Highly sensitive free-base-porphyrin-based thin-film optical waveguide sensor for detection of low concentration NO2 gas at ambient temperature

Meso-5,10,15,20-tetrakis-(4-tertbutyl phenyl) porphyrin was synthesized using Adler–Longo method and was served as sensing material. Electronic absorption spectra of the porphyrin chloroform solution and its thin film were studied comparatively. An optical waveguide sensor based on free-base porphyrin was fabricated by spin coating method. Absorption variation of porphyrin film was studied before and after exposure to NO2, H2S, SO2, and volatile organic gases. XRD patterns of porphyrin film before and after exposure to analytes (NO2, SO2, and H2S) were provided, and light source of the OWG testing system was selected. This facile-prepared sensor exhibited high sensitivity and selectivity to NO2 with fast response time of 3 s and slow recovery time of 10 min or so and was capable of measuring NO2 down to 10 ppb at ambient temperature. Scanning electron microscopy was employed to characterize film morphology before and after contact with NO2. Film thickness was measured before (71.3 nm) and after (76.8 nm) exposure to NO2, and film thickness variation value (5.20 nm) was calculated. The sensing behavior of the studied sensing device was tested through mixture of H2S, SO2, and VOC gases with NO2 and without NO2 for determination of the selectivity of the device. Film stability was probed by UV–Vis spectra, and response values of sensing element to NO2 gas were detected after several days of film preparation, and its RSD value was 1.66%.