Down to the Wire

Conducting Polymers Enable New Sensors
Contributed editorial appearing in
Scientific Computing & Instrumentation 20:7, June 2003, pg. 14.

In general, sweeping generalizations are bad things. However, the hallmark of a good scientist is the ability to extract common generalities from a large collection of items that appear to be different. This ability leads to landmark developments such as the Second Law of Thermodynamics and "the mathematics reduce to a previous problem and are left as an exercise for the student."

At the opposite end of the spectrum is the artistic ability to differentiate among nearly identical items. Engineers exploit this talent during the identification of specific variables responsible for unique system behavior. Generalization also lends its utility to materials classification. For instance, metals are considered good conductors of electricity while polymers and ceramics are most often known for their insulating properties. Yet, important exceptions abound. The current record holder for high-temperature superconductivity is ceramic and polymer research in the 1970s launched the development of conducting polymers a.k.a. "synthetic metals."

One mechanism used to create conductors from traditionally insulating materials is to create a composite comprised of conducting particulates dispersed throughout an insulating matrix. Liquid water consists of uncharged molecules that are not very good at conveying electrons from a cathode to an anode. However, the autodissociation of water into positively charged hydronium ions and negatively charged hydroxide ions provides a small concentration of charge carriers leading to ultrapure water having a conductivity of 55 nano-Siemens per centimeter (nS/cm). Increasing the number of charge carriers by creating an aqueous solution of 31 percent nitric acid raises the conductivity to 0.87 S/cm. This is still much lower than the 1 MS/cm conductivity of copper, but I would not want to drop a hair dryer into it.

This change in composite conductivity as a function of charge carrier concentration forms the basis of the sensor array known as an "electronic nose." Conducting particles of carbon-black are mixed into a matrix of low-density insulating polymer and deposited onto the surface of a cathode/anode couple. The conductivity of the composite film is determined by measuring the voltage drop across the electrodes in much the same way the resistance of a thermistor is determined. The concentration of carbon-black is not modulated by the addition or removal of particles. Rather, the film swells or shrinks as sample vapors are reversibly absorbed into and out of the polymer. The effect of analyte identity on polymer swelling differs as a function of the polymer used. Typically, the conductivities of a dozen or so electrodes coated with different polymer composites are recorded as the sensor array is exposed to various calibration gasses. The resulting database of sensor responses represents the training rules used to identify an unknown analyte, akin to training a bomb-sniffing dog.

A second mechanism uses organic polymers that are native conductors. Conjugated polymers comprised of alternating single and double carbon-to-carbon bonds can be considered conducting polymers (CP), or at least semiconducting polymers, such as polyaniline having a conductivity of 4 S/cm. The overlapping pi-orbitals along the CP backbone provide an electron transport channel and, just like carbon's neighbor, silicon, located right below carbon in Group 14 of the Periodic Table, conjugated CPs behave like a material whose conductivity can be increased by doping. For example, polyacetylene doped with arsenic fluoride exhibits conductivity in the range of 150 kS/cm, an order of magnitude higher than liquid mercury at 10 kS/cm. Some dopants also can be added and removed reversibly, forming a redox "chemiresistor" whose resistance changes when exposed to specific analytes.

The reaction of bromine and iodine with polyaniline removes electrons from the conjugated backbone, yielding a p-type semiconductor while sodium can be used to create n-type CP. The resulting CP pn junctions can be used to fabricate organic light emitting diodes (OLEDs), chemical field effect transistors (ChemFETs), and chemically sensitive capacitors (ChemCAPs). In addition to the benefits of low-temperature materials processing, plastic, and elastic properties of these organic electronic devices, their fabrication utilizes standard methods of synthetic organic chemistry permitting the creation of designer sensors. The 2000 Nobel Prize in Chemistry was awarded for research in conducting polymers and this notoriety will spur additional research; generally speaking, of course.