Fluorescent gold nanoclusters stabilized by lysozyme: Synthesis and deposition kinetics on silica substrates

Gold nanoclusters suspension were effectively synthesized under alkaline conditions in a chemical reduction process involving gold(III) chloride trihydrate and lysozyme (LYZ) molecules. Their size determined by high-resolution transmission electron microscopy (HR-TEM) was equal to 1.9 ± 0.5 nm. The nanoclusters, referred to as LYZ-Au NCs, were stable at pH below 4 and above 8, exhibiting a hydrodynamic diameter between 8 and 11 nm. The isoelectric point of LYZ-Au NCs appeared at pH 5.0. The suspension showed a pronounced fluorescence characterized by the red-emitting band at 668 nm. The deposition kinetics and stability of LYZ-Au NCs on bare and poly(diallyldimethylammonium chloride) (PDADMAC)-modified silica sensors were studied using quartz crystal microbalance (QCM). The influence of ionic strength, pH, and suspension concentration on the kinetics of LYZ-Au NCs deposition was determined. The significant increase in the maximum coverage of LYZ-Au NCs with ionic strength was attributed to the decreasing range of electrostatic interactions between deposited clusters. Atomic force microscopy (AFM) confirmed the formation of homogeneous layers of LYZ-Au NCs with controlled coverage on bare silica at pH 3.5 and PDADMAC-modified silica. It was shown by confocal microscopy investigations, that these layers also exhibited pronounced fluorescent properties.