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A Systematic Study of the Stabilization in Water of Gold Nanoparticles by Poly(Ethylene Oxide)-Poly(Propylene Oxide)-Poly(Ethylene Oxide) Triblock Copolymers Kamil Rahme,† Fabienne Gauffre,*,† Jean-Daniel Marty,† Bruno Payre´ ,‡ and Christophe Mingotaud† Laboratoire des IMRCP, CNRS UMR 5623, UniVersite´ Paul Sabatier, 31062 Toulouse cedex 9, France, and Centre de Microscopie EÄ lectronique Applique´ e a` la Biologie, UniVersite´ Paul Sabatier-Toulouse III, 133 rte de Narbonne, 31062 Toulouse cedex 4, France ReceiVed: January 12, 2007; In Final Form: March 19, 2007
Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO)x-(PPO)y-(PEO)x triblock copolymers have been efficiently used for the stabilization of gold nanoparticles (AuNps) in NaCl aqueous solutions. The effects of concentration and structure of the block copolymer (i.e., PEO block length, PPO block length, and overall length of the polymer) have been systematically investigated, using well-defined AuNps (diameter ∼12 nm) synthesized through a standard citrate reduction procedure. Aggregation and precipitation processes were assessed separately using empirical parameters calculated from optical absorbance spectra. Increasing the concentration, PEO and PPO block lengths, or the overall length of the polymer enhanced the nanoparticles colloidal stability. Among all these parameters, the paramount one appeared to be the length of the PPO block. Different aggregation states were observed and characterized by the displacement of the plasmon resonance band. A correlation between the displacement of this band and precipitation of the nanoparticles was established.
Introduction Nanoparticles of metal and semiconductors exhibit various optical and electronic size-dependent properties that differ from the properties of the bulk material.1-3 The specific features of nanoobjects result from quantum size effects and also from a high surface to volume ratio. The many foreseen applications include for instance catalysis, optoelectronic devices,4 and fluorescent semiconductor probes (“quantum dots”) for biomedical research.5 To maintain these particular properties in solution, dispersion and colloidal stabilization of the nanoparticles should be achieved within the used solvent. Therefore, nanoparticles are usually coated by an organic or inorganic layer. This protective coating provides compatibility with the solvent and counterbalances the Van der Waals attraction occurring between nanoparticles and causing their aggregation. The most common strategy to achieve colloidal stability proceeds via chemical binding of ligands at the surface of the nanoparticles. Although this covalent strategy usually guarantees a stable linkage between the ligand and the nanoparticle, it may also alter the properties of the particles through a modification of the dielectric constant of the environmental medium and of the electronic density in the nanoparticles.6-7 To maintain the properties of nanomaterial, a strategy based on the physical adsorption of ligands over the surface of the nanoparticles may be preferred. The ligands used to achieve an efficient physical stabilization are usually surfactants8 or macromolecules, such as linear or hyperbranched polymers.9-12 Many stabilization studies have been based on gold nanoparticles (AuNps). Indeed, AuNps are easily obtained through * To whom correspondence should be addressed. E-mail: gauffre@ imrcp.ups-tlse.fr. Phone: (33) 561556143. Fax: (33) 561558155. † Laboratoire des IMRCP. ‡ Centre de Microscopie E Ä lectronique Applique´e a` la Biologie.
simple synthetic procedures.13-15 Similar to other metal nanoparticles such as Ag and Cu, they strongly absorb in the visible region (the surface plasmon resonance absorption) that can be used to characterize their size16-19 as well as the interparticle distance.20-21 In addition, AuNps find many applications in various fields22 including catalysis23-25 and biotechnology.26-29 In the context of biological applications,30 toxicity studies,31-32 and environmentally friendly catalysis,33-34 it is of special interest to produce colloidal suspensions of AuNps in water that exhibit long-term stability in a large domain of concentration and in the saline conditions of biological fluids. Sakai et al. achieved an efficient, environmentally benign, and economical protocol for the synthesis of AuNps via reduction of AuCl4- ions in aqueous solutions containing poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers35-36 without the use of any standard reducing agent such as citrate or NaBH4. The reduction occurred via metal ion complexation with the block copolymers (noted EO-PO-EO). In addition to acting as reducing agent, it was observed that these polymers can be very efficient in stabilizing the colloidal dispersion of the obtained nanoparticles.36 By use of Sakai’s synthesis method with relatively high concentrations in polymers, Chen et al. investigated the effect of the microenvironment within EO-PO-EO block copolymers micelles over the structure of the so-formed nanoparticles and colloidal stability.37 The EO-PO-EO block copolymers have also been used in association with NaBH4 to produce small (2-3 nm) AuNps.38 However, a systematic investigation of the role of the molecular structure and of the concentration of the polymer upon aggregation and subsequent decantation was never reported. In the former studies,35-38 the polymer plays an active role in the formation of the nanoparticles. As a consequence, the structure and properties of the particle might depend of the
10.1021/jp070274+ CCC: $37.00 © 2007 American Chemical Society Published on Web 05/01/2007
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TABLE 1: Series of EO-PO-EO Block Copolymers Used in This Study and Their Properties (All Data Are Extracted from Ref 36 and the cmc Values Are Given at 25 °C, except for F68 and L62, Given Respectively at 27 and 20 °C and Extracted from Refs 41 and 42) name
Mw (g mol-1)
total no. EO units per polymera
no. PO units per polymer
cmc (mmol L-1)
L62 L64 P65 P84 P85 P123 P104 F68 F88 F127 F108 PEG
2500 2900 3400 4200 4600 5750 5900 8400 11400 12600 14600 8000
12 26 38 38 52 38 54 152 206 200 264 182
34 30 29 43 40 69 61 29 39 65 50 0
9.6 26 38 6.2 8.7 0.05 0.51 18 12 0.56 3.1
a The number of EO units per PEO blocks is half of the total number of EO units per polymer.
polymer used, and in addition, the polymer might be (at least partially) modified through the reaction. Since size, shape, and surface state of the particles are important parameters in colloidal stability, a full comprehension of the stabilizing effect of the EO-PO-EO block copolymer would be somewhat difficult to assess. For such a reason, we propose in the present work to uncouple the nanoparticles formation and their stabilization process. Our strategy is to synthesize well-controlled AuNps (diameter ≈ 12 nm) via a standard citrate reduction method, prior to the addition of the EO-PO-EO block copolymers. Thus, comparison of the stabilizing effect of each polymer can be achieved using exactly the same nanoparticle material. The effect of PEO blocks length, PPO block length, and overall polymer molecular weight were then separately addressed using six series of polymers. With the EO-PO-EO triblock copolymers, colloidal stability of AuNps in water was easily achieved over more than several weeks. To get access to shorter experimental time scales, concentrated NaCl solution was added to the polymer-stabilized AuNp suspensions. In the strategy proposed herein, no covalent bond is formed between the polymers and the nanoparticles. Thus, we expect that the results of this study could be readily extended to other types of nanomaterial. Experimental Section Materials. Water was purified through a filter and ionexchange resin (resistivity ∼16 MΩ cm), using a Purite device. All glassware was cleaned before use with an acid solution (3 parts of concentrated HCl and 1 part of concentrated HNO3), rinsed by triply distilled water, ethanol, and acetone and ovendried. The tetrachloroauric acid trihydrate (HAuCl4,3H2O), sodium citrate (C6H5Na3O7,2H2O), sodium chloride (NaCl), Pluronics F127, F68, and polyethyleneglycol PEG (Mw ) 8000 g mol-1) were purchased from Sigma Aldrich. Pluronics P123, P104, P84, F88, L62, L64, F108, P85, and P65 were a gift from BASF. Some of their molecular characteristic features are listed in Table 1. All products were used as received, without further purification. Preparation of Sodium Citrate AuNps. Colloidal gold with diameter ca. 12 nm was reproducibly obtained by reduction of tetrachloroauric acid trihydrate with sodium citrate as follows: 39 50 mL of an aqueous solution of HAuCl ‚3H O (1 mmol 4 2 L-1) was brought to reflux with vigorous stirring. A volume of 5 mL of a 38.8 mmol L-1 sodium citrate aqueous solution was
Figure 1. Characterization of AuNps obtained via citrate reduction. (a) TEM micrograph (bar ) 40 nm) and size distribution of the nanoparticles (total number of counted particles over 5500). (b) Absorption spectra of the pristine AuNp suspension.
rapidly added to the vortex of the solution. The color of the solution changed from pale yellow to dark blue and then to deep red burgundy within about 90 s. Stirring and boiling was maintained during 12 min after addition of sodium citrate. The solution was then removed from heat and kept under vigorous stirring, until cooled to room temperature. This pristine solution was diluted 5 times for the stability experiments. Stability Experiments. The absorbance value of AuNp aqueous suspensions just before salt addition was typically 0.65 ( 0.03 at 520 nm in a 1-cm path length cell (see Figure 1b). For each series of polymers, AuNPs from a same synthesis batch were used. Concentrated polymer solutions were added to samples of AuNP suspensions in order to reach the final desired concentration of polymer. Then, the ionic strength of the suspensions was adjusted to 0.9 mol L-1 by adding the appropriate amount of a 2.25 mol L-1 NaCl solution. The AuNp/ copolymer suspensions were constantly kept under stirring. The colloidal stability of AuNps was investigated by optical observation, absorption spectroscopy and TEM. UV-Visible Absorption. UV-visible spectra of the colloidal gold aqueous suspensions were measured by a diode array (HP 8452A, Hewlett-Packard) or a double beam (Cary 400 bio, Varian) UV-visible spectrophotometer, equipped with a temperature control system and magnetic stirring. Transmission Electron Microscopy (TEM). AuNps were observed using a HITACHI HU12A microscope at an accelerating voltage of 75 kV. A drop of AuNp suspensions was deposited on a TEM carbon grid and dried in air. TEM micrographs were analyzed using Image J software (http:// rsb.info.nih.gov/ij/) (more than one thousand particles were counted and measured for each sample). Results and Discussion AuNps used as starting materials for stabilization experiments were synthesized with a good reproducibility via reduction in water of HAuCl4 by sodium citrate.39 The nanoparticles were approximately spherical and well dispersed as shown by TEM (see Figure 1a). The mean particle diameter, determined from analysis of the TEM micrographs, was 12 nm with a standard deviation of 2 nm. The nanoparticle suspensions obtained via this procedure exhibited a broad absorption band around 520 nm (surface plasmon resonance40), resulting in a pink-red color of the solutions (Figure 1b). Negatively charged citrate species at the surface of the nanoparticles induced a short-term stability of the AuNps in water. To investigate the stabilization effect of block copolymers on those nanoparticles, the electrostatic repulsions (and stabilization) of the citrate species should be strongly screened. Therefore, the present study was conducted under high ionic strength conditions. More precisely, after synthesis of the nanoparticle as described previously, the desired
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Figure 3. Decantation of the AuNps: typical evolution of the absorbance spectrum of an AuNp suspension containing 2.8 mmol L-1 of P84, 22 and 54 h after NaCl addition.
samples made from such dried solutions, when AuNps were well isolated in the sample made from the pristine AuNp suspension (see Figure 1). In an effort to investigate this possible aggregation process, we defined the following parameter noted ∆λ
∆λ ) λmax - 520 nm Figure 2. (a) Evolution of the absorbance spectrum of AuNp suspensions containing 0.7 mmol L-1 of F68, before (t ) 0 h) and 2 h (t ) 2 h) after NaCl addition. (b) TEM micrograph of the sample 2 h after salt addition. Scale bar ≈ 100 nm.
amount of EO-PO-EO polymer was mixed to the suspension. Then, a concentrated NaCl solution was added to reach a final concentration of 0.9 mol L-1 in NaCl. Under such conditions and without any copolymers, AuNps instantaneously aggregated (with a change in the color of the solution) upon addition of NaCl. Then, the AuNps totally precipitated in few hours. Various nonionic EO-PO-EO triblock copolymers were used in order to provide steric stabilization of the AuNps. To precisely determine the role of the EO and PO blocks, as well as of the overall molecular weight of the copolymers, six series of compounds were investigated separately. To investigate the effect of the length of the EO block, we used two series named EO1 (containing polymers L62, P65, P85, F68, and F88) and EO2 (P123, P104, F127). In both series, the number of EO units was changed, whereas the number of PO units was kept constant (at ∼34 ( 6 and ∼65 ( 4, respectively). The series PO1 (P65, P84, P123) and PO2 (F88, F127, PEG) corresponds to a variation of the number of PO units, keeping constant the number of EO units (at 38 and ∼194 ( 12, respectively). Finally, the series M1 (L64, P84, P104) and M2 (F68, F88, F108) containing, respectively, ca. 47 and 84% of EO units (vs the total number of EO and PO units) were used to address the effect of molecular weight on the nanoparticle stabilization. Whatever the EO-PO-EO copolymer may be, its concentration in the AuNps solutions was kept below its critical micelle concentration (cmc, see Table 1). Depending on the exact nature of the polymer used, various behaviors and changes in the absorption spectra of the aqueous mixtures were observed. A first typical modification of the UV-visible spectra was the appearance and growth of a broad absorbance peak between 600 and 850 nm (Figure 2a). This growth was associated to a decrease of the intensity of the initial plasmon resonance band. Such a new absorption band corresponds to a red shift of the plasmon resonance band and should be related to the aggregation of some AuNps in the solution. Theoretical and experimental studies have previously described the displacement of the plasmon resonance band when gold nanoparticles spacing is decreased.36 TEM experiments confirmed this analysis (Figure 2b). Indeed, large clusters of AuNps were observed in
∆λ indicates the location of the maximum of the emerging absorbance band (λmax) relative to the plasmon band of the initial isolated nanoparticles (λ ) 520 nm). Presumably, a small ∆λ value (i.e., few tens of nanometers) may just correspond to a change in the surroundings of the AuNps or a very slight aggregation. The second typical modification of the UV spectra was a common decrease of the absorbance at all wavelengths: the shape of the spectrum was therefore more or less maintained vs time (see Figure 3). Such an evolution suggested that the aggregation state of the AuNps was not strongly modified in the solution, but that a decantation process occurred, decreasing the number of nanoparticles (aggregated or not) in the solution and therefore the absorbance. To follow this kind of behavior, we defined the D parameter by
D ) 1 - ((A520 + 0.216Amax)/A520 (t ) 0)) where A520 and Amax were the absorbances, respectively, at 520 nm and at λmax, and A520 (t ) 0) the initial absorbance at 520 nm of the pristine AuNps solution. The 0.216 constant was empirically defined to take into account the variation of the molar extinction coefficient between the two bands at 520 nm and at λmax. As shown below, D values were found close to unity when the AuNp precipitation was quantitative. On the contrary, small values of D could correspond just to minor changes in the absorption spectrum (due to aggregation for example) or to a slight decantation process (see below). Keeping in mind the previous limitations of these rough but simple parameters, we may assume that D characterizes the quantity of precipitated nanoparticles, whereas ∆λ allows one to follow the aggregation state of the AuNps in the solution. Influence of the Hydrophilic Blocks (PEO). The influence of the length of the PEO blocks upon stabilization of the nanoparticles was investigated using two series of EO-POEO copolymers, EO1 and EO2, each having a constant number of PO units and an increasing number of EO units. The evolution vs time of the absorption spectra for the polymers of the series EO1 (with a polymer concentration of 0.45 mmol L-1, this value was arbitrarily chosen close to 1/20 of the lowest cmc of the polymers of the series) are reported in Figure 4. In this series, the number of PO units was kept constant (at ∼34 ( 6) and the number of EO units goes from 12 to 206 in the order L62 < P65 < P85 < F68 < F88. Except for F88, a new absorbance band appeared at values higher than 520 nm, indicating that
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Figure 5. Time evolution of the parameters ∆λ (a) and D (b) for selected EO-PO-EO polymers from the series EO1 at a polymer concentration of 0.45 mmol L-1: (9) P65, (0) F68, (b) F88. Full lines correspond to an exponential fit of the experimental data.
In the case of the three previous polymers, the characteristic time τ∆λ was estimated to be around 2-5 h. In all the series of polymers, a similar time scale was generally found for the aggregation process (a couple of experimental conditions led to a higher time scale, see Supporting Information). In most cases, no systematic evolution of τ∆λ vs the experimental parameters (such as the number of PO or EO units in the polymer structure) could be extracted from the available data. Figure 5b displays the kinetic evolution of the precipitation parameter D. Those experimental data were also satisfactorily fitted by an empirical monoexponential function of time
D ) Dmax(1 - exp(-t/τD)) Figure 4. Time evolution of the optical absorption spectra of AuNp suspensions containing EO-PO-EO triblock copolymer for the series EO1. Arrows are oriented from the first to the last measurements, respectively, 9 min and 2 days after salt addition. Polymer concentration, 0.45 mmol L-1; [NaCl] ) 0.9 mol L-1.
nanoparticles did aggregate. This process seemed to be fast in the case of P65 and L62 (few minutes after addition of NaCl, the shapes of the UV-visible spectra of the corresponding solutions were constant), when few hours were needed for F68 and P85 to reach a stable aggregation state. Also for all the copolymers except for F88, the spectra exhibited an overall decrease on the long term, which demonstrated that AuNPs were precipitating. Time scales of the precipitation process were not the same for all EO-PO-EO polymers. With P65 and L62, all the gold nanoparticles precipitated after a couple of hours. For F68 and P85, this decantation needed tens of hours to be quantitative. Finally, in the case of F88, only a slight decrease of the plasmon band at 520 nm was observed with the growing of a slight shoulder around 620 nm. The values of ∆λ and D were extracted from all spectra and plotted as a function of time. A typical example of those curves is given for polymers F88, F68, and P65 (series EO1) in Figure 5. With concern for the aggregation parameter, a sharp increase of ∆λ was observed within the first 5 h after salt addition followed by a plateau, whatever the copolymer may be. The value corresponding to the plateau was found to increase as the length of the hydrophilic block decreased. For P65, which is the triblock copolymer with a short EO block length, we were unable to estimate ∆λ after 5 h because the precipitation was quantitative. The experimental kinetic data were satisfactorily fitted by a monoexponential equation (see Figure 5)
∆λ ) ∆λmax(1 - exp(-t/τ∆λ)) where ∆λmax is the plateau value and τ∆λ is a characteristic time describing the aggregation kinetics.
where Dmax represents the plateau value and τD is a characteristic time describing the decantation kinetics. Clearly, the fraction of precipitated nanoparticles increased with decreasing the length of the hydrophilic block in the previous example. The Dmax values extracted from the fit of the curves in Figure 5 were found around 1, 0.86, and 0.15. Therefore, we can conclude that with P65, a quantitative precipitation quickly took place, whereas for F68 precipitation was not total and clearly slower. Finally, the low value of D found for F88 should indicate the lack of precipitation (the corresponding τD value is therefore meaningless). The τD values obtained for P65 and F68 are 2 and 13 h, respectively. Again, no systematic evolution of τD vs structural parameters can be extracted from the available data. However, τD values were found always within the same order of magnitude or larger than τ∆λ (see Supporting Information) when clear aggregation and decantation processes were detected in the samples. Such a correlation suggests that nonaggregated nanoparticles do not precipitateswhich is an expected behaviorsbut also that aggregation can occur without being followed by precipitation when the interparticle distance within the aggregate remains large enough (characterized by ∆λmax < 170 nm) within the aggregates. A decrease of this interparticle distance (∆λmax > 170 nm)spossibly by coalescencesleads to the precipitation of the aggregates. For EO1 and EO2 series, ∆λmax and Dmax values extracted from the absorption spectra are reported in Figure 6. Within both series, ∆λmax decreased when the number of EO units in the hydrophilic blocks increased, showing that longer hydrophilic blocks increased the colloidal stability of Pluronics/AuNps suspensions (Figure 6a). In addition, whatever the length of the hydrophilic group may be, the values of ∆λmax for copolymers belonging to the EO2 series remained significantly lower than for those belonging to EO1. This demonstrates a strong effect of the length of the hydrophobic block to limit aggregation. Note that increasing the concentration has just a small effect
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Figure 6. Effect of the length of the hydrophilic groups, expressed in terms of number of EO units, (a) on the maximal value ∆λmax of the aggregation parameter and (b) on the maximal value Dmax of the decantation parameter. Series of polymers EO1, (9) 0.45 mmol L-1; series EO2, (0) 0.02 mmol L-1, (O) 0.04 mmol L-1, (+) 0.1 mmol L-1.
upon the value of ∆λmax. Indeed, the concentrations used in EO2 led to the best stabilizing effect, although they were lower (e0.1 mmol L-1) than in EO1 (0.45 mmol L-1). In addition, the concentration of the samples in EO2 was varied from 0.02 mmol L-1 to 0.1 mmol L-1, with very little effect on the variation of ∆λmax. For all polymers and concentrations in these series, the decantation parameter D took either a value above 0.8, indicating that most nanoparticles precipitated, or below 0.2 corresponding to no or very few precipitation (Figure 6b). It appears that the variation of the length of the hydrophilic block (in the range ca. 10-200) has almost no influence upon the precipitation behavior. Indeed, except for the largest polymer within series EO1 (i.e., F88), increasing the number of EO units did not modify strongly the Dmax value. Conversely the influence of number of PO units seemed important, since all samples in series EO1 exhibited almost full precipitation, with again the exception of F88 having the longest hydrophilic blocks, whereas in series EO2 all samples showed very little precipitation. As for aggregation, concentration in the range 0.02-0.45 mmol L-1 has only a small effect upon decantation. Influence of the Hydrophobic Block (PPO). The influence of the length of the PPO block upon stabilization of the nanoparticles was more deeply investigated using the two series of Pluronics, PO1 and PO2. The length of the hydrophilic blocks of these polymers are constant within both series and respectively made of 19 and 97 ( 6 EO units for PO1 and PO2 series. The time evolution of the absorption spectra corresponding to these polymer series are given in Figure 7. Excepted for P123 in the series PO1 and F127 in the series PO2, a significant second plasmon band appeared in all solutions, as well as a significant decrease of the overall spectra with time. ∆λmax and Dmax values extracted from the absorption spectra are reported in Figure 8. For hydrophobic blocks shorter than 50 PO units, ∆λmax values remained close to 300 nm, suggesting a strong aggregation, whereas increasing the length of the hydrophobic block led to a sharp decrease of ∆λmax. In general, ∆λmax and Dmax values obtained in both series superimposed on the same curve, demonstrating that the length of the hydrophobic block is one of the main parameters controlling aggregation. It appears that at 0.04 mmol L-1, all polymers with less than 50 PO units exhibited full aggregation whereas it was totally inhibited by longer hydrophobic groups. Even if the PPO block has clearly a stronger effect (compared to the PEO blocks) on the AuNps stability, one should keep in mind that the copolymers stabilized the nanoparticles through steric repulsion. Therefore, the overall length of the triblock polymers had an influence on the nanoparticle stability. As an example, the values of ∆λmax obtained for the series M1 at
Figure 7. Time evolution of the of AuNp suspensions containing EOPO-EO triblock copolymers for series PO1 and PO2. Arrows are oriented from the first to the last measurements, respectively, ∼7 min and 1 day after salt addition. Polymer concentration: 0.04 mmol L-1. [NaCl] ) 0.9 mol L-1.
Figure 8. Effect of the length of the hydrophobic group, expressed in terms of number of PO units, (a) on the maximal value ∆λmax of the aggregation parameter and (b) on the maximum value Dmax of the precipitation parameter. (9) Series PO1; (O) Series PO2. Polymer concentration: 0.04 mmol L-1.
0.07 and 0.42 mmol L-1 and M2 at 0.3 mmol L-1 are reported in Figure 9. Whatever the nature and the concentration of the polymer may be, the displacement of the absorption band (or the Dmax value, data not shown) decreased when the length of the polymer increased. In this case as well as in the previous ones, the concentration had little effect on the final aggregation and precipitation states of the AuNps. This suggested that, in these concentration conditions, there was a sufficient amount of polymer in the solution to ensure that the gold nanoparticles were in equilibrium and covered (more or less) by the copolymers. In other words, the change in stability and aggregation state described above should not be due to a lack of polymers within the solution. Various hypotheses can be proposed to explain the high influence of the hydrophobic block PPO on the stabilization of AuNps. The nanoparticles surface could interact more strongly with the PPO block than with the PEO block. However, the
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Figure 9. Effect of the overall length of the triblock copolymer, expressed in terms of monomer units on the maximal value of the aggregation parameter (∆λmax). Series M1, (O) 0.07 mmol L-1, (+) 0.42 mmol L-1; series M2, (9) 0.3 mmol L-1.
Figure 10. Effect of the polymer concentration over the characteristic times of aggregation τ∆λ (b) and decantation τD (O) for polymer F68.
gold nanoparticles obtained by citrate reduction are negatively charged (and therefore highly polar), so that it would be difficult to understand the nature of such interactions between hydrophobic groups and the hydrophilic surface. A more likely hypothesis is that the hydrophobic central block of the Pluronics plays a stabilizing role in the layer of polymers surrounding the nanoparticles, through Van der Waals interaction, just as it is the case in the formation of micelles.23 Therefore, longer PPO blocks would induce an increased stability of the polymer layer and so a better stabilization of the nanoparticles. Also, one can expect that an all hydrophilic polymer, such as PEG, adsorbed at the surface of the nanoparticle would adopt a rather globular conformation. In contrast, amphiphilic triblock copolymers coating the nanoparticles might adopt a more stretched conformation due to self-exclusion between the hydrophilic and hydrophobic parts of the polymers. Such a self-exclusion may increase the average thickness of the polymer layer around the AuNps, enhancing therefore the steric repulsion between nanoparticles and therefore their stabilities. Influence of the Concentration. In the previous experiments, concentration has little effect upon the aggregation and decantation parameters. However, the range of concentrations studied was limited, from 0.02 to 0.1 mmol L-1 for series EO2 and from 0.07 to 0.42 mmol L-1 for series M1. To gain a more comprehensive insight upon the effect of concentration, we
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Figure 11. Effect of the polymer concentration over the maximal value of the aggregation parameter (∆λmax) for polymer F68. The full line is a linear fit of the data (in semilogarithmic representation).
Figure 12. Correlation between the values of Dmax and ∆λmax for all polymers and conditions used in this study. The dashed line materializes the value ∆λmax ) 170 nm.
investigated the stabilization of nanoparticles in a larger range of concentrations. We used F68 which among all polymers used in this work is of intermediate molecular weight (Mw ) 8400 g mol-1) and possesses a relatively high cmc value (18 mmol L-1), and investigated various concentrations from 0.02 to 2.8 mmol L-1. As previously, the absorption spectra of the F68/AuNps suspensions were recorded and the values of ∆λ and D were extracted from the spectra and fitted by a monoexponential equation. Interestingly, although no systematic evolution of τ∆λ and τD could be determined by varying the number of PO or of EO units within the polymer structure, a clear evolution is observable for both kinetic parameters in function of the concentration (represented over a logarithmic scale Figure 10). Over ca. 1 mmol L-1, aggregation and decantation became very weak so that the corresponding characteristic times could not be determined. Below this limit, aggregation and decantation time scales were within the same order of magnitude, consistently with the cases of the other polymers. The increase of the characteristic times with increasing the polymer concentration is a clear evidence of the stabilizing effect of the polymer. The ∆λmax data values are reported Figure 11, again using a logarithmic scale, and fitted by a linear equation. In the whole concentration range, ∆λmax decreased when the concentration
Stabilization in Water of Au Nanoparticles increased, showing that increasing the polymer concentration has an increasing stabilization efficiency. At low concentrations the ∆λmax values seems to tend toward a maximal value of ca. 285 nm. In addition, it appears that, at high values, there should be a concentration (approximately 3 mmol L-1) above which no noticeable aggregation occurs. Concerning decantation, it was again observed that the nanoparticles either fully precipitates for concentrations C < 0.5 mmol L-1sor remains almost totally in solution beyond this limit (data not shown). The EO-PO-EO/AuNp system present a last interesting feature. Indeed, examination of the aggregation and decantation parameters revealed a strong correlation between the values of ∆λmax and Dmax. This is illustrated by Figure 12, where the results for all samples used in this study have been gathered. Clearly, decantation becomes significant only when ∆λmax overcomes 170 nm. This result suggests that the AuNp aggregates can be more or less dense, due to a more or less compact structure or to coalescence of the AuNps within the aggregates, and only those which are dense enough precipitate. Conclusions The effect of EO-PO-EO block copolymers on the stability of gold nanoparticles has been clearly demonstrated using nanoparticles formed and characterized before adding the copolymers. Analysis of the UV-visible spectra of the copolymer/ gold nanoparticles dispersions allowed one to describe the aggregation and the decantation of the nanoparticles separately, through two parameters. Decantation process is strongly related to the aggregation process and occurs on a time scale equivalent or larger than the characteristic time of the aggregation. The stabilization of the gold nanoparticles by the copolymers occurs below the cmc. It depends on the length of the copolymers, the most important factor being the length of the hydrophobic segment. As a result, F127 appears to be the most efficient toward colloidal stabilization, since at the lowest concentration studied (0.02 mmol L-1) the aggregation remains at a very low level (∆λmax < 75 nm) and decantation was not observed. Since the interactions between the polymers and the nanoparticules are weak physical interactions, we believe that these results may directly apply to other types of nanoparticles. Work is in progress in order to characterize the organization of the polymer at the surface of the gold nanoparticles. Acknowledgment. The authors whish to thank the French Ministry of Research for funding through an ACI “Jeune Chercheur”, and BASF for kindly providing some of the Pluronics. Supporting Information Available: Table of the values of the aggregation and decantation parameters ∆λmax, τ∆λ, D, and τD extracted from the fits of the experimental data for all polymers and concentrations used in this study, with an estimation of the corresponding experimental errors. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Zhang, L. X.; Sun, X. P.; Song, Y. H.; Jiang, X.; Dong, S. J.; Wang, E. A. Langmuir 2006, 22, 2838-2843.
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