Insights into the recognition of dimethomorph by disulfide bridged β–cyclodextrin and its high selective fluorescence sensing based on indicator displacement assay

In this work, the molecular recognition of dimethomorph by disulfide bridged β–cyclodextrin (SS–β–CD) was studied by UV spectroscopy , 2 D NMR, and molecular modeling . The results indicated that the SS–β–CD/dimethomorph was more stable than β–CD/dimethomorph, which is ascribed to the fact that the disulfide chain plays an important role in stabilizing the appropriate dual-CD conformation and also promoting the inclusion of the host and guest. In addition, a robust fluorescence method for dimethomorph sensing has been developed based on competitive host–guest interaction by selecting safranine T (ST) as optical indicator and SS–β–CD functionalized reduced graphene oxide (SS–β–CD–RGO) as the receptor. Upon the presence of dimethomorph to the pre-formed SS–β–CD–RGO·ST complex, the ST molecule is displaced by dimethomorph, leading to a “switch–on” fluorescence response. That is due to the fact that the binding constant of the dimethomorph/SS–β–CD complex was more than 5 times greater than that of ST/SS–β–CD. The fluorescence intensity of SS–β–CD–RGO·ST complex increased linearly with increasing concentration of dimethomorph ranging from 0.50 to 20.0   μM. The proposed method showed a detection limit of 0.11   μM for dimethomorph, and was successfully applied for the determination of dimethomorph residues in vegetables (cabbage, spinach) and environmental samples (water, soil) with good precision and recoveries from 96.5% to 104%.