Luminescence Linewidth Broadening and Nonradiative Energy Transfer Studies of Solid UO+-Datura HUEI-YANG D. KE and GARY D. RAYSON* Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003 The emission spectra and fluorescence decay curves of solid UO~+-Datura at liquid nitrogen temperature have been measured. The linewidth of the emission peaks of UO~ ÷ ions in UO~÷-Datura decreases with the UO~ ÷ concentration. This linewidth broadening phenomenon can be ex- plained by the existence of resonance interactions between adjacent UO]+-Datura species. The analysis of the emission peak position of the bound ions has been used to provide a measure of the electronic factors contributing to the interaction between the uranyl ion and phosphoryl and dicarboxyl moieties on the cell wall material. An observed blue shift of the uranyl fluorescence spectrum as a function of solution pH has been ascribed to a distortion of the normally linear O-U-O bond. An inter- and intra-molecular nonradiative energy transfer model has been successfully used to interpret the measured lifetime data of UO~ ÷- Datura. Index Headings: Laser-induced fluorescence spectroscopy; Uranyl lu- minescence; Fluorescence lifetime; Linewidth broadening; Resonance interaction; Luminescence spectral analysis; Nonradiative energy trans- fer; Biomass material. INTRODUCTION Nonliving biomass materials have been successfully used to remove heavy toxic and economically important metal ions from wastewaters and mining effluents by means of biosorption (adsorption). ~-~ The majority of these investigations have been focused on algal bio- masses. However, previous studies 6-9 have demonstrated that the same kind of biosorption could also occur on cultural cells from a higher-order plant, Datura innoxia. D. innoxia was selected for these studies because it belongs to the well-studied family Solanaceae (e.g., po- tatoes, tomatoes, and tobacco). To increase the proba- bility of isolating a pure cell line, we selected anther cells from the plant's flower. Due to the inherent complexity of higher-ordered (i.e., multicelled) plants, it was desir- able to investigate the binding of metals to cell-wall func- tionalization without the complication of multiple types of cells. Previously, phosphoryl and dicarboxyl groups have been demonstrated to be the dominant functional groups responsible for the binding of uranyl ions on D. innoxia cell walls. 9 The goal of this paper is to investigate the linewidth broadening phenomena and nonradiative en- ergy transfer in solid UO2+-Datura samples. This infor- mation will enable the ion-ion interactions between met- al-containing macromolecules in a complicated biological system to be better understood. The ground state of the uranyl ion is a singlet because of the even number of electrons and the diamagnetism. Thus, a transition of one electron could produce two unpaired electron spins, giving a triplet excited state. ~° Received 13 April 1992. * Author to whomcorrespondenceshouldbe sent. It has been reported 1°-12that the absorption spectrum of the UO 2+ ion is composed of seven bands which extend from the visible to the ultraviolet. These absorption tran- sitions have been postulated to result from electron transfers within, or from, the nearly linear, covalent O-U-O bond having a significant 7r-bond contribution. 1° The fluorescence emission spectrum of UO 2+ is mainly emitted from two excited levels at 21,270 and 20,502 cm -1 to five vibrational levels in the ground state? °~12 The five lowest-energy fluorescence emission bands have an av- erage spacing of 855 cm-L These spectral features have been reported for all uranyl compounds in solution or the solid state. This observation suggests that the main features of the spectra of the uranyl compounds arise from the structure of the uranyl ion. The differences in the structure of the spectra are ascribed to the interac- tion of U022+ with other components of the molecule complexes and the influence of the corresponding crystal fields, n The energy gap between the ground multiplet and the lower-lying excited state is governed by the in- terelectronic repulsions within the UO 2+ ion. Therefore, any subtle change in energy gap due to these effects could be directly detected by examining the emission peak po- sition shift of the U022+ ion. In this paper, the interaction between UO 2+ and phosphoryl, or dicarboxyl function- alities, on D. innoxia cell walls will be investigated with the use of U022+ luminescence. EXPERIMENTAL Materials and Methods. UO~+-containing solutions were directly prepared from solid uranyl nitrate (E.M. Science). A 0.1 M 2-(N-morpholino) ethanesulfonic acid (MES) (Sigma Chemical Company) was used as a non- complexing buffer. 13,14 Sodium hydroxide (Mallinckrodt) and nitric acid (Baker) were used to adjust solutions to the desired pH. Distilled, deionized water was used throughout the work. The preparation procedure for the D. innoxia biomass material has been described previ- ously2 Solutions containing various concentrations of UO~+ with 5 mg of D. innoxia mL -1 in 0.1 M MES buffer at pH 5 were directly prepared from the uranyl nitrate stock solutions. After agitation of the suspensions for 1 h to establish equilibrium, the solutions were centrifuged. The solid UO~+-Datura samples were air dried and stored for subsequent UO~+ luminescence measurements. Solid UO~+-Datura samples prepared from 1 mM UO~+ so- lution at different pH conditions were similarly dried and stored. Uranyl hydroxide samples were prepared by directly reacting a concentrated sodium hydroxide so- lution with uranyl nitrate stock solution. The solid sam- ples were similarly dried and stored. 1376 Volume 46, Number 9, 1992 0003-7028/92/4609-137652.00/0 APPLIED SPECTROSCOPY © 1992 Society for Applied Spectroscopy