Self-assembled polyoxometalate–dendrimer structures for selective photocatalysis

A. Kutz a, G. Mariani a, R. Schweins b, C. Streb c and F. Gröhn *a
aDepartment of Chemistry and Pharmacy, Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany. E-mail: franziska.groehn@fau.de
bInstitut Laue-Langevin, 71 Avenue des Martyrs – CS 20156, 38042 Grenoble Cedex 9, France
cInstitute of Inorganic Chemistry 1, Ulm University, Albert-Einstein-Allee, 89081 Ulm, Germany

Received 23rd September 2017 , Accepted 3rd November 2017

First published on 6th November 2017


We present a novel, self-assembled nanostructure with selective photocatalytic activity formed from anionic polyoxometalate clusters and cationic dendrimers by electrostatic self-assembly. The association of the components in aqueous solution is driven by ionic interaction and steric factors yielding stable aggregates of a defined size with a coil-like structure. The assemblies show high potential for the application in solar-energy conversion systems due to their enhanced and substrate specific photocatalytic activity.


Self-assembly bears great potential, because it allows for the design of complex nanostructures in a versatile manner.1 In particular, functional supramolecular structures can serve as smart nano-objects ranging from carrier systems to sensors and are of interest in medicine, materials science and nanotechnology. Given the increasing energy consumption on Earth, developing novel concepts for solar energy conversion is currently of major importance.2 Inspiring are natural systems where solar energy is exploited in photosynthesis through catalysis by small species inside a larger non-covalent assembly. Thus, it is of great promise to synthetically design self-organized assemblies for photocatalysis. Of particular value in this context are versatile systems that rely on general binding forces rather than very specific synthetic binding motifs. Association based on amphiphilicity, hydrogen bridging or metal coordination can yield a variety of fascinating structures.3 Supramolecular concepts have been applied to catalysis, for example as enzyme mimics based on coordination chemistry and hydrogen bridging. These act by creating cage-like, allosteric or dissymmetric sites through well-designed local binding motifs or by bringing two reaction partners into close proximity.4 Yet, hydrogen-bridged entities often are unstable in polar solutions and the synthetic access to specific binding motifs is often tedious.5 Hence, it is attractive to use the concept of electrostatic self-assembly to interconnect macroions and oppositely charged catalytically active species into novel supramolecular nanostructures.

Recently, we have shown that the assembly of macroions and organic aromatic counterions leads to well-defined supramolecular nanoparticles with various shapes, such as spheres, cylinders, hollow spheres and networks in aqueous solution.6 The key factor in the structural control in these systems was the combination of electrostatic macroion–counterion interactions and mutual π–π interactions of the counterions. This also has led to light-switchable, host-triggerable and catalytically active assemblies,7 and more recently to gold and CdS containing assemblies interconnected by a combination of ionic, Hamaker and π–π interactions.8 Flexible non-aromatic counterions, in contrast, did not yield defined structures.9

For homogeneous photocatalysis, polyoxometalates (POMs), anionic early transition metal oxo-clusters, are the candidates of choice as their photoreactivity can be tuned over a wide range and they offer various opportunities to develop concepts for energy conversion. Heteropolyoxometalates have been particularly successful homogeneous catalysts for photoredox reactions and are used in catalytic water oxidation, hydrogen production and the oxidation of organic substrates.10 The promising photocatalytic properties of POMs are largely due to their ability to undergo stepwise, multielectron redox reactions while retaining structural integrity. Furthermore, POMs can be chemically and structurally modified, giving access to a number of compounds with diverse properties.11

In this study, we present a new type of photocatalytically active self-assembled nanostructure which forms spontaneously in aqueous solution from the association of anionic polyoxometalate clusters and cationic dendrimers.

We demonstrate that the self-assembly of Keggin-type polyoxotungstates with macroions results in aggregates with increased photocatalytic activity and novel substrate selectivity. The underlying assembly represents a novel type of electrostatic self-assembly that forms stable nanoscale structures in solution based on ionic interactions and geometric constraints. Building blocks and the concept of self-assembly and catalysis are shown in Scheme 1.


image file: c7nr07097g-s1.tif
Scheme 1 Schematic representation of POM–dendrimer self-assembly and photocatalysis in aqueous solution as investigated in this study. Catalyzed model reaction is the photo-degradation of the dye methyl red.

Structure formation was achieved by combining aqueous solutions of a cationic poly(amidoamine) (PAMAM) dendrimer of generation 4 (G4) and of the tetravalent Keggin-type polyoxometalate cluster K4[SiW12O40] (POM), both set to pH 3.5. The formation of aggregates is evident by looking at the scattering intensity of a laser beam, and dynamic light scattering (DLS) yields information on the size and size distribution. The results for a sample with a charge ratio of l = 0.7 – that is the molar ratio of anionic POM charges (with −4 per POM) to cationic dendrimer charges (with +64 per dendrimer) – according to a molecular ratio of POM-clusters to dendrimers of 11.2, are shown in Fig. 1a.


image file: c7nr07097g-f1.tif
Fig. 1 Scattering characterization of POM–dendrimer assemblies: (a) dynamic light scattering: electric field autocorrelation function g1(τ) and distribution of relaxation times A(τ) for POM–dendrimer assemblies with l = 0.7; (b) static light scattering and SANS of POM–dendrimer assemblies with l = 0.7; filled symbols: SLS data points, open symbols: SANS data points, black line at high q: flexible cylinder fit; SLS, DLS: c(dendrimer) = 1.69 × 10−5 mol L−1; SANS: c(dendrimer) = 3.52 × 10−5 mol L−1.

Two sizes of aggregates coexist, showing hydrodynamic radii of RH = 33 nm and RH = 170 nm. The solution is stable with particles of this size for at least several weeks. The same size distribution results when the mixing order of the stock solutions is inverted, indicating that equilibrium structures rather than kinetic structures are present. Evidently, due to the ionic interaction of the multiply charged components, aggregation occurs. Remarkably, it stops at a finite particle size yielding nanoscale structures that are stable in solution.

To understand this stability, ζ-potential measurements were performed, which yield a positive value (ζ = +51 mV) demonstrating the charge stabilization of the particles.

This is consistent with the excess of positively charged dendrimers in the solution (l < 1) or, in other words, there are not sufficient tungsten oxide ions to allow for a complete macroscopic interconnection of the dendrimers. This scenario can further be confirmed by preparing samples with varying loading ratio. In the range 0.03 ≤ l ≤ 0.85, corresponding to 1 to 27 POM-clusters per dendrimer molecule, stable assemblies are formed. Above this charge ratio, precipitation occurs. Within the stable range, the larger assemblies possess sizes in the range of 120 nm < RH < 180 nm. Smaller sized aggregates of about RH = 30 nm are also found in the loading ratio range 0.03 ≤ l ≤ 0.85 with an approximately constant fraction and may correspond to smaller assemblies consisting of dendrimer macroions and cluster counterions; i.e., an intermediate building block aggregate.

To further elucidate the structure of these assemblies, small-angle neutron scattering (SANS) was performed. Fig. 1b shows the scattering curve resulting from the combination of static light scattering and SANS. It represents a Gaussian coil structure of a chain with a diameter of 6.2 nm. Here, the coil structure is evident from the −2 slope in the double logarithmic plot and confirmed by the linear dependence in a cross-section Guinier plot, the linearity of which demonstrates the presence of locally cylindrical structures (ESI). In addition, a coil structure is in agreement with the shape sensitive ratio of radius of gyration to hydrodynamic radius obtained from static and dynamic light scattering which is RG/RH = 1.66 typical of a coil. From the SANS cross-section Guinier plot, the cross-section radius of gyration is determined as RG,C = 2.0 nm. In more detail, a fit of the high q internal structural data with a flexible cylinder model (included as a line in Fig. 1b) yields a total cylinder diameter of 6.2 nm. This thickness is consistent with the G4 dendrimers’ diameter of 4.5 nm with POM-clusters of about 1.1 nm attached and slightly penetrating the dendrimer on both sides. In addition, POM-clusters must also be located in-between dendrimers serving as ionic connectors.§ The supramolecular coil structure is sketched in Scheme 1.

Hence, the combination of a multivalent polyoxometalate with oppositely charged dendrimeric macroions can yield stable nano-assemblies in aqueous solution. While our previous studies used π–π interactions between aromatic counterions as secondary attractive forces, here, for the first time we show that employing large multivalent inorganic ions as “counterions” of a polyelectrolyte represents a new route to defined supra-molecular ionic structures. In the formation of these charge-stabilized assemblies, steric factors likely play an important role.

This was discussed for biomolecule bundle formation through finite counterions (but infinite association for point-like counterions) by Pincus.12 In that regard, the structure formation reported here might be seen as an analogy to DNA-assembly induced by non-stacking stiff organic counterions;13 these DNA aggregates, however, represent a far more “specialized” case with respect to building block structures than the POM–dendrimer assemblies discussed here. Thus, on the one hand, it is understandable that assemblies with defined structure and size can be formed by charge-stabilization in solution. On the other hand, the formation of specifically a chain-like coil structure cannot be predicted at this point. One additional driving force might be the formation of a dipole upon attachment of POMs to the dendrimer.

It is of great interest whether the photocatalytic activity of the POM-cluster can be retained or even modified through self-assembly with macroions. For this purpose, we have investigated the photocatalytic degradation of organic dye molecules with different charges as a model reaction. In general, the POM-catalyzed degradation of dye molecules in aqueous solution using UV-light irradiation occurs via the formation of radicals, which, for example, initiates the N-de-alkylation and cleavage of the azo bond.14 Our first studies used the cationic dye methyl red as a substrate. Fig. 2 shows solutions using the POM–dendrimer assembly (A, B) or POM-cluster only (C, D) as the photocatalyst; samples are shown before and after 20 min irradiation with UV-light at a wavelength of 254 nm/366 nm. A comparison shows that the dye is almost completely degraded in the POM–dendrimer sample but not with the neat POM solution. In addition, a methyl red solution only shows almost no degradation under the same conditions (not shown). Hence, the self-assembly of the POM with the dendrimer has substantially increased the photocatalytic activity.


image file: c7nr07097g-f2.tif
Fig. 2 Photocatalytic degradation of methyl red by POM–dendrimer assemblies and POM in aqueous solution; A: POM–dendrimer before irradiation (l = 0.7), B: POM–dendrimer after 20 min UV-irradiation, C: POM before irradiation, D: POM after 20 min UV-irradiation; λirradiation = 266 nm/354 nm, c(MR) = 3.80 × 10−5 mol L−1. The dye degrades much more with the POM–dendrimer assembly (A → B) as compared to the cluster only solution (C → D).

Fig. 3 shows corresponding kinetic data from UV/Vis spectroscopy. Analysis of the non-irradiated reaction samples provides insight into the interaction of the dendrimer, POM-cluster and the methyl red dye molecule in solution (Fig. 4). Upon mixing a methyl red solution with a dendrimer solution, the methyl red spectrum remains virtually unchanged. This is expected since no strong attractive interaction should occur between two hydrophilic positively charged components.


image file: c7nr07097g-f3.tif
Fig. 3 Photocatalytic methyl red degradation by POM–dendrimer assemblies and POM-clusters: (a) time-dependent UV/Vis spectra of methyl red degradation in the presence of POM-clusters; (b) UV/Vis spectra of methyl red degradation in the presence of POM–dendrimer assemblies (l = 0.7); both c(MR) = 3.80 × 10−5 mol L−1, c(POM) = 1.35 × 10−4 mol L−1; pH 3.5, λirradiation = 266 nm/354 nm; (c) time-dependent relative methyl red concentration in comparison with the cluster only (POM, red curve) and the POM–dendrimer assembly (POM–dendrimer, l = 0.7, black curve).

image file: c7nr07097g-f4.tif
Fig. 4 UV/Vis spectra of pure methyl red (MR, black curve), POM–dendrimer assemblies of l = 0.7 with methyl red (blue curve) and methyl red in the presence of POM (red curve) and G4 dendrimer (green curve); c(MR) = 3.80 × 10−5 mol L−1, c(POM) = 1.35 × 10−4 mol L−1.

In contrast, a clear spectral change is evident upon combination with the POM-cluster (red curve). This change is partially due to mutual dye–dye interactions, specifically π–π stacking.9,15 It is likely that this association and interaction complicate the dye degradation. The POM-dye aggregation is further confirmed by the observation of larger aggregates in light scattering (not shown). In contrast, in the POM–dendrimer sample, assemblies as described are pre-formed and stable and no further expressed aggregation occurs when methyl red is added. This is evident from the virtually unchanged UV/Vis spectrum of methyl red in the POM–dendrimer sample (blue line in Fig. 4). In addition, dynamic light scattering shows a nearly constant assembly size upon addition of methyl red and irradiation (ESI). Thus, it is demonstrated that the association of POMs and dendrimers can be used to control the photocatalytic activity of POMs, presumably by inhibiting cluster-dye aggregation which would otherwise lead to lower reaction rates. Based on the magnitude of the non-covalent interactions in play it is expected that the cluster preferably associates with the dendrimer rather than the dye in the mixed system: the dendrimer carries 128 positive charges (with tertiary and primary amines protonated at pH 3.5) and methyl red only one. In addition, any attractive π–π interactions induced by ionic binding of dye to the cluster are also not expected to outweigh attractive electrostatic interactions between the cluster and dendrimer.

Furthermore, the influence of the POM/dendrimer loading ratio on the catalytic activity was investigated. It was found that at a low loading ratio with l = 0.03 – that is a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 molecular ratio of the cluster to dendrimer – a significant increase in photocatalytic activity is observed.

After 20 min irradiation time under identical reaction conditions, 83% dye degradation was found for the POM–dendrimer assembly whereas only 11% methyl red degradation was observed with POM only (ESI). At least two photocatalytic cycles can be performed via re-addition of fresh dye to the assembly (data not shown).

For a constant cluster concentration, we observed an activity increase with increasing cluster/dendrimer ratio up to a loading ratio of l = 0.7 where almost quantitative methyl red degradation was observed (98% after 20 min irradiation). For l = 0.85 the degradation kinetics are approximately the same as for l = 0.7.

Based on these promising results, it is illuminating to investigate the degradation of further dyes, which are shown in Scheme 2.


image file: c7nr07097g-s2.tif
Scheme 2 Dyes used in the photo-catalyzed degradation with POM and POM–dendrimer assemblies (in addition to methyl red as shown in Scheme 1).

For methylene blue, also a positively charged dye, the reactivity behavior is comparable to the methyl red results; methylene blue degradation is considerably faster with the dendrimer present in the solution. Under identical conditions and 280 min irradiation time, 83% dye degradation was observed with the POM–dendrimer assembly (l = 0.7) as compared to 7% with POM only and no degradation is observed without POM (ESI). Interestingly, when mixing aqueous solutions of alizarin yellow R, an uncharged dye, and the POM cluster at pH 3.5, instant precipitation occurs. Hence, no homogeneous photocatalytic reaction in solution is possible with the POM only for this dye.||[thin space (1/6-em)]16 In contrast, the solution remains stable when alizarin yellow R is added to the POM–dendrimer assembly. Here, homogeneous alizarin yellow R degradation was observed and followed by UV/Vis spectroscopy (ESI). Next, the photocatalytic degradation of a negatively charged dye, xylenol orange (XO), was investigated.

At pH 3, XO carries multiple charges with a net charge of −1.17 Time dependent UV/Vis data are shown in Fig. 5a. Here XO degradation is significantly faster when no dendrimer is present, opposite to the above cases. It can be suggested that the reversed reactivity is caused by the anionic charge of the XO molecule: as the charge of the POM-cluster and dye are the same, and a degradation-hindering association with the POM-cluster does not occur here. This is indeed evident from a comparison of the initial UV/Vis spectra in Fig. 5b. While no association occurs with the cluster alone, the XO shows spectral changes upon addition of the dendrimer (not shown), which again can be understood because of the opposite charge of the two components. It is likely that upon ionic binding to the dendrimer, dye molecules preferably bind adjacently so that they can also mutually interact as described previously,9 and a similar interaction occurs with the POM–dendrimer sample. Hence, in this case the dendrimer protects the dye from photo-degradation through electrostatic association. Protection occurs throughout the studied loading ratio range. This is of particular interest for XO, as here a substantial degradation of XO under irradiation is observed even without POM (52% after 130 min), which is avoided by the protection through assembly with the dendrimer.


image file: c7nr07097g-f5.tif
Fig. 5 UV/Vis spectroscopy of the photocatalytic degradation of xylenol orange (XO): (a) time-dependent relative XO concentration for the POM-cluster (red curve) and the POM–dendrimer assembly (l = 0.7, black curve); (b) UV/Vis spectra of pure XO (black curve) and XO in the presence of POM (green curve); c(XO) = 3.80 × 10−5 mol L−1, c(POM) = 1.35 × 10−4 mol L−1.

Finally, it should be mentioned that we have also found catalytic activity for assemblies of a dendrimer with a trivalent Keggin anion, K3PW12O40, confirming that a general principle rather than a very specific effect is reported.

In conclusion, we have described the formation of a new type of hundred-nanometer scale self-assembled structure from ionic POM-clusters and dendrimeric macroions in aqueous solution. We have demonstrated that these assemblies show substantially enhanced catalytic activity in the photo-degradation of certain dyes, while other dyes are protected from degradation. Our results suggest that substrate selectivity is controlled by the electrostatic charge of the substrate and is tunable via the POM/dendrimer ratio. As the self-assembly concept is based on general electrostatic interaction rather than special building blocks, this establishes a modular concept for the formation of versatile catalytically active assemblies with modified activity. Different ionic clusters and different polyelectrolytes might be used in the assembly formation to tune activity. While the dye degradation is a revealing model reaction, the concept presented here is, moreover, very promising as an application for other catalytic reactions with potential in solar energy conversion and solar technologies.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the German Science Foundation (DFG), the Interdisciplinary Center for Molecular Materials (ICMM), and Solar Technologies go Hybrid (SolTech) is gratefully acknowledged. The Institut Laue-Langevin (ILL), France is gratefully acknowledged for the beam time.

Notes and references

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Footnotes

Electronic supplementary information (ESI) available: Experimental section, SANS and DLS data of POM–dendrimer assemblies, time-dependent UV/Vis analysis of methylene blue and alizarin yellow R degradation. See DOI: 10.1039/c7nr07097g
In addition, it was possible to form stable aggregates of somewhat different sizes via a different procedure; i.e., it is also possible to control structures kinetically via the choice of the preparation route. This is currently of interest in designing more complex structures through non-covalent interactions.3c In this study, however, we will focus on the assemblies prepared through the mentioned routes.
§ Cryo-TEM of POM–dendrimer assemblies confirmed the elongated wormlike assembly structure. Investigations including the interaction with a staining agent and decoration with metal nanoparticles will be discussed in a forthcoming publication.
Results for loading ratio dependent methylene blue degradation using W4–G4 assemblies are shown in Table S1.
|| Most likely, the nitro-group attached to the W4 cluster.16

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