Article pubs.acs.org/JPCC
Self-Assembly of Gold Nanoparticles into 2D Arrays Induced by Bolaamphiphilic Ligands Jan Paczesny,*,† Michał Wójcik,‡ Krzysztof Sozański,† Kostyantyn Nikiforov,† Carsten Tschierske,§ Anne Lehmann,§ Ewa Górecka,‡ Józef Mieczkowski,‡ and Robert Hołyst*,† †
Polish Academy of Sciences, Institute of Physical Chemistry, Kasprzaka 44/52, 01-224 Warsaw, Poland Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland § Institute of Chemistry, Organic Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes Str. 2, D-06120 Halle, Germany ‡
S Supporting Information *
ABSTRACT: We performed synthesis and investigated the self-assembly properties of gold nanoparticles (NPs) with covalently attached bolaamphiphilic ligands (B-AuNPs). The judiciously designed coating rendered the NPs amphiphilic and induced their self-assembly. The B-AuNPs formed ordered two-dimensional structures over large areas upon simple dropcasting. The films exhibited an uncommon and applicable topography, consisting of densely packed rings of inner diameter of around 30 nm, with the B-AuNPs at the rim and an empty interior. We introduced and proved experimentally an explanation of how the structures were formed. The model involved elements of geometric packing and ligand reorganization. Upon contact with the hydrophilic surface, ligands rearranged at the surface of the metallic cores of the B-AuNPs so that the bolaamphiphilic moieties (constituting ca. 50% of the coating) were in proximity to the surface, while the hexanethiol moieties moved away from it. The described mechanism is of general relevance for the design of functional NPs capable of self-assembly.
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obtained in the case of AuNPs,8 as well as AgNPs9 or CoNPs.10 In a review by Srivastava and Kotov,11 the authors described the possible applications of unique optical (spectroscopy), magnetic (bioimaging), and electronic (single electron transistors) properties of such structures. A number of applications of nanoparticle arrays, such as catalysts for nanowire growth for dye-sensitized solar cells,12 catalyst layers in fuel cells,13 electrical transducers of chemical or biological binding processes,14 etc. require control over interparticle spacing in the 10−50 nm regime. This is difficult to execute. Variation of the length of the ligand alkyl chain allows the adjustment of spacing in the sub-5 nm range.15 The diblock copolymer methods ensure proper spacing control in a range above 25 nm.16 Precisely controlled self-assembly based on judicious molecular design may fulfill this apparent gap. In this article, we propose a method of obtaining two-dimensional arrays of NPs, with patterning length scale covering the intermediate range between the molecular assembly and topdown engineering. What induces the ordering is the presence of bolaamphiphilic moieties in the coating of the NPs.
INTRODUCTION With the requirements of nanotechnology constantly increasing in terms of device miniaturization, reproducibility, and cost efficiency, the “top-down” approach is gradually reaching its limits. Fluctuations of the size of manufactured nanodevices often produce a significant spread in characteristics, affecting the key parameters, such as, e.g., threshold voltage and on/off currents in the case of transistors.1 A lot of effort is therefore dedicated to the development of the “bottom-up” approach, which is based on self-assembly processes. In such methods, atoms, molecules, or nanoparticles spontaneously assemble into complex nanostructures due to specific interactions. In the case of nanoparticles (NPs), there are several ways of producing ordered structures, including self-organization due to thiol coating, assembly on modified substrates, or application of prefabricated templates.2 Self-assembly of ligand-protected nanoparticles, based on ligand−ligand interactions, is a wellestablished technique. It may utilize different monofunctional ligands,3 as well as particles bearing aromatic domains,4 liquid crystalline moieties,5 or domains allowing for multiple Hbonding.6 Thin films of periodically organized nanoporosity are among the most important research topics for the development of nanoengineering,7 with particular reference to one- and twodimensional networks of nanoparticles. Cellular structures were © 2013 American Chemical Society
Received: September 3, 2013 Revised: October 7, 2013 Published: October 11, 2013 24056
dx.doi.org/10.1021/jp408826f | J. Phys. Chem. C 2013, 117, 24056−24062
The Journal of Physical Chemistry C
Article
Bolaamphiphiles may be regarded as dimeric, end-to-end connected amphiphiles. They stabilize membranes (e.g., monolayer lipid membranes of archaebacteria)17 and form other self-assembled structures, such as liquid crystalline phases in the bulk state18 and gels or helical fibers in aqueous systems.19 It has been shown that bolaamphiphiles may assemble into nanowells and pores in membranes.20 In our recent work, we presented unique self-assembly properties of bolaamphiphiles in 2D systems.21,22 However, very few papers have been published until now on composite bolaamphiphileNP systems. A noteworthy example is a study by Sistach et al.,23 who reported that adsorption of a bolaamphiphilic surfactant at the surface of AuNPs increased the stability of the system and allowed for reversible aggregation upon pH changes. Meister et al.24 showed also the utility of helical nanofibers of bolalipids as a template for the formation of 1D assemblies of AuNPs. Here, we report how self-assembly properties of bolaamphiphiles covalently bound to metallic nanoparticles can be used to obtain regular, complex structures. We describe the synthesis of the bolaamphiphile-thiol conjugates and the exchange reaction25 allowing for their introduction into the AuNP coating. We also present an analysis of the mechanism of pattern formation, which is of general relevance for the design of functional NPs capable of self-assembly.
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MATERIALS AND METHODS B-AuNP Synthesis and Characterization. The scheme of the synthetic route of the studied material is shown in Figure 1. The starting point was the acetonide-protected 4,4″-glycerolfunctionalized terphenyl-2′-ol (compound [1]), whose synthesis had already been described previously.26 In the first step, [1] was etherified with 11-bromoundecan-1-ol in a Mitsunobu reaction to yield the bromine-terminated T-shaped molecule [2]. Afterward, the bromine was exchanged in a reaction with hexamethyldisilthiane (HMDST) to yield the desired thiol [3] after hydrolytic deprotection of the silyl group with tetrabutylammonium fluoride (TBAF) in tetrahydrofuran (THF). The yields of the reactions, referring to chromatographically and spectroscopically (1H NMR) homogeneous materials, were 68% and 74%, respectively. AuNPs were synthesized according to a well-described protocol27 based on the Brust method; details on the AuNP synthesis and purification can be found in the Supporting Information. n-Hexane thiol was used to passivate the metal surface. In the next step, the hexanethiol molecules of the primary grafting layers were partially exchanged by the prepared bolathiol [3] in a ligand exchange reaction.25,28 The exchange ratio is known to depend mainly on the starting concentration of the thiols and the duration of the reaction.29 These parameters were kept constant throughout all of the performed experiments. For the chosen reaction conditions, an exchange ratio of ca. 50% was obtained. The ratio of mesogenic units to n-alkanethiols at the nanoparticle surface was determined from integration and comparison of the 1H NMR signals characteristic for both molecules. Because of significant peak broadening for NP-conjugated thiols, the uncertainty of determination of the ligand ratio in the NP coating was up to 10%. In the final step, the acetonide groups were removed with HCl in THF and methanol. The precipitated bolaamphiphilecovered gold nanoparticles [4] were purified by repeated washing with pure THF. Deprotection of the glycerol groups was carried out as the very last step due to the insolubility of
Figure 1. Synthetic pathway used to obtain the bolaamphiphilecovered gold nanoparticles [4]. Abbreviations: TPP, triphenylphosphine; DIAD, diisopropylazodicarboxylate; HMDST, hexamethyldisilthiane; THF, tetrahydrofuran; AuNP@SC6H13, hexanethiol-covered gold nanoparticles. Reagents and conditions: [1] → [2], TPP, DIAD, THF, 35 °C; [2] → [3], HMDST, TBAF, −10 °C; [3] → [4], (AuNP@SC6H13), toluene; HCl(aq), CH3OH, THF.
the deprotected compound in the solvents used during the exchange reaction. As the efficiency of deprotection of compound [3] (observed by thin layer chromatography) was almost 100%, it was assumed that the same efficiency should apply to the reaction of B-AuNP hybrids. All of the reactions performed during the synthesis of ligands and nanoparticles were carried out under nitrogen atmosphere in dried glassware, with efficient magnetic stirring. Purification of the reaction products was carried out by column chromatography, using silica gel from Merck (pore size 60 Å). Analytical thin-layer chromatography (TLC) was performed using Silica Gel 60 Å F254 (Merck) precoated glass plates (0.25 mm thickness) and visualized using iodine vapor 24057
dx.doi.org/10.1021/jp408826f | J. Phys. Chem. C 2013, 117, 24056−24062
The Journal of Physical Chemistry C
Article
Langmuir film at chosen surface pressure. Most uniform monolayers were obtained in drop-casting for samples where the area per particle corresponded to the point at the isotherm around 40 mN/m, i.e., just before the bend, where the slope of the isotherm was maximal. XRR and SAXS Measurements. X-ray reflectivity (XRR) measurements were performed on a Bruker D8 Discover diffractometer, operating at a wavelength of 1.54 Å, in which the monochromatic parallel beam was formed by a parabolic Goebel mirror. The system was equipped with an Eulerian cradle and a reflectometry sample-stage, which ensured precise sample positioning. A scintillation counter together with an automatic absorber of the primary beam allowed for a linear dynamic range better than 108 cps. In the kinematical approximation, the reflected X-ray intensity is related to the electron density profiles. These profiles are estimated according to the expected structure of the film. Distribution of electron density as a function of the z-coordinate, i.e., the distance from the substrate, gives the starting parameters for fitting procedure which matches the simulated profile with the measured curve of intensity vs the 2θ angle. The software package Leptos 4.02 (available from Bruker-AXS, Karlsruhe, Germany) was used to evaluate the layer thicknesses from the angular interval between the Kiessig fringes recorded for the layer. Such fringes are caused by the interference between X-ray waves reflected from both interfaces of the thin layer and depend on the refractive indices of its constituents. Further details on the XRR experiments can be found in the Supporting Information. For small-angle X-ray scattering (SAXS) experiments, performed to determine the AuNP and B-AuNP sizes, a Bruker Nanostar system was used. The system was equipped with a parallel point beam of CuKα radiation formed by cross-coupled Goebel mirrors and a 3-pinhole collimator, an area detector, VANTEC 2000, and a thermostatted sample holder. AFM, SEM, and TEM Imaging. Atomic force microscopy (AFM) images were acquired in both noncontact (FM) and tapping (AM) modes with the UHV-350 AFM microscope at room temperature, at a base pressure in the low 10−10 mbar range. The AFM tips were Al-coated silicon Nanosensors PPPNCHR with a radius of curvature
