While possible analogies between peptide-mediated biosilica formation and alkylammonium cation templated zeolite crystallization have been acknowledged for some time, only recently have we identified the formation of nanoparticles as small as 4 nm in diameter, similar to those in TPA-silica systems, in basic amino acid sols.¹ Much like the alkylammonium-silica system, nanoparticle formation via hydrolysis of tetraethylorthosilicate (TEOS) in aqueous lysine solutions (Lys-Sil sols) occurs rapidly once the silica solubility limit is exceeded. Until only recently^1,2 the paradigm for silica nanoparticle formation was limited to hydrolysis of tetraethylorthosilicate (TEOS) in highly basic solutions in the presence of tetrapropylammonium (TPA) cations. The conditions of the lysine-mediated silica nanoparticle synthesis (i.e., more neutral pH) in comparison to the highly basic ones of the TPA-silica system open new avenues for more benign synthesis of many current and novel biocompatible silica-based materials.

We will discuss how the novelty of the Lys-Sil approach is enhanced by several nanoparticle characteristics, including 1) their narrow size distribution, 2) the tunability of their size (i.e., 4-20 nm) through adjustment of synthesis conditions (e.g., pH, silica content, temperature), and 3) their facile assembly into nanoparticle-crystals.³ Each has interesting implications in technologies like, for example, coatings and nanolithography, where tuned particle size, porosity, order, and mono- and multilayer continuity are critical. Moreover, exciting applications in the biological arena become more feasible as a result of the benign Lys-Sil synthesis conditions. As an example application, we will show how further neutralization of the lysine-silica sols by addition of compatible peptide oligomers (e.g., di-lysine) can lead to the formation of clear silica gels with tunable porosity and morphology. We will also show how these porous gels can be employed directly for cell encapsulation, harnessing the tunable gel porosity to facilitate nutrient regulation and to promote cell viability. This example serves as proof-of-concept for realizing a theoretically implantable matrix of “cellular reactors” capable of producing a metabolite of interest while being protected from immunological attack.

References

1. Davis, T. M.; Snyder, M. A.; Krohn, J. E.; Tsapatsis, M., Chemistry of Materials 2006, 18, 5814.

2. Yokoi, T.; Sakamoto, Y.; Terasaki, O.; Kubota, Y.; Okubo, T.; Tatsumi, T., Journal of the American Chemical Society 2006, 128, 13664.

3. Snyder, M. A.; Lee, J. A.; Davis, T. M.; Scriven, L. E.; Tsapatsis, M., 2007, Submitted.