Molecular Structure Encodes Nanoscale Assemblies: Understanding Driving Forces in Electrostatic Self-Assembly

Abstract

Supramolecular nanoparticles represent a key field in recent research as their synthesis through self-assembly is straightforward and they often can respond to external triggers. A fundamental understanding of structure-directing factors is highly desirable for a targeted structure design. This contribution demonstrates a quantitative relation between the size of supramolecular self-assembled nanoparticles and the free energy of association. Nanoparticles are prepared by electrostatic self-assembly of cationic polyelectrolyte dendrimers as model macroions and oppositely charged di- and trivalent organic dye molecules relying on the combination of electrostatic and π-π-interactions. A systematic set of sulfonate-group carrying azo-dyes was synthesized. Light scattering and ζ-potential measurements on the resulting nanoparticles yield hydrodynamic radii between 20 nm < R(H) < 50 nm and positive ζ-potential values indicating a positive particle charge. Studies on dye self-aggregation and dendrimer-dye association by isothermal titration calorimetry (ITC) and UV-vis spectroscopy allow for the correlation of the thermodynamic parameters of dendrimer-dye association with the size of the particles, showing that at least a free energy gain of ΔG ≈ - 32 kJ mol(-1) is necessary to induce dendrimer interconnection. Structural features of the azo dyes causing these to favor or prevent nanoparticle formation have been identified. The dye-dye-interaction was found to be the key factor in particle size control. A simple model yields a quantitative relation between the free energy and the particle sizes, allowing for predicting the latter based on thermodynamic measurements. Hence, a set of different molecular "building bricks" can be defined where the choice of building block determines the resulting assembly size.

Structure Tuning of Electrostatically Self‐Assembled Nanoparticles through pH: The Role of Charge Ratio

“…Good agreement with the experimental results was obtained using 0.3 for all the models. As observed in a previous study of us, no interparticle structure factor features affect the scattering curve at the amount of added low molecular salt present due to pH adjustment …”

“…Additionally, secondary interactions such as π–π stacking of the dye molecules or geometric effects occur during the process. The choice of the building blocks allows to form nanoparticles switchable by external triggers: for example, nanoparticles responsive to light and pH have been built . To achieve control over the process via pH, polyamidoamine (PAMAM) dendrimers have been used as macroions; their charge can be adjusted by regulating the solution.…”

“…The choice of the building blocks allows to form nanoparticles switchable by external triggers: for example, nanoparticles responsive to light and pH have been built. [19][20][21] To achieve control over the process via pH, polyamidoamine (PAMAM) dendrimers have been used as macroions; their charge can be adjusted by regulating the solution. Thereby, assembly on-off switchability was achieved via protonation and deprotonation of the dendrimer molecules.The effects of the pH on the stability and structure on the self-assembled nanoparticles have been investigated recently keeping constant the molar ratio of dye sulfonate groups to dendrimer primary amines.…”

Water‐Soluble Spiropyrans with Inverse Photochromism and Their Photoresponsive Electrostatic Self‐Assembly

“…In our previous studies with azo dyes two sulfonate groups usuallyw ere sufficient to cause electrostatic self-assembly with polyelectrolytes like PAMAMd endrimer [14,74] or linear PDADMAC. We also varied the pH, concentrations and used different loading ratios, but no assembly formationw as observed, as detected by light scattering.…”

Inducing Hetero‐aggregation of Different Azo Dyes through Electrostatic Self‐Assembly

“…The existence of dye‐only stacks can be excluded because, at loading ratio l =1.8, all the dye is bound to the dendrimers . The hypothesis of the possible presence of dyestuff–dendrimer complexes without any alignment of the dyestuffs can be discarded, because π–π stacking is essential to form the larger nano‐assemblies rather than guest–host systems only . Because the SANS data do not show any small particles in solution, their presence and therewith the presence on non‐interacting dye molecules can be excluded.…”