The electrochemical sensing system has been recognized as a promising and facile approach to detect dopamine (DA), a crucial neurotransmitter in the human body. However, the rational design of electrode materials is important in order to selectively detect DA in the presence of coexisting interferents, such as ascorbic acid (AA) and uric acid (UA). In this study, a water-stable and nanoporous zirconium-based metal−organic framework (MOF), UiO-66, is synthesized with a tunable degree of missing-linker defects, and the corresponding crystallinity, morphology, porosity, and degree of defects are characterized. Although all the UiO-66 synthesized here is electrically insulating and electrochemically inactive, the thin film of defective UiO-66 deposited on the electrode surface can significantly amplify the electrochemical sensing signal for DA. The effect of the degree of defects on the resulting sensing response for DA is examined, and the origin of such a signal amplification effect, which is relevant to the hopping-based electrochemical process of the irreversibly adsorbed DA in the defective MOF, is investigated. By serving the defective UiO-66 as a signal amplifier casted on top of another electrically conductive active material capable for selective DA detection, the modified electrode for sensing DA with enhanced performances can be developed. As a proof-of-concept demonstration, the defective UiO-66 thin film was coated on the graphene oxide (GO) modified electrode. With the use of such a bi-layer thin film for DA sensing, a much higher sensitivity (6.4-fold), a much smaller limit of detection (0.23-fold), and a better selectivity toward DA against AA and UA (2.6-fold and 1.2-fold, respectively) can be achieved compared to those of the GO thin film. Experimental results here shed light on the use of such porous MOF coatings to effectively enhance the sensing performances of other developed electrochemical sensing systems for DA.

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.

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