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

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.