Utilization of a new gold/Schiff-base iron(iii) complex composite as a highly sensitive voltammetric sensor for determination of epinephrine in the presence of ascorbic acid

Article abstract of DOI:10.1039/c6ra22028b

The preparation of new materials that can act as systems capable of sensing biologically relevant molecules constitutes a significant modern challenge as well as a necessity oriented towards disease prevention. Subcomponent self-assembly of 2-(methylhydrazino)benzimidazole, 4-tert-butyl-2,6-diformylphenol and Fe(ClO₄)₂(H₂O)₆ leads to a new, bimetallic iron(iii) complex of the following formula: [Fe₂(H₃L)₂(MeOH)₂(μ-OMe)₂](ClO₄)₄ (1), as established by ESI-MS, FTIR and single crystal X-ray analysis. It is important to note that ligand H₃L was also successfully synthesized and characterized for the first time. Compound 1 was successfully deposited on a gold electrode and applied as a voltammetric sensor with respect to epinephrine (EP). Cyclic voltammograms (CVs) proved the catalytic activity of the new, electrochemically prepared composite Au/1 for the oxidation of EP in the presence of ascorbic acid (AA). The respective current peaks were clearly separated from each other, thus enabling selective detection of these compounds coexisting in a mixture. For the prepared sensor a linear relationship between the current response of EP electrooxidation at the potential of peak maximum (i_p) and the concentration of EP in solution (c_EP) in the presence of constant AA concentration was found in the broad range of c_EP (R² ≥ 0.9997, 1.0 × 10^?8 M to 9.0 × 10^?4 M) with a high detection limit (7.4 × 10^?9 M), excellent reproducibility as well as high stability.

Full text of DOI:10.1039/c6ra22028b

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Utilization of new gold/schiff-base iron(III) complex composite as
highly sensitive voltammetric sensor for determination of
epinephrine in the presence of ascorbic acid
Received 00th January 20xx,
Accepted 00th January 20xx
Adam Gorczyński,a Maciej Kubicki,a Klaudia Szymkowiak,a Teresa Łuczak*a and Violetta Patroniak*a
DOI: 10.1039/x0xx00000x
Preparation of new materials that can act as systems capable of sensing biologically relevant molecules constitutes
significant modern challenge as well as necessity oriented towards disease preventions. Subcomponent self-assembly of 2-
(methylhydrazino)benzimidazole, 4-tert-butyl-2,6-diformylphenol and Fe(ClO4)2(H2O)6 leads to new, bimetallic iron(III)
complex of the following formula: [Fe2(H3L)2(MeOH)2(ꢀ-OMe)2](ClO4)4 (1), as established by ESI-MS, FTIR and single crystal
X-ray analysis. It is important to notice that ligand H3L was also successfully synthesized and characterized for the first
time. Compound 1 was successfully deposited on gold electrode and applied as voltammetric sensor with respect to
epinephrine (EP). The cyclic voltammograms (CVs) proved the catalytic activity of the new, electrochemically prepared
composite Au/1 for the oxidation of EP in the presence of ascorbic acid (AA). The respective current peaks were clearly
separated from each other, thus enabling selective detection of these compounds coexisting in a mixture. For prepared
sensor a linear relationship between the current response of EP electrooxidation at the potential of peak maximum (ip) and
the concentration of EP in solution (cEP) in the presence of constant AA concentration was found in the broad range of cEP
(R2≥0.9997, 1.0 x 10-8 M – 9.0 x 10-4 M) with high detection limit (7.4 x 10-9 M), excellent reproducibility as well as high
stability.
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oxidizes at bare electrodes in almost the same potential
region.4,5 It is thus necessary to separate the oxidation
potentials of these compounds to enable accurate
1. Introduction
Epinephrine (adrenaline) (EP) belongs to a class of biogenic
aliphatic catecholamines that plays an important role in
functioning of the mammalian, hormonal, central nervous and
cardiovascular systems.1 In similarity to other hormones, EP
exhibits a suppressive effect on the immune system and is thus
used as a drug to treat cardiac arrest or dysrhythmias, as a
bronchodilator for asthma and for sepsis treatment.2
Development of a simple method for EP detection and
determination is an important task from both fundamental
and practical point of view in bioscience as well as in medicinal
chemistry - especially in neurochemistry - and has resulted in
plenty of papers published in the last years. Catecholamines
are easily oxidized, thus electrochemical methods appear to be
suitable for their quantitative detection.3 However, the final
product of epinephrine oxidation - epinephrinechrome - easily
transforms into polymer, which makes a serious problem when
epinephrine is oxidized directly at the bare electrode due to
poisoning and blocking of the electrode surfaces. Another
problem is that epinephrine exists in natural environments
together with small biomolecules like ascorbic acid, which
determination of epinephrine. Among various attempts to
overcome the above mentioned problems, much attention has
been paid to application of chemically or electrochemically
modified electrode surfaces. The most papers have been
concerned with the voltammetric behavior of bare carbon,
carbon fiber and glassy carbon based electrodes. For example
a
pyrolytic graphite electrode modified with carbon
nanotubes6-8
,
a
graphite/mesoporous Al-containing SiO2
electrode9, carbon fiber electrodes covered with overoxidized
polypyrrole and DNA10 as well as carbon surface with
deposited gold nanoparticles (Au-NPs)11-13 have been tested
for selective detection of EP in the presence of interfering
compounds. Moreover, glassy carbon electrodes modified with
an overoxidized polypyrrole14 or dopamine15, polycaffeic
acid16, luminal17, Zn-Al hydroxide18, 5-amino-1,3,4-thiadiazole-
2-thiol19 and poly rutin20 films were reported. A glassy carbon
electrode covered with self-assembled monolayers (SAMs) of
cysteamine21 as well as with osmium and cobalt
hexacyanoferrate, hexacyanoruthenate or phtalocyanine
complexes8,22-25 have been prepared and investigated upon
the electrooxidation of EP. Aside from different carbon
materials used as
a template for preparing modified
electrodes, a promotion of electron transfer in the
electrochemical reaction during EP electrooxidation was
achieved after application of gold electrodes covered with
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spontaneously formed SAMs of alkanetiols or other S- methanolic solution with
containing organic compounds.26-33 It has been proven that the Micromass
c
=
~10-4
The
M
using
a Waters
ZQ spectrometer.
electrochemical
latter electrodes effectively protect the metal surface against measurements were carried out in a conventional three-
adsorption of reaction products that are poisonous for the compartment cell separated by glass frits using an Autolab
electrode surface.
potentiostat/galvanostat analyzer (AUTOLAB PGSTAT 302N,
As a consequence of facile and efficient synthetic protocols, Eco Chemie, B. V., Utrecht, The Netherlands). The scanning
we have recently focused our attention on Schiff-base electron microscopy (SEM) analysis was carried out using ZEISS
scaffolded metallosupramolecular architectures, which EVO 40 instrument (Germany) with the acceleration voltage of
renders them suitable systems for viable industrial 20 kV. The transmission electron microscopy (TEM) image was
applications.34-36 As a result, novel voltammetric sensor has obtained using JEM - 1200 EX2 (Japan) instrument, operating
been developed (Au/[Mn2(H2
L
)2](ClO4)2 composite, where H3
L
at 80 kV, after drying the gold colloidal Fe-complex solution
is ligand - 2,6-bis((2-(1H-benzo[d]imidazol-2-yl)-2- covered with the foil strengthened with carbon.
methylhydrazono)methyl)-4-(tert-butyl)phenol) and applied
for quantitative detection of dopamine and its interfering
2.3 Synthesis of ligand H3L.
biogenic compounds.36 It has struck us that such systems have Synthesis of new Schiff-base ligand H3
L
was performed under
been recognized as an unexplored niche with regard to nitrogen to eliminate the detrimental effect of moisture on the
deposition onto gold electrodes and sensing of biologically condensation efficiency. 2-(1-Methylhydrazinyl)-1H-
relevant compounds and in turn inspired us to pursue this benzo[d]imidazole (56.0 mg, 0.347 mmol)36 and 4-tert-butyl-
subject with regard to the other metal ions.37
2,6-diformylphenol (43.2 mg, 0.17 mmol) were transferred to
Despite that a lot of experimental data mentioned above is two-necked round bottom flask equipped with the reflux
present, development of novel and easy to synthesize condenser that was previously flushed with inert gas. 10 ml of
materials
capable
of
simultaneous
detection
of absolute ethanol was added via syringe and the mixture was
neurotransmitters in the presence of biogenic molecules, such stirred on magnetic stirrer, resulting in formation of
as AA, is still a challenging task for electrochemists. Motivated transparent yellow solution. The mixture was heated for a few
by successful application of hardly explored Schiff-base hours under reflux what followed precipitation of yellow solid.
metallosupramolecular architectures as sensors36, we have The content was cooled to room temperature, left to stir
prepared new bimetallic Fe(III) complex - [Fe2(H3
OMe)2](ClO4)4 ) (Fig. 1) – successfully deposited it at gold washed well with ethyl alcohol and diethyl ether respectively.
template and evaluated prepared material as electrochemical Recrystallization from DMSO/water system resulted in
L
)2(MeOH)2(
ꢀ
-
overnight and the precipitate was subsequently filtered and
(
1
sensor of EP in the presence of interfering AA.
microcrystalline yellow solid that was dried under vacuum in
50oC. Yield: 73.15 mg (87.0 %)
1HNMR (500 MHz, d6-DMSO):
δ = 1.42 (s, 9H), 3.71 (s, 6H),
7.00–7.06 (m, 4H), 7.35–7.37 (m, 4H), 7.90 (s, 2H), 8.21 (s, 2H),
10.53 (s, 1H), 11.79 (s, 2H) ppm.13CNMR (125 MHz, d6-DMSO):
δ
= 31.4, 31.8, 34.1, 109.9, 116.1, 119.7, 120.4, 120.7, 125.6,
Fig. 1. Schematic formation of new Fe(III) bimetallic complex via subcomponent self-
134.1, 136.5, 142.0, 142.6, 151.8, 153.4 ppm; Elemental
analysis calc. for [C28H30N8O] (494.59): C, 68.00; H, 6.11; N,
22.66; found: C, 66.09; H, 6.21; N, 22.61 %. ESI-MS(+) m/z: 495
assembly approach.
[(H4L)]+, 517 [H3
(O-H) 3637(w),
H)ar 3085(m), 3055(m);
s(CH3) 2905(w), 2868(w);
1561(vs), 1480 (s), 1461(s), 1448(s), 1414(m), 1385(m),
1353(w), 1317(m), 1280(m), 1224(m), (C-O) 1264(m); (C-H)ar,
(C-H)ar 1211(s), 1156(m), 1044(m), 1005(m), 945(s), 821(w),
743(s), 699(w), 677(w), 533(w), 433(w).
L
+Na+]+, 533 [H3
(N-H) 3500-2200 (max. at 3158) (m. br);
as(CH3) 2962(m), as(CH3) 2929(m);
(C=C)ar, (C=N) 1626(vs), 1597(vs),
L
+K+]+. Selected IR (KBr, cm-1):
2. Experimental
ν
ν
ν(C-
2.1 Reagents and physical measurements
ν
ν
All chemicals were of analytical grade and used as received
without further purification: tetrabutylammonium perchlorate
(TBAClO4), lithium perchlorate (LiClO4), NaH2PO4, K2HPO4,
NaOH and acetonitrile (ACN) from Merck, epinephrine (EP)
from Fluka; L-ascorbic acid (AA) and Fe(ClO4)2(H2O)6 from
Sigma; H2SO4, H2O2, EtOHabs, MeOHabs from POCh (Gliwice,
Poland). All synthetic manipulations were performed under
aerobic conditions using solvents and chemicals as received
unless otherwise stated.
ν
ν
ν
ν
γ
ρ
Crystals suitable for X-ray diffraction were obtained from
acidified solution of MeOH/toluene at lowered temperature.
2.4 Synthesis of iron(III) complex
Bimetallic iron(III) complex
1 was synthesized in two different
2.2 Instrumentation
ways: method A (similar manner to the recently published
one-pot template reaction36 with some notable changes),
Microanalysis of ligand H3
was performed using
microanalyser. IR spectrum of
L as well as bimetallic Fe(III) complex
1
a
Perkin-Elmer 2400 CHN
was obtained with a Perkin-
method B (direct complexation with ligand H3L).
1
Method A: 2-(1-Methylhydrazinyl)-1H-benzo[d]imidazole (52.3
mg, 0.324 mmol) and 4-tert-butyl-2,6-diformylphenol (40.4
mg, 0.162 mmol) were placed in 50 ml round bottom flask,
Elmer 580 spectrophotometer and peak positions are reported
in cm-1. ESI mass spectra of ligand and
1 were determined in
2 | J. Name., 2012, 00, 1-3
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dissolved upon addition of 20 ml of MeOH and stirred for 1 h. crystals of
1
(maybe due to the disordered solvent) were of
Yellow precipitate of formed ligand H3L was observed and relatively poor quality, and in consequence weak restraints
without isolation transferred to 15 ml of MeCN/MeOH (1:1 v:v) were applied to the displacement parameters of some atoms.
solution of Fe(ClO4)2(H2O)6 (58.8 mg, 0.162 mmol)), what The crystals of protonated ligand turned out to be twinned,
resulted in formation of black solution. Such system was (which was taken into account during both data reduction and
stirred for 48 h at room temperature without sealing, refinement); the BASF factor, describing the content of one of
providing direct access of air. The reaction mixture was the component41 was refined at 79.0(3)%.
subsequently concentrated in vacuo, precipitated with diethyl Crystal data:
ether, and recrystallized from MeOH/Et2O at lowered
temperature. Single crystal suitable for X-ray analysis was a= 10.6236(17) Å,
obtained by means of vial-to-vial diffusion at 4oC from α=66.459(15)°, β=87.591(12)°, γ=78.066(14)°, V= 2077.2(6) Å3,
(
1)
C60H74Fe2N16O64+·4(ClO4)-, Mr=1624.85, triclinic, P-1,
b=13.530(2) Å, c= 16.128(2) Å,
MeOH/iPr2O
system.
(cf.
crystallographic
): Yield: 85.4 mg, 61.1 %. reflections
) (1624.85): (Rint=13.94%), 2549 with I>2σ(I). Final R[I>2σ(I)]=9.86%,
C, 44.35; H, 4.59; N, 13.79; found: C, 44.13; H, 4.73; N, 13.73 wR2[I>2σ(I)]=15.47%, R[all reflections]=20.02%, wR2[all
%. ESI-MS(+) m/z (%): 548 (100) [Fe(HL)]+, 1095 (10) [Fe2(H )2– reflections]=18.33%, S=0.93, max/min Δρ=0.80/-0.40 e·Å-3.
H+]+, 1127 (<5) [Fe2(H )2–H++(MeOH)]+, 1195 (<5) [H5 ](ClO4)2 - 2(C28H32N8O)2+·4(ClO4)-·C7H8·CH3OH, Mr=1515.21,
{[Fe2(H (O-H) 3700 – 2200 triclinic, P-1, a=11.4426(5) Å, b=13.0011(5) Å, c= 24.5126(10)
)2](ClO4)}+. Selected IR (KBr, cm-1):
(m, br), (C-H)ar 3095(w), Å, α= 90.991(3)°, β= 91.953(4)°, γ= 92.005(4)°, V= 3641.7(3) Å3,
3066(w), 3015 (w); as(CH3) 2952(m);
s(CH3) Z=2, dx=1.38 g·cm-3, μ=2.16 mm-1, F(000)=1584. 25627
2905(w), 2867(w);
data). Z=1, F(000)=842, dx=1.308g·cm-3, μ=0.55mm-1. 15018
measured, 7301 symmetry-independent
[Fe2(H3L)2(MeOH)2(
ꢀ
-OMe)2](ClO4)4 (1
Elemental analysis calc. for [C60H86Cl4Fe2N16O28] (
1
L
L
L
L
ν
ν
(O-H) 3638(w),
ν
(N-H) 3352(m); ν
νas(CH3) 2962(m),
ν
ν
ν(C=C)ar, ν(C=N) 1659(vs), 1610(vs), reflections collected up to 2Θ = 67.5°, 12871 symmetry
1592(vs), 1556(m), 1546(m), 1494(m), 1474 (s), 1467(s), independent, (Rint=8.60%), 7644 with I>2σ(I). Final
1449(s), 1394(m), 1364(s), 1331(w), 1288(m), 1232(m), R[I>2σ(I)]=10.20%, wR2[I>2σ(I)]=23.13%, R[all refl.]=14.69%,
1220(m),
1108(vs), 1087(vs),
929(m), 826(m), 745(m), 636(s), 529(w), 501(w);
855(w), (ClO) 626(s).
ν
(C-O) 1257(m);
δ
(OClO) 1146(vs), 1121(vs), wR2[all refl.]=26.03%, S=1.05, max/min Δρ = 1.52/-0.58 e·Å-3.
γ
(C-H)ar,
ρ
(C-H)ar 1052(s), 1008(m), 958(m), Crystallographic data (excluding structure factors) for the
as(FeOFe) structural analysis has been deposited with the Cambridge
Crystallographic Data Centre, No. CCDC-1407322 ( ) and CCDC-
ν
γ
1
Method B: Schiff-base ligand H3L (20.2 mg, 0.041 mmol) was 1482453 ([H5L](ClO4)2). Copies of this information may be
placed in 50 ml round bottom flask and suspended in 20 ml of obtained free of charge from: The Director, CCDC, 12 Union
MeOH.
5 ml of MeCN/MeOH (1:1 v:v) solution of Road, Cambridge, CB2 1EZ, UK. Fax: +44(1223)336-033, e-
Fe(ClO4)2(H2O)6 (14.9 mg, 0.041 mmol)) was added what mail:deposit@ccdc.cam.ac.uk, or www: www.ccdc.cam.ac.uk.
resulted in formation of black solution. Such system was
2.6 Electrode activation and modification
stirred for 48
h at room temperature without sealing,
providing direct access of air. Work-up was identical to the one All solutions under investigation were prepared prior to each
presented in method A. Formed product was determined to experiment, using Millipore Milli-Q-water or acetonitrile (ACN)
be identical to the one formed by the template approach.
0.1 M TBAClO4, 0.1 M LiClO4 or phosphate buffer (0.6 M. pH
=7) served as a supporting electrolyte solution. All solutions
were deaerated by argon (99.998 %) before measurements
2.5 X-ray crystallography
Diffraction data were collected at 100(1) K by the ω-scan performed at room temperature.
technique on Agilent Technologies XCalibur four-circle The working electrode was either the bare gold electrode
diffractometer with Eos CCD detector, equipped with Nova (99.999 %, Polish State Mint) or the gold electrode modified
microfocus MoKα radiation source (λ = 0.71073 Å) for 1, and with the Fe-complex 1
of 0.03 cm2 geometric area. A gold
for [H5L](ClO4)2 at room temperature on Agilent Technologies sheet was used as an auxiliary electrode (10 cm2), whereas the
SuperNova four-circle diffractometer with Atlas CCD detector saturated calomel electrode (SCE) and the Ag/Ag+ in ACN
and mirror-monochromated CuKα radiation (λ=1.54178 Å) The electrodes were used as reference electrode in aqueous and
data were corrected for Lorentz-polarization as well as for ACN solution, respectively.
absorption effects.38 Precise unit-cell parameters were Before any experiment, the bare gold electrode was polished
determined by a least-squares fit of reflections of the highest with aluminium slurries of successively decreasing final grades
intensity, chosen from the whole experiment. The structures down to 0.05
were solved with SIR9239 and refined with the full-matrix least- the polished electrode was rinsed carefully with acetone and
squares procedure on F2 by SHELXL-2013.40 The structure of
water, respectively. After that the electrode was
ꢀm (Buehler) on polishing cloths (Buehler). Next,
1
contains voids filled with diffused electron density therefore electrochemically activated by cycling in the potential range
the SQUEEZE procedure41 was applied. All non-hydrogen between E = -0.7 V and E = 0.6 V vs SCE in the phosphate
atoms were refined anisotropically, hydrogen atoms were buffer (0.06 M, pH = 7), with the potential scan rate, dE/dt=
placed in idealized positions and refined as ‘riding model’ with 0.1 V s-1 until a stable cyclic voltammogram (acronyms CV or i-
isotropic displacement parameters set at 1.2 (1.5 for methyl or E curve) was obtained. This procedure avoids structural
hydroxyl groups) times Ueq of appropriate carrier atoms. The changes on the gold surface.42
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In order to obtain a gold electrode modified with SAM made of 3.1 Crystallographic data
Journal Name
1, the activated Au template was electrochemically cycled in
Both systems that comprise the H3L Schiff-base ligand have
0.1 M TBAClO4 supporting electrolyte solution in ACN with
addition of 0.001M Fe-complex at dE/dt = 0.1 Vs-1, until a
stable CV was obtained. The electrochemical properties of the
modified electrode were checked over two weeks and were
been crystallized and characterized in the solid state, either in
2+
the form of dicationic salt [H5
).
L] or bimetallic Fe(III) complex
(
1
found to be stable. The consecutive series of voltammetric 3.1.1. [H5L](ClO4)2
experiments performed each day were separated by rest
There are two symmetry-independent cations in the
asymmetric part of the unit cell, and they have very similar
geometry (Fig. 2). One can however note, that relative
orientation of OH and isopropyl groups are reverse in both
molecules (OH hydrogen in each case makes reasonable
hydrogen bonds).
periods at open circuit in without solution. Before re-use in
electrochemical measurements electrode was washed with
acetone and dried in an argon stream. The self-assembled
layers can be removed from the modified Au surface by
dipping it (for about 5 minutes) in a Piranha solution, i.e. in a
mixture 3:1 of aqueous solutions of 96% H2SO4 and 30% H2O2.
In order to recover the proposed sensors, the above described
procedure of polishing, activation and modification of the bare
Au electrodes must be repeated.
3. Results and discussion
Subcomponent self-assembly approach43 allowed us to
Fig. 2. Anisotropic ellipsoid representation of the molecule A of a [H5L]2+ cation (a);
ellipsoids are drawn at the 50% probability level, hydrogen atoms are shown as spheres
of arbitrary radii, hydrogen bond is shown as thin blue line; comparison of two
synthesize new, bimetallic iron(III) complex, metal ions bound
within N’N’O’N’N Schiff-base scaffold H3L in unprecedented
manner (cf. section 3.1 and Fig. 1). During the course of the symmetry-independent cations (fitting of the central rings – b).
reaction, native Fe(II) metal ions are oxidized by air to Fe(III)
The cations are almost planar, the dihedral angles between the
mean planes of the terminal ring systems are 13.5° and 10.2°
(the relevant geometrical features of the ligands are listed in
the Table S1, Supporting material). Intramolecular OH···N
hydrogen bonds help in adopting such conformation. In the
crystal structure, besides four perchlorate anions, methanol
and (heavily disordered) toluene molecules were found. The
species and coordinate with in situ formed ligand H3
L to form
[Fe2(H3L)2(MeOH)2(ꢀ-OMe)2](ClO4)4 ). Electrospray ionization
(1
mass spectrometry, elementary analysis as well as FT-IR
spectroscopy confirmed formation of FeOFeO bridged
bimetallic species, whereas its structure in the solid state has
been
unambiguously
confirmed
via
single
X-ray
crystallography. Despite the fact that the major species found
under ESI-MS studies are monometallic [Fe(HL)]+ positive ions,
we believe the solid state structure is retained in solution on
the basis of the following premises: (i) it is still possible to find
crystal structure results
– apart from the electrostatic
interactions between charged species) – from the extensive
network of hydrogen bonds, involving all strong hydrogen
bond donors (Fig. 3). The details of the hydrogen bonds are
listed in Table S2, Supporting material).
signals
[Fe2(H
are amenable to the applied cone-voltage. It is also of notice
that observed -oxo Fe2O2 motif is of relevance with regard to
from
bimetallic
species
[Fe2(HL
)2–H+]+,
L
)2(MeO)]+, {[Fe2(H
L
)2](ClO4)}+ (ii) all observed systems
ꢀ
nonheme iron-containing enzymes44-46 and quite unexpectedly
herein, it is formed via coordination with adventitious
methoxy groups rather than with exploitation of the phenoxy
moieties of H3L ligand. The latter situation was observed in
[Mn2(H2L)2](ClO4)2 manganese(II) complex, which we have
recently demonstrated to form electrochemical sensor at the
Au template with regard to sensing of dopamine in the
presence of interfering biogenic compounds.36 Herein
however, we also demonstrated that the template approach is
not the only one that leads to the desired product. It was
possible to isolate the new Schiff-base ligand and subdue it to
the full spectroscopic characterization. Furthermore, it was
possible to transform it into a diprotonated cationic salt
[H5L](ClO4)2 and unambiguously confirm its structure in the
solid state (vide infra). We were able to complexate H3L with
iron(II) salt and the final product turned out to be identical
with the one obtained via the subcomponent self-assembly
approach.
Fig. 3. A crystal structure of [H5L](ClO4)2 cation as seen along b-direction. Anions and
solvent molecules are shown in van der Waals spheres representation in order to
visualize their filling-structure role.
3.1.2. [Fe2(H3L)2(MeOH)2(ꢀ-OMe)2](ClO4)4
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The complex molecule is centrosymmetric (Ci-symmetrical),
the middle point of the Fe···Fe segment lies at the center of
symmetry in the space group P-1 (Fig. 4).
Fig. 5 Crystal lattice of [Fe2(H3
L
)2(MeOH)2(
ꢀ
-OMe)2](ClO4)4 complex as seen along
a direction; voids present within the crystal lattice are filled by perchlorate
counterions (omitted in a).
3.2 Electrooxidation of prepared complex on a gold electrode
Fig. 4. Anisotropic ellipsoid representation of the cation; ellipsoids are drawn at
the 30% probability level, hydrogen atoms are shown as spheres of arbitrary and its characterization
radii. The primed atoms are related with the unprimed ones by symmetry
The cyclic voltammograms of the bare polycrystalline gold
electrode recorded in 0.1 tetrabutyloammonium
operation 1-x,1-y,1-z.
M
perchlorate (TBAClO4) in acetonitrile (ACN) as a supporting
electrolyte solution is presented on Fig. 6a (curve 1). CV
obtained in 0.1M LiClO4 in ACN solution is attached to Fig 6a
(curve 2) for comparative issues.
Table 1 lists the relevant geometrical parameters. Fe atoms are
six-coordinated, by three atoms (NNO) from H3L molecule: two
oxygen atoms from bridging methoxy anion, and one oxygen
atom from additional methanol. The coordination is closed to
octahedral (cf. Table 1), and the central FeOFeO ring is planar.
This was also supported by SHAPE measurement
calculations47,48, which indicate the level of deviation from the
perfect polyhedron (S = O) by means of continuous shape
measurements (CShM). Indeed, the lowest value was
encountered for octahedron (S = 2.04), showing that the site
symmetry is best described as Oh. We did also compare the
symmetry of the previously synthesized [Mn2(H2L)2](ClO4)2
complex (Table S3, Supporting material) and concluded that
the trigonal prism geometry of Mn(II) cations (D3h site
symmetry) might indirectly confer to different conformation of
ligand H3
L and thus different chemisorption onto the Au
electrode (Figures S1 and S2). The ligands are slightly twisted
(dihedral angles between mean planes of the terminal ring
systems are 18.3(2)°.
Table 1. Relevant geometrical parameters (Å, °) with s.u.’s in parentheses.
Bond length [Å]
Bond angle [o]
O16A-Fe1-N29A
Fe1-O16A
1.879(16)
2.052(7)
2.185(8)
1.957(6)
2.101(7)
2.002(7)
157.1(3)
168.8(3)
169.5(3)
103.0(3)
---
Fe1-N29A
Fe1-N26A
Fe1-O1C
Fe1-O1B
Fe1-O1Ci
O1C-Fe1-N26A
O1B-Fe1-O1Ci
Fe1-O1C-Fe1’
---
Fig. 6. a) Cyclic voltammograms of polycrystalline gold electrode recorded in: (1)
0.1 M TBAClO4 and (2) in 0.1 M LiClO4 ACN solution. b) (1) The same solution as
in 1a) with (2) depicting the potential range of the electrical double layer of gold
electrode. dE/dt =0.1 Vs-1.
---
---
In the crystal structure there are voids which are filled by some
electron density, interpreted as disordered water molecules.
Hydrogen bonds (N-H···O and O-H···O) connect the complex
with perchlorate anions, and – together with the electrostatic
interactions between charged species – might be regarded as a
driving force for the crystal structure (Fig. 5).
A pair of anodic/cathodic peaks, observed at curve 1 in Fig 6a,
at 1.65 V vs Ag/Ag+ in the anodic scan and at 1.15 V vs Ag/Ag+
in the cathodic scan respectively is responsible for oxidation
and reduction of a gold oxide layer. Moreover, a couple of
peaks visible at -1.75 V and -1.35 V vs Ag/Ag+ is assigned to the
electrochemical behavior of TBA cation. The last conclusion is
very important from the electrochemical point of view, since in
certain papers that concern complexes studied in the same
supporting electrolyte (ACN), the latter pair of peaks was
incorrectly considered as coming from the studied complex
instead of TBA cation. It is also worth noting that this pair of
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_
peaks should be not assigned to the ClO4 anion. This is
confirmed by the shape of curve 2 presented in Fig. 6a. Curves
1 and 2 differ considerably despite being recorded in the
solutions of supporting electrolyte with different cations. A
characteristic feature of the bare gold electrode (Fig 6b, curve
1) is a wide potential region where no faradic reactions occur
(Fig 6b, curve 2). In this potential region only a charging and
discharging of the electrical double layer of the gold electrode
occurs (Fig. 6b, curve 2), therefore other electrochemical
processes can be studied in this electrode potential window.
The representative set of cyclic voltammograms recorded at
the gold electrode in the supporting electrolyte solution and in
the presence of the increasing concentration of
Fig. 8. Conformation changes of ligand H3
[Fe2(H3 )2(MeOH)2( -OMe)2](ClO4)4 complex;
cisoid, – cisoid-cisoid.
L
that outcome the final structure of
L
ꢀ
A – transoid-transoid, B – transoid-
C
H
3L ligand was found to exhibit conformation
published36 electrocatalyst [Mn2(H2L)2](ClO4)2, whereas herein
presented [Fe2(H3L)2(MeOH)2( -OMe)2](ClO4)4 complex
exhibits conformation . Bearing in mind that both Mn(II) and
C in the recently
ꢀ
[Fe2(H3L)2(MeOH)2(ꢀ-OMe)2](ClO4)4 complex is depicted in Fig.
7.
B
Fe(III) ions are isoelectronic, such situation is unprecedented
and is of prime importance, thus resulting in two important
implications. Firstly, we have established that previously
formed conformation
bridged bimetallic complexes is not mandatory to form stable
composites at gold electrode, since surface coverage of at Au
C that results in shielded, ꢀ-phenoxo
1
template has been achieved herein (no hydrolysis of iron was
observed) as well as is permanent under ambient conditions
(vide infra). Secondly, the mode of adsorption may be
anticipated to differ, since relative orientation of the NH
groups from benzimidazole moieties in iron and manganese
electrocatalysts varies. Please also note that despite being
isoelectronic, the central metal ions differ in the symmetry as
revealed by SHAPE calculations (Section 3.1.) As a result,
Fig. 7. Cyclic voltammograms of polycrystalline gold electrode recorded in: (1) a
supporting electrolyte solution: 0.1 M TBAClO4 in ACN and with addition of (2):1
planar chemisorption of complex
1
is anticipated, contrary to
)2](ClO4)2 i.e.
ꢀ
[Fe2(H3 )2(MeOH)2(
M; (3) 10
ꢀ
M
and (4) 50
ꢀM; (5) 100 ꢀM; (6) 1000 ꢀM of the
pendant-type chemisorption in [Mn2(H2
L
L
ꢀ
-OMe)2](ClO4)4. dE/dt = 0.1 Vs-1
.
perpendicular with respect to the orientation of Mn(II)-Mn(II)
molecular axis. (supporting – Fig. S1) Such situation would
result in ca. 9 times lower concentration of Fe-complex than
Mn-complex at the surface of the electrode. This would be the
reason that ratio of the Fe concentration to the concentration
of all complex in the supporting electrolyte solution is very
small, firmly too low, to give peak which may be assigned to
the Fe(III) ion.
As can be seen, a successive increase of the amount of Fe-
complex in solution results in strong adsorption and blocking
of the gold surface in the potential range of gold oxide
oxidation/reduction. The range of the electric double layer
almost does not change and, again the electrode surface is
covered by the studied complex in the region of TBA cation. It
should be noted that on the recorded i-E curves, peaks which
would demonstrate the presence of Fe(III) in the used complex
molecules are not present. This stands in contrast with
previously presented Mn-complex within the same Schiff base
By taking into account the crystallographic structure of the
studied [Fe2(H3L)2(MeOH)2(ꢀ-OMe)2](ClO4)4 complex, it is
evident that it can be considered as a good candidate to form
SAMs on the bare gold surface. It is commonly known that
gold is characterized by a good affinity to nitrogen.49,50 It is
generally accepted that chemisorption of modified
functionalized compounds on gold is preceded by the oxidative
abstraction of hydrogen in our case from N-H bonds, whereas
the “rest” of modified molecules create an ordered structure
at the electrode turned at a tilt angle towards the electrolyte
solution.51 The firm attachment of Fe-complex to the gold
electrode surface (Au) herein is realized via the covalent bonds
between the Au and N atoms from Schiff base ligand studied
scaffold
H
3L as dopamine sensor36, but becomes
understandable when comparison of their structures is made.
Formation of self-assembled monolayers (SAMs) from
metallosupramolecular architectures on the solid surfaces is
dependent on: (i) information encoded within organic ligand
and chosen metal ion that affect the final structure of the
complex; (ii) nature of the substrate (template) that influences
the adsorption behavior of the system under study i.e. the
electrocatalyst. Presented bicompartmental Schiff-base ligand
has the potential to form various metal ion complexes, in
accordance to the exhibited conformation (Fig.8).
complex molecule as well as π-stacking interactions.
(supporting – Fig. S2) This has been also confirmed by SEM
images of the formed [Fe2(H3L)2(MeOH)2(ꢀ-OMe)2](ClO4)4/Au
composite (supporting – Fig. S3).
6 | J. Name., 2012, 00, 1-3
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A
strong adsorption at the gold electrode surface was undergo a 1,4 Michael addition, which results in a cyclization
confirmed in separate experiments in which the electrode was reaction.48 Therefore, in the next step o-epinephrinequinone
cycled in solution with the Fe-complex, and then rinsed transforms into leucoepinephrinechrome via chemical
a
copiously with ACN, drying and dipped in the fresh supporting intramolecular 1,4-Michael addition.3,50 A subsequent couple
electrolyte. The resulting voltammogram was the same as of peaks at 0.05 V vs Ag/Ag+ and 0.11 V vs Ag/Ag+, appearing
before electrode washing. Such an experiment, with the same during the following cathodic and anodic scans, is related to
electrode has been repeated each day during two weeks and the reversible electrooxidation of leucoepinephrinechrome to
the recorded i - E curve did not change in shape as compared epinephrinechrome and its regeneration after reversal of the
to that obtained at the beginning of the presented project. We potential scan direction.
also managed to isolate and characterize the new Schiff-base
ligand H3L to check if the deposition of the sole ligand would
be possible on the bare Au electrode. Remarkably, the
electrochemically mediated deposition of ligand does not
occur, what is another proof that formation of SAMs – here in
the form of [Fe2(H3L)2(MeOH)2(ꢀ-OMe)2](ClO4)4/Au composite
material – is possible, provided that H3L was complexated with
metallic centers.
3.3 Oxidation of epinephrine at bare and modified gold
electrodes
From literature it is well know that formation of self-
assembled monolayers (SAMs) are one of the means for
functionalization of various electrodes. They are used in the
cases when investigation of some compounds on unmodified
electrodes disappoints or becomes troublesome. For the
above described reasons, such a case is an immanent trait
during epinephrine electrooxidation on the bare gold
electrode.
Electrochemical oxidation of epinephrine goes through three
steps, according to the ECE mechanism (where: E and C
denotes the electrochemical and the chemical step,
respectively) presented below in Figure 93:
Fig. 9. Electrooxidation of epinephrine on electrodes.
Fig. 10
0.1 M TBAClO4 in ACN (1) and with addition of (2) 500
voltammograms of polycrystalline gold electrodes recorded in 0.1 M TBAClO4 in
ACN and with addition of: (2) 1000 of the [Fe2(H3 )2(MeOH)2(
OMe)2](ClO4)4; (3) the same as in (2) after addition of 500 M EP. c) Cyclic
voltammograms of polycrystalline gold electrode recorded in 0.1 M TBAClO4 in
ACN covered with 1000 M of the Fe-complex (1) and (2) the same as in (1) after
addition of 0.01 M EP; (2) 0.1 M EP, (3) 1 M EP; (4) 10 M EP; (5) 100 M EP;
(6) 200 M EP; (7) 400 M EP; (8) 500
M EP. dE/dt =0.1 Vs-1.
.
a) Cyclic voltammograms of polycrystalline gold electrodes recorded in
ꢀ
M of EP. b) (1) Cyclic
Figure 10a shows the stable voltammetric response (after the
5th scan) of the bare gold electrode in the presence of EP. The
epinephrine oxidation at the bare gold electrode proceeds in
two potential ranges. The anodic peak with a maximum at E =
0.5 V vs Ag/Ag+ corresponds to irreversible electrooxidation of
EP to o-epinephrinequinone after exchange of 2 electrons and
2 protons. Epinephrinequinone contains both an electron-
deficient ring and an electron-donating amine group. When
the latter group becomes deprotonated, the molecule can
ꢀ
M
L
ꢀ-
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
From comparison of the results presented in Figs. 10a and 10b,
it is evident that a remarkable catalytic effect in epinephrine
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electrooxidation to o-epinephrinequinone can be achieved on
gold electrode modified with [Fe2(H3L)2(MeOH)2(
ꢀ
-
OMe)2](ClO4)4. This is manifested as a substantial shift of the
corresponding voltammetric peaks towards the more negative
direction accompanied by an increase in the current response
at the potential of oxidation peak maximum in comparison
with that of the bare gold electrode. Both these effects
confirmed electrocatalytic improvement of the kinetics of the
electrode reactions at electrochemically modified electrodes.52
The i - E curves recorded on the Au/1 modified electrode with
increasing concentration of EP in the supporting electrolyte
solution (Fig. 10c) suggest the presence of specific interactions
of the epinephrine with the electrode material and/or with
SAM of
1, which results in a decrease of the energy barrier for
the oxidative electron transfer. This conclusion is supported by
the Tafel slopes comprised between 140 mV for Au/
electrode and 220 mV for the bare Au electrode.52
1
As mentioned above and visualized in Fig. 11a, the detection of
epinephrine in the presence of other biogenic compounds like
ascorbic acid on the bare gold electrode is precluded. For
example, values of the peak separation (∆EEP-AA) for the 500
ꢀ
M EP concentration in the mixture with 5000 ꢀM AA
observed on unmodified gold electrode is only 0.05 V. As a
result, the cyclic voltammetric responses of these two
compounds practically overlap. Remarkably, this problem is
removed when the modified gold electrode developed in this
work is applied in analysis of the mixture containing EP and AA
(Fig. 11b). In the latter case, the peak potential separation
∆EEP-AA increases to 0.45 V. Based on the crystallographic
structure of [Fe2(H3L)2(MeOH)2(ꢀ-OMe)2](ClO4)4, it seems
reasonable to conclude that its chemisorption on Au template
leads to a SAM matrix with an excess of negative charge in the
form of free electron pairs localized on N atoms of the Fe-
complex in the chemisorbed layer. Thus, the electrode surface
covered by the prepared [Fe2(H3L)2(MeOH)2(ꢀ-OMe)2](ClO4)4
complex film should repel anionic species through electrostatic
repulsion while cationic species should be electrostatically
attracted. Taking into account the values of pKa = 9.9 and pKa =
4.1 of epinephrine and ascorbic acid, respectively53, it is
reasonable to conclude that under presented experimental
conditions, EP exists in its cationic form while AA in its anionic
form. It should be noted that the above given pKa values were
determined in water, alas the absolute pKa values will be
significantly higher in non-aqueous solvent54,55, yet it will not
affect final electrostatic forms of EP and AA. Thus, the positive
potential shift of the EP peak current may be attributed to the
electrostatic attraction between the positively charged EP and
the negatively charged self-assembled layer. Moreover a
repulsive interaction between the negatively charged of AA
and the negatively charged SAM would be the reason for the
movement of the peak current of AA towards the negative
potential values, as is observed on applied modified electrode
(Fig. 11b).
Fig. 11. a) Cyclic voltammograms recorded in 0.1 M TBAClO4 in ACN in the region
of the electric double layer of polycrystalline gold electrode (1); and the same as
in (1) after addition of 500
Cyclic voltammograms recorded in 0.1 M TBAClO4 in ACN in the region of the
electric double layer of polycrystalline gold electrode covered with 1000 M of
the [Fe2(H3 )2(MeOH)2( -OMe)2](ClO4)4 (1) and the same as in (1) after addition
of the mixture of 500 M EP + 5000
M AA. dE/dt = 0.1 Vs-1. c) The relationship
of the peak current of epinephrine oxidation to epinephrinequinone in the
presence of ascorbic acid at constant concentration (5000 M) versus
ꢀM EP (2) and 5000 ꢀ
M AA (3). dE/dt = 0.1 Vs-1. b)
ꢀ
L
ꢀ
ꢀ
ꢀ
ꢀ
epinephrine concentration in 0.1 M TBAClO4 in ACN for modified gold electrodes
with SAMs of Fe-complex; dE/dt = 0.1 V s-1, (R2 ≥0.9997).
The latter behavior confirms that the modified gold electrode
prepared in this paper may be successfully used for in vitro
quantitative determination of EP in the presence of excess of
AA. It should be emphasized as well that electrostatic
interactions are reciprocal to dielectric constant of the solvent
medium, therefore effect of solvent would be diminished, thus
resulting in enhancement of interactions between adsorbed
SAMs of [Fe2(H3L)2(MeOH)2(ꢀ-OMe)2](ClO4)4 and EP/AA
compounds. This may in turn contribute to the observed
electrocatalytic oxidation effect.
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Information on the detection sensitivity of new voltammetric concentrations over which the EP content can be reliably
sensor towards determination of EP in the presence of estimated on the basis of the above regression equation and
interfering AA was obtained from experiments performed by the EP detection limit is comparable or even higher than those
changing the concentration of EP with that of AA kept reported in some literature for different modified electrodes
constant, Fig 11c. Analysis was carried out over the range of (for comparison see 59). It should be however noted that the
0.01 μM to 1000 μM of EP at constant AA concentration equal data presented in Table 1 from supporting information of
to 5000 μM. A linear relationship of the oxidation current reference59 are estimated to EP behavior in water solution, but
versus epinephrine concentration (cEP) with the correlation as determination is made in vitro condition the replacement of
coefficient R2 ≥0.9997 has been observed over the range of water by ACN will have no effect on such a comparison.
0.01
following regression equations: ip [μM] = (845 ± 9) 10-2 +(740 ± 3.4 Electrode reproducibility and stability of the modified
8) 10-4 cEP [μM]. The detection sensitivity (ip/cEP) of the electrode
ꢀM to 900 ꢀM, Fig 11c and may be described by the
prepared sensor for epinephrine determination was
determined as 2.83 A cm-2 M-1. Moreover, the detection limit In the reproducibility tests, it was found that the relative
(it should be emphasized that the lower value of the detection standard deviations (RDS) of ten replicate voltammetric
limit means the possibility of the detection of the lower responses of 500 μM epinephrine solution on the novel Au/Fe-
analyte concentration) of epinephrine was estimated as complex SAM modified gold electrode was less than 3%
0.0074 μM according to the formula recommended by IUPAC: indicating excellent reproducibility of the prepared sensor. The
cL = k×sb/S56 (where sb is the standard deviation of the current stability of the [Fe2(H3L)2(MeOH)2(
ꢀ-OMe)2](ClO4)4 modified
density recorded in the supporting electrolyte solution electrode was checked daily during two weeks. Before each
evaluated on the basis of 10 times repeated measurements, S subsequent measurement, the electrically active composite
is the slope of the linear ip vs cEP dependence and k = 3 is a was only washed with ACN. In two weeks, the voltammetric
numerical constant).
responses for the 500 μM epinephrine solution decreased by
One might also consider the effect of other interfering about 5% of their initial response.
substances in the biological media. In the previous work36 we
showed that both uric and ascorbic acids are effectively
4. Conclusions
separated from dopamine when measured with Mn/Au
composite electrode. Herein, Fe/Au composite was
experimentally proved to be capable of measuring the
concentration of epinephrine in the presence of AA. The
common feature of both composite systems is that their
sensing-like properties are dependent on the electrostatic
interactions, therefore the acidity of detected compounds
determines the potential values at which they are detected.
Per analogy, one might expect that pKa = 4.1 and pKa = 5.4
values of ascorbic and uric acids are amenable for the
successive separation of the latter ones from EP herein as well.
One might consider though if dopamine (DA) and epinephrine
– which belong to the same family – could be effectively
separated i.e. that the former one would not hinder the
detection of the latter one. Oxidation of both compounds at
the bare gold electrode happens at E = 0.27 V/SCE (DA) and E =
0.31 V/SCE (EP) respectively, alas the value of the peak
separation (∆EDA-EP) is marginal. However, pKa values of DA
and EP are 8.7457 and 9.9058 respectively. This means that both
molecules would be present in their cationic form and would
The results of the study demonstrate that the newly
synthesized
[Fe2(H3
L
)2(MeOH)2(
ꢀ
-OMe)2](ClO4)4
complex
effectively forms SAMs on gold electrodes and exhibits
valuable properties with respect to the electrochemical activity
of epinephrine – a compound of biological importance. We
have shown that previously published, shielded, ꢀ-phenoxo
bridged ‘closed’ architecture of [Mn2(H2 )2](ClO4)2 is not
L
mandatory to form stable composites at gold electrode, as
herein studied Fe-complex exhibits different conformation of
H3L ligand and thus structure. This finds reflection in different
adsorption mode at the gold surface, yet still retains its
electrocatalytic properties and thus offers a new insight into
electrochemical modification of gold. We have also isolated
and characterized H3L ligand and could thus demonstrate that
not only complex
1 may be formed via direct reaction but also
that the electrochemically mediated deposition of the sole
ligand does not occur. It is crucial to note that present study
paves the way towards design of a broader array of Schiff base
metallosupramolecular architectures as electrochemical
sensors towards neurotransmitters. The modified electrode
shows good selectivity, sensitivity, reproducibility and high
stability. There is no doubt that the modified electrode
prepared in this paper is suitable for precise determination of
EP concentration without any interference of AA. The
prepared voltammetric sensor may be used – upon extraction
of the analite from the real sample – for in vitro determination
of epinephrine, as it is not important if the EP in such analysis
is prepared in water or in ACN. What is of significant notice is
that utilization of our sensor does not cause ACN to be
introduced into the human body. The preparation procedure
be positively attracted to the (1)/Au composite electrode. The
most important however is that over an order of magnitude
difference in acidity between EP and DA is enough for their
effective separation to occur, hence EP can be successfully
sensed in the presence of either AA, UA or DA.
The results observed prove that the modified gold electrode
enhances the sensitivity towards EP detection since at the bare
gold the linearity between the current density and the EP
concentration was obtained in a narrower concentration range
(0.1 μM to 500 μM) with the detection limit cL= 0.13 μM.31
Moreover, it is worth noting that both the range of
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incorporated SiO2 modified electrode, Electrochim. Acta,
2008, 53, 4615.
of the modified electrode with the Fe-complex SAM is simple
and has promising application for detection of EP in
physiological environment. An explanation of the detailed
mechanism of EP oxidation on the electrode modified with the
10 X. Jiang and X. Lin, Immobilization of DNA on carbon fiber
microelectrodes by using overoxidized polypyrrole template
for selective detection of dopamine and epinephrine in the
presence of high concentrations of ascorbic and uric acid,
Analyst, 2005, 130, 391.
11 B. Jin and H. Zhang, Nano-gold modified glassy carbon
electrode for selective determination of epinephrine in the
presence of ascorbic acid, Anal. Lett., 2002, 35, 1907.
12 N. F. Atta, A. Galal and S. M. Azab, Electrochemical
determination of neurotransmitters using gold nanoparticles
on nafion/carbon paste modified electrode, J. Electrochem.
Soc., 2012, 159, H765.
[Fe2(H3L)2(MeOH)2(
and interaction between obtained SAM layer and EP will be
the task of our future study. Furthermore, isolation of H3
ꢀ-OMe)2](ClO4)4 complex, of its structure
L
ligand will allow us to synthesize new architectures that
comprise other d-metal ions, when the template approach was
not successful.
13 N. F. Atta, A. Galal, F.M. Abu-Attia and S. M. Azab,
and
Acknowledgements
Simultaneous
determination
of
paracetamol
This research was carried out as a part of the Polish Ministry of
Science and Higher Education project (Grant No.
0111/DIA/2012/41) in the frame of “Diamond Grant”
programme. AG: I declare that I am a scholarship holder: (i)
funded by the Polish National Science Center (Grant No.
2016/20/T/ST5/00187) in the frame of ETIUDA programme; (ii)
within the project “Scholarship support for Ph.D. students
specializing in majors strategic for Wielkopolska’s
development”, Sub-measure 8.2.2 Human Capital Operational
Programme, co-financed by European Union under the
European Social Fund as well as (iii) of The Adam Mickiewicz
University Foundation in Poznań, 2014/2015 year edition. The
Ministry of Scientific Research, Poland is gratefully
acknowledged by T. Łuczak and K. Szymkowiak for financial
support of this work.
neurotransmitters in biological fluids using a carbon paste
sensor modified with gold nanoparticles, J. Mater. Chem.,
2011, 21, 13015.
14 J. Li and X. Q. Lin, Electrodeposition of gold nanoclasters on
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222.
,
15 Y. Zhang, W. Ren and S. Zhang, Simultaneous determination
of epinephrine, dopamine, ascorbic acid and uric acid by poly
dopamine-nanogold composites modified electrode, Int. J.
Electrochem. Sci., 2013, 8, 6839.
16 W. Ren, H. Q. Luo and N. B. Li, Simultaneous voltammetric
measurements of ascorbic acid, epinephrine and uric acid at
a glassy carbon modified with caffeic acid, Biosens.
Bioelectron. 21 (2006) 1086-1092.
17 S. M. Chen and K. C. Lin, The electrocatalytic properties of
biological molecules using polymerized luminal fim-modified
electrodes, J. Electronal. Chem. 2002, 523, 93.
18 F. Ni, Y. Wang, D. Zhang, F. Gao and M. Li, Electrochemical
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10 | J. Name., 2012, 00, 1-3
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ARTICLE
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DOI: 10.1039/C6RA22028B
New voltammetric sensor based on iron(III) Schiff-base complex/Au composite is synthesized and applied
for in vitro detection of epinephrine