Integration of Nanostructured Dielectrophoretic Device and Surface-Enhanced Raman Probe for Highly Sensitive and Rapid Pathogen Detection

Rapid and highly sensitive is critical for food safety and environmental protection. To achieve this, it requires the combination of both novel sample handling techniques and extremely sensitive detection methods. In previous studies, we have demonstrated the effective capture and quick concentration of pathogenic particles such as viral and bacterial particles in a fluidic chip using dielectrophoresis (DEP) enhanced with the highly focused electric field at the tip of an embedded nanoelectrode array. This work reports a further development with a synergistic approach to the concentration, detection and kinetic monitoring of pathogens through the integration of the nanostructured DEP device with nanotag-labelled Surface Enhanced Raman Scattering (SERS) for specific bacterial identification. A nanoelectrode array made of embedded Vertically Aligned Carbon Nanofibers (VACNFs) at the bottom of a microfluidic chip is used to effectively capture and concentrate nanotag-labelled E. coli DHα5 cells into a 200 micron x 200 micron area at which the Raman laser probe is focused. The SERS nanotags are based on iron oxide-gold (IO-Au) core-shell nanoovals (NOVs) of ~50 nm in size, which are coated with QSY21 Raman reporters and attached to E. coli cells through specific antibodies. The combination of the greatly enhanced Raman signal by the SERS nanotags and the effective DEP concentration significantly improves the detection limit and speed. The SERS signal has been measured with both a confocal Raman microscope and a portable Raman probe during DEP capture, and has been fully validated with fluorescence microscopy measurements under all DEP conditions. The SERS measurements are sensitive enough to detect a single bacterium. A concentration detection limit as low as 210 cfu/mL using a portable Raman system has been obtained with a DEP capture time of only ~50 s. The detection has been validated in purified solutions as well as complex samples including chicken broth, soil solution and apple juice. These results demonstrate the potential to develop a compact portable system for rapid and highly sensitive detection of specific pathogens. Furthermore, the AC impedance can be used to monitor the pathogen capture with slightly higher detection limit. Such system is reusable, requires minimum sample preparation, and is ideal for field applications.