Water- and pH-Stable Methylthio-Containing Metal–Organic Frameworks as Luminescent Sensors for Metal-Ion Detection

Currently, metal–organic frameworks (MOFs) with thiol functionalities are used as advanced robust materials to solve the problems of pollution and damage caused by metal ions in water. Herein, we report a framework assembled by reacting cadmium nitrate with a new ligand, 4-methylthiopyridine-2,6-dicarboxylic acid, which possesses methylthio and carboxyl functional groups. The resultant MOFs exhibit 5-c net, fnb-type, uninodal net topology with {3³·4·6⁴·7²} point symbols. The thiol-functionalized MOF features good thermal, water, and pH stabilities and luminescence emission at different temperatures. The methylthio-containing cadmium MOF (Cd-MOF) selectively detects the heavy-metal ions Cu²⁺, Hg²⁺, and Pb²⁺ in water by luminescence quenching, with low limits of detection (LODs) of 22.60 × 10^–7, 11.07 × 10^–6, and 9.58 × 10^–6 M, respectively. The highly selective response and performance of the Cd-MOF probe toward Cu²⁺, Hg²⁺, and Pb²⁺ ions were determined by investigating the effects of other interfering ions and different pH values, and the reusability performance was evaluated.