Sr-Doped NiO3 nanorods synthesized by a simple sonochemical method as excellent materials for voltammetric determination of quercetin

Article abstract of DOI:10.1039/c9nj05660b

The simple and novel surfactant-free synthesis of flower-like strontium-doped nickel oxide nanorods (SNO NRs) via a simple sonochemical co-precipitation method was used for electrochemical sensing of quercetin (QCT). The structure and morphology of the as

Full text of DOI:10.1039/c9nj05660b

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Sr-doped NiO nanorods synthesized by simple sonochemical method
3
as excellent materials for voltammetric determination of quercetin†
Venkatachalam Vinothkumar,a Arumugam Sangili,a Shen-Ming Chen,*a Pitchaimani
Veerakumar* and King-Chuen Lin*ab
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ab
aDepartment of Chemical Engineering and Biotechnology, National Taipei University of
Technology, No. 1, Chung-Hsiao East Road, Section 3, Taipei 10608, Taiwan, ROC
bDepartment of Chemistry, National Taiwan University, No. 1, Roosevelt Road, Section 4, Taipei,
1
0617, Taiwan, ROC
cInstitute of Atomic and Molecular Sciences, Academia Sinica, No. 1, Roosevelt Road, Section 4,
Taipei, 10617, Taiwan, ROC
*Corresponding Authors
E-mail address: smchen78@ms15.hinet.net (S.-M. Chen)
E-mail address: spveerakumar@gmail.com (P. Veerakumar)
E-mail address: kclin@ntu.edu.tw (K.-C. Lin)
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Abstract
The simple and novel surfactant-free synthesis of flower-like strontium doped nickel oxide
nanorods (SNO NRs) via simple co-precipitation method was used for electrochemical sensing of
quercetin (QCT). The structure and morphology of as-synthesized flower-like SNO NRs were
characterized by using various spectroscopic techniques. Then, CV, EIS, and DPV were used to
examine its electrochemical properties. The effective loading concentration, pH, scan rate and
stability of the SNO NRs modified electrode were studied. Under optimized condition, the
electrochemical detection of QCT demonstrated a low detection potential of 0.3 V (vs. Ag/AgCl),
and achieved a higher oxidation peak current compared to other modified electrodes in PB (pH
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.0). The voltammetric current response was found to linearly increase with increasing
concentration range from 0.0168.53 µM, along with a low detection limit of 1.98 nM, and high

2
sensitivity of 2.1055 µA mM cm . The sensor also shows good selectivity and satisfactory
recovery for real sample (apple and grape juice) analysis.
2

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1
. Introduction
In recent years, much interest has been focused on the synthesis of nanostructured materials
with different morphology, size, and porosity, because of their unique optical, electrical and
physicochemical properties.16 Among them, nickel oxide (NiO) is a very promising transition
metal oxide for the characteristics of higher electrochemical activity, non-toxicity, low cost as well
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as easy preparation. Currently, perovskite-type metal oxide nanostructured materials and their
derivatives have been investigated extensively in fundamental and high prospective application
such as gas sensors, solar cells, supercapacitor, and electrochemical sensors,11,12 and
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1
3
photocatalysis. These materials exhibit attractive physical and chemical properties such as high
specific conductivity, large surface area, long-life chemical stability,14,15 and high electrocatalytic
1
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activity. Particularly, great efforts have been devoted to the synthesis and research of nickelates
of the perovskite (RNiO
3
where R = La, Nd, Eu, Dy, Sm, and Sr) for enhanced catalytic
17
performance and various technological application. Various methods have been developed for
the synthesis of RNiO nanostructures with different size and morphology, such as electrospinning
3
method,18 simple hydrothermal techniques,19,20 thermal decomposition method,21 and low-
temperature synthesis.22 However, the above techniques are connected with complicated
procedures, hard for large-scale synthesis, and are costly tools. Commonly, the co-precipitation
method is one of the most effective and easiest methods for preparing perovskite metal oxides
nanostructures with high electrocatalytic activity. To the best of our knowledge, very few studies
on the fabrication of sensor-based perovskite-type strontium doped nickel oxide (SNO) have been
reported to date. Perovskite structure SrNiO
3
have not been reported for electrochemical
determination of quercetin (QCT) so far.
Quercetin (QCT), natural antioxidant pentahydroxyflavone, widely exists in medicinal plants,
leaves, fruits, and vegetables antibacterial and is an essential part of the human diet,2326 showing
anti-allergic characteristics and potential for anti-cancer application.27,28 It also acts as an anti-
mutagen to accelerate cell oxidative damage and also products human colonocyte DNA from
oxidative attack in vitro. However, an overdose of QCT may lead to adverse effects to cause kidney
26
failure and headache. Therefore, the detection of QCT is significant in biochemistry, natural
pharmaceutical chemistry, and clinical medicine. For the determination of QCT, various methods
including high performance liquid chromatography (HPLC),29 ultraviolet (UV) detection,30,31
capillary electrophoresis (CE),32 HPLC-spectrophotometry,33 spectrofluorometry,34 and gas
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chromatography35 have been developed. As alternatives to the accurate detection of QCT,
numerous electrochemical methods have been described.36
Herein, perovskite structure SrNiO
precipitation method (Scheme 1). Then SrNiO
as a novel nanostructure for the electrochemical detection of quercetin. The resultant perovskite
SrNiO NRs and electrochemical sensor were characterized by XRD, BET, FE-SEM, TEM, EIS,
3
NRs were synthesized with surfactant-free co-
3
perovskite NRs were employed for the first time
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CV, and DPV. The perovskite materials have large surface area to enhance the electron transfer
between modified electrode surface and electrolyte solution in QCT. Furthermore, the SNO
material exhibits high catalytic activity and excellent sensing performance of QCT detection. The
proposed QCT sensor displayed low detection limit, high sensitivity, and good selectivity, which
could be successfully applied to detect QCT in apple and grape juice samples.
Scheme 1 Schematic illustration of the synthesis of SNO nanorods and their electrocatalytic
activity towards the determination of QCT.
2. Experimental
2
.1. Material and reagents
Strontium chloride hexahydrate (SrCl
2
. 6H
2
O, 99%), nickel(II) chloride hexahydrate (NiCl .
2
6
H
2
O, ≥98%), liquid ammonia (liq. NH ) solution (25 wt%), quercetin (QCT, ≥95% (HPLC), solid)
3
were purchased from Sigma Aldrich. The supporting electrolytes 0.05 M phosphate buffer saline
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(
PB) was prepared by mixing disodium hydrogen phosphate (Na
2
HPO
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, 99%) and sodium
dihydrogen phosphate (NaH
2
PO
4
, 98%); the pH of the PB was adjusted by adding H
2
SO /NaOH.
4
All chemicals were of analytical reagent grade and were used without further purification.
Ultrapure water (≥ 18.2 MΩ·cm) produced by Millipore system was used for solution preparation.
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2.2. X-ray diffraction (XRD)
The powder XRD studies were attained on PANalytical X’Pert PRO diffractometer using Cu
Kα (λ= 1.5417 Å) radiation in the 2θ range from 10-80º.
2.3. Surface area measurements
The textural properties and porosity of the samples were analyzed from nitrogen (N )
2
º
physisorption isotherms obtained at −196 C on a Quantachrome Autosorb-1 volumetric analyzer.
Surface area was determined by adsorption data in the relative pressure (P/P ) range between 0.48
0
and 0.99 using the Brunauer–Emmett–Teller (BET) method. The total pore volume was based on
the quantity of nitrogen adsorbed at relative pressure of 0.99. The pore size distribution (PSD)
was evaluated through the Quenched Solid Density Functional Theory (QSDFT) method using
nitrogen adsorption data.
2
.4. Field emission-scanning electron microscopy (FE-SEM)
The surface morphology was examined by FE-SEM (JEOL JSM-6500F). The powder sample
was directly put on the surface of carbon tab and platinum was sputtering for 32 sec.
2
.5. Transmission electron microscopy
The TEM (JEOL JEM-2100F) was operated at an acceleration voltage of 200 kV. Scanning
transmission electron microscopy (STEM) was studied to determine elemental mappings, while
the energy-dispersive X-ray analysis (EDX) was used to analyze the elemental compositions of
as-prepared material.
2
.6. Electrochemical analysis
Electrochemical experiments were performed in a three-electrode cell system with embracing
2
a modified glassy carbon electrode (GCE: 0.0729 cm ) as the working electrode, Ag/AgCl act as
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a reference electrode, and platinum (Pt) wire acting as a counter electrode using electrochemical
workstation (CHI900 and CHI1205b, Shangai Chenhua, China). QCT sensor was performed by
cyclic voltammetry (CV) and different pulse voltammetry (DPV) methods.
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.7. Electrochemical impedance spectroscopy
3

Electrochemical impedance spectroscopy (EIS) was examined by 5.0 mM of [Fe(CN) ] and
6
/4
[Fe(CN) ] containing 0.1 M KCl between the frequency range from 100 mHz to 100 kHz. IM6ex
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ZAHNER elektrik (version, Thales 4.01, Z Simp Win EIS DATA analysis software) was used to
analyze the observed EIS data.
2
.8. Synthesis of SrNiO
Perovskite type SrNiO
Typically, powder of SrCl
dissolved in 100 mL of ultrapure water into a 250 mL beaker, respectively, and sonicated for 20
3
nanorods
NRs were prepared by simple surfactant free co-precipitation method.
.6H O (0.663 g, 0.05 M) and NiCl .4H O (1.18 g, 0.1 M) were
3
2
2
2
2
min. Then, 8.0 mL of liquid ammonia (liq. NH ) solution (25 wt%) was added dropwise into the
3
º
above solution under sonication at 80 C for 2 h. The obtained green color precipitate was
centrifuged with 6000 rpm and washed with Millipore water several times. The obtained product
º
º
º
was dried in an oven at 50 C for 12 h. Finally, SrNiO was calcinated at 500 C for 4 h with 4 C
3

1
min . The product was denoted as SNO (see Scheme 1).
∆
ꢀꢁꢂꢃ + ꢄꢅꢂꢃ + ꢆꢁꢇ. ꢀꢈ → ꢄꢅ ꢀꢁꢋ ¯
(1)
ꢉ
ꢊ
ꢊ
2
.9. Fabrication of SNO NRs/GCE
The glassy carbon electrode (GCE) was fabricated with SNO via a simple drop-casting method.
Firstly, GCE was treated with 0.05 µm alumina slurry and washed with Millipore water and ethanol.
Then the GCE was dried in at room temperature. Next, 5.0 mg of SNO NRs were dispersed in 1.0
mL of Millipore water with ultra-sonication for 10 min. Afterward, 6.0 µL of the suspension was
º
drop-casted on the pretreated GCE surface and dried at 50 C to complete fabrication of SNO
modified GCE.
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. Results and discussion
3
.1. Structural characterization
The crystallographic phase and purity of SNO nanorods were characterized by XRD. Fig. 1a,
displayed the XRD pattern of as-prepared SNO NRs, showing the diffraction peaks at 19.52, 25.81,
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º
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9.68, 33.7, 36.22, 44.9, 45.61, 46.58, 49.87, 57.26, 58.86, and 59.88 to be attributed to the plane
(200), (101), (220), (105), (110), (002), (025), (109), (117), (207), (215), (300), and (0013),
respectively, which can be easily indexed to the standard hexagonal phase SNO NRs (JCPDS. 00-
5-0904, 00-033-1347).37,38 The peaks found at 2θ = 25.18, 31.54, and 35.15 correspond to
º
2
strontium oxide (SrO) and diffraction peaks at 2θ of 37.15, 43.29, 62.99, 75.45, and 79.46º
correspond to nickel oxide (NiO) crystalline phase. The peaks were measured with standard XRD
pattern (JCPDS No. 27-1304 and JCPDS No.47-1049).3941
Fig. 1 (a) XRD patterns and (b) N
2
-sorption isotherm of SrNiO NRs (inset: BJH pore size
3
distribution.
The porous structure of SNO NRs was investigated by N adsorption/desorption isotherms.
2
Fig. 1b displayed the obtained results analyzed by BET for surface area and BJH pore size
distribution in inset Fig. 1b. Moreover, the isotherm could be identified as type-IV, demonstrating
the existent of the abundant mesoporous structure of the SNO NRs. The surface area and pore
2
1
diameter (D ) of the as-prepared flower-like SNO NRs is 15.265 m g and 23 nm, which is
p
4
2
higher than that of the reported hydrothermal synthesis of NiO nanosheets and nanowire. The
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pore size distribution of the SNO NRs sample was calculated by QSDFT (Fig. 2b inset). The
extreme peak of pore size distribution was concentrated at around 4.2 nm, verifying that these
samples were primarily composed of microspores and a considerable amount of mesoporous.
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Fig. 2 FE-SEM images of SNO NRs (a-d) at different magnifications, (e) Overlay image of the
flower-like SNO NRs and FE-SEM-EDX mapping analysis of (f) O, (g) Ni, (h) Sr.
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The morphology and size of as-prepared SNO NRs were characterized by FE-SEM and TEM
analysis as shown in Fig. 2 and Fig. 3. As illustrated Fig. 2a-d, the different magnification SEM
images of SNO indicate the unique flower-like morphology. The low magnification of image (Fig.
2
d) reveals that the material was clearly visualized to be the nano-rods morphology. Note that the
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flower-like shapes are assembled by nano-rods with the size less than 100 nm. Fig. 2e-h shows the
mapping analysis of as prepared SNO NRs; these results clearly indicated that the all elements are
evenly spread.
Fig. 3a-e, different magnification of TEM image further shows the interiors of the nanorods.
Fig. 3d,e shows the high-resolution of TEM analysis and the interplane spacing of 0.47 nm
corresponding to (111) plane of SNO. Furthermore, the elemental composition present in SNO
NRs has clearly confirmed through EDAX analysis as shown in Fig. 3g. The EDS analysis
indicates the existences of Sr, Ni, and O elements with the contents of 7.38%, 30.29%, and 62.33%,
respectively, indicating the formation of SrNiO NRs. In addition, the small amount of Sr metal
3
present in the sample indicates the few amounts of NiO compound observed. This outcome is
consistent with the findings of XRD result.
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1
2
3
4
5
6
7
8
9
0
Fig. 3 TEM images of different magnifications (a-e), (f) TEM-SEAD pattern, and (g) FE-SEM-
EDAX analysis of SNO NRs,
3
.2. Electrochemical characterizations
The electrochemical performance of the SNO NRs modified glassy carbon electrode was
investigative by using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry
10

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9
(
CV) measurements in 5.0 mM [Fe(CN)
]3‒/4‒/0.1 M KCl redox probe. EIS can be used as a
6
powerful method to examine the interfacial electron transfer resistance properties (Rct) of
chemically modified electrodes. The EIS of bare GCE, NiO and SNO NRs modified GCE (Fig 4a)
with different loading concentration of SNO NRs (4, 6 and 8 µL), respectively. For bare GCE and
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
NiO NPs, the Rct value was 745 Ω, and 980 Ω. At 4.0, 6.0 and 8.0 µL (5.0 mg mL ) loading
concentration of SNO NRs modified electrode (Fig. S1a), Rct was changed to 395, 276, and 410
Ω, suggesting that the SNO was efficiently coated on the surface of the bare electrode. The smallest
R
ct value was observed when Sr was doped NiO NRs (276 Ω). There is no doubt that Sr could
1
improve the conductivity of the NiO NPs and 6.0 µL (5.0 mg mL ) of SNO NRs coated on the
surface of GCE was observed at low Rct value, indicating the highest electron transfer efficiency.
Fig. 4b and Fig. S1b show the CVs of NiO NPs and different loading concentration of SNO
NRs (4, 6 and 8 µL (5.0 mg/mL)) modified GCE using the redox probe in 5.0 mM of [Fe(CN)
6
]3‒
0.1 M KCl at a scan rate of 50 mV s . All modified electrodes exhibited the redox features of
5.0 mM of [Fe(CN)
]3‒/4‒, suggesting their good electrocatalytic activity. The 6.0 µL loading
SNO/GCE exhibits higher peak to peak potential separation (ΔE ) value of about 115 mV, which
shows a quasi-reversible process. In contrast, a pair of redox peaks potential were observed for the
bare GCE, NiO NPs and 4.0, 8.0 µL of SNO NRs modified electrodes with a slightly higher ΔE
value about 169, 150, 142, and 145 mV. In addition, the oxidation peaks current of 6.0 µL (5.0 mg
/4‒/
‒1
6
p
p

1
2
mL ) loading concentration of SNO NRs/GCE was 88 µA cm , about 1.5, 1.0 and 3.0 times
higher than those of 4.0, 8.0 µL of SNO NRs and NiO modified electrodes, respectively. The
higher oxidation peaks current and ΔE
the electron transfer process is significantly facilitated at the electrode. The corresponding diagram
of ΔE and Rct values versus different modified electrodes is given in Fig. 4f.
To understand the electrochemical active surface area, the CV curve response of SNO
p
value of 6.0 µL loading of SNO NRs/GCE indicates that
p
NRs/GCE at different scan rate ranges from 20200 mV s were recorded in 5.0 mM [Fe(CN)
]3‒

1
6
/4‒/0.1 M KCl solution (Fig 4c). Obviously, the increasing scan rate increased the redox peak
current response gradually. The modified electrode surface area (A) was determined from the
1/2
linear plot of redox peak current versus square root of scan rate ( ) (see Fig. 4d). In addition, the
electroactive surface area of the modified electrodes was calculated by using the Randles-Sevcik
equation as follows (Eq. (2)).43
11

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2
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ꢐ
ꢂ
ꢉ
ꢂ
ꢐ
ꢂ
ꢍ
ꢁ = 2.69 × 10 ꢎ × ꢏ
ꢑ
ꢒꢓ
(2)
ꢌ
2
‒1
where i is the peak current (A), D is the diffusion coefficient (cm s ), n is the number of electrons
p

‒3
transfer (1 e ), ν is the scan rate, C is the concentration (mol cm ), and constant parameters C, D,
n. The electroactive surface area for SNO NRs/GCE was calculated from the slop value (Fig. 4d)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
to be 0.53 cm , which is higher than the geometric area of bare GCE and NiO NPs which are 0.072
and 0.32, respectively. It is clearly displayed in Fig. 4e. The result reveals that SNO NRs modified
GCE could be a greater electroactive material for the electrochemical sensor and was used for
further electrochemical studies. Consequently, given the obtained Rct, the charge transfer rate (k )
s
of bare GCE, NiO NPs and SNO NRs modified GCE were determined using the eq. (3),44
ꢔꢗ
ꢔ =
ꢕꢖ
(3)
ꢂ
ꢂ
ꢑ ꢘ ꢙ ꢒ
ꢚ
‒
1
‒1
where, k
s
= charge transfer rate, R = gas constant (8.314 J mol K ), T = temperature (25 ˚C), F
‒1
=
Faraday constant (96,485 C mol ), n = number of electron, and C = concentration (5.0 mM of
[
Fe(CN)
NRs/GCE yields 9.24 × 10 , 7.34 × 10 , 1.34 × 10 , 1.92 × 10 and 1.29 × 10 cm s ,
respectively. The observed results displayed a higher A, Ipa, small ΔE , and k represent the high
6
]3‒/4‒). The k
s
value for bare GCE, NiO NPs/GCE and 4, 6, and 8 µL loading SNO
‒
8
‒8
‒7
‒7
‒7
‒1
p
s
catalytic activity which renders the SNO NRs the best modifier amongst others. Hence, the 6.0 µL
loaded SNO NRs modified electrode is most suitable for electrocatalytic application.
12

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1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 4 (a) Nyquist plot, (b) CV curves of bare, NiO, SNO NRs modified GCE recorded in 5.0 mM
Fe(CN) vs different modified
]3‒/4‒/0.1 M KCl, (c) corresponding calibration plot for Rct and ΔE
[
6
p
‒1
electrodes, (d) CV curve of SNO NRs/GCE at different scan rate from 20 to 200 mV s , (e)
13

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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
calibration plot of oxidation and reduction current peaks vs square root of scan rate, (f) bar diagram
between active surface area (cm2).
3
.3. Electrochemical determination of QCT
The electrocatalytic activity of SNO was studied to quantify QCT by CV in PB (0.05 M,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0

1
pH 5.0) solution at 50 mV s . Fig. 5a shows the CV peak current response for the absence and
presence 50 µM of QCT buffered at pH 5.0 on the surface of bare GCE, NiO NPs, and SNO NRs
1
modified GCE at a scan rate of 50 mV s . At bare GCE, there is no redox peak was obtained in
absence of QCT and in the presence of QCT, the reversible reaction with an oxidation potential at
0
.33 V and the oxidation peak current at 0.946 µA. It can be seen that besides a pair of reversible
obtained at SNO NRs/GCE at 0.33 V and oxidation peak current at 3.07 µA. further the displayed
reduction peak at 0.039 V, the redox peak has been known as two-proton and two-electron
process of the catechol moiety (i.e. 3,4-dihydroxyl electron-donating groups) of QCR proceeded
to the quercetin-O-quinone (QCT-O-Q) product. The form of whose is referred to as the other
reduction group but are not of interest in this research. Obviously, from the obtained results (Fig.
5
a), SNO NRs-modified GCE demonstrates superior oxidation peak current response compared to
bare GCE and NiO NPs/GCE (Epa = 0.34 V, Ipa = 2.58 µA).
For examining the effect of the loading amount of SNO NRs on the GCE was studied by

1
varying dosage from 4.0, 6.0, 8.0 and 10.0 mg mL in DI water (Fig. 5b). The result displayed
1
that the oxidation peak current response (25 µM QCT) reached the maximum at 6.0 mg mL and
1
then decreased with increasing the amount 8.0 to 10.0 mg mL . The resulting bar diagram of
oxidation peak current versus various dosage loaded SNO NRs modified electrodes is shown in

1
Fig. 5b (inset). These results indicated that the current response at 6.0 mg mL provides the highest
performance-sensing condition for QCT.
3.4. Effect of pH
The electrochemical detection of QCT and the reaction in relation to faradic processes were
significantly influenced by pH. We monitor the pH influence (from 3.0  9.0) on the
electrochemical determination of 50 µM QCT at the SNO NRs/GCE in 0.05 M PB in Fig. 5c. The
oxidation peak potentials and peak currents of QCT were obtained at SNO NRs/GCE in various

1
pH solution at 50 mV s . Fig. 5d illustrates the variation of oxidation peak currents and peak
potentials with different pH. At pH 3, the cathodic peak was more prominent due to easy
14

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9
+
+
availability of H ions to cause faster kinetics. The H
3
O concentration significantly affects the
electro-oxidative mechanism. The oxidation potential of QCT kept increasing up to pH 5.0 and
then decreased slowly with increase of pH. This is probably because of the difficulty of the
modified electrode to repel the electro-active species towards itself at higher pHs. Maximum
current was observed at pH 5.0. However, at pH 9, the overpotential decreases further along with
the decrease of the anodic current. We chose physiological pH (5.0) for our further experiments
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
on account of better voltammograms and higher peak currents In addition, the electrochemical
.
oxidation peaks potential shifts negatively with the increased pH, indicating that proton is involved
in the electrode reactions. The linear equation can be expressed as Epa (V) = ‒0.0752 pH + 0.7037
(
R2 = 0.999, Fig. 5d).
(ꢋꢟ)ꢜ
(ꢔ)ꢠ
0.0591
ꢑ
0.0591ꢣ
ꢑ
ꢝ
ꢛꢌꢜ = ꢛ +
. log ꢞ
ꢡ − ꢢ
ꢤ ꢥꢈ
(4)
26
Eq. (4) indicates the Nernst equation, describes the relationship between pH and Epa. m and n
denote the number of proton and electrons involved in the reaction, a and b is the coefficients of
oxidant and reductant in the reaction. However, the slope value differs from the theoretical value
of 0.0591 V. This result suggests equal number of proton and electrons involved in the
electrochemical oxidation process. A probable electrochemical redox reaction mechanism for QCT
determination was demonstrated in Fig. 5e.23,26,45
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8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 5 (a) CV profiles of the bare GCE, NiO NPs, and SNO NRs-modified GCE in the absence (a-
c) and presence (d-f) of 50 µM QCT, (b) optimization of different loading SNO on surface of GCE,
(c) SNO NRs/GCE at different electrolyte pH, (d) the corresponding oxidation peak current
responses for QCT at various pH. All measurements performed under N -saturated in PB (pH 5.0,
2
0.05 M), and (f) plausible electrochemical oxidation mechanism of QCT at SNO NRs-modified
GCE.
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3
.5. Effect of concentration
As shown in Fig. 6a, the electrochemical response of SNO NRs/GCE by CV was acquired in
the presence of a different concentration of QCT from 0  125 µM in PB (0.05 M, pH 5.0) at 50
‒1
mV s . The oxidation peaks current response of QCT was found to increase linearly with
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
increasing concentration of QCT from 0  125 µM. Fig. 6b shows the linear plot between
concentration of QCT versus oxidation peak current, yielding the following linear equation of Ipa
2
[
µA] = 0.0515[µM] + 0.332, (R = 0.9617). The result suggests the excellent electrochemical
activity of SNO NRs/GCE towards the determination of QCT.
3
.6. Effect of scan rate
Fig. 6b displayed the CVs of the SNO NRs/GCE in PB (pH 5.0, 0.05 M) in the presence of 25
–
1
µM at different scan rates from 20180 mV s . The redox peak currents increased with the
increase of scan rates yielding a small positive shift in the oxidation peak current. The oxidation
and redox peak currents were linearly related to the square root of the scan rates as shown in Fig.
–
1
2
6c. The linear equation is found to be Ipa (µA) = 0.3776 (mV s ) – 0.8204 (R = 0.998) and Ipc
–
1
2
(µA) = –0.3437 (mV s ) + 1.2343 (R = 0.9948), indicating that the electrocatalytic reaction
kinetics should be controlled by a diffusion-controlled process. In order to obtain further
information on the rate-determining step, the Laviron’s equation (7) was used to estimate electron
transfer rate constant ks. Logarithmic peak currents were correlated with the logarithmic of scan
rate resulting in a straight line as in Fig.6d. These facts reveal that the electrochemical oxidation
of QCT was controlled by diffusion process, and the k
s
value was calculated from the slope of
Fig.6d.4648
ꢔꢗ
ꢧ
ꢝ
ꢛꢌꢦ = ꢛ +
ln ꢓ
(5)
(6)
(1 − ꢨ)ꢑꢘ
ꢔꢗ
ꢝꢩ
ꢛꢌꢜ = ꢛ − ꢢ
ꢤ lnν
ꢨnF
ꢔꢗ
ꢪꢫꢬꢙ = ꢨ log(1 − ꢨ) + (1 − ꢨ)logꢨ − log ꢢ
ꢤ
ꢭ
ꢑꢘꢓ
ꢨ(1 − ꢨ )ꢑꢘꢮꢛ
−
(7)
2
.3ꢔꢗ
17

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2
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4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
where, k
s
, E, and  are electron transfer rate constant, peak to peak separation and charge transfer
coefficient (assumed to be 0.5), respectively. According to Eq. (7) k
s
value was calculated to be
‒1
1.88 s . These results show that the SNO NRs modified GCE could greatly promote the k to
s
facilitate the electro-chemical oxidation of QCT.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 6. (a) CV profile of different concentration of QCT (10–100 µM) (inset: corresponding linear
plot between concentration of QCT and oxidation peak current), (b) CV profile SNO NRs-
modified GCE with N -saturated PBS (pH = 5.0) containing 25 µM of QCT recorded at different
2
−
1
scan rate at 20–180 mV s (-0.1 to 0.7 V) (vs Ag/AgCl)), (c) corresponding linear plot between
oxidation peak current and square root of scan rate, and (d) corresponding linear plot between log
oxidation peak current and oxidation peak potential.
18

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3
.7. Different pulse voltammetry
The electrochemical determination of QCT using SNO NRs/GCE was studied by the DPV
technique to examine the analytical parameter such as limit of detection, sensitivity, and linear
range. Fig. 7a shows the DPV curves of the SNO NRs/GCE at different concentration of QCT.
The corresponding oxidation peak currents increased with increasing concentration of QCT from
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
.01 to 68.5 µM in 0.05 M PB (pH 5.0). A well sharp oxidation peak was observed at +0.31 V.
Fig. 7b shows the relationship between the concentration of QCT and oxidation peak currents,
suggesting that the peak currents increase linearly with the QCT concentration following the linear
regression equation: Ipa (µA) = 0.1516 [µM] – 1.2627 (R² = 0.9826). The lower limit of detection
–
2
(
LOD) was calculated to be 1.98 nM with the sensitivity 2.1055 µA cm using the equation LOD
3σ/S, where S is the slope value of calibration curve and σ is the standard deviation (RSD). The
=
SNO NRs/GCE prepared in the study exhibited a greater analytical parameter such as lower limit
of detection, higher sensitivity and a wide linear range for the detection of QCT. The SNO-
NRs/GCE showed superior analytical performance for QCT determination in comparison with
some already surveyed electrochemical sensors26,36,45,49–60, as shown in Table 1. It is also
noteworthy that the LOD of QCT observed over the SNO NRs/GCE is much lower than those
4
9
50
i
51
55
other modified electrodes, such as Pt-PDA@SiO
2
, Co
3
O
4
, MIP /GO, Fe
3
O @ZnO/CP, Mn-
4
5
7
58
doped ZnS QDs, and poly(AMT), as summarized in Table 1. It is well-known that the transition
metals doped metal oxides have good catalytic activity towards various electrocatalysis/sensing
applications. However, Ni or NiO has advantages over most transition metals such as high stability
5
0
in neutral pH, lower capacitance current and high chemical resistance. For instance, Co is less
stable in physiological pH while Fe
is less corrosive resistance.55
3
O
4
In addition, Sr is recognized as potentially effective materials that are able to enhance the
specific capacity electrode, and is frequently taken into account mainly because of economic
4
5
52
consideration and its large abundance. It is actually less expensive, as compared with Ag, Pd,
and Pt49,59. However, Sr-doped NiO exhibits unique optical, electronic, and catalytic properties
that have drawn considerable attention in the scientific research community as well as in
prospective industrial applications.1722 In the present work, we have synthesized Sr-doped NiO
NRs via surfactant free co-precipitation method and has been used for QCT determination.
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 1. Comparison of QCT determination over various modified electrodes
Linear range Sensitivity
LOD
Electrode
Reference

1
(µM)
(µA µM )
(µM)
a
b
CTAB /Fe-MWCNTs /CPE
0.06–3000
0.0005–7
0.01–120
0.050.383
0.5330

0.0012
0.0064
0.006
0.016
0.100
0.048
0.008
0.012
0.06
26
36
45
49
50
51
52
53
54
55
56
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
c
d
CD /AuNPs /MWCNT/GCE

e
f
AgNPs @g-CN /GCE
0.1871
6.47
g
Pt-PDA @SiO
2
/GCE
Co
3
O
4
h/GCE

i
MIP /GO/GCE
0.6–15

j
Pd-MoS
2
-IL -OMC/GCE
0.02–10

Activated silica gel/CPEk
0.0165–0.3309
0.1–10.5
0.79–61

MIP/MIL-101(Cr)/MoS
2
/GCE
0.285
0.04
0.003
l
Fe
3
O
4
@ZnO/CP /GCE
0.16
MnWO
4
m/GCE
16.7–74.4

Mn-doped ZnS QDsn
0.33–20
0.3316.54
0.0491
0.050
0.16
57
58
59
60

Poly(AMT)o
2.2

p
PtNPs/PEDOT -MeOH/GCE

0.0052
0.002
g-C
3
N
4
/NiO/GCE

SNO NRs/GCE
0.0168.5
2.1055
0.00198
This work
a
b
c
d
Cetyltrimethylammonium bromide. Multiwalled carbon nanotubes. Cyclodextrin. Gold
e
f
g
h
nanoparticles. Silver nanoparticles. Graphitic carbon nitride. Platinum-polydopamine. Cobalt
i
j
k
l
oxide. Molecularly-imprinted polymer. Ionic liquid. Carbon paste electrode. Carbon paste.
mManganese tungstate. nQuantum dots. oPoly(5-amino-2-mercapto-1,3,4-thiadiazole).
pPoly(3,4-ethylenedioxythiophene) polystyrene sulfonate.
20

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5
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 7 (a) DPV profiles for various concentrations of QCT oxidation at SNO NRs/GCE in the range
of 0.01–68.53 µM, (b) The corresponding linear calibration curve of peak current versus QCT

1
concentration with a scan rate 50 mV s , (c) interference study of the simultaneous detection of
QCT and RU, (d) Interference study with other interferents such as RU, AA, CAF, CA, L-Cys,
2
+
+
2+
–
, Cl , and SO
2–. All measurements
–
Gly, Glu, and common ions such as Mg , Na , Ca , NO
3
4
were recorded in 0.05 M PB (pH 5.0).
3
.8. Selectivity, stability, and reproducibility
The selectivity of the fabricated SNO NRs/GCE sensor towards QCT was investigated. QCT
and rutin (RU) is well known to have electrochemical detection response with a close potential
difference. In Fig. 7c shows the CV response of selective detection of 4.7 µM QCT with RU,
21

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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
whereas the signal of QCT is no altered with addition of 50 µM RU. The observed result confirms
that RU has less than 3% interference-effect. Furthermore, the other common organic compound
such as ascorbic acid (AA), caffeine (CAF), citric acid (CA), L-cysteine (L-Cys), glycine (Gly),
2
+
+
2+
–
–
2–
glucose (Glu) and common ions such as Mg , Na , Ca , NO
3
, Cl , and SO
4
are examined
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
towards the detection of QCT. In Fig. 7d shows the interference relative error (%) current of other
organic compounds and common ions with QCT, wherein, the interference signal was observed
only less than 3%. This value is acceptable for practical applications. The stability of fabricated
sensor was explored to the stability test for 150 cycles in 0.05 M PBS (pH 7.0) in the presence of
2
5 µM QCT (Fig. 8a). The CV studies in the absence of QCT of SNO NRs/GCE appeared no
change in the current response after 150 cycles. The CV studies of QCT sensor at SNO NRs/GCE
exhibited the highly stable response for 150 cycles with the current retention of 93% from initial
value. The storage stability of modified electrode was investigated by CV analysis in 12 days. The
observed results only less than 10% indicated good storage stability of modified electrode (Fig.
8
b). The reproducibility of fabricated sensor was studied by detection of the oxidation peak current
response of 50 µM QCT at six different modified electrodes (Fig. 8c). the RSD value of 2.82%
was obtained for fabricated sensor. Furthermore, the repeatability of the proposed sensor was
examined in ten successive DPV measurements of 50 µM QCT with RSD value of 3.7%,
demonstrating good repeatability of the fabricated sensor as shown in Fig. 8d. The SNO NRs
modified GCE showed excellent stability, resistance against interference, reproducibility, and
repeatability test towards the QCT detection.
22

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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 8. (a) Stability of 150 CV cycles curves of SNO NRs-modified GCE in the presence of 25 µM
−
1
QCT recorded at 50 mV s . CV response of (b) storage stability of modified electrode for electro-
oxidation of QCT response after being kept for 12 days, (c) reproducibility of modified electrodes,
and (d) current with variation of repeated time. All measurements recorded in N -saturated PB (pH
2
‒
1
5.0, 0.05 M) with 50 µM QCT at 50 mV s .
.9. Application
The SNO NRs modified electrode was used for the detection of QCT in apple and grape juice
3
using the standard addition method (Fig. 9). Apple and grape juice samples (denoted as A. Juice
and G. Juice) were purchased from local market in Taiwan. The real samples were diluted by 1:10
ratio to prepare with pH 5.0 (0.05 M PB), and then 500 µM QCT (10 mM) standard solution was
added. The spiked solution was added into the electrolyte solution and studied. From Table 2, the
23

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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
recovery values of apple and grape real samples range from 99–101.7% and 97.3–98.72%
indicated that the SNO NRs modified GCE can be successfully applied for the determination of
QCT in real-time application.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 9 DPV profiles of SNO NRs modified electrode in real-time applications (a) A. Juice, (b) G.
Juice samples with and without spiked different concentration of QCT in PB (0.05 M, pH 5.0)
Table 2. Determination of QCT in real samples (n = 3)
Sample
Spiked (µM)
A.Juice-10
A.Juice-30
A.Juice-50
Found (µM) RSD (%)
Recovery (%)
Apple juice
NDa
ND




ND


1
3
4
.5
.0
.5
1.48
2.92
4.41
5.48
6.46
3.4
2.3
1.2
1.4
1.9
98.66
97.33
98.0
99.6
99.38
5.5
.5
6
Grape juice
G.Juice-10
G.Juice-30
G.Juice-50
1.0
ND
ND
ND
0.93






3.3
93.0
24

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9
2
.0
1.96
3.89
6.48
7.97
2.9
98.0
97.2
99.6
99.6
4.0
1.9
6
8
.5
.0
2.84
3.1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
aNot detected.
4. Conclusion
Herein, we have successfully synthesized the flower-like SrNiO
3
nanorods through an
ultrasound assisted-calcination process from a metal precursor. The SNO NRs modified GCE was
characterized by XRD, SEM, TEM with EDX, CV, and EIS studies. As-prepared strontium doped
NiO modified electrode enhanced electrochemical conductivity compared with the pristine NiO
3
NPs. The SNO NRs modified GCE has excellent electrochemical oxidation of quercetin sensing.
Hence this work demonstrated a good linear range from 0.01–68.5 µM, low limit of detection of
−
1
−2
1
.98 nM, with good sensitivity of 2.1055 µA µM cm . In addition, the SNO NRs based senor
owns many merits including rapid response, excellent reproducibility, stability, and selectivity for
the detection of QCT, and satisfactory recoveries for sensing various real samples.
†Electronic supplementary information (ESI) available: Electrochemical test details of
Nyquist plots and CV studies.
Conflics of interest
There are no conflicts of interest to declare
Acknowledgments
The authors are grateful for the financial support (MOST 106-2113-M-027-003-MY3 to S.M.C;
MOST-102-2113-M-002-009-MY3 to KCL) from the Ministry of Science and Technology
(MOST), Taiwan.
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3
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New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ05660B
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Table of Content
Sr-doped NiO nanorods synthesized by simple sonochemical method
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as excellent materials for voltammetric determination of quercetin†
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Venkatachalam Vinothkumar,a Arumugam Sangili,a Shen-Ming Chen,*a Pitchaimani
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ab
*ab
Veerakumar and King-Chuen Lin
The fabricated Sr-doped NiO nanorods-modified GCE was developed for the electrochemical
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sensing towards quercetin.
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