Determination of heavy metal ions based on quenching of the rare earth luminescence

Sensors and Actuators B 108 (2005) 832–835

Determination of heavy metal ions based on quenching of the rare earth luminescence Tsuyoshi Arakawa∗ , Akiko Muraki, Mizuki Hashimoto Department of Biological and Environmental Chemistry, School of Humanity-oriented Science and Engineering, Kinki University, Kayanomori 11-6, Iizuka, Fukuoka 820-8555, Japan Received 13 July 2004; received in revised form 15 November 2004; accepted 19 November 2004 Available online 9 January 2005

Abstract The determination of heavy metal ions based on quenching of the rare earth luminescence was investigated. A few heavy metal ions efficiently quenched the luminescence of Ce3+ in polymerized cellulose films, which were obtained by Na-CMC (sodium carboxymethyl cellulose) solution and CeCl3 solution or the mixed solution of CeCl3 and a heavy metal chloride. Also, the decay time of Ce3+ ions (τ) in Ce3+ –Cu+ , Ce3+ –Cu2+ , Ce3+ –Zn2+ and Ce3+ –Pb2+ system was shorter than that of only Ce3+ system (τ 0 ). The linear relationship between the concentration of Cu2+ , Zn2+ or Pb2+ ions andτ 0 /τ reflected the dynamic quenching, as predicted by the Stern–Volmer equation. © 2004 Elsevier B.V. All rights reserved. Keywords: Heavy metal ions; Rare earth; Luminescence; Decay time

1. Introduction Many rare earth complexes attract the interest as fluorescent probes in detection applications. The structural optimization of the rare earth complexes enhanced the availability in the molecular recognition and chirality sensing of biological substances [1,2]. It was reported that terbium chelate was used as a label in fluorescent immunoassays [3]. The polymers including the rare earth complex were synthesized, and the fluorescence was examined with the aim of the application to laser materials, etc. [4]. The hydrolysis product of the nerve agent Soman in water could be measured by the luminescence of Eu3+ -polymer complexes containing methyl-3,5-dimethylbenzoate ligand [5]. We reported that the copper ions (Cu+ or Cu2+ ) could be detected using the fluorescence property of rare earth element (Eu3+ ) in cellulose films, which was prepared from sodium carboxymethyl cellulose (Na-CMC) solution and the mixed solution of heavy metal chloride and europium chloride [6]. In this paper, we ∗

Corresponding author. Tel.: +81 948 22 5655; fax: +81 948 23 0536. E-mail address: [email protected] (T. Arakawa).

0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.11.060

demonstrated the ability to detect heavy metal ions (Pb2+ or Zn2+ ) using the luminescence of cellulose films by assisting of rare earth ions (Ce3+ ).

2. Experimental The cellulose film was prepared by the use of the 30 ml glass bottle container of which two parts was separated by a 100-mesh nylon cloth. The rare earth chloride (CeCl3 ) solution (10–100 mM, 5 ml) or the mixed solution of heavy metal chloride and rare earth chloride was put on the bottom of container. There was the viscous Na-CMC solution (4 mM, 2 ml) on a nylon cloth. Then the container was turned up side down and the solution contacted with each other through nylon cloth for 1 min, followed by the formation of cellulose film containing Ce3+ ions and heavy metal ions. Moreover, the fluorescence spectrum of a film prepared from mixed solution of GdCl3 (100 mM) and CuCl2 (0–30 mM) was also prepared in order to examine the cause of the decrease of the fluorescence intensity. The nylon mesh cloth (20 mm ∅ × 0.3 mm) attaching cellulose thin film was removed from the container,

T. Arakawa et al. / Sensors and Actuators B 108 (2005) 832–835

833

washed and dried in a desiccator. The emission and excitation spectra were recorded by a Hitachi recording absolute spectrofluorophotometer (F-4500) at room temperature. The life times were measured with Nd:YAG laser (266 nm) having pulses of less than 5 ns.

3. Results and discussion 3.1. The luminescence of Ce3+ in the cellulose film The luminescence of Ce3+ ions (5d → 4f transition) was strong and simple. Also the photo-excitation energy of Ce3+ is larger than that of Eu3+ . In this paper, europium ion changed to cerium ion as fluorescent probes. Photoluminescence spectra of Ce3+ for a Ce3+ -cellulose film and some Ce3+ –Mn+ (M: heavy metal) cellulose films under the excitation of 255 nm are shown in Fig. 1. The optimum excitation wavelength was same in every case. The emission band centered at 350 nm, which is well explained in terms of the dipole allowed 5d → 4f transition, is observed. The effect of varying Ce3+ ion concentration of cellulose films on the emission band is presented in Fig. 2. The abscissa indicates the concentration of CeCl3 when a cellulose film was prepared. At first, the intensity of emission band increased logarithmically with the concentration of Ce3+ ions in cellulose film up to ca. 0.25 mM/g (this point was corresponded to the concentration of solution for preparing a film), and then became almost constant. If the Ce3+ -containing cellulose film was prepared together with PbCl2 , ZnCl2 or CuCl2 , the intensity of emission band of Ce3+ ions decreased as shown in Fig. 1. Moreover, the intensity of the emission band decreased with the concentration of these heavy metal chlorides as shown in Fig. 3. The concentration of Pb2+ , Zn2+ , and Cu2+ in the cellulose film became almost constant over 50 mM for the concentration of metal chloride solutions under experimental conditions. When the concentration of heavy metal ions and Ce3+ ions in a Ce3+ –M2+ cellulose film was measured

Fig. 1. Emission spectra of cellulose films containing Ce3+ ions. The film was prepared by the use of (a) CeCl3 (100 mM) solution, (b) the mixed solution of CeCl3 (100 mM) and PbCl2 (50 mM) and (c) the mixed solution of CeCl3 (100 mM) and ZnCl2 (50 mM).

Fig. 2. Emission intensity and the content of Ce3+ ions in a cellulose film vs. the concentration of CeCl3 solution.

by X-ray fluorescence analysis, the cerium ion concentration was almost constant about 0.25 mM/g for every case. On the other hand, there was the increase of the M2+ concentration in the cellulose film during the increase of M2+ ions of solution and then the M2+ ion concentration reaches almost constant value (ca. 0.20 mM/g) at 50 mM solution, after which the concentration slightly increased up to ca. 0.24 mM/g at 100 mM solution. There was not large difference of M2+ concentration in a Ce3+ –M2+ cellulose film between three heavy metal ions. The quenching effect was stronger in the lower concentration range of these heavy metal ions. The effect on the intensity of luminescence under coexistence of heavy metal ions were summarized in Table 1, together with the results of decay time as described later. When Cu2+ , Fe3+ , Pb2+ , Zn2+ coexisted in the cellulose film, the luminescence was extremely decreased. In general, the luminescence of rare earth ion was strongly affected by Fe3+ ions as large absorption by Fe3+ ions exists from near 450 nm. Therefore, in this paper the Ce3+ –Fe3+ system was not discussed. In the case of the copper ion, the decrease of fluorescence intensity of Ce3+ for the concentration of Cu2+ was almost equal to that of Eu3+ of Eu3+ –Cu2+ system as shown in Fig. 4. However, in the case of Gd3+ ion with the emission peak of 310 nm under the excitation of 276 nm, it was found that the decrease of Gd3+ fluorescence intensity was largest in lower concentration range

Fig. 3. Dependence of emission intensity of Ce3+ for Ce3+ –Zn2+ , Ce3+ –Pb2+ and Ce3+ –Cu2+ cellulose films on the content of cation. The cellulose films prepared using aqueous solution of which changed the concentration of the heavy metal chloride by fixing the concentration of CeCl3 (100 mM).

834

T. Arakawa et al. / Sensors and Actuators B 108 (2005) 832–835

Table 1 The relative intensity (I0 /I) of 350 nm emission band and the relative decay time (τ 0 /τ) of Ce3+ ions for Ce3+ –Mn+ system Mn+

I0 /Ia

τ 0 /τ b

Cu2+

2.5 1.0 1.0 1.0 5.0 1.7 1.7

3.1 1.0 1.0 1.1 2.3 1.7 1.7

Co2+ Mn2+ Cr3+ Fe3+ Zn2+ Pb2+ a

The values were compared at 0.25 mM/g of Ce3+ and 0.2 mM/g of Mn+ . The value, τ 0 was 3.4 × 10 ns. These values were compared at 0.25 mM/g of Ce3+ and 0.2 mM/g of Mn+ , except for Ce3+ –Cr3+ (0.1 mM/g) and Ce3+ –Fe3+ (0.1 mM). b

of Cu2+ among three systems. These results were discussed later. On the other hand, it is well known that the intensity of luminescence for the cerium complexes is affected by the ligand [7]. Also, heavy metals are known to be strong quenchers of luminescence [8]. The quenching of perdeuterated naphthalene by paramagnetic ions (Co2+ , Cr3+ , Cu2+ , Ni2+ ) was explained as the exchange-type interaction between the triple state molecules and the paramagnetic ions. But, the decrease in the luminescence of Ce3+ –Co2+ and Ce3+ –Cr3+ was hardly observed, although Co2+ and Cr3+ are paramagnetic. Since Zn2+ or Pb2+ is not paramagnetic, it could not induce quenching by interaction from the viewpoint of Hoijtink’s mechanism [8], but it may cause quenching by spin–orbit interaction due to its large atomic number. In our measurements with Zn2+ or Pb2+ , the decrease of fluorescence intensity of Ce3+ was observed. 3.2. The decay time of Ce3+ in the cellulose film The decay times and amplitudes of cerium luminescence for a Ce3+ –Cu2+ and Ce3+ system are shown in Fig. 5. In the presence of Cu2+ ions, the decay time τ (slope) was smaller than that of only Ce3+ system (τ 0 ). At the same time, the decrease of the amplitude was almost consistent with that

Fig. 4. Dependence of emission intensity of rare earth ions for (1) Ce3+ –Cu2+ , (2) Eu3+ –Cu2+ and (3) Gd3+ –Cu2+ cellulose films on the content of Cu2+ ions. The cellulose films prepared using aqueous solution, which changed the concentration of the CuCl2 by fixing the concentration of LnCl3 (100 mM, Ln:Ce, Eu, Gd).

Fig. 5. Luminescence decay curves of a Ce3+ cellulose film and a Ce3+ –Cu2+ cellulose film.

of the luminescence intensity estimating by I0 /I. The same phenomena were observed in the Ce3+ –Pb2+ and Ce3+ –Zn2+ system. In comparison of Ce3+ –Pb2+ with Ce3+ –Zn2+ system, the difference was hardly appeared in the decay time. The changes in decay time and amplitude indicate the coexistence of static and dynamic quenching. The correlation between the concentration of these ions and τ 0 /τ are shown in Fig. 6. That is, the linear relationship in the range of lower concentration reflects the dynamic process, as predicted by the Stern–Volmer equation [9], whereas the decreasing of amplitude (or luminescence intensity) is mainly a result of static quenching. The relative decay times (τ 0 /τ) for other heavy metal ions are summarized in Table 1. According to the Stern–Volmer equation, the extent of quenching is proportional to the decay time of the cerium luminescence. The absorption spectrum of Fe3+ , which was a dynamic quencher, has a higher overlap with the emission spectrum of the cerium ion. On the other hand, it was expected that in the Ce3+ –Pb2+ and Ce3+ –Zn2+ system the dynamic quenching and the decrease of the decay time were not observed, because none of the completely colorless ions showed dynamic quenching. In the Eu3+ –Pb2+ and Eu3+ –Zn2+ system, quenching and the decrease of decay time were not clearly observed as reported elsewhere [6]. Although the mechanism of quenching

Fig. 6. Calibration graph for Ce3+ –Cu2+ , Ce3+ –Pb2+ and Ce3+ –Zn2+ in the linearized form of a Stern–Volmer plot.

T. Arakawa et al. / Sensors and Actuators B 108 (2005) 832–835

835

luminescence depended on the kind of the heavy metal ion. In the Ce3+ –Cu2+ , Ce3+ –Pb2+ and Ce3+ –Zn2+ system, the relative decay time linearly varied with the concentration of these heavy metal ions in the range of lower concentrations. This method based on quenching of cerium luminescence may be useful for the determination of the heavy metal.

References

Fig. 7. The energy levels and the fluorescent wavelength of Eu3+ (4f7 ), Ce3+ (4f1 ) and Gd3+ (4f7 ) ions. The value ∼320 nm is the wavelength of the adsorption edge got from UV spectrum of the CuCl2 aqueous solution (100 mM).

for Ce3+ –M2+ system could not be clarified, the clear quenching of Ce3+ –Pb2+ and Ce3+ –Zn2+ system may be due to the larger photo-excitation energy; the photo-excitation energy of Ce3+ –M2+ system (Ex = 255 nm) was larger than that of Eu3+ –M2+ system (Ex = 395 nm). Fig. 7 shows the energy levels of Eu3+ (4f7 ), Ce3+ (4f1 ) and Gd3+ (4f7 ) ions. The fluorescent wavelength of Ce3+ , Eu3+ and Gd3+ is consistent with Figs. 1, 2 and 4 of Ref. [6], respectively. In the figure, the wavelength (320 nm) of adsorption edge for Cu2+ ion is also indicated. For Gd3+ –Cu2+ system of which the emission level and the excited level are higher than the energy level of the adsorption edge, the quenching effect was strongest in the lower concentration range of Cu2+ among three systems. In the case of Eu3+ –Cu2+ and Ce3+ –Cu2+ system, a part of the excited level or the light emission level overlap with the energy of the absorption edge of Cu2+ as described before. Thus, it would be suggested that the quenching effect of the fluorescence of the rare earth ion was dependent on the excited level and light emission level in comparison with the energy level of adsorption edge of heavy metal ions. In Pb2+ or Zn2+ ions which the d orbital has satisfied, one of 3d or 4d electron would be excited in 4 or 5 s orbital when Pb2+ or Zn2+ ions was excited by 255 nm light. In the Ce3+ –Pb2+ and Ce3+ –Zn2+ system the early energy transfer process going through excitation state of Pb2+ or Zn2+ ions would predominantly occur. The operation of the method presented is simple. These results suggest that the present method has the ability of the detection of transition metal ions in environmental waters, including river water and industrial wastewater.

4. Conclusion The luminescence and the decay time of Ce3+ ions in cellulose polymer film which was prepared by Na-CMC solution and CeCl3 solution or the mixed solution of CeCl3 and a heavy metal chloride, was studied. The quenching of cerium

[1] E.P. Diamandis, T.K. Christopoulos, Europium chelate labels in timeresolved fluorescence immunoassays and DNA hybridization assays, Anal. Chem. 62 (1990) 1149A–1157A. [2] T. Yamada, S. Shinoda, J. Uenishi, H. Tsukube, Stero-controlled substitution on tris(2-pyridylmethyl) amine ligands and chirality tuning of luminescence in their lanthanide complexes, Tetrahedron Lett. 42 (2001) 9031–9033. [3] M.P. Baily, B.F. Rocks, C. Riley, Terbium chelate for use as a label in fluorescent immunoassays, Analyst 109 (1984) 1449–1450. [4] Y. Ueba, E. Banks, Y. Okamoto, Investigation on the synthesis and characterization of rare earth metal-containing polymers. II. Fluorescence properties of Eu3+ -polymer complexes containing ␤-diketone ligand, J. Appl. Polym. Sci. 25 (1980) 2007–2017. [5] A.L. Jenkins, O.M. Uy, G.M. Murray, Polymer-based lanthanide luminescence sensor for detection of the hydrolysis product of the nerve agent Soman in water, Anal. Chem. 71 (1999) 373–378. [6] T. Arakawa, M. Akamine, Determination of transition metal ions based on quenching of the rare earth luminescence, Sens. Actuators B 91 (2003) 252–255. [7] C.O. Hill, S.H. Lin, Quenching of phosphorescence by paramagnetic molecules in rigid media. II. Quenching of perdeuderated naphthalene by Co2+ , Cr3+ , Cu2+ , and Ni2+ , J. Chem. Phys. 53 (1970) 608– 612. [8] G.J. Hoijtink, The influence of paramagnetic molecules on singlet–triplet transitions, Mol. Phys. 3 (1960) 67–70. [9] O.S. Wolfbeis, in: Schulman (Ed.), Methods and Applications. Part II. Molecular Luminescence Spectroscopy, Wiley, New York, 1988, pp. 202–212.

Biographies Tsuyoshi Arakawa received his BE in industrial chemistry in 1971 from Miyazaki University and PhD in engineering in 1978 from Kyushu University. He had been an assistant professor at Osaka University since 1977 and moved to Kinki University, Kyushu. He has been a professor there since 1992. His current research is focused on optical chemical sensors based on laser-excited surface plasmon resonance, detection of chemical species with luminescent rare earth complexes, semiconductive gas sensors having heterocontacts and preparation of new intermetallic compounds by a mechanical alloying method.

Akiko Muraki received her BE in biological and environmental chemistry in 2002 from Kinki University, Kyushu. She was a student in Graduate School of Industrial Engineering Science, Kinki University, and was interested in determination of heavy metal ions based on quenching of the rare earth luminescence.

Mizuki Hashimoto received her BE in biological and environmental chemistry in 2004 from Kinki University, Kyushu. She was a student in Graduate School of Industrial Engineering Science, Kinki University, and was interested in determination of heavy metal ions based on quenching of the rare earth luminescence.