Spectrofluorometric Determination of Mercury and

Journal of Dispersion Science and Technology

ISSN: 0193-2691 (Print) 1532-2351 (Online) Journal homepage: http://www.tandfonline.com/loi/ldis20

Spectrofluorometric Determination of Mercury and Lead by Colloidal CdS Nanomaterial Manmohan L. Satnami, Sandeep K. Vaishanav, Rekha Nagwanshi & Kallol K. Ghosh To cite this article: Manmohan L. Satnami, Sandeep K. Vaishanav, Rekha Nagwanshi & Kallol K. Ghosh (2016) Spectrofluorometric Determination of Mercury and Lead by Colloidal CdS Nanomaterial, Journal of Dispersion Science and Technology, 37:2, 196-204, DOI: 10.1080/01932691.2015.1039020 To link to this article: http://dx.doi.org/10.1080/01932691.2015.1039020

Accepted author version posted online: 15 Jul 2015.

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Date: 12 January 2016, At: 22:20

Journal of Dispersion Science and Technology, 37:196–204, 2016 Copyright # Taylor & Francis Group, LLC ISSN: 0193-2691 print=1532-2351 online DOI: 10.1080/01932691.2015.1039020

Spectrofluorometric Determination of Mercury and Lead by Colloidal CdS Nanomaterial Manmohan L. Satnami,1 Sandeep K. Vaishanav,1 Rekha Nagwanshi,2 and Kallol K. Ghosh1 1 2

School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur, C.G., India Department of Chemistry, Govt. Madhav Science P. G. College, Ujjain, M.P., India

Downloaded by [PT Ravi Shankar Shukla University] at 22:20 12 January 2016

GRAPHICAL ABSTRACT

Heavy metal ions such as Hg and Pb are hazardous due to very high toxicity, mobility, and ability to accumulate through the food chain or atmosphere in the environment system. Therefore, ultrasensitive determination of mercury and lead is important to provide an evaluation index of ions in aqueous environment. This paper describes the investigation of surface modified quantum dots (QDs) as a sensing receptor for Hg2þ and Pb2þ ion detection by optical approach. Water-soluble L-cysteine-capped CdS QDs have been synthesized in aqueous medium. These functionalized nanoparticles were used as a fluorescence sensor for Hg2þ and Pb2þ ions, involved in the fluorescence quenching. The effect of foreign ions on the intensity of CdS QDs showed a low interference response toward other metal ions except Cu2þ and Fe2þ ions. The limit of detection (LOD) of this system is found to be 1.0 and 3.0 nM for Hg2þ and Pb2þ ions, respectively. Keywords Fluorescence quenching, fluorescence sensor, LOD, surface modified QDs

1. INTRODUCTION Heavy metal ions, such as mercury and lead, are harmful to human health and other living being due to their acute toxicity, mobility, and ability to accumulate through food chains or atmosphere in the environment.[1–3] Various Environmental and biological communities have intensively studied cell toxicity of mercury and lead over the years. High toxicity at very low concentration makes mercury Received 17 March 2015; accepted 6 April 2015. Address correspondence to Manmohan L. Satnami, School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur, C.G. 492010, India. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ldis.

contamination a global environmental problem.[4–6] Similarly, Pb2þ is also a severe threat to human health and the ecological system.[7] Lead poisoning is responsible for several diseases associated with environmental pollution.[8] To prevent leaking of hazardous chemical waste to the groundwater, the European parliament is regulating lead usage in electronics.[9] The US Environmental Protection Agency (EPA) set the safety limit of lead in drinking water as 15 mg=L. Because of acute toxicity, health concerns, and legal restrictions, it is important to have probes that can provide rapid on-site assessment of heavy metal contents. Therefore, ultrasensitive determination of mercury and lead is important to provide evaluation index of mercury and lead ions in aqueous environment. However, development of facile and efficient methods for rapid

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SPECTROFLUOROMETRIC DETERMINATION OF MERCURY AND LEAD

quantitative analysis of mercury and lead ions compared to analysis methods based on sophisticated and timeconsuming instruments[10,11] remains a challenge. In the past decade, the field of fluorescent semiconducting material-based biological and chemical nanosensors has witnessed an explosion because of unique optical properties of fluorescent semiconducting materials.[12–16] Quantum dots (QDs) have several advantages over conventional organic fluorescent dyes, such as higher photoluminescence (PL), excellent quantum yield (QY), size-dependent tunable optical properties, wide continuous absorption, narrow fluorescence band, and high photostability. Over the past two decades, great efforts have been focused on the development of QD-based detection of metal ions.[17–21] Some of the scientists have realized the specific detection of metal ions through modification of QDs with different surface capping ligands.[22–26] Some of the systems have been developed, such as the detection of Cu2þ ions through thioglycerol-capped CdS QDs,[22] and mercaptopropionic acid-coated core=shell CdTe=CdSe QDs,[23] Zn2þ ions through L-cysteine-capped CdS QDs,[9] Agþ ions through thioglycolic acid-coated CdSe QDs,[24] Cu2þ and Agþ ions through peptide-coated CdS QDs,[25] and Pb2þ ions through glutathione-capped ZnCdSe and CdTe QDs.[26] In the present investigation, an ultrasensitive detection strategy has been developed for the detection of mercury and lead ions utilizing the metal sulfide chemistry. It is reported that metal ions could interact with colloidal nanoparticles by coordinate bond such as metal-sulfide bond, which consequently results in drastic fluctuation and quenching of fluorescent intensity due to electron transfer and even aggregation of colloidal nanoparticles.[23,27–29] In light of such signal fluctuation, an effective approach has been used to detect heavy metal ions through the interactions between colloidal nanoparticles and heavy metal ions. 2. EXPERIMENTAL SECTION 2.1. Materials and Reagents Cadmium chloride pentahydrate (CdCl2 5H2O), Lcysteine hydrochoride (SHCH2NH2CH2COOH), and thiourea (NH2CSNH2) were purchased from Sigma Aldrich, Bangalore, India, and used without further purification. All the experiments were performed using Millipore water obtained from milliQ water purification system. 2.2. Synthesis of L-Cysteine–Capped CdS Quantum Dots Cadmium chloride (5.56 mg) and 3.13 mg of thiourea were dissolved in 30 ml of water to prepare Cd2þ=thiourea precursor solution. To this solution 20 ml of L-cysteine hydrochloride (10.68 mg) was added and pH of the reaction mixture was adjusted to 10.5 with 1.0 M NaOH (Scheme 1). The typical molar ratio of Cd2þ:thiourea=L-cysteine was

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SCH. 1. Cartoon representation of L-cysteine-capped CdS quantum dots.

1:2:3. The solution was nitrogen-bubbled for 30 minutes and then transferred to three-necked round bottom flask. The completely nitrogen-bubbled solution was then set up on the reflux for the nucleation and growth of CdS QD. The growth of CdS QDs were monitored by measuring the absorption of UV-Visible spectrum of the reaction mixture at certain interval of time. After being refluxed for 12 hours the yellow green QD was removed from the mantle and cooled down to room temperature. Then solution were precipitated with acetone and kept for 3–4 hours for complete precipitation. The precipitate were washed 3–4 times with acetone and dried to get crystalline powder. This crystalline powder was used for the further studies. 2.3. Characterization UV-Visible Spectrophotometer (Thermoscientific, Evolution 300, Medison USA) operated at a resolution of 2 nm. Fluorescence spectra were recorded using Varian Cary Eclipse Fluorescence Spectrophotometer (Agilent, G9800A, Singapur). The FTIR spectra of L-cysteine and L-cysteinecapped CdS QDs were measured with IR Affinity FTIR Spectrophotometer (Shimadju, IR Affinity 8400S, Kyoto Japan). TEM measurements were performed on a JEOL Transmission Electron Microscope (JEOL, JEM-2100F, Massachusetts, USA), operated at accelerating voltage 200 kV. SEM analysis has been carried out by ZEISS Scanning Electron Microscope (ZEISS, EVO-18, Germany). X-ray diffraction study has been performed on X’Pert3 Powder XRD Multifunctional (PANalytical, X’Pert3 Powder, Perth, Australia). 2.4. Determination of Hg2þ and Pb2þ For Pb2þ and Hg2þ determination, 1 mM synthesized QDs solution was added to phosphate buffer (pH ¼ 7.4) and was transferred to 2.0 mL eppendorf. An appropriate volume of standard Pb2þ and Hg2þ solution was then added. The fluorescence intensity of the solution was recorded as analytical signal. The equimolar (9.0 nM) solution of analyte (Pb2þ, Hg2þ) and interference ion (Analyte

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ion=interfering ion ¼ 1:1) were mixed and added into a eppendorf containing 1 mM of L-cysteine-capped CdS QDs solution. The fluorescence spectrum of the CdS QDs was recorded after thoroughly mixing the solution. Similar procedure was followed to record the spectrum for the study of the interference effects using other concentration ratio of interfering ions. 3. RESULTS AND DISCUSSION L-cysteine is a nontoxic and one of the very important amino acid in living beings and is used for the preparation of water-soluble nanofluorescent probe for chemical sensing. L- cysteine can bind with CdS by covalent bonds and form L-cysteine-capped CdS QDs.[30] The thiol group of L-cysteine can bind to the cadmium atom of the CdS QDs through covalent bond, and polar carboxylic acid group renders the nanocrystal water-soluble. Additionally, the carboxylic acid group will also be available for complexing to various metal ions. 3.1. Spectral Characteristics of L-Cysteine-Capped CdS QDs L-cysteine-capped CdS QDs are optically characterized by UV–Vis absorption spectroscopy and fluorescence measurement. The absorption and PL spectra of L-cysteine-capped CdS QDs are shown in Figure 1. The time-dependent absorption bands show the growth of the QDs (Figure 1A). The absorption band shifts toward longer wavelength with the reaction time and the final absorption band is located at 402 nm. Initially the absorption bands (t ¼ 1–7 hours) are broad; this broadness indicates the onset of conventional Ostwald ripening which causes a slow defocusing of size distribution.[31] Later on, the bands become sharper as reaction time elapsed. The sharp absorption features are indicative of monodispersed particles. The variation of the absorption bands indicates that the particles grow rapidly as the reaction time increased. The corresponding absorption

edges (obtained by the intersection of the sharply decreasing region of the spectrum with the baseline)[32] are located from 315 to 423 nm for reactions performed during 1 to 7 hours refluxing, respectively. The Fluorescence spectra of L-cysteine-capped CdS QDs corresponding to absorption spectra are shown in Figure 1B. It can be seen that the PL emission peak position varies with refluxing time from 475 to 650 nm in a similar manner as the absorption band position and CdS QDs diameter varies. The considerable variation between the emission peak position and the absorption edge indicates that the present CdS QDs exhibit trap-state emission rather than band-edge emission. Figure 2 shows the FTIR spectra of free L-cysteine and L-cysteine-capped CdS QDs. The IR absorption band around 1550–1600 cm1 (nCOO), 1400 cm1 (nCOO), and 3500–3000 cm1 (nOH, COOH) indicate the –COOgroup. The band at 2900–3420 cm1 (nN–H) indicates –NH2 group and 600–800 cm1 (nC–S) indicates the C–S group, while the band at 2550–2750 cm1 (nS–H) represents –S–H group. There are coexisting IR absorption bands of –COO, –NH2 observed on both L-cysteine and L-cysteine-capped CdS QDs. Therefore carboxylic acid and amino group are present on the surface of the CdS QDs, while the S–H group vibration (2550–2670 cm1 nS–H) is absent in L-cysteine-capped CdS QDs due to the formation of covalent bonds between thiols and the surface of CdS QDs. L-cysteine-capped CdS QDs were further characterized by powder x-ray diffraction (XRD) and transmission electron microscopy (TEM). Figure 3 shows the XRD pattern of L-cysteine-capped CdS QDs. The peaks are located at 2h ¼ 26.7 , 43.8 , and 51.3 , oriented along the (111), (220), and (311) directions and are in good agreement with the JCPDS file 10454 suggesting that the QDs are in cubic zinc blende form.[33] TEM and SEM images of the CdS QDs prepared by taking Cd2þ:L-cysteine 1:3 are shown in Figures 4 and 5. Insets of Figure 4 show the corresponding selected area

FIG. 1. A) Time-dependent absorption spectra of L-cysteine-capped CdS QDs. B) PL emission peaks of CdS QDs with the refluxing time.


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that the particles of size range 3–4 nm are present in greater extent (Figure 4A). The average sizes obtained from UV Vis spectra and TEM resemble each other reasonably well. A HR-TEM image of QDs prepared after 7 hours of reaction is indicative of well-resolved lattice planes (Figure 4B). From the lattice fringes of the micrograph, the interplanar spacing (d) was estimated to be 0.36 nm, which is correlated to the d value of (111) plane, that is, 0.36 nm of the cubic phase. The cubic zinc blend structure of the produced CdS nanoparticles was further evidenced by the SAED pattern. Scanning electron microscopic image of CdS QDs shown in Figure 5 reveals that CdS QDs are spherical in shape except few particles, which are clumped together.

FIG. 2.

FTIR spectra of pure L-cysteine and L-cysteine-capped CdS

QDs.

FIG. 3. XRD pattern of L-cysteine-capped CdS QDs. electron diffraction (SAED) pattern and frequency percentage of the total nanoparticles for L-cysteinecapped. The images show that L-cysteine-capped CdS QDs are spherical, well-defined, and uniform in size and shape. The size distribution histogram of CdS QDs reveals

3.2. Effect of pH on Fluorescence Intensity Generally, pH is one of the important parameters that affects the fluorescence of QDs.[34] Therefore it is of keen important to investigate the effect of pH on the fluorescence response of QD based nanosensor. We investigated the stability of L-cysteine-capped CdS QDs under different pH conditions. The Derjagui–Landau and Verwey–Overbeek theory (D.L.V.O theory) explained the stability of colloidal nanoparticles in water. The theory states that the stability of colloidal nanoparticles depends on the interaction balance between van der Waals attractive forces and electrostatic repulsive forces among these colloidal nanoparticles.[35,36] In addition to this, the surface potential of colloidal nanoparticles could also influence the stability of colloidal nanoparticles.[37,38] To confirm the correlation, QDs sample introduced into solution with different pH values to tune the surface potential of colloidal nanoparticles.[39,40] We observed that the fluorescent intensity of QDs is directly proportional to the pH (Figure 6). These results show that pH has significant influence on physical and chemical properties of colloidal nanoparticles. When pH values altered from 11 to 7, the dissociation reaction of thiol functional groups (–SH) played an important role in tuning the surface potential of colloidal nanoparticles. When pH value of the solution was decreased to 5.0, the deprotonized thiol (–S) and carboxylate (–COO) were

FIG. 4. HRTEM image of L-cysteine-capped CdS QDs, A) showing monodisperse particles and B) closer look of QDs showing lattice fringes. Inset A) histogram of QDs B) SAED and interplaner distance.

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FIG. 5.

SEM image of L-cysteine-capped QDs at different magnifications A) at 20.02 KX and B) at 14.02 KX magnification.

protonized and as a result, the ligands were separated from the surface of colloidal nanoparticles due to the dissociation reaction in the solution.[41] As a result, surface traps were exposed, which reduces the possibility of irradiative recombination.

FIG. 6.

Effect of pH on fluorescence spectra of CdS QDs.

3.3. Interaction of L-Cysteine-Capped CdS QDs with Hg2þ and Pb2þ Ion Herein, the selective fluorescence quenching experiments were performed with L-cysteine-capped CdS QDs (emission max 557 nm) which emitted maximum fluorescence intensity by excitation at 345 nm. Upon the addition of Hg2þ and Pb2þ ions into water-soluble colloidal nanoparticles, apparent changes in the fluoroscence spectra can be observed from Figure 7, which shows the fluorescence

FIG. 7. Fluorescence quenching of L-cysteine-capped CdS QDs A) by Hg2þ, B) by Pb2þ, C) Stern–Volmer plot for FL quenching of QDs by Pb2þ, and D) Stern–Volmer plot for FL quenching of QDs by Hg2þ. Inset D) Stern–Volmer plot for FL quenching of QDs by Hg2þ within the linear range.

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intensity of L-cysteine-capped colloidal nanoparticles in the presence of Hg2þ and Pb2þ ions (0–20 nM) in the phosphate buffered saline (PBS, 10 mM, 7.4) solution. The emission intensity of colloidal nanoparticles was quenched obviously with the addition of Hg2þ and Pb2þ. In general, the thiolated ligands act as useful protective and stabilizing agent for colloidal nanoparticle, it tailors the size shape and PL characteristics of colloidal nanoparticles. These ligands covalently bound to the surface of CdS QDs and prevent the nonradiative recombination; however, in the presence of Hg2þ and Pb2þ, the ligand displacement takes place as Hg and Pb have comparable affinity toward the thiol group as compared to cadmium that leads to the uncovered surface states. These results in the formation of trap states resulting in the nonradioactive recombination. The aggregation of colloidal nanoparticles is, therefore, attributed to the separation of the organic passivation layer from the surface of colloidal nanoparticles in the presence of Hg2þ and Pb2þ(Figure 7). The emission signal response to Hg2þ and Pb2þ ions could thus be utilized to detect mercury ions in aqueous solution. For the as-fabricated chemosensor, the limit of detection (LOD) is 1.0 nM for Hg2þ and 3.0 nM for Pb2þ which is lower than those of chemosensors based on QDs reported in previous studies by other research groups.[27,41,42] To provide a detailed comparison with previous studies, we compiled Table 1 for the specific comparison of this work and selected fluorimetric methods in the linear range and detection limit. It was found that Hg2þ and Pb2þ quench the fluorescence of QDs in a concentration-dependent manner that was best described by the Stern–Volmer relationship, which is given by the following equation: F0 =F ¼ 1 þ KSV ½Q

½1

F0 and F are the fluorescence intensities of QDs in the presence and absence of Hg2þ and Pb2þ, respectively. [Q] is the Hg2þ and Pb2þ concentration and KSV is the Stern–Volmer constant. Under the optimum condition, the linearity of the Stern–Volmer (SV) plot spans the range between 1.0 nM and 9.0 nM for Hg2þ with a correlation coefficient of 0.9237. The KSV was found to be 3.68 107 M1 while in case of Pb2þ linearity of the SV is in the range of 3.0–15 nM with correlation coefficient of 0.9743 and the Ksv was 9.86 107 M1 (Table 2). 3.4. Effect of Different Metal Ions on Luminescence Intensity In order to adopt the functionalized CdS QDs for the sensing probe for the Hg2þ and Pb2þ ions, it is important to evaluate the interference effect of some common foreign ions which is necessary for practical applications. This interference study was focused on cations that are naturally abundant in the environment. The study of three different ratio of 1:1, 1:100, 1:1000 [(Hg2þ=Pb2þion=interference ion)] was carried out and the results are presented in Table 3. It can be noted that of all three ratios investigated did not show any significant interference by metal ions except by Fe2þ ions (Figure 8). The relative standard deviation (RSD) for five replicate determinations of Hg2þ and Pb2þ ion at a concentration of 9 nM was estimated as 3.2% and 2.6%, respectively. The presence of foreign ion in 1:1 and 1:100 ratios produced lower interference than this RSD value. Thus, ions causing errors more than 3.2% and 2.6% are considered interferents. From Table 3, it can be noted that only Fe2þ and Cu2þ at 1:1000 ratio cause some interference. This result suggests that compared with Hg2þ and Pb2þ ions these metal ions only slightly quenched the photoluminescence intensity of colloidal nanoparticles due to the weak affinity between these metal ions and the

TABLE 1 Comparison of the main characteristics of the selected fluorimetric methods for the determination of Hg2þ and Pb2þ Probes a. Hg2þ DOB-CdTe RhB-CdTe=SiO Chitosan-CdTe=CdS C-dots=GDTC-CdSe=ZnS MPA-CdTe=CdS This Work b. Pb2þ GSH-ZnCdSe TGA-CdTe Xylene-CdSe=ZnS This work

Linear range (mM)

LOD (nM)

References

0.008–3 2–10 0–1.80 0.2–1.0 0.005–0.30 0–0.009

4.20 260 5.60 100 3.10 1.00

[46] [47] [48] [49] [50] –

0–0.20 2–100 0.05–6.0 0–0.015

20.0 270 20.0 3.0

[44] [51] [52] –

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TABLE 2 Analytical characteristics of the method of determination of Hg2þ and Pb2þ Pb2þ

Hg2þ

0–15 9.62 107 0.7180 0.9743 5 3.6 (9 nM)

0–9 3.68 107 0.9689 0.9237 5 2.9 (9 nM)

Parameters


Linear Range (nM) Slope Intercept Correlation coefficient (r) Number of points RSD% (n ¼ 5)

TABLE 3 Analytical characteristics of the method of determination of Hg2þ and Pb2þ Interfering % in Hg Determination

Interfering % in Pb Determination

Interfering ions ratio (Hg: Interfering ion)

Interfering ions ratio (Pb: Interfering ion)

Interfering ions

1:1

1:100

1:1000

1:1

1:100

1:1000

Liþ Naþ Kþ Mg2þ Ca2þ Sr2þ Ba2þ Al3þ Fe2þ Co2þ Ni2þ Cu2þ Agþ

0.73 0.19 0.46 0.31 0.14 0.18 0.38 0.44 1.56 0.53 0.56 0.92 0.82

0.93 0.59 0.96 0.73 0.47 0.53 0.76 0.89 1.96 0.84 0.79 1.70 1.30

1.34 1.20 1.60 1.23 0.98 1.18 1.36 1.56 5.53 1.33 1.59 3.90 1.90

0.69 0.29 0.41 0.36 0.26 0.36 0.32 0.42 1.32 0.42 0.67 0.98 0.88

0.95 0.65 0.92 0.79 0.59 0.75 0.71 0.98 1.84 0.83 0.88 1.81 1.47

1.54 1.32 1.51 1.39 1.09 1.46 1.43 1.69 4.89 1.29 1.86 3.40 2.07

FIG. 8. Comperative fluorescence response of interfering metal ions. A) compared to Pb2þ, and B) compared to Hg2þ.



thiol. The relative bond strength of metal sulfides is determined by their respective Ksp value.[43,44] The slight quenching of emission by adding other metal ions shows that the fluorescence of QDs also partially or completely quenched due to adsorption of metal ions to the surface of QDs when high concentration of other metal ions were added into thiol-coated QDs.[45] Furthermore, these infrared characteristic peaks of ligands did not disappear even though other heavy metal ions were excessively added into the probe solution. 4. CONCLUSION Water-soluble functionalized L-cysteine-capped CdS QDs was synthesized in one step process to develop a fluorescence sensor for Hg2þ and Pb2þ ion. This sensor is based on the fluorescence quenching by mercury and lead ions, which interacts with functionalized CdS QDs. Under the optimum conditions, the calibration plot was linear in the range of 1–9 nM in case of Hg2þ and 115 nM in case of Pb2þ with correlation coefficient of 0.9237 and 0.9743, respectively. The detection limit of this sensor is 1 and 3 nM, respectively, for Hg2þ and Pb2þ, respectively. There is a little or no interference from many metal ions that normally coexist with Hg2þ and Pb2þ ion. Therefore, this method can be used to detect the mercury ion at nanomolar levels. In addition to its good sensitivity, other advantages of this method include its simplicity, rapidity, high resistivity to chemicals, and metabolic degradation. ACKNOWLEDGMENTS We are thankful to Sophisticated Analytical Instrument Research Facility (AIRF) Jawahar Lal Nehru University (JNU), New Delhi, for TEM analysis. We are also grateful to the head, School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur, for FTIR analysis. Authors are thankful to the head, School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur, for providing laboratory facilities. FUNDING Financial assistance from DST (New Delhi) Fast track project (SR=FT=CS-39=2011) is acknowledged with appreciation. The Authors (SKV) are grateful to Pt. Ravishankar Shukla University, Raipur, for University research scholarship. REFERENCES [1] Campbell, L.M., Dixon, D.G., and Hecky, R.E. (2003) J. Toxicol. Env. Heal. B, 6: 325–356. [2] Robinson, S.A., Lajeunesse, M.J., and Forbes, M.R. (2012) Environ. Sci. Technol., 46: 7094–7101.

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