Study of pH Role in the Green Synthesis of Gold Nanoparticles: From Reduction to Size and Shape Control

This study investigates the influence of pH and extract concentration on the green synthesis of gold nanoparticles (AuNPs) using Camellia sinensis extract as a reducing and stabilizing agent. Synthesis was performed at pH 1–12 using undiluted and diluted extracts (2:5 and 1:5). The colloids were characterized by UV–Vis spectroscopy, TEM, SAED, and elemental mapping. Both parameters significantly affected nanoparticle formation, morphology, and optical properties. At extreme pH values and low extract concentrations, AuNP formation was suppressed. The pH range of 3–6 produced diverse morphologies: spherical (~18 nm), triangular (~21 nm), rod-shaped (~25 nm), and anisotropic flower-like structures (~53 nm). At higher pH values, smaller, more uniform spherical nanoparticles formed, including red (~18 nm) and yellow colloids (~6 nm). These changes were reflected in distinct UV–Vis spectra and color variations. Toxicity tests using Chlorella kessleri revealed no inhibition of algal growth. In contrast, selected AuNPs colloids reduced germination and root growth of Sinapis alba, with germination falling below 50% for spherical AuNPs smaller than 18 nm and root length decreasing by more than 60% compared to the positive control. Under the tested conditions, controlled adjustment of pH and extract concentration allows tuning of AuNP properties through a simple green synthesis approach.