Reconfiguring Self-Assembly of Photoresponsive Hybrid Colloids

Chemically active colloidal superstructures

Mimicking biological systems, artificial active colloidal motors that continuously dissipate energy can dynamically self-assemble to form active colloidal superstructures with specific spatial configurations and complex functionalities, which offers a promising pathway for developing new active soft matter materials with adaptability, self-repair, and reconfigurability. Beyond merely propelling their own motion, chemically driven colloidal motors can also induce phoretic effects and osmotic flows to affect the motion of neighboring colloidal motors through local fluid fields generated by chemical reactions, thereby achieving spontaneous chemical communication and promoting dynamic self-assembly between motors. This review summarizes the latest progress in the dynamic self-assembly of chemically driven colloidal motors, ranging from single chemically driven colloidal motors to chemically driven colloidal motors with passive colloidal particles and then to different chemically driven colloidal motors, ultimately forming active colloidal superstructures with complex dynamic behaviors. Not only are the interactions between chemically driven colloidal motors with different self-propulsion mechanisms and passive colloidal particles focused on, but also the communication behaviors between chemically driven colloidal motors are explored. We explain the fundamental physicochemical mechanisms that regulate the assembly behavior of chemically driven colloidal motors, propose general strategies for the controlled construction of active colloidal superstructures, and discuss the potential applications that may emerge from the directed dynamic self-assembly of these superstructures.

Preparation and assembly of SiO2@TiO2 photoresponsive colloidal rings

We report the synthesis of SiO2 colloidal rings coated with TiO2, which enable photoresponsive self-assembly into porous ordered structures and hybrid assembly for the capture and release of non-active colloids in a hydrogen peroxide (H2O2) solution. Such assemblies originate from a concentration gradient generated by the TiO2-mediated decomposition of H2O2 under ultraviolet (UV) irradiation. The work provides a straightforward strategy to fabricate active colloids with ring-like shapes for the controlled assembly of open crystalline colloidal materials.

Shape-Directed Dynamic Assembly of Active Colloidal Metamachines

Modularly organizing active micromachines into high-grade metamachines makes a great leap for operating the microscopic world in a biomimetic way. However, modulating the nonreciprocal interactions among different colloidal motors through chemical reactions to achieve the controllable construction of active colloidal metamachines with specific dynamic properties remains challenging. Here, we report the phototactic active colloidal metamachines constructed by shape-directed dynamic self-assembly of chemically driven peanut-shaped TiO2 colloidal motors and Janus spherical Pt/SiO2 colloidal motors. The long-range diffusiophoretic attraction generated by the photocatalytic reaction dominates the sensing and collision of peanut TiO2 motors with Janus Pt/SiO2 motors. The coupling of local chemical concentration gradient fields between the two types of motors generates short-range site-selective interactions, promoting the shape-directed assembly toward active colloidal metamachines with well-defined spatial configurations. Metamachines, made of colloidal motors, exhibit configuration-dependent kinematics. The colloidal metamachines can be reversibly reconstructed by adjusting lighting conditions and can move phototactically along a predetermined path under the structured light field. Such chemically driven colloidal metamachines that integrate multiple active agents provide a significant avenue for fabricating active soft matter materials and intelligent robotic systems with advanced applications.

Siamese neural network improves the performance of a convolutional neural network in colloidal self-assembly state classification

Identifying the state of the colloidal self-assembly process is critical to monitoring and controlling the system into desired configurations. Recent application of convolutional neural networks with unsupervised clustering has shown a comparable performance to conventional approaches, in representing and classifying the states of a simulated 2D colloidal batch assembly system. Despite the early success, capturing the subtle differences among similar configurations still presents a challenge. To address this issue, we leverage a Siamese neural network to improve the accuracy of the state classification. Results from a Brownian dynamics-simulated electric field-mediated colloidal self-assembly system and a magnetic field-mediated colloidal self-assembly system demonstrate significant improvement from the original convolutional neural network-based approach. We anticipate the proposed improvement to further pave the way for automated monitoring and control of colloidal self-assembly processes in real time and real space.

In-situ isomerization and reversible self-assembly of photoresponsive polymeric colloidal molecules enabled by ON and OFF light control

Photocatalytic colloids enable light-triggered nonequilibrium interactions and are emerging as key components for the self-assembly of colloidal molecules (CMs) out of equilibrium. However, the material choices have largely been limited to inorganic substances and the potential for reconfiguring structures through dynamic light control remains underexplored, despite light being a convenient handle for tuning nonequilibrium interactions. Here, we introduce photoresponsive N,O-containing covalent organic polymer (NOCOP) colloids, which display multi-wavelength triggered fluorescence and switchable diffusiophoretic interactions with the addition of triethanolamine. Our system can form various flexible structures, including ABn-type molecules and linear chains. By varying the relative sizes of active to passive colloids, we significantly increase the structural diversity of A2B2-type molecules. Most importantly, we demonstrate in-situ transitions between different isomeric configurations and the reversible assembly of various structures, enabled by on-demand light ON and OFF control of diffusiophoretic interactions. Our work introduces a new photoresponsive colloidal system and a novel strategy for constructing and reconfiguring colloidal assemblies, with promising applications in microrobotics, optical devices, and smart materials.

Advanced materials for micro/nanorobotics

Autonomous micro/nanorobots capable of performing programmed missions are at the forefront of next-generation micromachinery. These small robotic systems are predominantly constructed using functional components sourced from micro- and nanoscale materials; therefore, combining them with various advanced materials represents a pivotal direction toward achieving a higher level of intelligence and multifunctionality. This review provides a comprehensive overview of advanced materials for innovative micro/nanorobotics, focusing on the five families of materials that have witnessed the most rapid advancements over the last decade: two-dimensional materials, metal-organic frameworks, semiconductors, polymers, and biological cells. Their unique physicochemical, mechanical, optical, and biological properties have been integrated into micro/nanorobots to achieve greater maneuverability, programmability, intelligence, and multifunctionality in collective behaviors. The design and fabrication methods for hybrid robotic systems are discussed based on the material categories. In addition, their promising potential for powering motion and/or (multi-)functionality is described and the fundamental principles underlying them are explained. Finally, their extensive use in a variety of applications, including environmental remediation, (bio)sensing, therapeutics, etc., and remaining challenges and perspectives for future research are discussed.

Light-Driven Microrobots for Targeted Drug Delivery

As a new micromanipulation tool with the advantages of small size, flexible movement and easy manipulation, light-driven microrobots have a wide range of prospects in biomedical fields such as drug targeting and cell manipulation. Recently, microrobots have been controlled in various ways, and light field has become a research hotspot by its advantages of noncontact manipulation, precise localization, fast response, and biocompatibility. It utilizes the force or deformation generated by the light field to precisely control the microrobot, and combines with the drug release technology to realize the targeted drug application. Therefore, this paper provides an overview of light-driven microrobots with drug targeting to provide new ideas for the manipulation of microrobots. Here, this paper briefly categorizes the driving mechanisms and materials of light-driven microrobots, which mainly include photothermal, photochemical, and biological. Then, typical designs of light-driven microrobots with different driving mechanisms and control strategies for multiple physical fields are summarized. Finally, the applications of microrobots in the fields of drug targeting and bioimaging are presented as well as the future prospects of light-driven microrobots in the biomedical field are demonstrated.

Synthesis and Assembly of Photoresponsive Colloidal Tubes

Inspired by the sophisticated multicomponent and multistage assembly of proteins and their mixtures in living cells, this study rationally designs and fabricates photoresponsive colloidal tubes that can self-assemble and hybrid-assemble when mixed with colloidal spheres and rods. Time-resolved observation and computer simulation reveal that the assembly is driven by phoretic attraction originating from osmotic pressures. These pressures are induced by the chemical concentration gradients generated by the photochemical reaction caused by colloidal tubes in a H2O2 solution under ultraviolet (UV) irradiation. The assembled structure is dictated by the size and shape of the constituent colloids as well as the intensity of the UV irradiation. Additionally, the resulting assembly can undergo self-propelled motion originating from the broken symmetry of the surrounding concentration gradients. This motion can be steered by a magnetic field and used for microscale cargo delivery. The study demonstrates a facile synthesis method for colloidal tubes and highlights their unique potential for controlled, hierarchical self-assembly and hybrid-assembly.

Spearheading a new era in complex colloid synthesis with TPM and other silanes

Colloid science has recently grown substantially owing to the innovative use of silane coupling agents (SCAs), especially 3-trimethoxysilylpropyl methacrylate (TPM). SCAs were previously used mainly as modifying agents, but their ability to form droplets and condense onto pre-existing structures has enabled their use as a versatile and powerful tool to create novel anisotropic colloids with increasing complexity. In this Review, we highlight the advances in complex colloid synthesis facilitated by the use of TPM and show how this has driven remarkable new applications. The focus is on TPM as the current state-of-the-art in colloid science, but we also discuss other silanes and their potential to make an impact. We outline the remarkable properties of TPM colloids and their synthesis strategies, and discuss areas of soft matter science that have benefited from TPM and other SCAs.

Uniform, convex structuring of polymeric colloids via site-selected swelling

Non-spherical, polymeric colloids serve as building blocks for advanced functional materials. We propose a novel method to produce morphologically controlled, non-spherical particles by generating site-selected, convex structures on polystyrene (PS) particles. It consists of two simple procedures: a monolayer of PS particles is illuminated with UV light and is subsequently immersed in a fluorinated solvent (HFIP). UV irradiation generates site-selected, oxidized domains on PS particles with a different solvent affinity than unoxidized PS, and HFIP immersion preferentially swells the oxidized domains. Such swelling gives rise to site-selected, convex structures on PS particles. By adjusting UV irradiation conditions, including incident and azimuth angles, the oxidized sites, i.e., the swelled portions, can be accurately situated, allowing us to produce various convex shapes, including chiral shapes at desired positions on PS particles.

Reconfigurable self-assembly of photocatalytic magnetic microrobots for water purification

The development of artificial small-scale robotic swarms with nature-mimicking collective behaviors represents the frontier of research in robotics. While microrobot swarming under magnetic manipulation has been extensively explored, light-induced self-organization of micro- and nanorobots is still challenging. This study demonstrates the interaction-controlled, reconfigurable, reversible, and active self-assembly of TiO2/α-Fe2O3 microrobots, consisting of peanut-shaped α-Fe2O3 (hematite) microparticles synthesized by a hydrothermal method and covered with a thin layer of TiO2 by atomic layer deposition (ALD). Due to their photocatalytic and ferromagnetic properties, microrobots autonomously move in water under light irradiation, while a magnetic field precisely controls their direction. In the presence of H2O2 fuel, concentration gradients around the illuminated microrobots result in mutual attraction by phoretic interactions, inducing their spontaneous organization into self-propelled clusters. In the dark, clusters reversibly reconfigure into microchains where microrobots are aligned due to magnetic dipole-dipole interactions. Microrobots' active motion and photocatalytic properties were investigated for water remediation from pesticides, obtaining the rapid degradation of the extensively used, persistent, and hazardous herbicide 2,4-Dichlorophenoxyacetic acid (2,4D). This study potentially impacts the realization of future intelligent adaptive metamachines and the application of light-powered self-propelled micro- and nanomotors toward the degradation of persistent organic pollutants (POPs) or micro- and nanoplastics.

Shape-Dictated Self-Assembly of Photoresponsive Hybrid Colloids

The shape-dictated self-assembly of hybrid colloids induced by chemical concentration gradients generated by photocatalytic reactions of the colloids is studied. Different shapes enable the formation of assemblies with distinct lattice structures including hexagons, distorted hexagons, and squares, which are corroborated by computer simulations. Furthermore, assemblies change from lattices to chains when increasing the attraction between the colloids. The results show that photoresponsive hybrid colloids possess a unique capability for shape-dependent self-assembly, offering a practical and versatile approach to manipulate self-assembly at the microscale.

Advances in Colloidal Building Blocks: Toward Patchy Colloidal Clusters

The scalable synthetic route to colloidal atoms has significantly advanced over the past two decades. Recently, colloidal clusters with DNA-coated cores called "patchy colloidal clusters" have been developed, providing a directional bonding with specific angle of rotation due to the shape complementarity between colloidal clusters. Through a DNA-mediated interlocking process, they are directly assembled into low-coordination colloidal structures, such as cubic diamond lattices. Herein, the significant progress in recent years in the synthesis of patchy colloidal clusters and their assembly in experiments and simulations is reviewed. Furthermore, an outlook is given on the emerging approaches to the patchy colloidal clusters and their potential applications in photonic crystals, metamaterials, topological photonic insulators, and separation membranes.

Highly sensitive and simultaneous detection of ctDNAs related to non-small cell lung cancer in serum using a catalytic hairpin assembly strategy in a SERS microfluidic chip

Circulating tumor DNA (ctDNA) is an ideal biomarker for cancer diagnosis based on liquid biopsy, so there is an urgent need for developing an efficient, rapid, and ultrasensitive detection method to meet clinical needs. In this paper, a novel surface-enhanced Raman scattering (SERS) microfluidic chip combined with a catalytic hairpin assembly (CHA) was proposed to detect two non-small cell lung cancer (NSCLC)-related ctDNA (TP53 and PIK3CA-Q546K) simultaneously. The chip consists of six channels for parallel detection. In the reaction region, the CHA reaction between HP₁ of the SERS probe and HP₂ of the capture substrate was triggered by ctDNAs to form HP₁-HP₂ duplexes. As the reaction proceeds, more and more SERS probes are captured on the substrate. The gathered reaction products continuously form a lot of hot spots, which greatly enhance the SERS signal. This reaction was completed within 5 minutes. Through this method, the detection limits of TP53 and PIK3CA-Q546K in human serum were as low as 2.26 aM and 2.34 aM, respectively. The microfluidic chip also exhibited high specificity, reproducibility and stability. The clinical feasibility of the SERS microfluidic chip was verified by analyzing the serum samples of healthy subjects and NSCLC patients. The reliability of the experimental results was verified by the qRT-PCR test. The constructed SERS-based analytical micro-platform has great potential in dynamic monitoring of cancer staging and could be used as a clinical tool for early cancer screening.

Chain Assembly Kinetics from Magnetic Colloidal Spheres

Magnetic colloidal chains are a microrobotic system with promising applications due to their versatility, biocompatibility, and ease of manipulation under magnetic fields. Their synthesis involves kinetic pathways that control chain quality, length, and flexibility, a process performed by first aligning superparamagnetic particles under a one-dimensional magnetic field and then chemically linking them using a four-armed maleimide-functionalized poly(ethylene glycol). Here, we systematically vary the concentration of the poly(ethylene glycol) linkers, the reaction temperature, and the magnetic field strength to study their impact on the physical properties of synthesized chains, including the chain length distribution, reaction temperature, and bending modulus. We find that this chain fabrication process resembles step-growth polymerization and can be accurately described by the Flory-Schulz model. Under optimized experimental conditions, we have successfully synthesized long flexible colloidal chains with a bending modulus, which is 4 orders of magnitude smaller than previous studies. Such flexible and long chains can be folded entirely into concentric rings and helices with multiple turns, demonstrating the potential for investigating the actuation, assembly, and folding behaviors of these colloidal polymer analogues.