Acoustic Wave Sensors by unknow

Author:unknow
Language: eng
Format: epub
ISBN: 978-0-08-052333-0
Publisher: Elsevier Science
Published: 1997-03-14T16:00:00+00:00

Figure 5.2 Response of lead phthalocyanine-coated SAW to NO2 with (decoupled) and without (coupled) a metal subcoating. The data indicate that the response of the LiNbO3 SAW is due to acoustoelectric effects, rather than mass loading. The “single” designation indicates that the response is from an individual SAW sensor (i.e., no reference sensor was employed). (Reprinted with permission. See Ref. [67]. © 1985 Elsevier Publishers.)

More recent studies have focussed on the characterization of the conductive polymer, polypyrrole, and its use as a coating for acoustic wave vapor sensors [2–8]. One advantage of polypyrrole is that it can be generated directly on an electrode surface by electropolymerization, which makes it particularly suited for the TSM (quartz resonator) devices. The conductance of the polypyrrole film changes upon sorption of organic vapors, with nucleophilic vapors producing an increase in film resistance and electrophilic vapors causing a decrease in resistance [77]. The oxidation state of the deposited film also appears to influence its sorptive capabilities [75]. The observed response of polypyrrole-coated TSM sensors, however, is likely dominated by physical effects (e.g., mass loading and/or stiffness changes). The polypyrrole films are particularly well suited for hybrid sensor configurations. For example, the use of a SAW sensor (for mass measurements) in tandem with a chemiresistor (for conductivity measurements) is feasible; such hybrid devices have been previously reported using phthalocyanine coatings [78]. A unique advantage of the TSM devices is the presence of an electrode that can function as a working electrode in an electrochemical cell. Thus, sorption of vapors into the film can produce a mass loading response and, if the vapor is electroactive, the vapor may also be involved in redox reactions (current flow) with the film at the electrode surface [76]. Such a configuration could provide additional means of discriminating between sorbed vapors.

An uncoated substrate can also be affected by the interaction between polar analytes and the electric field associated with the propagating acoustic wave. This effect may have contributed to SAW sensor responses to high relative humidity levels by Huang [79]. Using lithium niobate substrates, increased attenuation of the SAW was observed in the RH range of 89% to 98%; minimal frequency shifts were observed during these studies. Increases in attenuation may have occurred as a result of electrical leakage between electrode finger pairs in the IDTs. Other studies have reported liquid sensing/detection of ionic species using uncoated acoustic devices, where the frequency of the device changes in response to the conductance of the solution. Yao et al. reported the use of a TSM device to monitor acid-base [80] and compleximetric titrations [81]. The frequency of the TSM changed with the specific conductance of the solution, which depends strongly on the ion(s) present and their respective concentrations. By plotting frequency shift vs titrant volume, the endpoint was determined as a change in slope. This sensor was subsequently used for the multi-component determination of aspirin and salicylic acid in solution [82]. Nomura and Kanazawa also used an uncoated TSM to analyze for metal ions in solution [83].

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.