Facile synthesis of concentrated gold nanoparticles with low size-distribution in water: temperature and pH controls
© Li et al; licensee Springer. 2011
Received: 14 April 2011
Accepted: 6 July 2011
Published: 6 July 2011
The citrate reduction method for the synthesis of gold nanoparticles (GNPs) has known advantages but usually provides the products with low nanoparticle concentration and limits its application. Herein, we report a facile method to synthesize GNPs from concentrated chloroauric acid (2.5 mM) via adding sodium hydroxide and controlling the temperature. It was found that adding a proper amount of sodium hydroxide can produce uniform concentrated GNPs with low size distribution; otherwise, the largely distributed nanoparticles or instable colloids were obtained. The low reaction temperature is helpful to control the nanoparticle formation rate, and uniform GNPs can be obtained in presence of optimized NaOH concentrations. The pH values of the obtained uniform GNPs were found to be very near to neutral, and the pH influence on the particle size distribution may reveal the different formation mechanism of GNPs at high or low pH condition. Moreover, this modified synthesis method can save more than 90% energy in the heating step. Such environmental-friendly synthesis method for gold nanoparticles may have a great potential in large-scale manufacturing for commercial and industrial demand.
Gold nanoparticles (GNPs), also named as gold colloids, have attracted increasing attention due to their unique properties in multidisciplinary research fields [1, 2]. Although GNPs are defined by tiny size, significant quantities of GNPs are likely required in many commercial and industrial applications. Remarkably, novel emerging applications bring a huge growth of the global demand of GNPs. For instance, (1) biomolecule- and/or biopolymer-conjugated GNPs are largely used as biomarkers and biodelivery vehicles in the medicine/pharmacy, and in cosmetic products, GNPs are employed as anti-aging components for skin protection [3–5]; (2) GNPs are used to treat wool or cotton fibers for a permanent coloration  of value textiles; (3) various polymer/gold nanocomposites display a high potential for novel coatings and paintings [7–11]; (4) GNPs are used to enhance the performance of non-volatile memory devices  and low temperature printing metal inks in electronics ; and (5) GNPs as catalysts are developed in novel usages [14–18]. Therefore, more attention should be paid on effective synthesis methods to match the enlarging demand of GNPs.
In the past decades, though many synthetic strategies have been developed to prepare GNPs in organic or aqueous solvents [19–24], the citrate reduction method has remained the best candidate to fit the enlarging demand of GNPs due to its advantages such as inexpensive reductant, non-toxic water solvent, and low pollution in the reaction [25–28]. The simple operation of pouring rapidly a certain amount of sodium citrate solution into a boiling solution of 0.25 mM chloroauric acid produces narrowly distributed GNPs which are biocompatible and easily handled in applications [29–31]. So, this method is extensively used in GNP-based bioassays and biomedicine systems [5, 32–34] and even in structured/assembled nanomaterials [35–41]. In the pioneering work on the citrate reduction method, Turkevich in 1951 reported the basic experimental approach and the effect of temperature and reagent concentration upon the nanoparticle size and size distribution , and in 1973, Frens published the control of size variation of GNPs by changing the concentration of sodium citrate . Then, in 1994, Zukoski published a sol formation mechanism and a particle growth model . Recently, the decisive role of sodium citrate on the pH value of the reaction mixture and the nanoparticle size was demonstrated based on experimental and theoretical modeling results [27, 43, 44]. On the other hand, in the majority of the published citrate reduction works, GNPs were synthesized from a dilute solution of 0.25 mM chloroauric acid, such a concentration yields aqueous GNPs with low weight content (0.005%) as a disadvantage. The low nanoparticle content asks for abundant water to be used in the preparation and consumes a lot of energy in the heating step. Sometimes, such dilute gold colloids cannot fulfill the requirement of high concentration. Thus, the classical citrate method will be limited in large-scale manufacturing. Considering the abovementioned advantages and disadvantages, we expected that the citrate reduction method should have been developed to produce concentrated aqueous GNPs already from several years ago. However, simply increasing the reactant concentration will change the systemic pH and salt concentration with drastic influence on the nanoparticle size polydispersity and the colloidal stability.
Herein, to meet the need of high concentrations, we modified the classical citrate reduction method and synthesized uniform GNPs from tenfold concentrated precursor (2.5 mM HAuCl4) via adding sodium hydroxide and controlling the temperature. We demonstrated that adding a proper amount of sodium hydroxide to the reaction mixture could produce uniform GNPs with a narrow size distribution after the reduction by sodium citrate at boiling sate. The low reaction temperature was helpful to control the nanoparticle formation rate, and uniform GNPs could be obtained at different temperature by adding a proper amount of alkali. The pH change resulting from the addition of alkali showed a critical role in the influence on the particle size distribution, which might be related to the different formation mechanism of GNPs under different pH conditions.
Hydrochloroauric acid trihydrate (HAuCl4 3H2O, 99.9%) was purchased from Sigma-Aldrich Shanghai Trading Co Ltd, Shanghai, China, while sodium citrate (Na3C6H5O7 2H2O, > 99%) and sodium hydroxide (NaOH, > 98%) were obtained from Shanghai Chemical Co., Shanghai, China. Deionized water (resistance > 18.2 MΩ) was prepared by an ultrapure water system in our laboratory. All chemicals were used as received without any purification.
Synthesis of concentrated nanoparticle dispersions via simply increasing reactant concentration
GNPs were first synthesized from HAuCl4 solution with gradually increased concentration of the reactant. In detail, 50 ml deionized water in a round-bottom flask was added to 5, 10, 20, 30, 40, and 50 mg chloroauric acid, respectively. After heating to boiling state, 0.3, 0.6, 1.2, 1.8, 2.4, and 3.0 ml sodium citrate solution (50 mg/ml) were rapidly introduced into the flask with drastic stirring, respectively. The mixtures were continuously heated for a certain period till a ruby-red color appeared.
Synthesis of concentrated GNPs under alkali control and different temperature
The concentrations of chloroauric acid and sodium citrate in the final mixture were respectively fixed to 2.5 and 5.0 mM, while that of NaOH was changed. The reaction temperature was selected to be boiling state, 85°C and 70°C. For example, 2.0 mL chloroauric acid (25 mM) was mixed with 5.3 to 10.2 mL of 20 mM NaOH solution, followed by adding the calculated volume of water to a total volume of 20 mL. The flask was put into an oil bath at 110°C for 30 min to balance the reaction mixture to 85°C. Then, 0.6 mL sodium citrate solution (50 mg/ml) was rapidly introduced into the flask under vigorous stirring. After different reaction time, samples were taken out for characterization. The reaction at the boiling state and 70°C was similarly performed, respectively.
Detecting the nanoparticle formation process
In the synthesis process of GNPs, a portion of the reaction mixture (0.5 to 1 mL) was taken out from the flask at different reaction time and immediately poured into 9 mL ice-cooled water at 0°C. Such an operation can basically cease the formation process of GNPs due to the low temperature surrounding and the dilution effect, so it was called here as a "sample-frozen" operation. Then, the transmission electron microscopy (TEM) samples were prepared at the earliest time and the ultraviolet-visible (UV-vis) spectra were recorded.
Characterization and instrumentation
UV-vis spectra were recorded on a U-3010 UV-visible spectrophotometer (Hitachi High-Technologies Co., Tokyo, Japan) to collect the surface plasmon resonance (SPR) information of GNPs, in which the highly concentrated samples were diluted pro rata by deionized water to adapt the measurement limitation. TEM samples were prepared by dropping the diluted gold colloids on carbon-coated copper grids, followed by natural drying; then, the samples were observed on a JEM-2010 microscope (JOEL Ltd, Tokyo, Japan).
Results and discussion
Size distribution enlarging of GNPs at high reactant concentration
Controlling the size distribution by adding sodium hydroxide
Decreasing reaction rate by lowering temperature
Optimal experimental parameters for GNP synthesis at different temperature
Reaction time (min)
pH analysis of the reaction mixture at different conditions
Figure 5 displays the pH values of the reaction mixture mixed at room temperature and those as-obtained gold colloids prepared at various conditions. The pH value shows a linear change with respect to the addition of NaOH both before and after the reaction, which is due to the buffer behavior of the sodium citrate and the low alkali dosage. When the reaction was performed at boiling state, the optimal NaOH dosage (6.6 mM) corresponds to pH 6.7. At 85°C, the pH of the best colloids prepared in presence of 7.7 mM NaOH is 6.8, while at 70°C the final pH for the best colloids is 7.5. The pH values of the acceptable GNPs with a narrow size distribution are listed in Table 1. It is found that the pH values for uniform gold colloids are slightly different at different reaction temperatures and a higher pH value is indicated at lower temperature. These pH values are very close to the neutral condition (between 6.5 and 7.5), which is in accordance with the literature .
Analysis of the pH influence on the nanoparticle size distribution
In this work, uniform GNPs with low size polydispersity can be synthesized from the chloroauric acid precursor at high concentration (2.5 mM) by the citrate reduction method via combined temperature and pH controls. The addition of a proper amount of sodium hydroxide can produce uniform GNPs with a narrow size distribution. The low reaction temperature is helpful to control the nanoparticle formation rate, and uniform GNPs can be obtained at different temperatures in presence of an optimized NaOH dosage. The pH analysis demonstrates that uniform GNPs can be obtained at around neutral conditions. The modified citrate reduction method can produce concentrated gold colloid dispersions and save more than 90% energy in the heating step. Such environmental-friendly synthesis method for gold nanoparticles may have a great potential in large-scale manufacturing to match the increasing commercial and industrial demands.
DL is a Ph.D. major in Physical Chemistry, Shandong University, China. He has focused his research interest on the gold nanomaterials especially on the polymer modified gold nanoparticles for more than 6 years from his postdoc careers in Institute of Chemistry, Chinese Academy of Sciences, China and in the Max-Planck Institute of Colloids and Interfaces, Germany. His published papers involved the core/shell nanostructures of the thermosensitive/pH-responsive polymer and amphiphilic polymer grafted gold nanoparticles toward multifunctional nanocarriers and nanosupports.
We thank Prof. Dr. Helmuth Möhwald (Max-Planck Institute of Colloids and Interfaces, Germany) for suggestions and editing of the English of this paper. This work has been supported by the National Natural Science Foundation of China (No. 21073102), as well as the Taishan Scholar Foundation (ts20070713) of Shandong Province, China.
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