Amino-functionalized MCM-41 for the simultaneous electrochemical determination of trace lead and cadmium

Electrochimica Acta 144 (2014) 161–167

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Amino-functionalized MCM-41 for the simultaneous electrochemical determination of trace lead and cadmium Xingxin Dai a , Fagui Qiu b , Xuan Zhou a , Yumei Long a,c, *, Weifeng Li a, *, Yifeng Tu a,c a

College of Chemistry, Chemical engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, PR China Department of Quartermaster Engineering, Jilin University, No, 5333, Xi'an Road, Changchun City 130062, PR China c The Key Lab of Health Chemistry and Molecular Diagnosis of Suzhou b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 17 May 2014 Received in revised form 24 July 2014 Accepted 12 August 2014 Available online 2 September 2014

In this work, a new strategy for the simultaneous determination of Pb2+ and Cd2+ was described based on amino-functionalized mesoporous silica (NH2-MCM-41) as sensing mediator. NH2-MCM-41 was prepared using a post-grafting process and the successful amino-functionality was confirmed by Fourier transform infrared (FTIR) and X-ray energy dispersive (EDS) spectra. It was found that both MCM-41- and NH2-MCM-41-modified glassy carbon (GC) electrode exhibit simultaneous response to Pb2 + and Cd2+. However, the NH2-MCM-41 modified electrode showed higher sensitivity than that of the MCM-41-modified one, which was attributed to the good chelating ability of amino groups to metal ions besides the high surface area and special mesoporous morphology of MCM-41. As a result, simultaneous assay of Pb2+ and Cd2+ was realized using anodic stripping voltammetric (ASV) method. Under the optimum experimental conditions, the linear response ranges for Pb2+ and Cd2+ ions are 0.5-250 mgL 1 and 50-450 mgL 1, respectively. The corresponding detection limits are 0.2 mgL 1 for Pb2+ and 1.0 mgL 1 for Cd2+, with good electrode renewability, which is defined as the complete removal of the accumulated metals from the electrode surface. In addition, the NH2-MCM-41 modified electrode was demonstrated for the successful determination of Pb2+ and Cd2+ in real samples, including tap water, lake water and tea. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: NH2-MCM-41 Sensor Anodic stripping voltammetry Lead Cadmium

1. Introduction With the increasing environmental problems caused by heavy metal pollution, rapid and accurate monitoring of heavy metal ions in ecological system is of great importance in assessing environmental damage and possible strategies for remediation. Many techniques have been developed for heavy metal ion detection and they are atomic absorption spectroscopy [1,2], atomic fluorescence spectrometry [3], inductively coupled plasma-mass spectrometry [4], colorimetry [5], fluorimetry [6] and electrochemical voltammetry [7,8]. Electrochemical method has several advantages over other techniques in that they are sensitive, selective, rapid, inexpensive and the possibility of in-field application. This strategy is based on chemically-modified electrodes with designable molecules so that the target compounds such as heavy metal ions will selectively adsorb onto and enrich the modified surface, making ultra-trace detection possible. Therefore, application of

* Corresponding author. Tel.: +86 512 65880089; fax: +86 512 65880089. E-mail addresses: [email protected] (Y. Long), [email protected] (W. Li). http://dx.doi.org/10.1016/j.electacta.2014.08.093 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.

proper electrode modifier materials is one the most efficient ways for the improvement of electrochemical performance. In the recent years, nano-sized or mesoporous materials have been widely employed for the construction of electrochemical sensors [9–18]. These materials often show unique chemical and physical properties, which are usually different from their bulk counterparts due to their small size and special structure. Some nano-sized or mesoporous materials, especially functionalized materials, possess high sensitivity, special selectivity and good stability to adsorbates, which can endow the modified electrode with dramatically enhanced electrochemical properties. Nevertheless, searching for suitable functional materials is still a challenging work in the sensor development for detecting ultra-trace heavy metal ions. Since mesoporous molecular sieves M41S was reported in 1992 [19], mesoporous silica has gained great interests owing to their large surface area, well-defined pore size and pore shape. The well-defined pore structure and high flexibility for surface modification make these materials very attractive for application as catalytic supports, sensors and adsorbent. For example, many functionalized mesoporous silica has demonstrated high capacity and excellent selectivity of metal ion adsorption [20–24]. On the

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Amino-functionalized MCM-41 was achieved by refluxing a dry toluene solution containing 0.5 g of the calcined MCM-41 and 2 mL of APS at 110  C. The modified MCM-41 was collected, rinsed with toluene via vacuum filtration, and dried. The resulting sample was denoted as NH2-MCM-41. 2.3. Apparatus The obtained products were characterized by XRD (PANalytical, X-Pert-Pro MPD with Cu-Ka radiation, l=1.540598 Å). The morphologies and microstructures of the as-prepared samples were analyzed by TEM (FEI Tecnai G20, an acceleration voltage of 200 kV) equipped with an EDS system. FTIR spectra (Nicolet 550, USA) with KBr as a diluting agent were recorded in the range of 4000-400 cm 1. Element analysis was examined by inductively coupled plasma optical emission spectrometry (ICP-OES) on a Varian 710-ES system (USA). Electrochemical measurements were performed on a CHI611D electrochemical workstation (Chenhua Fig. 1. XRD patterns of MCM-41 and NH2-MCM-41.

other hand, superior mass transport will be realized in this class of material due to their special pore structure. Moreover, silica-based materials are environment friendly and stable. It is therefore expected that mesoporous silica can be an excellent candidate for the fabrication of electrochemical sensor to detect ultra-trace heavy metals. In this study, amino-functionalized MCM-41was prepared and then employed as sensing materials for the electrochemical stripping analysis of Pb2+ and Cd2+, which are well-known hazardous environmental pollutants with toxic effects on living organisms. The properties of the electrochemical sensor were investigated and the modified electrode showed great affinity towards Pb2+ and Cd2+. Consequently, the simultaneous detection of Pb2+ and Cd2+ was realized. The experimental conditions related to characteristic of the sensing system have also been studied and then the sensor was applied in real sample assay. 2. Experimental 2.1. Materials 3-aminopropyltriethoxysilane (APS) was purchased from Sigma-Aldrich. Cetyltrimethylammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), acetic acid, cupric acetate and sodium acetate were obtained from Sinopharm Chemical Reagent Co. Ltd. (China). All the chemicals were of analytical grade and used without further purification. All solutions were freshly prepared using doubly deionized water and acetic acid buffer solution (NaAc-HAc, 0.2 M, pH 4.1) was chosen as the supporting electrolyte. 2.2. Preparation of NH2-MCM-41 MCM-41 was synthesized by sol-gel method modified from previous report [23]. Briefly, 0.3 g of cetyltrimethylammonium bromide (CTAB) was dissolved in 150 mL distilled water and stirred to form a homogeneous solution. Then, 1.5 mL of ammonia solution (28-30 W%) and 1.7 mL of tetraethyl orthosilicate (TEOS) were added in sequence to the solution under vigorous stirring. After about 12 h, the solid product was filtrated, washed with distilled water several times and dried in air at 110  C for 24 h. Finally, calcination was conducted in air at 550 C for 5 h to remove the template.

Fig. 2. TEM images of MCM-41 (a) and NH2-MCM-41 (b). Inset is the EDS of MCM-41 and NH2-MCM-41

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Fig. 3. FTIR spectra of MCM-41 and NH2-MCM-41.

Fig. 5. Effect of pH on the LSASV responses of Pb2+ (10 mg L 1) and Cd2+ (200 mg L 1) in 0.2 M NaAc-HAc solution. Deposition potential: -0.6 V; deposition time: 200 s.

Instruments Co., Shanghai, China). All electrochemical analysis was conducted in a conventional three-electrode system, in which a NH2-MCM-41-Nafion/GC, a platinum plate, and a saturated calomel electrode (SCE) served as working electrode, counter electrode and reference electrode, respectively.

electrode (MCM-41-Nafion/GCE) was also fabricated with the same procedure as described above.

2.4. Preparation of electrodes GCEs (diameter 3 mm) were first polished with 1.0 mm, 0.3 mm and 0.05 mm alumina slurries in sequence and then rinsed ultrasonically with doubly deionized water. After sonicating in absolute ethanol and water, respectively, the cleaned GCEs were dried with nitrogen. The electrode was modified by a simple casting method. Firstly, an appropriate amount of the as-received NH2-MCM-41 was dispersed into ethanol (5 mg/mL) containing Nafion (1%) to form a homogeneous suspension with the aid of ultrasonication. Subsequently, 15 mL of the dispersion was spread onto the clean GCE surface and left it dry at room temperature (NH2-MCM-41-Nafion/GCE). For comparison, a MCM-41 modified

2.5. Preparation of practical samples Tap water was taken from our research lab without pretreatment. Lake water was collected from Dushu Lake of Suzhou and the samples were purified using a filter paper to remove some of impurities. For tea sample, 1.0 g of tea powder was weighed and placed into beaker. Then 10 mL mix acid (volume ratio: nitric acid/ perchloric acid = 5) was added and evaporated to near dryness. The obtained white sample was re-diffused into water and heated. Finally, the solution was cooled, filtered and quantitatively transferred into a 50 mL volumetric flask for further analysis. Prior to measurement, all samples were diluted to meet the detecting concentration range of the proposed method. 2.6. Electrochemical measurements Electrochemical measurements were performed using linear sweep anodic stripping voltammetry (LSASV). Firstly, the NH2-MCM-41-Nafion/GC electrode was immersed in 10 mL of NaAc-HAc buffer solution, containing Pb2+ and Cd2+ at various concentrations. Then, the pre-concentration step was carried out in stirred solution at the potential of 1.2 V for 200 s. After an equilibration period of 10 s, LSASV responses were recorded from 1.0 to 0.2 V with a scan rate of 0.1 Vs 1. To ensure the complete removal of the residual metals from the surface, the electrode was cleaned at potential of +0.8 V for 300 s after each measurement. 3. Results and discussion 3.1. Characteristics of the NH2-MCM-41

Fig. 4. LSASV responses of 10 mg L 1 of Pb2+ and 200 mg L 1 of Cd2+ in 0.2 M NaAcHAc solution on bare, MCM-41-Nafion/GC, NH2-MCM-41-Nafion/GC electrodes (Deposition potential and time are -0.6 V and 200 s, respectively.).

XRD patterns of the samples are displayed in Fig. 1. Three well-resolved diffraction peaks of (100), (110) and (200) suggest good crystallinity with the hexagonal MCM-41 phase and no structural alteration can be found due to NH2-functionization. Fig. 2 shows the TEM images of MCM-41 (A) and APS functional MCM-41 (B). It can be clearly seen from Fig. 2A that native MCM-41 has the highly parallel channel-like porous structure packed in a hexagonal symmetry. The order mesoporous structure of the MCM-41 remained after modification with APS (Fig. 2B). EDS measurements (inset of Fig. 2A and 2B) demonstrate

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Fig. 8. LSASV responses of NH2-MCM-41-Nafion/GC electrode a) before and b) after pre-concentration in the NaAc-HAc solution containing 10 mg L 1 of Pb2+ and 200 mg L 1 of Cd2+, c) the renewed NH2-MCM-41-Nafion/GC electrode in NaAc-HAc solution.

Fig. 6. Effect of deposition potential (A) and deposition time (B) on the LSASV responses of Pb2+ (10 mg L 1) and Cd2+ (200 mg L 1) in 0.2 M NaAc-HAc solution with pH of 4.1.

that there exists Si and O signal for MCM-41 while the APS-modified MCM-41 is composed of Si, O and N elements. The N element signal is caused by the incorporation of amino groups into MCM-41 structure after modification, which was verified by FTIR results. As shown in Fig. 3, both the samples have a strong absorption band around 1075 cm 1 and 800 cm 1, which are attributed to asymmetric stretching vibration and symmetric stretching vibration of Si-O-Si, respectively. After grafting by APS, several new absorption bands can be found. The absorption bands around 1550 cm 1 and 685 cm 1 correspond to–NH2 symmetric bending vibration and N-H bending vibration, respectively [25]. C-H stretches can be clearly identified at 2930 cm 1 and 2858 cm 1 in the NH2-MCM-41 sample. These results confirm the existence of APS functional group in the modified MCM-41 sample. 3.2. Electrochemical response of lead and cadmium on modified electrodes Fig. 4 displays the LSASV responses of various electrodes toward Pb2+ and Cd2+ in 0.2 M NaAc-HAc solution (pH 4.1). There are no appreciable electrochemical features for bare GC electrode. However, two well-defined stripping peaks can be clearly observed for MCM-41-Nafion/GC electrode, one peak emerges at -0.496 V and the other is around -0.661 V, which corresponds to the reoxidaion of lead and cadmium, respectively. Furthermore, the stripping peak currents significantly increase when the NH2-MCM41-Nafion/GC electrode was employed. This affirms that the metal ion (Pb2+ and Cd2+) adsorption ability of electrode surface is greatly enhanced resulting from NH2-MCM-41 modification. The high affinity of the modified electrode could be related to the high specific surface area of MCM-41 and strong chelating ability of amino-groups.

Table 1 Interference study. (The concentration of Pb2+ and Cd2+ are 10 and 200 mg L 1, respectively.) Fig. 7. LSASV responses after pre-concentration in 0.2 M pH 4.1 NaAc-HAc solution containing Pb2+ and Cd2+ with different concentration. The inset shows the calibration curve. Conditions: deposition potential and time are -0.6 V and 200 s, respectively.

Tolerance limit (mg L

Interfering ions 3+

2+

2+

2+

2+

+

Al , Ca , Cu , Mg , Ni , K Mn2+, Co3+, Zn2+, Sn2+, Bi3+, Sb3+

20000 7500

1

)

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165

Table 2 Comparison of the NH2-MCM-41-Nafion/GC electrode and other electrodes for the determination of Pb2+ and Cd2+. Electrodes

Linear range (mg L

Detection limit (mg L

Pb (II)

Cd (II)

Pb (II)

Cd (II)

SbFME MPTMS BiFEs NHgFE BiBE NH2-MCM-41

20-100 1-21 6-80 1-16 10-100 0.5-250

20-100 0.6-11 2-60 1-13 10-100 50-450

3.1 1.3 2 0.05 0.1 0.2

1.9 0.4 2 0.01 0.05 1.0

1

)

1

)

Refs

[28] [29] [30] [31] [32] Here

SbFME: antimony film microelectrode. MPTMS: (3-mercaptopropyl) trimethoxysilane. BiFEs: Bismuth film electrodes. NHgFE: Nafion–mercury coated glassy carbon electrode. BiBE: bismuth bulk electrode.

3.3. Optimization of experimental parameters

3.4. Analytical performances

The effect of pH on the stripping voltammetric responses of the NH2-MCM-41-Nafion/GC electrode towards lead and cadmium is shown in Fig. 5. The stripping peak currents of lead and cadmium are weak at pH 3.6 and a maximum is achieved around pH 4.1. While further increasing the pH value, both the stripping peak currents for lead and cadmium notably decrease, which could result from the hydrolysis of Pb2+ and Cd2+. Thereby, a pH value of 4.1 was selected for all the subsequent experiments. Fig. 6A illustrates the influence of deposition potential on the stripping peak currents of lead and cadmium at NH2-MCM-41Nafion/GC electrode in the range from -1.6 to -0.6 V. The stripping currents for lead and cadmium increase with the deposition potential shifting from -0.6 to -1.2 V, and then decrease rapidly at more negative potentials. The decrease in peak currents at deposition potential more negative than -1.2 V may be due to the hydrogen evolution in the buffer solution (pH 4.1) [26]. Therefore, a deposition potential of -1.2 V was chosen in this work. Fig. 6B gives the relationship between the stripping response and the deposition time for the NH2-MCM-41-Nafion/GC electrode. The stripping peak current increases proportionally with the deposition time between 50 s and 200 s. For longer pre-concentration times, stripping currents were found to level off due to the saturation loading of active sites at the electrode surface. Consequently, the deposition time of 200 s was chosen for later measurements.

The simultaneous detection of Pb2+ and Cd2+ has been investigated and Fig. 7 presents LSASVs at NH2-MCM-41-Nafion/ GC electrode towards Pb2+ and Cd2+ at various concentrations. The stripping peak currents linearly increase as Pb2+ and Cd2+ concentrations increase. The linear concentration ranges are 0.5-250 mg L 1 for Pb2+ (inset up) and 50-450 mg L 1 (inset down) for Cd2+ with the detection limits (3s) of 0.2 mgL 1 (Pb2+) and 1.0 mgL 1 (Cd2+), respectively. These detection limits are well below the guideline values of Pb2+ and Cd2+ in drinking water recommended by world health organization [27]. It is therefore expected that the proposed sensor can be employed for real sample assay. The reproducibility of the NH2-MCM-41-Nafion/GC electrode was found to be excellent as evidenced by a relative standard deviation of 1.21% (Pb2+) and 3.19% (Cd2+) for eight consecutive measurements of 10 mg L 1 of Pb2+ and 200 mg L 1 of Cd2+. After each measurement, a regenerated NH2-MCM-41-Nafion/ GC electrode surface was realized by electrochemical cleaning process at +0.8 V for 300 s. As illustrated in Fig. 8 (curve a), no stripping current peak is observed before the proposed electrode immersed in the 0.2 M NaAc-HAc solution containing 10 mg L 1 of Pb2+ and 200 mg L 1 of Cd2+. After pre-concentration, the modified electrode show two well-defined stripping peaks (curve b). When the electrode was cleaned, the stripping peaks disappear again, indicating the accumulated metals have been completely removed from the surface. The renewed electrode surface allows to

Table 3 Determination of Pb2+ and Cd2+ in real samples using the proposed sensor and ICP-OES method (n = 3). Sample

Ions

Tap water

Pb2+

Cd2+

Lake water

Pb2+

Cd2+

Tea

Pb2+

Cd2+

ND: not detected.

Added (mg/L)

Found (mg/L)

Recovery

ICP-OES (mg/L)

0

1.75  0.05

–

1.9  0.06

2 2 0 130 100 0

3.68  0.08 5.81  0.07 ND 128  4.8 224  8.6 1.48  0.07

96.6% 101.5% – 98.5% 97.4% –

– – ND – – 1.43  0.03

2 2 0 130 100 0 50 50 0 130 100

3.43  0.15 5.57  0.12 ND 125  5.2 232  8.1 92  3.5 145.2  3.2 187  3.6 ND 133  5.3 228  8.7

97.5% 102.3% – 96.2% 100.9% – 106.4% 95% – 102.3% 99.1%

– – ND – – 101  2.8 – – ND – –

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eliminate the contamination of the surface and to minimize the memory effects. As a result, each modified electrode was confirmed to be used over several weeks for determining Pb2+ and Cd2+ without notable loss of sensitivity, illustrating good storage stability. 3.5. Interference studies Since other metal ions might affect the detection of Pb2+ and Cd in real sample analysis, the selectivity of NH2-MCM-41Nafion/GC electrode was evaluated by monitoring the peak current in the potential window from -1.0 V to -0.2 V vs SCE. The measurements were conducted using 10 mg L 1 of Pb2+ and 200 mg L 1 of Cd2+ in the presence of foreign ions. Table 1 summarized the corresponding tolerance limits for different ions in the detection of Pb2+ and Cd2+ under the optimized conditions. In addition, the interference between Pb2+ and Cd2+ each other was investigated in different proportions. The stripping peak currents did not interfere with each other due to the well-separated stripping peak between Pb2+ and Cd2+ (about 166 mV) at the NH2-MCM-41-Nafion/GC electrode. These results indicate high selectivity of the NH2-MCM-41-Nafion/GC electrode for simultaneous Pb2+ and Cd2+ detection, and the sensor can be used for analysis in complex system. The high selectivity of the present electrochemical sensor could result from the specific interaction of the modifier amino-functionality with Pb2+ and Cd2+. The further comparison of NH2-MCM-41-Nafion/GC electrode with other electrodes for the determination of Pb2+ and Cd2+ is listed in Table 2. As it can be seen, the proposed biosensor exhibited a wide linear response range and acceptable sensitivity. Moreover, the construction procedure of NH2-MCM-41-Nafion/GC electrode is simple, environment-friendly, and economical. 2+

3.6. Applications The proposed sensor was applied to detect Pb2+ and Cd2+ in tap water, lake water, and tea samples. A reference method based on ICP-OES was also performed and the results are listed in Table 3. The results obtained by our method were in good agreement with the routine ICP-OES measurements, indicating the proposed method is reliable. In addition, the acceptable recovery rates for metal detection were also obtained using standard addition method. The average recoveries ranged from 95% to 106.4% for Pb (II) and from 96.2% to 102.3 for Cd (II), respectively. Considering the environmental friend of mesoporous silica, functionalized mesoporous silica materials make great promise for developing the electrochemical methods. 4. Conclusions This paper reported a promising electrochemical sensor for the simultaneous determination of Pb2+ and Cd2+ based on NH2-functionalized MCM-41. The developed sensor showed wide linear range, low detection limit, and good renewability. The excellent analytical performances of the modified electrode were ascribed to the high surface area and special mesoporous morphology of MCM-41, as well as good chelating ability of amine group to metal ions. Moreover, the method was employed to simultaneously determine Pb2+ and Cd2+ in real samples with satisfactory results. Acknowledgments This work was supported by the National Natural Science Foundation of China (21005053), the Priority Academic Program Development of Jiangsu Higher Education Institutions and the

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