Enhancing the Performance of Surface-based Biosensors by AC Electrokinetic Effects - a Review

Miniaturized surface based biosensors are a cost effective and portable means for the sensing of biologically active compounds.With advents in micro-and nanotechnology, the design of surface based biosensors can be adapted for various detection goals and for integration with multiple detection techniques.In particular, the issue of pathogen detection is an important challenge with applications in defence, health care, food safety, diagnostics and clinical research.The research of micro-fluidic analytical systems, such as surface based biosensors or "lab-on-a-chip" designs, have gained increasing popularity, not only due to the enhancement of the analytical performance, but also due to their reduced size, decreased consumption of reagents and the ability to integrate multiple technologies within a single device.Although conventional pathogen detection methods are well established, they are greatly restricted by the assay time.For pathogens that typically occur at low concentrations, the mass transfer required for detection is diffusion limited and incubation is often needed in order to enhance the concentration of the target analyte.AC electrokinetic effects provide a means for biosensors to detect pathogens quickly and at lower concentrations, thus overcoming these bottlenecks. Overview of AC electrokinetic phenomenaAC electrokinetics deals with the movement of a particle and/or the fluid by means of an AC electric field and has received considerable attention for improving the capture of analytes.An example of an AC electrokinetic force is dielectrophoresis (DEP) where a nonuniform electric field acts on an uncharged particle.When acting on a fluid, AC electrokinetic forces can induce AC electroosmosis and AC electrothermal effects.These forces can create non-uniform streamlines to convex and mix (Li, 2004), or even to separate a mixture of particle sizes (Green & Morgan, 1998) .Most bioparticles, such as cells and viruses, behave as dielectrically polarized particles in the presence of an external field.Using AC electric fields for particle manipulation offers several advantages, such as allowing operation at low voltages, which is important for portable devices and minimizing electrolysis and chemical reactions.The following will provide a brief overview of AC electrokinetic forces with applications for use in biosensors, as comprehensive reviews of AC electrokinetic forces in general are available elsewhere (Ramos et al., 1998). www.intechopen.com Biosensors -Emerging Materials and Applications 244DEP is a force acting on the induced dipole of a polarizable particle in a suspending fluid in the presence of a non-uniform electric field (Pohl, 1951).It was first defined by Pohl in 1951, and was used to remove suspended particles from a polymer solution.Pethig & Markx (1997) provides a review of applied DEP in the field of biotechnology.In brief, if a particle, such as a bacterium or virus, is more polarizable than the surrounding medium, the particle undergoes positive DEP (pDEP) and tends towards areas of high electric field strength (Fig 1a-Left).If a particle is less polarizable than the surrounding medium, it undergoes negative DEP (nDEP) and tends towards areas of electric field minima (Fig 1a-Right).www.intechopen.comEnhancing the Performance of Surface-based Biosensors by AC Electrokinetic Effects -a Review 245

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