Improvement of the gas cluster ion beam-(GCIB)-based molecular secondary ion mass spectroscopy (SIMS) depth profile with O2(+) cosputtering.

Over the last decade, cluster ion beams have displayed their capability to analyze organic materials and biological specimens. Compared with atomic ion beams, cluster ion beams non-linearly enhance the sputter yield, suppress damage accumulation and generate high mass fragments during sputtering. These properties allow successful Secondary Ion Mass Spectroscopy (SIMS) analysis of soft materials beyond the static limit. Because the intensity of high mass molecular ions is intrinsically low, enhancing the intensity of these secondary ions while preserving the sample in its original state is the key to highly sensitive molecular depth profiles. In this work, bulk poly(ethylene terephthalate) (PET) was used as a model material and analyzed using Time-of-Flight SIMS (ToF-SIMS) with a pulsed Bi3(2+) primary ion. The optimized hardware of a 10 kV Ar2500(+) Gas Cluster Ion Beam (GCIB) with a low kinetic energy (200-500 V) oxygen ion (O2(+)) as a cosputter beam was employed for generating depth profiles and for examining the effect of beam parameters. The results were then quantitatively analyzed using an established erosion model. It was found that the ion intensity of the PET monomer ([M + H](+)) and its large molecular fragment ([M - C2H4O + H](+)) steadily declined during single GCIB sputtering, with distortion of the distribution information. However, under an optimized GCIB-O2(+) cosputter, the secondary ion intensity quickly reached a steady state and retained >95% intensity with respect to the pristine surface, although the damage cross-section was larger than that of single GCIB sputtering. This improvement was due to the oxidation of molecules and the formation of -OH groups that serve as proton donors to particles emitted from the surface. As a result, the ionization yield was enhanced and damage to the chemical structure was masked. Although O2(+) is known to alter the chemical structure and cause damage accumulation, the concurrently used GCIB could sufficiently remove the surface layer and allow the damage to be masked by the enhanced ionization yield when the ion-solid interaction volume was kept shallow with a low O2(+) energy. This low O2(+) energy (200 V) cosputtering also produced a smoother surface than a single GCIB. Because the oxidized species were produced by O2(+) and removed by GCIB simultaneously, a sufficiently high O2(+) current density was required to produce adequate enhancements. Therefore, it was found that 10 kV with 2 × 10(-6) A per cm(2) Ar2500(+) and 200 V with 3.2 × 10(-4) A per cm(2) O2(+) produced the best profile.

Effect of surface potential on extracellular matrix protein adsorption.

Extracellular matrix (ECM) proteins, such as fibronectin, laminin, and collagen IV, play important roles in many cellular behaviors, including cell adhesion and spreading. Understanding their adsorption behavior on surfaces with different natures is helpful for studying the cellular responses to environments. By tailoring the chemical composition in binary acidic (anionic) and basic (cationic) functionalized self-assembled monolayer (SAM)-modified gold substrates, variable surface potentials can be generated. To examine how surface potential affects the interaction between ECM proteins and substrates, a quartz crystal microbalance with dissipation detection (QCM-D) was used. To study the interaction under physiological conditions, the ionic strength and pH were controlled using phosphate-buffered saline at 37 °C, and the ζ potentials of the SAM-modified Au and protein were determined using an electrokinetic analyzer and phase analysis light scattering, respectively. During adsorption processes, the shifts in resonant frequency (f) and energy dissipation (D) were acquired simultaneously, and the weight change was calculated using the Kelvin-Voigt model. The results reveal that slightly charged protein can be adsorbed on a highly charged SAM, even where both surfaces are negatively charged. This behavior is attributed to the highly charged SAM, which polarizes the protein microscopically, and the Debye interaction, as well as other short-range interactions such as steric force, hydrogen bonding, direct bonding, charged domains within the protein structure, etc., that allow adsorption, although the macroscopic electrostatic interaction discourages adsorption. For surfaces with a moderate potential, proteins are not significantly polarized by the surface, and the interaction can be predicted through simple electrostatic attraction. Furthermore, surface-induced self-assembly of protein molecules also affects the adsorbed structures and kinetics. The adsorbed layer properties, such as rigidity and packing behaviors, were further investigated using the D-f plot and phase detection microscopy (PDM) imaging.

Enhancing the sensitivity of molecular secondary ion mass spectrometry with C60+-O2+ cosputtering.

In the past decade, the C60-based ion gun has been widely utilized in the secondary ion mass spectrometry (SIMS) analysis of organic and biological materials because molecular secondary-ions of high masses could be generated by cluster-ion bombardment. This technique furthers the development of SIMS in bioanalysis by eliminating the need for either heteroatom or isotope labeling. However, the intensity of high-mass parent ions was usually low and limited the sensitivity of the analysis, thus requiring an enhancement in the intensity of these molecular ions to widen the application of SIMS. In this work, the aim was to preserve samples in their original state while using a low kinetic energy O2(+) beam cosputtered with high-energy C60(+) to enhance the ion intensity through the depth-profile. Although O2(+) is generally used to enhance ion intensities in positive SIMS, it is known to alter the chemical structure and primarily provide elemental information; hence, it is not suitable for profiling organic and biological specimens. Nevertheless, owing to its high sputtering yield, cluster C60(+) ion removes and masks the structural damage, hence O2(+) may be used to enhance the ion intensity. The characteristic molecular ions of polyethylene terephthalate (PET), trehalose, and a peptide (papain inhibitor) are enhanced by 35×, 12×, and 3.5× with the use of the auxiliary O2(+) beam, respectively. This significant enhancement in ionization yield is attributed to the oxidation of molecules and formation of a hydroxyl group that serves as a proton donator. In addition to enhancing molecular SIMS signals, C60(+)-O2(+) cosputtering could also alleviate several problems, including sputtering rate decay, carbon deposition, and surface roughening, that are associated with C60(+) bombardment and produced better depth profiles.

Effect of cosputtering and sample rotation on improving C60(+) depth profiling of materials.

In the past decade, buckminsterfullerene (C(60))-based ion beams have been utilized in surface analysis instruments to expand their application to profiling organic materials. Although it had excellent performance for many organic and biological materials, its drawbacks, including carbon deposition, carbon penetration, continuous decay of the sputtering rate, and a rough sputtered surface, hindered its application. Cosputtering with C(60)(+) and auxiliary Ar(+) simultaneously and sample rotation during sputtering were proposed as methods to reduce the above-mentioned phenomena. However, the improvement from these methods has not been compared or studied under identical conditions; thus, the pros and cons of these methods are not yet known experimentally. In this work, a series of specimens including bulk materials and thin films were used to explore the differences between cosputtering and sample rotation on the analytical results. The results show that both of these methods can alleviate the problems associated with C(60)(+) sputtering, but each method showed better improvement in different situations. The cosputtering technique better suppressed carbon deposition, and could be used to generally improve results, especially with continuous spectra acquisition during sputtering (e.g., dynamic secondary ion mass spectrometry (SIMS) depth profiling). In contrast, for the scheme of sputter-then-acquire (e.g., alternative X-ray photoelectron spectrometry or dual-beam static SIMS depth profiling), a better result was achieved by sample rotation because it resulted in a flatter sputtered surface. Therefore, depending on the analytical scheme, a different method should be used to optimize the experimental conditions.

Dramatically enhanced oxygen uptake and ionization yield of positive secondary ions with C60+ sputtering.

To explore C(60)(+) sputtering beyond low-damage depth profiling of organic materials, X-ray photoelectron spectrometry (XPS) and secondary ion mass spectrometry (SIMS) were used to examine metallic surfaces during and after C(60)(+) sputtering. During C(60)(+) sputtering, XPS spectra indicated that the degrees of carbon deposition were different for different metallic surfaces. Moreover, for some metals (e.g., Al, W, Ta, Ti, and Mo), the intensity of the O 1s photoelectron increased significantly during C(60)(+) sputtering, even though the instrument was under ultrahigh vacuum (<5 × 10(-7) Pa). This result indicated that the rate of oxygen uptake was greater than the rate of C(60)(+) sputtering. This behavior was not observed with the commonly used Ar(+) sputtering. To measure the oxygen uptake kinetics, pure oxygen was leaked into the chamber to maintain a 5 × 10(-6) Pa oxygen environment. The C(60)(+)-sputtered surface had a clearly increased rate of oxygen uptake than the Ar(+)-sputtered surface, even for moderately reactive metals such as Fe and Ni. For relatively nonreactive metals such as Cu and Au, a small amount of carbon was implanted and no oxygen uptake was observed. High-resolution XPS spectra revealed the formation of metal carbides on these reactive metals, and the carbon deposition and enhanced uptake of oxygen correlated to the carbide formation. Because oxygen enhances the secondary ion yield through surface passivation, the enhanced oxygen uptake due to C(60)(+) sputtering could be beneficial for SIMS analysis. To examine this hypothesis, C(60)(+) and Ar(+) were used as primary ions, and it was found that the intensity enhancement (because of the oxygen flooding at 5 × 10(-6) Pa) was much higher with C(60)(+) than with Ar(+). Therefore, oxygen flooding during C(60)(+) sputtering has a great potential for enhancing the detection limit due to the enhanced oxygen uptake.

Rational shape-controlled synthesis of rhombic copper-based mesostructures.

Solution synthesis of optoelectronic components has the advantages of processability, bandgap tuning, and large-scale manufacturing potential. The synthesis of monodispersed rhombs in solution, however, has rarely been reported, even though rhombs are promising for realizing unique optical functions in integrated optoelectronics. We present in this article our success in developing a facile chemical method that used two polyols to generate nearly monodispersed metal-organic rhombic platelets. The success lies on the careful selection of precursors of proper oxidation states and the optimization of both the thermodynamic and the kinetic conditions for synthesis. Cuprous acetate, which acted as a heterogeneous nucleation agent, was dispersed in ethylene glycol, which acted as a stabilizer, a ligand, and a monomer for the formation of polymeric glycolates. By adjusting the volume ratio of polyethylene glycol (PEG) to ethylene glycol and the polymer size of PEG, rhombic platelets of 200-580 nm in side length and 170-240 nm in thickness were synthesized with aid of suitable structure-directing and dispersing agents. Energy-dispersive X-ray spectroscopy and FT-IR analyses revealed that the rhombic platelets were mainly composed of copper glycolate polymer chains. Knowledge obtained from this study can be expected to be applied to and to shed light on broad research topics concerning novel metal-organic nanostructure syntheses.

Electron tomography of HEK293T cells using scanning electron microscope-based scanning transmission electron microscopy.

Based on a scanning electron microscope operated at 30 kV with a homemade specimen holder and a multiangle solid-state detector behind the sample, low-kV scanning transmission electron microscopy (STEM) is presented with subsequent electron tomography for three-dimensional (3D) volume structure. Because of the low acceleration voltage, the stronger electron-atom scattering leads to a stronger contrast in the resulting image than standard TEM, especially for light elements. Furthermore, the low-kV STEM yields less radiation damage to the specimen, hence the structure can be preserved. In this work, two-dimensional STEM images of a 1-µm-thick cell section with projection angles between ±50° were collected, and the 3D volume structure was reconstructed using the simultaneous iterative reconstructive technique algorithm with the TomoJ plugin for ImageJ, which are both public domain software. Furthermore, the cross-sectional structure was obtained with the Volume Viewer plugin in ImageJ. Although the tilting angle is constrained and limits the resulting structural resolution, slicing the reconstructed volume generated the depth profile of the thick specimen with sufficient resolution to examine cellular uptake of Au nanoparticles, and the final position of these nanoparticles inside the cell was imaged.

Molecular dynamic-secondary ion mass spectrometry (D-SIMS) ionized by co-sputtering with C60+ and Ar+.

Dynamic secondary ion mass spectrometry (D-SIMS) analysis of poly(ethylene terephthalate) (PET) and poly(methyl methacrylate) (PMMA) was conducted using a quadrupole mass analyzer with various combinations of continuous C(60)(+) and Ar(+) ion sputtering. Individually, the Ar(+) beam failed to generate fragments above m/z 200, and the C(60)(+) beam generated molecular fragments of m/z ~1000. By combining the two beams, the auxiliary Ar(+) beam, which is proposed to suppress carbon deposition due to C(60)(+) bombardment and/or remove graphitized polymer, the sputtering range of the C(60)(+) beam is extended. Another advantage of this technique is that the high sputtering rate and associated high molecular ion intensity of the C(60)(+) beam generate adequate high-mass fragments that mask the damage from the Ar(+) beam. As a result, fragments at m/z ~900 can be clearly observed. As a depth-profiling tool, the single C(60)(+) beam cannot reach a steady state for either PET or PMMA at high ion fluence, and the intensity of the molecular fragments produced by the beam decreases with increasing C(60)(+) fluence. As a result, the single C(60)(+) beam is suitable for profiling surface layers with limited thickness. With C(60)(+)-Ar(+) co-sputtering, although the initial drop in intensity is more significant than with single C(60)(+) ionization because of the damage introduced by the auxiliary Ar(+), the intensity levels indicate that a more steady-state process can be achieved. In addition, the secondary ion intensity at high fluence is higher with co-sputtering. As a result, the sputtered depth is enhanced with co-sputtering and the technique is suitable for profiling thick layers. Furthermore, co-sputtering yields a smoother surface than single C(60)(+) sputtering.

Effect of surface chemical composition on the work function of silicon substrates modified by binary self-assembled monolayers.

It has been shown that the application of self-assembled monolayers (SAMs) to semiconductors or metals may enhance the efficiency of optoelectronic devices by changing the surface properties and tuning the work functions at their interfaces. In this work, binary SAMs with various ratios of 3-aminopropyltrimethoxysilane (APTMS) and 3-mercaptopropyltrimethoxysilane (MPTMS) were used to modify the surface of Si to fine-tune the work function of Si to an arbitrary energy level. As an electron-donor, amine SAM (from APTMS) produced outward dipole moments, which led to a lower work function. Conversely, electron-accepting thiol SAM (from MPTMS) increased the work function. It was found that the work function of Si changed linearly with the chemical composition and increased with the concentration of thiol SAMs. Because dipoles of opposite directions cancelled each other out, homogeneously mixing them leads to a net dipole moment (hence the additional surface potential) between the extremes defined by each dipole and changes linearly with the chemical composition. As a result, the work function changed linearly with the chemical composition. Furthermore, the amine SAM possessed a stronger dipole than the thiol SAM. Therefore, the SAMs modified with APTMS showed a greater work function shift than did the SAMs modified with MPTMS.

Dewetting of copper nanolayers on silica in oxygen: towards preparation of copper meso/nanowires by self-organization.

The present study has examined the thermal behavior of copper on silicon oxide to clarify the diffusion of copper on dielectrics in an oxygen environment. Films of copper-deposited silicon oxide were prepared on silicon wafers and then annealed in oxygen. Self-organization of copper occurred to form line structures of multiple strips in a specific oxygen pressure range. The line orientation of the produced structures was related to the line defects formed from termination of stacking faults and dislocations at the wafer surface. The line density was determined by the oxygen pressure used. The results underline a possibility of synthesizing copper meso/nanowires on dielectrics via self-organization.

Prevention of enamel demineralization with a novel fluoride strip: enamel surface composition and depth profile.

There is no topically applicable low concentration fluoride delivery device available for caries prevention. This study was aimed to assess the use of a low concentration (1450 ppm) fluoride strip as an effective fluoride delivery system against enamel demineralization. The enamel surface composition and calcium-deficient hydroxyapatite or toothpaste treatments were investigated using X-ray photoelectron spectroscopy. In vitro enamel demineralization was assayed using a pH cycling model and the dissolution of calcium ions from the treated specimens was quantified using ion chromatography. After 24-hr fluoride-strip treatment, the enamel was covered with a CaF2 layer which showed a granular morphology of 1 µm in size. Below the CaF2 layer was a region of mixed fluorapatite and CaF2. Fluoride infiltrated extensively in enamel to produce highly fluorinated fluorohydroxyapatite. In comparison, low-fluoride-level fluorinated fluorohydroxyapatite was formed on the enamel specimen exposed to toothpaste. The treatments with the fluoride strip as short as 1 hr significantly inhibited enamel demineralization. The fluoride strip was effective for topical fluoride delivery and inhibited in vitro demineralization of enamel by forming CaF2 and fluoride-containing apatites at the enamel surface. It exhibited the potential as an effective fluoride delivery device for general use in prevention of caries.

Rapid label-free determination of ketamine in whole blood using secondary ion mass spectrometry.

A fast and accurate drug screening to identify the possible presence of a wide variety of pharmaceutical and illicit drugs is increasingly requested in forensic and clinical toxicology. The current first-line screening relies on immunoassays. They determine only certain common drugs of which antibodies are commercially available. To address the issue, a rapid screening using secondary ion mass spectrometry (SIMS) has been developed. In the study, SIMS directly analyzed ketamine in whole blood without any pretreatment. While the untreated blood has a complicated composition, principal-components analysis (PCA) is used to detect unknown specimens by building up an analytical model from blank samples which were spiked with ketamine at 100 ng mL(-1), to simulate the presence of ketamine. Each characteristics m/z is normalized and scaled by multiplying the root square of intensity and square of corresponding m/z, developed by National Institute of Standards and Technology (NIST). Using linear regression and the result of PCA, this study enables to correctly distinguish ketamine positive and negative groups in an unknown set of specimens. The quantity of ketamine in an unknown set was determined using gas chromatography-mass spectrometry (GC-MS) as the reference methodology. Instead limited by commercially available antibodies, SIMS detects target molecules straight despite the label-free detection capabilities of SIMS, additional data processing (here, PCA) can be used to fully analyse the produced data, which extends the range of analytes of interest on drug screening. Furthermore, extremely low sample volume, 5 µL, is required owing to the high spatial resolution of SIMS. In addition, while the whole blood is analyzed within 3 min, the whole analysis has been shortened significantly and high throughput can be achieved.

Binary self-assembled monolayers modified Au nanoparticles as carriers in biological applications.

Gold nanoparticles (AuNPs) are good nonviral carriers because of their ease of synthesis and conjugation in biochemistry, and self-assembled monolayers (SAMs) provide a tunable system to change their interfacial properties. Using homogeneously mixed carboxylic acid and amine functional groups, a series of surface potentials and isoelectric points (IEPs) could be obtained and allow systematic study of the effect of surface potential. In this work, the result of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay revealed that binary-SAM modified AuNPs have high biocompatibility with HEK293T cells. The amount of AuNPs ingested by the cells was found to increase with increasing surface potential and the difference was also confirmed with a scanning transmission electron microscope. The ability of binary-SAM modified AuNPs as carriers was examined, and the plasmid deoxyribose nucleic acid (DNA)-containing eGFP reporter gene was used as the model cargo. Fluorescence imaging revealed that the transfection efficiency generally increased with increasing surface potential. More importantly, when the IEP of the AuNPs was higher than that of the environment of the endosome but lower than that of the cytoplasm, the plasmid DNA can be protected better and released more easily during the endocytosis process hence higher efficiency is obtained with 60% NH2 and 40% COOH in the binary-SAM.

Effects of the temperature and beam parameters on depth profiles in X-ray photoelectron spectrometry and secondary ion mass spectrometry under C60(+)-Ar(+) cosputtering.

Polymethylmethacrylate (PMMA) is widely used in various fields, including the semiconductor, biomaterial and microelectronic fields. Obtaining the correct depth profiles of PMMA is essential, especially when it is used as a thin-film. There have been many studies that have used earlier generation of cluster ion (SF5(+)) as the sputtering source to profile PMMA films, but few reports have discussed the use of the more recently developed C60(+) in the PMMA sputtering process. In this study, X-ray photoelectron spectroscopy (XPS) and dynamic secondary ion mass spectroscopy (D-SIMS) were used concurrently to monitor the depth profiles of PMMA under C60(+) bombardment. Additionally, the cosputtering technique (C60(+) sputtering with auxiliary, low-kinetic-energy Ar(+)) was introduced to improve the analytical results. The proper cosputtering conditions could eliminate the signal enhancement near the interface that occurred with C60(+) sputtering and enhance the sputtering yield of the characteristic signals. Atomic force microscopy (AFM) was also used to measure the ion-induced topography. Furthermore, the effect of the specimen temperature on the PMMA depth profile was also examined. At higher temperatures (+120°C), the depolymerization reaction that corresponded to main-chain scission dominated the sputtering process. At lower temperatures (-120°C), the cross-linking mechanism was retarded significantly due to the immobilization of free radicals. Both the higher and lower sample temperatures were found to further improve the resulting depth profiles.

Ultraviolet electroluminescence from nitrogen-doped ZnO-based heterojuntion light-emitting diodes prepared by remote plasma in situ atomic layer-doping technique.

Remote plasma in situ atomic layer doping technique was applied to prepare an n-type nitrogen-doped ZnO (n-ZnO:N) layer upon p-type magnesium-doped GaN (p-GaN:Mg) to fabricate the n-ZnO:N/p-GaN:Mg heterojuntion light-emitting diodes. The room-temperature electroluminescence exhibits a dominant ultraviolet peak at λ ≈ 370 nm from ZnO band-edge emission and suppressed luminescence from GaN, as a result of the decrease in electron concentration in ZnO and reduced electron injection from n-ZnO:N to p-GaN:Mg because of the nitrogen incorporation. The result indicates that the in situ atomic layer doping technique is an effective approach to tailoring the electrical properties of materials in device applications.

Parallel detection, quantification, and depth profiling of peptides with dynamic-secondary ion mass spectrometry (D-SIMS) ionized by C60(+)-Ar(+) co-sputtering.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) using pulsed C(60)(+) primary ions is a promising technique for analyzing biological specimens with high surface sensitivities. With molecular secondary ions of high masses, multiple molecules can be identified simultaneously without prior separation or isotope labeling. Previous reports using the C(60)(+) primary ion have been based on static-SIMS, which makes depth profiling complicated. Therefore, a dynamic-SIMS technique is reported here. Mixed peptides in the cryoprotectant trehalose were used as a model for evaluating the parameters that lead to the parallel detection and quantification of biomaterials. Trehalose was mixed separately with different concentrations of peptides. The peptide secondary ion intensities (normalized with respect to those of trehalose) were directly proportional to their concentration in the matrix (0.01-2.5 mol%). Quantification curves for each peptide were generated by plotting the percentage of peptides in trehalose versus the normalized SIMS intensities. Using these curves, the parallel detection, identification, and quantification of multiple peptides was achieved. Low energy Ar(+) was used to co-sputter and ionize the peptide-doped trehalose sample to suppress the carbon deposition associated with C(60)(+) bombardment, which suppressed the ion intensities during the depth profiling. This co-sputtering technique yielded steadier molecular ion intensities than when using a single C(60)(+) beam. In other words, co-sputtering is suitable for the depth profiling of thick specimens. In addition, the smoother surface generated by co-sputtering yielded greater depth resolution than C(60)(+) sputtering. Furthermore, because C(60)(+) is responsible for generating the molecular ions, the dosage of the auxiliary Ar(+) does not significantly affect the quantification curves.

The roughness, microhardness, and surface analysis of nanocomposites after application of topical fluoride gels.

OBJECTIVES: Application of acidulated phosphate fluoride (APF) gels has long been considered to cause deterioration of composite surfaces. The aims of this study were to demonstrate that nanocomposite surfaces were not affected by some APF gels and to investigate the possible underlying mechanisms. METHODS: The elemental composition and viscosity of 3 acidulated phosphate fluoride (APF) agents (60 Second Taste Gel, Topex, and Zap) and 1 neutral fluoride agent (pH7 Gel) were analyzed. Subsequently, 320 specimens of 3 nanocomposites (Premisa, Filtek Z350, and Grandio) and a microhybrid composite (Estelite Sigma) with 80 specimens for each composite were randomly divided into 5 groups (n=16) and treated with 4 fluoride gels as well as distilled water which served as the control. Fluoride gels were applied on composite resin surfaces 4 times, 30 min each time. The roughness and microhardness were measured after treatments. Qualitative examination of the surface degradation of the composites was carried out with Fourier transforming infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). RESULTS: Topex and Zap did not cause surface changes of composite resins, the possible reason being ascribed to the presence of magnesium aluminum silicate (MAS) clays. In contrast, 60 Second Taste Gel treatments caused significant roughness increase, microhardness decrease, more prominent filler dissolution, and IR spectral changes of Premisa, Filtek Z350, and Grandio. Estelite Sigma was less affected by the 4 fluoride gels. SIGNIFICANCE: The composite surfaces were not affected by Topex or Zap even after extended treatments. These two APF gels may be more suitable for clinical applications.