Magneto-Biosensor for the Detection of Uric Acid Using Citric Acid-Capped Iron Oxide Nanoparticles

In this work, a magneto-biosensor based on iron (II, III) oxide (magnetite, Fe₃O₄) nanoparticles for the detection of uric acid is developed and demonstrated. These Fe₃O₄ nanoparticles are successfully synthesized by a co-precipitation method comprising Fe²⁺ and Fe³⁺ with ammonium hydroxide, NH4OH, and using citric acid as a surfactant. Comparative studies of Fe₃O₄ nanoparticles with and without surfactant are also carried out to examine their characteristics. Both types of synthesized iron oxide nanoparticles are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, field emission-scanning electron microscopy, transmission electron microscopy, and vibrating sample magnetometry. The images obtained by field emission-scanning electron microscopy show an average diameter of 30 nm for citric acid-Fe₃O₄ nanoparticles. The Fourier transform infrared spectra indicate that the carboxylate groups of citric acid are bound onto the surface of magnetite nanoparticles by chemical bonds. For sensing experiments, the synthesized nanoparticles are used to modify the glassy carbon electrode, and the resultant citric acid-Fe₃O₄ modified glassy carbon electrode is used for the detection of uric acid through cyclic voltammetry. In the case of the citric acid-Fe₃O₄ nanoparticles-modified glassy carbon electrode, uric acid is oxidized at a less positive potential compared to oxidation using the naked Fe₃O₄ nanoparticles and a bare glassy carbon electrode. The citric acid-Fe₃O₄ nanoparticles-modified glassy carbon electrode exhibit a good linear response range for the detection of uric acid of 7.5 μM–0.18 mM, with a lower detection limit of 7.5 μM uric acid. This excellent performance of the fabricated biosensor is attributed to the larger surface area availability of citric acid-Fe₃O₄ nanoparticles, which promotes constant electron transfer between the modified glassy carbon electrode and the biomolecules (uric acid). The improved electrocatalytic activity of this modified electrode clearly proves that the proposed method is promising for the development of other electrochemical biosensors.