Selective and simultaneous detection of Zn2+, Cd2+, Pb2+, Cu2+, Hg2+ and Sr2+ using surfactant modified electrochemical sensors

Journal Pre-proof Selective and simultaneous detection of Zn2+, Cd2+, Pb2+, Cu2+, Hg2+ and Sr2+ using surfactant modified electrochemical sensors

Afzal Shah, Aamir Hassan Shah PII:

S0013-4686(19)31440-9

DOI:

https://doi.org/10.1016/j.electacta.2019.134592

Article Number:

134592

Reference:

EA 134592

To appear in:

Electrochimica Acta

Received Date:

23 April 2019

Accepted Date:

24 July 2019

Please cite this article as: Afzal Shah, Aamir Hassan Shah, Selective and simultaneous detection of Zn2+, Cd2+, Pb2+, Cu2+, Hg2+ and Sr2+ using surfactant modified electrochemical sensors, Electrochimica Acta (2019), https://doi.org/10.1016/j.electacta.2019.134592

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.

Journal Pre-proof

Selective and simultaneous detection of Zn2+, Cd2+, Pb2+, Cu2+, Hg2+ and Sr2+ using surfactant modified electrochemical sensors Afzal Shah a,* and Aamir Hassan Shah b,* 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] (Dr. Afzal Shah) and [email protected] (Dr. Aamir Hassan Shah)

1

Journal Pre-proof

Abstract The 1-(3-chlorophenyl)-3-dodecanoylthiourea (Cl-DPTU) surfactant modified GCE (ClDPTU/GCE) was developed for the selective and sensitive simultaneous detection of six metal ions. The as developed electrochemical sensing platform was characterized using electrochemical impedance spectroscopy (EIS) and chronocoulometry to determine the charge transfer characteristics. The charge transfer behavior of bare GCE and Cl-DPTU/GCE was compared and the results revealed the modifier to significantly improve the charge transduction between the redox probe and transducer. The square wave anodic stripping voltammetry (SWASV) was then performed to investigate the electrochemical catalytic activity of the modifier. The sensing limits of Cl-DPTU/GCE under optimized experimental conditions for Cd2+, Cu2+, Hg2+, Pb2+, Zn2+, and Sr2+ ions were obtained as 6.45 nM, 7.85 nM, 9.15 nM, 11.0 nM, 16.9 nM, and 178.8 nM respectively. The selectivity of Cl-DPTU/GCE was examined from the influence of interfering agents on the signals of the targeted metal ions. Likewise, the applicability of the SWASV was checked in real water samples. Interestingly the recovery percentages of nearly 100 % for all water samples evidenced authenticity of Cl-DPTU/GCE for multiple metal ions determination in real water samples. Moreover, computational studies were performed and the computed parameters supported the experimental results. Key words: Electrochemical sensor; Multiple metal toxins; Simultaneous detection; Selectivity; Computational studies

2

Journal Pre-proof

1.

Introduction The industrial and agricultural development have resulted in severe heavy metals (HMs)

pollution from mining, automobile industries, pesticides, urban and industrial wastes in the aqueous environment [1]. The HMs pollution is a serious threat to living organisms even at trace level concentration by entering into the cycle of food chain in ecological system. Along with other organisms, the human health is primarily affected by accumulation, and low clearance rate of these HMs pollution from the body. For instance, the hazardous effects of long term exposure to mercury damages the central nervous system, kidneys, endocrine system, vision problems and delayed development of children [2, 3]. Similarly, the presence of cadmium (Cd), mercury (Hg), lead (Pb) and copper (Cu) is the major cause of chronic diseases even at parts per million concentration [4]. Although the presence of trace amount of some of these metals is important for human bodies, for example, the unnecessary intake of Cu appeared to abolish the tissues and cause damage in DNA. These hazardous effects of copper are attributed to its affinity with hydroxyperoxides present in lipids resulting in the formation of malondialdehyde and 4-hydroxynonenal [5, 6]. The World Health Organization (WHO) has taken an important step to create awareness in public about lowest intake of metals and threshold concentration of heavy metal ions in regular drinking water [7]. However, the identification and removal of these pollutants from the water bodies are basic requirements for public safety. In this regard, various analytical methods have been developed and tested for the HMs detection i.e. fluorescence spectroscopy [8], flow injection method [9], inductively coupled plasma, optical emission spectrometry [10], anodic stripping voltammetry (ASV) [11, 12], differential pulse stripping voltammetry [13], macrocyclic extractants method [14], mass high resolution surface plasma resonance coupled with anodic stripping voltammetry [15] and liquid 3

Journal Pre-proof

chromatography [16]. However, the application of many of these methods is limited to the laboratory scale owing to their high cost, large size and the tedious procedure for complex sample and/or substrate preparation [17]. Therefore, the electrochemical methods are considered as better alternative of the above gadgets due to their sensitive, selective and rapid detection of metal ions owing to their cost effectiveness, small size and easy operations [18]. Among the electrochemical methods, the most commonly employed method for rapid simultaneous sensing of various HMs is anodic stripping voltammetry [19]. In the present study, SWASV was selected as primary method for the detection of simultaneous multiple metal ions with addition of a few other electrochemical methods due to its excellent sensitivity, fast speed of analysis and wide dynamic range. The development of electrochemical sensors primarily involves electrode fabrication that plays a crucial role in providing functionality to the sensors. During the electrode modification, host molecules are first anchored on the electrode surface which then forms host-guest inclusion complexes with the target metal ions in the analyte solution [20]. Typically, the compounds used for electrode fabrication are polymers, biological molecules (DNA and enzymes) and ligands [21]. The mercury film electrodes (MFEs) was previously used as a modifier but the long-term usage of it became limited due to toxicity of mercury. Bismuth film electrodes (BiFEs) was later employed as a promising alternative for MFEs because of their better analytical response and less toxic nature [22, 23]. Currently, surfactants are selected for electrode fabrication owing to their special chelating ability to form complexes with metal ions, hence, exhibit better detection ability [24]. The electron donor groups in the chemical structures of these surfactants possess strong binding ability for metal ions [25]. Impressed from the sensing ability of electron donor groups containing surfactants, 1-(3-chlorophenyl)-3-dodecanoylthiourea (Cl-DPTU) was selected in the present study for selective and sensitive detection of HMs. The binding affinity and electrocatalytic role

4

Journal Pre-proof

of these sensors are attributed to the presence of electron donating thiourea group having amine and sulfur groups along with carbonyl group in their molecules [26]. To the best of our knowledge this is the first report of using Cl-DPTU modified GCE as electrochemical sensor for the simultaneous detection of six metal ions in aqueous medium.

2.

Experimental

2.1.

Chemicals and Instrumentation The three surfactants namely 1-dodecanoyl-3-phenylthiourea (DPTU), 1-dodecanoyl-3-(2-

(trifluoromethyl)phenyl)thiourea (CF3-DPTU) and 1-(3-chlorophenyl)-3-dodecanoylthiourea (ClDPTU) procured from Sigma Aldrich were used as electrode modifiers. While, zinc acetate, strontium nitrate, copper chloride, cadmium chloride and mercuric chloride with 99% purity were used as analyte solutions. All the supporting electrolytes (Britton Robinson (BR) buffer (pH-4), phosphate buffer (pH-7), 0.1mM HCl, 0.1mM NaCl, 0.1mM NaOH and 0.1mM KCl) and interfering agents (nickel nitrate, cesium chloride, chromium chloride, cobalt chloride, copper sulphate, Leucine, Tryptophan, 3-hydroxy benzaldehyde and 3-bromo benzaldehyde) were obtained from Sigma Aldrich and used as received. Surfactants solutions were prepared in dimethyl sulphoxide (DMSO) while all the analyte solutions were prepared in doubly distilled water. The poor solubility of the selected surfactants helped in the designing of stable sensors. Moreover, samples of real water were collected from different sources in Islamabad, Pakistan to check the sensing ability of the designed sensor in real water samples. Electrochemical techniques were carried out on Metrohm Autolab PGSTAT302N. The cell assembly for electrochemical measurements consisted of bare and modified glassy carbon as a

5

Journal Pre-proof

working, Ag/AgCl (3 M KCl) as a reference and Pt wire as an auxiliary electrode. The pH of the solution was adjusted by a 620 lab pH meter.

2.2.

Electrode pretreatment and fabrication The pretreatment of glassy carbon electrode surface was performed both physically and

chemically before its fabrication with surfactants. In the first step, physical polishing of electrode was done on a polishing pad with alumina powder slurry (1 µm particle size) until the achievement of a shiny plane surface. It was then ultrasonicated and rinsed thoroughly with doubly distilled water. While in the second step, electrochemical pretreatment was carried out by passing the electrode through several polarization cycles within -1.4 V to +0.9 V range of potential ten times in phosphate buffer to achieve reproducible cyclic voltammograms [27]. After the physical and electrochemical cleaning of electrode, a 5 µL droplet of 20 µM surfactant solution prepared in DMSO was applied on the pretreated electrode surface followed by drying of the droplet in vacuum oven. The surfactant immobilized electrode was then washed carefully with doubly distilled water to remove any loosely bound surfactant molecule. The modified electrode was then subjected to square wave anodic stripping voltammetry (SWASV) in the solution mixture of heavy metal ions. All the solutions were purged with N2 gas for 30 minutes at room temperature before each experiment to remove oxygen and the nitrogen atmosphere was kept over the solution during the whole experiment to avoid any oxygen intake. In the deposition step of SWASV technique, a predefined deposition potential of –1.0 V was applied for 100 s to electroplate metal cations onto the electrode surface. Then the stripping step of SWASV involving oxidation of the electroplated metal atoms from negative to positive potentials during scanning of the square wave (SW) voltammogram. For each experiment, a newly prepared surfactant fabricated electrode was used to ensure the reproducible results. 6

Journal Pre-proof

3.

Results and Discussion

3.1.

Electrode modifier (Sensor) selection Electrode modification was assessed from the results of EIS and chronocoulometry (CC)

carried out in a 5 mM K3[Fe(CN)6] solution and 0.1M KCl. The EIS was performed to analyze the fabrication and charge transfer property of surfactant modified GC electrodes (DPTU/ GCE, ClDPTU/ GCE, CF3-DPTU/ GCE). Data for the Nyquist plots were obtained at bare and surfactant modified GC electrodes as shown in Figure 1a. The charge transfer resistance (RCT) can be qualitatively estimated from the semicircle diameter [28]. It was observed that the diameter of the semicircle is larger in case of CF3-DPTU/GCE as compared to bare GCE, thus displaying limited electron transfer across the electrode surface. However, the RCT was found to decrease by modifying the GCE with DPTU and Cl-DPTU as presented by the smaller diameter of semicircular arc. The equivalent circuit used for fitting impedance data is shown as the inset of Figure 1a. The parameters obtained from this equivalent circuit are listed in table 1. The difference in the EIS parameters of bare and surfactant modified GCE suggested successful fabrication of GC electrode with selected surfactant leading to effective charge transduction through the modified electrode surface. The charge transfer resistance was found to be the least in case of Cl-DPTU/ GCE (Rct = 10.1 kΩ) which indicates the fast electron transfer and higher conductivity on the electrode’s surface as compared to higher resistive values of DPTU/ GCE (Rct = 11.7 kΩ), bare GCE (Rct = 12.0 kΩ) and CF3-DPTU/ GCE (Rct = 14.6 kΩ). Therefore the Cl-DPTU/ GCE was selected as the best sensor among other candidates. Chronocoulometry (CC) was then performed to find out the surface properties of the modified electrode i.e. adsorbed charge (Qads) on surface with time, surface area of the adsorbed molecule and surface coverage (Γ) on the electrode surface. The chronocoulograms for Cl-DPTU/ 7

Journal Pre-proof

GCE showed increase in charge with respect to time than that of bare GCE as shown in Figure 1b. These results ensure the increased charge transfer of charge on modified electrode as compared to bare GCE. The working area and other parameters of the bare and Cl-DPTU/GCE was calculated from Cottrell equation are tabulated in table 2. The calculated surface area of the Cl-DPTU/GCE was found to be much higher (A = 0.123 cm2) than the bare GCE (A = 0.089 cm2) ensuring its better sensing ability. Likewise, the comparable enhancement in charge adsorbed and surface coverage was also observed for the Cl-DPTU/GCE. The calculated area occupied by single surfactant molecule on electrode surface was observed to be smaller suggesting greater lodging of molecules on the electrode’s surface. The coulometric parameters enlisted in table 2 show increase in the accessibility of active sites which strengthen the appearance of increase in analytical response of electrode after adsorption of selected surfactant. The Square wave voltammetry (SWV) was performed at bare and surfactant modified electrodes (DPTU/ GCE, Cl-DPTU/ GCE, CF3-DPTU/ GCE) for the simultaneous metals ions detection (Zn2+, Cd2+, Pb2+, Cu2+, Hg2+, Sr2+) in BRB (pH 4) at 100 mVs-1 as depicted by six oxidation signals (Figure 2). The oxidative signatures of Zn2+, Cd2+, Pb2+, Cu2+, Hg2+ and Sr2+ metal ions appeared at -1.06, -0.67, -0.43, 0.03, 0.27 and 0.87 V in SWVs. A distinct increase in current signals was found with surfactant modified electrodes as compared to bare GCE. This enhancement in current signal is attributed to the thiourea group present in the surfactant molecules which is suggested to interact with metal ions. Infact, the mercury ions are supposed to form covalent bond with sulfur of thionyl (C=S) groups of two adjacent DPTU molecule with the removal of two protons while metal ions can also form coordinate covalent bond between nitrogen and sulphur of thiourea group of three adjacent DPTU molecules to form complexes. Hence, the increase in binding sites of metal ions by surfactant leading to complexation of metal ions with

8

Journal Pre-proof

thiourea group in surfactant is responsible for this significant enhancement in the oxidation current signals. In case of CF3-DPTU modified electrode, the anodic current response was observed to decrease owing to the electron withdrawing effect of trifluoromethyl group attached to DPTU molecule leading to decrease in the electronegativity of thiourea group. Thus, the binding of metal ions on modified electrode decreased resulting in decrease of surface activity. While the oxidation signals for all six metal ions were found to be largest for Cl-DPTU/GCE owing to the enhancement of electronegativity of thiourea by electron donating effect of Chloro group attached to it which results in the increment of binding of metal ions and enhancement of surface activity of the ClDPTU/ GCE (Figure 2). Taking into account these observations, the Cl-DPTU/GCE was further investigated for sensing of metal ions.

3.2.

Parameters optimization As the experimental conditions were observed to affect the intensity and shape of peak, the

sensing conditions were optimized for the establishment of promising sensor for the simultaneous sensing of heavy metal ions. The stripping solvent or supporting electrolyte, medium pH, the concentration of recognition layer (surfactant), deposition time, deposition potential and deposition temperature are considered as the critical factors that influence the sensor performance. The Ohmic/iR drop and the migration currents are observed to vary with variation in stripping solvent. Thus, the effect of supporting electrolyte was determined on the voltammetric signals of the target metal ions in various electrolyte solutions including BRB (pH- 4), phosphate buffer (pH - 7), 0.1mM NaCl, 0.1mM HCl, 0.1mM KCl and 0.1mM NaOH using the Cl-DPTU /GCE. The electro-oxidation signals of all metal ions were examined to be the most enhanced in the BRB, and it was selected as the sensing medium. (Figure 3a). Moreover, as the pH of the medium also influenced the electrode reaction [29], the voltammetric signals of targeted metal ions

9

Journal Pre-proof

were examined over a wide pH range from 2.0 to 12.0 in the BRB solution. The square wave voltammograms appeared to show variable signals in response to change in pH as displayed in Figure 3b. The plot of peak current (Ipa) as a function of pH was established showing the maximum peak current at pH~4 in BRB. The reduction of oxidation signals in basic medium could be the result of the formation of hydroxides of metal ions. However, pH~4 solution was taken as the standard pH amongst all the studied pH media for further analysis of metal ions by forming complex of metal ions with fully deprotonated Cl-DPTU surfactant. The deposition step is a crucial step in terms of stripping of electrochemical species which determines the deposition of metal ions on the working electrode surface [30]. The standard metal ions coverage on electrode surface was displayed by the analysis of the effect of deposition parameters i.e. potential, time and temperature on oxidation signals. Keeping these considerations in mind, a study of the influence of deposition potential was performed on the oxidative signals of selected metal ions at the Cl-DPTU/GCE by setting the deposition potential from 0 V to -1.4 V (Figure 4a). The stripping signals were noticed to increase with enhancing negative deposition potential showing maximum peak currents at -1.0 V which is attributed to the saturation of electrode. Thus, -1.0 V was used as standard deposition potential for further experimental work. Similarly, the influence of deposition time on oxidation signatures of metal ions was determined by keeping the deposition potential constant at -1.0 V. The stripping currents of all metal ions were observed to increase with every 20 s rise in deposition time up till 100 s (Figure 4b). The maximum boost of peak currents was observed at 100 s with a corresponding decrease in its value after 100 s as a result of complete coverage of electrode surface and formation of multiple layers of electrochemically reduced metal ions at the Cl-DPTU modified electrode. Hence, 100 s

10

Journal Pre-proof

was taken as the standard deposition time for further electroanalytical study in order to detect multiple ions.

3.3.

Evaluation of detection and quantification limits The limit of sensitivity and quantification of Cl-DPTU/GCE was then investigated under

optimized conditions for the simultaneous detection of six metal ions as shown in Figure 5. The sensitivity limits and quantification limits of metals were calculated using equations i.e. 3𝜎/𝑚 and 10 𝜎/𝑚 respectively, where 𝜎 corresponds to the standard deviation of the blank solution (repeated measurements of electrochemical response in the electrolyte solution in the absence of analyte) and m designates to the slope of current-concentration plot [31, 32]. The sensitivity limit of detection for Cd2+, Cu2+, Hg2+, Pb2+, Zn2+, and Sr2+ ions were calculated as 6.45 nM, 7.85 nM, 9.15 nM, 11.0 nM, 16.9 nM, and 178.8 nM respectively which are lower than the threshold limits given by WHO and EPA (United States Environmental Protection Agency) [33]. The sensitivity and quantification limits of these metal ions reveal considerably good sensitivity of the Cl-DPTU surfactant based sensor as tabulated in table 3. The calculated sensitivity limits of the selected metal ions displayed a lower value than the sensitivity limits of various sensing mediums reported in literature [34-40] as presented in table 4. The sensitivity of our surfactant based sensor is although not more than PTE based modifier, the stability of our sensor is much greater due to its insolubility in medium for detection of metal ions i.e. water. This improved stability is further indicated by enhanced reproducibility.

3.4.

Validation of the proposed methodology Repeatability is the test–retest consistency of a sensor described as the discrepancy in

measurements attained on the same working electrode under same experimental environment [41]. 11

Journal Pre-proof

The repeatability of our designed sensor was recorded by six successive recordings of voltammetric signals on same working electrode with fixed Cl-DPTU surfactant concentration applied on fresh electrode surface each time as shown in Figure 6a. The oxidation signatures were observed to be almost identical showing no signal variation in all the measurements. It was also confirmed by the average current values of the metal ions by six repeated sensors showing relative standard deviation (RSD) value below 5% (see Figure 6b). On the contrary, reproducibility is the element of precision in any measurement system [42]. The reproducibility of the Cl-DPTU/GCE was determined by independent modification of six working electrodes and measuring their stripping anodic response for all the selected metals as shown in Figure 6c. The voltammetric signals came out to be similar in each recording while the RSD % values for Zn2+, Cd2+, Pb2+, Cu2+, Hg2+ and Sr2+ ions were found to be 3.57, 1.68, 2.31, 3.78, 2.05 and 3.80 (below 5%) respectively by the Cl-DPTU/GCE as displayed in table 3. Hence the results showed that our designed sensor is highly stable and durable due to its great repeatability and reproducibility. An important property of a sensor is its ability to discriminate between target metal ions and interfering agents present in the same environment [43]. So, the voltammetric current signals of metal ions was further investigated to determine the effect of interfering agents on the analyte solutions having fixed amount of selected metal ions (15 µM for Zn2+, 7.5 µM for Cd2+, 7.5 µM for Pb2, 10 µM for Cu2+, 7.5 µM for Hg2+, 30 µM for Sr2+) in the presence of interfering agents under similar experimental environment as conducted vide supra. Various interfering agents used in the study were metal cations (Ni2+, Cs2+, Cr3+, Co3+), anions (SO42-), amino acids (Tryptophan, Leucine), and organic compounds (3-bromo benzaldehyde, 3-hydroxy benzaldehyde). The SWV current signals of selected metal ions appeared not to deviate much in the presence of interfering 12

Journal Pre-proof

agents which is also characterized by a bar graph (Figure 7a and 7b). However, these results revealed that our designed sensor can maintain its robustness and abstract the metal ions from solution even in the presence of several folds higher concentration of interfering agents than the metal ions (see table 5). The Cl-DPTU/GCE was employed for the measurement of selected HMs in real water specimens to illustrate the feasibility of the proposed method for practical applications. In this regard, known concentration of metal salts was added to the samples of real water collected from different sources of drinking and tap water. The recovery of selected metal ions in these water samples was measured by comparing their peak currents values with the calibration curves. The percentage recoveries of these metal ions in five different real water samples appeared to be in between 95.78 % to 99.85 % with percentage RSD of 0.93 % to 4.80 % as listed in table 6. The recovery of analyte in these measurements revealed the precision and practical application of the designed surfactant based sensor for the sensing of multiple toxic HMs in real water samples.

3.5.

Computational Analysis The experimental results were further verified by computational calculations using Hyper

Chem 7.5 software. The computational analysis involved the calculation of energy values, band gap, chemical hardness, chemical softness, electronegativities and QSAR properties for all the selected sensors i.e. Cl-DPTU, DPTU and CF3-DPTU. The energy values include the values of HOMO, LUMO, binding energies and heat of formation [44]. The geometry of surfactant molecules was first optimized before the molecular orbital calculations. These calculations were performed by semi-empirical (AM1 method) as given in table 7. The HOMO and LUMO energy values were employed for the evaluation of ionization energy and electron affinity respectively. The pictorial representation of orbitals of HOMO and LUMO energy levels for selected modifiers

13

Journal Pre-proof

are presented in Figure 8. The band gap values calculated for all the modifiers determined the extent of promising interactions between surfactant molecules and metal ions. The highest value of band gap (-8.311 eV) was observed for Cl-DPTU as compared to band gap values of -7.851 eV for DPTU and -7.760 eV for CF3-DPTU. These calculations were completely in accordance with experimental findings that Cl-DPTU surfactant molecule (selected sensor in present study) can easily develop interactions with metal ions than other surfactant molecules [45]. The quantitative structure activity relationship (QSAR) properties are the physico-chemical properties of a molecule which are specifically calculated to determine the stability and efficiency. Thus, these calculations are required for the confirmation of the optimized groups that enhance the efficiency of a molecule [46, 47]. The QSAR properties calculated for all the three surfactant modifiers includes refractivity, Log P and polarizability. Similarly, the polarizability of a compound determines the amount of distortion in the electronic cloud of its molecule. It is greater for the molecules having electron donating and electron withdrawing groups. As expected, the polarizability of Cl-DPTU (42.25 Å3) and CF3-DPTU (41.89 Å3) was found to be larger than DPTU (40.32 Å3). Furthermore, log P, another QSAR property helped us to determine the hydrophobicity of the molecules. The higher log P value is symbolic of decrease in hydrophobicity and vice versa. The log P value of 4.53 for Cl-DPTU was the lowest value among the other studied surfactant molecules revealing its least attraction with water. This hydrophobic character of ClDPTU explains its higher stability as a modifier because it does not come off in the analyte solution [48]. The computational studies of our selected compounds showed complete correspondence with the experimental results.

14

Journal Pre-proof

Conclusions In summary, we have introduced a novel binder free electrochemical sensor based on thiourea based surfactant for the efficient sensing of multiple metal ions. The electrochemistry was performed by variety of electrochemical techniques i.e. EIS, CC, SWASV and SWV to establish the electrochemical response of peak currents. These electrochemical results were then verified theoretically by computational calculations of the interfacial properties of selected surfactants. Six metals ions i.e. Zn2+, Cd2+, Pb2+, Cu2+, Hg2+ and Sr2+ ions were sensed simultaneously using three different surfactants as modifiers on the electrode surface. A significant boost in the peak current response was observed with these surfactants based modifiers except CF3-DPTU/GCE as compared to bare GCE. The anodic peak response was found to be highest for Cl-DPTU/GCE to detect metal ions effectively. The optimum conditions i.e. supporting electrolyte, pH, deposition potential and deposition time were selected for the best functioning of the designed sensor. The selected sensor displayed lower limit of detection than the toxicity level of metal toxins recommended by EPA and WHO. The results revealed that our present fabricated electrochemical sensor have great discrimination ability and selectivity for selected metal ions even in the presence of several hundred folds higher concentration of interfering agents in the same environment. Moreover, the practical applicability of designed sensor in real samples (tap water and drinking water) was ensured by good recoveries with RSD less than 5%. We expect that our fabricated sensor could be considered as a promising candidate for successful detection of metal toxins in contaminated aqueous environments.

15

Journal Pre-proof

Acknowledgements Authors acknowledges the support of Quaid-i-Azam University and Higher Education Commission of Pakistan.

References [1] M. Shaban, A. Hussein, Detection of Heavy Metal Ions in Water by PAA/CNTs Nanosensor, Journal of Chemica Acta, 1 (2012) 49-51. [2] F. Zahir, S.J. Rizwi, S.K. Haq, R.H. Khan, Low dose mercury toxicity and human health, Environmental toxicology and pharmacology, 20 (2005) 351-360. [3] T.W. Clarkson, L. Magos, G.J. Myers, The toxicology of mercury—current exposures and clinical manifestations, New England Journal of Medicine, 349 (2003) 1731-1737. [4] J.O. Duruibe, M. Ogwuegbu, J. Egwurugwu, Heavy metal pollution and human biotoxic effects, International Journal of physical sciences, 2 (2007) 112-118. [5] S. Flora, M. Mittal, A. Mehta, Heavy metal induced oxidative stress & its possible reversal by chelation therapy, Indian Journal of Medical Research, 128 (2008) 501. [6] F. Gao, N. Gao, A. Nishitani, H. Tanaka, Rod-like hydroxyapatite and Nafion nanocomposite as an electrochemical matrix for simultaneous and sensitive detection of Hg2+, Cu2+, Pb2+ and Cd2+, Journal of Electroanalytical Chemistry, 775 (2016) 212-218. [7] B. Devadas, M. Sivakumar, S.M. Chen, M. Rajkumar, C.C. Hu, Simultaneous and selective detection of environment hazardous metals in water samples by using flower and christmas tree like cerium hexacyanoferrate modified electrodes, Electroanalysis, 27 (2015) 2629-2636. [8] M. Serry, A. Gamal, M. Shaban, A. Sharaf, High sensitivity optochemical and electrochemical metal ion sensor, Micro & Nano Letters, 8 (2013) 775-778.

16

Journal Pre-proof

[9] 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 (2012) 1105-1125. [10] L.K. Kumawat, N. Mergu, A.K. Singh, V.K. Gupta, A novel optical sensor for copper ions based on phthalocyanine tetrasulfonic acid, Sensors and Actuators B: Chemical, 212 (2015) 389394. [11] G.H. Hwang, W.K. Han, J.S. Park, S.G. Kang, Determination of trace metals by anodic stripping voltammetry using a bismuth-modified carbon nanotube electrode, Talanta, 76 (2008) 301-308. [12] S. Bernard, A. Enayati, L. Redwood, H. Roger, T. Binstock, Autism: a novel form of mercury poisoning, Med Hypotheses, 56 (2001) 462-471. [13] V.K. Gupta, R. Prasad, A. Kumar, Preparation of ethambutol-copper(II) complex and fabrication of PVC based membrane potentiometric sensor for copper, Talanta, 60 (2003) 149160. [14] V.K. Gupta, R. Mangla, U. Khurana, P. Kumar, Determination of Uranyl Ions Using Poly(vinyl chloride) Based 4-tert-Butylcalix[6]arene Membrane Sensor, Electroanalysis, 11 (1999) 573-576. [15] E. Bakker, E. Pretsch, Potentiometric sensors for trace-level analysis, Trends in analytical chemistry : TRAC, 24 (2005) 199-207. [16] A.K. Jain, V.K. Gupta, L.P. Singh, J.R. Raisoni, A comparative study of Pb2+ selective sensors based on derivatized tetrapyrazole and calix[4]arene receptors, Electrochimica Acta, 51 (2006) 2547-2553.

17

Journal Pre-proof

[17] G.D. O’Neil, M.E. Newton, J.V. Macpherson, Direct Identification and Analysis of Heavy Metals in Solution (Hg, Cu, Pb, Zn, Ni) by Use of in Situ Electrochemical X-ray Fluorescence, Analytical Chemistry, 87 (2015) 4933-4940. [18] W.-J. Li, X.-Z. Yao, Z. Guo, J.-H. Liu, X.-J. Huang, Fe3O4 with novel nanoplate-stacked structure: Surfactant-free hydrothermal synthesis and application in detection of heavy metal ions, Journal of Electroanalytical Chemistry, 749 (2015) 75-82. [19] A. Bobrowski, M. Putek, J. Zarębski, Antimony Film Electrode Prepared In Situ in Hydrogen Potassium Tartrate in Anodic Stripping Voltammetric Trace Detection of Cd(II), Pb(II), Zn(II), Tl(I), In(III) and Cu(II), Electroanalysis, 24 (2012) 1071-1078. [20] J. Li, S. Guo, Y. Zhai, E. Wang, Nafion–graphene nanocomposite film as enhanced sensing platform for ultrasensitive determination of cadmium, Electrochemistry Communications, 11 (2009) 1085-1088. [21] Z. Zou, A. Jang, E. MacKnight, P.-M. Wu, J. Do, P.L. Bishop, C.H. Ahn, Environmentally friendly disposable sensors with microfabricated on-chip planar bismuth electrode for in situ heavy metal ions measurement, Sensors and Actuators B: Chemical, 134 (2008) 18-24. [22] J. Ping, Y. Wang, J. Wu, Y. Ying, Development of an electrochemically reduced graphene oxide modified disposable bismuth film electrode and its application for stripping analysis of heavy metals in milk, Food Chemistry, 151 (2014) 65-71. [23] H. Bagheri, A. Afkhami, H. Khoshsafar, M. Rezaei, A. Shirzadmehr, Simultaneous electrochemical determination of heavy metals using a triphenylphosphine/MWCNTs composite carbon ionic liquid electrode, Sensors and Actuators B: Chemical, 186 (2013) 451-460. [24] G. Aragay, J. Pons, A. Merkoçi, Recent Trends in Macro-, Micro-, and Nanomaterial-Based Tools and Strategies for Heavy-Metal Detection, Chemical Reviews, 111 (2011) 3433-3458.

18

Journal Pre-proof

[25] I. Ullah, A. Shah, A. Badshah, U.A. Rana, I. Shakir, A.M. Khan, S.Z. Khan, Zia-ur-Rehman, Synthesis, Characterization and Investigation of Different Properties of Three Novel ThioureaBased Non-ionic Surfactants, Journal of Surfactants and Detergents, 17 (2014) 1013-1019. [26] I. Ullah, A. Naveed, A. Shah, A. Badshah, Zia-ur-Rehman, G.S. Khan, A. Nadeem, High Yield Synthesis, Detailed Spectroscopic Characterization and Electrochemical Fate of Novel Cationic Surfactants, Journal of Surfactants and Detergents, 17 (2014) 243-251. [27] T. Nagaoka, T. Yoshino, Surface properties of electrochemically pretreated glassy carbon, Analytical Chemistry, 58 (1986) 1037-1042. [28] A.H. Shah, Y. Wang, A.R. Woldu, L. Lin, M. Iqbal, D. Cahen, T. He, Revisiting Electrochemical Reduction of CO2 on Cu Electrode: Where Do We Stand about the Intermediates?, The Journal of Physical Chemistry C, 122 (2018) 18528-18536. [29] 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. [30] W. Yang, J. Justin Gooding, D. Brynn Hibbert, Characterisation of gold electrodes modified with self-assembled monolayers of l-cysteine for the adsorptive stripping analysis of copper, Journal of Electroanalytical Chemistry, 516 (2001) 10-16. [31] S. Niu, M. Zhao, R. Ren, S. Zhang, Carbon nanotube-enhanced DNA biosensor for DNA hybridization detection using manganese(II)-Schiff base complex as hybridization indicator, J Inorg Biochem, 103 (2009) 43-49. [32] J.J. Gooding, D.B. Hibbert, W. Yang, Electrochemical Metal Ion Sensors. Exploiting Amino Acids and Peptides as Recognition Elements, Sensors, 1 (2001) 75-90.

19

Journal Pre-proof

[33] K. Pokpas, N. Jahed, O. Tovide, P.G. Baker, E.I. Iwuoha, Nafion-graphene nanocomposite in situ plated bismuth-film electrodes on pencil graphite substrates for the determination of trace heavy metals by anodic stripping voltammetry, Int. J. Electrochem. Sci, 9 (2014) 5092-5115. [34] C. Hao, Y. Shen, J. Shen, K. Xu, X. Wang, Y. Zhao, C. Ge, A glassy carbon electrode modified with bismuth oxide nanoparticles and chitosan as a sensor for Pb (II) and Cd (II), Microchimica Acta, 183 (2016) 1823-1830. [35] N. Ratner, D. Mandler, Electrochemical detection of low concentrations of mercury in water using gold nanoparticles, Analytical chemistry, 87 (2015) 5148-5155. [36] I. Cesarino, G. Marino, J.d.R. Matos, É.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. [37] D.S. Rajawat, S. Srivastava, S.P. Satsangee, Electrochemical determination of mercury at trace levels using eichhornia crassipes modified carbon paste electrode, Int. J. Electrochem. Sci, 7 (2012) 11456-11469. [38] C. Martínez-Huitle, N.S. Fernandes, M. Cerro-Lopez, M. Quiroz, Determination of trace metals by differential pulse voltammetry at chitosan modified electrodes, Portugaliae Electrochimica Acta, 28 (2010) 39-49. [39] H. Xu, R. Miao, Z. Fang, X. Zhong, Quantum dot-based “turn-on” fluorescent probe for detection of zinc and cadmium ions in aqueous media, Analytica chimica acta, 687 (2011) 82-88. [40] M. Jarczewska, E. Kierzkowska, R. Ziółkowski, Ł. Górski, E. Malinowska, Electrochemical oligonucleotide-based biosensor for the determination of lead ion, Bioelectrochemistry, 101 (2015) 35-41.

20

Journal Pre-proof

[41] N. Serrano, A. González-Calabuig, M. del Valle, Crown ether-modified electrodes for the simultaneous stripping voltammetric determination of Cd(II), Pb(II) and Cu(II), Talanta, 138 (2015) 130-137. [42] G.A. Saleh, H.F. Askal, I.H. Refaat, A.H. Naggar, F.A.M. Abdel-aal, Adsorptive square wave voltammetric determination of the antiviral drug valacyclovir on a novel sensor of copper microparticles–modified pencil graphite electrode, Arabian Journal of Chemistry, 9 (2016) 143151. [43] M.A. Abu-Dalo, A.A. Salam, N.S. Nassory, Ion imprinted polymer based electrochemical sensor for environmental monitoring of copper (II), Int. J. Electrochem. Sci, 10 (2015) 6780-6793. [44] R.A.P. Ribeiro, S.R. de Lazaro, S.A. Pianaro, Density Functional Theory applied to magnetic materials: Mn3O4 at different hybrid functionals, Journal of Magnetism and Magnetic Materials, 391 (2015) 166-171. [45] A.H. Shah, A. Shah, U.A. Rana, S.U.-D. Khan, H. Hussain, S.B. Khan, R. Qureshi, A. Badshah, Redox Mechanism and Evaluation of Kinetic and Thermodynamic Parameters of 1,3Dioxolo[4,5-g]pyrido[2,3-b]quinoxaline Using Electrochemical Techniques, Electroanalysis, 26 (2014) 2292-2300. [46] J.D. Prades, A. Cirera, J.R. Morante, Ab initio calculations of NO2 and SO2 chemisorption onto non-polar ZnO surfaces, Sensors and Actuators B: Chemical, 142 (2009) 179-184. [47] R. Joseph, B. Ramanujam, A. Acharya, C.P. Rao, Lower rim 1,3-di{bis(2-picolyl)}amide derivative of calix[4]arene (l) as ratiometric primary sensor toward Ag+ and the complex of Ag+ as secondary sensor toward Cys: experimental, computational, and microscopy studies and INHIBIT logic gate properties of L, J Org Chem, 74 (2009) 8181-8190.

21

Journal Pre-proof

[48] M. Ibrahim, N.A. Saleh, W.M. Elshemey, A.A. Elsayed, Fullerene derivative as anti-HIV protease inhibitor: molecular modeling and QSAR approaches, Mini Rev Med Chem, 12 (2012) 447-451.

22

Journal Pre-proof

List of Figures 32

24

(b)

Bare / GCE Cl-DPTU / GCE

6.0 4.5

Q / C

Z'' (k)

7.5

Bare / GCE Cl-DPTU / GCE DPTU / GCE CH3-DPTU / GCE

(a)

16

8

3.0 1.5 0.0

0 0

8

16

Z' (k)

24

0.0

32

0.1

0.2

0.3

0.4

0.5

t / sec

Figure 1. (a) Nyquist plots obtained from the data recorded at bare and surfactant modified GCE in a medium of pH 4.0 containing 0.1M KCl and 5 mM K3[Fe(CN)6] (Inset shows equivalent circuit model with resistors, capacitors, and Warburg impedance elements) (b) Chronocoulograms measured in a solution containing 10 µM Hg2+ and BRB of pH 4. 80

Bare GCE Cl-DPTU / GCE DPTU/GCE CF3-DPTU/GCE

I / (A)

60 Cd2+ 2+ 40 Zn

Cu2+ Hg2+

20 Sr2+

Pb2+

0 -0.75

0.00

0.75

1.50

E / (V) vs Ag/ AgCl Figure 2. SWASV obtained at bare and surfactant-modified GCE in a solution mixture containing 30 µM Zn2+, 15 µM Cd2+, 15 µM Pb2+, 20 µM Cu2+, 15 µM Hg2+ and 60 µM Sr2+ in BRB (pH 4) at 100 mV s-1 scan rate, -1.0 V deposition potential and 100 s deposition time.

Journal Pre-proof

2+ Zn 2+ Cd 2+ Pb 2+ Cu 2+ Hg 2+ Sr

50

I / (A)

40 30

(b)

60

Ip (Zn

2+

Ip (Cd

50

I / (A)

(a)

Ip (Pb

)

2+

2+

)

)

2+

40

Ip (Cu

30

2+ Ip (Sr )

)

2+ Ip (Hg )

20

20

10

10

0 0 BRB

NaCl

HCl

KCl

PBS

NaOH

2

Suppporting Electrolytes

4

6

8

pH

10

12

Figure 3. (a) Effect of various stripping solvents (supporting electrolytes) i.e. BRB (pH = 4), phosphate buffer (pH = 7), 0.1mM KCl, 0.1mM NaCl, 0.1mM HCl and 0.1mM NaOH on the SWASV peak currents of 30 µM of Zn2+, 15 µM of Cd2+, 15 µM of Pb2+ , 20 µM of Cu2+ , 15 µM of Hg2+ and 60 µM of Sr2+ ions using Cl-DPTU/GCE at scan rate 100 mV/s. (b) Plot of SWASV peak currents of metal ions as a function of pH of BRB obtained on a Cl-DPTU/GCE at 100 mV s-1.

I / (A)

60 40

Zn2+ Cd2+

Zn2+

Pb2+ Cu2+

45

Hg2+ Sr2+

30

-1.2

Cu2+

-0.8

-0.4

0.0

0.4

Deposition Potential (V)

40

Pb2+

Sr2+

0.00

0.75

Pb2+ Cu2+

30

Hg2+ Sr2+

15

40

-1.6

E / (V) vs Ag/ AgCl

80

120

160

200

Deposition Time (s) Cu2+

Pb2+ Hg2+

0 1.50

Zn2+ Cd2+

45

Zn2+

Hg2+

0 -0.75

20 s 40 s 60 s 80 s 100 s 120 s 2+ 140 s Cd 160 s

0

20

20

-1.50

60

15

0 -1.6

Cd2+

(b)

I / ( A )

80

80

60

I / (A)

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

(a)

I / ( A )

100

-0.8

0.0

0.8

Sr2+

1.6

E / (V) vs Ag/ AgCl

Figure 4. (A) SWASV recorded in solution mixture of 30 µM Zn2+, 15 µM Cd2+, 15 µM Pb2+, 20 µM Cu2+, 15 µM Hg2+ and 60 µM Sr2+ ions in BRB of pH = 4; (inset: Plot of Ip vs Ed) (B) Effect of deposition time at a constant deposition potential of -1.0 V; (inset: Plot of Ip vs td).

Journal Pre-proof

100 15 M

I / (A)

80 30 M

60

20

1.1 nM

1 nM

1 nM 60 M Hg2+

Pb2+

Zn2+

15 M

Cu2+

2 nM Cd2+

40

20 M

8 nM

Sr2+

0 -1.50

-0.75

0.00

E / (V) vs Ag/ AgCl

0.75

Figure 5. (a) SWASV recorded for the sensing of Zn2+, Cd2+, Pb2+, Cu2+, Hg2+ and Sr2+ ions at ClDPTU/GCE in BRB (pH 4); Scan rate 100 mV/s, deposition potential -1.0 V, deposition time of 100 s. The concentration ranges are presented above each peak

Journal Pre-proof

(a)

3rd Scan 4th Scan

I / (A)

30

20

2+ Pb

2+ Zn

40

1st Scan 2nd Scan

2+ Cu

2+ Cd

Average Current value    A

40

5th Scan 6th Scan

2+ Hg

2+ Sr

10

0 -1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

(b)

30

20

10

0 Zn2+

E / (V) vs Ag/ AgCl 50

Cd2+

(c)

Cu2+

Hg2+

Sr2+

1st Measurement 2nd Measurement

2+ Cu 2+ Cd

3rd Measurement 4th Measurement 5th Measurement 6th Measurement

30 20

Pb2+

Metal ions

40

I / (A)

2+ Zn 2+ Cd 2+ Pb 2+ Cu 2+ Hg 2+ Sr

2+ Zn

2+ Pb 2+ Hg

10

2+ Sr

0 -1.5

0.0

E / (V) vs Ag/ AgCl

1.5

Figure 6. SWASV showing (a) repeatability of the Cl-DPTU/GCE (n=6) (b) Corresponding bar graph and (c) SWASV showing reproducibility of Cl-DPTU/GCE (n=6).

Journal Pre-proof

no interfering 2+ Ni 2+ Cs 3+ Cr 3+ Co 2SO4

(a) Cu2+

I / (A)

20

Cd2+ Hg2+

Zn2+

Leucine Tryptophan 3-bromo benzaldehyde 3-hydroxy benzaldehyde

Pb2+

10

Sr2+

40

(b)

Zn2+ Cd2+ Pb2+ Cu2+ Hg2+ Sr2+

30

I / (A)

30

20

10 0 -1.5

-1.0

-0.5

0.0

0.5

1.0

E / (V) vs Ag/ AgCl

1.5

2.0

0 No I.ANi2+ Cs2+ Cr3+ Co3+SO4 2-Leu Trp Br B OH B

Interfering Agents

Figure 7. (a) SWASV obtained with Cl-DPTU/GCE in a solution mixture of 30 µM of Zn2+, 15 µM of Cd2+, 15 µM of Pb2+, 20 µM of Cu2+, 15 µM of Hg2+ and 60 µM of Sr2+ ions along with one of the following interfering agents (I.A): No I.A, 1.5 mM Ni2+, 3 mM Cs2+, 5 mM Cr3+, 4.5 mM Co3+, 2.5 mM SO42-, 3.3mM Leucin, 2.5 mM Tryptophan, 2.5 mM 3-bromo benzaldehyde, 5 mM 3-hydroxy benzaldehyde and (b) Corresponding bar graph.

Journal Pre-proof

Figure 8. Pictorial representation of HOMO (Highest occupied molecular orbital) of optimized structures of Cl-DPTU, DPTU and CF3-DPTU surfactants by Hyper Chem 7.5 software using Semi-empirical AM1 method.

Journal Pre-proof

List of Tables Table I. Parameters calculated for bare and surfactant modified GC electrode from electrochemical impedance spectroscopy. Modifiers

CPE (µF)

Rs (Ω)

Rct (kΩ)

CF3-DPTU/GCE

1.89

825

14.6

Bare GCE

1.21

772

12.0

DPTU/GCE

1.07

724

11.7

Cl-DPTU/GCE

0.79

691

10.4

Table 2. Parameters calculated for bare and Cl-DPTU modified GCE from chronocoulometry. Electrode

Area / cm2

Qads / μC

Γ / 10-11 molcm-2

Ơ / Å2

Bare GCE

0.089

0.109

0.635

0.260

Cl-DPTU /GCE

0.123

0.483

2.815

0.059

Table 3. Figures of merits of the Cl-DPTU modified GCE. Metal

Limit of

Limit of

%RSD

%RSD

Linearity range

Ions

detection

Quantification

(Reproducibility)

(Repeatability)

(nM)

(nM)

(n=6)

(n=6)

Zn2+

16.9

56.5

3.57

4.70

2 nM-10 M

Cd2+

6.45

21.5

1.68

3.77

1 nM-5 M

Pb2+

11.0

36.8

2.31

3.40

1 nM-5 M

Cu2+

7.85

26.2

3.78

3.40

1.1 nM-10 M

Hg2+

9.15

30.4

2.05

4.36

1 nM-5 M

Sr2+

178.8

596

3.80

3.79

8 nM-5 M

Journal Pre-proof

Table 4. Comparison of some figures of merit related to the different reported modified electrodes for sensing ability of Zn2+, Cd2+, Pb2+, Cu2+, Hg2+ and Sr2+ ions. Electrode

Measurement

LOD (nM)

References

substrate

technique

Zn2+

Cd2+

Pb2+

Cu2+

Hg2+

Sr2+

BiONPs-CS/GCE

DPASV

N.M.

500

N.M

N.M.

N.M.

N.M

[34]

AuNPs/GCE

DPSV

N.M.

N.M.

N.M

N.M.

1000

N.M

[35]

SBA 15-silica/CPE

DPASV

N.M.

N.M.

40

200

400

N.M

[36]

WBMCPE

DPASV

N.M.

N.M.

N.M

N.M.

720

N.M

[37]

Chitosan/GCE

DPASV

N.M.

N.M.

N.M.

8900

N.M.

N.M.

[38]

CdTe QDs

PL

1200

N.M.

N.M.

N.M.

N.M.

N.M.

[39]

Oligonucleotide/

EIS

N.M.

N.M.

34.7

N.M.

N.M.

N.M.

[40]

SWASV

16.9

6.45

11.0

7.85

9.15

178.8 Present Work

Biosensor Cl-DPTU/GCE

BiONPs-CS: bismuth oxide nanoparticles-chitosan; GCE: glassy carbon electrode; AuNPs: gold nanoparticles; SBA 15-silica: silica organofunctionalised with 2-benzothiazolethiol; WBMCPE: Water hyacinth biomass modified carbon paste electrodes; CdTe QDs: cadmium telluride quantum dots; PL: photoluminescence; EIS: Electrochemical impedance spectroscopy; Cl-DPTU: 1-(3-chlorophenyl)-3dodecanoylthiourea. *N.M. = not measured

Journal Pre-proof

Table 5. Tolerance limit of interfering agents in the presence of of 30 µM of Zn2+, 15 µM of Cd2+, 15 µM of Pb2+, 20 µM of Cu2+, 15 µM of Hg2+ and 60 µM of Sr2+ ions Interfering

Tolerance Limit (Fold higher concentration than the analyte)/ µM

Agents

Zn2+

Cd2+

Pb2+

Cu2+

Hg2+

Sr2+

Ni2+

41.66

83.33

83.33

62.50

83.33

20.83

Cs2+

100

200

200

150

200

50

Cr3+

166.7

333.3

333.3

250

333.3

83.33

Co3+

150

300

300

225

300

75

SO4

83.33

166.7

166.7

125

166.7

41.67

Leucine

110

220

220

165

220

55

Tryptophan

83.33

166.7

166.7

125

166.7

41.67

3-bromo

83.33

166.7

166.7

125

166.7

41.67

166.7

333.3

333.3

250

333.3

83.33

2-

benzaldehyde 3-hydroxy benzaldehyde

Table 6. Results for Zn2+, Cd2+, Pb2+, Cu2+, Hg2+ and Sr2+ determination in real water samples obtained under optimum experimental conditions. Metal

Sample

Ions Zn2+ Cd2+ Pb2+ Cu2+ Hg2+ Sr2+

Initially

Spiked

found (µM) amount (µM)

Found

RSD (3) (%)

Recovery (%)

(µM)

Tap water

0

28

27.23

1.32

97.25

Drinking water

0

28

26.99

1.56

96.39

Tap water

0

14

13.63

4.29

97.35

Drinking water

0

14

13.54

3.84

96.71

Tap water

0

14

13.95

4.16

99.64

Drinking water

0

14

13.90

4.55

99.28

Tap water

0

19

18.92

1.86

99.57

Drinking water

0

19

18.94

1.34

99.68

Tap water

0

14

13.71

3.28

97.92

Drinking water

0

14

13.62

4.54

97.28

Tap water

0

60

57.62

3.34

96.03

Drinking water

0

60

58.72

4.22

97.86

Journal Pre-proof

Table 7. Comparative data showing chemical reactivity descriptors of Cl-DPTU, DPTU and CF3-DPTU surfactants. Parameters

Cl-DPTU

DPTU

CF3-DPTU

Total energy (kcal/mol)

-95369.03

-87079.80

-123300.19

Heat of formation (kcal/mol)

-42.47

-50.58

-205.85

Binding energy (kcal/mol)

-5181.29

-5212.51

-5543.23

EHOMO (eV)

-8.5048

-8.6068

-8.8314

ELUMO (eV)

-0.1937

-0.7553

-1.0753

I.E= -EHOMO (eV)

8.5048

8.6068

8.8314

E.A= -ELUMO (eV)

0.1937

0.7553

1.0753

Band Gap=EHOMO-ELUMO (eV)

-8.3111

-7.8514

-7.7601

Electronegativity /χ (eV)

4.3492

4.7116

4.9553

Chemical Hardness η (eV)

4.1555

3.9257

3.8801

Chemical Softness σ (eV)

0.2406

0.2547

0.2630

Refractivity (Å3)

111.23

101.71

107.68

Polarizability (Å3)

42.25

40.32

41.89

Log P

4.53

5.94

6.82