In situ SEM and ToF-SIMS analysis of IgG conjugated gold nanoparticles at aqueous surfaces

An evaluation of static ToF-SIMS analysis of environmental organics

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been extensively used in surface analysis due to its high mass resolution, sensitivity, and mass spectral imaging capabilities. Static ToF-SIMS has mainly been used for solid material analysis; however, its application in environmental organics is limited. During SIMS spectral analysis, relative mass accuracy and measurement repeatability are key factors for obtaining reliable speciation and acquiring chemical insights of the specimens. Herein, we provide an evaluation of four environmentally relevant organic systems, including glyoxal, pyruvic acid, oil-in-water emulsion, and carbon dioxide (CO2) capture solvent (i.e., N-2-ethoxyethyl-3-morpholinopropan-1-amine, EMMPA), to show the spectral measurement repeatability when using static ToF-SIMS. First, sample preparation is essential in acquiring accurate and reproducible results in ToF-SIMS analysis. The mass spectral results show that characteristic peaks observed can be distinguished with reasonable confidence by comparing the observed mass to charge ratios (m/z) to theoretical ones. The statistical analysis of peak areas indicates that the peak area and/or peak height measurement ratios are satisfactory among replicates. Compared with previous studies, the bismuth cluster primary ion beam, namely Bi3 +, has less fragmentation than Bi+. Therefore, Bi3 + is deemed more suitable for organic analysis using static SIMS. Our results show that ToF-SIMS offers a viable approach to study environmental organics including but not limited to aqueous aerosols, wastewater emulsions, and CO2 capture solvents. It is expected that future studies will expand organic speciation with high fidelity due to the continued advancement of SIMS as a sensitive analysis technique.

In situ liquid SEM imaging analysis revealing particle dispersity in aqueous solutions

A quantitative description on dispersity of boehmite (γ-AlOOH) particles, a key component for waste slurry at Hanford sites, can provide useful knowledge for understanding various physicochemical nature of the waste. In situ liquid scanning electron microscopy (SEM) was used to evaluate the dispersity of particles in aqueous conditions using a microfluidic sample holder, System for Analysis at Liquid Vacuum Interface (SALVI). Secondary electron (SE) images and image analyses were performed to determine particle centroid locations and the distance to the nearest neighbour particle centroid, providing reliable rescaled interparticle distances as a function of ionic strength in acidic and basic conditions. Our finding of the particle dispersity is consistent with physical insights from corresponding particle interactions under physicochemical conditions, demonstrating delicate changes in dispersity of boehmite particles based on novel in situ liquid SEM imaging and analysis. LAY DESCRIPTION: In situ liquid scanning electron microscopy (SEM) was used to determine the interparticle distance of boehmite (γ-AlOOH) particles, a key component for waste slurry at Hanford sites. This type of quantitative measurement is important to understand various physicochemical nature of the radiological waste containing boehmite. In situ liquid SEM was enabled by a unique vacuum compatible microfluidic cell, System for Analysis at Liquid Vacuum Interface (SALVI). We collected secondary electron (SE) images and performed image analyses to determine particle centroid locations and the distance to the nearest neighbour particle centroid to arrive at the interparticle distances in acidic and basic conditions. Our results show that delicate changes occur among boehmite particles under different pH conditions using novel in situ SEM imaging.

In situ molecular imaging of adsorbed protein films in water indicating hydrophobicity and hydrophilicity

In situ molecular imaging of protein films adsorbed on a solid surface in water was realized by using a vacuum compatible microfluidic interface and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Amino acid fragments from such hydrated protein films are observed and identified in the positive ion mode and the results are in agreement with reported works on dry protein films. Moreover, water clusters from the hydrated protein films have been observed and identified in both the positive and negative ion mode for a series protein films. Thus, the detailed composition of amino acids and water molecules in the hydrated protein films can be characterized, and the protein water microstructures can be revealed by the distinct three-dimensional spatial distribution reconstructed from in situ liquid ToF-SIMS molecular imaging. Furthermore, spectral principal component analysis of amino acid fragment peaks and water cluster peaks provides unique insights into the water cluster distribution, hydrophilicity, and hydrophobicity of hydrated adsorbed protein films in water.

Enabling liquid vapor analysis using synchrotron VUV single photon ionization mass spectrometry with a microfluidic interface

Vacuum ultraviolet (VUV) single photon ionization mass spectrometry (SPI-MS) is a vacuum-based technique typically used for the analysis of gas phase and solid samples, but not for liquids due to the challenge in introducing volatile liquids in a vacuum. Here we present the first demonstration of in situ liquid analysis by integrating the System for Analysis at the Liquid Vacuum Interface (SALVI) microfluidic reactor into VUV SPI-MS. Four representative volatile organic compound (VOC) solutions were used to illustrate the feasibility of liquid analysis. Our results show the accurate mass identification of the VOC molecules and the reliable determination of appearance energy that is consistent with ionization energy for gaseous species in the literature as reported. This work validates that the vacuum-compatible SALVI microfluidic interface can be utilized at the synchrotron beamline and enable the in situ study of gas-phase molecules evaporating off the surface of a liquid, which holds importance in the study of condensed matter chemistry.

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.

An investigation of the beam damage effect on in situ liquid secondary ion mass spectrometry analysis

RATIONALE

During in situ liquid secondary ion mass spectrometry (SIMS) analysis, the primary ion beam is normally scanned on a very small area to collect signals with high ion doses (10¹⁴ to 10¹⁶ ions/cm² ). As a result, beam damage may become a concern when compared with the static limit of SIMS analysis, in which the dose is normally less than 10¹² ions/cm² . Therefore, a comparison of ion yields in in situ liquid SIMS analysis versus traditional static SIMS analysis of corresponding dry samples is of great interest.

METHODS

In this study, a dipalmitoylphosphatidylcholine (DPPC) liposome solution was used as a model system. Both liquid sample and dry sample were examined. Secondary ion yields using three primary ion species (Bi⁺ , Bi₃⁺ and Bi₃⁺⁺ ) with various beam currents were investigated.

RESULTS

Usable ion yields for both positive and negative characteristic signals (including molecular ions and characteristic fragment ions) were achievable based on optimized experimental conditions for in situ liquid SIMS analysis. The ion yield of the key DPPC molecular ion was comparable to that of traditional static SIMS, and unexpected low fragmentation was observed. The flexible structure of the liquid plays an important role for these observations.

CONCLUSIONS

Therefore, beam damage may not be a concern in in situ liquid SIMS analysis if proper experimental conditions are used.

Through a Window, Brightly: A Review of Selected Nanofabricated Thin-Film Platforms for Spectroscopy, Imaging, and Detection

We present a review of the use of selected nanofabricated thin films to deliver a host of capabilities and insights spanning bioanalytical and biophysical chemistry, materials science, and fundamental molecular-level research. We discuss approaches where thin films have been vital, enabling experimental studies using a variety of optical spectroscopies across the visible and infrared spectral range, electron microscopies, and related techniques such as electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and single molecule sensing. We anchor this broad discussion by highlighting two particularly exciting exemplars: a thin-walled nanofluidic sample cell concept that has advanced the discovery horizons of ultrafast spectroscopy and of electron microscopy investigations of in-liquid samples; and a unique class of thin-film-based nanofluidic devices, designed around a nanopore, with expansive prospects for single molecule sensing. Free-standing, low-stress silicon nitride membranes are a canonical structural element for these applications, and we elucidate the fabrication and resulting features-including mechanical stability, optical properties, X-ray and electron scattering properties, and chemical nature-of this material in this format. We also outline design and performance principles and include a discussion of underlying material preparations and properties suitable for understanding the use of alternative thin-film materials such as graphene.

In situ nuclear magnetic resonance microimaging of live biofilms in a microchannel

The firstin situnuclear magnetic resonance microimaging of live biofilms in a transferrable microfluidic platform.

Improving the Molecular Ion Signal Intensity for In Situ Liquid SIMS Analysis

In situ liquid secondary ion mass spectrometry (SIMS) enabled by system for analysis at the liquid vacuum interface (SALVI) has proven to be a promising new tool to provide molecular information at solid-liquid and liquid-vacuum interfaces. However, the initial data showed that useful signals in positive ion spectra are too weak to be meaningful in most cases. In addition, it is difficult to obtain strong negative molecular ion signals when m/z>200. These two drawbacks have been the biggest obstacle towards practical use of this new analytical approach. In this study, we report that strong and reliable positive and negative molecular signals are achievable after optimizing the SIMS experimental conditions. Four model systems, including a 1,8-diazabicycloundec-7-ene (DBU)-base switchable ionic liquid, a live Shewanella oneidensis biofilm, a hydrated mammalian epithelia cell, and an electrolyte popularly used in Li ion batteries were studied. A signal enhancement of about two orders of magnitude was obtained in comparison with non-optimized conditions. Therefore, molecular ion signal intensity has become very acceptable for use of in situ liquid SIMS to study solid-liquid and liquid-vacuum interfaces. Graphical Abstract ᅟ.

Dynamic imaging of Au-nanoparticles via scanning electron microscopy in a graphene wet cell

High resolution nanoscale imaging in liquid environments is crucial for studying molecular interactions in biological and chemical systems. In particular, electron microscopy is the gold-standard tool for nanoscale imaging, but its high-vacuum requirements make application to in-liquid samples extremely challenging. Here we present a new graphene based wet cell device where high resolution scanning electron microscope (SEM) and energy dispersive x-rays (EDX) analysis can be performed directly inside a liquid environment. Graphene is an ideal membrane material as its high transparancy, conductivity and mechanical strength can support the high vacuum and grounding requirements of a SEM while enabling maximal resolution and signal. In particular, we obtain high resolution ([Formula: see text] nm) SEM video images of nanoparticles undergoing Brownian motion inside the graphene wet cell and EDX analysis of nanoparticle composition in the liquid enviornment. Our obtained resolution surpasses current conventional silicon nitride devices imaged in both a SEM and transmission electron microscope under much higher electron doses.

Two-dimensional and three-dimensional dynamic imaging of live biofilms in a microchannel by time-of-flight secondary ion mass spectrometry

A vacuum compatible microfluidic reactor, SALVI (System for Analysis at the Liquid Vacuum Interface), was employed for in situ chemical imaging of live biofilms using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Depth profiling by sputtering materials in sequential layers resulted in live biofilm spatial chemical mapping. Two-dimensional (2D) images were reconstructed to report the first three-dimensional images of hydrated biofilm elucidating spatial and chemical heterogeneity. 2D image principal component analysis was conducted among biofilms at different locations in the microchannel. Our approach directly visualized spatial and chemical heterogeneity within the living biofilm by dynamic liquid ToF-SIMS.

In situ chemical probing of the electrode-electrolyte interface by ToF-SIMS

A portable vacuum interface allowing direct probing of the electrode-electrolyte interface was developed. A classical electrochemical system consisting of a gold working electrode, platinum counter electrode, platinum reference electrode, and potassium iodide electrolyte was used to demonstrate real-time observation of the gold iodide adlayer on the electrode and chemical species as a result of redox reactions using cyclic voltammetry (CV) and time-of-flight secondary ion mass spectrometry (ToF-SIMS, a vacuum-based surface technique) simultaneously. This microfluidic electrochemical probe provides a new way to investigate the surface region with adsorbed molecules and the region of the diffused layer with chemical speciation in liquids in situ by surface sensitive techniques.

In situ molecular imaging of a hydrated biofilm in a microfluidic reactor by ToF-SIMS

A novel microfluidic reactor for biofilm growth was constructed to enablein situchemical imaging of hydrated biofilms using ToF-SIMS.