Supramolecular interaction of cucurbit[n]urils and coptisine by spectrofluorimetry and its analytical application

The supramolecular interaction of a homologous series of cucurbitn]uril (CBn], n = 5, 6, 7, 8) hosts and coptisine (COP) was studied by spectrofluorimetry. All of the CBn]s were found to react with COP to form 1:1 host-guest stable complexes and the fluorescence intensity of the complexes was greatly enhanced. The apparent association constants of the complexes were 1.44 × 10⁴, 1.28 × 10⁴, 1.86 × 10⁴ and 1.26 × 10⁴ L mol⁻¹ for CB5], CB6], CB7] and CB8], respectively. In addition, CB5] and CB7] exhibited a higher fluorescence signal than CB6] and CB8]. The fluorescence intensity of the complex with CB7] was enhanced 70-fold compared to that of the studied drug itself. Based on the significant enhancement of fluorescence intensity of supramolecular complex, a simple, rapid, highly sensitive, and selective spectrofluorimetric method was developed for the determination of COP in aqueous solution in the presence of CB7]. At the optimum reaction conditions, a linear relationship was obtained in the range from 0.05 to 1700 ng mL⁻¹ with a detection limit of 0.012 ng mL⁻¹. The proposed method was successfully applied for the determination of the drug in urine and serum samples.