Fluorescent Detection of Tadalafil Based on Competitive Host–Guest Interaction Using p‑Sulfonated Calix[6]arene Functionalized Graphene

A competitive fluorescence method toward tadalafil detection has been developed based on host–guest recognition by selecting rhodamine B (RhB) and p-sulfonated calix[6]­arene functionalized graphene (CX6–Gra) as the “reporter pair”. Upon the presence of tadalafil to the performed CX6–Gra–RhB complex, the RhB molecules are displaced by tadalafil, leading to a “switch–on” fluorescence signal. The observed fluorescence signal can be used for quantitative detection of tadalafil ranging from 1.00 to 50.00 μM with a detection limit of 0.32 μM (S/N = 3). The inclusion complex of tadalafil and CX6 was studied by molecular docking and the results indicated that a 1:1 host–guest stoichiometry had the lowest ΔG value of −7.18 kcal/mol. The docking studies demonstrated that the main forces including π–π interactions, electrostatic interactions, and hydrophobic interactions should be responsible for the formation of this inclusion compound. The mechanism of the competitive host–guest interaction was clarified. The binding constant (K) of the tadalafil/CX6 complex was more than 5 times greater than that of RhB/CX6.