Functional Multilayered Polyelectrolyte Assemblies on Biological Cells - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 6

polyanion is adsorbed. This process of multilayer formation is based on the
attraction of opposite charges, and thus requires a minimum of two op-
positely charged molecules. Consequently, one is able to incorporate more
than two molecules into the multilayer, simply by immersing the substrate
in as many solutions of polyeletrolytes as desired, as long as the charge is
reversed from layer to layer. Even aperiodic multilayer assemblies can easily
be prepared. 4

d n 8 y 4 n g | 4

2.1.3.2 Hydrogen-Bond-Mediated LbL Assembly
The concept of electrostatically driven assembly of multilayer structures
allows for the incorporation of a wealth of different materials. However,
polymers as multifunctional materials also offer the choice of building up
layered structures through other types of interaction. One of the most
commonly studied, nonelectrostatic interactions used in LbL assembly to
date is hydrogen bonding. By exploiting this interaction, uncharged ma-
terials can be successfully incorporated into multilayer films. The pioneering
studies in LbL multilayer assembly based on hydrogen bonding were re-
ported independently by Stockton and Rubner, 14 and Zhang's group 15 in
1997. Stockton and Rubner investigated the use of polyaniline in alternation
with a variety of water-soluble macromolecules, such as poly(vinyl-
pyrrolidone) (PVPON), poly(vinyl alcohol) (PVA), polyacrylamide, and PEO,
in which the oxygen atoms on the polymer backbone can be hydrogen-
bonding acceptors, or poly(N-isopropylacrylamide), in which both an ac-
ceptor (carbonyl) and donor (amide) are present. Besides, film deposition is
possible with polymer pairs containing side groups with carbazole and
dinitrophenyl units that can form charge-transfer complexes. 16,17

.

2.1.3.3 Covalent Bonding Based LbL Assembly
Covalent bonds can be used to assemble LbL films having high stability due
to the covalent bonds formed, and therefore these do not disassemble with
changes in pH or ionic strength. Bergbreiter and coworkers performed the
first example of sequential covalent assembly of polymers, using a co-
polymer of maleic anhydride reacted in alternation with a polyamidoamine
dendrimer. 18 Blanchard and coworkers also investigated approaches to
prepare multilayer films using a sequential covalent strategy. 19,20
Inspired by dopamine self-polymerizing at alkaline pH to form adherent
polymer coatings on a large variety of substrates, 21 synthetic polymers with
catechol and amine functionalities may be useful as ''universal'' LbL pri-
mers. The synthetic catecholamine polymers adsorb to virtually all surfaces
and can serve as a platform for LbL assembly in a surface-independent
fashion. 22 Besides, click chemistry refers to a set of covalent reactions with
high reaction yields that can be performed under extremely mild conditions.
This technique provides a simple and general method for the assembly of