Comparative effect of cyclodextrin nanocavities versus organic solvents on the fluorescence of carbamate and indole compounds

Abstract

The effect of the addition of different amounts of organic solvents ( S) on the fluorescence of aromatic compounds ( C) and their inclusion complexes with β-cyclodextrin ( βCD) and hydroxypropyl-β-cyclodextrin ( HPCD) has been examined using steady-state measurements. Carbamate pesticides with different aromatic moiety, such as carbofuran ( CF), promecarb ( PC), carbaryl ( CY) and bendiocarb ( BC) were used, as well as indole derivatives with different polarity in their lateral chains, such as melatonin ( M, neutral), 5-methoxytryptamine ( MT, cation) and auxin ( IA, anion). Their complexes in water show a fluorescence signal higher than that obtained for the free substrates in solvent:water mixtures (30%, v/v n-propanol or acetonitrile, and 50%, v/v methanol). The isofluorescent point ( IF), the %IF and the F 85% are defined in order to evaluate the use of CD nanocavities as a non-polluting alternative for the analysis of the compounds analyzed. Apparent formation constants ( K AP, M −1) for the complexes of C:HPCD at different solvent percentages were determined for CF and PC with methanol ( MeOH), n-propanol ( ProOH) and acetonitrile ( ACN), and for indole compounds with ACN. A decrease in the K AP values for the CF:HPCD (120–30) and PC:HPCD (2000–400) complexes occurs in accordance with the solvent affinities for CDs ( MeOH < ACN < ProOH). Nevertheless, in the indolic series, the polar characteristics of MT, IA and M determine their behaviour in the presence of ACN. For the neutral substrate M, K AP decreases with the increasing percentage of ACN (100–10). In contrast, for IA and MT (ionic substrates) K AP increases (10–100). These results may be accounted for by two different mechanisms: the competition between C and S for the cavity of the receptor or the formation of ternary complexes C:S:CD with additional stabilization.