Control of Teflon AF 2400 Permeability in a Liquid-Core Waveguide by an Ultra-Thin Crosslinked Polyamide Coating

Abstract

In the last few years Teflon AF has emerged as the leading material for implementing waveguiding with an aqueous core because of its low refractive index (nD = 1.29). This low index should make it possible for very low limits of detection to be achieved in Teflon AF as a result of the ability to excite with laser light over an increased area. Detection limits have remained high, however, due in part to the porosity of the material. In this communication we report a significant reduction in the permeability of Teflon AF 2400 capillary walls with the deposition and subsequent treatment of polyelectrolyte multilayers. Alternating layers of polycations and polyanions on a bare Teflon AF surface are sufficient to reduce its permeability to small molecules such as methanol and benzene. Crosslinking and deprotonation of these multilayers further reduces permeability to less than 10% of the permeability value through uncoated TAF. As a consequence, detection limits are reduced. Evidence of these results is presented with gas chromatography-mass spectrometry (GC/MS) and Raman spectroscopic measurements.

A Chromatographic Approach for Fluorescence Reduction in Liquid Raman Analysis

“…First, the LCW material used in this study, Teflont AF-2400, has a hydrophobic nature, and polymer nanovoids make the material permeable to small molecules in solution. 19 The hydrophobicity and permeability features make Teflont AF-2400 suitable for preconcentration purposes, 20 but for LC-LCW Raman approaches these features may lead to unwanted chromatographic band broadening and formation of micro-bubbles, which reduce the efficiency of the optical waveguide. In the present study, however, these unwanted effects were avoided mainly by the choice of aqueous acetonitrile (20%) as the solvent.…”

Quantitative Analysis of Raman Signal Enhancement from Aqueous Samples in Liquid Core Optical Fibers

“…We calculate, using the model presented, that an upgrade to R ϭ 0.995 would increase the Raman signal strength at 888 cm Ϫ1 by a factor of two. In addition to improved fabrication of the waveguides themselves, it is possible to improve reflectivity by additional coating of the inside walls, for example with poly(allylamine hydrochloride) and poly(acrylic acid) as described by Inya-Agha et al 14 We note that, in the limit of high reflectivity, light attenuation becomes asborption-dominated and mono-exponential theory becomes sufficient to predict the Raman enhancement factor.…”