13918 DOI: 10.1021/la900507w Langmuir 2009, 25(24), 13918–13925 Published on Web 05/21/2009 pubs.acs.org/Langmuir © 2009 American Chemical Society Headgroup-Dependent Lipid Self-Assembly on Zirconium Phosphate-Terminated Interfaces † B. P. Oberts and G. J. Blanchard* Michigan State University, Department of Chemistry, East Lansing, Michigan 48824-1322 Received February 10, 2009. Revised Manuscript Received April 14, 2009 We report on the self-assembly of selected phospholipids on a Zr phosphate-terminated thiol self-assembled monolayer (SAM) formed on a planar Au surface. The gold substrates were first reacted with 6-mercapto-1-hexanol and then treated with POCl 3 and ZrOCl 2 (aq) prior to exposure to phospholipids. The phospholipids used for adlayer formation were 1,2-dimyristoyl-sn-glycero-3-phosphatidic acid (DMPA), 1,2-dimyristoyl-sn-glycero-3-phosphatidyl- choline (DMPC), 1,2-dimyristoyl-sn-glycero-3-phosphatidylethanolamine (DMPE), 1,2-dimyristoyl-sn-glycero-3- [phospho-rac-(1-glycerol)] (DMPG), and 1,2-dimyristoyl-sn-glycero-3-[phospho-L-serine] (DMPS), and deposition was accomplished through vesicle fusion. The resulting interfaces were characterized using optical ellipsometry and water contact angle measurements, and cyclic voltammetry was used to interrogate the quality of the phospholipid adlayers. Our data indicate that the strongest lipid-interface interaction is with DMPA, whereas DMPC produces a slightly less organized adlayer. Phospholipids DMPE, DMPG, and DMPS were all found to interact relatively weakly with the zirconated interface, and we understand these results in the context of steric and hydrogen bonding effects in the adlayer that are dominated by the phospholipid headgroup. Introduction Lipid bilayers have been the subject of a great deal of investi- gation for both fundamental and practical reasons. The basis for the formation of a lipid bilayer structure is the balance of intermolecular interactions between the lipid nonpolar acyl chain regions and the polar headgroup interactions with the (aqueous) medium with which the bilayers are in contact. Lipid bilayers in biological systems are composed of many constituents and are structurally complex. It is thought that this complexity plays a role in stabilizing the folding of transmembrane proteins, thus mediating their function. There is a significant research effort involved with chemical sensing based on the use of biomolecules as the chemically selective elements. To succeed in using certain biomolecules as chemical sensing elements, an interface is required that can stabilize the structure of the biomolecule and at the same time function as part of a transduction system to relay the chemical signal of interest to instrumentation. Supported lipid bilayers are an appropriate choice for such purposes. The bilayer compo- sition and the manner in which the bilayer interacts with the interface to which it is bound need to be investigated as the initial step in this effort. We are interested in binding selected phos- pholipids to chemically modified interfaces, and one way to perform this binding is through interactions between the phos- pholipid headgroup moieties and the supporting surface. The work we discuss here is focused on the deposition and characterization of lipid monolayers, not bilayers. We have investigated lipid monolayers because we are interested in under- standing the interactions between the substrate and the lipid headgroups. It is these interactions that ultimately will govern the bilayer integrity and physical properties. Once the sub- strate-lipid interactions are understood, we can add an outer phospholipid leaflet by Langmuir-Schaefer deposition, 1-5 for example. Zr bisphosphonate and Zr bisphosphate (ZP) chemistry is a type of self-assembly that has been used in the formation of inter- facial adlayers for some time. 6-19 The primary motivation for the use of ZP chemistry is that the Zr phosphate/phosphonate associ- ation is energetically very favorable, 20 resulting in an essentially irreversible complexation that is characterized by fast reaction kinetics. 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