Tailoring Rhodamine Probes: Oxyethylene Chains, Conjugation, and End Substituents in the Quest for Superior Hg2+ Sensing

This study explores the impact of incorporating oxyethylene (OE) chains into rhodamine-based dye molecules for the enhancement of photophysical properties in the context of chemical sensing. The introduction of OE chains provides several advantages, including improved water solubility, photostability and resistance to self-assembled nanoaggregates formation. Despite these, the compound with oxyethylene chains showed poor response (similar to 2-fold) towards Hg2+ ions than that of dimeric probe (similar to 20 fold) with no such substituents. Notably, dimeric compound shows a remarkable three-fold larger response to Hg2+ ion compared to monomeric compound. This intriguing outcome is attributed to the extended conjugation within the dimeric structure, which can induce conformational change of the overall molecule. Moreover, the study investigates the effects of end substituents, such as electron-donating (-OMe, improved response) and electron-withdrawing (-NO2, poor response) groups, on the interaction with Hg2+ ions. The excellent sensitivity of the probe molecules is also evident in significantly low detection limits for Hg2+ ion (5-8 ppb) in real-life water samples. The high recovery values (>95 %) with sufficient low standard deviation (<5%) indicates that quantitative analysis of Hg2+ is achievable using the current protocol. Beyond that, we have employed the probe 1 for intracellular (HEK293T cells) fluorescence imaging of Hg2+.