Commentary
Introduction to nanocoatings produced by layer-by-layer (LbL) self-assembly

https://doi.org/10.1016/j.addr.2011.05.011Get rights and content

Abstract

Studies on the adsorption of oppositely charged colloidal particles ultimately resulted in multilayered polyelectrolyte self-assembly. The inception of layer-by-layer constructed particles facilitated the production of multifunctional, stimuli-responsive carrier systems. An array of synthetic and natural polyelectrolytes, metal oxides and clay nanoparticles is available for the construction of multilayered nanocoats on a multitude of substrates or removable cores. Numerous substrates can be encapsulated utilizing this technique including dyes, enzymes, drugs and cells. Furthermore, the outer surface of the particles presents and ideal platform that can be functionalized with targeting molecules or catalysts. Some processing parameters determining the properties of these successive self-assembly constructs are the surface charge density, coating material concentration, rinsing and drying steps, temperature and ionic strength of the medium. Additionally, the simplicity of the layer-by-layer assembly technique and the availability of established characterization methods, render these constructs extremely versatile in applications of sensing, encapsulation and target- and trigger-responsive drug delivery.

Section snippets

Historical perspective on layer-by-layer self-assembly

Novel materials have always been sought and the employment of surface modification at the molecular level realized this goal. Surface modification resulted in a multitude of new properties that were previously not associated with the native material. These changes include modifications of the electrical, optical, magnetic, physicochemical and biological properties of the material in question. As a consequence, several disciplines of natural science have experienced the impact of surface

Basic principles of the layer-by-layer technique

The formation of nanocoatings using LbL self-assembly technique distinguishes itself in its simplicity from other surface modification methods such as spin-coating, solution casting, thermal deposition, chemical self-assembly and the LB technique and will be discussed next.

Coating materials and substrates

Several polyelectrolytes and nanoparticles can be utilized to form the ultrathin multilayer structures using the LbL self-assembly technique. Furthermore, several substrates can be coated with nanothin multilayers.

Experimental parameters and LbL adsorption

The formation of polyelectrolyte multilayer self-assembly is usually reliant on the electrostatic adsorption between the substrate and subsequent layers [14], [53]. A two-stage process is envisioned by which (1) an initial anchoring of the coating material to the surface is followed by (2) a slow relaxation to form a densely-packed structure on the surface [89]. Some processing parameters that influence the adsorption steps of LbL assembly are briefly discussed.

Layer-by-layer disassembly

In several cases, intact LbL constructed systems are required to remain stable in order to control the release of substances i.e. drugs [42], [44] by posing a barrier to release with the possibility of variation of barrier permeability for water-soluble drugs and dyes substances under the influence of external stimuli such as changes in temperature or ultrasonic treatment [123].

The alternative approach to release the captured content in an LbL-assembled system relies on the disassembly of the

Characterization of LbL constructs

The combination of several techniques can be used to study the construction, disassembly or release of captured content from LbL PEMs. Some of the more common methods will be briefly discussed (Fig. 3).

Advantages of LbL-assembled multilayers

LbL self-assembly offers several advantages to other methods of encapsulation, coating or fixation of substances: (1) the wall thickness of capsules can be tailored in the nm–μm range, (2) several types of synthetic/natural colloids are available for LbL, (3) the location and sequence of the layers can be controlled, (4) surface labeling with targeting molecules is possible, (5) stabilization of submicron particles is possible [167], (6) LbL avoids the use of thermodynamically unstable

Conclusions

The evolvement and simplicity of LbL construction provided a simple, robust platform, independent of precise stoichiometry, for the modification of material surfaces or the encapsulation of various substrates. The technique facilitated a ‘library’ approach to create novel carrier systems, especially in the field of drug delivery. A myriad of colloids can be used to create novel hybrid-coated materials by use of different intermolecular forces.

Multifunctionality of LbL systems can be introduced

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    This commentary is part of the Advanced Drug Delivery Reviews theme issue on “Layer-by-Layer Self-Assembled Nanoshells for Drug Delivery”.

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