Complexation study of brilliant cresyl blue with β-cyclodextrin and its derivatives by UV–vis and fluorospectrometry

The complexation reactions of brilliant cresyl blue (BCB) with β-cyclodextrin (β-CD), mono[2-O-(2-hydroxypropyl)]-β-CD (2-HP-β-CD), mono[2-O-(2-hydroxyethyl)]-β-CD (2-HE-β-CD), and heptakis(2,6-di-methyl) -β-CD (DM-β-CD) were investigated using UV–vis and fluorospectrometry. The complexation between BCB and CDs could inhibit the aggregation of BCB molecules and could cause its absorbance at 634 nm gradually increasing. The fluorescence of BCB was also enhanced with the addition of CDs. The fluorescence enhancement was more notable in neutral and acidic media than in basic media. Hildebrand–Benesi equation was used to calculate the formation constants of β-CDs with BCB based on the fluorescence differences in the CDs solution. The stoichiometry ratio was found to be 1:1. The complexing capacities of β-CD and its three derivatives were compared and the results followed the order: 2-HP-β-CD > 2-HE-β-CD > DM-β-CD > β-CD. The effect of temperature on the formation of BCB-β-CD inclusion complexes has also been examined. The results revealed that the formation constants decreased with the increase of temperature from 1038.9 to 491.6 l/mol. Enthalpy and entropy values were calculated and the values were −25.77 kJ/mol and 35.04 J/kmol, respectively. The thermodynamic measurements suggest that the inclusive process was enthalpic favor. The release of high-energy water molecules and Van der Waals force played an important role in the inclusive process.