Analytical Chemistry Chemical Cytometry Quantitates Superoxide

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