Preparations And Applications Study Of External-field Responsive Active Colloid

Active colloids are a new class of colloidal systems that can utilize energy from the surrounding environment to achieve self-driven motion.Due to the advantages of controlled motion,rich assembly behavior,and the possibility of diverse functional modifications,active colloids not only have a wide range of application prospects in biological,material,and environmental fields,but also can be used as physical models to study important topics in condensed matter physics such as phase transitions and assemblies.The external-field responsive active colloids are a hot spot for research,mainly because these active colloids can be remotely manipulated by external fields to achieve real-time response and for in situ tracking studies,and thus have great advantages in applications.In recent years,progresses have been made in the design,preparation and modification of external-field responsive active colloids.In addition,the applications of these active colloids in two major fields,condensed matter physics and functional materials,have been explored.However,there is still a lack of:(a)active colloids suitable for constructing model experimental systems,(b)sufficient exploration of active colloids for condensed matter structure probing and colloidal assembly,and(c)a simple method for suitable biofunctional modification of active colloids.Based on these unsolved issues,this thesis designs two types of external-field responsive active colloids,including optical-field and magnetic-field,and studies the preparation of active colloids,functionalization modification,the applications of active colloids to probe colloidal glasses and induce colloidal self-assembly.These research works are helpful not only for understanding the physics of active colloids,but also for promoting the practical applications of active colloids in different fields.The main works of this paper are divided into the following four aspects:(1)Preparation of light-field responsive Ag3PO4 active colloids.Active colloids absorb energy from their environment and convert it into the kinetic energy of motion,so the active colloids are in a thermodynamically non-equlibrium state.By using active colloids as model systems,non-equilibrium properties of condensed physics systems can be studied.However,there is still a lack of suitable active colloids for constructing such model experimental systems.In this chapter,we have prepared UV-responsive Ag3PO4 active colloids with complex morphologies through a simple chemical synthesis method.By controlling the ratio of reactants,we have successful prepared Ag3PO4 cubes and tetrapods.We find that these two types of Ag3PO4 active colloids move in two different modes.Specifically,Ag3PO4 cubes move in a translational mode and Ag3PO4 tetrapods move in rotational mode.In addition,we can change the motion mode of tetrapods from rotational mode to translational mode by changing the irradiation direction of UV light.The size,morphology,activity and motion mode of Ag3PO4 active colloid can be adjusted,so it is expected to become a great choice for constructing model active colloidal system.(2)Perterbation of colloidal glasses using light-field responsive active colloidsIn this chapter,we probe the dynamical and mechanical characteristics of colloidal glass systems at the microscopic level using large,square-shaped Ag3PO4 active colloids.The photocatalytic reaction of Ag3PO4 active colloids can drive not only their own motion but also the local fluid flow around the particles,i.e.,electroosmotic flow.This electroosmotic flow can break the thermodynamic equilibrium of the colloidal system and thus perturb the system,providing an experimental model for microscopic perturbation studies of various physical systems.Here,we selected a colloidal glass as the system being perturbed.By adding Ag3PO4 active colloid to the colloidal glass and using the electroosmotic effect mentioned above,we locally perturbe the glass system and probe its micromechanical and dynamical response behaviors.The results show that,as the packing fraction of colloidal particles increases,the number of particles being perturbed decreases and the mean displacement of particles due to perturbation gradually decreases.This indicates that at high packing fractions the system exhibits rigid mechanical characteristics of a solid.This behavior is realted to the "cage" effect of the glassy system.Sepcifically,for a colloidal glass,the higher the packing fraction,the more difficult it is for the colloidal particles to escape from their "cage".And in fact the particles can only vibrate in the cage that is similar to the vibration of particles in lattices in a solid,and thus the system exhbits rigid mechanical characteristics.When the perturbation is removed,for the system with low packing fraction,the average displacement of the particles does not change with time,and the particles cannot recover to their initial positions,indicating that the colloidal system has fluid-like viscous behavior.On the contrary,as the packing fraction increases,the average displacement of the particles in the system gradually decreases when the perturbation is stoped,and the particles resume their intitial positions,which indicates that the colloidal system has similar elastic properties of a solid.However,the particles do not fully recover to their initial positions at high packing fractions,indicating that the system not only has the mechanical characteristics of an elastic solid,but also retains certain degree of viscous properties of a fluid.These results reveal the mechanical characteristics of the colloidal glass system,and the correlation of such mechanical characteristics with the dynamical properties.In addition,such a microscopic perturbation with active colloids can be employed to study the viscoelastic behavior of glass systems,as well as to study the mechanical properties of other colloidal model systems,such as colloidal crystals and gels.(3)Colloidal assembly controlled by light-responsive active colloidsThe chemical reaction of active colloids can not only propel their motion and probe colloidal glass but also can control the assembly of surrounding passive colloids.We explored the light responsive assembly behavior of passive colloids by adding Ag3PO4 active colloids into a colloidal system composed of passive polystyrene(PS)colloids.The results show that the Ag3PO4 active colloids can induce PS particles to assemble into rich assembly structures,such as crystal clustes,colloidal chains and colloidal gels.The assembly of PS microsphere is found to be orignated for the electroosmotic flow generated from the photocatalytic chemical reaction of Ag3PO4 with water.We find that these assemble structures depend on the intensity of UV light,the packing density of PS microspheres and the concentration of Ag3PO4 active colloids.At low light intensity,the PS microspheres experience a liquid-to-crystallite transition with increasing concentration of the Ag3PO4 active colloids.When the light intensity is strong,richer phase behaviors are found.For example,besides the liquid-to-crystallite transition,there is a structural transition from compact crystal clusters to branched colloidal chains at a high concentration of the Ag3PO4 active colloids.Furthermore,one single chain will eventually percolate the whole system leading to a colloidal gel phase with increasing packing density of the PS microspheres.These observations thereby provide a simple and easy strategy to manipulate the assembly of colloidal particles,and can be exploited as a powerful tool for the assembly of targeted microstructures and the fabrication of colloidal materials.(4)Preparation and modification of magnetic field responsive bowl-shaped active colloids.In addition to being used as a model to study colloidal systems,active colloids can also be used as a new type of functional materials.For this purpose,we have designed a novel and unconventional method to synthesize composite active colloids with a s bowl-shaped morphology and magnetic response.This composite bowl-shaped colloid is composed of a Fe2O3 ellipsoid as the bottom and 3-methacryloxypropyltrimethoxysilane(TPM)as the wall.Furthermore,we turn them into selfpropelled active colloids by coating a thin layer of catalytic material,palladium(Pd),on the concave side of the bowl.The bowl-shaped particles can move based on self-diffusion mechanism in the H2O2 solution.We can precisely control the trajectory of the bowl-shaped particles and achieve external field responsiveness movement by applying a magnetic field.In addition,since the TPM molecule contains double bonds,we can introduce functional monomers on the surface of TPM through the copolymerization reaction.We successfully achieved the biological functional modification of the bowl-shaped particles through the copolymerization of glucose monomers and TPM.This magnetic-field responsive bowl-shaped active colloid holds promise for applications in biomedicine for its unique concave structure,flexible control of motion and convenient functional modification.In conclusion,two types of active colloidal particles with external field response,Ag3PO4 active colloidal particles with optical field response and Fe2O3@TPM bowl-shaped active colloidal particles with magnetic field response,were prepared in this thesis.For Ag3PO4 active colloidal particles,we prepared Ag3PO4 active colloids with different sizes,morphologies,and modes of motion,and explored the application of Ag3PO4 active colloids as experimental model systems in the study of colloidal glasses and the controlled assembly.For Fe2O3@TPM bowl-shaped active colloidal particles,we propose a facile method for the synthesis of bowl-shaped active colloidal particles,and then explore their surface modifications for the regulation of motion behavior and their bio-functionalization for imaging and bacteria encapsulations.Our work is important for exploring the applications of active colloids in both fudenmental research of condensed matter physics and functional materials.