Homochiral Optical Fibers Coated with Nanoscale Films of Metal–Organic Frameworks and Double-Plasmonic Ag and Au for In Situ Enantioselective Detection

Abstract

Chiral recognition and detection of organic compounds represent an important task in the pharmaceutical, biological, and chemical industries since the difference between organic isomers strictly determines their biological activity and medical impact. Common enantioselective analysis performed with sophisticated equipment requires specially trained staff and a longer period of time. In this work, we propose a functional plasmonic fiber for online enantioselective detection and recognition of small organic molecules–cysteine, tyrosine, and omeprazole. Two nanothick layers of plasmon-active metals (Au and Ag) were deposited on the core(s) of a single optical fiber, giving rise to two plasmon absorption bands, which are well evident in transmitted light. The metal surface(s) was grafted with homochiral metal–organic frameworks (HMOFs), with subnanometer chiral pores, which ensured the enantioselective capture of organic enantiomers from their solution. The capture of organic moieties by grafted HMOF(s) (l- or d-MOF-6) results in their preconcentration near the plasmon-active metals, i.e., in the space of plasmonic “evanescent plasmon wave” excitation. As a result of the local enantiomer capture and corresponding changes in the shrouding refractive index, the wavelength shift of the plasmon absorption band(s) position occurs (for one plasmon absorption band in the case of one enantiomer presence or for both plasmon absorption bands in the case of an enantiomer mixture). Such a design of functional double-plasmon-active optical fiber allows the qualitative and quantitative detection of the enantiomers of small organic molecules. The proposed enantioselective detection approach is simple, fast, and based on low-cost equipment.