ToF-SIMS imaging and spectroscopic analyses of PEG-conjugated AuNPs

Gold nanoparticles (AuNPs) and poly(ethylene glycol) (PEG) are known safe materials to be used in vivo, and so, are becoming increasingly important in biomedical applications. Here, the stability of PEG-conjugated AuNPs in an aqueous solution was monitored as a function of time using several techniques, namely dynamic light scattering (DLS)-size analysis, UV-visible absorption spectrometry and ToF-SIMS. Roughly 15-nm-sized AuNPs were synthesized and pegylated with mPEG (MW = 5 kDa). Four different samples were synthesized, depending on whether or not a dialysis procedure was performed right after synthesis of the AuNPs or pegylation of the AuNPs. The DLS results show that size of mPEG-conjugated AuNPs and standard deviation increased in cases where a dialysis procedure was performed after pegylation of the AuNPs, indicating aggregation of the AuNPs due to the detachment of mPEG ligands from the surface of the AuNPs. Also, results from UV-visible absorption spectra and ToF-SIMS images point to the likelihood that free mPEG ligands in solution help prevent aggregation by maintaining a dynamic equilibrium between the conjugated ligands on the surface of the AuNPs and the free ligands in the aqueous solution. Conversely, once free, mPEG ligands were eliminated from the solution by dialysis and conjugated mPEG ligands detached from the surface of AuNPs into the solution, suspension stability lowered and the AuNPs showed signs of aggregation. We believe that the ToF-SIMS imaging analysis with DLS and UV-visible absorption analyses would be useful to study the stability of PEG-conjugated AuNPs in different environmental conditions.