Comparison of the peroxidase activities of iron oxide nanozyme with DNAzyme and horseradish peroxidase

Peroxidase-based assays are the most extensively used in bioanalytical sensors because of their simple colorimetric readout and high sensitivity owing to enzymatic signal amplification. To improve the stability, modification, and cost of protein-based enzymes, such as horseradish peroxidase (HRP), various enzyme mimics, such as DNAzymes and nanozymes, have emerged over the last few decades. In this study, we compared the peroxidase activities of HRP, a G-quadruplex (G4)-hemin DNAzyme, and Fe₃O₄ nanozymes in terms of activity and stability under different conditions. The reactions were much slower at pH 7 than at pH 4. At pH 4, the turnover rate of HRP (375 s^-1) was faster than that of G4 DNAzyme (0.14 s^-1) and Fe₃O₄ (6.1 × 10^-4 s^-1, calculated by surface Fe concentration). When normalized to mass concentrations, the trend was the same. Through observation of the reaction for a long time of 2 h, the changes in the color and UV-vis spectra were also different for these catalysts, indicating different reaction mechanisms among these catalysts. Moreover, different buffers and nanozyme sizes were found to influence the activity of the catalysts. Fe₃O₄ showed the highest stability compared to HRP and G4 DNAzyme after a catalytic reaction or incubation with H₂O₂ for a few hours. This study helps to understand the properties of catalysts and the development of novel catalysts with enzyme-mimicking activities for application in various fields.

Novel rapid coordination of ascorbic acid 2-phosphate and iron(III) as chromogenic substrate system based on Fe2O3 nanoparticle and application in immunoassay for the colorimetric detection of carcinoembryonic antigen

This work for the first time reports on a simple and rapid colorimetric immunoassay with rapid coordination of ascorbic acid 2-phosphate (AAP) and iron (III) for determination of carcinoembryonic antigen (CEA, used as a model) by using Fe₂O₃ nanoparticle based-chromogenic substrate system. The signal was produced rapidly (1 min) from the coordination of AAP and iron (III) with color development of colorless to brown. TD-DFT calculation methods were employed to simulate the UV-Vis spectra of AAP-Fe²⁺ and AAP-Fe³⁺ complexes. Moreover, Fe₂O₃ nanoparticle could be dissolved with the aid of acid, thereby releasing free iron (III). Herein, a sandwich-type immunoassay was established based on Fe₂O₃ nanoparticle as labels. As target CEA concentration increased, the number of Fe₂O₃ labelled-antibodies (bound specifically) increased, resulting in loading more Fe₂O₃ nanoparticle on platform. The absorbance increased as the number of free iron (III), derived from Fe₂O₃ nanoparticle, increased. So, the absorbance of reaction solution is positively correlated with antigen concentration. Under optimal conditions, the current results showed good performance for CEA detection in the range 0.02-10.0 ng/mL with a detection limit of 11 pg/mL. Moreover, the repeatability, stability, and selectivity of the colorimetric immunoassay were also acceptable.

A switchable electrochemical hairpin-aptasensor for ochratoxin A detection based on the double signal amplification effect of gold nanospheres

An OTA electrochemical sensor based on h-DNA and the double effect of gold nanospheres that can be applied for actual sample detection.