Dual Function Based on Switchable Colorimetric Luminescence for Water and Temperature Sensing in Two-Dimensional Metal–Organic Framework Nanosheets

A simple, rapid, highly selective, and real-time determination of water is urgently required for preventing danger from water contamination in materials. Herein, the excited-state proton transfer (ESPT) concept-based luminescent sensor [Cd2(2,5-tpt)(4,5-idc)(H2O)4] (1) (2,5-tpt = 2,5-dihydroxyterephthalic acid and 4,5-idc = 4,5-imidazoledicarboxylic acid) has been designed for discriminative detection via enol–keto tautomerism. To improve the sensitivity, two-dimensional (2D) nanosheets of 1 have been synthesized by top-down liquid ultrasonic exfoliation technology for sensing water in dimethylformamide, which lead to fast detection (<30 s), high selectivity, broad-range detection (0–50% v/v), and a low detection limit value (0.25% v/v). This sensor can serve dual sensing mechanisms along with a luminescent color change via shifted emission (green→yellow) in low water content and a turn-off method in high water content. For ease of use, the test-strip paper-based 2D nanosheets of 1 have been prepared and applied for water detection with long-term stability, pH stability, and good reusability. On-site water detection in real time can be evaluated using a smartphone color-scanning application for quantitative scanometric assays coupled with test-strip paper-based 2D nanosheets of 1. Also, 1 can be utilized for a colorimetric luminescent thermometer in the ranges of physiological and high temperature with good linearity and recyclability.