Hollow spheres of iron oxide as an “enzyme-mimic”: preparation, characterization and application as biosensors

Abstract

Soft-template assisted preparation of iron oxide nanoparticles using carbon derived from glucose as a template has been carried out in this work. The formation of hollow spheres of iron oxide (HSFe₂O₃) nanomaterial has been characterized using spectroscopic and microscopic techniques. Thus, the obtained nanomaterial has been used as a “nanozyme”, an enzyme-mimic substrate, to demonstrate its application in the field of biosensors. As a proof of concept, the colorimetric sensing and electrochemical sensing of important biomarkers such as hydrogen peroxide and cholesterol have been demonstrated. Using the spectrophotometric assay, the linear concentration ranges for peroxide detection have been determined to range from 0.01 mM to 3 mM at 652 nm and 0.005 mM to 0.70 mM at 452 nm. The steady-state kinetic analysis carried out in the presence of hydrogen peroxide as a substrate revealed the kinetic parameter values of the enzyme-mimic as follows: k_m = 1.00 × 10⁻³ M and V_max = 7.13 × 10⁻⁶ M s⁻¹. The electrochemical detection of hydrogen peroxide using HSFe₂O₃ enzyme mimic in the presence of a chromogenic substrate, 3,3′,5,5′-tetramethylbenzidine, at a near physiological pH led to a novel sensing mechanism and such a peroxide sensor exhibited the detection range from 0.05 mM to 6 mM at E_app = 0.22 V. The sensitivity of this particular biosensor has been estimated to be 25.25 μA mM⁻¹ cm⁻² (R² = 0.9871). Further, this nanozyme has been extended for the detection of cholesterol by entrapping the enzyme, cholesterol oxidase, into HSFe₂O₃. The linear concentration ranges for cholesterol detection using the spectrophotometric assay and the electrochemical method have been determined to range from 0.05 mM to 0.8 mM and 0.5 mM to 8 mM. Thus, the proposed method shows the possibility of obtaining various metal oxide nanoparticles of controlled size and shape that can also be used as a bio-mimic material to demonstrate their potential applications in the fields of electro-analysis and biosensors.