Separation and Tracing of Anthropogenic Magnetite Nanoparticles in the Urban Atmosphere

Magnetic iron oxide nanoparticles: An emerging threat for the environment and human health

Magnetic iron oxide nanoparticles (FexOy NPs, mainly Fe3O4 and γ-Fe2O3) are nanomaterials ubiquitously present in aquatic, terrestrial, and atmospheric environments, with a high prevalence and complex sources. Over the past decade, numerous reports have emerged on the presence of exogenous particles in human body, facilitated by the rapid development of separation and detection methods. The health risk associated with magnetic FexOy NP have garnered escalating attention due to their presence in human blood and brain tissues, especially for their potential association with neurodegenerative diseases like Alzheimer's disease. In this paper, we provide a comprehensive overview of sources, analysis methods, environmental impacts, and health risks of magnetic FexOy NP. Currently, most researches are primarily based on engineered FexOy NP, while reports about magnetic FexOy NP existing in real-world environments are still limited, especially for their occurrence levels in various environmental matrices, environmental transformation behavior, and biotoxic effects. Our study reviews this emerging pollutant, providing insights to address current research deficiencies and chart the course for future studies.

Natural, Incidental, and Engineered Nanomaterials in Surface Waters: Occurrence and Catalytic Reactivity Influences on Micropollutant Degradation Plus Nutrient Turnover

Nanomaterials (NMs)─whether natural, incidental, or engineered─are now documented to occur in aquatic environments, with concentrations of elements such as titanium, cerium, and palladium exceeding tens of parts per billion. While prior research has emphasized the toxicology of engineered NMs, their broader geochemical roles remain underexplored. Catalytically active NMs can influence key environmental processes, such as nutrient cycling and pollutant degradation, through photocatalytic, hydrolytic, and hydrogenation mechanisms. Reactive oxygen species (ROS) (including hydroxyl radicals, singlet oxygen, and hydrogen peroxide) produced via UVA or visible light photocatalysis on NM surfaces drive important transformations. Steady-state ROS concentrations (0.1-10 fM) are comparable to those from excited-state dissolved organic matter (DOM*). NMs can hydrolyze and facilitate the conversion of organic phosphorus to bioavailable inorganic forms or organic carbon to low-molecular-weight compounds, potentially fueling microbial food webs. However, major gaps remain regarding NM diversity, reactivity, and persistence. Addressing these requires integrating advanced nanoanalytical tools (e.g., ICP-MS, electron microscopy with EELS) and functional reactivity assays applied to environmentally sourced NMs over ecologically relevant time scales of days to months. This Perspective highlights NMs as dynamic, transient components of aquatic systems introduced through deposition, runoff, and biogenic activity, with implications for global biogeochemical cycling.

Ferrocene-derived magnetic fiber-particles from diesel exhaust: enhanced pulmonary toxicity via Bach1-SAT1-polyamine depletion

BACKGROUND AND AIM

Magnetic nanoparticles are key components of air pollution. The combustion of diesel engine fuels, especially with ferrocene doping to reduce emissions, may increase exposure to these particles and related health risks. This study aimed to reveal the generation and characterization of ferrocene-derived magnetic particles (FMP) in ferrocene-doped diesel exhaust, and to investigate its toxicities and associated mechanisms in an avian model.

METHODS

FMP was observed in ferrocene-doped diesel exhaust particles, and extracted with neodymium magnets. Extracted FMP was characterized, and exposed to hatchling chickens via aerosol inhalation. Pulmonary toxicities were assessed with pathological and molecular methods. Associated mechanisms were investigated with RNA-seq, in vitro cell culture, and in vivo gene silencing.

RESULTS

FMP was characterized to be fibrous, magnetic iron-containing carbon particles. Extracted FMP could directly induce pulmonary toxicity. Mechanistic investigations revealed molecular mechanism associated with ferroptosis via Bach1, SAT1 and polyamines depletion, and further confirmed with ferroptosis inhibitor treatment, Bach1 inhibitor treatment, supplementation of polyamines or SAT1 silencing.

CONCLUSIONS

Ferrocene doping could result in formation of magnetic particles in diesel exhaust. For the first time, magnetic fiber-like particles were extracted from ferrocene-doped DE particles, which is a potential source of magnetic particles in air pollution. To better balance emission control and health effects, further investigations are necessary.

Deposition of Air Pollution-Derived Magnetic Nanoparticles in Human Kidney Revealed by High-Resolution Microstructural Characterization

Exposure to air pollutants, especially fine particulate matter (PM2.5), has been recognized as a major contributor to the increasing prevalence of kidney diseases. However, until now, evidence for the translocation of airborne nanoparticles (NPs) in the human kidney has been lacking, hindering the understanding of the relationships between PM2.5 exposure and kidney diseases. Here, we report the discovery and analysis of airborne magnetite nanoparticles in human kidney stones (with mass concentrations ranging from 363 to 740 ng/g dry tissue weight) by high-resolution microstructural characterization. Notably, we established a methodology for highly selective extraction and accurate characterization of distinctive magnetite NPs and identified the abundant presence of these NPs with a distinctive core-shell structure of Fe3O4/SiO2 in both kidney stones and human blood. We demonstrate that such distinctive core-shell magnetite NPs are indicative of a coal-burning source. Hence, magnetite NPs deposited in the human kidneys in this study area most likely derived from air pollution emissions from coal-fired power plants and were transported via blood circulation to the kidney. Our results provide compelling evidence for understanding the systemic health risks of exposure to nanoparticulate, Fe-bearing air pollution and the associations observed between kidney diseases and PM2.5 exposure.

On the origin of metal species in the human brain: a perspective on key physicochemical properties

Normal functioning of the human brain is dependent on adequate regulation of essential metal nutrients. However, it is also highly sensitive to metal-mediated toxicity, linked to various neurodegenerative disorders. Exposure to environmental metal sources (especially to particulate air pollution) can stimulate toxicity and neuropathologic effects, which is particularly evident in populations chronically exposed to high levels of air pollution. Identifying the sources of metal-rich deposits in the human brain is important in not only distinguishing the effects of environmentally acquired metals from endogenous metal dysregulation, but also for tracing pollutant sources which may be subject to exposure control. This perspective reviews evidence for key physicochemical properties (size/morphology, chemical composition, oxidation state, magnetic properties, and isotopic composition) concerning their capacity to distinguish sources of metals in the brain. The scope for combining analytical techniques to study properties in tandem is also discussed.

Sand and dust storms: a growing global health threat calls for international health studies to support policy action

Sand and dust storms increasingly threaten global environmental and public health. To date, 150 countries are directly affected, with more than 100 classified as non-dust source regions. With climate change, these storms are expected to become more frequent and severe. Despite international awareness and initiatives, such as those led by the UN, crucial knowledge gaps continue to hinder effective, evidence-based public responses to sand and dust storms. In this Viewpoint, we review existing gaps in health research and highlight four key research priorities: the comprehensive health effects of sand and dust storms, including short-term and long-term exposures, diseases, regions, and health outcomes; the key particle sizes and toxic components of particles during sand and dust storms; the design of multicentre studies accounting for region-specific exposure patterns; and research on health outcomes attributable to particulate matter mixtures dominated by windblown dust versus other sources. We urgently call for international, collaborative, and multidisciplinary health studies considering sand and dust storm exposure characteristics and for the adoption of scientifically robust epidemiological methods in these studies.

Magnetite nanoparticles from representative coal fired power plants in China: Dust removal capture and their final atmospheric emission

Information on the emission of coal combustion-sourced magnetite nanoparticles (MNPs) is lacking, which is critical for their health-related risks. In this study, MNPs in coal fly ashes (CFAs) from various coal-fired power plants (CFPPs) in China equipped with various dust removal devices were extracted and quantified using single particle ICP-MS. The number concentrations of MNPs in CFAs captured by dust removal increased with stage, while their size decreased. Among all the dust removal devices, electrostatic-fabric-integrated precipitators showed the best removal of MNPs. Furthermore, throughout all the coal combustion by-products in a typical CFPP, MNPs in EFA (fly ash escaped from the stack) showed the highest number concentration (1.2 × 107 particles/mg) and lowest size (78 nm). Although the mass of CFA escaping through the stack is extremely low, it still had an emission rate of 1.9 × 1015 particles/h, contributing 3.56 % of the total emissions of MNPs in number. In addition, the purity of MNPs and their associated toxic metals showed a size-dependent variation pattern. As the particle size of MNPs decreased, the proportion of Fe in MNPs increased from 43 % in bottom ash (BA) to 84 % in EFA, while the abundance of trace toxic metals in EFA was 3.3 times higher than that of BA. These MNPs with the highest purity can adsorb elevated concentrations of toxic metals, and can be discharged directly into the atmosphere, posing a risk of synergistic toxicity.

Size-Dependent Elemental Composition in Individual Magnetite Nanoparticles Generated from Coal-Fired Power Plant Regulating Their Pulmonary Cytotoxicity

High-resolution characterization of magnetite nanoparticles (MNPs) derived from coal combustion activities is crucial to better understand their health-related risks. In this study, size distribution and elemental composition of individual MNPs from various coal fly ashes (CFAs) collected from a representative coal-fired power plant were analyzed using a single-particle inductively coupled plasma time-of-flight mass spectrometry technique. Majority (61-80%) of MNPs were identified as multimetal (mm)-MNPs, while the contribution of single metal (sm)-MNPs to the total increased throughout all the CFAs, reaching the highest in fly ash escaped through the stack (EFA). Among Fe-rich MNPs, Fe-sole and Fe-Al matrices were predominant, and Fe-sole MNPs were identified as the important carrier for toxic metals, with the highest mass contributions of toxic metals therein. Toxic potency results showed that the oxidative stress induced by MNPs was 1.2-2.2 times greater than those of <1 μm fractions in CFAs, while the reduction in cell viability showed no significant difference, elucidating that these MNPs can induce more distinct oxidative stress compared to cell toxicity. Based on structural equation model, MNP size can both directly and indirectly regulate the toxic potency, and the indirect regulation is through a size-dependent elemental composition of MNPs, including toxic metals. sm-MNPs and Fe-rich MNPs with Fe-sole, Fe-Cr, and Fe-Zn matrices can regulate the oxidative stress, whereas Cr, Zn, and Pb associated with Fe-sole, Fe-Al, Si-Fe, and Al-Fe MNPs showed significant effects on cell viability.

Foliar Uptake and Distribution of Metallic Oxide Nanoparticles in Maize (Zea mays L.) Leaf

The foliar uptake of Fe3O4, Cr2O3, CuO, and ZnO nanoparticles (NPs) by maize (Zea mays L.) was studied in a lab-scale experiment. The significant increase of Fe concentrations in leaves exposed to Fe3O4 was observed in both stomatal closing and stomatal opening treatments, suggesting the presence of a nonstomatal uptake. In parallel treatments with equal doses of Fe3O4 (∼200 nm), Cr2O3 (∼300 nm), CuO (∼30 nm), and ZnO (∼40 nm) (20-200 μg), the retention percentage of Fe in the leaves (21.0-69.0%) was higher than that of Cr, Cu, and Zn (0.5-14.0%). The steric hindrance effect seems more important for NPs of >200 nm, while hydrophobic surface and negative charge promote the foliar uptake of NPs smaller than 200 nm. The accumulation of NPs in the cuticle was observed through dark-field hyperspectral microscopy. Cr2O3, Fe3O4, and CuO NPs were difficult to penetrate the cuticle. In comparison, ZnO further migrated and distributed within the extracellular space of epidermal and mesophyll cells of the exposed leaf, possibly due to its comparatively higher solubility and hydrophilicity. The findings highlight the potential of the nonstomatal uptake, which might be a critical route for metallic oxide NPs to enter the food chain.

Metal-containing nanoparticles in road dust from a Chinese megacity over the last decade: Spatiotemporal variation and driving factors

As a crucial sink of metal-containing nanoparticles (MNPs), road dust can record their spatiotemporal variations in urban environments. In this study, taking Shanghai as a representative megacity in China, a total of 272 dust samples were collected in the winter and summer of 2013 and 2021/2022 to understand the spatiotemporal variations and driving factors of MNPs. The number concentrations of Fe-, Ti-, and Zn-containing NPs were 3.8 × 106 - 8.4 × 108, 2.3 × 106-1.4 × 108, and 6.0 × 105-2.3 × 108 particles/mg, respectively, according to single particle (sp)ICP-MS analysis. These MNPs showed significantly higher number concentrations in summer than in winter. Hotspots of Fe-containing NPs were more concentrated in industrial and traffic areas, Zn-containing NPs were mainly distributed in the central urban areas, while Ti-containing NPs were abundant in areas receiving high rainfall. The structural equation model results indicates that substantial rainfall in summer can help remove MNPs from atmospheric PM2.5 into dust, while in winter industrial and traffic activities were the primary contributors for MNPs. Moreover, the contribution of traffic emissions to MNPs has surpassed industrial one over the last decade, highlighting the urgency to control traffic-sourced MNPs, especially those from non-exhaust emissions by electric vehicles.

In Vivo Exposure Pathways of Ambient Magnetite Nanoparticles Revealed by Machine Learning-Aided Single-Particle Mass Spectrometry

Nanosized ultrafine particles (UFPs) from natural and anthropogenic sources are widespread and pose serious health risks when inhaled by humans. However, tracing the inhaled UFPs in vivo is extremely difficult, and the distribution, translocation, and metabolism of UFPs remain unclear. Here, we report a label-free, machine learning-aided single-particle inductively coupled plasma mass spectrometry (spICP-MS) approach for tracing the exposure pathways of airborne magnetite nanoparticles (MNPs), including external emission sources, and distribution and translocation in vivo using a mouse model. Our results provide quantitative analysis of different metabolic pathways in mice exposed to MNPs, revealing that the spleen serves as the primary site for MNP metabolism (84.4%), followed by the liver (11.4%). The translocation of inhaled UFPs across different organs alters their particle size. This work provides novel insights into the in vivo fate of UFPs as well as a versatile and powerful platform for nanotoxicology and risk assessment.

Precisely Identifying the Sources of Magnetic Particles by Hierarchical Classification-Aided Isotopic Fingerprinting

Magnetic particles (MPs), with magnetite (Fe3O4) and maghemite (γ-Fe2O3) as the most abundant species, are ubiquitously present in the natural environment. MPs are among the most applied engineered particles and can be produced incidentally by various human activities. Identification of the sources of MPs is crucial for their risk assessment and regulation, which, however, is still an unsolved problem. Here, we report a novel approach, hierarchical classification-aided stable isotopic fingerprinting, to address this problem. We found that naturally occurring, incidental, and engineered MPs have distinct Fe and O isotopic fingerprints due to significant Fe/O isotope fractionation during their generation processes, which enables the establishment of an Fe-O isotopic library covering complex sources. Furthermore, we developed a three-level machine learning model that not only can distinguish the sources of MPs with a high precision (94.3%) but also can identify the multiple species (Fe3O4 or γ-Fe2O3) and synthetic routes of engineered MPs with a precision of 81.6%. This work represents the first reliable strategy for the precise source tracing of particles with multiple species and complex sources.

Impact of airborne iron oxide nanoparticles on Tillandsia usneoides as a model plant to assess pollution in heavy traffic areas

Due to the increasing evidence of widespread sub-micron pollutants in the atmosphere, the impact of airborne nanoparticles is a subject of great relevance. In particular, the smallest particles are considered the most active and dangerous, having a higher surface/volume ratio. Here we tested the effect of iron oxide (Fe3O4) nanoparticles (IONPs) with different mean diameter and size distribution on the model plant Tillandsia usneoides. Strands were placed in home-built closed boxes and exposed to levels of airborne IONPs reported for the roadside air, i.e. in the order of 107 - 108 items m-2. Plant growth and other morpho-physiological parameters were monitored for two weeks, showing that exposure to IONPs significantly reduced the length increment of the treated strands with respect to controls. A dose-dependence of this impairing effect was found only for particles with mean size of a few tens of nanometers. These were also proved to be the most toxic at the highest concentration tested. The IONP-induced hamper in growth was correlated with altered concentration of macro- and micronutrients in the plant, while no significant variation in photosynthetic activity was detected in treated samples. Microscopy investigation showed that IONPs could adhere to the plant surface and were preferentially located on the trichome wings. Our results report, for the first time, evidence of the negative effects of airborne IONP pollution on plant health, thus raising concerns about related environmental risks. Future research should be devoted to other plant species and pollutants to assess the impact of airborne pollution on plants and devise suitable attenuation practices.

Airborne magnetite nanoparticles induced early vascular pathologies by disrupting lipid metabolism under high-fat dietary patterns

Magnetite nanoparticles (MNPs) have been extensively detected in the atmospheric environment and implicated as a prominent threat to atherosclerosis, a chronic vascular inflammatory disease. Due to globalization and economic development, the dramatic shift in diet from traditional to high-fat dietary patterns aggravated atherosclerosis progression induced by environmental factors. However, limited knowledge is available regarding vascular risks and underlying mechanisms of airborne MNPs in high-risk populations with high-fat dietary habits. Herein, we demonstrated that MNPs exerted a proatherogenic effect under high-fat dietary patterns, leading to aortic wall thickening, elastic fiber disorganization, macrophage infiltration, and local inflammation. Based on the correlation analysis between MNPs and PM group, we identified that MNPs might be a key PM component in atherogenic toxicity. MNPs exposure disturbed the dynamic process of lipid metabolism, manifested as aortic lipid accumulation, dyslipidemia, and hepatic lipid metabolism disorder, which was modulated by the JAK-STAT pathway. Overall, these findings provide new insight into understanding the cardiovascular risks and mechanisms of MNPs among high-risk populations.

Deep Neural Network-Based Electron Microscopy Image Recognition for Source Distinguishing of Anthropogenic and Natural Magnetic Particles

Deep learning models excel at image recognition of macroscopic objects, but their applications to nanoscale particles are limited. Here, we explored their potential for source-distinguishing environmental particles. Transmission electron microscopy (TEM) images can reveal distinguishable features in particle morphology from various sources, but cluttered foreground objects and scale variations pose challenges to visual recognition models. In this proof-of-concept work, we proposed a novel instance segmentation model named CoMask to tackle these issues with atmospheric magnetic particles, a key species of PM2.5. CoMask features a densely connected feature extraction module to excavate multiscale spatial cues at the single-particle level and enlarges the receptive field size for improved representation capability. We also employed a collaborative learning strategy to further improve performance. Compared with other state-of-the-art models, CoMask was competitive on benchmark and TEM data sets. The application of CoMask not only enables the source-distinguishing of magnetic particles but also opens up a new vista for machine learning applications.

Combustion conditions and feed coals regulating the Fe- and Ti-containing nanoparticles in various coal fly ash

Quantitative characteristics and sizes of nanoparticles (NPs) in coal fly ash (CFA) produced in coal-fired power plants as a function of coal type and plant design will help reveal the NP emission likelihood and their environmental implications. However, little is known about how combustion conditions and types of coal regulate the NP abundance in CFAs. In this study, based on single particle (SP)-ICP-MS technology, particle number concentrations (PNCs) and sizes of Fe- and Ti-containing NPs in CFAs were determined for samples collected from power plants of different designs and burning different types of coal. The PNCs of Fe- and Ti-containing NPs in all CFAs measured were in the range of 1.3 × 10⁷ - 3.4 × 10⁸ and 6.8 × 10⁶ - 2.2 × 10⁸ particles/mg, with the average particle sizes of 111 nm and 87 nm, respectively. The highest Fe-NP PNCs likely relate to the highest contents of Fe and pyrite in the feed coal. In addition, high TOC in CFAs are associated with metal-containing NPs, resulting in elevated abundances of these NPs with relatively large sizes. Moreover, elevated PNCs of NPs were found in CFAs produced by coal-fired power plants burning low-rank coals and with small installed capacity (especially those under 100-MW units). Compared to cyclone filters, ESPs and FFs with higher removal efficiency typically retain more Fe-/Ti- containing NPs with smaller sizes. Based on a structural equation (SE) model, raw coal properties (coal rank and Fe/Ti content), boiler types, and efficiency of particulate emission control devices likely indirectly affect PNCs of Fe- and Ti-containing NPs by influencing TOC contents and their corresponding metal concentrations of CFAs. This study provides the first analytic and comprehensive information concerning the direct and indirect regulating factors on NPs in various CFAs.

Systematic and Bibliometric Analysis of Magnetite Nanoparticles and Their Applications in (Biomedical) Research

Recent reports show air pollutant magnetite nanoparticles (MNPs) in the brains of people with Alzheimer's disease (AD). Considering various field applications of MNPs because of developments in nanotechnology, the aim of this study is to identify major trends and data gaps in research on magnetite to allow for relevant environmental and health risk assessment. Herein, a bibliometric and systematic analysis of the published magnetite literature (n = 31 567) between 1990 to 2020 is completed. Following appraisal, publications (n = 244) are grouped into four time periods with the main research theme identified for each as 1990-1997 "oxides," 1998-2005 "ferric oxide," 2006-2013 "pathology," and 2014-2020 "animal model." Magnetite formation and catalytic activity dominate the first two time periods, with the last two focusing on the exploitation of nanoparticle engineering. Japan and China have the highest number of citations for articles published. Longitudinal analysis indicates that magnetite research for the past 30 years shifted from environmental and industrial applications, to biomedical and its potential toxic effects. Therefore, whilst this study presents the research profile of different countries, the development in research on MNPs, it also reveals that further studies on the effects of MNPs on human health is much needed.

The foam cell formation associated with imbalanced cholesterol homeostasis due to airborne magnetite nanoparticles exposure

Fine particulate matter (PM) is a leading environmental cause for the increased morbidity and mortality of atherosclerosis (AS) worldwide, but little is known about the toxic component and disturbance of PM exposure on foam cell formation, a crucial pathological process in AS. Airborne magnetite nanoparticles (NPs) have been reported to be detected in human serum, which inevitably encounter with macrophages in atherosclerotic plaques, thus throwing potential disturbance on the formation of macrophage-derived foam cells. Here we comprehensively unveiled that the environmental concentrations of PM exposure triggered and potentiated the formation of macrophage-derived foam cells using both real-ambient PM exposed mice and atherosclerosis mice models, including high-fat diet (HFD)-fed mice and apolipoprotein E (ApoE)-deficient mice. The in vitro model further defined the dose-dependent response of PM treatment on foam cell formation. Interestingly, airborne magnetite NPs rather than non-magnetic NPs at the same concentration were demonstrated to be the key toxic component of PM in the promoted foam cell formation. Furthermore, magnetite NPs exposure led to abnormal cholesterol accumulation in macrophages, which was attributed to the attenuation of cholesterol efflux and enhancement of lipoprotein uptake, but independent of cholesterol esterification. The in-depth data revealed that magnetite NPs accelerated the protein ubiquitination and subsequent degradation of SR-B1, a crucial transporter of cholesterol efflux. Collectively, these findings for the first time identified magnetite NPs as one key toxic component of PM-promoted foam cell formation, and provided new insight of abnormal cholesterol metabolism into the pathogenesis of PM-induced atherosclerosis.

Inflammation and accompanied disrupted hematopoiesis in adult mouse induced by rare earth element nanoparticles

Rare earth element nanoparticles (REE NPs) or agents have been used extensively in various fields. Human exposure to REE NPs is an increasing concern. To date, REE NP-mediated comprehensive immune responses after incorporation into the body remain unclear. In our study, using gadolinium oxide NPs (Gd₂O₃) as a typical REE NP, we systematically investigated immune responses in vivo. The liver and spleen were the main sites where Gd₂O₃ retained and accumulated, while Gd₂O₃ content per unit tissue mass in the spleen was 4.4 times higher than that in the liver. Gd₂O₃ increased the number of monocyte-derived macrophages and myeloid-derived dendritic cells (M-DCs) in the liver. In the spleen, Gd₂O₃ caused infiltration of neutrophils, M-DCs, and B cells. The accumulation of Gd₂O₃ in the liver or spleen also contributed to an increased concentration of cytokines in peripheral blood. In both the bone marrow and spleen, Gd₂O₃ led to increased populations of hematopoietic stem cells (HSCs), multipotent progenitors, and common lymphoid progenitors. Compared to the decreased monocytes in peripheral blood on day 2, a significant decrease of circulating lymphocytes on day 7 was still observed, suggesting the exposure duration led to variable effects. This might be explained by the sustained accumulation of Gd₂O₃ in the liver and spleen. Together, our study systemically depicted the alterations in mature immune alterations together with hematopoiesis in both myeloid and lymphoid lineages induced by Gd₂O₃ exposure. Our findings will facilitate a comprehensive understanding of the interactions of immune system with REE NPs in vivo.

Current Methods and Prospects for Analysis and Characterization of Nanomaterials in the Environment

Analysis and characterization of naturally occurring and engineered nanomaterials in the environment are critical for understanding their environmental behaviors and defining real exposure scenarios for environmental risk assessment. However, this is challenging primarily due to the low concentration, structural heterogeneity, and dynamic transformation of nanomaterials in complex environmental matrices. In this critical review, we first summarize sample pretreatment methods developed for separation and preconcentration of nanomaterials from environmental samples, including natural waters, wastewater, soils, sediments, and biological media. Then, we review the state-of-the-art microscopic, spectroscopic, mass spectrometric, electrochemical, and size-fractionation methods for determination of mass and number abundance, as well as the morphological, compositional, and structural properties of nanomaterials, with discussion on their advantages and limitations. Despite recent advances in detecting and characterizing nanomaterials in the environment, challenges remain to improve the analytical sensitivity and resolution and to expand the method applications. It is important to develop methods for simultaneous determination of multifaceted nanomaterial properties for in situ analysis and characterization of nanomaterials under dynamic environmental conditions and for detection of nanoscale contaminants of emerging concern (e.g., nanoplastics and biological nanoparticles), which will greatly facilitate the standardization of nanomaterial analysis and characterization methods for environmental samples.

Internal Exposure and Distribution of Airborne Fine Particles in the Human Body: Methodology, Current Understandings, and Research Needs

Exposure to airborne fine particles (PM_2.5, particulate matter with aerodynamic diameter <2.5 μm) severely threatens global human health. Understanding the distribution and processes of inhaled PM_2.5 in the human body is crucial to clarify the causal links between PM_2.5 pollution and diseases. In contrast to extensive research on the emission and formation of PM_2.5 in the ambient environment, reports about the occurrence and fate of PM_2.5 in humans are still limited, although many studies have focused on the exposure and adverse effects of PM_2.5 with animal models. It has been shown that PM_2.5, especially ultrafine particles (UFPs), have the potential to go across different biological barriers and translocate into different human organs (i.e., blood circulation, brain, heart, pleural cavity, and placenta). In this Perspective, we summarize the factors affecting the internal exposure of PM_2.5 and the relevant analytical methodology and review current knowledge about the exposure pathways and distribution of PM_2.5 in humans. We also discuss the research challenges and call for more studies on the identification and characterization of key toxic species of PM_2.5, quantification of internal exposure doses in the general population, and further clarification of translocation, metabolism, and clearance pathways of PM_2.5 in the human body. In this way, it is possible to develop toxicity-based air quality standards instead of the currently used mass-based standards.

Characteristics of iron-containing magnetic particles in household dust from an urban area: A case study in the megacity of Shanghai

In order to characterize the magnetic properties and trace sources of household dust particles, magnetic measurements, geochemical and SEM/TEM analyses were performed on vacuum dust from 40 homes in Shanghai, China. Iron-containing magnetic particles (IMPs) in the household dust were dominated by magnetite, while maghemite, hematite and metallic iron were also present. The IMPs were mainly composed of coarse-grained particles (e.g., >0.1 µm). Ultrafine superparamagnetic (SP) grains (<30 nm) increased proportionately with the abundance of the total IMPs. Household dust had more and coarser IMPs than background soil, but less and finer IMPs than street dust and industrial emissions (coal combustion and metallurgy). Metallic Fe and spherical IMPs, originating from brake wear abrasion and coal combustion, respectively, have been observed using the SEM/TEM. Contents of magnetic particles were positively correlated to Mo, Ni and Sb, while HIRM was associated with As, Mo, Pb and Sb. The multiple lines of evidence including magnetic measurements, geochemical and SEM/TEM analyses suggested that industrial and traffic emissions and street dust were dominant contributors to the IMPs. Such an approach can help to establish more precisely the sources of household dust particles and could be applied to other indoor contexts and further urban environments.

Dry Sliding Behavior and Particulate Emissions of a SiC-graphite Composite Friction Material Paired with HVOF-Coated Counterface

Dry sliding wear tests and corresponding particulate matter (PM) analysis were conducted on a newly developed SiC-graphite-based composite friction material, paired with two types of HVOF counterface/discs: WC-CoCr and WC-FeCrAlY coatings, with a conventional martensitic stainless steel counterface as a reference. The trials were conducted on a pin-on-disc testing equipment at room temperature and a constant sliding velocity and contact pressure of 7 m/s and 0.5 MPa, respectively. The coefficient of friction (CoF) curves with the uncoated disc exhibited considerable fluctuations. On the other hand, the coated discs featured an increase in the CoF at the beginning of the tests, followed by either a continuous reduction until the end of the testing duration or the attainment of a steady state regime. The pin wear and emissions with both coatings were appreciably lower when compared to the trials with the uncoated disc. The evaluation of the friction layer observed a significant contribution of the counterface for all the pairings. The PM analysis was conducted on the particles that were lying in the range of 10 μm and 2.5 μm on a scanning electron microscope (SEM), and particles from 2.5 μm and 1 μm on transmission electron microscope (TEM), with an emphasis on the particles that were detached from the pin surface and friction layer to explain the wear mechanisms for each pairing. Through this, the need for the proper selection of both friction material and counterface to avoid the emission of harmful compounds in the environment was highlighted.

Size-resolved, quantitative evaluation of the magnetic mineralogy of airborne brake-wear particulate emissions

Exposure to particulate air pollution has been associated with a variety of respiratory, cardiovascular and neurological problems, resulting in increased morbidity and mortality worldwide. Brake-wear emissions are one of the major sources of metal-rich airborne particulate pollution in roadside environments. Of potentially bioreactive metals, Fe (especially in its ferrous form, Fe²⁺) might play a specific role in both neurological and cardiovascular impairments. Here, we collected brake-wear particulate emissions using a full-scale brake dynamometer, and used a combination of magnetic measurements and electron microscopy to make quantitative evaluation of the magnetic composition and particle size of airborne emissions originating from passenger car brake systems. Our results show that the concentrations of Fe-rich magnetic grains in airborne brake-wear emissions are very high (i.e., ~100-10,000 × higher), compared to other types of particulate pollutants produced in most urban environments. From magnetic component analysis, the average magnetite mass concentration in total PM₁₀ of brake emissions is ~20.2 wt% and metallic Fe ~1.6 wt%. Most brake-wear airborne particles (>99 % of particle number concentration) are smaller than 200 nm. Using low-temperature magnetic measurements, we observed a strong superparamagnetic signal (indicative of ultrafine magnetic particles, < ~30 nm) for all of the analysed size fractions of airborne brake-wear particles. Transmission electron microscopy independently shows that even the larger size fractions of airborne brake-wear emissions dominantly comprise agglomerates of ultrafine (<100 nm) particles (UFPs). Such UFPs likely pose a threat to neuronal and cardiovascular health after inhalation and/or ingestion. The observed abundance of ultrafine magnetite particles (estimated to constitute ~7.6 wt% of PM_0.2) might be especially hazardous to the brain, contributing both to microglial inflammatory action and excess generation of reactive oxygen species.

Magnetic Fractions of PM2.5, PM2.5-10, and PM10 from Coal Fly Ash as Environmental Pollutants

Characterization of magnetic particulate matter (PM) in coal fly ashes is critical to assessing the health risks associated with industrial coal combustion and for future applications of fine fractions that will minimize solid waste pollution. In this study, magnetic narrow fractions of fine ferrospheres related to environmentally hazardous PM_2.5, PM_2.5-10, and PM₁₀ were for the first time separated from fly ash produced during combustion of Ekibastuz coal. It was determined that the average diameter of globules in narrow fractions is 1, 2, 3, and 7 μm. The major components of chemical composition are Fe₂O₃ (57-60) wt %, SiO₂ (25-28 wt %), and Al₂O₃ (10-12 wt %). The phase composition is represented by crystalline phases, including ferrospinel, α-Fe₂O₃, ε-Fe₂O₃, mullite, and quartz, as well as the amorphous glass phase. Mössbauer spectroscopy and magnetic measurements confirmed the formation of nanoscale particles of ε-Fe₂O₃. Stabilization of the ε-Fe₂O₃ metastable phase, with quite ideal distribution of iron cations, occurs in the glass matrix due to the rapid cooling of fine globules during their formation from mineral components of coal.

A new sample preparation protocol for SEM and TEM particulate matter analysis

A new methodology has been developed to prepare electron microscopy, both SEM and TEM, specimens starting from particulate matter collected using environmental sampling systems. The approach is based on the extraction of the particles to be analyzed from the harvesting substrates. The extracted particles can be directly observed in an SEM, possibly in low-vacuum mode to prevent electrical charging. In order to prepare electron transparent samples, TEM observations require a further step, consisting in embedding the particles in an electron transparent carbon film deposited before dissolving the acetate extracting substrate. The protocol has been tested by analyzing particles collected during bench tests on brake pads and discs, carried out on a dynamometer equipped with a particulate matter sampling apparatus. The main advantages of the approach are: the complete extraction of the particulate matter specimens from the original substrates, that in this way do not interfere with the analyses; the extracted samples retain the topological information of the collection in the specimens prepared for SEM; possibility to be applied to any kind of particulate matter harvesting substrates.

Separation and Analysis of Nanoscale Zero-Valent Iron from Soil

Nanoscale zero-valent iron (nZVI) has become one of the most used engineered nanoparticles for soil remediation. However, isolating nZVI particles from a complex soil matrix for their accurate particle characterizations and transport distance measurements is still challenging. Here, this study established a new analysis approach combining ultrasound-assisted solvent extraction, magnetic separation, and single particle inductively coupled plasma mass spectrometry (SP-ICP-MS) analysis to isolate nZVI particles from soils and quantify their concentration and size. The interference from natural Fe-containing substances on nZVI analysis could be efficiently minimized by magnetic separation and dilution. After the optimization of extraction solvent type/concentration (i.e., 2.5 mM tetrasodium pyrophosphate) and ultrasonication time (i.e., 30 min), acceptable recoveries in both particle number (62.0 ± 10.8%-96.1 ± 4.8%) and Fe mass (70.6 ± 12.0%-119 ± 18%) could be achieved for different sizes (50 and 100 nm) and concentrations (50, 100, and 500 μg g^-1) of spiked nZVI from six soils. The detection limits of particle size and concentration were approximately 43.1 nm and 50 μg nZVI per gram soil, respectively. These results provide a feasible approach to quantify the nZVI concentration and size in complex soil matrices, which will allow the improvements to characterize and track the nZVI particles in the field, promote the use of nZVI particles for soil remediation, and better assess their environmental implications.

Metal-Containing Nanoparticles in Low-Rank Coal-Derived Fly Ash from China: Characterization and Implications toward Human Lung Toxicity

Characterization of nanoparticles (NPs) in coal fly ashes (CFAs) is critical for better understanding the potential health-related risks resulting from coal combustion. Based on single-particle (SP)-inductively coupled plasma mass spectrometry (ICP-MS) coupled with transmission electron microscopy techniques, this study is the first to determine the concentrations and sizes of metal-containing NPs in low-rank coal-derived fly ashes. Despite only comprising a minor component of the studied CFAs by mass, NPs were the dominant fraction by particle number. Fe- and Ti-containing NPs were identified as the dominant NPs with their particle number concentration ranging from 2.5 × 10⁷ to 2.5 × 10⁸ particles/mg. In addition, the differences of Fe-/Ti-containing NPs in various CFAs were regulated by the coalification degree of feed coals and combustion conditions of all of the low-rank CFAs tested. In the cases where these NPs in CFAs become airborne and are inhaled, they can be taken up in pulmonary interstitial fluids. This study shows that in Gamble's solution (a lung fluid simulant), 51-87% of Fe and 63-89% of Ti (ratio of the mass of Fe-/Ti-containing NPs to the total mass of Fe/Ti) exist in the NP form and remain suspended in pulmonary fluid simulants. These NPs are bioavailable and may induce lung tissue damage.

Prolific shedding of magnetite nanoparticles from banknote surfaces

Here, we use magnetic methods first to quantify the content of strongly magnetic particles of banknotes (US dollars, USD, and British pounds sterling, GBP), and then examine the possibility of their release from handled banknote surfaces. The content of magnetic particles, from magnetic remanence measurements, for the USD and paper GBP banknotes is high; greater, for example, than that in vehicle engine-exhaust emissions, and similar to that for airborne roadside particulate matter (PM). Our magnetic analyses of USD and GBP banknotes, and of the ink pigment widely used in their printing, reveal not only that the banknotes are highly magnetic, but also that strongly magnetic, nano-sized particles are readily and prolifically shed from their surfaces (especially from the USD banknotes). A common practice, prior to increased automation, was for bank tellers to count banknotes by licking a finger to adhere to each successive counted note, and thus speed up the manual counting process. Given the rate of particle shedding reported here, this traditional manual counting procedure must have resulted in prolific transfer of iron-rich nanoparticles both to the fingers and thence to the tongue. We hypothesise that, pre-automation, magnetite and other metal-bearing nanoparticles were repetitively and frequently ingested by bank tellers, and subsequently entered the brain directly via the taste nerve pathway, and/or indirectly via the systemic circulation and the neuroenteric system. This hypothesis may plausibly account for the reported and currently unexplained association between elevated neurodegeneration-related mortality odds ratios and this specific occupation.

Variation in the concentration and regional distribution of magnetic nanoparticles in human brains, with and without Alzheimer's disease, from the UK

The presence of magnetic nanoparticles (MNPs) in the human brain was attributed until recently to endogenous formation; associated with a putative navigational sense, or with pathological mishandling of brain iron within senile plaques. Conversely, an exogenous, high-temperature source of brain MNPs has been newly identified, based on their variable sizes/concentrations, rounded shapes/surface crystallites, and co-association with non-physiological metals (e.g., platinum, cobalt). Here, we examined the concentration and regional distribution of brain magnetite/maghemite, by magnetic remanence measurements of 147 samples of fresh/frozen tissues, from Alzheimer's disease (AD) and pathologically-unremarkable brains (80-98 years at death) from the Manchester Brain Bank (MBB), UK. The magnetite/maghemite concentrations varied between individual cases, and different brain regions, with no significant difference between the AD and non-AD cases. Similarly, all the elderly MBB brains contain varying concentrations of non-physiological metals (e.g. lead, cerium), suggesting universal incursion of environmentally-sourced particles, likely across the geriatric blood-brain barrier (BBB). Cerebellar Manchester samples contained significantly lower (~ 9×) ferrimagnetic content compared with those from a young (29 years ave.), neurologically-damaged Mexico City cohort. Investigation of younger, variably-exposed cohorts, prior to loss of BBB integrity, seems essential to understand early brain impacts of exposure to exogenous magnetite/maghemite and other metal-rich pollution particles.

Magnetic nanoparticles: An indicator of health risks related to anthropogenic airborne particulate matter

Due to their small dimensions, airborne particles are able to penetrate through inhalation into many human organs, from the lungs to the cardiovascular system and the brain, which can threaten our health. This work establishes a novel approach of collecting quantitative data regarding the fraction, the composition and the size distribution of combustion-emitted particulate matter through the magnetic characterization and analysis of samples received by common air pollution monitoring. To this end, SQUID magnetometry measurements were carried out for samples from urban and suburban areas in Thessaloniki, the second largest city of Greece, taking into consideration the seasonal and weekly variation of airborne particles levels as determined by occurring traffic and meteorological conditions. The level of estimated magnetically-responding atmospheric particulate matter was at least 0.5 % wt. of the collected samples, mostly being present in the form of ultrafine particles with nuclei sizes of approximately 14 nm and their aggregates. The estimated quantities of magnetic particulate matter show maximum values during autumn months (0.8 % wt.) when increased commuting takes place, appearing higher in the city center by up to 50% than those in suburban areas. In combination with high-resolution transmission electron imaging and elemental analysis, it was found that Fe₃O₄ and similar ferrites, some of them attached to heavy metals (Co, Cr), are the dominant magnetic contributors arising from anthropogenic high-temperature processes, e.g. due to traffic emissions. Importantly, nasal cytologic samples collected from residents of both central and suburban areas showed same pattern in what concerns magnetic behavior, thus verifying the critical role of nanosized magnetic particles in the assessment of air pollution threats. Despite the inherent statistical limitations of our study, such findings also indicate the potential transmission of infectious pathogens by means of pollution-derived nanoparticles into the respiratory system of the human body.