R. Krustev

Gold-nanoparticle/organic linker films: self-assembly, electronic and structural characterisation, composition and vapour sensitivity

Gold-nanoparticle/organic films were prepared via layer-by-layer self-assembly using dodecylamine-stabilised Au-nanoparticles and poly(propyleneimine) (PPI) dendrimers of generation one to five (G1-G5) or hexadecanedithiol (HDT) as linker compounds. TEM and FE-SEM images revealed that the bulk of the films consisted of nanoparticles with diameters of about 4 nm. XPS was used to study the chemical composition of the films. The C 1s and N 1s signals of an AuPPI-G4 film were interpreted qualitatively according to the dendrimer structure. The absence of the nitrogen signal in case of an AuHDT film indicated that the dodecylamine ligands were quantitatively exchanged during film assembly. About 76% of the sulfur atoms were bound to the nanoparticles. the remainder being present as free thiol (S H) groups. All films displayed linear current voltage characteristics and Arrhenius-type activation of charge transport. The conductivities of the AuPPI films decreased exponentially over approximately two orders of magnitude (6.8 x 10(-2) to 1.0 x 10(-3) ohms(-1) cm(-1)) with a five-fold increase of the dendrimer generation number. Dosing the films with solvent vapours caused their resistances to increase. Using different solvent vapours demonstrated that the sensitivity of this response was determined by the solubility properties of the linker compounds. Microgravimetric measurements showed that absorption of analyte was consistent with a Langmuir adsorption model. These measurements also revealed a linear correlation between the electrical response (deltaR/Rini) and the concentration of absorbed analyte. The absorption of d4-methanol from a saturated vapour atmosphere was studied by neutron reflectometry with an AuPPI-G4 film. This measurement indicated condensation of methanol on top of the film and a uniform distribution of the analyte across the film thickness.

X-ray diffraction and foam film investigations of PC head group interaction in water/ethanol mixtures.

The influence of ethanol on single phospholipid monolayers at the water/air interface and in foam films has been investigated. Grazing incidence X-ray diffraction investigations (GIXD) of Langmuir monolayers from 1,2-distearoyl-phosphatidylcholine (DSPC) spread on water subphases with different amounts of ethanol were performed. The thickness and free specific energy of formation of foam films stabilized by 1,2-dimyristoyl-phosphatidylcholine (DMPC) at different concentrations of ethanol in the film forming dispersions were measured. The GIXD investigations show that the tilt angle of the alkyl chains in the PC lipid monolayer decreases with increasing concentration of ethanol caused by a decrease of the diameter of the head groups. With increasing ethanol content of the solution also the thickness of the aqueous core of PC lipid foam films decreases. We assume that ethanol causes a decreasing probability for the formation of hydrogen bonds of water molecules to the PC head groups. The distinct difference between the effects of ethanol on lipid bilayers as described in the literature and on monolayers and foam films found in this study is discussed. Whereas PC monolayers at the water/air interface become unstable above 25 vol.% ethanol, the PC foam films are stable up to 50 vol.% ethanol. This is related to the decrease of the surface excess energy per lipid molecule by the interaction between the two film surfaces.

Influence of Na+, Ca2+ on the thickness and free energy of dmpc foam films.

Abstract Foam films prepared from dispersions of dimyristoylphophatidylcholine (DMPC) in water and a water–ethanol mixture have been investigated. The thickness of the films and their contact angle with the surrounding meniscus of liquid have been measured as a function on the concentration of NaCl and CaCl2. The measurements show that the preparation of the lipid dispersions influences long and short-range interactions. The binding of Ca2+-ions onto the DMPC headgroups leads to charging of the monolayers and electrical double-layer repulsion in the film. There is also some indication for a weak binding of Na+-ions. The free energy of film formation determined from contact angle measurements provides additional information on the estimation of the different contributions to the interaction in the film.

Influence of ethanol on the thickness and free energy of film formation of DMPC foam films

Foam films prepared from 1,2-dimirystoil-sn-glycero-3-phosphorylcholine (DMPC) dispersions in water–ethanol mixtures were investigated. Their thickness and contact angle (foam film/meniscus) were measured. Experimental results show that an increase of EtOH concentration in the film forming dispersions leads to a decrease in the film thickness. At EtOH concentrations above 40% v/v the foam films have a bilayer structure without any noticeable core of solvent. The film thickness remains constant with the further increasing of the EtOH concentration up until 50% v/v. This behaviour is corroborated by the strong increase in the contact angles (a decrease in the free energy of film formation) with increasing EtOH concentration. Ellipsometric measurements on the thickness of adsorbed DMPC monolayers and surface pressure isotherms of DMPC spread on the water–ethanol subphases show that the effective area per lipid molecule decreases, resulting in a larger monolayer thickness, when the EtOH concentration in the subphase is increased. An increase of the EtOH concentration leads to a dehydration of the DMPC molecules and a reduction in strength and range of the repulsive hydration force between the film monolayers. The film thickness and the free energy of film formation are governed by the balance of the van der Waals attraction and the repulsive hydration force between the foam film surfaces.