Manuel D. Montaño

Improvements in the detection and characterization of engineered nanoparticles using spICP-MS with microsecond dwell times

The imminent release of engineered nanomaterials (ENPs) into the environment has raised several questions regarding their fate, transport, and toxicity. However, their small size, expected low concentrations (ng L−1), and the high environmental background of naturally occurring nanomaterials make detection and characterization difficult. In recent years, single particle ICP-MS (spICP-MS) has been developed as a promising technique to detect and characterize engineered nanoparticles in biological and environmental matrices. Improvements in the spICP-MS technique were made in this study by employing 100 microsecond dwell times. Commercially available hardware and software were developed to fully capture multiple data points over the fast transient (~500 μs) nanoparticle events, which provides accurate particle sizing and counting. Reducing the background signal facilitated the characterization of Ag NPs even in the presence of ten-fold higher Ag+ concentration. By improving the time resolution between particle events, the upper limit of the dynamic range of Au NP concentration was increased to several μg L−1. These short dwell times also provide detection of two elements in the same nanoparticle, opening the door for possible environmental applications with the prospect of obtaining particle-by-particle elemental compositions. These improvements help further establish spICP-MS as a leading analytical technique for the detection and characterization of metal-containing ENPs, and introduces new possibilities for differentiating engineered nanomaterials from their naturally occurring analogues.

Current status and future direction for examining engineered nanoparticles in natural systems

Environmental context The detection and characterisation of engineered nanomaterials in the environment is essential for exposure and risk assessment for this emerging class of materials. However, the ubiquitous presence of naturally occurring nanomaterials presents a unique challenge for the accurate determination of engineered nanomaterials in environmental matrices. New techniques and methodologies are being developed to overcome some of these issues by taking advantage of subtle differences in the elemental and isotopic ratios within these nanomaterials. Abstract The increasing manufacture and implementation of engineered nanomaterials (ENMs) will continue to lead to the release of these materials into the environment. Reliably assessing the environmental exposure risk of ENMs will depend highly on the ability to quantify and characterise these materials in environmental samples. However, performing these measurements is obstructed by the complexity of environmental sample matrices, physiochemical processes altering the state of the ENM and the high background of naturally occurring nanoparticles (NNPs), which may be similar in size, shape and composition to their engineered analogues. Current analytical techniques can be implemented to overcome some of these obstacles, but the ubiquity of NNPs presents a unique challenge requiring the exploitation of properties that discriminate engineered and natural nanomaterials. To this end, new techniques are being developed that take advantage of the nature of ENMs to discern them from naturally occurring analogues. This paper reviews the current techniques utilised in the detection and characterisation of ENMs in environmental samples as well as discusses promising new approaches to overcome the high backgrounds of NNPs. Despite their occurrence in the atmosphere and soil, this review will be limited to a discussion of aqueous-based samples containing ENMs, as this environment will serve as a principal medium for the environmental dispersion of ENMs.

Single Particle ICP-MS: Advances toward routine analysis of nanomaterials

AbstractFrom its early beginnings in characterizing aerosol particles to its recent applications for investigating natural waters and waste streams, single particle inductively coupled plasma-mass spectrometry (spICP-MS) has proven to be a powerful technique for the detection and characterization of aqueous dispersions of metal-containing nanomaterials. Combining the high-throughput of an ensemble technique with the specificity of a single particle counting technique and the elemental specificity of ICP-MS, spICP-MS is capable of rapidly providing researchers with information pertaining to size, size distribution, particle number concentration, and major elemental composition with minimal sample perturbation. Recently, advances in data acquisition, signal processing, and the implementation of alternative mass analyzers (e.g., time-of-flight) has resulted in a wider breadth of particle analyses and made significant progress toward overcoming many of the challenges in the quantitative analysis of nanoparticles. This review provides an overview of spICP-MS development from a niche technique to application for routine analysis, a discussion of the key issues for quantitative analysis, and examples of its further advancement for analysis of increasingly complex environmental and biological samples. Graphical AbstractSingle particle ICP-MS workflow for the analysis of suspended nanoparticles

Methods for the Detection and Characterization of Silica Colloids by Microsecond spICP-MS

The rapid development of nanotechnology has led to concerns over their environmental risk. Current analytical techniques are underdeveloped and lack the sensitivity and specificity to characterize these materials in complex environmental and biological matrices. To this end, single particle ICP-MS (spICP-MS) has been developed in the past decade, with the capability to detect and characterize nanomaterials at environmentally relevant concentrations in complex environmental and biological matrices. However, some nanomaterials are composed of elements inherently difficult to quantify by quadrupole ICP-MS due to abundant molecular interferences, such as dinitrogen ions interfering with the detection of silicon. Three approaches aimed at reducing the contribution of these background molecular interferences in the analysis of (28)Si are explored in an attempt to detect and characterize silica colloids. Helium collision cell gases and reactive ammonia gas are investigated for their conventional use in reducing the signal generated from the dinitrogen interference and background silicon ions leaching from glass components of the instrumentation. A new approach brought on by the advent of microsecond dwell times in single particle ICP-MS allows for the detection and characterization of silica colloids without the need for these cell gases, as at shorter dwell times the proportion of signal attributed to a nanoparticle event is greater relative to the constant dinitrogen signal. It is demonstrated that the accurate detection and characterization of these materials will be reliant on achieving a balance between reducing the contribution of the background interference, while still registering the maximum amount of signal generated by the particle event.

Multi-day diurnal measurements of Ti-containing nanoparticle and organic sunscreen chemical release during recreational use of a natural surface water

Clear Creek in Golden, Colorado sees a large number of recreational users during summer, which is expected to result in release of sunscreen chemicals to the water. In this study, water samples were collected hourly for 72 hours over a busy holiday weekend, and were analyzed for the organic chemical – based (oxybenzone) and inorganic colloidal (titanium dioxide) active sunscreen constituents. An increase in oxybenzone concentration was observed daily during each days peak recreational use, approximately 12:00 to 19:00 h. This corresponded with an increase in titanium concentration. Metals naturally co-occurring with titanium such as aluminum and iron also showed an increase of these elements during bathing periods as well, suggesting the titanium increase may also be partially the result of sediment resuspension, consistent with the shallow water depth. The ratio of titanium to both aluminum and iron increases relative to the background elemental ratios during peak recreational use. Estimates of titanium mass loading suggested that sunscreen use only could not explain the observed Ti : Al and Ti : Fe ratios and that resuspended sediments likely have an elevated titanium metal ratio compared to natural suspended sediments. Single particle ICP-MS (spICP-MS), used to analyze water samples for Ti-containing particles, did not show diurnal trends in total particle number. Overall, this is the first consecutive-multi-day monitoring study for compounds released from sunscreen to a natural water system, and it highlights the challenges in dealing with detection of NPs above a natural background.

Measurement of the Density of Engineered Silver Nanoparticles Using Centrifugal FFF-TEM and Single Particle ICP-MS

A methodology has been developed to measure nanoparticle mass and density, by combining centrifugal field-flow fractionation (CeFFF; more commonly called sedimentation FFF or SdFFF) and transmission electron microscopy (TEM). Particle effective mass obtained from CeFFF retention data and particle size obtained from the TEM images were used to calculate the nanoparticle density. The method was initially applied to measure the density of monodispersed polystyrene latex nanoparticles. Measured densities for latex nanoparticles of 160-300 nm in diameter were in the range of 1041-1063 kg m-3 with standard deviations of 0.6-1.1%. Densities of engineered silver nanoparticles with nominal diameters of 30, 60, 75, and 100 nm were measured using this methodology. For all four silver nanoparticle samples, the measured densities were 18-24% lower than the nominal density of metallic silver, with an overall mean value of 7900 ± 675 kg m-3. Density values calculated using nanoparticle mass values obtained from single particle inductively coupled plasma-mass spectrometry (spICP-MS) measurements, corroborated the CeFFF-TEM results. The difference in the density of the silver nanoparticles compared to that of bulk silver suggests that the synthesis process could impart 20-37% porosity in silver nanoparticles. The data has important implications in the fields of nanomaterial, nanomedicine and nanotoxicology, where assumption of the bulk density for nanoparticles can result in erroneous estimation of parameters such as mass, size, porosity, and dosage. The presented methodology provides a straightforward and reproducible means for measurement of the density and porosity of engineered nanoparticles with a wide range of density and size.

Chapter 3 - Size Distributions

Because size and size distribution play an important role in the environmental fate and impacts of nanomaterials (NMs), accurate determination of particle size distributions (PSDs) remains a priority need. Despite a diverse range of methods to measure PSD, several challenges remain in the forefront. At the most fundamental level, the different ways of presenting the central size value, and the distribution of particles around this value, lead to potential for ambiguity. Number-, area-, mass- and intensity-based distributions will all appear different for particles other than very monodisperse samples. This may lead to the need to interconvert these distributions. Size-dependant analytical limitations of each method may lead to errors in this conversion, especially at the tails of the distributions. The presence of interfering particles in natural samples leads towards the need to develop tools with some degree of specificity that can lead to discrimination between engineered and naturally occurring particle types. Inductively coupled plasma–mass spectrometry-based methods seem to hold significant promise in this regard but further method development is needed. Finally it is important to recognize the inherent instability of NMs in solution that may lead to aggregation and alter the representation of particle size. This process may be at least in part reversible. Thus PSD is not a static number but is likely very dynamic changing in time and space. Since most methods for PSD analysis have not been validated for aggregates, considerable variability in results from different research groups is to be expected. Until the degree to which aggregation state can be maintained through sampling, storage and analysis is determined, rigorous disaggregation and PSD analysis of primary particles will provide the most robust and comparable size data.