  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.
