Novel Hg²⁺-Selective Signaling Probe Based on Resorufin Thionocarbonate and its μPAD Application

In this study, a novel resorufin thionocarbonate-based Hg²⁺-selective signaling probe (RT) for microfluidic paper-based analytical device (μPAD) applications is reported. The designed probe, RT, was readily synthesized by the one-step reaction of resorufin with phenyl thionochloroformate. The RT probe displayed a prominent color change from yellow to pink and a marked turn-on fluorescence signaling behavior exclusively toward the Hg²⁺ ion. The signaling of RT was due to Hg²⁺-induced hydrolysis of the phenyl thionocarbonate moiety to form the parent resorufin dye, which restored its spectroscopic properties. In addition, RT exhibited the Hg²⁺-selective signaling behavior without interference by coexisting environmentally relevant metal ions. The detection limit for Hg²⁺ in simulated wastewater samples was estimated to be 5.8 × 10^-8 M. In particular, an RT-equipped μPAD prepared using a wax printing technique enabled simple and convenient determination of Hg²⁺ ions in simulated wastewater samples, with a detection limit of 5.9 × 10^-6 M.