Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
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Figure 3. Equilibrium response as a function of the volume fraction<br />
of absorbed analyte, giving a master transduction curve that is<br />
dependent only on the characteristics of the particular sensor. (The<br />
data for pentane, octane, decane, TMP, TBT, and undecane are<br />
reprinted from Figure 8 of ref 19. Copyright 2010 American <strong>Chemical</strong><br />
Society.) Here the response midpoint, φ1/2, is 5.64 × 10 -3 , and Γ is<br />
2.75. a1/2 and � values for each analyte are reported in ref 19.<br />
approximation, the equilibrium sorption is proportional to the<br />
analyte activity, a, and is given by the linearized Flory-Huggins<br />
equation 17,19<br />
φ ) ae -(�+1)<br />
Here � is the Flory interaction parameter, which is a measure of<br />
an analyte’s affinity for the polymer. This transduction curve is of<br />
great importance in this paper, as we will use it to convert<br />
nonequilibrium response curves, such as that in Figure 1, into<br />
the nonequilibrium sorption curves from which we obtain kinetics<br />
data.<br />
EXPERIMENTAL SECTION<br />
Chemiresistor Fabrication. The chemiresistors used in this<br />
research consist of five identically fabricated FSCRs. The FSCR<br />
composite is composed of 15 vol % 3-7 µm gold-plated carbonylnickel<br />
particles (Goodfellow Inc., product no. NI06021) and a<br />
two-part, addition-cure poly(dimethylsiloxane) (PDMS) (Gelest<br />
Inc. optical encapsulant 41, PP2-OE41). The particles are immersion-gold-plated<br />
using Enthone Inc. Lectroless Prep (PCN 210004-<br />
001). The particles are mixed in the PDMS precursors, and a<br />
volume of hexane equal to the volume of PDMS is then added to<br />
thin the viscous composite precursor. A ∼5 µL volume of solventcast<br />
composite precursor is then deposited onto a glass substrate,<br />
forming a ∼3 mm diameter film, which spansa1mmgapbetween<br />
the two gold electrode pads. The sensors are cured at 40 °C for<br />
24hina∼750 G uniaxial magnetic field. The composite is placed<br />
in the magnetic field such that a conductive chainlike particle<br />
network is formed across the two electrodes. 18,22<br />
Analytes. The model analytes used in this research are<br />
acetone, toluene, p-xylene, mesitylene, and undecane. All of the<br />
analytes are from either Fisher <strong>Chemical</strong>s (ACS certified reagent<br />
grade) or Aldrich <strong>Chemical</strong>s (ReagentPlus grade). Pertinent<br />
physical data for the analytes (saturation vapor pressures and �<br />
(3)<br />
Table 1. Room Temperature Saturation Vapor<br />
Pressures 24 and � Parameters for the Analytes 19<br />
analyte P*(25 °C) (Torr) �<br />
toluene 28.97 1.268<br />
p-xylene 8.80 1.258<br />
mesitylene 2.55 1.424<br />
acetone 228.19 2.226<br />
undecane 0.39 1.126<br />
parameters) are included in Table 1. 19,24 The aromatic compounds<br />
toluene, xylene, and mesitylene and undecane were chosen for<br />
use as analytes due to their similar � parameters for with PDMS<br />
and dissimilar vapor pressures.<br />
Sensor Testing. The sensors are enclosed in a shielded flow<br />
cell with gas inlet and outlet ports and electrical throughputs.<br />
Known concentrations of analyte vapors are produced by mixing<br />
a controlled flow rate of analyte-saturated nitrogen from a<br />
temperature-controlled bubbler system with a controlled flow of<br />
pure nitrogen. 19 Conductance measurements are made by applying<br />
a constant 10 mV dc voltage across the electrodes with a power<br />
supply (Hewlett-Packard model 6552A) and measuring the current<br />
through the chemiresistors with picoammeters (Keithley Instruments<br />
Inc. model 6485).<br />
Sorption Kinetics. To study diffusion, it is necessary to<br />
determine analyte mass sorption as a function of time. This is<br />
accomplished by using the transduction curve in eq 2 to transform<br />
the FSCR response curves, such as that in Figure 1, into the<br />
volume fraction of absorbed analyte. Solving eq 2 for φ(t) gives<br />
φ(t) ) φ1/2 Γ ln[ 1 + (eΓ - 1) G0 - G(t)<br />
G(t) ] (4)<br />
Using the equilibrium transduction curve to relate the nonequilibrium<br />
response to the nonequilibrium sorption is an approximation,<br />
since it is an unproven assumption that the sensor<br />
conductance depends only on the total mass sorption and is<br />
insensitive to the swelling gradients that accompany diffusion. For<br />
direct mass uptake experiments gradients pose no problems, but<br />
gradients could potentially foil our approach, yet as we show in<br />
the following section, we obtain a response time that is clearly<br />
independent of the analyte activity. Perhaps this assumption is<br />
reasonable because gradients would have no effect except to<br />
second order: If the equilibrium sensor conductance is locally<br />
linear in the analyte activity, then only the curvature of the gradient<br />
would lead to a conductance response that is different from the<br />
zero gradient response at the same analyte uptake. The utility of<br />
this approximation can only be judged by the quality of the final<br />
experimental results shown in the following section.<br />
Figure 4 illustrates a time-dependent mass sorption curve<br />
obtained from the sensor response. In this figure the equilibrium<br />
swelling, calculated from applying eq 4 to the raw conductance<br />
data, differs from the prediction of the linearized Flory-Huggins<br />
equation (eq 3) by ∼10%. This experimental error does not cause<br />
an error in the computed sorption time. Diffusion into a finite slab<br />
(24) Yaws, C. L.; Narasimhan, P. K.; Gabbula, C. Yaws’ Handbook of Antoine<br />
Coefficients for Vapor Pressure [Online], 1st electronic ed.; Knovel: New York,<br />
2005. URL: http://www.knovel.com/web/portal/browse/display?_EXT_<br />
KNOVEL_DISPLAY_bookid)1183, accessed 3/5/2009.<br />
<strong>Analytical</strong> <strong>Chemistry</strong>, Vol. 82, No. 16, August 15, 2010<br />
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