Anal. Chem. 1997, 69, 718-723
Poly(N-methylpyrrole)-Modified Electrodes. Amperometric Response to Trace Chlorocarbons in Aqueous Solution Susan M. Hendrickson, Michael Krejcik,† and C. Michael Elliott*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872
Poly(N-methylpyrrole) films grown electrochemically on glassy carbon electrodes have been studied for their amperometric, flow injection mode response to chlorocarbons in aqueous solution. This system has been shown to quickly and reversibly respond to the presence of dichloromethane in water solution over a range of 5 orders of magnitude in concentration, from 10 ng/mL to 1 mg/mL. Various parameters, including applied potential, flow rate, and amount of polymer deposited, were examined in order to optimize the charge response of the polymer film to injections of saturated dichloromethane (employed as a prototypical chlorocarbon example) in aqueous electrolyte solution. Other small, chlorinated compounds, such as chloroform and carbon tetrachloride, were found to cause similar amperometric responses from poly(N-methylpyrrole), whereas non-chlorinated compounds caused significantly smaller responses. This system offers several possible advantages over current halocarbon detection methods, including the ability to sample directly from aqueous solutions. The accepted standard method for detection and quantitation of volatile halocarbons in water involves a purge-trap scheme followed by gas chromatographic analysis of the trapped volatiles employing a halogen-specific detector or a mass spectrometer interface.1 This method exhibits good detection limits and a relatively wide dynamic range for many small, volatile halocarbons (0.5 -1500 µg/L) but is often impractical for field operation.1 As a consequence, sample collection, storage, and transport are necessary and may result in analysis errors due to sample contamination or degradation. The development of a simple, inexpensive, and portable technique that would allow on-site detection of halocarbons directly in water samples would be a significant addition to current methodology. Direct electrochemical detection is one of the simplest and least expensive detection methods when it is applicable. Unfortunately, the direct voltammetric detection of most halocarbons at bare electrodes in protic solvents is virtually impossible, even at nontrace levels. Indirect electrochemical analysis schemes have been devised for several classes of molecules that are not amenable to direct electrochemical detection. These often involve the chemical generation of an electrochemically detectable
product, as, for example, in the electrochemical detection of glucose.2 Alternatively, the analytical response can be based on changes in the electrochemistry of an electroactive “indicator” species brought about by its interaction with the analyte. In the present example, we exploit the changes in the doping levels of an electrochemically grown conducting polymer film which occur as a result of its interaction with aqueous solutions containing dissolved chlorocarbons. Conducting polymers are currently being investigated for their interactions with neutral organics in both the vapor and liquid phases.3-13 Other groups have reported detection of various organic compounds in the vapor phase through monitoring the spectroscopy,3-6 work function,3,4,7,8 and conductivity9-13 of the film. However, few attempts have been made to use amperometric responses of conducting polymers in solution to sense the presence of neutral molecules. In some of our previous work, we have described the fundamental interaction that occurs between poly(N-methylpyrrole) (PNMP) and dichloromethane (DCM) in aqueous solution.14,15 Briefly, in the presence of low concentrations of dissolved DCM (∼1 mg/mL), both the oxidative and reductive peak potentials of PNMP shift to more positive values. As shown in Figure 1, the solid curve is the cyclic voltammogram for a PNMP film in aqueous 0.1 M NaClO4 solution, while the dashed curve is that for the same film after saturating the solution with DCM by purging it with DCM-saturated N2. This shift is reversible upon purging the solution with pure N2. Through quartz crystal microbalance studies performed in flow injection mode, it was determined that a preferential partitioning of DCM into the reduced PNMP film accompanies the shift in voltammetry.14,15 The observed shift in voltammetry can be rationalized through this difference in partitioning in the following way. Beginning with a reduced PNMP film, DCM preferentially
† Current address: EKOL s.r.o., ul.28.rijna 450, 584 01 Lednec n.saz., Czech Republic. (1) Standard Methods for Examination of Water and Wastewater, Franson, M. A. H., Ed.; American Public Health Association Publishers: Washington, DC, 1985; pp 591-618.
(2) Chi, Q.; Dong, S. Anal. Chim. Acta 1993, 278, 17-23. (3) Topart, P.; Josowicz, M. J. Phys. Chem. 1992, 96, 8662-8666. (4) Topart, P.; Josowicz, M. J. Phys. Chem. 1992, 96, 7824-7830. (5) Josowicz, M.; Janata, J.; Ashley, K.; Pons, S. Anal. Chem. 1987, 59, 253258. (6) Blackwood, D.; Josowicz, M. J. Phys. Chem. 1991, 95, 493-502. (7) Josowicz, M.; Janata, J. Anal. Chem. 1986, 58, 514-517. (8) Janata, J.; Langmaier, J. Anal. Proc. 1991, 28, 372-373. (9) Bartlett, P. N.; Archer, P. B. M.; Ling-Chung, S. K. Sens. Actuators 1989, 19, 125-140. (10) Bartlett, P. N.; Ling-Chung, S. K. Sens. Actuators 1989, 19, 141-150. (11) Bartlett, P. N.; Ling-Chung, S. K. Sens. Actuators 1989, 20, 287-292. (12) Slater, J. M.; Paynter, J.; Watt, E. J. Analyst 1993, 118, 379-384. (13) Slater, J. M.; Watt, E. J. Anal. Proc. 1992, 29, 53-56. (14) Feldheim, D. L.; Krejcik, M.; Hendrickson, S. M.; Elliott, C. M. J. Phys. Chem. 1994, 98, 5714-5720. (15) Feldheim, D. L.; Hendrickson, S. M.; Krejcik, M.; Elliott, C. M.; Foss, C. A. J. Phys. Chem. 1995, 99, 3288-3293.
718 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
S0003-2700(96)00924-9 CCC: $14.00
© 1997 American Chemical Society
Figure 1. Cyclic voltammograms of a 20 mC poly(N-methylpyrrole) film on glassy carbon obtained in 0.1 M NaClO4(aq) at 20 mV/s. The solid curve was obtained in dichloromethane-free electrolyte, and the dashed curve was obtained in dichloromethane-saturated electrolyte. The shaded areas of the curves are proportional to the doping levels of the polymer at Ef during the anodic potential scans.
partitions into the neutral polymer. Upon oxidation, DCM must be expelled to reach the lower partitioning level of the ionic, oxidized form of the polymer. The additional free energy needed to expel the excess DCM results in the higher positive potential for oxidation of PNMP (relative to that in the absence of DCM). In this article we describe how this preferential partitioning and resultant shift in redox potentials can be adapted to produce an amperometric response to DCM and other similar chlorocarbons present in aqueous samples. EXPERIMENTAL SECTION Chemicals. N-Methylpyrrole (Aldrich) was freshly distilled under nitrogen atmosphere prior to use. Sodium perchlorate (Aldrich) was used as received after drying overnight at 75 °C. Acetonitrile (Burdick and Jackson), dichloromethane (Mallinckrodt), and all other solvents were used as received. Finally, 18 MΩ water (Millipore) was used for all experiments. Polymer Growth. All polymer films were grown at a constant potential of +0.8 V vs Ag/Ag+ (0.1 M AgNO3, DMSO) from an acetonitrile solution containing 0.1 M NaClO4 and 1 M Nmethylpyrrole. After film growth, the acetonitrile solution was replaced with an aqueous solution containing 0.1 M NaClO4, and the potential applied to the film was cycled between +0.5 and -0.5 V vs SSCE until stable voltammograms were obtained. Films exposed to DCM prior to use in the flow cell were found to have more reproducible responses than films that were not pretreated. The pretreatment procedure involved purging the electrolyte solution with DCM-saturated N2 while cycling the potential until a stable, shifted voltammogram occurred. Subsequently, pure N2 was used to purge the solution until the original voltammetry returned. The pretreated film was then placed in the flow cell for flow injection measurements. When films were not in use, they were stored submerged in aqueous 0.1 M NaClO4. Electrochemical Instrumentation. Cyclic voltammetry was performed employing a EG&G PAR Model 173 potentiostat/ galvanostat in conjunction with a PAR 175 programmer. Data collection was performed with a Yokogawa 3023 X-Y recorder. The working electrode was a 3 mm diameter glassy carbon disk, the counter electrode was a platinum grid, and the reference electrode was an SSCE. The cell was half of a U-tube, to which
was clamped the working electrode of the electrochemical detector. The thin-layer flow cell employed was a BAS LC-44 containing dual 7 mm2 glassy carbon disk working electrodes. A 50 µm Teflon spacer separated the working electrodes from the stainless steel auxiliary electrode, which forms the entire opposing wall, relative to the working electrodes. An Ag/AgCl reference electrode (BAS RE-4) was located externally to the thin-layer cell upstream from the working electrode. The shape of the thinlayer compartment in its macroscopic dimensions, as determined by the Teflon spacer, was a diamond. The solution entrance and exit were situated on the auxiliary electrode side of the cell and at opposite ends of the diamond-shaped channel along its longer axis. The dual working electrodes were located in the center of the cell, symmetrically on either side of the diamond’s longer axis. Typically, one working electrode or the other was employed at a time. The total volume of the thin-layer compartment was ∼6.7 µL. The sample loop employed in these studies had a total volume of 100 µL; consequently, during a given experiment, the flow cell was flushed with ∼15 times its total volume by analyte solution. At higher flow rates, i.e., >0.1 mL/min, an FMI metered reciprocating pump equipped with a homemade pulse dampener was employed to flow solution through the cell. At lower flow rates, gravity flow was utilized. Sample injection was accomplished using a six-port injection valve (Rheodyne Model 7125), fitted with a 100 µL stainless steel sample loop. Data were collected using Asyst software. Profilometry. The glassy carbon disk electrodes supplied with the BAS LC-44 flow system are typically recessed by ∼1.5 µm into the surrounding insulating matrix. Consequently, a procedure involving partial masking of the electrode surface was employed to allow accurate profilometry measurements of the polymer thickness. This was achieved by dropping a concentrated solution of polystyrene dissolved in dichloromethane onto the electrode surface so as to cover approximately half of the electrode area. The dichloromethane was then allowed to evaporate at room temperature, leaving a layer of polystyrene partially covering the electrode surface. PNMP films were then grown on these electrodes as described above from the acetonitrile growth solution. Afterward, the masking material was redissolved with dichloromethane to uncover the bare glassy carbon surface. This procedure produced a good step edge from the glassy carbon to the PNMP film, allowing accurate profilometry measurements to be made employing a Tencor Instruments Alpha-Step profilometer. RESULTS AND DISCUSSION Polymer Growth. A number of possible modes exist for electrochemically forming films of PNMP directly on an electrode surface (e.g., constant potential or potential cycling); however, only constant potential was employed in these studies. While the mode of polymer growth is a parameter that might have been considered, preliminary studies indicated that constant potential growth produced films more reproducibly than potential cycling. Flow Cell Characteristics. To determine the flow characteristics of the thin-layer cell, the mass transport-limited currenttime response for 100 µL injections of 1 mM K3Fe(CN)6 in 0.1 M NaClO4(aq) was investigated. These experiments were conducted while the working electrode potential was held at -0.15 V vs Ag/ AgCl, several hundred millivolts negative of the E1/2 for the Fe(II/ III) couple. From these current-time curves, it is possible to determine, as a function of flow rate, the approximate time Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
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required for the concentration of analyte within the flow cell to change from zero to a constant value as the sample initially enters the cell, and from that constant value to zero as the sample is flushed from the cell. For flow rates of 0.05-0.29 mL/min (which bracket those used in all of the experiments employing DCM reported below), the time required to reach constant solution composition within the cell on the leading edge ranged from 18 to 5 s, respectively, and varied roughly linearly with flow rate. The time required to deplete the cell of analyte, once constant composition was achieved, also increased roughly linearly with decreasing flow rate but was found to be considerably longer than for the leading edge, varying from 120 to 31 s over the same range of flow rates. Description of Theoretical Response. The amperometric response to injections of aqueous DCM results from the shift in the redox potentials of PNMP described above. Theoretically, when a fixed potential (Ef in Figure 1) is applied to a polymer film in contact with a solution of 0.1 M NaClO4(aq), current will flow in the circuit until the equilibrium doping level is obtained. At equilibrium, the charge on the film should be approximately equal to the sum of the two areas shaded in Figure 1. If the electrolyte solution is then saturated with DCM, the equilibrium charge on the film at Ef is equal to only the lightly shaded area of Figure 1. Consequently, to reach charge equilibrium, a reductive current must flow through the circuit until charge equal to the heavily shaded area has passed. If the solution is then purged of DCM, an oxidative current must pass until the equilibrium charge value is regained (equal, again, to the sum of the two areas). PNMP Response to DCM Injections. A representative current-time response of a PNMP film to a 100 µL injection of saturated aqueous DCM is shown in Figure 2A. As the injection plug enters the flow cell, a reductive (upward) current spike occurs. When the PNMP/DCM equilibrium is reached, the current decays back to zero and remains so until the DCM concentration in the cell compartment begins to decrease. An oxidative current is then passed as the DCM is flushed from the cell compartment, shown by the broader downward current peak. The corresponding charge-time curve is shown in Figure 2B. Comparisons between the charge-time response of these polymers and the current-time response of the cell to injection of K3Fe(CN)6, vide supra, are instructive. The important conclusion from these comparisons is that, in every case examined, the time required for the PNMP/DCM interaction to reach equilibrium on the leading edge of the injection is at least twice the time required for the solution in the cell to reach constant composition. Consequently, the current-time and charge-time data for the leading edge does reflect the kinetics of the PNMP/DCM interaction.15 In contrast, the time required to flush the cell of analyte (the trailing edge) is, in all cases, comparable with the chargetime response of the PNMP films and, thus, represents a convolution of both solution mixing and desorption kinetics. This suggests that there is no useful kinetic information extractable from the trailing edge data, save establishing an upper limit to the rate of response. Reproducibility of Response. The charge response for a saturated DCM injection was found to increase linearly with film thickness between 135 and 850 nm as measured by profilometry. The linear increase in charge response is also evident from the integrated areas under the cyclic voltammograms with and without DCM present (vide infra). A maximum, limiting response is 720 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
Figure 2. (A) Current response for a 1 mC poly(N-methylpyrrole) film held at +0.3 V vs Ag/AgCl to a 100 µL injection of dichloromethane-saturated 0.1 M NaClO4(aq) at a flow rate of 0.1 mL/min. (B) Charge response for the same injection obtained by the time integration of the current obtained in (A).
anticipated at film thicknesses when the concentration of DCM in the cell is the limiting factor and the amount of polymer is in excess. However, this limit was not investigated during these studies. The response to DCM was quite reproducible from injection to injection on a given film (∼(7%). However, the magnitude of the charge response of the film decreased slowly over time. This was true irrespective of the number of DCM injections made. For example, the charge response of a typical newly grown film decreased by ∼20% over the course of the first 24 h. Subsequently, the response diminished by an average of ∼14%/day. As stated above, this decrease in response did not correlate with the number of DCM injections made; however, some decrease in film response was observed, which correlated with multiple injections of other analytes, e.g., carbon tetrachloride (vide infra). Finally, there was also some indication that, when the film was operated at more positive potentials (e.g., >+0.3 V vs Ag/AgCl), the decrease in the charge response of the film was accelerated. Changes in Film Thickness. For the 3 mm diameter glassy carbon electrode employed in these studies, each millicoulomb of charge passed while polymerizing PNMP at constant potential was found by profilometry to correspond to a thickness of approximately 50 nm (vide supra, see Table 1). These results allow for an estimate of the amount of polymer deposited on the surface by monitoring charge passed as it is being grown chronocoulometrically instead of performing profilometry meas-
Table 1. Charge Passed during Film Growth and Film Thickness Obtained by Profilometry from Representative Films charge passed (mC)
film thickness (nm)
1 2.5 3 5 7.5 8
135 250 250 509 616 850
Figure 4. Data analogous to those presented in Figure 3 for a single 1 mC poly(N-methylpyrrole) film at different values of applied potential. (A) Charge response calculated from the difference in areas under the cyclic voltammograms collected during pretreatment of the film vs applied potential, Ef (open circles and left axis). (B) Limiting charge response in the flow injection analysis of a poly(N-methylpyrrole) film to 100 µL injections of dichloromethane-saturated 0.1 M NaClO4(aq) vs applied potential, Ef (closed circles and right axis).
Figure 3. Plot of the maximum charge response of poly(Nmethylpyrrole) films to 100 µL injections of dichloromethane-saturated 0.1 M NaClO4(aq) vs the difference in areas under the cyclic voltammograms collected during pretreatment of the films.
urements on every film. For all of the studies reported herein, films were grown with thicknesses of approximately 50-850 nm. The maximum charge response of the polymer films to a saturated DCM injection was found to scale roughly linearly with the number of millicoulombs passed when growing the film and, thus, film thickness. However, a more quantitative correlation was found between the difference in the integrated areas of the cyclic voltammograms (CVs) collected during pretreatment of the film and the maximum charge response in the flow injection experiment (cf. Figure 3). Integration of the anodic portion of the CVs from -0.5 V (fully reduced) to some positive potential value, Ef, corresponds to the total charge accumulated in the film at Ef. The difference in this charge for a film in the presence and absence of DCM (again obtained from integration of the respective CVs) ideally should equal the charge passed at Ef when DCM (at the same concentration) is injected into the flow system. When charge data obtained from the integrated CVs are plotted vs charge obtained from flow injection experiments for a series of different film thicknesses, a linear correlation is obtained (Figure 3). However, the slope is not unity, and, in fact, the flow injection charges are 2.6 times those obtained from the integrated CVs. This difference probably arises from the fact that, at a scan rate of 20 mV/s, the films are not in true partition equilibrium in the CV experiments, resulting in the lower relative charge. Changes in Applied Potential. Consideration of voltammograms such as those given in Figure 1 suggests that a maximum
response for DCM should occur at one value of Ef. In Figure 4A, the difference in area under the cyclic voltammetric curves (obtained by integration of the current from -0.5 V to the applied potential, Ef) for a 50 nm thick film were plotted vs Ef. For this specific film, the maximum difference in integrated area occurred at a potential of +0.3 V. The maximum charge response to DCM of this same film as a function of Ef in the flow cell was also obtained, and the results are shown in Figure 4B. The greatest experimental charge response was obtained at +0.3 V, in agreement with the voltammetry prediction from Figure 4A. As mentioned above, the experimental charge values from the flow injection experiments are always higher than those predicted from the integrated currents obtained from the CVs. Despite these quantitative differences, it appears that voltammetric data such as those given in Figure 4A can provide good estimates of the Ef value that will yield an optimum charge response. Changes in Flow Rate. For PNMP films grown by passing 1 mC of charge, (i.e., ∼50 nm thick), it was necessary to employ a flow rate of ∼0.1 mL/min or slower in order to allow doping equilibrium of the film to be achieved during the residence time of a 100 µL injection of analyte solution in the cell. This is illustrated in Figure 5 for injections of saturated DCM. Similar results were found for injections of saturated chloroform solution. The charge response at 0.7 mL/min (Figure 5B, solid curve) did not reach a constant value; however, at a flow rate of 0.1 mL/min (Figure 5B, dashed curve), the charge curve plateaued for at least 40 s. The current response at 0.1 mL/min (Figure 5A, dashed curve) clearly indicates that the reductive process was complete and the current had decayed back to baseline before the oxidative process began. In other words, equilibrium partitioning of DCM into the polymer and the associated change in the polymer doping level was achieved before the concentration of DCM in the cell began to decrease. Furthermore, the maximum charge reached for the faster flow rate (0.7 mL/min) is lower than that reached at 0.1 mL/min, consistent with the fact that an equilibrium interaction was not achieved at the faster flow rate. Flow rates Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
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Figure 6. Log-log plot of the maximum charge response vs dichloromethane concentration. Maximum charge values have been divided by the maximum charge response due to a dichloromethanesaturated 0.1 M NaClO4(aq) injection, and concentration values have been divided by the concentration of saturated dichloromethane, 1 mg/mL. Each point represents a 10-fold change in dichloromethane concentration.
Figure 5. (A) Current responses and (B) charge responses for a 1 mC poly(N-methylpyrrole) film held at +0.3 V vs Ag/AgCl to a 100 µL injection of dichloromethane-saturated 0.1 M NaClO4(aq) at flow rates of 0.7 mL/min (solid curves) and 0.1 mL/min (dashed curves).
slower than 0.1 mL/min exhibited the same maximum charge response for 1 mC films, with simply a longer plateau. Different film thicknesses were also investigated as a function of flow rate. As might be expected, with thicker films, slower flow rates (e.g., 0.07 mL/min for a 10 mC film) are needed to produce a plateau in the charge-time response. Therefore, a trade-off exists between speed of sampling and maximum charge response. A 1 mC PNMP film sampled at 0.1 mL/min produced a complete response in 140 s (baseline-to-baseline), while a 10 mC PNMP film sampled at 0.07 mL/min required 200 s. Thinner films, thus, allow more samples to be run per unit time, but at the expense of the charge response magnitude. The maximum charge response to 100 µL of saturated DCM for the 10 mC film was 6 times that of the 1 mC film considered above (i.e., the thicker film had a higher sensitivity). Dilutions of Saturated DCM Solution. In general, the rate of evolution of the charge-time response was independent of DCM concentration. However, the magnitude of the charge response was found to have a strong correlation to the DCM concentration. Figure 6 is the log of the maximum charge response of a 1 mC film plotted vs the log of the DCM concentration divided by the saturated DCM concentration obtained from the literature.16 These data cover 5 orders of magnitude in DCM concentration. The concentration range (16) Organic Solvents: Physical Properties and Methods of Purification, 3rd ed.; Riddick, J. A., Bunger, W. B., Eds.; Wiley-Interscience: New York, NY, 1970.
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represented here is 1 mg/mL through 10 ng/mL (10 ppb). Interestingly, while this log-log plot is linear, the slope is significantly less than unity. Therefore, in absolute terms, the relative film response became more sensitive as the DCM concentration decreased. For example, a 1000× decrease in concentration between 1 mg/mL and 1 µg/mL produces only a 24× decrease in charge response. Other Chlorinated Compounds. Previously, we have shown that similar changes in the voltammetry for PNMP occur when the polymer is exposed to other small chlorinated molecules, e.g., chloroform, carbon tetrachloride, and 1,2-dichloroethane.14 Physically larger molecules and many small, non-chlorinated molecules (e.g., benzene, acetone, and 1,2-dichlorobenzene) were also observed to cause little or no modification of the PNMP voltammetry at comparable concentrations.14 These data suggested that these other small, chlorinated molecules should produce amperometric responses similar to that produced by DCM. Moreover, larger and non-chlorinated molecules would be expected to cause little or no response at similar concentrations. Carbon tetrachloride, chloroform, and 1,2-dichloroethane in addition to dichloromethane were investigated employing the flow injection system. Saturated solutions of each were prepared by adding an excess of the compound to a vial containing aqueous 0.1 M NaClO4 and allowing it to come to equilibrium. Injections (100 µL) were performed at Ef values of +0.3 and +0.1 V with 1 mC films and at +0.3 V with a 5 mC film. Table 2 summarizes these results. In an effort to deconvolute the charge response from the differences in solubilities of the various compounds, the charge responses listed in Table 2 have been divided by literature values for their saturated concentration in pure water.16 Very likely the absolute solubility of each compound will be different in 0.1 M NaClO4(aq) than in pure water, but it is reasonable to assume that the solubilities for each will be affected similarly.
Table 2. Maximum Charge Responses of Poly(N-methylpyrrole) Films to 100 µL Injections of 0.1 M NaClO4(aq) Saturated with Small, Chlorinated Compounds Normalized by Their Molarities and as Ratios to the Dichloromethane Response 1 mCb (+0.1 Vc) compound dichloromethane chloroform carbon tetrachloride 1,2-dichloroethane
solubilitya 0.153 0.068 0.005 0.082
1 mCb (+0.3 Vc)
µC/M
ratiod
53.6 ((4.8) 88.2 ((6.2) 128 ((8.7) 78.5 ((5.4)
1 1.6 2.4 1.5
(M)
5 mCb (+0.3 Vc)
µC/M
ratiod
µC/M
ratiod
110 ((16) 138 ((16) 1410 ((110) 112 ((3.3)
1 1.2 13 1.0
873 ((19) 1780 ((34) 4270 ((550) 1430 ((44)
1 2.0 4.9 1.6
a Values are solubilities in pure water from ref 16. b Number of millicoulombs passed during film growth at +0.8 V vs Ag/Ag+. c Potential applied to the film during the injection, Ef. d Ratio of each concentration-normalized charge response to that of dichloromethane.
Table 3. Maximum Charge Response of a 1 mC Poly(N-methylpyrrole) Film Held at +0.3 V vs Ag/Ag+ to 100 µL Injections of 0.5 mg/mL of Each Compound in 0.1 M NaClO4(aq) compound
maximum charge response (µC)
dichloromethane ether tetrahydrofuran acetone ethanol methanol
5.27 ((0.11) 3.08 ((0.04) 1.62 ((0.07) 0.94 ((0.08) 0.24 ((0.06) 0.14 ((0.01)
Attempts were made to correlate maximum charge response with a number of physical parameters such as dielectric constant, boiling point, density, solubility, and molecular weight. No good correlation was found for any of these parameters, although there was a qualitative trend of increasing solubility-normalized charge response to molecular weight. When the absolute charge responses were compared, the relative trends for the four compounds were found to depend both on film thickness and applied potential. In general, however, DCM, 1,2-dichloroethane, and chloroform each produced similar charge responses (within a factor of 2). The absolute response for carbon tetrachloride was generally considerably less and could be as much as an order of magnitude smaller on thin films at more negative potentials. Non-Chlorinated Compounds. The results from small nonchlorinated compounds investigated as 0.5 mg/mL solutions in 0.1 M NaClO4(aq) with a 1 mC film held at +0.3 V are shown in Table 3. As was predicted from the changes in voltammetry reported earlier (vide supra),14 each of the non-chlorinated compounds studied produced considerably smaller absolute charge responses than did DCM. Unlike for the four chlorocarbons considered above, there does seem to be a general trend in charge response with molecule polarity. The most polar
compound (methanol) produced a response roughly 40× smaller than that of DCM. CONCLUSIONS Poly(N-methylpyrrole) films grown electrochemically on glassy carbon electrodes produced a charge response to dichloromethane and other small-molecule chlorocarbons dissolved in aqueous solution. A wide dynamic range of 5 orders of magnitude of dichloromethane concentration, from 10 ng/mL to 1 mg/mL, was detected without preconcentration or dilution. Applied potential, flow rate, and amount of polymer deposited were varied to obtain the maximum response for a given sample. It was found that cyclic voltammograms obtained with and without dichloromethane present may be used to predict the applied potential for optimum film response. Flow rates of 0.1 mL/min or less were necessary to allow equilibrium partitioning of DCM into a 1 mC polymer film. Additionally, for larger amounts of deposited polymer, even slower flow rates were necessary, but larger charge responses were obtained. Other chlorinated compounds were studied, and qualitatively similar responses were obtained for chloroform, 1,2dichloroethane, dichloromethane, and carbon tetrachloride. Small, non-chlorinated compounds investigated gave 2-40 times smaller responses than DCM, indicating that some selectivity exists for the small, chlorinated compounds, particularly relative to more polar organic molecules. ACKNOWLEDGMENT The authors thank the National Science Foundation (CHE9311694) for financial support. S.M.H. gratefully acknowledges the Electrochemical Society for additional financial support. Special thanks to E. R. Fisher and C. R. Martin for use of the profilometer. Received for review September 11, 1996. December 4, 1996.X
Accepted
AC960924Y X
Abstract published in Advance ACS Abstracts, January 15, 1997.
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