polyaniline nanocomposite electrode

Synthetic Metals 257 (2019) 116185

Contents lists available at ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Ultra-selective detection of Cd2+ and Pb2+ using glycine functionalized reduced graphene oxide/polyaniline nanocomposite electrode

T

⁎

Farzana Hanifa, Amiza Tahira, Mehwish Akhtara, , Muhammad Waseemb, Sajjad Haiderc, ⁎ Mohamed F. Aly Aboudd, Imran Shakird, Muhammad Imrane, Muhammad Farooq Warsie, a

Department of Chemistry, The Govt. Sadiq College Women University, Bahawalpur, 63100, Pakistan Department of Chemistry, COMSATS University, Islamabad Campus, Park Road, Chak Shahzad, Islamabad, 44000, Pakistan c Department of Chemical Engineering, College of Engineering, King Saud University, P.O. Box 800, Riyadh, 11421, Saudi Arabia d Sustainable Energy Technologies (SET) Center, College of Engineering, King Saud University, PO-BOX 800, Riyadh, 11421, Saudi Arabia e Department of Chemistry, Baghdad-ul-Jadeed Campus, The Islamia University of Bahawalpur, Bahawalpur, 63100, Pakistan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Electrochemical sensor Graphene Polyaniline Composite Toxic metal ions Sensitivity and selectivity

An efficient electrochemical sensor with high sensitivity, selectivity and fast detection ability was developed for the trace level detection of Cd2+ and Pb2+ using glycine functionalized reduced graphene oxide/ polyaniline nanocomposite as a recognition layer. The morphological and structural studies of synthesized receptor nanocomposites were verified via X-ray diffraction, Fourier transform infrared spectroscopy, UV/Visible spectroscopy, Raman studies, SEM analysis and BET studies. Electrochemical features were explored via electrochemical impedance spectroscopy and cyclic voltammetry whereas the stripping voltammetric behavior of modified electrodes was analyzed by utilizing square wave anodic stripping voltammetry. The chemical and electrochemical parameters which influence the performance of modified sensors towards the accumulation and stripping of metal ions, for instance, pH of the medium, accumulation potential, scan rate, stripping medium and foreign species were carefully studied to achieve best sensing response. The designated sensor exhibited the excellent sensing properties towards heavy metal ions with quite low limit of detection 0.07 nM for Cd2+ and 0.072 nM for Pb2+ respectively. The proposed methodology allowed a robust and sensitive inspection of Pb2+ and Cd2+ in tap water, making this novel platform a promising candidate for field applications to monitor heavy metal contaminants. Most significantly, this methodology can be applicable for the ultrasensitive detection of other noxious pollutants due to excellent properties of receptor species such as fast electron transfer kinetics, broad surface area, wide conduction path, and exceptional electro-catalytic properties.

1. Introduction Heavy metal ions (HMI) have been rated as the most hazardous environmental pollutants being a high risk for the humans and other biological organisms of the ecosystem. Owing to the exponentially growing industry and agriculture along with anthropogenic resources, the environment is far more likely to be exposed to the HMI [1]. Although humans encounter to HMIs can transpire in multiple ways apart from exposure via portable water entails the highest threat of developing a disease [2]. Biomagnification of heavy metals in living systems interfere with the biological metabolism of the cell, exhibiting an immense susceptibility to form complexes when binding to the ligands via nitrogen, sulfur, and oxygen by replacing monovalent (Na2+) and bivalent ions (Ca2+, Mg2+, and Fe2+) of biological matter. The resulting

⁎

complexes may lead to neurotoxicity, hepatotoxicity, genotoxicity, nephrotoxicity, carcinogenicity and others by causing damage to DNA and proteins [3–5]. Heavy metals are being non-biodegradable pose serious hazards, making their detection and removal indispensable. For the quantitative analysis of the heavy metals ions in diversified matrices (such as air, water, soil and sediment), various analytical approaches have been employed falling under two major categories i.e. (i) spectral and (ii) electrochemical techniques. Atomic absorption / emission spectroscopy (AAS/AES) [6], inductively coupled plasma mass spectrometry (ICP-MS), mass spectroscopy (MS) [7], cold-vapor atomic fluorescence spectrometry (CV-AFS), ultraviolet–vis (UV) spectroscopy and X-ray fluorescence spectroscopy [8,9] enumerated as spectral techniques. These sophisticated methods are employed owing to their high sensitivity and low detection limit however being highly

Corresponding authors. E-mail addresses: [email protected] (M. Akhtar), [email protected] (M.F. Warsi).

https://doi.org/10.1016/j.synthmet.2019.116185 Received 11 July 2019; Received in revised form 5 September 2019; Accepted 23 September 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.

Synthetic Metals 257 (2019) 116185

F. Hanif, et al.

2.2. Instrumentation

expensive, time-consuming and complicated operational management/ procedures, these techniques are considered as less competent as compared to other electrochemical sensing platforms [10]. Electrochemical procedures with excellent detection properties have already been established as highly cost-effective, relatively simple, robust and sensitive for point-of-care analysis of heavy metal ions in diversified matrices [11–13]. A remarkable deal of efficiency with the electrochemical sensors can be succeeded by layering the surface of the working electrode with film of active electro-catalytic species. Various organic, inorganic and biological materials (metal NPs, carbon-based NPs, biomolecules, conducting polymers, surfactants, ionic liquids, etc.) are being employed frequently to boost the effectiveness of electrodes for high adsorption of toxic metal ions. In recent days, graphene has conquered enormous attention due to its excellent conductive pathway together with large surface area and high electro-catalytic properties [14–21]. However, following π-π stacking in addition to van der Waals interactions, the phenomenon of irreversible agglomeration and restacking tend to form graphite in return which limits the sensing traits of graphene [22,23]. To inhibit the phenomenon of restacking and agglomeration, graphene is grafted with other materials [24] such as conducting polymers and amino acids. Several amino acids including cysteine, glutathione, glycine have been utilized for the electrochemical detection of heavy metals. These amino acids incorporate the identification of HMIs through the formation of metal-ligand complexes. HMIs robustly bind with the electro-active species of glycine such as eNH2 and eCOO− moieties, which favor the development of metal-chelates complexes [25,26]. Conducting polymers such as polyaniline (PANI), polyacetylene (PA), poly(p-phenylene) (PPP), polypyrrole (PPy), poly(3,4ethylenedioxythiophene) (PEDOT) and polythiophene (PTh) has earned great attention owing to their excellent electrical properties and good environmental stability [25]. Thus, to anchor more recognition sites on the surface of the electrode and to achieve high affinity for the binding of heavy metal ions, polyaniline is combined with the rGO and Glycine. Polyaniline enhances the collective capacity for metal ions via nitrogen functionalities for instance amine (-NH-) and imine (=N-) functional groups [27]. In this work, we combine the high electron transfer, electrocatalytic, chelating traits of polyaniline and glycine with the huge surface area and conducting properties of graphene to fabricate an efficient electrochemical sensing platform. The as-synthesized sensing material is applied to modify glassy carbon electrode for the simultaneous detection of Cd2+ and Pb2+ in water samples by square wave anodic stripping voltammetry (SWASV) for the first time (to the best of our knowledge). Under optimal conditions, the designed sensor exhibits good electrochemical sensitivity and selectivity on to the modified electrode. The voltammetric behavior of modified electrode to determine the presence of Cd2+ and Pb2+ in real samples is also affirmed. The proposed sensing platform presented valued results in comparison to previously reported work.

Optical absorption spectra of nanocomposites were measured in 200–800 nm by UV–vis spectrophotometer (Agilent Technologies Cary 60 UV–Vis). Powder X-ray diffraction patterns of as prepared samples were recorded using 3040/60 diffractometer with Cu-Kα as a radiation resource (Philips X pert PRO). Fourier transform infrared (FT-IR) spectra were carried out by using spectrophotometer (Nexus 470). All the electrochemical measurements were executed with a Metrohm Autolab PGSTAT302 N (FRA module, NOVA 1.11 software, The Netherlands) using three electrode assembly comprising Ag/AgCl/Sat. KCl as the reference electrode, platinum wire as the counter electrode and glassy carbon electrode (GCE) was employed as the working electrode. The performance and defining traits of the developed sensors were verified by conducting CV and EIS measurements in 5 mM [Fe(CN) 6]3−/4− and 0.1 M KCl solution. Further, the proposed sensor was efficiently explored for the simultaneous measurement of Cd2+ and Pb2+ by using 0.1 M sodium acetate buffer as supporting electrolyte. For the effective measurements, all the studies were carried out under the optimum performance values of the sensor via SWASV. BET adsorption data was collected on Nova Station B. 2.3. Synthesis of GO and rGO Graphene oxide (GO) was chemically manufactured by oxidizing pristine graphite powder via modified Hummers method [28]. In the typical pattern of the procedure, 1 g of graphite powder is mixed with 1 g of sodium nitrate (NaNO3) functioning as a strong oxidizing agent. This mixture was kept in an ice bath whilst 50 ml of concentrated sulfuric acid (H2SO4) was added slowly under constant stirring of 30 min. Afterward, 6 g of potassium permanganate (KMnO4) was added to the resulting black colored mixture under vigorous stirring within three hours. During the addition of potassium permanganate (KMnO4), the mixture was kept in an ice bath in order to maintain a low temperature to avoid any unwanted explosion. The ice bath was removed after an hour whereas the stirring was continued for 48 h. The obtained slurry was treated with 300 ml of slightly warm distilled water followed by the addition of 20 ml of H2O2 to eliminate the excess amount of KMnO4 and synthesized MnO2 remains by converting them into water-soluble MnSO4. The resulting yellow colored suspension was then treated with the solution of H2SO4 (6%) and H2O2 (1%) to remove the remaining KMnO4. Subsequently, the reaction mixture was washed with the distilled water for several times to attain neutral pH. The as-synthesized GO was reduced to rGO by using hydrazine and ammonia as a reducing agent. In a typical procedure, 10 ml of GO-suspension was diluted with 0.09 L of deionized water, which then subjected to ultrasonication for 30 min. Consequently, in yellow-brown homogenous solution, 5 μL of hydrazine and 35 μL of ammonia were added by means of a micropipette and heated the solution at 95 °C for one hour. The resultant black suspension of rGO was dried and manually grinded to acquire rGO-powder.

2. Experimental 2.4. Synthesis of Glycine functionalized rGO 2.1. Chemicals and reagents To prepare Gly/rGO nanocomposite, 2.5 g of rGO was dispersed in 90 ml of 0.1 M glycine solution and ultrasonicated for half-an-hour. The obtained reaction mixture was kept unperturbed for the 12 h at the ambient temperature and subsequently treated with suitable amount of acetone to prevent further reaction. The resultant black colored precipitates were washed three-time with the distilled water followed by the final washing with acetone. The as-prepared nanocomposite was dried-up at 70 °C.

All the chemicals such as graphite powder (99%, Sigma-Aldrich), potassium permanganate (99%, Riedel-deHain), sodium nitrate (99%, Riedel-deHain), concentrated. hydrogen peroxide (35%, MERCK), sulfuric acid (95%, MERCK), aqueous ammonia (35%, BDH), and aniline (≥ 99.5%, Sigma-Aldrich) were purchased of analytical grade. Ammonium peroxydisulfate [APS (NH4)2S2O8] (≥ 98%), glycine (≥ 98.5%), hydrazine (80%), hydrochloric acid (37%), ethanol (≥99.5%), acetone (≥99.5%) cadmium nitrate tetrahydrate (98%), lead nitrate (≥99%), sodium hydroxide, nitrobenzene, cobalt chloride, copper chloride and other required chemicals were purchased from SigmaAldrich and used as received.

2.5. Synthesis of PANI Polyaniline was synthesized for comparison studies via chemical 2

Synthetic Metals 257 (2019) 116185

F. Hanif, et al.

was polished to a mirror-like shiny finish via 0.05 μm alumina slurry on a polishing pad followed by sonication in 1:1 ethanol-water mixture for 10 min to remove remaining alumina particles. After that electrode was thoroughly washed with ethanol, ultrapure water and dried at ambient temperature resulting in a smooth and shiny surface. The polishing step was repeated subsequently to every measurement in order to strip off the remaining modifier molecules. To prepare the sensor, modifiers were attached to the surface of the cleaned electrode via the dropcasting method. For this purpose, 5 μL drop of each modifier (rGO, Glycine, PANI, Gly/rGO and Gly/rGO/PANI) suspension (1 mg/1 ml) in 2:1 ethanol-water was deposited on to the surface of polished GCE and dried at room temperature. The resulting electrodes were designated as rGO/GCE, Gly/GCE, PANI/GCE, Gly/rGO/GCE and Gly/rGO/PANI/ GCE. Owing to the synergistic complexation effect of eNH2 and eCOO− groups of glycine and high surface area of rGO along with the conducting polymer (PANI) which reinforce the attachment of metal ions onto its surface via nitrogen functionalities for instance amine (-NH-) and imine (=N-) functional groups. All the adhered probing species effectively allocated extended active surface area, enhanced electrocatalytic activity, powerful adsorption capability, increased attachment sites and good electron tunneling properties to electrode [23,29,30]. The overall mechanisms of accumulation and anodic stripping of Pb2+ and Cd2+ on the surface of functionalized electrode during SWASV measurements can be described below, Where, M2+ = Pb2+ and Cd2+. The detailed sensing principle is stated in Scheme 2.

polymerization process by using ammonium peroxydisulfate as oxidative agent. In typical methodology, 20 ml of distilled aniline monomer was placed in a conical flask to which 40 ml of 1 M HCl was added under vigorous stirring for 20 min. The temperature of the above solution was retained at 0–5 °C by keeping the reaction vessel in an ice bath. To this mixture, 0.5 M solution of ammonium peroxydisulfate in 0.1 M HCl was added dropwise under continuous stirring of 2 h whilst maintaining the temperature of reaction medium lower than 5 °C. The subsequent suspension was agitated for 5 h at ambient temperature. The dark green colored precipitates confirmed the formation of polyaniline (PANI-HCl) which further deprotonated/neutralized. For this purpose, dark-green colored precipitates were stirred for several hours in 0.1 M aqueous solution of ammonia followed by numerous washings with distilled water to eliminate unreacted APS and solvent fractions. The final precipitates were dried at 40 °C to obtain a homogenous powder form. 2.6. Synthesis of Glycine functionalized rGO/PANI nanocomposite In situ oxidative polymerization process was employed to manufacture amino acid based nanocomposite, Gly/rGO/PANI NCs, in which ammonium peroxydisulfate (APS) was utilized as an oxidative agent. Specifically, the solution of glycine (0.1 M) was prepared by dispersing 1 g of glycine in 90 ml of distilled water at room temperature followed by the addition of 250 mg powder rGO. The resulting suspension was ultrasonicated for half-an-hour. To this mixture, 1 ml of aniline monomer was added during continuous agitation of 10 min on a magnetic stirrer. At the same time, 20 ml of 3 M APS solution was dropwise mixed to the above suspension following consistence stirring. Subsequently, the reaction mixture was unfastened from the stirrer and incubates the solution for the 12 h at room temperature. Afterwards, the collected black colored precipitates were dispersed in a suitable amount of acetone in order to stop the reaction. The final product was washed three-time with the distilled water followed by the final washing with acetone. The as-synthesized nanocomposites were kept at 70 °C for drying and manually grinded to obtain a powder. The entire procedure has been illustrated in Scheme 1.

3. Result and discussion 3.1. Characterization of Gly/rGO/PANI composites 3.1.1. XRD analysis Powder XRD diffraction patterns of graphite powder, GO, rGO, Gly/ rGO, and Gly/rGO/PANI are illustrated in Fig. 1A and B. Pure graphite flakes exhibit a sharp peak at 2θ = 26.4° which is shifted at 2θ = 10.07° corresponds to d-spacing of 0.34 nm and 1 nm respectively. This extensive change in the interlayer d-spacing is ascribed to the conversion of multilayer stacks of graphite into single layers in addition to the attachment of the oxygenated functionalities on the edges and basal planes of graphite sheets. Furthermore, the sharp peak of GO at 2θ = 10.07° disappeared on the exfoliation of the GO with hydrazine

2.7. Electrode polishing and sensor preparation Firstly, the working electrode, bare Glassy carbon electrode (3 mm)

Scheme 1. Schematic illustration of the entire procedure of synthesis of Gly/rGO/PANI. 3

Synthetic Metals 257 (2019) 116185

F. Hanif, et al.

Scheme 2. Representation of sensing reactions occurring at the working electrode surface for Cd2+ and Pb2+ detection.

3.1.3. FTIR analysis The Fourier transform infrared spectroscopy (FT-IR) is effectively utilized to study the diverse functional moieties of the substrate. FTIR spectra of graphite powder, GO, rGO, Gly, Gly/rGO and Gly/rGO/PANI are shown in Fig. 3(A–B). Graphene oxide (GO) is synthesized via hummers method from graphite flakes exhibits various intense peaks corresponds to diverse functionalities at 3350 cm−1, 1690 cm−1, 1560 cm−1, 1204 cm−1, and 1030 cm−1. The broad peaks at 3350 cm−1 and 1204 cm−1 are attributed to the eOH stretching and bending vibrations that stipulate the absorption of water molecules and occurrence of oxygen functionalities such as carboxylic groups. The remaining absorption bands at 1690 cm−1, 1560 cm−1, and 1030 cm−1 represents the C]O stretching vibrations of carbonyl and carboxylic groups, aromatic C]C vibrations, and CeO stretching vibrations respectively [37,38]. After the reduction of GO into rGO using hydrazine, obvious diminish in the peaks correlated with the oxygenated functionalities (carbonyl, epoxy, hydroxyl and carboxylic groups) are observed with a characteristic peak on 1524 cm−1 owing to restoration of the sp2 carbon networks in the aromatic domains of graphene sheets. Vanishing of characteristic broad band in rGO at 3350 cm−1 confirmed to the successful reduction of the hydroxyl groups attached on the graphene nanosheets [37–39]. In the spectrum of glycine, a range of peaks is observed with the obvious bands at 530 cm−1, 606 cm−1, 683 cm−1, 898 cm−1, 920 cm−1, 1040 cm−1, 1124 cm−1, 1320 cm−1, 1490 cm−1, 1580 cm−1, 2171 cm−1, 2601 cm−1, and 3105 cm-1 as seen in Fig. 3B. The vibrations at 530 cm−1, 606 cm−1, and 683 cm−1 confer to the carboxylic groups whereas the bands at 898 cm−1 and 1040 cm−1 are assigned to the CCN asymmetric and symmetric vibrations. The emergence of two peaks at 1124 cm−1 and 1490 cm−1 are evident of NH3+ group of zwitter ionic glycine together with the stretching vibrations of NH3+ group at 2601 cm−1 and 3105 cm−1. CH2 rocking and twisting vibrations appear at 920 cm−1 and 1320 cm-1 together with CO2-asymmetric vibrations at 1580 cm−1. The vibration band around 2171 cm-1 is ascribed to the combination bonds of the glycine [40]. After the modification of rGO with glycine to form Gly/ rGO an additional absorption peak to the peaks of glycine appears in the spectra at 1535 cm−1 due to the skeletal vibrations of reduced graphene nanosheets whereas the peak 656 cm-1 assigned to the carboxylic groups of glycine. The other absorption peaks are CeCeN asymmetric and CeCeN symmetric vibrations (872 cm−1 and

hydrate. After the reduction of the GO, the peak is shifted to 2θ = 24.04° holding the interplanar d-spacing of 0.36 nm. The observed peak in the rGO (2θ = 24.04°) was in close proximity with the diffraction peak of the graphite powder (2θ = 26.4°) which confirmed the removal of the oxygenated functionalities in the interspaces of the graphite. In case of Gly-rGO, the peaks are recorded at 19.5°, 24.06°, and 43.98°. The diffraction peaks at 19.5°and 43.98° with interrelated d-spacing of 0.45 nm and 0.20 nm are attributed to the peaks of glycine as reported in the literature [31,32] whereas the sharp peak which observed at 2θ = 24.06° with respective d-spacing of 0.36 nm interplanar, is assigned to the rGO. Comparative to the Gly-rGO, the composite Gly-rGO-PANI presented additional peaks at 26.14° (d-spacing =0.34 nm) and 30.36° (d-spacing =0.29 nm) which confirmed the formation of polyaniline from the aniline monomers on treatment with the APS. 3.1.2. UV/Visible studies Fig. 2(A–B) represents the UV/VIS patterns of graphite powder, GO, rGO, Glycine, Gly/rGO and Gly/rGO/PANI. The characteristic absorption peak of graphite powder at 268 nm is shifted to 230 nm in GO which exhibits п-п* transitions of CeC bonds in an aromatic ring. An additional flat peak is observed at 300 nm which is attributed to the characteristic n-п* transitions of C]O bonds. The peak in GO at 230 nm shows a red shift to approximately 260 nm after reduction with the hydrazine hydrate and ammonia which shows the elimination of the oxygen functionalities at the exterior of GO-nanosheets and the electronic conjugations of C]C in the basal planes of the GO nanosheets are recovered [33]. Glycine exhibits a peak at approximately 211 nm which is relatively in immediate proximity to the reported figures of glycine at 210 nm [32]. Two kinds of the characteristic peaks were observed in the UV-VIS spectra of the PANI; the first Plasmon peak at 371 nm owing to п-п* transitions and the other at 622 nm is corresponds to the n-п* electronic transitions between quinoid and benzenoid units [34,35]. In the case of Gly-rGO, there are two peaks at 229 nm and 266 nm which are attributed to the glycine and rGO respectively. The bathochromic shift in the peaks of both rGO and glycine is to the arising interactions in them and change in the particle size [36]. The UV–vis spectra of GlyrGO-PANI showed strong absorption peaks at 229 nm, 269 nm and 280 nm together with an additional flat peak at 590 nm correspond to glycine, rGO and polyaniline respectively.

Fig. 1. Powder XRD diffraction patterns of (A) graphite Powder, GO, rGO and (B) Gly/rGO, Gly/rGO/PANI. 4

Synthetic Metals 257 (2019) 116185

F. Hanif, et al.

Fig. 2. UV/VIS patterns of (A) Graphite powder, GO, rGO and (B) Glycine, Gly/rGO, Gly/rGO/PANI.

Fig. 3. FTIR spectra of (A) Graphite powder, GO, rGO and (B) Glycine, Gly/rGO, Gly/rGO/PANI.

1024 cm−1), NH3+ group (1123 cm−1 and 1472 cm−1), CO2-asymmetric vibrations (1598 cm−1), combination bonds of the glycine (2154 cm−1) and the stretching vibrations of NH3+ group (2602 cm−1, 3086 cm−1). The spectra of Gly/rGO/PANI interspersed with some additional absorption peaks (3231 cm−1, 1671 cm−1, 1589 cm−1, 1356 cm−1, 1275 cm−1, 1158 cm−1, 836 cm−1) matches with the characteristic peaks of polyaniline owing to the vibrations of various specific moieties. Various peak around 3231 cm-1 comparable with the NeH stretching vibrations. Strong CeN stretching vibrations arise at 836 cm−1, 1275 cm-1, and 1356 cm−1 together with the characteristic peak of the benzenoid ring in the PANI backbone at 1589 cm−1. The aromatic CeH bending bands and C]N stretching peaks of PANI marked at 1158 cm−1 and 1685 cm−1 respectively [41,42]. Hence, the designated FTIR spectra of Gly/rGO/PANI present the effective combination of precursors (rGO, Glycine, and polyaniline).

to the oxidation and reduction process can be investigated via the intensity ratio of the D and G bands (ID/IG) [45,46]. A high ID/IG ratio of 0.95 is obtained for graphene oxide which exhibits the successful installation of the oxygen functional moieties within the planes of the graphite sheets. After the process of reduction, low value of ID/IG ratio (0.83) is acquired this is credited to the removal of oxygen functionalities, reduction in the defects, and the restoration of sp2 structure of carbon. The in-plane sp2 crystallite size (La) for both GO and rGO is calculated via Tuinstra and Koening relation. The calculated crystallite size was slightly shifted from 20 nm in GO to 23.16 nm in rGO which indicates the restoration of sp2 domains in rGO [45,47]. Furthermore, two other broad bands for both GO and rGO were observed at higher wavenumber of ˜2710 cm−1 and 2930 cm−1. The bands at ˜2710 cm−1 are called 2D bands which arise due to the formation of monolayers of graphene in GO and rGO. Along with 2D bands, second order S3 bands were also acquired at 2930 cm−1 in GO and rGO owing to the combination of DeG bands [43,44,48]. The field emission scanning electron microscopy (FESEM) image of Gly/rGO/PANI composite is shown in Fig. 4B. The figure clearly reveals the coating of glycine and polyaniline (PANI) on thin folded sheets of reduced graphene oxide [29,30]. To determine the Brunauer –Emmett-Teller (BET) specific surface area and porosity of Gly/rGO/PANI nanocomposite, we measured N2 adsorption isotherm using N2 as an adsorbent. Fig. 4C exhibits BET isotherm curves of pure Gly/rGO/PANI nanocomposite. BET surface parameters determined from BET and BJH adsorption curves are given in Table 1. The BET calculated specific surface area of Gly/rGO/PANI nanocomposite is 162 m2/g. The enhanced surface area of Gly/rGO/PANI NC can be accredited to the preferential fictionalization of glycine and polyaniline on the large sheets of reduced graphene oxide (rGO). The high surface area and electrical conductivity of Gly/rGO/PANI nanocomposite may

3.1.4. Raman, SEM and BET analysis Raman spectroscopy is frequently employed to examine the crystallinity of substances by using monochromatic light. Fig. 4A shows Raman spectra of GO and rGO. In the typical Raman spectra of graphene oxide and reduced graphene oxide, two characteristic vibrational bands are observed including a G band which corresponds to the E2g phonon of the sp2 carbon atoms and a D vibrational band corresponding to the out-of-plane breathing mode of ĸ-point phonons of A1g symmetry [43,44]. The D band in the spectra of GO and rGO is ascertained at 1349 cm−1 and 1353 cm−1 along with the G band at 1604 cm−1 and 1598 cm−1 respectively. Due to installation of oxygen functionalities on the basal planes and edges of graphite, defects/disorders, vacancies, and change in grain size is observed in the graphene oxide which is depicted in the form of D vibrational bands. The distortion occurs due 5

Synthetic Metals 257 (2019) 116185

F. Hanif, et al.

Fig. 4. (A) Raman Spectra of GO, rGO (B) SEM image of composite (C) BET adsorption curve of composite.

electrocatalytic process. In case of rGO/GCE, redox peaks with much higher current are observed owing to the enhancement in the electrochemical binding sites, exceptional electrical conductivity and fast electron transfer rate due to the coated rGO film on GCE. Well-defined redox peak with an excellent increase in current is obtained on Gly/rGO modified GCE surface. In this case, high current of redox peak is assigned to the combined electrical properties of rGO and glycine which improved the charge transfer at the interface of the electrode and [Fe (CN)6] 3−/4− by increasing the binding sites and electrocatalytic behavior. The peak current of [Fe(CN)6] 3−/4− redox couple exhibits further improvement which symbolize enhanced electrocatalytic behavior, good electrical conductivity, large surface area and fast charge transfer rate of Gly/rGO/PANI/GCE. The electrochemically effective surface area of modified and unmodified electrodes was measured by cyclic voltammetry using Fe(CN)6] 3−/4− via Randles–Sevcik equation.

Table 1 BET parameters of Gly/rGO/PANI/ composite. Sample

Surface Area (BET)

Surface Area (BJH)

Pore Volume (BJH)

Pore Radius (BJH)

Gly/rGO/ PANI

162 m2/g

45 m2/g

0.041 cm3/g

1.5 nm

facilitate fast electron transfer rate resulting in enhanced redox peaks with much higher current. The pore radius and pore volume values are 1.5 nm and 0.041 cm3/g respectively. 3.2. Electrochemical measurements 3.2.1. EIS and CV responses The conductivity of unmodified and modified electrodes was inspected by using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). To get a wider understanding of the rate of charge transfer at the interface of electrodes and solution, cyclic voltammetry is executed. The CV-experiments were conducted in 5 mM of redox couple [Fe(CN)6]3−/4− and 0.1 M KCl solution at a scan rate of 100 mV/s for both unmodified and modified electrodes (rGO/GCE, Gly/ GCE, PANI/GCE, Gly/rGO/GCE, Gly/rGO/PANI/GCE). Cyclic voltammograms of bare GCE, rGO/GCE, Gly/GCE, PANI/GCE, Gly/rGO/GCE, Gly/rGO/PANI/GCE are displayed in Fig. 5A. The CVs of all modified electrodes exhibited the reversible electrochemical signals of the Fe (CN)63−/4− ions which shows higher current densities at the modified electrodes as compared to the unmodified GCE. A slight shift in the current is observed on the surface of PANI/GCE in comparison to the bare GCE which implies that the PANI amplified the electron transfer rate by increasing the active sites on GCE. In this case, an insignificant increase in current can be attributed to the semiconductive traits of polyaniline [49]. Gly/GCE showed an enhanced current peak which could be ascribed to the small particle size of the glycine [26] together with the contribution of other active groups (eNH2 and eCOOH) in

ip=2.69×105n3/2AC0D1/2v1/2

(1)

Where ip, C0 and D are the peak current, bulk concentration and diffusion coefficient of the Fe[(CN)6] 3−/4−redox probe, respectively. Whereas v represents the scan rate, n is the number of electron transfer (n = 1), and A signifies the electroactive surface area of the electrode. The calculated effective surface area for bare GCE, rGO/GCE, Gly/GCE, PANI/GCE, Gly/rGO/GCE, and Gly/rGO/PANI/GCE were 0.05 cm2, 0.075 cm2, 0.08 cm2, 0.1 cm2, 0.13 cm2, and 0.17 cm2. The electroactive surface area of Gly/rGO/PANI/GCE is approximately 3.4 times larger than that of bare GCE, demonstrating the up-graded charge transfer kinetics of modified electrode. These results ensure the improved sensitivity of Cd2+ and Pb2+ detection on the surface of Gly/ rGO/PANI/GCE as compared to the other modified electrodes such as rGO/GCE, Gly/GCE, PANI/GCE, Gly/rGO/GCE. Electron impedance spectroscopy aimed at inspecting the surface of electrode i.e. interfacial characteristics of electrode surface, in addition to the electron transfer ability of the system. EIS spectra of rGO/GCE, Gly/GCE, PANI/GCE, Gly/rGO/GCE and Gly/rGO/PANI/GCE are recorded in 5 mM of redox probe [Fe(CN)6]3−/4− and 0.1 M KCl solution 6

Synthetic Metals 257 (2019) 116185

F. Hanif, et al.

Fig. 5. (A) CV curves of bare GCE, rGO/GCE, Gly/GCE, PANI/GCE, Gly/rGO/GCE, and Gly/rGO/PANI/GCE in 5.0 mM redox probe [Fe(CN)6]3−/4− and 0.1 M KCl at a scan rate of 100 mV/s, (B) A Nyquist plot for bare GCE, rGO/GCE, Gly/GCE, PANI/GCE, Gly/rGO/GCE, and Gly/ rGO/PANI/GCE in 5.0 mM redox probe [Fe(CN)6]3−/4− and 0.1 M KCl. Inset illustrates Randel’s Equivalent circuit corresponds to the Nyquist diagrams.

3.2.2. Determination of Cd2+ and Pb+2 on modified electrode Square wave anodic Stripping voltammetry (SWASV) is a promising technique for the detection of toxic metal ion contaminants. In the present work, SWASV is employed for simultaneous determination of Cd2+ and Pb2+ utilizing an unmodified glassy carbon electrode (GCE) and 5 nanomaterials based modified sensors (PANI/GCE, Gly/GCE, rGO/GCE, Gly/rGO/GCE, and Gly/rGO/PANI/GCE). All the typical SWASV voltammograms of bare GCE and modified electrodes are shown in Fig. 6. For the bare GCE, weak signal of the metal ions was observed. The obvious rise in the current value is recorded with the modifiers (PANI/GCE, Gly/GCE, rGO/GCE, Gly/rGO/GCE, and Gly/ rGO/PANI/GCE) owing to the extended effective surface area of electrode as the modifiers adsorb on the surface of GCE resulting in the increase of the attachment sites of the electrode. Among the electrodes studied, Gly/rGO/PANI/GCE electrode has shown a better electrochemical response in the determination of target metal ions. The response attained on modified electrode is 8 fold higher than that of bare GCE. This enhanced current signal can be attributed to the synergistic complexation effect of eNH2 and eCOO− groups of glycine and high surface area of rGO [30,50]. Furthermore, the electrical conductivity and sensitivity of the electrode are improved by the conducting polymer (PANI) which also assist in the attachment of metal ions onto its surface via nitrogen functionalities for instance imine (]Ne) and amine (eNHe) functional groups [27]. The resulting oxidation peak of lead (Pb) and cadmium (Cd) emerged around −0.50 V and −0.76 V respectively.

with the AC frequency swept from 10 kHz to 0.1 Hz. As depicted inset of Fig. 5B, the typical impedance spectrum is observed in the form of the Nyquist plot, further fitted with Randle’s circuit model. Randle’s circuit model contains various components such as Warburg impedance (W), solution resistance (Rs), a constant phase element (CPE) and charge transfer resistance (Rct) instigated by the diffusion of ions and charge transfer reaction at the interface of electrode and electrolyte. Each Nyquist plot comprise a semicircle at higher frequencies correlated to electron transfer limited process whereas a straight line across low frequencies which ascribed to diffusion limited process. In Nyquist plot, the diameter of the semicircle section is corresponding to the electron transfer resistance Rct. The electron transfer resistance Rct is expressly employed as a basic parameter to assess the interfacial features of surface-functionalized electrodes. The obtained EIS experimental data explicitly revealed facile electron conductivity and electrochemical kinetics of Gly/rGO/PANI/GCE in respect of the bare GCE. The reduction in the Rct value with the modification of the electrode with Gly/rGO/ PANI nanocomposites may well attributed to the enhanced electroactive surface area, excellent electrocatalytic behavior, good electron tunneling properties, and fast electron transfer rate. The empirical results of CV and EIS were in agreement with each other signifying the successful development of the glassy carbon electrode with receptor materials. To further reveal the properties of the modified electrode, the heterogeneous electron transfer rate constant ko was computed for redox probe [Fe(CN)6]3−/4− at the surface of unmodified and modified electrodes by applying following equation; Rct = RT/ (nF)2Ak0C0

(2)

Where Rct refers to the electron transfer resistance from the corresponding impedance plots of the functionalized electrodes, R shows the ideal gas constant (j K−1 mol−1), whereas the T is the temperature measured in K, and F represents the faraday constant. A mark the geometric surface area of the electrode (cm2), Co is the molar concentration of the [Fe(CN)6]3−/4− and n is the electrons transferred per molecule of the redox couple. The calculated k0 values were 1.45 × 104 cms−1, 1.8 × 10-4 cms−1, 2.0 × 10-4 cms−1, 2.5 × 10-4 cms−1, 4.4 × 10-4 cms−1, and 6.3 × 10-4 cms−1 for bare GCE, rGO/GCE, Gly/ GCE, PANI/GCE, Gly/rGO/GCE, and Gly/rGO/PANI/GCE respectively. The k0 value of Gly/rGO/PANI/GCE is greatly enhanced as compared to the rGO/GCE, Gly/GCE, PANI/GCE, and Gly/rGO/GCE demonstrating the fast electron transfer reaction occur at the interface of the Gly/rGO/ PANI/GCE and the redox couple [Fe(CN)6]3−/4−. Consequently, reduced Rct value and phase angle value together with the amplified k0 value after immobilization of Gly/rGO/PANI is attributed to the synergetic complexation phenomenon of the sensing species at the electrode surface which improve the electron transfer rate of [Fe(CN)6]3−/ 4− redox couple and electrical conductivity within the grafted film of nanocomposites.

Fig. 6. SWASVs of 1.4 μM of Cd2+ and Pb2+ in 0.1 M NaA C-H Ac (pH 5) at Bare GCE, PANI/GCE, Gly/GCE, rGO/GCE, Gly/rGO/GCE, and Gly/rGO/PANI/ GCE, Deposition potential: -1.2 V, Deposition time: 120 s. 7

Synthetic Metals 257 (2019) 116185

F. Hanif, et al.

Fig. 7. (A) Effect of deposition potential on the stripping peak current of 1.4 μM Pb+2 and Cd+2 in 0.1 M NaA C-H Ac (pH 5.0) at Gly/ rGO/PANI/GCE. (B) Effect of deposition time on the stripping peak current of 1.4 μM Pb2+ and Cd2+ in 0.1 M NaA C-H Ac (pH 5.0) on Gly/rGO/PANI/GCE. (C) Effect of various supporting electrolytes (BRB (pH = 5), 0.1 M HCl, 0.1 M NaOH and NaA C-H Ac (pH = 5) at Gly/rGO/PANI/GCE in 1.4 μM Pb2+ and Cd2+. All the data was collected via SWASV responses.

respect to the response toward Pb2+ and Cd2+ on the Gly/rGO/PANI/ GCE electrode using SWASV. Optimum sensitivity with the highest current response of the anodic stripping peaks of Pb2+ and Cd2+ was obtained in 0.1 M NaAC–HAc solution. Thus, for further electroanalysis, HAc–NaAc was preferred as stripping electrolyte. The pH-value have significant influence onto the size of the square wave voltammetric peaks and also assists in the hydrolysis of metal ions, therefore, it is crucial to choose a suitable pH-value for the sensing of metal ions. For simultaneous sensing of Cd2+ and Pb2+ ions, the stripping peak response is measured in 0.1 M HAc–NaAc buffer in the pH range of 4.0–6.0 as shown in Fig. 8A. The Fig. 8B represents the response of Ip as a function of the pH values. In the simultaneous detection process of Cd2+ and Pb2+ ions, the maximum peak current was recorded at pH 5.0. At the highly acidic pH values, less peak current is related to the protonation of hydrophilic groups on the surface of sensing material which leads to the decrease in the attachment sites for the adsorption of the heavy metal ions [51]. Contrarily, the reduction in the stripping peaks at high pH value exhibits an obvious decrease due to the precipitation of metal ions in the form of metal hydroxide complexes which considerably decrease the existing number of metal ions in the solution that may possibly adsorb onto the surface of electrode [56]. Hence, the optimal pH-value was determined to be 5 for further studies.

3.2.3. Optimization of performance parameters To prevent the overlapping of metal ions in solution and to meet the highest performance of modified electrodes, numerous approaches have been designated to resolve these issues. In the current research article, we carefully assessed the influence of scan rates, deposition potential, pH, supporting electrolytes, and interfering agents on the performance of modified electrolytes. The effect of deposition potential on the anodic peak current response of Cd2+ and Pb2+ was investigated between −1.4 to −1.1 V in optimal pH medium and the results are shown in Fig. 7A. In the metal ions solution, the stripping peak current response remarkably elevated with the negative shift of applied deposition potential having the highest value at -1.2 V whereas diminishes at higher negative potentials. The decrease in stripping current is attributed to the inadequate accumulation of the metal ions at lower negative potential and the initiation of hydrogen evolution reaction at a higher negative potential that may damage the surface of the electrode [51,52]. Consequently, for preferential reduction and deposition of metal ions onto the surface of the electrode, -1.2 V is selected as the optimum depositional potential. The dependence of stripping peaks response on the deposition time was examined in the range of 30–210 s and obtained results are shown in Fig. 7B. The stripping peak currents showed a linear increase with a prolonged period holding the maximum peak current at 120 s. But at a deposition time beyond 120 s, the curved patterns of plot begin to exhibits an obvious decline with no oblivious change of peak current, indicating the saturation of all the possible attachment sites on the Gly/ rGO/PANI/GCE electrode by the adsorption of the heavy metal contaminants [53]. Thus, 120 s was preferred as the ideal deposition time for further studies. Having an impact on the sensitivity of the sensor, the deposition potential is measured as an imperative parameter. Supporting electrolytes are intended to purge off the electro-migration effect [54,55]. Therefore, the stripping voltammetric response of the peaks of current for the metal ions determination was also assessed by varying the nature of the stripping medium as illustrated in Fig. 7(C). The HAc–NaAc, BRB, HCl, and NaOH were compared in

3.2.4. Analytical features and stripping behavior of modified electrode SWASV electroanalytical responses for the simultaneous determinations of Cd2+ and Pb2+ were recorded at varying concentrations from 1.0 μM - 0.0001 μM were illustrated in Fig. 9A. The consequential experimental responses of the current peaks are extremely valuable in calculating the performance of the sensor and its analytical features. The current of both Cd2+ and Pb2+ exhibited a linear increase with the increase in the concentration of target metal ions over the range of 1.0 μM-0.0001 μM (1000 nM-0.1 nM). The linear regression coefficient (R2) is calculated from the linear calibration curves. The linear correlation curves were fitted by, I / μA = 3.97e−7 + 6.16 C / μM (R2 = 0.995) for Cd2+ at S/N = 3 8

Synthetic Metals 257 (2019) 116185

F. Hanif, et al.

Fig. 8. (A) The influence of pH on the SWASV stripping peak 1.4 μM of both Pb2+ and Cd2+ in 0.1 M NaA C-H Ac (pH 5.0) at Gly/rGO/PANI/GCE Error bar: n = 5. (B) Calibration Plots of I/μA vs. pH, Error bar: n = 5, of each of target metal ion.

3.2.5. Estimation of selectivity, stability, interferences, reproducibility, and repeatability The reproducibility and repeatability of the designated electrode are determined by performing a variety of the experiment of SWASV in 1.4 μM solution of metal ions by applying optimal parameters. To ensure the repeatability/stability of the modified sensor (Gly/rGO/PANI/ GCE), 10 successive cycles were operated on the same modified electrode which resulted in the RSD (%) of 3.20% and 4.13% for Cd2+ and Pb2+ respectively. In an effort to measure the reproducibility of the modified Gly/rGO/PANI/GCE sensor, 5 electrodes were developed via identical procedures. The maximum recorded deviations were 1.24% for Cd2+ and 1.84% for Pb2+. The RSD values for stability and reproducibility are less than 5% which showed the developed sensor exhibited good repeatability and reproducibility. Owing to the presence of the various ionic species and organic compounds in the drinking water, selective detection of metal ions become a challenge. Therefore, influence of diverse existing cations and anions onto the peak signals of Cd2+ and Pb2+ is examined through the SWASV responses. The obtained results at the Gly/rGO/PANI/GCE electrode for 100 folds higher concentration of Cu2+, Co2+, Cl1− and nitrobenzene (NB) are represented in Fig. 10A & B. The peak current for oxidation of the Cd(0) and Pb(0) on the surface of Gly/rGO/PANI/GCE modified electrode exhibited no obvious change despite other coexisting ions and NB. The anodic peak currents of the Cd2+ and Pb2+ in the presence and absence of foreign species and their relative signal changes are presented in the Table 3. As illustrated in Fig. 10B, Cu2+ is the most interfering agent. The main reason of which is the high affinity of Cu2+ ions towards the amine groups (eNH2) of glycine and aniline which are employed to modify the electrode [57]. Except Cu2+ ions, all

and I / μA = 4.50 e−7 + 7.03 C / μM (R2 = 0.995) for Pb2+ at S/N = 3 Various electroanalytical parameters such as the linear range (1.0 μM-0.0001 μM), Limit of detection (LOD = 3S.D./m) and limit of quantification (LOQ = 10*S.D./m); (where S.D. refers to the standard deviation of the blank and m represents the slope of the calibration line) were also determined by using the calibration curve as found in Fig. 9B. The LOD for Cd2+ was estimated to be 7.0 × 10−11 M and 7.2 × 10−11 M for Pb2+ with the LOQ values of 2.3 × 10-10 M and 2.4 × 10-10 M for Cd2+ and Pb2+ and respectively. The proposed sensor (Gly/rGO/PANI/ GCE) demonstrated to have excellent potential to detect the Cd2+ and Pb2+ in water which is considerably lower than the permitted limit for potable water set by the WHO (Cd2+ = 5 μg/L and Pb2+ = 10 μg/L). This can be attributed to the combined effect of modifiers including the effective binding of metal ions on the functional active groups for instance, eC]O, eNH2 and eCOOH, eOH groups of glycine and polyaniline together with the high surface area of rGO [27,30,50]. The analytical features of the modified sensor (Gly/rGO/PANI/GCE) were also compared with the previous reports for sensitive measurement of Cd2+ and Pb2+ (Table 2). Hence, the analytical performance of the modified sensor (Gly/rGO/PANI/GCE) showed an obvious lower LOD in contrast to most of the reported modified electrodes as listed in Table 2.

Fig. 9. (A) SWASV curves at Gly/rGO /PANI/GCE with different concentration of Cd2+ and Pb2+ and (B) calibration curves obtained for various Cd2+ and Pb2+ concentrations by SWASV in 0.1 M NaA C-H Ac (pH 5.0) at Gly/rGO/PANI/GCE. 9

Synthetic Metals 257 (2019) 116185

F. Hanif, et al.

Table 2 Statistical comparison of the analytical performance of stripping Cd2+ and Pb2+ ions using Gly/rGO/PANI/GCE with the other reported figures from literature. Sensing Substrate

Electrode

Electroanalytical Technique

Linear Range Cd

GO/ĸ-Car/L-cys G/PANI/PS nanoporous fiber G/PANI RGO-CS/PLL PA/PPy/GO Gly Nafion/G/PANI L-cys/GR-CS Bi/Au-GN-Cys MnFe2O4@Cys L-cys/AuNPs/NG Au@SiO2@Fe3O4/NG sGO/PPy L-cys-rGO Gly/rGO/PANI

GCE SPCE SPE GCE GCE GCE SPCE GCE GCE GCE GCE GCE SPE GCE GCE

SWASV SWASV SWASV DPASV DPV DPASV SWASV DPASV SWASV SWASV SWV SWV DPASV DPASV SWASV

18

A

5 to 50 nM 10-500 μg/L 1-300 μg/L 0.05-10 μg/L 5–150 μg/L – 1-300 μg/L 1.04-62.1 μg/L 0.50-40 μg/L 0.2-1.1 M 1–80 μg/L 5-80 μg/L 1.4 − 14,000 ppb 0.4-1.2 μM 0.0001-1 μM

0.58 nM 4.43 μg/L 0.1 μg/L 0.01 μg/L 2.13 μg/L 0.48 nM 0.1 μg/L 0.45 μg/L 0.10 μg/L 0.221 μM – – – 0.366 μg/L 0.07 nM

1.08 nM 3.30 μg/L 0.1 μg/L 0.02 μg/L 0.14 μg/L – 0.1 μg/L 0.12 μg/L 0.05 μg/L 0.0607 μM 0.056 μg/L 0.6 μg/L 0.07 ppb 0.416 μg/L 0.072 nM

[41] [47] [25] [48] [49] [22] [25] [50] [51] [52] [53] [54] [26] [55] This Work

Peak current (μA)

Relative signal change (%)[(I0/Ii – 1) × 100]

Cd2+

Pb2+

Cd2+

Pb2+

12.5 12.3 11.9 12.1 12.2

16.5 16.3 15.0 16.0 16.1

– 1.6 4.8 3.2 2.4

– 1.2 9 3 2.4

Lead (Pb) Cadmium (Cd)

B

16 14

Co2+ Pb2+

Ip / ( μ A )

Cd2+

Pb

on inexpensive materials (rGO, glycine, and polyaniline) with excellent electrochemical characteristics. We effectively synthesized glycine functionalized reduced graphene oxide/polyaniline nanocomposite (Gly/rGO/PANI) by using in-situ oxidative polymerization method. Fourier transform infrared spectroscopy, X-ray diff ;raction, UV/VIS spectroscopy, SEM, BET and Raman studies verified the successful incorporation of glycine into rGO/PANI compoiste. The electrochemical aspects of glassy carbon electrode decorated with Gly/rGO/PANI nanocomposite were elucidated by electrochemical impedance spectroscopy and cyclic voltammetry. The findings of these techniques verified the successful immobilization of sensing material on the surface of GCE. The stripping properties of the sensor were analyzed via square wave anodic stripping voltammetry. This novel platform demonstrated to have good sensitivity, low limit of detection, high selectivity, good reproducibility and repeatability with excellent practical applicability

Cu2+

20

Cd

No interference ions Co2+ Cu2+ Cl1− Nitrobenzene (NB)

This article demonstrates an efficient electrochemical sensor based

30

2+

Pb

Interference ions

4. Conclusion

NB Cu2+

References

2+

Table 3 Effect of interfering ions on the detection of Cd2+ and Pb2+ and their relative signal change.

3.2.6. Analysis of real sample to validate the proposed methodology The proposed methodology is verified by the detection of traces of cadmium and lead in tap water at Gly/rGO/PANI/GCE electrode via the standard addition method under the optimal experimental conditions. Recovery percentages together with RSD (%) of the metal ions (Cd2+ and Pb2+) are listed in the Table 4. By spiking the sample tap water with a known amount of metal ions Cd2+ and Pb2+, the recovery for the simultaneous detection was estimated to be 102% and 105% for Cd2+ and Pb2+ with the RSD value of 1.4% and 3.45%, respectively. Thus, there were no considerable differences recorded between the actual and calibrated concentrations of the metal ions, demonstrating the accuracy and precision of the Gly/rGO/PANI/GCE electrode which can be effectively applied to analyze water polluted by Cd2+ and Pb2+ ions.

Co2+ No interference Cl1-

2+

5 to 50 nM 10-500 μg/L 1-300 μg/L 0.05-10 μg/L 5–150 μg/L 0.005-5 nM 1-300 μg/L 0.56-67.2 μg/L 0.50-40 μg/L 0.2-1.1 M – – – 0.4-0.2 μM 0.0001-1 μM

other coexisting interfering agents presented a small signal change with the absolute value of relative signal change less than 4% imply the greater selectivity of the Gly/rGO/PANI/GCE modified electrode for Cd2+ and Pb2+. These results may well credited to the selective attachment of metal ions (Cd2+ and Pb2+) from the sample solution onto the surface of electrode owing to the strong affinity of Gly/rGO/PANI composite for these ions. Therefore, the procedure can be efficiently applied to detect metal ions (Cd2+ and Pb2+) even in the presence of high content of interfering species.

40

LOD

2+

12 10 8 6

10

4 2

0 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

0 A

B

C

D

E

Fig. 10. Study of influence of various interfering agents (A = Co2+, B = Cu2+, C = NB, D = Cl1−, and E = No interference) on Cd2+ and Pb2+ in 0.1 M NaA C-H Ac (pH 5) at Gly/rGO/PANI/GCE. The concentration of each interfering ions was 100 folds greater than Cd2+ and Pb2+. Data were collected via SWASV responses. 10

Synthetic Metals 257 (2019) 116185

F. Hanif, et al.

Table 4 Statistical comparison of Cd2+ and Pb+2 detection in tap water under optimal factors using Gly/rGO/PANI/GCE. Sample Analyte Tap water Tap water-1

Actual Concentration (μM) 2+

Pb 0 1.0

2+

Cd 0 1.0

Calibrated Concentration (μM) 2+

Pb N.D. 1.05 ± 0.09

2+

Cd N.D. 1.02 ± 0.15

and long-term stability in field application to monitor heavy metal contents. The enhancement of electrical conductivity, electron tunneling and ion-trapping properties of functionalized electrode displayed the excellent electrocatalytic activity and synergistic complexation effect of active moieties of glycine and polyaniline such as –NH2, −COOH, =N- and, -NH along with the increased active surface area. The designed sensor demonstrated excellent recovery percentages (102% for Cd2+ and 105% for Pb2+) and remarkable sensitivity of 41.3 μA μM−1 cm-2 for Pb2+ and 36 μA μM−1 cm-2 for Cd2+. The detection limits of 0.07 nM Cd2+ and 0.072 nM for Pb2+ were estimated to be below than the suggested figures by world health organization (WHO) and environmental protection agency (EPA). The efforts taken in this work will turn-up an innovative pathway in the implementation of a modified electrode for the sensitive detection of heavy Metal ions (HMIs).

Relative Standard Deviation (%) 2+

Pb – 3.45

2+

Cd – 1.40

Recovery (%) Pb2+ – 105

Cd2+ – 102

[17] S.J. Rowley-Neale, et al., An overview of recent applications of reduced graphene oxide as a basis of electroanalytical sensing platforms, Appl. Mater. Today 10 (2018) 218–226. [18] Y. Zuo, et al., Voltammetric sensing of Pb (II) using a glassy carbon electrode modified with composites consisting of Co3O4 nanoparticles, reduced graphene oxide and chitosan, J. Electroanal. Chem. 801 (2017) 146–152. [19] H. Xing, et al., Highly sensitive simultaneous determination of cadmium (II), lead (II), copper (II), and mercury (II) ions on N-doped graphene modified electrode, J. Electroanal. Chem. 760 (2016) 52–58. [20] Y. Zuo, et al., Utilization of AuNPs dotted S-doped carbon nanoflakes as electrochemical sensing platform for simultaneous determination of Cu (II) and Hg (II), J. Electroanal. Chem. 794 (2017) 71–77. [21] Y. Zuo, et al., Poly (3, 4-ethylenedioxythiophene) nanorods/graphene oxide nanocomposite as a new electrode material for the selective electrochemical detection of mercury (II), Synth. Met. 220 (2016) 14–19. [22] M. Zhou, Y. Zhai, S. Dong, Electrochemical sensing and biosensing platform based on chemically reduced graphene oxide, Anal. Chem. 81 (14) (2009) 5603–5613. [23] C. Gao, et al., AlOOH-reduced graphene oxide nanocomposites: one-pot hydrothermal synthesis and their enhanced electrochemical activity for heavy metal ions, ACS Appl. Mater. Interfaces 4 (9) (2012) 4672–4682. [24] C. Xu, X. Wang, J. Zhu, Graphene− metal particle nanocomposites, J. Phys. Chem. C 112 (50) (2008) 19841–19845. [25] L. Cui, J. Wu, H. Ju, Electrochemical sensing of heavy metal ions with inorganic, organic and bio-materials, Biosens. Bioelectron. 63 (2015) 276–286. [26] A. Nisar, et al., Sensitive and selective detection of multiple metal ions using amino acids modified glassy carbon electrodes, J. Electrochem. Soc. 165 (3) (2018) B67–B73. [27] S. Muralikrishna, et al., Hydrogels of polyaniline with graphene oxide for highly sensitive electrochemical determination of lead ions, Anal. Chim. Acta 990 (2017) 67–77. [28] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (6) (1958) p. 1339-1339. [29] N. Ruecha, et al., Sensitive electrochemical sensor using a graphene–polyaniline nanocomposite for simultaneous detection of Zn (II), Cd (II), and Pb (II), Anal. Chim. Acta 874 (2015) 40–48. [30] R. Seenivasan, W.-J. Chang, S. Gunasekaran, Highly sensitive detection and removal of lead ions in water using cysteine-functionalized graphene oxide/polypyrrole nanocomposite film electrode, ACS Appl. Mater. Interfaces 7 (29) (2015) 15935–15943. [31] K. Pal, A.K. Banthia, D.K. Majumdar, Polyvinyl alcohol–glycine composite membranes: preparation, characterization, drug release and cytocompatibility studies, Biomed. Mater. 1 (2) (2006) 49. [32] J.S. McConnell, R.M. McConnell, L.R. Hossner, Ultraviolet spectra of acetic acid, glycine, and glyphosate, J. Ark. Acad. Sci. 47 (1) (1993) 73–76. [33] Z. Ciplak, N. Yildiz, A. Calimli, Investigation of graphene/Ag nanocomposites synthesis parameters for two different synthesis methods, Fuller. Nanotub. Carbon Nanostruct. 23 (4) (2015) 361–370. [34] D. Sindhimeshram, M. Gupta, Transport Properties of Substituted Derivatives of Poly, aniline (1995). [35] J. Stejskal, P. Kratochvil, N. Radhakrishnan, Polyaniline dispersions 2. UV—vis absorption spectra, Synth. Met. 61 (3) (1993) 225–231. [36] S.V. Kumar, et al., One-step size-controlled synthesis of functional graphene oxide/ silver nanocomposites at room temperature, Chem. Eng. J. 219 (2013) 217–224. [37] C. Manoratne, S. Rosa, I. Kottegoda, XRD-HTA, UV Visible, FTIR and SEM interpretation of reduced graphene oxide synthesized from high purity vein graphite, Mater. Sci. Res. India 14 (1) (2017) 19–30. [38] J. He, L. Fang, Controllable synthesis of reduced graphene oxide, Curr. Appl. Phys. 16 (9) (2016) 1152–1158. [39] F.T. Johra, W.-G. Jung, Effect of pH on the synthesis and characteristics of RGO–CdS nanocomposites, Appl. Surf. Sci. 317 (2014) 1015–1021. [40] S.A.C. Azhagan, S. Ganesan, Effect of zinc acetate addition on crystal growth, structural, optical, thermal properties of glycine single crystals, Arab. J. Chem. 10 (2017) S2615–S2624. [41] M. Ibrahim, E. Koglin, Spectroscopic study of polyaniline emeraldine base: modelling approach, Acta Chim. Slov. 52 (2) (2005) 159–163. [42] S. Shahabuddin, R. Khanam, M. Khalid, N.M. Sarih, J.J. Ching, S. Mohamad, R. Saidur, Synthesis of 2D boron nitride doped polyaniline hybrid nanocomposites for photocatalytic degradation of carcinogenic dyes from aqueous solution, Arab. J. Chem. 11 (2018) 1000–1016. [43] F.T. Johra, J.-W. Lee, W.-G. Jung, Facile and safe graphene preparation on solution based platform, J. Ind. Eng. Chem. 20 (5) (2014) 2883–2887. [44] N. Hidayah, et al., Comparison on graphite, graphene oxide and reduced graphene oxide: synthesis and characterization, AIP Conference Proceedings (2017). [45] G. Eda, M. Chhowalla, Chemically derived graphene oxide: towards large‐area thin‐film electronics and optoelectronics, Adv. Mater. 22 (22) (2010) 2392–2415.

Acknowledgements We are thankful to the GSCWU-Bahawalpur (Pakistan), The Islamia University of Bahawalpur (Pakistan) and Higher Education Commission (HEC) of Pakistan. Authors from King Saud University (Riyadh, Saudi Arabia) sincerely appreciate the King Saud University for their contribution in this reserach through Researchers Supporting Project RSP2019/49. References [1] A.S. Mohammed, A. Kapri, R. Goel, Heavy metal pollution: source, impact, and remedies, Biomanagement of Metal-Contaminated Soils, Springer, 2011, pp. 1–28. [2] E. Rosenberg, Heavy metals in water: presence, removal and safety, Johnson Matthey Technol. Rev. 59 (4) (2015) 293–297. [3] A. Jan, et al., Heavy metals and human health: mechanistic insight into toxicity and counter defense system of antioxidants, Int. J. Mol. Sci. 16 (12) (2015) 29592–29630. [4] M. Jaishankar, et al., Toxicity, mechanism and health effects of some heavy metals, Interdiscip. Toxicol. 7 (2) (2014) 60–72. [5] S. Mishra, A.K.M. Tiwari, A.A. Mahdi, Impact of heavy metal carcinogens on human health, in: M. Rai, A.P. Ingle, S. Medici (Eds.), Biomedical Applications of Metals, Springer International Publishing: Cham., 2018, pp. 277–295. [6] M. Tuzen, Determination of heavy metals in soil, mushroom and plant samples by atomic absorption spectrometry, Microchem. J. 74 (3) (2003) 289–297. [7] E.P. Nardi, et al., The use of inductively coupled plasma mass spectrometry (ICPMS) for the determination of toxic and essential elements in different types of food samples, Food Chem. 112 (3) (2009) 727–732. [8] A. Prange, A. Knochel, W. Michaelis, Multi-element determination of dissolved heavy metal traces in sea water by total-reflection X-ray fluorescence spectrometry, Anal. Chim. Acta 172 (1985) 79–100. [9] G. Zarazua, et al., Analysis of total and dissolved heavy metals in surface water of a Mexican polluted river by total reflection X-ray fluorescence spectrometry, Spectrochim. Acta Part B Atomic Spectrosc. 61 (10–11) (2006) 1180–1184. [10] X. Dai, S. Wu, S. Li, Progress on electrochemical sensors for the determination of heavy metal ions from contaminated water, J. Chin. Adv. Mater. Soc. 6 (2) (2018) 91–111. [11] G. March, T.D. Nguyen, B. Piro, Modified electrodes used for electrochemical detection of metal ions in environmental analysis, Biosensors 5 (2) (2015) 241–275. [12] B. Bansod, et al., A review on various electrochemical techniques for heavy metal ions detection with different sensing platforms, Biosens. Bioelectron. 94 (2017) 443–455. [13] Y. Lu, et al., A review of the identification and detection of heavy metal ions in the environment by voltammetry, Talanta 178 (2018) 324–338. [14] J. Chang, et al., Graphene-based sensors for detection of heavy metals in water: a review, Anal. Bioanal. Chem. 406 (16) (2014) 3957–3975. [15] S. Roy, et al., Graphene oxide for electrochemical sensing applications, J. Mater. Chem. 21 (38) (2011) 14725–14731. [16] D. Chen, H. Feng, J. Li, Graphene oxide: preparation, functionalization, and electrochemical applications, Chem. Rev. 112 (11) (2012) 6027–6053.

11

Synthetic Metals 257 (2019) 116185

F. Hanif, et al.

[46] S. Perumbilavil, et al., White light Z-scan measurements of ultrafast optical nonlinearity in reduced graphene oxide nanosheets in the 400–700 nm region, Appl. Phys. Lett. 107 (5) (2015) 051104. [47] S.M. Hafiz, et al., A practical carbon dioxide gas sensor using room-temperature hydrogen plasma reduced graphene oxide, Sens. Actuators B Chem. 193 (2014) 692–700. [48] H. Wang, et al., Solvothermal reduction of chemically exfoliated graphene sheets, J. Am. Chem. Soc. 131 (29) (2009) 9910–9911. [49] Y. Fu, et al., Preparation of polyaniline-encapsulated carbon/copper composite nanofibers for detection of polyphenol pollutant, Colloids Surf. A Physicochem. Eng. Asp. 559 (2018) 289–296. [50] Ge.K. Ramesha, S. Sampath, Exfoliated graphite oxide modified electrode for the selective determination of picomolar concentration of lead, Electroanalysis 19 (23) (2007) 2472–2478. [51] H. Huang, et al., Ultrasensitive and simultaneous detection of heavy metal ions based on three-dimensional graphene-carbon nanotubes hybrid electrode materials, Anal. Chim. Acta 852 (2014) 45–54. [52] S. Zhang, et al., A self-supported electrochemical sensor for simultaneous sensitive

[53]

[54] [55]

[56]

[57]

12

detection of trace heavy metal ions based on PtAu alloy/carbon nanofibers, Anal. Methods 9 (48) (2017) 6801–6807. T. Priya, et al., A novel voltammetric sensor for the simultaneous detection of Cd2+ and Pb2+ using graphene oxide/κ-carrageenan/l-cysteine nanocomposite, Carbohydr. Polym. 182 (2018) 199–206. E.-R.E. Mojica, et al., Voltammetric determination of lead (II) ions at carbon paste electrode modified with banana tissue, J. Appl. Sci. 7 (9) (2007) 1286–1292. L. Oularbi, M. Turmine, M. El Rhazi, Preparation of novel nanocomposite consisting of bismuth particles, polypyrrole and multi-walled carbon nanotubes for simultaneous voltammetric determination of cadmium(II) and lead(II), Synth. Met. 253 (2019) 1–8. Y. Song, et al., The graphene/l-cysteine/gold-modified electrode for the differential pulse stripping voltammetry detection of trace levels of cadmium, Micromachines 7 (6) (2016) 103. S.L. Jiokeng, et al., Sensitive stripping voltammetry detection of Pb (II) at a glassy carbon electrode modified with an amino-functionalized attapulgite, Sens. Actuators B Chem. 242 (2017) 1027–1034.