Abstract
Raman spectroscopy represents a widely used analytical technique providing unique information about the vibrational motion of molecules. The great advantage for the study of biological molecules and their interaction including membrane models is the absence of any markers or probes that could have undesirable effects.
Unfortunately, the conventional Raman spectroscopy approach to liquid samples is limited to highly concentrated solutions or suspensions and to overcome this, drop coating deposition Raman (DCDR) method can be used. DCDR is based on evaporation of solvent from a small droplet of studied solution or suspension.
Evaporation of a solvent from droplet with a pinned contact line deposited on hydrophobic substrate can lead to the accumulation of solute in the ring-shaped pattern caused by a capillary flow in the droplet during the drying. The Raman spectra are then acquired by confocal Raman microspectrometer from the pre-concentrated analyte ring.
Because of this, the DCDR technique enables to measure samples at very low initial concentrations and small volume with high sensitivity in comparison with the conventional Raman measurements. The sensitivity of the spectra and the dried patterns are influenced not only by the properties of the solution but it depends also on the substrate characteristic such wettability and roughness that governs the droplet drying dynamics.
We focused our attention on the spherical membrane structures, liposomes, which are often used as a real membranes models. Liposome suspensions, dried on nanostructured hydrophobic substrates with different roughness, were studied by DCDR.
Substrates are prepared by a novel method that is based on the deposition of arrays of nanoparticles (Cu or Ag) fusing a gas aggregation source and subsequent overcoating of such prepared substrates by sputter-deposited thin C:F film. The nanoroughness and with it connected wettability of produced coatings is controlled by the number of nanoparticles in the base layer including an option of a gradient surface coverage.
It was shown that in the case of liposome suspension the nanoroughness leads to stronger preconcentration as well as to the reduction of the ring diameter, i.e. two parameters crucial for the DCDR measurements.