Electrostatic Aggregation and Formation of Core−Shell

NANO LETTERS

Electrostatic Aggregation and Formation of Core−Shell Suprastructures in Binary Mixtures of Charged Metal Nanoparticles

2006 Vol. 6, No. 9 1896-1903

Alexander M. Kalsin,† Anatoliy O. Pinchuk,‡ Stoyan K. Smoukov,† Maciej Paszewski,† George C. Schatz,‡ and Bartosz A. Grzybowski*,† Department of Chemical and Biological Engineering and The Northwestern Institute on Complex Systems, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208, and Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208 Received April 29, 2006; Revised Manuscript Received June 16, 2006

ABSTRACT Electrostatic aggregation of oppositely charged silver and gold nanoparticles leads to the formation of core−shell clusters in which the shell is formed by the nanoparticles, which are in excess. Arguments based on Debye screening of interactions between like-charged particles help explain why these clusters are stable despite possessing net electric charge. The core−shell aggregates exhibit unusual optical properties with the resonance absorption of the shell particles enhanced by the particles in the core and that of the core suppressed by the shell. Experimental UV−vis absorption spectra are faithfully reproduced by Mie theory. The modeling allows for estimation of the numbers of particles forming the shell and of the shell’s effective thickness. These theoretical predictions are substantiated by experiments using nanoparticles covered with different combinations of charged groups and performed at different values of pH.

Rapid progress in the synthesis of nanoparticles (NPs) of various sizes,1 forms,2 and compositions,3,4 has shifted the focus of nanoscience research toward the preparation and characterization of larger architectures composed of various types of nanoscopic components.5-7 In this context, the ability to tailor and understand interactions between NPs and the phenomena these interactions lead to is a prerequisite for developing novel nanoelectronic materials,8-10 devices,11-13 sensors,14,15 and detection schemes.16,17 While most strategies for NP assembly developed to date have relied on (bio)molecular interactions, dispersion forces, or differences in NP sizes, the electrostatic forces18swhich Nature uses so flexibly to maintain complex structures on molecular19 and macromolecular scales20,21shave only rarely22-25 been applied and have not been investigated in detail. The present work is a systematic study of electrostatic interactions in binary solutions of charged metal particles (here, gold and silver) and of the optical properties of aggregates that these interactions give rise to. We show that when nanoparticles of a given type are titrated with those bearing oppositely charged groups, they aggregate into clusters. These clusters * Corresponding author. E-mail: [email protected]. Tel: 847.491.3024. † Department of Chemical and Biological Engineering and The Northwestern Institute on Complex Systems. ‡ Department of Chemistry. 10.1021/nl060967m CCC: $33.50 Published on Web 08/22/2006

© 2006 American Chemical Society

possess net charge butsowing to efficient Debye screening of the electrostatic interactions between like-charged NPssare stable in solution and precipitate only when the mixture reaches overall electroneutrality. Importantly, aggregation is accompanied by very pronounced changes in the UV-vis spectra that cannot be expressed as linear combinations of the spectra of the constituent particles. Both experiments and calculations based on Mie theory suggest that the observed effects are due to the formation of coreshell-like suprastructures, in which the shells are made of NPs constituting the major component of the mixture. Shellforming particles control the overall optical response of the system and cause large changes in the intensity and/or wavelength of the surface plasmon resonance (SPR) band of the core NPs. Such changes are most pronounced in the case of silver-core aggregates, for which the silver SPR band is completely “extinguished”, while that of gold is markedly “enhanced”. This phenomenon is reminiscent of the recently reported optical screening in separated gold-shell Au/Agalloy-core structures.26 As we show, an analogous, continuous-shell approximation can be used to model our discrete clusters and reproduce their UV-vis spectra. Finally, we discuss and explain how these and other optical properties of the aggregates are affected by changes in solution properties and/or in the chemical/material characteristics of the NPs.

Figure 1. Representative UV-vis spectra illustrating the progress of titrations with charged NPs: (like charged) (a) AgMUA added to AuMUA, (b) AgTMA added to AuTMA; (oppositely charged) (c) AgTMA added to AuMUA, (d) AgMUA added to AuTMA, (e) AuMUA added to AgTMA, and (f) AuTMA added to AgMUA. Concentrations of all solutions used were 2 mM, and the legends give the values of the mole fractions, χAg. Dashed lines show the spectra of pure solutions of silver (red) and gold (blue) NPs. For solutions of like-charged particles, the spectra are additive in the sense that both bands change in proportion to the mole fraction of a given type of NPs present in solution. In contrast, for oppositely charged, interacting particles the spectra are not additive. Note that when precipitation occurs (at χAg∼ 0.47, 0.51, 0.53 and 0.62 for (c-f), respectively), the intensities of these spectra decrease markedly due to precipitation but grow again when the precipitate redisperses beyond the point of electroneutrality (e.g., for χAg > 0.47 for (c)). Experimental details of the titration procedures are given in Supporting Information.

We used gold (average size 〈d〉 ) 5.1 nm; dispersity σ ) 20%) and silver (〈d〉 ) 4.8 nm and σ ) 30%, skewed toward smaller particles) particles prepared by a modified literature procedure (cf. Supporting Information) and covered with either HS(CH2)11NMe3+Cl- (henceforth, TMA) or HS(CH2)10COOH (MUA) ω-functionalized alkanethiols (high-purity grade, ProChimia, Poland). Formulations of both types of NPs were estimated according to the method by Stoeva and co-workers,27 and were Au4100L380 (L stands for a thiol ligand) for gold nanoparticles and Ag3400L340 for silver ones corresponding to the ratio of the magnitude of charges |Q(AgTMA)/Q(AuMUA)| ) 0.90. Nano Lett., Vol. 6, No. 9, 2006

UV-vis spectra of monocomponent solutions of either positively (AgTMA, pH 7.3) or negatively charged (AgMUA, pH 10.0) silver particles showed a strong absorption/SPR band centered at λmax,Ag ) 424 nm and characterized by an extinction coefficient  ∼ 5900. For solutions of gold particles at the same pH, λmax,Au was 520 nm for AuMUAs and 517 nm for AuTMAs with extinction coefficients  ∼ 3100 similar for both cases. We note that for particles covered with MUA, the positions of the maxima depended on pH. Specifically, when the pH was lowered, λmax red shifted (e.g., from 520 nm for pH 10 to ∼650 nm at pH 3) due to hydrogen bonding between protonated carboxylic groups on different NPs and the ensuing formation of NP 1897

Figure 2. Titration curves for the absorption coefficients of gold Au(λmax,Au) and silver Ag(λmax,Ag) as a function of the mole fraction of silver, χAg. In all legends the titrant is given secondsfor instance, Au(-) + Ag(+) means that AgTMAs were added to AuMUAs. Unless otherwise indicated, curves give the values of Au(λmax,Au). Concentrations of all solutions used were 2 mM. (a) For titrations of likecharged NPs, both  values are nearly constant, implying no strong interactions between gold and silver NPs. (b-f) Titrations of oppositely charged particles: (b) AgTMA added to AuMUA. At low content of added AgNPs, AuNPs present in excess surround them to form isolated core-shell structures. With χAg increasing, largersbut still overall charged and solublesaggregates form. When the ratio Ag/Au is close to unity (χAg ∼ 0.42) large, electrically neutral aggregates start precipitating. Upon further addition of charged AgTMA NPs, these aggregates redisperse in solution. The red dot on the titration curve corresponds to the titration point (χTAg) 0.45), defined as the midpoint min between the maximum and minimum (i.e., complete precipitation), χTAg ) (χ max Ag + χ Ag )/2. The insert gives the red shift in λmax,Au as a function of χAg. (c) Changes in Au(λmax,Au) upon addition of AgTMA to AuMUA (blue, solid diamonds, χTAg ) 0.45) and AgMUA to AuTMA (red, open circles, χTAg ) 0.51). (d) Titration of AuMUA with AgTMA at pH 10 (solid diamonds) and at pH 8 (open circles). (e) Titration of AuTMA with AgMUA at pH 10 (solid diamonds) and at pH 8 (open circles). (f) Changes in Ag(λmax,Ag) during reverse titrations: AuMUA added to AgTMA (solid diamonds) and AuTMA added to AgMUA (open circles). Insert shows concomitant changes in Au(λmax,Au).

aggregates.28 To minimize the effects of such aggregation, we performed our experiments at basic values of pH. Experimental Observations. The UV-vis spectra for the binary solutions depended on the relative polarities of the NPs. When like-charged particles (i.e., AuMUA and AgMUA or AuTMA and AgTMA) were mixed, the spectra showed both silver and gold NP bands of relative intensities proportional to the content of the corresponding NPs in solution (Figure 1a,b). For oppositely charged particles, however, the spectra were not additive and depended on the sequence of NP addition (Figure 1c-f). Specifically, when gold NPs were titrated with silver ones of opposite polarity, the SPR band of silver was absent, while that of gold was enhanced and red shifted (significantly for AuMUA/AgTMA, Figure 1c; only slightly for AuTMA/AgTMA, Figure 1d). Conversely, when silver was titrated with gold (Figure 1e,f), the intensity of the silver band decreased gradually, and that of the gold band increased and also moved to longer wavelength. 1898

For both scenarios, the trends held only until the NPs started to precipitate when the mole fraction, χAg, reached ∼0.4-0.5. Finally, further addition of one type of NPs beyond the titration point resulted in redispersion of the precipitate and the “recovery” of both bands (when AgNPs were added) or only the gold band (when AuNPs were added). To study these phenomena in a quantitative manner, we sought an analytical measure that would standardize the UVvis spectra with respect to the concentrations of particles used and would capture the effects of electrostatic interactions/ aggregation alone. On the basis of the Beer-Lambert law, we derived (cf. Supporting Information) two absorption coefficients, Au(λmax,Au) and Ag(λmax,Ag), defined at the maxima of the gold λmax,Au and silver λmax,Ag plasmon bands, respectively, in such a way that (i) if particles do not interact (aggregate), the values of  should not depend on concentration and should be constant over the entire range of mole fractions studied; (ii) if particles interact, the magnitude of Nano Lett., Vol. 6, No. 9, 2006

Figure 3. (a) (left) Schematic illustration of a cluster formed from a NP of titrant (here, silver in the core) and oppositely charged NPs of titrate (here, gold in the shell). Counterion “atmospheres” (right) around particles are indicated as gray halos. The aggregate is stable because the electrostatic repulsions are screened above d > 2κC-1 ∼ 2 nm and the overall, favorable energy is mostly due to contacts between oppositely charged NPs. The optical properties of the composite structure are analogous to those of a spherically symmetric core-shell particle (middle). (b) The ζ-potential for a typical titration of AuMUAs with equally sized AuTMAs. The negative and roughly constant value of the ζ-potential before the precipitation point indicates that only AuMUAs are present on the surface (i.e., in the shells) of the forming aggregates. Dashed vertical line gives the precipitation/electroneutrality point expected on the basis of NP compositions. (c) Calculated spectra compare the optical responses of (red curVe) a shell made of 12 isolated Ag NPs 5 nm in diameter, spaced by 9 nm, and arranged in the vertexes of an icosahedron with (blue line) a continuous shell of the thickness 3.4 nm and internal radius of 2.5 nm (blue). (d) Experimental UV-vis spectra for the titration of 5 nm AgMUAs with 5 nm AgTMAs. (e) Theoretical simulations of these spectra derived from exact electrodynamic calculations.

the deviation from this constant level should be proportional to the degree of aggregation/precipitation. As shown in Figure 2a, the values of Au(λmax,Au) and Ag(λmax,Ag) for like-charged Au and Ag particles are nearly constant over the entire range of χAg, from 0 to 1 (Figure 2a). This confirms that particles of the same polarity do not interact or aggregate. At the same time, this result indicates that the pronounced, nonadditive changes in the spectra of oppositely charged NPs are due to electrostatic interactions between them. In the following, we discuss these changes according to the order of NP addition. For the addition of AgNPs to AuNPs, we quantify only the trends for the Au(λmax,Au) coefficient since either there is no Ag band (for AgTMA added to AuMUA) or it is very weak (for AgMUA added to AuTMA) and since Ag(λmax,Ag) remains roughly constant (corresponding to the absorption of Au at 424 nm). Nano Lett., Vol. 6, No. 9, 2006

For the opposite order of addition, we consider both Ag(λmax,Ag) and Au(λmax,Au) since both bands are changing perceptibly. (i) When AgNPs were added to AuNPs, the value of Au(λmax,Au) initially increases rapidly with increasing χAg (Figure 2b). At the same time, the maximum of the gold SPR band, λmax,Au, shifts to longer wavelength (Figure 2b, insert). These observations indicate that oppositely charged particles aggregate. Furthermore, when χAg reaches ∼0.42, the NPs start precipitating, and the values of  drop abruptly. Importantly, the titration point χTAg ∼ 0.45 (cf. Figure 2b for definition) corresponds to the overall electroneutrality of the solution determined by the condition ([AgL1]/[AuL2])neutral ) (NAg/NAu)|QAuL2/QAgL1| ) (3400/ 4100)/0.9 ) 0.92, where L1 and L2 denote oppositely charged 1899

Figure 4. (a) UV-vis spectra calculated from Mie theory for 2 mM mixtures of like-charged, 5 nm gold and silver NPs at various values of χAg. Simulations reproduce faithfully the experimental spectra shown in Figure 1a. The RMSD between the calculated and experimental curves are between 4.8% (for pure AgNP solution) and 8.9% (for pure AuNP solution). (b) Calculated spectra of the silver-core/gold-shell structures. Different curves correspond to different shell thicknesses, d. Diameter of the core is 5 nm for all curves. (c) Comparison of the experimental spectrum (solid line) characterizing titration of AuMUA with AgTMA at χAg ) 0.14 with the calculated spectrum (dashed line) for the core-shell structures with an effective gold shell thickness of 2.5 nm. Since the calculated absorption cross section of the 2.5-nm-thick, continuous shell is six times that of an isolated gold nanoparticle, we estimate that the shell is composed of six AuNPs, which also corresponds to χAg ) 0.14. (d) Calculated spectra of the gold-core/silver-shell structures. (e) Comparison of the experimental spectrum (solid curve) characterizing the titration of AgTMA with AuMUA at χAg ) 0.80 with the simulated one (dashed curve). The latter was fitted as a 1:3 linear combination of (i) the spectrum of an Au-Ag core-shell structure with shell thickness of 0.7 nm (content of silver ∼52%) and (ii) the spectrum of free Ag NPs remaining in solution. (f) Suggested mechanism for the aggregation of clusters, consisting of gold NPs in the core and polydispersed silver NPs in the shell.

ligands, and NAg and NAu are the numbers of Ag and Au atoms in one AgL1 and one AuL2 particle, respectively. From these experiments, we draw a conclusion that small, charged aggregates are stable in solution, while large, electroneutral ones precipitate. Next, we note that a mixture, in which gold nanoparticles are charged positively (i.e., a pair AuTMA/AgMUA), has a lower maximum value of Au(λmax,Au) and lower initial slope, dAu(λmax,Au)/dχAg, but a higher titration point (χTAg ∼ 0.51) than a mixture, in which AuNPs bear negative charges (i.e., AuMUA/AgTMA); the comparison shown in Figure 2c is based on the same pH values and concentrations. Finally, when pH is decreased, the AuMUA/AgTMA solutions are less stable (χTAg decreases) and enhancement of the gold band is less pronounced (lower Au(λmax,Au)) (Figure 2d). Similar trends were observed for AuTMA/AgMUA with the exception that the silver band is not fully extinguished (Figure 2e). The results of these experiments suggest that the structure and stability of the aggregates that form change when the coating alkanethiols are interchanged and that the protonation state of the charged groups might also play an important role. 1900

(ii) For the reverse order of titrationsthat is, when AuNPs were added to AgNPssthe silver band and the value of Ag(λmax,Ag) decrease monotonically (Figure 2f), while a relatively weak band appears at Au(λmax,Au). When gold particles are negatively charged (i.e., AuMUA is added to AgTMA), the value of Au(λmax,Au) initially increases, whereas for positively charged AuTMAs added to AgMUA, it stays constant and high before decreasing upon precipitation (Figure 2f, insert). Also, in the former case, the solution is stable until a higher content of silver NPs is reached (Figure 2f). Theoretical Explanation. To explain these trends, we considered and modeled several different scenarios of how particle interactions could produce the observed changes in the UV-vis spectra. The calculations ruled out possible electron delocalization and resonant energy transfer because it should producesin disagreement with experimentsa band between the SPR bands of “pure” gold and silver. Similarly, an electron transfer mechanism is not plausible since it can produce only minor shifts in the positions of the SPR bands (up to ∼10-20 nm in our calculations) and very small (few percent) changes in the bands’ intensities. In addition, since Nano Lett., Vol. 6, No. 9, 2006

the Fermi energies for gold and silver particles are nearly identical, no “driving force” required for ET is present. Both theory and experiment, however, corroborate a mechanism which assumes that oppositely charged NPs form core-shell-like aggregates in whichsdepending on the order of titrationseither silver or gold particles are positioned at the core (Figure 3a). We emphasize that such structures possessing an overall charge are not energetically unfavorable, since the electrostatic interactions between like-charged particles in the shell are strongly screened by counterions present near the surface of the NPs and by the metal cores of other particles. In particular, the Debye screening length between the particles forming a shell can be estimated from κC-1 ) (0kBT/2ce2)1/2, where c denotes the number density of ions, e is the charge of an electron,  is the static dielectric constant of the medium, and 0 is the permittivity of vacuum. Assuming that the counterions from the shell’s NPs not compensated by those of the core are evenly dispersed in the free volume of the shell, c ∼ 0.1 M and κC-1 ∼ 1 nm. This means that only particles separated by less than 2κC-1, ∼2 nm, repel one another significantly and that relatively tight shells can form without increasing overall electrostatic energy and can thus be stable. Also, chemical interactions (e.g., hydrogen bonding, vide infra) between thiol headgroups can further lower the aggregate’s energy. In addition to these arguments, the formation of the coreand-shell structures is experimentally supported by the measurements of the ζ (“zeta”) surface potential during titrations. Figure 3b illustrates this for the case of titration of AuMUA s with AgTMAs, where despite the addition of positively charged NPs to the solution, ζ remains negative and approximately constant. This observation indicates that only the negatively charged AuMUAs are present at the surface of the forming aggregates and that these NPs “envelop” the positively charged ones. For the addition of negatively charged NPs to the positively charged ones, the situation is reversed, and the values of ζ are positivesin this case, the aggregates have only positive NPs in the shells, as expected. The optical properties of the charged, core-shell clusters formed by discrete NPs are well approximated by those of “continuous,” spherically symmetric, core-shell particles. This simplification, which is confirmed by recent experimental observations,29 arises because the dimensions of the shell particles are small compared to the wavelength of light. The intrinsic size effect in the optical absorption of silver and gold NPs due to limitations in the mean free path of the conduction electrons inside the particles30 can be modeled using the size-corrected Drude approximation for the conduction electrons31 together with experimental data for the contribution of interband transitions.32 The absorption cross section of a core-shell nanoparticle according to Mie theory33 is then calculated as Cabs )

2π k2

where Nano Lett., Vol. 6, No. 9, 2006

∞

(2j + 1)Re{aj + bj} ∑ j)1

aj ) ψj(y)[ψ′j(n2y) - Ajχ′j(n2y)] - n2ψ′j(y)[ψj(n2y) - Ajχj(n2y)] ξj(y)[ψ′j(n2y) - Ajχ′j(n2y)] - n2ξ′j(y)[ψj(n2y) - Ajχj(n2y)] bj ) n2ψj(y)[ψ′j(n2y) - Bjχ′j(n2y)] - ψ′j(y)[ψj(n2y) - Bjχj(n2y)] n2ξj(y)[ψ′j(n2y) - Ajχ′j(n2y)] - ξ′j(y)[ψj(n2y) - Bjχj(n2y)] Aj ) Bj )

n2ψj(n2x)ψ′j(n1x) - n1ψ′j(n2x)ψj(n1x) n2χj(n2x)ψ′j(n1x) - m1χ′j(n2x)ψj(n1x) n2ψj(n1x)ψ′j(n2x) - n1ψj(n2x)ψ′j(n1x) n2χ′j(n2x)ψj(n1x) - n1χj(n2x)ψ′j(n1x)

In these expressions, n1 and n2 are the refractive indices of, respectively, the core and shell relative to the surrounding medium, x ) kRcore and y ) kRshell, ψ, ψ′, and ξ, ξ′ are Riccati-Bessel functions and their derivatives; k ) 2π/λ, where λ is the wavelength. Qualitatively, this model predicts that the wavelength of light passing through the shell becomes shorter by a factor which is determined by the shell’s effective refractive index (cf. Figure 3a, middle), as a result, the core is resonantly excited at a longer effective wavelength, λeff ) λintrinsicnshell. The magnitude of this effect depends on the effective thickness of the shell. It is worth noting that approximating the shell as continuous is further corroborated by a more sophisticated theoretical treatment that accounts for the granularity of the shell NPs and for the electrodynamic coupling (including retardation effects and multipole interactions) between the nanoparticles within the forming aggregates. This model35 is an extension of the rigorous theory pioneered by Gerardy and Ausloos34 and shows that the optical response of discontinuous clusters is the same as that of the corresponding continuous shells of the same mass (Figure 3c). It also reproduces the experimental UV-vis spectra of the clusters formed by oppositely charged NPs with the same type of a metal core (e.g., Au/Au or Ag/Ag; cf. Figure 3d,e). The extension of this approach to Au/Ag or Ag/Au core-and-shell structures requires nontrivial matching of Bessel functions describing EM fields around NPs of different dielectric propertiesswe are currently working on such an extension and hope to be able to report the results for all types of aggregates soon.35 In the following, we restrict ourselves to the simpler and more intuitive continuous-shell model to explain the results of the particular NP titrations. (i) For the titration of gold NPs with silver ones, the silver core is resonantly excited at λeff ∼ 520 nm. Consequently, the silver band at 424 nm appears “extinguished” while that of gold at ∼520 nm is “enhanced”. These effects are most pronounced when AgTMAs are added to AuMUAs, and the shell-forming particles covered with MUA can interact favorably with each other either by hydrogen bonds between the remaining protonated COOH groups or by divalent binding of COO- to NMe4+ cations present in solution.36 Here, the shell is relatively “tight” and provides 1901

an efficient screen for the silver inside. Modeling (Figure 4b) shows that in order to observe complete disappearance of the Ag band and to reproduce the shape of the experimental spectra, the thickness of the shell should be at least 2.5 nm, which corresponds to at least N min shell ) 6 gold NPs in the shell (Figure 4c). An upper bound on the number of core particles, N max shell, can be estimated from the condition that the separation between like-charged neighbors be larger than twice the Debye length kC-1 ∼ 1 nmsthis gives N max shell ∼ 11 gold NPs in the shell. In contrast, when the added gold particles are positively charged, the TMA groups interact purely repulsively and the shell is less tight and effectively thinner (1.5 nm and N max shell < ∼4)sexperimentally, this manifests itself by incomplete “extinguishing” of the silver SPR band and a smaller increase in Au(λmax,Au) (cf. Figure 1d). (ii) For the titration of silver NPs with gold ones, an important issue is that the former are more polydisperse and their distribution is skewed toward smaller particles. Consequently, when AuNPs are added, the silver shell is effectively thin (Figure 4d), and not all of the gold band is extinguished. Modeled spectra reproduce this effect for a shell thickness of at most 1 nm (Figure 4e, note that in this case the numbers of particles cannot be easily estimated because of their polydispersity). What the calculations for isolated core-shell aggregates do not reproduce, however, is the rapid growth of Au(λmax,Au) and concomitant decrease in Ag(λmax,Ag). This effect can be rationalized by the formation of higher-order aggregates, in which AgNPs constituting the shells are found between the large AuNPs (Figure 4f). In such “reverse” core-shells, the absorption wavelength of silver is shifted to higher values near the gold SPR, leading to the previously discussed “extinguishing” of the silver SPR. While this scenario would account for the appearance of the experimental UV-vis spectra, the mechanism of aggregation of extended structures is not obvious. One possibility here is that the small particles constituting the shells are labile and that aggregation of individual core-shells is entropically driven and accompanied by the release of smaller, silver NPs (Figure 4f). A similar entropic effect for the aggregation of like-charged NPs mediated by expulsion of small, multivalent ions has been described in a recent theoretical work.37 Effect of pH Changes. Finally, we briefly discuss changes in the spectra that accompany the changes in pH. Since TMA groups are fully ionized (pKb ∼ 0), pH changes in the regime we are working in (pH 7-10) affect only the protonation state of the MUA groups (the pKa of MUA in a SAM on gold varies between 4.5 and 5.7 depending on ionic strength38). Therefore, for solutions in which the titrated NPs are coated with MUA, increasing the pH causes more surface carboxylic groups to be deprotonated so that more oppositely charged titrant NPs are needed to reach overall electroneutrality. For example, in the case of the titration of AuMUA with AgTMA shown in Figure 2d, χTAg is higher at pH 10 than at pH 8. Conversely, when MUA is on the titrant NPs, increasing the pH results in fewer of these particles being needed to give an electroneutral solution. For the titration 1902

of AuTMA with AgMUA (Figure 2e), χTAg is lower at pH 10 than at pH 8. In conclusion, we have studied experimentally, modeled, and rationalized the properties of binary mixtures of charged gold and silver nanoparticles. These mixtures have several unique optical characteristics that cannot be deduced from the properties of the individual NPs, but result from the formation of core-shell aggregates. Since the changes in UV-vis spectra accompanying aggregation are very pronounced and sensitive to the chemical properties of the SAMs coating the NPs, they could provide a basis for new detection schemes in which NPs of different types would carry interacting analytes. On the fundamental level, this work leaves open several intriguing questions. Why, for example, do the solutions precipitate only when reaching overall electroneutrality and why do the neutral precipitates redisperse upon addition of excess charged NPs? While we believe these effects are entropic and due to ordering/ disordering of solvent molecules around charged/neutral NPs, this hypothesis needs further verification. In a wider context, quantitative understanding of the interactions between charged nanoscopic objects is a prerequisite for the rational use of electrostatics as a flexible and potentially powerful method for self-assembly of new nanostructured materials.25 Acknowledgment. B.A.G. gratefully acknowledges financial support from the Camille and Henry Dreyfus New Faculty Awards Program, the National Science Foundation (Grant No. 0503673), and the ACS Petroleum Research Fund (Award # 42953-AC5). A.O.P. and G.C.S. were supported by the Northwestern MRSEC (NSF Grant DMR-0076097) and by DARPA. Supporting Information Available: Experimental details on nanoparticles syntheses and UV-vis titrations and definition of the absorption coefficients. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13, 3843-3858. (2) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. D. Angew. Chem., Int. Ed. 2004, 43, 3673-3677. (3) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (4) Kamata, K.; Lu, Y.; Xia, Y. N. J. Am. Chem. Soc. 2003, 125, 23842385. (5) Stellacci, F. Nat. Mater. 2005, 4, 113-114. (6) Redl, F. X.; Cho, K. S.; Murray, C. B.; O’Brien, S. Nature 2003, 423, 968-971. (7) Son, D. H.; Hughes, S. M.; Yin, Y. D.; Alivisatos, A. P. Science 2004, 306, 1009-1012. (8) Xia, Y. N.; Gates, B.; Li, Z. Y. AdV. Mater. 2001, 13, 409-413. (9) Hoinville, J.; Bewick, A.; Gleeson, D.; Jones, R.; Kasyutich, O.; Mayes, E.; Nartowski, A.; Warne, B.; Wiggins, J.; Wong, K. J. Appl. Phys. 2003, 93, 7187-7189. (10) Grunes, J.; Zhu, J.; Anderson, E. A.; Somorjai, G. A. J. Phys. Chem. B 2002, 106, 11463-11468. (11) Tucker, J. R. J. Appl. Phys. 1992, 72, 4399-4413. (12) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 1852. (13) Seker, F.; Malenfant, P. R. L.; Larsen, M.; Alizadeh, A.; Conway, K.; Kulkarni, A. M.; Goddard, G.; Garaas, R. AdV. Mater. 2005, 17, 1941-1945. (14) Zayats, M.; Kharitonov, A. B.; Pogorelova, S. P.; Lioubashevski, O.; Katz, E.; Willner, I. J. Am. Chem. Soc. 2003, 125, 16006-16014.

Nano Lett., Vol. 6, No. 9, 2006

(15) Li, J. S.; He, X. X.; Wu, Z. Y.; Wang, K. M.; Shen, G. L.; Yu, R. Q. Anal. Chim. Acta 2003, 481, 191-198. (16) Sonnichsen, C.; Reinhard, B. M.; Liphard, J.; Alivisatos, A. P. Nat. Biotechnol. 2005, 23, 741-745. (17) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547-1562. (18) Grzybowski, B. A.; Winkleman, A.; Wiles, J. A.; Brumer, Y.; Whitesides, G. M., Nat. Mater. 2003, 2, 241-245. (19) Kittel, C. Introduction to solid-state physics, 8th ed.; Wiley: Hoboken, NJ, 2005. (20) Simonson, T. Rep. Prog. Phys. 2003, 66, 737-787. (21) Williams, L. D. Annu. ReV. Biophys. Biomol. Struct. 2000, 29, 497521. (22) Kolny, J.; Kornowski, A.; Weller, H. Nano Lett. 2002, 2, 361-364. (23) Leunissen, M. E.; Christova, C. G.; Hynninen, A. P.; Royall, C. P.; Campbell, A. I.; Imhof, A.; Dijkstra, M.; van Roij, R.; van Blaaderen, A. Nature 2005, 437, 235-240. (24) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55-59. (25) Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A. Science 2006, 312, 420-424. (26) Sun Y. Wiley B.; Li Z.-Y.; Xia Y. J. Am. Chem. Soc. 2006, 126, 9399-9406. (27) Stoeva, S.; Klabunde, K. J.; Sorensen, C. M.; Dragieva, I., J. Am. Chem. Soc. 2002, 124, 2305-2311.

Nano Lett., Vol. 6, No. 9, 2006

(28) Li, G. T.; Wang, T. Y.; Bhosale, S.; Zhang, Y.; Fuhrhop, J. H. Colloid Polym. Sci. 2003, 281, 1099-1103. (29) Hubenthal, F.; Ziegler, T.; Hendrich, C.; Alschinger, M.; Trager, F. Eur. Phys. J. D 2005, 34, 165-168. (30) Mie, G. Ann. Phys. (Leipzig) 1908, 25, 377-445. (31) Pinchuk, A.; Kreibig, U. New J. Phys. 2003, 5, 151. (32) Johnson, P. B.; Christy, R. W. Optical-Constants of Noble-Metals. Phys. ReV. B 1972, 6, 4370-4379. (33) Bohren, C. F.; Huffman, D. R. Absorption and scattering of light by small particles; Wiley Professional Paperback Edition; Wiley: New York, 1998. (34) Gerardy, J. M.; Ausloos, M.Phys. ReV. B 1982, 25, 4204-4229. (35) Pinchuk A. O.; Kalsin, A. M.; Schatz, G. C.; Grzybowski, B. A. 2006, in preparation. (36) Su, C. H.; Wu, P. L.; Yeh, C. S. Bull. Chem. Soc. Jpn. 2004, 77, 189-193. (37) Wu, J. Z.; D. B., Blanch, H. W.; Prausnitz, J. M. J. Chem. Phys. 1999, 111, 7084-7094. (38) Smalley, J. F.; Chalfant, K.; Feldberg, S. W.; Nahir, T. M.; Bowden, E. F., J. Phys. Chem. B 1999, 103, 1676-1685.

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