Citric Acid-Functionalized Rhodium-Platinum Nanoparticles as Peroxidase Mimics for Determination of Cholesterol

Nanostructured artificial enzymes, also called nanozymes, are considered effective substitutes for natural enzymes. In this study, citric acid-functionalized rhodium-platinum nanoparticles (CA-RhPt NPs) were synthesized by a simple one-pot synthetic method. Transmission electron microscopy images showed the uniform structure of the CA-RhPt NPs, with an average diameter of 3.5 +/- 0.5 nm, and X-ray photoelectron spectroscopy and zeta potential measurements demonstrated the presence of citric acid in the CA-RhPt NPs. The CA-RhPt NPs exhibited enhanced peroxidase-like activity that catalyzed the reaction between 3,3',5,5'-tetramethylbenzidine (TMB) and hydrogen peroxide (H2O2) due to the synergistic effects of Rh and Pt. The catalytic mechanism of CA-RhPt NPs was found to decompose H2O2 to a hydroxyl radical (center dot OH), and then, the center dot OH oxidizes the TMB, generating a blue product. The excellent peroxidase-like CA-RhPt NPs were then applied to the colorimetric determination of cholesterol. The CA-RhPt NP-based colorimetry showed sensitive cholesterol detection in the range from 0.08 to 5.0 mM, with a detection limit of 25.7 mu M. The recoveries of cholesterol concentrations from the spiked human serum samples were 96.8% and 106%, which were comparable to those from the commercialized cholesterol detection kit. These results demonstrate the possibility of CA-RhPt NPs as alternatives for natural enzymes and applicability for sensitive cholesterol detection.