Nanoporous Thin-film Waveguide Resonance Sensors with Wavelength Interrogation over a Broad Bandwidth

A TiO2 nanoporous thin film based leaky waveguide resonance sensor operating in the wavelength interrogation mode was characterized. The resonance wavelengths (lambda(R)) and the induced shifts in lambda(R) were determined directly from the reflected light intensity spectra measured at a given incident angle. A considerable broadening of the resonance band was observed with the sensor, mainly resulting from a large bulk scattering of the nanoporous TiO2 film. The best fit to the experimental data with the Fresnel equations leads to a pore-volume fraction of 0.474 for the TiO2 film. The sensor is responsive to adsorption of small molecules such as glutathione (GSH, MW = 307.3 Da). A change of the surrounding liquid from water to aqueous GSH solution (0.1 mM) results in a redshift of Delta lambda(R) approximate to 3 nm from lambda(R) = 593.6 nm, corresponding to a volume fraction of 0.05 for the GSH molecules adsorbed on the porous film based on simulations. According to the theoretical analyses, a small change in refractive index of the surrounding liquid can induce a large blueshift of lambda(R) under proper conditions.