Chemistry Reference
In-Depth Information
Fig. 41
Δ
RAS, defined as the difference
Δ
R
/
R
after
Δ
R
/
R
before
between RAS spectra measured
before (
R
/
R
after
) the exposure of a 30-ML-thick Zn-HepOTTP LS film to
increasing quantities of volatile compounds.
Top
: ethanol 3% (
full line
), 9% (
dotted line
), 18%
(
dashed line
).
Middle
: hexane 3% (
full line
), 9% (
dotted line
), 18% (
dashed line
).
Bottom
:
trimethylamine 3% (corresponding to 90 ppm,
full line
), 9% (corresponding to 270 ppm,
dotted
line
). Significant wavelengths (see text) have been evidenced by
vertical dashed lines. Inset
:
Δ
Δ
R
/
R
before
) and after (
Δ
RAS signal amplitudes (from spectra) for ethanol and hexane at characteristic wavelengths
versus nominal concentration of VOCs. The
lines
have been drawn to guide the eyes. The error bar
in this case is evaluated as
0.0002 (From Bussetti et al. [
86
]. Reproduced with permission.
Copyright 2009, AIP Publishing LLC.)
modulates the optical anisotropy via the variation in the interaction between
adjacent molecules. This information can be read out, leading to the possible
chemical recognition of the single analytes.
LS systems offer in general a large surface-to-volume ratio, meaning a wide
sensing area for adsorbing volatile molecules [
85
]. However, the higher structural
order of LB layers is expected to provide a larger anisotropy signal, that is, a higher
sensitivity to the adsorption process monitored by RAS. New experiments were then
performed on an LB thin film of the same porphyrin used in [
86
], to exploit the more
ordered arrangement of the molecules due to this deposition technique. In particular,
