Microplasma assisted synthesis of gold nanoparticles mediated by ultrasound

Gold nanoparticles (AuNPs) exhibit unique optical, electronic and catalytic properties that allow their potential application in many areas. The performance of the nanoparticles depends closely on their size distribution. One of the recent developments of the synthesis of AuNPs involves the utilization of atmospheric pressure microplasmas. This approach quickly attracted much attention due to its low cost and high throughput. Yet, there is still a certain degree of difficulty to control the nanoparticles size effectively. In this study, we attempted to mediate the size distribution of AuNPs by coupling ultrasound into the microplasma-assisted nanoparticle synthesis system. The system was modeled after Mohan Sankaran and was comprised of a traditional electrochemical cell with a platinum foil as the anode, and the physical cathode being replaced by an atmospheric pressure microplasma. The microplasma was approximately 1 mm in length and was produced between a copper capillary tube (7 cm long with inner diameter of 0.355 mm) and the surface of the electrolyte precursor (containing l.214 mM HAuCI₄ and 34 mM sodium citrate). Research grade argon (Ar) gas (99.99%) was used to pressurize the capillary tube at a constant flow rate of 50 SCCM. During the synthesis of AuNPs, the microplasma discharge was sustained at a voltage around 600 V with an operating current of -30 rnA. The ultrasonic reactor was operated at 40 kHz. The nanoparticle synthesis time was set to as 3 min, 5 min, 10 min respectively. UV -vis absorption spectroscopy results indicated that the average size of AuNPs prepared under ultrasound irradiation is greater than that of without ultrasound. With ultrasound assistance, the average size of AuNPs increases over reaction time. The ultrasound assisted process can provide an effective strategy to adjust the size of microplasma induced AuNPs.