Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing
Reexamination Certificate
2000-08-29
2004-09-14
Noguerola, Alex (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
C205S789000
Reexamination Certificate
active
06790341
ABSTRACT:
BACKGROUND OF THE INVENTION
Testing of various types of samples, for example, drinking water, waste water, and biological fluids such as blood and urine, can be performed electrochemically. Assessment of water quality for human health, environmental, and industrial concerns has become increasingly important over the past few decades. Analysis of pollutants, e.g., trace metals in aqueous solution is particularly important because many of these metals (e.g., Hg, Pb) are toxic in low concentrations.
Microelectrodes are useful in analysis of fluids containing electro-active analytes, particularly metals. Microelectrodes can have various geometries, e.g., hemispheres, disks, bands, tubes, rings, and cylinders and generally have one or more dimension on the order of 0.1 to 20 micrometers (Morris, R. B., Franta, D. J. and White, H. S. J. Phys. Chem. 1987, 91, 3559-3564). A microelectrode is an electrode with a dimension (thickness, T or width, W or radius, r) substantially less than the characteristic diffusion length of an analyte of interest. The characteristic diffusion length of an analyte is a function of the duration of the measurement, i.e., it is the square root of the product of the analyte's diffusion coefficient multiplied by the time of the measurement. As is known to those in the art, small metal cations, for example, have typical diffusion lengths which range from about 0.1 to about 1 cm
2
/second, depending, of course, on ionic mobility, among other factors.
As the dimensions of an electrode are made smaller, a number of advantages are gained. The mass transport rate increases, the electrode surface is covered more uniformly, diffusion layer capacitance decreases, the effects of solution resistance decrease, the signal to noise ratio increases, and the need for supporting electrolyte and deoxygenation of the sample is reduced or obviated.
The signal from microelectrodes consists of two components: a faradaic component 25 and a non-faradaic component. The faradaic component represents a chemical reaction occurring on the electrode surface. The non-faradaic component represents the capacitive charging unrelated to the chemistry occurring on the electrode surface. The faradaic component is usually proportional to the periphery of the electrode. The non-faradaic component is proportional to the surface area of the electrode. Electrode geometries which maximize the periphery to surface area ratio also maximize the ratio of the faradaic component to the non-faradaic component and produce readily producible signal. Microelectrodes produce higher periphery to area ratios. The area contacting the sample determines the non-faradaic component, while the periphery determines the faradaic (desired) component.
U.S. Pat. No. 5,120,421 teaches that conventionally sized electrodes often have large uncompensated resistance, making them useless in solutions of low conductivity, e.g., for detecting very low concentrations of analytes.
The diffusion layer (boundary layer), as will be understood by those of ordinary skill in the art, is that volume of fluid sample between where the analyte is at bulk concentration and where the analyte concentration approaches zero (i.e., the fluid layer immediately adjacent to the analyte-covered electrode). It is known to those in the art that the diffusion layer is related to the capacitance of the electrode and the non-faradaic component of the signal, and the diffusion layer can be measured with an electrocapacitance meter (A. Bard and L. R. Faulkner,
Electrochemical Methods: Fundamentals and Applications
, John Wiley, (1980); and P. t. Kissinger and W. Heineman, Eds.,
Laboratory Techniques in Electroanalytical Chemistry
, 2nd ed., Marcell Dekker, (1990) which are incorporated in its entirety by reference herein). Hence, the diffusion layer acts as a capacitor and any change in applied voltage produces non-faradaic current (component that interferes with the signal). Decreasing the size (dimensions) of the electrode leads to a decrease in the charge stored in the diffusion layer because the electrode surface area is decreased.
The only disadvantage of microelectrodes is the difficulty in measuring low currents. This problem can be overcome by fabrication of an array of identical microelectrodes, so that current signals from multiple microelectrodes can be added together to make a large enough signal for accurate measurements to be made. If the individual electrodes are spaced sufficiently far apart from one another, the currents from each individual electrode are additive and non-interfering.
In recent years many attempts have been made to develop more useful electrode sensors, some with multiple electrodes (arrays) and some with microelectrodes, and methods for making them. U.S. Pat. No. 5,437,999 by Diebold et al. describes an electrode sensor and methods for making such a sensor. The method utilizes photolithographic and screen-printing techniques.
U.S. Pat. No. 5,393,399 by Van den Berg et al. describes an amperometric sensor having a planar structure obtained by photolithographic techniques, useful for measuring the content of an oxygen reducible substance in a fluid.
U.S. Pat. No. 5,670,03 by Hintsche et al. describes an electrochemical sensor with multiple interdigital microelectrodes with structure widths * in the sub-micron range. The spaces between the interdigitated electrodes is about 700 nm, “which are small relative to the distances traveled by the molecules to be detected, in the measuring time.”
U.S. Pat. No. 5,217,112 by Almon describes an electrode sensor comprising an auxiliary electrode, a reference electrode and five working electrodes, methods for making such a sensor and voltammetric methods of using such a sensor.
U.S. Pat. No. 5,437,772 by De Castro et al. describes an electrode sensor with multiple interdigitated electrodes which can be coated with mercury forming an array useful for trace metal analysis of samples, especially those containing lead. The sensor is particularly useful in anodic stripping voltammetry techniques.
U.S. Pat. No. 5,103,179 by Thomas et al. describes a water analyzer having several electrodes of the type which normally interfere with one another (active and passive sensors). The water analyzer includes a first electrode (active) which perturbs the sample solution, a second electrode whose reading is affected by the operation of the first electrode, and a sequencing means which activates the first electrode at a time different from the reading of the second electrode.
U.S. Pat. No. 4,874,500 by Madou et al. describes a microelectrochemical electrode structure wherein a monolithic substrate has a well extending into the substrate from the front surface and a passage extending into the substrate from the back surface to the bottom of the well, and an electrode wholly between the front and back surfaces, and a conductor in the passage for electrically communicating the electrode to the back surface.
U.S. Pat. Nos. 5,296,125 and 5,120,421 by Glass et al. describe an electrochemical detection system including a multielement microelectrode array detector capable of acquiring a plurality of signals and electronic means for receiving these signals and converting them into a readout or display providing information about the nature and concentration of elements present in the sample solution.
U.S. Pat. No. 5,676,820 by Wang et al. describes an electrochemical sensor for remote detection, particularly useful for metal contaminants and organic or other compounds. The microelectrode is connected to a long communications cable, allowing convenient measurements of samples as far away as ten to more than 100 feet.
U.S. Pat. No. 5,292,423 by Wang describes a method and apparatus for trace metal testing using mercury-coated screen printed electrodes. Voltammetric and potentiometric stripping analyses are used. Screen printing allows for formation of electrodes with smallest dimensions of about 25 micrometers.
U.S. Pat. No. 5,254,235 by Wu describes a microelectrode array, contructed from a woven a minigrid, pre
Darling Robert B.
Saban Steven
Yager Paul
Greenlee Winner and Sullivan P.C.
Noguerola Alex
University of Washington
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