~o Paulo Coelho,† Gloria Tardajos,† Vladimir Stepanenko,‡ Alexander Ro¨dle,‡ Gustavo Ferna´ndez,*,‡ and Joa Andre´s Guerrero-Martı´nez*,†
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Cooperative Self-Assembly Transfer from Hierarchical Supramolecular Polymers to Gold Nanoparticles †
Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Avda. Complutense s/n, 28040, Madrid, Spain and Institut für Organische Chemie and Center for Nanosystems Chemistry, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
‡
ABSTRACT The transfer of information encoded by molecular
subcomponents is a key phenomenon that regulates the biological inheritance in living organisms, yet there is a lack of understanding of related transfer mechanisms at the supramolecular level in artificial multicomponent systems. Our contribution to tackle this challenge has focused on the design of a thiolated π-conjugated linking unit, whose hierarchical, cooperative self-assembly in nonpolar media can be efficiently transferred from the molecular to the nanoscopic level, thereby enabling the reversible self-assembly of gold nanoparticle (AuNP) clusters. The transfer of supramolecular information by the linking π-system can only take place when a specific cooperative nucleation-elongation mechanism is operative, whereas low-ordered noncooperative assemblies formed below a critical concentration do not suffice to extend the order to the AuNP level. To the best of our knowledge, our approach has allowed for the first time a deep analysis of the hierarchy levels and thermodynamics involved in the self-assembly of AuNPs. KEYWORDS: cooperativity . self-assembly . supramolecular polymerization . gold nanoparticle . π-conjugated systems
T
he self-assembly of colloidal nanoparticles (NPs) has emerged as a powerful concept for devising novel nanomaterials, becoming a natural starting point for the bottom-up fabrication of devices in nanotechnology.1,2 In this context, self-assembly refers to the spontaneous organization of nanocrystals as discrete units through noncovalent interactions between molecules located at the NPs interfaces.3 5 In particular, colloidal gold nanoparticles (AuNPs) feature unique optical responses due to the excitation of localized surface plasmon resonances (LSPRs),6 which arise from the interaction between light and the conduction electrons at the AuNP. The intense and highly confined electromagnetic fields induced by the LSPRs in self-assembled AuNPs provide a very sensitive tool to detect small changes in their dielectric environments,7 9 property that is particularly attractive for sensing applications.10 12 Although there are extensive studies on AuNPs self-assembly by COELHO ET AL.
using different types of noncovalent interactions between their ligands, such as van der Waals, π π, electrostatic interactions and hydrogen bonds,13 18 mechanistic and thermodynamic insights into AuNPs self-assembly pathways remain thus far unknown. However, that understanding may be guidance to develop new switchable plasmonic nanostructures with adaptive functionalities by rational selection of capping agents. To tackle this challenge, we envisaged that the introduction of tailor-made small π-conjugated moieties as linking units would enable a close inspection of the hierarchy levels involved in the AuNPs selfassembly. Such discrete π-systems have the advantage that their self-assembly can be readily monitored by a variety of spectroscopic techniques, which in turn allows a deep understanding of their supramolecular polymerization mechanisms.19 23 Oligophenyleneethynylene (OPE) derivatives are ideally suited to meet the above criteria due VOL. XXX
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* Address correspondence to
[email protected],
[email protected]. Received for review August 4, 2015 and accepted October 21, 2015. Published online 10.1021/acsnano.5b04841 C XXXX American Chemical Society
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ARTICLE Scheme 1. Chemical structure of the thiolated OPE derivative (OPESH, 1), and schematic representation of its hierarchical selfassembly in MCH/THF (75:25) (a) and its impact on AuNPs cooperative self-assembly (b).
to their excellent optical, photophysical and supramolecular properties.24 26 As a proof-of-concept structure, we have designed an unsymmetrically substituted, thiolated (OPE) derivative (OPESH, 1, Scheme 1) that is expected to self-associate in solution via cooperative π π and hydrogen bonding interactions (see Scheme 1, for synthetic and characterization details see the Supporting Information). In addition to a relatively extended π-surface, our target OPE features an amide group that is thought to participate in H-bonding, an alkyl side chain to provide enough solubility in nonpolar media and a thioacetate group to ensure the coordination to the surface of previously synthesized AuNPs (for synthesis and ligand replacement see the Supporting Information). RESULTS AND DISCUSSION The self-assembly behavior of OPESH 1 has been initially tested by UV vis spectroscopy. 1 is fairly soluble in moderately polar solvents such as chloroform, dichloromethane, and tetrahydrofuran (THF), and exists in a monomeric state in these media. Alternatively, moving to nonpolar solvents strongly increases the aggregation tendency of the system, however to the point that the aggregates precipitate rapidly out of solution. We found out that a mixture of methylcyclohexane COELHO ET AL.
(MCH)/THF (75:25) was optimal both in terms of solubility and significant degree of aggregation. Figure 1a depicts the temperature-dependent UV vis experiments of 1 in the above solvent mixture between 323 and 273 K at a selected concentration of 7 10 5 M (for full details regarding the concentration range used for these studies see the Supporting Information). On cooling, the absorption bands at 332 and 353 nm sharpen and increase their intensity whereas in the region between 225 and 300 nm the absorbance slightly decreases. This behavior is indicative of the planarization of the alkyne-aryl groups of the OPE backbone,26 29 and subsequent involvement in π π interactions, which is typically accompanied by moderate spectral changes.30,31 Additionally, the appearance of an isosbestic point at 310 nm suggests an equilibrium state between the OPESH monomeric and aggregated species in solution. To analyze in depth the self-assembly mechanism of 1, cooling curves at different concentrations (cooling rate: 1 K min 1) were monitored at 331 nm, as shown in Figure 1b and c. Surprisingly, we noticed that the shape of the curves largely depends on the chosen concentration (see the Supporting Information). Below a critical concentration of ∼1 10 4 M, the monomerto-aggregate transition is defined by a shallow, sigmoidal VOL. XXX
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TABLE 1. Thermodynamic Parameters of the Isodesmic Model31 Obtained by the Boltzmann Equation Fitting of the Temperature-Dependent UV Vis Spectra of OPESH at Low Concentrations (< 1 10 4 M) at 298 K ΔH C [M 1]
4.0 10 5.0 10 6.0 10 7.0 10
Tm [K] DPn 5 5 5 5
291 293 295 297
1.43 1.23 1.31 1.36
Ka [M 1]
6.3 103 5.6 103 6.7 103 7.1 103
ΔS
[kJ mol 1] [J mol
98.9 110.5 112.9 115.7
1
ΔG K 1] [kJ mol 1]
259 303 299 315
21.7 19.9 23.8 21.9
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Figure 1. (a) Temperature-dependent UV vis studies of 1 in MCH/THF (75:25) at 7 10 5 M. Arrows show spectral changes upon cooling. (b,c) Cooling curves at 7 10 5 M (b) and 1.4 10 4 M (c) at a cooling rate of 1 K min 1. (d h) TEM (d,f,h), AFM (inset in d and panel e) and SEM (g) micrographs on a carbon-coated copper grid (TEM) or silicon wafer (SEM, AFM) showing the spherical and nanotubular aggregates formed by 1 depending on the concentration.
curve (Figures 1b and S4), characteristic of an isodesmic process. By contrast, sharp nonsigmoidal cooling curves are obtained above 1 10 4 M (Figures 1c and S6), suggesting the formation of better-ordered cooperative assemblies32 when the concentration is increased. The shallow sigmoidal curves observed at lower concentrations were successfully fitted to the isodesmic model (see the Supporting Information),33 obtaining melting temperatures (Tm) between 291 and 297 K, association constants (Ka) of 5.6 7.1 103 M 1 and values of free Gibbs energy (ΔG) ranging from 19.9 to 23.8 kJ mol 1 (Table 1). The large and negative values of enthalpy (ΔH) ( 98.9 to 115.7 kJ mol 1) indicate that the process is exothermically enthalpy-driven. On the other hand, the nonsigmoidal cooperative curves observed at higher concentrations were fitted using a nucleation-elongation model developed by ten Eikelder, Markvoort, Meijer and co-workers (see the Supporting Information).34,35 This model distinguishes between a thermodynamically unfavorable dimerization step and a more favorable elongation process that is activated below a critical elongation temperature (Te). Table 2 illustrates the calculated thermodynamic parameters, showing Te values of ∼319 323 K, values of dimerization constants (Kd) in the range of 5 56 M 1 and elongation constants (Ke) of 7.1 9.1 103 M 1. The large difference between the Kd and Ke values yields a degree of cooperativity (σ), defined as the quotient of Kd and Ke, ranging from 0.0007 to 0.002. These low σ values for the supramolecular polymerization of 1 imply a relatively high degree of cooperativity, comparable to other π-systems reported in the literature.20 Atomic force microscopy (AFM) as well as transmission (TEM) and scanning (SEM) electron microscopy studies (Figures 1d h) provide some insight into the distinct morphologies of the self-assembled species formed by 1 in MCH/THF (75:25) below and above 1 10 4 M. To allow a better comparison, both samples at 7 10 5 M and 1.4 10 4 M were initially heated to the monomeric state followed by a slow cooling process (1 K min 1) into aggregate species. The corresponding solutions were then drop-casted either
32,33 TABLE 2. Thermodynamic Parameters of the Cooperative Mechanism Model Obtained by the Fitting the Temperature-Dependent UV Vis Spectra of OPESH at Various Concentrations of 1 (> 1 10 4 M) in the Absence and Presence of AuNPs
CNP [M 1]
0 0 0 0 3.5 10 3.5 10 3.5 10 3.5 10
COPESH [M]
9 9 9 9
1.1 10 1.2 10 1.3 10 1.4 10 1.1 10 1.2 10 1.3 10 1.4 10
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ΔH°nucl [kJ mol 1] 4 4 4 4 4 4 4 4
13.8 ( 0.7 19.5 ( 1.1 16.6 ( 1.3 13.6 ( 1.0 15.0 ( 1.0 18.5 ( 1.4 20.2 ( 1.8 16.7 ( 1.1
ΔH° [kJ mol 1]
82.7 ( 2.8 70.5 ( 2.2 90.5 ( 3.7 100.0 ( 4.7 58.8 ( 3.0 56.9 ( 2.3 57.5 ( 2.1 71.1 ( 2.6
ΔS° [kJ mol 1]
0.1846 ( 0.0089 0.1440 ( 0.0069 0.2062 ( 0.0117 0.2346 ( 0.0148 0.1153 ( 0.01 0.1076 ( 0.077 0.1081 ( 0.0067 0.1509 ( 0.0083
Te [K]
Kd [M 1]
Kel [M 1]
51.0 5.4 16.7 56.8 26.0 6.7 3.4 12.7
7.1 10 7.7 103 8.3 103 9.1 103 9.1 103 8.3 103 7.7 103 7.1 103
319.9 ( 0.3 322.3 ( 0.2 321.8 ( 0.3 322.7 ( 0.3 307.8 ( 0.4 311.7 ( 0.3 315.1 ( 0.3 316.6 ( 0.3
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7.1 10 0.7 10 2.0 10 6.3 10 2.9 10 0.8 10 0.4 10 1.8 10
3 3 3 3 3 3 3 3
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forces will then create a sheet-like structure that would ultimately roll up to yield a hollow nanotubular assembly (Scheme 1). This is evident from both SEM and TEM micrographs, in which some partially coiled fragments of tubes and planar sheets can be recognized (see the Supporting Information). We questioned whether this intriguing hierarchical self-assembly process could be transferred36,37 to link AuNPs in a cooperative fashion. Our strategy initially aimed at coating the surface of spherical AuNPs of 12.7 ( 1.2 nm average in diameter with OPESH units (see the Supporting Information for synthesis and characterization) via Au S bonding.38 As a control experiment, we tested the self-assembly behavior of the modified gold nanoparticles (OPES-AuNPs) and found out that their propensity to aggregate in MCH/ THF (75:25) is negligible even at high AuNPs concentrations (∼1.0 10 7 M) and low temperatures (273 K) (Figure S2c). We thus envisaged that the information encoded in the self-assembled structures of 1 could be exploited to initiate the self-assembly of the modified AuNPs. To that end, we dissolved a mixture of the thiolated OPE 1 and OPES-AuNPs in THF and subsequently added MCH up to a solvent ratio MCH/THF (75:25), reaching final concentrations of 7 10 5 M and 1.0 10 9 M for 1 and modified AuNPs, respectively. Previously, we observed that 1 in isolation is prone to form spherical associates under identical conditions of solvents and concentrations. In the mixture with the OPES-AuNPs, however, the weak; noncooperative;tendency of 1 to self-assemble does not suffice to trigger the aggregation of the OPESAuNPs (Figure S2d). In fact, the competition between self-assembly of 1 and coassembly on the surface of the modified AuNPs results in the disintegration of the molecular aggregates. By contrast, the behavior of the system turned out to be entirely different when the concentration of 1 was increased to 1.4 10 4 M. Figure 2a shows the temperature-dependent UV vis spectra of a mixture of 1 (1.4 10 4 M) and modified AuNPs (3.5 10 9 M) in MCH/THF (75:25). The spectra reveal contributions from both OPESH (transitions at ∼332 and 353 nm) and AuNPs (LSPR band between ∼500 700 nm) components. Upon decreasing the temperature from 323 to 273 K, the absorption bands of the OPESH units sharpen whereas the LSPR band of AuNPs (initially at ∼540 nm) broadens and shifts toward higher wavelengths up to 585 nm due to LSPR coupling (inset in Figure 2a).7 At 323 K, the dispersions are red in color (Figure 2b) and perfectly stable, implying that the AuNPs are well separated from each other in solution. Below 298 K, the AuNPs are consistently found to aggregate, as confirmed by the color change of the dispersions to violet/purple.39 Contrary to the behavior of 1 in isolation under equivalent conditions, the isosbestic point of the molecular component has now VOL. XXX
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on silicon wafer (for AFM and SEM) or carbon-coated copper grid (for TEM) and investigated by the respective technique. As shown in Figure 1d, the low-concentration aggregate species correspond to polydisperse spherical aggregates with an average diameter of 35 ( 10 nm (see also Figures S9 and S10). These are most likely formed as follows: due to the amphiphilic character of the OPESH, the monomeric units would initially form small spherical associates in which the polar thioacetate groups are shielded from the nonpolar environment. These individual micelles most likely further agglomerate via van der Waals and solvophobic interactions between the alkyl chains into larger clusters (Scheme 1). Regarding the self-assembly behavior of 1 at higher concentrations (>1 10 4 M), AFM imaging reveals a new molecular rearrangement into considerably larger and better ordered 1D assemblies with a diameter corresponding to 40 ( 6 nm and length of 700 ( 50 nm (Figures 1e h and S11 13). Interestingly, combined SEM and TEM investigations clearly demonstrate that the fiber-like assemblies are indeed hollow in nature (Figures 1f h). The external diameter of the nanotubes was measured to be 35 ( 2 nm, whereas the wall thickness corresponds to 4.2 ( 0.2 nm (Figures 1h and S11 S13). Cross section analysis of a broken fragment of a nanotube identified by AFM yields a value of membrane thickness of 5.0 ( 0.2 nm, which is in good agreement with TEM considering than in the latter technique only the high electron density of antiparallel-arranged OPE moieties within the bilayer structure are visualized. This dimension is appreciably larger than the length of the geometry-optimized structure of 1 assuming a fully outstretched conformation of the alkyl chain (∼3.5 nm based on molecular modeling, Figure S24). On this basis, we propose a molecular model based on a bilayer membrane in which the polar thioacetate groups are shielded from the nonpolar environment in the inner part of the membrane, whereas the alkyl chains are directed toward the periphery and interior of the nanotubes (Scheme 1). Therefore, initial dimerization of the system driven by weak H-bonding involving the thioacetate groups and solvophobic interactions will cause the activation of the self-assembly by simultaneous π π interactions of the aromatic fragments and H-bonding between the amide functional groups. This assumption is fully supported by the large downfield shifts observed for the thioacetate (Δδ = 0.45 ppm) and amide (Δδ = 0.37 ppm) protons in temperaturedependent 1H NMR studies on cooling from 323 to 273 K (see Figure S19). Furthermore, the absence of coupling signals between spatially distant protons in the 2D ROESY spectrum of the assemblies suggests a symmetric aggregate structure and rules out the formation of antiparallel stacks within the same monolayer (Figure S20). The cooperative nature of these
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been appreciable displaced to 355 nm. According to the overall results, a temperature decrease has promoted the self-assembly of OPESH units, and as a result the aggregation of OPES-AuNPs. In order to monitor whether the cooperative selfassembly behavior of 1 is maintained in the mixture, the absorption at 331 nm at various concentrations above 1 10 4 M was plotted versus the temperature (Figures 2c and S8). The resulting curves (cooling rate: 1 K min 1) are nonsigmoidal and can be accurately fitted to the nucleation-elongation model.34,35 The incorporation of AuNPs leads to a decrease in the Te (∼9 K) values with respect to the isolated nanotubular assemblies of 1. On the other hand, the Kd (6 26 M 1) and Ke (7.1 9.1 103 M 1) values are nearly identical to those calculated in the absence of AuNPs, leading to a degree of cooperativity between 0.0004 and 0.003. These results suggest that the cooperative information encoded in the nanotubular assemblies of 1 has been efficiently transferred from the molecular to the nanoscopic (AuNP) scale. An insight into the precise stages involved in this self-assembly process was gained by comparing the temperature values at which the absorption bands of COELHO ET AL.
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Figure 2. (a) Temperature-dependent UV vis studies of a mixture of 1 (1.4 10 4 M) and OPES-AuNPs (3.5 10 9 M) in MCH/THF (75:25). Arrows show spectral changes upon cooling whereas the star denotes an isosbestic point. Inset: Shift of the LSPR band of the AuNPs vs temperature (the dashed line points 298 K). (b) Color change associated with AuNPs self-assembly. (c) Cooling curve of a mixture of 1 (1.4 10 4 M) and OPES-AuNPs (3.5 10 9 M) monitored at 331 nm (cooling rate of 1 K min 1). (d) Reversibility of the OPESH-triggered AuNPs self-assembly process.
the OPE fragments (331 nm, Figure 2c) and the LSPR of the AuNPs (540 nm, inset of Figure 2a) undergo an abrupt change in intensity and position, i.e., the activation step. As evident in Figure 2c, the molecular elongation step at 331 nm commences when the temperature falls below the Te, i.e., between 308 and 316 K depending on the concentration. Concomitantly, the LSPR band undergoes a small red-shift of ∼5 nm from 323 to 298 K that is ascribed to the formation of small AuNP clusters. In sharp contrast, the starting point for the abrupt displacement of the LSPR band occurs at remarkably lower temperatures (see inset in Figure 2a). We calculated that at 298 K, the degree of aggregation of 1 corresponds to around 0.8. According to these findings, the initial stage of the overall selfassembly involves the self-association of AuNPs into clusters, promoted by initial cooperative self-assembly of units of 1 (Scheme 1b). Subsequently, at lower temperatures the LSPR band abruptly shifts from 550 to 585 nm, indicating a growth of the AuNPs initial nuclei into larger aggregates. Although the changes in absorption for the LSPR band against temperature do not reach a clear end, i.e., a plateau in the plot of λLSPR vs T is not reached at the lowest temperature (273 K) (see inset in Figure 2a), the steep slope observed is in agreement with a cooperative process. Thus, both consecutive clustering and aggregation steps may be considered as nucleation and elongation steps of a cooperative self-assembly of AuNPs transferred from the hierarchical supramolecular assembly. The required reversibility of such cooperative selfassembly was tested by recording UV vis spectra during six cycle times between 273 and 323 K. Figure 2d plots the position of the LSPR maxima for both temperatures, further demonstrating the complete reversibility by temperature switching. The relatively small drop-off in intensity and broadening of the bands after each cycle is caused by the precipitation of a small fraction of assembled structures formed in solution (see Figure S22). The OPESH-triggered clustering of the AuNPs has been closely followed by AFM (Figures 3a and S14). The sample was prepared by spin-coating a mixture of 1 (1.4 10 4 M) and OPES-AuNPs (3.5 10 9 M) at 298 K. Typical images reveal the presence of small AuNPs clusters formed by ∼3 8 self-assembled nanocrystals, which is in good agreement with the small red-shift of 5 nm observed from 323 to 298 K (see simulated optical properties of AuNPs clusters in Figure S25). Interestingly in coexistence with AuNPs clusters, sheet-like lamellar structures formed by molecular stripes of self-assembled OPE units with typical periodic distances of 5.5 ( 0.3 nm are observed (Figure 3b). This result suggests that the molecular layers grow in the presence of AuNPs through a combination of weak H-bonding involving the thioacetate groups and alkyl chain packing of face-to-face molecules in different
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layers, and simultaneous π π interactions of the aromatic fragments and H-bonding between the amide bonds within the same layer. Although the selfassembly mode changes from nanotubes to planar sheets in the presence of AuNPs, the chemical environment of the OPE moiety may be similar in both proposed models, which is in good agreement with the observed downfield shifts for the thioacetate and amide protons in temperature-dependent 1H NMR and ROESY analyses (see Figure S21). Complementary TEM, SEM and AFM studies shed some light on the hierarchical self-assembly of the AuNPs. Under drop-casting conditions of the mixture, large aggregates of several hundreds of nanometres in size are observed, in which the AuNPs are embedded in the sheet-like structures formed by 1 (Figures 3c and d and S15 S17). The minimum distance between the surface of neighboring AuNPs is ∼4.0 nm (Figure 3d), which closely corresponds to twice the length of the aromatic backbone of 1 (see Figure S24). To test the strength of the degree of cooperativity of 1 in the presence of OPES-AuNPs, a small fraction of stabilized nanocrystals were added to a previously prepared solution of OPESH nanotubes at room temperature. SEM micrographs, in contrast to the typical self-assembly COELHO ET AL.
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Figure 3. (a,b) AFM images of a mixture of 1 (1.4 10 4 M) and OPES-AuNPs (3.5 10 9 M) on silicon wafer under spincoating conditions showing the initial stages of the OPESHtriggered AuNPs self-assembly: (a) formation of small AuNPs nuclei embedded in lamellar structures (see panel b) formed by 1. (c,d) TEM micrographs on a carbon-coated copper grid, and (e) SEM micrograph on silicon wafer, showing the clustering of the AuNPs under drop-casting conditions. (f) SEM micrograph of partially disassembled molecular nanotubes after addition of OPES-AuNPs at room temperature.
transfer of OPES-AuNPs in which any tubular nanostructure is observed (Figure 3e), show the partial disassembly of the molecular nanotubes due the formation of self-assembled AuNPs within sheet-like lamellar structures (Figure 3f and Figure S18), confirming the previously observed high degree of cooperativity (see Tables 1 and 2). Finally, the specificity of the cooperative selfassembly transfer process from supramolecular polymers to AuNPs was checked by mixing AuNPs (∼1.0 10 7 M), perfectly stabilized by a thiol polyoxyethylene glycol polymer in the solvent mixture of MCH/THF (75:25) at the temperature range of interest (see the Supporting Information), with OPE molecules (1.4 10 4 M). The addition of the supramolecule induces the nonreversible aggregation of the polyoxyethylene glycol-stabilized AuNPs (see Figure S23), even at high temperatures, which suggests that the transfer of cooperative information is mainly driven by specific interactions between the capping ligands and the supramolecular polymer precursors. Therefore, the interactions that direct the self-assembly of 1 into nanotubular aggregates at high concentrations (simultaneous π π interactions of the aromatic fragments and H-bonding between the amide functional groups) are responsible for the clustering and selfassembly of AuNPs, indicating that nanocrystal capping agents with specific functionalities, for instance the thiol derivative supramolecular polymeric precursor, are needed in order to obtain an efficient cooperative self-assembly transfer process. CONCLUSION In summary, we have shown that the hierarchy levels exhibited by cooperative supramolecular polymers can be efficiently transferred to trigger the self-assembly of metal nanoparticles in a fully reversible manner. Spectroscopic and microscopic studies have allowed a deep analysis of the thermodynamics and pathways involved in the self-assembly of AuNPs, thus demonstrating that the cooperative self-assembly of AuNPs is preceded by a thermodynamically unfavorable nucleation event. Our findings should pave the way for a new realm of hierarchical self-assembly processes of metal nanoparticles. We thus anticipate the use of such supramolecular-nanoparticle polymerization as a powerful methodology for the preparation of new type of superstructures for nanoplasmonics applications by selecting the appropriate dimensions, morphology, and supramolecular functionalization of the metal nanoparticles. We are currently focusing our efforts on the preparation of larger and nonspherical gold nanoparticles stabilized with multifunctional interactive ligands, with the aim of obtaining gold nanocomposites with defined intense and anisotropic plasmonic properties. Conflict of Interest: The authors declare no competing financial interest.
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Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b04841. Synthetic details, compound and nanoparticle characterizations, supramolecular polymerization models, UV vis analyses and fits, TEM, AFM, SEM, temperature-dependent 1 H NMR and ROESY experiments, and compound and nanoparticle simulations. (PDF)
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Acknowledgment. This work has been funded by the Spanish MINECO (MAT2014-59678-R) and the Madrid Regional Government (S2013/MIT-2807). J.P.C. acknowledges receipt of a Ci^ encia sem Fronteiras fellowship from the CNPq of Brazil. A.G.-M. acknowledges receipt of a Ramón y Cajal Fellowship from the Spanish MINECO. G. F. thanks the Humboldt Foundation (Sofja Kovalevskaja Award) and the BMBF for financial support.
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