Thiol-Induced Assembly of Au Nanoparticles into Chainlike Structures

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Thiol-Induced Assembly of Au Nanoparticles into Chainlike Structures and Their Fixing by Encapsulation in Silica Shells or Gelatin Microspheres Eun Chul Cho, Sung-Wook Choi, Pedro H. C. Camargo, and Younan Xia* Department of Biomedical Engineering, Washington University, St. Louis, Missouri 63130 Received January 11, 2010. Revised Manuscript Received February 25, 2010 This article describes a simple method for controlling the assembly of gold nanoparticles (Au NPs) into chainlike structures with tunable lengths and interparticle separations. The chainlike assemblies were induced to form by adding HS(CH2)nCOOH (n = 2, 10, and 15) into a suspension of Au NPs in a mixture of ethanol and water (98:2 by volume). The number of Au NPs in the chainlike assemblies could be altered by varying the concentration of the thiol while the interparticle distance between Au NPs in the chain could be adjusted by using thiols of different chain lengths. The chainlike assemblies of Au NPs were structurally unstable during storage and purification. We solved this problem by fixing the chainlike assemblies with silica coating via the St€ober method or by encapsulating them inside gelatin microspheres with the use of a fluidic device. After fixing, the chainlike assemblies of Au NPs could be preserved for a long period of time, during which their characteristic optical properties remained unchanged.

Introduction There are two major approaches for manipulating the properties of metal nanoparticles: (i) control over their size, shape, composition, and structure (solid vs hollow)1-4 and (ii) control *Corresponding author. E-mail: [email protected]. (1) (a) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257–264. (b) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41, 1578–1586. (2) (a) Mohamed, M. B.; Volkov, V.; Link, S.; El-Sayed, M. A. Chem. Phys. Lett. 2000, 317, 517–523. (b) Hu, M.; Chen, J.; Li, Z.-Y.; Au, L.; Hartland, G. V.; Li, X.; Marqueze, M.; Xia, Y. Chem. Soc. Rev. 2006, 35, 1084–1094. (3) (a) Wiley, B.; Sun, Y.; Xia, Y. Acc. Chem. Res. 2007, 40, 1067–1076. (b) Xiong, Y.; Wiley, B.; Chen, J.; Li, Z.-Y.; Yin, Y.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 7913–7917. (4) (a) Metraux, G. S.; Cao, Y. C.; Jin, R.; Mirkin, C. A. Nano Lett. 2003, 3, 519– 522. (b) Skrabalak, S. E.; Au, L.; Li, X.; Xia, Y. Nat. Protoc. 2007, 2, 2182–2190. (5) Bishop, K. J.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Small 2009, 5, 1600– 1630. (6) (a) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335– 1338. (b) Fan, H.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, H.; Lopez, G. P.; Brinker, J. Science 2004, 304, 567–571. (c) Korgel, B. A.; Fitzmaurice, D. Adv. Mater. 1998, 10, 661–665. (d) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188–1191. (e) Petit, C.; Taleb, A.; Pileni, M.-P. Adv. Mater. 1998, 10, 259–261. (7) (a) Wei, Y.; Bishop, K. J. M.; Kim, J.; Soh, S.; Grzybowski, B. A. Angew. Chem., Int. Ed. 2009, 48, 9477–9480. (b) Klajn, R.; Bishop, K. J. M.; Fialkowski, M.; Paszewski, M.; Campbell, C. J.; Gray, T. P.; Grzybowski, B. A. Science 2007, 316, 261– 264. (c) Sidhaye, D. S.; Kashyap, S.; Sastry, M.; Hotha, S.; Prasad, B. L. V. Langmuir 2005, 21, 7979–7984. (d) Wang, T.; Zhang, D.; Xu, W.; Li, S.; Zhu, D. Langmuir 2002, 18, 8655–8659. (8) (a) Mirkin, C. A.; Retsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607–609. (b) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078–1081. (c) Li, M.; Kim, K.; Wong, W.; Mann, S. Chem. Mater. 1999, 11, 23–26. (d) Sastry, M.; Lala, N.; Patil, V.; Chavan, S. P.; Chittiboyina, A. G. Langmuir 1998, 14, 4138–4142. (9) Kohut, A.; Voronov, A.; Peukert, W. Part. Part. Syst. Charact. 2005, 22, 329–335. (10) (a) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515–7520. (b) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262–1269. (11) (a) Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A. Science 2006, 312, 420–424. (b) Kalsin, A. M.; Grzybowski, B. A. Nano Lett. 2007, 7, 1018–1021. (c) Kalsin, A. M.; Pinchuk, A. O.; Smoukov, S. K.; Paszewski, M.; Schatz, G. C.; Grzybowski, B. A. Nano Lett. 2006, 6, 1896–1903. (d) Kalsin, A. M.; Kowalczyk, B.; Smoukov, S. K.; Klajn, R.; Grzybowski, B. A. J. Am. Chem. Soc. 2006, 128, 15046–15047. (12) Maheshwari, V.; Kane, J.; Saraf, R. F. Adv. Mater. 2008, 20, 284–287. (13) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237–240. (14) Zhang, H.; Wang, D. Angew. Chem., Int. Ed. 2008, 47, 3984–3987. (15) Rycenga, M.; McLellan, J. M.; Xia, Y. Adv. Mater. 2008, 20, 2416–2420. (16) Glotzer, S. C.; Solomon, M. Nat. Mater. 2007, 6, 557–562.

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over their assembly (or aggregation) into larger structures.5-16 The assembly can be induced by solvent evaporation6 or through dedicated procedures involving chemical reactions,7 special molecular interactions between oligonucleotides or biotin-avidin,8,9 van der Waals forces,10 electrostatic forces,11,12 dipole-dipole interaction,13 solvent exchange,14 modification of selected facets of nanoparticles with organic molecules,15,16 magnetic fields,17 lipid (surfactant) assembly,18,19 and polymer assistance.20 The assemblies can be constructed in one to two dimensions to provide the nanoparticles with various new properties. Recently, 1-D assemblies of gold nanoparticles (Au NPs) have received considerable attention because their optical properties are similar to those of Au nanorods or nanowires.9-15,18,20-23 As with 2-D or 3-D structures, specific interaction between particles is required for the1-D assembly, but the interaction must be spatially specific for the Au NPs to grow one-dimensionally.12-15,21-24 To date, most reported studies have been focused on creating the 1-D assemblies by adjusting the concentrations of ligands which could induce the assembly. However, this approach leaves out many other parameters, such as the interparticle separation, for precisely controlling the optical properties of resultant assemblies. Despite the relative easiness of 1-D assembly with Au NPs, the assembled structures are often structurally unstable during (17) Ahniyaz, A.; Sakamoto, Y.; Bergstrom, L. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 17570–17574. (18) (a) Bakshi, M. S.; Possmayer, F.; Petersen, N. O. J. Phys. Chem. C 2007, 111, 14113–14124. (b) Bakshi, M. S.; Possmayer, F.; Petersen, N. O. Chem. Mater. 2007, 19, 1257–1266. (c) Zhang, Y. X.; Zeng, H. C. Langmuir 2008, 24, 3740–3746. (19) Li, X.; Li, Y.; Yang, C.; Li, Y. Langmuir 2004, 20, 3734–3739. (20) Kang, Y.; Erickson, K. J.; Taton, T. A. J. Am. Chem. Soc. 2005, 127, 13800– 13801. (21) (a) Nie, Z.; Fava, D.; Kumacheva, E.; Zou, S.; Walker, G. C.; Rubinstein, M. Nat. Mater. 2007, 6, 600–614. (b) Fava, D.; Nie, Z.; Winnik, M. A.; Kumacheva, E. Adv. Mater. 2008, 20, 4318–4322. (c) Pramod, P.; Thomas, K. G. Adv. Mater. 2008, 20, 4300–4305. (22) (a) Bakshi, M. S.; Possmayer, F.; Petersen, N. O. J. Phys. Chem. C 2007, 111, 14113–14124. (b) Bakshi, M. S.; Possmayer, F.; Petersen, N. O. Chem. Mater. 2007, 19, 1257–1266. (23) Chen, C.-L.; Zhang, P.; Rosi, N. L. J. Am. Chem. Soc. 2008, 130, 13555– 13557. (24) (a) Murugadoss, A.; Chattopadhyay, A. J. Phys. Chem. C 2008, 112, 11265– 11271. (b) Zhang, H.; Fung, K.-H.; Hartmann, J.; Chan, C. T.; Wang, D. J. Phys. Chem. C 2008, 112, 16830–16839. (c) Zhu, L.; Xue, D.; Wang, Z. Langmuir 2008, 24, 11385– 11389. (d) Polavarapu, L.; Xu, Q.-H. Langmuir 2008, 24, 10608–10611.

Published on Web 03/10/2010

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Cho et al. 17.7 ( 1.6 nm, as obtained by transmission electron microscopy (TEM) imaging. Chainlike Assemblies of Au NPs. The as-prepared Au NPs were centrifuged at 13 200 rpm to remove the excess sodium citrate and HAuCl4. After the supernatant had been removed, the precipitate was redispersed with ethanol. During the purification process, a small amount of water remained (∼2%, v/v). The chainlike assemblies of Au NPs were obtained by adding a solution of HS(CH2)nCOOH into the suspension of Au NPs in a mixture of ethanol and water. The number of Au NPs in the assemblies was controlled by varying the concentration of the thiol. We also used different thiols to investigate the effect of alkyl chain length (n = 2, 10, and 15) on the spacing between the Au NPs in the assemblies and their optical properties.

Fixing the Assemblies of Au NPs by Silica Coating.

Figure 1. Schematic illustrating the preparation of chainlike assemblies of Au NPs. A typical procedure involves the addition of a HOOC-terminated alkanethiol, SH-(CH2)n-COOH, into a suspension of Au NPs in a mixture of ethanol and water. The number of Au NPs in the assemblies can be controlled by adjusting the concentration of the thiol while the separation between adjacent Au NPs in the assemblies can be tuned by the number (n) of methylene units in the thiol.

storage and purification. They become gradually aggregated into larger structures with time and finally precipitated out from the suspension medium. In addition, the original 1-D structures are not easily recovered after purification process by centrifugation. Under these conditions, the optical properties of the 1-D assemblies are often changed to undesired one. Therefore, it is practically important to develop methods for permanently fixing the structures and thus their optical properties. In this article, we describe a simple method for controlling the assemblies of Au NPs into chainlike structures with tunable lengths and interparticle separation. A schematic illustration is shown in Figure 1. The chainlike assemblies were induced to form by adding HS(CH2)nCOOH (n = 2, 10, and 15) into a suspension of Au NPs in a mixture of ethanol and water. The number of Au NPs in the chainlike assemblies could be controlled by adjusting the concentration of the thiol while the interparticle separation between the Au NPs could be tuned by using thiols with different alkyl chain lengths, n. We could then fix the structure of these assemblies by coating them with silica shells using the St€ober method or encapsulating them inside gelatin microspheres with a simple fluidic device, as shown in Figure 2. After fixing, the assemblies of Au NPs could be preserved for a long period of time without any changes to their optical properties.

Experimental Section Materials. Gold(III) chloride trihydrate (HAuCl4 3 3H2O, g99.9%), 3-mercaptopropionic acid (MPA, HS(CH2)2COOH, g99%), 11-mercaptoundecanoic acid (MUA, HS(CH2)10COOH, 99%), 16-mercaptohexadecanoic acid (MHA, HS(CH2)15COOH, 99%), sodium citrate, tetraethyl orthosilicate (TEOS, g99%), ammonia (g99.99%), propanol (99.7%), and toluene (99.9%) were all obtained from Aldrich (Milwaukee, WI) and used as received. Ethanol (100%) was obtained from AAPER Alcohol and Chemical (Philadelphia, PA). Preparation of Au NPs. Au NPs were prepared by reduction of HAuCl4 with sodium citrate. In a typical synthesis, 2 mg of HAuCl4 3 3H2O was dissolved in 19 mL of deionized water, and this solution was heated up to 100 °C. After the addition of 1 mL of 0.5 wt % sodium citrate, the solution turned pink within 5 min. The resulting Au NPs had an average diameter of 10006 DOI: 10.1021/la100127w

Figure 2A shows a schematic illustrating the fixing procedure. Fixing was performed by adding 1 mL of an ethanol/water (98:2 by volume) mixture containing the assemblies of Au NPs into 9 mL of 2-propanol, followed by consecutive addition of 0.5 mL of ammonia and 15 μL of TEOS to the dispersion. After 3 h, the silica-coated assemblies of Au NPs were harvested by repeated centrifugation and wash with deionized water. The final products were stored in deionized water. We also prepared silica-coated Au NPs the same way, except 1 mL of an ethanol/water containing Au NPs was added into 9 mL of 2-propanol, instead of adding Au NPs assemblies dispersion.

Encapsulation of the Assemblies of Au NPs in Gelatin Microspheres. For encapsulating the assemblies of Au NPs within gelatin microspheres, we employed a fluidic device fabricated with a PVC tube, a syringe needle, and a glass capillary tube (Figure 2B).25 First, we prepared chainlike assemblies by adding MPA of various concentrations into an aqueous dispersion of Au NPs. After reaction for 8-24 h, the chainlike assemblies were mixed with 20 wt % gelatin aqueous solution at a ratio of 1:1 by volume. A glass syringe filled with a mixture of the assemblies and gelatin solution was heated at 40 °C using a heating band to prevent gelation of the mixture. The mixture (served as a discontinuous phase) was then introduced into the fluidic device to form waterin-oil emulsion, where toluene containing 3 wt % of Span 80 served as continuous and collection phases. The flow rates of the water and toluene phases were 0.05 and 0.5 mL/min, respectively. The gelatin emulsion containing the assemblies of Au NPs flew into the collection phase that was cooled with an ice bath and then gently stirred overnight to allow solvent to evaporate. After washing several times with methanol, the resultant microspheres were stored in methanol. We also encapsulated Au NPs in gelatin microspheres. We first added 10 μL of 0.1 wt % poly(vinyl alcohol) (PVA) aqueous solution to 1 mL of the Au NPs dispersion to prevent the Au NPs from aggregation, and then the PVA-coated Au NPs were mixed with 20 wt % gelatin aqueous solution at a ratio of 1:1 by volume. The subsequent procedures were the same as those for encapsulating the Au NPs assemblies. Characterization of the Assemblies of Au NPs. We characterized the Au NPs, chainlike assemblies of Au NPs, silicacoated assemblies, and gelatin-encapsulated assemblies using a number of tools include TEM (Hitachi H-7500), scanning electron microscope (SEM, FEI Nova NanoSEM 2300), and optical microscope (Carl Zeiss Axio, Germany). We investigated the optical properties of these structures using a UV-vis spectrometer (Varian 50Bio). The concentration of Au NPs in the suspension was determined using inductively coupled plasma mass spectroscopy (ICP-MS, ICP-MS 7500CS, Agilent Technologies) in conjunction with TEM imaging.

Results and Discussion Formation of Chainlike Structures from Au NPs. Figure 3 shows TEM images of Au NPs and their chainlike assemblies (25) Choi, S.-W.; Cheong, I. W.; Kim, J.-H.; Xia, Y. Small 2009, 5, 454–459.

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Figure 2. (A) Schematic showing how to fix chainlike assemblies of Au NPs by coating with silica shells or by encapsulating them in gelatin microspheres. The chainlike assemblies of Au NPs were induced to form by adding 3-mercaptopropionic acid (MPA) into a suspension of Au NPs in water or a mixture of ethanol and water. (B) Schematic of a simple fluidic device used for the preparation of gelatin microspheres containing chainlike assemblies of Au NPs. The chainlike assemblies of Au NPs were mixed with an aqueous gelatin solution, and the mixture was then injected into the oil phase to generate a water-in-oil (W/O) emulsion. The resultant microspheres were collected by cooling the gelatin emulsion and washing the microspheres with methanol.

Figure 3. TEM images showing chainlike assemblies of Au NPs formed with MPA (n = 2) at different concentrations: (A) 0, (B) 0.1, (C) 1, and (D) 2 mM. The reaction time was 24 h for all samples.

formed by adding MPA (n = 2) into the dispersion of Au NPs. As shown in Figure 3A, the Au NPs were well dispersed in a mixture of ethanol and water before adding MPA, and they remained well separated on the TEM grid until the concentration of MPA had reached 10-4 mM. When the concentration of MPA was further increased, the Au NPs began to assemble into chainlike structures. At 0.1 mM, we observed short chains containing a few Au NPs (Figure 3B). The chains became longer at 1 mM MPA (Figure 3C), and the chains tended to be connected to form a network of chains and the assemblies Langmuir 2010, 26(12), 10005–10012

became a 2-D web (Figure 3D) at 2 mM MPA. The same result was also observed for the reaction of Au NPs with 1 mM MPA for 3 days. We also obtained chainlike assemblies by adding MUA (n = 10) into a dispersion of Au NPs (Figure 4). The number of Au NPs increased with respect to the concentration of MUA, and the Au NPs finally reached a network of chains. However, the growth rate of the chains treated with MUA was slower than that with MPA. It took 2 weeks to obtain the chainlike network structure with 1 mM MUA, while it took only 2-3 days to obtain a similar structure with 1 mM MPA. At a much higher concentration of MUA (4 mM, Figure 4C), it took only 24 h to obtain the chain network. We determined the concentration of Au NPs in the suspension using ICP-MS, and the results showed that 1 mL of suspension contained (8 ( 0.2)  1014 Au NPs. On the basis of this data, we estimated that 5.6  10-3 mM of thiol was required to provide enough thiol molecules to fully cover the surface of all the Au NPs in the suspension. Our experimental results showed that the chains began to grow significantly at g10-2 mM (Figure 5A). In addition, we also found that the pH value of the mixture of ethanol and water changed from 7.5 at 10-4 mM to 6.1 at 1 mM of MPA. These results imply that the chainlike structures of Au NPs were induced by the slightly charged thiols remained after their chemisorption onto the Au NPs in the mixture. It has been reported that the excess amount of charged surfactants could affect the stabilities of colloids by screening charges on colloids or formation of micelles.26-28 Since MPA could not form micelles in the mixture of ethanol and water (98:2 by volume), it could only (26) Mondain-Monval, O.; Leal-Calderon, F.; Phillip, J.; Bibette, J. Phys. Rev. Lett. 1995, 75, 3364–3367. (27) Pashley, R. M.; Ninham, B. W. J. Phys. Chem. 1987, 91, 2902–2904. (28) Zaccone, A.; Wu, H.; Lattuada, M.; Morbidelli, M. J. Phys. Chem. B 2008, 112, 1976–1986.

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Figure 5. (A) Average number of Au NPs in the chainlike assemblies formed with MPA for 24 h as a function of concentration of MPA. For each data point, 40 chainlike assemblies were taken from TEM images to count the number of Au NPs. (B) The Debye length (κ-1) for the Au NPs in a mixture of ethanol and water (98:2 by volume) as a function of the concentration of MPA. Figure 4. TEM images showing chainlike assemblies of Au NPs formed with MUA (n = 10) at different concentrations: (A) 0.1 mM for 2 weeks, (B) 1 mM for 2 weeks, and (C) 4 mM for 24 h.

screen the charges of Au NPs. We estimated the Debye length (κ-1) for the dispersions of Au NPs at g10-2 mM of MPA (Figure 5B). At these concentrations, it is expected that an excess amount of free MPA existed in the dispersion. The κ-1 was calculated by using an equation in refs 25 and 26 (see the Supporting Information). The value of κ-1 was decreased significantly with increasing the concentration of MPA. This indicates that the contact between Au NPs became easier at high concentrations of MPA, which could be ascribed to the screening of charges on the Au NPs. The anisotropic growth of Au NPs was explained by Zhang and Wang.14 Since the electrostatic potential of an Au dimer is higher at its lateral side than its end side, the Au NPs preferred attaching to the end of the dimer. This was experimentally supported by recent work of Akbulut and co-workers,29 which is based on measurement of surface forces acting on charged nanospheres, nanorods, and nanowires. They concluded that long-range onset distances of the repulsive forces were increased in the following order: nanowires > nanorods > nanospheres. In addition, they also commented that the larger dimension (the length) of the particles determines the long-range force, while the smaller dimension (the width or rod/wire diameter) determines the short-range force. Likewise, the formation of our

chainlike assemblies of Au NPs can also be explained by above mechanism. Control of Optical Properties of the Chainlike Assemblies. Parts A and B of Figure 6 show the photographs of dispersions containing the chainlike assemblies of Au NPs when they were induced to form with MPA and MUA, respectively. After being reacted with 1 mM MPA (Figure 6A), the as-prepared Au NPs (left) turned their colors from pinkred to violet and finally to dark blue with time (middle). After 3 days, the assemblies were precipitated (right). For the assemblies formed with 1 mM MUA (Figure 6B), the colors of asprepared Au NPs were slowly changed to dark red at 24 h and finally violet in 2 weeks. It is interesting to point out that the chainlike assemblies of Au NPs had different colors when they were induced to form with the thiols of different chain lengths. We also found that the two chainlike assemblies of Au NPs had different UV-vis spectra. For the assemblies formed with 1 mM MPA (Figure 6C), the peak at 520 nm was slightly redshifted and a new peak was generated at 683 nm. By contrast, for those formed with 1 mM MUA (Figure 6D), the assemblies had a new peak appeared at a shorter wavelength (586 nm) than that with 1 mM MPA. Previous simulation studies suggested that the localized surface plasmon resonance (LSPR) of chainlike assemblies of Au NPs could be controlled by varying the interparticle separation in the chains.30 Shorter spaced chains aligned along the polarization direction interact more strongly and have more significantly

(29) Akbulut, M.; Godfrey, A. A. R.; Min, Y.; Belman, N.; Reynolds, M.; Golan, Y.; Israelachvili, J. Langmuir 2007, 23, 3961–3969.

(30) (a) Lazarides, A. A.; Schatz, G. C. J. Phys. Chem. B 2000, 104, 460–467. (b) Pecharroman, C. Phys. Chem. Chem. Phys. 2009, 11, 5922–5929.

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Figure 6. (A) A photograph of a suspension of Au NPs before (left) and after being treated with 1 mM MPA (middle, 24 h; right, 72 h). (B) A photograph of a suspension of Au NPs after being treated with 1 mM MUA (left, 24 h; right, 2 week). (C, D) UV-vis spectra of the asprepared Au NPs and chainlike assemblies of Au NPs formed with (C) 1 mM MPA and (D) 1 mM MUA. (E, F) TEM images of the chainlike assemblies of Au NPs formed with (E) 1 mM MPA for 24 h and (F) 1 mM MUA for 2 weeks. The insets in (E) and (F) are schematics showing the different interparticle separations between Au NPs in the assemblies.

shifted LSPR peaks than the chains with more separated nanoparticles. Experimentally, the interparticle separations in 2-D or 3-D assemblies of Au NPs could be regulated by using thiols of different alkyl chain lengths.31 In the present work, we used the thiols with two different alkyl chain lengths: n = 2 for MPA and n = 10 for MUA. We estimated the interparticle separations of the two types of assemblies from the TEM images shown in Figure 6E,F. The chainlike assemblies formed with MPA had an average interparticle separation of 0.6 nm while the assemblies formed with MUA had 1.4 nm. The results indicate that the Au NPs in the chains formed with 1 mM MPA were more closely connected each other than those formed with 1 mM MUA, thereby displaying the different LSPR spectra and colors. To clarify the effect of alkyl chain length on the interparticle separation in the chainlike assemblies, we further investigated the LSPR and interparticle separation of the assemblies formed with 1 mM MHA (n = 15, Figure 7A). In this case, the UV-vis spectrum showed a peak at 568 nm after 3 weeks, and the interparticle separation was increased to 2.6 nm (see the inset). In addition, the rate of change in the spectra and color was much slower than (31) (a) Martin, J. E.; Wilcoxon, J. P.; Odinek, J.; Provencio, P. J. Phys. Chem. B 2000, 104, 9475–9486. (b) Chen, C.-F.; Tzeng, S.-D.; Chen, H.-Y.; Lin, K.-J.; Gwo, S. J. Am. Chem. Soc. 2008, 130, 824–826.

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those formed with the thiols of shorter alkyl chains. Figure 7B plots the interparticle separations and the wavelengths of newly generated LSPR peaks for the chainlike assemblies of Au NPs as a function of alkyl chain lengths (n) of HS(CH2)nCOOH. The peak positions were more red-shifted as n was decreased, in other words, as the interparticle separation was decreased. Fixing the Assemblies of Au NPs by Silica Coating. A major advantage of the chainlike assemblies is they can be easily prepared by simply mixing Au NPs and additives, thereby easily controlling the LSPR of Au NPs. However, the assemblies often become aggregated with time and finally precipitated. For example, the chainlike assemblies formed with 1 mM MPA were unstable after 72 h (Figure 6A), and those with 1 mM MUA tended to precipitate after 2 months. In addition, the original 1-D nanostructures could not be preserved after the assemblies being centrifuged. Under these conditions, the LSPR of the 1-D assemblies are easily ruined. For this reason, it is practically necessary to develop methods to fix these assemblies. We first fixed chainlike assemblies of Au NPs by coating them with silica. For this coating, we modified the procedures reported by Lu and co-workers for Au@silica core-shell particles.32 (32) Lu, Y.; Yin, Y.; Li, Z.-Y.; Xia, Y. Nano Lett. 2002, 2, 785–788.

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Figure 7. (A) UV-vis spectra of the as-prepared Au NPs (black), and chainlike assemblies of Au NPs formed with 1 mM MHA (n = 15) for 24 h (red) and 3 weeks (blue). The inset shows a TEM image of the chainlike assemblies of Au NPs formed with 1 mM MHA for 3 weeks. (B) Peak maxima (left y-axis) and interparticle separations of the chainlike assemblies of Au NPs (right y-axis) as a function of the number (n) of methylene units in HS(CH2)nCOOH. The peak positions and interparticle separations were obtained from UV-vis spectra and TEM images, respectively.

Coating was performed by first adding 1 mL of the ethanol/water (98:2 by volume) suspension containing assemblies of Au NPs into 9 mL of 2-propanol. In this step, the composition of cosolvent was quite important, both in preserving the structure of the assemblies and in controlling the thickness of the silica shell. It was also important to rapidly add ammonia and TEOS right after dispersing the chainlike assemblies into 2-propanol in order to prevent aggregation of the assemblies. After being coated with silica, the assemblies were stably dispersed in both water and ethanol, even with repeated wash and centrifugation. We also prepared the silica-coated Au NPs with the same method. We observed that the colors of the dispersions containing the silicacoated assemblies of Au NPs or Au NPs remained unchanged during storage for over one year. We compared the structures of the chainlike assemblies before and after being coated with silica. Figure 8A,C,E shows TEM images for the Au NPs and the chainlike assemblies of Au NPs induced by MPA, and Figure 8B,D,F shows SEM and TEM (inset) images of the corresponding Au NPs and their assemblies after being coated with silica. These figures indicate that each assembly or Au NP was mostly coated independently with silica (40-80 nm in thickness) without any aggregation of the chains or nanoparticles. We also compared UV-vis spectra of the chainlike assemblies of Au NPs before and after coating with silica (Figure 9). We observed a small red shift in the spectra (2030 nm) after coating, which is probably attributed to a change to the refractive index by silica coating.32 However, there was no significant change in the peaks, and the colors of the dispersions 10010 DOI: 10.1021/la100127w

Figure 8. (A, C, E) TEM images of chainlike assemblies of Au NPs formed with MPA at different concentrations for 24 h. (B, D, F) SEM images showing silica-coated, chainlike assemblies of Au NPs. The concentration of MPA used for the assemblies was (A, B) 0, (C, D) 0.05, and (E, F) 0.2 mM. The insets show TEM images of the silica-coated, chainlike assemblies of Au NPs (scale bars: 50 nm).

before and after coating (insets) were essentially the same. The results clearly demonstrate that the original chainlike structures were well preserved after coating with silica. Encapsulation of Chainlike Assemblies of Au NPs with Gelatin Microspheres. As shown in Figure 2B, we employed a simple fluidic device to encapsulate the chainlike assemblies of Au NPs with polymer. Since Au NPs and chainlike assemblies of Au NPs were well dispersed in water, we used a water-in-oil (W/O) emulsion system to make the microspheres. The two chainlike assemblies of Au NPs were prepared in water by adding 0.4 and 1 mM MPA into aqueous dispersion of Au NPs, and the resulting colors of dispersions were violet and dark blue, respectively. We also used as-prepared Au NPs to encapsulate them in polymer microspheres. We chose gelatin as encapsulating material because this polymer was transparent and the structures of the assemblies could be easily fixed by simply cooling a mixture of gelatin and the chainlike assemblies to below room temperature. We used toluene with oil-soluble surfactant (Span 80) as the continuous phase. In water phase, the chainlike assemblies of Au NPs or Au NPs were dispersed in warm (∼40 °C) aqueous solution of gelatin. We directly mixed the chainlike assemblies of Au NPs with gelatin solution. On the other hand, for the gelatin microspheres containing Au NPs, we pretreated the as-prepared Au NPs with PVA prior to mixing them with gelatin solution, which prevents the aggregation of the Au NPs. The concentration of gelatin in the mixture was 10 wt %. After the aqueous mixture of gelatin and the assemblies or Au NPs was introduced into the fluidic device, cooling of the emulsion in the ice bath below 10 °C was necessary to induce gelation of the gelatin. Langmuir 2010, 26(12), 10005–10012

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Figure 10. (A-C) Photographs showing gelatin microspheres containing: (A) PVA-coated Au NPs, (B) chainlike assemblies of Au NPs formed with 0.4 mM MPA, and (C) chainlike assemblies of Au NPs formed with 1 mM MPA. (D) An optical micrograph of a multilayered lattice crystallized from the gelatin microspheres containing chainlike assemblies of Au NPs (with 0.4 mM MPA).

Figure 9. UV-vis spectra of the chainlike assemblies of Au NPs before (black) and after (red) silica coating. The chainlike assemblies of Au NPs were prepared by adding MPA of different concentrations: (A) 0, (B) 0.05, and (C) 0.2 mM. The insets are photographs of the suspensions of the assemblies before (left) and after (right) silica coating.

After washing several times with methanol, we obtained microspheres of ∼150 μm in diameter. As shown in Figure 10A-C, the three microspheres have different colors (pinkred, violet, and blue), representing the colors of the Au NPs and the assemblies of Au NPs. We could also construct a multilayered opal structure with these microspheres (Figure 10D), which can be considered as direct evidence of good uniformity in size and shape Figure 11A shows the UV-vis spectra for the assemblies of Au NPs and Au NPs before (solid lines) and after (dotted lines) mixing with gelatin. Although there were minor red shifts in the peak position as in the case of silica coating, it was clearly seen that the Au NPs and the assemblies of Au NPs retained their characteristic spectra after mixing with gelatin. We also tried to obtain the UV-vis spectra for the three types of microspheres (Figure 11B). Since UV-vis light could not penetrate into the close-packed microspheres, we aligned the sample cuvetts in order for UV-vis light to pass through the top regions where the microspheres were less densely packed (inset). It could be seen Langmuir 2010, 26(12), 10005–10012

Figure 11. (A) UV-vis spectra taken from the Au NPs and chainlike assemblies of Au NPs before (solid lines) and after (dotted lines) mixing with gelatin aqueous solution. (B) UV-vis spectra taken from the gelatin microspheres containing Au NPs and chainlike assemblies of Au NPs. The gelatin aqueous solution or gelatin micropsheres contained (i) PVA-coated Au NPs, (ii) chainlike assemblies of Au NPs formed with 0.4 mM MPA, and (iii) chainlike assemblies of Au NPs formed with 1 mM of MPA. In (B), because the light cannot penetrate into the close-packed microspheres, we adjusted the positions of the samples so that the light can pass through the regions where the microspheres were less densely packed (see inset).

that the UV-vis spectra of the microspheres showed similar patterns to those of the corresponding assemblies/gelatin mixtures. DOI: 10.1021/la100127w

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Conclusion In this research, we developed a method for controlling the assembly of Au NPs into chainlike structures with tunable lengths and interparticle separations. The number of Au NPs in the chainlike assemblies can be controlled with thiol solutions of different concentrations while the interparticle separations between the Au NPs could be adjusted by using thiols of different alkyl chain lengths. The chainlike assemblies of Au NPs were fixed either by silica coating or by encapsulating them with uniform gelatin microspheres. After fixing, the structures of chainlike assemblies could be preserved for a long period of time against, during which their characteristic optical properties remained unchanged. In addition, the assemblies of Au NPs encapsulated in silica or gelatin were also found to be stable in aqueous solutions when the pH, temperature, or salt concentration was varied. We expect that these core-shell type structures and microspheres can be applied as molecular probes or as building blocks of photonic crystals whose optical properties

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can be finely tuned by varying the structures of the chainlike assemblies. Acknowledgment. This work was supported in part by a grant from the NSF (DMR-0804088) and start-up funds from Washington University in St. Louis. ECC was partially supported by a postdoc fellowship through the Korea Research Foundation (KRF-2007357-D00070) funded by the Korean Government (MOEHRD). S.-W.C. was partially supported by a Korea Research Foundation Grant funded by the Korean Government (KRF-2007-357-D00080). P.H.C.C. was partially supported by the Fulbright Program and the Brazilian Ministry of Education (CAPES). This work was performed in part at the Nano Research Facility (NRF), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the NSF under Award ECS-0335765. Supporting Information Available: Explanation of Figure 5B. This material is free of charge via Internet at http://pubs. acs.org.

Langmuir 2010, 26(12), 10005–10012