Reconfigurable assembly of colloidal motors towards interactive soft materials and systems

Arbitrary Construction of Versatile NIR-Driven Microrobots

Emerging light-driven micro/nanorobots (LMNRs) showcase profound potential for sophisticated manipulation and various applications. However, the realization of a versatile and straightforward fabrication technique remains a challenging pursuit. This study introduces an innovative bulk heterojunction organic semiconductor solar cell (OSC)-based spin-coating approach, aiming to facilitate the arbitrary construction of LMNRs. Leveraging the distinctive properties of a near-infrared (NIR)-responsive organic semiconductor heterojunction solution, this technique enables uniform coating across various dimensional structures (0D, 1D, 2D, 3D) to be LMNRs, denoted as "motorization." The film, with a slender profile measuring ≈140 nm in thickness, effectively preserves the original morphology of objects while imparting actuation capabilities exceeding hundreds of times their own weight. The propelled motion of these microrobots is realized through NIR-driven photoelectrochemical reaction-induced self-diffusiophoresis, showcasing a versatile array of controllable motion profiles. The strategic customization of arbitrary microrobot construction addresses specific applications, ranging from 0D microrobots inducing living crystal formation to intricate, multidimensional structures designed for tasks such as microplastic extraction, cargo delivery, and phototactic precise maneuvers. This study advances user-friendly and versatile LMNR technologies, unlocking new possibilities for various applications, signaling a transformative era in multifunctional micro/nanorobot technologies.

A Clinically Feasible Diagnostic Electrochemical Micronano Motors Biosensor Built on Miniature Swimmer for Multiplex Detection and Grading of Breast Cancer Biomarkers

Estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor-2 (HER2), and Ki67 are four crucial biomarkers used in the clinical diagnosis of breast cancer. Accurate detection of these biomarkers is essential for an effective diagnosis and treatment. MOF-based micronano motors (MOFtors) are promising for various applications, including environmental remediation, targeted nanosurgery, and biomarker detection. This paper presents a clinically feasible diagnostic electrochemical micronano motor biosensor, built on a miniature swimmer, for the multiplex detection and grading of breast cancer biomarkers. We designed a biosensor, named MOFtor-MSEM, incorporating aptamers and antibodies functionalized on SiO2@Co-Fe-MOF, which acts as a miniature swimmer in solution. The SiO2@Co-Fe-MOF serves as the body, while complementary double-chain-linked antibodies function as paddles. In a homogeneous solution, when a positive voltage is applied to the working electrode, the electrostatic interaction between the neutral SiO2@Co-Fe-MOF and the negatively charged complementary double-linked antibody causes the antibody to move toward the electrode and then regress due to water resistance. This back-and-forth motion propels the miniature swimmer, enabling it to move the target analyte through the solution. The sensor features an automatic "sample-amplifying signal-output" process, achieving simultaneous signal amplification and output of four electrochemical signals on a single nanomaterial, a significant challenge in electrochemical sensing. The biosensor boasts a short detection time of 40 min, compared to approximately 1 week for current clinical tissue testing. Additionally, the bioplatform selectively detects HER2, ER, Ki67, and PR in the range of 0-1500 pg/mL, with detection limits of 0.01420, 0.03201, 0.01430, and 0.01229 pg/mL, respectively.

Self-propelled object that generates a boundary with amphiphiles at an air/aqueous interface

A benzoic acid (BA) disk was investigated as a novel self-propelled object whose driving force was the difference in surface tension. 4-Stearoyl amidobenzoic acid (SABA) was synthesized as an amphiphile to control the nature of motion based on intermolecular interactions between BA and SABA. The BA disk exhibited characteristic motion depending on the surface density of the SABA on the aqueous phase, that is, reciprocating motion as a one-dimensional motion and restricted and unrestricted motion as a two-dimensional motion. The trajectory of the reciprocating motion was determined by the initial direction of motion, and the boundary between an aqueous surface and the BA-SABA condensed molecular layer was used as the field's boundary. The presented results indicate that the characteristic nature of motion can be designed at the molecular level based on the intermolecular interactions between an energy-source molecule and an amphiphile.

Reconfigurable Assembly of Planar Colloidal Molecules via Chemical Reaction and Electric Polarization

Colloidal molecules, ordered structures assembled from micro- and nanoparticles, serve as a valuable model for understanding the behavior of real molecules and for constructing materials with tunable properties. In this work, we introduce a universal strategy for assembling colloidal molecules consisting of a central active particle surrounded by several passive particles as ligands. During the assembly process, active particles attract the surrounding passive particles through phoresis and osmosis resulting from the chemical reactions on the surface of the active particles, while passive particles repel each other due to the electric polarization induced by an alternating current (AC) electric field. By carefully selecting particles of varying structures and sizes, we have assembled colloidal molecules of symmetric and asymmetric dimers, trimers, and multimers. Furthermore, the coordination number of these colloidal molecules can be regulated in real time and in situ by tuning the interaction forces between the constituent particles. Brownian dynamics simulations reproduced the formation of the colloidal molecules and validated that the self-assembly arises from chemically induced attraction and electrical dipolar repulsion. This strategy for reconfigurable colloidal assemblies poses the potential for designing adaptive micro-nanomachines.

Open Questions of Chemically Powered Nano- and Micromotors

Chemically powered nano- and micromotors are microscopic devices that convert chemical energy into motion. Interest in these motors has grown over the past 20 years because they exhibit interesting collective behaviors and have found potential uses in biomedical and environmental applications. Understanding how these motors operate both individually and collectively and how environments affect their operation is of both fundamental and applied significance. However, there are still significant gaps in our knowledge. This Perspective highlights several open questions regarding the propulsion mechanisms of, interactions among, and impact of confinements on nano- and micromotors driven by self-generated chemical gradients. These questions are based on my own experience as an experimentalist. For each open question, I describe the problem and its significance, analyze the status-quo, identify the bottleneck problem, and propose potential solutions. An underlying theme for these questions is the interplay among reaction kinetics, physicochemical distributions, and fluid flows. Unraveling this interplay requires careful measurements as well as a close collaboration between experimentalists and theoreticians/numerical experts. The interdisciplinary nature of these challenges suggests that their solutions could bring new revelations and opportunities across disciplines such as colloidal sciences, material sciences, soft matter physics, robotics, and beyond.

Motion of magnetic motors across liquid-liquid interface

HYPOTHESIS

In a number of applications related to chemical engineering and drug delivery, magnetic nanoparticles should move through a liquid-liquid interface in the presence of surfactant molecules. However, due to the action of capillary forces, this is not always possible. The mechanism of particle motion through the interface essentially depends on the intensity of the Marangoni flow, which is induced on the interface during its deformation.

EXPERIMENTS

In this paper we study the motion of nanoparticles Fe3O4 through the water-tridecane interface under the action of a nonuniform magnetic field when using different surfactants.

FINDINGS

If the linear size of the magnetic motor turns out to be less than a certain critical value, then it is not able to move between phases due to the action of capillary forces on the interface. Depending on the type and concentration of the surfactant used, various mechanisms for the motor motion through the liquid-liquid interface can be carried out. In one of them, a liquid phase is transferred through the interface along with a movable motor, while in the other, it is not.

Spatial Patterning of Micromotor Aggregation and Flux

Using a spatially varying light pattern with light activated semi-conductor based magnetic TiO2 micromotors, we study the difference in micromotor flux between illuminated and non-illuminated regions in the presence and absence of an applied magnetic field. We find that the magnetic field enhances the flux of the motors which we attribute to a straightening of the micromotor trajectories which decreases the time they spend in the illuminated region. We also demonstrate spatially patterned light-induced aggregation of the micromotors and study its time evolution at various micromotor concentrations. Although light induced aggregation has been observed previously, spatial patterning of aggregation demonstrates a further means of control which could be relevant to swarm control or self-assembly applications. Overall, these results draw attention to the effect of trajectory shape on the flux of active colloids as well as the concentration dependence of aggregation and its time dependence within a spatially patterned region, which is not only pertinent to self-assembly and swarm control, but also provides insight into the behavior of active matter systems with spatially varying activity levels.

Recursively positive and negative chemotaxis coupling with reaction kinetics in self-organized inanimate motion

Reconstructing recursive chemotaxis in inanimate self-propelled objects is inevitable in the development of recursively and autonomously artificial mass transport systems. However, the fabrication of inanimately recursive chemotaxis has been extremely challenging because of the difficulty in introducing competitive positive and negative feedback into an inanimate self-propelled object. Herein, a coumarin derivative (coumarin, 4-methylcoumarin (4-MC), or 6-methylcoumarin (6-MC))-based disk floated on water as a self-propelled object exhibited characteristic features of motion; these features include continuous motion, repetition between positive and negative chemotaxis to the Na₃PO₄ powder as a base stimulus, and oscillatory motion above the Na₃PO₄ powder depending on the Na₃PO₄ density of the powder and the functional group of coumarin derivatives. The mechanism of the characteristic features of motion to the base stimulus is discussed in relation to the surface tension of the coumarin derivatives as the driving force of motion and the reaction rate of the hydrolysis between coumarin derivatives and OH^- obtained from Na₃PO₄. This study suggests a novel avenue for developing a recursive chemotactic system coupled with reaction kinetics in self-organized motion.

Height-dependent oscillatory motion of a plastic cup with a camphor disk floated on water

We have developed a self-propelled object, which is composed of a plastic cup and a camphor disk, on water to reflect its three-dimensional shape in the nature of motion. The self-propelled object, of which the driving force of motion is the difference in the surface tension, exhibited oscillatory motion between motion and rest. The period and the maximum speed of oscillatory motion increased and decreased depending on the height of the cup, h, respectively. Two types of diffusion coefficients were estimated based on the diffusion of camphor molecules which were indirectly visualized using 7-hydroxycoumarin. The experimental result on the period of oscillatory motion depending on h could be reproduced by the numerical calculation based on the diffusion of camphor molecules around the object and the diffusion coefficients which were experimentally estimated. The experimental results suggest that characteristic features of motion can be created based on the three-dimensional shape of the object.

Electric and Magnetic Field-Driven Dynamic Structuring for Smart Functional Devices

The field of soft matter is rapidly growing and pushing the limits of conventional materials science and engineering. Soft matter refers to materials that are easily deformed by thermal fluctuations and external forces, allowing for better adaptation and interaction with the environment. This has opened up opportunities for applications such as stretchable electronics, soft robotics, and microfluidics. In particular, soft matter plays a crucial role in microfluidics, where viscous forces at the microscale pose a challenge to controlling dynamic material behavior and operating functional devices. Field-driven active colloidal systems are a promising model system for building smart functional devices, where dispersed colloidal particles can be activated and controlled by external fields such as magnetic and electric fields. This review focuses on building smart functional devices from field-driven collective patterns, specifically the dynamic structuring of hierarchically ordered structures. These structures self-organize from colloidal building blocks and exhibit reconfigurable collective patterns that can implement smart functions such as shape shifting and self-healing. The review clarifies the basic mechanisms of field-driven particle dynamic behaviors and how particle-particle interactions determine the collective patterns of dynamic structures. Finally, the review concludes by highlighting representative application areas and future directions.

Nanoparticle Emulsions Enhance the Inhibition of NLRP3

Antibacterial delivery emulsions are potential materials for treating bacterial infections. Few studies have focused on the role and mechanism of emulsions in inflammation relief. Therefore, based on our previous analysis, in which the novel and natural Pickering emulsions stabilized by antimicrobial peptide nanoparticles were prepared, the regulation effect of emulsion on inflammasome was explored in silico, in vitro and in vivo. Firstly, the interactions between inflammasome components and parasin I or Pickering emulsion were predicted by molecular docking. Then, the inflammasome stimulation by different doses of the emulsion was tested in RAW 264.7 and THP-1 cells. Finally, in Kunming mice with peritonitis, NLRP3 and IL-1β expression in the peritoneum were evaluated. The results showed that the Pickering emulsion could combine with ALK, casp-1, NEK7, or NLRP3 to affect the assembly of the NLRP3 and further relieve inflammation. LPNE showed a dose–dependent inhibition effect on the release of IL-1β and casp-1. With the concentration of parasin I increased from 1.5 mg/mL to 3 mg/mL, the LDH activity decreased in the chitosan peptide-embedded nanoparticles emulsion (CPENE) and lipid/peptide nanoparticles emulsion (LPNE) groups. However, from 1.5 to 6 mg/mL, LPNE had a dose–dependent effect on the release of casp-1. The CPENE and parasin I-conjugated chitosan nanoparticles emulsion (PCNE) may decrease the release of potassium and chloride ions. Therefore, it can be concluded that the LPNE may inhibit the activation of the inflammasome by decreasing LDH activity, potassium and chloride ions through binding with compositions of NLRP3.

Propulsion, deformation, and confinement response of hollow nanocellulose millimotors

HYPOTHESIS

Micromotor and nanomotor particles are typically made using dense solid particles that can sediment or be trapped in confined flow environments. Creation of much larger motors should be possible if a very low-density system is used with sufficient strength to carry liquid and still experience propulsive motion. Light, dense millimotors should also be able to deform more than dense solid ones in constrictions.

EXPERIMENTS

Millimotors are created from permeable capsules of bacterial cellulose that are coated with catalse-containing metal-organic frameworks, enabling reactive propulsion in aqueous hydrogen peroxide. The motion of the motors is quantified using particle tracking and the deformation is measured using microcapillary compression and flow through confined channels.

FINDINGS

Two different propulsion mechanisms are dominant depending on the motor surface chemistry: oxygen bubbles are expelled from hydrophilic millimotors, driving motion via recoil force and buoyancy. Hydrophobic millimotors remain attached to growing bubbles and move by buoyancy alone. Despite their large size, the low-density capsules compress to pass through contractions that would impede and be blocked by solid motors. The sparse structure but relatively large size of the motors enables them to transport significant volumes of liquid using minimal solid mass as a motor support structure.

Gas generation due to photocatalysis as a method to reduce the resistance force in the process of motors motion at the air-liquid interface

HYPOTHESIS

The problem of the development of miniature motors able to move on the air-liquid interface at low Reynolds numbers is a crucial challenge due to dominating role of viscous force. To solve this problem the chemical generation of gas can be used. Generated gas pushes liquid out from the surfer surface, so the resistance force is reduced.

EXPERIMENTS

Surfer composed of TiO₂ nanoparticles and ferromagnetic cobalt microparticles moves at the interface of an aqueous solution of hydrogen peroxide under the action of magnetic force. After irradiation with UV or visible light, the gas cavern is formed at the surfer surface due to photo-catalytic decomposition of hydrogen peroxide. As a result, the area of surfer contact with liquid is reduced.

FINDINGS

The resistance force acting on the surfer is reduced due to the liquid pushing out from the surfer surface. This effect is strengthened with the increase in the intensity of gas generation. The resistance force is increased when increasing the liquid viscosity or using a surfactant. The proposed method allows control of the velocity of the motors in a rather wide range by changing the gradient of the magnetic field and parameters of light.