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Role of Salt in the Spontaneous Assembly of Charged Gold Nanoparticles in Ethanol Xiaogang Han, James Goebl, Zhenda Lu, and Yadong Yin* Department of Chemistry, University of California, Riverside, California 92521, United States ABSTRACT: This paper investigates the role of salt in the spontaneous linear assembly of charged gold nanoparticles in ethanol and attempts to clear up a misunderstanding on the role of ethanol in this process. Many prior reports have noted that the addition of ethanol to an aqueous solution of gold nanoparticles causes their aggregation into linear assemblies. It was therefore believed that ethanol plays the determining role during the assembly process. In this work, we carried out systematic studies which indicate that residual salt in conjunction with ethanol, instead of ethanol itself, induces the assembly of gold nanoparticles in ethanol. In the absence of salt, gold nanoparticles can be well dispersed in an ethanol solution. Furthermore, we find that the chainlike assemblies can disassemble upon dilution of the salt or the evaporation of ethanol if the gold nanoparticles are protected with a sufficiently strong ligand.
’ INTRODUCTION Low-dimensional gold nanoparticle (AuNP) assemblies have recently attracted considerable attention because of their potential applications in optical, electronic, and sensor components.110 One of their main attributes lies in the electronic coupling between adjacent particles which leads to many new properties. For example, this coupling effect has been widely regarded as the cause of hot spots that can dramatically increase efficiency in surface enhanced Raman scattering (SERS).1114 Although twoand three-dimensional assemblies of AuNPs have been produced and studied by many groups during the past two decades,1519 one-dimensional (1D) assemblies have only been reported very recently due to the difficulties arising from their preparation, which stem from the perception of the isotropic structure and spherical morphology of AuNPs.20,21 Besides the use of crosslinking molecules, e.g., mercaptoethanol,22 1,9-nonanedithiol,23 specific proteins, and oligodeoxynucleotides,24 linear assembly of AuNPs has most often been induced by the addition of destabilizing species such as salt to the nanoparticle aqueous dispersion.25,26 It is believed that the 1D assembly of AuNPs in aqueous solution is triggered by the combined effect of shortrange anisotropic dipoledipole attractions and the long-range electrostatic charge repulsion between particles.25,26 Many groups have also reported that similar linear assemblies formed during the addition of ethanol and other polar organic solvents to aqueous AuNP solutions.25,2730 Gu and co-workers, for example, reported that AuNPs spontaneously organize into linear assemblies after being transferred from a postreaction aqueous solution into ethanol after centrifugation.27,30 While the assembly mechanism of AuNPs in water has been well r 2011 American Chemical Society
studied,25,31 the linear assembly in ethanol was either not clearly explained or it was solely attributed to ethanol. In one of the proposed mechanisms, it was believed that the low polarity of the solvent causes an asymmetrical distribution of dipole moment in the particles and subsequent directional assembly.27,29 However, such a mechanism is not consistent with some experimental observations. For example, in the above example reported by Gu et al., a clear trend was found in which shorter AuNP chains resulted when an increasing amount of ethanol was added,27 which has been confirmed in our laboratory. This result contradicts the expectation that the assembled chains would grow longer in the presence of more ethanol. In this paper, we systematically investigate the assembly of AuNPs in ethanol and conclude that the formation of linear aggregations can be mainly attributed to the presence of salt, which is often inadvertently brought into the system from the residual supernatant of centrifuged samples. Ethanol promotes nanoparticle aggregation when the system also contains salt. However, AuNPs can disperse well in a pure ethanol solution. This understanding allows us to clarify the phenomena observed but unexplained or misinterpreted previously.27 In addition, we report that when the AuNPs are capped with sufficiently strong ligands, the assembly process is reversible: the linear chain structures form upon the addition of salt, and dissociate after the dilution of salt or the evaporation of ethanol. We expect that Received: February 3, 2011 Revised: March 23, 2011 Published: April 05, 2011 5282
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this work can help to further elucidate the mechanism of the 1D assembly of charged nanoparticles in nonaqueous polar solvents.
’ EXPERIMENTAL SECTION Chemicals. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 3 3H2O, 99.9þ%) was purchased from Acros Organics, ammonium hydroxide aqueous solution (28%) from Fluka, ethanol (200 proof) from Gold Shield, and tannic acid, sodium citrate tribasic dihydrate (99%), bis (psulfonatophenyl) phenylphosphine dihydrate dipotassium salt (BSPP, 97%), tetraethyl orthosilicate (TEOS, 98%) from Sigma-Aldrich. Other chemicals used here were purchased from Fisher Scientific. All chemicals were used without further purification. All solutions were prepared in deionized water (DI water, 18 MΩ-cm) from a Milli-Q water purification system. Preparation of AuNPs and Ligand Exchange. AuNPs were prepared by following published procedures.27,32 Briefly, a 20 mL aqueous solution containing 4 mL of 1% (w/w) trisodium citrate and 0.08 mL of 1% (w/w) tannic acid was rapidly added to an 80 mL aqueous solution containing 1 mL of 1% (w/w) hydrogen tetrachloroaurate(III) trihydrate after all were heated to 60 °C. Then the mixed solution was boiled for about 10 min under magnetic stirring. The solution was subsequently cooled to room temperature (RT) with chilled water. The average diameter of as-synthesized AuNPs was ca. 10 nm based on transmission electron microscopy (TEM) measurements. A ligand exchange process was performed to enhance the stability of the assynthesized AuNPs. In brief, the AuNPs solution was mixed with an excess quantity of BSPP (with a final concentration of 0.2 mg/mL),33,34 and then shaken overnight. The resulting negatively charged AuNPs are denoted as AuNP/BSPP. The samples of AuNPs and AuNP/BSPP were kept at RT, and purified before use. The particle concentrations were quantified according to their extinction coefficient of 1.05 108 M1 3 cm1 at 520 nm. Assembly in Ethanol. In a typical process, 1.0 mL AuNP solution was first washed carefully with water after centrifugation at 10 000 rpm (12 745g) for 6 min, then to the precipitate (ca. 1 μL) was added 10 μL of 4.0 mM NaCl aqueous solution before the addition of ethanol (100 μL), and finally the mixture was allowed to sit for 30 min. The final solution contains AuNP assemblies, ∼ 0.4 mM NaCl, and 90% (v/v) ethanol. Characterization. UVvisible (UVvis) spectra were collected using a Varian Cary 50 UVvis spectrophotometer with cuvettes of 1 or 0.5 cm path length. TEM imaging was performed on a Philips TECNAI 12 operated at 120 kV. ζ-Potential measurements were recorded on a Brookhaven Instruments Zeta Potential Analyzer at a scattering angle of 90° and a temperature of 25 °C.
’ RESULTS AND DISCUSSION The Role of Salt. The surface of the as-synthesized AuNPs is covered primarily with citrate ligands and a small amount of tannic acid. For simplicity, we denote such particles AuNP/ citrate. Consistent with observations reported in the literature, direct addition of ethanol to the aqueous reaction solution may cause aggregation of AuNPs. To investigate the factors behind the assembly of charged AuNPs in ethanol, we began with a typical assembly experiment. The AuNP solution (1.0 mL) was centrifuged after being synthesized in the water phase, as much of its supernatant was removed as possible, and the remaining AuNP precipitation (ca. 1.0 μL) was transferred to ethanol (100 μL). During that process, the solution color changed gradually from red to bluish purple and finally blue in about 6 h (inset in Figure 1a), indicating the aggregation of the AuNPs. This result is
Figure 1. Normalized UVvis extinction spectra of dispersions of centrifuged AuNP precipitation in EtOH (100 μL) with addition of (a) different volumes of the supernatant of the original synthesis solution (0, 2, 5, 10 μL); (b) DI water (10 μL), supernatant of synthesis solution (0 and 10 μL), and NaCl aqueous solution (10 μL, final concentration of 0.4 mM); (c) nothing, but the AuNP centrifuged precipitations were pretreated with (green) or without (red) water washing before adding EtOH. The AuNPs in panels a and b were not washed with water before adding EtOH. The spectrum of control samples dispersed in water is also presented in the panels for comparison. The inset in panel a is a digital photo of the EtOH solution of AuNPs showing color change during the salt-induced assembly process. All the samples here were incubated for 30 min at RT before their spectra were recorded.
in agreement with observations in the literature.27 Note that the AuNP precipitation was not extensively washed with water before being transferred to ethanol, implying that some supernatant (ca. 1.0 μL) may be left along with the AuNPs after centrifugation. It is therefore reasonable to suspect that in addition to ethanol, perhaps the residual salt is responsible for the aggregation. 5283
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Figure 2. TEM images of AuNPs incubated in ethanol without (a) and with (b) 0.4 mM NaCl added at RT for 30 min.
To test this hypothesis, we purposely injected different volumes of additional supernatant (0, 2, 5, 10 μL) from the original synthesis solution into the precipitations before the addition of ethanol, and then recorded their UVvis spectra after incubation for 30 min at RT. The results shown in Figure 1a reveal that the extinction spectrum of AuNPs red-shifts with increasing amounts of added supernatant, indicating that the supernatant affects the aggregation of AuNPs in ethanol. In particular, the addition of 10 μL of supernatant solution leads to a very quick color transition from red to blue which can complete within 30 min. The aqueous supernatant primarily contains sodium ions (from trisodium citrate) and chloride ions (from HAuCl4). Thus it is likely that sodium chloride in the supernatant could be directly responsible for the aggregation of AuNPs. To verify this speculation, we designed two AuNP assembly tests in which different “additives” were injected. Instead of 10 μL of supernatant, identical volumes of aqueous NaCl (4.0 mM) and DI water were added to the centrifuged AuNPs while controlling other conditions. The NaCl solution was used to imitate the supernatant, while the 10 μL of DI water was used as a NaCl-free control. If our speculation above is true, the sample with the NaCl added should display the same plasmon peak shift as if actual supernatant was added since both contain similar concentrations of NaCl, while the sample with only DI water added should exhibit only a minimal change in extinction. As shown in Figure 1b, the results are nearly in accordance with our prediction, confirming the hypothesis that the residual salt may contribute to AuNP assembly in the ethanol solution. It is noteworthy that while the spectrum of the sample with only DI water added is noticeably different from that of the sample without any “additive” (red line in Figure 1a,b), it is nearly identical to the spectrum of AuNPs dispersed in pure DI water (black dashed line in Figure 1b). We believe this difference is caused by the presence of a small amount of salt in the former two samples, with a concentration of ∼0.04 mM estimated by assuming 1.0 μL of the original reaction solution is left in the precipitation. It is very possible that when the extra DI water is added, the overall salt concentration deceases, reducing the degree of AuNP aggregation. In principle, the salt effect can be confirmed if we can completely remove the residual salt solution from the centrifuged precipitates and then test the dispersibility of the nanoparticles in ethanol. However, simply drying the precipitates cannot remove the salt, and it also creates irreversible aggregates that do not disperse even in DI water. To solve this problem, we removed as much residual salt as possible from the AuNP precipitation by careful washing with DI water and centrifugation before
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performing the assembly in ethanol.35 The UVvis record of the final assemblies shown in Figure 1c displays a spectrum with a profile (green solid line) almost identical to that of AuNPs dispersed in water (black dashed line), indicating that AuNPs cannot assemble when there is insufficient salt present. Note from Figure 1c that the plasmon peak of AuNPs in ethanol is positioned at ∼520 nm, which is a slight deviation from the approximate value of 516 nm calculated according to the Mie equation.36 This is mainly ascribed to the adsorption of ligands and solvent molecules on the AuNP surfaces. Similar to those in previous literature,37 the linear assemblies exhibit a red-shift of their plasmon bands in extinction spectra (Figure 1a,b). Besides the original plasmon peak at 520 nm, a new feature above 600 nm evolves into a notable band upon increasing salt concentration (Figure 1a), which represents a 1D longitudinal plasmon coupling between AuNPs, revealing the chain structure of the assemblies.20 TEM observation confirms the chainlike interparticle connections. In consideration of possible aggregation due to the drying process during sampling, we examined TEM images for AuNPs dispersed in ethanol with 1% (v/v) water as a control sample undergoing the same TEM sampling procedure. The images in Figure 2 clearly confirm that the chainlike assemblies were formed in solution before undergoing the drying process. To further support the hypothesis that the presence of salt plays an important role in the assembly of AuNPs in ethanol, we carried out systematic studies by observing the assembly behavior in the presence of different concentrations of NaCl. A series of NaCl solutions with final concentrations ranging from 0.1 to 0.4 mM were injected into a mixed solvent of ethanol/water (9:1, v/v) containing cleaned AuNPs (final concentration 24 nM). Figure 3a shows that with increasing NaCl concentration, the intensity of the initial band at 520 nm decreases and a new band develops with enhancing intensity and a gradual red-shift in band position. We plot the position of the second plasmon band against the concentration of NaCl in Figure 3b, which reveals a positive dependence with a gradually decreasing slope at higher concentrations. Concentration of AuNPs. Next, to find other important factors for the spontaneous assembly of AuNPs in ethanol, we examined the effects of the AuNP concentration and the ratio of ethanol to water on assembly in the presence and absence of salt. Figure 4 shows UVvis spectra illustrating the effect of varying particle concentration. In the series of samples containing no salt with different concentrations of AuNPs (0.5, 1.0, 2.0, 3.0 C, C = 24 nM), the extinction spectra in Figure 4a reveal that none of the samples assembled at all, even after incubating overnight. For easier comparison, normalized results are inset in Figure 4a, which unambiguously shows that the spectra are identical with a sole peak at 520 nm (also marked with a dashed line in Figure 4a) for all the samples without salt added. When NaCl (final concentration 0.4 mM) was added, aggregation occurred in each of the above samples within several minutes. Figure 4b contains the UVvis results obtained 20 min after the salt addition. The spectrum of every sample has a dramatic red shift compared with its counterpart without salt shown in Figure 4a, indicating that the addition of salt triggered the change of AuNPs in ethanol regardless of AuNPs concentration. Hence, at least in the test range, the AuNP concentration has no significant effect on the assembly if no salt is involved. However, when there is enough salt in the system, higher concentrations of AuNPs lead to more extensive assembly. As can be seen in Figure 4b, the second peak 5284
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Figure 3. (a) Normalized UVvis spectra of the AuNP assemblies produced by adding different concentrations of NaCl salt to the ethanol dispersion of nanoparticles. All the samples were incubated for 50 min. (b) Plots of the second peak position as a function of the concentration of NaCl.
red-shifts gradually from 610 to 633 nm with increasing AuNP concentration (marked with dashed lines). Figure 4c indicates that a high concentration of AuNPs enables a quicker start of assembly (especially in the first 10 min) while decreasing the time required to reach its end-point. Particularly, for samples with lower concentrations of 1.0 or 0.5 C, the reaction is so slow that the second peak does not appear until 5 min after mixing (Figure 4c). Note for the UVvis measurements here, we used cuvettes with a 0.5 cm optical path length to avoid signal distortion due to the high concentration of the solution. Ethanol-to-Water Ratio. While examining the effects of the ethanol-to-water ratio, we obtained similar results. Figure 5a shows that if no NaCl is added, no noticeable changes of optical extinction occur for samples in an ethanolwater solvent system with the ethanol fraction ranging from 0 to 0.99. Only after the addition of a certain amount of NaCl will the AuNPs spontaneously assemble, with the extinction spectrum red-shifting in 10 min (Figure 5bd). In Figure 5b it can be seen that for samples with an ethanol fraction higher than 0.8, notable redshifts emerge after the addition of 0.3 mM NaCl, while for the samples with lower ethanol fractions from 0.8 to 0.1, no perceptible changes occur in their spectra. Further increasing the NaCl concentration to 1.0 mM and 12.0 mM causes AuNPs to assemble, and Figures 5c and 5d display the corresponding red-shift of their spectra. For the sample without ethanol added (its fraction is 0), much more NaCl (40 mM) was necessary to induce the assembly because of the higher solvation power of
Figure 4. (a,b) UVvis extinction spectra of different concentrations of AuNPs incubated in ethanol without (a) and with (b) addition of 0.4 mM NaCl. (c) The evolution of second peak position as a function of assembly time for different concentrations of the AuNPs incubated in ethanol with 0.4 mM NaCl. Inset in panel a is a normalized result of the spectra in panel a. The concentration “C” in all the panels here refers to 24 nM.
pure water to ions. Regardless of the ratio of ethanol to water (from 0 to ca. 1), AuNP assembly could not be triggered until there was enough salt in the system. If no salt is dissolved, a mixed solvent with ethanol fractions ranging from 0 to 0.99 allows good dispersion of charged AuNPs, as opposed to the aggregation commonly observed for solutions of AuNPs and ethanol. Rather, the effect of ethanol in this process seems to be an enhancement of the sensitivity of the aggregation to salt, i.e., if more ethanol is present in the mixed solvent, less salt is needed to induce assembly. This can be seen from Figure 5e, which depicts the relation between the NaCl concentration and the ethanol fraction at which the AuNP solutions start to turn purple/blue within 10 min. The zone above the line linking the data points in Figure 5e is termed the “aggregation zone” because AuNPs under the conditions within this zone become aggregated; conversely, the area below this line is called the “dispersion zone.” 5285
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Figure 5. (ad) UVvis extinction spectra of AuNPs in the mixed solvent of ethanolwater at different volume ratios after incubation without (a) and with (bd) NaCl of various concentrations (0.3, 1.0, and 12.0 mM) at RT. The samples in panel a were incubated overnight, those in bd were incubated for 30 min. (e) The dependence of dispersion/aggregation states on the concentration of NaCl and volume fraction of ethanol in the ethanol/ water mixed solvents. The dispersion/aggregation transition points were determined by the solution color turning purple in 10 min. The data were obtained from three parallel tests.
It is now safe to conclude that the concentration of dissolved salt, instead of the AuNP concentration or the ethanol-to-water ratio, plays the dominant role in the assembly of AuNPs; the latter can influence the assembly only with sufficient salt involved in the system. In this context, the phenomenon described by Gu et al. can be clearly explained. When the AuNPs precipitation was not washed before being transferred into ethanol, the residual salt would trigger the assembly. Figure 6a shows the extinction spectra of dispersions prepared by adding various amounts of ethanol to the same amount of AuNP precipitations. The more ethanol is added, the lower the degree of nanoparticle aggregation (shorter chain structures) and the less the spectrum red-shifts. This seems to contradict the above conclusion that ethanol has the ability to enhance the nanoparticle aggregation. Here we want to point out that this conclusion is valid when we compare samples containing the same amount of salt but different volumes of ethanol. Adding more ethanol to a fixed
sample will dilute the concentrations of both salt and AuNPs, actually leading to a weaker tendency toward aggregation. Control experiments with washed AuNPs did not indicate the occurrence of assembly upon the addition of different amounts of ethanol, as shown in Figure 6b, which is consistent with our prior observations. Mechanism of Linear Assembly. Consistent with the case in the aqueous phase, the aggregation of charged AuNPs in ethanol is also caused by a reduction of electrostatic force due to the addition of ionic species. AuNPs can be dispersed in both ethanol and water as long as there is sufficient surface protection from the capping ligands. Because the solvation power of ethanol is relatively lower than that of water, the thinner electrical double layer makes the colloidal nanoparticles less stable against the increase of ionic strength in the system. In fact, we measured the ζ-potential of AuNPs in EtOH-H2O (100:1, v/v) to be 30 to 33 mV at 25 °C, which is smaller than the value in DI water 5286
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Figure 6. UVvis extinction spectra of the AuNP solutions formed by adding different volumes of EtOH (200, 500, 1000 μL) to the same amount of centrifuged precipitate of AuNPs (1.0 μL) without (a) and with (b) water washing. The incubation time was 30 min at RT.
(50 mV), supporting the conclusion that the nanoparticle dispersions are stable in such mixed solvents but have a higher susceptibility to the effects of the addition of salt than purely aqueous solutions. Since the assembly is due to high ionic strength, which is not an exclusive property of NaCl, it is expected that other salts would be able to cause similar aggregation. We therefore tested several other salts, such as NaNO3, NH4Cl, MgCl2, and AlCl3, all of which showed the ability to induce spontaneous aggregation of AuNPs in ethanol solutions (data not shown). It is noted that salts with high valence ions, such as Mg2þ, are more effective at inducing AuNP aggregation in ethanol than monovalent salts. This is in agreement with observations made in water.31 When the added salt reaches a critical concentration in the system, the nanoparticles become so unstable that they are apt to aggregate due to the reduced electrostatic repulsion. In this process it makes sense to first form dimers as triggered by the short-range dipolar interactions, and then grow into larger aggregates by adding more AuNPs through collision. In their studies on aqueous systems, Wang et al. pointed out that additional particles are added to the ends to form chains as driven by the long-range electrostatic repulsion forces,25,38 because a particle approaching from the side of the dimer would undergo stronger electrostatic repulsion than one colliding at the ends.26 We believe that this mechanism is also responsible for the linear assembly in systems containing ethanol. As the size of the assemblies enlarges, the electrostatic repulsion between them increases rapidly, resulting in a decrease of the chance for an
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Figure 7. (a) UVvis extinction spectra of AuNP/BSPP dispersed in ethanol recorded after 1 day, 2 days, 1 week, and 4 weeks. (b) UVvis extinction spectra of AuNP/BSPP dispersed in various polar solvents recorded after 2 days. The dashed lines denoting 520 nm of wavelength are used to facilitate the comparison of peak positions. No salt is added for all the samples here.
end-to-end collision. As a result, the growth tends to cease at a certain size depending on the concentration of salt (Figure 3a,b). Effects of Capping Ligands and Solvents. The capping ligand contributes significantly to the stability of the AuNPs. In addition to citrate, we tested BSPP as another type of ligand commonly applied for aqueous AuNPs.24 Citrate was linked to the particle surface directly during synthesis, while BSPP was introduced through a ligand exchange process. AuNPs capped with both ligands could disperse in ethanol with 1% (v/v) of water and remain stable for a long time. Figure 7a indicates that AuNP/BSPP can maintain a good dispersion for at least one month. Since citrate is a relatively weak ligand to AuNPs, it can gradually detach from the particle surface after extensive washing and centrifugation steps. The weak ligands can also be partially replaced by the solvent molecules when ethanol is added, leading to reduced stability. BSPP, on the other hand, is a relatively strong ligand which renders AuNPs a high stability in both water and ethanol solutions. The stability and aggregation behavior of AuNPs were also tested in polar solvents other than ethanol, including acetone, acetonitrile, dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). To enable mixing with water at any ratio, the test solvents were chosen due to complete miscibility in water. Figure 7b shows optical extinction spectra of AuNP/BSPP in the polar solvents with 1% (v/v) of water indicating that these solvents have effects similar to ethanol that allow good dispersion 5287
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Figure 9. Dissociation of the AuNP/BSPP assemblies in ethanol (90%, v/v, 10 mM NaCl) through addition of water or volatilization of ethanol at RT. The assembly and dissociation processes are reversible.
Figure 8. Normalized optical extinction spectra of salt-induced AuNP assemblies in ethanol (90%, v/v) before (green) and after (red) dilution of the systems three times with water: (a) AuNP/Citrate; (b) AuNP/ BSPP. The spectra of dispersions without the addition of NaCl were included as black curves for comparison. The incubation time is 30 min at RT.
of AuNPs. The plasmon band of AuNPs in acetonitrile shows an almost identical position to that in water because of their similar refractive indices (1.333 for water and 1.344 for acetonitrile at 20 °C), while a very small red-shift of ∼12 nm can be observed for both acetone and ethanol due to their slightly higher refractive indices (1.359 for acetone and 1.361 for ethanol at 20 °C). For solvents with even higher refractive indices such as DMF and DMSO (1.431 and 1.479 at 20 °C, respectively), 3 and 6 nm red shifts can be clearly noticed. The AuNPs protected by citrate can also be dispersed well in DMF, acetone and acetonitrile, but they aggregate in DMSO, possibly because the weakly bound citrate ligands are stripped from the AuNP surface by DMSO molecules which contain nucleophilic sulfur centers. Reversibility of Linear Assembly. An interesting question arises after concluding the determining effect of the salt concentration in AuNP assembly: can the assemblies dissociate upon dilution of the salt in the solution? We tested separately the assemblies of AuNP/citrate and AuNP/BSPP through addition of water. After diluting the solution three times, the assemblies comprised of AuNP/citrate did not dissociate (Figure 8a), but the AuNP/BSPP assemblies did (Figure 8b). From Figure 8a it can be seen that the normalized extinction spectra are almost identical before (green line) and after (red line) the dilution, suggesting that linear assemblies remain without dissociation. However, in Figure 8b, the spectrum recorded after dilution (red line) blue-shifts back to overlap perfectly with that of the sample
before aggregation (black line), indicating that the assemblies of AuNP/BSPP completely dissociate after dilution. The difference in the reversibility lies in the ligand binding stability. For AuNPs capped with citrate, the ligands are partially detached from the particle surface during assembly so that the nanoparticles in the assemblies are held strongly by the van der Waals force. Reducing the ionic strength by dilution cannot produce enough electrostatic force to counter the van der Waals force, making it difficult to disperse the assemblies. On the other hand, BSPP binds to the Au surface strongly even during the assembly process. The electrosteric force prevents the permanent aggregation of the nanoparticles. Dilution of the solvent reduces the ionic strength of the system and enhances the electrostatic interaction, thereby pushing the nanoparticles away from each other and producing a stable nanoparticle dispersion again. For AuNP/BSPP assemblies, the reverse process can be initiated by not only solvent dilution, but also the volatilization of ethanol from the system. The volatilization increases the concentration of water in solution such that the system can bear much higher ionic strength, and thus the assemblies dissociate. Figure 9 depicts a demonstration of this process. An assembled sample, dark blue in color, was placed in the wells (upper), and became red colored at the moment of addition of some water or evaporation of the proper amount of ethanol at RT (lower). Adding salt or ethanol to the system will cause AuNP assembly and changes the solution color to blue. Such a reversible process that allows assembly and disassembly of AuNP linear assemblies has not been noted previously in the literature.26
’ CONCLUSIONS We conclude that the key factor determining the aggregation of AuNPs in ethanol or ethanol/water mixtures is residual salt instead of ethanol itself. A higher degree of assembly will occur if the AuNP/ethanol mixture contains a higher concentration of salts, which can be brought into the system either intentionally or unintentionally. Because AuNPs solutions often contain some residual salts when they are mixed with ethanol, it has become a common misconception that ethanol can cause the aggregation of AuNPs. When the residual salt is removed by careful washing, AuNPs can disperse well in ethanol. For a sample with fixed amounts of AuNPs and salt, ethanol acts similarly to water and can dilute the solution and reduce the tendency of aggregation. We found that assemblies of AuNPs triggered by salt in an 5288
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Langmuir ethanol/water mixture can dissociate upon dilution of the system with water or evaporation of ethanol, if the nanoparticles are originally protected by a sufficiently strong ligand, such as BSPP. Besides clarification of the misunderstanding of the role of ethanol in AuNP aggregation, we expect our findings can cast some light upon the mechanism of 1D assembly of charged nanoparticles in mixed polar organic solvents.
’ ACKNOWLEDGMENT Y.Y. thanks the US Army Research Office (Grant No. W911NF-10-1-0484), the 3M Nontenured Faculty Grant, and the DuPont Young Professor Grant for support of this research. Y.Y. is a Cottrell Scholar of the Research Corporation for Science Advancement. The use of the Central Facility for Advanced Microscopy and Microanalysis at UCR is acknowledged. ’ REFERENCES (1) Wang, X. J.; Li, G. P.; Chen, T.; Yang, M. X.; Zhang, Z.; Wu, T.; Chen, H. Y. Nano Lett. 2008, 8, 2643. (2) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699. (3) Chen, G.; Wang, Y.; Yang, M. X.; Xu, J.; Goh, S. J.; Pan, M.; Chen, H. Y. J. Am. Chem. Soc. 2010, 132, 3644. (4) Sidhaye, D. S.; Prasad, B. L. V. Chem. Phys. Lett. 2008, 454, 345. (5) Chen, G.; Wang, Y.; Tan, L. H.; Yang, M. X.; Tan, L. S.; Chen, Y.; Chen, H. Y. J. Am. Chem. Soc. 2009, 131, 4218. (6) Ren, S. Q.; Lim, S. K.; Gradecak, S. Chem. Commun. 2010, 46, 6246. (7) Tam, J. M.; Murthy, A. K.; Ingram, D. R.; Nguyen, R.; Sokolov, K. V.; Johnston, K. P. Langmuir 2010, 26, 8988. (8) Wang, M. H.; Li, Y. J.; Xie, Z. X.; Liu, C.; Yeung, E. S. Mater. Chem. Phys. 2010, 119, 153. (9) Xing, S. X.; Tan, L. H.; Yang, M. X.; Pan, M.; Lv, Y. B.; Tang, Q. H.; Yang, Y. H.; Chen, H. Y. J. Mater. Chem. 2009, 19, 3286. (10) Liu, X. F.; He, X. R.; Jiu, T. G.; Yuan, M. J.; Xu, J. L.; Lv, J.; Liu, H. B.; Li, Y. L. ChemPhysChem 2007, 8, 906. (11) Camden, J. P.; Dieringer, J. A.; Wang, Y. M.; Masiello, D. J.; Marks, L. D.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2008, 130, 12616. (12) Porter, M. D.; Lipert, R. J.; Siperko, L. M.; Wang, G.; Narayanan, R. Chem. Soc. Rev. 2008, 37, 1001. (13) Li, T.; Liu, D.; Wang, Z. Biosens. Bioelectron. 2009, 24, 3335. (14) Maher, R. C.; Maier, S. A.; Cohen, L. F.; Koh, L.; Laromaine, A.; Dick, J. A. G.; Stevens, M. M. J. Phys. Chem. C 2010, 114, 7231. (15) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. Rev. Phys. Chem. 1998, 49, 371. (16) Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 2735. (17) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545. (18) Han, X. G.; Li, Y. L.; Wu, S. G.; Deng, Z. X. Small 2008, 4, 326. (19) Sastry, M.; Lala, N.; Patil, V.; Chavan, S. P.; Chittiboyina, A. G. Langmuir 1998, 14, 4138. (20) Pramod, P.; Thomas, K. G. Adv. Mater. 2008, 20, 4300. (21) Tang, Z. Y.; Kotov, N. A. Adv. Mater. 2005, 17, 951. (22) Lin, S.; Li, M.; Dujardin, E.; Girard, C.; Mann, S. Adv. Mater. 2005, 17, 2553. (23) Hussain, I.; Brust, M.; Barauskas, J.; Cooper, A. I. Langmuir 2009, 25, 1934. (24) Coomber, D.; Bartczak, D.; Gerrard, S. R.; Tyas, S.; Kanaras, A. G.; Stulz, E. Langmuir 2010, 26, 13760. (25) Zhang, H.; Wang, D. Y. Angew. Chem., Int. Ed. 2008, 47, 3984. (26) Yang, M.; Chen, G.; Zhao, Y.; Silber, G.; Wang, Y.; Xing, S.; Han, Y.; Chen, H. Phys. Chem. Chem. Phys. 2010, 12, 11850.
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dx.doi.org/10.1021/la200459t |Langmuir 2011, 27, 5282–5289
