Metal-Clad Waveguide Resonance Sensor Using a Mesoporous TiO₂ Thin Film as the Chemical Sensitive Core Layer

A metal-clad optical waveguide sensor based on guided-mode resonance spectroscopy has been developed using a mesoporous TiO₂ film as the chemical-sensitive core layer. The sol-gel block copolymer templated mesoporous TiO₂ films are ∼275 nm thick, supporting both the transverse electric (TE) and transverse magnetic (TM) modes and thus providing the sensor with the TE- and TM-related dual sensitivities. Both the TE- and TM-mode resonance wavelengths (_R) at the same incident angle were obtained by broadband attenuated total reflection spectroscopy. The film porosity was determined to be ∼0.535 by using the combination of Fresnel formula and Bruggeman approximation to fit the measured values of _R. The TE- and TM-related sensitivities of the sensor to the two analytes of elaidic acid (EA) and Cr⁶⁺ were measured at the same incident angle. For each analyte, the TM-related sensitivity is higher than that with the TE-related one. The two sensitivities can be calibrated with each other. Moreover, by adjusting the block copolymer concentration in the coating solution, the mesoporous TiO₂ films with different porosities were prepared for investigating the porosity dependence of the sensor's sensitivity. The findings indicate that a larger-porosity TiO₂ film can lead to higher sensitivity. The work demonstrated that the metal-clad mesoporous TiO₂ film-based guided-mode resonance sensor is advantageous over a conventional SPR sensor for in situ detection of small molecules.