Microfabricated Liquid Junction Ag/AgCl Reference Electrode and Its Application to a One-Chip Potentiometric Sensor

Micro analysis system for pH and protease activities with an integrated sample injection mechanism

A micro analysis system for the electrochemical determination of the activity of protease along with pH sensing was fabricated aiming for its use in telemetric micro analysis systems targeting the testing of the stomach and intestines. The system consisted of a pH-sensing site and two protease assay sites formed in polydimethylsiloxane (PDMS) micro flow channels. To introduce sample solutions, valves were formed with gold electrodes in the inlets, which functioned on the basis of electrowetting. An external sample solution could be introduced into the sensing sites by switching on the valves at appropriate times. In the pH-sensing site, a pH-indicator electrode changed its electrode potential immediately after a sample solution reached an internal liquid-junction reference electrode. The slope of the calibration plot was -74.5 mVpH(-1). Bovine serum albumin (BSA) was used as the substrate for the enzyme and was spotted on the wall of the flow channel that faced the pH-indicator electrode of the protease assay sites. The release of protons accompanying the hydrolysis of BSA by the enzyme was detected using the pH-indicator electrode. When trypsin was contained in the sample solution as a test enzyme, a distinct decrease in pH, which was dependent on the trypsin activity, was observed, indicating that enzymatic hydrolysis was proceeding. The initial rate of potential change varied in proportion to the activity in a range between 1.0 and 51.7 Uml(-1). The integration of the microfluidic and sensing functions provides significant advantages for the use of this system as an isolated telemetric micro system that might operate with small batteries.

A system-on-chip digital pH meter for use in a wireless diagnostic capsule

This paper describes the design and implementation of a system-on-chip digital pH meter, for use in a wireless capsule application. The system is organized around an 8-bit microcontroller, designed to be functionally identical to the Motorola 6805. The analog subsystem contains a floating-electrode ISFET, which is fully compatible with a commercial CMOS process. On-chip programmable voltage references and multiplexors permit flexibility with the minimum of external connections. The chip is designed in a modular fashion to facilitate verification and component re-use. The single-chip pH meter can be directly connected to a personal computer, and gives a response of 37 bits/pH, within an operating range of 7 pH units.

Needle-Type Ultra Micro Silver/Silver Chloride Reference Electrode for Use in Micro-Electrochemistry

A needle-type ultra micro silver/silver chloride reference electrode having a micro capillary with outer and inner diameters of 1.0 microm and 0.5 +/- 0.2 microm, respectively, was constructed. This micro reference electrode can be stuck into a living cell, and is applicable to use in very small environments, such as an electrochemical cell of an electrochemical scanning tunneling microscope or the detection portion of a micro-TAS. Excellent stability and repeatability of the micro reference electrode potential could be obtained by filling the micro capillary with agar gel containing 3.33 mol/L potassium chloride as a salt bridge, by covering the bare part of the silver wire on which silver chloride was deposited, and by electromagnetic shielding of the measurement cell and wire lead from the electromagnetic waves. The electrode showed stable potential for 7 days after its fabrication using 3.3 mol/L potassium chloride aqueous solution containing silver chloride as an internal electrolyte solution. The electrode exhibited constant electrode potential and excellent stability in test solutions of pH 5-9. The electrode potential of a commercial pH glass electrode measured against the micro reference electrode in standard pH buffer solutions was in good accordance with the Nernst equation.

Chip-scale universal detection based on backscatter interferometry

An on-chip detector based on backscatter interferometry has been developed to perform subnanoliter-volume refractive index measurements. The detection system consists of a simple, folded optical train based on the interaction of a laser beam and an etched channel, consisting of two radii joined by a flat portion, thus defining a curved surface in the shape of a hemisphere in a silica (glass) plate. The backscattered light from the channel takes on the form of a high-contrast interference pattern that contains information related to the bulk properties of the fluid contained within the probe volume. Positional changes of the interference pattern (fringes) allow for the determination of deltan at the 10(-6) level, corresponding to 743 microM or 139 x 10(-15) mol or 12.8 x 10(-12) g of sucrose, in a probe volume of only 188 x 10(-12) L. A theoretical model of the on-chip backscatter interferometric detector has also been developed, evaluated, and found to be in agreement with experimental data. It is shown that the model can be used to predict general system performance for changes in the optical train such as the chip's wall thickness and channel diameter.