Magnetic nanoparticles (MNP) comprised of magnetite are fast gaining interest in the scientific research community due to their vast and widely varied applications in industry. These applications include waste water treatments, carbon capture, data storage, magnetic inks, and importantly, the biomedical industry. For biomedical applications, it is important for particles to be synthesised in a manner that makes them uniform in shape and size, and is suitable for industrial scale-up.
Currently many synthetic methodologies for magnetite rely on energy intensive processes and toxic precursors to synthesise high quality magnetic nanoparticles with narrow size distributions and consistent defined shapes. In an age of increased environmental awareness, it is critical to address these concerns before the large scale manufacture of magnetite nanoparticles is further established. Room temperature co-precipitation (RTCP) is a synthetic methodology which does not require environmentally harsh conditions, instead using water as a solvent and non-toxic iron salts as reagents, with no additional heating costs and expenditure required in the process. This process however does not currently offer fine control over the properties of particles formed, with wide variation in shape and size observed between reactions.
In Nature, the synthesis of sophisticated biominerals is commonplace with the nucleation and directed assembly of these materials being facilitated and templated by various proteins and biomolecules. The presence of these biological entities allows for bespoke high quality biominerals to be formed under ambient conditions, such as neutral pH and low temperature. The identification and study of how these ‘bioadditives’ function has allowed herein an investigation into the use of simpler compounds and molecules to aid the mineralisation of magnetite under greener reaction conditions.
This thesis covers the bioinspired application of additives to producing tailored nanomaterials, spanning three different systems of particle synthesis to ascertain the effect of batch and continuous production, as well as lay the groundwork for the scale-up of MNP production with the addition of additives. The papers within have explored the use of additives, as well as two fluidic systems to tune the shape and size of MNP in an environmentally sustainable manner.
A screening study based on the active functionalities from bioadditives was conducted to search for chemical functional groups which may control the shape and size distribution of particles formed. A series of ethylenediamine (EDA) based additives capable of tightly modulating the shape of particles formed were identified, acting as a starting point for further study. The addition of the longer chain EDA-based additives triethylenetetramine (TETA), tetraethylenepentamine (TEPA), and entaethylenehexamine (PEHA) produced 96, 97, and 91 % faceted particles respectively, showcasing the excellent morphological control these additives display.
This initial screening was built upon, with molecular dynamics modelling (MD) of the EDA series of amines revealing a preference for adsorption to the [111] crystal facet of magnetite. This face-specific adsorption enables the formation of primarily octahedral Fe3O4 particles, opening the door to the formation of shape-mediated particles under ambient conditions. TEPA was identified as the most effective of the EDA series at modulating the shape of particles formed, with a concentration study showing a 1:62.5 - 1:125 additive to Fe ratio produces the most highly faceted particles.
An iterative Design of Experiment (DoE) process was then conducted on the effect of TEPA, investigating the significance of three factors influencing the shape and magnetism of MNP formed with the addition of TEPA: i) the Fe/additive ratio, ii) the ferric/ferrous iron ratio, and iii) the timepoint of additive addition. Three rounds of DoE were conducted, comprised of two factorial designs (FFD), and a path of steepest ascent optimisation (PSA). The time-point of additive addition was found to be insignificant, suggesting TEPA acts to interact with forming magnetite particles rather than with aqueous Fe ions. Further FD narrowed down the ideal ferric and iron to additive ratios which produce MNP with the highest proportion of faceted particles and saturation magnetisation. The use of a PSA optimisation design allowed a compromise to be found between percentage of faceted particles formed and the saturation magnetisation, with ideal conditions found at a 1:50 and 1:59 additive to Fe ratio to produce near homogeneously faceted MNP.
A co-axial macrofluidic system was investigated for its potential to produce highly reproducible particles utilising highly controlled laminar flow. The biomineralisation protein Mms6 was found to be unable to exert control over the size of particles formed within this system. The millifluidic system however, allows for the tuning of particle size between a 20.5 and 6.5 nm range via simple adjustment of the ferric ion ratio. This millifluidic system was then used alongside the the EDA series of additives (EDA-PEHA) finding particle shape morphology was controllable, with a 1:100 ratio of TEPA to iron producing 58 % faceted particles.
Furthermore, a continuous flow static mixer was designed capable of producing 311 g day-1 (where day covers a 24 hour period) of MNP under co-precipitation conditions, five times higher than previously reported for MNP synthesis. The EDA series of additives was found to be highly effective within this system, producing 84 % faceted particles on addition of a 1:100 ratio of TEPA to iron. Optimisation was conducted on the continuous flow static mixer, varying the Fe and NaOH feed concentrations, and ferric ratio to tune the size of particles produced. TEPA was used as an additive across a 0.4 - 0.6 ferric range, and was found to consistently produce a high proportion of faceted particles (73 - 81 %), showcasing the robustness of TEPA as an additive across multiple systems and ferric ranges.
As this thesis is presented in paper format, the introductions to each paper may be similar and use the same references due to the need to present each paper as a stand alone body of work for publication.
