Abstract
Platinum nanozymes exhibiting highly efficient and robust oxidase-like activity are successfully synthesized and modified using sodium alginate (SA-PtNPs). According to a steady-state dynamic assay, Michaelis-Menton constant (Km) is calculated as 11.6 μM, indicating that the affinity of SA-PtNPs toward the substrate, 3, 3′, 5, 5′-tetramethylbenzidine (TMB), is high. It shows in the paper that SA-PtNPs exhibit a significant oxidant effect on substrate-O2 to produce as an oxidase mimic. Moreover, the oxidase-like activity fluctuated slightly under changes in environmental pH and incubation time, implying that SA can increase the dispersibility and stability of PtNPs. A colorimetric assay for oligomeric proanthocyanidins (OPC) was realized given how few explorations of the former there are. We found that the significant inhibitory effect of OPC on the oxidase-like activity is due to the competitive effect between OPC and TMB for binding to the active site of SA-PtNPs, resulting in a color change. Under optimal conditions, the logarithmic value of the chromogenic difference (ΔA450nm) to OPC concentration was linear (4–32.5 μM, r = 0.999) with a limit of detection (LOD) of 2.0 μM. The antioxidant capacity of OPC obtained by the Soxhlet extraction method from grape seeds was 2.85 U/mg. The recovery from the experiment in which OPC was added to grape seeds ranged from 97.0 to 98.6% (RSDs of 0.5–3.4%), suggesting a high accuracy in OPC detection. These findings are important because OPC is an internationally recognized antioxidant that eliminates free radicals in the human body and, therefore, may prevent a variety of diseases. Thus, we envisage that this Pt nanozyme-based assay may be prevalent for antioxidant capacity evaluation and analytical applications.
Highlights
Proanthocyanidin (PC) is a general term for a large class of polyphenolic compounds widely found in plants, red wine, as well as grape seeds (Da Silva et al, 1991; Fracassetti et al, 2013)
The highresolution transmission electron microscope (TEM) image shows that the crystal plane spacing of the Sodium alginate (SA)-platinum nanoparticles (PtNPs) is 0.19 nm
When the concentration of oligomeric proanthocyanidin (OPC) in the reaction system reached 1 mM, the oxidation of TMB by SA-PtNPs was drastically inhibited. These phenomena fully illustrate that OPC could distinctly inhibit the color response of the SA-PtNPs-TMB system, namely inhibiting the oxidase-like activity of SA-PtNPs
Summary
Proanthocyanidin (PC) is a general term for a large class of polyphenolic compounds widely found in plants, red wine, as well as grape seeds (Da Silva et al, 1991; Fracassetti et al, 2013). The use of natural products as ligands instead of chemicals to stabilize nanoparticles can greatly improve biocompatibility and broaden their biomedical applications. To this end, efforts have been made to adjust the size and morphology of PtNPs to mimic enzyme-like activity by applying natural products (Deng et al, 2017, 2019; You et al, 2017; He et al, 2019a,b). This study presents a new preparation method for Pt nanozymes and shows a good prospect of application based on SA-PtNPs. On the other hand, this assay provides a new insight for antioxidant capacity evaluation and opens a new avenue for OPC analysis.
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