Strategies to Optimize Biosensors Based on Impedance Spectroscopy to Detect Phytic Acid Using Layer-by-Layer Films

Impedance spectroscopy has been proven a powerful tool for reaching high sensitivity in sensor arrays made with nanostructured films in the so-called electronic tongue systems, whose distinguishing ability may be enhanced with sensing units capable of molecular recognition. In this study we show that for optimized sensors and bio-sensors the dielectric relaxation processes involved in impedance measurements should also be considered, in addition to an adequate choice of sensing materials. We used sensing units made from layer-by-layer (LbL) films with alternating layers of the polyeletrolytes, poly(allylamine) hydrochloride (PAH) and poly(vinyl sulfonate) (PVS), or LbL films of PAH alternated with layers of the enzyme phytase, all adsorbed on gold interdigitate electrodes. Surprisingly, the detection of phytic acid was as effective in the PVS/PAH sensing system as with the PAH/phytase system, in spite of the specific interactions of the latter. This was attributed to the dependence of the relaxation processes on nonspecific interactions such as electrostatic cross-linking and possibly on the distinct film architecture as the phytase layers were found to grow as columns on the LbL film, in contrast to the molecularly thin PAH/PVS films. Using projection techniques, we were able to detect phytic acid at the micromolar level with either of the sensing units in a data analysis procedure that allows for further optimization.

The role of an insulating surface layer on the relaxation time of the ionic redistribution in an electrolytic cell

We analyze the influence of a surface dielectric layer on the transient phenomena related to the ionic redistribution in an electrolytic cell submitted to a step-like external voltage. The adsorption-desorption phenomenon is taken into account in the famework of the Gouy-Chapman approximation, where the ions are assumed dimensionless. In the limit of small amplitude of the applied voltage, where the equations of the problem can be linearized, we obtain an analytical solution for the surface densities of ions, for the electrical potential and for the relaxation time for the transient phenomena. In the general case, when the linearized analysis is no longer valid, the solution of the problem is obtained numerically. The role of the thickness of the dielectric layer on the relaxation time is also discussed.

Ionization and Structural Changes of the DMPG Vesicle along Its Anomalous Gel-Fluid Phase Transition: A Study with Different Lipid Concentrations

Dispersions of saturated anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG) have been extensively studied regarding their peculiar thermostructural behavior. At low ionic strength, the gel-fluid transition is spread along nearly 17 degrees C, displaying several thermal events in the calorimetric profile that is quite different from the single sharp peak around 23 degrees C found for higher ionic strength DMPG dispersions. To investigate the role of charge in the bilayer transition, we carefully examine the temperature dependence of the electrical conductivity of DMPG dispersions at different concentrations, correlating the data with the corresponding differential scanning calorimetry (DSC) traces. Electrical conductivity together with electrophoretic mobility measurements allowed the calculation of the dependence of the degree of ionization of DMPG vesicles on lipid concentration and temperature. It was shown that there is a decrease in vesicle charge as the lipid concentration increases, which is probably correlated with the increase in the concentration of bulk Na(+). Apart from the known increase in the electrical conductivity along the DMPG temperature transition region, a sharp rise was observed at the bilayer pretransition for all lipid concentrations studied, possibly indicating that the beginning of the chain melting process is associated with an increase in bilayer ionization. It is confirmed here that the gel-fluid transition of DMPG at low ionic strength is accompanied by a huge increase in the dispersion viscosity. However, it is shown that this measured macroviscosity is distinct from the local viscosity felt by either charged ions or DMPG charged aggregates in measurements of electrical conductivity or electrophoretic mobility, Data presented here give support to the idea that DMPG vesicles, at low ionic strength, get more ionized along the temperature transition region and could be perforated and/or deformed vesicle structures.

Ion dependence of magnetic anisotropy in MFe(2)O(4) (M = Fe, Co, Mn) nanoparticles synthesized by high-temperature reaction

The influence of different M(2+) cations on the effective magnetic anisotropy of systems composed of MFe(2)O(4) (M Fe, Co and Mn) nanoparticles was investigated. Samples were prepared by the high-temperature (538 K) solution phase reaction of Fe (acac) 3, Co (acac) 2 and Mn (acac) 2 with 1,2 octanodiol in the presence of oleic acid and oleylamine. The final particles are coated by an organic layer of oleic acid that prevents agglomeration. Transmission electron microscopy (TEM) images show that particles present near spherical form and a narrow grain size distribution, with mean diameters in the range of 4.5 - 7.6 nm. Powder samples were analyzed by ac susceptibility and Mossbauer measurements, and K(eff) for all samples was evaluated using both techniques, showing a strong dependence on the nature of the divalent cation. (C) 2008 Elsevier B.V. All rights reserved.

In situ impedance spectroscopy study of the electrochemical corrosion of Ti and Ti-6Al-4V in simulated body fluid at 25 degrees C and 37 degrees C

The corrosion resistance of Ti and Ti-6Al-4V was investigated through electrochemical impedance spectroscopy, EIS, potentiodynamic polarisation curves and UV-Vis spectrophotometry. The tests were done in Hank solution at 25 degrees C and 37 degrees C. The EIS measurements were done at the open circuit potential at specific immersion times. An increase of the resistance as a function of the immersion time was observed, for Ti (at 25 degrees C and 37 degrees C), and for Ti-6Al-4V (at 25 degrees C), which was interpreted as the formation and growth of a passive film on the metallic surfaces. (C) 2009 Elsevier Ltd. 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.

Molecular Ordering of Layer-by-Layer Polyelectrolyte Films Studied by Sum-Frequency Vibrational Spectroscopy

The molecular arrangement in organic thin films is crucial for their increasing technological applications. Here, we use vibrational spectroscopy by sum-frequency generation (SFG) to study the ordering of polyelectrolyte layers adsorbed on silica for all steps of layer-by-layer (LbL) self-assembly. In situ measurements during adsorption and rinsing showed that the adsorbed polymer has a disordered conformation and confirmed surface charge overcompensation upon polyelectrolyte adsorption by probing the interfacial electric field. In dry films, the polymer chains acquired a net orientational ordering, which was affected, however, by the adsorption of subsequent layers. Such a detailed characterization may allow the control of LbL film structure and functionality with unprecedented power.

Controlled fabrication of gold nanoparticles biomediated by glucose oxidase immobilized on chitosan layer-by-layer films

The control of size and shape of metallic nanoparticles is a fundamental goal in nanochemistry, and crucial for applications exploiting nanoscale properties of materials. We present here an approach to the synthesis of gold nanoparticles mediated by glucose oxidase (GOD) immobilized on solid substrates using the Layer-by-Layer (LbL) technique. The LbL films contained four alternated layers of chitosan and poly(styrene sulfonate) (PSS), with GOD in the uppermost bilayer adsorbed on a fifth chitosan layer: (chitosan/PSS)(4)/(chitosan/GOD). The films were inserted into a solution containing gold salt and glucose, at various pHs. Optimum conditions were achieved at pH 9, producing gold nanoparticles of ca. 30 nm according to transmission electron microscopy. A comparative study with the enzyme in solution demonstrated that the synthesis of gold nanoparticles is more efficient using immobilized GOD. (C) 2009 Elsevier B.V. All rights reserved.

Modeling Adsorption Processes of Poly-p-phenylenevinylene Precursor and Sodium Acid Dodecylbenzenesulfonate onto Layer-by-Layer Films Using a Langmuir-type Metastable Equilibrium Model

The adsorption kinetics curves of poly(xylylidene tetrahydrothiophenium chloride) (PTHT), a poly-p-phenylenevinylene (PPV) precursor, and the sodium salt of dodecylbenzene sulfonic acid (DBS), onto (PTHT/DBS)(n) layer-by-layer (LBL) films were characterized by means of UV-vis spectroscopy. The amount of PTHT/DBS and PTHT adsorbed on each layer was shown to be practically independent of adsorption time. A Langmuir-type metastable equilibrium model was used to adjust the adsorption isotherms data and to estimate adsorption/desorption coefficients ratios, k = k(ads)/k(des), values of 2 x 10(5) and 4 x 10(6) for PTHT and PTHT/DBS layers, respectively. The desorption coefficient has been estimated, using literature values for poly(o-methoxyaniline) desorption coefficient, as was found to be in the range of 10(-9) to 10(-6) s(-1), indicating that quasi equilibrium is rapidly attained.

Molecular Ordering in Layer-by-Layer Polyelectrolyte Films Studied by Sum-Frequency Vibrational Spectroscopy: The Effects of Drying Procedures

Sum-Frequency Vibrational Spectroscopy (SFVS) has been used to investigate the effect of nitrogen-flow drying on the molecular ordering of Layer-by-Layer (LbL) films of poly(allylamine hydrochloride) (PAH) alternated with poly(styrene sulfonate) (PSS). We find that films dried by spontaneous water evaporation are more ordered and homogeneous than films dried by nitrogen flow. The latter are quite inhomogeneous and may have regions with highly disordered polymer conformation. We propose that drying by spontaneous water evaporation reduces the effect of drag by the drying front, while during nitrogen-flow drying the fast evaporation of water ""freezes"" the disordered conformation of adsorbed polyelectrolyte molecules. These findings are important for many applications of LbL films, since device performance usually depends on film morphology and its molecular structure.

Thickness and Annealing Temperature Effects on the Optical Properties and Surface Morphology of Layer-by-Layer Poly(p-phenyline vinylene) plus Dodecylbenzenesulfonate Films

The fabrication of controlled molecular architectures is essential for organic devices, as is the case of emission of polarized light for the information industry. In this study, we show that optimized conditions can be established to allow layer-by-layer (LbL) films of poly(p-phenylene vinylene) (PPV)+dodecylbenzenesulfonate (DBS) to be obtained with anisotropic properties. Films with five layers and converted at 110 degrees C had a dichroic ratio delta = 2.3 and order parameter r = 34%, as indicated in optical spectroscopy and emission ellipsometry data. This anisotropy was decreased with the number of layers deposited, with delta = 1.0 for a 75-layer LbL PPV + DBS film. The analysis with atomic force microscopy showed the formation of polymer clusters in a random growth process with the normalized height distribution being represented by a Gaussian function. In spite of this randomness in film growth, the self-covariance function pointed to a correlation between clusters, especially for thick films. In summary, the LbL method may be exploited to obtain both anisotropic films with polarized emission and regular, nanostructured surfaces. (c) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 206-213, 2011

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.

Adsorption kinetics and charge inversion in layer-by-layer films from nickel tetrasulfonated phthalocyanine and poly(allylamine hydrochloride)

Layer-by-layer (LBL) films of nickel tetrasulfonated phthalocyanine (NiTsPc) alternated with poly(allylamine hydrochloride) (PAH) have been prepared, whose surface charge has been evaluated using surface potential measurements. From adsorption kinetics results, we obtained the immersion time of similar to 40 s, which was used to assemble layers of NiTsPc. The effect of gold (Au) and aluminum (Al) electrodes on the charge behavior was examined. We found that the surface potential (i.e. surface charge) was inverted each time a layer of PAH was alternated with another of NiTsPc molecules for the two types of electrodes, which was attributed to charge overcompensation between positive charges of PAH molecules, and negative charges from NiTsPc molecules. (C) 2009 Elsevier B.V. All rights reserved.

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.

Quartz recrystallization regimes, c-axis texture transitions and fluid inclusion reequilibration in a prograde greenschist to amphibolite facies mylonite zone (Ribeira Shear Zone, SE Brazil)

The crystal-plastic behavior of quartz mylonites from the Ribeira Shear Zone (SE Brazil), a major strike-slip structure that was active during a prograde metamorphic phase related to the Neoproterozoic Brasiliano-Pan African Orogeny, was investigated using a multi-method approach. Geothermobarometry results indicate deformational conditions ranging from similar to 300 to similar to 630 degrees C and 500-700 MPa. A strong correlation between mapped metamorphic zones and a dominance of different dynamic recrystallization mechanisms of quartz occurs within the mylonite zone. Bulging recrystallization (BLG) dominates within the chlorite zone between 300 and 410 degrees C, subgrain rotation recrystallization (SGR) operates within the biotite zone from 410 to 520 degrees C, and grain boundary migration recrystallization (GBM) dominates in the garnet zone above 520 degrees C. The development of quartz c-axis textures is mainly governed by temperature and dynamic recrystallization mechanisms. Textures from BLG zone mylonites are characterized by maxima around Z; SGR zone mylonites display single girdles or asymmetric type I crossed girdles; and GBM zone mylonites comprise maxima around Y and intermediate between X and Z. The scarcity or absence of water-bearing fluid inclusions in quartz mylonites from the SGR and GBM zones, which are dominated by carbonic inclusions, suggests water-deficient conditions, whereas BLG zone mylonites are dominated by water-bearing inclusions. This evidence indicates that water was available in the protoliths but has been eliminated with increasing deformation and deformation temperature. No effect of the water content variation on the quartz microstructural and recrystallized grain size evolution was detected, and little influence on c-axis texture development was observed. Most of the fluid inclusion densities were reequilibrated during the shear zone exhumation history, recording a decompression in the range of 300-500 MPa, while microstructural reequilibration effects related to the prograde metamorphism are largely preserved. Fluid inclusion microstructures and densities from two SGR zone samples preserved evidence for a near isothermal compression within the interior of the Ribeira Shear Zone during the prograde metamorphism. (C) 2009 Elsevier B.V. All rights reserved.