Electrohydrodynamically enhanced drying droplets for concentration of Salmonella bacteria prior to their detections using antibody-functionalized SERS-reporter submicron beads

Abstract Gas ions can be ejected at the tip of a corona needle to form an ionic wind. When the corona needle is close to one hemispherical droplet sitting on flat glass slide, the ionic wind wiping across the liquid surface can induce micro-centrifugal vortex inside the droplet to drag suspended particles to the droplet bottom to trap at the stagnant point. Besides using this electrohydrodynamically generated ionic wind, the droplet evaporation can be enhanced because the eddy air streams around the droplet substantially increase the moisture removal rate. The liquid evaporation can induce internal fluid circulation to assemble particles on the surface. When the surface tension is significantly altered to stretch the contact line, the droplet is flatten as a liquid film to accelerate drying processes. The droplet is finally dried down. The trapped particles then become solid deposits and concentrate on the glass slide. This paper describes that the ionic wind flows were applied to evaporate a droplet of 10 μL as a liquid film within 15 min to further concentrate vortex-trapped Salmonella at the level of 1000 colony forming units (CFU) per mL (or lower), when the bacteria were bound with antibody-conjugated Raman tags made of silica-coated gold nanoparticle aggregates containing reporter molecules. Using this evaporation device, the concentration effects was concluded to be 300-fold higher than using the previous ionic wind-driven micro-centrifugal device. Using 30 μL sample volume, single CFU of Raman tag-bound Salmonella was detected, which was equivalent to about 30 CFU mL−1.