Optical characterisation of growth dynamics for colloidal gold nanoparticles in citrate environment

Abstract

This study emphasises on detection and characterisation of gold nanoparticles based on its optical response. Water dispersed gold nanoparticles are synthesised from citrate reduction with dilute gold chloride under external energy sources. Three methods (i.e. heat-reflux, UV photoreaction, and UV photoinitiation) were used as reaction energy source to produce the nanoparticles. Firstly, exact solution of Mie equations was vigorously tested by varying particle diameter for monodisperse and polydisperse size distributions to investigate its limitations and effects on extinction cross-sections respectively. Changes in peak extinction cross-sections due to effect of temperature are negligible even at 100 °C. Simulated fragmentation mechanism in the particle size distribution produced calculated extinction cross-section that are comparable to time-resolved experiments with λpk blue-shifted from approximately 610 nm to 520 nm. Secondly, verification of size distribution derived from photon correlation spectroscopy and small angle X-ray scattering was carried out with extinction profile used as comparison; the results shows general agreement between these instruments with mode particle size ranged from 20 nm to 80 nm. However, size distribution derived from small angle X-ray scattering produced expected extinction lower than measured values for λ > 600 nm. Thus, the following characteristics of the colloid are as follows: in general, heat-refluxed nanoparticles are smaller compared to UV synthesised particles. Weaker incident photon energy produced consistently larger particles with broader size distribution and emerged later when compared reactants exposed to high energy UV. The isoelectric point for our colloidal system was measured at pI = 2.31 ± 0.56. Lastly, time-resolved scattering measurements were carried out in three separate experiments involving photon correlation spectroscopy and home-built configurations. They collectively indicate the likelihood of fragmentation mechanism in particle size distribution during the time as the colloidal colour turn from transparent to blue.