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].