Experimental factors controlling analyte ion generation in laser desorption/ionization mass spectrometry on porous silicon

Desorption/ionization on porous silicon (DIOS) is a relatively new laser desorption/ionization technique for the direct mass spectrometric analysis of a wide variety of samples without the requirement of a matrix. Porous silicon substrates were fabricated using the recently developed nonelectrochemical H2O2-metal-HF etching as a versatile platform for investigating the effects of morphology and physical properties of porous silicon on DIOS-MS performance. In addition, laser wavelength, mode of ion detection, pH, and solvent contributions to the desorption/ionization process were studied. Other porous substrates such as GaAs and GaN, with similar surface characteristics but differing in thermal and optical properties from porous silicon, allowed the roles of surface area, optical absorption, and thermal conductivities in the desorption/ionization process to be investigated. Among the porous semiconductors studied, only porous silicon has the combination of large surface area, optical absorption, and thermal conductivity required for efficient analyte ion generation under the conditions studied. In addition to these substrate-related factors, surface wetting, determined by the interaction of deposition solvent with the surface, and charge state of the peptide were found to be important in determining ion generation efficiency.     

High sensitivity and analyte capture with desorption/ionization mass spectrometry on silylated porous silicon

Silylation chemistry on porous silicon provides for ultrahigh sensitivity and analyte specificity with desorption/ionization on silicon mass spectrometry (DIOS-MS) analysis. Here, we report that the silylation of oxidized porous silicon offers a DIOS platform that is resistant to air oxidation and acid/base hydrolysis. Furthermore, surface modification with appropriate hydrophobic silanes allows analytes to absorb to the surface via hydrophobic interactions for direct analyte extraction from complex matrixes containing salts and other nonvolatile interferences present in the sample matrix. This enables rapid cleanup by simply spotting the sample onto the modified DIOS target and removing the liquid phase containing the interferences. This approach is demonstrated in the analysis of protein digests and metabolites in biofluids, as well as for the characterizing of inhibitors from their enzyme complex. An unprecedented detection limit of 480 molecules (800 ymol) for des-Arg(9)-bradykinin is reported on a pentafluorophenyl-functionalized DIOS chip.     

Enzyme kinetics by directly imaging a porous silicon microfluidic reactor using desorption/ionization on silicon mass spectrometry

Enzyme kinetics were obtained in a porous silicon microfluidic channel by combining an enzyme and substrate droplet, allowing them to react and deposit a small amount of residue on the channel walls, and then analyzing this residue by directly ionizing the channel walls using a matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) laser source. The porous silicon of the channel walls functions in a manner analogous to the matrix in MALDI-MS, and is referred to as a desorption/ionization on silicon mass spectrometry (DIOS-MS) target when used in this configuration. Mass spectrometry signal intensity of substrate residue correlates with relative concentration, and position in the microchannel correlates with time, thus allowing determination of kinetic parameters. The system is especially suitable for initial reaction velocity determination. This microreactor is broadly applicable to time-resolved kinetic assays as long as at least one substrate or product of the reaction is ionizable by DIOS-MS.     

Surfactant-enhanced desorption/ionization on silicon mass spectrometry

Perfluorinated surfactants are demonstrated to dramatically enhance desorption/ionization on fluorinated silicon (DIOS) mass spectrometry. Perfluorooctanesulfonic acid improved the signal-to-noise ratio of tryptic digests and gave a 3-fold increase in the number of peptides identified. Similar results were also obtained using perfluoroundecanoic acid; yet among the seven different surfactants tested, controls such as nonfluorinated sodium dodecyl sulfate or fluorinated molecules with minimal surfactant activity did not enhance the signal. The same surfactants also enhanced the DIOS-MS signal of amino acids, carbohydrates, and other small organic compounds. The signal enhancement may be facilitated by the high surface activity of the perfluorinated surfactants on the fluorinated silicon surfaces allowing for a higher concentration of analyte to be absorbed.     

电偶腐蚀法制备多孔硅的研究EI

用电偶腐蚀法制备多孔硅,主要研究了铂电极的优化制备工艺以及腐蚀条件对多孔硅厚度的影响,并且结合SEM,AFM等测试手段对所制备的多孔硅的表面形貌进行了分析。实验发现,在相同的腐蚀条件下,多孔硅的厚度随铂电极的厚度以及铂电极与腐蚀硅片的面积比的增大而增大。     

Ordered silicon nanocavity arrays in surface-assisted desorption/ionization mass spectrometry

We report here a simple method to generate ordered nanocavity arrays on a Si wafer and use it in surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). A close-packed SiO2 nanosphere array was first deposited on a low-resistivity Si wafer using a convective self-assembly method. The nanoparticle array was then used as a mask in a reactive ion etching (RIE) process to selectively remove portions of the Si surface. Subsequent sonication removed those physically adsorbed SiO2 nanoparticles and exposed an ordered nanocavity array underneath. The importance of this approach is its capability of systematically varying surface geometries to achieve desired features, which makes detailed studies of the impacts of surface features on the desorption/ionization mechanism feasible. We demonstrated that the in-plane width and out-of-plane depth of the cavities were adjustable by varying etching times, and the intercavity spacing was controllable by varying the number of particle layers deposited. MS detection of small peptides on these substrates showed comparable sensitivity to conventional porous Si substrates (DIOS, desorption/ ionization on porous silicon). The desorption and ionization efficiency of these roughened surfaces exhibited a nonmonotonic relationship to the increased total surface area. Several possible factors contributing to the observed phenomenon are speculated upon. The application of this arrayed surface in metabolite detection of Arabidopsis thaliana root extracts is also demonstrated.     

Desorption ionization of biomolecules on metals

Direct desorption ionization of various types of biomolecules on metal substrates without the need of matrices was observed by a time-of-flight mass spectrometer. It provides a new convenient method for detection of small biomolecules without the confusion of ion peaks from matrix compounds. Simple commercial Al foil can be used as the substrate to obtain mass spectra of biomolecules without the need of an etching process to produce a porous surface such as with direct ionization on silicon (DIOS). The desorption and ionization mechanism is also discussed.     

Photoluminescence studies of thermal impurity diffused porous silicon layers

Porous silicon layers were formed on diffused layers. Both boron and phosphorus impurities were thermally diffused using solid sources in n-Si, p-Si, n-epi/Si and p-epi/Si substrates of various resistivities. Porous silicon on these layers was formed by electrochemical and chemical etching under various etching conditions. Strong visible luminescence was observed from these porous silicon structures. Infrared absorption studies indicated that surface molecule identities are immaterial to the enhancement or degradation of photoluminescence. This revised version was published online in July 2006 with corrections to the Cover Date.     

Quantitative analysis with desorption/ionization on silicon mass spectrometry using electrospray deposition

Desorption/ionization on silicon mass spectrometry (DIOS-MS) is demonstrated as a quantitative analytical tool when coupled to electrospray deposition (ESD). In this study, we illustrate the utility of DIOS-MS in the quantitative analysis of a peptide and two amino acids with deuterated and structural analogues used as internal standards. An important feature of this approach is the incorporation of ESD to improve sample homogeneity across the porous silicon surface. ESD allowed for a marked improvement in quantitative analysis due to its applicability to LC-DIOS, and because of the absence of matrix, sample can be deposited at very low flow rates (150 nL/min). Experiments comparing the traditional dried droplet and ESD methods show that ESD samples exhibit significantly improved quantitation and much higher sample-to-sample reproducibility.     

Microstructural characterisation of metallurgical grade porous silicon nanosponge particles

Porous silicon finds numerous applications in the areas of bio-technology, drug delivery, energetic materials and catalysis. Recent studies by Vesta Sciences have led to the development of porous silicon nanosponge particles from metallurgical grade silicon powder through their own patented chemical etching process (Irish patent no. IE20060360). This discovery paves the way for a more economical production method for porous silicon. The study presented here studies the structural morphology of the porous silicon nanosponge particles using high resolution electron microscopy techniques combined with porisometry type measurements, where appropriate. The related surface pore structure is examined in detail using Scanning Electron Microscopy and Transmission Electron Microscopy techniques while the internal pore structure is explored using Focused Ion Beam milling and ultramicrotomed cross-sections. Three samples of the silicon particles were analysed for this study which include the starting metallurgical grade silicon powder and two samples that have been chemically etched. Analysis of the etched samples indicates a disordered pore structure with pore diameters ranging up to 15 nm on porous silicon particles ranging up to 5 μm in size. Crystallographic orientation did not appear to affect the surface pore opening diameter. Internal pore data indicated pore depths of up to 1 μm dependant on the particle size and etching conditions applied.     

多孔硅的制备及其吸杂处理对电学性能的影响

多孔硅吸杂是减少晶体硅中杂质和缺陷,提高太阳能电池转换效率的有效方法。采用电化学腐蚀方法在单晶硅片上制备多孔硅,通过观察多孔硅的形貌、结构及单晶硅片的电阻率变化,研究不同电流密度制备的多孔硅对吸杂效果的影响,并从多孔硅的结构出发探究多孔硅吸杂的机理。结果表明,随电流密度增加,孔隙率明显增加,多孔硅在电流密度为100mA/cm2时,孔隙率最大;电流密度越大,多孔硅伴随所产生的弹性机械应力增加,晶格常数相应增加,这两个因素都有利于缺陷和金属杂质在多孔硅层-基底界面处迁移和富集,导致单晶硅吸杂后电阻率增大。     

多孔硅对单晶硅少子寿命影响状况的研究

以p型单晶硅片为研究对象,在单晶硅片表面采用化学腐蚀方法制备多孔硅层,通过实验选取制备多孔硅的最佳工艺条件,采用SEM观察多孔硅表面形貌,以及用微波光电导法测试少子寿命的变化情况。结果表明,在相同的腐蚀溶液配比条件下腐蚀11min得到的多孔硅层的表面形貌最好,孔隙率最大。在850℃下热处理150min时样品少子寿命的提高达到最大,不同腐蚀时间的样品少子寿命提高程度不同,腐蚀11min的样品少子寿命提高最大,约有10%左右。多孔层的形成伴随着弹性机械应力的出现,引起多孔层-硅基底界面处产生弹性变形,这有利于缺陷和金属杂质在界面处富集。另外,多孔硅仍具有晶体结构,但其表面方向上的晶格参数要比初始硅的晶格参数大,也有利于金属杂质向多孔层迁移。     

永冲电化学腐蚀制备n型多孔硅及其发光性能

采用脉冲电化学腐蚀法,以n型单晶硅为衬底制备多孔硅（n—PS）,通过扫描电镜（SEM）、室温500—700nm范围内荧光光谱,系统研究腐蚀时间、占空比和脉冲频率对n-PS的结构形貌和可见光区室温光致发光特性（PL）的影响,结果表明,相比直流电化学腐蚀方法,脉冲腐蚀能获得孔径分布均匀且发光强度更高的多孔硅;随腐蚀时间、占空比和脉冲频率等腐蚀条件的变化,其发光峰位及发光强度均有明显改变;当等效腐蚀时间为30min、占空比为0．5、脉冲频率为10Hz时,制备的n—PS的PL强度较高,发光性能较好。     

Desorption/ionization on porous silicon mass spectrometry studies on pentose-borate complexes

Desorption/ionization on porous silicon mass spectrometry (DIOS-MS) was used to investigate the binding affinities between aldopentose isomers and boron. Boron has been recognized for its importance in pentose synthesis and stabilization in prebiotic conditions. Boron may also account for the fact that ribose, among other aldopentoses, is the favored building block in RNA synthesis. This research started with the detection of aldopentoses in the positive mode through cationization and the aldopentose-borate complexes in the negative mode. Then two competition schemes, one using a pentose structure analogue and the other using 13C-labeled ribose, were designed to compare the relative binding affinities of four aldopentoses (xylose, lyxose, arabinose, and ribose) to boron. Both approaches determined the binding preference to be ribose > lyxose > arabinose > xylose. This work illustrates the potential of DIOS-MS in the analyses of nonvolatile, small molecules in delicate chemical equilibria. Without externally introduced matrices, background signals are not a limiting factor. Furthermore, the possible dramatic change of pH associated with the matrix introduction, which may disturb the equilibria of interest, is avoided.     

电化学腐蚀多孔硅表面形貌的结构特性

多孔硅作为微电子机械系统中重要的热绝缘层和牺牲层材料,其表面形貌结构特性是影响多孔硅上薄膜器件性能的重要因素,为此,利用双槽电化学腐蚀方法制备了多孔硅薄膜,并通过原子力显微镜和场发射扫描电子显微镜对制备多孔硅的表面形貌和孔径大小分布进行了观察．结果发现：腐蚀初期,在硅表面会有大量的硅柱形成,硅柱的直径、高度、分布密度与电流密度成正比关系;硅柱在进一步腐蚀过程中会消失,多孔硅的表面粗糙度随着腐蚀的进行,先减小再增大,最后达到稳定值0．52nm;多孔硅孔径大小分布区间随腐蚀时间增加变窄．     

A Raman spectroscopic study of photoluminescent porous silicon fibres

Porous silicon fibres are obtained from the topmost layers of porous silicon prepared by anodically etching p-type silicon–silicon epitaxial wafers, and exhibite, like black porous silicon, very intense light emission at room temperature under UV or visible laser excitation. The new material was characterised using several techniques in particular Raman spectroscopy. Silicon fibre is shown to be porous and highly ordered. A reversible thermal effect on the Raman characteristics of silicon fibre as well as on those of porous silicon is observed and is correlated with the change in the hole sizes of these materials and suggests a tuning method to reversibly change the band gap and other physical properties.     

Porous silicon-based direct hydrogen sulphide fuel cells

In this paper, the use of Au/porous silicon/Silicon Schottky type structure, as a direct hydrogen sulphide fuel cell is demonstrated. The porous silicon filled with hydrochlorid acid was developed as a proton conduction membrane. The Au/Porous Silicon/Silicon cells were fabricated by first creating the porous silicon layer in single-crystalline Si using the anodic etching under illumination and then deposition Au catalyst layer onto the porous silicon. Using 80 mM H2S solution as fuel the open circuit voltage of 0.4 V was obtained and maximum power density of 30 W/m2 at room temperature was achieved. These results demonstrate that the Au/Porous Silicon/Silicon direct hydrogen sulphide fuel cell which uses H2S:dH2O solution as fuel and operates at room temperature can be considered as the most promising type of low cost fuel cell for small power-supply units.     

Porous silicon nanoparticles containing neurotropic drugs

Porous silicon samples with a pore diameter under 100 nm have been prepared by a two-sided anodic electrochemical etching of single-crystal silicon. We have studied vinpocetine and afobazol sorption on porous silicon. The drugs have been shown to have no effect on the structure of the porous silicon. Comparison of the degree of afobazol and vinpocetine release from the drug delivery systems produced in this study and from microcapsules demonstrates that porous silicon nanoparticles can be used as a means of prolonged drug delivery, suggesting that further pharmacological research is warranted.     

Desorption/ionization on silicon time-of-flight/time-of-flight mass spectrometry

Desorption/ionization on silicon (DIOS) tandem time-of-flight (TOF/TOF) mass spectrometry (MS) provides high accuracy and significant fragmentation information, particularly in the characterization of biomolecules. DIOS TOF/TOF offers a high-throughput surface-based ionization platform as well as complete fragmentation through high collision energies. The absence of matrix interference in DIOS allows for the MS and MS/MS analysis of small molecules well below m/z 300. In addition, sample preparation is minimal, and the DIOS chips can be stored and reanalyzed for fragmentation information or accurate mass measurements. The combined benefits of robustness, minimal sample preparation, good sensitivity, high throughput, and sequencing capability make DIOS TOF/TOF a powerful tool for small molecule characterization and protein identification.     

Characterization of nanoporous silicon layer to reduce the optical losses of crystalline silicon solar cells

Reduction of optical losses in crystalline silicon solar cells by surface modification is one of the most important issues of silicon photovoltaics. Porous Si layers on the front surface of textured Si substrates have been investigated with the aim of improving the optical losses of the solar cells, because an anti-reflection coating and a surface passivation can be obtained simultaneously in one process. We have demonstrated the feasibility of a very efficient porous Si AR layer, prepared by a simple, cost effective, electrochemical etching method. Silicon p-type CZ (100) oriented wafers were textured by anisotropic etching in sodium carbonate solution. Then, the porous Si layers were formed by electrochemical etching in HF solutions. After that, the properties of porous Si in terms of morphology, structure and reflectance are summarized. The structure of porous Si layers was investigated using SEM. The formation of a nanoporous Si layer on the textured silicon wafer result in a reflectance lower than 5% in the wavelength region from 500 to 900 nm. Such a surface modification allows improving the Si solar cell characteristics. An efficiency of 13.4% is achieved on a monocrystalline silicon solar cell using the electrochemical technique.