Effect of Alkylation on the Cellular Uptake of Polyethylene Glycol-Coated Gold Nanoparticles

Alkyl groups (C_nH_2n+1) are prevalent in engineered bionanomaterials used for many intracellular applications, yet how alkyl groups dictate the interactions between nanoparticles and mammalian cells remains incomprehensively investigated. In this work, we report the effect of alkylation on the cellular uptake of densely polyethylene glycol-coated nanoparticles, which are characterized by their limited entry into mammalian cells. Specifically, we prepare densely PEGylated gold nanoparticles that bear alkyl chains of varying carbon chain lengths (n) and loading densities (termed "alkyl-PEG-AuNPs"), followed by investigating their uptake by Kera-308 keratinocytes. Strikingly, provided a modest alkyl mass percentage of 0.2% (2 orders of magnitude lower than that of conventional lipid-based NPs) in their PEG shells, dodecyl-PEG-AuNPs (n = 12) and octadecyl-PEG-AuNPs (n = 18) can enter Kera-308 cells 30-fold more than methoxy-PEG-AuNPs (no alkyl groups) and hexyl-PEG-AuNPs (n = 6) after 24 h of incubation. Such strong dependence on n is valid for all serum concentrations considered (even under serum-free conditions), although enhanced serum levels can trigger the agglomeration of alkyl-PEG-AuNPs (without permanent aggregation of the AuNP cores) and can attenuate their cellular uptake. Additionally, alkyl-PEG-AuNPs can rapidly enter Kera-308 cells via the filipodia-mediated pathway, engaging the tips of membrane protrusions and accumulating within interdigital folds. Most alkyl-PEG-AuNPs adopt the "endo-lysosomal" route of trafficking, but ∼15% of them accumulate in the cytosol. Regardless of intracellular location, alkyl-PEG-AuNPs predominantly appear as individual entities after 24 h of incubation. Our work offers insights into the incorporation of alkyl groups for designing bionanomaterials for cellular uptake and cytosolic accumulation with intracellular stability.