(521b) Integrated Dielectrophoretic Chip For Continuous Sorting, Trapping, And Detecting Using Surface-Enhanced Raman Scattering

We report an integrated dielectrophoretic chip that can continuously sort, trap and detect pathogens with a throughput of 3 μL/min. A processing rate of 500 bacteria per second is estimated when 100 μl of a heterogeneous colony of 10⁷ CFU(colony forming units)/ml is processed in a single pass within 30 minutes. Using planar electrodes to form three-dimensional dielectrophoretic (DEP) gates, the pathogens can be deflected to different channels on the chip where they are individually concentrated. Individual bacteria have different DEP characteristics based on their surface properties, permittivities, and whether they are alive or dead. Therefore, a mixture of bacteria can be sorted into individual channels using DEP based on these inherent characteristics. A DEP gate is also used to trap and concentrate the bacteria as they flow through the channel, forming a concentrated mound. Upon concentration Raman spectroscopy can detect and identify the concentrated mounds of bacteria. With the addition of silver nanoparticles into the pathogen mixtures, the Raman shift can be enhanced orders of magnitude through surface-enhance Raman scattering creating an rapid on-chip detection method. With electrodes on both the top and the bottom of the channel, the field can penetrate across the entire height of the channel. Consequently the DEP force will be exerted across the entire height of the channel in contrast to being restricted to the electrode surface as typically seen in parallel planar electrodes. These electrode gates can deflect and trap the pathogens as they pass through the channel when an AC field is applied at a frequency such that the particle exerts a negative DEP force. Thus the pathogens will be repelled from the high field region, which is the electrode itself, and be forced to cross fluid streamlines and deflect towards individual channels. However, there is a balance in the forces between the projected DEP force and the Stokes drag on the particle in the flow direction. A DEP gate perpendicular to the streamline requires a DEP force larger than the Stokes drag to prevent the particle from passing through the gate. At the high fields needed for this strong of a DEP force, reactions, heating or other damaging side effects to the electrodes and to the pathogens themselves can occur. However, an angled electrode will only need to exert a DEP force large enough deflect and slow the pathogen's velocity such that instead a sudden stop in the particle flow, the particle can translate along the electrode edge. Thus the pathogens can be sorted by these angled electrodes such that they are deflected to different channels on the chip. Concentration of the pathogens in the trapping electrode gate is also investigated using three different electrode designs, a crescent, a pointed tip, and a multi-curved ?flower? shape. The angle of the electrode with respect to the streamline can be optimized for higher throughput of the pathogens without increasing the electric field to the point of damaging them or the integrity of the electrodes. Mixtures of Escherichia coli Nissle, Lactobacillus, and Candida albican are sorted, concentrated and detected using surface-enhanced Raman scattering. Differing spectra in each concentration will allow the rapid identification of the concentrated pathogen at the trapped gates. It is hence possible to expand the mixtures to include more than three types of bacteria, such as those found in a realistic gastrointestinal sample: Escherichia coli Nissle, Lactobacillus plantarum, Enterobactor cloacae, Citrobacter rodentium, Proteus mirablis, and Bacillus subtilis. These samples cannot rely on traditional culturing techniques and thus this integrated chip can be employed for rapid diagnostics of laboratory samples.