Microalgae-Mediated Synthesis of Functionalized Gold Nanoparticles with High Photothermal Stability

Gold nanoparticles (AuNPs) exhibit localized surface plasmon resonance and are therefore promising photothermal agents. However, the aggregation propensity and poor colloidal stability of AuNPs limit their biomedical applications, particularly in photothermal therapy. Here, a one-pot green method is employed to synthesize AuNPs with superior colloidal and photothermal stabilities using aqueous extracts of Coelastrella thermophila (CT) and Arthrospira sp. (SP) as reducing and stabilizing agents. These biogenic AuNPs (CT@AuNPs and SP@AuNPs, respectively), as well as their counterpart synthesized using a conventional procedure (Chem@AuNPs), exhibit narrow size distributions, spherical shapes, and negatively charged surfaces. CT@AuNPs and SP@AuNPs feature surface functional groups derived from algal proteins, carbohydrates, and fatty acids and exhibit greater colloidal and solution-phase photothermal stabilities than Chem@AuNPs. Moreover, CT@AuNPs and SP@AuNPs exhibit a concentration-dependent toxicity toward HeLa cells but a low toxicity toward Vero cells and effectively kill the former cells upon photothermal stimulation, which highlights the potential of these biogenic AuNPs as photothermal agents. The AuNP-induced cell death is attributed to hyperthermia and the oxidative stress caused by the photogenerated reactive oxygen species. Overall, our alternative route to highly stable AuNPs addresses the persistent challenges posed by the use of conventionally synthesized AuNPs in photothermal therapy and other fields.