Selectively Confined Poly(3,4-Ethylenedioxythiophene) in the Nanopores of a Metal–Organic Framework for Electrochemical Nitrite Detection with Reduced Limit of Detection

Poly(3,4-ethylenedioxythiophene) (PEDOT) selectively generated in the nanopores of a zirconium-based porphyrinic metal–organic framework (MOF), NU-902, is synthesized by in situ polymerization with the coexistence of MOF crystals and excessive poly(sodium 4-styrenesulfonate) (PSS) followed by the successive washing steps to remove the well-dispersed PEDOT:PSS from the MOF-based solid. For comparison, PEDOT-NU-902 composite with PEDOT solely present between MOF crystals and that containing both pore-confined PEDOT and interparticle PEDOT are also synthesized by physical blending method and the in situ polymerization without adding PSS, respectively. Crystallinity, morphology, porosity, and electrochemical behavior of these PEDOT-NU-902 composites are investigated. Since both PEDOT and the porphyrinic linkers of NU-902 are active electrocatalysts for nitrite oxidation, these composites along with the pristine NU-902 and PEDOT are applied for electrochemical nitrite sensors in aqueous electrolytes. The composite with PEDOT solely confined within the MOF pores can effectively reduce the nonfaradaic current originating from PEDOT while achieving a moderate catalytic faradaic current for nitrite, which results in its smallest limit of detection (LOD) for nitrite determination compared to other PEDOT-NU-902 composites and the pristine materials. The electrochemical nitrite sensor based on pore-confined PEDOT achieves a sensitivity of 133 μA/mM cm2, a linear range of up to 1.6 mM, and a LOD of 1.71 μM. By utilizing nitrite detection as a proof-of-concept demonstration here, the findings suggest the unique role of the pore-confined conducting polymers within structurally rigid MOFs in electrochemical sensors and shed the light on the design and applications of such nanocomposites for a range of electroanalytical purposes.