Enhanced fluorescence of triphenylmethane dyes in aqueous surfactant solutions at supramicellar concentrations--effect of added electrolyte

The colour change of triphenylmethane (TPM) dyes induced by surfactants at concentrations much greater than their critical micellar concentrations is found to be accompanied by enhanced fluorescence. Thus, the otherwise weak fluorescence of TPM dyes can be detected using supramicellar surfactant concentrations. In this respect, the nonionic polyoxyethylene (POE) chain-containing surfactants are found to be more efficient compared with ionic surfactants. The POE surfactants, Triton X-100, Tween-20 and Tween-60 present a polymer-like surface to the dyes, which can thus easily bind to them. At supramicellar concentrations, the hydrophobic environment formed in these micelles is effective in preventing nonradiative relaxation processes of the dyes. As a result, there is enhanced fluorescence for even micromolar concentrations of the dyes. Among the Tween series, Tween-60 being more hydrophobic leads to greater fluorescence enhancement than Tween-20. From the fluorescence properties, binding constants for dye binding to the surfactants can be determined. Thus the relative efficiency of these surfactants as binding substrates can be assessed. Another interesting observation is that the electrolyte LiCl in presence of the surfactants leads to even larger fluorescence enhancement than the surfactants alone.