Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Figure 1. Response of an FSCR to a 2000 s exposure of p-xylene<br />
at an activity of 0.019. The conductance drops exponentially and then<br />
recovers on the same time scale during the nitrogen purge.<br />
for most vapor detection applications), twice the analyte vapor<br />
pressure gives twice the swelling, yet also gives twice the number<br />
density of analyte molecules, and thus twice the diffusive flux. A<br />
swelling time that is independent of the analyte concentration is<br />
highly desirable, since it would simplify the interpretation of<br />
kinetics data. In fact, the raw data we collect are sensor response<br />
versus time, so the response must be converted to polymer<br />
swelling before the swelling time can be computed. This is<br />
accomplished using the transduction curve of the particular<br />
chemiresistor, which is the relation between its change in<br />
conductance and equilibrium swelling. In a previous paper we have<br />
shown that this transduction curve is independent of the analyte,<br />
and so can be determined from testing the chemiresistor with<br />
any particular analyte, regardless of its affinity for the polymer. 18,19<br />
The kinetics data we present were taken with the fieldstructured<br />
chemiresistors (FSCRs) we have developed over the<br />
past several years. 18-23 These sensors differ from traditional<br />
carbon black chemiresistors in that the particle phase is composed<br />
of Au-coated Ni particles that are structured into conducting<br />
pathways using magnetic fields. Field-structuring consistently<br />
brings the particle phase to a conducting threshold over a wide<br />
range of particle volume fractions, and the Au-coated particles do<br />
not adsorb typical VOCs. As a result, FSCRs have significantly<br />
increased baseline stability, sensitivity, reversibility, and sensorto-sensor<br />
reproducibility. 18<br />
BACKGROUND<br />
Response Curve. The response of a chemiresistor exposed<br />
to a step increase in analyte concentration is given in Figure 1.<br />
The conductance drops to its equilibrium value as the polymer<br />
swells and then recovers on the same time scale as the polymer<br />
deswells. FSCR response is defined as the conductance ratio G/G0,<br />
where G and G0 are the conductances in the presence and<br />
absence of analyte, respectively. Plotting the equilibrium value<br />
of G/G0 as a function of analyte concentration gives a sigmoidal<br />
(18) Read, D. H.; Martin, J. E. Adv. Funct. Mater. 2010, 20 (11), 1–8.<br />
(19) Read, D. H.; Martin, J. E. Anal. Chem. 2010, 82 (12), 5373–5379.<br />
(20) Martin, J. E.; Anderson, R. A.; Odinek, J.; Adolf, D.; Williamson, J. Phys.<br />
Rev. B 2003, 67, 094207.<br />
(21) Martin, J. E.; Hughes, R. C.; Anderson, R. A. Sensor Devices Comprising<br />
Field Structured Composites. U.S. Patent 6,194,769, Feb 27, 2001.<br />
(22) Martin, J. E.; Hughes, R. C.; Anderson, R. A. Field-Structured Material Media<br />
and Methods for Synthesis Thereof. U.S. Patent 6,290,868, Sept 18, 2001.<br />
(23) Read, D. H.; Martin, J. E. Anal. Chem. 2010, 82, 2150–2154.<br />
6970 <strong>Analytical</strong> <strong>Chemistry</strong>, Vol. 82, No. 16, August 15, 2010<br />
Figure 2. The FSCR equilibrium conductance versus analyte activity<br />
plot forms a sigmoidal curve, which is parametrized by Γ and the<br />
response midpoint, a1/2. For example, the acetone curve has a<br />
response midpoint of ∼0.15, and the detection range is from an<br />
activity of ∼0.01 to an activity of 0.36. Response curves for this single<br />
FSCR exposed to various analytes illustrate selectivity for more<br />
hydrophobic analytes. As evident, these equilibrium data do not<br />
discriminate between chemically similar analytes.<br />
equilibrium response curve, as in Figure 2. Because the relative<br />
resistance change, ∆R/R0, is exponential with the analyte<br />
concentration, the relative conductance can be fit by 18<br />
G<br />
)<br />
G [ 0<br />
1 + eΓa/a1/2 - 1<br />
e Γ -1<br />
- 1 ]<br />
Here a is the analyte vapor activity, Γ is a fit parameter related to<br />
the abruptness of the conductor-insulator transition (typically<br />
∼4), and a1/2 is the response midpoint, defined as the analyte<br />
activity that reduces G0 by half.<br />
Figure 2 shows that a poly(dimethylsiloxane) chemiresistor<br />
is much more sensitive to the hydrophobic analytes toluene,<br />
p-xylene, mesitylene, and undecane than to the hydrophilic analyte<br />
acetone. In fact, the response midpoint for propanol is nearly 10×<br />
that of toluene or p-xylene. Although this polymer is selective for<br />
hydrophobic analytes, the equilibrium response cannot be used<br />
to discriminate between different analytes unless the analyte<br />
activity is known. More typically, these data would be used to<br />
determine the activity of a known analyte.<br />
Transduction Curve. We have previously shown that the<br />
response of an FSCR is a universal function of polymer swelling,<br />
regardless of the chemical nature of the analyte. 18,19 The response<br />
curve in Figure 2 can thus be thought of as a combination of a<br />
solely device-dependent transduction curve (conductance as a<br />
function of swelling) and the solely analyte-dependent mass<br />
sorption isotherm that relates polymer swelling to the vapor<br />
concentration. The transduction curve for a typical sensor is shown<br />
in Figure 3, with the volume fraction of absorbed analyte<br />
determined gravimetrically. This curve is given by<br />
G<br />
)<br />
G [ 0<br />
1 + eΓφ/φ1/2 - 1<br />
e Γ -1<br />
- 1 ]<br />
where φ is the volume fraction of absorbed analyte and φ1/2 is<br />
the response midpoint. The transduction curve is parametrized<br />
by φ1/2 and Γ, and these are strongly dependent on the<br />
fabrication process used to make the sensor. 18,19,23 To a good<br />
(1)<br />
(2)