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.
Highlights
High-quality colloidal nanocrystals play a critical and expanding role in numerous fields including nanomedicine, catalysis, optoelectronics, imaging, and sensors
Approaches based on reduced reactor volumes via droplet microfluidics have received attention as a route to continuous and rapid nanomaterial synthesis[16,17] while electron beam techniques, such as TEM in liquid[18,19] and pulse radiolysis, have shown rapid synthesis capability
In this work we have merged the concepts of microfluidics with electron irradiation to create a segmented flow microfluidic device based on metal precursor loaded liquid droplets in a gaseous carrier
Summary
Letter order of minutes, and stable colloids can be formed without additives. With aqueous precursors, under certain conditions, metal ion reduction is due to reactions with plasma-generated chemical species dissolved from the gas phase, e.g., H2O2.28 Alternatively, energetic electrons from the plasma are known to become solvated[29] at the liquid surface where direct reduction of the metal ions can occur.[27,30] In this work we have merged the concepts of microfluidics with electron irradiation to create a segmented flow microfluidic device based on metal precursor loaded liquid droplets in a gaseous carrier. The plasma-induced reduction of Au3+ and the formation of Au0 in nanoparticles proceeds at a rate that is ∼103 times greater than the fastest rates previously observed with high dose rate TEM and over 107 faster than gamma radiolysis, for similar dose rates, Figure 4 (see Supporting Information, SI-3). Such a remarkable enhancement of the reaction efficiency must depend on factors in addition to that of reduced volume,. The supply of precursor from the interior to the surface may be a limiting factor
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