Standard Reference Materials: Comparison of Redox Standards

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In the absence of a reference method or a higher order standard, The principal disadvantage of accelerated studies is that reference materials, like any other material, can degrade for unexpected reasons over time, or can degrade following different kinetic models ; predictions can then become unreliable. In most stability studies, real-time or accelerated, a few units of the reference material are tested at intervals.

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If the measurement system used for testing the materials is not perfectly stable, this can generate imprecise data or can be mistaken for instability of the material. To overcome these difficulties, it is often possible to move RM units, at intervals, to some reference temperature where they remain stable, and then test all the accumulated units - which have undergone different exposure times - at the same time.

This is referred to as an isochronous study. This strategy has the advantage of improving the precision of data used in assessing stability at the cost of delaying results until the end of the stability study period. From Wikipedia, the free encyclopedia. Three steps in producing a natural water CRM. Archived from the original PDF on 8 July Retrieved 30 May Paul International Laboratory Accreditation Cooperation. Retrieved 1 June Geneva: International Organization for Standardization.

International Organization for Standardization. National Institute of Standards and Technology. Geneva: World Health Organization. British Pharmacopoiea. Retrieved 3 June Fassett; Greenberg, R. January Accreditation and Quality Assurance. BAM, Germany. Retrieved 13 June AgNP-instability was attributed to the failure of the protective coatings on the NPs to prevent aggregation in the biological fluids both of high ionic strength.

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Loza et al. In their study, they hypothesized that the release of silver ions led directly to silver toxicity and confirmed this via cell culture-, microbiological-, and reactive oxygen species experiments. Researchers have also demonstrated that AgNPs in blood readily interact with surrounding biomolecules such as proteins and lipids, leading to the formation of protein coronas on the NP surface Walczyk et al. On the other hand, it has been shown that the release of silver ions can be potentially suppressed by the addition of humic and fulvic acids, dissolved oxygen, natural and low salt sea water, and other organic matter Liu and Hurt, In the past two decades, a large research effort has been devoted to the aspects of the toxicity of AgNPs, covering investigations of environmental fate, and including a plethora of in vivo and in vitro studies Marambio-Jones and Hoek, ; Fabrega et al.

Published cytotoxicity tests and in vivo assays lend limited evidence to claims that silver is carcinogenic in any tissue U. Department of Health and Human Resources, However, a plethora of in vitro studies have provided evidence that AgNPs are not only transported into cells and internalized, but target endosomes and lysosomes Asharani et al.

Exposing A cells human alveolar basal epithelial cells to AgNPs resulted in not only reactive oxygen species generation, but reductions in cell viability and mitochondrial membrane potential Chairuangkitti et al. Conversely, exposure to AgNPs at high concentrations up to 6. While there is evidence that AgNPs are toxic Maurer and Meyer, , the full mechanisms of toxicity are still not well-understood and research efforts should be devoted to gaining more clarity. The main drawbacks to establishing a systematic comparison of the current published studies are the lack of uniformity in terms of size and shape in the synthesis and the purification procedures of AgNPs, varying size distributions, coatings, and precursors, a lack of particle characterization, and the lack of implementation of validation with reference materials Gliga et al.

Nonetheless, increased oxidative stress, apoptosis, and genotoxicity have been highlighted as the main in vitro outcomes of AgNP exposure Kim and Ryu, These confounding differences in methodology have often lead to contradictory findings in in vitro studies. Studies that compare AgNPs of varying sizes show a greater toxic effect for particles of smaller diameter Carlson et al. Oxidative stress has been the main link to the toxicity of AgNPs themselves Kim et al.

Although Ag ion release has often been highlighted as the main cause of cytotoxicity and toxic effects, researchers find difficulty in determining the extent of the toxicity of AgNPs when Ag ions are also present in solution the Ag ion induced effects often mask the effect of AgNPs at high metal ion concentrations. Foldbjerg et al. To date, the weight researchers must place on ion release when discussing AgNP toxicity is still a difficult concept to discern. While AgNPs have been shown to be toxic to bacteria, hence their main use in the formulation of antibacterial products, significant evidence is present to support the toxicity of AgNPs to other organisms.

Marambio-Jones and Hoek provide comprehensive evidence that AgNPs cause inactivity not only in bacterial cells, but also fungi, virii, and algae.

AgNPs have also been found to be toxic to models such as zebrafish Yeo and Yoon, , Drosophila melanogaster Ahamed et al. Yeo and Yoon found that nano-silver ions penetrated the skin and blood tube of zebrafish larvae in the form of aggregates, while Ahamed et al. Further, silver nanowires were not only toxic to Daphnia magna , but Scanlan et al.

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In correlation with the effect that AgNPs have on soil and soil ecosystems, toxic effects have also been reported on a diverse range of soil invertebrates which include Eisenia fetida, Enchytraeus albidus, Eisenia andrei, Porcellio scaber , and Folsomia candida Tkalec et al. As can be seen in the aforementioned sections, AgNPs have been shown to have toxic effects to both in vitro and in vivo models , however there is a limited number of studies that report the impacts of AgNPs on human health Korani et al.

Currently silver, present in the human body in low concentrations via inhalation of air particulate and through diet and drinking water, is considered relatively harmless to humans and is not regarded as toxic to the immune, cardiovascular, nervous, or reproductive systems ATSDR, ; Lansdown, Even though the benefits of the Ag on human health are yet to be proven, colloidal silver suspensions are being incorporated into health supplements Fabrega et al.

Occupational health studies have found that long-term exposures to Ag have led to irreversible conditions such as argyria, wherein the skin turns bluish in color as a response to the accumulation of Ag in body tissues Hill, ; Wadhera and Fung, It is worthy to note that the critical oral dosage that elicits this effect is not known and may vary from individual to individual.

Silver and nanosilver accumulation in the skin, liver, kidneys, corneas, gingiva, mucous membranes, nails, and spleen are also possible Rosenman et al. An extensive review of the exposure-related health effects of silver and silver related compounds was conducted by Drake and Hazelwood in and later by Lansdown in Drake and Hazelwood, ; Lansdown, Studies have listed the liver as the primary organ for silver accumulation and elimination.

Even though the majority of Ag-containing consumer products are designated for topical application, the risk of percutaneous absorption of silver is very low as the human epidermis is a relatively impenetrable barrier the exception being dermal abrasions, wounds, and cuts. Lansdown also reasons that although there is an increasing use of Ag in silver thread and textile fibers, there has been no evidence of increased blood silver or accumulation of silver precipitates in the skin in chronic exposure and the risks of argyria in these cases have been deemed negligible.

In the same vein, the toxic risks associated with silver ingestion are low, as most products releasing Ag ions for oral or gastrointestinal hygiene were removed from pharmacopeias and permitted lists in most countries, in light of the risks of argyria Lansdown, More comprehensive studies and research efforts are necessary to clearly aid risk assessment, identify the toxic mechanisms of AgNPs and their toxicological effects where areas of human health are concerned.

Generally, the synthesis of NPs can be classified in two main categories: Top-down, where the procedure involves the use of bulk materials, such as metallic silver, that are reduced to form NPs using physical, chemical, or mechanical processes; or bottom-up, where the procedure requires starting from molecules, atoms, or ions to obtain NPs Hornyak et al. In recent years, the development of methods for the synthesis of AgNPs has been the subject of significant interest Tran et al.

Generally, AgNPs are synthetized in liquid phase using chemical methods such as: Classical reduction with citrate Turkevich et al. However, despite the myriad of AgNP synthesis methods, few offer the capability to achieve shape and size control. Therefore, it is necessary to control and establish reaction conditions that facilitate reproducible synthesis of spherical NPs with uniform size distributions.

In this context, some of the variables that can be tuned in the chemical synthesis process to control the size and shape of AgNPs are:. Alternatively, other synthesis routes employ seed methods, where small NPs serve as seed or nucleation centers that allow control of the shape and particle size of the AgNPs Jana et al. The most common methods used for the synthesis of uniform and spherical AgNPs are summarized in Table 1.

Table 1. Chemical methods for the synthesis of monodisperse and quasi-spherical AgNPs in liquid phase. Another important factor to consider for the synthesis of AgNPs in liquid phase is their subsequent stabilization.

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The stabilization of AgNPs is necessary for their compatibility across the range of applications described above Kang and Haider, and will impact the interaction in the environment. Figure 7. An illustration of some selected surface chemistries and conjugation strategies that are applied to NPs.

Reproduced from Sperling and Parak with permission of the Royal Society. Depending on the type of NP i. Molecules with low molar mass have been used as stabilizing agents Warner et al.

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Van Hyning and Zukoski, Alternatively, synthetic polymers can be used for the stabilization of NPs. In this context, amphiphilic polymers have been employed to stabilize NPs Mayer, On the other hand, hydrophilic polymer chain interactions generate external loops which can interact with the solvent and sterically stabilize NPs, see Figure 8. Figure 8. Steric stabilization of AgNPs. Reproduced from Zamiri et al. CC BY 3. Stabilization will directly impact the physical and chemical properties of AgNPs, and subsequently may limit their applications. However, besides capping agents, storage temperatures are also critical to the stability of these materials.

These processes are unintended in the synthesis of spherical and uniform size distributions of AgNPs; therefore, it is key to control the temperature of these colloidal systems to avoid thermodynamically the formation of such structures. Also, AgNPs can be modified and destabilized by photochemical reactions. Gorham et al.

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Other factors to take into account with regard to the destabilization of AgNPs are post-synthesis residues and incorrect purification procedures. Recently, it has been discovered that the use of biopolymers as a capping agents foster biocompatibility and safety from toxicological points of view Jena et al. Specifically, different carbohydrates and their derivatives such as a guar gum Vanamudan and Sudhakar, , carboxymethyl cellulose CMC Velusamy et al. Proteins have also been employed for the stabilization of AgNPs. Darroudi et al. Furthermore, some studies demonstrate the good stability of capped biopolymer-AgNPs.

Chen et al. Shanmugaraj and Ilanchelian later demonstrated that AgNPs capped with chitosan were stable for more than 4 months. All the studies described above, show that biopolymers can be used as capping agents and provide evidence for employing these macromolecules as stabilizing agents for NPs in liquid phase. Overall, the stabilization of AgNPs and other NPs in liquid phase is still considered a chemical challenge, mainly due to the complexity of some liquid media biological, environmental, organic, etc.

As previously mentioned, NPs constitute a focus of interest in nanoscience and nanotechnology Kang and Haider, ; Sharma et al. Particularly, there is an interest in establishing controlled chemical e. Moreover, due to advancements in the production and applications of nanomaterials, scientists are developing new, and adapting classic, analytical techniques for the detection, characterization, and quantification of NPs.

An extensive discussion of the fundamentals and analytical capabilities of the most common techniques for the characterization of NPs specifically metal, metal oxide and metalloid has been thoroughly reviewed Gunsolus and Haynes, ; Costa-Fernandez et al. The main current measurement techniques MTs for the characterization of NPs in general, and AgNPs in particular, and the requisite information they provide are listed in Table 2.

Table 2. Common measurement techniques MT used for the characterization of NPs. In others words, the measurements made using a metrological approach allow the establishment of extremely important variables in the quality of the measurements such as bias, precision and traceability to International System of Units S. Consequently, it allows accurate and concrete conclusions of the chemical or physical property studied at the nanoscale. In the last decade, some institutions and standardization bodies have been working to establish standards, protocols, guidelines, and procedures for the correct measurement and characterization of NPs see Table 3.

Table 3. Representative standards, guides, and protocols developed in the recent years for the characterization of NPs.

Standard Reference Materials: Comparison of Redox Standards Standard Reference Materials: Comparison of Redox Standards
Standard Reference Materials: Comparison of Redox Standards Standard Reference Materials: Comparison of Redox Standards
Standard Reference Materials: Comparison of Redox Standards Standard Reference Materials: Comparison of Redox Standards
Standard Reference Materials: Comparison of Redox Standards Standard Reference Materials: Comparison of Redox Standards
Standard Reference Materials: Comparison of Redox Standards Standard Reference Materials: Comparison of Redox Standards
Standard Reference Materials: Comparison of Redox Standards Standard Reference Materials: Comparison of Redox Standards
Standard Reference Materials: Comparison of Redox Standards Standard Reference Materials: Comparison of Redox Standards

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