Smartphone-assisted "on-off-on" boric acid-functionalized Eu-MOF sensing platform for dual-mode detection of Hg2+ and glutathione.

A fluorescent switch nanoprobe based on boric acid-functionalized europium-based metal-organic framework (BA-Eu-MOF) integrated with a visual analysis platform assisted by smartphone was established for the sensitive detection of Hg2+ and glutathione (GSH). The probe was fabricated with 5-boronoisophthalic acid as ligand and europium ions as metal node, which enables specific recognition of Hg2+ through a transmetalation mechanism. Upon the introduction of Hg2+, the red fluorescence of the probe was quenched ("off" state). When GSH is added, because its thiol groups have a stronger binding affinity for Hg2+, it competitively extracts Hg2+ from the BA-Eu-MOF/Hg2+ complex, leading to fluorescence recovery ("on" state). Based on this "on-off-on" mode, the quantitative detection of Hg2+ and GSH was achieved with detection limits of 0.67 nM and 0.087 µM. Furthermore, the intelligent sensing platform for quantitative detection of analytes was developed by utilizing a smartphone Color Grab App with detection limits of 9.57 µM and 2.32 µM, respectively. Both modes were successfully employed to the detection of Hg2+ and GSH in actual samples with spiked recoveries ranging from 88.9% to 108.0%. Benefiting from the porous characteristics of MOF and the specific recognition capability of the boric acid group, the proposed probe exhibits notable advantages including facile preparation, operational simplicity, high sensitivity, and excellent selectivity. This study provides a facile, reliable and field-applicable novel detection strategy for environmental monitoring and the investigation of interactions between heavy metals and biomolecules.