Human serum albumin encapsulated gold nanoclusters: effects of cluster synthesis on natural protein characteristics

Abstract

The differences in the physiochemical properties between native Human Serum Albumin (HSA) and HSA encapsulated gold nanoclusters (HSA-AuNCs) are characterised. The light absorbance (UV-Vis), electrophoretic mobility, dynamic viscosity, density, hydrodynamic radius (DLS), absorption (QCM) and chemical bonding (XPS) characteristics of the molecules were studied. The UV-Vis and DLS data show the formation of large aggregates for HSA-AuNCs between pH 4–6 which is not observed for native HSA. This observation was further supported by QCM measurements showing a large increase in mass adsorbed at pH 6 between HSA and HSA-AuNCs. The DLS data also revealed a hydrodynamic diameter of 12 nm for HSA-AuNCs, nearly double that of 7 nm for native HSA at pH higher than 6, suggesting the formation of compact HSA-AuNCs dimers. The electrophoretic mobility data for both HSA-AuNCs and HSA were converted to zeta potentials. The zeta potential of HSA-AuNCs was seen to be more negative between pH 6–12, suggesting that the protein surface interacts with unreacted gold salt anions. Measurements of density and viscosity were also found to be in agreement with previous data suggesting that HSA-AuNCs form aggregates. XPS data also suggest that not all reactants are used up during the HSA-AuNC synthesis and positive side chains play a part in the initial synthesis stages. It was concluded that HSA-AuNCs most likely form dimers at natural and high pH. Between pH 4–6 HSA-AuNCs form very large aggregates limiting their use as fluorescent probes in this pH range. It was also found that the native characteristics of HSA are altered upon HSA-AuNC synthesis which needs to be taken into consideration when applying HSA-AuNCs as fluorescent probes in all fluorescent imaging and sensing.