Adsorption Patterns of Gold Nanoparticles on Methyl-Terminated Self

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Adsorption Patterns of Gold Nanoparticles on Methyl-Terminated Self-Assembled Monolayers Sungwoon Lee, Jun Hee Yoon, and Sangwoon Yoon* Department of Chemistry, Institute of Nanosensor and Biotechnology, Dankook University, 152 Jukjeon-ro, Suji-gu, Yongin, Gyeonggi 448-701, Korea

bS Supporting Information ABSTRACT: Understanding the adsorption of gold nanoparticles (AuNPs) on self-assembled monolayers (SAMs) is important because an assembly of the AuNPs-SAM-gold substrate provides easily controllable metal
metal junctions, in which fascinating phenomena such as electron tunneling and surfaceenhanced Raman scattering can occur. In this work, we report strikingly different adsorption patterns of AuNPs on methylterminated SAMs. In contrast to the general belief that the terminal functional groups determine the surface properties of SAMs, the AuNPs adsorb on the surfaces of SAMs of 4-methylbenzenethiols (MBT) in a uniform, dispersed fashion, whereas aggregated adsorption is observed on the surfaces of SAMs of alkanethiols. We explore the effects of the terminal methyl group orientation, structural properties of SAMs, such as orderliness and packing density, and surface energies on the adsorption pattern differences. Dispersed or aggregated adsorption is determined by whether the citrate anions on the AuNPs are retained or removed by the SAM surfaces during the adsorption and thus, is critically dependent on the extent of the interactions between the AuNPs and SAMs. Direct interactions between the AuNPs and hydrophobic surfaces of alkanethiol SAMs strip the AuNPs of their citrate layers, leading to aggregated adsorption. For the less hydrophobic MBT SAMs, water mediates and softens the adsorption of the AuNPs. As a result, the citrate anions are retained on the AuNP surfaces, leading to dispersed adsorption of AuNPs. Forced interactions between the AuNPs and the MBT SAM surfaces by vigorous stirring yield aggregated adsorption, supporting our model.

1. INTRODUCTION Self-assembled monolayers (SAMs) provide well-defined surfaces of which properties can be tailored with ease and flexibility.1,2 Spontaneous assemblies of thiolate molecules on gold substrates, driven by intermolecular forces, produce highly ordered structures, and the surface properties are determined by the terminal functional group of the thiolate molecules. Macroscopic interfacial properties such as wetting,3
5 friction,6 adhesion,7 and corrosion resistance8,9 of the surfaces are easily controlled by modifying the terminal groups of SAMs. Interfacial interactions between the surfaces of SAMs and the external environment are of paramount significance in utilizing SAMs. One of the most widely studied systems is the use of SAMs as biomimetic cell membranes via the adsorption of proteins and bacteria. Whitesides and co-workers varied the hydrophilicity of surfaces using mixed SAMs of ω-functionalized longchain alkanethiolates to find that the more hydrophobic surfaces adsorb greater quantities of proteins.10 Molecular conformation and solvation of oligo(ethylene glycol)-terminated SAMs also influence the resistance to protein adsorption.11,12 Tengvall and co-workers have compared the affinity of human plasma proteins to five different functionalities of SAMs (i.e., methyl, trifluoromethyl, ester, sulfate, and carbonyl).13 r 2011 American Chemical Society

Another interesting and equally important system to the adsorption of proteins on SAMs is the adsorption of nanoparticles on SAMs. Nanoparticles are increasingly important due to their unique properties and ever-expanding applicability ranging from electronics to biomedicine.14
17 Adsorbed nanoparticles on SAMs present a new opportunity to explore the novel phenomena occurring in nanogaps. When metal nanoparticles are adsorbed on the surfaces of SAMs, metal
metal junctions are formed between the nanoparticles and the metal substrates. The metal
metal junctions, where fascinating phenomena, such as electron tunneling and nonlinear optical properties (e.g., surface-enhanced Raman scattering) occur, are easily modified by changing the molecules constituting the SAMs.18
24 Thus, understanding the nature of interactions between metal nanoparticles and SAMs is key to engineering junction structures. Despite its significance, only a few studies have been directed toward this goal.25
28 Liu and coworkers have shown that electrostatic interactions drive the adsorption of negatively charged gold nanoparticles (AuNPs) on amine-terminated SAMs.25 The contribution of hydrophobic interactions to AuNP adsorption has been explored by Fan and Lopez.26 Received: March 2, 2011 Revised: May 10, 2011 Published: June 07, 2011 12501

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They varied the surface hydrophobicities of AuNPs and planar gold films using a mixture of ω-substituted alkanethiols and then examined the number of adsorbed AuNPs on the SAMs. In our previous study, we compared the surface densities of AuNPs on SAMs of SHC6H4X (X = CH3, OCH3, NH2, NO2, OH, or COOH).27 As discussed above, previous studies have been focused on the extent of the adsorption of AuNPs and its dependence on the functionalities of SAMs. In this work, we report the adsorption patterns of AuNPs on SAMs. The adsorption patterns are strikingly different between SAMs of 4-methylbenzenethiols (MBT) and SAMs of alkanethiols although both possess methyl-terminated surfaces. This is contrary to a general belief that the terminal group determines the surface properties of the SAMs. We experimentally explore what factors are most critical in determining the adsorption patterns of AuNPs on methyl-terminated SAMs.

2. EXPERIMENTAL SECTION 2.1. Preparation of SAMs. SAMs of the following molecules were prepared on gold substrates: 4-methylbenzenethiol (MBT), 4-ethylbenzenethiol (EBT), 1-nonanethiol (C9SH), 1-decanethiol (C10SH), 1-undecanethiol (C11SH), 1-dodecanethiol (C12SH), 11-mercapto-1-undecanol (SHC11OH), 4-mercaptophenol (SHPhOH), 10-(p-tolyloxy)decane-1-thiol (SHC10PhC1), and 4-(decyloxy)benzenethiol (SHPhC10). All chemicals were purchased from Aldrich and TCI except for SHC10PhC1 and SHPhC10, which were synthesized in our laboratory (Supporting Information). Thermally evaporated polycrystalline gold films on silicon wafers (Korea Materials & Analysis Corp.) were cut into 5  5 mm pieces, cleaned in piranha solutions (H2SO4:H2O2, 3:1), and washed with distilled water and pure ethanol, repeatedly. To prepare the SAMs, the gold substrates were immersed in 10 mM ethanolic solutions containing the chemicals listed above for 24 h and were then washed with ethanol and dried with nitrogen gas. The contact angles of the SAMs were measured using the static sessile drop method (Pheonix 300, Surface Electro Optics Co.). 2.2. Synthesis of AuNPs. The AuNPs were synthesized using reduction of Au3þ by citrate.29 Briefly, a trisodium citrate solution (50 mL, 34 mM) was added to a solution of HAuCl4 (950 mL, 270 mM) at 80 °C with vigorous stirring. Heating for 30
40 min changed the color of the solution from yellow to wine red, which is characteristic of AuNPs. Transmission electron microscopy (TEM) measurements revealed that the AuNPs have a diameter of 13.8 ( 1.5 nm. The UV
vis spectrum displayed a strong extinction band at 521 nm, attributed to the surface plasmon resonance of the dispersed AuNPs.30,31 2.3. Adsorption of AuNPs on SAMs. The prepared SAMs on gold substrates were immersed in a solution containing AuNPs (2 mL, 3.1 nM) for 12 h to promote the physisorption of AuNPs onto the SAM surfaces. After incubation, the samples were washed with water and dried with nitrogen gas, repeatedly. The AuNP adsorption densities and patterns were investigated using scanning electron microscopy (SEM, Supra 55 Carl Zeiss, S-4300 Hitachi, or S-4800 Hitachi). To confirm the surface states of AuNPs adsorbed on the SAMs, the AuNPs were desorbed from the SAM surfaces by ultrasonication and subjected to UV
vis spectroscopy (Lambda 25, Perkin
Elmer).

Figure 1. SEM images of the AuNPs adsorbed on SAMs of (a) MBT and (b) C10SH. The insets show magnified images of (a) and (b). A schematic illustration (not to scale) of the AuNPs adsorbed on methylterminated SAMs of MBT and C10SH is presented above the corresponding SEM images. The scale bars in the main and inset images are 200 and 80 nm, respectively.

3. RESULTS AND DISCUSSION 3.1. Adsorption Patterns of AuNPs on SAMs of MBT and C10SH. We examined the adsorption of citrate-stabilized AuNPs

with a diameter of 13.8 nm onto methyl-terminated SAMs. SAMs of MBT and C10SH were prepared on gold substrates and subsequently immersed in aqueous AuNP solutions for adsorption to the surfaces. The surface properties of SAMs are governed by the terminal functional group of the SAM molecules. Both MBT and C10SH SAMs present methyl groups from the surface; therefore, it is expected that these SAM surfaces would exhibit the same reactivity toward AuNPs, leading to similar adsorption densities and patterns. Contrary to this expectation, the SEM images of the adsorbed AuNPs show strikingly different adsorption patterns (Figure 1). While the surface densities of the adsorbed AuNPs are similar (∼1140/μm2, MBT; ∼1220/μm2, C10SH), the AuNPs are uniformly distributed on the MBT SAMs, whereas the AuNPs are clustered on the C10SH SAMs. These adsorption patterns are invariant with experimental details such as immersion time allowed for the AuNPs to adsorb onto the SAMs and washing/drying method (Supporting Information). The acquired images are also representative. Considering that SEM is a local probe technique, we obtained images at nine different spots on a 5  5 mm sample and found that the images are similar regardless of the positions (Supporting Information). It is intriguing that markedly different adsorption patterns for the AuNPs are observed for the MBT and C10SH SAMs, although the SAM surfaces commonly present methyl groups. 3.2. Effect of Methyl Group Orientation. We first note on the terminal methyl group orientation of the two SAMs. Because arenethiols form SAMs in an upright geometry, the terminal methyl groups of the MBT SAMs orient straight upward. In contrast, the projection of the terminal methyl groups of alkanethiol SAMs changes with even or odd numbers of methylene groups due to a tilted geometry of alkyl chains.32
34 The surface properties of alkanethiol SAMs (e.g., wetting) are critically dependent on the number of methylene groups within the alkyl chain. Therefore, it is possible that the differences in the 12502

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Figure 2. SEM images of the AuNPs adsorbed on SAMs of (a) MBT, (b) EBT, (c) C9SH, (d) C10SH, (e) C11SH, and (f) C12SH. The insets display magnified SEM images of (a)
(f). The scale bars in the main and inset images are 200 and 40 nm, respectively.

AuNP adsorption may have risen from the differently oriented terminal methyl groups of the arenethiol and alkanethiol SAMs. To test this possibility, we varied the number of methylene groups at the termini of the SAMs. Figure 2 presents the SEM images of the AuNPs adsorbed on SAMs of MBT, EBT, and CnSH (n = 9, 10, 11, or 12). Figure 2 shows that the adsorption patterns are not affected by the orientation of the terminal methyl group. The AuNPs adsorb uniformly on the EBT SAMs, similar to those on the MBT SAMs. Aggregated adsorption of the AuNPs is observed for the alkanethiol SAMs regardless of the number of methylene groups (Figure 2c
f). Therefore, dispersed or aggregated adsorption of the AuNPs on methylterminated SAMs is not related to the orientation effect. 3.3. Influences of SAM Structures. We further investigated the effects of SAM structures on the adsorption pattern of the AuNPs. The lateral interaction of the arenethiol SAMs is different from that of alkanethiol SAMs; the former is dominated by the π
π interactions between the benzene rings, while the latter is driven by the van der Waals interactions between the CH2 groups.2,35
40 Possible differences in the orderliness and packing density of the two SAMs may influence the adsorption of AuNPs. Alkanethiols form highly ordered, densely packed SAMs with the sulfur headgroup adsorbed on the 3-fold hollow sites of the Au(111) √ surfaces √ and with the alkyl chain canted by 28°.2,41
43 The ( 3  3)R30° overlayer structure of alkanethiol SAMs yields an occupation area of 0.216 nm2, corresponding to a surface coverage of 0.33 monolayer (ML).44 In contrast to alkanethiols, conflicting reports exist as to the detailed structure of arenethiol SAMs. For SAMs of thiophenol (TP), well-ordered monolayers with an upright phenyl ring geometry,45,46 as well as poorly defined disordered structures,47
51 have been reported. Compared to TP, MBT

Figure 3. SEM images of the AuNPs adsorbed on SAMs of (a) SHC10PhC1 and (b) SHPhC10. The insets represent magnified SEM images of (a) and (b). The scale bars in the main and inset images are 200 and 40 nm, respectively.

forms a more highly organized monolayer. In an X-ray photoelectron spectroscopy (XPS) study, Pugmire and co-workers found the surface coverage of MBT SAMs (0.28 ML) to be higher than that of TP (0.21 ML), yet lower than that of alkanethiols (0.33 ML).52 Allara and co-workers reported highly ordered monolayer structures of MBT, based on the measurements of ellipsometry, infrared spectroscopy, atomic force microscopy, and XPS.53 12503

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Figure 5. UV
vis spectra of the AuNPs desorbed from the SAMs of MBT (red line) and C12SH (blue line). The inset shows the visual color of the AuNP solutions prepared by the desorption of AuNPs from the SAMs of MBT and C12SH.

Figure 4. SEM images of the AuNPs adsorbed on SAMs of SHC11OH. The inset shows a magnified SEM image. The scale bar in the main and inset image is 200 and 40 nm, respectively.

To explore the effect of SAM structures on the AuNP adsorption patterns, we prepared the two SAMs shown in Figure 3. A long alkyl chain spacer was inserted in SHC10PhC1 (Figure 3a) to form SAMs with a structure similar to that of C10SH SAMs but with the surface functionality of MBT. In fact, arenethiol SAMs with even a single methylene spacer (4methylbenzylthiol (SHC1Ph)) have the identical packing density to that of C10SH SAMs.52 Despite such similarity in the SAM structure between SHC10PhC1 and C10SH, Figure 3a shows that the adsorption pattern of AuNPs on SHC10PhC1 SAMs more closely resembles that of AuNPs on MBT SAMs rather than on C10SH SAMs. In contrast, the SAM structure of SHPhC10 is contributed by the lateral interactions between the benzene rings, similar to MBT. However, Figure 3b shows that the AuNPs adsorbed on SHPhC10 SAMs are aggregated, as on the SAMs of C10SH. These results suggest that the possible differences in the SAM structures between MBT and alkanethiols are not responsible for the uniform or aggregated adsorption of the AuNPs. More likely is that the surface group or the surface molecular entity influences the adsorption pattern of the AuNPs. The significance of surface groups, rather than the SAM structures, is supported by the adsorption of AuNPs on SAMs of 11-mercapto-1-undecanol (SHC11OH). If the SAM structure solely determines the adsorption pattern, then the AuNPs on the SAMs of SHC11OH should exhibit the same aggregated adsorption pattern as those observed on the SAMs of alkanethiols because both SAMs are formed by the van der Waals interactions between the long chain alkyl spacers. The only difference between the two SAMs lies in the terminal group exposed to outer environments. Figure 4 shows that the AuNPs are adsorbed in a uniform distribution on the SAMs of SHC11OH, which is contrary to the case of C12SH SAMs. This result suggests that interactions of the surface terminal groups with the AuNPs play a definitive role in the adsorption of AuNPs on SAMs. Beyond the significance of the surface group or the surface molecular entity, the effect of other structural factors of SAMs (e.g., flexibility/rigidity of molecular chains, domains or defects of SAMs) on the adsorption of AuNPs is not clear yet. Flexibility

or rigidity of SAMs can influence the adsorption of AuNPs onto the SAMs. Alkanethiols are more flexible with many conformational degrees of freedom while arenethiols are rigid and constrained due to the benzene backbone. When alkanethiols form SAMs, however, flexibility of individual molecules should be greatly reduced due to van der Waals interchain interactions. As described above, it is well-known that alkanethiols form densely packed, highly ordered rigid pseudocrystalline SAM structures. In particular, a Raman spectroscopy study by Pemberton and co-workers shows that the alkyl chain of adsorbed alkanethiols is mostly all-trans conformation, suggesting that alkanethiol SAMs are conformationally rigid.54 Arenethiols are expected to form even more rigid SAMs than alkanethiols due to larger van der Waals dimensions if only interchain interactions are taken into account. However, the lower packing density of arenethiols, which counteracts the rigidity of the SAMs, makes an intuitive estimation of the rigidity of the two SAM structures difficult.52 Therefore, quantitative comparison of the rigidity of SAMs should be precedent to connect the adsorption patterns of AuNPs to the flexibility or rigidity of the SAMs. Detailed surface structures of SAMs can be another factor affecting the adsorption patterns of AuNPs onto the SAMs. In particular, domains and defects, rather than the molecular level structures, might influence the adsorption of AuNPs due to the size of the nanoparticles interacting with the SAM surfaces. Noteworthy are the scanning tunneling microscopy (STM) studies by Liu and co-workers.51 They observed that the density and distribution of vacancy and adatom islands differs between decanethiol and benzenethiol SAMs. It is not clear yet how these differences will affect the adsorption of AuNPs onto each SAM. Further systematic investigation using STM and SEM will be necessary to correlate the detailed surface structures of SAMs with the adsorption patterns of AuNPs. 3.4. Origin of Aggregated and Dispersed Adsorption of AuNPs onto SAMs. From the results discussed above, we focus on the effects of interactions between AuNPs and the surfaces of SAMs. Dispersed or aggregated adsorption of the AuNPs onto the surfaces of SAMs can be understood using the DLVO theory.55 The stability of AuNPs is determined by the balance between the electrostatic repulsion induced by the citrate anions surrounding the AuNPs and van der Waals attraction between the AuNPs. If the citrate anions are somehow removed, then the attraction term dominates the total interaction potential and aggregation occurs. For example, addition of p-aminothiophenol (pATP) to an aqueous solution of citrate-capped AuNPs replaces 12504

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Figure 6. Schematic illustration of the (a) dispersed and (b) aggregated adsorption of AuNPs on higher and lower energy SAM surfaces, respectively.

the citrate anions on the surfaces of the AuNPs with neutralcharged pATP through the formation of strong Au
S bonds, leading to the formation of AuNP aggregates.56 Similarly, we reason that the observed aggregated adsorption of the AuNPs on the alkanethiol SAMs is induced by the elimination of the weakly bound citrate anions from the surfaces of AuNPs during the adsorption process. In contrast, the citrate layers should remain intact on the surfaces of AuNPs during the adsorption of the AuNPs onto MBT SAMs, causing dispersed, uniform adsorption. The state of the citrate layers on the adsorbed AuNPs can be measured by desorbing the AuNPs from the SAMs into a solution and subsequently characterizing the AuNPs using optical spectroscopy. The AuNPs were desorbed from the surfaces of MBT and C12SH SAMs into water by ultrasonication. The color inspection and UV
vis spectra of desorbed AuNPs reveal the characteristic features of dispersed and aggregated AuNPs, suggesting that indeed the dispersed and aggregated adsorption is due to the differences in the surface charges of the AuNPs. Figure 5 demonstrates that AuNPs desorbed from the MBT SAMs are red, whereas those desorbed from the C12SH SAMs are blue; these are characteristic colors of dispersed and aggregated AuNPs, respectively. Furthermore, the UV
vis spectrum of the AuNPs from the MBT SAMs shows a surface plasmon band at 519 nm, indicating that the AuNPs are well separated from each other due to the electrostatic repulsion of the citrate anions remaining on the AuNPs. In contrast, the AuNPs from the C12SH SAMs exhibit a broad extinction in the longer wavelength region (∼680 nm), which is attributed to the multiple coupling of the surface plasmons among the AuNPs in close proximity.57,58 These observations suggest that the AuNPs on the MBT SAMs possess intact citrate anions, which causes the AuNPs to be dispersed in the aqueous solution upon desorption, whereas citrate anions have been lost during the adsorption of the AuNPs on the C12SH SAMs, which leads to the aggregated AuNPs upon desorption from the C12SH SAMs. 3.5. Interactions of AuNPs with SAM Surfaces. We next questioned why then the citrate anions on the AuNPs are retained or removed by the MBT or C12 SH SAMs during

adsorption. We propose that the extent of interaction between the surfaces of SAMs and AuNPs determines the fate of citrate anions on the AuNPs. Stronger interactions of AuNPs with the SAM surfaces strip the AuNPs of the citrate anions, leading to the aggregated adsorption. In contrast, weaker interactions keep the citrate anions on the surfaces of AuNPs, leading to the dispersed adsorption. What makes a difference in the extent of interactions between the SAM surfaces and AuNPs? One possibility is the difference in the electronic structures between MBT and C12SH. However, our density functional theory calculations (B3LYP/LANL2DZ) show that electron densities at the terminal methyl groups are not significantly different between the two molecules (Supporting Information). We believe that solvents and the surface energies of SAMs play a significant role in determining the extent of interactions between the SAMs and AuNPs. As illustrated in Figure 6b, hydrophobic surfaces (i.e., low surface energies) are more likely to interact with the AuNPs directly, stripping the AuNPs of the weakly bound citrate anions from the surfaces upon collision, resulting in the aggregated adsorption. In contrast, water should mediate and soften the interactions between the less hydrophobic surfaces and AuNPs, causing the citrate anions to remain intact on the AuNP surfaces, and thus leading to the dispersed adsorption (Figure 6a). This picture is supported by the comparison between the adsorption patterns and contact angles of SAMs. In Figure 7, we plotted the contact angles of the SAMs used in our experiments. We also marked the adsorption patterns (dispersed vs aggregated) that each SAMs yield in the plot. Figure 7 shows that dispersed adsorption of AuNPs is observed on higher free energy surfaces, whereas aggregated adsorption occurs on lower energy surfaces. Although further investigation is required to find a more quantitative relation between the surface energies and the adsorption pattern, this result shows that these two are closely related. The significance of the extent of interactions between AuNPs and the SAM surfaces in determining the adsorption pattern is 12505

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suggest that the extent of interaction between the AuNPs and SAM surfaces determines the adsorption patterns. The possibility of controlling the AuNP adsorption patterns with identical terminal groups of SAMs opens a new avenue to the facile fabrication of nanoparticle-decorated two- or three-dimensional assembly structures.

Figure 7. Static contact angles of the SAM surfaces used in our experiments. Error bars indicate one standard deviation (1σ) from three measurements. We observed the dispersed adsorption of AuNPs on SAMs of SHC11OH, MBT, EBT, and SHC10PhC1. The aggregated adsorption of AuNPs was observed on SAMs of SHPhC10 and C12SH.

4. CONCLUSIONS We observed strikingly different adsorption patterns for AuNPs on methyl-terminated SAMs. The AuNPs were adsorbed in a uniform and dispersed fashion on the surfaces of MBT SAMs. In contrast, aggregated adsorption was observed for the AuNPs on SAMs of alkanethiols. We examined the effects of terminal methyl group orientation and the SAM organization and packing density and discovered that these factors are not responsible for the different adsorption patterns. The dispersed or aggregated adsorption behavior of the AuNPs on methylterminated SAMs was induced by the retention or removal of citrate anions on and from the AuNP surfaces, respectively, during the adsorption processes. Direct interactions between the AuNPs and hydrophobic surfaces of alkanethiol SAMs stripped the AuNPs of their citrate layers, leading to the aggregated AuNP adsorption. For the less hydrophobic MBT SAMs, water mediated and softened the adsorption of the AuNPs. As a result, the citrate anions were retained on the AuNP surfaces, leading to the dispersed adsorption. Forced interactions between the AuNPs and MBT SAMs by vigorous stirring yielded aggregated AuNP adsorption, supporting our model. ’ ASSOCIATED CONTENT

bS

Supporting Information. Synthesis of SHC10PhC1 and SHPhC10; dependence of the AuNP adsorption patterns on immersion time of SAMs in AuNP solutions; test of representativeness of acquired images; density functional theory calculations of electronic structures of MBT and C12SH SAMs; control experiments of the effect of stirring on the adsorption of AuNPs onto MBT SAMs. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 8. SEM images of the AuNPs adsorbed on MBT SAMs. The MBT SAMs were immersed in a solution containing the AuNPs for adsorption in (a), while the solution was stirred vigorously in (b) to increase the interactions between the AuNPs and MBT SAMs. The scale bars in the main and inset images are 200 and 40 nm, respectively.

further corroborated by the following pattern-changing experiment. We deliberately increased the extent of interactions between the AuNPs and MBT SAM surfaces by vigorously stirring the AuNP solution while the MBT SAMs were immersed for the AuNP adsorption, as shown in the scheme in Figure 8b. The resulting SEM images show that vigorous stirring causes the aggregated adsorption of AuNPs on MBT SAMs in contrast to the dispersed adsorption observed in the absence of agitation. This observation confirms that the forced interaction of AuNPs with MBT SAMs removes the citrate anions from the AuNP surfaces, resulting in the aggregated adsorption, consistent with our proposal. Stirring may increase the AuNP
AuNP interactions as well as the AuNP
SAM interactions. However, the increased interparticle interactions did not lead to aggregated adsorption (see Supporting Information). These results strongly

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ82-31-8005-3152; Fax: þ82-31-8005-3148; E-mail: [email protected].

’ ACKNOWLEDGMENT This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2009-0074477 and 2010-0007764). ’ REFERENCES (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (3) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (4) Colorado, R., Jr.; Lee, T. R. Langmuir 1998, 14, 6337. (5) Pemberton, J. E. Langmuir 2003, 19, 6422. (6) Leggett, G. J. Anal. Chim. Acta 2003, 479, 17. (7) Houston, J. E.; Kim, H. I. Acc. Chem. Res. 2002, 35, 547. 12506

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