Investigations into the Electrostatically Induced Aggregation of Au

Langmuir 2000, 16, 8789-8795

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Investigations into the Electrostatically Induced Aggregation of Au Nanoparticles† Andrew N. Shipway, Michal Lahav, Rachel Gabai, and Itamar Willner* Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received March 2, 2000. In Final Form: June 19, 2000 The aggregation of Au nanoparticles in solution is induced and influenced by cationic and oligocationic species. This solution-state aggregation bears similarities to multilayer formation on surfaces but is more facile because of the nanoparticles’ intrinsic instability in solution. Aggregation is followed by transmission electron microscopy (TEM) and the appearance of features at λ ) 600-900 nm in the absorbance spectrum. It is found that these features are a function of factors such as the aggregant size, charge, and concentration, and the method of mixing the components, and they can be related to aggregate morphology. It seems that there are two mechanisms that can act to cause aggregation. Multiply charged aggregants can bind nanoparticles together into dense aggregates, displaying a defined absorbance at ca. λ ) 700 nm, whereas singly charged aggregants cause a slower aggregation into string-like aggregates with a less defined absorbance. Whereas multiply charged aggregants can “cross-link” the layers in a multilayer structure on a surface, singly charged aggregants cannot.

Introduction Extensive recent research effort has been directed toward the organization of chemical assemblies of nanoscale dimensions.1 In addition to new concepts for the engineering of miniaturized devices and their possible impact on nanotechnology, materials of nanoscale dimensions can give rise to quantum effects, resulting in unique electronic, optical, and nonlinear properties. To construct such devices, however, new fabrication techniques must be developed, and new building blocks must be utilized. With this goal in mind, microfabrication methodologies have steadily improved “top-down” engineering, and the world of supramolecular chemistry2 has approached the need from the “bottom up”. Despite these advances, however, the intermediate scale of 1-100 nm remains relatively unexploited. The world of colloid and cluster science has risen to this challenge and offers a wide range of nanoparticles with various sizes and properties.3 With this opportunity in hand, we are left with a need to control the organization of these nanoparticles at the smallest possible scale. This challenge has been addressed to some extent by a large body of recent work describing the construction of colloid dyads and triads,4 “strings”,5 clusters,6 and multilayer architectures7 among other examples.6,8 These assemblies have been built by a variety of techniques, such as the utilization of specific interactions † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. * To whom correspondence should be addressed. Telephone: 9722-6585272. Fax: 972-2-6527715. E-mail: [email protected].

(1) Nanosystems, Molecular Machinery, Manufacturing and Computation; Drexler, K. E., Ed.; Wiley: New York, 1992. Carter, F. L.; Schultz, A.; Duckworth, D. In Molecular Electronic Devices; Carter, F. L., Ed.; Marcel Dekker: New York, 1987; pp 183-199. Bradley, D. Science 1993, 259, 890-892. (2) Lehn, J.-M. In Supramolecular Chemistry; VCH: Weinheim, Germany, 1995. Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89-112. (3) Schmid, G. Chem. Rev. 1992, 92, 1709-1727. Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18-52. (4) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr.; Schultz, P. G. Nature 1996, 382, 609-611. (5) Marinakos, S. M.; Brousseau, L. L., III; Jones, A.; Feldheim, D. L. Chem. Mater. 1998, 10, 1214-1219. (6) Antonietti, M.; Go¨ltner, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 910-928.

of functionalized colloids, the use of a mold, the use of micellar “nanoreactors”, and the use of stepwise surface treatments, respectively, for the four examples above. In addition, research by Whitesides on self-organization at the millimeter scale has cast light on the possibilities at smaller dimensions.9 The phenomenon of aggregation or flocculation of the particles in a Au colloid solution upon the addition of a “cross-linking” agent is well documented.10-12 The process begins with a color change, which is eventually followed by the precipitation of the nanoparticles. This effect is generally considered a nuisance to the scientist, who would like to have the colloid in a stable solution along with other solutes, for instance, for the study of surfaceenhanced Raman scattering (SERS)13 or surface plasmon spectroscopy (SPS).14 Attempts at controlling the aggregation process have centered on carboxylate-functionalized colloids, which display flocculation behavior as a function of pH.15 Although at high (completely dissociated) (7) Vossmeyer, T.; DeIonno, E.; Heath, J. R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1080-1083. Blonder, R.; Sheeney, L.; Willner, I. Chem. Commun. 1998, 1393-1394. Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61-65. Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499-1501. (8) Baker, B. E.; Kline, N. J.; Treado, P. K.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 8721-8722. Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396-5401. Sato, T.; Ahmed, H. Appl. Phys. Lett. 1997, 70, 2759-2761. Feldheim, D. L.; Keating, C. D. Chem. Soc. Rev. 1998, 27, 1-12. (9) Terfort, A.; Bowden, N.; Whitesides, G. M. Nature 1997, 386, 162-164. Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M. Science 1997, 276, 233-235. Huck, W. T.; Tien, J.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 8267-8268. Tien, J.; Breen, T. L.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 12670-12671. (10) Olivier, B. J.; Sorensen, C. M. J. Colloid Interface Sci. 1990, 134, 139-146. Olivier, B. J.; Sorensen, C. M. Phys. Rev. A. 1990, 41, 2093-2100. (11) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J.; Anal. Chem. 1995, 67, 735-743. Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795-797. (12) Fe´lidj, N.; Le´vi, G.; Pantigny, J.; Aubard, J. New J. Chem. 1998, 725-732. (13) Campion, A.; Kambhampati, P. Chem. Soc. Rev. 1998, 27, 241250. Moskovits, M. Rev. Mod. Phys. 1985, 57, 783-826. (14) Mulvaney, P. Langmuir 1996, 12, 788-800. (15) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 39443947. Sastry, M.; Mayya, K. S.; Bandyopadhyay, K. Colloids Surf. A: Phisiochem. Eng. Aspects 1997, 127, 221-228.

10.1021/la000316k CCC: $19.00 © 2000 American Chemical Society Published on Web 08/19/2000

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pH these colloids are stable, a lowering of the pH leads to some protonation of the carboxylic functions, allowing interactions between the particles and, thus, aggregation. These aggregates can often be redispersed by a subsequent rise in pH. Similar reversible aggregation can be observed for DNA-coated colloids, for which the temperature of the single particle-aggregate transition is indicative of the degree of DNA complementarity.16 The close contact of nanoparticles that display a plasmon absorbance leads to the appearance of an absorbance band attributed to the coupling of the plasmon absorbances of the particles (the color change noted above). This property can be modeled theoretically17 and has been shown by a Langmuir-Blodgett technique to emerge at interparticle separations below ca. 5 nm.18 Integration of the spectral region containing this feature has been used as a semiquantitative assay of the degree of flocculation within a solution,19 and it has also been used as an indicator in the DNA sensors noted above. This characteristic absorbance also appears when multilayers of colloidal particles are constructed on surfaces as a consequence of their proximity to each other.20 This appearance of an additional plasmon absorbance is of particular interest to the field of SERS and has led to research effort directed at the stabilization of small colloid aggregates in solution,12 either by the modification of the nanoparticles or by their encapsulation in a matrix. Here, we present an analysis of the aggregation behavior of citrate-stabilized Au colloids (ca. 12 nm) as provoked by the addition of cationic molecules or cationic nanoparticles. These agents cause the cross-linking of the nanoparticles as a consequence of electrostatic interactions between the positive charges and the negatively charged citrate coating on the Au particles. The aggregates are characterized by absorbance spectroscopy and transmission electron microscopy (TEM) and are compared to multilayer assemblies constructed on glass supports. Experimental Section Materials. Cyclobis(paraquat-p-phenylene)21 was synthesized according to literature procedures. Twelvenanometer Au colloids22 were synthesized by the addition of AuHCl4‚3H2O (50 mg) to a refluxing, rapidly stirred, solution of sodium citrate (100 mg) in water (500 mL), which was stirred under reflux for an additional 15 min before being allowed to cool. Particle size was verified by absorbance spectroscopy and TEM. Amine-functionalized silica nanoparticles were synthesized by a literature method.23 Tetraethyl orthosilicate (1.5 mL) was added to

Shipway et al.

a solution of ammonium hydroxide (2 mL for 100-nm particles, 3 mL for 250-nm particles) in ethanol (50 mL) and stirred for 24 h to yield silica nanoparticles. 3-Aminopropyl triethoxysilane (25 µL) was added, and stirring was continued for an additional 24 h, after which the mixture was heated to reflux for 1 h. The cooled mixture was washed by centrifugation (3 times with ethanol, and then 3 times with water) to give amino-substituted SiO2 nanoparticles. The amino-nanoparticles were modified by N-carboxyundecyl-N′-methyl-4,4′-bipyridyl (50 mg) in the presence of 1-[3-(dimethylamino)-propyl]-3-ethylcarbodiimide hydrochloride (EDC) (35 mg) in a 1:1 mixture of water-ethanol (50 mL) for 3 h, after which they were washed by centrifugation (3 times with ethanol, and then 3 times with water). The presence of bipyridinium (viologen) groups on the silica nanoparticles was verified by their electrochemical response. Ultrapure water was used throughout this work and was obtained from a Nanopure system (Barnstead). All other materials were used as purchased from Aldrich. Glass Modifications. Cleaning and silylation of the electrodes were performed according to literature procedures.24 The electrodes were washed with ethanol, sonicated for 15 min in 5% HCl, and washed with ethanol and then with water. Silylation was performed by immersion of the glass surface in a 2-3% solution of (3-aminopropyl)triethoxysilane for ca. 5 min, after which they were washed thoroughly with ethanol, and then heated to 110 °C for 10 min. Finally, the silylated electrodes were washed with water. Colloid treatment of the surface consisted of immersion in the colloid solution for 2 h, and “cross-linker” treatment with immersion in a ca. 30 mM solution of the chloride salt of the cross-linker for 30 min. After each treatment, the glass was washed three times with water. Special attention was taken to avoid drying of the slide, and functionalized slides were stored in pure water. Instrumentation. Absorbance spectra were recorded on a Uvikon 860 (Kontron) spectrometer. Transmission electron micrographs (TEMs) were obtained on a JEOL 100 CX instrument. Formvar/300-mesh carbon-coated grids were pretreated by the evaporation of a drop of aqueous (3-aminopropyl)triethoxysilane (1 ppm) and were then washed with water. The sample was deposited on the grid by interaction for 10 min followed by washing with water to remove any unbound particles and excess salts. Dynamic light scattering experiments were performed on a Mastersizer µ+ instrument manufactured by Malvern Instruments Ltd., U.K. Results and Discussion

(16) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607-609. Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078-1081. Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959-1964. (17) Quinton, M.; Kreibig, U. Surf. Sci. 1986, 172, 557-577. Blatchord, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 120, 435-455. (18) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978-1981. (19) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763-3772. (20) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353-3261. (21) Odell, B.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1547-1550. Asakawa, M.; Dehaen, W.; L’abbe, G.; Menzer, S.; Nouwen, J.; Raymo, F. M.; Stoddart, J. F.; Williams, D. J. J. Org. Chem. 1996, 61, 9591-9595. (22) Turkevich, J.; Stevenson, P. C.; Hiller, J. Discuss. Faraday Soc. 1951, 11, 55-57. (23) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396-5401.

We have previously described the construction of multilayered Au nanoparticle assemblies on aminefunctionalized glass substrates by the alternate treatment of the substrate with solutions of (negatively charged) citrate-stabilized nanoparticles and (positively charged) oligocationic molecules (Scheme 1).25 Although the nanoparticle assemblies are formed only by electrostatic interactions between the organic cross-linker and the nanoparticle’s surface capping, these assemblies are highly stable, only being removed by physical scratching or (24) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313-1317. (25) Shipway, A. N.; Lahav, M.; Blonder, R.; Willner, I. Chem. Mater. 1999, 11, 13-15. Lahav, M.; Gabriel, T.; Shipway, A. N.; Willner, I. J. Am. Chem. Soc. 1999, 121, 258-259. Lahav, M.; Shipway, A. N.; Willner, I. J. Chem. Soc., Perkin Trans. 2 1999, 1925-1931. Lahav, M.; Shipway, A. N.; Willner, I.; Nielsen, M. B.; Stoddart, J. F. J. Electroanal. Chem. 2000, 482, 217-221. Lahav, M.; Gabai, R.; Shipway, A. N.; Willner, I. Chem. Commun. 1999, 1937-1938. Kharitonov, A. B.; Shipway, A. N.; Willner, I. Anal. Chem. 1999, 71, 5441-5443.

Electrostatically Induced Aggregation of Au Nanoparticles

Figure 1. Absorbance spectra of Au colloid multilayer assemblies cross-linked with (A) cyclobis(paraquat-p-phenylene) (1) or (B) methyl viologen (2). (a) 1 colloid layer, (b)-(e) 2-5 layers, respectively.

extreme electrochemical conditions. When constructed on conductive (ITO) glass, the electrochemical analysis of the Au nanoparticle surface and the cross-linker molecule allows the quantitative analysis of the arrays, demonstrating that the films are built up in a linear manner, each layer containing the same amount of material. Figure 1 shows absorbance spectra for 1-5-layer assemblies of 12-nm Au nanoparticles “cross-linked” by either (A) cyclobis(paraquat-p-phenylene) (1) or (B) N,N′-dimethy4,4′-bipyridinium (methyl viologen) (2). The plasmon absorbance of the Au nanoparticles is clearly visible at ca. 520 nm, as well as a second feature that begins to appear (at around 600 nm) after the second layer and strengthens and red-shifts as more layers are added. This absorbance is attributed to the coupled plasmon absorbances of nanoparticles that are in close contact and is expected to strengthen (particularly at long wavelengths) as interactions become longer-range. In these oligocation-cross-linked structures, the Au nanoparticles come into very close contact as a consequence of the small size of the cross-linker. This close contact is believed to bring about the additional absorbances above 600 nm. To test this hypothesis, we undertook the preparation of Au-nanoparticle multilayer structures in which the Au nanoparticles are more widely spaced. The ability to control this property could also allow the construction of Au-nanoparticle matrices with tunable optical and electronic properties. Silica nanoparticles are ideal candidates for these larger cross-linkers. Spherical silica nanoparticles can be synthesized as monodisperse suspensions and are nonabsorbing at wavelengths from 500 to 900 nm. Figure 2A shows how silica nanoparticles bearing positively charged amine and viologen units were synthesized. Particles with diameters of 100 and 250 nm

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were synthesized (as confirmed by TEM), and the presence of the viologen moieties was qualitatively confirmed by electrochemical analysis of the suspension. The particles were used as cross-linkers for the construction of 5-layer assemblies of Au nanoparticles. Figure 2C shows the absorbance spectra for structures cross-linked by (a) 1, (b) 100-nm SiO2 particles, and (c) 250-nm SiO2 particles. As the cross-linker size increases, the interparticle plasmon absorbance decreases. In the case of the 250-nm SiO2 cross-linker, only the single-particle plasmon absorbance at ca. 520 nm is visible. The appearance of the coupled plasmon band upon multilayer build-up is in common with the changes observed upon the addition of a cross-linker to a solution of Au nanoparticles. Figure 3 shows spectra of Au nanoparticle solutions 15 min after the addition of various quantities of 1. Figure 3A shows photographs and Figure 3B shows absorbance spectra of colloid solutions that have been mixed with small quantities of 1. Cuvette 0 contains no 1, cuvette 1 contains ca. 17 molecules of 1 for each colloid particle, and the others contain incrementally ca. 17 molecules per colloid particle more, so that cuvette 6 contains 100 molecules per particle (