Silver Nanoparticles: Technological Advances, Societal Impacts, and Metrological Challenges

The applications of copper (Cu) and Cu-based nanoparticles, which are based on the earth-abundant and inexpensive copper metal, have generated a great deal of interest in recent years, especially in the field of catalysis. The possible modification of the chemical and physical properties of these nanoparticles using different synthetic strategies and conditions and/or via postsynthetic chemical treatments has been largely responsible for the rapid growth of interest in these nanomaterials and their applications in catalysis. In addition, the design and development of novel support and/or multimetallic systems (e.g., alloys, etc.) has also made significant contributions to the field. In this comprehensive review, we report different synthetic approaches to Cu and Cu-based nanoparticles (metallic copper, copper oxides, and hybrid copper nanostructures) and copper nanoparticles immobilized into or supported on various support materials (SiO2, magnetic support materials, etc.), along with their applications in catalysis. The synthesis part discusses numerous preparative protocols for Cu and Cu-based nanoparticles, whereas the application sections describe their utility as catalysts, including electrocatalysis, photocatalysis, and gas-phase catalysis. We believe this critical appraisal will provide necessary background information to further advance the applications of Cu-based nanostructured materials in catalysis.

The rapid advancement of nanotechnology has created a vast array of engineered nanomaterials (ENMs) which have unique physical (size, shape, crystallinity, surface charge) and chemical (surface coating, elemental composition and solubility) attributes. These physicochemical properties of ENMs can produce chemical conditions to induce a pro-oxidant environment in the cells, causing an imbalanced cellular energy system dependent on redox potential and thereby leading to adverse biological consequences, ranging from the initiation of inflammatory pathways through to cell death. The present study was designed to evaluate size-dependent cellular interactions of known biologically active silver nanoparticles (NPs, Ag-15 nm, Ag-30 nm, and Ag-55 nm). Alveolar macrophages provide the first defense and were studied for their potential role in initiating oxidative stress. Cell exposure produced morphologically abnormal sizes and adherence characteristics with significant NP uptake at high doses after 24 h. Toxicity evaluations using mitochondrial and cell membrane viability along with reactive oxygen species (ROS) were performed. After 24 h of exposure, viability metrics significantly decreased with increasing dose (10-75 microg/mL) of Ag-15 nm and Ag-30 nm NPs. A more than 10-fold increase of ROS levels in cells exposed to 50 microg/mL Ag-15 nm suggests that the cytotoxicity of Ag-15 nm is likely to be mediated through oxidative stress. In addition, activation of the release of traditional inflammatory mediators were examined by measuring levels of cytokines/chemokines, including tumor necrosis factor (TNF-alpha), macrophage inhibitory protein (MIP-2), and interleukin-6 (IL-6), released into the culture media. After 24 h of exposure to Ag-15 nm nanoparticles, a significant inflammatory response was observed by the release of TNF-alpha, MIP-2, and IL-1beta. However, there was no detectable level of IL-6 upon exposure to silver nanoparticles. In summary, a size-dependent toxicity was produced by silver nanoparticles, and one predominant mechanism of toxicity was found to be largely mediated through oxidative stress.