Fluorescent Fe3O4 Quantum Dots for H2O2 Detection

A universal green synthesis approach of several quantum dots (QDs), including iron oxide (Fe3O4) QDs, gold (Au) QDs, zirconium oxide (ZrO2) QDs, and graphene (Gr) QDs, was demonstrated in this study. Nanozyme-based signal amplification strategy of those QDs was also examined, which may have significant advantages for the development of biosensors with visual read-outs. A synergetic peroxidase-like activity of Fe3O4 QDs was observed for the facile, visual, field-portable, and sensitive detection of H2O2. The fluorescence emission, UV–vis spectrum, and circular dichroism spectra of the Fe3O4 QDs were located at 419, 395, and 335 nm, respectively. The band gap energy of synthesized Fe3O4 QDs was estimated to be 2.0 eV based on the Tauc plot. The X-ray photoelectron spectroscopic analysis revealed that the Fe(II) to Fe(III) ratio was increased when the Fe3O4 NPs were converted to the Fe3O4 QDs. The nonenzymatic activity of those QDs was further investigated using a mixture of 3,3′,5,5′-tetramethylbenzidine (TMB) and H2O2. The Fe3O4 QDs possessed high peroxidase-like activity and exhibited Michaelis–Menten kinetics behavior. Kinetic studies revealed that the Fe3O4 QDs demonstrated a higher affinity toward TMB than the standard enzyme horseradish peroxidase. This prolonged peroxidase-like Fe3O4 QD catalytic activity was successfully applied for the detection of H2O2 with a limit of detection (LOD) of 3.87 nM, which was calculated based on the standard deviation method. While similar approach was examined with Au QDs, ZrO2 QDs, and Gr QDs in this study, no characteristic enzymatic activity was observed, which confirmed the unique properties of the Fe3O4 QDs. The facile synthesis approach and the unique nanozymatic activity of the Fe3O4 QDs described in the present study open a new horizon in materials chemistry and the development of colorimetric biosensors for environmental, energy, and medical applications.