A system of FRET dyes designed to assess the degree of nano-scale contact between surfaces for interfacial adhesion

Interfacial adhesion caused by intermolecular forces only occur between surfaces at nano-scale contact (NSC), i.e., 0.1-0.4 nm and can be evaluated using Forster resonance energy transfer spectroscopy (FRET). For this, a suitable pair of fluorescent dyes must be selected, which spectroscopic properties will determine the FRET system performance. Here, we present a brand-new FRET dye system specifically designed to measure NSC in the distance range relevant for van-der-Waals and hydrogen bonding, i.e., below 1 nm.

We propose the FRET pair: 7-Amino-4-methyl-cumarin (C120) and 5(6)-Carboxy-2',7'-dichlor-fluorescein (CDCF) with high quantum yield (QY, QY_C120 = 0.91 and QY_CDCF = 0.64) and a distance detection range of 0.6-2.2 nm (0.1 mM). Adhered-thin films with increasing NSC degrees are produced with ascending pressure from 1.5 to 150 bar. To validate the proposed FRET measurement, we are correlating the bonded films interfacial adhesion (separation energy) to the measured FRET intensity, indicating its degree of NSC.

We find that the proposed dyes are producing the desired FRET signal in adhered-thin films, for an interaction range of 0.6-2.2 nm, with high sensitivity due to the dye's high quantum yields. The increasing adhesion in these films is only caused by its increase in NSC. We find that the adhesion strength, measured as the separation energy between the films, is correlated to the measured FRET signal. Hence, the introduced FRET system is accurately able to measure the degree of NSC between soft surfaces.