Characterization of Fluorescent Phospholipid Liposomes Entrapped in Sol−Gel Derived Silica

Abstract

Bilayer lipid membranes (BLMs) have been widely examined as sensing elements for a variety of analytes, in both the vapor and solution phases, using electrochemical, acoustic wave, and fluorescence methods. For successful development of stable sensing devices, it is necessary to be able to immobilize the BLMs in a manner that allows long-term retention of the membrane structure and still permits large-scale structural reorganizations such as phase transitions. In this work, small unilamellar liposomes were formed from either 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or l-a-phosphatidylcholine (egg PC) and were doped with 1−5 mol % of the fluorescent probes diphenylhexatriene (DPH) or nitrobenzoxadiazole-labeled dipalmitoylphosphatidylethanolamine (NBD-PE). The liposomes were entrapped in a series of different sol−gel derived silicate materials and the stability and phase-transition behavior of the liposomes was characterized. DPPC was observed to undergo reversible phase transitions when entrapped in glasses derived from either sodium silicate or a diglyceryl silane precursor; however, liposomes did not undergo phase transitions when entrapped in tetraethyl orthosilicate derived glasses, indicating that they had likely ruptured during the encapsulation process. As a practical demonstration of the use of the immobilized membranes for sensing applications, we have examined the use of pH-induced phase transitions as a means of generating a fluorescence signal that is based on changes in self-quenching of NBD-PE within liposomes composed of DPPC and dipalmitoylphosphatidic acid (DPPA). The results show that such pH-induced phase transitions occur for the entrapped vesicles and that the fluorescence responses follow the pH dependence of DPPA.

Analysis of Lipid Phase Behavior and Protein Conformational Changes in Nanolipoprotein Particles upon Entrapment in Sol–Gel-Derived Silica

“…21 This is analogous to effects observed in work with thermal unfolding of proteins in silica gel, where a higher free energy (thus higher unfolding temperature) was imposed upon entrapped, folded proteins due to decreased volume available for the unfolded form. 40,41 Since the lipid tails become less dense when undergoing a gel phase to liquid crystalline transition, 42 it is reasonable to believe that the excluded volume effect can be involved.…”

“…21 Their work demonstrated that the use of DGS- and SS-derived gels permitted the DPPC liposomes to exhibit phase transitions as they would in solution, while the use of TEOS-derived gels did not. Also, they depicted, via confocal fluorescence imaging, that the liposomes are capable of undergoing a variety of conformational changes once trapped inside of the gel, forming aggregates or bicelles.…”

Sol-gel Chemistry in Medicinal Science

“…Other metal oxides, such as alumina Al 2 O 3 [32] or vanadium oxide V 2 O 5 [33] were also shown to be suitable for biological macromolecules encapsulation. The process was extended to antibodies [34], nucleic acid [35], phospholipids [36], and poly-saccharides [37]. Additionally, several biophysical studies were devoted to the understanding of the behaviour of entrapped species [38,39].…”

Biophysical and Functional Characterization of an Ion Channel Peptide Confined in a Sol−Gel Matrix

“…A detailed characterization of the physical properties and stability of the immobilized lipid systems was carried out later by the Brennan group. 13 They incorporated liposomes composed of dipalmitoyl phosphatidylcholine (DPPC) in a TEOS sol-gel matrix and observed that the lipid bilayer did not exhibit phase transition suggesting that encapsulation resulted in rupture of these structures. This behavior was attributed to interactions between the lipid and the ethanol resulting as a byproduct of the chemical reactions involved in the formation process of the silica matrix.…”