A visual film sensor based on silole-infiltrated SiO₂ inverse opal photonic crystal for detecting organic vapors

The reversible color change of the silole-infiltrated SiO₂ inverse opal photonic crystal (IOPC) film can be obtained by alternating its exposure to different vapor environments. When the film was put in diethyl ether or petroleum ether vapor, a stopband red shift of more than 100 nm could be clearly observed, while the color changed from green to red. When exposed to air, the stopband underwent a blue shift and the color changed back to green. The result is attributed to the silole molecules, hexaphenylsilole (HPS), which can be transformed reversibly between the crystal and amorphous state when alternately exposed to air and vapors of diethyl ether/petroleum ether. When crystal HPS changed to amorphous HPS in an atmosphere of organic vapors, both the specific surface area and refractive index of HPS increased. The higher specific surface area of HPS improved the adsorption behavior of organic vapors. Both the improved adsorption and higher refractive index of HPS increased the effective refractive index of HPS-infiltrated SiO₂ IOPC, which resulted in the red shift of the stopband and the color change, according to the Bragg Law. Based on the reversible aggregation state transfer and the adsorption-desorption of organic vapors, the effective refractive index of the film varied repeatedly, which caused the reversible stopband shift and color change. The visual detection of organic vapors can be realized because of the remarkable color change of the IOPC film, which provides a simple route for monitoring volatile organic compounds, and is important for chemical and biological sensors.