Detection of extremely low- and high-concentration organic acids by a highly-aligned lamellar block copolymer photonic crystal film

Abstract

Detection of organic acids was carried out using lamella-forming polystyrene-block-poly (4-vinylpyridine) (PS–P4VP) block copolymer (BCP) photonic crystals. PS-P4VP photonic crystal thin films featuring highly-aligned lamellar microstructures parallel to the substrate were prepared through solvent annealing method by chloroform. When the lamella-forming PS-P4VP films were immersed in low-concentration organic acids, including formic acid (FA), acetic acid (AA), and propionic acid (PA), red-shifting reflectance wavelengths could be observed with the increase of the acid concentration. In contrast to the AA and PA systems, the lamella-forming PS-P4VP thin film was able to detect extremely low-concentration FA (5 × 10−5 vol%) due to its strongly protonated ability. When the PS-P4VP thin film was immersed in high-concentration FA and AA solutions, the longer and red-shifting reflectance wavelengths were observed with the increase of the acid concentration. In addition, the unexpected blue-shifting reflectance wavelength was observed when the film was immersed in a high-concentration PA solution. This is attributed to the alternation of segregation strength resulted from the compatibility effect. As a result, well-aligned lamellar PS-P4VP photonic crystal thin films are highly sensitive to the stimuli of organic acids for detecting extremely low- and high-concentration organic acids due to the change of protonated ability and compatibility, respectively.