Inorganic and <i>Erythroxylum coca</i> Leaf Extract-Mediated Synthesis of Gold Nanoparticles: A Comparative Study of Size, Surface Chemistry, and Colloidal Stability.

Abstract

Gold nanoparticles (AuNPs) were synthesized via two complementary routes, an inorganic surfactant-mediated method and a plant-extract-assisted biosynthesis, to elucidate how synthesis pathways influence nanoparticle physicochemical properties. In the inorganic route, hexadecyltrimethylammonium bromide (CTAB)-stabilized AuNPs were prepared using CTAB dissolution temperatures of 70-90 °C. UV-Vis spectroscopy showed localized surface plasmon resonance (LSPR) bands at 554-556 nm, while dynamic light scattering (DLS) indicated a decrease in hydrodynamic diameter from 110 to 97 nm with increasing dissolution temperature. Zeta potentials above +40 mV indicated strong electrostatic stabilization, and transmission electron microscopy (TEM) revealed ultrasmall Au cores with a narrow size distribution (2.4-3.0 nm) and a face-centered cubic crystal structure. In the biosynthetic route, AuNPs were obtained using aqueous <i>Erythroxylum coca</i> leaf extracts (1-4% <i>w</i>/<i>v</i>). The extracts exhibited a concentration-dependent red shift (~380 to ~420 nm), and biosynthesized AuNPs displayed LSPR bands in the 550-580 nm range. DLS yielded hydrodynamic diameters of 270-390 nm, with pronounced aggregation (3341 nm) at the lowest extract concentration. Under optimized conditions (HC5, n = 5), reproducible plasmonic and colloidal properties were obtained (maximum absorbance, localized surface plasmon resonance wavelength (λ_max) = 569.6 ± 1.7 nm; hydrodynamic diameter (<i>D</i>_H) = 237.6 ± 24.3 nm; absolute zeta potential (|ζ|)= 32.2 ± 2.6 mV). TEM analysis indicated predominantly quasi-spherical particles with a broader, log-normal size distribution, consistent with extract-mediated growth under heterogeneous organic capping environments.