Preparation and characterization of casein-stabilized gold nanoparticles for catalytic applications

Casein micelle stabilized sub-10 nm gold nanoparticles (GNPs) which are hydrophilic and biocompatible, were synthesized in aqueous medium by chemical reduction of HAuCl4 in the presence of amphiphilic protein aggregates, casein micelles. The prepared GNPs were evidenced by UV–vis spectroscopy, TEM, FTIR, and XRD analysis. The size of the GNPs can be controlled easily by adjusting casein concentration and the environment pH in the system. Furthermore, casein micelle stabilized GNPs function as effective catalyst to activate the reduction of 4-nitrophenol (to form 4-aminophenol) in the presence of NaBH4. The catalytic activity of GNPs depends upon the nanoparticle size, the casein concentration and the pH used for synthesis. On the basis of their excellent colloidal stability and catalytic function, and biocompatible surface, the casein micelle-stabilized GNPs hold great promise for being used in nanoscience and biomedical applications.

Graphical abstractThe catalytic rate constant is the highest for GNPs with size below 5.0 nm synthesized in 5.0 mg/mL casein solution at pH 2.0. However, the catalytic rate constant decreases sharply for the same GNPs with pH adjusted to 9.0.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Sub-10 nm sized gold nanoparticles (GNPs) stabilized by protein are prepared. ► Size of GNPs can be controlled by pH and casein concentration. ► GNPs exhibit catalytic activity for the reduction of aromatic nitro compounds. ► Catalytic activity of GNPs depends on GNP size, the casein concentration and pH used for synthesis.