Analysis of Compounds Dissolved in Nonpolar Solvents by Electrospray Ionization on Conductive Nanomaterials

Recent Developments in Single-Cell Metabolomics by Mass Spectrometry─A Perspective

Recent advancements in single-cell (sc) resolution analyses, particularly in sc transcriptomics and sc proteomics, have revolutionized our ability to probe and understand cellular heterogeneity. The study of metabolism through small molecules, metabolomics, provides an additional level of information otherwise unattainable by transcriptomics or proteomics by shedding light on the metabolic pathways that translate gene expression into functional outcomes. Metabolic heterogeneity, critical in health and disease, impacts developmental outcomes, disease progression, and treatment responses. However, dedicated approaches probing the sc metabolome have not reached the maturity of other sc omics technologies. Over the past decade, innovations in sc metabolomics have addressed some of the practical limitations, including cell isolation, signal sensitivity, and throughput. To fully exploit their potential in biological research, however, remaining challenges must be thoroughly addressed. Additionally, integrating sc metabolomics with orthogonal sc techniques will be required to validate relevant results and gain systems-level understanding. This perspective offers a broad-stroke overview of recent mass spectrometry (MS)-based sc metabolomics advancements, focusing on ongoing challenges from a biologist's viewpoint, aimed at addressing pertinent and innovative biological questions. Additionally, we emphasize the use of orthogonal approaches and showcase biological systems that these sophisticated methodologies are apt to explore.

Arc-Induced Electrospray Ionization Mass Spectrometry

Arc-induced electrospray ionization mass spectrometry (AESI-MS) was developed during which alternating current electrospray is simply achieved through the arc plasma. The AESI source exploits the arc's temperature and charge properties to generate aerosols consisting of charged microdroplets. The electrospray region, in which organic molecules are contained within microdroplets, partially overlaps with the arc plasma region. Guided by the electric field, these molecules undergo ionization, yielding ionic target analytes. AESI represents a soft ionization method that combines the mechanisms of atmospheric pressure chemical ionization and electrospray ionization, facilitating the ionization of analytes with wide ranging polarities. The precisely targeted spraying area enhances ion entry into the mass analyzer, thereby enabling excellent ionization efficiency. The AESI source exhibits several notable advantages over the electrospray ionization source, including an elevated but comparable level of active species concentrations and types, simplified mass spectra for direct amino acid analysis, high salt tolerance, versatile analysis of compounds with varying polarities, and reliable quantitative analysis of amino acids in complex matrices. Overall, AESI broadens the methodologies employed to generate microdroplets, providing a technological and scientific framework for creating distinctive electrospray ionization techniques.

Ultrasensitive and Green Bubbling Extraction Strategies: An Extensible Solvent-Free Re-Enrichment Approach for Ultratrace Pollutants in Aqueous Samples

Ultratrace organic pollutants in the environment pose severe threats to human health; hence, their accurate detection is essential. In this study, we develop a secondary solvent-free enrichment strategy based on bubbling extraction (BE). Especially, we used BE solid-phase microextraction and BE carbon nanotube paper absorption to capture aerosols from a liquid water surface, desorb analytes, and analyze the analytes using mass spectrometry. The application of a solvent-free enrichment strategy helps overcome technical challenges in implementing BE technology, including reproducibility, quantification, and sensitivity. This approach objectively demonstrates the enrichment efficiency of BE, resulting in improved mass spectrometry response and quantification. It effectively tackles the difficulties in detecting and quantifying ultratrace environmental pollutants in mass spectrometric analysis. The present study successfully conducted a quantitative analysis of 16 polycyclic aromatic hydrocarbons and 7 antibiotics in 48 environmental water samples. This strategy proved effective in detecting the presence and distribution of polar and nonpolar environmental pollutants in rivers and lakes. Moreover, this strategy has several advantages, such as ultrahigh sensitivity at the femtograms per liter level, good greenness, multiplexed quantitation, low sample consumption, and ease of operation. Overall, the utilization of the ultrasensitive and environmentally friendly BE approach presents a reliable and adaptable method for the identification of ultratrace environmental pollutants in water specimens, thereby enabling early monitoring of pollutant levels.

Does the Red Shift in UV-Vis Spectra Really Provide a Sensing Option for Detection of N-Nitrosamines Using Metalloporphyrins?

N-nitrosamines are widespread cancerogenic compounds in human environment, including water, tobacco products, food, and medicinal products. Their presence in pharmaceuticals has recently led to several recalls of important medicines from the market, and strict controls and tight limits of N-nitrosamines are now required. Analytical determination of N-nitrosamines is expensive, laborious, and time-inefficient making development of simpler and faster techniques for their detection crucial. Several reports published in the previous decade have demonstrated that cobalt porphyrin-based chemosensors selectively bind N-nitrosamines, which produces a red shift of characteristic Soret band in UV-Vis spectra. In this study, a thorough re-evaluation of metalloporphyrin/N-nitrosamine adducts was performed using various characterization methods. Herein, we demonstrate that while N-nitrosamines can interact directly with cobalt-based porphyrin complexes, the red shift in UV-Vis spectra is not selectively assured and might also result from the interaction between impurities in N-nitrosamines and porphyrin skeleton or interaction of other functional groups within the N-nitrosamine structure and the metal ion within the porphyrin. We show that pyridine nitrogen is the interacting atom in tobacco-specific N-nitrosamines (TSNAs), as pyridine itself is an active ligand and not the N-nitrosamine moiety. When using Co(II) porphyrins as chemosensors, acidic and basic impurities in dialkyl N-nitrosamines (e.g., formic acid, dimethylamine) are also UV-Vis spectra red shift-producing species. Treatment of these N-nitrosamines with K2CO3 prevents the observed UV-Vis phenomena. These results imply that cobalt-based metalloporphyrins cannot be considered as selective chemosensors for UV-Vis detection of N-nitrosamine moiety-containing species. Therefore, special caution in interpretation of UV-Vis red shift for chemical sensors is suggested.

Controlling morphology in electrosprayed methylcellulose nanowires via nanoparticle addition: coarse-grained modeling and experiments

Electrospray deposition (ESD) has shown great promise for manufacturing micro- and nanostructured coatings at scale on versatile substrates with complex geometries. ESD exhibits a broad spectrum of morphologies depending upon the properties of spray fluids. Among them are nanowire forests or foams obtained via the in-air gelation of electrospray droplets formed from methylcellulose (MC) solutions. In this study, we explored MC ESD loaded with nanoparticles of various shapes and uncovered the effects of particle fillers on morphology evolution using coarse-grained simulations and physical experiments. Utilizing electrostatic dissipative particle dynamics, we modeled the electrohydrodynamic deformation of particle-laden MC droplets undergoing in-flight evaporation. The simulations quantitatively predict the suppression of droplet deformation as the size or concentration of spherical nanoparticles increases. While small particles can be readily encapsulated into the nanowire body, large particles can arrest nanowire formation. The model was extended to nanoparticles with complex topologies, showing MC nanowires emerging from particle edges and vertices due to curvature-enhanced electric stress. In all cases, strong agreements were found between simulation and experimental results. These results demonstrate the efficacy of the coarse-grained model in predicting the morphology evolution of electrosprayed droplets and lay the groundwork for employing MC nanowires for developing nanostructured composites.