Dissipative Quartz Crystal Microbalance Research Articles

Adsorption Behaviors of DPPC/MO Aggregates on SiO2 Surfaces

The adsorption kinetics of extruded 1,2-dipalmitoyl- sn-glycero-3-phosphatidylcholine (DPPC)/1-(cis-9-octadecenoyl)- rac-glycerol (monoolein, MO) aggregates on SiO 2 surface at 25 degrees C is investigated in real time, using the dissipative quartz crystal microbalance (QCM) technique. Four adsorption pathways have been identified depending on the molar fraction of MO in the DPPC/MO system: (I) intact vesicle adsorption, (II) vesicle reorganization on a SiO 2 surface, (III) supported lipid bilayer (SLB) formation, and (IV) cubosome adsorption. The results can be understood by the fact that DPPC is a lamellar phase-forming lipid, whereas MO prefers the cubic phase. Therefore, the incorporation of MO in DPPC increases the packing parameter. Equally important, MO also increases the mobility of lipid molecules and lateral pressure in the bilayers as a result of the presence of a unique cis- double bond. Before extrusion, the vesicles size increases with the MO content when X MO <or= 0.7 and cubosomes are formed for X MO >or= 0.8. The extruded DPPC/MO suspensions consist of reformed vesicles for X MO <or= 0.7 and filtered cubosomes for X MO >or= 0.8, all with a uniform diameter of approximately 100 nm. Differential scanning calorimetry (DSC) further indicates that the addition of MO lowers the main phase transition temperature of DPPC and thus makes the hydrophobic interior more fluid.

Application of PEDOT layers for the electrogravimetric detection of sulphate and phosphate in aqueous media

Poly(3,4-ethylenedioxythiophene), PEDOT, films were electrochemically prepared from aqueous solutions containing different supporting electrolytes (ClO 4 −, SO 4 2− and H 2PO 4 −). Characterization of the resulting films was carried out using cyclic voltammetry and potential step experiments combined with the dissipative quartz crystal microbalance technique. The ion incorporation caused an increase of the film mass and below 10 −2 M depended on the concentration of the solutions. Potential step experiments showed that the pH of the solutions also has an influence on the mass change of the films, which can only be partially attributed to the pH dependence of the charge of the anions. In principle, the ratio of the mass and the passed electric charge (apparent molar mass) during the redox switching of the polymer can be used to identify the ionic species.

Interactions of fusidic acid and elongation factor G with lipid membranes

Fusidic acid (FA) is a potent antibiotic and blocks the protein synthesis by binding to elongation factor G (EF-G) directly. Here we hypothesized that the antibiotic activity of FA would be potentiated by several orders of magnitude if both FA and EF-G would be residing in the lipid membranes and, hence, the probability of interaction would transform from three-dimensional to two-dimensional. Such detailed information could lead to more effective therapeutic interventions if they are understood on a molecular level. Interactions between FA and various lipid membranes composed of 1-palmitoyl-2-oleyl- sn-glycero-3-phosphocholine (POPC) and cholesterol (Chol) were studied by capillary electrochromatography (CEC). The influence of the lipid vesicle size—sonicated liposomes and liposomes extruded through 30-, 50-, and 100-nm filters—on the packing of vesicles on the silica capillary surface was investigated by CEC and dissipative quartz crystal microbalance. The CEC results evidenced that FA interacts with and resides in phospholipid membranes. Likewise, monolayer, asymmetrical flow field flow fractionation, and CEC studies confirmed that EF-G is hydrophobic and incorporated into POPC and POPC/Chol membranes. Including EF-G in phospholipid vesicles did not improve the binding of FA to the membranes.

Construction of Viscoelastic Biocompatible Films via the Layer-by-Layer Assembly of Hyaluronan and Phosphorylcholine-Modified Chitosan

Films of hyaluronan (HA) and a phosphorylcholine-modified chitosan (PC-CH) were constructed by the polyelectrolyte multilayer (PEM) deposition technique and their buildup in 0.15 M NaCl was followed by atomic force microscopy, surface plasmon resonance spectroscopy (SPR), and dissipative quartz crystal microbalance (QCM). The HA/PC-CH films were stable over a wide pH range (3.0-12.0), exhibiting a stronger resistance against alkaline conditions as compared to HA/CH films. The loss and storage moduli, G' and G", of the films throughout the growth of eight bilayer assemblies were derived from an impedance analysis of the QCM data recorded in situ. Both G' and G" values were one order of magnitude lower than the moduli of HA/CH films. The fluid gel-like characteristics of HA/PC-CH multilayers were attributed to their high water content (50 wt %), which was estimated by comparing the surface coverage values derived from SPR and QCM measurements. Given the versatility of the PEM methodology, HA/PC-CH films are attractive tools for developing biocompatible surface coatings of controlled mechanical properties.

Structure of Anionic Phospholipid Coatings on Silica by Dissipative Quartz Crystal Microbalance

The adsorption of anionic phospholipids on silica was investigated by the dissipative quartz crystal microbalance (QCM) technique. Liposomes composed of 1 mM 80:20 mol % of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphatidylcholine (POPC)/phosphatidic acid, POPC/phosphatidylglycerol, or POPC/phosphatidylserine in N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid) buffer at pH 7.4 (with or without 3 mM of CaCl2) were examined. We have previously demonstrated that similar phospholipid coatings can be used in capillary electrochromatography as a stationary phase for the separation of analytes. In this work, we focus on the formation of the coatings and on the type of lipid structure formed on silica. The QCM investigation comprised qualitative results based on changes in frequency and resistance, and quantitative modeling of the obtained results. The latter was performed using the dissipative QCM, which measures the quartz crystal impedance, combined with equivalent circuit analysis. A previously developed coating and cleaning procedure for phospholipid-coated fused silica capillaries was adopted in this study, and the same silica-coated crystal was used throughout the QCM study. We will demonstrate in this work that the type of lipid structure formed on silica, that is, a rather rigid supported lipid bilayer or a viscoelastic supported vesicle layer (SVL), is highly dependent on the lipid and solvent composition. We also show for the first time that the modeling of the dissipative QCM data can be used to extract a more quantitative picture of an adsorbed SVL, because, so far, published studies have merely used the QCM data in a qualitative sense.

Simple dissipative quartz crystal microbalance and methods for determining dissipation decay constants

We describe a simple dissipative quartz crystal microbalance (QCM) and two simple methods for determining the dissipation factor. The microbalance consists of an oscillator circuit interfaced with a personal computer. The oscillation voltages are undersampled through a low speed data acquisition card. Both methods for determining the resonant frequency and the dissipation factor assume a limited variation of the resonant frequency, which is the case for general applications of QCMs. The first method directly fits the undersampled data with a nonlinear function. The second method determines the resonant frequency of a quartz crystal by Fourier transformation of the acquired data. The dissipation factor is obtained by rectifying the undersampled data and then fitting them with an exponential function.