Tackling the water solubility dilemma of spiroring-closing rhodamine: Sulfone-functionalization enabling rational designing water-soluble probe for rapid visualizing mercury ions in cosmetics.

Rhodamine derivatives possessing spiroring-closing structures exhibit colorlessness, while the induction of spiroring-opening by metal ions results in notable color changes, rendering them as ideal platform for the development of functional probes with broad applications. However, the spiroring-closing form of rhodamine-based probes exhibits limited water solubility due to its neutral character, necessitating the incorporation of organic solvents to enhance solubility, which may adversely affect the natural system. Designing rhodamine probes with high solubility in both the zwitterionic and neutral form is of utmost importance and presents a significant challenge. This study presents a sulfone-rhodamine-based probe that exhibits good water solubility both in the spiroring opening and closing for detecting Hg. Upon the presence of Hg, the color undergoes a noticeable change from colorless to pink, with a response time of less than 1 min. probe 1 demonstrates an excellent linear relationship with Hg concentrations within the range of 0-8 μM, and achieves a detection limit is 17.26 nM. The effectiveness of probe 1 was confirmed through the analysis of mercury ions in cosmetic products. Utilizing this probe, test paper strips have been developed to enhance the portability of Hg detection naked eyes.