Development of grating light reflection spectroscopy for chemical sensing applications

Abstract

Grating Light Reflection Spectroscopy (GLRS) exploits the interaction of light with a transmission diffraction grating in contact with a sample. With this technique, light reflected from the grating exhibits a redistribution of spectral intensity and phase dependent upon the bulk dielectric properties (refractive index and absorbance) of the sample. The use of a transmission diffraction grating makes GLRS unique within the field of reflection-based optical measurements, in that it can operate at the critical point of reflection sensitivity to sample properties regardless of the state of the reflection interface. In addition, recent theoretical advances have demonstrated the ability of GLRS to make absolute determinations of the size and concentration of scattering inhomogeneities in the sample matrix. These properties make GLRS attractive as a sensor in complex or difficult analytical environments due to its relative insensitivity to fouling layers and by its ability to characterize the bulk properties of optically dense and/or heterogeneous matrices.Recent efforts have implemented GLRS as an on-column detector for samples flowing through rectangular micro-channels 100 microns wide by 10 microns deep. This has resulted in a significant enhancement of measurement precision, so that the technique may take full advantage of its inherent sensitivity. The coupling of this performance enhancement to the theoretical development mentioned above has resulted in subnanometer determinations of the radii of dendrimeric particles in aqueous solvent at concentrations that prohibit the use of transmission-based particle sizing techniques. These experiments led to the use of GLRS as an "in-column" liquid chromatography detector, where it proved useful in the identification and quantitation of absorbing and non-absorbing analytes. Building upon these advances, initial steps toward the embodiment of GLRS in a self-contained probe have shown great promise with respect to the creation of a rugged, sensitive, and versatile chemical sensor.