- Nano Express
- Open Access
A Facile pH Controlled Citrate-Based Reduction Method for Gold Nanoparticle Synthesis at Room Temperature
© The Author(s). 2016
- Received: 6 May 2016
- Accepted: 5 August 2016
- Published: 15 August 2016
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.
- Surface Plasmon Resonance
- Gold Nanoparticles
- Trisodium Citrate
- Transmission Electron Microscopy Characterization
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 , optical resonances in the near infrared region , and orientation-dependent plasmon excitation . Among these, gold nanoparticles (AuNPs) exhibit a strong absorption of wavelengths due to surface plasmon resonance (SPR) in the visible range . The first synthesis of Au colloids was reported 150 years ago by Michael Faraday using phosphorous to reduce AuCl4 − ions . For efficient and effective use in applications, synthesis routes must render monodisperse AuNPs of tailored size and shape. There exist several synthesis routes [8–11], which could be used to obtain gold nanoparticles with desired structural and physical characteristics. One of the most celebrated among these has been the Turkevich method , wherein a mild reducing agent trisodium citrate is added to a boiling aqueous solution of HAuCl4 to obtain monodisperse AuNPs. Later, Frens demonstrated that the control over the size of the gold nanoparticles in Turkevich synthesis could be readily achieved by varying the relative concentration of trisodium citrate . Controlling the structural characteristics of the nanoparticle needs the manipulation of kinetic and thermodynamic parameters of the systems using various additives, light and thermal energies, and their various combinations [14–16]. To achieve this control, gold nanoparticles are usually grown through a fast nucleation process followed by a diffusion-controlled growth [17, 18]. The size and polydispersity of the resulting nanoparticles are thus controlled in a way similar to the well-known LaMer model , which is also known as “focusing of size distribution” in the field of nonaqueous solution synthesis of nanoparticles at elevated temperatures . However, from the green chemistry point of view, such nonaqueous synthesis routes are not ideal and to make AuNPs biocompatible, lengthy, and laborious process of surface functionalization is generally required .
The AuNPs synthesized via Turkevich approach can be size-controlled in an 8- to 100-nm range, but the polydispersity increases with particle size . Besides, the standard approach yields spherical particles, but it has been shown that other geometrical shapes can be obtained by applying minimum modifications to existing protocol [21–23]. Controlling the pH and precursor to reductant concentration ratio is an integral part of such modifications. Herein, we study the effect of the pH on the synthesis of gold nanoparticle at room temperature. The synthesis of gold nanoparticles via room temperature approach is simple and convenient and gives a narrow particle size distribution (11.7 ± 2.2 nm). There exists an optimal pH for every colloidal solution at which the particles formed are fairly stable and monodisperse without an extra stabilizing agent. At pH values other than the optimal value, there is uncontrolled nucleation and growth which results either in anisotropic shape of particles or in coagulation of particles, thus giving polydispersity in size distribution. The synthesis of AuNPs via Turkevich method involves several steps among which citrate reduction of Au3+ species is the first rate-determining step. We calculate the reaction rate (k c) of this step at room temperature in Turkevich approach. It is found that this rate depends critically on the pH and precursor ratio of the reaction mixture. The large value of k c (7655 M−1 s−1) for the optimized case of 2:1 citrate to AuCl3 (pH 5) helps in quick synthesis of AuNPs at room temperature. We use the different rate values in simulation program developed in lines of theoretical model incorporating nucleation, growth, and aggregation processes. The simulation predicts that the mean particle size under various experimental conditions and theoretical predictions agrees well with the corresponding experimentally obtained sizes of AuNPs. Such comparison further substantiates the similarities of intermediate reaction steps in the Turkevich process at 373 K and room temperature. The room temperature synthesis of AuNPs via citrate-based reduction of gold(III) chloride represents a bio-friendly approach and gives us an insight into pH controlled reduction of Au3+ ions. The study reveals a novel and easy way to manipulate Turkevich like method carried out at room temperature. Though the literature regarding citrate based reduction of AuCl3 to obtain AuNPs is quite extensive, our method introduces a fundamental change in existing method in terms of temperature conditions.
Optical absorption spectra were taken using Lambda 950 (Perkin Elmer) in the wavelength range 200–800 nm at room temperature using quartz cell. Transmission electron microscopy (TEM) was carried out using JEOL JEM-2000F. Samples for TEM characterization were prepared by placing a drop of gold colloidal solution on carbon coated copper grid and dried under IR lamp. For further characterization, Fourier transform infrared spectroscopy (FTIR) was performed using MAGNA 550, Nicolet Instruments Corporation, USA. X-ray diffraction (XRD) of gold nanoparticles was carried out using “PANalytical Xpert PRO.” XRD sample was prepared by drop-casting colloidal gold nanoparticles on glass slides.
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. There have been recent reports where researchers have been able to decrease the reaction temperature up to 343 K by optimizing the pH and reactant ratio of reaction mixture . According to Turkevich et al. , 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. This implies that at 300 K, reaction shall take around 2600 min to complete. However, if appearance of red wine color is to be taken as an indicator of near completion of reaction, for a particular case of molar ratios 2:1 and 5:1 (pH 5), the reaction is over within 8 h. In contrast to this, for other molar ratios, no further change in blue-violet color is observed after 24 to 48 h. Interestingly, these observations suggest that at room temperature, a fivefold reduction in reaction time is possible through adjustments of precursor ratio and initial pH condition, i.e., at pH 5, 2:1 and 5:1 precursor molar ratio.
Synthesis and “Colloidal” Stability of Gold Nanoparticles
Nucleation and Growth of AuNPs (Case of 300 K)
Role of pH for the Control Over Nucleation and Growth of Nanoparticles
In order to explain the effect of pH on reaction in Turkevich process, we must look at role of H+ and OH− ions in the standard reaction mechanism [12, 27]. In this reaction, there are three parameters which are responsible for the controlled synthesis of gold nanoparticles, namely, concentration of gold chloride and sodium citrate, and the pH of the solution. The reaction takes place in various steps in between the final product and reactants. First, gold chloride decomposes into gold and chloride ions, while sodium and citrate ions are produced by the dissolution of sodium citrate. During the reaction, trisodium citrate plays the role of reducing as well as stabilizing agent and if concentration relative to gold precursor is high, it act as a buffering agent also [21, 22].
As the H+ ions increase in the solution after addition of HCl, the availability of electron to reduce the gold(III) chloride becomes lesser and reaction rate becomes slower in this case. In accordance with Le Chatelier’s principle [29, 30] and pKa1 being 3.1 for citrate , the tendency of citrate to oxidize decreases at a low pH (<3). Hence, there is a lack of electron for the reduction of gold(III) chloride at such minimum pH. For this pH condition, no color change is observed in the solution, and hence, we conclude that reaction is not taking place at very low pH conditions.
Similarly, for higher pH values (pH >9), the diluted NaOH (NaOH = Na+ + OH−) has been added to provide the OH− ionic species in the solution. Hydroxyl ions directly react with AuCl4 − and convert it into AuCl3(OH−), hence making it less reactive. At higher pH (>9) values, there is no color change in the solution, and hence, the reaction does not take place at higher pH conditions [10, 27]. It is important to note that while best particle size distribution was obtained at pH >6 by Ji et al. , pH should be lesser than 6 to get monodisperse AuNPs if citrate-based reduction is performed at room temperature. At high temperature, both nucleation and growth are faster processes in Turkevich reaction and a high pH restricts the nucleation and growth processes rendering monodisperse particles. In comparison, when reaction is carried out at room temperature, nucleation in our studies is a slow process at higher pH conditions, and simultaneous nucleation and growth may introduce inhomogeneity in AuNP sizes. However, in our case interestingly at low pH conditions, fraction of AuCl4 − in reaction mixture increases which improves the nucleation rate and results in formation of monodisperse particles. The hydroxylation of Au3+ species thus determines the size distribution of AuNPs at room temperature in a quite contrary way as compared to high temperature reactions. Moreover, the tendency of citrate to oxidize decreases with decrease in pH, and hence, its role as a stabilizing agent becomes more prominent. A better nucleation rate achieved at room temperature via lowering of pH and slow growth thus yields monodisperse AuNPs.
Mechanism of Nucleation and Growth (at Room Temperature)
The simulation models based on DLVO theory and Monte Carlo approach where coulombic charges play the most crucial role, are not suitable to model Turkevich synthesis since this is not a burst nucleation process (followed by Ostwald ripening) [32, 35]. It rather involves simultaneous nucleation, growth, and aggregation processes whereby the particle surfaces catalyze the growth, and the number of particles affects the course of chemical reactions. The theoretical model proposed by Sanjeev et al. successfully takes into account these simultaneous processes and we used it to understand the nucleation and growth process of gold nanoparticles at room temperature [39, 40]. A simulation code based on the algorithms necessary to model theoretical predictions has been written for this purpose.
The kinetics of reaction at room temperature will be different when compared to classical Turkevich synthesis. This implies that the values of rate constants (of intermediate reactions) at 373 and 300 K are different. Moreover, we account for reduction of AuCl4−x(OH)x species by assuming experimentally obtained rate constant “k c” to be average of all Au3+ reductions by citrate ions. A similar assumption is made for AuCl2−x(OH)x species while incorporating “k h” and “k n” in our calculations.
We used the following parameteric values to simulate different experimental conditions:
Temperature = 300 K; k n = (1.67 × 109)N avg M−5 L −1 s −1; k h = 1.8 × 10−3 cm−2 L−1 s−1; k s = 1/1300 s−1; k d = 400 M−1 s−1; k c = variable (in M−1 s−1); T = conc. of AuCl3 (variable); and C = conc. of trisodium citrate (variable).
Data comparing simulated and experimental mean particle size for variable ratio (fixed pH = 5) case. The experimental trend in size variation is in close conformity with mean particle size from simulation. The precursor ratio 2:1 and 5:1 give minimum sized particles
Citrate to AuCl3 ratio
k c (M−1 s−1)
Plasmon peak (nm)
Experimental particle size (nm)
Simulated particle size (nm)
Data comparing simulated and experimental mean particle size for variable pH (fixed ratio 2:1) case. The experimental trend in size variation is in close conformity with mean particle size from simulation
k c (M−1 s−1)
Plasmon peak (nm)
Experimental particle size (nm)
Simulated particle size (nm)
The pH dependence at a given ratio 2:1 of mean AuNP size obtained from simulation agrees well with the one observed experimentally (Table 2). The predicted values of mean AuNP diameter at pH 3 and pH 5 are in good agreement with the ones obtained from UV-vis spectroscopy and TEM data. In sharp contrast to all other pH values, the pH 6 case estimates a simulated size (108 nm) which differs significantly from experimental size (30 nm). This might be due to aggregation processes in reaction where particles are conjoined together (Fig. 6b). The simulation strategy as proposed herein, or to the best of our knowledge in the literature, does not take into account the conjoining of particles. Instead, it will account the conjoined particle as a single big size particle. Furthermore, at pH 4 condition also we find an apparent disagreement between experimental (20 nm) and predicted particle size (10 nm). The deviation can be attributed to evolution of anisotropic particles as seen in TEM image (Fig. 6a). The simulation calculates volume of particles and assuming a spherical shape yields its diameter. Hence, the apparent anomaly in diameter stems from anisotropic particles which will have a higher particle size compared to spherical geometry for same volume. When we compare results for fixed pH (pH = 5) and variable precursor ratios, the experimental and simulated results agree satisfactorily (Tables 1 and 2). The predicted particle size (tens of nm) for pH values other than 4–5 is also in agreement with experiments. It is to be noted that predicted particle sizes are well within the range of sizes obtained experimentally. The trends of increase and decrease in mean particle size with pH and precursor ratio variation match the experimental data. The agreements between experimental findings and results from simulation suggest that the theoretical model that incorporates simultaneous nucleation, growth and aggregation in a synthesis holds well for the room temperature process. Moreover, the effect of pH is accurately accounted by calculating rate constant “k c” for each case.
The Turkevich synthesis of gold nanoparticles at room temperature is explained, and the role of the initial pH of the solution and the concentration ratio of sodium citrate to the gold chloride is discussed in detail. The study on role of initial pH of the solution of AuCl3 and trisodium citrate for size control shows that at optimal pH, the particle size is uniform and the particles are monodisperse. At pH values lower and higher than the optimal pH for a given precursor solution, non-uniform shape and size of the nanoparticles is obtained. The long shelf life of the gold nanoparticles without using any stabilizing or capping agent is the main outcome of this room temperature synthesis approach, and the nanoparticles remain in colloidal form for a year at room temperature. We used modified nucleation-growth model of standard Turkevich reaction to simulate our reaction and included the appropriate variations in kinetic constants using experimental results. The agreement between mean particle size from simulation and the experiments show that the reaction steps in room temperature reaction and standard Turkevich reaction are essentially same.
We gratefully acknowledge the Industrial Research and Consultancy Center (IRCC) of IIT Bombay, Council of Scientific and Industrial Research (CSIR, New Delhi) and National Center for Photovoltaic Research and Education (NCPRE-project funded by MNRE, the Government of India) for the financial support of this work.
The manuscript was written through contributions of all authors. The experiments and characterization were performed by HT, AKus and AKum. The simulations of nucleation and growth were performed by HT. MA contributed through research guidance, discussion, and manuscript modifications. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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