Luminescent Coordination Polymer with Ultrasensitive “Turn-On” Hg²⁺ Sensing Ability and Its Portable Hydrogel Test Kit

Hg2+ has serious effects on ecosystems and human health. Thus, it is very important to develop simple and efficient methods to detect Hg2+. A luminescent coordination polymer (LCP) [Cd(L)(1-NA)2(H2O)·(L)(H2O)0.5]n (1) was constructed by 5,5′-((9H-fluoren-9-ylidene)-methylene)dipyrimidine (L) and 1-naphthoic acid (1-NA) with rigid groups. 1 demonstrates a 1-D wave-like chain structure. Its fluorene and naphthalene groups from adjacent chains exist in π–π interaction. 1 has excellent water, pH, and thermal stability. It exhibits very rare and ultrahigh sensitive "turn-on" Hg2+ sensing ability. Its detection limit toward Hg2+ is extremely low (8.86 × 10–8 M). Particularly, the fluorescence intensity of the water suspension of 1 (2.5 mL, 0.05 mg mL–1) can be rapidly enhanced by approximately 80 times after only adding Hg2+ solution (200 μL, 1 mmol). Accordingly, 1 shows the lowest LOD and the highest emission enhancement in all LCP-based "turn-on" Hg2+ sensors. Furthermore, 1 was immobilized in a hydrogel to fabricate a portable test kit to realize the convenient, fast, and quantitative Hg2+ detection. This study provides an excellent sensor material and the first on-site platform for environmental monitoring of Hg2+.