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Chemical modification of the PtO electrode by naphthoquinone using aminosilane
53
J. Electrounul. Chem., 260 (1989) 53-62 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Chemical modification of the PtO electrode by naphthoquinone using aminosilane Eugenii Yu. Katz, Anatolii Ya. Shkuropatov, Shuvalov Institute of Soil Science and Photosynthesrs, 142292 (U.S.S.R.)
Olga I. Vagabova and Vladimir A.
USSR Academy of Sciences, Pushchino, Moscow Region
(Received 23 June 1988; in revised form 3 October 1988)
ABSTRACT A new, simple method is described for the chemical modification of PtO electrodes by a naphthoquinone aminoderivative based on the heterogeneous reaction of naphthoquinone with aminosilane bound to the electrode. Comparison of electrochemical properties of the quinonoid modified electrode and modelling water-soluble aminonaphthoquinone with various degrees of amino group nitrogen substitution, has shown that only the primary silane amino group takes part in the quinone binding to the electrode. A cyclic voltammetric study showed that the surface concentration of the quinone immobilized at the electrode was ca. 10-i’ mol/cm*, corresponding to monolayer density. Quasi-reversible quinone reduction with a rate constant of about 1 s-’ could be maintained over many hundreds of electron transfer cycles.
INTRODUCTION
Electrodes chemically modified by redox groups [l], including quinonoid ones [2], are used in the processes of electrochemical and photoelectrochemical catalysis. In particular, immobilized redox groups can function as an electron transfer mediator between protein redox sites and the electrode [3]. To develop high efficiency photoelectrochemical energy converters on the basis of the reaction centers (RCs) of photosynthetic bacteria, we have studied the functioning of a number of quinones as electron transfer mediators from the RC acceptor site to the electrode [4,5]. These mediators must meet the following requirements simultaneously: they should function in the RC instead of the removed native quinones and should be immobilized easily on the electrodes as a monolayer with electrochemical reversibility. Modification processes and the electrochemical properties of the PtO electrode modified covalently by naphthoquinone are described in the present paper. The reaction, which has not been used for electrode chemical modification previously, allows the quinonoid modified electrode to be obtained with quasi-reversible electrochemical 0022-0728/89/$03.50
0 1989 Blsevier Sequoia S.A.
54
properties by a new, experimentally simple, method. This electrode is of special interest due to the ability of the water-soluble analogue of the immobilized aminonaphthoquinone to function in the RC from Rhodobacter sphaeroides R-26 bacteria as a primary quinone with a recombination time of the photoinduced separated charges similar to the native one [5]. This allows the quinonoid modified electrode to be used for the coupling of the photoinduced charge separation process in the immobilized RCs with an electron transfer to the electrode. EXPERIMENTAL
Electrodes made of Pt wire of ca. 0.15 cm* geometrical area were used for chemical modification. The real area obtained from the electrochemical hydrogen adsorption in 0.5 M H,SO, [6] was 2.2 times larger than the geometrical one. The electrodes with a PtO layer were prepared according to Lenhard and Murray [7] and silanized by refluxing in a ca. 7% (v/v) solution of 3-(2aminoethylamino)propyltrimethoxysilane, en-silane, (Fluka) in very dry toluene for about 4 min. The silanized electrodes were rinsed thoroughly with dry toluene and then with ethanol. The reaction of silanized electrodes with 2,3-dichloro-l&naphthoquinone, Q, (Fluka) was carried out in a saturated solution of Q for ca. 1 h at room temperature. In order to model the electrochemical properties of the immobilized quinone, the following water-soluble aminonaphthoquinones were synthesized: 2-chloro-3-[[2-(dimethylbutylammonium bromide)ethyl]amino]-1,4_naphthoquinone (QN,) obtained according to ref. 8, 2-chloro-3-diethylamino-1,Cnaphthoquinone hydrochloride (QN,) obtained by the reaction of Q with diethylamine in ethanolic solution [9] and isolated by preparative thin-layer chromatography on silica gel Silufol plates (Czechoslovakia) developed in chloroform (R f = 0.45). Elemental analysis (Carlo Erba Strumentazione, Italy) for the synthesized quinones was satisfactory. Anal. Calcd for QN, (C,,H2,N202ClBr): C, 52.00; H, 5.82; N, 6.74%; and for QN2 (C,,H,,N02Cl. HCl): C, 56.02; H, 4.70; N, 4.67%. Found for QN,: C, 52.22; H, 5.84; N, 6.72%; and for QN,: C, 56.23; H, 4.72; N, 4.65%. Electrochemical measurements were performed using a GWP-673 electrochemical analyzer (GDR) in a three-electrode cell with a mercury dropping or Pt pin working electrode, a glassy carbon auxiliary electrode isolated by a frit and a molar calomel reference electrode; all potentials are reported with respect to this reference. Positive CH3
0
I
NH-_(CH212
--+N
-(CH,),-CH,
Br-
* HCI
I C”3 Cl
(QN, 1
(QN,)
55
feedback circuitry was used for 80% ohmic drop compensation during the cyclic voltammetric study. The measurements were carried out at ambient laboratory temperature (22 f 2 o C) in solutions freed of oxygen by bubbling with argon. RESULTS AND DISCUSSION
Naphthoquinone immobilization at the electrode using en&lane Immersion of the silanized PtO electrode into a solution of Q leads to nucleophilic addition of the en-silane amino group to the quinone [lo], and as a result it is immobilized covalently at the electrode as aminoquinone:
Pt
0-Si
\
Q -KH213-NH-
(CH2)2-NH2
-HCI
/
Pt
0-Si
\
-(CH2)3--NH-(CH212-NH
/
Pt
I-/
0
\
0-Si-(CH2)3-NH-_(CH2)2-NH / 11; Cl :QC7
In addition to covalent immobilization of aminoquinone, excess Q and hydroquinone, QH2, formed as a by-product are adsorbed at the modified electrode surface. Two pairs of the coupled cathodic-anodic peaks are observed on the voltammetric curve of the quinonoid modified electrode obtained immediately after its modification in Q solution (Fig. 1, curve a). The cathodic and anodic peaks at more positive potentials differ in peak current value (1, > I,), decrease rapidly on cyclic scanning of the potential and disappear completely during rinsing of the electrode with ethanol. However, they appear again after dipping the modified electrode in ethanolic Q solution. Thus they can be attributed to the redox process of the adsorbed quinone: Qad + 2 e- + 2 Hf P OH,,,
56
The second pair of cathodic and anodic peaks, with equal peak current values (Z, = Z,) and at more negative potentials, is not changed during rinsing of the electrode with ethanol (Fig. 1, curve b) and decreases slowly on cyclic scanning of the potential (by lo-20% for one hundred cycles). Therefore it can be attributed to the redox process of the quinone immobilized at the electrode. For comparison we modified the electrode by a known method [8] resulting in immobilization of the same naphthoquinone bound to the electrode surface via the amine group: CH3
Pt
OH
+
0
+I (CH30)3Si(CH2)3-N-(CH2)2
-NH
I C’43
Cl 0
Pt
I--
\
+i”’
0-Si-(CH2)3-N /
0
-(CH2)2-NH
I CH3 Cl
The electrochemical properties of the quinones covalently immobilized by either method turned out to be practically the same. However, the latter method includes multistep chemical synthesis of silanoquinone [8] and therefore it is considerably more complex than that developed in the present study. It is well known [ll] that there are no significant changes in the redox potential for various substances during their covalent immobilization at electrodes if the redox center of the molecule is not changed. However, the potential shift observed in the present study for the covalently bound quinone by ca. 200 mV in the negative direction compared with the adsorbed parent quinone Q, is accounted for by the electron-donating properties of the amino group inserted into the quinoid ring during its immobilization. Electrochemical properties of modeling aminonaphthoquinones To clarify the nature of quinone binding to the electrode functionalized by en-silane, the electrochemical properties of the parent Q and its amino derivatives QN, and QN2 modeling the Q bond with the primary and secondary en-silane amino groups, have been studied. The absorption spectra obtained for the synthesized modeling quinones (Fig. 2) coincide with those published earlier for QN, [8] and for 2-chloro-3-piperidino-1,4naphthoquinone, which is similar to QN2 [12], and
-0.6
-0 4
-02
0 E/V
J
/cm-
Fig. 1. Cyclic voltammograms of the quinonoid modified electrode after its modification in Q ethanolic solution (a), after rinsing with ethanol (b) obtained in 10 mM Tris+ HCl pH 8.0 water buffer solution &d in the presence of 3 X 10e4 M QN, in the solution (c); u = 40 mV/s. Fig. 2. Absorption spectra of 5 X 10e4 M quinone solutions in 10 m M Tris + HCl pH 8.0 ethanol + water 9: 1 buffer with path length 0.1 cm; (a) Q; (b) QN,; (c) QN2.
have peaks with maxima of about 470 and 500 nm, respectively, resulting in the red color characteristic of aminonaphthoquinones. The polarographic properties of the parent Q and its aminoderivatives QN, and QN2 were studied in ethanolic + water solution (9 : l), since Q is poorly soluble in water (Fig. 3). Polarographic half-wave potentials, E,,2, equal to -0.25, -0.34 and -0.44 V were obtained for Q, QN, and QN,, respectively, at pH 8.0. It is known that substitution of an alkylamino group for a hydrogen atom in naphthoquinone results in a shift of the redox potential by 250 to 280 mV in the negative direction [13]. In the case of QN, the substitution of an alkylamino group for a chlorine atom leads to a redox potential shift by 190 mV. It should be noted that the diethylamino group in QN2 results in a smaller potential shift - by only 90 mV. A similar effect is known for p-benzoquinone amino derivatives [14] and can be explained by a decrease of the nitrogen electron-donor effect resulting from the fact that the secondary amino groups are not coplanar with the quinonoid ring for steric reasons [15]. Reduction of QN, and QN2 in aqueous solution occurs at the same potentials found for ethanolic + water solution. The electrochemical process for both quinones is reversible at the Hg
58
Fig. 3. Sampled dc polarograms of 5 X10e4 M quinone solutions in 10 mM Tns+HCl pH 8.0 ethanol+water 9: 1 buffer: (a) Q; (b) QN2; (c) QN,; (d) background solution. Potential scan rate, 5 mV/s; drop time, 1 s. Fig. 4. Cyclic voltammograms of 1 X10e4 M quinone solutions in 10 mM Tris+HCl pH 8.0 water buffer registered by using the hanging mercury drop electrode, o = 170 mV/s (A) and the bare PC electrode, u = 40 mV/s (B): (a) QN,; (b) QN2.
electrode (Fig. 4A) and corresponds to a 2 e- + 2 H+ reduction characteristic of aqueous quinone solutions [16]. However, the reduction of aminonaphthoquinones at the unmodified Pt electrode differs considerably in reversibility (Fig. 4B). Whereas quasi-reversible reduction is observed for QN, (peak-to-peak separation AE is 40 mV at a scan rate, u, of 40 mV/s), QN2 reduction is electrochemically irreversible (AE = 220 mv). The differences in the potentials of the cathodic and anodic peaks are decreased and increased, respectively, as a result of the different electrochemical reversibility of QNI and QN2 at the bare Pt electrode. As has been shown by Hubbard and co-workers [17,18] ‘the structure of a quinone molecule affects the orientation of its ring when it is adsorbed at the surface of the Pt electrode. This results in various degrees of electrochemical reversibility of quinones with different structures. Probably it accounts for the difference in electrochemical reversibility of QN, and QN,.
59
Differences in the electrochemical properties of the modeling quinones QN, and QN2 at the bare Pt electrode allow the conclusion that in the case of the quinonoid modified electrode the immobilized quinone is bound only via the primary amino group of en-silane. The voltammetric curves obtained for the electrode modified covalently by naphthoquinone bound to it via the en-silane amino group and for the bare Pt electrode in QN, solution, are very similar (Fig. 1, curve b and Fig. 4B, curve a). The cyclic voltammetric curve obtained for the quinonoid modified electrode in the presence of QN2 in the solution has one cathodic and two anodic peaks (Fig. 1, curve c). Since the electrochemical process of QN2 at the Pt electrode is irreversible, its cathodic peak is shifted in the negative direction, approaching the cathodic peak of the quinone immobilized at the electrode. Due to this, the reversible reduction of the immobilized quinone as well as the irreversible reduction of QN2 occur at the same potential and are recorded as a single cathodic peak. The reduction of QN2 at the Pt electrode is likely to be both direct and mediated by the immobilized quinone. However, oxidation of the reduced forms of the immobilized quinone and QN2 occurs at considerably different potentials, resulting in two peaks on the anodic part of the voltammetric curve and in unequal values of the cathodic and anodic peaks corresponding to the reversible redox process. This voltammetric curve models the electrochemical properties which the chemically modified electrode would have if the quinone bond with en-silane were via both primary and secondary amino groups. Since in reality for the chemically modified electrode, only one pair of coupled cathodic and anodic peaks, equal in height and corresponding to a quasi-reversible redox process, is observed, one can exclude participation of the en-silane secondary amino group in quinone binding and confirm the structure of the quinonoid modified electrode shown above. The result obtained is consistent with the suggestion of a non-active state of the secondary amino group of the immobilized en-silane due to the formation of a hydrogen bond with a silanol group [19]. Thus, this considerably simpler method of electrode modification results in a structure analogous to that of the quinonoid modified electrode described in ref. 8, excluding a complicated synthesis of silanoquinone. Electrochemical kinetics of the redox process for the immobilized aminonaphthoquinone Since the redox process for the quinone is accompanied by protonation, the effect of pH on the voltammetric curve of the quinonoid modified electrode was studied. The theoretically expected value for the slope of an E O-pH plot is 59 mV for a 2 e- + 2 H+ system, e.g., quinone in aqueous solution [16]. However, at low potential scan rates within the pH range from 2.0 to 9.5 this slope for the covalently immobilized quinone was slightly smaller (aE O/apH = 50 mV at u = 10 mV/s, Fig. 5). This result, as well as a further decrease of the 3E “/apH value with increasing potential scan rate (Fig. 5, inset), can be accounted for by the existence of quinone and semiquinone in various states of protonation contributing differently to the electrochemical kinetics [20]. The surface concentration of the immobilized quinone was measured by integration of the cyclic voltammetric.peaks at pH 9.2 (Fig. 6, curve a) taking into account
60
E"lV -0 5-
-04.
-03.
-0 2.
-01. I 2
. 3
4
5
6
18
9 PH
-0
6
Fig. 5. pH dependence of the formal potential for the covalently Inset shows the variation of &P/apH with scan rate.
-04
-0.2
0 EIV
immobilized
Fig. 6. Cyclic voltammograms of the quinonoid modified electrode rinsed Britton-Robbinson buffer background: (a) pH 9.2, (b) pH 2.0; u = 50 mV/s.
quinone;
with
u =lO mV/s.
ethanol
against
that the 2 e- + 2 H+ reduction led to the formation of hydroquinone. This value is ca. 2 X lOPro mol/cm* and corresponds approximately to a monolayer coverage [LX]. However, with decreasing pH the peaks on the voltammetric curve decrease dramatically and at pH 2.0 the surface concentration of the electrochemically active quinone is only ca. 1 X 10-l’ mol/cm* (Fig. 6, curve b). With increasing pH the peaks increase to their initial height. The decrease in the amount of electrochemically active quinone in acid medium was also observed in other work [2]. For a multilayer redox coating it was attributed to the fact that only the monolayer adjacent to the electrode participated in the redox process in the acid medium [2]. For the electrode modified by a quinone monolayer the decrease in the amount of electrochemically active quinone in acid media cannot be interpreted simply. To study the electrochemical kinetics of the redox process of the covalently immobilized quinone, voltammetric curves were obtained at various potential scan rates (Fig. 7). The peak currents, I, and I,, varied linearly with scan rate at low scan rates (u G 50 mV/s) but deviated from linearity at higher rates (Fig. 8) corresponding to the properties of the immobilized redox groups [2]. The width of the current peak at half height is 90 mV at u = 50 mV/s, which exceeds the theoretical value of 90.6/n mV (n = 2) expected for an ideal reversible process [21]. The difference between the peak potential of the anodic and cathodic peaks, AE,, is
61
60 60
40-
-08
-0 6
-0.4
-
-02
E/V
0
a
.__._/
,O/
-3
-2
-1
0 log(v/V s-7
Fig. 7. Cyclic voltammograms of the quinonoid modified electrode rinsed with ethanol against Britton-Robbinson buffer background, pH 9.2, at different scan rates: (a) 10; (b) 20; (c) 50; (d) 100; (e) 200 mV/s. Fig. 8. Dependences of peak current IP (A) and peak-to-peak for the covalently immobilized quinone (pH 9.2).
separation
AE (B) on potential
scan rate
small and independent of u at low scan rates (AE, = 40 mV at u < 20 mV/s); however it differs from the zero expected for a reversible electron transfer [21]. At higher scan rates AE, is proportional to log u (Fig. 8). The formal potential, E O, is calculated as the average of the anodic and cathodic peak potentials of cyclic voltammograms, (E, + E,)/2, and remains constant at various potential scan rates. The deviations of the cathodic and anodic peaks from E” are the same (aE,/a log u = i3E,/a log u = 0.03 V) which suggests that the anodic and cathodic transfer coefficients (a and j3) are about 0.5. Though the theory does not allow the rate constant K to be determined for this electrochemical process, a very rough estimate of the order of magnitude of the apparent rate constant can be made using the tabulated parameter m for CI= 0.5 [21]: m = (RT/nF)(
K/u)
The rate constant obtained is equal to ca. 1 s-’ and indicative of a slow electron transfer. The order of magnitude is typical for electrodes chemically modified by redox groups, being reduced with protonation [22].
62
Unfortunately, the apparent time of electron transfer between the immobilized quinone and the electrode (r = l/K) is ca. 1 s which exceeds the recombination time of photoinduced separated charges in the reaction centers containing aminonaphthoquinone as a primary quinone (ca. 60 ms [5]). Therefore, it is necessary to increase the reversibility of the redox process of the immobilized quinone for efficient coupling of the photoinduced charge separation with the electron transfer to the quinonoid modified electrode. REFERENCES 1 R.W. Murray in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 13, Marcel Dekker, New York, 1984, p. 191. 2 C. Degrand, Ann. Chim., 75 (1985) 1. 3 F.A. Armstrong, H.A.O. Hill and N.J. Walton, Q. Rev. Biophys., 18 (1985) 261. 4 E.Yu. Katz, A.Ya. Shkuropatov, 0.1. Vagabova and V.A. Shuvalov, Zh. Fiz. Khim. (USSR), 61 (1987) 3009. 5 E.Yu. Katz, A.Ya. Shkuropatov, 0.1. Vagabova and V.A. Shuvalov, Biofizika (USSR), 33 (1988) 66. 6 R. Woods in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 9, Marcel Dekker, New York, 1980, p. 1. 7 J.R. Lenhard and R.W. Murray, J. Electroanal. Chem., 78 (1977) 195. 8 G.S. Calabrese, R.W. Buchanan and MS. Wrighton, J. Am. Chem. Sot., 105 (1983) 5594. 9 N. Buu-Hoi, R. Royer and B. Eckert, Reel. Trav. Chim. Pays-Bas, 71 (1952) 1059. 10 J.M. Bruce in S. Coffey (Ed.), Rodd’s Chemistry of Carbon Compounds, Vol. 3, Part B, Elsevier, Amsterdam, 1974, p. 82. 11 J.R. Lenhard, R. Rocklin, H. Abruna, K. Willnan, K. Kuo, R. Nowak and R.W. Murray, J. Am. Chem. Sot., 100 (1978) 5214. 12 V.A. Koptjug (Ed.), Atlas of Spectra of Aromatic and Heterocyclic Compounds, Vol. 10, Institute of Organic Chemistry of the USSR Academy of Sciences, Novosibirsk, 1976, p. 27 (in Russian). 13 D.J. Currie and H.L. Holmes, Can. J. Chem., 44 (1966) 1027. 14 D.W. Cameron, R.G.F. Giles and M.H. Pay, Tetrahedron Lett., (1970) 2049. 15 T.I. Temnikova, Course of Theoretical Principles of Organic Chemistry, Khimiya, Leningrad, 1968, p. 159 (in Russian). 16 J.Q. Chambers in S. Patai (Ed.), The Chemistry of the Quinonoid Compounds, Interscience, New York, 1974, p. 739. 17 M.P. Soriaga, J.L. Stickney and A.T. Hubbard, J. Electroanal. Chem., 144 (1983) 207. 18 J.H. White, M.P. Soriaga and A.T. Hubbard, J. Electroanal. Chem., 185 (1985) 331. 19 P.R. Moses, L.M. Wier, J.C. Lennox, H.O. Finklea, I.R. Lenhard and R.W. Murray, Anal. Chem., 50 (1978) 576. 20 M.R. Tarasevich, S.N. Suslov and V.A. Bogdanovskaya, Elektrokhimiya, 20 (1984) 1202. 21 E. Laviron, J. Electroanal. Chem., 101 (1979) 19. 22 E. Laviron and L. Roullier, J. Electroanal. Chem., 115 (1980) 65.
J. Electrounul. Chem., 260 (1989) 53-62 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Chemical modification of the PtO electrode by naphthoquinone using aminosilane Eugenii Yu. Katz, Anatolii Ya. Shkuropatov, Shuvalov Institute of Soil Science and Photosynthesrs, 142292 (U.S.S.R.)
Olga I. Vagabova and Vladimir A.
USSR Academy of Sciences, Pushchino, Moscow Region
(Received 23 June 1988; in revised form 3 October 1988)
ABSTRACT A new, simple method is described for the chemical modification of PtO electrodes by a naphthoquinone aminoderivative based on the heterogeneous reaction of naphthoquinone with aminosilane bound to the electrode. Comparison of electrochemical properties of the quinonoid modified electrode and modelling water-soluble aminonaphthoquinone with various degrees of amino group nitrogen substitution, has shown that only the primary silane amino group takes part in the quinone binding to the electrode. A cyclic voltammetric study showed that the surface concentration of the quinone immobilized at the electrode was ca. 10-i’ mol/cm*, corresponding to monolayer density. Quasi-reversible quinone reduction with a rate constant of about 1 s-’ could be maintained over many hundreds of electron transfer cycles.
INTRODUCTION
Electrodes chemically modified by redox groups [l], including quinonoid ones [2], are used in the processes of electrochemical and photoelectrochemical catalysis. In particular, immobilized redox groups can function as an electron transfer mediator between protein redox sites and the electrode [3]. To develop high efficiency photoelectrochemical energy converters on the basis of the reaction centers (RCs) of photosynthetic bacteria, we have studied the functioning of a number of quinones as electron transfer mediators from the RC acceptor site to the electrode [4,5]. These mediators must meet the following requirements simultaneously: they should function in the RC instead of the removed native quinones and should be immobilized easily on the electrodes as a monolayer with electrochemical reversibility. Modification processes and the electrochemical properties of the PtO electrode modified covalently by naphthoquinone are described in the present paper. The reaction, which has not been used for electrode chemical modification previously, allows the quinonoid modified electrode to be obtained with quasi-reversible electrochemical 0022-0728/89/$03.50
0 1989 Blsevier Sequoia S.A.
54
properties by a new, experimentally simple, method. This electrode is of special interest due to the ability of the water-soluble analogue of the immobilized aminonaphthoquinone to function in the RC from Rhodobacter sphaeroides R-26 bacteria as a primary quinone with a recombination time of the photoinduced separated charges similar to the native one [5]. This allows the quinonoid modified electrode to be used for the coupling of the photoinduced charge separation process in the immobilized RCs with an electron transfer to the electrode. EXPERIMENTAL
Electrodes made of Pt wire of ca. 0.15 cm* geometrical area were used for chemical modification. The real area obtained from the electrochemical hydrogen adsorption in 0.5 M H,SO, [6] was 2.2 times larger than the geometrical one. The electrodes with a PtO layer were prepared according to Lenhard and Murray [7] and silanized by refluxing in a ca. 7% (v/v) solution of 3-(2aminoethylamino)propyltrimethoxysilane, en-silane, (Fluka) in very dry toluene for about 4 min. The silanized electrodes were rinsed thoroughly with dry toluene and then with ethanol. The reaction of silanized electrodes with 2,3-dichloro-l&naphthoquinone, Q, (Fluka) was carried out in a saturated solution of Q for ca. 1 h at room temperature. In order to model the electrochemical properties of the immobilized quinone, the following water-soluble aminonaphthoquinones were synthesized: 2-chloro-3-[[2-(dimethylbutylammonium bromide)ethyl]amino]-1,4_naphthoquinone (QN,) obtained according to ref. 8, 2-chloro-3-diethylamino-1,Cnaphthoquinone hydrochloride (QN,) obtained by the reaction of Q with diethylamine in ethanolic solution [9] and isolated by preparative thin-layer chromatography on silica gel Silufol plates (Czechoslovakia) developed in chloroform (R f = 0.45). Elemental analysis (Carlo Erba Strumentazione, Italy) for the synthesized quinones was satisfactory. Anal. Calcd for QN, (C,,H2,N202ClBr): C, 52.00; H, 5.82; N, 6.74%; and for QN2 (C,,H,,N02Cl. HCl): C, 56.02; H, 4.70; N, 4.67%. Found for QN,: C, 52.22; H, 5.84; N, 6.72%; and for QN,: C, 56.23; H, 4.72; N, 4.65%. Electrochemical measurements were performed using a GWP-673 electrochemical analyzer (GDR) in a three-electrode cell with a mercury dropping or Pt pin working electrode, a glassy carbon auxiliary electrode isolated by a frit and a molar calomel reference electrode; all potentials are reported with respect to this reference. Positive CH3
0
I
NH-_(CH212
--+N
-(CH,),-CH,
Br-
* HCI
I C”3 Cl
(QN, 1
(QN,)
55
feedback circuitry was used for 80% ohmic drop compensation during the cyclic voltammetric study. The measurements were carried out at ambient laboratory temperature (22 f 2 o C) in solutions freed of oxygen by bubbling with argon. RESULTS AND DISCUSSION
Naphthoquinone immobilization at the electrode using en&lane Immersion of the silanized PtO electrode into a solution of Q leads to nucleophilic addition of the en-silane amino group to the quinone [lo], and as a result it is immobilized covalently at the electrode as aminoquinone:
Pt
0-Si
\
Q -KH213-NH-
(CH2)2-NH2
-HCI
/
Pt
0-Si
\
-(CH2)3--NH-(CH212-NH
/
Pt
I-/
0
\
0-Si-(CH2)3-NH-_(CH2)2-NH / 11; Cl :QC7
In addition to covalent immobilization of aminoquinone, excess Q and hydroquinone, QH2, formed as a by-product are adsorbed at the modified electrode surface. Two pairs of the coupled cathodic-anodic peaks are observed on the voltammetric curve of the quinonoid modified electrode obtained immediately after its modification in Q solution (Fig. 1, curve a). The cathodic and anodic peaks at more positive potentials differ in peak current value (1, > I,), decrease rapidly on cyclic scanning of the potential and disappear completely during rinsing of the electrode with ethanol. However, they appear again after dipping the modified electrode in ethanolic Q solution. Thus they can be attributed to the redox process of the adsorbed quinone: Qad + 2 e- + 2 Hf P OH,,,
56
The second pair of cathodic and anodic peaks, with equal peak current values (Z, = Z,) and at more negative potentials, is not changed during rinsing of the electrode with ethanol (Fig. 1, curve b) and decreases slowly on cyclic scanning of the potential (by lo-20% for one hundred cycles). Therefore it can be attributed to the redox process of the quinone immobilized at the electrode. For comparison we modified the electrode by a known method [8] resulting in immobilization of the same naphthoquinone bound to the electrode surface via the amine group: CH3
Pt
OH
+
0
+I (CH30)3Si(CH2)3-N-(CH2)2
-NH
I C’43
Cl 0
Pt
I--
\
+i”’
0-Si-(CH2)3-N /
0
-(CH2)2-NH
I CH3 Cl
The electrochemical properties of the quinones covalently immobilized by either method turned out to be practically the same. However, the latter method includes multistep chemical synthesis of silanoquinone [8] and therefore it is considerably more complex than that developed in the present study. It is well known [ll] that there are no significant changes in the redox potential for various substances during their covalent immobilization at electrodes if the redox center of the molecule is not changed. However, the potential shift observed in the present study for the covalently bound quinone by ca. 200 mV in the negative direction compared with the adsorbed parent quinone Q, is accounted for by the electron-donating properties of the amino group inserted into the quinoid ring during its immobilization. Electrochemical properties of modeling aminonaphthoquinones To clarify the nature of quinone binding to the electrode functionalized by en-silane, the electrochemical properties of the parent Q and its amino derivatives QN, and QN2 modeling the Q bond with the primary and secondary en-silane amino groups, have been studied. The absorption spectra obtained for the synthesized modeling quinones (Fig. 2) coincide with those published earlier for QN, [8] and for 2-chloro-3-piperidino-1,4naphthoquinone, which is similar to QN2 [12], and
-0.6
-0 4
-02
0 E/V
J
/cm-
Fig. 1. Cyclic voltammograms of the quinonoid modified electrode after its modification in Q ethanolic solution (a), after rinsing with ethanol (b) obtained in 10 mM Tris+ HCl pH 8.0 water buffer solution &d in the presence of 3 X 10e4 M QN, in the solution (c); u = 40 mV/s. Fig. 2. Absorption spectra of 5 X 10e4 M quinone solutions in 10 m M Tris + HCl pH 8.0 ethanol + water 9: 1 buffer with path length 0.1 cm; (a) Q; (b) QN,; (c) QN2.
have peaks with maxima of about 470 and 500 nm, respectively, resulting in the red color characteristic of aminonaphthoquinones. The polarographic properties of the parent Q and its aminoderivatives QN, and QN2 were studied in ethanolic + water solution (9 : l), since Q is poorly soluble in water (Fig. 3). Polarographic half-wave potentials, E,,2, equal to -0.25, -0.34 and -0.44 V were obtained for Q, QN, and QN,, respectively, at pH 8.0. It is known that substitution of an alkylamino group for a hydrogen atom in naphthoquinone results in a shift of the redox potential by 250 to 280 mV in the negative direction [13]. In the case of QN, the substitution of an alkylamino group for a chlorine atom leads to a redox potential shift by 190 mV. It should be noted that the diethylamino group in QN2 results in a smaller potential shift - by only 90 mV. A similar effect is known for p-benzoquinone amino derivatives [14] and can be explained by a decrease of the nitrogen electron-donor effect resulting from the fact that the secondary amino groups are not coplanar with the quinonoid ring for steric reasons [15]. Reduction of QN, and QN2 in aqueous solution occurs at the same potentials found for ethanolic + water solution. The electrochemical process for both quinones is reversible at the Hg
58
Fig. 3. Sampled dc polarograms of 5 X10e4 M quinone solutions in 10 mM Tns+HCl pH 8.0 ethanol+water 9: 1 buffer: (a) Q; (b) QN2; (c) QN,; (d) background solution. Potential scan rate, 5 mV/s; drop time, 1 s. Fig. 4. Cyclic voltammograms of 1 X10e4 M quinone solutions in 10 mM Tris+HCl pH 8.0 water buffer registered by using the hanging mercury drop electrode, o = 170 mV/s (A) and the bare PC electrode, u = 40 mV/s (B): (a) QN,; (b) QN2.
electrode (Fig. 4A) and corresponds to a 2 e- + 2 H+ reduction characteristic of aqueous quinone solutions [16]. However, the reduction of aminonaphthoquinones at the unmodified Pt electrode differs considerably in reversibility (Fig. 4B). Whereas quasi-reversible reduction is observed for QN, (peak-to-peak separation AE is 40 mV at a scan rate, u, of 40 mV/s), QN2 reduction is electrochemically irreversible (AE = 220 mv). The differences in the potentials of the cathodic and anodic peaks are decreased and increased, respectively, as a result of the different electrochemical reversibility of QNI and QN2 at the bare Pt electrode. As has been shown by Hubbard and co-workers [17,18] ‘the structure of a quinone molecule affects the orientation of its ring when it is adsorbed at the surface of the Pt electrode. This results in various degrees of electrochemical reversibility of quinones with different structures. Probably it accounts for the difference in electrochemical reversibility of QN, and QN,.
59
Differences in the electrochemical properties of the modeling quinones QN, and QN2 at the bare Pt electrode allow the conclusion that in the case of the quinonoid modified electrode the immobilized quinone is bound only via the primary amino group of en-silane. The voltammetric curves obtained for the electrode modified covalently by naphthoquinone bound to it via the en-silane amino group and for the bare Pt electrode in QN, solution, are very similar (Fig. 1, curve b and Fig. 4B, curve a). The cyclic voltammetric curve obtained for the quinonoid modified electrode in the presence of QN2 in the solution has one cathodic and two anodic peaks (Fig. 1, curve c). Since the electrochemical process of QN2 at the Pt electrode is irreversible, its cathodic peak is shifted in the negative direction, approaching the cathodic peak of the quinone immobilized at the electrode. Due to this, the reversible reduction of the immobilized quinone as well as the irreversible reduction of QN2 occur at the same potential and are recorded as a single cathodic peak. The reduction of QN2 at the Pt electrode is likely to be both direct and mediated by the immobilized quinone. However, oxidation of the reduced forms of the immobilized quinone and QN2 occurs at considerably different potentials, resulting in two peaks on the anodic part of the voltammetric curve and in unequal values of the cathodic and anodic peaks corresponding to the reversible redox process. This voltammetric curve models the electrochemical properties which the chemically modified electrode would have if the quinone bond with en-silane were via both primary and secondary amino groups. Since in reality for the chemically modified electrode, only one pair of coupled cathodic and anodic peaks, equal in height and corresponding to a quasi-reversible redox process, is observed, one can exclude participation of the en-silane secondary amino group in quinone binding and confirm the structure of the quinonoid modified electrode shown above. The result obtained is consistent with the suggestion of a non-active state of the secondary amino group of the immobilized en-silane due to the formation of a hydrogen bond with a silanol group [19]. Thus, this considerably simpler method of electrode modification results in a structure analogous to that of the quinonoid modified electrode described in ref. 8, excluding a complicated synthesis of silanoquinone. Electrochemical kinetics of the redox process for the immobilized aminonaphthoquinone Since the redox process for the quinone is accompanied by protonation, the effect of pH on the voltammetric curve of the quinonoid modified electrode was studied. The theoretically expected value for the slope of an E O-pH plot is 59 mV for a 2 e- + 2 H+ system, e.g., quinone in aqueous solution [16]. However, at low potential scan rates within the pH range from 2.0 to 9.5 this slope for the covalently immobilized quinone was slightly smaller (aE O/apH = 50 mV at u = 10 mV/s, Fig. 5). This result, as well as a further decrease of the 3E “/apH value with increasing potential scan rate (Fig. 5, inset), can be accounted for by the existence of quinone and semiquinone in various states of protonation contributing differently to the electrochemical kinetics [20]. The surface concentration of the immobilized quinone was measured by integration of the cyclic voltammetric.peaks at pH 9.2 (Fig. 6, curve a) taking into account
60
E"lV -0 5-
-04.
-03.
-0 2.
-01. I 2
. 3
4
5
6
18
9 PH
-0
6
Fig. 5. pH dependence of the formal potential for the covalently Inset shows the variation of &P/apH with scan rate.
-04
-0.2
0 EIV
immobilized
Fig. 6. Cyclic voltammograms of the quinonoid modified electrode rinsed Britton-Robbinson buffer background: (a) pH 9.2, (b) pH 2.0; u = 50 mV/s.
quinone;
with
u =lO mV/s.
ethanol
against
that the 2 e- + 2 H+ reduction led to the formation of hydroquinone. This value is ca. 2 X lOPro mol/cm* and corresponds approximately to a monolayer coverage [LX]. However, with decreasing pH the peaks on the voltammetric curve decrease dramatically and at pH 2.0 the surface concentration of the electrochemically active quinone is only ca. 1 X 10-l’ mol/cm* (Fig. 6, curve b). With increasing pH the peaks increase to their initial height. The decrease in the amount of electrochemically active quinone in acid medium was also observed in other work [2]. For a multilayer redox coating it was attributed to the fact that only the monolayer adjacent to the electrode participated in the redox process in the acid medium [2]. For the electrode modified by a quinone monolayer the decrease in the amount of electrochemically active quinone in acid media cannot be interpreted simply. To study the electrochemical kinetics of the redox process of the covalently immobilized quinone, voltammetric curves were obtained at various potential scan rates (Fig. 7). The peak currents, I, and I,, varied linearly with scan rate at low scan rates (u G 50 mV/s) but deviated from linearity at higher rates (Fig. 8) corresponding to the properties of the immobilized redox groups [2]. The width of the current peak at half height is 90 mV at u = 50 mV/s, which exceeds the theoretical value of 90.6/n mV (n = 2) expected for an ideal reversible process [21]. The difference between the peak potential of the anodic and cathodic peaks, AE,, is
61
60 60
40-
-08
-0 6
-0.4
-
-02
E/V
0
a
.__._/
,O/
-3
-2
-1
0 log(v/V s-7
Fig. 7. Cyclic voltammograms of the quinonoid modified electrode rinsed with ethanol against Britton-Robbinson buffer background, pH 9.2, at different scan rates: (a) 10; (b) 20; (c) 50; (d) 100; (e) 200 mV/s. Fig. 8. Dependences of peak current IP (A) and peak-to-peak for the covalently immobilized quinone (pH 9.2).
separation
AE (B) on potential
scan rate
small and independent of u at low scan rates (AE, = 40 mV at u < 20 mV/s); however it differs from the zero expected for a reversible electron transfer [21]. At higher scan rates AE, is proportional to log u (Fig. 8). The formal potential, E O, is calculated as the average of the anodic and cathodic peak potentials of cyclic voltammograms, (E, + E,)/2, and remains constant at various potential scan rates. The deviations of the cathodic and anodic peaks from E” are the same (aE,/a log u = i3E,/a log u = 0.03 V) which suggests that the anodic and cathodic transfer coefficients (a and j3) are about 0.5. Though the theory does not allow the rate constant K to be determined for this electrochemical process, a very rough estimate of the order of magnitude of the apparent rate constant can be made using the tabulated parameter m for CI= 0.5 [21]: m = (RT/nF)(
K/u)
The rate constant obtained is equal to ca. 1 s-’ and indicative of a slow electron transfer. The order of magnitude is typical for electrodes chemically modified by redox groups, being reduced with protonation [22].
62
Unfortunately, the apparent time of electron transfer between the immobilized quinone and the electrode (r = l/K) is ca. 1 s which exceeds the recombination time of photoinduced separated charges in the reaction centers containing aminonaphthoquinone as a primary quinone (ca. 60 ms [5]). Therefore, it is necessary to increase the reversibility of the redox process of the immobilized quinone for efficient coupling of the photoinduced charge separation with the electron transfer to the quinonoid modified electrode. REFERENCES 1 R.W. Murray in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 13, Marcel Dekker, New York, 1984, p. 191. 2 C. Degrand, Ann. Chim., 75 (1985) 1. 3 F.A. Armstrong, H.A.O. Hill and N.J. Walton, Q. Rev. Biophys., 18 (1985) 261. 4 E.Yu. Katz, A.Ya. Shkuropatov, 0.1. Vagabova and V.A. Shuvalov, Zh. Fiz. Khim. (USSR), 61 (1987) 3009. 5 E.Yu. Katz, A.Ya. Shkuropatov, 0.1. Vagabova and V.A. Shuvalov, Biofizika (USSR), 33 (1988) 66. 6 R. Woods in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 9, Marcel Dekker, New York, 1980, p. 1. 7 J.R. Lenhard and R.W. Murray, J. Electroanal. Chem., 78 (1977) 195. 8 G.S. Calabrese, R.W. Buchanan and MS. Wrighton, J. Am. Chem. Sot., 105 (1983) 5594. 9 N. Buu-Hoi, R. Royer and B. Eckert, Reel. Trav. Chim. Pays-Bas, 71 (1952) 1059. 10 J.M. Bruce in S. Coffey (Ed.), Rodd’s Chemistry of Carbon Compounds, Vol. 3, Part B, Elsevier, Amsterdam, 1974, p. 82. 11 J.R. Lenhard, R. Rocklin, H. Abruna, K. Willnan, K. Kuo, R. Nowak and R.W. Murray, J. Am. Chem. Sot., 100 (1978) 5214. 12 V.A. Koptjug (Ed.), Atlas of Spectra of Aromatic and Heterocyclic Compounds, Vol. 10, Institute of Organic Chemistry of the USSR Academy of Sciences, Novosibirsk, 1976, p. 27 (in Russian). 13 D.J. Currie and H.L. Holmes, Can. J. Chem., 44 (1966) 1027. 14 D.W. Cameron, R.G.F. Giles and M.H. Pay, Tetrahedron Lett., (1970) 2049. 15 T.I. Temnikova, Course of Theoretical Principles of Organic Chemistry, Khimiya, Leningrad, 1968, p. 159 (in Russian). 16 J.Q. Chambers in S. Patai (Ed.), The Chemistry of the Quinonoid Compounds, Interscience, New York, 1974, p. 739. 17 M.P. Soriaga, J.L. Stickney and A.T. Hubbard, J. Electroanal. Chem., 144 (1983) 207. 18 J.H. White, M.P. Soriaga and A.T. Hubbard, J. Electroanal. Chem., 185 (1985) 331. 19 P.R. Moses, L.M. Wier, J.C. Lennox, H.O. Finklea, I.R. Lenhard and R.W. Murray, Anal. Chem., 50 (1978) 576. 20 M.R. Tarasevich, S.N. Suslov and V.A. Bogdanovskaya, Elektrokhimiya, 20 (1984) 1202. 21 E. Laviron, J. Electroanal. Chem., 101 (1979) 19. 22 E. Laviron and L. Roullier, J. Electroanal. Chem., 115 (1980) 65.