Multimodal microscopy-based identification of surface nanobubbles.

HYPOTHESIS: Surface nanobubbles, which were controversially discussed in the literature, promise a number of outstanding applications, and their presence may hamper nanoscale processes at solid-aqueous interfaces. A most crucial and yet unsolved question is the rapid and conclusive identification of gas-filled (surface) nanobubbles. We hypothesize that surface nanobubbles and oil nanodroplets can be conclusively differentiated in co-localization experiments with atomic force microscopy (AFM) and time-resolved fluorescence microscopy by localizing tracer fluorophores and analyzing their fluorescence lifetimes. EXPERIMENTS: Combined AFM and fluorescence lifetime imaging microscopy (FLIM) were conducted to localize the various interfaces labelled by the reporter dye rhodamine 6G (Rh6G). The dependence of the fluorescence lifetime of Rh6G on its local environment was determined for air/water, water/glass and polysiloxane/water interfaces under different conditions. FINDINGS: In in situ co-localization experiments, surface nanobubbles labeled with Rh6G were probed by AFM with high spatial resolution and were differentiated from polysiloxane droplets as well as contamination originating from lubricant-coated syringe needles owing to the characteristic short fluorescence lifetime of Rh6G at the gas/water interface observed in FLIM. In particular, this approach lends itself to conclusively identify and rapidly differentiate these gas-filled entities from adsorbed contamination, such as siloxane-based oil nanodroplets.

Surface Nanobubbles Studied by Time-Resolved Fluorescence Microscopy Methods Combined with AFM: The Impact of Surface Treatment on Nanobubble Nucleation.

The impact of surface treatment and modification on surface nanobubble nucleation in water has been addressed by a new combination of fluorescence lifetime imaging microscopy (FLIM) and atomic force microscopy (AFM). In this study, rhodamine 6G (Rh6G)-labeled surface nanobubbles nucleated by the ethanol-water exchange were studied on differently cleaned borosilicate glass, silanized glass as well as self-assembled monolayers on transparent gold by combined AFM-FLIM. While the AFM data confirmed earlier reports on surface nanobubble nucleation, size, and apparent contact angles in dependence of the underlying substrate, the colocalization of these elevated features with highly fluorescent features observed in confocal intensity images added new information. By analyzing the characteristic contributions to the excited state lifetime of Rh6G in decay curves obtained from time-correlated single photon counting (TCSPC) experiments, the characteristic short-lived (<600 ps) component of could be associated with an emission at the gas-water interface. Its colocalization with nanobubble-like features in the AFM height images provides evidence for the observation of gas-filled surface nanobubbles. While piranha-cleaned glass supported nanobubbles, milder UV-ozone or oxygen plasma treatment afforded glass-water interfaces, where no nanobubbles were observed by combined AFM-FLIM. Finally, the number density of nanobubbles scaled inversely with increasing surface hydrophobicity.

Enzyme degradable polymersomes from hyaluronic acid-block-poly(ε-caprolactone) copolymers for the detection of enzymes of pathogenic bacteria.

We introduce a new hyaluronidase-responsive amphiphilic block copolymer system, based on hyaluronic acid (HYA) and polycaprolactone (PCL), that can be assembled into polymersomes by an inversed solvent shift method. By exploiting the triggered release of encapsulated dye molecules, these HYA-block-PCL polymersomes lend themselves as an autonomous sensing system for the detection of the presence of hyaluronidase, which is produced among others by the pathogenic bacterium Staphylococcus aureus. The synthesis of the enzyme-responsive HYA-block-PCL block copolymers was carried out by copper-catalyzed Huisgen 1,3-dipolar cycloaddition of ω-azide-terminated PCL and ω-alkyne-functionalized HYA. The structure of the HYA-block-PCL assemblies and their enzyme-triggered degradation and concomitant cargo release were investigated by dynamic light scattering, fluorescence spectroscopy, confocal laser-scanning microscopy, scanning and transmission electron, and atomic force microscopy. As shown, a wide range of reporter dye molecules as well as antimicrobials can be encapsulated into the vesicles during formation and are released upon the addition of hyaluronidase.

Hydrodynamic effects of the tip movement on surface nanobubbles: a combined tapping mode, lift mode and force volume mode AFM study.

We report on an Atomic Force Microscopy (AFM) study of AFM tip-nanobubble interactions in experiments conducted on argon surface nanobubbles on HOPG (highly oriented pyrolytic graphite) in water in tapping mode, lift mode and Force Volume (FV) mode AFM. By subsequent data acquisition on the same nanobubbles in these three different AFM modes, we could directly compare the effect of different tip-sample interactions. The tip-bubble interaction strength was found to depend on the vertical and horizontal position of the tip on the bubble with respect to the bubble center. The interaction forces measured experimentally were in good agreement with the forces calculated using the dynamic interaction model. The strength of the hydrodynamic effect was also found to depend on the direction of the tip movement. It was more pronounced in the FV mode, in which the tip approaches the bubble from the top, than in the lift mode, in which the tip approaches the bubble from the side. This result suggests that the direction of tip movement influences the bubble deformation. The effect should be taken into account when nanobubbles are analysed by AFM in various scanning modes.

Unraveling supported lipid bilayer formation kinetics: osmotic effects.

Solid-supported lipid bilayers are used as cell membrane models and form the basis of biomimetic and biosensor platforms. The mechanism of their formation from adsorbed liposomes is not well-understood. Using membrane-permeable solute glycerol, impermeable solutes sucrose and dextran, and a pore forming peptide melittin, we studied experimentally how osmotic effects affect the kinetics of the adsorbed liposome-to-bilayer transition. We find that its rate is enhanced if adsorbed liposomes are made permeable but is not significantly retarded by impermeable solutes. The results are explained in terms of adsorbed liposome deformation and formation of transmembrane pores.