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Sensors and Actuators B: Chemical, Volume 164, Issue 1, 31 March 2012, Pages 1-6
M.J. Schertzer, R. Ben Mrad, P.E. Sullivan
Dept. of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada M5S 3G8
Abstract
Real-time measurement of electrical properties are used to perform the first real-time detection of particle concentration and chemical reactions in electrowetting on dielectric devices without the need for optical access. For particle laden droplets, the change in both resistance and capacitance was found to be linear from zero to five-hundred particles with a resolution of approximately six particles for both measurements. Electrical properties were also measured for mixtures of alkaline phosphatase and p-Nitrophenyl Phosphate; reagents commonly used in immunoassays as the resultant chemical reaction produces a yellow precipitate. Experiments were performed with mixtures created off-chip and in droplet that were statically mixed on chip. The difference between the measured and expected capacitance was found to increase with the concentration of alkaline phosphatase and chemical reactions could be positively identified in mixtures made both on and off the chip. Real-time measurements of reagents mixed in a four electrode electrowetting on dielectric mixer were also taken. In these experiments, chemical equilibrium was reached after approximately 20 cycles and the difference between the measured and expected capacitance was over 8.5 times greater than the experimental uncertainty.
Additional Information:
This work builds on the author’s previous publication which demonstrated that mixing of inert fluids in digital microfluidic devices can be monitored in real time using capacitance measurements. The current work furthers this initial principle in two important ways: (1) it demonstrates that electrical resistance (and therefore impedance) can be used as a feedback signal and (2) that the utility of this sensory technique can be extended to the measurement of particle concentration and the chemical reactions in digital microfluidic devices. The specific reaction examined here occurs between Alkaline Phosphatase and Para-Nitrophenyl Phosphate. These reagents were chosen as they are common reagents that generate a visual signal in many macroscale and microfluidic immunoassay applications. Further study is being performed to examine the utility of this technique for detecting other chemical reactions. Since every addressable position in a digital microfluidic device is essentially a parallel plate capacitor, it is possible to use this method to receive real-time information on fluid composition at every addressable position without increasing fabrication complexity or requiring optical access. Although this technique is demonstrated for digital microfluidic devices, it can be also be implemented in other microfluidic devices where electrical properties of the fluid can be measured.
