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Stensberg et al. Page 7 In summary, most of the information available on the mechanisms of toxicity and associated effects of Ag NPs comes from in vitro studies, with only limited information from in vivo studies. Three main mechanisms of toxicity of Ag NPS have been proposed: oxidative stress, DNA damage and cytokine induction. Results from in vivo studies have shown that exposure of Ag NPs can result in effects in different major organs. It is important to mention that the studies summarized here used different formulations of Ag NPs (most were generated in the laboratory and some were purchased commercially). Very few studies have evaluated the mechanisms and associated toxicity effects of Ag NPs ‘leached’ from current commercial products. Physicochemical assessment techniques The physicochemical properties of Ag NPs are relevant to their toxicology; namely size, shape and surface chemistry. For example, a number of studies have correlated the size and shape of Ag NPs with their bactericidal properties [17,23,90,91]. A variety of methods are available to quantify these properties, several of which are summarized in Table 1; other approaches have been described in recent articles and reviews [92,93]. It has been noted, however, that while the physicochemical characterization of Ag NPs is necessary for a comprehensive ana lysis of its biological uptake and interactions with cells, such measurements are not sufficient for predicting nanotoxicological effects [94]. Indeed, this issue should be viewed in the opposite direction: toxicological studies are critical for correlating the physicochemical properties of nanoparticles and their interactions with living systems. Transmission electron microscopy Transmission electron microscopy (TEM) is the most common method of characterizing NP size and shape [91,95]. TEM images of NPs are typically acquired in brightfield mode, based on the contrast generated by electron scattering from heavy atoms, then subjected to image ana lysis software to obtain statistical distributions in particle size and ellipticities. Individual NPs can also be assessed for uniformity in shape or lack of thereof [17,96]. However, TEM ana lysis is limited by sample size (typically <200 particles) and also by variability in sample preparation methods, which are typically performed by casting and drying droplets of NPs in solution onto polymer-coated grids. This practice can skew the true size distribution of particles in suspension, as well as their state of agglomeration [97]. For these reasons, size ana lysis by TEM is best conducted in conjunction with other methods that c haracterize particles in their equilibrium states. Dynamic light scattering Dynamic light scattering (DLS) is a method of particle size ana lysis based on light scattering and the Brownian motion of particles in solution [91,95,98]. Whereas TEM defines particle size by differences in electron scattering, DLS measures the hydrodynamic radius of particles based on their rates of translational diffusion. DLS measurements will often produce larger values than those obtained by TEM, because they include the influence of the organic surface coating in the size estimates and the particle size distribution includes aggregates as well as individually dispersed particles. For these reasons, DLS is considered by many to be a more accurate estimate of the effective size of particles in solution [99,100]. However, DLS also has some drawbacks: the values are dependent on particle concentration, due to its sensitivity to aggregation effects, and it cannot provide an accurate assessment of particle shape without considerable parameterization. Furthermore, nonspherical particles are subject to additional motional behaviors that are easily misinterpreted. For example, DLS measurements of monodispersed gold nanorods can Nanomedicine (Lond). Author manuscript; available in PMC 2012 May 24. NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptPDF Image | Toxicological studies on silver nanoparticles
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