Characterisation of gold agglomerates: size distribution, shape and optical properties

The slow agglomeration of gold colloids of approximate diameter 30 nm in the presence of a small concentration of L-cysteine?HCl has been followed by multiple techniques, namely particle tracking analysis (PTA), differential centrifugal sedimentation (DCS), UV-visible spectroscopy (UV), second order spectroscopy (SOS) and transmission electron microscopy (TEM). The citrate-stabilized Au nanoparticles were characterized by PTA, DCS, UV and dynamic light scattering (DLS) prior to exposure to the cysteine. Hydrodynamic forces during centrifugation can cause the disintegration of weakly held agglomerates. TEM reveals small linear agglomerates that become open linked chains of fractal-like structures after several hours of agglomeration. Second order Rayleigh scattering observed at the harmonic wavelength of 680 nm was resonantly enhanced by surface plasmon excitation of the growing agglomerates. During the initial stage of the agglomeration process, when chainlike or quasi-chainlike agglomerates were the dominant species, the SOS signal goes up by a factor of about three before reaching saturation. This study of a model nanoparticle system provides insights into the information obtained from a range of measurement techniques, with recommendations for characterisation of agglomerating nanoparticles under end-use conditions.