Surface-assisted laser desorption/ionization mass spectrometry on titania nanotube arrays

Surface-Assisted Laser Desorption/Ionization Mass Spectrometry with a Two-Dimensional Au Nanoparticle Array for Soft Ionization

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) is a valuable technique for detecting small molecules in environmental and medicinal studies. We investigated dot-like and 2D-array gold nanoparticle-based SALDI-MS substrates that excite surface plasmons and enhance the desorption/ionization of sample molecules via charge transfer between the substrate and sample molecules. We aimed to optimize the nondissociative detection of sample molecules by efficiently transferring energy while suppressing excess internal energy. SALDI-MS measurements using crystal violet (CV) molecules revealed ion intensity and spectral pattern differences between the dot-like and 2D-array substrates. SALDI-MS measurements using dot-like substrates suggested two desorption/ionization processes: internal energy supply and charge transfer between the substrate and sample molecules. However, SALDI-MS measurements using 2D-array substrates suggested that the internal energy supply was suppressed. As a result, the dot-like substrate provided higher desorption/ionization efficiency but increased fragmentation, whereas the 2D-array substrate was suitable for highly sensitive and nondissociative SALDI-MS measurements. This study contributes to the optimization of SALDI-MS measurements and advances our understanding of energy transfer and sample molecule dissociation.

Substrate-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry of Some Small Biomolecules Using TiO2-Nanotubes: The Effect of Nanotube Diameter and Salt Addition

Substrate-assisted laser desorption/ionization (SALDI) is a kind of soft ionization method that is most suitable for the analysis of low molecular weight analytes when it is coupled with a time-of-flight mass spectrometer. Unlike the conventional matrix-assisted laser desorption/ionization, there is no interference in the SALDI with matrices for the low mass analyte peaks (m/z < 700). The focus of this work is to develop substrates based on nanomaterials to obtain higher sensitivity, better reproducibility, and easier preparation. The mass spectra of some small molecules (capecitabine, hemin, methadone, noscapine, oxycodone, thebaine, malathion, chlorpyrifos, ethion, permethrin, and phosalone) deposited on the TiO₂-nanotube (TiO₂-NTs) plate by the SALDI-TOF-MS technique are reported. The nanotubes are synthesized in different diameter sizes of nanotubes via the anodizing method. The intensity of the analyte peaks and the softness of ionization are optimized by varying the diameter of nanotubes and adding relevant alkali salts to the analytes. In addition, the reproducibility of the signal intensity of analytes is optimized by changing the surface hydrophilicity of the TiO₂-NT plate.

Surface-Assisted Laser Desorption/Ionization Imaging Mass Spectrometry (SALDI-IMS)-Based Detection of Vinca Alkaloids Distribution in the Petal of Madagascar Periwinkle

The surface-assisted laser desorption/ionization (SALDI) technique uses inorganic materials to aid desorption and ionization of molecules. SALDI is suitable for analyzing small molecules due to the absence of interfering signals in the low m/z range originating from the organic matrix. Imaging mass spectrometry (IMS) is a versatile imaging approach with high spatial resolution for analyzing various molecular species, but its application depends heavily on the ionization method. We have developed a functionalized titanium dioxide (TiO₂) nanowire as a solid substrate for SALDI-MS detection of low-molecular-weight molecules. We apply this novel substrate for imprinting fragile specimens such as petals and further SALDI-IMS analysis. The TiO₂ nanowire substrate is prepared from a commercial Ti plate by a hydrothermal process and subsequently chemically modified to improve the quality and selectivity of imprinting as well as the sensitivity of SALDI-IMS analysis. Here, the functionalized TiO₂ nanowire substrate is applied to visualize the distribution of vinca alkaloids in the petal of Madagascar periwinkle (Catharanthus roseus).

Matrix-Free High-Resolution Atmospheric-Pressure SALDI Mass Spectrometry Imaging of Biological Samples Using Nanostructured DIUTHAME Membranes

Applications of mass spectrometry imaging (MSI), especially matrix-assisted laser desorption/ionization (MALDI) in the life sciences are becoming increasingly focused on single cell analysis. With the latest instrumental developments, pixel sizes in the micrometer range can be obtained, leading to challenges in matrix application, where imperfections or inhomogeneities in the matrix layer can lead to misinterpretation of MS images. Thereby, the application of premanufactured, homogeneous ionization-assisting devices is a promising approach. Tissue sections were investigated using a matrix-free imaging technique (Desorption Ionization Using Through-Hole Alumina Membrane, DIUTHAME) based on premanufactured nanostructured membranes to be deposited on top of a tissue section, in comparison to the spray-coating of an organic matrix in a MALDI MSI approach. Atmospheric pressure MALDI MSI ion sources were coupled to orbital trapping mass spectrometers. MS signals obtained by the different ionization techniques were annotated using accurate-mass-based database research. Compared to MALDI MSI, DIUTHAME MS images captivated with higher signal homogeneities, higher contrast and reduced background signals, while signal intensities were reduced by about one order of magnitude, independent of analyte class. DIUTHAME membranes, being applicable only on tissue sections thicker than 50 µm, were successfully used for mammal, insect and plant tissue with a high lateral resolution down to 5 µm.

Visualizing vinca alkaloids in the petal of Catharanthus roseus using functionalized titanium oxide nanowire substrate for surface-assisted laser desorption/ionization imaging mass spectrometry

Imaging mass spectrometry (IMS) is a powerful technique that enables analysis of various molecular species at a high spatial resolution with low detection limits. In contrast to the matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) approach, surface-assisted laser desorption/ionization (SALDI) can be more effective in the detection of small molecules due to the absence of interfering background signals in low m/z ranges. We developed a functionalized TiO₂ nanowire as a solid substrate for IMS of low-molecular-weight species in plant tissues. We prepared TiO₂ nanowires using an inexpensive modified hydrothermal process and subsequently functionalized them chemically with various silane analogs to overcome the problem of superhydrophilicity of the substrate. Chemical modification changed the selectivity of imprinting of samples deposited on the substrate surface and thus improved the detection limits. The substrate was applied to image distribution of the metabolites in very fragile specimens such as the petal of Catharanthus roseus. We observed that the metabolites are distributed heterogeneously in the petal, which is consistent with previous results reported for the C. roseus plant leaf and stem. The intermediates corresponding to the biosynthesis pathway of some vinca alkaloids were clearly shown in the petal. We also performed profiling of petals from five different cultivars of C. roseus plant. We verified the semi-quantitative capabilities of the imprinting/imaging approach by comparing results using the LC-MS analysis of the plant extracts. This suggested that the functionalized TiO₂ nanowire substrate-based SALDI is a powerful technique complementary to MALDI-MS.

Direct nanoimprinting of nanoporous organosilica films consisting of covalently crosslinked photofunctional frameworks

Nanoimprinting methods have been used widely to prepare various patterned or nanostructured thin films from inorganic or organic components. However, the accumulation of large functional aromatic groups in covalently crosslinked nanoimprints is challenging, due to the difficulty in controlling the fluidity and reactivity of the precursor films. In this work, nanoimprinting of naphthalimide-silica sol-gel films results in vertically oriented nanoporous structures consisting of covalently crosslinked UV-absorbing frameworks. The nanoimprinted films demonstrate potential as robust analytical substrates for laser desorption/ionization mass spectrometry (LDI-MS). The sol-gel polycondensation behavior of the precursors is examined using 29Si NMR spectroscopy to determine reaction conditions suitable for nanoimprinting. The inorganic-organic hybrid frameworks containing a high density of naphthalimide groups exhibit small volume shrinkage during the polycondensation reactions, which leads to desired nanoimprinting. Various bio-related compounds on the order of picomole to femtomole quantities are detectable by LDI-MS measurements using the nanoimprinted substrates. To improve their user-friendliness and signal intensity in LDI-MS analysis, the nanoimprinted substrates are patterned with surface-modified silica nanoparticles. The direct formation of surface nanostructures by nanoimprinting of functional organosilica films may open a new path to developing optically and electronically functional materials, thereby widening their utility.

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.

Small- and large-sized iron(II, III) oxide nanoparticles for surface-assisted laser desorption/ionization mass spectrometry of small biomolecules

RATIONALE

Common surface-assisted laser desorption/ionization (SALDI) surfaces are functionalized to improve mass spectrometric detection. Such surfaces are selective to certain group(s) of compounds. The application of universal and sensitive SALDI surfaces with appropriate size/surface area is paramount. In this study, two different sizes/surface areas of Fe3 O4 are compared as SALDI surfaces.

METHODS

For accurate surface area comparisons, the physical properties of the Fe3 O4 nanoparticles used as SALDI surfaces were determined using scanning electron microscopy, X-ray diffractometry, and N2 Brunauer-Emmet-Teller adsorption techniques. SALDI mass spectrometry (MS) data were acquired using a time-of-flight (TOF) mass spectrometer operated in the linear mode and equipped with a 50-Hz pulsed nitrogen laser (at 337 nm). Small biomolecules (adenosine, glucose, sucrose, tryptophan, and tripeptide) and a real sample (human serum) were analyzed.

RESULTS

The average sizes/specific surface areas of the SALDI surfaces of the small- and large-sized Fe3 O4 nanoparticles were ~21 nm/~82 m2 /g and ~39 nm/~38 m2 /g, respectively. An overall ~2.0-fold enhancement in signal-to-noise ratios was observed for the ionic species of the analyzed biomolecules in SALDI-MS using small-sized Fe3 O4 in comparison to large-sized Fe3 O4 nanoparticles. MS sensitivity from adenosine calibration curves (concentration between 0.05 and 10.0 mM) was ~2.0-fold higher for small-sized than large-sized Fe3 O4 nanoparticles as SALDI surfaces.

CONCLUSIONS

We have shown that transition-metal oxides such as Fe3 O4 nanoparticles are suitable and efficient surfaces for SALDI-TOF-MS analysis of small biomolecules. We observed improvement in signal-to-noise ratios and detection sensitivity for the analyzed samples from SALDI surfaces using small-sized (possessing larger surface area) than large-sized Fe3 O4 nanoparticles.

A novel laser desorption/ionization method using through hole porous alumina membranes

RATIONALE

A novel matrix-free laser desorption/ionization method based on porous alumina membranes was developed. The porous alumina membranes have a two-dimensional (2D) ordered structure consisting of closely aligned straight through holes of sub-micron in diameter that are amenable to mass production by industrial fabrication processes.

METHODS

Considering a balance between the ion generating efficiency and the mechanical strength of the membranes, the typical values for the hole diameter, open aperture ratio and membrane thickness were set to 200 nm, 50% and 5 μm, respectively. The membranes were coated with platinum on a single side that was exposed to the laser. Evaluation experiments were conducted on the feasibility of this membrane structure for an ionization method using a single peptide and mixed peptides and polyethylene glycol samples and a commercial matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometer in the positive ion mode.

RESULTS

Results showed a softness of ionization and no sweet spot nature. The capillary action of the through holes with very high aspect ratio enables several loading protocols including sample impregnation from the surface opposite to the laser exposure side.

CONCLUSIONS

The feasibility study indicates that the through hole porous alumina membranes have several advantages in terms of usefulness over the conventional surface-assisted laser desorption ionization (SALDI) methods. The proposed novel ionization method is termed Desorption Ionization Using Through Hole Alumina Membrane (DIUTHAME).

Nanomaterials and continuous wave laser-based efficient desorption for atmospheric pressure mass spectrometric imaging of live hippocampal tissue slices

Micrometer-resolution mass spectrometric imaging of live hippocampal tissue is achieved with a highly efficient desorption of biomolecules using a 532 nm continuous wave laser and gold nanoparticles or graphene oxide as an energy transporter, which enables clear identification of the distributions of monoacylglycerol, adenine, cholesterol, sphingosine and ceramide.

Micrometer-resolution mass spectrometric imaging of live hippocampal tissue is achieved with a highly efficient desorption of biomolecules using a 532 nm continuous wave laser and gold nanoparticles or graphene oxide as an energy transporter.

Atmospheric pressure mass spectrometric imaging of live hippocampal tissue slices with subcellular spatial resolution

We report a high spatial resolution mass spectrometry (MS) system that allows us to image live hippocampal tissue slices under open-air atmospheric pressure (AP) and ambient temperature conditions at the subcellular level. The method is based on an efficient desorption process by femtosecond (fs) laser assisted with nanoparticles and a subsequent ionization step by applying nonthermal plasma, termed AP nanoparticle and plasma assisted laser desorption ionization (AP-nanoPALDI) MS method. Combining the AP-nanoPALDI with microscopic sample scanning, MS imaging with spatial resolution of 2.9 µm was obtained. The observed AP-nanoPALDI MS imaging clearly revealed the differences of molecular composition between the apical and basal dendrite regions of a hippocampal tissue. In addition, the AP-nanoPALDI MS imaging showed the decrease of cholesterol in hippocampus by treating with methyl β-cyclodextrin, which exemplifies the potential of AP-nanoPALDI for live tissue imaging for various biomedical applications without any chemical pretreatment and/or labeling process.

Ambient mass spectrometry-based approaches have found application in biology and medicine. Here the authors report a mass spectrometric imaging method (ambient nanoPALDI) for live hippocampal tissues, based on gold nanorodassisted femtosecond laser desorption and subsequent non-thermal plasma induced ionization.

Highly oriented Langmuir-Blodgett film of silver cuboctahedra as an effective matrix-free sample plate for surface-assisted laser desorption/ionization mass spectrometry

The design of a homogeneous sample plate to solve the sweet heating spot issues is the key step to expand the applicability of surface-assisted laser desorption/ionization mass spectrometry (SALDI MS). Herein, large-scale and highly oriented Langmuir-Blodgett (LB) films of uniform silver nanocrystals have been fabricated as a highly efficient and matrix-free sample plate for SALDI MS. Three individual silver nanocrystals (cubes, cuboctahedra and octahedra) assembled LB films have been applied as the sample plates for glucose detection by SALDI MS without an additional matrix. The results show that the signal intensity, background noise, signal-to-noise ratio and reproducibility have been significantly improved using LB films as the sample plate in comparison with commercial matrixes of CHCA and DHB. In particular, a relative signal of 5.7% was obtained for LB films of silver cuboctahedra. The significant improvement in the SALDI MS measurement could be attributed to the homogenous dissipation of laser irradiation energy to create a large area of the sweet heating spot on well-oriented silver cuboctahedra-based LB film. This ready-to-use sample plate has the potential for widespread commercial applications in SALDI MS.

A new approach to surface activation of porous nanomaterials using non-thermal helium atmospheric pressure plasma jet treatment

Non-thermal helium atmospheric pressure plasma jet treatment is applied to the surface activation of porous TiO₂ nanoparticle assemblies. Treatment conditions such as the working distance of the plasma discharge, helium gas flow rate, and treatment time are optimized for effective removal of contaminants from the assembly surface. Laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) is applied to detect trace amounts of contaminants on assembly surfaces. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations confirm that the nanoparticle assemblies retain their original perfect spherical structures as well as their ultra-fine convex-concave nano-surfaces even after the plasma jet treatment. N₂ adsorption/desorption and X-ray diffraction (XRD) measurements show no significant changes in their BET specific surface areas and crystal structures, respectively. The plasma jet-treated TiO₂ nanoparticle assemblies show a 3.8 fold improvement in their reaction rate constants for methylene blue degradation and a 2 fold enhancement of their photocurrents under UV irradiation when compared with untreated TiO₂.

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'.

Recent advances in inorganic materials for LDI-MS analysis of small molecules

In this review, various inorganic materials were summarized for the analysis of small molecules by laser desorption/ionization mass spectrometry (LDI-MS).

Surface-assisted laser desorption ionization mass spectrometry techniques for application in forensics

Matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) is an excellent analytical technique for the rapid and sensitive analysis of macromolecules (>700 Da), such as peptides, proteins, nucleic acids, and synthetic polymers. However, the detection of smaller organic molecules with masses below 700 Da using MALDI-MS is challenging due to the appearance of matrix adducts and matrix fragment peaks in the same spectral range. Recently, nanostructured substrates have been developed that facilitate matrix-free laser desorption ionization (LDI), contributing to an emerging analytical paradigm referred to as surface-assisted laser desorption ionization (SALDI) MS. Since SALDI enables the detection of small organic molecules, it is rapidly growing in popularity, including in the field of forensics. At the same time, SALDI also holds significant potential as a high throughput analytical tool in roadside, work place and athlete drug testing. In this review, we discuss recent advances in SALDI techniques such as desorption ionization on porous silicon (DIOS), nano-initiator mass spectrometry (NIMS) and nano assisted laser desorption ionization (NALDI™) and compare their strengths and weaknesses with particular focus on forensic applications. These include the detection of illicit drug molecules and their metabolites in biological matrices and small molecule detection from forensic samples including banknotes and fingerprints. Finally, the review highlights recent advances in mass spectrometry imaging (MSI) using SALDI techniques.

Phosphopeptide separation using radially aligned titania nanotubes on titanium wire

Phosphoproteomic analysis offers a unique view of cellular function and regulation in biological systems by providing global measures of a key cellular regulator in the form of protein phosphorylation. Understanding the phosphorylation changes between normal and diseased cells or tissues offers a window into the mechanism of disease and thus potential targets for therapeutic intervention. A key step in these studies is the enrichment of phosphorylated peptides that are typically separated and analyzed by using liquid chromatography mass spectrometry. The mesoporous titania beads/particles (e.g., Titansphere TiO2 beads from GL Sciences Inc., Japan) that are widely used for phosphopeptide enrichment are expensive and offer very limited opportunities for further performance improvement. Titiania nanotube arrays have shown promising characteristics for phosphopeptide separation. Here we report a proof-of-concept study to evaluate the efficacy of nanotubes on Ti-wire for phosphoproteomics research. We used titania nanotubes radially grown on titanium wires as well as the commercial beads to separate phosphopeptides generated from mouse liver complex tissue extracts. Our studies revealed that the nanotubes on metal wire provide comparable efficacy for enrichment of phophopeptides and offer an ease of use advantage versus mesoporous beads, thus having the potential to become a low cost and more practical material/methodology for phosphopeptide enrichment in biological studies.

Gold-decorated titania nanotube arrays as dual-functional platform for surface-enhanced Raman spectroscopy and surface-assisted laser desorption/ionization mass spectrometry

In this report, we demonstrate gold-decorated titania nanotube arrays (Au-TNA substrate) as a dual-functional platform for surface-enhanced Raman spectroscopy (SERS) and surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). The Au nanoparticles are grown on the substrate using vapor deposition of Au. The resulting substrates perform better than Au colloids in terms of the reproducibility of the SERS measurements, long-term stability of the fabricated structures, and clean surface of the Au. The nanostructure of the Au-TNA substrate was designed to optimize the SALDI-MS and SERS performance. Excellent reproducibility of the SERS measurements using the Au-TNA substrate was obtained, with a standard error less than 6 %. SALDI activity was also demonstrated for the same Au-TNA substrates. Finally, the Au-TNA substrate was used for combined SERS and SALDI-MS analysis (i) to discriminate the structural isomers of pyridine compounds (para-, meta-, and ortho-pyridinecarboxylic acid) and (ii) to detect polycarbamate, a dithiocarbamate fungicide. These results are difficult to obtain using either approach alone.

Novel galvanic nanostructures of Ag and Pd for efficient laser desorption/ionization of low molecular weight compounds

A simple approach for synthesis of palladium and silver nanostructures with readily adjustable morphologies was developed using galvanic electrochemical deposition, for application to surface-assisted laser desorption/ionization (SALDI) of small biological molecules. A range of fatty acids, triglycerides, carbohydrates, and antibiotics were investigated to assess the performance of the new materials. Intense analyte cations were generated from the galvanic surfaces upon UV laser irradiation such as potassium adducts for a film thickness <100 nm (originating from impurities of the electrolyte solution) and Pd and Ag cluster ions for films with a thickness >120 nm. Possible laser desorption/ionization mechanisms of these galvanic structures are discussed. The films exhibited self-organizing abilities and adjustable morphologies by changing electrochemical parameters. They did not require any stabilizing agents and were inexpensive and very easy to produce. SALDI analysis showed that the materials were stable under ambient conditions and analytical results with excellent measurement reproducibility and detection sensitivity similar to MALDI were obtained. Finally, we applied the galvanic surfaces to fast screening of natural oils with minimum sample preparation.

Applications of nanomaterials in mass spectrometry analysis

Mass spectrometry (MS) based analyses have received intense research interest in a series of rapidly developing disciplines. Although current MS techniques have enjoyed great successes, several key challenges still remain in practical applications, especially for the detection of biomolecules in biological systems. The use of nanomaterials in MS based analysis provides a promising approach due to their unique physical and chemical properties. In this review, nanomaterials with different compositions and nanostructures employed in MS applications are summarised and classified by their functions. Such an integrated and wide reaching review will provide a comprehensive handbook to researchers with various backgrounds working in this exciting interdisciplinary area.

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.

Colloidal TiO2 nanoparticles as substrates for M(S)ALDI mass spectrometry of transition metal complexes

RATIONALE

Nanoparticles as substrates for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) have advantages over organic matrices, since they enable acquisition of spectra in the low-mass range. It has been previously shown that TiO(2) nanoparticles can be used as substrate for MALDI-TOF MS analysis of phospholipids and for other types of molecules, but none of them was applied to the analysis of transition metal complexes.

METHODS

The MALDI-TOF mass spectra of potential anti-tumor drugs [AuCl(2)(bipy)]Cl, [PtCl(4)(bipy)], and [RuCl(2)(bipy)(2) ]Cl acquired with organic matrices have been compared with spectra acquired with colloidal titanium dioxide nanoparticles. Colloidal TiO(2) nanoparticles (NPs) with average diameter of 5 nm were synthesized and characterized by microscopy. For some experiments, the TiO(2) NPs were treated at 60 °C. Suspensions of matrix and the analyte were premixed, applied to the MALDI target and left at room temperature. Mass spectra were acquired with a 50-Hz pulsed nitrogen laser emitting at a wavelength of 337 nm.

RESULTS

The MALDI spectra of transition metal complexes acquired with TiO(2) NPs exhibited somewhat lower sensitivity than those with organic matrices; on the other hand, they are characterized by significantly lower number of signals arising from the tested complexes than the organic matrices. Whereas adducts between organic matrices and the analytes were detectable in the spectra, this was not the case for the TiO(2)-assisted mass spectra.

CONCLUSIONS

We have shown that colloidal TiO(2) NPs can be used as substrates for MALDI-TOF MS of transition metal complexes. Although the sensitivity of this approach in comparison with the use of organic matrices might still be a problem, the potential of the applications of NPs for the mass spectrometric characterization of transition metal complexes is clearly demonstrated.

3-Mercaptopropionic acid modified ZnSe quantum dots as the matrix for direct surface-assisted laser desorption/ionization mass spectrometric analysis of peptides/proteins from sodium salt solution

This study describes a strategy of using zinc selenium quantum dots (ZnSe QDs) modified with 3-mercaptopropionic acid (3-MPA) as the matrix for direct analysis of peptides and proteins from sodium salt solution in surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). The enhancement of detection sensitivity for these biomolecules was due to the adsorption of positively charged peptides or proteins onto the surfaces of negatively charged ZnSe-3MPA QDs via electrostatic interactions resulting in an increase in ionization efficiency for sodium adduct ions ([M+Na](+)). The applicability of the current approach was demonstrated for a variety of peptides, including leucine-enkephalin, methione-enkephalin, HW6, substance P and angiotensin II, and proteins (cytochrome c, myoglobin and lysozyme). Signal intensities of these peptides or proteins can be enhanced by 25-95 times compared with those obtained by LDI-MS in the absence of ZnSe-3MPA QDs. Applying ZnSe-3MPA QDs to serve as the matrix in SALDI-MS is a simple and effective approach for direct analysis of peptide and protein molecules from sodium salt solution without any pretreatment as the peptides and proteins can be successfully detected as sodium adduct ions ([M+Na](+)).

Nanomaterial-assisted laser desorption ionization for mass spectrometry-based biomedical analysis

Nanomaterials have been widely used to assist laser desorption ionization of biomolecules for mass spectrometry analysis. Compared with classical matrix-assisted laser desorption ionization, strategies based on nanomaterial-assisted ionization generate a clean background, which is of great benefit for the qualitative and quantitative analysis of small biomolecules, such as therapeutic and diagnostic molecules. As label-free platforms, they have successfully been used for high-throughput enzyme activity/inhibition monitoring and also for tissue imaging to map in situ the distribution of peptides, metabolites and drugs. In addition to widely used porous silicon nanomaterials, gold nanoparticles can be easily chemically modified by thiol-containing compounds, opening novel interesting perspectives. Such functionalized nanoparticles have been used both as probes to extract target molecules and as matrices to assist laser desorption ionization for developing new enzyme immunoassays or for studying DNA hybridization. More recently, semiconductor nanomaterials or quantum dots acting as photosensitive centers to induce in-source redox reactions for proteomics and to investigate biomolecule oxidation for metabolomics have been shown to offer new analytical strategies.

On-plate desalting and SALDI-MS analysis of peptides with hydrophobic silicate nanofilms on a gold substrate

We report the use of silicate nanofilms for on-plate desalting and subsequently direct laser desorption/ionization-mass spectrometric (LDI-MS) analysis of peptides. A hydrophobic octadecyltrichlorosilane (OTS) monolayer is formed on a calcinated nanofilm on a gold substrate to facilitate sample deposition and interaction with the surface that allows effective removal of MS-incompatible contaminants such as salts and surfactants by simple on-plate washing while the peptides are retained on the spot. By elimination of interferences from matrix-related ions and contaminants, sensitivity of MS analysis has been enhanced over ca. 20 times, leading to improved detection of peptides at the low-femtomolar level. A high recovery rate of the peptides is obtained by using relatively rough nanofilms, which are prepared through a modified layer-by-layer deposition/calcination process. The performance of the films has been investigated with peptide samples in the presence of high salts (NaCl and sodium acetate) and urea. Compared to matrix-assisted laser desorption/ionization analysis with CHCA matrix, LDI with on-plate desalting offers marked improvement for analysis of peptides due to low background ions and reduction of sample complexity. Additionally, selective capture of the hydrophobic components of a protein can be achieved, providing a highly useful strategy for specific peptide enrichment. LDI with on-plate desalting approach has also been successfully applied to peptide analysis from protein digests.

Ultrathin calcinated films on a gold surface for highly effective laser desorption/ionization of biomolecules

We report a nanoscale calcinated silicate film fabricated on a gold substrate for highly effective, matrix-free laser desorption ionization mass spectrometry (LDI-MS) analysis of biomolecules. The calcinated film is prepared by a layer-by-layer (LbL) deposition/calcination process wherein the thickness of the silicate layer and its surface properties are precisely controlled. The film exhibits outstanding efficiency in LDI-MS with extremely low background noise in the low-mass region, allowing for effective analysis of low mass samples and detection of large biomolecules including amino acids, peptides, and proteins. Additional advantages for the calcinated film include ease of preparation and modification, high reproducibility, low cost, and excellent reusability. Experimental parameters that influence LDI on calcinated films have been systemically investigated. Presence of citric acid in the sample significantly enhances LDI performance by facilitating protonation of the analyte and reducing fragmentation. The wetting property and surface roughness appear to be important factors that manipulate LDI performance of the analytes. This new substrate presents a marked advance in the development of matrix-free mass spectrometric methods and is uniquely suited for analysis of biomolecules over a broad mass range with high sensitivity. It may open new avenues for developing novel technology platforms upon integration with existing methods in microfluidics and optics.

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

A novel matrix-free LDI MS platform using a thin film of patterned nanostructured gold, capped with methyl- and carboxy-terminated self-assembled monolayers (SAMs) is presented. Calibration on the matrix-free LDI surface was performed using a peptide standard mixture available for MALDI analysis. MS analysis for limit of detection was performed using angiotensin I peptide. Peptide fragments from standard protein digests of bovine serum albumin, bovine catalase, and bovine lactoperoxidase were used to carry out peptide mass fingerprinting analysis. Sequence coverage of each protein digest and the number of detected peptide fragments were compared with conventional MALDI MS on a standard MALDI plate. Versatility of the nanostructured gold LDI substrate is illustrated by performing MS analysis on a protein digest using different enzymes and by small molecule MS analysis.

Detection of proteins and protein-ligand complexes using HgTe nanostructure matrixes in surface-assisted laser desorption/ionization mass spectrometry

We have analyzed peptides, proteins, and protein-drug complexes through surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) using HgTe nanostructures as matrixes. We investigated the effects of several parameters, including the concentration of the HgTe nanostructures, the pH of the buffer, and the concentration of salt, on the performance of this system. When HgTe nanostructures are used as matrixes, [M + H](+) ions were the dominant signals. Relative to other commonly used nanomaterials, HgTe nanostructures provided lower background signals from metal clusters, fewer fragment ions, less interference from alkali-adducted analyte ions, and a higher mass range (up to 150,000 Da). The present approach provides limits of detection for angiotensin I and bovine serum albumin of 200 pM and 14 nM, respectively, with great reproducibility (RSD: <25%). We validated the applicability of this method through the detections of (i) the recombinant proteins that were transformed in E. coli, (ii) the specific complex between bovine serum albumin and l-tryptophan, and (iii) a carbonic anhydrase-acetazolamide complex. Our results suggest that this novel and simple SALDI-MS approach using HgTe nanostructures as matrixes might open several new ways for proteomics and the analysis of drug-protein complexes.

Ultrasonication-assisted spray ionization mass spectrometry for the analysis of biomolecules in solution

In this paper, we describe a novel technique--ultrasonication-assisted spray ionization (UASI)--for the generation of singly charged and multiply charged gas-phase ions of biomolecules (e.g., amino acids, peptides, and proteins) from solution; this method employs a low-frequency ultrasonicator (ca. 40 kHz) in place of the high electric field required for electrospray ionization. When a capillary inlet is immersed into a sample solution within a vial subjected to ultrasonication, the solution is continually directed to the capillary outlet as a result of ultrasonication-assisted capillary action; an ultrasonic spray of the sample solution is emitted at the outlet of the tapered capillary, leading to the ready generation of gas-phase ions. Using an ion trap mass spectrometer, we found that singly charged amino acid and multiply charged peptides/proteins ions were generated through this single-step operation, which is both straightforward and extremely simple to perform. The setup is uncomplicated: only a low-frequency ultrasonicator and a tapered capillary are required to perform UASI. The mass spectra of the multiply charged peptides and proteins obtained from sample solutions subjected to UASI resemble those observed in ESI mass spectra.

Mass spectrometry imaging of pharmacological compounds in tissue sections

The use of MS imaging (MSI) to resolve the spatial and pharmacodynamic distributions of compounds in tissues is emerging as a powerful tool for pharmacological research. Unlike established imaging techniques, only limited a priori knowledge is required and no extensive manipulation (e.g., radiolabeling) of drugs is necessary prior to dosing. MS provides highly multiplexed detection, making it possible to identify compounds, their metabolites and other changes in biomolecular abundances directly off tissue sections in a single pass. This can be employed to obtain near cellular, or potentially subcellular, resolution images. Consideration of technical limitations that affect the process is required, from sample preparation through to analyte ionization and detection. The techniques have only recently been adapted for imaging and novel variations to the established MSI methodologies will further enhance the application of MSI for pharmacological research.

Effect of urea surface modification and photocatalytic cleaning on surface-assisted laser desorption ionization mass spectrometry with amorphous TiO2 nanoparticles

We have investigated the effect of urea surface modification and the photocatalytic cleaning on surface-assisted laser desorption ionization mass spectrometry (SALDI-MS) with amorphous TiO2 nanoparticles for the reduction of the background noise and the improvement of the sensitivity. In the use of nanoparticles of high surface area, chemical background signals arising from ambient environments and organic contaminants can frequently be serious problems below 500 Da, possibly reducing the advantages of the matrix-free approach. In this study, removal of contaminants and enhanced SALDI efficiency were easily achieved with UV irradiation via the photocatalyst effect of TiO2 before SALDI-MS measurements. The surface cleaning achieved by the UV photocatalytic procedure reduced the background noise and increased the peak intensities of peptides. In addition, we found that urea surface modification of TiO2 nanoparticles increased the performance of the TiO2-SALDI-MS. (1) The urea-surface modification of TiO2 made it possible to produce proton-adduct forms without citrate buffer, resulting in low background noises below 500 Da, in contrast to the essential use of a citrate buffer in the bare TiO2-SALDI-MS. (2) The detection sensitivity of angiotensin I increased to 0.3 fmol with the urea-surface modification, as compared to the use of bare TiO2 nanoparticles (6 fmol). The urea-TiO2 could ionize proteins of more than 20,000 Da such as trypsinogen (600 fmol). (3) The urea modification of TiO2 had the advantage of selective detection of phosphopeptides without sample clean up, or prefractionation in tryptic digest products of bovine hemoglobin.

Surface assisted laser desorption/ionization on two-layered amorphous silicon coated hybrid nanostructures

Matrix-free laser desorption/ionization was studied on two-layered sample plates consisting of a substrate and a thin film coating. The effect of the substrate material was studied by depositing thin films of amorphous silicon on top of silicon, silica, polymeric photoresist SU-8, and an inorganic-organic hybrid. Des-arg(9)-bradykinin signal intensity was used to evaluate the sample plates. Silica and hybrid substrates were found to give superior signals compared with silicon and SU-8 because of thermal insulation and compatibility with amorphous silicon deposition process. The effect of surface topography was studied by growing amorphous silicon on hybrid micro- and nanostructures, as well as planar hybrid. Compared with planar sample plates, micro- and nanostructures gave weaker and stronger signals, respectively. Different coating materials were tested by growing different thin film coatings on the same substrate. Good signals were obtained from titania and amorphous silicon coated sample plates, but not from alumina coated, silicon nitride coated, or uncoated sample plates. Overall, the strongest signals were obtained from oxygen plasma treated and amorphous silicon coated inorganic-organic hybrid, which was tested for peptide-, protein-, and drug molecule analysis. Peptides and drugs were analyzed with little interference at low masses, subfemtomole detection levels were achieved for des-arg(9)-bradykinin, and the sample plates were also suitable for ionization of small proteins.

CHCA-modified Au nanoparticles for laser desorption ionization mass spectrometric analysis of peptides

Gold nanoparticles (AuNPs) have been studied as a potential solid-state matrix for laser desorption/ionization mass spectrometry (LDI-MS) but the efficiency in ionization remains low. In this report, AuNPs are capped by a self-assembled monolayer of cysteamine and modified with alpha-cyano-4-hydroxycinnamic acid (CHCA) for effective MALDI measurements. CHCA-terminated AuNPs offer marked improvement on peptide ionization compared with citrate-capped or cysteamine-capped AuNPs. The coating also effectively suppresses formation of Au cluster ions and analyte fragment ions, leading to cleaner mass spectra. Addition of glycerol and citric acid to the peptide/AuNPs sample further improves the performance of these AuNPs for LDI-MS analysis. Glycerol appears to enhance the dispersion of AuNPs in sample spots, increasing the sample ionization and shot-to-shot reproducibility, while citric acid serves as an external proton donor, providing high production of protonated analyte ions and reducing fragmentation of peptides on the nanoparticle-based surface. Optimal ratios of citric acid, glycerol, and AuNPs in sample solution have been systematically studied. A more than 10-fold increase for desorption ionization of peptides can be achieved by combining 5% glycerol and 20 mM citric acid with the CHCA-terminated AuNPs. The applicability of the CHCA-AuNPs for LDI-MS analysis of protein digests has also been demonstrated. This work shows the potential of AuNPs for SALDI-MS analysis, and the improvement with chemical functionalization, controlled dispersion, and use of an effective proton donor.

Matrix-less laser desorption/ionisation mass spectrometry of polyphenols in red wine

Matrix-assisted laser desorption/ionisation (MALDI) of small molecules is challenging and in most cases impossible due to interferences from matrix ions precluding analysis of molecules <300-500 Da. A common matrix such as ferulic acid belongs to an important class of compounds associated with antioxidant activity. If the shared phenolic structure is related to the propensity as an active MALDI matrix then it follows that direct laser desorption/ionisation should be possible for polyphenols. Indeed matrix-less laser desorption/ionisation mass spectrometry is achieved whereby the analyte functions as a matrix and was used to monitor low molecular weight compounds in wine samples. Sensitivity ranging from 0.12-87 pmol/spot was achieved for eight phenolic acids (4-coumaric, 4-hydroxybenzoic, caffeic, ferulic, gallic, protocatechuic, syringic, vanillic) and 0.02 pmol/spot for trans-resveratrol. Additionally, 4-coumaric, 4-hydroxybenzoic, caffeic, ferulic, gallic, syringic, vanillic acids and trans-resveratrol were identified in wine samples using accurate mass measurements consistent with reported profiles based on liquid chromatography (LC)/MS. Minimal sample pre-treatment make the technique potentially appropriate for fingerprinting, screening and quality control of wine samples.