Sol-gel processes applied to micro- and nanotechnology

In this thesis, we intend to develop the sol-gel technology (a chemical way to make glass) from the traditional "raw" materials side to a novel technology allowing realization of innovative products in the field of micro- and nanotechnology. We first present the development of a process for the realization of 60 µm thick and 100 µm wide microlens arrays in sol-gel glass, based on an innovative photoresist replication and softening method. We synthesize and characterize our hybrid organic/inorganic sol-gel glass solution. The structural and optical characterizations indicate the reproducibility and high optical quality of our sol-gel glass micro-lens arrays. Then, we demonstrated a novel non-aqueous synthesis route to obtain a borosilicate solution used for the in situ modification of a poly dimethylsiloxane (PDMS) microchannel. Our modification process is producing a covalently bonded smooth and sub-micron thick (800 nm) crack-free borosilicate glass coating. The efficiency of the coating is demonstrated by a negligible diffusion of fluorescent Rhodamine B in the PDMS, when flowing water- and toluene-based solutions through the microchannel. Moreover, the coating prevents swelling and consequent structural damage of the PDMS when exposed to harsh chemicals like toluene. Finally, we discovered and report on an original process for the generation of borosilicate nanoparticles by mixing a non-aqueous Si-B binary oxide solution with water. Our experiments allow us to assess the borosilicate nature of the generated particles, and also to propose a novel formation mechanism via exothermic phase separation. Our discovery has high potential; not only for life science applications, but monodisperse borosilicate particles could be key to the realization of photonic bandgap devices with high optical contrast, contrast agents for ultrasonic microscopy or chemical filtration membranes. To sum up, our work demonstrates the possibility of innovative and versatile use of the sol-gel process in the micro- and nanotechnology field.

From Particles to Pillars

Josias Basil Wacker

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.

EPFL2012

On the relationship between the microstructure and the mechanical properties of an ODS ferritic/martensitic steel

Amuthan Ramar

In the quest of materials for the first wall of the future fusion reactor, it has been shown that oxide dispersion strengthened (ODS) ferritic / martensitic (F/M) steels appear to be promising candidates. The inherent good mechanical properties supported by a good thermal conductivity, swelling resistance and low radiation damage accumulation of the base material, such as EUROFER97, are further enhanced by the presence of a fine dispersion of nanometric oxide particles. They would allow in principle for a higher operating temperature of the fusion reactor, above 550°C, which improves its thermal efficiency. In effect, their strength remains higher than the base material at high temperatures. There are however limitations to their application in the reactor, which relate to the intimate link between the microstructure and the mechanical properties. These limitations, such as an increased ductile to brittle transition temperature above room temperature relative to the base material, come directly from the role played by the oxide particles but also from their indirect impact on the overall microstructure, stemming from the procedure of elaboration of these materials. In addition, there is a lack of knowledge on their response to irradiation, which, even though it appears to be promising with a reduced irradiation induced hardening, is not very well understood. In this work we have developed and evaluated ferritic/martensitic steel, EUROFER97, strengthened by a dispersion of yttria particles. (1) The powder metallurgical process used to obtain it was optimized, (2) the basic mechanisms of hardening were identified and (3) the impact of irradiation on its microstructure and mechanical properties were investigated. In order to understand the behaviour of this class of material other oxide dispersion strengthened (ODS) ferritic/martensitic steels based on EUROFER97 and model alloys were also investigated. For the elaboration of the ODS steel the powders of atomized EUROFER97 and yttria are ball milled in a planetary ball mill and the material is compacted by hot isostatic pressing (HIP). The mechanical properties are evaluated by uniaxial tensile test, Charpy test and micro-indentation while the microstructure is investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The focused ion beam (FIB) technique is also used. In the study of the ball milling it appears that the optimal time is 20 hours, as the X-ray peak broadening, crystallite size and hardness of the particles remain unchanged at longer times. Yttria nanoparticles clusters are no more detectable by X-ray after 2 hrs of milling, which is attributed here to their incorporation in the matrix, as opposed to dissolution in the matrix as is often suggested in the open literature. HR-TEM performed on the yttria particles before and after incorporation in the steel by ball milling shows that the yttria maintain its bcc equilibrium structure throughout the process. This could suggest that they are not dissolved during ball milling. We have clarified this point by a cross sectional examination of the EUROFER97 ball milled particles with 0.3wt% of yttria in an FIB. By X-ray EDS analysis it shows nanometric regions reach in yttrium and oxygen, which are the yttria particles embedded in the EUROFER97 particles, proving that they are not dissolved by ball milling. The mechanism at the origin of the alloy compaction during HIP was elaborated. The alloy compaction was done through HIPing at 1150°C for 2.5 hrs at a pressure of 190 MPa. This condition is shown to provide the highest bulk density, to a value close to 99.9%. In the optimization process, it is found that temperature plays a larger role than pressure for alloy compaction. The compaction appears to be mainly due to diffusion. Also, it appears that the ball milled particle size plays a major role in the alloy compaction. It is shown that smaller particles size induces better compaction, provided that the densification proceeds through diffusion flow. This further suggests to terminate the ball milling process by 20 hrs as the particles size reaches its minimum. The impact of the ball milling process on the mechanical properties was evaluated. EUROFER97 with and without ball milling, with and without oxide addition presents an average density around 99.8%. In the as received state, casted EUROFER97, atomized EUROFER97, ball milled EUROFER97 and ODS Yttria present microstructure morphology typical of tempered martensite, whereas ODS Ti presents a nano sized grain microstructure with an average grain size of 200 nm. Ball milling and HIPping does have a slight influence up to 400°C, due to the higher dislocation density induced by ball milling relative to casted EUROFER97. It becomes negligible above 400°C, since both casted and ball milled EUROFER97 present similar hardness and microstructure. In ODS Yttria, yttria presents an average particle size of 25 nm. In ODS Ti, Y-Ti-O presents an average particle size of 7 nm. In the following the identification of the hardening mechanisms in ODS F/M steels. A new model was developed to quantify the collective hardening due to different obstacles, assuming that they provide independent hardening mechanisms. The model helped identifying the dominating source of hardening in ODS F/M steels. The base of the model is the dispersion barrier strengthening model, which is given by Δσsparticle or defects = Mαμb(Nd)1/2 for the hardening due small defects or oxide particles and is given by Δσd-d = Mαμb(ρ)1/2 for the hardening due to dislocations. In this approximation, the total hardening is then given by Δσtotal = {(Δσd-d)2 + (Δσparticle or defects)2}1/2 . The model is used to explain the hardening due to oxides, dislocations and irradiation induced defects. It appears that dispersed oxides is dominating hardness up to 800°C, while following a heat treatment at 1000°C it is the dislocations that then dominate the hardness. Deformation mechanisms were investigated using differential strain rate experiments. The activation parameters, i.e. the activation volume and activation energy, were calculated. It appears that there is a decrease in the activation volume from room temperature to 400°C, which is less than 3%. At about 500°C there is a sudden drop in the activation volume that reaches a minimum at 600°C. With further increase in temperature, there is a significant raise in the activation volume. This abrupt decrease between 400°C and 600°C hints at a change in dislocation-rate controlling mechanism, marking two regimes, a low temperature one and a high temperature one. In situ TEM observations show that at low temperature the deformation is mainly due to the formation of the Orowan loops around the particle. At high temperature the activation energy is about 3.4 eV for ODS, whereas it is 1.3 eV for EUROFER97. The calculated activation energy corresponds well to the one of dislocation climb mechanism. This indicates that at high temperature the yttria particles could be overcome by climb, while at low temperature they are overcome by the Orowan mechanism. The peculiar response of the yield stress to irradiation for this class of material was rationalized using the total hardening model we developed. At a dose of 2 dpa, irradiation induced defects are higher in number density than the dispersed yttria particles, which although they are weaker than the yttria particles they can explain the significant hardening observed in the material. Conversely, the shallow impact of irradiation on ODS F/M steels at low doses, relative to base F/M steels, is explained by the fact that the amount of dispersoid is found to be higher than the density of irradiation induced defects.

EPFL2008

Functionalization of polymer optical fibers for medical application

Jialuo Luo

The increasing challenges caused by the growing and aging population and the fast pace of life are calling for a change of our current medical care system towards being rapid, easy, and on-demand self-care. Empowerment with the self-caring components such as early diagnosis, treatment, and monitoring of individuals will sharply release the social burden. In this context, fibrous materials stand out for their small size, processing flexibility, designable functionalities, and the possibility to be further integrated into im-plants or wearable devices. As we know, optical fiber is widely used in the biomedical field. For example, conventional silica optical fiber has mature applications such as light guides for surgery and catheter-based endoscopy while novel types of optical fiber (hydrogel, polymer, biomedical material, and elastomer) open the door to various new application scenarios like optogenetics, active monitoring, and continuous sensing, etc. While it remains a hot topic, the use of optical fibers for drug delivery is still in lack of exploration and suggests an ideal starting point for us. Optical fiber allows light propagation and thus gives the possibility to build up a light-responsive delivery system if combined with light-responsive drug carriers. Mesoporous silica nanoparticles(MSNP) were selected to host drug substances due to their high surface area, big pore volume, and versatility of functionalization and were coated on the fiber. Optical fiber will work as a light source to trigger the release from carriers. In this thesis, we constructed the hybrid system to give an optical fiber-based release system and elucidate the interaction between light from the optical fiber and drug-hosted particles with a further expectation to integrate with sensing or monitor-ing function to have an advanced delivery device. The first part of the work involved the synthesis of mesoporous silica nanoparticles using non-surfactants as templating agents. For the first time, a systematic study has been carried out to unravel the morphogenetic mechanism of mesoporous silica nanoparticles by polyphenols. Our study has, on one side, suggested that the pKa value of the polyphenols is responsible for the particle size due to the effect on nucleation and particle growth process, and on the other side, demonstrated that the formation of the supramolecular network contributes to the porous structure by serving as the skeleton for the silica pre-cursors to polymerize around. The second part of the work is to establish a method to generate a particle-polymer optical fiber(POF) complex matrix. A robust and controlled strategy for the fabrication of particle-coated surfaces that applies to multiple genres of substrates and coating materials was proposed based on layer-by-layer(LbL) assembly. The approach enables a high drug loading as well as a low structural complexity en-dowed by a dense and homogenous monolayer coverage of particles on the substrates. The computer vision method was employed to quantitatively evaluate the particle coating, increasing dramatically the efficiency and accuracy of the coating evaluation. The final part of the work investigates the light-responsive release of photolabile coumarin-chlorambucil conjugates from optical fibers. To the best of our knowledge, it is the first study to explore the release of drugs from the side of optical fibers. Overall, this novel particle-optical fiber-based one-dimensional drug deliver

EPFL2022