From Particles to Pillars

Metal oxide nanoparticles and their applications (e.g. in nano-electronics, nano-optics or drug delivery) have seen a growing interest in the past decades. Traditional procedures to synthesize, functionalize and integrate nanoparticles in devices mostly happen in mL to L-sized chemical reactors. An upcoming method uses microfluidic chips, on which these operations are done in pL to nL-sized droplets. The miniaturization of the reactor volumes has several advantages, such as reduced waste and very precise metering of reagents, high degree of controllability of reaction parameters, and high reaction speeds due to short diffusion lengths. The first part of this thesis deals with the generation of simple and complex (interleaved) microdroplets in microfluidic chips. Simple droplets are produced in different oils using a microfluidic flow focusing device. A mathematical model is derived from these experiments, which relates the size of the highly uniform droplets to the oil and water flow rates. I then introduce and discuss a procedure to produce complex double droplets in a uniformly hydrophilic microfluidic chip. These droplets are built up of a spherical unit (consisting of fluorocarbon oil) and an elongated or crescent-shaped non-spherical silicone oil droplet which partially engulfs the fluorocarbon moiety. In the second part of this thesis, simple microfluidic droplets (0.4 nL volume each) are continuously generated on-chip from reagents used for the synthesis of fluorescent silica nanoparticles. The fluorescent dye is embedded in the silica network of the nanoparticles via chemical bonds, which reduces dye leakage from the nanoparticles and increases the stability of the dye, as evidenced by a reduced photochemical bleaching compared to a pure dye solution. The size of the nanoparticles (50–350 nm in diameter) can be easily adjusted by controlling the reaction time and the flow rates of the reactant solutions that constitute the microdroplets. The droplet-based continuous synthesis is contrasted with the layer-wise growth of silica nanoparticles in a batch process inside a mL-sized reactor. Droplet-based synthesis enables a faster reaction and allows improved nanoparticle size uniformity when compared to conventional methods. To exemplify droplet-based surface functionalization of nanoparticles, a very important process in nanoparticle engineering, the grafting of titania particles with gold nanoparticles is shown in a microfluidic chip. To this end, porous titania particles are loaded with a reducing agent and then suspended in water and mixed with chlorauric acid in a droplet-by-droplet manner on-chip. The third part treats the assembly of gold-titania nanocomposites and silica nanoparticles inside droplets into micrometer- and submillimeter-sized superstructures. Two different ways of assembling nanoparticles in droplets are presented in this thesis. The first method relies on the convection-driven merging of aqueous colloidal microdroplets. The microdroplets are generated in the bulk and contain about one nanoparticle each. After the complete evaporation of water, spherical superstructures of silica or gold-titania nanoparticles are obtained. In the second method, a microfluidic device is used to produce colloidal water droplets in oil. Each one of the droplets contains all the nanoparticles of the final assemblies. After water evaporation, superstructures are obtained which range from doublets to porous spherical or rod-like micro-objects. These assemblies can be doped with magnetite nanoparticles and then easily manipulated with a hand magnet. The superstructures possess the catalytic activity endowed by the single nanoparticles and have the advantage of being much easier to manipulate than nanoparticles. To show these characteristics, the assemblies are used as catalysts in addition reactions of aromatic amines and thiophenols in a microfluidic environment. Furthermore, magnetic porous assemblies are functionalized with an enzyme that catalyzes the deposition of a fluorescent dye through the reduction of hydrogen peroxide. The approaches presented here use nanoparticles whose surfaces were pre-activated with a gel-layer of silica or titania. During a heat treatment of the superstructures at moderate temperatures, the gel solidifies, which results in mechanically very stable assemblies, in which the single nanoparticles are linked together via metal oxide bridges.