In this work, molecular-level events accompanying the photobleaching of carbon dots (CDs) synthesized from ethylenediamine and citric acid (CACDs) were identified using solution-phase NMR. By combining quantitative 13 C NMR data with fluorescence measurements we show that this new approach is capable of identifying not only molecular fluorophores in CACDs, but also their contribution to the overall CACD fluorescence and their relative photostability. Specifically, imidazo[1,2-a]pyridine-7-carboxylic acid (IPCA) is found to be the dominant (84 %) species responsible for fluorescence in CACDs along with a second undetermined source, most likely associated with the aromatic core which is significantly (approximately 20 times) more photostable than IPCA. The presence of these two fluorescent species with different photostabilities rationalizes not only the photobleaching kinetics but also the evolution of the fluorescence spectrum during photobleaching Diffusion-ordered spectroscopy (DOSY) also reveals that all of the IPCA molecules are trapped within or covalently bound to the CACD, but are not present as isolated molecules freely rotating in solution. Singlet oxygen is confirmed as a key ROS responsible for photobleaching, with prototypical photoproducts identified from mass spectrometry studies of citrazinic acid. Quantitative 13 C NMR of CACDs is possible because their extremely high colloidal stability enables high concentrations (667 mg/mL) to remain stable in solution. The approach described in this study could be extended to identify the structure of chromophores in other CDs and interrogate molecular level processes that accompany CD sensing.

Microplastics and nanoplastics (collectively, MNPs) are increasingly entering soils, with potential adverse impacts to agriculture and groundwater. Environmental detection, characterization, and quantification of MNPs is difficult and subject to artifacts, often requiring labor-intensive separation from environmental matrices. These analytical challenges make it difficult to conduct experiments investigating specific MNP characteristics influencing their transport and fate, particularly when examining multiple plastic types at low concentrations. By synthesizing a suite of metal-tagged polymers, which are cryomilled to create polydisperse fragmented particle suspensions, single particle ICP-MS (spICP-MS) can be used to quantify MNP particle size and concentration in controlled fate and transport studies. Use of unique metal-polymer pairs enables accurate, simultaneous analysis of multiple MNP types which can be used to track total particle transport and retention within a variety of environmental matrices. This was demonstrated using saturated sand column transport experiments to quantify the movement of two plastics having different properties: tin-tagged polystyrene (Sn-PS) and tantalum-tagged polyvinylpyrrolidone (Ta-PVP). The behavior of these polydisperse, fragmented MNPs was compared to that of fluorescent, carboxylated monodisperse PS spherical microspheres (Fl-PS). Mobility of all MNP types increased with decreasing particle size, and hydrophilic Ta-PVP particles migrated more effectively than the hydrophobic Sn-PS particles. Furthermore, the addition of humic acid (HA) to the carrier solution increased the colloidal stability of both metal-tagged MNP suspensions, resulting in much greater elution from the column than in HA-free deionized water or moderately- hard water (ionic strength = 5mM). This combination of particle synthesis and spICP-MS analysis provides insights into the transport of MNP having physical properties that are representative of environmental MNPs and opens up a broad range of applications for study of MNP environmental fate and transport.

Top-down fabrication method to prepare metal-tagged nanoplastics with irregular shapes and diverse sizes for lab-based studies using spICP-MS.

Although nanomaterials (NMs), both natural (clays, mineral dust aerosols, etc.) and anthropogenic (nanoplastics, tire wear particles, etc.) have been recognized as a key component of environmental processes, their characterization and quantification in environmental matrices remains difficult. In contrast to monodisperse NM standards, environmentally-sampled NMs can contain a broad, continuous, polydisperse particle size distribution which creates analytical difficulties for many single particle counting techniques, including single particle inductively coupled plasma mass spectrometry (spICP-MS). By employing environmentally relevant, nanoplastics tagged with metals, as well as Al-bearing stream mineral colloids, we demonstrate the deleterious effects of particle-based backgrounds caused by coincident small particles on spICP-MS analysis of particle number concentration (PNC) and particle size distribution (PSD). A novel methodology is presented that successfully minimizes the effects of particle coincidence using serial dilutions to identify distortion-free segments of the PSD, which are combined and modeled using a power law distribution. The physical relevance of the parameters derived from power law modeling are demonstrated using suspensions of two different nanoplastics tagged with metals. Finally, this new methodology is applied to analysis of Al-bearing colloids sampled during a storm event in order to highlight the possible distortions present in a single dilution analysis. This comparison demonstrates the value of our proposed methodology for environmental NM and colloid analysis. Multiple dilutions allow artifact-free analysis of regions of the particle size distribution. Power law modeling leads to larger size analysis range.

Nanocellulose has attracted widespread interest for applications in materials science and biomedical engineering due to its natural abundance, desirable physicochemical properties, and high intrinsic mineralizability (i.e., complete biodegradability). A common strategy to increase dispersibility in polymer matrices is to modify the hydroxyl groups on nanocellulose through covalent functionalization, but such modification strategies may affect the desirable biodegradation properties exhibited by pristine nanocellulose. In this study, cellulose nanofibrils (CNFs) functionalized with a range of esters, carboxylic acids, or ethers exhibited decreased rates and extents of mineralization by anaerobic and aerobic microbial communities compared to unmodified CNFs, with etherified CNFs exhibiting the highest level of recalcitrance. The decreased biodegradability of functionalized CNFs depended primarily on the degree of substitution at the surface of the material rather than within the bulk. This dependence on surface chemistry was attributed not only to the large surface area-to-volume ratio of nanocellulose but also to the prerequisite surface interaction by microorganisms necessary to achieve biodegradation. Results from this study highlight the need to quantify the type and coverage of surface substituents in order to anticipate their effects on the environmental persistence of functionalized nanocellulose.

Carbon nanotubes (CNTs) have unique physical and chemical properties that drive their use in a variety of commercial and industrial applications. CNTs are commonly oxidized prior to their use to enhance dispersion in polar solvents by deliberately grafting oxygen-containing functional groups onto CNT surfaces. In addition, CNT surface oxides can be unintentionally formed or modified after CNTs are released into the environment through exposure to reactive oxygen species and/or ultraviolet irradiation. Consequently, it is important to understand the impact of CNT surface oxidation on the environmental fate, transport, and toxicity of CNTs. In this review, we describe the specific role of oxygen-containing functional groups on the important environmental behaviors of CNTs in aqueous media (e.g., colloidal stability, adsorption, and photochemistry) as well as their biological impact. We place special emphasis on the value of systematically varying and quantifying surface oxides as a route to identifying quantitative structure-property relationships. The role of oxygen-containing functional groups in regulating the efficacy of CNT-enabled water treatment technologies and the influence of surface oxides on other carbon-based nanomaterials are also evaluated and discussed.