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The effects of adsorbed anions on the oxidation of D -glucose on gold single crystal electrodes
Electrochimica Acra, Vol. Printed in Great Britain.
37, No.
THE
2, pp. 357463,
0013-4686/92 $5.00 + 0.00 Pergamon Pres plc.
1992
EFFECTS
OF
OXIDATION
OF
ADSORBED
D-GLUCOSE
CRYSTAL
ANIONS ON
ON
GOLD
THE
SINGLE
ELECTRODES
M. W. HSIAO,R. R. ADZIC* and E. B. YEAGER Case Center for Electrochemical Sciences and the Chemistry Department, Case Western Reserve University, Cleveland, OH 44106, U.S.A. (Received 18 September 1990) Abstract-The effects of various anions on the oxidation of glucose on single crystal gold electrodes were studied in HClO,, CF,SOIH, HNO,, H,SO,, H,PO, and HCl solutions. Contrary to common belief, glucose can be oxidized on gold in acid solutions, but only in the absence of strongly adsorbed anions such as chlorides, sulfates and phosphates. The reaction is partly or completely inhibited in solutions containing specifically adsorbed anions. The data show some indication of an (OH),, layer on gold at potential cathodic to + 1.2V vs rhe in HClO, and CF, SOjH solutions. Based on the rates of oxidation of glucose, the following sequence has been found for the inhibition of the glucose oxidation by adsorbed anions: ClO; u CF,SOF 4 NO? 4 HSO; (SO:-) < H,POc (HPO:-) < Cl-. Key words: glucose, single crystal gold, oxidation.
INTRODUCTION The electro-oxidation of glucose has been studied in recent years principally because of the interest in the development of a glucose sensor[ 1,2]. An implantable miniature, accurate and reliable sensor to monitor the glucose concentration in the body is needed to control an insulin pump in the treatment of diabetes mellitus. The understanding of the reaction kinetics of glucose oxidation is far from complete. Vassilyev et a1.[3,4] have reported that the oxidation of glucose is a typical electrode reaction whose kinetics depend on the nature of the electrode materials. The dependence on the particular crystal plane has also been established for the electro-oxidation of glucose on gold[5] and platinum[6]. A significant difference in the reaction kinetics on gold and platinum electrodes for glucose electro-oxidation in phosphate buffer has been found[5], contrary to an earlier report[4]. Some authors[4,7] believed that in acid solutions gold shows little catalytic activity except in the region of oxide formation. This conclusion was based on data obtained with sulfuric acid solutions and did not take into account the adsorption of HSO; and SOianions. This paper presents evidence that the adsorption of these and other anions suppresses the catalytic activity of gold for the glucose oxidation. EXPERIMENTAL Gold single crystals were purchased from Metal Crystals and Oxides, Ltd. (Cambridge, U.K.), oriented and cut to better than 1”. The details of preparation of single crystals have been described[5].
The final preparation involved annealing in a hydrogen flame followed by cooling down to about 100°C in air. Protected by a drop of ultra-pure water, the crystals were transferred into the cell. Measurements were done using 0.1 M solutions of HC104, H,PO,, TFMSA, HN03 and HCl, and 0.05 M H#O., . The HClO,, HNOs, HCl and H,SO, were Baker’s Ultrex reagents. H,PO, (85%) was purchased from Mallinckrodt Inc. and purified according to the literature[l]. TFMSA and glucose (A.C.S. grade) were purchased from Aldrich Chemical Company. The concentration of glucose was 0.01 M. The ultra-pure water was produced from tap water by reverse osmosis and distillation under nitrogen[9]. A glass cell was used with three separate compartments for the working, counter and reference electrodes. The counter electrode compartment was separated by a fine glass frit from the working electrode compartment. Gold served as a counter electrode while a reversible hydrogen electrode (rhe) served as a reference. All potentials are given vs this reference electrode. The kinetic studies were carried out under potentiostatic control using a model 173 potentiostat-galvanostat, and a model 175 programmer (EG&G Princeton Applied Research Corp.) and model 3033 X-Y recorder (Yokogawa, Tokyo, Japan). All measurements were made at room temperature. The experiments were run in nitrogen-saturated solutions with nitrogen (Ultra purity 99.999%, Matheson Gas Products, Inc.) flowing above the solution to minimize oxygen contamination during the measurements. RESULTS
*Permanent address: Institute of Electrochemistry, ICTM, University of Belgrade, Belgrade, Yugoslavia. 351
AND DISCUSSION
Figure 1 displays the voltammetry curves for the low index planes of gold in 0.05 M H2S04 and
M. W. HS~AOet al.
358
0.01 M Glucose 0.05 M 6%
I
0
Au (, , ,)
1
I
I
I
0.5
1
1.5
-0.3
I
E/V(vsfhe)
current in addition to the double-layer charging current. In 0.1 M HC104 solution the glucose oxidation reaction occurs on all three low index planes of gold at a considerable rate (Fig. 3). The onset of the reaction takes place in the double-layer region at rather high positive potentials, E > 0.5 V. The most positive onset potential and peak potential are on the Au(ll1) surface (Fig. 3). The highest peak current and the most negative onset potential are on the Au(l00) surface (Fig. 3). The formation of anodic film II on gold at potentials greater than approximately 1.3 V is substantially changed in the HC104 solution in the presence of glucose, unlike in H,SO, and H,PO, solutions. The initial stages of gold oxidation, especially on Au( 111) and Au(lOO), are effectively blocked. This is rarely observed in the oxidation of small organic molecules. There is no indication of the oxidation of glucose on gold oxide. Some small amounts of strongly bound species may be oxidized, which is indicated by the activation of the surface seen in the reversed sweep. Upon sweep reversal, the oxidation of glucose after anodic film II reduction occurs at a similar rate as in the positive going sweep. The surface appears blocked if the sweep is reversed before the onset of formation anodic film II (Fig. 4). The origin of the blocking of the surface will be discussed below. The structural sensitivity of this reaction on single crystal gold in acid solution (Fig. 3) is similar to that observed in the phosphate buffer at pH 7.4[5] in regard to the anodic going sweep. It has been shown
Fig. 1. Electra-oxidation of glucose (10m2M) on Au(llO), Au(l11) and Au(100) in 0.05 M H, SO., (solid line) and voltammetry curves in the absence of glucose (dashed line).
Scan rate = 50 mV s-l. 0.05 M H2S04 with 0.01 M D-glucose. The inserts show the voltammetry curves in the double-layer region of gold. The shapes of the curves for gold single crystal electrodes in 0.05 M H, SO0 are in agreement with those reported in the literature[lO, 111.The Au( 111) and Au( 110) surfaces appear ineffective for the oxidation of glucose throughout the double-layer region of gold (Fig. 1). This also applies to the Au(l00) plane, except at potentials more positive than 1.0 V, where a small anodic current is observed (Fig. 1). In the region of formation of anodic film II, the reaction occurs at a small rate on Au(l00) and Au( 110) and at a negligible rate on Au( 111). To make a distinction between the anodic film formed at E > + 1.3 V and a precursor of that film, which in some electrolytes occurs at much more negative potentials, the terms anodic films I and II will be used in this paper. The reaction is even more inhibited in 0.1 M H,PO, (Fig. 2). Glucose is essentially inert on gold in 0.1 M H,PO, even in the region of formation of anodic film II. This is surprising. It is believed that anions are not adsorbed on the anodic film II covered gold surface[l2, 131, since the formation of anodic film II breaks up the anion overlayer lattice. On the Au(ll1) surface at E = +0.65 to +0.85 V, following the sharp peak associated with anion adsorption at +0.65 V, there is some evidence of a small anodic
Fig. 2. Electra-oxidation of glucose (IO-’ M) on Au(l IO), Au(ll1) and Au(100) (solid line) in 0.1 M H,PO, and voltammetry curves in the absence of glucose (dashed line). Scan rate = 50 mV s-l.
Oxidation of n-glucose
I 0.2-
0.01 M Glucose 0.1 M HCIQ
.1
.l
v g
359
an adsorbed reaction intermediate during glucose electro-oxidation. This adsorbed species probably blocks the surface. Separate experiments with gluconolactone showed that these molecules are essentially inert on Au(100) in acid solution in this potential region[ 151. The adsorption of perchlorate anions is usually considered as very weak. This view is in agreement with the present data. The weak chemisorption of ClO; and the proposition of its full discharge at single crystal gold electrodes in the doublelayer region have been discussed by AngersteinKozlowska et aZ.[131.The fast oxidation of glucose in HClO, does not support the hypothesis that ClO; is adsorbed on the surfaces. In siru FTIRRAS measurements also have not detected adsorption of ClO; on Au( 100) at potentials cathodic to + 1.3 V vs rhe[ 151. The overall mechanism of glucose oxidation on gold in neutral and acid solutions can be written in the following form[5, 151 Au+H,O-AuOH”-“‘+H++he-
[)ZfHz [ [>]
.l
(1)
+2H++2e-
(4 0
-I
0.5
1
1.5
0.6
-.--
Au(110)
.-------Au
E/V(vsrhe)
-
Fig. 3. Electra-oxidation of glucose (lo-* M) on Au(llO), Au(ll1) and Au(100) (solid line) in 0.1 M HClO, and voltammetry curves in the absence of glucose (dashed line). Scan rate=50mVs-‘.
0.01 M Glucose
(111)
Au (100)
0.4
that in phosphate buffer the onset of the reaction coincides with the (OH),, layer formation, the anodic film I. The phosphate adsorption on gold in this solution on Au( 100) surface takes place at potentials more positive than the (OH),,, layer formation, as clearly seen by in situ FTIR spectroscopy[l4, IS]. Figure 4 displays the voltammetry curves for the reaction in 0.1 M HClO, solution with the positive potential limit E = + 1.2 V. The onset of the formation of the anodic film II on gold occurs at anodic potential higher than 1.2 V. The currents in anodic going sweep (Fig. 4) are similar to those obtained when the sweep included the anodic film II (Fig. 3). The cathodic going sweep (Fig. 4) shows negligible currents, indicating that the surface is completely blocked. The maxima observed in the curves in the anodic sweeps in Fig. 4 are not caused by diffusion control. These curves do exhibit some sensitivity to agitation, however, as will be discussed in a later paper[l5]. The anodic peaks, together with the inhibition of the reaction in the cathodic going sweep (Fig. 4), can be interpreted in two ways. The blocking effect can be caused by adsorption of the reaction product or intermediate(s), or by increased anion coverage in this potential region. In situ FTIRRAS measurements[l5] show that gluconolactone is detected as
N
e -
0.2
C (W
Fig. 4. Ektro-oxidation of glucose (lo-* M) on the low index planes of gold in 0.1 M HClO, (A) and voltammetry curve in the absence of glucose (B). Scan rate = 50 mV s-‘. Au(ll0) -‘-; Au(lll) ..‘; Au(100) -.
M. W. HSIAOet al.
360
followed by hydrolysis of gluconolactone
0.01 M
o..,_
+ Hz0 -
R-COOH.
0.1 M
Glucose
HNO,
(3) 0.2 -
The most probably first step in glucose oxidation appears to be the oxidatin of hydrogen atom bound to C, atom[5, 15, 161, oiz.
0.1 -
o-
Evidence for such has been observed by replacing the U-H with deuterium in the glucose molecule. The rate of the overall oxidation reaction is slowed down by a factor 2.3 and this supports the involvement of the cc-H in the molecule. Angerstein-Kozlowska et a/.[ 131 have reported the slow oxidation of H, on single crystal gold electrodes
I
I
I
I
0
0.5
1
..
I 1.6
E/V(vsrhe)
Fig. 6. Electra-oxidation of glucose (lo-* M) on Au(l lo), Au( 111) and Au( 100) (solid line) in 0.1 M HNO, and voltammetry curves in the absence of glucose (dashed line). Scan rate = 50 mV s-r.
I
I
I
I
0
0.5
1
1.5
E/V(vsrhe)
Fig. 5. Electra-oxidation of glucose (10-r M)on Au(1 lo), Au(l11) and Au(100) in 0.1 M TFMSA (solid line) and voltammetry curves in the absence of glucose (dashed line). Scan rate = 50 mV s-r.
in acid solutions. The H, oxidation was found to take place on the surface presumably covered by chemisorbed anions and (OH),,, species. One might try to look for a parallelism between the oxidation of H, and glucose on Au(hk1). Surprisingly, the most active surface for H, oxidation, ie Au( 1lo), is the least active for glucose oxidation. It is, however, interesting that to occur the reactions require the formation of the anodic film I. In an earlier work it has been shown that the onset of glucose oxidation coincides with (OH),,, layer formation on gold in phosphate buffer[S]. This may be operative also in HCIO, solutions. Figure 4 shows that the onset of the oxidation of glucose coincides with the first peak in the “double-layer” region of gold. On the basis of FTIRRAS data[l5] and the behavior of glucose in various acids, it appears that the peaks in the double-layer region of gold in the HC104 solutions (Fig. 4B) may be due to the formation of a partially charged (OH),,, layer rather than the adsorption of anions or mixed adsorption of (OH),,, and anions. Further work is needed to obtain quantitative information on this reaction which will be the subject of a separate paper. Figure 5 shows that the behavior in 0.1 M trifluoromethanesulfonic acid (TFMSA) is similar to that in 0.1 M HCIO,. The voltammetry curves for the oxidation of single crystal gold surfaces in HC104 and TFMSA show almost identical features. This indicates that CFJSO; ion is not adsorbed or only weakly adsorbed. When the positive potent al limit is set at 1.2 V vs rhe, the current in the cathodic going sweep is very low as in the curve in HCIO., (Fig. 4).
Oxidation of ~glucose
361
-
-4
Y c
-c
Y -C
--e
I
2
I
0
1
I
zi
I
0
z
0
M. W. HUO et al.
362
Figure 6 shows the voltammetry curves for gold and the oxidation of glucose in 0.1 M HNO, solution. The oxidation of glucose takes place only at very positive potentials with the rates much smaller than those found in HCIOl and TFMSA solutions. This is probably due to a higher adsorbability of NO; ions than of ClO; and CF,SO; ions. According to the supplier of this acid, the Cl- concentration in 0.1 M HN09 is 0.0008 ppm ( _ lo-* M), which is negligible and it is not a substantial cause of slower kinetics in this reaction. In order to explore further the origin of the inhibition of glucose oxidation in H2S0, and H,PO, solutions, experiments have been performed involving additions of these acids to 0.1 M HC104 solution. Figures 7 and 8 clearly show that the additions of 10m3M H, SO, and 1O-3 M H,PO,, respectively, cause a pronounced inhibition of glucose oxidation. The greater effects of the phosphates indicate stronger adsorption of these anions. The most pronounced inhibition in both cases is seen with the Au(ll0) surface, in agreement with a view that anions with a tetrahedral structure are the most strongly bound on the surface with Au(ll0) symmetry[l7].
1
:P Au (110)
0.01 M Glucose i
-0.1 M HCI
E /V(vsrhe)
Fig. 9. Electra-oxidation of glucose (10e2 M) on Au(l IO), Au( 111) and Au( 100) in 0.1 M HCl (solid line) and voltammetry curves in the absence of glucose (dashed line). Scan rate = 50 mV s-l.
0.01 M Glucose in Au (100)
E /V(vsrhe)
Fig. 10. A comparison of the rates of oxidation of glucose (1O-2 M) on Au(lOO) in various solutions which are given in the graph. All electrolytes are 0.1 N, except H,FQ, which is O.lM. Scan rate=50mVs-‘.
The least inhibition is seen for the Au( 111) surface, even though the symmetry of this plane should favor the adsorption of tetrahedral anions. A molecular adsorption of H,PO, certainly plays a role when 10m3M H3P0, is added to 0.1 M HCIO,, since its dissociation will be suppressed by the presence of stronger HCIOl acid. Figure 9 displays the curves for the three single crystal gold surfaces in 0.1 M HCl solution in the absence and in the prersence of 10m2M glucose. The presence of glucose has no effect on the curves for 0.1 M HCl except in the region of Cl- desorption. The oxidation of glucose is completely inhibited in this solution. A comparison of the rates of the oxidation of glucose on the Au( 100) surface in various electrolyte solutions is shown in Fig. 10. The rates are similar in HClO, and TFMSA solutions, considerably lower in HNO, , and almost completely suppressed in H, SO, and H, PO,. The oxidation of glucose may be taken as a probe of the adsorbability of anions on gold for the following reasons. Specifically adsorbed anions reduce the adsorption of glucose to the gold surface, and, more importantly, suppress the formation of (OH),,,. On the basis of the present data, and the reaction in the neutral phosphate buffer solution, it appears that the (OH),,, on the surface of gold is necessary for the oxidation of glucose to take place. Based on the results in Fig. 10, the following sequence in the adsorbability of anions can be inferred: ClO; _ CF,SO; 6 NO; < HSO;(SO:-) < H,PO;(HPO:-) < Cl-. The data show a greater adsorbability of the phosphoric acid anions than the sulfuric anions. In conclusion, the present data show that the oxidation of glucose readily occurs on gold in acid solutions in the absence of adsorbed anions. This corrects the erroneous conclusion about the lack of
Oxidation of D-glucose activity of gold for this reaction in acid solutions, which was based on data in sulfuric acid. The reaction is highly or completely suppressed in solutions with specifically adsorbed anions such as HSO; (SO:-) or H,PO: (HPO$-). Chlorides cause a complete blocking .of this reaction on gold. Acknowledgements-The
authors acknowledge the support of the U.S. Natural Institute of Health, the State of Ohio through the Edison Sensor Technology Center and the U.S. OtTice of Naval Research.
REFERENCES 1. S. J. Updike and G. Hicks, Nature 214, 986 (1967). 2. H. M. Free and A. H. Free, Anal. Chem. 56, 664A (1984). 3. Yu. B. Vassilyev, 0. A. Khazova and N. N. Nikolaeva, J. electroanal. Chem. 1%. 105 (1985). 4. Yu. B. Vassilyev, 0. A. Khazova and N. N. Nikolaeva, J. electroanal. Chem. 196, 127 (1985). 5. R. R. Adzic, M. W. Hsiao and E. B. Yeager, J. electroanal. Chem. 260, 475 (1989). 6. G. Kokkinidis, J. M. Leger and C. Lamy, J. elecfrounal. Chem. 242, 221 (1988).
363
7. B. Beden, I. Cetin, A. Kahyaoglu, D. Takky and C. Lamy, J. Catalysis 104, 37 (1987). 8. D. Ferrier, K. Kinoshita, J. McHardy and P. Stonehart,
9 J. electroanal. Chem. 61. 233 (197%. B. Cahan, M. VIllules and E. B. Yeager, J. electrounal. ’ Chem. in press. 10. A. E. Bolzon. T. Iwasita and W. Vielstich. J. elecrroanal. Chem. 242, 221 (1988). 11. A. Hamelin, in Modern Aspects of Electrochemistry, Vol. 16 (Edited by B. E. Conway, R. E. White and J. O’M Bockris), Ch. 1 and references therein. Plenum Press, New York (1985). 12. S. Strbac. R. R. Adzic and A. Hamelin. J. electroanal. Chem. 249, 291 (1988). 13. H. Angerstein-Kozlowska, B. E. Conway, A. Hamelin and L. Stoicoviciu, J. electroanal. Chem. 277, 233 (1990). 14. M. Razaq, M. W. Hsiao, D. Gervasio, R. Adzic and E. B. Yeager, E.C.S. Meeting, Los Angeles, CA, May 1989. Extended Abstract. Vol. 89-l. 639. 15. M. W. Hsiao, Ph.D. Thesis, Case Western Reserve University (1990). 16. M. W. Hsiao, R. R. Adzic, D. Gervasio and E. B. Yeager, E.C.S. Meeting, Montreal, Canada, May 1990. Extended Abstract, Vol. 90-1, 661. 17. N. Markovic, A. Tripkovic, N. Marinkovic and R. Adzic, in Molecular Phenomena at Electrode Surface (Edited by M. P. Sariaga) Am. Chem. Sot., Symposium Series 378, 447 pp. American Chemical Society, Washington (1988).
37, No.
THE
2, pp. 357463,
0013-4686/92 $5.00 + 0.00 Pergamon Pres plc.
1992
EFFECTS
OF
OXIDATION
OF
ADSORBED
D-GLUCOSE
CRYSTAL
ANIONS ON
ON
GOLD
THE
SINGLE
ELECTRODES
M. W. HSIAO,R. R. ADZIC* and E. B. YEAGER Case Center for Electrochemical Sciences and the Chemistry Department, Case Western Reserve University, Cleveland, OH 44106, U.S.A. (Received 18 September 1990) Abstract-The effects of various anions on the oxidation of glucose on single crystal gold electrodes were studied in HClO,, CF,SOIH, HNO,, H,SO,, H,PO, and HCl solutions. Contrary to common belief, glucose can be oxidized on gold in acid solutions, but only in the absence of strongly adsorbed anions such as chlorides, sulfates and phosphates. The reaction is partly or completely inhibited in solutions containing specifically adsorbed anions. The data show some indication of an (OH),, layer on gold at potential cathodic to + 1.2V vs rhe in HClO, and CF, SOjH solutions. Based on the rates of oxidation of glucose, the following sequence has been found for the inhibition of the glucose oxidation by adsorbed anions: ClO; u CF,SOF 4 NO? 4 HSO; (SO:-) < H,POc (HPO:-) < Cl-. Key words: glucose, single crystal gold, oxidation.
INTRODUCTION The electro-oxidation of glucose has been studied in recent years principally because of the interest in the development of a glucose sensor[ 1,2]. An implantable miniature, accurate and reliable sensor to monitor the glucose concentration in the body is needed to control an insulin pump in the treatment of diabetes mellitus. The understanding of the reaction kinetics of glucose oxidation is far from complete. Vassilyev et a1.[3,4] have reported that the oxidation of glucose is a typical electrode reaction whose kinetics depend on the nature of the electrode materials. The dependence on the particular crystal plane has also been established for the electro-oxidation of glucose on gold[5] and platinum[6]. A significant difference in the reaction kinetics on gold and platinum electrodes for glucose electro-oxidation in phosphate buffer has been found[5], contrary to an earlier report[4]. Some authors[4,7] believed that in acid solutions gold shows little catalytic activity except in the region of oxide formation. This conclusion was based on data obtained with sulfuric acid solutions and did not take into account the adsorption of HSO; and SOianions. This paper presents evidence that the adsorption of these and other anions suppresses the catalytic activity of gold for the glucose oxidation. EXPERIMENTAL Gold single crystals were purchased from Metal Crystals and Oxides, Ltd. (Cambridge, U.K.), oriented and cut to better than 1”. The details of preparation of single crystals have been described[5].
The final preparation involved annealing in a hydrogen flame followed by cooling down to about 100°C in air. Protected by a drop of ultra-pure water, the crystals were transferred into the cell. Measurements were done using 0.1 M solutions of HC104, H,PO,, TFMSA, HN03 and HCl, and 0.05 M H#O., . The HClO,, HNOs, HCl and H,SO, were Baker’s Ultrex reagents. H,PO, (85%) was purchased from Mallinckrodt Inc. and purified according to the literature[l]. TFMSA and glucose (A.C.S. grade) were purchased from Aldrich Chemical Company. The concentration of glucose was 0.01 M. The ultra-pure water was produced from tap water by reverse osmosis and distillation under nitrogen[9]. A glass cell was used with three separate compartments for the working, counter and reference electrodes. The counter electrode compartment was separated by a fine glass frit from the working electrode compartment. Gold served as a counter electrode while a reversible hydrogen electrode (rhe) served as a reference. All potentials are given vs this reference electrode. The kinetic studies were carried out under potentiostatic control using a model 173 potentiostat-galvanostat, and a model 175 programmer (EG&G Princeton Applied Research Corp.) and model 3033 X-Y recorder (Yokogawa, Tokyo, Japan). All measurements were made at room temperature. The experiments were run in nitrogen-saturated solutions with nitrogen (Ultra purity 99.999%, Matheson Gas Products, Inc.) flowing above the solution to minimize oxygen contamination during the measurements. RESULTS
*Permanent address: Institute of Electrochemistry, ICTM, University of Belgrade, Belgrade, Yugoslavia. 351
AND DISCUSSION
Figure 1 displays the voltammetry curves for the low index planes of gold in 0.05 M H2S04 and
M. W. HS~AOet al.
358
0.01 M Glucose 0.05 M 6%
I
0
Au (, , ,)
1
I
I
I
0.5
1
1.5
-0.3
I
E/V(vsfhe)
current in addition to the double-layer charging current. In 0.1 M HC104 solution the glucose oxidation reaction occurs on all three low index planes of gold at a considerable rate (Fig. 3). The onset of the reaction takes place in the double-layer region at rather high positive potentials, E > 0.5 V. The most positive onset potential and peak potential are on the Au(ll1) surface (Fig. 3). The highest peak current and the most negative onset potential are on the Au(l00) surface (Fig. 3). The formation of anodic film II on gold at potentials greater than approximately 1.3 V is substantially changed in the HC104 solution in the presence of glucose, unlike in H,SO, and H,PO, solutions. The initial stages of gold oxidation, especially on Au( 111) and Au(lOO), are effectively blocked. This is rarely observed in the oxidation of small organic molecules. There is no indication of the oxidation of glucose on gold oxide. Some small amounts of strongly bound species may be oxidized, which is indicated by the activation of the surface seen in the reversed sweep. Upon sweep reversal, the oxidation of glucose after anodic film II reduction occurs at a similar rate as in the positive going sweep. The surface appears blocked if the sweep is reversed before the onset of formation anodic film II (Fig. 4). The origin of the blocking of the surface will be discussed below. The structural sensitivity of this reaction on single crystal gold in acid solution (Fig. 3) is similar to that observed in the phosphate buffer at pH 7.4[5] in regard to the anodic going sweep. It has been shown
Fig. 1. Electra-oxidation of glucose (10m2M) on Au(llO), Au(l11) and Au(100) in 0.05 M H, SO., (solid line) and voltammetry curves in the absence of glucose (dashed line).
Scan rate = 50 mV s-l. 0.05 M H2S04 with 0.01 M D-glucose. The inserts show the voltammetry curves in the double-layer region of gold. The shapes of the curves for gold single crystal electrodes in 0.05 M H, SO0 are in agreement with those reported in the literature[lO, 111.The Au( 111) and Au( 110) surfaces appear ineffective for the oxidation of glucose throughout the double-layer region of gold (Fig. 1). This also applies to the Au(l00) plane, except at potentials more positive than 1.0 V, where a small anodic current is observed (Fig. 1). In the region of formation of anodic film II, the reaction occurs at a small rate on Au(l00) and Au( 110) and at a negligible rate on Au( 111). To make a distinction between the anodic film formed at E > + 1.3 V and a precursor of that film, which in some electrolytes occurs at much more negative potentials, the terms anodic films I and II will be used in this paper. The reaction is even more inhibited in 0.1 M H,PO, (Fig. 2). Glucose is essentially inert on gold in 0.1 M H,PO, even in the region of formation of anodic film II. This is surprising. It is believed that anions are not adsorbed on the anodic film II covered gold surface[l2, 131, since the formation of anodic film II breaks up the anion overlayer lattice. On the Au(ll1) surface at E = +0.65 to +0.85 V, following the sharp peak associated with anion adsorption at +0.65 V, there is some evidence of a small anodic
Fig. 2. Electra-oxidation of glucose (IO-’ M) on Au(l IO), Au(ll1) and Au(100) (solid line) in 0.1 M H,PO, and voltammetry curves in the absence of glucose (dashed line). Scan rate = 50 mV s-l.
Oxidation of n-glucose
I 0.2-
0.01 M Glucose 0.1 M HCIQ
.1
.l
v g
359
an adsorbed reaction intermediate during glucose electro-oxidation. This adsorbed species probably blocks the surface. Separate experiments with gluconolactone showed that these molecules are essentially inert on Au(100) in acid solution in this potential region[ 151. The adsorption of perchlorate anions is usually considered as very weak. This view is in agreement with the present data. The weak chemisorption of ClO; and the proposition of its full discharge at single crystal gold electrodes in the doublelayer region have been discussed by AngersteinKozlowska et aZ.[131.The fast oxidation of glucose in HClO, does not support the hypothesis that ClO; is adsorbed on the surfaces. In siru FTIRRAS measurements also have not detected adsorption of ClO; on Au( 100) at potentials cathodic to + 1.3 V vs rhe[ 151. The overall mechanism of glucose oxidation on gold in neutral and acid solutions can be written in the following form[5, 151 Au+H,O-AuOH”-“‘+H++he-
[)ZfHz [ [>]
.l
(1)
+2H++2e-
(4 0
-I
0.5
1
1.5
0.6
-.--
Au(110)
.-------Au
E/V(vsrhe)
-
Fig. 3. Electra-oxidation of glucose (lo-* M) on Au(llO), Au(ll1) and Au(100) (solid line) in 0.1 M HClO, and voltammetry curves in the absence of glucose (dashed line). Scan rate=50mVs-‘.
0.01 M Glucose
(111)
Au (100)
0.4
that in phosphate buffer the onset of the reaction coincides with the (OH),, layer formation, the anodic film I. The phosphate adsorption on gold in this solution on Au( 100) surface takes place at potentials more positive than the (OH),,, layer formation, as clearly seen by in situ FTIR spectroscopy[l4, IS]. Figure 4 displays the voltammetry curves for the reaction in 0.1 M HClO, solution with the positive potential limit E = + 1.2 V. The onset of the formation of the anodic film II on gold occurs at anodic potential higher than 1.2 V. The currents in anodic going sweep (Fig. 4) are similar to those obtained when the sweep included the anodic film II (Fig. 3). The cathodic going sweep (Fig. 4) shows negligible currents, indicating that the surface is completely blocked. The maxima observed in the curves in the anodic sweeps in Fig. 4 are not caused by diffusion control. These curves do exhibit some sensitivity to agitation, however, as will be discussed in a later paper[l5]. The anodic peaks, together with the inhibition of the reaction in the cathodic going sweep (Fig. 4), can be interpreted in two ways. The blocking effect can be caused by adsorption of the reaction product or intermediate(s), or by increased anion coverage in this potential region. In situ FTIRRAS measurements[l5] show that gluconolactone is detected as
N
e -
0.2
C (W
Fig. 4. Ektro-oxidation of glucose (lo-* M) on the low index planes of gold in 0.1 M HClO, (A) and voltammetry curve in the absence of glucose (B). Scan rate = 50 mV s-‘. Au(ll0) -‘-; Au(lll) ..‘; Au(100) -.
M. W. HSIAOet al.
360
followed by hydrolysis of gluconolactone
0.01 M
o..,_
+ Hz0 -
R-COOH.
0.1 M
Glucose
HNO,
(3) 0.2 -
The most probably first step in glucose oxidation appears to be the oxidatin of hydrogen atom bound to C, atom[5, 15, 161, oiz.
0.1 -
o-
Evidence for such has been observed by replacing the U-H with deuterium in the glucose molecule. The rate of the overall oxidation reaction is slowed down by a factor 2.3 and this supports the involvement of the cc-H in the molecule. Angerstein-Kozlowska et a/.[ 131 have reported the slow oxidation of H, on single crystal gold electrodes
I
I
I
I
0
0.5
1
..
I 1.6
E/V(vsrhe)
Fig. 6. Electra-oxidation of glucose (lo-* M) on Au(l lo), Au( 111) and Au( 100) (solid line) in 0.1 M HNO, and voltammetry curves in the absence of glucose (dashed line). Scan rate = 50 mV s-r.
I
I
I
I
0
0.5
1
1.5
E/V(vsrhe)
Fig. 5. Electra-oxidation of glucose (10-r M)on Au(1 lo), Au(l11) and Au(100) in 0.1 M TFMSA (solid line) and voltammetry curves in the absence of glucose (dashed line). Scan rate = 50 mV s-r.
in acid solutions. The H, oxidation was found to take place on the surface presumably covered by chemisorbed anions and (OH),,, species. One might try to look for a parallelism between the oxidation of H, and glucose on Au(hk1). Surprisingly, the most active surface for H, oxidation, ie Au( 1lo), is the least active for glucose oxidation. It is, however, interesting that to occur the reactions require the formation of the anodic film I. In an earlier work it has been shown that the onset of glucose oxidation coincides with (OH),,, layer formation on gold in phosphate buffer[S]. This may be operative also in HCIO, solutions. Figure 4 shows that the onset of the oxidation of glucose coincides with the first peak in the “double-layer” region of gold. On the basis of FTIRRAS data[l5] and the behavior of glucose in various acids, it appears that the peaks in the double-layer region of gold in the HC104 solutions (Fig. 4B) may be due to the formation of a partially charged (OH),,, layer rather than the adsorption of anions or mixed adsorption of (OH),,, and anions. Further work is needed to obtain quantitative information on this reaction which will be the subject of a separate paper. Figure 5 shows that the behavior in 0.1 M trifluoromethanesulfonic acid (TFMSA) is similar to that in 0.1 M HCIO,. The voltammetry curves for the oxidation of single crystal gold surfaces in HC104 and TFMSA show almost identical features. This indicates that CFJSO; ion is not adsorbed or only weakly adsorbed. When the positive potent al limit is set at 1.2 V vs rhe, the current in the cathodic going sweep is very low as in the curve in HCIO., (Fig. 4).
Oxidation of ~glucose
361
-
-4
Y c
-c
Y -C
--e
I
2
I
0
1
I
zi
I
0
z
0
M. W. HUO et al.
362
Figure 6 shows the voltammetry curves for gold and the oxidation of glucose in 0.1 M HNO, solution. The oxidation of glucose takes place only at very positive potentials with the rates much smaller than those found in HCIOl and TFMSA solutions. This is probably due to a higher adsorbability of NO; ions than of ClO; and CF,SO; ions. According to the supplier of this acid, the Cl- concentration in 0.1 M HN09 is 0.0008 ppm ( _ lo-* M), which is negligible and it is not a substantial cause of slower kinetics in this reaction. In order to explore further the origin of the inhibition of glucose oxidation in H2S0, and H,PO, solutions, experiments have been performed involving additions of these acids to 0.1 M HC104 solution. Figures 7 and 8 clearly show that the additions of 10m3M H, SO, and 1O-3 M H,PO,, respectively, cause a pronounced inhibition of glucose oxidation. The greater effects of the phosphates indicate stronger adsorption of these anions. The most pronounced inhibition in both cases is seen with the Au(ll0) surface, in agreement with a view that anions with a tetrahedral structure are the most strongly bound on the surface with Au(ll0) symmetry[l7].
1
:P Au (110)
0.01 M Glucose i
-0.1 M HCI
E /V(vsrhe)
Fig. 9. Electra-oxidation of glucose (10e2 M) on Au(l IO), Au( 111) and Au( 100) in 0.1 M HCl (solid line) and voltammetry curves in the absence of glucose (dashed line). Scan rate = 50 mV s-l.
0.01 M Glucose in Au (100)
E /V(vsrhe)
Fig. 10. A comparison of the rates of oxidation of glucose (1O-2 M) on Au(lOO) in various solutions which are given in the graph. All electrolytes are 0.1 N, except H,FQ, which is O.lM. Scan rate=50mVs-‘.
The least inhibition is seen for the Au( 111) surface, even though the symmetry of this plane should favor the adsorption of tetrahedral anions. A molecular adsorption of H,PO, certainly plays a role when 10m3M H3P0, is added to 0.1 M HCIO,, since its dissociation will be suppressed by the presence of stronger HCIOl acid. Figure 9 displays the curves for the three single crystal gold surfaces in 0.1 M HCl solution in the absence and in the prersence of 10m2M glucose. The presence of glucose has no effect on the curves for 0.1 M HCl except in the region of Cl- desorption. The oxidation of glucose is completely inhibited in this solution. A comparison of the rates of the oxidation of glucose on the Au( 100) surface in various electrolyte solutions is shown in Fig. 10. The rates are similar in HClO, and TFMSA solutions, considerably lower in HNO, , and almost completely suppressed in H, SO, and H, PO,. The oxidation of glucose may be taken as a probe of the adsorbability of anions on gold for the following reasons. Specifically adsorbed anions reduce the adsorption of glucose to the gold surface, and, more importantly, suppress the formation of (OH),,,. On the basis of the present data, and the reaction in the neutral phosphate buffer solution, it appears that the (OH),,, on the surface of gold is necessary for the oxidation of glucose to take place. Based on the results in Fig. 10, the following sequence in the adsorbability of anions can be inferred: ClO; _ CF,SO; 6 NO; < HSO;(SO:-) < H,PO;(HPO:-) < Cl-. The data show a greater adsorbability of the phosphoric acid anions than the sulfuric anions. In conclusion, the present data show that the oxidation of glucose readily occurs on gold in acid solutions in the absence of adsorbed anions. This corrects the erroneous conclusion about the lack of
Oxidation of D-glucose activity of gold for this reaction in acid solutions, which was based on data in sulfuric acid. The reaction is highly or completely suppressed in solutions with specifically adsorbed anions such as HSO; (SO:-) or H,PO: (HPO$-). Chlorides cause a complete blocking .of this reaction on gold. Acknowledgements-The
authors acknowledge the support of the U.S. Natural Institute of Health, the State of Ohio through the Edison Sensor Technology Center and the U.S. OtTice of Naval Research.
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