Mass Spectrometry of Nanoparticles is Different

Nanoprojectile Secondary Ion Mass Spectrometry for Nanometrology of Nanoparticles and Their Interfaces

Nanoscale molecular characterization plays a crucial role in enhancing our insights into fundamental and materials processes occurring at the nanoscale. However, for many traditional techniques, measurements on different ensembles are mixed and the analytical result reflects the average surface composition or arrangement. Advances in nanometrologies that allow for measurements to be differentiated based on the chemical environment examined are critical for accurate analysis. Here, we present a variant of secondary ion mass spectrometry, SIMS, termed nanoprojectile SIMS, NP-SIMS, capable of nanoscale molecular analysis. The technique examines the sample with a suite, 10⁶-10⁷, of individual gold nanoprojectiles (e.g., Au₄₀₀⁴⁺) which stochastically probe the surface. Analysis of coemitted ions from each impact allows for the inspection of colocalized moieties within the ejected volume of a single projectile impact (10-15 nm in diameter). If some of these 10⁶-10⁷ measurements arise from nanodomains of similar composition, data can be grouped based on the detected secondary ions. We applied the method to examine a mixture of three different-sized nanoparticles with identical metal cores (3-5 nm in diameter), differing in the length of the attached ligand (decanetiol, tetradecanethiol, and hexadecanethiol). Using NP-SIMS, we determined the relative abundance of the three particles on the surface and isolated measurements based on the impact parameter between the impacting nanoprojectile and the surface particle, demonstrating that measurements occurring near the center of the particle can be differentiated from those at the particle-particle and particle-substrate interfaces. The results suggest that the described methodology is well-suited for molecular analysis of nanoassemblies and may be applied for tracking defects. Here we demonstrate that, using NP-SIMS, ensemble averaging can be avoided and molecular analysis can be undertaken at a scale below 5 nm, allowing for nanoscale molecular analysis of nano-objects and their interfaces.

Identification and characterisation of individual nanoplastics by scanning transmission X-ray microscopy (STXM)

Nanoplastics (NP) are of environmental and human health concern. We tested a novel NP extraction method and scanning transmission X-ray spectro-microscopy (STXM) in combination with near-edge X-ray absorption fine-structure spectroscopy (NEXAFS) to image and identify individual NP in environmental and food matrices. We (1) discussed the potential of STXM compared to other methods potentially suitable for NP analysis, (2) applied the method on NP suspensions of eight of the most common polymers, (3) analyzed environmental water and soil samples spiked with NP and (4) characterized NP in tea water infused in plastic teabags and unspiked soil samples. Here we show that STXM has methodological advantages and that polymers give characteristic spectra, which allows NP identification in environmental and food matrices. For soils we deliver a visual and spectroscopic characterization of NP, proving their presence and highlighting their diversity. Thus, STXM, can be used for the detection and characterisation of NP in different types of matrices.

Nanoparticle Analysis in Biomaterials Using Laser Ablation-Single Particle-Inductively Coupled Plasma Mass Spectrometry

In the past decade, the development of single particle–inductively coupled plasma mass spectrometry (SP-ICPMS) has revolutionized the field of nanometallomics. Besides differentiation between dissolved and particulate metal signals, SP-ICPMS can quantify the nanoparticle (NP) number concentration and size. Because SP-ICPMS is limited to characterization of NPs in solution, we show how solid sampling by laser ablation (LA) adds spatial-resolution characteristics for localized NP analysis in biomaterials. Using custom-made gelatin standards doped with dissolved gold and commercial or synthesized gold nanoparticles, LA-SP-ICPMS conditions such as laser fluence, beam size, and dwell time were optimized for NP analysis to minimize NP degradation, peak overlap, and interferences from dissolved gold. A data-processing algorithm to retrieve the NP number concentration and size was developed for this purpose. As a proof-of-concept, a sunflower-root-sample cross-section, originating from a sunflower plant exposed to gold NPs, was successfully imaged using the optimized LA-SP-ICPMS conditions for localized NP characterization.

Characterization techniques for nanoparticles: comparison and complementarity upon studying nanoparticle properties

Nanostructures have attracted huge interest as a rapidly growing class of materials for many applications. Several techniques have been used to characterize the size, crystal structure, elemental composition and a variety of other physical properties of nanoparticles. In several cases, there are physical properties that can be evaluated by more than one technique. Different strengths and limitations of each technique complicate the choice of the most suitable method, while often a combinatorial characterization approach is needed. In addition, given that the significance of nanoparticles in basic research and applications is constantly increasing, it is necessary that researchers from separate fields overcome the challenges in the reproducible and reliable characterization of nanomaterials, after their synthesis and further process (e.g. annealing) stages. The principal objective of this review is to summarize the present knowledge on the use, advances, advantages and weaknesses of a large number of experimental techniques that are available for the characterization of nanoparticles. Different characterization techniques are classified according to the concept/group of the technique used, the information they can provide, or the materials that they are destined for. We describe the main characteristics of the techniques and their operation principles and we give various examples of their use, presenting them in a comparative mode, when possible, in relation to the property studied in each case.

The collision of a hypervelocity massive projectile with free-standing graphene: Investigation of secondary ion emission and projectile fragmentation

We present here the study of the individual hypervelocity massive projectiles (440-540 keV, 33-36 km/s Au₄₀₀⁴⁺ cluster) impact on 1-layer free-standing graphene. The secondary ions were detected and recorded separately from each individual impact in the transmission direction using a time-of-flight mass spectrometer. We observed C_1-10^± ions emitted from graphene, the projectiles which penetrated the graphene, and the Au_1-3^± fragment ions in mass spectra. During the projectile-graphene interaction, the projectile loses ∼15% of its initial kinetic energy (∼0.18 keV/atom, 72 keV/projectile). The Au projectiles are neutralized when approaching the graphene and then partially ionized again via electron tunneling from the hot rims of the holes on graphene, obtaining positive and negative charges. The projectile reaches an internal energy of ∼450-500 eV (∼4400-4900 K) after the impact and then undergoes a ∼90-100 step fragmentation with the ejection of Au₁ atoms in the experimental time range of ∼0.1 μs.

SIMS of transfer ribonucleic acid molecules encapsulated between free-standing graphene sheets

In this study, the authors used cluster-secondary ion mass spectrometry method to investigate the preserved transfer ribonucleic acid (tRNA) encapsulated between two free-standing graphene sheets. Single impacts of 50 keV C60 (2+) projectiles generated the emission of tRNA fragment ions in the transmission direction for mass selection and detection in a time-of-flight mass spectrometer. Ribonucleic acid (RNA) is extremely unstable and prone to rapid enzymatic degradation by ribonucleases. Employing graphene to isolate RNA from the environment, the authors prevent the aforementioned process. Encapsulation was achieved by drop casting a solution of tRNA, prepared using deuterated water, onto one graphene sheet and covering it with another. The event-by-event bombardment/detection mode allowed us to use colocalization analysis method to characterize the tRNA and its immediate environment. The authors found that upon drying, tRNA agglomerated into nanostructures ∼60 nm in diameter via formation and subsequent drying of aqua cells. The tRNA nanoagglomerates had a density of ∼42 structures per μm(2) with coverage of ∼12% of the surface area. In addition, trace amounts of water remained mostly around the tRNA nanoagglomerates, probably in the form of hydration.