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

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

Utilizing AgNPt-SALDI to Classify Edible Oils by Multivariate Statistics of Triacylglycerol Profile

Edible oils are valuable sources of nutrients, and their classification is necessary to ensure high quality, which is essential to food safety. This study reports the establishment of a rapid and straightforward SALDI-TOF MS platform used to detect triacylglycerol (TAG) in various edible oils. Silver nanoplates (AgNPts) were used to optimize the SALDI samples for high sensitivity and reproducibility of TAG signals. TAG fingerprints were combined with multivariate statistics to identify the critical features of edible oil discrimination. Eleven various edible oils were discriminated using principal component analysis (PCA). The results suggested the creation of a robust platform that can examine food adulteration and food fraud, potentially ensuring high-quality foods and agricultural products.

Analysis of Synthetic Monodisperse Polysaccharides by Wide Mass Range Ultrahigh-Resolution MALDI Mass Spectrometry

Carbohydrates, such as oligo- and polysaccharides, are highly abundant biopolymers that are involved in numerous processes. The study of their structure and functions is commonly based on a material that is isolated from complex natural sources. However, a more precise analysis requires pure compounds with well-defined structures that can be obtained from chemical or enzymatic syntheses. Novel synthetic strategies have increased the accessibility of larger monodisperse polysaccharides, posing a challenge to the analytical methods used for their molecular characterization. Here, we present wide mass range ultrahigh-resolution matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) as a powerful platform for the analysis of synthetic oligo- and polysaccharides. Synthetic carbohydrates 16-, 64-, 100-, and 151-mers were mass analyzed and characterized by MALDI in-source decay FT-ICR MS. Detection of fragment ions generated from glycosidic bond cleavage (or cross-ring cleavage) provided information of the monosaccharide content and the linkage type, allowing for the corroboration of the carbohydrate compositions and structures.

Ratiometric and sensitive cyanide sensing using dual-emissive gold nanoclusters

The detection of cyanide anion (CN^-), a highly toxic pollutant, has attracted growing attention in the past years. In this work, a nanosensor composed of hyperbranched polyethyleneimine (hPEI)-assisted dual-emissive gold nanoclusters (DE-Au NCs) is proposed for ratiometric detection of CN^- based on surface valence state-driving etch. The ratiometric color change of fluorescence is based on a fact that the red-emissive Au NCs with a high content of surface Au(I) can be easily etched by CN^-, while the blue-emissive Au NCs with nearly neutral character can resist CN^-. Because of the specific gold-CN^- chemistry and electrostatic attraction between the positively charged hPEI protecting layer and the negatively charged CN^-, the DE-Au NC-based nanosensor provides high selectivity toward CN^- over other anions with a limit of detection of 10 nM. Practical application of the proposed DE-Au NC nanosensor is verified by satisfying recoveries of CN^- determination in river water and urine samples. Graphical abstract.

New Strategy to Preserve Phosphate by Ionic Liquid Matrices in Matrix-Assisted Laser Desorption/Ionization: A Case of Adenosine Nucleotides

Adenosine -5′-triphosphate (ATP) plays a valuable role in metabolic activity to produce adequate energy in a biosystem. A high ATP/AMP ratio has a correlation with diabetes that induces suppression of AMP-activated protein kinase (AMPK). Matrix-assisted laser desorption/ionization (MALDI)–mass spectrometry (MS) has outstanding potential in determining the ratio of several types of adenosine phosphates in a sample to rapidly understand the primary energy transfer in metabolism. Although MALDI is viewed as a soft ionization technique for MS analysis, excess photon energy might crack the phosphate bonds leading to misinterpretation of the ATP level. In this work, ionic liquid matrices (ILMs) were employed to reduce fragmentation and increase the detection efficiency during the MALDI process. This study demonstrated for the first time that 2,5-dihydroxybenzoic acid pyridine (DHBP) is one of the most effective matrices for further quantitative analysis of adenosine nucleotides. This systematic screening of ILMs also enhances the fundamental understanding of MALDI.

Soft Matrix-Assisted Laser Desorption/Ionization for Labile Glycoconjugates

Since its introduction, matrix-assisted laser desorption/ionization (MALDI) has been widely used for the mass analysis of biomolecules. The "soft ionization" of MALDI enables accurate mass determination of intact biomolecules. However, the ionization and desorption processes of MALDI are not adequately soft as many labile biomolecules, such as glycoconjugates containing sialic acid or the sulfate functional group, easily dissociate into fragments and sometimes, no intact molecules are observed. In this study, we compared the conventional matrix of MALDI, namely 2,5-dihydroxybenzoic acid, to various soft matrices of MALDI-specifically, 5-methoxysalicylic acid, diamond nanoparticle trilayers, HgTe nanostructures, ionic liquid, and droplets of frozen solutions-by using three labile glycoconjugates as analytes: gangliosides, heparin, and pullulan. We demonstrated that droplets of frozen solution are the softest matrices for gangliosides and heparin. In particular, droplets of frozen solution do not generate fragments for gangliosides and can be used to determine the relative abundance of various gangliosides, whereas ionic liquid 2,5-dihydroxybenzoic acid butylamine is the most suitable matrix for pullulan mass analysis. Graphical Abstract.

A flexible and multifunctional metal–organic framework as a matrix for analysis of small molecules using laser desorption/ionization mass spectrometry

Fast screening of multi-functional MOFs as SALDI-MS matrices for the detection of small molecules and simultaneous enrichment and detection of analytes.

Functionalized HgTe nanoparticles promote laser-induced solid phase ionization/dissociation for comprehensive glycan sequencing

Glycoconjugates are ubiquitously present and play a critical role in various biological processes. Due to their low stability and incredibly high degree of structural diversity, the structural characterization of glycan generally requires chemical derivatization and sophisticated instrumentation. Herein, we report a method for complicated glycan characterization in a single assay by employing 2,5-dihydroxybenzoic acid functionalized mercury telluride nanoparticles (HgTe@DHB NPs) as a dual ionization-dissociation element in matrix-assisted laser desorption/ionization mass spectrometry. Using a linear glycan, HgTe@DHB NPs promote laser-induced extensive and intense dissociation of the glycan, superior to HgTe microparticles and other inorganic nanoparticles (TiO₂, ZnO, and Mn₂O₃ NPs). Abundant generation of diagnostic glycosidic (Y-, and B-type ions) and cross-ring cleavage (A-type ions) ions permits unambiguous determination of the composition, sequence, branching, and linkage of labile sialylated glycans. The general utility of this approach was demonstrated by the characterization of labile sialylated glycans and two sets of complicated isomeric glycans. This phenomenon was delineated further by investigating the NP's physico-chemical characteristics, revealing that their nanoscale-dependent thermodynamic properties, including UV absorption, photoelectron release dynamics and thermal energy, were the key to levitate temperature synergistically, thus inducing spontaneous glycan decomposition during the nanoparticle-assisted laser desorption-ionization process. Our results show that this "pseudo-MS/MS" obtained by HgTe@DHB can be beneficial for the analysis of biologically relevant and more complicated carbohydrates, without the need for chemical pre-derivatization and conventional tandem mass spectrometry.

CeO2-CB nanocomposite as a novel SALDI substrate for enhancing the detection sensitivity of pharmaceutical drug molecules in beverage samples

SALDI-MS analysis of pharmaceutical drug molecules (amitriptyline, imipramine and promazine) using carbon-based substrates, namely, activated charcoal (AC), carbon nanotubes (CNTs), carbon black (CB), graphene (rGO), graphene oxide (GO) and graphite, was explored and compared with the conventional organic matrix of MALDI. CB exhibited superior performance with respect to the other substrates in terms of detection sensitivity. Despite the effectiveness of CB to detect all drug molecules, it demonstrated a number of background signals, which may be an issue for the analysis of other molecules in the future. Therefore, for the first time, a CeO₂-CB nanocomposite was synthesized and applied as a novel SALDI substrate to minimize the background signals and stabilize CB when exposed to high laser power. The nanocomposite was characterized using XRD, TEM, FTIR, UV-Vis and N₂ sorpometry. The spectrum obtained using the novel nanocomposite in the absence of the drug molecules showed minimal background signals compared to CB. Additionally, the CeO₂-CB nanocomposite enhanced the detection sensitivity of the drug molecules with a limit of detection (LOD) of 100 ng/mL. This active substrate nanocomposite was further applied for the analysis of drug-spiked beverages without sample pretreatment or extraction, mimicking cases encountered by forensic toxicologists. All of the drugs and/or their adducts were detected in the drug-spiked beverage samples.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

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

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

Revisiting the quantitative features of surface-assisted laser desorption/ionization mass spectrometric analysis

Surface-assisted laser desorption/ionization (SALDI) coupled with mass spectrometry (MS) is frequently used to analyse small organics owing to its clean background. Inorganic materials can be used as energy absorbers and the transfer medium to facilitate the desorption/ionization of analytes; thus, they are used as SALDI-assisting materials. Many studies have demonstrated the usefulness of SALDI-MS in quantitative analysis of small organics. However, some characteristics occurring in SALDI-MS require certain attention to ensure the reliability of the quantitative analysis results. The appearance of a coffee-ring effect in SALDI sample preparation is the primary factor that can affect quantitative SALDI-MS analysis results. However, to the best of our knowledge, there are no reports relating to quantitative SALDI-MS analysis that discuss or consider this effect. In this study, the coffee-ring effect is discussed using nanoparticles and nanostructured substrates as SALDI-assisting materials to show how this effect influences SALDI-MS analysis results. Potential solutions for overcoming the existing problems are also suggested.This article is part of the themed issue 'Quantitative mass spectrometry'.

Quantum dots assisted laser desorption/ionization mass spectrometric detection of carbohydrates: qualitative and quantitative analysis

A quantum dots (QDs) assisted laser desorption/ionization mass spectrometric (QDA-LDI-MS) strategy was proposed for qualitative and quantitative analysis of a series of carbohydrates. The adsorption of carbohydrates on the modified surface of different QDs as the matrices depended mainly on the formation of hydrogen bonding, which led to higher MS intensity than those with conventional organic matrix. The effects of QDs concentration and sample preparation method were explored for improving the selective ionization process and the detection sensitivity. The proposed approach offered a new dimension to the application of QDs as matrices for MALDI-MS research of carbohydrates. It could be used for quantitative measurement of glucose concentration in human serum with good performance. The QDs served as a matrix showed the advantages of low background, higher sensitivity, convenient sample preparation and excellent stability under vacuum. The QDs assisted LDI-MS approach has promising application to the analysis of carbohydrates in complex biological samples.

Simultaneous detection of phosphatidylcholines and glycerolipids using matrix-enhanced surface-assisted laser desorption/ionization-mass spectrometry with sputter-deposited platinum film

Matrix-assisted laser desorption/ionisation (MALDI) imaging mass spectrometry (IMS) allows for the simultaneous detection and imaging of several molecules in brain tissue. However, the detection of glycerolipids such as diacylglycerol (DAG) and triacylglycerol (TAG) in brain tissues is hindered in MALDI-IMS because of the ion suppression effect from excessive ion yields of phosphatidylcholine (PC). In this study, we describe an approach that employs a homogeneously deposited metal nanoparticle layer (or film) for the detection of glycerolipids in rat brain tissue sections using IMS. Surface-assisted laser desorption/ionisation IMS with sputter-deposited Pt film (Pt-SALDI-IMS) for lipid analysis was performed as a solvent-free and organic matrix-free method. Pt-SALDI produced a homogenous layer of nanoparticles over the surface of the rat brain tissue section. Highly selective detection of lipids was possible by MALDI-IMS and Pt-SALDI-IMS; MALDI-IMS detected the dominant ion peak of PC in the tissue section, and there were no ion peaks representing glycerolipids such as DAG and TAG. In contrast, Pt-SALDI-IMS allowed the detection of these glycerolipids, but not PC. Therefore, using a hybrid method combining MALDI and Pt-SALDI (i.e., matrix-enhanced [ME]-Pt-SALDI-IMS), we achieved the simultaneous detection of PC, PE and DAG in rat brain tissue sections, and the sensitivity for the detection of these molecules was better than that of MALDI-IMS or Pt-SALDI alone. The present simple ME-Pt-SALDI approach for the simultaneous detection of PC and DAG using two matrices (sputter-deposited Pt film and DHB matrix) would be useful in imaging analyses of biological tissue sections.

Quantification of saccharides in honey samples through surface-assisted laser desorption/ionization mass spectrometry using HgTe nanostructures

Quantification of monosaccharides and disaccharides in five honey samples through surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) using HgTe nanostructures as the matrix and sucralose as an internal standard has been demonstrated. Under optimal conditions (1× HgTe nanostructure, 0.2 mM ammonium citrate at pH 9.0), the SALDI-MS approach allows detection of fructose and maltose at the concentrations down to 15 and 10 μM, respectively. Without conducting tedious sample pretreatment and separation, the SALDI-MS approach allows determination of the contents of monosaccharides and disaccharides in honey samples within 30 min, with reproducibility (relative standard deviation <15%). Unlike only sodium adducts of standard saccharides detected, sodium adducts and potassium adducts with differential amounts have been found among various samples, showing different amounts of sodium and potassium ions in the honey samples. The SALDI-MS data reveal that the contents of monosaccharides and disaccharides in various honey samples are dependent on their nectar sources. In addition to the abundant amounts of monosaccharides and disaccharides, oligosaccharides in m/z range of 650 - 2700 are only detected in pomelo honey. Having advantages of simplicity, rapidity, and reproducibility, this SALDI-MS holds great potential for the analysis of honey samples.

Using surface-assisted laser desorption/ionization mass spectrometry to detect ss- and ds-oligodeoxynucleotides

We applied surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) with HgTe nanostructures as the matrix for the detection of single- and double-stranded oligodeoxynucleotides (ss-ODNs and ds-ODNs). The concentrations of surfactant and additives (metal ions, an amine) and the pH and ionic strength of the sample matrix played significantly different roles in the detection of ss- and ds-ODNs with various sequences. In the presence of Brij 76 (1.5 %), Hg(2+) (7.5 μM), and cadaverine (10 μM) at pH 5.0, this SALDI-MS approach allowed the simultaneous detection of T15, T20, T33, and T40, with limits of detection at the femtomole-to-picomole level and sample-to-sample intensity variation <23 %. In the presence of Ag(+) (1 μM) and cadaverine (10 μM) at pH 7.0, this technique allowed the detection of randomly sequenced ss- and ds-ODNs at concentrations down to the femtomole level. To the best of our knowledge, this paper is the first to report the detection of ss-ODNs (up to 50-mer) and ds-ODNs (up to 30 base pairs) through the combination of SALDI-MS with HgTe nanostructures as matrices. We demonstrated the practicality of this approach through analysis of a single nucleotide polymorphism that determines the fate of the valine residue in the β-globin of sickle cell megaloblasts.