(335e) Rapid Chip-Scale Detection by Micro-Spiral Flow and Surface Enhanced Raman Scattering

We report a new chip-scale bio-particle concentration technique that can rapidly (< 15 min) produce strong surface-enhanced Raman (SERS) signals from a dilute 0.2 ml sample with low pathogen counts (~ 10⁴ CFU/ml). This concentration technique relies on convection by a long-range converging micro-spiral flow to rapidly concentrate pathogens into a packed mound about 100 microns in radius on the bottom of the liquid chamber. This lateral dimension is commensurate with that of a typical Raman laser beam which now sees an effective concentration roughly of 10⁸ CFU/ml. Despite such dramatic concentration, analysis of the Raman shift alone produces minimal characteristic signals due to the weakened laser intensity through the water sample. However, with the addition of silver nanoparticles to enhance the pathogens' Raman shift, signals due to SERS at similar concentrations are seen to enhance the signal intensity over an order of magnitude. Sensitive pathogen detection can hence be achieved within 15 minutes. The micro-spiral flow is generated by an ionic wind imparting force on the interface of the liquid, driving a vortex. A secondary flow results from the fast rotation of the primary vortex, similar to that of a viscometer, such that the bulk liquid results in a downward spiral. The resulting flow at the bottom of the chamber creates a converging flow stagnation point. Pathogens suspended in the liquid are swept to the stagnation point by convective forces and trapped by external body forces, such as gravity. Latex particles and yeast suspensions form packed mounds at the stagnation point, increasing the local concentration. Thus samples as low as 10⁴ CFU/ml can form a packed mound where locally the concentration is higher than the initial suspension concentration. A laser can then be focused at the packed mound to measure the intensity and the Raman shift of the dilute sample. Since this concentration technique relies on convection, trapping can occur within minutes. We see noticeable concentration of yeast and particles at the stagnation point without magnification within 15 minutes of the initiation of the spiral flow. An additional dielectrophoretic trap is fabricated on the bottom of the liquid chamber to enhance the capture of bacteria and other submicron pathogens. The micro dimension of the pathogens enhances Brownian effects limiting the concentration and stability of the packed mound, which is necessary for accurate Raman spectroscopy. Coupling a dielectrophoretic trap with the converging stagnation flow hence aids in the trapping and stabilization of the pathogens onto a substrate, providing consistent Raman spectra for them. In contrast to optical tweezers, this form of trapping is non-obtrusive and hence is not detrimental to the viability of the pathogens, thus allowing for further differentiating between live and dead pathogens. It is hence possible to obtain spectra for suspensions of pathogens viz. E. coli, yeast and sepsis causing bacteria at low concentrations in mixtures with silver nanoparticles for SERS enhancement in a short time frame. These advantages of rapid concentration and detection can lead to the development of a field applicable diagnostic kit. Future efforts are being directed towards the integration of this concentration technique with a portable Raman unit.