Roles of Silver-Chloride Complexations in Sunlight-Driven Formation of Silver Nanoparticles

Synthesis and Characterization of Lignin-Silver Nanoparticles

Metal nanoparticle synthesis via environmentally friendly methods is gaining interest for their potential advantages over conventional physico-chemical approaches. Herein, we propose a robust green synthesis route for lignin-modified silver nanoparticles, utilizing the recovery of lignin as a renewable raw material and exploring its application in valuable areas. Through a systematic approach combining UV-Vis spectroscopy with AAS and DLS, we identified repeatable and scalable reaction conditions in an aqueous solution at pH 11 for homogeneous silver nanoparticles with high uniformity. The TEM median sizes ranged from 12 to 15 nm with circularity between 0.985 and 0.993. The silver nanoparticles yield exceeded 0.010 mol L-1, comparable with traditional physico-chemical methods, with a minimal loss of silver precursor ranging between 0.5 and 3.9%. Characterization by XRD and XPS revealed the presence of Ag-O bonding involving lignin functional groups on the pure face-centered cubic structure of metallic silver. Moreover, the lignin-modified silver nanoparticles generated a localized thermal effect upon near-infrared laser irradiation (808 nm), potentially allowing for targeted applications in the biomedical field. Our study showcases the potential of lignin as a renewable reducing and capping agent for silver nanoparticle synthesis, addressing some shortcomings of green synthesis approaches and contributing to the development of suitable nanomaterials.

Improving the sensing sensitivity of silver nanoparticle-based colorimetric biosensors from the point of salt

The sensing sensitivity was improved for silver nanoparticles (AgNPs)-based colorimetric biosensors by using the most suitable salt to induce AgNPs aggregation. As for the salt composed of low-affinity anion and monovalent cation, the cation-dependent charge screening effect was the driving force for AgNPs aggregation. Apart from the charge screening effect, both the bridging of multivalent cation to the surface ligand of AgNP and the interaction between anion and Ag contributed to inducing AgNPs aggregation. Considering the higher aggregation efficiency of AgNPs resulted in a narrower sensing range, salt composed of low-affinity anion and monovalent cation was recommended for AgNPs-based colorimetric analysis, which was confirmed by fourfold higher sensitivity of DNA-21 detection using NaF than NaCl. This work inspires further thinking on improving the sensing performance of metal nanomaterials-based sensors from the point of colloidal surface science.

Development of mechanistic models for the description of radionuclide retention onto calcareous gravels

The retention capacity of natural calcareous gravels used as a filling material between the packages containing radioactive waste materials in the disposal cells was evaluated for Sr, U, Ni, Ag, 14C, 99Tc, 239Pu, 228Th and 152Eu. The thermodynamic calculations performed considering repository conditions indicated that the chemistry of most of these elements was dominated by neutral or cationic species, this is the case of 238Pu 152Eu, 228Th, Ni and Ag. The studied gravels presented high sorption capacities for these cationic or neutral species, but they are not efficient on retaining anionic species as in the case of 99Tc or Sr, which is already present in the composition of the studied gravels. For those elements where the predominant species are carbonated (14C and U) low distribution coefficients were obtained. A preliminary mechanistic sorption model was developed for each radionuclide considering carbonate (>CO3H) and calcium hydroxide (>CaOH) sites. Our preliminary model allowed to successfully reproduce the experimental trend of the data obtained in this work.

Unveiling the Role of Precursors in the Byproduct Formation of AgCl-Replicated Bimetallic Nanostructures and Their Stability-Dependent Photothermal Properties

AgCl nanomaterials recently attracted scientific interest as useful structural building blocks for producing metallic nanomaterials owing to their facile synthesis, controllable morphology, and ease of removal under ambient conditions. However, their complex chemical reactivity has primarily been studied in association with water solubility or reducibility. This study investigates the pivotal role of precursor ligands in the photochemical synthesis of metallic cubic mesh nanostructures on the AgCl templates. The side reactions between AgCl and Au precursors with different ligands are thoroughly discussed along with their influence on the byproduct formation and the structural stability of the resulting metallic nanostructures. Importantly, we introduce for the first time the partial destruction of AgCl and the formation of undesirable byproducts caused by the presence of highly oxidizing and Cl-containing AuCl₄^-. In addition, a synthetic route for producing highly pure and stable metallic nanostructures using a halogen-free Au precursor or Pt-priming is proposed. Further, the photothermal properties of these replicated metallic nanostructures are presented as a new evaluation tool for analyzing their overall structural stability. Discovering the role of precursor ligands in the reaction system will prove useful as a guide for the synthesis of functional noble metal nanomaterials using silver halide templates.

Nanoscale Hydrophobicity and Electrochemical Mapping Provides Insights into Facet Dependent Silver Nanoparticle Dissolution

Metal or metallic nanoparticle dissolution influences particle stability, reactivity, potential fate, and transport. This work investigated the dissolution behavior of silver nanoparticles (Ag NPs) in three different shapes (nanocube, nanorod, and octahedron). The hydrophobicity and electrochemical activity at the local surfaces of Ag NPs were both examined using atomic force microscopy (AFM) coupled with scanning electrochemical microscopy (AFM-SECM). The surface electrochemical activity of Ag NPs more significantly affected the dissolution than the local surface hydrophobicity did. Octahedron Ag NPs with dominant surface exposed facets of {111} dissolved faster than the other two kinds of Ag NPs. Density functional theory (DFT) calculation revealed that the {100} facet elicited greater affinities toward H₂O than the {111} facet. Thus, poly(vinylpyrrolidone) or PVP coating on the {100} facet is critical for stabilizing and prevent the {100} facet from dissolution. Finally, COMSOL simulations demonstrated consistent shape dependent dissolution as we observed experimentally.

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.

Antibacterial silver and gold complexes of imidazole and 1,2,4-triazole derived N-heterocyclic carbenes

A series of gold(I) (4a-4h, 5a-5b) and silver(I) (3a-3h) complexes of 1,2,4-triazolylidene and imidazolylidene based N-heterocyclic carbene ligands were prepared and the antibacterial activities of these complexes have been evaluated. The complexes were characterised using ¹H-NMR, ¹³C-NMR, HRMS and in the cases of 3a, 3c, 4b and 5b by X-ray crystallography. The gold(I) complexes with phenyl substituents (4a-4d) were found to have potent antibacterial activity against Gram-positive bacteria, with the complexes of the 1,2,4-triazolylidene ligands being more active (4c, MIC = 4-8 μg mL^-1 against Enterococcus faecium and 2 μg mL^-1 against Staphylococcus aureus) than the analogous imidazolylidene complexes 4a and 4b (4a, MIC = 64 μg mL^-1 against E. faecium and 2-4 μg mL^-1 against S. aureus). Two of the silver(I) complexes have promising antibacterial activity against Acinetobacter baumannii (3f, MIC = 2-4 μg mL^-1 and 3g, MIC = 2 μg mL^-1). Silver(I) complex 3f and gold(I) complex 4c were tested against multi-drug resistant bacterial strains and high levels of antibacterial activity were observed. The potential for antibacterial resistance to develop against these metal containing complexes was investigated and significantly, no resistance was observed upon continuous treatment, whilst resistance was developed against the widely used broad-spectrum antibiotic ciprofloxacin in the same bacterial strains, under the conditions tested. The solution and gas phase stabilities of the complexes have been investigated using a combination of ¹H-NMR, HRMS and detailed computational mechanistic studies were undertaken to gain insights into the possible decomposition reactions for silver complexes in aqueous solution.

Recent advances in responses of arbuscular mycorrhizal fungi - Plant symbiosis to engineered nanoparticles

The application of engineered nanomaterials (ENMs) is increasing in all walks of life, inevitably resulting in a high risk of ENMs entering the natural environment. Recent studies have demonstrated that phytoaccumulation of ENMs in the environment may be detrimental to plants to varying degrees. However, plants primarily assimilate ENMs through the roots, which are inevitably affected by rhizomicroorganisms. In this review, we focus on a group of common rhizomicroorganisms-arbuscular mycorrhizal fungi (AMF). These fungi contribute to ENMs immobilization and inhibition of phytoaccumulation, improvement of host plant growth and activation of systematic protection in response to excess ENMs stress. In present review, we summarize the biological responses of plants to ENMs and the modulatory mechanisms of AMF on the immobilization of ENMs in substrate-plant interfaces, and indirectly regulatory mechanisms of AMF on the deleterious effects of ENMs on host plants. In addition, the information of feedback of ENMs on mycorrhizal symbiosis and the prospects of future research on the fate and mechanism of phyto-toxicity of ENMs mediated by AMF in the environment are also addressed. In view of above, synergistic reaction of plants and AMF may prove to be a cost-effective and eco-friendly technology to bio-control potential ENMs contamination on a sustainable basis.

Evaluation of antibacterial activities of silver nanoparticles on culturability and cell viability of Escherichia coli

Nanoparticles released into the environment are attracting increasing concern because of their potential toxic effects. Conventional methods for assessing the toxicity of nanoparticles are usually confined to cultivable cells, but not applicable to viable but non-culturable (VBNC) cells. However, it remains unknown whether silver nanoparticles (AgNPs), a typical antimicrobial agent, could induce bacteria into a VBNC state in natural environments. In this work, the viability of E. coli, an indicator bacterium widely used for assessing the antibacterial activity of AgNPs, was examined through coupling plate counting, fluorescence staining and adenosine triphosphate (ATP) production. AgNPs were found to have a considerable antibacterial ability, which resulted in less than 0.0004% of culturable cells on plates. However, more than 80% of the cells still maintained their cell membrane integrity under the stress of 80 mg/L AgNPs. Meanwhile, the residue of ATP production (0.6%) was 1500 times higher than that of the culturable cells (< 0.0004%). These results clearly demonstrate that when exposed to AgNPs, most of cells fell into a VBNC state, instead of dying. Environmental factors, e.g., Cl^- and illumination, which could change the dissolution, hydrophilicity and zeta potential of AgNPs, eventually influenced the culturability of E. coli. Inhibition of dissolved Ag⁺ and reactive oxygen species was found to facilitate the mitigation of the strain into a VBNC state. Our findings suggest the necessity of re-evaluating the environmental effects and antibacterial activities of AgNPs.

Silver nanomaterials: synthesis and (electro/photo) catalytic applications

In view of their unique characteristics and properties, silver nanomaterials (Ag NMs) have been used not only in the field of nanomedicine but also for diverse advanced catalytic technologies. In this comprehensive review, light is shed on general synthetic approaches encompassing chemical reduction, sonochemical, microwave, and thermal treatment among the preparative methods for the syntheses of Ag-based NMs and their catalytic applications. Additionally, some of the latest innovative approaches such as continuous flow integrated with MW and other benign approaches have been emphasized that ultimately pave the way for sustainability. Moreover, the potential applications of emerging Ag NMs, including sub nanomaterials and single atoms, in the field of liquid-phase catalysis, photocatalysis, and electrocatalysis as well as a positive role of Ag NMs in catalytic reactions are meticulously summarized. The scientific interest in the synthesis and applications of Ag NMs lies in the integrated benefits of their catalytic activity, selectivity, stability, and recovery. Therefore, the rise and journey of Ag NM-based catalysts will inspire a new generation of chemists to tailor and design robust catalysts that can effectively tackle major environmental challenges and help to replace noble metals in advanced catalytic applications. This overview concludes by providing future perspectives on the research into Ag NMs in the arena of electrocatalysis and photocatalysis.

Metabolic profiling of nanosilver toxicity in the gills of common carp

Studies have shown silver nanoparticles (AgNPs) exposure can result in a series of toxic effects in fish gills. However, it is still unclear how AgNPs affect metabolite expression and their related molecular metabolic pathways in fish gills. In this study, we employed untargeted metabolomics to study the effects of AgNPs and silver supernatant ions on fish gill metabolites. The results showed that AgNPs can induce significant changes in 96 differentially expressed metabolites, which mainly affect amino acid metabolism and energy metabolism in fish gills. Among these metabolites, AgNPs specifically induce significant changes in 72 differentially expressed metabolites, including L-histidine, L-isoleucine, L-phenylalanine, and citric acid. These metabolites were significantly enriched in the pathways of aminoacyl-tRNA biosynthesis, ABC transporters, and the citrate cycle. In contrast, Ag⁺ supernatant exposure can specifically induce significant changes in 14 differentially expressed metabolites that mainly interfere with sphingolipid metabolism in fish gills. These specifically regulated fish gill metabolites include sphinganine, sphingosine, and phytosphingosine, which were significantly enriched in the sphingolipid metabolism pathway. Our results clearly reveal the effects and potential toxicity mechanisms of AgNPs on fish gill metabolites. Furthermore, our study further determined the unique functions of released silver ions in AgNPs toxicity in fish gills.

Silver nanoparticles in aquatic sediments: Occurrence, chemical transformations, toxicity, and analytical methods

Sediments represent the major sink for released silver nanoparticles (AgNPs) in aquatic environments. It is well known that the environmental behavior and toxicity of AgNPs in sediments are governed by their specific chemical species instead of their total concentration. This review focuses on various chemical transformations of AgNPs in sediments, which have not been well outlined before. We first outline the concentrations of AgNPs in sediments. The predicted concentrations are 1-5 µg kg^-1 in most model studies. Once enter sediments, AgNPs are transformed to different species (e.g., Ag₂S, Ag-humic substance complexes, AgCl, and Ag⁺) during multiple chemical transformations, such as oxidative dissolution, sulfidation, chlorination, and complexation. Those chemical behaviors mitigate the toxicity of AgNPs by reducing their availability and decreasing Ag⁺ release. Benthic invertebrates and microbes are prone to be affected by AgNPs. AgNPs are found to be accumulated in sediment-dwelling organisms and transferred to higher trophic levels along the food web. Besides X-ray absorption spectroscopy, reliable separation procedures coupled with detection techniques, are powerful tools that characterize the speciation of AgNPs in sediments. More research is needed to investigate diverse chemical transformations in various sediments through development of novel techniques and mathematical models.

Combined impact of silver nanoparticles and chlorine on the cell integrity and toxin release of Microcystis aeruginosa

Silver nanoparticles (AgNPs) have shown to be toxic to freshwater cyanobacterial species, and sodium hypochlorite (NaOCl) is a common oxidant for the treatment of cyanobacterial cells. AgNPs have a high possibility of co-existing with the cyanobacterial cells in the aqueous environments leading to its exposure to NaOCl during water treatment; however, their combined effects on the cyanobacterial cells are largely undocumented. This work compares the individual and combined effect of AgNP and NaOCl on the integrity and toxin (microcystins) release of Microcystis aeruginosa at varying levels. The results show that the AgNP (0.2-0.6 mg/L) alone has negligible effects on the cell lysis, while NaOCl alone shows concentration-dependent (0.2 < 0.4 < 0.6 mg/L) rupturing of cells. In contrast, the AgNP + NaOCl (0.2-0.6 mg/L) samples show increasing loss in cell integrity at higher AgNP (0.4 and 0.6 mg/L) levels than the NaOCl only samples. NaOCl exposure results in increasing dissolution of AgNPs with time, releasing silver ions (Ag⁺), affecting its size and morphology. The cell-associated total Ag declines over time with an increase in NaOCl levels, maybe due to increasing cell-lysis or NaOCl induced oxidative dissolution of AgNPs. The cell-associated total Ag and released Ag⁺ possibly weaken the cellular membrane, thus assisting NaOCl in faster cell-lysis. The combined exposure of AgNP and NaOCl also results in a higher release of toxin from the cells. This work collectively reveals that the AgNPs combined with NaOCl can enhance the cell lysis and release of toxins.

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

Elucidating the interactions between metal ions and dissolved organic matter and deciphering mechanisms for their mineralization in the aquatic environment are central to understanding the speciation, transport, and toxicity of nanoparticles (NPs). Herein, we examine the interactions between Ag⁺ and Au³⁺ ions in mixed solutions (χ_Ag = 0.2, 0.5, and 0.8) in the presence of humic acids (HAs) under simulated sunlight; these conditions result in the formation of bimetallic Ag-Au NPs. A key distinction is that the obtained alloy NPs are compositionally and morphologically rather different from NPs obtained from thermally activated dark processes. Photoillumination triggers a distinctive plasmon-mediated process for HA-assisted reductive mineralization of ions to bimetallic alloy NPs which is not observed in its dark thermal reduction counterpart. The initial nucleation of bimetallic NPs is dominated by differences in the cohesive energies of Ag and Au crystal lattices, whereas the growth mechanisms are governed by the strongly preferred incorporation of Ag ions, which stems from their greater photoreactivity. The bimetallic NPs crystallize in shapes governed by the countervailing influence of minimizing free energy through the adoption of Wulff constructions and the energetic penalties associated with twin faults. As such, assessments of the stability and the potential toxic effects of bimetallic NPs arising from their possible existence in aquatic environments will depend sensitively on the origins of their formation.

Proteomic profiling reveals the differential toxic responses of gills of common carp exposed to nanosilver and silver nitrate

Although the toxic effects of silver nanoparticles (AgNPs) on fish gills have been reported, the underlying mechanism of toxicity remains unclear. The present study aimed to elucidate the mechanism of toxicity in the gills of common carp following exposure to AgNPs and silver nitrate (AgNO₃) using histopathology and proteomics. Histopathological findings revealed that both AgNPs and AgNO₃ caused telangiectasia and epithelial cell hyperplasia in fish gills; however, the pathological features and location of lesions caused by the two forms of silver were markedly different. Proteomics revealed that AgNPs and AgNO₃ induced 139 and 185 differential expression proteins (DEPs) in gills, respectively, and the two forms of silver induced only 42 shared proteins. AgNPs specifically induced 87 DEPs which mainly involved signaling mechanisms, cytoskeleton, and the arachidonic acid metabolism processes. AgNO₃ specifically induced 125 DEPs that were mainly clustered in the glutathione metabolism and protease processes. These results suggested that the toxic effects of AgNPs and AgNO₃ were dramatically different in terms of protein expression in fish gills, which may provide novel perspectives for understanding the toxicity mechanism of silver nanoparticles in fish gills.

A Review on the Environmental Fate Models for Predicting the Distribution of Engineered Nanomaterials in Surface Waters

Exposure assessment is a key component in the risk assessment of engineered nanomaterials (ENMs). While direct and quantitative measurements of ENMs in complex environmental matrices remain challenging, environmental fate models (EFMs) can be used alternatively for estimating ENMs' distributions in the environment. This review describes and assesses the development and capability of EFMs, focusing on surface waters. Our review finds that current engineered nanomaterial (ENM) exposure models can be largely classified into three types: material flow analysis models (MFAMs), multimedia compartmental models (MCMs), and spatial river/watershed models (SRWMs). MFAMs, which is already used to derive predicted environmental concentrations (PECs), can be used to estimate the releases of ENMs as inputs to EFMs. Both MCMs and SRWMs belong to EFMs. MCMs are spatially and/or temporally averaged models, which describe ENM fate processes as intermedia transfer of well-mixed environmental compartments. SRWMs are spatiotemporally resolved models, which consider the variability in watershed and/or stream hydrology, morphology, and sediment transport of river networks. As the foundation of EFMs, we also review the existing and emerging ENM fate processes and their inclusion in recent EFMs. We find that while ENM fate processes, such as heteroaggregation and dissolution, are commonly included in current EFMs, few models consider photoreaction and sulfidation, evaluation of the relative importance of fate processes, and the fate of weathered/transformed ENMs. We conclude the review by identifying the opportunities and challenges in using EFMs for ENMs.

Environmental effects of stratospheric ozone depletion, UV radiation and interactions with climate change: UNEP Environmental Effects Assessment Panel, update 2019

This assessment, by the United Nations Environment Programme (UNEP) Environmental Effects Assessment Panel (EEAP), one of three Panels informing the Parties to the Montreal Protocol, provides an update, since our previous extensive assessment (Photochem. Photobiol. Sci., 2019, 18, 595-828), of recent findings of current and projected interactive environmental effects of ultraviolet (UV) radiation, stratospheric ozone, and climate change. These effects include those on human health, air quality, terrestrial and aquatic ecosystems, biogeochemical cycles, and materials used in construction and other services. The present update evaluates further evidence of the consequences of human activity on climate change that are altering the exposure of organisms and ecosystems to UV radiation. This in turn reveals the interactive effects of many climate change factors with UV radiation that have implications for the atmosphere, feedbacks, contaminant fate and transport, organismal responses, and many outdoor materials including plastics, wood, and fabrics. The universal ratification of the Montreal Protocol, signed by 197 countries, has led to the regulation and phase-out of chemicals that deplete the stratospheric ozone layer. Although this treaty has had unprecedented success in protecting the ozone layer, and hence all life on Earth from damaging UV radiation, it is also making a substantial contribution to reducing climate warming because many of the chemicals under this treaty are greenhouse gases.

Nanomaterial Transformation in the Soil-Plant System: Implications for Food Safety and Application in Agriculture

Engineered nanomaterials (ENMs) have huge potential for improving use efficiency of agrochemicals, crop production, and soil health; however, the behavior and fate of ENMs and the potential for negative long-term impacts to agroecosystems remain largely unknown. In particular, there is a lack of clear understanding of the transformation of ENMs in both soil and plant compartments. The transformation can be physical, chemical, and/or biological, and may occur in soil, at the plant interface, and/or inside the plant. Due to these highly dynamic processes, ENMs may acquire new properties distinct from their original profile; as such, the behavior, fate, and biological effects may also differ significantly. Several essential questions in terms of ENMs transformation are discussed, including the drivers and locations of ENM transformation in the soil-plant system and the effects of ENM transformation on analyte uptake, translocation, and toxicity. The main knowledge gaps in this area are highlighted and future research needs are outlined so as to ensure sustainable nanoenabled agricultural applications.