Solution-based Supramolecular Hierarchical Assembly of Frenkel Excitonic Nanotubes Driven by Gold Nanoparticle Formation and Temperature.

Translating nature's successful design principle of solution-based supramolecular self-assembling to broad applications─ranging from renewable energy and information technology to nanomedicine─requires a fundamental understanding of supramolecular hierarchical assembly. Though the forces behind self-assembly (e.g., hydrophobicity) are known, the specific mechanism by which monomers form the hierarchical assembly still remains an open question. A crucial step toward formulating a complete mechanism is understanding not only how the monomer's specific molecular structure but also how manifold environmental conditions impact the self-assembling process. Here, we elucidate the complex correlation between the environmental self-assembling conditions and the resulting structural properties by utilizing a well-characterized model system: well-defined supramolecular Frenkel excitonic nanotubes (NTs), self-assembled from cyanine dye molecules in aqueous solution, which further self-assemble into bundled nanotubes (b-NTs). The NTs and b-NTs inhabit distinct spectroscopic signatures, which allows the use of steady-state absorption spectroscopy to monitor the transition from NTs to b-NTs directly. Specifically, we investigate the impact of temperature (ranging from 23 °C, 55 °C, 70 °C, 85 °C, up to 100 °C) during in situ formation of gold nanoparticles to determine their role in the formation of b-NTs. The considered time regime for the self-assembling process ranges from 1 min to 8 days. With our work, we contribute to a basic understanding of how environmental conditions impact solution-based hierarchical supramolecular self-assembly in both the thermodynamic and the kinetic regime.

Emerging Structural and Interfacial Features of Particulate Polymers at the Nanoscale.

Particulate polymers at the nanoscale are exceedingly promising for diversified functional applications ranging from biomedical and energy to sensing, labeling, and catalysis. Tailored structural features (i.e., size, shape, morphology, internal softness, interior cross-linking, etc.) determine polymer nanoparticles' impact on the cargo loading capacity and controlled/sustained release, possibility of endocytosis, degradability, and photostability. The designed interfacial features, however (i.e., stimuli-responsive surfaces, wrinkling, surface porosity, shell-layer swellability, layer-by-layer surface functionalization, surface charge, etc.), regulate nanoparticles' interfacial interactions, controlled assembly, movement and collision, and compatibility with the surroundings (e.g., solvent and biological environments). These features define nanoparticles' overall properties/functions on the basis of homogeneity, stability, interfacial tension, and minimization of the surface energy barrier. Lowering of the resultant outcomes is directly influenced by inhomogeneity in the structural and interfacial design through the structure-function relationship. Therefore, a key requirement is to produce well-defined polymer nanoparticles with controlled characteristics. Polymers are amorphous, flexible, and soft, and hence controlling their structural/interfacial features through the single-step process is a challenge. The microfluidics reaction strategy is very promising because of its wide range of advantages such as efficient reactant mixing and fast phase transfer. Overall, this feature article highlights the state-of-the-art synthetic features of polymer nanoparticles with perspectives on their advanced applications.

Single-Step In Situ Assembling Routes for the Shape Control of Polymer Nanoparticles.

Controlling shapes of polymer nanoparticles via single-step process is a challenge due to their amorphous chemical nature. Precise regulation of interfacial interactions, electrical charging and reaction dynamics during ongoing polymerization process provides an environment where uniform nucleation, growth and in situ assembling can be realized, and hence nanoparticles of complex shapes can be obtained. In this work, it is investigated how in situ assembling of the growing nanoparticles succeeds and specifically in different manners by using cationic, anionic, polyionic, and nonionic surface-active agents in a time-dependent blended form. Micelle of molecular surfactants leads the spheres, but long chained polyelectrolytes support in situ assembling of growing spheres to form the nonspherical polymer nanoparticles in order to minimize the surface energy of a system. Similarly, a nonionic polymer promotes the movement of growing species in solution and allows tunable aggregation-based growth which produces more complexed nanoparticles. Furthermore, the application of acid, base and salt solution also contribute specific effect where unexpected size and shape of nanoparticles can be obtained. Overall, the roles of limited polarizability, solvation power, mobility, ionic strength, pH, and microfluidics for the synthesis of various shape-controlled polymer nanoparticles are presented here.