TECHNICAL FIELD

This invention relates to electrochemical detection and analysis using a microelectrode array. In specific embodiments, the invention relates to applications for in-situ analysis of gaseous chemicals in air or other gaseous environments. In specific embodiments, the invention relates to in-situ analysis performed with the assistance of chemically selective preconcentrators.

ART BACKGROUND

Electrochemical analysis is a highly sensitive, chemically selective method for identifying and quantifying many different ionized chemicals in water. Sensitivity levels of 1 ppb or better are achievable for many chemicals, including those regulated by the U.S. Environmental Protection Agency (EPA). However, electrochemical analysis has historically required that field samples be transported back to an analytical laboratory. There, additional laboratory chemicals are typically added to a sample before analysis is performed using bench-scale equipment. Such analysis requires skilled laboratory technicians and causes often undesirable time delays.

We previously developed a microelectrode array technology that helps to overcome these problems. Our microelectrode array technology is described in U.S. Pat. No. 7,625,469, which issued to William G. Yelton et. al. on Dec. 1, 2009 under the title, “Nanoelectrode Array for Electrochemical Analysis,” which is commonly owned herewith, and which is hereby incorporated herein by reference in its entirety. U.S. Pat. No. 7,625,469 will hereinafter be referred to as “the '469 patent”.

The electrochemical sensor of the '469 patent can be built on a substrate of choice such as silicon, sapphire, alumina, or glass. With available fabrication techniques, the areal density of individual sensor units can reach tens of billions per square centimeter. Scaling the number of sensor units in the microarray provides an additive increase in signal response. As a consequence, this technology offers detection limits on the order of 1 ppb.

The microarrays enable higher sensitivity by providing greater signal response than macroscopic disk electrodes, due to enhanced mass transport from radial diffusion. Appropriate material selection for the working and counter electrodes offers an initial degree of analyte selectivity through control of the applied voltage. By combining multiple microelectrode arrays having different working electrode materials, the sensor can be designed to have unique fingerprint responses to target species and to suppress false-positive responses to similar chemical species.

Due to the microscale geometry of the sensor, low microampere operating powers are sufficient for data storage and monitoring. Because of its high redundancy, the microelectrode array is robust against sensor fouling.

There is still a need for new approaches to electrochemical sensing that increase sensitivity while minimizing false positive responses. In particular, there is a need for new approaches that are both more sensitive and more selective for target analytes.

There is also a need for new sensing techniques that can be practiced from relatively great distances where analyte concentrations are low out of concern, for example, for personnel safety. One approach that addresses this need extends electrochemical detection technology to gas-phase chemical species, for example to remotely detect gas-phase contaminants emitted from hazardous manufacturing activities or geological events. Providing real-time detection and analysis of such gas species could greatly benefit the health and security of the general population, improve the safety of first responders, and enhance the monitoring of manufacturing processes.

SUMMARY OF THE INVENTION

We have developed a new microelectrode array sensor concept. According to various embodiments, the sensor includes an array of working microelectrodes referred to here as the “MEA”, and it includes one or more electrodes to perform the functions of auxiliary and reference electrodes. (In some embodiments, as explained below, a single electrode can perform both the auxiliary and reference functions.) The sensor also includes a microporous material that, in operation, maintains a liquid environment between the working electrode and the electrode or electrodes serving as the auxiliary and reference electrodes.

For purposes of the present discussion, the working electrode is the electrode at which an electrochemical reaction is driven by applied voltage and a current signal is measured. To maintain ionic balance in the electrochemical cell, the auxiliary electrode generates a current equal in magnitude of current but opposite in sign to the current generated at the working electrode. The reference electrode is the electrode against which the working electrode potential is measured. As noted, the reference electrode may be the same electrode as the auxiliary electrode in some circumstances.

For purposes of the present discussion, microscale structures are structures having at least one significant dimension, such as diameter, that is 10 μm or less. This definition includes within its scope nanoscale structures, which are here defined as having at least one significant dimension that is 100 nm or less. Accordingly, we would regard an electrode as a microelectrode if it has a diameter or mean diameter that is 10 μm or less, and as a nanoelectrode if its diameter or mean diameter is 100 nm or less. Similarly, we would regard a material as microporous if it is permeated with pores having an average diameter of 10 μm or less, and as nanoporous if the average pore diameter is 100 nm or less.

The prefix micro or the prefix nano as used below in terms such as “micropore”, “microchannel”, “microwell”, etc., thus signifies that the structure being referred to is a microscale or nanoscale structure, as appropriate. As explained above, nanostructures are to be understood as a subspecies of microstructures.

In specific embodiments, each working microelectrode is a recessed electrode built into an individual microwell. The microwells can be filled with an aqueous electrolyte such as water or a buffer solution of specified pH, or even a nonaqueous electrolyte such as an organic solvent or ionic liquid. Because of the microdimensionality, the microwells can extract sufficient moisture for operation from the environment via capillary condensation, even in low-humidity, desert-like climates. The microgeometry also inhibits the electrolyte from escaping the microwells by evaporation, making it possible for the electrolyte to subsist for the electrochemical detection of captured and dissolved atmospheric gases.

For at least some applications, the electrolyte can be selected specifically for the analyte that is to be detected. For example, an aqueous electrolyte may be chosen for capturing halogen gases from the atmosphere. Halogen gases will dissolve in aqueous solutions and hydrolyze into ionic species that may be detected by their electrochemical reduction or oxidation at the working electrode.

In other embodiments, the working microelectrodes are not recessed, but instead are coplanar, i.e., they terminate at ends (by an adopted convention, we refer to these ends, without loss of generality, as “top” ends) that are flush with a substrate surface that contacts the electrolyte. In yet other embodiments, the working microelectrodes are prominent, that is, they are raised above the substrate surface that contacts the electrolyte.

Accordingly, recessed microelectrodes may be formed directly within the pores of the microporous material (in which case each pore constitutes a microwell), or in other embodiments an array of coplanar or prominent working microelectrodes may be used. In either case, a microporous material in some form is provided for maintaining the adjacent liquid environment.

In further developments, we add a metal film that can selectively capture and preconcentrate a dissolved or hydrolyzed analyte (or more than one such analyte) that serves as a marker species for the gas targeted for detection. Such a film may be within the pores of the microporous material, or in any other location that permits it to be in contact with the electrolyte. The capture mechanism is controlled by applying a voltage to the metal film. The voltage is chosen to drive a desired electrochemical reaction between the film and the marker species. The reaction produces a solid phase that retains the marker species (with or without application of voltage, depending on the specific embodiment) until it is released for detection by applying a voltage sufficient to drive the reverse electrochemical reaction.

In further developments, we add a bed of preconcentrator material that can selectively preconcentrate a target gas by adsorption over a period of time from the ambient atmosphere and can then release the gas on command into the microelectrode array for sensing. Suitable preconcentrator materials include zeolites, metal-organic framework (MOF) materials, and activated carbons. The controlled release process may be driven, for example, by temperature, pressure, or electrochemical potential.

Preconcentration can increase sensitivity by integrating the collection of the target species over time. Tailoring the preconcentrator material to the target analyte can reduce false positive detections by making the collection more chemically selective.

