Quantification and Imaging of Nanoscale Contact with Förster Resonance Energy Transfer

Abstract

Adhesion is caused by molecular interactions that only take place if the surfaces are in nanoscale contact (NSC); i.e., the distance between the surfaces is in the range of 0.1–0.4 nm. However, there are several difficulties measuring the NSC between surfaces, mainly because regions that appear to be in full contact at low magnification may show no NSC when observed at higher magnifications. Thus, the measurement area of NSC is very small with imaging techniques, and an experimental technique to evaluate NSC for large contact areas has not been available thus far. Here, we are proposing Förster resonance energy transfer (FRET) spectroscopy/microscopy for this purpose. We demonstrate that NSC in a distance range of 1–10 nm can be evaluated. Our experiments reveal that, for thin films pressed under different loads, NSC increases with the applied pressure, resulting in a higher FRET signal and a corresponding increase in adhesion force/energy when separating the films. Furthermore, we show that local variations in molecular contact can be visualized with FRET microscopy. Thus, we are introducing a spectroscopic technique for quantification (FRET spectroscopy) and imaging (FRET microscopy) of NSC between surfaces, demonstrated here for the application of surface adhesion. This could be of interest for all fields where adhesion or nanoscale surface contact are playing a role, for example, soft matter, biological materials, and polymers, but also engineering applications, like tribology, adhesives, and sealants.

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

“…4−8 Therefore, the adhesion between two surfaces is proportional to its NSC area, and can only be properly investigated if the associated degree of NSC is correctly evaluated. 9 However, we still cannot gain insight into it due to the physical difficulties of looking directly at the NSC. Here, we tailored a FRET-based experimental technique to enable access to NSC and provide a quantification approach.…”

“…Recently it has been demonstrated that the NSC measured with FRET correlates to the adhesion between soft materials. − These studies showed that for bonded thin films pressed under different loads, the FRETeff and adhesion increase with the applied pressure, caused by the correspondent increase of the degree of NSC . Thus, FRET has been introduced as a proper technique to quantify the degree of NSC and establish a relation between FRETeff and adhesion in bonded films.…”

“…FRET (Förster Resonance Energy Transfer) has recently been introduced as an experimental technique for the measurement of NSC between soft surfaces. − FRET takes advantage of the fluorescence properties of specific compatible molecules to determine their exact nanometric distance (0–20 nm) . Thus, if these molecules, named Donor and Acceptor, are in NSC, a nonradiative Donor → Acceptor energy transfer will occur and a FRET signal can be detected. , The FRET distance range depends on the Förster Radius (R 0 ) of the FRET dyes utilized and can only be properly sensed between 0.5R 0 and 2R 0 .…”

“…To employ this technique for NSC between surfaces, each interface needs to be labeled with the Donor or Acceptor dye (Figures and S1). Due to the natural roughness of any surface, the NSC is not complete . Instead, areas that look to be in full contact at the microscale end up revealing opens gaps at the nanometric scale .…”

Protein/Protein and Quantum Dot/Protein Organization in Sequential Monolayer Materials Studied Using Resonance Energy Transfer

“…In this way, we built a simple but highly ordered system, with Förster resonance energy transfer (FRET). Simões et al recently showed the application of a similar idea, with the assembly of two thin films composed of simple organic fluorophores for the quantification of nanoscale contacts [22]. FRET networks have been proposed for broadening the possible utilization of FRET-based biosensors [23].…”