- Email: [email protected]
Sensitive metal ions (II) determination with resonance Raman method
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 52–56
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Sensitive metal ions (II) determination with resonance Raman method Zhi Yu a, Lucas A. Bracero a,b, Lei Chen a, Wei Song a, Xu Wang a, Bing Zhao a,⇑ a b
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China Department of Chemistry, West Virginia University, Morgantown, WV 26506, United States
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Highly chelation between zincon
Zincon can chelate with divalent metal ions (zincon-Cu2+ and zincon-Ni2+) shows different resonance Raman spectra which are powerful evidence for the metal ions determination.
and metal ions (II) possess high RR activity. " Fingerprint information from RR spectra made the detection more convenient. " This method realized the detection of blend metal ions successfully.
a r t i c l e
i n f o
Article history: Received 7 September 2012 Received in revised form 15 November 2012 Accepted 30 November 2012 Available online 20 December 2012 Keywords: Zincon Resonance Raman Cu2+ and Ni2+ Quantitative evaluation
a b s t r a c t In this paper, a new proposal for the quantitative evaluation of divalent metal ions (M2+) is developed by the use of the competitive resonance Raman (RR)-based method. Upon excitation with light of the appropriate wavelength (532 nm), a strong electric field is generated that couples with the resonance of the complex (zincon-M2+), increasing the character signals of these complexes, resulting in sensitive detection. Herein, the RR probe, zincon-M2+ complex that the RR intensity gets lower with the decreasing of the M2+ concentration, which leads to the transformation of the Raman information. As a result, by using the proposed RR-based method, we could find the liner calibration curves of Cu2+ and Ni2+, which show the potential in quantitative evaluation of an unknown sample. In addition, the abundant fingerprint information shows that RR leads to the successful analysis of a blended solution, which contains two ions: Cu2+ and Ni2+. Ó 2012 Elsevier B.V. All rights reserved.
Introduction Heavy metal ions analyses are an important part of studies in analytical chemistry. The determinations of these ions in the various materials from the industrial samples to environmental samples have been performed continuously [1–8]. Some metal ions appear together in many real samples. The determination of traces of metal ions has long been an important subject in environmental analysis, many types of industrial processes, and chemical reac⇑ Corresponding author. Tel.: +86 431 85168473. E-mail address: [email protected] (B. Zhao). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.11.112
tions [1–4]. Meanwhile, uncontrolled disposal of wastes is contaminating food and water that come from natural sources to alarming levels with the industrialization and urbanization in developing countries. Higher concentration of metal ions will cause environmental and foods pollution. Metal ions and their complex that are a potentially toxic and carcinogenic metal, have been determined by a variety of complexing agents. For example, when the concentration of the copper and nickel in the soil is higher, it will be diluted with the flow of water to the river and lake, this cause ecological harm. In addition, in the wastewater, there are often different kinds of metal ions mixed together, so finding a proper way to detect the content of the mental ions among many others is very
Z. Yu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 52–56
necessary [5–8]. So it is necessary to develop a simple, fast, and accurate method for monitoring the levels of contaminants. Traditional optical techniques for metal ion assay in solution include mass spectroscopy [9], X-ray fluorescence [10], fluorescence spectrometry [11], atomic absorption spectroscopy [12], etc. have all been used for the simultaneous determination of these ions in different samples. Among the most widely used analytical methods, colorimetric [13,14] and UV–vis spectrophotometry [4,15– 19] techniques are widely used, due to the resulting experimental rapidity, simplicity. However, the simultaneous determination of these ions by the use of the traditional spectrophotometry techniques are difficult, because, generally, the absorption spectra overlap in a bright region and the superimposed curves are not suitable for quantitative evaluation [19,20]. Raman spectroscopy is a promising and ultrasensitive analytical tool due to its nondestructive analysis and high sensitivity to the molecular structure of samples [21–23]. And it has realized many analyses among different territories, such as the monitoring of the environment pollutants, constituent detection of liquid coal, and identification of synthetic diamond [24,25]. Resonance Raman (RR) spectroscopy, in which the Raman intensity of some molecular vibrations are strongly enhanced when the excitation wavelength of the Raman scattering approaches an absorbance maximum of an electronic transition, is an important analytical technique with high sensitivity and selectivity [26,27]. RR spectroscopy, which can provide a larger amount of fingerprint information for an individual component of a mixture, has recently demonstrated promise for biomolecular studies and biomedical diagnostics [24,25,28]. Nowadays RR spectroscopy technique was employed as a quantitative method for its high sensitivity and selectivity. Recently, our previous work proposed the idea of zincon reagent-based protein assay using the RR method. By examining the remaining probes that are not bound to the protein with RR method to achieve protein assay indirectly [20]. For many years, zincon was used as a chromogenic reagent, which has been extensively used for the colorimetric and spectrophotometric determination of many metals, because of the formation of water-soluble metal ion complexes (Scheme 1) [4,17,29]. Herein, we employed zincon-M2+ complex as a RR probe for evaluating the concentration of metal ions in the solution. This assay happens when just mixing the zincon with the divalent metal ions, the color of the solution will change which can be identified by the UV–vis spectra. Meanwhile, zincon compounds with different kinds of metal ions exhibit different RR bands. RR was then used as a detection method to realize the evaluation of the mixture system based on the different resonance band. It exhibits high sensitivity and good selectivity during the detection among the blend reagent of different mental ions.
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Experimental section Materials Copper (II) chloride (CuCl2), nickel chloride (NiCl2), were purchased from Sigma Aldrich Co. Ltd., and zincon monosodium salt (2-carboxy-20-hydroxy-50-sulfoformazylbenzene) was purchased from Fluka Co., Ltd. at highest purity available and used as received without further purification. Ultrapure water (18.0 MX cm 1) was used throughout the present study. Pretreatment of metal ions samples To determine the quantitative M2+ (Cu2+ , Ni2+) concentrations, the Cu2+ and Ni2+ with the concentration of 1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6, 2.0 10 6, 1.0 10 6, 6.7 10 7, 3.3 10 7, 2.0 10 7, 1.0 10 7, 6.7 10 8, and 0 (blank sample) mol/L were placed in each cuvette with the concentration of zincon (8 10 5 mol/L) kept constant and keep constant of pH at 4.5. To determine the mixture of metal ions (Cu2+ and Ni2+ mixture), the concentrations of Cu2+ and Ni2+ were set as sample a to sample d (a: 1.7 10 5 and 3.3 10 5, b: 1.3 10 5 and 6.7 10 5, c: 1.0 10 5 and 1.0 10 4, d: 6.7 10 6 and 1.3 10 4, e: 3.3 10 6 and 1.7 10 4 mol/L. These samples were each placed in individual cuvettes with zincon, which had a concentration of 8 10 5 mol/L. Instruments RR spectra of zincon-M2+ systems were recorded using a JobinYvon/HORIBA LabRam ARAMIS Raman spectrometer equipped with a liquid cell hold. The radiation from an air-cooled frequency doubled Nd:Yag laser (532 nm) was used as an excitation source. UV–vis spectra were recorded on a UV-3600 spectrophotometer (Shimadzu). Results and discussion The determination of metal ions concentration is of great importance in food and environment analysis. Previous work of determining metal ion concentrations with zincon has been based on colorimetric and absorption spectra assays. Fig. 1 shows optical image of the solutions containing two metal ions (Cu2+ and Ni2+). Herein, zincon can be used as a colorimetric sensor with the ‘‘naked eye’’ for determining different metal ions. As shown in Fig. 1A and B, the color of the zincon-M2+ solution containing different metal ions were completely different, which indicates the variation of the conformation of zincon. Meanwhile, the addition of Cu2+ or Ni2+ produce a change of color (orange to blue for Cu2+
Scheme 1. Structures of zincon in its free and the zincon-M2+ forms.
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Z. Yu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 52–56
Fig. 1. Photos of zincon (8 10 5 M) solutions containing two kinds of metal ions (Cu2+, A and Ni2+, B) with the different concentrations (from left to right are: 1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6 mol/L and blank sample).
and orange to brown for Ni2+) corresponding to the formation of the respective zincon-metal complex. In addition, the color of zincon solutions containing Cu2+ or Ni2+ was directly related to the M2+ concentration. However, a noticeable color change was not observed at lower metal ions concentrations (below 2.0 10 5 mol/L for Cu2+ and 3.3 10 5 mol/L for Ni2+). UV–vis spectroscopy was also widely used for quantitative evaluation of the target. Fig. 2 shows the UV–vis absorbance spectra for the zincon-M2+ (zincon-Cu2+ and zincon-Ni2+) system with increasing concentrations of M2+. The absorption maximum at 465 nm of zincon decreased gradually and shifted to longer wavelengths as the concentration of Cu2+ and Ni2+ increased, and it eventually formed a new band near 600 nm and 666 nm, which can be assigned to the free chelator of zincon and M2+. When the concentration of Cu2+ was higher than 6.7 10 5 mol/L, the peak at 465 nm disappeared completely. This is confirmation that the complex of zincon and Cu2+ was formed completely. Further, when the concentrations of Cu2+ and Ni2+ were lower than 3.3 10 6 mol/L, no significant variation of the peak at 465 nm occurred compared to the absorbance of zincon. Interestingly, it was difficult to detect a significant change in the absorption spectra at much lower M2+ concentrations (below 3.3 10 6 mol/L for Cu2+ and Ni2+).
Fig. 2. Concentration-dependent UV–vis spectra of zincon-M2+ complex with different concentration of (A) Cu2+ and (B) Ni2+ (1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6, 2.0 10 6, 1.0 10 6, 6.7 10 7, 3.3 10 7, 2.0 10 7, 1.0 10 7, 6.7 10 8 mol/L and blank sample).
RR of zincon-copper (II) system To successfully detect M2+ at lower concentrations, RR spectroscopy was performed. RR can provide a larger amount of fingerprint information for an individual component of a mixture, which is helpful for us to quantitatively analyze a target. Fig. 3 shows the RR spectra of zincon-Cu2+ complex with different concentrations of Cu2+. As shown in Fig. 3, the intensities of the Raman bands at 1387 cm 1 assigned to zincon decreased with increasing Cu2+ concentrations. When the concentration of Cu2+ is higher than 2.0 10 5 mol/L, the RR spectra of zincon-Cu2+ complex are very different from zincon. Further, some new RR bands appeared at 1499, 1404, 1355, 1060, 1007, 876, 862 and 630 cm 1, which are assigned to RR effect of zincon-Cu2+ complex. Meanwhile, RR bands at 1387, 1301 and 1174 cm 1 are decreased with the increasing the concentration of Cu2+ (from 6.7 10 8 to 1.0 10 5 mol/L). To quantitatively evaluate Cu2+ with our proposed RR-based method, 1387 cm 1 was selected. Concentration-dependent RR intensities
Fig. 3. RR spectra of zincon-Cu2+ complex containing different concentrations of Cu2+ (from up to bottom are 1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6, 2.0 10 6, 1.0 10 6, 6.7 10 7, 3.3 10 7, 2.0 10 7, 1.0 10 7, 6.7 10 8 mol/L and blank sample).
Z. Yu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 52–56
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Fig. 5. RR spectra of zincon-Ni2+ complex containing different concentrations of Ni2+ (from up to bottom are 1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6, 2.0 10 6, 1.0 10 6, 6.7 10 7, 3.3 10 7, 2.0 10 7, 1.0 10 7, 6.7 10 8 mol/L and blank sample).
selected and plotted in Fig. 4B. As shown in Fig. 4B, the Raman intensity varied monotonically with changes in the concentration of Ni2+. The relationship between the intensity and the concentration were found to be linear (Y = 6551.9–113.3X, R2 = 0.980) at 1387 cm 1 in the range from 3.3 10 5 to 2 10 6 mol/L of Ni2+. The error bars indicate the batch-to-batch variability of the RR intensity with five independent measurements. Thus, the RR effect of zincon could be a promising method with high sensitivity and selectivity for analyses of M2+. RR of zincon-copper (II) and zincon-nickel (II) mixture system
Fig. 4. Concentration-dependent of RR intensity of the band at 1387 cm 1 for zincon-M2+ containing different concentrations of (A) Cu2+ and (B) Ni2+ (from right to left are 1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6, 2.0 10 6, 1.0 10 6, 6.7 10 7, 3.3 10 7, 2.0 10 7, 1.0 10 7, 6.7 10 8 mol/L and blank sample).
of zincon at 1387 cm 1 are plotted in Fig. 4A. The error bars indicate the batch-to-batch variability of the RR intensity with five independent measurements. As shown in Fig. 4A, the Raman intensity varied monotonically with changes in the concentration of Cu2+. The relationship between the intensity and the concentration was found to be linear (Y = 6470.8–184.7X, R2 = 0.982) at 1387 cm 1 in the range from 2 10 5 to 2 10 6 mol/L of Cu2+. RR of zincon-nickel (II) system In order to quantitatively evaluate Ni2+, the RR-based method was also performed. Fig. 5 shows the concentration-dependent RR spectra of zincon-Ni2+ complex with different concentrations of Ni2+. Similar to the zincon-Cu2+ spectra, as shown in Fig. 5, the intensities of the Raman bands of zincon decreased with increasing Ni2+ concentrations in the zincon-Ni2+ complex. When the concentration of Ni2+ is higher than 3.3 10 5 mol/L, the RR spectra of zincon-Ni2+ complex are very different from zincon. Further, new RR bands appeared at 1482, 1394, 1193, 1155, 875, 829, 728 and 640 cm 1, which are assigned to the RR effect of the zincon-Ni2+ complex. Meanwhile, the RR bands at 1387, 1301 and 1174 cm 1 decrease with an increase in the Ni2+ concentration (from 6.7 10 8 to 1.0 10 5 mol/L). The band at 1387 cm 1 was also
Metal ions are always appearing together in many real samples, thus it is necessary to find an effective and rapid method for the determination of metal ions. As a widely used method, UV–vis spectroscopy was performed for determination of the proposed Cu2+ and Ni2+ mixture system. Fig. 6 shows the UV–vis spectra of zincon mixed with the two metal ions (Cu2+ and Ni2+). As shown in Fig. 6, this copper–nickel mixture is an extremely difficult complex system due to the high spectral overlapping observed between the absorption spectra for these components. In view of this, the RR method was also performed on our mixture system. All of the RR-based results indicate that zincon as RR probe for the detection of M2+ is a promising quantitative and determination method. In addition, zincon is a widely accepted reagent for the determination of divalent metal ions, such as Cu2+ and Ni2+ et al. with a color change and possesses a high stability constant towards Cu2+ (log K = 7.5 ± 0.1) and Ni2+ (log K = 10 ± 0.3) [29,30]. Figs. 3 and 5 provide a large amount of fingerprint information for an individual component of zincon-Cu2+ and zinconNi2+ complex via RR effect. For the different resonance effect of zincon-Cu2+ and zincon-Ni2+, their RR spectra are significantly different. This finding is useful and helpful for the determination of metal ions in the mixture system. Therefore, RR-based method can also be performed for the determination of metal ions in this mixture system. Fig. 7 shows RR spectra of the mixture (zinconCu2+ and zincon-Ni2+ complex) with different concentration of Cu2+ and Ni2+. As shown in Fig. 7, by decreasing and increasing the concentrations of Cu2+ and Ni2+ concomitantly, their RR bands assigned to zincon-Cu2+ complex transformed to the RR bands assigned to zincon-Cu2+ and zincon-Ni2+ mixture system, and finally transformed to the RR bands assigned to zincon-Ni2+ complex. Therefore, this RR-based method is good candidate for metal ions
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Z. Yu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 52–56
centration as 1.63 10 5 mol/L, which is close to Cu2+ concentration that inputted in sample a (1.7 10 5 mol/L). Conclusions We have successfully utilized this new method for the detection of the zincon-M2+ system by RR, and thus realized a sensitive and fast quantitative evaluation of divalent metal ions. The large amount of fingerprint information that was provided by the RR effect can be utilized to determine the mixture. Meanwhile, unknown concentrations of metal ions can be quantified with the calibration curves. Furthermore, this method has been successfully used in a mixture system containing Cu2+ and Ni2+, which leads to the detection of metal ions in a mixture system for real samples. The proposed RR-based method exhibited great potential in highly sensitive concentration determination of metal ions and a mixture of metal ions, which exhibited great potential in a complex system. Fig. 6. UV–vis spectra of mixture system (Cu2+ and Ni2+) with different concentrations (a, 1.7 10 5 and 3.3 10 5, b, 1.3 10 5 and 6.7 10 5, c, 1.0 10 5 and 1.0 10 4, d, 6.7 10 6 and 1.3 10 4, e, 3.3 10 6 and 1.7 10 4 mol/L, respectively).
Acknowledgments This study was supported by the National Natural Science Foundation (Grant Nos. 20921003, 20903044, 20973074, 21073072) of PR China. Specialized Research Fund for the Doctoral Program of Higher Education (20110061110017), the 111 project (B06009) and the Development Program of the Science and Technology of Jilin Province (20110338). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.11.112. References
Fig. 7. RR spectra of zincon-Cu2+ and zincon-Ni2+ mixture system with different concentrations of Cu2+ and Ni2+ (from up to bottom are 2 10 4 and 0, sample a: 1.7 10 5 and 3.3 10 5, sample b: 1.3 10 5 and 6.7 10 5, sample c: 1.0 10 5 and 1.0 10 4, sample d: 6.7 10 6 and 1.3 10 4, sample e: 3.3 10 6 and 1.7 10 4, 0 and 2 10 4 mol/L, respectively).
(Cu2+ and Ni2+) which have significantly change of RR spectra after chelating with zincon compared to the RR spectrum of zincon. This RR-based detection protocol for the determination of Cu2+ and Ni2+ is a promising method. Using the characteristic peaks, it is easier to identify the composition of the mixture system due to the difference of the chelator (zincon-Cu2+ and zincon-Ni2+). The RR-based method was established with much more fingerprint information to identify metal ions in the mixture system. The proposed method with the calibration of characteristic peak is a powerful technique for determinating the concentration of the mixture sample. As shown in Fig. 3, the Raman intensity varied monotonically with change in concentration of Cu2+. A characteristic peak at 1355 cm 1 of zincon-Cu2+ was chosen to plot the calibration curves (Fig. S1). The relationship between the intensity and the concentration was found to be linear (Y = 1941.1 + 56.1X, R2 = 0.984) at 1355 cm 1 in the range from 3.3 10 5 to 6.7 10 6 mol/L. Based on this calibration curve and the intensity of the band at 1355 cm 1 in RR spectra of the mixture sample a (Fig. 7), we can quantitative evaluate Cu2+ con-
[1] T. Khayamian, A.A. Ensafi, B. Hemmateenejad, Talanta 49 (1999) 587–596. [2] G. Aragay, J. Pons, A. Merkoçi, Chem. Rev. 111 (2011) 3433–3458. [3] G. Aragay, G. Alarcón, J. Pons, Mercè Font-Bardía, A. Merkoçi, J. Phys. Chem. C 116 (2012) 1987–1994. [4] J. Ghasemi, S. Ahmadi, K. Torkestani, Anal. Chim. Acta 487 (2003) 181–188. [5] P.C. Onianwa, I.G. Adetola, C.M.A. Iwegbue, M.F. Ojo, O.O. Tella, Food Chem. 66 (1999) 275–279. [6] A.K. Salama, M.A. Radwan, Emir. J. Agric. Sci. 17 (2005) 34–42. [7] E. Somer, J. Food Sci. 39 (1974) 215–217. [8] A. Maceda-Veiga, M. Monroy, A. Sostoa, Ecotoxicol. Environ. Saf. 76 (2012) 93– 101. [9] R. Flamin, A. Panighel, Mass Spectrom. Rev. 25 (2006) 741–774. [10] O.W. Lau, S.Y. Ho, Anal. Chim. Acta 280 (1993) 269–277. [11] C. Cano-Raya, M.D. Fernández-Ramos, L.F. Capitán-Vallvey, Anal. Chim. Acta 555 (2006) 299–307. [12] M. Soylak, U. Divrikli, L. Elci, M. Dogan, Talanta 56 (2002) 565–570. [13] S.K. Sun, K.X. Tu, X.P. Yan, Analyst 137 (2012) 2124–2128. [14] Y. Zhou, S. Wang, K. Zhang, X. Jiang, Angew. Chem. Int. Ed. 47 (2008) 7454– 7456. [15] H. Eskandari, A.G. Saghseloo, M.A. Chamjangali, Turk J. Chem. 30 (2006) 49–63. [16] F.M. Fernández, M.B. Tudino, O.E. Troccoli, Anal. Chim. Acta 433 (2001) 119– 134. [17] C.E. Säbel, J.M. Neureuther, S. Siemann, Anal. Biochem. 397 (2010) 218–226. [18] J.H. Kalivas, Anal. Chim. Acta 428 (2001) 31–40. [19] J. Ghasemi, A. Nizazi, Microchem. J. 71 (2002) 1–8. [20] L. Chen, Y. Park, H. Seo, W. Hong, Y.M. Jung, B. Zhao, J. Raman Spectrosc. 42 (2011) 1963–1966. [21] S. Šašic´, Pharmaceutical Applications of Raman Spectroscopy, John Wiley & Sons, Ltd., New Jersey, 2008. [22] V.A. Shashilov, I.K. Lednev, Chem. Rev. 110 (2010) 5692–5713. [23] S.P. Xu, X.H. Ji, W.Q. Xu, X.L. Li, L.Y. Wang, Y.B. Bai, B. Zhao, Y. Ozaki, Analyst 129 (2004) 63–68. [24] E. Smith, G. Dent, Modern Raman Spectroscopy – A Practical Approach, John Wiley & Sons, Ltd., Chichester, 2005. [25] F. Siebert, P. Hildebrandt, Vibrational Spectroscopy in Life Science, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008. [26] A.B. Myers, Chem. Rev. 96 (1996) 911–926. [27] A.B. Myer, Acc. Chem. Res. 30 (1997) 519–527. [28] K.A. Kane, L. Jensen, J. Phys. Chem. C 114 (2010) 5540–5546. [29] X. Lou, L. Zhang, J. Qin, Z. Li, Chem. Commun. 44 (2008) 5848–5850. [30] E.Y. Hashem, M.S. Abu-bakr, S.M. Hussain, Ann. Chim. 94 (2004) 571–579.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Sensitive metal ions (II) determination with resonance Raman method Zhi Yu a, Lucas A. Bracero a,b, Lei Chen a, Wei Song a, Xu Wang a, Bing Zhao a,⇑ a b
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China Department of Chemistry, West Virginia University, Morgantown, WV 26506, United States
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Highly chelation between zincon
Zincon can chelate with divalent metal ions (zincon-Cu2+ and zincon-Ni2+) shows different resonance Raman spectra which are powerful evidence for the metal ions determination.
and metal ions (II) possess high RR activity. " Fingerprint information from RR spectra made the detection more convenient. " This method realized the detection of blend metal ions successfully.
a r t i c l e
i n f o
Article history: Received 7 September 2012 Received in revised form 15 November 2012 Accepted 30 November 2012 Available online 20 December 2012 Keywords: Zincon Resonance Raman Cu2+ and Ni2+ Quantitative evaluation
a b s t r a c t In this paper, a new proposal for the quantitative evaluation of divalent metal ions (M2+) is developed by the use of the competitive resonance Raman (RR)-based method. Upon excitation with light of the appropriate wavelength (532 nm), a strong electric field is generated that couples with the resonance of the complex (zincon-M2+), increasing the character signals of these complexes, resulting in sensitive detection. Herein, the RR probe, zincon-M2+ complex that the RR intensity gets lower with the decreasing of the M2+ concentration, which leads to the transformation of the Raman information. As a result, by using the proposed RR-based method, we could find the liner calibration curves of Cu2+ and Ni2+, which show the potential in quantitative evaluation of an unknown sample. In addition, the abundant fingerprint information shows that RR leads to the successful analysis of a blended solution, which contains two ions: Cu2+ and Ni2+. Ó 2012 Elsevier B.V. All rights reserved.
Introduction Heavy metal ions analyses are an important part of studies in analytical chemistry. The determinations of these ions in the various materials from the industrial samples to environmental samples have been performed continuously [1–8]. Some metal ions appear together in many real samples. The determination of traces of metal ions has long been an important subject in environmental analysis, many types of industrial processes, and chemical reac⇑ Corresponding author. Tel.: +86 431 85168473. E-mail address: [email protected] (B. Zhao). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.11.112
tions [1–4]. Meanwhile, uncontrolled disposal of wastes is contaminating food and water that come from natural sources to alarming levels with the industrialization and urbanization in developing countries. Higher concentration of metal ions will cause environmental and foods pollution. Metal ions and their complex that are a potentially toxic and carcinogenic metal, have been determined by a variety of complexing agents. For example, when the concentration of the copper and nickel in the soil is higher, it will be diluted with the flow of water to the river and lake, this cause ecological harm. In addition, in the wastewater, there are often different kinds of metal ions mixed together, so finding a proper way to detect the content of the mental ions among many others is very
Z. Yu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 52–56
necessary [5–8]. So it is necessary to develop a simple, fast, and accurate method for monitoring the levels of contaminants. Traditional optical techniques for metal ion assay in solution include mass spectroscopy [9], X-ray fluorescence [10], fluorescence spectrometry [11], atomic absorption spectroscopy [12], etc. have all been used for the simultaneous determination of these ions in different samples. Among the most widely used analytical methods, colorimetric [13,14] and UV–vis spectrophotometry [4,15– 19] techniques are widely used, due to the resulting experimental rapidity, simplicity. However, the simultaneous determination of these ions by the use of the traditional spectrophotometry techniques are difficult, because, generally, the absorption spectra overlap in a bright region and the superimposed curves are not suitable for quantitative evaluation [19,20]. Raman spectroscopy is a promising and ultrasensitive analytical tool due to its nondestructive analysis and high sensitivity to the molecular structure of samples [21–23]. And it has realized many analyses among different territories, such as the monitoring of the environment pollutants, constituent detection of liquid coal, and identification of synthetic diamond [24,25]. Resonance Raman (RR) spectroscopy, in which the Raman intensity of some molecular vibrations are strongly enhanced when the excitation wavelength of the Raman scattering approaches an absorbance maximum of an electronic transition, is an important analytical technique with high sensitivity and selectivity [26,27]. RR spectroscopy, which can provide a larger amount of fingerprint information for an individual component of a mixture, has recently demonstrated promise for biomolecular studies and biomedical diagnostics [24,25,28]. Nowadays RR spectroscopy technique was employed as a quantitative method for its high sensitivity and selectivity. Recently, our previous work proposed the idea of zincon reagent-based protein assay using the RR method. By examining the remaining probes that are not bound to the protein with RR method to achieve protein assay indirectly [20]. For many years, zincon was used as a chromogenic reagent, which has been extensively used for the colorimetric and spectrophotometric determination of many metals, because of the formation of water-soluble metal ion complexes (Scheme 1) [4,17,29]. Herein, we employed zincon-M2+ complex as a RR probe for evaluating the concentration of metal ions in the solution. This assay happens when just mixing the zincon with the divalent metal ions, the color of the solution will change which can be identified by the UV–vis spectra. Meanwhile, zincon compounds with different kinds of metal ions exhibit different RR bands. RR was then used as a detection method to realize the evaluation of the mixture system based on the different resonance band. It exhibits high sensitivity and good selectivity during the detection among the blend reagent of different mental ions.
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Experimental section Materials Copper (II) chloride (CuCl2), nickel chloride (NiCl2), were purchased from Sigma Aldrich Co. Ltd., and zincon monosodium salt (2-carboxy-20-hydroxy-50-sulfoformazylbenzene) was purchased from Fluka Co., Ltd. at highest purity available and used as received without further purification. Ultrapure water (18.0 MX cm 1) was used throughout the present study. Pretreatment of metal ions samples To determine the quantitative M2+ (Cu2+ , Ni2+) concentrations, the Cu2+ and Ni2+ with the concentration of 1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6, 2.0 10 6, 1.0 10 6, 6.7 10 7, 3.3 10 7, 2.0 10 7, 1.0 10 7, 6.7 10 8, and 0 (blank sample) mol/L were placed in each cuvette with the concentration of zincon (8 10 5 mol/L) kept constant and keep constant of pH at 4.5. To determine the mixture of metal ions (Cu2+ and Ni2+ mixture), the concentrations of Cu2+ and Ni2+ were set as sample a to sample d (a: 1.7 10 5 and 3.3 10 5, b: 1.3 10 5 and 6.7 10 5, c: 1.0 10 5 and 1.0 10 4, d: 6.7 10 6 and 1.3 10 4, e: 3.3 10 6 and 1.7 10 4 mol/L. These samples were each placed in individual cuvettes with zincon, which had a concentration of 8 10 5 mol/L. Instruments RR spectra of zincon-M2+ systems were recorded using a JobinYvon/HORIBA LabRam ARAMIS Raman spectrometer equipped with a liquid cell hold. The radiation from an air-cooled frequency doubled Nd:Yag laser (532 nm) was used as an excitation source. UV–vis spectra were recorded on a UV-3600 spectrophotometer (Shimadzu). Results and discussion The determination of metal ions concentration is of great importance in food and environment analysis. Previous work of determining metal ion concentrations with zincon has been based on colorimetric and absorption spectra assays. Fig. 1 shows optical image of the solutions containing two metal ions (Cu2+ and Ni2+). Herein, zincon can be used as a colorimetric sensor with the ‘‘naked eye’’ for determining different metal ions. As shown in Fig. 1A and B, the color of the zincon-M2+ solution containing different metal ions were completely different, which indicates the variation of the conformation of zincon. Meanwhile, the addition of Cu2+ or Ni2+ produce a change of color (orange to blue for Cu2+
Scheme 1. Structures of zincon in its free and the zincon-M2+ forms.
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Fig. 1. Photos of zincon (8 10 5 M) solutions containing two kinds of metal ions (Cu2+, A and Ni2+, B) with the different concentrations (from left to right are: 1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6 mol/L and blank sample).
and orange to brown for Ni2+) corresponding to the formation of the respective zincon-metal complex. In addition, the color of zincon solutions containing Cu2+ or Ni2+ was directly related to the M2+ concentration. However, a noticeable color change was not observed at lower metal ions concentrations (below 2.0 10 5 mol/L for Cu2+ and 3.3 10 5 mol/L for Ni2+). UV–vis spectroscopy was also widely used for quantitative evaluation of the target. Fig. 2 shows the UV–vis absorbance spectra for the zincon-M2+ (zincon-Cu2+ and zincon-Ni2+) system with increasing concentrations of M2+. The absorption maximum at 465 nm of zincon decreased gradually and shifted to longer wavelengths as the concentration of Cu2+ and Ni2+ increased, and it eventually formed a new band near 600 nm and 666 nm, which can be assigned to the free chelator of zincon and M2+. When the concentration of Cu2+ was higher than 6.7 10 5 mol/L, the peak at 465 nm disappeared completely. This is confirmation that the complex of zincon and Cu2+ was formed completely. Further, when the concentrations of Cu2+ and Ni2+ were lower than 3.3 10 6 mol/L, no significant variation of the peak at 465 nm occurred compared to the absorbance of zincon. Interestingly, it was difficult to detect a significant change in the absorption spectra at much lower M2+ concentrations (below 3.3 10 6 mol/L for Cu2+ and Ni2+).
Fig. 2. Concentration-dependent UV–vis spectra of zincon-M2+ complex with different concentration of (A) Cu2+ and (B) Ni2+ (1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6, 2.0 10 6, 1.0 10 6, 6.7 10 7, 3.3 10 7, 2.0 10 7, 1.0 10 7, 6.7 10 8 mol/L and blank sample).
RR of zincon-copper (II) system To successfully detect M2+ at lower concentrations, RR spectroscopy was performed. RR can provide a larger amount of fingerprint information for an individual component of a mixture, which is helpful for us to quantitatively analyze a target. Fig. 3 shows the RR spectra of zincon-Cu2+ complex with different concentrations of Cu2+. As shown in Fig. 3, the intensities of the Raman bands at 1387 cm 1 assigned to zincon decreased with increasing Cu2+ concentrations. When the concentration of Cu2+ is higher than 2.0 10 5 mol/L, the RR spectra of zincon-Cu2+ complex are very different from zincon. Further, some new RR bands appeared at 1499, 1404, 1355, 1060, 1007, 876, 862 and 630 cm 1, which are assigned to RR effect of zincon-Cu2+ complex. Meanwhile, RR bands at 1387, 1301 and 1174 cm 1 are decreased with the increasing the concentration of Cu2+ (from 6.7 10 8 to 1.0 10 5 mol/L). To quantitatively evaluate Cu2+ with our proposed RR-based method, 1387 cm 1 was selected. Concentration-dependent RR intensities
Fig. 3. RR spectra of zincon-Cu2+ complex containing different concentrations of Cu2+ (from up to bottom are 1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6, 2.0 10 6, 1.0 10 6, 6.7 10 7, 3.3 10 7, 2.0 10 7, 1.0 10 7, 6.7 10 8 mol/L and blank sample).
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Fig. 5. RR spectra of zincon-Ni2+ complex containing different concentrations of Ni2+ (from up to bottom are 1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6, 2.0 10 6, 1.0 10 6, 6.7 10 7, 3.3 10 7, 2.0 10 7, 1.0 10 7, 6.7 10 8 mol/L and blank sample).
selected and plotted in Fig. 4B. As shown in Fig. 4B, the Raman intensity varied monotonically with changes in the concentration of Ni2+. The relationship between the intensity and the concentration were found to be linear (Y = 6551.9–113.3X, R2 = 0.980) at 1387 cm 1 in the range from 3.3 10 5 to 2 10 6 mol/L of Ni2+. The error bars indicate the batch-to-batch variability of the RR intensity with five independent measurements. Thus, the RR effect of zincon could be a promising method with high sensitivity and selectivity for analyses of M2+. RR of zincon-copper (II) and zincon-nickel (II) mixture system
Fig. 4. Concentration-dependent of RR intensity of the band at 1387 cm 1 for zincon-M2+ containing different concentrations of (A) Cu2+ and (B) Ni2+ (from right to left are 1.0 10 4, 6.7 10 5, 3.3 10 5, 2.0 10 5, 1.0 10 5, 6.7 10 6, 3.3 10 6, 2.0 10 6, 1.0 10 6, 6.7 10 7, 3.3 10 7, 2.0 10 7, 1.0 10 7, 6.7 10 8 mol/L and blank sample).
of zincon at 1387 cm 1 are plotted in Fig. 4A. The error bars indicate the batch-to-batch variability of the RR intensity with five independent measurements. As shown in Fig. 4A, the Raman intensity varied monotonically with changes in the concentration of Cu2+. The relationship between the intensity and the concentration was found to be linear (Y = 6470.8–184.7X, R2 = 0.982) at 1387 cm 1 in the range from 2 10 5 to 2 10 6 mol/L of Cu2+. RR of zincon-nickel (II) system In order to quantitatively evaluate Ni2+, the RR-based method was also performed. Fig. 5 shows the concentration-dependent RR spectra of zincon-Ni2+ complex with different concentrations of Ni2+. Similar to the zincon-Cu2+ spectra, as shown in Fig. 5, the intensities of the Raman bands of zincon decreased with increasing Ni2+ concentrations in the zincon-Ni2+ complex. When the concentration of Ni2+ is higher than 3.3 10 5 mol/L, the RR spectra of zincon-Ni2+ complex are very different from zincon. Further, new RR bands appeared at 1482, 1394, 1193, 1155, 875, 829, 728 and 640 cm 1, which are assigned to the RR effect of the zincon-Ni2+ complex. Meanwhile, the RR bands at 1387, 1301 and 1174 cm 1 decrease with an increase in the Ni2+ concentration (from 6.7 10 8 to 1.0 10 5 mol/L). The band at 1387 cm 1 was also
Metal ions are always appearing together in many real samples, thus it is necessary to find an effective and rapid method for the determination of metal ions. As a widely used method, UV–vis spectroscopy was performed for determination of the proposed Cu2+ and Ni2+ mixture system. Fig. 6 shows the UV–vis spectra of zincon mixed with the two metal ions (Cu2+ and Ni2+). As shown in Fig. 6, this copper–nickel mixture is an extremely difficult complex system due to the high spectral overlapping observed between the absorption spectra for these components. In view of this, the RR method was also performed on our mixture system. All of the RR-based results indicate that zincon as RR probe for the detection of M2+ is a promising quantitative and determination method. In addition, zincon is a widely accepted reagent for the determination of divalent metal ions, such as Cu2+ and Ni2+ et al. with a color change and possesses a high stability constant towards Cu2+ (log K = 7.5 ± 0.1) and Ni2+ (log K = 10 ± 0.3) [29,30]. Figs. 3 and 5 provide a large amount of fingerprint information for an individual component of zincon-Cu2+ and zinconNi2+ complex via RR effect. For the different resonance effect of zincon-Cu2+ and zincon-Ni2+, their RR spectra are significantly different. This finding is useful and helpful for the determination of metal ions in the mixture system. Therefore, RR-based method can also be performed for the determination of metal ions in this mixture system. Fig. 7 shows RR spectra of the mixture (zinconCu2+ and zincon-Ni2+ complex) with different concentration of Cu2+ and Ni2+. As shown in Fig. 7, by decreasing and increasing the concentrations of Cu2+ and Ni2+ concomitantly, their RR bands assigned to zincon-Cu2+ complex transformed to the RR bands assigned to zincon-Cu2+ and zincon-Ni2+ mixture system, and finally transformed to the RR bands assigned to zincon-Ni2+ complex. Therefore, this RR-based method is good candidate for metal ions
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centration as 1.63 10 5 mol/L, which is close to Cu2+ concentration that inputted in sample a (1.7 10 5 mol/L). Conclusions We have successfully utilized this new method for the detection of the zincon-M2+ system by RR, and thus realized a sensitive and fast quantitative evaluation of divalent metal ions. The large amount of fingerprint information that was provided by the RR effect can be utilized to determine the mixture. Meanwhile, unknown concentrations of metal ions can be quantified with the calibration curves. Furthermore, this method has been successfully used in a mixture system containing Cu2+ and Ni2+, which leads to the detection of metal ions in a mixture system for real samples. The proposed RR-based method exhibited great potential in highly sensitive concentration determination of metal ions and a mixture of metal ions, which exhibited great potential in a complex system. Fig. 6. UV–vis spectra of mixture system (Cu2+ and Ni2+) with different concentrations (a, 1.7 10 5 and 3.3 10 5, b, 1.3 10 5 and 6.7 10 5, c, 1.0 10 5 and 1.0 10 4, d, 6.7 10 6 and 1.3 10 4, e, 3.3 10 6 and 1.7 10 4 mol/L, respectively).
Acknowledgments This study was supported by the National Natural Science Foundation (Grant Nos. 20921003, 20903044, 20973074, 21073072) of PR China. Specialized Research Fund for the Doctoral Program of Higher Education (20110061110017), the 111 project (B06009) and the Development Program of the Science and Technology of Jilin Province (20110338). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.11.112. References
Fig. 7. RR spectra of zincon-Cu2+ and zincon-Ni2+ mixture system with different concentrations of Cu2+ and Ni2+ (from up to bottom are 2 10 4 and 0, sample a: 1.7 10 5 and 3.3 10 5, sample b: 1.3 10 5 and 6.7 10 5, sample c: 1.0 10 5 and 1.0 10 4, sample d: 6.7 10 6 and 1.3 10 4, sample e: 3.3 10 6 and 1.7 10 4, 0 and 2 10 4 mol/L, respectively).
(Cu2+ and Ni2+) which have significantly change of RR spectra after chelating with zincon compared to the RR spectrum of zincon. This RR-based detection protocol for the determination of Cu2+ and Ni2+ is a promising method. Using the characteristic peaks, it is easier to identify the composition of the mixture system due to the difference of the chelator (zincon-Cu2+ and zincon-Ni2+). The RR-based method was established with much more fingerprint information to identify metal ions in the mixture system. The proposed method with the calibration of characteristic peak is a powerful technique for determinating the concentration of the mixture sample. As shown in Fig. 3, the Raman intensity varied monotonically with change in concentration of Cu2+. A characteristic peak at 1355 cm 1 of zincon-Cu2+ was chosen to plot the calibration curves (Fig. S1). The relationship between the intensity and the concentration was found to be linear (Y = 1941.1 + 56.1X, R2 = 0.984) at 1355 cm 1 in the range from 3.3 10 5 to 6.7 10 6 mol/L. Based on this calibration curve and the intensity of the band at 1355 cm 1 in RR spectra of the mixture sample a (Fig. 7), we can quantitative evaluate Cu2+ con-
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