Turn-on fluorescent capsule for selective fluoride detection and water purification

Area of Science:

• Supramolecular Chemistry
• Analytical Chemistry
• Materials Science

Background:

• Developing selective anion sensors for aqueous environments remains a significant challenge.
• Organic molecular capsules offer potential for host-guest chemistry in water.
• Fluoride detection is crucial for environmental monitoring and health applications.

Purpose of the Study:

• To design and synthesize a novel organic molecular capsule for selective fluoride recognition in water.
• To investigate the binding mechanism and fluorescence response of the capsule towards fluoride.
• To explore the practical application of the capsule in fluoride removal from water.

Main Methods:

• Synthesis of a hemicryptophane capsule composed of a cyclotriveratrylene (CTV) cap and a tris(2-aminoethyl)amine (tren) binding site.
• Characterization of fluoride encapsulation using NMR (1H, 19F), LC-MS, and X-ray crystallography.
• Fluorescence spectroscopy for determining binding constants and detection limits.

Main Results:

• Triple protonation of the hemicryptophane (L) created a capsule ([3H-L]3+) capable of selective aqueous fluoride binding.
• The capsule exhibited a 'turn-on' fluorescence response at 324 nm upon fluoride addition.
• An association constant (KA) of (7.5 ± 0.4) × 10^4 M^-1 and a detection limit of 570 nM were determined.
• The protonated capsule immobilized on silica gel demonstrated efficient removal of fluoride from water.

Conclusions:

• The [3H-L]3+ capsule is the first metal-free system reporting fluoride binding in water via fluorescence.
• This work presents a promising platform for developing fluorescent sensors and adsorbents for fluoride.
• The findings highlight the potential of organic host-guest chemistry for real-world environmental applications.