Tris buffer-accelerated ligand exchange rate for instant fluorescence detection of trivalent chromium ion

Functional nucleic acids (FNAs) have attracted a lot of attention for the rapid detection of metal ions. Cr³⁺ is one of the major heavy metal ions in natural waters. Due to the slow ligand exchange rate of Cr³⁺, the FNA-based Cr³⁺ sensors require long assay times, limiting the on-site applications. In this study, we report that the good's buffers containing amino and polyhydroxy groups greatly increase the ligand exchange rate of Cr³⁺. Using EDTA as a model coordinate ligand, the Tris buffer (100 mM, pH 7.0) showed the best acceleration effect among the eight buffers. It improved the rate constant ∼20-fold, shorten the half-time 19-fold, and lowered the activation energy ∼70% at 40 °C. The Tris buffer was then applied for sensor based on the Cr³⁺-binding induced fluorescence quenching of fluorescein (FAM)-labeled and single-stranded DNA (ssDNA), which shortened the assay time from 1 h to 1 min. The Tris buffer also ∼100% enhanced the fluorescence intensity of FAM, achieving the 11.4-fold lower limit of detection (LOD = 6.97 nM, S/N = 3). By the combination use of the Tris buffer and ascorbic acid, the strong interference from Cu²⁺, Pb²⁺, and Fe³⁺ suffered in many previous reported Cr³⁺ sensors was avoided. The practical application of the sensor for the detection of Cr³⁺ spiked in the real water samples were demonstrated with high recovery percentages. The Tris buffer could be applied for other metal ions with slow ligand exchange rate (such as V²⁺, Co³⁺ and Fe²⁺) to solve diverse issues such as long assay time and low synthesis yield of metal complexes, without the need of heating treatment.