chem 76 8-12... 

While planar optical sensors exist in various forms, the focus of this chapter has been on planar waveguide-based platforms that employ evanescent wave effects as the basis for sensing. The advantages of evanescent wave interrogation of thin film optical sensors have been discussed for both optical absorption and fluorescence-based sensors. These include the ability to increase device sensitivity without adversely affecting response time in the case of absorption-based platforms and the surface-specific excitation of fluorescence for optical biosensors, the latter being made possible by the tuneable nature of the evanescent field penetration depth. 

Figure 1. The specific binding of analytes to immobilized receptors induces a response of the optical biosensor due to changes in the evanescent wave at the sensor surface.

Seo et al. (1999) used a planar optic biosensor that measures the phase shift variation in refractive index due to antigen binding to antibody. In this method, they were able to detect S. enterica serovar T) himurium with a detection limit of 1 x 10 cfu/ml. When chicken carcass fluid was inoculated with 20 cfu/ml, the sensor was able to detect this pathogen after 12 h of nonselective enrichment. A compact fiber optic sensor was also used for detection of S. T) himurium at a detection limit of 1 X 10" cfu/ml (Zhou et al., 1997, 1998) however, its efficacy with food samples is unproven. Later, Kramer and Lim (2004) used the fiber optic sensor, RAPTOR , to detect this pathogen from spent irrigation water for alfalfa sprouts. They showed that the system can be used to detect Salmonella spiked at 50 cfu/g seeds. An evanescent wave-based multianalyte array biosensor (MAAB) was also employed for successful testing of chicken excreta and various food samples (sausage, cantaloupe, egg, sprout, and chicken carcass) for S. T) himurium (Taitt et ah, 2004). While some samples exhibited interference with the assay, overall, the detection limit for this system was reported to be 8 x 10 cfu/g. 

The most common principle used in optical biosensors for detection in the real part of the refractive index is evanescent wave detection, where the transducer optics are modified by changes in optical parameters of the medium in contact with the sensor surface via the interaction with the evanescent light wave penetrating into the ambient medium. As the evanescent wave decays exponentially from the surface, the most sensitive detection is just at the transducer surface. 

MEMS-Based Biosensor, Figure 7 (a) Schematic of an evanescent wave traveling in an optical waveguide. The traveling light in the waveguide is influenced by analyte-induced changes of the refraction index in the adjacent sensitive layer, (b) Schematic of a conventional Mach-Zehnder interferometer The cross-section shows the separate sensor and reference branches [12]... 

Optical biosensor technology has realized remarkable developments in the last decades [126-129]. This section will specifically focus on IR fiber sensors based on ChG materials. We will first describe the mechanism of evanescent wave spectroscopy and summarize the essential properties of ChG glasses when used in the framework of fiber-based sensing. Finally we will present several techniques commonly employed for the shaping of ChG glasses into fiber-optic sensors and survey their applications. 

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