Linking Adhesion Force to Area in Molecular Contact measured by FRET spectroscopy

Abstract

Adhesion depends on the area in molecular contact (AMC). Though, is extremely difficult to measure AMC, mostly because areas that appear to be in full contact, show no AMC, when observed at higher magnification. Here, we show how AMC can be measured using Förster resonance energy transfer (FRET) spectroscopy, a technique capable of measuring the nanometric distance between interactive surfaces. FRET uses the non-radiative energy transferred between fluorescence molecules (donor and acceptor) to study their exact molecular distance. If the molecules are close enough to each other (molecular contact), a FRET signal can be detected. Through a FRET signal, it is possible to calculate the energy transfer efficiency which indicates the degree of molecular contact between the surfaces. For the experiments, poly(2-Hydroxyethyl methacrylate) thin films labelled with the fluorescence molecules, 7-(diethylamino)coumarin-3-carbohydrazide (donor) and fluorescein-5-thiosemicarbazide (acceptor), were produced at the same dye(s) molar concentration and bonded using different pressure loads. Our results reveal that AMC increases with the pressure applied in the bonded thin films, resulting in higher FRET signal and adhesion force. Hence, in this work, we implement an experimental measuring method for AMC and validate the relationship between AMC (measured with FRET spectroscopy) and adhesion (force energy) within polymeric thin films.