A transparent TMPyP/TiO2 composite thin film as an HCl sensitive optochemical gas sensor

Tetracationic porphyrin (TMPyP) molecules were incorporated into an optically transparent TiO2 thin film, prepared by Glancing Angle Physical Vapour Deposition (GAPVD), by simple infiltration (at pH 6.4). The preparation of optically transparent TMPyP/TiO2 composite thin films provides a method for the integration of the porphyrin molecules into photonic devices for direct monitoring of gases. Previously, UV–visible and fluorescence spectral techniques have been used to study the reversible protonation of TMPyP in aqueous solution. The optical spectrum of TMPyP shows an intense Soret band at 423 nm with a 22 nm red shift upon protonation by HCl. The experimental conditions for monitoring the concentration of HCl gas by absorption spectroscopy have been optimized. The maximum absorbance change was observed at the Soret band wavelength. A selected temperature of 80 °C and a 300 s recovery period were found to be the optimum operating parameters (response time t50 = 16.8 ± 0.7 s). The composite with smaller surface concentration of TMPyP (Г = 0.3 × 10−9 mol cm−2) presented the best detection limit (0.1 ppm). The response of the composite sensor was highly stable for several months.