Matrix-free LDI mass spectrometry platform using patterned nanostructured gold thin film

Laser desorption/ionization mass spectrometry of L-thyroxine (T4) using combi-matrix of α-cyano-4-hydroxycinnamic acid (CHCA) and graphene

An optimal combi-matrix for MALDI-TOF mass spectrometry was presented for the analysis of L-thyroxine (T4) in human serum. For the selection of the optimal combi-matrix, several kinds of combi-matrices were prepared by mixing the conventional organic matrix of CHCA with nanomaterials, such as graphene, carbon nanotubes, nanoparticles of Pt and TiO2. In order to select the optimal combi-matrix, the absorption at the wavelength of laser radiation (337 nm) for the ionization of sample was estimated using UV–Vis spectrometry. And, the heat absorption properties of these combi-matrices were also analyzed using differential scanning calorimetry (DSC), such as onset temperature and fusion enthalpy. In the case of the combi-matrix of CHCA and graphene, the onset temperature and fusion enthalpy were observed to be lower than those of CHCA, which represented the enhanced transfer of heat to the analyte in comparison with CHCA. From the analysis of optical and thermal properties, the combi-matrix of CHCA and graphene was selected to be an optimal combination for the transfer of laser energy during MALDI-TOF mass spectrometry. The feasibility of the combi-matrix composed of CHCA and graphene was demonstrated for the analysis of T4 molecules using MALDI-TOF mass spectrometry. The combi-matrix of CHCA and graphene was estimated to have an improved limit of detection and a wider detection range in comparison with other kinds of combi-matrices. Finally, the MALDI-TOF MS results of T4 analysis using combi-matrix were statistically compared with those of the conventional immunoassay.

Covalent Organic Framework Nanofilm-Based Laser Desorption/Ionization Mass Spectrometry for 5-Fluorouracil Analysis and Tissue Imaging

Although matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has become a ubiquitous and effective tool for macromolecules, direct analysis of small molecules by MALDI-MS using conventional organic matrices poses a challenge. Herein, a large-area, uniform, and stable covalent organic framework (COF) nanofilm prepared directly on indium-tin oxide (ITO) glass was first introduced as a substrate for LDI-MS, which showed enhanced sensitivity, no background interference, and high reproducibility in the analysis of diverse small molecules. Taking into account all these merits, an attractive approach of COF nanofilm-based LDI-MS was developed to quantitatively evaluate the pharmacokinetics of 5-fluorouracil (5-FU) in mouse plasma. A good linear relationship (10-20,000 ng/mL) and a low limit of detection (LOD) for 5-FU (∼100 pg/mL) were achieved. In view of the fact that the COF nanofilm was uniform and without the requirement of additional matrix spraying, it was further extended for LDI-MS imaging (LDI-MSI) to visualize the spatial distribution of 5-FU in mouse liver at different interval times after intravenous and intragastric administrations. The results indicated that the decay of 5-FU in mouse liver obtained with the COF nanofilm-based LDI-MSI was consistent with the tendency of 5-FU pharmacokinetics. This work not only offers an alternative solution for LDI-MS/MSI analysis of small molecules but also extends the application fields of COF nanofilm in MS research.

Mass Spectrometry Imaging for Glycome in the Brain

Glycans are diverse structured biomolecules that play crucial roles in various biological processes. Glycosylation, an enzymatic system through which various glycans are bound to proteins and lipids, is the most common and functionally crucial post-translational modification process. It is known to be associated with brain development, signal transduction, molecular trafficking, neurodegenerative disorders, psychopathologies, and brain cancers. Glycans in glycoproteins and glycolipids expressed in brain cells are involved in neuronal development, biological processes, and central nervous system maintenance. The composition and expression of glycans are known to change during those physiological processes. Therefore, imaging of glycans and the glycoconjugates in the brain regions has become a “hot” topic nowadays. Imaging techniques using lectins, antibodies, and chemical reporters are traditionally used for glycan detection. However, those techniques offer limited glycome detection. Mass spectrometry imaging (MSI) is an evolving field that combines mass spectrometry with histology allowing spatial and label-free visualization of molecules in the brain. In the last decades, several studies have employed MSI for glycome imaging in brain tissues. The current state of MSI uses on-tissue enzymatic digestion or chemical reaction to facilitate successful glycome imaging. Here, we reviewed the available literature that applied MSI techniques for glycome visualization and characterization in the brain. We also described the general methodologies for glycome MSI and discussed its potential use in the three-dimensional MSI in the brain.

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.

Application of Graphdiyne in Surface-Assisted Laser Desorption Ionization Mass Spectrometry

Graphdiyne (GD) is a new kind of carbon nanomaterial which has carbon-carbon triple bonds to form a layered structure. Here, we report the application of GD as the matrix for small molecule analysis in laser desorption ionization mass spectrometry (LDI MS). The GD matrix displayed two advantages: little background in the low mass range and good molecular ion signal in negative ion mode for many small molecules, e.g., fatty acids, amino acids, peptides, and drugs can be obtained in negative ion mode. By comparing the signal intensity of tetraphenylborate and juglone with and without GD existing, it was found that GD can enhance both of the desorption efficiency and ionization efficiency in LDI. Through analysis of the serum samples from liver cancer patients and healthy people, the GD-assisted LDI MS results showed that fatty acids could be used as potential biomarkers for the early diagnosis of liver cancer.

Influence of buried oxide layers of nanostructured SOI surfaces on matrix-free LDI-MS performances

In this paper, we report on the nanostructuration of the silicon crystalline top layer of different "home-made" SOI substrates presenting various buried oxide (BOx) layer thicknesses. The nanostructuration was achieved via a one-step metal assisted chemical etching (MACE) procedure. The etched N-SOI substrate surfaces were then characterized by AFM, SEM and photoluminescence. To investigate their laser desorption/ionization mass spectrometry performances, the different surfaces have been assessed towards peptide mixtures. We have shown that the matrix-free LDI process occurred from surface heating after laser irradiation and was fostered by thermal confinement in the thin nanostructured Si surface layer. This thermal confinement was enhanced with the increase of the buried oxide layer thickness until an optimal thickness of 200 nm for which the best results in terms of signal intensities, peptide discrimination and spot to spot and surface to surface variations were found.

Nanostructured TiO2 Materials for Analysis of Gout-Related Crystals Using Laser Desorption/Ionization Time-of-Flight (LDI-ToF) Mass Spectrometry

Crystals of monosodium urate monohydrate (MSU) and calcium pyrophosphate dihydrate (CPPD) are known to induce arthropathic diseases called gout and pseudogout, respectively. These crystals are deposited in various joints or tissues, causing severe pain. Correct identification of crystals is crucial for the appropriate treatment of gout and pseudogout, which exhibit very similar symptoms. Herein, a novel approach of laser desorption/ionization time-of-flight (LDI-ToF) mass spectrometry (MS) was introduced to analyze MSU and CPPD crystals with three different types of nanostructured TiO₂ materials including TiO₂ nanoparticles (P25), TiO₂ nanowires synthesized from wet-corrosion method, and the mixture of P25 and TiO₂ nanowires (P25/TiO₂ nanowires) as inorganic solid matrices. Furthermore, the feasibility of LDI-ToF MS based on these TiO₂ nanostructures for the analysis of the two arthropathy-related crystals was tested using spiked samples in synovial fluid at known crystal concentrations. The mass analysis results of MSU and CPPD crystals demonstrated that (1) the electrostatic interaction between analytes and solid matrices was key for the analyte ionization and (2) LDI-ToF MS with nanostructured TiO₂ materials has the potential to be a practical approach for the diagnosis of gout and pseudogout.

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

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

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

RATIONALE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Complementarity of Matrix- and Nanostructure-Assisted Laser Desorption/Ionization Approaches

In recent years, matrix-assisted laser desorption/ionization (MALDI) has become the main tool for the study of biological macromolecules, such as protein nano-machines, especially in the determination of their molecular masses, structure, and post-translational modifications. A key role in the classical process of desorption and ionization of the sample is played by a matrix, usually a low-molecular weight weak organic acid. Unfortunately, the interpretation of mass spectra in the mass range of below m/z 500 is difficult, and hence the analysis of low molecular weight compounds in a matrix-assisted system is an analytical challenge. Replacing the classical matrix with nanomaterials, e.g., silver nanoparticles, allows improvement of the selectivity and sensitivity of spectrometric measurement of biologically important small molecules. Nowadays, the nanostructure-assisted laser desorption/ionization (NALDI) approach complements the classic MALDI in the field of modern bioanalytics. In particular, the aim of this work is to review the recent advances in MALDI and NALDI approaches.

Laser desorption ionization mass spectrometry: Recent progress in matrix-free and label-assisted techniques

The MALDI-based mass spectrometry, over the last three decades, has become an important analytical tool. It is a gentle ionization technique, usually applicable to detect and characterize analytes with high molecular weights like proteins and other macromolecules. The earlier difficulty of detection of analytes with low molecular weights like small organic molecules and metal ion complexes with this technique arose due to the cluster of peaks in the low molecular weight region generated from the matrix. To detect such molecules and metal ion complexes, a four-prong strategy has been developed. These include use of alternate matrix materials, employment of new surface materials that require no matrix, use of metabolites that directly absorb the laser light, and the laser-absorbing label-assisted LDI-MS (popularly known as LALDI-MS). This review will highlight the developments with all these strategies with a special emphasis on LALDI-MS.

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

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

Morphology-Driven Control of Metabolite Selectivity Using Nanostructure-Initiator Mass Spectrometry

Nanostructure-initiator mass spectrometry (NIMS) is a laser desorption/ionization analysis technique based on the vaporization of a nanostructure-trapped liquid "initiator" phase. Here we report an intriguing relationship between NIMS surface morphology and analyte selectivity. Scanning electron microscopy and spectroscopic ellipsometry were used to characterize the surface morphologies of a series of NIMS substrates generated by anodic electrochemical etching. Mass spectrometry imaging was applied to compare NIMS sensitivity of these various surfaces toward the analysis of diverse analytes. The porosity of NIMS surfaces was found to increase linearly with etching time where the pore size ranged from 4 to 12 nm with corresponding porosities estimated to be 7-70%. Surface morphology was found to significantly and selectively alter NIMS sensitivity. The small molecule (<2k Da) sensitivity was found to increase with increased porosity, whereas low porosity had the highest sensitivity for the largest molecules examined. Estimation of molecular sizes showed that this transition occurs when the pore size is <3× the maximum of molecular dimensions. While the origins of selectivity are unclear, increased signal from small molecules with increased surface area is consistent with a surface area restructuring-driven desorption/ionization process where signal intensity increases with porosity. In contrast, large molecules show highest signal for the low-porosity and small-pore-size surfaces. We attribute this to strong interactions between the initiator-coated pore structures and large molecules that hinder desorption/ionization by trapping large molecules. This finding may enable us to design NIMS surfaces with increased specificity to molecules of interest.

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

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

"RaMassays": Synergistic Enhancement of Plasmon-Free Raman Scattering and Mass Spectrometry for Multimodal Analysis of Small Molecules

SiO₂/TiO₂ core/shell (T-rex) beads were exploited as "all-in-one" building-block materials to create analytical assays that combine plasmon-free surface enhanced Raman scattering (SERS) and surface assisted laser desorption/ionization (SALDI) mass spectrometry (RaMassays). Such a multi-modal approach relies on the unique optical properties of T-rex beads, which are able to harvest and manage light in both UV and Vis range, making ionization and Raman scattering more efficient. RaMassays were successfully applied to the detection of small (molecular weight, M.W. <400 Da) molecules with a key relevance in biochemistry and pharmaceutical analysis. Caffeine and cocaine were utilized as molecular probes to test the combined SERS/SALDI response of RaMassays, showing excellent sensitivity and reproducibility. The differentiation between amphetamine/ephedrine and theophylline/theobromine couples demonstrated the synergistic reciprocal reinforcement of SERS and SALDI. Finally, the conversion of L-tyrosine in L-DOPA was utilized to probe RaMassays as analytical tools for characterizing reaction intermediates without introducing any spurious effects. RaMassays exhibit important advantages over plasmonic nanoparticles in terms of reproducibility, absence of interference and potential integration in multiplexed devices.

Calcinated gold nanoparticle arrays for on-chip, multiplexed and matrix-free mass spectrometric analysis of peptides and small molecules

A patterned gold nanoparticle microarray, functionalized with a nanoscale silicate coating, has been developed for on-chip, high-throughput mass spectrometric analyses of biomolecules with minimal sample preparation and reagent costs. Fabrication was realized by the combination of layer-by-layer functionalization of the nanoparticles with suitable polyelectrolytes, followed by fluidic patterning of the glass microarray support and calcination for permanent fixation of the nano-coating. Performance of the microarray was evaluated for surface-assisted laser-desorption/ionization mass spectrometry (SALDI-MS), where the nano-silicate coating was found to enhance SALDI efficiency, resulting in comparable performance to some common organic matrices for small and medium sized molecules. Performance contributing factors of this material have been discussed; heat confinement and interband transition/plasmonic resonance may play important roles. Taking the accessibility of fabrication, performance, and reusability of this substrate together, the material developed here provides a new tool for multiplexed and chip-based mass spectrometric analysis.

Simultaneous and direct analysis of multiple types of organic contaminants in water based on a MOF decorated with a suitable quantity of Au nanoparticles, using SALDI-TOF MS

Simultaneous, fast and sensitive analysis of multiple types of organic contaminants using SALDI-TOF MS was realized for the first time.

Matrix-enhanced surface-assisted laser desorption/ionization mass spectrometry (ME-SALDI-MS) for mass spectrometry imaging of small molecules

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS), a parallel technique to matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), utilizes inorganic particles or porous surfaces to aid in the desorption/ionization of low-molecular-weight (MW) analytes. As a matrix-free and "soft" LDI approach, SALDI offers the benefit of reduced background noise in the low MW range, allowing for easier detection of biologically significant small MW species. Despite the inherent advantages of SALDI-MS, it has not reached comparable sensitivity levels to MALDI-MS. In relation to mass spectrometry imaging (MSI), intense efforts have been made in order to improve sensitivity and versatility of SALDI-MSI. We describe herein a detailed protocol that utilizes a hybrid LDI method, matrix-enhanced SALDI-MS (ME-SALDI MS), to detect and image low MW species in an imaging mode.

Nanoglassified, optically-active monolayer films of gold nanoparticles for in situ orthogonal detection by localized surface plasmon resonance and surface-assisted laser desorption/ionization-MS

Localized surface plasmon resonance (LSPR) represents a sensitive and versatile method for detection of biomolecules in a label-free fashion, but identification of bound analytes can be challenging with LSPR alone, especially for samples in a complex medium. We report the fabrication of an optically active, plasmonic film of gold nanoparticles by using a self-assembly and calcination process, which offers orthogonal measurements enabling multifaceted characterization on the same surface with LSPR and surface-assisted laser desorption/ionization mass spectrometry. This proof-of-concept study involves plasmonic characterization of the fabricated nanofilm, real-time monitoring of vesicle-surface interactions toward formation of fluid lipid bilayer, and mass spectrometric analysis of peptides and cytochrome c digest. This multifunction-enabling surface material can yield complementary analytical information, providing new tools for comprehensive analysis of biomolecular samples.

The role of physical and chemical properties of Pd nanostructured materials immobilized on inorganic carriers on ion formation in atmospheric pressure laser desorption/ionization mass spectrometry

Fundamental parameters influencing the ion-producing efficiency of palladium nanostructures (nanoparticles [Pd-NP], nanoflowers, nanofilms) during laser irradiation were studied in this paper. The nanostructures were immobilized on the surface of different solid inorganic carrier materials (porous and mono-crystalline silicon, anodic porous aluminum oxide, glass and polished steel) by using classical galvanic deposition, electroless local deposition and sputtering. It was the goal of this study to investigate the influence of both the nanoparticular layer as well as the carrier material on ion production for selected analyte molecules. Our experiments demonstrated that the dimensions of the synthesized nanostructures, the thickness of the active layers, surface disorders, thermal conductivity and physically or chemically adsorbed water influenced signal intensities of analyte ions during surface-assisted laser desorption/ionization (SALDI) while no effects such as plasmon resonance, photoelectric effect or catalytic activity were expected to occur. Excellent LDI abilities were seen for Pd-NPs immobilized on steel, while Pd nanoflowers on porous silicon exhibited several disadvantages; viz, strong memory effects, dependency of the analytical signal on amount of physically and chemically adsorbed water inside porous carrier, reduced SALDI activity from unstable connections between Pd and semiconductor material, decrease of the melting point of pure silicon after Pd immobilization and resulting strong laser ablation of metal/semiconductor complex, as well as significantly changed surface morphology after laser irradiation. The analytical performance of Pd-NP/steel was further improved by applying a hydrophobic coating to the steel surface before galvanic deposition. This procedure increased the distance between Pd-NPs, thus reducing thermal stress upon LDI; it simultaneously decreased spot sizes of deposited sample solutions.

Matrix-assisted laser desorption/ionization, nanostructure-assisted laser desorption/ionization and carbon nanohorns in the detection of antineoplastic drugs. 1. The cases of irinotecan, sunitinib and 6-alpha-hydroxy paclitaxel

The development of surface-assisted laser desorption/ionization (SALDI) methodologies in mass spectrometry allows, in principle, the development of new analytical approaches to qualitative and quantitative measurements on small molecules. Some of these methods have been applied to characterize two antineoplastic drugs: irinotecan (1) and sunitinib (2), and also 6-α-hydroxy-paclitaxel (3), the main metabolite of paclitaxel. Three different SALDI approaches have been tested employing nanostructure- assisted laser desorption/ionization (NALDI), carbon nanohorns (NHs) and carbon nanohorns covered by liquid additives. The results so obtained have been compared to those observed under matrix-assisted laser desorption/ionization (MALDI) conditions. Compounds 1 and 2 show the easy formation of protonated molecular species under all the experimental conditions, but the highest absolute intensity was achieved by NALDI. On the contrary, ionic species of low intensity are present for 3, among which are those that exhibit the highest intensity caused by [M+K](+) ions. After a critical evaluation of the obtained data, the linear response of the [M+H](+) ion intensity of 1 versus different deposited sample amounts was investigated, and the best results (R(2) = 0.9889) were obtained under MALDI conditions. The analysis of plasma samples spiked with 1 showed, again, that the MALDI approach was the best one (R(2) = 0.9766). The failure of NALDI measurements could be rationalized by the presence of ion suppression effects.

Biomimetic antireflective silicon nanocones array for small molecules analysis

Biomimetic antireflective silicon nanocones array is used for analysis of small molecules by mass spectrometry. The role of the absorbed laser energy and its distribution in the laser desorption/ionization process has been investigated by varying the antireflective features precisely. By optimizing the antireflective silicon array, the absorbed laser energy can be channeled completely into the desorption/ionization of analytes. The optimized silicon array exhibits excellent performance to detect peptide, amino acid, drug molecule, and carbohydrate without any interference in the low-mass region.

Commercial silicon-on-insulator (SOI) wafers as a versatile substrate for laser desorption/ionization mass spectrometry

We report here that a commercial silicon-on-insulator (SOI) wafer offers an opportunity for laser desorption/ionization (LDI) of peptide molecules, which occurs directly from its flat surface without requiring special surface preparation. The LDI-on-SOI exhibits intact ionization of peptides with a good detection limit of lower than 20 fmol, of which the mass range is demonstrated up to insulin with citric acid additives. The LDI process most likely arises from laser-induced surface heating promoted by two-dimensional thermal confinement in the thin Si surface layer of the SOI wafer. As a consequence of the thermal process, the LDI-on-SOI method is also capable of creating post-source decay (PSD) of the resulting peptide LDI ions, which is suitable for peptide sequencing using conventional TOF/TOF mass spectrometry.

Pulsed-laser desorption/ionization of clusters from biofunctional gold nanoparticles: implications for protein detections

In this paper, we describe a pulsed-laser desorption/ionization mass spectrometry (LDI-MS) approach for the detection of proteins with femtomolar sensitivity through the analysis of gold (Au) clusters desorbed from aptamer-modified gold nanoparticles (Apt-AuNPs) on a nitrocellulose membrane (NCM). After the target protein (thrombin) was selectively captured by the surface-bound 29-mer thrombin-binding aptamer (TBA(29)), the thrombin/TBA(29)-AuNP complexes were concentrated and deposited onto the NCM to form a highly efficient background-free surface-assisted LDI substrate. Under pulsed laser irradiation (355 nm), the binding of thrombin decreased the desorption and/or ionization efficiencies of the Au atoms from the AuNP surfaces. The resulting decreases in the intensities of the signals for Au clusters in the mass spectra provided a highly amplified target-labeling indicator for the targeted protein. Under optimized conditions, this probe was highly sensitive (limit of detection: ca. 50 fM) and selective (by at least 1000-fold over other proteins) toward thrombin; it also improved reproducibility (<5%) of ion production by presenting a more-homogeneous substrate surface, thereby enabling LDI-based measurements for the accurate and precise quantification of thrombin in human serum. This novel LDI-MS approach allows high-speed analyses of low-abundance thrombin with ultrahigh sensitivity; decorating the AuNP surfaces with other aptamers also allowed amplification of other biological signals.

Free and immobilized matrix molecules: impairing ionization by quenching secondary ion formation in laser desorption MS

Within the last 25 years, matrix-assisted laser desorption ionization (MALDI) has become a powerful analytical tool in mass spectrometry (MS). While the method has been successfully applied to characterize large organic molecules such as proteins, sugars and polymers, its utilization for small molecules (≤ 600 Da) is significantly impaired by the coformation of matrix ions. Reducing or eliminating matrix-related signals has been subject of many studies. Some of which propose the enhancement of so-called matrix suppression effects, while others suggest the replacement of matrix molecules by materials such as microporous silicon. Alternatively, the immobilization of matrix molecules by utilizing them as self-assembled monolayers (SAMs) has been discussed. In continuation of this research, the current manuscript focuses on the elucidation of ion formation processes occurring on the surface of light absorbing SAMs. Ion yields obtained by free and immobilized matrix molecules as well as those generated by matrix-free gold film-assisted laser desorption ionization (GF-LDI) were compared. Experiments showed that the formation of strong analyte signals essentially required the presence of free matrix molecules, while the immobilization of the latter severely impaired ionization. The observed effect inversely correlated with the surface coverage of SAMs determined by cyclic voltammetry (CV). Based on these findings, the MS signal generated on light absorbing SAMs could be used supplementary to CV for determining the surface coverage of light absorbing SAMs.