Direct Measurement of Aqueous Mercury(II): Combining DNA-Based Sensing with Diffusive Gradients in Thin Films

A highly specific DNA-functionalized hydrogel sensing layer was integrated with the diffusive gradients in thin films (DGT) technique for the direct determination of aqueous mercury(II). The DNA-functionalized layer in the DGT unit exhibited both high affinity (complexation constant K_c = 10^19.8 at 25 °C) and high binding capacity (9.5 mg Hg disk^-1) toward Hg²⁺. The diffusion coefficient for Hg²⁺ complexed with common inorganic ligands was an order of magnitude higher than that for Hg²⁺ complexed with natural dissolved organic matter: 9.0 × 10^-6 versus 9.8 × 10^-7 cm² s^-1 at 25 °C. The performance of the DNA-DGT sensor was further assessed under variable pH (3-10) and temperature (5-40 °C) conditions, as well as across a range of hydrochemically diverse artificial and natural freshwaters. The observed effects of the environmental and solution compositional variables on Hg²⁺ binding to the DNA in the sensing layer were successfully accounted for by equilibrium speciation calculations and temperature-corrected, multicomponent diffusion coefficients for aqueous Hg(II). The results therefore support the use of the DNA-DGT sensor as an alternative to traditional sampling and analysis methods for measuring aqueous Hg(II) concentrations down to the nanomolar level in freshwater environments.