Pencil lead microelectrode and the application on cell dielectrophoresis

A microelectrode was fabricated by electrochemical etching of a pencil lead (0.5mm in diameter) in 1.0M NaOH aqueous solution. The pencil lead was dipped into the solution and then an ac voltage (3.0Vrms for 10min) was imposed against a stainless plate under mild stirring (450rpm). The electrochemically sharpened pencil tip was about 10μm in diameter (12±3μm, n=5), and the lateral part was insulated within a polypropylene micro-pipette tip (2–200μL volume range). The cyclic voltammograms conducted in 2.0mM ferricyanide/ferrocyanide buffer solution (pH 7.0) are with low capacitive current and a typical sigmoidal signal of micro-sized electrodes. The microelectrode was used to perform dielectrophoresis of polystyrene latex microbeads (nominal diameter of 3μm) and human red blood cells. A conducting glass (indium tin oxide coated glass, 40mm×40mm×1mm) served as the counter electrode (0.5mm beneath the microelectrode) to generate the asymmetrical electric field and also as the window for microscopic observation. With the sinusoidal bias voltage (30Vrms) ranged from 20Hz to 2MHz, positive and negative dielectrophoretic phenomena were identified.