Analytical Chemistry Chemical Cytometry Quantitates Superoxide

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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)
Figure 3. Equilibrium response as a function of the volume fraction of absorbed analyte, giving a master transduction curve that is dependent only on the characteristics of the particular sensor. (The data for pentane, octane, decane, TMP, TBT, and undecane are reprinted from Figure 8 of ref 19. Copyright 2010 American Chemical Society.) Here the response midpoint, φ1/2, is 5.64 × 10 -3 , and Γ is 2.75. a1/2 and � values for each analyte are reported in ref 19. approximation, the equilibrium sorption is proportional to the analyte activity, a, and is given by the linearized Flory-Huggins equation 17,19 φ ) ae -(�+1) Here � is the Flory interaction parameter, which is a measure of an analyte’s affinity for the polymer. This transduction curve is of great importance in this paper, as we will use it to convert nonequilibrium response curves, such as that in Figure 1, into the nonequilibrium sorption curves from which we obtain kinetics data. EXPERIMENTAL SECTION Chemiresistor Fabrication. The chemiresistors used in this research consist of five identically fabricated FSCRs. The FSCR composite is composed of 15 vol % 3-7 µm gold-plated carbonylnickel particles (Goodfellow Inc., product no. NI06021) and a two-part, addition-cure poly(dimethylsiloxane) (PDMS) (Gelest Inc. optical encapsulant 41, PP2-OE41). The particles are immersion-gold-plated using Enthone Inc. Lectroless Prep (PCN 210004- 001). The particles are mixed in the PDMS precursors, and a volume of hexane equal to the volume of PDMS is then added to thin the viscous composite precursor. A ∼5 µL volume of solventcast composite precursor is then deposited onto a glass substrate, forming a ∼3 mm diameter film, which spansa1mmgapbetween the two gold electrode pads. The sensors are cured at 40 °C for 24hina∼750 G uniaxial magnetic field. The composite is placed in the magnetic field such that a conductive chainlike particle network is formed across the two electrodes. 18,22 Analytes. The model analytes used in this research are acetone, toluene, p-xylene, mesitylene, and undecane. All of the analytes are from either Fisher Chemicals (ACS certified reagent grade) or Aldrich Chemicals (ReagentPlus grade). Pertinent physical data for the analytes (saturation vapor pressures and � (3) Table 1. Room Temperature Saturation Vapor Pressures 24 and � Parameters for the Analytes 19 analyte P*(25 °C) (Torr) � toluene 28.97 1.268 p-xylene 8.80 1.258 mesitylene 2.55 1.424 acetone 228.19 2.226 undecane 0.39 1.126 parameters) are included in Table 1. 19,24 The aromatic compounds toluene, xylene, and mesitylene and undecane were chosen for use as analytes due to their similar � parameters for with PDMS and dissimilar vapor pressures. Sensor Testing. The sensors are enclosed in a shielded flow cell with gas inlet and outlet ports and electrical throughputs. Known concentrations of analyte vapors are produced by mixing a controlled flow rate of analyte-saturated nitrogen from a temperature-controlled bubbler system with a controlled flow of pure nitrogen. 19 Conductance measurements are made by applying a constant 10 mV dc voltage across the electrodes with a power supply (Hewlett-Packard model 6552A) and measuring the current through the chemiresistors with picoammeters (Keithley Instruments Inc. model 6485). Sorption Kinetics. To study diffusion, it is necessary to determine analyte mass sorption as a function of time. This is accomplished by using the transduction curve in eq 2 to transform the FSCR response curves, such as that in Figure 1, into the volume fraction of absorbed analyte. Solving eq 2 for φ(t) gives φ(t) ) φ1/2 Γ ln[ 1 + (eΓ - 1) G0 - G(t) G(t) ] (4) Using the equilibrium transduction curve to relate the nonequilibrium response to the nonequilibrium sorption is an approximation, since it is an unproven assumption that the sensor conductance depends only on the total mass sorption and is insensitive to the swelling gradients that accompany diffusion. For direct mass uptake experiments gradients pose no problems, but gradients could potentially foil our approach, yet as we show in the following section, we obtain a response time that is clearly independent of the analyte activity. Perhaps this assumption is reasonable because gradients would have no effect except to second order: If the equilibrium sensor conductance is locally linear in the analyte activity, then only the curvature of the gradient would lead to a conductance response that is different from the zero gradient response at the same analyte uptake. The utility of this approximation can only be judged by the quality of the final experimental results shown in the following section. Figure 4 illustrates a time-dependent mass sorption curve obtained from the sensor response. In this figure the equilibrium swelling, calculated from applying eq 4 to the raw conductance data, differs from the prediction of the linearized Flory-Huggins equation (eq 3) by ∼10%. This experimental error does not cause an error in the computed sorption time. Diffusion into a finite slab (24) Yaws, C. L.; Narasimhan, P. K.; Gabbula, C. Yaws’ Handbook of Antoine Coefficients for Vapor Pressure [Online], 1st electronic ed.; Knovel: New York, 2005. URL: http://www.knovel.com/web/portal/browse/display?_EXT_ KNOVEL_DISPLAY_bookid)1183, accessed 3/5/2009. Analytical Chemistry, Vol. 82, No. 16, August 15, 2010 6971
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Anal. Chem. 2010, 82, 6745-6750 Let
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purchased from Invitrogen-Molecular
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Figure 3. Electropherograms of TPP-
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Anal. Chem. 2010, 82, 6751-6755 Res
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(pH 8.0) with cysteine and cystamin
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Figure 4. Ion mobility mass spectra
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GOx glucose + O298 gluconic acid +
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coater at 2000 rpm for 20 s, and th
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Figure 5. Comparison of cyclic volt
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in channels with either no grooves
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indicators of atmospheric processin
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Figure 1. GC/MS total ion chromatog
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Table 2. Concentrations and Stable
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on a substrate are preferred. 20-24
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Figure 4. SERS analysis of NAADP co
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Anal. Chem. 2010, 82, 6775-6781 Hig
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tion, 2 µL of proprionaldehyde wer
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Figure 4. Analysis of 2a by HPLC-MS
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eaction of 1a with PBH can be condu
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Numerous references had demonstrate
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Figure 1. TEM images of the prepare
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Figure 4. Schematic representation
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esult in a big SPR signal change wi
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also reduces chemical noise, which
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Table 1. Extraction Yields, Liquid
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scanning of AMPP amides of the anal
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Anal. Chem. 2010, 82, 6797-6806 δ
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Figure 1. Schematic view of the pre
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from the specimen and enclosed in a
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Figure 4. (A) IRMS mass-44 chromato
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Table 3. Ambient Measurement Result
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Anal. Chem. 2010, 82, 6807-6813 Dir
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Polymerase (1 U per sample). Reacti
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of which was constant for all ampli
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analytically useful signals at less
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carbon black and RP-C18 for the ext
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solution in an equal volume, and 1
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Table 2. Concentrations and Ratios
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Anal. Chem. 2010, 82, 6821-6829 Mac
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mg/mL protein, followed by separati
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Figure 2. Productivity of SEQUST an
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Figure 4. High-resolution MS/MS spe
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Figure 6. Characterization of PSMs
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containing T-T mismatches. 23 Based
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Figure 1. Extinction spectra of sol
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Figure 3. (A) The value of Ex650 nm
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Figure 5. Extinction spectra and co
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wished to explore the dehydration o
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constant medium for separation; we
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Figure 2. Standards of (Pi)n, n ) 1
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Figure 5. Quantitative calibration
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Anal. Chem. 2010, 82, 6847-6853 Met
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Figure 1. 226 Ra spectrum by liquid
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Table 1. Counting Properties and De
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Table 3. Analysis of 226 Ra in Sedi
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mercial microarray scanner and fabr
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Figure 2. Optical transmission meas
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Figure 5. Volcano plots detailing t
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data as well. The 41 genes in Table
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corresponding compound if its chemi
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Figure 1. Schematic illustration of
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Table 1. Absolute Quantification Re
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ment. Therefore, the long-time drea
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Figure 1. Chemically actuated micro
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Figure 2. Influence of a surfactant
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Figure 5. Device to eject and mix s
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Anal. Chem. 2010, 82, 6877-6886 Imm
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NaCl, phosphate buffer saline (PBS)
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Figure 2. Product ion mass spectra
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-70 °C resolved the problem, givin
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Table 1. Intraday Precision and Acc
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Anal. Chem. 2010, 82, 6887-6894 Fer
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Figure 1. Infrared spectra of (A) u
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Figure 3. Cyclic voltammograms obta
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Figure 6. Calculated charge from ch
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Anal. Chem. 2010, 82, 6895-6903 Ele
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Table 1. Chemical Structure, pKa Va
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pH with a tilted baseline (Figure 1
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Table 2. Linearity and Detection Li
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ascorbic acid (AA), uric acid (UA),
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the multielement capabilities, the
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RESULTS AND DISCUSSION Sulfur Detec
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Table 2. Molecular Properties and C
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Anal. Chem. 2010, 82, 6911-6918 Dir
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dilution and hybridization buffer.
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solution under appropriate incubati
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Figure 4. Standardization curve for
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Page 179 and 180: Figure 2. With a suitable removal o
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Page 183 and 184: Scheme 1. Reactions of Selenium Rea
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Page 193 and 194: Figure 4. Comparison between simula
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Page 199 and 200: Figure 2. Normalized product ion ab
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Page 211 and 212: Figure 5. Extracted ion current ESI
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Page 229 and 230: Figure 6. The average measured resp
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Page 241 and 242: Figure 2. Configuration editor wind
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Page 251 and 252: Table 2. Capillary Electrophoresis
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Page 263 and 264: CONCLUSIONS In this paper we have i
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Anal. Chem. 2010, 82, 7021-7026 Cel
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luciferase (T7 control vector) was
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Figure 3. Inhibitory effects of luc
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Anal. Chem. 2010, 82, 7027-7034 Pat
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Electrochemistry. Electrochemical m
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Figure 2. SEM images of Au substrat
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Table 3. Film Thickness Measurement
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Anal. Chem. 2010, 82, 7035-7043 Qua
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Figure 1. (A) FL spectra of BSPOTPE
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Figure 4. (A) Variation in the FL i
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Figure 6. (A) FL spectra of BSPOTPE
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Scheme 1. Proposed Mechanism for Fl
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one or more drawbacks including poo
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Figure 2. Free fluorophores do not
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Anal. Chem. 2010, 82, 7049-7052 Dev
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Figure 2. Comparative analysis of d
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