Patent Grant
8272250
1. Field of the Invention
This invention relates to a sensor for use in an indicator to provide a warning of exposure to a toxic gas, and to a method of producing such a sensor.
2. Description of Related Art
Personal badge-type exposure indicators are critical components of next generation protective gear. Ideally, such indicators not only warn of an exposure event but also quantify the extent of exposure and provide a stream of data in real time so that informed decisions can be made regarding ambient toxicity.
The inventors have determined that a film of naked nanoparticles on a non-conductive substrate such as glass or polyethylene is a suitable sensor for use in an indicator of the type for use with protective gear.
The current flow between metal nanoparticles interconnected by molecules is a fundamental process underlying single electron transistors and much of the field of molecular electronics. When the distance between nanoparticles is greater than 2 nm and the barrier to charge transfer greater than 1 eV, current flow between particles occurs via single-electron tunneling. Under these conditions, the residence time of the electron on a nanoparticle is relatively long and electric current flow occurs via a series of discrete tunneling “hops” of electrons from nanoparticle to nanoparticle. In this regime, the rate of current flow depends on a number of factors including the bias applied, the electronic structure of the interparticle molecules, the goodness of the electrical contact between the molecules and the surface of the nanoparticles, the distance between nanoparticles and the charging energy of the nanoparticles.
Current flow through monolayers of close-packed metal nanoparticles have been extensively studied. Examples studied to date include films of thiol-capped 2.7-4.8 nm diameter Ag nanoparticles, and monolayer-protected gold nanoparticles. The nanoparticles in such films are typically encapsulated in monolayer coatings, which prevent particle coalescence as well as retain a constant and well defined interparticle spacing. The formation of films from the coated nanoparticles occurs via self-assembly. The resulting bilayer of molecules between the nanoparticles in such films provides a barrier to direct charge transport between particles, ensuring that interparticle, single-electron tunneling of charge across the molecular bridge between the nanoparticles is the dominant charge transfer mechanism. In this configuration, the conduction characteristics of the nanoparticle film are expected to be especially sensitive to the nature of the molecular bridge. Self-assembly methods, however, are not ideally suited for study of the molecular bridge because changing the type of bridge also changes the interparticle spacing so the results are convoluted.
To circumvent the above mentioned problem the inventors focused on films of naked nanoparticles. Using a gas-phase deposition approach, monolayers of ligand-free nanoparticles can be generated in which the average interparticle distance is controllable. When the interparticle distance is small enough, these naked nanoparticle films also display conduction behaviors characteristic of single-electron tunneling through the spaces between the particles. Because the electrons necessarily tunnel through the interparticle space, the addition of molecular material to these spaces (most likely as an adsorbate on the nanoparticle surfaces) impacts the tunneling rate and current flow observed. Thus, the medium, through which the electron tunnels, can be changed without changing the interparticle spacing.
As mentioned above, the inventors have determined that a film of naked metal nanoparticles on a glass or polyethylene substrate is a suitable sensor for use in an indicator of the type for use with protective gear. As a specific example, the resistance across an Ag nanoparticle film changes when the film is exposed to a toxic gas such as 2-chloroethyl ethyl sulfide (CIEES), which is a simulant for mustard gas. The same is true when the film is exposed to sulfur mustard gas or HCN warfare agent.
In accordance with one aspect, the present invention provides a method of producing a sensor for use as an indicator of exposure to a toxic gas comprising the steps of: generating nanoparticles of a conductive metal; depositing the nanoparticles on a non-conductive inert substrate to yield a two-dimensional film of nanoparticles, wherein the spacing between the nanoparticles is small enough to permit electron tunneling between particles and a current can be made to flow across the film; and connecting an electrode to each end of the film, whereby, when an electrical current is passed through the film and the sensor is exposed to a toxic gas, changes in the electrical resistance of the film will provide an indication of the presence of such toxic gas.
In accordance with a second aspect, the present invention provides a sensor for use as an indicator of exposure to a toxic gas comprising: a non-conductive, inert substrate; a two-dimensional film of nanoparticles of a conductive metal on said substrate, wherein the spacing between the nanoparticles is small enough to permit electron tunneling between particles and a current can be made to flow across the film, whereby, when an electrical current is passed through the film and the sensor is exposed to a toxic gas, changes in the electrical resistance of the film will provide an indication of the presence of such toxic gas.
Using the above defined method, arrays of naked nanoparticles have been made with interparticle spacing small enough that electrons can tunnel between particles and a current can be made to flow through the nanoparticle film. The rate of electron tunneling across the film is extremely sensitive to the nature of the material between the nanoparticles. Adsorption of any species on or near the nanoparticles causes a large change in conductance of the interparticle gaps. The measured resistance of the film or tunneling current is a sensitive means of sensing the presence of adsorbate.
The particles are naked and the particle spacing is controlled. Because the particles are naked, there is dependence on matrix material, and tailor-designing matrices that respond to specific chemicals is not required. Any gases that adsorb to the nanoparticles can be detected, and it should be possible to determine the nature of the adsorbed gas from changes in conductance characteristics of the film.
The invention is described in greater detail below with reference to the accompanying drawings, wherein:
The inventors deposited nanoparticles on a substrate using a deposition apparatus described elsewhere (see Pedersen, D. B. et al, J. Phys. Chem. C., 111 (15), 5592-5598). Nanoparticles were first generated in the gas phase using a magnetron DC-sputtering source. Application of a 280 V bias between an anode cap and a metal target caused a discharge in the 0.17 Torr pressure of Ar gas maintained between them. The current flow to the discharge was kept to 200 mA. Any Ar+ ions generated in the discharge were accelerated toward the negatively biased metal target which they struck with force, thus liberating metal atoms to the gas phase. These atoms were swept up in the flow of Ar leaving the discharge region. Upon leaving the sputtering region the atoms passed through an aggregation zone where the collision frequency between metal atoms was high, and formation of nanoparticles occurred. The nanoparticles thus generated them moved downstream into the expansion zone, which was evacuated by a 500 L s−1 turbo pump (Varian V-550). The nanoparticles then passed through an orifice into the neighboring deposition chamber where a pressure of <10−4 Torr was maintained during deposition by a 300 L s−1 turbo pump (Varian TV-301). The size of the nanoparticles could be varied by varying parameters such as Ar and He gas flow rates, aggregation zone length and discharging current. A substrate (polyethylene or glass) with painted silver electrodes positioned in front of the orifice collected the nanoparticles which deposited as 2D films of naked nanoparticles. The distance between the particles varied with deposition time pseudo-continuously; at longer times more particles reside on the surface and the average interparticle distance is decreased accordingly. The resistance between electrodes was monitored during deposition with an Agilent digital multimeter (34401A) connected to a computer via HPIB interface.
Exposure experiments were conducted in a fume hood. A nanoparticle-coated polyethylene film was placed on a stand. Light exiting an optic fiber connected to a halogen lamp passed through the sample and was collected by a collimating lens attached to a second optic fiber, on the other side of the sample, that carried the light to the CCD array of a UV-vis spectrometer (Ocean Optics SD2000). In this configuration, the resistance between electrodes and the spectrum of the nanoparticles between electrodes could be monitored simultaneously during exposure of the nanoparticle film to CIEES. Exposure was effected by opening a bottle of CIEES (Aldrich, 98%) 5 cm from the film and letting the vapors diffuse in the fume hood.
The nanoparticle sensor was also exposed to sulfur mustard gas and HCN warfare agent, and the sensor responded well to both. The sensor was exposed to CO and there was no response which demonstrates some selectivity.
The deposition of Ag nanoparticles generated by the sputtering source onto substrates yielded two dimensional arrays of nanoparticles. A sample scanning tunneling microscope (STM) image of a film deposited on highly ordered pyrolitic graphite (HOPG) revealed particles appearing as white shapes against a darker background. The outline of each particle is discernible and the size easily determined. From such images the 2D nature of the films was established and the diameter of the nanoparticles was found to be 2.8±0.5 nm. The distance between particles could be varied by varying the deposition time. The distance between the nanoparticles was found to be >10 nm but smaller interparticle separation was possible by increasing the deposition time. In general, the interparticle separation has a well defined average value because the deposition is a random process. It is straightforward to show that a random deposition yields an average interparticle separation that varies inversely with t1/2, where t is the deposition time. Accordingly, plots of the interparticle distance versus the inverse of the square root of the deposition time are linear (see Pedersen et al supra). The linearity combined with STM data and trends in the optical properties of such films establish that the films are 2D arrays of nanoparticles with interparticle distances that decrease steadily as deposition time is increased.
For a 15 min deposition of Ag nanoparticles on a glass slide or polyethylene film, the average interparticle distance is small enough that current can flow between two silver electrodes situated at either end of the nanoparticle film. When the particle density is low enough, such current is expected to flow via tunneling of electrons across the interparticle gaps. Controlling the distance between adjacent nanoparticles affords an opportunity to examine the distance dependence of the through-space tunneling current between nanoparticles. A plot of the resistance, measured between two silver electrodes spaced 3 mm apart on the surface of a polyethylene film, versus t is shown in
The tunneling current, or rate of tunneling, is given by
I=I0e−βd
where I0 is the pre-exponential factor, d is the interparticle separation, and β is the fall-off or attenuation factor. A fit of this equation to the inverse of the resistance is shown in the inset of
The addition of molecules to the interparticle spaces is expected to change the rate of tunneling and thus the resistance of the nanoparticle film. To effect such change, the nanoparticles were exposed to CIEES. A sample of the change in film resistance that resulted during the exposure is shown in
To gauge the sensitivity, the change in resistance and the change in the optical properties of some nanoparticle films were monitored simultaneously. Some sample results of the optical and resistance data obtained are shown in
Following exposure to CIEES, the resistance of the nanoparticle films decreased significantly, as seen in
The method described above has been used to deposit copper nanoparticles on an inert substrate, i.e. glass and polyethylene. Sensors can also be produced using any noble metal such as gold, platinum and palladium. The particle size and spacing of the nanoparticles are listed hereinbefore as 3.2 and 6.0, respectively. However, it has been determined that the particle size can be 1-100 nm and preferably 2-50 nm, and the edge to edge spacing can be 4 to 50 nm and preferably 5 to 25 nm.
This application claims priority on U.S. Provisional Application 61/071,172 filed Apr. 16, 2008.
| Number | Name | Date | Kind |
|---|---|---|---|
| 6537498 | Lewis et al. | Mar 2003 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20100263435 A1 | Oct 2010 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61071172 | Apr 2008 | US |
