Three-dimensional localization of polymer nanoparticles in cells using ToF-SIMS

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.

Depth correction of 3D NanoSIMS images using secondary electron pixel intensities

Strategies that do not require additional characterization to be performed on the sample or the collection of additional secondary ion signals are needed to depth correct 3D SIMS images of cells. Here, we develop a depth correction strategy that uses the pixel intensities in the secondary electron images acquired during negative-ion NanoSIMS depth profiling to reconstruct the sample morphology. This morphology reconstruction was then used to depth correct the 3D SIMS images that show the components of interest in the sample. As a proof of concept, we applied this approach to NanoSIMS depth profiling data that show the 15N-enrichment and 18O-enrichment from 15N-sphingolipids and 18O-cholesterol, respectively, within a metabolically labeled Madin-Darby canine kidney cell. Comparison of the cell morphology reconstruction to the secondary electron images collected with the NanoSIMS revealed that the assumption of a constant sputter rate produced small inaccuracies in sample morphology after approximately 0.66 μm of material was sputtered from the cell. Nonetheless, the resulting 3D renderings of the lipid-specific isotope enrichments better matched the shapes and positions of the subcellular compartments that contained 15N-sphingolipids and 18O-cholesterol than the uncorrected 3D SIMS images. This depth correction of the 3D SIMS images also facilitated the detection of spherical cholesterol-rich compartments that were surrounded by membranes containing cholesterol and sphingolipids. Thus, we expect this approach will facilitate identifying the subcellular structures that are enriched with biomolecules of interest in 3D SIMS images while eliminating the need for correlated analyses or additional secondary ion signals for the depth correction of 3D NanoSIMS images.

Mechanisms of Fluorine-Induced Separation of Mass Interference during TOF-SIMS Analysis

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is one of very few analytical techniques allowing sample chemical structure to be characterized in three-dimensional (3D) with nanometer resolution. Due to the excellent sensitivity in the order of ppm-ppb and capability of detecting all ionized elements and molecules, TOF-SIMS finds many applications for analyzing nanoparticle-containing systems and thin films used in microdevices for new energy applications, microelectronics, and biomedicine. However, one of the main drawbacks of this technique is potential mass interference between ions having the same or similar masses, which can lead to data misinterpretation. In this work, we present that this problem can be easily solved by delivering fluorine gas to a sample surface during TOF-SIMS analysis and we propose mechanisms driving this phenomenon. Our comprehensive studies, conducted on complex thin films made of highly mass-interfering elements, show that fluorine modifies the ionization process, leading to element-specific changes of ion yields (which can vary by several orders of magnitude), and affects the efficiency of metal hydride and oxide formation. In conjunction, these two effects can efficiently induce separation of mass interference, providing more representative TOF-SIMS data with respect to the sample composition and significant enhancement of chemical image resolution. Consequently, this can improve the chemical characterization of complex multilayers in nanoscale.

Nanoprojectile Secondary Ion Mass Spectrometry for Analysis of Extracellular Vesicles

We describe a technique based on secondary ion mass spectrometry with nanoprojectiles (NP-SIMS) for determining the protein content of extracellular vesicles, EVs, via tagged antibodies. The technique uses individual gold nanoprojectiles (e.g., Au₄₀₀⁴⁺ and Au₂₈₀₀⁸⁺), separated in time and space, to bombard a surface. For each projectile impact (10-20 nm in diameter), the co-emitted molecules are mass analyzed and recorded as an individual mass spectrum. Examining these individual mass spectra for co-localized species allows for nanoscale mass spectrometry to be performed. The high lateral resolution of this technique is well suited for analyzing nano-objects. SIMS is generally limited to analyzing small molecules (below ∼1500 Da); therefore, we evaluated three molecules (eosin, erythrosine, and BHHTEGST) as prospective mass spectrometry tags. We tested these on a model surface comprising a mixture of all three tags conjugated to antibodies and found that NP-SIMS could detect all three tags from a single projectile impact. Applying the method, we tagged two surface proteins common in urinary EVs, CD63 and CD81, with anti-CD63-erythrosine and anti-CD81-BHHTEGST. We found that NP-SIMS could determine the relative abundance of the two proteins and required only a few hundred or thousand EVs in the analysis region to detect the presence of the tagged antibodies.

Mass spectrometry as a tool to advance polymer science

In contrast to natural polymers, which have existed for billions of years, the first well-understood synthetic polymers date back to just over one century ago. Nevertheless, this relatively short period has seen vast progress in synthetic polymer chemistry, which can now afford diverse macromolecules with varying structural complexities. To keep pace with this synthetic progress, there have been commensurate developments in analytical chemistry, where mass spectrometry has emerged as the pre-eminent technique for polymer analysis. This Perspective describes present challenges associated with the mass-spectrometric analysis of synthetic polymers, in particular the desorption, ionization and structural interrogation of high-molar-mass macromolecules, as well as strategies to lower spectral complexity. We critically evaluate recent advances in technology in the context of these challenges and suggest how to push the field beyond its current limitations. In this context, the increasingly important role of high-resolution mass spectrometry is emphasized because of its unrivalled ability to describe unique species within polymer ensembles, rather than to report the average properties of the ensemble.

Graphene quantum dots enhanced ToF-SIMS for single-cell imaging

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) has shown promising applications in single-cell analysis owing to its high spatial resolution molecular imaging capability. One of the main drawbacks hindering progress in this field is the relatively low ionization efficiency for biological systems. The complex chemical micro-environment in single cells typically causes severe matrix effects, leading to significant signal suppression of biomolecules. In this work, we investigated the signal enhancement effect of graphene quantum dots (GE QDs) in ToF-SIMS analysis. A × 160 magnification of ToF-SIMS signal for amiodarone casted on glass slide was observed by adding amino-functionalized GE QDs (amino-GE QDs), which was significantly higher than adding previously reported signal enhancement materials and hydroxyl group-functionalized GE QDs (hydroxyl-GE QDs). A possible mechanism for GE QD-induced signal enhancement was proposed. Further, effects of amino-GE QDs and hydroxyl-GE QDs on amiodarone-treated breast cancer cells were compared. A significant signal improvement for lipids and amiodarone was achieved using both types of GE QDs, especially for amino-GE QDs. In addition, ToF-SIMS chemical mapping of single cells with better quality was obtained after signal enhancement. Our strategy for effective ToF-SIMS signal enhancement holds great potential for further investigation of drug metabolism pathways and the interactions between the cell and micro-environment.

Single-cell assay on microfluidic devices

Advances in microfluidic techniques have prompted researchers to study the inherent heterogeneity of single cells in cell populations.

Observation of endoplasmic reticulum tubules via TOF-SIMS tandem mass spectrometry imaging of transfected cells

Advances in three-dimensional secondary ion mass spectrometry (SIMS) imaging have enabled visualizing the subcellular distributions of various lipid species within individual cells. However, the difficulty of locating organelles using SIMS limits efforts to study their lipid compositions. Here, the authors have assessed whether endoplasmic reticulum (ER)-Tracker Blue White DPX^®, which is a commercially available stain for visualizing the endoplasmic reticulum using fluorescence microscopy, produces distinctive ions that can be used to locate the endoplasmic reticulum using SIMS. Time-of-flight-SIMS tandem mass spectrometry (MS²) imaging was used to identify positively and negatively charged ions produced by the ER-Tracker stain. Then, these ions were used to localize the stain and thus the endoplasmic reticulum, within individual human embryonic kidney cells that contained higher numbers of endoplasmic reticulum-plasma membrane junctions on their surfaces. By performing MS² imaging of selected ions in parallel with the precursor ion (MS¹) imaging, the authors detected a chemical interference native to the cell at the same nominal mass as the pentafluorophenyl fragment from the ER-Tracker stain. Nonetheless, the fluorine secondary ions produced by the ER-Tracker stain provided a distinctive signal that enabled locating the endoplasmic reticulum using SIMS. This simple strategy for visualizing the endoplasmic reticulum in individual cells using SIMS could be combined with existing SIMS methodologies for imaging intracellular lipid distribution and to study the lipid composition within the endoplasmic reticulum.

Visualizing molecular distributions for biomaterials applications with mass spectrometry imaging: a review

Mass spectrometry imaging (MSI) is a rapidly emerging field that is continually finding applications in new and exciting areas. The ability of MSI to measure the spatial distribution of molecules at or near the surface of complex substrates makes it an ideal candidate for many applications, including those in the sphere of materials chemistry. Continual development and optimization of both ionization sources and analyzer technologies have resulted in a wide array of MSI tools available, both commercially available and custom-built, with each configuration possessing inherent strengths and limitations. Despite the unique potential of MSI over other chemical imaging methods, their potential and application to (bio)materials science remains in our view a largely underexplored avenue. This review will discuss these techniques enabling high parallel molecular detection, focusing on those with reported uses in (bio)materials chemistry applications and highlighted with select applications. Different technologies are presented in three main sections; secondary ion mass spectrometry (SIMS) imaging, matrix-assisted laser desorption ionization (MALDI) MSI, and emerging MSI technologies with potential for biomaterial analysis. The first two sections (SIMS and MALDI) discuss well-established methods that are continually evolving both in technological advancements and in experimental versatility. In the third section, relatively new and versatile technologies capable of performing measurements under ambient conditions will be introduced, with reported applications in materials chemistry or potential applications discussed. The aim of this review is to provide a concise resource for those interested in utilizing MSI for applications such as biomimetic materials, biological/synthetic material interfaces, polymer formulation and bulk property characterization, as well as the spatial and chemical distributions of nanoparticles, or any other molecular imaging application requiring broad chemical speciation.

Label-free visualization of nilotinib-functionalized gold nanoparticles within single mammalian cells by C60- SIMS imaging

Obtaining a comprehensive grasp of the behavior and interaction of pharmaceutical compounds within single cells provides some of the fundamental details necessary for more effective drug development. In particular, the changes ensuing in the carrier, drug, and host environment in targeted drug therapy applications must be explored in greater detail, as these are still not well understood. Here, nilotinib-functionalized gold nanoparticles are examined within single mammalian cells with use of imaging cluster secondary ion mass spectrometry in a model study designed to enhance our understanding of what occurs to these particles once that have been internalized. Nilotinib, several types of gold nanoparticles, and the functionalized combination of the two were surveyed and successfully imaged within single cells to determine uptake and performance. Both nilotinib and the gold particle are able to be distinguished and visualized in the functionalized nanoparticle assembly within the cell. These compounds, while both internalized, do not appear to be present in the same pixels of the chemical image, indicating possible cleavage of nilotinib from the particle after cell uptake. The method provided in this work is a direct measurement of uptake and subcellular distribution of an active drug and its carrier within a framework. The results obtained from this study have the potential to be applied to future studies to provide more effective and specific cellular delivery of a relevant pharmaceutical compound.

Three-dimensional imaging of cholesterol and sphingolipids within a Madin-Darby canine kidney cell

Metabolic stable isotope incorporation and secondary ion mass spectrometry (SIMS) depth profiling performed on a Cameca NanoSIMS 50 were used to image the (18)O-cholesterol and (15)N-sphingolipid distributions within a portion of a Madin-Darby canine kidney (MDCK) cell. Three-dimensional representations of the component-specific isotope distributions show clearly defined regions of (18)O-cholesterol and (15)N-sphingolipid enrichment that seem to be separate subcellular compartments. The low levels of nitrogen-containing secondary ions detected at the (18)O-enriched regions suggest that these (18)O-cholesterol-rich structures may be lipid droplets, which have a core consisting of cholesterol esters and triacylglycerides.

C60-SIMS imaging of nanoparticles within mammalian cells

To achieve successful drug delivery via nanoparticles the interactions between the nanoparticle and the chemistry of the surrounding biological environment is of central importance. A thorough understanding of these interactions is necessary in order to better elucidate information regarding drug pathways and mechanisms of action in treatment protocols. As such, it is important to identify the location of the nanoparticle, the state of its functionalization, as well as any changes in the cellular environment. The use of cluster secondary ion mass spectrometry (SIMS) using C60 (+) primary ions makes simultaneous acquisition of this information possible. Here, SIMS has been successfully used to chemically image gold nanoparticles (AuNPs) within a model, single cell system involving macrophage-like RAW 264.7 cells. The macrophage-like properties of this cell line make it extremely well-suited for cell-uptake studies. Both AuNPs and two pharmaceutical compounds, amiodarone and elacridar, were successfully imaged within a cellular system using cluster SIMS. To verify that SIMS can also be used to detect functionalization and nanoparticles simultaneously, fluorophore-functionalized AuNPs were studied as a model system. The fluorescent characteristics of these functionalized nanoparticles enabled the visual confirmation of the presence and location of the particles within the cell.