5.1. The Process of Layer-by-Layer (LbL) Nanocoating
LbL self-assembly of polyelectrolytes took its origin in the 1990s [78]. Poly(styrene-4-sulfonate), PSS, was one of the first polyanions employed for LbL self-assembly and remains widely utilized today. As polycation, an ammonium-containing polymer, poly(N,N-dimethylallylamine), PDDA, was successfully employed to create a multilayer structure comprising of alternating polyanion and -cation layers [79].
A series of proteins were also successfully employed as polycations namely cytochrome c, lysozyme, histone f3, myoglobin, and hemoglobin. By adjustment of the pH of the medium, amylase, glucose oxidase, and catalase were employed as polyanions [80]. DNA was also employed successfully as a polyelectrolyte for LL self-assembly [81].
LbL coating has also been employed to modify inorganic surfaces. Although many applications for these surface modifications are possible, only some antimicrobial examples are mentioned. Stainless steel surfaces were primed with an acrylate-based surfactant via electrografting. Subsequently, PSS and PDDA layers were coated in an alternating fashion. Lastly, a layer of chitosan was coated as an antibacterial layer against E. coli and S. aureus [82]. Silicone-based intraocular lenses (IOL) are commonly employed to replace the natural eye lens when it is damaged. The IOL can, however, allow adhesion of many kinds of bacteria and lead to post-operative infections with catastrophic effects in some patients. LbL nanocoating of the lenses with hyaluronic and chitosan had significant anti-adhesion and bactericidal effects that reduced the risk of postoperative infections [83,84].
The technique of LbL nanocoating is uncomplicated and requires relatively low concentrations of the polyelectrolytes to produce an efficient coat, often in the low nanometer range. A substrate for coating is required, a polycation and separate polyanion solution, and clean water as the washing liquid. Figure 3 illustrates the technique. Numerous polysaccharides, especially GAGs, are charged polyelectrolytes and the next section will elaborate on this.