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

Abstract

Gold nanoparticles (AuNPs) exhibit localized surface plasmon resonance (LSPR) and are promising photothermal agents. However, the aggregation susceptibility and poor colloidal stability limit their biomedical applications, particularly in photothermal therapy. Here, a one-pot green method is employed to synthesize AuNPs using aqueous extracts of Coelastrella thermophila (CT) and Arthrospira sp. (SP) as reducing and stabilizing agents. These biogenic AuNPs (CT@AuNPs and SP@AuNPs), and their counterpart synthesized using a conventional procedure (Chem@AuNPs) have quasispherical shapes with hydrodynamic diameters of 26.8 ± 10.3, 19.2 ± 6.1, and 11.4 ± 3.3 nm, respectively, and negatively charged surfaces. Fourier transform infrared spectroscopy reveals that CT@AuNPs and SP@AuNPs feature surface functional groups derived from algal proteins, carbohydrates, and fatty acids, which are associated with their greater colloidal stability (48 h) than Chem@AuNPs. These AuNPs elevate solution temperature up to ∼8 °C under continuous green laser irradiation for 1 h. Cytotoxicity of CT@AuNPs and SP@AuNPs was lower toward Vero cells than toward HeLa cells, and under our experimental conditions, they induced cell death in ∼70% of the latter upon green laser irradiation. Analyses using the 2′,7′-dichlorodihydrofluorescein diacetate and morphology suggest that the AuNP-induced cell death is attributed to LSPR-mediated hyperthermia and reactive oxygen species. Our alternative route to stable AuNPs addresses the persistent challenges posed by conventionally synthesized AuNPs in photothermal therapy and other fields.