The Macromolecular, Supramolecular, and Nanochemistry and Chemical Measurement and Imaging Programs jointly support Professor Reginald M. Penner of the Department of Chemistry at the University of California at Irvine to investigate chemical sensors based upon metal nanowires. Three new modes for detecting molecules using metal nanowires are being developed in this project, which coincide with three objectives: 1) the use of surface electron scattering to detect gas phase molecules using noble metal nanowires. A specific example is a single gold nanowire that is capable of rapidly detecting 1 ppm NO2 and NO in air at elevated temperatures. 2) To exploit the heat of molecular chemisorption as means for detecting adsorbates in the gas phase using arrays of suspended metal nanowires. These investigations focus on ethylene on platinum and ethanol on palladium. 3) To create a family of nanoscopic chemical sensors based upon nanogaps in metal nanowires that are modified by the insertion of an affinity medium, such as a conductive organic polymer imprinted for the analyte molecule of interest, as well as a conductive polymer with incorporated antibodies during polymerization from solution. A broader impact of this research activity is a project involving outreach to economically disadvantaged high school students in the Santa Ana, California school district. Each year this provides a student from Santa Ana High School ten weeks of full-time paid summer research in Professor Penner's laboratory.
As a chemical sensor is made progressively smaller, it responds more rapidly to changing concentrations of the molecule that is being measured. In this project, a family of highly miniaturized chemical sensors are created using metal nanowires. These nanowires are prepared using a process that is closely related to those used in the semiconductor industry to prepare integrated circuits. The resulting nanowires, composed of gold, palladium, or platinum, are approximately 1/100th the diameter of a human hair, but of similar lengths on the order of millimeters or centimeters. The electrical resistance of these diminutive wires increases when molecules attach to the surfaces of the wire, and this process provides a means for detecting these molecules very rapidly, within a few seconds. The molecules that are being detected in these studies are nitrogen dioxide, sulfur dioxide, carbon monoxide, and hydrocarbons like methane and ethane. In addition to a rapid response, a second advantage of these nanowire-based chemical sensors is their simplicity: A chemical sensor consists of a single metal nanowire, two electrical contacts to this nanowire, and an instrument for measuring its electrical resistance. Consequently, these sensors can be very inexpensive.
