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Cytotoxicity of ruthenium-N,N-disubstituted-N'-acylthioureas complexes.
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3481
A nanocomposite of NiFe2O4–PANI as a duo active
electrocatalyst toward the sensitive colorimetric
and electrochemical sensing of ascorbic acid†
D. Navadeepthy,
M. Thangapandian, C. Viswanathan
and N. Ponpandian
*
A non-enzymatic ascorbic acid sensor using a nickel ferrite/PANI (NF–PANI) nanocomposite and based on
colorimetric and electrochemical sensing methods was investigated in this study. The nanocomposite was
prepared by an in situ polymerization and utilized as an electrocatalyst to sense ascorbic acid (AA) through
the peroxidase mimic sensing of H2O2 in the presence of 3,5,3,5-tetramethylbenzidine (TMB) as a coloring
agent. It was also utilized to detect AA present in real samples prepared from fruit extracts, commercial
beverages, and vitamin-C tablets. The limit of detection (LoD) for AA sensing by the peroxidase mimic
method was found to be 232 nM. The relative standard deviation (RSD) calculated for analysis of the real
samples analysis ranged from 1.7–3.2%. Similarly, the electrochemical sensing of AA by NF–PANI was
examined by cyclic voltammetric, chronoamperometric, and differential pulse voltammetric analyses.
Received 9th April 2020
Accepted 21st June 2020
The LoD for the electrochemical method applied to AA sensing was 423 nM. The nanocomposite
functioned as an effective electrocatalytic sensing agent in both methods to selectively detect AA due to
DOI: 10.1039/d0na00283f
the combined effect of NF and PANI. Thus, it was shown that the nanocomposites could be utilized for
the laboratory-based detection of AA by various methods and could give rapid results.
rsc.li/nanoscale-advances
1. Introduction
Ascorbic acid (AA), one of the soluble vitamins present in the
human body, is widely used as a preservative in the food,
cosmetic, and pharmaceutical industries.1,2 It plays a vital role
in various physiological and biochemical processes, and its
deciency can affect the metabolism and cause serious illness
in the human body. The ideal range of AA in human serum is
50–70 mM. An imbalance in the level of AA may induce variation
in the production of ROS and antioxidants which leads to tissue
damage and related diseases.3 There is an urgent need to design
a reliable and highly sensitive sensor for the on-site and rapid
detection of AA for healthcare and food quality and security. A
number of analytical techniques are employed for the detection
of AA, such as liquid chromatography, spectrophotometry,
uorescence, chemiluminescence, and electrochemical
methods,4 but even though they are prominently used and have
demonstrated low detection limits with good accuracy, they are
time-consuming, expensive, and require skilled personnel to be
performed. Among the far-ranging analytical techniques,
peroxidase mimic colorimetric sensing and electrochemical
methods have attracted much attention. Colorimetric sensing
Department of Nanoscience and Technology, Bharathiar University, Coimbatore
641046, India. E-mail: ponpandian@buc.edu.in; Fax: +91-422-2422397; Tel: +91422-2426-421
† Electronic supplementary
10.1039/d0na00283f
information
(ESI)
available.
This journal is © The Royal Society of Chemistry 2020
See
DOI:
strategies show results that are visible to the naked-eye and are
not primarily dependent on the instrument utilized.5 Similarly,
electrochemical methods are well established and known to
have high accuracy in terms of both selectivity and sensitivity.
This provides for ease of analysis, which is required in eld tests
to meet laboratory purposes.6
It is well-known that uric acid (UA) and dopamine co-exist
along with AA in biouids. Both UA and dopamine represent
potential interfering species in the electrochemical detection of
AA as they undergo oxidation at a potential close to that of AA.
Similarly, the oxidative products from the interfering
compounds may further oxidize AA, which can lead to an
inaccurate analysis. Hence, it is necessary to develop a selective
and sensitive method for the determination of AA.7
Recently, nanomaterial-based enzyme mimics have attracted
much attention owing to their advantages over natural enzymes,
such as easy preparation procedure, excellent stability, low cost,
and high catalytic activity. In this context, tremendous efforts
have been carried out to develop a nanozyme using various
nanomaterials and composites with metals,8 metal oxides,9
carbon based materials,10–12 polymers13 and polymer nanocomposites14 and layered hydroxides15,16 are studied. However,
they show poor catalytic activity, owing to their decreased
specic surface area and aggregation in solution. Peroxidase,
a natural enzyme, is conventionally used as a catalyzing agent
and though it possesses excellent oxidizability, it has some
inherent drawbacks, such as easy denaturation, easily digestible
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by protease, and expensive preparation and purication are
needed.
Recently, magnetic nanoparticles have been widely
employed in solution-based sensing applications since they are
easily separable aer the process.17,18 In particular, ferrites, such
as ZnFe2O4,19,20 CoFe2O4,21 and MnFe2O4 (ref. 22) have been
investigated and reported to be excellent peroxidase mimicking
agents. The Fe3+ ions in the ferrite structure play a vital role in
the peroxidase mimic property since the Fenton reactions are
the major backbone for the degradation of organic substrates.19
Nickel ferrite (NiFe2O4), an inverse spinel ferrite, is a biocompatible, non-toxic, and ferrimagnetic material that is easy to
prepare and very stable at ambient conditions.23 The composition of Fe3+ and Ni2+ in the composite makes them prominently
used for catalytic reactions. Nowadays, conducting polymers are
applied in many diverse applications due to their semiconducting properties, compatible nature, and fast
responses.24,25 Among the various conducting polymers, polyaniline (PANI) is the most widely studied conducting polymer
due to its low cost of production, stability, high conductivity,
and environmentally benign nature.26,27 Also, the dopant ions in
the PANI structure offer favorable sites for transferring the
electrons to biomolecules and can accelerate the electron
transfer between an electrode surface and electroactive
molecules.24,28,29
Herein, we report the efficacy of the composite NiFe2O4–
PANI (NF–PANI) for the sensing of AA by two orthogonal
methods, namely as a peroxidase mimic and in electrochemical
sensing. Though electrochemical methods are accurate and
sensitive, colorimetric methods are preferable for a fast and
cost-effective analysis. Thus, combining both advantages of
colorimetric and electrochemical sensing is desired but still
remains a challenge in the eld of sensors. The present work
elaborates the formations of nanocomposites of metal oxides
and conducting polymers for use in colorimetric and electrochemical methods for sensing AA.
2.
Experimental methods
2.1
Materials
The reagents hydrochloric acid (HCl), nickel chloride hexahydrate (NiCl2$6H2O), ferric chloride hexahydrate (FeCl3$6H2O),
hydrogen peroxide (H2O2), urea, glucose, fructose, glycine,
aspartic acid, 3,5,3,5-tetramethylbenzidine (TMB), ammonium
persulfate (APS), aniline, L-cystine, and also the chemicals for
the phosphate and sodium buffer preparations were procured
from Himedia (P) Ltd., India. Commercially available orange
juices were used for the real sample analyses. All the reagents
were used as received without any further purication.
Paper
slowly into the mixture, which was stirred for another 2 h. The
homogeneous mixture was transferred to the stainless steel
autoclave and heated at 180 C for 12 h. Further, the autoclave
was allowed to cool naturally, and the precipitate was ltered.
The obtained sample was washed several times with distilled
water and nally with ethanol and then dried at 60 C. The
obtained nanoparticles were well ground and then calcined at
500 C for 4 h. The nal product was again well ground and used
for further analysis.
2.3
The NF–PANI nanocomposites were prepared by an in situ
polymerization with as-prepared NF nanoparticles obtained
through a hydrothermal process. Typically, 0.1 M of monomer
aniline was initially dispersed in 25 mL of 0.1 M HCl dopant
solution. Next, 2 g of as-prepared NF nanoparticles were added
to the above solution, which was then sonicated to get
a uniform dispersion. Simultaneously, the solution was cooled
to an ice-cold condition between 0 C to 5 C. To the above
mixture of monomer and nanoparticles, 0.15 M of APS prepared
in 25 mL of 1 M HCl was added slowly and the mixture was then
sonicated while maintaining the temperature below 5 C. The
mixture was le overnight and allowed to settle. A dark green
precipitate was obtained, which was then washed several times
with water and nally with methanol to remove the unreacted Cl
ions present in the solution. The ltered product was dried
below 70 C without any further modication. PANI was
synthesized by a polymerization method using the same process
as the earlier described one without the addition of NF to the
monomer solution.
2.4
Synthesis of NF nanoparticles
First, 0.3244 g (0.1 M) ferric chloride was dissolved in 20 mL
distilled water, and 0.356 g (0.05 M) nickel chloride was dissolved in 20 mL of distilled water separately and stirred for
20 min. The above solutions were mixed and 0.0567 g (0.1 M) of
sodium borohydride was dissolved in 50 mL of water and added
3482 | Nanoscale Adv., 2020, 2, 3481–3493
Peroxidase mimic sensing
The catalytic activity of NF–PANI was evaluated by the colorimetric method using the chromogenic substrate TMB in the
presence of H2O2. The reactions were carried out in sodium
acetate buffer optimized at pH 3.5. Here, 100 mL of a xed
concentration of TMB was initially added to 2 mL of the buffer
followed by the addition of 100 mL of a 10 mg/10 mL sample and
a xed concentration of H2O2, respectively. The solution was
shaken well and allowed to react for 5 min. Then, the kinetic
measurements for all the reactions were monitored in time
course mode at 652 nm. The apparent kinetic parameters were
estimated by using the Michaelis–Menten equation: 1/n ¼ (Km/
Vmax)(1/[S]) + 1/Vmax, where n is the initial velocity, Vmax is the
maximum intensity denoting the velocity, [S] is the concentration of the substrate, and Km is the Michaelis constant.19,30 All
the studies were carried out with the same procedure with
varying the concentrations of the variant of interest.
2.5
2.2
Synthesis of NF–PANI nanocomposites
Ascorbic acid sensing
Ascorbic acid detection was carried out with the same buffer
with xed concentrations of catalyst, TMB, and H2O2. Typically,
aer 5 min of adding the reactants, catalyst, TMB, and H2O2, 25
mL of AA with different concentrations was added to the reaction
liquid. Finally, the catalytic kinetics was investigated for each
concentration. Several potentially interfering compounds, such
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as urea, glucose, fructose, sucrose, starch, glycine, aspartic acid,
dopamine, and cysteine were also prepared in the same
concentration as that of AA and subjected to peroxidase mimic
activity. The selectivity of the catalyst was examined through
repeating the peroxidase activity with the interfering
compounds separately and along with the presence of AA. To
investigate the efficacy of the catalyst for sensing AA, we used
commercial beverages and natural fruit juices. The orange juice
(Minute Maid) and vitamin C tablets containing AA were
procured from the open market and extracts from oranges and
lemons were freshly prepared in the lab. The extract and
commercial juices were diluted for obtaining various concentrations before analysis. The tablet was crushed and diluted for
obtaining different concentrations. All the dilutions were made
in the buffer solution.
2.6
Electrochemical sensing
The electrochemical investigations were performed through
a three-electrode cell assembly. All the measurements, such as
cyclic voltammetry (CV), chronoamperometry (CA), and differential pulse voltammetry (DPV) were carried out using a PAR
analytical instruments, USA. A glassy carbon electrode (GCE)
was employed as the working electrode and Ag/AgCl (3 M KCl)
and Pt wire were employed as the reference and counter electrodes, respectively. The studies were performed by modifying
the GCE using the NF–PANI nanocomposite as an electrocatalyst. GCE was cleaned with various grades of alumina (0.3
micron and 1 micron) prior to the modication. Here, 1 mg of
NF–PANI nanocomposite was dispersed in a 2 mL ethanol + 5 mL
Naon mixture and 10 mL of the sample was drop-cast on the
surface of GCE and dried overnight before analysis.
3.
Results and discussions
3.1
Formation mechanism
Nickel ferrite consists of Fe ions in its outer structure, which
makes the compound positively charged in 1 M of HCl.
Generally, metal oxides possess a positive surface charge when
the pH is below the point of zero charge (PZC), while they
become negative above the PZC. The surface of magnetite has
its PZC at pH 6, so it is positively charged in the monomer
solution when it contains more protons than the hydroxyl
groups. Therefore, Cl present in the solution gets absorbed on
Scheme 1
the surface of the NF nanoparticles and compensates the
positive charge with ferrite ions. Thus, in the acidic solution,
aniline monomers get converted to cationic anilinium ions.
Thereby, electrostatic interactions occur between anionic Cl
adsorbed on the nanoparticle's surface and cationic anilinium
ions in the solution. The APS added into the mixture initiates
the polymerization of the monomers on the nanoparticle's
surface. This makes the composite nanostructure NF–PANI. A
schematic illustration of the polymerization process of the
nanocomposite is shown in Scheme 1.31
3.2
Characterization results and discussions
The X-ray diffraction (XRD) pattern of PANI, NF, and the NF–
PANI nanocomposite are shown in Fig. 1(a). The XRD peaks
correspond to the typical inverse spinel structure (JCPDS # 10m space group.32 The average crystallite size
0325) with the Fd3
was found to be 20 nm using the Scherrer formula. The polyaniline peak was obtained between 20–30 and the composite
material also showed the peak at the same angle.24 There were
no other impurity peaks observed in the XRD pattern that
conrmed the formation of the amorphous form with the
partially crystalline PANI. Instead, the XRD patterns of the
nanocomposite NF–PANI showed all the major peaks of nickel
ferrite with a hint of the formation of PANI with the XRD peak at
25 . This clearly depicted that the composite formation does not
destroy the spinel structure of NF as well as the amorphous
nature of PANI.
Raman spectroscopic analysis was carried out to strengthen
the structural properties of the samples. Fig. 1(b) shows the
Raman spectra for pure NF, PANI, and the NF–PANI nanocomposite. In pure nickel ferrite, only the Eg and A1g modes of
vibrations at 473 and 692 cm1 could be observed.33 In PANI and
the NF–PANI nanocomposites, the defective and graphitic peaks
were present at 1300 and 1560 cm1, which are the characteristic peaks of the C]C bonds of quinonoid and benzenoid units
of PANI and the sp2 hybridized carbon atoms of carbon present
in the polymer. The Eg and A1g peaks were diminished in
composites due to the composite formation. This may be due to
the polymer formed over the surface of NF and the polymer
matrix completely covering the surface of the ferrite. There was
a signicant change in the intensity of the D and G bands of the
composites, which was due to the composite formation.
Formation mechanism of the NF–PANI nanocomposite.
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Fig. 1 Structural, morphological, and elemental analyses of NF–PANI. (a) XRD analysis of pristine PANI, NF, and the composite NF–PANI; (b)
Raman spectra of PANI, NF, and the composite NF–PANI; FESEM analysis of (c) NF, (d) PANI, (e) NF–PANI; and (f) elemental analysis of NF–PANI;
with the inset table showing the % of elements present in NF–PANI.
Fig. 1(c)–(e) show the eld emission scanning electron
microscopy (FESEM) images of NF, PANI, and the NF–PANI
nanocomposites. Fig. 1(c) shows the hydrothermally synthesized nickel ferrite nanoparticles, which were spherical in shape
with an average diameter ranging from 200–250 nm. The NaBH4
added in the reaction mixture acted as a reducing agent as well
3484 | Nanoscale Adv., 2020, 2, 3481–3493
as a stabilizing agent and facilitated the Ostwald ripening
process. Fig. 1(d) shows the FESEM micrographs of pure polyaniline in an aggregated form. The traditional oxidative chemical polymerization route is known to yield granular
polyaniline.34 Fig. 1(e) shows the morphology of nickel ferrite
covered with polyaniline. The aniline polymerization takes
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place in the presence of nickel ferrite nanoparticles. Hence, the
polyaniline nanostructures take nickel ferrite as an active
nucleation point and cover their surface during polymerization.
Thus, nickel ferrite particles could not be seen on the surface of
polyaniline.31 The elemental analysis of the NF–PANI is shown
Fig. 1(f), with the inset showing the atomic percentage of
elements present in the sample. The EDAX results conrmed
that the composite consisted of C, O, Fe, Ni, and miniscule
percentages of S and Cl. The S and Cl were due to the leover
ions from HCl and APS aer the reaction. No other impurity was
present in the composite, which conrmed the elemental purity
of the sample.
4. Sensing analysis of H2O2
To investigate the peroxidase mimic activity of NF–PANI and
to colorimetrically detect H2O2, TMB was used as a chromogenic substrate. It oxidizes in the presence of NF–PANI as
a catalyst to change the colorless solution to a blue color.
When the redox catalytic reaction occurred in the solution, it
Nanoscale Advances
could be observed by the naked-eye and qualitatively analyzed
by its UV-Visible absorbance spectrum. The reactions were
carried out by adding xed concentration of TMB in the
acetate buffer (pH 3.5 optimized) of the required volume followed by the addition of the catalyst and varying concentrations of H2O2. Similarly, the sensing studies were repeated by
varying the concentration of TMB with xed concentrations of
H2O2. The UV-Visible (UV) absorption spectrum of the solution aer 10 min of reaction was recorded. The catalytic
activity of NF, PANI, and NF–PANI were separately analyzed.
Initially, the concentration of TMB was varied from 10 nM to
30 mM and the concentration of H2O2 was kept constant at
10 mM. The sample concentration was also kept constant as
100 mL from a 10 mg/10 mL dilution. The results conrmed
that the NF–PANI composite had better activity than PANI and
NF, as shown in Fig. 2(a) and (b), when varying the concentrations of both TMB and H2O2. The PANI samples showed
very poor redox behavior in both the analyses; whereas the
addition of NF on the PANI matrix enormously increased the
peroxidase activity, which was due to the catalytic activity of
(a) Comparison of the catalytic activity upon varying the concentration of TMB, (b) comparison of the catalytic activity of all the samples at
varying concentrations of H2O2, (c) change in the intensity of absorbance upon varying the concentration of TMB with the NF–PANI sample, and
(d) intensity variations with the increase in the concentration of H2O2.
Fig. 2
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Ni and Fe ions in the ferrite. Normally, in ferrites, their
peroxidase enzyme mimic activity is ascribed to the Fenton
reactions taking place in the solution. The H2O2 adsorbed on
the nanoparticles is easily reduced by the electrons donated
by the Fe3+ ions in the structure. The OH radicals are
produced in either cases when Fe3+ or Fe2+ ion reacts with
H2O2.35,36 This makes the composite a better catalytic agent in
the peroxidase mimic sensing of H2O2.
The catalytic activity of NF–PANI at varying concentrations of
TMB (30, 20, 10, 5, 1 mM, and 750, 500, 250, 100, 90, 80, 70, 60,
50, 40, 30, 20, 10 nM) and H2O2 (600, 550, 500, 450, 400, 350,
300, 250, 200, 150, 100, 50, 25, 10, 5, 1 mM, and 900, 800, 700,
600, 500, 400, 300, 200, 100 nM) is shown in Fig. 2(c) and (d) and
was used to determine the linearity in the sensing and the limit
of detection (LoD) of the sample. There was a linear increase in
the intensity of absorbance with the increase in the concentrations of TMB and H2O2.
The kinetic analysis of the catalytic redox reactions by the
nanocomposite was further performed by linear tting of the
concentration-dependent variation in the absorbance spectrum. Fig. 3(a) and (b) show the linearly tted graphs and their
insets show the tted results with the R2 values of 0.9961 and
0.9813 for TMB and H2O2, respectively. The rate of the reactions Vmax and the Michaelis–Menten constant Km were
determined for NF, PANI, and NF–PANI and the values are
given in Table 1, calculated utilizing the same calculation
mentioned in our earlier work report. The Km value indicates
the enzyme affinity, where usually a lower Km value indicates
a higher affinity toward the substrate. Also, Vmax represents
the velocity of the reaction.18 The tted parameters conrmed
that the NF–PANI showed a higher affinity toward the
substrate with a high Vmax. The kinetic studies clearly
conrmed that the composite showed a better sensing ability
for the sensing of H2O2. Similarly, the comparative activities
of the catalysts with other sensing systems in the existing
literature are also given in Table 2. The LoD of TMB was found
to be 10 nM and 132 nM for H2O2. The lower LoD proves that
Fig. 3
Paper
Kinetic analysis parameters for NF, PANI, and NF–PANI
Table 1
Catalyst
Substance
Km [mM]
Vmax [108 M s1]
NiFe2O4
TMB
H2O2
TMB
H2O2
TMB
H2O2
39.7931
22.1265
69.859
36.859
13.5045
5.898
7.662
1.4513
0.7092
0.5432
9.9512
1.4684
PANI
NiFe2O4/PANI
the catalytic activity was efficient with even lower concentrations of TMB and H2O2.
5.
Sensing of ascorbic acid
The sensing of AA was carried out by utilizing the peroxidase
mimic property of the nanozyme. At optimal conditions, the
sensing of AA was carried out using the NaAc buffer of pH 3.5.
Fig. 4(a) and (b) show the gradual decrease in absorbance
intensity with the increase in the concentrations of AA. The
inset in Fig. 4(b) shows the linear relationship within the range
of 10–100 mM for the absorbance and the concentrations of AA
with the value of R2 ¼ 0.99501 and the LoD ¼ 232 nM through
the relationship 3s(standard deviation)/slope. This clearly
shows that the catalyst was efficient for detecting AA to a lower
range compared to the other reported values detected by other
systems.39,40 In the peroxidase mimic sensing of H2O2, TMB
oxidizes to produce a blue colored solution, whereas in AA
sensing, when AA is added to the reaction solution, TMB
reduces and oxidizes AA to become colorless. The increase in
the concentration of AA decreases the color of the solution,
which becomes transparent. This also suggests that AA forms
a complex with the TMB substrate. It has already been reported that the substrate containing a nitrogen center has
strong affinity toward AA,1,41 whereby it forms a strong covalent
bond with nitrogen. In our study, the possible binding centers
Linear peak fitting for the catalytic analysis with varying concentrations of (a) TMB and (b) H2O2.
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Table 2
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Comparison of the LoDs with the existing systems in the literature for sensing H2O2
Catalyst
Method
Limit of detection
of H2O2
Reference
Fe3O4/CoFe–LDH hybrid
Polyallylamine–IrO2/GO
Fe2+, Co2+ and Ni2+
HTCPP–ZnS
NGZF
NF–PANI
Colorimetric method
Colorimetric method
Electrochemical method
Colorimetric method
Colorimetric method
Colorimetric method
0.2 mM
324 nM
7.3 mM
0.01 mM
0.025 mM
132 nM
33
34
37
38
19
Present work
were terminal amine groups (–NH2) of the two TMB substrates.
Thus, there was a decreasing intensity of absorbance with the
increase in the concentration of AA in the reacting solution.
Thus the number of effective TMB molecules to be oxidized by
H2O2 in the presence of the catalyst became less. Further, the
selectivity toward AA sensing was investigated by testing in the
Ascorbic acid sensing (a) and (b) absorbance decrease with the increase in the concentration of AA, (b) inset shows the linearity with
increasing the concentration of AA, (c) selectivity of the sensing of AA in the presence of potentially interfering compounds compared with AA at
a 10 mM concentration each and (d) change in color with the addition of AA and with other interfering compounds with and without adding AA.
Fig. 4
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Table 3
Paper
Comparison of the LoD with systems in the existing literatures for the sensing of AA
Catalyst
Method
Limit of detection
of ascorbic acid
Reference
Gr/CuPc/PANI
PANI/HNTs
Graphene-based 3D nanocomposites
Polymer-coated electrodes
PANI/MnO2–Sb2O3
S,N co-doped graphene quantum
Carbon dots on CoOOH
MoS2-decorated N-doped carbon nanotubes
Nickel ferrite nanoparticles on a carbonaceous matrix
Mustard seeds
3,4:9,10-perylene tetracarboxylic acid-modied zinc ferrite
PANI–MnO2
NF–PANI
Electrochemical method
Electrophoretic deposition method
Electrochemical method
Electrochemical method
Electrochemical method
Fluorescence method
Fluorescence method
Colorimetric method
Colorimetric method
Colorimetric method
Colorimetric method
Colorimetric method
Colorimetric method
6.3 108 M
0.21 mM
460 mM
0.0267 mM
0.12 mM
1.2 mM
25 nM
0.12 mM
260 nM
3.26 mM
0.834 mM
26 nM
0.232 nM
7
39
40
43
44
45
46
47
48
49
50
29
Present work
presence of potentially interfering compounds under optimized conditions. About 5 mM concentrations of various
interferences, such as urea, glucose, fructose, sucrose, dopamine, starch, arginine, aspartic acid, cysteine, and glycine
were considered.42 The same reaction conditions were maintained as for 5 mM of AA. There was no color change in the
TMB solution. Further, when AA was introduced to each
interfering solution mixture, the blue color of the TMB disappeared. Fig. 4(d) shows the changes in color with and
without the addition of AA. The sensing ability was also
compared with other existing literature reports and the results
are given in Table 3. The sensitivity of the sample reached
the nM concentration level, whereas the reported ones showed
LoDs in the mM concentration range. Thus, the present work
emphasizes the sensing of H2O2 to a very low concentration,
which may be applicable for biological applications.
5.1
Real sample analysis
The practical viability of the present AA sensor was also investigated with real samples. Orange and lemon extracts and
vitamin C tablet were diluted to three different concentrations
(25, 50, and 100 mM) for the present investigation.1,51 As listed in
Table 4, the recoveries of a known amount of AA in the 500-fold
diluted real samples were between 97.5% and 106.97% with the
RSD ranging from 1.7% to 3.1%. The results clearly indicated
that the proposed sensor is applicable for the quantication of
AA in natural as well as in commercial AA-containing foods and
beverages.52
6. Electrochemical sensing of AA
6.1
Electro-oxidation of AA by NF–PANI-modied GCE
The electrochemical sensing of AA was performed by analyzing
the oxidative property of the nanocomposite. Herein, we
investigated the redox capability of AA on the surface of NF–
PANI with the help of cyclic voltammetry (CV), chronoamperometry (CA), and differential pulse voltammetry
(DPV). All the measurements were performed at various pH
values and it was found that the activity was high at pH 7 of
3488 | Nanoscale Adv., 2020, 2, 3481–3493
Table 4 Real sample analysis for the sensing of AA present in fresh and
commercially available fruit juices and tablets
Sample
Added (mM)
Found (mM)
Recovery (%)
RSD (%,
n ¼ 3)
Orange juice
25
50
100
25
50
100
25
50
100
24.37
50.12
103.56
26.24
48.89
101.84
25.34
49.35
98.61
97.48
100.24
103.56
106.97
97.78
101.84
97.36
98.7
98.61
2.8
3.1
2.6
2.2
1.7
1.9
2.1
1.9
2.3
Lemon extract
Vitamin C
PBS, which was chosen as the optimized pH for further
studies. Fig. 5(a) shows the cyclic voltammograms of bare GCE
and NF–PANI in PBS compared with the NF–PANI upon the
addition of 500 nM of AA. It can be clearly seen that the bare
GCE and NF–PANI did not show any oxidation or reduction
peaks before the addition of AA, whereas upon the addition of
a xed concentration of AA to the reaction solution, an
oxidation peak at 602 mV corresponding to the change of AA
from dehydroascorbic acid could be observed. To strengthen
the analysis of the oxidative property of NF–PANI toward AA,
scan rate and concentration-dependent analyses were performed. Fig. 5(b) shows the increase in peak current in CV for
different scan rates with a xed concentration of AA, which
conrmed the inuence of the catalytic activity of the NF–PANI
nanocomposite.3,4 The inset of Fig. 5(b) shows the linear tting
of the peak currents at different scan rates of the CV. The
sensing tests of AA with varying concentrations from 100 nm
to 1 mM were also performed with CV, CA, and DPV. The voltammograms of NF–PANI with varying the concentrations of
AA are shown in Fig. 5(c). The peak current was increased by
increasing the concentrations of AA, which clearly conrmed
that the catalyst improved the oxidation of incoming AA in
contact with the catalyst. The linearity of the sensing was
analyzed by linear tting the data between 200–1000 nM and
this is shown in an inset graph with R2 ¼ 0.9943. Fig. 5(d)
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Nanoscale Advances
Fig. 5 Electrochemical oxidation of ascorbic acid: (a) CV analysis of bare GCE, and NF–PANI-modified GCE with and without the addition of AA,
(b) CV analysis of NF–PANI at different scan rates for the detection of AA; inset shows a linear fit of the scan rate, (c) voltammograms showing the
increase in peak current with the increase in concentration of AA; inset shows its linear fit, and (d) chronoamperometric graph of different
concentrations of AA at a fixed potential of 602 mV.
shows the chronoamperometric investigation with varying the
concentrations of AA with the xed potential of 602 mV,
showing that the catalyst maintained a stable oxidation
current for all the concentrations. This suggests that the NF–
PANI nanocomposite showed excellent sensing performances.26,53 A schematic illustration for the sensing of AA by
the electro-oxidation of NF–PANI in PBS is shown in Scheme 2.
The peak current of ascorbic acid oxidation at the surface of
the NF–PANI can be used to detect the concentration of ascorbic
acid. To further study the catalytic property of NF–PANI, DPV
analysis of the electrode was carried out and the results are
shown in Fig. 6(a). The oxidation of AA occurred at 0.401 V,
where the peak current increased with the concentration of AA
added.54 The tting of the peak current gave a straight line, as
shown in Fig. 6(b). The linear tting gave an R2 value of 0.98759,
which suggests the linearity of the sensing. The oxidation peak
currents of ascorbic acid at the electrode surface were proportional to the concentration of the ascorbic acid in the range of
This journal is © The Royal Society of Chemistry 2020
0.1 to 1 mM. The detection limit (3s) of ascorbic acid was found
to be 0.423 106 M.43,53
6.2
Repeatability and stability of the nanocomposite
The long-term stability of the NF–PANI nanocomposite was
established through analysis of the catalyst over a 7 day period.
The modied electrode was stored for a week at room
temperature without any further modication and the experiments were performed again. According to the voltammograms, there was no change in the peak potential for ascorbic
acid oxidation, whereas the intensity of the peaks decreased by
1.2% compared with the initial response. This may be ascribed
to the ageing effect of the electrodes. Similarly, the repeatability of the samples was studied by preparing the electrode
three times and analyzing separately, and the results are
attached in the ESI 1.† According to the results, the NF–PANImodied electrodes had increased sensitivity and stability
toward the oxidation of AA.
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Scheme 2
Paper
Schematic illustration for the electrochemical sensing of ascorbic acid.
(a) DPV analysis of NF–PANI-modified GCE; the peak current increases with the increasing concentration of AA and (b) linear fitting of the
concentration-dependent increase in the peak current with R2 ¼ 0.9875.
Fig. 6
7. Conclusions
In summary, a nanocomposite NF/PANI with excellent peroxidase
mimic activity and electrochemical behavior toward the sensing
of AA was reported. The composite nanosensor combining the
inuence of the Fenton reactions of NF and the high surface area
of PANI's conductive network provided excellent catalytic performance in both sensing methods, electrochemical and colorimetric. Also, the AA present in the real samples was detected with
an RSD ranging from 1.7% to 3.1%. Moreover, the samples
showed good selectivity toward the interfering compounds with
an LoD of 48 nM. Similarly, the electrochemical studies revealed
the sensing ability of AA with high accuracy, and the LoD of this
method was found to be 423 nM. Though the compound showed
a low limit of detection in the peroxidase method, the electrochemical activity also showed comparable results with existing
3490 | Nanoscale Adv., 2020, 2, 3481–3493
electrochemical sensors. Thus, the nanocomposite has the
potential to mimic peroxidase in the sensing of chemical
compounds as well as can act as an efficient electrochemical
sensor in selectively detecting AA. The obtained results suggest
that the nanocomposite can be utilized as a real-time sensor for
instant lab analysis and also as an electrochemical sensor. This
will invoke new attempts by researchers to study the dual property
of the material to develop novel duo-sensors.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors would like to thank DST-FIST, DST-PURSE and
UGC-SAP, Government of India for the support of
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instrumentation facilities. One of the authors DN would like to
thank Bharathiar University for her University Research
Fellowship.
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