Colloidal Self-Assembly: Particle Synthesis, Functionalisation and Applications

Colloids, broadly defined as particles small enough to not sediment under gravitational force while dispersed in a continuous medium, are of great scientific and industrial importance. They are present in foods, cosmetics, paints and other materials, often in a jammed or a gelled state. This dissertation explores the self-assembly of hard and soft colloidal particles into mesoscopic structures through modification and control of their surface potentials. Due to its selective binding properties and thermal reversibility, deoxyribonucleic acid (DNA) was used to coat spherical polymeric colloids and subsequently have them assemble into high density colloidal gels. The formation and morphology of one- and two-component colloidal gels was investigated; the use of in-house synthesised fluorinated latex colloids (n = 1.36) allowed for refractive indexmatching of such systems in aqueous solutions, greatly aiding the imaging process deep inside dense gels. Additionally, the diffusion of tracer particles in the confinement of the gel structure was investigated. Furthermore, using the same coating technique, oil droplets (ODs) were functionalised with DNA: various sizes and configurations of DNAcoated ODs were investigated with the goal of creating functional oil-in-water emulsions for possible industrial and biotechnological applications. Taking a different assembly approach, 200 nm diameter fluorinated latex particles without any DNA coating were assembled into photonic crystals which, rather than showing iridescence, displayed strong single colour reflectance along with high transmittance, with the reflection colour varying with interparticle distance. Jumping back to colloidal assembly via DNA, rod-sphere structures made from DNA-coated gold nanoparticles and long DNA-coated virions were explored, demonstrating how anisotropic building blocks could create composite structures with increased porosity. Finally, an external magnetic field was shown to aid the assembly of superparamagnetic DNA-coated colloids into long linear constructs, while an additional coating of colloids was shown to stabilise the constructs after the magnetic field had been switched off

Colloidal motion under the action of a thermophoretic force

We present thermophoretic measurements in aqueous suspensions of three different polystyrene (PS) particles of varying negative charge, size, and surface coating. Our measurement technique is based on the observation of the colloidal steady-state distribution using conventional bright-field microscopy, which avoids undesirable effects such as laser-induced convection or local heating. We find that the colloids with the weakest zeta potential exhibit the strongest thermophoretic effect, suggesting that the Soret coefficient has a more intricate dependence on surface functionality than predicted by existing theoretical approaches. We also study the relaxation of the colloids to steady-state and propose a model to quantify the relaxation speed, based on the time evolution of the colloidal center of mass. Our observations are well described by this model and show that the relaxation speed tends to increase with the magnitude of the thermophoretic force.This work was funded by the Winton Programme for the Physics of Sustainability, Unilever Case and EPSRC (Grant No. 1353070)

DNA-coated Functional Oil Droplets

Many industrial soft materials often include oil-in-water (O/W) emulsions at the core of their formulations. By using tuneable interface stabilizing agents, such emulsions can self-assemble into complex structures. DNA has been used for decades as a thermoresponsive highly specific binding agent between hard and, recently, soft colloids. Up until now, emulsion droplets functionalized with DNA had relatively low coating densities and were expensive to scale up. Here a general O/W DNA-coating method using functional non-ionic amphiphilic block copolymers, both diblock and triblock, is presented. The hydrophilic polyethylene glycol ends of the surfactants are functionalized with azides, allowing for efficient, dense and controlled coupling of dibenzocyclooctane functionalized DNA to the polymers through a strain-promoted alkyne-azide click reaction. The protocol is readily scalable due to the triblock's commercial availability. Different production methods (ultrasonication, microfluidics and membrane emulsification) are used with different oils (hexadecane and silicone oil) to produce functional droplets in various size ranges (sub-micron, $\sim 20\,\mathrm{\mu m}$ and $> 50\,\mathrm{\mu m}$), showcasing the generality of the protocol. Thermoreversible sub-micron emulsion gels, hierarchical "raspberry" droplets and controlled droplet release from a flat DNA-coated surface are demonstrated. The emulsion stability and polydispersity is evaluated using dynamic light scattering and optical microscopy. The generality and simplicity of the method opens up new applications in soft matter and biotechnological research and industrial advances.Comment: 7 pages, 2 figures, 1 tabl

Microrheology of DNA hydrogels

A key objective in DNA-based material science is understanding and precisely controlling the mechanical properties of DNA hydrogels. We perform microrheology measurements using diffusing wave spectroscopy (DWS) to investigate the viscoelastic behavior of a hydrogel made of Y-shaped DNA nanostars over a wide range of frequencies and temperatures. Results show a clear liquid to equilibrium-gel transition as the temperature cycles up and down across the melting-temperature region for which the Y-DNA bind to each other. Our measurements reveal a crossover between the elastic G'(ω) and loss modulus G"(ω) around the melting temperature Tm of the DNA building blocks, which coincides with the systems percolation transition. This transition can be easily shifted in temperature by changing the DNA-bond length between the Y-shapes. Employing also bulk rheology, we further demonstrate that by reducing the flexibility between the Y-shaped DNA bonds we can go from a semi-flexible transient network to a more energy-driven hydrogel with higher elasticity while keeping the microstructure the same. This level of control in mechanical properties will facilitate the design of more sensitive molecular sensing tools and controlled release systems.Z. X. receives financial supports from National University of Defense Technology Scholarship at Cambridge, and NanoDTC Associate Programme. E. E. and A. C. acknowledge support from the ETN-COLLDENSE (H2020- MCSA-ITN-2014, grant no. 642774). E. E. and T. W. thank the Winton Program for Sustainable Physics. T. C. and D. L. thank the National Basic Research Program of China (973 program, No. 2013CB932803), the National Natural Science Foundation of China (No. 21534007), and the Beijing Municipal Science & Technology Commission for financial supports. I. S. and R. L. acknowledge support from EPSRC, No. RG90425 and 135307. M. Z. is funded by a joint EPSRC and Unilever CASE award RG748000

DNA-Coated Functional Oil Droplets

Many industrial soft materials include oil-in-water (O/W) emulsions at the core of their formulations. By using tuneable interface stabilizing agents, such emulsions can self-assemble into complex structures. DNA has been used for decades as a thermoresponsive, highly specific binding agent between hard and, recently, soft colloids. Up until now, emulsion droplets functionalized with DNA had relatively low coating densities and were expensive to scale up. Here, a general O/W DNA-coating method using functional nonionic amphiphilic block copolymers, both diblock and triblock, is presented. The hydrophilic poly(ethylene glycol) ends of the surfactants are functionalized with azides, allowing for efficient, dense, and controlled coupling of dibenzocyclooctane-functionalized DNA to the polymers through a strain-promoted alkyne–azide click reaction. The protocol is readily scalable due to the triblock’s commercial availability. Different production methods (ultrasonication, microfluidics, and membrane emulsification) are used with different oils (hexadecane and silicone oil) to produce functional droplets in various size ranges (submicron, ∼20 and >50 μm), showcasing the generality of the protocol. Thermoreversible submicron emulsion gels, hierarchical “raspberry” droplets, and controlled droplet release from a flat DNA-coated surface are demonstrated. The emulsion stability and polydispersity is evaluated using dynamic light scattering and optical microscopy. The generality and simplicity of the method opens up new applications in soft matter, biotechnological research, and industrial advances