Facile synthesis of biocompatible sub-5 nm alginate-stabilised gold nanoparticles with sonosensitising properties

The use of ultrasmall gold nanoparticles (usAuNPs, diameters <5 nm) in cancer diagnosis and therapy has been increasing due to their unique tumour tissue/cell penetrating traits and renal-excretable properties. In order to produce usAuNPs with narrow polydispersity, toxic stabilising agents were normally coated on usAuNP surface during their synthesis. This has necessitated post-synthesis surface modification to minimise stabilising-agent-related toxicity in order to qualify usAuNPs for in vivo use. Nevertheless, the average sizes of usAuNPs that were surface-modified using the reported methods were often found to be above the renal filtration threshold (5 nm), hence limiting their renal excretion and suitability for in vivo use. Here, we report a novel and facile synthesis of biocompatible and renal-clearable usAuNPs using sodium alginate as the stabilising agent. The alginate-stabilised usAuNPs (usAuNPs^ALG) had an average size of 4.5 ± 0.2 nm with narrow polydispersity (0.21), a zeta potential of −30.4 mV, and were stable in water for up to 45 days. They were haemocompatible and non-toxic in vitro at concentrations up to 100 μg/ml (4T1 breast cancer cells). Further, usAuNPs^ALG were found to generate reactive oxygen species and exert dose-dependent sonotoxicity to 4T1 and MDA-MB-231 cells in vitro at a level equivalent to that of the larger AuNPs (>5 nm), but upon ultrasound irradiation at 2–4-fold lower frequency (1 MHz) and intensity (0.5 W/cm²) than those previously reported in AuNP-mediated SDT. This suggests the potential use of the usAuNPs^ALG as sonosensitising agents for anti-cancer adjunctive therapy. Overall, this study reported the novel method for the synthesis of biocompatible usAuNPs with considerable sonosensitising properties and the potential of these sub-5 nm nanoparticles for in vivo applications.