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Potentiometric and Fiber Optic Sensors for pH Based on an Electropolymerized Cobalt Porphyrin Timothy L. Blair,+John R. Allen, Sylvia Daunert,’ and Leonidas G. Bachas* Department of Chemistry and Center of Membrane Sciences, University of Kentucky, Lexington, Kentucky 40506-0055
Potentiometric and fiber optic sensors based on polymer films were prepared by electrodeposition of the ion-carrier monomer cobalt(I1)tetrakis ( p hydrosyphenyl)porphyrin, [Co(pOH)TPP]. Fiber optic sensors were developed by electropolymerization of the porphyrin on the surface of indium(tin)oxide glass slides. The optical sensors were highly selective to pH, presenting minimal interferences from anions. They had a linear response from pH 8 to 12. Potentiometric sensors were prepared from porphyrin films electrodeposited on glassy-carbon disk electrodes. These electrodes gave a near-Nernstian response to pH from pH 2 to 12 with minimal interferences.
OH
Q OH
INTRODUCTION
Flguro 1. Structure of cobalt(1I) tetrakis(phydroxyphenyl)porphyrln.
Several reports have described the successful use of electropolymerized membranes in potentiometric sensors.1-@ These sensors offer advantages over electrodes based on plasticized poly(viny1chloride) (PVC) membranes. Because the film consists of covalently linked ionophore monomers with no additional polymer matrix or plasticizer, leaching of the membrane components into the sample solution is greatly diminished. This results in ion-selective electrodes (ISEs) that have increased lifetimes.6 Recently, we have reported that electropolymerized porphyrin films result in functional potentiometric anion-selectiveelectrodes.6 Specifically, electrodes prepared by electropolymerization of a cobalt(I1) tetrakis(o-aminopheny1)porphyrin monomer on the surface of glassy-carbon electrodeswere selective for thiocyanate and had detection limits of 5 X 10-7 M. The selectivity and lifetimes of the ISEs based on this polymeric porphyrin were improved with respect to PVC membrane electrodes containing the same ion carrier.1”12 In addition to ISEs, polymer membrane films have also been employed in optical sensors. Munkholm et al. used
acryloylfluorescein copolymerized with acrylamide or 2-hydroxyethyl methacrylate to produce pH-sensitivefilms13that were later incorporated into COZ sensors.14 Carey and Jorgensen used the fluorescent polymers poly(pheny1quinoline),poly(biphenylquinoline),and poly(pheny1quinoxaline) to develop fiber optic sensors for high acidities.16 There are also numerous reports of optical sensors based on the immobilization of reagents on or in polymer films.16J7 However, to the best of our knowledge there is no report on the use of electropolymerization to prepare films that are suitable for the development of optical sensors for ions. Given the advantages of electropolymerization in preparing ISEs with improved characteristics, we decided to investigate the feasibility of using electropolymerizationin the development of fiber optic sensors. This paper describes a fiber optic sensor for pH based on an electropolymerized porphyrin film. Specifically, cobalt(11)tetrakis@-hydroxyphenyl)porphyrin, [Co@-OH)TPP], was prepared and polymerized electrochemically on indium(tin) oxide glass slides (Figure 1). The electropolymerized porphyrin films were then used as sensing elements for the development of fiber optic sensors. Studies were also conducted to elucidate the response Characteristics of these sensors. In addition, comparisons were made between these sensors and a potentiometric sensor based on electropolymerization of the same monomer on glassy-carbon disk electrodes.
t Current addrees: Department of Chemistry, The University of the South, Sewanee, TN 37375. (1)Heineman, W. R.; Wieck,H. J.;Yacynych, A.M. Anal. Chem. 1980, 52,346346. (2)Dong, S.;Wang, Y. Electroanalysie 1989,1,94.106. (3)Cheek, G.;Wales, C. P.; Nowark, R. J. Anal. Chem. 1983,55,380381. (4)Ohnuki, Y.; Matauda, H.; Ohsaka, T.; Oyama, N. J. Electroanal. Chem. 1988,158,6647. (5)O h d a . T.: Hirokawa.. T.:. Mivamota. . H.:. OYama, _ . N. Anal. Chem. EXPERIMENTAL SECTION 1987;69,176d-1761. (6) Daunert, S.; Wallace, S.; Florido, A.;Bachas, L. G.Anal. Chem. Reagents. Potassium thiocyanate and acetonitrile (stored 1991,63,1676-1679. over 3-A molecular sieves) were purchased from Aldrich (Mil(7)Karag&ler,A.E.;Ataman,O.Y.;Galal,A.;Xue,Z.-L.;Zimmer,H.; waukee, WI). Tetraethylammonium perchlorate (TEAP)was Mark, H. B.,Jr. AM^. Chim. Acta 1991,248,163-172. (8) A M ,N.;Zimmer, H.;Mark,H. B., Jr.Ana1. Lett. 1991,24,14311443. (13)Munkholm, C.;Walt, D. R.; Milnnovich, F. P.; Klainer, S. M. (9)Kliza, D. M.;Meyerhoff, M. E. Electroanalysis 1992,4,841-849. Anal. Chem. 1986,58,1427-1430. (14)Munkholm, C.; Walt, D. R.; Milanovich, F. P. Talanta 1988,35, (10)Ammann, D.; Huser, M.; KrHutler, B.;Rusterholz, B.; Schulthess, 109-112. P.; Lindemann, B.; Halder, E.; Simon, W. Helv. Chim. Acta 1986,69, (15)Carey, W. P.;Jorgensen, B. S. Appl. Spectrosc. 1991,45,834-838. 849-854. (16)Wolfbeis, 0.S.And. Chim. Acta 1991,250,181-201. (11)Hodinar, A.;Jyo, A. Chem. Lett. 1988,993-996. (17)Arnold, M. A. Anal. Chem. 1992,64,1015A-1025A. (12)Hodinar, A,;Jyo, A. Anal. Chem. 1989,61,1169-1171.
0003-2700/93/0365-2155$04.00/0
0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
It
common end of bifurcated fiber optic bundle
electropolymerized fil 2-mm spacer Figure 2. Schematic of the construction of the fiber optic sensor.
obtained from Southwestern Analytical Chemicals (Austin,TX). Cobalt acetate was acquired from Merck (Rahway, NJ) and [Rum(NHa)e]Cla from Strem Chemicals (Newburyport, MA). 2-(N-Morpholino)ethanesulfonicacid (MES),sodium salicylate, sodium acetate, and all inorganic salts used were purchased from Fisher (Fair Lawn, NJ) or Sigma (St. Louis, MO). Distilled, deionized (Milli-Qwater purification system;Millipore, Bedford, MA) water was used to prepare all aqueous solutions. Monomer Synthesis. The synthesis of tetrakis@-hydroxyphenyl)porphine, @-OH)TPP,was accomplished by following a two-step procedure reported by Little et al.la The lH NMR spectrum (deuterated acetone, TMS) had peaks at 6 9.0 (8H, &pyrrole),8.1(8H,2,6arylprotons),and7.3 (8H,3,5arylprotons). The metalation of tetrakis@-hydroxypheny1)porphine was carried out in a mixture of chloroform and methanol, 9.0 and 3.0 mL, respectively. A 100-mgportion of @-0H)TPPwas dissolved in the solvent, and a solution of cobalt acetate (42mg in 1.0 mL of methanol)was added. The mixture was allowedtoreflux under argon for 45 min, concentrated, and extracted three times with 100mL of acetone. The three portionsof acetonewere combined, filtered, and evaporated. The solid was then washed with warm water and dried at 100 O C . The metalation step yielded 86 mg of [Co@-OH)TPP]. The metalation caused shifta of the lH NMR (deuterated acetone, TMS) to 13 11.6 (8H),9.9 (8H),and 8.0 (8H). [Co@-OH)TPP]is now commerciallyavailablefrom Midcentury (Posen, IL), and some of the described experiments were performed with this product. Apparatus. An EG&G Princeton Applied Research potentiostat/galvanoetat (Model 273) was used in conjunction with a Houston Instrument X-Y recorder (Model 200) for the electropolymerization process. Spectroecopic measurements were made using an Oriel (Stratford,CT) modular spectrophotometer. A 100-Wtungsten filament lamp (Model 6333)was powered by an Oriel power supply (Model68736). The light from the source passed through the monochromator (Model 77296) and was focused on one leg of a bifurcated bundle of randomized glass fibers (Oriel Model 77533). Changes in signal were monitored at 470 nm. Variable slits (Model 77269) on each side of the monochromator were set at 0.12 mm. Fiber optic seneors were prepared by placing an indium(tin) oxide glass slide coated with the electropolymerizedporphyrin film at the common end of the bifurcated fiber optic bundle (Figure 2). Light from the source paaeedthroughthe film, reflected off the PTFEplate, and p d throughthe film again before returningto the fiber optic bundle. The other leg of the bifurcated bundle carried the light to the photomultiplier tube (Model 77761). The PMT readout device (Model 7070)was interfaced with a strip-chart recorder (Linear Model 1200). Initial evaluation of the polymer f i b was performed on a Perkin-Elmer Lambda-6 UV-Vis spectrophotometer (Norwalk, CT). Preparation and Evaluation of the Fiber Optic Sensors. A conventional three-electrode setup was employed for the electropolymerizationof [Co@-0H)TPPl. The porphyrin films were prepared using an indium(tin)oxide-coatedglass slide (Delta Technologies, Stillwater,MN) as the working electrode, while a silver wire and a platinum wire were the “pseudoreference”and (18) Little, R. G.; Anton,J. A.; Loach, P. A.; Ibers, J. A. J . Heterocycl. Chem. 1976,12, 343-349.
counter electrodes, respectively. An acetonitrile solution containing 1.0mM [Cow-OH)TPP] and 0.10 M TEAP was used for the electropolymerization. The potential of the cell was scanned at 200 mV/s from 0 to 1.3 V. The poly[Co@-OH)TPP]films were prepared using 200 potential cycles. The potentiostat/ galvanostat was then set at a constant final potential of 0.0 V for 2 min. The slides were washed with water and stored in a 0.010 M thiocyanate solution. The sensor was calibrated by placing its sensing tip in a phosphoric acid solution and adjusting the pH with a NaOH solution. The pH of the solution was also monitored using a Fisher glass pH electrode (Model 13-620-287)and an Accumet pH meter (Model 925). The sensor was stored in a solution of pH -10 between experiments. Preparation and Evaluation of ISEs. Porphyrin films were coated on a glassy-carbon disk electrode (BioanalyticalSystems, West Lafayette, IN; Model MF2012) using the same system described above. The pH response of the electrodes was evaluated by adding aliquota of NaOH to 25.00 mL of 1.00 X 1W M phosphoric acid. The polymerized electrodes were conditioned in a 0.100 M phosphate buffer (pH 7.0)before and between pH experiments. The anionic response of the electrodes was evaluatedby adding different volumes of standard anion solutions to 10.00 mL of a buffered sample solution (0.100M MES-NaOH (pH 5-60),0.100 M MES-NaOH (pH 6.00),or 0.100 M MESNaOH (pH 6.50)).
RESULTS AND DISCUSSION The spectrochemical properties of porphyrins have been extensively studied, and their spectral characteristics are wellknown. Porphyrins absorb in the visible region, and present bands, like the Soret band, that are characteristic of each porphyrin molecule. It has been demonstrated that the spectra of [Cob-0H)TPPl in solution present significant differences depending on the pH of the solution. Rather recently, the electropolymerization of cobalt tetraphenylporphyrins has been reported,*g*mand in particular, cobalt(I1) tetrakis(o-aminopheny1)porphyrinfibswere used to develop ISEs with improved selectivity for thiocyanate.6 Given the ability to form polyporphyrin films by electrochemical means, it was envisioned that fiber optic sensors based on such films could also be developed. [Cob-0H)TPPI was electropolymerized on transparent indium(tin) oxide glass slides by using cyclic voltammetry, and the resulting polymer filmswere evaluatedas ion-selective material in the development of fiber optic sensors. Two hundred cycles were used in the electropolymerization to produce f i i with an absorbance of about 0.5 at 445nm. The polymer-coated slide was cut and positioned at the common tip of the bifurcated fiber optic bundle. Although the Soret band is centered around 445nm,it was found that, for a given change in pH, the maximum difference in sensor signal was observed at 470 nm. This difference in wavelength is controlled by instrumental factors, e.g., the transmittance characteristics of the glass fibers and the spectral distribution of the irradiance of the lamp. The potential range was also optimized for the optical sensor. The potential range used to prepare the ISEs previously reported by u s 6 (0-1.2 V) resulted in films that gave a good response to changing pH, but had a very short lifetime (
