Simultaneous determination of ascorbic acid, dopamine, uric acid, and tryptophan by nanocrystalline ZSM-5 modified electrodes

Article abstract of DOI:10.1166/jnn.2014.9344

Nanocrystalline ZSM-5 was prepared by using propyltriethoxysilane as an additive in the conventional ZSM-5 synthesis composition. Materials were characterized by a complementary combination of X-ray diffraction, nitrogen sorption, and Scanning electron microscopy. Transition metal ion exchanged nanocrystalline ZSM-5 (M-Nano-ZSM-5, where M = Cu₂₊, Ni₂₊, Co₂₊, Fe₂₊, and Mn₂₊) modified electrodes were constructed for the simultaneous determination of ascorbic acid (AA), dopamine (DA), uric acid (UA), and tryptophan (Trp). Electrochemical studies were carried out by using cyclic voltammetry, linear sweep voltammetry, and chronoamperometry in buffer solution at pH 3.5. Fe-Nano-ZSM-5 modified electrode exhibited excellent electrocatalytic activity with wellseparated oxidation peaks towards AA, DA, UA, and Trp in their simultaneous determination. Among the M-Nano-ZSM-5 and transition metal ion-exchanged ZSM-5 (M-ZSM-5) materials investigated in this study, Fe-Nano-ZSM-5 exhibited the highest catalytic activities towards the oxidation of AA, DA, UA, and Trp with good stability, sensitivity, and selectivity. The analytical performance of this sensor was demonstrated for the simultaneous determination of AA, DA, UA, and Trp in blood serum and UA concentration in urine samples.

Full text of DOI:10.1166/jnn.2014.9344

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 6539–6550, 2014
Copyright © 2014 American Scientific Publishers
All rights reserved
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Printed in the United States of America
Simultaneous Determination of Ascorbic Acid,
Dopamine, Uric Acid, and Tryptophan by
Nanocrystalline ZSM-5 Modified Electrodes
Balwinder Kaur and Rajendra Srivastava∗
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India
Nanocrystalline ZSM-5 was prepared by using propyltriethoxysilane as an additive in the conven-
tional ZSM-5 synthesis composition. Materials were characterized by a complementary combina-
tion of X-ray diffraction, nitrogen sorption, and Scanning electron microscopy. Transition metal ion
exchanged nanocrystalline ZSM-5 (M-Nano-ZSM-5, where M = Cu2+, Ni2+, Co2+, Fe2+, and Mn2+
)
modified electrodes were constructed for the simultaneous determination of ascorbic acid (AA),
dopamine (DA), uric acid (UA), and tryptophan (Trp). Electrochemical studies were carried out by
using cyclic voltammetry, linear sweep voltammetry, and chronoamperometry in buffer solution at
pH 3.5. Fe-Nano-ZSM-5 modified electrode exhibited excellent electrocatalytic activity with well-
separated oxidation peaks towards AA, DA, UA, and Trp in their simultaneous determination. Among
the M-Nano-ZSM-5 and transition metal ion-exchanged ZSM-5 (M-ZSM-5) materials investigated in
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this study, Fe-Nano-ZSM-5 exhibited the highest catalytic activities towards the oxidation of AA, DA,
IP: 172.79.99.67 On: Mon, 28 Dec 2015 11:14:23
UA, and Trp with good stability, sensitivity, and selectivity. The analytical performance of this sensor
Copyright: American Scientific Publishers
was demonstrated for the simultaneous determination of AA, DA, UA, and Trp in blood serum and
UA concentration in urine samples.
Keywords: Nanocrystalline ZSM-5, Ion-Exchange, Simultaneous Determination, Electrocatalytic
Oxidation, Bio-Molecules.
1. INTRODUCTION
the maintenance of muscle mass and body weight in
humans.1 To correct possible dietary deficiencies, Trp is
added to dietary and food products as a food fortifier
and to pharmaceutical formulations. HPLC and spectro-
photometric procedures are known to determine the Trp
concentration; however these methods are complex and
time consuming.2 Uric acid (UA) is the end product of
purine metabolism and its concentration in the body should
be maintained.3 Its abnormal concentration level leads to
several diseases such as hyperuricemia, gout, leukemia,
and pneumonia.4 Dopamine (DA) is an important neu-
rotransmitter in the mammalian central nervous system.5
Low levels of DA may cause neurological disorders such
as schizophrenia and Parkinson’s disease.6 Ascorbic acid
(AA) prevents scurvy and is known to take part in sev-
eral biological reactions. It has been widely used in foods
and drinks as an antioxidant and also for the prevention
and treatment of common cold, mental illness, infertil-
ity, cancer, and AIDS.7 AA, DA, UA, and Trp usually
coexist in blood serum. Therefore, the development of a
Medical scientists have made significant contribution to
understand the chemistry of human body. The deficiency
or maladjustment of various bio-molecules present in body
may lead to many diseases. Thousands of different bio-
molecules exist in the body and it is almost impossible to
detect them in one run, because of their different chem-
ical properties. It would be very interesting if we can
detect at least a class of compounds in one run, then
it is a great achievement in the medical science. In this
study, nanocrystalline zeolite based modified electrodes
were constructed and find their potential application as
electrochemical sensor in the simultaneous detection of
four different physiologically important bio-molecules.
Tryptophan (Trp) is an essential amino acid required
for the biosynthesis of proteins (precursor molecules
of hormones, neurotransmitters and other relevant bio-
molecules) and find importance in nitrogen balance and
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 9
1533-4880/2014/14/6539/012
doi:10.1166/jnn.2014.9344
6539

Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Tryptophan
Kaur and Srivastava
sensitive and selective method for their simultaneous deter-
mination is highly desirable for analytical application and
diagnostic research. AA, DA, UA, and Trp are electro-
chemically active and therefore, in principle, it is possi-
ble to detect them simultaneously using electrochemical
method. However, it was found that they are oxidized at
nearly the same potential with poor sensitivity at unmodi-
fied electrodes.8 To overcome this problem, various mod-
ified electrodes have been constructed and investigated
in their simultaneous determination.9ꢀaꢁ–ꢀdꢁ These modified
electrodes are successful in detecting either two or three
such species simultaneously.10ꢀaꢁ–ꢀfꢁ Simultaneous detection
of all four species is rarely reported.11 Very recently, we
have reported the simultaneous determination of AA, DA,
UA, and Trp using transition metal exchanged mesoporous
polyaniline.12
NaOH were obtained from Spectrochem Pvt. Ltd., whereas
dopamine, L-ascorbic acid, uric acid, and tryptophan were
obtained from Himedia Pvt. Ltd. The standard phosphate
buffer solutions with different pH values were prepared by
adding 7.5 M aqueous NaOH solution to aqueous solution
of ortho phosphoric acid [H3PO4 = 10ꢃ11 mL (85% aque-
ous solution) in 1 L of aqueous solution] while magneti-
cally stirring until the pH of the aqueous solution reached
the desired value (1.0, 3.5, 5.0, 7.0, and 9.0). The electro-
chemical measurements were performed in buffer solution
(pH 3.5). Deionized water from Millipore Milli-Q sys-
tem (Resistivity 18 Mꢄcm) was used in the electrochemi-
cal studies. Blood serum samples and urine samples were
obtained from Parmar Hospital, Ropar, Punjab, India.
2.2. Sample Preparation
Microporous crystalline aluminosilicate, zeolites, are
widely used as shape-selective catalysts, ion-exchanged
materials, and adsorbents for organic compounds.13
Adsorptive and ion-exchange properties of zeolite were
explored for obtaining modified electrodes.14 Their supe-
rior performance can often be attributed to the existence
of a well-defined system of micropores (size below 1.5 nm
in diameter) with uniform shape and size, typically of
molecular dimensions. However, for some applications,
the presence of such micropores alone can also result
in an unacceptably slow diffusion of reactants and prod-
Nano-ZSM-5 was synthesized by following the reported
procedure.19 In a typical synthesis, 1.2 g of sodium alu-
minate (53 wt.% Al2O3, 43 wt.% Na2O) was dissolved in
25 mL of distilled water (Solution A). 2.06 g of PrTES
was mixed with 25 mL of TPAOH (1 M aq. solution)
(Solution B). Solution A and solution B were mixed, and
the resultant solution was stirred for 15 minutes at ambi-
ent condition, until it became a clear solution. 18.7 g
of TEOS was added into the resultant solution and stir-
ring was continued for 6 h. The molar composition of
the gel mixture was 90 TEOS/10 PrTES/2.5 Al O /3.3
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2
3
ucts to and from the active sites located inside the zeo-
lite crystals.15 To overcome this diffusion limitation, two
approaches have been adopted; either to minimize the
crystal size of zeolite catalysts (nanocrystalline zeolite)
or prepare a zeolite with interconnected micropores and
mesopores (hierarchical zeolite).16 In recent years, zeolites
with interconnected intra- or inter-crystalline mesoporos-
ity have attracted much attention due to improved diffu-
sion, catalytic activity, selectivity, and retardation against
deactivation.17ꢂ18 Several attempts have been made for the
preparation of nanosized zeolites.19ꢂ20ꢀaꢁ–ꢀdꢁ There are only
a few reports available in literature, where zeolites mod-
ified electrodes have been used for the detection of bio-
logical compounds.21ꢀaꢁꢂꢀbꢁ In this study, transition metal
exchanged nanocrystalline ZSM-5 (M-Nano-ZSM-5 where
M = Cu2+, Ni2+, Co2+, Fe2+, and Mn2+) based electrodes
were constructed for the detection of AA, DA, UA, and
Trp and results are compared with the transition metal
exchanged ZSM-5 (M-ZSM-5) based electrodes.
IP: 172.79.99.67 On: Mon, 28 Dec 2015 11:14:23
Na2O/25 TPAOH/2500 H2O. This mixture was transferred
Copyright: American Scientific Publishers
to a Teflon-lined autoclave, and hydrothermally treated at
443 K for 3 days under static conditions. The final prod-
uct was filtered, washed with distilled water and dried at
373 K. Material was calcined at 823 K for 4 h under
flowing air. ZSM-5 was synthesized at 443 K using the
same synthesis composition as mentioned above for the
Nano-ZSM-5, but without PrTES additive. Nano-ZSM-5
and ZSM-5 (1 g) were cation exchanged into M-Nano-
ZSM-5 or M-ZSM-5 (where M = Cu, Ni, Co, Mn, Fe) by
repeating the ion-exchange, three times with a 50 mL of
1 M aqueous solution of metal source at 343 K for 4 h.
Metal contents were obtained using the atomic absorption
spectrophotometer (AAS).
2.3. Electrode Fabrication
Cyclic voltammetry (CV), linear sweep voltammetry (LSV)
and chronoamperometric studies of zeolite samples were
performed using Potentiostat-Galvanostat BASi EPSILON,
USA. A three electrode electrochemical cell was employed
with Ag/AgCl as the reference electrode (3 M KCl), zeo-
lites mounted glassy carbon (3 mm diameter) as the work-
ing electrode, and Pt foil as the counter electrode. Before
modification, the polished electrode was ultrasonicated in
ethanol and deionized water for 5 minutes, respectively. 20
ꢅL aliquot of zeolite suspension (a homogenous sonicated
solution of 2 mg of zeolite, 10 ꢅL nafion, and 1 mL of
deionized water) was placed onto the electrode surface, the
2. EXPERIMENTAL DETAILS
2.1. Chemicals
All chemicals used in the study were of A.R. grade and
used as received without further purification. Tetraethy-
lorthosilicate (TEOS), propyltriethoxysilane (PrTES), and
tetrapropylammonium hydroxide (TPAOH) were pur-
chased from Aldrich. CuCl2 ·2H2O, MnCl2 ·4H2O, NiCl2 ·
6H2O, CoCl2 · 6H2O, FeCl2 · 4H2O, D-(+)-Glucose, and
6540
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Kaur and Srivastava
Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Tryptophan
Table I. Physico-chemical characteristics of zeolites investigated in this
study.
electrode was dried in air leaving the material mounted onto
the glassy carbon surface.
Total
surface
area
External
surface
area
Total
pore
volume
(cm3/g)
Transition metal
content in zeolite
(mg/g)
2.4. Characterization
X-ray diffraction (XRD) patterns were recorded in the
range of 5–80ꢀ on a PANalytical X’PERT PRO diffrac-
tometer using Cu Kꢆ radiation (ꢇ = 0ꢃ1542 nm, 40 kV,
20 mA) and a proportional counter detector. Nitro-
gen adsorption measurement at 77 K was performed
by Autosorb-IQ Quantachrome Instruments volumetric
adsorption analyzer. Samples were out-gassed at 573 K
for 2 h in the degas port of the adsorption appara-
tus. The specific surface area was determined by BET
method using the data points of P/P0 in the range of
about 0.05–0.3. The pore diameter was estimated using
the Barret–Joyner–Halenda (BJH) model. Scanning elec-
tron microscopy (SEM) measurements were carried out
on a JEOL JSM-6610LV to investigate the morphology
of the materials. During the SEM investigation, energy-
dispersive X-ray spectroscopy (EDS) was utilized in the
sample characterization and the EDS elemental maps were
obtained. The chemical composition of the samples was
estimated by atomic absorption spectrophotometer (Ana-
lytic Jena Model AAS 5EA, Germany).
Sample
(m2/g)
(m2/g)
ZSM-5
–
–
286
492
476
483
475
480
470
266
273
50
246
234
240
237
243
230
46
0.201
0.412
0.400
0.407
0.400
0.405
0.396
0.194
0.198
Nano-ZSM-5
Cu-Nano-ZSM-5
Ni-Nano-ZSM-5
Co-Nano-ZSM-5
Fe-Nano-ZSM-5
Mn-Nano-ZSM-5
Ni-ZSM-5
123.8
123.2
119.0
114.5
109.9
124.4
127.0
Cu-ZSM-5
52
nature, confirming the nanocrystalline nature of the mate-
rial. M-Nano-ZSM-5 and M-ZSM-5 also exhibited XRD
pattern corresponding to a highly crystalline MFI struc-
ture. The N2-adsoption isotherm for Nano-ZSM-5 showed
a type-IV isotherm similar to the mesoporous silica mate-
rials (Fig. 1(b)). The major difference in the isotherm
of Nano-ZSM-5 when compared to ZSM-5 is a distinct
increase of N2 adsorption in the region 0ꢃ4 < P/Po < 0ꢃ95,
which is interpreted as capillary condensation in mesopore
void spaces. The mesopores show a pore size distribution
in the range of 1–10 nm. Surface area and pore volume
3. RESULTS AND DISCUSSION
3.1. Physicochemical Characterizations
Nano-ZSM-5 exhibited MFI framework structure with
high phase purity, which was confirmed by using XRD
(Fig. 1(a)). The XRD pattern of Nano-ZSM-5 is broad in
of M-Nano-ZSM-5 and M-ZSM-5 were found to be sim-
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ilar when compared to Nano-ZSM-5 and ZSM-5, respec-
tively (Table I). The SEM images show that the spheroid
shaped Nano-ZSM-5 particles are composed of very small
nanocrystals (Fig. 2(a)). Zeolite particles with capsule-like
Copyright: American Scientific Publishers
450
1.4
(b)
(a)
1.2
400
1.0
0.8
350
0.6
Nano-ZSM-5
0.4
300
0.2
0.0
0
10 20 30 40 50
Pore size (nm)
250
200
150
100
50
ZSM-5
Conv-ZSM-5
Nano-ZSM-5
0.0
0.2
0.4
0.6
0.8
1.0
10
20
30
40
50
Relative pressure (P/P0)
2θ
Figure 1. (a) XRD patterns of ZSM-5 and Nano-ZSM-5 and (b) N2-adsorption isotherms of ZSM-5 and Nano-ZSM-5 (inset shows pore size
distribution).
J. Nanosci. Nanotechnol. 14, 6539–6550, 2014
6541

Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Tryptophan
Kaur and Srivastava
Figure 2. (a) SEM images of ZSM-5 and Nano-ZSM-5 and (b) EDS elemental maps of representative elements in Fe-Nano-ZSM-5.
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Copyright: American Scientific Publishers
crystal morphology with large crystal were obtained for
ZSM-5 (Fig. 2(a)). Mesopores are created by the crys-
tal packing of zeolite nanocrystals. Mesopore size distri-
bution obtained from N2-adsorption studies confirms that
they are in the range of 2–10 nm. Based on SEM and N2-
adsorption studies, one can conclude that spheroid shaped
Nano-ZSM-5 is composed of several nanozeolite particles
of approximately 10 nm. EDS mapping confirms the exis-
tence of finely dispersed metal species in the Nano-ZSM-5
matrix (Fig. 2(b)). PrTES contains only three hydrolyzable
moieties (with one hydrophobic propyl group), which is
unfavorable for the formation of extended tetrahedral SiO2
linkages. Consequently, the zeolite growth is significantly
retarded at the organic and inorganic interfaces, resulting
in the formation of nanocrystalline zeolites. Mesopores are
formed due to the crystal packing of these nanosized zeo-
lite particles. In this study, M-Nano-ZSM-5 and M-ZSM-5
were investigated in the electrochemical oxidation of AA,
DA, UA, and Trp. Metal contents obtained from AAS for
various transition metal exchanged zeolites investigated in
this study were found to be similar (Table I).
methods. However, in-depth investigation was made using
Fe-Nano-ZSM-5 modified electrode. For comparison, study
was also performed using M-ZSM-5 modified electrodes.
To explore the electrochemical behavior of the Fe-Nano-
ZSM-5 modified electrode, Fe-ZSM-5 modified electrode,
and bare glassy carbon electrode, CV was taken in 10 mL
buffer solution (pH 3.5). The electrochemical response of
Fe-Nano-ZSM-5 modified electrode and Fe-ZSM-5 modi-
fied electrode exhibited a pair of well defined redox peaks
at 0.29 V and 0.14 V versus Ag/AgCl, and potential peak
separation ꢈEp = 0ꢃ15 V (Fig. 3), which is attributed to
the electron transfer between Fe(II) and Fe(III) in the Fe-
Nano-ZSM-5 modified electrode.22 No redox peak was
observed for the bare glassy carbon electrode (Fig. 3). Fe-
Nano-ZSM-5 modified electrode exhibits higher current
response compared to Fe-ZSM-5 modified electrode.
The influence of the scan rate on the anodic and
the cathodic peak currents of Fe-Nano-ZSM-5 modified
electrode was studied. CVs at various scan rates (10–
600 mV s−1) obtained at the Fe-Nano-ZSM-5 modified
electrode show that the anodic and the cathodic peak cur-
rent increases with the increase in the scan rate (Fig. 4).
The plot of the peak currents against the scan rates shows
an excellent linear relationship (Fig. 4, inset), suggesting
that the electrochemical process is a diffusion-controlled
electron transfer process rather than surface controlled
at these scan rates.23 The concentration of electroactive
3.2. Electrochemical Characterizations of
M-Nano-ZSM-5 Modified Electrodes
In this study, electro-catalytic oxidation of AA, DA, UA,
and Trp were investigated using M-Nano-ZSM-5 modi-
fied electrodes using CV, LSV, and chronoamperometric
6542
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Kaur and Srivastava
Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Tryptophan
5ꢃ02×10−7 mol cm−2. The calculated surface concen-
trations of electroactive species in Mn-Nano-ZSM-5,
Co-Nano-ZSM-5, Ni-Nano-ZSM-5, and Cu-Nano-ZSM-5
Fe-Nano-ZSM-5 modified electrode
Fe-ZSM-5 modified electrode
Bare glassy carbon electrode
modified electrode are found to be 1ꢃ7×10−7, 4ꢃ7×10−7
,
2
1
5ꢃ3×10−7, and 4ꢃ9×10−7 mol cm−2, respectively.
Before the determination of AA, DA, UA, and Trp,
amperometry was used to evaluate the antifouling prop-
erty of the Fe-Nano-ZSM-5 modified electrode, since the
oxidized products of these analytes could strongly absorb
onto the electrode surface that induce electrode fouling and
poor reproducibility. The response of the Fe-Nano-ZSM-
5 electrode remained quite stable throughout the entire
period (40 minutes), with only less current diminutions for
the oxidation of AA, DA, UA, and Trp, respectively. Metal
ions exchanged on Nano-ZSM-5 matrix catalyze the elec-
trochemical oxidation of AA, DA, UA, and Trp at the elec-
trode surface and the oxidized products of these analytes
are diffuse out from electrode surface after the reaction
through the inter-crystalline mesopores of zeolite matrix
and they are not absorbed at the electrode surface. The
synergistic contribution provided by the highly dispersed
metal ions (for catalysis) and inter-crystalline mesopores
(for enhance diffusion of reactant and product molecules)
present in M-Nano-ZSM-5 prevents the electrode from
passivation.21b
0
–1
–2
0.0
0.2
0.4
0.6
0.8
1.0
Potential (V)
Figure 3. CV responses of Fe-Nano-ZSM-5 modified electrode, Fe-
ZSM-5 modified electrode and bare glassy carbon electrode in 10 mL
buffer solution (pH 3.5) at a scan rate 50 mV s−1
.
species (ꢉ) can be estimated by plotting the peak currents
3.3. Individual Electrocatalytic Oxidation of
AA, DA, UA, and Trp
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against the scan rates by following Eq. (1).
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The Fe-Nano-ZSM-5 modified electrode was employed
to study the electrocatalytic oxidation of AA, DA, UA,
and Trp. Prior to the implementation of Fe-Nano-ZSM-5
modified electrode for the oxidation of analytes, a compar-
ison of electrochemical behavior of Fe-Nano-ZSM-5 mod-
ified electrode, Fe-ZSM-5 modified electrode, and bare
glassy carbon electrode (GCE) towards determination of
AA, DA, UA, and Trp individually, was first investigated
using CV in 10 mL buffer solution (pH 3.5) (Fig. 5). The
results show that analytes are oxidized with well-defined
and distinguishable oxidation peaks with peak potentials
at 0.19 V, 0.34 V, 0.41 V, and 0.80 V for AA, DA,
UA, and Trp, respectively at the Fe-Nano-ZSM-5 mod-
ified electrode (Fig. 5(a)). Fe-ZSM-5 modified electrode
showed distinguishable oxidation peaks with peak poten-
tials at 0.34 V, 0.41 V, and 0.80 V corresponding to DA,
UA, and Trp respectively, but was unable to detect AA
(Fig. 5(b)). On the other hand, bare GCE fail to distin-
guish the oxidation peaks of AA and merged voltammetric
anodic peaks appear at 0.36 V, 0.47 V, and 0.81 V for DA,
UA, and Trp, respectively (Fig. 5(c)). Compared with the
bare GCE, the peak potentials of these analytes at the Fe-
Nano-ZSM-5 modified electrode and Fe-ZSM-5 modified
electrode are shifted negatively (decrease in oxidation over
potential) along with significant increase in oxidation peak
current, which indicate that the modified electrode plays
a catalytic effect on the oxidation of AA, DA, UA, and
Trp. The enlarged separation of the anodic peak potential
Copyright: American Scientific Publishers
Ip = n2F 2Aꢉꢊ/4RT
(1)
The slope of the anodic peak current against the scan
rate is found to be 0.033 (Fig. 4, inset). From the
slope of anodic peak currents versus scan rates, the
calculated surface concentration of electroactive species
in Fe-Nano-ZSM-5 modified electrode is found to be
12
8
4
0
600 mV s–1
12
6
–4
0
–6
–8
–12
10 mV s–1
0
100 200 300 400 500 600
Scan rate (mV s–1
0.8
)
–12
0.0
0.2
0.4
0.6
1.0
Potential (V)
Figure 4. CVs of Fe-Nano-ZSM-5 modified electrode in 10 mL buffer
solution (pH 3.5) at various scan rates (10–600 mV s−1). Inset shows plot
of peak currents versus scan rates.
J. Nanosci. Nanotechnol. 14, 6539–6550, 2014
6543

Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Tryptophan
Kaur and Srivastava
40
30
20
20
(b)
(a)
(c)
30
20
15
10
5
10
10
0
0
0
–10
–20
–30
–40
–5
–10
–15
AA
DA
UA
Trp
–10
–20
–30
AA
DA
UA
Trp
AA
DA
UA
Trp
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
0.2
0.4
0.6
0.8
1.0
Potential (V)
Potential (V)
Potential (V)
Figure 5. CVs of 10 mL buffer solution (pH 3.5) containing 100 ꢅM each of AA, DA, UA, and Trp at a scan rate of 50 mV s−1 at (a) Fe-Nano-ZSM-5
modified electrode, (b) Fe-ZSM-5 modified electrode, and (c) bare glassy carbon electrode.
O
HO
OH
HO
O
O
HO
2H+
2e-
+
+
O
OH
O
OH
O
AA
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HO
HO
Copyright: American Scientific Publishers
2H+
2e-
+
+
O
NH 3
NH 3
DA
O
O
OH
OH
H
N
H
N
NH
NH
2H +
2e-
O
O
+
+
N
H
N
H
N
H
O
N
H
O
UA
O
O
O
OH
OH
2H +
2e-
+
+
NH 2
NH2
N
N
H
Trp
Scheme 1. Electrochemical oxidations of AA, DA, UA, and Trp over ZSM-5 modified electrodes investigated in this study.
6544
J. Nanosci. Nanotechnol. 14, 6539–6550, 2014

Kaur and Srivastava
Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Tryptophan
80
coupled with the increased sensitivity makes it possi-
ble to effectively determine the individual concentrations
of AA, DA, UA, and Trp on Fe-Nano-ZSM-5 modified
electrode.
3.0
2.5
2.0
1.5
60 (a)
(b)
50
40
30
60
40
20
0
The individual electrocatalytic oxidation of AA, DA,
UA, and Trp on the Fe-Nano-ZSM-5 modified electrode
was investigated by CV and LSV in 10 mL buffer solu-
tion (pH 3.5) at a scan rate of 50 mV s−1 by varying
their concentrations. In the case of AA, with increase in
the concentration of AA, anodic peak current increases,
which correspond to the oxidation of hydroxyl groups of
the furan ring in AA to carbonyl groups (Scheme 1).
No corresponding reduction peak appears, revealing that
the oxidation of AA is irreversible. DA undergoes a
two-electron oxidation process resulting in the forma-
tion of dopamine-o-quinone (DOQ) (Scheme 1). With
increase in the concentration of DA, anodic peak current
increases, while cathodic peak current decreases, sug-
gesting that the oxidation of DA is reversible. The cou-
ple of redox peaks is corresponding to the two-electron
oxidation of DA to dopamine-o-quinone (DOQ) and the
subsequent reduction of DOQ to DA. With increase in
the concentration of UA, anodic peak current increases,
which is due to the oxidation of bridging double bond
of uric acid to hydroxyl groups (uric acid 4,5 diol)
(Scheme 1). Similar enhancement of anodic peak cur-
rent is also observed for Trp by increasing its concen-
tration, which is due to the oxidation of phenyl ring of
Trp (Scheme 1). The modified electrode showed excel-
lent electrocatalytic activity towards AA, DA, UA, and
Trp oxidation, individually, in a buffer solution (pH 3.5).
The electrochemical response of AA, DA, UA, and Trp
increases linearly with the increase in their concentrations.
The electrochemical sensor shows a linear response for
AA, DA, UA, and Trp in the concentration range of 10 ꢅM
to 1 mM.
The diffusion coefficients (D) for these analytes were
calculated using chronoamperometry (Fig. 6). Chronoam-
perograms were obtained at different concentrations of
analytes at a desired potential step (250, 400, 550, and 850
mV for AA, DA, UA, and Trp, respectively). The plots of
I versus t−1/2 exhibited straight lines for different concen-
trations of analytes. Cottrell equation (Eq. (2)) was used
to calculate the diffusion coefficient for various analytes
investigated in this study.24
20
1.00 1.25 1.50 1.75 2.00
0.4 0.5 0.6 0.7 0.8 0.9 1.0
Time–1/2 (s–1/2
)
Time1/2 (s1/2
)
d
c
b
a
0
10
20
30
40
50
60
Time (s)
Figure 6. Chronoamperograms obtained at the Fe-Nano-ZSM-5 mod-
ified electrode in the absence (a) and in the presence of (b) 100 ꢅM,
(c) 200 ꢅM, and (d) 500 ꢅM of AA in 10 mL buffer solution (pH 3.5).
Inset: (a) Dependence of current on the time−1/2 derived from the
chronoamperogram data. (b) Dependence of IC /IL on time1/2 derived
from the data of chronoamperograms.
Based on the slope of IC /IL versus t1/2 plot; k can be
obtained for a given analyte concentration. From the values
of the slopes, an average value for k was obtained for
the oxidation of analyte. The rate constants for electro-
Delivered by Publishing Technology to: Chinese University of Hong Kong
catalytic oxidation of AA, DA, UA, and Trp were found
IP: 172.79.99.67 On: Mon, 28 Dec 2015 11:14:23
as 2ꢃ2×102, 3ꢃ7×103, 1ꢃ7×103, and 2ꢃ6×103 M−1 s−1
,
Copyright: American Scientific Publishers
respectively.
For comparison, individual electrocatalytic oxidation of
AA, DA, UA, and Trp was also performed on Fe-ZSM-5
modified electrode using CV in buffer solution (pH 3.5) at
a scan rate of 50 mV s−1. It may be noted that AA was
not possible to detect using Fe-ZSM-5 modified electrode.
CV measurements show that for the same concentration
of analyte, Fe-Nano-ZSM-5 modified electrode exhibits
higher current response compared to Fe-ZSM-5 modified
electrode.
3.4. Simultaneous Catalytic Oxidation of AA, DA, UA,
and Trp at Fe-Nano-ZSM-5 Modified Electrode
Previous section demonstrated that the Fe-Nano-ZSM-5
modified electrode displayed very good electrochemical
catalytic activities towards AA, DA, UA, and Trp oxida-
tion, individually. However, the objective of this study is
to determine the concentration of AA, DA, UA, and Trp,
simultaneously, from their quaternary mixture. One of such
real system is the blood serum, in which all these four
species coexist. Following section demonstrates the appli-
cability of Fe-Nano-ZSM-5 modified electrode towards the
simultaneous determination of AA, DA, UA, and Trp in
their mixture.
Ip = nFAD1/2c/ꢋ1/2t1/2
(2)
The diffusion coefficients for AA, DA, UA, and Trp, were
found to be 21ꢃ5×10−4, 15ꢃ5×10−4, 3ꢃ6×10−4, and 3ꢃ8×
10−4 cm2 s−1, respectively.
The rate constant for the chemical reaction between
the analytes and redox sites of Fe-Nano-ZSM-5 modified
electrode was evaluated by chronoamperometry through
Eq. (3).25
For simultaneous determination of AA, DA, UA, and
Trp; LSV was carried out at a scan rate of 50 mV s−1
in 10 mL buffer solution (pH 3.5) at the Fe-Nano-ZSM-5
IC /IL = ꢋ1/2ꢌ1/2 = ꢋ1/2ꢀkC0tꢁ1/2
(3)
J. Nanosci. Nanotechnol. 14, 6539–6550, 2014
6545

Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Tryptophan
Kaur and Srivastava
modified electrode (Fig. 7). The results show that all four
analytes are oxidized with well-defined and distinguishable
peaks. An apparent enhancement in the oxidation peak cur-
rents was observed, when the concentrations of analytes
were increased, indicating that all these analytes has been
oxidized at their corresponding potentials by the active Fe-
Nano-ZSM-5 modified electrodes via a cyclic mediation
redox process. Fe2+ is oxidized to Fe3+ on the electrode
surface at 0.29 V. Fe3+ oxidizes the analytes and it under-
goes reduction producing Fe2+ again. This redox cycle
causes an increase in the anodic peak current.26–28
AA, DA, UA, and Trp, respectively (Fig. 7, inset). These
results demonstrate that individual or simultaneous deter-
mination of these bio-molecules on the Fe-Nano-ZSM-5
modified electrode can be achieved with high sensitivity
and selectivity.
Influence of the pH on the simultaneous electrocatalytic
oxidation of AA, DA, UA, and Trp was investigated on
the Fe-Nano-ZSM-5 modified electrode by varying the pH
of the supporting electrolyte (buffer) by LSV at a scan rate
of 50 mV s−1 in a solution containing equal concentra-
tions of AA, DA, UA, and Trp in a wider pH range (pH
1.0–9.0). The anodic peak current increases by increasing
the pH of the medium, with a maximum anodic peak cur-
rent at pH 3.5 (Fig. 8(a)). This behavior was observed for
all bio-molecules investigated in this study. Peak potentials
for the oxidation of AA, DA, UA, and Trp shifted nega-
tively with higher pH value, indicating that protons take
part in their electrode reaction processes. The maximum
separation of peak potentials for AA-DA, UA-DA, Trp-UA
is observed at pH 3.5 (Fig. 8(b)). Therefore, in order to
obtain high sensitivity and selectivity, pH 3.5 was selected
as an optimum pH value for the determination of AA, DA,
UA, and Trp in their quaternary mixture. Slope obtained
from the linear behavior between the applied potential and
the pH for AA, DA, UA, and Trp were found to be 0.063,
0.050, 0.053, and 0.058 V/pH (Eqs. (4)–(7)), which are
close to the anticipated Nernstian value (0.059 V/pH) for
Four distinguished anodic peaks at potentials corre-
sponding to the oxidation of AA, DA, UA, and Trp were
observed at the Fe-Nano-ZSM-5 modified electrode in the
LSV plot during the simultaneous measurement experi-
ments using quaternary mixture having equal concentra-
tions (Fig. 7), which matches well with their individual
anodic peak potential as discussed in the above section.
LSV studies show that the anodic peak responses for qua-
ternary mixture containing AA, DA, UA, and Trp, are well
separated from each other with a potential difference of
140 mV, 120 mV, and 340 mV for the oxidation peak
potentials of AA-DA, DA-UA and UA-Trp, respectively,
which is large enough to simultaneously determine the
concentration of AA, DA, UA, and Trp in their mixture
solution. With increase in the concentration of quaternary
mixture, the response current corresponding to the ana-
Delivered by Publishing Technology to: Chinese University of Hong Kong
a two electrons/two protons reaction.29
lytes oxidation increases. The anodic peak current obtained
was found to be linearly dependent on the concentration
IP: 172.79.99.67 On: Mon, 28 Dec 2015 11:14:23
Copyright: American Scientific Publishers
Ep (AA) = −0ꢃ063pH+0ꢃ332 R2 = 0ꢃ995
Ep (DA) = −0ꢃ050pH+0ꢃ485 R2 = 0ꢃ994
Ep (UA) = −0ꢃ053pH+0ꢃ650 R2 = 0ꢃ998
Ep (Trp) = −0ꢃ058pH+0ꢃ919 R2 = 0ꢃ991
(4)
(5)
(6)
(7)
of analytes (in the range of 10 ꢅM–400 ꢅM for DA, UA,
and Trp, whereas 10 ꢅM–300 ꢅM for AA) with the sensi-
tivity of 0.0500, 0.1200, 0.0890, and 0.1000 ꢅA/ꢅM and
the lower detection limit of 5.8, 7.2, 3.6, and 5.1 ꢅM for
40
40
30
20
10
0
DA
AA
UA
Trp
For comparison, simultaneous determination studies were
also performed using Fe-ZSM-5 modified electrode by
LSV in buffer solution (pH 3.5) at a scan rate of 50 mV
DA
20
0
Trp
s
−1. Fe-ZSM-5 modified electrode show the anodic peaks
UA
f
corresponding to DA, UA, and Trp (No AA was detected).
LSVs were carried out using Fe-Nano-ZSM-5 and Fe-
ZSM-5 modified electrodes in buffer solution (pH 3.5) at a
scan rate of 50 mV s−1 in the presence and in the absence
of analytes (Figs. 9(a), (b)). Oxidation peak currents in the
case of Fe-ZSM-5 were found to be less when compared to
Fe-Nano-ZSM-5 modified electrode. This improved ana-
lytical performance is attributable to highly dispersed Fe2+
active centers on Nano-ZSM-5, which facilitates the elec-
tron transfer to the electrode. These observations confirm
that Fe-Nano-ZSM-5 has high electro-catalytic capability
than Fe-ZSM-5 modified electrode. Enhancement of the
AA, DA, UA, and Trp oxidation currents at Fe-Nano-
ZSM-5 modified electrode can be correlated with the
accessibility of more number of Fe2+ active centers in the
Fe-Nano-ZSM-5 due to its large specific surface area when
compared with Fe-ZSM-5.
0
100
200
300
400
Concentration (µM)
AA
a
–0.4
0.0
0.4
0.8
1.2
Potential (V)
Figure 7. LSVs of the quaternary mixture (having equal concentrations
of AA, DA, UA, and Trp) of different concentrations in 10 mL buffer
solution (pH 3.5) at Fe-Nano-ZSM-5 modified electrode at a scan rate of
50 mV s−1: (a) blank, (b) 10 ꢅM, (c) 50 ꢅM, (d) 100 ꢅM, (e) 200 ꢅM,
and (f) 400 ꢅM. Inset shows calibration curve for the sensor response
towards simultaneous electrocatalytic oxidation of AA, DA, UA, and Trp.
6546
J. Nanosci. Nanotechnol. 14, 6539–6550, 2014

Kaur and Srivastava
Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Tryptophan
AA
DA
1.0
0.8
0.6
0.4
0.2
0.0
(b)
(a)
AA
DA
UA
Trp
20
UA
Trp
16
12
8
4
0
0
2
4
6
8
10
0
2
4
6
8
10
pH
pH
Figure 8. Effect of the pH on (a) the peak current and (b) the peak potential; containing 100 ꢅM each of AA, DA, UA, and Trp at a scan rate 50
mV s−1
.
The high electrocatalytic activity of Fe-Nano-ZSM-5
encouraged us to investigate the influence of different M-
Nano-ZSM-5 materials towards AA, DA, UA, and Trp
oxidation. LSVs were carried out using different M-Nano-
ZSM-5 modified electrodes by increasing the concentra-
they were found to be insensitive towards AA oxidation.
A comparison of different M-Nano-ZSM-5 modified elec-
trodes towards the electrocatalytic oxidation of AA, DA,
UA, and Trp is summarized (Fig. 10 and Table II). Based
Delivered by Publishing Technology to: Chinese University of Hong Kong
on the experimental evidence, one can conclude that Fe-
IP: 172.79.99.67 On: Mon, 28 Dec 2015 11:14:23
Nano-ZSM-5 exhibited superior sensing ability and current
tions of AA, DA, UA, and Trp in 10 mL buffer solution
(pH 3.5) at a scan rate of 50 mV s−1. Although different
M-Nano-ZSM-5 modified electrodes exhibited high elec-
trocatalytic oxidation capability for DA, UA, and Trp but
Copyright: American Scientific Publishers
sensitivity compared to other M-Nano-ZSM-5 modified
electrodes and Fe-ZSM-5 modified electrode, investigated
in this study. High catalytic activity of Fe-Nano-ZSM-5
5
DA
(a)
(b)
Trp
20
15
10
5
4
3
2
1
0
UA
AA
Trp
DA
UA
AA
0
–1
Fe-Nano-ZSM-5
Fe-Nano-ZSM-5
Fe-ZSM-5
Fe-ZSM-5
–2
0.0
0.3
0.6
0.9
1.2
–0.3
0.0
0.3
0.6
0.9
1.2
Potential (V)
Potential (V)
Figure 9. Comparison of LSV of the Fe-Nano-ZSM-5 and Fe-ZSM-5 modified electrodes (a) in the absence and (b) in the presence of AA, DA, UA,
and Trp from their quaternary mixture having equal concentration (100 ꢅM) of these analytes in 10 mL buffer solution at a scan rate of 50 mV s−1
.
J. Nanosci. Nanotechnol. 14, 6539–6550, 2014
6547

Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Tryptophan
Kaur and Srivastava
3.5. Reproducibility, Stability, and Anti-Interference
Property of the Fe-Nano-ZSM-5
AA
DA
UA
Trp
Modified Electrode
The reproducibility and stability of the sensor was evalu-
ated in these sensing studies. Five Fe-Nano-ZSM-5 mod-
ified electrodes were made and their current responses to
50 ꢅM concentration of AA, DA, UA, and Trp mixture
were investigated. The relative standard deviation (RSD)
was found to be 2.5%, confirming that the fabrication
method was highly reproducible. The long-term stability
of the sensor was evaluated by measuring its sensitivity to
50 ꢅM concentration of AA, DA, UA, and Trp mixture
)
–1
M
µ
A
µ
Sensitivity
(
E l e c t r o d e
ꢀ
for 20 days. The sensor was stored in refrigerator at 5 C
and its sensitivity was tested at the interval of 5 days. The
LSV response of the electrode to the same concentration
of AA, DA, UA, and Trp decreased less than 3.5% indicat-
ing that the electrode has good reproducibility and excel-
lent stability. In order to investigate the selectivity of the
Fe-Nano-ZSM-5 modified electrode towards simultaneous
determination of AA, DA, UA, and Trp; LSV measure-
ments were performed in the presence of various interfering
agents. The tolerance limit was taken as the maximum con-
centration of foreign substances that caused a relative error
of approximately + 5% for the determination of quaternary
mixture containing equimolar concentration (50 ꢅM) of
analytes (AA, DA, UA, and Trp) plus the potential inter-
fering substances at pH 3.5. Tolerance limits for Na+, K+,
Figure 10. Comparison of the sensitivity for various analytes at
different M-Nano-ZSM-5 modified electrodes investigated in this
study.
modified electrode towards the oxidation of these ana-
lytes may be due the inherent high oxidation capability
of Fe2+ compared to other M2+ in first transition metal
atom series and its high electroactive surface concentra-
tion. The results obtained using Fe-Nano-ZSM-5 modi-
fied electrode for the electrocatalytic oxidation of AA,
DA, UA, and Trp were also compared with the previ-
ous work reported in the literature and are summarized in
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Mg2+, Zn2+, and glucose were found to be 250, 200, 200,
Table III.9–12
IP: 172.79.99.67 On: Mon, 28 Dec 2015 11:14:23
Copyright: American Scientific Publishers
Table II. Comparison of the electrocatalytic activities of different M-Nano-ZSM-5 and Fe-ZSM-5 modified electrodes for the simultaneous oxidation
of AA, DA, UA, and Trp.
Electroactive surface
concentration (mol cm−2
Lower detection
limit (ꢅM)
Sensitivity
(ꢅA/ꢅM)
Sample
)
Analyte
Linear range (ꢅM)
Fe-Nano-ZSM-5
5ꢃ02×10−7
1ꢃ67×10−7
4ꢃ7×10−7
5ꢃ3×10−7
4ꢃ9×10−7
–
AA
DA
UA
Trp
10–300
10–400
10–400
10–400
5ꢃ8
7ꢃ2
3ꢃ6
5ꢃ1
0.0500
0.1200
0.0890
0.1000
Mn-Nano-ZSM-5
Co-Nano-ZSM-5
Ni-Nano-ZSM-5
Cu-Nano-ZSM-5
Fe-ZSM-5
AA
DA
UA
Trp
0
–
–
10–300
10–300
10–400
8ꢃ3
5ꢃ9
3ꢃ4
0.0740
0.0770
0.0890
AA
DA
UA
Trp
0
–
–
10–300
10–300
10–400
6ꢃ3
3ꢃ9
6ꢃ7
0.0610
0.0680
0.0810
AA
DA
UA
Trp
0
–
–
10–300
10–300
10–400
2ꢃ2
5ꢃ4
2ꢃ3
0.0630
0.0740
0.0800
AA
DA
UA
Trp
0
–
–
10–100
10–100
10–100
2ꢃ1
5ꢃ5
3ꢃ2
0.0320
0.0260
0.0330
AA
DA
UA
Trp
0
10ꢃ0
9ꢃ4
10ꢃ0
9ꢃ1
–
10–200
10–200
10–200
0.0690
0.0660
0.0780
6548
J. Nanosci. Nanotechnol. 14, 6539–6550, 2014

Kaur and Srivastava
Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Tryptophan
Table III. Comparison of electrocatalytic activities for the simultaneous oxidation of AA, DA, UA, and Trp using different modified electrodes.
S. no.
1.
Electrochemical sensor
Fe-Nano-ZSM-5 modified electrode
Analyte
Linear range
LOD
pH
Reference
AA
DA
UA
Trp
10–300 ꢅM
10–400 ꢅM
10–400 ꢅM
10–400 ꢅM
5.8 ꢅM
7.2 ꢅM
3.6 ꢅM
5.1 ꢅM
3.5 Present work
2.
3
MWCNT modified carbon-ceramic electrode
AA
DA
UA
15–800 ꢅM
0.50–100 ꢅM
0.55–90 ꢅM
7.71 ꢅM 4.5
0.31 ꢅM
0.42 ꢅM
[9a]
[9b]
Lanthanum–MWCNT nanocomposites modified electrode
AA
DA
UA
NO−2
0.40 ꢅM–0.71 mM
0.04 ꢅM–0.89 mM
0.04 ꢅM–0.81 mM
0.40 ꢅM–0.71 mM
140 nM
13 nM
15 nM
130 nM
6.0
4.
5.
6.
7.
8.
Nitrogen doped graphene modified electrode
Chitosan-Graphene modified electrode
AA
DA
UA
5.0–1300 ꢅM
0.5–170 ꢅM
0.1–20 ꢅM
2.2 ꢅM
0.25 ꢅM
0.045 ꢅM
6.0
7.0
4.5
[9c]
[10a]
[10b]
[10c]
[11]
AA
DA
UA
50–1200 ꢅM
1.0–24 ꢅM
2.0–45 ꢅM
50 ꢅM
1.0 ꢅM
2.0 ꢅM
Palladium nanoparticle-loaded carbon nanofibers modified electrode
Helical carbon nanotubes modified electrode
AA
DA
UA
0.05–4 mM
0.5–160 ꢅM
2–200 ꢅM
15 ꢅM
0.2 ꢅM
0.7 ꢅM
AA
DA
UA
25–1045 ꢅM
2.5–10 ꢅM
5–175 ꢅM
0.12 ꢅM 7.4
0.08 ꢅM
0.22 ꢅM
Gold nanoparticles/overoxidized polyimidazole composite modified glassy
carbon electrode
AA
DA
210–1010 ꢅM
5–268 ꢅM
2 ꢅM
0.08 ꢅM
4
Delivered by Publishing Technology to: ChineUsAe University of Hong K0o.5ngꢅM
6–486 ꢅM
IP: 172.79.99.67 On: Mon, 28 Dec 2015 11:14:23
Trp
3–464 ꢅM
0.7 ꢅM
Copyright: American Scientific Publish1e0r0s–300 ꢅM
9.
Fe-Meso-PANI modified electrode
AA
6.5 ꢅM
9.8 ꢅM
5.3 ꢅM
5.2 ꢅM
3.5
[12]
DA
UA
Trp
100–300 ꢅM
100–300 ꢅM
100–300 ꢅM
Table IV. Determination of AA, DA, UA, and Trp in the blood serum
and urine samples.
150, and 75 ꢅM, respectively. The relative standard devia-
tions (RSD) with respect to the measurement of peak cur-
rents for AA, DA, UA, and Trp in the presence of these
interference species were found to be 3.5%, 3.0%, 3.3%,
4.0%, respectively; confirming that no significant interfer-
ence for these common species occurred.
Spiking
(ꢅM)
Detected
(ꢅM)a
Clinical
value (ꢅM)
RSD
(%)a
Sample
Analyte
Serum 1
AA
DA
UA
Trp
60
30
10
10
65ꢃ7
30ꢃ8
24ꢃ6
9ꢃ6
–
–
14.0
–
1.9
2.3
2.6
1.7
3.6. Determination of AA, DA, UA, and Trp in
Standard Sample and Real Samples
Serum 2
Urine 1
Urine 2
AA
DA
UA
Trp
70
40
30
60
78ꢃ1
39ꢃ4
45ꢃ7
59ꢃ3
–
–
16.5
–
1.5
1.4
1.8
2.2
Before exploring the potential applicability of Fe-Nano-
ZSM-5 modified electrode for real samples (blood serum
and urine), a known concentration of quaternary mixture,
having different concentrations of individual species, were
analyzed. For example: A quaternary mixture containing
AA, DA, UA, and Trp having concentrations 60 ꢅM,
10 ꢅM, 80 ꢅM, and 10 ꢅM was prepared and LSV
was performed. Concentration of AA, DA, UA, and Trp
based on the anodic peak currents and calibration profile
obtained previously (Fig. 7) were found to be approxi-
mately identical (58.1, 9.5, 78.4, and 9.3 for AA, DA, UA,
and Trp respectively) to the original concentrations taken
for the analysis. Having confirmed the concentrations of
AA
DA
UA
Trp
–
–
–
–
–
–
35ꢃ6
–
–
–
36.5
–
–
–
2.5
–
AA
DA
UA
Trp
–
–
–
–
–
–
16ꢃ5
–
–
–
15.8
–
–
–
2.4
–
Notes: For the analysis of the blood serum, samples were spiked with different
levels of the known amount of analytes; aAverage of three replicates.
J. Nanosci. Nanotechnol. 14, 6539–6550, 2014
6549

Simultaneous Determination of Ascorbic Acid, Dopamine, Uric Acid, and Tryptophan
Kaur and Srivastava
various species in the standard quaternary mixture, the
scope of Fe-Nano-ZSM-5 modified electrode was extended
for real samples. Two different kinds of blood serum and
urine samples were analyzed using LSV. The results indi-
cate that Fe-Nano-ZSM-5 modified electrode has a good
reliable operational ability (Table IV). The interference
of other molecules in the samples for the determination
of the AA, DA, UA, and Trp was not found. Therefore,
the Fe-Nano-ZSM-5 based sensor provides an economical,
simple, and efficient protocol for the simultaneous deter-
mination of these compounds in biological and pharma-
ceutical samples.
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4. CONCLUSIONS
In this work, transition metal exchanged nanocrytsalline
ZSM-5 modified electrodes were constructed and used
in the electrochemical oxidation of four important bio-
molecules of physiological relevance. Transition metal
exchanged nanocrystalline ZSM-5 modified electrode
exhibited high electrocatalytic activities towards the oxi-
dation of AA, DA, UA, and Trp by significantly decreas-
ing their oxidation over potentials and enhancing the
anodic peak currents. The modification of transition metal
exchanged nanocrystalline ZSM-5 not only improves the
electrochemical catalytic activities towards the oxidation
of AA, DA, UA, and Trp, but also resolves the merged
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oxidation peaks of AA, DA, UA, and Trp into four well-
IP: 172.79.99.67 On: Mon, 28 Dec 2015 11:14:23
defined peaks, which is very important for simultaneous
Copyright: American Scientific Publishers
determination of these analytes. The results demonstrate
that Fe-Nano-ZSM-5 has higher catalytic activities towards
the oxidation of AA, DA, UA, and Trp with good stabil-
ity, sensitivity, and selectivity. High selectivity and good
antifouling property encouraged us to use Fe-Nano-ZSM-5
modified electrode for the simultaneous determination of
AA, DA, UA, and Trp in human blood serum and UA in
urine samples. Highly dispersed metal ions on large sur-
face area Nano-ZSM-5 matrix and inter-crystalline meso-
pores (for enhance diffusion of reactants and products
molecules) are responsible for the high electrocatalytic
activity of M-Nano-ZSM-5. The analytical performance of
this sensor can also be evaluated for electrochemical detec-
tion of other electroactive bio-molecules.
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Acknowledgments: Authors thank Department of Sci-
ence and Technology, New Delhi for financial assistance
(DST grant SB/S1/PC-91/2012). Balwinder Kaur is grate-
ful to CSIR, New Delhi for JRF fellowship.
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Received: 7 March 2013. Accepted: 11 April 2013.
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