Electrochimica Acta 145 (2014) 209–216

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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta

Electrooxidation of morin on glassy carbon electrode modified by

carboxylated single-walled carbon nanotubes and surfactants
Guzel Ziyatdinova ∗ , Endzhe Ziganshina, Herman Budnikov
Analytical Chemistry Department, A.M. Butlerov Institute of Chemistry, Kazan Federal University, Kremlyevskaya, 18, Kazan, 420008, Russian Federation

a r t i c l e i n f o a b s t r a c t

Article history: Voltammetric characteristics of morin on glassy carbon electrode (GCE) modified by carboxylated single-
Received 22 July 2014 walled carbon nanotubes (SWNT-COOH) and surfactants in phosphate buffer have been found. Cationic
Received in revised form 18 August 2014 cetylpyridium bromide (CPB), nonionic Triton X100 and anionic sodium dodecylsulfate surfactants
Accepted 20 August 2014
under different concentrations have been tested as modifier of SWNT-COOH/GCE. The form of CVs and
Available online 28 August 2014
oxidation potentials are not changed significantly in the presence of all type surfactants on the elec-
trode surface that confirms negligible influence of surfactant on electron transfer rate. Morin oxidation
Keywords:
currents are increased on surfactant-modified electrodes. The best characteristics are observed on CPB
Voltammetry
Chemically modified electrodes
(1 ␮M)/SWNT-COOH/GCE when 1.8-fold increase of oxidation currents has been observed in compari-
Carbon nanotubes son with SWNT-COOH/GCE. Mechanism of morin oxidation on CPB/SWNT-COOH/GCE is suggested on
Surfactants the basis of relationship between oxidation potential and pH of supporting electrolyte. Electrooxida-
Morin. tion is adsorption-controlled irreversible two-step process with participation of one electron and one
proton on each step. The linear dynamic ranges of morin determination under conditions of differential
pulse voltammetry are 0.1-100 and 100-750 ␮M with the limits of detection and quantification 28.9 and
96.0 nM of morin, respectively. The developed approach applied for morin quantification in mulberry
leaves using preliminary extraction with ethanol.
© 2014 Elsevier Ltd. All rights reserved.

1. Introduction Antioxidant properties of morin are caused by ability to elec-

tron transfer that allows to use electrochemical methods for their
Morin (3,2 ,4 ,5,7-pentahydroxyflavone) is one of the natural investigation. Electrochemical measurements leading to the deter-
flavonoids that is presented in plants, fruits, flowers and plant mination of physicochemical parameters for antioxidants (e.g.,
derived materials [1,2]. It belongs to flavonol subclass and consists redox potential, number of electrons transferred, electrode reac-
of two aromatic rings (A and B in Scheme 1) which are linked by an tion rate constant, etc), are relevant also for understanding the
oxygen-containing heterocycle (ring C). reaction mechanisms. On the other hand, the electroanalytical
Morin possesses various biological and biochemical effects techniques have advantages over other analytical methods, such as
including anti-inflammatory, antineoplastic, cardioprotective rapid response, higher sensitivity and low detection limits, as well
activities [3–5] and chemopreventive effect against oral carcino- the possibility to improve the selectivity by using suitable electrode
genesis in vitro and in vivo [6]. Moreover, it shows antioxidant conditions.
properties that realized via different mechanisms: scavenging of Therefore, a number of electrochemical methods using
reactive oxygen species, inhibition of the enzymes participating in bare glassy carbon (GCE) [11,12], platinum [13,14] and hang-
reactive oxygen species production, chelation of low valent metal ing mercury dropping [15] electrodes as well as various
ions such as Fe2+ or Cu2+ and regeneration of membrane bound modified electrodes based on graphene oxide/silver nanoparti-
antioxidants such as ␣-tocopherol [7–10]. cles [16], poly(tetrafluroethylene)-deoxyribonucleate acid film-
modified GCE [17], polyvinylpyrrolidone-doped carbon paste
electrode [18], multi-walled carbon nanotubes-paraffin oil paste
electrode [19] and nujol-graphite or diphenylether-graphite paste
∗ Corresponding author. Analytical Chemistry Department A.М. Butlerov Institute electrodes [20] have been developed for morin determina-
of Chemistry Kazan Federal University Kremlyevskaya, 18 Kazan Russia 420008 tion. Application of chemically modified electrodes increases the
Tel.: +7 843 2337736; fax: +7 843 2387901.
E-mail address: Ziyatdinovag@mail.ru (G. Ziyatdinova).
sensitivity and selectivity of quantification. Different types of

http://dx.doi.org/10.1016/j.electacta.2014.08.062
0013-4686/© 2014 Elsevier Ltd. All rights reserved.
210 G. Ziyatdinova et al. / Electrochimica Acta 145 (2014) 209–216

Scanning electron microscopy (SEM) of the electrode surfaces

was performed using tabletop scanning electron microscope TM-
1000 (Hitachi, Japan).
“Expert-001” pH meter (Econix-Expert Ltd., Russia) equipped
with the glass electrode was used for pH measurements.

2.3. Preparation of the modified electrodes

The GCE was carefully polished with alumina (0.05 ␮m) on pol-
ishing cloth. Then, it was rinsed with acetone and double distilled
Scheme 1. Structure of morin.
water before use. Homogeneous suspension of SWNT-COOH with
final concentration of 0.5 mg mL−1 was got by ultrasonic disper-
voltammetry have been applied for the evaluation of morin-DNA sion for 10 min in double distilled water. Electrodes modification
interactions [17,21] that clarifies the pharmacokinetics of morin. was performed by coverage of the GCE with 4 ␮L SWNT-COOH sus-
Surfactants are used in electroanalytical chemistry as sub- pension without any electrochemical precondition of the electrode
stances improving the electrochemical characteristics of biolog- surface and evaporation to dryness.
ically active substances [22–25]. Two important properties of Surfactant/SWNT-COOH/GCE was prepared in the same way
surfactants are advantageously used in electroanalysis namely as SWNT-COOH/GCE with additional drop casting of 2 ␮L sur-
adsorption at the interface and self-aggregation into organized factant solution of corresponding concentration in the range of
structures [26]. Therefore, surfactants are able to modify and con- 1-10000 ␮M.
trol the properties of electrode surfaces leading to changes in
reaction rates and pathways. Combination of surfactants with 2.4. Procedures
carbon nanomaterials as electrode surface modifier is another
opportunity to affect on electrochemical processes and not inves- 2.4.1. Scanning electron microscopy (SEM)
tigated enough. SEM images of the electrode surfaces were performed at room
The present work is focused on electrochemical behavior of temperature in ambient conditions. The 4 ␮L of SWNT-COOH sus-
morin on GCE modified with carboxylated single-walled carbon pension were dropped on the GCE surface and allowed to evaporate
nanotubes (SWNT-COOH) and surfactants being investigated for to dryness. Then SEM-image of the surface was scanned at accel-
the first time and development of differential pulse voltammetric erating voltage of 15 kV and emission current 35.7 mA. The same
procedure for its quantification. procedure was applied for Surfactant/SWNT-COOH/GCE.

2. Experimental 2.4.2. Cyclic voltammetry

10.0 ml of supporting electrolyte containing 5% ethanol or morin
2.1. Reagents model solution were added in electrochemical cell and cyclic
voltammograms were recorded at potential range from 0.1 to 1.0 V
Morin hydrate (85% purity) was purchased from Sigma and scan rate of 100 mV s−1 .
(Germany). Its 0.02 M stock solution was prepared daily dissolving
a definite amount of the substance in 5.0 mL of ethanol (rectifi- 2.4.3. Differential pulse voltammetry (DPV)
cate). More dilute solutions (model solutions) were prepared before 10.0 ml of supporting electrolyte containing 5% ethanol or morin
measurements in 10.0 mL volumetric flasks by dilution of the stock model solution were added in electrochemical cell and anodic DP
solution with supporting electrolyte (0.1 M phosphate buffer (PB)). voltammograms were registered within the potential range from 0
The ethanol portion was reduced to 5%. to 1.0 V using the pulse amplitude of 50 mV, pulse width 50 ms and
Carboxylic acid functionalized single-walled carbon nanotubes scan rate 10 mV s−1 .
(SWNT-COOH) with OD 4-5 nm and length 500-1500 nm (90%
purity) were obtained from Sigma-Aldrich (Germany). 2.4.4. Sample preparation and morin quantification
Surfactants Triton X100 from Sigma (Germany), sodium dodecyl Raw mulberry leaves (Morus nigra L.) were used. Preliminary
sulfate (SDS) from Panreac (Spain) and cetylpyridinium bromide extraction of morin with ethanol was applied. A representative
(CPB) from Aldrich (Germany) were used. Their stock solutions portion of the milled leaves (0.1 g) was accurately weighted and
(0.1 M) were prepared by dissolving a definite amount of the appro- quantitatively transferred into separating funnel. Then, 2 mL of
priate substance in 10.0 mL of water. More dilute solutions were ethanol were added and shaked for 15 min. The extract was col-
prepared by exact dilution of the stock solution. lected and used for further measurements.
All other chemicals were of analytical reagent grade purity and The standard addition method was employed for the quantifica-
used as received. Double distilled water was used for the measure- tion of the morin. 0.25 mL of the extract was transferred into the cell
ments. The experiments were carried out at laboratory temperature with 9.75 mL of the supporting electrolyte. The additions of morin
(25 ◦ C). All solutions were kept in glass vessels at +4 ◦ C. stock solution were 2.5-7.5 ␮M. DP voltammograms in the range
of 0-1 V at scan rate 10 mV s−1 were recorded. Morin content was
2.2. Apparatus recalculated per 1 g of raw leaves.

Voltammetric measurements were performed on potentio- 2.4.5. Statistical analysis

stat/galvanostat ␮Autolab type III with the software GPES, version All measurements were performed 5 times. Statistical evalua-
4.9.005 (Eco Chemie B.V., Netherlands). The electrochemical cell tion was performed at significance level of 5% by SPSS for Windows
consisted of working GCE (SWNT-COOH/GCE or surfactant/SWNT- software (SPSS Inc., USA). All data were expressed as the X ± X
COOH/GCE) with 3.14 mm2 geometric surface area, silver-silver where X was the average value and X was the confidence interval.
chloride saturated KCl reference electrode and counter electrode Regression analysis was performed using OriginPro 8.0 (Origin-
(platinum wire). Lab, USA) software.
 G. Ziyatdinova et al. / Electrochimica Acta 145 (2014) 209–216 211

Fig. 1. SEM images of bare GCE (A), SWNT-COOH/GCE (B) and CPB/SWNT-COOH/GCE (C).

3. Results and discussion presence of carboxylic groups. The importance of the functionaliza-
tion appears to be with regards to the number of oxygenated species
3.1. Characterization of modified electrode that present on the tubes ends and walls, as far as the more oxygen-
containing groups lead to the higher rate of electron transfer that
The surface of GCE and modified electrodes has been checked agrees well with the data reported earlier [30–32].
by SEM. Fig. 1 displays the typical topography of bare GCE, SWNT-
COOH/GCE and CPB/SWNT-COOH/GCE.
3.3. Voltammetric behavior of morin on
GCE shows unstructured smooth surface. On SWNT-COOH mod-
Surfactant/SWNT-COOH/GCE
ified electrode, the top view changes significantly. SWNTs are
randomly distributed as aggregates with linear sizes of 5-112 ␮m
Cationic (CPB), anionic (SDS) and nonionic (Triton X100) surfac-
indicating immobilization of nanomaterial on the electrode surface.
tants have been investigated as modifiers of SWNT-COOH/GCE. All
Addition of CPB layer does not effect on the surface topography and
present as very thin film that is caused by initial concentration of
surfactant solution of 1 ␮M.

3.2. CV of morin on GCE and SWNT-COOH/GCE

Voltammetric behavior of morin on GC-based electrodes in

0.1 M PB рН 7.0 containing 5% of ethanol has been studied. Morin
is irreversibly oxidized on GCE at 0.41 V (Fig. 2, curve 2) that is con-
firmed by the absence of cathodic step on the backward branch of
CV. Modification of the electrode surface with SWNT-COOH leads
to significant changes in the form of morin CV showing two well-
defined irreversible oxidation steps at 0.29 and 0.79 V (Fig. 2, curve
3).
So, the decrease of morin overpotential on 0.12 V means higher
electron transfer rate. Oxidation currents on SWNT-COOH/GCE are
7-fold higher than that on bare GCE due to increase of the effective
surface area of modified electrode. Such changes in voltammet-
ric characteristics of morin indicate catalytic effect of SWNT-COOH
that corresponds to data reported earlier for other phenolic antiox-
Fig. 2. CVs of 100 ␮M morin on GCE (curve 2), SWNT-COOH/GCE (curve 3) and
idants on multi-walled carbon nanotube modified GCE [27–29]. CPB(1 ␮M)/SWNT-COOH/GCE (curve 4) in 0.1 M phosphate buffer pH 7.0 (curve 1
Moreover, electrocatalytic effect of SWNT-COOH is caused by on GCE) containing 5% of ethanol. Potential scan rate is 100 mV s−1 .
212 G. Ziyatdinova et al. / Electrochimica Acta 145 (2014) 209–216

Fig. 3. Effect of surfactant nature and concentration on potential (A) and current (B) of morin electrooxidation on Surfactant/SWNT-COOH/GCE in phosphate buffer solution
pH 7.0. Potential scan rate is 100 mV s−1 .

of them are electrochemically inactive at anodic potentials under anionic SDS micelles, i.e. the lack of their adsorption on electrode
investigation. surface.
Morin is electrochemically active on surfactant-modified The best parameters of morin oxidation are obtained on 1 ␮M
SWNT-COOH/GCE. The voltammetric characteristics are indepen- CPB/SWNT-COOH/GCE.
dent on the way of electrode preparation (layer-by-layer drop It should be noted, that morin oxidation product is strongly
casting or mixture of modifiers). Surfactant/SWNT-COOH/GCE has adsorbed on the electrode surface leading to it blockage that is
been used for further investigations. confirmed by substantial decrease of morin oxidation currents on
Effect of surfactant nature and concentration on morin voltam- 30-45% during the second scan. Therefore, new electrode has been
metric characteristics is presented on Fig. 3. The form of CVs and prepared before each measurement.
oxidation potentials do not change significantly in the presence
of all type surfactants on the electrode surface. This fact confirms 3.3.1. Effect of pH
negligible influence of surfactant layer on SWNT-COOH/GCE on Effect of supporting electrolyte pH in the range of 4.8-9.0 has
electron transfer rate that agrees well with data for morin behavior been investigated. There are two irreversible oxidation steps (the
in surfactant micellar media [33,34]. first one is the peak) on CVs of morin in the whole range of pH.
Morin oxidation currents on Surfactant/SWNT-COOH/GCE are Morin oxidation potentials decrease as pH increases while oxida-
1.5-fold increased vs. SWNT-COOH/GCE in the presence of high con- tion currents grow up until pH 8.0 is reached (Fig. 4). Ionization
centrations (1-10 mM) of SDS and Triton X100 when the critical of morin molecule (pKa1 = 4.99, pKa2 = 8.29, pKa3 = 10.33) leads to
micelle concentration is achieved. Spherical and rodlike micelles easier electron detachement from anion as well as increase of
of SDS and Triton X100 occuring on the electrode surface at sur- its oxidation currents. Further increase of pH leads to decrease
factant concentrations noted above able to include morin inside of morin oxidation currents on 13% due to partial oxidation of
the micelles (in the palisade layer or outsides it for spherical and substrate by dissolved molecular oxygen that is typical for all easy-
rodlike micelles, respectively) due to hydrophobic interactions oxidisable compounds. So, pH 8.0 has been chosen for further
[33,34], i.e. providing its concentration. At lower concentrations investigations.
when the pre-micelles present, a static repulsion between SDS Mechanism of morin electrooxidation on CPB/SWNT-COOH/GCE
and the morin anion takes place that corresponds well with spec- is suggested on the basis of relationship between oxidation poten-
troscopy data [34]. So, the SDS pre-micelle leads to insignificant tial and pH of supporting electrolyte. There is linear dependence
decrease of morin oxidation currents. In the case of CPB, the oppo-
site effect is observed. Morin oxidation currents do not change
significantly in the whole range of surfactant concentration exclud-
ing 1 ␮M of CPB for which 1.8-fold increase of oxidation currents
has been observed in comparison with SWNT-COOH/GCE. This
can be explained by two factors. The first one is hydrophobic
interaction of morin aromatic rings with alkyl- and aromatic frag-
ments of CPB molecules. At the same time, morin (pKa1 = 4.99)
is ionized and negatively charged in PB pH 7.0 that provides its
electrostatic interaction with positively charged film of CPB on
the SWNT-COOH/GCE surface. Both types of interactions leads
to concentration of morin on the electrode surface and, there-
fore, increase of oxidation currents. The higher CPB concentration
results to rising thickness and density of CPB film delaying the
electron transfer that confirms by anodic shift of morin oxida-
tion potentials. The opposite effect is observed in the case of SDS
and Triton X100 high concentrations. The morin oxidation currents
increase due to the absence of electrostatic interaction between Fig. 4. Effect of supporting electrolyte pH on voltammetric characteristics of morin
ionized carboxylic groups of SWNTs and nonionic Triton X100 and oxidation on CPB/SWNT-COOH/GCE. Potential scan rate is 100 mV s−1 .
 G. Ziyatdinova et al. / Electrochimica Acta 145 (2014) 209–216 213

Scheme 2. Morin electrooxidation on CPB/SWNT-COOH/GCE.

Fig. 5. A) CVs on CPB/SWNT-COOH/GCE in phosphate buffer solution pH 8.0 in the absence (curve 1) and in the presence of 100 ␮M morin at the following scan rates (mV
s−1 ): 25 (curve 2), 50 (3), 100 (4) and 250 (5). B) Relationship between morin oxidation current and potential scan rate.

of morin first oxidation peak potential (E1 ) vs. pH value. The corre- electrode in organic media [13]. On contrary, one two-
sponding Equation 1 is electronic peak of morin oxidation has been observed on
renewable pencil electrode [38], GCE in acidic media [21] and
E1 [V ] = (0.70 ± 0.01) − (0.060 ± 0.002)pH R2 = 0.9952 (1) poly(tetrafluroethylene)-deoxyribonucleate acid film-modified
GCE [17].
The slope of 60 mV per pH unit indicates that the process
involves a proton and the number of protons and electrons par-
ticipating in the electrode reaction is the same. 3.3.2. Effect of scan rate
The plot of E2 vs. pH for the second oxidation step of morin is The influence of potential scan rate in the range of 10-250 mV
described by the Equation 2 s−1 on the voltammetric behavior of morin has been tested (Fig. 5A).
The first peak oxidation current is proportional to the scan rate
E2 [V ] = (1.251 ± 0.009) − (0.065 ± 0.001)pH R2 = 0.9993 (2)
(Fig. 5B) and described by Equation 3:
These data show the equal number of electrons and protons
involved in the second oxidation step of morin meaning that dur- Ip [A] = −(0.23 ± 0.07) + (0.051 ± 0.006)[mVs−1 ] R2 = 0.9995
ing the reaction both electrons and protons are removed from the (3)
molecule.
According to Laviron [35], for a totally irreversible peak the
width at mid-height of the anodic peak can be parametrized as This linear fit reflects the adsorption-controlled feature of the
E1/2 [mV] = (62.5/(1-␣)n). In general, ˛ for a totally irreversible electrochemical process [39]. From the intercept of the Ep vs. ln
electrode process is assumed to be 0.5 [36]. The E1/2 of the first plot, the value of standard rate constant ks = 150 ± 1 s−1 for morin
peak of morin at pH 8.0 is 101 ± 1 mV, hence, the number of elec- oxidation in PB pH 8.0 has been calculated. This value is 2-fold
trons involved in the morin oxidation process is 1.24 ± 0.03. The higher than reported earlier for morin adsorbed on GCE [12].
first oxidation peak of morin corresponds to the oxidation of the
2 ,4 -dihydroxy moiety at ring B and involves one electron and one
proton. 3.4. Analytical characterization
The second oxidation step of morin at pH 8.0 is character-
ized by E1/2 is 95 ± 2 mV and the number of electrons equals DP mode of electrode polarization allows to increase the sen-
to 1.31 ± 0.06. This voltammetric response is associated with the sitivity of determination and decrease the limits of detection for
oxidation of hydroxyl group at position 3 in ring C [37]. electrochemically active compounds [39]. So, DPV has been applied
The first step oxidation current is higher than that one for the for morin quantification.
second step corresponding to the antioxidant activity of flavonoids There are two steps on morin DPVs on CPB/SWNT-COOH/GCE
associated with the oxidation of hydroxyl groups of the ring B. at 0.176 and 0.75 V in PB pH 8.0. The first one has the form of peak
Based on the results obtained and literature data [13], the scheme and has been used for analytical purposes.
of morin oxidation is suggested (Scheme 2). Effect of pulse parameters on amperometric response of morin
Thus, morin oxidation on CPB/SWNT-COOH/GCE is irreversible has been evaluated. The best results have been observed at pulse
two-step process with participation of one electron and one amplitude of 50 mV and modulation time of 50 ms (Fig. 6).
proton on each step that agrees well with data reported earlier for The oxidation current linearly depends on morin concentra-
GCE with mechanically attached morin hydrate [11] and Pt tion (Fig. 7). The corresponding calibration plots are described by
214 G. Ziyatdinova et al. / Electrochimica Acta 145 (2014) 209–216

Table 1
Analytical characteristics of morin voltammetric determination.

Electrode Method LOD/␮mol Analytical Ref.

dm−3 range/␮mol
dm−3

MWNT-paraffin oil paste electrode AdCV 0.001 0.005÷0.1 19

HMDE SWCAdSV 0.00719 0.019÷0.193 15
Renewable pencil electrode SWAAdSV 0.025 0.095÷1.33 38
GCE AdSWV 0.0125 0.127÷25.0 12
AgNPs-AETGO/GCE SWV 0.0033 0.001÷5.0 16
Polyvinylpyrrolidone-doped CPE Sequential 0.19 1.0÷10.0 18
injection 10.0÷400.0
lab-on-valve
with AD
CPB/SWNT-COOH/GCE DPV 0.0289 0.1÷100 Current
100÷750

HMDE–hanning mercury drop electrode

SWCAdSV - square-wave cathodic adsorptive stripping voltammetry
SWAAdSV - square-wave anodic adsorptive stripping voltammetry
AdSWV - adsorptive square-wave voltammetry
AgNPs-AETGO/GCE - glassy carbon electrode (GCE) modified with silver nanoparticles self-assembled onto the surfaces of 2-aminoethanethiol functionalized graphene oxide
sheets
AD–amperometric detection
AdCV–adsorptive cyclic voltammetry

Table 2
Voltammetric determination of morin in model solutions using CPB/SWNT-
COOH/GCE in 0.1 М PB pH 8.0 (n = 5; P = 0.95).

Added/␮g Found/␮g RSD/%

1.51 1.52 ± 0.02 0.86

15.11 15.0 ± 0.2 0.90
151.1 152 ± 2 1.01
755.6 753 ± 6 0.71
1511 1509 ± 25 1.35

Equations 4 and 5:

Ip [A] = (−0.011 ± 0.008) + (20.8 ± 0.2) × 103 cmorin [M],

R2 = 0.9996 (4)

Ip [A] = (1.0 ± 0.1) + (10.2 ± 0.3) × 103 cmorin [M],

Fig. 7. Baseline-corrected DP voltammograms of 50 (curve 1), 100 (2), 250 (3) and
500 (4) ␮M morin on CPB/SWNT-COOH/GCE in phosphate buffer solution pH 8.0.
R2 = 0.9954 (5)
Potential scan rate is 10 mV s-1. Pulse amplitude is 50 mV, pulse width is 50 ms.
Insert: DP voltammograms for 0.5 (curve 1), 1.0 (2), 5.0 (3) and 10.0 (4) ␮M of
morin.
Two concentration ranges with different slopes are probably
caused by changing contributions of adsorptive and diffusion cur-
rent components that is often observed for wide concentration Validation of the procedure for the quantitative assay of morin
range (3-4 orders). has been tested via evolution of limits of detection (LOD) and quan-
tification (LOQ), the linearity range and repeatability.
The linear dynamic ranges for 0.1-100 and 100-750 ␮M of morin
are observed. The limits of detection (LOD) and quantification
(LOQ) has been calculated using statistic treatment (3SDa /b) and
(10SDa /b), respectively, where SDa is the standard deviation of
the average arithmetic of 10 voltammograms of the blank solu-
tion obtained at the potential of morin oxidation and b is the slope
of the calibration graph. The LOD and LOQ are 28.9 and 96.0 of
morin, respectively. Both parameters indicate satisfactory sensi-
tivity of the developed approach. The analytical ranges and LOD
observed in present work are better or comparable with reported
earlier using other electrodes and different voltammetric modes

Table 3
Morin recovery in mulberry leaves extract (n = 5; P = 0.95).

Spiked/␮g Expected/␮g Found/␮g RSD/% Recovery/%

0.00 20.1 ± 0.2 0.97

7.56 27.66 27.4 ± 0.2 0.67 99.2 ± 0.8
Fig. 6. Effect of pulse parameters on oxidation current of 100 ␮M morin on 15.1 35.2 35.05 ± 0.07 0.17 99.6 ± 0.2
CPB/SWNT-COOH/GCE in phosphate buffer solution pH 8.0. Potential scan rate is 22.7 42.8 43.4 ± 0.5 0.85 101 ± 1
10 mV s−1 .
 G. Ziyatdinova et al. / Electrochimica Acta 145 (2014) 209–216 215

Table 4
Morin contents in mulberry leaves.

Sample Found by DPV/mg g−1 RSD/% Found by HPLC/mg g−1 RSD/% t-testa F-testb

1 1.59 ± 0.02 0.96 1.61 ± 0.05 1.22 1.94 1.71

2 1.71 ± 0.04 1.70 1.75 ± 0.06 1.44 1.80 1.34
a
ttab = 2.45 at P = 0.05 and df =6
b
Ftab = 6.94 at P = 0.05 and df1 = 4, df2 = 2

and summarized in Table 1. It should be noted, that low detection

limits are usually achieved by combination of preliminary adsorp-
tive concentration with SWV characterized by higher sensitivity
than DPV.
The intra-day repeatability of morin determination has been
evaluated by five measurements at five various concentration lev-
els. The added-found method has been applied (Table 2). The
average recovery of 100 ± 1% (n = 25) has been achieved that shows
high accuracy of the determination. The obtained relative standard
deviation does not exceed 1.4%. The inter-day repeatability has
been checked on 50 ␮M level by 5 measurements with RSD 1.7%
that is almost the same as intra-day parameter. So far as the
electrode has been prepared before each measurement, the repro-
ducibility of signal is reflected by repeatability indicating very
good results.The robustness of the method developed has been
investigated by evaluating voltammetric response under influence
Fig. 8. Baseline-corrected DP voltammograms of mulberry leaves extract on
of morin standard solution concentration, preparation of SWNT-
CPB/SWNT-COOH/GCE in phosphate buffer solution pH 8.0: 1–0.25 mL of extract;
COOH suspension, change of GCE to another one with the same 2–extract + 5.00 ␮M of morin; 3 - extract + 7.50 ␮M of morin. Pulse amplitude is
geometric area. The highest RSD of corresponding measurements 50 mV, pulse width is 50 ms. Potential scan rate is 10 mV s−1 .
is 4.9% showing good robustness of the approach.
homogeneous and there are no significant differences in precision
3.5. Interference study of voltammetry and chromatography.

The selectivity of the modified electrode was evaluated by test- 4. Conclusion

ing the influences of several interfering substances on the detection
of 30.0 ␮M morin in 0.1 M PB pH 8.0. The electrochemical results GCE modified with SWNT-COOH and surfactant CPB has shown
indicate that 1000-fold higher concentrations of inorganic ions (K+ , high electrocatalytic activity towards morin oxidation that has been
Na+ , Mg2+ , Ca2+ , Br- , NO3 - , Cl- and SO4 2− ), and 100-fold higher explained by hydrophobic and electrostatic interaction between
concentrations of glucose, rhamnose, sucrose, ascorbic and ferulic morin and CPB. Electrooxidation of morin is adsorption-controlled
acids do not affect on morin determination. Quercetin oxidation irreversible two-step process with participation of one electron and
peak overlaps with morin signal complicating its quantification. one proton on each step. The first oxidation peak of morin cor-
Gallic and sinapic acids as well as rutin affect on morin analytical responds to the oxidation of the 2 ,4 -dihydroxy moiety at ring B
signal at concentrations higher than 50 ␮M but the changes in peak while the second oxidation step is associated with the oxidation of
currents do not exceed 10%. hydroxyl group at position 3 in ring C. Convenient and efficient DPV
method has been developed for morin quantification with limits of
3.6. Real sample analysis detection and quantification of 28.9 and 96 nM of morin, respec-
tively, indicating its satisfactory sensitivity. The approach applied
The approach developed has been applied to morin quantifi- to mulberry leaves analysis.
cation in mulberry leaves. The efficiency of extraction has been
checked by DPV. The effect of extractant volume (1-3 mL), time (5- Acknowledgments
20 min) and order of extraction (1-3) has been evaluated. The best
results have been observed for single extraction for 15 min at plant The financial support of Russian Foundation for Basic Research
material/ethanol ratio 1:20. (grant 14-03-31173) is gratefully acknowledged.
There is well-defined signal at 0.18 V on the DPV of extract corre-
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