Dual-Function Sensing Platform for Hg2+ Based on a Redox-Active Thiosemicarbazone Receptor

Abstract

Recently, our group developed a platform for adsorptive stripping voltammetric analysis of heavy metals based on electrodes modified with “clickable” thiosemicarbazone ligands, achieving detection limits for Hg2+ in the nM range1. Here, we present an extension of our previous results towards obtaining an innovative, dual-function sensing platform suitable for both rapid screening and accurate quantitation of mercury. For this purpose, we designed a novel ferrocene-based redox-active thiosemicarbazone receptor having an intramolecular signalling pathway between the binding site and redox-active center, a prerequisite for obtaining efficient redox-active receptors3. The metalloligand, which also contains an azido functional group, was synthesized by reacting 4-(6-azidohexyl)-3-thiosemicarbazide1 with ferrocenealdehyde. The binding properties of the ligand towards several heavy metal ions (Cd2+, Cu2+, Hg2+, Ni2+, Pb2+, Zn2+) were evaluated by UV-VIS spectroscopy, ESI-MS and cyclic voltammetry. Notably, the voltammograms recorded in the presence of sub-stoechiometric amounts of Hg2+ displayed a “two-wave” behavior4 for the Fc/Fc+ redox couple (ΔEp~100 mV), suggesting a high binding constant for this particular ion. Next, modified electrodes were prepared by grafting glassy carbon substrates with triisopropylsilyl-protected phenylethynyl groups through the electrochemical reduction of the corresponding diazonium salt5, followed by deprotection of the alkyne groups and „click” immobilization of the thiosemicarbazone receptor. The electrode surface functionalization was further assessed using electrochemical techniques and X-ray photoelectron spectroscopy, and the complexation behavior of the modified electrodes in the presence of heavy metal ions was investigated using Osteryoung square-wave voltammetry6. As expected, the Fc/Fc+ couple from the surface-immobilized ligand displayed the same voltammetric behavior as in solution, and the appearance of a new voltammetric wave shifted by ~100 mV allows the rapid qualitative screening of Hg2+. Furthermore, the modified electrodes were employed for the accurate quantitation of Hg2+ ions using the accumulation at open circuit-anodic stripping voltammetry technique, achieving a LOD of 5.0 nM. Finally, we demonstrate that electrodes modified with the thiosemicarbazone metalloligand have a very good reproducibility, reusability and stability. Acknowledgements This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS-UEFISCDI, project number PN-III-P1-1.1-TE-2021-1163, within PNCDI III.