DendrimerâDye Assemblies as Templates for the Formation of Gold

Article pubs.acs.org/Macromolecules

Dendrimer−Dye Assemblies as Templates for the Formation of Gold Nanostructures Jasmin Düring,† Wiebke Alex,† Alexander Zika,† Robert Branscheid,‡ Erdmann Spiecker,‡ and Franziska Gröhn*,† †

Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials and ‡Institute of Micro- and Nanostructure Research & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander-University Erlangen-Nürnberg, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: Dendrimer−dye assemblies are used as novel supramolecular nanoreactors for the formation of various gold nanostructures. The organic−inorganic hybrid systems are investigated with dynamic light scattering, UV−vis spectroscopy, and transmission electron microscopy (TEM) as well as cryo-TEM and high-resolution TEM (HRTEM). We show that the shape of the hybrid assemblies is determined by the choice and the ratio of the building blocks. Shape and size of the gold nanoparticles within the assemblies are controlled by the reducing agent. The accessible range of gold morphologies extends from small and spherical, over ellipsoidal, faceted, and large to highly anisotropic. The approach may open the way to new hybrid systems with applications in catalysis or in the biomedical field.

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thermodynamically controlled templating process.21 Lower generation dendrimers, in difference, function as stabilizing molecules attaching to the gold surface like a classical “colloid stabilizer”.21 More recently, gold/polymer composite particles have been developed using amine-rich core−shell polymer particles as both reductant and nanoreactor to generate, encapsulate, and stabilize gold nanoparticles.18,19 Three-dimensional dendritic gold nanostructures can be prepared by reacting pyrrole monomer with tetrachloroauric acid (HAuCl4) in the presence of poly(sodium 4-styrenesulfonate) (PSS).20 Not only size and shape alter the optical properties of gold nanostructures but also their mutual distances.4,22−29 For example, the aggregation of spherical gold nanoparticles induced by the presence of butyric acid causes a change in the color of the solution, which is used to determine the presence of certain bacteria.30 It was shown that aggregated gold nanoparticles can create more plasmonic heat and thereby work more efficiently in killing cancer cells as single gold nanoparticles.31 Thus, studying gold nanoparticle self-assembly is very promising, and a lot of recent research has been conducted in this respect.32−35 Yet, it remains desirable to develop effective routes to more complex systems with tunable gold morphology and the possibility to combine the nanoparticles with other functional entities. In general, self-assembly offers an elegant approach of creating well-ordered structures from individual compo-

INTRODUCTION The unique optical and photophysical properties of noble metal nanoparticles, particularly gold nanoparticles, show high potential in different fields such as in the photothermal treatment of cancer or in biomedical imaging.1,2 These properties can be triggered by manipulating size and shape of the nanostructures.3−5 Spherical 10−20 nm gold nanoparticles can be prepared using the classical citrate method,6,7 while for smaller particles (1−3 nm) the Brust−Schiffrin method can be applied.8 Promising rodlike gold nanoparticles are obtainable with a seed-mediated growth in the presence of silver nitrate, CTAB (cetyltrimethylammonium bromide), and ascorbic acid.9−13 Yet the controlled formation of more complex anisotropic gold nanostructures remains challenging, though some progress has been made in this regard. Antonietti and Gröhn et al. used a hydrophilic polyelectrolyte microgel as an ionic nanoreactor for the formation of nonclassical gold nanostructures.14 Inspired by biomineralization,15 the microgel serves as organic template, directing the growth of the inorganic structure. In extension of this concept, a thermoresponsive polymer can be used for the design of microgels and the encapsulation and growth of gold nanoparticles.16,17 Dendrimers as well-defined macromolecules are good tools for the formation of gold nanoparticles via two mechanisms: Higher generation dendrimers act as “electrostatic nanotemplates” where the number of Au-precursor ions ionically associated with an oppositely charged dendrimer molecule determines the size of the gold nanoparticle formed upon reduction, e.g., 1024 gold ions become reduced into a 3.5 nm gold particle in the inside of a G8 polyamidoamine (PAMAM) dendrimer in a © XXXX American Chemical Society

Received: April 10, 2017 Revised: August 24, 2017

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DOI: 10.1021/acs.macromol.7b00752 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Overview of the Assembly Formation from Dendrimer and Ionic Dye Molecules and Loading of the Assembly Nanoreactor with Gold Salta

a

The reduction of the gold salt with different reducing agents induces the formation of various gold−dendrimer−dye hybrid assemblies.

nents.36−39 Structure formation is driven by mutual, noncovalent interactions of the building blocks, which can cover the whole range from inorganic nanoparticles over macromolecules and polymers to small organic molecules.40 Here again, dendrimers represent well-defined and versatile building blocks for self-assembly.41−45 For example, multivalent ionic dye molecules can interconnect oppositely charged dendrimer macroions into a variety of nano-objects with different shapes and functionalities in solution.43−45 In a corresponding gold system, a large generation 8 (G8) PAMAM dendrimer was first used as nanotemplate for the formation of well-defined gold nanoparticles within the dendrimer as described above (“pregold particle-loaded G8 PAMAM”),21 and these gold− dendrimer hybrid particles were then interconnected with an aromatic diazo dye to form well-defined and size tunable gold− dendrimer−dye hybrid assemblies.47 The delicate interplay of electrostatic and coordination interactions together with the π−π stacking of dye molecules was recently discussed for a dendrimer−dye system loaded with CdS.46 It is thus highly promising to investigate in how far organic electrostatically selfassembled structures can act as templates for the formation of metal nanoparticles and to develop a fundamental understanding of structure directing effects and possibilities. The goal of this study is to use a double-electrostatic selfassembly and templating approach as a route to hybrid nanostructures. The objective is to test self-assembled systems, specifically different supramolecular dendrimer−dye assemblies, as ionic nanoreactors for the formation of gold nanostructures by “electrostatic nanotemplating” (Scheme 1). The role of dendrimer generation, molecular dye structure, reducing agent, and pH will be discussed, showing the broad variety of tuning the ternary gold−dendrimer−dye structures, which may be of importance in the biomedical field or in catalysis.

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prior to use via recrystallization (96%). The dye disodium 4-((4-(tertbutyl)phenyl)diazenyl)-3-hydroxynaphthalene-2,7-disulfonate (tBARAc) was synthesized in our group.44 The deionized water was filtered with two hydrophilic PTFE membranes in a row, which had a pore size of 0.2 μm. Sample Preparation. Aqueous stock solutions of all chemicals were prepared at pH 3.5. The NaBH4 stock solution was always prepared freshly before use with neutral or basic water. The samples were prepared by subsequently mixing water, dendrimer and the dye via stirring at 990 rpm. Then the aqueous stock solution of the gold salt HAuCl4 was added and reduced after few minutes with respective amounts of reducing agent, both via stirring at 990 rpm under ambient conditions. The final concentration of the polyelectrolyte in the samples was kept constant (c(G4) = 3.5 × 10−6 mol L−1; c(G8) = 1.9 × 10−7 mol L−1) in a volume of 3−4 mL. The concentrations of the other components varied according to the anticipated ratios. A stock solution of gold containing G8 (Au-G8) was prepared according to ref 21. The solution had a concentration of c(G8) = 3.22 × 10−6 mol L−1 and a pH of 9.6. Light Scattering. The measurements were performed at an instrument equipped with an ALV 5000 correlator with 320 channels (ALV GmbH, Langen, Germany), an ALV CGS 3 goniometer, and a red HeNe laser (λ = 632.8 nm, 20 mW). The measurements covered a scattering angle range of 30° ≤ Θ ≤ 150°. The intensity autocorrelation function g2(τ) was for each angle transferred into the electric field autocorrelation function g1(τ) via the Siegert relation. The electric field autocorrelation function g1(τ) was successively transformed into the distribution of relaxation times A(τ) by a regularized inverse Laplace transformation using the program CONTIN developed by Provencher.48 From the distribution of relaxation times the apparent diffusion coefficient was calculated. Via extrapolation to zero scattering vector square the diffusion coefficient was obtained, with which via Stokes−Einstein relation the hydrodynamic radius was calculated. UV−Vis Spectroscopy. Absorption spectra were recorded on a SHIMADZU UV spectrophotometer (UV-1800) with a slit width of 1.0 nm. TEM. The transmission electron microscopy (TEM) images were acquired with a Zeiss EM 900 microscope, a Zeiss EM 912 microscope (both operated at 80 kV), a Phillips CM300 microscope operated at 300 kV, or a Phillips CM30 microscope equipped with a cryo-sample holder and operated at 200 kV. The specimens were prepared by depositing 5 μL of the diluted sample solution onto carbon-coated copper grids, 300 mesh, and air-dry the grids. Cryo-TEM samples were

EXPERIMENTAL SECTION

Chemicals. The poly(amido amine) dendrimer of generation 8 (G8) was supplied by Dendritech, Midland; the poly(amido amine) dendrimer of generation 4 (G4) and all other chemicals except for the dyes were purchased from Sigma-Aldrich. The Acid Red dyes (Ar26, Ar27) were obtained from Acros, Geel, Belgium, and were purified B


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Figure 1. Analysis of a gold−G4−Ar26 assembly, reduced with hydrazine. (a) Dynamic light scattering: electric field autocorrelation function g1(τ) and distribution of relaxation times A(τ) at a scattering angle of θ = 90°; width of distribution σ = 0.18, hydrodynamic radius RH = 64 nm. (b, c) TEM of a gold−G4−Ar26 assembly, reduced with hydrazine.

Figure 2. Analysis of gold−G4−Ar26 assemblies after reduction with NaBH4: (a, b) Dynamic light scattering: electric field autocorrelation functions g1(τ) and distributions of relaxation times A(τ) at a scattering angle of θ = 90°; (a) gold-free G4−Ar26 assemblies, width of distribution σ = 0.20; (b) gold−G4−Ar26 assemblies, width of distribution σ = 0.25. (c) UV−vis spectrum of G4−Ar26 assemblies (red) and gold−G4−Ar26 assemblies after reduction (black). (d) Cryo-TEM picture of a gold−G4−Ar26 assembly. prepared by the use of a Vitrobot and glow-discharged Quantifoil grids.

aqueous solution at pH 3.5. At pH 3.5 both the primary and the tertiary amine groups of the dendrimer are positively charged. Ar26 bears two anionic sulfonate groups, making it capable of building well-defined supramolecular G4−Ar26 assemblies through electrostatic interactions and mutual π−π interactions of the dye molecules. Addition of HAuCl4 leads to gold salt loaded structures as AuCl4− can enter the cationic dendrimer. Further addition of a reducing agent causes the formation of metallic gold nanostructures within the G4−Ar26 assembly as is depicted in Scheme 1. There, the organic assembly functions as a nanoreactor for the gold formation. Unless stated otherwise, the ratios of the components are kept constant at a charge ratio of rcharge = 0.5 (dye/dendrimer) and (Au/dendrimer). The charge ratio is defined as the ratio of the number of negative charges of the dye counterion to the

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RESULTS AND DISCUSSION First, a G4 PAMAM dendrimer with Ar26 dye is investigated with different charge ratios in terms of dye/dendrimer ratio or the initial loading with gold salt. Then the influence of the dye structure and valence is shown. The starting dendrimer G4 is exchanged by a much larger G8 and further a pre-gold particleloaded G8 PAMAM. Each of these building-block constellations is investigated with different reducing agents. Finally, the influence of pH will be discussed, G4 Dendrimer−Ar26 Assemblies as Gold Templates. In the first part of this study, generation 4 (G4) PAMAM dendrimer is mixed with the azo dye Acid Red 26 (Ar26) in C


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Figure 3. Investigation of a gold−G4−Ar26 assembly, reduced with ascorbic acid after nucleation seed formation with NaBH4. (a) Dynamic light scattering: electric field autocorrelation function g1(τ) and distribution of relaxation times A(τ) at a scattering angle of θ = 90°; width of distribution σ = 0.16. Inset: TEM of a gold−G4−Ar26 hybrid assembly. (b) UV−vis spectrum of G4−Ar26 assemblies (red) and gold−G4−Ar26 assemblies after reduction (black).

Again, for comparison, without dendrimer and dye, reduction leads to a precipitation of the reduced gold. With RH = 70 nm the dendrimer−dye−gold assemblies are in the same size range as the initial gold-free assemblies (RH = 66 nm) (Figure 2a,b). The absorption band in the UV−vis spectrum of the dye disappears within a short time after adding the strong reducing agent (Figure 2c), which is likely due to the destruction of the dye. TEM measurements show small individual spherical gold nanoparticles within the assemblies (Figure 2d). The gold nanoparticles are monodisperse with an average size of 2.4 ± 0.5 nm. Despite the disappearance of the dye, the assemblies are held together, which was proven by cryo-TEM and DLS (Figure 2). This is most likely due to the attractive Hamaker interactions between the colloidal gold particles, preventing the dendrimers from disassembling.47 Previously, it was shown that the reduction of gold salt with NaBH4 in the presence of G4 PAMAM yields gold colloids of about 1.8 nm.21 There, in contrast to the templating with the G8 dendrimer, the dendrimer acts as surface stabilizing molecule similar to low molar mass molecules. In the present case the slightly increased particle size could be explained by additional dye molecules contributing to the stabilization of the gold. Reducing Agent: Ascorbic Acid. To prevent the destruction of one of the building blocks, ascorbic acid, a weaker reducing agent, was used. Ascorbic acid alone is not sufficiently strong to reduce the gold salt at ambient conditions. Therefore, 1/100 of the NaBH4 amount necessary for complete gold salt reduction is added to induce nucleation seeds. Once nucleation seeds are present, ascorbic acid is able to reduce the remaining gold salt.10 The more gentle reduction results in well-defined gold assemblies with preserved dye molecules proven by UV−vis measurements (Figure 3b). With RH = 62 nm the assembly size is only slightly smaller than the gold-free assemblies (RH = 66 nm) (Figure 3a). The gold morphology is mostly spherical with partially intergrown particles. The sizes are in a range of 2−11 nm with an average size of 6.2 nm. As the assembly size is nearly the same before and after gold formation, it can be concluded that the G4−Ar26 assemblies function as a nanoreactor, where only the gold ions within one assembly participate in the formation of gold nanoparticles. Because of the weaker reducing agent, the gold nanoparticles are larger and more polydisperse as in the case of the reduction with NaBH4. Once more, this is in contrast to the precipitating sample without organic stabilizer.

number of positive charges of the dendrimer. G4 carries 64 positive charges at the surface and 126 in total. G8 has 1024 surface charges and at pH 3.5 2046 positive charges in total. This means, in the case of a 2-fold anionic dye, at a charge ratio rcharge(dye/dendrimer) = 0.5 on average 32 dye molecules are attached to one G4 dendrimer and 512 to a G8 dendrimer. At a charge ratio of rcharge(Au/dendrimer) = 0.5 on average every dendrimer is loaded with 63 gold ions for G4 and with 1023 in the case of G8. At both charge ratios rcharge = 0.5 the charges of the dendrimer in theory are fully neutralized, and the dendrimer is fully loaded. Reducing Agent: Hydrazine. Results for the reduction with hydrazine are given in Figure 1. Hydrazine is a weak reducing agent but strong enough to reduce the gold salt in the absence of nucleation seeds at ambient conditions. The assembly sizes are nearly identical before and after gold formation (RH = 66 nm before, RH = 64 nm after) (Figure 1a), which is in contrast to the reduction of gold in the absence of dendrimer and dye under otherwise same conditions where a black precipitation is observed. In the hybrid sample, UV−vis spectra indicate nanoscale gold formation and confirm that the dye is not destroyed (Figure S1). In transmission electron microscopy (TEM) anisotropic and partly faceted gold nuggets can be found within the assemblies. At higher magnification an organic shell surrounding the gold structures is visible, as indicated by the arrows in Figure 1b,c. The gold nuggets are in a size range of 20−100 nm. The organic layer has an average thickness of 4 nm. Thus, it can be concluded that in this approach the G4− Ar26 assemblies act as nanoreactors for the gold formation. Because of the slow reduction, only few large gold nuggets are formed within the assemblies. The organic part is ousted by the growing gold nanostructures forming a shell around the final gold nuggets. The formation of anisotropic gold nanostructures with hydrazine is known.14,49 For example, with microgels branched gold nuggets were obtained. The microgel served as nanotemplate, directing the gold growth whereby the final gold structure can be influenced by the cross-linking density. This is most likely not the case in the present system. The G4−Ar26 assemblies are not covalently interconnected but only held together by electrostatic interaction and π−π stacking. These interactions are obviously too weak to direct the growth of the gold nanostructures. Reducing Agent: NaBH4. Light scattering measurements reveal the presence of well-defined assemblies before and after the reduction step with the strong reducing agent NaBH4. D


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Figure 4. Analysis of gold−G4−Ar26 assemblies, reduced with oxalic acid: (a) UV−vis spectrum of G4−Ar26 assemblies (red) and gold−G4−Ar26 assemblies after reduction with 1/100 NaBH4 and oxalic acid at rcharge(Au/G4) = 0.75 (black); (b) TEM of a gold−G4−Ar26 assembly at rcharge(Au/ G4) = 0.75.

Figure 5. Analysis of gold−G4−Ar26 assemblies for reduction approach 1/100 NaBH4 and oxalic acid. (a) Dependence of the assembly size on the charge ratio rcharge(Ar26/G4). Red: gold-free G4−Ar26 assemblies; black: hybrid gold−G4−Ar26 assemblies. The blue arrows indicate the size change. (b) Dynamic light scattering on a sample with rcharge(Ar26/G4) = 0.9 and rcharge(Au/G4) = 0.5: electric field autocorrelation function g1(τ) and distribution of relaxation times A(τ) at a scattering angle of θ = 90°. Inset: TEM of the same sample.

Reducing Agent: Oxalic Acid. Variation of rcharge(Au/G4). Next, the weak reducing agent oxalic acid is tested. Like ascorbic acid, oxalic acid can only reduce the gold salt in the presence of nucleation seeds, which again are provided by the addition of 1/100 of the NaBH4 amount necessary for complete gold salt reduction. The formation of nanoscale gold colloids can be followed with UV−vis and is completed within 60 min. By using this approach, the dye is not destroyed and the assembly size stays constant and well-defined (RH = 67 nm, σ = 0.19 after reduction). This approach is used to further investigate the system by varying the ratio of gold to dendrimer. Higher amounts of gold salt (rcharge(Au/G4) = 0.75) increase the size of the hybrid assemblies to RH = 86 nm while charge ratios ≥1.0 lead to a precipitation of the sample. UV−vis and TEM confirm the formation of larger gold particles (Figure 4), again in difference to the corresponding instable sample without dendrimer. The resulting gold morphology is spherical to slightly elongated (aspect ratio = 1.1; Figure 4b). The sizes range from about 4 to 25 nm with an average size of 14 ± 6.4 nm. The increase compared to rcharge(Au/G4) = 0.5 is expected as more gold salt is available, and the larger assembly sizes can be understood by the overloading of the dendrimer. Once the charge stoichiometry is exceeded, the assemblies grow, as was shown previously in G4−dye systems.50 For higher overstoichiometric charge ratios the samples usually lack stability

and precipitate within short time. The gold salt here functions as counterion additionally to the anionic dye. Variation of rcharge(Ar26/G4). The variation of rcharge(Ar26/ G4) in a range of 0.3 < rcharge < 0.9 results in assemblies with sizes 48 nm ≤ RH ≤ 84 nm. The assembly sizes decrease after gold formation compared to the initial gold-free assemblies, except for rcharge(Ar26/G4) = 0.3 (Figure 5a). Figure 5b depicts DLS of a gold−G4−Ar26 assembly at rcharge(Ar26/G4) = 0.9. The small peak at lower decay times indicates the presence of smaller assemblies or single dendrimers. It is significantly larger for the gold−dendrimer−dye assemblies than for the gold-free assemblies. This means that upon gold formation a part of the assembly dissolves, adding free dendrimer or small dendrimer− dye assemblies to the solution. This behavior is more pronounced for overstoichiometric loading, i.e., for rcharge(Ar26/G4) > 0.5. For both charge ratios, Au/G4 and Ar26/G4 rcharge = 0.5, the assembly sizes stay nearly constant and the error bars are smallest; i.e., the assemblies are welldefined, and the initial G4-Ar26 assemblies function as nanoreactors for the gold formation. At the lowest ratio rcharge(Ar26/G4) = 0.3 the hybrid assemblies are poorly defined, as is evident from the large error in Figure 5a. Here, initially a lot of free dendrimer coexists with the assemblies. Upon loading with gold salt and reduction, the free dendrimer likely incorporates into the hybrid assemblies, resulting in larger and polydisperse hybrid structures. Starting from rcharge(Ar26/G4) ≥ 0.5, the supramolecular hybrid structures are better defined E


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with RH = 53 nm nearly unchanged (Figure 7e). In TEM spherical superstructures are visible containing spherical gold nanoparticles (Figure 7b). TEM yields average radii of about 900 nm, much larger than DLS suggests. Cryo-TEM, in contrast, shows some “bubble”-like gold containing assemblies of up to several hundred nanometers as can be seen in Figure 7c. The main structure is in the same size range as the DLS results (Figure 7d). In this structure only few gold nanoparticles of an average size of 19 ± 5 nm are surrounded by an organic matrix of low electron contrast. Additionally, small (about 4 nm) gold nanoparticles can be found in the assemblies. This indicates that within one spherical G4−Ar27 assembly only few gold nanoparticles are formed in contrast to the above systems. The large spherical, gold containing aggregates found in regular TEM thus represent a drying effect, and the combination of DLS and cryo-TEM yields the structural information here. Thus, the choice of the dye clearly influences the overall shape of the hybrid assemblies, but not the shape of the gold nanostructures. The gold architecture is mainly influenced by the choice of the reducing agent. This behavior can be understood when considering the involved forces for assembly formation. Dendrimer and dye are held together by electrostatic and mutual π−π interactions. Those are all noncovalent interactions, and the system is dynamic and flexible. The growing gold nanoparticle is a hard and unyielding part of the assembly that is hardly influenced by the comparatively weak interaction forces around it. Within the confined space of the nanoreactor the growth is consequently governed by the speed of the reducing reaction. Therefore, the same reducing agents result in similar gold morphologies for different supramolecular assemblies. When more gold precursor ions are present in an assembly due to higher rcharge(Au/G4), the resulting gold structures are larger, but the morphology is still mainly determined by the growth speed. G8 Dendrimer−Dye Assemblies as Gold Templates. Considering the dendrimer only (without dye) in the gold nanoparticle formation, the G8 dendrimer behaves fundamentally different as compared to the G4 dendrimer: For G8 possessing a diameter of 11 nm, upon gold salt loading and reduction, 3 nm gold nanoparticles are formed within its interior following a “fixed loading law” in a templating mechanism.21,47 TEM of these gold nanoparticle−containing G8 dendrimers from the original studies of Gröhn et al. is reproduced in Figure S2.21 In contrast, when gold is formed in the presence of G4, several G4 molecules attach to one gold nanoparticle, thereby stabilizing it.21 Hence, it is highly interesting now to investigate how the roles of the low and high generation dendrimers differ in the dye−dendrimer templating approach. Thus, with the G8 polyelectrolyte dendrimer and the dye Ar26 assemblies are formed, loaded with gold salt and reduced with different reducing agents, as will be discussed in the following. Influence of Reducing Agents. Hydrazine yields faceted gold nuggets within the assemblies, featuring an average size of 77 ± 14 nm with an average aspect ratio of 1.23 (Table 1 and Figure 8a). Compared to the G4 dendrimer system, here the gold colloids mostly are hexagonally shaped and less anisotropic, likely owing to the different stabilizing behavior of the larger and less flexible G8 dendrimer. With NaBH4 as reducing agent small spherical gold nanoparticles are formed within the hybrid assemblies (Figure 8b). The gold structures have an average size of 3 ± 1.5 nm, which is in a similar range

(Figure 5b). For a charge ratio rcharge(Ar26/G4) = 0.9 they feature mainly spherical gold nanoparticles of different sizes (5−20 nm) (inset in Figure 5b). The gold architectures at this charge ratio are similar to the ones with both charge ratios rcharge = 0.5. Hence, the charge ratio rcharge(Ar26/G4) allows a tuning of the assembly sizes while the morphology of the gold nanoparticles stays unchanged. Oppositely, the reducing agents have a strong influence on the gold shape. A slow reduction with hydrazine facilitates the growth of large and anisotropic gold nuggets while during a fast reaction with NaBH4 small (2.4 nm) spherical gold nanoparticles are stabilized by G4 and Ar26. The weaker reducing agents oxalic acid and ascorbic acid, each in the presence of nucleation seeds, tend to generate polydisperse spherical to slightly elongated gold nanoparticles, where the overall gold size can be tuned by the Au/dendrimer ratio. G4 Dendrimer−Dye Assemblies as Gold Templates: Variation of Dyes. Assemblies with TBARAc. To probe the influence of the building blocks, different dyes were tested for the electrostatic assembly formation. tBARAc is a divalent anionic dye like Ar26 with a different molecular structure and causes assemblies which are only slightly larger (RH = 70 nm) than the G4−Ar26 structures (RH = 66 nm). When hydrazine is used as reducing agent, the gold architectures resemble those of the Ar26 system (Figure 6a), and the assembly sizes before and

Figure 6. TEM of gold−G4−tBARAc assemblies: (a) reduction with hydrazine; (b) reduction with 1/100 NaBH4 and oxalic acid.

after reduction do not change. The gold structure is anisotropic and faceted. Again, at higher magnification an organic shell surrounding the gold is visible, as emphasized by the arrow in Figure 6a. The sizes of the assemblies in TEM correspond well to the data obtained by DLS. The reduction with oxalic acid in the presence of nucleation seeds results in spherical polydisperse gold nanoparticles similar to the ones in the G4−Ar26 system (Figure 6b). The average gold colloid size is 7.9 ± 3.3 nm. Again the assembly sizes are nearly identical before and after reduction, demonstrating the templating role of the supramolecular assembly. These results point to a similar conclusion as in the Ar26 system: the shape of the gold nanoparticles is not influenced by the assembly structure but mainly by the choice of the reducing agent. Assemblies with Ar27. Ar27 (Scheme 1) is a trivalent anionic dye and expected to cause different, more spherical assembly structures.43,50 With RH = 49 nm G4−Ar27 assemblies are smaller than the G4−Ar26 assemblies. With hydrazine for the reduction of the gold salt, the resulting gold architectures are similar to those with the other systems reduced by hydrazine (Figure 7a). The sizes of the hybrid assemblies are consistent in DLS and TEM, and indeed a more spherical structure is found. Upon reduction with small amounts of NaBH4 and oxalic acid, the assembly size remains F


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Figure 7. Investigation of gold−G4−Ar27 assemblies: (a) TEM after reduction with hydrazine; (b) TEM of spherical superstructures after reduction with 1/100 NaBH4 and oxalic acid; (c) cryo-TEM of “bubble”-like assemblies after reduction with 1/100 NaBH4 and oxalic acid; (d) cryo-TEM of the main structure after reduction with 1/100 NaBH4 and oxalic acid; (e) dynamic light scattering on gold−G4−Ar27 assemblies after reduction with 1/100 NaBH4 and oxalic acid: electric field autocorrelation function g1(τ) and distribution of relaxation times A(τ) (scattering angle θ = 90°), width of distribution σ = 0.15.

Table 1. Comparison of the Gold Nanoparticle Sizes and Shapes in the Gold−G4−Ar26 System and in the Gold−G8− Ar26 System in Dependence on the Reducing Agents gold−G4−Ar26

gold−G8−Ar26

size (nm)

shape

size (nm)

shape

hydrazine NaBH4 oxalic acid

20−100 2.4 ± 0.5 8.7 ± 2.8

anisotropic spherical spherical, slightly elongated

77 ± 14 3.0 ± 1.5 35 ± 9

ascorbic acid

6.2 ± 3

spherical

15 ± 4

often hexagonal spherical spherical, slightly angular spherical

compared to the gold−G4−Ar26−NaBH4 system and corresponds to the size that can be stabilized within one G8 dendrimer.21 In the presence of small amounts of NaBH4 oxalic acid reduces the gold salt, yielding spherical to slightly angular gold nanoparticles of an average size of 35 ± 9 nm (see Figure 8c). Thus, in the case of G8 the resulting gold nanoparticles are larger than in the comparable G4 system (8.7 ± 2.8 nm, Table 1). With ascorbic acid and NaBH4, mainly spherical gold nanoparticles with an average size of 15 ± 4 nm are obtained (Figure 8d). The resulting gold colloids here are also larger than in the case of the gold−G4−Ar26 system reduced with ascorbic acid (d = 6.2 ± 3 nm). Thus, in the case of weak reducing agents the larger polyelectrolyte G8 enables larger gold structures. G4 is relatively flexible and can partly “wrap around” the growing gold nanoparticle. Therefore, the resulting colloids are smaller than in the comparable G8 system, where

Figure 8. TEM of the gold−G8−Ar26 system: (a) after reduction with hydrazine; (b) after reduction with NaBH4; inset shows HRTEM of one single crystalline spherical gold nanoparticle with atomic planes being visible; (c) after reduction with oxalic acid and small amounts of NaBH4; (d) after reduction with ascorbic acid and small amounts of NaBH4.

the dendrimer is rather stiff. The shape of the gold structures is again mainly dependent on the reducing agent. Variation of rcharge(Ar26/G8) with the Reducing Agent Hydrazine. The variation of the charge ratio rcharge(Ar26/G8) yields well-defined assemblies of different sizes, as can be seen G


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Figure 9. Analysis of the gold−G8−Ar26 system reduced with hydrazine: (a) Dependence of the hydrodynamic radii on the charge ratio rcharge(Ar26/G8) for G8−Ar26 assemblies (black) and gold−G8−Ar26 assemblies (red). (b) TEM of an assembly with rcharge(Ar26/G8) = 0.3. (c) TEM of an assembly with rcharge(Ar26/G8) = 0.5. (d) TEM of an assembly with rcharge(Ar26/G8) = 0.7.

be found for reduction with ascorbic acid and oxalic acid, which might be attributed to the less flexible dendrimer geometry at high generation. The smaller generation dendrimer can wrap around the gold surface, stabilizing the nanoparticle better and facilitating smaller gold structures. Hydrazine causes faceted anisotropic gold nuggets within both assembly systems, but for G8 the gold nuggets are more regular and often hexagonally shaped. Changing the ratios of dye and polyelectrolyte and thus changing the size and composition of the nanoreactors allows to tune on the size and the stability of the resulting hybrid gold−dendrimer−dye assemblies, but not on the shape of the gold architecture, likely because the environment of the growing gold crystal is still determined by weak noncovalent forces. The case of rcharge(Ar26/G8) = 0.3 is an exception where the gold formation does not solely take place inside of preformed nanoreactors, but free, gold-salt-loaded dendrimer causes the gold morphology to grow uncoordinatedly. Multistep Approach: Gold−G8 Dendrimer−Dye Assemblies as Templates for Further Gold Growth. Next, a double-templating approach is exploited: supramolecular structures are formed with G8 dendrimers already containing Au nanoparticles. First, a 3 nm gold nanoparticle is formed inside of G8 PAMAM dendrimer via the established route of nanotemplating (Figure S2).21 This pre-gold−G8 hybrid particle is then used as the polyelectrolyte in a similar way as in the approaches described above. Pre-gold−G8 is interconnected with Ar26 forming pre-gold−G8−Ar26 hybrid assemblies, followed by a loading with gold salt and a subsequent reduction with differing reducing agents, yielding hybrid assemblies with a higher gold content than in the previous approaches. The stability of the initial pre-gold−G8−Ar26 assemblies before adding gold salt significantly differs from the G8−Ar26 assemblies. Because of the presence of 3 nm gold nanoparticles within the G8 dendrimer the assemblies are larger (RH = 126 nm) compared to the gold-free G8−Ar26 assemblies (RH = 56 nm). This is caused by the Hamaker interaction between the

in Figure 9a (red). The gold free assemblies are in a size range of 36 nm < RH < 72 nm when changing the dye to G8 ratio rcharge(Ar26/G8) from 0.3 to 0.7. The subsequent loading with gold salt and reducing with hydrazine result in hybrid assemblies the sizes of which are depicted in Figure 9 (black). For small charge ratios (rcharge = 0.3) the sample is bimodal after reduction, with one peak in the same size range as the initial assemblies and the other peak indicating much larger structures. These larger structures can be found in TEM, as evident in Figure 9b. The other ratios yield well-defined monomodal samples, where the assemblies are slightly larger in the case of rcharge(Ar26/G8) = 0.5 and slightly smaller for rcharge(Ar26/G8) = 0.7 compared to the gold free G8-Ar26 assemblies. TEM reveals again anisotropic and faceted gold nuggets surrounded by an organic layer (Figure 9c,d). These results can be understood when considering the goldfree assemblies. For the ratio rcharge(Ar26/G8) = 0.3 not only assemblies are present in solution, but in analogy to the previous system also individual dendrimer molecules. These dendrimers are also loaded with gold salt. Upon reduction, the gold ions within these free G8 dendrimers also contribute to the gold-nanostructure formation, causing ill-defined gold architectures much larger than the initial assemblies. At the ratio rcharge(Ar26/G8) = 0.5 fewer free gold-salt-loaded dendrimers are present in solution, which was confirmed by DLS. Thus, the hybrid assemblies are much better defined and only slightly larger than the gold-free G8−Ar26 assemblies. For rcharge(Ar26/G8) = 0.7 the sizes of the gold free assemblies and gold containing assemblies are nearly the same, as at this ratio the amount of free dendrimer is negligible. Concluding the results so far, the gold morphology can be substantially influenced by the speed of the reduction which is controlled through the choice of the reducing agent. A different dye changes the overall assembly shape, but not significantly the gold structure, because the noncovalent interactions within the assembly are too weak to influence the growing gold crystal. With a larger polyelectrolyte an influence on the gold size can H


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facilitated by Hamaker interaction (Figure 12a). Thus, the weak reducing agent oxalic acid in this case acts similar to hydrazine.

gold colloids, which is a short-range attractive interaction, thus affecting the assemblies. For higher ratios rcharge(Ar26/G8) ≥ 0.5 the long-term stability of the assemblies is reduced, causing precipitation within 1 h. Adding gold salt and reducing with hydrazine stabilizes the assemblies. The resulting assemblies are in a size range of 126−146 nm and feature spherical welldefined gold nanoparticles of an average size of 7.3 ± 1.8 nm. In TEM the gold nanoparticles are densely packed within the assemblies, whereby organic material is visible along some edges (Figure 10). In contrast to the initially gold-free G8− Ar26 system, here no faceted anisotropic gold morphologies are formed.

Figure 12. Reduction with oxalic acid at two different gold-todendrimer ratios. (a) TEM of a part of an assembly at rcharge(Au/G8) = 0.5. (b) Cryo-TEM image of an assembly at rcharge(Au/G8) = 0.7, displaying the assembly structure in solution.

Cryo-TEM on a sample with the charge ratio rcharge(Au/G8) = 0.7 displays the assembly structure in solution (Figure 12b), which is network-like and in accordance with the gold-free G8− Ar26 system.43 For the higher loading ratios, the gold nanoparticles exhibit an average size of 8.2 ± 4.6 nm, slightly larger and more polydisperse. Finally, the pH dependence of the dendrimer is exploited. So far, all approaches were performed at pH 3.5, where all amine groups are protonated. At pH 7 only the primary surface amine groups are protonated; therefore, the G8 dendrimer carries 1024 positive charges. The stability range shifts to lower ratios, giving stable samples at rcharge(Ar26/G8) = 0.35. After adding gold salt (1024 gold ions per dendrimer, in contrast to 2048 gold ions at pH 3.5) to the assemblies, the supramolecular size is well-defined with a hydrodynamic radius of RH = 232 nm (Figure 13a). Because of the higher pH and the initial presence of gold nanoparticles, the assemblies are significantly larger compared to the approaches at pH 3.5. The reduction with NaBH4 causes the initial 3 nm gold nanoparticles to grow and to partially interconnect, yielding anisotropic gold morphologies as evident in Figure 13. Only few nanoparticles can be found that are smaller than 3 nm. Hence, the NaBH4 causes only few new nucleation seeds while growth of the present nanoparticles is preferred. The growth is highly anisotropic and leads to the interconnection and coalescence of several nanoparticles (Figure 13b,c). The presence of different crystallographic planes indicates that the particles do not undergo directed growth but a statistical interconnection. This is expected as nucleation seeds are present before growth. With RH = 355 nm the assembly size after reduction is larger than before reduction, which is most likely caused by free dendrimer contributing to the assembly formation. More importantly, the final hybrid assemblies are stable in solution and well-defined.

Figure 10. TEM analysis of the gold−G8−Ar26 system with pregold−G8 after reduction with hydrazine: (a) TEM of an assembly with rcharge(Ar26/G8) = 0.5; (b) higher magnification of an assembly of the same approach.

When reducing gold-salt-loaded pre-gold−G8−Ar26 assemblies with NaBH4 the assembly sizes stay constant and welldefined. Within the assemblies polydisperse, mostly spherical and partly anisotropic gold morphologies of an average size of 9.5 ± 3 nm can be found (Figure 11a). A higher loading with gold salt (rcharge(Au/G8) = 0.7) yields more interconnected gold structures (Figure 11b,c). The average size is with 8.1 ± 3.4 nm slightly smaller and has a higher standard deviation as more small colloids are present. In both cases very small spherical gold nanoparticles (up to 3 nm) can be found additionally, resulting from the nucleation seed formation by the strong reducing agent NaBH4. Thus, to prevent the formation of new nucleation seeds and promote the growth of the gold nanoparticles within the G8, the weak reducing agent oxalic acid is applied. Here, the presence of gold nanoparticles allows to omit the use of NaBH4. For the ratio rcharge(Au/G8) = 0.5 assemblies and gold nanoparticles are well-defined. The mainly spherical gold colloids have an average size of 7.5 ± 1.3 nm. Thus, it can be concluded that the initial 3 nm gold nanoparticles grow uniformly. In analogy to the system with hydrazine the gold nanoparticles within the assemblies appear to be quite densely packed on the TEM grid which is likely a drying effect

Figure 11. TEM of gold−G8−Ar26 assemblies with pre-gold−G8 after reduction with NaBH4: (a) rcharge(Au/G8) = 0.5; (b, c) rcharge(Au/G8) = 0.7. (c) Higher magnification of intergrown gold nanoparticles. I


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Figure 13. Hybrid gold−G8−Ar26 assemblies at pH 7, reduced with NaBH4. (a) Dynamic light scattering on samples before (black) and after (blue) gold salt reduction: electric field autocorrelation functions g1(τ) and distributions of relaxation times A(τ) at scattering angles θ = 90°. (b, c) HRTEM of a part of a gold−G8−Ar26 assembly: (c) higher magnification of (b) with visible atomic planes.

reducing agents oxalic acid and hydrazine induce a growth of the initially 3 nm gold nanoparticles, yielding spherical, welldefined larger gold nanoparticles. Finally, the pH of the assembly solution was altered. If the dendrimer is only 50% charged instead of 100%, larger assemblies of RH > 200 nm result. The loading with gold salt and reduction with NaBH4 causes an assembly size increase to RH > 300 nm and anisotropic, interconnected gold morphologies together with spherical nanoparticles. Thus, supramolecular dendrimer−dye assemblies as nanoreactors for the formation of gold have been established as novel route to complex multicomponent nanostructures with a tunable assembly size and morphology. While gold has served as a model system here to yield a fundamental view on structure-directing effects, the concept that is based on ionic interaction is general and can be widely applied. The versatility of the approach may open the way to new functional materials from hybrid nanoparticles and dyes with possible applications in the fields of optoelectronics, catalysis, and phototherapy.

Thus, a different pH opens the way to further hybrid assembly sizes and gold architectures, demonstrating the versatility of the dendrimer−dye system in combination with gold.

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CONCLUSION In this study, we have presented a novel approach to form supramolecular organic−inorganic hybrid structures. Key is to use organic electrostatically self-assembled supramolecular structures in aqueous solution again for electrostatic nanotemplating of inorganic nanoparticles. This double-electrostatic approach was herein used to form supramolecular polymer− dye−gold nanostructures in aqueous solution, representing a proof of concept. Specifically, cationic PAMAM dendrimers (G4 or G8) and anionic azo-dyes (Ar26, Ar27, or tBARAc) have been combined. Organic−inorganic hybrid architectures have been created by first forming dendrimer−dye assemblies by electrostatic self-assembly and then loading these assemblies which still carry charges with oppositely charged gold ions (HAuCl4), again through ionic interaction. Within these gold ion−macroion−dye assemblies, subsequent chemical reduction of the gold salt results in gold nanostructures; that is, the dendrimer−dye assemblies function as a nanoreactor for the formation of gold nanoparticles. The shape of the gold nanoparticles can be tuned by the reducing agent: The strong reducing agent NaBH4 preferentially causes small (2−3 nm) spherical gold nanoparticles, while the weaker reducing agents ascorbic acid and oxalic acid in the presence of small amounts of NaBH4 yield polydisperse spherical to slightly ellipsoidal gold nanoparticles of larger average sizes. With hydrazine highly anisotropic gold nuggets of up to 100 nm form. The choice of the molecular dye structure determines the shape of the 70− 300 nm assemblies while the gold particle shape is independent of the dye. Varying the polyelectrolyte, the gold particles are slightly larger for the G8 dendrimer. When using hydrazine as reducing agent, the gold nanonuggets are better defined with a preferentially hexagonal shape as compared to the highly anisotropic shape in the case of G4 dendrimer. Moreover, the G8 dendrimer can function in a multistep process: it first served as a nanotemplate for the formation of well-defined and stabilized 3 nm gold nanoparticles in its interior, and in a next step, the gold−G8 hybrid particles were then used as the “polyelectrolyte” for assembly and further gold loading and reduction. Here again, the different reducing agents influence the gold morphology. In particular, NaBH4 causes a partial interconnection of the gold nanoparticles, while the weaker

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00752. UV−vis spectroscopy of G4−Ar26 assemblies and gold− G4−Ar26 assemblies after reduction with hydrazine showing the dye stability and TEM of gold−G8 hybrid nanoparticles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.G.). ORCID

Franziska Gröhn: 0000-0003-1016-2583 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of Deutsche Forschungsgemeinschaft (DFG), the Interdisciplinary Center for Molecular Materials (ICMM, University Erlangen-Nürnberg), and Solar Technologies go Hybrid (SolTech) is gratefully acknowledged. We also thank J


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