Thermodynamic characteristics and Langmuir-Blodgett deposition behavior of mixed DPPA/DPPC monolayers at air/liquid interfaces

The role of dipalmitoylphosphatic acid (DPPA) as a transfer promoter to enhance the Langmuir-Blodgett (LB) deposition of a dipalmitoylphosphatidylcholine (DPPC) monolayer at air/liquid interfaces was investigated, and the effects of Ca2+ ions in the subphase were discussed. The miscibility of the two components at air/liquid interfaces was evaluated by surface pressure-area per molecule isotherms, thermodynamic analysis, and by the direct observation of Brewster angle microscopy (BAM). Multilayer LB deposition behavior of the mixed DPPA/DPPC monolayers was then studied by transferring the monolayers onto hydrophilic glass plates at a surface pressure of 30 mN/m. The results showed that the two components, DPPA and DPPC, were miscible in a monolayer on both subphases of pure water and 0.2 mM CaCl2 solution. However, an exception occurs between X(DPPA)=0.2 and 0.5 at air/CaCl2-solution interface, where a partially miscible monolayer with phase separation may occur. Negative deviations in the excess area analysis were found for the mixed monolayer system, indicating the existence of attractive interactions between DPPA and DPPC molecules in the monolayers. The monolayers were stable at the surface pressure of 30 mN/m for the following LB deposition as evaluated from the area relaxation behavior. It was found that the presence of Ca2+ ions had a stabilization effect for DPPA-rich monolayers, probably due to the association of negatively charged DPPA molecules with Ca2+ ions. Moreover, the Ca2+ ions may enhance the adhesion of DPPA polar groups to a glass surface and the interactions between DPPA polar groups in the multilayer LB film structure. As a result, Y-type multilayer LB films containing DPPC could be fabricated from the mixed DPPA/DPPC monolayers with the presence of Ca2+ ions.     

Miscibility of gramicidin A-ethyl nonadecanoate in langmuir monolayers in the presence of salts dissolved in the subphase

The behavior of binary mixed Langmuir monolayers from gramicidin A (GA) and ethyl nonadecanoate (EN), spread on aqueous subphases containing NaCl and CaCl2, was investigated on the basis of the analysis of surface pressure-average area per molecule (pi-A) isotherms complemented with Brewster angle microscopy (BAM) images. Compression modulus versus surface pressure (C(S-1)-pi) curves indicate the existence of interactions in the GA-EN mixed monolayers at low surface pressures (below 5 mN m(-1)). However, for mixtures in which the ester is the predominant component, both GA and EN are miscible within regions from fully expanded to collapse. To examine the interactions between both components in the studied system, values of the mean molecular area per molecule (A12) were plotted as a function of molar fraction of gramicidin A (X(GA)). A12-X(GA) plots exhibit negative deviations from ideality at high surface pressures, wherein beta-helices of GA are vertically oriented in respect to the interface. However, at surface pressures below the plateau transition, which is due to reorientation of GA, the binary system obeys the additive rule. Brewster angle microscopy (BAM) was applied for a direct visualization of the monolayers morphologies. The obtained images prove that for molar ratios of GA > or = 0.3 and at surface pressures above 5 mN m(-1), both components are immiscible at the interface. The observed negative deviations from the additively rule were attributed to the formation of a three-dimensional phase in the mixed film, which provokes its contraction at the interface.     

Halofantrine-phospholipid interactions: monolayer studies.

The antimalarial agent halofantrine penetrates dipalmitolylphosphatidylcholine (DPPC) monolayers resulting in an increase in surface pressure and an expansion in area occupied by the lipid components of the monolayer. This phenomenon is observed at concentrations (0.05-0.2 microm) of halofantrine that have no surface activity. Penetration increases with drug concentration and is greatest at low initial surface pressures of the monolayer. A critical surface pressure of the DPPC monolayer has been determined from constant area and constant pressure conditions. The magnitude of these values support the hypothesis that halofantrine readily penetrates the DPPC monolayers. The presence of cholesterol in the DPPC monolayer hampers penetration and a lower critical surface pressure is obtained under such conditions. Even then, a slower rate of penetration is observed only in monolayers maintained at high initial surface pressures (10, 15 mN/m), corresponding to the liquid condensed phase of the monolayer, and not at low surface pressures (2.5, 5.0 mN/m). These results help to give a better understanding of the dynamics of the halofantrine-phospholipid interaction as well as the pharmacodynamic character of the drug.     

Effect of Ester Moiety on Structural Properties of Binary Mixed Monolayers of Alpha-Tocopherol Derivatives with DPPC

Phospholipid membranes are ubiquitous components of cells involved in physiological processes; thus, knowledge regarding their interactions with other molecules, including tocopherol ester derivatives, is of great importance. The surface pressure–area isotherms of pure α-tocopherol (Toc) and its derivatives (oxalate (OT), malonate (MT), succinate (ST), and carbo analog (CT)) were studied in Langmuir monolayers in order to evaluate phase formation, compressibility, packing, and ordering. The isotherms and compressibility results indicate that, under pressure, the ester derivatives and CT are able to form two-dimensional liquid-condensed (LC) ordered structures with collapse pressures ranging from 27 mN/m for CT to 44 mN/m for OT. Next, the effect of length of ester moiety on the surface behavior of DPPC/Toc derivatives’ binary monolayers at air–water interface was investigated. The average molecular area, elastic modulus, compressibility, and miscibility were calculated as a function of molar fraction of derivatives. Increasing the presence of Toc derivatives in DPPC monolayer induces expansion of isotherms, increased monolayer elasticity, interrupted packing, and lowered ordering in monolayer, leading to its fluidization. Decreasing collapse pressure with increasing molar ratio of derivatives indicates on the miscibility of Toc esters in DPPC monolayer. The interactions between components were analyzed using additivity rule and thermodynamic calculations of excess and total Gibbs energy of mixing. Calculated excess area and Gibbs energy indicated repulsion between components, confirming their partial mixing. In summary, the mechanism of the observed phenomena is mainly connected with interactions of ionized carboxyl groups of ester moieties with DPPC headgroup moieties where formed conformations perturb alignment of acyl chains, resulting in increasing mean area per molecule, leading to disordering and fluidization of mixed monolayer.     

Effects of surface pressure on the structure of the monolayer formed at the air/water interface by a non-ionic surfactant

The monolayer formed at an air/water interface by the synthetic non-ionic surfactant, 1,2-di-O-octadecyl-rac-glyceryl-3-(omega-methoxydodecakis (ethylene glycol)) (2C18E12) has been characterized using Langmuir trough measurements, Brewster angle microscopy (BAM), and neutron reflectometry. The BAM and reflectometry studies were performed at four different surface pressures (pi) in the range 15-40 mN/m. The BAM studies (which give information on the in-plane organisation of the surfactant layer) demonstrate that the 2C18E12 molecules are arranged on the water surface to form distinct, approximately circular, 5 microm diameter domains. As the surface pressure is increased these domains retain their size and shape but are made progressively more close-packed, such that the monolayer is made more or less complete at pi=40 mN/m. The neutron reflectometry measurements were made to determine the structure of the interfacial surfactant layer at pi=15, 28, 34 and 40 mN/m, providing information on the thickness of the 2C18E12 alkyl chains', head groups' and associated solvent distributions (measured along the surface normal), along with the separations between these distributions, and the effective interfacial area per molecule. Partial structure factor analyses of the reflectivity data show that the effective interfacial area occupied decreases from 217 A2 per 2C18E12 molecule at pi=15 mN/m down to 102 A2 at pi=40 mN/m. There are concomitant increases in the widths of the surfactant's alkyl chains' and head groups' distributions (modelled as Gaussians), with the former rising from 12 A (at pi=15 mN/m) up to 19 A (at pi=40 mN/m) and the latter rising from 13 A (at pi=15 mN/m) up to 24 A (at pi=40 mN/m). The compression of the monolayer is also shown to give rise to an increased surface roughness, some of which is due to the thermal roughness caused by capillary waves, but with a significant contribution also coming from the intrinsic/structural disorder in the monolayer. At all surface pressures studied, the alkyl chains and head groups of the 2C18E12 are found to exhibit a significant overlap, and this increases with increasing pi. Given the various trends noted on how the structure of the 2C18E12 monolayer changes as a function of pi, we extrapolate to consider the structure of the monolayer at pi>40 mN/m (making comparison with its single chain (CnEm) counterparts) and then relate these findings to the observations recorded on the structure and solute entrapment efficiency of 2C18E12 vesicles.     

Mode of interaction of two fluorinated-hydrogenated hybrid amphiphiles with dipalmitoylphosphatidylcholine (DPPC) at the air-water interface

Two-component Langmuir monolayers formed on 0.02M Tris buffer solution (pH 7.4) with 0.13M NaCl at 298.2K were investigated for two different fluorinated-hydrogenated hybrid amphiphiles (F6PH5PPhNa and F8PH5PPhNa or F6 and F8, respectively) with DPPC. Surface pressure (pi), surface potential (DeltaV) and dipole moment (mu( perpendicular)) as a function of molecular surface area (A) were measured by employing the Whilhelmy method and an ionizing electrode method. From the A- and DeltaV-X(F6) (or X(F8)) curves, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined as a function of surface mole fraction (X(Fn)) at discrete surface pressures. Then, the behavior of occupied surface areas and surface potentials of the respective components could be made clearer. Compressibility (C(s)), elasticity (C(s)(-1)), and excess Gibbs energy (DeltaG((ex))) as a function of X(F6) (or X(F8)) were estimated at definite pressures. These physico-chemical parameters were found to reflect the mechanical strength of monolayer films formed. The regular solution theory being applied to DeltaG((ex)), the activity coefficients (f) as well as the interaction parameter (I(p)) between DPPC and two hybrid amphiphiles in the binary monolayers were evaluated. I(p) values thus obtained indicated that F8 molecules interact more strongly with DPPC molecules than F6. Moreover, in order to better understand the morphological monolayer state, Langmuir-Blodgett (LB) films made from DPPC and fluorinated-hydrogenated hybrid amphiphiles were examined by atomic force microscopy (AFM). The miscibility of the two components in the monolayer state is evidenced by these thermodynamic quantities and AFM observations. Furthermore, AFM images demonstrated that F8 could more effectively disperse the ordered domains of DPPC than F6.     

Influence of hydrophobic alkylated gold nanoparticles on the phase behavior of monolayers of DPPC and clinical lung surfactant

The effect of hydrophobic alkylated gold nanoparticles (Au NPs) on the phase behavior and structure of Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC) and Survanta, a naturally derived commercial pulmonary surfactant that contains DPPC as the main lipid component and hydrophobic surfactant proteins SP-B and SP-C, has been investigated in connection with the potential implication of inorganic NPs in pulmonary surfactant dysfunction. Hexadecanethiolate-capped Au NPs (C(16)SAu NPs) with an average core diameter of 2 nm have been incorporated into DPPC monolayers in concentrations ranging from 0.1 to 0.5 mol %. Concentrations of up to 0.2 mol % in DPPC and 16 wt % in Survanta do not affect the monolayer phase behavior at 20 °C, as evidenced by surface pressure-area (π-A) and ellipsometric isotherms. The monolayer structure at the air/water interface was imaged as a function of the surface pressure by Brewster angle microscopy (BAM). In the liquid-expanded/liquid-condensed phase coexistence region of DPPC, the presence of 0.2 mol % C(16)SAu NPs causes the formation of many small, circular, condensed lipid domains, in contrast to the characteristic larger multilobes formed by pure lipid. Condensed domains of similar size and shape to those of DPPC with 0.2 mol % C(16)SAu NPs are formed by compressing Survanta, and these are not affected by the C(16)SAu NPs. Atomic force microscopy images of Langmuir-Schaefer-deposited films support the BAM observations and reveal, moreover, that at high surface pressures (i.e., 35 and 45 mN m(-1)) the C(16)SAu NPs form honeycomb-like aggregates around the polygonal condensed DPPC domains. In the Survanta monolayers, the C(16)SAu NPs were found to accumulate together with the proteins in the liquid-expanded phase around the circular condensed lipid domains. In conclusion, the presence of hydrophobic C(16)SAu NPs in amounts that do not influence the π-A isotherm alters the nucleation, growth, and morphology of the condensed domains in monolayers of DPPC but not of those of Survanta. Systematic investigations of the effect of the interaction of chemically defined NPs with the lipid and protein components of lung surfactant on the physicochemical properties of surfactant films are pertinent to understanding how inhaled NPs impact pulmonary function.     

Effect of lysozyme adsorption on the interfacial rheology of DPPC and cholesteryl myristate films

A model tear film lipid layer composed of a binary mixture of cholesteryl myristate (CM) and 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) was characterized using surface tension measurements, Brewster angle microscopy (BAM) and interfacial stress rheology (ISR). Isotherms showed that films containing >or=90 mol % CM have a 17-fold greater % area loss between the first and second compressions than the films with less CM. BAM images clearly showed that CM films did not expand after compression, and solid-like regions extending 1-2 mm were observed at low pressures (1 mN/m). Lipid films with or=50 mol % CM became elastic at higher surface pressures. Increasing CM content reduced the surface pressure at which the mixed film became elastic. Lysozyme adsorption into a CM film increased the compressibility and resulted in a more expanded film. Lysozyme increased the ductility of the CM/DPPC films with no film breakdown occurring up to the highest pressure measured (40 mN/m). In summary, CM increased the elasticity of the lipid films, but also caused them to become brittle and incapable of expansion following compression. Lysozyme adsorption increased the ductility and decreased the isotherm hysteresis for CM/DPPC films.     

A monolayer study on interactions of docetaxel with model lipid membranes

Docetaxel (DCT) is an antineoplastic drug for the treatment of a wide spectrum of cancers. DCT surface properties as well as miscibility studies with l-alpha-dipalmitoyl phosphatidylcholine (DPPC), which constitutes the main component of biological membranes, are comprehensively described in this contribution. Penetration studies have revealed that when DCT is injected under DPPC monolayers compressed to different surface pressures, it penetrates into the lipid monolayer promoting an increase in the surface pressure. DCT is a surface active molecule able to decrease the surface tension of water and to form insoluble films when spread on aqueous subphases. The maximum surface pressure reached after compression of a DCT Langmuir film was 13 mN/m. Miscibility of DPPC and DCT in Langmuir films has been studied by means of thermodynamic properties as well as by Brewster angle microscopy (BAM) analysis of the mixed films at the air-water interface, concluding that DPPC and DCT are miscible and they form non-ideally mixed monolayers at the air-water interface. Helmholtz energies of mixing revealed that no phase separation occurs. In addition, Helmholtz energies of mixing become more negative with decreasing areas per molecule, which suggests that the stability of the mixed monolayers increases as the monolayers become more condensed. Compressibility values together with BAM images indicate that DCT has a fluidizing effect on DPPC monolayers.     

Directed assembly of binary monolayers with a high protein affinity: infrared reflection absorption spectroscopy (IRRAS) and surface plasmon resonance (SPR)

Infrared reflection absorption spectroscopy (IRRAS) and surface plasmon resonance (SPR) techniques have been employed to investigate human serum albumin (HSA) binding to binary monolayers of zwitterionic dipalmitoylphosphatidylcholine (DPPC) and cationic dioctadecyldimethylammonium bromide (DOMA). At the air-water interface, the favorable electrostatic interaction between DPPC and DOMA leads to a dense chain packing. The tilt angle of the hydrocarbon chains decreases with increasing mole fraction of DOMA (X(DOMA)) in the monolayers at the surface pressure 30 mN/m: DPPC ( approximately 30 degrees ), X(DOMA) = 0.1 ( approximately 15 degrees ), and X(DOMA) = 0.3 ( approximately 0 degrees ). Negligible protein binding to the DPPC monolayer is observed in contrast to a significant binding to the binary monolayers. After HSA binding, the hydrocarbon chains at X(DOMA) = 0.1 undergo an increase in tilt angle from 15 degrees to 25 approximately 30 degrees , and the chains at X(DOMA) = 0.3 remain almost unchanged. The two components in the monolayers deliver through lateral reorganization, induced by the protein in the subphase, to form multiple interaction sites favorable for protein binding. The surfaces with a high protein affinity are created through the directed assembly of binary monolayers for use in biosensing.     

The production and verification of pristine semi-fluorinated thiol monolayers on gold

In this work the interaction between human serum albumin (HSA) and a monofluorinated phospholipid, 1-palmitoyl-2-[16-fluoropalmitoyl-phosphatidylcholine] (F-DPPC), was studied by using Langmuir monolayer and Brewster angle microscopy (BAM) techniques. Different amounts of F-DPPC were spread on a previously formed HSA monolayer located at the air/water interface at 25 °C and the mixed monolayers thus obtained showed the existence of a liquid expanded-liquid condensed (LE-LC) phase transition (at 14 mN/m), attributed to the pure F-DPPC monolayer, coexisting with a second transition (at 22-24 mN/m) corresponding to the protein conformational change from an unfolded state to another in “loops” configuration. Relative thickness measurements recorded during the compression of the mixed monolayers showed the existence of an “exclusion” surface pressure (π_exc), above which the protein is squeezed out the interface, but not totally. BAM images reveal that some protein molecules in a packed “loops” configuration remain at the interface at surface pressures higher than the “exclusion” surface pressure. The application of the Defay-Crisp phase rule to the phase diagram of the F-DPPC/HSA system can explain the existence of certain regions of surface pressure in which the mixed monolayer components are miscible, as well as those others that they are immiscible.     

Degradation behaviors of polyester monolayers at the air/water interface: Alkaline and enzymatic degradations

The degradation kinetics of Langmuir monolayer films of a series of biodegradable polyesters has been studied to investigate the effect of degradation medium, alkalinity and enzymes. The degradation behavior of polyester monolayers strongly depended on both degradation medium and surface pressure. As the surface pressure was increased, the degradation rates of poly(l-lactide) (PLLA) and poly[(R)-3-hydroxybutyrate] (P(3HB)) increased in both degradation media. When monolayers were exposed to an alkaline subphase, the degradation of PLLA monolayers occurred at relatively low surface pressures; the PLLA monolayers were hydrolyzed at pH 10.5 regardless of surface pressure, while the alkaline degradation of P(3HB) monolayer occurred over a constant surface pressure of 7 mN/m at pH 11.8. These results have been explained by the difference in hydrophilic/hydrophobic balance of the polymers; PLLA is more hydrophilic than P(3HB). In contrast, the enzymatic degradations of both polymer monolayers occurred at higher constant surface pressures than those of the alkaline treatment; 7 mN/m for PLLA and 10 mN/m for P(3HB). This behavior was attributed to the enzymes being much larger than the alkaline ions: the enzymes need a larger contact area with the submerged monolayers to be activated.     

Infrared reflection absorption spectroscopy coupled with Brewster angle microscopy for studying interactions of amphiphilic triblock copolymers with phospholipid monolayers

Novel water-soluble amphiphilic triblock copolymers poly(glycerol monomethacrylate)-b-poly(propylene oxide)-b-poly(glycerol monomethacrylate) (PGMA-b-PPO-b-PGMA) were synthesized because of their expected enhanced ability to interact with biological membranes compared to the well-known poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-b-PPO-b-PEO) block copolymers. Their bulkier hydrophilic PGMA blocks might induce a disturbance in the packing of liquid-crystalline lipid bilayers in addition to the effect caused by the hydrophobic PPO block alone. To gain a better insight into the polymer-membrane interactions at the molecular level, the adsorption kinetics and concomitant interactions of (PGMA14)(2-)PPO(34) with model membranes of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were monitored using infrared reflection absorption spectroscopy (IRRAS) coupled with Brewster angle microscopy (BAM) and surface pressure (pi) measurements. The maximum penetration surface pressure of ca. 39 mN/m suggests that (PGMA14)(2-)PPO(34) is able to insert into lipid monolayers even above the so-called monolayer-bilayer equivalent pressure of 30-35 mN/m. Copolymer adsorption to a liquid-expanded DPPC-d62 monolayer proceeds in a two-step mechanism: (i) initially only the more hydrophobic PPO middle block penetrates the lipid monolayer; (ii) following the liquid-expanded-liquid-condensed (LE-LC) phase transition, the bulky PGMA hydrophilic blocks are dragged into the headgroup region as the PPO block inserts further into the fatty acid region. The adsorption kinetics is considerably faster for DMPC-d54 monolayers due to their higher fluidity. Copolymer adsorption to an LC-DPPC-d62 monolayer leads to a change in the monolayer packing by forcing the lipid alkyl chains into a more vertical orientation, their tilt angle with respect to the surface normal being reduced from initially 30 degrees +/- 3 degrees to 18 degrees +/- 3 degrees. BAM images rule out macroscopic phase separation and show that coalescence of DPPC-d62 LC domains takes place at relatively low surface pressures of pi > or = 23 mN/m, suggesting that (PGMA14)(2-)PPO (34) partitions into both LE as well as LC domains.     

Comparison of paclitaxel penetration in normal and cancerous cervical model monolayer membranes

The aim of the present study was to evaluate the penetration of paclitaxel in normal as well as cancerous human cervical monolayer membranes and to compare these results with the paclitaxel penetration in a model dipalmitoylphosphatidylcholine (DPPC) monolayer. At physiologically relevant surface pressures of 30 mN/m, equilibrium drug penetration was observed in DPPC model membrane, whereas in cervical lipid model membranes exclusion of the drug and destabilization of the membrane was observed. The maximum surface pressure increment due to penetration (Δπ_max) of 600 nM paclitaxel, for DPPC monolayer was found to be 3.6, 5.4 and 5.0 times higher than those for penetration in the cancerous monolayer at surface pressures 10, 20 and 30 mN/m, respectively. At initial surface pressure 10 mN/m, the maximum surface pressure increment, for 600 nM paclitaxel penetration, of normal cervical lipid membrane was double that of the cancerous cervical lipid membrane. At 30 mN/m initial surface pressure the representative IC₅₀ concentration of the drug produced negligible drug penetration and significant membrane destabilization in cervical lipid model membranes. The difference in penetration profile could be due to differences in composition of the model membranes. The cholesterol level in cancerous cervical membrane was 1.5-folds higher than that in the normal cervical membrane. Apart from PC, another constituent present in 20–32% in cancerous and normal membranes is sphingomyelin (SM). Introduction of 70% SM to the DPPC monolayer decreased the Δπ_max from 4.7 to 1.1 mN/m, revealing the rigidifying effect of SM which was directly proportional to the amount of SM added. Modulation of fluidity of the membranes can alter the penetration of paclitaxel in biological membranes and hence its toxicity profile.     

Mixing behavior of a poly(ethylene glycol)-grafted phospholipid in monolayers at the air/water interface

Mixed phospholipid monolayers hosting a poly(ethylene glycol) (PEG)-grafted distearoylphosphatidylethanolamine with a PEG molecular weight of 5000 (DSPE-PEG5000) spread at the air/water interface were used as model systems to study the effect of PEG-phospholipids on the lateral structure of PEG-grafted membrane-mimetic surfaces. DSPE-PEG5000 has been found to mix readily with distearoylphosphoethanolamine-succinyl (DSPE-succynil), a phospholipid whose structure resembles closely that of the phospholipid part of the DSPE-PEG5000 molecule. However, properties of mixed monolayers such as morphology and stability varied significantly with DSPE-PEG5000 content. In particular, our surface pressure, epifluorescence microscopy (EFM), and Brewster angle microscopy (BAM) studies have shown that mixtures containing 1-9 mol % of DSPE-PEG5000 form stable condensed monolayers with no sign of microscopic phase separation at surface pressures above approximately 25 mN/m. Yet, at 1 mol % of DSPE-PEG5000 in mixed monolayers, the two components have been found to behave nearly immiscibly at surface pressures below approximately 25 mN/m. For monolayers containing 18-75 mol % of DSPE-PEG5000, a high-pressure transition has been observed in the low-compressibility region of their isotherms, which has been identified on the basis of continuous BAM imaging of monolayer morphology, as reminiscent of the collapse nucleation in a pure DSPE-PEG5000 monolayer. Thus, the comparative analysis of our surface pressure, EFM, and BAM data has revealed that there exists a rather narrow range of mixture compositions with DSPE-PEG5000 content between 3 and 9 mol %, where somewhat homogeneous distribution of DSPE-PEG5000 molecules and high pressure stability can be achieved. This finding can be useful to "navigating" through possible mixture compositions while developing guidelines to the rational design of membrane-mimetic surfaces with highly controlled bio-nonfouling properties.     

Patterning of magnetic bimetallic coordination nanoparticles of Prussian blue derivatives by the Langmuir-Blodgett technique

We report a novel method to prepare patterns of nanoparticles over large areas of the substrate. This method is based on the adsorption of the negatively charged nanoparticles dispersed in an aqueous subphase onto a monolayer of the phospholipid dipalmitoyl-l-α-phosphatidylcholine (DPPC) at the air-water interface. It has been used to prepare patterns of nanoparticles of Prussian blue analogues (PBA) of different size (K(0.25)Ni[Fe(CN)(6)](0.75) (NiFe), K(0.25)Ni[Cr(CN)(6)](0.75) (NiCr), K(0.25)Ni[Co(CN)(6)](0.75) (NiCo), Cs(0.4)Co[Cr(CN)(6)](0.8) (CsCoCr), and Cs(0.4)Co[Fe(CN)(6)](0.9) (CsCoFe)). The behavior of DPPC monolayer at the air-water interface in the presence of the subphase of PBA nanoparticles has been studied by the compression isotherms and Brewster angle microscopy (BAM) images. Atomic force microscopy (AFM) of the transferred films on mica substrates shows that patterns of the nanoparticles are observed for a 10(-4) M concentration of the subphase, based on the nanoparticle precursors, at surface pressures between 1 and 6 mN/m and transfer velocities from 10 to 80 mm/min. Vertical, horizontal, or tilted fringes of the nanoparticles with respect to the transfer direction can be obtained depending on the transfer velocity and surface pressure.     

Topography of dipalmitoyl-phosphatidyl-choline monolayers penetrated by β-casein

In this work we have analyzed the topography by atomic force microscopy (AFM) of dipalmitoyl-phosphatidyl-choline (DPPC) monolayers previously spread at the air–water interface and penetrated by β-casein. AFM images of β-casein–DPPC monolayers were taken from Langmuir–Blodgett films deposited onto hydrophilic mica substrates at different initial surface pressures (π_i) and after the compression of the mixed films. The monolayer topography depends on the initial structure of the phospholipid:liquid expanded (LE) at 3 mN/m, coexistence between LE and liquid condensed (LC) structures at 7 mN/m, at the end of the LE–LC transition at 10 mN/m, and with a LC structure at 15 mN/m. The area occupied by DPPC domains in the mixed film increases with the π_i value, especially for DPPC with a LC structure at 15 mN/m. At this surface pressure the thickness of the film is at a maximum. After the film compression at 25 mN/m, which is above the equilibrium spreading pressure of β-casein (), this protein is displaced from the interface by DPPC and the topography of the mixed monolayer depends on the initial structure of the DPPC monolayer. A notable feature of the topography of these mixed monolayers is the presence of multilayers of β-casein and DPPC of high thickness (50–70 nm) at the lower π_i values. Although the film is dominated by DPPC at the highest surface pressures (at 25 mN/m), β-casein is not displaced totally from the interface and coexists as β-casein collapsed domains within the network of the DPPC structure.     

New protocols for preparing dipalmitoylphosphatidylcholine dispersions and controlling surface tension and competitive adsorption with albumin at the air/aqueous interface

The adsorption behavior of dipalmitoylphosphatidylcholine (DPPC), which is the major component of lung surfactant, at the air/aqueous interface and the competitive adsorption with bovine serum albumin (BSA) were studied with tensiometry, infrared reflection absorption spectroscopy (IRRAS), and ellipsometry. Dynamic surface tensions lower than 1 mN/m were observed for DPPC dispersions, with mostly vesicles, prepared with new protocols, involving extensive sonication above 50 °C. The lipid adsorbs faster and more extensively for DPPC dispersions with vesicles than with liposomes. For DPPC dispersions by a certain preparation procedure at T > T_c, when lipid particles were observed on the surface, dynamic surface tensions as low as 1 mN/m were measured. Moreover, IRRAS intensities and ellipsometric δΔ values were found to be much higher than the values for other DPPC dispersions or spread DPPC monolayers, suggesting that a larger amount of liposomes or vesicles adsorb on the surface. For DPPC/BSA mixtures, the tension behavior is controlled primarily by BSA, which prevents the formation of a dense DPPC monolayer. When BSA is injected into the subphase with a spread DPPC monolayer or into a DPPC dispersion with preadsorbed layers, little or no BSA adsorbs and the DPPC layer remains on the surface. When a DPPC monolayer is spread on a BSA solution at 0.1 wt% at 25 °C, then DPPC lipid can displace the adsorbed BSA molecules. The lack of BSA adsorption, and the expulsion of BSA by DPPC monolayer is probably due to the strong hydrophilicity of the lipid polar headgroup. When a DPPC dispersion is introduced with Trurnit's method or when dispersion drops are sprayed onto the surface of a DPPC/BSA mixture, the surface tension becomes lower and is controlled by DPPC, which can prevent the adsorption of BSA. The results may be important in understanding inhibition of lung surfactants by serum proteins and in designing efficient protocols of surfactant preparation and administration.     

Comparison of paclitaxel penetration in normal and cancerous cervical model monolayer membranes

The aim of the present study was to evaluate the penetration of paclitaxel in normal as well as cancerous human cervical monolayer membranes and to compare these results with the paclitaxel penetration in a model dipalmitoylphosphatidylcholine (DPPC) monolayer. At physiologically relevant surface pressures of 30 mN/m, equilibrium drug penetration was observed in DPPC model membrane, whereas in cervical lipid model membranes exclusion of the drug and destabilization of the membrane was observed. The maximum surface pressure increment due to penetration (Δπ_max) of 600 nM paclitaxel, for DPPC monolayer was found to be 3.6, 5.4 and 5.0 times higher than those for penetration in the cancerous monolayer at surface pressures 10, 20 and 30 mN/m, respectively. At initial surface pressure 10 mN/m, the maximum surface pressure increment, for 600 nM paclitaxel penetration, of normal cervical lipid membrane was double that of the cancerous cervical lipid membrane. At 30 mN/m initial surface pressure the representative IC₅₀ concentration of the drug produced negligible drug penetration and significant membrane destabilization in cervical lipid model membranes. The difference in penetration profile could be due to differences in composition of the model membranes. The cholesterol level in cancerous cervical membrane was 1.5-folds higher than that in the normal cervical membrane. Apart from PC, another constituent present in 20–32% in cancerous and normal membranes is sphingomyelin (SM). Introduction of 70% SM to the DPPC monolayer decreased the Δπ_max from 4.7 to 1.1 mN/m, revealing the rigidifying effect of SM which was directly proportional to the amount of SM added. Modulation of fluidity of the membranes can alter the penetration of paclitaxel in biological membranes and hence its toxicity profile.     

Adsorption behavior of glucose oxidase on a dipalmitoylphosphatic acid monolayer and the characteristics of the mixed monolayer at air/liquid interfaces

A dipalmitoylphosphatic acid (DPPA) monolayer at the air/liquid interface is used as a binding layer to incorporate glucose oxidase (GOx) from the subphase. The effects of the adsorption time of GOx on the behavior of the mixed DPPA/GOx monolayer and the relevant structure of the mixed LB film were studied using the characteristics of the pressure-area (pi-A) isotherm, Brewster angle microscopy (BAM), and atomic force microscopy (AFM). The experimental results show that two equilibrium states of GOx adsorption exist in the presence of a DPPA monolayer. The first equilibrium stage occurs at tens of minutes after spreading of DPPA, and a surface pressure of ca. 7.5 mN/m is obtained. The second equilibrium stage approaches slowly, and a higher equilibrium surface pressure (ca. 16 mN/m) was obtained at ca. 8 h after the first stage. The BAM and AFM images show that, after the second equilibrium stage is reached, a more condensed phase and rough morphology are obtained on the mixed DPPA/GOx monolayer, indicating a higher amount of GOx incorporated into the mixed film. For the first equilibrium stage of GOx adsorption, DPPA molecules can still pack regularly and closely under compression, suggesting that GOx molecules are mainly located beneath the DPPA monolayer at the compressed state. A more uniform phase was detected on a film prepared after the first equilibrium stage was reached. The present result indicates that distinct structures and properties of mixed DPPA/GOx films can be prepared from the various stages of GOx adsorption.