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.
Keywordsgold nanoparticles concentrated sodium citrate
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.
- Daniel MC, Astruc D: Gold nanoparticles: Assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem Rev 2004, 104: 293. 10.1021/cr030698+View Article
- Sardar R, Funston AM, Mulvaney P, Murray RW: Gold Nanoparticles: Past, Present, and Future. Langmuir 2009, 25: 13840. 10.1021/la9019475View Article
- Boisselier E, Astruc D: Gold nanoparticles in nanomedicine: preparations, imaging, diagnostics, therapies and toxicity. Chem Soc Rev 2009, 38: 1759. 10.1039/b806051gView Article
- Sperling RA, Rivera Gil P, Zhang F, Zanella M, Parak WJ: Biological applications of gold nanoparticles. Chem Soc Rev 2008, 37: 1896. 10.1039/b712170aView Article
- Zhao WA, Gao Y, Kandadai SA, Brook MA, Li YF: DNA polymerization on gold nanoparticles through rolling circle amplification: Towards novel scaffolds for three-dimensional periodic nanoassemblies. Angew Chem-Int Ed 2006, 45: 2409. 10.1002/anie.200600061View Article
- Richardson MJ, Johnston JH: Sorption and binding of nanocrystalline gold by Merino wool fibres-An XPS study. J Colloid Interface Sci 2007, 310: 425. 10.1016/j.jcis.2007.01.075View Article
- Iwakoshi A, Nanke T, Kobayashi T: Coating materials containing gold nanoparticles. Gold Bull 2005, 38: 107. 10.1007/BF03215244View Article
- Jans H, Jans K, Lagae L, Borghs G, Maes G, Huo Q: Poly(acrylic acid)-stabilized colloidal gold nanoparticles: synthesis and properties. Nanotechnology 2010, 21: 455702. 10.1088/0957-4484/21/45/455702View Article
- Ohno K, Koh K, Tsujii Y, Fukuda T: Synthesis of gold nanoparticles coated with well-defined, high-density polymer brushes by surface-initiated living radical polymerization. Macromolecules 2002, 35: 8989. 10.1021/ma0209491View Article
- Ohno K, Koh K, Tsujii Y, Fukuda T: Fabrication of ordered arrays of gold nanoparticles coated with high-density polymer brushes. Angew Chem-Int Ed 2003, 42: 2751. 10.1002/anie.200250850View Article
- Freudenberger R, Zielonka A, Funk M, Servin P, Haag R, Valkova T, Landau U: Recent developments in the preparation of nano-gold composite coatings. Gold Bull 2010, 43: 169. 10.1007/BF03214984View Article
- Lee JS: Recent progress in gold nanoparticle-based non-volatile memory devices. Gold Bull 2010, 43: 189. 10.1007/BF03214986View Article
- Bishop PT, Ashfield LJ, Berzins A, Boardman A, Buche V, Cookson J, Gordon RJ, Salcianu C, Sutton PA: Printed gold for electronic applications. Gold Bull 2010, 43: 181. 10.1007/BF03214985View Article
- Hughes MD, Xu YJ, Jenkins P, McMorn P, Landon P, Enache DI, Carley AF, Attard GA, Hutchings GJ, King F, Stitt EH, Johnston P, Griffin K, Kiely CJ: Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions. Nature 2005, 437: 1132. 10.1038/nature04190View Article
- Hvolbaek B, Janssens TVW, Clausen BS, Falsig H, Christensen CH, Norskov JK: Catalytic activity of Au nanoparticles. Nano Today 2007, 2: 14.View Article
- Piccolo L, Daly H, Valcarcel A, Meunier FC: Promotional effect of H 2 on CO oxidation over Au/TiO 2 studied by operando infrared spectroscopy. Appl Catal B-Environ 2009, 86: 190. 10.1016/j.apcatb.2008.08.011View Article
- Zhou XC, Xu WL, Liu GK, Panda D, Chen P: Size-Dependent Catalytic Activity and Dynamics of Gold Nanoparticles at the Single-Molecule Level. J Am Chem Soc 2010, 132: 138. 10.1021/ja904307nView Article
- Wallace WT, Whetten RL: Coadsorption of CO and O 2 on selected gold clusters: Evidence for efficient room-temperature CO 2 generation. J Am Chem Soc 2002, 124: 7499. 10.1021/ja0175439View Article
- Volkert AA, Subramaniam V, Haes AJ: Implications of citrate concentration during the seeded growth synthesis of gold nanoparticles. Chem Commun 2011, 47: 478. 10.1039/c0cc02075cView Article
- Oh E, Susumu K, Goswami R, Mattoussi H: One-Phase Synthesis of Water-Soluble Gold Nanoparticles with Control over Size and Surface Functionalities. Langmuir 2010, 26: 7604. 10.1021/la904438sView Article
- Martin MN, Basham JI, Chando P, Eah SK: Charged Gold Nanoparticles in Non-Polar Solvents: 10-min Synthesis and 2D Self-Assembly. Langmuir 2010, 26: 7410. 10.1021/la100591hView Article
- Brust M, Walker M, Bethell D, Schiffrin DJ, Whyman R: Synthesis of Thiol-derivatised Gold Nanoparticles in a Two-phase Liquid-Liquid System. J Chem Soc, Chem Commun 1994, 801.
- Templeton AC, Hostetler MJ, Kraft CT, Murray RW: Reactivity of monolayer-protected gold cluster molecules: Steric effects. J Am Chem Soc 1998, 120: 1906. 10.1021/ja973863+View Article
- Waters CA, Mills AJ, Johnson KA, Schiffrin DJ: Purification of dodecanethiol derivatised gold nanoparticles. Chem Commun 2003, 540.
- Turkevich J, Stevenson PC, Hillier J: A Study of the Nucleation and Growth Processes in the Synthesis of Colloidal Gold. Discuss Faraday Soc 1951, 55.
- Frens G: Controlled Nucleation for Regulation of Particle-Size in Monodisperse Gold Suspensions. Nature-Physical Science 1973, 241: 20.View Article
- Ji XH, Song XN, Li J, Bai YB, Yang WS, Peng XG: Size control of gold nanocrystals in citrate reduction: The third role of citrate. J Am Chem Soc 2007, 129: 13939. 10.1021/ja074447kView Article
- Biggs S, Chow MK, Zukoski CF, Grieser F: The Role of Colloidal Stability in the Formation of Gold Sols. J Colloid Interface Sci 1993, 160: 511. 10.1006/jcis.1993.1430View Article
- Nguyen DT, Kim DJ, So MG, Kim KS: Experimental measurements of gold nanoparticle nucleation and growth by citrate reduction of HAuCl 4 . Adv Powder Technol 2010, 21: 111. 10.1016/j.apt.2009.11.005View Article
- Uppal MA, Kafizas A, Ewing MB, Parkin IP: The effect of initiation method on the size, monodispersity and shape of gold nanoparticles formed by the Turkevich method. New J Chem 2010, 34: 2906. 10.1039/c0nj00505cView Article
- Uppal MA, Kafizas A, Lim TH, Parkin IP: The extended time evolution size decrease of gold nanoparticles formed by the Turkevich method. New J Chem 2010, 34: 1401. 10.1039/b9nj00745hView Article
- Li DX, Li CF, Wang AH, He Q, Li JB: Hierarchical gold/copolymer nanostructures as hydrophobic nanotanks for drug encapsulation. J Mater Chem 2010, 20: 7782. 10.1039/c0jm01059fView Article
- Kim YP, Oh YH, Kim HS: Protein kinase assay on peptide-conjugated gold nanoparticles. Biosens Bioelectron 2008, 23: 980. 10.1016/j.bios.2007.10.001View Article
- Liu SH, Zhang ZH, Han MY: Nanometer-sized gold-loaded gelatin/silica nanocapsules. Adv Mater 2005, 17: 1862. 10.1002/adma.200500124View Article
- Xia H, Wang D: Fabrication of Macroscopic Freestanding Films of Metallic Nanoparticle Monolayers by Interfacial Self-Assembly. Adv Mater 2008, 20: 4253. 10.1002/adma.200702978View Article
- Elghanian R, Storhoff JJ, Mucic RC, Letsinger RL, Mirkin CA: Selective colorimetric detection of polynucleotides based on the distance-dependent optical properties of gold nanoparticles. Science 1997, 277: 1078. 10.1126/science.277.5329.1078View Article
- Storhoff JJ, Elghanian R, Mucic RC, Mirkin CA, Letsinger RL: One-pot colorimetric differentiation of polynucleotides with single base imperfections using gold nanoparticle probes. J Am Chem Soc 1998, 120: 1959. 10.1021/ja972332iView Article
- Zhang C, Zhang ZY, Yu BB, Shi JJ, Zhang XR: Application of the biological conjugate between antibody and colloid Au nanoparticles as analyte to inductively coupled plasma mass spectrometry. Anal Chem 2002, 74: 96. 10.1021/ac0103468View Article
- Li DX, He Q, Yang Y, Möhwald H, Li JB: Two-stage pH response of poly(4-vinylpyridine) grafted gold nanoparticles. Macromolecules 2008, 41: 7254. 10.1021/ma800894cView Article
- Wilson R: Haptenylated mercaptodextran-coated gold nanoparticles for biomolecular assays. Chem Commun 2003, 108.
- Li DX, Cui Y, Wang KW, He Q, Yan XH, Li JB: Thermosensitive nanostructures comprising gold nanoparticles grafted with block copolymers. Adv Funct Mater 2007, 17: 3134. 10.1002/adfm.200700427View Article
- Chow MK, Zukoski CF: Gold Sol Formation Mechanisms-Role of Colloidal Stability. J Colloid Interface Sci 1994, 165: 97. 10.1006/jcis.1994.1210View Article
- Yang SC, Wang YP, Wang QF, Zhang RL, Ding BJ: UV irradiation induced formation of Au nanoparticles at room temperature: The case of pH values. Colloid Surf A-Physicochem Eng Asp 2007, 301: 174. 10.1016/j.colsurfa.2006.12.051View Article
- Kumar S, Gandhi KS, Kumar R: Modeling of formation of gold nanoparticles by citrate method. Ind Eng Chem Res 2007, 46: 3128. 10.1021/ie060672jView Article
- Enustun BV, Turkevich J: Coagulation of Colloidal Gold. J Am Chem Soc 1963, 85: 3317. 10.1021/ja00904a001View Article
- Kimling J, Maier M, Okenve B, Kotaidis V, Ballot H, Plech A: Turkevich method for gold nanoparticle synthesis revisited. J Phys Chem B 2006, 110: 15700. 10.1021/jp061667wView Article
- Pong BK, Elim HI, Chong JX, Ji W, Trout BL, Lee JY: New insights on the nanoparticle growth mechanism in the citrate reduction of Gold(III) salt: Formation of the au nanowire intermediate and its nonlinear optical properties. J Phys Chem C 2007, 111: 6281. 10.1021/jp068666oView Article
- Polte J, Ahner TT, Delissen F, Sokolov S, Emmerling F, Thunemann AF, Kraehnert R: Mechanism of Gold Nanoparticle Formation in the Classical Citrate Synthesis Method Derived from Coupled In Situ XANES and SAXS Evaluation. J Am Chem Soc 2010, 132: 1296. 10.1021/ja906506jView Article
- Ojea-Jimenez I, Romero FM, Bastus NG, Puntes V: Small Gold Nanoparticles Synthesized with Sodium Citrate and Heavy Water: Insights into the Reaction Mechanism. J Phys Chem C 2010, 114: 1800. 10.1021/jp9091305View Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.