Preparation of polyaniline grafted graphene oxide–WO₃ nanocomposite and its application as a chromium(III) chemi-sensor

Abstract

A nanohybrid of polyaniline grafted graphene oxide@WO₃ (PANI-g-rGO@WO₃) was prepared by oxidative polymerization of aniline in the presence of tungsten oxide and reduced graphene oxide. The physicochemical characterization of PANI-g-rGO@WO₃ was done by FESEM, X-ray diffraction microscopy, FTIR, UV-Vis spectroscopy, thermogravimetric analysis and XPS. The composite was analyzed for hazardous metal selectivity and was found to be highly sensitive for trivalent chromium (Cr³⁺). For potential trivalent chromium (Cr³⁺) cation sensor development, the PANI-g-rGO@WO₃ nanocomposite was deposited on a flat silver electrode (AgE, surface area, 0.0216 cm²) with conducting binders coating to give a sensor with a fast selective response towards trivalent chromium cations in the liquid phase. The sensor also exhibits good sensitivity, long-term stability, and enhanced electrochemical responses. The calibration plot is linear (r² = 0.9815) over a 0.1 nM to 0.01 M Cr³⁺ concentration range. The sensitivity is ∼4.4251 μA mM⁻¹ cm⁻² and the detection limit is 0.031 ± 0.010 nM (signal-to-noise ratio, at a SNR of 3). The development of a sensitive sensor for hazardous and carcinogenic chromium cations is also commenced using the PANI-g-rGO@WO₃ nanocomposite and the I–V method for promising potential future applications in the environmental and healthcare fields.

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1 Introduction

Nanocomposites were first investigated in 1950 and their industrial importance was recognised forty years later through the reports of Toyota Motor Corporation that disclosed new applications for nylon-6 polymers using a montmorillonite filler. Nanocomposites with polymers are very attractive because they reinforce the polymer and introduce new electronic interactions due to morphological or electronic modifications of the two components. On the basis of the components and the synthetic methods used for the preparation of nanocomposites, it is expected that significant properties would be obtained. Nanocomposites with conducting polymers were prepared using various techniques, such as the sol–gel, colloidal dispersion, electrochemical encapsulation, and cation-exchange techniques, which opened new horizons for the materials field.^1–3

When carbon materials like CNTs or graphite/graphene oxide/graphene are combined with conducting polymers, the composite possesses increased thermal and conducting properties. This is because a conduction combination occurs in the long range. Conductive polymer composites together with CNTs and metals/metal oxides are also much studied.

In this regard PANI has received special attention because of its intriguing properties such as its facile polymerization in aqueous media^4–7 or non-aqueous media,⁸ versatile redox behavior, good stability in air, low cost, high conductivity, and high pseudo-capacitance.^9,10 The promising potential applications of PANI-based composites,^11–15 for example graphene/PANI, for supercapacitors,^16–18 energy, and environmental remediation,^19–21 have been recently explored and investigated. Carbon/inorganic composites are in the spotlight of researchers owing to their remarkable properties.^22–24 WO₃ is the most investigated among the many inorganic materials owing to its many advantages including genuine color switching, good chemical stability, and strong adherence to substrates.^25–29 WO₃ has been successfully utilized in photoelectrocatalytic processes, electrochromic devices, dye-sensitized solar cells, gas sensors, and electrocatalysis. WO₃ of different structures (nanowire,³⁰ nanorod,³¹ and mesoporous^32,33) has been prepared by different methods including a hydrothermal process and sol–gel derived spin or dip coating methods.^34,35

Polyaniline is a conducting polymer with –NH₂ functional groups, which are particularly suitable for orientation into the matrices of WO₃ because the number of amino units in this polymer enables stronger bonding with inorganic moieties.³⁶ Nano-WO₃ has gained tremendous attention in recent years owing to its excellent electrical, chemical and optical properties³⁷ so it would be very interesting to introduce a polyaniline functionalized graphene oxide/WO₃ nanocomposite for potential chemical sensor applications.

The design and synthesis of new chemi-sensors for environmentally and biologically important toxic metal ions is an important subject in the field of chemical sensors.³⁸ Chromium is an important metal for human and animal biology as it is involved in several biochemical processes at the cellular level.^39,40 The chromium ion is not only an essential nutrient for humans and animals, but also plays an important role in the metabolism of carbohydrates, lipids, proteins, and nucleic acids.⁴¹ Chromium deficiency can increase the risk factors associated with diabetes and cardiovascular diseases.⁴² In addition, chromium is a known environmental pollutant that accumulates due to agricultural and industrial activity.⁴³ High levels of chromium may affect cellular structures.⁴⁴ Therefore, great importance is attached to developing selective chemi-sensors for chromium. Due to its simplicity, selective chemi-sensor detection of Cr³⁺ has great advantages over other detection techniques such as electrochemical⁴⁵ and potentiometric methods.⁴⁶ Trivalent chromium is one of the most effective fluorescent quenchers known owing to its paramagnetic nature, which also makes it difficult to develop a turn-on sensor for it. In the recent past, several attempts have been made to develop chemi-sensors to detect paramagnetic species.⁴⁷ The build-up of chromium due to various industrial and agricultural activities is a matter of concern due to environmental pollution.⁴⁸ The trivalent chromium ion is extremely toxic and usually causes serious damage to the health and environment, and therefore its detection using a reliable method is immediately required. Herein, we report its detection using a PANI-g-rGO@WO₃ nanocomposite with a conducting binder fabricated flat silver electrode. The PANI-g-rGO@WO₃ nanocomposite thin film deposited on the AgE will be prepared and studied in detail as ionic sensors for the detection of Cr³⁺. An easy coating method for the construction of a PANI-g-rGO@WO₃ nanocomposite thin layer within the conducting binding agents will be executed for the preparation of the films on the flat AgE. In this approach, the PANI-g-rGO@WO₃ nanocomposite fabricated films with conducting binders will be utilized towards the sensing of the target carcinogenic trivalent chromium cations using the reliable I–V method. It is confirmed that the fabricated chromium sensor with the active PANI-g-rGO@WO₃ nanocomposite material on the AgE is unique and valuable for ultra-sensitive recognition with a short response time.

2 Experimental

The main reagents (aniline monomer from Merck, tungstic acid from CDH and ammonium persulphate from Loba Chemie) were used as provided. All other reagents (hydrochloric acid, nitric acid, sulfuric acid, sodium nitrate, potassium nitrate, cadmium nitrate, lead nitrate etc.) and chemicals were of analytical grade. Analytical grade aluminum sulphate, disodium phosphate barium nitrate, calcium chloride, stannous chloride, cadmium sulphate, potassium dichromate, cerium nitrate, cobalt nitrate, butyl carbitol acetate, chromium chloride, mercury chloride, magnesium chloride, nickel chloride, antimony chloride, ethyl acetate, and monosodium phosphate were used and purchased from Sigma-Aldrich Company, USA. The stock solution of 1.0 M chromium chloride was prepared from the purchased chemical.

2.1 Preparation of PANI-g-rGO@WO₃

The polyaniline functionalized reduced graphene oxide–WO₃·nH₂O nanocomposite was synthesized in three steps.

First, rGO was synthesized by a modified Hummers method using conc. H₂SO₄, NaNO₃ and KMnO₄ and it was then refluxed overnight for acylation in the presence of excess SOCl₂ to convert the surface bound carboxylic acid groups to acyl chloride groups. After this, the solid product was separated by distillation. The solid obtained was dispersed in THF and reacted for a further 24 h with protected N-Boc-4-aminophenol. Amine deprotection was performed through hydrolysis of N-(tert-butoxycarbonyl) using trifluoroacetic acid and the remaining product was isolated by centrifugation and washing. This amine terminated rGO is called GO-NH₂.

PANI–rGO was prepared by in situ polymerization of the aniline monomer by APS and –NH₂ terminated rGO in the form of the initiator.⁴⁹ rGO-NH₂ was sonicated in 100.0 mL of 1.0 M HCl for 5 h to get a high dispersion and then it was cooled to 0–5 °C with continuous stirring. Then 100.0 mL of 1.0 M aniline monomer solution, prepared in 1 M HCl, was added dropwise to a cooled 500 mL conical flask containing dispersed rGO-NH₂ with continuous stirring for 2 h.

The mixture was then cooled, kept for 4 h and then 100.0 mL tungstic acid (1.0 M) prepared in double distilled water was added and stirring was continued for 40 minutes. This slurry was kept overnight at room temperature and filtered, washed with double distilled water to remove excess impurities and then dried in an oven at 45.0 °C overnight. In the composite, tremendous changes in the morphology would be expected which may be basically due to the interactions of aminophenol on the surface of the rGO. Therefore, during the formation of the composite, WO₃·nH₂O was found when blending Na₂WO₄ in the presence of PANI–rGO in the 1.0 M HCl medium. The formation of WO₃·nH₂O may be due to metal coordination and hydrogen bonding.

2.2 Fabrication of electrodes

0.1 M phosphate buffer solution (PBS) at pH 7.0 was prepared by mixing an equimolar concentration of 0.2 M Na₂HPO₄ and 0.2 M NaH₂PO₄ solution in 100.0 mL deionized water under ambient conditions. The silver electrode (AgE) was fabricated with the PANI-g-rGO@WO₃ nanocomposite using butyl carbitol acetate (BCA) and ethyl acetate (EA) as conducting binders. Then it was kept in the oven at 40.0 °C for 1 h until the film was completely dried, stable, and smooth. A cell was assembled with the PANI-g-rGO@WO₃ nanocomposite/binder/AgE and Pd-wire as the working and counter-electrodes respectively. Trivalent chromium was diluted to various concentrations (full concentration range: 0.1 nM to 1.0 M) in DI water and used as the target analyte. The ratio of the current versus concentration (slope of the calibration curve) was used to calculate the Cr³⁺ sensitivity. The detection limit was evaluated from the ratio of 3N/S (ratio of noise × 3 vs. sensitivity) from the linear dynamic range of the calibration curve. An electrometer was used as a constant voltage source for the I–V measurements in the simple two electrode system. The amount of 0.1 M PBS was kept constant at 10.0 mL in the beaker throughout the chemical investigation. The I–V response was measured with the PANI-g-rGO@WO₃ nanocomposite/AgE film.

2.3 Characterization

The following instruments were used for characterization during the present research work: electronic balance (digital, ae-ADAM-PW124, UK). Aqueous solutions of different concentrations were prepared by dilution of standard salt solutions in deionized water (18.6 MΩ cm⁻¹, Milli-Q plus, Millipore, USA). A field-emission scanning electron microscope (FESEM, JEOL JSM 6300, Japan) with a Link-Oxford-Isis X-ray microanalysis system (EDX) was used for the surface studies. An FTIR spectrophotometer, model 2000 (Perkin-Elmer, USA), was used for functional group analysis and an Elma Elmasonic 120v P180H instrument was used for sonication. The X-ray photoelectron spectroscopy (XPS) measurements were executed on a Thermo Scientific K-Alpha KA1066 spectrometer for the PANI-g-rGO@WO₃ nanocomposite. A monochromatic Al Kα X-ray radiation source was used as the excitation source, with the beam-spot size kept at 300.0 μm. The spectra were recorded in the fixed analyzer transmission mode, with the pass energy kept at 200.0 eV. The scanning of the spectra was performed at pressures less than 10⁻⁸ Torr. The I–V technique was executed using an electrometer (Keithley, 6517A, Electrometer, USA) for measuring the current responses in the two electrode systems for the target trivalent chromium cations based on the PANI-g-rGO@WO₃ nanocomposite in the buffer phase under ambient conditions, where the flat AgE and Pd wire were used as the working and counter electrodes respectively.

The PANI, rGO, WO₃ and PANI-rGO@WO₃ sample materials were dried completely at between 40–50 °C in an oven. Then 200 mg portions were finely ground in a mortar pestle and pellets were made at room temperature with the help of a hydraulic pressure instrument at 25 kN pressure for 20 min. The thickness of pellet was measured by a micrometer. Electrical conductivity measurements for the samples were performed on pressed pellets by using four-probe technique.

3 Results and discussions

In this study, we have prepared the hybrid PANI–rGO@WO₃ nanocomposite by grafting PANI on rGO and simultaneous reactions with inorganic matrices of WO₃. Since it is a new nanostructure with a higher surface-to-volume ratio, it is expected that the nanocomposite could give better efficiency, reproducible behavior, and chemical and thermal as well as electrical stability. Here we observe the functionalization of the N-Boc-protected 4-aminophenol to acylated graphene oxide followed by reduction with in situ polymerization with the aniline monomer that would produce a highly conducting network. Graphene oxide is easily dispersed in the aqueous medium due to the presence of groups containing oxygen on the surface of graphene oxide that work in the form of nucleation sites for the reaction with polyaniline on the surface of GO. WO₃ coordinates with the nitrogen groups in the network to assist conduction in the composite.⁵⁰ A schematic diagram for the preparation is given in Fig. 1(A–C).

Fig. 1 A schematic representation for the preparation of the PANI-g-rGO@WO₃ composite.

3.1 Characterization

The FESEM images of the PANI–rGO@WO₃ composite are shown in Fig. 2 at different magnifications. A dense surface with shapes of small morphology was seen in the composite and in the lower resolution image it was seen that aggregation of bundles of the smaller shapes also takes place in spite of separated structure. Thus the morphology of the material has been changed due to interactions in the formation of the organic–inorganic composite materials after grafting with rGO and the inorganic precipitate. As in the present case, it is the increased surface area and porosity that leads to easy ion diffusion through the materials. The presence of a whitish luster in the images is due to the presence of metal oxides.

Fig. 2 Scanning electron micrographs of the PANI-g-rGO@WO₃ nanocomposite at different magnifications.

It is clear from the thermogravimetric analysis (TGA) curve (Fig. 3) of the material that, up to 100 °C, only 6.5% weight loss was observed, which may be due to the removal of external H₂O molecules present at the surface of the composite.⁵¹ A further weight loss of approximately 24.5% mass between 100 and 700 °C may be due to the slight decomposition of the organic part. Two broad peaks at ∼50 °C and ∼800 °C in the DTA curve show the reaction is exothermic during the change of phase of the material.

Fig. 3 Thermogravimetric analysis of the PANI-g-rGO@WO₃ nanocomposite under a nitrogen atmosphere up to 900 °C.

The FTIR spectrum of the PANI-g-rGO@WO₃ nanocomposite is shown in Fig. 4. Within the spectrum of the composite there are high-intensity peaks in the range of 1574–1488 cm⁻¹ that can be attributed to the aromatic C=N and C=C stretching vibrations of the quinoid and benzenoid vibrations of PANI.^52,53 The peak at 780 cm⁻¹ is usually ascribed to the C–H out-of-plane deformation mode, whereas other peaks in this region are attributed to the ring stretching and C–H in-plane deformation modes.^54,55 The WO₃ spectrum shows nearly identical numbers and positions of the IR bands in the range of 600–3200 cm⁻¹. Only the ring-deformation modes are shifted because of polaron–polaron interactions between the planar polyaniline backbone and WO₃, indicating that some of them do not fully interact with the polyaniline molecules because of bulky groups.

Fig. 4 FTIR spectrum of the PANI-g-rGO@WO₃ nanocomposite.

X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic method that determines the elemental composition, empirical formula, chemical state and electronic state of the elements present within materials. XPS spectra are attained by irradiating materials with a beam of X-rays while simultaneously determining the kinetic energy and number of electrons that escape from the top 1.0 to 10.0 nm of the material being analyzed. Here, XPS measurements were measured for the PANI-g-rGO@WO₃ nanocomposite to investigate the chemical states of nitrogen, carbon, oxygen, and tungsten. XPS was used to determine the chemical states of the PANI-g-rGO@WO₃ nanocomposite and their depth. The full XPS spectrum of the PANI-g-rGO@WO₃ nanocomposite is presented in Fig. 5(a). In Fig. 5(b), the spin–orbit peak of the W4f_7/2 binding energy for the samples appears at around 35.9 eV, which is in good agreement with the reference data for tungsten.⁵⁶ The C1s spectrum shows peaks at 285.2 eV and is presented in Fig. 5(c). The C1s peaks indicate the presence of carbon in the nanocomposite.⁵⁷ In Fig. 5(d), the binding energy of N1s is 399.3 eV in the nanocomposite.⁵⁸ The O1s spectrum in Fig. 5(e) shows a distinguishing peak at 532.6 eV. The peak at 532.6 eV is assigned to oxygen, which indicates the presence of oxygen (i.e., O₂⁻) in the PANI-g-rGO@WO₃ nanocomposite.⁵⁹ Therefore, it is concluded that the PANI-g-rGO@WO₃ nanocomposite has three different elements.

Fig. 5 X-ray photoelectron spectroscopy study of the PANI-g-rGO@WO₃ nanocomposite. (a) Full spectrum of the PANI-g-rGO@WO₃ nanocomposite, (b) W4f_7/2 level, (c) C1s level, (d) N1s level, and (e) O1s level acquired with Mg Kα1 radiations. X-ray beam-spot size is 300.0 μm; pass-energy is 200.0 eV; pressures less than 10⁻⁸ Torr.

3.2 Applications

The potential application of the PANI-g-rGO@WO₃ nanocomposite assembled on the flat silver electrode as a cationic sensor (especially for Cr³⁺ cations) for measuring and detecting the target analyte has been studied. The nanocomposite/AgE assemblies have advantages such as stability in air, non-toxicity, chemical inertness, electrochemical activity, simplicity of assembly, ease of fabrication, and chemically safe characteristics. As in the case of Cr³⁺ cations, the rationale is that the current response from the I–V method of the PANI-g-rGO@WO₃ nanocomposite/AgE changes considerably when the aqueous chromium analyte is adsorbed. Here, the PANI-g-rGO@WO₃ nanocomposite was employed for the detection of chromium in the liquid phase. I–V responses were measured with the nanocomposite coated thin-film (in the two electrode system). In the experimental section, the Cr³⁺ sensing protocol was already outlined using the nanocomposite/AgE modified electrode. The concentration of Cr³⁺ was varied from 0.1 nM to 1.0 M by adding de-ionized water in different proportions. Here, Fig. 6(a) shows the I–V responses for the uncoated AgE (gray dots) and the nanocomposite coated AgE (red dots). In the PBS system, the nanocomposite/AgE shows that the reaction is reduced slightly due to the presence of the PANI-g-rGO@WO₃ nanocomposite on the bare AgE surface. A control experiment was also performed with the PANI-g-rGO@WO₃ nanocomposite/AgE assembly in the presence of trivalent (Cr³⁺) and hexavalent chromium (Cr⁶⁺) ions (each concentration was kept at 0.1 μM) and the results are presented in the ESI (Ω).† A considerable enhancement of the current with the applied potential is demonstrated for the fabricated composite/AgE in the presence of the target Cr³⁺ analyte, which is shown in Fig. 6(b). In the presence of the target analyte, the current response linearly increases with respect to the applied potential. In the lower region, the linear portion of the I–V curve exhibits the unsaturation of the sensor surface, whereas in the higher region it is quite stable due to unavailable sites on the nanocomposite. The red-dotted and blue-dotted curves show the response of the fabricated film before and after injection of 25.0 μL Cr³⁺ in 10.0 mL PBS solution, respectively, measured for the fabricated nanocomposite/AgE films. Significant increases of current are obtained after injection of the target component at regular intervals. The I–V responses (calibration curves) to the varying chromium concentration (0.1 nM to 1.0 M) on the thin nanocomposite/AgE were investigated (time delay 1.0 s) and the results are presented in Fig. 6(c). The calibration curve is also plotted in terms of the current density versus analyte concentration and presented in the ESI (ς).† Analytical parameters (such as the sensitivity, detection limit, linearity, and linear dynamic range etc.) were calculated from the calibration curve (current vs. concentration), which is presented in Fig. 6(d). A wide chromium concentration range was selected to study the possible detection limit (from the calibration curve), which was examined from 0.1 nM to 1.0 M. The sensitivity was calculated from the calibration curve, which was close to 4.4251 μA mM⁻¹ cm⁻². The linear dynamic range of the composite/AgE sensor was employed from 0.1 nM to 0.01 M (linearly, r² = 0.9815; Y = mX + C), where the detection limit was calculated to be about 0.031 ± 0.010 nM (ratio 3N/S). The nanocomposite/AgE exhibited porous behavior, with the electrical resistance decreasing (the current response increases) in the presence of the target chromium cations in the PBS phase. The film resistance decreased gradually (increasing the resultant current) upon the Cr³⁺ concentration increasing in the bulk system. The electrical I–V response of the sensor increased gradually from +0.1 V to +0.9 V by increasing the analyte Cr³⁺ concentration. As the concentration of the analyte increased, the value of the current increased with respect to the applied potential. The calibration curve is enhanced from the lower concentration value (unsaturated area) and after a certain concentration value (>0.01 M), the current response starts to stabilize, which suggests the saturation of the sensor surfaces at higher concentration values. The lower part of the calibration curve of the sensor is the receptor (nanocomposite) for Cr³⁺ ions, which sensed the target analytes. Then the current signal is gradually increased with the concentration of the analytes. At very high levels of analyte concentration, the receptor sensing decreased, which indicated saturation of the sensor surface (at trace levels of detection). The designed sensor would be beneficial at lower concentrations of Cr³⁺ ions.

Fig. 6 Analysis of Cr³⁺ responses. I–V responses of (a) bare AgE and PANI-g-rGO@WO₃ nanocomposite/AgE; (b) PANI-g-rGO@WO₃ nanocomposite/AgE in the absence of Cr³⁺ and PANI-g-rGO@WO₃ nanocomposite/AgE in the presence of Cr³⁺; (c) concentration variations (0.1 nM to 1.0 M) of Cr³⁺, (d) calibration plot of the PANI-g-rGO@WO₃ nanocomposite fabricated AgE electrode (at +0.5 V).

In the two-electrode system, the I–V characteristics of the PANI-g-rGO@WO₃ nanocomposite/AgE are activated as a function of chromium concentration under ambient conditions, where an improved current response is observed. The current response of the nanocomposite film increases with the increasing concentration of Cr³⁺. However similar phenomena for toxic chemical detection have been reported previously.^60–64 For a low concentration of Cr³⁺ in a liquid medium, there is a smaller surface coverage of chromium molecules on the nanocomposite/AgE film and hence the surface reaction proceeds steadily. By increasing the chromium concentration, the surface reaction is increased significantly (the response gradually increases as well) owing to the large surface area contact with the chromium molecules. A further increase of the Cr³⁺ concentration on the nanocomposite/AgE surface (low-dimensional crystalline size and low-lattice disorder) causes a more rapid increase in the current responses, due to a larger surface being covered by the chromium cations. Usually, the surface coverage of Cr³⁺ molecules on the nanocomposite/AgE surface reaches saturation, based on the regular enhancement of the current responses.

The I–V characteristics of the PANI-g-rGO@WO₃ nanocomposite are sensed and activated as a function of the target analyte Cr³⁺ ion concentration under ambient conditions, where an improved current response is observed in the I–V responses. The schematic diagram in the ESI (Scheme Πb)† presents the mechanism of the π–π interactions between the PANI-g-rGO@WO₃ nanocomposite and the target Cr³⁺ ions. The current response of the PANI-g-rGO@WO₃ film increases with the increasing concentration of Cr³⁺ ions in the unsaturated area (lower region of Cr³⁺ concentration). Similar phenomena for toxic chemical detection have been reported previously.^65,66 At low concentrations of Cr³⁺ ions in the liquid medium, there is a smaller surface coverage of Cr³⁺ ions on the PANI-g-rGO@WO₃/AgE film (uncovered receptor) and the surface reaction proceeds steadily. By increasing the Cr³⁺ ion concentration, the surface reaction is increased significantly (gradually increased response) due to surface area (assembly of PANI-g-rGO@WO₃/AgE) contact with the Cr³⁺ ions molecules (in the non-saturated area). By increasing the analyte concentration, it is found that the current response is almost stable over the concentration of chromium at 0.01 M. With a further increase of the Cr³⁺ ion concentration on the PANI-g-rGO@WO₃/AgE surface, a more rapid increase in the current response is exhibited, due to the larger area covered by the Cr³⁺ ions and the π–π interactions of the functional groups (lone pairs of nitrogen). The π–π interactions could be approached as per the schematic view (Scheme Πa†), or intermolecular and intramolecular (Scheme Πb†) interactions of the electron rich system as explained in the published works.^67,68 Usually, the surface coverage of Cr³⁺ ions on the PANI-g-rGO@WO₃/binder/AgE surface reaches saturation, based on the regular enhancement of the current responses.

The selectivity for Cr³⁺ in the presence other chemicals such as Al³⁺, Ba²⁺, Ca²⁺, Cd²⁺, Ce²⁺, Co²⁺, Hg²⁺, Mg²⁺, Ni²⁺, Sb³⁺ and Sn²⁺, and a blank (DI water) was studied using the PANI-g-rGO@WO₃ nanocomposite embedded on the flat AgE, as presented in Fig. 7(a). The concentrations of all analytes were kept constant at the 0.1 μM level in the PBS system. From the current response of each individual analyte, the current responses were calculated at +0.5 V with the nanocomposite/AgE sensors. Here, it is clearly demonstrated that the nanocomposite/AgE sensor is most selective for Cr³⁺ compared with the other chemicals. For comparison, the I–V responses were also performed for the tri- and tetravalent chromium (i.e., Cr(III) and Cr(IV)) using the PANI-g-rGO@WO₃ nanocomposite/AgE sensor with 0.1 μM analyte concentration. It was observed that trivalent chromium exhibited a higher current response compared to tetravalent chromium, as presented in the ESI (Φ).†

Fig. 7 Selectivity and reproducibility study with all analytes (0.1 μM) by PANI-g-rGO@WO₃ nanocomposite/AgE. (a) I–V responses for various cations and (b) I–V responses for all reproducible signals (Run-1 to Run-6). All analyte concentrations were taken as 0.1 μM. Potential range: 0 to +1.5 V; delay time: 1.0 s.

To check the reproducibly and storage stabilities, the I–V response for the PANI-g-rGO@WO₃ nanocomposite/AgE sensor was examined and presented in Fig. 7(b). After each experiment (each run), the fabricated nanocomposite/AgE substrate was washed thoroughly with the phosphate buffer solution and it was observed that the current response did not significantly decrease. The average current loss in each experiment was calculated as ∼0.0642 μA. Here it is observed that the current loss for each reading is negligible compared to the initial response of the sensors using PANI-g-rGO@WO₃ nanocomposite/AgE. The sensitivity was retained as almost the same as the initial sensitivity up to 7 days and after that the response of the fabricated nanocomposite/AgE electrode gradually decreased. The Cr³⁺ sensor based on the nanocomposite/AgE displayed good reproducibility and stability for over a week and no major changes in the ionic responses were found. After a week, the chemical sensor response of the nanocomposite/AgE slowly decreased, which may be due to the weak interactions between the fabricated PANI-g-rGO@WO₃ nanocomposite’s active surfaces and the chromium cations.

A significant result was achieved for the PANI-g-rGO@WO₃ nanocomposite/AgE, which can be employed as an efficient electron mediator for the development of efficient and selective Cr³⁺ cation sensors. A response time around 10.0 s was necessary for the fabricated nanocomposite/AgE to reach the saturated steady-state level. The higher sensitivity of the fabricated nanocomposite/AgE could be attributed to the excellent absorption (porous surfaces of the PANI-g-rGO@WO₃ nanocomposite/binder/AgE) and adsorption ability and high catalytic decomposition activity of the nanocomposite. The estimated sensitivity of the fabricated sensor is relatively higher and the detection limit is comparatively lower than previously reported chromium sensors based on other composites or material modified electrodes measured by electrochemical approaches.^69–71

The I–V responses of PANI, rGO and WO₃ compared to the PANI-g-rGO@WO₃ nanocomposite for the analysis of Cr³⁺ ions was also studied and the results are presented in the ESI section (Ψ).† The current response in the I–V curve for the PANI-g-rGO@WO₃ nanocomposite is higher than the single phase of the individual polymer (PANI) and materials (rGO and WO₃). This is due to the high specific surface area, in that the PANI-g-rGO@WO₃ nanocomposite provides a favorable nano-environment for Cr³⁺ detection in good quantities. The high sensitivity of the nanocomposite/AgE provides high electron communication features which enhances direct electron transfer between the active sites of the PANI-g-rGO@WO₃ nanocomposite and the binder coated AgE. The PANI-g-rGO@WO₃ nanocomposite/AgE system is demonstrated as a simple and reliable method for the detection of toxic cations using flat electrodes. It also allows significant access to a large group of chemicals for a wide range of ecological and biomedical applications in the environmental and healthcare fields respectively. Table 1 shows some selected applications of nanocomposite materials for sensing inorganic cations using electrochemical methods.^72–75

Table 1 Some selected applications of nanocomposite materials for sensing inorganic cations by electrochemical approaches

Electrode materials	Sensitivity (μA mM^−1 cm^−2)	Detection limit	Linear range	Ref.
Rhodamine-based chemosensors	—	0.023 μM	0–10 μM	72
AgNPs	—	64.51 nM	10–370 μM	73
Hydrazide-based AgNPs	—	0.45 μM	1.0–50 μM	74
Fluorescent chemosensor–pyrene moiety	—	0.23 μM	—	75
Poly(aniline)–GO–WO3/AgE	∼4.4251	0.031 nM	0.1 nM to 0.01 M	This work

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By evaluation of the conductivity it was found that PANI, rGO, WO₃ and PANI-g-rGO@WO₃ have a conductivity of about 4.60 × 10⁻⁵ S cm⁻¹, 1 × 10⁻³ S cm⁻¹, 1.3 S cm⁻¹ and 8.0 S cm⁻¹ respectively. The above results showed that the value of conductivity for PANI, rGO and WO₃ are nearly same and within the semiconductor range while the conductivity of PANI-g-rGO@WO₃ was found in the range of conductor that's why the I–V results for PANI, rGO and WO₃ are nearly same as compared to the PANI-g-rGO@WO₃.

4 Conclusions

In this study, we synthesized a low cost, environmentally friendly PANI-g-rGO@WO₃ nanocomposite with a continuous and repeatable electrical conducting network of PANI and rGO in WO₃. The PANI-g-rGO@WO₃ nanocomposite showed improved thermogravimetric stability in comparison with the homopolymer. The physicochemical characterization indicated that the poly(aniline) molecules were adsorbed onto WO₃ and then polymerized inside the matrices, which was used as the core in the formation of the hybrid composites. PANI-g-rGO@WO₃ was used as a sensor that was fabricated on a flat silver electrode and evaluated by electrochemical approaches for toxic chromium cations using the I–V method. Analytical performances of the Cr³⁺ sensor using the nanocomposite/AgE were investigated by the reliable I–V method in terms of the sensitivity (∼4.4251 μA mM⁻¹ cm⁻²), detection limit (0.031 ± 0.010 nM) and short response time as well as the reproducibility. Hence, this approach introduces a new route for efficient cationic sensor development in the environmental and healthcare fields. Here, we highlight its analytical potential as a toxic chemical sensor with the PANI-g-rGO@WO₃ nanocomposite/binder/AgE assembly and suggest avenues for further research.

Acknowledgements

This Project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, under grant no. (130-926-D1435). The authors, therefore, acknowledge with thanks DSR technical and financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17925d

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