Chemical Transformations of Nanosilver in Biological Environments

Abstract

The widespread use of silver nanoparticles (Ag-NPs) in consumer and medical products provides strong motivation for a careful assessment of their environmental and human health risks. Recent studies have shown that Ag-NPs released to the natural environment undergo profound chemical transformations that can affect silver bioavailability, toxicity, and risk. Less is known about Ag-NP chemical transformations in biological systems, though the medical literature clearly reports that chronic silver ingestion produces argyrial deposits consisting of silver-, sulfur-, and selenium-containing particulate phases. Here we show that Ag-NPs undergo a rich set of biochemical transformations, including accelerated oxidative dissolution in gastric acid, thiol binding and exchange, photoreduction of thiol- or protein-bound silver to secondary zero-valent Ag-NPs, and rapid reactions between silver surfaces and reduced selenium species. Selenide is also observed to rapidly exchange with sulfide in preformed Ag2S solid phases. The combined results allow us to propose a conceptual model for Ag-NP transformation pathways in the human body. In this model, argyrial silver deposits are not translocated engineered Ag-NPs, but rather secondary particles formed by partial dissolution in the GI tract followed by ion uptake, systemic circulation as organo-Ag complexes and immobilization as zero-valent Ag-NPs by photoreduction in light affected skin regions. The secondary Ag-NPs then undergo detoxifying transformations into sulfides, and further into selenides or Se/S mixed phases through exchange reactions. The formation of secondary particles in biological environments implies that Ag-NPs are not only a product of industrial nanotechnology, but have long been present in the human body following exposure to more traditional chemical forms of silver.

Silver Nanoparticles Induced RNA Polymerase-Silver Binding and RNA Transcription Inhibition in Erythroid Progenitor Cells

“…For example, a recent study by George et al showed that Ag nanoplate had less cellular uptake by cells than spherical nAg particles. 20 In agreement with this finding, we observed greater inhibitory effect on R-globin and β-globin expression for nAg in spherical shape (i.e., 10,25,40, and 110 nm spherical nAg particles), compared to nAg in plate shape ( Figure S1, P < 0.05). Furthermore, the size of nAg is also a crucial determinant for its bioavailability, and nAg particles with smaller size often revealed greater uptake by cells than those with larger size.…”

Selenium biochemistry and its role for human health

“…Theoretically, many metal cations may form insoluble colloids or complexes with selenide, as has been observed in yeast cultures, 299 but only silver (Ag) has been documented to accumulate in mammalian cells in this way. 300 Conversely, Se is generally associated to a decreased bioaccumulation of arsenic (As) and cadmium (Cd), so that excretion mediated by conjugation of these elements with organic Se-species may be gathered as the favoured route. 298 The antioxidant action of Se-proteins mitigates the cellular damage induced by metal-generated ROS.…”

“…The biological transformation of Ag nanoparticles has been proposed to occur by means of gastric dissolution and ions absorption, circulatory thiol transport, photoreduction to secondary Ag 0 particles and superficial sulfidation. 300 Reduced Se-species have also been shown to react with the surface of Ag nanoparticles. Kinetic and thermodynamic evidence support the hypothesis that Se cannot compete with the initial sulfidation, but Se/S exchange reactions occur afterwards, leading to the formation of Ag/S/Se particulate deposits in tissues.…”

“…Kinetic and thermodynamic evidence support the hypothesis that Se cannot compete with the initial sulfidation, but Se/S exchange reactions occur afterwards, leading to the formation of Ag/S/Se particulate deposits in tissues. 300 Further studies are strongly needed to shed light on the exact metabolic pathways through which individual selenospecies influence the biological transformation and toxicity of heavy metals in vivo.…”

Impact of Sulfidation of Silver Nanoparticles on Established&lt;i&gt; P. aeruginosa Biofilm&lt;/i&gt;

“…Thus, the fate, transport and toxicity of transformed Ag-NPs in biofilms should be considered alongside studies that investigate the pristine form of the nanoparticles. A number of studies reveal that the prevalent route of Ag-NPs transformation in biological media is sulfidation [22] [23] [24]. The surface oxidized silver ions react with inorganic sulfide to form Ag 2 S, which eventually transforms into Ag 2 S-NPs [23].…”