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Mechanical properties of PEM films

The knowledge of the mechanical properties of polyelectrolyte multilayer films is of primary importance for their applications, especially with regard to bacterial adhesion prevention as cells prefer to adsorb on ‘‘hard’’ materials. Mechanical properties are studied both at planar films and at PEM capsules. Measurements on planar films are commonly carried out with measuring the indentation of an AFM tip or colloidal probe within the films upon application of a given force (normal to the PEM surface). Capsules elasticity is usually evaluated by osmotic pressure measurements and AFM. In the case of multilayers, both osmotic pressure studies and AFM measurements were carried out at capsules with resulting Young modulus in the range of 300-400 MPa [38, 48]. ) multilayers are characterized by a Young modulus up to 100 MPa, which is of the same order of magnitude as the value for the capsules.
Hence, crude comparison between different experiments shows that neither the method nor the geometry has an effect on the value of the Young modulus for a given material composition. The relevance of the internal structure in determining the mechanical properties then becomes obvious, especially with respect to the water content. Dried films show a modulus of one to two orders of magnitude higher than their wet homologues. As an example, elasticity of dried ) films is ca. 6 GPa [58] and for ) about 10 GPa. Another important parameter is the polymer charge density. For instance, if ) multilayers are prepared at intermediate pH values where both polyelectrolytes are charged, the chains are more or less stretched and the Young modulus is high (more than 10 GPa in the dry state). At lower charged densities (adjusted by pH) the chains are more coiled and can deform to accommodate the moving indenter AFM tip with greater ease. The Young modulus is then reduced to about 50% as compared to that of PEM with higher charge densities. The type of salt has also an effect on the mechanical properties of the multilayers even though opposite conclusion was drawn by Salomaki et al. from QCM studies. Multilayers with F ions were rather ‘‘rubber-like”, while films with Br are rather glassy. This difference is not clearly understood so far. Exponentially growing films are less structured and much more hydrated than linearly growing films. Therefore their Young moduli are much lower in general than for the linearly growing films. For instance, multilayers have only a young modulus in the order of several tens of kPa [34].

Ion permeability of polyelectrolyte multilayers

The LBL technique allows control of the net charge density of the film and also its ion permselectivity. Several studies showed that ion permeability of PEM is governed by many factors including deposition conditions, film composition and thickness, and amount of cross-linking in the film. Based on this, films can be highly permeable, selectively permeable, or nearly impermeable to ions. These factors that govern permeability in PEMs also offer a means to control the permeability of these films. This should allow tailoring of PEM for several applications. Studies in this area focus mainly on the movement of highly charged redox couples or redox-active species through PEM films.
Cyclic voltammetry experiments show that the first few layers of and film are highly permeable to redox-active species such as and [26, 43]. Results of cyclic voltammograms of at -coated electrodes and of at rotating disk electrodes coated with are consistent with the fact that the first few bilayers on substrates generally have a different structure than layers deposited after the charge distribution of the polymer film is fully established [12]. The first few layers may not fully cover the electrode or they may swell and thus be more permeable. After deposition of four or five bilayers, the blocking ability of PEM films increases slowly with adsorption of additional layers [26]. Experiments were also carried to test the effect of the presence of an electrolyte on the permeability of PEM films. Electrodes coated with films prepared in the absence of salt gave a quasi-reversible voltammogram with a peak current that was 80-fold larger than that obtained at electrodes coated with films prepared in the presence of salt (Figure 1.16) [43]. Those findings show that the permeability of PEM films can be simply controlled by varying the amount of electrolyte in the deposition solutions. The increased permeability in the absence of a supporting electrolyte is explained by the fact that the salt present during deposition is responsible for the screening of charges on the polyelectrolyte chains resulting in an intertwined structure with many loops and tails [61, 62] which is less permeable than structures in films prepared in the absence of salt.
It has been also shown that the concentration of supporting electrolyte present during electrochemical measurements affects transport of redox-active species through PEM films. Farhat and Schlenoff showed that at rotating-disk electrodes coated with films, limiting currents increased with the ionic strength [26]. This is explained by the extrinsically charge-compensated sites in films occurring due to high concentrations of and – ions. The transport of becomes easier by hopping between these sites. Increasing ionic strength might also affect transport by decreasing Donnan exclusion [63] and/or by swelling the film [64].

Incorporation of Nanoparticles and Biomolecules into PEM Films

Earlier attempts to incorporate molecules and particles into PEM films were based on the incorporation during the film buildup itself. We hereby mention the loading of particles and molecules by postdiffusion into already prepared multilayer films. Results showed that factors affecting the incorporation of nanoparticles and biomolecules in PEM films includes internal structure of the film and its ability to swell upon incorporation of charged or uncharged species, size and shape of the colloidal particles to be incorporated in the film, charge density and charge distribution within the film, and the influence of external parameters like the ionic strength, pH and temperature of the solution containing the nanoparticles and the concentration of the nanoparticles in the solution plays a role as well.

Incorporation of bioactive molecules and ions

Several studies showed that PEMs of weak polyelectyrolytes are viable systems for the controlled loading and release of small hydrophilic molecules. multilayer films were loaded by two water-soluble dyes; the cationic dye Indoine Blue and the anionic dye Chromotrope 2R [97]. The incorporation of the two molecules into multilayer films and their release from the films showed a strong dependence on solution pH. The maximum loading was achieved under pH conditions that resulted in the largest electrostatic attraction between the dye molecules and the film, while the release of the molecules from the films was achieved when repulsive forces between the dye molecules and the films were dominant. It was shown also that the swelling of the film helped to facilitate the release of the dye molecules by transporting counterions into the film to screen the electrostatic interactions and also by creating pores through which the molecules can travel. One of the most interesting findings in this study is that it is possible to trap the small molecules in the film at a certain solution pH and to release the trapped molecules simply by changing the pH. So the reversible loading is controlled by a complex interplay between the physico-chemical properties of the films and of the loaded molecules.

Loading of nanoparticles in PEM films

The first work introducing the reliability of the concept of loading PEM films with nanoparticles was performed with the exponentially growing films by Srivastava and coworkers [95]. This study showed the possibility of loading and unloading exponentially growing PEM films with NPs by simple diffusion and thus introducing the use of PEM films as carriers of nanoparticles for biological, optoelectronic, and environmental applications. The films were constructed from poly-(diallyldimethylammonium chloride) (PDDA) and from poly-(acrylic acid) (PAA) and were loaded with Cadmium telluride (CdTe) nanoparticles capped either with thioglycollic acid (NP1) or 2-(dimethylamino) ethanethiol (NP2) (Figure 1.24). The reversible loading of NPs was investigated with UV−vis studies and then confirmed by confocal microscopy (Figure 1.25). Results showed a homogeneous loading of the NPs throughout the films thickness.

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Advances in nanodendrimers research and applications

Several of studies have been carried out to investigate the physicochemical and structural properties of dendrimers in solution either theoretically by applying computer simulations and or experimentally by analytical techniques. For the aim of the optimization of the computer models to give a realistic picture, a large amount of comparative studies have been carried out between prediction based theoretical calculations and experimental results [136]. In general, the physicochemical properties of dendrimers mainly depend on the generation number, surface functionalities and core structure. For carboxylated dendrimers with acidic end groups, SANS (small angle neutron scattering) and NMR (nuclear magnetic resonance) measurements of self-diffusion coefficients at different pH values were carried. Results showed that at pH 2, due to the electrostatic repulsion between the positively charged protonated tertiary amines of the core, the most extended conformation of the core is observed which leads to a larger core radius [137]. At pH 6, the amount of positively charged amines is equal to the amount of negatively charged carboxylic groups (isoelectric point) which gives rise to a dense core conformation more prone to shrinking as a result of electrostatic interactions between the negatively charged surface C – groups and the in the inner shells of the dendrimer. This shows that shrinking is not only a result of weak forces leading to a uniform molecular density of the dendrimer (entropy), but may also be mediated by attractive forces (enthalpy) between inner parts of the dendrons and surface groups. This conformation minimizes the repulsion between the negatively charged surface carboxylates and the positively charged inner shell amines leading to a lower repulsive energy of the system. At basic pH (close to 11), the electrostatic repulsion between the negative charged forces of the surface groups again results in more extended conformation with a highly expanded surface area (swelling) (Figure 1.34).

Investigations of dendrimer toxicity

The recent explosion of interest in dendrimers research accompanied with their growing range of applications makes it so critical to investigate their biocompatibility and toxicity [177]. However, currently little information exists regarding the risks such materials pose to human health and the environment. To date, the cytotoxicity of dendrimers has been primarily studied in vitro; however, a few in vivo studies have been recorded [178].
Several studies showed the significant generation-dependent cytotoxicity of amino-terminated PAMAM dendrimers on human intestinal adenocarcinoma Caco-2 cells [179, 180], with higher generation dendrimers being the most toxic [179, 181]. In the same context, Fisher et al. showed that the degree of substitution as well as the type of amine functionality affects the degree of toxicity, with primary amines being more toxic than secondary or tertiary amines [181].
Malik et al. observed a generation-dependent haemolytic effect of PAMAM-NH2 dendrimers on a solution of rat blood cells [182]. However, the biocompatibility of dendrimers was also dependent on the chemistry of the core, but is most strongly influenced by the nature of the dendrimers surface. Dendrimers containing an aromatic polyether core and anionic carboxylate surface groups have shown to be haemolytic on a solution of rat blood cells after 24 h. Comparative toxicity studies on anionic (carboxylate-terminated) and cationic (amino-terminated) PAMAM dendrimers using Caco-2 cells have shown a significantly lower cytotoxicity of the anionic ones [179]. The cationic dendrimers are prone to destabilize cell membranes and cause cell lysis. Lee et al. observed down-regulation of mitochondrial DNA-encoded genes involved in the maintenance of mitochondrial membrane for human lung cells after exposure to G4-NH2 PAMAM dendrimers. Results showed damage in the mitochondria and a decrease of cell viability resulting in apoptosis [183].
However, only few in vivo studies have reported the toxicity of dendrimers. Concerning animal models, Roberts et al. showed that upon injection into mice, doses of 10 mg/kg of PAMAM dendrimers (up to G5), with amino-terminated surfaces, did not appear to be toxic, while at very high concentrations, they induced some inhibition of cell growth in vitro but upon injection into mice, no acute or long-term toxicity problems were observed [184]. Li et al. studied the molecular link between exposure to cationic PAMAM dendrimers and lung damage in mice. The authors observed in vivo toxicity due to acute lung injury because PAMAM triggers cell death by deregulating a signaling pathway [185]. Chauhan et al. studied the toxicity of G4-NH2 and G4-OH PAMAM dendrimers in Swiss albino mice. They suggested a probable interference of the dendrimers with glucose metabolism as well as toxic effects on kidney and liver [186].
Concerning aquatic organisms, Petit et al. investigated the toxicity of G2, G4 and G5 PAMAM dendrimers to the green alga Chlamydomonas reinhardtii. The results indicated a toxicity increase with generation number [187]. Moreover, Heiden et al. [188] studied the toxicity of G3.5 and G4 PAMAM dendrimers towards zebrafish embryos. While G4 dendrimers were toxic towards growth and development of zebrafish embryos, G3.5 carboxylated dendrimers did not exhibit toxicity. Suarez et al. followed the toxicity of G1-NH2, G4-NH2, and G4-OH PAMAM dendrimers to the microalga Pseudokirchneriella subcapitata [189]. Findings showed a high toxicity particularly for G4-NH2 which support previous results indicating increased toxicity for higher dendrimer generation probably as a consequence of a larger surface area for interaction with living organisms [190].

Table of contents :

I. Polyelectrolyte multilayer (PEM) films
I.1. Polyelectrolyte multilayer (PEM) films obtained by electrostatic interactions
I.1.1. Properties of polyelectrolytes
I.1.2. Strong polyelectrolytes versus weak polyelectrolytes
I.1.3. History of polyelectrolyte multilayer films
I.1.4. Principles of the Layer by Layer (LBL) deposition of PEM films
I.1.5. Interactions in multilayer LBL films
I.1.6. Charge balance in polyelectrolyte multilayers
I.1.7. Growth modes of multilayer films
I.1.7.1. Films with linear growth
I.1.7.2. Films with exponential growth
I.1.8. Physico-chemical parameters affecting the growth of PEM films
I.1.8.1. Solution pH
I.1.8.2. Ionic strength of the buffer and polyelectrolyte solutions
I.1.8.3. Buildup temperature
I.1.8.4. Effect of salt anion on the film construction
I.1.9. Mechanical properties of PEM films
I.1.10. Water content in PEM films
I.2. Polyelectrolyte multilayer films obtained by hydrogen bonding
I.3. Electrochemical properties of polyelectrolyte multilayers
I.3.1. Ion Permeability of polyelectrolyte multilayers
I.3.2. Donnan Potential in PEM films
I.4. Applications of PEM films
I.5. Incorporation of Nanoparticles and Biomolecules into PEM Films
I.5.1. Incorporation of bioactive molecules and ions
I.5.2. Incorporation of Nanocolloids
I.5.3. Incorporation of proteins and peptides
I.5.4. Loading with Multivalent ions
I.5.5. Loading of nanoparticles in PEM films
II. Electrokinetic Investigations of Nanodendrimers
II.1. Concept of soft particles
II.2. Electrokinetics of soft particles
II.3. Polyamidoamine (PAMAM) carboxylated nanodendrimers
II.3.1. Synthesis and structure
II.3.2. Advances in nanodendrimers research and applications
II.3.3. Investigations of dendrimer toxicity
I. Materials and samples preparation
I.1. The polyelectrolyte solutions
I.2. Construction of the films
I.3. Heating and aging of the films
I.4. Electrokinetics of the nanodendrimers
I.5. Loading the films with nanodendrimers
II. Characterization Methods
II.1. Atomic force microscopy (AFM)
II.1.1. AFM technique and principles
II.1.2. AFM cantilevers and probes
II.1.3. Different AFM modes
II.1.4. Force measurements
II.1.5. Determining film elastic modulus using AFM
II.2. Quartz Crystal microbalance-dissipation (QCM-D)
II.3. Confocal Raman Spectroscopy (CRS)
II.4. Dynamic light scattering (DLS) and Phase Analysis light Scattering (PALS)
II.4.1. Size measurements by DLS
II.4.2. Electrophoretic mobility measurements by PALS
I. Characterization of (PDADMAC-PAA) films
II. Loading of the films with G6.5 PAMAM nanodendrimers
II.1. Effect of G6.5 dendrimers concentration on the morphology of P A MAC PAA films
II.2. Effect of G6.5 concentration on Young modulus of P A MAC PAA films
III. Conclusions
III. References


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