1576
Anal. Chem. 1983, 5 5 , 1576-1579
Electrocatalysis of Ascorbate Oxidation with Electrosynthesized, Surface-Bound Mediators Kenneth J. Stutts and R. Mark Wightman” Department of Chemistty, Indiana University, Bloomington, Indiana 47405
The electrooxldatlon of ascorbate has been examined at carbon electrodes. EiectrocatalyslsIs observed wlth homogeneous redox mediators: either substituted catechols or amlnophenols. Modlfled surfaces were prepared to examine the rates of eiectrocataiysis of ascorbate wlth monolayer coverages of these compounds. Surface attachment of aromatic amines vla amldiration leads to surface moieties that, when oxldlred, react to become weaker oxldants. Surface amidiratlon of aromatic compounds via alkylamine slde chalns does not affect the subsequent electrochemistry of these compounds. However, with the use of methoxy and nitro groups as aromatic substltuents, aromatic hydroxy and amlne groups can be electrochemlcaily generated after surface attachment. Surface mediators generated by uslng these techniques provided attached specles wlth Eo’s of +0.18, +0.05, and -0.02 V, respectlvely (vs. Ag/AgCI). Eiectrocataiysls of ascorbate Is only observed wlth the surface covered with the strongest oxidant.
Recently, several papers have appeared concerning the electrocatalysis of the oxidation of ascorbate at carbon electrodes. There have been two different approaches to this problem-surface modification of the electrode via the attachment of redox mediators (1-3) or an alteration of the chemical nature of the carbon surface (4-7). This paper concerns the former method with a special emphasis on the use of electrosynthesis to form the desired redox mediator after a suitable starting material has been attached to the surface. The electrochemistry of ascorbate in pH 7.4 solutions can be characterized as very irreversible both chemically (8) and, a t carbon electrodes, electrochemically. In contrast, the apparent heterogeneous charge transfer reactions are much more rapid at gold or mercury electrodes (9, IO). At carbon electrodes, the apparent rate can be accelerated with the use of electron-transfer mediators in solution-catechols have been shown to be particularly efficient in this regard (2). Surface attachment of catechols to carbon electrodes by adsorption or chemical reaction has also been shown to catalyze the reaction (2, 3). Ascorbate and various catechols are found in mammalian brain, and discrimination and identification of these compounds in mammalian brain with microvoltammetric electrodes is a topic of current interest (11). Electrocatalysis of the oxidation of ascorbate at pH 7.4, therefore, is of particular interest to us, so that the voltammetry of ascorbate can be examined in the presence of, but without interference from, catechols. As described in this paper, we have examined the homogeneous electrocatalysis of the oxidation of ascorbate at carbon paste electrodes with two substituted catechols and two aminophenols to determine whether this approach can be used to move the ascorbate oxidation wave to sufficiently negative potentials that resolution from catechols is possible. Analogues of the solution species employed have been “electrosynthesized” on the surface and have also been examined in the electrooxidation of ascorbate. Our results 0003-2700/83/0355-1576$01.50/0
demonstrate that surface electrosynthesis is a convenient way to generate delicate surface-bound species from rugged precursors, but the data also suggest that catalysis by monolayer amounts of these materials is not sufficiently rapid to achieve our goals.
EXPERIMENTAL SECTION Chemicals. Ascorbic acid (I, MCB), (3,4-dihydroxypheny1)ethylamine hydrochloride (11, Sigma), (3,4-dihydroxybenzyl)amine hydrobromide (111, Aldrich), (3,4,5-trihydroxyphenyl)ethylamine hydrochloride (IV, Sigma), and (3,4-dimethoxyphenyl)ethylamine (Aldrich, were used as received. (2-Amino-4,5-dihydroxypheny1)ethylamine dihydrobromide (V) was a generous gift from hydroC. L. Blank. (2-Nitro-4,5-dimethoxyphenyl)ethylamine chloride (VI, mp 235-238 OC) was synthesized in a manner outlined by Stone (12). 3,3’-Dimethoxybenzidine (VII, Eastman) was recrystallized from acetonitrile. p-Aminophenol (VIII, MCB) and p-anisidine (IX, Baker) were sublimed prior to use. NAcetyl-p-aminophenol (X) was synthesized (13)and recrystallized twice from methanol (mp 167 “C). The pH 7.4 phosphate buffer (0.1 M) used throughout was made from K2HP04(Fisher) and KHzPOl (MCB). Water was distilled from alkaline permaganate. All solutions were deoxygenated with a nitrogen purge. &COH
I
I
njqiv, v
VI VU vn1 I X x Electrodes and Instrumentation. The carbon paste electrode was fabricated as described previously (14) and had a geometric area of 0.0219 cm2, The electrode was used in a 30-mL glass bottle fitted with a plastic screw-on top which had holes drilled for the reference, auxiliary, and working electrodes and nitrogen purging. Glassy carbon (GC) sheets (Tokai Ltd., Japan) were cut into 2.5 X 0.8 cm sheets and were polished with decreasingly abrasive emery cloths and with 5.O-wm, 0.3-pm, and 0.05-wm alumina polishes (Buehler) in sequence. These electrodes formed the floor of an electrochemical cell and the geometric area employed was 0.038 cm2. The cell was designed specifically for the examination of chemically modified electrodes and allowed quick assembly and rapid flushing of its contents. Separation of the auxiliary and reference electrodes from the working electrode was accomplished by a fine frit. All potentials are reported with respect to the Ag/AgCl (4 M KC1) reference electrode. A Princeton Applied Research (PAR) 174A potentiostat was used in conjunction with an Omnigraphic X-Y recorder (Houston Instruments) Homogeneous Catalysis. Rate constants of homogeneous catalysis were determined by the method of Andrieux et al. (15) 0 1983 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983 1577 Table I. Formal Potentials and Homogeneous Rate Constants for the Elect,rocatalysis of I by Electron Transfer Mediators no. I1 VI11 IV
mediator (( 3,4-dihydroxyphenyl)et hylamine ) (paminophenol)
E"' ( V vs. Ag/AgCl)' -i- 0.1 8
-10.095
((3,4,5-trihydroxyphenyl)- -t 0.015b
ethylamine)
k , , IL
mol-' 5.'
lo6 5 x lo3
4x
2X
lo3
-0.081 3 X 10' phenyl )ethylamine ) The formal potentials are the average of E,, and Epc From ref 17. from the cyclic voltammograms. V
((2-amino-4,5-1dihydroxy-
with cyclic voltammetry The diffusion-controlled peak current was first recorded at a carbon paste electrode for the oxidation of the reduced form of the molecule to be used as a redox mediator. Subsequently, an aliquot of I was added to the cell to give a 101 concentration ratio of ]:mediator. (The injected volume was sufficiently small, so that the change in mediator concentration was
