A quercetin-modified biosensor for amperometric determination of uric acid in the presence of ascorbic acid.

Abstract

The present work reports a quercetin-modified wax-impregnated graphite electrode (Qu/WGE) prepared through an electrochemical oxidation procedure in quercetin-containing phosphate buffer solution (PBS), for the purpose of detecting uric acid (UA) in the presence of ascorbic acid (AA). During modification quercetin was oxidized to the corresponding quinonic structure, and in the blank buffer solution the electrodeposited film exhibits a voltammetric response anticipated for the surface-immobilized quercetin. Retarding effect of the film towards the reaction of anionic species was found; therefore the pH of sample solutions was selected to ensure the analyte in molecular form. At suitable pHs the Qu/WGE shows excellent electrocatalytic effect towards the oxidation of both AA and UA, and separates the voltammetric signal of UA from AA by about 280 mV, allowing simultaneous detection of these two species. A linear relation between the peak current and concentration was obtained for UA in the range of 1-50 microM in the presence of 0.5 mM AA, with a detection limit 1.0 microM (S/N=3). This sensor was stable, reproducible and outstanding for long-term use.

Full text

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A quercetin-modified biosensor for amperometric determination
of uric acid in the presence of ascorbic acid
Jian-Bo He∗, Guan-Ping Jin, Qun-Zhi Chen, Yan Wang
School of Chemical Engineering, Hefei University of Technology, Hefei 230009, PR China
Abstract
The present work reports a quercetin-modified wax-impregnated graphite electrode (Qu/WGE) prepared through an electrochemical oxidation
procedure in quercetin-containing phosphate buffer solution (PBS), for the purpose of detecting uric acid (UA) in the presence of ascorbic acid
(AA). During modification quercetin was oxidized to the corresponding quinonic structure, and in the blank buffer solution the electrodeposited
filmexhibitsavoltammetricresponseanticipatedforthesurface-immobilizedquercetin.Retardingeffectofthefilmtowardsthereactionofanionic
species was found; therefore the pH of sample solutions was selected to ensure the analyte in molecular form. At suitable pHs the Qu/WGE shows
excellent electrocatalytic effect towards the oxidation of both AA and UA, and separates the voltammetric signal of UA from AA by about 280mV,
allowing simultaneous detection of these two species. A linear relation between the peak current and concentration was obtained for UA in the
range of 1–50?M in the presence of 0.5mM AA, with a detection limit 1.0?M (S/N=3). This sensor was stable, reproducible and outstanding
for long-term use.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Quercetin-modified electrode; Uric acid; Ascorbic acid; Biosensor; Spectroelectrochemistry
1. Introduction
As the primary end product of purine metabolism, uric acid
(UA) is a greatly important analyte in clinical field. In a healthy
human being, the typical concentration of UA in urine is in the
millimolar range (around 2mM), whereas that in blood is in the
micromolar range (120–450?M) [1,2]. Abnormalities of UA
level are symptoms of several diseases, such as gout, hype-
ruricaemia and Lesch-Nyhan syndrome [3]. Electrochemical
sensingofUAisoneofthefeasiblemethods,butamajorobstacle
in monitoring UA is interferences from other electroactive con-
comitants,suchasascorbicacid(AA),whichcanbeoxidizedata
potentialclosetothatofUAonvarioustypesofelectrodes[4–6].
In recent years, many different electrode modification strategies
have been employed for improving the selectivity and sensitiv-
ity of the sensor, such as electrochemical polymerization [7–9],
monolayer bonding [10–12], melioration of carbon structure
[13–15] and electrochemical pretreatment [1,16] etc. Various
inorganic and organic materials, such as poly(vinyl alcohol)
∗Corresponding author. Tel.: +86 551 2901450; fax: +86 551 2901450.
E-mail address: jbhe@hfut.edu.cn (J.-B. He).
[7], poly(N,N-dimethylaniline) [8], caffeic acid [9], propionyl-
choline [10], heteroaromatic thiol [11], dimercaptothiadiazole
[12], palladium [17], ferrocene carboxylic acid [18], cyclodex-
trin [19], benzoquinone and the derivatives [20–22], oracet blue
[23], 3,4-dihydroxybenzaldehyde [24], norepinephrine [25] and
rutin[26]havebeenusedaselectrodemodifierfortheirpotential
utility in the simultaneous determination of UA and AA.
Among the electrode modifiers reported in literatures, we
notice that some organic molecules with quinonoid [20–23] or
o-dihydroxy catechol [24–26] structures are emerging as potent
electron transfer mediator for the electrocatalytic oxidation of
AA and/or UA. Flavonoids are benzo-?-pyrone derivatives with
several hydroxyl groups attached to ring structures C6–C3–C6;
some of flavonoids contain o-dihydroxy catechol (the B-ring),
such as quercetin, rutin, catechin and luteolin, etc. To our best
knowledge, few reports for the biosensors with flavonoid as
modifier are available until now. As an instance reported in lit-
eratures, rutin was reported to show electrocatalytic properties
for the mediated oxidation of ascorbic acid on the DPPC films
modified GC electrode [26].
Quercetin (Scheme 1) is a natural antioxidant present in the
plantkingdomandthecommonhumandiets.Ithasvariousben-
eficial effects on human health due to their polyphenolic nature

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Scheme 1. The molecular structure of quercetin.
with radical-scavenging activity and metal-chelating properties
[27–29]. Therefore, common interest is dealing with develop-
ing biosensors for quercetin detection [30,31]. We report here
a quercetin-modified electrode (Qu/WGE) prepared by electro-
chemical deposition of quercetin at a wax-impregnated graphite
electrode(WGE).Thiselectrodeexhibitsanattractiveabilityfor
the determination of UA in the presence of AA. In our previous
works, the WGE showed advantages of low background cur-
rents, low noise, fast base line stabilization [32], and especially
the strong adsorption effect on quercetin. Therefore, the WGE
wasselectedasthesubstratefortheimmobilizationofquercetin
for fabrication of biosensors.
2. Experimental
2.1. Chemicals and solutions
Quercetinisreagent-gradematerialfromShanghaiChemical
Works (Shanghai, China). Uric acid and ascorbic acid were pur-
chasedfromChemicalRegentCompanyofShanghai(Shanghai,
China) and were used as received. All other chemicals were
of analytical grade. Doubly-distilled water from an all-glass
distillatory apparatus was used in all cases. Phosphate buffer
solutions (PBS, 0.1M) with different pH were prepared by mix-
ing four solutions of 0.1M H3PO4, KH2PO4, K2HPO4 and
K3PO4. Quercetin stock solution of 1.0mM was prepared in
ethanol with the aid of ultrasonic agitation. UA stock solution
of 1.0mM was prepared in 0.1M NaOH, and AA of 50mM
was prepared in water. All stock solutions were kept at 4◦C in a
refrigerator, and were diluted to various desired concentrations
with buffer supporting electrolyte before use.
2.2. Apparatus and procedures
Electrochemical measurements including cyclic voltamme-
try (CV), linear scan voltammetry (LSV), electrochemical
impedance spectrum techniques (EIS) and differential pulse
voltammetry (DPV) were performed on a computer-controlled
electrochemical analyzer (CHI660B, Shanghai, China). A con-
ventional three-electrode electrochemical cell was used for
electrochemical measurements. The working electrode was
a wax-impregnated graphite disk with a geometric area of
0.125cm2,eithermodifiedwithquercetinornotforcomparison.
A saturated calomel electrode (SCE) and a platinum electrode
were used as reference and auxiliary electrodes, respectively. In
situ UV spectroelectrochemical experiment was carried out on
anUV-2550spectrophotometer(Shimadzu,Japan)withahome-
made long path-length thin-layer electrochemical cell, to clarify
the molecular structure of quercetin oxidization product. All
experiments were conducted at room temperature. Sample solu-
tionswerebubbledwithhighpureN2forabout15mintoremove
dissolved oxygen before each measurement. Considering the
adsorptionofanalytesontheQu/WGE,apre-accumulationstep
was always performed in open circuit. The accumulation time
of 60s was selected in all cases.
2.3. Electrode preparation
The bare graphite disk electrode was polished successively
withemerypaperstillbeingamirrorface,andthenwashedwith
doubled-distilled water and ethanol for 15min under ultrasonic
condition, respectively. The electrode then was impregnated
with molten wax under infrared light to form wax-impregnated
graphite electrode (WGE). After cooling down, it was first
polished mechanically to a mirror like finish with fine wet
emery paper (grain size 600, 1200 and 3000) and then soni-
cated cleaning in ethanol and water for 15min, respectively.
The electrochemical activation of the electrode was performed
by a continuous potential cycling from −0.2 to 0.8V at a scan
rate 100mVs−1in 0.1M PBS until a stable voltammogram
was obtained. Afterwards, the activated electrode was placed
in 0.1M PBS (pH 7.0) containing 0.5mM quercetin, and it was
modified by 4 cycles of potential scan between −0.2 and 1.0V
at the scan rate 50mVs−1. During this stage, a deposited layer
of quercetin is bound to the surface of anodized WGE. The pre-
pared electrode was rinsed with water to remove any physically
adsorbedsubstances,andthenwasstoredin0.1MPBS(pH7.0)
in a refrigerator at 4◦C for next use.
3. Results and discussion
3.1. Modification of electrode
The effect of various experimental variables on the immo-
bilization of quercetin was investigated to optimize the
modification. The surface coverage was evaluated via the cyclic
voltammograms[25]recordedat50mVs−1inablankpH7PBS
andbyintegrationofanodicpeakcurrent.Theoptimizedexperi-
mentalconditionsareshowninTable1.Thenumberofpotential
cycles was optimized for surface modification on the basis of a
pre-set scan rate of 50mVs−1. The maximal surface coverage
was found around four cycles of potential, and therefore four
cycles of potential scan is considered as the optimum in the
subsequent modification process. The effect of the scan poten-
tial range on the surface coverage was investigated by fixing
Table 1
The optimized modification conditions
Experimental variable Optimal result
Number of potential cycles
Anodic switching potential
Solution pH
Quercetin concentration
4
>1.0V
7.0
0.5mM

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negative switching potential at −0.2V and changing the anodic
switching potential. The favorable coverage was obtained when
the anodic switching potential was set beyond 1.0V, therefore
modification by cycling the potential between −0.2 and 1.0V
was selected as optimal and used in all subsequent studies. The
variationofthesurfacecoverageasafunctionofquercetinsolu-
tion pH showed that neutral solution is more favorable for the
modification than acid or alkaline media; therefore pH value of
7.0 was selected as the optimal. Finally, the effect of quercetin
concentrationonthesurfacecoveragewasinvestigated.Thebest
coverage was obtained when the modification was carried out in
0.5mM quercetin solution.
It should be stressed that the anodic switching potential
has a strong impact on the immobilization of quercetin at the
WGE. The favorable result was obtained when the positive scan
extended into the oxygen-evolution potential range. It is well
known that the surface anodization process may result in an
increaseofcarboxylandoxygencontentatcarbonelectrodesur-
face [33]. The quercetin modification on the WGE surface can
thus be realized as a reaction of the surface carboxyl, hydroxyl
and/or other oxygen containing groups with the oxidation prod-
uct of quercetin.
Themolecularstructureoftheoxidationproductofquercetin
was verified by in situ thin-layer UV spectroelectrochemistry.
As illustrated in Fig. 1, quercetin exhibits two characteristic
absorption bands of flavonoid compounds at 397nm (Band
I) and 260nm (Band II), along with a very weak band at
332nm (Band III). Spectral changes accompanying oxidation
of quercetin at 0.75V show that both Band I and Band II dra-
matically decreased, while Band III largely increased with the
time of oxidation. Such spectral changes have been also found
for quercetin after electro-oxidation in aprotic medium [34], or
after chemical oxidation by Cu(II) [35] or H2O2[36]. The Band
III is indicative that the quinonic structure is the chromophore
present in the final species [34].
The deposition mechanism of o- and p-quinones onto
anodized carbon electrode has been studied in some detail
[37,38]. The anodically formed quinones can enter into a reac-
tion as Michael acceptors with active functional groups existing
at the surface of anodized carbon electrodes which act as nucle-
ophiles with quinone rings in position 4 or 5 [39]. In the case of
quercetin, position 6?in the o-quinone ring is suitable for nucle-
ophilicattack.Thecarboxylorhydroxylgroupsontheanodized
Fig. 1. Thin-layer UV absorption spectra of quercetin during potentiostatic oxi-
dation at 0.75V vs. SCE in 0.1mM quercetin (pH 7.0). Spectral tracing was
repeated every 40s after the potential was applied. The first line was recorded
before electrolysis.
WGE surface behaves as nucleophiles against the o-quinone
ring formed from quercetin oxidation, leading to bond forma-
tion between quercetin and surface active groups and hence to
the deposition of quercetin on the WGE surface. A possible
mechanism for the quercetin modification can be proposed as
shown in Scheme 2.
As for the pH effect on the modification step, as mentioned
above, it seems that in acidic media the protonation of elec-
trodesurfacefunctionalgroupsinhibitsthenucleophilicreaction
between the latter groups and the oxidized form of quercetin,
while in alkaline media an increase in hydroxyl ion concen-
tration enhances the nucleophilic attack of modifier by solvent
molecules which minimize the apparent reactivity of functional
groups at the surface of the electrode.
3.2. Electrochemical responses of Qu/WGE
Theelectroactivityandstabilityofelectrodepositedquercetin
film was examined by repetitive potential scans in a blank sup-
porting electrolyte solution after careful ultrasonic cleaning in
water. The result was illustrated in Fig. 2. As can be seen, the
quercetin immobilized on the WGE still exhibited CV response
similar to that of quercetin in solution [40]. Three anodic peaks
A1, A2 and A3 appeared successively at 0.208, 0.310 and
0.618V, corresponding to the oxidation of 3?,4?-dihydroxyl at
Scheme 2. A possible mechanism for the quercetin immobilization on the WGE.

Page 4

Fig. 2. Multi-cycle voltammogram obtained at the Qu/WGE in blank pH 7.0
PBS with scan rate 100mVs−1. The electrode was prepared in optimal condi-
tions: 0.5mM quercetin solution (pH 7.0); scan range: −0.2 to 1.0V; scan rate:
50mVs−1; number of cycles: 4.
B-ring, 3-hydroxyl at C-ring and the hydroxyls at A-ring of
quercetin, respectively, among which the former is a two elec-
tron two proton reaction, leading to a corresponding o-quinonic
structure. A cathodic peak C1 was observed at 0.185V after the
scan was reversed, which is the counterpart peak of A1. This
redox couple showed a small peak-to-peak separation (?Ep) of
23mV, less than the value of 29mV expected for an ideally
reversible 2e−reaction, indicating a couple of voltammetric
peaks of surface species. Fig. 2 shows a significant decrease
in anodic current during the first 2 cycles, however, no con-
siderable decay in peak height was observed in the subsequent
repetitive scans, indicating good stability of Qu/WGE during
repeated operation.
The cyclic voltammetric behaviors of the WGE before and
after the quercetin modification were examined also using
Fe(CN)64−as an electrochemical probe. Fig. 3(A) shows that
at bare WGE a couple of quasi-reversible CV peaks appeared
at Em((Epa+Epc)/2) of 0.210V with ?Epof about 110mV at
100mVs−1(curve a), but only a small sigmoid wave appeared
after the modification (curve b), indicating a retarding effect of
the deposited layer on the redox reaction of Fe(CN)64−/3−. In
comparisonwiththeCVatbareWGE,apositiveshiftofEmcan
beseenaftermodification,from0.210to0.246V.Thisprobably
suggeststhatthedepositedquercetiniselectronegativeduetothe
electron withdrawing effect of the anodically formed carbonyls,
which repels the anions approach.
Corresponding electrochemical impedance spectra of this
system before and after modification were measured at the for-
mal potential of [Fe(CN)6]4−/3−redox couple. Fig. 3(B) shows
the EIS of the bare WGE (a) and the Qu/WGE (b). EIS is an
effective method to probe the features of a surface-modified
electrode, and a modified Randle’s equivalent circuit has been
used for data fitting [10,41–43]. This model includes the solu-
tion resistance (Rsol), charge-transfer resistance (Rct), Warburg
impedance(ZW),doublelayercapacitance(Cdl),aswellasaddi-
tional membrane parameters, membrane resistance (Rm) and
capacitance (Cm). For a simple estimation, Rct value can be
estimateddirectlyfromthediameterofthehighfrequencysemi-
circle. As can be seen in Fig. 3(B), at the bare WGE a small
semicircle of about 3k? diameter with an almost straight tail
line is presented (curve a), which is characteristic of a diffusion-
limiting step of the electrochemical process. The diameter of
the high frequency semicircle was significantly enlarged by the
surface deposition of the quercetin layer (curve b); a Rctvalue
of about 16k? can be estimated, which indicates an increased
resistance to the anion redox couple of Fe(CN)64−/3−at the
Qu/WGE. This is in good agreement with the CV behaviors as
shown in Fig. 3(A).
3.3. Electrocatalytic effect of Qu/WGE on oxidation of UA
and AA
It was found that the Qu/WGE is catalytically active to the
oxidation reaction of both UA and AA. Fig. 4 shows the CVs
of the both analytes on the Qu/WGE in comparison with that
at bare WGE. UA only shows a small anodic peak at about
0.40VattheWGE,whileattheQu/WGEgivesaanodicpeakat
about 0.327V with a greatly increased peak current (Fig. 4A).
The anodic overpotential decreased by about 73mV indicates
a strong catalytic activity towards the oxidation of UA. Also,
AA shows a broad and small anodic peak at about 0.305V
at the WGE, while at the Qu/WGE gives an anodic peak at
Fig.3. Cyclicvoltammogram(A)andImpedancespectrum(B)atbareWGE(a)
and Qu/WGE (b) in 1.0mM Fe(CN)64−/3−+0.1M PBS (pH 7.0). (A) scan rate:
100mVs−1; (B) frequency range: 100kHz–0.05Hz; amplitude: 5mV; initial
potential: 0.215V vs. SCE.

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Fig. 4. Cyclic voltammograms of 0.1mM UA (A) and 0.5mM AA (B) in 0.1M
PBS at the WGE (a) and the Qu/WGE (b). Solution pH (optimum): 6.0 (A) and
4.0 (B); scan rate: 50mVs−1.
about 0.275V with a greatly increased peak current (Fig. 4B).
The anodic overpotential decreased by about 30mV indicates a
catalytic activity towards the oxidation of AA.
The scan rate dependence of the LSV response of UA
was examined at the Qu/WGE. A linear relationship between
the anodic peak current and the scan rate was found as
ipa/?A=−13.3+41.8v/(mVs−1) (r2=0.9985) in the range of
10 to 300mVs−1, indicating a current peak of surface species.
This suggests that UA was first adsorbed on the deposited
quercetin layer, followed by catalytic oxidization at the elec-
trode.
Differential pulse voltammograms were measured in a UA
and AA mixture. It is clearly shown in Fig. 5 that UA and AA
both are catalytically oxidized at the Qu/WGE. At bare WGE
onlyarathersmallandbroadanodicDPVpeakappearedatabout
0.28V.Incontrast,theQu/WGEgivesawellshapedanodicpeak
at 0.33V for UA and another broader peak at 0.05V for AA,
respectively.About280mVpeakseparationallowstodetermine
UA in the presence of a large amount of AA, or to determine
UA and AA simultaneously.
It is observed that the pH of solution has a significant influ-
ence on the electro-oxidation of UA and AA at the Qu/WGE, by
altering both the peak current and peak potential. UA showed a
maximal peak current at a pH value of about 6, while AA did
Fig.5. DifferentialpulsevoltammogramsobtainedattheWGE(a)andQu/WGE
(b) in a mixture (0.1mM UA+0.5mM AA+0.1M PBS, pH 6.0). Pulse ampli-
tude: 50mV; pulse width: 50ms; pulse period: 200ms.
also at pH of about 4 (Fig. 6A). Afterward the peak currents
of both species decreased largely until the pH was increased
up to 8. Both UA and AA exist mostly in anionic form at pH
higherthan5.4and4.1,whicharethepKavaluesofUAandAA,
respectively, at 25◦C [44]. In Section 3.2 we have described the
Fig. 6. (A) pH-dependences of DPV peak currents of UA and AA. (B) pH-
dependences of DPV peak potentials of UA and AA (Inset: pH-dependence of
the peak separation). The DPVs were obtained at the Qu/WGE in mixtures of
0.1mM UA and 0.5mM AA with different pHs. Pulse amplitude: 50mV; pulse
width: 50ms; pulse period: 200ms.

Page 6

retarding effect of the modified quercetin on the redox reaction
of anion couple of Fe(CN)64−/3−, and attributed it to the elec-
trostatic repulsion from the electronegative modification layer.
The same repulsion effect probably occurs between the modifi-
cation layer and UA/AA anions, leading to the decrease in DPV
peak current. By contraries, at lower pH where UA and AA
exist mainly in their molecular forms, a favorable electrostatic
attraction between the analytes and the modification layer prob-
ably plays an important role for the adsorption and the catalytic
activities. Fig. 6(A) also shows a dramatic increase in the peak
current of both UA and AA at pH of 9, which could be related
to the pH effect on the modified quercetin, since the pKavalue
of quercetin was found at about 9 [40].
The effect of solution pH on the peak potentials of AA and
UAisshownin Fig.6(B).BothAAandUAoxidationpotentials
shifted negatively with the increasing pH. The peak potential
showsaslopeof−59mV/pHforUAoxidationand−53mV/pH
forAA,bothindicatingthatthenumberofelectronsandprotons
involved in the oxidation mechanisms is the same. It has been
knownthatUAisoxidizedviaatwoelectrontwoprotonprocess,
followed by a hydrolyzation reaction to produce allantoin as
shown in Scheme 3(A) [45]. As for AA, its electrochemical
oxidation generates the dehydroascorbic acid (DAA), with the
loss of two electrons and the consequent loss of hydrogen ions
[17] (Scheme 3(B)).
The peak separation between AA and UA is also pH-
dependent. The worst peak separation of about 0.21V was seen
in the pH range of 7–8 (Fig. 6(B), inset). Considering the favor-
able peak current for UA and the large peak separation between
these two compounds, a befitting pH range of 4.0–6.0 can be
selected for the selective detection of UA in the presence of AA.
3.4. Analytical application for the selective determination
of UA
Fig. 7 shows the differential pulse voltammograms recorded
at the Qu/WGE in solutions containing various concentrations
of UA and 0.5mM AA. The linear relationship between peak
current ip(after background correction) and UA concentration
is found as ip/?A=0.09056+0.35153cUA/?M (r2=0.99914)
in the range of 1.0–50?M (Fig. 7, inset), with a detection limit
of 1.0?M (S/N=3). The peaks are reproducible for subsequent
Fig. 7. Differential pulse voltammograms obtained at the Qu/WGE in pH 6.0
UA solutions containing 0.5mM AA. UA concentration (from the bottom up):
1.0, 5.0, 10.0, 30.0, 50.0, 70.0, 90.0, 100.0?M. Pulse amplitude: 50mV; pulse
width: 50ms; pulse period: 200ms. Inset: The linear relationship between DPV
peak current and UA concentration.
DPV scans, indicating a good stability and antifouling ability
in the presence of AA. The detection limit of 1.0?M UA in
0.5mM AA solution means that 500-fold concentration of AA
cannotinterferewiththedetectionofUA,whichislargeenough
for application in physiological conditions. It shows that this
electrode can be applied to the quantificational detection of UA
in the present of high concentration AA or for a simultaneous
determination of UA and AA.
A real urine sample was detected by DPV method at bare
WGE and Qu/WGE, respectively, after diluted 10 times with
the pH 6 PBS. As shown in Fig. 8 the bare WGE showed an
extremely low peak (curve a). In the case of Qu/WGE, a well-
shaped peak with good intensity was observed at about 0.30V
(curveb),correspondingtotheoxidationofUA.Onlyaverylow
and broad peak was observed at around −0.03V for AA oxida-
tion. The data show, in principle, the potential for the selective
determination of UA in the presence of AA.
The stability of the electrode was tested. During the first 3
days the signal showed 3% decrease, in the 7 days the current
responsedecreasedbyabout5%ofitsinitialresponseandinthe
following1monthsby10%.Theelectrodeshouldbewelltreated
topreventfromabsorptioncontaminationstomaintaintherepro-
ducibility.TheadsorbedUAandAAcanbeeasilyremovedfrom
Scheme 3. Oxidation mechanisms of UA (A) and AA (B).

Page 7

Fig. 8. Differential pulse voltammograms for the detection of real urine sample
(diluted 10 times with pH 6 PBS) at (a) bare WGE and (b) Qu/WGE. Pulse
amplitude: 50mV; pulse width: 50ms; pulse period: 200ms.
the surface by potential scanning between −0.2 and 0.8V in
pH 7.0 PBS for several circles until the background CV curve
was obtained. In this way the experimental repeatability can be
maintained.
4. Conclusions
The oxidation of quercetin on an anodized wax-impregnated
graphite electrode gives rise to a redox-active electrodeposited
film. The redox response of the film is that anticipated for the
surface-immobilized substance. This film exhibits potent and
persistent electrocatalytic behavior toward UA and AA oxi-
dation. In amperometric determination by DPV, the limit of
detectionofUAwasestimatedtobeofintheorderof1.0?Mand
the calibration plot was linear in the range of 1–50?M. Unlike
the bare electrode the quercetin-modified electrode could sep-
arate the oxidation peak potentials of UA and AA present in
mixture solutions. The electrode surface can be renewed eas-
ily by potential cycling and then maintain their stability and
reproducibility. This study has also demonstrated the practical
analytical utility of the modified electrode for the detection UA
in human urine. Thus, this electrode can be used for individual
or simultaneous electroanalysis of UA and AA.
Acknowledgements
The authors are grateful to the Open Fund of the Key Lab of
Bio-processofEducationMinistryofChina(HFUT)andDoctor
Fund of Hefei University of Technology, for financial support
to this work.
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