Damilos, Spyridon Panagiotis; (2020) Development of flow reactors for tuneable gold nanoparticle synthesis: Towards continuous manufacturing. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Gold nanoparticles (Au NPs) are gaining increasing attention in various sectors, such as electronics, catalysis and biomedicine. However, nanomaterial synthesis is considered a bottom-up process, which means that the desired particle structure, size and yield depend on the thermodynamics and kinetics of the fabrication process. Therefore the current challenges in nanomaterial manufacturing are related to understanding, controlling and integrating different operating units for continuous operation, while maintaining narrow particle size distribution, uniform size and high yield. This thesis is aimed at investigating the use of microwave heating for the flow synthesis of gold nanoparticles and developing a manufacturing platform for the continuous synthesis of high quality Au NPs of desired sizes. Initially, an alternative form of heating using microwaves was investigated to address the control of the longitudinal temperature profile and evaluate the heating efficiency in a milli-scale tube by varying the process parameters. The study was conducted both experimentally and computationally – by developing a finite element method (FEM) model – in a commercial single-mode microwave applicator. Varying the input microwave power, the medium (water) temperature inside the tube increased. However, the temperature profile plateaued and remained unaffected by the increase of the input microwave power beyond a certain value, as there was an excess of microwave power which could not be absorbed by the system. Microwave efficiency increased by increasing the inlet flow rate, due to the volumetric nature of microwave heating, without decreasing the temperature profile significantly. Changing the tube orientation both the microwave absorption and temperature profile decreased, while increasing the system pressure (from 1 bara to 2.3 bara), the temperature of the medium increased (by ~ 20 oC). The outcomes of this study provide in-depth understanding of the operating parameters on the temperature profile and energy efficiency of microwave-assisted flow synthesis systems. Then, the potential of microwave heating for the synthesis of spherical Au NPs via the reduction of the gold precursor (tetrachloroauric acid) by trisodium citrate (namely Turkevich method) was investigated. For that reason, two synthesis platforms were developed, using microwave heating only (one-stage system) or using microwaves for reaction initiation and conventional heating in series as growth stage (two-stage system). In the latter system, increasing the microwave power from 0 (only conventional heating) to 25 W, the particle size varied between 20 – 25 nm. A Welch’s unpaired t-test was conducted indicating that the particle sizes obtained at different input microwave powers were statistically significant. Varying the residence time under microwave heating had no significant effect on final particle size, despite a small change on the temperature profile inside the microwave reactor. The large particle size distribution (> 18 %) of the synthesised Au NPs in flow under microwave heating was attributed to the non-isothermal longitudinal temperature profile, the particle deposition on reactor walls and the residence time distribution deviating from a plug flow reactor profile. The aforementioned results indicate the shortcomings of the microwave technology for nanoparticle synthesis in flow. Existing mathematical models describing the kinetics of Au NPs synthesis via the Turkevich method were not applicable for designing continuous milli-fluidic flow reactors due to the complexity of the synthesis route. Therefore, two statistical correlations were developed for the estimation of the particle size and reaction time for the targeted synthesis of 10 – 20 nm Au NPs. The influence of order of reactant addition and synthesis pH on the particle size and reaction time was investigated. The developed models take into account the reactant speciation, concentrations and temperature, while the final pH of the colloidal solution was ~ 5.6, to ensure highly reproducible synthesis, yielding targeted spherical and monodisperse Au NPs. The developed statistical correlations could be used as a tool to engineer the flow process for a tuneable Au NPs synthesis. Finally, based on the developed statistical correlations a continuous milli-scale manufacturing platform was constructed for citrate-capped Au NPs synthesis (Turkevich method) under conventional heating. Using a two-phase flow system (using heptane as continuous phase) prevented the fouling in the reactor walls. A membrane separator and a UV-Vis spectrometer were integrated into a single standalone flow platform. The in-house designed membrane separator allowed the downstream separation of the organic and the aqueous streams, while the inline UV-Vis spectrometer ensured the quality control in flow via online monitoring of the synthesised particles (size, yield and process stability). Recycling of the heptane was tested, showing that re-using the collected heptane without further purification, increased the polydispersity of the colloidal solutions (~ 23 %). This platform is considered as a viable option towards scalable continuous manufacturing of nanoparticles.

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