ARTICLE pubs.acs.org/JPCA
Ionic Quenching of Naphthalene Fluorescence in Sodium Dodecyl Sulfate Micelles Alessandra F. Silva, Haidi D. Fiedler,* and Faruk Nome Department of Chemistry, National Institute of Catalysis, INCT-Cat., Federal University of Santa Catarina, Florianopolis SC 88040-900, Brazil
bS Supporting Information ABSTRACT: Micellar effects on luminescense of organic compounds or probes are well established, and here we show that quenching is highly favored in aqueous sodium dodecyl sulfate (SDS) micelles, which concentrate a naphthalene probe and cations of lanthanides, transition metals, and noble metals. Interactions have been studied by steady state and timeresolved fluorescence in examining the fluorescence suppression of naphthalene by metal ions in anionic SDS micelles. The quenching is collisional and correlated with the unit charge and the reduction potential of the metal ion. The rate constants, calculated in terms of local metal ion concentrations, are close to the diffusion control limit in the interior of SDS micelles, where the microscopic viscosity decreases the transfer rate, following the Stokes-Einstein relation.
’ INTRODUCTION Micellar binding of probes accelerates their fluorescence quenching by counterions in aqueous solutions and increases in local concentrations and medium effects may enhance the efficiency of quenching kinetically and thermodynamically. In a typical example, several metal ions were used as fluorescence quenchers of naphthalene in aqueous micellar sodium dodecyl sulfate (SDS), with quenching by metal ions showing typical Stern-Volmer behavior, and apparent Stern-Volmer constants decrease in the order Fe3þ > Cu 2þ > Pb2þ > Cr3þ > Ni2þ. Similarly, efficient energy transfer from naphthalene to terbium chloride in aqueous micellar sodium dodecyl sulfate was observed and was examined in detail.1,2 Fluorescent probes have been used for quantitative identification of metals in natural waters and biological systems and metalprobe interactions can enhance or decrease fluorescence.1,2 Surfactants can form micelles that act as “microreactors”, compartmentalizing, concentrating, or separating and diluting reactants and thereby altering, sometimes dramatically, apparent rate and equilibrium constants of chemical reactions.2-4 Micelles bind and solubilize a wide variety of compounds, ranging from hydrophobic hydrocarbons to inorganic ions and hydrated electrons.5,6 Micellar effects on luminescent proprieties of organic compounds or probes are well established.7 However, in the presence of metal ions, luminescence results showing either enhancement or quenching are difficult to interpret because photophysical processes and effective mechanisms in these systems are not well characterized.1,2,8 In this work we show that quenching is highly favored in aqueous SDS micelles, which effectively concentrate both a naphthalene probe and cations of lanthanides, transition metals, r 2011 American Chemical Society
and noble metals. Interactions have been studied by steady state and time-resolved fluorescence measurements to examine the dynamics of the fluorescence quenching.
’ EXPERIMENTAL SECTION Reagents. Ampoule standard solutions containing 1000 mg of Fe3þ, Al3þ, Pb2þ, and Cu2þ (Fluka, Steinheim, Germany) were prepared by dilution into 1000 mL containing 2% v/v nitric acid (65% v/v Suprapur, Merck, Darmastadt, Germany). Standard solutions containing 1000 mg L-1 of Cr3þ (Acros Organics, Geel, Belgium) and Zn2þ, Ni2þ, Cd2þ, and Sr2þ (Merck, Darmstadt, Germany) were used with appropriate dilutions. A Fe2þ solution was prepared by weighing FeSO4 3 7H2O (Merck, Darmstadt) to obtain 1000 mg dm-3 of Fe2þ with equimolar ascorbic acid (Vetec, Rio de Janeiro), to keep the metal in a reduced state. LaCl3, SmCl3, ErCl3, and TbCl3 were from Across Organics (Geel, Belgium) and EuCl3, AuCl3, and PdCl2 from Sigma-Aldrich (Steinheim). All other reagents were of the best available analytical grade. Doubly deionized water with conductance
