Effect of Ligand Shell Structure on the Interaction between Monolayer

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Effect of Ligand Shell Structure on the Interaction between Monolayer-Protected Gold Nanoparticles Ying Hu, Oktay Uzun, Cedric Dubois, and Francesco Stellacci* Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed: October 10, 2007; In Final Form: NoVember 29, 2007

Here we present a comparative analysis of two types of equally monodisperse gold nanoparticles (∼8 nm in diameter), one coated only with dodecanethiol ligands and the other coated with a mixture of dodecanethiol and 4-methylbenzenethiol ligands. The former particles (‘homoligand’) have a homogeneous coating, while the latter ones (‘rippled’) show a striated structure composed of phase-separated ribbon-like domains of alternating composition. A combined scanning tunneling and transmission electron microscopy study shows that homoligand nanoparticles interdigitate into one another less than rippled particles and readily form hexagonally packed supracrystals, while rippled particles are trapped in more disordered ‘glassy’ arrangements.

Introduction Gold nanoparticles coated with a self-assembled monolayer (SAM) of thiolated molecules1-3 have attracted considerable interest because of their potential applications in fields that range from electronics3-5 to nanomedicine.6-10 These particles are easy to synthesize and can be coated readily with mixtures of thiolated ligand molecules.1,2,11 The ligand shell mediates the interactions between the outside environment and the nanoparticle’s core. Hence, multiple types of ligands can provide nanoparticles with different and complementary properties, ranging from solubility12-16 to catalysis8-10,17-20 or directed assembly properties.21-23 Recently we investigated, with scanning tunneling microscopy (STM)12,24 and infrared spectroscopy,25 gold nanoparticles coated with a mixture of ligands that were known to phase separate into randomly sized and shaped domains when co-assembled on flat gold surfaces.26,27 We found that, on gold nanoparticles, these molecules assembled into phase-separated ordered ribbon-like domains of alternating composition (see cartoon in Figure 1f).12,24 These striated phases have an average width that is less than 5 Å and form due to a delicate interplay between enthalpic and entropic driving forces, as recently shown through simulations.28-30 Such a molecularly defined arrangement has drastic effects on the nanoparticle properties, generating some ‘structural’ effects in the ligand shell interaction with the environment. For example, we showed that these particles (hereafter called rippled) had a non-monotonic dependence of solubility12,31 on ligand shell composition. They were also effective at repelling protein nonspecific adsorption.12 We have also shown that due to topological reasons ‘rippled’ particles have two diametrically opposite defects (where the circular ribbon-like domains collapse into points) that are more reactive toward place exchange reactions with a pseudo-secondorder kinetic rate 2 orders of magnitude greater than what is expected for similar particles.32 As a consequence we showed that it is possible to selectively place ligand molecules (of a type different from the two already present on the particles) at the two ‘polar’ extremes of rippled particles. We used this property to link particles in chains but also to assemble them * To whom correspondence should be addressed. E-mail: [email protected].

onto surfaces through these polar molecules and hence with a specific relation between the plane of the substrate and the plane of the ligand phases (see Figure 2b).32 It is obvious that the presence of ‘ripples’ on nanoparticles reduces the quasi-spherical symmetry and introduces a specific directionality.29,30 The effect of this phenomenon on particleparticle interactions and the ability of nanoparticles to form ordered assemblies has yet to be investigated. In our previous studies we had always been hindered by the fact that mixedligand particles are polydisperse in size and hence difficult to study for properties that depend on the average interactions of many particles. Recent advances in nanoparticle synthesis33 and a careful choice of ligand molecules allowed us to perform this study on the effect of ripple structure on particle-particle interactions. Hereafter we start by presenting a discussion on the opportunities and challenges involved in STM studies of nanoparticles and then show a combined STM and transmission electron microscopy (TEM) analysis that illustrates how ripple particles interdigitate into one another more strongly than their homoligand counterparts and are considerably less able to form ordered supracrystals. Experimental Section All nanoparticles (NPs) used here were synthesized using a newly developed method.33 Briefly, 0.25 mmol of (triphenylphosphine)gold(I) chloride salt and 0.5 mmol of the desired thiolated molecule or molecule mixture were dissolved in 20 mL of benzene to which 2.50 mmol of tert-butylamine-borane complex (20 mL of benzene) was added. The reaction was performed for 1 h at 100 °C and for 4 h at room temperature. A 100 mL amount of ethanol was then added to the reaction mixture to precipitate the NPs as black solid powder. The solid product was separated by centrifugation, and the recovered powder was washed with excess ethanol and dried under vacuum. The nanoparticles’ core size was characterized using TEM. Homoligand particles were coated with dodecanethiol (DDT) and had an average diameter (as measured by TEM) of 7.8 nm with a standard deviation of 1.2 nm. Mixed-ligand ‘rippled’ particle coated with a 2:1 mixture (stoichiometric ratio

10.1021/jp709895z CCC: $40.75 © 2008 American Chemical Society Published on Web 03/29/2008

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Figure 1. STM height images of gold nanoparticles coated with (a and b) DDT (homoligand) and (d and e) a 2:1 mixture of DDT and MBT (mixed ligands). The cartoons in c and f are meant to schematically exemplify the arrangement of the ligands on the nanoparticles. Specifically, f shows the ribbon-like domains that form when immiscible ligands are placed on the ligand shell of nanoparticles.

Figure 2. Cartoons exemplifying two of the possible relative positions that rippled particles can take relative to the substrate depending on the deposition procedure. In (a) we represent the “sideway” position and in (b) the “pole-up” one.

used in the reaction) of DDT and 4-methylbenzenethiol (MBT) had an average diameter of 8.0 nm with a standard deviation of 1.4 nm. All STM images of the particles were obtained using a Veeco Multimode Scanning Probe Microscope. Samples were prepared by placing layers of these particles onto Au (111) thermally evaporated on freshly cleaved mica substrates (Molecular Imaging, AZ). Pt-Ir mechanically cut tips were used (Veeco, CA). Different imaging conditions, such as tip current, bias between the sample and the tip, and tip scanning speed, were used. The set point current ranged from 200 to 700 pA, the bias voltage ranged from 600 to 1500 mV, and the tip speed varied from 0.4 to 1.3 µm/s. Imaging gains used varied from 0.7 to 0.5 for the integral and 0.5 to 0.2 for the proportional. The TEM samples are prepared by dissolving nanoparticles in toluene with an approximate optical density of 0.1, and one drop of this solution was placed on top of a carbon-coated 200 mesh copper grid (Ladd Research, VT) laid over a Kimwipe (Kimtech Science, WE). TEM images of these substrates were

Figure 3. Two images (height and current) of a group of five nanoparticles coated with a 2:1 mixture of nonanethiol:MBT captured at two different speeds (1.22 and 1.36 µm/s). (Left) Height images; (right) corresponding current image. Of the five nanoparticles, three (C, D, E) consistently showed ripple structure and the other two (A, B) showed none. This was repeatedly observed for five different speeds in this series (see Figure S4 in Supporting Information).

analyzed using ImageJ (http://rsb.info.nih.gov/ij/index.html) software, and size distributions were obtained by counting at least 200 nanoparticles. Results Two approaches were used to prepare the sample. The first consisted of placing a few drops of nanoparticle solutions directly onto the substrate and then drying under a nitrogen stream; we call this approach “sideway” assembly because it tends to produce samples with particles arranged on surfaces as illustrated in Figure 2a.12,24,32 An alternative method consisted of preparing both a SAM of 5-amino-2-mercaptobenzimidazole (AMB) and reacting rippled nanoparticles with a 40 times excess (relative to the number of poles) of 11-mercaptoundecanoic acid (MUA) at room temperature for 30 min in toluene.32 The SAM surface was then soaked in the NPs solutions for 3 h to allow

Effect of Ligand Shell Structure

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Figure 4. (a) TEM images of DDT nanoparticles showing hexagon packing. (b) TEM images of 2:1 DDT:MBT nanoparticles showing a more distorted packing.

the MUA molecules to react with the AMB. The substrate was then dried and imaged. We call this approach “pole-up” assembly (see Figure 2b).32 In STM metallic nanoparticles appear as circular objects (to a first approximation).24,34-37 The diameter of these objectss for the most partscorresponds to the diameter of the particles measured in TEM plus twice the extended length of the ligand molecules (in the case of DDT ∼1.5 nm).24 An analysis of the images shows that the height of these objects is not always equal to their diameter as one would expect for spherical objects. Both literature34-37 and our images of ligand-coated nanoparticles of similar size show that isolated nanoparticles appear “flat” in STM, i.e., their height is typically less than 3 nm (Figure S1a, in Supporting Information, SI). Similarly, particles in packed layers have a height that corresponds to their diameter (Figure S1b, in Supporting Information). Interestingly, we noticed that when an individual nanoparticle adsorbs on top of a packed layer, its height is between these two situations (3-4 nm; Figure S1c). The reasons leading to this phenomenon are not entirely clear; multiple factors can contribute to the observed height variation. It is known that height in STM images is always a combination of topological and electronic properties38 mediated by a set of feedback parameters. Furthermore, it has been shown that the electronic properties of nanoparticles (as probed by STM) vary depending on the particle’s aggregation state.34,39 An isolated particle shows a pronounced Coulomb blockade (thus effectively lowering the applied bias), while films of particles have a more Ohmic behavior.34 When imaging nanoparticles free of unbound ligands and other impurities it is possible to obtain images that show features on the particles’ ligand shell.24 For all of the nanoparticles studied we obtained a series of images at different imaging tip speeds and roughly within the same image window. In order to determine whether the structures observed on the particles were real or a product of instrumental noise, plots of average feature spacing versus tip speed were generated (Figure S2, Supporting Information). In the case of noise such plots are made of points that can be extrapolated by a line that passes through the origin.24 On the contrary, real features show only a weak dependence on tip speed.24 For the first time, we present a partial explanation of the observed variation of average spacing with tip speed. To address this problem we imaged a SAM of mercaptopropionic acid molecules. We observed the textbook x3 ×x3R30° and c (4 × 2) reconstructions.40-42 When imaging this SAM at different tip speeds we noticed that the tip/sample thermal drift had a significant effect on the images. As speed increased the

number of molecules observed vertically decreased (Figure S3). As a consequence, spacing measurements taken along a given crystallographic plane led to an average spacing with a measurable tip speed dependence (from 0.49 nm at 0.41 µm/s to 0.53 nm at 1.22 µm/s). When studying mixed-ligand “rippled” particles we do not have true molecular resolution because of the nanoparticle curvature (and overall sample cleanness), and we only observe one of the two types of molecules present in the mixed SAM. Hence, variations are amplified because the spacings measured are larger, as we measure across multiple headgroups. Moreover, given that we mostly observe crystallographic planes perpendicular to the fast scan axis and consistently measure along the horizontal direction (as opposed to along a crystallographic axis), we have variations that are due to the changes in angle of our crystallographic planes. Recently, we developed other means to evaluate images. It is possible to selectively functionalize particles at the two polar defects and use the ‘polar’ molecules as chemical handles to direct the assembly of the particles onto a surface in a way that keeps the planes of the ripples parallel to the substrate (Figure 2b). STM images of particles assembled in this way do not show ‘ripples’. This is probably due to the fact we are observing the areas around the pole for these particles where ‘polar’ molecules have significant mobility. Obviously, in our “pole-up” assembly method only a fraction of the nanoparticle has the pole occupied, and there is a limited efficiency of the overall assembly process, thus generating surfaces that have a mixture of “pole-up” and “sideways” particles.32 As show in Figure 3, sequential images (taken at different speeds) of these surfaces consist of particles that consistently show (or do not show) ripples: a strong indication of images made of real features. To validate the image interpretation for this assembly approach we compared images obtained for two types of particles which had markedly different reactivity at the polar defects. At 20 times excess of MUA, 60% of nonanethiol:MBT particles were able to make chains while only 20% of DDT:MBT particles produced chains.32 Similarly, 57% of nonanethiol:MBT particles assembled onto a surface through the pole-up method showed no ripples, while only 13% of DDT:MBT particles assembled in the same way showed no ripples. Discussion Recently there has been significant interest in particle assembly into supracrystals.43,44 To the best of our knowledge, present studies in literature have focused on homoligand

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Figure 5. STM height images of DDT nanoparticles. (Left) Multilayer of nanoparticles packed one layer above another. (Right) Enlarged picture of the hexagon packing of the nanoparticles; the arrows in green represent a measurement of the angle between neighboring nanoparticles performed to outline the packing quality of these particles.

Figure 6. STM height images of DDT:MBT (2:1) nanoparticles assembled in a sideway matter. (Left) Some order in the nanoparticle packing; (Right) the enlarged picture shows the lack of hexagonal packing with the particles mostly in a distorted cubic lattice. The arrows in green represent a measurement of the angle between neighboring nanoparticles performed to outline the packing quality of these particles.

nanoparticles.44,45 The effect of ribbon-like domains in the particles’ ligand shell on their assembly properties has yet to be addressed. Here we focused on two samples that are equally ‘monodisperse’, synthesized with the newly developed Stucky method.33 Homoligand particles were coated with dodecanethiol (DDT) and had an average diameter of 7.8 ( 1.2 nm. ‘Rippled’ particles, coated with a 2:1 mixture of DDT and MBT, had an average diameter of 8.0 ( 1.4 nm. This small difference in size and polydispersity can be attributed to the presence of different thiolated molecules during the reaction. Given the similarity in size and polydispersity these particles should have formed supracrystals of equal quality. STM measurements on these particles were used to characterize the ligand shell as well as the packing ability. Relative to the former property we find that DDT particles are coated with hexagonally packed alkane molecules with an average headgroup spacing of 0.78 ( 0.15 nm (see Figure S5 in the Supporting Information for an example of the cross-section measurement). This number is slightly larger than what would be expected (0.74 nm) applying the continuous model46 to our previous data24 and to recently published molecular dynamic simulations.30 Yet we have to state that this measurement may be severely affected by experimental errors. As stated previously,24 measuring headgroup spacing in our system is challenging and measurements are affected by the fact that we often do not image all the headgroups. In any way we can affirm

that the measured headgroup spacing is larger than the one that we previously measured on nanoparticles coated with octanethiols and confirming that there is a splay in the ligand molecules of nanoparticles. STM images on mixed-ligand particles were significantly different from those of their homoligand counterparts showing stripes instead of dots (see Figure 1). The average spacing of these stripes was 1.0 ( 0.2 nm in line with previous measurements on similar systems.12,24 It is interesting to compare the size of the particles measured in TEM with that measured in STM. DDT particles have a TEM size of 7.8 ( 1.2 nm (Figure 4a). STM images of isolated particles (or particles on top of particle monolayer) showed an average size of 11.9 ( 1.9 nm. This is close to the expected value of 10.9 nm derived from adding to the TEM diameter twice the length of an all-trans DDT molecule (∼1.5 nm).47 A careful analysis of the STM images of packed layers shows an average particle diameter of 10.0 ( 0.6 nm. We believe that this value is lower than the isolate particle value because of the interdigitation of the ligand of neighboring particles. The ease of interdigitation agrees with the large degree of splay observed when measuring headgroup spacing. Our STM images show an interparticle average distance of 2.2 nm, which is in agreement with the measured TEM interparticle distance of 1.74 ( 0.17 nm (it should be noted that TEM images are a twodimensional projection of a three-dimensional object). A similar analysis was performed for the DDT:MBT nanoparticles. In this

Effect of Ligand Shell Structure case the TEM average diameter was 8.0 ( 1.5 nm (Figure 4b), while the STM diameter of isolated particles was 10.1 ( 1 nm. The diameter measured for packed nanoparticles was 8.9 ( 0.9 nm. Also in this case the average interparticle distance measure with STM (0.9 nm) is in agreement with the one measured with TEM (1.34 ( 0.12 nm). It is immediately evident that these particles get closer than their homoligand counterpart, something that could be explained thanks to their ordered stripes and to a significant overlap between these stripes, possibly with the long ligand of one particle placing itself on top of the short ligand of another. It should be mentioned that this could also be reasonable for particles coated with random mixtures of long and short ligands where most of the long ligands arrange themselves onto the longer ones. ‘Ripples’ are the thermodynamic equilibrium form for mixed-ligand-coated nanoparticles; hence, it is impossible to make a true control experiment to understand which is the true case. Yet it should be noted that the degree of interdigitation is significant, and an ordered structure is certainly more conducive to this tight packing. The average particle distance is smaller that the DDT extended length, probably indicating that the molecule is richer in gauche defects and has a bent head. In both cases the degree of interdigitation between particles is significant and should be considered as a major driving force toward their assembly. In the case of ripple particles the degree of overlap is larger than in the case of homoligand ones; similar proportion should be expected when comparing the strength of the particle-particle interactions. The similarities and differences highlighted above produce different assembly behaviors for these two types of particles. Care was taken to process these particles in identical ways and to use solutions for concentration so as to minimize differences due to different solubility. For the homoligand nanoparticles, both in TEM and STM we observed relatively large hexagonally packed supracrystals. Due to the size variation the coherence length (defined as the average size of a single defect-free domain) never exceeded a few unit cells but the area covered by ordered arrangements of these particles exceeded the hundreds of nanometers (see Figure 5). In STM we often observed a second ordered layer of particles on top of the first one; this was not the case in TEM most likely because of differences in sample preparation and substrates used. An analysis of the STM and TEM images of the supracrystal shows that the angle between three neighboring nanoparticles is 60° ( 10°, in good agreement with a closed-packed model (see Figure S6a, Supporting Information). In stark contrast, mixed-ligand nanoparticles assembled in the same way show a more disordered arrangement where almost no hexagonally packed region was observed. Interestingly, regions with distorted cubic cells (angles varying from 80° to 50°, see Figure 6 and Figure S6b, Supporting Information) were present in both TEM and STM images. Let us reiterate that the core size of these two types of particles is approximately the same; hence, the great difference observed in the assembly of the particles has to be explained either in terms of a difference in ligand-ligand interactions or substantially different geometric packing constrains for the ligand shells. The large interdigitation observed for rippled particles should translate into strong particle-particle interactions that ‘freezes’ the particles in offequilibrium assemblies. In the case of homoligand particles, weaker particle-particle interactions allow for a more rapid assembly into the closed-packed form. Effectively the assemblies observed in this study for rippled particles are the nanoscale

J. Phys. Chem. C, Vol. 112, No. 16, 2008 6283 equivalent of a glass where ‘bond’ lengths are approximately constant but bond angles vary over a wide range. Conclusions We presented a discussion of STM images of homoligand and mixed-ligand 8 nm gold particles. We have shown that ripples in the ligand shell enhance particle-particle interactions, leading to a stronger degree of interdigitation. This leads to a significantly less ordered assembly of particles into a nanoscale glassy state. Acknowledgment. This work was supported in part by the NIH TPEN Center. We acknowledge the Packard Foundation and the NHLBI TPEN program (U01-HL080731) awards. This work used some of the facilities supported by the MRSEC Program at MIT (DMR 02-13282). Supporting Information Available: Six figures highlighting some of the paper’s results. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Templeton, A. C.; Wuelfing, M. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (2) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (3) Thomas, K. G.; Kamat, P. V. Acc. Chem. Res. 2003, 36, 888. (4) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (5) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (6) You, C. C.; Chompoosor, A.; Rotello, V. M. Nano Today 2007, 2, 34. (7) Han, G.; Ghosh, P.; Rotello, V. M. Nanomedicine 2007, 2, 113. (8) Verma, A.; Nakade, H.; Simard, J. M.; Rotello, V. M. J. Am. Chem. Soc. 2004, 126, 10806. (9) Pasquato, L.; Pengo, P.; Scrimin, P. Supramol. Chem. 2005, 17, 163. (10) Pasquato, L.; Pengo, P.; Scrimin, P. J. Mater. Chem. 2004, 14, 3481. (11) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (12) Jackson, A. M.; Myerson, J. W.; Stellacci, F. Nat. Mater. 2004, 3, 330. (13) Levy, R.; Thanh, N. T. K.; Doty, R. C.; Hussain, I.; Nichols, R. J.; Schiffrin, D. J.; Brust, M.; Fernig, D. G. J. Am. Chem. Soc. 2004, 126, 10076. (14) Lucarini, M.; Franchi, P.; Pedulli, G. F.; Gentilini, C.; Polizzi, S.; Pengo, P.; Scrimin, P.; Pasquato, L. J. Am. Chem. Soc. 2005, 127, 16384. (15) Stellacci, F.; Bauer, C. A.; Meyer-Friedrichsen, T.; Wenseleers, W.; Alain, V.; Kuebler, S. M.; Pond, S. J. K.; Zhang, Y. D.; Marder, S. R.; Perry, J. W. AdV. Mater. 2002, 14, 194. (16) Akthakul, A.; Hochbaum, A. I.; Stellacci, F.; Mayes, A. M. AdV. Mater. 2005, 17, 532. (17) Briggs, C.; Norsten, T. B.; Rotello, V. M. Chem. Commun. 2002, 1890. (18) Lucarini, M.; Franchi, P.; Pedulli, G. F.; Pengo, P.; Scrimin, P.; Pasquato, L. J. Am. Chem. Soc. 2004, 126, 9326. (19) Manea, F.; Houillon, F. B.; Pasquato, L.; Scrimin, P. Angew. Chem., Int. Ed. 2004, 43, 6165. (20) Verma, A.; Simard, J. M.; Worrall, J. W. E.; Rotello, V. M. J. Am. Chem. Soc. 2004, 126, 13987. (21) Shipway, A. N.; Katz, E.; Willner, I. Chemphyschem 2000, 1, 18. (22) Barsotti, R. J.; Stellacci, F. J. Mater. Chem. 2006, 16, 962. (23) Glotzer, S. C. Science 2004, 306, 419. (24) Jackson, A. M.; Hu, Y.; Jacob Silva, P.; Stellacci, F. J. Am. Chem. Soc. 2006, 128, 11135. (25) Centrone, A.; Hu, Y.; Jackson, A. M.; Zerbi, G.; Stellacci, F. Small 2007, 3, 814. (26) Smith, R. K.; Reed, S. M.; Lewis, P. A.; Monnell, J. D.; Clegg, R. S.; Kelly, K. F.; Bumm, L. A.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 1119. (27) Stranick, S. J.; Atre, S. V.; Parikh, A. N.; Wood, M. C.; Allara, D. L.; Winograd, N.; Weiss, P. S. Nanotechnology 1996, 7, 438.

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