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

Systems that can be controlled through external triggers are of interest for a variety of applications from medicine to energy conversion. Here, the effects of pH on the electrostatic self-assembly of generation four poly(amidoamine) dendrimers and a divalent azo dye, keeping constant the charge ratio, are discussed. Nanoparticles stabilized by an excess of dendrimer charge form at 3.5 <= pH <= 8.5. The nanoparticle structure strongly depends on pH: at 7 <= pH <= 8.5, where only the dendrimer primary amines are protonated, spherical particles form, while at 3.5 <= pH < 7 where also the tertiary amines are protonated, cylinders form. Not only the nanoparticle structure but also the interaction of the molecular building blocks is controlled by pH: the dye stacking changes and the twist angle of the dye molecules increases as the pH decreases. Results are related to changes in the dye-dendrimer and dye-dye interaction, in particular spherical nanoparticles are formed when the interaction is small. Finally, the structures at different pH and constant charge ratio are compared to results at constant loading ratio to understand the overall role of the pH in electrostatic self-assembly. A pH-responsive shape is of great interest, for example, for drug delivery.