Fast and Sensitive Detection of Bacteria by Means of AC Electrokinetics and Micro-Raman Spectroscopy
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Rapid diagnosis of pathogens requires the ability to detect the presence of a target pathogen in a sample at as low concentrations as possible. In the present thesis, a surface-based method that can provide early and label-free detection of target microorganisms in water is presented. The method combines three key features, namely, (a) accelerated transportation to, and focusing of bacteria on, the detection surface by means of alternating current (AC) electrokinetic effects; (b) selective capture of the target microorganism using surface-immobilized antibodies; (c) sensitive detection of target bacteria with micro-Raman spectroscopy. The non-uniform electric field is created by an AC signal-driven planar, quadrupolar, gold microelectrode array that is deposited on the detection surface (oxidized silicon wafer). The combined effects of AC electroosmosis and dielectrophoresis concentrated the bacteria onto a surface within a few minutes. Bacterial detection is subsequently, accomplished through Raman spectra acquired at pre-determined locations near the electrodes after sample removal and rinsing of the capture surface. Detection was successful at concentrations as low as 102 bacteria/mL using a sample volume of 50 μL. The method is sensitive enough to detect the capture of a single bacterium. The bacterial capture efficiency was found to be strongly dependent on the selection of the applied voltage and AC frequency values. Tests performed in the range 1-20 Vpp and 1 kHz-1 MHz showed that the most effective operating conditions was (12 Vpp, 10 kHz). Overall, high bacteria capture efficiencies were observed at low frequencies (≤10 kHz). The mechanism of bacteria capture was investigated with the aid of numerical simulations. It was found that the capture efficiency was very high under electric field conditions that cause a strong AC electroosmotic effect. The simulations results were in qualitative agreement with the experimental observations.