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.

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This study presents electrostatically self‐assembled nanoparticles from linear flexible polyelectrolytes (poly(diallyldimethylammonium chloride or quarternized poly(4‐vinylpyridine)) and an ionic photo‐isomerizable azo dye (Acid Yellow 38) that can change their size upon UV‐light irradiation. Assemblies with narrow size distribution are stable in aqueous solution. For samples with under‐stoichiometric dye load, UV‐light exposure triggers a size decrease, e.g. from a hydrodynamic radius of Rh = 94 nm to Rh = 62 nm for an Ay38‐PDADMAC sample with a charge ratio of lcharge = 0.7. Size changes are caused by trans‐cis isomerization of the dye, accompanied by a change in hydrophilicity, binding enthalpy and entropy. Assemblies are characterized by static and dynamic light scattering, atomic force microscopy, UV–vis spectroscopy and isothermal titration calorimetry. Zeta potential measurements give insight into the electrostatic stabilization and size‐control of the ionic nano‐assemblies, revealing a master curve of effective surface charge density versus hydrodynamic radius. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys., 2013

A 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.

A light-triggered shape change of supramolecular nanostructures is achieved through electrostatically self-assembly of linear polyelectrolytes and oppositely charged dyes in aqueous solution: Upon UV-irradiation, 1-µm-long, flexible cylinders with a cross-section of 10 nm convert into ellipsoids of 400 nm × 40 nm. The nano-object shape is encoded in the molecular dye structure.