Article pubs.acs.org/Macromolecules
Electrostatic Self-Assembly of Dendrimer Macroions and Multivalent Dye Counterions: The Role of Solution Ionic Strength Giacomo Mariani,†,‡ Ralf Schweins,‡ and Franziska Gröhn*,† †
Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany ‡ Institut Laue-Langevin DS/LSS, 71 Avenue des Martyrs, F-38000 Grenoble, France ABSTRACT: 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.
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INTRODUCTION The synthesis of nanoparticles with suitable properties for different applications is crucial in material science. Among the different synthesis strategies, self-assembly has special importance in the organization of soft matter.1−15 Great potential lies in nature-inspired self-assembled systems such as carrier systems for drug delivery.2,3,16−18 The key for this application is the possibility to form nontoxic nanoparticles with a size and shape tunable via external triggers such as pH or light. For example, it has been demonstrated that nanoparticle shape and size strongly influence the circulation lifetime once intravenously administered.19−21 Among the other self-assembly strategies, electrostatic self-assembly is of special importance due to the wide range of different building blocks available and the striking number of different morphologies that can be formed.8,22−29 Theoretical and experimental studies have revealed that physicochemical parameters such pH and ionic strength are of crucial importance to control the nanoparticle structure and stability in electrostatically self-assembled systems. In the case of charged diblock copolymers, the morphology of the assemblies depends on the added salt or pH: spherical or cylindrical micelles and vesicles, or even more complex structures such as toroids, are formed.30−37 Hence, fundamental understanding of the self-assembly has special importance for the design of novel systems and for the improvement of the modern synthesis strategies. In addition, self-assembly in aqueous solution is of special interest for medical application. © XXXX American Chemical Society
For this reason, the driving forces and the basic principles of self-assembly have been investigated theoretically in the past years.38−40 Recently, the combination of different noncovalent interactions has come into focus for a more versatile nanostructure design.2,8,9 For example, we have introduced a new concept of electrostatic self-assembly leading to the formation of responsive supramolecular nanoparticles in aqueous solution with narrow size distribution and varying shape.41−44 The process is based on a general combination of interactions rather than on specific binding motifs; in particular, macroions are interconnected through structural multivalent organic counterions that can mutually interact through secondary interactions such as π−π stacking or geometric factors.45,46 Using appropriate building blocks, supramolecular nanoparticles sensitive to light and pH have been formed. For instance, with a dye that isomerizes upon UV irradiation the control of the nanoassembly dimensions via irradiation has been achieved.44,47−50 Particular potential lies in porphyrin− polyelectrolyte assemblies as novel functional photocatalytic nanosystems for light conversion.51 Recently, we focused on the driving factors of the self-assembly since they are the key for making tailored nanoparticles with desired shape and dimensions. It has been shown that the delicate balance of different interactions plays a central role. Thermodynamics has Received: March 22, 2016 Revised: October 14, 2016
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DOI: 10.1021/acs.macromol.6b00565 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Self-Assembly and Molecular Building Block Structures
Characterization of the Azo Dyes. Yields were calculated on the basis of carbon content from elemental analysis. NMR spectra showed that the product is salt-free except for
