Continuous In-Flight Synthesis for On-Demand Delivery of Ligand-Free Colloidal Gold Nanoparticles

Abstract

We demonstrate an entirely new method of nanoparticle chemical synthesis based on liquid droplet irradiation with ultralow (<0.1 eV) energy electrons. While nanoparticle formation via high energy radiolysis or transmission electron microscopy-based electron bombardment is well-understood, we have developed a source of electrons with energies close to thermal which leads to a number of important and unique benefits. The charged species, including the growing nanoparticles, are held in an ultrathin surface reaction zone which enables extremely rapid precursor reduction. In a proof-of-principle demonstration, we obtain small-diameter Au nanoparticles (∼4 nm) with tight control of polydispersity, in under 150 μs. The precursor was almost completely reduced in this period, and the resultant nanoparticles were water-soluble and free of surfactant or additional ligand chemistry. Nanoparticle synthesis rates within the droplets were many orders of magnitude greater than equivalent rates reported for radiolysis, electron beam irradiation, or colloidal chemical synthesis where reaction times vary from seconds to hours. In our device, a stream of precursor loaded microdroplets, ∼15 μm in diameter, were transported rapidly through a cold atmospheric pressure plasma with a high charge concentration. A high electron flux, electron and nanoparticle confinement at the surface of the droplet, and the picoliter reactor volume are thought to be responsible for the remarkable enhancement in nanoparticle synthesis rates. While this approach exhibits considerable potential for scale-up of synthesis rates, it also offers the more immediate prospect of continuous on-demand delivery of high-quality nanomaterials directly to their point of use by avoiding the necessity of collection, recovery, and purification. A range of new applications can be envisaged, from theranostics and biomedical imaging in tissue to inline catalyst production for pollution remediation in automobiles.

Plasmas for additive manufacturing

“…Recently, vapor precursors have been replaced by liquid precursors that are introduced in the microplasma as aerosol droplets by atomization. [ 67 ] The inspiration for this strategy was plasma‐liquid electrochemistry where metal salts dissolved in aqueous solution are reduced in solution by a cathodic plasma formed at the solution surface. [ 68 ] Assuming a similar mechanism, the precursor is reduced inside the droplet to nucleate metal nanoparticles.…”

“…Examples of direct‐write aerosol deposition processes: (a) measured nanoparticle synthesis rate for Au from HAuCl 4 in plasma‐droplet reactor versus other chemical methods (references indicated by (a)–(o) symbols are noted in text) reprinted with permission from Maguire et al, [ 67 ] (b) fully‐integrated system for inkjet printing by in‐flight plasma treatment of droplets during deposition reprinted with permission from Tsumaki et al, [ 84 ] and (c) various oxidation states of in‐flight plasma‐treated copper oxide ink on nanocellulose (unreduced), polyethylene terephthalate (reduced), and polyimide (reduced); reprinted with permission from Dey et al [ 86 ] …”

Microplasmas for Advanced Materials and Devices

“…The other aspect of practical interest relates to the possibility of microplasmas to accommodate the injection of liquid droplets in a flow configuration, which can be challenging or certainly with limited scope for other atmospheric pressure plasmas. It is worth mentioning that the metal NP production rates at a small laboratory scale (≈10 23 atoms L −1 s −1 )) reported for this type of microplasmas are much higher not only compared to other plasma types but also compared to advanced chemical microreactors.…”

“…d) HRTEM image of Au NPs synthesized by a droplet‐plasma microreactor shown in (c). c,d) Reproduced with permission . Copyright 2018, American Chemical Society.…”

“…This work exhibits a production rate of 90 mL h −1 for a Au@Ag CSNP solution. An interesting example of continuous‐flow synthesis of ligand‐free colloidal Au NP with the average size of 4 nm, was demonstrated using micro‐droplets passing through the plasma generated in a narrow channel (Figure c) . Importantly, nonequilibrium and high energy density conditions lead to nearly complete precursor reduction.…”