Novel in situ sensing surface forces apparatus for measuring gold versus gold, hydrophobic, and biophysical interactions

Abstract

The surface forces apparatus (SFA) was developed in the late 1960s as a powerful tool for investigating molecular interactions across apposing surfaces including the first measurement of van der Waals forces and interactions in biologic and liquid media. However, the SFA has two major disadvantages. First, it traditionally uses white light interference between back-silvered muscovite mica surfaces to measure distances and to infer forces from distance shifts during interaction of two surfaces. Hence, distance shifts and force measurement are not decoupled. Second, productive SFA interferometers are so far limited to measuring across mica versus mica or mica versus metal. Direct gold-gold configurations were suggested in the late 1990s but not experimentally achieved as proof-of-principle until recently using a templating technique. In this work, we show how we solve these two disadvantages. First, we present a new SFA design that decouples force and distance measurements with similar resolution. The presented SFA design is inexpensive and can be home-built with mostly commercially available parts. Second, we present an alternative physical vapor deposition approach to construct a stable gold-gold interferometer and demonstrate its performance showing hydrophobic interactions, bubble formation, hemifusion of bilayers, and friction experiments. The presented system is easy to use. The obtained results show excellent reproducibility, indicating that the designed SFA and the three-mirror gold-gold interferometer functions as well as or even better than the traditional interferometer configurations used in SFA. This opens SFA to a wide range of options for various possible applications. Specifically, the gold-gold configuration allows a broad range of surface modifications for studying biophysical interactions as demonstrated in this work.

Cohesion Gain Induced by Nanosilica Consolidants for Monumental Stone Restoration

“…Breaking, shearing, and normal forces were measured with the in-house-modified surface forces apparatus (SFA), equipped with two strain gauge-type force sensors (ME-Meßsysteme GmbH; coupled with the GSV8 controller), detecting forces (>1 μN) in the normal and shearing directions in real time. The detailed design of the apparatus is presented in Wieser et al 30 Owing to the high-resolution and distance-independent force signal (now decoupled from the interferometric surface separation measurement unlike in traditional SFA and surface force balance (SFB) experiments 36 ), large adhesive forces can be precisely and quickly measured even at very high surface separation velocities, >1 μm s –1 . Traditional data analysis of such strongly adhesive forces would require tracing of the interferometric fringes of equal chromatic order (FECO) after the adhesive jump-out events (to hundreds of micrometers), back to the contact position at zero separation, leading to a time-consuming adhesion force calculations.…”

“…Here, we present a facile and robust method to study the strength of solvent-dispersed consolidants at single mineral contacts quantitatively using in-house-modified surface forces apparatus (SFA) with real-time force sensing. 30 Complementary structural information about the nanosilica suspensions during drying, aggregation, and solidification is obtained from X-SFA experiments. 31 Force sensing introduced in SFA allows measuring of high adhesion induced by consolidants confined between two mineral surfaces.…”

Understanding and Controlling Mode Hybridization in Multicavity Optical Resonators Using Quantum Theory and the Surface Forces Apparatus

“…In the standard SFA setup, the fluid (D) is confined between two molecularly smooth sheets of transparent mica (T), coated on the outside with partially reflecting Ag layers (M). , The fluid thickness can be determined as a function of the resonance wavelength using an analytic expression valid for a single cavity (MT 1 DT 2 M) with a composite dielectric layer (T 1 DT 2 ) . To apply an electric field, metal electrodes such as gold layers can be introduced at the fluid-mica interfaces. − However, these layers act as additional mirrors and create a multicavity resonator, producing additional features that should be analyzed using more advanced theoretical tools such as quantum theory.…”

“…We anticipate that the SFA can be used to study virtually any planar optical multilayer as a function of the thickness of one or more layers, for example, to characterize the coupling of epsilon-near-zero modes with excitons embedded in a fluid or create planar optical metamaterials with tunable optical response. Our findings also have implications in the design and interpretation of SFA experiments on the electrochemistry of surfaces and the electrification of nanoscale fluid films, typically involving multicavity metal-dielectric resonators. − …”

Visualization of Ion|Surface Binding and In Situ Evaluation of Surface Interaction Free Energies via Competitive Adsorption Isotherms

“… 51 It can measure the distance between the mirrors with subnanometer resolution and the interaction force, obtained from an independent force sensor, has a detection limit of ≃0.1 μN/m. 52 …”