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This study concerns the development of a maghemite (γ-Fe2O3)-coupled tetra sulfophenyl porphyrin (TSPP) thin film–based optical waveguide (OWG) sensor (TSPP-γ-Fe2O3-OWG) for the detection of ethylenediamine (EDA). For ease of manufacture, we fabricated sensors using a spin-coating technique, and characterized their response based on their ultraviolet-visible (UV-vis) absorption spectra and the response intensity of OWG sensor. We have observed significant changes in the location of the peaks in the sensor’s UV-vis absorption spectra after exposed to EDA gas, indicating aggregation of TSPP. Optical characterization confirmed that the combination of TSPP with maghemite (TSPP-γ-Fe2O3-OWG) had improved device response, with twice as high as those of the acid-treated TSPP-OWG sensor, and 30 times higher than that of the pure γ-Fe2O3-OWG sensor. At ambient temperature, the composite sensor exhibited a linear response to EDA gas in the concentration range of 0.1–1000 ppm with signal-to-noise ratio (S/N) 8.33, confirming that the improved stability of the composite TSPP-γ-Fe2O3 thin-film OWG enables the detection of trace amounts of gas with high sensitivity and selectivity.

Substituent effect on optical gas sensing performance in porphyrin-based optical waveguide detection system was studied by molecular dynamics simulation (MDS), absorption/emission spectrum analysis, and optical waveguide (OWG) detection. The affinities of porphyrin with seven types of substituents (–H, –OH, –tBu, –COOH, –NH2, –OCH3, –SO3−) on para position of meso-phenyl porphyrin toward gas molecules in adsorption process were studied in different size of boxes with the same pressure and concentration. Analyte gases (CO2, H2S, HCl, NO2) were exposed to porphyrin film in absorption spectrophotometer, and in OWG with evanescent field excited by a guiding laser light with 670 nm wavelength. The extent of interaction between host molecule and the guest analytes was analyzed by the number of gas molecules in vicinity of 0.3 nm around substituents of porphyrin molecules. Optical waveguide results reveal that sulfonate porphyrin is mostly responsive to hydrochloride, hydrosulfide gas and nitrogen dioxide gases with strong response intensity. Molecular dynamics and spectral analysis provide objective information about the molecular state and sensing properties. Molecular rearrangements induced by gas exposure was studied by spectral analysis and surface morphology before and after gas exposure taking hydrosulfide gas as an example. Film-gas interaction mechanism was discussed in terms of each gas and substituent group characters.

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