When considering new sensory technologies one should look to nature for guidance. Indeed, living organisms have developed the ultimate chemical sensors. Many insects can detect chemical signals with perfect specificity and incredible sensitivity. Mammalian olfaction is based on an array of less discriminating sensors and a memorized response pattern to identify a unique odor. It is important to recognize that the extraordinary sensory performance of biological systems does not originate from a single element. In actuality, their performance is derived from a completely interactive system wherein the receptor is served by analyte delivery and removal mechanisms, selectivity is derived from receptors, and sensitivity is the result of analyte-triggered biochemical cascades. Clearly, optimal artificial sensory sys-

Conjugated polymer-based chemical sensors.

An analyte monitor includes a sensor, a sensor control unit, and a display unit. The sensor has, for example, a substrate, a recessed channel formed in the substrate, and conductive material disposed in the recessed channel to form a working electrode. The sensor control unit typically has a housing adapted for placement on skin and is adapted to receive a portion of an electrochemical sensor. The sensor control unit also includes two or more conductive contacts disposed on the housing and configured for coupling to two or more contact pads on the sensor. A transmitter is disposed in the housing and coupled to the plurality of conductive contacts for transmitting data obtained using the sensor. The display unit has a receiver for receiving data transmitted by the transmitter of the sensor control unit and a display coupled to the receiver for displaying an indication of a level of an analyte. The analyte monitor may also be part of a drug delivery system to alter the level of the analyte based on the data obtained using the sensor.

Analyte Monitoring Device And Methods Of Use

Color Control in π-Conjugated Organic Polymers for Use in Electrochromic Devices

Application of conducting polymers to biosensors.

A chemosensor is a molecular device designed to detect a specific molecule or class of molecules.1a,b Research in this field is poised for considerable advances in the coming years with the advent of diverse methods for analyte detection and new developments in the field of molecular recognition. To date, the most common signal transduction schemes utilize optical or electrical methods.1c Fluorescence is a highly sensitive optical transduction method, and analyte binding events that produce an attenuation, enhancement, or wavelength shift in the emission can be used to produce a functional sensor.1d Changes in absorption spectra, while less sensitive, have also been extensively used.1e Redox processes are widely used in electrical-based transduction methods, and typical systems function in either potentiometric or amperometric modes.1f Conductometric detection schemes based on SnO2, conducting polymers,1h and phthalocyanines1i have also been investigated. As shown in Scheme 1, a chemosensor is composed of two functional elements, a receptor and a reporter group, which need not be separate in identity. When the equilibrium between the analyte and receptor is rapid, sensors can be produced that provide a real-time response, which continuously varies with the concentration of the analyte. The detection sensitivity is determined by both the ability to measure the transduction event and the association constant of the receptor-analyte complex. As a result, when pursuing higher sensitivity one may seek instrumentation improvements and/or endeavor to increase the magnitude of the association constant of the receptor-analyte complex. The standard approach to higher association constants is to design highly preorganized receptors that do not pay a high entropic penalty for complexation. The downside of this approach is that preorganization and high association constants generally result in slow dissociation kinetics. A molecular chemosensor with slow kinetics or an irreversible response cannot yield a reversible real-time response. Molecular systems displaying irreversible or slow behavior are nonetheless useful, but are properly called dosimeters or indicators. Methods may be developed to allow irreversible systems to function in a sensory device. For example, the device can be “reset” by chemical, electrochemical, photochemical, or physical events. These processes can cause the analyte to dissociate from the receptor or can result in the replacement of the indicator (chemosensor) molecules. Such approaches have the disadvantage of introducing additional complexity into the sensor devices. This Account describes an approach to enhancing the sensitivity of chemosensors in effect by “wiring chemosensory molecules in series” (Scheme 2). Recent work from my research laboratory has shown that this molecular wire approach provides a universal method by which to obtain signal amplification relative to single molecule systems. I will use the term molecular wire interchangeably with conducting polymer. For the sake of clarity, some representative conducting polymers are shown in Scheme 3. These materials are insulators in their neutral (undoped) Timothy M. Swager is a native of Montana and received a B.S. from Montana State University in 1983 and a Ph.D. from the California Institute of Technology in 1988. After a postdoctoral fellowship at the Massachusetts Institute of Technology (1988-1990), he began his independent academic career at The University of Pennsylvania and was promoted to Professor in 1996. He is currently a Professor of Chemistry at the Massachusetts Institute of Technology. His interests in supramolecular chemistry include the design of electronic polymers, sensors, and liquid crystals, molecular recognition, and catalysis. Scheme 1

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The Molecular Wire Approach to Sensory Signal Amplification

: This document reports the fabrication of a chemically derivatized microelectrode array that can function as a transistor when immersed in an electrolyte solution. The key finding is that a small signal (charge) needed to turn on the device can be amplified. The device to be described MIMICS the fundamental characteristics of solid state transistor, since the resistance between to contacts can be varied by a signal to be amplified. The chemical transistor is the set of three (drain, gate, and source) Au microelectroces covered by polypyrrole.

Chemical derivatization of an array of three gold microelectrodes with polypyrrole: Fabrication of a molecule-based transistor

: An array of eight Au microelectrodes, each approximately 0.12 microns thick, 3 microns wide, and 140 microns long separated from each other by a distance of 1.4 micron has been fabricated on a 0.45 micron thick Si02 layer grown on a single crystal Si substrate using standard microfabrication techniques. Each electrode can be individually addressed and characterized electrochemically. The individual electrodes can b functionalized with polypyrrole or with poly-N-methylpyrrole by oxidation of pyrrole or N-methylpyrrole, respectively, using conditions similar to those for macroscopic electrodes. The amount of polymer deposited can be controlled and it is possible to electrically connect adjacent microelectrodes with deposited polymer. Since the reduced forms of these polymers are insulating and the oxidized forms are electronically conducting it is possible to prepare electronic devices that are analogous to diodes and transistors using adjacent microelectrodes connected with polymer.

Chemical derivatization of microelectrode arrays by oxidation of pyrrole and n-methylpyrrole: fabrication of molecule-based electronic devices.

Diffusion to arrays of closely spaced (1.2-0.2 pm) ultramlcroelectrodes (50 pm X 2.3 w) was studied by dlgltal dmuiation and by examining the redox behavior of Ru(NH,)~+ in H,O. Cyiindrlcai diffusion of solution species resulted in quasi-steady-state currents at the microband electrodes. Generation-collectlon experlments, analogous to rotating ring-disk collection experhnents, resulted in larger generator currents than those observed at a single microelectrode due to the back diffusion of products to the neighboring mlcroe lectrode. A collection efficiency of 93% was observed for the reoxidation of RU(NH,):+ generated at a central microelectrode 0.2 pm from two fianklng collector mlcroelectrodes. This experlmenl as well as generator-shrgle collector electrode pairs was simulated at a two-dimensional rectangular expandlng grld and yielded results in good agreement wlth the experiment. Predictions of the model that the collection efflclency principally depends on the gap dze, rather than electrode wldth, were tested experimentally. The novel application of microelectrode arrays to the study of the foilow-up reactions of electrogenerated Intermediates is demonstrated. The digital simulation of electrochemistry of ultramicroelectrode arrays is shown here to be successful in predicting the effect of variations in electrode geometry on the current response. Our results on the properties of arrays of closely spaced microelectrodes represent the most complete study where theory can be tested with experiment.

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Digital Simulation of the Measured Electrochemical Response of Reversible Redox Couples at Microelectrode Arrays: Consequences Arising from Closely Spaced Ultramicroelectrodes

Several types of new microelectronic devices including diodes, transistors, sensors, surface energy storage elements, and light-emitting devices are disclosed. The properties of these devices can be controlled by molecular-level changes in electroactive polymer components. These polymer components are formed from electrochemically polymerizable material whose physical properties change in response to chemical changes, and can be used to bring about an electrical connection between two or more closely spaced microelectrodes. Examples of such materials include polypyrrole, polyaniline, and polythiophene, which respond to changes in redox potential. Each electrode can be individually addressed and characterized electrochemically by controlling the amount and chemical composition of the functionalizing polymer. Sensitivity of the devices may be increased by decreasing separations between electrodes as well as altering the chemical environment of the electrode-confined polymer. These very small, specific, sensitive devices provide means for interfacing electrical and chemical systems while consuming very little power.

Gregg P. Kittlesen

Papers

Chemical derivatization of an array of three gold microelectrodes with polypyrrole: Fabrication of a molecule-based transistor

Chemical derivatization of an array of three gold microelectrodes with polypyrrole: Fabrication of a molecule-based transistor

Chemical derivatization of microelectrode arrays by oxidation of pyrrole and n-methylpyrrole: fabrication of molecule-based electronic devices.

Digital Simulation of the Measured Electrochemical Response of Reversible Redox Couples at Microelectrode Arrays: Consequences Arising from Closely Spaced Ultramicroelectrodes

Molecule-based microelectronic devices