Apparent diffusion coefficients and electron propagation mechanisms in viologen polyelectrolyte coatings containing multiply-charged anions

Self-Assembling Bipyridinium Multilayers

The identification of strategies to assemble nanostructured films with engineered properties on solid supports can lead to the development of innovative functional materials. In particular, the self-assembly of electroactive multilayers from simple molecular building blocks on metallic electrodes can offer the opportunity to regulate the exchange of electrons between the underlying substrate and solution species. In this context, we designed an experimental protocol to prepare electroactive films from bipyridinium bisthiols. Specifically, we found that a compound incorporating two bipyridinium dications at its core and terminal thiol groups self-assembles into remarkably stable multilayers on polycrystalline gold. The surface coverage of the resulting films can be regulated by adjusting the exposure time of the gold substrate to the bipyridinium solution. Control experiments with appropriate model compounds demonstrate that both bipyridinium dications as well as both thiol groups must be present in the molecular skeleton to encourage multilayer growth. The resulting films transport electrons efficiently from the electrode surface to the film/solution interface. Indeed, they mediate the reduction of Ru(NH(3))(6)(3+) in the electrolyte solution but prevent the back oxidation of the resulting Ru(NH(3))(6)(2+). Furthermore, these polycationic bipyridinium films capture electrostatically Fe(CN)(6)(4-) tetraanions, which can also be exploited to transport electrons across the interfacial assembly. In fact, electrons can travel through the bipyridnium(2+/1+) couples to redox probes in solution and then back to the electrode through the Fe(CN)(6)(4/3-) couples. Thus, our original approach to self-assembling multilayers can produce stable electroactive films with unique electron transport properties, which can be regulated with a careful choice of the anionic components.

Ca-alginate microspheres encapsulated in chitosan beads

Chitosan beads (CBs) incorporating Ca-alginate microspheres (CAMs), containing a drug, were prepared as an oral sustained delivery system. Stable and monodisperse Ca-alginate microspheres loaded with drug were obtained by a membrane emulsification method. The Ca-alginate microspheres were encapsulated in chitosan beads by the ionotropic gelation method with a polyelectrolyte complex reaction between two oppositely charged polyions. The surface and internal characteristics of the beads were improved by ionic cross-linking in tripolyphosphate (TPP) solution adjusted to pH 5.0. The release experiments were performed using lidocaine.HCl (cationic drug) and sodium salicylate (anionic drug) as model drugs. Initial release of drugs depended on the degree of swelling. Ca-alginate microspheres encapsulated in chitosan beads were superior to both drug-loaded CBs and CAMs beads for sustained release because they had a three-layer composition; a calcium alginate core bounded by an inter-phasic chitosan-alginate membrane, which itself was surrounded by a layer of chitosan-TPP.

Electron transport in bipyridinium films

Bipyridinium dications are versatile building blocks for the assembly of functional materials. In particular, their reliable electrochemical response has encouraged the design of electroactive films. Diverse and elegant experimental strategies to coat metallic and semiconducting electrodes with bipyridinium compounds have, in fact, emerged over the past two decades. The resulting interfacial assemblies span from a few nanometers to several micrometers in thickness. They incorporate from a single molecular layer to large collections of entangled polymer chains. They transport electrons efficiently from the electrode surface to the film/solution interface and vice versa. Electron self-exchange between and the physical diffusion of the bipyridinium building blocks conspire in defining the charge transport properties of these fascinating electroactive assemblies. Often, the matrix of electron-deficient bipyridinium dications can be exploited to entrap electron-rich analytes. Electrostatic interactions promote the supramolecular association of the guests with the surface-confined host matrix. Furthermore, chromophoric sites can be coupled to the bipyridinium dications to produce photosensitive arrays capable of harvesting light and generating current. Thus, thorough investigations on the fundamental properties of these functional molecule-based materials can lead to promising applications in electroanalysis and solar energy conversion, while contributing to advances in the basic understanding of electron transport in interfacial assemblies.