pH optimization for high-efficiency PEGylation of gold nanorods

The use of methoxy polyethylene glycol thiol (mPEG-SH), in conjunction with additives, is one of the most straightforward, versatile, and widely used methods for modifying gold nanorods (GNRs), especially for applications in the field of biomedicines. The decreased toxicity of PEGylated GNRs is critical for investigating cell-nanomaterial interactions. Most techniques suffer from low PEGylation efficiency. Several investigations of the PEGylation process have been conducted to determine its mechanism. Herein, we systematically investigated the qualitative and quantitative effects of the pH value on GNR PEGylation efficiency. The PEGylation process was applied to modify the GNRs at pH 0.3–14. PEGylation of GNRs at pH 2.3 and 9.9 resulted in better modification efficiency than under other pH conditions, achieving average PEGylation efficiencies of 95.5% and 83.6%, respectively. In both cases, the resulting GNRs could survive centrifugation after surface modification as demonstrated by measuring their longitudinal surface plasmon resonance (LSPR) UV-Vis absorption peaks. The aggregation index correlated to the PEGylation efficiency at the various pH levels. The origin of the differences observed at different pH values was also elucidated. These findings significantly expand the potential use of the aggregation index, as well as the understanding of processes used to modify gold nanoparticles.