ZnO–ZnFe₂O₄/Fe₃O₄/Carbon Nanocomposites for Ultrasensitive and Selective\nDopamine Detection

Abstract

Electrochemical detection of submicromolar\nlevels of dopamine (DA)\nby iron–carbon-based redox mediators requires the synergistic\neffect of facile Fe²⁺ ↔ Fe³⁺ redox chemistry,\nhigh DA adsorption capacity, and fast electron-transfer kinetics.\nIts absence in most reported mediators has led to detection limits\nthat are well above the lower threshold [DA] in healthy humans (i.e.,\n0.01 μM). Herein, we report a ZnO–ZnFe₂O₄/Fe₃O₄/carbon nanocomposite, which possesses\nthe aforementioned synergy and therefore displays impressive sensing\ncapabilities. ZnO–ZnFe₂O₄/Fe₃O₄/carbon is synthesized <i>in situ</i> via\nthe carbothermal reduction of Congo red (CR)-decorated ZnO–ZnFe₂O₄ nanofibers. This synthesis approach allows CR-derived\nCO­(g) to consume the lattice oxygen at the edges of ZnFe₂O₄ and generate oxygen vacancy (O_V)-rich-Fe₃O₄/ZnO interfaces embedded in mesoporous graphitic\ncarbon. Differences in work function cause interfacial electron transfer\nfrom Fe₃O₄ to ZnO, which improves the Fe²⁺ ↔ Fe³⁺ redox chemistry and increases the\ncharge-carrier concentration and electron-transfer rate. Meanwhile,\nthe lattice vacancies and surface polarization increase the surface\nenergy, which improves DA adsorption. Benefiting from these physicochemical\nadvantages, a nafion/ZnO–ZnFe₂O₄/Fe₃O₄/carbon-modified glassy carbon electrode (GCE)\ndisplays a low detection limit of 1.57 nM, a high sensitivity of 2.7186\nAM^–1 cm^–2, and a rapid response\ntime of 13 s. Crucially, it selectively detects DA in the presence\nof 100 times more ascorbic acid, uric acid, urea, and potassium chloride\nand similar levels of serotonin. In addition, it is stable, reproducible,\nand active in biological fluids. These properties put nafion/ZnO–ZnFe₂O₄/Fe₃O₄/carbon/GCE on the\nsame pedestal as the current state-of-the-art and could therefore\npotentially be used for the practical diagnosis of DA-related diseases\nin biomedical applications. Therefore, our results demonstrate that\nthe <i>in situ</i> carbothermic synthesis of Fe₃O₄ from organic-dye-decorated zinc ferrite nanofiber is\na useful method for improving its electrocatalytic properties. This\nknowledge could potentially be applied to the synthesis of an electrocatalyst\nfor other electrochemical applications.