Non-enzymatic electrochemical sensing of glucose and hydrogen peroxide using a bis(acetylacetonato)oxovanadium(iv) complex modified gold electrode

Article abstract of DOI:10.1039/c5ra26534g

A non-enzymatic electrochemical sensor, bis(acetylacetonato)oxovanadium(iv) complex, [VO(acac)₂], fabricated on a self-assembled 4-(pyridine-4'-amido)thiophenol (PATP) monolayer modified gold electrode, was developed for the detection of glucose and hydrogen peroxide (H₂O₂) at neutral pH. The modified electrode was characterized by electrochemical and microscopic techniques. The non-enzymatic sensor exhibited a remarkable catalytic performance for glucose oxidation and H₂O₂ reduction. Chronoamperometry was used for the electrochemical determination of glucose and H₂O₂. The non-enzymatic sensing of glucose was realized with a linear response range from 0.001 to 0.5 mM with a detection limit of 0.1 μM (S/N = 3). The sensor also has a good performance for the electrocatalytic reduction of H₂O₂ with a linear response range from 0.02 to 0.9 mM with a detection limit of 0.03 μM (S/N = 3). In addition, [VO(acac)₂]-PATP-Au showed a good selectivity for glucose and H₂O₂ detection in the presence of potential interfering agents such as ascorbic acid, uric acid, l-dopa, l-cysteine and different ions like Na⁺, K⁺, Cl^-etc. The kinetic parameters such as the electron transfer coefficient and the catalytic reaction rate constant were also determined for glucose and H₂O₂. Finally, the modified electrode was used to achieve quantitative detection of glucose and H₂O₂ in blood and milk, respectively for practical applications.

Full text of DOI:10.1039/c5ra26534g

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Non-enzymatic electrochemical sensing of glucose
and hydrogen peroxide using
Cite this: RSC Adv., 2016, 6, 20800
a bis(acetylacetonato)oxovanadium(IV) complex
modified gold electrode†
Received 12th December 2015
Accepted 12th February 2016
*
Koushik Barman and Sk Jasimuddin
DOI: 10.1039/c5ra26534g
www.rsc.org/advances
A non-enzymatic electrochemical sensor, bis(acetylacetonato)oxo- manage the blood glucose level patients need to monitor the
vanadium(IV) complex, [VO(acac)2], fabricated on a self-assembled 4- blood glucose level on a regular basis. On the other hand,
(pyridine-40-amido)thiophenol (PATP) monolayer modified gold elec- hydrogen peroxide (H2O2) is a simple but very important
trode, was developed for the detection of glucose and hydrogen molecule in nature, and is extensively used as an oxidizing
peroxide (H2O2) at neutral pH. The modified electrode was charac- agent in the food and chemical industries.4 Moreover, H2O2 is
terized by electrochemical and microscopic techniques. The non- one of the most important markers for oxidative stress and also
enzymatic sensor exhibited a remarkable catalytic performance for acts as a precursor in the formation of highly reactive and
glucose oxidation and H2O2 reduction. Chronoamperometry was used potentially harmful hydroxyl radicals.5,6 Therefore, the accurate
for the electrochemical determination of glucose and H2O2. The non- determination of glucose and H2O2 is of practical importance.
enzymatic sensing of glucose was realized with a linear response Several analytical techniques have been carried out for the
range from 0.001 to 0.5 mM with a detection limit of 0.1 mM (S/N ¼ 3). determination of glucose and H2O2 viz., titrimetry, spectrom-
The sensor also has a good performance for the electrocatalytic etry, uorometry, chemiluminescence and electrochemical
reduction of H2O2 with a linear response range from 0.02 to 0.9 mM methods.7–10 Amongst them, the electrochemical approach is
with a detection limit of 0.03 mM (S/N ¼ 3). In addition, [VO(acac)2]– promising because of its higher sensitivity and selectivity, lower
PATP–Au showed a good selectivity for glucose and H2O2 detection in detection limit, faster response time, better long term stability
the presence of potential interfering agents such as ascorbic acid, uric and chip.11 For the detection of glucose and H2O2 both enzy-
acid, L-dopa, L-cysteine and different ions like Na+, K+, Clꢀ etc. The matic12,13 and non-enzymatic14,15 sensor have been developed.
kinetic parameters such as the electron transfer coefficient and the First, second and third generation enzymatic glucose sensors
catalytic reaction rate constant were also determined for glucose and has been developed due to overcome the disadvantages. The
H2O2. Finally, the modified electrode was used to achieve quantitative third generation sensor still in their infancy, yet some of them
detection of glucose and H2O2 in blood and milk, respectively for based on nano-mesoporous electrode surface show some
practical applications.
promise.16 There are still some disadvantages of enzyme-based
determination. Examples include complicated enzyme immo-
bilization, critical operating conditions viz. optimum tempera-
ture and pH, chemical instability, poor reproducibility and high
cost.17 To solve these problems, fourth generation enzyme-free
sensors have been developed for glucose oxidation and H2O2
reduction. In general, these electroactive analytes can be
oxidized or reduced directly at ordinary solid electrodes.
However, owing to their high over-potential, slow electrode
kinetics and poor measurement stability caused by poisoning
from the intermediate products restricts the performance of
this electrodes.18 Therefore; current efforts have mainly focused
on discovering new materials with high catalytic activity and
good stability in order to construct non-enzymatic sensors. The
fabrication of a wide variety of nanomaterials have been intro-
duced for the selective and sensitive detection of glucose19,20 as
well as H2O2.21,22 On the other hand very limited numbers of
Introduction
Green plants produce glucose by the reduction of carbon
dioxide and the metabolic oxidation of glucose sustains all
living beings.1 Glucose is transported to cells via insulin in the
bloodstream. The human body maintains blood glucose levels
at a concentration of 4–8 mM (70–120 mg dLꢀ1).2 An abnormal
blood sugar level causes diabetes which represents a leading
cause of several complications for human health like compli-
cations to the retina, circulatory system, kidneys etc.3 To
Department of Chemistry, Assam University, Silchar, Assam-788011, India. E-mail: j.
seikh@gmail.com
† Electronic supplementary information (ESI) available: Figures, table etc. See
DOI: 10.1039/c5ra26534g
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metal complexes have been used so far for the electrochemical Sigma Aldrich, India, K4[Fe(CN)6]3$H2O were purchased from
sensing of glucose and H2O2. Complexes with reversible redox Merck (India). All the reagents and solvents were analytical
capabilities such as cobalt phthalocyanine,23 nickel curcumin,24 grade and were used without further purication. 0.1 M phos-
nickel porphyrine,25 copper hexacyanoferrate26 have been used phate buffer solution (PBS) was prepared by mixing 0.1 M
for the effective electrocatalytic sensing for glucose whereas NaClO4 and 0.01 M H3PO4 and the pH's were adjusted by the
cobalt tetrasulfophthalocyanine,27 cobalt tetraruthenated addition of 0.11 M NaOH using Smalley's method.44 Double
porphyrin,28 cobaltoxyhydroxide,22 DNA–Cu2+ complex29 for distilled water was used throughout the course of the
H2O2 sensing.
experiment.
Instead of cobalt, nickel and copper containing complexes no
other earth abundant transition metal complexes have been re-
ported for the electrochemical sensing of glucose and H2O2.
Among rst d-block transition-metal series, vanadium has critical
roles in various chemical and biological processes.30 Presently the
catalytic role of vanadium in higher oxidation states (IV and V) has
received much attention aer the discovery of vanadium depen-
dent enzymes such as vanadiumiron nitrogenase of Azotobacter
vinelandii and vanadium haloperoxidases in marine algae.31 Several
oxovanadium and dioxovanadium complexes acts as functional
models of haloperoxidases and catalyze oxyhalogenation of various
aromatic substrates.32 Instead of these, oxovanadium complexes
have also been used to catalyze several reactions such as the
oxidation of olens, alcohols,33 aldehydes,34 tertiary amine,35
thiols,36 hydrogen peroxide,37 epoxidation38 and oxidative coupling
reaction.39 Apart from their role as catalyst, there is a widespread
interest on the biological chemistry of vanadium compounds
because of its perceived potential for the development as a phar-
macologic agent for the treatment of diabetes mellitus.40 Extensive
literature review shows that among large number of oxovanadium
compounds, only bis(acetylacetonato)oxovanadium, [VO(acac)2],
exhibits the greatest capacity to enhance insulin receptor kinase
activity in cells associated with a signicant decrease in plasma
glucose concentration.41 Posner and co-workers showed that
vanadate reacted with H2O2 stimulated the phosphorylation of the
insulin receptor in endosomes with efficiency comparable to that
of insulin.42 Makinen and Brady showed that [VO(acac)2] stimulate
the uptake of glucose by serum-starved 3T3-L1 adipocytes in the
presence of bovine serum albumin.43
These rich catalytic and pharmacological properties of oxo-
vanadium complexes encourage us to prepare the [VO(acac)2]
complex modied gold electrode and study the non-enzymatic
electrochemical sensing behaviour for glucose and hydrogen
peroxide. To the best of our knowledge, this is the rst report of
sensing both glucose and hydrogen peroxide by the same metal
complex modied electrode at neutral pH. The oxovanadium(IV)
complex modied gold electrode shows excellent electro-
catalytic activity and exhibit notable sensing performance
towards glucose and H2O2. The kinetics of glucose oxidation
and hydrogen peroxide reduction was also examined in detail.
More importantly, we demonstrate successfully its application
for the quantitative detection of glucose in human blood
sample and H2O2 in processed milk.
Apparatus and instrumentations
Electrochemical measurements were performed on a CHI 660C
Electrochemical workstation (CH Instrument, USA). A three
electrode system was employed with gold or modied gold
electrode as working electrode (2 mm diameter, 0.031 cm2 area),
Pt wire as a counter electrode and Ag/AgCl (3 M KCl) as refer-
ence electrode. All experiments were performed at ambient
temperature and inert atmosphere. The eld emission scanning
electron microscopy (FE-SEM) images were obtained using FE
SEM, FEI INSPECT F50 operated at an acceleration voltage of 20
kV. pH measurement of solutions were carried out on a pH
meter (Macro Scientic Works (Regd), New Delhi).
Construction of [VO(acac)2]–PATP modied gold electrode
A gold electrode was polished with wet a-alumina (0.5 mm) on
a at polishing pad for 10 minutes and rinsed several times with
doubly distilled water. The cleanliness of the gold electrode
surface was ascertained by recording the repetitive cyclic vol-
tammograms in 0.5 M H2SO4 between ꢀ0.2 and +1.5 V versus
Ag/AgCl with 0.1 V sꢀ1 scan rate until a steady characteristic
gold oxide cyclic voltammogram was obtained.45 The electrode
was then rinsed with doubly distilled water and immersed in 1.0
mM ethanolic solution of 4-aminothiophenol (4-ATP) for 24
hours. The 4-ATP was self-assembled over the gold electrode
surface via gold–sulfur interaction and the modied electrode
4-ATP–Au was thoroughly washed with double distilled water.
Thereaer, the modied gold electrode was dipped into 1.0 mM
isonicotinic acid solution for 4 hours under stirring condition
and 4-(pyridine-40-amido)thiophenol modied gold electrode
(PATP–Au) was formed. Aer washed with double distilled water
PATP–Au electrode was immersed into an ethanolic solution of
1.0 mM [VO(acac)2] and stirred for 2 hours so that the pyridine
nitrogen of PATP–Au was able to form adduct with the vacant
coordination site of vanadium in [VO(acac)2]. The nally
modied electrode [VO(acac)2]–PATP–Au was washed thor-
oughly with distilled water and dried in air for further use.
Results and discussion
Characterization of modied gold electrode
The step wise modication and surface morphology of the bare
gold electrode was characterised by FE-SEM. [VO(acac)2]–PATP
modied gold electrode shows very rough and porous surface
which is favourable for the electrocatalytic activity. Elemental
mapping images conrms the immobilization of [VO(acac)2]
Experimental
Chemicals and reagents
4-Aminothiophenol, isonicotinic acid, [VO(acac)2], D-(+)-glucose over self-assembled monolayer 4-PATP modied gold elec-
and hydrogen peroxide (30 wt% in H2O) were procured from trode.46 The modication process was also monitored by cyclic
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voltammetry and electrochemical impedance spectroscopy
using [Fe(CN)6]3ꢀ/4ꢀ as redox probe in 0.1 M PBS solution at pH
7.0. The cyclic voltammogram of 0.5 mM [Fe(CN)6]4ꢀ exhibits an
electrochemically reversible redox couple on bare electrode.
Aer modication the gold electrode with 4-aminothiophenol,
the cyclic voltammogram of [Fe(CN)6]4ꢀ exhibit an irreversible
couple with low current height than bare gold electrode. The
current height decreased even more when 4-(pyridine-40-amido)
thiophenol (PATP) modied Au was used as working electrode.
These CV results indicated that the electronic communication
between gold and [Fe(CN)6]4ꢀ is blocked due to PATP lm
formation.46 In the Nyquist plot, the diameter of the semi-circle
increases gradually when stepwise modication on the gold
electrode surface was carried out. The observed trend is due to
the fact that the modied electrode blocked the electron
transfer for the redox reaction of [Fe(CN)6]4ꢀ. Electrochemical
impedance measurement supports the CV results. The fabrica-
tion of [VO(acac)2] over PATP–Au electrode was conrmed by
taking a comparative cyclic voltammogram for PATP–Au and
[VO(acac)2]–PATP–Au in 0.1 M PBS buffer at pH 7.0. A quasir-
eversible [VVO(acac)2]+/[VIVO(acac)2] redox couple at +0.33 V
supports the fabrication of [VO(acac)2]–4-PATP–Au electrode.46
Fig.
2
Cyclic voltammograms obtained with bare, PATP and
[VO(acac)2]–PATP modified gold electrode in 0.5 mM hydrogen
peroxide in 0.1 M PBS solution (pH 7.0).
initiated by the non-covalent interaction of glucose with surface
bound [VO(acac)2]. During anodic scan [VIVO(acac)2]0 is
oxidized to the catalytically active [VVO(acac)2]+ complex over
PATP modied gold electrode.
[VIVO(acac)2]0 / [VVO(acac)2]+ + eꢀ.
(a)
Electrocatalytic oxidation of glucose and reduction of H2O2
Once [VVO(acac)2]+ is formed, glucose is oxidized on the
Fig. 1 and 2 shows the cyclic voltammograms (CV) of 0.1 mM
glucose and 0.5 mM hydrogen peroxide, respectively in 0.1 M
PBS at pH 7.0 using bare Au, PATP–Au and [VO(acac)2]–PATP–Au
electrodes. An irreversible oxidation of glucose occurred at
+0.65 V by the [VO(acac)2]–PATP–Au electrode with large
increase of current whereas no such prominent peak was
observed with bare and PATP–Au electrode (Fig. 1). In absence
of glucose no such oxidation peak was observed at [VO(acac)2]–
PATP–Au electrode (Fig. S1†). This behaviour indicate the elec-
trocatalytic activity of [VO(acac)2]–PATP modied gold electrode
towards glucose oxidation. DPV experiment gives a prominent
glucose oxidation peak only at [VO(acac)2]–PATP–Au electrode
under similar condition and supports the results obtained by
CV (Fig. S2†). The catalytic pathway of glucose oxidation can be
describe on assuming that the electrochemical process is
modied electrode surface via the following reactions.
[VVO(acac)2]+ + glucose / intermediate + [VIVO(acac)2]0 (b)
[VVO(acac)2]+ + intermediate /
gluconolactone + [VIVO(acac)2]0.
(c)
The cyclic voltammogram of H2O2 in 0.1 M PBS (pH 7.0)
shows a cathodic response at around ꢀ0.24 V at bare gold
electrode. When the gold electrode was modied by 4-(pyridine-
40-amido)thiophenol, no current response was observed. Aer
modication with [VO(acac)2], a sharp peak was observed
around ꢀ0.11 V with sufficiently high current response during
the cathodic scan. In absence of H2O2 no such reduction peak
was obtained at [VO(acac)2]–PATP–Au electrode under similar
condition (Fig. S3†). Through these observations it was clear
that oxovanadium(IV) complex exhibit enhanced electrocatalytic
efficiency by their adhesion on the PATP–Au electrode. This is
rationalized by a high ability of the [VO(acac)2] to transfer
electrons involved in the catalytic reaction and sense the pres-
ence of H2O2 electrochemically. A common two electron redox
mechanism is proposed for hydrogen peroxide reduction at the
[VO(acac)2]–PATP–Au electrode surface in which a substantial
interaction with H2O2 and vanadium promotes the electron
transfer and is shown by the following reactions.
[VIVO(acac)2]0 + H2O2 / H2O + [VVO(acac)2]+
[VVO(acac)2]+ + eꢀ / [VIVO(acac)2]+
(d)
(e)
Fig.
1 Cyclic voltammograms obtained with bare, PATP and
Electrochemical impedance spectroscopy was also carried
out for glucose and H2O2 using bare and modied gold
[VO(acac)2]–PATP modified gold electrode in 0.1 mM glucose in 0.1 M
PBS solution (pH 7.0).
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electrodes. The diameter of the semicircle observed in the method and other modied electrodes reported for the elec-
Nyquist plot corresponds to charge transfer resistance, Rct; the trocatalytic oxidation of glucose and reduction of H2O2. It can
smaller the semi-circle, faster is the charge transfer. Fig. S4 and be seen that the detection limit obtained in the present system
S5† shows that the diameter of the semicircle (Rct) changes are comparable with some reported metal complex modied
upon modication of gold electrode surface. The Rct values for electrodes and quite better than the metal nanoparticle/
0.1 mM glucose oxidation and 0.5 mM H2O2 reduction at nanocomposite modied electrodes.
[VO(acac)2]–4-PATP–Au electrode in 0.1 M PBS at pH 7.0 were 1.3
ꢁ 104 U and 1.2 ꢁ 105 U respectively, which were quite smaller
Effect of scan rate and kinetic analysis for glucose oxidation
than the Rct obtained at PATP modied and bare gold electrode.
and hydrogen peroxide reduction
The observed results are due to the fact that the [VO(acac)2]-
modied electrode ease the electron transfer rate for the The inuence of the scan rate on the electrocatalytic oxidation
oxidation of glucose and reduction of H2O2 where as the 4-PATP of glucose (Fig. S12a†) and the reduction of H2O2 (Fig. S13a†) at
modied gold electrode blocked the electron transfer. Electro- [VO(acac)2]–4-PATP–Au were investigated using cyclic voltam-
chemical impedance measurements clearly indicate that metry. The results showed that on increasing the scan rates the
[VO(acac)2]–4-PATP modied gold electrode has lower resis- oxidation peak potential of glucose and the reduction potential
tance as compared to bare or 4-PATP modied gold electrodes. of hydrogen peroxide shis to more positive and more negative
This study supports the CV results and reveals that the values, respectively, conrming the kinetic limitation of the
[VO(acac)2]–4-PATP–Au electrode is an efficient electrocatalyst electrochemical reaction.27 Moreover, a plot of scan rate–
for the oxidation of glucose and reduction of hydrogen normalized current (Ipa/n1/2) versus scan rate (Fig. S14†) shows
peroxide.
a shape typical of EC catalytic process for glucose oxidation.47 In
addition, a plot of the peak current (Ipa) versus the square root of
pffiffi
the scan rate ( n) (Fig. S12b†) in the range of 50–100 mV sꢀ1 was
Determination of glucose and H2O2
found to be linear following the linear regression equation Ipa
(mA) ¼ 0.3392n (mV sꢀ1) ꢀ 1.1978 (R2 ¼ 0.9945), revealing that
the electrooxidation reaction of glucose at [VO(acac)2]–4-PATP–
Au electrode was followed diffusion controlled electron transfer
Based on optimized conditions, determination of glucose and
H2O2 were performed using chronoamperometry. Fig. S6†
shows the chronoamperometry curves of glucose with different
concentration. The oxidation peak current of glucose was linear
with its concentration in the range of 0.1–0.5 mM (Fig. 3a). The
regression equation was I ¼ 3.717C + 1.646 (R2 ¼ 0.99), with
a detection limit of 0.1 mM (S/N ¼ 3). Exactly same detection
limit was obtained in the lower concentration range of glucose
(1.0 mM to 5.0 mM) (Fig. S8†). The detection limit was further
process. The diffusion coefficient (D) for glucose was 9.5 ꢁ 10ꢀ6
pffiffi
cm2 sꢀ1 and is calculated using the plot (Ipa vs. n) and Ran-
dles–Sevcik equation48
Ip ¼ 2.69 ꢁ 105n3/2AD1/2Cn1/2
(1)
conrmed by differential pulse voltammetry (Fig. S10†). Fig. S7 where, Ip is the peak current, n is the number of electrons
and S9† shows the chronoamperogram response for H2O2 in the transferred, A is the electrode area, C is the concentration of
concentration range of 0.5–0.9 mM and 20–40 mM. The current electroactive species, and n is the scan rate, considering
was linearly proportional to its higher concentration range with a temperature of 298 K. The electron transfer coefficient for the
a linear regression equation I ¼ 9.81C + 18.61 (R2 ¼ 0.99) totally irreversible oxidation of glucose at [VO(acaca)2]–4-PATP–
(Fig. 3b) and in the low concentration range with a linear Au can be determined from equation48
regression equation I ¼ 9.81C + 0.02 (R2 ¼ 0.99) (Fig. S9b†). The
Ep ꢀ Ep/2 ¼ 1.857RT/aF ¼ 47.7/a mV
(2)
detection limit for H2O2 was 0.03 mM (S/N ¼ 3). The detection
limit was further conrmed using CV results (Fig. S11†). Table
S1† shows a comparison of the proposed electrochemical
where Ep and Ep/2 represent the peak potential and the half-
height peak potential, respectively in cyclic voltammetry
experiment where R, T and F have their usual meaning. For
glucose oxidation, Ep ꢀ Ep/2 ¼ 38 mV, hence electron transfer
coefficient (a) is calculated to be 0.63.
The standard heterogeneous rate constant (ks) for the irre-
versible oxidation of glucose at [VO(acac)2]–4-PATP–Au elec-
trode was calculated by using the Velasco equation49
ks ¼ 1.11D1/2(Ep ꢀ Ep/2
)
ꢀ1/2n1/2
(3)
The estimated ks values for totally irreversible oxidation of
glucose at [VO(acac)2] modied electrodes was found to be 5.5 ꢁ
10ꢀ3 cm sꢀ1. The observed higher ks value for glucose at the
modied electrode indicates that the oxidation of glucose was
faster at the [VO(acac)2]–PATP modied gold electrode. The
kinetic parameters for the reduction of H2O2 were also
Fig. 3 (a) Plot of resulting current in chronoamperometry at 30
seconds versus concentration of glucose (0.1–0.5 mM). (b) Plot of
resulting current in chronoamperometry at 30 seconds versus
concentration of H2O2 (0.5–0.9 mM).
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calculated (as described for glucose): n ¼ 2, a ¼ 0.69, D ¼ 10.6 ꢁ made with ve days intervals (when not in use, the sensor was
10ꢀ6 cm2 sꢀ1 and k ¼ 3.3 ꢁ 10ꢀ3 cm sꢀ1
.
stored at room temperature using a rubber cap). The sensor
retained 100% of its original current response aer 20 days both
for glucose (1.0 mM) and H2O2 (0.5 mM) in 0.1 M PBS at
[VO(acac)2]–PATP–Au (Fig. S20†).
Effect of accumulation potential and time
The effect of accumulation time and potential on the oxidation
behavior of glucose and reduction of H2O2 at [VO(acac)2]–PATP–
Au electrode was investigated. Fig. S15† shows that the oxida-
tion peak current of glucose and H2O2 which were remaining
constant with increasing accumulation time from 0 to 300 s.
Therefore the accumulation time of 60 s was chosen as the
optimum time for further study in both cases. In addition, the
inuence of accumulation potential on the peak current was
examined over the potential range 0.0 to 6.0 V for glucose and
0.0 to ꢀ0.5 V for H2O2 (Fig. S16†). The peak current for glucose
was decreased by changing accumulation potential to more
positive value and is due to the oxidation of glucose during the
accumulation step at potential higher than that 0.35 V
(Fig. S16a†) where as in case of H2O2, by changing accumulation
potential to more negative value and is due to the reduction of
H2O2 during the accumulation step at potential lower than that
ꢀ0.02 V (Fig. S16b†). In fact, the maximum observed current
were equal to those observed for open circuit accumulation.
Interference study
In the electrochemical detection of glucose and H2O2, the
elimination of interferences is a real challenge. Ascorbic acid,
uric acid, citric acid, levodopa, cysteine, and different common
ions are the major potential interfering agents in the physio-
logical system. In the present study 0.1 mM glucose in presence
of 10 fold excess interferents were used at +0.65 V. The resulting
amperograms are shown in Fig. 4. There is no obvious current
response observed with the addition of these interfering
substances, however, an obvious current response with the
addition of glucose was appeared. In addition, the inuence of
those co-existing electroactive species in the amperometric
determination of H2O2 was also studied. The working potential
was held at ꢀ0.11 V. The amperogram (Fig. 5) shows that all the
potential interferents mentioned did not affect the sensor
selectivity for H2O2. These results suggest that the interfering
effect caused by these electroactive species is quite negligible,
indicating the highly selective detection of glucose and H2O2 at
the oxovanadium complex modied electrode.
Effect of pH
The effect of pH on the electrooxidation of glucose and H2O2
ware also investigated in the range of pH 5.0–10.0. As shown in
Fig. S17† the oxidation peak potential of glucose were pH
dependent and was shied towards more negative potential
with increments in solution pH following the linear regression
equation of Epa (V) ¼ ꢀ0.0625 pH + 1.094 (R2 ¼ 0.9957). The
slope of 62.5 mV pHꢀ1 indicated that equal numbers of protons
and electrons were involved in the electrode reaction process.50
Similarly, in Fig. S18† the reduction peak potential of H2O2 were
also pH dependent and that they shied toward more positive
potential with increments in solution pH following the linear
regression equation of Epa (V) ¼ ꢀ0.026 pH + 0.068 (R2 ¼ 0.976).
Investigation of the inuence of pH on the peak current of
glucose and H2O2 at the modied electrode revealed that peak
current of glucose and hydrogen peroxide reached a maximum
at pH 7.0 and then decreased by increasing pH of the solution
(Fig. S19a and b†).
Real sample analysis
To testify the feasibility of [VO(acac)2]–PATP–Au in real sample
analysis, human blood sample (aer fasting) was taken for
glucose determination whereas processed milk was chosen for
the determination of H2O2. Before testing, the blood and milk
samples were half diluted by 0.1 M phosphate buffer solution.
Fig. S21† shows the overlaid DPV of blood sample solution in
PBS (pH 7.6) and aer addition of standard glucose solution in
blood sample solution. The DPV of blood sample clearly shows
the oxidation of glucose at +0.56 V. The content of glucose in
Reproducibility, sensitivity and stability
A reproducible and long-term stable electrochemical sensor is
highly desirable for the practical application and commerciali-
zation. The reproducibility of the [VO(acac)2]–PATP–Au elec-
trode was examined by 10 repetitive measurements for glucose
and H2O2 in 0.1 M PBS solution. The results showed that the
anodic peak current for glucose and cathodic peak current for
hydrogen peroxide remains same with a relative standard
deviation (RSD) of 0.2 and 0.3%, respectively, indicating that the
modied electrode has a good reproducibility. The modied
electrode was highly sensitive towards glucose and H2O2 and
the sensitivity was 120.24 mA cmꢀ2 mMꢀ1 and 326.66 mA cmꢀ2
mMꢀ1 for glucose and hydrogen peroxide, respectively. To
Fig. 4 Amperometric response at [VO(acac)2]–4PATP–Au electrode
with an applied potential of +0.65 V on subsequent addition of 0.1 mM
glucose, 1.0 mM AA, 1.0 mM UA, 1.0 mM Cys, 1.0 mM L-dopa, 1.0 mM
CA, 1.0 mM NaCl, 1.0 mM KCl, 0.1 mM glucose under stirring condition
(supporting electrolyte: 0.1 M PBS (pH 7.0), brown curve shows
further explore the long-term stability, measurements were background current).
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good recovery. The new non-enzymatic sensor can be useful for
clinical diagnosis and food industry in near future.
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20806 | RSC Adv., 2016, 6, 20800–20806
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