# Inorganic and Erythroxylum coca Leaf Extract-Mediated Synthesis of Gold Nanoparticles: A Comparative Study of Size, Surface Chemistry, and Colloidal Stability

## TL;DR

This study compares two methods for making gold nanoparticles, showing how each affects their size, surface properties, and stability.

## Contribution

A direct comparison of inorganic and plant-based synthesis methods for gold nanoparticles, focusing on physicochemical outcomes.

## Key findings

- Inorganic synthesis produced smaller, more uniform gold nanoparticles with strong electrostatic stabilization.
- Biosynthesis using Erythroxylum coca leaf extract resulted in larger particles with broader size distributions and concentration-dependent aggregation.
- Both methods yielded colloidal stability, but with distinct surface chemistry and plasmonic properties.

## 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 Erythroxylum coca leaf extracts (1–4% w/v). 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 (λmax) = 569.6 ± 1.7 nm; hydrodynamic diameter (DH) = 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.