Investigating the Influence of Experimental Parameters on the Formation of Gold Nanoparticles: A Green Synthesis and Design of Experiments Approach

Abstract

This thesis examines the green synthesis of gold nanoparticles using catechin, a naturally occurring plant-derived compound, as both a reducing and capping agent. The synthesis process was guided by a structured Design of Experiments (DOE) methodology to ensure systematic investigation and optimization. A two-phase experimental approach was adopted. First, a full factorial screening design was used to evaluate the effects of catechin concentration, NaOH concentration, and pre-reaction time. This was followed by an optimization phase using a face-centered composite design to identify conditions that produce nanoparticles with ideal characteristics. The nanoparticles were analyzed using UV-visible spectroscopy and dynamic light scattering (DLS), which provided data on surface plasmon resonance (SPR λmax), hydrodynamic diameter (Z-average), and polydispersity index (PDI). The screening results identified NaOH concentration, along with its interaction with catechin concentration, as key variables influencing nanoparticle formation. Mechanistic interpretation suggested that higher alkalinity improves catechin’s effectiveness in reducing gold ions, thereby facilitating nanoparticle formation. These findings were confirmed in the optimization phase, where NaOH emerged as the most influential factor. The model also predicted optimal conditions that would produce small, uniform particles with low PDI values. However, these conditions have not yet been experimentally tested. A comparison of two synthesis approaches, based on the order of reagent mixing, showed that adding catechin to NaOH before combining with the gold solution led to more reliable and stable nanoparticle formation. In contrast, the opposite sequence, introducing NaOH to the gold solution before adding catechin, often failed, especially at low concentrations. When a delay (pre-reaction time) was introduced to the alternative synthesis procedure, the results of the gold nanoparticles were larger-sized nanoparticles. This suggests that both reagent order and timing significantly affect the outcome. In conclusion, the purpose of this study is to synthesize and optimize the production of gold nanoparticles using catechin and NaOH under environmentally friendly conditions through a combination of screening and optimization experimental designs. This study also highlights how both the chemical environment and mixing sequence impact nanoparticle quality