High-aspect-ratio silica nozzle fabrication for nano-emitter electrospray applications

Abstract

With the trend in chemical and biological analysis for a reduction in sample size, the demand is for smaller and more finely controlled droplet emission from electrospray micronozzles. One of the key issues is to make reliable electrospray emitters with smaller diameters and high aspect ratio (length to diameter) to ensure a high value of electric field at the nozzle tip. Silica nozzles with aspect-ratio of over 20 were designed and fabricated in both single and array formats. An investigation of silicon deep etch by DRIE was performed and achieved a depth greater than 200 μm for 10 μm diameter channels. The surface wettability was also studied and the result showed that a thin gold film with low surface energy would offer the desired level of hydrophobic performance and be sufficiently robust for most micronozzle applications. The microfabricated single and linear-array nozzles were tested in the operation modes of on-line pumped and off-line isolated. The on-line tests for the single nozzles resulted in stable cone-jet electrospray with flow rates as low as 100 nl/min at voltage of a few kV. The off-line tests for the linear-array nozzles showed cone-jet pulsed electrospray with the production of picolitre droplets.