Accepted Manuscript Title: Surfactant modified glassy carbon electrode as an efficient sensing platform for the detection of Cd (nullnull) and Hg (nullnull) Authors: Anum Zahid, Afzal Shah, Faiza Jan Iftikhar, Aamir Hassan Shah, Rumana Qureshi PII: DOI: Reference:
S0013-4686(17)30600-X http://dx.doi.org/doi:10.1016/j.electacta.2017.03.120 EA 29154
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
15-11-2016 20-2-2017 16-3-2017
Please cite this article as: Anum Zahid, Afzal Shah, Faiza Jan Iftikhar, Aamir Hassan Shah, Rumana Qureshi, Surfactant modified glassy carbon electrode as an efficient sensing platform for the detection of Cd (x4cf;x4cf;) and Hg (x4cf;x4cf;), Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.03.120 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Surfactant modified glassy carbon electrode as an efficient sensing platform for the detection of Cd (ӏӏ) and Hg (ӏӏ) * * Anum Zahida, Afzal Shaha , Faiza Jan Iftikhara, Aamir Hassan Shaha,b and Rumana Qureshia
a
Department of Chemistry, Quaid-i-Azam University, 45320, Islamabad, Pakistan
b
University of Chinese Academy of Sciences, Beijing 100049, China
*To whom correspondence should be addressed Tel: +92-5190642110 Fax: +92-5190642241 E-mail:
[email protected] (Afzal Shah),
[email protected] (Aamir Hassan Shah)
1
Abstract A highly sensitive electrochemical sensor using a novel surfactant 1-cyclohexyl-3dodecanoylthiourea (CDT) as a recognition layer was developed for the detection of Cd2+ and Hg2+ using electrochemical impedance spectroscopy, chronocoulometry and a number of voltammetric techniques such as cyclic, square wave and differential pulse stripping voltammetry. For optimization, the effect of several experimental factors such as concentration of the surfactant, scan rate, pH, accumulation time, number of cycles and supporting electrolytes were investigated. The limits of detection quite lower than the danger level suggested by world health organization (WHO), Environmental protection agency (EPA) and European water quality (EWQ) suggested the suitability of our designed sensor for monitoring metal based water toxins. Theoretical studies carried out for the calculation of interaction energy of CDT with Hg2+ and Cd2+ supported the experimental findings. The validity of the proposed method for real water sample analysis was ensured from reasonable percentage recoveries and less than 4 % RSD values. Moreover, the designed sensor demonstrated excellent discrimination ability for the target analytes in the presence of interfering agents.
Keywords: Electrochemical sensor; Mercuric and cadmium ions; Electrochemical impedance spectroscopy; Chronocoulometry; Computational study
2
1.
Introduction
The presence of heavy metal (HM) ions in aqueous systems is a serious threat to living organisms and water dwelling species. Some HM ions are highly toxic due to their strong affinity for reacting with the chelating agents or ligands to form complexes and hence, cause deleterious effects to human health as they change the molecular structure of DNA, proteins and also act as an inhibitor for various biocatalytic reactions [1-3]. Cadmium and mercuric ions are considered as the most lethal and chronic among the top twelve substances declared highly toxic by United Nations environmental program. Cadmium has also been classified as a cancer causing substance by the international agency for research on cancer [2]. While mercury has been reported to cause autism, vestibular dysfunction, kidney failure, birth defects, respiratory track damage, speech impairment and skin diseases [4-7]. Cadmium causes liver damage, changes the constitution of blood and bones as it can replace calcium and causes many skeletal disorders, destruction of RBCs and high blood pressure. So, Cd (II) and Hg (II) are toxic even at very low concentration, hence, there is a dire need of developing such analytical tools which could detect cadmium and mercury below the contaminant level of these metal ions suggested toxic by world health organization and environmental protection agency [4, 7-10]. To contribute in this domain, we have developed 1cyclohexyl-3-dodecanoylthiourea (CDT) based environment friendly electrochemical sensor for the trace level detection of heavy metal toxins. The development of sensor using N, S and O containing surfactant as a recognition layer is an innovative approach of sequestering toxic metal ions from aqueous systems. The ill managed wastewater of underdeveloped agricultural countries badly affects the fertility of agricultural land. So designing thiourea containing surfactants is a smart approach of addressing this problem as their wash out water can increase the fertility of soil.
3
Moreover, using surfactant CDT for electrode modification is a step forward for avoiding the chances of secondary pollution [11]. The methods like Fourier transform infrared spectroscopy and electron spin resonance spectroscopy [12-13], ion chromatography [14, 15], chemiluminescence [16, 17], capillary electrophoresis [18], extended X-ray absorption fine structure spectroscopy [19] and spectropolarimetry [20] have been used for the detection and analysis of transition metal ions. Similarly, various other conventional methods including inductively coupled plasma mass spectroscopy [21], instrumental neutron activation analysis [22], atomic absorption spectroscopy [23] and X-ray fluorescence spectroscopy [24] have also been employed for the detection of environmental toxins. Albeit various conventional methods are employed for the detection of water toxins, however, electrochemical sensors have privilege over the conventional methods because of their faster speed of analysis, easy handling, accuracy and higher sensitivity. Electrochemical sensors cover the major class of chemical sensors and are ideally suited for the analysis of analytes of interest due to their convenient field-based size, cost affordability, absolute descrimination ability and rapid response. These electrochemical techniques have enhanced the enactment of the conventionaly used analytical devices/tools, as they are simple to operate, portable, and have no problems related to the usage of expensive reagents and slow preparation processes. In this perspective, the current research work is aimed at the fabrication of electrode surface for the development of ultra-sensitive electrochemical sensor that can respond to heavy metal ions in the concentration ranges suggested toxic by Environmental Pollution Agency and World Health Organization. This article presents the development of surfactant based electrochemical sensor by immobilizing 1-cyclohexyl-3-dodecanoylthiourea (CDT) on glassy carbon electrode (GCE) for the detection of mercury (Hg2+) and cadmium (Cd2+) ions. The performance of the designed sensor
4
was tested by electrochemical impedance spectroscopy, cyclic voltammetry, differential pulse and square wave stripping voltammetry. The effect of different electrolytes on the performance of the sensor was also examined. Under suitable experimental conditions, linear calibration plots in wide concentration ranges were obtained with detection limits of 0.084 µg/L and 0.090 µg/L for Cd2+ and Hg2+ respectively. The sensitivity of the proposed method was found to be 0.034 µAL/ µg for Hg2+ and 0.010 µAL/µg for Cd
2+
. Theoretical studies were also carried out for supporting the
experimental findings.
2.
Experimental
Electrochemical impedance spectroscopy (EIS) and voltammetric experiments including cyclic voltammetry (CV), differential pulse anodic stripping voltammetry (DPASV) and square wave anodic stripping voltammetry (SWASV) were performed by using Metrohm Auto lab PGSTAT302N, equipped with FRA and NOVA 1.11. All the electrochemical experiments were performed by using a three electrode system. The bare and modified glassy carbon electrode was used as working, (Ag/AgCl) as a reference and platinum wire as an auxiliary electrode. Prior to every experiment, the surface of GCE was polished by rubbing it over a buffing pad having diamond powder of 1 µm particle size followed by thorough rinsing with a jet of doubly distilled water. The thiourea based surfactant was immobilized on the surface of glassy carbon electrode. For immobilization, a 10 micro liter solution of known concentration of surfactant was taken in a micropipette and its droplet was placed over the surface of electrode followed by drying the droplet in a desiccator. The surfactant coated electrode was then slowly washed with doubly distilled water to remove any loosely attached surfactant molecules. The modified electrode was then placed in to the solution of target metal ions in an electrochemical cell, followed by electrochemical investigation. To avoid the diffusion of atmospheric oxygen into the solution of analytes, a 5
constant flux of nitrogen was kept over the solution during all the voltammetric measurements. Negative deposition potential was applied in order to electroplate metal ions on the electrode surface. After this treatment several anodic stripping square wave voltammetric scans were run and the current was measured during the stripping steps. For comparison with modified electrode, electroplating and stripping analysis were also performed over bare GCE. Amsterdam density functional (ADF) software using density functional tight binding method (DFTB) was applied for computational measurements. In this study, interaction energy of surfactant (CDT) binding to cadmium and mercuric ions was evaluated. Single point energy of metal ions (Hg2+ and Cd2+) and CDT were computed first individually and then for the merged respective metal ions and surfactant CDT as a one unit system using DFTB method.
3.
Result and discussion
Electrochemical impedance spectroscopy was employed for probing the charge transfer ability of the designed electrochemical sensor as shown in Fig. 1. The charge transfer resistance corresponding to the diameter of semicircle obtained at unmodified glassy carbon electrode was found greater for [Fe (CN) 6]4-/3- as compared to CDT modified GCE. This EIS result ensures higher electrical conductivity and heterogeneous electron transfer kinetics at CDT modified electrode compared to bare GCE. Cyclic voltammetry was carried out to test the electrochemical response of the modified and unmodified GCE for the redox behavior of cadmium and mercuric ions. The voltammograms shown in Fig. S1 demonstrate intense current signals of the analytes on modified GCE as compared to bare GCE. Thus, CDT modified GCE is a sensitive platform for the trace level detection of heavy metal ions.
6
Differential pulse voltammetry was performed in order to certify the electrocatalytic role of the modifier from the enhanced current signals of the analytes on CDT GCE in comparison to bare GCE. It can be seen from the differential pulse voltammograms shown in Fig.2 that the peak height of 1 mM solution of cadmium and mercuric ions at CDT GCE are almost 10 folds higher than bare GCE. A double step choronoamperometric experiment was also carried out for 2 mM solution of cadmium and mercuric ions (see Fig. S2) and several useful parameters were calculated using the following form of Anson equation. Q = 2nFACD1/2 π−1/2 t1/2 + nFAГ0 + Qdl … … … … … … … … … … … … (1) Where Q represents total charge, n the number of electrons transferred per molecule, F the Faraday constant, C the concentration of analyte, D the diffusion coefficient, t the time, A the area of the electrode, Г0 the surface concentration of adsorbed species and Qdl the double layer charging. The parameters calculated from chronocoulometry are listed in Table 1 and observation of the data reveal that the surface area, diffusion coefficient and surface coverage values of CDTGCE are higher than the unmodified GCE electrode due to increase in charge density and charge transfer of electrons on the surface of CDT modified GCE. The influence of the amount of surfactant on peak current response of the analytes was also investigated. The magnitude of peak currents of the analytes was found directly proportional to the concentration of CDT until the saturation of the electrode surface as shown in Fig. 3. The intensification of current signals of cadmium and mercuric ions with increase in CDT concentration can be related to the electrocatalytic role of CDT and increase in surface area of modified GCE. The maximum current values of Hg2+ and Cd2+ were observed at 1 mM
7
(saturation point) concentration of CDT. After the saturation point, increase in concentration of surfactant led to the decrease in peak current intensity of the analytes which might be due to multilayer formation of CDT molecules over the surface of GCE that may cause passivation of the electrode and thus cause hindrance to the electron transfer process [25-27]. The influence of scan rate on the peak currents of cadmium and mercuric ions was studied by cyclic voltammetry using CDT modified GCE (Fig. S3). The anodic peak current increased linearly with the increase of the square root of scan rate which suggests the diffusion controlled nature of the oxidation processes of the analytes occurring at the surface of CDTGCE. The value of diffusion coefficient obtained using the Randles-Sevcik equation (2) was concurrent with the values obtained from the Anson equation. 𝐼𝑝 = (2.99 × 105 )𝑛(𝛼𝑛)1/2 𝐴𝐶𝐷1/2 1/2 … … … … … … …
(2)
Where n denotes the no. of electron participating in the redox reaction, the scan rate, 𝛼 the charge transfer coefficient, A the surface area of the electrode and C the concentration of the analyte. The effective surface area of the unmodified and modified electrodes was estimated to be 0.071 cm2 and 0.135 cm2. The magnitude of D for cadmium and mercuric ions was calculated as 2.0 × 10-6 and 7.3 × 10-6 cm2 s-1 respectively. Different functional groups are present in the chemical structure of surfactant and ionization of these groups in media of different pH may influence the metal ions binding ability of surfactant. So cadmium and mercuric ions sensing ability of the CDT was examined in solution of different pH using differential pulse voltammetric technique. The maximum current response of cadmium and mercuric ions was observed at pH 4 and pH 8 respectively as shown in Fig. 4. The maximum peak currents under these conditions ensure the high metal ions
8
complexation ability of CDT with cadmium and mercury. The decrease in current signals at higher pH conditions can be related to the formation of Cd (OH)2 and Hg(OH)2 that may lower the accumulated level of metal ions on the electrode surface [28, 29]. The effect of supporting electrolytes on the peak height of cadmium and mercury was quantified as shown in Fig. S4. Solutions of potassium chloride, hydrochloric acid, sulphuric acid, phosphoric acid, Brinton Robinson buffer (BRB) of pH 7 and sodium hydroxide were used as supporting electrolytes. The results revealed that peak position, shape and height are greatly affected by the variation in supporting electrolyte. The maximum current intensity of the analytes was registered in solution of hydrochloric acid and thus this electrolyte was selected as the most suitable supporting electrolyte for the determination of cadmium and mercuric ions. The effect of temperature on the peak current of the targeted metal ions was also investigated. Peak current was found to increase with the rise in temperature (Fig. 5). This effect can be attributed to the possible decrease in viscosity of the solution, thus leading to faster suppleness of the analyte with concomitant rapid electron transfer rate [30]. The effect of accumulation time on the electroreduction of cadmium and mercuric ions on the electrode surface was also explored as shown in Fig. 6. The intensification of peak currents as a function of time indicates the increase in accumulation of mercury and cadmium on CDT modified electrode surface up to 90 s and 120 s respectively. The decrease in peak height with further accumulation time points to saturation of the working electrode surface. Hence, 90 and 120 seconds were selected as optimum accumulation times for mercury and cadmium respectively.
9
The presence of other metal ions in real samples may influence the current signals of the analytes. Hence, the effect of interfering agents on the peak currents of cadmium and mercury was also investigated. In the present work, silver, copper, aluminum, lead, zinc and cobalt were studied as possible interfering species. It can be seen in Fig. 7 that the peak currents of mercury and cadmium exhibit only a slight variation in the presence of 2 fold higher concentration of interfering ions, thus, suggesting the selectivity of the developed electrochemical sensor for mercury and cadmium ions. Analytical parameters of the CDT modified GCE for the detection of target metal ions were evaluated under optimum conditions (Fig. 8). Linear calibration curves of cadmium and mercury were obtained in the concentration ranges of 12 to 90 µg/L and 14-156 µg/L with correlation coefficients of 0.998 and 0.994 respectively using HCl as a supporting electrolyte. The limit of detection (LOD) was measured according to equation (2) 3𝜎
LOD = ( ) -------------- (3) 𝑚
Where, 𝜎 is the standard deviation of the blank solution and m the slope of current versus concentration plot (Fig. S5). The standard deviation 𝜎 was determined based on the peak current of the blank solutions. The limits of detection of cadmium and mercury were found to be
0.084
µg/L and 0.090 µg/L respectively which are quite below the maximum contamination level of Cd2+ (0.005 mg/L) and Hg2+ (0.001 mg/L) proposed by WHO and 0.003 mg/L and 0.002 mg/L for the respective metal ions by EPA [4, 7-10]. The sensitivity of the proposed method was found to be 0.010 µAL/µg for cadmium and 0.034 µAL/µg for mercuric ions using square wave voltammetry. An observation of Table 2 reveals that compared to reported methods, our designed
10
sensor holds great promise as a sensitive analytical tool for the trace level detection of cadmium and mercuric ions [31-37]. In order to check the validity of the proposed method, known concentrations of cadmium and mercuric ions with added amount of HCl were used in test solutions followed by the determination of percentage recoveries of the targeted metal ions using the direct calibration method. The recovery percentages of cadmium and mercury ions using CDT modified GCE in water samples are presented in Table 3. An examination of the data reveals that reasonable percentage recoveries with relative standard deviation of less than 4% are obtained, thus suggesting the practical applicability of the proposed method. The binding affinities of the cadmium and mercuric ions with surfactant CDT were studied by density functional based tight binding (DFTB) method integrated in the Amsterdam density functional (ADF) program. The binding energy was calculated to confirm the interaction between CDT and metal ions (Hg2+ and Cd2+). The energy levels obtained from theoretical studies using ADF single point energy calculations show that p orbitals of the sulphur atom have a maximum contribution in the highest occupied molecular orbitals (HOMO) of CDT. The HOMO and lowest unoccupied molecular orbitals (LUMO) of CDT are presented in Fig. S6. Major contribution in the LUMO of CDT comes from the p orbitals of oxygen, carbon, nitrogen and sulphur atoms as shown in Fig. S7. The data corresponding to molecular orbital properties of CDT are listed in Table S1. While Table 4 shows the parameters of CDT obtained from the molecular orbital calculations. DFT calculations were performed for the interaction energy after merging the CDT molecules with the metal ions (Cd2+ and Hg2+) individually as a one unit system. The optimized structures of CDT merged with Cd2+ and Hg2+ are shown in Fig. 9. The interaction energy Eint was calculated as; energy of interaction = binding energy of merge structure – total binding energy. 11
The negative value of Eint in both cases ensures that strong interaction occurs between CDT and metal ions (Hg2+ and Cd2+).
4.
Conclusion
Thiourea surfactant CDT was successfully employed for the first time as a recognition layer in the development of electrochemical sensor for the trace level detection Cd (II) and Hg(II) ions in water samples. The modified electrode gave a 10% boost to the current signals of the analytes as compared to unmodified glassy carbon electrode. Electrochemical impedance spectroscopy was performed to check the difference in the behavior of modified and bare electrodes and the results revealed that CDT modified electrode is a preferred sensing platform as its charge transfer resistance was found less than the GCE. A number of voltammetric experiments revealed CDT as an effective electrode modifier for sequestering cadmium and mercuric ions from aqueous system. The sensitivity of the proposed method was found to be 0.034 µAL/µg for Hg2+ and 0.010 µAL/µg for Cd2+ with relative standard deviation of less than 4%. The limits of detection obtained for the selected metal ions are quite below the guidelines for maximum contamination level suggested by WHO, EPA and European water quality. Interaction energy of CDT with mercury and cadmium ions obtained from
the theoretical studies supported the experimental results. The designed sensor showed pH and temperature responsiveness, demonstrated good percentage recoveries and exhibited remarkable electrocatalytic activity and anti-interference ability. Moreover, the sensor showed its applicability in different electrolytes with HCl as the best supporting electrolyte.
Acknowledgements The authors gratefully acknowledge the financial support of Quaid-i-Azam University Islamabad and Higher Education Commission of Pakistan through project number 20–3070.
12
13
References [1] A. Afkhami, T. Madrakian, S.J. Sabounchei, M. Rezaei, S. Samiee, M. Pourshahbaz, Construction of a modified carbon paste electrode for the highly selective simultaneous electrochemical determination of trace amounts of mercury (II) and cadmium (II), Sensors and Actuators B: Chemical, 161 (2012) 542-548. [2] J. Gong, T. Zhou, D. Song, L. Zhang, Monodispersed Au nanoparticles decorated graphene as an enhanced sensing platform for ultrasensitive stripping voltammetric detection of mercury (II), Sensors and actuators B: Chemical, 150 (2010) 491-497. [3] V.K. Gupta, M.L. Yola, N. Atar, A.O. Solak, L. Uzun, Z. Üstündağ, Electrochemically modified sulfisoxazole nanofilm on glassy carbon for determination of cadmium (II) in water samples, Electrochimica Acta, 105 (2013) 149-156. [4] F. Xuan, X. Luo, I.M. Hsing, Conformation-dependent exonuclease III activity mediated by metal ions reshuffling on thymine-rich DNA duplexes for an ultrasensitive electrochemical method for Hg2+ detection, Analytical chemistry, 85 (2013) 4586-4593. [5] D.S. Rajawat, S. Srivastava, S.P. Satsangee, Electrochemical determination of mercury at trace levels using eichhornia crassipes modified carbon paste electrode, International Journal of Electrochemical Science, 7 (2012) 11456-11469. [6] L.E. Kreno, K. Leong, O.K. Farha, M. Allendorf, R.P. Van Duyne, J.T. Hupp, Metal–organic framework materials as chemical sensors, Chemical Reviews, 112 (2011) 1105-1125. [7] K. Farhadi, M. Forough, R. Molaei, S. Hajizadeh, A. Rafipour, Highly selective Hg2+ colorimetric sensor using green synthesized and unmodified silver nanoparticles, Sensors and Actuators B: Chemical, 161 (2012) 880-885.
14
[8] C. Hu, X. Chen, S. Hu, Water-soluble single-walled carbon nanotubes films: Preparation, characterization and applications as electrochemical sensing films, Journal of Electroanalytical Chemistry, 586 (2006) 77-85. [9] J. Li, S. Guo, Y. Zhai, E. Wang, High-sensitivity determination of lead and cadmium based on the Nafion-graphene composite film, Analytica Chimica Acta, 649 (2009) 196-201. [10] W. Yantasee, Y. Lin, T.S. Zemanian, G.E. Fryxell, Voltammetric detection of lead (II) and mercury (II) using a carbon paste electrode modified with thiol self-assembled monolayer on mesoporous silica (SAMMS), Analyst, 128 (2003) 467-472. [11] A. Shah, S. Shahzad, A. Munir, M.N. Nadagouda, G.S. Khan, D.F. Shams, D.D. Dionysiou, U.A. Rana, Micelles as soil and water decontamination agents, Chemical Reviews, 116 (2016) 6042-6074. [12] M. Salmain, A. Vessières, S. Top, G. Jaouen, I.S. Butler, Analytical potential of near‐infrared fourier transform Raman spectra in the detection of solid transition metal carbonyl steroid hormones, Journal of Raman Spectroscopy, 26 (1995) 31-38. [13] D.T. Burns, B.G. Dalgarno, B.D. Flockhart, Optimization of the electron spin resonance spectrometric determination of transition-metal ions by variation of temperature, Analytica Chimica Acta, 218 (1989) 93-100. [14] F. Nordmeyer, L. Hansen, D. Eatough, D. Rollins, J. Lamb, Determination of alkaline earth and divalent transition metal cations by ion chromatography with sulfate-supressed barium and lead eluents, Analytical Chemistry, 52 (1980) 852-856. [15] R. Mohamed, A. Abdel-Lateef, H. Mahmoud, A. Helal, Determination of trace elements in water and sediment samples from Ismaelia Canal using ion chromatography and atomic absorption spectroscopy, Chemical Speciation & Bioavailability, 24 (2012) 31-38.
15
[16] P. Jones, T. Williams, L. Ebdon, Development of a novel multi-element detection system for trace metal determination based on chemiluminescence after separation by ion chromatography, Analytica Chimica Acta, 237 (1990) 291-298. [17] S.J. Ussher, A. Milne, W.M. Landing, K. Attiq-ur-Rehman, M.J. Séguret, T. Holland, E.P. Achterberg, A. Nabi, P.J. Worsfold, Investigation of iron (III) reduction and trace metal interferences in the determination of dissolved iron in seawater using flow injection with luminol chemiluminescence detection, Analytica Chimica Acta, 652 (2009) 259-265. [18] R. Zhu, W.T. Kok, Determination of trace metal ions by capillary electrophoresis with fluorescence detection based on post-column complexation with 8-hydroxyquinoline-5-sulphonic acid, Analytica Chimica Acta, 371 (1998) 269-277. [19] H.D. Dewald, J. Watkins, R. Elder, W.R. Heineman, Development of extended x-ray absorption fine structure spectroelectrochemistry and its application to structural studies of transition-metal ions in aqueous solution, Analytical Chemistry, 58 (1986) 2968-2975. [20] R.J. Palma, P. Reinbold, K.H. Pearson, Spectropolarimetric determinations of the 3d transition metals utilizing a stereospecific ligand, Analytical Chemistry, 42 (1970) 47-51. [21] L. Tormen, R.A. Gil, V.L. Frescura, L.D. Martinez, A.J. Curtius, The use of electrothermal vaporizer coupled to the inductively coupled plasma mass spectrometry for the determination of arsenic, selenium and transition metals in biological samples treated with formic acid, Analytica Chimica Acta, 717 (2012) 21-27. [22] M. Almeida-Silva, N. Canha, M.D.C. Freitas, H. Dung, I. Dionísio, Air pollution at an urban traffic tunnel in Lisbon, Portugal—An INAA study, Applied Radiation and Isotopes, 69 (2011) 1586-1591.
16
[23] D.I. Bannon, C. Murashchik, C.R. Zapf, M.R. Farfel, J. Chisolm, Graphite furnace atomic absorption spectroscopic measurement of blood lead in matrix-matched standards, Clinical Chemistry, 40 (1994) 1730-1734. [24] E. Marguí, I. Queralt, M. Hidalgo, Application of X-ray fluorescence spectrometry to determination and quantitation of metals in vegetal material, TrAC Trends in Analytical Chemistry, 28 (2009) 362-372. [25] R.G. Compton, C.E. Banks, Understanding voltammetry, World Scientific, 2007. [26] H. Wan, F. Zhao, B. Zeng, Direct electrochemistry and voltammetric determination of midecamycin at a multi-walled carbon nanotube coated gold electrode, Colloids and Surfaces B: Biointerfaces, 86 (2011) 247-250. [27] S.B.D. Borah, S. Baruah, W.S. Mohammed, J. Dutta, Heavy metal ion sensing by surface plasmon resonance on gold nanoparticles, ADBU Journal of Engineering Technology, 1 (2014). [28] X. Hao, J.L. Lei, N.B. Li, H.Q. Luo, An electrochemical sensor for sodium dodecyl sulfate detection based on anion exchange using eosin Y/polyethyleneimine modified electrode, Analytica Chimica Acta, 852 (2014) 63-68. [29] M.B. Gholivand, G. Malekzadeh, M. Torkashvand, Enhancement effect of sodium-dodecyl sulfate on the anodic stripping voltammetric signal of phenylephrine hydrochloride at carbon paste electrode, Journal of Electroanalytical Chemistry, 704 (2013) 50-56. [30] A.H. Shah, A. Shah, S.U.D. Khan, U.A. Rana, H. Hussain, S.B. Khan, R. Qureshi, A. Badshah, A. Waseem, Probing the pH dependent electrochemistry of a novel quinoxaline carboxylic acid derivative at a glassy carbon electrode, Electrochimica Acta, 147 (2014) 121-128.
17
[31] N. Daud, N.A. Yusof, S.M.M. Nor, Electrochemical characteristic of biotinyl somatostatin14/Nafion modified gold electrode in development of sensor for determination of Hg (II), International Journal of Electrochemical Science, 8 (2013) 10086-10099. [32] H. Huang, T. Chen, X. Liu, H. Ma, Ultrasensitive and simultaneous detection of heavy metal ions based on three-dimensional graphene-carbon nanotubes hybrid electrode materials, Analytica Chimica Acta, 852 (2014) 45-54. [33] D.S. Rajawat, S. Srivastava, S.P. Satsangee, Electrochemical determination of mercury at trace levels using eichhornia crassipes modified carbon paste electrode, International Journal of Electrochemical Science, 11 (2012) 11456-11469. [34] I.Cesarino, G. Marinodo Rosário Matos, J. Cavalheiro, E.T.G. Cavalheiro, Evaluation of a carbon paste electrode modified with organofunctionalised SBA-15 nanostructured silica in the simultaneous determination of divalent lead, copper and mercury ions, Talanta, 75 (2008) 15-21. [35] D.S. Rajawat, N. Kumar, S.P. Satsangee, Trace determination of cadmium in water using anodic stripping voltammetry at a carbon paste electrode modified with coconut shell powder, Journal of Analytical Science and Technology, 5 (2014) 19. [36] W.Yantasee, Y. Lin, T.S. Zemanian, G.E. Fryxell, Voltammetric detection of lead (II) and mercury (II) using a carbon paste electrode modified with thiol self-assembled monolayer on mesoporous silica (SAMMS), Analyst, 128 (2003) 467-472. [37] A. Zahid, A. Lashin, U.A. Rana, N. Al-Arifi, I. Ullah, D.D. Dionysiou, R. Qureshi, A. Waseem, H.B. Kraatz, A. Shah, Development of surfactant based electrochemical sensor for the trace level detection of mercury, Electrochimica Acta, 190 (2016) 1007-1014. Figure Caption
18
19
Figures 6
-2
-Z"(kΩcm )
Un modified GCE CDT modified GCE
4
2
0
0.0
5.0 10.0 15.0 -2 Z'(kΩcm )
20.0
Fig.1. Nyquist plot showing impedance response at a bare and CDT modified glassy carbon electrodes in a 5 mM K3Fe(CN)6 solution. Frequency range is from 1 Hz to 14 kHz.
(A) (B) (C)
I (µA)
16 12 8 4 0 -1.0
-0.5 0.0 0.5 E / V vs Ag/AgCl
1.0
Fig.2. Differential pulse voltammograms of 1 mM Cd2 and Hg2 solution at a (A) bare GCE and (B) CDTGCE. (C) Blank DPV shows response of CDTGCE in BRB of pH 7.0.
20
2.8 0.09 mM 0.2 mM 0.3 mM 0.6 mM 0.9 mM 1.0 mM 1.1 mM
I (A)
2.0
(A)
I (µA)
2.4
1.6 1.2 0.8 0.4 0.0 -1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.2 E / V vs Ag/AgCl
3.0 2.7 2.4 2.1 1.8 1.5 1.2 0.9 0.6 0.3 0.0
(B) Ip of Cd2+ Ip of Hg2+
0.2 0.4 0.6 0.8 1.0 Concentration of DAH (mM)
1.2
Fig. 3. (A) Effect of concentrations of CDT on SWASV peak current of 100 µg/L of cadmium and mercury in HCl (B) Peak current of cadmium and mercury versus concentration of adsorbed surfactant (CDT).
7
30
15 10 5
5 0 1 2 3 4 5 6 7 8 9 10 pH pH
0 52 --1 .
E/
-0 .
8
Vv
4 -0 .
s.A
g/A 0 .0 gC l
2
3
4
5
6
7
8
9
4 3 2
5 4 3 2 2
1 0 -1 -1.0
10
pH
4
6 pH
8
10
12
1 2
1 0
8
6
E / V vs.Ag/AgCl
pH
4
1.0
10
0.5
15
(B)
6
0.0
6 5
20
-0. 5
I (µA)
25
I (µA)
(A)
I ( A)
I ( A)
30 25 20
Fig. 4. (A) DPASV of 1 mM solution of cadmium ions using CDTGCE in BRB of different pH and (B) influence of pH on the peak current of 0.5 mM mercuric ions solution using CDTCE in BRB of pH 2-10 at accumulation time of 360 s and scan rate of 50 mV/s.
21
90 80 70 60 50 40 30 20 10 0 -10
I (µA)
300 K 325 K 330 K 348 K 358 K
-0.9 -0.6 -0.3 0.0 0.3 E / V vs Ag/AgCl
0.6
Fig.5. Effect of temperature on the SWASV response of cadmium and mercuric ions using CDTGCE. Scan rate for SWASV was kept 50 mV/s.
30
Ip of Cd2+ Ip of Hg2+
I (A)
28 26 24 22 20 30
60
90
120
150
180
accumulation time/sec Fig.6. Effect of accumulation time on the SWASV peak current of cadmium and mercuric ions solution using CDTGCE. Scan rate for SWASV was kept 50 mV/s.
22
Cd2+ Hg2+
20
I / (A)
15 10 5 0
None
Cu2+
Ag1+ Pb2+ Al3+
Co2+ Zn2+
Fig. 7. The effect of interfering agents on the SWASV peak current of the analytes using CDTGCE. 6 3.0
12 mg/L
(A)
28 mg/L
56
I ()
2.0
5
40 mg/L mg/L
4
78 mg/L 90
I ()
2.5
mg/L
1.5
3
1.0
2
0.5
1
0.0
0 0.0
-1.0
-0.8
-0.6
-0.4
E/V vs. Ag/AgCl
156g/L 140 g/L 90g/L 66g/L 55 g/L 40 g/L 28g/L 14g/L
0.1 0.2 0.3 E/V vs. Ag/AgCl
0.4
Fig.8. (A) SWASVs obtained at CDTGCE in cadmium ions solution using HCl as supporting electrolyte by keeping accumulation time of 120 s and scan rate of 50 mV/s (B) SWASVs of mercuric ions solution obtained at CDTGCE in HCl as supporting electrolyte, keeping accumulation time of 90 s and scan rate of 50 mV/s.
23
A
B
Fig.9. Merged structures of (A) Cd2+ and (B) Hg2+ crystal with surfactant CDT.
24
Tables Table 1. Parameters obtained from chronocoulometry using GCE and CDTGCE. Electrode Area/cm2 D/10-6 (cm2 s- Qads/10-4 C 1
Γ0/10-8
)
Area occupied per molecule
GCE
0.071
1.3
5.1
2.6
0.638
CDTGCE
0.125
6.25
6.0
3.77
0.44
Table 2. Comparison of the proposed sensor with the reported methods for the detection of cadmium and mercuric ions.
Metal ions
Electrode substrate
Measurement technique
LOD µg/L
Ref.
Hg2+ Cd2+
CDTGCE CDTGCE Biotinyl Somatostatin14/Nafion modified gold electrode G-MWCNTs WBMCPE SBA-15 with 2benzothiazolethiol CS-MCPE SH-SAMMSCPE DANGCE
SWASV SWASV
0.090 0.084
This work This work
Cyclic Voltammetry
0.4
Ref No. 31
DPASV DPASV
0.1 195
Ref No. 32 Ref No. 33
DPASV
100
Ref No.34
DPASV SWASV SWASV
105 3 0.64
Ref No.35 Ref No.36 Ref No.37
Hg2+ Cd2+ Hg2+ Hg2+ Cd2+ Hg2+ Hg2+
Table 3. Recovery data of Cd2+ and Hg2+ in water samples using CDTGCE in HCl as supporting electrolyte. 25
Metal ions
Sample name
Spiked(µg/L)
Found(µg/L)
RSD (%)
Recovery (%)
Hg2+
Drinking water
60
53
3.163
88.33
15
11
2.13
73.33
1 Cd2+
Drinking water 2
Hg2+
Tap water 1
95
100
2.66
95.0
Cd2+
Tap water 2
60
64
3.5S
106.6
26
Table 4. The energy values of the CDT and CDT merged with cadmium and mercury ions. Binding Energy of CDT (ECDT)
-96408.17 kcal/mol
Binding Energy of cadmium (ECd2+)
228089.97 kcal/mol
Total Binding Energy (ECDT + ECd2+)
-131681.87 kcal/mol
Binding Energy of merge structure (Emerge)
100093.89 kcal/mol
Energy of Interaction with Hg2+ (Eint)
-31587.91 kcal/mol
Binding Energy of mercury (EHg2+)
906552.99kcal/mol
Total Binding Energy (ECDT + EHg2+)
-810144.82kcal/mol
Bonding Energy of merge structure (Emerge)
-805524.07kcal/mol
Energy of Interaction with Cd2+ (Eint)
-4620.75 kcal/mol
27
