Synthesis, characterization, and immobilization of Prussian blue-modified Au nanoparticles: application to electrocatalytic reduction of H2O2

Au nanoparticles modified with electroactive Prussian blue (PB) were for the first time synthesized by a simple chemical method. Transmission electronic microscopy showed that the average size of the Prussian blue shell/Au core hybrid composite (PB@Au) was about 50 nm, and Fourier transform IR, UV-vis spectra, and cyclic voltammetry confirmed the existence of PB on the surface of Au nanoparticles. Using the LbL technique, multilayer thin films of PB@Au nanoparticles were prepared by the alternate adsorption of oppositely charged linear polyelectrolyte poly(allylamine hydrochloride) (PAH) onto ITO glass for the construction of a hydrogen peroxide sensor. The novel multilayer films were characterized by SEM, cyclic voltammetry, and UV-visible absorption spectroscopy. The {PAH/PB@Au}n multilayer-modified electrode showed a well-defined pair of redox peaks and dramatic catalytic activity toward the reduction of hydrogen peroxide.     

Effect of Poly(amine)s on the Redox Properties of Carboxymethylcellulose and Ferrocene‐modified Poly(amine) Layer‐by‐Layer Films

Multilayer films consisting of carboxymethylcellulose (CMC) and ferrocene‐modified poly(ethyleneimine) (Fc‐PEI) or poly(allylamine hydrochloride) (Fc‐PAH) were successfully prepared on a gold electrode to examine their redox properties. The redox current of (Fc‐PEI/CMC)_n film‐coated electrodes increased with the number of layers, while the (Fc‐PAH/CMC)_n film‐coated electrodes exhibited increased response only for the first eight bilayers. The (Fc‐PEI/CMC)_n and (Fc‐PAH/CMC)_n films deposited on the surface of Fc‐free multilayer film‐coated electrodes also showed a redox response. The (PEI/CMC)₅ film‐coated electrode showed redox responses in Fc‐PEI and Fc‐PAH solutions, confirming the uptake of the Fc‐polymers in the inner film. In contrast, the uptake of the Fc‐polymers in the (PAH/CMC)₅ film was severely suppressed, suggesting that different permeability of (PEI/CMC)₅ and (PAH/CMC)₅ films.     

Preparation and resistivity study of silver nanocomposite films using spin‐assisted layer‐by‐layer assembly

Nanocomposite films [Ag/(PAH‐PSS)_nPAH]_m were fabricated on a silicon substrate using a time‐ and cost‐efficient spin‐assisted layer‐by‐layer (SA‐LbL) self‐assembly technique. A virtually monolayer‐like layer of self‐assembled silver nanoparticles was formed when deposition time increased to 30 min. It was found that polymer multilayers could effectively decrease the resistivity of silver nanoparticle monolayer, which was far higher than that of bulk silver metal; however, the resistivity of Ag/(PAH‐PSS)_nPAH multilayer films increased along with the increasing of the number of polymer bilayers. XPS investigations showed that silver nanoparticles were partially oxidized, which might be the major cause of the high resistivity of silver nanoparticle monolayer. Copyright © 2008 John Wiley & Sons, Ltd.     

Construction of Polycation‐Based Non‐Viral DNA Nanoparticles and Polyanion Multilayers via Layer‐by‐Layer Self‐Assembly

Summary: The multilayers of polycation‐based non‐viral DNA nanoparticles and biodegradable poly(L ‐glutamic acid) (PGA) were constructed by a layer‐by‐layer (LbL) technique. Poly(ethyleneimine) (PEI) was used to condense DNA to develop non‐viral DNA nanoparticles. AFM, UV‐visible spectrometry, and TEM measurements revealed that the PEI‐DNA nanoparticles were successfully incorporated into the multilayers. The well‐structured, easily processed multilayers with the non‐viral DNA nanoparticles may provide a novel approach to precisely control the delivery of DNA, which may have great potential for gene therapy applications in tissue engineering, medical implants, etc.

A TEM image of the cross section of a (PGA/PEI‐DNA nanoparticle)₂₀ multilayer.     

Interaction and adhesion properties of polyelectrolyte multilayers

The growth, morphology, and interaction/adhesion properties of supported poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH) and DNA/PAH multilayers were investigated by means of surface plasmon resonance spectroscopy, atomic force microscope (AFM) imaging, and AFM-related force measurements. Multilayers were assembled on a prelayer of poly(ethylenimine) (PEI) both with and without drying. SPR results showed a linear growth of the assembly in the case of PSS/PAH multilayers and nonlinear growth for DNA/PAH multilayers. Measurements of forces acting between a bare glass sphere and a multilayer-coated surface indicated repulsive or attractive forces, depending on surface charge, which suggests that, on approach, electrostatic forces dominate. On separation, we observed large pull-off forces in the case of positively charged multilayers and weak pull-off forces in the case negatively charged multilayers. Multiple adhesions and plateau regions observed on separation were interpreted in terms of a bridging of multiple polymer chains between the glass particle and the multilayer and a stretching of the polyelectrolyte loops. The dependence of the pull-off force on the number of deposited layers shows regular oscillations.     

Raman and surface enhanced Raman microscopy of microstructured polyethylenimine/DNA multilayers

We analyze microstructured multilayer films of poly(ethyleneimine) (PEI) and DNA by employing Raman and surface enhanced Raman spectroscopy (SERS). The microstructuring of the samples allows a simultaneous measurement of signal and reference in a single analytic process. Silver nanoparticles are implemented in the microstructured multilayers for SERS measurements. The recorded SERS spectra of PEI/DNA are dominated by the Raman bands of the DNA bases which show a larger mean enhancement than bands belonging to DNA backbone vibrations. Our results show that the combination of SERS and microstructured multilayer films provides an adapted way to characterize the polyelectrolytes as well as to measure the enhancement factor and the distance dependence for the SERS active silver nanoparticles. Furthermore, microstructured polyelectrolyte films containing SERS active nanoparticles are used for sensing molecules.     

Layer—by—layer Multilaryer Films Self—assembled from a Rare—earth—containing Poly—oxometalate,Na9[Eu（W5O18）2] and Poly （allylamine hydrochloride） and Their Photoluminescent Properties

Ultrathin multilayer films of a rare-earth-containing polyoxometalate Na9[Eu(W5O18)2](EW) and poly (allymamine hydrochloride)(PAH) have been prepared by layer-by-layer self-assembly from dilute aqueous solution.The fabrication process of the EW/PAH multilaryer films was followed by UV-vis spectroscopy and ellipsometry,which show that the deposition process is linear and highly reproducible from layer to layer.An average EW/PAH bilayer thickness of ca.2.1nm was determined by ellipsometry.In addition,the scanning electron microscopy(SEM) image of the EW/PAH film indicates that the film surface is relatively uniform and smooth.The photoluminescent properties of these films were also investigated by fluorescence spectroscopy.     

Photo-cross-linkable polyelectrolyte multilayers for 2-D and 3-D patterning

We report the synthesis of poly(acrylic acid-ran-vinylbenzyl acrylate) (PAArVBA), a photo-cross-linkable weak polyelectrolyte, and its incorporation into polyelectrolyte multilayer (PEM) films. PEM films assembled from PAArVBA and poly(allylamine hydrochloride) (PAH) are found to exhibit similar thickness trends with assembly pH as those previously reported for poly(acrylic acid) (PAA)/PAH multilayers. Swelling properties of the as-built and photo-cross-linked films are studied by in situ ellipsometry. Two-dimensional masking techniques are used to pattern regions of high and low swelling, as confirmed by atomic force microscopy (AFM), and to provide spatial control over the low-pH-induced microporosity transition exhibited by PAH/PAA PEMs. Films containing alternating blocks of PAH/PAArVBA bilayers and PAH/PAA bilayers were assembled, laterally photopatterned, and exposed to low-pH solution to generate nanoporosity leading to patterned Bragg reflectors, thereby demonstrating three-dimensional control over film structure in these weak PEM assemblies.     

Flexible NH3 sensors fabricated by in situ self-assembly of polypyrrole

Novel flexible NH₃ gas sensors were formed by the in situ self-assembly of polypyrrole (PPy) on plastic substrates. A negatively charged substrate was prepared by the formation of an organic monolayer (3-mercapto-1-propanesulfonic acid sodium salt—MPS) on a polyester (PET) substrate using a pair of comb-like Au electrodes. Two-cycle poly(4-styrenesulfonic acid) sodium salt/poly(allylamine hydrochloride) (PSS/PAH) bilayers (precursor layer) were then layer-by-layer (LBL) deposited on an MPS-modified substrate. Finally, a monolayer of PPy self-assembled in situ and PPy multilayer thin films self-assembled LBL in situ on a (PSS/PAH)₂/MPS/Au/Cr/PET substrate. The thin films were analyzed by atomic force microscopy (AFM). The effects of the precursor layer (PSS), the deposition time of the monolayer of PPy and the number of PPy multilayers on the gas sensing properties (response) and the flexibility of the sensors were investigated to optimize the fabrication of the film. Additionally, other sensing properties such as sensing linearity, reproducibility, response and recovery times, as well as cross-sensitivity effects were studied. The flexible NH₃ gas sensor exhibited a strong response that was comparable to or even greater than that of sensors that were fabricated on rigid substrate at room temperature.     

Characteristics of polyelectrolyte multilayers: effect of PEI anchoring layer and posttreatment after deposition

The influence of a first (anchoring) layer and film treatment on the structure and properties of polyelectrolyte multilayer (PEM) films obtained from polyallylamine hydrochloride (PAH) and polysodium 4-styrenesulfonate (PSS) was studied. Branched polyethyleneimine (PEI) was used as an anchoring layer. The film thickness was measured by ellipsometry. Complementary X-ray reflectometry and AFM experiments were performed to study the change in the interfacial roughness. We found that the thickness of the PEM films increased linearly with the number of layers and depended on the presence of an anchoring PEI layer. Thicker films were obtained for multilayers having PEI as the first layer comparing to films having the same number of layers but consisting of PAH/PSS only. We investigated the wettability of PEM surfaces using direct image analysis of the shape of sessile water drops. Periodic oscillations in contact angle were observed. PAH-terminated films were more hydrophobic than films with PSS as the outermost layer. The effect of long time conditioning of PEM films in solutions of various pH's or salt (NaCl) concentrations was also examined. Salt or base solutions induced modification in wetting properties of the polyelectrolyte multilayers but had a negligible effect on the film thickness.     

Preparation and characterization of the nanoporous ultrathin multilayer films based on molybdenum polyoxometalate (Mo38)n

Ultrathin multilayer films of the wheel-shaped molybdenum polyoxometalate cluster (Mo₃₈)_n and poly(allylamine hydrochloride)(PAH) have been prepared by the layer-by-layer (LbL) self-assembly method. The ((Mo₃₈)_n/PAH)_m multilayer films have been characterized by X-ray photoelectron spectra (XPS) and atomic force microscopy (AFM). UV-VIS measurements reveal regular film growth with each (Mo₃₈)_n adsorption. The electrochemistry behavior of the film at room temperature was investigated.     

Assembly of multilayer films from polyelectrolytes containing weak and strong acid moieties

Multilayer films were assembled from a copolymer containing both weakly and strongly charged pendant groups, poly(4-styrenesulfonic acid-co-maleic acid) (PSSMA), deposited in alternation with poly(allylamine hydrochloride) (PAH). The strongly charged groups (styrene sulfonate, SS) are expected to form electrostatic linkages (to enhance film stability), while the weakly charged groups (maleic acid, MA) can alter multilayer film properties because they are responsive to external pH changes. In this study, we varied several assembly conditions such as pH, SS/MA ratio in PSSMA, and the ionic strength of the polyelectrolyte solutions. The multilayer films were also treated by immersion into pH 2 and 11 solutions after assembly. Quartz crystal microgravimetry and UV-visible spectrophotometry showed that the thickness of PSSMA/PAH multilayers decreases with increasing assembly pH regardless of whether salt was present in the polyelectrolyte solutions. When no salt was added, the multilayers are thinner, smoother, and grow less regularly. Atomic force microscopy images indicate that the presence of salt in polyelectrolyte solutions results in rougher surface morphologies, and this effect is especially significant in multilayers assembled at pH 2 and pH 11. When both polyelectrolytes are adsorbed at conditions where they are highly charged, salt was necessary to promote regular multilayer growth. Fourier transform infrared spectroscopy studies show that the carboxylic acids in the multilayers are essentially ionized when assembled from different pHs in 0.5 M sodium chloride solutions, whereas some carboxylic acids remain protonated in the multilayers assembled from solutions with no added salt. This resulted in different pH stability regimes when the multilayers were exposed to different pH solutions, post assembly.     

The effect of support material and conditioning on wettability of PAH/PSS multilayer films

An efficient method for characterizing wetting properties of heterogeneous surfaces produced by sequential adsorption of polyelectrolytes was developed. Three types of polyelectrolytes were used: polyallylamine hydrochloride (PAH), polyethyleneimine (PEI), both of a cationic type, and polysodium 4-styrenesulfonate (PSS), of an anionic type. Multilayer films were prepared by 'layer-by-layer' (LbL) deposition technique. Natural ruby mica, glass, titanium foil and silicon wafers were used as the support material for PE films. Wetting of polyelectrolyte films was determined experimentally by contact angle measurements, using technique of direct image analysis of shape of sessile drops. Periodic oscillations in contact angle values were observed for multilayers terminated by polycation and polyanion, respectively, and the variations in contact angle values strongly depended on the conditions of adsorption and multilayer treatment after deposition. Therefore, the influence of ionic strength of polyelectrolyte solution used for deposition on wetting of multilayer films was considered and also the effect of conditioning in different environments was investigated. It is usually assumed that film properties and stability strongly depend on the first layer which is used to anchor a multilayer at the surface of support material. To investigate influence of the first layer, PAH/PSS films were compared with more complex ones having PEI as the first layer with a sequence of PSS/PAH deposited on top of it.     

Preparation of polypyrrole (PPy)-derived polymer/ZrO2 nanocomposites

New polypyrrole (PPy)-derived polymer/ZrO₂ nanocomposite materials are prepared by single-step oxidative polymerization of pyrrole (Py) and/or N-methylpyrrole (mPy) in the presence of HCl-functionalized ZrO₂ nanoparticles and ammonium persulfate. The physicochemical features of the PPy–ZrO₂, poly(Py-co-mPy)–ZrO₂ and PmPy–ZrO₂ hybrids were analyzed by XPS, FTIR, XRD and UV–Vis techniques. To explore the advantages of these nanocomposites for potential applications, their thermal, conductive and electrochemical properties were investigated. The characterization reveals that a chemical bonding, based on electrostatic interactions, is established between the polymers and the ZrO₂ nanoparticles. Interestingly, it is found that the growth of polymer on the surface of Cl-functionalized ZrO₂ becomes more significant as the Py moiety (–NH– species) content in the polymer increases. The thermal stability and conductivity of the polymers increase by hybridization with the ZrO₂ nanoparticles. This is assigned to the affective interaction of the polymers with the ZrO₂ nanoparticles. Particularly, the resulting nanocomposites keep high conductivities, ranging between 0.323 and 0.929 S cm⁻¹. Finally, voltammetric characterization shows that the PPy–ZrO₂ and poly(Py-co-mPy)–ZrO₂ nanocomposites are electroactive, thus demonstrating their capability for electrochemical applications. These results highlight the great influence of the nanoparticle interface and the nature of monomer on the nanocomposite formation and properties.

     

Critical surface tension of poly(vinyl butyral) and poly(vinyl benzal) multilayers and its relation to their surface orientation. I. Effect of surface pressures during deposition

Critical surface tensions γ_c of poly(vinyl butyral) and poly(vinyl benzal) multilayers built up by the Langmuir-Blodgett method were measured with polyhydric alcohols and n-alkanes. The γ_c values of both polymer multilayers increased with increasing pressures of the piston oils used to control pressures of polymers on the water surface during deposition. The γ_c value of poly(vinyl butyral) multilayers built up to lower pressure of the piston oil was approximately consistent with a crystalline hydrocarbon surface, while the γ_c value of the multilayer built up to higher pressure of the piston oil was approximately consistent with a—CH₃ rather than a ? CH₂ ? CH₂? surface. All results for γ_c values of poly(vinyl benzal) multilayers were very close to the γ_c value of benzene ringrich surface. The γ_c value of the multilayer built up to lower pressure of the piston oil almost coincided with the γ_c value for amorphous polystyrene, while the γ_c value for the multilayer built up to higher pressure of the piston oil was in fair agreement with γ_c for an aromatic ring edge in the crystalline state. Values of γ_s^d, the dispersion force contribution to the surface free energy of multilayers calculated by Fowkes' relation, were in fair agreement with the corresponding observed γ_c values, respectively. It is concluded from these measurements that orientations and surface structures in both polymer multilayers are affected by pressure change of piston oils. The properties on monolayers of two polymers at a air-water interface and on barium stearate multilayers are also presented.     

Effective charge propagation and storage in hybrid films of tungsten oxide and poly(3,4-ethylenedioxythiophene)

Hybrid (composite) electroactive films consisting of such an organic conducting polymer as poly(3,4-ethylenedioxythiophene), PEDOT, and such a polynuclear inorganic compound as amorphous tungsten oxide, WO₃/H _x WO₃ were fabricated on carbon electrodes through electrodeposition by voltammetric potential in acid solution containing EDOT monomer and sodium tungstate. Electrostatic interactions between the negatively charged tungstic units (existing within WO₃) and the oxidized positively charged conductive polymer (oxidized PEDOT) sites create a robust hybrid structure which cannot be considered as a simple mixture of the organic and inorganic components. It is apparent from scanning electron microscopy that hybrid structures are granular but fairly dense. Because PEDOT and mixed-valence tungsten oxides are electronically conducting, the resulting hybrid films are capable of fast propagation. The reversible and fast redox reactions of tungsten oxide component lie in the potential range where PEDOT matrix is conductive. Furthermore, the hybrid films exhibit good mediating capabilities towards electron transfers between model redox couples such as cationic iron(III,II) and anionic hexacyanoferrate(III,II). Since the films accumulate effectively charge and show high current densities at electrochemical interfaces, they could be of importance to electrocatalysis and to construction of redox capacitors.     

Crystallization behavior and electroactive properties of PVDF,P(VDF-TrFE) and their blend films

PVDF, poly(vinylidene fluoride), as a semi-crystalline polymer, has interesting electroactive properties but usual melt and solution processing techniques result in its thermodynamically favored non-polar α-phase. By comparison, poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE), PT for short, directly crystallizes in the polar β-phase under the same conditions as PVDF. In this study, blend thin films comprising PVDF and P(VDF-TrFE) were prepared by solvent casting method. The difference in the crystallization behavior is comprehensively investigated between the polymers: PVDF, P(VDF-TrFE), and the resulting blend films. It is found that replacement of the fluoride atom in TrFE monomer induces a strong steric hindrance that may alter the crystallization process to become more favorable for nucleation of the PVDF β-phase. To figure out the effect of TrFE content on the crystallization behavior and electroactive properties, films with different blend ratios of PVDF and P(VDF-TrFE) were prepared. We found that the PVDF films exhibit higher crystallization activation energy (ΔE) as PT content increases. The atomic force microscopy (AFM) in the piezoresponse force microscopy (PFM) mode illustrated that P5T5 films with equal contents of PVDF and P(VDF-TrFE) induced the highest d₃₃ values.     

Synthesis, multilayer film assembly, and capsule formation of macromolecularly engineered acrylic acid and styrene sulfonate block copolymers

We report the use of copolymers synthesized with specific block ratios of weakly and strongly charged groups for the preparation of stable, pH-responsive multilayers. In this study, we utilized reversible addition-fragmentation chain transfer (RAFT) polymerization in the synthesis of novel pH-sensitive copolymers comprising block domains of acrylic acid (AA) and styrene sulfonate (SS) groups. The PAA x- b-SS y copolymers, containing 37%, 55%, and 73% of AA groups by mass (denoted as PAA 37- b-SS 63, PAA 55- b-SS 45, and PAA 73- b-SS 27, respectively), were utilized to perform stepwise multilayer assembly in alternation with poly(allylamine hydrochloride), PAH. The ratio of AA to SS groups, and the effect of the pH of both anionic and cationic adsorption solutions, on multilayer properties, were investigated using ellipsometry and atomic force microscopy. The presence of SS moieties in the PAA x- b-SS y copolymers, regardless of the precise composition, lead to films with a relatively consistent thickness. Exposure of these multilayers to acidic conditions postassembly revealed that these multilayers do not exhibit the characteristic large swelling that occurs with PAA/PAH films. The film stability was attributed to the presence of strongly charged SS groups. PAA x- b-SS y/PAH films were also formed on particle substrates under various adsorption conditions. Microelectrophoresis measurements revealed that the surface charge and isoelectric point of these core-shell particles are dependent on assembly pH and the proportion of AA groups in PAA x- b-SS y. These core-shell particles can be used as precursors to hollow capsules that incorporate weak polyelectrolyte functionality. The role of AA groups in determining the growth profile of these capsules was also examined. The multilayer films prepared may find applications in areas where pH-responsive films are required but large film swelling is unfavorable.     

Counterions and water in polyelectrolyte multilayers: a tale of two polycations

Attenuated total internal reflectance Fourier transform infrared, ATR-FTIR, spectroscopy was used to compare the water uptake and doping within polyelectrolyte multilayers made from poly(styrene sulfonate), PSS, and a polycation, either poly(allylamine hydrochloride), PAH, or poly(diallyldimethylammonium chloride), PDADMAC. Unlike PDADMA/PSS multilayers, whose water content depended on the solution ionic strength, PAH/PSS multilayers were resistant to doping by NaCl to a concentration of 1.2 M. Using (infrared active) perchlorate salt, the fraction of residual counterions in PDADMA/PSS and PAH/PSS was determined to be 3% and 6%, respectively. The free energy of association between the polymer segments, in the presence of NaClO4, was about 5 kJ mol-1 and -10 kJ mol-1, respectively, for PDADMA/PSS and PAH/PSS, indicating the relatively strong association between the polymer segments in the latter relative to the former. Varying the pH of the solution in contact with the PAH/PSS multilayer revealed a transition to a highly swollen state, interpreted to signal protonation of PAH under much more basic conditions than the pKa of the solution polymer. The increase in the multilayer pKa suggested an interaction energy for PAH/PSS in NaCl of ca. 16 kJ mol-1.     

Absorption of light and heavy water vapours in polyelectrolyte multilayer films

We studied the swelling and the uptake of water (H₂O or D₂O) vapours in polyelectrolyte (PE) multilayer (PEM) samples deposited on solid support (Si wafers) as a function of the isotope nature of the vapour and the charge of the last polymer layer. The samples were prepared with deuterated poly(sodium 4-styrenesulfonate) (dPSS) and poly(allylamine hydrochloride) (PAH). Two types of samples were studied. The sample with a structure Si/PEI/(dPSS/PAH)₆/dPSS was negatively charged. A positively charged sample was PAH terminated and had the structure Si/PEI/(dPSS/PAH)₆. The film thickness and scattering length density were estimated from neutron reflectometry (NR) experiments and the results were complemented with in-situ QCM measurements.We demonstrate that the swelling of PEM in H₂O and D₂O vapours is similar. However, the amount of adsorbed D₂O is around 10% more than the adsorbed H₂O. Such isotope effect correlates well with the rough estimation that the isotope effect usually scales with the difference in the mass density of the different isotope forms of the substances. For precise analysis of the NR data we assumed existence of empty voids in the structure of the PEM. These voids might be filled with “condensed” water when the samples are exposed to water vapors. We show that the layers we studied consist of up to 25% of such voids.We showed that the amount of sorbed water depends on the nature of the last layer which builds the PEM thus confirming the “odd-even effect” already shown in the literature.