Selective Sensing of Peroxynitrite by Hf-Based UiO-66-B(OH)<sub>2</sub> Metal–Organic Framework: Applicability to Cell Imaging

The first boronic acid functionalized Hf-based UiO-66 (UiO = University of Oslo) metal–organic framework (MOF) having the ability to detect both extracellular and intracellular peroxynitrite is presented. The Hf-UiO-66-B­(OH)<sub>2</sub> material (<b>1</b>) was synthesized under solvothermal conditions from a mixture of HfCl<sub>4</sub> and 2-borono-1,4-benzenedicarboxylic acid [H<sub>2</sub>BDC–B­(OH)<sub>2</sub>] ligand in DMF in the presence of formic acid (modulator) at 130 °C for 48 h. The desolvated material (<b>1′</b>) was utilized as a fluorescent turn-on probe for the rapid sensing of extracellular peroxynitrite (ONOO<sup>–</sup>) under conditions mimicking those of biological medium (10 mM HEPES buffer, pH 7.4). Selective sensing of ONOO<sup>–</sup> over other ROS/RNS was also achieved by <b>1′</b>. The oxidative cleavage of attached boronic acid groups forming corresponding hydroxy-functionalized ligands can be accounted for the fluorescent increment phenomenon in the presence of ONOO<sup>–</sup>. The probe showed extraordinary sensitivity (detection limit = 9.0 nM) toward ONOO<sup>–</sup> in 10 mM HEPES buffer at pH 7.4. Probe-loaded cells did not exhibit cytotoxicity and morphological deformities. It is remarkable that the probe inside the cells responded toward the peroxynitrite solution to give an intense blue fluorescent signal. The fluorescence microscopy study with J774A.1 macrophage cells unambiguously demonstrated that probe <b>1′</b> is suitable to image peroxynitrite in living cells.