Biomedical Engineering Reference
In-Depth Information
Penza
. [113] demonstrated an SAW LB CNT nanocomposite
film for VOC detection at room temperature. Figure 9.37a shows
a typical time response of two ST-cut quartz-based SAW 433 MHz
sensors, one coated by two monolayers of LB CdA film and other
coated by two monolayers of LB nanocomposite of 27.5 wt.% SWCNTs
embedded in a CdA host matrix. These devices are exposed, at room
temperature and relative humidity of about 10%, to 5 min pulses of
ethanol decreasing spot concentrations ranging from 173 to 7 ppm.
The SAW sensor response decreases upon a given concentration of
analyte for both detectors due to mass loading of vapor molecules
adsorbed into nanomaterial. A comparison of the SAW sensitivity
to ethanol between nanocomposite LB film at 27.5 wt.% of SWCNTs
embedded in CdA and LB CdA film has been performed. The results
obtained, shown in Fig. 9.37b, indicate that the frequency shift as
a function of ethanol concentration is fitted by a linear regression
for both SAW sensors, coated by a two-monolayer thick LB film. The
ethanol sensitivity of 39 Hz/ppm of SWCNT nanocomposite is about
twice that of unembedded LB CdA film. This better vapor sensitivity
of the SWCNT nanocomposite could be attributed to an enhanced
acousto-electrical interaction between gaseous analyte and higher
electrical conductivity nanocomposite (e.g., measured conductance
of the 55.4 nm thick LB 27.5 wt.% nanocomposite film was as high as
25
et al
µ
S) compared to lower conductivity CdA matrix (e.g., measured
conductance of the 55.6 nm thick CdA film was as low as 0.1
S).
This better sensitivity of the SWCNT nanocomposite toward the
various tested VOCs of ethanol, ethylacetate, and toluene has been
also measured for polar and nonpolar organic solvents, as reported
in Fig. 9.37c, considering the slope of the SAW frequency change vs.
vapor concentration linear relationship for each analyte under test.
Moreover, the vapor sensitivity of the nanocomposite of SWCNTs
embedded in the CdA host matrix can be enhanced by increasing
the weight ratio of SWCNTs in CdA [179]. Also, the sensitivity of a
given nanocomposite at fixed weight ratio of the CNT-filler increases
with the SAW resonant frequency from 315 to 433 MHz, as shown in
Fig. 9.37d, and the highest sensitivity has been measured for toluene
vapor as 43.7 Hz/ppm using the SAW 433 MHz device. The enhanced
gas sensitivity with increasing frequency is attributed to the well-
known mass-frequency relationship in a vibrating body, where the
frequency shift due to mass loading is proportional to the resonant
fundamental frequency squared.
µ
