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

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 (3 σ) 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).