Evanescent sensing

Scorsone E., Christie S., Persaud K.C., Simon P., Kvasnik F., Fibre-optic evanescent sensing of gaseous ammonia with two forms of a new near-infrared dye in comparison to phenol red, Sensor. Actual. B-Chem. 2003 90 37-45. 

Conventional evanescent sensing works exceedingly well for relatively small biomolecules such as proteins and DNA molecules whose size is much smaller than the decay length. However, it becomes less sensitive when detecting biospecies, such as cells, with dimensions over 1 pm. In Chap. 15, deep-probe waveguide sensors are developed to overcome this limitation, which have a decay length comparable to the size of the biospecies of interest. 

Figure 7.2 A schematic diagram of nanometer position sensing. Light from the evanescent field scattered by the microparticle is measu red with a quadrant photodiode detector, whose differential outputs correspond to the x and y displacements and the total intensity depends exponentially on the distance z between the particle and the glass plate.

The major advantage of TIRF is that fluorophores outside the evanescent wave (typically more than 200 nm away from the surface) are not excited. Hence, TIRF has an intrinsic sectioning capability. Of interest is that the section capability (z-resolution) is far better than for confocal microscopy systems, which typically have a z-resolution of about 1 /mi. In addition and in contrast to confocal microscopy, TIRF does not cause out-of-focus bleaching because only the molecules at the surface will sense the evanescent wave. However, in comparison with confocal microscopy, a clear limitation of TIRF is that only one z-plane can be imaged the molecules immediately adjacent to the surface. As a consequence,... 

Kvasnik F., McGrath A.D., Distributed chemical sensing utilizing evanescent wave interactions, Proc. SPIE-Int. Soc. Opt. Eng. 1990 1172 75. 

Schwotzer G. et. al., Optical sensing of hydrocarbons in air or in water using UV absorption in the evanescent field of fibers, Sensors Actuators B 1997 38 150-153. 

Matejec V., Kasik I., Chomat M., Ctyroky J., Berkova D., Huttel I., Modified inverted graded-index fibers for evanescent-wave chemical sensing, Proc. SPIE 3860 (1999), Boston, pp.443-451. 

Matejec V., Chomat M., Pospisilova M., Hayer M., Kasik I. Optical fiber with novel geometry for evanescent-wave sensing, Sensors Actuators (1995) B 29, pp. 416-422. 

The absorption-based platforms described previously employed evanescent wave interrogation of a thin sensing layer coated onto a planar waveguide. A sensitivity enhancement strategy for optical absorption-based sensors based on planar, multimode waveguides was developed recently by us18. The objective was to apply this theory to the development of low-cost, robust and potentially mass-producible sensor platforms and the following section outlines the assumptions and predictions of this theoretical model. 

Figure 6 illustrates the platform under consideration in this analysis. The principle of sensor operation is as described previously for absorption-based optical sensors employing evanescent wave interrogation of the sensing layer. 

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. 

In addition, typical methods of sensing are total internal reflection fluorescence or monitoring of fluorescence resonance energy transfer6,7. The second class is a direct optical detection principle which relies either on reflectometry or refractometry. The latter is connected to evanescent field... 

The waveguide is split into two arms one arm (the reference arm) is shielded and not affected by the environment. At the interface of the other arm, the evanescent field will be influenced by any sensing effect. Thus, the two partial beams will propagate in a different way within the two arms, in dependence on the difference in the refractive indices adjacent to the two arms. 

Nearly all integrated optical sensors4 6 rely on evanescent field sensing. This principle can be explained using Figure 7. 

Figure 7. Evanescent field sensing the black part of the field profile is the operational evanescent field.

The fabrication and characterization of a fiber optic pH sensor based on evanescent wave absorption was presented by Lee63. The unclad portion of a multi-mode optical fibre was coated with the sol-gel doped with pH sensitive dye. The sensitivity of the device increased when the multiple sol-gel coatings were used in the sensing region. The dynamic range and the temporal response of the sensor were investigated for two different dyes -bromocresol purple and bromocresol green. 

Blue R., Stewart G., Fibre-optic evanescent wave ph sensing with dye doped sol-gel films, Intern. J. Optoelectron. 1995 10 211-222. 

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