Origin of the Differential Nanoscale Reactivity of Biologically and Chemically Formed Green Rust Crystals Investigated by Chemical Force Spectroscopy

Iron-containing nanoparticles, such as green rusts, can be formed by either chemical (c-GR) or biological (b-GR) pathways. It is known that c-GRs display very high reactivity toward organic and inorganic contaminants and thus have great potential for the remediation of contaminated environments, whereas b-GRs are very weakly reactive. This reactivity difference is usually attributed to much higher surface/volume ratio of c-GR compared to b-GR. Using atomic and chemical force microscopy to probe the reactivity at the nanoscale of both types of nanoparticles, we are able to show that the primary reason for the low reactivity of b-GR is not the low surface/volume ratio but the passivation of the surface due to the presence of biological exopolymers (EPS). This conclusion should hold true for many biological nanoparticles and allows us to explain their often observed low, yet unexplained, reactivity

New 2-in-1 Polyelectrolyte Step-by-Step Film Buildup without Solution Alternation: From PEDOT-PSS to Polyelectrolyte Complexes

Although never emphasized and increasingly used in organic electronics, PEDOT-PSS (poly­(3,4-ethylenedioxythiophene)-poly­(styrene sulfonate)) layer-by-layer (lbl) film construction violates the alternation of polyanion and polycation rule stated as a prerequisit for a step-by-step film buildup. To demonstrate that this alternation is not always necessary, we studied the step-by-step construction of films using a single solution containing polycation/polyanion complexes. We investigated four different systems: PEDOT-PSS, bPEI-PSS (branched poly­(ethylene imine)-poly­(sodium 4-styrene sulfonate)), PDADMA-PSS (poly­(diallyl dimethyl ammonium)-PSS), and PAH-PSS (poly­(allylamine hydrochloride)-PSS). The film buildup obtained by spin-coating or dipping-and-drying process was monitored by ellipsometry, UV–vis-NIR spectrophotometry, and quartz-crystal microbalance. The surface morphology of the films was characterized by atomic force microscopy in tapping mode. After an initial transient regime, the different films have a linear buildup with the number of deposition steps. It appears that, when the particles composed of polyanion-polycation complex and complex aggregates in solution are more or less liquid (case of PEDOT-PSS and bPEI-PSS), our method leads to smooth films (roughness on the order of 1–2 nm). On the other hand, when these complexes are more or less solid particles (case of PDADMA-PSS and PAH-PSS), the resulting films are much rougher (typically 10 nm). Polycation/polyanion molar ratios in monomer unit of the liquid, rinsing, and drying steps are key parameters governing the film buildup process with an optimal polycation/polyanion molar ratio leading to the fastest film growth. This new and general lbl method, designated as <i>2-in-1 method</i>, allows obtaining regular and controlled film buildup with a single liquid containing polyelectrolyte complexes and opens a new route for surface functionalization with polyelectrolytes

Morphogen Electrochemically Triggered Self-Construction of Polymeric Films Based on Mussel-Inspired Chemistry

Inspired by the strong chemical adhesion mechanism of mussels, we designed a catechol-based electrochemically triggered self-assembly of films based on ethylene glycol molecules bearing catechol groups on both sides and denoted as bis-catechol molecules. These molecules play the role of morphogens and, in contrast to previously investigated systems, they are also one of the constituents, after reaction, of the film. Unable to interact together, commercially available poly­(allylamine hydrochloride) (PAH) chains and bis-catechol molecules are mixed in an aqueous solution and brought in contact with an electrode. By application of defined potential cycles, bis-catechol molecules undergo oxidation leading to molecules bearing “reactive” quinone groups which diffuse toward the solution. In this active state, the quinones react with amino groups of PAH through Michael addition and Schiff’s base condensation reaction. The application of cyclic voltammetry (CV) between 0 and 500 mV (vs Ag/AgCl, scan rate of 50 mV/s) of a PAH/bis-catechol solution results in a fast self-construction of a film that reaches a thickness of 40 nm after 60 min. The films present a spiky structure which is attributed to the use of bis-functionalized molecules as one component of the films. XPS measurements show the presence of both PAH and bis-catechol cross-linked together in a covalent way. We show that the amine/catechol ratio is an important parameter which governs the film buildup. For a given amine/catechol ratio, it does exist an optimum CV scan rate leading to a maximum of the film thickness as a function of the scan rate

Stable Bioactive Enzyme-Containing Multilayer Films Based on Covalent Cross-Linking from Mussel-Inspired Adhesives

The use of immobilized enzymes is mandatory for the easy separation of the enzyme, the unreacted substrates, and the obtained products to allow repeated enzymatic assays without cumbersome purification steps. The immobilization procedure is however critical to obtain a high fraction of active enzyme. In this article, we present an enzyme immobilization strategy based on a catechol functionalized alginate. We demonstrate that alkaline phosphatase (ALP) remains active in multilayered films made with alginate modified with catechol moieties (AlgCat) for long duration, that is, up to 7 weeks, provided the multilayered architecture is cross-linked with sodium periodate. This cross-linking reaction allows to create covalent bonds between the amino groups of ALP and the quinone group carried by the modified alginate. In the absence of cross-linking, the enzymatic activity is rapidly lost and this reduction is mainly due to enzyme desorption. We also show that NaIO₄ cross-linked (AlgCat-Alp)_<i>n</i> films can be freeze-dried and reused at least 3 weeks later without lost in enzymatic activity

Mobility of Proteins in Highly Hydrated Polyelectrolyte Multilayer Films

The lateral diffusion of a protein (human serum albumin labeled with fluorescein isothiocyanate) within a highly hydrated polyelectrolyte film is studied. The film is built up with poly­(l-lysine) as polycation and hyaluronate as polyanion. Fluorescence recovery after photobleaching is used to evaluate the mobility of the labeled protein. Spatial Fourier transformation is applied to the fluorescence intensity recorded at various times after bleaching of a narrow rectangular area within an image representative of the film. This approach necessitates no hypothesis on the intensity distribution at the end of the bleaching provided that the bleach has not appreciably changed the concentration ratios of the different diffusing species. Furthermore, under the hypothesis that molecules move according to Fick’s law, we represent the Fourier transform by a weighted sum of exponentials each containing another diffusion coefficient and evaluate the proportion attached to each term of this sequence using the simulated annealing method. A criterion, combining goodness-of-fit and the entropy characterizing the diffusion coefficient spectrum, is proposed to avoid overinterpretation of the experimental data. The optimum spectrum of the diffusion coefficient is then extracted from the time evolution of the light intensity at various albumin concentrations within the films. It appears that the mobility, quantified by the amount of tracer molecules having a diffusion coefficient smaller than, e.g., 0.1 μm²/s, undergoes a transition between 20 and 2000 μg/mL of internal concentration. This suggests that the mutual interactions of the albumin molecules and the interactions between fluorescently labeled albumin and the film network become increasingly important in the reduction of the albumin mobility as the albumin concentration increases

Layer-by-Layer Enzymatic Platform for Stretched-Induced Reactive Release

An original “all-in-one” platform combining polymers, enzymes, and enzymatic substrates in a unique film is designed. A polymeric barrier stratum prevents any contact between enzymes adsorbed on top of the film and substrates loaded in an underlying reservoir. Upon stretching of the film, a continuous diffusion of substrates through the barrier is triggered, followed by a catalytic reaction. This leads to the formation of products that are released from the film. This new platform acts as a stretch-induced reactive release system and emerges as an innovative concept in mechano-responsive materials

Electrotriggered Confined Self-assembly of Metal–Polyphenol Nanocoatings Using a Morphogenic Approach

Supramolecular metal-phenolic thin films attract an increasing interest since they allow the design of new types of self-assembling materials, such as tunable electronics or biomaterials. In this study, a new electrotriggered self-assembly of tannic acid-Fe­(III) (TA-Fe­(III)) nanocoatings was developed using the morphogenic approach with Fe­(III) ions as a morphogen. Morphogens are molecules or ions produced locally that diffuse into the solution and induce a chemical reaction or interaction in a confined space near a surface. Using a mixture of TA and Fe­(II) ions in contact with an electrode, a confined electrogenerated gradient of Fe­(III) was obtained by application of an anodic current to locally form TA-Fe­(III) coordination complexes. TA-Fe­(III) nanocoatings, based on di- and tri-coordinated complexes, were thus obtained. Both the film thickness and its self-assembly kinetics were tuned by controlling the Fe­(II)/TA molar ratio of the building solution, the intensity, and the duration of the applied current. We showed that this strategy can be applied to two other polyphenols (gallic acid and rosmarinic acid). This new electrotriggered confined self-assembly of metal–polyphenol gives new perspectives in applications such as antioxidant coating

Efficient Gas and Water Vapor Barrier Properties of Thin Poly(lactic acid) Packaging Films: Functionalization with Moisture Resistant Nafion and Clay Multilayers

Poly­(lactic acid) (PLA) represents one of the most promising and attractive biobased polymer for the industrial development of environmentally sustainable packaging. However, oxygen and water barrier properties of PLA based films cannot compete with those of commercially available composite multilayers. To fill this gap, we used the layer-by-layer deposition technique on commercially used PLA thin films (30 μm thick) in order to increase their barrier properties to oxygen and water vapor. Nanometric films were grown by alternating branched poly­(ethylene imine) (BPEI), hydrophobic fluorinated polymer (Nafion), and montmorillonite clay (MMT) layers with the aim of obtaining low gas permeability in both dry and moist conditions as well as low water vapor permeability. Two different kinds of architectures were designed and successfully prepared, based on a 4 layer repeating unit (BPEI/MMT/BPEI/Nafion), represented here as quadlayer (QL), and on a 6 layer repeating-unit ((BPEI/Nafion)₂/BPEI/MMT), hexalayer (HL). Reduction in oxygen and water permeabilities is observed for films based on both types of repeat units. The reduction of the permeabilities increases with the number of quad and hexalayers achieving reductions in terms of oxygen permeability in both dry and humid conditions up to 98% and 97% respectively for 10 HL and QL. Furthermore, a reduction of 78% of water vapor transmission rate through the functionalized film was obtained for these films. As far as oxygen permeability is concerned, HL films are more efficient than QL films for smaller numbers of deposition units. These properties are shown to result from the complementarity between the presence of BPEI/Nafion and MMT layers

Stretch-Induced Biodegradation of Polyelectrolyte Multilayer Films for Drug Release

The design of stimuli-responsive polymer assemblies for the controlled release of bioactive molecules has raised considerable interest these two last decades. Herein, we report the design of mechanically responsive drug-releasing films made of polyelectrolyte multilayers. A layer-by-layer (LbL) reservoir containing biodegradable polyelectrolytes is capped with a mechanosensitive LbL barrier and responds to stretching by a total enzymatic degradation of the film. This strategy is successfully applied for the release in solution of an anticancer drug initially loaded within the architecture

Morphological aspect of differentiated cell.

<p>Optical phase contrast microscopy visualization of differentiated cells seeded on type I collagen (A, B) and polyelectrolyte multilayer films (PEMs) (C, D) until confluence under normoxic (A, C) and hypoxic (B, D) environment. Objective×20, scale bar 55 µm. The morphological examination of the confluent cells showed cobblestone shape (A, C) in normoxia and a spindle like (B, D) shape in hypoxia.</p