Making and Breaking of DNA-Metal Base Pairs: Hg²⁺ and Au Nanocluster Based Off/On Probe

Making the “DNA-metal base pairs” is one of the promising avenues for designing DNA based optical sensors and other potential applications. Herein, particular emphasis has been given on designing an off/on optical probe using DNA-metal base pairs. DNA minor groove binder dye, Hoechst (HOE) is used to probe the construction and breaking of DNA-metal base pairs. The HOE dye binds with the adenine-thymine (AT) rich part of the DNA by hydrophobic interaction, which is evidenced by the blue shifting of emission from 496 to 452 nm. Study reveals that Hg 2+ ion binds to DNA and form DNA-metal base pairs via strong T–Hg 2+ bond formation by replacing the HOE dye. Steady state and time-resolved spectroscopic studies are being used to understand the DNA-metal interaction and consequent change in conformation of DNA is being studied by circular dichroism (CD). It is to be noted that 87% PL quenching (turn off) of HOE dye is found due to DNA-metal base pairs formation. The turn off of the PL signal is being used to calculate the limit of detection (LOD) of Hg 2+, which is found to be 0.87 μM. Furthermore, PL intensity (turn on) recovery up to 91.04% is achieved by the addition of Au nanoclusters (NCs). Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) studies confirm the aggregation of Au NC and formation of S–Hg 2+ bond during the interaction of Au nanocluster with DNA-metal base pairs. It reveals that Au NC plays an important role for breaking of DNA-metal base pairs and retaining the canonical Watson–Crick base by switching the conformation and recovery of PL. Thus, this switchable off/on system may pave imperative application in the field of optical sensor, gene therapy, and cell biology.