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

Collective dynamics of Ca atoms encapsulated in C60 endohedral fullerenes

Endohedral C60 fullerenes with up to four encapsulated Ca atoms were investigated by ab initio molecular dynamics simulations (AIMD). The relatively long runs allow us to describe the correlated movement of the Ca atoms inside the fullerene cage. For the systems with one or two Ca atoms a relatively unimpeded rotation was conjectured by earlier nuclear magnetic resonance experiments and supported by previous ab initio calculations used to sample the potential energy landscape. Here, by AIMD calculations, we confirm not only the circular motion, but also the correlated movement of the two Ca atoms, which is due to electric dipole interactions on the inner surface of the C60 molecule. Furthermore, systems with three and four Ca atoms present highly symmetric configurations of the embedded atoms, which are shown to rotate consistently within the fullerene cage, while more complex charge density patterns emerge. Employing artificial neural network models we perform a force-field mapping, which enables us to reproduce the main characteristics of the actual dynamics, such as the circular motion and the correlated movement of the Ca atoms.

What Are the Key Factors for the Detection of Peptides Using Mass Spectrometry on Boron-Doped Diamond Surfaces?

We investigated the use of boron-doped diamond (BDD) with different surface morphologies for the enhanced detection of nine different peptides by matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). For the first time, we compared three different nanostructured BDD film morphologies (Continuous, Nanograss, and Nanotips) with differently terminated surfaces (-H, -O, and -F) to commercially available Ground Steel plates. All these surfaces were evaluated for their effectiveness in detecting the nine different peptides by MALDI-MS. Our results demonstrated that certain nanostructured BDD surfaces exhibited superior performance for the detection of especially hydrophobic peptides (e.g., bradykinin 1-7, substance P, and the renin substrate), with a limit of detection of down to 2.3 pM. Further investigation showed that hydrophobic peptides (e.g., bradykinin 1-7, substance P, and the renin substrate) were effectively detected on hydrogen-terminated BDD surfaces. On the other hand, the highly acidic negatively charged peptide adrenocorticotropic hormone fragment 18-39 was effectively identified on oxygen-/fluorine-terminated BDD surfaces. Furthermore, BDD surfaces reduced sodium adduct contamination significantly.

Au@BN-enhanced laser desorption/ionization mass spectrometry and imaging for determination of fipronil and its metabolites in food and biological samples

Gold nanoparticles (AuNPs) represent an attractive inorganic matrix for laser desorption/ionization mass spectrometry (LDI-MS) detection of low-molecular-weight analytes; however, their direct use is hindered by severe aggregation. To limit AuNPs aggregation, hexagonal boron nitride nanosheets (h-BNNs) were employed as supports to improve their desorption/ionization efficiency. Thus, Au@BN was synthesized and systematically characterized. It showed low background noise and high sensitivity for LDI-MS of fipronil and its metabolites. Au@BN-assisted LDI-MS was validated using complex samples including blueberry juice, green tea beverage, and fish muscle, achieving low detection limits (0.05-0.20 µg·L^-1 for liquid media, 0.82-1.25 ng·g^-1 for fish muscle), wide linear ranges (0.2-100 µg·L^-1 for liquid media, 3.00-1000 ng·g^-1 for fish muscle), high reproducibility (7.55%-13.7%), and satisfactory recoveries (82.62%-109.1%). Furthermore, spatial distributions of analytes in strawberries and zebrafish were successfully imaged. This strategy allows for the quantitative analysis of other small molecules in complex substrates.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2017-2018

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2018. Also included are papers that describe methods appropriate to glycan and glycoprotein analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, new methods, matrices, derivatization, MALDI imaging, fragmentation and the use of arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Most of the applications are presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. The reported work shows increasing use of combined new techniques such as ion mobility and highlights the impact that MALDI imaging is having across a range of diciplines. MALDI is still an ideal technique for carbohydrate analysis and advancements in the technique and the range of applications continue steady progress.

Silver Nanostructured Substrates in LDI-MS of Low Molecular Weight Compounds

Mass spectrometric techniques can provide data on the composition of a studied sample, utilizing both targeted and untargeted approaches to solve various research problems. Analysis of compounds in the low mass range has practical implications in many areas of research and industry. Laser desorption ionization techniques are utilized for the analysis of molecules in a low mass region using low sample volume, providing high sensitivity with low chemical background. The fabrication of substrates based on nanostructures to assist ionization with well-controlled morphology may improve LDI-MS efficiency for silver nanoparticles with plasmonic properties. In this work, we report an approach for the preparation of silver nanostructured substrates applied as laser desorption ionization (LDI) plates, using the chemical vapor deposition (CVD) technique. Depending on the mass of used CVD precursor, the approach allowed the synthesis of LDI plates with tunable sensitivity for various low molecular weight compounds in both ion-positive and ion-negative modes. Reduced chemical background and sensitivity to small biomolecules of various classes (fatty acids, amino acids and water-soluble metabolites) at nanomolar and picomolar detection levels for lipids such as triacylglycerols, phosphatidylethanolamines and lyso-phosphatidylcholines represent an emerging perspective for applications of LDI-MS plates for the collection of molecular profiles and targeted analysis of low molecular weight compounds for various purposes.

Diamond formation mechanism in chemical vapor deposition

Artificial diamond plays a vital role in the manufacturing industry, jewelry, and future photoelectronic devices, but it is a key challenge to prepare the required large-area diamonds. A distinctive way to solve this problem possibly hides in the undiscovered formation mechanism of thermodynamically metastable diamond compared to graphite in low-pressure chemical vapor deposition. We design a series of short-term growth on the margins of cauliflower-like nanocrystalline diamond particles and find that diamond is formed by the transformation from graphite, not by the piling up of sp³ carbon. Atomically dispersed Ta atoms let the transition spontaneously occur. This subverts the general knowledge and supplies a way to prepare large-area diamonds based on large-sized graphite under normal pressure.

It is a key challenge to prepare large-area diamonds by using the methods of high-pressure high-temperature and normal chemical vapor deposition (CVD). The formation mechanism of thermodynamically metastable diamond compared to graphite in low-pressure CVD possibly implies a distinctive way to synthesize large-area diamonds, while it is an intriguing problem due to the limitation of in situ characterization in this complex growth environment. Here, we design a series of short-term growth on the margins of cauliflower-like nanocrystalline diamond particles, allowing us to clearly observe the diamond formation process. The results show that vertical graphene sheets and nanocrystalline diamonds alternatively appear, in which vertical graphene sheets evolve into long ribbons and graphite needles, and they finally transform into diamonds. A transition process from graphite (200) to diamond (110) verifies the transformation, and Ta atoms from hot filaments are found to atomically disperse in the films. First principle calculations confirm that Ta-added H- or O-terminated bilayer graphene spontaneously transforms into diamond. This reveals that in the H, O, and Ta complex atmosphere of the CVD environment, diamond is formed by phase transformation from graphite. This subverts the general knowledge that graphite is etched by hydrogen and sp³ carbon species pile up to form diamond and supplies a way to prepare large-area diamonds based on large-sized graphite under normal pressure. This also provides an angle to understand the growth mechanism of materials with sp² and sp³ electronic configurations.

Nanoporous Substrates with Molecular-Level Perfluoroalkyl/Alkylamide Surface for Laser Desorption/Ionization Mass Spectrometry of Small Proteins

The rapid detection of biomolecules greatly contributes to health management, clinical diagnosis, and prevention of diseases. Mass spectrometry (MS) is effective for detecting and analyzing various molecules at high throughput. However, there are problems with the MS analysis of biological samples, including complicated separation operations and essential pretreatments. In this study, a nanostructured organosilica substrate for laser desorption/ionization mass spectrometry (LDI-MS) is designed and synthesized to detect peptides and small proteins efficiently and rapidly. The surface functionality of the substrate is tuned by perfluoroalkyl/alkylamide groups mixed at a molecular level. This contributes to both lowering the surface free energy and introducing weak anchoring sites for peptides and proteins. Analyte molecules applied onto the substrate are homogeneously distributed and readily desorbed by the laser irradiation. The organosilica substrate enables the efficient LDI of various compounds, including peptides, small proteins, phospholipids, and drugs. An amyloid β protein fragment, which is known as a biomarker for Alzheimer's disease, is detectable at 0.05 fmol μL^-1. The detection of the amyloid β at 0.2 fmol μL^-1 is also confirmed in the presence of blood components. Nanostructured organosilica substrates incorporating a molecular-level surface design have the potential to enable easy detection of a wide range of biomolecules.

Yolk-shell Ni/NiO anchored on N-doped graphene synthesized as dual-ion MALDI matrix for detecting and imaging bioactive small molecules

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is an attractive tool to analyze the bioactive small molecules but remains a great challenge owing to the serious background interference from conventional matrix with m/z < 1000. Herein, we reported a dual-ion MALDI matrix of yolk-shell Ni/NiO nanoparticles anchored on nitrogen-doped graphene (Ni/NiO/N-Gr) to enhance MALDI performance. The Ni/NiO/N-Gr was synthesized via the pyrolysis and controllable oxidation strategy based on the nanoscale regulation of Kirkendall effect. The novel matrix showed the superior behavior for the analysis of various small molecular metabolites (amino acids, saccharides, spermidine, creatinine, hippuric acid, dopamine, and ascorbic acid) with high sensitivity, excellent salt tolerance, and favorable reproducibility in dual-ion modes compared to the traditional α-cyano-4-hydroxycinnamic acid (CHCA) and control substances (Ni/N-Gr and NiO/N-Gr). Meanwhile, we have realized accurate quantitation of blood glucose in mice with a linearity concentration range of 0.2-7.5 mM and qualitative detection of various endogenous small molecular metabolites in mice serum and urine samples. Especially, the Ni/NiO/N-Gr assisted LDI MS imaging (MSI) has exhibited the excellent spatial distribution of lipids in hippocampus region of mice brain. These results may provide an approach to explore the MALDI MS and MSI applications in clinical diagnosis.

Control of electrostatic self-assembly seeding of diamond nanoparticles on carbon nanowalls

Seeding of diamond nanoparticles on vertically-aligned multi-layer graphene, the so-called carbon nanowalls (CNWs), is studied by using deionized water, ethylene glycol, ethanol, and formamide as dispersion mediums. Detonation nanodiamond particles show the smallest mean size and size distribution with a high positive zeta potential when dispersed in ethanol. The contact angle of ethanol on CNWs is almost zero degree, confirming highly wetting behaviour. The diamond nanoparticles dispersed in ethanol are distributed the most uniformly with minimal aggregation on CNWs as opposed to those dispersed in other liquids. The resulting diamond nanoparticle-seeded CNWs, followed by short-term growth in microwave plasma chemical vapor deposition, show a marked decrease in field emission turn-on field down to 1.3 Vμm^-1together with a large increase in current density, compared to bare CNWs without diamond seeding. The results provide a way to control the density, size, and uniformity (spacing) of diamond nanoparticles on CNWs and should be applied to fabricate hybrid materials and devices using nanodiamond and nanocarbons.

Hybrid Surface Design of Organosilica Films for Laser Desorption/Ionization Mass Spectrometry: Low Free Energy Surface with Interactive Sites

Laser desorption/ionization mass spectrometry (LDI-MS) assisted by solid substrates is useful for the facile and rapid analysis of low-molecular-weight compounds. The LDI performance of solid substrates depends on not only a surface morphology but also the surface functionalities dominating the surface-analyte interactions. In this study, we propose a hybrid surface design for LDI substrates, realizing both weak surface-analyte interaction and homogeneous distribution of analytes. The hybrid surface consisted of a mixture of fluoroalkylsilane (FAS), SiO₂, and TiO₂ and was formed on organosilica substrates containing UV-laser-absorbing naphthalimide moieties. To investigate the surface interactions, the hybrid surface as well as conventional hydrophobic surfaces treated with FAS only were prepared on flat organosilica films. Contact angle measurements and surface free energy analysis showed that the hybrid surface exhibited the highest hydrophobicity, while the contribution of the polar and hydrogen bonding terms in the surface free energy was clearly observed. The organosilica film with the hybrid surface demonstrated significant LDI performance for the detection of biorelated compounds (e.g., peptides, phospholipids, and medicines), and a high detection ability was particularly observed for peptides. The substrate surface promoted the desorption/ionization of analytes through a low surface free energy and uniform distribution of the analytes due to the interactive sites. The hybrid surface design was then applied to a nanostructured organosilica substrate consisting of a base film and a nanoparticle layer. The signal intensity of a peptide was further improved approximately 3-fold owing to the increased surface area of the nanostructured substrate, and the limit of detection reached the subfemtomole order. Our hybrid surface design is expected to improve the LDI performance of various nanostructured solid substrates.

Effects of Carbon Nanowalls (CNWs) Substrates on Soft Ionization of Low-Molecular-Weight Organic Compoundsin Surface-Assisted Laser Desorption/Ionization Mass Spectrometry (SALDI-MS)

Carbon nanowalls (CNWs), which are vertically oriented multi-layer graphene sheets, were employed in surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) measurements to detect low-molecular-weight organic compounds. CNWs substrates with widely different wall-to-wall distances from 142 to 467 nm were synthesized using a radical-injection plasma-enhanced chemical vapor deposition (RI-PECVD) system with nanosecond pulse biasing to a sample stage. When survival yield (SY) values of N-benzylpyridinium chloride (N-BP-Cl) were examined, which is commonly used to evaluate desorption/ionization efficiency, a narrower wall-to-wall distance presented a higher SY value. The highest SY value of 0.97 was realized at 4 mJ/cm² for the highest-density CNWs with a wall-to-wall distance of 142 nm. The laser desorption/ionization effect of arginine, an amino acid, was also investigated. When CNWs with a narrower wall-to-wall distance were used, the signal-to-noise (SN) ratios of the arginine signals were increased, while the intensity ratios of fragment ions to arginine signals were suppressed. Therefore, the CNWs nanostructures are a powerful tool when used as a SALDI substrate for the highly efficient desorption/ionization of low-molecular-weight biomolecules.

Surface-enhanced laser desorption/ionization mass spectrometry for rapid analysis of organic environmental pollutants by using polydopamine nanospheres as a substrate

Polydopamine nanospheres (PDA) were designed to serve as a new substrate for surface-enhanced desorption/ionization mass spectrometry (SELDI-MS). Compared with conventional organic matrices, the PDA substrate showed superior LDI performance for analyzing a wide variety of environmental pollutants, including polycyclic aromatic hydrocarbons, bisphenols, benzophenones, sulfonamides, perfluorinated compounds and estrogens. Benzoapyrene was used to evaluate the ability of quantitative analysis and its corresponding limit of detection (LOD) of as low as 0.1 ng was achieved. High sensitivity and good reproducibility of PDA-based SELDI-MS allowed us to determine ultratrace PAHs in airborne particulate matters (PM_2.5), and the corresponding concentration of BaP in different PM_2.5 were 5.32, 8.97 and 9.79 ng m^-3. Significantly, PDA exhibits the characteristics of simple synthesis, low cost, non-toxicity and less matrix interference, which provides the possibility for the sensitive analysis of organic small molecule pollutants at low concentrations in complex environmental samples.

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.

Atmospheric Pressure Plasma-Treated Carbon Nanowalls’ Surface-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (CNW-SALDI-MS)

Carbon nanowalls (CNWs), vertically standing highly crystallizing graphene sheets, were used in the application of a surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF-MS). The CNW substrates solved the issues on interferences of matrix molecules and alkali metal addition ions in low-weight molecule detection. Before SALDI sample preparations, the hydrophobic CNW was treated by atmospheric pressure plasma for exposing hydrophilicity to the CNWs’ surface. Detection of water soluble amino acids, arginine, was demonstrated.

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.

Single-step growth of graphene and graphene-based nanostructures by plasma-enhanced chemical vapor deposition

The realization of many promising technological applications of graphene and graphene-based nanostructures depends on the availability of reliable, scalable, high-yield and low-cost synthesis methods. Plasma enhanced chemical vapor deposition (PECVD) has been a versatile technique for synthesizing many carbon-based materials, because PECVD provides a rich chemical environment, including a mixture of radicals, molecules and ions from hydrocarbon precursors, which enables graphene growth on a variety of material surfaces at lower temperatures and faster growth than typical thermal chemical vapor deposition. Here we review recent advances in the PECVD techniques for synthesis of various graphene and graphene-based nanostructures, including horizontal growth of monolayer and multilayer graphene sheets, vertical growth of graphene nanostructures such as graphene nanostripes with large aspect ratios, direct and selective deposition of monolayer and multi-layer graphene on nanostructured substrates, and growth of multi-wall carbon nanotubes. By properly controlling the gas environment of the plasma, it is found that no active heating is necessary for the PECVD growth processes, and that high-yield growth can take place in a single step on a variety of surfaces, including metallic, semiconducting and insulating materials. Phenomenological understanding of the growth mechanisms are described. Finally, challenges and promising outlook for further development in the PECVD techniques for graphene-based applications are discussed.

Tailoring Electro/Optical Properties of Transparent Boron-Doped Carbon Nanowalls Grown on Quartz

Carbon nanowalls (CNWs) have attracted much attention for numerous applications in electrical devices because of their peculiar structural characteristics. However, it is possible to set synthesis parameters to vary the electrical and optical properties of such CNWs. In this paper, we demonstrate the direct growth of highly transparent boron-doped nanowalls (B-CNWs) on optical grade fused quartz. The effect of growth temperature and boron doping on the behavior of boron-doped carbon nanowalls grown on quartz was studied in particular. Temperature and boron inclusion doping level allow for direct tuning of CNW morphology. It is possible to operate with both parameters to obtain a transparent and conductive film; however, boron doping is a preferred factor to maintain the transparency in the visible region, while a higher growth temperature is more effective to improve conductance. Light transmittance and electrical conductivity are mainly influenced by growth temperature and then by boron doping. Tailoring B-CNWs has important implications for potential applications of such electrically conductive transparent electrodes designed for energy conversion and storage devices.

Dual-Ion-Mode MALDI MS Detection of Small Molecules with the O-P,N-Doped Carbon/Graphene Matrix

It is challenging to realize a dual-ion mode of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for detecting small molecules. Herein, graphene coated by porous amorphous carbon with P-O surface group and codoped by phosphorus and nitrogen (O-P,N-C/G) was synthesized from an aerogel formed by phytic acid, polyaniline, and electrochemically exfoliated graphene. The carbon material synthesized has the feature of large surface area (583 m²/g), good electrical conductivity, strong UV absorption, heteroatom doping, and surface functional groups suitable for laser- induced desorption/ionization. It was employed as a novel matrix suitable for both positive-ion and negative-ion modes in MALDI-TOF MS for the analysis of various small molecules including amino acids, small peptides, saccharides, drugs, and environmental pollutants, significantly outperforming control materials and a traditional CHCA (α-cyano-4-hydroxycinnamic acid) or 2,5-dihydroxybenzoic (DHB) matrix. Remarkably, the detection limit of the anticancer drugs (5-fluorouracil and ellagic acid) reaches 50 pmol. In addition, nice MALDI-TOF MS images can be mapped to detect mixed amino acids corresponding to homogeneous distribution of ion intensity. The monosaccharides and disaccharides can be distinguished by using the new matrix. Last but not least, it can be used to quantitatively detect glucose in human serum and soft drinks (glucose/fructose, 203.1 mM) without adding standards.

Disposable MoS2-Arrayed MALDI MS Chip for High-Throughput and Rapid Quantification of Sulfonamides in Multiple Real Samples

In this work, we demonstrate, for the first time, the development of a disposable MoS₂-arrayed matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) chip combined with an immunoaffinity enrichment method for high-throughput, rapid, and simultaneous quantitation of multiple sulfonamides (SAs). The disposable MALDI MS chip was designed and fabricated by MoS₂ array formation on a commercial indium tin oxide (ITO) glass slide. A series of SAs were analyzed, and clear deprotonated signals were obtained in negative-ion mode. Compared with MoS₂-arrayed commercial steel plate, the prepared MALDI MS chip exhibited comparable LDI efficiency, providing a good alternative and disposable substrate for MALDI MS analysis. Furthermore, internal standard (IS) was previously deposited onto the MoS₂ array to simplify the experimental process for MALDI MS quantitation. 96 sample spots could be analyzed within 10 min in one single chip to perform quantitative analysis, recovery studies, and real foodstuff detection. Upon targeted extraction and enrichment by antibody conjugated magnetic beads, five SAs were quantitatively determined by the IS-first method with the linear range of 0.5-10 ng/mL ( R² > 0.990). Good recoveries and repeatability were obtained for spiked pork, egg, and milk samples. SAs in several real foodstuffs were successfully identified and quantified. The developed method may provide a promising tool for the routine analysis of antibiotic residues in real samples.