Directional Supramolecular Polymerization in a Dynamic Microsolution: A Linearly Moving Polymer's End Striking Monomers

Although directional chain reactions are common in nature's self-assembly processes and in covalent polymerizations, it has been challenging to perform such processes in artificial one-dimensional self-assembling systems. In this paper, we describe a system, employing perylene bisimide (PBI) derivatives as monomers, for selectively activating one end of a supramolecular polymer during its growth and, thereby, realizing directional supramolecular polymerization. Upon introduction of a solution containing only a single PBI monomer into the microflow channel, nucleation was induced spontaneously. The dependency of the aggregation efficiency on the flow rate suggested that the shear force facilitated collisions among the monomers to overcome the activation energy required for nucleation. Next, by introducing a solution containing both monomer and polymer, we investigated how the shear force influenced the monomer-polymer interactions. In situ fluorescence spectra and linear dichroism revealed that growth of the polymers was accelerated only when they were oriented under the influence of shear stress. Upon linear motion of the oriented polymer, polymer growth at that single end became predominant relative to the nucleation of freely diffusing monomers. When applying this strategy to a two-monomer system, the second (less active) monomer reacted selectively at the forward-facing terminus of the first polymer, leading to the creation of a diblock copolymer through formation of a molecular heterojunction. This strategy-friction-induced activation of a single end of a polymer-should be applicable more generally to directional supramolecular block copolymerizations of various functional molecules, allowing molecular heterojunctions to be made at desired positions in a polymer.

Microfluidic enhancement of self-assembly systems

Dynamic, kinetically-controlled, self-assembly processes are commonly observed in nature and are capable of creating intricate, functional architectures from simple precursors. However, notably, much of the research into molecular self-assembly has been performed using conventional bulk techniques where the resultant species are dictated by thermodynamic stability to yield relatively simple assemblies. Whereas, the environmental control offered by microfluidic systems offers methods to achieve non-equilibrium reaction conditions capable of increasingly sophisticated self-assembled structures. Alterations to the immediate microenvironment during the assembly of the molecules is possible, providing the basis for kinetically-controlled assembly. This review examines the key mechanism offered by microfluidic systems and the architectures required to access them. The mechanisms include diffusion-led mixing, shear gradient alignment, spatial and temporal confinement, and structural templates in multiphase systems. The works are selected and categorised in terms of the microfluidic approaches taken rather than the chemical constructs which are formed.

Microflow system promotes acetaminophen crystal nucleation

It is known that interfaces have various impacts on crystallization from a solution. Here, we describe crystallization of acetaminophen using a microflow channel, in which two liquids meet and form a liquid-liquid interface due to laminar flow, resulting in uniform mixing of solvents on the molecular scale. In the anti-solvent method, the microflow mixing promoted the crystallization more than bulk mixing. Furthermore, increased flow rate encouraged crystal formation, and a metastable form appeared under a certain flow condition. This means that interface management by the microchannel could be a beneficial tool for crystallization and polymorph control.

In Situ Photophysical Characterization of π-Conjugated Oligopeptides Assembled via Continuous Flow Processing

Bioinspired materials have been developed with the aim of harnessing natural self-assembly for precisely engineered functionality. Microfluidics is poised to play a key role in the directed assembly of advanced materials with ordered nano and mesoscale features. More importantly, there is a strong need for understanding the kinetics of continuous assembly processes. In this work, we describe a continuous microfluidic system for the assembly and alignment of synthetic oligopeptides with π-conjugated cores using a three-dimensional (3D) flow focusing of inlet reactant streams. This system facilitates in situ confocal fluorescence microscopy and in situ fluorescence lifetime imaging microscopy (FLIM), which can be used in unprecedented capacity to characterize the integrity of peptides during the assembly process. To achieve continuous assembly, we integrate chevron patterns in the ceiling and floor of the microdevice to generate a 3D-focused sheath flow of the reactant peptide. Consequently, the peptide stream is directed toward an acidic triggering stream in a cross-slot geometry which mediates assembly into higher-order fiber-like structures. Using this approach, the focused peptide stream is assembled using a planar extensional flow, which ensures high degrees of microstructural alignment within the assembled material. We demonstrate the efficacy of this approach using three different synthetic oligopeptides, and in all cases, we observe the efficient and continuous assembly of oligopeptides. In addition, finite element simulations are used to guide device design and to validate 3D focusing. Overall, this approach presents an efficient and effective method for the continuous assembly and alignment of ordered materials using microfluidics.

Molecular Synchronization Enhances Molecular Interactions: An Explanatory Note of Pressure Effects

In this study, we investigated a unique aspect of the supramolecular polymerization of tetrakis (4-sulfonatophenyl) porphyrin (TPPS), a self-assembling porphyrin, under non-equilibrium conditions by subtracting the effects of back-pressure on its polymerization. We focused on the enhanced self-assembly abilities of TPPS under a process of rapid proton diffusion in a microflow channel. Rapid protonation caused synchronization of many sets of protonation/deprotonation equilibria on the molecular scale, leading to the production of many sets of growing suparmolecular spices. Pressure effects in the microflow channel, which could potentially promote self-assembly of TPPS, were negligible, becoming predominant only when the system was in the synchronized state.

Tubulation of liposomes via the interaction of supramolecular nanofibers

We achieved tubulation of self-assembled lipid membranes, liposomes, via the interaction of supramolecular nanofibers, porphyrin J-aggregates. This structural change was reversible, and the deformation of the porphyrin J-aggregates caused reconstruction of the liposomes from the tubes. We discussed the tubulation mechanism and calculated the force provided by porphyrin J-aggregates for tubulation.

Reversible Vesicle-to-Disk Transitions of Liposomes Induced by the Self-Assembly of Water-Soluble Porphyrins

Structural control of lipid membranes is important for mechanisms underlying biological functions and for creating high-functionality soft materials. We demonstrate the reversible control of vesicle structures (liposomes) using supramolecular assemblies. Specifically, water-soluble anionic porphyrin molecules interact with positively charged lipid membrane surfaces to form one-dimensional self-assembled structures (J-aggregates) under acidic conditions. Cryogenic transmission electron microscopy revealed that porphyrin J-aggregates on the membrane surface induced an extensive structural change from vesicles to layered disks. Neutralization of the solution deformed the porphyrin J-aggregates, thereby reforming nanosized liposomes from the layered disks.

Effect of Hydrodynamic Forces on meso-(4-Sulfonatophenyl)-Substituted Porphyrin J-Aggregate Nanoparticles: Elasticity, Plasticity and Breaking

The J aggregates of 4-sulfonatophenyl meso-substituted porphyrins are non-covalent polymers obtained by self-assembly that form nanoparticles of different morphologies. In the case of high aspect-ratio nanoparticles (bilayered ribbons and monolayered nanotubes), shear hydrodynamic forces may modify their shape and size, as observed by peak force microscopy, transmission electron microscopy of frozen solutions, small-angle X-ray scattering measurements in a disk-plate rotational cell, and cone-plate rotational viscometry. These nanoparticles either show elastic or plastic behaviour: there is plasticity in the ribbons obtained upon nanotube collapse on solid/air interfaces and in viscous concentrated nanotube solutions, whereas elasticity occurs in the case of dilute nanotube solutions. Sonication and strong shear hydrodynamic forces lead to the breaking of the monolayered nanotubes into small particles, which then associate into large colloidal particles.