pubs.acs.org/NanoLett
Precision Assembly of Oppositely and Like-Charged Nanoobjects Mediated by Charge-Induced Dipole Interactions David A. Walker,† Christopher E. Wilmer,† Bartlomiej Kowalczyk,†,‡ Kyle J. M. Bishop,† and Bartosz A. Grzybowski*,†,‡ †
Department of Chemical and Biological Engineering, ‡ Department of Chemistry, and Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208 ABSTRACT The range of electrostatic interactions controls precisely the mutual orientations of assembling charged nanoobjects. For nonspherically symmetric particles, polarization effects and induced dipoles can dominate charge-charge interactions. These charge-induced dipole interactions mediate orientation-specific aggregation of both oppositely and like-charged particles. KEYWORDS Electrostatics, self-assembly, like-charge attraction, charge-induced dipoles
G
iven the serial nature and limited precision of the existing methods for “nanotweezing”,1-3 self-assembly (SA) offers a potentially more convenient route to parallel manipulation and precise positioning of nanoscopic components. In this context, electrostatic interactions provide a basis for a flexible SA modality capable of organizing charged nanoobjects into larger structures that would be hard to prepare with other types of interactions. Indeed, electrostatic forces can assemble a variety of nanoparticle supracrystals,4-7 in which the ordering of the constituent particles ranges from simple cubic to diamond-like. When charge-charge interactions are combined with dipolar effects, the latter can mediate the assembly of spherical nanoparticles into one-dimensional nanowires8 or twodimensional free-floating sheets.9 To date, however, these effects have been investigated mostly in systems comprising spherically symmetric components (i.e., nanoparticles), and there are relatively few experimental examples10-14 of nanoscale electrostatics involving particles of lower symmetry. Here, we show that with appropriate control of the screening length, electrostatic forces can be used for noninvasive positioningswith precision down to ca. 7 nmsand assembly of nanoobjects of different shapes (e.g., nanoparticles, nanorods, or nanotriangles) and charge polarities. Remarkably, a competition between charge-charge (due to the charged organic ligands coating the particles) and chargeinduced dipole (due to the orientation-specific polarization of the particles’ metal cores) interactions can bring together and orient both oppositely and like-charged objects. The likecharge attraction does not derive from quantum15 or electrohydrodynamic effects16,17 but is controlled solely by the range of the counterion-screened electrostatic interactions.
If extended to objects of different material compositions, electrostatic positioning could be useful in the synthesis of the fabled “nanomolecules”18 or for the tailoring of orientation-dependent plasmonic properties.19 Gold nanoparticles (NPs, Figure 1a,b) 5.9 ( 0.6 nm in diameter were synthesized as previously described20 and were stabilized with a self-assembled monolayer (SAM) of either positively charged N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride (TMA, ProChimia, Poland) or negatively charged (at pH ) 10) mercaptoundecanoic acid (MUA, ProChimia). The functionalized NPs were characterized by an electrophoretic mobility of µ ) 1.9 × 10-8 m2/V s, corresponding to a surface charge density of σ ≈ +0.01 C/m2 for TMA AuNPs and σ ≈ -0.01 C/m2 for MUA AuNPs21 (see Supporting Information, section S2). Gold nanorods (NRs, 34 ( 3.3 nm long; 10.9 ( 1.2 nm in cross-sectional diameter) were synthesized from CTABstabilized seed particles in the presence of Ag(I) ions,22 purified by three washing cycles with water, centrifuged at 10000g for 45 min, and redispersed in deionized water. Subsequently, the rods were functionalized with either TMA or MUA thiols (Figure 1a,b) and had surface charge densities of σ ≈ +0.005 C/m2 and σ ≈ -0.005 C/m2, respectively (the latter, at pH ) 10). We note that for the growth method used here, the ends of the rods are known to expose Au {100} facets and the density of the adsorbed thiols (and hence, of charge) is approximately constant over the entire rod’s area (as opposed to the growth method without Ag ions where the rod’s {111} ends adsorb thiols preferentially; see section S6 in Supporting Information for details). Gold nanotriangles (NTs, 158 ( 14 nm sides, 8.7 ( 0.8 nm thickness) were synthesized by sequential seeded growth from gold citrate stabilized seeds23 and were purified by two sedimentation and washing steps to remove the isotropic products. The NTs were then functionalized with TMA
* Correspondence to
[email protected]. Received for review: 04/6/2010 Published on Web: 05/25/2010 © 2010 American Chemical Society
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solutions.29 In addition, control experiments were performed with otherwise identical solutions of uncharged particles (e.g., stabilized by ethylene glycol alkane thiols)sin this case, no aggregation was observed. From the above, we conclude that the formation of the aggregates requires the presence of electrostatic interactions. The structures formed by the charged particles depend on their polarities and on the concentration of salts in the solutions. Parts a-c of Figure 2 illustrate self-assembly of oppositely charged NPs and NRs (e.g., TMA NPs and MUA NRs). Without the addition of NMe4+Cl-, when the screening is due only to the neutralizing counterions and residual base (salt concentration, cS ≈ 1 mM, and screening length, κ-1 ) (ε0εkBT/2cSe2)1/2 ≈ 10 nm), the NPs localize preferentially onto the “tips” of the NRs (Figure 2a). When, however, salt is added and the screening length decreases (e.g., cS ≈ 250 mM and κ-1 ≈ 0.6 nm), the NPs aggregate onto the sides of the NRs (Figure 2b). Similar behavior is observed in collections of oppositely charged NTs and NRs: under low-salt conditions, the rods attach to the tips of the triangles (Figure 2d); when salt concentrations are high, the NRs orient parallel to and near the centers of triangles’ edges (Figure 2e). Analysis of multiple samples indicates that the precisionsas quantified by the standard deviation of the frequency distributions shown in parts c and f of Figure 2swith which the centers of the particles orient in the “tip” configuration is (6.9 nm for the NP/NR system and (27 nm for the NR/NT system. For the “side” configuration, the standard deviation from the perfect alignment of a NP at the NR’s center is (6.8 nm; for the NR/NT pair, this value is ∼(24 nm. We note that in both NP/NR and NR/NT systems changing the screening length does not result in gradual changes of the small particle’s orientationsinstead, a sharp transition between the “side” and the “tip” configurations is observed for cS ∼2.5 mM and κ-1 ≈ 6 nm. Interestingly, these effects also extend to like-charge objects. In this case, however, only the “tip” configuration is stable and is observed under low-salt conditions (Figure 3a); for high-salt concentrations, particles do not aggregate at all (Figure 3b). To rationalize the positional trends during SA, we focus on the NP/NR system which is easier to treat theoretically. Under the high-salt conditions, the electrostatic interactions are screened and short ranged, and the energy of the NP/ NR pair is well approximated by the so-called Derjaguin approximation28,30,31
FIGURE 1. Scheme and dimensions of Au NRs, NPs, and NTs coated with charged SAMs: (a) AuNR and AuNP coated with positively charged TMA thiols; (b) AuNR and AuNP coated with negatively charged, deprotonated MUA thiols; (c) AuNT coated with positively charged TMA thiols. Note: MUA functionalized AuNTs were not prepared, as the interactions between negatively charged MUA and the positively charged hexadecyltrimethylammonium bromide (CTAB) surfactant (which stabilizes the as-synthesized AuNTs) lead to rapid and irreversible aggregation of the triangles. The graph in the right column illustrates that the magnitude of the electric potential, φ, around particles of both polarities decreases in an exponential manner. Depending on the degree of ionic screening, the potential extends approximately 0.5-20 nm from the particles’ surfaces. Red and blue colors are used to indicate charge polarity and the sign of the potential. The lower scheme in the right column specifies the structures of the thiols and counterions in (a-c).
(Figure 1c) and had a surface charge density of σ ≈ +0.001 C/m2. Assembly experiments were based on binary NP/NR or NR/NT mixtures. The solutions of NPs and NRs were diluted to 1.5 × 1013 NPs/mL and 2.0 × 1011 NRs/mL, mixed together in a 5:95 v/v ratio (ca. 4 NPs per 1 NR), and drop cast onto transmission electron microscopy (TEM) grids (see section S7 in Supporting Information for further details). In experiments with NTs, a small aliquot of TMA NT stock solution was mixed with MUA NRs to give final concentrations of 5 × 109 NRs/mL and 2.5 × 108 NTs/mL (ca. 20 NRs per 1 NT). In cases where higher salt concentrations were desired, small aliquots of concentrated tetramethylammonium chloride (NMe4+Cl-) were added. All types of assemblies described later in the paper were imaged by two methods: (i) TEM of evaporated NP/NR or NR/NT samples; and (ii) cryo-TEM of the frozen NP/NR or NR/NT solutions (see section S7 in Supporting Information for further details). The results of these two methods agreed indicating that the observed modes of NP/NR and NR/NT assembly cannot be attributed to either capillary24-26 or entropic27,28 effects such as those seen previously during evaporation of relatively concentrated NP24 and/or NR25,26 © 2010 American Chemical Society
UDA(r) ) 2πF
∫r∞ UFlat(z) dz
(1)
where UFlat is the energy of the two charged planar surfaces and F is a geometrical factor correcting for the local curvature of the real surfaces. For a sphere of radius as and a cylindrical rod with hemispherical ends (length Lr and radius ar), the curvature factor is Fside ) asar1/2/(as + ar)1/2 for the 2276
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FIGURE 2. Site-selective self-assembly of oppositely charged particles. Schemes and representative TEM images of (a) the “tip” arrangements observed for large-screening length where charge-induced dipole interactions are appreciable (here, for TMA NPs/MUA NRs pair with κ-1 ≈ 10 nm) and (b) the “side” arrangement observed for small screening lengths (TMA NPs/MUA NRs, κ-1 ≈ 0.6 nm). In the schemes, the screening length is proportional to the thickness of the halos around the particles; red and blue colors indicate particle polarity. The first image in each column is from cryo-TEM. Scale bar ) 20 nm. (c) Histograms quantifying the observed positions of NPs along the NR’s contour (coordinate defined in the inset scheme). Dark-gray bars with a maximum at 0 nm correspond to the “tip” arrangement; light-gray bars with a maximum at 20 nm correspond to the “side” configuration. Statistics are based on the analysis of 52 TEM images for each screening length; dark-gray and light-gray arrows indicate standard deviations of the distributions (i.e., precision of positioning). TEM images of assemblies formed by oppositely charged TMA NTs and MUA NRs under (d) low-salt conditions (cS ≈ 1 mM, κ-1≈ 10 nm) and under (E) high-salt conditions (cS ≈ 100 mM, κ-1≈ 1 nm). Scale bar ) 50 nm. (f) Histograms quantify the observed positions of NRs around the perimeter of the NTs as described by the inset. Dark-gray bars with maximum at 0 nm correspond to the “tip” arrangement at low-salt conditions; light-gray bars with peak at 80 nm correspond to the “side” configuration at high-salt conditions. Statistics are based on the analysis of 45 TEM images for each screening length; dark-gray and light-gray arrows indicate standard deviations of the distributions (i.e., precision of positioning).
“side” arrangement, and Ftip ) asar/(as + ar) for the “tip” arrangement. It follows that the magnitude of the relative energies of these configurations is Uside/Utip ) (1 + as/ar)1/2sthat is, the “side” configuration is more energetically favorable (since Uside < 0 and Utip < 0). In contrast, when screening is weak, electrostatic effects are long-ranged and reflect the entire geometry of the particles rather than just their local curvature near the point of contact. In the limit of infinite screening length (i.e., zero salt concentration), the PB equation reduces to the Laplace equation ∇2φ ) 0, and the energies of interaction can be approximated by the net charges on the particles: QS ) σAS for the spherical NP and QR ) σAR for the rod, where A is the surface area (note that because the SAMs are tethered onto the particles covalently, the charges within them cannot redistribute as in conductors; also see section S6 in Supporting Information for details on the charge density distribution on the NRs). Even under these conditions, however, the charge-charge interactions © 2010 American Chemical Society
alone cannot account for the “tip” arrangement since the electrostatic energy
U(r) ≈ QsQr /4πε0εr is always lower for the “side” configuration (for which the distance between the centers of charge, r, is smaller than for the “tip” configuration). The “tip” arrangement can become energetically favorable by taking into account interactions between the higher-order multipolessnotably, that between the charge on the sphere and the dipole induced in the rod’s metallic core. According to this improved approximation
U(r) ≈ QsQr /4πε0εr - Qs2Vrα/8πε0εr4 where Vr is the volume of the rod’s core and R is the rod’s dimensionless polarizability, calculated as described in ref 32 (see section S4 in Supporting Information). Importantly, the energetic preference for the “tip’ configuration is due to the polarizability being significantly higher along than across the rod (R ∼ 9.6 along the long axis of the 2277
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FIGURE 4. Qualitative phase diagrams of (a) oppositely charged and (b) like-charged NR-NP assemblies calculated using the simple electrostatic model accounting for the charge-charge and chargeinduced dipole interactions but not for counterion screening (see main text). QS/QR is the ratio of charges on the sphere and on the rod, respectively; R|/R⊥ is the relative polarizability along vs across the rod. (c) The relative energies of the “tip” and “side” configurations calculated numerically (see SOM, section 3) and plotted against dimensionless parameter (κa)-1 (where a ) 2.95 nm is the NP radius). The “tip” configuration becomes favorable for large screening lengths, when the charge on the NP can induce a significant dipole within the rod’s metallic core. (d) Numerically calculated scaled energies of the NR/NP interaction as a function of the position of the NP along NR’s contour (see inset) Solid line with global minimum at 0 nm (“tip” configuration) corresponds to the low-salt conditions (here, (κa)-1 ) 10). Dashed line with global minimum at 19.6 nm (“side” configuration) corresponds to the high-salt conditions ((κa)-1 ) 0.26).
FIGURE 3. Attraction between like-charged particles. (a) Schemes and representative TEM images of positively charged TMA NPs assembled onto the end of positively charged TMA NRs. The assembly occurs at weak screening (i.e., large screening length, here cS ≈ 1 mM, κ-1≈ 10 nm) when the attractive charge-induced dipole interaction overcomes the like-charge repulsion. Note: similar structures are observed for negatively charged MUA NPs and NRs. (b) When the screening is stronger (i.e., shorter screening length, here cS ≈ 250 mM, κ-1≈ 0.6 nm), large dipoles in the NRs are not induced and the particles remain unaggregated. The first representative image in each column is from cryo-TEM. Scale bars )20 nm.
rod vs. R ∼ 2.2 perpendicular to this axis) and is illustrated in the qualitative phase diagram in Figure 4a. Also, the same simple model can be extended to the case of likecharged particles under low-salt conditions. Figure 4b gives the corresponding phase diagram where specific regions indicate whether
section S3 in Supporting Information, these simulations derive the electrostatic potential between interacting particles, φ, from the linearized Poisson-Boltzmann (PB) equation, ∇2φ ) κ2φ (applicable because eφ < kBT in our system), and calculate energies of different particle configurations as
U(r) ≈ QsQr /4πε0εr - Qs2Vrα/8πε0εr4
U)
is lower in the “side” or in the “tip” arrangement. As expected, the “tip” arrangement is preferred for larger relative charges of the NPs polarizing the NRs, and for higher values of the rods polarizability, Rsthat is, when chargeinduced dipole attractions can overcome the charge-charge repulsions. We make two more comments concerning the above reasoning. First, our argument neglects van der Waals forces since their magnitudes are not only relatively small33,34 (few times kBT at contact; cf. section S5 in Supporting Information for discussion) but also proportional to the effective area of contact between the particles such that they always favor the “side” arrangement of the NPs on the NRs.28 Second, the results of the simple model above agree with more rigorous numerical simulations accounting for finite screening length (here, 0.5 nm < κ-1 < 20 nm). As detailed in © 2010 American Chemical Society
1 2
∑ σi ∫ φ dSi
where the integral is carried out over the surface Si of particle i, and the sum is over all particles.35 An example of such a calculation is shown in Figure 4c which confirms that for the NP/NR system, the “side” configuration is preferred for short screening lengths (i.e., high salt concentration), while the “tip” arrangement is favored for long screening lengths (low salt concentration). The numerical model also explains the experimentally observed transition between the “side” and the “tip” configurationssthis is illustrated in Figure 4d which shows the energies of the NP/NR pair plotted as a function of NP’s position along the rod’s contour. For both low- and high-salt conditions, there are two minima separated by an energy barriershowever, for the low-salt case, the “tip” minimum is deeper, while for the high-salt case, the energy of the “side” configuration is lower. 2278
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We emphasize that the importance of charge-induced dipole forces and the consequent attraction of like-charged particles are due to the nanoscopic dimensions of the assembling particles and the commensurability of the particles’ dimensions with the thickness of the screening layer. In particular, the conditions necessary to observe the “tip” configuration are (i) highly polarizable NRs/NTs (ideally, having conducting cores) and (ii) long-ranged electrostatic interactions, κLr e 1, where Lr is the characteristic dimension of a rod. In practice, the latter condition is met only when Lr is nanoscopic as the screening length, κ-1 is typically less than ∼100 nm in water at room temperature (for µM salt concentrations). For example, if the only ions in solution are the counterions needed to balance the charge on the particles, the screening length is given by κ-1 ) (ε0εkBT/ZPcPe2)1/2, with an “effective” salt concentration of (1/2)ZPcP where ZP is the number of (dissociated) charges per particle and cP is the particle concentration (see section S1 in Supporting Information). In the present system, (1/2)ZPcP ≈ 0.1 mM, corresponding to the maximal screening length of ∼30 nm. While this value could be further increased by decreasing the charge or concentration of particles in solution, weakly charged particles become increasingly ineffective at inducing dipoles in neighboring particles and, for entropic reasons, are unlikely to assemble from increasingly dilute solutions.28 Finally, the combination of monopolar and dipolar interactions can mediate formation of larger, low-symmetry assemblies. One example is shown in Figure 5a,b, where the partial stacking of the like-charged NRs is facilitated by the oppositely charged NPs attached at the rods’ tips. Qualitatively, this configuration represents a trade-off between (i) the attractive tendency of the rod-particle “dipoles” to assemble side-to-side and (ii) the repulsive interactions due to the like-charge repulsions between rod’s surfaces. The energy of the structure is due to charge-charge, chargedipole, and dipole-dipole interactions, and in the limit of infinite screening length can be approximated as
(
QsQr Qr2 QsQr Qs2 1 U(∆) ) 2 + +2 + 4πε0ε R1 R2 R3 R4 2
Qs2Vrα 2R14
-
p2 3p2 cos θ + R23 R23
)
FIGURE 5. “Dimeric” NP-NR assemblies. (a) Scheme (left) and dimensions (right) of the assemblies formed by “dimerization” of NR-NP “dipoles” at higher particle concentrations (3.0 × 1015 NPs/ mL and 7.6 × 1014 NRs/mL) and in the regime of large screening length (salt concentration cS ≈ 1 mM). (b) Representative TEM images (scale bar )20 nm). (c) Dimensionless electrostatic energy (normalized with respect to the energetic minimum) calculated using eq 2 in the main text and plotted as a function of the “overlap” parameter, ∆ ) 2ar/[tan(θ)(2Lr + as)]. The minimum of energy, ∆ ) 0.37, agrees well with the configuration observed in experiments, ∆ ) 0.43.
structures that would be difficultsif not impossiblesto realize using other types of interparticle potentials. From a fundamental point of view, the singular feature of nanoscale electrostatics is the importance of polarization effects that can often dominate the familiar charge-charge interactions and can lead to such counterintuitive phenomena as like-charge attraction. Design of nanostructured materials based on electrostatic forces will therefore require a deeper understanding and appreciation of the many-body polarization effects present in assemblies of charged, nonspherical nanoobjects.
(2)
Acknowledgment. This work was supported by the Nonequilibrium Energy Research Center (NERC) which is an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0000989.
where ∆ is the scaled horizontal distance between the centers of mass of the two rods, p is the dipole moment induced in the NRs, and other dimensions are defined in the right scheme of Figure 5a. The energy diagram constructed using this formula (Figure 5c) predicts the energetic minimum at ∆ ≈ 0.37, which agrees well with experimental results (∆ ≈ 0.43). In summary, electrostatic interaction at the nanoscale can be “engineered” to assemble small nanoscopic objects into © 2010 American Chemical Society
Supporting Information Available. Detailed discussions of electrostatic calculations, polarizability, van der Waals interactions, nanoparticle synthesis, and assembly experiments. This material is available free of charge via the Internet at http://pubs.acs.org. 2279
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REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18)
(19) Hao, E.; Schatz, G. C. J. Chem. Phys. 2004, 120, 357–366. (20) Kalsin, A. M.; Kowalczyk, B.; Wesson, P.; Paszewski, M.; Grzybowski, B. A. J. Am. Chem. Soc. 2007, 129, 6664–6665. (21) Kalsin, A. M.; Kowalczyk, B.; Smoukov, S. K.; Klajn, R.; Grzybowski, B. A. J. Am. Chem. Soc. 2006, 128, 15046–15047. (22) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957– 1962. (23) Millstone, J. E.; Wei, W.; Jones, M. R.; Yoo, H. J.; Mirkin, C. A. Nano Lett. 2008, 8, 2526–2529. (24) Lin, X. M.; Jaeger, H. M.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 2001, 105, 3353–3357. (25) Ming, T.; Kou, X. S.; Chen, H. J.; Wang, T.; Tam, H. L.; Cheah, K. W.; Chen, J. Y.; Wang, J. F. Angew. Chem., Int. Ed. 2008, 47, 9685–9690. (26) Mitamura, K.; Imae, T.; Saito, N.; Takai, O. J. Phys. Chem. B 2007, 111, 8891–8898. (27) Dogic, Z.; Fraden, S. Curr. Opin. Colloid Interface Sci. 2006, 11, 47–55. (28) Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Small 2009, 5, 1600–1630. (29) Other works investigating assembly of NRs and NPs via evaporative effects report concentrations ranging from ∼1012 to 1013 NRs/ mL or NPs/mL necessary for the development of ordered structures. Here we use concentrations 1 to 2 orders of magnitude below these critical concentrations. (30) Deryaguin, B. V. 1934, 69, 155. (31) White, L. R. J. Colloid Interface Sci. 1983, 95, 286–8. (32) Venermo, J.; Sihvola, A. J. Electrost. 2005, 63, 101–117. (33) Bishop, K. J. M.; Grzybowski, B. A. ChemPhysChem 2007, 8, 2171– 2176. (34) Bishop, K. J. M.; Kowalczyk, B.; Grzybowski, B. A. J. Phys. Chem. B 2009, 113, 1413–1417. (35) Verwey, E. J. W.; Overbeek, J. T. G., Theory of the stability of lyophobic colloids; Dover: New York, 1948.
Kim, P.; Lieber, C. M. Science 1999, 286, 2148–2150. Boggild, P.; Hansen, T. M.; Tanasa, C.; Grey, F. Nanotechnology 2001, 12, 331–335. Xie, H.; Haliyo, D. S.; Regnier, S. Nanotechnology 2009, 20, 9– 14. Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A. Science 2006, 312, 420–424. Kolny, J.; Kornowski, A.; Weller, H. Nano Lett. 2002, 2, 361–364. 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. Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55–59. Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237–240. Tang, Z. Y.; Zhang, Z. L.; Wang, Y.; Glotzer, S. C.; Kotov, N. A. Science 2006, 314, 274–278. Correa-Duarte, Miguel A.; Perez-Juste, J.; Sanchez-Iglesias, A.; Giersig, M.; Liz-Marzan, Luis M. Angew. Chem., Int. Ed. 2005, 44, 4375–4378. Gole, A.; Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20, 7117–22. Niidome, Y.; Takahashi, H.; Urakawa, S.; Nishioka, K.; Yamada, S. Chem. Lett. 2004, 33, 454–455. Pramod, P.; Joseph, S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2007, 129, 6712–6713. Vinod, T. P.; Yang, M.; Kim, J.; Kotov, N. A. Langmuir 2009, 25, 13545–13550. Cooper, L. N. Phys. Rev. 1956, 104, 1189–1190. Aranson, I. S.; Blair, D.; Kalatsky, V. A.; Crabtree, G. W.; Kowk, W. K.; Vinokur, V. M.; Welp, U. Phys. Rev. Lett. 2000, 84, 3306– 3309. Trau, M.; Sankaran, S.; Saville, D. A.; Aksay, I. A. Nature 1995, 374, 437–439. Wei, Y. H.; Bishop, K. J. M.; Kim, J.; Soh, S.; Grzybowski, B. A. Angew. Chem., Int. Ed. 2009, 48, 9477–9480.
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