Resonance-Amplified Diffraction Sensors

The need to improve sensing technology for widespread application in everyday life is fueling the development of diffraction-based sensing, an optical sensing technique. Selectivity and sensitivity issues must first be resolved. Both selectivity and sensitivity can be increased by using resonance conditions — that is, using a probe wavelength where the system absorbs. The resonance effects are based on a change in absorbance that produces a surprisingly large but indirect change in the real part of the refractive index, as modeled by the Kramers-Kronig equation. In the two approaches studied here, the systems undergo absorbance changes upon analyte exposure. The first approach was aimed at development of a sensitive alcohol sensor through incorporation of CR-546, a chromoreactand dye, into a polymeric diffraction grating. This attempt was abandoned once theoretical results from the Kramers-Kronig equation showed the overall change in the real part of the refractive index was too small for detection. The second approach involved incorporation of silver nanospheres into an elastomer grating. These spheres have promising universal application due to the fact that absorbance changes in the quadrupolar coupling peak are accomplished with modulation in the refractive index. Theoretical results, based on unpatterned PDMS membranes, are promising. Incorporation of silver nanospheres in a diffraction grating was successfully attempted, but time restrictions prevented collection of diffraction data.