Nile Red-Adsorbed Gold Nanoparticle Matrixes for Determining Aminothiols through Surface-Assisted Laser Desorption/Ionization 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).

Laser desorption ionization of small molecules assisted by tungsten oxide and rhenium oxide particles

Inorganic metal oxides have shown potential as matrices for assisting in laser desorption ionization with advantages over the aromatic acids typically used. Rhenium and tungsten oxides are attractive options due to their high work functions and relative chemical inertness. In this work, it is shown that ReO3 and WO3 , in microparticle (μP) powder forms, can efficiently facilitate ionization of various types of small molecules and provide minimized background contamination at analyte concentrations below 1 ng/µL. This study shows that untreated inorganic WO3 and ReO3 particles are valid matrix options for detection of protonatable, radical, and precharged species under laser desorption ionization. Qualitatively, the WO3 μP showed improved detection of apigenin, sodiated glucose, and precharged analyte choline, while the ReO3 μP allowed better detection of protonated cocaine, quinuclidine, ametryn, and radical ions of polyaromatic hydrocarbons at detection levels as low as 50 pg/µL. For thermometer ion survival yield experiments, it was also shown that the ReO3 powder was significantly softer than α-cyano-4-hydroxycinnaminic acid. Furthermore, it provided higher intensities of cocaine and polyaromatic hydrocarbons, at laser flux values equal to those used with α-cyano-4-hydroxycinnaminic acid.

A turn-on near-infrared fluorescent chemosensor for selective detection of lead ions based on a fluorophore-gold nanoparticle assembly

A turn-on fluorescent chemosensor of Pb(2+) in the near-infrared (NIR) region, which is based on the Pb(2+)-tuned restored fluorescence of a weakly fluorescent fluorophore-gold nanoparticle (AuNPs) assembly, has been reported. In this fluorophore-AuNP assembly, NIR fluorescent dye brilliant cresyl blue (BCB) molecules act as fluorophores and are used for signal transduction of fluorescence, while AuNPs act as quenchers to quench the nearby fluorescent BCB molecules via electron transfer. In the presence of Pb(2+), fluorescent BCB molecules detached from AuNPs and restored their fluorescence due to the formation of a chelating complex between Pb(2+) and glutathione confined on AuNPs. Under the optimal conditions, the present BCB-AuNP assembly is capable of detecting Pb(2+) with a concentration ranging from 7.5 × 10(-10) to 1 × 10(-8) mol L(-1) (0.16-2.1 ng mL(-1)) and a detection limit of 0.51 nM (0.11 ng mL(-1)). The present BCB-AuNP assembly can be used in aqueous media for the determination of Pb(2+) unlike common organic fluorescent reagents, and also shows advantages of NIR fluorescence spectrophotometry such as less interference, lower detection limit, and higher sensitivity. Moreover, the present method was successfully applied for the detection of Pb(2+) in water samples with satisfactory results.

Detection of posaconazole by surface-assisted laser desorption/ionization mass spectrometry with dispersive liquid-liquid microextraction

A simple, rapid, and sensitive method for the detection of posaconazole using dispersive liquid-liquid microextraction (DLLME) coupled to surface-assisted laser desorption/ionization mass spectrometric detection (SALDI/MS) was developed. After the DLLME, posaconazole was detected using SALDI/MS with colloidal gold and α-cyano-4-hydroxycinnamic acid (CHCA) as the co-matrix. Under optimal extraction and detection conditions, the calibration curve, which ranged from 1.0 to 100.0 nM for posaconazole, was observed to be linear. The limit of detection (LOD) at a signal-to-noise ratio of 3 was 0.3 nM for posaconazole. This novel method was successfully applied to the determination of posaconazole in human urine samples.

CuFe2O4 magnetic nanocrystal clusters as a matrix for the analysis of small molecules by negative-ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

CuFe₂O₄ MNCs were proposed as a new matrix for negative ion MALDI-TOF MS, which exhibited interference-free background, high salt tolerance and good reproducibility for analysis of small molecules.

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.

A lithium-rich composite metal oxide used as a SALDI-MS matrix for the determination of small biomolecules

A lithium-rich composite metal oxide material used as a SALDI matrix for high throughput analysis of small molecules.

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.

Layer-by-layer thin film of reduced graphene oxide and gold nanoparticles as an effective sample plate in laser-induced desorption/ionization mass spectrometry

This work demonstrated a simple platform for rapid and effective surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS) measurements based on the layer structure of reduced graphene oxide (rGO) and gold nanoparticles. A multi-layer thin film was fabricated by alternate layer-by-layer depositions of rGO and gold nanoparticles (LBL rGO/AuNP). The flat and clean two-dimensional film was served as the sample plate and also functioned as the matrix in SALDI-TOF MS. By simply one-step deposition of analytes onto the LBL rGO/AuNP sample plate, the MS measurements of various homogeneous samples were ready to execute. The optimization of MS signal was reached by the variation of the layer numbers of rGO and gold nanoparticles. Also, the small molecules including amino acids, carbohydrates and peptides were successfully analyzed in SALDI-TOF MS using the LBL rGO/AuNP sample plate. The results showed that the signal intensity, S N(-1) ratio and reproducibility of SALDI-TOF spectra have been significantly improved in comparison to the uses of gold nanoparticles or α-cyano-4-hydroxy-cinnamic acid (CHCA) as the assisted matrixes. Taking the advantages of the unique properties of rGO and gold nanoparticles, the ready-to-use MS sample plate, which could absorb and dissipate laser energy to analytes quite efficiently and homogeneously, has shown great commercial potentials for MS applications.

Synthesis of graphene-ZnO-Au nanocomposites for efficient photocatalytic reduction of nitrobenzene

A simple hydrothermal method of preparing highly photocatalytic graphene-ZnO-Au nanocomposites (G-ZnO-Au NCs) has been developed. Zinc acetate and graphene oxide are reduced by catechin to form graphene-zinc oxide nanospheres (G-ZnO NSs; average diameter of (45.3 ± 3.7) nm) in the presence of ethylenediamine (EDA) as a stabilizing agent and gold nanorods (Au NRs) at 300 °C for 2 h. Then Au NRs are deposited onto as-formed G-ZnO NSs to form G-ZnO-Au NCs. Upon ultraviolet light activation, G-ZnO-Au NCs (4 mg mL(-1)) in methanol generates electron-hole pairs. Methanol (hydroxyl group) assists in trapping holes, enabling photogenerated electrons to catalyze reduction of nitrobenzene (NB) to aniline with a yield of 97.8% during a reaction course of 140 min. The efficiency of G-ZnO-Au NCs is 3.5- and 4.5-fold higher than those provided by commercial TiO2 and ZnO NSs, respectively. Surface assisted laser desorption/ionization mass spectrometry has been for the first time applied to detect the intermediates (nitrosobenzene and phenylhydroxylamine) and major product (aniline) of NB through photoelectrocatalytic or photocatalytic reactions. The result reveals that the reduction of NB to aniline is through nitrosobenzene to phenylhydroxylamine in the photoelectrocatalytic reaction, while via nitrosobenzene directly in the photocatalytic reaction. G-ZnO-Au NC photocatalyst holds great potential in removal of organic pollutants like NB and in the production of aniline.

Stable label-free fluorescent sensing of biothiols based on ThT direct inducing conformation-specific G-quadruplex

In this work, a new, label-free, turn-on fluorescent sensor for biothiols detection based on ThT direct inducing conformation-specific G-quadruplex is developed. The sensing approach is based on a conformational switch of oligonucleotide controlled by Hg(2+) and a commercially available water-soluble fluorescent dye, Thioflavin T (ThT). A noticeable fluorescence light-up in ThT on binding to the G-quadruplex grants the sensor excellent sensitivity. The specific quadruplex conformation induced directly by ThT and pronounced structural selectivity of ThT for G-quadruplexes could generate more stable luminescence and make sure high specificity in complex biological samples. The present assay allows for the selective determination of cysteine and glutathione in the range of 2.0 × 10(-8)-2.5 × 10(-6)M and 3.0 × 10(-8)-2.0 × 10(-6)M with a detection limit of 8.4 nM and 13.9 nM respectively. The diagnostic capability and potential in practical applications of this method have been demonstrated by detecting biothiols in human blood serum.

Furoic and mefenamic acids as new matrices for matrix assisted laser desorption/ionization-(MALDI)-mass spectrometry

The present study introduces two novel organic matrices for matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) for the analysis of small molecules. The first matrix is "2-amino-4,5-diphenylfuran-3-carboxylic acid" (also called furoic acid, FA) which was synthesized and then characterized by ultraviolet (UV), infrared (FTIR), nuclear magnetic resonance NMR ((1)H and (13)C) and mass spectrometry. The compound has organic semiconductor properties and exhibits intense UV-absorption which is suitable for the UV-MALDI laser (N2 laser, 337 nm). The second matrix is mefenamic acid (MA). The two matrices can be successfully applied for various classes of compounds including adenosine-5'-triphosphate (ATP, 0.5 µL(10.0 nmol)), spectinomycin (spect, 0.5 µL(14.0 nmol)), glutathione (GSH, 0.5 µL(9.0 nmol)), sulfamethazole (SMT, 0.5 µL(2.0 nmol)) and mixture of peptides gramicidin D (GD, 0.5µL (9.0 nmol)). The two matrices can effectively absorb the laser energy, resulting in excellent desorption/ionization of small molecules. The new matrices offer a significant enhancement of ionization, less fragmentation, few interferences, nice reproducibility, and excellent stability under vacuum. Theoretical calculations of the physical parameters demonstrated increase in polarizability, molar volume and refractivity than the conventional organic matrices which can effectively enhance the proton transfer reactions between the matrices with the analyte molecules. While the reduction in density, surface tension and index of refraction can enhance homogeneity between the two new matrices with the analytes. Due to the sublimation energy of mefenamic acid is (1.2 times) higher than that of the DHB, it is more stable to be used in the vacuum.

Human serum albumin-modified Fe3O4 magnetic nanoparticles for affinity-SALDI-MS of small-molecule drugs in biological liquids

Here, we report on the use of human serum albumin (HSA)-modified Fe(3)O(4) nanoparticles (NPs) (HSA-Fe(3)O(4) NPs) for affinity-SALDI-MS of small drugs in human biological liquids. We demonstrated that HSA-Fe(3)O(4) NPs effectively captured small drugs from human urine and serum via the interactions between HSA and these drugs. The drugs adsorbed on HSA could then be identified by directly introducing the HSA-Fe(3)O(4) NPs into a mass spectrometer for SALDI-MS analysis. The ability of HSA to interact with multiple small drugs facilitated the simultaneous detection of a 4-drug-mixture in serum, viz., phenytoin, ibuprofen, camptothecin, and warfarin sodium, by affinity-SALDI-MS using HSA-Fe(3)O(4) NPs. In contrast, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) with an organic matrix could detect only warfarin sodium. We also demonstrated the capacity of affinity-SALDI-MS to quantify warfarin sodium in urine samples across a range of 50 - 1000 μM (R(2) = 0.998) when using HSA-Fe(3)O(4) NPs. The detection sensitivity was further improved to a range of 5 - 100 μM (R(2) = 0.999) by using denatured HSA. The open structure of denatured HSA may enhance the effective extraction of small drugs from biological liquids, and increase the detection-sensitivity of affinity-SALDI-MS. Affinity-SALDI-MS using protein-modified Fe(3)O(4) NPs can open up new approaches to the analytical detection of small drugs in biological liquids by SALDI-MS.

Quantum dot applications endowing novelty to analytical proteomics

This review surveys all the state-of-art applications of quantum dots (QDs) in conventional and modern analytical methods in proteomic studies. A brief introduction of QDs and their properties is initially presented followed by outlining the application of QDs in fluorescence, MS, imaging, and cancer-based proteomics. The in-depth application of QDs in MALDI-MS and surface assisted laser desorption/ionization-MS has been elaborately discussed, summarizing the speculated mechanism behind the protein-QDs interactions during QD matrix applications leading to enhanced detection sensitivity.

Fast nucleotide identification through fingerprinting using gold nanoparticle-based surface-assisted laser desorption/ionisation

We report a method centred on gold nanoparticle-based surface-assisted laser desorption/ionisation for analysis of deoxynucleotides and alkylated nucleobases. Gold nanoparticles allow for enhanced analysis capability by eliminating undesired signature peaks; thus more elegant mass spectra can be attained that allow identification by nucleotide mass fingerprint. The resulting fingerprinting patterns on the spectra are compared and associated with the presence of different nucleotides in the sample. This method can be easily extended to modified nucleotides implicated in genome lesions due to exposure to environment chemicals, such as DNA adducts (e.g. guanine adducts). The use of gold nanoparticles for surface-assisted laser desorption/ionisation can be an useful tool to resolve common issues of background noise when analysing nucleic acids samples.

Functionalized graphene-coated cobalt nanoparticles for highly efficient surface-assisted laser desorption/ionization mass spectrometry analysis

Graphene-coated cobalt nanoparticles surface-functionalized with benzylamine groups (CoC-NH(2) nanomagnets) were shown to effectively enrich analytes for surface-assisted laser desorption/ionization mass spectrometry (affinity SALDI-MS) analysis. These CoC-NH(2) nanomagnets are highly suited for use with affinity SALDI-MS because their mean diameter of 30 nm, high specific surface area of 15 m(2) g(-1), and high-strength saturation magnetization of 158 emu g(-1) led to efficient extraction of analytes by magnetic separation, which in turn enabled excellent SALDI-MS performance. Surface modification of CoC nanomagnets with benzylamine groups increased the yield of peptide ions and decreased fragmentation of benzylpyridinium ions, so-called "thermometer ions" formed through soft ionization. The CoC-NH(2) nanomagnets were used to extract perfluorooctanesulfonate from large volumes of aqueous solutions by magnetic separation, which was identified directly by SALDI-MS analysis with high sensitivity even at the sub-part-per-trillion level (∼0.1 ng/L). The applicability of CoC-NH(2) nanomagnets in conjunction with SALDI-MS for the enrichment and detection of pentachlorophenol, bisphenol A, and polyfluorinated compounds (PFCs) with varying chain length, which are environmentally significant compounds, as well as small drugs, was also evaluated.

Development of nanomaterials for SALDI-MS analysis in forensics

Within the last decade, the escalation of research output in the field of nanotechnology has spurred the development of new nanomaterials for use as assisting agents in surface assisted laser desorption ionization mass spectrometry (SALDI-MS). Specifically modified nanomaterials, coupled with mass spectrometry, have improved the detection sensitivity, specificity, flexibility and reproducibility of SALDI-MS analysis. The technological advancement of LDI-MS has in turn, propelled the use of the analytical technique in the field of forensics. In this report, the various roles and applications of metal-, silicon- and carbon-based nanostructured materials as SALDI matrices in the analysis of forensic samples are described. The advantages of SALDI-MS as an analytical tool for forensic sample analysis are also discussed.

Detection of melamine in infant formula and grain powder by surface-assisted laser desorption/ionization mass spectrometry

We have developed a method for the determination of melamine (MEL), ammeline (AMN), and ammelide (AMD) by surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) using gold nanoparticles (Au NPs). The major peaks for MEL, AMN, and AMD at m/z 127.07, 128.05, and 129.04 are assigned to the [MEL + H](+), [AMN + H](+), and [AMD + H](+) ions. Because the three tested compounds adsorb weakly onto the surfaces of the Au NPs through Au-N bonding, they can be easily concentrated from complex samples by applying a simple trapping/centrifugation process. The SALDI-MS method provides limits of detection of 5, 10, and 300 nM for MEL, AMN, and AMD, respectively, at a signal-to-noise ratio of 3. The signal variation for 150-shot average spectra of the three analytes within the same spot was 15%, and the batch-to-batch variation was 20%. We have validated the practicality of this approach by the analysis of these three analytes in infant formula and grain powder. This simple and rapid SALDI-MS approach holds great potential for screening of MEL in foods.

Using surface-assisted laser desorption/ionization mass spectrometry to detect proteins and protein-protein complexes

In this study, we combined surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) with HgTe nanostructures as matrix for the detection of several proteins (α1-antitrypsin, trypsin, IgG, protein G) and their complexes. We investigated the effects of several parameters (the concentration and nature of surfactants and metal ions, the pH, and concentration of the analytes in the sample matrixes) on the sensitivity of the detection of these proteins and their complexes. The presence of stabilizing Brij 76 surfactant and Zn(II) ions allowed the detection of weak protein complexes, such as α1-antitrypsin-trypsin and IgG-protein G complexes, at the picomole level. We observed multiply charged states at m/z 72,160 ([α1-antitrypsin + trypsin + H](+)) and 86,585 ([IgG + protein G + 2H](2+)) for the α1-antitrypsin-trypsin and IgG-protein G complexes, respectively. To the best of our knowledge, detection of weak protein complexes and determination of their stoichiometry have not been demonstrated previously when a combination of SALDI-MS and nanostructures were used. This simple and reproducible SALDI-MS approach using HgTe nanostructures holds great potential for the detection of other proteins and their complexes.

Detection of biothiols in cells by a terbium chelate-Hg (II) system

Great efforts have been devoted to the development of sensitive and specific analysis methods for biothiols because of their important roles in biological systems. We present a new detection system for biothiols that is based on the reversible quenching and restoration of fluorescence of terbium chelate caused by Hg(2+) and thiol species. In the presence of biothiols, a restoration of fluorescence of terbium chelate after quenching by Hg(2+) was observed due to the interaction of Hg(2+) with thiol groups, and the restored fluorescence increased with the concentration of biothiols. This method was sensitive and selective for biothiols. The detection limit was 80 nM for glutathione, 100 nM for Hcy, and 400 nM for Cysteine, respectively. The terbium chelate-Hg (II) system was successfully applied to determine the levels of biothiols in cancer cells and urine samples. Further, it was also shown to be comparable to Ellman's assay. Compared to other fluorescence methods, the terbium chelate probe is advantageous because interference from short-lived nonspecific fluorescence can be efficiently eliminated due to the long fluorescence lifetime of terbium chelate, which allows for detection by time-resolved fluorescence. The terbium chelate probe can serve as a diagnostic tool for the detection of abnormal levels of biothiols in disease.