Elucidating the Mechanism of Silica Nanoparticle PEGylation Processes Using Fluorescence Correlation Spectroscopies

Surface modification with polyethylene glycol (PEG; PEGylation) is a widely used technique to improve nanoparticle (NP) stability, biocompatibility, and biodistribution profiles. In particular, PEGylation of silica surfaces and coatings plays a pivotal role across various classes of NPs. Despite the use of numerous protocols there is limited fundamental understanding of the mechanisms of these processes for NPs. Here, after reaction optimization for particle stability, we employ fluorescence correlation and crosscorrelation spectroscopy (FCS, FCCS) on ultrasmall (<10 nm) fluorescent silica nanoparticles (SNPs) in water as a test bed. We show unexpected fast reaction kinetics in successful PEGylation observed even at nanomolar concentrations and attributed this to instant noncovalent adsorption of PEG molecules to the SNP surface preceding covalent attachment. Further studies of various reaction conditions enable the elucidation of process design criteria for NP PEGylation and surface modification with functional ligands, which may be applicable to a broad range of NPs thereby accelerating progress in fields ranging from biosensing to nanomedicine.