Light- and Dark-Directed Morphological Control in N -Methyl- d -Glucamine-Mediated Gold Nanostar Synthesis: Implications for Photothermal Performance

We report a systematic investigation of light-dependent morphological control in the seedless, N-methyl-d-glucamine (NMGA)-mediated synthesis of gold nanostars (AuNSs). This green, aqueous, surfactant-free route reveals that external illumination plays a decisive role during nucleation and early anisotropic growth. Under otherwise identical conditions (RNMGA = 12 and [Au]f = 0.20 mM, where RNMGA and [Au]f denote the NMGA-to-Au molar ratio and final analytical gold concentration, respectively), light exposure accelerates Au3+ reduction, generating a higher density of nuclei and yielding smaller nanostars with reduced core diameters (61.0 ± 11.6 nm) and shorter tips (23.2 ± 5.6 nm). In contrast, dark synthesis suppresses excessive nucleation and promotes directional tip elongation, producing larger, more anisotropic structures with larger cores (94.1 ± 17.4 nm) and longer tips (54.2 ± 10.9 nm). Systematic variation of illumination time, color temperature, RNMGA, ethanol content (0–50 vol %), and precursor concentration (0.20–0.50 mM) establishes clear relationships among photon energy, reduction kinetics, and morphological evolution. These structural differences translate directly into functional effects: dark-grown AuNSs consistently exhibit higher near-infrared extinction and enhanced photothermal conversion efficiencies, reaching ∼34% at [Au]f = 0.50 mM and RNMGA = 14. Across RNMGA values of 8–14, the photothermal conversion efficiency of dark-grown NMGA-AuNSs is, on average, 45% higher than that of the corresponding light-grown samples. Together, these results identify controlled light exclusion as a simple yet powerful design parameter for producing highly anisotropic AuNSs with a superior photothermal performance.