Detection of sulfadimethoxine using optical images of liquid crystals

A label-free method for sulfadimethoxine (SDM) detection using an aptamer-based liquid crystal biosensor is developed. The sensor probe is fabricated by immobilizing amine-functionalized aptamers onto the glass slide decorating mixed self-assembled layers of triethoxysilylbutyraldehyde (TEA) and N,N-dimethyl-n-octadecyl-3-aminopropyltrimethoxysilylchloride (DMOAP). Liquid crystals (LCs) are supported on the surface and serve as response elements, which assume the homeotropic alignment and cause a dark optical appearance under crossed polarizers. In the presence of SDM, the formation of SDM–aptamer compounds induces a notable change in the topographical structure of the surface, which disturbs the original homeotropic orientation of LCs and results in a bright optical appearance. A detection limit of 10 μg L−1 is obtained, which is far lower than the maximum residue limit (100 μg L−1 in China). This facile method shows good specificity for SDM detection and may have great potential for detecting other small molecules.