Toxicity of Silver Nanoparticles Increases during Storage Because of Slow Dissolution under Release of Silver Ions

Abstract

The dissolution of citrate-stabilized and poly(vinylpyrrolidone)-stabilized silver nanoparticles in water was studied by dialysis for up to 125 days at 5, 25, and 37 °C. The particles slowly dissolve into ions on a time scale of several days. However, in all cases, a limiting value of the released silver was observed, i.e., the particles did not completely dissolve. In some cases, the nanoparticles released up to 90% of their weight. Formal kinetic data were computed. Rate and degree of dissolution depended on the functionalization as well as on the storage temperature. The release of silver led to a considerably increased toxicity of silver nanoparticles which had been stored in dispersion for several weeks toward human mesenchymal stem cells due to the increased concentration of silver ions. Consequently, "aged" (i.e., immersed) silver nanoparticles are much more toxic to cells than freshly prepared silver nanoparticles.

Controlled Evaluation of Silver Nanoparticle Dissolution Using Atomic Force Microscopy

“…The rate constants increased linearly between 0.01 and 0.10 d -1 over the 10-550 mM NaCl range. These first-order rate constants are lower than rate constants reported in previous studies on AgNP dissolution, which is contrary to expectations because the catalytic effect of NaCl is absent from the previous studies; [18][19][20] however, mass-based rate constants are not always comparable for surface-mediated reactions.…”

“…It has also been reported that AgNPs achieve a pseudo-equilibrium that is dependent on the initial AgNP concentration and size, a hypothesis explained by citing the size-dependent thermodynamic properties of AgNPs, 19,20 and that mass-based AgNP dissolution rate constants decreased as the concentration of AgNPs increased. 18,19 Both these phenomena could be caused by the depletion of molecular oxygen and hydrogen ions; 18,20 however, this possibility does not appear to hold in at least one case where these trends are apparent. 19 Another interpretation is that aggregation and dissolution act as competing processes, both of which depend on initial nanoparticle concentration and size.…”

“…AgNPs can be re-adsorbed to the particle surface, 13 12,19,20 Another study showed that AgNPs in the size range 5-46 nm were small enough to diffuse through chorion pore canals of zebrafish embryos, which may lead to a stronger toxic effect than what would be observed for particles that are too large to enter the embryos in a similar manner. 21 Furthermore, polystyrene particles in the nanometer size range have been shown to have a longer retention time in the gut of two suspension-feeding bivalves than micron-sized particles, suggesting that smaller particles may have more potential to bioaccumulate than larger particles.…”

“…Kittler et al observed that poly(vinylpyrrolidone) (PVP)-coated AgNPs dissolved more rapidly than citrate-coated AgNPs, but no explanation for this result was given. 20 PVP stabilizes AgNPs against aggregation more effectively than citrate, 42 so the increased dissolution rate of PVP-coated AgNPs could be due to decreased aggregation. It has also been reported that AgNPs achieve a pseudo-equilibrium that is dependent on the initial AgNP concentration and size, a hypothesis explained by citing the size-dependent thermodynamic properties of AgNPs, 19,20 and that mass-based AgNP dissolution rate constants decreased as the concentration of AgNPs increased.…”

Silver Release from Silver Nanoparticles in Natural Waters

“…To our knowledge, only three studies evaluated AgNP dissolution over a time course of several months. 12,13,29 The first study predicts that AgNPs will not persist in an oxic solution and will undergo complete oxidative dissolution. 12 Thermodynamic calculations suggest that the oxidative dissolution of AgNPs should proceed to completion under oxic conditions.…”