Evaluating the degree of molecular contact between cellulose fiber surfaces using FRET microscopy

Abstract: The degree of molecular contact, i.e. the contact area on the nanometer scale, between paper fibers is crucial for the van-der-Waals and hydrogen bond adhesion between the fibers and thus for the fiber-fiber bond strength. We apply Förster resonance energy transfer (FRET) to investigate the degree of contact in the distance range of 1–10 nm between pulp fiber bonds and between thin films. The FRET system with DCCH and FTSC as fluorescence dyes has been validated for spectrophotometry and for local imaging with widefield microscopy, using pHema thin films. Bonding between thin films can be detected with this system, however it has not been possible to achieve a significant FRET signal between bonded pulp fibers. Therefore, we conclude that in principle it is possible to quantify the degree of contact between two surfaces on the nanometer scale with the investigated FRET system. For further work on pulp fibers we recommend an exclusively surface active dyeing, as bulk dyeing massively deteriorates the signal to noise ratio which is likely the reason for the low FRET signal found in this work. Graphic Abstract: [Figure not available: see fulltext.].