Conjugated Polymers as a New Class of Dual-Mode Matrices for MALDI Mass Spectrometry and Imaging

Coulomb Field-Driven Desorption/Ionization by Femtosecond Laser for Mass Spectrometry Detection and Imaging

Surface-assisted laser desorption/ionization (SALDI) offers promising prospects for mass spectrometry detection and imaging of small biomolecules, as it addresses most of the matrix-related issues encountered in conventional matrix-assisted laser desorption/ionization (MALDI). Currently, nearly all of the fundamental aspects and applications of SALDI depend on nanosecond (ns) lasers, whereas few efforts have been made to integrate ultrafast femtosecond (fs) lasers with SALDI. Therefore, the intrinsic fundamental principle remains poorly understood. Herein, a novel surface-assisted femtosecond laser desorption/ionization mass spectrometry (fs-SALDI-MS) platform was developed, which significantly reduces analyte fragmentation and preserves molecular integrity. Spectral interferences from surface-assisted materials and alkali-metal adducts are absent in fs-SALDI mass spectra. Ion survival yields continuously increase with decreasing laser pulse widths from 5 ns to 600 fs, highlighting a gradual transition from thermal to nonthermal effects. A lower absolute limit of detection down to ∼3 amol for representative antifungal and psychotropic drugs and clearer visualization of ultratrace drug residues on latent fingerprints can be achieved, indicating that fs-SALDI results in gentler and more efficient detection/ionization processes than mainstream ns-SALDI. The biological applicability of this method was further validated through 10 μm-spatial-resolution lipid imaging of mouse brain sections. In short, a novel Coulomb field-driven desorption/ionization mechanism is proposed for fs-SALDI, opening new avenues for the development of emerging fs-SALDI techniques with superior analytical performance.

Decoding Sugars: Mass Spectrometric Advances in the Analysis of the Sugar Alphabet

Monosaccharides play a central role in metabolic networks and in the biosynthesis of glycomolecules, which perform essential functions across all domains of life. Thus, identifying and quantifying these building blocks is crucial in both research and industry. Routine methods have been established to facilitate the analysis of common monosaccharides. However, despite the presence of common metabolites, most organisms utilize distinct sets of monosaccharides and derivatives. These molecules therefore display a large diversity, potentially numbering in the hundreds or thousands, with many still unknown. This complexity presents significant challenges in the study of glycomolecules, particularly in microbes, including pathogens and those with the potential to serve as novel model organisms. This review discusses mass spectrometric techniques for the isomer-sensitive analysis of monosaccharides, their derivatives, and activated forms. Although mass spectrometry allows for untargeted analysis and sensitive detection in complex matrices, the presence of stereoisomers and extensive modifications necessitates the integration of advanced chromatographic, electrophoretic, ion mobility, or ion spectroscopic methods. Furthermore, stable-isotope incorporation studies are critical in elucidating biosynthetic routes in novel organisms.

Conjugate Polymer Anchor Enhancing Matrix Vacuum Stability and Improving MALDI MSI via Ion Bond

Poor vacuum stability limits the application of many matrices in matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) that requires long-term measurement duration in high vacuum. In this study, a new approach using conjugate polymer anchor to protect unstable matrix from volatilizing in MALDI source based on ion bond is provided. Unlike strong covalent bonds which often introduce unnecessary groups, the weaker ion bonds are more conducive to breaking under laser radiation while effectively preventing matrix volatilization in a vacuum environment. The results confirm that conjugate polymer anchor will neither introduce additional ion peaks nor affect signal intensity, yet maintains comparable quantification properties. Vacuum stability of three kinds of typical matrices is enhanced using polymer anchors, and the in situ MALDI MS imaging of mouse brain and liver cancer is improved significantly.

Organic metal chalcogenide-assisted metabolic molecular diagnosis of central precocious puberty

Metabolic analysis in biofluids based on laser desorption/ionization mass spectrometry (LDI-MS), featuring rapidity, simplicity, small sample volume and high throughput, is expected to be a powerful diagnostic tool. Nevertheless, the signals of most metabolic biomarkers obtained by matrix-assisted LDI-MS are too limited to achieve a highly accurate diagnosis due to serious background interference. To address this issue, nanomaterials have been frequently adopted in LDI-MS as substrates. However, the "trial and error" approach still dominates the development of new substrates. Therefore, rational design of novel LDI-MS substrates showing high desorption/ionization efficiency and no background interference is extremely desired. Herein, four few-layered organic metal chalcogenides (OMCs) were precisely designed and for the first time investigated as substrates in LDI-MS, which allowed a favorable internal energy and charge transfer by changing the functional groups of organic ligands and metal nodes. As a result, the optimized OMC-assisted platform satisfyingly enhanced the mass signal by ≈10 000 fold in detecting typical metabolites and successfully detected different saccharides. In addition, a high accuracy diagnosis of central precocious puberty (CPP) with potential biomarkers of 12 metabolites was realized. This work is not only expected to provide a universal detection tool for large-scale clinical diagnosis, but also provides an idea for the design and selection of LDI-MS substrates.

A reactive matrix for in situ chemical derivatisation and specific detection of cis-diol compounds by matrix-assisted laser desorption/ionisation mass spectrometry

Analysis of cis-diol compounds is essential, because they play important roles in cosmetics, food, pharmaceuticals, and living organisms. Herein, we describe the development of a matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS) method to analyse cis-diol compounds. In this method, a 6-borono-1-methylquinoline-1-ium (BMQI) reactive matrix was designed for in situ derivatisation of cis-diol compounds based on the boronate affinity interaction between boronic acid and cis-diol groups. Compared to traditional commercial matrices and other boronic acid reagents, BMQI can significantly accelerate the desorption/ionisation process, improve reproducibility, exhibit free background interference, and enhance signal intensity in the analysis of various cis-diol compounds even for amounts as low as 1 nmol. The BMQI-assisted laser desorption/ionisation mass spectrometry (LDI-MS) was successfully applied to the rapid screening and identification of sugar alcohols in different sugar-free foods. This work provides an alternative method to the LDI-MS analysis of cis-diol-containing molecules, and the method can be extended to other food samples and biofluids.

Nanometer Resolution Mass Spectro-Microtomography for In-Depth Anatomical Profiling of Single Cells

Visually identifying the molecular changes in single cells is of great importance for unraveling fundamental cellular functions as well as disease mechanisms. Herein, we demonstrated a mass spectro-microtomography with an optimal voxel resolution of ∼300 × 300 × 25 nm³, which enables three-dimensional tomography of chemical substances in single cells. This mass imaging method allows for the distinguishment of abundant endogenous and exogenous molecules in subcellular structures. Combined with statistical analysis, we demonstrated this method for spatial metabolomics analysis of drug distribution and subsequent molecular damages caused by intracellular drug action. More interestingly, thanks to the nanoprecision ablation depth (∼12 nm), we realized metabolomics profiling of cell membrane without the interference of cytoplasm and improved the distinction of cancer cells from normal cells. Our current method holds great potential to be a powerful tool for spatially resolved single-cell metabolomics analysis of chemical components during complex biological processes.

Polyacrylamide gel as a new embedding medium for the enhancement of metabolite MALDI imaging

In this study, polyacrylamide gel (PAAG) was successfully used as a new embedding medium to provide the more effective maintenance of biological tissues during the sectioning process, enhancing the tissue imaging of metabolites via matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). Herein, PAAG, agarose, gelatin, optimal cutting temperature compound (OCT), and ice media were used to embed rat liver and Atlantic salmon (Salmo salar) eyeball samples. These embedded tissues were then sectioned into thin slices and thaw-mounted on conductive microscope glass slides for MALDI-MSI detection to evaluate the embedding effects. The results showed that PAAG embedding has characteristics superior to those of commonly-used embedding media (e.g., agarose, gelatin, OCT, and ice) with the advantages of one-step operation without heating, a better performance of morphology maintenance, the absence of PAAG polymer-ion-related interference below m/z 2000, and the more efficient in situ ionization of metabolites, providing a significant enhancement of both the numbers and intensities of the metabolite ion signals. Our study demonstrates the potential of PAAG embedding as a standard practice for metabolite MALDI tissue imaging, which will lead to an expanded application scope of MALDI-MSI.

Surface molecularly imprinted-based matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for highly selective and sensitive direct analysis of paraquat in complicated samples

Herein, a novel surface molecularly imprinted-based matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (SMI-MALDI-TOF MS) method for direct target paraquat (PQ) analysis in complicated samples is reported. Notably, a captured analyte-imprinted material can be directly detected via MALDI-TOF MS by using imprinted material as nanomatrix. Using this strategy, the molecular specific affinity performance of surface molecularly imprinted polymers (SMIPs) and the high-sensitivity detection capability of MALDI-TOF MS was integrated. The introduction of SMI endowed the nanomatrix with the capacity for rebinding the target analyte and ensuring specificity, prevented the interfering organic matrix, and enhanced the analyzing sensitivity. By using paraquat (PQ) as a template, dopamine as a monomer, and covalent organic frameworks with a carboxyl group (C-COFs) as a substrate, polydopamine (PDA) was decorated on C-COFs via a simple self-assembly procedure to generate an analyte-based surface molecularly imprinted polymer (C-COF@PDA-SMIP), which served the dual function of SMIP capturing the target analytes and high-efficiency ionization. Thus, a reliable MALDI-TOF MS detection PQ with high selectivity and sensitivity as well as an interference-free background was achieved. The synthesis and enrichment conditions of C-COF@PDA-SMIPs were optimized, and its structure and property were characterized. Under optimal experimental conditions, the proposed method achieved highly selective and ultrasensitive detection of PQ from 5 to 500 pg mL^-1, and the limit of detection was as low as 0.8 pg mL^-1, which is at least three orders of magnitude lower than that achieved without enrichment. In addition, the specificity of the proposed method was superior to that of C-COFs and nonimprinted polymers. Moreover, this method exhibited reproducibility, stability, and high salt tolerance. Lastly, the practical applicability of the method was successfully verified by analyzing complicated samples, such as grass and orange.

2-Hydrazinoterephthalic Acid as a Novel Negative-Ion Matrix-Assisted Laser Desorption/Ionization Matrix for Qualitative and Quantitative Matrix-Assisted Laser Desorption/Ionization-Mass Spectrometry Analysis of N-Glycans in Peach Allergy Research

Glycans recently attracted considerable attention as the proposal of cross-reactive carbohydrate determinants for food allergy. Matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) is powerful in analyzing biomolecules, while its applications in glycans are still challenging. Herein, a novel reactive matrix-assisted laser desorption/ionization (MALDI) matrix, 2-hydrazinoterephthalic acid, was rationally designed and synthesized. It provides uniform co-crystallization with glycans and only produces deprotonated ions with high intensities in the negative-ion mode. In combination with sinapic acid, a rapid and high-throughput method was established for on-target analysis of glycans with a superior limit of detection at the femtomole level and a good linearity (R² > 0.999). Furthermore, the established method was successfully applied to quantify N-glycans in different cultivars and tissues of peach [Prunus persica (L.) Batsch]. Our work suggests the potential role of N-glycans as biomarkers for food-borne allergy and lays a methodological foundation for the elucidation of the possible relationship between carbohydrate epitopes and food allergy.

Sample Preparation for Metabolite Detection in Mass Spectrometry Imaging

Metabolites reflect the biological state of cells and tissue, and metabolomics is therefore a field of high interest both to understand normal physiological functions and disease development. When studying heterogeneous tissue samples, mass spectrometry imaging (MSI) is a valuable tool as it conserves the spatial distribution of analytes on tissue sections. A large proportion of metabolites are, however, small and polar, making them vulnerable to delocalizing through diffusion during sample preparation. Here we present a sample preparation method optimized to limit diffusion and delocalization of small polar metabolites in fresh frozen tissue sections. This sample preparation protocol includes cryosectioning, vacuum frozen storage, and matrix application. The methods described were primely developed for matrix-assisted laser desorption/ionization (MALDI) MSI, but the protocol describing cryosectioning and vacuum freezing storage can also be applied before desorption electrospray ionization (DESI) MSI. Our vacuum drying and vacuum packing approach offers a particular advantage to limit delocalization and safe storage.

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

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

Rapid methylation of valsartan in human plasma using evaporative derivatization reagent to improve its sensitivity in MALDI-TOF mass spectrometry

Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) is a rapid and low-solvent-consumption technique. However, almost every mass in the low mass-to-charge-ratio region of the mass spectrum appears as strongly fluctuating matrix background signals. Thus, it is difficult to identify small molecules using this technique. In this study, we used methanol to methylate valsartan, an angiotensin II receptor blocker that is commonly used to treat high blood pressure and heart failure. The methylation derivatization of valsartan enhanced the detection sensitivity and transformed the detection m/z ratio. The liquid-phase microextraction of valsartan in human plasma (20 μL) was achieved by acidifying valsartan with HCl aqueous solution and extracting it with toluene. An acetyl chloride/anhydrous methanol mixture was added for methylation derivatization, which was completed within 30 min at 30 °C. Finally, the residue was re-dissolved in irbesartan methanolic solution, which together with the matrix 2-mercaptobenzothiazole was spotted on an AnchorChip target plate for MALDI-TOF MS analysis. Liquid-phase microextraction was performed and the methylation-derivatization parameters were investigated. The valsartan calibration range was 0.2-10 μg mL^-1 with good linearity in human plasma. In the within- and between-run analyses, the relative standard deviation and relative error were both <11.32%. This method was successfully applied to determine the valsartan concentration in the plasma of 10 patients with hypertension.

Developing a Noncontact Heating Matrix Spraying Apparatus with Controllable Matrix Film Formation for MALDI Mass Spectrometry Imaging

Matrix deposition plays an important role in obtaining high-quality and reliable molecular spatial location information for matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). To control the matrix film formation, an automatic matrix spraying apparatus was developed with the introduction of a noncontact heating lamp. Compared with the unheated condition, the noncontact heating lamp suppressed the coffee-ring effect and the diffusion phenomenon of the analyte effectively by controllable matrix film formation. Meanwhile, the signal intensity was increased by 2-5 fold. To prove the ability of the matrix deposition apparatus, the apparatus combined with metabolomics analysis was used to show the spatial distribution of the substance in sprouted potato tubers. The potential biomarkers at m/z 868.5049 and m/z 852.5101 were identified as α-solanine and α-chaconine, and the synthesis pathways were further searched. To further demonstrate the quality of MALDI images including localization and spatial resolution, lipid distribution in rat brain tissue was investigated by the developed noncontact heating matrix spraying apparatus. An excellent match with distinguishable compartments of lipids in the rat brain was obtained between the H&E-stained sections and MALDI-MSI images. These results indicate that the developed noncontact heating matrix spraying apparatus is reliable and provides a low-cost, high-quality, rapid approach for MALDI-MSI.

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.

Top-Down Rational Engineering of Heteroatom-Doped Graphene Quantum Dots for Laser Desorption/Ionization Mass Spectrometry Detection and Imaging of Small Biomolecules

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) is widely applied in mapping macrobiomolecules in tissues, but it is still limited in profiling low-molecular-weight (MW) compounds (typically metabolites) due to ion interference and suppression by organic matrices. Here, we present a versatile "top-down" strategy for rational engineering of carbon material-based matrices, by which heteroatom-doped graphene quantum dots (HGQDs) were manufactured for LDI MS detection and imaging of small biomolecules. The HGQDs derived from parent materials inherited the π-conjugated networks and doping sites for promoting energy transfer and negative ion generation, while their extremely small size guaranteed the matrix uniformity and signal reproducibility in LDI MSI. Compared to other HGQDs, nitrogen-doped graphene quantum dots (NGQDs) exhibited superior capability of assisting LDI of various small molecules, including amino acids, fatty acids, saccharides, small peptides, nucleobases, anticancer drugs, and bisphenol pollutants. Density functional theory simulations also corroborated that the LDI efficiency was markedly raised by the proton-capturing pyridinic nitrogen species and compromised by the electron-deficient boron dopants. NGQDs-assisted LDI MS further enabled label-free investigation on enzyme kinetics using an ordinary short peptide as the substrate. Moreover, due to the high salt tolerance and signal reproducibility, the proposed negative-ion NGQDs-assisted LDI MSI was able to reveal the abundance and distribution of low-MW species in rat brain tissue and achieved the imaging of low-MW lipids in coronally sectioned rat brains subjected to traumatic brain injury. Our work offers a new route for customizing nanomaterial matrices toward LDI MSI of small biomolecules in biomedical and pathological research.

A sensitive environmental forensic method that determines bisphenol S and A exposure within receipt-handling through fingerprint analysis

As human beings have been consistently exposed to bisphenol A (BPA) and bisphenol S (BPS) derived from various products, the intake of BPS/BPA to humans has been extensively studied. However, using conventional biological matrices such as urine, blood, or dissected skin to detect BPS/BPA in the human body system requires longer exposure time to them, hardly defines the pollutant source of the accumulated BPS/BPA, and is often invasive. Herein, our new approach i.e. fingerprint analysis quantitatively confirms the transfer of BPS/BPA from receipts (specific pollution source) to human skin only within receipt-handling of "20 s". When receipts (fingertip region size; ~1 cm²) containing 100-300 μg of BPS or BPA are handled, 20-40 μg fingerprint^-1 of BPS or BPA is transferred to human skin (fingertip). This transferred amount of BPS/BPA can still be toxic according to the toxicity test using water fleas. As a visual evidence, a fingerprint map that matches the distribution of the absorbed BPS/BPA is developed using a mass spectrometry imaging tool. This is the first study to analyze fingerprints to determine the incorporation mechanism of emerging pollutants. This study provides an efficient and non-invasive environmental forensic tool to analyze amounts and sources of hazardous substances.

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

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

Unravel the Local Complexity of Biological Environments by MALDI Mass Spectrometry Imaging

Classic metabolomic methods have proven to be very useful to study functional biology and variation in the chemical composition of different tissues. However, they do not provide any information in terms of spatial localization within fine structures. Matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI MSI) does and reaches at best a spatial resolution of 0.25 μm depending on the laser setup, making it a very powerful tool to analyze the local complexity of biological samples at the cellular level. Here, we intend to give an overview of the diversity of the molecules and localizations analyzed using this method as well as to update on the latest adaptations made to circumvent the complexity of samples. MALDI MSI has been widely used in medical sciences and is now developing in research areas as diverse as entomology, microbiology, plant biology, and plant–microbe interactions, the rhizobia symbiosis being the most exhaustively described so far. Those are the fields of interest on which we will focus to demonstrate MALDI MSI strengths in characterizing the spatial distributions of metabolites, lipids, and peptides in relation to biological questions.

4-Hydrazinoquinazoline acting as a reactive matrix for the rapid and sensitive analysis of neutral and sialylated glycans using MALDI MS

The direct analysis of glycans by matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) presents limited sensitivity due to the lower ionization efficiency of glycans. Various chemical derivatization methods have been developed to improve the detection sensitivity of glycans, but most of them need tedious preparation and cleanup procedures. Herein, a reactive matrix, 4-hydrazinoquinazoline (4-HQ), was developed for the rapid and sensitive detection of both neutral and sialylated glycans by MALDI MS. With 4-HQ as the reactive matrix, the detection limits of maltoheptaose and A3 glycan decreased 100-fold and 20-fold, respectively, compared with the conventional matrix. Moreover, 4-HQ formed homogeneous crystals and therefore showed good shot-to-shot reproducibility. Finally, the reactive matrix was successfully applied for the analysis of glycans released from glycoproteins and human serum. Importantly, the application of 4-HQ is the same as that of a conventional matrix with the additional advantage of on-target reaction at room temperature. Thus, 4-HQ can be used for the routine analysis of glycans by MALDI MS due to its simple use, great reproducibility, and enhanced detection of both neutral and sialylated glycans.

Material-assisted mass spectrometric analysis of low molecular weight compounds for biomedical applications

Low molecular weight compounds play an important role in encoding the current physiological state of an individual. Laser desorption/ionization mass spectrometry (LDI MS) offers high sensitivity with low cost for molecular detection, but it is not able to cover small molecules due to the drawbacks of the conventional matrix. Advanced materials are better alternatives, showing little background interference and high LDI efficiency. Herein, we first classify the current materials with a summary of compositions and structures. Matrix preparation protocols are then reviewed, to enhance the selectivity and reproducibility of MS data better. Finally, we highlight the biomedical applications of material-assisted LDI MS, at the tissue, bio-fluid, and cellular levels. We foresee that the advanced materials will bring far-reaching implications in LDI MS towards real-case applications, especially in clinical settings.

Recent advances in the application of isoindigo derivatives in materials chemistry

In this review, the data on the application of isoindigo derivatives in the chemistry of functional materials are analyzed and summarized. These bisheterocycles can be used in the creation of organic solar cells, sensors, lithium ion batteries as well as in OFET and OLED technologies. The potentials of the use of polymer structures based on isoindigo as photoactive component in the photoelectrochemical reduction of water, as matrix for MALDI spectrometry and in photothermal cancer therapy are also shown. Data published over the past 5 years, including works published at the beginning of 2021, are given.

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.

Design of Multi-Shelled Hollow Cr2 O3 Spheres for Metabolic Fingerprinting

Schizophrenia (SZ) detection enables effective treatment to improve the clinical outcome, but objective and reliable SZ diagnostics are still limited. An ideal diagnosis of SZ suited for robust clinical screening must address detection throughput, low invasiveness, and diagnosis accuracy. Herein, we built a multi-shelled hollow Cr₂ O₃ spheres (MHCSs) assisted laser desorption/ionization mass spectrometry (LDI MS) platform for the direct metabolic profiling of biofluids towards SZ diagnostics. The MHCSs displayed strong light absorption for enhanced ionization and microscale surface roughness with stability for the effective LDI of metabolites. We profiled urine and serum metabolites (≈1 μL) with the enhanced LDI efficacy in seconds. We discriminated SZ patients (SZs) from healthy controls (HCs) with the highest area under the curve (AUC) value of 1.000 for the blind test. We identified four compounds with optimal diagnostic power as a simplified metabolite panel for SZ and demonstrated the metabolite quantification for clinic use. Our approach accelerates the growth of new platforms toward a precision diagnosis in the near future.

MALDI Matrices for the Analysis of Low Molecular Weight Compounds: Rational Design, Challenges and Perspectives

The analysis of low molecular weight (LMW) compounds is of great interest to detect small pharmaceutical drugs rapidly and sensitively, or to trace and understand metabolic pathways. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) plays a central role in the analysis of high molecular weight (bio)molecules. However, its application for LMW compounds is restricted by spectral interferences in the low m/z region, which are produced by conventional organic matrices. Several strategies regarding sample preparation have been investigated to overcome this problem. A different rationale is centred on developing new matrices which not only meet the fundamental requirements of good absorption and high ionization efficiency, but are also vacuum stable and "MALDI silent", i. e., do not give matrix-related signals in the LMW area. This review gives an overview on the rational design strategies used to develop matrix systems for the analysis of LMW compounds, focusing on (i) the modification of well-known matrices, (ii) the search for high molecular weight matrices, (iii) the development of binary, hybrid and nanomaterial-based matrices, (iv) the advance of reactive matrices and (v) the progress made regarding matrices for negative or dual polarity mode.

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.

Interactive Estimation of Heterogeneity from Mass Spectrometry Imaging

In this work, we demonstrate a new approach for interactively assessing hyperspectral data spatial structures for heterogeneity using mass spectrometry imaging. This approach is based on the visualization of the cosine distance as the similarity levels between mass spectra of a chosen region and the rest of the image (sample). The applicability of the method is demonstrated on a set of mass spectrometry images of frontal mouse brain slices. Selection of the reference pixel of the mass spectrometric image and a further view of the corresponding cosine distance map helps to prepare supporting vectors for further analysis, select features, and carry out biological interpretation of different tissues in the mass spectrometry context with or without histological annotation. Visual inspection of the similarity maps reveals the spatial distribution of features in tissue samples, which can serve as the molecular histological annotation of a slide.

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.

Toward the Rational Design of Universal Dual Polarity Matrix for MALDI Mass Spectrometry

A series of novel anthranilic acid derivatives I-IV, of which COOH-NH₂ (I) and COOH-NHMe (IV) are endowed with acid and base bifunctionality, were designed and synthesized for matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry applications in dual polarity molecular imaging of biological samples, particularly for lipids. The heat of protonation, deprotonation, and proton transfer reaction as well as the capability of analyzing biomolecules in both positive and negative ion modes for I-IV were systematically investigated under standard 355 nm laser excitation. The results indicate correlation between dual polarity and acid-base property. Further, COOH-NHMe (IV) showed a unique performance and was successfully applied as the matrix for MALDI-TOF mass spectrometry imaging (MSI) for studying the mouse brain. Our results demonstrate the superiority of COOH-NHMe (IV) in detecting more lipid and protein species compared to commercially available matrices. Moreover, MALDI-TOF MSI results were obtained for lipid distributions, making COOH-NHMe (IV) a potential next generation universal matrix.

On-Tissue Chemical Derivatization of Catecholamines Using 4-( N-Methyl)pyridinium Boronic Acid for ToF-SIMS and LDI-ToF Mass Spectrometry Imaging

The analysis of small polar compounds with ToF-SIMS and MALDI-ToF-MS have been generally hindered by low detection sensitivity, poor ionization efficiency, ion suppression, analyte in-source fragmentation, and background spectral interferences from either a MALDI matrix and/or endogenous tissue components. Chemical derivatization has been a well-established strategy for improved mass spectrometric detection of many small molecular weight endogenous compounds in tissues. Here, we present a devised strategy to selectively derivatize and sensitively detect catecholamines with both secondary ion ejection and laser desorption ionization strategies, which are used in many imaging mass spectrometry (IMS) experiments. Chemical derivatization of catecholamines was performed by a reaction with a synthesized permanent pyridinium-cation-containing boronic acid molecule, 4-( N-methyl)pyridinium boronic acid, through boronate ester formation (boronic acid-diol reaction). The derivatization facilitates their sensitive detection with ToF-SIMS and LDI-ToF mass spectrometric techniques. 4-( N-Methyl)pyridinium boronic acid worked as a reactive matrix for catecholamines with LDI and improved the sensitivity of detection for both SIMS and LDI, while the isotopic abundances of the boron atom reflect a unique isotopic pattern for derivatized catecholamines in MS analysis. Finally, the devised strategy was applied, as a proof of concept, for on-tissue chemical derivatization and GCIB-ToF-SIMS (down to 3 μm per pixel spatial resolution) and LDI-ToF mass spectrometry imaging of dopamine, epinephrine, and norepinephrine in porcine adrenal gland tissue sections. MS/MS using collision-induced dissociation (CID)-ToF-ToF-SIMS was subsequently employed on the same tissue sections after SIMS and LDI mass spectrometry imaging experiments, which provided tandem MS information for the validation of the derivatized catecholamines in situ. This methodology can be a powerful approach for the selective and sensitive ionization/detection and spatial localization of diol-containing molecules such as aminols, vic-diols, saccharides, and glycans along with catecholamines in tissue sections with both SIMS and LDI/MALDI-MS techniques.