Developing an Aptamer-Based Colorimetric Method for Highly Specific Discrimination of Hg2+ and MeHg

Highly toxic inorganic mercury (Hg2+) and methylmercury (MeHg) are widespread heavy metal pollutants that negatively affect human health and ecosystems. Thus, developing a method for the rapid, facile, and visual detection of Hg2+ and MeHg to prevent mercury poisoning is urgent and challenging. In this study, a colorimetric biosensor using a thymine-rich (T-rich) HT9 deoxyribonucleic acid (DNA) aptamer serving as a recognition probe (RP) labeled with a thiol group (-SH) and poly diallyldimethyl ammonium chloride (PDDAC)-coated gold nanorods (AuNRs) as a signal probe (SP) was developed for the specific detection of Hg2+ and MeHg. In the presence of Hg2+, Ag0/Hg0 amalgam nanoparticles (ANPs) were formed and bound to the HT9 DNA aptamer, producing a light lavender color. Due to its stronger binding ability, MeHg showed priority binding to the HT9 DNA aptamer, yet reduced Ag0 NPs were deposited on the surface of PDDAC-coated AuNRs. Interestingly, this produced a color ranging from olive to atrovirens. The method exhibited a good linear relationship in ranges of 0.30–1.00 μM and 0.50–5.00 μM for Hg2+ and MeHg, respectively, with corresponding visual limits of detection (LODs) of 0.30 μM (for Hg2+) and 0.50 μM (for MeHg). Their theoretical LODs (3σ/S) were calculated to be 0.071 μM (for Hg2+) and 0.12 μM (for MeHg). The method used in this work was successfully applied to the colorimetric detection of Hg2+ and MeHg in real seawater samples, with a recovery (Rec.) of 98–107% and a relative standard deviation (RSD, n = 6) ≤ 8%. This method provides a useful technical reference for the semi-quantitative determination of Hg2+ and MeHg in drinking water and aquatic products, especially in the case of accident-level contamination.