Visible-laser desorption/ionization on gold nanostructures

In this report, we describe the visible-laser desorption/ionization of biomolecules deposited on gold-coated porous silicon and gold nanorod arrays. The porous silicon made by electrochemical etching was coated with gold using argon ion sputtering. The gold nanorod arrays were fabricated by electrodepositing gold onto a porous alumina template, and the subsequent partial removal of the alumina template. A frequency-doubled/tripled Nd : YAG laser was used to irradiate the gold nanostructured substrate, and the desorbed molecular ions were mass-analyzed by a time-of-flight mass spectrometer. The desorption/ionization of biomolecules for both substrates was favored by the use of the 532-nm visible-laser, which is in the range of the localized surface plasmon resonance of the gold nanostructure. The present technique offers a potential analytical method for low-molecular-weight analytes that are rather difficult to handle in the conventional matrix-assisted laser desorption/ionization (MALDI) mass spectrometry.     

纳米金淀积的多孔硅靶增强样品的激光解吸/电离质谱信号

硅片类型和多孔硅结构的多样性影响了多孔硅表面的激光解吸/离子化质谱(DIOS)(无辅助基质的激光解吸/电离飞行时间质谱(LDI-TOF-MS))数据的重复性和靶的耐储时间。本工作通过在多孔硅的表面淀积金纳米颗粒并将其作为目标靶来增强软物质分子如聚乙二醇和多肽的激光解吸/电离质谱信号。纳米金的淀积钝化了多孔硅表面的Si-H活性基团,增加了靶的耐储时间。用场发射扫描电镜表征了多孔硅淀积金纳米颗粒前后的形貌,用X射线能量色散光谱法分析金的百分含量,结果表明其含量随沉积时间的延长而增加。激光解吸/电离质谱信号的增强可能是由多孔硅及其支持的金纳米颗粒的光学和物理性质引起的,该类型的样品靶在激光解吸/电离飞行时间质谱的应用上结合了多孔硅和金纳米颗粒的双重优势。     

Facile Electrochemical Method for the Fabrication of Stable Corrosion-Resistant Superhydrophobic Surfaces on Zr-Based Bulk Metallic Glasses

Both surface microstructure and low surface energy modification play a vital role in the preparation of superhydrophobic surfaces. In this study, a safe and simple electrochemical method was developed to fabricate superhydrophobic surfaces of Zr-based metallic glasses with high corrosion resistance. First, micro–nano composite structures were generated on the surface of Zr-based metallic glasses by electrochemical etching in NaCl solution. Next, stearic acid was used to decrease surface energy. The effects of electrochemical etching time on surface morphology and wettability were also investigated through scanning electron microscopy and contact angle measurements. Furthermore, the influence of micro–nano composite structures and roughness on the wettability of Zr-based metallic glasses was analysed on the basis of the Cassie–Baxter model. The water contact angle of the surface was 154.3° ± 2.2°, and the sliding angle was <5°, indicating good superhydrophobicity. Moreover, the potentiodynamic polarisation test and electrochemical impedance spectroscopy suggested excellent corrosion resistance performance, and the inhibition efficiency of the superhydrophobic surface reached 99.6%. Finally, the prepared superhydrophobic surface revealed excellent temperature-resistant and self-cleaning properties.     

Nanostructured pyramidal black silicon with ultra-low reflectance and high passivation

In this study, nanostructured pyramidal black silicon is prepared by metal assisted chemical etching method, in which the silver nitrate (AgNO₃) is used as the metal catalyst. Effects of the concentration of AgNO₃ on passivation and optical properties of the black silicon are investigated. The experimental results show that at the AgNO₃ concentration of 0.03 M, the nanostructure length is about 300 nm, and the reflectance of the black silicon with a stack of silicon nitride (SiN_x) and aluminum oxide (Al₂O₃) is 0.8%, which is comparable to that of the conventional black silicon with micrometer-long nanowires. In addition, an acceptably low surface recombination rate of 42 cm/s can be obtained. Plasma chemical vapor deposited SiN_x is deposited well on the top of nanostructures of black silicon, but shows poor coverage at the bottom region. Spatial atomic layer deposited Al₂O₃ can conformally cover the nanostructures with high passivation quality. Simulation result indicates an improvement of 5.5% of conversion efficiency for the nanostructured pyramidal black silicon solar cell compared to industrial silicon solar cell. The short nanostructured pyramidal surface with low reflectance and high passivation is expected to be helpful for black silicon technology applied to photovoltaic applications.     

Matrix-free infrared soft laser desorption/ionization

Infrared soft laser desorption/ionization was performed using a 2.94 µm Er : YAG laser and a commercial reflectron time-of-flight mass spectrometer. The instrument was modified so that a 337 nm nitrogen laser could be used concurrently with the IR laser to interrogate samples. Matrix-assisted laser desorption/ionization (MALDI), laser desorption/ionization and desorption/ionization on silicon with UV and IR lasers were compared. Various target materials were tested for IR soft desorption ionization, including stainless steel, aluminum, copper, silicon, porous silicon and polyethylene. Silicon surfaces gave the best performance in terms of signal level and low-mass interference. The internal energy resultant of the desorption/ionization was assessed using the easily fragmented vitamin B₁₂ molecule. IR ionization produced more analyte fragmentation than UV-MALDI analysis. Fragmentation from matrix-free IR desorption from silicon was comparable to that from IR-MALDI. The results are interpreted as soft laser desorption and ionization resulting from the absorption of the IR laser energy by the analyte and associated solvent molecules. Copyright © 2004 John Wiley & Sons, Ltd.     

Mass spectrometric analysis of low molecular mass polyesters by laser desorption/ionization on porous silicon

A low molecular mass polyester was analyzed by desorption/ionization on porous silicon (DIOS) mass spectrometry. The results were compared with those of matrix-assisted laser desorption ionization (MALDI) mass spectrometry using matrixes of alpha-cyano-4-hydroxycinnamic acid (CHCA) and 10,15,20-tetrakis(pentafluorophenyl)porphyrin (F20TPP). The CHCA matrix was not suitable for characterization of low molecular mass components of the polyester because the matrix-related ions interfered with the component ions. On the other hand, the F20TPP matrix showed no interference because no matrix-related ions appeared below m/z 822. However, the solvent selection for determining optimal conditions of sample preparation was limited, because F20TPP does not dissolve readily in any of the available organic solvents. In the DIOS spectra, the polymer ions were observed at high sensitivity without a contaminating ion. No matrix is needed for DIOS spectra of low molecular mass polyesters, facilitating sample preparation and selectivity of a precursor ion in post-source decay measurements.     

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. Copyright © 2014 John Wiley & Sons, Ltd.     

Chemical structure and surface morphology evolution of glow discharge polymer films by Ar‐ions intermittent etching

The Ar‐ions intermittent‐etching technique was successfully incorporated during the deposition of glow discharge polymer (GDP) films. The ionic components and ion energy distributions (IEDs) of C₄H₈/H₂ and C₄H₈/H₂/Ar plasma were diagnosed by an energy‐resolved mass spectrometer, respectively. The Fourier transform infrared spectroscopy, scanning electron microscope, and white‐light interferometer were used to studying the chemical structure, surface morphology, and roughness of the GDP films, which are deposited with the various time of Ar‐ions intermittent etching. With the introduction of Ar into the chamber, the intensity of the C H absorption peaks becomes weak and the large‐mass C H species were ionized and dissociated from the mass spectrometer results. The surface roughness of GDP films are decreased with Ar‐ions intermittent etching, the lowest surface roughness (Rq) is only 33.6 nm when the intermittent cycle is 60 minutes/15 minutes. The highest sp³CH₃ (sym) absorption peaks are attributed to samples also with 60‐minute/15‐minute intermittent cycle, which shortens the length of the carbon chain and reduces the probability of the cluster formations.     

Coffee-ring effects in laser desorption/ionization mass spectrometry

This report focuses on the heterogeneous distribution of small molecules (e.g. metabolites) within dry deposits of suspensions and solutions of inorganic and organic compounds with implications for chemical analysis of small molecules by laser desorption/ionization (LDI) mass spectrometry (MS). Taking advantage of the imaging capabilities of a modern mass spectrometer, we have investigated the occurrence of “coffee rings” in matrix-assisted laser desorption/ionization (MALDI) and surface-assisted laser desorption/ionization (SALDI) sample spots. It is seen that the “coffee-ring effect” in MALDI/SALDI samples can be both beneficial and disadvantageous. For example, formation of the coffee rings gives rise to heterogeneous distribution of analytes and matrices, thus compromising analytical performance and reproducibility of the mass spectrometric analysis. On the other hand, the coffee-ring effect can also be advantageous because it enables partial separation of analytes from some of the interfering molecules present in the sample. We report a “hidden coffee-ring effect” where under certain conditions the sample/matrix deposit appears relatively homogeneous when inspected by optical microscopy. Even in such cases, hidden coffee rings can still be found by implementing the MALDI-MS imaging technique. We have also found that to some extent, the coffee-ring effect can be suppressed during SALDI sample preparation.     

MoS2/Ag nanohybrid: A novel matrix with synergistic effect for small molecule drugs analysis by negative-ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

This paper reports a facile synthesis of molybdenum disulfide nanosheets/silver nanoparticles (MoS₂/Ag) hybrid and its use as an effective matrix in negative ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The nanohybrid exerts a strong synergistic effect, leading to high performance detection of small molecule analytes including amino acids, peptides, fatty acids and drugs. The enhancement of laser desorption/ionization (LDI) efficiency is largely attributed to the high surface roughness and large surface area for analyte adsorption, better dispersibility, increased thermal conductivity and enhanced UV energy absorption as compared to pure MoS₂. Moreover, both Ag nanoparticles and the edge of the MoS₂ layers function as deprotonation sites for proton capture, facilitating the charging process in negative ion mode and promoting formation of negative ions. As a result, the MoS₂/Ag nanohybrid proves to be a highly attractive matrix in MALDI-TOF MS, with desired features such as high desorption/ionization efficiency, low fragmentation interference, high salt tolerance, and no sweet-spots for mass signal. These characteristic properties allowed for simultaneous analysis of eight different drugs and quantification of acetylsalicylic acid in the spiked human serum. This work demonstrates for the first time the fabrication and application of a novel MoS₂/Ag hybrid, and provides a new platform for use in the rapid and high throughput analysis of small molecules by mass spectrometry.     

Comparative characterisation by atomic force microscopy and ellipsometry of soft and solid thin films

Ellipsometry and atomic force microscopy (AFM) were used to study the film thickness and the surface roughness of both ‘soft’ and solid thin films. ‘Soft’ polymer thin films of polystyrene and poly(styrene–ethylene/butylene–styrene) block copolymer were prepared by spin‐coating onto planar silicon wafers. Ellipsometric parameters were fitted by the Cauchy approach using a two‐layer model with planar boundaries between the layers. The smooth surfaces of the prepared polymer films were confirmed by AFM. There is good agreement between AFM and ellipsometry in the 80–130 nm thickness range. Semiconductor surfaces (Si) obtained by anisotropic chemical etching were investigated as an example of a randomly rough surface. To define roughness parameters by ellipsometry, the top rough layers were treated as thin films according to the Bruggeman effective medium approximation (BEMA). Surface roughness values measured by AFM and ellipsometry show the same tendency of increasing roughness with increased etching time, although AFM results depend on the used window size. The combined use of both methods appears to offer the most comprehensive route to quantitative surface roughness characterisation of solid films. Copyright © 2007 John Wiley & Sons, Ltd.     

Mechanism of etching and of surface modification of polyimide in RF and LF SF6-O2 discharges

Etch rates of Kapton H polyimide film in SF₆-O₂ plasmas (0.25 torr) were studied as a function of the input gas mixture, the excitation frequency (25–450 kHz; 13.56 MHz), and the biasing mode. The treated surface was examined by X ray photoelectron spectroscopy (ESCA), scanning electron microscopy (SEM), and contact angle measurement. The ion and neutral species of the plasma were sampled and analyzed by mass spectrometry. Etch rates are found to depend on the positive ion flux and the degree of dissociation of neutral molecules. Plasma-treated surfaces are always covered with a deposited material (C_nH_mO_xF_y) which partially obstructs the etching reaction by a masking effect and causes surface roughness. A proposed kinetic analysis of the etching mechanism is in good agreement with the experimental data.     

A quantitative model of ultraviolet matrix-assisted laser desorption/ionization

A quantitative model of primary ionization in ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) is presented. It includes not only photochemical processes such as exciton pooling, but also the effects of the desorption event. The interplay of these two is found to be a crucial aspect of the MALDI process. The desorbing plume is modeled as an adiabatic expansion with entrained clusters. The parameters in the model are defined as much as possible via experiment or by analogy with known effects. The model was applied to the matrix 2,5-dihydroxybenzoic acid and found to reproduce the fluence dependence of the fluorescence yield and key features of the picosecond two-pulse ion generation efficiency curves. In addition, the model correctly predicts a fluence rather than irradiance threshold, the magnitude of the threshold, the magnitude of the ion yield, laser wavelength effects, plume temperatures, plume expansion velocities and the spot size effect.     

利用胶体探针技术研究多巴与纳米、微米及微纳复合结构表面之间的相互作用

通过在硅片表面有机蒸镀不同厚度的二十九烷制备了不同晶体密度的仿生旱金莲叶面蜡质纳米结构表面,采用端基修饰多巴的原子力显微镜胶体探针,对各纳米结构表面进行了粘附性能测试,发现蒸镀200 nm厚度二十九烷结晶的纳米结构表面具有较低粘附力。采用反应离子刻蚀方法制备了不同高度的硅材质仿生鲨鱼皮微米结构表面,并选择了200 nm厚度二十九烷在仿生鲨鱼皮表面进行有机蒸镀制备了微纳复合结构表面,通过胶体探针的研究发现多巴与高度为1、3、5μm微纳复合结构表面的粘附力均小于与200 nm厚度二十九烷结晶的纳米结构表面之间的粘附力,说明微纳复合结构表面具有很强的抗多巴粘附能力,并且这种复合结构表面相对于硅材质的仿生鲨鱼皮微米结构表面还兼有旱金莲叶面的强疏水性和极佳的抗水粘附能力。     

Two-laser infrared and ultraviolet matrix-assisted laser desorption/ionization

Matrix-assisted laser desorption/ionization (MALDI) was performed using two pulsed lasers with wavelengths in the IR and UV regions. A 10.6 micro m pulsed CO(2) laser was used to irradiate a MALDI target, followed after an adjustable delay by a 337 nm pulsed nitrogen laser. The sample consisted of a 2,5-dihydroxybenzoic acid matrix and bovine insulin guest molecule. The pulse energy for both of the lasers was adjusted so that the ion of interest, either the matrix or guest ion, was not produced by either of the lasers alone. The delay time for maximum ion yield occurs at 1 micro s for matrix and guest ions and the signal decayed to zero in approximately 400 micro s. A mechanism is presented for enhanced UV MALDI ion yield following the IR laser pulse based on transient heating.     

High surface water interaction in superhydrophobic nanostructured silicon surfaces: convergence between nanoscopic and macroscopic scale phenomena

In the present work, we investigate wetting phenomena on freshly prepared nanostructured porous silicon (nPS) with tunable properties. Surface roughness and porosity of nPS can be tailored by controlling fabrication current density in the range 40-120 mA/cm(2). The length scale of the characteristic surface structures that compose nPS allows the application of thermodynamic wettability approaches. The high interaction energy between water and surface is determined by measuring water contact angle (WCA) hysteresis, which reveals Wenzel wetting regime. Moreover, the morphological analysis of the surfaces by atomic force microscopy allows predicting WCA from a semiempiric model adapted to this material.     

发光多孔硅表面的STM研究

自从Canham首次报道了室温下多孔硅的光致发光现象以来^[1],多孔硅已成为半导体光电化学及材料领域内最为热门的研究课题^[2].     

Reduction of spectral interferences using ultraclean gold nanowire arrays in the LDI-MS analysis of a model peptide

The surface chemistry of gold nanowires (AuNWs) has been systematically assessed in terms of contamination and cleaning processes. The nanomaterial’s surface quality was correlated to its performance in the matrix-free laser desorption ionization mass spectrometry (LDI-MS) analysis of low molecular weight analytes. Arrays of AuNWs were deposited on glass slides by means of the lithographically patterned nanowire electrodeposition technique. AuNWs were then characterized in terms of surface chemical composition and morphology using X-ray photoelectron spectroscopy, scanning electron microscopy and atomic force microscopy. AuNWs were subjected to a series of well-known cleaning procedures with the aim of producing the best performing surfaces for the LDI-MS detection of leucine enkephalin, chosen as a model analyte with a molar mass below 1,000 g/mol. Prolonged cyclic voltammetry in 2 M sulfuric acid and, most of all, oxygen plasma cleaning for 5 min provided the best results in terms of simpler (interference-free) and more intense mass spectrometry spectra of the reference compound. The analyte always ionized as the sodiated adduct, and leucine enkephalin limits of detection of 0.5 and 2.5 pmol were estimated for the positive and negative analysis modes, respectively. This study points out the tight correlation existing between the chemical status of the nanostructure surface and the AuNW-assisted LDI-MS performance in terms of reproducibility of spectra, intensity of analyte ions and reduction of interferences.

Isoliquiritigenin (4,2′,4′-trihydroxychalcone): A new matrix-assisted laser desorption/ionization matrix with outstanding properties for the analysis of neutral oligosaccharides

A novel matrix of isoliquiritigenin (ISL), a flavonoid with a chalcone structure (4,2′,4′-trihydroxychalcone), was demonstrated to be advantageous in the analysis of neutral oligosaccharides by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). With ISL as a matrix, adequate signal for an analyte can be obtained in much lower matrix concentrations and laser intensity compared to commonly used MALDI matrices. Four different sample preparation methods were tested, and the dried droplet method exhibited the best performance on MALDI-TOF-MS analysis of oligosaccharides with ISL as a matrix. For the analysis of carbohydrates, compared with popular matrices such as 2,5-dihydroxybenzoic acid (DHB) and 2,4,6-trihydroxyacetophenone (THAP), ISL exhibited outstanding matrix properties as follows: (1) higher homogeneity of crystallization thus allowing automatic data acquisition, (2) better spectral quality in terms of resolution and signal to noise ratio (S N⁻¹), (3) better salt tolerance, (4) higher sensitivity, and (5) enough fragmentation yield to use LIFT-TOF/TOF MS to get richer structural information. In addition, reliable quantitative analysis of oligosaccharides with a good linearity over two concentration orders (1–100 pmol μL⁻¹) and good reproducibility of the signal intensity (RSD less than 15%) were achieved using this matrix. These results give a new outlook on high-speed analysis of neutral carbohydrates by MALDI-TOF MS.     

Hydrogen bubbles and formation of nanoporous silicon during electrochemical etching

Many nanoporous Si structures, including those formed by common electrochemical etching procedures, produce a uniformly etched nanoporous surface. If the electrochemical etch rate is slowed down, details of the etch process can be explored and process parameters may be varied to test hypotheses and obtain controlled nanoporous and defect structures. For example, after electrochemical etching of heavily n‐doped (R = 0.05–0.5 Ω·cm) silicon 〈100〉 single crystals at a current density of 10 mA cm^?2 in buffer oxide etch (BOE) electrolyte solution, defect craters containing textured nanopores were observed to occur in ring‐shaped patterns. The defect craters apparently originate at the hydrogen/BOE bubble interface, which forms during hydrogen evolution in the reaction. The slower hydrogen evolution due to low current density and high BOE viscosity allows sufficient bubble residence time so that a high defect density appears at the bubble edges where local reaction rates are highest. Current‐carrying Si? OH species are most likely responsible for the widening of the craters. Reducing the defect/doping density in silicon lowers the defect concentration and thereby the density of nanopores. Measurements of photoluminescence lifetime and intensity show a distinct feature when the few nanopores formed at the ring edges are isolated from each other. Overall features observed in the photoluminescence intensity by XPS strongly emphasize the role of surface oxide that influences these properties. Copyright © 2005 John Wiley & Sons, Ltd.