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 (SA_NO = 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 (SA_NO = 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 k_j,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.