Polymer Monolayers Research Articles

Self-assembled monolayers of polythiophene conductive polymers improve biocompatibility and electrical impedance of neural electrodes

There is continued interest in the development of conductive polymer coatings to improve the electrical properties and biocompatibility of electrodes for neural prostheses. We present here a new type of coating, based on mixed self-assembled monolayers (SAMs) of thiolated poly(alkylthiophene)s and functionalized alkanethiols. When assembled as a SAM on electrodes designed for in vitro electrophysiology, these polymers are able to significantly lower electrode impedance at 1 kHz. The same mixed formulation is able to promote the outgrowth of neurites from primary mouse cortical neurons when the alkanethiol component is functionalized with a neural cell adhesion molecule (NCAM) binding antibody. Atomic force microscopy of the SAMs shows that they exert their effect through the well-known mechanism of increasing electrode surface area. These new covalently bound films have the potential to be more robust and are more controllable in their composition than existing electrodeposited conductive polymer coatings.

Kinetics of Photoinduced Electron Transfer in Polythiophene−Porphyrin−Fullerene Molecular Films

Photoinduced electron transfer (ET) processes were studied by the time-resolved Maxwell displacement charge (TRMDC) method in bilayer structures consisting of an electron donor-acceptor and conductive polymer monolayers, porphyrin-fullerene dyad and polyhexylthiophene, respectively, both layers prepared by the Langmuir-Blodgett (LB) method. The charge separation involves two fast steps: an intramolecular ET in the dyad molecule followed by an interlayer ET from the polymer to the formed porphyrin radical cation. These fast vertical intra- and interlayer processes could not be time-resolved by the TRMDC method. The lifetime of the charge separated state in the system was extended to hundreds of milliseconds by lateral electron and hole transfers in fullerene and polymer sublayers. The kinetics of the system was described by a model involving two long-living energetically different complete charge separated states. The data analysis indicates that the charge separation has a recombination time of 0.5 s. This is a promising result for possible applications.

Estimation of polymer–surface interfacial interaction strength by a contact AFM technique

Atomic force microscopy (AFM) measurements were employed to assess polymer–surface interfacial interaction strength. The main feature of the measurement is the use of contact-mode AFM as a tool to scratch off the polymer monolayer adsorbed on the solid surface. Tapping-mode AFM was used to determine the depth of the scraped recess. Independent determination of the layer thickness obtained from optical phase interference microscopy (OPIM) confirmed the depth of the AFM scratch. The force required for the complete removal of the polymer layer with no apparent damage to the substrate surface was determined. Polypropylene (PP), low-density polyethylene (PE), and PP-grafted-maleic anhydride (PP-g-ma) were scraped off silane-treated glass slabs, and the strength of surface interaction of the polymer layer was determined. In all cases it was determined that the magnitude of surface interaction force is of the order of van der Waals (VDW) interactions. The interaction strength is influenced either by polymer ability to wet the surface (hydrophobic or hydrophilic interactions) or by hydrogen bonding between the polymer and the surface treatment.

The influence of lead (II) ions introduced into the subphase on the stability of mixed "polyamic acid + surfactant" monolayers and manufacturing of dye-containing Langmuir-Blodgett polymeric films

The conditions for the preparation of monolayers of polyamic acid (PA) with one COOH group per repeating unit on aqueous subphases are described. It is revealed that the presence of Pb2+ ions in the aqueous subphase favors keeping up the polymer on the water surface, while the monolayers are unstable on the pure aqueous subphase. The behavior of PA films on water/air interfaces is examined mainly at pH 5.8-6.0 and 1x10-5 mol L-1 lead nitrate. The influence of n-octadecyl alcohol and n-octadecylpyridinium bromide on the character of isotherms and stability of monolayers is studied. The limiting area per repeating unit, Sm, increases along with the increase of surfactant fraction in the initial (PA + n-octadecyl alcohol) and (PA + n-octadecylpyridinium bromide) mixtures. The Sm values, collapse pressures, and compressibilities of the films are tabulated. Basing on these polyamic monolayers, Langmuir - Blodgett (60-80)-monolayer films containing various dyes (Rhodamine dye, rose Bengal B, and quinaldine red) are fabricated by depositing on glass (quartz) support by Schaefer method. Absorption and emission spectra of these films are registered. The dye content was as a rule 6 to <FONT FACE=Symbol>»</FONT>30 mole%. Vis absorption spectra of polymeric (60-80) monolayer films containing rose Bengal B and quinaldine red, measured after soaking in aqueous buffer solution, appeared to be sensitive to the pH values of the latter. The 'apparent' pKaª value of quinaldine red in (PA + octadecyl alcohol) films, as determined by standard spectroscopic procedure, equals 2.3±0.3. Such pH response of polymeric multilayers can be used in sensors.

In situ preparation of hydrophobic CaCO3 in the presence of sodium oleate

Hydrophobic CaCO3 particles were directly prepared via carbonation of Ca(OH)2 slurry in the presence of sodium oleate at room temperature. Sodium oleate was used to modify the surface property of CaCO3 particles. The measurement of relative contact angle and active ratio indicated that CaCO3 samples were hydrophobic. DTG, FT-IR and TEM analysis of the obtained product indicated that the hydrophobic property was attributed to the deposition of calcium oleate, produced in the reaction mixture, onto the surface of calcium carbonate particles. They were covered on the CaCO3 crystals surface and modified their surface property; at the same time they own CC bonds and could be polymerized or copolymerized later to give a polymeric monolayer.

Stable Blue Emission from a Polyfluorene/Layered‐Compound Guest/Host Nanocomposite

AbstractIn this study a blue‐light‐emitting conjugated polymer, poly(9,9‐dioctylfluorene), is confined to the interlayer space of inorganic, layered metal dichalcogenide materials, metallic MoS2, and semiconducting SnS2. The nanocomposites are prepared through Li intercalation into the inorganic compound, exfoliation, and restacking in the presence of the polymer. X‐ray diffraction and optical absorption measurements indicate that a single conjugated polymer monolayer, with an overall extended planar morphology conformation, is isolated between the inorganic sheets, so that polymer aggregation or π–π interchain interactions are significantly reduced. Photoluminescence (PL) measurements show that the appearance of the undesirable green emission observed in pristine polymer films is suppressed by incorporating the polymer into the inorganic matrix. The blue emission of the intercalated polymer is stable for extended periods of time, over two years, under ambient conditions. Furthermore, the green emission is absent in the PL spectra of nanocomposite films heated at 100 °C for 7 h in air with direct excitation of the keto defect. Finally, no green emission was observed in the electroluminescence spectrum of light‐emitting devices fabricated with a polymer‐intercalated SnS2 nanocomposite film. These results support the proposed hypothesis that fluorenone defects alone are insufficient to generate the green emission and that interchain interactions are also required.

Electrochemical Properties of Self-Assembled Monolayers of Polyaniline: Effects of the Thiol Substituent and Reduced Dimensionality

Self-assembled monolayers and bulk films of thioanilines, polymerized on gold and platinum surfaces, have been characterized and compared to bulk polyaniline (PANI) films. In a previous study [Kuwabata; et al. Langmuir 1999, 15, 6807-6812], only one redox couple was observed in the cyclic voltammetric profile of a polymerized monolayer of thioaniline on gold, in contrast to the known profiles of bulk PANI, which exhibit two couples. We observe two couples in both a polymerized thioaniline monolayer and a bulk polythioaniline (S-PANI) film, but the 200 mV window between the couples (the width of the region of high conductivity) in the S-PANI films is much smaller than the 600 mV window in bulk PANI. We ascribe this difference to the presence of the thiol substituent. The windows of high conductivity of the polymerized thioaniline monolayer and the bulk S-PANI film are the same within the limits of our experiment, implying that the difference in the dimensionality of the films (a 2D monolayer vs 3D bulk films) has a limited effect on the films' voltammetric profiles.

Poly(ethylene oxide)s hydrophobically modified. Adsorption and spreading at the air–water interface

A comparative study of spread and adsorbed monolayer of poly(ethylene oxide)s of different molecular weight hydrophobically modified with alkyl isocyanates of different length chain is reported. The modification of the polymer was carried out according to reported procedures. The polymers obtained were studied at the air–water interface by Langmuir isotherms for spread monolayers and by Gibbs isotherms for the adsorption process. Isotherms obtained are interpreted in terms of the hydrophobic and hydrophilic balance of the polymers. Limiting area per repeating unit ( A 0 ) and collapse pressure ( π c ) from spread monolayers were obtained. Spread monolayers of the hydrophobically modified polymers show larger collapse pressure values than unmodified polymer monolayers. In the adsorption process the excess surface concentration Γ ∞ , area per repeat unit σ, and efficiency of the adsorption were determined. The values of the area occupied per repeat unit in adsorbed monolayer ( σ) were larger than those of the spread monolayer. The efficiency of the adsorption of poly(ethylene oxide)s increases with the hydrophobic modification and with the alkyl chain length.

Ordering and dewetting in spin-coated films of a liquid crystalline main chain polymer

The ordering of spin-coated films of a main-chain smectic liquid crystalline polymer with an azobenzene group connected by flexible spacers to a phenyl malonate unit has been studied as a function of film thickness using scanning force microscopy and UV/Vis spectroscopy. Already the as-prepared films are characterized by a significant out-of-plane orientation of the azobenzene groups. Annealing of films thicker than 7 nm results in aggregation and homeotropical alignment of azobenzene groups forming smectic layers parallel to the substrate. An optically isotropic mesophase was found upon annealing in the temperature range of smectic mesophase. Films thinner than the bilayer thickness of 7 nm exhibit a completely different behavior, whereby spin-coating results in a spontaneously homeotropic alignment of the azobenzene units. A subsequent annealing generates a structure of holes and rims. Within the holes a polymer monolayer remains, planarly aligned to substrate.

Adsorption and electrically stimulated desorption of the triblock copolymer poly(propylene sulfide– bl-ethylene glycol) (PPS–PEG) from indium tin oxide (ITO) surfaces

Protein-resistant triblock copolymers, poly(propylene sulfide– bl-ethylene glycol) (PPS–PEG) have been previously demonstrated to chemisorb onto gold surfaces forming monolayers that resist non-specific protein adsorption and are stable against oxidation. In this paper, we report on the adsorption of PPS–PEG onto a transparent and electrically conductive substrate, indium tin oxide (ITO). In addition, we demonstrate the controlled desorption of PPS–PEG by applying an electrical stimulus. We have used three complementary surface characterization techniques: variable angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to analyze the adsorption and electro-desorption of PPS–PEG from an ITO surface. All three methods confirmed the formation of PPS–PEG adlayers on the ITO surfaces. Based on our experimental XPS and ToF-SIMS results as well as former publications, we postulate that the chemisorption of the PPS–PEG on ITO involves direct sulfide–indium (or tin) interactions. When an ascending anodic electrical stimulus was applied to the surface of the modified samples, a gradual and steady polymer removal was observed, with complete loss of the polymeric monolayer at a potential of 2000 mV (referenced to Ag electrode). Anodic polarization did not result in oxidation of the thioether function of the PPS–PEG adlayers, indicating excellent oxidation resistance of PPS–PEG on ITO surfaces. This work is focused on exploiting electrical stimuli for the in situ surface modification under dynamic control.

Migration and transfer of excitation energy in homogeneously dispersed porphyrin monolayers prepared from amphiphilic copolymers

Photophysical processes of tetraphenylporphyrin (TPP) units incorporated into a polymer monolayer have been studied by fluorescence spectroscopy with emphasis on energy migration and aggregate formation of TPPs in the two-dimensional sheet. Although UV–VIS absorption spectra showed few of TPP aggregates, the fluorescence was largely quenched in the high-TPP samples due to efficient energy migration and following energy transfer to the aggregates. With the aid of computer simulation, time-resolving decay analysis revealed the large diffusion coefficient of excitation energy and a low concentration of aggregate, suggesting that the TPP moieties were placed in the plane not with a random distribution, but with an ordered arrangement, probably due to some mutual repulsive forces which prevented overlap of the TPP units at the air–water interface. These results suggest that the polymer monolayer containing TPP units can be utilized as a component of molecular assemblies for designing efficient energy harvesting systems.

UV Polymerization of Self-Assembled Monolayers of a Novel Diacetylene on Silver: A Spectroscopic Analysis by Surface Plasmon Resonance and Surface Enhanced Raman Scattering

UV polymerization of self-assembled monolayers of a novel carbazolyl-diacetylene (CDS9) chemisorbed on silver films was demonstrated by surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS) experiments. SPR tests performed during UV exposure permitted one to observe the growth of the absorption coefficient, associated with the formation of the polymeric backbone. The Raman spectra of polymerized monolayers exhibited the bands associated with the C=C stretching modes of the conjugated backbone, typical of the blue and red polymeric phases usually present in polydiacetylenes, with a clear predominance of the red form. Moreover, the strong surface enhancement of the Raman band corresponding to the aromatic C=C stretching modes suggested that carbazolyl groups arrange nearly perpendicularly to the metal surface. In contrast, the absence of a SERS signal in the region of conjugated C[triple bond]C bond stretchings confirmed a polymerization scheme with conjugated triple bonds nearly parallel to the plane of the metal.

Photopolymerization of Mixed Monolayers and Black Lipid Membranes Containing Gramicidin A and Diacetylenic Phospholipids

We formed monolayers and black lipid membranes (BLMs) of photopolymerizable lipids mixed with the channel-forming protein gramicidin A to evaluate their miscibility and the potential for improved stability of the BLM scaffold through polymerization. Analyses of surface pressure vs area isotherms indicated that gramicidin A dispersed with three different synthetic, polymerizable, diacetylene-containing phospholipids, 1,2-di-10,12-tricosadiynoyl-sn-glycero-3-phosphocholine (DTPC), 1,2-di-10,12-tricosadiynoyl-sn-glycero-3-phosphoethanolamine (DTPE), and 1-palmitoyl-2,10,12-tricosadiynoyl-sn-glycero-3-phosphoethanolamine (PTPE) to form mixed monolayers at the air-water interface on a Langmuir-Blodgett (LB) trough. Conductance measurements across a diacetylenic lipid-containing BLM confirmed dispersion of the gramicidin channel with the lipid layer and demonstrated gramicidin ion-channel activity before and after UV exposure. Polymerization kinetics of the diacetylenic films were monitored by film pressure changes at constant LB trough area and by UV-vis absorption spectroscopy of polymerized monolayers deposited onto quartz. An initial increase in film pressure of both the pure diacetylene lipid monolayers and mixed films upon exposure to UV light indicated a change in the film structure. Over the time scale of the pressure increase, an absorbance peak indicative of polymerization evolved, suggesting that the structural change in the lipid monolayer was due to polymerization. Film pressure and absorbance kinetics also revealed degradation of the polymerized chains at long exposure times, indicating an optimum time of UV irradiation for maximized polymerization in the lipid layer. Accordingly, exposure of polymerizable lipid-containing black lipid membranes to short increments of UV light led to an increase in the bilayer lifetime.

Fabrication of the periodic nanopillar arrays by heat-induced deformation of 2D polymer colloidal monolayer

A strategy based on heat-deformable monolayer polymer (polystyrene) colloidal template and precursor solution-dipping is presented to fabricate large-scale nanopillar arrays. In the method, the appropriate heating time of the polymer colloidal templates is a key factor. Fe 2O 3 and silica hexagonal periodic triangular prism nanopillar arrays with an aspect ratio of about 1.5:1 were successfully fabricated. This route is also applicable for synthesis of other material nanopillar arrays and opens a new way to create patterned 1D nanostructures for applications in sensor arrays, piezoelectric antenna arrays, optoelectronic devices, superhydrophobic and self-cleaning surfaces.

Shear and compression viscoelasticity in polymer monolayers

Poly-vinlyacetate (PVAc) forms very stable and reproducible monolayers on the surface ofwater, a model system to understand polymer physics on two dimensions. A recentlyintroduced technique is applied here to to study viscoelasticity of PVAc monolayers. Themethod is based on measurement of surface tension in two orthogonal directions duringanisotropic deformation. Compression and shear moduli are explored over a verylarge concentration range, highlighting a series of four different regimes. At lowconcentration the polymers are in a dilute gas. Above the overlap concentrationΓ* there is a fluid semi-dilute region, where the monolayer propertiesare described by scaling laws. At a threshold concentrationΓ**, a decrease in the gradient of pressure with concentration is observed, and we argue thatthere is still a large fraction of free area on the surface. Compressing further, we thenidentify close packing as the point where the pressure gradient rises sharply and a shearmodulus emerges. This is interpreted as a transition to a soft-solid due to thekinetic arrest of close-packed monomers. The rheological properties of PVAc aboveΓ** have not been studied previously. Discussion includes possible explanations for the observedbehaviour in terms of both equilibrium and non-equilibrium conditions, and therelation to microscopic chain properties. Temperature dependent effects aroundΓ** are also observed and described.

Variations of wettability and protein adsorption on solid siliceous carriers grafted with poly( N-isopropylacrylamide)

Poly( N-isopropylacrylamide), a thermally responsive polymer, was end-grafted to mercaptopropyl derivatives of silica gel, plane glass sheets and glass capillary tubing by free radical polymerization of the monomer in 1,4-dioxane at 100 °C. The polymer monolayer attached to the glass carriers provided them with thermally controlled wettability registered by two independent methods: direct measurements of the water contact angle and capillary rise. The water contact angle changed from 54 ± 3 ° to 68 ± 3 ° in the temperature range from 20 to 50 °C. The polymer grafting to silica gel (pore diameter 100 Å, particle size 5 μm) resulted in 15–30-fold reduction in protein adsorption on the carrier at 35 °C. Adsorption isotherms of myoglobin indicate completely different characters of the protein adsorption to silica gel and its polyNIPAM-grafted derivative. Cooling of the grafted carrier containing adsorbed myoglobin to 9 °C led to a partial release of the protein to the contacting solution, whereas heating of the system to 35 °C resulted in reversible binding of the protein. Adsorption of myoglobin on polyNIPAM-coated silica was ca. 2-fold higher at 35 than at 9 °C, most probably due to steric repulsion displayed by the swollen copolymer at the lower temperature.

Nitrilotriacetic Acid Functionalized Graft Copolymers: A Polymeric Interface for Selective and Reversible Binding of Histidine‐Tagged Proteins

AbstractA series of novel graft copolymers, poly(L‐lysine)‐graft‐poly(ethylene glycol) (PLL‐g‐PEG) with 28, 46, and 96 % of the PEG chains carrying a terminal nitrilotriacetic acid (NTA) group, have been synthesized. Through electrostatic interactions, these polycationic graft copolymers assemble spontaneously from aqueous solution onto negatively charged surfaces, forming polymeric monolayers that present NTA groups at controlled surface densities. Such NTA‐presenting surfaces on a highly PEGylated background are attractive sensor platforms for the reversible binding, through Ni2+ ions, of histidine (His)‐tagged biomolecules such as proteins and antibodies. With these three polymers, NTA‐ligand surface densities of 16, 23, and 37 pmol cm–2, respectively, have been obtained. The surface assembly of the polymers, as well as their sensing performance, has been monitored quantitatively using in‐situ optical‐waveguide light‐mode spectroscopy. The NTA‐functionalized PLL‐g‐PEG surfaces prove to be highly resistant to non‐specific adsorption in contact with human serum, while allowing the specific and reversible surface binding of the 6 × His‐tagged green fluorescent protein (GFP)uv‐6His in its native conformation. The amount of GFPuv‐6His immobilized on the polymeric surface increases with increasing NTA surface density. Furthermore, micropatterns consisting of NTA‐functionalized PLL‐g‐PEG in a background of PLL‐g‐PEG are produced using the “molecular assembly patterning by lift‐off” technique. Exposure to Ni2+ and GFPuv‐6His results in a protein pattern of excellent contrast, as judged by fluorescence microscopy. The NTA‐functionalized PLL‐g‐PEG surface is considered to be a promising sensor platform for binding 6 × His‐tagged proteins, thanks to the simplicity and cost‐effectiveness of the surface modification protocol, the high specificity and nearly quantitative reversibility of the protein binding, and the potential for fabricating microarrays of multiple capture molecules.

Cell adhesion on a polymerized peptide–amphiphile monolayer

We report the synthesis and characterization of a stable polymerized monolayer of peptide–amphihiles on a planar solid support that promotes mouse fibroblast cell adhesion and spreading. Peptide–amphiphiles consisting of a polymerizable fatty acid attached to a short RGD containing peptide sequence are self-assembled and polymerized at the water–air interface by means of the Langmuir– Blodgett technique. The surface concentration of the peptide–amphiphile is varied by co-spreading the peptide–amphiphile with an analogous non-modified polymerizable amphiphile at the water/air interface, prior to UV light-induced polymerization. The polymerized monolayer is transferred onto a hydrophobized smooth mica surface and the resulting surfaces have been investigated with respect to directing the cell adhesion and spreading of mouse fibroblast cells in a serum-free medium. Fibroblast cells adhere and spread on surfaces exposing the bioactive ligand but do not spread on reference surfaces without peptide. We find a maximum number of adherent cells at rather high peptide surface concentrations of about 10 mol% in the mixed monolayer, equivalent to more than 50 pmol/cm 2 peptide on the surface of the film. We attribute this finding to a limited accessibility of the ligands by the integrins. Because of the stability of the polymerized peptide–amphiphile monolayer, these surfaces can be re-seeded multiple times with cells, i.e. adherent cells can be removed from the surface, the surface can be sterilized and cells can be re-attached.

Biodegradable poly( dl-lactide)/poly(ε-caprolactone) mixtures: Miscibility at the air/water interface

Mixtures of biodegradable polymers, poly( dl-lactide) and poly(ε-caprolactone) monolayers at the air/water interface have been studied. Surface pressure–area isotherms of poly( dl-lactide), poly(ε-caprolactone) and their mixtures were obtained by monolayer compression at constant temperature. The behavior of the mixed monolayers was analyzed according to the classical addition rule. Good agreement was observed between experimental and ideal behavior except for one composition where a negative deviation was observed. The polymer monolayer miscibility was corroborated by comparison between the surface pressure–area isotherms of the random copolymers ( dl-lactide-co-ε-caprolactone) and their mixtures at the same compositions. Brewster angle microscopy (BAM) shows homogeneity in the monolayers in the whole range of compositions. These results also confirm the miscibility of the mixtures.

Equilibrium Behavior and Dilational Rheology of Polyelectrolyte/Insoluble Surfactant Adsorption Films: Didodecyldimethylammonium Bromide and Sodium Poly(styrenesulfonate)

The surface pressure of monolayers of an insoluble surfactant, didodecyldimethylammonium bromide (DODAB), has been measured onto subphases with different concentrations of poly(styrenesulfonate) (PSS) and at different temperatures. The presence of PSS in the subphase shifts the surface-pressure (Pi) curves to larger areas per DODAB molecule, A, and shifts the surface phase transition to higher Pi's. The presence of PSS chains decreases the surface electric potential; the decrease is higher than expected from the formation of a double layer between the DODAB molecules and the PSS segments. Increasing the temperature shifts the surface-pressure curves to higher areas and also increases the values of Pi of the surface phase transition. The effect of the PSS chains on the Pi versus A curves is contrary to the one induced by the presence of inert electrolytes in the subphase. The behavior is consistent with the existence of a dense layer of PSS segments beneath the DODAB monolayer at low PSS concentrations, c. Two PSS layers exist at higher concentrations, a dense layer immediately below the DODAB and a less-dense layer, below the first one, that protrudes deep into the subphase. The surface-pressure relaxation curves have been found to be bimodal through the whole range of surface pressures and at all the values of polymer concentration studied. These results point out that the adsorption layers behave mainly as elastic bodies, with zero-frequency elasticity, epsilon(omega = 0), which agrees with the equilibrium compressibility modulus. The increase [epsilon(omega = 1) - epsilon(omega = 0)] has been found to be independent of both polymer concentration and molecular weight. The zero-frequency-dilational viscosity, kappa(omega = 0), strongly increases with Pi in the two-dimensional condensed-liquid region. The surface viscosity strongly decreases with increasing frequency; the decreasing rate is higher than the one found for the monolayers of nonionic insoluble polymers. kappa(omega = 0) has also been found to be independent of both polymer concentration and molecular weight. These results seem to indicate that it is the film formed by the DODAB molecules and the first dense polymer layer that determines the surface viscoelastic moduli of this system.