A one-pot synthesis of colloidal Ag–Au nanoparticles with controlled composition
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We report a one-pot synthesis method for a production of a stable Ag–Au bimetal colloidal solutions (Ag–Au hydrosols) by simply mixing disodium salt of ethylenediaminetetraacetic acid (Na2EDTA) with AgNO3 and HAuCl4 in aqueous alkaline medium at 80 °C. We found that Na2EDTA act not only as a reductant of metal ions and a potent stabilizer of metal nanoparticle size, but also as an effective agent for controlling the composition of the formed nanoparticles, owing to the high complexing affinity of Na2EDTA to silver ions. It was found that by controlling Na2EDTA concentration in the reaction mixture it is possible to control not only size and size distribution of a synthesized colloidal bimetallic Ag–Au nanoparticles, but bimetallic Ag–Au nanoparticles composition (Ag/Au ratio) as well.
KeywordsBimetal Ag–Au synthesis Nanoparticles Ag Au Hydrosols Na2EDTA
Currently, systems consisting of precious metal-based nanoparticles with intensive absorption bands in the visible optical region are widely employed in a vast variety of practical applications. Gold and silver nanoparticles exhibiting surface plasmon resonance (SPR) are of practical interest due to the possibility to tune the spectral position and SPR amplitude by changing composition, size, shape, structure and dielectric surrounding of the nanoparticles [1, 2, 3, 4, 5]. From the practical standpoint, aside from the morphology and size of such nanoparticle their composition is also a crucial parameter. Composition control opens an additional way for controlling optoelectronic properties and also may enhance the practically important nanoparticle characteristics. For example, it was demonstrated in Ref.  that the composition of bimetallic nanoparticles plays an essential role in nonenzymatic detection.
Methods for the preparation of bimetallic Ag–Au nanoparticles using various reducing agents are the subject of a numerous papers [7, 8, 9, 10, 11, 12]. For example, in Ref.  authors used trimethylamine borane (a reducing agent with relatively low activity) for the preparation of bimetallic nanoparticles with controlled size distribution by exploiting the different reduction rates of silver and gold. In Ref.  authors reported the preparation of Ag–Au nanoparticles with controlled composition by the reduction of precursors with oleylamine in the organic medium at different temperatures. In most cases composition control is achieved by varying the relative molar concentration of reactants [15, 16, 17, 18] and at the same time a linear relationship between this reaction parameter and the Ag/Au ratio in the final bimetallic nanoparticles is noted.
Regardless of the current progress made in developing of the synthetic procedures, the choice of the reported approaches to the preparation of bimetallic nanoparticles still remains somewhat limited and involved protocols are usually relatively sophisticated. Also, in Ref.  authors suggest that the synthetic protocols for high-quality bimetallic nanoparticles with controllable size, morphology, composition and structure have not been sufficiently well-developed thus far.
In this paper we report a simple and single-step approach to the preparation of bimetallic Ag–Au colloids that employs Na2EDTA, which simultaneously acts as a reductant and an effective agent for regulating the composition of obtained nanoparticles. While synthetic protocols for the preparation of nanoparticles consisting of individual gold or silver using Na2EDTA (or EDTA) are relatively well-known [20, 21, 22, 23, 24, 25], Na2EDTA–based syntheses of bimetallic Ag–Au nanoparticles have not yet been reported in the literature.
2 Experimental part
Colloidal Ag–Au nanoparticles were synthesized using 0.008 M Na2EDTA, 0.075 M NaOH, 0.01 M AgNO3 and 0.023 M HAuCl4 solutions. In the major set of experiments (except specifically noted cases) the molar ratios of (Ag + Au)/Na2EDTA and Ag/Au were set equal 1. Total metal concentration (Ag + Au) in all experiment was 5 × 10−4 M.
The synthesis was carried according to the following protocol. Under the intensive stirring, in a 100 ml reaction flask 39.1 ml of distilled water was mixed with 6 ml of NaOH solution and with 3.1 ml of Na2EDTA solution; then this solution with pH 12.0 alkalinity was heated to 80 ℃. The sequence of the further addition of reagents is of principal importance, since it is known that when both silver nitrate and hydrogen tetrachloroaurate are simultaneously present in the solution, insoluble silver chloride may form . In order to avoid this adverse side reaction, 1.25 ml of AgNO3 solution was added first to the hot pH 12.0 alkaline Na2EDTA basic solution to produce a clear solution of the strong silver complex with EDTA. Almost immediately after that, 0.55 ml of HAuCl4 solution was added under vigorous stirring and after 5–10 s the reaction mixture changed colour, so that the final volume of the reaction mixture was 50 ml. For comparison, colloidal solutions of individual silver and gold nanoparticles were prepared under identical conditions.
Samples of colloidal nanoparticles were characterized using optical spectroscopy, transmission electron microscopy (TEM). Optical absorption spectra in 200–800 nm wavelength range were acquired using Cary 500 double-beam spectrophotometer. The composition of nanoparticles was determined by inductively coupled plasma optical emission spectrometry using Prodigy Plus analyser.
3 Results and discussion
The presence of only one absorption band (Fig. 1, curve c) in the spectrum of the bimetallic sol (λmax = 472 nm), located almost in the middle relative to the absorption bands of silver (λmax = 418 nm) and gold (λmax = 520 nm), indicates that alloy particles of two metals are formed.
At the high excess of Na2EDTA, i.e. when the molar concentration of Na2EDTA is 4 times higher than the sum of molar concentrations of starting metals (Fig. 4, line 1), a completely different behaviour is observed. As indicated by the visually noticeable colour change, the reaction starts several seconds after the injection of all reagents.
Judging by value of λmax = 517 nm, from the very beginning of the reaction, almost pure gold nanoparticles are formed. But over time scene changes rapidly, position of λmax shifts into the short wavelength region and stabilizes approximately after 20 min. Further increase of the reaction time (>20 min) does not result in significant changes of the λmax at all investigated Na2EDTA concentrations (Fig. 4, lines 1–3).
At lower Na2EDTA concentration the average size of Ag–Au nanoparticles increases and the size distribution becomes broader. It is worth pointing out that the stability of the prepared colloidal solutions also depends on the Na2EDTA concentration: at [Na2EDTA] ≥ 5 × 10−4 M solutions remain stable for more than 1 year, while al lower concentration ([Na2EDTA] = 1.25 × 10−4 M) solution are stable only for several weeks.
Characteristics of the colloidal solutions of Ag–Au bimetallic nanoparticles as a function of Na2EDTA concentration (Ag/Au molar ratio = 1, reaction time—20 min)
Na2EDTA concentration in the reaction mixture (M)
Position of λmax (nm)
Ag/Au molar ratio in the bimetal (%)
Average diameter of the bimetallic nanoparticles (nm)
2 × 10−3
5 × 10−4
1.25 × 10−4
As one can see from the table, at the same molar ratio of the starting reagents (Ag/Au = 1) as the Na2EDTA concentration decreases, the nanoparticles become enriched in silver and at the same time their size increases. It is interesting to note that such a trend, i.e. the simultaneous increase in the silver content and nanoparticle size is similar to the one reported in Ref. , although the synthetic protocol for bimetallic nanoparticles employed in that paper is completely different (reduction with oleylamine in organic medium).
Obtained experimental results demonstrate that the composition of bimetallic nanoparticles, their size and size distribution is determined by the concentration of Na2EDTA. Theoretical modelling  shows that the λmax in the absorption spectra of the Ag–Au spherical nanoparticles is almost linearly dependent on their composition, i.e. the molar ratio of metals in the particle. It should be emphasized that in the majority of works reporting the synthesis of Ag–Au nanoparticles using a range of different reducing agents [15, 16, 17, 18] a good correlation with theoretical calculations was observed. However, our results differ noticiably from the data mentioned above.
Such a discrepancy from the existing data may be explained in the following manner. In our case Na2EDTA plays several different roles. Firstly, it is a reducing agent ensuring the formation of metallic nanoparticles. Secondly, it acts as a stabilizing agent providing small size of the prepared particles (bimetallic nanoparticles are known to reach 100 nm and more in size in the absence of stabilizing agents ). Thirdly, Na2EDTA forms complex compounds with metal ions, and especially strong ones with Ag+ (pК1 = 7.31, pК2 = 11.31 ). In addition to that Na2EDTA is known to act as a reducing agent for Au3+ ions in the broad range of Au/Na2EDTA ratios from 1/0.3 to 1/100 [18, 28]. As we have demonstrated earlier , silver colloids are formed at Ag/Na2EDTA = (1/1 ÷ 2/1), that is, when the concentration of Na2EDTA is less than the concentration of Ag+. While at higher Na2EDTA concentrations (Ag/Na2EDTA < 1), that is, when the concentration of Na2EDTA is higher than the concentration of Ag+, no reaction occurs. Taking into account all the presented above considerations, the observed peculiar effect of the Na2EDTA concentration on the λmax dependence of the metals Ag/Au molar ratio is related to Na2EDTA silver chelating ability. As a result, silver complex compounds of different composition are formed, and a change in the composition of these complexes occurs during the reaction. Thereby silver complex compounds of different composition are forming and transforming over the course of the reaction.
At low concentrations (1.25 × 10−4 M, ratio Ag/Na2EDTA = 2/1) complexing ability of Na2EDTA is not too significant and thus the relationship between λmax position of Ag–Au colloids and the Ag/Au molar ratio is almost linear, except for the low silver concentration region (Fig. 3a). On the contrary, at significant Na2EDTA concentrations (2 × 10−3 M, ratio Ag/Na2EDTA = 1/8) complexing ability of Na2EDTA is significant and therefore Na2EDTA almost entirely hinder silver nanoparticles formation and as a result at first primarily gold nanoparticles are formed (Fig. 4, initial area of line 1). In this case, the absence of the zero point on the line (Fig. 3c) is very significant and indicates that at high Na2EDTA concentrations (2 × 10−3 M), in the absence of Au, the synthesis reaction does not occur. In the case of high Na2EDTA concentrations (2 × 10−3 M) we assume, that gold nanoparticles are formed on early stages of the reaction and then serve as a catalyst for the reduction of silver ions and thus as a result final bimetallic nanoparticles are “enriched” with gold (see Table 1). To test this assumption, we conducted the following experiment. We added 2% molar mass (to the molar mass of silver) of pre-synthesized gold nanoparticles to the reaction medium with high concentration of Na2EDTA (2 × 10−3 M) and obtained a silver colloid with a pronounced plasmon peak at λmax = 418 nm, which, in our opinion, indicates the formation of silver nanoparticles. As we established earlier , under such conditions (in the presence of high concentration of Na2EDTA) without gold nanoparticles the synthesis reaction of silver nanoparticles does not occur.
This paper for the first time demonstrates a method of one-pot preparation of stable colloids of bimetallic Ag–Au nanoparticles using Na2EDTA without any polymeric stabilizing agents. Na2EDTA was found to act not only as a reductant of metal ions but also as an effective agent for controlling the composition of the formed nanoparticles, which may be explained by the high complexing ability of Na2EDTA to silver ions. It was found that by controlling Na2EDTA concentration in the reaction mixture when other reaction parameters are fixed (most importantly the concentration of Ag and Au salts) it is possible to control composition (Ag/Au ratio), size and size distribution of a synthesized colloidal bimetallic Ag–Au nanoparticles.
This work has been supported by the Belarusian State Program of Scientific Research “Photonics, Opto- and Microelectronics” (Task No. 1.2.03).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interests.
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