Combined Atomic Force Microscopy and Fluorescence Correlation Spectroscopy Measurements to Study the Dynamical Structure of Interfacial Fluids

Abstract

We have studied the dynamic structure of thin (approximately a few nanometers) liquid films of a nearly spherical, nonpolar molecule tetrakis(2-ethylhexoxy)silane (TEHOS) by using a combination of atomic force microscopy (AFM) and fluorescence correlation spectroscopy (FCS). Ultra-sensitive interferometer-based AFM was used to determine the stiffness (force gradient) and the damping coefficient of the liquid film. The experiments show oscillations in the damping coefficient with a period of approximately 1 nm, which is consistent with the molecular dimension of TEHOS as well as previous X-ray reflectivity measurements. Additionally, we performed FCS experiments for direct determination of the molecular dynamics within the liquid film. From the fluctuation autocorrelation curve, we measured the translational diffusion of the probe molecule embedded within the fluid film formed on a solid substrate. The autocorrelation function was best fitted with two components, which indicate that the dynamics are heterogeneous in nature. However, the heterogeneity is not as pronounced as had been previously observed for molecularly thin liquid films sandwiched between two solid substrates.

Simultaneous normal and shear measurements of nanoconfined liquids in a fiber-based atomic force microscope

“…11 and 13͒ and TEHOS. 7 Here, we present simultaneous normal and lateral stiffness measurements, which were performed on TEHOS and OMCTS with Si cantilevers purchased from MikroMasch ͑Ref. 24͒ and NT-MDT.…”

“…1 Geometrically induced layering has been observed by a variety of instruments and experimental techniques such as surface force apparatus, 2,3 atomic force microscopy, 4 and new spectroscopic techniques 5 including fluorescence correlation spectroscopy. 6,7 From molecular packing arguments, we would expect that whenever the distance between the confining surfaces is an integer number of the respective molecular size, the layering of the molecules is in a high density state. At other separations, molecular packing is disrupted and the liquid assumes a low density state.…”

Recent advances in hybrid measurement methods based on atomic force microscopy and surface sensitive measurement techniques

“…149 2.2.4 AFM coupled to uorescence correlation spectroscopy (FCS). Only a few studies were published for combinations of AFM with FCS, [150][151][152][153][154] although combinations of AFM and confocal uorescence microscopy are utilized in the past 20 years (see above). This may be related to the application of FCS, which is utilized to measure quantitatively dynamic parameters, such as chemical reaction rates, 155 diffusion coefficients, 156 hydrodynamic radii, 157 (average) concentration 158 and singlettriplet transitions.…”

“…A combination of these two techniques was utilized typically in the analysis of liquid lms and membranes. 150,151 The rst study utilizing AFM-FCS was done by Burns et al in 2005. It was utilized to analyze the relative partitioning and the mobility of labeled probes in gel and disordered liquid phase of bilayers consisting of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid domains in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC).…”

“…The FCS analysis showed that two components are present in the DOPC uid phase (fast and slow), which was ascribed to membrane heterogeneities. 150 Subsequently, in the group of Schwille, the AFM-FCS technique was utilized to characterize dynamic and structural organization of phase-separated membranes 161 and to show coexistence of the liquid disordered and liquid ordered phase for ceramides, which forms gel-like domains. 153 The formation of these ceramide-rich domains was found to be dependent on the acyl chain length of the ceramide.…”