Phase control of nanostructured iron oxide for application to biosensor

We report results of the studies relating to the phase transformation of bare Fe₃O₄ nanoparticles (NPs) to α-Fe₂O₃ NPs obtained during electrophoretic film deposition onto indium-tin oxide coated glass plates. The in situ oxidation of NPs during electrophoretic deposition can be circumvented using surface passivation of the Fe₃O₄ NPs with an organic shell (carbon) as well as an inorganic shell (silica), while retaining the biocompatibility of the Fe₃O₄ NPs. XRD and XPS studies reveal the transformation of Fe₃O₄ NPs to α-Fe₂O₃ NPs upon electrophoretic deposition, and the retention of the phase of the Fe₃O₄ NPs upon encapsulation with carbon and silica, respectively. The results of SEM studies indicate decreased agglomeration of the Fe₃O₄ NPs upon encapsulation during film deposition. Attempts have been made to compare the characteristics of cholesterol biosensors fabricated using Fe₃O₄@C and α-Fe₂O₃ NPs, respectively. The Fe₃O₄@C NPs based cholesterol biosensor shows response time of 60 s, a linearity range of 25-500 mg dl^-1, a sensitivity of 193 nA mg^-1 dl cm^-2 and a Michaelis-Menten constant of 1.44 mg dl^-1.