Encapsulation of microalgae in silica and their potential use as bioreactors for the synthesis of metallic nanoparticles

CECILIA SPEDALIERI; MATÍAS JOBBÁGY; SARA A. BILMES

Madrid

Congreso; XVII International Sol-Gel Conference; 2013

International Sol-Gel Society

Encapsulation of microorganisms in silica sol-gel matrices provides an interesting approach towards developing bioactive hybrid materials. The optical and mechanical properties of the inorganic matrix along with the capability of microorganisms for releasing biomolecules or modifying their surroundings can provide many novel characteristics to this type of material. Moreover, the capability of several microorganisms of biosynthesizing metallic nanoparticles enables the possibility of developing bioreactors of nanoentites with controlled size and shape. In this work we present the biosynthesis of gold nanoparticles by microalgae -mainly Chlorella vulgaris- confined in cavities within a silica matrix. The encapsulation methodology consists in a two-step process in which the algae are fixed in a crosslinked alginate beads, and the SiO2 matrix is then formed around the bead by a sol-gel. Once the gel is formed a citrate solution is added to dissolve the calcium alginate, generating a small cavity where cells can grow. Two sol-gel routes were employed: (i) aqueous pathway, using sodium silicate and Ludox, and (ii) from TEOS precursor. In both cases the synthesis conditions were adjusted in order to minimize the cellular stress and to provide optimized transport of the biosynthesized nanoparticles. Gels obtained from TEOS gave better optical properties, which is fundamental for the growth of photosynthetic microalgae inside the cavities. The biosynthesis of gold nanoparticles by the encapsulated microalgae from gold cations was made by adding Au precursors either in the pre-encapsulation process or after the gel is formed. The formation of alginate beads using HAuCl4 as the cation precursor produced biosynthesized 10-15nm Au NPs, demonstrating bioavailability of gold cations which do not participate in the cross-linking of alginate. On the other hand, Au NPs are also biosynthesized when HAuCl4 solution added on top of gels with algae encapsulated in alginate beads crosslinked with Ca2+ or H+. The production of Au NPs by the encapsulated algae is comparable to that obtained by the free species. In both cases, NPs are produced inside the cell and further expulsed to the external membrane. The matrix structure ?which in turn is given by the synthesis conditions-, has strong influence in the diffusion of AuCl4 - to the encapsulated cells, as well as in the transport of biosynthesized NPs from the cell containing cavities.