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16(9) (2000), 4352-4357
Films consisting of three cross-linked molecular layers of human serum albumin (HSA) or three HSA layers alternating with three heparin layers were immobilized on surfaces of grating coupler sensors and hydrophobized glass. Optical responses of the coated sensors to pH changes were observed. A decrease in the effective refractive index, Delta N-eff, measured by a grating coupler was observed when the pH of buffers contacting the films was increased. The process was quite reversible, which indicated that the mass of the film remained constant when the buffers were repeatedly exchanged. Theoretical treatment of the grating coupler optics related the decrease in Delta N-eff to expansion of the films. The interpretation of sensor measurements was confirmed by atomic force microscopy which revealed reversible changes in the thickness of NSA/ heparin film induced by pH changes of the buffers.

[105] Influence of a Base Electrolyte on the Adsorption Behavior of an Anionic Surfactant as Observed by 2nd-Harmonic Generation

G. Moller, S. Schrader, H. Motschmann and D. Prescher

Langmuir 16(10) (2000), 4594-4598

The adsorption process of sodium-[4-(4'- trifluormethylphenylazo)phenyl-1-oxyethylsulfonate] (NaTAPOS) at the water/1,2-dichlorethane interface was investigated by second harmonic generation and ellipsometry. The combination of both techniques yields the surface excess and the orientation of the amphiphiles within the adsorption layer. These data were used for an investigation of equilibrium properties of the system. The average tilt angle of the chromophore within the adsorption layer remains constant in the whole concentration range. The mean tilt angle was found to be 39 +/- 3 degrees and the area per molecule at maximum surface coverage is 45.2 +/- 6 Angstrom(2). The present study focuses on the impact of a base electrolyte on the adsorption process. Neither the limiting area per molecule nor the orientational order was changed by base electrolyte dissolved in the aqueous phase. The maximum surface coverage with and without base electrolyte is achieved at the same bulk concentration and is within the error limit identical; however, at lower concentrations a significantly higher surface excess is observed in the absence of base electrolyte. This is counterintuitive since one might expect an electrostatic shielding of the prevailing interaction. The adsorption isotherms with and in absence of base electrolyte can be described by Frumkin's equation of state. This model accounts for regular surface behavior and takes into account the lateral interaction of the absorbed species. The model fit yields two quantities: the free enthalpy of adsorption and the interaction parameter. Only slight changes of the free enthalpy of adsorption were observed by the presence of base electrolyte; however, a significant increase of the corresponding interaction parameter was observed. These findings are in contradiction to the Debye-Huckel theory and are explained by a model in analogy to the polyelectrolyte layer-by-layer adsorption.

[106] Covalently Attached Multilayer Assemblies by Sequential Adsorption of Polycationic Diazo-Resins and Polyanionic Poly(Acrylic Acid)

J. Q. Sun, T. Wu, F. Liu, Z. Q. Wang, X. Zhang and J. C. Shen

Langmuir 16(10) (2000), 4620-4624

Diazo-resins (DAR) as polycation and poly(acrylic acid) (PAA) as polyanion were alternately assembled into a multilayer structure by using the layer-by-layer self-assembly technique. PAA as a weak polyelectrolyte was very sensitive to the pH value of the solution, and that pH could be used to tune the thickness of the PAA layer. Upon UV irradiation, a fraction of the carboxylate groups reacted with diazonium groups at the adjacent interfaces of the multilayer, so partially covalently attached multilayer assemblies could be formed. Their stability improved greatly compared with the fraction that was not UV- irradiated. The photoreaction occurring between the layers was confirmed by means of UV-visible and Fourier transform infrared spectroscopy. The thickness of the W-irradiated films was characterized with X-ray diffraction, results from which showed that the thickness of the films can be adjusted in nanometer scale by simply changing the pH value of the solutions.

[107] Electroactive Myoglobin Films Grown Layer-by-Layer with Poly(Styrenesulfonate) on Pyrolytic-Graphite Electrodes

H. Y. Ma, N. F. Hu and J. F. Rusling

Langmuir 16(11) (2000), 4969-4975

Stable multilayer electroactive films were grown on rough pyrolytic graphite (PG) electrodes by alternate adsorption of layers of polyanion poly(styrenesulfonate) (PSS) and positively charged myoglobin (Mb) from their aqueous solutions. Incorporation of large amounts of electroactive Mb was facilitated by high electrode surface areas and by adsorbing coiled PSS from 0.5 M NaCl solutions. Cyclic voltammetry of {PSS/Mb}(n), films showed a pair of well-defined, chemically reversible peaks at about -0.25 V vs SCE at pH 5.5, characteristic of the Mb heme Fe-III/Fe-II redox couple. Electroactivity was extended to 7 {PSS/Mb} bilayers on rough PG surfaces, in comparison to 2 electroactive layers on smooth gold coated with mercaptopropanesulfonic acid. Square wave voltammograms of {PSS/Mb}, films gave good fits by nonlinear regression analysis to a model featuring dispersion of formal potentials, providing average formal potentials and an apparent rate constant. Oxygen and trichloroacetic acid were catalytically reduced by Mb in {PSS/Mb}(n) films with significant decreases in the electrode potential required. Making such films on PG rather than on gold provides a larger number of electroactive layers with no need for chemical pretreatment of the electrode.

[108] Fabrication of Microporous Thin-Films from Polyelectrolyte Multilayers

J. D. Mendelsohn, C. J. Barrett, V. V. Chan, A. J. Pal, A. M. Mayes and M. F. Rubner

Langmuir 16(11) (2000), 5017-5023

A simple process has been developed to create large area, highly uniform microporous thin films. Multilayers of weak polyelectrolytes were assembled onto silicon substrates by the sequential adsorption of poly(acrylic acid) and poly(allylamine) from aqueous solution. These multilayers were then immersed briefly into acidic solution (pH approximate to 2.4) to effect a substantial and irreversible transformation of the film morphology. The resulting microporous structures are 2- 3 times the thickness of the original films, possess a correspondingly reduced relative density of 1/2 to 1/3, and are stable against further rearrangement under ambient; conditions. In addition, the microporous films may undergo a secondary reorganization in neutral water, leading to a morphology with more discrete throughpores. A mechanism is proposed for these transformations based on interchain ionic bond breakage and reformation in this highly protonating environment;, leading to an insoluble precipitate on the substrate which undergoes spinodal decomposition with the solvent. FTIR (Fourier transform infrared spectroscopy) analysis supports the underlying chemical basis of this pH-induced phase separation, and AFM (atomic force microscopy), in situ ellipsometry, and SEM (scanning electron microscopy) have been used to monitor the morphological changes. The unique combination of properties exhibited by these microporous films makes them potential candidates for microelectronic and biomaterial applications.

[109] Mobile Phospholipid-Bilayers on a Polyion/Alkylthiol Layer Pair

L. Q. Zhang, M. L. Longo and P. Stroeve

Langmuir 16(11) (2000), 5093-5099

The formation of phospholipid bilayers on a polymer/alkylthiol layer pair was investigated by surface plasmon resonance (SPR). The organic layer pair between the lipid membrane and the solid gold surface consisted of a self-assembled monolayer of 11- mercaptoundecanoic acid (MUA) on a gold surface followed by a thin layer of hydrated cationic poly(diallyldimethylammonium chloride) (PDDA). The lipid layers were formed by vesicular fusion of small unilamellar vesicles of an anionic lipid 1- stearoyl-2-oleoyl-phosphatidylserine (SOPS), a zwitterionic lipid 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), and mixtures of these two lipids with different compositions. In the case of pure SOPS and lipid mixtures with a POPC composition below 25%, the lipid layer thickness was approximately that of a single bilayer, while multilayers tended to build up with higher concentrations of POPC. The electrostatic interaction of the cationic PDDA with the negatively charged lipid membrane is most probably the driving force for the adsorption of a single bilayer. Fluorescence recovery after photobleaching (FRAP) experiments showed that single bilayers supported on the PDDA/alkylthiol layer pair were mobile at room temperature with lateral diffusivities of approximately (1-2) x 10(-9) cm(2)/s. A membrane fusion peptide wt-20 of the influenza virus was bound to the supported bilayers as detected by SPR. This suggests the potential of this model membrane system for use as a biosensor.

[110] Self-Assembled Monolayers and Multilayered Stacks of Lyotropic Chromonic Liquid-Crystalline Dyes with Inplane Orientational Order

T. Schneider and O. D. Lavrentovich

Langmuir 16(12) (2000), 5227-5230

A simple method to create large areas of in-plane oriented molecular mono- and multilayers is presented. The method is based on layer-by-layer adsorption of polyions and a lyotropic chromonic (nonamphiphilic) liquid crystal, which allows one to orient the molecules uniformly in the plane of deposition. The long-range in-plane orientation of the chromonic molecules determines the unique structural and optical properties of the films. Control of the orientational order allows one to use the films as polarizers, alignment layers, optical compensators, retarders, filters, etc.

[111] Freestanding Layer-by-Layer Assembled Films of Magnetite Nanoparticles

A. A. Mamedov and N. A. Kotov

Langmuir 16(13) (2000), 5530-5533

A new technique for preparation of free-standing ultrathin films is presented. These films were made by layer-by-layer (LBL) deposition process, which is utilized for coatings on solid substrates and colloids. A film composed of alternating layers of magnetite nanoparticles and poly(diallyldimethylammonium bromide) was assembled on cellulose acetate, which was subsequently dissolved in acetone. From the suspended state, the LBL film can be transferred onto any solid or porous support. The strength of the film was observed to significantly increase when every other layer of magnetite was replaced with a layer of alumosilicate sheets serving as a molecular framework for the assembly.

[112] Imaging Nanoscale Patterns on Biologically Derived Microstructures

Y. M. Lvov, R. R. Price, J. V. Selinger, A. Singh, M. S. Spector and J. M. Schnur

Langmuir 16(14) (2000), 5932-5935

We have demonstrated a new approach for imaging nanoscale patterns on three-dimensional submicrometer structures using charged particles. Nanoparticle structures were assembled onto lipid tubules through the sequential adsorption of oppositely charged polymers and silica spheres. For tubules of the zwitterionic lipid DCB8,11PC, this process leads to the formation of caps on the ends of the tubules, with 50-100 silica spheres in each cap. For tubules of DC8,11PC mixed with 2% of the charged lipid DC8,9PEOH, the sequential adsorption leads to both end caps and helices of nanoparticles winding around the interior of the tubules. These results give new insight into the pattern of charge in lipid tubules.

[113] Construction of Multilayer Thin-Films Containing Avidin by a Layer-by-Layer Deposition of Avidin and Poly(Anion)S

J. Anzai, T. Hoshi and N. Nakamura

Langmuir 16(15) (2000), 6306-6311

A layer-by layer deposition of avidin and poly(anion)s including poly(styrenesulfonate) (PSS), poly(vinyl sulfate) (PVS), and dextransulfate (DS) afforded multilayer thin films on the surface of a quartz slide, through electrostatic force of attraction between positive charges on the avidin molecule and negative charges on the polymer chains. It was possible to regulate the thickness of the films arbitrarily by changing the number of depositions. The loading of avidin in the multilayer thin films depended on the concentration of avidin and poly(anion)s in the bathing solutions from which the films were deposited. In the PVS- and PSS-based multilayer films, a large amount of avidin was immobilized in each deposition, depending on the concentrations of avidin and poly(anion) in the bathing solutions. In contrast, nearly a monomolecular layer was formed in each layer in the avidin-DS films independent of the concentration of bathing solutions. Avidin molecules assembled into the thin film retained its binding activity to biotin and analogues. The binding properties of avidin in the multilayer films were nearly comparable to those in solution; the films bound biotin, desthiobiotin, iminobiotin, and 2-(4'- hydroxyphenyl azo)benzoic acid effectively.

[114] Sequential Electrostatic Assembly of Amine-Derivatized Gold and Carboxylic Acid-Derivatized Silver Colloidal Particles on Glass Substrates

A. Kumar, A. B. Mandale and M. Sastry

Langmuir 16(17) (2000), 6921-6926

The formation of alternating layers of positively charged gold and negatively charged silver colloidal particles on glass substrates via electrostatic interaction is described. The charging of the gold and silver colloidal particles is accomplished by self-assembly of 4-aminothiophenol (4-ATP) and 4-carboxythiophenol (4-CTP) monolayers on the colloidal particles respectively and subsequent ionization of the functional groups at appropriate pH values of the colloidal solution. Glass substrates, which are negatively charged at pH > 3, are immersed first in the positively charged amine- derivatized gold solution leading to the formation of a monolayer of the gold particles and charge reversal of the glass surface. Thereafter, the gold particle covered glass surface is immersed in the negatively charged carboxylic acid- derivatized colloidal silver solution and the silver particles electrostatically self-assembled on the glass surface. This process map be continued to yield multilayer structures of the colloidal particles. The kinetics of electrostatic self- assembly of the colloidal particles on glass, the formation of the multilayer films, and their thermal stability have been followed with UV-vis spectroscopy, X-ray diffraction, ellipsometry, and X-ray photoemission spectroscopy measurements.

[115] Conductance and Capacitance of Polyelectrolyte and Lipid- Polyelectrolyte Composite Capsules as Measured by Electrorotation

R. Georgieva, S. Moya, S. Leporatti, B. Neu, H. Baumler, C. Reichle, E. Donath and H. Möhwald

Langmuir 16(17) (2000), 7075-7081

Polyelectrolyte capsules were fabricated in aqueous media by stepwise adsorption of polyelectrolytes onto fixed erythrocytes and subsequent template dissolution. Lipid polyelectrolyte composite capsules were prepared assembling lipid layers on these polyelectrolyte capsules. Dipalmitoyl phosphatidyl acid (DPPA), dipalmitoyl phosphatidyl choline (DPPC), and a mixture of both were used. Confocal laser scanning microscopy showed that the Lipids form a homogeneous coverage on the capsule surface. An electrorotation technique was used to study the electrical properties of polyelectrolyte and lipid- polyelectrolyte composite capsules. A conductivity of 1 S/m for polyelectrolyte capsule walls was found. Lipid-polyelectrolyte composite capsules yielded conductivities in the range from 10(- 4) to 10(-1) mS/m and capacities of 2.7 mu F/cm(2) for DPPA and 0.5 mu F/cm(2) for DPPC. These conductivities of lipid- polyelectrolyte composite capsules were much higher than for black lipid membranes. They increased with the bulk electrolyte concentration, which was attributed to the presence of pores or defects in the lipid structures. The effective area of pores was estimated as 0.01% of the total capsule surface.

[116] Nucleation and Growth of Cobalt Hydroxide Crystallites in Organized Polymeric Multilayers

L. Q. Zhang, A. K. Dutta, G. Jarero and P. Stroeve

Langmuir 16(17) (2000), 7095-7100

In this paper, we report the hydrolysis of Co2+ ions absorbed in organized, multilayered polymer films to form cobalt hydroxide nanocrystals. Polymer films were prepared using the layer-by-layer deposition technique that consists of adsorbing polycations and polyanions alternately on a quartz substrate. By monitoring the UV-vis absorbance of the polymer films as a function of the number of absorption-hydrolysis cycles, it was shown that the crystals continued to grow with additional cycles. It is also found in this study that using more polymer layer pairs results in more crystal growth. Hydrolysis of the Co2+ ions in a nitrogen-rich environment gives rise to mainly needlelike crystallites of alpha-Co(OH)(2) that are initially about 100 nm in length and th en increase in size with the number of absorption-hydrolysis cycles. However, in an oxygen- rich microenvironment, hexagonal crystallites were found to be predominant. X-ray diffraction and transmission electronmicroscopic studies revealed that these hexagonal crystallites mainly consist of beta-Co(OH)(2) with a contribution of CoOOH. Our studies suggest that by varying the nitrogen-to-oxygen ratio the formation of alpha-Co(OH)(2) and beta-Co(OH)(2) can be controlled.

[117] Stepwise Assembly of Isotactic Poly(Methyl Methacrylate) and Syndiotactic Poly(Methacrylic Acid) on a Substrate

T. Serizawa, K. Hamada, T. Kitayama, K. Katsukawa, K. Hatada and M. Akashi

Langmuir 16(18) (2000), 7112-7115

Stepwise assembly of isotactic poly(methyl methacrylate) and syndiotactic poly(methacrylic acid) from acetonitrile and mixed acetonitrile/water solutions, respectively, onto a gold substrate was studied. Quantitative quartz crystal microbalance analysis of the molar ratio (monomer unit) between assembled polymers suggested Stepwise stereocomplex formation. The assembled amount and the assembly ratio between PMAA and PMAA were dependent on the acetonitrile content of the poly(methacrylic acid) solution. Infrared spectra of the assembly and cast films of each polymer also showed stereocomplex formation in the assembly. Atomic force microscopic observation showed the assembled polymer to have a molecularly smooth surface. The solvent species used was an important factor in stereocomplex formation at the liquid- polymer film interface.

[118] Preparation and Characterization of Ultrathin Films Layer-by- Layer Self-Assembled from Graphite Oxide Nanoplatelets and Polymers

T. Cassagneau, F. Guerin and J. H. Fendler

Langmuir 16(18) (2000), 7318-7324

Graphite, oxidized by HNO3 and NaClO3 to 25% (GO-25), 42% (GO- 42), and 33% (GO-33), has been dispersed to partially exfoliated nanoplatelets and, along with poly(diallyldimethylammonium chloride) (PDDA) and poly(ethylene oxide) (PEO), layer-by-layer self-assembled to ultrathin S- (PDDA/GO)(10), S-PDDA/(GO/PEO)(10), and S-(PDDA/GO/PEO)(10) films (where S is the indium tin oxide- or chromium/gold-coated glass or quartz substrate and GO refers to platelets of oxidized graphite). The GO powders have been characterized by X- ray diffraction prior and by transmission electron microscopy after their dispersions. Thicknesses of each of the successively adsorbed layers were comparatively determined by absorption spectrophotometry, surface plasmon resonance spectroscopy, quartz crystal microbalance, and scanning force microscopy. Good agreements were obtained by these three different techniques.

[119] Formation of Polyelectrolyte Multilayers

M. Castelnovo and J. F. Joanny

Langmuir 16(19) (2000), 7524-7532

A mechanism for the formation of polyelectrolyte multilayers is proposed. All of the experiments on such systems show that there is strong interpenetration of consecutive layers. We explain the very strong stability of the multilayers by the complexation between polyelectrolytes of opposite signs. Scaling laws on bulk polyelectrolyte complexation and their applications to the case of multilayers are derived in analogy with the description of neutral polymers under poor solvent conditions. Qualitative agreement is reached with some recent experiments.

[120] Nanocomposites by Electrostatic Interactions - 1 - Impact of Sublayer Quality on the Organization of Functionalized Nanoparticles on Charged Self-Assembled Layers

F. Auer, M. Scotti, A. Ulman, R. Jordan, B. Sellergren, J. Garno and G. Y. Liu

Langmuir 16(20) (2000), 7554-7557

Bis-benzamidines were studied as linkers for the formation of gold nanoparticle assemblies on planar gold surfaces modified with omega-mercaptohexadecanoic acid. In situ ellipsometry, external reflection Fourier transform spectroscopy, and atomic force microscopy showed that the packing density and order of the bis-benzamidine layer exerted a pronounced influence on the thickness and density of the nanoparticle layers. These layers exhibited optical properties similar to those of solid gold.

[121] Selective Self-Organization of Colloids on Patterned Polyelectrolyte Templates

K. M. Chen, X. P. Jiang, L. C. Kimerling and P. T. Hammond

Langmuir 16(20) (2000), 7825-7834

Submicron-sized colloidal particles have been self-organized into patterned arrangements on a substrate using a novel technique. At the substrate, a polyelectrolyte multilayer film has been deposited onto a chemically patterned surface; subsequently, the polyelectrolyte surface is immersed in an aqueous colloidal suspension of bare SiO2 microspheres or functionalized polystyrene latex particles. The colloids self- organize at the surface, driven by the spatially varied electrostatic and secondary interactions between the colloid and the substrate. The polyelectrolyte platform provides a strong bond to the colloids, imparting mechanical robustness which enables postprocessing of the patterned assemblies. An important advantage to this approach is that the use of a polyelectrolyte multilayer platform opens up the possibility of introducing functionality into the underlying layers. We have demonstrated control over the density and selectivity of particle adsorption. Three mechanisms have been used to control adsorption: (i) pH of the colloid suspension, which determines the ionization of the uppermost surface of the polyelectrolyte multilayer; (ii) ionic strength of the suspension, which determines the extent of charge screening about the colloid and polyelectrolyte; and (iii) concentration of added surfactant, which causes charge screening and introduces hydrophobic interactions between the surfactant and polyelectrolyte. Finally, an energy adsorption model is presented.

[122] Layer-by-Layer Assembly of CdSe Nanoparticles Based on Hydrogen- Bonding

E. C. Hao and T. Q. Lian

Langmuir 16(21) (2000), 7879-7881

We report the synthesis of nonaqueous dispersion of CdSe nanoparticles and the layer-by-layer assembly of hybrid poly(vinylpyridine) (PVP)/CdSe multilayer thin films based on hydrogen bonding. The multilayer buildup is monitored by W-vis spectra, which showed a linear increase of the film absorbance with the number of adsorbed CdSe layers. The resulted films have a high degree of flatness and smoothness as indicated by small-angle X-ray diffraction. Hydrogen bond interaction, the driving force for the formation of PVP/CdSe multilayer thin film, between the pyridine group of PVP and the carboxylic acid group on CdSe was verified by FTIR spectroscopy.

[123] Selective Deposition in Layer-by-Layer Assembly - Functional Graft-Copolymers as Molecular Templates

X. P. Jiang and P. T. Hammond

Langmuir