Preparation and measurement methods for studying nanoparticle aggregate surface chemistry

Abstract

Despite best efforts at controlling nanoparticle (NP) surface chemistries, the environment surrounding nanomaterials is always changing and can impart a permanent chemical memory. We present a set of preparation and measurement methods to be used as the foundation for studying the surface chemical memory of engineered NP aggregates. We attempt to bridge the gap between controlled lab studies and real-world NP samples, specifically TiO(2), by using well-characterized and consistently synthesized NPs, controllably producing NP aggregates with precision drop-on-demand inkjet printing for subsequent chemical measurements, monitoring the physical morphology of the NP aggregate depositions with scanning electron microscopy (SEM), acquiring "surface-to-bulk" mass spectra of the NP aggregate surfaces with time-of-flight secondary ion mass spectrometry (ToF-SIMS), and developing a data analysis scheme to interpret chemical signatures more accurately from thousands of data files. We present differences in mass spectral peak ratios for bare TiO(2) NPs compared to NPs mixed separately with natural organic matter (NOM) or pond water. The results suggest that subtle changes in the local environment can alter the surface chemistry of TiO(2) NPs, as monitored by Ti(+)/TiO(+) and Ti(+)/C(3)H(5)(+) peak ratios. The subtle changes in the absolute surface chemistry of NP aggregates vs. that of the subsurface are explored. It is envisioned that the methods developed herein can be adapted for monitoring the surface chemistries of a variety of engineered NPs obtained from diverse natural environments.

Measurement Methods for the Oral Uptake of Engineered Nanomaterials from Human Dietary Sources: Summary and Outlook

“…Provided that other techniques such as SEM and TEM have been used to locate samples rich in ENMs and ENM aggregates (perhaps after cryomicrotoming of the sample of interest), surface‐chemistry‐based methods such as X‐ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), low‐energy ion scattering (LEIS), atomic force microscopy (AFM), scanning probe microscopy (SPM), and scanning tunneling microscopy (STM) can be used to provide very specific chemical information at a range of analytical figures of merit for ENM detection (Baer and others ). For example, XPS is a surface‐sensitive tool that can be used for examining compound‐identifying ENM functional groups (Gorham and others ), and time of flight (ToF)‐SIMS has recently been used in concert with specific sample preparation methodologies to discern subtle differences in ENM aggregate coatings (Szakal and others ). For compositional analysis alone, however, it is difficult to envision widespread deployment of surface‐chemistry–based methods for ENM detection because of their high learning curves and considerable expense.…”

“…Specific questions can also be answered with tailored methods such as fluorescence imaging and confocal laser scanning microscopy (CLSM) for studying trophic transfer of ENMs in the invertebrate food web (Holbrook and others ). However, there is no question that the majority of ENM‐related imaging studies have employed SEM or TEM to answer specific ENM‐related size information (Powell and others ; Luykx and others ; Tiede and others ; Pipan‐Tkalec and others ; Powell and others ; Huang and others ; Szakal and others ; Weir and others ). Regardless of ENM shape, the appropriate sample benefits from nanometer and subnanometer lateral resolution that can be offered with SEM and TEM, respectively, can answer critical questions regarding core–shell thicknesses, fates of ENM sizes, and extent of integration (for example, “Was the ENM integrated exactly as we expected/planned in a particular matrix?”).…”

“…For example, combining SEM with energy‐dispersive spectroscopy (EDS) gives high‐resolution composition maps with percent‐level detection of various elements, which is particularly important for inorganic‐based ENM analyses (Huang and others ). In addition, TEM coupled with electron energy loss spectroscopy (EELS) (Powell and others ) or selected area electron diffraction (SAED) (Szakal and others ) can provide pertinent information about the imaged ENM. If the lateral imaging resolution can be on the order of 200 nm to 1 μm, such as with aggregated forms of ENMs, then much more chemical information can be provided with a ToF‐SIMS analysis, including simultaneous organic/inorganic information.…”

Overview of Nanomaterial Characterization and Metrology

“…For example, SIMS has been employed in conjunction with inkjet printing to better understand the chemical characteristics of titanium dioxide nanomaterials deposited on a surface. 116 In addition, it has been used to study the drying characteristics and related dissolution states of silver nanomaterial suspensions in order to infer the nature of the initial solution. 116,117 Finally, the primary ion beam can be used to remove material from the specimen surface so that SIMS data can be collected in a depth profiling mode.…”

“…116 In addition, it has been used to study the drying characteristics and related dissolution states of silver nanomaterial suspensions in order to infer the nature of the initial solution. 116,117 Finally, the primary ion beam can be used to remove material from the specimen surface so that SIMS data can be collected in a depth profiling mode. Depending on material, this technique can reliably be utilized to provide depth information from a single nanometer to several micrometers, depending on mode of operation and targeted analyte.…”