Deposition-induced phenomena in lipid monolayers

Langmuir-Blodgett (LB) deposition is used to transfer lipid monolayers onto solid substrates for atomic force microscopy (AFM) and other analytical techniques. The fundamental requirement is the fidelity of molecular and mesoscopic morphology following transfer; however, evidence of unaltered transfer is lacking as LB deposition limits direct observation of the monolayer during transfer.

To address this, in situ fluorescence optical microscopy was used to image two-phase dipalmitoylphosphatidylcholine (DPPC) monolayers during and following transfer to mica or silicon substrates via Inverse Langmuir-Schaefer (ILS) deposition. In ILS deposition, the subphase liquid is drained to lower the DPPC monolayer onto an immersed mica substrate, providing continuous access for fluorescence microscopy during the deposition process.

Immediately prior to deposition, small increases in surface pressure occur, likely due to electrostatic interactions between substrate and monolayer as the subphase depth decreases. This induces domain growth variations consistent with surface pressure induced morphology changes. The extent of monolayer rearrangement varies with the initial monolayer surface pressure and is most pronounced during liquid-condensed (LC) - liquid-expanded (LE) coexistence. On contact with the substrate, rapid nucleation and dendritic growth of the LC phase occurs at the expense of the LE phase. AFM analysis reveals sub-optical LC features within the LE phase after deposition that are not found at the air-water interface. These features can be explained by contact-line pinning and evaporative dewetting, along with substrate-induced ordering, similar to what is observed during nanoparticle deposition on solid substrates. Analogous features are common to many deposited lipid/protein AFM images in the literature and occur for many typical monolayer compositions, subphases, substrates and deposition methods.