Study of Spectral Sensitivity to Small Molecule Analytes Using TiO2 Nanoporous Thin-film Leaky Waveguide Resonance Sensors

Leaky-mode resonance (LMR) sensors with the Kretschmann configuration and wavelength interrogation were prepared using gold-cladded nanoporous TiO2 thin-film waveguides. Resonance wavelengths (λR) for the leaky modes in the LMR sensor relies on the angle (θ) of the collimated incident beam with respect to the lateral surface normal of the prism coupler. With a given value of θ λR linearly increases with increasing refractive index (nc) of liquid in a small range. The refractive index (RI) sensitivity at θ = 6° (was measured to be ΔλR/nc = 2532.1 nm RIU−1 using aqueous sucrose solutions whose RI range from 1.333 to 1.356. The nanoporous structure of the LMR sensor greatly increases the depth of interaction between the guided wave and adsorbed target molecules, which makes its sensitivity to small-molecule adsorption higher than that of the conventional surface plasmon resonance (SPR) device. The measurements with the cetyltrimethylammonium bromide and bromothymol blue adsorptions demonstrated that the values of ΔλR for the LMR sensor are 2 times larger than those for the SPR sensor that contain a naked gold layer.