Negative Ion Laser Desorption/Ionization Time-of-Flight Mass Spectrometric Analysis of Small Molecules Using Graphitic Carbon Nitride Nanosheet Matrix

Molecularly Imprinted Heterostructure-Assisted Laser Desorption Ionization Mass Spectrometry Analysis and Imaging of Quinolones

Quinolone residues resulting from body metabolism and waste discharge pose a significant threat to the ecological environment and to human health. Therefore, it is essential to monitor quinolone residues in the environment. Herein, an efficient and sensitive matrix-assisted laser desorption/ionization mass spectrometry (MALDI/MS) method was devised by using a novel molecularly imprinted heterojunction (MIP-TNs@GCNs) as the matrix. Molecularly imprinted titanium dioxide nanosheets (MIP-TNs) and graphene-like carbon nitrides (GCNs) were associated at the heterojunction interface, allowing for the specific, rapid, and high-throughput ionization of quinolones. The mechanism of MIP-TNs@GCNs was clarified using their adsorption properties and laser desorption/ionization capability. The prepared oxygen-vacancy-rich MIP-TNs@GCNs heterojunction exhibited higher light absorption and ionization efficiencies than TNs and GCNs. The good linearity (in the quinolone concentration range of 0.5-50 pg/μL, R2 > 0.99), low limit of detection (0.1 pg/μL), good reproducibility (n = 8, relative standard deviation [RSD] < 15%), and high salt and protein resistance for quinolones in groundwater samples were achieved using the established MIP-TNs@GCNs-MALDI/MS method. Moreover, the spatial distributions of endogenous compounds (e.g., amino acids, organic acids, and flavonoids) and xenobiotic quinolones from Rhizoma Phragmitis and Rhizoma Nelumbinis were visualized using the MIP-TNs@GCNs film as the MALDI/MS imaging matrix. Because of its superior advantages, the MIP-TNs@GCNs-MALDI/MS method is promising for the analysis and imaging of quinolones and small molecules.

Research progress on nanomaterial-based matrices for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis

Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is a novel soft ionization bio-mass spectrometry technology emerging in the 1980s, which can realize rapid detection of non-volatile, highly polar, and thermally unstable macromolecules. However, the analysis of small molecular compounds has been a major problem for MALDI-TOF MS all the time. In the MALDI analysis process based on traditional matrices, large numbers of interference peaks in the low molecular weight area and "sweet spots" phenomenon are produced, so the detection method needs to be further optimized. The promotion of matrix means the improvement of MALDI performance. In recent years, many new nanomaterial-based matrices have been successfully applied to the analysis of small molecular compounds, which makes MALDI applicable to a wider range of detection and useful in more fields such as pharmacy and environmental science. In this paper, the newly developed MALDI matrix categories in recent years are reviewed initially. Meanwhile, the potential applications, advantages and disadvantages of various matrices are analyzed. Finally, the future development prospects of nanomaterial-based matrices are also prospected.

On-tissue derivatization for isomer-specific mass spectrometry imaging and relative quantification of monosaccharides in biological tissues

Spatially-resolved profiling of tissue monosaccharides not only gives an insight into the spatial heterogeneity of monosaccharides, but also helps to decipher the possible roles of monosaccharides in biological processes. Here, we develop an on-tissue derivatization method, coupled with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to image and quantify the aldose and ketose isomers of monosaccharide in biological tissues. A new derivatization reagent, 1-naphthaleneacethydrazide (NAH) was synthesized for the on-tissue derivatization of monosaccharides, and it significantly enhanced the imaging sensitivity of monosaccharides. Moreover, the NAH-derivatized aldose and ketose can generate isomer-specific diagnostic ions during MALDI-MS/MS analysis, and thus paves way for the isomer-specific MS imaging of aldose and ketose monosaccharides. On this basis, we further constructed a quantitative MALDI-MS imaging model based on isomer-specific diagnostic ions, and calculated the expression contents of aldose and ketose monosaccharide isomers in different tissue regions of carrot section. We expect that the development of this method should provide more precise view on the spatial distributions and contents of different monosaccharides in heterogeneous biological tissues.

In Vitro Diagnostic Examination and Prognosis Surveillance by Hierarchical Heterojunction-Assisted Metabolic Analysis

High-throughput metabolic analysis based on laser desorption/ionization mass spectrometry exhibits broad prospects in the field of large-scale precise medicine, for which the assisted ionization ability of the matrix becomes a determining step. In this work, the gold-decorated hierarchical metal oxide heterojunctions (dubbed Au/HMOHs) are proposed as a matrix for extracting urine metabolic fingerprints (UMFs) of primary nephrotic syndrome (PNS). The hierarchical heterojunctions are simply derived from metal-organic framework (MOF)-on-MOF hybrids, and the native built-in electric field from heterojunctions plus the extra Au decoration provides remarkable ionization efficiency, attaining high-quality UMFs. These UMFs are employed to realize precise diagnosis, subtype classification, and effective prognosis evaluation of PNS by appropriate machine learning, all with 100% accurate ratios. Moreover, a high-confidence marker panel for PNS diagnosis is constructed. Interestingly, all panel metabolite markers present obviously uniform downregulation in PNS compared to healthy controls, shedding light on mechanism exploration and pathway analysis. This work drives the application of metabolomics toward precision medicine.

Nanomaterials-assisted metabolic analysis toward in vitro diagnostics

In vitro diagnostics (IVD) has played an indispensable role in healthcare system by providing necessary information to indicate disease condition and guide therapeutic decision. Metabolic analysis can be the primary choice to facilitate the IVD since it characterizes the downstream metabolites and offers real-time feedback of the human body. Nanomaterials with well-designed composition and nanostructure have been developed for the construction of high-performance detection platforms toward metabolic analysis. Herein, we summarize the recent progress of nanomaterials-assisted metabolic analysis and the related applications in IVD. We first introduce the important role that nanomaterials play in metabolic analysis when coupled with different detection platforms, including electrochemical sensors, optical spectrometry, and mass spectrometry. We further highlight the nanomaterials-assisted metabolic analysis toward IVD applications, from the perspectives of both the targeted biomarker quantitation and untargeted fingerprint extraction. This review provides fundamental insights into the function of nanomaterials in metabolic analysis, thus facilitating the design of next-generation diagnostic devices in clinical practice.

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.

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.

Fullerenol as a water-soluble MALDI-MS matrix for rapid analysis of small molecules and efficient quantification of saccharin sodium in foods

Due to the strong background interferences in the low-mass region and poor reproducibility of conventional organic matrices, it is of great importance to develop a novel matrix for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to qualitatively and quantitatively analyze small molecules. In this work, water-soluble fullerenol C₆₀(OH)_24-26 was selected as a MALDI matrix for the analysis of low-molecular-weight compounds in consideration of optical absorption property, water solubility and stability. Compared with the traditional matrices, fullerenol demonstrated lower background interference and stronger peak intensity. In addition, the hydrophilic fullerenol could avoid the heterogeneous crystallization in sample preparation, increase the reproducibility and sensitivity of MALDI-MS, and ameliorate quantitative analysis of small molecules. With saccharin as model analyte, quantitative analysis was carried out using fullerenol as matrix. The results demonstrated satisfying reproducibility and good tolerance to salt. The limit-of-detection of the quantitative analysis was as low as 4 pmol, and the linear range is 1-100 μg mL^-1 with R² greater than 0.99. The analytical results also showed excellent precision and accuracy, low matrix effect and good recovery rate. Fullerenol as a potential matrix was further validated in the quantification of saccharin sodium in different real food samples, such as nuts and drinks. This work not only confirms the potential of fullerenol for the quantitative analysis in food field, but also provides a new technique for rapid analysis of small molecules.

Graphene oxide-metal oxide nanocomposites for on-target enrichment and analysis of phosphorylated biomolecules

The surface of matrix-assisted laser desorption/ionization mass spectrometry target is modified for improved signal strength and detection of analytes. The developed method includes on-target enrichment and detection of phosphopeptides/phospholipids using graphene oxide-lanthanide metal oxides (samarium, gadolinium, dysprosium, and erbium) nanocomposites. Enriched phosphopeptides are detected using material enhanced laser desorption/ionization mass spectrometry and phospholipids by laser desorption/ionization-mass spectrometry. Nanocomposites are prepared using graphene oxide with respective metal salts at high pH. They are characterized for nano-morphology, chemistry, porosity, composition, crystallinity, and thermal stability. Phosphopeptides enrichment protocol is developed and optimized for tryptic β-casein digest and that of phospholipids by phosphatidylcholine standard. Statistical analyses of phosphopeptides and phospholipids from milk show overlapping results for gadolinium, dysprosium, and erbium oxide nanocomposites. GO-Gd₂ O₃ has better enrichment efficiency and application as LDI material. Selectivity for GO-Dy₂ O₃ is 1:2500, for GO-Sm₂ O₃ is 1:3500, and 1:4000 for GO-Gd₂ O₃ . GO-Er₂ O₃ has a sensitivity of 25 fmol, whereas the highest sensitivity is down to 0.5 fmol for GO-Gd₂ O₃ . On-target enrichment is batch to batch reproducible with a standard deviation of <1, reduced time of enrichment to 10 min, and ease of operation compared to solid-phase batch extraction. The developed method enriches serum phosphopeptides characteristic of cancer-related phosphoproteins.

Sulfonic acid functionalized hierarchical porous covalent organic frameworks as a SALDI-TOF MS matrix for effective extraction and detection of paraquat and diquat

Design and construction of a matrix with specific adsorption on the target compounds can effectively reduce the detection limit of surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS) analysis. Sulfonic acid functionalized hierarchical porous covalent organic frameworks (H-COF-SO₃H) was synthesized by defect-structure and post-modification method, and then used as matrix and adsorbent for the determination of quaternary ammonium herbicides paraquat (PQ) and diquat (DQ). N₂ adsorption-desorption experiments confirmed that H-COF-SO₃H possesses hierarchical porosity with pore widths concentrated at 1.3,1.5, and 2.8 nm. The strong UV absorption at 200-450 nm and good thermal stability made H-COF-SO₃H being a promising matrix without background interference. H-COF-SO₃H can efficiently enrich PQ and DQ via electrostatic attraction, and the key role of -SO₃H group on specific adsorption was confirmed by density functional theory (DFT) calculations. The limits of detection (LODs) for PQ and DQ with H-COF-SO₃H enrichment were 0.5 and 0.1 ng·mL^-1, respectively, which were 20 and 60 times higher than those without H-COF-SO₃H enrichment, respectively. The spiked recoveries of PQ and DQ for the three food samples were 92.0-113.2% and 80.1-102.6% with RSDs of 2.2-9.2% and 2.0-8.7%, respectively. This work provides an analyte-oriented approach for fabricating SALDI-TOF MS matrix.

A Polymer-Based Matrix for Effective SALDI Analysis of Lipids

Surface-assisted laser desorption/ionization (SALDI) has become an attractive branch of matrix-assisted laser desorption/ionization (MALDI) and has been successfully applied for the detection of small molecules due to the lack of the interference of matrix. Herein, the low-cost and highly accessible polyvinylidene fluoride (PVDF) was modified using a facile alkali treatment and investigated as a SALDI matrix. The modified PVDF has a strong optical absorption and can be applied as a dual-mode substrate for both SALDI MS and SALDI imaging analysis. Modified PVDF powder showed superior performance in SALDI MS analysis of lipids, with good reproducibility, high sensitivity, and low background interference, especially for triacylglycerols (TAGs) and fatty acids. Additionally, the lipids in raw and extracted serum were both successfully determined with modified PVDF powder. A modified PVDF membrane (m-PVDF-m) showed excellent ability in lipids imaging in tissues due to its flat surface, mass signal enhancement, and elimination of matrix coating. The distribution of several TAGs and cholesteryl esters on mouse kidney section was presented by SALDI imaging directly on m-PVDF-m. These results demonstrated that modified PVDF materials presented exciting opportunities as matrices for the first time in SALDI MS acquisition and SALDI imaging.

Thiol functionalized covalent organic framework for highly selective enrichment and detection of mercury by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A spherical thiol-functionalized covalent organic framework (COF-SH) was designed via a facile thiol-yne click reaction of a alkynyl-terminated COF and pentaerythritol tetra(3-mercaptopropionate). The COF-SH was explored as a new adsorbent for the selective enrichment of Hg2+. The as-prepared COF-SH exhibited a uniform mesoporous structure, a high abundance of binding sites, and good chemical stability, which endow it with great performance for the adsorption of Hg2+ and its corresponding maximum adsorption capacity was up to 617.3 mg g-1. Furthermore, the adsorption behavior of Hg2+ on the COF-SH wasin good agreement with the Langmuir and pseudo-second-order models. The influences of adsorbent dosage, pH, selectivity, and reusability of the COF-SH on Hg2+ adsorption were also investigated. Besides this, the COF-SH showed high selectivity towards Hg2+ even in the presence of a high concentration of K+, Na+, Ca2+, Mg2+ and Zn2+ metal ions. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), the corresponding limit of detection (LOD) of Hg2+ was determined at very low concentrations of 80 pg mL-1 (equal to 396 amoL μL-1). In addition, the COF-SH was successfully applied to rapidly enrich and sensitively detect Hg2+ in industrial sewage, with recoveries in the range of 101.8-103.4%, demonstrating the promising potential of COF-SH as an effective adsorbent for use in environmental sample pretreatment.

Experimental and Computational Study of Aminoacridines as MALDI(-)-MS Matrix Materials for the Analysis of Complex Samples

Monoaminoacridines (1-, 2-, 3-, 4-, and 9-aminoacridine) were studied for suitability as matrices in the negative ion mode matrix-assisted laser desorption/ionization mass spectrometry (MALDI(-)-MS) analysis of various samples. This is the first study to examine 1-, 2-, and 4-aminoacridine as potential matrix material candidates for MALDI(-)-MS. In addition, spectral (UV-Vis absorption and fluorescence), proton transfer-related (basicity and autoprotolysis), and crystallization properties of these compounds were characterized experimentally and/or computationally. For testing the capabilities of these aminoacridines as matrix materials, four samples related to cultural heritage materials-stearic acid, colophony resin, dyer's madder dye, and a resinous case-study sample from a shipwreck-were analyzed with MALDI(-)-MS. A novel algorithm (implemented as an executable Python script) for MS data analysis was developed to compare the five matrix materials and to help mass spectrometrists rapidly identify peaks originating from the sample and matrix material. It was determined that all five of the studied aminoacridines can successfully be used as matrix materials in MALDI(-)-MS analysis. As an interesting finding, in several cases, the best mass spectra were obtained by using a relatively small amount of matrix material mixed with an excess amount of sample. 3- and 4-aminoacridine outperformed the other aminoacridines in the ease of obtaining acceptable spectra, average number of ions identified in the mass spectra, and low dependence of the sample-to-matrix mass ratio on experimental results.

4-Mercaptobenzoic acid as a MALDI matrix for highly sensitive analysis of metals

4-Mercaptobenzoic acid (MBA) is introduced as a matrix for laser desorption/ionization time-of-flight mass spectrometry (MS) analysis of metals, exhibiting matrix-interference-free background, greatly enhanced MS signal intensity, and excellent reproducibility. The developed method was successfully extended for the rapid screening and sensitive determination of ultratrace metals in fine particulate matter (PM_2.5).

Self-Assembled Au Nanoparticle Arrays for Precise Metabolic Assay of Cerebrospinal Fluid

Precise and rapid monitoring of metabolites in biofluids is a desirable but unmet goal for disease diagnosis and management. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) exhibits advantages in metabolite analysis. However, the low accuracy in quantification of the technique limits its transformation to clinical usage. We report herein the use of Au nanoparticle arrays self-assembled at liquid-liquid interfaces for mass spectrometry (MS)-based quantitative biofluids metabolic profiling. The two-dimensional arrays feature uniformly and closely packed Au nanoparticles with 3 nm interparticle gaps. The experimental study and theoretical simulation show that the arrays exhibit high photothermal conversion and heat confinement effects, which enhance the laser desorption/ionization efficacy. With the nanoscale roughness, the AuNP arrays as laser desorption/ionization substrates can interrupt the coffee-ring effect during droplet evaporation. Therefore, high reproducibility (RSD <5%) is obtained, enabling accurate quantitative analysis of diverse metabolites from 1 μL of biofluids in seconds. By quantifying glucose in the cerebrospinal fluid (CSF), it allows us to identify patients with brain infection and rapidly evaluate the clinical therapy response. Consequently, the method shows potential in advanced metabolite analysis and biomedical diagnostics.

Polyacrylonitrile-based carbon nanofibers as a matrix for laser desorption/ionization time-of-flight mass spectrometric analysis of small molecules under both positive and negative ionization modes

Carbon fiber (CNF), prepared by carbonization of electrospun polyacrylonitrile (PAN) fibers, is systematically investigated as a mediator to replace conventional organic matrices for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-MS). CNF exhibits a high salt tolerance, sensitivity, and resolution for organic matrix-free laser desorption/ionization time-of-flight mass spectrometry (LDI-MS) analysis of various analytes under both positive and negative ionization modes. Especially, saccharides, a neutral molecule having low negative ionization efficiency, are successfully detected with CNF. Taken together, this study clearly demonstrates CNF is a promising material to develop an efficient and universal platform for LDI-MS analysis regardless of preferential ionization modes of analytes. Graphical abstract.

Hybrid MoS2/g-C3N4-assisted LDI mass spectrometry for rapid detection of small molecules and polyethylene glycols and direct determination of uric acid in complicated biological samples

A novel matrix-assisted laser desorption/ionization time-of-flight mass spectrometric method (MALDI-TOF MS) for determination of highly sensitive small molecular compounds was developed based on molybdenum disulfide nanosheets hybridized with ultrathin graphitic carbon nitride (MoS₂/g-C₃N₄) as the matrix. With this approach, the synergistic effects of MoS₂ and g-C₃N₄ enhance the UV absorption of MoS₂/g-C₃N₄, increase both desorption and ionization efficiency in LDI MS, and induce higher signal-to-noise ratio of analytes when compared with the bare MoS₂ and g-C₃N₄ matrix in the determination of amino acids, antibiotics, neutral oligosaccharides, uric acid, and polyethylene glycols (PEGs). The detection limits of these small molecular compounds are in the ranges 0.1 to 10 μg mL^-1, 1*10^-3 to 1.0 μg mL^-1, 1.0 to 10 μg mL^-1, and 2*10^-4 μg mL^-1, respectively, and the polydispersity index of these PEGs is less than 1.02. Moreover, high salt tolera`nce and homogeneous deposition on the spot results in good reproducibility. The relative standard deviations (RSDs) of shot-to-shot and spot-to-spot (n = 15) of these compounds are less than 10.1% and 12.5%, respectively. With MoS₂/g-C₃N₄, the uric acid in complicated biological samples can be directly determined in combination with LDI-TOF MS. We synthesized MoS₂/g-C₃N₄ nanohybrid as an efficient matrix for MALDI-TOF MS analysis of small molecules as well as quantitative detection of uric acid in human urine.

Surface Siloxane-Modified Silica Materials Combined with Metal-Organic Frameworks as Novel MALDI Matrixes for the Detection of Low-MW Compounds

Surface siloxane (3-aminopropyl triethoxysilane hydrolyzates)-modified silica materials were used as "initiators", which resulted in the release and desorption of intact molecules adsorbed on the surface of a matrix. A covalently cross-linked MIL-53(Al) material was used to enhance the ionization of analytes. Herein, we have provided an efficient matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS) matrix strategy, which responded to both ion and laser irradiation with low background interference in the low-molecular-weight (MW) region. The matrixes MIL-53(Al), SBA-15@APTES, SiO₂@APTES, SBA-15@APTES@MOF, and SiO₂@APTES@MOF were synthetized and used for the analysis of a series of low-MW compounds to verify the effectiveness of the strategies. Compared to conventional matrixes, the surface-modified SBA-15@APTES@MOF and SiO₂@APTES@MOF had low background, high sensitivity, extensive applicability, good stability, and ultrahigh tolerance of salt concentrations. The detection limits of standard analytes were determined to range from 0.1 to 1 × 10^-5 mg/mL for 16 amino acids as well as citric acid, reserpine, tetraethylammonium chloride, melamine, bisphenol A, and malachite green. These results could help in designing more efficient nanostructure-initiator materials and further promote the application of MALDI-TOF MS.

Ultrafast Photocatalytic Reaction Screening by Mass Spectrometry

Here we report a semiconductor-assisted laser desorption/ionization mass spectrometry (SA-LDI MS) platform to monitor photocatalytic reactions online and apply it for ultrafast reaction screening. In this method, we use photocatalytic nanomaterials as the substrate for LDI and then initiate and monitor the reactions simultaneously. The features of our method include the following: (i) It has a reaction acceleration effect: only seconds are needed in our interfacial reactions vs hours in conventional bulk phase. (ii) The reaction trend in our system agrees with that in bulk phase. (iii) By adding a stable analogue of reactant as internal standard, a quantification of the reaction can be achieved. (iv) The sensitivity is high: for 500 amol of reactant, the photocatalytic reaction can still be initiated and detected. This platform has advantages in ultrafast reaction screening (e.g., screening of nine catalysts requires 24 h by the UPLC-MS system but only 10 min by SA-LDI MS). Furthermore, the high specificity of MS enables the screening of catalytic selectivity of A-TiO₂ nanoparticles for a methyl red (MR) and acid yellow (AY) mixture, whose absorption wavelengths are overlapped and thus cannot be discriminated by conventional optical methods. Furthermore, by using SA-LDI MS, we also monitored reductive debrominations during the degradation process of polybrominated diphenyl ethers (PBDEs), which is a type of important pollutant that is difficult to degrade and detect in liquid phase, and the photocatalytic reduction of CO₂. Overall, SA-LDI MS realizes ultrafast photocatalytic reaction screening for the first time and provides practical analytical value in the field of catalyst screening.

Fe3O4-assisted laser desorption ionization mass spectrometry for typical metabolite analysis and localization: Influencing factors, mechanisms, and environmental applications

Fe₃O₄ has been suggested as an efficient matrix for small-molecule analysis by laser desorption ionization mass spectrometry (LDI-MS), but thus far there has been no systematic study exploring the influencing factors of nano-Fe₃O₄ on the detection of typical metabolites, or the mechanism by which nano-Fe₃O₄ assists the desorption and ionization of analytes after receiving laser energy. In this study, Fe₃O₄ nanoparticles with different physicochemical properties were synthesized and characterized. The results revealed that smaller particle size and greater surface hydroxyl amount of nano-spherical Fe₃O₄ could improve the intensity and relative standard deviation of typical metabolites by LDI-MS. The thermally driven desorption process played a vital role in LDI performance, but the chemical interactions between nano-Fe₃O₄ and analytes did not. Good intra- or inter-spot repeatability and linearity of analytes were obtained by the optimum Fe₃O₄-assisted LDI-MS. Finally, the developed method was successfully used for the rapid analysis and localization of endogenous metabolites in biofluids and whole zebrafish tissue section samples. Our results not only elucidate the influencing factors and mechanisms of nano-Fe₃O₄ for the detection of typical metabolites in LDI-MS but also reveal an innovative tool for the imaging of chemicals in the regions of interest in terms of eco-toxicological research.

A spherical covalent-organic framework for enhancing laser desorption/ionization mass spectrometry for small molecule detection

A spherical vinyl-functionalized covalent-organic framework (COF-V) was prepared at room temperature by a facile method and applied as a novel substrate for surface-enhanced laser desorption/ionization mass spectrometry (SELDI-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.

A boronic acid modified binary matrix consisting of boron nitride and α-cyano-4-hydroxycinnamic acid for determination of cis-diols by MALDI-TOF MS

A MALDI-TOF mass spectrometric method is described for the determination of small molecule compounds with cis-diol. It is based on the use of a binary matrix consisting of boron nitride (BN) and α-cyano-4-hydroxycinnamic acid that was modified with the derivatization reagent of (3-(acridin-9-ylamino)phenyl)boronic acid which can recognize cis-diols. The binary matrix is used for desorption/ionization (DI) in the positive ion mode. The mechanism leading to DI enhancement was investigated. The results imply that BN is beneficial for the DI because it induces an enhancement in the positive ion mode. The boronic acid-functionalized binary matrix was successfully applied to capture the glucose, shikimic acid and quinic acid. The method was applied to the determination of 3-chloro-1,2-propanediol in plant oil. Graphical abstract Schematic representation of a method for detecting the cis-diol compounds on matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) using the binary matrix of boron nitride (BN)/α-cyano-4-hydroxycinnamic acid (CHCA) that was modified with (3-(acridin-9-ylamino)phenyl) boronic acid (AYPBA).

Ultrafast intramolecular vibrational energy transfer in carbon nitride hydrocolloid examined by femtosecond two-dimensional infrared spectroscopy

In this work, ultrafast vibrational and structural processes in a graphitic carbon nitride hydrocolloid system were studied using a combination of linear infrared and nonlinear two-dimensional infrared (2D IR) spectroscopies. The experimentally observed three IR line shapes in the C=N stretching vibration frequency region were analyzed and attributed to the rigid and conjugated molecular frame of the prepared g-CN molecular species, which is believed to be a dimeric tris-s-triazine, as well as attributed to insignificant solvent influence on the delocalized C=N vibrations. Vibrational transition density cubes were also computed for the proposed g-CN dimer, confirming the heterocyclic C=N stretching nature of the three IR absorption peaks. Intramolecular vibrational energy transfer dynamics and spectral diffusion of the g-CN system were characterized by examining a series of time-dependent 2D IR spectra. A picosecond intramolecular vibrational energy redistribution process was found to occur among these delocalized C=N stretching modes, acting as an efficient vibrational energy transfer channel. This work reasonably connects the experimentally observed IR signature to a specific g-CN structure and also provides the first report on the ultrafast intramolecular processes of such carbon nitride systems. The obtained results are fundamentally important for understanding the molecular mechanisms of such carbon-nitride based functional materials.

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.

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.

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.