Detection of Cadmium(II) in Aquatic Products Using a Rolling-Circle Amplification-Coupled Ratio Fluorescent Probe...

The existing aptamers for cadmium (Cd²⁺), the common toxic heavy metal contaminant in food, cannot meet the requirements for detecting Cd²⁺ in rapid detection methods. In previous work, we found that coupling aptamer-peptide conjugates (APCs) with peptides and aptamers can provide a less disruptive method with a significantly improved affinity. Moreover, we found that the spatial conformation of aptamers and peptides is crucial for obtaining proper affinity in APC. Therefore, we describe a simple design strategy to obtain a series of APCs with different affinities by designing peptide orientations (N-terminal, C-terminal). The best affinity was found for APC(C1-N) with a binding constant (K_a) of 2.23 × 10⁶ M^-1, indicating that the APC(C1-N) affinity was significantly increased by 829.17% over aptamer. Finally, a rolling-circle amplification (RCA)-coupled ratio fluorescence-based biosensor for Cd²⁺ detection was established with a detection limit of 0.0036 nM, which has great potential for practical aquatic product detection.