Effects of ionic liquids on fluorescence properties of 4-t-octylphenol in the presence of surfactant or β-cyclodextrin

Both the surfactant and β-cyclodextrin (β-CD) had the sensitizing effect on the fluorescence intensities (FIs) of 4-tert-octylphenol (4-t-OP). β-CD and 4-t-OP could form 1:1 inclusion complex, the association constant was 9.8×103 L/mol and the change of free energy was −5.44 kJ/mol, indicating that the complex formation of 4-t-OP-β-CD was thermodynamically favorable. The absolute fluorescence-quenching values (ΔFa) of the 4-t-OP-ILs system were higher in the presence of β-CD (ΔFa, 113-144 a.u.) than those (ΔFa, 28-33 a.u.) in the absence of β-CD, suggesting the sensitizing effect of β-CD on 4-t-OP-ILs quenching system. With the addition of ILs, the included probabilities of 4-t-OP by β-CD were greatly decreased, leading to the weakly protecting effects of β-CD from the collision quenching of 4-t-OP molecules in aqueous solution, and thus the FIs of 4-t-OP-β-CD were markedly quenched. The bimolecular quenching constants (Kq) by four ILs were all less than 2.0×1010 L/mol, and the fluorescence lifetimes (FLs) of 4-t-OP-β-CD ranged from 4.86 to 3.15 ns in ILs concentration range of 0-0.6 mM. Based on the consistent interpretations from FL, FI and temperature effect data, it can be concluded that the quenching mode of 4-t-OP-β-CD system by ILs is a dynamic quenching mechanism. The results obtained in this study lay theoretical foundation for the detection of trace 4-t-OP or ILs by means of the sensitizing fluorescence-quenching method.