Controlling Ion Transport through Multilayer Polyelectrolyte Membranes by Derivatization with Photolabile Functional Groups

NIR-Light-Activated Ratiometric Fluorescent Hybrid Micelles for High Spatiotemporally Controlled Biological Imaging and Chemotherapy

Intelligent-responsive imaging-therapy strategy has shown great significance for biomedicine. However, it is still a challenge to construct spatiotemporally controlled imaging-therapy systems triggered by near infrared (NIR) light. In this work, NIR-light-activated ratiometric fluorescent hybrid micelles (RFHM) are prepared via the co-assembly of upconversion nanoparticles (UCNPs), doxorubicin (DOX), and UV-light-responsive amphiphilic block copolymer for the spatiotemporally controlled imaging and chemotherapy. Upon NIR light irradiation, UCNPs can convert NIR light to UV light. The emitted UV light induces the photoreaction of copolymer to further trigger ratiometric fluorescence imaging and degradation of hybrid micelles, resulting in rapid DOX release from hybrid micelles for antitumor therapy. The animal experiments reveal that NIR light can not only remotely regulate the ratiometric fluorescence imaging of RFHM in tumor tissue, but also trigger DOX release from RFHM to inhibit tumor growth. Therefore, this study provides a new strategy to achieve high spatial-temporal-controlled biological imaging and chemotherapy.

Dual Monitoring of Surface Reactions in Real Time by Combined Surface-Plasmon Resonance and Field-Effect Transistor Interrogation

By combining surface plasmon resonance (SPR) and electrolyte gated field-effect transistor (EG-FET) methods in a single analytical device we introduce a novel tool for surface investigations, enabling simultaneous measurements of the surface mass and charge density changes in real time. This is realized using a gold sensor surface that simultaneously serves as a gate electrode of the EG-FET and as the SPR active interface. This novel platform has the potential to provide new insights into (bio)adsorption processes on planar solid surfaces by directly relating complementary measurement principles based on (i) detuning of SPR as a result of the modification of the interfacial refractive index profile by surface adsorption processes and (ii) change of output current as a result of the emanating effective gate voltage modulations. Furthermore, combination of the two complementary sensing concepts allows for the comparison and respective validation of both analytical techniques. A theoretical model is derived describing the mass uptake and evolution of surface charge density during polyelectrolyte multilayer formation. We demonstrate the potential of this combined platform through the observation of layer-by-layer assembly of PDADMAC and PSS. These simultaneous label-free and real-time measurements allow new insights into complex processes at the solid-liquid interface (like non-Fickian ion diffusion), which are beyond the scope of each individual tool.

Continuous assembly of supramolecular polyamine-phosphate networks on surfaces: preparation and permeability properties of nanofilms

Supramolecular self-assembly of molecular building blocks represents a powerful "nanoarchitectonic" tool to create new functional materials with molecular-level feature control. Here, we propose a simple method to create tunable phosphate/polyamine-based films on surfaces by successive assembly of poly(allylamine hydrochloride) (PAH)/phosphate anions (Pi) supramolecular networks. The growth of the films showed a great linearity and regularity with the number of steps. The coating thickness can be easily modulated by the bulk concentration of PAH and the deposition cycles. The PAH/Pi networks showed chemical stability between pH 4 and 10. The transport properties of the surface assemblies formed from different deposition cycles were evaluated electrochemically by using different redox probes in aqueous solution. The results revealed that either highly permeable films or efficient anion transport selectivity can be created by simply varying the concentration of PAH. This experimental evidence indicates that this new strategy of supramolecular self-assembly can be useful for the rational construction of single polyelectrolyte nanoarchitectures with multiple functionalities.

Intrinsic Properties of Polyelectrolyte Multilayer Membranes: Erasing the Memory of the Interface

Polyelectrolyte multilayers (PEMUs) are ultrathin membranes made by alternating adsorption of oppositely charged polyelectrolytes on substrates. Although PEMUs have shown exceptional selectivity for certain ion-filtering applications, they usually contain an excess of one of the polyelectrolytes due to the history- and condition-dependent mode of PEMU assembly. This excess charge provides fixed sites for ion exchange, enhancing the concentration of oppositely charged ions. Thus, the ion-permselective properties of PEMUs cannot be compared unless they are assembled under identical conditions. This work demonstrates the enhanced permeability of PEMUs as-made from poly(diallyldimethylammonium) (PDADMA), and poly(styrene sulfonate) (PSS) to ferricyanide as an example of an anion. Annealing by NaCl followed by pairing of excess PDADMA with additional PSS produces an almost stoichiometric film that better reflects the intrinsic transport properties of PEMUs. This pairing, observed in real time using electrochemical methods, occurs at the PEMU/solution interface under countercurrent transport of PSS from solution and excess PDADMA paired with a counterion, termed PDADMA*, from the PEMU bulk. A quantitative comparison of PSS and PDADMA* diffusion reveals the conditions under which PEMU assembly depends on PSS molecular weight and concentration.

Stimuli-Responsive Free-Standing Layer-By-Layer Films

Free-standing, stimuli-responsive polyelectrolyte multilayer films enabled by light-induced degradation of sacrificial compartments are introduced. Two examples are described: i) a triple responsive film that uses light, redox, and pH for different functions, and ii) different wavelengths of light for different functions. This approach to multiresponsive materials offers simple design and chemical synthesis while enabling different stimuli to perform separate functions in the same material.

Toward ion-free polyelectrolyte multilayers: cyclic salt annealing

Polyelectrolyte multilayers (PEMUs) are made from various combinations of polyanions and polycations. It is now understood that these ultrathin films of polyelectrolyte complex may also incorporate counterions derived from the solutions from which the PEMU was deposited or exchanged into the film postassembly. If these ions are required to compensate nonstoichiometric ratios of polycation and polyanion they cannot leave the film and exert considerable influence on film properties, such as modulus and permeability. These "extrinsic" charges also complicate fundamental studies on PEMUs. We report a method to remove almost all ionic content from a PEMU made of poly(diallyldimethylammonium chloride), PDADMAC, and poly(styrenesulfonate), PSS. In this method, a high salt concentration plasticizes the multilayer past its glass transition, dispersing all the buried excess PDADMA throughout the film. Exposure to a solution of PSS in a lower salt concentration consumes excess PDADMA near the surface without overcompensating with PSS. The process is repeated in a cyclic fashion, removing >95% of the ions charge present in the as-made PEMU.

pH-responsive ion transport in polyelectrolyte multilayers of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(4-styrenesulfonic acid-co-maleic acid) (PSS-MA) bearing strong- and weak anionic groups

We describe the creation of interfacial architectures displaying pH-dependent ionic transport properties which until now have not been observed in polyelectrolyte multilayers.

UV-photo graft functionalization of polyethersulfone membrane with strong polyelectrolyte hydrogel and its application for nanofiltration

A strong polyelectrolyte hydrogel was graft copolymerized on a polyethersulfone (PES) ultrafiltration (UF) membrane using vinyl sulfonic acid (VSA) as the functional monomer, and N,N'-methylenbisacrylamide (MBAA) as the cross-linker monomer. This was carried out in one simple step using the UV photoirradiation method. The effect of the polymerization conditions on the degree of grafting (DG) was investigated using the gravimetric method which measures the total hydrogel grafted on the membrane, and with ATR-FTIR spectroscopy which indicates the functional monomer fraction in the hydrogel layer. The VSA could not graft polymerize without the cross-linker as comonomer. An increase in the cross-linker fraction from 0.25 to 2.5 mol % (relative to the functional monomer VSA) resulted in a higher DG. Although the surface morphology changed upon modification, the resulting surface roughness as measured by AFM was very low. From the monitoring of DG with UV time (4.5-30 min) at constant conditions, it was deduced that during the early stages of the polymerization mainly the cross-linker was grafted, thus inducing the graft copolymerization of the functional monomer. Polymerization using a higher monomer concentration (12.5-40% VSA) at constant monomer/cross-linker ratio resulted in a higher VSA fraction in the grafted hydrogel, although the gravimetric DG was similar. Ion exchange capacity and X-ray photoelectron spectroscopy measured after modification under the different conditions supported these findings. The new membranes were tested under nanofiltration (NF) conditions. A NF membrane could be obtained when the MBAA fraction was above 0.25%. The Na2SO4 rejection was 90-99% and the permeability 10-1 L m(-2) h(-1) bar(-1) when the MBAA fraction increased from 0.75 to 2.5%. The order of rejection of single salts solution was Na2SO4 > MgSO4 ≈ NaCl > CaCl2, as expected on the basis of Donnan exclusion for negatively charged NF membranes. An increase in the salts rejection with increasing degree of cross-linking and VSA fraction was attributed to an increase in the membrane charge density and to steric exclusion that also resulted in an increase of rejection for uncharged solutes such as sucrose or glucose. The new membrane presented a high, essentially unchanged Na2SO4 rejection (>97%) in the range of salt concentrations up to 4 g/L, and only slightly reduced rejection (>92%) at a concentration of 8 g/L; this can be related to its high barrier layer charge density measured by ion exchange capacity. In addition, because poly(vinyl sulfonic acid) (PVSA) is a strong polyelectrolyte the membrane separation performance was stable in the range of pH 1.5 to pH 10.

Dynamic aspects of films prepared by a sequential deposition of species: perspectives for smart and responsive materials

The deposition of surface coatings using a step-by-step approach from mutually interacting species allows the fabrication of so called "multilayered films". These coatings are very versatile and easy to produce in environmentally friendly conditions, mostly from aqueous solution. They find more and more applications in many hot topic areas, such as in biomaterials and nanoelectronics but also in stimuli-responsive films. We aim to review the most recent developments in such stimuli-responsive coatings based on layer-by-layer (LBL) depositions in relationship to the properties of these coatings. The most investigated stimuli are based on changes in ionic strength, temperature, exposure to light, and mechanical forces. The possibility to induce a transition from linear to exponential growth in thickness and to change the charge compensation from "intrinsic" to "extrinsic" by controlling parameters such as temperature, pH, and ionic strength are the ways to confer their responsiveness to the films. Chemical post-modifications also allow to significantly modify the film properties.

Dynamics of water in polyelectrolyte multilayers: restricted diffusion and cross-relaxation

The diffusion properties of water in polyelectrolyte multilayers (PEM) are investigated by pulsed field gradient diffusion NMR. The dependence of the mean-square displacement on the observation time does not agree to Gaussian diffusion, suggesting restricted diffusion in a porous structure. However, the extraction of a pore size in a model of restricted diffusion yields a very large pore size of several micrometers. The additional influence of cross-relaxation of water and polymer spins is investigated in Goldman-Shen experiments. These demonstrate a strong influence of cross-relaxation rates on diffusion echo decays, such that pore sizes obtained from the model of restricted diffusion have to be corrected. Corrected pore sizes are about 4 microm and reflect the existence of domains of lower polymer density and thus faster water diffusion. These heterogeneities occur upon PEM preparation at high salt content for large layer numbers and are detected in the surface morphology, too.

Influence of an electric field on oriented films of DMPC/gramicidin bilayers: a circular dichroism study

A film of oriented bilayers containing a mixture of gramicidin and dimyristoylphosphatidylcholine (DMPC) has been deposited on a fused-silica window coated with a 10 nm thick gold layer. The thin layer of gold allows the application of an electric potential across the film and the study of its influence on the structure and integrity of the bilayers. Electrochemical measurements, ellipsometry, and far-UV circular dichroism (CD) were employed to characterize the properties of the film of bilayers as a function of the potential applied to the gold electrode. For potentials across the film that are within the range approximately +300 to -150 mV the oriented film of bilayers is stable, and no change in the CD spectra of gramicidin molecule is observed. At more negative potentials, an increase in the film thickness and water content measured by ellipsometry indicated that the film swells and incorporates water, which causes a change in the circular dichroism spectrum of gramicidin molecules in the film. This transformation was interpreted as a change in the average orientation of gramicidin molecules within the film due to a decrease in the ordering of the molecules upon swelling.

Modulating the release kinetics through the control of the permeability of the layer-by-layer assembly: a review

The layer-by-layer (LbL) self-assembly technique has emerged as a simple and versatile method for coating biological and non-biological templates for various biomedical applications. A promising avenue of this technique lies in the encapsulation of drugs and other biological substances for controlled release. Fundamental studies of LbL assembly on flat surfaces have provided a sound understanding of film deposition theory and its pertinence to ionic and molecular transport and diffusion through polyelectrolyte multilayer (PEM) films. However, there is a lack of information on the permeability of three-dimensional PEM shell systems. In either PEM films or shells, it has been shown that drug release is a function of the ionic strength, pH and/or multilayer thickness. This report aims to provide an overview of the physicochemical parameters affecting the permeability of two- and three-dimensional multilayer shells, including ionic strength, layer number and pH. Furthermore, their synergic effect on loading and release of biologically active molecules from LbL multilayers are discussed.

Reversible disulfide cross-linking in layer-by-layer films: preassembly enhanced loading and pH/reductant dually controllable release

This paper describes the fabrication of polyelectrolyte multilayer film which combines preassembly of poly(allylamine hydrochloride) (PAH) and 5,10,15,20-tetraphenyl-21H,23H-porphine-p,p',p' ',p' ''-tetrasulfonic acid tetrasodium hydrate (TPPS) in aqueous solution with the layer-by-layer (LbL) assembly of the PAH-TPPS complex and cross-linkable polyelectrolyte, PAASH60, which is a poly(acrylic acid) with 60% of its carboxylic acid grafted of thiol groups. During preassembly, TPPS was incorporated into PAH chains. After oxidative cross-linking to form disulfide bonds in between the layers, the multilayer with preassembly of the PAH-TPPS complex allowed for release and loading of TPPS in a reproducible way. The release of TPPS from the loaded film was a pH-controlled process. To compare with the conventional multilayer, the reloading capacity was greatly enhanced, which was related to the charge binding sites that formed by release of TPPS from the multilayer. Moreover, the release of TPPS could also be achieved by breaking off the cross-linking through reduction of disulfide bonds, and the release rates could be controlled by the reductive efficiency of the reductants in the media. In this way, the release of TPPS is pH/reductant dually controllable, thereby facilitating a new route to multistimuli controllable materials.

Layer-by-Layer Enzyme/Polyelectrolyte Films as a Functional Protective Barrier in Oxidizing Media

The influence of a catalase (Cat) layer located at different depths in the layer-by-layer hemoglobin/polystyrene sulfonate films with an (Hb/PSS)(20)(-)(x)/(Cat/PSS)/(Hb/PSS)(x) (x = 0-20) architecture on kinetics of hemoglobin degradation under treatment with hydrogen peroxide solutions of different concentrations and features of H(2)O(2) decay in surrounding solutions has been studied. While assembled on the top of the multilayers, the catalase layer shows the highest activity in hydrogen peroxide decomposition. Hemoglobin in such films retains its nativity for a longer period of time. The effect of catalase layers is compared with that of protamine, horseradish peroxidase, and inactivated catalase. Positioning an active layer with catalytic properties as an outer layer is the best protection strategy for layer-by-layer assembled films in aggressive media.

Water distribution in multilayers of weak polyelectrolytes

The water localization in thin polyelectrolyte multilayers assembled from poly(acrylic acid) and poly(allylamine hydrochloride) was investigated with neutron reflectivity in an atmosphere of controlled humidity and with bulk water. Water was found to be distributed asymmetrically within the multilayer and to localize preferentially at the polymer surface. The diffusion of water into the multilayer did not completely penetrate to the substrate, but instead there appeared to be an exclusion zone near the Si substrate. These results help to explain previous observations of anomalous water transport kinetics in weak polyelectrolyte systems.

Polyelectrolyte multilayer capsules as vehicles with tunable permeability

This review is devoted to a novel type of polymer micro- and nanocapsules. The shell of the capsule is fabricated by alternate adsorption of oppositely charged polyelectrolytes (PEs) onto the surface of colloidal particles. Cores of different nature (organic or inorganic) with size varied from 0.1 to 10 mum can be used for templating such PE capsules. The shell thickness can be tuned in nanometer range by assembling of defined number of PE layers. The permeability of capsules depends on the pH, ionic strength, solvent, polymer composition, and shell thickness; it can be controlled and varied over wide range of substances regarding their molecular weight and charge. Including functional polymers into capsule wall, such as weak PEs or thermosensitive polymers, makes the capsule permeability sensitive to correspondent external stimuli. Permeability of the capsules is of essential interest in diverse areas related to exploitation of systems with controlled and sustained release properties. The envisaged applications of such capsules/vesicles cover biotechnology, medicine, catalysis, food industry, etc.

Pore assembled multilayers of charged polypeptides in microporous membranes for ion separation

In this study, highly permeable ion-selective membranes are prepared via immobilization of polyelectrolyte multilayer networks within the inner pore structure of a microporous (pore size = 0.2 microm) support. Electrostatic layer-by-layer assembly is achieved through alternate adsorption of cationic and anionic polyelectrolytes under convective flow conditions. To initiate pore assembly, the first layer consists of covalently bound charged polypeptides (poly(L-glutamic acid) (PLGA) or poly(L-lysine) (PLL)) establishing a charged support for subsequent adsorption. Nonstoichiometric immobilization of charged multilayers within a confined pore geometry leads to an enhanced volume density of ionizable groups in the membrane phase. This overall increase in the effective charge density allows for Donnan exclusion of ionic species (especially divalent co-ions) using microporous materials characterized by permeability values that exceed conventional membrane processes. Multilayer assemblies are fabricated using both PLGA/PLL and synthetic polyelectrolytes (poly(styrenesulfonate)/poly(allylamine)) in an attempt to compare the level of adsorption and separation properties of the resulting materials. The role of salt concentration in the carrier solvent on overall polyelectrolyte adsorption was examined to determine its effect on both solute (Cl-, SO4(2-), As(V)) and water transport. Constriction of the pore size induced by multilayer propagation was monitored through permeability measurements and dextran rejection studies at each stage of the deposition process.

Effect of layer integrity of spin self-assembled multilayer films on surface wettability

We investigated the correlation between surface wettability and internal structure of polyelectrolyte (PE)/PE and PE/inorganic multilayer films prepared by the spin self-assembly (SA) method. Spin self-assembled poly(allylamine hydrochloride) (PAH)/poly(sodium 4-styrenesulfonate) (PSS) multilayer films deposited from PE solutions of 10 mM show the distinct oscillation in contact angles with variation of the outermost PE layer, representing the saturated values in contact angles of individual PAH and PSS layers. These contact angles are also well consistent with the angles measured from respective PE layers (i.e., PAH and PSS) of the spin SA (PAH/CdS-COO-) and (CdS-NH3+/PSS) films carrying the flat interface between PE and inorganic CdS nanoparticle layers as confirmed by X-ray reflectivity. Furthermore, based on the contact angle of CdS-NH3+ layer in the ordered (CdS-NH3+/PSS) films, the change in surface wettability of CdS-NH3+ layers of two different spin SA (CdS-NH3+/poly(methacrylic acid) (PMAA)) multilayer films with ordered and disordered internal structure is also investigated. The films with ordered and disordered internal structure were fabricated by the pH adjustment of PMAA. The CdS-NH3+ layer in both CdS-NH3+/PSS and CdS-NH3+/PMAA multilayer films with the ordered internal structure has the contact angle of about 25 +/- 2 degrees irrespective of the PSS or PMAA sublayer. As a result, the same surface wettability of PE or inorganic layers, despite different sublayers, strongly indicates that the spin SA method in optimum condition allows the top surface to be completely covered with a low level ofinterdigitation with a sublayer at each deposition step, and this leads to the conclusion that physical and chemical characteristics of the sublayers have no significant influence on those of the outermost layer.

Optically active polyelectrolyte multilayers as membranes for chiral separations

Ultrathin films of chiral polyelectrolyte complex, prepared by the multilayering process, exhibit selectivity in the membrane separations of optically active compounds, such as l- and d-ascorbic acid. The flux through these polyelectrolyte multilayers, PEMUs, is exceptionally high and may be controlled by the concentration of salt present in the permeating solutions. Both in-situ ATR-FTIR and chiral capillary electrochromatography indicate that flux selectivity is mainly kinetically controlled, stemming from a difference in diffusion rates of various enantiomers through PEMUs, rather than a difference in partitioning.