ECOF 6. European Conference on Organised Films. Sheffield UK, 1996.
PREPARATION OF HIGHLY-STRUCTURIZED LANGMUIR-BLODGETT FILMS OF SOLUBLE PHOSPHOLIPIDS USING THE MATRIX REPLICATION TECHNIQUE
P.P. Karageorgiev, V.V. Karageorgieva, V.V. Luchinin
Micro technology Centre of Si-Petersburg Electrotechnical Slate University, Professor Popov str. 5, St.-Petersburg, 197376, Russia
Our choice of amphiphilic material was single-chain phospholipid: hexadecylphosphocholine, which is partly water-soluble with the critical micelle concentration Ccmf ~ 10 -6 M. Preparation of a soluble phospholipid - based multilayered structure, that would possess the long-range order, involves two problems of critical importance, One relates to the first phase of the LBF synthesis, i.e. to the many times repeated transfer of the phospholipid monolayer from the subphase surface onto the solid substrate. The obtained LBF structure is devoid of a long-range order in this since its component monolayers are in a dynamic liquid state. The second problem applies to the second phase synthesis and is concerned with creating such conditions which would inheriting structural information of the ordered matrix (substrate) by overlaying monolayers.
We attacked these problems in the following way.
The first challenge was in providing adequate conditions to suppress direct massexchange between the subphase and the substrate since the amphiphilic molecule adsorption has been established cause a microcrystalline phase formation in the substrate-overlaying monolayer which can result in disintegrating multilayered LBF structure; while the molecular desorption would cause dissolution of the previously applied monolayers. The on-substrate adsorption was restricted by us through decreasing the amphiphilic substance content of the subphase. However, the resulting disbalance between the subphase and its surface monolayer can also bring about a partial dissolution of the superficial layer. To avoid this problem as well as to ensure the continuity of the so-formed monolayer, we had to steadily compress it by means of a movable barrier. Accordingly, both the monolayer surface pressure and continuity, while going from the subphase to the substrate were made well above those at the equilibrium state. It should be emphasized that the suggested approach is feasible solely at a sufficiently low rate of desorption from the surface to the bulk.
Therefore, a bivalent metal salt was admixed to the subphase with the aim of limiting the applied film dissolution rate. Heteropolar bonding of phospholipid molecules through metal cations (salt bridges) caused the barrier height to increase, just preventing the substance desorption.
The second phase of the synthesis involved creating a buffer monolayer at the substrate surface to act as an ordered matrix for the overlain phospholipid monolayer. By this means, the structural information could be transmitted from the matrix to the first monolayer through interaction between polar fragments of phospholipid molecules and active centers of the matrix. Further, this structural information can be communicated to the next monolayer of phospholipid due to penetrating non-polar molecular fragments of the second monolayer into voids between hydrocarbon radicals of the first monolayer.
The obtained films were examined using various structural analysis techniques, such as low angle X-ray diffractometry and reflection high-energy election diffractometry. A general conclusion drawn from these analyses is that the films have had a layered structure with the identity parameter along the normal to the substrate approximately 3.6 nm. According to RHEED data, the arrangement of aliphatic HPhCh chains is similar to the molecular arrangement in the buffer matrix layer for which LBF of stearic acid was used.
P.P. Karageorgiev, V.V. Karageorgieva, V.V. Luchinin. Preparation of highly-structurized Langmuir-Blodgett films of soluble phospholipids using the matrix replication technique // European Conference on Organised Films ECOF 6. Sheffield UK, 1996, p. 6.07