Arrays of Covalently Bonded Single Gold Nanoparticles on Thiolated

Ligand Adsorption and Exchange on Pegylated Gold Nanoparticles. Kumudu Siriwardana , Manuel Gadogbe , Siyam M. Ansar , Erick S. Vasquez , Willard E. C...
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Arrays of Covalently Bonded Single Gold Nanoparticles on Thiolated Molecular Assemblies Melvin T. Zin, Hin-Lap Yip, Ngo-Yin Wong, Hong Ma, and Alex K.-Y. Jen* Department of Materials Science and Engineering, UniVersity of Washington, Seattle, Washington 98195-2120 ReceiVed December 1, 2005. In Final Form: April 25, 2006 A simple approach to form arrays of covalently bonded single gold nanoparticles (AuNPs) is demonstrated. Asymmetric molecular assemblies composed of two layers of rigid aromatic molecules with different structures, arranged in hexagonal arrays on a template produced by edge-spreading lithography, are used to guide the assembly of AuNPs. Arrays of single AuNPs are achieved by taking advantage of the interplay of electrostatic interactions and covalent bonding in conjunction with the positional constraint on the template. Schiff base chemistry is highlighted in the surface chemical reaction to selectively modify nanoscale surface features with high yield.

Introduction Gold nanoparticles (AuNPs) have been investigated as building blocks to create novel material constructs1a and as active components in fabricating sensing,1b electronic,1c and optoelectronic devices.1d In many cases, the desired configuration involves two-dimensional (2-D) arrays of AuNPs supported on a substrate with defined interspacing and symmetries. Although numerous methods have been used to pattern AuNPs for fundamental studies, as well as technological applications,2-5 generation of nanoparticle arrays remains a significant challenge and offers an opportunity for innovation. Because of the possibility to integrate AuNPs with organic or biological systems, we have been exploring the potential of employing genetically engineered polypeptides and functional molecular assemblies to manipulate the organization of AuNPs.6 Recently, we reported the assembly of AuNPs into arrays of lines and squares using gold-binding polypeptides through biomolecular recognition.7 Here, using functional molecular assemblies, we report a simple method to assemble AuNPs through covalent bonding. Previously, several approaches have been reported using flexible aliphatic molecules and electron beam lithography3 or scanning probe techniques4,5 to produce the desired templates for the assembly of AuNPs. However, the results from structural investigations have revealed that the self-assembled monolayers (SAMs) from alkyl derivatives are easily perturbed upon * To whom correspondence should be addressed. E-mail: ajen@ u.washington.edu. Tel: (206) 543-2626. Fax: (206) 543-3100. (1) (a) Boyen, H. G.; Kastle, G.; Weigl, F.; Koslowski, B.; Dietrich, C.; Ziemann, P.; Spatz, J. P.; Riethmuller, S.; Hartmann, C.; Moller, M.; Schmid, P.; Garnier, M. G.; Oelhafen, P. Science 2002, 297, 1533. (b) Taton, T. A.; Mirkin, C. A.; Letsinger, R. L. Science 2000, 289, 1757. (c) Hicks, J. F.; Zamborini, F. P.; Osisek, A. J.; Murray, R. W. J. Am. Chem. Soc. 2001, 123, 7048. (d) Maier; S. A., Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. AdV. Mater. 2001, 13, 1501. (2) McMillan, R. A.; Paavola, C. D.; Howard, J.; Chan, S. L.; Zaluzec, N. J.; Trent, J. D. Nat. Mater. 2002, 1, 247. (3) Mendes, P. M.; Jacke, S.; Critchley, K.; Plaza, J.; Chen, Y.; Nikitin, K.; Palmer, R. E.; Preece, J. A.; Evans, S. D.; Fitzmaurice, D. Langmuir 2004, 20, 3766. (4) (a) Demers, L.; Ginger, D. S.; Park, S.-J.; Li, Z.; Chung, S.-W.; Mirkin, C. A. Science 2002, 296, 5574. (b) Schwartz, P. V. Langmuir 2001, 17, 5971. (5) (a) Garno, J. C.; Yang, Y.; Amro, N. A.; Cruchon-Dupeyrat, S.; Chen, S.; Liu, G. Nano Lett. 2003, 3, 389. (b) Li, Q.; Zheng, J.; Liu, Z. Langmuir 2003, 19, 166. (c) Liu, S.; Maoz, R.; Sagiv, J. Nano Lett. 2004, 4, 845. (d) Fresco, Z. M.; Frechet, J. M. J. Am. Chem. Soc. 2005, 127, 8302. (6) Sarikaya, M.; Tamerler, C.; Jen, A. K-Y.; Schulten, K.; Baneyx, F. Nat. Mater. 2003, 2, 577. (7) Zin, M. T.; Ma, H.; Sarikaya, M.; Jen, A. K-Y. Small 2005, 1, 698.

interacting with another phase (for example, organic or biological molecules, nanoparticles, and quantum dots) and the introduction of ω-functional groups at chain termini can lead to a severe disorder within the film.8-11 This problem can potentially be solved by taking advantage of strong intermolecular π-π interactions in the SAMs from aryl derivatives to enhance the integrity of the film and reduce the disorder. In addition, these SAMs from rigid aromatic molecules possess interesting electrical and optical properties which can be explored for possible applications in molecular electronics and optoelectronics.12-17 As part of our effort on functional molecular assemblies,12-17 in this work, we demonstrate the use of SAMs of (10-mercaptomethyl-9-anthryl)-(4-aldehyde-phenyl)acetylene (MMAPA) molecules as a robust foundation to support an overlayer with thiol end-groups for covalent bonding with AuNPs. We establish the control over the structure of molecular assemblies via a combination of self-assembly processes in a layer-by-layer scheme and surface chemical reaction. As a proof-of-concept demonstration, asymmetric molecular assemblies were formed by two layers of rigid aromatic molecules with different structures. Schiff base chemistry was employed in the surface chemical reaction to selectively modify nanoscale surface features with high yield. In the literature, using a neutral (-CH3) background, numerous studies have described the assembly of negatively charged AuNPs onto positively charged (NH2/NH3+) regions through electrostatic attraction5b-c or onto thiolated (-SH) regions through covalent bonding.5d However, to our knowledge, electrostatic repulsion from a negatively charged (COOH/COO-) background has not been capitalized in the assembly of negatively charged AuNPs. (8) Ong, T. H.; Ward, R. N.; Davies, P. B.; Bain, C. D. J. Am. Chem. Soc. 1992, 114, 6243. (9) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156. (10) Kacker, N.; Kuman, S. K.; Allara, D. L. Langmuir 1997, 13, 6366. (11) Hautman, J.; Klein, M. L. Phys. ReV. Lett. 1991, 67, 1763. (12) Ma, H.; Kim, K.-S.; Li, H.; Horwitz, J. S.; Zin, M. T.; Zareie, M. H.; Sarikaya, M.; Jen, A. K.-Y. Polymer Preprints 2003, 44, 239. (13) Krapchetov, D. A.; Ma, H.; Jen, A. K.-Y.; Fischer, D. A.; Loo, Y.-L. Langmuir 2005, 21, 5887. (14) Zareie, M. H.; Ma, H.; Reed, B. W.; Jen, A. K-Y.; Sarikaya, M. Nano Lett. 2003, 3, 139. (15) Kang, S.-H.; Ma, H.; Kang, M.-S.; Kim K.-S.; Jen, A. K.-Y.; Zareie, M. H.; Sarikaya, M. Angew. Chem. 2004, 116, 1538; Angew. Chem., Int. Ed. 2004, 116, 1512. (16) Kim, K.-S.; Kang; M.-S.; Ma, H.; Jen, A. K.-Y. Chem. Mater. 2004, 16, 5058. (17) Ma, H.; Kang, M.-S.; Xu, Q.-M.; Kim, K.-S.; Jen, A. K.-Y. Chem. Mater. 2005, 17, 2896.

10.1021/la053256x CCC: $33.50 © 2006 American Chemical Society Published on Web 06/13/2006

CoValently Bonded Single Gold Nanoparticles

Figure 1. (A) Schematic illustration of ESL to produce templates for lateral structuring of molecular assemblies. (B) Top: Nanopatterned gold substrate contains a hexagonal array of nanoholess unmodified gold surfaces in MUA background. Below: Crosssectional profile along three nanoholes indicated by arrows. Their depth is in agreement with the height of surrounding MUA molecules.

We achieve arrays of single AuNPs by taking advantage of the interplay of long-range electrostatic interactions (both between AuNPs themselves and between AuNPs and a negatively charged background) and short-range covalent bonding. In addition, we show that edge-spreading lithography (ESL) can be used to produce templates for assembly of AuNPs and for lateral structuring of molecular assemblies. Experimental Section Materials. All solvents and reagents were purchased from Aldrich and used as received unless otherwise stated. 11-Mercaptoundecanoic acid (MUA) and 5-amino-2-mercaptobenzoimidazole (MBIZ) were purchased from Aldrich and were used as received. MMAPA was designed and synthesized in our laboratories. Molecular structures of MUA, MBIZ, and MMAPA are provided in the Supporting Information. Absolute (200 proof) ethanol (Aaper Alcohol and Chemical Company) was used for making thiol solutions. Substrates. Polycrystalline gold substrates were prepared by electron-beam evaporation of gold (23 nm thick; 99.999%, Kurt J. Lesker Company) onto Si(100) wafers that had been primed with a layer of titanium (2 nm thick; 99.995%, Kurt J. Lesker Company) to promote adhesion between silicon and gold. Electron-beam evaporation (SEC 6000, CHA Industries) of gold was performed under high vacuum with pressure of ∼1 × 10-6 Torr at a rate of 0.1 nm/s. Gold substrates have an excellent reproducibility regarding their physical and chemical characteristics, and were characterized to have a root-mean-square (RMS) roughness of 0.6 ( 0.2 nm and an average grain diameter of 20-40 nm. Edge-Spreading Lithography (ESL). Colloidal suspensions (0.2-0.3 wt %) of silica beads (1.6 ( 0.06 µm) were prepared from dilution of stock solutions purchased from Duke Scientific. Before the assembly of beads, diced Au substrates (∼1 cm2) were treated with oxygen plasma (∼2 min) to remove any contaminants and to render the surface hydrophilic to facilitate the deposition of a colloidal suspension. A drop of colloidal suspension was pipetted onto the gold substrate and was allowed to evaporate slowly. Capillary forces bring the silica beads into a 2-D lattice during the evaporation of the colloidal suspension (Figure 1A, step 1). Large domains of hexagonal-close-packed beads can be generated over areas of 1 cm2 by adjusting the concentration and volume of colloidal suspension deposited on gold substrate. The quality of a 2-D lattice of silica beads was checked by optical microscopy. Typical defects include slip dislocations, missing beads, and regions of multiple layers or a submonolayer. Planar poly(dimethylsiloxane) (PDMS) slabs were prepared from Sylgard 184 (Dow Corning). The mixture (10:1 v/v ratio) of PDMS and cross-linking agent was degassed to prevent the

Langmuir, Vol. 22, No. 14, 2006 6347 formation of bubbles before curing under ambient conditions in a polystyrene Petri dish to obtain a flat slab of PDMS having a thickness of ∼3 mm. MUA solution (1 mM in ethanol) was pipetted onto a PDMS slab (∼1 cm2) for inking. After ∼2 min, the PDMS slab was dried with nitrogen and placed on the 2-D lattice of silica beads (Figure 1A, step 2). Due to small molecular weight and low vapor pressure of MUA molecules, formation of SAMs occurred in less than 7 min on the entire gold substrate except the regions masked by beads. Beads were lifted off by ultrasonication of the sample in deionized water for ∼15 min, producing a nanopatterned Au substrate containing a hexagonal array of nanoholes defined by the surrounding MUA molecules (Figure 1A, step 3). Surface Chemical Reaction and Self-Assembly of MMAPA. Because the aldehyde (-CHO) end-groups of MMAPA molecules can be easily oxidized by oxygen, MMAPA solution needs to be degassed and kept under nitrogen at all times. After the nanopatterned Au substrate was immersed into a 2 mL solution of MMAPA (0.05 mM in ethanol), 2 µL of ammonium hydroxide (28.0-30.0% NH3) was added to hydrolyze the acetyl protecting group. Under nitrogen, self-assembly of MMAPA proceeded for 5 h. Then, the sample was taken out, rinsed thoroughly in ethanol, and immersed immediately into MBIZ solution (0.1 mM in ethanol) for 1.5 h under nitrogen in order to have the surface chemical reaction complete. After the formation of asymmetric molecular assemblies, the sample was washed successively in ethanol (∼3 min), chloroform (∼3 min), and ethanol (∼3 min) to remove any unreacted MBIZ molecules. Assembly of AuNPs. To obtain the saturated coverage of AuNPs on thiolated molecular assemblies, assembly of AuNPs was carried out by immersing the sample (∼1 cm2) in the AuNP suspension (pH 6.5 or 9.0) for 3-5 min. Physically immobilized AuNPs were removed by rinsing (∼5 min) and ultrasonication (∼2 min) in deionized water. The sample was dried in a gentle flow of nitrogen before SEM characterizations and AFM measurements. Because of the repulsive interparticle forces, the AuNPs are prevented from forming a complete coverage. At the saturated coverage on thiolated molecular assemblies, the spatial distribution among AuNPs was 30-120 nm (see Supporting Information for details). Atomic Force Microscopy (AFM). Imaging and measurements were made in the tapping mode using a Nanoscope III AFM (Digital Instruments) operating in ambient conditions at a scan rate of 0.51.0 Hz. Silicon cantilevers with spring constants ranging from 12 to 103 N/m were used. Image resolution was 512 × 512 pixels. Roughness measurements and cross-sectional analysis were performed using algorithm contained in the AFM software.

Results and Discussion Templates for lateral structuring of molecular assemblies were produced by ESL as schematically illustrated in Figure 1A. Similar to nanosphere lithography (NSL) developed by van Duyne et al., ESL relies on the use of 2-D lattice of spherical colloids as a mask to create arrays of molecular assemblies with hexagonal symmetry. While NSL allows the fabrication of nanostructures by deposition of materials (typically, metals such as silver),18 ESL dictates the formation of SAMs into patterns on the metal substrate by exploiting the reactive spreading of thiols around the edges of spherical colloids.19 As the first step, ESL begins with the assembly of silica beads by depositing a colloidal suspension onto a thin film of gold supported on a silicon wafer. During the slow evaporation of the colloidal suspension, the beads are brought into a 2-D lattice through capillary forces (Figure 1A, step 1). By adjusting the concentration and volume of colloidal suspension deposited onto the substrate, it was possible to routinely obtain hexagonal-close-packed beads covering areas (18) (a) Hulteen, J. C.; Van Duyne, R. P. J. Vac. Sci. Technol. A 1995, 13, 1553. (b) Hulteen, J. C.; Treichel, D. A.; Smith, M. T.; Duval, M. L.; Jensen, T. R.; Van Duyne, R. P. J. Phys. Chem. B 1999, 103, 3854. (19) (a) McLellan, J. M.; Geissler, M.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 10830. (b) Geissler, M.; McLellan, J. M.; Chen, J.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 3596.

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Figure 2. Formation of thiolated molecular assemblies for the covalent attachment of single AuNPs. (i) Edge-spreading lithography of MUA molecules produced a nanopatterned gold substrate that contains hexagonal arrays of nanoholes. (ii) These nanoholes are backfilled with MMAPA molecules to create a reactive template presenting aldehyde (-CHO) end-groups. (iii) This reactive template allowed surface chemical reaction with MBIZ molecules through Schiff base (-CdN-) formation to form thiolated molecular assemblies. (iv) Under the conditions at which MUA molecules are deprotonated, assembly of negatively charged AuNPs onto thiolated molecular assemblies allowed the covalent attachment of single AuNPs as a result of the interplay of electrostatic interactions, covalent bonding, and positional constraint on the template.

of ∼30 µm × ∼30 µm within single domains, although larger domains were frequently obtained on the same sample. The second step is the placement on the beads of a flat slab of PDMS that has been saturated with MUA molecules and dried with nitrogen (Figure 1A, step 2). From the PDMS slab (serving as a reservoir), the MUA molecules diffuse along the surface of each bead and onto the substrate where they form into a SAM. As long as the supply of MUA molecules from the PDMS slab is not depleted, the formation of SAMs continues and expands radially around the footprint of each bead. In their demonstration, Xia and coworkers controlled the reactive spreading of molecules to produce rings of SAMs.19 In our application, we allowed the MUA SAMs to cover the entire gold substrate (except the areas masked by beads). Formation of multilayers or aggregations is commonly observed in the solution-phase self-assembly of MUA molecules as a result of the strong hydrogen-bonding between carboxyl (-COOH) end-groups.20 ESL ensures the formation of a monolayer of MUA molecules and facilitates the functionalization in a controlled fashion. As the final step, the PDMS slab is removed and the beads are lifted off by ultrasonication in deionized water to leave behind a hexagonal array of nanoholessunmodified gold surfaces in the matrix of MUA SAMs (Figure 1A, step 3). There are physical and chemical reasons for the selection of MUA in our approach. First, the heights of MMAPA (1.36 nm)7 and MUA (1.41 nm)20 are comparable, which minimizes steric hindrance for the subsequent surface chemical reaction. Second, unlike neutral backgrounds, electrostatic repulsive forces from the MUA-functionalized negatively charged regions influence the assembly of AuNPs. The nanopatterned gold substrate after the lift-off of beads can be seen in Figure 1B. The holes were regular in shape, and the interspacing between the holes in the 2-D array was 1.6 µm, corresponding to the center-to-center distance of hexagonalclose-packed beads. Their depth of ∼1.4 nm, as measured by the cross-sectional profile, was in agreement with the height of surrounding MUA molecules.20 Although their actual width could not be ascertained by AFM due to the convolution with the tip, their apparent width was ∼80.0 nm. The procedure’s reproducibility is mainly dependent on the assembly of beads; defects in the 2-D lattice of beads such as disorder, domain boundaries, (20) (a) Evans S. D.; Ulman, A.; Goppert-Berarducci, K. E.; Gerenser, L. J. Am. Chem. Soc. 1991, 113, 5866. (b) Arnold, R.; Azzam, W.; Terfort, A.; Wo¨ll, C. Langmuir 2002, 18, 3980.

and multiple or incomplete layers strongly affect the uniformity of holes, and therefore the quality of templates. Figure 2 outlines the formation of thiolated molecular assemblies for the assembly of AuNPs. After the nanopatterned gold substrate had been sequentially cleaned with deionized water and ethanol, it was immersed into the MMAPA solution. Deprotection of thioacetyl using ammonium hydroxide permitted the chemisorption of MMAPA molecules in the nanoholes.7 Unique features of MMAPA are the way in which the aldehyde (-CHO) functional group is exposed at the terminus of its ethynylphenyl moiety to promote surface chemical reaction with minimal steric hindrance,7 and the presence of the anthracene moiety, which allows parallel-displaced π-π stacking with its neighboring molecules to yield densely packed and highly organized SAMs.14,15 Noticeably, MMAPA possesses a sturdy framework to resist conformational instability that would inevitably be induced by the formation of layered architectures involving additional phases.7 Backfilling by MMAPA was carried out for 5 h to ensure the formation of densely packed and highly organized SAMs, which was essential as a foundation layer to ensure the formation of an overlayer of reasonably ordered MBIZ molecules. In control experiments where the backfilling by MMAPA is less than 5 h, nanoparticle arrays of poor quality were formed (see Supporting Information for details). Longer times for backfilling enable the ordering of MMAPA molecules through parallel-displaced π-π interactions. The adverse effect of longer times for backfilling is the displacement of MUA by MMAPA, which leads to the increase in size of regions of thiolated molecular assemblies (∼130 nm) compared to the width of nanoholes (∼80 nm). Liu and co-workers have noted that the matrix molecules at the perimeter of nanoholes are significantly displaced by the backfilling molecules in comparison to the matrix molecules away from the boundary of nanoholes.21 After 5 h, the sample was taken out of the MMAPA solution, copiously rinsed in ethanol, and immediately placed in the solution of MBIZ molecules for the surface chemical reaction to take place for 1.5 h as described elsewhere.7 There are two functional groups on MBIZ: the amine (-NH2) functionality for reacting with aldehyde (-CHO) end-groups of MMAPA and the thiol (-SH) (21) (a) X, S.; Laibinis, P. E.; Liu, G. J. Am. Chem. Soc. 1998, 120, 9356. (b) Xu, S.; Miller, S.; Laibinis, P. E.; Liu, G. Langmuir 1999, 15, 7244. (c) Liu, G.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457. Also see: Scott, J. R.; Baker, L. S.; Everett, W. R.; Wilkins, C. L.; Fritsch, I. Anal. Chem. 1997, 69, 2636. Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502.

CoValently Bonded Single Gold Nanoparticles

Figure 3. Hexagonal arrays of thiolated molecular assemblies at low (A) and high (B) magnifications.

functionality for covalent bonding with AuNPs. Schiff base (-Cd N-) chemistry was chosen for the surface chemical reaction based on the following reasons: (i) it proceeds under ambient conditions without activation; (ii) it produces water as the only byproduct, and therefore does not contaminate the surface; and (iii) it uses functional groups that are compatible with a wide range of molecules and solvents. Being a heterocyclic molecule containing moieties to form hydrogen-bonding, MBIZ introduces asymmetry to the structure of molecular assemblies and, at the same time, improves ordering and packing in the overlayer through intermolecular π-π stacking and hydrogen-bonding.12 After the surface chemical reaction, the sample was washed successively in ethanol, chloroform, and ethanol to remove excess MBIZ molecules and dried in nitrogen prior to the assembly of AuNPs. As presented in Figure 3, chemical coupling of MBIZ onto MMAPA occurs in a selective way and at a high yield. Formation of hexagonal arrays of thiolated molecular assemblies indicates that the surface chemical reaction is locally confined onto MMAPA and goes to completion. Identical images acquired from repeated scans of the same area by AFM suggest that the MBIZ molecules are securely anchored on MMAPA molecules and that the asymmetric molecular assemblies are stable. The AFM, with its sensitivity and resolution, provides a suitable tool not only to qualitatively assess the structural integrity of thiolated molecular assemblies but also to quantitatively analyze their dimensions. Before the surface chemical reaction, because the heights of MMAPA and MUA are comparable, the chemical patternssregions of MMAPA surrounded by MUAscould not be resolved in topography imaging. After the surface chemical reaction, the chemical patterns on the template became visible as a result of the difference in height between thiolated molecular assemblies and surrounding MUA molecules. As expected of the thickness of an overlayer of reasonably organized and vertically oriented MBIZ molecules, the increase in height was ∼1.5 nm. However, the size of thiolated regions (∼130 nm) appears larger than the apparent width of nanoholes (∼80 nm). This may be attributed to the displacement of MUA by MMAPA during the backfilling. Additional details on the characterization of surface chemical reaction and formation of thiolated molecular assemblies using grazing angle Fourier transform infrared (FTIR) spectroscopy and contact angle measurements are provided in the Supporting Information. The AuNPs used in this work have a nominal diameter of 10.0 nm and are in the size range of optical, electrical, and plasmonic properties useful for devices. As confirmed by the SEM

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characterization, they have a size variation of about 10% (ca. 10.0 ( 1.1 nm). While the vertical measurements by AFM were in agreement with the SEM characterization (ca. 10.0 ( 1.8 nm), the lateral measurements of the diameter of one AuNP was ∼29.0 ( 2.5 nm due to the convolution with the tip.22 The AuNPs are negatively charged due to adsorption of citrate and chloride anions and a coating of [AuCl2]- produced by incomplete reduction of [AuCl4]- during their synthesis.23 The pH value of the AuNP suspension was ∼6.5,23 which is at the higher end of the range of pKa (more accurately, pK1/2) values reported in the literature for the SAMs of ω-mercaptoalkanoic acids on gold.24-36 Various techniques have been applied to determine the pKa of SAMs carrying carboxyl (-COOH) terminal groups, and the results from different investigations vary from ∼5 to 11.24-35 Prior studies have shown that the surface pKa values fall alkaline of the solution pKa values due to several factors including Coulombic repulsion among carboxyl (-COOH) end-groups, hydrogen-bonding, and double-layer potential effects.24-27 Despite a large discrepancy, numerous studies have approximated that the pKa values of MUA SAMs range from 5 to 8.28-35 Thus, when the template containing the hexagonal arrays of thiolated molecular assemblies was immersed into the AuNP suspension (pH 6.5), the surrounding MUA molecules were predominantly deprotonated. Even though the pKa of surface-bound (as opposed to solution-dispersed) carboxyl (-COOH) end-groups is greatly elevated, the ionization of acid groups begins between pH 3 and 4, rising sharply to about pH 7 and leveling off around pH 8.32-34 To be certain that the carboxyl (-COOH) end-groups of MUA SAMs are fully dissociated, we have also performed the assembly of AuNPs at pH 9 (see Supporting Information for details). The assembly of AuNPs was carried out by immersing the template containing the hexagonal arrays of thiolated molecular assemblies in the AuNP suspension. After 3-5 min, the sample was withdrawn, rinsed and ultrasonicated in deionized water, and dried under a gentle flow of nitrogen. Since the chemisorption of thiol moiety onto gold is far stronger than the affinity between (22) (a) Schwarz, U. D.; Haefke, H.; Reimann, P.; Guntherodt, H.-J. J. Microsc. 1994, 173, 183. (b) Ramirez-Aguilar, K. A.; Rowlen, K. L. Langmuir 1998, 14, 2562. (23) (a) Frens, G. Nature Phys. Sci. 1973, 241, 20. (b) Handley, D. A. In Colloidal Gold: Principles, Methods and Applications; Hayat, M. A., Ed.; Academic Press: San Diego, 1989; Vol. 1, Chapter 1. (c) Thompson, D.; Collins, I. J. Colloid Interface Sci. 1992, 152, 197. (24) Fawcett, W. R.; Andreu, R. J. Phys. Chem. 1994, 97, 12753. (25) Aoki, K.; Kakiuchi, T. J. Electroanal. Chem. 1999, 478, 101. (26) (a) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741. (b) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (c) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675. (27) (a) White, H. S.; Peterson, J. D.; Cui, Q.; Stevenson, K. J. J. Phys. Chem. B 1998, 102, 2930. (b) Smith, C. P.; White, H. S. Langmuir 1993, 9, 1. (28) (a) Molinero, V.; Calvo, E. J. J. Electroanal. Chem. 1998, 445, 17. (b) Godinez, L. A.; Castro, R.; Kaifer, A. E. Langmuir 1996, 12, 5087. (c) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385. (29) (a) Sugihara, K.; Shimazu, K.; Uosaki, K. Langmuir 2000, 16, 7101. (b) Shimazu, K.; Teranishi, T.; Sugihara, K.; Uosaki, K. Chem. Lett. 1998, 669. (c) Wang, J.; Frostman, L. M.; Ward, M. D. J. Phys. Chem. 1992, 96, 5224. (30) Cheng, Q.; Braither-Toth, A. Anal. Chem. 1992, 64, 1998. (31) Smalley, J. F.; Chalfant, K.; Feldberg, S. W.; Nahir, T. M.; Bowden, E. F. J. Phys. Chem. B 1999, 103, 1676. (32) (a) He, H. X.; Huang, W.; Zhang H.; Li, Q. G.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 517. (b) Vezenov, D. V.; Noy, A.; Rosznyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006. (c) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114. (d) van der Vegte, E. W.; Hadziioannou, G. J. Phys. Chem. B 1997, 101, 9563. (33) Zhou, J.; Luo, L.; Yang, X.; Wang, E.; Dong, S. Electroanalysis 1999, 11, 1108. (34) Schweiss, R.; Welzel, P. B.; Werner, C.; Knoll, W. Langmuir 2001, 17, 4304. (35) (a) Kokkoli, E.; Zukoski, C. F. Langmuir 2000, 16, 6029. (b) Kane, V.; Mulvaney, P. Langmuir 1998, 14, 3303. (36) (a) Graber, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735. (b) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148.

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Figure 4. (A) Top: 3-D topographical plot. Below: Cross-sectional profile along the dotted line. The increase in height over the regions of thiolated molecular assemblies corresponds to the diameter of one AuNP and therefore confirms the covalent attachment of single AuNPs. (B) Arrays of AuNPs over a large area. The dimension of single AuNPs is uniform across the 2-D array. The difference in dimensions of single and clusters of AuNPs is readily distinguishable, as pointed out by the arrows. (C) Individually resolved AuNPs.

NH groups on MBIZ and gold, the assembly of AuNPs proceeds mostly through covalent bonding. Although deprotonation may be incomplete and reprotonation of carboxyl (-COOH) terminal groups could not be ruled out, the repulsion was sufficient to prevent the physical adsorption of negatively charged AuNPs onto MUA-modified negatively charged surfaces. As in the determination of pKa, the exact Debye length of MUA SAMs could not be assessed experimentally. However, several studies have characterized the exponential decay of the repulsion from MUA SAMs with the average Debye lenghs of 16 nm at pH 4.7 to 12 nm at pH 9.32-34 Thus, around the pH of AuNP suspension (pH 6.5), the Debye length of MUA SAMs is approximately in the range of 11-15 nm. At low pH (pH 1-3), the carboxyl (-COOH) end-groups are fully protonoted. As pH is raised (pH > 3), repulsion from MUA SAMs increases rapidly with the Debye length as large as 30 nm.33-35 On unpatterned samples of thiolated molecular assemblies, the scatter in the spatial distribution of surface-bound AuNPs ranges from 30 to 120 nm, indicating that the repulsive interparticle forces may be stronger than the Coulombic repulsion from MUA SAMs. In the absence of MUA SAMs, the nearest-neighbor distance between AuNPs is ∼30 nm while the average distance between them is ∼120 nm (see Supporting Information for details). The nearest-neighbor distance corresponds to about one-third the size of a patch of thiolated molecular assemblies, while the average distance is comparable to the size of thiolated patches. The nearest-neighbor distance between surface-bound AuNPs on thiolated molecular assemblies is governed by a combination of repulsive interactions and chemical linkages.35 Essentially, repulsive interparticle forces, which are responsible for keeping the AuNPs apart in solution, prohibits the additional immobilization of AuNPs in the vicinity (30-50 nm) of surface-bound AuNPs. By taking advantage of the interplay of electrostatic interactions and covalent bonding, in conjunction with the positional constraint on the template, we were able to achieve arrays of single AuNPs. A perfect array of AuNPs, in a 3-D topographical plot, is shown in Figure 4A. As measured by the cross-sectional profile, the increase in height of ∼10.2 nm over the regions of thiolated molecular assemblies (relative to surrounding MUA molecules) corresponds to the diameter of one AuNP and therefore confirms

Zin et al.

Figure 5. Schematic representation of the proposed mechanism for the covalent attachment of single AuNPs. When carboxyl (-COOH) end-groups of MUA SAMs are deprotonated, patches of thiolated molecular assemblies are in the matrix of negatively charged MUA SAMs. Thus, the outer areas of thioated patches are screened by the Coulombic repulsion from MUA SAMs, effectively decreasing the available area for the assembly of negatively charged AuNPs. Gold nanoparticles are driven away from the interfaces and directed toward the inner area of thiolated patches where their assembly is energetically favorable. Rearrangement of AuNPs on thiolated patches is greatly hindered by strong Au-S bonds, and electrostatic interactions between negatively charged AuNPs make possible the covalent attachment of single AuNPs on each thiolated patch. Please see text for details.

the covalent attachment of single AuNPs. Within areas as large as 20 µm2, the efficiency of covalent attachment of single AuNPs on thiolated patches is as high as 83%. A typical array of AuNPs is shown in Figure 4B. Out of 115 thiolated patches, 6 contained no AuNPs, 96 contained single AuNPs, and 2 contained AuNP clusters. Dimension of single AuNPs is uniform across the 2-D array. On thiolated molecular assemblies, the mobility of AuNPs is drastically reduced by strong Au-S bonds, and the repulsive interparticle forces inhibit the spontaneous coalescence of AuNPs on the surface. Thus, aggregation of AuNPs after their immobilization is very unlikely. This would imply an improbable mechanism for the aggregation of AuNPs in which a fixed number of particles are involved to form larger surface-bound particles with uniform dimensions across the 2-D array in a systematic manner. The difference in dimensions of single and clusters of AuNPs is readily distinguishable, as pointed out by the arrows in Figure 4B. Clusters of AuNPs, presumably formed by the aggregation of a few AuNPs in solution due to the loss or inadequate stabilizing ligands during their synthesis, represented a small fraction. As shown in Figure 4C, AuNPs can be individually resolved. Taking into account the nearest-neighbor distance between surface-bound AuNPs (∼30 nm on unpatterned samples), selective adsorption of AuNPs on thiolated patches is influenced by the electrostatic repulsive forces from MUA-functionalized negatively charged regions. Proposed mechanism for the covalent attachment of single AuNPs on thiolated molecular assemblies is schematically represented in Figure 5. Covalent attachment of single AuNPs implies that the effective area for the assembly of AuNPs is considerably smaller than the physical area of each thiolated patch (i.e., no larger than 100 nm, otherwise the assembly of two AuNPs would have been possible). This is because the outer areas of regions of thiolated molecular assemblies are screened by the Coulombic repulsion from MUA SAMs and therefore are not accessible for the assembly of AuNPs. To allow the covalent attachment of single AuNPs, the diameter of thiolated patches needs to decrease from ∼130 to ∼100 nm or less, suggesting that the Debye length of MUA SAMs is ∼15 nm or more. This is consistent with the estimates of the Debye length (11-15 nm) of MUA SAMs in the AuNP suspension (pH 6.5) based on previous studies (16 nm at pH 4.7 to 12 nm at pH 9).33-35

CoValently Bonded Single Gold Nanoparticles

Due to the Coulonbic repulsion from MUA SAMs (Debye length ∼15 nm), AuNPs are driven from the interfaces and directed toward the inner areas of thiolated patches where their assembly is energetically favorable. Chemical adsorption of AuNPs toward the centers of the patches of thiolated molecular assemblies is clearly evident by the interspacing (∼1.6 µm, see Figure 4C) between the individually well-resolved AuNPs, which corresponds to the center-to-center distance between thiolated patches. After the assembly, reorganization of AuNPs is vastly restricted by strong Au-S bonds, limiting the available area for additional AuNPs and making the accommodation of more AuNPs very difficult.

Conclusion The covalent bonding of AuNPs onto functional molecular assemblies in a site-specific fashion is of importance to many disciplines and constitutes an essential step in the formation of 2-D arrays of hybrid architectures. At present, in-depth investigations on the factors that govern the assembly of nanoparticles into 2-D arrays have been limited. In particular, the complexity of interparticle forces at or near the surface and the profound influence of interactions with functional molecular assemblies employed as templates within a confined configuration such as in the 2-D arrays have not been addressed. Using self-assembly

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processes and surface chemical reaction, we have demonstrated a simple approach to form arrays of covalently bonded single AuNPs based on the interplay of electrostatic interactions and covalent bonding. This work establishes an experimental framework to systematically understand the assembly of AuNPs through the manipulation of interfacial interactions. Acknowledgment. This work was supported by the Air Force Office of Scientific Research (AFOSR) under the Bioinspired Concept Program and the Army Research Office (ARO) under the DURINT program. A.K-Y.J. thanks the Boeing-Johnson Foundation for its support. H.-L.Y. and M.T.Z. thank the Center for Nanotechnology at the University of Washington for its Nanotechnology UIF Fellowships. Supporting Information Available: Chemical structures of molecules. Results of control experiments: assembly of AuNPs in the absence of MUA molecules; assembly of AuNPs at pH 9; backfilling by MMAPA for less than 5 h. Characterization of surfacess polycrystalline gold substrate, SAM of MMAPA, and asymmetric molecular assemblies (after coupling of MBIZ onto SAM of MMAPA)s by AFM and water contact angle. Grazing angle FTIR spectrum of thiolated molecular assemblies. This material is available free of charge via the Internet at http://pubs.acs.org. LA053256X