Articles

Anal. Chem. 1995, 67, 1654-1660

Nit rat emselective Elect rode Developed by Electrochemically Mediated Imprinting/Doping of Polypyrrole

Richard S. Hutchins and Leonidas 0. Bachas*

Department of Chemistry and Center of Membrane Sciences, Universify of Kentucky, Lexington, Kentucky 40506-0055

Nitrate-selective electrodes have been developed by elec- tropolymerizing pyrrole onto glassy-carbon electrodes in the presence of NaN03. Electrochemical variables were used to optimize the potentiometric response of the electrodes and to maximhe the selectivity for nitrate over potentially interferent anions. Calibration plots with near- Nernstian slopes for nitrate were observed, -56 f 1 mV/ decade (n = 18), over a linear range of four decades of concentration (5.0 x 10-5-0.50 M nitrate). The elec- trodes had detection limits of (2 f 1) x M nitrate (n = 18). The commercially available nitrate-selective electrodes s d e r from interference by lipophilic anions, such as perchlorate and iodide. Compared to these electrodes, the polypyrrole-based nitrate electrodes dem- onstrated improved selectivity c d c i e n t s for perchlorate and iodide by as much as 4 orders of magnitude (KG3-,c10,- = 5.7 x low2; G,-3-,l- = 5.1 x An electrochemically mediated templating mechanism is proposed to explain the observed high selectivity for nitrate.

The use of chemical recognition principles in the design of ionophores has been shown to be of great importance and utility in the development of ion-selective electrodes OSEs) .1-3 Anion- selective electrodes that employ ionophores that possess no anion recognition functionalities, other than a positive charge, respond simply on the basis of the anion's lipophilicity. In these sensors, the ionophore is contained in a hydrophobic membrane, and anions from an aqueous sample solution tend to partition into the nonaqueous membrane solely on the basis of lipophilicity, with the more lipophilic anions responding the best. This gives rise to the following selectivity order, which is known as the Hofmeis ter series: large lipophilic anions =. C104- =. SCN- > I- > NOS- > Br- > C1- > H2P04-.4 Therefore, it is necessary to incorporate

(1) Pretsch, E.; Badertschen, M.; Welti, M.; Marnizumi, T.; Mol?, W. E.; Simon, W. Pure Appl. Chem. 1988, 60, 567-574.

(2) Chang, Q.; Park, S. B.; Kliza, D.; Cha, G. S.; Tun, H.; Meyerhoff, M. E.Am. Biotech. Lob. 1990, 8, 10-21.

(3) Wotring, V. J.; Johnson, D. M.; Daunert, S.; Bachas, L G. In Immunochemical Assays and Biosensor Technologyfir the 1990's; Nakamura, R M., Kasahara, Y., Rechnitz, G. A, Eds.; American Society of Microbiology: Washington, DC, 1992; pp 355-376.

(4) Hofmeister, F. Arch. E@. Pathol. Phannakol. 1888, 24, 247-260.

1654 Analytical Chemistty, Vol. 67, No. 10, May 15, 1995

selective interactions between ionophores and the targeted anion, using chemical recognition principles, if a truly selective anion sensor is to be achieved for any of the less lipophilic anions in the Hofmeister series.

There are several examples in the literature that demonstrate the success of using chemical recognition principles in the development of selective ISEs. Among these, crown and biwrown ether ionophores have been incorporated into polymer membranes to develop selective electrodes for many cations. The size of the host cavity, responsible in part for the complexing ability of the crown ethers, is one type of chemical recognition that has been used to induce a desired selectivity in ISES.~-~ Another trpe of recognition element that can be utilized in ISE development involves specific metal-ligand interactions. Such interactions have been used in the development of phosphate-,@ salicylate-,'O and nitriteselective electrodes"J2 based on organ~tin?~~ porphy- rin,l0 and c ~ r r i n ~ ~ J ~ ionophores. Additionally, bis(diethy1dithio- carbamato)mercury(ID, In(IID porphyrins, and Schiff base com- plexes of C O O have been used in developing selective electrodes for sulfite,13 chloride,14 and iodide,l5 respectively. Another a p proach that has been used for selective recognition is based on proton uptake by macrocyclic polyamine ionophores.16J7 Finally, a carbonateselective electrode, employing (tduoroacetyl)-p butylbenzene in conjunction with quaternary ammonium salts in the membrane, has been developed which takes advantage of an

(5) Kimura, K; Shono, T. In Ion-Selediue Electrodes, 4; Pungor, E., Ed.; Elsevier New York, 1985; pp 155-177.

(6) Tbth, K; Lindner, E.; Pungor, E.; &ai, B.; Bitter, I.; Tbke, L. In IonSelective Electrodes, 4; Pungor, E., Ed.; Elsevier: New York, 1985; pp 231-244.

(7) Ma, Y. L.; Galal, A; Zimmer, H.; Mark, H. B., Jr.; Huang, Z. F.; Bishop, P. L. Anal. Chim. Acta 1994,289, 21-26.

(8) Glazier, S. A; Amold, M. A Anal. Chem. 1991, 63, 754-759. (9) Chaniotakis, N. A; Jurkschat, K; Ruhlemann, A Anal. Chim. Acta 1993,

(10) Kibbey, C. E.; Park, S. B.; DeAdwyler, G.; Meyerhoff, M. E. 1. Electroanal.

(11) Schulthess, P.; Ammann, D.; Wiutler, B.; Caderas, C.; Stephek, R; Simon,

(12) O'Reilly, S. A; Daunert, S.; Bachas, L G. Anal. Chem. 1991, 63, 1278-

(13) Pranitis, D. M.; Meyerhoff, M. E. Anal. Chim. Acta 1989,217, 123-133. (14) Park, S. B.; Matuszewski, W.; Meyerhoff, M. E.; Liu, Y. H.; Kadish, K. M.

(15) Yuan, R; Chai, Y. Q.; Lin, D.; Gao, D.; Li, J. Z.;Yu, R Q.Anal. Chem. 1993,

(16) Umezawa, Y.; Kataoka, M.; Takami, W. Anal. Chem. 1988,60,2392-2396. (17) Carey, C. M.; Riggan, W. B., Jr. Anal. Chem. 1994, 66, 3587-3591.

282,345-352.

Chem. 1992,335, 135-149.

W. Anal. Chem. 1985, 57, 1397-1401.

1281.

Electroanalysis 1991, 3, 909-916.

65, 2572-2575.

0003-2700/95/0367-1654$9.00/0 0 1995 American Chemical Society

adduct formation with carbonate to achieve selective carbonate recognition.I8

The focus of the present work has been the development of a truly selective nitrate electrode. There are currently several commercial nitrateselective electrodes that are limited in their utility because they are actually selective for anions more lipophilic than nitrate (e.g., perchlorate, iodide, et^.).'^ In this work, we use a different approach toward achieving selective chemical recognition of nitrate that is based on electrochemically mediated molecular imprinting. This templating approach involves elec- trochemical polymerization of an appropriate monomer under controlled conditions that allow the generation of selective recognition sites in the formed polymer film during the doping process. It has been reported that the electropolymerization conditions, specifically the size of the counterion used, can control the sizeexclusion selectivity in polypyrrole (PPy) film^.^ Further studies have illustrated that there is a linear relationship between the d-spacing in polypyrrole films, observed by X-ray diffraction, and the size of the counterion present in the electropolymerization solution.21 Dong and cc-workers reported the development of chloride-selective electrodes fashioned by doping chloride into the polypyrrole film.22323 In one case, the PPy-based chloride elec- trodes had films that were prepared using cyclic voltammetry with chloride as the doped counterion.22 The selectivity of these electrodes for chloride over nitrate was poor (K .. 1). In the other case, using the same counterion but a dif- ferent imprinting procedure (fixed potential electropolymeriza- tion), a slight preference for chloride over nitrate was observed (~i$g!,~~,- = 5 x 10-9.23 This illustrates the importance of controlling both the doped anion and the morphology of the imprinted PPy films, the latter through an optimization of the various electrochemical parameters during electropolymerization. While not all of the available electrochemical parameters were adjusted, Dong's work demonstrated that there is potential for preparing PPy films which can discriminate against anions that are more lipophilic than the doping ion by imprinting the film using optimized electrochemical conditions.

Herein an electrochemical templating technique was employed in the development of a polymer-based nitrateselective electrode. By polymerizing pyrrole in the presence of NaN03, a film was produced with pores that are complementary to the size of the targeted analyte ion. Both the size of the pore and the charge distribution within the polymerized film form a host cavity for nitrate, which provides additional selectivity over conventional nitrateselective electrodes. Despite previous reports of polypyr- role electrode~,2~-~~ to our knowledge this work represents the

(18) Meyerhoff, M. E.; Pretsch, E.; Weltis, D. H.; Simon, W. Anal. Chem. 1987,

(19) Umezawa, Y. In CRC Handbook of Ion-Selective Electrodes: Selectivity Coefficients; CRC Press: Boca Raton, FL, 1990; pp 89,99.

(20) Wang, J.; Chen, S. P.; Lin, M. A J Electroanal. Chem. 1989, 273, 231- 242.

(21) Yamaura, M.; Hagiwana, T.; Iwata, IC Synth. Met. 1988, 26, 209-224. (22) Dong, S.; Sun, 2.; Lu, 2. Analyst 1988, 113, 1525-1528. (23) Dong, S.; Che, G. Talanta 1991, 38, 111-114. (24) Lu, 2.; Sun, Z.; Dong, S. Electroanalysis 1989, 1, 271-277. (25) Kang, S. C.; Lee, K S.; Kim, J. D.; Kim, K J. Bull. Korean Chem. SOC. 1990,

(26) Daunert, S.; Wallace, S.; Florido, A; Bachas, L. G. Anal. Chem. 1991,63,

(27) Hulanick, A; Michalska, A; Lewenstam, A Talanta 1994, 41, 323-325. (28) Gao, Z.; Bobacka, J.; Lewenstam, A; Ivaska, A Electrochim. Acta 1994,

(29) Dong, S.; Sun, Z.; Lu, 2. J. Chem. Soc., Chem. Commun. 1988, 993-995.

59, 144-150.

11, 124-126.

1676-1679.

39, 755-762.

first comprehensive study of the effects various electropo!ymer- imtion parameters have on the potentiometric response properties of the electrodes to nitrate. Because the electropolymerization variables affect the film morphology, the chemical recognition properties of the polypyrrole film will also be affected. A careful study of these conditions enables optimization of the response of the prepared electrodes to the targeted anion, as well as aiding in the development of strategies toward the rational design of ISEs. The selectivity coefficients for the optimal polypyrrolebased nitrateselective electrode were improved by more than 4 orders of magnitude for some interferents, when compared with those for commercially available nitrateselective electrodes. Addition- ally, a selective response to nitrate over all the anions tested has been achieved. The unique advantage of this approach to ion- selective electrode design is the ability to exercise electrochemical control of the molecular imprinting process.

EXPERIMENTAL SECTION Reagents. The pyrrole monomer and the sodium salts of

perchlorate, salicylate, and phosphate were obtained from Sigma (St. Louis, MO). The remainder of the inorganic sodium salts were procured from the following sources: nitrate and sulfate from Aldrich (Milwaukee, WI); nitrite and chloride from Fisher Scientitic (Cincinnati, OH); iodide from J. T. Baker (Phillips- burg, NJ); thiocyanate from Matheson Coleman & Bell (Cincinnati, OH); and bromide from Mallinckrodt Chemical Works (St. Louis, MO). AU the aqueous solutions were prepared using deionized (Milli-Q water puriiication system; Millipore, Bedford, MA) distilled water.

Apparatus and Electrodes. The electropolymerization was performed using an EG&G Princeton Applied Research poten- tiostat/galvanostat (Model 362; Princeton, NJ). A threeelectrode cell was used for preparing the modified electrodes, with a glassy- carbon rod electrode (Model MF2012; Bioanalytical Systems, West Lafayette, IN) as the working electrode and silver and platinum wires as the reference and counter electrodes, respectively. Potentiometric studies were carried out using Fisher Accumet 810 and 825 digital pH/mV meters, in conjunction with Fisher Recordall (Series 5000) strip-chart recorders. A jacketed beaker, online with a Fisher Isotemp circulator bath (Model 9500), was used to maintain the temperature at 25 "C during all experiments. Potential differences were measured between the prepared poly- pyrrolecoated indicator electrodes and the Orion Ag/AgCl double junction reference electrodes (Model 13-641-900). The reference electrodes were prepared with an inner filling solution saturated in AgCl (Orion 9@0@02) and an outer compartment possessing the same buffer used in the sample solution (when water was used in place of buffer, 1.00 x M NaH2P04, pH 5.00, was used as the outer filling solution).

The thickness of the optimized polypyrrole films was deter- mined using a Mitutoyo electronic height gage (Model 192-656, Aurora, IL). Electrodes were prepared by applying tape to a portion of the electrode surface and then removing it following film deposition. The step across the surface was measured with contact using a Brown and Sharpe indicator (BesTest 0.0001 in.) and without contact using a CyberOptics laser trigger (Model LT loo).

Procedure. Before electrodeposition onto the glassycarbon electrodes, the surface of the electrodes was polished using Alpha (30) Pei, Q.; Qian, R Electrochim. Acta 1992,37, 1075-1081. (31) Bobacka, J.; Gao, 2.; Ivaska, A J Electroanal. Chem. 1994, 368, 33-41.

Analytical Chemistry, Vol. 67, No. 10, May 15, 1995 1655

A cloth (Mark V Laboratory, East Granby, 0 and GAMAL alumina, particle size finer than 0.1 pm (Fisher). The electrodes were then sonicated in water for 5 min before use. Five minutes of sonication was found to be optimal as electrodes prepared without sonication, and those sonicated for longer periods of time, responded poorly in comparison. S i l a r time-dependence effects were recently reported that support our experimental observa- t i o n ~ . ~ ~ Sonication must proceed long enough to adequately clean the electrode surface, but extensive sonication results in adverse electrode surface pitting.

Pyrrole was purified by distillation on the day of use and was protected from light until the electropolymerization solution was prepared. The electropolymerization solution, composed of an aqueous solution of the pyrrole monomer and sodium nitrate salt (concentrations were varied), was deoxygenated by purging with Nz for 10 min before the experiment and flowing the gas over the solution during the experiment. Both fixed potential and constant current techniques were used to coat the working electrodes with polypyrrole. The electropolymerization variables optimized in- cluded potential/current used, final potential applied (the potential at which the working electrode was poised during the final 2 min of fixed potential electropolymerization), and time of electropc- lymerization.

The prepared electrodes were rinsed with water and stored in an aqueous solution of sodium nitrate for conditioning (the time of conditioning and the concentration of the conditioning solution were both optimized). Electrodes were calibrated by adding different standard solutions to a jacketed beaker, initially contain- ing 5.00 mL of water or buffer, with stirring. The potentiometric response was monitored using the pH/mV meter and recorded with the stripchart recorder. The selectivity coefficients were determined using the IUPAC recommended Nikolsky-Eisenman equation.33 The potentiometric slopes, determined by fitting the linear portion of the calibration plot to the Nernst equation, and the detection limits of the prepared electrodes are reported in this article as the average value i 1 standard deviation. Between experiments, the electrodes were stored in the conditioning solution at room temperature.

RESULTS AND DISCUSSION The electrochemically prepared polypyrrole films were tested

for their potentiometric responses. Many of the conditions used to prepare the polypyrrole-coated glassycarbon electrodes were varied to determine the effects, if any, on the potentiometric responses (slopes of calibration plots, detection limits, and linear range of response) of the films to nitrate. The potential responses of electrodes prepared with the nitratedoped polypyrrole polymer, [PPy(NO3-)], were tested after 2 and 24 h of conditioning in 1.00 x M NaNO3. Figure 1 illustrates the importance of the conditioning time on the observed potentiometric slopes. Regard- less of the polymerization time used to prepare the electrodes, a variable that controls film thickness, the slopes obtained using 24 h of conditioning were closer to the theoretical slopes expected on the basis of the Nernst equation. Thus, all electrodes were conditioned for at least 24 h before use. Longer conditioning times produced no further improvements in response.

The concentration of the conditioning electrolyte was varied from 1.00 x to 1.00 M NaN03 to determine how the response

(32) B a n g , H.; Coury, L. A, Jr. Anal. Chem. 1993, 65, 1552-1558. (33) Buck, R P.; Lindner, E. Pure Appl. Chem. 1994, 66, 2527-2536.

0

-1 0 h P) 0

0 a a -20

? > -30 E

P) CL -40 0 v)

v

-

,g 24h -60 I

5 15 20 30

Electropolymerlzatlon Time (mln)

Figure 1. Effect of varying the electropolymerization and condition- ing time of the ISE on the potentiometric nitrate response (slopes of calibration plots). Electrodes were prepared using fixed potential (+0.90 V) polymerization in a solution of 0.10 M pyrrole and 0.100 M NaN03. The electrodes were conditioned in 1 .OO x M NaN03.

of the PF'y electrodes would be affected. One recent report about a PPy(Cl-)doped electrode showed that the baseline potential (the potential before addition of analyte) was markedly higher for electrodes conditioned in high electrolyte concentration.3l On the contrary, another study indicated that a PPy(Br-)doped electrode had a lower baseline potential as the conditioning concentration of NaBr increased.28 Neither a lowering nor a raising of the baseline potential was observed herein; however, our work and the two previous studies have all been conducted with polymer films prepared under different conditions. The baseline potential of the electrode conditioned in 1.00 x M NaN03 was within 5.0 mV of the electrode conditioned in 1.00 M NaN03. It was also observed that electrodes conditioned in 1.00 x or 1.00 x M NaN03 gave substantially longer response times during the initial calibration experiment and overall slightly worse detection limits (2.7 x and 2.4 x M, respectively) than those conditioned in more concentrated solutions (detection limits of 1.4 x 10-5-1.6 x M). The optimal nitrate conditioning solution was determined to have a concentration of M NaN03; the concentration was fixed at 1.00 x M NaN03 for all remaining studies.

Electrodes were first prepared using the fixed potential method, and the effect of the film thickness was observed by varying the potential applied as well as the time of polymerization. Figure 1 illustrates the effect of the time of polymerization used in preparing the electrodes on the slopes of the calibration curves for nitrate. Thicker films (longer electropolymerization times) resulted in slopes that were closer to theoretical. No signiicant improvement was observed for polymerization times beyond 20 min. This result is consistent with that reported in testing the potentiometric response of a @F4-)doped PPy electrode to tetratluoroborate.3l Both the time of polymerization and the value of the fixed potential affect film thickness, and in this sense it can be said that they are interrelated variables. Yet the film morphology (i.e., density and porosity) can be expected to change, regardless of the thickness of the film, as a function of the applied

1656 Analytical Chemistry, Vol. 67, No. 10, May 15, 1995

c -25 1

G I t 0 (d 0 0) 2 -35 > E - al 0 " - B

_ _ 0.5 0.6 0.7 0.8 0.9 1 .o

Electropolymerization Potent ial (V ) Flgure 2. Effect of varying the potential used during a fixed potential electropolymerization (20 min) on the potentiometric nitrate response (slopes of calibration plots). For the polymerization and conditioning solutions used, see legend to Figure 1. The electrodes were conditioned 24 h before use (n = 3 for electropolymerization potentials from 0.75 to 0.95 V).

potential.34 The effect of various applied potentials on the potentiometric nitrate response can be seen in Figure 2. A potential of at least f0.60 V (vs Ag wire) is required for deposition of the polymer onto the glassy-carbon electrodes. This minimum oxidation potential is required for film formation in aqueous solutions, as was shown in a previous study of polypyrrole films, regardless of the electrolyte used for d0ping.3~ In Figure 2 it can be seen that the potentiometric response to nitrate improves as the applied potential increases above +0.60 V up to +0.95 V. It was also observed in this study that further increases in potential (Le., 2+1.00 V) resulted in a slight degradation of the nitrate response. Electrodes prepared at f0.95 V gave nitrate calibration plots with the best slopes (closest to the theoretical value of -59 mV/decade change in activity).

The effect of varying the final potential applied to the polypyr- role films after electropolymerization at a fixed potential of +0.95 V was also tested. During the final 2 min of the 20 min electropolymerization time, the applied potential was changed from $0.95 V to values between 0 and +1.30 V. Although the effect on the potentiometric response was small, as the h a 1 applied potential increased or decreased relative to +0.95 V, the slopes of the calibration plot for nitrate decreased. The slopes decreased with decreasing final potential, reaching -44.7 f 0.6 mV/decade at 0.0 V. At +1.3 V, the upper limit of the final applied potential used, the slope was -45.3 f 0.3 mV/decade (compared to -49.8 f 0.9 mV/decade at f0.95 V; n = 3 in each case). Two other studies, one using a PPy(C1-)22 and the other a PPy(13F4-)31 film, reported that the only effect of the final applied potential was to shift the absolute potential values of the calibration plots (i.e., the plots shifted parallel to one another, but the slopes remained constant), However, in both cases the electrodes were prepared using cyclic voltammetry and not k e d potential polymerization. The method used for electropolymerization should affect the film morphology and may be the reason the potentiometric response in the present case is dependent on the final potential.

(34) Zagorska, M.; Wycislik, H.; Przyluski, J. Synth. Met. 1987, 20, 259-268. (35) Asarapiriyanont, S.; Chandler, G. IC; Gunawardena, G. A; Pletcher, D. J

Electroanal. Chem. 1984, 177, 229-244.

Table I. Effect of Changes in the aalvanostatic Current Used during Electropolymeriutlon on the Potentiometrlc Slopes for Nitrate.

current (u&

5 25 100 500 1000 5000

slope (mV/decade) -37 -36 -49 -21 -31 -24 SD (mV/decade) i~3 i6 &1 &3 i3 i7

'The average slopes of the electrodes and the corresponding standard deviations are shown (n = 3).

Similar experiments were also performed using the constant current (galvanostatic) technique for polymer film formation. Glassy-carbon electrodes, 3 mm in diameter, with films prepared under different applied currents were compared in an attempt to find the ideal current for film growth. The results of this study, given in Table 1, demonstrate that a current of 100 p A not only produced films that responded better to nitrate (i.e., calibration plots with slopes closer to theoretical), but also produced electrodes that had a very reproducible response. At currents less than 100 ,uA, film growth was slow and the slopes and reproducibility were worse. Using larger currents caused rapid film growth with films that were visibly rough. At a current of 5000 the films adhered poorly to the surface of the electrode, resulting in highly irreproducible potentiometric responses. This behavior of the prepared films can be related to the potential observed during film growth. For the films prepared using 100 ,uA, the potential at the start of the experiment was consistent at +0.80 V. M e r about 30 s, it dropped to +0.75 V and remained essentially unchanged for the remainder of the electropolymer- ization. However, when a current of 500 pA was used, the potential at the start of the experiment was +1.00 V and quickly jumped to f 2 . 2 V. This indicated that at a current of 500 PA, overoxidation of the polymer was occurring, and a ragged, poorly adhered film resulted. To achieve electrodes with optimal and reproducible potentiometric responses, a current of 100 pA was employed in all further constant current film preparations.

The concentration of pyrrole and electrolyte in the electropo- lymerization solution has also been shown to influence the resultant iilm morphology and performance of doped PPy f i h ~ s . ~ ~ ~ ~ One study found that varying the pyrrole concentration from 0.01 to 0.05 M produced an improved response to chloride in PPy(CI-) electrodes;22 however, films prepared with 0.20 M pyrrole mono- mer showed no continued improvement. Knowing that the response of the ISEs worsens with limited pyrrole present in the electropolymerized solution, the concentration of pyrrole was varied in our study between 0.10 and 1.0 M; aqueous 0.100 M NaN03 was used as the electrolyte. The polymer films were prepared galvanostatically using different electropolymerization times. The prepared electrodes illustrate that, when all other variables were kept constant, increasing the pyrrole concentration above 0.10 M results in ISEs with higher slopes (see Table 2). A monomer concentration of 1.0 M was used to prepare all further electrodes. This concentration study was performed using both 2 and 10 min electropolymerization times. The thicker films (10 min) gave ISEs with higher slopes compared to those of ISEs with the thinner films at every pyrrole concentration used.

Subsequently, the film thickness was optimized by varying the time of electropolymerization using a fixed current of 100 pA and 1.0 M pyrrole monomer. The response of these films to nitrate

Analytical Chemisfv, Vol. 67, No. 10, May 15, 1995 1657

Table 2. Effect of the Concentratlon of Pyrrole In the Electropolymerlzation Solution on the Response of PPy(NOa-) Electrodes to Nitrate for Electropolymerlzation Times of 2.0 and 10.0 Yln (n = 3)

2.0 10.0

Pyrrole slope detection slope detection concn (M) (mV/decade) limit (M) (mV/decade) limit (M)

0.10 - 2 6 f 5 2 x 10-5 - 4 2 f 9 7 x 0.50 - 3 5 f 1 5 10-5 - 5 2 f 4 5 10-5 1.0 - 4 2 f 4 3 x 10-5 -53 f 1 3 x 10-5

i ; -10

?

U a -20

1

0 10 20 30 Electropolymer i ta t lon Time (min)

Flgure 3. Effect of the time of electropolymerization using the constant current method (100 PA) on the slopes of the calibration plots for nitrate. Electrodes were prepared in a solution of 1.0 M pyrrole and 0.100 M NaN03 and then conditioned for 24 h in 1 .OO x lo-' M NaN03 before use ( n = 3).

(Figure 3) demonstrates that 20 min is sufficient time to produce electrodes with slopes of calibration plots that are close to the Nernstian value. Except when electropolymerizing for very short periods of time (52 min), the constant current method of preparing PPy films proved to be highly repeatable in terms of the potentiometric performance of the electrodes. The thick- nesses of the films prepared with a 20 min deposition time were determined using a Mitutoyo electronic height gage. The thick- nesses of the films deposited were measured using two separate techniques (with contact using a stylus, and without contact using a laser trigger) on each of three glassycarbon electrodes. The film thicknesses were found to be 13 pm in each of the six measurements.

It has been observed previously that the concentration of electrolyte used during the electropolymerization affects the response of polypyrrole-based chloride ISES .~~ In particular, it was observed that the ISEs gave better responses when higher concentrations of electrolyte were used. In the present study, the NaN03 concentration was varied from 0.100 to 1.00 M to ascertain the effects of excess electrolyte present during electropolymer- ization. No difference in electrode performance was observed over this electrolyte concentration range (data not shown); a concentra- tion of 0.100 M was selected for the preparation of all remaining nitrate-doped films.

The selectivity pattern of electrodes prepared under the optimized electropolymerization conditions described above for constant current film formation is shown in Figure 4. The electrodes demonstrate a significant deviation in selectivity from the Hofmeister series. Specifically, the electrodes were selective

0

-50

n > -100 E

w -150 6

W

-200 t \\

0 - 2 5 0 - * 1 I 1 * I

-6 -5 -4 -3 -2 -1

log (activity) Figure 4. Selectivity pattern of PPy(NO3-) electrodes (preparation conditions as described in Figure 3, using 20 min for the electropo- lymerization). The anions tested include (1) nitrate, (2) thiocyanate, (3) bromide, (4) iodide, (5) chloride, (6) perchlorate, (7) salicylate, (8) sulfate, and (9) phosphate (n = 18 for nitrate; n = 3 for all other ions). A € is the difference between the steady-state potential and the starting potential (Le., potential of the cell before any addition of anions). Electrodes were conditioned in 1 .OO x 1 O-' M NaN03 for a minimum of 1 h between experiments.

for nitrate and thiocyanate over the other anions tested. The response to nitrate was near-Nernstian, with slopes of -56 f 1 mV/decade (n = 18), and the linear range of response extended down to 5.0 x M nitrate. AU the experiments described to this point were performed in distilled water, as has been the precedent for testing PPy films for potentiometric response.22828+29 None of the anions tested changed the pH of the unbuffered solution to any significant level; the maximum change observed during the experiments was 50.3 pH units. Based on the weak response of the polymer-based ISEs to phosphate, the sensors were also tested for their response to nitrate in buffered phosphate (5.00 x M) solutions of both pH 5.00 and 7.00. It was found that the response at pH 7.00 was poorer with respect to both the slopes (-44 f 1 mV/decade) and detection limits ((8 f 2) x

M nitrate) than that observed at pH 5.00 (-50 f 1 mV/ decade and (1.7 f 0.3) x 10-5 M nitrate, respectively: n = 3 at each pH). Subsequently, the effect of hydroxide interference was examined, the results of which are displayed in Figure 5. The ISE responded linearly to changes in pH between pH 6 and 10. Thus, by working at pH 5.00 (where the hydroxide concentration is minimal), the detection limits achieved for nitrate were found to be improved over those observed at pH 7.00.

The electrodes responded quickly to additions of nitrate in the sample solution, with response times ranging from 24 s at concentrations above 6 x M to <6 s at lower concentrations. The response time is dehed as the period between the time of addition of analyte to the sample solution and the time when a steady-state potential with less than 0.2 mV/min change has been achieved.

The useful lifetimes of several optimized electrodes were determined under two different storage conditions. In one case, the prepared electrodes were placed in 1.00 x M NaN03 solution in a clear glass vial. In the other, electrodes were similarly conditioned while being kept in the dark. The results of this experiment are displayed in Figure 6. Electrodes stored

1658 Analytical Chemistry, Vol. 67, No. 10, May 15, 1995

250 9 2oo t 150 1 n > t

-E v 1001

4.5 5.5 6.5 7.5 8.5 9.5

PH Figure 5. Response of a PPy(N03.) ISE (prepared as described in Figure 3, using 20 min for the electropolymerization) to pH.

al 0 a 0 -20 U - a! -'I i 9) -40 a 0 ' -50

0 20 40 60 80 100 Days

Figure 6. Lifetime study of PPy(N03-) ISEs (prepared as described in Figure 3, using 20 min for the electropolymerization). The electrodes were all stored in 1 .OO x lo-* M NaN03 and were either exposed to (0) or protected from (0) the light. The slopes of the calibration plots for nitrate (n = 3) over 3 months illustrate the effects of exposure to light.

in the light demonstrated useful lifetimes of about 9 days, followed by a rapid deterioration in response. When electrodes were stored in the dark, the potentiometric response remained unchanged over at least 55 days. The slope of the nitrate calibration plot on day 90 was found to be 90% of the original response, indicating only a slow deterioration in the response of the sensors. In fact, after more than 5 months (159 days), the slope of the nitrate calibration plot was still 90% of the original response. The pyrrole monomer polymerizes when exposed to light; thus, it requires fresh distillation on the day it is used for electropolymerization. In a similar manner, the polypyrrole film is sensitive to light and undergoes deterioration when exposed to light for an extended period of time. The electrodes stored under dark conditions were exposed to light only during the duration of each potentiometric experiment (1-2 h). It is possible that such electrodes may demonstrate lifetimes that signilicantly exceed those of traditional ISEs based on ionophoreimpregnated polymer films under conditions where the electrodes are both tested and stored in the dark.

A fixed interference study was performed for many of the common inorganic and organic anions to determine the selectivity

Table 3. Selectivity Coefficlents of Qalvanostatically Prepared PPy(N03-) Electrodes Using the Fixed Interference Method with an Interferent Concentration of 1.00 x lo-* Ma

anion K G - X SD (n = 3)

thiocyanate bromide perchlorate chloride iodide salicylate phosphate sulfate

3.6 x 10-' 7.6 x 5.7 x 10-2 5.7 x 10-2 5.1 x 5.1 x

6 x 9 10-4

0.6 x lo-' 0.4 x 0.2 x 10-2 0.3 x 0.5 x 0.5 x 3 10-4 2 10-4

Experiments were performed in unbuffered solutions.

Table 4. Comparlson of the KCAOd,-,x Values Observed for Several Anions Using Two Commercial Nitrate Electrodes and the Qalvanostatically Prepared PPy(NOa-) Electrodes

anion electrode perchlorate iodide chloride

Orion Model 92-07 lo00 20 0.006 Coming Model 477316 800 17 0.005 P&(Nos-) electrode 0.057 0.051 0.057

coefficients of the optimized PPy(NO3-) electrode. Table 3 contains the selectivity coefficients of the optimized PPy(NO3-) ISE. The observed selectivity is markedly altered from the Hofmeister series and that of other reported nitrate ISEs. The selectivity observed is obtained as a result of the electrochemical nitrate templating procedure used to prepare the electrodes. To our knowledge, this molecular imprinting approach using poly- pyrrole and nitrate represents the only polymer-based nitrate ISE system that has been found to be selective for nitrate over perchlorate, iodide, and bromide. The selectivity coefficients of two commercial nitrateselective electrodeslg are compared to the galvanostatically prepared PPy(NO3-) electrode in Table 4. Both commercial electrodes suffer major interference from perchlorate and iodide and respond preferentially to these anions rather than to nitrate. Several studies have demonstrated that these PPy- based electronically conducting polymers suffer from redox interferents and that the two types of responses (Le., redox and potentiometric) cannot be separated. However, the selectivity observed herein still makes them of utility in matrices where no strong redox species are present. Additionally, it has been previously shown that it is possible to prepare freestanding PPy film~.3O3-~~ These films could be used as membranes in conven- tional ISE configurations, which allow for only ionic conductivity in the internal filling solution. Thus, it should be possible to couple the preparation of freestanding PpY films with that of electrochemically imprinting selective films to eliminate potential redox interferences.

The use of molecular imprinting in the development of novel ISEs to improve selectivity has been demonstrated in the case of

(36) Burgmayer, P.; Murray, R W. J. Electroanal. Chem. 1983, 147, 339-344. (37) Burgmayer, P.; Murray, R W. J. Phys. Chem. 1984, 88,2515-2521. (38) Cai, 2.; Martin, C. R J. Am. Chem. SOC. 1989, 111, 4138-4139. (39) Rosatzin, R; Andersson, L. I.; Simon, W.; Mosbach, K J. Chem. SOC., Perkin

Trans. 1991,2, 1261-1265.

Analytical Chemistry, Vol. 67, No. 10, May 15, 1995 1659

nitrate. However, while traditionally strong interferents, such as perchlorate and iodide, can be discriminated against on the basis of sizeexclusion using this technique, other ions Q.e., thiocyanate) that are similar in size to nitrate cannotH Because the more lipophilic anions can enter the hydrophobic PPy films easier, a Hofmeister-type selectivity is superimposed over the nitrate imprinting selectivity. Thus, while anions more lipophilic than nitrate can be sterically hindered from entering the nitratedoped PPy films if their radii are larger than that of nitrate (i.e., Clod-, I-), more lipophilic anions, such as thiocyanate, of equal or smaller radii, will still pose an interference problem. One approach to further improve the selectivity achieved using this templating approach is to combine the electropolymerized imprinting rec- ognition with a selective coordination interaction. This would provide size-exclusion selectivity coupled with the selectivity arising from the ability of ions to coordinate favorably with the ionophore within the membrane. This is also consistent with a study by Rosatzin et al., where a calcium ionophore was polym- erized in the presence and absence of calcium metal using 2,Y- azoisobutyronitrile as the initiator. Although the formed polymers were not used in ion-selective electrodes, both the selectivity for calcium over other ions and the binding strength of the polym- erized ionophore improved dramatically in the calcium-imprinted polymers over reference polymers prepared in the absence of

calcium ions.39 Combining recognition elements in this manner to develop highly selective ISEs is an approach we are currently investigating in our laboratory.

Another advantage of ISE films prepared in this fashion is their insolubility in organic solvents, media in which conventional ISEs are unusable. In summary, by using a molecular imprinting/dop ing approach, nitrateselective electrodes were prepared that pos- sess near-Nernstian slopes of -56 f 1 mV/decade (n = 18) over a linear range of four decades of concentration (5 x 10-5-0.50 M). These electrodes demonstrate a detection limit of (2 f 1) x

M (n = 18) and a selectivity that is one of the best reported to date for nitrate determination using a polymer-based ISE.

ACKNOWLEDGMENT The authors would like to thank the Kentucky Space Grant

Consortium and the National Aeronautics and Space Administra- tion as well as the National Science Foundation (EHR-9108764) for hancial support of this work.

Received for review November 7, 1994. Accepted February 17, 1995.@

AC941078Z

@ Abstract published in Advance ACS Abstracts, April 1, 1995.

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