NOTES
3508
contributions. Similar dramatic structural changes are expected for divalent boron if sufficiently nonelectronegative substituents may be used.
Acknowledgment. Grateful acknowledgment is made to the Sational Science Foundation; to the donors of the Petroleum Research Fund, administered by the American Chemical Society; and to the Faculty Research Fund of the Horace H. Rackham School of Graduate Studies of the University of Michigan for their partial support of this research.
Electrocatalytic Oxidation of D-Glucose in Neutral Media with Electrodes Catalyzed by 4,4’,4,’’4’’’-Tetrasulfophthalocyaninemolybdenum Dioxide Salts
by Gregory G. Arzoumanidis’ and John J. O’Connell Monsanto Research Corporation, Boston Laboratory, Everett, Massachusetts 08149 (Received March 3,196Q)
D-Glucose, the most abundant blood carbohydrate, plays a key role as an energy source in the metabolic process of living organisms. A program on the electrocatalytic oxidation of this and related sugars and their derivatives is under investigation to study the feasibility of an implantable fuel cell for an artificial heartala A number of naturally occurring oxidation-reduction processes are catalyzed by metal chelate complexes. The majority of these chelates are porphyrin-ring derivatives. 2b These porphyrins are the catalytic sites for many enzymes, and their catalytic activity occurs through the central metal atom in the planar porphyrin structure. To this family of compounds belong the easily prepared metal phthalocyanines. These materials are unusually stable, simple-to-purify semiconductors. Their water solubility is substantially increased by substitution with hydrophilic groups such as -SO& -COOH, etc. Our experiments were designed to test the catalytic activity of several MoO2-4,4’,4’’,4”’-tetrasulfophthalocyanine (Mo02Pc4S4-) salts, deposited on carbon electrodes, for the electrooxidation of D-glucose. The tests were carried out in a solution of 0.5 D-glucose in 0.5 M NaCl plus 0.5 M phosphate buffer at the blood pH of 7.4 at room temperature. The MoOzz+ central group was selected because of its affinity for sugars at neutral pH condition^.^ This catalyst was prepared according to the procedure of Weber and Busch5 and was purified three times by dissolving it in water and reprecipitating it with NaC1. The electrode substrates used were highly porous composites prepared from a conductive carbon black and polytetrafluorethylThe Journal of Phpical Chemistry
ene. The hydrophilicity of the substrates was increased substantially by boiling them in concentrated H N 0 3 for 1 hr, rinsing with water, and soaking in a saturated solution of (NHJzSzOsfor 24 hr. The electrodes were prepared by impregnation of the above substrate with a 0.5% solution of Mo02-4,4’,4”, 4”’-tetrasulfophthalocyanine in concentrated HzS0.i. This was followed by a water rinse, treatment with hot 40% KOH solution, and finally a water wash. Although the catalyst is water soluble, it was adsorbed firmly on the electrode substrate and no color was observed in the electrolyte solution during testing. As a result the electrocatalytic process is heterogeneous, and dextrose is adsorbed and oxidized on the electrode surface. The reported electrooxidation of CHSOH and HCHOe is catalytically effected on a platinum electrode by molybdates in sulfuric acid solution. Table I lists the electrodes and room temperature anodic half-cell reaction performance values, obtained by using a Kordesch-Narko bridge.’ A standard calomel electrode (sce) served as the reference electrode. It was immersed in the electrolyte through a Luggin capillary situated at the working electrode. For comparison, the open circuit potential (OCP) of an identical electrode without the catalyst is 0 f 0.1 V (8s. sce). Cu2+shows a marked effect on the activity which is not very well understood at the present time. The suggested shuttling between Cu2+and Cu+ or the formation of an initial complex with dextrose would prob-
Or
P l o mA/cm‘
4 1
-0.8
I
0
10
I 20
I
I
I
30 TIme ( m i n u t e s )
40
50
mA/cmz
I SO
Figure 1. Polarization of ~oO2-4,4’,4’’,4’~’-tetrasulfophthalocyanine-Ca2+ catalyzed carbon electrode in 0.5 M n-glucose 0.5 M NaCl phosphates (G) buffer p H 7.4 solution at room temperature.
+
+
(1) T o whom inquiries should be sent at American Cyanamid Co., Central Research Division, Stamford, Conn. 06904. (2) (a) National Heart Institute Contract PH43-66-976with Monsanto Research Corp., Boston Laboratory, Everett, Mass. (b) E. L. Kropa, Texas Rept. Biol. Med., 18,543 (1960). (3) F.H. Moser and A . L. Thomas, “Phthalocyanine Compounds,” American Chemical Society lMonograph Beries, No. 157, Reinhold Publishing Corp., New York, N. Y ., 1963. (4) E. Bayer and W-.Voelter, Ann., 696,194(1966). (6) J. H.Weber and D. 1%. Busch, Inorg. Chem., 4,469 (1966). (6) J. A. Shropshire, J . Electrochem. Soo., 112, 465 (1965). (7) K. Kordesch and A.Marko, ibid., 107,480 (1960).
NOTES
3509
_
~
Table I : Mo02Pc4S~---Catalyaed, D-Glucose Oxidation Electrodes and Constant-Current Measurements in Neutral Media
No.
1 2 3 4 5 6 7 8 9 10 a
-
% Teflon
-Catalyst-Chelate anion
7
Cation
10 5 2
Ce4+ Sr2+
3
-0.61 -0.69 -0.58
Sr2 +
20
-0.69
10
-0.49
Sr2+
10
-0.69
10
[-0.691
K+
RIoOr4,4’,4’ ’,4‘”-tetrasulfophthalocyanine anion.
Current density, mA/om*
-0.64 -0.65 -0.60 -0.69 -0.17 -0.60 -0.66 -0.61
Srz+ Sr2 + Ba2+ cu2+
+ +
OCPb US. ace/ V
20 10 20 20 20 20 20 30
Ca2 +
MoO~PC~S‘-~ MoOnPc4S4MoOzPc4S‘h‘loOzPc4S‘MoOZPc4S4MoOzPc4S4MoOzPc4S4M002Pc4S’MoOzPc ( 4 : l ) MoOzPc4S‘MoO~PC’ ’(3 :2) M002Pc4S4MoSz as promoter
on substrate
Potential, V us. wec measd after 5 min
* Open-circuit potential.
5 1 1 5
-0.67
-0.45 -0.56 -0.68
-0.20
Standard calomel electrode.
an Erwin Halstrup motor potentiostat. The curve from an identical carbon electrode without the catalyst, treated with HSOs, (“&SzOs, and hot 40% KOH and tested under the same conditions, is a straight line a t ,.)IO zero current. The results obtained in these tests con-0,6 2 0 firm the catalytic activity of these electrodes in neutral - - l o .D-glucose solution at low temperatures. Under these -20 conditions the electrode maintains the reversible oxidation potential at current densities below 1 mA/cm2. -30 Above this, however, the electrode gradually polarizes toward positive values. Its performance becomes irreversible and thus time limited. r30 r The low concentration* (about 0.022%) of the active, open-chain form of D-glucose in neutral solutions is one of the limiting factors. Another limitation of the elec- 0 .,6 trodes appears to be the slow diffusion processes. The oxidation products (gluconic acid, saccharic acid, etc.) are weak acids with low ionization constants. Their tendency to adsorb on the electrode surface9 competes with the reaction of these acids with the phosphate buffer. This neutralization becomes slow in the enFigure 2. Cyclic current-potential (us. sce) curves of vironment of the electrode, resulting in an erroneous Mo0~-4,4‘,4”,4’’’-tetrasulfophthalocyanine-Ca2 catalyzed carbon electrode, of area 7.5 cm2, a t 20 T‘/hr (a) in 0.5 M degrading shift of the potential due to a change in the n-glucose + 0.5 M NaCl + 0.5 M phosphates buffer pH 7.4 pH at the catalytic site. The polarized electrodes can at 37” unstirred and (b) in 0.5 M NaCl + 0.5 M phosphates be regenerated by treatment with hot 40% KOH solubuffer pH 7.4 a t 37”. tion and rinse. We believe the mechanism of this heterogeneous ably lead to an increase of the activity, in contrast to the electrocatalytic process is similar to that reported for decrease shown by the data. CH30H and HCH0.6 I n simplified terms, the reaction The metal salts of M00~-4,4’,4’’,4’’’-tetrasulfo- starts with chemical oxidation of D-glucose. This is phthalocyanine were formed in situ on the electrode by followed by the electrochemical path, involving the impregnation with the corresponding metal nitrate system Mae+ e- e Mo6+, and the cycle continues. solution and rinsing. Potential-time curves for MoO2-4,4’,4’’,4’’’-tetrasul(8) M . L. Wolfram and R. S. Tipson, Ed., Advan. Carbohydrate fophthalocyanine-ea2+catalyzed electrodes are shown Chem., 14, 101 (1959). in Figure 1. Figure 2 shows the voltage sweep, from a 116, 334 (9) M.L.B.llao and R. F. Drake, J. Electrochem. SOC., Wenking potentiostat, with programed potential from (1966).
A
t20 +30
N
u
I
+
+
Volume 79, Number 10 October 1969
-
NOTES
35 10
Acknowledgment. This work was supported by the National Institutes of Health under Contract No. PH43-66-976. The authors wish to thank Dr. S. P. Terpko for the preparation of the catalyst and Mr. R. F. Drake for his assistance in the interpretation of the test results.
Spin-Lattice Relaxation of Solvents Containing Nitroxide Spin Labels'.
by
S.B. W. Roeder, W. Wun, and 0. H. Griffithlb
Department of Chemistry, University of Oregon, Eugene, Oregon Q7g03 (Received September 3, 1968)
Useful information regarding the structure of micelles has been obtained using nitroxide free radicals (spin labels).2ta The experimental procedure in these earlier studies consisted of introducing a small amount of spin label into the micelle-containing solution and observing the electron spin resonance spectrum of the spin label. From the esr data and some supporting optical data it was possible to test a number of models of micelle solubilization. Water-insoluble or nearly water-insoluble spin labels were used to avoid interfering signals arising from equilibrium concentrations of label in the aqueous phase. The next logical step is to extend this work to spin labels soluble in both water and hydrocarbon solvents. Using these labels, information is likely to be gained by measuring changes in the equilibrium constants in addition to the familiar rotational and solvent polarity information obtained from spin label studies. Unfortunately, the esr spectra of the labels in the aqueous phase and micelle phase usually overlap severely, and it is worthwhile examining the system using some other technique in addition to esr. ISuclear magnetic resonance is an obvious choice. It is well known that paramagnetic metal ions4t5and stable free radicals6 shorten the relaxation times and, therefore, broaden the nmr absorption lines of nearby nuclei. This effect can be exploited using either highresolution nmr or pulsed (spin-echo) nrnr technique^.^-^ The former approach can be applied directly by observing the broadening of previously assigned nmr lines as the spin label is introduced into the system.2 I n the pulsed nmr approach, however, it would be desirable to have reference information such as the relative effectiveness of nitroxides vs. paramagnetic metal ions, the concentration dependence of the effects in aqueous and long-chain hydrocarbon solvents, the influence of pH on the spin-lattice relaxation time (TI) of water protons, and the temperature dependence of the effects. The purpose of this note is to report the results of a The Journal of Physical Chemistry
brief study aimed a t obtaining these data. The three nitroxide free radicals investigated are OH
.O
0 I
0
0
I1
I11
The nitroxide free radicals I, 11, and I11 were prepared by standard method^.^!^ The source of ferric ion was Mallinckrodt analytical reagent ferric nitrate. The measurements were made on an NMR Specialties PS-60B spin-echo spectrometer operating at 25 RIIHz. Oxygen was removed from the aqueous samples by bubbling nitrogen gas through the samples. Pure water degassed in this fashion was found to give a T1 comparable to that obtained from a sample degassed by the freeze-thaw method. The hydrocarbon solutions were degassed by the freeze-thaw method. Most T1 measurements were made by plotting the recovery of the magnetization using two 90" pulses. A few additional points were taken with the 180-90" 7 null method. Temperature was regulated with a Varian V4540 temperature controller. The effect of the alcohol nitroxide (I) and the ketone nitroxide (11) on shortening the TI of water protons is presented in Figure 1. For comparison, measurements made on ferric ion-water solutions are also given in Figure 1. It is clear from these data that nitroxides I and I1 are much less effective a t shortening TI than are ferric ions. To make this observation more quantitative, the data were fitted to the functional relation
where the proportionality constant4 C is equal to 4a2y2q/kT, pelf is the effective magnetic moment in Bohr magnetons, N is the number of paramagnetic molecules per cubic centimeter, y is the gyromagnetic ratio, q is the viscosity coefficient, and TI0 is the spinlattice relaxation time for the solvent protons in the (1) (a) This work was supported by Public Health Research Grant No. CA 10337-02 from the National Cancer Institute; (b) Alfred P. Sloan Fellow. (2) A. S. Waggoner, 0. H. Griffith, and C. R. Christensen, Proc. Nat. Acad. Sci. U.S.,57, 1198 (1967). (3) A. 8. Waggoner, A. D. Keith, and 0. H. Griffith, J . Phys. Chem., 72,4129 (1968). (4) N.Bloembergen, E.M. Purcell, and R. V. Pound, Phys. Rev.,73, 679 (1948). (5) J. A.Pople, W. G. Schneider, and H. J. Bernstein, "High-Resolution Nuclear Magnetic Resonance," McGraw-Hill Book Co., Inc., New York, N. Y., 1969,p 207. (6) H.S. Gutowsky and J. C. Tai, J . Chem. Phys., 39, 208 (1963). (7) R. Briere, H. Lemaire, and A. Rassat, Bull. SOC.C h i m Fr., 3273 (1966). (8) E.G. Rozantzev and M. B. Neiman, Tetrahedron, 20, 131 (1964).
