4.2 Coating uniformity
Nanomaterial properties are controlled in part by the coating uniformity
or smoothness of the surface of metal deposited on biotemplate. This
uniformity relies on the molecular interactions between the deposited
metal and residues on the surface or in the channel of the biotemplate.
Coating agents such as citric acid are added to the deposition reaction
to alter the charge of surface-exposed residues and increase
metal-biotemplate interactions during the metal deposition process
[39, 68, 69]. However, these pH-sensitive electrostatic interactions
can be disrupted by other reagents of the deposition process leading to
poor nanoparticle synthesis and nonuniform coating.
To address this challenge, covalent bonds that are resistant to modest
pH changes can be introduced to drive nanoparticle synthesis. Site
specific mutagenesis of the capsid protein to introduce cysteine and
lysine residues has been demonstrated to improve nanoparticle synthesis
[30, 70]. These residues may be introduced either on the surface or
inner channel of TMV VLPs (e.g. S3C, T103C, T158K) or on either terminus
of the CP [70–72]. These modifications allow for metal deposition
under a wider range of pHs and promote high density nucleation sites for
nanoparticle synthesis, unlike wildtype whose nanoparticle synthesis is
driven by weaker surface drying effects. For example, two cysteine
residues have been genetically added to the amino-terminus of TMV CP,
resulting in 4260 available thiol groups per VLP for metal reduction and
deposition [30]. The introduced thiol groups on the TMV surface
serve as a reaction site for nanomaterials such as gold and palladium to
reduce metal ions or form covalent bonds with non-metal materials via
thiol coupling. Cysteine enhances the deposition of several metals
including gold, silver, and palladium, and also enables novel
attachments for device manufacture (e.g. nanoforests for battery anode
synthesis) [30]. Lysine substitution has also served as a functional
group for chemical conjugation of silica coatings [70].