Abstract

Recently AC electroosmosis (ACEO) has received increasing interest to control and manipulate particles and fluids at the micro/nano-scale. ACEO can operate at relatively low voltages, which is suitable for integrated lab-on-a-chip systems. Particle trapping by ACEO has no dependence on particle properties, so nano particle trapping can be possible. Micro-cantilevers are finding wide applications in detecting nano-particles.  However, current real-time detection typically has a detection limit several orders of magnitude higher than an infectious dose.  The time required for detection is generally much longer than desired, due to time involved in the diffusion process of those agents.  Consequently, pre-concentrating biological analytes such as proteins, viruses, and bacteria, is important in real-time detection.  This is especially true for the detection of bio-analytes that occur at very low concentration, for example biowarfare agents.  The sensitivity as well as the detection time could be improved by orders of magnitude if a concentration trap could be embedded with cantilevers.  For this reason we have integrated micro-cantilever with AC electroosmotic trap in a microfluidic chamber.

ACEO is a newly discovered microfluidic phenomenon, referring to the microfluidic motions generated at electrode surfaces when AC signals are applied.  A nanometer layer of charges/ions is induced by an electric field at the interfaces of electrolytes and solids, whether the solid is a metal or a cell membrane.  If there also exist electric fields parallel to the electrodes, the induced ions will migrate under the influences of the tangential fields, and produce osmotic microflows due to fluid viscosity.  The generated microflows convey particles from the bulk of the fluid onto the electrode surface. 

Nano particle trapping is conducted adjacent to an electrolyte/electrode interface on microcantilever. The conductive gold (Au) layer on microcantilever is required to generate microfluidic convection of nano-particles from solution bulk onto microcantilever surfaces that enhances sensitivity of the system. By combining both experimental investigation and theoretical analysis, this research work demonstrates a microcantilever particle trap at nano-scale. Preliminary experiments concentrating latex nano-particles are conducted and the particle concentration effect has been experimentally verified by measuring the resonance frequency using atomic force microscope (AFM).

Keyword: AC electroosmosis, micro-cantilever, microfluidic, nano-particle trapping, atomic force microscope, resonance frequency

(a)	(b)	(c)
Figure: Image sequence of 200 nm fluorescent particles trapped on the micro-cantilever for AC Electroosmosis; (a) t = 10 min, (b) t = 20 min, (c) t = 30 min.