Uranyl fluorescence in acidic solution: quenching effects by tetramethylammonium (TMA⁺)

The quenching of uranyl luminescence by various cation species was studied in aqueous media at low pH. Solutions with different nitrate salts, held at constant uranyl nitrate, nitric acid, and ion concentration, were tested to examine the quenching effects of the cations from the nitrate salts. Alkali metal (Li⁺, Na⁺, Rb⁺) and quaternary ammonium cations (NH₄⁺), (CH₃)₄N⁺ (TMA⁺), (C₂H₅)₄N⁺ (TEA⁺) were investigated. Solutions containing TMA⁺ reduced the lifetime of uranyl fluorescence significantly more than the other cations. Uranyl emission spectra also showed that TMA⁺ increased the complex formation between uranyl and nitrate ions. Fluorescence decay lifetime measurements for most solutions yielded values between 1.4-1.9 μs at 20 °C, while 1.8 M TMA⁺ reduced the lifetime of uranyl fluorescence to 0.6 μs. Decay rate versus concentration data (Stern-Volmer plots) indicated a dynamic quenching process with increasing fluorescence decay rates at higher cation concentrations for Li⁺, TMA⁺, and TEA⁺. The temperature dependencies of the decay rates and the kinetics in D₂O were also examined.