A pair of FRET dyes designed to measure nano-scale contact and the associated adhesion force

Interfacial adhesion is caused by intermolecular forces that only occur between surfaces at nano-scale contact (NSC) i.e., 0.1-0.4nm. To evaluate NSC and its influence on adhesion, F\"orster resonance energy transfer (FRET) spectroscopy has been used. FRET is a technique capable to measure nanometric distances between surfaces by taking advantage of the interaction amid some specific fluorescence molecules, named donor and acceptor. The F\"orster radius (R0) of the FRET pair indicates the distance detection range (0.5R0-2R0) of the system and, must be selected considering the final purpose of each study. Here, we propose a new FRET pair: 7-Amino-4-methyl-cumarin (C120) and 5(6)-Carboxy-2',7'-dichlor-fluorescein (CDCF) with high quantum yield (QY, QYC120=0.91 and QYCDCF=0.64) and a distance range of 0.6-2.2nm (0.1 mM) specifically developed to measure NSC between soft surfaces. For this, polymeric thin films were bonded using different loads, from 1.5 to 150 bar, to create different degrees of NSC, analyzed by FRET spectroscopy, and later pulled apart to measure their interfacial separation energy (adhesion force). Our experiments showed that NSC increases with the applied pressure in the bonded thin films, leading to higher FRET intensity and adhesion force/separation energy. Thus, we have validated a new FRET pair, suitable to measure the degree of NSC between surfaces and establish a linear relationship between FRET and adhesion force; which can be of interest for any type of study with soft materials interfaces that include NSC and its influence on adhesion, as sealants, adhesives or sensors.