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 (λR) 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 λR. 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.