Employing Triphenylene-Based, Layered, Conductive Metal–Organic Framework Materials as Electrochemical Sensors for Nitric Oxide in Aqueous Media

This paper describes the first use of conductive metal–organic frameworks as the active material in the electrochemical detection of nitric oxide in aqueous solution. Four hexahydroxytriphenylene (HHTP)-based MOFs linked with first-row transition metal nodes (M = Co, Ni, Cu, Zn) were compared as thin-film working electrodes for promoting oxidation of NO using voltammetric and amperometric techniques. Cu- and Ni-linked MOF analogs provided signal enhancement of 5- to 7-fold over a control glassy carbon electrode (SANO = 6.7 ± 1.2 and 5.7 ± 1.1 for Ni3(HHTP)2 and Cu3(HHTP)2, respectively) for detecting micromolar concentrations of NO. Zinc-based MOF electrodes offered more limited enhancement (SANO = 3.1 ± 0.5), while the cobalt-based MOF analog had intrinsic redox activity at potentials close to NO oxidation, which interfered with sensing. Combining MOFs with a conductive polymer improved electrode stability under repeated electrochemical scanning (14 ± 3% decrease in signal over 10 scans). The stabilized Ni3(HHTP)2@polymer-coated electrodes were able to detect NO at physiologically relevant concentrations (LOD = 9.0 ± 4.8 nM) in amperometric sensing experiments, and exhibited moderate selectivity against ascorbic acid and nitrite (log kj,NO = −1.3 ± 0.3 and −0.83 ± 0.68 for ascorbic acid and nitrite, respectively). This study demonstrates that layered, conductive 2D MOFs have promising applicability for NO detection in aqueous environments.