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PubMed2005

Photocurrent generation in heterostructured ultrathin films fabricated by layer-by-layer deposition of polyelectrolytes bearing tris(2,2'-bipyridine)ruthenium(II) and ferrocene moieties.

The photoelectrochemical properties of single-component and heterostructured layer-by-layer deposited films bearing tris(2,2'-bipyridine)ruthenium(II) (Ru) moieties were investigated by photocurrent measurements in solutions in the presence of sacrificial reagents. The photocurrent increased with an increase in the thickness of the films and then had a maximum at a thickness of 10 nm. This increase demonstrates a light-harvesting effect based on excitation energy migration among the Ru moieties to the film/electrolyte interface. A cathodic photocurrent was observed for a heterostructured film where bilayers bearing ferrocene (Fc) moieties and bilayers bearing Ru moieties were deposited on an indium tin oxide (ITO) substrate in the order (ITO/Fc/Ru). On the other hand, an anodic photocurrent was observed for the reverse order film (ITO/Ru/Fc). These results show that the direction of the photocurrent is determined by the gradient of the redox potentials formed in the heterostructured films. The internal quantum efficiency for the ITO/Ru/Fc film was twice that for the single-component film (ITO/Ru). This enhancement of the quantum efficiency is due to suppression of charge recombination by successive electron transfers in the heterostructured film.

Journal Article↗

PubMed2004

Novel solid-state polymer electrolyte consisting of a porous layer-by-layer polyelectrolyte thin film and oligoethylene glycol.

A novel solid-state polymer electrolyte was constructed using layer-by-layer (LbL) polyelectrolyte assembly of linear poly(ethylenimine) (LPEI) and poly(acrylic acid) (PAA), combined with a plasticization step using oligoethylene glycol dicarboxylic acid (OEGDA). This composite film exhibits a relatively high ionic conductivity of 9.5 x 10(-5) S/cm at 25 degrees C and 22% relative humidity. Detailed characterization of the composite was undertaken using grazing-angle Fourier transform infrared (GA-FTIR), atomic force microscopy (AFM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and impedance spectroscopy. After immersing the LPEI/PAA films into OEGDA aqueous solutions, the films exhibited a swelling behavior and increased surface roughness indicative of porosity induced by reorganization of ionic interactions between LPEI and PAA in acidic solution. This internal porous structure allows inclusion of OEGDA within the multilayer and increased ionic conductivity under ambient conditions due to the combined effects of plasticization of the LbL matrix by atmospheric water as well as the added mobility of ions in molten OEGDA within the composite.

Journal Article↗

PubMed2004

Hydrogen-bonding-directed layer-by-layer assembly of poly(4-vinylpyridine) and poly(4-vinylphenol): effect of solvent composition on multilayer buildup.

This paper describes the buildup of hydrogen-bonding-directed poly(4-vinylpyridine)/poly(4-vinylphenol) (PVPy/PVPh) multilayer film that was fabricated by layer-by-layer (LbL) assembly of PVPy and PVPh from an ethanol solution. UV-visible spectroscopy and Fourier transform infrared (FT-IR) spectroscopy revealed a uniform deposition process. The interaction between PVPy and PVPh was identified as hydrogen bonding through FT-IR spectroscopy and temperature-dependent IR spectral changes of the hydrogen-bonded multilayer. Notably, we discussed the effect of solvent conditions on the growth of PVPy/PVPh multilayer films monitored by UV-visible spectroscopy. It was found that increasing the ratio of N,N-dimethylformamide (DMF) in the mixed ethanol/DMF solvents resulted in a marked decrease of the amount of polymers adsorbed, which was attributed to the increased polarity of the adsorption solutions. Furthermore, the solvent stability of PVPy/PVPh multilayer film in mixed ethanol/DMF solvents with different DMF ratios was also investigated. As a result, a new method for tuning the structure of hydrogen-bonding-directed multilayer film was developed.

Ethanol↗

PubMed2004

In situ kinetics of layer-by-layer assembled nonlinear-optical-active amphiphiles from dynamic surface force measurements.

We report the synthesis and layer-by-layer (LBL) deposition of a class of azo-benzene surfactants with the polycation poly(ethylenimine) (PEI). The different surfactants of the type X-azo-(CH2)10-SO3-, where X = -NO2, -CN, and -COCH3 in the azo-benzene moiety, have decreasing electron-withdrawing strengths. We use dynamic surface force measurements to study the in situ kinetics of adsorption of the amphiphiles onto PEI. Ex situ kinetics data obtained by adsorption-paused UV-visible spectroscopy validate the surface force results. These measurements describe the first application of dynamic force measurements to follow adsorption in LBL systems. UV-visible spectroscopy, second harmonic generation (SHG), and single-wavelength ellipsometry were also used to characterize the films. The observed blue shift upon adsorption of the amphiphiles suggests H-type aggregation within the multilayer. Two of the surfactants studied within the LBL films follow Langmuir adsorption behavior with equilibrium adsorption times under 200 s. The SHG results are consistent with the expected trends in the hyperpolarizabilities of the amphiphiles.

Journal Article↗

PubMed2004

Counterions in poly(allylamine hydrochloride) and poly(styrene sulfonate) layer-by-layer films.

The amount of counterions in layer-by-layer (LBL) films of poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) has been determined with X-ray photoelectron spectroscopy (XPS) for films prepared from solutions with various NaCl concentrations. Sodium and chloride counterions are present in LBL films produced from salt solutions, which are located at the surface and in the bulk of the films. The percentage of bulk counterions increases with the ionic strength of the polyelectrolyte before reaching a constant value. The bulk sodium/sulfur percentage ratios tend to 0.8 for samples washed with pure water and for samples washed with NaCl aqueous solutions, while the bulk chlorine/nitrogen percentage ratios tend to 0.5 for the same samples. The ratio between the percentages of polyelectrolyte ionic groups lies close to unity for all samples, indicating that counterions do not contribute to charge compensation in the polyelectrolyte during the adsorption process. The presence of counterions in LBL films is explained by Manning condensation near the polyelectrolyte ionic groups, leading to inter-polyelectrolyte ionic bondings via ionic networks. It is believed that condensation leads to the formation of NaCl crystallites in these LBL films, which was confirmed by X-ray diffraction measurements.

Crystallization↗

PubMed2005

Layer by layer self-assembled polyelectrolyte multilayers with embedded phospholipid vesicles obtained by spraying: integrity of the vesicles.

In a previous paper (Michel, M.; Vautier, D.; Voegel, J.-C.; Schaaf, P.; Ball, V. Langmuir 2004, 20, 4835), we showed that phospholipid vesicles can be incorporated into poly(glutamic-acid)/poly(allylamine) (PGA/PAH) multilayered polyelectrolyte films built by the alternated dipping of a surface in polyanion and polycation solutions. AFM imaging, quartz crystal microbalance, and ellipsometry suggested that the vesicles remain intact when adhering on the surface. In the present paper, we show that such films can also be realized by spraying both the polyelectrolyte solutions and the vesicles onto the surface. Using such vesicles filled with ferrocyanide ions, we prove by cyclic voltammetry that the sprayed vesicles remain intact when embedded in the multilayers. We show that multilayers containing two distinct layers of intact vesicles separated by several polyanion/polycation bilayers can also be constructed. Polyelectrolyte multilayers containing layers of phospholipid vesicles could act as reservoirs for drug or other biologically active molecules in controlled release bioactive coatings.

Electrochemistry↗

PubMed2005

Layer-by-layer assembly of poly(ethyleneimine) and plasmid DNA onto transparent indium-tin oxide electrodes for temporally and spatially specific gene transfer.

The layer-by-layer assembly technique was used to adsorb alternately poly(ethyleneimine) and plasmid DNA onto the surface of a transparent electrode made of indium-tin oxide. The surface with adsorbed poly(ethyleneimine) and DNA was characterized by X-ray photoelectron spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy, and contact angle measurements. These analyses revealed that the alternate adsorption process generated a multilayered assembly of cationic poly(ethyleneimine) and anionic DNA. For the spatially and temporally specific gene transfer, cells were cultured on the plasmid-loaded electrode and then a short electric pulse was applied to the cell-electrode system. It was shown that, upon electric pulsing, the plasmid was released from the electrode and transferred into the cells, resulting in efficient gene expression even in primary cultured cells. Transfection could be effected for hippocampal neurons after 3-day culture on the plasmid-loaded electrode, which indicated the feasibility of selecting the time of transfection. Our results also showed that electroporation could be performed in a spatially specific manner by using a plasmid-arrayed electrode, demonstrating the feasibility of the method for the fabrication of transfected cell microarrays.

Animals↗

PubMed2005

Layer-by-layer fabrication and characterization of DNA-wrapped single-walled carbon nanotube particles.

Carbon nanotubes have been proposed as support materials for numerous applications, including the development of DNA sensors. One of the challenges is the immobilization of DNA or other biological molecules on the sidewall of carbon nanotubes. This paper introduces a new fabrication of DNA-carbon nanotubes particles using the layer-by-layer (LBL) technique on single-walled carbon nanotubes (SWCNTs). Poly(diallyldimethylammonium) (PDDA), a positively charged polyelectrolyte, and DNA as a negatively charged counterpart macromolecule are alternatively deposited on the water-soluble oxidized SWCNTs. Pure DNA/PDDA/SWCNTs particles can be prepared and separated by simple unltracentrifugation. The characterization of DNA/PDDA/SWCNTs particles was carried out by scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible spectroscopy, Raman spectroscopy, and thermogravimetric analysis (TGA). An electrode modified by the DNA/PDDA/SWCNTs particles shows a dramatic change of the electrochemical signal in solutions of tris(2,2'-bipyridyl)ruthenium(II) ((Ru(bpy)(3)2+) as a reporting redox probe. A preliminary application of the DNA-modified carbon nanotubes in the development of DNA sensors used in the investigation of DNA damage by nitric oxide is presented.

Carbon↗

PubMed2005

Single-walled carbon nanotube combing during layer-by-layer assembly: from random adsorption to aligned composites.

Oriented SWNTs in polymer composites have shown dramatic improvements in the physical properties of a composite because of the anisotropic shape and properties of SWNTs. Controlled alignment of SWNTs during composite fabrication implies better material function performance. This letter reports a new fabrication technique whereby aligned SWNTs and robust SWNT-polymer composites can be made using a fusion method of SWNT combing and layer-by-layer (LBL) assembly. As we previously reported, LBL assembly demonstrated exceptional processing ability in constructing the uniform distribution of a SWNT-polymer composite. Combined with this uniformity, this SWNT combing technique endows controlled alignment of single-stranded SWNTs in a SWNT-polymer composite system. SWNT combing employs air-water interfacial forces to change the molecular topography from the random adsorption state to the stretched alignment of SWNTs. More specifically, air-water interfacial forces are associated with an excess viscous drag force and an intrinsic dewetting rate along SWNTs. Moreover, the alignment efficiency of SWNTs is high enough to construct a multilayered LBL film with horizontal-linear weaving structures. This simple method also can be applied for aligning other nanowire materials because it utilizes simple geometric features of SWNTs.

Journal Article↗

PubMed2005

Electrostatic layer-by-layer deposition of photoactive dendrimers with triviologen-like cores on their surfaces. Synthesis and electrochemical and photocurrent generation measurements.

The stepwise assembly of Fréchet-type dendrimers with naphthalene peripheral groups and positively charged viologen-like cores on quartz and ITO surfaces utilizing the layer-by-layer approach was investigated. We were able to deposit only the (+6) charged dendrimers series on ITO. The number of assembled dendrimers was found to increase as we go to higher-generation dendrimers. This dendrimer generation effect was evident from the UV-vis and electrochemical measurements of the assembled dendrimers. The half-wave potentials (E1/2) of the dendrimers shift to less negative values as the dendrimer generation increases in acetonitrile and to more negative values when assembled on ITO. Anodic photocurrent generation was seen upon light irradiation of the second- and third-generation dendrimers, NB1V3+6 and NB2V3+6, assembled on ITO but not for the zero-generation one, NV3+6. This observation was attributed to a fast charge recombination process in NV3+6 when compared to that of NB1V3+6 and NB2V3+6 dendrimers.

Journal Article↗

PubMed2005

Dynamic sequential layer-by-layer deposition method for fast and region-selective multilayer thin film fabrication.

We present a newly devised technique, the dynamic layer-by-layer (LbL) deposition method, that is designed to take advantage of the LbL deposition method and fluidic devices. Polyelectrolyte solutions are sequentially injected through the fluidic LbL deposition device to quickly build well-defined multilayer films on a selected region with a linear increase in the material deposited. Multilayer film fabrication by this new method on a specific region was proven to be fast and effective. The effects on film quality of the processing parameters such as concentration of polyelectrolytes, flow rate, and contact time were investigated. A half-tethered self-standing film on a substrate was fabricated to demonstrate the effectiveness and the region-selective deposition capability of the devised dynamic LbL deposition method.

Adsorption↗

PubMed2006

Direct determination of the thermodynamics of polyelectrolyte complexation and implications thereof for electrostatic layer-by-layer assembly of multilayer films.

Interpolyelectrolyte complex (IPEC) formation between poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) has been studied over a range of ionic strengths by isothermal titration calorimetry (ITC), turbidity titration, and electrostatic layer-by-layer assembly (ELBL). The results indicate that IPEC formation of PSS/PAH in aqueous solution is predominantly entropy-driven. The thermodynamic parameters suggest the formation of different types of complexes and aggregates due to salt-induced conformational changes in the polyelectrolyte conformation. Differences in polyelectrolyte behavior in the different salt-concentration regimes are described in terms of changes in the Debye screening length of the polyelectrolytes. The relationship of the results to the effect of salt concentration on the assembly of polyelectrolyte multilayer films (PEMs) is discussed.

Calorimetry, Differential Scanning↗

PubMed2006

Fabrication of fluorescent nanotubes based on layer-by-layer assembly via covalent bond.

A pressure-filter-template approach was employed to prepare fluorescent nanotubes of polyethyleneimine (PEI) and 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) through covalent combination in the porous of alumina template based on the layer-by-layer (LbL) assembly technique. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images exhibited a tubular structure of the obtained samples. The wall thickness of the nanotubes is well controlled by varying the assembled cycle number, which is detected by UV-visible spectroscopy. Fourier transform infrared (FT-IR) spectroscopy confirmed the formation of covalent bonds between PEI and PTCDA in nanotubes.

Journal Article↗

PubMed2006

Use of a ruthenium-containing conjugated polymer as a photosensitizer in photovoltaic devices fabricated by a layer-by-layer deposition process.

Multilayer polymer films composed of a ruthenium terpyridine complex containing poly(p-phenylenevinylene) (Ru-PPV) and sulfonated polyaniline (SPAN) were prepared by a layer-by-layer electrostatic self-assembly deposition. The deposition process was carried out from SPAN solution in water and Ru-PPV in dimethylformamide (DMF). Optical-quality multilayer thin films were obtained. The film growth process was monitored by quartz crystal microbalance, and the surface morphology of the films was studied by atomic force microscopy. It was found that the properties of the multilayer films were dependent on deposition conditions such as the pH of the SPAN solution, the presence of salt in the polymer solutions, and the post-film-forming thermal annealing process. Cross-section transmission electron microscopic images suggested that there was no stratified structure formed in the multilayer films. Photovoltaic cells were fabricated by sandwiching the multilayer films between indium-tin-oxide and aluminum electrodes. The device performances were examined by illumination with AM 1.5 simulated solar light. The power conversion efficiencies of these devices were on the order of 10(-3)%. The maximum incident photon-to-electron conversion efficiency (IPCE) of the devices was found to be approximately 2% at 510 nm, which is consistent with the absorption maximum of the ruthenium complex. This indicates that the photosensitization process is due to the electronic excitation of the ruthenium complex.

Journal Article↗

PubMed2006

Layer-by-layer self-assembled polyelectrolyte multilayers with embedded liposomes: immobilized submicronic reactors for mineralization.

The development of chemical reactions in nanospaces is of paramount importance for the development of active nanodevices, particularly in nanofluidics. It has been shown in a previous paper that phospholipid vesicles can be incorporated without spontaneous bilayer rupture into poly-L-glutamic acid/poly(allylamine) (PGA/PAH) multilayered polyelectrolyte films. The aim of the present study was to use such a system as an "embedded submicronic reactor" able to trigger precipitation of calcium phosphates within closed spaces through an enzymatic reaction, the enzyme also being encapsulated in the vesicle interior. To this aim, large unilamellar vesicles (LUVs) were produced containing calcium ions as active ions in the mineralization process, spermine as an activator of crystal growth, and alkaline phosphatase as a catalyst to convert phosphate esters into phosphates. After stabilization by adding a layer of poly-(D-lysine), these vesicles were embedded in a (PGA-PAH)n film. A paranitrophenyl phosphate containing solution was then put in contact with this film. It is shown by means of infrared spectroscopy in the attenuated total reflection mode that, consecutively to this contact, calcium phosphates are growing inside the embedded vesicles. By using scanning near-field fluorescence microscopy, it is demonstrated that the alkaline phosphatase enzymes are most probably located inside the vesicles after their embedding. In addition, atomic force microscopy was used to show, after chemical removal of the organic top layer of the film, that the inorganic platelets produced after the precipitation reaction are localized in volumes of similar size and shape as that of the vesicles into which the phosphate ester hydrolysis and subsequent precipitation reaction did occur.

Journal Article↗

PubMed2006

A novel layer-by-layer approach for the fabrication of conducting polymer/RNA multilayer films for controlled release.

Poly(anilineboronic acid) (PABA)/ribonucleic acid (RNA) multilayer films were prepared under neutral condition using a layer-by-layer deposition of PABA and RNA. RNA was used both as a polyelectrolyte for multilayer formation as well as dopant for PABA. Photoelastic modulated infrared reflection absorption spectroscopy measurements suggest that PABA interacts covalently with RNA through the formation of a boronate ester, a boron-nitrogen dative bond, as well as electrostatic interactions of anionic phosphates with cationic amines. The deposition procedure was monitored with UV-vis absorption spectroscopy, showing a linear dependence of absorbance with the number of PABA/RNA bilayers deposited. The multilayer films were further characterized using X-ray photoelectron spectroscopy and ellipsometry, which yielded a PABA/RNA bilayer thickness of approximately 10 nm. The PABA/RNA multilayer films are redox-active at neutral pH, consistent with the formation of a self-doped polymer. Electrochemical control of PABA under these conditions allows potential-induced controlled release of RNA from a multilayer at neutral pH, suggesting that this may serve as a novel method for controlled release of RNA under physiological conditions.

Aniline Compounds↗

PubMed2006

The role of surface charging during the coadsorption of mercaptohexanol to DNA layers on gold: direct observation of desorption and layer reorientation.

We study the coadsorption of mercaptohexanol onto preimmobilized oligonucleotide layers on gold. Monitoring the position of the DNA relative to the surface by optical means directly shows the mercaptohexanol-induced desorption of DNA and the reorientation of surface-tethered strands in situ and in real time. By simultaneously recording the electrochemical electrode potential, we are able to demonstrate that changes in the layer conformation are predominantly of electrostatic origin and can be reversed by applying external bias to the substrate.

Adsorption↗

PubMed2006

Electrochemical and spectroelectrochemical behavior of the TCNQ(0/)(-) couple on a glassy carbon electrode. Layer-by-layer nucleation and growth.

On the basis of the electrochemical results obtained for thin films of 7,7,8,8- tetracyanoquinodimethane (TCNQ) on a glassy carbon electrode, the reduction and oxidation of the [TCNQ](0/)(-) couple in KCl aqueous media occurs via a mechanism involving layer-by-layer nucleation and growth. In situ recorded UV-visible spectroelectrochemical data allow two different crystal structures for the oxidized form of TCNQ to be discriminated.

Adsorption↗