Development of a fluorescent chelating ligand for scandium ion having a Schiff base moiety

Spectrochimica Acta Part A 90 (2012) 72–77

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Development of a fluorescent chelating ligand for scandium ion having a Schiff base moiety Hiroshi Yamada, Masahito Kojo, Tomomi Nakahara, Kumi Murakami, Takashi Kakima, Hideaki Ichiba, Takehiko Yajima, Takeshi Fukushima ∗ Department of Analytical Chemistry, Faculty of Pharmaceutical Sciences, Toho University, 2-2-1 Miyama, Funabashi-shi, Chiba 274-8510, Japan

a r t i c l e

i n f o

Article history: Received 28 September 2011 Received in revised form 5 January 2012 Accepted 8 January 2012 Keywords: Metal-ion complex Ligand Fluorescence Scandium ion Rare earth metal 1-(2-Hydroxy-3-methoxybenzaldehyde)-4aminosalicylhydrazone

a b s t r a c t A fluorescent ligand, 1-(2-hydroxy-3-methoxybenzaldehyde)-4-aminosalicylhydrazone (HMB-ASH), was newly designed and synthesized, and its fluorescence characteristics for metal ions were investigated in the pH range 3.0–10.5 (at a difference of 0.5 for each metal). After testing 31 different metal ions, it was found that HMB-ASH was able to emit fluorescence intensely at 512 nm with an excitation wavelength of 353 nm in the presence of Sc3+ , one of the rare earth metals, at pH values around 3.5 and 8.0. The other metal ions hardly showed fluorescence with HMB-ASH. The fluorescence was more intense at pH 8.0, and the detection limit of Sc3+ in a buffer solution (pH 8.0) was approximately 18.8 nmol L−1 (0.85 ppb). © 2012 Elsevier B.V. All rights reserved.

1. Introduction For the sensitive detection of trace metal ions in environmental samples, a number of fluorescent chelating ligands have been developed [1,2]. Owing to the industrial significance of metal ions, especially those of rare earth metals, the development of a superior ligand for selectively chelating the metal ion is required. Recently, we developed a fluorescent chelating ligand having two Schiff base moieties, 2,4-[bis-(2, 4-dihydroxybenzylidene)]dihydrazinoquinazoline (DBHQ) [3,4] and 2,4-[bis-(2-hydroxy-3methoxybenzylidene)]-dihydrazinoquinazoline (HBQZ) [5]. DBHQ and HBQZ can chelate gallium ions (Ga3+ ) and zinc ions (Zn2+ ), respectively, to emit fluorescence. However, both ligands have a small amount of intrinsic fluorescence because they possess a quinazoline moiety. To obtain a good chelating ligand, the intrinsic fluorescence originating from the ligand itself should preferably be minimal. In order to reduce the intrinsic fluorescence emitted from the ligand itself, aromatic groups other than quinazoline should be selected. Therefore, in the present study, 4-aminosalicylate (p-aminosalicylate), which is capable of chelating metal ions [6,7], was employed and connected to an o-vanillin moiety, which has also been used for chelating reagents via a Schiff

∗ Corresponding author. Tel.: +81 47 472 1504; fax: +81 47 472 1504. E-mail address: [email protected] (T. Fukushima). 1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2012.01.014

base [8–10]. Thus, in this paper, we report the fluorescence properties of a new fluorescent chelating ligand, 1-(2-hydroxy-3methoxybenzaldehyde)-4-aminosalicylhydrazone (HMB-ASH), in the presence of Sc3+ . 2. Experimental 2.1. Chemicals 4-Aminosalicylic acid and 80% hydrazine monohydrate were bought from Tokyo Chemical Industries Co., Ltd. (Tokyo, Japan). o-Vanillin was purchased from Kanto Chemical Co., Ltd. (Tokyo, Japan). Ethanol (EtOH, 99.0%) was obtained from Japan Alcohol Trading Co., Ltd. (Chiba, Japan) and was used after distillation. Water was used after purification using an Autopure WR600G system (Yamato Scientific Co., Ltd., Tokyo, Japan). Divalent or trivalent ions tested in this study, which were purchased from Wako Pure Chemicals Co., Ltd. (Osaka, Japan) in the form of salts with either NO3 − , SO4 2− , or Cl− ions, were of atomic absorption grade. 2.2. Apparatus Excitation and fluorescence spectra were measured by using the spectrofluorometer FP-6300 (Jasco Corporation, Tokyo, Japan) with slit widths of 10 nm. All fluorescence and absorption data were obtained in a temperature-controlled room (25 ◦ C). NMR spectra were measured by using a JEOL ECP1 spectrometer (JEOL Ltd.,

H. Yamada et al. / Spectrochimica Acta Part A 90 (2012) 72–77

Tokyo, Japan) (500 MHz for 1 H and 13 C) with tetramethylsilane as an internal standard. The infrared spectrum was obtained using an FT/IR-4100 spectrometer (Jasco). The mass spectrum was measured using a JMS 600H spectrometer (JEOL Ltd., Tokyo, Japan). 2.3. Synthesis of HMB-ASH 4-Aminosalicylic acid (10.0 g, 65 mmol) was dissolved in MeOH (200 mL) and was refluxed in the presence of 10 mL concentrated sulfuric acid for 4 h. After the reaction, the solution was concentrated to approximately 20 mL and was neutralized by adding a 10% NaHCO3 aqueous solution. The crude crystal was filtered and recrystallized from MeOH/H2 O (1/1) to obtain methyl 4aminosalicylate (yield 65.7%). Methyl 4-aminosalicylate (4.0 g, 23 mmol) was added to 80% hydrazine monohydrate (12 mL), and the mixture was refluxed for 90 min. After evaporation, the residue was added to distilled H2 O to obtain a crude crystal. The crude crystal was recrystallized from EtOH to obtain 4-aminosalicyl hydrazide (yield 78.4%). Subsequently, 4-aminosalicyl hydrazide (0.5 g, 3.0 mmol) in EtOH (100 mL) was made to react with o-vanillin (0.45 g, 3.0 mmol) for 2 h in a water bath at 100 ◦ C. After cooling to room temperature, the precipitate was collected by filtration and was recrystallized from MeOH to obtain the final product, HMB-ASH (yield 82.5%). FAB-MS: m/z 302 (M+H)+ ; Anal. Calcd. for C15 H15 O4 N3 : C, 59.80; H, 5.02; N, 13.95. Found: C, 60.02; H, 4.89; N, 13.77. 1 H NMR (DMSO-d6 ) ı (ppm): 12.27 (s,1H), 11.67 (s,1H), 11.00 (s,1H), 8.55 (s,1H), 7.59 (d,1H), 7.06 (d,1H), 6.96 (d,1H), 6.80 (t,1H), 6.09 (d,1H), 5.99 (s,2H), 4.32 (s, 3H). 13 C NMR (DMSO-d6 ) ı (ppm): 166.31, 162.33, 163.33, 155.25, 148.42, 148.02, 147.63, 129.44, 121.42, 119.46, 114.31, 106.41, 101.88, 99.86, 56.34. FT-IR (solid phase) (cm−1 ): 3352 (vOH ), 1595 (vC N ), 1249, 1200 (vAr O ). Melting point: 252–254 ◦ C. 2.4. Screening assay of binding metal ions to HMB-ASH Each metal ion (Li+ , Ag+ , K+ , Na+ , Mg2+ , Ca2+ , Sr2+ , Ba2+ , Co2+ , Be2+ , Zn2+ , Hg2+ , Ni2+ , Cu2+ , Cd2+ , Ti2+ , Mn2+ , Fe3+ , Ga3+ , Al3+ , In3+ , Pb2+ , Sn2+ , Tl3+ , Ce3+ , Bi3+ , Sb3+ , Sc3+ , Y3+ , Ge4+ , and Zr4+ ) was examined to determine whether it could be coordinated by HMB-ASH to emit fluorescence. Each solution containing 100 ␮L of the selected metal ion (1000 ppm), 100 ␮L of a 100 ␮mol L−1 solution of HMB-ASH in DMSO, and 100 ␮L of a buffer solution with varying pH values (pH 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0, and 10.5) was placed in a glass test tube. The buffer solutions of pH 3.0–5.0, 5.5–7.5, and 7.5–9.0 were prepared from 100 mmol L−1 CH3 CO2 Na CH3 CO2 H, CH3 CO2 H NH3 , and NH4 Cl NH3 , respectively. In a dark room, ultraviolet light from a D2 lamp was irradiated on each test tube. The sample emitting intense fluorescence was concluded to be the solution that contained the fluorescent complex of HMB-ASH. In this experiment, the fluorescent emission was evaluated by visual assessment. 2.5. Influence of pH and the excitation and emission spectra In a polypropylene test tube, 50 ␮L of each pH buffer solution (100 mmol L−1 CH3 CO2 Na CH3 CO2 H, CH3 CO2 H NH3 , and NH4 Cl NH3 ) was serially added to 50 ␮L of 44.4 ␮mol L−1 Sc3+ [Sc(NO3 )3 ] in H2 O and 50 ␮L of a 100 ␮mol L−1 solution of HMBASH in DMSO, and the resultant solution mixture was left to stand for 10 min at room temperature. Then, the solution was diluted 10fold with distilled EtOH, and its spectrum was measured by using a spectrofluorescence photometer, FP-6300 (Jasco).

73

2.6. Effect of coexisting ions on the fluorescence intensity In a test tube, 22.2 ␮mol L−1 (1.0 ppm) Sc3+ ions (50 ␮L) were added to 100 mmol L−1 ammonia buffer (pH 8.0) (50 ␮L) and 100 ␮M HMB-ASH (50 ␮L), and then, the tested ions (11.1 ␮M (0.5 eq.), 22.2 ␮M (1.0 eq.), 111 ␮M (5.0 eq.), 222 ␮M (10 eq.), 1,110 ␮M (50 eq.), and 2,220 ␮M (100 eq.)) were added and treated in a similar manner as described above. An error within ±5.0% of the measured fluorescence intensity was considered tolerable. To achieve masking of the 22.2, 44.4, 66.6, and 111 ␮mol L−1 Fe 3+ ions, 4.44 mM (200 eq. to Sc3+ ion) potassium thiocyanate (KSCN) (50 ␮L) was added and treated in a similar manner as described above (n = 3). 2.7. Binding ratio of Sc3+ to HMB-ASH Molar ratio method: Fifty microliters of ammonia buffer (pH 8.0) was added to 50 ␮L of varying concentrations (2.22, 6.67, 11.12, 15.57, 22.2, and 33.4 ␮mol L−1 ) of Sc3+ , followed by the addition of 50 ␮L of a 20 ␮mol L−1 solution of HMB-ASH in DMSO, and the solution mixture was treated in a manner similar to that described above. Next, 50 ␮L of ammonia buffer (pH 8.0) was added to 50 ␮L of 22.2 ␮mol L−1 Sc3+ , followed by the addition of 50 ␮L of varying concentrations (10, 15, 30, 40, and 60 ␮mol L−1 ) of HMB-ASH in DMSO, and the solution mixture was treated in a manner similar to that described above (n = 3). The binding ratio was obtained by using the molar ratio method; that is, the fluorescence intensities were plotted against the molar ratios of Sc3+ to HMB-ASH, and the molar ratio of HMB-ASH coordinated to Sc3+ was stoichiometrically determined from the plots. Continuous variation method: Fifty microliters of ammonia buffer (pH 8.0) was added to 10, 20, 30, 40, 50, 60, 70, 80, and 90 ␮L of 22.2 ␮mol L−1 Sc3+ , followed by the addition of 90, 80, 70, 60, 50, 40, 30, 20, and 10 ␮L of a 20 ␮mol L−1 solution of HMB-ASH in DMSO, and the solution mixture was treated in a manner similar to that described above (n = 3). The fluorescence intensity of each solution was measured at 512 nm at an excitation wavelength of 353 nm. The binding ratio was obtained by using the continuous variation method; that is, the fluorescence intensities were plotted against the ratio [Sc3+ ]/([Sc3+ ] + [HMB-ASH]) or the ratio [HMBASH]/([Sc3+ ] + [HMB-ASH]). Thereby, the molar ratio of HMB-ASH coordinated to Sc3+ was stoichiometrically determined from the plots. 2.8. Binding constant The apparent binding constant of HMB-ASH with Sc3+ was calculated using the modified Benesi–Hildebrand equation reported by Roy et al. [11]: 1 Fmax =1+ K[C] F

(1)

where F and Fmax are equal to F − F0 and Fmax − F0 , respectively. F0 , F, and Fmax are the fluorescence intensities of HMB-ASH (20 ␮M), HMB-ASH with a test concentration of Sc3+ (2.22, 6.67, 11.12, 15.6, 22.2, and 33.4 ␮mol L−1 ), and HMB-ASH with the maximum concentration (33.4 ␮mol L−1 ) of Sc3+ , respectively. K and [C] are the apparent binding constant and test concentration of Sc3+ , respectively. Fmax /F was plotted against 1/[C], and the value of K was obtained from the slope of the obtained line. 2.9. Calibration curve and detection limit A calibration curve was constructed by plotting the values of F − F0 against Sc3+ concentration. Fifty microliters of the Sc3+ solution (2.22, 4.44, 11.1, 15.5, 22.2, and 44.4 ␮mol L−1 ) was

74

H. Yamada et al. / Spectrochimica Acta Part A 90 (2012) 72–77

Fig. 1. Synthetic pathway of HMB-ASH.

added to 50 ␮L of a 100 ␮mol L−1 HMB-ASH solution and 50 ␮L of 100 mmol L−1 ammonia buffer (pH 8.0) (n = 3). The subsequent procedure was the same as that in the previous treatment. The detection limit (DL) was determined by the obtained calibration curve using the following equation: DL = 3.3

 , s

(2)

where  and s are the standard deviation of 22.2 nmol L−1 Sc3+ and slope of the calibration curve, respectively (n = 5). 2.10. Application to environmental water samples In polypropylene test tubes, 1.0 ppm (22.2 ␮mol L−1 ) Sc3+ solutions in a river water sample, taken from Shinkawa (Chiba, Japan), and a tap water sample (Chiba, Japan) were prepared. These samples were subjected to a solid-phase extraction (SPE) procedure. Four hundred microliters of these sample solutions were loaded on an anion exchange cartridge, MonospinTM SAX (GL Sciences Inc., Tokyo, Japan), and were centrifuged at 1800 × g for 30 min. This procedure was repeated twice. Fifty microliters of the obtained filtrate was serially added to 50 ␮L of 100 mmol L−1 ammonia buffer (pH 8.0) and 100 ␮mol L−1 HMB-ASH, and the solution mixture was treated in a similar manner as described in Section 2.5. In contrast, 50 ␮L of river and tap water samples was directly added to 50 ␮L of 100 mmol L−1 ammonia buffer (pH 8.0) and 100 ␮mol L−1 HMBASH (without SPE). The recovery of Sc3+ (%) was calculated by using the following equation: Recovery (%) =

Ftest − Fblank × 100, Fstd − Fblank

(3)

results showed that Sc3+ ions coordinated with HMB-ASH to emit intense fluorescence upon irradiation from a D2 lamp. Previously, Tang and his colleagues reported a fluorescent ligand, salicylaldehyde salicyloylhydrazone (SASH), and the chemical structure of SASH is quite similar to that of HMB-ASH, with the exception that HMB-ASH bears 3-methoxy and aromatic 4-amino groups attached to both phenyl moieties (Fig. 2). It has been reported that SASH (Fig. 2(b)) exhibits fluorescence in the presence of Al3+ , Sc3+ , and In3+ [12–14]. However, the structurally similar compound, HMB-ASH, showed fluorescence in the presence of only Sc3+ . 3.2. Influence of pH and the fluorescence spectrum Fig. 3 (a) shows the excitation and emission spectra of the HMB-ASH–Sc3+ complex obtained at pH 8.0. As shown in Fig. 3(a), the optimum wavelengths for the excitation and emission were 353 and 512 nm, respectively. The level of intense fluorescence exhibited by HMB-ASH in the presence of Sc3+ was approximately 30 times that exhibited by HMB-ASH alone, suggesting that the formation of the HMB-ASH–Sc3+ complex certainly enhanced the fluorescence. With regard to the optimum pH for the measurement, it was found that the fluorescence was intense in both weakly acidic (pH 3.5) and basic (pH 8.0) regions (Fig. 3(b)) and that the fluorescence was more intense in the basic pH region than in the acidic pH region. Among the organic solvents used for the dilution and fluorescence measurement of the HMB-ASH–Sc3+ complex, EtOH gave the most intense fluorescence (data not shown); therefore, EtOH was used for further study.

where Fstd and Fblank are the fluorescence intensities in the presence and absence of 1.0 ppm Sc3+ in the distilled H2 O, respectively, and Ftest is the fluorescence intensity of the 1.0 ppm Sc3+ solutions in the environmental water samples. 3. Results and discussion 3.1. Synthesis and fluorescence property of HMB-ASH HMB-ASH was designed and synthesized as shown in Fig. 1. 4-Aminosalicylic acid was first esterified to the methylester, and subsequently, a hydrazine group was introduced to the ester moiety. Next, an o-vaniline moiety was covalently bound to the hydrazine group to form a Schiff base moiety. Elemental analysis, FAB-MS, 1 H NMR, and 13 C NMR data showed that HMB-ASH was successfully synthesized. Next, to identify a metal ion that could coordinate with HMB-ASH to emit fluorescence, 31 types of metal ions (1000 ppm) in different pH-buffered solutions were tested. The

Fig. 2. Chemical structures of HMB-ASH (a) and SASH [11–13] (b).

H. Yamada et al. / Spectrochimica Acta Part A 90 (2012) 72–77

75

Table 1 Effect of coexisting ions on the detection of 22.2 ␮M Sc3+ using 50 ␮M HMB-ASH at pH 8.0 and 3.5 (tolerable error: ±5%). Tolerance ratio (m/m)

Coexisting ions (pH 8.0)

>100 50 10 5 2 1 0.5

Al3+ Ni2+ In3+ Ga3+ Fe3+

Mn2+ Ca2+

Mg2+

Co2+

Zn2+

Cd2+

Br−

Hg2+

Ba2+

Sr2+

Pb2+

Hg2+

F−

Cl−

H2 BO3 −

–

Tolerance ratio (m/m)

Coexisting ions (pH 3.5)

>100 50 10 5 2 1 0.5

Al3+ Sr2+ Cd2+ Ni2+

Mn2+ Ca2+ Pb2+ Hg2+

Br−

Cl−

Co2+ In3+

Fe3+

Zn2+ –

Ga3+

Ba2+

F−

H2 BO3 −

Tolerance ratio (m/m) was expressed as a ratio of the tested ion (mol) to the Sc3+ ion (mol).

3.3. Coexisting ions

3.4. Binding ratio of HMB-ASH to Sc3+ and the binding constant

Table 1 shows the effect of ions on the fluorescence intensity of the HMB-ASH–Sc3+ complex at pH 8.0 and 3.5. A difference was observed in the effect of coexisting ions on the fluorescence intensity at pH 3.5 and 8.0. Overall, very few ions interfered with the fluorescence of the HMB-ASH–Sc3+ complex at pH 8.0. In the case of SASH, trivalent cations such as Al3+ , In3+ , and Ga3+ interfered with the SASH–Sc3+ complex [13]. However, these trivalent cations could hardly influence the fluorescence in the case of the HMBASH–Sc3+ complex. As shown in Table 1, among the tested ions, Fe3+ intensively interfered with the fluorescence intensity of the HMB-ASH–Sc3+ complex (pH 8.0). In order to prevent the interference by Fe3+ , 44.4 mmol L−1 (200 eq. to Sc3+ ) thiocyanate ions (SCN− ) were added, which resulted in the masking of Fe3+ to form a soluble complex, Fe(SCN)2+ or Fe(SCN)6 3− . The masking effect of Fe3+ by SCN− was clearly observed until the experiment reached 5 eq. of Fe3+ to Sc3+ (Fig. 4).

Fig. 5(a) and (b) shows the plots of the fluorescence intensity against the molar ratio of Sc3+ to HMB-ASH (a) and of HMB-ASH to Sc3+ (b). Both plots (a) and (b) indicate that HMB-ASH could bind to Sc3+ at a molar ratio of 1:1. In addition, the continuous variation method (Job plot, Fig. 5(c) and (d)) also indicated the binding of HMB-ASH to Sc3+ at the 1:1 ratio. Therefore, similar to the SASH–Sc3+ [12] or SASH–In3+ complex [14], it was found that HMB-ASH could bind to Sc3+ at a molar ratio of 1:1. The binding association constant K was determined by the Benesi–Hildebrand method, and log K was found to be approximately 5.4. This log K value was considered to be sufficient for the fluorescence detection of Sc3+ in the application samples.

3.5. Calibration curve and the detection limit Fig. 6 shows a calibration curve for Sc3+ ions using the present fluorescent ligand, HMB-ASH. A linear calibration curve was obtained for the Sc3+ ions in the concentration range 22.2–444 nM (r2 = 0.998) (Fig. 6). The precision, given by the relative standard deviation (RSD) as a percentage, was in the range 2.22–11.8% (n = 3). The detection limit of Sc3+ in the buffer (pH 8.0) was approximately 18.8 nmol L−1 .

Fig. 3. Excitation and emission spectra of HMB-ASH in the presence (solid line) and absence (dotted line) of Sc3+ at pH 8.0 (a). Fluorescence intensity of the HMBASH–Sc3+ complex at 512 nm (ex. 353 nm) as a function of pH (b).

Fig. 4. Masking effect of 44.4 mmol L−1 thiocyanate ions on the fluorescence intensity of the HMB-ASH–Sc3+ complex in the presence of 22.2 ␮mol L−1 (1 eq.) to 111 ␮mol L−1 (5 eq.) Fe3+ . Filled and open columns represent results obtained with and without the thiocyanate ion, respectively. The mean value of the fluorescence intensity in the presence of 22.2 ␮mol L−1 (1 eq.) Fe3+ was designated as 100. Each column represents mean ± SD (n = 5).

76

H. Yamada et al. / Spectrochimica Acta Part A 90 (2012) 72–77

Fig. 5. Binding ratio of HMB-ASH to Sc3+ by molar ratio (a and b) and continuous variation (c and d) methods.

3.6. Preliminary application to an environmental sample Scandium is a rare earth metal and has been used for developing industrial materials such as alloys and manufacturing devices such as halide light bulbs [15]. Therefore, wastewater containing Sc3+ ions might be discharged from factories into natural water bodies; consequently, environmental water might be contaminated by Sc3+ ions. Steps should therefore be taken to prevent this contamination. The proposed method was used to test tap and river water samples for Sc3+ ions. No Sc3+ was detected in the tap and river samples by using the present method (data not shown). Next, 1.0 ppm (22.2 ␮M) Sc3+ solutions containing tap and river water samples were artificially prepared, and the recovery of Sc3+ was examined. When the tap and river water samples were directly used without SPE, Sc3+ was not sufficiently recovered (Table 2). In contrast, the recovery of Sc3+ from these water samples remarkably improved after SPE using an anion-exchange cartridge. Considering this result, an anion found in the environment, such as F− , might interfere with the detection of Sc3+ because F− was shown to interfere with the fluorescence of the HMB-ASH–Sc3+ complex (Table 1). Thus, the proposed method will be useful for Sc3+ detection in environmental water samples after SPE using an anion-exchange cartridge.

Table 2 Recovery of 1.0 ppm (22.2 ␮M) Sc3+ from tap and river water samples by the proposed method with or without the solid-phase extraction (SPE) procedurea (n = 3). Recovery (%) Tap water Without SPE With SPE River water Without SPE With SPE a

62.3 ± 2.37 95.1 ± 5.89 63.2 ± 2.31 103 ± 1.90

Solid-phase extraction with MonospinTM SAX.

4. Conclusion A fluorescent chelating ligand, HMB-ASH, was newly designed and synthesized. HMB-ASH exhibited selective fluorescence with the Sc3+ ion. Thus, the selective and sensitive determination of Sc3+ ions was achieved by using HMB-ASH as a fluorescent ligand. Acknowledgements The authors thank Dr. Y. Ohshima, Dr. M. Fujii, and Prof. K. Kato of Toho University for their kind advice on the measurement of NMR and IR spectra of HMB-ASH. Thanks are also due to Miss Miyuki Takeshima and Miss Haruyo Shibata of Toho University for their technical assistance with this research. References

Fig. 6. Calibration curve for Sc3+ using HMB-ASH as a fluorescent ligand. Each point represents mean ± SD (n = 5).

[1] J. Nils, J. Kai, ACS Chem. Biol. 2 (2007) 31–38. [2] T. Terai, T. Nagano, Curr. Opin. Chem. Biol. 12 (2008) 515–521. [3] J. Kimura, H. Yamada, H. Ogura, T. Yajima, T. Fukushima, Anal. Chim. Acta 635 (2009) 207–213. [4] J. Kimura, H. Yamada, T. Yajima, T. Fukushima, J. Lumin. 129 (2009) 1362–1365. [5] H. Yamada, A. Shirao, K. Kato, J. Kimura, H. Ichiba, T. Yajima, T. Fukushima, Chem. Pharm. Bull. 58 (2010) 875–878. [6] L. Wu, D.R. Williams, Chem. Spec. Bioavailab. 5 (1993) 61–66. [7] T. Murat, C. Seval, K. Yalcin, J. Therm. Anal. Calorim. 103 (2011) 995–1000. [8] D. Kara, A. Fisher, S.J. Hill, J. Environ. Monit. 9 (2007) 994–1000.

H. Yamada et al. / Spectrochimica Acta Part A 90 (2012) 72–77 [9] B. Tang, Z.Z. Chen, N. Zhang, J. Zhang, Y. Wang, Talanta 68 (2006) 575–580. [10] B. Tang, T.X. Yue, J.S. Wu, Y.M. Dong, Y. Ding, H.J. Wang, Talanta 64 (2004) 955–960. [11] P. Roy, K. Dhara, M. Manassero, J. Ratha, P. Banerjee, Inorg. Chem. 46 (2007) 6405–6412.

[12] [13] [14] [15]

77

C.Q. Jiang, B. Tang, C. Wang, X. Zhang, Analyst 121 (1996) 317–320. B. Tang, M. Du, Y. Wang, X. Zhang, C. Zhang, Analyst 123 (1998) 283–286. F. Hans, H.J. Wang, B. Tang, Chem. Res. Chin. Univ. 18 (2002) 183–188. D.J. Cordier, Mineral Commodity Summaries 2011: Scandium, US Geological Survey, 2011, pp. 140–141.