Elucidating the Role of Dissolved Organic Matter and Sunlight in Mediating the Formation of Ag-Au Bimetallic Alloy Nanoparticles in the Aquatic Environment

Catalytic and biological properties of Ag-Pt bimetallic nanoparticles: composition-dependent activity and cytotoxicity

Due to their unique elemental compositions and interface coupling effects, bimetallic nanoparticles (BNPs), a class of nanoalloys, have attracted significant attention for applications in biomedicine, environmental remediation, and catalysis. BNPs, formed via the combination of two metal ions under light or thermal conditions, exhibit enhanced catalytic properties due to synergistic interactions between constituent metals, which result in optimized electronic structures, increased active sites, and reduced activation energy for catalytic reactions. However, BNPs may pose potential toxicity risks to organisms through bioaccumulation and environmental exposure. In this study, Ag-Pt nanoparticles (AP NPs) with varying molar ratios were synthesized and characterized to elucidate the relationship between composition, catalytic activity, and cytotoxicity. Catalytic assays revealed that AP NPs exhibited remarkable oxidase-like activity. Cytotoxicity tests revealed dose- and composition-dependent effects, with the AP55 (Ag : Pt at 5 : 5 ratio) exhibiting the highest cytotoxicity compared to monometallic counterparts at equivalent concentrations. Notably, the proportion of Ag in the AP NPs was identified as the dominant factor influencing catalytic activity and cytotoxicity. Mechanistic investigations attributed this cytotoxicity to the interplay of peroxidase-like catalytic activity, oxidative stress, and lysosomal ion release, disrupting cellular redox homeostasis and triggering apoptosis. Enzymatic assays further confirmed reductions in antioxidant defenses, including superoxide dismutase (SOD) and catalase (CAT) activities, amplifying reactive oxygen species (ROS) generation and oxidative damage. These findings underscore the critical role of catalytic behavior in mediating biological interactions and cytotoxic effects of BNPs. We establish a relationship between composition, oxidase-like activity, and cytotoxicity, providing insights into their potential biomedical applications and paving the way for the rational design of multifunctional nanomaterials with tunable biological effects.

Enhanced adsorption of aqueous perfluorooctanoic acid on iron-functionalized biochar: elucidating the roles of inner-sphere complexation

Perfluorooctanoic acid (PFOA) is ubiquitously detected in various water bodies, which raises the urgent need for developing effective and economic remediation methods in response to its health risks. The adsorptive removal of PFOA by biochar (BC) is regarded as a simple, effective, and economical technique. However, engineered BCs, including FeCl3-activated BC, for PFOA removal and adsorption mechanisms have been ill-studied. In this study, a FeCl3-activated dairy manure-derived biochar (Fe@MBC) was prepared via one-step pyrolysis/activation, and its properties and adsorption characteristics were compared with a pristine manure-derived biochar (P-MBC). The FeCl3 activation largely increased the surface area of Fe@MBC and the deposition of FexOy minerals on surface of Fe@MBC while significantly elevating the surface roughness of Fe@MBC. The maximum adsorption capacity of Fe@MBC for PFOA (233 mg·g-1) was five times higher than that of P-MBC (46 mg·g-1). PFOA adsorption was favorable at low solution pH and was independent on ionic strength, which supported the major contribution by inner-sphere complexation rather than out-sphere complexation. This mechanism was further confirmed by the disappearance of FeO peak on Fourier transform infrared spectrum and the blue-shift of Fe binding energies on X-ray photoelectron Fe 2p spectrum of Fe@MBC after PFOA adsorption. Fe@MBC maintained a near 100% adsorption capacity for PFOA after 4 cycles of chemical regeneration. Fe@MBC also exhibited efficient removal for PFOA and other PFAS compounds at trace levels in the lake water and wastewater treatment plant effluent. Thus, this study highlights a promising insight for selectively eliminating PFASs from water.

Environmental behavior of silver nanomaterials in aquatic environments: An updated review

The increasing applications of silver nanomaterials (nano-Ag) and their inevitable release posed great potential risks to aquatic organisms and ecosystems. Considerable attention has been attracted on their behaviors and transformations, which were critically important for their subsequent biological toxicities and ecological effects. Therefore, the summary of the recent efforts on the environmental behavior of nano-Ag would be beneficial for understanding the environmental fate and accurate risk assessment. This review summarized the studies on various physical, chemical and biological transformations of nano-Ag, meanwhile, the influencing factors (including the intrinsic properties and environmental conditions) and related mechanisms were highlighted. Surface structure and facets of nano-Ag, abiotic conditions and natural freeze-thaw cycle processes could affect the transformations of nano-Ag under different environmental scenarios (including freshwater, seawater and wastewater). The interactions with co-present components, such as chemicals and other particles, impacted the multiple processes of nano-Ag. Besides, the contradictory effects and mechanisms by several environmental factors were summarized. Lastly, the key knowledge gaps and some aspects that deserve further investigation were also addressed. Therefore, the current review aimed to provide an overall analysis of transformation processes of nano-Ag, which will provide more available information and pave the way for the future research areas.

Novel insights into the multistep chlorination of silver nanoparticles in aquatic environments

Due to the increasing applications, silver nanoparticles (AgNPs) are inevitably released into the environments and are subjected to various transformations. Chloride ion (Cl^-) is a common and abundant anion with a wide range of concentration in aquatic environments and exhibits a strong affinity for silver. The results indicate that AgNPs experienced multistep chlorination, which was dependent on the concentration of Cl^- in a non-linear manner. The dissolution of AgNPs was accelerated at Cl/Ag ratio of 1 and the intensive etching effect of Cl^- contributed to the significant morphology changes of AgNPs. The dissolved Ag⁺ quickly precipitated with Cl^- to form an amorphous and passivating AgCl(s) layer on the surface of AgNPs, thus the dissolution rate of AgNPs decreased at higher Cl/Ag ratios (100 and 1000). As the Cl/Ag ratio further increased to 10,000, the overall transformation rate increased remarkably due to the complexation of Cl^- with AgCl(s) to form soluble AgCl_x^(x-1)- species, which was verified by the reaction of AgCl nanoparticles with Cl^-. Besides, several environmental factors (electrolytes, surfactants and natural organic matter) affected AgNPs dissolution and the following chlorination. These results will expand the understanding of the environmental fate and potential risks of AgNPs in natural chloride-rich waters.

Ambivalent effects of dissolved organic matter on silver nanoparticles/silver ions transformation: A review

The numerous applications of silver nanoparticles (AgNPs) lead to their spread in aquatic systems and the release of silver ions (Ag⁺), which brings potential risks to environment and human health. Owing to the different toxicity, the mutual transformations between AgNPs and Ag⁺ has been a hot topic of research. Dissolved organic matter (DOM) is ubiquitous on the earth and almost participates in all the reactions in the nature. The previous studies have reported the roles of DOM played in the transformation between AgNPs and Ag⁺. However, different experiment conditions commonly caused contradictory results, leading to the difficulty to predict the fate of AgNPs in specific reactions. Here we summarized mechanisms of DOM-mediated AgNPs oxidation and Ag⁺ reduction, and analyzed the effects of environmental parameters. Moreover, the knowledge gaps, challenges, and new opportunities for research in this field are discussed. This review will promote the understanding of the fate and risk assessments of AgNPs in natural water systems.

Reduction of Ionic Silver by Sulfur Dioxide as a Source of Silver Nanoparticles in the Environment

The natural formation of silver nanoparticles (AgNPs) via biotic and abiotic pathways in water and soil media contributes to the biogeochemical cycle of silver metal in the environment. However, the formation of AgNPs in the atmosphere has not been reported. Here, we describe a previously unreported source of AgNPs via the reduction of Ag(I) by SO₂ in the atmosphere, especially in moist environments, using multipronged advanced analytical and surface techniques. The rapid reduction of Ag(I) in the atmospheric aqueous phase was mainly caused by the sulfite ions formed from the dissolution of SO₂ in water, which contributed to the formation of AgNPs and was consistent with the Finke-Watzky model with a major contribution of the reduction-nucleation process. Sunlight irradiation excited SO₂ to form triplet SO₂, which reacted with water to form H₂SO₃ and greatly enhanced Ag(I) reduction and AgNP formation. Different pH values affected the speciation of Ag(I) and S(IV), which were jointly involved in the reduction of Ag(I). The formation of AgNPs was also observed in the atmospheric gas phase via direct reduction of Ag(I) by SO₂(gas), which occurred even in 50 ppbv SO₂(gas). The natural occurrence of AgNPs in the atmosphere may also be involved in silver corrosion, AgNP transformation and regeneration, detoxification of gaseous pollutants, and the sulfur cycle in the environment.