Kinetic Discrimination of Metal Ions Using DNA for Highly Sensitive and Selective Cr³⁺ Detection

Most metal sensors are designed for a strong binding affinity toward target metal ions, and the underlying principle relies on binding thermodynamics. The kinetic aspect of binding, however, was rarely explored for sensing. In this work, the binding kinetics of 19 common or toxic metal ions are compared based on a fluorescence quenching assay using DNA oligonucleotides as ligands. Among these metals, Cr³⁺ shows uniquely slow fluorescence quenching kinetics, and the quenched fluorescence cannot be recovered by EDTA or sulfide. Most other metals quenched fluorescence instantaneously and can be fully recovered by these metal chelators. Various factors such as DNA sequence and length, chelating agent, pH, and fluorophore type were studied to understand the binding mechanism, leading to a unique two-stage binding model for Cr³⁺. This system has a wide dynamic range of up to 50 μM Cr³⁺ and a low limit of detection of 80 nM. It is also useful for measuring Cr³⁺ in lake water. This work proposes a new metal sensor design by monitoring binding kinetics with Cr³⁺ being a primary example.