A liquid drop RC filter apparatus for detection

Abstract

A new analytical detector based on a liquid drop resistor-capacitor (RC) filter is described, in which transformed gain vs. frequency curves are used to analyze compounds. This detector can be used to detect either charged or neutral species (that are dielectrically different) which are dissolved in a liquid (e.g., water, alcohol, solvent mixtures, etc.). This device was fabricated by modifying an electrowetting on dielectric (EWOD)-based experimental setup. When a liquid drop is placed on a dielectric surface, the system acts as a RC filter. At a given frequency, gain is a function of conductivity, surface tension, dielectric constant, double-layer thickness of the solid-liquid drop interface, as well as the applied voltage. Since different liquids and solutions have different physical properties, each liquid/solution has a unique curve (peak) in gain vs. frequency plot. This is the basic principle behind the detector. Different amounts of zinc chloride dissolved in water, benzalkonium chloride in water, 1-methylimidazole in water, cetyltrimethyl-ammonium chloride (CTAC) in water, and CTAC dissolved in ethylene glycol solutions were tested with the detector as proof of principle. The device can be used as a stand-alone detector or can easily be coupled with droplet based microfluidic lab-on-a-chip systems such as EWOD-based microfluidic chips.