Ionic surfactants as molecular spacers at graphite electrodes

The results Indicate head-on surfactant-surface Interactions and formation of hemlmlcelles at the surface. Structural differences between RPG and GC r...
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370

A ~ I Chem. . 1993, 65,370-374

Ionic Surfactants as Molecular Spacers at Graphite Electrodes Ana Marino and Anna Brajter-Toth' Department of Chemistry, University of Florida, Gainesville, Florida 32611-2046

A surface modelfor rough pyroiyllcgaphtte (RPG) and glassy carbon(GC) electrodes has been developedin io& surfactant medla. Ionic surfactant aggregates enable the control and optlmlzatlon of response of these graphtte electrodes. The feadblitty of formlng surfactant aggregates directly from solution h demonstrated. The results Indicate head-on surfactant-surfaceinteractions and formation of hemlmldles at the surface. Structural differences between RPG and GC resutt in differencesin aggregation and consequentlydifferent effects of surfactants on probereqmnse. Therefore, the nature of the probe along with the surface and surfactant structure must be condderedfor optimizationof response. Furthermore, In surfactant soiutlons ImprovementsIn sensitivity of inactive GC and reproduclbillty of active RPG are demonstrated.

INTRODUCTION Graphite surfaces exhibit properties such as reasonably low residual currents and wide potential windows, rendering them well suited for electrochemical analysis. However, surface fouling, as a result of irreversible adsorption, and limited sensitivity can pose practical problems in analysis of biological molecules. Previous work in our laboratory demonstrated the utility of micellar media in achieving better reproducibility at rough pyrolytic graphite (RPG) and improved sensitivity at glassy carbon (GC) surfaces.' This is possible when favorable surface-surfactant and surfactantanalyte interactions occur, as observed in previous work.' A number of electrochemicalstudies in surfactant solutions have addressed the issue of interactions occurring between micelles and electroactive species.2-10 Other studies investigating structural features and electron-transfer properties of surfactants in a Langmuir trough have been reported.11-'5 Previous work by Bard and co-workers3 on the reductive electrochemistry of methylviologen in micellar solutions of anionic sodium dodecyl sulfate (SDS) at GC electrodes suggests that not only electrostatic but other effects such as hydrophobic ones can contribute to surfactant-probe inter-

* To whom correspondence should be addressed.

(1)Boyette, S. PbD. Thesis, University of Florida, 1991. (2) Kaifer, A. E.; Bard, A. J. J. Phys. Chem. 1985,89, 4876-4880. (3) Kaifer, A. E.; Bard, A. J. J. Phys. Chem. 1987, 91, 2006-2007. (4) Quintela, P. A.; Kaifer, A. E. Langmuir 1987, 3, 769-773. (5) Yeh, P.; Kuwana, T. J. Electrochem. SOC.,1976,123, 1334-1339. (6) McIntire, G. L.; Blount, H. N. J. Am. Chem. SOC.1979,101,77207721. (7) Owsawa, Y.; Shim Azaki, Y.; Aoyagui, S. J. Electroanal. Chem. Interfacial Electrochem. 1980, 114, 235-246. (8) Owsawa, Y.; Aoyagui, S. J. Electroanal. Chem. Interfacial Elmtrochem. 1982, 136, 353-360. (9) Owsawa, Y.; Aoyagui, S. J. Electroanal. Chem. Interfacial Electrochem. 1983, 145, 109. (10)Georges, J.; Desmettre, S. Electrochim. Acta 1984, 29, 521. (11)Zhang, X.; Bard, A. J. J. Am. Chem. SOC.1989,111, 8098-8105. (12) Daifuku, H.; Aoki, K.; Tkuda, K.; Mabuda, H. J. Electroanal. Chem. Interfacial Electrochem. 1985, 183, 1. (13) Lee, C . W.; Bard, A. J. J. Electroanal. Chem. Interfacial Electrochem. 1988,239, 441-447. (14) Widrig, C . A.; Miller, C. J.; Maida, M. J. Am. Chem. SOC.1988. iio,2009-zoii. (15) Yokota, T.; Itoh, K.; Fujishima, A. J. Electroanal. Chem. Interfacial Electrochem. 1987,216, 289-292. 0003-2700/93/0365-0370$04.00/0

actions. These results are consistent with other reports suggestingthat micellar effecta on probe response are strongly dependent on the surface properties of the substrate and the surfactant s t r u ~ t u r e . ~ JOf- ~particular interest to our work was a previous study by Bard and co-workersllin which cyclic voltammetry (CV)measurements were conducted to investigate electron-transferproperties of electroactivesurfactants. They observed an increase in peak currents when surfactants were adsorbed tail-head-tail-head on tin oxide and a decrease in peak currents when adsorbtion was head-tail-tail-head. The increase in current was attributed to the shorter separation between surfactant head groups (Ru(bpy)32+)and the surface in tail-on adsorption. However, since tin oxide is a hydrophilicsurface,head-tail-tail-head multilayerswere expected to be more stable. Layers formed by tail-on adsorption were expected to contain structural defects due to surfactant flipping as a result of unfavorable surfactantsurface interactions. Furthermore, they also observed a decrease in rate of electron transfer following an increase in surfactant layer thickness where electron transfer was still possible but sluggish at a multilayer coverage. In this work surfactants of different carbon chain lengths and different chargehead groups were employed to investigate their effect on the response of structurally different graphite surfaces. Unlike previous studies where surfactant layers were formed at hydrophilic surfaces,11this study was conducted at heterogeneous surfaces with both hydrophobicand hydrophilic properties. Furthermore, surfactants adsorbed spontaneously at the surfaces used in this work. Under Langmuir-Blodgett techniques used in previous studies, surfactant monolayers (or multilayers) are formed under controlled pressure. Compressed layers allow minimal penetration and result in electrode hindrance for electroactive species in solution. However, as reported by Bard and coworkers,ll the compressed surfactant monolayers and multilayers can relax following exposure to solution. Due to the heterogeneityof graphite surfaces,the surfactant aggregates formed at such surfaces can be expected to have structures different than those formed at homogeneous surfaces. Furthermore, surfactant effects on probe response will also depend on probe structure. Probe and surface structure will control multiple interactions: surface-probe, surfacesurfactant, and surfactant-probe. It is demonstrated that at graphite surfaces a decrease in response is observed when surfaceprobe interactions are sufficientlystrong where surfactant adsorption effectivelydecreasesprobe interactions with the surface. However, an enhanced electrochemical response can result if surfactant-probe and surfactant-surface interactions are favorable and probe-surface interactions are weak. Hydrophobic and electrostatic effects are considered in developinga model which explains the response at graphite surfaces in surfactant solutions. Results presented in this work demonstrate similarities between surfactant-glassy carbon (GC)and surfactant-rough pyrolytic graphite (RPG) interactions. At both surfaces the surfactant polar head group interacta with hydrophilicgroups at the surface. The results are consistent with the formation 0 1993 American Chemical Society

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of hemimicelles at both surfaces.16-20 In a hemimicelle, surfactant tail groups interact hydrophobically favoring a two-dimensional aggregate. The ability to optimize signal and reproducibility at graphite surfaces using surfactant solutions is discussed.

activity with sluggish rates of electron The lack of activity has been attributed to the properties of GC. Among these is relatively low hydrophilicity which has been related to the small amount of surface oxides and low density of edge plane orientation in the graphite lattice which has been related to low surface roughness.28 Low surface roughness in itself was shown not to account for poor electrode activity.29 EXPERIMENTAL SECTION Different pretreatment methods lead to increased hydroMaterials and Solutions. 3,4-Dihydroxyphenylaceticacid philicity (determined by contact angle studies),2*edge plane (DOPAC),3-hydroxytyramine (dopamine, DA), 12 carbon chain density (determined through Raman studies),30 and surface length sodium dodecyl sulfate (SDS), 14 carbon chain length roughness (determined through double-layer capacitance tetradecyltrimethylammonium bromide (?TAB), 12 carbon chain measurements).29 length dodecyltrimethylammonium bromide (DTAB) and 16 The electrochemical activity of GC is typically determined carbon chain length hexadecyltrimethylammonium bromide (CTAB)were obtained from Sigma. Sodium phosphate monobaby measurements of heterogeneous rate constants of outersic and dibasic were obtained from Mallinckrodt. All chemicals sphere systems such as hexacyanoferrate(III).29 At unactiwere used as received. Solutionswere prepared daily in doubly vated GC, rate constants are at least 1 order of magnitude distilled, deionized water. lower than the constants measured a t more active surfaces.29 Probe concentrations ranged between 0.05 and 0.5 mM in 0.5 In spite of the low density of oxides, the GC surface has M pH 7 phosphate buffer. At pH 7 DOPAC (pK,’s 4.22, 9.58, been reported to be capable of preconcentrating cations and 12.1521)is negatively charged and dopamine (DA)(pK, 8.92n) presumably a t the oxide sites.26.31932 Evidence of cationic is positively charged. Surfactant, solutions contained probes at preconcentration has been supported from results obtained concentrationsranging from 0.05 to 0.5 mM in 0.5 M pH 7.00 with systems such as cationic ruthenium(II1) hexaammine phosphate buffer along with surfactants at concentrations above and cobalt(II1) hexaammine which produced higher currents the critical micelle concentrations (cmc). Anionic surfactantSDS than predicted for fast redox couples.2s Anions have been (aggregationnumber 62,cmc = 2.25 mM2)solutions were lOmM, and cationic surfactant (aggregationnumber 78, cmc = 1.3mMZ3) reported to exhibit lower currents and slower kinetics at GC solutions were 3 mM. than predicted, presumably due to the same electrostatic Apparatus. All electrochemical measurements were made effech32 with a Bioanalytical Systems electrochemical analyzer (BASElectrochemical kinetics of anionic DOPAC and cationic loo), and data were downloaded to an IBM PS/2 Model 50 DA are slow a t GC.1831 This is apparent in CV experiments computer. A conventional three-electrodesetup was employed, from the large differences between anodic and cathodic peak with a glassy carbon (GC) or rough pyrolytic graphite (RPG) potentials (Up) which are shown in Table I. Upvalues of working electrode, platinum auxiliary electrode, and a saturated ca. 30 mV are expected for a fast two-electron (reversible) calomel (SCE) reference electrode. All potentials are reported system.% Under the experimental conditions of Table I, peak at room temperature versus SCE. currents (i,) for DOPAC and DA are expected to be equal (Do Electrode Preparation. The GC electrodeswere constructed = 5.9 X 10” cm2/s and 6.0 X lo+ cm%, respectively) and can as previously reported’ from 3-mm-diameter GC rods (Elecbe easily predicted from the equation for CV currents for an trosynthesis). The electrode was polished metallographically irreversible two-electron system.% The relevant i, values for prior to use with Gamma alumina/water slurry (Fisher) on a microcloth using a polishing wheel (Ecomet 1). After polishing, DOPAC and DA are listed in Table I. The results show that the electrodes were ultrasonicated for at least 5 min in distilled i, for DA is larger than predicted, indicating that some water. The RPG (Pfiier) electrodeswere made from arectangular preconcentration a t the surface may in fact be occurring. The rod of pyrolytic graphite with an exposed surface of ca. 3 X 3 mm. i, values for DOPAC are, however, significantly lower, To produce rough pyrolytic graphite (RPG), the electrode was suggesting repulsion from the surface. Cationic preconcenresurfaced on 600-grit silicon carbide paper (Fisher) using a tration of catechols has been previously reported at inactive polishing wheel. RPG was then rinsed with distilledwater prior GC.31932 Surface interactions of DA are not detected from to use. The electrode areas were determined by chronocoulometry of log i, vs log scan rate (v) plots listed in Table I. For using 0.3 mM potassium ferricyanide (Do 7.63 X lo+ ~ m ~ /ins ) ~ slopes ~ adsorbing species theoretical slope values of ca. 1.0 are 1 M KC1. Typical electrode areas were ca. 0.05 cm2. expected.33 The slope values listed in Table I are below 0.5, Methods of Analysis. Cyclicvoltammetry (CV) experiments the theoretical value for a diffusion-controlled pr0cess.3~The were conducted at a scan rate (v) of 100 mV s-l to measure peak currents (i,) and anodic to cathodic peak separations (All,). CV values below 0.5 may indicate some hindrance or partial experiments were also conducted at v ranging from 10 to lo00 blocking of electrodes.11*33 It has been reported that when mV 8-1 to measure i, as a function of v. hydrophobic interactions a t GC are strong, adsorption is detected.2~3 Furthermore hydrophobic interactions, when RESULTS AND DISCUSSION present, have been reported to dominate over electrostatic.3 The results indicate that hydrophobic interactions of DA and Characteristic Features of Unactivated GC. UnactiDOPAC must not be strong a t GC since no adsorption was vated glassy carbon (GC) surfaces have poor electrochemical evident from slopes of plots of log i, vs log v. (16) Rosen, M. J. Surfactants and InterfacialPhenomena; John Wiley and Sons: New York, 1989. (17) Chandar, P.; Somasundaren, P.; Torro, N. J. Interface Sci. 1987, 117. 31-46. --, -(18) Scamenorn,J. F.;Schechter,R.S.;Wade, W. H. J. Colloidlnterface Sci. 1982,85, 463-478. (19) Kunjappu,J. T.; Somasundaren, P. J. Colloid Interface Sci. 1989, 38,305-311. (20) Ma, C.; Li, C. J. Colloid Interface Sci. 1989, 131, 485-492. (21) Hawley, M. D.; Tatawawadi, S. V.; Piekarski, S.;Adams, R. N. J. Am. Chem. Soc. 1967,89, 447-450. (22) Ishimitau, T.; Hirose, S.; Sakurai, H. Talanta 1977,24,555-560. (23) Cline-Love, L. J.; Habarta, J. G.; Dorsey, J. G. Anal. Chem. 1984, 35,447-452. (24) Stackelber, M.; Pilgram, M.; Toome, V. Z . Electrochem. 1953,57, 342.

(25) Kovach, P.M.;Deakin, M. R.; Wightman, R. M. J . Phys. Chem. 1986,90,4612-4617. (26) Wightman, R. M.; Deakin, M. R.; Kovach, P. M.; Kuhr, W. G.; Stutts, K. J. J. Electrochm. SOC.1984,131, 1578-1583. (27) Engstrom, R. C. Anal. Chem. 1982,54, 2310-2314. (28) Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984,56,136-141. (29) Bodalbhai, L.; Brajter-Toth, A. Anal. Chim. Acta 1990,231,191201. (30) Bard, A. Electroanalytical Chemistry;Marcel Dekker, Inc.: New York, 1991; Vol. 17, pp 221-364. (31) Deakin, M. R.; Kovach, P. M.; Stutta, K. J.; Wightman, R. M. Anal. Chem. 1986,58,1474-1480. (32) Saraceno, R. A.; Ewing, A. G. Anal. Chem. 1988,60,2016-2020. (33) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley and Sons: New York, 1980; pp 213-242,488-531, 538-546.

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Table 1. Cyclic Voltammetry Result. at GC solutions* Zb Upc (mV) DOPACiBUFFER -1 250*3 DOPACiDTAB -1/+1 204*7 DOPACITTAB -li+l 131 12 DOPACICTAB -1/+1 110 f 9 DAiBUFFER +1 189 33 DAiSDS +1/-1 156 11 DAiCTAB +1/+1 257 f 8 DOPACiSDS -11-1 512 + 42

;d-lC*-l d (A em-2 M-1)

*

0.24 f 0.03 0.41 & 0.01 0.43 0.03 0.43 0.05 0.39 f 0.02 0.40 0.03

**

0.20 0.17

*

*+ 0.06 0.05

;d-lC*-l e (A -2, 0.33

M-1)

m, 0.45 0.05 0.48 0.04 0.48 0.02 0.43 f 0.03 0.42 f 0.02 0.38 0.01

*

0.33

a Buffersolutions were prepared with a 0.5 mM probe and 0.5 M pH 1.00 phosphate buffer. Surfactant solutions were prepared 88 buffer solutions with surfactant concentration > cmc. Charge of probelcharge of surfactant. E CV experiments run at B scan rate of 100 mV 8.'. Measured peak currents normalized over an electrode area of 0.062 cm2 and 0.5 x 10-3M prohe concentration. e Calculated peak currents from i, = (2.99 X 105)n(nn.)'/2 33 with en. = 0.5. I Slopes for CV plots of log i, YS log Y with correlation coefficients ranging between 0.98 and

1.00.

1

-SURFACTANT

Schematic representation of hemlmlcelle fmnation in aqueous media at RPG and GC surfaces. This figure b fw visual purposes with no quantitative or exact structural infwmatbn implled. Figure 1.

Effect of Surfactants on Probe Response at GC. Previous work with hydrophilic surfaces in aqueous media has shown adsorption of surfactants to occur preferentially through the hydrophilic head group, producing assemblies a t the surface.1lJs20 In such assemblies the hydrophobic tails of the surfactants interact, and consequently, surfactant head groups also face the solution, as seen in Figure 1. If these types of assemblies form a t GC and if electrostatic effects are important in surfactant-surface interactions, greater cationic surfactantGC interactions should occur. Anionic SDS and cationic DTAB were the surfactants used in this work. SDS and DTAB hoth have 12 carbon chain lengths and singly charged head groups ((-1) and (+l), respectively). The electroactive probes used in testing surfactant interactions were DOPAC and DA. A t pH 7 the probes are negatively (-1) and positively (+1) charged, respectively.21,22 Both probes also have some degree of hydrophobicity. Table I lists i, and AE, values from CV experiments, As shown in Table I, i, values for DOPAC in the presence of DTAB are much greater than predicted (for an electrochemically irreversibleprobe) and, therefore, much greater than the values a t bare GC. In addition, the AE, value for DOPAC decreases following DTAB addition. The results point to the presence of DTAB in solution favoring DOPAC reaction a t GC. Table I also lists i, and AE, values for DOPAC in the presence of SDS. As shown in Table I, a significant increase in AE, and a decrease in i, follows where i, values are in fact below the calculated and measured values at hare GC, confirming some additional electrode hindrance by the SDS surfactant. Although the AE, and i, values of DA are not significantly affected hy the presence of SDS, SDS improves reproducibility (of AE, values) for DA. Previous work with surfactants in a Langmuir trough provides evidence of decreased rates of electron transfer following increasing aggregate thickness." The observed effect on the electrochemical response of GC supports a somewhat different model for the effect of increasing surfactant. chain length on response. Results in Table I for DOPAC show sluggish kinetics a t bare GC with no evidence for adsorption. However, the addition of 12 carbon chain length surfactant (DTAB) results in improved kinetics, as seen from the decrease in Up.

Increasing carhon chain length of the cationic Surfactant from 12 (DTAB) to 14 ('lTAB) and finally to 16 carbons (CTAB) results in further improvement in kinetics of DOPAC, as is clear from the decrease in AE, values (Table I). Unlike what has been previously reported for other surfaces where the surfactant hindered the response of electroactive probes, the presence of cationic surfactant a t GC has rendered the surface more active for otherwise nonadsorbing anionic DOPAC. Furthermore, even though anionic SDS does not alter sensitivity of the response of DA, it increases reproducibility of AE, measurements and significantly decreases DOPAC response. Together, the results indicate that both anionic and cationic surfactants affect the response of GC. However, only cationicsurfactantsimprove the response. The response enhancement is most significant with cationic surfactants of longer chain length. Characteristic Features of Active RPG. Adsorption of DOPAC and DA a t RPG has been reported to occur irreversibly causing problem with reproducibility.' AEp values a t RPG in buffer listed in Table I1 are less than 30 mV (foradiffusion-controlled two-electron process).33 The slope values in Table I1 of log i, vs log Y plots are greater than 0.5 (for a diffusion-controlled process).33 These results provide supporting evidence for DOPAC and DA adsorption a t RPG. The interactions involved in the adsorption are not totally clear. RPG is known to he more active than GC." There are differences in wettability and surface roughness between RPG andGC. Thecontactangle,whichisameasureofwettability, has been determined to be 66 and 49' for GC and RPG, respectively, where smaller contact angles are indicative of a more wettable surface.2s Using double-layer capacitance measurementasand theassumption thatthemicroscopicarea of alumina-polished GC is 1.3 times its geometric the roughness factor of RPG has been determined. Reported values of roughness factors were 6.5and 1.3for RPG and GC, respectively,29indicating RPG is a rougher surface, although roughnessitselfhas been shownnot tocontrihute toincreased ad~orption.3~ Differencesin electrochemicalactivity between GC and RPG have been determined from measurements of heterogeneous rate constants. The rate constant for hexacyanoferrate(II1) is 0.01 cm s-l a t GC and 0.13 cm sd a t RPG." RPG is a more hydrophilic surface with a greater density of surface oxides and, therefore, greater anionic charge density than GC. If electrostatic effects were controlling adsorption of DA and DOPAC, negatively charged DOPAC should not adsorb a t RPG. However, the results in Table I1 confirm DOPAC adsorption. In view of similar AE,values for DOPAC and DA a t RPG, indicating their kinetics are similar, the currents for both (34) Bodalbhai,L.;Biajte~-Toth,A.;Anol. Chem. 1988,60,2551-2~1. (35) Kinoahits. K. Carbon; Electrmhemieol ond Physicochemical Properties;Wiley Interscience: New York. 1988; Chapter 5.

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Table 11. Cyclic Voltammetry Results at RPG solution" DOPAC/BUFFER DOPAC/DTAB DOPAC/TTAB DOPAC/CTAB DA/BUFFER DA/SDS DA/CTAB DOPAC/SDS

Zb -1 -l/+l -l/+l -l/+l +1 +1/-1 +1/+1 -1/-1

i&-lC*-l d (A cm-2 M-1)

AEPc(mV) 20 f 9 32f 1 33 f 1 43 f 4 27 f 2 43 f 4 68 f 5 220 16

1.01 f 0.11 1.04 f 0.08 1.07 f 0.04 0.83 f 0.04 1.46 f 0.12 0.80 f 0.05 0.55 f 0.04 0.26 f 0.02

*

me 0.65 f 0.02 0.50 f 0.05 0.53 f 0.03 0.57 & 0.02 0.62 & 0.01 0.56 f 0.02 0.51 f 0.03 0.46 f 0.01

0.5 M pH 7.00 phosphate buffer solutions contained 0.5 mM probe. Surfactant solutions were prepared as buffer with 10 mM SDS and 3 mM cationic surfactant. Charge of probe/charge of surfactant. e Measured a t a scan rate of 100 mV 8-l. d Peak currents normalized over an electrode area of 0.05 cm2 and 0.5 X 10-3 M probe concentrations. e Slopes for log i, vs log v.

probes should be similar. However, i, of positively charged DA is significantly larger than i, of DOPAC probably as a result of preferred electrostatic interactions. Surface interactions, other than electrostatic ones, must account for the observed adsorption of DOPAC. Effect of Surfactants on Probe Response at RPG. DOPAC and DA both adsorb at RPG, and therefore it was expected that the addition of surfactants could limit their adsorption. Table I1 shows their i, and AE, values in surfactant solutions. The presence of surfactants results in a small increase in AE, for the probes. No change in peak currents for DOPAC is observed in the presence of 12 and 14 carbon chain length cationic surfactants which indicates that the concentration of DOPAC at the surface does not significantly change from that at the bare RPG. The i, decreases when the 16 carbon chain length CTAB is added. In CTAB solutionsAI?, values increase above those measured in shorter carbon chain length cationic surfactant solutions but still remain relatively small, indicating reasonably fast kinetics. The AE, values in Table 11 for DOPAC in the cationic surfactant solutions are representative of values for a quasireversible two-electron system. Slopes of ca. 0.5 of log i, vs log Y plots provide evidence of a decrease in adsorption. Along with AE, values this indicates that the surfactant does not significantly change probe kinetics and the increase in AE, is largely due to the decrease in adsorption. In solutions containing 12 carbon chain length anionic surfactant (SDS) a significant decrease in peak current and an increase in AE, was measured for both probes. The increase in AE, was especially significant for DOPAC. For DA, AE, and slope values of plots of log i, vs log Y in Table I1are typical for a diffusion-controlledquasi-reversiblesystem and are comparable to values for DOPAC in CTAB solutions, indicating a decrease in probe-surface interactions without a significant change in rate of electron transfer. SDS causes an irreversibleelectrochemicalresponse of DOPAC; the effect of CTAB on DA response is less severe. The addition of surfactant was expected to alter currents and peak potentials at RPG only if the surface was altered to prevent probe-surface interactions. It is apparent from the results that anionic and long-chain cationic surfactants do limit DA and DOPAC interactionswithRPG, consequently reducing probe response. The cationicsurfactants, especially of the short chain lengths are, however, less effective. The effects of surfactant charge and carbon chain length on probe response are similar to those observed at GC with anionic surfactant significantly decreasing the response of anionic probe and with the cationic surfactants of longer chain length having the biggest enhancing effect. Analytical Significance of the Results. In spite of different quantitative effects on response, surfactants consistently improve the precision of i, values at RPG, rendering the surface more useful for analysis. Since adsorption has been determined to foul RPG, a decrease in adsorption

Table 111. Cyclic Voltammetry Results as a Function of Time of RPG Exposure solution i&-lC*-I (A cm-2 M-l) hE, (mV) td(min) DOPAUBUFFER" DOPACICTABb DA/BUFFERO DAISDSb

1.48 f 0.11 1.21 f 0.13 1.00 f 0.04 0.76 f 0.04 0.77 f 0.05 0.76 f 0.05 1.76 f 0.13 1.41 i 0.18 1.18 f 0.10 0.65 i 0.04 0.66 f 0.05 0.65 f 0.03

21 f 2 27 f 5 29 f 1 39 f 3 46f8 50 f 6 27 f 1 29 f 1 31 f 1 42 f 3 44f2 45 f 3

0 2 5 0 2 5 0 2 5 0 2 5

a 0.5 mM probe in 0.5 M pH 7.00phosphate buffer. Aa in footnote a with the addition of 3 mM CTAB and 10 mM SDS, respectively. c Scan rate of 100 mV s-l. Time of exposure of RPG to solution before CV.

Table IV. CV Peak Currents as a Function of Concentration at GC [DOPAC] (mM) i,. (bufferp (NA) i,, (CTAB)b (1A) 0.05 no signal 0.96 f 0.05 0.20 no signal 4.5 f 0.1 0.35 0.50

1.0 f 0.03 2.8 f 0.2

6.8 f 0.6 10.4 f 0.3

a Anodic peak currents for DOPAC solutions in 0.5 M pH 7.00 phosphate buffer measured a t CV scan rate of 100 mV 8-l. b Anodic peak currents for DOPAC solutions with 3 mM CTAB also a t a CV scan rate of 100 mV a-1.

contributes to the improved reproducibility. In fact, the addition of surfactants improvessurfacestability80that RPG can be reused without resurfacing. This, of course, occurs with some sacrifice in sensitivity due to the elimination of adsorption. The results summarized in Table I11 confiim this, where long-time exposure of RPG to DOPAC and DA in surfactant-free solutions is followed by a decrease in peak currents, while the response in CTAB and SDS Solutions, respectively, remains constant. Apparently, surfactants compete effectively for the surface, producing a modified surfacewhich can be dynamicallyrenewed. The modification limits electrode fouling by decreasing irreversible probesurface interactions. At GC the improvement in precision is not observed due to the already good reproducibility of current measurements at the bare surface where the probes investigated do not adsorb. However, due to the lack of activity of GC, sensitivity of this surface is poor. Peak currents for DOPAC listed in Table IV indicate that the addition of CTAB improves the limits of detection at this surface. Therefore, the addition of surfactant to solution can increase the sensitivity of GC surfaces.

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Surface Model. Due to the presence of anionic sites on graphite at neutral pH, cationic surfactants should interact more favorably with these surfaces than anionic Surfactants. In addition, structural differences between surfaces are expected to result in structurally different surfactant assemblies. Therefore, different effects of surfactants on probe response can be expected depending on the surface, probe, and surfactant. Sensitive, oxide-rich RPG is excellent for testing surfactant-graphite interactions. Even though response decreases at RPG in surfactant solutions, with an exception of a response of anionic probe with anionic surfactant, AE, and i, values indicate reasonablyfast processes which are significantly faster than at GC. Because of the reasonably high density of anionic sites at RPG, anionicsurfactants should hinder the response of probes of the same charge at RPG more than cationic surfactants of equal chain length. This in fact is evident from the decrease in DA response in SDS (12 carbon). By comparison, DOPAC response is not significantly altered in 12-carbon cationic DTAB. Short chain length cationic surfactant aggregates can access the RPG surface, facilitating probe access, while anionic surfactants of equal chain length do not access the surface efficiently, decreasing probe response. Increasing carbon chain length should increasehydrophobic interactions between neighboring surfactant tail groups and increase the order of the aggregates. The increased order and dimensions can decrease surface activity. Furthermore, a more hydrophobic surface environment is expected. This can be an improvement which can, however, be overcome by the increase in probe-electrode distance. The decrease in response in fact occurs for DOPAC at RPG where larger AE, and smaller i, result following an increase in cationic surfactant chain length. Since long-chain cationic surfactant aggregates, similarly to anionic aggregates, are expected to facilitate probe access less effectively, AE, and i, values for DOPAC in CTAB and DA in SDS are comparable in magnitude. As a result of the poor response observed at the bare GC surfaces, surfactants which can interact favorably with the surface and the probe can be expected to improve response. As at RPG, at GC, this will be expected of cationic surfactants. However, since at smoother GC short chain length surfactant aggregates should be more organizedthan at RPG, short chain length cationic surfactants should be able to alter (improve) response. This is confirmed by the results. Reactivity of DA at GC does not significantly change in the presence of SDS, even though SDS-DA are expected to interact. Since, the DA surface concentration remains unchanged with SDS present this suggests DAaurface interactions are significant in SDS solutions. The increase in AE, for DOPAC in SDS solutions provides clear evidence for the presence of SDS at the surface. Increasing carbon chain length creates a more hydrophobic environment at GC. The accompanying decrease in Upfor DOPAC suggests further improvement in electrode access which must occur through a combination of electrostatic/ hydrophobic interactions.

The nature of the probes clearly determines how their electrochemical response is affected by surfactants. DOPAC and DA both have hydrophobic and hydrophilic properties which can contribute to their interactions with the surface and with the surfactant aggregates. Both probes exhibit a decreasein response in surfactant solutionsof the same charge, indicating unfavorable electrostatic interactions contribute to a decrease in response. This decrease is more severe for DOPAC in SDS solutions because of the added unfavorable surfactantaurface interactions. Furthermore, the decrease is more marked at oxide-rich, rough, and active RPG than at GC. Both cationic and anionic surfactants modify RPG and GC surfaces,altering probe response. The greater interactions of cationic surfactants with both surfaces indicate similar surfactantaurface interactions. In aqueous media this means head-on interactions with hemimicelle formation. Although GC is considered as a relatively hydrophobic surface, the reported contact angle for GC is 6 6 O 28 compared to 90° for hydrophobic surfaces, suggesting some degree of surface hydrophilicity making it feasible for head-on interactions of surfactants to occur.

CONCLUSIONS We have demonstrated the feasibility of forming reproducible, modified graphite surfaces in surfactant solutions without the need for conditions such as those used in forming a film with Langmuil-Blodgett methods. On the basis of the electrochemical results, a model for surface-surfactant interactions has been developed at two types of graphite surfaces which enables prediction, and consequently control, of response. The results are consistent with head-on surfadantsurface interactions at inactive GC and active RPG. The effect of surfactants on probe response varies at these surfaces because of structural differences between the surfaces. Therefore, the structure of the surface and the nature of the probe must be considered in optimizing response. Our study indicates that along with controlled surface modification,optimization of probe response occurs. We have shown improvementa in sensitivity of inactive GC and reproducibility of RPG with minimal sacrifice in sensitivity. The information gained in the study has been used to identify several key properties of graphite surfaces which must be considered to control and optimize response at these surfaces.

ACKNOWLEDGMENT This work was supported in part by the US.Army Research Office through Grants No. DAAA 15-85-C-00034 and No. DAA103-86-0-0001administered by Battelle.

RECEIVED for review June 29, 1992. Accepted November 9, 1992.