- Email: [email protected]
522—Catalytic oxydation of biological components on platinum electrodes modified by adsorbed metals
Bi&&~&her$trj
.-
Ad Bioenerge&s, 9 (1382) 57 I -582
571
A_sq%ion ofJ. E&&zna~~Cheti, and’co&itutingVol~- 141(1982) ..P+vier *u.cia S& ~&sann e 1 Print& ti’Th& Netherlands
._.-._ .
.’
._ -:.--
.-
522~_CATALtiC OJUQA;IION.OF BIOLOGICAL &%PONENTS PLATINUM-ELEWODES MODiFiED BY ADSORBED METALS . ANbDIC
OXiDA~ON
ON
.OF.GLUCOSE
MIZUHO SAICAMOTO and KIYOKO TAKAMURA Tokyo College or Pharmacy, Horitrouchi 1432-l. Hachioji, Tokyo 192-03 (Japan)
(Manuscript received March 1st 1982)
SUMMARY The catalytic otidation of glucose on Pt electrodes modified by adsorbed metals was studied in 1 M HCIO, by linear sweeP-voltammetry. The adsorbed metals (denoted as M,,. such as Ri,, and Phi,,) formed on Pt in the Potential region more positive than the reversible potential of an M’+/M’ couple. lead to a marked increase in the anodic &urrent of glucose by about one order of magnitude. The catalytic activity depends on the surface coverage by the M,, . The strondy adsorbed species of lactone type, which are responsible for blocking the successive oxidation, are formed on the electrode surface in the anodic processes of glucose on a bare Pi electrode. The formation of such poisonous species is accelerated in the presence of adsorbed hydrogen on Pt. The effects of M_, were discussed on the basis that M,, plays its major role on the Pt electrode surface in removal of the adsorbed hydrogen which initiates the formation of the poisonous species.
INTRODUCTION
The adsorbed species on the electrode surface play very important roles in electrocatal$tic processes. It has recently been shown that some adsorbed metals (denoted as M,,) formed on solid electrodes at underpotentials (the potential region more positive than. the reversible potential of an Mr+/Mo couple) [1] exert high catalytic activity in certain redox systems: for example, the anodic oxidation of some carbonaceous materials [2- 141, oxygen reduction [ 15,161, hydrogen evolution [ 17, IS] and the metal redox couples [19,20] such as Fe2+/Fe3? or Ti3+/Ti4’. In particular, the-striking c&lytic.activity of M,> at a Pt electrode on the oxidation of simple carbonaceous materials such as CO [2?5,6,8], .CH,OH [2,7] and HCQOH [2,4,10,13] bccornes a center of attraction’ from the viewpoint- of electrochemical energyconver..-. sioii_--:, _. :’ .~ ‘-. _, _.- = : It’appearsof.inte&t to extend the iwestigation of the catalytic activity of M,, to biological. redo& ‘s$+ks, since the ‘use of modified electrodes by M,, in biological
syste,F
will._. &niribute to..the -&provem&
..,.-. 0302498/8~~tlOOO-~/$02.~5
of ‘bioftie! cells and the development of -.
a 4982 Plsevier Se&oia~S.A.
‘.
572
-heterogeneous redox models for oxidtie enzymes. However, Little:attention has so far been, paid to the effect of M,, on the electrochemical reaction of biological components, until the catalytic activity of M,, for the oxidation of ascorbic acid at a Pt electrode was shown by the present authors [ZL]. The anodic oxidation of glucose on Pt electrode has been investigated by several authors [22-271 with respect to medical applications to a cardiac pacemaker or an implantable glucose sensor. However, the Pt electrode itself does not show enough activity and stability for such practical uses, because of the production of selfpoisonous intermediates which block the successive oxidation of glucose f25-271. Thus, it seems worth while to study the oxidation of glucose using Pt electrodes modified by M,,. The purpose of the present study is to discover how the oxidation of glucose is affected by M,, formed on the Pt electrode surface and to ascertain the major role of M,, in the oxidation processes. EXPERIMENTAL
Anhydrous glucose was obtained from Wako Pure Chemical Industries. The base electrolyte solution of 1 iW HCIO, was prepared by dissolving ultrapure grade of 60% I-ICIO, in twice-distilled water. Adequate amounts of metal oxides (PbO, Bi,O,, Tl,O, AgzO and CuO of reagent grade). were dissolved in the base solution, which were used as stock solutions of each metal ions. Pt plate used as working electrode was 99.99% pure, 0.1 mm thick and 1.O cm2 in area. The Pt plate was polished with 0.3 pm alumina polishing powder, washed thoroughly with twice-distilled water and finally cleaned electrochemically [28] in the base solution by applying a triangular potential sweep. The electrode potential was measured relative to a saturated calomel electrode (s.c.e.). Electrolysis was carried out by a linear potential sweep method, the potential being applied from a Hokuto Denko potentiostat (Model HA-101) in connection with a Hokuto Denko function generator (Model HB-107A) at a voltage sweep rate of 100 mV s-‘. Before each run, to record a potential scan diagram, a sufficient amount of pure nitrogen was bubbled in the electrolytic solution to remove the dissolved oxygen. All the measurements were carried out at (25 C I)“C. RESULTSAND DISCUSSION Faradaic adsorption of metal ions on Pt electrode Figure 1 shows the current-potential (I-U) curves in 1 M HClO, containing 2.0 X 10m4M Pb’+ (A) or 1.0 X 10A4M Bi 3+ (B). The current peaks are seen in a more positive potent&I region than the reversible Nernst potential of M’+/MO couple (marked as Ul, on the I-U curves in Fig. 1). -The peaks are attributed t0 reaction (1) [If.
U
YS
s.c.e.W 0.5
cl
1.0
Fig. 1. Current-potential (I-U) curves of (A) 2.0X low4 M Pb2+, and (B) LOX 10e4M Bi3” in 1 N HCIlO,. (- - - - - -) Without metal ions.
The cathodic peaks correspond to the formation of adsorbed metal (denoted as Mad), and the anodic peaks to the dissolution of M,,. By such adsorption phenomena (so-called faradaic adsorption), a submonolayer or monolayer amount of metals is adsorbed on solid electrodes [I]. The current peaks due to faradaic adsorption are also obtained in 1 Af HCIO, containing Ag+ , TI”+ and Cu2* . The surface coverage of M,, (8,) is given by Q=
QM Q,=,
(2)
where QM and Q,=, denote the charge required for the anodic dissolution of M,, formed on the electrode surface and that for the monolayer formation of M,, respectively. The former value is obtained by subtracting the charge in the absence of M,, from that in- the presence of M,, (i.e. corresponding to the shaded parts in Fig. 1. Each charge is determined by integrating the currents between the potentials U, and U,). The latter vAue is obtained by equation (3).
Qe=, =
2102 X 1.81A &
cm-’
(3)
where t and A. denote the ionic, valence of metal and an apparent electrode area respectively. ‘De value qf 210 PC. cmm2 is generally employed as ‘the -charge associated with .t& ‘adsorption of a monolayer of hydrogen f29], i.e. the charge recjtied for the tionolayer formation of monovalent atoms. The roughness factor of Pt plate is e&mat&d as 1.81 1391, therefore 1.81Arepresents the real area of the ekctrode skrface.
574 TABLE
I
Surface coverage (0) at various concentrations of Pb’*
and Bi3’.
[Pb’* ]
hb
fBi3+ ]
6Ei
1.0x lo+ 4.0x 10-s 2.0x lo+
0.14 0.20 0.49
5.0x lo+ 1.0x m-5 1.4x IO-+
0.23 0.46 0.52
5.0x lo-’
0.55
2.5x 10-J mx 10-S
0.80
1.0x 10-3
0.54
Negative potential &it: _.
-2SG
mV,
-,-
_.
I.50
Lead and bismuth are the typical metals which induce the striking catalytic effek in the adsorbed states on the oxidation of some carbonaceous materials [&8,13] or.
ascorbic acid [21] on Pt electrode. The surface coverages of Pb,, and Bi,, &timated by the above procedure are given in Table 1. The& values depend on both the bulk concentration of metal ions and the negative limit of the pote&ial sweep [1], and; increase with an increase in the metal ion concentration when the potential sweep is reversed at the definite negative potential. The amount of Pb,, on the electrode. surface is less than that of Bi,, at the same bulk concentration, and 8,, already reaches the limiting value (ca. 0.5) at 2.0 X 10e4 N of Pb*’ in solution. This would be caused by the less positive U, of Pb” (Bi’+ lBi”: +0.04 V, Pb” tPb”: -0.43 V). Since it has been found that the maximum effect of M,, for the oxidation of mcorbic acid or formic acid is attained at 8, = 0.5 -[9,10,21], the I-U curves of glucose were measured in the solutions containing 2.0 X 10m4M Pb** or 1.0 X 1W4 M Bi3+ _ Catalytic effects of Mu, on the oxidation of glucose
In Fig. 2 the I-U curves for glucose are shown using two kinds of Pt electrode:one is an ordinary electrode and another is covered with Pb,, (denoted as Pb,,Pt electrode). On the Pb,, Pt electrode, a marked increase in the anodic cutient by about one order of magnitude is observed in the double-layer region (+200 mV < U < f 500 mV versftss.c.e_)_ During the anodic sweep on Pt ekctrode , glucose is oxidized stepwise in three potential regions [26,27], and the resulting 1-U curves are quite complicated as.shown in Fig. 2. f 1) In the potential region of hydrogen adsorption (-200 mV =ZUG i 50. mV uzxs~~s.c.e.), the oxidation of glucose begins by an interaction between an adsorbed hydrogen atom on the electrode surface and a free electron pair of. the hemizicetal OH group, and results in weak&&g the adjacent C:,-H bond (equation 4). .- _
.,
,. ~.,,.,
_.> m::yif; .:
‘_
-.
Fig. 2. I-U curves of 0.5 M glucose in 1 M HCIO, with (- - - - - -) and without ( -) Pb=* *( - - - - - -) Obtained in base electrolyte.
2.0x 10+&f
A stable adsorbate of glkonolactone type is produced. This requires two sites on Pt and inhibits. the subsequent oxidation of glucose. (2) -In the double-layer potential region (t 100 mV G US f500 mV), glucose is oxidized to gluco~olatone throtigh the processes of equations (5) and (6) OH
(R’
e
OH
-0,l
cl-H
‘-e
Pt
\I
G==
,Ct-Pt
R
;(3) hi the $dt&& of &ko~e
.-.’ .., ..-_.__
..;_
.&x
_ -: . . -.:.
$6
+
ii*+e-
t9
k&on of -thePi oxide formation (above -G 800 mV);a reflction to Fur, but .the rektioti has.not_yet
P&-O layef.:is &sti&~ -: .-: _
: 1.
-. :___
576 :
,_
been found in detail [27]. In this ppt&tial region-the poisonin~.s$&s ,of. l&one type adsorbed on the surface are decomposed SOL@ @rther:oxidatio~. .-. ; ‘The peak current (fp) at i-O,QO V in the. d@&-layer r&$0+ .on. a && Pt: elec-trode remains very small with increasing glucose conqmt&ti@‘~&. shown- iti Fig. 3, and is directly proportional to the sweep -rate (t)) as.Shoti’in Fig:C Th&e’ facts suggest the formation of strongly adsorbed ~@cies (so-&led p&sonous s+SGs) during the anodic sweep on the electrode surface [26,27]; :On the other. hand,: a marked enhancement of I’ is observed on the Pb,kPt electr&Ie.witli an increase-in the concentration of glucose up to about 0.2 M, and it then ap$r&hesa limking
value (Fig. 3). The ~ffusion~ontro~led
process on @e PbixPt
electrode is demon-
strated by the linear relationship between Ip-and ul/’ (Fig.4). The standard redox potential Ud (the; mid-point potential at pG7) for_ thk : glucose/gluconolactone couple is ~0.364 V uersus n.h.e. [31]; the&fore the value of U, corresponding to the present experimental condition is -$I64 \r uersus-s.c.& The fact that the anodic oxidation of gl&ose on Pt electrode beg$s :&~-the hydrogen adsorption region ( - 0.20 V S US +O.OS V oerstrs s&e,), sug&sts a. rather--l&w overvoltage for its oxidation on the Pt surface_ However,-the. cur&t ‘at les&positi~e potentials than the Pt oxide region remains very small. Accord&&y, the-c&lytic effect of glucose in the double layer region on the Pt electrode surface is large_ A similar effect was also found in the presence of-other I+&,,, such as Bi,, (Fig. 5) and Tl,,. However, CuGd or Ag,, displays no catalytic action. That is, there is a difference in the catalytic action
.
Fig.I.Peak Pbat-
againstconcentration~fgi~~k:(~4B) Z at 4-O .20V . withoutPbz+ (OR .P.
O)I,-at
current (I,>
Pt); (O-
p1)_
:
:+0.30V~+~~PbzC~(&~ ._
..
.I ; -_. :--
_--_
.' :
..- : ._
.. ..
-’
:.: : _:
9
. . : 2.
:
-4
: I--_
3 -.
50
;_: .
.6.
~,.
.---
-.g
100
Fig. 4: Peak current of glucose against sweep rate (c) and square root of sweep rate ( o*/~): (0 -0) v&h 2.6X lo-* M Pb’+ (on Pb_,-Pt); (e -e) without Pb2+ (on Pt).
377
578
consideration the partiahy charged properties of the adsorbed metals on a platinum substrate 1321. Consideration of the-role of M,,
in the camytic iixidatiotr
of gltccose
By reversing a cathodic sweep at Iess negative potential limits on a bare- Pt electrode, the current due to the oxidation of glucose in the subsequent anodic sweep increases significantly (Fig. 6). The result suggests that the formation of the strorigly adsorbed species of lactone type (equation 4) is accelerated in the presence of adsorbed hydrogen on Pt. In the doubie-layer potential region (the catalytic effect of M,, is observed), the electrode surface is partly covered with the poisonous species. Accordingly, if the oxidation of glucose takes place only on the Pt ektrode surface free from the poisonous species 1271,oxidation in, this potential region must be inhibited. After the decomposition of the poisonous species at more positive potential, glucose is oxidized during the repeated cathodic sweep on the freshly reduced electrode surface resulting in a large peak current (Fig. 2). The catalytic effects of M,, on the oxidation of glucose are similar to that on. the oxidation of formic acid, as shown in Fig. 7. The oxidation of formic acid on a Pt electrode most probably begins with oxidative dehydrogenation [33] and carboxyl ad-radical may react with adsorbed hydrogen to form the strongly adsorbed species which occupy the two or three sites on Pt as follows [33]:
Fig. 6. Z-U CU~Y~Sof 0.5 M gbose on Pt electrode: with the sweet, reversed al different negative potential limits: (a) -0.05 v; (b) -0.1 Y; (c) -0.2 v; (d) -0.25 V.
-
579
_.
I/ vs s.c.e.W 0
1.U
u.3
Fig. 7. I-U curves of 0.13 M formic acid in 1 M HCIO, with (- - - - - -) Ad without ( -) Pb’“.
HCOOH +$OOH,, GOOH,,+
+ H,,
2 H,,-+C_OH,d+
1.0x IO-‘M
(7) H,O
(8)
The symbols x represent the sites occupied by adsorbed species. The species COH,, is known as a poisonous species which causes the decrease in the reaction rate. Thus AdZi& et al. [lo] bave already proposed the assumption that M,, prevents the adsorbed hydrogen from inhibiting. the formation of the poisonous species, for the catalytic oxidation of formic acid ou the M,,Pt electrode (Pb,,, Biod, TIad, etc.). The adsorbed metals ~suppressthe. hydrogen adsorption peak as shown in Fig. 1, indicating-ihe block of-H,, site on the Pt surface. At less negative potential limit on the 1-b curve of glucose on the, Pb,= Pt electrode, the. catalyzed anodic current is reduced (Fig. I%).This would be dtte to the.decrease of the surface coverage of Pb,, as the-p.otential syeep reverses at the. more positive side, On the Pb,,Pt electrode, surface, the’ formation of poisono.us species. must be. hindered as a result of the ‘. . removal of adsorbed hydrogerr.by: Ivf=,; .I Su&$~ro~e~of Mad is similar-to. ihat in t& ‘catalytic oxidatfon of H.COOH [IO], .-a& Cap also;be: suppOrted by .ti&,Wult.ca@ytic effect. on the. _. ko .__ . i&al: PAad-gave -,. . : ._ .. : . .; : ‘. : .. -“.’
U
I
vs
s.c.e.W
0
0.5
Fig. 8. I-U curves of 0.5 M glucose on Pb,, -Pt electrode at different negative potential limits: (a) 0 V: V; (d) -0.25 V. Pb2+ concentration 2.0X IO-’ M.
(b) -0.1 V; (c) -0.2
anodic
oxidation
of CH,OH,
in which
by the loss of hydrogen [33]. The catalytic effect is correlated
the strongly
adsorbed
to the surface coverage
species of M,,
forms-dirktly (8,)
[S-14,21].
Fig. 9. Dependence of peak current (I,) on surface coverage of Bi,,. Zp were obtained on Z-U curves of in the presence of various concentrations of Ri3+.
0.1 M glucose
.~.
.., :_-
i81 -.
.. .l$e @?I._of.:ihe &&:ctirqent- (:I;) again& ‘8,. is illustrated in Fig. 9. ?he.maximal tkhti~meqt’ f& th&-&cifla~ck~ o$: &l&&e is attained ‘at’ & * 0.5; that is; Bi _, -. &hibits th& catal$iti eff+t &h&h iis-.a&&& is’ & ihe Subm&olayer region. Excessive .coie&ge of MAPYouId lead @l@_tieri& of it&&vn &t&y& activity as a res&of the decre&& in +zt;ve sites on. Pi for the adsorption of &xcose. At 8,i c-2.0, the peak current fails to that obtained without-M ,,.]This f&t suggests that the bxidation of @u&se iS blocked’ by M id. lyin’g on thenPt. Site. An adsorption step accompanying dehydiogenation (cf. equation 5) on a bare Pt site will still be necessary to initiate
the godic oxidation of gllicose. In concIu$gn, ihe major role of Mad in the catalytic oxidation of glucose on Pt tilcctrode will be ass’kned to remove the hydrogen adsorbed on the Pt surface which initiates the formation of the poisonous specks. In the present experiment, the catalytic. oxidation of glucose on the M,,Pt electrode was performed in acid solution to discuss the role of +fad in the catalytic processes. On the other hand, the data in neutral media will be desirable for obtaining information closer to the physiological kditions. Our brief experimental results in 0.1 M NaCIO, show that the adsorbed metal still displayed a catalytic effect but somewhat smaller compared to that in acidic solution. Accordingly, the present results would provide a clue to make an efficient biofuel cell. REFEREN& ! M. Kolb in Advances in Electrochemistry
and Electrochemical
Engineering. H. Gerischer and C.W.
Tobias(Editors),Wiley-Interscience, New York, 1978,Vol. II, p. 125. 2 3 4 5 6 7 8 9 10 1I 12 13 14
M. Breiter, J. Elec~oanal. Chem., 23 (1969) 173. V.N. Adreev and V.E. Kazarinov,Electrokhimiya, IO (1974) 841. B. Conway and H. Kozlowska, J. Electrochem.~Soc., 120 (1973) 756. M. Watanabe and S. Motoo:J. Electroanal. Chem., 60 (1975) 259. S. Motoo tid M. Watanabe, J. Electroanal. Chem., 69 (1976) 429. M. Watanabe and S. Motoo, J. Electroanal. Chem., 60 (1975) 267. S. Motoo.and M. Watiabe. J_ Elec~roanal. Chem_, 111 (1980) 261. S. Motoo and M. Watanabe, J. Electroan& Chem.. 98 (1979) 203. R.R. Ad%%, D.N. Siimic; A.R. DcspiC tid D.M. Drazic. J. Elestroanal. Chem., 80 (1977) 81. A.A. Mikhailova, N.V. Osenzova &d Y.B. Vasilyeu, Electrokhimiya, 13 (1977) 518. RR. AdZi& M.B. Spasojevic and A.R. DespiL; J. Electroanal. Chem., 92 (1978) 31. RR AdZi& and. A-V. Tripkovii; J. Electroanal. Chem., 99 (1979) 43. -M.D. Sp-jeviq RR. AdZi: and A,R &spit; J. Electroanal. Chem., 109 (1980) 261.
IS RR. Ad&5 4-V. Tripkohi:and RT.~Atanasoski, J. Electroanal. Chem, 94 (1978)231. 16 RR @ii= by; Tripkovii: [email protected]. lM+rkoviir, J. Electroanal. Chem., 114 (1980) 37. 17 R.R. Adz%, i$D:q,lSpasojeti;ii: &d A.R Desp+, Electrochim. Acta. 24 (1979) 569. -. 18 N. Furuya and S:Motoo, J. Electroanal. &em., 98 (1979) 195. 19 -N.V. Korovin, N.& _Kozlova and_O.N..Savelyeva, Electrokhimiya, 15 (1979) 1575. 20 R&.-Ad% and A;R. Despik J_~Chem: Pliys.. 61 (1974) 3!82.~, 21 K Takam&a and-M. Sakamot$ J. Electrotial; Chem., 113 (1980) 273. .?2_ J.R %&, GJ. _Richt.er,.F: *on Sturm and.E Weidlich, Bioeleccrochem. Bicienerg.. 3 (1976) 139. 23 M;L.fi.-Rae an$,.R.F.‘~&J. Electrochem. Sot., 116. (1969) 334. -24 H:Lerner; i Giitir, J.S. ‘&kine; -&d_C.K. Colton,,& Electrochem. s0c.i 126 (1976) 237. 25.E.S@& Electrocjlim.-Acia;22-(i977) 313: ..
582
..
26 S. Ernst. J. Heitb&n and C.H. Hamman, J. ElectroanaL Chem:\~& (1979).1731’ ’ 27 S. Ernst, J. Heitbaum and CH. H amnan, Ber. Bunsenges. Wys. Chem, 84 (1980) 50.
28 29 30 31 32 33
J. Clavilier and J-P. Chauvixkau, J. Ele&anaL Chem.. 100 (i979)461.. T. Biegler, D.A.J. Rand id R. Woods, J. Ekctroan~l. Chein.. 29 (1971) 269. .’ D.A.J. Rand and R Woods, J. EIectroamL Che&., 31 (197lj.29. K. Burton, Ergeb. Physiol., 49 (1957) 275. K. Takamura, E Watana-be and T. Takamuk~, Ekctrochinz. Acta, 26 (1981) 979. A. Capon and R Parsons, J. EkctroanaL Chem., 45 (1973) 205.
:
-.
-. _.
:
.-
Ad Bioenerge&s, 9 (1382) 57 I -582
571
A_sq%ion ofJ. E&&zna~~Cheti, and’co&itutingVol~- 141(1982) ..P+vier *u.cia S& ~&sann e 1 Print& ti’Th& Netherlands
._.-._ .
.’
._ -:.--
.-
522~_CATALtiC OJUQA;IION.OF BIOLOGICAL &%PONENTS PLATINUM-ELEWODES MODiFiED BY ADSORBED METALS . ANbDIC
OXiDA~ON
ON
.OF.GLUCOSE
MIZUHO SAICAMOTO and KIYOKO TAKAMURA Tokyo College or Pharmacy, Horitrouchi 1432-l. Hachioji, Tokyo 192-03 (Japan)
(Manuscript received March 1st 1982)
SUMMARY The catalytic otidation of glucose on Pt electrodes modified by adsorbed metals was studied in 1 M HCIO, by linear sweeP-voltammetry. The adsorbed metals (denoted as M,,. such as Ri,, and Phi,,) formed on Pt in the Potential region more positive than the reversible potential of an M’+/M’ couple. lead to a marked increase in the anodic &urrent of glucose by about one order of magnitude. The catalytic activity depends on the surface coverage by the M,, . The strondy adsorbed species of lactone type, which are responsible for blocking the successive oxidation, are formed on the electrode surface in the anodic processes of glucose on a bare Pi electrode. The formation of such poisonous species is accelerated in the presence of adsorbed hydrogen on Pt. The effects of M_, were discussed on the basis that M,, plays its major role on the Pt electrode surface in removal of the adsorbed hydrogen which initiates the formation of the poisonous species.
INTRODUCTION
The adsorbed species on the electrode surface play very important roles in electrocatal$tic processes. It has recently been shown that some adsorbed metals (denoted as M,,) formed on solid electrodes at underpotentials (the potential region more positive than. the reversible potential of an Mr+/Mo couple) [1] exert high catalytic activity in certain redox systems: for example, the anodic oxidation of some carbonaceous materials [2- 141, oxygen reduction [ 15,161, hydrogen evolution [ 17, IS] and the metal redox couples [19,20] such as Fe2+/Fe3? or Ti3+/Ti4’. In particular, the-striking c&lytic.activity of M,> at a Pt electrode on the oxidation of simple carbonaceous materials such as CO [2?5,6,8], .CH,OH [2,7] and HCQOH [2,4,10,13] bccornes a center of attraction’ from the viewpoint- of electrochemical energyconver..-. sioii_--:, _. :’ .~ ‘-. _, _.- = : It’appearsof.inte&t to extend the iwestigation of the catalytic activity of M,, to biological. redo& ‘s$+ks, since the ‘use of modified electrodes by M,, in biological
syste,F
will._. &niribute to..the -&provem&
..,.-. 0302498/8~~tlOOO-~/$02.~5
of ‘bioftie! cells and the development of -.
a 4982 Plsevier Se&oia~S.A.
‘.
572
-heterogeneous redox models for oxidtie enzymes. However, Little:attention has so far been, paid to the effect of M,, on the electrochemical reaction of biological components, until the catalytic activity of M,, for the oxidation of ascorbic acid at a Pt electrode was shown by the present authors [ZL]. The anodic oxidation of glucose on Pt electrode has been investigated by several authors [22-271 with respect to medical applications to a cardiac pacemaker or an implantable glucose sensor. However, the Pt electrode itself does not show enough activity and stability for such practical uses, because of the production of selfpoisonous intermediates which block the successive oxidation of glucose f25-271. Thus, it seems worth while to study the oxidation of glucose using Pt electrodes modified by M,,. The purpose of the present study is to discover how the oxidation of glucose is affected by M,, formed on the Pt electrode surface and to ascertain the major role of M,, in the oxidation processes. EXPERIMENTAL
Anhydrous glucose was obtained from Wako Pure Chemical Industries. The base electrolyte solution of 1 iW HCIO, was prepared by dissolving ultrapure grade of 60% I-ICIO, in twice-distilled water. Adequate amounts of metal oxides (PbO, Bi,O,, Tl,O, AgzO and CuO of reagent grade). were dissolved in the base solution, which were used as stock solutions of each metal ions. Pt plate used as working electrode was 99.99% pure, 0.1 mm thick and 1.O cm2 in area. The Pt plate was polished with 0.3 pm alumina polishing powder, washed thoroughly with twice-distilled water and finally cleaned electrochemically [28] in the base solution by applying a triangular potential sweep. The electrode potential was measured relative to a saturated calomel electrode (s.c.e.). Electrolysis was carried out by a linear potential sweep method, the potential being applied from a Hokuto Denko potentiostat (Model HA-101) in connection with a Hokuto Denko function generator (Model HB-107A) at a voltage sweep rate of 100 mV s-‘. Before each run, to record a potential scan diagram, a sufficient amount of pure nitrogen was bubbled in the electrolytic solution to remove the dissolved oxygen. All the measurements were carried out at (25 C I)“C. RESULTSAND DISCUSSION Faradaic adsorption of metal ions on Pt electrode Figure 1 shows the current-potential (I-U) curves in 1 M HClO, containing 2.0 X 10m4M Pb’+ (A) or 1.0 X 10A4M Bi 3+ (B). The current peaks are seen in a more positive potent&I region than the reversible Nernst potential of M’+/MO couple (marked as Ul, on the I-U curves in Fig. 1). -The peaks are attributed t0 reaction (1) [If.
U
YS
s.c.e.W 0.5
cl
1.0
Fig. 1. Current-potential (I-U) curves of (A) 2.0X low4 M Pb2+, and (B) LOX 10e4M Bi3” in 1 N HCIlO,. (- - - - - -) Without metal ions.
The cathodic peaks correspond to the formation of adsorbed metal (denoted as Mad), and the anodic peaks to the dissolution of M,,. By such adsorption phenomena (so-called faradaic adsorption), a submonolayer or monolayer amount of metals is adsorbed on solid electrodes [I]. The current peaks due to faradaic adsorption are also obtained in 1 Af HCIO, containing Ag+ , TI”+ and Cu2* . The surface coverage of M,, (8,) is given by Q=
QM Q,=,
(2)
where QM and Q,=, denote the charge required for the anodic dissolution of M,, formed on the electrode surface and that for the monolayer formation of M,, respectively. The former value is obtained by subtracting the charge in the absence of M,, from that in- the presence of M,, (i.e. corresponding to the shaded parts in Fig. 1. Each charge is determined by integrating the currents between the potentials U, and U,). The latter vAue is obtained by equation (3).
Qe=, =
2102 X 1.81A &
cm-’
(3)
where t and A. denote the ionic, valence of metal and an apparent electrode area respectively. ‘De value qf 210 PC. cmm2 is generally employed as ‘the -charge associated with .t& ‘adsorption of a monolayer of hydrogen f29], i.e. the charge recjtied for the tionolayer formation of monovalent atoms. The roughness factor of Pt plate is e&mat&d as 1.81 1391, therefore 1.81Arepresents the real area of the ekctrode skrface.
574 TABLE
I
Surface coverage (0) at various concentrations of Pb’*
and Bi3’.
[Pb’* ]
hb
fBi3+ ]
6Ei
1.0x lo+ 4.0x 10-s 2.0x lo+
0.14 0.20 0.49
5.0x lo+ 1.0x m-5 1.4x IO-+
0.23 0.46 0.52
5.0x lo-’
0.55
2.5x 10-J mx 10-S
0.80
1.0x 10-3
0.54
Negative potential &it: _.
-2SG
mV,
-,-
_.
I.50
Lead and bismuth are the typical metals which induce the striking catalytic effek in the adsorbed states on the oxidation of some carbonaceous materials [&8,13] or.
ascorbic acid [21] on Pt electrode. The surface coverages of Pb,, and Bi,, &timated by the above procedure are given in Table 1. The& values depend on both the bulk concentration of metal ions and the negative limit of the pote&ial sweep [1], and; increase with an increase in the metal ion concentration when the potential sweep is reversed at the definite negative potential. The amount of Pb,, on the electrode. surface is less than that of Bi,, at the same bulk concentration, and 8,, already reaches the limiting value (ca. 0.5) at 2.0 X 10e4 N of Pb*’ in solution. This would be caused by the less positive U, of Pb” (Bi’+ lBi”: +0.04 V, Pb” tPb”: -0.43 V). Since it has been found that the maximum effect of M,, for the oxidation of mcorbic acid or formic acid is attained at 8, = 0.5 -[9,10,21], the I-U curves of glucose were measured in the solutions containing 2.0 X 10m4M Pb** or 1.0 X 1W4 M Bi3+ _ Catalytic effects of Mu, on the oxidation of glucose
In Fig. 2 the I-U curves for glucose are shown using two kinds of Pt electrode:one is an ordinary electrode and another is covered with Pb,, (denoted as Pb,,Pt electrode). On the Pb,, Pt electrode, a marked increase in the anodic cutient by about one order of magnitude is observed in the double-layer region (+200 mV < U < f 500 mV versftss.c.e_)_ During the anodic sweep on Pt ekctrode , glucose is oxidized stepwise in three potential regions [26,27], and the resulting 1-U curves are quite complicated as.shown in Fig. 2. f 1) In the potential region of hydrogen adsorption (-200 mV =ZUG i 50. mV uzxs~~s.c.e.), the oxidation of glucose begins by an interaction between an adsorbed hydrogen atom on the electrode surface and a free electron pair of. the hemizicetal OH group, and results in weak&&g the adjacent C:,-H bond (equation 4). .- _
.,
,. ~.,,.,
_.> m::yif; .:
‘_
-.
Fig. 2. I-U curves of 0.5 M glucose in 1 M HCIO, with (- - - - - -) and without ( -) Pb=* *( - - - - - -) Obtained in base electrolyte.
2.0x 10+&f
A stable adsorbate of glkonolactone type is produced. This requires two sites on Pt and inhibits. the subsequent oxidation of glucose. (2) -In the double-layer potential region (t 100 mV G US f500 mV), glucose is oxidized to gluco~olatone throtigh the processes of equations (5) and (6) OH
(R’
e
OH
-0,l
cl-H
‘-e
Pt
\I
G==
,Ct-Pt
R
;(3) hi the $dt&& of &ko~e
.-.’ .., ..-_.__
..;_
.&x
_ -: . . -.:.
$6
+
ii*+e-
t9
k&on of -thePi oxide formation (above -G 800 mV);a reflction to Fur, but .the rektioti has.not_yet
P&-O layef.:is &sti&~ -: .-: _
: 1.
-. :___
576 :
,_
been found in detail [27]. In this ppt&tial region-the poisonin~.s$&s ,of. l&one type adsorbed on the surface are decomposed SOL@ @rther:oxidatio~. .-. ; ‘The peak current (fp) at i-O,QO V in the. d@&-layer r&$0+ .on. a && Pt: elec-trode remains very small with increasing glucose conqmt&ti@‘~&. shown- iti Fig. 3, and is directly proportional to the sweep -rate (t)) as.Shoti’in Fig:C Th&e’ facts suggest the formation of strongly adsorbed ~@cies (so-&led p&sonous s+SGs) during the anodic sweep on the electrode surface [26,27]; :On the other. hand,: a marked enhancement of I’ is observed on the Pb,kPt electr&Ie.witli an increase-in the concentration of glucose up to about 0.2 M, and it then ap$r&hesa limking
value (Fig. 3). The ~ffusion~ontro~led
process on @e PbixPt
electrode is demon-
strated by the linear relationship between Ip-and ul/’ (Fig.4). The standard redox potential Ud (the; mid-point potential at pG7) for_ thk : glucose/gluconolactone couple is ~0.364 V uersus n.h.e. [31]; the&fore the value of U, corresponding to the present experimental condition is -$I64 \r uersus-s.c.& The fact that the anodic oxidation of gl&ose on Pt electrode beg$s :&~-the hydrogen adsorption region ( - 0.20 V S US +O.OS V oerstrs s&e,), sug&sts a. rather--l&w overvoltage for its oxidation on the Pt surface_ However,-the. cur&t ‘at les&positi~e potentials than the Pt oxide region remains very small. Accord&&y, the-c&lytic effect of glucose in the double layer region on the Pt electrode surface is large_ A similar effect was also found in the presence of-other I+&,,, such as Bi,, (Fig. 5) and Tl,,. However, CuGd or Ag,, displays no catalytic action. That is, there is a difference in the catalytic action
.
Fig.I.Peak Pbat-
againstconcentration~fgi~~k:(~4B) Z at 4-O .20V . withoutPbz+ (OR .P.
O)I,-at
current (I,>
Pt); (O-
p1)_
:
:+0.30V~+~~PbzC~(&~ ._
..
.I ; -_. :--
_--_
.' :
..- : ._
.. ..
-’
:.: : _:
9
. . : 2.
:
-4
: I--_
3 -.
50
;_: .
.6.
~,.
.---
-.g
100
Fig. 4: Peak current of glucose against sweep rate (c) and square root of sweep rate ( o*/~): (0 -0) v&h 2.6X lo-* M Pb’+ (on Pb_,-Pt); (e -e) without Pb2+ (on Pt).
377
578
consideration the partiahy charged properties of the adsorbed metals on a platinum substrate 1321. Consideration of the-role of M,,
in the camytic iixidatiotr
of gltccose
By reversing a cathodic sweep at Iess negative potential limits on a bare- Pt electrode, the current due to the oxidation of glucose in the subsequent anodic sweep increases significantly (Fig. 6). The result suggests that the formation of the strorigly adsorbed species of lactone type (equation 4) is accelerated in the presence of adsorbed hydrogen on Pt. In the doubie-layer potential region (the catalytic effect of M,, is observed), the electrode surface is partly covered with the poisonous species. Accordingly, if the oxidation of glucose takes place only on the Pt ektrode surface free from the poisonous species 1271,oxidation in, this potential region must be inhibited. After the decomposition of the poisonous species at more positive potential, glucose is oxidized during the repeated cathodic sweep on the freshly reduced electrode surface resulting in a large peak current (Fig. 2). The catalytic effects of M,, on the oxidation of glucose are similar to that on. the oxidation of formic acid, as shown in Fig. 7. The oxidation of formic acid on a Pt electrode most probably begins with oxidative dehydrogenation [33] and carboxyl ad-radical may react with adsorbed hydrogen to form the strongly adsorbed species which occupy the two or three sites on Pt as follows [33]:
Fig. 6. Z-U CU~Y~Sof 0.5 M gbose on Pt electrode: with the sweet, reversed al different negative potential limits: (a) -0.05 v; (b) -0.1 Y; (c) -0.2 v; (d) -0.25 V.
-
579
_.
I/ vs s.c.e.W 0
1.U
u.3
Fig. 7. I-U curves of 0.13 M formic acid in 1 M HCIO, with (- - - - - -) Ad without ( -) Pb’“.
HCOOH +$OOH,, GOOH,,+
+ H,,
2 H,,-+C_OH,d+
1.0x IO-‘M
(7) H,O
(8)
The symbols x represent the sites occupied by adsorbed species. The species COH,, is known as a poisonous species which causes the decrease in the reaction rate. Thus AdZi& et al. [lo] bave already proposed the assumption that M,, prevents the adsorbed hydrogen from inhibiting. the formation of the poisonous species, for the catalytic oxidation of formic acid ou the M,,Pt electrode (Pb,,, Biod, TIad, etc.). The adsorbed metals ~suppressthe. hydrogen adsorption peak as shown in Fig. 1, indicating-ihe block of-H,, site on the Pt surface. At less negative potential limit on the 1-b curve of glucose on the, Pb,= Pt electrode, the. catalyzed anodic current is reduced (Fig. I%).This would be dtte to the.decrease of the surface coverage of Pb,, as the-p.otential syeep reverses at the. more positive side, On the Pb,,Pt electrode, surface, the’ formation of poisono.us species. must be. hindered as a result of the ‘. . removal of adsorbed hydrogerr.by: Ivf=,; .I Su&$~ro~e~of Mad is similar-to. ihat in t& ‘catalytic oxidatfon of H.COOH [IO], .-a& Cap also;be: suppOrted by .ti&,Wult.ca@ytic effect. on the. _. ko .__ . i&al: PAad-gave -,. . : ._ .. : . .; : ‘. : .. -“.’
U
I
vs
s.c.e.W
0
0.5
Fig. 8. I-U curves of 0.5 M glucose on Pb,, -Pt electrode at different negative potential limits: (a) 0 V: V; (d) -0.25 V. Pb2+ concentration 2.0X IO-’ M.
(b) -0.1 V; (c) -0.2
anodic
oxidation
of CH,OH,
in which
by the loss of hydrogen [33]. The catalytic effect is correlated
the strongly
adsorbed
to the surface coverage
species of M,,
forms-dirktly (8,)
[S-14,21].
Fig. 9. Dependence of peak current (I,) on surface coverage of Bi,,. Zp were obtained on Z-U curves of in the presence of various concentrations of Ri3+.
0.1 M glucose
.~.
.., :_-
i81 -.
.. .l$e @?I._of.:ihe &&:ctirqent- (:I;) again& ‘8,. is illustrated in Fig. 9. ?he.maximal tkhti~meqt’ f& th&-&cifla~ck~ o$: &l&&e is attained ‘at’ & * 0.5; that is; Bi _, -. &hibits th& catal$iti eff+t &h&h iis-.a&&& is’ & ihe Subm&olayer region. Excessive .coie&ge of MAPYouId lead @l@_tieri& of it&&vn &t&y& activity as a res&of the decre&& in +zt;ve sites on. Pi for the adsorption of &xcose. At 8,i c-2.0, the peak current fails to that obtained without-M ,,.]This f&t suggests that the bxidation of @u&se iS blocked’ by M id. lyin’g on thenPt. Site. An adsorption step accompanying dehydiogenation (cf. equation 5) on a bare Pt site will still be necessary to initiate
the godic oxidation of gllicose. In concIu$gn, ihe major role of Mad in the catalytic oxidation of glucose on Pt tilcctrode will be ass’kned to remove the hydrogen adsorbed on the Pt surface which initiates the formation of the poisonous specks. In the present experiment, the catalytic. oxidation of glucose on the M,,Pt electrode was performed in acid solution to discuss the role of +fad in the catalytic processes. On the other hand, the data in neutral media will be desirable for obtaining information closer to the physiological kditions. Our brief experimental results in 0.1 M NaCIO, show that the adsorbed metal still displayed a catalytic effect but somewhat smaller compared to that in acidic solution. Accordingly, the present results would provide a clue to make an efficient biofuel cell. REFEREN& ! M. Kolb in Advances in Electrochemistry
and Electrochemical
Engineering. H. Gerischer and C.W.
Tobias(Editors),Wiley-Interscience, New York, 1978,Vol. II, p. 125. 2 3 4 5 6 7 8 9 10 1I 12 13 14
M. Breiter, J. Elec~oanal. Chem., 23 (1969) 173. V.N. Adreev and V.E. Kazarinov,Electrokhimiya, IO (1974) 841. B. Conway and H. Kozlowska, J. Electrochem.~Soc., 120 (1973) 756. M. Watanabe and S. Motoo:J. Electroanal. Chem., 60 (1975) 259. S. Motoo tid M. Watanabe, J. Electroanal. Chem., 69 (1976) 429. M. Watanabe and S. Motoo, J. Electroanal. Chem., 60 (1975) 267. S. Motoo.and M. Watiabe. J_ Elec~roanal. Chem_, 111 (1980) 261. S. Motoo and M. Watanabe, J. Electroan& Chem.. 98 (1979) 203. R.R. Ad%%, D.N. Siimic; A.R. DcspiC tid D.M. Drazic. J. Elestroanal. Chem., 80 (1977) 81. A.A. Mikhailova, N.V. Osenzova &d Y.B. Vasilyeu, Electrokhimiya, 13 (1977) 518. RR. AdZi& M.B. Spasojevic and A.R. DespiL; J. Electroanal. Chem., 92 (1978) 31. RR AdZi& and. A-V. Tripkovii; J. Electroanal. Chem., 99 (1979) 43. -M.D. Sp-jeviq RR. AdZi: and A,R &spit; J. Electroanal. Chem., 109 (1980) 261.
IS RR. Ad&5 4-V. Tripkohi:and RT.~Atanasoski, J. Electroanal. Chem, 94 (1978)231. 16 RR @ii= by; Tripkovii: [email protected]. lM+rkoviir, J. Electroanal. Chem., 114 (1980) 37. 17 R.R. Adz%, i$D:q,lSpasojeti;ii: &d A.R Desp+, Electrochim. Acta. 24 (1979) 569. -. 18 N. Furuya and S:Motoo, J. Electroanal. &em., 98 (1979) 195. 19 -N.V. Korovin, N.& _Kozlova and_O.N..Savelyeva, Electrokhimiya, 15 (1979) 1575. 20 R&.-Ad% and A;R. Despik J_~Chem: Pliys.. 61 (1974) 3!82.~, 21 K Takam&a and-M. Sakamot$ J. Electrotial; Chem., 113 (1980) 273. .?2_ J.R %&, GJ. _Richt.er,.F: *on Sturm and.E Weidlich, Bioeleccrochem. Bicienerg.. 3 (1976) 139. 23 M;L.fi.-Rae an$,.R.F.‘~&J. Electrochem. Sot., 116. (1969) 334. -24 H:Lerner; i Giitir, J.S. ‘&kine; -&d_C.K. Colton,,& Electrochem. s0c.i 126 (1976) 237. 25.E.S@& Electrocjlim.-Acia;22-(i977) 313: ..
582
..
26 S. Ernst. J. Heitb&n and C.H. Hamman, J. ElectroanaL Chem:\~& (1979).1731’ ’ 27 S. Ernst, J. Heitbaum and CH. H amnan, Ber. Bunsenges. Wys. Chem, 84 (1980) 50.
28 29 30 31 32 33
J. Clavilier and J-P. Chauvixkau, J. Ele&anaL Chem.. 100 (i979)461.. T. Biegler, D.A.J. Rand id R. Woods, J. Ekctroan~l. Chein.. 29 (1971) 269. .’ D.A.J. Rand and R Woods, J. EIectroamL Che&., 31 (197lj.29. K. Burton, Ergeb. Physiol., 49 (1957) 275. K. Takamura, E Watana-be and T. Takamuk~, Ekctrochinz. Acta, 26 (1981) 979. A. Capon and R Parsons, J. EkctroanaL Chem., 45 (1973) 205.
:
-.
-. _.
: