Novel porous iron phthalocyanine based metal-organic framework electrochemical sensor for sensitive vanillin detection

Article abstract of DOI:10.1039/d0ra06783k

Vanillin is widely used as a flavor enhancer and is known to have numerous other interesting properties, including antidepressant, anticancer, anti-inflammatory, and antioxidant effects. However, as excess vanillin consumption can affect liver and kidney

Full text of DOI:10.1039/d0ra06783k

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PAPER
Novel porous iron phthalocyanine based metal–
organic framework electrochemical sensor for
sensitive vanillin detection†
Cite this: RSC Adv., 2020, 10, 36828
a
Liying Wei,ab Yuxia Liu,c Wenfeng Zhuge,a Qing Huang,a
*
Jinyun Peng,
Wei Huang,a Gang Xianga and Cuizhong Zhanga
Vanillin is widely used as a flavor enhancer and is known to have numerous other interesting properties,
including antidepressant, anticancer, anti-inflammatory, and antioxidant effects. However, as excess
vanillin consumption can affect liver and kidney function, simple and rapid detection methods for vanillin
are required. Herein, a novel electrochemical sensor for the sensitive determination of vanillin was
fabricated using an iron phthalocyanine (FePc)-based metal–organic framework (MOF). Scanning
electron microscopy and transmission electron microscopy showed that the FePc MOF has a hollow
porous structure and a large surface area, which impart this material with high adsorption performance.
A glassy carbon electrode modified with the FePc MOF exhibited good electrocatalytic performance for
the detection of vanillin. In particular, this vanillin sensor had a wide linear range of 0.22–29.14 mM with
a low detection limit of 0.05 mM (S/N ¼ 3). Moreover, the proposed sensor was successfully applied to
the determination of vanillin in real samples such as vanillin tablets and human serum.
Received 6th August 2020
Accepted 22nd September 2020
DOI: 10.1039/d0ra06783k
rsc.li/rsc-advances
accuracy of these methods, they are time-consuming and costly,
and most require complicated sample pretreatment before
1. Introduction
analysis. In recent years, electrochemical (EC) methods have
attracted extensive attention because of their high sensitivity,
high selectivity, low cost, fast response, and operational
simplicity. As vanillin has electrochemically active groups, it
can be detected electrochemically. However, there are few
studies on the electrochemical detection of vanillin, mainly
because the overpotential for vanillin oxidation on a bare elec-
trode is too high and the easy adsorption of the oxidation
products on the electrode surface causes electrode poisoning.13
These issues could be overcome via electrode modication.
Therefore, it is important to develop new chemically modied
electrodes for the sensitive detection of vanillin.
Metal–organic frameworks (MOFs) are porous crystalline
materials with periodic network structures formed by metal
ions/clusters and organic ligands connected through coordi-
nation bonds.14–16 MOFs, which have the advantages of large
specic surface areas, high porosities, and adjustable pore
sizes, have been widely studied in elds such as energy storage,
gas storage and separation, catalysis, and sensors.16–20 The
advantageous characteristics of MOFs combined with the
simplicity, low cost, and high sensitivity of electrochemical
sensors has made the development of MOF-based electro-
chemical sensors of particular recent interest.21–24
Vanillin (4-hydroxy-3-methoxybenzaldehyde), the major
component of Vanilla planifolia, a large orchid plant, is mainly
derived from vanilla beans or pods.1 Due to the unique
aromatic properties, vanillin is one of the most commonly
produced spices globally, and it is widely used as a avor
enhancer for candy, ice cream, wine, and other food products.2
In addition, vanillin has been found to have antidepressant,
anticancer, anti-inammatory, and antioxidant effects, and
shows potential as a food preservative.3–6 However, excessive
vanillin intake can affect liver and kidney function, causing
symptoms such as headaches, nausea, and vomiting.7 There-
fore, it is necessary to develop simple and rapid detection
methods for vanillin.
Various analytical methods have been developed for the
determination of vanillin, including gas chromatography–mass
spectrometry, high-performance liquid chromatography, thin-
layer chromatography, capillary electrophoresis, and UV-Vis
spectrometry.8–12 Despite the good selectivity and high
aCollege of Chemistry and Chemical Engineering, Guangxi Normal University for
Nationalities, Chongzuo 532200, China. E-mail: pengjinyun@yeah.net; Fax: +86 771
7870799; Tel: +86 771 7870653
bSchool of Pharmacy, Henan University of Traditional Chinese Medicine, Zhengzhou
450046, China
Phthalocyanine, one of the most investigated functional
organic materials, has a two-dimensional conjugated macrocy-
clic structure with 18p electrons that is composed of four iso-
indole rings connected by nitrogen atoms.25–27 The
cCollege of Physics and Electronic Engineering, Guangxi Normal University for
Nationalities, Chongzuo 532200, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06783k
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phthalocyanine structure is similar to the porphyrin structure,
but phthalocyanine is easier to synthesize. In addition, the
application range of phthalocyanine exceeds that of porphyrin
owing to the enhanced stability, spectral characteristics, coor-
dination characteristics, and structural exibility of phthalocy-
anine.28 Phthalocyanine can coordinate different metals to form
metal phthalocyanine (MPc) complexes, which have unique
electronic and optical properties, including good electron-
transfer performance and redox activity.28–31 In addition,
phthalocyanine derivatives, in which hydrogen on the phtha-
locyanine benzene rings is substituted with functional groups
such as amino carboxyl, or nitro groups, can exhibit increased
reactivity for MPc formation and expand the application
eld.32–34 Owing to their unique structures and various func-
tional properties, including excellent semiconductor properties
and catalytic activities, MPc derivatives have been widely used
in various elds including catalysis, sensors, and photodynamic
therapy.35–42 Due to its redox activity, catalytic activity, and
carrier transport ability result from the extended p-system, MPc
complexes have been used as the linking units in 2D MOFs,
which show good stability, conductivity, redox activity, and
electrocatalytic activity.43–45
Here, we developed a novel electrochemical sensor based on
an iron phthalocyanine (FePc) MOF for the sensitive detection
of vanillin. The electrochemical behavior of vanillin on a glassy
carbon electrode (GCE) modied with the FePc MOF was
systematically investigated, and the developed sensor was
applied to the detection of vanillin in real samples such as
vanillin tablets and human serum. To the best of our knowl-
edge, this is the rst example of an electrochemical sensor
based on a phthalocyanine derived MOF.
2.2 Synthesis of materials
2.2.1 Synthesis of tetracarboxylic iron phthalocyanine
(TCFePc). TCFePc was synthesized using a previously re-
ported method with some improvements.30 Typically, tri-
mellitic anhydride (33 g), urea (60 g), anhydrous FeCl3 (7.80
g), and ammonium molybdenate (0.50 g) were ground and
ꢀ
mixed thoroughlꢀy. The mixture was reacted at 140 C for 1 h
and then at 190 C for 4 h. Subsequently, boiling water was
added to the product. Aer ltration, the precipitate was
washed with boiling water and methanol, and then dried
ꢀ
under vacuum at 50 C for 24 h to give tetraformamido iron
phthalocyanine. This product was mixed with 1 M hydro-
chloric acid in a saturated sodium chloride solution (500 mL)
and boiled for 3 min. Aer cooling, suction ltration was
performed and the lter cake was transferred to a round-
bottom ask. Aer adding 2 M sodium hydroxide in a satu-
rated sodium chloride solution (500 mL), the mixture was
reuxed at 90 ꢀC for 12 h. The product was diluted 5 times
water and subjected to suction ltration. The pH of the
ltrate was adjusted to <3 by adding concentrated hydro-
chloric acid. Aer standing overnight, the precipitate was
collected by suction ltration, washed with boiling water,
methanol, and ether, and nally dried under vacuum to
obtain TCFePc.
2.2.2 Synthesis of [Fe3O(OOCCH3)6OH]$2H2O. [Fe3-
O(OOCCH3)6OH]$2H2O was synthesized by dissolving
Fe(NO3)3$9H2O (8 g) and Na(OOCCH3)$3H2O (11 g) in 9 mL of
deionized water and allowing the obtained solution to stir
overnight at room temperature. A red precipitate was obtained
by ltration, washed once with cold deionized water, and dried
in an oven at 100 ꢀC. Finally, the crystalline product was ob-
tained by recrystallization from DMF.
2. Experimental
2.1 Reagents and apparatus
2.2.3 Synthesis of FePc MOF. The FePc MOF material was
synthesized using
a
solvothermal method.46 [Fe3-
O(OOCCH3)6OH]$2H2O (80 mg) and TCFePc (80 mg) were
ultrasonically dissolved in 16 mL of DMF, and then 2.4 mL of
TFA was added. The resulting solution was placed in a 50 mL
Teon-lined stainless steel reaction kettle and heated at 150 ꢀC
for 12 h. Aer cooling to room temperature, the reaction
mixture was centrifuged at 7000 rpm for 5 min, washed 3 times
with acetone, and then dried under vacuum to obtain the FePc
MOF.
Trimellitic anhydride, urea, anhydrous FeCl3, ammonium
molybdate, triuoroacetic acid (TFA), iron(III) nitrate non-
ahydrate (Fe(NO3)3$9H2O), sodium acetate trihydrate
(NaOOCCH3$3H2O), and N,N-dimethylformamide (DMF)
were purchased from Aladdin Reagent Co., Ltd. (Shanghai,
China). Britton–Robinson (B–R) buffer solutions (0.1 M) with
different pH values were prepared by mixing stock solutions
of 0.04 M H3PO4, H3BO3, CH3COOH, and 0.2 M NaOH. All
reagents were of analytical grade and used directly without
further purication. Double distilled water was used in all
experiments.
All electrochemical measurements were performed on a CHI
760E electrochemical workstation (Shanghai Chenhua Instru-
ment Co., Ltd., Shanghai, China) using a GCE working elec-
trode, Pt counter electrode, and Ag/AgCl reference electrode.
The FePc MOF material was characterized using scanning
electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan; 15 kV),
transmission electron microscopy (TEM, JEM 2100, JEOL,
Japan; 200 kV), Fourier transform infrared (FT-IR) spectroscopy
(Spectrum 65, PerkinElmer, Waltham, USA), X-ray diffraction
(XRD, D8 advance, Bruker, Germany) and X-ray photoelectron
spectroscopy (XPS, K-Alpha, Thermo Scientic, USA).
Scheme 1 Fabrication of FePc MOF/GCE modified electrode and
electrochemical determination of vanillin.
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FePc (Fig. 1D, red), the peak at 1698 cmꢁ1 corresponds to the
telescopic vibration absorption peak of C]O, whereas the
peaks at 1333, 1147, 1086, 857, and 739 cmꢁ1 are assigned to the
characteristic absorption peaks of the phthalocyanine macror-
ing. Similar peaks are observed for the FePc MOF (Fig. 1D,
black) but the intensities are reduced.
XPS was used to analyze the chemical composition of FePc
MOF, and the results showed the presence of C, N, O, Fe, and F
elements in the sample (Fig. S1A†). In the Fe 2p spectrum
(Fig. S1B†), two sets of peaks at 710.9 and 724.8 eV, assignable
to Fe 2p3/2 and Fe 2p1/2, respectively, revealing the oxidation
state of +3 for Fe in the FePc MOF.47 Moreover, the two satellite
peaks at 718.8 and 733.1 eV, which further proves the existence
of Fe3+. Fig. S1D† shows The XRD pattern of FePc MOF (black
curve). The diffraction peaks are consistent with the simulation
results (red curve).
2.3 Preparation of modied electrode
Prior to modication, the GCE was polished with 0.3 and 0.05
mm alumina slurries to remove adsorbed organic substances
and then washed by sonication in ethanol and double distilled
water. The FePc MOF (1.5 mg) was dispersed in 1 mL of DMF
under sonication for 40 min. Next, 3 mL of 1.5 mg mLꢁ1 FePc
MOF suspension was dropped onto the surface of GCE and
dried under infrared light to prepare FePc MOF/GCE. Scheme 1
illustrates the Fabrication of FePc MOF/GCE modied electrode
and electrochemical determination of vanillin.
2.4 Preparation of samples
Vanillin tablets were obtained from China Pharmaceutical
University Pharmaceutical Co., Ltd. (0.2 g ꢂ 36 tablets). Twenty
tablets were ground into a ne powder. Then, a precisely
weighed amount of powder (ꢃ0.2 g vanillin) was dissolved in
ethanol. Aer ltering the solution, 5 mL of the ltrate was
diluted 50 times with ethanol. The human serum samples,
which were collected from Chongzuo People's Hospital
(Guangxi, China), were mixed with anhydrous ethanol at 1 : 3
(v/v) and centrifuged at 10 000 rpm for 10 min.
3.2 Electrochemical behavior of FePc MOF
3.2.1 Electrochemical impedance spectroscopy (EIS). EIS
can be used to study the resistance of different electrodes.
Nyquist plots in the frequency range from 0.1 to 105 Hz were
obtained for a bare GCE and the FePc MOF/GCE in a 1.0 mM
[Fe(CN)6]3ꢁ/4ꢁ solution (Fig. 2A). The Nyquist plots consist of
two main regions, namely, a linear region at lower frequencies
and a semicircular region at higher frequencies, which repre-
3. Results and discussion
3.1 Characterization of FePc-based MOF
The surface morphology of the FePc MOF was characterized sent the diffusion process and the charge-transfer resistance
using SEM and TEM (Fig. 1). The SEM image (Fig. 1A) indicates (Rct), respectively.48 As shown in Fig. 2A, in the frequency range
that the FePc MOF material has a hollow porous structure, of 1 to 105 Hz, the Rct of the bare GCE is ꢃ950 ohm. The
which was further conrmed by the hollow structure (Fig. 1B) diameter of the semicircle in the Nyquist plot is signicantly
and pore structure (Fig. 1C) observed in the TEM images. larger for the bare GCE than for the modied electrode, which
Fig. 1D shows the FT-IR spectra of FePc and the FePc MOF. For indicates that the bare GCE has a smaller surface area and lower
Fig. 1 (A) SEM and (B and C) TEM images of FePc MOF. (D) FT-IR spectra of FePc (red) and FePc MOF (black).
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Fig. 2 (A) Nyquist plots for bare GCE (a) and FePc MOF/GCE (b) in 1.0 mM [Fe(CN)6]3ꢁ/4ꢁ (K3Fe(CN)6/K4Fe(CN)6 ¼ 1 : 1, containing 0.1 M KCl). (B)
Cyclic voltammograms of 65.46 mM vanillin on bare GCE (a) and FePc MOF/GCE (b) in B–R buffer (pH 7.0). Scan rate ¼ 0.1 V sꢁ1. (C) Differential
pulse voltammograms of 15 mM vanillin in 0.1 M B–R buffer at different pH values (2.0–9.0) obtained using the FePc MOF/GCE and (D) changes in
the oxidation peak current and potential as a function of pH.
3.2.2 Enhanced oxidation of vanillin at FePc MOF/GCE.
The electrochemical behavior of vanillin at the bare GCE and
FePc MOF/GCE was studied using cyclic voltammetry (CV) in
B–R buffer (pH 7.0) containing 65.46 mM vanillin. Within the
potential window of 0.2–1.0 V, a weak oxidation peak is
observed with the bare GCE (Fig. 2B). However, with the FePc
MOF/GCE, a signicant oxidation peak is observed at ꢃ0.62 V,
indicating the good electron-transfer performance of the FePc
MOF. Moreover, Fig. S2† shows the CV of without and with
vanillin on FePc MOF/GCE. It can be seen that FePc MOF/GCE
has no oxidation peak in blank buffer solution, indicating
that the FePc MOF/GCE can be used to determine vanillin. A
comparison of the oxidation peak currents of vanillin on the
bare GCE and FePc MOF/GCE reveals that the FePc MOF
imparts enhanced sensitivity and catalyzes the electro-oxidation
of vanillin on the GCE. We attribute the high peak current of
FePc MOF/GCE to its porous structure, as vanillin can pass
through these pores and reach the electrode surface more
easily.
Fig. 3 Cyclic voltammograms of FePc MOF/GCE in 0.1 M B–R buffer
(pH 7.0) containing 15 mM vanillin collected at different scan rates
(0.01–0.1 V sꢁ1). Inset a: linear relationship between the oxidation
current (Ip) and the scan rate (v). Inset b: linear relationship between
the oxidation peak potential (Ep) and the natural logarithm of the scan
rate (ln v).
3.3 Effect of pH
The electrochemical responses of vanillin on the FePc MOF/GCE in
B–R buffer solutions at pH 2.0–9.0 were studied using differential
pulse voltammetry (DPV), as shown in Fig. 2C. The oxidation peak
current of vanillin increases slowly from pH 2.0 to 6.0, reaches
a maximum value at pH 7.0, and then gradually decreases from pH
7.0 to 9.0. Therefore, pH 7.0 B–R buffer was selected for vanillin
analysis. In addition, as the pH value increases from 2.0 to 9.0, the
electron conductivity. The lower Rct of the FePc MOF/GCE (ꢃ328
ohm) suggests that the redox reaction of [Fe(CN)6]3ꢁ/4ꢁ is
signicantly accelerated on the surface of the modied elec-
trode. This result clearly demonstrates that the introduction of
the FePc MOF material onto the GCE surface can reduce the
electron-transfer resistance.
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Scheme 2 Proposed mechanism for the oxidation of vanillin at FePc MOF/GCE.
oxidation peak potential (Ep, V) obviously decreases, indicating vanillin on the surface of the FePc MOF/GCE is a typical
that protons participate in the oxidation reaction. A linear rela- adsorption-controlled process.
tionship was observed between Ep and pH, as described by the
In addition, the oxidation peak potential (Ep) is shied
equation Ep ¼ 0.9186 ꢁ 0.0444 pH (R2 ¼ 0.9932) (Fig. 2D). The positively as the scan rate increased, which indicates that the
slope of ꢁ44.4 mV pHꢁ1 demonstrated that the numbers of oxidation process is irreversible. The linear relationship
transferred protons and electrons are the same during the elec- between the oxidation peak potential and the natural logarithm
trochemical oxidation of vanillin.
of the scan rate (ln v) can be expressed as Ep ¼ 0.0219(ln v) +
0.7044 (R2 ¼ 0.9933). For an irreversible and adsorption-
controlled oxidation process, the correlation between the Ep
and ln v can be described using Laviron's theory:49
3.4 Effect of scan rate
Determining the inuence of the scan rate on electrochemical
behavior is an effective method for exploring the mechanism of
an electrode surface reaction. The effect of the scan rate on the
electrochemical performance of the FePc MOF/GCE for the
oxidation of vanillin was investigated using CV (Fig. 3). In B–R
buffer (pH 7.0), the oxidation peak current (Ip) of vanillin
exhibits a good linear relationship with the scan rate (v) in the
range of 0.01–0.1 V sꢁ1, as described by the equation Ip ¼ 0.3233
+ 5.630v (R2 ¼ 0.9911). This result indicates that the oxidation of
RT RTk0 RT
Ep ¼ E00 þ
ln
þ
ln v
anF
anF
anF
here, E00 is the formal standard potential, R is the gas constant, T is
the absolute temperature, a is the charge transfer coefficient, n is
the number of transferred electrons, k0 is the standard rate
constant of the reaction, v is the scanning rate, and F is the Faraday
constant. Using this equation, an can be calculated as 1.17. As a is
generally assumed to be 0.5 for an irreversible electrochemical
Fig. 4 (A) Q–t curves of bare GCE (a) and FePc MOF/GCE (b). (B) Linearized Q–t1/2 plots for bare GCE (a) and FePc MOF/GCE (b). (C) Differential
pulse voltammograms of FePc MOF/GCE in 0.1 M B–R buffer with 0.22–29.14 mM vanillin. (D) Linear relationship between the oxidation peak
current and the vanillin concentration.
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Table 1 Comparison of different electrochemical electrodes for vanillin determination
Electrode
Technique
Linear range (mmol Lꢁ1
)
LOD (mmol Lꢁ1
)
Reference
Au–Ag alloy NPa/GCE
BDDb
Amperometryj
SWVk
0.2–50
3.3–98
2–70
0.07–20
0.2–10
0.01–200
0.3–135
4–15 and 20–70
0.5–56
13–660 and 0.66–33
0.22–29.14
0.04
0.16
1
0.02
0.1
0.03
0.15
1.29
0.07
3.9 and 0.32
0.05
53
55
56
57
58
59
60
61
Arg-Gc/GCE
DPV
Al–TiO2-NPs/SPCEd
MWNTs-PDA@MIP/SWNTs-COOHe/GCE
Aptamer-AuNPs/FcKB/ZIF-8f@GCE
MoS2–CNFg/GCE
LSVl
DPV
SWV
I–Tm
CTABMGPEh
CoS NR@naon-GCE
G-QDi@Naon/AuNPSPCE
FePc MOF/GCE
DPV
DPV
LSV and DPV
DPV
62
63
This work
a
b
c
d
e
Nanoparticles. Boron-doped diamond electrode. Arginine-functionalized graphene. Screen-printed carbon electrode. Polydopamine-
f
functionalized multi-walled carbon nanotubes, MIP and carboxyl single-walled carbon nanotubes composite. Ketjen black/ferrocene dual-
doped zeolite-like MOFs and electrodeposited gold nanoparticles coupled with DNA aptamer. Carbon nanobers. CTAB-modied graphene
paste electrode. Graphene-quantum dots. Amperometry: amperometric measurements. Square wave voltammetry. Linear sweep
voltammetry. Current–time.
g
h
i
j
k
l
m
reaction, the value of n is approximately 2. Furthermore, from the increases with increasing vanillin concentrations, and a good
relationship between Ep and pH, the numbers of transferred linear relationship (Ip ¼ 0.027c ꢁ 0.023, R2 ¼ 0.9978) is observed
protons and electrons are the same. Therefore, it can be concluded between the vanillin concentration and its oxidation peak
that the oxidation process of vanillin at FePc MOF/GCE involves current in the range of 0.22–29.14 mM. The limit of detection
two protons and two electrons, which is consistent with previously (LOD, S/N ¼ 3) was calculated as 0.05 mM.
reported results.50–52 The proposed mechanism for vanillin oxida-
tion is shown in Scheme 2.
The proposed method was compared with previous reported
electrochemical methods for vanillin detection (Table 1). It can
be seen that the FePc MOF/GCE provides a reasonable linear
range for vanillin detection and has a lower LOD, which is
comparable to other sensors.
3.5 Chronocoulometry analysis
The effective surface areas of the bare GCE and FePc MOF/GCE
were analyzed by chronocoulometry. Fig. 4A shows the Q–t
curves of the bare GCE and FePc MOF/GCE in 1 mM K3Fe(CN)6
containing 0.1 M KCl. Fig. 4B shows the linearized Q–t1/2 plots,
where gave equations of Q (mC) ¼ 38.96t1/2 + 1.438 (R2 ¼ 0.9998)
and Q (mC) ¼ 54.04t1/2 + 13.739 (R2 ¼ 0.9962) for bare GCE and
FePc MOF/GCE, respectively. The slopes of the Q–t1/2 plots can
be expressed as 2nFAcD1/2/p1/2, as in the Anson equation:53
3.7 Reproducibility, stability, and interference
To evaluate the reproducibility of the sensor, the DPV current
responses to 1.5 mM vanillin were used. Eleven consecutive
measurements with the same electrode gave a relative standard
deviation (RSD) for the response peak current of 2.02%.
Measurements with ve independently prepared electrodes
gave an RSD of 1.25%. These results indicate that the sensor has
good reproducibility. To evaluate stability, the modied elec-
trode was stored at 4 ꢀC for 1 week. Aer week, the sensor
maintained 93% of the initial peak current, indicating good
storage stability.
2nFAcD1=2t1=2
Q ¼
þ Qdl þ Qads
p1=2
where A (cm2) is the surface area of the electrode; c is the
vanillin concentration; D is the apparent diffusion coefficient; F
is the Faraday constant; n is the number of electrons trans-
ferred, Qdl (C) is the double-layer charge, which can be elimi-
nated by background subtraction; and Qads (C) is Faradaic
charge due to the oxidation of adsorbed vanillin. For 1 mM
K3Fe(CN)6, n can be considered equal to 1 and D equal to 7.6 ꢂ
Table 2 Effects of interfering substances on the sensor to vanillin
Interferent
na
Er (%)
Interferent
na
Er (%)
10ꢁ6 cm2 sꢁ1 54 Thus, using the slopes of the linear plots, the A
.
Glucose
Sucrose
Maltose
Starch
Dextrin
Glycine
300
300
100
50
50
50
100
50
50
10
200
100
ꢁ1.49
0.70
Na+
100
50
50
ꢁ0.82
ꢁ3.92
2.17
Mg2+
Zn2+
Cu2+
Fe3+
Al3+
values for bare GCE and FePc MOF/GCE were calculated as
0.129 and 0.180 cm2, respectively. These results indicate that
modication with FePc MOF increases the specic surface area
of the electrode, thus providing more reaction sites and
improving the detection sensitivity.
ꢁ1.96
ꢁ2.27
ꢁ3.80
ꢁ2.12
1.40
50
2.27
500
500
50
500
100
100
50
ꢁ0.03
2.33
+
L-Leucine
L-Gystein
L-Phenylalanine
Ascorbic acid
K+
NH4
ꢁ1.55
ꢁ0.03
ꢁ0.82
ꢁ3.92
1.55
2ꢁ
0.45
SO4
ꢁ2.34
Clꢁ
3.6 Vanillin detection
ꢁ
4.92
HCO3
2ꢁ
ꢁ2.30
1.19
CO3ꢁ
Under the optimal conditions, the electrochemical response of
FePc MOF/GCE to vanillin at different concentrations was
studied using DPV. As shown in Fig. 4C and D, the peak current
Ca2+
NO3
50
0.80
a
Molar ratio (interfering substances/vanillin).
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The selectivity of the sensor was evaluated by examining the
interference of possible coexisting substances in drug and
biological samples. The results are shown in Table 2, using a 1.5
mM vanillin standard solution, no interference (deviation <5%)
with 500-fold Fe3+, Al3+, and SO42ꢁ; 300-fold glucose and
sucrose; 200-fold K+; 100-fold maltose, L-leucine, Ca2+, Na+,
HCO3ꢁ, and Clꢁ; 50-fold starch, dextrin, L-gystein, L-phenylala-
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nine, Mg2+, Zn2+, Cu2+, NH4 , CO32ꢁ, NO3ꢁ; and 10-fold ascorbic
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GCE when coexisting with 150 mM UA and DA. It can be seen
that owing to the oxidation potential differences, the oxidation
peak of vanillin (oxidation peak: 0.5–0.70 V) is signicantly
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3.8 Analysis of real samples
To evaluate the practicality of the developed sensor, the feasi-
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was investigated using DPV under the optimized conditions. As
shown in Table S1,† satisfactory vanillin recoveries were ob-
tained using the FePc MOF/GCE. Moreover, the vanillin
contents determined using the FePc MOF/GCE were in good
agreement with those obtained by UV analysis, indicating that
the sensor has good application potential for determining the
vanillin content of real samples (Table S2†).
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4. Conclusions
In this work, a novel electrochemical sensor was developed for
the determination of vanillin by modifying a GCE with FePc
MOF. The good electrocatalytic performance of the FePc MOF
may be due to its pore structure, which can absorb vanillin and
accelerate the electron-transfer rate, thus increasing the oxida-
tion rate. The fabricated sensor exhibited a linear response
range for vanillin of 0.22–29.14 mM with an LOD of 0.05 mM.
Moreover, the prepared sensor was successfully applied to
detect the content of vanillin in tablets and human serum.
22 Z. Zeng, X. Fang, W. Miao, Y. Liu, T. Maiyalagan and S. Mao,
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
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˙
This work was supported by the Natural Science Foundation of
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Province
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no.
2016GXNSFAA380113,
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China [grant no. 21465004].
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