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

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 (n_D = 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.