Hg(II) Adsorption on Gold Nanoparticles Dominates DNA-Based Label-Free Colorimetric Sensing

Hg(II) can specifically bind between two thymine bases forming a T–Hg(II)–T base pair, which was often used for the detection of mercury, including using gold nanoparticles (AuNPs) to design label-free colorimetric biosensors. In this work, we focused on the interaction between Hg(II) and AuNPs and their effect on DNA adsorption. A series of experiments including fluorescence, UV–vis, dynamic light scattering, and Raman spectroscopy were performed. A blue-to-red color change of the as-prepared AuNPs was observed with increasing concentration of Hg(II), whereas an opposite red-to-blue trend occurred for the washed AuNPs with free citrate removed. T15 DNA as well as other DNA sequences all showed a similar color change in the presence of Hg(II). Hence, the interaction of Hg(II) and the AuNPs, instead of DNA/Hg(II) binding, dominated this system. Moreover, with up to 1 μM Hg(II), Hg(II) stabilized the AuNPs against salt-induced aggregation and promoted DNA adsorption. In the presence of Hg(II), DNA was adsorbed more strongly on the AuNPs. Our work indicated the need to consider target and AuNP interactions when designing such label-free sensors.