Functionalizing Surfaces by Physical Vapor Deposition To Measure the Degree of Nanoscale Contact Using FRET

Adhesion between solid materials is caused by intermolecular forces that only take place if the adhering surfaces are at nanoscale contact (NSC) (i.e., 0.1–0.4 nm. To study adhesion, NSC can be evaluated with Förster Resonance Energy Transfer (FRET). FRET uses the interaction of compatible fluorescence molecules to measure the nanometer distance between bonded surfaces. For this, each surface is labeled with one fluorescence dye, named the Donor or Acceptor. If these molecules are in NSC, a nonradiative Donor–Acceptor energy transfer will occur and can be detected using FRET spectroscopy. Here, for the first time, we introduce an innovative concept of a FRET-based NSC measurement employing dye-nanolayer films prepared by a physical vapor deposition (PVD). The dye nanolayers were prepared by PVD from the vaporization of the Donor and Acceptor molecules separately. The selected molecules, 7-Amino-4-methyl-cumarin (C120) and 5(6)-Carboxy-2′,7′-dichlor-fluorescein (CDCF), present high quantum yields (QY, QY_D = 0.91 and QY_A = 0.64) and a low FRET distance range of 0.6–2.2 nm, adequate for the study of NSC. The produced dye-nanolayer films exhibit a uniform dye distribution (verified by atomic force microscopy) and suitable fluorescence intensities. To validate the NSC measurements, FRET spectroscopy experiments were performed with bonded dye-nanolayer films prepared under different loads (from 1.5 to 140 bar), thus creating different degrees of NSC. The results show an increase in FRET intensity (R² = 0.95) with the respective adhesion energy between the films, which is directly related to the degree of NSC. Hence, this work establishes FRET as an experimental technique for the measurement of NSC, and its relation to surface adhesion. Additionally, thanks to the FRET dye-nanolayer approach, the method can be employed on arbitrary surfaces. Essentially, any sufficiently transparent substrate can be functionalized with FRET compatible dyes to evaluate NSC, which represents a breakthrough in contact mechanics investigations of soft and hard solid materials.