Multimethod approach for the detection and characterisation of food-grade synthetic amorphous silica nanoparticles

Definition, detection, and tracking of nanowaste in foods: Challenges and perspectives

Commercial applications of nanotechnology in the food industry are rapidly increasing. Accordingly, there is a simultaneous increase in the amount and diversity of nanowaste, which arise as byproducts in the production, use, disposal, or recycling processes of nanomaterials utilized in the food industry. The potential risks of this nanowaste to human health and the environment are alarming. It is of crucial significance to establish analytical methods and monitoring systems for nanowaste to ensure food safety. This review provides comprehensive information on nanowaste in foods as well as comparative material on existing and new analytical methods for the detection of nanowaste. The article is specifically focused on nanowaste in food systems. Moreover, the current techniques, challenges as well as potential use of new and progressive methods are underlined, further highlighting advances in technology, collaborative efforts, as well as future perspectives for effective nanowaste detection and tracking. Such detection and tracking of nanowaste are required in order to effectively manage this type ofwasted in foods. Although there are devices that utilize spectroscopy, spectrometry, microscopy/imaging, chromatography, separation/fractionation, light scattering, diffraction, optical, adsorption, diffusion, and centrifugation methods for this purpose, there are challenges to be overcome in relation to nanowaste as well as food matrix and method characteristics. New technologies such as radio-frequency identification, Internet of things, blockchain, data analytics, and machine learning are promising. However, the cooperation of international organizations, food sector, research, and political organizations is needed for effectively managing nanowaste. Future research efforts should be focused on addressing knowledge gaps and potential strategies for optimizing nanowaste detection and tracking processes.

Biocompatibility Analysis of Bio-Based and Synthetic Silica Nanoparticles during Early Zebrafish Development

During the twenty-first century, engineered nanomaterials (ENMs) have attracted rising interest, globally revolutionizing all industrial sectors. The expanding world population and the implementation of new global policies are increasingly pushing society toward a bioeconomy, focused on fostering the adoption of bio-based nanomaterials that are functional, cost-effective, and potentially secure to be implied in different areas, the medical field included. This research was focused on silica nanoparticles (SiO2-NPs) of bio-based and synthetic origin. SiO2-NPs are composed of silicon dioxide, the most abundant compound on Earth. Due to their characteristics and biocompatibility, they are widely used in many applications, including the food industry, synthetic processes, medical diagnosis, and drug delivery. Using zebrafish embryos as in vivo models, we evaluated the effects of amorphous silica bio-based NPs from rice husk (SiO2-RHSK NPs) compared to commercial hydrophilic fumed silica NPs (SiO2-Aerosil200). We evaluated the outcomes of embryo exposure to both nanoparticles (NPs) at the histochemical and molecular levels to assess their safety profile, including developmental toxicity, neurotoxicity, and pro-inflammatory potential. The results showed differences between the two silica NPs, highlighting that bio-based SiO2-RHSK NPs do not significantly affect neutrophils, macrophages, or other innate immune system cells.

Stimuli-responsive biodegradable silica nanoparticles: From native structure designs to biological applications

Due to their inherent advantages, silica nanoparticles (SiNPs) have greatly potential applications as bioactive materials in biosensors/biomedicine. However, the long-term and nonspecific accumulation in healthy tissues may give rise to toxicity, thereby impeding their widespread clinical application. Hence, it is imperative and noteworthy to develop biodegradable and clearable SiNPs for biomedical purposes. Recently, the design of multi-stimuli responsive SiNPs to improve degradation efficiency under specific pathological conditions has increased their clinical trial potential as theranostic nanoplatform. This review comprehensively summaries the rational design and recent progress of biodegradable SiNPs under various internal and external stimuli for rapid in vivo degradation and clearance. In addition, the factors that affect the biodegradation of SiNPs are also discussed. We believe that this systematic review will offer profound stimulus and timely guide for further research in the field of SiNP-based nanosensors/nanomedicine.

Characterization of nanoparticles in silicon dioxide food additive

Silicon dioxide (SiO2), in its amorphous form, is an approved direct food additive in the United States and has been used as an anticaking agent in powdered food products and as a stabilizer in the production of beer. While SiO2 has been used in food for many years, there is limited information regarding its particle size and size distribution. In recent years, the use of SiO2 food additive has raised attention because of the possible presence of nanoparticles. Characterization of SiO2 food additive and understanding their physicochemical properties utilizing modern analytical tools are important in the safety evaluation of this additive. Herein, we present analytical techniques to characterize some SiO2 food additives, which were obtained directly from manufacturers and distributors. Characterization of these additives was performed using dynamic light scattering (DLS), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), and single-particle inductively coupled plasma mass spectrometry (spICP-MS) after the food additive materials underwent different experimental conditions. The data obtained from DLS, spICP-MS, and electron microscopy confirmed the presence of nanosized (1-100 nm) primary particles, as well as aggregates and agglomerates of aggregates with sizes greater than 100 nm. SEM images demonstrated that most of the SiO2 food additives procured from different distributors showed similar morphology. The results provide a foundation for evaluating the nanomaterial content of regulated food additives and will help the FDA address current knowledge gaps in analyzing nanosized particles in commercial food additives.

Mesoporous Silica Particles Retain Their Structure and Function while Passing through the Gastrointestinal Tracts of Mice and Humans

Mesoporous silica particles (MSPs) can be used as food additives, clinically for therapeutic applications, or as oral delivery vehicles. It has also been discussed to be used for a number of novel applications including treatment for diabetes and obesity. However, a major question for their possible usage has been if these particles persist structurally and retain their effect when passing through the gastrointestinal tract (GIT). A substantial breaking down of the particles could reduce function and be clinically problematic for safety issues. Hence, we investigated the biostability of MSPs of the SBA-15 kind prepared at large scales (100 and 1000 L). The MSPs were orally administered in a murine model and clinically in humans. A joint extraction and calcination method was developed to recover the MSPs from fecal mass, and the MSPs were characterized physically, structurally, morphologically, and functionally before and after GIT passage. Analyses with N2 adsorption, X-ray diffraction, electron microscopy, and as a proxy for general function, adsorption of the enzyme α-amylase, were conducted. The adsorption capacity of α-amylase on extracted MSPs was not reduced as compared to the pristine and control MSPs, and adsorption of up to 17% (w/w) was measured. It was demonstrated that the particles did not break down to any substantial degree and retained their function after passing through the GITs of the murine model and in humans. The fact the particles were not absorbed into the body was ascribed to that they were micron-sized and ingested as agglomerates and too big to pass the intestinal barrier. The results strongly suggest that orally ingested MSPs can be used for a number of clinical applications.

Size effect and mucus role on the intestinal toxicity of the E551 food additive and engineered silica nanoparticles

The E551 food additive is composed of synthetic amorphous silica particles. The current regulation does not mention any specifications regarding their size and granulometric distribution, thus allowing the presence of silica nanoparticles despite their potential toxicity. The digestion process could modify their physicochemical properties and then influence their toxicological profile. After physicochemical characterization, subacute toxicity of engineered silica nanoparticles from 20 to 200 nm, native and digested E551 additives were evaluated from in vitro models of the intestinal barrier. Single cultures and a co-culture of enterocytes and mucus-secreting cells were established to investigate the mucus role. Toxicological endpoints including cytotoxicity, ROS production, intestinal permeability increase, and actin filament disruption were addressed after a 7-day exposure. The results showed a size-dependent effect of silica nanoparticles on cytotoxicity and intestinal permeability. A time-dependent disruption of actin filaments was observed in Caco-2 cells. The mucus layer spread on the HT29-MTX single culture acted as an efficient protective barrier while in the co-culture, small nanoparticles were able to cross it to reach the cells. From a hydrodynamic diameter of 70 nm, nanoparticles were not internalized in the intestinal cells, even in mucus-free models. Digestion did not affect the physicochemical properties of the additive. Due to a mean hydrodynamic diameter close to 200 nm, both native and digested E551 additives did not induce any toxic effect in intestinal barrier models. This study emphasized a cutoff size of 70 nm from which the interactions of the E551 additive with intestinal cells would be limited.

The safety of nanomaterials in food production and packaging

Nanotechnology involves developing, characterising, and applying structures ranging in size from 1 to 100 nm. As a key advanced technology, it has contributed to a substantial impact across engineering, medicine, agriculture and food. With regards to their application in food, nanomaterials posses the ability to lead the quantitative and qualitative development of high-quality, healthier, and safer foods by outperforming traditional food processing technologies for increasing shelf life and preventing contaminations. Although rapid progress has been made in nanotechnology in food products, the toxicity of nanoparticles and nanomaterials is not very well known. As a result, nanomaterials are potentially toxic, therefore, considering the constantly increasing employment in food science, they need to be further characterised, and their use must be better regulated. We may face a crisis of nanotoxicity if the molecular mechanisms by which nanoparticles and nanomaterials interact with food and within living organisms is not fully understood. Food safety can be guaranteed only if we are thoroughly aware of nanomaterial properties and potential toxicity. Therefore, it is urgently necessary to have in the food sector a regulatory system capable of managing nanofood risks and nanotechnology, considering the health effects of food processing techniques based on nanotechnology. This present review discusses the impact and role nanotechnology play in food science. The specific application of Nanomaterials in food science, their advantages and disadvantages, the potential risk for human health and the analysis to detect nanocomponents are also highlighted.

Genome-Wide DNA Methylation Alterations and Potential Risk Induced by Subacute and Subchronic Exposure to Food-Grade Nanosilica in Mice

The intensive application of nanomaterials in the food industry has raised concerns about their potential risks to human health. However, limited data are available on the biological safety of nanomaterials in food, especially at the epigenetic level. This study examined the implications of two types of synthetic amorphous silica (SAS), food-grade precipitated silica (S200) and fumed silica Aerosil 200F (A200F), which are nanorange food additives. After 28-day continuous and intermittent subacute exposure to these SAS via diet, whole-genome methylation levels in mouse peripheral leukocytes and liver were significantly altered in a dose- and SAS type-dependent manner, with minimal toxicity detected by conventional toxicological assessments, especially at a human-relevant dose (HRD). The 84-day continuous subchronic exposure to all doses of S200 and A200F induced liver steatosis where S200 accumulated in the liver even at HRD. Genome-wide DNA methylation sequencing revealed that the differentially methylated regions induced by both SAS were mainly located in the intron, intergenic, and promoter regions after 84-day high-dose continuous exposure. Bioinformatics analysis of differentially methylated genes indicated that exposure to S200 or A200F may lead to lipid metabolism disorders and cancer development. Pathway validation experiments indicated both SAS types as potentially carcinogenic. While S200 inhibited the p53-mediated apoptotic pathway in mouse liver, A200F activated the HRAS-mediated MAPK signaling pathway, which is a key driver of hepatocarcinogenesis. Thus, caution must be paid to the risk of long-term exposure to food-grade SAS, and epigenetic parameters should be included as end points during the risk assessment of food-grade nanomaterials.

Oral intake of silica nanoparticles exacerbates intestinal inflammation

Nanoparticles, i.e., particles with a diameter of ≤100 nm regardless of their composing material, are added to various foods as moisturizers, coloring agents, and preservatives. Silicon dioxide (SiO2, silica) nanoparticles in particular are widely used as food additives. However, the influence of SiO2 nanoparticle oral consumption on intestinal homeostasis remains unclear. The daily intake of 10-nm-sized SiO2 nanoparticles exacerbates dextran sulfate sodium (DSS)-induced colitis, whereas the daily intake of 30-nm-sized SiO2 nanoparticles has no influence on intestinal inflammation. The exacerbation of colitis induced by consuming 10-nm-sized SiO2 nanoparticles was abolished in mice deficient in apoptosis-associated speck-like protein containing a CARD (ASC). Our study indicates that the oral intake of small SiO2 nanoparticles poses a risk for worsening intestinal inflammation through activation of the ASC inflammasome.

An effective approach for size characterization and mass quantification of silica nanoparticles in coffee creamer by AF4-ICP-MS

Silicon dioxide (SiO₂) has been used as a food additive (E551) for decades. However, some safety concerns have been raised recently due to the detection of silica nanoparticles (SiO₂ NPs) in a variety of foodstuffs and their unknown long-term health risk to humans. In order for risk assessment to be conducted, it is essential to establish a reliable, valid, and pragmatic method for analysis of SiO₂ NPs in foods for estimation of exposure. This paper presents an effective approach for both size characterization and mass quantification of SiO₂ NPs in commercial high-fat coffee creamer using asymmetric flow field-flow fractionation (AF4) coupled to inductively coupled plasma mass spectrometry (ICP-MS). SiO₂ NPs from coffee creamer were well extracted after cleanup with hexane in a two-phase (hexane vs. water) aqueous environment. Size determination of SiO₂ NPs was performed by on-line AF4-ICP-MS based on calibration with monodispersed standards. The dominant primary size of SiO₂ NPs in the studied sample was 36.5 nm. The mass percentages of SiO₂ NPs (vs. total SiO₂) were 18.6% for the dominant primary nano-silica particles by prechannel calibration and 35.7% for total SiO₂ NPs (≤ 100 nm) by postchannel calibration, with recoveries of 89-96% for the former and 75% for the latter. The established approach was demonstrated to be efficient and practical for routine analysis of polydispersed SiO₂ NPs with wide nano-size distribution in coffee creamer. This method may be extended to monitor the presence of SiO₂ NPs in other similar complex food matrices. Graphical abstract.

Silver, copper, and copper hydroxy salt decorated fumed silica hybrid composites as antibacterial agents

Decoration of matrices such as silicates, graphene, etc. is an efficient technique in order to develop multifunctional materials with enhanced properties, which are of use for microbial control. Consequently, it leads to an increased search for alternative matrices and synthesis methods for decoration. Herein, decoration of a fumed silica is proposed, with structures that consisted of silver (Ag@FS), copper hydroxy salt (CuHS@FS), and copper (Cu@FS), for antibacterial applications. With the simple combination of the metal precursor salt, the appropriate solvent, and the fumed silica, the composites were obtained by one-pot solvothermal (200 °C for 1 h), rapid (2 min) microwave assisted precipitation, and by ascorbic acid chemical reduction, respectively. Characterization by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), and field emission scanning electron microscopy (FE-SEM) proved the successful decoration of the fumed silica with layered copper hydroxy salt (90 width x 970 length nm) and round-like metallic Ag (210 nm) and Cu (370 nm) particles. Fourier transformed infrared (FTIR) and Raman spectroscopy evidenced the presence of SiOMetal interactions. The antibacterial activity was evaluated against the Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus, giving inhibition and bactericidal values between 3-12 mg/ mL and 12-24 mg/ mL, respectively, with a maximum ion liberation ratio of 1.4 %. The application of the fumed silica presented here, is an attractive alternative to existing matrices, in order to fabricate multifunctional materials, as it is ready-to-use and feasible for large-scale production. Moreover, the applied synthesis routes provide rapid approaches for decoration, creating composites useful for antibacterial applications.

Determination of the fate and biological responses of food additive silica particles in commercial foods

Synthetic amorphous silica (SAS) is widely added to commercial foods as an anticaking agent. Concern about the potential application of nanosized silica in foods has increased as nanomaterials are not intended for use as food additives. This study evaluated the particle size distributions and biological responses of food additive SAS. An accurate, sensitive, and cost-effective analytical method for probing SAS was established, and quantitative analysis of its presence in commercial foods was performed. The results demonstrate that food additive SAS is an aggregated material composed of nanosized particles with nanosized aggregates of silica particles identified in commercial foods. Food additive SAS did not exhibit acute cytotoxicity compared to both general-grade nano (G-nano) and bulk (G-bulk) silica. Moreover, intestinal transport amounts of food additive SAS were significantly lower than for G-nano. Taken together, we find that food additive SAS does not exhibit acute toxicity resulting from nanosized materials.

Determination of Total Silicon and SiO2 Particles Using an ICP-MS Based Analytical Platform for Toxicokinetic Studies of Synthetic Amorphous Silica

Synthetic amorphous silica (SAS), manufactured in pyrogenic or precipitated form, is a nanomaterial with a widespread use as food additive (E 551). Oral exposure to SAS results from its use in food and dietary supplements, pharmaceuticals and toothpaste. Recent evidence suggests that oral exposure to SAS may pose health risks and highlights the need to address the toxic potential of SAS as affected by the physicochemical characteristics of the different forms of SAS. For this aim, investigating SAS toxicokinetics is of crucial importance and an analytical strategy for such an undertaking is presented. The minimization of silicon background in tissues, control of contamination (including silicon release from equipment), high-throughput sample treatment, elimination of spectral interferences affecting inductively coupled plasma mass spectrometry (ICP-MS) silicon detection, and development of analytical quality control tools are the cornerstones of this strategy. A validated method combining sample digestion with silicon determination by reaction cell ICP-MS is presented. Silica particles are converted to soluble silicon by microwave dissolution with mixtures of HNO3, H2O2 and hydrofluoric acid (HF), whereas interference-free ICP-MS detection of total silicon is achieved by ion-molecule chemistry with limits of detection (LoDs) in the range 0.2-0.5 µg Si g-1 for most tissues. Deposition of particulate SiO2 in tissues is assessed by single particle ICP-MS.

Small silica nanoparticles transiently modulate the intestinal permeability by actin cytoskeleton disruption in both Caco-2 and Caco-2/HT29-MTX models

Amorphous silica nanoparticles are widely used as pharmaceutical excipients and food additive (E551). Despite the potential human health risks of mineral nanoparticles, very few data regarding their oral toxicity are currently available. This study aims to evaluate and to understand the interactions of silica particles at 1 and 10 mg mL^-1 with the intestinal barrier using a Caco-2 monolayer and a Caco-2/HT29-MTX co-culture. A size- and concentration-dependent reversible increase of the paracellular permeability is identified after a short-term exposure to silica nanoparticles. Nanoparticles of 30 nm induce the highest transepithelial electrical resistance drop whereas no effect is observed with 200 nm particles. Additive E551 affect the Caco-2 monolayer permeability. Mucus layer reduces the permeability modulation by limiting the cellular uptake of silica. After nanoparticle exposure, tight junction expression including Zonula occludens 1 (ZO-1) and Claudin 2 is not affected, whereas the actin cytoskeleton disruption of enterocytes and the widening of ZO-1 staining bands are observed. A complete permeability recovery is concomitant with the de novo filament actin assembly and the reduction of ZO-1 bands. These findings suggest the paracellular modulation by small silica particles is directly correlated to the alteration of the ZO-actin binding strongly involved in the stability of the tight junction network.

Multivariate optimization of field-flow fractionation of nanoscale synthetic amorphous silica in processed foods supported by computational modelling

Separation and size characterization of SiO₂ in a laminar flow mode on the surface of cellulose membrane.

Analyzing the Interaction between Two Different Types of Nanoparticles and Serum Albumin

Two different types of nanoparticles (silicon dioxide and titanium dioxide) were selected within this study in order to analyze the interaction with bovine and human serum albumin. These particles were characterized by transmission and scanning electron microscopy (TEM and SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDXS). In addition, the hydrodynamic size and the zeta potential were measured for all these nanoparticles. The serum proteins were incubated with the nanoparticles for up to one hour, and the albumin adsorption on the particle surface was investigated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The effect induced on the secondary structure of proteins was analyzed by Fourier transform infrared spectroscopy (FTIR). The results showed that albumin adsorbed on the surface of both types of nanoparticles, but in different quantities. In addition, we noticed different changes in the structure of albumin depending on the physicochemical properties of each type of particle tested. In conclusion, our study provides a comparative analysis between the different characteristics of nanoparticles and the protein corona formed on the particle surface and effects induced on protein structure in order to direct the development of “safe-by-design” nanoparticles, as their demands for research and applications continue to increase.

Chronic Oral Exposure to Synthetic Amorphous Silica (NM-200) Results in Renal and Liver Lesions in Mice

Introduction

Silicon dioxide, produced as synthetic amorphous silica (SAS), is made of nanoparticles (NPs), either present as such or as agglomerates and aggregates, and is widely used in many types of food processes and products as an additive. To assess whether repeated, long-term exposure to SAS NPs may result in adverse effects, mice were exposed for 18 months via drinking water to NM-200, one of the reference nanostructured silica used for applications related to food, at 4.8 mg NM-200/kg body weight per day, a dose relevant to the estimated dietary exposure to SAS in humans.

Methods

The experiment focused on the kidney and liver as target organs and was carried out in parallel using 3 mouse lines (wild type and transgenic) differing for the expression of α-synuclein, that is, murine and human mutated (A53T). Sensitive determination of silicon revealed higher contents in liver and kidneys of NM-200–exposed mice compared with unexposed aged-matched controls.

Results

Histological abnormalities, such as vacuolization of tubular epithelial cells, were detected in all kidneys, as well as inflammatory responses that were also detected in livers of exposed animals. Less frequent but more deleterious, amyloidosis lesions were observed in glomeruli, associated with perivascular amyloid accumulation in liver.

Conclusion

These histological findings, in conjunction with the observation of detectable deposition of silica, highlight that chronic oral intake of SAS may pose a health risk to humans and need to be examined further.

Challenges in isolating silica particles from organic food matrices with microwave-assisted acidic digestion

Synthetic amorphous silica is widely used in food processing as a food additive (E551) due to its properties as a flavour carrier and anti-caking agent. The direct measurement of E551 suspended or embedded in complex matrices is difficult without prior removal of the matrix components. The isolation of nanoparticles from the matrix is hence the first step towards their comprehensive characterization. Due to its complexity, matrix removal is frequently not trivial and may cause modification of the number-size distribution of the silica particles. The isolation of engineered silica nanoparticles by removal of the matrix with microwave-assisted acidic digestion is demonstrated methodologically using both monodisperse (size standards) and polydisperse (E551) particles spiked into ultrapure water and tomato sauce. For the characterization of the isolated nanoparticles, asymmetric field flow fractionation (AF₄) coupled to multi-angle laser light scattering (MALS) and inductively coupled plasma mass spectrometry (ICP-MS) were chosen. The combination of ICP-MS and ultracentrifugation allowed for the rapid and reliable measurement of the dissolved fraction of SiO₂. The results show that microwave-assisted acidic digestion partially dissolves silica nanoparticles. Moreover, the digestion conditions, in particular the low pH value, lead to strong agglomeration of the particles. A complete deagglomeration is not achieved, even when exposing the suspension to elevated sonication doses. The consequence of these two findings is a size distribution of particles after acidic digestion that is different from the original distribution before digestion. This result may have an impact on the evaluation of whether the material is a nanomaterial according to the recommended definition of the European Commission. Graphical abstract.

Microfiltration of Submicron-Sized and Nano-Sized Suspensions for Particle Size Determination by Dynamic Light Scattering

Dynamic light scattering (DLS) is commonly used for the determination of average particle diameters and suspension stability and popular in academics and industry. However, DLS is not considered suitable for polydisperse samples. The presence of little quantities of micrometre particles in nano and submicrometre suspensions especially affect the reliability of DLS results. Microfiltration might be a suitable method for the removal of unwanted large particles. This study investigates the effect of microfiltration on the diameter distributions as measured by DLS. Polystyrene standards (40–900 nm diameter), and monomodal silica suspensions were filtered with polytetrafluoroethylene (PTFE) membranes (0.1–1.0 µm pore size) to investigate retention properties and grade efficiency. Non-ideal materials were used to prove the results. Experiments showed that a mono-exponential decay can be achieved by filtration. A size safety factor of at least three between labeled pore size and average diameter was found to keep separation as low as possible. Filtration in order to enhance DLS for particulate submicrometre materials was considered suitable for narrowly distributed coated titania and kaolin powder. In a regulatory context, this might have an impact on considering a substance false positive or false negative according to the European Commission (EC) recommendation of a definition of the term nanomaterial.

On the Operational Aspects of Measuring Nanoparticle Sizes

Nanoparticles are defined as elementary particles with a size between 1 and 100 nm for at least 50% (in number). They can be made from natural materials, or manufactured. Due to their small sizes, novel toxicological issues are raised and thus determining the accurate size of these nanoparticles is a major challenge. In this study, we performed an intercomparison experiment with the goal to measure sizes of several nanoparticles, in a first step, calibrated beads and monodispersed SiO₂ Ludox®, and, in a second step, nanoparticles (NPs) of toxicological interest, such as Silver NM-300 K and PVP-coated Ag NPs, Titanium dioxide A12, P25(Degussa), and E171(A), using commonly available laboratory techniques such as transmission electron microscopy, scanning electron microscopy, small-angle X-ray scattering, dynamic light scattering, wet scanning transmission electron microscopy (and its dry state, STEM) and atomic force microscopy. With monomodal distributed NPs (polystyrene beads and SiO₂ Ludox®), all tested techniques provide a global size value amplitude within 25% from each other, whereas on multimodal distributed NPs (Ag and TiO₂) the inter-technique variation in size values reaches 300%. Our results highlight several pitfalls of NP size measurements such as operational aspects, which are unexpected consequences in the choice of experimental protocols. It reinforces the idea that averaging the NP size from different biophysical techniques (and experimental protocols) is more robust than focusing on repetitions of a single technique. Besides, when characterizing a heterogeneous NP in size, a size distribution is more informative than a simple average value. This work emphasizes the need for nanotoxicologists (and regulatory agencies) to test a large panel of different techniques before making a choice for the most appropriate technique(s)/protocol(s) to characterize a peculiar NP.

Re-evaluation of silicon dioxide (E 551) as a food additive

The EFSA Panel on Food Additives and Nutrient Sources added to Food (ANS) provides a scientific opinion re-evaluating the safety of silicon dioxide (E 551) when used as a food additive. The forms of synthetic amorphous silica (SAS) used as E 551 include fumed silica and hydrated silica (precipitated silica, silica gel and hydrous silica). The Scientific Committee on Food (SCF) established a group acceptable daily intake (ADI) 'not specified' for silicon dioxide and silicates. SAS materials used in the available biological and toxicological studies were different in their physicochemical properties; their characteristics were not always described in sufficient detail. Silicon dioxide appears to be poorly absorbed. However, silicon-containing material (in some cases presumed to be silicon dioxide) was found in some tissues. Despite the limitations in the subchronic, reproductive and developmental toxicological studies, including studies with nano silicon dioxide, there was no indication of adverse effects. E 551 does not raise a concern with respect to genotoxicity. In the absence of a long-term study with nano silicon dioxide, the Panel could not extrapolate the results from the available chronic study with a material, which does not cover the full-size range of the nanoparticles that could be present in the food additive E 551, to a material complying with the current specifications for E 551. These specifications do not exclude the presence of nanoparticles. The highest exposure estimates were at least one order of magnitude lower than the no observed adverse effect levels (NOAELs) identified (the highest doses tested). The Panel concluded that the EU specifications are insufficient to adequately characterise the food additive E 551. Clear characterisation of particle size distribution is required. Based on the available database, there was no indication for toxicity of E 551 at the reported uses and use levels. Because of the limitations in the available database, the Panel was unable to confirm the current ADI 'not specified'. The Panel recommended some modifications of the EU specifications for E 551.

Quantification and size characterisation of silver nanoparticles in environmental aqueous samples and consumer products by single particle-ICPMS

Single particle-inductively coupled plasma mass spectrometry (SP-ICPMS) is a promising technique able to generate the number based-particle size distribution (PSD) of nanoparticles (NPs) in aqueous suspensions. However, SP-ICPMS analysis is not consolidated as routine-technique yet and is not typically applied to real test samples with unknown composition. This work presents a methodology to detect, quantify and characterise the number-based PSD of Ag-NPs in different environmental aqueous samples (drinking and lake waters), aqueous samples derived from migration tests and consumer products using SP-ICPMS. The procedure is built from a pragmatic view and involves the analysis of serial dilutions of the original sample until no variation in the measured size values is observed while keeping particle counts proportional to the dilution applied. After evaluation of the analytical figures of merit, the SP-ICPMS method exhibited excellent linearity (r²>0.999) in the range (1-25) × 10⁴ particlesmL^-1 for 30, 50 and 80nm nominal size Ag-NPs standards. The precision in terms of repeatability was studied according to the RSDs of the measured size and particle number concentration values and a t-test (p = 95%) at the two intermediate concentration levels was applied to determine the bias of SP-ICPMS size values compared to reference values. The method showed good repeatability and an overall acceptable bias in the studied concentration range. The experimental minimum detectable size for Ag-NPs ranged between 12 and 15nm. Additionally, results derived from direct SP-ICPMS analysis were compared to the results conducted for fractions collected by asymmetric flow-field flow fractionation and supernatant fractions after centrifugal filtration. The method has been successfully applied to determine the presence of Ag-NPs in: lake water; tap water; tap water filtered by a filter jar; seven different liquid silver-based consumer products; and migration solutions (pure water and sweat simulant) from plasters. Results obtained by SP-ICPMS were supported by transmission electron microscopy and energy dispersive spectroscopy characterisation, suggesting that the proposed methodology can be applied as a positive screening test in the simultaneous quantification and size characterisation of Ag-NPs in samples of environmental interest.

Nanomaterials in Food Products: A New Analytical Challenge

This chapter focuses on the problem of detecting, characterizing, and determining the concentration of nanomaterials in foods and other biological matrices. After providing an overview of the unique challenges associated with nanoparticle metrology in complex media, sample pretreatment methods (including extraction, digestion, and inline chromatographic separation), imaging analysis, and nanomaterial quantification methods are presented in detail. The chapter also addresses numerous methods under development, including atmospheric scanning electron microscopy, single-particle inductively coupled plasma mass spectrometry, immunological detection methods, and optical techniques such surface plasmon resonance. The chapter concludes with an overview of the research needs in this area.

The safety of nanostructured synthetic amorphous silica (SAS) as a food additive (E 551)

KEY MESSAGES

Particle sizes of E 551 products are in the micrometre range. The typical external diameters of the constituent particles (aggregates) are greater than 100 nm. E 551 does not break down under acidic conditions such as in the stomach, but may release dissolved silica in environments with higher pH such as the intestinal tract. E 551 is one of the toxicologically most intensively studied substances and has not shown any relevant systemic or local toxicity after oral exposure. Synthetic amorphous silica (SAS) meeting the specifications for use as a food additive (E 551) is and has always been produced by the same two production methods: the thermal and the wet processes, resulting in E 551 products consisting of particles typically in the micrometre size range. The constituent particles (aggregates) are typically larger than 100 nm and do not contain discernible primary particles. Particle sizes above 100 nm are necessary for E 551 to fulfil its technical function as spacer between food particles, thus avoiding the caking of food particles. Based on an in-depth review of the available toxicological information and intake data, it is concluded that the SAS products specified for use as food additive E 551 do not cause adverse effects in oral repeated-dose studies including doses that exceed current OECD guideline recommendations. In particular, there is no evidence for liver toxicity after oral intake. No adverse effects have been found in oral fertility and developmental toxicity studies, nor are there any indications from in vivo studies for an immunotoxic or neurotoxic effect. SAS is neither mutagenic nor genotoxic in vivo. In intact cells, a direct interaction of unlabelled and unmodified SAS with DNA was never found. Differences in the magnitude of biological responses between pyrogenic and precipitated silica described in some in vitro studies with murine macrophages at exaggerated exposure levels seem to be related to interactions with cell culture proteins and cell membranes. The in vivo studies do not indicate that there is a toxicologically relevant difference between SAS products after oral exposure. It is noted that any silicon dioxide product not meeting established specifications, and/or produced to provide new functionality in food, requires its own specific safety and risk assessment.