We demonstrated that embedding AgNPs efficiently reduces the interactions with the buffered water media, protecting the AgNPs from fast oxidation. This study also reveals that the release of bio-available silver (impacting on the algal photosynthesis) is controlled by the depth at which AgNPs are located for a given host matrix see (figure below), i.e., the silica cover layer. The release of the Ag+ ions, at the origin of the biocidal effect, can be completely annihilated when the thickness of the silica layer is greater than 7 nm.
The toxicity of silver released from these nanocomposites to algal photosynthesis is comparable to similar concentrations of Ag+ released from AgNO3. To reveal the origin of the observed silver toxicity to algae (silver ions Ag+ and/or AgNPs), we performed fluorometry measurements of solutions in presence of cysteine, known as a strong silver ligand. Addition of cysteine abolishes the Ag+ toxicity to the algal photosynthesis. This finding confirms that all the toxicity exerted by the suspension after contact with our samples with embedded AgNPs is due to the presence of Ag+ in the solution. The TEM observations after immersion in water indicate that the Ag release is accompanied by shrinkage of the AgNPs that are located in the immediate proximity of the free surface while the AgNPs density remains unchanged, suggesting that these AgNPs are the source of the ionic silver released into the solution.
Initial Ag reservoir (grey bar) and released silver (green bar) and associated cross-sectional TEM images for different samples with the AgNPs with (a) and (b) decreasing distances from the free surface and, (c) and (d) increasing Ag+ reservoir.
Assessing bio-available silver released from silver nanoparticles embedded in silica layers using the green algae Chlamydomonas- reinhardtii as bio-sensors, A. Pugliara, K. Makasheva, B. Despax, M. Bayle, R. Carles, P. Benzo, G. BenAssayag, B. Pécassou, M. Sancho, E. Navarro, Y. Echegoyen and C. Bonafos, Science of the Total Environment 565, 863–871 (2016).