Redox-active gold nanoparticle-encapsulated poly(amidoamine) dendrimer for electrochemical sensing of 4-aminophenol

Article abstract of DOI:10.1016/j.molliq.2020.115131

Metal nanoparticle-encapsulated with dendrimer networks have gained enormous interest towards fabrication of various electrochemical devices, including electrochemical sensors. In this work, we have designed a redox active electrochemical sensor using fer

Full text of DOI:10.1016/j.molliq.2020.115131

Journal of Molecular Liquids 325 (2021) 115131
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Redox-active gold nanoparticle-encapsulated poly(amidoamine)
dendrimer for electrochemical sensing of 4-aminophenol
Mari Elancheziyan, Sellappan Senthilkumar⁎
Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology (VIT), Vellore 632014, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2020
Received in revised form 15 December 2020
Accepted 18 December 2020
Available online 30 December 2020
Metal nanoparticle-encapsulated with dendrimer networks have gained enormous interest towards fabrication
of various electrochemical devices, including electrochemical sensors. In this work, we have designed a redox ac-
tive electrochemical sensor using ferrocene terminated poly(amidoamine) (PAA) dendrimers encapsulated with
gold nanoparticles (AuNPs) for the detection of 4-aminophenol (4-AP). The redox mediator, ferrocene (as ferro-
cene carboxaldehyde), was covalently attached to an amine terminated third generation of PAA dendrimer
through Schiff base condensation to form a stable imine bond. Thereafter, AuNPs were entrapped in the dendritic
network to yield a Fc-PAA-AuNPs, which was deposited over a glassy carbon electrode (GCE) to form Fc-PAA-
AuNPs/GCE. The cyclic voltammogram of 0.1 M phosphate buffer at the fabricated electrode displayed a set of dis-
tinct redox peaks with an oxidation peak at 0.427 V and reduction peak at 0.345 V corresponding to ferrocene/
ferrocenium (Fc/Fc+) redox couple. Further, the fabricated electrode has shown prominent electrocatalytic re-
sponse for the detection of 4-AP based on which an electrochemical sensor towards the determination of 4-AP
has been developed. Noticeably, the newly developed electrochemical sensor has shown impressive performance
Keywords:
PAA dendrimer
Gold nanoparticles
4-Aminophenol
Ferrocene
Nonenzymatic electrochemical sensor
towards detection of 4-AP with a linear range from 30 to 1064 μM, with a sensitivity of 87.8 μA mM− cm and a
limit of detection of 7.61 μM. Besides, the developed sensor exhibited good selectivity and excellent stability to-
wards the detection of 4-AP. The superior efficacy of the developed sensor could be due to the covalent immobi-
lization of Fc and effective encapsulation of AuNPs in the hyperbranched Fc terminated PAA dendritic network,
which augumented their electronic conductivity as well as operational stability.
1
−2
©
2020 Elsevier B.V. All rights reserved.
1. Introduction
quantification of 4-AP becomes necessary in the healthcare and envi-
ronmental sectors.
Development of facile, robust and reliable analytical techniques for
Applications of various analytical techniques including spectropho-
tometry [14], colorimetric [15], capillary electrophoresis [16], gas chro-
matography [17], chemiluminescence [18], liquid chromatography [19]
to the detection of 4-AP yielded satisfactory results. However, these
techniques are time consuming, expensive, tedious and require skilled
operators for sample preparation and operation of sophisticated instru-
mentation, which restrict them in real time use and onsite determina-
tion. On the contrary, electrochemical methods are simple, robust,
rapid, portable, robust, involve no sample preparation, which could
favor the sensitive and selective determination of 4-AP [10,20,21]. The
performance of the electrochemical sensors manifestly relies on the na-
ture of electrode modifying materials, which stipulate the sensitivity,
selectivity and stability of the fabricated electrode. Therefore, enormous
effort has been devoted towards the fabrication of suitable platforms,
which include carbon nanomaterials, metal organic frameworks, poly-
mers, redox mediators, metal oxides, carbides, complexes and nanopar-
ticles [22–25]. Despite all these efforts, these sensors suffer from one or
more of the demerits such as narrow linear range, limited sensitivity
and long-term stability. These challenges could be addressed with
the selective and sensitive determination of crucial analytes is a compel-
ling requirement in human healthcare and environmental safety [1,2].
One such analyte is 4-aminophenol (4-AP), which is a derivative of phe-
nol with an amine substitution in the para position and it is formed dur-
ing the degradation of azo dyes, pesticides and paracetamol (widely
used analgesic and antipyretic drug) [3–5]. 4-AP has adverse effects on
human health causing skin and eye irritation, respiratory problems, ter-
atogenic and nephrotoxicity and affects other life forms as well [6,7].
Thus, the maximum amount of 4-AP in pharmaceutics is restricted to
5
0 ppm by the European Pharmacopeias and United States [8,9]. More-
over, 4-AP is a commonly used synthetic material in a variety of fields
such as plastics, petroleum additives, pharmaceutical ingredients, dye-
stuffs, rubber, chemical inhibition. As a result, tremendous amounts of
-AP are inevitably released into the atmosphere and poses a serious
threat to the environment [10–13]. Therefore, the detection and
4
⁎
Corresponding author.
E-mail address: senthilkumar.s@vit.ac.in (S. Senthilkumar).
https://doi.org/10.1016/j.molliq.2020.115131
167-7322/© 2020 Elsevier B.V. All rights reserved.
0

M. Elancheziyan and S. Senthilkumar
Journal of Molecular Liquids 325 (2021) 115131
appropriate choice of materials which in turn could afford firm immobi-
lization, proper wiring, improved surface area and high conductivity.
Accordingly, we intended to develop an electrochemical sensor for the
determination of 4-AP by judicious modifying the electrode in order to
circumvent the above-mentioned challenges.
Dendrimers belong to a special type of synthetic polymer materials,
which have high surface functionality, well-defined three-dimensional
and highly branched architectures [26,27]. In particular, the poly(ami-
doamine) (PAA) dendrimer is most commonly studied due to its good
biocompatibility, high structural flexibility and functionality, excellent
mono dispersity, tunable porosity, high physical and chemical stability
from Sigma-Aldrich, India. 4-AP (98%), sodium borohydride, sodium
sulfate anhydrous, disodium hydrogen phosphate, sodium dihydrogen
phosphate and sulfuric acid were received from Merck, India. All other
chemicals and solvents used were of high purity analytical grade. All
aqueous solutions were prepared using Milli-Q water obtained from
Merck Millipore.
2.2. Instrumentation
The prepared Fc-PAA-AuNPs were characterized using high resolu-
tion transmission electron microscopy (HR-TEM; FEI-TECNAI G2 20
Twin) at 200 kV. Energy dispersive X-ray analysis (EDX) was carried
out using a Bruker Xflash 6 T130 spectrometer with a liquid nitrogen
free detector. Absorption spectra of synthesized Fc-PAA dendrimer
and corresponding AuNPs were measured in a Jasco V-670 UV–visible
spectrophotometer. The functional groups of PAA and Fc-PAA
dendrimers were confirmed by Fourier-transform infrared spectros-
copy (FTIR) using a Shimadzu, IR Affinity-1 spectrophotometer. Electro-
chemical experiments were performed in a three-electrode type
electrochemical cell with using a CHI760E electrochemical workstation.
Modified/unmodified GCEs were used as a working electrode, a plati-
num coil as a counter electrode and an aqueous Ag/AgCl (3 M KCl) elec-
trode as a reference electrode. Autolab PGSTAT 204 potentiostat
(Metrohm Autolab, Netherlands) was used for performing the electro-
chemical impedance measurements and the results were processed
using Autolab NOVA 2.1.2 software.
[28,29]. Hence, PAA dendrimers have found a wide range of applications
from simple coating and adhesion to complex membrane chemistry,
biomimetic, nanotechnology, electrochemical sensing, biosensing and
immunosensing [30–33]. Further, the pores of PAA can accommodate
suitable metal nanoparticles to enhance the electrical conductivity of
PAA dendrimers. Among various metal nanoparticles, gold nanoparti-
cles (AuNPs) have received extensive attention in electrochemical sen-
sors due to their high electrocatalytic activity, good biocompatibility
and unique optical and electronic properties [34–37]. AuNP encapsu-
lated PAA dendrimers have received massive interest among re-
searchers for fabricating novel electrode platforms for various
electrochemical applications [38].
Another advantage of PAA dendrimer is their synthetic versatility
and affordability in tailor-made architect. PAA dendrimers coated on
an electrode surface can be used to immobilise desirable redox media-
tors or biomolecules to develop novel electrochemical sensors. Redox
mediators such as azure-A, toluidine blue, thionine, ferrocene, pheno-
thiazine, viologen have often been used in electrochemical sensing be-
cause of their well-resolved redox mediating properties [39–42].
Therefore, a variety of redox mediators and their derivatives are fre-
quently being explored for the immobilization over a PAA network in
order to develop redox active platforms for facile electrochemical sen-
sors. There is still a demand for the immobilization of different mole-
cules on PAA dendrimers for improving their performance. Ferrocene
2.3. Synthesis of ferrocene terminated PAA dendrimer (Fc-PAA)
Fc terminated PAA dendrimers were synthesized using a simple
Schiff base condensation reaction as displayed in Fig. 1A. A PAA den-
dritic network was synthesized using the procedure previously re-
ported by us [47] and the synthesis scheme is provided in the
supplementary information (scheme S1). Thereafter, a toluene solution
of Fc carboxaldehyde (96 mmol) was dropwise added into a stirred tol-
uene solution of PAA dendrimer (2.9 mmol). This mixture was refluxed
at 70 °C for 24 h, after which the solvent was removed to obtain a resi-
due. The obtained residue was dissolved using ethanol (30 mL) to which
(
Fc) is an organometallic molecule widely used as a redox mediator
for the voltammetric determination of various substrates due to its
unique electrochemical properties [43,44]. However, Fc has poor adhe-
sion on the surface of bare electrodes with a low solubility in aqueous
medium which limits its inclusive application in voltammetric determi-
nation. Many approaches have been used to improve the adhesion of Fc
onto an electrode such as composite matrix, mechanical immobiliza-
tion, self-assembled monolayers and covalent bonding [45,46].
4
NaBH was added and the mixture was refluxed for 6 h to reduce the
-C=N- double bond in imine groups [48]. After completion of reaction,
ethanol was removed and the resulting residue was washed using 3%
(w/v) aqueous NaHCO
3
2 2
and extracted using CH Cl . The combined ex-
tracts were dried using anhydrous Na
2
SO and then filtered. The filtrate
4
In the present work, we sought to develop a redox-active dendrimer,
encapsulated with AuNPs as an electrochemical sensor for 4-AP by
harnessing the synthetic versatility of PAA dendrimer. Initially, an
amine terminated third generation PAA dendrimer was synthesized,
to which ferrocene carboxaldehyde (Fc-CHO) was covalently attached
to the terminal amine of PAA and free aldehyde of Fc-CHO to form the
Fc terminated PAA dendrimer (Fc-PAA). Further, AuNPs were encapsu-
lated in the Fc-PAA dendritic network to form Fc-PAA-AuNPs. The PAA
dendrimers assist towards the covalent immobilization of Fc and also
the preparation of AuNPs with reasonable monodispersity and are
thus imperative in this setup. The electrochemical sensor was fabricated
by immobilising the prepared Fc-PAA-AuNPs on a glassy carbon elec-
trode to yield a Fc-PAA-AuNPs/GCE. The fabricated Fc-PAA-AuNPs/GCE
showed promising electrocatalytic behaviour towards the oxidation of
was concentrated under reduced pressure to obtain Fc functionalised
PAA dendrimers (Fc-PAAs).
2.4. Synthesis of Fc-PAA encapsulated with gold nanoparticles (Fc-PAA-
AuNPs)
Fc-PAA encapsulated with gold nanoparticles were synthesized
based on the procedure reported earlier [49,50] with minor modifica-
tion (Fig. 1B). In brief, 5 mL of freshly prepared aqueous HAuCl solution
4
(5.5. mM) was added dropwise to a constantly stirred solution of 5 mL
of Fc-PAA dendrimer in ethanol to form a pale-yellow solution, which
was further continuously stirred for 30 min. Subsequently, a fresh solu-
4
tion of 0.1 M NaBH in water was injected and the solution immediately
changed its color from pale yellow to red, which indicates the reduction
of Au(III) to Au(0). The solution was continuously stirred for another
15 min. The Fc-PAA-AuNPs obtained were stored at 5 °C in refrigerator.
4
-AP. In addition, the fabricated electrochemical sensor has shown a
broad linear range, high selectivity and good reproducibility.
2. Experimental section
2.5. Fabrication of Fc-PAA-AuNPs/GCE
2.1. Chemicals and reagents
Initially, the GCE was carefully polished until free from any physi-
cally adsorbed materials to mirror-like surface with 1.00, 0.30 and
Ferrocene carboxaldehyde (98%), methyl acrylate (99%), ethylene
2
0.05 μm alumina slurries and sonicated in an EtOH/H O mixture (1:1)
diamine (99.5%), gold(III) chloride trihydrate (99.9%) were procured
and then dried under the stream of N . Different volumes of Fc-PAA-
2
2

M. Elancheziyan and S. Senthilkumar
Journal of Molecular Liquids 325 (2021) 115131
Fig. 1. Scheme for (A) Synthesis of ferrocene terminated PAA dendrimer and (B) Synthesis of Fc-PAA encapsulated with gold nanoparticles.
AuNPs (in 1 μL increment) have been immobilized by dropcasting on
the surface of GCE and the cyclic voltammetric response was recorded.
The current response was found to increase up to 5 μL of Fc-PAA-
AuNPs. Slightly higher currents were obtained at higher volumes,
whereas the CV response was unstable. Hence, 5 μL of Fc-PAA-AuNPs
was chosen as the optimum amount for the fabrication of Fc-PAA-
AuNPs/GCE. Subsequently, 5 μL of Fc-PAA-AuNPs was dropcasted on
the surface of a GCE and dried at room temperature to form Fc-PAA-
AuNPs/GCE. Similarly, Fc-PAA/GCE and PAA/GCE were also fabricated
for comparison.
2
stretching band of terminal free amine group (−NH ) was found to dis-
appear. These observations ensure the covalent attachment of ferrocene
carboxaldehyde to amine terminated PAA dendrimer to form Fc-PAA.
Additionally, new sharp bands appeared at 823 and 485 cm− which
were found to be similar to that of C-H bending and Fe-C out of the
plane bending vibration of Fc rings, reported for a covalently
immobilized on silane [43]. These two new bands further affirm that
ferrocene has been attached to the PAA dendritic network. Fig. 2B de-
picts the UV–vis spectra of synthesized PAA dendrimer, Fc-PAA dendri-
mer and Fc-PAA-AuNPs recorded in ethanol. Typically, the PAA
dendrimer, Fc-PAA dendrimer and Fc-PAA-AuNPs exhibited two ab-
sorption bands at 217 and 285 nm, similar to that observed for π → π*
transition (at 250 and 275 nm) in the -C=O group present in dendritic
network reported earlier [56]. On the other hand, Fc-PAA and Fc-PAA-
AuNPs showed a strong peak around 365 nm. The observed peak is in
agreement with literature report on Fc terminated dendrimer [56] and
thus ascribed to the charge transfer band in ferrocene unit. This further
witnesses the attachment of ferrocene to the dendrimer network. A
broad band observed at 535 nm for Fc-PAA-AuNPs, which is in accor-
dance with the surface plasmon resonance (SPR) of AuNPs entrapped
in PAA network reported earlier [57]. Hence, this SPR band at 535 nm
evidences the encapsulation of AuNPs in the Fc-PAA dendrimer net-
work. Fig. 2C displays the transmission electron micrograph of a gold
nanoparticles encapsulated Fc-PAA dendritic network, which clearly
shows the formation of spherical shaped AuNPs with particle sizes rang-
ing from 5 to 8 nm. Further, the EDX spectra of the Fc-PAA-AuNPs exhib-
ited peak at 2.15 keV (Fig. 2D) indicating the presence of AuNPs.
1
3. Results and discussion
3.1. Characterization of PAA, Fc-PAA and Fc-PAA-AuNPs dendrimers
The FTIR spectra of synthesized PAA dendrimer (pink) and ferrocene
terminated PAA dendrimer (red) are shown in Fig. 2A. The spectrum of
−1
PAA dendrimer shows a sharp intense band at 1636 cm and the value
is in agreement with that reported in literatures [51,52] for -C=O
−1
stretching band in the PAA network. Thus, this band at 1636 cm has
been assigned to the stretching vibration of -C=O groups in the PAA
dendritic network. The strong absorption bands appeared at 1546 and
−1
3
282 cm were in accordance with the bending and stretching vibra-
tion of N-H groups, as reported in literature [53] and this indicated the
presence of primary amine groups in PAA dendrimer. Further, a new
−1
sharp peak appeared at 1243 cm , which is similar to that observed
for C-N stretching vibration in literature [54,55] and also the N-H
3

M. Elancheziyan and S. Senthilkumar
Journal of Molecular Liquids 325 (2021) 115131
Fig. 2. (A) FTIR spectra of PAA (brown) and Fc-PAA (red). (B) UV–Vis spectra of PAA, Fc-PAA and Fc-PAA-AuNPs (blue). (C) Transmission electron micrograph of Fc-PAA-AuNPs. (D) Energy
dispersive X-ray (EDX) analysis of Fc-PAA-AuNPs/GCE.
]4 and 2.5 mM [Fe(CN)
−
]3−at various modified electrodes in 0.1 M KCl. Inset: Proposed equivalent circuit (B) Cyclic voltammograms of 0.1 M phosphate
Fig. 3. (A) EIS of 2.5 mM [Fe(CN)
6
6
−1
buffer (pH 7.0) at a bare GCE (black), PAA/GCE (pink), Fc-PAA/GCE (red) and Fc-PAA-AuNPs/GCE (blue) at a scan rate of 50 mV s
.
4

M. Elancheziyan and S. Senthilkumar
Journal of Molecular Liquids 325 (2021) 115131
3
.2. Electrochemical behaviour of Fc-PAA-AuNPs/GCE modified electrode
Fc-PAA-AuNPs/GCE, respectively. It becomes obvious that the incorpo-
ration of AuNPs in the electrode setup has considerably enhanced the
electroactive surface area.
The electrochemical behaviour of the assembled electrode was in-
vestigated by electrochemical impedance spectroscopy (EIS) and cyclic
voltammetry (CV). EIS gives substantial information regarding the
impedimetric changes that occur between the electrode and electrolyte
interface during the electrode assembly process. EIS was recorded at an
open circuit potential of 0.2 V, over the frequency range of 1 mHz to
To probe the effect of scan rate on the redox reaction of Fc at Fc-PAA-
AuNPs/GCE, cyclic voltammetry of 0.1 M phosphate buffer (pH 7.0) was
performed by increasing the scan rate from 10 to 200 mV s− . The re-
sults are shown in Fig. S1. The oxidation and reduction peak currents
of Fc-PAA-AuNPs/GCE progressively increased with increase in scan
rate. Both the peak currents varied linearly with the square root of
scan rate (Fig. S2), denoting a diffusion controlled redox process [42].
Thereafter, cyclic voltammetry of 0.1 M phosphate buffer of increasing
pH from 4.0 to 10.0 was conducted to study the effect of pH and the ob-
tained results are shown in Fig. S3. On increasing the pH from 4.0 to 7.0,
the oxidation peak current at the Fc-PAA-AuNPs/GCE also increased
gradually and on further increasing the pH beyond 7.0, the peak current
at Fc-PAA-AuNPs/GCE began to decrease. The maximum current re-
sponse was achieved at pH 7.0 and hence, this pH was chosen as the op-
timum pH for further electrocatalytic applications.
1
1
00 mHz with an AC amplitude of 0.01 V. Fig. 3A portrays the EIS spectra
4−
3−
of 2.5 mM [Fe(CN)
6
]
and 2.5 mM [Fe(CN)
6
]
at a bare GCE (black), a
PAA/GCE (pink), a Fc-PAA/GCE (red) and a Fc-PAA-AuNPs/GCE (blue) in
.1 M KCl. An equivalent circuit (inset to Fig. 3A) consisting of a solution
resistance (R ) in series with the parallel combination of a constant
0
s
phase element (CPE) and the impedance of the faradaic reaction was
used to fit the experimental EIS results. The impedance of the faradaic
reaction includes a charge transfer resistance (Rct) in series with War-
burg impedance (W). EIS spectra of bare GCE displays an Rct (corre-
sponding to the diameter of a fitted semicircle) of 559 Ω, indicative of
feeble electron transfer process on the surface of bare electrode. After
immobilization of PAA dendrimer on bare GCE, the Rct value has
reduced to 385 Ω, indicating that PAA dendrimer improves electron
transfer ability [58]. On the other hand, immobilization of Fc-PAA on
GCE (Fc-PAA/GCE) significantly reduces the Rct to 252 Ω due to the in-
clusion of Fc on the dendritic network, which improved electron shut-
tling due to the redox nature of the Fc mediator. Further,
encapsulation of AuNPs on the Fc-PAA dendritic network dramatically
reduces the Rct to a lowest value of 145 Ω, signifying the highly
conducting behaviour of the newly developed Fc-PAA-AuNPs/GCE. The
increased conductivity of the fabricated sensor could be due to the cova-
lent anchoring of highly redox active Fc as well as the effective encapsu-
lation of AuNPs on the dendritic network, which augmented the
electron transfer process.
3.3. Electrocatalytic oxidation of 4-AP at Fc-PAA-AuNPs/GCE
The electrocatalytic oxidation of 4-AP at a Fc-PAA/GCE and a Fc-PAA-
AuNPs/GCE in 0.1 M phosphate buffer (pH 7.0) was investigated by cy-
−1
clic voltammetry at a scan rate of 50 mV s . The results obtained are
depicted in Fig. 4A and B, respectively. Initially, cyclic voltammetry of
0.5 mM 4-AP at a bare GCE and a PAA/GCE was performed and the re-
sults obtained are presented in the inset to Fig. 4A and Fig. 4B, respec-
tively. It is evident that both bare GCE and PAA/GCE have shown a low
oxidation peak current for the oxidation of 4-AP.
Interestingly, Fc-PAA/GCE has shown good electrocatalytic activity
upon sequential addition of 0.1 mL aliquots of 0.01 M 4-AP, as evidenced
by the increasingly larger oxidation peak at 0.420 V in the cyclic voltam-
mograms in both Fig. 4A and Fig. 4B. These results indicate that the
modified electrode has the ability to electrocatalytically oxidise 4-AP.
Further, inclusion of AuNPs into the Fc-PAA dendritic network (Fc-
PAA-AuNPs/GCE) yielded a 188% increase in the oxidation current of
4-AP compared to that at a Fc-PAA/GCE. On applying the oxidation po-
tential of 0.427 V, Fc in the Fc-PAA-AuNPs/GCE was electrochemically
Fig. 3B illustrates the cyclic voltammograms of 0.1 M phosphate
buffer (pH 7.0) at a bare GCE (black), PAA/GCE (pink), Fc-PAA/GCE
(
red) and Fc-PAA-AuNPs/GCE (blue) at a scan rate of 50 mV s−1. As an-
ticipated, the bare and PAA/GCE did not show any redox response due to
their non-redox nature within the given potential window. After the co-
valent attachment of ferrocene on PAA dendrimer (Fc-PAA/GCE), a set
of distinct redox peak at 0.416 V (anodic, Epa) and 0.328 V (cathodic,
+
oxidised to ferrocenium ion (Fc ), which in turn chemically oxidizes
E
pc) with the respective formal potential (E°′) and peak-to-peak separa-
tion (ΔE ) of 0.372 V and 88 mV was observed. The obtained redox
peaks are in accordance with the literature reports on immobilized Fc
45,59], which correspond to the characteristic ferrocene/ferrocenium
the target analyte 4-AP to quinoneimine and gets reduced to Fc. The
+
p
electrochemical oxidation of Fc to Fc and the chemical oxidation of
+
4-AP by Fc , occur in a cycle as long as the analyte is available at the
[
electrode surface, which result in an increased current response for
every addition of 4-AP. Noticeably, this mediated electrocatalytic oxida-
tion of 4-AP occurred at a moderately low onset potential with a larger
oxidation current at the Fc-PAA-AuNPs/GCE than at the Fc-PAA/GCE.
The increased current response for Fc-PAA-AuNPs/GCE could be again
due to the incorporation of a large number of AuNPs in the highly
branched Fc-PAA dendritic network, leading to a 23% increase in the
electrode surface area, as well as sensitivity by improving the conductiv-
ity of the developed sensor.
Fig. 4C depicts the amperometric response of 0.45 V in the presence
of increasing 4-AP concentration at a bare GCE (black), a Fc-PAA/GCE
(red) and a Fc-PAA-AuNPs/GCE (blue) modified electrodes in a contin-
uously stirred 0.1 M phosphate buffer (pH 7.0). Initially, the operating
potential for the amperometric measurements has been optimized by
performing experiments at various potentials from 0.40 V to 0.55 V
(Fig. S4). The response current towards the detection of 4-AP at Fc-
PAA-AuNPs/GCE was found to increase with the working potentials up
to 0.45 V. While a marginally improved response was attained even be-
yond 0.45 V, the increase in current was non-linear at higher potentials
and thus we have chosen 0.45 V as optimum potential for the detection
of 4-AP. It can be observed that the bare GCE displays a small and non-
linear current response after each addition of 4-AP. On the other hand,
Fc-PAA/GCE and Fc-PAA-AuNPs/GCE showed a rapid increase in current
after successive addition of 4-AP, which stabilized within 3 s, indicating
+
(
Fc/Fc ) redox couple. This further confirms that the ferrocene moieties
have been successfully attached to the PAA dendritic network. Further,
encapsulation of AuNPs in the Fc-PAA dendritic network (Fc-PAA-
AuNPs/GCE) has exhibited a similar redox behaviour with an E°’and
p
ΔE of 0.386 V and 82 mV, respectively. Distinct increase in the peak cur-
rents of Fc-PAA-AuNPs/GCE could be due to the encapsulation of a large
number of AuNPs within the dendritic network, which improved their
surface area and conductivity with Fc synergistically. This electrode
setup with integration of redox-active ferrocene and highly conductive
AuNPs in the dendrimer network could be effectively utilized in electro-
chemical sensing.
The electroactive surface area of Fc-PAA/GCE and Fc-PAA-AuNPs/
GCE has been estimated using Randles-Sevick Eq. (1).
ꢀ
ꢁ
5
n3=2AD1=2Cv1=2
Ip ¼ 2:69 ꢀ 10
ð1Þ
p
where I is the peak current (A), n is the number of electrons
involved in the redox reaction, A is the electroactive surface area
of the modified electrode (cm ), D is the diffusion coefficient
2
−6
2 −1
(
4 6
6.2 × 10 cm s for 1 mM K [Fe(CN)] in 0.1 M KCl) [60], C is the
−3
−1
concentration (mol cm ) and v is scan rate (V s ). The surface areas
were determined to be 0.077 and 0.095 cm for Fc-PAA/GCE and
2
5

M. Elancheziyan and S. Senthilkumar
Journal of Molecular Liquids 325 (2021) 115131
Fig. 4. Cyclic voltammograms obtained at (A) Fc-PAA/GCE and (B) Fc-PAA-AuNPs/GCE after sequential addition of 0.1 mM 4-AP to 0.1 M phosphate buffer (pH 7.0). Inset to Fig. 4A & B:
Cyclic voltammograms of 0.0 and 0.5 mM 4-AP at a bare GCE and PAA/GCE. (C) Amperometric i-t response of Fc-PAA-AuNPs/GCE (blue), Fc-PAA/GCE (red) and bare GCE (black) for
different concentration of 4-AP in 0.1 M phosphate buffer (pH 7.0) at +0.45 V. (D) Corresponding calibration plot for detection of 4-AP.
1
−1
the superior electrocatalytic behaviour of the newly assembled elec-
trodes. Fig. 4D displays the calibration plot for the determination of
better sensitivities of 87.8 μA mM− cm−2 and 72.2 μA mM cm−2
,
−1
−2
whereas the bare GCE exhibited a low sensitivity of 35.9 μA mM cm
.
4
-AP, wherein the sensitivity was estimated from the slope (m) of the
calibration plot and LOD (S/N = 3) was calculated using the formula
σ/m, where ‘σ’ denotes the standard deviation of the blank. Fc-PAA-
The higher current response observed with Fc-PAA-AuNPs/GCE could
be attributed to the inclusion of AuNPs into the dendritic network,
which improved their surface area and enhanced their electronic con-
ductivity. Moreover, Fc-PAA-AuNPs/GCE showed a wide linear range
3
AuNPs/GCE and Fc-PAA/GCE showed linear current response with
Table 1
Comparison of the performances of different 4-AP electrochemical sensors.
Electrodes
Methods
Linear range (μM)
Sensitivity
LOD (μM)
Ref.
μA mM− cm−2
1
Pt/ZnO/ITO
AuPt/ZnO/ITO
G-PANI/CPE
CS/Au/Pd/rGO/GCE
ZnO/NPC/GCE
AP
AP
AP
DPV
AP
-
-
12.55
14.31
-
4.11
3.60
15.68
0.12
0.014
0.023
0.05
0.1
0.01
0.32
0.013
7.61
[61]
[61]
[7]
[8]
[9]
[13]
[62]
[25]
[63]
[64]
[20]
This work
50–500
1–300
5–120
0.4–200
0.04–17
0.5–1.6
0.2–200
0.5–400
0.05–20
30–1064
-
31.02
27.2
4.278
35.84
14.60
-
RGO/P-
L
-GSH/GCE
AP
MoS /GCE
2
DPV
DPV
DPV
DPV
DPV
AP
CuO-Au/MWCNTs/GCE
ZIF-67/MWCNT-COOH/Nf/GCE
VMSF/ITO
MoS
2
@NHCSs/GCE
-
Fc-PAA-AuNPs/GCE
87.8
ITO – Indium tin oxide, PANI – Polyaniline, CPE – Carbon paste electrode, rGO/RGO – Reduced graphene oxide, NPC – Nitrogen doped porous carbons, P-
L
-GSH – poly-L-glutathione, ZIF-
67 – Zeolitic imidazolate framework −67, VMSF – Vertically-ordered mesoporous silica-nanochannel films.
6

M. Elancheziyan and S. Senthilkumar
Journal of Molecular Liquids 325 (2021) 115131
for the electrocatalytic determination of 4-AP from 30 to 1064 μM with a
term durability of Fc-PAA-AuNPs/GCE was tested for 30 long days at
regular interval of time in the absence and presence of 0.4 mM 4-AP
(Inset to Fig. 5A). These studies revealed that the fabricated Fc-PAA-
AuNPs/GCE retains 97.8% and 93.6% of its initial response in the absence
and presence of 4-AP, respectively at the end of 30 days. The excellent
stability as well as the good durability of Fc-PAA-AuNPs/GCE could be
due to the stable covalent anchoring of Fc on the PAA dendritic network,
which significantly improved the performance of the sensor. Addition-
ally, the reproducibility of the developed sensor was examined with
five exclusively fabricated electrodes (using same) the protocol and
evaluated their response with and without 0.4 mM 4-AP (Fig. 5B). The
relative standard deviation (RSD) of the current response was found
to be 2.83% in the presence of 4-AP and 1.78% in the absence of 4-AP, in-
dicating the excellent reproducibility among the individually developed
sensors. Further, the repeatability of the developed Fc-PAA-AuNPs/GCE
modified electrode was tested using cyclic voltammetry in the presence
of 0.4 mM of 4-AP (5 times) with the same modified electrode (Fig. S5).
The RSD was found to be 2.13%, which suggested good repeatability of
the developed modified electrode. The prolonged storage and cyclic sta-
bility along with excellent reproducibility of Fc-PAA-AuNPs/GCE indi-
cate that it is promising sensor for the detection 4-AP.
high sensitivity and low limit of detection (LOD). The sensitivity and
−1
−2
LOD were found to be 87.8 μA mM cm and 7.61 μM, respectively.
Table 1 compares the obtained parameters such as linear range, LOD
and sensitivity of the proposed electrochemical sensor with the recently
published 4-AP sensor and it is found to be comparable or better than
the previous reports. The improved performances in terms of current
response and rapidity of Fc-PAA-AuNPs/GCE could be attributed to the
covalently attached Fc as well as encapsulation of AuNPs in the PAA net-
work. The covalent attachment of Fc would have afforded enhanced
electrical wiring for better electron transport and the AuNPs encapsu-
lated in the highly branched dendritic network is expected to provide
increased surface area and conductivity and thus resulting in better
sensing performance.
3.4. Selectivity, stability and reproducibility of Fc-PAA-AuNPs/GCE
Stability of the fabricated sensor was evaluated by continuous po-
tential cycling at a scan rate of 50 mV s−1 in 0.1 M phosphate buffer
(
0
pH 7.0). Fig. 5A displays 50 consecutive cyclic voltammograms of
.1 M phosphate buffer at the Fc-PAA-AuNPs/GCE. Evidently, there
were no obvious changes in the peak behaviour with respect to peak
current and peak potential in these repeated scans. Furthermore, long
Further, the selectivity is an imperative parameter for the any devel-
oped sensors in real time monitoring and other practical utilizations. To
Fig. 5. (A) Fifty consecutive cyclic voltammograms of 0.1 M phosphate buffer at a Fc-PAA-AuNPs/GCE at a scan rate of 50 mV s−1. Inset: Day to day current response of the fabricated sensor
in the absence and presence of 0.4 mM 4-AP for 30 days. (B) Current response obtained at five individually fabricated sensors with and without 0.4 mM 4-AP. (C) Amperometric response
obtained at Fc-PAA-AuNPs/GCE for each 0.1 mM 4-AP in the presences of 1 mM of various interferents. (D) Current response columnar diagram of interference studies. Electrolyte: 0.1 M
phosphate buffer (pH 7.0).
7

M. Elancheziyan and S. Senthilkumar
Journal of Molecular Liquids 325 (2021) 115131
Table 2
Declaration of Competing Interest
Determination of 4-AP spiked in water samples.
Samples
Spiked (μM)
Found (μM)
Recovery (%)
RSDa (%)
The authors declare that there is no conflict of interest.
Lake water
75
73.08
97.4
2.55
2.30
1.85
1.20
Acknowledgement
1
50
100
00
154.61
101.97
200.06
103.1
102.0
100.0
Tap water
2
This work was financially supported by the Science and Engineering
Research Board (SERB), Government of India (Sanction No. SB/FT/CS-
a
Three replicates were performed.
0
78/2014).
evaluate the effect of interference at the Fc-PAA-AuNPs/GCE,
amperometry was performed (Fig. 5C) in the presence of 0.1 mM of
Appendix A. Supplementary data
4
-AP with 10 times excess concentrations of the coexisting species at
an operating potential of 0.45 V and their current response is presented
in Fig. 5D. Reagents that are commonly found in pharmaceutical indus-
tries and environmental samples including glucose (Glu), dopamine
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2020.115131.
(
DA), ascorbic acid (AA), uric acid (UA), 2-nitrophenol (2-NP), 2-
aminophenol (2-AP), catechol (CC), paracetamol (PA), chlorophenol
CP) and quercetin (QR) were chosen for this investigation. It is ob-
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