Desorption/Ionization on Silicon Nanowires

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.

Characterization of individual nano-objects with nanoprojectile-SIMS

Secondary ion mass spectrometry (SIMS) applied in the event-by-event bombardment/detection mode is uniquely suited for the characterization of individual nano-objects. In this approach, nano-objects are examined one-by-one, allowing for the detection of variations in composition. The validity of the analysis depends upon the ability to physically isolate the nano-objects on a chemically inert support. This requirement can be realized by deposition of the nano-objects on a Nano-Assisted Laser Desorption/Ionization (NALDI™) plate. The featured nanostructured surface provides a support where nano-objects can be isolated if the deposition is performed at a proper concentration. We demonstrate the characterization of individual nano-objects on a NALDI™ plate for two different types of nanometric bacteriophages: Qβ and M13. Scanning electron microscope (SEM) images verified that the integrity of the phages is preserved on the NALDI™ substrate. Mass spectrometric data show secondary ions from the phages are identified and resolved from those from the underlying substrate.

Laser-induced thermal desorption facilitates postsource decay of peptide ions

We investigated the thermal mechanism involved in laser desorption/ionization (LDI) of thermally labile molecules from the flat surfaces of amorphous Si (a-Si) and crystalline Si (c-Si). a-Si was selected for this study because of its thermal property, such as low thermal conductivity; thus, it was predicted to be highly susceptible to laser-induced surface heating. By virtue of lack of surface nanostructures, the flat surfaces offer a simple model system to focus on the thermal mechanism, avoiding other effects, including possible non-thermal contributions that can arise from the physical existence of surface nanostructures. For the energetics study, the internal energies of substituted benzylpyridinium ions produced by LDI on the bare and coated surfaces of a-Si and c-Si were obtained using the survival yield method. The results, including LDI thresholds, ion yields, and internal energies all suggested that the LDI mechanism would be indeed thermal, which is most likely promoted by thermal desorption caused by laser-induced surface heating. In addition, the LDI process driven by laser-induced thermal desorption (LITD) was also found to be capable of depositing an excessive internal energy in resulting LDI ions, which underwent a dissociation. It exhibited the essentially same features as in postsource decay (PSD) in MALDI-TOF/TOF mass spectrometry. We report that the LDI process by LITD offers not only a way of intact ionization but also a facile means for PSD of peptide ions, which this work demonstrates is well suited to peptide sequencing using TOF/TOF mass spectrometry.

Diamond nanowires for highly sensitive matrix-free mass spectrometry analysis of small molecules

This paper reports on the use of boron-doped diamond nanowires (BDD NWs) as an inorganic substrate for matrix-free laser desorption/ionization mass spectrometry (LDI-MS) analysis of small molecules. The diamond nanowires are prepared by reactive ion etching (RIE) with oxygen plasma of highly boron-doped (the boron level is 10(19) B cm(-3)) or undoped nanocrystalline diamond substrates. The resulting diamond nanowires are coated with a thin silicon oxide layer that confers a superhydrophilic character to the surface. To minimize droplet spreading, the nanowires were chemically functionalized with octadecyltrichlorosilane (OTS) and then UV/ozone treated to reach a final water contact angle of 120°. The sub-bandgap absorption under UV laser irradiation and the heat confinement inside the nanowires allowed desorption/ionization, most likely via a thermal mechanism, and mass spectrometry analysis of small molecules. A detection limit of 200 zeptomole for verapamil was demonstrated.

Fullerenes, nanotubes, and graphite as matrices for collision mechanism in secondary ion mass spectrometry: determination of cyclodextrin

A technique for improving the sensitivity of high mass molecular analysis is described. Three carbon species, fullerenes, single walled carbon nanotubes, and highly ordered pyrolytic graphite are introduced as matrices for the secondary ion mass spectrometry analysis of cyclodextrin (C(42)H(70)O(35), 1134 u). The fullerene and nanotubes are deposited as single deposition, and 10, 20, or 30 deposition films and cyclodextrin is deposited on top. The cyclodextrin parent-like ions and two fragments were analyzed. A 30 deposition fullerene film enhanced the intensity of cationized cyclodextrin with Na by a factor of 37. While the C(6)H(11)O(5) fragment, corresponding to one glucopyranose unit, increased by a factor of 16. Although fragmentation on fullerene is not suppressed, the intensity is twice as low as the parent-like ion. Deprotonated cyclodextrin increases by 100× and its C(8)H(7)O fragment by 10×. While the fullerene matrix enhances secondary ion emission, the nanotubes matrix film generates a basically constant yield. Graphite gives rise to lower intensity peaks than either fullerene or nanotubes. Scanning electron microscopy and atomic force microscopy provide images of the fullerene and nanotubes deposition films revealing flat and web structured surfaces, respectively. A "colliding ball" model is presented to provide a plausible physical mechanism of parent-like ion enhancement using the fullerene matrix.

Quantitative analysis of phospholipids using nanostructured laser desorption ionization targets

Since its introduction as an ionization technique in mass spectrometry, matrix-assisted laser desorption ionization (MALDI) has been applied to a wide range of applications. Quantitative small molecule analysis by MALDI, however, is limited due to the presence of intense signals from the matrix coupled with non-homogeneous surfaces. The surface used in nano-structured laser desorption ionization (NALDI) eliminates the need for a matrix and the resulting interferences, and allows for quantitative analysis of small molecules. This study was designed to analyze and quantitate phospholipid components of liposomes. Here we have developed an assay to quantitate the DPPC and DC(8,9)PC in liposomes by NALDI following various treatments. To test our method we chose to analyze a liposome system composed of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and DC(8,9)PC (1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine), as DC(8,9)PC is known to undergo cross-linking upon treatment with UV (254 nm) and this reaction converts the monomer into a polymer. First, calibration curves for pure lipids (DPPC and DC(8,9)PC) were created using DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) as an internal standard. The calibration curve for both DPPC and DC(8,9)PC showed an R(2) of 0.992, obtained using the intensity ratio of analyte and internal standard. Next, DPPC:DC(8,9)PC liposomes were treated with UV radiation (254 nm). Following this treatment, lipids were extracted from the liposomes and analyzed. The analysis of the lipids before and after UV exposure confirmed a decrease in the signal of DC(8,9)PC of about 90%. In contrast, there was no reduction in DPPC signal.

Analysis of deprotonated acids with silicon nanoparticle-assisted laser desorption/ ionization mass spectrometry

Chemically modified silicon nanoparticles were applied for the laser desorption/negative ionization of small acids. A series of substituted sulfonic acids and fatty acids was studied. Compared to desorption ionization on porous silicon (DIOS) and other matrix-less laser desorption/ionization techniques, silicon nanoparticle-assisted laser desorption/ionization (SPALDI) mass spectrometry allows for the analysis of acids in the negative ion mode without the observation of multimers or cation adducts. Using SPALDI, detection limits of many acids reached levels down to 50 pmol/µl. SPALDI of fatty acids with unmodified silicon nanoparticles was compared to SPALDI using the fluoroalkyl silylated silicon powder, with the unmodified particles showing better sensitivity for fatty acids, but with more low-mass background due to impurities and surfactants in the untreated silicon powder. The fatty acids exhibited a size-dependent response in both SPALDI and unmodified SPALDI, showing a signal intensity increase with the chain length of the fatty acids (C12-C18), leveling off at chain lengths of C18-C22. The size effect may be due to the crystallization of long chain fatty acids on the silicon. This hypothesis was further explored and supported by SPALDI of several, similar sized, unsaturated fatty acids with various crystallinities. Fatty acids in milk lipids and tick nymph samples were directly detected and their concentration ratios were determined by SPALDI mass spectrometry without complicated and time-consuming purification and esterification required in the traditional analysis of fatty acids by gas chromatography (GC). These results suggest that SPALDI mass spectrometry has the potential application in fast screening for small acids in crude samples with minimal sample preparation.

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.

Dihydrobenzoic acid modified nanoparticle as a MALDI-TOF MS matrix for soft ionization and structure determination of small molecules with diverse structures

Efficient structural characterization is important for quality control when developing novel materials. In this study, we demonstrated the soft ionization capability of the hybrid of immobilized silica and 2,5-dihydrobenzoic acid (DHB) on iron oxide magnetic nanoparticles in MALDI-TOF MS with a clean background. The ratio between SiO(2) and DHB was examined and was found to affect the surface immobilization of DHB on the nanoparticle, critically controlling the ionization efficiency and interference background. Compared with commercial DHB, the functionalized nanoparticle-assisted MALDI-TOF MS provided superior soft ionization with production of strong molecular ions within 5 ppm mass accuracy on a variety of new types of synthetic materials used for solar cells, light emitting devices, dendrimers, and glycolipids, including analytes with either thermally labile structures or poor protonation tendencies. In addition, the enhancements of the molecular ion signal also provided high-quality product-ion spectra allowing structural characterization and unambiguous small molecule identification. Using this technique, the structural differences among the isomers were distinguished through their characteristic fragment ions and comprehensive fragmentation patterns. With the advantages of long-term stability and simple sample preparation by deposition on a regular sample plate, the use of DHB-functionalized nanoparticles combined with high-resolution MALDI-TOF MS provides a generic platform for rapid and unambiguous structure determination of small molecules.

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.

Matrix-free laser desorption/ionization mass spectrometry using silicon glancing angle deposition (GLAD) films

Glancing angle deposition (GLAD) was used to fabricate nanostructured silicon (Si) thin films with highly controlled morphology for use in laser desorption/ionization mass spectrometry (DIOS-MS). Peptides, drugs and metabolites in the mass range of 150-2500 Da were readily analyzed. The best performance was obtained with 500 nm thick films deposited at a deposition angle of 85 degrees . Low background mass spectra and attomole detection limits were observed with DIOS-MS for various peptides. Films used after three months of dry storage in ambient conditions produced mass spectra with negligible low-mass noise following a 15 min UV-ozone treatment. The performance of the Si GLAD films was as good as or better than that reported for electrochemically etched porous silicon and related materials, and was superior to matrix-assisted laser desorption/ionization (MALDI)-MS for analysis of mixtures of small molecules between 150-2500 Da in terms of background chemical noise, detection limits and spot-to-spot reproducibility. The spot-to-spot reproducibility of signal intensities (100 shots/spectrum) from 21 different Si GLAD film targets was +/-13% relative standard deviation (RSD). The single shot-to-shot reproducibility of signals on a single target was +/-19% RSD (n = 7), with no indication of sweet spots or mute spots.

Fabrication of nanocluster silicon surface with electric discharge and the application in desorption/ionization on silicon-mass spectrometry

This study presents a new, simple, and low-cost technique to fabricate a nanocluster silicon (NCSi) surface on planar silicon using a micro-scale direct current (DC) discharge under ambient conditions. The method requires no masks, chemicals, vacuum environment, or laser, but only a high-voltage supply. The NCSi surfaces, characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy, consist of oxidized silicon nanoclusters 50-200 nm in diameter, likely formed by melting due to high temperatures in the discharge. The minimum size of the NCSi spot is determined by the size of the discharge tip (approximately 90 microm). Arbitrary NCSi areas can be produced on a silicon wafer by moving the discharge needle on the surface with the help of a computer-controlled xyz stage. NCSi surfaces can also be formed on three-dimensional (3D) surfaces, as demonstrated with silicon micropillars. NCSi surfaces can be used, for example, in various analytical applications. In this study, we demonstrate their use as sample plates in the analysis of drugs and peptides with desorption/ionization on silicon-mass spectrometry (DIOS-MS).

Analysis of various organic and organometallic compounds using nanostructure-assisted laser desorption/ionization time-of-flight mass spectrometry (NALDI-TOFMS)

Nanostructure-assisted laser desorption/ionization time-of-flight mass spectrometry (NALDI-TOFMS) has been developed recently as a matrix-free/surface-assisted alternative to matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The NALDI surface of silicon nanowires is already very effective for the analysis of small to medium sized, polar organic molecules in positive ion mode. The current study examined this technology for the analysis of several nonpolar organic, organometallic, and ionic compounds in positive ion mode, as well as a fluorinated compound and various acids in negative ion mode. NALDI data are compared and contrasted with MALDI data for the same compounds, and the higher sensitivity of NALDI is highlighted by the successful characterization of two porphyrins for a sample amount of 10 amol per spot.

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.

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.

Longitudinal surface plasmon resonance based gold nanorod biosensors for mass spectrometry

A "strategy" for analyte capture/ionization based on chemical derivatization of gold nanorods and infrared laser desorption ionization (IR-LDI) is described. This is the first example of laser desorption/ionization of biomolecules using gold nanorods irradiated with an IR laser. LDI is performed at wavelengths (1064 nm) that overlap with the longitudinal surface plasmon resonance (LSPR) mode of gold nanorods. The absorbed energy from the laser facilitates desorption and ionization of the analyte. The wavelength of the LSPR band can be tuned by controlling the aspect ratio (length-to-diameter) of the nanorod. For example, the SPR band for Au nanorods having an aspect ratio of 5:1 is centered at approximately 840 nm, and this band overlaps with the 1064 nm output of a Nd:YAG laser. We show that a variety of biomolecules can be efficiently desorbed and ionized by 1064 nm irradiation of nanorods. We also show that analyte capture can be controlled by surface chemistry of the nanorods. The results of these studies are important for designing nanomaterial-based capture assays for mass spectrometry and interfacing nanomaterials with imaging/spatial profiling mass spectrometry experiments.

Lithographically patterned silicon nanowire arrays for matrix free LDI-TOF/MS analysis of lipids

Silicon nanowire arrays were patterned onto silicon chips by a combination of lithography and chemical vapor deposition using the vapor-liquid-solid growth method. Thus, highly reproducible sample deposition zones were obtained that were used for laser desorption ionization (LDI) mass spectrometric analysis of lipidic species with lithium salts as dopants. Using a conventional UV laser (337 nm), hydrocarbons and numerous lipids (triglycerides, diglycerides, wax esters) could be effectively lithiated yielding [M + Li](+) ions. Upon doping with lithium hydroxide the SiNW arrays yielded high signal-to-noise ratios with low limits of detection (e.g. 750 pg tripalmitin on target with S/N 5) and efficient ionization for a range of fatty acids (FA), mono-, di- and triglycerides and hydrocarbons (HC), in the form of [FA-H + 2Li](+), [mono- or diglyceride-H(2)O + Li](+), or [triglyceride + Li](+) and [HC + Li](+), respectively. It is expected that these chips will find a broad range of applications in the analysis of natural compounds and food control.

Matrix-free laser desorption/ionization mass spectrometry on silicon nanowire arrays prepared by chemical etching of crystalline silicon

This paper reports on the use of silicon nanowires (SiNWs), easily prepared in a single step by chemical etching of crystalline silicon in HF/AgNO(3) aqueous solution, as a highly sensitive substrate for laser desorption/ionization mass spectrometry (LDI-MS) analysis. The SiNWs' diameter and length depend on the etchant concentration and dissolution time. Optimized LDI substrate consists of nanowires with an average diameter in the range of 20-100 nm and 2.5 mum in length. The optimized SiNWs' surface morphology coupled to a controlled surface chemistry allowed a significant LDI-MS performance through measurements of a broad range of analytes, including small molecules, peptides, and a bovine serum albumin (BSA) digest. A signal-to-noise ratio of 250 was ascertained for a 10 fmol bradykinin pick, in reflector mode acquisition. Likewise, the sutent, a small tyrosine kinase inhibitor, could be observed down to 10 fmol, as compared to 500 fmol limit detection using the classical matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). We have further investigated the optical properties of the nanowires, and our results suggest that they have a small or no effect on the desorption/ionization (D/I) process. On the contrary, the surface morphology and thermal properties of the silicon nanostructures are found to be the essential features contributing to the D/I performance.

Electrochemical aspects of electrospray and laser desorption/ionization for mass spectrometry

Soft-ionization methods, namely electrospray ionization and laser desorption/ionization, are widely used to transfer large molecules as intact gas-phase ions either from a solution or from a solid substrate. During both processes, in-source electrochemical and photoelectrochemical reactions occur. These electrode reactions, which take place at interfaces, play important roles in influencing the ionization products, but they have received little attention. We show that having good control over both types of electrochemical reactions can lead to new analytical applications. Examples include online tagging by grafting of mass tags and in-source photooxidation of peptides.

A novel strategy for MALDI-TOF MS analysis of small molecules

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) does not work efficiently on small molecules (usually with molecular weight below 500 Da) because of the interference of matrix-related peaks in low m/z region. The previous methods developed for this problem focused on reducing the peaks caused by the traditional matrices. Here, we report a novel strategy to analyze small molecules in a high and interference-free mass range by using metal-phthalocyanines (MPcs) as matrices which should be capable of forming matrix-analyte adducts. The mass of the target analyte was calculated by subtracting the mass of MPc from the mass of the MPc-analyte adduct. MPcs were also detectable and could serve as internal standards. Various MPcs with aromatic or aliphatic groups and different metal centers were then synthesized and explored. Aluminum-phthalocyanines (AlPcs), gallium-phthalocyanines (GaPcs), and indium-phthalocyanines (InPcs) were efficient matrices to form MPc-analyte adducts in either the positive or negative ion mode. The detection limits varied from 17 to 75 fmol, depending on analyte types. The mechanism of adducts formation was also proposed. Collectively, our strategy provides a novel and efficient way to analyze small molecules by MALDI-TOF MS.

Quantum dots improve peptide detection in MALDI MS in a size dependent manner

Laser Desorption Ionization Mass Spectrometry employs matrix which is co-crystallised with the analyte to achieve "soft ionization" that is the formation of ions without fragmentation. A variety of matrix-free and matrix-assisted LDI techniques and matrices have been reported to date. LDI has been achieved using ultra fine metal powders (UFMPs), desorption ionisation on silicon (DIOS), sol-gel assisted laser desorption/ionization (SGALDI), as well as with common MALDI matrices such as 2,5-dihydroxy benzoic acid (DHB), 3,5-dimethoxy-4-hydroxycinnamic acid (SA), α-cyano-4-hydroxycinnamic acid (CHCA) to name a few. A variety of matrix additives have been shown to improve matrix assisted desorption, including silicon nanowires (SiNW), carbon nanotubes (CNT), metal nanoparticles and nanodots. To our knowledge no evidence exists for the application of highly fluorescent CdSe/ZnS quantum dots to enhance MALDI desorption of biological samples. Here we report that although CdSe/ZnS quantum dots on their own can not substitute matrix in MALDI-MS, their presence has a moderately positive effect on MALDI desorption, improves the signal-to-noise ratio, peak quality and increases the number of detected peptides and the overall sequence coverage.

Engineered nanoparticle surfaces for improved mass spectrometric analyses

Engineering of nanoparticle surface functionality provides controlled interactions with biomolecules such as cell membrane lipids, proteins and nucleic acids. Concurrently, this surface chemistry control also opens up new avenues for improving mass spectral analyses. In this Minireview, we highlight some of the emerging work that integrates surface-engineered nanoparticles with mass spectrometry to improve the analysis of a wide variety of chemical and biological systems.

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.

Comparison of inert supports in laser desorption/ionization mass spectrometry of peptides: pencil lead, porous silica gel, DIOS-chip and NALDI target

In the search for alternative inert surfaces replacing silicon chips in Desorption/Ionization On porous Silicon (DIOS)-like mass spectrometry analyses, nanostructured silicon-based NALDI chips were evaluated in Laser Desorption/Ionization (LDI) of peptides. Comparisons were made using commercially available DIOS chips (MassPREP-DIOS-target), amorphous carbon powder from lead pencil and porous silica gel used for chromatographic purposes as reference supports. A set of synthetic model peptides presenting variable amino acid sequences of various lengths was analyzed under all conditions. The LDI responses of the four 'matrix-free' techniques were compared, especially in terms of peptide detection sensitivity and overall experiment robustness.

Surface cleaning of germanium nanodot ionization substrate for surface-assisted laser desorption/ionization mass spectrometry

In surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS), a chemical background signal, arising from organic contaminants such as plasticizers, is frequently observed mainly under m/z ca. 600, which impairs the advantages of the matrix-free approach. Silver salts, which are used for the cationization of aromatic compounds, are also difficult to remove completely after the measurements. In this study, surface cleaning techniques used in semiconductor processing were used to clean our developed silicon-based SALDI substrate on which self-assembled germanium nanodots (GeNDs) had been deposited (termed a GeND chip). An immersion cleaning method using acetone with sonication, and a sulfuric-peroxide mixture (SPM) cleaning method using a mixture of H(2)SO(4)/H(2)O(2)/deionized water, were examined for their effectiveness in removing organic compounds and residual silver salts. Removal of both types of contaminants was successfully performed by SPM cleaning. The limit of detection for glutathione was improved from ca. 5 pmol without cleaning to ca. 50 fmol after the SPM cleaning. Since GeND chips can tolerate acidic cleaning and sonication due to their chemical inertness and rigid nanodot structures, they appear to be an ideal reusable SALDI substrate.