Cell-Mimic Directional Cargo Transportation in a Visible-Light-Activated Colloidal Motor/Lipid Tube System.

Active tether and transportation of cargoes on cytoskeletal highway enabled by molecular motors is key for accurate delivery of vesicles and organelles in the complex intracellular environment. Here, a hybrid system composed of colloidal motors and self-assembled lipid tubes is designed to mimic the subcellular traffic system in living cells. The colloidal motors, composed of gold-coated hematite, display light-activated self-propulsion tunable by the light intensity and the concentration of hydrogen peroxide fuel. Importantly, the motors show light-switchable binding with lipid cargoes and attachment to the lipid tubes, whereby the latter act as the motor highways. Upon assembly, the colloidal motor/lipid tube system demonstrates directional delivery of lipid vesicles, emulating intracellular transportation. The assembly and function of the hybrid system are rationalized by a cooperative action of light-triggered electrophoretic and hydrodynamic effects, supported by finite element analysis. A synthetic analog of the biological protein motor/cytoskeletal filament system is realized for the manipulation and delivery of different matter at the microscale, which is expected to be a promising platform for various applications in materials science, nanotechnology, microfluidics, and synthetic biology.

Active self-assembly of colloidal machines with passive rotational parts via coordination of phoresis and osmosis.

The organization of microscopic objects into specific structures with movable parts is a prerequisite for building sophisticated micromachines with complex functions, as exemplified by their macroscopic counterparts. Here we report the self-assembly of active and passive colloids into micromachinery with passive rotational parts. Depending on the attachment of the active colloid to a substrate, which varies the degrees of free freedom of the assembly, colloidal machines with rich internal rotational dynamics are realized. Energetic analysis reveals that the energy efficiency increases with the degrees of freedom of the machine. The experimental results can be rationalized by the cooperation of phoretic interaction and osmotic flow encoded in the shape of the active colloid, which site-specifically binds and exerts a torque to passive colloids, supported by finite element calculations and mesoscale simulations. Our work offers a new design principle that utilizes nonequilibrium interfacial phenomena for spontaneous construction of multiple-component reconfigurable micromachinery.

Ionic Effects in Ionic Diffusiophoresis in Chemically Driven Active Colloids.

We study experimentally the effect of added salt in the phoretic motion of chemically driven colloidal particles. We show that the response of passive colloids to a fixed active colloid, be it attractive or repulsive, depends on the ionic strength, the ζ potential, and the size of the passive colloids. We further report that the direction of self-propulsion of Janus colloids can be reversed by decreasing their ζ potential below a critical value. By constructing an effective model that treats the colloid and ions as a whole subjected to the concentration field of generated ions and takes into account the joint effect of both generated and background ions in determining the Debye length, we demonstrate that the response of the passive colloids and the velocity of the Janus colloids can be quantitatively captured by this model under the ionic diffusiophoresis theory beyond the infinitely-thin-double-layer limit.

Magnetic Manipulation and Assembly of Nonmagnetic Colloidal Rods in a Ferrofluid.

We investigated experimentally and theoretically the interactions and assembly of rodlike colloids in a ferrofluid confined at solid/liquid interface by the gravity under external magnetic fields. We first derived analytical expressions for the interaction energy of a single rod with the external magnetic field and the interaction between two rods using classical electromagnetism. The theory well captured the experimentally observed alignment of a single rod along the field direction under an in-plane field and switching between the horizontal and the vertical configurations in an out-of-plane field due to the competition between the magnetic energy and the gravitational energy. The theory can also predict the symmetric position fluctuations of a free rod on a fixed one at 90° and the gradual bias toward the end of the fixed rod as the angle was reduced to 0°, favoring the tip-toe arrangement. Finally, we showed that this anisotropic interaction led to the formation of chain-like structures, whose growth kinetics followed a simple scaling behavior with time. This work provides a theoretical framework for understanding the behaviors of rodlike colloids in ferrofluids and highlights the importance of shape anisotropy in manipulating colloids and their self-assembly.

Mechanistic understanding of nanoparticle interactions to achieve highly-ordered arrays through self-assembly for sensitive surface-enhanced Raman scattering detection of trace thiram.

Over the last few decades, there is increasing worldwide concern over human health risks associated with extensive use of pesticides in agriculture. Developing excellent SERS substrate materials to achieve highly sensitive detection of pesticide residues in the food is very necessary owing to their serious threat to human health through food chains. Self-assembled metallic nanoparticles have been demonstrated to be excellent SERS substrate materials. Hence, alkanethiols-protected gold nanoparticles have been successfully prepared for forming larger-scale two-dimensional monolayer films. These films can be disassembled into a fluid state and re-assembled back to crystallized structure by controlling surface pressure. Further investigations reveal that their self-assembled structures are mainly dependent on the diameter of gold nanoparticles and ligand length. These results suggest that the size ratio of nanoparticle diameter/ligand length within the range of 4.45-2.35 facilitates the formation of highly ordered 2D arrays. Furthermore, these arrays present excellent Surface-Enhanced Raman Scattering performances in the detection of trace thiram, which can cause environmental toxicity to the soil, water, animals and result in severe damage to human health. Therefore, the current study provides an effective way for preparing monodispersed hydrophobic gold nanoparticles and forming highly ordered 2D close-packed SERS substrate materials via self-assembly to detect pesticide residues in food. We believe that, our research provides not only advanced SERS substrate materials for excellent detection performance of thiram in food, but also novel fundamental understandings of self-assembly, manipulation of nanoparticle interactions, and controllable synthesis.

High salinity effects on the depletion attraction in colloid-polymer mixtures.

HYPOTHESIS: Depletion attraction induced by polymers can be employed across multiple disciplines. Previous studies implied that besides screening the electrostatic repulsion between colloids, adding salt may also affect the polymers even in a nonpolar solvent. Here, we study the depletion-induced gelation of a colloid-polymer model system, focusing on the salt effects on the depletion attraction. EXPERIMENTS: Confocal microscopy was used to quantitatively characterize the colloidal gels formed by the polymethylmethacrylate (PMMA)/polystyrene (PS) model system. The attraction experienced by colloids was estimated by correlating the colloidal dynamics with the local structure. Correspondingly, static light scattering was employed to systematically investigate the polymers. The resulting radius of gyration Rg and osmotic pressure were used to evaluate the depletion attraction offered by polymers. FINDINGS: Salt was discovered to lower the strength of inter-particle attraction, which can be attributed to the salt-induced decrease in Rg. The depletion attraction grew sublinearly with c, owing to the considerable decrease in Rg in the good solvent as c increased. We demonstrated how the close form equations in the framework of renormalization group theory can be employed to predict the depletion interaction using the properly determined zero-concentration radius of gyration.

Corrigendum: Self Organization of Binary Colloidal Mixtures via Diffusiohporesis.

[This corrects the article DOI: 10.3389/fchem.2022.803906.].

Self-Organization of Binary Colloidal Mixtures via Diffusiophoresis.

Catalytic activity of the colloids and chemotactic response to gradients of the chemicals in the solution leads to effective interaction between catalytic colloids. In this paper, we simulate mixtures of active and passive colloids via a Brownian dynamics algorithm. These particles interact via phoretic interactions, which are determined by two independent parameters, surface activity and surface mobility. We find rich dynamic structures by tuning passive colloids' surface mobility, size, and area fractions, which include schools of active colloids with exclusion zone, yolk/shell cluster, and stable active-passive alloys to motile clusters. Dynamical cluster can also be formed due to the nonreciprocity of the phoretic interaction. Increasing the size ratio of passive colloids to active colloids favors the phase separation of active and passive colloids, resulting in yolk/shell structure. Increasing the area fraction of active colloids tends to transfer from dynamical clusters into stable alloys. The simulated binary active colloid systems exhibit intriguing nonequilibrium phenomena that mimic the dynamic organizations of active/passive systems.

Estimating the velocity of chemically-driven Janus colloids considering the anisotropic concentration field.

The application of the active colloids is strongly related to their self-propulsion velocity, which is controlled by the generated anisotropic concentration field. We investigated the effect of this anisotropy on velocity induced by numerical treatments and size of Janus colloids. The far-field approximation is effective in estimating the velocity, even though it neglects the shape effect on the anisotropy of the concentration field. If the surface mobility contrast between the active and the inert part is moderate, the spherical approximation is feasible for sphere-like Janus colloids. Legendre expansion of the concentration field causes artificial anisotropy. Raising the order of the expansion can suppress this effect, but also distorts the concentration field at the top of active part. Thus, the order of the expansion should be chosen carefully depending on the goal of the study. Based on the verified Legendre expansion method and ionic-diffusiophoresis model, we show that due to the size-effect on both the concentration field and the surface mobility, increasing size of colloids can lower the self-propulsion velocity. Our finding is consistent with previous experimental observations without fitting parameter, shedding new light on the self-propulsion mechanism of chemically-driven active colloids. We further show a velocity reversal at high overall ζ potential induced by increasing size, providing a new way for controlling the dynamics of acitve colloids.

Highly efficient chemically-driven micromotors with controlled snowman-like morphology.

We report the synthesis of silver-based Janus micromotors that self-propel at 3.5 µm s-1 and speed up to 45 µm s-1 in 0.044 and 1.5 mM of H2O2, respectively, via ionic diffusiophoresis. Morphology optimization further accelerates the speed to 90 µm s-1, which leads to a force of 1 pN and a power of 0.1 fW, similar to biomolecular motors. Their efficiency reaches 10-5, at least two orders of magnitude higher than other chemically-driven micromotors. These micromotors hold great promises in various applications.

Design and One-Pot Synthesis of Capsid-like Gold Colloids with Tunable Surface Roughness and Their Enhanced Sensing and Catalytic Performances.

Viral capsid-like particles tiled with mosaic patches have attracted great attention as they imitate nature's design to achieve advanced material properties and functions. Here, we develop a facile one-pot soft-template method to synthesize biomimetic gold capsid-like colloids with tunable particle size and surface roughness. Uniform submicron-to-micron-sized hollow gold colloidal particles are successfully achieved by using tannic acids as soft templates and reducing agents, which first self-assemble into spherical complex templates before the reduction of Au3+ ions via their surface hydroxyl groups. The surface roughness, the size, and the total number of the patches of the prepared gold particles are further tuned, utilizing a mechanism that offers morphology control by varying the number of surface hydroxyl groups participating in the reduction reactions. Among different capsid-like gold colloids, those possessing a rough surface display superior catalytic properties and show promising results as surface-enhanced Raman spectroscopy (SERS) solid substrates for detecting small organic molecules and biomimetic enzymes in a liquid phase for sensing biomolecules in real samples. These capsid-like gold colloids are also expected to find practical applications in delivery systems, electronics, and optics. We believe that our strategy of imitating nature's design of capsid-like structures should also be used in the design and fabrication of other functional colloidal particles.

Phase diagram of the Pluronic L64-H2O micellar system from mechanical spectroscopy.

The linear viscoelastic properties of aqueous Pluronic L64 solutions have been investigated at high copolymer concentrations (25-62 wt%) using our modified low-frequency mechanical spectroscopy. The concentration-temperature phase diagram of the L64/H(2)O system was constructed by studying the evolution of the loss modulus and loss tangent as temperature is increased at a fixed frequency. A particular attention was focused on the dynamics approaching the beginning and ending points (39% and 60%) of the fusiform gel region in the phase diagram. The dynamics is found to have a similar viscoelastic behavior at the low and high concentrations, where a frequency scaling expected for a static percolated network is exhibited. Moreover, with increasing temperature, the system above the critical gel concentration undergoes a transition from a viscoelastic liquid to a solid gel through a percolated particle network. Therefore, our results suggest that the formation of the gel is dominated by the percolation of the particle clusters.