Determining enediol compounds in tea using surface-assisted laser desorption/ionization mass spectrometry with titanium dioxide nanoparticle matrices

Steroid hormones analysis with surface-assisted laser desorption/ionization mass spectrometry using catechin-modified titanium dioxide nanoparticles

This paper describes the application of catechin-modified titanium dioxide nanoparticles (TiO(2) NPs) as matrices to analyze four steroid hormones by surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). The catechin-modified TiO(2) NPs have high absorbance at 337 nm and are effective SALDI matrices when using a nitrogen laser. Four test steroid hormones-cortisone, hydrocortisone, progesterone, and testosterone-were directly analyzed by SALDI-MS. The limits of detection at a signal-to-noise ratio of 3 for cortisone, hydrocortisone, progesterone, and testosterone were 1.62, 0.70, 0.66, and 0.23 μM, respectively. This approach provides good quantitative linearity for the four analytes (R(2)>0.986) with good reproducibility (the shot-to-shot and batch-to-batch variations for the four analytes were less than 10% and 15%, respectively). We validated the practicality of this approach-considering its advantages in sensitivity, repeatability, rapidity, and simplicity-through the analysis of testosterone in a urine sample.

On particle ionization/enrichment of multifunctional nanoprobes: washing/separation-free, acceleration and enrichment of microwave-assisted tryptic digestion of proteins via bare TiO2 nanoparticles in ESI-MS and comparing to MALDI-MS

A simple, rapid, straightforward and washing/separation free of in-solution digestion method for microwave-assisted tryptic digestion of proteins (cytochrome c, lysozyme and myoglobin) using bare TiO(2) nanoparticles (NPs) prepared in aqueous solution to serve as multifunctional nanoprobes in electrospray ionization mass spectrometry (ESI-MS) was demonstrated. The current approach is termed as 'on particle ionization/enrichment (OPIE)' and it can be applied in ESI-MS, atmospheric pressure-matrix-assisted laser desorption/ionization mass spectrometry (AP-MALDI-MS) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). The bare TiO(2) NPs can assist, accelerate and effectively enhance the digestion efficiency, sequence coverage and detection sensitivity of peptides for the microwave-assisted tryptic digestion of proteins in ESI-MS. The reason is attributed to the fact that proteins or partially digested proteins are easily attracted or concentrated onto the surface of TiO(2) NPs, resulting in higher efficiency of digestion reactions in the microwave experiments. Besides, the TiO(2) NPs could act as a microwave absorber to accelerate and enrich the protein fragments in a short period of time (40-60 s) from the microwave experiments in ESI-MS. Furthermore, the bare TiO(2) NPs prepared in aqueous solution exhibit high adsorption capability toward the protein fragments (peptides); thus, the OPIE approach for detecting the digested protein fragments via ESI and MALDI ionization could be achieved. The current technique is also a washing and separation-free technique for accelerating and enriching microwave-assisted tryptic digestion of proteins in the ESI-MS and MALDI-MS. It exhibits potential to be widely applied to biotechnology and proteome research in the near future.

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.

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.

Nanomaterial-based surface-assisted laser desorption/ionization mass spectrometry of peptides and proteins

We have investigated six nanomaterials for their applicability as surfaces for the analyses of peptides and proteins using surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). Gold nanoparticles (NPs) were useful nanomaterials for small analytes (e.g., glutathione); Pt nanosponges and Fe(3)O(4) NPs were efficient nanomaterials for proteins, with an upper detectable mass limit of ca. 25 kDa. Nanomaterials have several advantages over organic matrices, including lower limits of detection for small analytes and lower batch-to-batch variations (fewer problems associated with "sweet spots"), when used in laser desorption/ionization mass spectrometry.

Effects of ZnO nanowire length on surface-assisted laser desorption/ionization of small molecules

The effects of nanowire (NW) length on surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) of small molecules were investigated using ZnO NWs of 50 nm diameter with a broad range of lengths ranging from 25 to 1600 nm. Characterization of the ZnO NWs revealed that the length was the only parameter that varied in this study, while other properties of the NWs remained essentially the same as the bulk properties. Experiments on SALDI efficiency exhibited that the SALDI processes on NWs have a certain length window. In the present case of ZnO NWs, the SALDI efficiency was found to be enhanced on the nanowires of 250 nm length, corresponding to an aspect ratio of 5. The roles of NW length in the SALDI processes were discussed from the viewpoint of efficient energy-transfer media as well as physical obstacles screening laser irradiation and preventing the escape of nascent ions from NW surfaces. The existence of the length window may provide valuable insight for tailoring new nanostructures for efficient SALDI of small molecules.

Quantification of captopril in urine through surface-assisted laser desorption/ionization mass spectrometry using 4-mercaptobenzoic acid-capped gold nanoparticles as an internal standard

We have developed a new internal standard method for the determination of the concentration of captopril (CAP) through surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) using gold nanoparticles (Au NPs). This approach provided linearity for CAP over the concentration range 2.5-25 microM (R(2) = 0.987), with a limit of detection (signal-to-noise ratio = 3) of 1.0 microM. The spot-to-spot variations in the concentration of CAP through SALDI-MS analyses performed in the absence and presence of the internal standard were 26% and 9%, respectively (15 measurements). This approach provides simplicity, accuracy, precision, and great reproducibility to the determination of the levels of CAP in human urine samples.

Accurate quantitation of glutathione in cell lysates through surface-assisted laser desorption/ionization mass spectrometry using gold nanoparticles

We developed a method for the determination of three aminothiols--cysteine, glutathione (GSH), and homocysteine--using surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). The analytes were first captured using the unmodified 14-nm gold nanoparticles; N-2-mercaptopropionylglycine-modified gold nanoparticles serving as internal standard were sequentially added, and then the sample was analyzed using SALDI-MS. This approach provided good quantitative linearity of the three analytes (R(2) = approximately 0.99), with good reproducibility (relative standard deviations: <10%), in the analyses of GSH in the lysates of human red blood cells and MCF-7 cancer breast cells in the presence and absence of the anti-inflammatory drug sulfasalazine. The internal-standard SALDI-MS approach provides simplicity, accuracy, and precision to the determination of GSH in cells under drug invasion, to open an avenue for SALDI-MS to be used for the precise quantitative determination of a variety of analytes. From the clinical editor: This paper reports the development of a surface assisted laser desorption/ionization mass spectrometry method to precisely determine aminothiols-cysteine (Cys), glutathione (GSH), and homocysteine (HCys).

Functionalized pyrolytic highly oriented graphite polymer film for surface-assisted laser desorption/ionization mass spectrometry in environmental analysis

The pyrolytic highly oriented graphite polymer film (PGS) was first employed to analyze low-mass analytes in environmental analysis by surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). PGS is a synthetic uniform and highly oriented graphite polymer film with high thermal anisotropic conductivity. We have found that negative ion mode SALDI-MS using oxidized PGS (PGS-SALDI-MS) can be used to detect [M-H]- ions from perfluorooctanoic acid (PFOA) and other perfluoroalkylcarboxylic acids when the PGS surface is modified with the cationic polymer polyethyleneimine (PEI). The signal intensity of PFOA when employing the PEI modification showed a ten-fold increase over that obtained from desorption/ionization on porous silicon (DIOS). PFOA was quantified using PGS-SALDI-MS and the calibration curve showed a wide linear dynamic range of response (20-1000 ppb). The combination of atmospheric pressure ionization and PGS (AP-PGS-SALDI) showed greater signal intensity than vacuum PGS-SALDI for deprotonated PFOA. Several other environmentally important chemicals, including perfluoroalkylsulfonic acid, pentachlorophenol, bisphenol A, 4-hydroxy-2-chlorobiphenyl, and benzo[a]pyrene, were also successfully used to evaluate PGS-SALDI-MS. In addition, we found that nonafluoro-1-butanesulfonic acid was able to produce protonated peptides in positive ion PGS-SALDI-MS, but that perfluoropentanoic acid and trifluoroacetic acid were not. It is suggested that perfluoroalkylsulfonic acids are better protonating agents than perfluoroalkylcarboxylic acids in SALDI-MS.

Detection of aminothiols through surface-assisted laser desorption/ionization mass spectrometry using mixed gold nanoparticles

We have employed mixtures of two differently sized (average diameters: 3.5 and 14 nm) gold nanoparticles (Au NPs) as selective probes and matrices for the determination of aminothiols using surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). When using 38 and 150 pM solutions of the 3.5- and 14-nm Au NPs, respectively, as the probe and matrix, SALDI-MS provided limits of detection (signal-to-noise ratio = 3) of 2, 20, and 44 nM for 1.0 mL solutions of glutathione (GSH), cysteine (Cys), and homocysteine, respectively. The signal intensities of these analytes varied by less than 20% for SALDI-MS analyses recorded over 50 sample spots; in contrast, they varied by as much as 60% when using a conventional matrix (2,5-dihydroxybenzoic acid). We validated the practicality of this approach - with its advantages of sensitivity, reproducibility, rapidity, and simplicity - through the analysis of GSH in MCF-7 cell lysates and Cys in plasma.

Silver-coated gold nanoparticles as concentrating probes and matrices for surface-assisted laser desorption/ionization mass spectrometric analysis of aminoglycosides

A novel method for the determination of aminoglycosides by surface-assisted laser desorption/ionization mass spectrometry (SALDI MS) with the aid of silver-coated gold nanoparticles (Au@AgNPs) has been developed. The Au@AgNPs with surface capped by anionic citrate were used as concentrating probes as well as matrices in SALDI MS. Adsorption of aminoglycosides onto the nanoparticles was mainly through electrostatic attraction. The aminoglycoside-adsorbed nanoparticles were directly characterized by SALDI MS after a simple washing. Using Au@AgNPs to preconcentrate the aminoglycosides from 500 microL buffer solution, the limits of detection (LODs) at signal-to-noise ratio of 3 were 3, 25, 15, 30, and 38 nM for paromomycin, kanamycin A, neomycin, gentamicin, and apramycin, respectively. This method was successfully applied to the determination of aminoglycosides in human plasma samples. The LODs of aminoglycosides in plasma samples were 9, 130, 81, and 180 nM for paromomycin, kanamycin A, neomycin, and gentamicin, respectively. Recoveries of aminoglycosides in plasma samples were about 80%.

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.

DNA functionalized gold nanoparticles for bioanalysis

Gold nanoparticles (Au NPs) have become one of the most interesting sensing materials because of their unique size- and shape-dependent optical properties, high extinction coefficients, and super-quenching capability. Au NPs that are bioconjugated with DNA (DNA-Au NPs) have been demonstrated for selective and sensitive detection of analytes such as mercury(ii) ions, platelet-derived growth factor (PDGF), and adenosine triphosphate (ATP). This review focuses on approaches using DNA-Au NPs for colorimetric, fluorescent, and scattering detection of biopolymers and small solutes. We highlight the important roles that the size and concentration of Au NPs, the length and sequence of DNA, the nature of the capping agents, and the ionic strength and pH of solution play in determining the specificity and sensitivity of the nanosensors for the analytes. The advantages and disadvantages of different detection methods for sensing of interesting analytes using DNA-Au NPs will be discussed.

Determining estrogens using surface-assisted laser desorption/ionization mass spectrometry with silver nanoparticles as the matrix

We describe the application of silver nanoparticles (Ag NPs) as matrices for the determination of three estrogens using surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). Because Ag NPs have extremely high absorption coefficients (1.2 x 10(8) M(-1) cm(-1)) at 337 nm, they are effective SALDI matrices when using a nitrogen laser. Three tested estrogens--estrone (E1), estradiol (E2), and estriol (E3)--adsorb weakly onto the surfaces of the Ag NPs, through van der Waals forces. After centrifugation, the concentrated analytes adsorbed on the Ag NPs were subjected directly to SALDI-MS analyses, with the limits of detection for E1, E2, and E3 being 2.23, 0.23, and 2.11 microM, respectively. The shot-to-shot and batch-to-batch variations for the three analytes were less than 9% and 13%, respectively. We validated the practicality of this present approach through the quantitation of E2 in human urine. Using this approach, we determined the concentration of E2 in a sample of a pregnant woman's urine to be 0.16+/-0.05 microM (n=10).

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) of low molecular weight organic compounds and synthetic polymers using zinc oxide (ZnO) nanoparticles

We have developed surface-assisted laser desorption/ionization mass spectrometry using zinc oxide (ZnO) nanoparticles with anisotropic shapes (ZnO-SALDI-MS). The mass spectra showed low background noises in the low m/z, i.e. less than 500 u region. Thus, we succeeded in SALDI ionization on low molecular weight organic compounds, such as verapamil hydrochloride, testosterone, and polypropylene glycol (PPG) (average molecular weight 400) without using a liquid matrix or buffers such as citric acids. In addition, we found that ZnO-SALDI has advantages in post-source decay (PSD) analysis and produced a simple mass spectrum for phospholipids. The ZnO-SALDI spectra for synthetic polymers of polyethylene glycol (PEG), polystyrene (PS) and polymethylmethacrylate (PMMA) showed the sensitivity and molecular weight distribution to be comparable to matrix-assisted laser desorption/ionization (MALDI) spectra with a 2,5-dihydroxybenzoic acid (DHB) matrix. ZnO-SALDI shows good performance for synthetic polymers as well as low molecular weight organic compounds.