Label-free liquid crystal-based detection of As(III) ions using ssDNA as a recognition probe

We report a label-free liquid crystal (LC)-based biosensor for sensitive detection of arsenic (III) ions (As3+) in aqueous solutions, using an arsenic-binding aptamer (Ars-3 aptamer) as a molecular recognition element. In this sensing system, the cationic surfactant, cetyltrimethylammonium bromide (CTAB), was employed to induce a homeotropic orientation of the LCs at the aqueous/LC interface, a result of the self-assembly of CTAB molecules. In the absence of As3+, the addition of Ars-3 aptamers disturbed the self-assembly of CTAB at the aqueous/LC interface, due to interactions between CTAB and the Ars-3 aptamers, causing an orientational transition of LCs from homeotropic to planar. In the presence of As3+, the specific binding of the Ars-3 aptamers with As3+ led to the formation of an aptamer-As3+ complex, resulting in a conformational change of the aptamer. This change weakened the interaction between CTAB and the aptamer at the interface, causing the orientation of the LCs to remain unchanged in a homeotropic state. The changes in the orientation of the LCs caused by the interactions between the Ars-3 aptamer, As3+, and CTAB were simply converted and observed under a polarized light microscope as a shift between a bright and a dark image. A low detection limit of 50 nM (similar to 3.7 ppb) was obtained, which is well below the maximum permissible level of As3+ in drinking water (133 nM) set by the US Environmental Protection Agency and the World Health Organization. Other chemical species (heavy metal ions) did not induce these changes, and thus, the biosensor was highly specific for As3+ ion sensing. The potential application of the developed sensor for As3+ detection in tap water was also demonstrated. Therefore, this LC-based biosensor is a promising platform for simple, rapid, label-free determination of As3+ concentration in aqueous samples with high selectivity and sensitivity.