Review on liquid-core waveguide technology and its application for spectroscopic analysis

Liquid core waveguide (LCW) is well known as a sampling, light path and signal enhancement tool for miniaturized or portable instrumentation of spectroscopic analysis such as absorption, fluorescence, and Raman spectroscopy. Total internal reflection (TIR) occurs in various LCW apparatus to increase the optical path and thereby enhance detection sensitivity by several orders of magnitude. Teflon AF is made up of tetrafluoroethylene and 2,2-difluoromethyl-4,5-difluoro-1,3-dioxide. In this review, the working principle, different types, and characteristics of Teflon AF LCW were systematically discussed. The different detection methods based on Teflon AF LCW, including absorbance spectrophotometry, fluorescence spectrophotometry, and Raman spectroscopy, were then discussed in detail. The application fields for Teflon AF LCW were also summarized, involving metallic ions, N, S, pesticide and veterinary residues, protein-based medicines, and other compounds in food, environmental, biological, medicine samples. The current challenges and potential future opportunities for Teflon AF LCW were emphasized to improve their study and application.