Refractive index, light sensors

From a general point of view, a chemical sensor is a device capable of continuously monitoring the concentration of an analyte. The two main classes are electrochemical sensors and optical chemical sensors. The latter are based on the measurement of changes in an optical quantity refractive index, light scattering, reflectance, absorbance, fluorescence, chemiluminescence, etc. For remote sensing, an optical fiber is used, and the optical sensor is then called an optode because of... 

The interaction phenomenon of the analyte with radiation is governed or modifies the principle on which light is transmitted through the waveguide. Depending on that interaction, we can differentiate three basic types of intrinsic optical sensors [82] refractive-index-type sensors, luminescence-based sensors and absorption-type sensors. 

Surface plasmon resonance (SPR) sensors are optical sensing devices that take advantage of the sensitivity of a special type of electromagnetic field, a surface plasmon, to changes in refractive index. SPR sensors can be classified based on the method for optical excitation of surface plasmons and the measured characteristic of the light wave interacting with the surface plasmon. 

Figure 1. Schematic of the optical fiber system. Excitation light is launched into the fiber. Due to the refractive index differences between the fiber core and cladding materials, the light is internally reflected and travels through the fiber with minimal loss (see inset). The emitted light is carried back from the fluorescent sensor located on the tip of the fiber to a CCD camera detector. Reprinted with permission from Science, 2000, 287, 451-452. Copyright 2000 AAAS.

Fig. 10. A SPR Detection realized in a BIAcore system. A fan of polarized light passes a prism and is focused at the interface to an aqueous phase under conditions of total reflection. An evanescent wave enters the solvent phase. If the prism is coated with a thin gold layer at the interface the free electrons in the metal absorb energy from the evanescent wave for a distinct angle, depending on the refractive index of the solvent near the interface. B The gold layer can be modified with, e.g., a carboxydextrane matrix, where catcher molecules can be immobilized by standard chemistry. If a ligand is applied with the aqueous phase it may interact with the catcher and accumulate in the matrix, causing a shift in the resonance angle. If no specific binding occurs the refractive index in proximity of the sensor is less affected...

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