Colorimetric detection of hydrogen peroxide and cholesterol using Fe3O4-brominated graphene nanocomposite

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.