Laser desorption/ionization on the layer of graphene nanoparticles coupled with mass spectrometry for characterization of polymers

Covalent organic framework film-based laser desorption/ionization mass spectrometry for rapid and sensitive quantification of homocysteine in human serum

RATIONALE

Homocysteine (hcy) is a metabolite in the human body and an important determinant of cardiovascular health. To assist in the assessment of human cardiovascular safety, a rapid and accurate quantitative analysis method must be developed. Laser desorption/ionization mass spectrometry (LDI-MS) has been widely used in biological, chemical, and medical analyses due to its excellent characteristics of high throughput, low sample consumption, and high speed. Here, an LDI-MS method based on covalent organic framework (COF) film was developed for the rapid and sensitive determination of hcy in human serum using an isotope-labeled internal standard.

METHODS

We tried to cultivate COFs on indium-tin oxide substrate at room temperature to form a thin film, which was used in LDI-MS. Compared with the traditional organic matrices, the COF film had a clean background and a high signal response for the detection of hcy. In addition, using COF film as the substrate, with a high signal intensity and a clean background, can be helpful to analyze a series of small molecules such as amino acids, bisphenols (Bps), estrogens, and drugs. We evaluated the limit of detection (LOD) and the reproducibility of this method. Finally, the calibration curve was constructed for the quantification of serum hcy using an isotope-labeled internal standard and was applied for the rapid determination of hcy in clinical human serum.

RESULTS

COF film-assisted LDI-MS had a higher response signal and a cleaner background compared to four conventional organic matrices. The LOD of the method for hcy was 0.5 μmol/L, equivalent to 500 fmol. This method exhibited excellent performance for small-molecule compounds, including amino acids, Bps, drugs, and estrogens. The reproducibilities of this method for shot-to-shot and dot-to-dot were determined to be good. This method was applied for the rapid analysis of hcy in clinical human serum, showing good correlation with those obtained by hospital testing.

CONCLUSION

The COF film-based LDI-MS method had simple sample preparation, short analysis time, clean background, and good reproducibility for hcy analysis. As an important indicator of human health, the detection of serum hcy is of great significance to human health.

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.

Synergistic Effect of Metal-Organic Framework/Gallic Acid in Enhanced Laser Desorption/Ionization Mass Spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has become an indispensable tool for high-throughput analysis of macromolecules, but many challenges still remain in detection of small molecules due to the severe matrix-related background interference in the low-molecular-weight ranges (MW < 700 Da). Herein, a gallic acid (GA)-functionalized zirconium 1,4-dicarboxybenzene metal-organic framework (MOF) (denoted as UiO-66-GA) was designed to serve as a new substrate, and a novel strategy on the basis of the synergistic effect of MOF and GA was developed to enhance the LDI process. In comparison with conventional organic matrices, the UiO-66-GA substrate showed superior LDI performance in the analysis of a wide variety of molecules including amino acids, unsaturated fatty acids, bisphenols (BPs), oligosaccharides, peptides, protein, and polyethylene glycol (PEG) of various average molecular weights from 200 to 10000. Perfluorooctanoic sulfonate (PFOS) was used to evaluate the ability of quantitative analysis, and its corresponding limit of detection as low as 1 fmol was achieved. High sensitivity and good salt tolerance of the UiO-66-GA-assisted LDI-MS were allowed to determine ultratrace PFOS in the spiked human urine and serum samples. In addition, the synergistic mechanism of MOF and GA in the enhanced LDI process was investigated by comprehensively comparing GA- and its analogue-functionalized UiO-66, and the results revealed that two aspects contributed to the enhanced LDI process: (1) an enhancement in the metal-phenolic coordination system of UiO-66-GA promoted laser absorption and energy transfer; (2) introduction of carboxyl and hydroxyl groups of GA onto UiO-66 facilitated the LDI process in both positive and negative ion modes. This work expands a new domain for the MOF applications and provides a promising alternative for various molecule analyses.

The improved performance of MALDI-TOF MS on the analysis of herbal saponins by using DHB-GO composite matrix

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is an excellent analytical technique for rapid analysis of a variety of molecules with straightforward sample pretreatment. The performance of MALDI-TOF MS is largely dependent on matrix type, and the development of novel MALDI matrices has aroused wide interest. Herein, we devoted to seek more robust MALDI matrix for herbal saponins than previous reported, and ginsenoside Rb1, Re, and notoginsenoside R1 were used as model saponins. At the beginning of the present study, 2,5-dihydroxybenzoic acid (DHB) was found to provide the highest intensity for saponins in four conventional MALDI matrices, yet the heterogeneous cocrystallization of DHB with analytes made signal acquisition somewhat "hit and miss." Then, graphene oxide (GO) was proposed as an auxiliary matrix to improve the uniformity of DHB crystallization due to its monolayer structure and good dispersion, which could result in much better shot-to-shot and spot-to-spot reproducibility of saponin analysis. The satisfactory precision further demonstrated that minute quantities of GO (0.1 μg/spot) could greatly reduce the risk of instrument contamination caused by GO detachment from the MALDI target plate under vacuum. More importantly, the sensitivity and linearity of the standard curve for saponins were improved markedly by DHB-GO composite matrix. Finally, the application of detecting the Rb1 in complex biological sample was exploited in rat plasma and proved it applicable for pharmacokinetic study quickly. This work not only opens a new field for applications of DHB-GO in herbal saponin analysis but also offers new ideas for the development of composite matrices to improve MALDI MS performance.

Graphene-Coated Glass Substrate for Continuous Wave Laser Desorption and Atmospheric Pressure Mass Spectrometric Imaging of a Live Hippocampal Tissue

The atmospheric pressure mass spectrometric (AP-MS) imaging technology combined with an inverted optical microscopic system is a powerful tool for determining the presence and spatial distributions of specific biomolecules of interest in live tissues. Efficient desorption and ionization are essential to acquire mass spectrometric (MS) information in an ambient environment. In this study, we demonstrate a new and efficient desorption process using a graphene-coated glass substrate and a continuous wave (CW) laser for high-resolution AP-MS imaging of a live hippocampal tissue. We found that desorption of biomolecules in a live tissue slice was possible with the aid of a graphene-coated glass substrate and indirect application of a 532 nm CW laser on the graphene substrate. Interestingly, the desorption efficiency of a live tissue on the graphene-coated substrate was strongly dependent on the number of graphene layers. Single-layer graphene was found to be the most sensitive substrate for efficient desorption and reproducible high-resolution hippocampal tissue imaging applications. The subsequent ionization process using nonthermal plasma generated sufficient amounts of molecular ions to obtain high-resolution two-dimensional MS images of the cornu ammonis and the dentate gyrus regions of the hippocampus. Therefore, graphene-coated substrates could be a promising platform to induce an efficient desorption process essential for highly reproducible ambient MS imaging.

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.

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.

Matrix-assisted laser desorption/ionization mass spectrometry for the analysis of polyamines in plant micro-tissues using cucurbituril as a host molecule

In this study, a matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) strategy using cucurbit[n]uril (CB[n]) as a host molecule is proposed for the analysis of low molecular weight (LMW) compounds in complex samples. As a proof-of-concept, CB[6] was selected as the host molecule, and endogenous polyamines in plant tissue were chosen as the target analytes. Due to the molecular recognition and mass shifting properties of CB[6], the ionic signals associated with polyamines were moved to the higher mass region (>1000 Da) after specifically binding to CB[6], while signal interference derived from the conventional organic matrix and the complex sample matrix remained in the low mass region because of the incompatibility of their molecular size with CB[6] cavities. The strategy not only facilitated the analysis of LMW compounds in complex samples by MALDI MS, but also offered high throughput by accomplishing the entire analytical procedure within 10 min. The detection of polyamine concentration showed good linearity in the range of 0.02-10.0 ng/μL with correlation coefficients (R) greater than 0.9915. The limits of detection were 8.8-28.8 pg. The good reproducibility and reliability of the method were demonstrated by excellent intraday and interday precisions with relative standard deviations less than 7.9%, and the recovery ranged from 92.1% to 117.1%. Finally, the good sensitivity of the method allowed for the quantitative analysis of endogenous polyamine concentrations in various micro-tissues of Arabidopsis thaliana (20.0-740.0 μg fresh weight for each sample).

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.

Improved MALDI imaging MS analysis of phospholipids using graphene oxide as new matrix

Matrix assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) is an increasingly important technique for detection and spatial localization of phospholipids on tissue. Due to the high abundance and being easy-to-ionize of phosphatidylcholine (PC), therefore, selecting matrix to yield signals of other lipids has become the most crucial factor for a successful MALDI-IMS analysis of phospholipids. Herein, graphene oxide (GO) was proposed as a new matrix to selectively enhance the detection of other types of phospholipids that are frequently suppressed by the presence of PC in positive mode. Compared to the commonly used matrix DHB, GO matrix significantly improved signal-to-noise ratios of phospholipids as a result of its high desorption/ionization efficiency for nonpolar compounds. Also, GO afforded homogeneous crystallizations with analytes due to its monolayer structure and good dispersion, resulting in better reproducibility of shot-to-shot (CV < 13%) and spot-to-spot (CV < 14%) analysis. Finally, GO matrix was successfully applied to simultaneous imaging of PC, PE, PS and glycosphingolipid in the mouse brain, with a total of 65 phospholipids identified.

Citric Acid Capped Iron Oxide Nanoparticles as an Effective MALDI Matrix for Polymers

A new matrix-assisted laser desorption ionization (MALDI) mass spectrometry matrix is proposed for molecular mass determination of polymers. This matrix contains an iron oxide nanoparticle (NP) core with citric acid (CA) molecules covalently bound to the surface. With the assistance of additives, the particulate nature of NPs allows the matrix to mix uniformly with polar or nonpolar polymer layers and promotes ionization, which may simplify matrix selection and sample preparation procedures. Several distinctively different polymer classes (polyethyleneglycol (PEG), polywax/polyethylene, perfluoropolyether, and polydimethylsiloxane) are effectively detected by the water or methanol dispersed NPCA matrix with NaCl, NaOH, LiOH, or AgNO₃ as additives. Furtheremore, successful quantitative measurements of PEG1000 using polypropylene glycol 1000 as an internal standard are demonstrated. Graphical Abstract ᅟ.

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.

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.

Graphene oxide embedded sol–gel (GOSG) film as a SALDI MS substrate for robust metabolite fingerprinting

A graphene oxide embedded sol–gel (GOSG) film was utilized as a substrate for surface-assisted laser desorption/ionization mass spectrometry (SALDI MS).

Laser engineered graphene paper for mass spectrometry imaging

A pulsed laser engineering approach is developed to prepare novel functional graphene paper with graphitic nanospheres homogeneously decorated on the surface and the superior performance of engineered paper is revealed in matrix-free mass spectrometry (MS) detection and imaging. We demonstrate that the stability of graphene paper under intense irradiation can be dramatically increased through a designed laser engineering process by forming densely packed graphitic nanospheres on the paper surface. Moreover, the surface hydrophobicity is enhanced and electric conductivity is improved. The engineered graphene paper can image the invisible micro-patterns of trace amount molecules and increases the detection limit towards diverse molecules by over two orders of magnitude compared to the pristine graphene paper and commercial products in MS analysis.

Advances in MALDI mass spectrometry in clinical diagnostic applications

The concept of matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) was first reported in 1985. Since then, MALDI MS technologies have been evolving, and successfully used in genome, proteome, metabolome, and clinical diagnostic research. These technologies are high-throughput and sensitive. Emerging evidence has shown that they are not only useful in qualitative and quantitative analyses of proteins, but also of other types of biomolecules, such as DNA, glycans, and metabolites. Recently, parallel fragmentation monitoring (PFM), which is a method comparable to selected reaction monitoring, has been reported. This highlights the potentials of MALDI-TOF/TOF tandem MS in quantification of metabolites. Here we critically review the applications of the major MALDI MS technologies, including MALDI-TOF MS, MALDI-TOF/TOF MS, SALDI-TOF MS, MALDI-QqQ MS, and SELDI-TOF MS, to the discovery and quantification of disease biomarkers in biological specimens, especially those in plasma/serum specimens. Using SELDI-TOF MS as an example, the presence of systemic bias in biomarker discovery studies employing MALDI-TOF MS and its possible solutions are also discussed in this chapter. The concepts of MALDI, SALDI, SELDI, and PFM are complementary to each other. Theoretically, all these technologies can be combined, leading to the next generation of the MALDI MS technologies. Real applications of MALDI MS technologies in clinical diagnostics should be forthcoming.

Functionalized graphene-coated cobalt nanoparticles for highly efficient surface-assisted laser desorption/ionization mass spectrometry analysis

Graphene-coated cobalt nanoparticles surface-functionalized with benzylamine groups (CoC-NH(2) nanomagnets) were shown to effectively enrich analytes for surface-assisted laser desorption/ionization mass spectrometry (affinity SALDI-MS) analysis. These CoC-NH(2) nanomagnets are highly suited for use with affinity SALDI-MS because their mean diameter of 30 nm, high specific surface area of 15 m(2) g(-1), and high-strength saturation magnetization of 158 emu g(-1) led to efficient extraction of analytes by magnetic separation, which in turn enabled excellent SALDI-MS performance. Surface modification of CoC nanomagnets with benzylamine groups increased the yield of peptide ions and decreased fragmentation of benzylpyridinium ions, so-called "thermometer ions" formed through soft ionization. The CoC-NH(2) nanomagnets were used to extract perfluorooctanesulfonate from large volumes of aqueous solutions by magnetic separation, which was identified directly by SALDI-MS analysis with high sensitivity even at the sub-part-per-trillion level (∼0.1 ng/L). The applicability of CoC-NH(2) nanomagnets in conjunction with SALDI-MS for the enrichment and detection of pentachlorophenol, bisphenol A, and polyfluorinated compounds (PFCs) with varying chain length, which are environmentally significant compounds, as well as small drugs, was also evaluated.