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

The supramolecular interaction of a homologous series of cucurbit[n]uril (CB[n], n = 5, 6, 7, 8) hosts and coptisine (COP) was studied by spectrofluorimetry. All of the CB[n]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 CB[5], CB[6], CB[7] and CB[8], respectively. In addition, CB[5] and CB[7] exhibited a higher fluorescence signal than CB[6] and CB[8]. The fluorescence intensity of the complex with CB[7] 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 CB[7]. 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.