Assembly of Polygonal Nanoparticle Clusters Directed by Reversible

NANO LETTERS

Assembly of Polygonal Nanoparticle Clusters Directed by Reversible Noncovalent Bonding Interactions

2009 Vol. 9, No. 9 3185-3190

Mark A. Olson,† Ali Coskun,† Rafal Klajn,†,‡ Lei Fang,† Sanjeev K. Dey,† Kevin P. Browne,†,‡ Bartosz A. Grzybowski,*,†,‡ and J. Fraser Stoddart*,† Department of Chemistry, Department of Chemical and Biological Engineering, Northwestern UniVersity, EVanston, Illinois 60208 Received April 30, 2009; Revised Manuscript Received July 23, 2009

ABSTRACT The reversible molecular template-directed self-assembly of gold nanoparticles (AuNPs), a process which relies solely on noncovalent bonding interactions, has been demonstrated by high-resolution transmission electron microscopy (HR-TEM). By employing a well-known host-guest binding motif, the AuNPs have been systemized into discrete dimers, trimers, and tetramers. These nanoparticulate twins, triplets, and quadruplets, which can be disassembled and reassembled either chemically or electrochemically, can be coalesced into larger, permanent polygonal structures by thermal treatment using a focused HR-TEM electron beam.

Twenty-seven years before the mass spectrometric identification of Buckminsterfullerene (C60) in 1986, Richard Feynman imagined a world in which matter can be manipulated and controlled at the atomic and molecular scale, anticipating the advent of the era of nanotechnology. Arguably for the synthetic chemist, subtle transformations at the atomic and molecular level are an age-old practice, and so it is not surprising that template-directed protocols1 have found much success in the hands of molecular nanotechnologists. The template-directed assembly of metal nanoparticles (MNPs)2 exemplifies the precision with which molecular templates can now be used to build nanometer-scale entities with increasing architectural complexity for device fabrication3 and applications in molecular electronics,4 plasmonics,5 photonics,6 and surface-enhanced spectroscopies.7 At the very foundation of this bottom-up approach8 to nanofabrication using MNPs lies a much more elegant and challenging display of structural control, looking to the simplest arrangement of MNPs possible, namely, the reversible organization9 of dimeric, trimeric, and tetrameric MNP assemblies. The template-directed assembly of MNPs into dimeric, trimeric, and tetrameric arrangements was first reported by Schultz et al.10 using DNA hybridization. This seminal work was followed by subsequent successes employing dynamic DNA templates11 and rigid phenylacetylene derivatives.12 The * To whom correspondence should be addressed. (J.F.S.) Tel: (+1)-847491-3793. Fax: (+1)-847-491-1009. E-Mail: [email protected]. (B.A.G.) Tel: (+1)-847-491-3113. Fax: (+1)-847-491-3728. E-Mail: [email protected]. † Department of Chemistry. ‡ Department of Chemical and Biological Engineering. 10.1021/nl901385c CCC: $40.75 Published on Web 08/20/2009

 2009 American Chemical Society

application of reversible noncovalent bonding interactions13 brings with it a highly modular and tunable platform from which to launch an approach, limited only by the extent of the creativity of the synthetic chemist. Such interactions have been largely applied in the template-directed synthesis of macrocycles,14 cyclophanes,14 knots,15 and, more recently, mechanically interlocked molecular switches,16 that is, bistable rotaxanes and catenanes. These same molecular templates can be designed in a systematic fashion, relying on the free energy of binding as the driving force to organize Au metal nanoparticles (AuNPs) into precise predetermined spatial arrangements. A strong host-guest complex has elements of both preorganization and complementarity17 between the host and the guest. A pseudorotaxane,18 for example, is a supramolecular entity, consisting of a linear guest that is threaded and bound within a macrocyclic host using one or more kinds of noncovalent bonding interactions. Pseudorotaxanes based on the preferential binding of the cyclobis(paraquat-pphenylene) (CBPQT4+) tetracationic cyclophane19 with the electrochemically active recognition unit, diethyleneglycoldisubstituted tetrathiafulvalene (TTF-DEG), benefit from having strong binding (high Ka value) as well as displaying reversible complexation.20 The pseudorotaxanes form as a result of [π· · · π], [C-H· · ·π] and [C-H· · ·O] interactions21 between the π-electron deficient CBPQT4+ host and the π-electron rich TTF-DEG guest. The TTF unit undergoes22 a sequential and reversible two-electron oxidation process (TTF f TTF·+ f TTF2+), generating a dicationic species, triggering the dethreading of the complex on account of the

Coulombic repulsion between the CBPQT4+ ring and the TTF2+ dication. Reduction of TTF-DEG back to the neutral state regenerates the 1:1 complex. This process can be initiated21 both chemically and electrochemically. Structural modifications to either the guest and/or the host provide the opportunity to expand the structural complexity of the molecular templates with ease. Herein, we describe the reversible template-directed assembly of AuNPs into homodimers, trimers, and tetramers using solely noncovalent bonding interactions provided by a clutch of CBPQT4+/TTF-DEG binding interactions. Following the templated organization of AuNPs into discrete low-entropy arrangements, electrons can then be used to remove the template and simultaneously initiate the coalescence of the AuNP arrangements into rod-, triangular-, and square-like nanostructures, a process that would otherwise be unlikely in the absence of the molecular templating interactions. A trio of CBPQT4+-based derivatives, the dimer 18+, the trimer 212+, and the tetramer 316+ (Figure 1), were prepared (see Supporting Information) in high yield, using Cu(I)catalyzed Huisgen 1,3-dipolar cycloadditions23 and starting from an alkyne- and an azide-functionalized CBPQT4+ ring appended at a single ortho position on one of the p-xylyl rings. We have reported4 recently the functionalization of MNPs with supramolecular switches, in which the TTF-DEG dithiolane derivative 5 in a mixed monolayer with 4 affords (Figure 1) soluble TTF-DEG functionalized AuNPs (d ) 4.58 ( 1.23 nm) that are capable of forming pseudorotaxanes reversibly with the CBPQT4+ ring. The zeta (ζ)-potentials of the TTF-DEG-coated AuNPs were monitored (Figure 2) to confirm the redox behavior of the surface-bound electroactive TTF units and to confirm their binding by the CBPQT4+ ring. Upon addition of Fe(ClO4)3, the surfacebound TTF units are oxidized to TTF2+ dications leaving the AuNPs with a ζ-potential of +21.9 ((1.9) mV in DMF (Figure 2b). Following the addition of ascorbic acid, the TTF2+ dications are reduced back to their neutral state, and the ζ-potential decreases to -0.5 ((2.1) mV (Figure 2a).

Figure 2. ζ-Potential measurements on TTF-DEG-functionalized AuNPs, illustrating the sequential and reversible two-electron oxidation of 5 from the neutral state (a) (TTF f TTF·+ f TTF2+) generating the dicationic species (b) and the reversible 1:1 complexation with CBPQT4+ on the AuNP surface (c).

The formation of pseudorotaxanes with the CBPQT4+ rings adhering to the surface of AuNPs is observed following the addition of excess of the cyclophane, as indicated by the ζ-potential increasing to +29.2 ((0.7) mV (Figure 2c). In order to induce the dethreading of the surface-bound 1:1 complexes, Fe(ClO4)3 is added to the mixture, leading to a decrease in the ζ-potential down to a level characteristic of the TTF2+ dicationic state. These results indicate that AuNP surface-bound TTF units can bind CBPQT4+ rings, thereby increasing the surface potential of the MNPs. Following oxidation of the TTF unit, the dethreading of the tetracationic cyclophane decreases the surface charge, resulting in moderately charged TTF2+-coated MNPs. These threading and dethreading events can be repeated numerous times by addition of chemical oxidants and reductants or electrochemically by reversibly applying22 +900 mV, the potential

Figure 1. Structural formulas of the CBPQT4+ dimer 18+, trimer 212+, tetramer 316+, and the dithiolane-functionalized triethyleneglycol 4 and TTF-DEG 5 (green station ) TTF). 3186

Nano Lett., Vol. 9, No. 9, 2009

Figure 3. The TTF-DEG monofunctionalized AuNPs undergo template-directed self-assembly into homodimers and trimers when combined with the templates 18+ and 212+, respectively. AuNPs functionalized with 5% TTF-DEG can self-assemble into tetrameric patterns when combined with the template 316+. All of these template-directed assemblies undergo reversible complexation/ decomplexation when two electrons are removed from the TTF recognition unit generating the TTF2+ dicationic species, at which point Coulombic repulsion with the tetracationic cyclophane (CBPQT4+) initiates dethreading of the pseudorotaxane and removal of the templates.

at which the TTF2+ dications are generated. The ζ-potentials of AuNPs covered solely with 4 are close to zero and are not affected by the addition of Fe(ClO4)3, ascorbic acid, or the CBPQT4+ ring. To prevent formation of network-like NP aggregates and to ensure the binding of one TTF-DEG-functionalized AuNP to only one CBPQT4+ ring, the NPs were functionalized in highly dilute solutions of 4 and 5 (molar ratio of 3000:1). This procedure gave a ∼1:13 mixture of “monofunctionalized” AuNPs (χTTF ) 0.0065)24 and nonfunctionalized25 AuNPs, that is, covered entirely by 4 with χTTF ) 0.0. When mixed with solutions of the host templates 1·8PF6 and 2·12PF6 in MeCN, the monofunctionalized AuNPs undergo (Figure 3) a template-directed self-assembly into homodimers and trimers in solution in roughly 10% yields for each of these aggregate structures, as determined by TEM. In order to obtain tetrameric arrangements, we used AuNPs prepared using a 20:1 molar ratio of 4 and 5 which had26 an average of 11.5 TTF units per particle. When combined with 3·16PF6 in MeCN, these AuNPs self-assembled into tetrameric arrangements in around 5% yield. In all three cases, we observed the presence of many27 unbound single and illdefined AuNP assemblies, resulting from nanoparticles prepared by the high dilution technique in order to circumvent cross-linking and network formation. In this system, the free energy (-∆Go) of binding is the major driving force behind the template-directed selfNano Lett., Vol. 9, No. 9, 2009

assembly of AuNPs functionalized with CBPQT4+ rings. To study the thermodynamics of pseudorotaxane formation, isothermal titration microcalorimetry (ITC) measurements22c were performed (see Supporting Information) to determine the thermodynamic parameters (∆Ho, ∆So, ∆Go) and ultimately the association constants (Ka values) characterizing the binding of TTF-DEG with the alkyne-functionalized CBPQT4+ derivative (see Supporting Information). For the complexation of TTF-DEG with the alkyne-functionalized CBPQT4+ derivative, the value of Ka was found to be 1.15 × 105 M-1, giving rise to a ∆Ho value of -11.4 ( 0.1 kcal·mol-1 and a ∆So value of -15.1 ( 0.2 cal·mol-1·K-1, corresponding to a ∆Go value of -6.92 ( 0.05 kcal·mol-1 at 298 K in MeCN. The thermodynamic parameters obtained by ITC provide evidence that pseudorotaxane formation for this host/guest system is a favorable process, and points to the formation of discrete homodimeric, trimeric, and tetrameric polygonal clusters by way of host-guest templating interactions. Following the formation of the self-assembled dimer, trimer, and tetramer arrangements, removal of two electrons from the TTF unit generates the TTF2+ dications at which point dethreading of the template occurs on account of Coulombic repulsion between the charged species (Figure 3). Reduction of the TTF2+ dication back to the neutral TTF state regenerates the templated-AuNPs. High-resolution transmission electron microscopy (HR-TEM) of TTF monofunctionalized AuNPs, when mixed in MeCN with the hosts 1·8PF6 and 2·12PF6, revealed the presence of homodimeric (Figure 4a,b) and trimeric (Figure 4c,d) ensembles, respectively. The 5%-functionalized TTF AuNPs, when mixed in MeCN with the host 3·16PF6, form tetrameric assemblies (Figure 4e,f) reflecting the 4-fold symmetry of the pseudo-

Figure 4. Representative HR-TEM images of the templatedirected assemblies of the homodimers (a,b), trimers (c,d), and tetramers (e,f) obtained after mixing the appropriate template with TTF-DEG-functionalized AuNPs. Scale bars ) 2 nm (a, b, d-f), 5 nm (c). Bottom: The OPLS-2005 minimized geometries for 18+, 212+, and 316+ (left to right). Hydrogen atoms have been omitted for clarity. 3187

Figure 5. Representative high-resolution transmission electron microscopy (TEM) images of the template-directed coalesced homodimers (a), trimers (b-d), and tetramers (e-h) into rod-, triangular-, and square-like nanostructures, following electron ablation, and magnified images of the voids created during thermal treatment (c, f, h). Scale bars ) 2 nm (a, b, d, e, and g) and 1 nm (c, f, h).

rotaxane. For all types of structures, no higher-order oligomers are observed. Also, in each case the clusters can be disassembled into isolated NPs (observed by HR-TEM) by the addition of excess of Fe(ClO4)3 to the stock solution, oxidizing the TTF unit, and reconstituted (observed by HRTEM) by the addition of ascorbic acid to the same stock solution, effectively reducing the oxidized TTF units. In all three cases, we can cycle the redox chemistry many times using the same stock solutions, leading to aggregate formations only in the reduced solutions and only single NPs from the oxidized solutions. In other words, the assembly process is fully reversible in solution. To further rationalize the shapes and dimensions of the assemblies, we used the Optimized Potentials for Liquid Simulations (OPLS-2005)28 force field to minimize the geometries for 18+, 212+, and 316+ (Figure 4 bottom, left to right) to reveal the precise dimensions of the molecular templates. By reference to crystallographic data (see Supporting Information) previously obtained29 for CBPQT4+ complexes with TTF and diethyleneglycol-disubstituted 1,5dihydroxynaphthalene guests, along with the crystal structure of thioctic acid,30 it has been estimated that each CBPQT4+ ring is positioned approximately 0.8 nm from the AuNP surface to which it is bound by a TTF dithiolane ligand. Overall, these dimensions correlate well with the distances observed by HR-TEM for the template-directed AuNP assemblies, where the molecular template may sit in numerous positions and still remain in contact and bound with the AuNPs. Finally, we investigated the possibility of processing the reversible assemblies into permanent clusters. Specifically, we have used the phenomenon of nanoparticle coalescence upon removal of the self-assembled monolayers protecting the proximal NPs. Thermal treatment31 at temperatures above 95 °C leads to desorption of the thiolates from the gold surface and fusing of nanoparticles, the process being driven by minimization of the overall surface tension.31 Here, we used a focused electron beam to achieve32 temperatures on the order of several hundred degrees Celsius, sufficient to melt the templated assemblies rapidly into rod- (Figure 5a), triangular- (Figure 5b-d), and square-like nanostructures33 3188

(Figure 5e-h). This process is an example of a templatedirected one that is not present in the absence of the molecular templating interactions, as proximity and precise spatial arrangements are key components. In every case, we observe coalescence of the AuNP ensembles with time as we witness the joining of separate Au lattices with atomic resolution (Figure 5, see Supporting Information). The remarkable feature of this coalescence process is the formation of very small internal voids in the trimeric and tetrameric structures (Figure 5b, c, e-h). As heating proceeds, the void diameter gradually decreases until it becomes only a few atoms wide (Figure 5g,h, indicated by arrows). Particles presenting such narrow voids could be interesting candidates for sensing applications based on plasmonic enhancement.34 On reflection, we have demonstrated the reversible molecular template-directed self-assembly of gold nanoparticles, relying solely upon reversible noncovalent bonding interactions. By capitalizing on the favorable free energy of binding of a well-established binding motif, gold nanoparticles have been systemized into discrete dimers, trimers, and tetramers that can be assembled/disassembled both chemically and electrochemically, and subsequently have been fused into larger, permanent structures by thermal treatment. The subtle interplay between preorganization, molecular recognition, and complementarity, starting with small molecular components, helps to exert control and order by way of molecular templation. In the fullness of time, this protocol could provide an avenue by which matter can be manipulated more precisely on the nanoscale. Acknowledgment. This research is supported by the MRSEC program of the NSF (DMR-0520513) at the Materials Research Center of Northwestern University. L.F. gratefully acknowledges the support of a Ryan Fellowship from Northwestern University. B.A.G. gratefully acknowledges the financial assistance from the Alfred P. Sloan Fellowship and from the Dreyfus Teacher-Scholar Award. Supporting Information Available: Description of the synthesis, characterization, and methods. This material is available free of charge via the Internet at http://pubs.acs.org. Nano Lett., Vol. 9, No. 9, 2009

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