1H-NMR characterization of the nanoparticles

The PEG content in the nanoparticle and in the outside shell were determined by ¹H-NMR spectroscopy by modification of a method described elsewhere¹³. Generally, the methylene protons of PEG (3.6 p.p.m.) were quantitatively compared to the protons of the lactic (1.6 and 5.2 p.p.m.) and glycolic repeating units (4.8 p.p.m.) to determine how much PEG was present in the blends forming the nanoparticles. To determine the amount of PEG in the shell, a first reading was done in D₂O and compared to the spectra obtained in D₂O/d₃-ACN solvent mixtures. In the former, only the PEG protons (3.6 p.p.m) are observed, while in the latter, the whole polymer being soluble, all protons are visible. To compare spectra, 1 wt% trimethylsilyl propanoic acid (TMSP) was used as an internal standard. The density of the PEG shell was calculated as described elsewhere¹⁴, using the PEG content (percent by mass), a polymer density of 1.2 g cm⁻³, a PEG molecular weight of 5000 g mol⁻¹, and the surface (cm²) and volume (cm³) of a nanoparticle calculated from the Z-average. Results from the nanoparticle synthesis and characterization are presented in Supplementary Table ³.