Modifiable porous silica microsphere optical sensor for chemical detection

This paper reports a novel Extrinsic Fabry-Perot Interferometer (EFPI) sensor platform based on similar to 50 mu m-diameter porous silica microspheres attached to the ends of single-mode optical fibers. The glass spheres, with 45% internal void volumes, act as geometrically well-defined Fabry-Perot (FP) cavities that produce interferograms that only depend on the index of refraction of guest molecule types and loadings. The primary advantage of the sensor is that the silica micropores inside the glass spheres present inherent surface hydroxyl groups, which can be chemically modified using a wide selection of silanization reactions. Silanized silica microspheres provide a novel and broad sensor platform where myriad silane coupling agents act as bridges connecting organic and inorganic materials. Commercially available silanization reagents are diverse and afford silica pores with selectivity for sensing chemicals and biochemicals. When guest molecules are adsorbed in the pores of the microspheres, a proportional change in the light path length can be calculated and measured. A gas sample generator consisting of vapor generators, analyte permeation tubes, and flow controllers were configured to characterize the sensor response to various volatile organic compounds. An optical interrogator with a 1 Hz scan frequency and 80 nm wavelength range was employed for full spectral scanning and data acquisition. Experimental results demonstrate shifts of the interferogram when an EFPI glass microsphere is exposed to different vapors and vapor concentrations. Future work will compare EFPI results of guest molecule adsorptions by unaltered versus silanized porous glass microspheres.