The use of a sorptive layer to collect and concentrate vapor molecules at a sensor's surface is shown schematically in Fig. 1. The presence of these sorbed vapor molecules can then be detected in a variety of ways, depending on the transduction mechanism of the sensor device. Acoustic wave devices, such as the thickness shear mode (TSM) device or the surface acoustic wave (SAW) device, transduce the presence of the vapor as an increase in mass detected by a change in the acoustic wave velocity.[5-8] Typically, the acoustic wave velocity change is measured as a change in oscillator frequency.
A schematic diagram of an MPN as an individual particle, its component parts, and a film is shown in Fig. 2. The film represents a cross section where gold cores are separated by the organic monolayers, with areas of lower density suggested between the MPNs. As a sorptive layer, molecules could sorb into the thiol material and/or the free volume associated with areas of lower density.
Chemiresistor devices measure a change in the resistance of the applied thin films as a function of the sorbed gar or vapor.[9-12] Because an MPN film on such a device carries the current, it participates directly in the transduction mechanism. These films consist of metal particles separated from one another, insulating monolayers on their surfaces (Fig. 2). The current observed as electrons travel through a film from particle core to particle core can be modulated by the sorption of vapor molecules in the insulating regions. These insulating regions are of molecular dimensions, hence the sorption of molecules can represent a significant perturbation. Wohltjen and Snow described MPN-sensing films as metal-insulator-metal ensembles (MIMEs) in recognition of the nanostructure of the films.
Regardless of the transduction mechanism chosen, the sorption of a vapor into the MPN film represents a significant aspect of the sensor response. The amount of vapor uptake and the rate of vapor uptake by these film materials provide useful information for understanding and interpreting sensor response behavior. Differences in the sorption of various vapors (i.e.,
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