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PubMed1980

Different regional specializations of neurons in the ganglion cell layer and inner plexiform layer of the California horned shark, Heterodontus francisci.

We have described a population of neurons in the retinal of a shark, Heterodontus francisci, which is precisely aligned within the inner plexiform layer (IPL) and which differs from neurons in the ganglion cell layer (GCL) in soma size and topographical distribution. GCL neurons are relatively small and form a horizontally oriented visual streak; IPL neurons are significantly larger and form a circular specialization in the far temporal retina. Thus, it appears that there are two distinct retinal specializations in Heterodontus: one subserving frontal vision and one which provides a panoramic view of the lateral visual field.

Animals↗

PubMed2004

Layer-by-layer deposition of hyaluronic acid and poly-L-lysine for patterned cell co-cultures.

A novel method for patterning cellular co-cultures that uses the layer-by-layer deposition of ionic biopolymers is described. Non-biofouling hyaluronic acid (HA) micropatterns were used to immobilize cells and proteins to glass substrates. Subsequent ionic adsorption of poly-L-lysine (PLL) to HA patterns was used to switch the HA surfaces from cell repulsive to adherent thereby facilitating the adhesion of a second cell type. The utility of this approach to pattern co-cultures of hepatocytes or embryonic stem cells with fibroblasts was demonstrated. In addition, the versatility of this approach to generate patterned co-cultures irrespective of the primary cell seeding and relative adhesion of the seeded cells was demonstrated. Thus, the proposed method may be a useful tool for fabricating controlled cellular co-cultures for cell-cell interaction studies and tissue engineering applications.

Adsorption↗

PubMed2006

Disassembly of layer-by-layer films of plasmid DNA and reducible TAT polypeptide.

This paper reports the disassembly of layer-by-layer (LbL) films of plasmid DNA and a reducible cationic polypeptide. To utilize a reducing microenvironment of cellular plasma membrane as a potential trigger, LbL films are assembled to contain both DNA and the TAT-based polypeptide (PTAT) with reducible disulfide bonds in the backbone. The assembly and disassembly processes are monitored by goniometry, ellipsometry, and atomic force microscopy (AFM). The structure of the PTAT films is compared with that of non-reducible poly(L-lysine) (PLL) films. Both PTAT and PLL films exhibit exponential growth but with the contact angle alternating between characteristic values. Ellipsometry and AFM show a gradual and complete disassembly of the PTAT but not the PLL films in a 24h period in the reducing environment in vitro. This study suggests a potential of using reducible LbL films for controlled DNA delivery.

DNA↗

PubMed2005

Layer-by-layer assembled carbon nanotubes for selective determination of dopamine in the presence of ascorbic acid.

Multilayer films of shortened multi-walled carbon nanotubes (MWNTs) are homogeneously and stably assembled on glassy carbon (GC) electrodes using layer-by-layer (LBL) method based on electrostatic interaction of positively charged poly(diallyldimethylammonium chloride) (PDDA) and negatively charged shortened MWNTs. The assembled MWNT multilayer films were studied with respect to the electrocatalytic activity toward ascorbic acid (AA) and dopamine (DA) and were further applied for selective determination of DA in the presence of AA. Scanning electron microscopy (SEM) used for characterization of MWNT films indicates that the assembled MWNTs are almost in a form of small bundles or single nanotubes on the electrodes. Cyclic voltammetric results with assembled MWNT electrode indicate that the strategy based on the LBL method for assembling the MWNT multilayer films on substrate well retains the electrochemical catalytic activity of the MWNTs toward AA and DA, offering some advantages particularly attractive for analytical applications, such as the form of MWNTs assembled on the substrate, i.e., small bundles or single tubes, homogeneity and stability of the as-assembled MWNT films. These features make the assembled MWNTs relatively potential for selective and sensitive determination of DA in the presence of AA.

Ascorbic Acid↗

PubMed2006

Direct electron transfer and electrocatalysis of hemoglobin adsorbed on mesoporous carbon through layer-by-layer assembly.

Using chitosan as an effective linker between CMK-3 and glassy carbon electrode surface, {Hb/CMK-3}n multilayer film-modified electrodes were constructed through layer-by-layer assembly. The morphology of thus-formed {Hb/CMK-3}n film was characterized by scanning electron micrographs, and the interaction of hemoglobin (Hb) with CMK-3 was studied by UV-vis spectroscopy and electrochemical methods. Under optimal conditions, {Hb/CMK-3}6 film showed a couple of stable and well-defined redox peaks at about -377 and -296 mV in pH 7.0 buffers. Furthermore, the {Hb/CMK-3}6 film displayed excellent electrocatalysis to the reduction of both H2O2 and O2. Based on thus-formed film and its direct electron transfer behavior, a novel biosensor was presented for the determination of H2O2 ranging from 1.2 to 57 muM with the detection limit of 0.6microM at S/N=3. CMK-3 provided a desirable matrix for protein immobilization and biosensor preparation.

Adsorption↗

PubMed2006

Layer-by-layer hydroxymethyl ferrocene modified sensor for one-step flow/stop-flow injection amperometric immunoassay of alpha-fetoprotein.

A rapid one-step flow/stop-flow injection amperometric immunoassay for alpha-fetoprotein (AFP) using a novel home-produced electrochemical sensor was proposed. The sensor was prepared using layer-by-layer adsorption of positively charged poly(allylamine) (PAA) and negatively charged hydroxymethyl ferrocene on a screen-printed electrode (SPE). The electrochemistry of the immobilized ferrocene moieties showed a surface-controlled electrode process. Based on an electrochemical enzyme-linked immunoassay with the immobilized ferrocene moieties as an electron transfer mediator between the electrode and the horseradish peroxidase (HRP)-labeled anti-AFP antibody, a calibration curve with two linear ranges from 5 to 20 and 20 to 150 ng ml-1 and a detection limit of 2 ng ml-1 for AFP determination was obtained under the optimized conditions of 0.891 ml min-1 flow rate, 20 microl injection volume and +25 mV applied potential. The sensor showed good repeatability and reproducibility and retained more than 95% of its original signal after 15 days of storage. The proposed method eliminated the need for washing and addition of any substrate or mediator. The complete assay could be handled in less than 25 min with a one-step injection of a 40 microl sample solution. The proposed method would be valuable for the diagnosis and monitoring of carcinoma and its metastasis.

Biosensing Techniques↗

PubMed2006

Preparation of polyaniline-modified electrodes containing sulfonated polyelectrolytes using layer-by-layer techniques.

Polyaniline (PAni) has been used frequently for the construction of biosensors. However, a prime limitation is its instability at basic or neutral pH because of the loss of its electrochemical activity and conductivity. In this study, three available sulfonated polyanions: Nafion, poly(vinyl sulfonate) (PVS), and poly(styrene sulfonate) (PSS) serving as the counterion and providing an acidic microenvironment to stabilize PAni, are used to fabricate a sensor for ammonium ion detection. Nafion used to be a common ion-sensitive membrane due to its high proton conductivity. However, its high cost and limited solubility has constrained its uses. PVS and PSS are water-soluble polymers, easily incorporating with PAni to form the composites. Surface analysis by electron spectroscopy for chemical analysis (ESCA) and scanning electron microscope (SEM), and the electrochromic property for the PAni composites provided the convenient tools to characterize the electrode fabrication. On the aspect of sensing the ammonium ions, the modified electrodes exhibited electroactivity of PAni in ammonium ion detection and also showed the linear dependence of reduction current on the ammonium ion concentration. The pH effect on the sensing response was also evaluated and found insignificant to the response (ranging from pH 6.9-7.6). For increasing the stability of the electrodes, the diazo-resin (DAR) was introduced to the coat on the outmost layer and then cured by UV irradiation, giving the covalent network between the layers of polyelectrolytes. The PSS-doped PAni electrode was found to perform detection sensitivity in the linear range of 0-100mM of ammonium ion concentration.

Adsorption↗

PubMed2007

Layer-by-layer self-assembly aluminum Keggin ions/Prussian blue nanoparticles ultrathin films towards multifunctional sensing applications.

In this study we described the nanocomposites films of specially synthesized inorganic Prussian blue (PB) nanoparticles and polyoxocation Al(13) Keggin ions that possess the excellent sensing activities. Film fabrication using layer-by-layer (LBL) self-assembly technique was followed by electrochemical characterization. The assembled multilayer Al(13)/PB films as sensor devices for both the catalytic reduction of H(2)O(2) and detecting the change of relative humidity were also investigated. The sensitivity of the biosensor was 0.886 mA cm(-2)mM(-1), and about two orders of magnitude change in resistance was observed as the relative humidity increasing from 5 to 95%. Both sensors exhibited good reproducibility, wide linear range. The performance and multifunctional abilities of these nanocomposites promise potential applications in biosensors, environmental controlling system and biomedical devices.

Aluminum Compounds↗

PubMed2006

Direct electron transfer between hemoglobin and pyrolytic graphite electrodes enhanced by Fe(3)O(4) nanoparticles in their layer-by-layer self-assembly films.

Alternate adsorption of negatively charged Fe(3)O(4) nanoparticles from their pH 8.0 aqueous dispersions and positively charged hemoglobin (Hb) from its pH 5.5 buffers on solid substrates resulted in the assembly of {Fe(3)O(4)/Hb}(n) layer-by-layer films. Quartz crystal microbalance (QCM), UV-vis spectroscopy, and cyclic voltammetry (CV) were used to monitor and confirm the film growth. A pair of well-defined, nearly reversible CV peaks for HbFe(III)/Fe(II) redox couples was observed for {Fe(3)O(4)/Hb}(n) films on pyrolytic graphite (PG) electrodes. Although the multilayered films grew linearly with the number of Fe(3)O(4)/Hb bilayers (n) and the amount of Hb adsorbed in each bilayer was generally the same, the electroactive Hb could only extend to 6 bilayers. This indicates that only those Hb molecules in the first few bilayers closest to the electrode surface are electroactive. The electrochemical parameters such as the apparent heterogeneous electron transfer rate constant (k(s)) were estimated by square wave voltammetry (SWV) and nonlinear regression. The Soret absorption band position of Hb in {Fe(3)O(4)/Hb}(6) films showed that Hb in the films retained its near native structure in the medium pH range. The {Fe(3)O(4)/Hb}(6) film electrodes also showed good biocatalytic activity toward reduction of oxygen, hydrogen peroxide, trichloroacetic acid, and nitrite. The electrochemical reduction overpotentials of these substrates were lowered significantly by {Fe(3)O(4)/Hb}(n) films.

Animals↗

PubMed2005

Selective transport of ions and molecules across layer-by-layer assembled membranes of polyelectrolytes, p-sulfonato-calix[n]arenes and Prussian Blue-type complex salts.

Our recent studies in the field of ultrathin membranes prepared upon layer-by-layer assembly of various polyionic compounds such as polyelectrolytes, calixarenes and polyelectrolytes, and metal hexacyanoferrate salts such as Prussian Blue are reviewed. It is demonstrated that polyelectrolyte multilayers can be used (a) as nanofiltration and reverse osmosis membranes suitable for water softening and seawater desalination and (b) as molecular sieves and ion sieves for size-selective separation of neutral and charged aromatic compounds. Furthermore, hybrid membranes of p-sulfonato-calixarenes and cationic polyelectrolytes showing specific host-guest interactions with permeating ions are described. The membranes exhibit high selectivities for distinct metal ions. Finally, it is demonstrated that purely inorganic membranes of Prussian Blue (PB) and analogues can be prepared upon multiple sequential adsorption of transition metal cations and hexacyanoferrate anions. Due to the porous lattice of PB, the membranes are useful as ion filters able to separate cesium from sodium ions, for example.

Calixarenes↗

PubMed2006

Layer-by-layer polyelectrolyte coating of low molecular weight poly(lactic acid) nanoparticles.

Low molecular weight (M(w)) poly(L-lactic acid) (PLA) nanoparticles were coated with polyelectrolytes (PEs) by layer-by-layer (LbL) technique using a filtration approach. Poly(allylamine hydrochloride) and poly(sodium 4-styrenesulfonate) were applied as PEs in coating. LbL coating is aimed to use in producing (nano)particulate drug delivery systems with improved biocompatibility and sustained or targeted release of drug substances. Nanoparticles of rapidly biodegradable polymers, like the low M(w) PLA, open up a possibility to control the release of the encapsulated substance by the coating, but set challenges to the coating process due to increased aggregation tendency and degradation rate of the polymer. When the core PLA nanoparticles were prepared by nanoprecipitation, surface properties of the nanoparticles were affected by solvent selection. Successful LbL coating of the PLA nanoparticles was obtained only with chloroform, but not with dichloromethane as the solvent during nanoprecipitation. Reason for this was found to be the more charged surface of the nanoparticles prepared with chloroform compared to the nanoparticles prepared with dichloromethane.

Electrolytes↗

PubMed2004

The pH stimulated reversible loading and release of a cationic dye in a layer-by-layer assembled DNA/PAH film.

Through the layer-by-layer (LbL) deposition method, DNA was assembled into an ultrathin film with a cationic poly(allylamine hydrochloride) (PAH). The loading and release of a typical cationic dye, 5, 10, 15, 20-tetrakis(4-N-methylpyridyl)porphine-tetra-(p-toluenesulfonate) (TMPyP), in the DNA/PAH films were investigated. It has been found that the LbL-assembled DNA/PAH film was very stable in both acidic and alkaline solutions. Stimulated by the pH change of the dye solution, the dye can be easily loaded into or released from the DNA/PAH film. In an alkaline solution, the dye could be rapidly loaded into the DNA/PAH film at room temperature, while in an acidic solution, the dye could be rapidly released. The mechanism of such pH-stimulated loading and release in the DNA/PAH film was discussed. It was further observed that the loading and release of the dye in the DNA/PAH film was reversible upon pH change and the process could be repeated many times.

Cations↗

PubMed2005

Silver nanocomposite layer-by-layer films based on assembled polyelectrolyte/dendrimer.

Silver nanocomposite multilayer films were prepared through the in situ method. Multilayer thin films, prepared through the sequential electrostatic deposition of a positively charged third-generation poly(amidoamine) dendrimer (PAMAM) and negatively charged poly(styrenesulfonate) (PSS) and poly(acrylic acid) (PAA), were utilized as nanoreactors for the formation of silver nanoparticles. The silver ions were preorganized in layer-by-layer (LBL) films composed of PAMAM dendrimers and subsequently reduced with hydrogen to prepare the silver nanoparticles. The UV-vis spectrum and profilometer were used to characterize the regular growth of bilayers. UV-vis absorption from plasmon resonance at 435 nm and TEM images indicated the formation of the silver nanoparticles in the multilayer films. The silver nanocomposite LBL films were also constructed on the indium tin oxide-glass and investigated using cyclic voltammetry. The silver nanoparticles in the multilayer films have a stronger negative redox potential. The silver nanocomposite LBL films may have a potential application in the catalysis of reduction of 4-nitrophenol with sodium borohydride.

Journal Article↗

PubMed2005

Electrical double layer interactions between dissimilar oxide surfaces with charge regulation and Stern-Grahame layers.

Models of surfaces with intrinsic ionisable amphoteric surface sites governed by the dissociation of acid-base potential determining ion species together with the capacity for the adsorption of anion and cations of the supporting electrolyte are required to describe both the results of electrokinetic and titration measurements of inorganic oxides. The Gouy-Chapman-Stern-Grahame (CGSG) model is one such model that has been widely used in the literature. The electrical double layer interaction between two dissimilar CGSG surfaces has been studied by Usui recently [S. Usui, J. Colloid Interface Sci. 280 (2004) 113] where erroneous discontinuities in the slope of the pressure-separation relation were observed. We revisit this calculation and provide a simple general methodology to analyse the electrical double layer interaction between dissimilar ionisable surfaces with ion adsorption.

Journal Article↗

PubMed2006

Fabrication of hollow multifunctional spheres containing MCM-41 nanoparticles and magnetite nanoparticles using layer-by-layer method.

Macroscopic mesoporous silica spheres have been fabricated by alternatively depositing preformed MCM-41 nanoparticles and polyelectrolytes onto polystyrene lattices. High surface area hollow mesoporous spheres were obtained by removal of the core by solvent or calcination. Further, the versatility of the layer-by-layer (LBL) method was extended to fabricate magnetite-mesoporous silica composites by depositing magnetite and MCM-41 nanoparticles onto polystyrene beads. Such high surface area composites are important since the mesopores can be used for encapsulation of varied materials like enzymes and drugs while the presence of magnetite ensures application in biocatalysis and separation under magnetic field.

Journal Article↗

PubMed2006

Layer-by-layer coating of degradable microgels for pulsed drug delivery.

Recently, we reported on "self-rupturing" microcapsules which consist of a biodegradable dextran-based microgel surrounded by a polyelectrolyte membrane. Degradation of the microgel increases the swelling pressure in the microcapsules which, when sufficiently high, ruptures the surrounding polyelectrolyte membrane. The membrane surrounding the microgels is deposited using the layer-by-layer (LbL) technique, which is based on the alternate adsorption of oppositely charged polyelectrolytes onto a charged substrate. In this paper, we characterize with confocal microscopy, electrophoretic mobility, scanning electron microscopy and atomic force microscopy in detail the deposition and the properties of the LbL coatings on the dextran microgels. We show that by fine-tuning the properties of both the microgel core and the LbL membrane the swelling pressure which is evoked by the degradation of the microgel is indeed able to rupture the surrounding LbL membrane. Further, we show that the application of an LbL coating on the surface of the microgels dramatically lowers the burst release from the microcapsules and results in massive release at the time the microcapsules rupture.

Delayed-Action Preparations↗

PubMed2003

Fabrication and photoelectric response of poly(allylamine hydrochloride)/PM thin films by layer-by-layer deposition technique.

Thin films of poly(allylamine hydrochloride) (PAH) and bacteriorhodopsin (bR) embedded in purple membrane (PM) have been prepared by layer-by-layer (LBL) self-assembly technique. The results obtained by UV-Vis spectroscopy and atomic force microscopy (AFM) analysis showed that the biological activity of bR was preserved and PM fragments could be well oriented onto the ITO substrate. A photo-electrochemical cell with the structure of ITO/(PAH/PM)(n)/electrolyte (0.5M KCl)/Pt was fabricated and studied. The photocurrent peaks of (PAH/PM)(6) corresponding to light-on and light-off were about 200 and 100 nA/cm(2), respectively, with the former enhanced 30% higher than that of the reference films made of (PDAC/PM)(6).

Bacteriorhodopsins↗

PubMed1982

Structural similarities between a membrane-like surface layer on the cell wall of some microbial cells and the lamellar layer of vesicles formed by short-chain phosphonic acids in water. A freeze-fracture study.

A thin layer on the cell wall of some microbial cells, among them such of medical importance (Aspergillus niger, Trichophyton sp.), is characterized by a mosaic-like rodlet pattern in freeze-fracture electron microscopy. A very similar texture could be found in freeze-fractured bilayer vesicles of n-octan phosphonic acid in presence of 50% glycerol. The formation of this structure in phosphonic acid vesicles is connected with the transformation of the bilayer into the solid crystalline state by cooling. The texture could be suppressed by a higher cooling velocity. In absence of glycerol by the normal quick freezing procedure predominantly intermediate stages were fixed. For the interpretation of the structures revealed a model is proposed, where trigonal subunits stick together, because they are hydrophobic on the one side. These subunits are small crystals with a diameter of 4--7 nm. They are in principle also able to construct a bilayer arrangement directly by a "subassembly-selfassembly" mechanism of selforganization. The membrane-like layer on the cell wall of microbial cells may consist of a crystalline bilayer arrangement of similar substances.

Aspergillus niger↗