Detection of 3,4-diaminotoluene based on Sr0.3Pb0.7TiO3/CoFe2O4 core/shell nanocomposite: Via an electrochemical approach

Article abstract of DOI:10.1039/d0nj01074j

Control of the sol-gel shell coating is important in the development of core-shell magnetic nanocomposites. Herein, we explored a scalable sol-gel method for the preparation of an Sr_0.3Pb_0.7TiO₃/CoFe₂O₄ core-shell magnetic nanocomposite (SPT/CFO MNc) with a finely controlled shell and evaluated its efficiency as an electrochemical sensor. Firstly, CoFe₂O₄ nanoparticles were obtained via a citrate sol-gel method and sintered at 700 °C. Subsequently, Sr_0.3Pb_0.7TiO₃ was applied to the CoFe₂O₄ nanoparticles to allow the formation of shells. The X-ray diffraction results indicated that the core nanoparticles have a cubic CoFe₂O₄ spinel structure and high-resolution transmission electron microscopy and scanning electron microscopy confirmed the successful formation of a uniform and thin Sr_0.3Pb_0.7TiO₃ shell. The magnetic hysteresis loops confirmed the ferromagnetic nature of the as-prepared magnetic nanocomposite, which exhibited a saturation magnetization of 4 emu g^-1 and coercive field of 600 Oe. An electrochemical sensor selective toward 3,4-diaminotoluene was fabricated by coating the synthesized Sr_0.3Pb_0.7TiO₃/CoFe₂O₄ core-shell magnetic nanocomposite onto a glassy carbon electrode. Detailed experimental analyses were performed to assess the analytical parameters of the proposed sensor. The calibration curve of 3,4-diaminotoluene was obtained by plotting the linear relation of current versus 3,4-DAT concentration. According to the slope of the calibration curve, the sensor sensitivity was calculated to be 24.3323 μA μM^-1 cm^-2 by considering the surface area of the glassy carbon electrode (GCE: 0.0316 cm²). The linear dynamic range was estimated by considering the linear part of the calibration curve, which was found to be 0.1 nM-0.01 mM (linear dynamic range). Based on the signal-to-noise ratio of 3, the detection limit (96.09 ± 4.80 pM) and limit of quantification (320.3 pM) were calculated. Furthermore, effective and satisfactory results were obtained in the analysis of environmental samples.

Full text of DOI:10.1039/d0nj01074j

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Detection of 3,4-diaminotoluene based on Sr0.3pb0.7TiO3/CoFe2O4 core/shell
nanocomposite by the electrochemical approach
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Ali B. Abou Hammad1, Amir Elzwawy2, A. M. Mansour1, M.M. Alam3, Abdullah M. Asiri4,
Mohammad Razaul Karim4, Mohammed M. Rahman4*, Amany M. El Nahrawy1*
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1Solid State Physics Department, Physics research division, National Research Centre, 12622
El-Bohouth Str., Cairo, Egypt
2Ceramics Department, National Research Centre, 12622 El-Bohouth Str., Cairo, Egypt
3Department of Chemical Engineering and Polymer Science, Shahjalal University of Science and
Technology, Sylhet 3100, Bangladesh
4Center of Excellence for Advanced Materials Research and Chemistry Department, Faculty of
Science, King Abdulaziz University, Jeddah 21589, P.O. Box 80203, Saudi Arabia
*Corresponding Authors:
Dr. M.M. Rahman (E-mail: mmrahman@kau.edu.sa; Tel. +966-596421830)
Dr. Amany M. Elnahrawy (E-mail: amany_physics_1980@yahoo.com)
Abstract
The control of the sol-gel coating shell is important in developing the applicability of core-shell
magnetic nanocomposites. Herein, we explore the scalable sol-gel for preparing
Sr0.3pb0.7TiO3/CoFe2O4 core-shell magnetic nanocomposite (SPT/CFO MNc) with finely
controlled shell and regard their efficiency as an electrochemical sensor. Firstly, CoFe2O4
nanoparticles derived from the citrate sol-gel method and sintered at 700oC. Sr0.3pb0.7TiO3 shells
were applied to CoFe2O4 nanoparticles to allow the formation of adherence shells. X-ray
diffraction implies the cubic CoFe2O4 spinel structure of the core nanoparticles and the high-
resolution transmission electron microscope and scanning electron microscope observation
confirmed the successful formation of uniform and finely Sr0.3pb0.7TiO3 shell. Magnetic
hysteresis loops confirmed the ferromagnetic nature of the prepared magnetic nanocomposite
owing a saturation magnetization of 4 emu/g and a 600 Oe coercive field. The electrochemical
sensor selective toward 3,4-diaminotoluene was fabricated by the coating of synthesized
Sr0.3pb0.7TiO3/CoFe2O4 core-shell magnetic nanocomposite onto a glassy carbon electrode. The
detailed experimental analyses were performed to assess the sensor analytical parameters of the
proposed sensor. The calibration curve of the 3,4-diaminotoluene was formed by plotting a linear
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relation of current versus 3,4-DAT concentration. From the slope of the calibration curve, the
sensor sensitivity (24.3323 µAµM-1cm-2) by considering the surface area of glassy carbon
electrode (GCE: 0.0316 cm2) was calculated. The linear dynamic range was estimated by
considering the linear part on the calibration curve and was found as 0.1nM0.01mM (Linear
Dynamic Range). From the signal to noise ratio of 3, the detection limit (96.09±4.80 pM) and
limit of quantification (320.3 pM) were calculated. Aside from this, it was effective and
satisfactory results were obtained in analyzing the environmental samples.
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Keywords: Multiferroic magnetoelectric; Sol-gel CoFe2O4; PbTiO3 nanocomposite; 3,4-diamine
toluene sensor; Glassy carbon electrode; Environmental safety
1. Introduction
Multiferroic magnetoelectric materials have attracted increasing consideration because of the
coexistence of ferromagnetism and ferroelectricity into a single domain and open a
multifunctional way for emergent device applications. CoFe2O4 (CFO) nanoparticles are a
promising magnetic structure because of their exclusive magnetic properties such as high
coercive force, strong anisotropy, chemical stabilization, reasonable saturation magnetization,
and mechanical coherence 1,2. Coupling could, in general, allow data to be written electrically
and read magnetically 3–5. The doping with different spinel structured magnetite compounds in
ferroelectric materials exhibits interesting properties depending upon different parameters, such
as the selection of material, composition, better stoichiometric ratio, size distribution, oxygen
vacancies, and synthesis techniques 6,7
.
Particularly, the ferrite-composites are promising candidates for progressing permanent magnetic
structures, due to their high Curie temperature, lower cost, high electrical resistance, excellent
corrosion resistance, and inherent magnetic property 8,9. A variety of ferroelectric specifications
can be justified by substituting the lead or titanium cations with rare-earth and transition-metal
ions. Generally, the effect of transition dopants on PbTiO3, such as Fe3+, resulting in the creation
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of oxygen vacancies (V0 ) for charge compensation, which is predicted to possess a major impact
on the properties and the performance of ferroelectric complexes 9–12. SrTiO3 is a cubic
paraelectric material with a perovskite structure. A polycrystalline samples of the PbSrTiO3
ceramic system were investigated and established a complete series of solid solutions from
PbTiO3 with Curie temperature of Tc= 763 K to SrTiO3 with Curie temperature of Tc = 20 K 13.
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These results suggest that the doping of Sr in PbTiO3 reduces the phase transition temperature as
well as lattice distortion (c/a), It was also observed that the (PbSr)TiO3 compound forms a
continuous solid solution system with reduced phase transition temperature as well as grain size
with Sr14. The decrease in grain size with increasing Sr content may be referred to slower grain-
growth rates due to a slower Sr2+ diffusion with Pb2+ ion 15.
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The small grains have elevated resistivity and diminished leakage current within the specimen,
which improves the electrical as well as magnetic order in the magnetoelectrically coupled
system. The mixed oxides of transition and semi-conductive nanostructured metal with various
shapes and sizes have versatile industrial and technological applications 16,17. The properties of
nanoparticles such as optical, magnetic, electronic and catalytic depend on the size and shape.
The nanoparticles show excellent performances compared to the bulk materials. It occurs due to
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the change of band structure of nanomaterial of semi-conductive and transition metals
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Therefore, the nanostructured mixed metal oxides due to the coexistence of different phases in
the same matric open the new windows for technological development 20,21
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Generally, a chemical sensing quest have been investigated with the metal oxide nanostructures
for the detection of different chemicals such as hydrazine, methanol, chloroform,
dichloromethane, acetone, ethanol etc, which are not environmental friendly. The sensing
mechanism with doped or un-doped metal oxides thin films utilized mainly the properties of
porous film formed by the physi-sorption and chemisorptions techniques. The chemical detection
is based on the current changes of the fabricated thin films caused by the chemical components
of the reacting system in aqueous medium.22–28 The main efforts are focused on detecting the
minimum quantity aqueous ammonia necessary for the fabricated doped sensors for
electrochemical investigation. Nanomaterials offer many opportunities of tuning the chemical
sensing properties. The health hazardous toxins are produced by living organisms, the toxic
chemicals from various industrial activities as by-products or raw materials are responsible for
the contamination of the environment. In the end, theses toxins affect humans, animals and living
organisms directly or indirectly 29. The amine 3,4-DAT used broadly in dye and pigment
formulation, is a savior toxin for the environment 30. The other important industrial uses of 3,4-
DAT are fungicide stabilizers, hydraulic fluids, antioxidants, impact resins, polyamides,
polyurethane foams and photographic developers 31. It has been reported that 3,4-DAT has
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strong enough toxicity to generate cancer in the liver and female mice . Thus, 3,4-DAT has
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been listed as a priority carcinogenic substance by the NCL (National Cancer Institute) and
IARC (International Agency of Research on Cancer). A reliable detection method for 3,4-DAT is
needed to save the human, animals, and organisms. Very recently, a simple and reliable
technique based on the electrochemical method in the electrochemical approach has been
introduced to detect numerous chemicals using the mixed-metal oxides of transition and semi-
conductive metals 30,33–39. The purpose of the present work is to expand the applicability of
CoFe2O4 ferrite nanoparticles through their incorporation in perovskite nanocomposite systems
that are prepared via the advanced sol-gel method to achieve superior magnetic, electrical
property, and exchange spring magnet. The structural, bandgap and magnetic properties of
Sr0.3pb0.7TiO3/CoFe2O4 magnetic nanocomposite are investigated.
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In this study, an electrochemical sensor probe was fabricated with glassy carbon electrode (GCE)
using prepared core-shell Sr0.3pb0.7TiO3/CoFe2O4 magnetic nanocomposite (SPT/CFO MNc) by
electrochemical approach. The stability of the sensing layer of Sr0.3pb0.7TiO3/CoFe2O4 MNc onto
GCE was enhanced by the addition of 5% Nafion. Even after several used, the fabricated
materials is still remained in the electrode surface. By detailed investigation for detection of 3,4-
Diaminotoluene (3,4-DAT) in phosphate buffer medium, the sensor parameters of the proposed
MNc sensor are evaluated such as the linear dynamic range (LDR), sensitivity, response time,
DL, and reproducibility and found as reliable and efficient on the detection of the target analyte.
This method is used to develop the electrochemical sensor by using a mixture of MNc, which is
very efficient in the detection of 3,4-DAT. It is introduced a new route with semi-conductor
materials for the safety of healthcare and environmental fields.
2. Experimental section
2.1. Materials and methods
Titanium (IV) isopropoxide (Ti[OCH(CH3)2]4; Sigma), Lead acetate trihydrate (Pb(CH3CO2)2 ·
3H2O; Merck), Strontium(II) nitrate (Sr(NO3)2-PubChem), citric acid (CA, 99.5%, Merck),
acetylacetone (CH3COCH2COCH3, (AA)) and ethanol absolute (EtOH). The environmental toxic
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chemicals in the form of analytical grades such as 3,4-diamine toluene (3,4-DAT), 4-
aminophenol (4-AP), Zimtaldehyde, 4-nitrophenol (4-NP), chlorzoxazone, 2,4-diaminophenol
(2,4-DAP), phenylhydrazine (PHyd), 3-chlorophenol (3-CP), toluene and acetonitrile were
obtained from Sigma-Andrich. The other necessary chemicals monosodium, disodium phosphate
buffer and Nafion (commercially available as 5% suspension in ethanol) were procured from the
similar Sigma-Andrich. The Keithley electrometer, which is the main source of potential (Volt)
was collected from the main company in the USA. Copper acetate (C4H6CuO4; Sigma) and Iron
Nitrate (Fe (NO3)3 · 9H2O; Sigma).
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The crystalline structure of the core (CFO) and core-shell SPT/CFO magnetic nanocomposite
was described using X-ray [Bruker- D8 advance diffractometer (XRD)] by the mono-
chromatized CuK radiation with wavelength; λ= 1.54056 Å in operation conditions 40 mA,
step size 0.05/1s and 40 kV. The reality of the surface morphology micrographs of the as-
prepared magnetic nano-powder was assessed by employing the Field emission-scanning
electron microscopy (FE-SEM; model JEM - 1230). The particle size of the magnetic
nanocomposite sample was detected using the HR-TEM (JEM-2100; Jeol-Japan). The Scanning
electron microscopy is measured using (Hitachi/S-4800, Japan) instrument with an image
resolution of 1nm at 15kV and a separate unit for Pt deposition of 10 nm thickness. X-ray
photoelectron spectroscopy was performed using (ESCALAB 250Xi, Thermo scientific, U.K.).
The obtained band gap (Eg) for the prepared sample, diffuse-reflectance (DR) data were
measured by (Jasco V-570; USA) spectrophotometer within the wavelength range (0.2–2.5 µm).
Magnetic hysteresis loops were attained by a vibrating sample magnetometer (VSM, model
7407, Lake Shore Cryotronics, Inc., USA) at room temperature.
2.2. Preparation of core-shell SPT/CFO magnetic nanocomposite
First step: CoFe2O4 (CFO) fine magnetic particles were prepared at a lower temperature by
citrate sol-gel reactions. The basic solution was prepared using a ferric nitrate and cobalt nitrate
with the ratio 2:1 mixed in distilled water/EtOH/citric acid as solvents and catalytic under
magnetic stirring for 2h at 70oC. The resultant mixture solution was dried at 200oC for 10 h to
form the xerogel CFO. The xerogel CFO was then sintered at the 700oC for 3 h to produce CFO
nanoparticles. In the second step: the evaluated stoichiometry for titanium (IV) isopropoxide,
lead acetate trihydrate, strontium (II) nitrate (AA) and EtOH were used for the sol-gel
preparation of SPT sol. Then the CFO is introduced in SPT sol and stirred for 2h, then aged for 3
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days to support the highly cross-linkages process for core-shell SPT/CFO magnetic
nanocomposite surface. The core-shell mixture was kept in a drying oven at 100oC to evaporate
different residual solvents. After this step, the resulting powder was sintered at 800oC.
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2.3. Fabrication of sensor using GCE and SPT/CFO MNc
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The working electrode of the desired 3,4-DAT sensor is the most significant element and was
fabricated using GCE. To fabricate the working electrode, the GCE was coated as a thin layer of
the film by the slurry of synthesized SPT/CFO MNc in ethanol and allowed to keep at the
ambient condition of the room to dry. Then, a drop of Nafion was added on the modified GCE to
boost the stability of the SPT/CFO MNc layer on GCE as per this study requirement. After
obtaining, the optimum dryness of the recently modified GCE, it was positioned inside an oven
at 35oC for an hour. For the assembling of the desired electrochemical sensor, SPT/CFO
MNc/Nafion/GCE and a simple Pt-wire were connected in series with the Keithley electrometer.
The selective 3,4-DAT with concentration ranging from 0.1nM to 0.1mM was subjected to
analysis by the assembled 3,4-DAT chemical sensor and analyzed data were used to form a plot
chart of current versus concentration labeled as the calibration curve. By identifying the
maximum linear segment on the calibration curve, the linear dynamic range (LDR) was
recognized. Using the slope of the calibration curve, the sensitivity of the sensor was calculated.
Applying the signal-to-noise ratio at 3, the lower limit of detection (DL) was estimated.
Equimolar concentrations of disodium phosphate and monosodium were mixed to form the
phosphate buffer medium. During the electrochemical investigation, the buffer solution in the
inspection beaker was taken 10.0 mL as constant throughout the complete study.
3.
Results and discussions:
3.1. Structural analysis of core-shell SPT/CFO MNc
The X-ray diffraction patterns of CFO NPs and Sr0.3pb0.7TiO3/CoFe2O4 core-shell magnetic
nanocomposite calcined at temperature 800oC are given in Fig. 1(a, b). For the core material, i.e.
CoFe2O4 in Fig. 1 (a), the diffraction peaks appeared at 2θ = 18.29o, 30.07o, 35.49o, 37.09o,
43.37o, 53.49o, 56.99o, 62.63o, and 74.09o which corresponds to the (111), (220), (311), (222),
(400), (422), (511), (440), and (533) planes of cubic CoFe2O4 lattice, respectively according to
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JCPDS card no. 022-1086 . Fig. 1 (b) represents the XRD patterns of the prepared SPT/CFO
core-shell nanocomposite, where the diffraction peaks show the presence of the shell material
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and the core material phases. The peaks at 2θ = 21.73o, 22.73o, 31.53o, 32.23o, 39.27o, 44.15o,
46.37o, 50.41o, 51.75o, 52.34o, 55.87o, 57.28o, 65.91o, 67.69o, 70.97o, 77.01o, and 77.34o are
respectively corresponding to (001), (100), (101), (110), (111), (002), (200), (102), (201), (210),
(112), (211), (202), (220), (212), (113) and (310) tetragonal planes of PbTiO3 phase in
accordance to JCPDS card no. 006-0452 41. Additionally, the second phase that was found in the
XRD chart was indexed as a phase of core material CoFe2O4, where the peaks at angles 2θ =
30.17o, 35.49o, 43.37o, and 63.43o were associated to (220), (311), (400), and (440) of CoFeO3
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cubic phase in accordance respectively, according to JCPDS card no. 022-1086 . Some peaks
appear broad due to the overlap between the peaks for the two phases.
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Fig. 1. The XRD patterns of (a) core CoFe2O4 (CFO) NPs and (b) core-shell
Sr0.3pb0.7TiO3/CoFe2O4 (SPT/CFO) magnetic nanocomposite.
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3.2. Morphological analysis of core-shell SPT/CFO MNc
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The nano-structures of the Sr0.3pb0.7TiO3/CoFe2O4 nanopowders were detected using HR-TEM.
Free aggregates image was observed to have formed from the fine and smaller nano-particles for
the CFO core (approximately 2-10 nm) and the SPT shell coated for the CFO nanoparticles was
approximately 12 nm, as indicated from the histogram images in Fig. 2(a-c).This result explains
the successful chemical growth in SPT nanogel that was assessed by more formation of the SPT
shell on the CFO surface. Thus, the advanced sol-gel method has accomplished the formation of
core-shell SPT/CFO with an excellent microstructure. Fig. 2(d, e) conveys the HR-SEM
micrographs of the specimen prepared through the sol-gel method with the chemical formula for
the core-shell of SPT/CFO magnetic nanocomposite. As illustrated before, the core-shell sample
prepared through SPT (sol) CFO nanoparticles composition was aged three days to support the
complete gelation process for SPT on the CFO surface; this is associated with the higher cross-
linked SPT contents in the SPT/CFO network before sintering at 800oC for 3h. Fig. 2(d, e) for
SPT/CFO shows the nanostructure formation of high nanopores, nanospheres, and the presence
of some dense agglomerated structure, as displayed by the HR-SEM micrographs for uncoated
(Fig.2 d) and coated with platinum (Fig. 2 e).
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Fig. 2. (a) HR-TEM image for core-shell Sr0.3pb0.7TiO3/CoFe2O4(SPT/CFO) magnetic
nanocomposite, (b, c) Histogram of particles distribution of bothe core and shell, and (d,e) FE-
SEM images of Pt-uncoated and Pt-coated sample.
3.3. Chemical composition (XPS)
The survey scan (XPS) spectrum of Sr0.3pb0.7TiO3/CoFe2O4 core-shell magnetic nanocomposite
is offered in Fig. 3. The spectrum appears the produced photoelectron peaks at different binding
energies corresponding to the chemical composition for the core-shell Sr0.3pb0.7TiO3/CoFe2O4
magnetic nanocomposite. As presented from the binding energy spectrum, the core-shell
magnetic nanocomposite was composed of the Pb, Co, Fe, Ti and O. From the spectrum the
binding energy attributed for Pb at ≈ 23 eV, 141 eV, 414 eV and 458 eV correspond to the Pb4f,
42
Pb4d3 and Pb4d5, respectively . The XPS spectrum of the core chemical composition (Fe2p
and Co2p) was estimated at the binding energy 712 eV, 727 eV, 763 eV and 783 eV
corresponded to Fe2p3/2, Fe2p1/2, Fe2p and Co2p1/2, which confirms the presence of both Fe and
Co cations in the chemical composition. It's widely conventional that the iron and cobalt cations
can cause observable shake up in photoelectron spectrum in the higher spin state, thus the
valence of iron and cobalt can be distinguished43,44
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Also, the three peaks that were detected at 436 eV, 586 and 645 eV matched to the binding
energies of Ti 2p, Ti 2s and Ti 2s, respectively, which signposted the actuality of Ti in the
composites45.
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Fig. 3. X-ray photoelectron spectroscopy (XPS) of Sr0.3pb0.7TiO3/CoFe2O4 (SPT/CFO) magnetic
nanocomposite.
3.4. Diffused reflectance
The optical absorption spectrum of semiconductors commonly displays a sharp increase at a
specific magnitude of the incident photon energy which can be regarded as the excitation of the
electrons from the valence to the conductive band. Energy and momentum conservation must be
fulfilled in procedures of optical absorption. KT= 0.026 eV at room temperature, so intrinsic
semiconductors where Eg > 0.5 eV are fundamentally insulators since the energy acquired by the
lattice at room temperature is inadequate to create any conductive electron-hole pairs 46. Eg
controls the semiconductor's light absorption features. Regarding the light with a shorter
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wavelength than a threshold wavelength, λg is absorbed to create charge carriers, while light with
longer wavelengths penetrates unimpeded through semiconductor λg = (1239.6/Eg) 47–49. Direct
versus indirect bandgap transitions are a more important element of the light-absorbing
characteristics of semiconductors. In direct band gaps, a photon with energy equivalent to the
original Eg is absorbed in a semiconductor by creating a hole and an electron, while in indirect
type gaps, an electron, a hole, and a phonon are generated by the equivalent absorption process
50. The indirect transition in many semiconductors has smaller energy than the direct one.
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Absorption coefficient (α) is related to the frequency (ν) by51,52
:
훼 = 퐴(ℎ휈 ― 퐸푔)푛 ℎ휈 (1)
2
where n = 1 and 4 for direct and indirect bandgap transition consecutively. For dense powder
pellets, the absorption coefficient is calculated from the diffusely reflected light. Units used to
measure the intensity of diffusely reflected light are termed as Kubelka-Munk. For using KM
units, the values of (αhv)2 acquired from the measured magnitudes of the absorption coefficients
(α) are multiplied by 53:
(1 ― 푟)2
퐾푀 =
(1 + 푟) (2)
where r is a symbol of the ratio of the initial intensity I0 of the incident ray to the reflected one I.
The prepared SrTiPbO/CoFeO core-shell samples were investigated optically by a diffuse
reflection (R) method at ambient, where the diffused reflectance (r) was recorded as a function of
wavelength. As revealed in Fig. 4, the resulting bandgap calculated by KM principal has a value
of 1.78 eV associated with direct or indirect allowed transition 54.
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0
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h (eV)
Fig. 4. Diffused reflectance of both core CoFe2O4 (CFO) and Sr0.3pb0.7TiO3/CoFe2O4
(SPT/CFO) core-shell magnetic nanocomposite.
3.5. Magnetic specifications
The vibrating sample magnetometer is a low-cost, versatile and reliable tool for studying the
magnetic specifications of powders, single crystals, and thin films 55. The magnetic hysteresis
loop of Sr0.3pb0.7TiO3/CoFe2O4 nanocomposite (SPT/CFO MNC) [Fig. 5] is measured at ambient
temperature and below curie temperature of the targeted magnetic nanocomposite material.
Separately depending on the varied behavior upon magnetic field exposure, it can be classified
into ferro-, antiferro- or paramagnetic material. Here it shows the conventional trend of a hard-
ferromagnetic material with increased coercivity and magnetic anisotropy with a preferred
orientation of the magnetization alignment as expected from cobalt ferrite existence. The
enclosed area of the M-H loop depicts the required energy loss upon magnetization and
demagnetization. This area is large as a property of hard magnetic materials; hence it is more
applicable for media recording and permanent magnets appliances.
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Fig. 5. The magnetic hysteresis loop of Sr0.3pb0.7TiO3/CoFe2O4 in the magnetic field range of
±10k Oe, the sample mass is 43 mg, right inset: maximized center area of the hysteresis loop.
The loop squareness (Mr/Ms), remnant magnetization (Mr), coercivity (Hc) and saturation
magnetization (Ms) parameters obtained from the hysteresis loop are tabulated in Table 1. The
small value of squareness (~ 0.3) might be attributed to the existence of inert single domain
particles or the fact that these particles are only interacting magnetoelastically 56. The coercivity
value is calculated as:
퐻푐(푟푖푔ℎ푡) ― 퐻푐(푙푒푓푡)
퐻퐶 = [
] (3)
2
and is related to the movement of domain walls as a change of magnetic field magnitude or
direction occurs. Hc(right) and Hc(left) are addressed as the loop intersection points with the
magnetic field axis 57, and was found to be at 600 Oe The saturation magnetization of the
nanocomposite represented by the upper limit of its magnetization, i.e., its polarization ability by
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the applied magnetic field , is moderate around ~ 4 emu/g which is less than the pure cobalt
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ferrite (~ 47-57 emu/g) based on altered chemical preparation methods 59. The saturation
magnetization value can be included in calculating the magnetic moment per unit formula in
Bohr magneton (µB) using the equation: 60,61
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휂퐵 = (푀.푤푡 × 푀 ) (4)
푠
5595
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A magnetic attitude is a prominent tool for an enormous range of applications, such as magnetic
59,62,63
resonance imaging, magnetic sensors, hyperthermia and drug delivery
.
Table 1. The magnetic parameters acquired from the M-H loop
Saturation
Remanent
Squareness
(Mr/Ms)
Coercivity (Hc)
Magnetic moment
magnetization magnetization
per unit formula ηB
(Ms)
(Mr)
(µB)
3.7 emu/g
1.3 emu/g
0.3
600 Oe
0.41
3.6.
Development of sensor based on SPT/CFO MNc/Nafion/GCE
The potential application of Sr0.3pb0.7TiO3/CoFe2O4 core-shell magnetic NCs as chemical sensor
has been explored for detecting and quantifying carcinogenic as well as hazardous chemicals,
which are not environmental friendly at all. Development of mixed metal oxide composite
materials as chemical sensor is in the initial stage and only a limited number of reports are
available.64–70 The Sr0.3pb0.7TiO3/CoFe2O4 core-shell magnetic NCs fabricated GCE-sensor
probes have advantages such as stability in air, non-toxicity, chemical stability, electrochemical
activity, and ease to fabricate. As in the case of chemical sensor, the principle of operation is that
the current response in electrochemical technique of SPT/CFO MNc/Naifon/GCE drastically
changes when are 3,4-DAT adsorbed into aqueous phase. Here, the slurry of synthesized
SPT/CFO MNc in ethanol was subjected to a deposit on the flat part of GCE (0.0316 cm2) as thin
film and the stability of this thin layer of NPs on GCE was long time stable by the addition of a
drop of Nafion. It should be mentioned that the Nafion is a conductive copolymer and has its
conductivity. Thus, the use of Nafion enhances the conductivity as well as the electron transfer
33–
rate of the working electrode. This similar observation has been reported in several articles
35,71–74. As per the authors knowledge, the synthesized SPT/CFO MNc on GCE is implemented
as the first-time chemical sensor for reliable detection of 3,4-DAT and regarding this, no other
reports are available.
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To begin this study, several chemicals that are hazardous for both the environment and public
health were analyzed in the electrochemical (I-V) approach by a recently assembled sensor as
illustrated in Fig. 6(a). As in Fig. 6(a), the electrochemical responses of 3,4-DAT, 4-AP,
Zimtaldehyde, 4-NP, chlorzoxazone, 2,4-DAP, PHyd, 3-CP, toluene, and acetonitrile are
explored and this experiment has been executed with 0.1 µM of each toxin in phosphate buffer
medium. Among the toxic chemicals displayed in Fig. 6(a), 3,4-DAT exhibits the maximum
affinity toward the synthesized NPs of SPT/CFO MNc in the electrochemical analysis by the
developed sensor. On that condition, 3,4-DAT is defined as selective for the sensor assembly.
After achieving the selectivity of the sensor based on SPT/CFO MNc/Nafion/GCE, the selective
3,4-DAT was analyzed with varying concentrations from lower to higher as explored in Fig.
6(b). The I-V responses are possible to distinguish between each other, and the intensity of I-V
responses is enhanced with the increasing concentration of 3.4-DAT. This nature of I-V
responses has been reported by the previous authors to detect the numerous toxic chemicals
using the mixture of transition and semi-conductive metals oxides on GCE 75–81. To establish the
calibration of this 3,4-DAT sensor with SPT/CFO MNc/Nafion/GCE, a plot known as calibration
curve plotted from the linear relation of current vs. concentration of 3,4-DAT is demonstrated in
Fig. 6(c). To this exploration, the current data are collected from Fig. 6(b) at applied potential
+1.5V. The current data are found to distribute linearly on the calibration curve from 0.01mM to
0.1nM and this concentration range of 3,4-DAP on the calibration curve is defined as a linear
dynamic range (LDR). As shown in Fig. 6(c), the linear exploration of the current data affords a
proof of the reliability of the sensor, and the LDR resulted in a wider range of concentrations.
The linearity of LDR is executed by plotting current vs. log (conc. of 3,4-DAT) as in Fig. 6(d),
which is fitted with the regression coefficient R2=0.9961 and provides evidence of the linearity
of LDR. The sensitivity of the projected sensor is calculated using the slope of the calibration
curve and surface area of GCE (0.0316 cm2) and the obtained sensitivity around 24.3323 µAµM-
1cm-2 is highly appreciable. The lowest limit of detection (DL) of the proposed 3,4-DAT is
calculated applying the signal to noise ratio of 3. The outstanding value of DL around
96.09±4.80 pM and limit of quantification (320.3 pM) were obtained.
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Fig. 6. The electrochemical investigation of the sensor based on SPT/CFO MNc/Nafion/GCE.
(a) I-V responses of toxic chemicals at 0.1µM of each, (b) the electrochemical response of 3,4-
DAT, (c) Calibration curve of 3,4-DAT sensor and (d) current vs. log (conc. of 3,4-DAT).
The response time of a sensor is an efficiency measuring parameter and is defined as the time
required to complete an electrochemical analysis. Fig. 7(a) illustrates the response time of a 3,4-
DAT sensor around 25.0 sec. The result might be appreciable. The reproducibility as well as
repeatibility is a parameter of reliability, and this experiment was performed with 0.1µM of 3,4-
DAT in phosphate buffer medium (identical conditions) and presented in Fig. 7(b). As it
perceived in Fig. 7(b), the electrochemical data are undistinguished in identical conditions, and
the efficiency of the reproducibility runs are not altered from washing the electrode after each
electrochemical analysis. Thus, it can be concluded that the proposed electrochemical sensor can
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perform with equal electrochemical response (I-V data) in an identical condition. It provides the
evidence around the reliability of the 3,4-DAT sensor. At applied potential +1.5V, the precision
of current data of a reproducibility test is executed in terms of relative standard deviation (RSD)
and high precious value around 1.54% (RSD) is observed. As per the requirement of this sensor
application, the long-time working capacity with high precision was tested with 0.1µM of 3,4-
DAT, demonstrated in Fig. 7(c) as similar as in reproductively test. The 3,4-DAT sensor was
found to work with precision through seven consecutive days. SPT/CFO MNc/Nafion/GCE
sensor probe was optimized in various pH (5.8, 6.5, 7.1, 7.9, and 8.5) in identical conditions and
presented in the Fig. 7(d). From this observation, it was exhibited the highest sensor response at
pH 7.1. So, during the electrochemical measurement pH 7.1 was maintained during the rest of
the electrochemical analysis. Therefore, it can be predicted that a proposed 3,4-DAT based on
SPT/CFO MNc/Nafion/GCE is capable enough to perform for a long period with a consistent
outcome.
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Fig. 7. (a) The response time of 3,4-DAT sensor with SPT/CFO MNc/Nafion/GCE, (b)
reproducibility parameter at 0.1µM of 3,4-DAT in phosphate buffer medium, (c) long time
performance test, and (d) pH effect, in 0.1µM of 3,4-DAT.
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The synthesized NMc of SPT/CFO were modified with various compositions and the
corresponding modified NPs on GCE were tested with 0.1µM of 3,4-DAT. The modified GCE
with SPT/CFO MNc shows the highest reactivity toward 3,4-DAT compared to other unmodified
or modified electrodes. Therefore, the proposed 3,4-DAT sensor exhibited higher sensitivity, a
broad LDR, considerably lower DL, short response time and precious reproducibility capacity.
Table 2 shows the comparison of the parameters in terms of sensitivity, LDR and DL of similar
recently published works to detect chemicals by electrochemical method.
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Table 2: The assessment of the analytical parameters of target chemical sensor with SPT/CFO
MNc/Nafion/GCE by electrochemical approach.
Modified GCE
DL
LDR
0.1pM1.0µM
0.1 nM0.1 mM
0.1 nM ∼
Sensitivity
Ref.
[30]
[36]
[72]
MnCoxOy NPs/GCE
0.26 pM
0.37 μA μM−1 cm−2
5.063 μA μM−1 cm−2
0.028 µAµM−1cm−2
ZnO/Yb2O3 NSs/GCE 19.0 pM
Ag2O/CuO NSs/GCE 3.31 pM
0.1mM
PANI/G/CNTs
NPs/GCE
63.4 pM
2.187 μA μM−1 cm−2
24.332 μA μM−1 cm−2
[82]
0.1 nM  0.01
M
SPT/CFO MNc /GCE
96.09
pM
This
0.1nM0.01mM
work
*DL (detection limit), LDR (linear dynamic range), pM(picomole), mM(millimole).
3.7. Validation of SPT/CFO MNc/Nafion/GCE sensor probe
To execute the analysis for the validity of real samples, it is extracted sample as well from
various sources of environmental locations, such as industrial effluents, extractions from a PVC
water bottle, or distilled water, and extractions from food packaging bags. Samples are extracted
in the PBS from the selective food packaging bags. The resulting data are analyzed by
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electrochemical method with SPT/CFO MNc/GCE sensor probe and included in Table 3, which
are quite acceptable and satisfactory.
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Table 3. The analysis of environmental samples using SPT/CFO MNc/GCE chemical sensor by
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recovery method.
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Sample
Added 3,4-
DAP conc.
(µM)
Measured 3,4-DAP conc.a by
Average RSDc
SPT/CFO MNc/Nafion/GCE (µM) recoveryb (%)
(%)
(n=3)
0.46
2.60
1.78
R1
R2
R3
Industrial effluent
PC- water bottle
PVC- food
0.0100
0.01031
0.00990
0.01123
0.01029 0.01022
0.00941 0.00957
0.01096 0.01085
102.73
96.27
0.01000
0.01000
110.13
packaging bag
aMean of 3 repeated determination (Signal-to-Noise ~ 3) SPT/CFO MNc/binder/GCE.
bConcentration of 3,4-DAP determined/Concentration taken. (Unit: nM)
cRelative standard deviation value indicates precision among three repeated measurements (R1, R2, R3).
3.8. Sensing mechanism of 3,4-DAT onto SPT/CFO MNc/Nafion/GCE
Due to the aggregated nanocomposites, SPT/CFO MNc have a large surface area that may be
responsible for such a sensitive oxidation of selective 3,4-DAT chemical at room temperature on
the surface of MNc. The rate of the 3,4-DAT reaction on the SPT/CFO MNc was higher than
other composites under identical conditions.30,83 Current responses in electrochemical technique
during 3,4-DAT detection largely depends on the dimensions, morphology, and nano-porosity of
the SPT/CFO MNc. Removal of the conducting electrons from the SPT/CFO MNc/Nafion/GCE,
it increases the surface conductance of the fabricated electrode the electrochemical system.84–86
This removal of electrons from the conduction band of nanocomposite increases the conductance
of the SPT/CFO MNc quickly. Aggregated SPT/CFO MNc onto nafion coated GCE is increased
the reaction ability of the SPT/CFO MNc for the activity of 3,4-DAT. The SPT/CFO
MNc/Nafion/GCE sensor requires approximately 25.0 s to achieve a constant current in the
electrochemical measurements. This excellent sensitivity and high electrochemical performances
of the SPT/CFO MNc is due to the Sr0.3pb0.7TiO3/CoFe2O4 core-shell magnetic NCs that
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enhances the oxidation of 3,4-DAT. A possible oxidation mechanism of 3,4-DAT is presented
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with SPT/CFO MNc/Nafion/GCE sensor probe in Scheme 1.
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Scheme 1. Proposed sensing mechanism for the determination of 3,4-DAT with SPT/CFO
MNc/Nafion/GCE by electrochemical approach
During the 3,4-DAT oxidation process, an enhancement of electrons in the buffer
medium is observed and that asked for amendment of electrochemical response. According to the
proposed oxidation mechanism, 3,4-DAT and H2O are adsorbed on SPT/CFO MNc/Nafion/GCE
surface, and ammonium ions, hydrogen ions and electrons are generated at the surface as
described in the reaction (5).87–90
푪ퟕ푯ퟔ(푵푯ퟐ)ퟐ + ퟏퟒ푯ퟐ푶 → ퟕ푪푶ퟐ + ퟐ푵푯ퟒ+ + ퟑퟎ푯 + + ퟑퟐ풆 ―
(5)
As the reaction (vii) produces electrons, the conductance of the Sr0.3pb0.7TiO3/CoFe2O4
core-shell working electrode-surface increases, and this is the main reason for the enhancement
of electrochemical response of 3,4-DAT.
Here, non-toxic nature, chemical stability, and electro-chemical activity make the
Sr0.3pb0.7TiO3/CoFe2O4 core-shell magnetic MNc as one of the best sensing material matrixes for
the 3,4-DAT by electrochemical approach at room conditions. 3,4-DAT gives a significant
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response upon contact with the SPT/CFO MNc in the electrochemical measurement. The interior
resistance of the fabricated Sr0.3pb0.7TiO3/CoFe2O4 core-shell magnetic MNc/Nafion/GCE sensor
decreases with the increasing electronic communication, which is an important characteristic
feature of the composites or nanomaterials at room conditions and vice versa.91–93 During the
reaction of 3,4-DAT, the number of electrons in the conduction band increases and hence
decreases the resistance of the Sr0.3Pb0.7TiO3/CoFe2O4 core-shell magnetic MNc. Reactions of
the 3,4-DAT on the SPT/CFO MNc surface is the main phenomenon involved in this proposed
3,4-DAT sensor.On the other hand, the aqueous 3,4-DAT sensing mechanism of
Sr0.3Pb0.7TiO3/CoFe2O4 core-shell magnetic MNc sensor probe is based on the metal oxides, due to
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oxidation or reduction of the mixed metal oxide itself, according to the dissolved O2 in bulk-solution or
surface-air of the surrounding atmosphere (Equation 6-7).
-
e- (Sr0.3Pb0.7TiO3/CoFe2O4 MNc) + O2 → O2
(6)
(7)
e- (Sr0.3Pb0.7TiO3/CoFe2O4 MNc) + O2 → 2O-
-
These reactions take place in bulk-solution or air/liquid interface or surrounding air due
to the low carrier concentration which increased the resistance. The 3,4-DAT sensitivity towards
Sr0.3Pb0.7TiO3/CoFe2O4 MNc could be attributed to the high oxygen deficiency and defect
density leads to increase oxygen adsorption. Larger the amount of oxygen adsorbed on the
surface, larger would be the oxidizing capability and faster would be the oxidation of 3,4-DAT.
The reactivity of 3,4-DAT would have been very large as compared to other chemical with the
surface under same condition. 34,94–96When 3,4-DAT reacts with the adsorbed O2 on the doped-
surface of the film of Sr0.3Pb0.7TiO3/CoFe2O4 MNc, it gets oxidized to carbon dioxide and
ammonium hydroxide, liberating free electrons in the conduction band could be expressed
through the following reactions (Equation 5): These reactions corresponded to oxidation of the
reducing carriers. These processes increased the carrier concentration and consequently reduced
the resistance on exposure to reducing liquids/analytes.97–99 At the room condition, the exposure
of mixed metal oxide surface of Sr0.3Pb0.7TiO3/CoFe2O4 MNc to reducing liquid/analytes results
in a surface mediated combustion process. The elimination of iono-sorbed oxygen enhances the
electron density as well as the surface conductance of the film.100–103 The reducing analyte (3,4-
DAT) donates electrons to Sr0.3Pb0.7TiO3/CoFe2O4 MNc surface. Therefore, resistance is
decreased, or conductance is increased. This is the cause why the analyte response (current
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response) increases with increasing potential. Thus produced electrons supply to rapid enhance
in conductance of the thick film of Sr0.3Pb0.7TiO3/CoFe2O4 MNc onto GCE. The chemical is
dispersed onto the contact surface of Sr0.3Pb0.7TiO3/CoFe2O4 MNc, which would enhanced the
ability to absorb more oxygen species by reducing the resistance of fabricated sensor probe in
aquesous system. The response time was around 25 sec for the Sr0.3Pb0.7TiO3/CoFe2O4 MNc
coated GCE electrode to reach saturated steady state current. The high sensitivity of
Sr0.3Pb0.7TiO3/CoFe2O4 MNc film on GCE can be attributed to the good absorption (porous
surfaces fabricated with MNc coating), adsorption ability, and high catalytic activity of the
Sr0.3Pb0.7TiO3/CoFe2O4 MNc. The estimated sensitivity of the fabricated sensor is relatively
higher than previously reported chemical sensors based on other composite or materials modified
electrodes.104–108 Due to large surface area, the mixed materials provided a favorable nano-
environment for the chemical detection with good quantity. The high sensitivity of
Sr0.3Pb0.7TiO3/CoFe2O4 MNc provided high electron communication features which improved
the direct electron transfer between the active sites of composite materials and GCE.109–112 The
modified thin film had a good stability. Additionally, due to high specific surface area, the
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nanocomposites of Sr0.3Pb0.7TiO3/CoFe2O4 impart a favorable environment for the 3,4-DAT
113–115
detection (by adsorption) with large quantity.
The high sensitivity of MNc provides high
electron communication aspects which promote the direct electron communication between the
active sites of Sr0.3Pb0.7TiO3/CoFe2O4 and GCE. Sr0.3Pb0.7TiO3/CoFe2O4 MNc reveals some
prospective in providing chemical based sensors, and substantial progress has been performed in
the research segment. As for the nanocomposites, Sr0.3Pb0.7TiO3/CoFe2O4 MNc provide a path to
a new generation of chemical sensors and to achieving high appliance density with accessibility
to individual sensors.77,116–121 Reliable methods for fabricating, assembling and integrating
building blocks onto chemical sensors need to be explored.
4. Conclusion:
In this approach, free aggregates CoFe2O4 (CFO) NPs with homogeneous size distribution
have been successfully prepared as core magnetic NPs by a citrate sol-gel method at 700oC. The
advance core-shell Sr0.3Pb0.7TiO3/CoFe2O4 (SPT/CFO) magnetic nanocomposite was
successfully prepared through controlled sol-gel reactions as a coating layer and in-situ process
followed by the sintering process at 800oC. The microstructures of both core and core-shell
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magnetic nanocomposites confirmed the excellent spinal magnetic structure. The SPT/CFO MNc
displayed a lower optical band-gap 1.78 eV associated with direct or indirect allowed transition.
Furthermore, the SPT/CFO MNc exhibited superior magnetic properties, which not only
promoted the coercivity and saturation magnetization of the core CoFe2O4 NPs but also
nominates them for a widespread series of magnetic recording media, drug delivery, and
magnetically actuated hyperthermia appliances. The application with the synthesized SPT/CFO
MNc onto GCE was for the development of an electrochemical sensor with selective 3,4-DAT.
The assembled 3,4-DAT sensor was studied in detail to execute the sensor parameters such as
sensitivity, LDR, DL, response time, and reproducibility and all the parameters were found as
satisfactory. Finally, the sensor was applied to analyze the samples under the recovery method,
and it was found quite appreciable. Therefore, the developed method of the fabrication of an
electrochemical sensor using Sr0.3Pb0.7TiO3/CoFe2O4 MNc onto GCE would be a key point in the
field of the environmental sector on an extensive scale.
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Acknowledgment:
Our thanks to Hayden Clement, who graciously revised our report for academic and
grammatical purposes. Center of Excellence for Advanced Materials Research (CEAMR) at
King Abdulaziz University, Jeddah, Saudi Arabia is highly acknowledged for sensor
development in this research report.
Conflict of interest
The authors declare that there is no conflict of interest to report.
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