Energy transfer—room temperature phosphorescence for the optosensing of transition metal ions

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Abstract

A room temperature phosphorescence (RTP) method for the optosensing of environmentally important transition metals (Hg(II), Cd(II), Pb(II), Zn(II), Cu(II), Ni(II), Co(II) and Fe(III)) in waters is described. The measurement principle is based on the radiationless energy transfer (ET) from the Al-ferron chelate immobilised on Dowex 1X2-200, and acting as the phosphorescent donor, to dithizone dye acting as the acceptor. Dithizone presents an absorption spectrum exhibiting adequate spectral overlap with the RTP emission spectrum of the donor and so giving rise to useful analytical RTP signals.

The method has been developed by means of a flow injection system, where the immobilised Al-ferron chelate is packed into a conventional flow cell placed inside the cell holder of a phosphorimeter. After measurement, the sensing phase can be regenerated using ethanol.

Working under optimized experimental conditions, including 0.1 M sodium acetate/acetic acid, pH 5.0, and an acceptor concentration of 5×10−5 M, the selected metal ions form metal–dithizone complexes. The increase of the RTP signal observed can be related to the increase of the concentrations of the analytes (i.e. to the decrease of the free dithizone concentration). Both RTP intensity and triplet lifetime measurements can be used as analytical signals in the proposed method.

Detection limits (3σ) of 15–33 μg l−1 were found for the metal ions under study using RTP intensity measurements. The precision (R.S.D.) at 0.5 mg l−1 for all analytes was <1.8% (n=5) and the calibration graphs were linear up to 1.5 mg l−1 (maximum concentration assayed).

Introduction

The determination of traces of metal ions in water samples has long been an important subject in environmental analysis. Atomic spectroscopic techniques, such as inductively coupled plasma mass spectrometry (ICP-MS) or atomic absorption spectrometry (AAS) are, nowadays, the most commonly used analytical techniques for such determinations. Unfortunately, those atomic techniques require a considerable cost of acquisition and maintenance and involve taking the samples to the laboratory, with a considerable waste of money and time. In practice, however, a large number of the water samples analysed in routine analysis may not be contaminated by transition metals. For these reasons, the development of cheap and rapid analytical methods that could indicate only those samples which are contaminated (i.e. providing a reliable yes/no response) are today of great interest. So-called screening tests are aimed at such a purpose.

A sample screening test can be considered as an analytical method to identify from a number of samples those which contain one or more analytes above a pre-set concentration level [1], [2]. Only those samples with a concentration level “similar to” or “higher than” an established limit will then be exhaustively analysed. One important aspect of the screening tests is that they can be used to detect either a single analyte or a group of analytes with a common property of interest (e.g. toxicity). This could be of great importance in screening tests for drugs [3], pesticides [4] or toxic metals [5].

Several screening methods for toxic transition metals have been described in the literature. Some are based on atomic spectrometric techniques, including atomic emission spectrometry [6] or mass spectrometry [7]. Unfortunately, as pointed out above, the use of those techniques are expensive. Molecular absorption or luminescence techniques would be cheaper and simpler in order to develop analytical methods for a group of transition metals [5], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Most of these methods used molecular absorption measurements. However, the development of analytical methods for transition metals based on luminescence techniques (e.g. fluorescence or phosphorescence spectrometry) offers great potential, as a consequence of their high sensitivity and selectivity. In this area, methods based on fluorescence measurements have been already reported for groups of transition metals [5], [16], [17], [18].

Room temperature phosphorescence (RTP) methods offer exceptional characteristics in terms of sensitivity and selectivity and some other advantages over fluorescence such as low background phosphorescent emission from the solid supports typically used to immobilise the phosphorescent molecules, and greater difference between excitation and emission wavelengths, etc. Moreover, the relatively long triplet lifetimes could allow the development of simple optical sensing systems based on decay time measurements. These measurements are expected to be more robust than those based on intensity because lifetimes are virtually independent of fluctuations in instrumental parameters, of the sensing layer thickness, and of losses of the luminophor by washing [19]. These advantages of phosphorimetry are most attractive for the use of RTP to develop new optosensing methods [20].

The development of a RTP method for the determination of a given analyte, however may not be easy. In some cases it is not possible to find the appropriate indicator (for example, to date there is no reliable direct phosphorescence method for Hg(II) determination [21]). Such a problem can be avoided by resorting to energy transfer (ET) processes. Provided there is a spectral overlap between the emission of an inert luminescent donor and the absorption spectrum of an acceptor (whose absorption spectral characteristics change in the presence of transition metals), radiationless energy transfer can occur. Methods based on this concept convert colour changes into luminescence (intensity or lifetime) information [21].

In this paper, an optosensor for transition metals (Hg(II), Cd(II), Pb(II), Zn(II), Cu(II), Ni(II), Co(II) and Fe(III)) based on an energy transfer process, has been developed. The phosphorescent Al-ferron chelate immobilised on a Dowex 1X2-200 resin has been chosen as donor, while dithizone has been selected as acceptor. This compound can form complexes with various metal ions and has been previously proposed by Romberg and Müller [10] for the development of a screening test for heavy metals using molecular absorbance measurements in an organic medium.

Section snippets

Reagents and materials

Analytical reagent grade chemicals were employed for the preparation of all the standards and solutions. The chelating reagent 8-hydroxy-7-iodo-5-quinolinesulphonic acid (ferron) was provided by Fluka AG (Switzerland). An Al-ferron chelate solution (1×10−4 M) was prepared as described elsewhere [22].

Dithizone was provided by Fluka. A stock solution of 1×10−4 M dithizone was prepared daily, following a procedure described by Singh et al. [23]. It is well known that dithizone and its metal

Immobilised Al-ferron chelate as a potential energy transfer donor for dithizone

As is well known, the Al-ferron molecule has a rigid structure with a heavy atom (iodine) present in the quinoline moeity. This chelate also contains sulphonic acid groups suitable for fixing the complex to an anionic solid support (Dowex 1X2-200 has been used to immobilise the Al-ferron complex, giving a very strong immobilisation as a consequence of the electrostatic attraction between the ammonium groups of the anionic resin and the sulfonate groups of the complex). As has previously been

Conclusions

The analytical potential of the energy transfer phenomena to overcome the lack of suitable luminophores for a variety of analytes not conventionally associated with RTP measurements has been demonstrated with the development of an optosensor for transition metals. The described optosensing system is based on the use of a phosphorescent sensing material consisting of an Al-ferron chelate (immobilised on Dowex 1X2-200) acting as ET donor and dithizone (sensitive to the presence of transition

Acknowledgements

Financial support from “Plan Nacional de I + D” (Spanish Ministry of Science and Technology) through the project Ref. PPQ2000-1291-C02-02 and from “Plan Nacional de I + D + I de Asturias” (FICYT) through the project Ref. PC-CIS01-13 is gratefully acknowledged.

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