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We present broadband vibrational sum-frequency generation (VSFG) spectra of Langmuir-Blodgett monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and different mixtures of them as model systems of pulmonary surfactants. The systematic study explored the dependence of the vibrational spectra as a function of surface tension and mixture ratio in various polarization combinations. The extremely short acquisition time and the high spectral resolution of our recently developed spectrometer helped minimize sample degradation under ambient conditions throughout the duration of the measurement and allowed the detection of previously unseen vibrational bands with unprecedented signal-to-noise ratio. The dramatically improved capability to record reliable vibrational spectra together with the label-free nature of the VSFG method provides direct access to native lipid structure and dynamics directly in the monolayer. The resulting data deliver quantitative information for structural analysis of multi-component phospholipid monolayers and may aid in the development of new synthetic pulmonary surfactants. [ABSTRACT FROM AUTHOR]

Contrast in confocal microscopy of phase-separated monolayers at the air-water interface can be generated by the selective adsorption of water-soluble fluorescent dyes to disordered monolayer phases. Optical sectioning minimizes the fluorescence signal from the subphase, whereas convolution of the measured point spread function with a simple box model of the interface provides quantitative assessment of the excess dye concentration associated with the monolayer. Coexisting liquid-expanded, liquid-condensed, and gas phases could be visualized due to differential dye adsorption in the liquid-expanded and gas phases. Dye preferentially adsorbed to the liquid-disordered phase during immiscible liquid-liquid phase coexistence, and the contrast persisted through the critical point as shown by characteristic circle-to-stripe shape transitions. The measured dye concentration in the disordered phase depended on the phase composition and surface pressure, and the dye was expelled from the film at the end of coexistence. The excess concentration of a cationic dye within the double layer adjacent to an anionic phospholipid monolayer was quantified as a function of subphase ionic strength, and the changes in measured excess agreed with those predicted by the mean-field Gouy-Chapman equations. This provided a rapid and noninvasive optical method of measuring the fractional dissociation of lipid headgroups and the monolayer surface potential. [ABSTRACT FROM AUTHOR]

Understanding interactions between inhaled nanoparticles and lung surfactants (LS) present at the air-water interface in the lung, is critical to assessing the toxicity of these nanoparticles. Specifically, in this work, we assess the impact of engineered carbon nanoparticles (ECN) on the ability of healthy LS to undergo reversible collapse, which is essential for proper functioning of LS. Using a Langmuir trough, multiple compression-expansion cycles are performed to assess changes in the surface pressure vs. area isotherms with time and continuous cyclic compression-expansion. Further, theoretical analysis of the isotherms is used to calculate the ability of these lipid systems to retain material during monolayer collapse, due to interactions with ECNs. These results are complemented with fluorescence images of alterations in collapse mechanisms in these monolayer films. Four different model phospholipid systems, that mimic the major compositions of LS, are used in this study. Together, our results show that the ECN does not impact the mechanism of collapse. However, the ability to retain material at the interface during monolayer collapse, as well as re-incorporation of material after a compression-expansion cycle is altered to varying extent by ECNs and depends on the composition of the lipid mixtures. [ABSTRACT FROM AUTHOR]