Preparation of a lead sensor based on porous multiwalled carbon nanotubes/thiolated chitosan composite materials

Thiolated chitosan (CS–TGA) was prepared using chitosan (CS) and thioglycolic acid (TGA). Then MWCNTs were added to the mixture of CS–TGA and CS to prepare the CS/CS–TGA/MWCNs porous composite by freeze-drying method and this composite was used to modify an indium tin oxide glass electrode. The electrode was used as a sensor for Pb2+. The morphology and structure of the composite were characterized by infrared spectroscopy and scanning electron microscope, and their electrochemical behavior was also studied.

Turkish Journal of Chemistry http://journals.tubitak.gov.tr/chem/ Research Article Turk J Chem (2014) 38: 182 188 ă ITAK c TUB ⃝ doi:10.3906/kim-1212-33 Preparation of a lead sensor based on porous multiwalled carbon nanotubes/thiolated chitosan composite materials Jun WAN∗, Ling XING, Wei WANG College of Environment and Safety Engineering, Key Laboratory of Eco-chemical Engineering, Ministry of Education, Qingdao University of Science and Technology, Qingdao, P.R China Received: 12.12.2012 • Accepted: 14.06.2013 • Published Online: 14.03.2014 • Printed: 11.04.2014 Abstract:Thiolated chitosan (CS–TGA) was prepared using chitosan (CS) and thioglycolic acid (TGA) Then MWCNTs were added to the mixture of CS–TGA and CS to prepare the CS/CS–TGA/MWCNs porous composite by freeze-drying method and this composite was used to modify an indium tin oxide glass electrode The electrode was used as a sensor for Pb 2+ The morphology and structure of the composite were characterized by infrared spectroscopy and scanning electron microscope, and their electrochemical behavior was also studied Under the optimized experimental conditions, the sensor showed a linear range of 2.0 × 10 −9 ∼ 2.0 × 10 −8 mol L −1 for Pb 2+ with a detection limit of 9.53 × 10 −10 mol L −1 according to the σ rule The prepared heavy ion sensor displayed excellent electrochemical response and high sensitivity Key words: Thiolated chitosan, composite materials, porous materials, lead sensor Introduction Chitosan is a kind of renewable natural biopolymer, and its amino and hydroxyl can easily form high charge density cationic polyelectrolyte in acidic solution, which means it has good complexation and adsorption properties In order to extend the scope of the application of chitosan and make it easy to separate and regenerate, cross-linking, grafting, acylation, and etherification were introduced to improve the nature of the chitosan 1,2 For example, after the reaction of carboxyl of thioglycolic acid and amino of thitosan, a new bioadhesive material, chitosan–thioglycolic acid (CS–TGA) conjugates, was obtained 3−6 The conjugates can effectively improve the adhesion, permeability, and swelling behavior of chitosan As a result, heavy metal ions can be firmly absorbed on the chitosan Due to their possessing unique electronic, mechanical, structural properties, and energy storage, carbon nanotubes (CNTs) have greatly attracted the attention of researchers all over the world since they were found in 1991 7−10 Their special properties such as good electrical conductivity, stable performance, and excellent catalytic activity make them one of the best electrode materials 11 Liu and Hu doped CNTs to cellulose, promoting the transfer of electrons in the thin film of cellulose 12 It has also been reported that CNTs were mixed with polymer to improve their hydrophobicity 13,14 Lau and Cooney prepared the porous composite MWCNTs-CS, which dispersed evenly with an average pore size of about 10∼15 µ m and good conductive properties 15 Han et al prepared the porous scaffolds of alginate–chitosan/hydroxyapatite polyelectrolyte complex, exhibiting higher mechanical strength and better thermal stability 16 ∗ Correspondence: 182 wanjundz@sohu.com WAN et al./Turk J Chem Lead and its compounds are toxic chemicals in the list of environmental pollutants They cannot be degraded in water and great harm to the environment and life Even at extremely low concentrations, lead can cause nerve dysfunction, kidney damage, and reproductive system damage Due to the great concern about lead contamination, there is still an urgent demand for lead ion detection techniques There are many ways to detect trace lead The electrochemical method is very effective Ping 17 reported that an ionic liquid modified carbon paste electrode exhibited well-defined and separate stripping voltammetric peaks for cadmium and lead, and the detection limit was 0.10 µ g L −1 for cadmium and 0.12 µ g L −1 for lead Pan et al modified nanomaterial/ionophore on a glassy carbon electrode, using nanosized hydroxyapatite to improve the sensitivity and establishing a sensitive electrochemical method of determination of lead, with a linear range of 5.0 nM to 0.8 µ M 18 Xu et al proposed a method using multiwall carbon nanotubes/Nafion composite film electrode to detect the heavy metals Pb 2+ and Cd 2+ in real water, and the determination limits were 100 ng L −1 for Pb and 150 ng L −1 for Cd 19 In this paper, chitosan/chitosan–thioglycolic acid/multiwalled carbon nanotube composites were prepared and were used to modify an indium tin oxide (ITO) glass electrode to detect Pb 2+ This method was simple and sensitive for the detection of heavy metal ions in a certain concentration range Experimental 2.1 Reagents and instruments Multiwalled carbon nanotubes (Φ 20∼ 30 nm) were purchased from Chengdu Chemistry Co., Ltd ITO glass electrodes were purchased from Shenzhen Nanbo Display Devices Co., Ltd Chitosan (CS), 1–ethyl–3–(3– dimethyllaminopropyl) carbodiimide hydrochloride (EDC), N–hydroxysuccinimide (NHS), and other reagents were of analytical grade and used as received without further purification All solutions were prepared with doubly distilled water All the electrochemical measurements were carried out on a CHI 832B electrochemical analyzer (Shanghai Chen Hua Instrument Co Ltd.) A platinum wire and an Ag/AgCl electrode were used as auxiliary electrode and reference electrode, respectively An ITO glass electrode and modified ITO glass electrodes were used as working electrode The as-prepared samples were analyzed by JSM-6700F scanning electron microscopy (SEM, Japan Electron, Japan) and Nicolet FT-IR 510P spectrophotometer (IR, Nicolet, USA) 2.2 Preparation of CS/CS–TGA/MWCNTs composite Appropriate amounts of EDC and NHS were added to CS–HCl solution under stirring Then thioglycolic acids were added to the solution to adjust pH to After stirring for h at room temperature, the reaction solutions were removed into a dialysis bag in order to obtain the thiolated chitosan The prepared thiolated chitosan was mixed with chitosan according to mass ratio of 7:3 20 in 0.2 mol L −1 acetic acid After MWCNTs were added, a black viscous liquid was obtained and it was freeze-dried at –10 ◦ C Then black powders were obtained 2.3 Preparation of CS/CS–TGA/MWCNTs/ITO electrode A glass electrode was cleaned in ethanol in an ultrasound cleaner for Then 10 µ L of the prepared composite–acetic acid solution was deposited directly onto the ITO electrode The electrode was then quickly transferred to a vacuum freeze-dryer, and frozen at –20 ◦ C for h, forming the modified electrode 183 WAN et al./Turk J Chem 2.4 Electrochemical determination of Pb 2+ on the modified electrode Before each test, the modified electrode was immersed into the solution for adsorption under stirring After enriching for a certain period of time, the electrode was removed from the solution and washed using ultrapure water Then the electrode was placed in the electrolytic cell containing the supporting electrolyte, and was reduced for 30 s under the reduction potential of –1.0 V Square wave voltammetry was used to detect Pb 2+ Results and discussion 3.1 The IR characterization of CS and CS–TGA Figure shows the infrared spectra of CS–TGA and CS Compared with the infrared spectrum of CS, a –SH Transmittance[%] stretching vibration appeared at 2459 cm −1 , which was consistent with the IR spectrum of the –SH peak characteristic frequency in organic sulfides, indicating that the as-prepared compound contained –SH functional groups 0 9 0 8 0 7 0 6 0 5 0 40 00 24 59 14 22 16 23 99 82 34 38 94 16 08 9.737 54 C S-TG A CS 30 00 20 00 100 Wavenumber cm–1 Figure IR-spectra of CS–TGA and CS 3.2 Characterization of the CS/CS–TGA/MWCNTs composite In this work, porous CS/CS–TGA/MWCNTs composite scaffolds were fabricated from aqueous mixtures of chitosan/thiolated chitosan and MWCNTs using a 2-step process in which the CS/CS–TGA/MWCNTs solution was frozen and then freeze-dried Based on the literature, 20 the mass ratio of CS–TGA and the CS was selected to be 7:3 The content of MWCNTs in the composites was optimized The resulting scaffold porous structure with different mass fractions of MWCNTs was characterized by SEM images (Figures 2a–2d) As the concentration of MWCNTs in the precipitate increases, the degree to which the tips of the MWCNTs puncture the precipitate’s surface increases At the lowest MWCNT concentrations (the content of MWCNTs was about 0.83 wt%, Figure 2a), the CS/CS–TGA surface was mostly smooth with few distinguishable MWCNTs visible, and the pore size distribution was not uniform (see Figure 2a) At higher MWCNT concentrations (the MWCNTs content was 3.5 wt% and 5.0 wt%, Figures 2c and 2d), the tips of the MWCNTs began to puncture the surface, the pore size was too large, and the distribution was uneven and not suitable for absorption (see Figures 2c and 2d) When the MWCNTs content was 2.5 wt% (Figure 2b), the pore size distribution was relatively uniform, and the pore size was suitable for absorption The amount of 2.5wt% MWCNTs was selected in the next experiment 184 WAN et al./Turk J Chem Figure SEM image of CS/CS–TGA/MWCNTs composite material with different content of MWCNTs: (a) the content of MWCNTs is 0.83 wt%; (b) the content of MWCNTs is 2.5 wt%; (c) the content of MWCNTs is 3.5 wt%; (d) the content of MWCNTs is 5.0 wt% 3.3 Optimization of the Pb 2+ determination conditions The effect of pH value of the substrate solution on the electrochemistry was examined, and the results are shown in Figure 3a It can be seen that the peak current increased as the pH increased from 3.5 to 5.0 and the maximum response was at pH 5.0; then it decreased Thus, the optional pH value was 5.0 and was chosen for all experiments Figure 3b was the adsorption time optimization curve of the electrode soaked in the solution containing µM of Pb 2+ for different times in acetate buffer (pH 5.0) electrolyte The figure shows that the peak current increased with adsorption time, and reached its maximum when the adsorption time was 35 min; it then decreased slowly This was probably because of the electrode adsorption reaching equilibrium at 35 We chose the adsorption time of 35 as the optimal time 185 WAN et al./Turk J Chem 2.4 a b 2.2 Current/10 A -6 -5 Current/10 A 2.0 1.8 1.6 1.4 1.2 3.5 4.0 4.5 5.0 5.5 6.0 10 15 Potential/V 20 25 30 35 40 45 T/min Figure Optimization conditions of (a) pH and (b) adsorption time 3.4 Amperometric response to Pb 2+ Pb 2+ was detected by square wave voltammetry (SWV) Figures 4a and 4b show the SWV curves and the linear relationship of the response of the modified electrode to Pb 2+ under the optimal conditions The linear response range of the sensor to Pb 2+ was 2.0 × 10 −9 ∼ 2.0 × 10 −8 mol L −1 and the linear regression equation was I (µ A) = 2390 C (µ mol L −1 ) + 17.746 (n = 7, r = 0.9980) with a detection limit of 9.53 × 10 −10 mol L −1 according to the σ rule The results indicated that the prepared sensor had a higher sensitivity for Pb 2+ Moreover, the detection limit was much lower than the acceptable Pb 2+ concentration in drinking water (0.01 mg/L) according to the World Health Organization (WHO) The modified electrodes display better electrochemical characteristics, high sensitivity, and much lower detection limits than reported before 21,23,25 -1.5 5.0 -2.0 4.5 a 4.0 -5 -3.0 Current/10 A -5 Current/10 A -2.5 g -3.5 -4.0 3.5 3.0 -4.5 a -5.0 -5.5 2.5 2.0 -1.0 -0.8 -0.6 -0.4 Potential/V -0.2 0.0 b 10 12 14 16 -9 Concentration/10 M Figure a) SWV curves obtained after immerging in different concentration of Pb 2+ (a) × 10 −9 ; (b) × 10 −9 ; (c) × 10 −9 ; (d) × 10 −9 ; (e) × 10 −8 ; (f) 1.2 × 10 −8 ; (g) 1.4 × 10 −8 in 0.1 mol L −1 acetate buffer solution (pH 5.0) of CS/CS–TGA/MWCNs/ITO electrode; b) Calibration plot of peak current versus Pb 2+ concentration (the detection condition was similar to a.) 186 WAN et al./Turk J Chem 3.5 Reproducibility and interference The reproducibility of the present method was then evaluated; the RSD for repeated measurements of 1.0 × 10 −8 mol L −1 Pb 2+ was 3.9% These experimental results confirmed that the reproducibility of the produced sensor was excellent The influence of Cu 2+ , Cd 2+ , and Hg 2+ was also tested under the same conditions The concentration of each interfering substance was 1.0 × 10 −6 mol L −1 , which was 100 times that of Pb 2+ under the detection conditions Figure exhibits the effect of the interferons The result showed that Cu 2+ , Cd 2+ , and Hg 2+ did not cause any observable interference, suggesting that, especially at low concentration of Pb 2+ , there was no interference from Cu 2+ , Cd 2+ , or Hg 2+ , which might lead to false Pb 2+ measurements We can see that the selectivity of the sensor is quite good Pb 2+ Pb 2+ 2+ Cu Pb 2+ 2+ Cd 2+ 2+ Pb , Hg Current/10-5 Figure The current response of the sensor in 1.0 × 10 −8 mol L −1 Pb 2+ solution and Pb 2+ with 1.0 × 10 −6 mol L −1 other different ions solutions To demonstrate the performance of the developed method, a comparison of the linear range and detection limit obtained by several methods for Pb 2+ detection was made in the Table The detection limit of this developed Pb 2+ sensor was lower than that of sensors based on bismuth-modified MWCNT, 21 OMC-IL chitosan/CILE, 23 and ionic-liquid/Schiff base/MWCNTs/nanosilica 25 In comparison with the results reported by other research groups, our results show that the detection limit is good Table Comparison of some properties in the present work with those in other studies Method Bismuth-modified MWCNT dsDNA/α–Fe2 O3 /GCE OMC-IL2 -chitosan/CILE 5-Br-PADAP/MWCNT ionic-liquid/Schiff base/MWCNTs/nanosilica CS/CS–TGA/MWCNTs/ITO Linear range 2∼100 µg L1 0.1240 nM 0.051.4 àM 0.9114.6 àg L1 ì 10−9 ∼1.0 × 10−1 M 2.0 × 10−9 ∼2.0 × 10−8 M Detection limit 1.3 µg L−1 0.1 nM 25 nM 0.1 àg L1 2.51 ì 109 M 9.53 ì 10−10 M Ref [21] [22] [23] [24] [25] This work OMC, ordered mesoporous carbon; IL , 1-ethyl-3-methylimidazolium tetrafluoroborate; CILE, carbon ionic-liquid electrode; 5-Br-PADAP, 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol Furthermore, the reproducibility of the electrode was also tested The catalytic current response could maintain about 95% of its original response over weeks when the modified electrode was stored and measured intermittently 187 WAN et al./Turk J Chem Conclusion In this work, the composite of CS/CS–TGA/MWCNs was prepared and was used to modify a glass ITO electrode to form a heavy metal ions sensor The prepared material had good adsorption properties for heavy metal ions and was suitable for a sensor Under the optimal conditions, the sensor showed a linear range of 2.0 × 10 −9 ∼ 2.0 × 10 −8 mol L −1 for Pb 2+ with a detection limit of 9.53 × 10 −10 mol L −1 according to the σ rule The sensor exhibited high sensitivity and good response to detect heavy metal ions Acknowledgments This work was supported by the National Natural Science Foundation of China (No 21175077 and 21105053), the Scientific and Technical Development Project of Qingdao (12-1-4-3-(4)-jch and 11-2-4-3-(8)-jch), and the Nature Science Foundation of Shandong Province (ZR2010BM025) References Karuppasamy, K.; 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