Comparison of on-line detectors for field flow fractionation analysis of nanomaterials

A multi-technique analytical approach to support (eco)toxicological investigation of zinc oxide nanoparticles

Understanding the mechanism of toxicity of nanoparticles and their behavior in biological environments is crucial for designing materials with reduced side effects and improved performance. Among the factors influencing nanoparticle behavior in biological environments, the release and bioavailability of potentially toxic metal ions can alter equilibria and cause adverse effects. In this study, we applied two on-line Field-Flow Fractionation (FFF) strategies and compared the results with off-line benchmarking centrifugal ultrafiltration to assess a key descriptor, namely the solubility of zinc oxide (ZnO) nanoparticles. We found that, at the highest nanoparticle concentrations, the nanoparticle-ion ratio quickly reaches equilibrium, and the stability is not significantly affected by the separation technique. However, at lower concentrations, dynamic, non-equilibrium behavior occurs, and the results depend on the method used to separate the solid from the ionic fraction, where FFF yielded a more representative dissolution pattern. To support the (eco)toxicological profiling of the investigated nanoparticles, we generated experimental data on colloidal stability over typical (eco)toxicological assay durations. The Zeta Potential vs pH curves revealed two distinct scenarios typical of surfaces that have undergone significant modification, most likely due to pH-dependent dissolution and re-precipitation of surface groups. Finally, to enhance hazard assessment screening, we investigated ion-dependent toxicity and the effects of exposure to fresh water. Using an in vitro human skin model, we evaluated the cytotoxicity of fresh and aged ZnO nanoparticles (exposed for 72 h in M7), revealing time-dependent, dose-dependent, and nanoparticle-dependent cytotoxicity, with lower toxicity observed in the case of aged samples.

Past progress in environmental nanoanalysis and a future trajectory for atomic mass-spectrometry methods

The development of engineered nanotechnology has necessitated a commensurate maturation of nanoanalysis capabilities. Building off a legacy established by electron microscopy and light-scattering, environmental nanoanalysis has now benefited from ongoing advancements in instrumentation and data analysis, which enable a deeper understanding of nanomaterial properties, behavior, and impacts. Where once environmental nanoparticles and colloids were grouped into broad 'dissolved or particulate' classes that are dependent on a filter size cut-off, now size distributions of submicron particles can be separated and characterized providing a more comprehensive examination of the nanoscale. Inductively coupled plasma-quadrupole mass spectrometry (ICP-QMS), directly coupled to field flow fractionation (FFF-ICP-QMS) or operated in single particle mode (spICP-MS) have spearheaded a revolution in nanoanalysis, enabling research into nanomaterial behavior in environmental and biological systems at expected release concentrations. However, the complexity of the nanoparticle population drives a need to characterize and quantify the multi-element composition of nanoparticles, which has begun to be realized through the application of time-of-flight MS (spICP-TOFMS). Despite its relative infancy, this technique has begun to make significant strides in more fully characterizing particulate systems and expanding our understanding of nanoparticle behavior. Though there is still more work to be done with regards to improving instrumentation and data processing, it is possible we are on the cusp of a new nanoanalysis revolution, capable of broadening our understanding of the size regime between dissolved and bulk particulate compartments of the environment.

Field-flow fractionation - an excellent tool for fractionation, isolation and/or purification of biomacromolecules

Field-flow fractionation (FFF) with its several variants, has developed into a mature methodology. The scope of the FFF investigations has expanded, covering both a wide range of basic studies and especially a wide range of analytical applications. Special attention of this review is given to the achievements of FFF with reference to recent applications in the fractionation, isolation, and purification of biomacromolecules, and from which especially those of (in alphabetical order) bacteria, cells, extracellular vesicles, liposomes, lipoproteins, nucleic acids, and viruses and virus-like particles. In evaluating the major approaches and trends demonstrated since 2012, the most significant biomacromolecule applications are compiled in tables. It is also evident that asymmetrical flow field-flow fractionation is by far the most dominant technique in the studies. The industry has also shown current interest in FFF and adopted it in some sophisticated fields. FFF, in combination with appropriate detectors, handles biomacromolecules in open channel in a gentle way due to the lack of shear forces and unwanted interactions caused by the stationary phase present in chromatography. In addition, in isolation and purification of biomacromolecules quite high yields can be achieved under optimal conditions.

Field-Flow Fractionation in Molecular Biology and Biotechnology

Field-flow fractionation (FFF) is a family of single-phase separative techniques exploited to gently separate and characterize nano- and microsystems in suspension. These techniques cover an extremely wide dynamic range and are able to separate analytes in an interval between a few nm to 100 µm size-wise (over 15 orders of magnitude mass-wise). They are flexible in terms of mobile phase and can separate the analytes in native conditions, preserving their original structures/properties as much as possible. Molecular biology is the branch of biology that studies the molecular basis of biological activity, while biotechnology deals with the technological applications of biology. The areas where biotechnologies are required include industrial, agri-food, environmental, and pharmaceutical. Many species of biological interest belong to the operational range of FFF techniques, and their application to the analysis of such samples has steadily grown in the last 30 years. This work aims to summarize the main features, milestones, and results provided by the application of FFF in the field of molecular biology and biotechnology, with a focus on the years from 2000 to 2022. After a theoretical background overview of FFF and its methodologies, the results are reported based on the nature of the samples analyzed.

On the Fractionation and Physicochemical Characterisation of Self-Assembled Chitosan-DNA Polyelectrolyte Complexes

Chitosan is extensively studied as a carrier for gene delivery and is an attractive non-viral gene vector owing to its polycationic, biodegradable, and biocompatible nature. Thus, it is essential to understand the chemistry of self-assembled chitosan-DNA complexation and their structural and functional properties, enabling the formation of an effective non-viral gene delivery system. In this study, two parent chitosans (samples NAS-032 and NAS-075; Mw range ~118-164 kDa) and their depolymerised derivatives (deploy nas-032 and deploy nas-075; Mw range 6-14 kDa) with degrees of acetylation 43.4 and 4.7%, respectively, were used to form polyelectrolyte complexes (PECs) with DNA at varying [-NH3+]/[-PO4-] (N/P) molar charge ratios. We investigated the formation of the PECs using ζ-potential, asymmetric flow field-flow fractionation (AF4) coupled with multiangle light scattering (MALS), refractive index (RI), ultraviolet (UV) and dynamic light scattering (DLS) detectors, and TEM imaging. PEC formation was confirmed by ζ-potential measurements that shifted from negative to positive values at N/P ratio ~2. The radius of gyration (Rg) was determined for the eluting fractions by AF4-MALS-RI-UV, while the corresponding hydrodynamic radius (Rh), by the DLS data. We studied the influence of different cross-flow rates on AF4 elution patterns for PECs obtained at N/P ratios 5, 10, and 20. The determined rho shape factor (ρ = Rg/Rh) values for the various PECs corresponded with a sphere morphology (ρ ~0.77-0.85), which was consistent with TEM images. The results of this study represent a further step towards the characterisation of chitosan-DNA PECs by the use of multi-detection AF4 as an important tool to fractionate and infer aspects of their morphology.

Critical metal geochemistry in groundwaters influenced by dredged material

This study investigated critical metal (CM) geochemistry including rare earth elements (REEs), Co, Ni, and Mn in groundwaters below and surrounding two dredged material placement facilities (DMPFs). Metal concentrations are elevated at both sites, spanning several orders of magnitude. The highest CM concentrations measured exceed many environments considered as aqueous resources (Co and Ni > 1 mg L^-1, REEs > 3 mg L^-1). Correlations between sulfur and iron, major cations, and CMs indicate that oxidation of sulfides present in the DM releases metals both directly from sulfide minerals and indirectly through acid dissolution of and/or desorption from additional minerals. REE fractionation patterns indicate that their mobility in the groundwaters may be influenced by interactions with silicate, carbonate, and phosphate minerals. Significant positive Gd and Eu anomalies were observed, which may be attributed to increased mobility of Eu²⁺ and anthropogenic Gd. Nanogeochemical analysis of filtered samples revealed several REE-bearing nanoparticulate (diameter < 100 nm) species, some of which co-occurred with aluminum, suggesting an (oxy)hydroxide or a clay mineral component. Further characterization of soluble and nano scale geochemical speciation is needed to fully assess the viability of CM recovery from DM-associated groundwater. CM recovery from DM-associated waters can provide a beneficial use, both offsetting costs associated with disposal, and supplementing domestic CM resources.

Detection, Identification and Size Distribution of Silver Nanoparticles (AgNPs) in Milk and Migration Study for Breast Milk Storage Bags

The engineered silver nanoparticles (AgNPs) have been widely used in various food contact materials (FCMs) based on their antibacterial properties. This widespread use of nanosilver has, however, increased the risk of exposure of AgNPs to human due to their migration from FCMs causing a potential hazard present in foods. Therefore, it is important to establish a reliable and practical method for the detection of AgNPs in food matrices to support risk assessment on AgNPs exposure. Taking the examples of milk and AgNPs-containing breast milk storage bags, this study established an approach for size characterization and quantification of AgNPs in milk and evaluated the relevant silver migration, based on enzymatic digestion and the analysis by asymmetric flow field–flow fractionation (AF4) hyphenated with inductively coupled plasma mass spectrometry (ICP-MS) and single particle inductively coupled plasma mass spectrometry (SP-ICP-MS). No migration of AgNPs was found from breast milk storage bags under various simulated storage conditions as well as extreme scenarios. The suitability and reliability of this method were also validated by the determination of multiple parameters, including accuracy, repeatability, limit of detection (LOD), limit of quantification (LOQ), and recovery, for AF4-ICP-MS and SP-ICP-MS, respectively, with good and overall acceptable evaluation results obtained for all. The established and validated approach was demonstrated to be suitable for the characterization and quantitation of AgNPs in milk as well as the analysis of their migration from breast milk storage bags.

The micro-, submicron-, and nanoplastic hunt: A review of detection methods for plastic particles

Plastic particle pollution has been shown to be almost completely ubiquitous within our surrounding environment. This ubiquity in combination with a variety of unique properties (e.g. density, hydrophobicity, surface functionalization, particle shape and size, transition temperatures, and mechanical properties) and the ever-increasing levels of plastic production and use has begun to garner heightened levels of interest within the scientific community. However, as a result of these properties, plastic particles are often reported to be challenging to study in complex (i.e. real) environments. Therefore, this review aims to summarize research generated on multiple facets of the micro- and nanoplastics field; ranging from size and shape definitions to detection and characterization techniques to generating reference particles; in order to provide a more complete understanding of the current strategies for the analysis of plastic particles. This information is then used to provide generalized recommendations for researchers to consider as they attempt to study plastics in analytically complex environments; including method validation using reference particles obtained via the presented creation methods, encouraging efforts towards method standardization through the reporting of all technical details utilized in a study, and providing analytical pathway recommendations depending upon the exact knowledge desired and samples being studied.

Incidence of metal-based nanoparticles in the conventional wastewater treatment process

Metal-based nanoparticles (NPs) can be found in wastewater streams, which are significant pathways for the release of NPs to the environment. Determination of the NPs concentration in wastewater streams is important for performing appropriate ecotoxicological evaluations. The aim of this work was to determine the incidence of NPs from 13 different elements throughout the wastewater treatment process by using single particle inductively coupled plasma mass spectrometry (spICP-MS). The incidence was determined in samples of the influent, post-primary treatment and effluent of the activated sludge process, as well as in the reclaimed water of a full-scale wastewater treatment plant (WWTP). In addition, concentration of NPs was determined in the waste activated sludge and in the anaerobic digester. The concentration of metal-based NPs in the influent wastewater were between 1,600 and 10,700 ng/L for elements such as Ti, Fe, Ce, Mg, Zn and Cu, while that for Ni, Al, Ag, Au, Co and Cd was below 100 ng/L. Concentrations in reclaimed water ranged between 0.6 and 721 ng/L, ranked as Mg > Ti > Fe > Cu > Ni > Ce > Zn > Mn > Al  > Co > Ag > Cd  > Au. Results indicated that the activated sludge process and reclaimed water system removed 84-99% of natural and engineered metal-based NPs from influent to reclaimed water, except for Mg, Ni and Cd where the removal ranged from 70 to 78%. The highest concentrations of NPs were found in the waste activated sludge and anaerobic sludge, ranging from 0.5 to 39,900 ng/L. The size distribution of NPs differed in different wastewater streams within the WWTP, resulting in smaller particles in the effluent (20-180 nm) than in the influent (23-233 nm) for most elements. Conversely, NPs were notably larger in the waste activated sludge samples than in the anaerobic sludge or wastewater, since conditions in the secondary treatment lead to precipitation of several metal-based NPs. The incidence of metal-based NPs from 13 elements in wastewater decreased significatively after the conventional wastewater treatment train. However, anaerobic digesters store high NPs concentrations. Hence, the disposal of sludge needs to take this into account to evaluate the risk of the release of NPs to the environment.

Limitations of Nanoparticles Size Characterization by Asymmetric Flow Field‑Fractionation Coupled with Online Dynamic Light Scattering

The development of reliable protocols suitable for the characterisation of the physical properties of nanoparticles in suspension is becoming crucial to assess the potential biological as well as toxicological impact of nanoparticles. Amongst sizing techniques, asymmetric flow field flow fractionation (AF4) coupled to online size detectors represents one of the most robust and flexible options to quantify the particle size distribution in suspension. However, size measurement uncertainties have been reported for on-line dynamic light scattering (DLS) detectors when coupled to AF4 systems. In this work we investigated the influence of the initial concentration of nanoparticles in suspension on the sizing capability of the asymmetric flow field-flow fractionation technique coupled with an on-line dynamic light scattering detector and a UV–Visible spectrophotometer (UV) detector. Experiments were performed with suspensions of gold nanoparticles with a nominal diameter of 40 nm and 60 nm at a range of particle concentrations. The results obtained demonstrate that at low concentration of nanoparticles, the AF4-DLS combined technique fails to evaluate the real size of nanoparticles in suspension, detecting an apparent and progressive size increase as a function of the elution time and of the concentration of nanoparticles in suspension.

How the use of a short channel can improve the separation efficiency of nanoparticles in asymmetrical flow field-flow fractionation

The use of a commercially available short length channel (14 cm length) is proposed to improve the efficiency associated to the separation by asymmetrical flow field-flow fractionation of particles in the nanometer range respect to a standard channel (27 cm length). The effect of channel length on elution times, separation efficiency and resolution have been studied. Polystyrene particles between 50 and 500 nm in size have been used to compare the behavior of both channels. Theoretical aspects based on the different contributions on particle diffusion inside the channel during the separation process have been considered to justify the results obtained. Non-equilibrium diffusion contribution to the efficiency has shown to be the most relevant aspect to be controlled during the separation. The increment of the field strength applied through the cross-flow velocityallows the reduction of diffusion while keep elution times constant. The use of the same cross-flow in a channel with a smaller area is the key factor that justifies the better efficiencies observed along the whole size range studied (improvements that reach factors up to 4.7 in experimental efficiency respect to the standard channel were achieved). The separation of polystyrene particles of 100 and 200 nm was achieved with a resolution of 1.20, whereas a 0.66 value was obtained with the standard channel at the same elution times. Channel recoveries have been also compared under optimized conditions to ensure that no side effects are produced, including the separation of mixtures of TiO₂ nanoparticles. Similar or even better values were obtained with the short length channel, with recoveries higher than 85% for all the polystyrene particles tested and 75% recovery for the TiO₂ nanoparticle mixture, which justifies its use for the separation of nanoparticles, providing better resolutions without compromise elution times or recoveries.

Genotoxicity of Nanomaterials: Advanced In Vitro Models and High Throughput Methods for Human Hazard Assessment-A Review

Changes in the genetic material can lead to serious human health defects, as mutations in somatic cells may cause cancer and can contribute to other chronic diseases. Genotoxic events can appear at both the DNA, chromosomal or (during mitosis) whole genome level. The study of mechanisms leading to genotoxicity is crucially important, as well as the detection of potentially genotoxic compounds. We consider the current state of the art and describe here the main endpoints applied in standard human in vitro models as well as new advanced 3D models that are closer to the in vivo situation. We performed a literature review of in vitro studies published from 2000-2020 (August) dedicated to the genotoxicity of nanomaterials (NMs) in new models. Methods suitable for detection of genotoxicity of NMs will be presented with a focus on advances in miniaturization, organ-on-a-chip and high throughput methods.

A field-deployable surface-enhanced Raman scattering (SERS) method for sensitive analysis of silver nanoparticles in environmental waters

The proliferation of silver nanoparticles (AgNPs) in the environment and resultant transport into aquatic systems have raised concerns regarding their potential toxicity to various organisms. These environmental and ecological concerns demand reliable AgNP detection methods which can measure environmentally relevant quantities of AgNPs in real aquatic systems. This study developed a method that couples a rapid vacuum filtration technique with a portable Raman spectrometer to achieve on-site detection of ultra-low levels of AgNPs in typical and complex aquatic systems. To extract and detect AgNPs, aluminum chloride and ferbam were added for AgNP aggregation and labelling, respectively. The AgNP aggregates were filtered through a membrane, and their presence and quantity were determined based upon the surface-enhanced Raman scattering (SERS) peak intensity of ferbam. Under the optimized conditions, the extraction efficiencies are 99 ± 0.001% in ultrapure water and 98 ± 0.025% in marine water for 1 mg/L AgNPs. This method enables simple volume adjustment and improves the consistency of AgNP distribution on the membrane. The performance of the method was evaluated in different environmental waters, including marine water, fresh waters (pond water, river water, and reservoir outlet water) and drinking waters (municipal tap water and well water), with highest signal intensity in marine water and lowest signals in fresh waters. The signal intensity difference was suggested to be caused by the amount of natural organic matter (NOM) in these environmental waters. Using pond water as an example, the interference was minimized by changing the aggregating salt from AlCl₃ to MgCl₂, and AgNPs as low as 5 μg/L were reliably detected with a volume of 100 mL. At the same volume, the developed method was sensitive enough to detect 1 μg/L AgNPs in marine water and also holds promise for assessing the time-dependent transformation of AgNPs.

A Comparative Study of Particle Size Distribution of Graphene Nanosheets Synthesized by an Ultrasound-Assisted Method

Graphene-based materials are highly interesting in virtue of their excellent chemical, physical and mechanical properties that make them extremely useful as privileged materials in different industrial applications. Sonochemical methods allow the production of low-defect graphene materials, which are preferred for certain uses. Graphene nanosheets (GNS) have been prepared by exfoliation of a commercial micrographite (MG) using an ultrasound probe. Both materials were characterized by common techniques such as X-ray diffraction (XRD), Transmission Electronic Microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). All of them revealed the formation of exfoliated graphene nanosheets with similar surface characteristics to the pristine graphite but with a decreased crystallite size and number of layers. An exhaustive study of the particle size distribution was carried out by different analytical techniques such as dynamic light scattering (DLS), nanoparticle tracking analysis (NTA) and asymmetric flow field flow fractionation (AF4). The results provided by these techniques have been compared. NTA and AF4 gave higher resolution than DLS. AF4 has shown to be a precise analytical technique for the separation of GNS of different sizes.

Nanotechnology in Food Packaging

Food packaging is an integral component of the global food supply chain, protecting food from dirt, chemical contaminants and microorganisms, and helping to maintain food quality during transport and storage. Much of this packaging relies on modern polymeric materials, which have been developed to help control the exposure of products to light, oxygen and moisture. These have the benefits of being lightweight, cost-effective, reusable, recyclable and resistant to chemical and physical damage. Although traditional polymeric materials can fulfill many of these requirements, efforts continue to maintain or improve packaging performance while reducing the use of raw materials, waste and costs. The use of nanotechnology to produce nanocomposite materials has great promise to improve the characteristics of food packaging, but many of the products are still in their infancy. Only a relatively small number of nanoenabled products have entered the market and many, but not all, occupy niche markets. This chapter briefly describes the areas where nanomaterials have been used in research and commercial products to improve mechanical and barrier properties and to create active and intelligent packaging materials. It also addresses the regulation of nanomaterials in food contact applications and migration when evaluating the safety of these materials.

Nanosilver conductive ink: A case study for evaluating the potential risk of nanotechnology under hypothetical use scenarios

Engineered nanomaterials (ENMs) are being incorporated into a variety of consumer products due to unique properties that offer a variety of advantages over bulk materials. Understanding of the nano-specific risk associated with nano-enabled technologies, however, continues to lag behind research and development, registration with regulators, and commercialization. One example of a nano-enabled technology is nanosilver ink, which can be used in commercial ink-jet printers for the development of low-cost printable electronics. This investigation utilizes a tiered EHS framework to evaluate the potential nano-specific release, exposure and hazard associated with typical use of both nanosilver ink and printed circuits. The framework guides determination of the potential for ENM release from both forms of the technology in simulated use scenarios, including spilling of the ink, aqueous release (washing) from the circuits and UV light exposure. The as-supplied ink merits nano-specific consideration based on the presence of nanoparticles and their persistence in environmentally-relevant media. The material released from the printed circuits upon aqueous exposure was characterized by a number of analysis techniques, including ultracentrifugation and single particle ICP-MS, and the results suggest that a vast majority of the material was ionic in nature and nano-specific regulatory scrutiny may be less relevant.

Physicochemical characterization of titanium dioxide pigments using various techniques for size determination and asymmetric flow field flow fractionation hyphenated with inductively coupled plasma mass spectrometry

Seven commercial titanium dioxide pigments and two other well-defined TiO₂ materials (TiMs) were physicochemically characterised using asymmetric flow field flow fractionation (aF4) for separation, various techniques to determine size distribution and inductively coupled plasma mass spectrometry (ICPMS) for chemical characterization. The aF4-ICPMS conditions were optimised and validated for linearity, limit of detection, recovery, repeatability and reproducibility, all indicating good performance. Multi-element detection with aF4-ICPMS showed that some commercial pigments contained zirconium co-eluting with titanium in aF4. The other two TiMs, NM103 and NM104, contained aluminium as integral part of the titanium peak eluting in aF4. The materials were characterised using various size determination techniques: retention time in aF4, aF4 hyphenated with multi-angle laser light spectrometry (MALS), single particle ICPMS (spICPMS), scanning electron microscopy (SEM) and particle tracking analysis (PTA). PTA appeared inappropriate. For the other techniques, size distribution patterns were quite similar, i.e. high polydispersity with diameters from 20 to >700 nm, a modal peak between 200 and 500 nm and a shoulder at 600 nm. Number-based size distribution techniques as spICPMS and SEM showed smaller modal diameters than aF4-UV, from which mass-based diameters are calculated. With aF4-MALS calculated, light-scattering-based “diameters of gyration” (Øg) are similar to hydrodynamic diameters (Øh) from aF4-UV analyses and diameters observed with SEM, but much larger than with spICPMS. A Øg/Øh ratio of about 1 indicates that the TiMs are oblate spheres or fractal aggregates. SEM observations confirm the latter structure. The rationale for differences in modal peak diameter is discussed.

Dispersive liquid-liquid microextraction of silver nanoparticles in water using ionic liquid 1-octyl-3-methylimidazolium hexafluorophosphate

Using the ionic liquid (IL) 1-octyl-3-methylimidazolium hexafluorophosphate as the extractant and methanol as the dispersion solvent, a dispersive liquid-liquid microextraction method was developed to extract silver nanoparticles (AgNPs) from environmental water samples. Parameters that influenced the extraction efficiency such as IL concentration, pH and extraction time were optimized. Under the optimized conditions, the highest extraction efficiency for AgNPs was above 90% with an enrichment factor of >90. The extracted AgNPs in the IL phase were identified by transmission electron microscopy and ultraviolet-visible spectroscopy, and quantified by inductively coupled plasma mass spectrometry after microwave digestion, with a detection limit of 0.01μg/L. The spiked recovery of AgNPs was 84.4% with a relative standard deviation (RSD) of 3.8% (n=6) at a spiked level of 5μg/L, and 89.7% with a RSD of 2.2% (n=6) at a spiked level of 300μg/L, respectively. Commonly existed environmental ions had a very limited influence on the extraction efficiency. The developed method was successfully applied to the analysis of AgNPs in river water, lake water, and the influent and effluent of a wastewater treatment plant, with recoveries in the range of 71.0%-90.9% at spiking levels of 0.11-4.7μg/L.

Size-exclusion chromatography of metal nanoparticles and quantum dots

This review presents an overview of size-exclusion chromatographic separation and characterization of noble metal nanoparticles (NPs) and quantum dots (QDs) over the past 25 years. The properties of NPs and QDs that originate from quantum and surface effects are size dependent; to investigate these properties, a separation technique such as size-exclusion chromatography (SEC) is often needed to obtain narrow distribution NP populations that are also separated from the unreacted starting materials. Information on the size distributions and optical properties of NPs have been obtained by coupling SEC to detection methods such as ultraviolet-visible and/or fluorescence spectroscopy. Problems associated with the sorption of NPs and QDs onto various SEC stationary phases, employing both aqueous and organic eluents, are also discussed here.

Gaining a Critical Mass: A Dose Metric Conversion Case Study Using Silver Nanoparticles

Mass concentration is the standard convention to express exposure in ecotoxicology for dissolved substances. However, nanotoxicology has challenged the suitability of the mass concentration dose metric. Alternative metrics often discussed in the literature include particle number, surface area, and ion release (kinetics, equilibrium). It is unlikely that any single metric is universally applicable to all types of nanoparticles. However, determining the optimal metric for a specific type of nanoparticle requires novel studies to generate supportive data and employ methods to compensate for current analytical capability gaps. This investigation generated acute toxicity data for two standard species (Ceriodaphnia dubia, Pimephales promelas) exposed to five sizes (10, 20, 30, 60, 100 nm) of monodispersed citrate- and polyvinylpyrrolidone-coated silver nanoparticles. Particles were sized by various techniques to populate available models for expressing the particle number, surface area, and dissolved fraction. Results indicate that the acute toxicity of the tested silver nanoparticles is best expressed by ion release, and is relatable to total exposed surface area. Particle number was not relatable to the observed acute silver nanoparticle effects.

Asymmetric flow field-flow fractionation coupled to inductively coupled plasma mass spectrometry for the quantification of quantum dots bioconjugation efficiency

Hyphenation of asymmetric flow field-flow fractionation (AF4) to an on-line elemental detection (inductively coupled plasma-mass spectrometry, ICP-MS) is proposed as a powerful diagnostic tool for quantum dots bioconjugation studies. In particular, conjugation effectiveness between a "model" monoclonal IgG antibody (Ab) and CdSe/ZnS core-shell Quantum Dots (QDs), surface-coated with an amphiphilic polymer, has been monitored here by such hybrid AF4-ICP-MS technique. Experimental conditions have been optimized searching for a proper separation between the sought bioconjugates from the eventual free reagents excesses employed during the bioconjugation (QDs and antibodies). Composition and pH of the carrier have been found to be critical parameters to ensure an efficient separation while ensuring high species recovery from the AF4 channel. An ICP-MS equipped with a triple quadropole was selected as elemental detector to enable sensitive and reliable simultaneous quantification of the elemental constituents, including sulfur, of the nanoparticulated species and the antibody. The hyphenated technique used provided nanoparticle size-based separation, elemental detection, and composition analysis capabilities that turned out to be instrumental in order to investigate in depth the Ab-QDs bioconjugation process. Moreover, the analytical strategy here proposed allowed us not only to clearly identify the bioconjugation reaction products but also to quantify nanoparticle:antibodies bioconjugation efficiency. This is a key issue in future development of analytical and bioanalytical photoluminescent QDs applications.

Progress and future of in vitro models to study translocation of nanoparticles

The increasing use of nanoparticles in products likely results in increased exposure of both workers and consumers. Because of their small size, there are concerns that nanoparticles unintentionally cross the barriers of the human body. Several in vivo rodent studies show that, dependent on the exposure route, time, and concentration, and their characteristics, nanoparticles can cross the lung, gut, skin, and placental barrier. This review aims to evaluate the performance of in vitro models that mimic the barriers of the human body, with a focus on the lung, gut, skin, and placental barrier. For these barriers, in vitro models of varying complexity are available, ranging from single-cell-type monolayer to multi-cell (3D) models. Only a few studies are available that allow comparison of the in vitro translocation to in vivo data. This situation could change since the availability of analytical detection techniques is no longer a limiting factor for this comparison. We conclude that to further develop in vitro models to be used in risk assessment, the current strategy to improve the models to more closely mimic the human situation by using co-cultures of different cell types and microfluidic approaches to better control the tissue microenvironments are essential. At the current state of the art, the in vitro models do not yet allow prediction of absolute transfer rates but they do support the definition of relative transfer rates and can thus help to reduce animal testing by setting priorities for subsequent in vivo testing.

Using light scattering to evaluate the separation of polydisperse nanoparticles

The analysis of natural and otherwise complex samples is challenging and yields uncertainty about the accuracy and precision of measurements. Here we present a practical tool to assess relative accuracy among separation protocols for techniques using light scattering detection. Due to the highly non-linear relationship between particle size and the intensity of scattered light, a few large particles may obfuscate greater numbers of small particles. Therefore, insufficiently separated mixtures may result in an overestimate of the average measured particle size. Complete separation of complex samples is needed to mitigate this challenge. A separation protocol can be considered improved if the average measured size is smaller than a previous separation protocol. Further, the protocol resulting in the smallest average measured particle size yields the best separation among those explored. If the differential in average measured size between protocols is less than the measurement uncertainty, then the selected protocols are of equivalent precision. As a demonstration, this assessment metric is applied to optimization of cross flow (V(x)) protocols in asymmetric flow field flow fractionation (AF(4)) separation interfaced with online quasi-elastic light scattering (QELS) detection using mixtures of polystyrene beads spanning a large size range. Using this assessment metric, the V(x) parameter was modulated to improve separation until the average measured size of the mixture was in statistical agreement with the calculated average size of particles in the mixture. While we demonstrate this metric by improving AF(4) V(x) protocols, it can be applied to any given separation parameters for separation techniques that employ dynamic light scattering detectors.

Closing the gap: accelerating the translational process in nanomedicine by proposing standardized characterization techniques

On the cusp of widespread permeation of nanomedicine, academia, industry, and government have invested substantial financial resources in developing new ways to better treat diseases. Materials have unique physical and chemical properties at the nanoscale compared with their bulk or small-molecule analogs. These unique properties have been greatly advantageous in providing innovative solutions for medical treatments at the bench level. However, nanomedicine research has not yet fully permeated the clinical setting because of several limitations. Among these limitations are the lack of universal standards for characterizing nanomaterials and the limited knowledge that we possess regarding the interactions between nanomaterials and biological entities such as proteins. In this review, we report on recent developments in the characterization of nanomaterials as well as the newest information about the interactions between nanomaterials and proteins in the human body. We propose a standard set of techniques for universal characterization of nanomaterials. We also address relevant regulatory issues involved in the translational process for the development of drug molecules and drug delivery systems. Adherence and refinement of a universal standard in nanomaterial characterization as well as the acquisition of a deeper understanding of nanomaterials and proteins will likely accelerate the use of nanomedicine in common practice to a great extent.

Critical experimental evaluation of key methods to detect, size and quantify nanoparticulate silver

Different analytical techniques, sedimentation flow field fractionation (SdFFF), asymmetrical flow field flow fractionation (AF4), centrifugal liquid sedimentation (CLS) and dynamic light scattering (DLS) have been used to give complementary size information about suspensions of silver nanoparticles (AgNPs) in the size range of 20-100 nm by taking advantage of the different physical principles on which are based. Particle morphology was controlled by TEM (Transmission Electron Microscopy). Both SdFFF and AF4 were able to accurately size all AgNPs; among sedimentation based techniques, CLS underestimated the average sizes of larger samples (70 and 100 nm), but it produced the best separation of bimodal mixtures Ag40/60 and Ag40/70 mix compared to SdFFF. On the contrary, DLS overestimated the average sizes of the smallest samples (20 and 30 nm) and it was unable to deal with bimodal mixtures. Quantitative mass and number particle size distributions were also calculated starting from UV-vis signals and ICP-MS data and the results evaluated as a means to address the issue of determining nanoparticle size distributions as required for implementation of European regulations relating to labeling of nanomaterials in consumer products. The results are discussed in light of possible particle aggregation state, analysis repeatability, size resolution and quantitative recoveries.

An insight into silver nanoparticles bioavailability in rats

A comprehensive study of the bioavailability of orally administered silver nanoparticles (AgNPs) was carried out using a rat model. The silver uptake was monitored in liver and kidney tissues, as well as in urine and in feces. Significant accumulation of silver was found in both organs, the liver being the principal target of AgNPs. A significant (∼50%) fraction of silver was found in feces whereas the fraction excreted via urine was negligible (< 0.01%). Intact silver nanoparticles were found in feces by asymmetric flow field-flow fractionation (AsFlFFF) coupled with UV-Vis analysis. Laser ablation-ICP MS imaging showed that AgNPs were able to penetrate into the liver, in contrast to kidneys where they were retained in the cortex. Silver speciation analysis in cytosols from kidneys showed the metallothionein complex as the major species whereas in the liver the majority of silver was bound to high-molecular (70-25 kDa) proteins. These findings demonstrate the presence of Ag(i), released by the oxidation of AgNPs in the biological environment.

Characterization and quantification of silver nanoparticles in nutraceuticals and beverages by asymmetric flow field flow fractionation coupled with inductively coupled plasma mass spectrometry

This study evaluated the feasibility of asymmetric flow field flow fractionation coupled with inductively coupled plasma mass spectrometry (AF4-ICP-MS) for separation, characterization and quantification of silver nanoparticles (AgNPs) in complex nutraceutical and beverage samples. For improved determination, different analysis conditions were proposed depending on the NP size, i.e. below 20 nm and in the 20-60 nm range. After optimization of the different experimental parameters affecting the AF4 separation process and the analyte detection, the proposed methods showed a wide dynamic linear range (i.e., in the 10-1000 μg L(-1)) and limits of detection below 28 ng L(-1). A previous probe ultrasonication for 90 s (corresponding to 45 pulses of 2 s) of the tested samples resulted in complete AgNPs disaggregation. As a result, a fast accurate determination was achieved (complete analysis was done in ca. 37 min). The practicality of the proposed methodology for the intended determination was demonstrated by successful determination of the AgNPs present in a variety of nutraceuticals and a beverage at concentration levels in the 0.7-29.5×10(3) μg L(-1) range. A good agreement was observed among these concentration data and those determined by more conventional sample preparation techniques, such as ultracentrifugation and acid digestion. Also, the estimated NP sizes using AF4 compared satisfactorily with those determined by image techniques, i.e. transmission electron microscopy (TEM). All together demonstrated the utility of this novel analytical methodology for the analysis of AgNPs of different size in complex matrices.

Detection and characterization of silver nanoparticles and dissolved species of silver in culture medium and cells by AsFlFFF-UV-Vis-ICPMS: application to nanotoxicity tests

A methodology based on Asymmetric Flow Field-Flow Fractionation (AsFlFFF) coupled with UV-Vis absorption spectrometry and ICP mass spectrometry (ICPMS) has been developed and applied to the study of silver nanoparticles (AgNPs) and dissolved species of silver in culture media and cells used in cytotoxicity tests. The effect of a nano-silver based product (protein stabilized silver nanoparticles ca. 15 nm average diameter) on human hepatoma (HepG2) cell viability has been studied. UV-Vis absorption spectrometry provided information about the nature (organic vs. nanoparticle) of the eluted species, whereas the silver was monitored by ICPMS. A shift towards larger hydrodynamic diameters was observed in the AgNPs after a 24 hour incubation period in the culture medium, which suggests a "protein corona" effect. Silver(I) associated with proteins present in the culture medium has also been detected, as a consequence of the oxidation process experimented by the AgNPs. However, the Ag(I) released into the culture medium did not justify the toxicity levels observed. AgNPs associated with the cultured HepG2 cells were also identified by AsFlFFF, after applying a solubilisation process based on the use of tetramethylammonium hydroxide (TMAH) and Triton X-100. These results have been confirmed by transmission electronic microscopy (TEM) analysis of the fractions collected from the AsFlFFF. The effect of AgNPs on HepG2 cells has been compared to that caused by silver(I) as AgNO3 under the same conditions. The determination of the total content of silver in the cells confirms that a much larger mass of silver as AgNPs with respect to AgNO3 (16 to 1) is needed to observe a similar toxicity.

Controlled evaluation of silver nanoparticle sulfidation in a full-scale wastewater treatment plant

Sulfidation of silver nanoparticles (AgNPs), which is known to alter AgNP toxicity, occurs during transport through wastewater treatment plants. In this study, arrays of immobilized AgNPs fabricated by nanosphere lithography (NSL) were used to study AgNP sulfidation in a full-scale wastewater treatment plant (WWTP). A detailed laboratory study preceded field deployment. The characteristic NSL pattern remained discernible by atomic force microscopy and transmission electron microscopy after both lab and field exposures. Growth of AgNPs due to an increase in density upon sulfidation permitted the study of sulfidation kinetics in the WWTP. Sulfidation occurred almost exclusively in anaerobic zones of the WWTP, where the initial sulfidation rate was 11-14 nm of Ag converted to Ag2S per day. Measurements of the chemical composition and crystallinity of AgNPs exposed to primary influent for ∼ 10 d confirmed that they had been converted almost entirely to Ag2S. Laboratory experiments revealed that the sulfidation process is not uniform and that partially sulfidized AgNPs retain the potential to release toxic Ag(+) ions. The results indicate that primary AgNPs are sulfidized directly without dissolving and forming secondary precipitates. This study demonstrates the utility of immobilized AgNPs for detailed, in situ investigations of nanomaterial tranformations.

Asymmetrical flow field-flow fractionation with multi-angle light scattering and quasi-elastic light scattering for characterization of polymersomes: comparison with classical techniques

Polymersomes formed from amphiphilic block copolymers, such as poly(ethyleneoxide-b-ε-caprolactone) (PEO-b-PCL) or poly(ethyleneoxide-b-methylmethacrylate), were characterized by asymmetrical flow field-flow fractionation coupled with quasi-elastic light scattering (QELS), multi-angle light scattering (MALS), and refractive index detection, leading to the determination of their size, shape, and molecular weight. The method was cross-examined with more classical ones, like batch dynamic and static light scattering, electron microscopy, and atomic force microscopy. The results show good complementarities between all the techniques; asymmetrical flow field-flow fractionation being the most pertinent one when the sample exhibits several different types of population.