Catalytic systems based on the organic nickel(ii) complexes in chronoamperometric determination of urea and creatinine

Article abstract of DOI:10.1007/s11172-009-0145-9

The organic nickel(ii) complexes with catalytic activity in the electrochemical oxidation of creatinine and urea were synthesized and studied. The signals of electrocatalytic oxidation of the studied carbonyl-containing amines in model solutions were obta

Full text of DOI:10.1007/s11172-009-0145-9

Russian Chemical Bulletin, International Edition, Vol. 58, No. 6, pp. 1119—1125, June, 2009
1119
Catalytic systems based on the organic nickel(II) complexes
in chronoamperometric determination of urea and creatinine*
A. N. Kozitsina,a Zh. V. Shalygina,a S. S. Dedeneva,a G. L. Rusinov,b S. G. Tolshchina,b
E. V. Verbitskiy,b and Kh. Z. Braininaa
aUral State Economical University,
62 ul. 8 Marta, 620219 Ekaterinburg, Russian Federation.
Fax: +7 (343) 257 2415. Eꢀmail: kan@usue.ru
bI. Ya. Postovsky Institute of Organic Synthesis, Ural Division, Russian Academy of Sciences,
22/20 ul. S. Kovalevskoi, 620041 Ekaterinburg, Russian Federation.
Fax: +7 (343) 374 1189. Eꢀmail: Rusinov@ios.uran.ru
The organic nickel(II) complexes with catalytic activity in the electrochemical oxidation
of creatinine and urea were synthesized and studied. The signals of electrocatalytic oxidation of
the studied carbonylꢀcontaining amines in model solutions were obtained. The detection limit
(by the 3σꢀcriterion) is 8.7•10–6 and 2.7•10–5 mol L–1 for urea and creatinine, respectively.
Key words: urea, creatinine, organic nickel(II) complexes, chronoamperometry, voltammetry,
electrocatalysis.
One of the key characteristics of human kidney and
liver activity is the content of urea (I) and creatinine (II)
in the blood serum. The content of these compounds in
the dialysis liquid defines the degree of completion of the
hemodialysis procedure.
Drawbacks of colorimetric enzymeꢀfree methods are
instability of colored complexes and complexity and
nonspecificity of analysis. Enzymatic optical methods are
specific and sensitive, but their use for urea and creatinine
determination is restricted by the instability of enzymes
during storage and exploitation, complexity of the analyꢀ
tical procedure, and high cost of equipment and expendꢀ
able materials.
Simplicity of electrochemical methods, on the one
hand, and high sensitivity and selectivity of enzymes, on
the other hand, increased the popularity of electrochemiꢀ
cal biosensors in clinical diagnostics.
The active center of urease contains nickel(II) ions,
and the active centers of creatininase and creatinine
hydrolase contain zinc and/or manganese(II) ions. Metal
ions in the enzyme composition act as cofactors, because
protein molecules are catalytically inactive and manifest
catalytic activity only in combination with metal ions.
Works on the electrocatalytic oxidation of phenols,
alcohols, vitamins, amino acids, insulin, and urea in the
presence of complexes of transition metal ions appeared
in the recent years.25—27 The Ni(OH)2 films formed on the
nickel anode in a strongly alkaline medium were used in
the electrocatalytic oxidation of selected amines and
alcohols.28 The glassy carbon electrodes (GCE) modified
by the macrocyclic nickel(II) complexes are used for the
determination of hydroxyꢀ and aminoꢀcontaining organic
compounds.27,29—31 The modifier, viz., nickel(II) complex,
is immobilized on the working surface of the GCE by the
Urea is determined using direct photometric methods
based on the reaction of urea with diacetyl monoxime or
the catalytic reactions with urease are used. The Jaffe
reaction1 or (very rarely) enzymatic hydrolysis by creatiꢀ
nine amidohydrolase using autoanalyzers2 are used for the
photometric determination of creatinine.
Conductometric,3 potentiometric,4—6 and amperoꢀ
metric variants of biosensors7—11 or field transistors12,13
with urease as a catalyst or recognizing compound are
used for the determination of urea in biological fluids.
In biosensors for creatinine determination, creatiniꢀ
nase14—16 or a threeꢀenzyme system consisting of creatiꢀ
nine amidohydrolase, creatinase, and sarcosine oxidase17—24
are often used.
* Dedicated to Academician O. N. Chupakhin on his 75th
birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1091—1097, June, 2009.
1066ꢀ5285/09/5806ꢀ1119 © 2009 Springer Science+Business Media, Inc.

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Kozitsina et al.

Catalytic oxidation of urea and creatinine
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 6, June, 2009
1121
carbon rod, respectively. Suspensions were prepared with a
UDꢀ20 automated ultrasonic disintegrator (Techpan, Poland).
Synthesis of the organic nickel(II) complexes. Complexes 2,
4, 5, 6, and 7 were synthesized at the I. Ya. Postovsky Institute
of Organic Synthesis (Ural Division, Russian Academy of
Sciences).35—38 Purity of the reagents was checked by IR and
NMR spectroscopy, mass spectrometry, and elemental analysis.
Bis(1,1,1,7,7,7ꢀhexafluoroheptaneꢀ2,4,6ꢀtrionate)nickel(II)
tetrahydrate (1). A solution of nickel(II) acetate (4.8 g, 0.012 mol)
in distilled water (25 mL) was added to 1,1,1,7,7,7ꢀhexafluoroꢀ
heptaneꢀ2,4,6ꢀtrione (4.8 g, 0.012 mol), and the reaction mixture
was stirred for 3—4 h until complex 1 precipitated. The
precipitate was filtered off, washed on the filter with several
portions of hot water, dried in air, and purified by sublimation
in vacuo. The yield was 4.2 g (85%), m.p. 102—104 °C. Found (%):
C, 27.31; H, 0.95; F, 37.12. C14H4F12Ni2O6. Calculated (%):
C, 27.41; H, 0.66; F, 37.16. IR, ν/cm–1: 1608 (C=O).
electropolymerization of the corresponding nickel(II)
complex from solution.27,29—31
The zinc complex of 1,4,7,10ꢀtetraazacyclododecane
(cyclen) incorporated into poly(ethylene glycol) dimethylꢀ
acrylate was used in the potentiometric determination of
creatinine.32
The voltammetric sensor sensitive to urea and conꢀ
sisting of the thickꢀfilm electrode modified by nickel(II)
diethyl thiocarbamate based on carbonꢀcontaining ink
“Metech” provides the stable and reproducible response
to the urea content in the blood serum.33
The purpose of the present work is the study of the
oxidation catalytic activity of some nickel(II) complexes
based on diꢀ and triketones, tetrazine derivatives, and
macrocyclic derivatives of the porphyrin and tetraazaꢀ
porphyrin series (1—9).
[(3,5ꢀDimethylpyrazolꢀ1ꢀyl)ꢀ6ꢀ(benzothiazolꢀ2ꢀylamino)ꢀ
sꢀtetrazinato](acetylacetonato)nickel(II) (3). A solution of nickel
acetylacetonate dihydrate (147 mg, 0.5 mmol) in benzene (10 mL)
was added with stirring to a heated solution of 3ꢀ(3,5ꢀdimethylꢀ
pyrazolꢀ1ꢀyl)ꢀ6ꢀ(benzothiazolꢀ2ꢀylamino)ꢀsꢀtetrazine (162 mg,
0.5 mmol) in benzene (10 mL). The mixture was stirred for 3 h
at 50 °C and then left to stand at room temperature for 8 h.
A black precipitate formed was filtered off and washed on the
filter with benzene and methanol. The yield was 191 mg (77%),
m.p. > 350 °C. Found (%): C, 45.19; H, 3.63; N, 22.36; Ni, 10.86;
S, 6.40. C19H18N8O2SNi•H2O. Calculated (%): C, 45.72;
H, 4.04; N, 22.45; Ni, 11.76; S, 6.42. IR, ν/cm–1: 1599, 1512,
1484, 1452, 1399, 1123, 1100, 1057, 1019, 986, 798, 754, 727.
Nickel(II) 2ꢀphenacylꢀ5,10,15,20ꢀtetraphenylporphyrinate
(8). A mixture of acetophenone (90 μL, 0.15 mmol) and Cs2CO3
(250 mg, 0.11 mmol) in DMF (20 mL) was stirred for 30 min.
Then nickel(II) 2ꢀnitroꢀ5,10,15,20ꢀtetraphenylporphyrinate
(110 mg, 0.77 mmol) was added, and the mixture was stirred for
72 h. The solvent was distilled off in vacuo, and the solid residue
was dissolved in CHCl3 (50 mL), washed with H2O (3×50 mL),
and dried with anhydrous Na2SO4. The residue was separated
on silica gel eluting with a CHCl3—hexane (1 : 1) mixture.
Compound 8 was obtained as a violet powder. The yield was
40 mg (33%). MS, m/z (Irel (%)): 787 [M – 3 H]+ (100), 788
[M – 2 H]+ (68), 789 [M – H]+ (55), 790 [M]+ (60), 791
[M + H]+ (15), 831 [M + CH3CN]+ (20).
Nickel(II) 2ꢀ(2ꢀnitrophenacyl)ꢀ5,10,15,20ꢀtetraphenylporꢀ
phyrinate (9). A mixture of 2ꢀnitroacetophenone (173 mg,
0.11 mmol) and Cs2CO3 (341 mg, 0.11 mmol) in DMF (10 mL)
was stirred for 30 min. Then 2ꢀnitroꢀ5,10,15,20ꢀtetraphenylꢀ
porphyrinato)nickel(II) (150 mg, 0.21 mmol) was added to the
reaction mixture, and the latter stirred for 2.5 h. The solvent was
distilled off in vacuo, and the solid residue was dissolved in
CHCl3 (50 mL), washed with H2O (3×50 mL), and dried with
anhydrous Na2SO4. The residue was separated on silica gel
eluting with CHCl3. Compound 7 was obtained as a violet
powder. The yield was 81 mg (47%). 1H NMR (CDCl3), δ: 2.65
(s, 2 H, CH2); 7.17—7.36 (m, 12 H, Ar); 7.59—7.80 (m, 10 H,
Ar); 7.81—7.88 (m, 5 H, Ar); 8.16—8.18 (m, 2 H, Ar); 8.28 (m,
1 H, Ar); 8.39 (m, 1 H, Ar). MS, m/z (Irel (%)): 833 [M – 2 H]+
(100), 834 [M – H]+ (37), 835 [M]+ (8), 836 [M + 2 H]+ (91).
Procedure of preparation of the working electrode. Preparation
of modifying solutions/suspensions. Solutions of the complexes
were prepared by the dissolution of exact weighed samples of 1
The compounds were chosen on the basis of available
literature data27,29—31 on the application of the macrocyꢀ
clic complexes in the catalytic determination of amines
and alcohols and due to the relative simplicity of the work
with these compounds and their accessibility and stability
in aqueous media.
Experimental
Bis(1,1,1,7,7,7ꢀhexafluoroheptaneꢀ2,4,6ꢀtrionate)dinickel(II)
tetrahydrate (1), bis(1ꢀphenylꢀ4,4,5,5,6,6,7,7,7ꢀnonafluoroꢀ
heptadionateꢀ1,3)nickel(II) dihydrate (2), [(3,5ꢀdimethylpyrazolꢀ
1ꢀyl)ꢀ6ꢀ(benzothiazolꢀ2ꢀylamino)ꢀsꢀtetrazinato](acetylacetonato)ꢀ
nickel(II) (3), Ni5(μ ꢀOH)2(μꢀOOCCMe3)4(μꢀN,N´,N´´ꢀ3,5ꢀ
3
Me2C3HN2C2(O)N4)4(MeCN)2 (4), nickel(II) tetrapyrazinoꢀ
porphyrazinate (5), nickel(II) 2,9,16,23ꢀtetra(4ꢀheptylphenyl)ꢀ
tetrapyrazinoporphyrazinate (6), nickel(II) mesoꢀtetraphenylꢀ
porphyrinate (7), nickel(II) 2ꢀphenacylꢀ5,10,15,20ꢀtetraphenylꢀ
porphyrinate (8), and nickel(II) 2ꢀ(2ꢀnitrophenacyl)ꢀ5,10,15,20ꢀ
tetraphenylporphyrinate (9) were synthesized at the I. Ya.
Postovsky Institute of Organic Synthesis (Ural Division, Russian
Academy of Sciences).
N,NꢀDimethylformamide, dimethyl sulfoxide, 1,2ꢀdimethꢀ
oxyethane, acetonitrile, and NaOH were commercially availꢀ
able from Ecros and used as received. Poly(vinyl chloride) (PVC)
(Mη = 75000, Dzerzhinsk, Russia) served as the polymeric basis
of the protective film. Commercial dibutyl phthalate was used as
a plasticiser. Weighed samples of sodium hydroxide, urea (Fluka),
or creatinine (Merck) were dissolved in triply distilled water.
The structure of complex 3 was proposed by analogy to the
earlier describe product of complex formation of 3ꢀ(5ꢀaminoꢀ
1,3,4ꢀthiadiazolꢀ2ꢀylamino)ꢀ6ꢀ(3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀ
sꢀtetrazine with nickel cations34 and was confirmed by the IR
spectral data.
Voltammetric and chronoamperometric studies were carried
out using an IVAꢀ5 inversion voltammetric analyzer (IVA,
Ekaterinburg) equipped with a threeꢀelectrode cell. The working
electrodes (WE) were transducers modified by the nickel(II)
complexes. Thickꢀfilm electrodes (IVA, Ekaterinburg) were used
as transducers, and reference and auxiliary electrodes were a
saturated silver—silver chloride electrode (SSCE) and a glassy

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Kozitsina et al.
(50 mg), 5 (10 mg), 8 (5 mg), or 9 (5 mg) in 1 mL of DMSO, and
weighed samples of 2 (200 mg) and 4 (4 mg) were dissolved in
1 mL of DMF and 1,2ꢀDME, respectively. Suspensions were
prepared by the ultrasonic treatment of exact weighed samples
of complexes 3 (50 mg), 5 (40 mg), or 6 (40 mg) in 2 mL of
DMF and complex 7 (23 mg) in 2 mL of THF.
Modification of transducers was carried out by the deposition
of a solution or suspension on the surface of the working zone of
the transducer followed by drying in air until the solvent
evaporated completely. To prevent physical washing down of
complex 1 with the supporting solution, a film of plasticized
PVC was deposited on the working surface of the WE by the
evaporation of 3 μL of polymer solution in THF.
I/μA
30
4
3
20
10
2
0
–10
–20
1
Electrochemical preparation of the working electrode. The
working surface of the WE was formed by cyclic voltammetry in
a threeꢀelectrode cell against the background of 0.25 M NaOH
as described earlier.33
0.1
0.2
0.3
0.4
0.5
0.6 E/V
Fig. 2. Cyclic voltammograms for the transducer in the absence
(1) and presence of 3•10–3 M urea in a solution (2) and for the
working electrode in the absence (3) and presence of 3•10–3
urea in a solution (4); catalyst 2, background 0.25 M NaOH, and
scan rate 0.1 V s–1
Chronoamperometric determination of the catalytic activity
of the organic nickel(II) complexes. To study the catalytic activity
of the organic nickel(II) complexes, 10 mL of the background
(0.25 M NaOH) and the WE were placed in the electrochemical
cell. The potential was cycled from 0 to 0.8 V to form the
working surface of the WE. The chronoamperogram of nickel
oxidation was detected in the supporting electrolyte at the
potential defined depending on the nature of the modifier
used at 0.568 0.013 (1), 0.566 0.013 (2), 0.549 0.015 (3),
0.658 0.080 (4), 0.696 0.027 (5), 0.621 0.024 (6), 0.641 0.050
(7), 0.546 0.013 (8), and 0.543 0.010 V (9). A 0.1 M solution
of urea or a 0.01 M solution of creatinine was added by portions
of 0.1 mL, and the chronoamperometric signal was repeatedly
detected.
M
.
shown in Fig. 1. The shape of the first cyclic voltamꢀ
mogram is typical of the transducer. The further conseꢀ
cutive potential cycling results in the appearance and
increase in the anodic—cathodic peak characteristic of
the NiII—NiIII system with Еа in the range from 0.45 to
0.60 V (anodic peak) and with ЕC in the range from 0.3
to 0.4 V (cathodic peak), depending on the nature of the
modifier used. Sixty cycles are enough for the complete
formation of the working surface of the WE.
The cyclic voltammograms detected using the transꢀ
ducer and WE in the absence and presence of urea or
creatinine are shown in Figs 2 and 3, respectively. The
response of the oxidation current of urea or creatinine is
observed only when the WE is used, indicating the
electrocatalytic oxidation of the corresponding amines.
In all cases, the current was measured 70 s after.
The efficiency of the catalytic systems was estimated using
the “added—found” method.33
Results and Discussion
Voltammetric study of the catalytic activity of the
nickel(II) complexes. The characteristic cyclic voltammoꢀ
grams obtained upon the formation of the WE surface are
I/μA
4
I/μA
40
60
4
3
30
3
15
2
20
10
1
1
0
2
1
0
–1
–2
–3
–10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
E/V
Fig. 3. Cyclic voltammograms for the transducer in the absence
(1) and presence of 0.02•10–3 M creatinine in a solution (2) and
for the working electrode in the absence (3) and presence of
0.02•10–3 M creatinine in a solution (4). Catalyst 2, background
0.1
0.2
0.3
0.4
0.5
0.6 E/V
Fig. 1. Characteristic cyclic voltammograms for the working
electrodes in 0.25 M NaOH (scan rate 0.1 V s–1). Catalyst 2;
1, 15, and 60: respective numbers of cycles.
0.25 M NaOH, and scan rate 0.1 V s–1
.

Catalytic oxidation of urea and creatinine
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 6, June, 2009
1123
A similar current increase in the presence of urea or
creatinine was observed for the chronoamperometric
determination of the corresponding analytes (Fig. 4).
Since the compounds containing the amino group form
radical cations during electrochemical oxidation,39 the
electrochemical catalytic oxidation of urea or creatinine
proceeds via Scheme 1.
I/μA
a
I/μA
10
8
R
2 = 0.999
2.5
1.5
8
6
0
2
4
6
C/mМ
4
7
Scheme 1
1
0
10
20
30
40
50
60
70
80 t/s
R(R´)NH2 is urea or creatinine, and L is an organic ligand.
b
I/μA
I/μA
Selection of the catalyst. Complexes 4 and 7 exhibit
no electrochemical catalytic activity in the presence
of urea. The phenyl substituents in 7 or 3ꢀhydroxyꢀ6ꢀ
(3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazine molecules in clusꢀ
ter 4 prevent the urea molecules to access the active sites
of the modifiers. In the case of complexes 8 and 9, the
coordination of the urea molecules near the Ni2+ center
through hydrogen bonding between the NH2 groups of
urea and the carbonyl group of the substituent competes
with the steric factor of the phenyl groups. The electroꢀ
chemical signal observed in this case in the presence of
urea is at the level of measurement error.
In the case of other nickel(II) complexes under study,
a distinct electrochemical response to the presence of urea
in the solution is observed.
The analytical characteristics of urea and creatinine
determination using the WE based on complexes 1—3, 5,
and 6 are listed in Table 1. The WE based on complexes 6
2.4
2.0
1.6
1.2
0.8
0.4
13
0.8
R
2 = 0.999
0.4
0
0.2
0.4
C/mМ
12
1
10
20
30
40
50
60
70
80
t/s
Fig. 4. Chronoamperograms of the oxidation of urea (a, curves
1—7) and creatinine (b, curves 1—12) obtained for the working
electrodes at different concentrations of the analyte; background
0.25 M NaOH; catalyst 2. Insets: dependences of the analytical
signal on the concentration of urea (a, 8) and creatinine (b, 13).
Table 1. Analytical characteristics of chronoamperometric determination of urea and creatinine using the working
electrodes against the background of 0.25 mol L–1 NaOH
Complex
NiII
Сamine/mol L–1
Added Found
Sr(%)
Parametersa
R
c/mmol L–1
a
b
Urea
1
2
3
5
6
1.11 0.09
1.09 0.09
0.98 0.05
0.85 0.12
0.95 0.08
9.4
8.5
5.5
6.7
7.0
5.46
0.264
18.706
5.729
0.040
9.08
0.547
20.618
9.565
0.647
0.9951
0.9991
0.9923
0.9987
0.9947
1
0.01—10
Creatinine
1
2
3
0.54 0.09
0.58 0.20
0.46 0.04
13.6
27.7
7.8
1.88
0.38
0.79
0.119
–0.04
0.18
0.9974
0.9877
0.9987
0.9544
0.9972
0.03—0.5
0.03—1
0.05—0.55
0.03—1
1.45
0.915
0.72
2.11
0.5
0.03—0.5
aParameters for the regression equation I = aC + b (I/μA, C/mol L–1).

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Russ.Chem.Bull., Int.Ed., Vol. 58, No. 6, June, 2009
Kozitsina et al.
I/μA
10
generate the least pronounced (among the presented group
of catalysts) analytical signals of urea oxidation in the
range of the analyte concentrations studied. This is due,
most likely, to the partial screening of the Ni2+ sites of the
catalyst from the urea molecules by the heptylphenyl
substituents.
8
6
R2 = 0.9997
4
The optimum results (the least value of the relative
standard deviation (RSD) and closest values of the found
additive) were obtained when the WE based on complexes
1—3 and 5 were used. The linear dependence of the
analytical signal on the urea concentration for the presented
2
0
v/mV s–1
50
100
150
WE ranges from 10–5 to 10–2 mol L–1
.
Fig. 5. Dependence of the maximum current of catalyst oxidaꢀ
tion in the absence of area on the potential sweep rate; backꢀ
ground 0.25 M NaOH; catalyst 2.
The detection limit is 8.7•10–6 mol L–1 (by the 3σꢀ
criterion), which is lower than the detection limit of urea
in the blood serum (Cmin = 0.2•10–3 mol L–1) determined
by the standard enzymatic method using a BUN/UREA
Vitrous System colorimetric analyzer (Johnson and Johnson,
Great Britain).33
I/μA
0.360
0.350
0.340
0.330
0.330
The WE based on complexes 1—3, 5, and 6 were used
for the electrochemical determination of creatinine. It is
known that the catalytic activity of the nickel(II) comꢀ
pounds in electrochemical oxidation reactions decreases
on going from the primary to secondary amine.28 Since
creatinine is a secondary amine, it is reasonable to expect
a decrease in the electrocatalytic response of creatinine
oxidation for the WE used compared to the electrocatalytic
response of the current corresponding to urea oxidation.
Indeed, in the case of using the WE based on complexes 5
and 6, well pronounced signals for creatinine oxidation
were not obtained.
0.11
0.13
0.15
t
–0.5/s–0.5
Fig. 6. Dependence of the analytical signal on , where t is the
detection time of the chronoamperogram at the content of urea
in the cell equal to 1•10–3 mol L–1; background 0.25 M NaOH;
catalyst 2.
The dependence of I on t–0.5 is linear in the range of
the detection time of the analytical signal from 30 to 70 s
and corresponds to the Cottrell equation. This indicates
that the oxidation of urea is limited by analyte diffusion
from the solvent bulk.
However, when the WE based on complexes 1—3
are used in electrochemical oxidation, the analytical
signal of creatinine is well pronounced. The optimum
results (the least RSD value) were obtained when using
these WE. The range of the linear dependence of the
analytical signal on the creatinine concentration is from
5•10–5 to 10–3 mol L–1. The detection limit equal to
2.7•10–5 mol L–1 is lower than the minimum content of
creatinine in the blood serum of the healthy adults
(4.4•10–5 mol L–1).
The study of the electrochemical oxidation of creatinine
also gave the linear dependence of I on t–0.5
.
It is most likely that the oxidation rate of the amines
under study is limited by the diffusion of the analyte to the
working surface of the WE.
The working electrodes with the supported nickel(II)
complexes make it possible to obtain an analytical signal
in the heterogeneous electrocatalytic oxidation of electrically
inactive urea (complexes 1—3, 5, and 6) or creatinine
(complexes 1—3). The electrochemical oxidation of the
carbonylꢀcontaining amines under study occurs presumably
with the intermediate formation of radical cations, and
the rateꢀdetermining step is analyte diffusion to the
working electrode surface. The linear dependence of
the analytical signal on the analyte concentration with
the correlation coefficient 0.999 and the low detection
limit (8.7•10–6 and 2.7•10–5 mol L–1 for the determinaꢀ
tion of urea and creatinine, respectively) make it possible
to use these WE for the determination of the concenꢀ
trations of studied amines in biological fluids.
The linear dependence of the analytical signal on the
creatinine concentration with the correlation coefficient
0.999 and the low detection limit confirm that the method
of additives can be used for the calculation of the creatinine
concentration in body fluids.
Kinetics of amine oxidation. The dependence of the
current of catalyst oxidation on the sweep rate obtained by
cyclic voltammetry is shown in Fig. 5. The linear dependꢀ
ence of I on v indicates that an electroactive substance
localized on the transducer surface participates in the rateꢀ
determining process. It is most likely that this process is
the reversible electrochemical oxidation of NiIIL to NiIIIL.
The dependence of the current of urea oxidation on
the detection time of the analytical signal obtained by
chronoamperometry is shown in Fig. 6.

Catalytic oxidation of urea and creatinine
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 6, June, 2009
1125
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This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 06ꢀ03ꢀ
08141ꢀofi, 07ꢀ03ꢀ96068ꢀr_ural_a, 07ꢀ03ꢀ96112ꢀr_ural_a,
and 07ꢀ03ꢀ96113ꢀr_ural_a).
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Received December 5, 2008;
in revised form April 27, 2009