Misplaced Emphases in Wars on Poverty

The development of the layer-by-layer (LbL) technique has turned out to be an efficient way to physically modify the surface properties of different materials, for example to improve the adhesive interactions between fibers in paper. The main objective of the work described in this thesis was to obtain fundamental data concerning the adhesive properties of wood biopolymers and LbL films, including the mechanical properties of the thin films, in order to shed light on the molecular mechanisms responsible for the adhesion between these materials. LbLs constructed from poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA), starch containing LbL films, and LbL films containing nanofibrillated cellulose (NFC) were studied with respect to their adhesive and mechanical properties. The LbL formation was studied using a combination of stagnation point adsorption reflectometry (SPAR) and quartz crystal microbalance with dissipation (QCM-D) and the adhesive properties of the different LbL films were studied in water using atomic force microscopy (AFM) colloidal probe measurements and under ambient conditions using the Johnson-Kendall-Roberts (JKR) approach. Finally the mechanical properties were investigated by mechanical buckling and the recently developed SIEBIMM technique (strain-induced elastic buckling instability for mechanical measurements). From colloidal probe AFM measurements of the wet adhesive properties of surfaces treated with PAH/PAA it was concluded that the development of strong adhesive joints is very dependent on the mobility of the polyelectrolytes and interdiffusion across the interface between the LbL treated surfaces to allow for polymer entanglements. Starch is a renewable, cost-efficient biopolymer that is already widely used in papermaking which makes it an interesting candidate for the formation of LbL films in practical systems. It was shown, using SPAR and QCM-D, that LbL films can be successfully constructed from cationic and anionic starches on silicon dioxide and on polydimethylsiloxane (PDMS) substrates. Colloidal probe AFM measurements showed that starch LbL treatment have potential for increasing the adhesive interaction between solid substrates to levels beyond those that can be reached by a single layer of cationic starch. Furthermore, it was shown by SIEBIMM measurements that the elastic properties of starch-containing LbL films can be tailored using different nanoparticles in combination with starch. LbL films containing cellulose I nanofibrils were constructed using anionic NFC in combination with cationic NFC and poly(ethylene imine) (PEI) respectively. These NFC films were used as cellulose model surfaces and colloidal probe AFM was used to measure the adhesive interactions in water. Furthermore, PDMS caps were successfully coated by LbL films containing NFC which enabled the first known JKR adhesion measurements between cellulose/cellulose, cellulose/lignin and cellulose/glucomannan. The measured adhesion and adhesion hysteresis were similar for all three systems indicating that there are no profound differences in the interaction between different wood biopolymers. Finally, the elastic properties of PEI/NFC LbL films were investigated using SIEBIMM and it was shown that the stiffness of the films was highly dependent on the relative humidity.QC 20110923</p

Immobilization of single strand DNA on solid substrate

Thin films based on Layer-by-Layer (LbL) self assembled technique are useful for immobilization of DNA onto solid support. This communication reports the immobilization of DNA onto a solid support by electrostatic interaction with a polycation Poly (allylamine hydrochloride) (PAH). UV-Vis absorption and steady state fluorescence spectroscopic studies exhibit the characteristics of DNA organized in LbL films. The most significant observation is that single strand DNA are immobilized on the PAH backbone of LbL films when the films are fabricated above the melting temperature of DNA. DNA immobilized in this way on LbL films remains as such when the temperature is restored at room temperature and the organization remains unaffected even after several days. UV-Vis absorption spectroscopic studies confirm this finding.Comment: Eight pages, five figure

Modulating Nanoparticle Film Assembly Using Amphiphiles

Nanocomposite thin films comprised of nanoparticles have shown great promise for use in electronics, photonics, biomedical as well as energy storage and conversion devices. One versatile method for fabricating such thin films is layer-by-layer (LbL) assembly, a process that involves sequential deposition of oppositely charged species to create conformal thin films. The advantage of LbL assembly lies in the fact that the properties and structure of films can be tuned by varying assembly conditions such as pH and ionic strength. Furthermore, a variety of nanomaterials with useful properties can be incorporated within LbL assembled thin films. Despite these advantages, there are a few limitations to using LbL assembly to fabricate nanoparticle films: (1) Favorable film growth of all-nanoparticle LbL assembly in aqueous phase occurs within a narrow processing window thus limiting the versatility of LbL assembly. (2) nanoparticle LbL assembly has generally been limited to aqueous phase due to the ease of charging nanomaterials in water. (3) The fabrication of nanoparticle films via LbL assembly is slow and typically takes several hours to complete. In this thesis, amphiphiles will be used to address these three limitations of nanoparticle LbL assembly. The first limitation is addressed by using a small amphiphilic molecule, hexylamine to broaden the narrow nanoparticle LbL assembly window. In addition, an array of experimental techniques is used to reveal the mechanism leading to a broad processing window. It will be demonstrated that the second limitation of nanoparticle LbL assembly to aqueous phase can be overcome by using a surfactant Aerosol-OT (AOT) to charge stabilize particles in toluene for non-polar LbL assembly. Furthermore, the effect of the surface chemistry of particles and dispersion moisture content on the charge of particles in non-polar media is probed along with the role of relative humidity on the LbL assembly process in non-polar media. Lastly, electrophoretic deposition (EPD) of surfactant-charged particles in a non-polar solvent is used to rapidly assemble nanocomposite films, thus overcoming the third limitation of nanoparticle LbL assembly

Biosensors Platform Based on Chitosan/AuNPs/Phthalocyanine Composite Films for the Electrochemical Detection of Catechol. The Role of the Surface Structure

Producción CientíficaBiosensor platforms consisting of layer by layer films combining materials with different functionalities have been developed and used to obtain improved catechol biosensors. Tyrosinase (Tyr) or laccase (Lac) were deposited onto LbL films formed by layers of a cationic linker (chitosan, CHI) alternating with layers of anionic electrocatalytic materials (sulfonated copper phthalocyanine, CuPcS or gold nanoparticles, AuNP). Films with different layer structures were successfully formed. Characterization of surface roughness and porosity was carried out using AFM. Electrochemical responses towards catechol showed that the LbL composites efficiently improved the electron transfer path between Tyr or Lac and the electrode surface, producing an increase in the intensity over the response in the absence of the LbL platform. LbL structures with higher roughness and pore size facilitated the diffusion of catechol, resulting in lower LODs. The [(CHI)-(AuNP)-(CHI)-(CuPcS)]2-Tyr showed an LOD of 8.55∙10−4 μM, which was one order of magnitude lower than the 9.55·10−3 µM obtained with [(CHI)-(CuPcS)-(CHI)-(AuNP)]2-Tyr, and two orders of magnitude lower than the obtained with other nanostructured platforms. It can be concluded that the combination of adequate materials with complementary activity and the control of the structure of the platform is an excellent strategy to obtain biosensors with improved performances.Ministerio de Ciencia, Innovación y Universidades - Fondo Europeo de Desarrollo Regional (project RTI2018-097990-B-100)Junta de Castilla y Leon - Fondo Europeo de Desarrollo Regional (project VA275P18)Infraestructuras Red de Castilla y León (grant UVA01

The Effect of Assembly Technique on Weak Polyelectrolyte Multilayer Film Morphology and Humidity Swelling/Deswelling Behavior

The goal of this research is to investigate the film morphology and humidity swelling/deswelling behavior of polyelectrolyte multilayers (PEMs) that are constructed from poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) by two assembly techniques: automated dipping layer-by-layer (LbL) and spin-assisted (SA) LbL. Two sets of pH polyelectrolyte solution conditions were tested: (PAH7.5/PAA3.5) and (PAH7.0/PAA7.0). It was found that when the films were constructed with the same number of bilayers (=20 bilayers), SA-LbL (PAH7.5/PAA3.5) has a greater thickness than (PAH7.0/PAA7.0). (PAH7.5/PAA3.5) thin films constructed by automated dipping LbL did not result in a uniform film morphology, making thickness comparisons between techniques for those assembly conditions difficult. On the other hand, the thickness of (PAH7.0/PAA7.0)20 thin films constructed by automated dipping LbL was a tenth of the thickness of (PAH7.0/PAA7.0)20 thin films constructed by SA-LbL. In addition, film morphology was studied for the uneven automated dipping LbL (PAH7.5/PAA3.5)20 thin film using contact mode AFM. Silane treatment and an adsorbed polyethylenimine (PEI) underlayer were attempted to produce a smooth film surface. However, neither surface treatments gave a smooth film surface for the uneven automated dip-coated LbL (PAH7.5/PAA3.5)20. Therefore, the assembly technique is an important factor on the thin film thickness and film morphology. Humidity swelling/deswelling tests were carried out on SA-LbL (PAH7.5/PAA3.5)20, SA-LbL (PAH7.0/PAA7.0)20, and automated dip-coated LbL (PAH7.0/PAA7.0)20. SALbL (PAH7.5/PAA3.5)20 and SA-LbL (PAH7.0/PAA7.0)20 demonstrated a maximum swelling of approximately 40% under a humid air environment. However, the humidity hysteresis effect for the automated dipping LbL (PAH7.0/PAA7.0)20 was hard to tell due to the high uncertainty in the film thickness

Graphene-based LbL deposited films: further study of electrical and gas sensing properties

Graphene-surfactant composite materials obtained by the ultrasonic exfoliation of graphite powder in the presence of ionic surfactants (either CTAB or SDS) were utilised to construct thin films using layer-by-layer (LbL) electrostatic deposition technique. A series of graphene-based thin films were made by alternating layers of either graphene-SDS with polycations (PEI or PAH) or graphene-CTAB with polyanions (PSS). Also, graphene-phthalocyanine composite films were produced by alternating layers of graphene-CTAB with tetrasulfonated nickel phthalocyanine. Graphene-surfactant LbL films exhibited good electric conductivity (about 0.1 S/cm) of semiconductor type with a band gap of about 20 meV. Judging from UV-vis spectra measurements, graphene-phthalocyanine LbL films appeared to form joint π-electron system. Gas sensing testing of such composite films combining high conductivity of graphene with the gas sensing abilities of phthalocyanines showed substantial changes (up to 10%) in electrical conductivity upon exposure to electro-active gases such as HCl and NH3

A closer physico-chemical look to the Layer-by-Layer electrostatic self-assembly of polyelectrolyte multilayers

The fabrication of polyelectrolyte multilayer films (PEMs) using the Layer-by-Layer (LbL) method is one of the most versatile approaches for manufacturing functional surfaces. This is the result of the possibility to control the assembly process of the LbL films almost at will, by changing the nature of the assembled materials (building blocks), the assembly conditions (pH, ionic strength, temperature, etc.) or even by changing some other operational parameters which may impact in the structure and physico-chemical properties of the obtained multi-layered films. Therefore, the understanding of the impact of the above mentioned parameters on the assembly process of LbL materials plays a critical role in the potential use of the LbL method for the fabrication of new functional materials with technological interest. This review tries to provide a broad physico-chemical perspective to the study of the fabrication process of PEMs by the LbL method, which allows one to take advantage of the many possibilities offered for this approach on the fabrication of new functional nanomaterials.Comment: Published Pape

Layer-by-Layer Thin Films and Coatings Containing Metal Nanoparticles in Catalysis

The layer-by-layer (LbL) technique is one of the most promising ways of fabricating multilayer thin films and coatings with precisely controlled composition, thickness, and architecture on a nanometer scale. This chapter considers the multilayer thin films and coatings containing metal nanoparticles. The main attention was paid to LbL films containing metal nanoparticles assembled by convenient methods based on the different intermolecular interactions, such as hydrogen bonding, charge transfer interaction, molecular recognition, coordination interactions, as driving force for the multilayer buildup. Much attention has paid to the LbL films containing metal nanocomposites for multifunctional catalytic applications, in particular, photocatalysis, thermal catalysis, and electrocatalysis. The preparation protocol of LbL-assembled multilayer thin films containing metal nanoparticles (such as Au, Ag, Pd, Pt), metal oxides (Fe3O4), and sulfides (CdS) that are supported on the various surfaces of nanotubes of TiO2, Al2O3 membranes, graphene nanosheets, graphene oxide and further applications as catalysts with respect to photocatalytic, electrocatalytic performances is discussed. The systematization and analysis of literature data on synthesis, characterization, and application of multilayer thin films and coatings containing metal nanoparticles on the diverse supports may open new directions and perspectives in this unique and exciting subject

Graphene-based LbL deposited films: further study of electrical and gas sensing properties

Graphene-surfactant composite materials obtained by the ultrasonic exfoliation of graphite powder in the presence of ionic surfactants (either CTAB or SDS) were utilised to construct thin films using layer-by-layer (LbL) electrostatic deposition technique. A series of graphene-based thin films were made by alternating layers of either graphene-SDS with polycations (PEI or PAH) or graphene-CTAB with polyanions (PSS). Also, graphene-phthalocyanine composite films were produced by alternating layers of graphene-CTAB with tetrasulfonated nickel phthalocyanine. Graphene-surfactant LbL films exhibited good electric conductivity (about 0.1 S/cm) of semiconductor type with a band gap of about 20 meV. Judging from UV-vis spectra measurements, graphene-phthalocyanine LbL films appeared to form joint π-electron system. Gas sensing testing of such composite films combining high conductivity of graphene with the gas sensing abilities of phthalocyanines showed substantial changes (up to 10%) in electrical conductivity upon exposure to electro-active gases such as HCl and NH3