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.

A new type of functional supramolecular assembly is formed by combining cationic porphyrin (TAPP) and the anionic generation 7.5 (G7.5) poly(amidoamine) (PAMAM) dendrimer in aqueous solution at basic pH. The interplay of noncovalent interactions is used as a key to control the formation of supramolecular structures and thereby to improve the photocatalytic activity of the porphyrin. The porphyrin–dendrimer assemblies show pH responsiveness. Their size, shape, and internal structure are controlled by varying the loading ratio. Structures were analyzed by light scattering, small-angle neutron scattering, ζ-potential measurements, UV/vis and fluorescence spectroscopy, and atomic force microscopy. The assemblies show structures with high quantum yield which makes them suitable for light harvesting processes, for example, in solar energy conversion. As a photocatalytic model reaction, the degradation of the anionic dye methyl orange is investigated, showing differing interactions with the assemblies based on their charge. A substantially enhanced photocatalytic activity was found.

AbstractA new type of light responsive nanoscale assemblies based on water-soluble spiropyrans is presented. We have synthesized four anionic spiropyrans bearing multiple sulfonate groups and investigated their photochromic behavior in aqueous solution. Depending on the pH, either inverse photochromism (acidic conditions) or normal photochromism (alkaline conditions) is found. Kinetic data for the interconversion of the spiropyran and merocyanine isomers including the subsequent slow hydrolysis have been obtained by UV/Vis spectroscopy. The results show that the spiropyrans undergo hydrolysis in both alkaline and acidic solution, while in the latter the rate is far slower than in the former. This prolonged hydrolytic stability together with the inverse photochromism under acidic conditions makes the sulfonated spiropyrans suitable to build photoresponsive nanostructures with cationic polyelectrolytes. We show how the self-assembly process is driven by electrostatic interactions and how the spiropyrans’ photochromic property allows the size control of the supramolecular objects by visible light. The assembly size is characterized by dynamic light scattering and TEM. In addition, UV/Vis and fluorescence spectroscopy and ζ-potential measurements help to explain the size change upon visible light irradiation.

We present a fundamental study on ZnO nanorod–porphyrin assembly formation in solution, providing the key to novel tunable hybrid assemblies with potential in solar energy conversion. The combination of 40 nm ZnO nanorods with ionic porphyrins – meso-tetra(4-carboxyphenyl)porphyrin (TCPP) and meso-tetra-(4-sulfonatophenyl)porphyrin (TPPS) – results in the formation of novel well-defined hybrid assemblies which are stable in solution and exhibit an adjustable size up to 500 nm. Structures have been characterized with dynamic light scattering (DLS), transmission electron microscopy (TEM) and absorption and emission spectroscopy. In particular, the combination of both porphyrins with ZnO in a ternary assembly yields a large stability range in terms of the TCPP/ZnO ratio and may be of significance as a hybrid system for solar cells or photocatalysis.

In this study, a new type of filamentous structures consisting of a generation 9 poly(amido amine) dendrimer (G9) and CdS is reported. The linearity of the interconnected dendrimers is a result of the electrostatic repulsion between the multiply charged dendrimer macroions. Structures have been investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The internal structure of the CdS-fibers reveals information on the mechanism of the fiber formation. In contrast to previous systems with smaller generation poly(propylene imine)-dendrimers, Cd2+ is here found to be responsible for the interconnection of G9. Furthermore, more complex supramolecular structures were built by associating the CdS–dendrimer hybrid fibers with different ionic dyes, displaying the versatility of this system for future nanotechnology applications such as optoelectronics or energy conversion.

The interplay of several non-covalent interaction forces is used as key to supramolecular structures. Combining cationic alkyltrimethylammonium bromide surfactants and the divalent anionic azo dye Acid Red 26 (Ar26) as small building blocks in aqueous solution, electrostatic interactions of the oppositely charged building blocks in combination with hydrophobic effect and π–π interactions play a major role in aggregate formation. Static and dynamic light scattering and small-angle neutron scattering (SANS) revealed different sizes of aggregates in the range of 2 nm ≤ RH ≤ 420 nm depending on surfactant length, concentration and of dye to surfactant loading ratio. A strong relationship of assembly size with surfactant concentration has been found, where initial surfactant monomers and micelles influence the aggregate formation differently. The stability of dye–surfactant aggregates which also shows a dependency on surfactant tail length has been related to ζ-potential measurements. Small-angle neutron scattering elucidated that dye–surfactant aggregates possess cylindrical shapes with different aspect ratios. UV/Vis spectroscopy gave information on the dye–dye π–π stacking geometry and extent, while the thermodynamic parameters for micellization and dye–surfactant binding ΔH, ΔG, and ΔS as well as stoichiometry and binding constant obtained by isothermal titration calorimetry revealed insight into the interplay of interactions.

The fundamental understanding of the driving forces in electrostatic self-assembly is highly desirable for the design of novel systems and of more effective synthesis strategies. The focus of this study is the effects of the electrostatic interaction on supramolecular self-assembled nanoparticles formed by cationic dendrimers as model polyelectrolytes and oppositely charged di- and trivalent dyes, elucidated by changing the solution ionic strength. Increasing ionic strength results in the formation of larger nanoparticles, although the screened electrostatic interaction of the building blocks may be expected to result in the formation of smaller particles. Yukawa potential and DLVO theory have been used to understand this phenomenon. The screened electrostatic potential decreases the nanoparticle repulsion resulting in larger aggregates, which also causes an increase of the nanoparticle charge leading to stabilization. Contrarily, the ionic strength has no effects on the nanoparticle shape and on the dye stacking due to their π–π interaction. This shows how the electrostatic interaction controls the dimensions of the nanoaggregates through the stabilization mechanism, while the secondary interactions, and in particular the π–π interaction, encode the nanoparticle shape. Revealing these relationships is a key step in understanding the ionic association of building blocks under secondary interactions.

pH can be used to tune the self-assembly of cationic polyelectrolyte dendrimers and oppositely charged dyes and to produce particles with a desired shape and size in aqueous solution. We present fundamental insight into the effect of pH on electrostatic self-assembly of poly(amidoamine) dendrimers of generation 4 and di- and trivalent anionic organic dyes. The solution pH is used as a key to turn on the interaction and to control the association by regulating the macroion charge. Stable and well-defined nanoparticles are formed in solution, being more stable at low pH where the dendrimer protonation is complete. Nanoparticle stability was correlated with ζ-potential measurements. We prove that the assemblies are electrostatically stabilized and elucidate the importance of the surface charge density. pH was also used as a key to nanoparticle dimension and shape. For example, smaller particles form at a lower pH. The nanostructures have been characterized using dynamic light scattering and small-angle neutron scattering. A “phase diagram” has been developed for each dye, showing the assembly size, shape, and instability regions dependent on the pH. Overall, a pH-responsive nanoparticle shape is a key step toward the design of novel smart therapeutic carrier systems.

Light stimulation was used to trigger the assembly of nanostructures by directly powering changes at the supramolecular level without incurring net chemical changes at the molecular level. Polyethylene imine, a polybase, was mixed in aqueous solution with sodium 1-naphthol-4-sulfonate, an aromatic alcohol, which, in the electronic excited-state, undergoes a short-lived increase in acidity. Excited-state proton transfers between these components were induced by photoexcitation, which led to the formation of hydrogen bonds in the ground-state. Ionic forces, π–π stacking, and hydrophobic effect provided further stabilization. The photoinduced formation of nanosized aggregates was detected by dynamic light scattering and atomic force microscopy. Absorption and emission spectroscopy were used to rule out photochemical reactions and elucidate the supramolecular arrangement.

The rational design of supramolecular nanoparticles by self-assembly is a crucial field of research due to the wide applications and the possibility of control through external triggers. Understanding the shape-determining factors is the key for tailoring nanoparticles with desired properties. Here, we show how the thermodynamics of the interaction control the shape of the nanoparticle. We highlight the connection between the molecular structure of building blocks, the interaction strength, and the nanoassembly shape. Nanoparticles are prepared by electrostatic self-assembly of cationic polyelectrolyte dendrimers of different generations and oppositely charged multivalent organic dyes relying on the combination of electrostatic and π–π interactions. Different building blocks have been used to vary interaction strength, geometric constraints, and charge ratio, providing insights into the assembly process. The nanoassembly structure has been characterized using atomic force microscopy, static light scattering, small angle neutron scattering, and UV–vis spectroscopy. We show that the isotropy/anisotropy of the nanoassemblies is related to the dye valency. Isothermal titration calorimetry has been used to investigate both dye–dye and dye–dendrimer interaction. The existence of a threshold value in entropy and enthalpy change separating isotropic and anisotropic shapes for both interactions has been demonstrated. The effects of the dye molecular structure on the interaction thermodynamics and therefore on the nanoparticle structure have been revealed using molecular modeling. The polar surface area of the dye molecule takes a key role in the dye self-interaction. This study opens the possibility for a priori shape determination knowing the building blocks structure and their interactions.

A novel concept for the formation of organic–inorganic hybrid nanostructures based on a tunable supramolecular templating mechanism involving several types of noncovalent interactions is presented. Different organic–inorganic hybrid structures with narrow size distributions composed of a cationic dendrimer, an oppositely charged divalent diazo dye, and CdS can be created: First, larger supramolecular structures consisting of several 100 to 1000 dendrimers is built based on ionic and π–π interaction. Second, with this self-assembled template, the hybrid nanostructure forms based on electrostatics and coordination interaction. The influence on shape and internal structure of the supramolecular hybrid architectures was investigated. A size control of the assemblies is possible in a range of 60–420 nm. The prospect to direct self-assembly and templating through the controlled combination of different noncovalent interactions potentially allows to create tailor-made systems for various applications.