291395 The Effects of Gold Nanoparticles On Lipid Langmuir Monolayers

Monday, October 29, 2012
Hall B (Convention Center )
Dane Christie, Chemical Engineering, The City College of New York, New York, NY

ABSTRACT: We report the effects of dodecanethiol ligated gold nanoparticles (Au nps) on the phase behavior and surface morphology of select Langmuir lipid monolayers. Surface pressure vs. molecular area isotherms and epifluorescence microscope (FM) images were used to analyze the systems. The lipids 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (sodium salt) (POPG) were chosen for study. DMPE models the rigid lipid DPPC which is a major component of lung surfactant and POPG is a fluid lipid component of the lung surfactant system. Considerable interest lies in investigating the interaction of the lung surfactant system with Au nps because of nanotoxicity concerns. Over the course of our experiments we found that the incremental addition of Au nps has the effect of condensing then fluidizing DMPE and POPG monolayers. In addition, Au nps induce dramatic changes in the domain morphology and collapse behavior of DMPE monolayers.

INTRODUCTION: Lungs are biological organs which facilitate gas exchange in mammalian species. They do so by expanding upon inhalation to intake oxygen rich air and contract to expel carbon dioxide rich air. At the exchange interface is a complex membrane called lung surfactant (LS). It is a thin membrane consisting of lipids, proteins, and carbohydrates that is one molecule thick on complete expansion (1). Upon exhalation, LS is compressed and forms reversible folds into the alveolar lining fluid. Its primary function is to reduce the surface tension in the lungs associated with breathing. It also serves as a filter for undesirable particulate matter. The lowered surface tension effect of LS has the disadvantage of making the displacement of particles from inhaled air to the aqueous sub phase more favorable (2). Particles in the subphase are then removed by macrophages (2).

 In a special case where nanoparticulate matter (10-100nm) is inhaled, it may be incorporated into the surfactant membrane and hinders the LS’s ability to achieve low surface tensions (3). The nanomaterial’s high surface area to volume ratio has an increased reactivity relative to a micron sized particle of similar composition. For this reason nanopollutants pose the danger of being more toxic than their micron sized counterparts. A 2002 study correlated an increased concentration in ultra fine combustion byproducts (<100 nm) in air with an increased risk in the probability of dying from a cardiopulmonary disease or lung cancer (4). Long term exposure to other sources of nanocontaminants (e.g. Industrial waste) may have similar outcomes. 

A model LS system may be used to elucidate how nanopollutants interact with real LS. A binary system of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and POPG in a 7:3 molar ratio is used as a model LS to study the collapse behavior of Langmuir monolayers (5). DPPC is a saturated zwitterionic lipid and is the major lipid component of LS. It is responsible for the ability of LS films to reach low values of surface tension (1 mN/m) (6). Upon compression, pure DPPC forms a rigid film. When compressed to the point of two-dimensional (2-D) instability, DPPC explores the third dimension via a fracture mechanism (7) at 25°C, whereas the 7:3 DPPC/POPG mixture folds on collapse. In the absence of a functional DPPC system during the summer internship due to sample issues, DMPE is a suitable replacement for investigating the effects of gold on a rigid film. DMPE and DPPC are structurally similar in that they are both saturated zwitterionic lipids but different in the length of their hydrocarbon chains and the size of their head groups (DPPC has a phosphocholine head group which is larger than DMPE’s phosphoethanolamine head group). When Langmuir monolayers of DMPE are compressed beyond their 2-D instability they explore the third dimension via a folding mechanism, similar to the binary DPPC/POPG system. DMPE films are also capable of achieving low levels of surface tension (≈ 5 mN/m). POPG is an unsaturated anionic lipid and forms a fluid film when compressed. Because of its fluid nature, POPG lacks the ability to resist high levels of compression and collapses at a surface tension value of (≈ 22 mN/m). POPG collapses via a vesiculation mechanism (7). The nanocontaminant of choice is a dodecanethiol ligated (Au nps). Gold is used because it is surface active and has the ability to scatter light. By exploring the interactions of gold with a fluid (POPG) film and a rigid (DMPE) film, we hope to get a glimpse of how gold may interact model LS.


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