Continuous flow scalable synthesis of nanoparticles
Nanomaterials, objects with one or more dimensions in the nanoscale regime, are expected to play a critical role in the field of nanotechnology, a field that aims to harness size-dependent physico-chemical properties. Scientific advances over the past three decades have highlighted the potential for controlling optical, magnetic and electronic properties of materials by varying their size in the nanoscale regime. The translation of these scientific findings into commercial products is being pursued vigourously in industry and academia, with several products at various stages of development.
Nanoparticles are the foremost contenders amongst nanomaterials that are likely to proliferate commercially, e.g. as drug delivery vehicles, energetic fuels etc. The use of nanoparticles in itself is not unique to the recent past; historical examples of their uses abound. For example, metal nanoparticles were used to fabricate stained glass windows in Europe, carbon black particles were used as ink pigments in China, and colloids of noble metal nanoparticles were part of traditional medicines/biocides in India. More recently, the use of carbon nano/sub-micron particles as reinforcement in rubber composites, the use of nanoscale alumina particles for polishing semiconductor wafers and the use of gold nanoparticles as contrast agents for electron microscopy are well-established examples of nanoparticle based commercial applications. What sets the current research and development efforts apart from such previous efforts at accidental/evolutionary nanotechnology is the ability to routinely characterize and manipulate the nanomaterials at the atomic scale and the resulting improved understanding of the science behind their distinctive properties.
Schematic diagram representing the interaction of light with metal nanoparticles. The interaction of electromagnetic field with the metal nanoparticles results in coherent oscillations of the conduction band electrons. The circles illustrate the collective displacement of the mobile electrons within the nanoparticles (plasmons) from the ionic cores, corresponding to two different phases of the incident radiation’s electric field.
Distinct colours exhibited by aqueous sols of gold nanoparticles synthesized during the course of our research. The cartoons depict the morphology of nanostructures present in the respective sols.
A primary objective of our project was to synthesize sub-10 nm metal nanoparticles in a continuous fashion using aqueous-phase processes. To achieve this, a simple co-axial flow microreactor setup was fabricated by molding PolyDiMethylSiloxane (PDMS) as shown in figure 13a. The metal salt was pumped through the inner tube, while tannic acid flowed around the core. The initial pH of the reagents was adjusted to ensure rapid reduction of metal salt by tannic acid molecules. Complete conversion of the metal salt into silver or gold nanoparticles was found to occur for residence times of the order of 10s, under the conditions of high reactivity in tune with earlier kinetics measurements. Interestingly, the nanoparticle size distribution was found to be similar to batch experiments at steady-state conditions, and were found to be insensitive to variations in flow velocities.
Microchannel flows can provide uniform and repeatable heat and mass transport conditions, which is purported to help in enhancing the reproducibility of nanoparticle synthesis; however, the widespread use and ‘numbering-up’ of microchannels is hindered by large pressure drops due to small channel sizes, leading to very high power requirements for pumping the reagents. As our earlier results suggested that speed of mixing and flow conditions had very little effect on the nanoparticle size distribution, as long as their reactivity was tuned by choosing appropriate feed conditions, we designed a simple gravity fed open channel reactor with multiple feed inlets to mimic the drop-wise addition protocol for synthesizing monodisperse gold nanoparticles . Salient features of our process design are the very high throughput (g/hr) vis-à-vis typical throughput values of microchannel reactors (mg/hr) and the negligible power-requirements for pumping liquids.
Another novel reactor set-up that has been succesfully implemented to scale up nanoparticle production includes a spinning bowl -disk reactor, designed by Prof. Sanjeev Gupta,wherein thin films are created using centrifugal action and contacted to minimize the diffusion length scale. Such a reactor has been used to synthesize metal nanaoparticles in the 10-100 g/hr throughput range.
