Single-Particle Analysis of the Photodegradation of Submicron Polystyrene Particles Using Infrared Photothermal Heterodyne Imaging

Sunlight irradiation is the predominant process for degrading plastics in the environment, but our current understanding of the degradation of smaller, submicron (<1000 nm) particles is limited due to prior analytical constraints. We used infrared photothermal heterodyne imaging (IR-PHI) to simultaneously analyze the chemical and morphological changes of single polystyrene (PS) particles (∼1000 nm) when exposed to ultraviolet (UV) irradiation (λ = 250-400 nm). Within 6 h of irradiation, infrared bands associated with the backbone of PS decreased, accompanied by a reduction in the particle size. Concurrently, the formation of several spectral features due to photooxidation was attributed to ketones, carboxylic acids, aldehydes, esters, and lactones. Spectral outcomes were used to present an updated reaction scheme for the photodegradation of PS. After 36 h, the average particle size was reduced to 478 ± 158 nm. The rates of size decrease and carbonyl band area increase were -24 ± 3.0 nm h-1 and 2.1 ± 0.6 cm-1 h-1, respectively. Using the size-related rate, we estimated that under peak terrestrial sunlight conditions, it would take less than 500 h for a 1000 nm PS particle to degrade to 1 nm.

The impact of biofilms and dissolved organic matter on the transport of nanoparticles in field-scale streams

The fate and transport of nanoparticles (NPs) in streams is critical for understanding their overall environmental impact. Using a unique field-scale stream at the Notre Dame-Linked Experimental Ecosystem Facility, we investigated the impact of biofilms and the presence of dissolved organic matter (DOM) on the transport of titanium dioxide (TiO₂) NPs. Experimental breakthrough curves were analyzed using temporal moments and fit using a mobile-immobile model. The presence of biofilms in the stream severely reduced the transport of the TiO₂ NPs, but this was mitigated by the presence of DOM. Under minimal biofilm conditions, the presence of DOM increased the mass recovery of TiO₂ from 4.2% to 32% for samples taken 50 m downstream. For thriving biofilm conditions only 0.5% of the TiO₂ mass was recovered (50 m), but the presence of DOM improved the mass recovery TiO₂ to 36%. The model was suitable for predicting early, peak, tail, and truncation time portions of the breakthrough curves, which attests to its ability to capture a range of processes in the mobile and immobile domains of the stream. The model outcomes supported the hypothesis that DOM changed the interaction of NP-biofilm from an irreversible to a reversible process. Collectively, these outcomes stress the importance of considering biogeological complexity when predicting the transport of NPs in streams.

Critical Review of Thermal Decomposition of Per- and Polyfluoroalkyl Substances: Mechanisms and Implications for Thermal Treatment Processes

Per- and polyfluoroalkyl substances (PFASs) are fluorinated organic chemicals that are concerning due to their environmental persistence and adverse human and ecological effects. Remediation of environmental PFAS contamination and their presence in consumer products have led to the production of solid and liquid waste streams containing high concentrations of PFASs, which require efficient and cost-effective treatment solutions. PFASs are challenging to defluorinate by conventional and advanced destructive treatment processes, and physical separation processes produce waste streams (e.g., membrane concentrate, spent activated carbon) requiring further post-treatment. Incineration and other thermal treatment processes are widely available, but their use in managing PFAS-containing wastes remains poorly understood. Under specific operating conditions, thermal treatment is expected to mineralize PFASs, but the degradation mechanisms and pathways are unknown. In this review, we critically evaluate the thermal decomposition mechanisms, pathways, and byproducts of PFASs that are crucial to the design and operation of thermal treatment processes. We highlight the analytical capabilities and challenges and identify research gaps which limit the current understanding of safely applying thermal treatment to destroy PFASs as a viable end-of-life treatment process.

Using Infrared Photothermal Heterodyne Imaging to Characterize Micro- and Nanoplastics in Complex Environmental Matrices

A key challenge for addressing micro- and nanoplastics (MNPs) in the environment is being able to characterize their chemical properties, morphologies, and quantities in complex matrices. Current techniques, such as Fourier transform infrared spectroscopy, provide these broad characterizations but are unsuitable for studying MNPs in spectrally congested or complex chemical environments. Here, we introduce a new, super-resolution infrared absorption technique to characterize MNPs, called infrared photothermal heterodyne imaging (IR-PHI). IR-PHI has a spatial resolution of ∼300 nm and can determine the chemical identity, morphology, and quantity of MNPs in a single analysis with high sensitivity. Specimens are supported on CaF₂ coverslips under ambient conditions from where we (1) quantify MNPs from nylon tea bags after steeping in ultrapure water at 25 and 95 °C, (2) identify MNP chemical or morphological changes after steeping at 95 °C, and (3) chemically identify MNPs in sieved road dust. In all cases, no special sample preparation was required. MNPs released from nylon tea bags at 25 °C were fiber-like and had characteristic IR frequencies corresponding to thermally extruded nylon. At 95 °C, degradation of the nylon chemical structure was observed via the disappearance of amide group IR frequencies, indicating chain scission of the nylon backbone. This degradation was also observed through morphological changes, where MNPs altered shape from fiber-like to quasi-spherical. In road dust, IR-PHI analysis reveals the presence of numerous aggregate and single-particle (<3 μm) MNPs composed of rubber and nylon.

Identification of transformation products to characterize the ability of a natural wetland to degrade synthetic organic pollutants

Natural wetlands have been recognized as a natural reactor for degradation and elimination of environmental pollutants. The Upo Wetland, the largest inland wetland in Korea, is mainly surrounded by agricultural lands and it is susceptible to contamination from excess nutrient loads and synthetic organic contaminants (SOCs) (e.g., pesticides). The aim of this study was to identify major SOCs in the wetland and evaluate their degradation. We used high resolution mass spectrometry (HRMS) with a two-step analysis approach (i.e., 1^st analysis for target measurement along with suspect and non-target screening (SNTS) and 2^nd analysis for complimentary suspect screening) to identify and quantify the transformation products (TPs) of the identified parent SOCs. Quantitative analysis of 30 targets, mainly including pesticides, showed that fungicides were the major SOCs detected in the wetland, accounting for about 50% of the composition ratio of the total SOCs quantified. Orysastrobin occurred at the highest mean concentration (>700 ng/L), followed by two other fungicides, carbendazim and tricyclazole. The first analysis (SNTS) tentatively identified 39 TPs (30 by suspect, 9 by non-target screening) of 14 parent pesticides. Additionally, the second analysis (complimentary suspect screening) identified 9 more TPs. Among the 48 total TPs identified, 7 were confirmed with reference standards. The identification of the remaining TPs had a high confidence level (e.g., level 2 or 3). Regarding transport though the wetland, most TPs showed greater peak area ratios (i.e., the relative portion of chromatographic area of the TPs to the parent compound) at the outlet point of the wetland compared to the inlet point. The risk quotient, which was calculated using the concentrations of parent compounds, decreased toward the outlet, demonstrating the degradation capacity of the wetland. The estimates for biodegradability, hydrophobicity, and toxicity by an in-silico quantitative structure-activity relationship (QSAR) model indicated a lower half-life, lower logD_OW, and greater effect concentration for most TPs compared to the parent compounds. Based on these results, we conclude that natural wetlands play a role as an eco-friendly reactor for degrading SOCs to form numerous TPs that are lower risk than the parent compounds.

A computational model for the catalytic hydrogel membrane reactor

The catalytic hydrogel membrane reactor (CHMR) is a promising new technology for hydrogenation of aqueous contaminants in drinking water. It offers numerous benefits over conventional three-phase reactors, including immobilization of nano-catalysts, high reactivity, and control over the hydrogen (H₂) supply concentration. In this study, a computational model of the CHMR was developed using AQUASIM and calibrated with 32 experimental datasets for a nitrite (NO₂^-)-reducing CHMR using palladium (Pd) nano-catalysts (~4.6 nm). The model was then used to identify key factors impacting the behavior of the CHMR, including hydrogel catalyst density, H₂ supply pressure, influent and bulk NO₂^- concentrations, and hydrogel thickness. Based on the model calibration, the reaction rate constants for the NO₂^- steady-state adsorption Hinshelwood reaction equation, k₁ and k₂, were 0.0039 m³ mole-Pd^-1 s^-1 and 0.027 (mole-H₂ m³)^1/2 mole-Pd^-1 s^-1, respectively. The reactant flux, which is the overall NO₂^- removal rate for the CHMR, is affected by the NO₂^- reduction rate at each catalyst site, which is in turn controlled by the available NO₂^- and H₂ concentrations that are regulated by their mass transport behavior. Reactant transport in the CHMR is counter-diffusional. So for thick hydrogels, the concurrent concentrations of NO₂^- and H₂ are limiting in the middle region along the x-y plane of the hydrogel, which results in a low overall NO₂^- removal rate (i.e., flux). Thinner hydrogels provide higher concurrent reactant concentrations throughout the hydrogel, resulting in higher fluxes. However, if the hydrogel is too thin, the flux becomes limited by the amount of Pd that can be loaded, and unused H₂ can diffuse into the bulk and promote biofilm growth. The hydrogel thickness that maximized the NO₂^- flux ranged between 30 and 150 μm for the conditions tested. The computational model is the first to describe CHMR behavior, and it is an important tool for the further development of the CHMR. It also can be adapted to assess CHMR behavior for other contaminants or catalysts or used for other types of interfacial catalytic membrane reactors.

Activity and stability of the catalytic hydrogel membrane reactor for treating oxidized contaminants

The catalytic hydrogel membrane reactor (CHMR) is an interfacial membrane process that uses nano-sized catalysts for the hydrogenation of oxidized contaminants in drinking water. In this study, the CHMR was operated as a continuous-flow reactor using nitrite (NO₂^-) as a model contaminant and palladium (Pd) as a model catalyst. Using the overall bulk reaction rate for NO₂^- reduction as a metric for catalytic activity, we evaluated the effect of the hydrogen gas (H₂) delivery method to the CHMR, the initial H₂ and NO₂^- concentrations, Pd density in the hydrogel, and the presence of Pd-deactivating species. The chemical stability of the catalytic hydrogel was evaluated in the presence of aqueous cations (H⁺, Na⁺, Ca²⁺) and a mixture of ions in a hard groundwater. Delivering H₂ to the CHMR lumens using a vented operation mode, where the reactor is sealed and the lumens are periodically flushed to the atmosphere, allowed for a combination of a high H₂ consumption efficiency and catalytic activity. The overall reaction rate of NO₂^- was dependent on relative concentrations of H₂ and NO₂^- at catalytic sites, which was governed by both the chemical reaction and mass transport rates. The intrinsic catalytic reaction rate was combined with a counter-diffusional mass transport component in a 1-D computational model to describe the CHMR. Common Pd-deactivating species [sulfite, bisulfide, natural organic matter] hindered the reaction rate, but the hydrogel afforded some protection from deactivation compared to a batch suspension. No chemical degradation of the hydrogel structure was observed for a model water (pH > 4, Na⁺, Ca²⁺) and a hard groundwater after 21 days of exposure, attesting to its stability under natural water conditions.

Effect of reactor configuration on the kinetics and nitrogen byproduct selectivity of urea electrolysis using a boron doped diamond electrode

Electrochemical systems have emerged as an advantageous approach for decentralized management of source-separated urine with the possibility of recovering or removing nutrients and generating energy. In this study, the kinetics and byproduct selectivity of the electrolytic removal of urea were investigated using a boron doped diamond working electrode under varied operational conditions with a primary focus on comparing undivided and divided reactors. The urea removal rate in the undivided and divided reactors was similar, but the divided reactor had an increased required cell voltage needed to maintain the equivalent current density. The current efficiency was similar for 0.1, 0.25, and 0.5 A (33.3, 83.3, 167 mA/cm²), suggesting no interference from competing reactions at higher potentials. In a divided reactor, increasing the anolyte pH reduced the urea removal rate presumably from hydroxyl radical scavenging by hydroxide. Further, for all divided reactor experiments, the final pH was less than 1, suggesting that the transport of protons across the ion exchange membrane to the cathode was slower than the oxidation reactions producing protons. The nitrogen byproduct selectivity was markedly different in the undivided and divided reactors. In both reactors, nitrate (NO₃^-) formed as the main byproduct at the anode, but in the undivided reactor it was reduced at the stainless steel cathode to ammonia. In the presence of 1 M chloride, the urea removal kinetics improved from the generation of reactive chlorine species, and the byproduct selectivity was shifted away from NO₃^- to presumably chloramines and N₂. Overall, these results indicate that the electrochemical reactor configuration should be carefully considered depending on the desired outcome of treating source-separated urine (e.g., nitrogen recovery, H₂ generation).

Stability of 2H- and 1T-MoS2 in the presence of aqueous oxidants and its protection by a carbon shell

Two-dimensional molybdenum disulfide (MoS₂) is emerging as a catalyst for energy and environmental applications. Recent studies have suggested the stability of MoS₂ is questionable when exposed to oxidizing conditions found in water and air. In this study, the aqueous stability of 2H- and 1T-MoS₂ and 2H-MoS₂ protected with a carbon shell was evaluated in the presence of model oxidants (O₂, NO₂⁻, BrO₃⁻). The MoS₂ electrocatalytic performance and stability was characterized using linear sweep voltammetry and chronoamperometry. In the presence of dissolved oxygen (DO) only, 2H- and 1T-MoS₂ were relatively stable, with SO₄²⁻ formation of only 2.5% and 3.1%, respectively. The presence of NO₂⁻ resulted in drastically different results, with SO₄²⁻ formations of 11% and 14% for 2H- and 1T-MoS₂, respectively. When NO₂⁻ was present without DO, the 2H- and 1T-MoS₂ remained relatively stable with SO₄²⁻ formations of only 4.2% and 3.3%, respectively. Similar results were observed when BrO₃⁻ was used as an oxidant. Collectively, these results indicate that the oxidation of 2H- and 1T-MoS₂ can be severe in the presence of these aqueous oxidants but that DO is also required. To investigate the ability of a capping agent to protect the MoS₂ from oxidation, a carbon shell was added to 2H–MoS₂. In a batch suspension in the presence of DO and NO₂⁻, the 2H–MoS₂ with the carbon shell exhibited good stability with no oxidation observed. The activity of 2H–MoS₂ electrodes was then evaluated for the hydrogen evolution reaction by a Tafel analysis. The carbon shell improved the activity of 2H–MoS₂ with a decrease in the Tafel slope from 451 to 371 mV dec⁻¹. The electrode stability, characterized by chronopotentiometry, was also enhanced for the 2H–MoS₂ coated with a carbon shell, with no marked degradation in current density observed over the reaction period. Because of the instability exhibited by unprotected MoS₂, it will only be a useful catalyst if measures are taken to protect the surface from oxidation. Further, given the propensity of MoS₂ to undergo oxidation in aqueous solutions, caution should be used when describing it as a true catalyst for reduction reactions (e.g., H₂ evolution), unless proven otherwise.

Two-dimensional molybdenum disulfide (MoS₂) is emerging as a catalyst for energy and environmental applications.

Catalytic Hydrogel Membrane Reactor for Treatment of Aqueous Contaminants

Heterogeneous hydrogenation catalysis is a promising approach for treating oxidized contaminants in drinking water, but scale-up has been limited by the challenge of immobilization of the catalyst while maintaining efficient mass transport and reaction kinetics. We describe a new process that addresses this issue: the catalytic hydrogel membrane (CHM) reactor. The CHM consists of a gas-permeable hollow-fiber membrane coated with an alginate-based hydrogel containing catalyst nanoparticles. The CHM benefits from counter-diffusional transport within the hydrogel, where H₂ diffuses from the interior of the membrane and contaminant species (e.g., NO₂^-, O₂) diffuse from the bulk aqueous solution. The reduction of O₂ and NO₂^- were investigated using CHMs with varying palladium catalyst densities, and mass transport of reactive species in the catalytic hydrogel was characterized using microsensors. The thickness of the "reactive zone" within the hydrogel affected the reaction rate and byproduct selectivity, and it was dependent on catalyst density. In a continuously mixed flow reactor test using groundwater, the CHM activity was stable for a 3 day period. Outcomes of this study illustrate the potential of the CHM as a scalable process in the treatment of aqueous contaminants.

Effect of Urine Compounds on the Electrochemical Oxidation of Urea Using a Nickel Cobaltite Catalyst: An Electroanalytical and Spectroscopic Investigation

Cyclic voltammetry (CV) and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were used to investigate the effect of major urine compounds on the electro-oxidation activity of urea using a nickel cobaltite (NiCo₂O₄ ) catalyst. As a substrate, carbon paper exhibited better benchmark potential and current values compared with stainless steel and fluorine-doped tin oxide glass, which was attributed to its greater active surface area per electrode geometric area. CV analysis of synthetic urine showed that phosphate, creatinine, and gelatin (i.e., proteins) had the greatest negative effect on the electro-oxidation activity of urea, with decreases in peak current up to 80% compared to that of a urea-only solution. Further investigation of the binding mechanisms of the deleterious compounds using in situ ATR-FTIR spectroscopy revealed that urea and phosphate weakly bind to NiCo₂O₄ through hydrogen bonding or long-range forces, whereas creatinine interacts strongly, forming deactivating inner-sphere complexes. Phosphate is presumed to disrupt the interaction between urea and NiCo₂O₄ by serving as a hydrogen-bond acceptor in place of catalyst sites. The weak binding of urea supports the hypothesis that it is oxidized through an indirect electron transfer. Outcomes of this study contribute to the development of electrolytic systems for treating source-separated urine.

Fabricating Optical-quality Glass Surfaces to Study Macrophage Fusion

Visualizing the formation of multinucleated giant cells (MGCs) from living specimens has been challenging due to the fact that most live imaging techniques require propagation of light through glass, but on glass macrophage fusion is a rare event. This protocol presents the fabrication of several optical-quality glass surfaces where adsorption of compounds containing long-chain hydrocarbons transforms glass into a fusogenic surface. First, preparation of clean glass surfaces as starting material for surface modification is described. Second, a method is provided for the adsorption of compounds containing long-chain hydrocarbons to convert non-fusogenic glass into a fusogenic substrate. Third, this protocol describes fabrication of surface micropatterns that promote a high degree of spatiotemporal control over MGC formation. Finally, fabricating glass bottom dishes is described. Examples of use of this in vitro cell system as a model to study macrophage fusion and MGC formation are shown.

Comparative analysis of the photocatalytic reduction of drinking water oxoanions using titanium dioxide

Regulated oxidized pollutants in drinking water can have significant health effects, resulting in the need for ancillary treatment processes. Oxoanions (e.g., nitrate) are one important class of oxidized inorganic ions. Ion exchange and reverse osmosis are often used treatment processes for oxoanions, but these separation processes leave behind a concentrated waste product that still requires treatment or disposal. Photocatalysis has emerged as a sustainable treatment technology capable of catalytically reducing oxoanions directly to innocuous byproducts. Compared with the large volume of knowledge available for photocatalytic oxidation, very little knowledge exists regarding photocatalytic reduction of oxoanion pollutants. This study investigates the reduction of various oxoanions of concern in drinking water (nitrate, nitrite, bromate, perchlorate, chlorate, chlorite, chromate) using a commercial titanium dioxide photocatalyst and a polychromatic light source. Results showed that oxoanions were readily reduced under acidic conditions in the presence of formate, which served as a hole scavenger, with the first-order rate decreasing as follows: bromate > nitrite > chlorate > nitrate > dichromate > perchlorate, corresponding to rate constants of 0.33, 0.080, 0.052, 0.0074, 0.0041, and 0 cm²/photons × 10¹⁸, respectively. Only bromate and nitrite were reduced at neutral pH, with substantially lower rate constants of 0.034 and 0.0021 cm²/photons × 10¹⁸, respectively. No direct relationship between oxoanion physicochemical properties, including electronegativity of central atom, internal bond strength, and polarizability was discovered. However, observations presented herein suggest the presence of kinetic barriers unique to each oxoanion and provides a framework for investigating photocatalytic reduction mechanisms of oxoanions in order to design better photocatalysts and optimize treatment.

A Facile Method for Separating and Enriching Nano and Submicron Particles from Titanium Dioxide Found in Food and Pharmaceutical Products

Recent studies indicate the presence of nano-scale titanium dioxide (TiO2) as an additive in human foodstuffs, but a practical protocol to isolate and separate nano-fractions from soluble foodstuffs as a source of material remains elusive. As such, we developed a method for separating the nano and submicron fractions found in commercial-grade TiO2 (E171) and E171 extracted from soluble foodstuffs and pharmaceutical products (e.g., chewing gum, pain reliever, and allergy medicine). Primary particle analysis of commercial-grade E171 indicated that 54% of particles were nano-sized (i.e., < 100 nm). Isolation and primary particle analysis of five consumer goods intended to be ingested revealed differences in the percent of nano-sized particles from 32%‒58%. Separation and enrichment of nano- and submicron-sized particles from commercial-grade E171 and E171 isolated from foodstuffs and pharmaceuticals was accomplished using rate-zonal centrifugation. Commercial-grade E171 was separated into nano- and submicron-enriched fractions consisting of a nano:submicron fraction of approximately 0.45:1 and 3.2:1, respectively. E171 extracted from gum had nano:submicron fractions of 1.4:1 and 0.19:1 for nano- and submicron-enriched, respectively. We show a difference in particle adhesion to the cell surface, which was found to be dependent on particle size and epithelial orientation. Finally, we provide evidence that E171 particles are not immediately cytotoxic to the Caco-2 human intestinal epithelium model. These data suggest that this separation method is appropriate for studies interested in isolating the nano-sized particle fraction taken directly from consumer products, in order to study separately the effects of nano and submicron particles.

"Insensitive" to touch: fabric-supported lubricant-swollen polymeric films for omniphobic personal protective gear

The use of personal protective gear made from omniphobic materials that easily shed drops of all sizes could provide enhanced protection from direct exposure to most liquid-phase biological and chemical hazards and facilitate the postexposure decontamination of the gear. In recent literature, lubricated nanostructured fabrics are seen as attractive candidates for personal protective gear due to their omniphobic and self-healing characteristics. However, the ability of these lubricated fabrics to shed low surface tension liquids after physical contact with other objects in the surrounding, which is critical in demanding healthcare and military field operations, has not been investigated. In this work, we investigate the depletion of oil from lubricated fabrics in contact with highly absorbing porous media and the resulting changes in the wetting characteristics of the fabrics by representative low and high surface tension liquids. In particular, we quantify the loss of the lubricant and the dynamic contact angles of water and ethanol on lubricated fabrics upon repeated pressurized contact with highly absorbent cellulose-fiber wipes at different time intervals. We demonstrate that, in contrast to hydrophobic nanoparticle coated microfibers, fabrics encapsulated within a polymer that swells with the lubricant retain the majority of the oil and are capable of repelling high as well as low surface tension liquids even upon multiple contacts with the highly absorbing wipes. The fabric supported lubricant-swollen polymeric films introduced here, therefore, could provide durable and easy to decontaminate protection against hazardous biological and chemical liquids.

Instillation versus inhalation of multiwalled carbon nanotubes: exposure-related health effects, clearance, and the role of particle characteristics

Inhaled multiwalled carbon nanotubes (MWCNTs) may cause adverse pulmonary responses due to their nanoscale, fibrous morphology and/or biopersistance. This study tested multiple factors (dose, time, physicochemical characteristics, and administration method) shown to affect MWCNT toxicity with the hypothesis that these factors will influence significantly different responses upon MWCNT exposure. The study is unique in that (1) multiple administration methods were tested using particles from the same stock; (2) bulk MWCNT formulations had few differences (metal content, surface area/functionalization); and (3) MWCNT retention was quantified using a specialized approach for measuring unlabeled MWCNTs in rodent lungs. Male Sprague-Dawley rats were exposed to original (O), purified (P), and carboxylic acid functionalized (F) MWCNTs via intratracheal instillation and inhalation. Blood, bronchoalveolar lavage fluid (BALF), and lung tissues were collected at postexposure days 1 and 21 for quantifying biological responses and MWCNTs in lung tissues by programmed thermal analysis. At day 1, MWCNT instillation produced significant BALF neutrophilia and MWCNT-positive macrophages. Instilled O- and P-MWCNTs produced significant inflammation in lung tissues, which resolved by day 21 despite MWCNT retention. MWCNT inhalation produced no BALF neutrophilia and no significant histopathology past day 1. However, on days 1 and 21 postinhalation of nebulized MWCNTs, significantly increased numbers of MWCNT-positive macrophages were observed in BALF. Results suggest (1) MWCNTs produce transient inflammation if any despite persistence in the lungs; (2) instilled O-MWCNTs cause more inflammation than P- or F-MWCNTs; and (3) MWCNT suspension media produce strikingly different effects on physicochemical particle characteristics and pulmonary responses.

Different shades of oxide: from nanoscale wetting mechanisms to contact printing of gallium-based liquid metals

Gallium-based liquid metals are of interest for a variety of applications including flexible electronics, soft robotics, and biomedical devices. Still, nano- to microscale device fabrication with these materials is challenging because, despite having surface tension 10 times higher than water, they strongly adhere to a majority of substrates. This unusually high adhesion is attributed to the formation of a thin oxide shell; however, its role in the adhesion process has not yet been established. In this work, we demonstrate that, dependent on dynamics of formation and resulting morphology of the liquid metal-substrate interface, GaInSn adhesion can occur in two modes. The first mode occurs when the oxide shell is not ruptured as it makes contact with the substrate. Because of the nanoscale topology of the oxide surface, this mode results in minimal adhesion between the liquid metal and most solids, regardless of substrate's surface energy or texture. In the second mode, the formation of the GaInSn-substrate interface involves rupturing of the original oxide skin and formation of a composite interface that includes contact between the substrate and pieces of old oxide, bare liquid metal, and new oxide. We demonstrate that in this latter mode GaInSn adhesion is dominated by the intimate contact between new oxide and substrate. We also show that by varying the pinned contact line length using varied degrees of surface texturing, the adhesion of GaInSn in this mode can be either decreased or increased. Lastly, we demonstrate how these two adhesion modes limit microcontact printing of GaInSn patterns but can be exploited to repeatedly print individual sub-200 nm liquid metal drops.

Characterization of food-grade titanium dioxide: the presence of nanosized particles

Titanium dioxide (TiO2) is widely used in food products, which will eventually enter wastewater treatment plants and terrestrial or aquatic environments, yet little is known about the fraction of this TiO2 that is nanoscale, or the physical and chemical properties of TiO2 that influence its human and environmental fate or toxicity. Instead of analyzing TiO2 properties in complex food or environmental samples, we procured samples of food-grade TiO2 obtained from global food suppliers and then, using spectroscopic and other analytical techniques, quantified several parameters (elemental composition, crystal structure, size, and surface composition) that are reported to influence environmental fate and toxicity. Another sample of nano-TiO2 that is generally sold for catalytic applications (P25) and widely used in toxicity studies, was analyzed for comparison. Food-grade and P25 TiO2 are engineered products, frequently synthesized from purified titanium precursors, and not milled from bulk scale minerals. Nanosized materials were present in all of the food-grade TiO2 samples, and transmission electron microscopy showed that samples 1-5 contained 35, 23, 21, 17, and 19% of nanosized primary particles (<100 nm in diameter) by number, respectively (all primary P25 particles were <100 nm in diameter). Both types of TiO2 aggregated in water with an average hydrodynamic diameter of >100 nm. Food-grade samples contained phosphorus (P), with concentrations ranging from 0.5 to 1.8 mg of P/g of TiO2. The phosphorus content of P25 was below inductively coupled plasma mass spectrometry detection limits. Presumably because of a P-based coating detected by X-ray photoelectron spectroscopy, the ζ potential of the food-grade TiO2 suspension in deionized water ranged from -10 to -45 mV around pH 7, and the iso-electric point for food-grade TiO2 (<pH 4) was significantly lower than that for P25. The presence of other elements in or on the TiO2 (Si content of 0.026-0.062% and Al content of 0.0006-0.810%) was also different from the case for P25 and would influence the environmental fate of TiO2. X-ray diffraction analysis confirmed the presence of anatase and/or rutile in the food-grade materials, and although the presence of amorphous TiO2 could not be ruled out, it is unlikely on the basis of Raman analysis. The food-grade TiO2 was solar photoactive. Cationic dyes adsorbed more readily to food-grade TiO2 than P25, indicating very different potentials for interaction with organics in the environment. This research shows that food-grade TiO2 contains engineered nanomaterials with properties quite different from those of P25, which has previously been used in many ecotoxicity studies, and because food-grade TiO2 is more likely than P25 to enter the environment (i.e., potentially higher exposure levels), there is a need to design environmental (and human) fate and toxicity studies comparing food-grade to catalytic TiO2.

Extraction and quantification of carbon nanotubes in biological matrices with application to rat lung tissue

Extraction of carbon nanotubes (CNTs) from biological matrices such as rat lung tissue is integral to developing a quantification method for evaluating the environmental and human health exposure and toxicity of CNTs. The ability of various chemical treatment methods, including Solvable (2.5% sodium hydroxide/surfactant mixture), ammonium hydroxide, nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric acid, hydrogen peroxide, and proteinase K, to extract CNTs from rat lung tissue was evaluated. CNTs were quantified using programmed thermal analysis (PTA). Two CNTs were used to represent the lower (500 °C) and upper (800 °C) PTA limit of CNT thermal stability. The recovery efficiency of each of the eight chemical reagents evaluated was found to depend on the ability to (1) minimize oxidation of CNTs, (2) remove interfering background carbon from the rat lung tissue, and (3) separate the solid-phase CNTs from the liquid-phase dissolved tissue via centrifugation. A two-step extraction method using Solvable and proteinase K emerged as the optimal approach, enabling a recovery of 98 ± 15% of a 2.9 ± 0.19 μg CNT loading that was spiked into whole rat lungs. Due to its high yield and applicability to low organ burdens of nanomaterials, this extraction method is particularly well suited for in vivo studies to quantify clearance rates and retained CNTs in lungs and other organs.

Disruption of Model Cell Membranes by Carbon Nanotubes

Carbon nanotubes (CNTs) have one of the highest production volumes among carbonaceous engineered nanoparticles (ENPs) worldwide and are have potential uses in applications including biomedicine, nanocomposites, and energy conversion. However, CNTs possible widespread usage and associated likelihood for biological exposures have driven concerns regarding their nanotoxicity and ecological impact. In this work, we probe the responses of planar suspended lipid bilayer membranes, used as model cell membranes, to functionalized multi-walled carbon nanotubes (MWCNT), CdSe/ZnS quantum dots, and a control organic compound, melittin, using an electrophysiological measurement platform. The electrophysiological measurements show that MWCNTs in a concentration range of 1.6 to 12 ppm disrupt lipid membranes by inducing significant transmembrane current fluxes, which suggest that MWCNTs insert and traverse the lipid bilayer membrane, forming transmembrane carbon nanotubes channels that allow the transport of ions. This paper demonstrates a direct measurement of ion migration across lipid bilayers induced by CNTs. Electrophysiological measurements can provide unique insights into the lipid bilayer-ENPs interactions and have the potential to serve as a preliminary screening tool for nanotoxicity.

Photocatalytic reduction of nitrate using titanium dioxide for regeneration of ion exchange brine

Nitrate is often removed from groundwater by ion exchange (IX) before its use as drinking water. Accumulation of nitrate in IX brine reduces the efficiency of IX regeneration and the useful life of the regeneration brine. For the first time, we present a strategy to photocatalytically reduce nitrate in IX brine, thereby extending the use of the brine. Titanium dioxide (Evonik P90), acting as photocatalyst, reduced nitrate effectively in both synthetic brines and sulfate-removed IX brine when formic acid (FA) was used as the hole scavenger (i.e., electron donor) and the initial FA to nitrate molar ratio (IFNR) was 5.6. Increasing the NaCl level in the synthetic brine slowed the nitrate reduction rate without affecting by-product selectivity of ammonium and gaseous N species (e.g., N(2), N(2)O). In a non-modified IX brine, nitrate removal was greatly inhibited owing to the presence of sulfate, which competed with nitrate for active surface sites on P90 and induced aggregation of P90 nanoparticles. After removing sulfate through barium sulfate precipitation, nitrate was effectively reduced; approximately 3.6 × 10(24) photons were required to reduce each mole of nitrate to 83% N Gases and 17% NH(4)(+). To make optimum use of FA and control the residual FA level in treated brine, the IFNR was varied. High IFNRs (e.g., 4, 5.6) were found to be more efficient for nitrate reduction but left higher residual FA in brine. IX column tests were performed to investigate the impact of residual FA for brine reuse. The residual FA in the brine did not significantly affect the nitrate removal capacity of IX resins, and formate contamination of treated water could be eliminated by rinsing with one bed volume of fresh brine.

Detection of carbon nanotubes in environmental matrices using programmed thermal analysis

Carbon nanotube (CNT) production is rapidly growing, and there is a need for robust analytical methods to quantify CNTs at environmentally relevant concentrations in complex organic matrices. Because physical and thermal properties vary among different types of CNTs, we studied 14 single-walled (SWCNTs) and multiwalled CNTs (MWCNTs). Our aim was to apply a classic analytical air pollution method for separating organic (OC) and elemental carbon (EC) (thermal optical transmittance/reflectance, TOT/R) to environmental and biological matrices and CNTs. The TOT/R method required significant modification for this analysis, which required a better understanding of the thermal properties of CNTs. An evaluation of the thermal properties of CNTs revealed two classes that could be differentiated using Raman spectroscopy: thermally "weak" and "strong." Using the programmed thermal analysis (PTA) method, we optimized temperature programs and instituted a set of rules for defining the separation of OC and EC to quantify a broad range of CNTs. The combined Raman/PTA method was demonstrated using two environmentally relevant matrices (cyanobacteria (CB) and urban air). Thermal evaluation of CB revealed it to be a complex matrix with interference occurring for both weak and strong CNTs, and thus a pretreatment method was necessary. Strong CNT masses of 0.51, 2.7, and 11 μg, corresponding to concentrations of 10, 54, and 220 μg CNT/g CB, yielded recoveries of 160 ± 29%, 99 ± 1.9%, and 96 ± 3.0%, respectively. Urban air was also a complex matrix and contained a significant amount (12%) of background EC that interfered greatly with weak CNTs and minimally with strong CNTs. The current detection limit at 99% confidence for urban air samples and strong CNTs is 55 ng/m(3) (0.33 μg). Overall, the PTA method presented here provides an initial approach for quantifying a wide range of CNTs, and we identify specific future research needs to eliminate potential interferences and lower detection limits.