Kinetically guided colloidal structure formation

Thermoresponsive oil-continuous gels based on double-interpenetrating colloidal-particle networks

Gels composed of multicomponent building blocks offer promising opportunities for the development of novel soft materials with unique and useful structures. While interpenetrating polymer networks have been extensively studied and applied in the creation of these gels, equivalent strategies utilizing colloidal particles have received limited scientific and technological attention. This study presents a novel class of thermo-responsive apolar double gels from interpenetrating networks of attractive colloidal silica and lipid particles. These double gels are easily assembled and suitable for the fabrication of 3D-printed edible soft constructs. Emphasis is focused on the rheological properties and structure emerging on the dilute regime (ϕ ≲ 0.1). Rheological investigations demonstrate that double gels exhibit greater stiffness and resilience to yielding compared to their single lipid gel counterparts. The scaling behavior of the oscillatory linear shear moduli and the critical strain for yielding with volume fraction remain comparable between single and double gels. Creep yielding in double gels exhibits two exponential decay regimes, suggesting the presence of thicker gel strands undergoing flow. Visualization and quantification of the quiescent microstructure confirms the existence of such denser aggregates devoid of larger clusters due to steric hindrance of interpenetrating networks in double gels. This is in stark contrast to lipid single gels where aggregates grow unrestrictedly into larger clusters. Our study constitutes the first demonstration on the assembly of apolar double gel networks as a promising avenue for the design of novel soft materials and foods with tailored structure and mechanics.

Kinetically assembled binary nanoparticle networks

Embedding percolating networks of nanoparticles (NPs) within polymers is a promising approach for mechanically reinforcing polymers and for introducing novel electronic, transport, and catalytic properties into otherwise inert polymers. While such networks may be obtained through kinetic assembly of unary system of NPs, the ensuing structures exhibit limited morphologies. Here, we investigate the possibility of increasing the diversity of NP networks through kinetic assembly of multiple species of NPs. Using lattice Monte Carlo simulations we show that networks obtained from co-assembly of two NP species of different sizes exhibit significantly more diverse morphology than those assembled from a single species. In particular, we achieved considerable variations in the particle spatial distribution, proportions of intra- and interspecies contacts, fractal dimension, and pore sizes of the networks by simply modulating the stoichiometry of the two species and their intra and inter-species affinities. We classified these distinct morphologies into "integrated", "coated", "leaved", and "blocked" phases, and provide relevant phase diagrams for achieving them. Our findings are relevant to controlled and predictable assembly of particle networks for creating multifunctional composites with improved properties.

Transient self-organisation of DNA coated colloids directed by enzymatic reactions

Dynamic self-organisation far from equilibrium is a key concept towards building autonomously acting materials. Here, we report the coupling of an antagonistic enzymatic reaction of RNA polymerisation and degradation to the aggregation of micron sized DNA coated colloids into fractal structures. A transient colloidal aggregation process is controlled by competing reactions of RNA synthesis of linker strands by a RNA polymerase and their degradation by a ribonuclease. By limiting the energy supply (NTP) of the enzymatic reactions, colloidal clusters form and subsequently disintegrate without the need of external stimuli. Here, the autonomous colloidal aggregation and disintegration can be modulated in terms of lifetime and cluster size. By restricting the enzyme activity locally, a directed spatial propagation of a colloidal aggregation and disintegration front is realised.

Two-Photon Lithographic Patterning of DNA-Coated Single-Microparticle Surfaces

The spatially defined functionalization of microparticles with asymmetric shape-controlled nucleic acid patterns is a major challenge in materials science. The asymmetric patterning of microparticles is important to allow the controlled fabrication of crystalline lattices or controlled aggregates of microparticles. We present the combination of two-photon lithography and photocleavable o-nitrobenzylphosphate ester nucleic acid coating-modified microparticles as a versatile means to asymmetrically pattern single microparticle surfaces. The two-photon patterning of microparticles with predesigned nucleic acid structures of different sizes (700 nm to 2.8 μm) and shapes (circles, rings, triangles, and squares) are demonstrated. In addition, complex patterned domains consisting of two different asymmetric nucleic acid domains are fabricated by the controlled Z-positioning of the microparticles in respect to the two-photon irradiation sources. In addition, the two-photon lithographic patterning of the photocleavable DNA coating allows the generation of functional nucleic acid domains for the photostimulated activation of the catalytic hybridization assembly (CHA) of branched nucleic acid structures on single microparticles.

Cage dynamics leads to double relaxation of the intermediate scattering function in a binary colloidal system

A system of binary colloids where one fraction of particles is aggregating by forming irreversible bonds and the other fraction of particles only interacts as hard spheres, is simulated using Brownian cluster dynamics. These aggregating species always formed percolating clusters for the case of diffusing hard spheres while for the static case, formation of percolating clusters depended on the fraction of static hard spheres in the system. The dynamics of the hard spheres inside the percolating clusters was studied by restarting the simulation after the kinetics of aggregation was arrested. Two cases were studied, one where the percolated particles moved within the bonds or cage dynamics was allowed and another where the movement within the bonds was not allowed or the cages were static. The hard spheres showed anomalous diffusion in both cases. The mean square displacement showed that for the case of dynamic cages we always had diffusive hard spheres irrespective of the fraction of hard spheres for volume fractions below 0.49. Static cages, depending on the fraction of hard spheres, showed either diffusive or arrested behavior of hard spheres. The intermediate scattering function of only the hard sphere particles showed double relaxation similar to the colloidal glass system for low volume fraction, where the fraction of hard sphere particles was small. For higher fractions we observed only a single stretched exponential. We could differentiate between slow and fast particles for both static and dynamic cages. For the case of static cages the hard spheres were permanently stuck inside the cages while for the case of dynamic cages almost all the hard spheres were moving in and out of the cages.

Robust and Biocompatible Functionalization of ZnS Nanoparticles by Catechol-Bearing Poly(2-methyl-2-oxazoline)s

Zinc sulfide (ZnS) nanoparticles (NPs) are particularly interesting materials for their electronic and luminescent properties. Unfortunately, their robust and stable functionalization and stabilization, especially in aqueous media, has represented a challenging and not yet completely accomplished task. In this work, we report the synthesis of colloidally stable, photoluminescent and biocompatible core-polymer shell ZnS and ZnS:Tb NPs by employing a water-in-oil miniemulsion (ME) process combined with surface functionalization via catechol-bearing poly-2-methyl-2-oxazoline (PMOXA) of various molar masses. The strong binding of catechol anchors to the metal cations of the ZnS surface, coupled with the high stability of PMOXA against chemical degradation, enable the formation of suspensions presenting excellent colloidal stability. This feature, combined with the assessed photoluminescence and biocompatibility, make these hybrid NPs suitable for optical bioimaging.

Molecular control over colloidal assembly

Contemporary chemical and material engineering often takes inspiration from nature, targeting for example strong yet light materials and materials composed of highly ordered domains at multiple different lengthscales for fundamental science and applications in e.g. sensing, catalysis, coating technology, and delivery. The preparation of such hierarchically structured functional materials through guided bottom-up assembly of synthetic building blocks requires a high level of control over their synthesis, interactions and assembly pathways. In this perspective we showcase recent work demonstrating how molecular control can be exploited to direct colloidal assembly into responsive materials with mechanical, optical or electrical properties that can be adjusted post-synthesis with external cues.

Lattice animals in diffusion limited binary colloidal system

In a soft matter system, controlling the structure of the amorphous materials has been a key challenge. In this work, we have modeled irreversible diffusion limited cluster aggregation of binary colloids, which serves as a model for chemical gels. Irreversible aggregation of binary colloidal particles leads to the formation of a percolating cluster of one species or both species which are also called bigels. Before the formation of the percolating cluster, the system forms a self-similar structure defined by a fractal dimension. For a one component system when the volume fraction is very small, the clusters are far apart from each other and the system has a fractal dimension of 1.8. Contrary to this, we will show that for the binary system, we observe the presence of lattice animals which has a fractal dimension of 2 irrespective of the volume fraction. When the clusters start inter-penetrating, we observe a fractal dimension of 2.5, which is the same as in the case of the one component system. We were also able to predict the formation of bigels using a simple inequality relation. We have also shown that the growth of clusters follows the kinetic equations introduced by Smoluchowski for diffusion limited cluster aggregation. We will also show that the chemical distance of a cluster in the flocculation regime will follow the same scaling law as predicted for the lattice animals. Further, we will also show that irreversible binary aggregation comes under the universality class of the percolation theory.

The mechanical properties of polymer-colloid hybrid hydrogels

The incorporation of monodisperse colloidal particles in hydrogels is a promising approach to create hybrid gels with unique structural, mechanical and functional properties. However, the colloidal structure formation within the hydrogels often remains uncontrolled, leaving behind possible mechanically synergetic effects of the polymeric and the colloidal system. Here we show that colloidal structure formation within the hybrid gels has a significant influence on the elasticity and toughness of the hybrid gels. We combine a polyacrylamide hydrogel with DNA coated colloids (DNAcc), where structure formation can be triggered independently at different points in time. Consequently, we are able to create hybrid gels that are composed of the same components, but do differ in explicit colloidal structure. While monodisperse colloids enhance the storage modulus of the gels, the yield strain is simultaneously drastically reduced. The toughness of these brittle hybrid gels is rescued by colloidal structure formation at higher polyacrylamide concentrations. The toughness is increased at lower polyacrylamide concentrations. We show that the toughness of the hydrogels at 10% (w/v) polyacrylamide and 4% (v/v) DNAcc can be increased by a factor of approx. 35, indicating that control over colloidal structure formation yields access to significant synergetic effects in polymer-colloid hybrid gels.

Colloidal joints with designed motion range and tunable joint flexibility

The miniaturization of machines towards the micron and nanoscale requires the development of joint-like elements that enable and constrain motion. We present a facile method to create colloidal joints, that is, anisotropic colloidal particles functionalized with surface mobile DNA linkers that control the motion range of bonded particles. We demonstrate quantitatively that we can control the flexibility of these colloidal joints by tuning the DNA linker concentration in the bond area. We show that the shape of the colloidal joint controls the range of motion of bonded particles through a maximisation of the bond area. Using spheres, cubes, and dumbbells, we experimentally realize spherical joints, planar sliders, and hinges, respectively. Finally we demonstrate the potential of the colloidal joints for programmable bottom-up self-assembly by creating flexible colloidal molecules and colloidal polymers. The reconfigurability and motion constraint offered by our colloidal joints make them promising building blocks for the development of switchable materials and nanorobots.