Nanomechanical Characterization of Micellar Surfactant Films via Atomic Force Microscopy at a Graphite Surface

Nanomechanics of self-assembled surfactants revealed by frequency-modulation atomic force microscopy

Surfactants play a critical role in bottom-up nanotechnologies due to their peculiar nature of controlling the interfacial energy. Since their operational mechanism originates from the molecular-scale formation and disruption processes of molecular assemblies (i.e., micelles), conventional static-mode atomic force microscopy has made a significant contribution to unravel the detailed molecular pictures. Recently, we have successfully developed a local solvation measurement technique based on three-dimensional frequency-modulation atomic force microscopy, whose spatial resolution is not limited by jump-to-contact. Here, using this novel technique, we investigate molecular nanomechanics in the formation and disruption processes of micelles formed on a hydrophobic surface. Furthermore, an experiment employing a hetero-nanostructure reveals that the nanomechanics depends on the form of the molecular assembly. Namely, the hemifusion and disruption processes are peculiar to the micellar surface and cause a higher energy dissipation than the monolayer surface. The technique established in this study will be used as a generic technology for further development of bottom-up nanotechnologies.

Kinetic stability modulation of polymeric nanoparticles for enhanced detection of influenza virus via penetration of viral fusion peptides

Specific interactions between viruses and host cells provide essential insights into material science-based strategies to combat emerging viral diseases. pH-triggered viral fusion is ubiquitous to multiple viral families and is important for understanding the viral infection cycle. Inspired by this process, virus detection has been achieved using nanomaterials with host-mimetic membranes, enabling interactions with amphiphilic hemagglutinin fusion peptides of viruses. Most research has been on designing functional nanoparticles with fusogenic capability for virus detection, and there has been little exploitation of the kinetic stability to alter the ability of nanoparticles to interact with viral membranes and improve their sensing performance. In this study, a homogeneous fluorescent assay using self-assembled polymeric nanoparticles (PNPs) with tunable responsiveness to external stimuli is developed for rapid and straightforward detection of an activated influenza A virus. Dissociation of PNPs induced by virus insertion can be readily controlled by varying the fraction of hydrophilic segments in copolymers constituting PNPs, giving rise to fluorescence signals within 30 min and detection of various influenza viruses, including H9N2, CA04(H1N1), H4N6, and H6N8. Therefore, the designs demonstrated in this study propose underlying approaches for utilizing engineered PNPs through modulation of their kinetic stability for direct and sensitive identification of infectious viruses.

Rheological Characterization of Mixed Surfactant Films at Droplet Interfaces via Micropipette Aspiration

Cationic and anionic surfactant mixtures can form viscous films that dominate the rheology and stability of micrometer-sized droplet suspensions. In this work, we use micropipette aspiration to study the mechanical properties of mixed surfactant surface films of anionic sodium dodecyl sulfate (SDS) and cationic dodecylamine hydrochloride (DAH) on alkane and lipid droplets. For octane droplets, SDS was found to decrease the surface tension until a minimum of 5 ± 1 mJ/m² was reached after the critical micelle concentration (cmc). The surface viscosity of the droplets was found to be on the order of 10^-3 mN s/m at an SDS concentration of 10 mM. An addition of 0.2 mM of DAH was found to increase this viscosity to a peak of 0.24 ± 0.01 mN s/m. Similar to octane, the surface tension of dodecane decreased to a value of 7.7 ± 0.4 mJ/m² at SDS concentrations above cmc. Unlike with octane, however, the dodecane droplets had a significant surface viscosity of 0.37 ± 0.01 mN s/m when only the 10 mM SDS film was present. An addition of DAH caused a decrease in this viscosity initially, before rising to a peak viscosity of 0.45 ± 0.01 mN s/m at a DAH concentration of 0.15 mM. We speculate that the peaks in viscosities were the result of the completions of a phase change associated with microcrystalline SDS/DAH grains growing in the film at the surface of the droplets. Fluorescence microscopy and visual observations provided further evidence that these films can show rigid microcrystalline-like structure. Further work done with soybean oil in the same conditions and with a lipid film, simulating biological lipid droplets, confirmed that lipid droplets behave rheologically similar to alkanes in the presence of these mixed surfactant and lipid films. These results imply that droplet mechanics may be heavily influenced by the presence of microcrystalline grains in the oil-water systems with complex surfactant mixtures.

Atomic Force Microscopy Force Mapping Analysis of an Adsorbed Surfactant above and below the Critical Micelle Concentration

Force curves collected using an atomic force microscope (AFM) in the presence of adsorbed surfactants are often used to draw conclusions about adsorbed film packing, rigidity, and thickness. However, some noteworthy features of such force curve characteristics have yet to be thoroughly investigated and explained. In this work, we collected force curves from tetradecyltrimethylammonium bromide films adsorbed on highly oriented pyrolytic graphite (HOPG), silica, and silica that had been hydrophobized by functionalization with dichlorodimethyl silane. Breakthrough events in the force curves from several different trials were compared to show that the breakthrough distance, often reported as the adsorbed film thickness, increased with concentration below the critical micelle concentration (CMC) but was approximately 3.5 nm on all surfaces between 2× and 10× CMC; an unexpected result because of the different surface chemistries for the three surfaces. We employed an AFM probe with a different force constant ( k) value as well as a colloidal probe and the breakthrough distance remained approximately 3.5 nm in all cases. Gradient mapping, a variant of force mapping, was also implemented on the three surfaces and resulted in a new technique for visualizing adsorbed surfactant in situ. The resulting maps showed patches of adsorbed surfactant below the CMC and revealed that with increasing concentration, the size of the patches increased resulting in full coverage near and above the CMC. These results are, to our knowledge, the first time force mapping has been used to spatially track patches of adsorbed surfactant. Finally, layers of surfactants on an AFM tip were investigated by collecting a force map on a single AFM tip using the tip of a separate AFM probe. A breakthrough event was observed between the tips, indicating that a layer of surfactant was present on at least one, if not both tips.

Characterization of Repulsive Forces and Surface Deformation in Thin Micellar Films via AFM

Here we examine how the force on an atomic force microscope (AFM) tip varies as it approaches micellar surfactant films, and use this information to discern the film's surface structure and Young's modulus. Rows of wormlike hemimicelles were created at a graphite interface using 10 mM sodium dodecyl sulfate (SDS). We found that the repulsive force on a silicon nitride tip as it approached the surface was exponential, with a decay length of 2.0 ± 0.1 nm. The addition of Na₂SO₄ was found to cause a change in this behavior, with a clear split into two exponential regions at concentrations above 1 mM. We also observed that the range of these forces increased with added salt from ∼15 nm in pure SDS to ∼20 nm at a Na₂SO₄ concentration of 1.34 mM. These forces were inconsistent with electrostatic repulsion, and were determined to be steric in nature. We show that the behavior at higher salt concentrations is consistent with the theory of polyelectrolyte brushes in the osmotic regime. From this, we hypothesize the presence of micellar brushes at the surface that behave similarly to adsorbed polymer chains. In addition, the Young's modulus of the film was taken from data near the interface using Sneddon's model, and found to be 80 ± 40 MPa. Similar experiments were performed with 10 mM dodecylamine hydrochloride (DAH) solutions in the presence of added magnesium chloride. The decay length for the pure DAH solution was found to be 2.6 ± 0.3 nm, and the addition of 1.34 mM of MgCl₂ caused this to increase to 3.7 ± 0.3 nm. No decay length splitting was observed for DAH. We conclude that the behavior at the surface resembles that of an uncharged polymer brush, as the ionic and surface charge densities are much lower for DAH than for SDS.