Improved Performance of Unmodified and Cobalt Phthalocyanine-Modified Carbon-Kel-F Composite Electrodes

J. Electrochem. Soc., Vol. 141, No. 2, February 1994 �9 The Electrochemical Society, Inc. 323

62. R. J. Gillepsie and J. Passmore, J. Chem. Soc., Chem. Commun., 1333 (1969), R. J. Gillepsie, J. Passmore, P.K. Ummat, and O. C. Vaidya, Inorg. Chem., 10, 1327 (1971); J. Barr, R. J. Gillepsie, and P. K. Ummat, J. Chem. Soc. Chem. Commun., 264 (1970); C. Davies, R. J. Gillepsie, J. J. Park, and J. Passmore, Inorg. Chem., 10, 2781 (1971).

63. J. J. Lingane, J. Am. Chem. Soc., 67, 1916 (1945).

64. J .J . Lingane, G. G. Swain, and M. Fields, J. Am. Chem. Soc., 65, 1348 (1943).

65. J. J. Lingane and I. M. Kolthoff, ibid., 61, 825 (1939). 66. D. Elothmani, Ph.D. Thesis, University of Rennes

(1993); G. Le Guillanton, D. Elothmani, Q. T. Do, and J. Simonet, Abstract 1655, p. 2276, The Electrochem- ical Society Extended Abstracts, Vol. 93-1, Honolulu, HI, Meeting, May 16-21, 1993.

Improved Performance of Unmodified and Cobalt Phthalocyanine-Modified Carbon-KeI-F Composite Electrodes

Jongman Park and Brenda R. Shaw* Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060

ABSTRACT

Composite electrodes comprising Ketjenblack (carbon black) in Kel-F [poly(chlorotrifluoroethylene)] were prepared by dispersing carbon black in a solution of Kel-F in carbon tetrachloride at 150~ under autologous pressure. (p-Xylene was used as an al ternative solvent for composite materials containing silver or mercury, which react violently with hot CC14.) After removal of the solvent, the dry mixture was pressure molded to give electrodes that had low background currents in aqueous and nonaqueous electrolyte solutions. Cobalt phthalocyanine-modif ied electrodes (Kel-CoPc) were prepared by adsorbing cobalt phthalocyanine onto carbon black particles before preparing electrode materials as described above. Electrochemical pretreatment gave composite electrodes that yielded low background currents and low overpoten- rials for hexammine ruthenium(III) chloride, hexacyanoferrate(III), and ferrocene. Electrochemically pre t reated Kel-CoPc electrodes performed well in electrochemical detection of L-cysteine in high pressure l iquid chromatography.

In this paper we describe an al ternative fabricat ion method for Kel-F composite electrodes, their improved electrochemical behavior toward various electroactive species and solvents, and an improvement in electrode performance as a result of electrochemical surface treatment. Cobal t-phthalocyanine-modif ied composite electrodes were also prepared and used in cyclic vo]tammetry and detection of L-cysteine by l iquid chromatography with electrochemical detection (LCEC). These solid bulk-modified electrodes showed the same electrocatalytic activity as cobalt phthalocyanine-modif ied electrodes made by surface treatment or bulk-modification of carbon paste or graphite epoxy, but were better in terms of sensitivity, longevity, and renewability.

Various composite electrodes have been studied because of their high faradaic- to-charging current ratios. 1-23 Another advantage of composite electrodes is the feasibil- i ty of bulk modification. 2~2~ The electrode matr ix can be easily modified during preparat ion by incorporat ing modifiers such as solid part icles (zeolites, clays, layered double hydroxide, ruthenium oxide),~'2~ vinyl monomers (vinylfer- rocene, vinylpyridine), 24'2~ or molecular catalysts (cobalt- phthalocyanine). 26 Unlike surface-modified electrodes, the modified composite electrodes are robust and have renew- able surfaces.

Among the various composite electrodes, carbon composite electrodes have been studied extensively because of their good electrochemical stability, availability, and low cost. There are many kinds of carbon (carbon black, graphite) composite electrodes, including those fabricated with epoxies, polyethylene, 7-12 Teflon, 2-4 Kel-F, 5-7'21'22'29'3~ silicone rubber, PVC, polypropylene, ~4 chloroprene, ~ copolymer of ethylenevinyl acetate and vinyl acetate, ~6 and cross-linked polystyrene with divinylbenzene. ~7 The characteristics of the electrodes are affected by the physical configuration of the conducting mater ial and the properties of the polymeric matrix, such as mechanical strength, hydrophobicity, and swelling behavior in solvents. How- ever, the electrochemical propert ies are determined largely by the nature of the conducting material.

* Electrochemical Society Active Member.

Most composite electrodes work well in their specific ap- plications, but still have several problems. First, the di- mensional s tabil i ty of the electrodes in nonaqueous solutions is often poor although their performance in aqueous solution is superior to solid electrodes. 2~'25 This problem is due to the affinity of the polymer matr ix toward organic solvents. The resulting swelling of the polymeric matr ix leads to a larger active area of the electrode surface, giving larger charging current. This swelling is a continuous process and depends on the extent of solvent-polymer interaction. Composite electrodes based on Teflon, Kel-F, polyethylene, and polypropylene are durable in some organic solvents. ~-~'~'1~'3~

The second problem is the lack of hardness as with polyethylene, polypropylene, and silicone rubber composite electrodes, which makes them difficult to machine or polish. The third problem is the difficulty in modification of composite electrodes that are resistant to swelling in organic solvents. They are too inert toward organic solvents to be dissolved readi ly for incorporat ion of modifiers, and their monomeric precursors are not easily copolymerized with modifiers.

Among existing composite electrodes Kel-F [poly (chlorotrifluoroethyiene)] is one of the most at t ract ive polymeric matrices for use in organic solvents because it is nearly inert toward swelling and has good mechanical properties. 6 Tallman and coworkers and Anderson and coworkers have made Kel-F composite electrodes with graphite, silver, gold, or p la t inum powder as the conducting component. 6'32"34 They used suspensions of Kel-F and graphite or metal powder for mixing the powders, and molded the mixtures into electrodes at elevated temperature and high pressure. They studied the electrochemical behavior of these electrodes extensively. 57'31-3~ It was shown recently that increasing the dispersion of graphite during preparat ion of Kel-Graf electrodes improved performance by decreasing active site radii. 29 The procedure outlined below was designed to further reduce active site radii.

Kel-F polymer can be dissolved in various organic solvents, such as carbon tetrachloride, methylene chloride, and xylene, at high temperature and pressure} 6 By dissolving Kel-F polymer in these solvents it is possible to disperse carbon black part icles thoroughly with Kel -F polymer. Af-

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.197.26.12Downloaded on 2014-09-28 to IP

http://ecsdl.org/site/terms_use

324 J. Electrochem. Soc., Vol. 141, No. 2, February 1994 �9 The Electrochemical Society, Inc.

Table I. The composition of the KeI-F composite electrodes.

Kel-F Carbon Electrodes polymer (w/o) black (w/o) Modifier (w/o)

A 94.0 6.0 NA B 92.0 8.0 NA C 90.0 10.0 NA D 88.0 12.0 NA E 85.0 15.0 NA

Kel-CoPc 87.9 9.7 2.4 (Co-phtha]ocyanine)

ter mixing carbon black and Kel-F polymer, the solvents can be removed easily from the mixture by evaporation under vacuum.

In this work carbon tetrachloride was selected as the solvent because of high solubility of Kel-F at low temperature (above 114~ in a sealed ampul). When the reactivity of CC14 is a problem (see safety note in the Experimental sec- tion), as with silver-containing composite electrodes, p- xylene can be used as the solvent for Kel-F. 37

The carbon black (Ketjenblack EC 600JD) used has a very small particle size (30 nm), and it is highly electrically conductive. 38 The pore volume of Ketjenblack EC 600JD is 495 ml/100 g (by dibutylphthalate adsorption). The appar- ent bulk density of this material is only 115 gaiter; surface area is 1250 m2/g (BET, N2)? 9 These values indicate irregu- lar structure that leads to the formation of chains of cup- shaped particles that are 30 nm across and result in high conductivity.

Literature from the supplier of the Ketjenblack EC- 600JC shows values of i to 10 ~-cm for the volume resistiv- ity of composites containing 10% carbon black. The high volume-to-weight ratio of the carbon black enabled the preparation of Kel-F electrodes having low weight per- centages of carbon black with high conductivity, compared to the case of Kel-Graf electrodes. 5 In the simple measure- ment using a digital multimeter, the ohmic resistance of 5 mm lengths of 3 mm diam Kel-F composite electrode rods ranged from several tens to several hundred ohms, which depends on the content of carbon black in the electrodes. The excellent performance of Kel-F composite electrodes can now be extended to include bulk-modified electrodes.

Experimental All reagents used in this work were reagent grade unless

specified otherwise. Ferrocene (Aldrich) was purified by sublimation; cobalt-phthalocyanine (Kodak) was used as received without further purification. Freshly opened HPLC grade methanol, acetonitrile, tetrahydrofuran (THF), and methylene chloride from Baker were used for nonaqueous solutions. Aqueous solutions were prepared with deionized water (18 Ml2-cm) from a Millipore Milli-Q system. In LCEC experiments the buffer solution was filtered with a MiUipore membrane filter prior to use. Sodium naphthalenide etching solution (1M) for chemical surface pretreatment of the electrodes was prepared by a method described elsewhere. 4~ Voltammetric experiments were carried out with a BAS 100 Electrochemical Analyzer (Bioan- alytical Systems). The BAS 400 HPLC used for LCEC experiments had a 20 microliter sample injection loop, an electrochemical detection system (LC 4B and 17A), a Phase II, ODS column (100 mm x 3.2 mm, 3 micrometer) from Bioanalytical Systems. Surface analysis was performed using an Amray 1810D scanning electron microscope.

Preparation of composite electrodes.--A mixture (see Table I) weighing about 10 g containing poly(chlorotrifluoroethylene) (3M Co., Kel-F 81) and well-ground carbon black (Ketjenblack EC 600JD, Akzo Chemie America) was placed in a Pyrex glass ampul (38 mm od, 200 mm long) with 140 ml carbon tetrachloride. The ampul was sealed with a flame after freezing the solvent in the ampul with liquid nitrogen. The sealed ampul was placed in a metal container with bumpers to prevent movement of the glass ampul within the metal housing. The tube was shaken vigorously for 2 h to disperse the carbon black. The temperature was then raised to 150~ for 5 to 8 h with vigorous

shaking to dissolve the Kel-F polymer and mix it with carbon black.

SAFETY NOTE: We experienced several explosions that were contained by the metal jacket surrounding the ampul. Explosions may occur due to flaws in the glass or poor temperature control combined with the autologous pressure of hot solvent in the ampul. We also naively attempted to prepare composites containing silver metal using this procedure. Violent explosions occurred as Ag reacted with CC14 at elevated temperature, p-Xylene was used as an alternative solvent for systems containing Ag or Hg to prevent an explosive reaction. The temperature has to be raised to 180~ when using p-xylene as the solvent for Kel-E

After cooling to room temperature, the ampul was opened and the solvent was removed from the mixture using a vacuum oven at 80~ with a trap. The resulting mixture was ground into fine powder with an analytical mill (Bel-Art products). The particle sizes were difficult to determine because of aggregation. Particles and aggregates ranged in size from less than 0.1 micrometer to greater than 12 micrometers after grinding, as observed using a light microscope. It is important to note that each particle was an intimate mixture of Kel-F and carbon black at this point in the procedure. All particles were black: there was no evidence of domains of pure Kel-F at the scale visible by a light microscope.

The molding process was adapted from "Technical Infor- mation for Kel-F 81 Plastic PCTFE" by 3M Co., 41 and modified as follows. The powder was placed in a homemade mold which has 21 holes (3 mm diam x 12 mm long) and heated to 260~ for 20 min in order to soften the mixture. Then the air inside the mold was removed by vacuum for 10 min in order to prevent the formation of voids caused by trapped air present during the molding process. The pressure of the mold was raised to 10,000 psi momentarily, then maintained at 5,000 psi for 10 min, and then the mold was cooled down to room temperature slowly under pressure. The resulting composite material was press fit into a Kel-F rod, or a Kel-F block (1 • 1 in. 2 face, donated by Bioanalyt- ical Systems) for LCEC experiments. Electrical connection was made with silver powder and a brass pin connector or brass wire.

For preparation of cobalt phthalocyanine-modified Kel- F composite electrodes cobalt phthalocyanine was dissolved in 100 ml of dimethylsulfoxide (DMSO, from Alfa), then carbon black was added to the solution and stirred vigorously overnight (see Table I for proportions). The mixture was filtered, and the colorless filtrate confirmed that nearly all of the cobalt phthalocyanine was adsorbed onto the carbon black particles. The particles were then washed with copious amounts of distilled water to remove DMSO. The mixture was dried thoroughly at 150~ and ground into fine particles using an analytical mill. The resulting cobalt- phthalocyanine-coated carbon black powder was used for the preparation of modified composite electrodes using the same procedure as for the plain composite electrodes.

The electrode surface was ground with 600 grit Si-C paper and then polished with 6 micrometer and i micrometer diamond paste (from Buehler) consecutively. Finally 0.05 micrometer alumina polishing suspension (from Buehler) was used to get a mirror-like surface. Between polishing steps the electrodes were washed thoroughly with copious amounts of deionized water. The polished electrodes were then sonicated for 5 min in order to remove the polishing particle residue from the electrode surface. The composi- tions of the electrodes used in this work are shown in Table I.

Chemical surface treatment of the electrodes was done by applying a drop of IM sodium naphthalenide THF solution in order to reduce hydrophobicity of the electrode sur- faceY After all of the sodium naphthalenide reacted with KeI-F polymer and moisture in the air, the electrodes were washed thoroughly with methanol and deionized water. Electrochemical surface treatment of the electrodes was done by applying a negative step potential from the rest




potential to -2 .0 V vs. Ag/Ag + (0.01M) reference electrode for appropriate periods (see Results and Discussion) in 0.05M tetrabutylammonium tetrafluoroborate (TBABF4) in acetonitrile solution.

Results and Discussion Unmodified Kel-F composite electrodes.--Cyclic voItam-

metric behavior of Kel-F electrodes in aqueous so lu t i on . - The background current of Kel-F electrode B (8% carbon black) in 0.05M KH2PO4 was about 15 times lower than that of a glassy carbon electrode which has the same geometric area (Fig. la). The background current was reduced because of the decreased active surface area of the electrode relative to glassy carbon. Lowering the carbon black content decreased the electrical conductivity of the electrodes (lower limit: 5% carbon black). Higher content of carbon black gave high conductivity but poor physical strength of the electrodes (upper limit: 20% carbon black). Electrodes containing 8 to 10% carbon black by weight gave the best results in terms of conductivity, physical strength, and electrode performance.

The peak currents and peak separation between cathodic and anodic peaks for Ru(Ntt3)~ + were almost the same as those of the glassy carbon electrode, which has the same geometric surface area (Fig. ib). The change of carbon black content did not alter the peak currents significantly. The carbon black particles at the surface of the electrode

~ . 1 1 1

/ ;

/

I f ~176176 ~

~ 0 . 0 - 0 . 5 - - " " - 1 . 0 - 1 . 3 0 0

i

b ~ / . . . . . . . . . . "

] , I . . . . , . . . . . . . : " : ' " 1 " " , ' " ' , ,

+t �9 :~ . - "'~:" 40 ~ , ~ ~ f -O.S - t . O -1 .300

. . . . . . . . . . . . .

/ ~176

v -0 .S -~..0 -1.3C0

�9 /

k...'

Fig. I. Cyclic voltammetric responses of KeI-F composite electrodes in aqueous solution (SCE reference electrode). (a) Background current for electrode B (8% C-black, solid line] and glossy carbon electrode (dotted line). (b) 1 mM Ru (NH~)~CI3 in 0.1M KH2P04, 100 mV/s, glassy carbon (dotted line], electrode B (8% C-black, solid line). (c) 2 mM K3Fe(CN)~ in 0.1M KH2P04 100 mV/s, electrode B (8% carbon black, solid line), electrode C (10% C-black, dolted line).

would be randomly distributed to give an ensemble of mi- croelectrodes, which give enhanced current density per unit active surface area because of radial diffusion.

Because of the overlap of the individual diffusion layers the overall flux of electroactive species from the bulk solution resembles the case of a continuous electrode such as a glassy carbon electrode, on the timescale of these voltammograms. So the peak currents of Kel-F electrodes are similar to that of the glassy carbon electrode and are not affected significantly by the content of carbon black. The increase in carbon black content caused only an increase in charging current.

Cyclic voltammetric behavior of Fe(CN)~- anion (Fig. ic) was somewhat different from Ru(NH3)~ + cation. The peak currents were lower than those of the glassy carbon electrode, and the separation between cathodic and anodic peaks was very large [500 mV for electrode B (8% carbon black)], and the reproducibility was poor. As the content of carbon black was increased the peak current increased and the peak separation decreased [295 mV for electrode C (10% carbon black)]. Still, low peak current and large peak separation were obtained, and they were not reproducible. This large overpotential for Fe(CN)~- is likely due to the interaction between the Fe(CN)~- or Fe(CN)~- and the Kel- F electrode matrix or the smeared Kel-F polymer on the electrode surface generated during the polishing process. The Kel-F polymeric matrix or smeared Kel-F polymer may have very high electronegative dipole characteristics, and affect the electrical double layer or electrode-solute interactions and increase the overpotential for anionic species and neutral species. 8 A large overpotential was also observed for the oxidation of acetaminophen, which is not shown here.

Surface treatment of Kel-F electrodes.--Chemical and electrochemical surface treatments were applied in order to change the characteristics of the electrode surface. The hydrophobic electrode surface posed no significant barrier to electron transfer in organic solvents. However, it was useful to treat the electrode surface to increase hy- drophilicity to reduce the barrier to heterogeneous electron transfer in water. A chemical surface treatment was tried by using sodium naphthalenide solution in order to reduce surface chlorine and fluorine of the polymer matrix. 42 Al- though it was possible to reduce the hydrophobicity of the matrix, it was hard to control the degree of surface treatment because the sodium naphthalenide is extremely air sensitive and reactive. This chemical treatment yielded a huge charging current in cyclic voltammetry because of the uncontrollable treatment process.

Electrochemical surface treatment was more efficient and controllable. The electrochemical treatment was done by applying a negative potential step [-2.0 V vs. Ag/Ag § (0.01M) reference electrode] for an appropriate period (see Fig. 2) in 0.05M TBABF4 acetonitrile solution. The surface of the Kel-F electrode could be reduced at potentials more negative than -1.3 V [vs. Ag/Ag + (0.01M)] in acetonitrile to produce a hydrophilic surface (see below).

Figure 2a shows the effect of electrochemical surface treatment on the cyclic voltammogram of Fe(CN)~- solution. Surface treatment reduced the overpotential dramati- cally, as well as increasing charging current. The duration of the potential step should be chosen carefully in order to minimize the increase in charging current due to surface roughening. The treatment time required for optimum results is dependent on the composition of the electrode.

Electrodes with a high content of KeI-F polymer required a longer potential step. In the case of electrode B, 1 s at -2.0 V [vs. Ag/Ag + (0.01M)] was optimal for the Fe(CN)~-/Fe(CN)~- couple. Longer treatment increased only charging current, but not peak current. Shorter treatment was not enough to decrease the effect of the hydrophobicity of the Kel-F polymer matrix, so the peak separation was still high. In the case of Ru(NH3)~+, the surface treatment increased only charging current as seen in Fig. 2b, even though the duration of treatment was short (250 ms).




a i " ,

I , I , ,/JJ~-~'-"r"~, , , , I

\ I \ I \ I

\ ! ~ J

u T

Fig. 2. Effect of electrochemical surface treatment at - 2 . 0 V vs.

A~/Ag+ (0.01M) reference electrode in 0.05M TBABF4 acetonitrile solution for electrode B. (a) 0.7 mM K3Fe(CN)6 in 0.1M KCI, 100 mV/s, SCE reference electrode. Solid line: before treatment. Dolted Line: after treatment for 1 s. (b) 0.5 mM RulNH3)6CI3 in 0.1M Kr 100 mV/s, SCE reference electrode. Solid line: before treatment. Dotted line: after treatment for 250 ms.

Scanning electron micrographs (Fig. 3) show the effect of the electrochemical surface treatment. The surface of the untreated electrode was very smooth (Fig. 3a). When the electrode was treated for less than 10 s, the appearance of the electrode surface was still reflective and smooth and no change in the electrode surface was visible by SEM. Fig- ure 3b shows an electrode surface treated for a long period of time (for 20 s) at -2.0 V [vs. Ag/Ag + (0.01M)] in 0.05M TBABF4 acetonitrile solution. The surface of the electrode appeared to be rough and have some cracks produced by the extreme surface treatment. The change of surface morphology of the electrode may be due to the removal of a layer of smeared KeI-F polymer (formed during thee polishing process) ~ by electrochemical reduction of the KeI-F matrix polymer. After removal of the smeared KeI-F polymer layer, a reduction of the electrode matrix might result in the cracks observed in Fig. 3b.

The electrochemically treated electrode surface was hydrophilic, which may be due to formation of hydrophilie functional groups. This electrochemical surface treatmen~ method was also applied to the cobalt phthalocyanine- modified KeI-F electrodes in order to improve performance for the catalytic detection of L-cysteine in the LCEC experiment, as described below. The mechanism of the surface change and the nature of the treated surface are under investigation.

Cyclic voltammetric behavior in nonaqueous solu t ions . - The Kel-F composite electrodes were tested in methanol, acetonitrile, THF, and methylene chloride in order to char- acterize their performance in organic solvents. Figure 4 shows the cyclic voltammetric behavior of Ke]-F electrodes in methanol. In the case of electrode E (15% C-black) the background current was about half of that for glassy carbon with the same geometric surface area (Fig. 4a and b). It was about one-fourth as large as for glassy carbon in the case of electrode B (8% carbon black).

The peak potential and peak current of the ferrocene couple and the usable potential window of the solvent were almost the same as those for the glassy carbon electrode (cf., Fig. 4c and d). When the scan rate of the electrode

potential was increased from 100 mV to 200, 1003, 5120 mV/s, the peak current was linearly proportional to the square root of scan rate of the electrode potential with a correlation coefficient of 0.9998, and the peak separation between the cathodic and anodic peak did not change significantly as can be seen in Fig. 4e.

In acetonitrile solution the electrochemical behavior of the electrode for the ferrocene couple was similar to that in methanol (Fig. 5d). The background current was very low compared to the glassy carbon electrode when electrode B was used in the usable potential window shown in Fig, 4c. When the electrode potential was scanned beyond -1.3 V, the background current increased gradually due to surface roughening by the electrochemical reaction of the electrode itself (Fig. 5b). This reaction is likely due to the electrochemical reduction of the Kel-F polymeric matrix.

Methylene chloride has a very high affinity toward Kel- E 3~'43 Figure 6 shows the cyclic voltammetric behavior of Kel-F electrodes for ferrocene in methylene chloride. The peak currents were almost the same, but the background current of electrode B (8% carbon black) was slightly higher than that of the glassy carbon electrode and increased slowly as the electrode potential was scanned re- peatedly, It is known that Kel-F swells very slowly in highly chlorinated or fluorinated solvents such as CH2C12, CC14, or hexafluorobenzene. It seems likely that the electrode surface swelled slowly causing increased active surface area, resulting in a high charging current contribution to the background current. But this swelling effect was much less than that observed for cross-linked polystyrene composite electrodes, and no change in physical appearance was observed after keeping the electrode in CH2C12 for two days. The swelling process would be faster for Kel-F electrodes than for pure Kel-F polymer because Kel-F

Fig. 3. Scanning electron micrographs of KeI-F composite electrode B (8% C-black). Above, untreated; below, the electrode surface was treated electrochemically for 20 s at - 2 . 0 V vs. Ag/Ag § {0.1M) in O.05M TBABF4 acetonitrile solution. (1 cm =- 10 I~m)




4-1 . ~ -O.S00

"v

4-*.~o , o ~ S *o.o -o.so0

Fig. 4. Cyclic voltammetric responses of KeI-F composite electrodes in 0.05M TBABF4 methanol solution (Ag/AgCI reference electrode). Background at glassy carbon (a) and electrode E (15% C-black) (b), 100 mV/s. Cyclic voltammogram for 1 mM ferrocene for glassy carbon (c) and electrode E (d), I00 mV/s. Electrode E (e), 200, 1003, 5120 mV/s.

might be degraded into low molecular weight chains during molding of the electrodes (about 260-280~ about I h), and because of the presence of carbon black particles. In order to increase the chemical inertness of Kel-F electrodes the molding process should be optimized by a using rapid heating and cooling procedure. 3~

In THF solution the available potential window was the same as for glassy carbon, but the background current increased slowly during consecutive scans, which means the electrode surface swelled in THF solution, too. After using a Kel-F electrode for several hours the electrode appeared unchanged physically, although the electrode body (Kel-F polymer) was cracked by THF because of the strain of press fitting the composite electrode material into the Kel-F rod.

Cobalt phthalocyanine-modified Kel-F composite electrode.--Cobalt phtha locyan ine was chosen as a modi f ie r because of its h igh the rmal s tabi l i ty and e lect rocata lyt ic activity. Cobalt ph tha locyan ine is known as an e lectro- chemical cata lyst for the ox ida t ion of var ious organic, inor- ganic, and biological compounds such as 02, SO2, H202, organothiols , hydrazine, carbohydrates , etc. 28'44-48 And it has been used as a bulk modi f ie r for carbon pas te 46-4s and graphi te epoxy 26 electrodes.

F igure 7 shows the e lec t rochemica l behav ior of the glassy carbon electrode, unmodi f i ed Ke l -F electrode, and cobal t ph tha locyan ine -modi f i ed Ke l -F composi te e lectrode for

oxidation of L-cysteine in a 0.05M phosphate buffer solution (pH 6). The oxidation of L-cysteine starts about 800 mV vs. SCE on the glassy carbon electrode because of the slow heterogeneous kinetics, and the current rise over- laps the background oxidation current increase (Fig. 7a). The behavior of the unmodified composite electrode was similar to that of glassy carbon (Fig. 7b). A large catalytic oxidation current was observed starting about 170 mV vs.

SCE on the cobalt phthalocyanine-modified Kel-F electrode, whose surface was treated electrochemically at -2.0 V vs. Ag/Ag + (0.01M) reference electrode for 5 s in acetonitrile solution (Fig. 7c). In the case of an untreated modified electrode, the catalytic oxidation started about 450 mV vs.

SCE because of the high overpotential resulting from the nature of the Kel-F matrix polymer (Fig. 7d). The catalytic current was about twenty times smaller than that of the treated electrode but significant current was observed at lower potentials than for the glassy carbon electrode.

The observed peak shape for the catalytic current in the cyclic voltammogram was unusual. The oxidation current appeared to have a steady-state contribution to the current, and the current was decreased markedly in the second consecutive scan, decreasing slowly in following scans. The electrode response was restored slowly by purging the solution with argon. The surface of the electrode is believed to have been covered and deactivated by the oxidation product. This effect was also observed in LCEC experiments.

Application of the cobalt phthaloeyanine-modi- fled electrode in electrochemical detection of L-cysteine was examined in a high pressure liquid chromatography (HPLC and LCEC) experiment. The electrode was mounted in an electrochemical thin layer flow cell. Hydrodynamic voltammograms are shown in

t ~ -o15 ' " ' . . . .

,f c [ ' ' : �9 �9 �9 J . ~ 1

+o .Tr~J '~.5 .I,0.0 -O.S -O.TOQ

d i . . . . . I , , +t.ooo, ___ ~ . s J f l~o.o -o .s -o.7oo

Fig. 5. Cyclic voltammetric responses of KeI-F composite electrode in 0.05M TBABF4 acetonitrile solution. Ag/Ag + (0.0IM) reference electrode, 100 mV/s. Backgrounds for glassy carbon (a) and elec- trade B (8% carbon black) (b,c). (d) 1 mM ferrocene for glassy carbon (dotted line) and electrode B (solid line).



328 J. Electrochem. Soc., Vol. 141, No. 2, F e b r u a r y 1994 �9 The Electrochemical Society, Inc.

a J +t .~0 ~ ~.e~ -'fl *i :..~. ~ - : .?00

/ S .

I . . . . I . . . . I , , ' . , , ~ r - - ~ . . . . I _ . . . ~ +t .50o +I .o ~ i f - 4.0.0 -o.$ -~ .zoo

/

d

I . . . . I . . . . , . / . . , ~ - - - . . . . . : . . , , - - ' ~ ' ~ +1 +o.0 -o .= -1 . ~ 0

Fig. 6. Cyclic voltommetric responses of KeI-F composite electrode B in 0.05M TBABF 4 methylene chloride solution (Ag wire pseudorefer- ece electrode). Bockgrounds for glossy carbon (o) andelectrode B (8% C-black} (b), 100 mV/s, Cyclic voltommogrom for I mM ferrocene for glossy carbon (c) and electrode B (d),

Fig. 8 for the glassy carbon electrode and the modified electrode whose surface was treated electrochemically in CH3CN solution for 5 s at -2 .0 V vs. Ag/Ag + (0.01M). In pH 6 buffer solution the catalytic oxidation of L-cysteine started at +0.2 V vs . Ag/AgC1 reference electrode and increased steadily to a maximum at 0.7 V. The current increased again after 0.8 V, possibly due to the normal oxidation of L-cysteine with high overpotential in addition to the catalytic current. The hydrodynamic behavior was similar with 50% acetonitrile added to the mobile phase (Fig. 8c).

The electrode response at 0.7 V was linear for L-cysteine concentrations from 0.08 to 100 micromolar (1.6 to 2000 pmol for 20 microliter injection) with a 40 nA/micromolar slope and a correlation coefficient R = 0.998. The sensitivity was 28.5 nA/micromolar (1.42 nA/pmol) at 0.4 V, which compares favorably with the value of roughly 2 nA/ micromolar at the same potential for cobalt phthalocyanine-modified graphite-epoxy composite electrodes with a 4 mm diam. 26 The signal-to-noise ratio was 15 for 0.08 micromolar solution (1.6 pmol). Figure 9 shows the stability and reproducibility of the cobalt-phthalocyanine modified Kel-F composite electrode�9 The response of the modified electrode was about five times higher than that of the glassy carbon electrode at 0.7 V (Fig. 9c).

The relative standard deviation for two sets of eight consecutive injections of 10 micromolar L-cysteine solution with a 3 h interval were 0.5% (Fig. 9a) and 0.7% (Fig. 9b), respectively. The response decrease was 1% after 3 h. These results compare favorable with the values of 3 to 5% relative standard deviation and 4% decrease in response for the surface-modified electrode with polymeric

cobalt phthalocyanine. ~9 In 50% acetonitrile in water (pH 6 buffer), the sensitivity dropped to about 0.2 nA/pmol at +0.55 V (ca. 0.4 nA/pmol at +0.7 u onset of diffusion-lim- ited current), compared with 0.145 nA/pmol at an electrode coated with PolYmeric cobalt phthalocyanine used under similar conditions (same electrode area, mobile phase, potential, flow rate). The cobalt phthalocyanine-modified electrode is stable, reproducible, sensitive, and offers a catalytic effect that lowers the oxidation potential of L-cysteine.

Since the Kel-F composite electrode is very resistant toward swelling in organic solvents such as methanol and acetonitrile, the cobalt-phthalocyanine-modified Kel-F composite electrode can be used as an electrode for electrochemical detection in mixed aqueous/organic solvent systems without a swelling problem, which occurs in other polymer-based modified composite electrodes.

Conclusion By utilizing the solubility of Kel-F polymer in CCI~ at

high temperature and high pressure, it was possible to disperse the carbon black particles in the polymeric solution. The composite electrodes molded from this mixture, after removal of solvent, behave like ensembles of microelec- trodes giving high signal-to-noise ratios. By using the Kel- F polymer as a composite electrode matrix, the resistance to swelling in organic solvents was improved, relative to other composite matrices. The Kel-F composite electrodes have high overpotentials for neutral or anionic redox cou- ples due to the nature of the Kel-F matrix polymer. The surface morphology or chemistry of the polished electrode can be changed by electrochemical surface treatment to enhance the reversibility of some electrode reactions. Work by other groups indicates that insulating polymers thought to be relatively inert, such as poly(ethyleneterephthalate) (PETP) and poly(tetrafluorethylene) (PTFE or Teflon) are electroactive. Electroactivity of these polymers ~1'~2 is of in- terest with respect to electrical breakdown of high voltage cable insulation. The electrocatalyst cobalt phthalocyanine was adsorbed onto the surface of carbon black to prepare a bulk modified KeI-F composite electrode. The modified Kel-F composite electrode has good stability, sensitivity, reproducibility, and resistance to swelling in organic solvents.

T 5~,R

1

d + t | " ' * I I * ' ~ | �9 ~ . ~ 4 0 � 9

Fig. 7. Cyclic voltammograms for 4 mM L-cysteine in 0.05M KH2PO4 (pH 6), 100 mV/s, SCE reference electrode. (a) Glassy carbon electrode, (b) unmodified KeI-F composite electrode, (c) cobalt phlhalocyanine-modified KeI-F composite electrode, electrochemically treated surface (-2.0 V vs. Ag/Ag § in 0.05M TBABF4 acetonitrile solution for 5 s). (d) Same as (c) but wilhaut pretreatment.




1000

800

600

O~ 400

200

0 0

o

o

o

o

o o o �9

o o �9

o

o �9 o �9

o o �9

~ ~ 1 7 6 �9 e �9 ~ e �9 o e ~

? 0 O , . . . , . . _ ~ . . . , . . . , . . . , . . .

200 400 600 800 1000 1200 1400

Electrode potential (mY)

100

80

6O

20

O o 0 0

0 0

0

O

O O at 700 mY

O RSD = 2 .3% (N=25 ) ~J - t n

- T - I " I '" I I

0 200 400 600 800 1000 Potential (mV vs. Ag/AgC1)

Fig. 8. Hydrodynamic voltammograms for 10 micromolar L-cysteine. 20 Microliter injection, 0.05M KH2PO4 buffer (pH 6), 1.0 ml/ rain. (a) Glassy carbon electrode (tap, solid circles); (b) CoPc-modified KeI-F electrode with electrochemically treated surface (5 s at -2 .0 V vs. Ag/Ag + in 0.05M TABABF4 acetonitrile solution) [bottom, open circles); (4 same conditions as (b), except mobile phase contains 50% acetanitrile.

Acknowledgment Donation of equipment and supplies by Bioanalytical

Systems is greatly appreciated. We wish to thank Akzo Chemie America and 3M Industr ia l Chemical Product Di- vision for donation of Ketjenblack and Kel-F 81 plastic, respectively. Assistance with SEM analysis given by Ed- ward J. Neth and Yan-Fei Shen is appreciated.

T 80 aA

!

a b c

Fig. 9. Chromatagrams showing reproducibility and stability of CaPc-modified KeI-F electrode and response comparison between glassy carbon electrode and CoPc-modified KeI-F electrode: 10 micramalar L-cystelne; 20 microliter injection, O.05M KH2PO4 buffer (pH 6), 1.0 ml/min, +0.70 V vs. Ag/AgCI reference electrode. (a) First eight injections, (b) eight injections after 3 h, (c) response of glassy carbon electrode.

Manuscript submitted March 4, 1991; revised manuscript received Nov. 19, 1993.

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Study of the Redox Behavior of Anthraquinone in Aqueous Medium

J. Revenga, F. Rodriguez, and J. Tijero Departamento Ingenieria Quimica, Facultad Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain

ABSTRACT

The electrochemical reduction mechanism of anthraquinone (AQ) has been studied in N,N-dimethyl formamide (DMF) and/or its water mixtures by means direct current polarography (DCP) and differential pulse polarography (DPP). The polarograms consisted of two well-defined waves in anhydrous DMF. The presence of small amounts of water does not effect the first wave but causes the second wave to shift toward more positive potentials; large amounts of water (solutions with at least 40% water) give a single wave. Half-wave potentials for the one-electron step in DMF are more negative than those in aqueous solutions and have been used to calculate the disproportionation constant, KD, of the radical anion in each solution. The reduction potentials in aqueous solutions have been analyzed on the basis of the Heyrovsky model. Results show that the electrode process is a two electron-two proton conversion of the AQ quinone to dihydroxyanthracene (in acidic media) or its dianion (in alkali media). From these results a reduction mechanism has been proposed for AQ in both aprotic and protic solvents.

Polarographic studies, particularly of organic compounds, are often difficult because the compounds are not soluble in water. For this reason most polarographic studies on such compounds have been done in organic solvents [DMF, acetonitrile (ACN)] or mixtures containing water.

Studies in aprotic solvents.--In DMF, anthraquinone (2~Q) produces two well-defined polarographic waves and the half-wave potentials are invariant with concentration changes. The double wave suggests a reduction reaction of AQ through the anthrasemiquinone radical anion (AQ'-) to 9,10-dihydroxyanthracene dianion (AQ-2). The formation of AQ'- and AQ -2 has been shown by electron spin resonance studies. I Polarographic studies of AQ have been reported and quantitative methods developed in the aprotic solvents; DMF, 2 ACN, 3,4 and chloroform. 5

Studies in protic solvents.--The electrochemical behavior of mono- and disulfonate anthraquinones has been examined previously in detail in strongly basic solutions of water and alcohols, 6 and also in acidic media, 7 where the electrode process is a two-electron two-proton conversion of the quinone to dihydroxyanthracene (in acidic media) or to dianion (in alkali media). Redox potentials have been determined for aqueous solutions of dihydroxy derivatives of anthraquinone, over the pH range 7 to 13. 8 Strongly chelating anthraquinones and their derivatives were stud- ted 9 by direct current and pulse polarography for their redox characteristics; all substances were reduced in a two- electron reversible process in both aqueous and 75% ethanolie solutions.

Anthraquinone is practically insoluble in water (0.006 g/liter at 50~ and nearly insoluble to soluble in organic solvents at room temperature. In this way, the first step of the present investigation was obtained in an aqueous solution of AQ. I~ Our aim is to clarify the reduction mechanism of AQ in water. The dielectric constants of the solvents, pH, and concentration have been examined to determine their influence on the equilibrium AQ/AQ'-/AQ -2. This study

shows that AQ reduction is highly sensitive to the presence of proton donors in the medium.

Experimental MateriaIs.--All reagents were of analytical reagent-

grade quality. DMF was obtained from Merck and used without further purification. AQ was purchased from Aldrich and purified by two successive ethanolic recrystal- lizations. To obtain different buffer solutions for pH 6-13 acetic acid, HsBO3, HsPO4, and NaOH were used. Tetraethylammonium perchlorate (TEAP) was prepared and purified according to the method of Given et al. ,1 Wa- ter was purified with a Milli-Q-filtering system. Aqueous solutions of AQ were prepared by dissolving different amounts of AQ in DMF and raising the volume to 50 cm 8 with a buffer solution. After this step the pH was adjusted to the desired value using a soda solution.

Apparatus.--Polarographic measurements were made with conventional experimental setups. A Polarecor E-506 (Metrohm) was used with an E-505 mercury drop electrode and a saturated calomel electrode (SCE) as a reference electrode. The pH measurements were carried out with an Orion pH meter. Standard glass equipment was used.

Procedure.--Two electrochemical techniques were used to study the redox behavior of AQ in DMF and DMF-H20 solutions.

DCR--Solutions with at least 60% DMF were studied with this technique. TEAP was used as the supporting electrolyte in both the DMF and the DMF-H20 solutions, (experiments 1-8). Conventional polarographie curves were measured in the ranges 1-9 - 1O-4M in AQ (experiments 1-6) and 5 �9 10-hM in AQ (experiments 7-10).

DPR--Aqueous solutions with less than 70% DMF were prepared in such a way that they were 0. IN in the principal buffer component, (experiments 9-34). DPP was carried out in the 5-50% DMF range with 1-5 �9 10-~M in AQ (experiments 11-34).



Documents

Improved Performance of Unmodified and Cobalt Phthalocyanine-Modified Carbon-Kel-F Composite Electrodes