Desorption-ionization mass spectrometry on porous silicon

Proteomics mining of cancer hallmarks on a single-cell resolution

Dysregulated proteome is an essential contributor in carcinogenesis. Protein fluctuations fuel the progression of malignant transformation, such as uncontrolled proliferation, metastasis, and chemo/radiotherapy resistance, which severely impair therapeutic effectiveness and cause disease recurrence and eventually mortality among cancer patients. Cellular heterogeneity is widely observed in cancer and numerous cell subtypes have been characterized that greatly influence cancer progression. Population-averaged research may not fully reveal the heterogeneity, leading to inaccurate conclusions. Thus, deep mining of the multiplex proteome at the single-cell resolution will provide new insights into cancer biology, to develop prognostic biomarkers and treatments. Considering the recent advances in single-cell proteomics, herein we review several novel technologies with particular focus on single-cell mass spectrometry analysis, and summarize their advantages and practical applications in the diagnosis and treatment for cancer. Technological development in single-cell proteomics will bring a paradigm shift in cancer detection, intervention, and therapy.

Self-Assembled Molecular Fibers Aligned by Compression in Water

Molecular self-assembly has attracted much attention as a potential approach for fabricating nanostructured functional materials. To date, energy-efficient fabrication of nano-objects such as nanofibers, nanorings, and nanotubes is achieved using well-designed self-assembling molecules. However, the application of molecular self-assembly to industrial manufacturing processes remains challenging because regulating the positions and directions of self-assembled products is difficult. Non-covalent molecular assemblies are also too fragile to allow mechanical handling. The present work demonstrates the macroscopic alignment of self-assembled molecular fibers using compression. Specifically, the macroscopic bundling of self-assembled nanofibers is achieved following dispersion in water. These fiber bundles can also be chemically crosslinked without drastic changes in morphology via trialkoxysilyl groups. Subsequently, vertically oriented porous membranes can be produced rapidly by slicing the bundles. This technique is expected to be applicable to various functional self-assembled fibers and can lead to the development of innovative methods of producing anisotropic nanostructured materials.

What Are the Key Factors for the Detection of Peptides Using Mass Spectrometry on Boron-Doped Diamond Surfaces?

We investigated the use of boron-doped diamond (BDD) with different surface morphologies for the enhanced detection of nine different peptides by matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). For the first time, we compared three different nanostructured BDD film morphologies (Continuous, Nanograss, and Nanotips) with differently terminated surfaces (-H, -O, and -F) to commercially available Ground Steel plates. All these surfaces were evaluated for their effectiveness in detecting the nine different peptides by MALDI-MS. Our results demonstrated that certain nanostructured BDD surfaces exhibited superior performance for the detection of especially hydrophobic peptides (e.g., bradykinin 1-7, substance P, and the renin substrate), with a limit of detection of down to 2.3 pM. Further investigation showed that hydrophobic peptides (e.g., bradykinin 1-7, substance P, and the renin substrate) were effectively detected on hydrogen-terminated BDD surfaces. On the other hand, the highly acidic negatively charged peptide adrenocorticotropic hormone fragment 18-39 was effectively identified on oxygen-/fluorine-terminated BDD surfaces. Furthermore, BDD surfaces reduced sodium adduct contamination significantly.

Mass Spectrometry Imaging Techniques: Non-Ambient and Ambient Ionization Approaches

Molecular information can be acquired from sample surfaces in real time using a revolutionary molecular imaging technique called mass spectrometry imaging (MSI). The technique can concurrently provide high spatial resolution information on the spatial distribution and relative proportion of many different compounds. Thus, many scientists have been drawn to the innovative capabilities of the MSI approach, leading to significant focus in various fields during the past few decades. This review describes the sampling protocol, working principle and applications of a few non-ambient and ambient ionization mass spectrometry imaging techniques. The non-ambient techniques include secondary ionization mass spectrometry and matrix-assisted laser desorption ionization, while the ambient techniques include desorption electrospray ionization, laser ablation electrospray ionization, probe electro-spray ionization, desorption atmospheric pressure photo-ionization and femtosecond laser desorption ionization. The review additionally addresses the advantages and disadvantages of ambient and non-ambient MSI techniques in relation to their suitability, particularly for biological samples used in tissue diagnostics. Last but not least, suggestions and conclusions are made regarding the challenges and future prospects of MSI.

Surface-Assisted Laser Desorption/Ionization Mass Spectrometry with a Two-Dimensional Au Nanoparticle Array for Soft Ionization

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) is a valuable technique for detecting small molecules in environmental and medicinal studies. We investigated dot-like and 2D-array gold nanoparticle-based SALDI-MS substrates that excite surface plasmons and enhance the desorption/ionization of sample molecules via charge transfer between the substrate and sample molecules. We aimed to optimize the nondissociative detection of sample molecules by efficiently transferring energy while suppressing excess internal energy. SALDI-MS measurements using crystal violet (CV) molecules revealed ion intensity and spectral pattern differences between the dot-like and 2D-array substrates. SALDI-MS measurements using dot-like substrates suggested two desorption/ionization processes: internal energy supply and charge transfer between the substrate and sample molecules. However, SALDI-MS measurements using 2D-array substrates suggested that the internal energy supply was suppressed. As a result, the dot-like substrate provided higher desorption/ionization efficiency but increased fragmentation, whereas the 2D-array substrate was suitable for highly sensitive and nondissociative SALDI-MS measurements. This study contributes to the optimization of SALDI-MS measurements and advances our understanding of energy transfer and sample molecule dissociation.

Single Cell mass spectrometry: Towards quantification of small molecules in individual cells

Studying cell heterogeneity can provide a deeper understanding of biological activities, but appropriate studies cannot be performed using traditional bulk analysis methods. The development of diverse single cell bioanalysis methods is in urgent need and of great significance. Mass spectrometry (MS) has been recognized as a powerful technique for bioanalysis for its high sensitivity, wide applicability, label-free detection, and capability for quantitative analysis. In this review, the general development of single cell mass spectrometry (SCMS) field is covered. First, multiple existing SCMS techniques are described and compared. Next, the development of SCMS field is discussed in a chronological order. Last, the latest quantification studies on small molecules using SCMS have been described in detail.

Spatial neurolipidomics-MALDI mass spectrometry imaging of lipids in brain pathologies

Given the complexity of nervous tissues, understanding neurochemical pathophysiology puts high demands on bioanalytical techniques with respect to specificity and sensitivity. Mass spectrometry imaging (MSI) has evolved to become an important, biochemical imaging technology for spatial biology in biological and translational research. The technique facilitates comprehensive, sensitive elucidation of the spatial distribution patterns of drugs, lipids, peptides, and small proteins in situ. Matrix-assisted laser desorption ionization (MALDI)-based MSI is the dominating modality due to its broad applicability and fair compromise of selectivity, sensitivity price, throughput, and ease of use. This is particularly relevant for the analysis of spatial lipid patterns, where no other comparable spatial profiling tools are available. Understanding spatial lipid biology in nervous tissue is therefore a key and emerging application area of MSI research. The aim of this review is to give a concise guide through the MSI workflow for lipid imaging in central nervous system (CNS) tissues and essential parameters to consider while developing and optimizing MSI assays. Further, this review provides a broad overview of key developments and applications of MALDI MSI-based spatial neurolipidomics to map lipid dynamics in neuronal structures, ultimately contributing to a better understanding of neurodegenerative disease pathology.

Serological Exosome Metabolic Biopsy of Hepatocellular Carcinoma via Designed Core-Shell Nanoparticles

Exosome metabolite-based liquid biopsy is a promising strategy for large-scale application in practical clinics toward precise medicine. Given the current challenges in successive isolation and analysis of exosomes and their metabolites in this field, we established a low-cost, high-throughput, and rapid platform for serological exosome metabolic biopsy of hepatocellular carcinoma (HCC) via designed core-shell nanoparticles. It starts with the efficient extraction of high-quality serum exosomes and exosome metabolic features, based on which significantly obvious sample clusters are observed by unsupervised cluster analysis. The following integration of feature selection and supervised machine learning enables the identification of six key metabolites and achieves high-performance prediction between HCC, liver cirrhosis, and healthy controls. Specifically, both sensitivity and accuracy achieve 100% among any pairwise intergroup discrimination in a blind test. The quality and reliability of six key metabolites are further evaluated and validated by using different machine learning algorithms and pathway exploration. Our platform contributes to the future growth of new liquid biopsy technologies for precision diagnosis and real-time monitoring of HCC, among other conditions.

Detection of Short Peptides as Putative Biosignatures of Psychrophiles via Laser Desorption Mass Spectrometry

Studies of psychrophilic life on Earth provide chemical clues as to how extraterrestrial life could maintain viability in cryogenic environments. If living systems in ocean worlds (e.g., Enceladus) share a similar set of 3-mer and 4-mer peptides to the psychrophile Colwellia psychrerythraea on Earth, spaceflight technologies and analytical methods need to be developed to detect and sequence these putative biosignatures. We demonstrate that laser desorption mass spectrometry, as implemented by the CORALS spaceflight prototype instrument, enables the detection of protonated peptides, their dimers, and metal adducts. The addition of silicon nanoparticles promotes the ionization efficiency, improves mass resolving power and mass accuracies via reduction of metastable decay, and facilitates peptide de novo sequencing. The CORALS instrument, which integrates a pulsed UV laser source and an Orbitrap™ mass analyzer capable of ultrahigh mass resolving powers and mass accuracies, represents an emerging technology for planetary exploration and a pathfinder for advanced technique development for astrobiological objectives. Teaser: Current spaceflight prototype instrument proposed to visit ocean worlds can detect and sequence peptides that are found enriched in at least one strain of microbe surviving in subzero icy brines via silicon nanoparticle-assisted laser desorption analysis.

Effects of Silane Monolayers on Lysophosphatidylcholine (LysoPC) Detection by Desorption Ionization on Silicon Mass Spectrometry (DIOS-MS) in Solution and Plasma

Desorption ionization on silicon mass spectrometry (DIOS-MS) enables high throughput analysis of low-molecular-weight biomolecules. However, detection of metabolite biomarkers in complex fluids such as plasma requires sample pretreatment, limiting clinical application. Here, we show that porous silicon, chemically modified using monolayers of n-propyldimethylmethoxysilane molecules, is a good candidate for fingerprinting lysophosphatidylcholine (lysoPC) in plasma, without sample pretreatment, for DIOS-MS-based diagnosis (e.g., sepsis). Results were correlated to lysoPC molecule location inside/outside the pores, determined by time-of-flight secondary ion mass spectrometry profiling, and to physicochemical properties.

Titania Nanosheet as a Matrix for Surface-Assisted Laser Desorption/Ionization Mass Spectrometry Analysis and Imaging

Surface-assisted laser desorption/ionization (SALDI) acts as a soft desorption/ionization technique, which has been widely recognized in small-molecule analysis owing to eliminating the requirement of the organic matrix. Herein, titania nanosheets (TiO₂ NSs) were applied as novel substrates for simultaneous analysis and imaging of low-mass molecules and lipid species. A wide variety of representative analytes containing amino acids, bases, drugs, peptides, endogenous small molecules, and saccharide-spiked urine were examined by the TiO₂ NS-assisted LDI mass spectrometry (MS). Compared with conventional organic matrices and substrates [Ag nanoparticles (NPs), Au NPs, carbon nanotubes, carbon NPs, CeO₂ microparticles, and P25 TiO₂], the TiO₂ NS-assisted LDI MS method shows higher sensitivity and less spectral interference. Repeatability was evaluated with batch-to-batch relative standard deviations for 5-hydroxytryptophan, glucose-spiked urine, and glucose with addition of internal standard, which were 17.4, 14.9, and 2.8%, respectively. The TiO₂ NS-assisted LDI MS method also allows the determination of blood glucose levels in mouse serum with a linear range of 0.5-10 mM. Owing to the nanoscale size and uniform deposition of the TiO₂ NS matrix, spatial distributions of 16 endogenous small molecules and 16 lipid species from the horizontal section of the mouse brain tissue can be visualized at a 50 μm spatial resolution. These successful applications confirm that the TiO₂-assisted LDI MS method has promising prospects in the field of life science.

Effects of High-Quality Carbon Nanowalls Ionization-Assisting Substrates on Surface-Assisted Laser Desorption/Ionization Mass Spectrometry Performance

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) is performed using carbon nanowalls (CNWs) for ionization-assisting substrates. The CNWs (referred to as high-quality CNWs) in the present study were grown using a radical-injection plasma-enhanced chemical vapor deposition (RI-PECVD) system with the addition of oxygen in a mixture of CH₄ and H₂ gases. High-quality CNWs were different with respect to crystallinity and C-OH groups, while showing similar wall-to-wall distances and a wettability comparable to CNWs (referred to as normal CNWs) grown without O₂. The efficiency of SALDI was tested with both parameters of ion intensity and fragmental efficiency (survival yield (SY)) using N-benzylpyridinuim chloride (N-BP-CI). At a laser fluence of 4 mJ/cm², normal CNWs had an SY of 0.97 and an ion intensity of 0.13, while 5-sccm-O₂- high-quality CNWs had an SY of 0.89 and an ion intensity of 2.55. As a result, the sensitivity for the detection of low-molecular-weight analytes was improved with the high-quality CNWs compared to the normal CNWs, while an SY of 0.89 was maintained at a low laser fluence of 4 mJ/cm². SALDI-MS measurements available with the high-quality CNWs ionization-assisting substrate provided high ionization and SY values.

Laser-Shock-Driven In Situ Evolution of Atomic Defect and Piezoelectricity in Graphitic Carbon Nitride for the Ionization in Mass Spectrometry

Nanostructures─coupled with mass spectrometry─have been intensively investigated to improve the detection sensitivity and reproducibility of small biomolecules in laser desorption/ionization mass spectrometry (LDI-MS). However, the impact of laser-induced shock wave on the ionization of the nanostructures has rarely been reported. Herein, we systematically elucidate the laser shock wave effect on the ionization in terms of the in situ development of atomic defects and piezoelectricity in two-dimensional graphitic carbon nitride nanosheets (g-C₃N₄ NS) by short laser pulses. The mass analysis results of immunosuppressive drugs verify the enhanced LDI-MS performance, structurally originating from anisotropic lattice distortions in g-C₃N₄ NS, i.e., in-plane extension (contraction) and out-of-plane contraction (extension) that modulate the charge carrier motion. Along with the experimental investigations, density functional theory calculations on Mulliken charges and dipole moments demonstrate the contribution of defect and piezoelectricity to the ionization. The results of this study provide a mechanistic understanding of the underlying ionization processes, which is crucial for revealing the full potential of laser shock waves in LDI-MS.

Composite PVK/SLGO As Matrix for MALDI-TOF MS Detection of Small Molecules in Dual-Ion Mode

Currently, most matrices developed for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS) for small-molecule detection are only suitable for the positive or negative ion mode and not the dual-ion mode, except for carbon-based nanomaterials. The lone-pair electrons on the N atom in poly n-vinylcarbazole (PVK) can serve as a Lewis base with strong electron-donation effects, which is favorable for negative ion mode detection. The surface of single-layer graphene oxide (SLGO) contains many oxygen atoms in carboxyl and hydroxyl groups that act as Lewis acids and thereby provides favorable protonation sites for positive ion mode detection. In this study, composite PVK/SLGO was prepared by combining the advantages of amorphous PVK and SLGO. PVK/SLGO was tested as a novel matrix for positive- and negative-ion-mode MALDI-TOF MS for the analysis of amino acids, nucleic acid bases, environmental endocrine disruptors, antibiotics, and various small molecules. PVK/SLGO was compared with PVK, SLGO, and commercially available matrices of 9-aminoacridine (9-AA) and α-cyano-4-hydroxycinnamic acid (CHCA). The PVK/SLGO matrix was demonstrated to be suitable for the positive and negative ion modes, exhibiting high signal intensity and detection sensitivity without background interference. The limits of detection of the aforementioned molecules ranged from 0.1 to 0.0001 and 0.01 to 0.0001 mg/mL in the positive and negative ion modes, respectively. The quantitative determination of enrofloxacin in milk was realized using an internal standard method with a linear range of 0.0001-0.1 mg/mL (R ² = 0.9991). Furthermore, the PVK/SLGO matrix exhibited high salt tolerance (up to 1000 mmol/L) and stability over 28 consecutive days. Studies regarding its ionization mechanism revealed that the good performance originates from the combined materials acting synergistically. This study provides a foundation for developing bimodal composite matrices and further expands the scope of PVK/SLGO applications.

Adsorption of CO2 by a novel zeolite doped amine modified ternary aerogels

Novel amine functionalized materials can capture greenhouse gas CO₂. In this study, SiO₂-Al₂O₃-ZrO₂ ternary composite aerogel was prepared by sol-gel method, supercritical drying, ultrasonic non-in-situ synthesis and other processes using aluminum chloride hexahydrate as aluminum source, ethyl orthosilicate as silicon source and tetrabbutyl zirconate as zirconium source. The composite material was used as the carrier material. By impregnation method, the modified agent bis - (3-trimethoxy-silpropyl) amine and the composite were fully mixed and modified, and the novel zeolite doped amine functionalized ternary composite aerogel was obtained by doping acidification activation zeolite. The results show that the prepared novel zeolite amine-modified ternary aerogels have rich microporous structure and ordered mesoporous structure. After loading different contents of amine-based materials (CAA-X) in the ternary aerogels, the comparison between CAAZ-X and zeolite amine-modified ternary aerogels is conducted. Zeolite doped CAAZ-30 material shows the best adsorption performance, with a maximum adsorption capacity of 5.30 mmol/g. In the presence of water vapor, CAAZ-30 material also showed the best adsorption performance, with a maximum adsorption capacity of 5.33 mmol/g. This can help us design suitable adsorbent materials for CO₂ capture in different practical applications.

Laser desorption/ionization on nanostructured silicon: morphology matters

The surface morphology of the silicon nanostructure plays a crucial role in the laser desorption/ionization (LDI) process. Understanding the correlation between the surface morphology and LDI performance is the foundation for creating silicon substrates with high LDI efficiency. Most of the present studies focus only on the structural parameters (such as porosity, depth, total surface area, dimension, etc.) of a single structure, but their effects on LDI efficiency vary with the types of silicon structures. Herein, two representative types of silicon nanostructures, porous silicon (PSi) and thorny silicon (TSi), were created to address this issue. The results indicate that the PSi substrate can generate a stronger heat effect and is beneficial to desorption; the TSi substrate can facilitate electron transfer and is favorable to ionization. Subsequently, the assertion was further confirmed by simultaneously detecting a dozen of standard samples and a real sample on both the TSi and PSi substrates, in which PSi can significantly enhance the detection signals of organic salts, whereas the TSi substrate can greatly increase the LDI efficiencies of neutral analytes. This finding provides a foundation for improving the LDI performance by tailoring silicon nanostructures, which is helpful for designing and creating substrates with high LDI performance.

Advances in mass spectrometry imaging for toxicological analysis and safety evaluation of pharmaceuticals

Safety issues caused by pharmaceuticals have frequently occurred worldwide, posing a tremendous threat to human health. As an essential part of drug development, the toxicological analysis and safety evaluation is of great significance. In addition, the risk of pharmaceuticals accumulation in the environment and the monitoring of the toxicity from natural medicines have also received ongoing concerns. Due to a lack of spatial distribution information provided by common analytical methods, analyses that provide spatial dimensions could serve as complementary safety evaluation methods for better prediction and evaluation of drug toxicity. With advances in technical solutions and software algorithms, mass spectrometry imaging (MSI) has received increasing attention as a popular analytical tool that enables the simultaneous implementation of qualitative, quantitative, and localization without complex sample pretreatment and labeling steps. In recent years, MSI has become more attractive, powerful, and sensitive and has been applied in several scientific fields that can meet the safety assessment requirements. This review aims to cover a detailed summary of the various MSI technologies utilized in the biomedical and pharmaceutical area, including technical principles, advantages, current status, and future trends. Representative applications and developments in the safety-related issues of different pharmaceuticals and natural medicines are also described to provide a reference for pharmaceutical research, improve rational clinical medicine use, and ensure public safety.

Identification of Epigallocatechin-3-Gallate (EGCG) from Green Tea Using Mass Spectrometry

In an era where humanity is reinstating its lost hope and expectation on natural products, green tea occupies quite a position for what it has proven to be, in its endeavors for human welfare and health. Epigallocatechin-3-gallate (EGCG) is the key to the vast biological activities of green tea. Green tea is no longer in the backdrop; it has emerged as the most viral, trending bioactive molecule when it comes to health benefits for human beings. This review focuses on the use of various analytical techniques for the analysis of EGCG. That which has been achieved so far, in terms of in vitro, pure component analysis, as well as those spikes in biological fluids and those in vivo in animal and human samples, was surveyed and presented. The use of MS-based techniques for the analysis of EGCG is elaborately reviewed and the need for improvising the applications is explained. The review emphasizes that there is plenty of room to explore matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) applications in this subject area.

Laser-induced electron transfer desorption/ionization on MoO3 and WO3 surfaces for the determination of dithiocarbamates

Surface layers of molybdenum oxide MoO₃ and tungsten oxide WO₃ produced by thermal oxidation of molybdenum and tungsten plates in the air were studied for the first time as a platform for laser-induced electron transfer desorption/ionization. High analytical performance of such layers for the determination of metal complexes with dithiocarbamates, such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, and diethyldithiocarbamate, has been demonstrated. All studied complexes are detected as radical cations, with no fragment ions. The ion yields from MoO₃ and WO₃ surfaces were found to be more than two orders of magnitude higher than those from nanocrystalline silicon surfaces. A novel method has been developed for the determination of trace amounts of dithiocarbamates based on the complexation of analytes with gold ions, followed by laser-induced electron transfer desorption/ionization. The limits of detection of dithiocarbamates were estimated to be about 1 ng/mL. The proposed method was successfully applied to the rapid screening of tetramethylthiuram disulfide residues in juice.

Surface-assisted laser desorption/ionization mass spectrometry imaging: A review

In the last decades, surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) has attracted increasing interest due to its unique capabilities, achievable through the nanostructured substrates used to promote the analyte desorption/ionization. While the most widely recognized asset of SALDI-MS is the untargeted analysis of small molecules, this technique also offers the possibility of targeted approaches. In particular, the implementation of SALDI-MS imaging (SALDI-MSI), which is the focus of this review, opens up new opportunities. After a brief discussion of the nomenclature and the fundamental mechanisms associated with this technique, which are still highly controversial, the analytical strategies to perform SALDI-MSI are extensively discussed. Emphasis is placed on the sample preparation but also on the selection of the nanosubstrate (in terms of chemical composition and morphology) as well as its functionalization possibilities for the selective analysis of specific compounds in targeted approaches. Subsequently, some selected applications of SALDI-MSI in various fields (i.e., biomedical, biological, environmental, and forensic) are presented. The strengths and the remaining limitations of SALDI-MSI are finally summarized in the conclusion and some perspectives of this technique, which has a bright future, are proposed in this section.

Tissue Imprinting on 2D Nanoflakes-Capped Silicon Nanowires for Lipidomic Mass Spectrometry Imaging and Cancer Diagnosis

Spatially resolved tissue lipidomics is essential for accurate intraoperative and postoperative cancer diagnosis by revealing molecular information in the tumor microenvironment. Matrix-free laser desorption ionization mass spectrometry imaging (LDI-MSI) is an emerging attractive technology for label-free visualization of metabolites distributions in biological specimens. However, the development of LDI-MSI technology that could conveniently and authentically reveal molecular distribution on tissue samples is still a challenge. Herein, we present a tissue imprinting technology by retaining tissue lipids on 2D nanoflakes-capped silicon nanowires (SiNWs) for further mass spectrometry imaging and cancer diagnosis. The 2D nanoflakes were prepared by liquid exfoliation of molybdenum disulfide (MoS₂) with nitrogen-doped graphene quantum dots (NGQDs), which serve as both intercalation agent and dispersant. The obtained NGQD@MoS₂ nanoflakes were then decorated on the tip of vertical SiNWs, forming a hybrid NGQD@MoS₂/SiNWs nanostructure, which display excellent lipid extraction ability, enhanced LDI efficiency and molecule imaging capability. The peak number and total ion intensity of different lipids species on animal lung tissues obtained by tissue imprinting LDI-MSI on NGQD@MoS₂/SiNWs were ∼4-5 times greater than those on SiNWs substrate. As a proof-of-concept demonstration, the NGQD@MoS₂/SiNWs nanostructure was further applied to visualize phospholipids on sliced non small cell lung cancer (NSCLC) tissue along with the adjacent normal tissue. On the basis of selected feature lipids and machine learning algorithm, a prediction model was constructed to discriminate NSCLC tissues from the adjacent normal tissues with an accuracy of 100% for the discovery cohort and 91.7% for the independent validation cohort.

SALDI Substrate-Based FeNi Magnetic Alloy Nanoparticles for Forensic Analysis of Poisons in Human Serum

In this study, FeNi magnetic alloy nanoparticles (MANPs) were employed for the forensic analysis of four poisons—dimethametryn, napropamide, thiodicarb, and strychnine—using surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). FeNi MANPs were prepared via coprecipitation using two reducing agents, sodium borohydride (NaBH4) and hydrazine monohydrate (N2H4·H2O), to optimize the prepared MANPs and investigate their effect on the performance of SALDI-MS analysis. Thereafter, SALDI-MS analysis was carried out for the detection of three pesticides and a rodenticide. The prepared substrate offered sensitive detection of the targeted analytes with LOD values of 1 ng/mL, 100 pg/mL, 10 ng/mL, and 200 ng/mL for dimethametryn, napropamide, thiodicarb, and strychnine, respectively. The relative standard deviation (%RSD) values were in the range of 2.30–13.97% for the pesticides and 15–23.81% for strychnine, demonstrating the good spot-to-spot reproducibility of the FeNi substrate. Finally, the MANPs were successfully employed in the analysis of poison-spiked blood serum using a minute quantity of the sample with an LOD of 700 ng/mL dimethametryn and napropamide, 800 ng/mL thiodicarb, and 500 ng/mL strychnine. This study has great potential regarding the analysis of several poisons that may be found in human serum, which is significant in cases of self-harm.

Recent advances in on-site mass spectrometry analysis for clinical applications

In recent years, mass spectrometry (MS) is increasingly attracting interests for clinical applications, which also calls for technical innovations to make a transfer of MS from conventional analytical laboratories to clinics. The system design and analysis procedure should be friendly for novice users and appliable for on-site clinical diagnosis. In addition, the analysis result should be auto-interpreted and reported in formats much simpler than mass spectra. This motivates new ideas for developments in all the aspects of MS. In this review, we report recent advances of direct sampling ionization and miniature MS system, which have been developed targeting clinical and even point-of-care analysis. We also discuss the trend of the development and provide perspective on the technical challenges raised by diseases such as coronavirus SARS-CoV-2.

The local electric field effect of onion-like carbon nanoparticles for improved laser desorption/ionization efficiency of saccharides

It is still a challenge to improve ionization efficiency of saccharides in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Herein, the highly curved onion-like carbon nanoparticles (OCS) were synthesized from the low-price candle raw via a facile strategy. The unique nanostructure of OCS showed large surface area with plentiful mesoporous architecture, highly curved sp² carbon with regulating electronic effect, and good hydrophilicity, which could be beneficial to facilitate the desorption and ionization efficiency in MS process. The prepared OCS material as MALDI matrix exhibited the superior performance for the detection of xylose, glucose, maltose monohydrate, and raffinose pentahydrate in positive-ion mode with low background noise, enhanced ion intensities, uniform distribution, excellent reproducibility, good salt-tolerance, and high sensitivity compared to control candle soot (CS) and traditional α-cyano-4-hydroxycinnamic acid (CHCA) matrices. This highly effective LDI of OCS matrix was attributed to its enhancing local electric field effect, strong UV absorption ability, and high photo-thermal conversion performance. Furthermore, the OCS-assisted LDI MS approach was employed to quantitatively detect glucose in rat serum. This LDI MS platform may have valuable for the analysis of metabolites in clinical research.

Co3O4 nanocrystals as matrices for the detection of amino acids, harmful additives and pesticide residues by MALDI-TOF MS

Research of detection of low molecular weight compounds on human health and biological systems become increasingly important. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), a soft ionization equipment, is a rapid, reliable, high-sensitivity, high-throughput and simple test instrument. However, the application of MALDI-TOF MS in the analysis of small molecules (<500 Da) has become a great challenge because of the interference from the conventional matrices in low mass region when using conventional matrices. In this research, tricobalt tetraoxide (Co₃O₄) nanocrystals with rich surface hydroxyl groups were synthesized and served as novel matrices for the detection of small molecules by MALDI-TOF MS. In comparison with conventional organic matrices, the use of as-prepared Co₃O₄ nanocrystal matrices showed little matrix background interference, good reproducibility and high signal intensity in the analyses of amino acids, harmful additives and pesticide residues. For the detection of most amino acids, Co₃O₄ nanocrystal matrices have good detection performance both in the positive and negative ion modes and have a unique decarboxylation peak in the positive ion mode, which is conducive to the identification of amino acids. In addition, Co₃O₄ nanocrystals are completely feasible to test triadimefon, pirimicarb and other pesticide residues, as well as additives such as bisphenol A and melamine in the positive ion mode. It is also feasible to detect small molecule compounds in practical samples using Co₃O₄ nanocrystals as matrices. We believe the work provides an alternative approach for the detection of small molecules and expands the application scope of Co₃O₄ nanocrystals.

Multiplexed nanomaterial-assisted laser desorption/ionization for pan-cancer diagnosis and classification

As cancer is increasingly considered a metabolic disorder, it is postulated that serum metabolite profiling can be a viable approach for detecting the presence of cancer. By multiplexing mass spectrometry fingerprints from two independent nanostructured matrixes through machine learning for highly sensitive detection and high throughput analysis, we report a laser desorption/ionization (LDI) mass spectrometry-based liquid biopsy for pan-cancer screening and classification. The Multiplexed Nanomaterial-Assisted LDI for Cancer Identification (MNALCI) is applied in 1,183 individuals that include 233 healthy controls and 950 patients with liver, lung, pancreatic, colorectal, gastric, thyroid cancers from two independent cohorts. MNALCI demonstrates 93% sensitivity at 91% specificity for distinguishing cancers from healthy controls in the internal validation cohort, and 84% sensitivity at 84% specificity in the external validation cohort, with up to eight metabolite biomarkers identified. In addition, across those six different cancers, the overall accuracy for identifying the tumor tissue of origin is 92% in the internal validation cohort and 85% in the external validation cohort. The excellent accuracy and minimum sample consumption make the high throughput assay a promising solution for non-invasive cancer diagnosis.

As cancer is increasingly considered a metabolic disorder, it is postulated that serum metabolite profiling can be a viable approach for detecting the presence of cancer. Here, the authors report a machine learning model using mass spectrometry-based liquid biopsy data for pan-cancer screening and classification.

Quantitative analysis of vitamin D using m/MALDI-TOF mass spectrometry based on a parylene matrix chip

Vitamin D deficiency is associated with various disorders and is diagnosed based on the concentration of 25-hydroxy vitamin D3 (25(OH)D3) in serum. The parylene matrix chip was fabricated to reduce the matrix background noise, and the homogenous distribution of the matrix was retained for the quantitative analysis of 25(OH)D3. The Amplex Red assay was performed to confirm that the sample-matrix mixing zone of the parylene matrix chip was formed below the surface of the parylene-N film. The homogeneous distribution of the matrix was verified from the fluorescence image. For effective analysis using a parylene matrix chip, 25(OH)D3 was modified through the nucleophilic addition of betaine aldehyde (BA) to form a hemiacetal salt. Such modified 25(OH)D3 with a positive charge from BA could be effectively analyzed using MALDI-TOF mass spectrometry. Serum 25(OH)D3 was extracted by liquid–liquid extraction (LLE) and quantified using MALDI-TOF mass spectrometry based on the parylene matrix chip. The intensity of the mass peak of 25(OH)D3 was linearly correlated (r2 = 0.992) with the concentration of 25(OH)D3 spiked in serum, and the LOD was 0.0056 pmol/μL. Energy drinks and vitamin D3 tablets were also employed for the real sample analysis. Finally, the results of the chemiluminescence binding assay and MALDI-TOF mass spectrometry were statistically analyzed to determine the applicability of the method using the Bland–Altman test and Passing–Bablok regression.

A new update of MALDI-TOF mass spectrometry in lipid research

Matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS) is an indispensable tool in modern lipid research since it is fast, sensitive, tolerates sample impurities and provides spectra without major analyte fragmentation. We will discuss some methodological aspects, the related ion-forming processes and the MALDI MS characteristics of the different lipid classes (with the focus on glycerophospholipids) and the progress, which was achieved during the last ten years. Particular attention will be given to quantitative aspects of MALDI MS since this is widely considered as the most serious drawback of the method. Although the detailed role of the matrix is not yet completely understood, it will be explicitly shown that the careful choice of the matrix is crucial (besides the careful evaluation of the positive and negative ion mass spectra) in order to be able to detect all lipid classes of interest. Two developments will be highlighted: spatially resolved Imaging MS is nowadays well established and the distribution of lipids in tissues merits increasing interest because lipids are readily detectable and represent ubiquitous compounds. It will also be shown that a combination of MALDI MS with thin-layer chromatography (TLC) enables a fast spatially resolved screening of an entire TLC plate which makes the method competitive with LC/MS.

Mass Spectrometry Imaging of Metabolites by Nanostructure Initiator Mass Spectrometry with Fluorinated Gold Nanoparticles

Nanostructure initiator mass spectrometry (NIMS) with fluorinated gold nanoparticles (f-AuNPs) enables the detection and spatial localization of a breath of polar metabolites and lipids with high spatial resolution and ultrasensitivity. Here we describe the methods and procedures for the synthesis and application of f-AuNPs for NIMS of small molecule metabolites and lipids in biological tissues, encompassing sample preparation, mass spectrometric detection, and data analysis and interpretation.

Mass Spectrometry Imaging of Biological Tissues by Laser Desorption Ionization from Silicon Nanopost Arrays

Mass spectrometry imaging (MSI) plays an expanding role in the label-free spatial mapping of hundreds of molecules simultaneously. Currently, matrix-assisted laser desorption ionization (MALDI) is among the most widely adopted MSI techniques. However, matrix application can impact the fidelity of spatial distributions, and matrix selection and related spectral interferences in the low mass range can lead to biased molecular coverage. Nanophotonic ionization from silicon nanopost arrays (NAPA) is an emerging matrix-free MSI platform with enhanced sensitivity for several molecular classes, for example, neutral lipids and biooligomers. Here, we describe a protocol with minimal sample preparation for NAPA-MSI of metabolites, lipids, and biooligomers from biological tissues.

Inorganic Matrices Assisted Laser Desorption/Ionization Mass Spectrometry for Metabolic Analysis in Bio-fluids

Metabolic analysis in bio-fluids interprets the end products in the bio-process, emerging as an irreplaceable disease diagnosis and monitoring platform. Laser desorption/ionization mass spectrometry (LDI MS) based metabolic analysis exhibits great potential for clinical applications in terms of high throughput, rapid signal readout, and minimal sample preparation. There are two essential elements to construct the LDI MS-based metabolic analysis: 1) well-designed nanomaterials as matrices; 2) machine learning algorithms for data analysis. This review highlights the development of various inorganic matrices to comprehend the advantages of LDI MS in metabolite detection and the recent diagnostic applications based on target metabolite detection and untargeted metabolic fingerprints in biological fluids.

Yolk-shell Ni/NiO anchored on N-doped graphene synthesized as dual-ion MALDI matrix for detecting and imaging bioactive small molecules

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is an attractive tool to analyze the bioactive small molecules but remains a great challenge owing to the serious background interference from conventional matrix with m/z < 1000. Herein, we reported a dual-ion MALDI matrix of yolk-shell Ni/NiO nanoparticles anchored on nitrogen-doped graphene (Ni/NiO/N-Gr) to enhance MALDI performance. The Ni/NiO/N-Gr was synthesized via the pyrolysis and controllable oxidation strategy based on the nanoscale regulation of Kirkendall effect. The novel matrix showed the superior behavior for the analysis of various small molecular metabolites (amino acids, saccharides, spermidine, creatinine, hippuric acid, dopamine, and ascorbic acid) with high sensitivity, excellent salt tolerance, and favorable reproducibility in dual-ion modes compared to the traditional α-cyano-4-hydroxycinnamic acid (CHCA) and control substances (Ni/N-Gr and NiO/N-Gr). Meanwhile, we have realized accurate quantitation of blood glucose in mice with a linearity concentration range of 0.2-7.5 mM and qualitative detection of various endogenous small molecular metabolites in mice serum and urine samples. Especially, the Ni/NiO/N-Gr assisted LDI MS imaging (MSI) has exhibited the excellent spatial distribution of lipids in hippocampus region of mice brain. These results may provide an approach to explore the MALDI MS and MSI applications in clinical diagnosis.

Lignin as a MALDI matrix for small molecules: a proof of concept

Driven by the interest in metabolomic studies and the progress of imaging techniques, small molecule analysis is booming, while it remains challenging to be realized by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Herein, lignin, the second most abundant biomass in nature, was applied as a dual-ion-mode MALDI matrix for the first time to analyze small molecules. The low ionization efficiency and strong optical absorption properties make lignin a potential MALDI matrix in small molecule analysis. A total of 30 different small molecules were identified qualitatively and six kinds of representative molecules were detected quantitatively with a good linear response (R² > 0.995). To verify the accuracy of our quantitative method in MALDI, myricitrin, a major bioactive component in Chinese bayberry, was analyzed in different cultivars and tissues. The myricitrin content in real samples detected by MALDI was highly consistent (R² > 0.999) with that detected by high-performance liquid chromatography, thus indicating the applicability of the lignin matrix. Further characterization by ultraviolet and nuclear magnetic resonance spectroscopy was carried out to explain the possible mechanism of lignin as a matrix and provide more theories for a rational matrix design.

Photothermal Structural Dynamics of Au Nanofurnace for In Situ Enhancement in Desorption and Ionization

Fundamental properties of nanostructured substrates govern the performance of laser desorption/ionization mass spectrometry (LDI-MS); however, limited studies have elucidated the desorption/ionization mechanism based on the physicochemical properties of substrates. Herein, the enhancement in desorption/ionization is investigated using a hybrid matrix of Au nanoisland-functionalized ZnO nanotubes (AuNI-ZNTs). The underlying origin is explored in terms of the photo-electronic and -thermal properties of the matrix. This is the first study to report the effect of laser-induced surface restructuring/melting phenomenon on the LDI-MS performance. AuNI plays a central role as a photothermal nanofurnace, which facilitates the internal energy transfer from the AuNI to the adsorbed analytes by reconstruction in the structurally dynamic AuNI and therefore favors the desorption process. Moreover, piezoelectricity is driven in situ in the AuNI-ZNT hybrid, which modulates the overall band structure and thereby promotes the ionization process. Ultimately, high LDI-MS performance is demonstrated by analyzing small metabolites of fatty acids and monosaccharides, which are challenged to be detected in conventional LDI-MS. This study emphasizing the understanding of matrix properties can provide insights into the design and development of a novel nanomaterial as an efficient LDI matrix. Furthermore, the developed hybrid matrix can overcome the major hurdles existing in conventional LDI-MS.

Lipid response of hepatocellular carcinoma cells to anticancer drug detected on nanostructure-assisted LDI-MS platform

High heterogeneity of hepatocellular carcinoma (HCC) tumor has become an obstacle to select effective therapy for the treatment of HCC patients. Methods that can guide the decision on therapy choice for HCC treatment are highly demanded. Evaluating the drug response of heterogeneous tumor cells at the molecular level can help to reveal the toxicity mechanism of anticancer drugs and provide more information than current cell-based chemosensitivity assays. In the present work, nanostructure-assisted laser desorption/ionization mass spectrometry (NALDI-MS) was used to investigate the lipid response of HCC cells to anticancer drugs. Three types of HCC cells (LM3, Hep G2, Huh7) were treated with sorafenib, doxorubicin hydro-chloride, and cisplatin. We found that the lipid profiles of HCC cells changed a lot after the drug treatment, and the degree of lipid changes was related to the cell viability. Two pairs of fatty acids C16:1/C16:0 and C18:1/C18:0 were found to be strongly related to the viability of HCC cells after drug treatment, and were more sensitive than Methyl-thiazolyl tetrazolium (MTT) assay. Accordingly, they can act as sensitive and comprehensive indexes to evaluate the drug susceptibility of HCC cells. In addition, the peak ratio of several neighboring phospholipids displayed high correlation with drug response of specific cell subtype to specific drug. The ratio of neighboring lipids may be traced back to the activity of enzyme and gene expression which regulate the lipidomic pathway. This method provides drug response of heterogenous tumor cells at molecular level and could be a potential candidate to precise tumor chemosensitivity assay.

Quantitative analysis of galactose using LDI-TOF MS based on a TiO2 nanowire chip

A novel method for quantifying galactose was developed to serve as a newborn screening test for galactosemia using laser desorption/ionization time-of-flight (LDI-TOF) mass spectrometry (MS) with a TiO2 nanowire chip. Herein, phosphate citrate buffer, serum, and dried blood spot (DBS) were employed for the quantitative analysis of galactose. To quantitatively analyze galactose, its reduction potential was used to oxidize o-phenylene diamine (OPD) into 2,3-diaminophenazine (DA), which were both detected using LDI-TOF MS with a TiO2 nanowire chip according to the concentration of galactose. The reproducibility and the interference of glucose were determined to demonstrate the applicability of this method. Moreover, mixtures of galactose, phenylalanine, and 17 α-OHP were analyzed to determine the interference induced by other biomarkers of metabolic disorders. The OPD oxidation of galactose was found to be selectively achieved under high-glucose conditions, similar to human blood, thereby showing good reproducibility. The intensities of the mass peaks of OPD and DA based on LDI-TOF MS with a TiO2 nanowire chip were linearly correlated in the galactose concentration range of 57.2–220.0 μg/mL (r2 = 0.999 and 0.950, respectively) for serum samples and 52.5–220.0 μg/mL (r2 = 0.993 and 0.985, respectively) for DBS after methanol precipitation/extraction. The enzyme immunoassay and LDI-TOF MS analysis results were statistically analyzed, and a mixture of phenylalanine, 17 α-OHP, and galactose was simultaneously investigated quantitatively at the cutoff level.

Mass Spectrometric Biosensing: A Powerful Approach for Multiplexed Analysis of Clinical Biomolecules

Rapid and sensitive detection of clinical biomolecules in a multiplexed fashion is of great importance for accurate diagnosis of diseases. Mass spectrometric (MS) approaches are exceptionally suitable for clinical analysis due to its high throughput, high sensitivity, and reliable qualitative and quantitative capabilities. To break through the bottleneck of MS technique for detecting high-molecular-weight substances with low ionization efficiency, the concept of mass spectrometric biosensing has been put forward by adopting mass spectrometric chips to recognize the targets and mass spectrometry to detect the signals switched by the recognition. In this review, the principle of mass spectrometric sensing, the construction of different mass tags used for biosensing, and the typical combination mode of mass spectrometric imaging (MSI) technique are summarized. Future perspectives including the design of portable matching platforms, exploitation of novel mass tags, development of effective signal amplification strategies, and standardization of MSI methodologies are proposed to promote the advancements and practical applications of mass spectrometric biosensing.

Nanostructured Substrates as Matrices for Surface Assisted Laser Desorption/Ionization Mass Spectrometry: A Progress Report from Material Research to Biomedical Applications

Within the past two decades, the escalation of research output in nanotechnology fields has boosted the development of novel nanoparticles and nanostructured substrates for use as matrices in surface assisted laser desorption/ionization mass spectrometry (SALDI-MS). The application of nanomaterials as matrices, rather than organic matrices, offers remarkable characteristics that allow the analysis of small molecules with fewer matrix interfering peaks, and share higher detection sensitivity, specificity, and reproducibility. The technological advancement of SALDI-MS has in turn, propelled the application of the analytical technique in the field of biomedical analysis. In this review, the properties and fabrication methods of nanostructured substrates in SALDI-MS such as metallic-, carbon-, and silicon-based nanostructures, quantum dots, metal-organic frameworks, and covalent-organic frameworks are described. Additionally, the latest progress (most within 5 years) of biomedical applications in small molecule, large biomolecule, and MS imaging analysis including metabolite profiling, drug monitoring, bacteria identification, disease diagnosis, and therapeutic evaluation are demonstrated. Key parameters that govern nanomaterial's SALDI efficiency in biomolecule analysis are also discussed. Finally, perspectives of the future development are given to provide a better advancement and promote practical application in clinical MS.

Reactive Desorption Electrospray Ionization Mass Spectrometry To Determine Intrinsic Degradability of Poly(lactic-co-glycolic acid) Chains

Because of its speed, sensitivity, and ability to scrutinize individual species, mass spectrometry (MS) has become an essential tool in analytical strategies aimed at studying the degradation behavior of polyesters. MS analyses can be performed prior to the degradation event for structural characterization of initial substrates or after it has occurred to measure the decreasing size of products as a function of time. Here, we show that MS can also be usefully employed during the degradation process by online monitoring the chain solvolysis induced by reactive desorption electrospray ionization (DESI). Cleavage of ester bonds in random copolymers of lactic acid (LA) and glycolic acid (GA) was achieved by electrospraying methanol-containing NaOH onto the substrates. Experimental conditions were optimized to generate methanolysis products of high abundance so that mass spectra can be conveniently processed using Kendrick-based approaches. The same reactive-DESI performance was demonstrated for two sample preparations, solvent casting for soluble samples or pressed pellets for highly crystalline substrates, permitting to compare polymers with LA/GA ratios ranging from 100/0 to 5/95. Analysis of sample fractions collected by size exclusion chromatography showed that methanolysis occurs independently of the original chain size, so data recorded for poly(LA-co-GA) (PLAGA) copolymers with the average molecular weight ranging from 10 to 180 kDa could be safely compared. The average mass of methanolysis products was observed to decrease linearly (R² = 0.9900) as the GA content increases in PLAGA substrates, consistent with the susceptibility of ester bonds toward solvolysis being higher in GA than in LA. Because DESI only explores the surface of solids, these data do not reflect bulk degradability of the copolymers but, instead, their relative degradability at the molecular level. Based on a "reactive-DESI degradability scale" such as that established here for PLAGA, the proposed method offers interesting perspectives to qualify intrinsic degradability of different polyesters and evaluate their erosion susceptibility or to determine the degradability of those polymers known to degrade via erosion only.

Using surfactants as matrix for the matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS) of amino acids: Sodium dodecyl sulfate (SDS) and sodium octyl sulfate (SOS)

In the present study, the applicability of two surfactants including sodium dodecyl sulfate (SDS) and sodium octyl sulfate (SOS) as the matrix for the Matrix-Assisted Laser Desorption/Ionization (MALDI) of several amino acids (phenylalanine (Phe), valine (Val), proline (Pro), alanine (Ala), and tyrosine (Tyr)) is investigated. Also, the effect of the material of the repeller plate of the ionization part of the used time of flight (TOF) mass spectrometer on the spectral patterns of the amino acids is studied. Furthermore, the recorded MALDI spectra of amino acids are compared with their corresponding direct laser desorption/ionization (direct-LDI) TOF mass spectra. It is observed that the SDS is an appropriate matrix for the Na⁺ transfer to the Phe and Val amino acids, especially, when the Ag metal is selected as the material of the repeller plate. In this case, the peaks of the [M + Na]⁺ and [M-H + 2Na]⁺ species are considerably more intense compared to when the NaF salt is used as a Na⁺ source in the LDI of these amino acids. Unlike Phe and Val, the SDS is not a good matrix for the other selected amino acids. The decrease of the carbonic chain length of the surfactant on the MALDI spectrum of Phe is investigated and it is seen that the mentioned important peaks disappeared in the presence of SOS as the matrix. The density functional theory (DFT) calculation is employed to characterize the structure of [M + Na]⁺ and [M-H + 2Na]⁺ species and determine the interaction sites of amino acids for the Na⁺ attachment. Also, the change in standard Gibbs free energy (∆G^°) of the M + Na⁺ → [M + Na]⁺ and [M + Na]⁺ + Na⁺ → [M-H + 2Na]⁺ + H⁺ reactions are calculated. Based on the values of ∆G^°, the attachment of the first Na⁺ to the amino acid takes place in the gas phase while the attachment of the second one to [M + Na]⁺ is not a favorable process in the gas phase.

Engineering of nanomaterials for mass spectrometry analysis of biomolecules

Mass spectrometry (MS) based analysis has received intense attention in diverse biological fields. However, direct MS interrogation of target biomolecules in complex biological samples is still challenging, due to the extremely low abundance and poor ionization potency of target biological species. Innovations in nanomaterials create new auxiliary tools for deep and comprehensive MS characterization of biomolecules. More recently, growing research interest has been directed to the compositional and structural engineering of nanomaterials for enriching target biomolecules prior to MS analysis, enhancing the ionization efficiency in MS detection and designing biosensing nanoprobes in sensitive MS readout. In this review, we mainly focus on the recent advances in the engineering of nanomaterials towards their applications in sample pre-treatment, desorption/ionization matrices and ion signal amplification for MS profiling of biomolecules. This review will provide a toolbox of nanomaterials for researchers devoted to developing analytical methods and practical applications in the biological MS field.

A general purpose MALDI matrix for the analyses of small organic, peptide and protein molecules

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) has been widely applied for the analysis of large biomolecules. The emergence of inorganic material substrates and new organic matrices extends the use of MALDI MS for small molecule analyses. However, there are usually preferred matrices for different types of analytes. Here, an organic compound, 4-hydroxy-3-nitrobenzonitrile, was found to be a general purpose matrix for the analyses of small organic, peptide and protein molecules. In particular, 4-hydroxy-3-nitrobenzonitrile has a strong UV absorption property, and it provides a clean background in the low mass range. Its analytical performances as a UV-laser matrix were demonstrated for different types of analytes, including organic drugs, peptides, proteins, mouse brain tissue and bacteria. Compared with commercial matrices, this new matrix has better performances when analyzing small molecules, such as drugs, peptides and lipids, while it has similar performances when analyzing proteins.

TiN nanopillar-enhanced laser desorption and ionization of various analytes

The present paper reports on the use of TiN nanopillars as a robust analytical substrate for laser desorption/ionization mass spectrometry (LDI-MS). TiN nanopillars were fabricated on silicon wafers through the dynamic oblique deposition of titanium, followed by thermal treatment in an ammonia atmosphere. The TiN nanopillars were readily applicable to a simple "dried-droplet" method in the LDI-MS without surface modification or pre-treatment. A broad range of analytes were investigated, including a small drug molecule, a synthetic polymer, sugars, peptides, and proteins. Intact molecular signals were detected with low noise interference and no fragmentation. The developed TiN-nanopillar-based approach extends the applicable mass limit to 150 kDa (immunoglobulin G) and was able to detect trypsinogen (24 kDa) at levels as low as 50 fmol μL-1 with adequate shot-to-shot signal reproducibility. In addition, we carried out MS analysis on biomolecule-spiked human blood plasma and a mixture of standard samples to investigate the promise of the TiN nanopillars for clinical research. The experimental observations are validated using electromagnetic and heat-transfer simulations. The TiN nanopillars show a reduced reflection and exhibit surges in the TiN surface temperature upon irradiation with electromagnetic radiation. Localization of thermal energy at the tips of the TiN pillars is likely to be responsible for the superior LDI performance. Our results suggest that the development of nanostructured TiN substrates will contribute to the widespread implementation of nanostructured solid substrates for biomedical and clinical applications with simple processes.

Hybrid Surface Design of Organosilica Films for Laser Desorption/Ionization Mass Spectrometry: Low Free Energy Surface with Interactive Sites

Laser desorption/ionization mass spectrometry (LDI-MS) assisted by solid substrates is useful for the facile and rapid analysis of low-molecular-weight compounds. The LDI performance of solid substrates depends on not only a surface morphology but also the surface functionalities dominating the surface-analyte interactions. In this study, we propose a hybrid surface design for LDI substrates, realizing both weak surface-analyte interaction and homogeneous distribution of analytes. The hybrid surface consisted of a mixture of fluoroalkylsilane (FAS), SiO₂, and TiO₂ and was formed on organosilica substrates containing UV-laser-absorbing naphthalimide moieties. To investigate the surface interactions, the hybrid surface as well as conventional hydrophobic surfaces treated with FAS only were prepared on flat organosilica films. Contact angle measurements and surface free energy analysis showed that the hybrid surface exhibited the highest hydrophobicity, while the contribution of the polar and hydrogen bonding terms in the surface free energy was clearly observed. The organosilica film with the hybrid surface demonstrated significant LDI performance for the detection of biorelated compounds (e.g., peptides, phospholipids, and medicines), and a high detection ability was particularly observed for peptides. The substrate surface promoted the desorption/ionization of analytes through a low surface free energy and uniform distribution of the analytes due to the interactive sites. The hybrid surface design was then applied to a nanostructured organosilica substrate consisting of a base film and a nanoparticle layer. The signal intensity of a peptide was further improved approximately 3-fold owing to the increased surface area of the nanostructured substrate, and the limit of detection reached the subfemtomole order. Our hybrid surface design is expected to improve the LDI performance of various nanostructured solid substrates.