SFM and TOF-SIMS Study of Novel Self-Organization phenomena in mixed LB monolayers
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SFM and TOF-SIMS Study of Novel Self-Organization phenomena in mixed LB monolayers
B. Pignataro*, L. Sardone, A. Licciardello and G. Marletta Dipartimento di Scienze Chimiche, Università di Catania, V.le A. Doria 6-95125, Catania, Italy. ABSTRACT Mixed monolayers of dimyristoylphosphatidylcholine (DMPC) and quercetin palmitate (QP) in a molar ratio of 25/75 have been transferred on mica and oxygen plasma cleaned silicon by the Langmuir-Blodgett (LB) technique at different subphase temperatures. Scanning Force Microscopy (SFM) in height, phase and lateral force modes has been employed to investigate the structural and mechanical features at nanoscopic level of these samples. Although the two molecules show a wide range of miscibility at 37 °C, they give rise to phase separation at 10 °C. This last system provides a new example of nanometric scale self-organization. In particular spiral shaped domains rising from the wrapping-up of nanoscopic fiber-like structures have been observed. The high resolution achieved by the use of the dynamic scanning force microscopy operating in the net attractive regime allow to visualize characteristic nanoscopic rupture points along the supramolecular fibers. High mass resolution Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) spectra showed DMPC- as well as QP-related peaks. The ToF-SIMS spectra from the nanostructured samples (10 °C) have been compared with those from the homogeneous ones (37 °C). The phase separated samples provides interesting secondary ions that highlight the QP supramolecular condensation within the fiber-like structures.
INTRODUCTION The study of self-organization processes of organic and biological molecules is a fundamental issue for the understanding of life science processes and applications in biotechnology [1]. A primary driver of progress in this field consists in the ability to investigate the structures and their peculiar properties at very localized scales. Advances in experimental tools are needed together with characterization strategies that combine either new techniques or established tools. Thus, for instance, the more and more refined Scanning Force Microscopies (SFMs) allowed to follow at the molecular scale several self-organization phenomena such as those related to DNA-STV oligomers [2], protein bidimensional crystal structures [3], domains in Langmuir-Blodgett (LB) monolayers [4] and so on. In addition, the combination of SFMs with surface-sensitive mass spectrometry tools like the Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) has been shown to be very useful in relating the indirect chemical information provided by scanning probes with the direct molecular information provided by surface mass spectrometry [5]. In this last period work in our lab dealt with the investigation of a new example of nanometric scale self-organization process leading to the formation of fiber-like supramolecules that wrap-up in spiral domains. This phenomenon was observed to arise from a phase separation process that occurs by mix
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