Fe3O4-brominated graphene (Fe3O4-GBR) nanocomposites were synthesized via an in situ method using the precursors FeSO4.7H2O and GBR in different (1:1, 1:2, 2:1, 1:5, 1:10, 1:20, and 5:1) weight ratios at pH 11.5. The Fe3O4-GBR (1:5) nanocomposite in combination with H2O2 and 3,3',5,5'-tetramethylbenzidine (TMB) showed swift and superior intrinsic peroxidase mimetic enzyme activity compared with the other Fe3O4-GBR composites, GBR and Fe3O4, as observed by colorimetry. It was characterized using high-resolution scanning electron microscopy (HRSEM), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (PXRD), and thermogravimetric analysis (TGA). Its catalytic activity was optimized by varying different parameters, and the optimum conditions for peroxidase mimetic activity were observed using 100 μL Fe3O4-GBR (1 mg/mL), 50 μL TMB (1 mg/mL), and 200 μL H2O2(1 mM) in 400 μL of acetate buffer of pH 2.3 at 30 °C temperature. Kinetic analysis has revealed the Michaelis-Menten kinetic behavior of peroxidase activity with Michaelis-Menten constants (Km) and maximum initial velocities (Vmax) of 0.082 mM and 14.1 nMs-1 respectively, for H2O2 and 0.086 mM and 5.1 nMs-1, respectively for TMB. The limit of detection and linear range were found to be 49.6 μM and 100-880 μM, respectively, for H2O2 and 41.9 μM and 47.6-952.3 μM, respectively, for cholesterol. On this basis, a simple, swift, sensitive, selective, and reproducible colorimetric assay to detect cholesterol levels in blood serum samples using Fe3O4-GBR nanocomposite has been developed. Thus, Fe3O4-GBR composite as compared to Fe3O4 and GBR has shown better peroxidase mimicking activity for biosensing.
Keywords: Cholesterol detection; Colorimetry; Fe3O4-brominated graphene nanocomposite; Hydrogen peroxide detection; Peroxidase mimetic.
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