A ratiometric green-to-blue emitting Hg2+ ion-responsive ionic liquid-based micro-optode†

This study presents an Hg²⁺ ion-responsive ratiometric green-to-blue emitting ionic liquid (IL), trihexyltetradecylphosphonium hydroxyproline-1,3,6-trisulfonate (HPTIL), and its micro-optode, mHPTIL. Neat HPTIL functioned as a photoluminescent ink without a solvent, highlighting its effectiveness as a safekeeping script material and served as a green-to-blue-emitting Hg²⁺ ion-responsive photoluminescent IL. Upon the introduction of heavy metal Hg²⁺ ions into the water-suspended mHPTIL system, a noticeable ratiometric shift from greenish to bluish fluorescence was observed, exhibiting detection and quantification limits in the nanomolar (nM) range with the lowest LOD of 1.67 nM. Additionally, a very convenient and portable solid-state paper-strip-based test kit was introduced to monitor the heavy metal, Hg²⁺ ions. This work proposes a novel approach for generating low-dimensional materials based on ILs that have exceptional properties for environmental and forensic monitoring.