Translation from batch to continuous processing of metal nanoparticle synthesis and application metallic nanostructures printed on flexible substrates
One of the key challenges in nanoparticle synthesis is the quality control on scaling up the operation from bench to plant scale, which is constrained by conventionally adopted batch operation. Translation from batch operation to continuous green synthesis (metal, bi-metallic core-shell, and alloy nanoparticles (NPs)) with low polydispersity index (PDI) could unlock potential applications of metallic nanostructures with a projected market of $40.6 Billion and more by 20271. However, the continuous large-scale production suffers from high polydispersity due to lack of process optimization. We attempt to address such scale-up challenges while using the green synthesis of metallic nanoparticles in this work. The objective of this thesis is to optimize continuous processing of metal nanoparticle synthesis and demonstrate application of metallic nanostructure printed of flexible substrate using inkjet printing technology. The first part of the thesis is motivated by the desire to translate the batch protocol for NPs synthesis (developed in our group earlier)2,3 to a continuous process, and hence increase the affordability of NPs for end users. In this work, nanoparticle colloids are synthesized using different designs of CFRs and steady-state synthesis of nanoparticles is achieved with insignificant variation in particle size. Our results further showed that a balance between engineering and chemical parameters are required to obtain desired particle size distribution (PSD) and morphology during green synthesis of NPs. We improved our reactor design from channel to pool-based to address poor reagent mixing and our results show that the pool reactors could produce uniform particles of sizeii 7.2±1.0 nm with the production rate of 7.1 mg/h. We later moved to a CSTR-based reactor to address variations in the particles’ morphology while changing the flow rate of precursor salt. We found that the CSTR-based reactor can synthesise colloids (Gold, Silver, bimetallic gold-silver core-shell, and gold-silver alloy) at higher (10 times) flow rates and offers a better and affordable route for continuous nanoparticles synthesis within numerous applications in the healthcare and energy sectors. For the first time, to the best of my knowledge, the steady-state synthesis of metal nanoparticles is demonstrated here. After attaining steady state, the particle size distribution does not vary significantly. Investigations are performed to find out the effect of engineering parameters as well as chemical parameters. Particle size distribution is more sensitive to chemical parameters in comparison to engineering parameters. Although, engineering parameters like reactor design, mixing, temperature are important parameters to tailor nanoparticle size in a controlled fashion. Hence, there must be a balance between both to get desired particle size and morphology. In the second part of the thesis, we demonstrated the application of in-situ fabricated silver nanowires on copier paper for non-enzymatic glucose, using a form of inkjet printing technology. The inkjet printing technique is another avenue of fabrication of nanostructure-based flexible substrates that can be scaled using roll to roll printing techniques. Using this technique, we could fabricate, and customize electroadhesive pads based on interdigitated electrode designs with an interelectrode distance of 1 mm on paper. It was observed that, if left as a residue, lateral silver ion migration on applying high voltage leads to lowering of the gaps between electrodes, which resultsiii in increased load capacity. Further electromigration can be controlled by chemical fixing of the sample, i.e., by immersing printed samples in 0.5 M Sodium thiosulphate. The advantage of this process is that different designs of electrodes can be easily fabricated depending upon the required application. The in-situ fabricated silver nanowires incorporated with CuO/Cu2O nanoparticles are used for non-enzymatic glucose detection. Non-enzymatic sensors (4th generation) can replace the enzymatic detectors (3rd generation)4, but major challenge associated with 4th generation detectors is that it requires alkaline pH for reaction to be initiated. Paper based standard electrode (Ag/AgCl); silver nanowire incorporated with CuO/Cu2O nanoparticles (synthesised by wet chemical method) are used as working electrode. Ag nanowires modified with CuO nanoparticles show a linear increment in current with increase in glucose concentration. Glucose detection is performed in the concentration range of normal sugar level in human body in the range from 2.2 – 6.6 mM.iv References: 1 Metal Nanoparticles - Global Market Trajectory & Analytics 2021, <https://www.researchandmarkets.com/reports/5030157/metal-nanoparticlesglobal-market-trajectory> (19/09/2021). 2 Sivaraman, S. K., Kumar, S. Santhanam, V. Room-temperature synthesis of gold nanoparticles; Size-control by slow addition. Gold Bulletin 43, 275-286 (2010). 3 Sivaraman, S. K., Kumar, S. Santhanam, V. Monodisperse sub-10nm gold nanoparticles by reversing the order of addition in Turkevich method – The role of chloroauric acid. Journal of Colloid and Interface Science 361, 543-547 (2011). 4 Toghill, K. E. & Compton, R. G. Electrochemical non-enzymatic glucose sensors: a perspective and an evaluation. Int. J. Electrochem. Sci 5, 1246-1301 (2010).