Abstract
Chemosensors have attracted considerable attention among the numerous strategies for detecting organic molecules in water. A turn-off mechanism was previously employed for the construction of a cyclodextrin (CD) chemosensor. This mechanism is greatly effective but has several shortcomings. In order to overcome these shortcomings, new fluorescent chemosensors NC0αCD, NC0βCD, and NC0γCD, which were (7-nitrobenz-2-oxa-1,3-diazol-4-yl)amine-modified α-CD, β-CD, and γ-CD, respectively, were prepared. Their guest selectivities were different from those of previously reported CD chemosensors. Here, the mechanism of new CD chemosensors was investigated using nuclear magnetic resonance (NMR) spectroscopy and molecular mechanics calculations. The fluorescence intensity of NC0βCD and NC0γCD slightly decreased and largely increased, respectively, upon the addition of ursodeoxycholic acid as a guest. This is due to the fact that the fluorophore of NC0βCD moved away to the hydrophilic bulk water to form hydrogen bonds between the host and the guest, while the fluorophore of NC0γCD remained located at the primary hydroxy side of the γ-CD unit to form a stable inclusion complex with hydrogen bonds between the fluorophore and the guest. NC0αCD also acted as a turn-on chemosensor for small guests, which could not be detected by the previous CD chemosensors. The motion restriction of the fluorophore through the generation of inclusion complexes could also contribute to increase in fluorescence intensity.
Highlights
The development of methods for the detection of organic molecules in water is an important avenue of research in many fields including environmental, chemical, biological, clinical, and security areas
Colorless neutral molecules can be detected in water by following changes in fluorescence intensity or color changes of chemosensors
Effects werewere not considered in the chemosensor-guest complexation process, Althoughthe thesolvent solvent effects not considered considered the chemosensor-guest chemosensor-guest complexation
Summary
The development of methods for the detection of organic molecules in water is an important avenue of research in many fields including environmental, chemical, biological, clinical, and security areas. Colorless neutral molecules can be detected in water by following changes in fluorescence intensity or color changes of chemosensors. Colored photophysically active guests can be detected by some methods using a photoinduced electron transfer, charge-transfer, etc., but the methods for the detection of colorless neutral molecules are quite few. A new method for the detection of colorless neutral molecules is desired. Ueno employed a turn-off mechanism for the construction of a cyclodextrin (CD) chemosensor to detect organic molecules. This turn-off mechanism was due to the positional change in a fluorophore from the inside to the outside of the CD cavity. CDs are cyclic oligosaccharides, consisting of six, seven, and eight D-glucopyranose units named as α-CD, β-CD, and γ-CD, respectively [13,14,15,16]
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