The degree of polydispersity in micellar dispersions: A sum rule approach

Statistical properties of a two-dimensional ideal dispersion of polydisperse micelles are derived by analyzing the convergence properties of a sum rule set by mass conservation. Internal micellar degrees of freedom are accounted for by a microscopic model describing small displacements of the constituting amphiphiles with respect to their equilibrium positions. The transfer matrix (TM) method is employed to compute internal micelle partition function. We show that the conditions under which the sum rule is saturated by the largest eigenvalue of the TM determine the value of amphiphile concentration above which the dispersion becomes highly polydisperse and micelle sizes approach a Schultz distribution. (C) 2011 Elsevier B.V. All rights reserved.

Lipid bilayer pre-transition as the beginning of the melting process

We investigate the bilayer pre-transition exhibited by some lipids at temperatures below their main phase transition, and which is generally associated to the formation of periodic ripples in the membrane. Experimentally we focus on the anionic lipid dipalmytoylphosphatidylglycerol (DPPG) at different ionic strengths, and on the neutral lipid dipalmytoylphosphatidylcholine (DPPC). From the analysis of differential scanning calorimetry traces of the two lipids we find that both pre- and main transitions are part of the same melting process. Electron spin resonance of spin labels and excitation generalized polarization of Laurdan reveal the coexistence of gel and fluid domains at temperatures between the pre- and main transitions of both lipids, reinforcing the first finding. Also, the melting process of DPPG at low ionic strength is found to be less cooperative than that of DPPC. From the theoretical side, we introduce a statistical model in which a next-nearest-neighbor competing interaction is added to the usual two-state model. For the first time, modulated phases (ordered and disordered lipids periodically aligned) emerge between the gel and fluid phases as a natural consequence of the competition between lipid-lipid interactions. (C) 2009 Elsevier B.V. All rights reserved.

Vesicles with charged domains

This work summarizes results obtained on membranes composed of the ternary mixture dioleoylphosphatidylglycerol (DOPG), egg sphingomyelin (eSM) and cholesterol (Chol). The membrane phase state as a function of composition is characterized from data collected with fluorescence microscopy on giant unilamellar vesicles. The results suggest that the presence of the charged DOPG significantly decreases the composition region of coexistence of liquid ordered and liquid disordered phases as compared to that in the ternary mixture of dioleoylphosphatidycholine, sphingomyelin and cholesterol. The addition of calcium chloride to DOPG:eSM:Chol vesicles, and to a lesser extent the addition of sodium chloride, leads to the stabilization of the two-phase coexistence region, which is expressed in an increase in the miscibility temperature. On the other hand, addition of the chelating agent EDTA has the opposite effect, suggesting that impurities of divalent cations in preparations of giant vesicles contribute to the stabilization of charged domains. We also explore the behavior of these membranes in the presence of extruded unilamellar vesicles made of the positively charged lipid dioleoyltrimethylammoniumpropane (DOTAP). The latter can induce domain formation in DOPG:eSM:Chol vesicles with initial composition in the one-phase region. (C) 2010 Elsevier B.V. All rights reserved.

Layer-by-Layer Assembly of Carbon Nanotubes Incorporated in Light-Addressable Potentiometric Sensors

The integration of carbon nanotubes in conjunction with a chemical or biological recognition element into a semiconductor field-effect device (FED) may lead to new (bio)chemical sensors. In this study, we present a new concept to develop field-effect-based sensors, using a light-addressable potentiometric sensor (LAPS) platform modified with layer-by-layer (LbL) films of single-walled carbon nanotubes (SWNTs) and polyamidoamine (PAMAM) dendrimers. Film growth was monitored for each layer adsorbed on the LAPS chip by Measuring current-voltage (IIV) curves. The morphology of the films was analyzed via atomic force microscopy (AFM) and field-emission scanning electron microscopy (FESEM), revealing the formation of a highly interconnected nanostructure of SWNTs-network into the dendrimer layers. Constant current (CC) Measurements showed that the incorporation of the PAMAM/SWNT LbL film containing LIP to 6 bilayers onto the LAPS Structure has a high pH sensitivity of ca. 58 mV/pH. The biosensing ability of the devices was tested for penicillin G via adsorptive immobilization of the enzyme penicillinase atop the LgL film. LAPS architectures modified with the LbL film exhibited higher sensitivity, ca. 100 mV/decade, in comparison to ca. 79 mV/decade for all unmodified LAPS, which demonstrates the potential application of the CNT-LbL Structure in field-effect-based (bio)chemical sensors.

Enhanced activity of horseradish peroxidase in Langmuir-Blodgett films of phospholipids

The immobilization of enzymes in nanostructured films has potential applications, e.g. in biosensing, for which the activity may not only be preserved, but also enhanced if optimized conditions are identified. Optimization is not straightforward because several requirements must be fulfilled, including a suitable matrix and film-forming technique. In this study, we show that horseradish peroxidase (HRP) has its activity enhanced when immobilized in Langmuir-Blodgett (LB) films, in conjunction with dipalmitoylphosphaticlylglycerol (DPPG). Incorporation of HRP into a DPPG monolayer at the air-water interface was demonstrated with compression isotherms, and Polarization-Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS). From the PM-IRRAS data, we inferred that HRP was not denatured when adsorbed on a pre-formed, low pressure DPPG monolayer. A change in orientation was induced by the phospholipid matrix, with the amide C=O and NH groups from HRP being oriented perpendicular to the surface, parallel to the DPPG acyl chains, i.e. the alpha-helix was inserted into the monolayer. The mixed DPPG-HRP monolayer could be transferred onto solid supports, to which HRP activity was ca. 23% higher than in solution. The control of molecular architecture and choice of a suitable phospholipid matrix allowed HRP-containing LB films to be used in sensing peroxide. (c) 2008 Elsevier B.V. All rights reserved.

Enzyme immobilisation on electroactive nanostructured membranes (ENM): Optimised architectures for biosensing

Electroactive nanostructured membranes have been produced by the layer-by-layer (LbL) technique, and used to make electrochemical enzyme biosensors for glucose by modification with cobalt hexacyanoferrate redox mediator and immobilisation of glucose oxidase enzyme. Indium tin oxide (ITO) glass electrodes were modified with up to three bilayers of polyamidoamine (PAMAM) dendrimers containing gold nanoparticles and poly(vinylsulfonate) (PVS). The gold nanoparticles were covered with cobalt hexacyanoferrate that functioned as a redox mediator, allowing the modified electrode to be used to detect H(2)O(2), the product of the oxidase enzymatic reaction, at 0.0 V vs. SCE. Enzyme was then immobilised by cross-linking with glutaraldehyde. Several parameters for optimisation of the glucose biosensor were investigated, including the number of deposited bilayers, the enzyme immobilisation protocol and the concentrations of immobilised enzyme and of the protein that was crosslinked with PAMAM. The latter was used to provide glucose oxidase with a friendly environment, in order to preserve its bioactivity. The optimised biosensor, with three bilayers, has high sensitivity and operational stability, with a detection limit of 6.1 mu M and an apparent Michaelis-Menten constant of 0.20 mM. It showed good selectivity against interferents and is suitable for glucose measurements in natural samples. (C) 2008 Elsevier B.V. All rights reserved.

Processing of electroactive nanostructured films incorporating carbon nanotubes and phthalocyanines for sensing

The use of carbon nanotubes (CNTs) combined with other materials in nanostructured films has demonstrated their versatility in tailoring specific properties. In this study, we produced layer-by-layer (LbL) films of polyamidoamine-PAMAM-incorporating multiwalled carbon nanotubes (PAMAM-NT) alternated with nickel tetrasulfonated metallophthalocyanine (NiTsPc), in which the incorporation of CNTs enhanced the NiTsPc redox process and its electrocatalytic properties for detecting dopamine. Film growth was monitored by UV-vis spectroscopy, which pointed to an exponential growth of the multilayers, whose roughness increased with the number of bilayers according to atomic force microscopy (AFM) analysis. Strong interactions between -NH3+ terminal groups from PAMAM and -SO3- from NiTsPc were observed via infrared spectroscopy, while the micro-Raman spectra confirmed the adsorption of carbon nanotubes (CNTs) onto the LbL film containing NiTsPc. Cyclic voltammograms presented well-defined electroactivity with a redox pair at 0.86 and 0.87 V, reversibility, a charge-transfer controlled process, and high stability up to 100 cycles. The films were employed successfully in dopamine (DA) detection, with limits of detection and quantification of 10(-7) and 10(-6) mol L-1, respectively. Furthermore, films containing immobilized CNTs could distinguish between DA and its natural interferent, ascorbic acid (AA).

Phytase immobilization on modified electrodes for amperometric biosensing

Phytase (myo-inositol hexaphosphate phosphohydrolase) and phytic acid (myo-inositol hexaphosphate) play an important environmental role, in addition to being a health issue in food industry. Phytic acid is antinutritional due to its ability to chelate metal ions and may also react with proteins decreasing their bioavailability. In this work, we produced biosensors with phytase immobilized in Layer-by-Layer (LbL) films, which could detect phytic acid with a detection limit of 0.19 mmol L-1, which is sufficient to detect phytic acid in seeds of grains and vegetables. The biosensosrs consisted of LbL films containing up to eight bilayers of phytase alternated with poly(allylamine) hydrochloride (PAH) deposited onto an indium-tin oxide (ITO) substrate modified with Prussian Blue. Amperometric detection was conducted in an acetate buffer solution (at pH 5.5) at room temperature, with the biosensor response attributed to the formation of phosphate ions. In subsidiary experiments with the currents measured at 0.0 V (vs. SCE), we demonstrated the absence of effects from some interferents, pointing to a good selectivity of the biosensor. (c) 2007 Elsevier B.V. All rights reserved.

Immobilization of Ibuprofen-Containing Nanospheres in Layer-by-Layer Films

Liposomes have been applied to many fields as nanocarriers, especially in drug delivery as active molecules may be entrapped either in their aqueous interior or onto the hydrophobic surface. In this paper we describe the fabrication of layer-by-layer (LbL) films made with liposomes incorporating the anti-inflammatory ibuprofen. The liposomes were made with dipalmitoyl phosphatidyl choline (DPPC), dipalmitoyl phosphatidyl glycerol (DPPG) and palmitoyl oleoyl phosphatidyl glycerol (POPG). LbL films were assembled via alternate adsorption of the polyamidoamine dendrimer (PAMAM), generation 4, and liposomes containing ibuprofen. According to dynamic light scattering measurements, the incorporation of ibuprofen caused DPPC and DPPG liposonnes to become more stable, with a decrease in diameter from 140 to 74 nm and 132 to 63 nm, respectively. In contrast, liposomes from POPG became less stable, with an increase in size from 110 to 160 nm after ibuprofen incorporation. These results were confirmed by atomic force microscopy images of LbL films, which showed a large tendency to rupture for POPG liposomes. Film growth was monitored using nanogravimetry and UV-Vis spectroscopy, indicating that growth stops after 10 bilayers. The release of ibuprofen obtained with fluorescence measurements was slower for the liposomes, with decay times of 9.2 and 8.5 h for DPPG and POPG liposomes, respectively, than for the free drug with a decay time of 5.2 h. Ibuprofen could also be released from the LbL films made with DPPG and POPG liposomes, which is promising for further uses in patches.

Interaction of a C-terminal peptide of Bos taurus diacylglycerol acyltransferase 1 with model membranes

Diacylglycerol acyltransferase 1 (DGAT1) catalyzes the final and dedicated step in the synthesis of triacylglycerol, which is believed to involve the lipids oleoyl coenzyme A (OCoA) and dioleoyl-sn-glycerol (DOG) as substrates. In this work we investigated the interaction of a specific peptide, referred to as SIT2, on the C-terminal of DGAT1 (HKWCIRHFYKP) with model membranes made with OCoA and DOG in Langmuir monolayers and liposomes. According to the circular dichroism and fluorescence data, conformational changes on SIT2 were seen only on liposomes containing OCoA and DOG. In Langmuir monolayers, SIT2 causes the isotherms of neat OCoA and DOG monolayers to be expanded, but has negligible effect on mixed monolayers of OCoA and DOG. This synergistic interaction between SIT2 and DOG + OCoA may be rationalized in terms of a molecular model in which SIT2 may serve as a linkage between the two lipids. Our results therefore provide molecular-level evidence for the interaction between this domain and the substrates OCoA and DOG for the synthesis of triacylglycerol. (C) 2009 Elsevier B.V. All rights reserved.

Silsesquioxane as a New Building Block Material for Modified Electrodes Fabrication and Application as Neurotransmitters Sensors

Nanostructured films comprising a 3-n-propylpyridiniunn silsesquioxane polymer (designated as SiPy(+)Cl(-)) and copper (II) tetrasulfophthalocyanine (CuTsPc) were produced using the Layer-by-Layer technique (LbL). To our knowledge this is the first report on the use of silsesquioxane derivative polymers as building blocks for nanostructured thin films fabrication. Deposition of the multilayers were monitored by UV-Vis spectroscopy revealing the linear increment in the absorbance of the Q-band from CuTsPc at 617 nm with the number of SiPy(+)Cl(-)/CuTsPc or CuTsPc/SiPy(+)Cl(-) bilayers. FTIR analyses showed that specific interactions between SiPy+Cl- and CuTsPc occurred between SO(3)(-) groups of tetrasulfophthalocyanine and the pyridinium groups of the polycation. Morphological studies were carried out using the AFM technique, which showed that the roughness and thickness of the films increase with the number of bilayers. The films displayed electroactivity and were employed to detection of dopamine (DA) and ascorbic acid (AA) using cyclic voltammetry, at concentrations ranging from 1.96 x 10(-4) to 1.31 x 10(-3) molL(-1). The number and the sequence of bilayers deposition influenced the electrochemical response in presence of DA and AA. Using differential pulse technique, films comprising SiPy(+)/CuTsPc were able to distinguish between DA and ascorbic acid (AA), with a potential difference of approximately with 500 mV, in the concentration range of 9.0 x 10(-5) to 2.0 x 10(-4) molL(-1), in pH 3.0.

The use of colloidal ferrofluid as building blocks for nanostructured layer-by-layer films fabrication

The electrostatic layer-by-layer technique has been exploited as an useful strategy for fabrication of nanostructured thin films, in which specific properties can be controlled at the molecular level. Ferrofluids consist of a colloidal suspension of magnetic grains (with only a few nanometers of diameter) with present interesting physical properties and applications, ranging from telecommunication to drug delivery systems. In this article, we developed a new strategy to manipulate ferrofluids upon their immobilization in nanostructured layered films in conjunction with conventional polyelectrolytes using the layer-by-layer technique. We investigated the morphological, optical, and magnetic properties of the immobilized ferrofluid as a function of number of bilayers presented in the films. Ferrofluid/polyelectrolyte multilayers homogeneously covered the substrates surface, and the magnetic and optical properties of films exhibited a linear dependence on the number of bilayers adsorbed.

Interactions between Bacteriophage DNA and Cationic Biomimetic Particles

The interaction between giant bacteriophage DNA and cationic biomimetic particles was characterized from sizing by dynamic light-scattering, zeta-potential analysis, turbidimetry, determination of colloid stability, visualization from atomic force microscopy (AFM), and determination of cytotoxicity against E. coli from colony forming unities counting. First, polystyrene sulfate (PSS) particles with different sizes were covered by a dioctadecyldimethylammonium bromide (DODAB) bilayer yielding the so-called cationic biomimetic particles (PSS/DODAB). These cationic particles are highly organized, present a narrow size distribution and were obtained over a range of particle sizes. Thereafter, upon adding lambda, T5 or T2-DNA to PSS/DODAB particles, supramolecular assemblies PSS/DODAB/DNA were obtained and characterized over a range of DNA concentrations and particle sizes (80-700 nm). Over the low DNA concentration range, PSS/DODAB/DNA assemblies were cationic, colloidally stable with moderate polydispersity and highly cytotoxic against E. coli. From DNA concentration corresponding to charge neutralization, neutral or anionic supramolecular assemblies PSS/DODAB/DNA exhibited low colloid stability, high polydispersity and moderate cytotoxicity. Some nucleosome mimetic assemblies were observed by AFM at charge neutralization (zeta-potential equal to zero).

Hybrid materials from intermolecular associations between cationic lipid and polymers

Intermolecular associations between a cationic lipid and two model polymers were evaluated from preparation and characterization of hybrid thin films cast on silicon wafers. The novel materials were prepared by spin-coating of a chloroformic solution of lipid and polymer on silicon wafer. Polymers tested for miscibility with the cationic lipid dioctadecyldimethylammonium bromide (DODAB) were polystyrene (PS) and poly(methyl methacrylate) (PMMA). The films thus obtained were characterized by ellipsometry, wettability, optical and atomic force microscopy, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and activity against Escherichia coli. Whereas intermolecular ion-dipole interactions were available for the PMMA-DODAB interacting pair producing smooth PMMA-DODAB films, the absence of such interactions for PS-DODAB films caused lipid segregation, poor film stability (detachment from the silicon wafer) and large rugosity. In addition, the well-established but still remarkable antimicrobial DODAB properties were transferred to the novel hybrid PMMA/DODAB coating, which is demonstrated to be highly effective against E. coli.

Immobilization of liposomes in nanostructured layer-by-layer films containing dendrimers

Artificial vesicles or liposomes composed of lipid bilayers have been widely exploited as building blocks for artificial membranes, in attempts to mimic membrane interaction with drugs and proteins and to investigate drug delivery processes. In this study we report on the immobilization of liposomes of 1,2-dipalmitoyi-sn-Glycero-3-[Phospho-rac-(1-glycerol)] (Sodium Salt) (DPPG) in layer-by-layer (LbL) films, alternated with poly (amidoamine) G4 (PAMAM) dendrimer layers. The average size of the liposomes in solution was 120 nm as determined by dynamic light scattering, with their spherical shape being inferred from scanning electron microscopy (SEM) in cast films. LbL films containing up to 20 PAMAM/DPPG bilayers were assembled onto glass and/or silicon wafer substrates. The growth of the multilayers was achieved by alternately immersing the substrates into the PAMAM and DPPG solutions for 5 and 10 min, respectively. The formation of PAMAM/DPPG liposome multilayers and its ability to interact with BSA were confirmed by Fourier transform infrared spectroscopy (FTIR). The structural features and film thickness were obtained using X-ray diffraction and surface plasmon resonance (SPR). (c) 2007 Elsevier B.V. All rights reserved.