A novel dinuclear Schiff-base copper(II) complex modified electrode for ascorbic acid catalytic oxidation and determination

Article abstract of DOI:10.1039/c1dt11370d

A new dinuclear copper salicylaldehyde-glycine Schiff-base complex [Cu ₂(Sal-Gly)₂(H₂O)₂] was synthesized and structurally characterized. [Cu₂(Sal-Gly)₂(H ₂O)₂] crysta

Full text of DOI:10.1039/c1dt11370d

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PAPER
A novel dinuclear Schiff-base copper(II) complex modified electrode for
ascorbic acid catalytic oxidation and determination†
a
a,b
c
d
a,b
a
a
Zhijun Zhang, Xi Li,* Chenggang Wang, Chaocan Zhang,* Peng Liu, Tingting Fang, Yan Xiong and
Wenjing Xu
a
Received 21st July 2011, Accepted 14th October 2011
DOI: 10.1039/c1dt11370d
A new dinuclear copper salicylaldehyde-glycine Schiff-base complex [Cu
synthesized and structurally characterized. [Cu (Sal-Gly) (H O) ] crystallized in the monoclinic system
in the P2 /c space group. The molecule is a dinuclear complex, formed by two [Cu(Sal-Gly)(H O)]
2
(Sal-Gly)
2
(H
2
O)
2
] was
2
2
2
2
1
2
units. The electropolymerization properties of the copper complex on a glass carbon electrode were
studied at different potential ranges. The electropolymerization occurred when the high scan potential
reached 1.4 V. The modified electrode exhibited good electrocatalytic oxidation properties to ascorbic
-
1
2
acid and showed a sensitivity of 22.9 nA mM (r = 0.9998) and detection limit of 0.39 mM (S/N = 3) in
the amperometric determination of ascorbic acid. The designed determination method can be used to
analyze vitamin C tablets.
Introduction
The electrocatalytic oxidation of ascorbic acid by the modified
metal complexes in aqueous solution is considered to be via
Ascorbic acid (vitamin C, H A) is one of the most important water-
soluble small-molecular-weight antioxidants and free-radical scav-
engers that is used in a large scale in food, beverages, cosmetic
and pharmaceutical formulations applications. Moreover, H
has been recognized as a neuromodulator for dopamine (DA)
and glutamate involved in physiological processes. Due to
the important applications and biofunctions, fast and accurate
determination of H
2
an electron-transfer mechanism. In most cases, the predominant
-
reactive species is the ascorbate ion HA which is oxidized by
electron transfer followed by deprotonation to give the ascorbyl
1
2
A
radical (A ). Then A is rapidly converted to dehydroascorbic
acid (dA).
∑-
∑-
10
2
Transition-metal salicylaldehyde Schiff-base complexes have
been broadly studied and showed a wide variety of properties,
11
2
A is required. Various methods have been
such as DNA binding and cleavage activities, antimicrobial and
3
employed for the analysis of ascorbic acid, including titration,
12
13
14
antitumor activities, antiradical activity, organic catalysts,
4
5
6
HPLC, UV, fluorimetry, etc. Because of the good analytical
15
and also a catalyst for H
2
A oxidation. Moreover, transition-
properties, electroanalytical techniques have also been frequently
metal salicylaldehyde Schiff-base complexes were reported to show
good electropolymerization properties on electrode surfaces to
7
used for H
2
A determination. However, a high overpotential
emerged, as well as a low detection current, for direct electroox-
idation of ascorbic acid at bare electrodes. This is because the
oxidative product adsorbs on the electrode surface and results
in electrode fouling. Therefore modified electrodes, especially
transition-metal complex modified electrodes, have largely been
used as electron mediators for ascorbic acid catalytic oxidation and
16
form a conductive film. Thus they are potential complexes that
can be used to modify electrodes for electrocatalytic oxidation and
detection of H A.
2
8
In the present work, we synthesized a new dinuclear copper
salicylaldehyde-glycine Schiff-base complex. Its electropolymer-
ization on a glass carbon electrode (GC) and the electrocat-
9
determination. The modification can lower the overpotential and
give a higher response current to improve the detection sensitivity.
alytic activity of the modified GC to the oxidation of H
2
A
were studied. Then the determination of H A with the modi-
2
fied GC by an amperometric method was performed. Finally
the proposed method was applied to analysis of vitamin C
tablets.
a
Department of Chemistry, School of Science, Wuhan University of Technol-
ogy, Wuhan, 430070, PR China
b
Institute of High Pressure and Temperature Physics, School of Science,
Wuhan University of Technology, Wuhan, 430070, PR China
c
College of Chemistry, Central China Normal University, Wuhan, 430079,
Results and discussion
PR China
d
School of Materials Science and Engineering, Wuhan University of
X-Ray crystal structure determination
Technology, Wuhan, 430070, PR China
†
Electronic supplementary information (ESI) available. CCDC reference
Details of the crystal parameters, data collection and refinements
number 833606. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1dt11370d
are listed in Table 1. [Cu
2
(Sal-Gly)
2
(H
2
2
O) ] was found to crystallize
1
252 | Dalton Trans., 2012, 41, 1252–1258
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◦
Table 1 Crystal data and structure refinement for [Cu
2
(Sal-Gly)
2
(H
2
O)
2
]
Table 2 Selected bond distances ( A˚ ) and angles ( ) for [Cu
Gly) (H O)
2
(Sal-
2
2
2
]
Empirical formula C H26Cu N O
24 2 2 12
M
r
661.55
298(2)
Cu1–N1
Cu1–O2
Cu1–O6
1.925(2)
1.9459(19)
2.544(3)
Cu1–O1
Cu1–O1¢
Cu1 ◊ ◊ ◊ Cu1¢
1.9556(19)
1.9911(18)
3.0141(7)
T/K
Crystal system
Monoclinic
P2 /c
Space group
1
a/A˚
13.3741(19)
16.9121(10)
14.204(2)
104.985(2)
1268.4(3)
2
1.732
1.747
676
2.97–28.22
-17 to 15, - 9 to 7, -17 to 18
3110/2324
0.0708
2324/3/188
0.956
N1–Cu1–O2
N1–Cu1–O1
O2–Cu1–O1
N1–Cu1–O1¢
O2–Cu1–O1¢
O2–Cu1–Cu1¢
O1–Cu1–Cu1¢
84.03(9)
93.64(9)
173.86(9)
173.69(9)
102.09(8)
141.58(6)
40.65(5)
O1–Cu1–O1¢
N1–Cu1–O6
O2–Cu1–O6
O1–Cu1–O6
N1–Cu1–Cu1¢
O1¢–Cu1–Cu1¢
O6–Cu1–Cu1¢
80.42(8)
87.93(9)
90.26(9)
83.97(8)
134.25(7)
39.77(5)
88.43(6)
b/A˚
c/A˚
◦
b/
V/A˚
3
Z
-3
D
c
/g cm
-1
m/mm
F(000)
◦
q range for data collection/
Limiting indices, hkl
◦
Table 3 Overview of hydrogen bond lengths (A) and angles ( )
˚
Reflections collected/unique
R
int
D–H ◊ ◊ ◊ A
d(D–H) d(H ◊ ◊ ◊ A) d(D ◊ ◊ ◊ A) ∠D–H ◊ ◊ ◊ A
Data/restraints/parameters
2
Goodness-of-fit on F
O6–H6A ◊ ◊ ◊ O3 (i)
O6–H6B ◊ ◊ ◊ O3¢(ii) 0.82(2)
0.82(3)
2.30(4)
1.99(3)
3.005(3)
2.784(3)
146(4)
162(3)
a
Final R indices [I > 2s(I)]
R
1
= 0.0651, wR
2
2
= 0.0924
= 0.0854
a
R indices (all data)
R
1
= 0.0461, wR
Drmax, min/e A˚
-3
0.560, -0.339
Symmetry code: i = 1 - x, -1/2 + y, 3/2 - z; ii = x, 3/2 - y, -1/2 + z.
ꢀ
ꢀ
ꢀ/ |F
2
= [ꢀ
) ]/ ) ]]
ꢀ[w(F
a
2
2
2
2
2
1/2
R
1
=
ꢀF
o
| - |F
c
o
|, wR
2
[w(F
o
- F
c
o
2
2
where w = q/s (F
o
) + (aP) + bP.
in the monoclinic system in the P2
is a dinuclear complex, formed by two [Cu(Sal-Gly)(H
1
/c space group. The molecule
O)] units.
2
The perspective view along the b axis of the structure is shown in
Fig. 1(A) and selected bond lengths and angles are summarized
II
in Table 2. The Cu ion is five-coordinated in a square-pyramidal
configuration, with one imine N atom (N1), one phenolate O atom
(
O1), one carboxylate O atom (O2) of the Schiff-base and one
phenolate O atom (O1¢) of the other Schiff-base moiety defining
the basal plane, and the O atom (O6) of a coordinated water
molecule occupying the apical position. The bond length of Cu1–
O6 is much longer than other Cu–O bond lengths indicating the
II
occurrence of a strong Jahn–Teller effect. The Cu atoms are
bridged by two phenolic O atoms (O1, O1¢), with a Cu ◊ ◊ ◊ Cu
˚
distance of 3.0141 (7) A, falling in the relatively small range of
2
17
˚
.93–3.10 A indicative of weak metal–metal interactions. The
two units are nearly coplanar, with the least-square plane Cu1,
O1, O2, N1, Cu1¢, O1¢, O2¢, N1¢ deviations ranging from -0.085(2)
˚
to 0.085(2) A. The Cu distance from the equatorial coordination
˚
˚
least-squares plane is only 0.0298(4) A for Cu1 and -0.0300(4) A
for Cu1¢. The structure is somewhat different from most of other
reported dinuclear copper salicylaldehyde-amino acid Schiff-base
complexes, in which the two coordination units of the molecules
18
are not in a plane. Only two similar structures were found for
this kind of dinuclear copper complex.
For [Cu (Sal-Gly) (H O) ], the packing structure view towards
19
2
2
2
2
the (1 0 0) plane is shown in Fig. 1B. In the crystal structure,
there are two kinds of hydrogen-bond interactions among the
molecules: O6–H6A ◊ ◊ ◊ O3i (i = 1 - x, -1/2 + y, 3/2 - z) and
O6–H6B ◊ ◊ ◊ O3¢ii (ii = x, 3/2 - y, -1/2 + z) (see Table 3). The
hydrogen bonds are formed between the coordinated water and
carboxyl oxygen. Through the hydrogen bonds, the four oxygen
atom (O6, O3¢ii, O6¢, O3ii), from the four adjacent molecules, are
connected to form a four-membered ring (see Fig. 1B), that was
also found in other reported dinuclear copper Sal-Gly Schiff-base
2 2 2 2
Fig. 1 (A) View of [Cu (Sal-Gly) (H O) ] along the b axis with the atom
numbering scheme. Displacement ellipsoids for non-hydrogen atoms are
drawn at 30% probability level. (B) View of crystal packing towards the (1
0
-
0) plane with hydrogen bonds. Symmetry code: i = 1 - x, -1/2 + y, 3/2
z; ii = x, 3/2 - y, -1/2 + z.
˚
Cu1–O3 and Cu1¢–O3¢ bonds (2.5572(21) A) forming an infinite
18c
complexes. The molecules are linked by these hydrogen bonds,
two-dimensional network in the (1 0 0) plane.
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Dalton Trans., 2012, 41, 1252–1258 | 1253

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Electrochemical polymerization
C = Q/nFA
(1)
Cyclic voltammetry (CV) curves for electropolymerization of
where Q/C is the charge under the metal oxidation peak, n (= 1)
is the number of electrons, A is the effective area of the electrode
(calculated from eqn (1)) and F is the Faraday constant (95 485 C
II
[Cu
2
(Sal-Gly)
2
(H
2
O)
2
]((L)Cu ) in DMSO at the potential range of
-
0.3 to 1.4 V are illustrated in Fig. 2A. The cyclic voltammogram
-
1
25
exhibits three anodic peaks at 0.22 (I), 0.53 (II) and 0.91 V (III).
Peaks I and II are ascribed to oxidation of (L)Cu to (L)Cu
and (L)Cu to (L)Cu , respectively. Peak III corresponds to the
electron transfer from predominantly ligand-centered orbitals
that was also present in previously reported anodic oxidation of
mol ).
A scan rate dependent study was conducted on poly(L)Cu
I
II
II
II
III
II
(P(L)Cu ) in a monomer-free DMSO solution containing 0.1 M
NaNO and is shown in Fig. 3. As shown in the inset of Fig.
16b,20
3
3, the peak currents increased linearly with sweep rate up to
16a,21
II
I
-1
metal-salen complexes.
The (L)Cu /(L)Cu reductive peak
200 mV s , which is consistent with the voltammetric response for
25
was observed at about 0 V (I¢). The redox peaks increase with
a surface confined film. This result indicates a strongly adsorbed
electroactive material that is not limited by the ionic flux of counter
subsequent scans for over 40 cycles, demonstrating that copper-
22
26
salen films are continuously deposited on the electrode surface.
The electropolymerization did not occur at low scan potentials
Fig. 2B) as found in the electropolymerization of tetraruthenated
ions and also implies conductivity of the polymer film.
(
23
II
nickel-porphyrin, indicating electropolymerization of (L)Cu
needs a high potential. Plots of the selected peak I¢ vs. the
cycle number are shown in inset b. The growth rate of peak I¢
decreased with the number of scanning passes, indicating that the
16a,24
conductivity decreased with deposition.
II
Fig. 3 P(L)Cu scan-rate dependence. Inset: plot of linear current
increase vs. scan rate.
Electrochemistry impedance spectrum (EIS) is a useful tool for
27
studying the interface properties of surface modified electrodes.
It can give information on the impedance changes in the modi-
fication process. The complex impedance can be presented as a
combination of the real impedance (Z¢) and imaginary impedance
(
Z¢¢), Nyquist plot. The Nyquist plots of EIS investigations of
II
bare GC and P(L)Cu modified GC registered in 0.1 M KCl
containing 5 mM mol equiv. of [Fe(CN)
3
-
4-
6
] /[Fe(CN) ] are shown
6
in Fig. 4. The inset figures show the Randle’s equivalent circuit
corresponding to the Nyquist plots, top for bare GC, bottom for
modified GC. In these circuits, R
the constant phase element referred as film capacitance, R
s
is the solution resistance, CPE
the
f
f
film resistance, CPEdl the constant phase element that represents
all the frequency dependent electrochemical phenomena, namely
double-layer capacitance, Rct the charge transfer resistance, and
W the Warburg impedance resulting from the diffusion of charged
species from the bulk of the electrolyte solution to the interface and
II
28
Fig. 2 Cyclic voltammetry for electropolymerization of 1 mM (L)Cu
through the interface layer. The values of the equivalent circuit
-1
in DMSO 0.1 M NaNO
3
(supporting electrolyte) on GC, 50 mV s . (A)
elements are shown in Table 4. Rct increased significantly when
Electropolymerization cycles from 1, 5, . . . , 40 at -0.3 to 1.4 V; inset (a)
is an amplified figure, insert (b) is the plot of reduction peak current vs.
scans. (B) Electropolymerization at different potential ranges: (a) -0.3 to
GC was modified as well as the CPEdl. And also the new elements
II
R
f
and CPE
f
were presented because of the P(L)Cu formation.
0
.5 V, (b) -0.3 to 0.8 V, (c) -0.3 to 1.1 V, (d) -0.3 to 1.4 V; dashed line is
Electrocatalytic oxidation of ascorbic acid
the first cycle, solid line is the 40th cycle.
The remarkable electrocatalytic properties of the modified elec-
trode towards the oxidation of ascorbic acid in pH 6.8 PBS (0.1
M KCl) are demonstrated in Fig. 5. At a bare GC, an irreversible
oxidation peak was observed at 0.28 V with a current of 20 mA
The apparent surface coverage (C) of modified electrode is 2.95 ¥
0- mol cm estimated by integrating the charge under the
9
-2
1
reduction peak of Cu(II)/Cu(I), using eqn (1).
254 | Dalton Trans., 2012, 41, 1252–1258
1
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Table 4 Fitting values of the equivalent circuit elements for bare and modified GC
5
6
R
s
/X
10 CPEdl/F
R
ct/X
W/X
R
f
/X
10 CPE
f
/F)
B-GC
M-GC
82.1
100.1
3.059
40.25
189.6
2399
0.00168
0.0032
1505
8.568
Scheme 1 Reaction mechanism of ascorbic acid at the modified electrode.
-
+
H
2
A ꢀ HA + H
(2)
(3)
(4)
(5)
-
II
∑-
+
I
HA + (L)Cu ꢀ A + H + (L)Cu
3-/4-
Fig. 4 Nyquist plots of bare and modified GC in 5 mM Fe(CN)
6
∑
-
II
I
A
+ (L)Cu ꢀ dA + (L)Cu
solution containing 0.1 M KCl, in the frequency range 0.1 to 20 kHz.
B-GC and M-GC are the experimental data for bare GC and modified
GC, respectively, Cal is calculated data. Insets show Randle’s equivalent
circuits, top for bare GC, bottom for modified GC
I
-
II
(L)Cu - e ꢀ (L)Cu
30b
Here, pKa1 for ascorbic acid is about 4.5, so the predominant
-
-
reactive species is the ascorbate ion HA . HA is initially oxidized
II
∑-
I
by (L)Cu to produce the ascorbate radical anion (A ) and (L)Cu ,
∑
-
II
A
is subsequently oxidized by (L)Cu to produce dehydroascor-
bic acid (dA) and (L)Cu . (L)Cu is finally electrochemically
I
I
oxidized to give an anodic peak shown in Fig. 5d, which indicated
I
both the Cu(I) centers in [Cu
oxidation at 0.16 V.
2
L
2
(H
2
2
O) ] undergo simultaneous
31
Amperometric determination of H
2
A
Amperometric experiments of H A were carried out using
2
II
P(L)Cu modified GC in a well-stirred PBS (pH 6.8) solution
with 0.1 M KCl as supporting electrolyte. The oxidation peak
currents were measured at 0.2 V and plotted against the bulk
Fig. 5 CVs of bare (a, c) and modified GC (b, d) electrodes in pH6.8 PBS
concentration of H A after background subtraction (Fig. 6A and
2
(
2
0.1 M KCl) solution: H A = 0 mM (a, b), 3 mM (c, d); scan rate 100 mV
-1
B). The modified GC electrode has high sensitivity and specificity
to ascorbic acid. It has almost no response in a 10 mM citric
s
(
Fig. 5c). At the modified GC, the oxidation peak shifted to 0.16
acid (CA), glucose and H
2
2
O solution but an obvious current
V and the current increased to 35 mA (Fig. 5d). The remarkable
increase when 2 mM H A was injected to the system (Fig. 6A).
2
enhancement in the peak current and the lowering of overpotential
The electrode also showed a relatively rapid response time with
the current increasing and reaching a steady-state within only
20 s. The amperometric response was found to be linear to the
II
provided clear evidence of the catalytic effect of P(L)Cu toward
7a,9a,29
H
2
A.
The electrochemical oxidation of H
2
A is considered to undergo
H A concentration over the range 2–500 mM with a sensitivity of
2
-
1 2
two consecutive one-electron transfer processes involving the
participation of a radical anion intermediate to form dehy-
22.9 nA mM , r = 0.9998. The detection limit is 0.39 mM (S/N =
II
3). The reconstructed P(L)Cu modified GC used for H A titration
2
30
-1
2
droascorbic acid (dA). This species subsequently undergoes a
hydration reaction characteristic of carbonyl groups to form an
electroinactive product. The electrocatalytic oxidation of ascorbic
acid at the modified electrode can be described by the following
equations and Scheme 1:
showed a sensitivity of 21.7 nA mM , r = 0.9999, indicating
II
that the P(L)Cu modified electrode had a good reproducibility.
The different aspects of proposed method compared with those
from recent references are listed in Table S1 (ESI†). From the
data shown in Table S1, our proposed set-up has a wide linear
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group, contains two symmetric units connected through Cu–
O bonds. Unlike the reported dinuclear copper salicylaldehyde-
amino acid Schiff-base complexes, the two units of the molecule
here are nearly coplanar. This copper complex can be elec-
tropolymerized on a glass carbon electrode when the high scan
potential reached 1.4 V, but at low scan potential no polymer-
ization occurred. The complex modified electrode showed good
electrocatalytic oxidation properties to ascorbic acid that leads
to a lower oxidation potential and higher oxidation peak than
for bare GC in CV scans. This modified electrode can show
amperometric determination of H A with a sensitivity of 22.9
2
-
1
2
nA mM (r = 0.9998) and detection limit of 0.39 mM (S/N = 3). It
showed acceptable RSD and recovery in the analysis of vitamin C
tablets showing that the present system could be used for sample
determination.
Experimental
Materials and instrumentation
All common reagents were of analytical grade and used as received.
The vitamin C tablets were purchased from Hubei Huazhong
Pharmaceutical Co., Ltd. (20101142) and Guangxi Zhengtang
Pharmaceutical Co., Ltd. (20100901) with the specified amount
of 100 and 49.5 mg per tablet, respectively.
The crystal structure was determined using a Bruker SMART
APEX CCD-based diffractometer. Infrared spectra were recorded
on a Nicolet AVATAR360 spectrometer as KBr pellets (4000–
-
1
Fig. 6 Typical amperometric curve obtained with a modified GC in pH
.8 PBS (0.1 M KCl) at an applied potential of 0.2 V vs. SCE: (A) successive
injection of H A in the range 2–500 mM, (B) the corresponding standard
addition plots.
400 cm ), elemental analysis was performed on a Vario EL-III
1
6
CHNSO Elementar, H NMR spectra were obtained on a Mercury
Plus 400 NMR spectrometer in CDCl solvent, the electronic
spectrum of DMSO solution of [Cu
0 M) was measured in the 200–800 nm region on a Lambda35
2
3
2
(Sal-Gly)
2
(H
2
O)
2
] (c = 1 ¥
-
4
1
range similar to that reported in the literature, with an oxidation
potential and detection limit lower than most of the reported
values.
The proposed method was further applied to analyze vitamin C
tablets. The recovery was studied by the corresponding tablets
spectrometer.
Synthesis of ligand
3-Formyl-4-hydroxybenzoic acid (C H O ). 3-Formyl-4-
hydroxybenzoic acid was prepared according to the literature
8
6
4
32
added with certain value of standard solution of H
2
A. The
determination results are shown in Table 5. The RSD and recovery
were in the range of 0.01–1.09% and 94.76–103.9%, respectively,
which were acceptable, showing that the proposed method can
be used for sample determination and presented a good stability.
with slight modification. 20.0 g of NaOH was dissolved in 40 mL
of water and 10 mL of methanol and 10 g of 4-hydroxybenzoic
acid were added. 20 mL of chloroform was added dropwise later
at 60 C. After refluxing at 60 C for 10 h, the solution was
sufficiently cooled with ice and filtered to obtain a slightly yellow
solid. The crude product was dissolved by concentrated ammonia
7a
◦
◦
Conclusion
and precipitated by 15% CuSO to obtain a green precipitate.
4
The new dinuclear copper salicylaldehyde-amino acid Schiff-base
Then, the green precipitate was dissolved in hot sulfuric acid
solution and cooled to obtain the white product, yield 30%. Anal.
complex [Cu
2
(Sal-Gly)
2
(H
2
O)
2
], crystallizing in the P2 /c space
1
2
Table 5 Detection of H A in vitamin C tablets
Sample
Specified (mg)
Detected (mg) (n = 5)
RSD (%)
Added (mM)
Detected (mM)
Recovery (%)
a
HZ-1
HZ-2
HZ-3
ZT-1
ZT-2
ZT-3
100
100
100
49.5
49.5
49.5
89
92
88
44.7
47.2
46.3
1.09
0.28
0.47
0.01
0.70
0.42
10
20
10
10
10
10
9.825
20.79
9.782
10.16
9.782
9.476
98.25
103.9
97.82
101.6
97.82
94.76
b
a
b
Tablets from Hubei Huazhong Pharmaceutical Co., Ltd. Tablets from Guangxi Zhengtang Pharmaceutical Co., Ltd.
1
256 | Dalton Trans., 2012, 41, 1252–1258
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Calc. for C
8
H
6
◦
O
4
: C 57.83, H 3.61. Found: C 57.85, H 3.62%.
Electrochemical measurements
-1
Mp 240–242 C. Significant IR bands (KBr, v cm ): 3550–3200
v(O–H), 1669 v(C O), 1319 v(C–H, aldehyde), 1288 v(C–O). H
NMR (CDCl
1
Electrochemical measurements were controlled on a CH In-
struments model 660B Electrochemical Workstation (Shanghai
Chenhua Instruments, Shanghai) using a three electrode set-up
3
) d/ppm: 7.08 (d, 1H, ph-H), 8.26 (d, 1H, ph-H),
8
.398 (s, 1H, ph-H), 9.98 (s, 2H, CHO), 11.49 (s, 1H, OH).
2
comprising of a glassy carbon disc (surface area of 0.071 cm )
working, platinum wire auxiliary and a saturated calomel reference
3
-Formyl-4-hydroxybenzoic acid ethyl ester (C10
H
10
4
O ). 3-
(SCE) electrode (All potentials relative to this electrode).
Formyl-4-hydroxybenzoic acid (0.02 mol, 3.32 g) was dissolved
in 20 mL ethanol and then 1 mL sulfuric acid was added with
stirring. After refluxing for 10 h the solution was added into 200
mL cold water to obtain the crude product. The crude product
was extracted by 0.01 M sodium carbonate aqueous solution and
then dried in vacuum to obtain a pure white solid (81%). Anal.
Electropolymerization was performed in nitrogen-saturated
DMSO solution containing 5 ¥ 10 M copper complex and 0.1 M
-4
NaNO as supporting electrolyte. The modified electrode was used
3
to perform the scan-rate dependent study in nitrogen saturated
DMSO solution. The EIS was performed in 5 mM equiv. molar
3-
4-
ratio of [Fe(CN)
6
] /[Fe(CN) ] solution containing 0.1 M KCl,
6
Calc. for C10
H
10
◦
O
4
: C 61.86, H 5.15. Found C 61.79, H 5.30%.
at 0.188 V and frequency ranges from 0.1 to 20 kHz.
Electrocatalytic oxidation of ascorbic acid and amperometric
-1
Mp 60.2–60.8 C. Significant IR bands (KBr, v cm ): 3401 v(O–
H), 1709 v(C O, aldehyde), 1661 v(C O, ester), 1281 v(Cph–O),
determination of H A were all performed in pH 6.8 PBS solutions
2
1
1
214, 1179 v(C–O–C). H NMR (CDCl
3
) d/ppm: 1.41 (t, 3H,
containing 0.1 M KCl with a volume of 8 mL. Amperometric
determination was performed at an applied potential of 0.2 V vs.
CH
3
), 4.39 (m, 2H, CH ), 7.04 (d, 1H, ph-H), 8.20 (d, 1H, ph-H),
2
8
.33 (s, 1H, ph-H), 9.96 (s, 1H, CHO), 11.39 (s, 1H, OH).
SCE and 8 mL of different concentrations of H A were injected to
2
the electrolytic cell successively with constant magnetic stirring.
For the vitamin C tablets analysis, the ground vitamin tablets were
dissolved in double distilled water. 8 mL of tablet solution was
injected to the determination system five times and then 8 mL of
Synthesis of complex [Cu
2
(Sal-GLy)
2
(H
2
O)
2
]
3
-Formyl-4-hydroxybenzoic acid ethyl ester (1 mmol, 0.194 g) and
glycine (1.2 mmol, 0.090 g) were dissolved in aqueous methanol
80%, 20 mL). The mixture was stirred at room temperature for 30
2
H A solution of known concentration was added.
(
min to give a clear yellow solution. Then an aqueous solution (10
mL) of copper acetate (1 mmol, 0.200 g) was added with stirring.
The mixture was stirred and refluxed at 323 K for 6 h, then cooled
to room temperature. After filtration, the filtrate was left to stand at
room temperature. Green crystals were obtained after two weeks,
Acknowledgements
We thank Dr. Jing Yang for helping us in the article’s modification.
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H
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N
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258 | Dalton Trans., 2012, 41, 1252–1258
This journal is © The Royal Society of Chemistry 2012