The role of incipient hydrous oxides in the oxidation of glucose and some of its derivatives in aqueous media

Wrerrochimicu Acta, Vol 37. No 8. pi Printed m Great Bntam

13634370,

1992

am-4686/92ssOo+oOO 01992 POIplOIlPW¶Ltd

THE ROLE OF INCIPIENT HYDROUS OXIDES IN THE OXIDATION OF GLUCOSE AND SOME OF ITS DERIVATIVES IN AQUEOUS MEDIA L D BURKEand T G RYAN Department of Chemistry, Umverslty College, Cork, Ireland (Recemed 25 June 1991,

m rewed form 22 October 1991)

Abstract--Gold m aqueous media, with its extensive workmg range apparently devoid of comphcatrons due to chemisorbed hydrogen and oxygen species, 1s frequently considered as the ideal electrode system for fundamental mvestlgations m electrochemistry However, tins system displays agmficant electrocatalytrc actlvlty m base, an effect that has been attnbuted to a phenomenon known as pre-monolayer oxidation Glucose oxidation at gold m base commences at cu 0 15 V (rhe), the reaction (evidently confined at low potent& to the aldehyde group) bemg mediated by an Au(I) mclpient hydrous oxide species m agreement with thrs approach to electrocatalysis the reactlon of other specs, eg formaldehyde and hmethylamme borane ondation and nitrate Ion reduction, commences m the same regon The alcohol groups m glucose (and Its derivatives) are more resIstant to oxidation, they only commence reaction above 0 65 V (rhe) where a different mediator, an Au(II1) hydrous oxide specks, IS present at the interface The glucose oxidation rate, which IS of slgmficant interest m the blosensor area, decreases as the pH of the solution IS reduced Key words

Gold, glucose oxldatlon, hydrous oxides, medlatlon, electrocatalysls

INTRODUCTION The electrooxldatlon of glucose has been the subject of considerable mvestigatlon m recent years as the process IS of interest m the development of blosensors, pacemakers and atificial hearts[l-121 The metals most commonly used are platmum and gold while both metals are highly resistant to Qssolution and monolayer oxide formation, their selection m other respects seems curious Platinum, with slgmficant vacancies m Its d-band, 1s a strong chemlsorber and should therefore, m terms of the activated chemlsorptlon model of electrocatalysls[l3], be a far more active anode matenal for the process involved, this 1s assuming (as 1s rarely the case Hrlth orgamcs) that deactivation due to polsonmg species such as CO,, 1s not too significant Gold, with no vacancies m its d-band and thus only a very weak chemlsorber, 1s not expected to be a good electrocatalyst for this type of reaction, this 1s not borne out m practice as gold 1s an active electrode matenal (under certain condlhons more active than platmum[2]) for carbohydrate oxldatlons m general, especially m base, and has been applied with success[5] as the basis of a pulsed amperometnc detector system m liquid chromatography Over the past few years a novel theory of electrocatalysis has been developed m this laboratory[14-161 According to a recently published generalized version of this approach[l7] metal adatoms at the electrode surface undergo pre-monolayer oxldatlon, le they oxidize to a very low coverage mclplent hydrous oxide state at qmte low potentials, usually significantly lower than that required to mitlate regular monolayer oxide growth on the metal

surface These hydrous oxide species are highly reactive, m many cases they tngger or mediate oxidation processes, which occur only at potentials above the adatom/maplent hydrous oxide transition However, they also mhlblt reduction processes, obviously m cases where the latter re.qmres the intervention of adatoms A summary of the pre-monolayer oxldatlon behavlour of gold m base[l8] has already been published, the objective of the present work 1s to survey the electrooxldation of glucose at this electrode system m terms of the new approach, some expenments were also carned out on mtrate ion reduction on gold m base to provide support for the validity of the mterfaclal mediator approach to the mechanism of the electrochenucal processes involved

EXPERIMENTAL The workmg electrode consisted of a length of gold wire (Goodfellow metals, 99 95% punty, 1 0 mm diameter, ca 0 8 cm2 exposed area) sealed drectly into glass It was generally cleaned before use by nuld abrasion with fine-grade emery paper, It was then washed wth acetone and tnply dlstllled water A gold counter electrode was used m the same compartment as the workmg electrode Potentials were measured, and are reported, Hnth respect to a hydrogen reference electrode m the same solution, a Luggm capdlary was used to mrnrnlze rR errors All solutions were made up using tnply Qstdled water, the first &stllatlon involved the use of KMnO, to destroy orgamcs and the later ones were carned out using quartz glassware All reagents used were of Analar grade Glucose (~-glucose monohydrate),

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L D BURKEand T G RYAN

function generator (Metrohm VA scanner, Model E612) Cychc voltammograms were recorded usmg a Bkadenkl, Model RW-ZlT, X-Y recorder The solution around the workmg electrode was purged wth a flow of mtrogen gas, all charge and current density values are gwen with respect to geometnc surface area

3

2

RESULTS

1

Typical examples of the responses observed for glucose oxidation on gold in base are shown m Fig 1 The reaction commenced (posltlve sweep) and terrntnated (negative sweep) Just below 0 2V On the positive sweep a first maxlmum was attained between ca 0 5 V (5 mV s-l) and 0 55 V (1OOmV s-l), depending on the sweep-rate This was followed by a mmlmum Just above 0 7 V after which the current agam mcreased to reach a second maximum between ca 1 03 V (5 mV s-l) and 1 10 V (100 mV s-l) Following a slight dip (mmlmum at 1 18 V) a third maximum was attained at ca 1 24V A sharp drop was observed above 1 25 V, the latter value comcldmg approximately with the onset of regular monolayer oxide formation on gold m base[l9] On the subsequent negative scan slgmficant reactlvatlon of the surface commenced at ca 1 2 V, and maxima (at 100 mV s-l) were recorded at cu 1 0 and 0 67 V An dlustratlon of the effects of altermg the glucose concentration and mcreasmg the anodlc hmlt IS gven m Fig 2 It can be seen here that although there were significant oxldatlon currents at the upper limit of the positive sweep, the surface was passlvated on the subsequent negative sweep until the monolayer film was reduced below 1 1 V The current density at 0 45 V on the positive scan was virtually a linear function of the glucose concentration, this was confirmed, Fig 3, for the values taken at the first peak maximum, E 'Y0 5V, m a more extensrve senes of expenments The oxldatlon on gold of several glucose denvattves was also mvestlgated The effect of adding potassmm glucarate (the salt of glucanc aad) to the base IS outlined m Fig 4a The onset of oxldatlon m

Y ;0

(b)

100 mV s-l

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2

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08

04 El

V vs

12

rhe

Rg 1 Typlcal cychc voltammograms (0 O-l 4 V, T = 25°C) for a clean gold electrode m N,-shrred 1 0 M NaOH, contammg glucose up to a level of 0 025 M, dlustratmg the effect of altenng the analytical sweep-rate (a) v = 5 mV s-l, (b) v = 100 mV s-’ gluc~ntc acid [D( +)-glucotuc acid-a-lactone], glucant acid (D-glucanc acid, monopotassmm salt) and glucoromc acid (sodium gluconorate monohydrate) were from Koch-Light Ltd Potassium mtrate was from BDH chemicals Structural formulae for glucose and its vanous derivatives used here are gven m the relevant figures The electrochemical equipment conslsted of a potentlostat (Wenkmg, Model PGS 81) controlled by a

FHO

I

H-Y-OH HO-f-H

8 c

I

I

H-f-OH

I

I

I

0

04

08

I

I

12

18

ElVvs rhe Fig 2 Typical cychc voltammograms (- 0 l-1 8 V, 20 mV s-l) for a clean gold electrode m N,-stirred 10 M and 0 02 M(---) glucose, T = 25°C NaOH contammg 0 01 M (-)

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Glucose oxldatton on Au

4

2

7 zl 2 =0

-

0

-2

0 05

0

[Glucose] 1 M

04

Fig 3 Varlabon of I_ (anodlc sweep, E = 0 45 V) with glucose concentration, for oxldatlon of glucose at a gold electrode m N,-degassed 10 M NaOH, T = 25°C Data taken from cychc voltammograms recorded at 20mV s-‘, -0 l-l 2v

NaOH, lllustratmg the effect of addition of gluwromc acid (sodium salt) (gluwromc acid structure shown) to the solution gluwromc acid concentration = 0 01 M, dashed =0 05 M, full hne (-)

65

V, a sharp peak was observed on and negative sweep at ca 1 1 V (a regon was observed, again on both negative sweep (see Fig l), wth as the sole reducing agent) The

(4

TooH H-C-OH

15

12

rhe

Fig 5 Typical cyclic voltammograms (-0 l-l 2 V, 20 mV s-l, T = 25°C) for a gold electrode m N,-stirred 1 0 M

hne (---),

this case was ca 0 both the positive peak m the same the posltlve and glucose present

08

ElVvs

10

response for glucomc acid lactone, Fig 4b, resembled that for the glucarate amon, reaction commenced on the positive sweep at ca 0 6 V and a maxlmum was again observed at ca 1 1 V Finally, the response for sodium gluconorate is shown m Fig 5, the behavlour m this case resembled that of glucose, m so far as oxldatlon commenced at a sq@%antly lower potential, ca 0 3 V m Fig 5, and two peaks, one at ca 0 58 V and the other at ca 1 1 V, were observed on the posltlve scan Glucose appeared to be inert Hrlth regard to ox+ datlon on gold m solutions of low pH, Fig 6a This Inertness 1s borne out by the fact that m expenments mvolvmg a more positive upper limit than that shown

05

0°5 (a)

r 6 4: E

t

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:OJ

(W

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r E

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Ii . -

010

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005

0 0 -0 05

-1 0 0

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EIVvs

rhe

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Rg 4 Typical cychc voltammograms (-0 l-1 3 V, 20 mV s-l, T = 25°C) for a gold electrode m N&rred 10 M NaOH, dlustratmg the effect of ad&bon of gh~~se denvatives to the solution (a) Ad&tlon of ~ghtcanc acid (monopotassmm salt) to a level of 0 05 M, (b) ad&bon of gluwmc acid la&one (gh~wmc acid structure shown) to a level of 0 05 M EA 3718-E

-0

10 0

08

04

ElVvs

12

rhe

Rg 6 Typical cycbc voltammograms (-0 l-l 3 V, 20 mV s-l, T = 25°C) for a gold electrode m N,-stirred solutions of varpng pH, wnmmng glucose to a level of 0 OSM (a) 10 M H2S0, solution (PH = 15) and (b) phosphate btier solution (PH = 6 76)

L D BURKEand T G

1366

m Fig 6a the monolayer oxide was reduced electrochemically, le by current from the external arcult, rather than by reaction Hrlth the dissolved organic compound The major effect of the presence of the glucose was the increase m cathodic current below 0 V, which probably arose due to aldehyde reduction to alcohol A similar lack of anodlc response was observed at pH = 3 5 At pH = 6 8, Fig 6b, oxldatlon commenced (positwe sweep) above 0 2 V and rose, almost linearly, to attain a maximum at ca 0 95 V A maximum m the same regon was observed on the subsequent negative sweep and this was followed by a short current plateau from cu 0 8 to 0 65 V and then a decrease to zero faradalc current at ca 0 4 V Cyclic voltammograms for glucose oxldatlon on gold at vanous alkaline pH values (8 1, 10 5 and 12 1) are shown m Fig 7 While the general behavlour pattern urlth regard to oxldatlon was not greatly &fferent to that observed unth 1 0 mol drne3 NaOH, some slgmficant differences are worth noting The rates of oxidation m Fig 7 are about an order of magnitude lower than m Fig 1 Current density values below 0 4 V m Fig 7 are particularly low, this 1s especially true for the negative sweep at pH values of 8 1 and 105 Typical responses for mtrate ron reduction on gold m base are shown m Fig 8 for three different levels of NO; concentration The cathodic current commenced (negative sweep) and terminated (posltlve sweep) at cu 0 2 V (rhe) The reduction current mcreased almost linearly with increasing potential (negative sweep) to reach a hmltmg value or plateau The current density m the case of the latter was found to be a linear function of the nitrate Ion concentration, at least up to a value of 1 0 M NO; (Fig 9)

(4

0

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I

I

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08

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ElVvs

rhe

Rg 7 Typical cychc voltammograms (-0 l-l 3 V, 20 mV s-l, T = 25°C) for a gold electrode m phosphate buffer solutions of varymg pH, contammg glucose to a level of 0 05 mol dm-” (a) pH = 8 1, (b) pH = 10 5, and (c) pH=121

RYAN

E

2 .

-4

-6

I -0 4

0

04

EIVvs rhe Fig 8 Typical cychc voltammograms for a gold electrode m N,-degassed 1 OM NaOH (-045-O 4V, 20mV s-l, T = 25°C) contammg vanous concentrahons of NO? (a) (- - -) 0 02 M, (b) (---) 0 06 M and (c) (-) 0 1M

The rate of reduction, even m the plateau Te&lon, was slgmficantly lower on the subsequent positive sweep

DISCUSSION The oxldatlon of glucose on gold m base may be explained, m terms of the mclpient hydrous oxide mediation model of electrocatalysq as follows For gold m pure base a total of five pre-monolayer oxldatlon peaks have been Identified[l8], it may be noted that further support for this approach was obtained recently from UCvoltammetry work[20] The first peak at cu 0 1 V (rhe) was attnbuted to generation of a catlomc Au(I) species, represented as [Au +(H20)n]sda The four remaining transitions (at

0

02

I

I

I

1

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[NO;] I M Rg 9 Vanatlon of k (E 2 -0 2V) mth NO,- concentration for NOT reduction at Au m N,-degassed 1 0 M NaOH at 25°C

Glucose oxldatlon

ca 0 35, 0 65, 105 and 1 15 V) were all attnbuted to formatron of amomc hydrous Au(II1) oxide species that have been formulated recently[lS, 201 as [Au,(OH):-I,, The spread m potential values may be due to Qfferences m the nature of the sttes involved and the effect of eiectrostatlc interaction between species of similar charge[ 181 The oxldatlon of glucose m base (Fig I), commencing at (or Just below) 0 2 V, is evidently medlated by the Au(I) mediator Other reductants exhlbltmg similar behavlour m the regon include formaldehyde[l8] and both dlmethylamme and tbutylamme borane[21,22], since the active states of these other compounds exist m base m anionic form it 1s assumed here that a similar feature arises m the case of glucose One posslblhty 1s a keto-enol transformation of the latter[8,23], with lomzatlon of the enol form m solutions of high pH, DIZ R - CHOH CHO+RCOH +RCOH

= CHOH = CHO- + H+

(1)

Electrostatic interaction between the catlomc mediator [-Au+(H,O),] and the amomc form of glucose 1s evidently a major factor m promoting oxldatlon of glucose at low potential The increase m oxldatlon rate over the range 0 2-O 45 V IS probably due to a combmatlon of (a) increased coverage of the Au(I) mediator, and (b) faster regeneration of the Au(I) state after reduction of the latter by glucose, at more positive potentials The attainment of a plateau, and even a slight decrease m rate, over the range 0 45-O 8 V may be attnbuted to conversion of a significant portion of the mediator from the Au(I) to the Au(II1) state The latter 1s normally assumed to commence formatlon[l8] at cu 0 35 V but this reaction 1s evidently retarded here due to presence of the reducing agent which probably favours coverage of the Au(I) state The increase above 0 8 V 1s assumed to be due to the partlclpatlon of a different mediator, namely the Au(III) incipient hydrous oxide species It 1s possible that Au(II1) mediation commences at a lower potential, ze at cu 0 65 V (m the regon of the third premonolayer oxldatlon peak[l8] as 1s the case with other orgamcs, eg ethylene glycol or pyrrohdme[24] on gold m base) The net current over the range 0 4 to 0 8 V may be a combmatlon of a decreasing component due to the Au(I) mediator and an mcreasmg component due to the Au(II1) mediator While reaction m the reDon below cu 0 4 V 1s assumed to involve only the aldehyde group (the latter being converted to carboxyhc acid), the mcrease m current above 0 8 V 1s evidently due to oxldatlon of an alcohol function It may be noted for instance that m Fig 4 both the glucarate and gluconate ions, m which the only oxidizable groups are alcohols, also commence reaction above 0 65 V In the case of glucose, reaction above 0 8 V probably mvolves a combmation of aldehyde and alcohol group oxldatlon The rate of glucose oxidation on the anodlc sweep reached a hmltmg value at cu 1 1 V, m some instances, eg Figs la and 2, there was a significant decrease above the latter value It was

on Au

1367

pointed out earher[l8] that two further mmor maxima, one at cu 105 V and the other at cu 1 15 V (sometimes only a single peak[l8] 1s observed m this regon) occurred above 1 OV In some cases a peak was observed m the presence of glucose at cu 12 V (Fig 1) however, this was mvanably followed by a rather sharp decrease of the rate of oxldatlon, though certainly not to a total cessation There may be significant loss of the mediator m this regon, the maplent oxide at this stage reacts rapidly wth the glucose resulting m reduction of the former to adatoms There may be loss of the latter due to incorporation of adatoms mto surface sites or formation of gold nuclei at the interface (both effects correspond to conversion of metal adatoms to lattice atoms and hence a decrease m mediator coverage) The further increase m current at cu 1 3 V 1s m the potential region where the mltlal stages of monolayer oxide formation occur for this system It was pointed out earher[l9] that this mltlal stage involved partlclpatlon of hydrous oxide species (the increase m glucose oxldatlon rate here 1s evidently due to the latter), but then the monolayer product becomes more anhydrous, and less reactive, Hrlth increasing surface coverage (hence the maximum, Fig 2, at cu 1 45 V) Further increase m current above 1 6 V may be attnbuted to oxygen gas evolution As pointed out recently[25], the latter process occurs by an unusual mechanism on gold m base with potentials below 2 0 V On the reverse sweep the surface was inactive with regard to glucose oxldatlon until the potential was decreased to Just below 1 2 V There was little mdlcation of a monolayer oxide reduction peak, the removal of this film evidently involved reaction Hrlth glucose via a local cell mechanism The high rate of reaction at cu 1 0 V on the negative sweep 1s apparently due to a high adatom (and hence mediator) coverage followmg reduction of the pamally placeexchanged monolayer oxide deposit The final decay of current as the potential dropped below 0 2 V 1s due to loss of the Au(I) mediator, the latter being reduced to the adatom state m this repon According to a kmetlc theory of mdatorcatalysed oxldatlon processes gven earher[l6], the rate of reaction should be first-order at low concentrations and zero-order at high concentrations of the dissolved reductant Only the first-order regon was observed here, Fig 3, with solutions of glucose concentration rangmg from 0 to 0 1 M The intercept on the current axis at zero reductant concentration was not quite zero, this 1s reasonable as no attempt was made here to subtract double layer chargmg currents The fact that the current density m Fig 3 did not attam a hnutmg value, or zero-order repon, may be due to a number of reasons The zero-order stage, as m enzyme-catalysed processes[l6], corresponds to the state where the Au(I) site 1s saturated urlth the dissolved reactant Since the amomc form of glucose may constitute only a low percentage of the latter, the concentration of active reductant may be far less than the glucose concentration The ability of the reductant to saturate the active site also depends upon onentation (favourable interaction depends on the aldehyde group commg mto direct contact urlth the Au+ site), and the reductant not being oxidized

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L D BURKEand T G RYAN

too readily It IS interesting to note that at low potentials the rate of oxldatlon on gold m 1 0 M NaOH IS somewhat lower in the case of formaldehyde[ 181as compared with glucose (both at 0 1 M), le glucose reacts rather rapidly at Au(I) mediator sites The gluconorate amon, which also contains an aldehyde group, Fig 5, reacts m a similar manner to glucose The mam difference is that the maJor peak m this case IS the one at cu 0 5 V, which IS assumed to be due to conversion of aldehyde to carboxyhc acid There was a slight increase m oxldatlon rate in the region above 0 8 V, however, the effect was not as dramatic as m the case of glucose A possible explanation for this effect 1s that the gluconorate anion expenences significant repulsion at the Au(II1) medlator sites, which are assumed[26] to bear a similar amomc charge Glucose 1s clearly less reactive, Figs 6 and 7, on gold m solutions of lower pH There is evidently no oxldatlon at low pH, Fig 6a, the ongm of minor peaks at ca 0 6 V may well be due to double layer phenomena, eg adsorptlon/desorption of anions or orgamc matenal It 1s unlikely to be due to glucose oxldatlon, as m such a case there should also be an oxldatlon peak on the negative sweep There was slgmficant oxldatlon at pH = 6 8, Fig 6b unlike the behavlour m base (Fig 1), where reaction continued to ca 0 2 V, oxldatlon ceased on the negative sweep in the case at ca 0 4 V In one respect this 1s surpnsmg as the Au*/Au(I) hydrous oxtde transition occurs at a lower potential m solutions of low pH[21], w the Au(I) mediator should be present at potentials well below 0 2V (this 1s borne out by the fact that dlmethylamme borane oxldlses on gold m a solution of pH = 7 0 at a potential as low as -0 05 V (rhe)[21]) The obvious explanation for the lack of reaction 1s the need for the solution species involved m reactlon at low potential to be amomc Evidently at pH = 6 8 the proton actlvlty 1s sufficiently high to suppress dlssoclatlon of the enol form of the glucose, equation 1, apparently only the dissociated form of the enol 1s medlated with regard to oxldatlon by the Au(I) species Reaction at pH values of 8 1 and 10 5, Fig 7, also appeared to be restncted (at least on the negative sweep) to potential values above 0 4 V The only case where slgmficant oxldatlon occurred at potentials below the latter value was m the case of the buffer solution of pH = 12 1, Fig 7c In all these buffer solutions the rate of oxldatlon at any point of the sweep was lower than m base, Figs 1 and 2. This may well be due to strong adsorption of the various types of phosphate amons (H,POi , HW- , or PO:-), which may either lower the mediator actwlty, or reduce the degree of interaction of the latter wth the glucose molecules The reduction of the mtrate ion on gold m base, Fig 8, provides useful support for the mterfaclal cychc redox mechamsm of electrocatalysls Above 0 2 V the mclplent hydrous Au(I) oxide mediates the oxidation of the glucose function, whereas below 0 2 V the gold adatoms mediate the reduction of the mtrate The latter process IS assumed to involve oxldatlon of the gold adatoms to the hydrous state[20] The resultmg mclplent oxide, below 0 2 V,

1sreduced electrochermcally back to the adatom, w an electrochenucal cycle IS also involved m the reduction Nitrate reduction IS a complex process which can yield various products[27], eg mtnte, ammoma, hydroxylamme and mtrogen All steps in such reduction processes may not involve adatoms or mterfacial redox cycles once the first step m mtrate reduction, vvlth electron transfer from the adatom to the ion, takes place, the transfer of subsequent electrons may be quite facile The lower current on the poslhve sweep 1s probably due to loss of adatoms on the reduced metal surface, ti the latter state strongly favours the conversion of adatoms to lattice atoms at the metal surface The linear vanatlon of reduction rate mth concentration, Fig 9, (or the absence of zero-order behavlour at high concentration) 1s surpnsmg It probably reflects a comhnatlon of weak adsorption of the nitrate Ion plus a rapld rate of reduction of the latter at the adatom sites The mtrate reduction process 1s important from a fundamental vlewpomt as, m combmation wth the aldehyde 0x1datlon reaction (both processes commence and terminate at ca 0 2 V), It provides conslderable support for the adatom/maplent hydrous oxide model of electrocatalysls[l7] where, m this case medlatlon 1s determined by the Au*/Au(I) transltlon The results reported m the present work are m general agreement with those pubhshed by other authors Vassdyev et al [2] for instance have observed that the oxldatlon of glucose on gold m acldlc media IS virtually neghglble Adzlc et al [3] reported that on low-index smgle crystal planes of gold m phosphate buffer solution (PH = 7 4), glucose commenced oxidation at ca -0 2 V (see), or cu 0 4 V (rhe) However, m weak base, 0 01 M NaOH, an addlhonal mmor peak commenced at cu -0 65 V (see) The authors attnbuted the first peak observed m base to oxldatlon on metallic gold, Hrlth the latter free. of adsorbed OH-, The second peak, which was observed m both media (although, as observed m the present case, the response was much more marked m base), was attnbuted to the partlapation of a pre.cursor oxide, represented as Au(OH)(’ -I)- Larew and Johnson[rl] attnbuted the low actlvlty of gold m acid to the absence of “catalytic hydrous gold oxide” (the structures for glucose and its denvatlves shown here m various diagrams are taken from their work) The various oxldatron mechanisms proposed by these authors[rl] assume that the mltial step mvolves 0x1datlon of the metal surface, w the gh~cose reacts only Hrlth surface OH groups, and not metal atoms Thus may well be correct, however, it IS worth beanng m mmd the minute fraction of the electrode surface that is covered by these active mediators, especially at low potentials (according to a recent estlmate[l7], m the case of sdver m base, the coverage mvolved m pre-monolayer oxldatlon 1s m the repon of 0 1% of a monolayer) A :unous feature of glucose oxldatlon on platmum m acid 1s that the response at low potentials generally commences on the poslhve sweep at cu 0 2 V (he) this 1s true[lO, 1I] almost lrrespectlve of the crystal plane involved at the electrode surface. It IS mterestmg to note that this IS also the potential postulated[ 141as that required to mltiate mclplent hydrous oxide growth on platinum at low pH, it IS also the

Glucose oxldatlon on Au

value at which qmte unrelated species, eg hydrazme[l4], commences oxldatlon on the metal The oxldatlon of glucose on platmum m base has been investigated by Lamy and coworkers[8], these authors @ve a useful summary of the complexity of the glucose system m solution and the dfficulty of detaded analysis (before or after electrolysis) by techniques such as HPLC They observed three oxldahon peaks on the positive scan, one arose m the hydrogen regon, another close to the onset of monolayer film formation (It was postulated earher[28] that the early stages of the latter process m the case of platinum also involves hydrous oxide species), and the major response on the posltlve sweep arose m the centre of the monolayer oxide formatlon regon (evidently at a coverage correspondmg approximately to monolayer OH,) On the subsequent negative sweep the current remained anodlc throughout the scan (suggesting a reduced level of surface deactlvatlon) until the potential reached a value correspondmg to the onset of the first hydrogen desorptlon peak on the anodlc sweep TUB bears out a point made earlier m connection with hydrazme and formaldehyde oxldatlon on platlnum m base[29], oxldatlon of these species (and glucose) commences at a lower potential m base than m acid (rhe scale) because the high OH- activity m base stabilizes the hydrous oxide mediator, re the adatom/hydrous oxide transltlon occurs at lower potentials m base (the latter effect was confirmed recently[30] for platinum)

CONCLUSIONS It was demonstrated m this work that the mterfacial cychc redox mechanism of electrocatalysls provldes a logcal mterpretatlon of the electrocatalytlc behavlour of glucose and related compounds at gold anodes m aqueous media In agreement wrth earlier proposals for this system at least two mclplent hydrous oxide mediator systems appear to be mvolved At low potentials the mediator 1s a catlomc Au(I) species that catalyses oxldatlon of reactive anionic solution species, le the enolate form of the aldehyde At higher potentials the mediator changes to an amomc Au(II1) maplent hydrous oxide species that catalyses oxldatlon of alcohol, and probably neutral aldehyde groups When the orgamc species bears an unusual degree of amomc charge the rate of oxldatlon m this upper repon 1s slow, which 1s evidently due to electrostatic repulsion between the mediator and the reactmg molecule This was observed m the case of the gluconorate amon which, m base, may well bear two negatrve charges, one on the carboxylate group and the other on the enolate In the case of species without the aldehyde group, eg glucarate and gluconate, only the second mediator was active with respect to oxldatlon Oxldatlon at mtermedlate pH values was slow this 1s apparently due to non-dissoclatlon of the enol plus mhlbltlon due to foreign amon adsorptlon at mediator sites It was also confirmed that the rate of glucose oxldatlon on gold at low pH was negligible Under these condltlons the enol species would agam exist only m the nondlssoclated form the mediator coverage may also be lower, the oxide being less stable m acid The

1369

mediator does exist, however, as compounds such as oxahc acid do oxtdze on gold m acid (this, and other related reations, are currently bemg investigated m this laboratory) The electrochermcal tezhmques employed in the present work gwe very httle specdic mformatlon wncemmg the nature of the mdators and mtermediates involved at the interface It has, however, provtded a useful source of electrochenucal data and ideas so that the system may be investigated by other means, SERS appears to be especially pronusmg The latter needs to be used Hnth caution as the active area of these electrodes is evidently quite small If, as suggested earlier for sdver, only cu 0 1% of the surface 1s actwe with respect to electrocatalysls, there seems to be considerable scope for lmprovmg performance simply by ralsmg the percentage of surface metal atoms m the adatom state, and maintaining these at the higher energy level Fmally, it may be noted that the idea of medlatlon of oxldatlon processes occurrmg at gold m base by anionic hydroxy species has now been postulated by a number of other authors[31, 321

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