Abstract
The synthesis of gold nanoparticles using citrate reduction process has been revisited. A simplified room temperature approach to standard Turkevich synthesis is employed to obtain fairly monodisperse gold nanoparticles. The role of initial pH alongside the concentration ratio of reactants is explored for the size control of Au nanoparticles. The particle size distribution has been investigated using UV-vis spectroscopy and transmission electron microscope (TEM). At optimal pH of 5, gold nanoparticles obtained are highly monodisperse and spherical in shape and have narrower size distribution (sharp surface plasmon at 520 nm). For other pH conditions, particles are non-uniform and polydisperse, showing a red-shift in plasmon peak due to aggregation and large particle size distribution. The room temperature approach results in highly stable “colloidal” suspension of gold nanoparticles. The stability test through absorption spectroscopy indicates no sign of aggregation for a month. The rate of reduction of auric ionic species by citrate ions is determined via UV absorbance studies. The size of nanoparticles under various conditions is thus predicted using a theoretical model that incorporates nucleation, growth, and aggregation processes. The faster rate of reduction yields better size distribution for optimized pH and reactant concentrations. The model involves solving population balance equation for continuously evolving particle size distribution by discretization techniques. The particle sizes estimated from the simulations (13 to 25 nm) are close to the experimental ones (10 to 32 nm) and corroborate the similarity of reaction processes at 300 and 373 K (classical Turkevich reaction). Thus, substitution of experimentally measured rate of disappearance of auric ionic species into theoretical model enables us to capture the unusual experimental observations.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-016-1576-5) contains supplementary material, which is available to authorized users.
Highlights
Noble metal nanoparticles have been intensively studied during the past two decades
Reaction Time of AuNP Synthesis A typical Turkevich synthesis of AuNPs at 373 K takes 20 min for the characteristic red wine color to appear in the solution
According to Turkevich et al [12], the solution temperature (373 K) plays an important role for the gold nanoparticle formation, and for every decrease of 10 K there is a twofold increase in time period necessary for the completion of reaction
Summary
Noble metal nanoparticles have been intensively studied during the past two decades. Metal nanoparticles show optical properties of significant technological interest, including enhanced fluorescence [1, 2], non-linear optical absorbance [3], optical resonances in the near infrared region [4], and orientation-dependent plasmon excitation [5]. Gold nanoparticles (AuNPs) exhibit a strong absorption of wavelengths due to surface plasmon resonance (SPR) in the visible range [6]. The first synthesis of Au colloids was reported 150 years ago by Michael Faraday using phosphorous to reduce AuCl4− ions [7]. For efficient and effective use in applications, synthesis routes must render monodisperse AuNPs of tailored size and shape.
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