Competitive fluorescence assay for Cd2+ based on aptamer structure-switching

Cadmium ion (Cd2+) is a heavy metal pollutant that causes serious effects on human health. Development of a simple and specific Cd2+ detection method is of great significance in food safety and environmental monitoring. In this work, we demonstrated a fluorescence assay for Cd2+ based on aptamer structure-switching. A short 17-mer aptamer against Cd2+ with fluorescein (FAM) labeled at the 5′ end was used as an affinity ligand, and a partially complementary DNA (cDNA) with black hole quencher 1 (BHQ1) labeled at the 3′ end was used as quencher. In the absence of Cd2+, the aptamer hybridized with the cDNA, forming a duplex helix structure that showed low fluorescence intensity as FAM was quenched by adjacent BHQ1. In the presence of Cd2+, the aptamer specially bound to Cd2+, and the FAM-labeled aptamer dissociated from the aptamer-cDNA duplex, showing high fluorescence intensity. We examined the effects of different lengths of cDNA, the composition of binding buffer, incubation time, and incubation temperature on the performance of the sensing system. Under the optimal experimental conditions, we achieved sensitive and selective detection of Cd2+ with a detection limit of 2.4 nM by using the 17-mer aptamer and 13-mer cDNA. The FAM-labeled aptamer exhibited a large fluorescence increase upon Cd2+, with a maximum change about 51.6-fold. The developed assay also showed good responses to Cd2+ in actual water samples, showing the potential for practical application.