Magnetite chitosan hydrogel nanozyme with intrinsic peroxidase activity for smartphone-assisted colorimetric sensing of thiabendazole

Although nanozymes proved a promising utility in sensing applications, they often lack a rational approach in their design. Herein, magnetite chitosan hydrogel (MCH) is reported as a rational peroxidase nanozyme for the smartphone-assisted colorimetric detection of thiabendazole (TBZ). Chitosan due to its polycationic nature, renders a microenvironment similar to amino acids in HRP enzyme. Characterizations of the nanozyme using SEM, XRD, and XPS confirmed the distribution of Fe3O4NPs on the chitosan matrix. The peroxidase activity was demonstrated using TMB and H2O2 as substrates which resulted in a characteristic absorption at 652 nm. MCH nanozyme showed a 24% higher peroxidase activity in acidic pH than the pristine Fe3O4 confirming the role of chitosan in boosting the electron transfer. Kinetics result suggested the catalytic reaction followed a Michalis-Menten model with Km and Vmax of 0.45 mM and 15 μM/min for TMB and 2.8 mM and 4.18 μM/min for H2O2 respectively. These values are competitive with natural HRP enzymes reported before. Further, MCH nanozyme showed improved thermal and temporal stability as well as reusability retaining 80% of its activity after the 4th cycle. TBZ showed concentration-dependent inhibition on the peroxidase activity. The degree of inhibition exhibited a linear relationship with the concentration of TBZ from 0.1 to 100 μM (R2 = 0.998) enabling the detection of TBZ down to 0.73 and 1.84 μM in a spectrometer and smartphone-based readouts respectively. The results show the potential of the prepared nanozyme as a point-of-need sensor for food safety monitoring.