Recent explorations in electrostatic multilayer thin film assembly
Introduction
The past decade has seen the use of non-covalent interactions as a key tool to the construction of new molecular and supramolecular architectures that cover a range of length scales. In the area of polymer thin films, the most influential area of growth in the use of non-covalent interactions is electrostatic layer-by-layer assembly, which was first introduced by Decher in 1991 [1], [2]. This technique, which is based on the alternating adsorption of multiply charged cationic and anionic species, has experienced an explosion of growth in both application and theoretical and experimental advances. The popularity of the method is due to the ability to create highly tailored polymer thin films with a nearly unlimited range of functional groups incorporated within the structure of the film. This flexibility is achieved through the use of a simple, yet elegant adsorption process that is highly inexpensive, and readily accessible. The area has progressed to include the incorporation of electro-active polymers, organic dyes, semiconductor quantum dots, electrochemically active species, and a wide variety of proteins and other biosystems [3], [4], [5].1
As the area of layer-by-layer (LBL) assembled films enters its 10th year, the impact on the use of layer-by-layer electrostatic adsorption in the polymer thin film area has become even more evident. New applications and a greater knowledge and control of the structure of the assembled films have been the thrust in this still-expanding research field. Closer examinations of the interactions within these systems have brought out the diversity of structures, film morphology and surface properties achieved by altering shielding and adsorption conditions. As shown schematically in Fig. 1, electrostatic interactions between the polyion in solution and the surface are the key to the final structure of the polyion layered thin film; however, secondary, shorter range forces also play a role in determining the film thickness, the final morphology of the film, the surface properties, and in some cases, can determine whether or not stable multilayers form at all. These secondary interactions can also play a role in the selective deposition of polymers on surfaces, the formation of acentric polar structures, and the nature of permeation and ion transport within the film. An understanding of these interactions, as well as an ability to combine polyions with other charged systems such as particles or low molar mass diamphiphiles, lead to the development of new ordered systems, dimensional polymer structures and patterns, selective membranes, and a range of functional organic and organic–inorganic hybrid composite thin films.
In the past 2 years, a number of new advances have contributed to both the fundamental understanding of polyion thin film assembly, and the use of these materials in the formation of mesoscale multifunctional materials systems. This review will address advancements gained from 1998 to the present. Areas that will be covered include fundamental studies, particularly gains made in understanding the role of electrostatics in combination with secondary interactions such as hydrophobicity, hydrogen bonding and dispersion forces in these systems. The incorporation and control of nano-to-micro scale particles in thin film composite architectures will also be covered, including the possibilities introduced by the incorporation of inorganic or organometallic functionalities. Finally, new approaches to the construction of two- and three-dimensional objects utilizing layering techniques will be addressed, and the use of electrostatic interactions, sometimes coupled with weaker non-specific interactions in directed assembly, will be discussed. Relevant examples of new applications and potential uses of multilayer films will also be addressed.
Section snippets
Advances in understanding: weak vs. highly charged systems
Until recently, a primary focus of fundamental studies on layer-by-layer adsorption was the behavior of highly charged strong polyelectrolyte chains, which can be shielded by altering ionic strength. Shielding and surface charge density effects on layer thickness and stability can be achieved in weak polyelectrolytes such as polyacids or polybases, by varying the pH and thus, affecting the relative amount of charge along the backbone. In the past 2 years, seminal works on the effects of pH on
Bringing molecular order to layer-by-layer thin films
Perhaps one of the fastest growing areas in the use of LBL techniques is the incorporation of electroactive and electro-optical moieties into the films. For this reason, there has been an increasing interest in the use of these films for non-linear optical devices, hole and ion transport, and other applications in which the ordering of the chromophore itself can be critical to final properties. For this reason, perhaps one of the bigger challenges for the electrostatic self assembly community
Nano- to microparticle composites: mesoscale assembly
Amongst the most rapidly expanding areas in materials science is the synthesis, functionalization and assembly of particles and particle arrays. Nanoscale inorganic particles such as quantum dots are known for their unique electrical and optical properties due to confinement effects. On the other end of the spectrum, polymer, silica and metallic submicron-to-micrometer sized spheres are of interest as ordered arrays for photonic bandgap applications. A major goal in the materials science
New dimensions: two-dimensional and three-dimensional structures from layer-by-layer approaches
The LBL technique has provided a route to a large number of functional organic and organic–inorganic composite thin films. A number of applications for these films can be further enabled by the creation of two- and three-dimensional microstructures. In these cases, layer-by-layer systems move beyond the conventional realm of planar films and become the basis for patterned devices, encapsulant particles, and dimensional objects. The ability to direct the deposition of layer-by-layer films onto
Other advancement areas
There are several other areas of new growth in the layer-by-layer assembly field, as well as important advancements in promising applications. Perhaps the greatest expansion in the field has been in the area of electro and photo-active thin films. In the past 5 years, several researchers have successfully demonstrated the use of this technique in the formation of electroluminescent polymers into layer-by-layer films [56], [57].
Conclusions
The use of electrostatic layer-by-layer thin films has continued to experience an accelerated growth in the past 2 years, as a greater understanding of the effects of adsorption conditions, polyion molecular structure, and surface interactions has been gained. A better fundamental understanding has been gained in general in this area, with more in-depth analysis of the adsorption process, and new experimental techniques to probe the structure of multilayer films; the adsorption of weakly
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