Carbon Nanodots As a Matrix for the Analysis of Low-Molecular-Weight Molecules in Both Positive- and Negative-Ion Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry and Quantification of Glucose and Uric Acid in Real Samples

Characterization of glycan isomers using magnetic carbon nanoparticles as a MALDI co-matrix

Matrix-assisted laser desorption ionization-in source decay (MALDI-ISD) analysis is a useful technique in the structural analysis of glycans. Our recent publication demonstrated that magnetic carbon nanoparticles (MCNPs), used as a MALDI co-matrix, significantly enhanced ISD efficiency for glycomic analysis by MALDI-TOF. In this study, MCNPs were used for the structural study of isomeric glycans. Results from the standard glycans confirmed easy distinction of positional and linkage isomers without the need for further derivatization of glycan molecules. Extensive glycosidic and cross-ring fragmented ions provided different fragment patterns for various glycan isomers. Core- and branch-fucosylated isomers were distinguished by several unique ions, and pseudo-MS3 data were used to recognize the fucosylated branch. Although no diagnostic fragment ion was observed for 2,3- and 2,6-linked sialic acid isomers, their MALDI-ISD patterns were found to be significantly different (P < 0.05). Furthermore, the method introduced in this study could not only be used for the identification of glycan isomers but has also proved effective for the isomeric structural confirmation of gangliosides. GD1a and GD1b gangliosides were easily distinguished by the diagnostic ion originated from GD1a, produced by Z4αZ2β cleavages. Moreover, liquid chromatography coupled with MALDI-TOF was applied to analyze N-glycan isomers derived from a pooled human blood serum sample, providing an alternative method of isomeric glycomic analysis of biological specimens.

Synergistic Effect of the Heterostructure of Au Nanoislands on TiO2 Nanowires for Efficient Ionization in Laser Desorption/Ionization Mass Spectrometry

A combination nanostructured matrix with metal Au nanoislands and semiconductor TiO₂ nanowires is presented to enhance both desorption and ionization efficiency in laser desorption/ionization (LDI) mass spectrometry. The heterostructure of Au nanoislands on TiO₂ nanowires was fabricated via (1) TiO₂ nanowire synthesis through a modified wet-corrosion method and (2) Au nanoisland formation through thermal annealing of a sputtered Au layer on the TiO₂ nanowires. Herein, the synergistic effect of this heterostructure for highly efficient ion production was experimentally elucidated in terms of the formation of high temperature on the surface of Au and the creation of a Schottky barrier at the Au-TiO₂ interface. Finally, four types of immunosuppressors were analyzed to demonstrate the improved ionization performance of the heterostructure for LDI mass spectrometry.

Selection of Internal Standards for Quantitative Matrix-Assisted Laser Desorption/Ionization Mass Spectrometric Analysis Based on Correlation Coefficients

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) has shown its great success in the qualitative analysis of a wide range of organic and biological molecules. However, its application in quantitative analysis is still limited by the difficulty in the availability of isotope-labeled internal standards. The present work investigates the relationship between the correlation coefficient of the peak intensities of analyte and candidate internal standard ions and the linearity of possible quantitative analysis. Based on the two analyte examples, ciprofloxacin and substance P, the results show that the performance of the selected nonisotope-labeled internal standard is greatly related to the correlation coefficient. A high positive correlation coefficient (>0.7) between the ions of analyte and candidate standard can result in a good linearity (R ² > 0.98) and vice versa. The results provide a new way to select nonisotope-labeled internal standards for MALDI analysis and thus can be potentially applied in the rapid quantitative mass spectrometry.

Fast and facile preparation of nanostructured silicon surfaces for laser desorption/ionization mass spectrometry of small compounds

RATIONALE

Many important biological processes rely on specific biomarkers (such as metabolites, drugs, proteins or peptides, carbohydrates, lipids, ...) that need to be monitored in various fluids (blood, plasma, urine, cell cultures, tissue homogenates, …). Although mass spectrometry (MS) hyphenated to liquid chromatography (LC) is widely accepted as a 'gold-standard' method for identifying such synthetic chemicals or biological products, their robust fast sensitive detection from complex matrices still constitutes a highly challenging matter.

METHODS

In order to circumvent the constraints intrinsic to LC/MS technology in terms of prior sample treatment, analysis time and overall method development to optimize ionization efficiency affecting the detection threshold, we investigated laser desorption/ionization mass spectrometry (LDI-MS) by directly depositing the sample under study onto cheap inert nanostructures made of silicon to perform straightforward sensitive and rapid screening of targeted low mass biomarkers on a conventional MALDI platform.

RESULTS

The investigated silicon nanostructures were found to act as very efficient ion-promoting surfaces exhibiting high performance for the detection of different classes of organic compounds, including glutathione, glucose, peptides and antibiotics. Achieving such broad detection was compulsory to develop a SALDI-MS-based pre-screening tool.

CONCLUSIONS

The key contribution of the described analytical strategy consists of designing inert surfaces that are fast (minute preparation) and cheap to produce, easy to handle and able to detect small organic compounds in matrix-free LDI-MS prerequisite for biomarkers pre-screening from body fluids without the recourse of any separation step.

Boric-Acid-Functionalized Covalent Organic Framework for Specific Enrichment and Direct Detection of cis-Diol-Containing Compounds by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry

Design and synthesis of a novel matrix that serves as highly selective adsorption material are significant for the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) analysis of small molecules in complicated biosamples. In this work, we presented a facile one-pot strategy for the synthesis of boric-acid-functionalized covalent organic frameworks (B-COFs) by using 2,4,6-trihydroxy-1,3,5-benzenetrialdehyde, benzidine, and 4-aminophenyl-boronic acid as ligands. Compared with bare COFs, the B-COFs have similar crystallinity, specific surface, and well-developed pore structure. The surface area and average pore size of B-COFs were 238.0 m²/g and 1.2 nm, respectively. The resulting material was used as an adsorbent for selective enrichment of cis-diol-containing compounds based on an affinity reaction between phenylboronic acid and cis-diol. Using luteolin, riboflavin, and pyrocatechol as model analytes, the enrichment ability of B-COFs as a matrix was examined by MALDI-TOF MS assay, and its high selectivity against target analytes was obtained in the presence of 100 times more anti-nonspecific compounds than that even in the complicated biosample. The limits of detection for luteolin, riboflavin, and pyrocatechol were as low as fg/mL with B-COF enrichment. The B-COFs were further employed and validated for specific enrichment and direct detection of target analytes with complex samples such as human serum, milk, and Capsicum samples. Large surface area, numerous boric-acid active sites, and super stability make B-COFs with high enrichment capacity, high selectivity and sensitivity, satisfying reproducibility, and excellent applicability in MALDI-TOF MS assays.

A robust and homogeneous porous poly(3,4-ethylenedioxythiophene)/graphene thin film for high-efficiency laser desorption/ionization analysis of estrogens in biological samples

Herein, a robust and homogeneous porous poly(3,4-ethylenedioxythiophene)/graphene (PEDOT/graphene) thin film surface-assisted laser desorption/ionization (SALDI) functional platform was prepared through a rapid and facile in-situ photopolymerization method. The graphene-embedded PEDOT skeleton well circumvented the aggregation-related problems in the traditional carbon-based SALDI method which combined with time-of-flight mass spectrometer (TOF MS). As a result, the reproducibility and quantitative capacity of the SALDI platform were significantly improved. Furthermore, the highly efficient adsorption performance of the PEDOT/graphene thin film was demonstrated in terms of in vitro and in vivo solid-phase microextraction (SPME) extraction. It showed that porous morphology with abundant graphene doping favored the adsorption and enrichment of target analytes. Owing to the excellent adsorption capability of the PEDOT/graphene thin film and the inherent strong laser absorption ability of graphene, expected SALDI effect (3-13 times higher than the commercial nanomaterial-assisted LDI plate) and quantitative analysis (linear range 0.5-100 μg L-1) of the PEDOT/graphene functional surfaces were achieved. As for the real-world applications, we deployed the PEDOT/graphene thin film SALDI platform for the analysis of five estrogens in biological samples at microliter-volume level, without tedious sample preparation procedures. Satisfactory recoveries ranging from 60.6% to 99.0% were obtained. The present study suggested that the graphene-embedded PEDOT skeleton with porous morphology would be developed as promising coating for the adsorption of analytes of interest. Additionally, the combination of PEDOT with graphene not only expanded the application fields of PEDOT, but also offered an efficient strategy for preparing homogeneous functional surfaces to realize the quantitative analysis in SALDI method.

Direct identification of forensic body fluids by MALDI-MS

The rapid identification of human body fluids is meaningful for forensic casework.

Monodispersed Ag Nanoparticle in Layered Double Hydroxides as Matrix for Laser Desorption/Ionization Mass Spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) in the low-molecular-weight (LMW) range is a long-standing challenge because of the fragments from the matrix and the heterogeneity of the matrix-analyte crystals. In this work, a homogeneous film with the monodispersed Ag nanoparticles (Ag NPs) in the confined interlayer of layered double hydroxides (LDHs) has been achieved. The Ag NPs with advantageous optical absorption could realize the energy capture and transfer process, and LDHs with abundant hydroxyl groups are beneficial for the deprotonated reaction. Therefore, the as-prepared film exhibited interference-free deprotonated signals in negative-ion mode with high ionization efficiency. The uniform matrix-analyte spots were constructed through the homogeneous assembly process, contributing to the high reproducibility for both the liquid and gaseous samples. Good linearities were successfully realized in the range from 0.1 μM to 1.0 mM for glucose with the relative standard deviation (RSD) of 3.8%, and 0.2-2.0 mM with the average RSD of 4.5% for psoralen samples, respectively. It is believed that the proposed matrix could exhibit competitive advantages for MALDI detection in the LMW region, which may provide new insight into development for MALDI mass detection.

Size-selected silver nanoparticles for MALDI-TOF mass spectrometry of amyloid-beta peptides

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is one of the most efficient mass spectrometric techniques for the analysis of high-molecular-weight compounds with superior selectivity and sensitivity. Common MALDI matrices are low molecular weight (LMW) organics and will therefore produce a large amount of matrix-related ion peaks, which limits the use of MALDI-MS for the detection of LMW molecules. A major breakthrough of this limitation was made by the introduction of surface assisted desorption/ionization techniques, with graphite particles firstly as the matrix, followed by expansion into other types of nanoparticles or nanostructures. However, previous studies failed to address well the optimum size and concentration of Ag NPs to be used as the MALDI matrix. In this study, to explore and compare the efficiency of different sized silver nanoparticles (Ag NPs) as the MALDI matrix for the detection of LMW molecules, three different sized Ag NPs (2.8 ± 1.0, 12.8 ± 3.2 and 44.2 ± 5.0 nm) have been successfully developed as the MALDI time-of-flight MS (MALDI-TOF MS) matrix and amyloid-beta (Aβ) peptides, crucially involved in Alzheimer's disease and a variety of cancers, were chosen as an example of LMW molecules in our MALDI-TOF MS analysis with Ag NPs as matrices. The results showed size-selected MS signals with the smallest (2.8 ± 1.0 nm) Ag NP matrix producing the highest spectral intensities, when compared with other larger sized Ag NP matrices and conventional matrices such as SA and DHB. Furthermore, the optimal concentrations for different sized Ag NPs as matrices were determined as follows: 0.125 nM (2.8 ± 1.0 nm Ag NPs), 0.0625 nM (12.8 ± 3.2 nm Ag NPs), and 0.03125 nM (44.2 ± 5.0 nm Ag NPs), respectively. These results not only corroborated that Ag NPs could act as a very suitable matrix to assist in the desorption/ionization of LMW molecules but also revealed size-selected mass spectrometry signals with smaller Ag NPs as the MALDI matrix bearing more advantages than their larger counterparts. These novel findings paved the way for wider applications of MALDI-MS using Ag NPs as matrices for the analysis of LMW molecules.

Dual-Ion-Mode MALDI MS Detection of Small Molecules with the O-P,N-Doped Carbon/Graphene Matrix

It is challenging to realize a dual-ion mode of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for detecting small molecules. Herein, graphene coated by porous amorphous carbon with P-O surface group and codoped by phosphorus and nitrogen (O-P,N-C/G) was synthesized from an aerogel formed by phytic acid, polyaniline, and electrochemically exfoliated graphene. The carbon material synthesized has the feature of large surface area (583 m²/g), good electrical conductivity, strong UV absorption, heteroatom doping, and surface functional groups suitable for laser- induced desorption/ionization. It was employed as a novel matrix suitable for both positive-ion and negative-ion modes in MALDI-TOF MS for the analysis of various small molecules including amino acids, small peptides, saccharides, drugs, and environmental pollutants, significantly outperforming control materials and a traditional CHCA (α-cyano-4-hydroxycinnamic acid) or 2,5-dihydroxybenzoic (DHB) matrix. Remarkably, the detection limit of the anticancer drugs (5-fluorouracil and ellagic acid) reaches 50 pmol. In addition, nice MALDI-TOF MS images can be mapped to detect mixed amino acids corresponding to homogeneous distribution of ion intensity. The monosaccharides and disaccharides can be distinguished by using the new matrix. Last but not least, it can be used to quantitatively detect glucose in human serum and soft drinks (glucose/fructose, 203.1 mM) without adding standards.

Hybrid CuCoO-GO enables ultrasensitive detection of antibiotics with enhanced laser desorption/ionization at nano-interfaces

The soaring concerns globally on antibiotic overuse have made calls for the development of rapid and sensitive detection methods urgent. Here we report that the hybrid CuCoO-GO matrix allows for sensitive detection of various antibiotics in combination with MALDI TOF MS. The new matrix is composed of few-layered GO nanosheets decorated with CuCoO nanoparticles with an average size of 10 nm, and exhibits excellent aqueous suspensibility. Accurate quantitation of the sulfonamide antibiotics in milk samples have been demonstrated using a CuCoO-GO matrix and a stable isotope (C¹³)-labeled analyte as the internal standard. Our experiments have achieved lower limits of detection (LOD) by several hundred fold for the detection of a panel of representative antibiotics, in comparison with the literature reports. Both intrabacterial and extrabacterial residual antibiotics can be sensitively detected with our method. We have further investigated the molecular mechanism of the enhanced desorption/ionization efficiency by the CuCoO-GO matrix with synchrotron radiation techniques for the first time. This work provides a sensitive matrix enabling MALDI-TOF MS to be applied in small molecular analysis, but also presents a distinct perspective on the mechanism behind the material functions.

Negative ion laser desorption/ionization time-of-flight mass spectrometric analysis of small molecules by using nanostructured substrate as matrices

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is an excellent analytical technique for rapid and sensitive analysis of macromolecules such as polymers and proteins. However, the main drawback of MALDI-TOF MS is its difficulty to detect small molecules with mass below 700 Da because of the intensive interference from MALDI matrix in the low mass region. In recent years there has been considerable interest in developing matrix-free laser desorption/ionization by using nanostructured substrates to substitute the conventional organic matrices, which is often referred as surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS). Despite these attractive features, most of the current SALDI-TOF MS for the analysis of small molecules employ positive ion mode, which is subjected to produce multiple alkali metal adducts, and thus increases the complexity of the analysis. Different from the complicated adducts produced in positive ion mode, mass spectra obtained in negative ion mode are featured by deprotonated ion peaks without matrix interference, which simplifies the interpretation of mass spectra and detection of unknown. In this review, we critically survey recent advances in nanostructured substrates for negative ion LDI-TOF MS analysis of small molecules in the last 5 years. Special emphasis is placed on the preparation of the nanostructured substrates and the results achieved in negative ion SALDI-MS. In addition, a variety of promising applications including environmental, biological, and clinical analysis are introduced. The ionization mechanism of negative ionization is briefly discussed.

Carbon Nanoparticles and Graphene Nanosheets as MALDI Matrices in Glycomics: a New Approach to Improve Glycan Profiling in Biological Samples

Glycomics continues to be a highly dynamic and interesting research area due to the need to comprehensively understand the biological attributes of glycosylation in many important biological functions such as the immune response, cell development, cell differentiation/adhesion, and host-pathogen interactions. Although matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) has proven to be suitable for glycomic profiling studies, there is a need for improved sensitivity in the detection of native glycans, which ionize inefficiently. In this study, we investigated the efficiencies of graphene nanosheets (GNs) and carbon nanoparticles (CNPs) as MALDI matrices and co-matrices in glycan profiling. Our results indicated an enhancement of signal intensity by several orders of magnitude upon using GNs and CNPs in MALDI analysis of N-glycans derived from a variety of biological samples. Interestingly, increasing the amounts of CNPs and GNs improved not only the signal intensities but also prompted in-source decay (ISD) fragmentations, which produced extensive glycosidic and cross-ring cleavages. Our results indicated that the extent of ISD fragmentation could be modulated by CNP and GN concentrations, to obtain MS² and pseudo-MS³ spectra. The results for glycan profiling in high salt solutions confirmed high salt-tolerance capacities for both CNPs and GNs. Finally, the results showed that by using CNPs and GNs as co-matrices, DHB crystal formation was more homogeneous which improved shot-to-shot reproducibility and sensitivity. Graphical Abstract ᅟ.

Nanoscale silicon surface-assisted laser desorption/ionization mass spectrometry: environment stability and activation by simple vacuum oven desiccation

Nanoscale silicon surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) is an emerging matrix-free, highly sensitive MS analysis method. An important challenge in using nanoscale silicon SALDI-MS analysis is the aging and stability of silicon after storage in various environments. No proper nanoscale silicon SALDI-MS activation procedure has been reported to solve this issue. This study investigated the sensitivity, wettability, and surface oxidation behavior of nanoscale silicon surface SALDI-MS in a room, an inert gas atmosphere, and a vacuum environment. A simple vacuum oven desiccation was proposed to activate the SALDI-MS surface, and the limit of detection was further enhanced 1000 times to a 500 attomole level using this approach. The long-term stability and desorption/ionization mechanism were also investigated.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2013-2014

This review is the eighth update of the original article published in 1999 on the application of Matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2014. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly- saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 37:353-491, 2018.

Disposable MoS2-Arrayed MALDI MS Chip for High-Throughput and Rapid Quantification of Sulfonamides in Multiple Real Samples

In this work, we demonstrate, for the first time, the development of a disposable MoS₂-arrayed matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) chip combined with an immunoaffinity enrichment method for high-throughput, rapid, and simultaneous quantitation of multiple sulfonamides (SAs). The disposable MALDI MS chip was designed and fabricated by MoS₂ array formation on a commercial indium tin oxide (ITO) glass slide. A series of SAs were analyzed, and clear deprotonated signals were obtained in negative-ion mode. Compared with MoS₂-arrayed commercial steel plate, the prepared MALDI MS chip exhibited comparable LDI efficiency, providing a good alternative and disposable substrate for MALDI MS analysis. Furthermore, internal standard (IS) was previously deposited onto the MoS₂ array to simplify the experimental process for MALDI MS quantitation. 96 sample spots could be analyzed within 10 min in one single chip to perform quantitative analysis, recovery studies, and real foodstuff detection. Upon targeted extraction and enrichment by antibody conjugated magnetic beads, five SAs were quantitatively determined by the IS-first method with the linear range of 0.5-10 ng/mL ( R² > 0.990). Good recoveries and repeatability were obtained for spiked pork, egg, and milk samples. SAs in several real foodstuffs were successfully identified and quantified. The developed method may provide a promising tool for the routine analysis of antibiotic residues in real samples.

In situ detecting changes in membrane lipid phenotypes of macrophages cultured in different cancer microenvironments using mass spectrometry

Macrophages, the important cells of immune system, have exhibited distinct gene phenotypes with diverse functions in different microenvironments. In the present study, macrophages RAW264.7 (M0 macrophages) and lipopolysaccharide (LPS) plus interferon gamma (INF-γ)-treated M0 macrophages (M1 macrophages) were cultured in different lung cell-derived culture supernatants (CSs) as imitative tumor microenvironments. The lipids (mainly from cell membrane) of intact macrophages were in situ detected by matrix-assisted laser desorption/ionization-Fourier transform ion cyclotron resonance mass spectrometry. Approximately 300 of small molecules were observed in negative ion mode. Partial least square-discriminant analysis (PLS-DA) suggested that two types of the macrophages have different membrane lipid phenotypes. Changes in the levels of phosphatidylethanolamine PE(16:1/18:0), PE(18:1/18:0), PE(36:2), PE-Cer(d36:1), and PE(P-16:0/18:1) were closely associated with membrane phenotypes of macrophages. The heatmap also revealed that directional induction to classically activated macrophages (M1 macrophages) in vitro had greater impact on the membrane lipid phenotypes of macrophages than different lung cell-derived CSs. The results are consistent with the data obtained by biological technologies.

DBDA as a Novel Matrix for the Analyses of Small Molecules and Quantification of Fatty Acids by Negative Ion MALDI-TOF MS

Matrix interference ions in low mass range has always been a concern when using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) to analyze small molecules (<500 Da). In this work, a novel matrix, N1,N4-dibenzylidenebenzene-1,4-diamine (DBDA) was synthesized for the analyses of small molecules by negative ion MALDI-TOF MS. Notably, only neat ions ([M-H]^-) of fatty acids without matrix interference appeared in the mass spectra and the limit of detection (LOD) reached 0.3 fmol. DBDA also has great performance towards other small molecules such as amino acids, peptides, and nucleotide. Furthermore, with this novel matrix, the free fatty acids in serum were quantitatively analyzed based on the correlation curves with correlation coefficient of 0.99. In addition, UV-Vis experiments and molecular orbital calculations were performed to explore mechanism about DBDA used as matrix in the negative ion mode. The present work shows that the DBDA matrix is a highly sensitive matrix with few interference ions for analysis of small molecules. Meanwhile, DBDA is able to precisely quantify the fatty acids in real biological samples. Graphical Abstract ᅟ.

Nanomaterials and continuous wave laser-based efficient desorption for atmospheric pressure mass spectrometric imaging of live hippocampal tissue slices

Micrometer-resolution mass spectrometric imaging of live hippocampal tissue is achieved with a highly efficient desorption of biomolecules using a 532 nm continuous wave laser and gold nanoparticles or graphene oxide as an energy transporter, which enables clear identification of the distributions of monoacylglycerol, adenine, cholesterol, sphingosine and ceramide.

Micrometer-resolution mass spectrometric imaging of live hippocampal tissue is achieved with a highly efficient desorption of biomolecules using a 532 nm continuous wave laser and gold nanoparticles or graphene oxide as an energy transporter.

N-Phenyl-2-naphthylamine as a Novel MALDI Matrix for Analysis and in Situ Imaging of Small Molecules

Due to its strong ultraviolet absorption, low background interference in the small molecular range, and salt tolerance capacity, N-phenyl-2-naphthylamine (PNA) was developed as a novel matrix in the present study for analysis and imaging of small molecules by matrix-assisted laser desorption/ionization mass spectrometry time-of-fight (MALDI-TOF MS). The newly developed matrix displayed good performance in analysis of a wide range of small-molecule metabolites including free fatty acids, amino acids, peptides, antioxidants, and phospholipids. In addition, PNA-assisted LDI MS imaging of small molecules in brain tissue of rats subjected to middle cerebral artery occlusion (MCAO) revealed unique distributions and changes of 89 small-molecule metabolites including amino acids, antioxidants, free fatty acids, phospholipids, and sphingolipids in brain tissue 24 h postsurgery. Fifty-nine of the altered metabolites were identified, and all the changed metabolites were subject to relative quantitation and statistical analysis. The newly developed matrix has great potential application in the field of biomedical research.

Hydroxyl-Group-Dominated Graphite Dots Reshape Laser Desorption/Ionization Mass Spectrometry for Small Biomolecular Analysis and Imaging

Small molecules play critical roles in life science, yet their facile detection and imaging in physiological or pathological settings remain a challenge. Matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) is a powerful tool for molecular analysis. However, conventional organic matrices (CHCA, DHB, etc.) used in assisting analyte ionization suffer from intensive background noise in the mass region below m/z 700, which hinders MALDI MS applications for small-molecule detection. Here, we report that a hydroxyl-group-dominated graphite dot (GD) matrix overcomes limitations of conventional matrices and allows MALDI MS to be used in fast and high-throughput analysis of small biomolecules. GDs exhibit extremely low background noise and ultrahigh sensitivity (with limit of detection <1 fmol) in MALDI MS. This approach allows identification of complex oligosaccharides, detection of low-molecular-weight components in traditional Chinese herbs, and facile analysis of puerarin and its metabolites in serum without purification. Moreover, we show that the GDs provide an effective matrix for the direct imaging or spatiotemporal mapping of small molecules and their metabolites (m/z < 700) simultaneously at the suborgan tissue level. Density functional theory calculations further provide the mechanistic basis of GDs as an effective MALDI matrix in both the positive-ion and negative-ion modes. Collectively, our work uncovered a useful matrix which reshapes MALDI MS technology for a wide range of applications in biology and medicine.

A layer-by-layer assembled MoS2 thin film as an efficient platform for laser desorption/ionization mass spectrometry analysis of small molecules

A chip-based platform for laser desorption/ionization mass spectrometry (LDI-MS) analysis of small molecules was developed by utilizing layer-by-layer (LBL) assembly of MoS2 nanoflakes and polyallylamine on an arbitrary substrate. The LDI-MS efficiency of small molecules on MoS2 films increased as a function of LBL assembly cycles until reaching a saturation point. The optimized MoS2 nanoflake film exhibits high LDI-MS efficiency, salt tolerance, reusability and uniform ionic signal distribution, and its performance was further enhanced by surface modification with perfluoroalkanes mimicking a clathrate nanostructure.

A redox-modulated fluorescent strategy for the highly sensitive detection of metabolites by using graphene quantum dots

In this paper, a redox-modulated fluorescent strategy based on the transformation of Fe²⁺/Fe³⁺ couple and enzymatic reaction for rapid monitoring glucose and uric acid using graphene quantum dots (GQDs) as fluorescent probe was developed. Hydrogen peroxide (H₂O₂) can be produced by the enzymatic reaction of a series of metabolites, such as glucose and uric acid. In the presence of hydrogen peroxide, Fe²⁺ can be oxidized and converted to Fe³⁺, which have a significant quenching difference in the fluorescence of graphene quantum dots (GQDs). Thus, a sensitive and label-free biosensor for the detection of uric acid and glucose was developed. Under the optimized experimental conditions, the fluorescence intensity was linearly correlated with the concentration of uric acid and glucose in the range of 0.1-45 μmolL^-1 and 0.1-30 μmolL^-1 with a detection limit of 0.026 μmolL^-1and 0.021 μmolL^-1, respectively. The proposed method was applied to the determination of uric acid and glucose in human serum samples with satisfactory results, which had potential application to detect metabolites associated with H₂O₂ release.

Carbon nanowalls: a new versatile graphene based interface for the laser desorption/ionization-mass spectrometry detection of small compounds in real samples

Carbon nanowalls, vertically aligned graphene nanosheets, attract attention owing to their tunable band gap, high conductivity, high mechanical robustness, high optical absorbance and other remarkable properties. In this paper, we report for the first time the use of hydrophobic boron-doped carbon nanowalls (CNWs) for laser desorption/ionization of small compounds and their subsequent detection by mass spectrometry (LDI-MS). The proposed method offers sensitive detection of various small molecules in the absence of an organic matrix. The CNWs were grown by microwave plasma enhanced chemical vapor deposition (MW-PECVD), using a boron-carbon gas flow ratio of 1200 in H₂/CH₄ plasma, on silicon <100> wafer. The hydrophobicity of the surface offers a straightforward MS sample deposition, consisting of drop casting solutions of analytes and drying in air. Limits of detection in the picomolar and femtomolar ranges (25 fmol μL^-1 for neurotensin) were achieved for different types of compounds (fatty acids, lipids, metabolites, saccharides and peptides) having clinical or food industry applications. This rapid and sensitive procedure can also be used for quantitative measurements without internal standards with RSDs <19%, as in the case of glucose in aqueous solutions (LOD = 0.32 ± 0.02 pmol), blood serum or soft drinks. Moreover, melamine (63 ± 8.19 ng μL^-1), a toxic compound, together with creatinine and paracetamol, was detected in urine samples, while lecithin was detected in food supplements.

Imaging mass spectrometry for metabolites: technical progress, multimodal imaging, and biological interactions

Imaging mass spectrometry (IMS) allows the study of the spatial distribution of small molecules in biological samples. IMS is able to identify and quantify chemicals in situ from whole tissue sections to single cells. Both vacuum mass spectrometry (MS) and ambient MS systems have advanced considerably over the last decade; however, some limitations are still hard to surmount. Sample pretreatment, matrix or solvent choices, and instrument improvement are the key factors that determine the successful application of IMS to different samples and analytes. IMS with innovative MS analyzers, powerful MS spectrum databases, and analysis tools can efficiently dereplicate, identify, and quantify natural products. Moreover, multimodal imaging systems and multiple MS-based systems provide additional structural, chemical, and morphological information and are applied as complementary tools to explore new fields. IMS has been applied to reveal interactions between living organisms at molecular level. Recently, IMS has helped solve many previously unidentifiable relations between bacteria, fungi, plants, animals, and insects. Other significant interactions on the chemical level can also be resolved using expanding IMS techniques. WIREs Syst Biol Med 2017, 9:e1387. doi: 10.1002/wsbm.1387 For further resources related to this article, please visit the WIREs website.

Dopamine-modified TiO2 monolith-assisted LDI MS imaging for simultaneous localization of small metabolites and lipids in mouse brain tissue with enhanced detection selectivity and sensitivity

Localization of metabolites using multiplexed mass spectrometry imaging (MSI) provides important chemical information for biological research. In contrast to matrix-assisted laser desorption/ionization (MALDI), TiO2-assisted laser desorption/ionization (LDI) for MSI improves detection of low molecular mass metabolites (<500 Da) by reducing matrix background. However, the low UV absorption of TiO2 nanoparticles and their ester hydrolysis catalytic activity hinder the detection of phospholipids and many low-abundance molecules. To address these challenges, we evaluated and optimized the material morphology and composition of TiO2. Dopamine (DA) was found to be an efficient ligand for TiO2, resulting in increased UV light absorption, higher surface pH, and formation of monolithic TiO2-DA structures. The sub-micron scale and higher surface pH of the TiO2 particle sizes led to improved detection of phospholipid signals. Compared to unmodified TiO2 sub-micron particles, the DA-modified TiO2 monolith led to 10- to 30-fold increases in the signal-to-noise ratios of a number of compound peaks. The TiO2-DA monolith-assisted LDI MSI approach has higher selectivity and sensitivity for Lewis basic compounds, such as fatty acids, cholesterols, ceramides, diacylglycerols, and phosphatidylethanolamine, when analyzed in positive mode, than traditional MALDI MS. Using this new method, over 100 molecules, including amino acids, alkaloids, free fatty acids, peptides, and lipids, were localized in mouse brain sections. By comparing the presence and localization of those molecules in young and old mouse brains, the approach demonstrated good performance in the determination of aging-related neurochemical changes in the brain.

Nanomaterials as Assisted Matrix of Laser Desorption/Ionization Time-of-Flight Mass Spectrometry for the Analysis of Small Molecules

Matrix-assisted laser desorption/ionization (MALDI), a soft ionization method, coupling with time-of-flight mass spectrometry (TOF MS) has become an indispensible tool for analyzing macromolecules, such as peptides, proteins, nucleic acids and polymers. However, the application of MALDI for the analysis of small molecules (<700 Da) has become the great challenge because of the interference from the conventional matrix in low mass region. To overcome this drawback, more attention has been paid to explore interference-free methods in the past decade. The technique of applying nanomaterials as matrix of laser desorption/ionization (LDI), also called nanomaterial-assisted laser desorption/ionization (nanomaterial-assisted LDI), has attracted considerable attention in the analysis of low-molecular weight compounds in TOF MS. This review mainly summarized the applications of different types of nanomaterials including carbon-based, metal-based and metal-organic frameworks as assisted matrices for LDI in the analysis of small biological molecules, environmental pollutants and other low-molecular weight compounds.

Mechanisms of Nanophase-Induced Desorption in LDI-MS. A Short Review

Nanomaterials are frequently used in laser desorption ionization mass spectrometry (LDI-MS) as DI enhancers, providing excellent figures of merit for the analysis of low molecular weight organic molecules. In recent years, literature on this topic has benefited from several studies assessing the fundamental aspects of the ion desorption efficiency and the internal energy transfer, in the case of model analytes. Several different parameters have been investigated, including the intrinsic chemical and physical properties of the nanophase (chemical composition, thermal conductivity, photo-absorption efficiency, specific heat capacity, phase transition point, explosion threshold, etc.), along with morphological parameters such as the nanophase size, shape, and interparticle distance. Other aspects, such as the composition, roughness and defects of the substrate supporting the LDI-active nanophases, the nanophase binding affinity towards the target analyte, the role of water molecules, have been taken into account as well. Readers interested in nanoparticle based LDI-MS sub-techniques (SALDI-, SELDI-, NALDI- MS) will find here a concise overview of the recent findings in the specialized field of fundamental and mechanistic studies, shading light on the desorption ionization phenomena responsible of the outperforming MS data offered by these techniques.

Enhanced Laser Desorption/Ionization Mass Spectrometric Detection of Biomolecules Using Gold Nanoparticles, Matrix, and the Coffee Ring Effect

Nanomaterials have been extensively used as alternate matrices to minimize the low molecular weight interferences observed in typical MALDI but such nanomaterials typically do not improve the spot-to-spot variability that is commonly seen. In this work, we demonstrate that nanoparticles and low matrix concentrations (<2.5 mg/mL) can be used to homogeneously concentrate analytes into a narrow ring by taking advantage of the "coffee ring" effect. Concentration of the samples in this way leads to enhanced signals when compared to conventional MALDI, with higher m/z analytes being enhanced to the greatest extent. Moreover, the ionization suppression often observed in samples with high salt concentrations can be overcome by preparing samples in this way. The ring that is formed is readily visible, allowing the laser to be focused only on spots that contain analyte. The coffee-ring effect represents a new mode by which nanomaterials can be used to enhance the MALDI-based detection of biomolecules.

Hexagonal boron nitride nanosheets as a multifunctional background-free matrix to detect small molecules and complicated samples by MALDI mass spectrometry

h-BN nanosheets were applied as a background-free matrix for the analysis and imaging of small molecules and as an adsorbent to enrich samples.

Water-soluble Au nanoclusters for multiplexed mass spectrometry imaging

Homogeneous Au nanoclusters were utilized for enhancing the detection sensitivity and lateral resolution of multiplexed mass spectrometry imaging due to their high ultraviolet adsorption, high water solubility, and high biocompatibility.

Cu2+1O/graphene nanosheets supported on three dimensional copper foam for sensitive and efficient non-enzymatic detection of glucose

Three dimensional copper foam/Cu₂₊₁O/graphene nanosheets for sensitive and efficient non-enzymatic detection of glucose.

Carbon Dots and 9AA as a Binary Matrix for the Detection of Small Molecules by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

Application of matrix-assisted laser-desorption/ionization mass spectrometry (MALDI MS) to analyze small molecules have some limitations, due to the inhomogeneous analyte/matrix co-crystallization and interference of matrix-related peaks in low m/z region. In this work, carbon dots (CDs) were for the first time applied as a binary matrix with 9-Aminoacridine (9AA) in MALDI MS for small molecules analysis. By 9AA/CDs assisted desorption/ionization (D/I) process, a wide range of small molecules, including nucleosides, amino acids, oligosaccharides, peptides, and anticancer drugs with a higher sensitivity were demonstrated in the positive ion mode. A detection limit down to 5 fmol was achieved for cytidine. 9AA/CDs matrix also exhibited excellent reproducibility compared with 9AA matrix. Moreover, by exploring the ionization mechanism of the matrix, the influence factors might be attributed to the four parts: (1) the strong UV absorption of 9AA/CDs due to their π-conjugated network; (2) the carboxyl groups modified on the CDs surface act as protonation sites for proton transfer in positive ion mode; (3) the thin layer crystal of 9AA/CDs could reach a high surface temperature more easily and lower transfer energy for LDI MS; (4) CDs could serve as a matrix additive to suppress 9AA ionization. Furthermore, this matrix was allowed for the analysis of glucose as well as nucleosides in human urine, and the level of cytidine was quantified with a linear range of 0.05-5 mM (R(2) > 0.99). Therefore, the 9AA/CDs matrix was proven to be an effective MALDI matrix for the analysis of small molecules with improved sensitivity and reproducibility. This work provides an alternative solution for small molecules detection that can be further used in complex samples analysis. Graphical Abstract ᅟ.

Facile Synthesis of N-Doped Carbon Dots as a New Matrix for Detection of Hydroxy-Polycyclic Aromatic Hydrocarbons by Negative-Ion Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry

N-doping carbon dots (N-CDs) were prepared by microwave-assisted pyrolysis of dl-malic acid and ethanolamine as precursors. The material served as an excellent matrix for the detection of the environmental pollutants hydroxy-polycyclic aromatic hydrocarbons (OH-PAHs) by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) in negative ion mode. The obtained N-CDs exhibited good UV absorption capacity and favorable solubility. The use of the N-CDs matrix exhibited low matrix background interference and was beneficial to improve the signal response due to the specific π-conjugated polyaromatic structure and the doping of nitrogen atoms. The developed method was found to have good reproducibility and sensitivity. The N-CDs as a new matrix also were employed for the detection of OH-PAHs in real PM2.5 samples. The mass concentrations of Σ-hydroxy-pyrene, Σ-dihydroxy-anthraquinone, and Σ-dihydroxy-benzo(a)pyrene on the collected PM2.5 samples ranged from 0.125 to 0.136 ng/m(3), 0.039 to 0.052 ng/m(3), and 0.053 to 0.072 ng/m(3), respectively. This work extends the application field of N-CDs and provides a good candidate of matrix for MALDI-TOF MS detection of environmental pollutants.

Imaging of Endogenous Metabolites of Plant Leaves by Mass Spectrometry Based on Laser Activated Electron Tunneling

A new mass spectrometric imaging approach based on laser activated electron tunneling (LAET) was described and applied to analysis of endogenous metabolites of plant leaves. LAET is an electron-directed soft ionization technique. Compressed thin films of semiconductor nanoparticles of bismuth cobalt zinc oxide were placed on the sample plate for proof-of-principle demonstration because they can not only absorb ultraviolet laser but also have high electron mobility. Upon laser irradiation, electrons are excited from valence bands to conduction bands. With appropriate kinetic energies, photoexcited electrons can tunnel away from the barrier and eventually be captured by charge deficient atoms present in neutral molecules. Resultant unpaired electron subsequently initiates specific chemical bond cleavage and generates ions that can be detected in negative ion mode of the mass spectrometer. LAET avoids the co-crystallization process of routinely used organic matrix materials with analyzes in MALDI (matrix assisted-laser desorption ionization) analysis. Thus uneven distribution of crystals with different sizes and shapes as well as background peaks in the low mass range resulting from matrix molecules is eliminated. Advantages of LAET imaging technique include not only improved spatial resolution but also photoelectron capture dissociation which produces predictable fragment ions.