Investigation of the oxidation of hydroquinone at the liquid/liquid interface

Article abstract of DOI:10.1016/j.cclet.2009.09.004

The oxidation of hydroquinone (QH₂) was investigated for the first time at liquid/liquid (L/L) interface by scanning electrochemical microscopy (SECM). In this study, electron transfer (ET) from QH₂ in aqueous to ferrocene (Fc) in nitrobenzene (NB) was probed. The apparent heterogeneous rate constants for ET reactions were obtained by fitting the experimental approach curves to the theoretical values. The results showed that the rate constants for oxidation reaction of QH₂ were sensitive to the changes of the driving force, which increased as the driving force increased. In addition, factors that would affect ET of QH₂ were studied. Experimental results indicated ion situation around QH₂ molecule could change the magnitude of the rate constants because the capability of oxidation of QH₂ would be affected by them.

Full text of DOI:10.1016/j.cclet.2009.09.004

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 225–228
www.elsevier.com/locate/cclet
Investigation of the oxidation of hydroquinone
at the liquid/liquid interface
*
Xiao Quan Lu , De Fang Dong, Xiu Hui Liu, Dong Na Yao,
Wen Ting Wang, Yu Mei Xu
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
Received 18 June 2009
Abstract
The oxidation of hydroquinone (QH
2
) was investigated for the first time at liquid/liquid (L/L) interface by scanning
in aqueous to ferrocene (Fc) in nitrobenzene
electrochemical microscopy (SECM). In this study, electron transfer (ET) from QH
NB) was probed. The apparent heterogeneous rate constants for ET reactions were obtained by fitting the experimental approach
curves to the theoretical values. The results showed that the rate constants for oxidation reaction of QH were sensitive to the
changes of the driving force, which increased as the driving force increased. In addition, factors that would affect ET of QH were
2
(
2
2
studied. Experimental results indicated ion situation around QH molecule could change the magnitude of the rate constants
2
because the capability of oxidation of QH would be affected by them.
2
#
2009 Xiao Quan Lu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: SECM; Electron transfer; Liquid/liquid interface; Hydroquinone
Quinones are very important compounds that are involved in a variety of biological energy metabolism [1]. They
are acting as a shuttle for electrons to move one place to another in photosynthesis or respiration, resulting an electron
transport process feasible [2,3]. As we all know, QH is one of the most important species in quinones, which is
2
involved in many metabolic processes where it acts as an electron donor. Because the reaction often occurs in
biomembranes or membrane surfaces, choosing the L/L interface to study the redox reaction and assess its specific
function or capability in biological electron transport processes is clearly the approach of choice. Furthermore, the L/L
interface has been suggested as a simple model for biological and artificial membranes to understand charge transfer
processes in biological systems, which include not only ion transfer but also ET. ET at the L/L interface is
fundamentally important for understanding the energy conversion in biomembranes [4,5], and the kinetics of ET at
immiscible electrolyte solutions (ITIES) could be probed directly by SECM. In the present report, the electrochemical
oxidation of QH has been studied by cyclic voltammetry and differential pulse voltammetry at glassy carbon
2
electrodes and platinum electrodes. However, up to now, there is little report about ET of QH at ITIES measured by
2
SECM. In this paper, ET process of QH was observed at the L/L interface by SECM.
2
*
Corresponding author.
E-mail address: luxq@nwnu.edu.cn (X.Q. Lu).
1001-8417/$ – see front matter # 2009 Xiao Quan Lu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.09.004

226
X.Q. Lu et al. / Chinese Chemical Letters 21 (2010) 225–228
SECM, a technique which has been invented and extensively applied by Bard and co-workers [4–9] is a powerful
tool for the characterization of EToccurring at the ITIES via a bimolecular reaction between redox species confined to
the two different solutions. The reactions both at the tip and at the ITIES can be summarized as follows:
þ
ꢀ
Fc ðoÞ þ e ! FcðoÞðtipÞ
(1)
(2)
þ
ꢁꢀ
þ
Fc ðoÞ þ QH2ðwÞ ! FcðoÞ þ Q ðwÞ þ 2H ðwÞðITIESÞ
ꢁ
S
where Q is the free radical of hydroquinone. In the electrochemical systems, QH has been known to undergo a
2
ꢀ
totally irreversible 2e oxidation at electrodes [10,11]. However, in our work, the ITIES is assumed to act as an
ꢁ
S
electrode and Q produced at the interface is relatively unreactive and decays by disproportion to QH /QHꢀ in
2
aqueous solution. So only the first one electron procedure played mainly role in the interface reaction, and the second
irreversible oxidation could not act as the dominating step in ET at ITIES.
The experimental system for investigation of ET at the interface can be represented as follows:
Ag/AgCl/10 mmol/L phosphate buffer (pH 7.0) 0.5 mL, 0.1 mol/L LiCl, x mmol/L NaClO , y mmol/L QH /NB
4
2
0
.5 mL, 10 mmol/L TBAClO , 1 mmol/L Fc/tip.
4
The SECM feedback mode has been well established and is applicable to the study of interfacial redox reactions. As
shown in Fig. 1, the approach curves (solid squares are experimental points) at several concentrations appeared as
+
different tip currents. When the concentration of QH in aqueous phase is higher, the reaction between Fc and QH
2
2
carried out at the interface and as the electrode approached the interface (where d = 0) the current had a sharp
enhancement, which showed a positive feedback. In Fig. 1 are feedback curves of the reacted system at interface with
various K (K = C /C ); K = 15, 10, 5, 2 and 1, which were all at the interface dynamic controlled region. The
r
r
W
O
r
ꢀ1
obtained k values within the range 0.0009–0.08 cms from a diffusion coefficient of Fc in nitrobenzene (NB) of
f
ꢀ
6
2
ꢀ1
5
.9 ꢂ 10 cm s gotten by steady-state voltammetry. In our research system, the diffusion coefficient of QH in
2
ꢀ ꢀ1
5
2
water obtained by steady-state voltammetry was 1.2 ꢂ 10 cm s . The experimental results demonstrated that the
k values depended on the potential drop across the ITIES, which increased with an increase of the concentration of
f
QH . Because the interface reaction is a bimolecular reaction and bimolecular reaction rate is proportional to the bulk
2
concentration of redox reactant in two phases, under constant composition model, as the concentration of reactant in
two phases increased, bimolecular reaction rate increased.
Ion transfer is often happening during the biological electron transport process. In our study, the concentration of
common ion was changed to stimulate the biological system and investigate whether the conditions of ion would
influence ET. In SECM measurements, the potential across the interface is adjusted by varying the concentration of
ꢀ
common ion (ClO4 ) in two liquid phases, which would provide a controllable driving force for ET reactions. And the
2
Fig. 1. Experimental SECM approach curves for the heterogeneous ET reaction between QH and tip generated organic phase reactants at the NB/
water interface for various reactant concentration ratios K
r
= 15, 10, 5, 2, 1 (from top to bottom). The supporting electrolyte in NB was 10 mmol/L
4
+ 0.1 mol/L LiCl. Solid lines show the simulated curves.
TBAClO and in aqueous phase were 0.1 mol/L NaClO
4

X.Q. Lu et al. / Chinese Chemical Letters 21 (2010) 225–228
227
4ꢀ
4
Fig. 2. Effects of various concentrations of common ion (ClO ) on the shape of SECM current-distance curves. The concentrations of NaClO
in
water was 0.0001 mol/L, 0.001 mol/L, 0.01 mol/L, 0.1 mol/L, and 1 mol/L (from top to bottom) with scanning rate of 1 mm/s. Solid lines represent
the theoretic values of the following k (cm/s): (1) 0.06, 0.02, 0.005, 0.001, 0.0004. The reactant concentration ratio K = 2.
f
r
ET rate constant is governed by the ratio of the common ion concentration in both of the aqueous phase and the organic
phase when the overpotentials are low. The relationship can be expressed as follows:
log k12 ¼ const ꢀ alog½CClO ꢀꢃW
(3)
4
ꢀ
ꢀ
4
where ½CClO ꢃW is the concentration of ClO in water, a is electron-transfer coefficient. The equation is obtained
4
ꢀ
from a series of equations (1)–(3) in Ref [7]. In our research system, ½C
ꢃW was changed when other conditions
ClO
4
ꢀ
unchanged. The corresponding approach curves of the system were given in Fig. 2. We found an increase in ClO of
4
water resulted in a decrease in the potential drop for ETand a decrease in rate constant of ET. ATafel type plot, that is,
ꢀ
log k vs log ½CClO ꢃW showed a good linear relationship. The transfer coefficient obtained from the slope of the line
f
4
was 0.56. According to the experimental results, we proposed that the various concentrations of common ion have
effect on the ET rate. In some situation, the low concentrations of common ion have to use to coordinate to ET, which
would result in a change of the fast ET. In a word, the biological membrane electron transport could be affected by the
around ion situation. Hence, the more suitable environment was chosen out, and the better QH could work.
2
In conclusion, the oxidation of QH was investigated at the L/L interface by SECM. To our knowledge, it is the first
2
time to study ET between QH and Fc at a simulated biological membrane. From the results of our study, we can see
2
that the only variation of the driving force is the potential drop across the interface, which can be precisely controlled
externally. Moreover, as the potential drop was increased, both the driving force and the rate constants increased.
Besides, we also found the ion conditions around QH could affect the value of the rate constants through affecting the
2
oxidation of QH . Thus, a comparatively more beneficial circumstance should be offered to keep the QH working
2
2
well.
. Experimental
A three-electrode configuration was employed with a 12.5-mm radius Pt ultramicroelectrode (UME), a Pt wire as
1
the counter electrode, and an Ag/AgCl reference electrode. The corresponding preparation and further treatment of Pt
UME were described previously [12–14] and RG = r /a at the range 5–6. All electrochemical measurements were
g
performed using a CHI 900 system (CH Instrument, USA). Experiments were operated at room temperature.
The working electrode-UME was placed in the upper phase (NB solution) throughout all the measurements, and the
feedback model was applied to record the electrical signals at the tip from the redox species at the interface. Aqueous
solutions with various concentrations were prepared from distilled water with LiCl and NaClO as supporting
4
electrolytes, while organic solutions were prepared from NB and with TBAClO as supporting electrolyte (Table 1).
4

228
X.Q. Lu et al. / Chinese Chemical Letters 21 (2010) 225–228
Table 1
Parameters obtained for the reacted systems at ITIES.
a
f
b
c
f
NB
d
e
ꢀ1
)
Reactant in H
2
O
E
ðmVÞ
Reactant in NB
E
ðmVÞ
Overall driving force (mV)
419
k
f
(cm s
H2O
QH
2
131
Fc
550
0.001
a
Supporting electrolyte: 0.1 mol/L NaClO
4
+ 0.1 mol/L LiCl.
b
Formal potential of the reactant redox couple in the H
O phase.
Supporting electrolyte: 10 mmol/L TBAClO
2
O phase vs. a saturated Ag/AgCl electrode as measured by cyclic voltammetry using Pt
UME in the H
c
2
4
.
d
2
Apparent formal potential of the reactant redox couple in the NB phase vs. a saturated Ag/AgCl electrode in the H O phase containing the
indicated supporting electrolyte as measured by cyclic voltammetry.
e
f
NB
f
The overall driving force of the reaction: ðE ꢀ E Þ:
H2O
Acknowledgments
This work was supported by the Natural Science Foundation of China (No. 20775060, No. 20875077 and No.
0927004), the Natural Science Foundation of Gansu (No. 0701RJZA109 and No. 0803RJZA105) and Key Projects of
Scientific Research Base of Department of Education, Gansu Province (No. 08zx-07).
2
References
[
[
[
[
[
[
[
[
[
1] R. Bentley, I.M. Campbell, in: S. Patai (Ed.), The Chemistry of Quinonoid Compounds, Wiley, New York, 1974, p. 715.
2] M.D. Porter, T.B. Bright, D. Allara, et al. J. Am. Chem. Soc. 109 (1987) 3559.
3] J.C. Love, L.A. Estroff, J.K. Kriebel, et al. Chem. Rev. 105 (2005) 1103.
4] P. Sun, F. Li, Y. Chen, et al. J. Am. Chem. Soc. 125 (2003) 9600.
5] A.L. Barker, P.R. Unwin, S. Amemiya, et al. J. Phys. Chem. B 103 (1999) 7260.
6] Y. Zu, F. Fan, A.J. Bard, J. Phys. Chem. B 103 (1999) 6272.
7] Z.F. Ding, B.M. Quinn, A.J. Bard, J. Phys. Chem. B 105 (2001) 6367.
8] B. Liu, M.V. Mirkin, J. Am. Chem. Soc. 121 (1999) 8352.
9] Z. Zhang, Y. Yuan, P. Sun, et al. J. Phys. Chem. B 106 (2002) 6713.
[
[
[
[
[
10] B.R. Eggins, Chem. Commun. (1969) 1267.
11] B.R. Eggins, Chem. Commun. (1969) 232.
12] A.J. Bard, F.R.F. Fan, J. Kwak, et al. Anal. Chem. 61 (1989) 1794.
13] D.O. Wipf, A.J.J. Bard, Electrochem. Soc. 138 (1991) 469.
14] X.Q. Lu, H.R. Zhang, L.N. Hu, et al. Electrochem. Commun. 8 (2006) 1027.