Sulfonate-Grafted Metal–Organic Framework─A Porous Alternative to Nafion for Electrochemical Sensors

Nafion, a polymer containing negatively charged sulfonate groups, is commonly used as a thin film cast on the electrode to adjust the selectivity of complicated electrochemical reactions involving charged reactants or analytes in the electrochemical sensing and electrolytic systems. Herein, a highly porous and chemically stable metal–organic framework (MOF) with enriched sulfonate groups in its pores is synthesized by performing the postsynthetic immobilization of the sulfonate-based ligand within a zirconium-based MOF, MOF-808, and the resulting sulfonate-grafted MOF (SO3-MOF-808) is proposed as a "porous Nafion"─an appealing alternative to the conventional Nafion coating for modulating the selectivity of electrochemical reactions. As a demonstration, the electrochemical sensing of dopamine (DA), which usually suffers from interference caused by the oxidation of the coexisting negatively charged ascorbic acid (AA) and uric acid (UA), is implemented. With the use of the SO3-MOF-808 thin film deposited on the active electrode surface, both the current signal for DA oxidation and the selectivity toward the oxidation of DA against those of AA and UA remarkably outperform those achieved by the Nafion thin film with an optimized film thickness. Findings here suggest the new role of such sulfonate-grafted MOFs as an advanced alternative to Nafion in a range of electrochemical sensing systems and other complicated electrochemical reactions.