Abstract:
Metal organic frameworks (MOFs) have been used as electrochemical sensors owing to their high surface area and porosity. Unfortunately, the low conductivity and redox activity of MOFs limit their application in electrochemical sensing. To address this problem redox-active materials can be incorporated in MOFs to produce MOF composites. In this work we report on the synthesis and characterization of Zeolitic imidazolate framework-8 (ZIF-8), Zirconium based MOF (UiO-66) and their corresponding phthalocyanine (Pc) composites. Solvothermal synthesis was used to obtain ZIF-8, UiO-66, and their composites in powder form. The morphology and elemental composition were attained using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis, respectively. The composites were composed of agglomerated rods surrounded by irregular polyhedral structures. The presence of MOF and Pc elements confirmed that the Pcs were within the MOFs. The crystallographic properties of the composites were probed using powder X-ray diffraction (PXRD) which showed that the composites were more crystalline than the pristine MOFs. The increase in order of the structure of the composites confirmed incorporation of the Pcs onto the MOFs. The surface area and pore size distribution of the composites were determined using Brunauer-Emmett-Teller (BET) analysis. The pore volume ranged from 0.10 to 0.37 cc/g. The decrease in surface area and pore volume of the composites indicated that the Pcs were adsorbed onto the MOF pores. The optical properties of the composites were obtained using ultraviolet-visible (UV-vis) spectroscopy. The composites exhibited the characteristic B and Q bands of phthalocyanines and showed metal-to-metal charge transfer between MOFs and Pcs confirming that the phthalocyanines were incorporated within the MOF structure. ZIF-8, UiO-66 and their corresponding phthalocyanine composites were immobilized on a glassy carbon electrode (GCE) using the drop drying method. The modified electrodes were used for the detection of 4-chlorophenol, and the electrochemical responses were recorded using cyclic voltammetry. The electrocatalytic behaviour of the electrodes modified with MOF composites showed improved oxidation compared to the ZIF-8 or UiO-66 modified electrodes. The FePc@ZIF-8 modified electrode showed the best catalytic behaviour with an anodic peak potential at 0.54 V while FePc@UiO-66 behaved best with respect to enhancing catalytic currents. The limit of detection ranged from 0.05 mM to 2.8 mM for all redox active composites. This work shows that phthalocyanine-MOF composites are effective electro-catalysts and may be developed for the detection of other environmental pollutants.
