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

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.