LAUSR

Silicon oxide was initially loaded on a Fe3O4 magnetic nanoparticle substrate (Fe3O4@SiO2) and then functionalized with ─NH2 group (Fe3O4@SiO2@NH2) to construct a novel hierarchical magnetic nanocomposite. A sensitive urea biosensor medium involving a dip-coated hierarchical magnetic nanocomposite on F-doped SnO2 conducting glass was designed (Fe3O4@SiO2@NH2/SnO2:F) to achieve an excellent platform for urease (Urs) enzyme immobilization via covalent linking to the exposed NH2 groups through glutaraldehyde (Urs/Fe3O4@SiO2@NH2/SnO2:F). The hierarchical magnetic nanocomposite selection criteria were based on enhancement of urea biosensing by Urs immobilization via covalent linking to the exposed NH2 groups, while the SnO2:F selection as substrate was based on its ability to afford high electronic density to the biosensor surface as an electrostatic repulsion layer for the anionic interferents in the biological environment. FE-SEM, TEM, FTIR, CV, EIS, and I–V techniques established the morphology of the modified electrode's surface and electrochemical behavior of urea on the fabricated Urs/Fe3O4@SiO2@NH2/SnO2:F biosensor. The sensing mechanism can be clarified on the basis of the two reactions, namely (1) catalytic reaction and (2) oxidation or reduction of metal oxides, same as in the case of solid-state gas sensors. The I–V results display high sensitivity for urea detection of within 5–210 mg/dL and a limit of detection of 3 mg/dL.