Gold nanoparticle-enhanced secondary ion mass spectrometry and its bio-applications

Enhancement of signals in time-of-flight secondary ion mass spectrometry (ToF-SIMS) studies is necessary to many biological applications. We have developed an efficient method of enhancing the signals of secondary ions from peptides using gold nanoparticles (AuNPs) attached to a well-controlled surface such as self-assembled monolayers (SAMs). AuNPs function as both signal enhancers and effective binding sites for peptides, which allow the high signal intensity from the peptides to produce well-contrasted ToF-SIMS images of peptides that are micropatterned on the surface of the AuNPs. For application, this AuNP-enhanced SIMS (NE-SIMS) provided the basis for the spectrum and images to assay protein kinases and their inhibitors. Phosphorylation efficiency and inhibitor effect were quantified by detecting mass change of the peptide substrates by kinase reaction. Maximum efficiency of phosphorylation was achieved from cysteine-tethered peptides with a spacer linker, indicating that accessibility of kinase was dependent on the surface orientation and length of the peptide substrate on the three-dimensional AuNPs. The activities of other enzymes such as phosphatase and protease could also be assayed using NE-SIMS. Our study shows that NE-SIMS can be applied as a useful tool for enzyme assay in biochip surfaces.     

Imaging subcellular features of a sectioned rat brain using time-of-flight secondary ion mass spectrometry and scanning probe microscopy

Coronal sections of unfixed rat brain samples were prepared on a flat substrate in order to reveal hippocampal formation (CA1-4 pyramidal neurons) and adjacent neocortical white matter. We demonstrate the feasibility of using surface sensitive techniques such as time-of-flight secondary ion mass spectrometry (ToF-SIMS) and scanning probe microscopy (SPM) to probe lipid distribution, as well as the subcellular features of neurons. In the same anatomical areas, the phase shift image in SPM is especially useful in revealing the cross-section of subcellular structures. We show that the phase shift images reveal distinctive subcellular features and ion images of CN⁻ and PO₂⁻ fragments from ToF-SIMS appear to define some of the subcellular features.     

Molecular ion yield enhancement induced by gold deposition in static secondary ion mass spectrometry

Static ToF-SIMS was used to evaluate the effect of gold condensation as a sample treatment prior to analysis. The experiments were carried out with a model molecular layer (Triacontane M = 422.4 Da), upon atomic (In⁺) and polyatomic (Bi₃⁺) projectile bombardment. The results indicate that the effect of molecular ion yield improvement using gold metallization exists only under atomic projectile impact. While the quasi-molecular ion (M+Au)⁺ signal can become two orders of magnitude larger than that of the deprotonated molecular ion from the pristine sample under In⁺ bombardment, it barely reaches the initial intensity of (M−H)⁺ when Bi₃⁺ projectiles are used. The differences observed for mono- and polyatomic primary ion bombardment might be explained by differences in near-surface energy deposition, which influences the sputtering and ionization processes.     

Improvement of sensitivity using principal component analysis by dual focused ion beam time-of-flight secondary ion mass spectrometry

It is important to develop and to practically use the method to analyze a micro-nanometer order area. Especially, three-dimensional microanalysis for minute structure that consists of the organic compounds and the polymer is difficult. We developed a novel three-dimensional microanalysis method by means of focused ion beam (FIB) for section processing and ToF-SIMS for mapping method. For the purpose of realization of three-dimensional microanalysis and a chemical and structural analysis of the organic matter, the sensitivity improvement of ToF-SIMS in the three-dimensional analysis device and the method of the spectral analysis are examined. To improve the sensitivity of ToF-SIMS, the sample stage was modified to arrange perpendicularly with the ToF optical axis, and the distortion of electric field was corrected. And, by analyzing the fragment ions by using the principal component analysis (PCA) to raise the efficiency of the spectrum analysis, spatial resolution has improved. As a result, the resolution of the device improved to sub micrometer order, and advanced to the achievement of the three-dimensional microanalysis.     

Imaging of aerosols using time of flight secondary ion mass spectrometry

Interest in environmental aerosol chemistry has grown over the last decade as a result of its role in both climate change and troposheric pollution. In this work, the combination of ToF-SIMS and SEM/EDX was employed to explore the surface chemistry of aerosols. The capabilities and limitations of ToF-SIMS were investigated using particles of known composition and size produced by a vibrating orifice aerosol generator (VOAG). Principal component analysis (PCA) proved to help in the distinction of particles of different types by consolidating the information generated by ToF-SIMS.     

Evaluation of secondary ion yield enhancement from polymer material by using TOF-SIMS equipped with a gold cluster ion source

We investigated the enhancement of the secondary ion intensity in the TOF-SIMS spectra obtained by Au⁺ and Au₃⁺ bombardment in comparison with Ga⁺ excitation using polymer samples with different molecular weight distributions. Since the polymer samples used in this experiment have a wide molecular weight distribution, the advantages of the gold cluster primary ion source over monoatomic ion could accurately be evaluated. It was observed that the degree of fragmentation decreased by the usage of cluster primary ion beam compared with monoatomic ion beam, which was observed as a shift of the intensity distribution in the spectra. It was also found out that the mass effect of Au⁺ and Ga⁺ as monoatomic primary ion, resulted in about 10-60 times of enhancement for both samples with different molecular distributions. On the other hand, the Au₃⁺ bombardment caused intensity enhancement about 100-2600 compared with Ga⁺ bombardment, depending on the mass range of the detected secondary ion species. The cluster primary ion effect of Au₃⁺, compared with Au⁺, therefore, was estimated to be about 10-45.     

Protein quantification on dendrimer-activated surfaces by using time-of-flight secondary ion mass spectrometry and principal component regression

Interaction between streptavidin and biotin on poly(amidoamine) (PAMAM) dendrimer-activated surfaces and on self-assembled monolayers (SAMs) was quantitatively studied by using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The surface protein density was systematically varied as a function of protein concentration and independently quantified using the ellipsometry technique. Principal component analysis (PCA) and principal component regression (PCR) were used to identify a correlation between the intensities of the secondary ion peaks and the surface protein densities. From the ToF-SIMS and ellipsometry results, a good linear correlation of protein density was found. Our study shows that surface protein densities are higher on dendrimer-activated surfaces than on SAMs surfaces due to the spherical property of the dendrimer, and that these surface protein densities can be easily quantified with high sensitivity in a label-free manner by ToF-SIMS.     

Optimization of a quadrupole ion storage trap as a source for time-of-flight mass spectrometry

Designs of a quadrupole ion trap (QIT) as a source for time-of-flight (TOF) mass spectrometry are evaluated for mass resolution, ion trapping, and laser activation of trapped ions. Comparisons are made with the standard hyperbolic electrode ion trap geometry for TOF mass analysis in both linear and reflectron modes. A parallel-plate design for the QIT is found to give significantly improved TOF mass spectrometer performance. Effects of ion temperature, trapped ion cloud size, mass, and extraction field on mass resolution are investigated in detail by simulation of the TOF peak profiles. Mass resolution (m/Δm) values of several thousand are predicted even at room temperature with moderate extraction fields for the optimized design. The optimized design also allows larger radial ion collection size compared with the hyperbolic ion trap, without compromising the mass resolution. The proposed design of the QIT also improves the ion-laser interaction volume and photon collection efficiency for fluorescence measurements on trapped ions.     

Determination of the spatial distribution of trace elements in stainless steel by imaging microprobe secondary ion mass spectrometry

Imaging microprobe secondary ion mass spectrometry (SIMS) using a liquid metal ion source (LMIS) has been used to determine the spatial distribution of metal ions in stainless steel. This scanning ion microprobe is used to achieve high resolution chemical maps of the surface of a sample. For conductive samples such as stainless steel, the practical spatial resolution approaches 0.2 μm and the sensitivity varies with the ion of interest. We have obtained important information relating to the distribution of surface contaminants, segregation, and corrosion phenomena of the elements in these alloys. In this report we hope to demonstrate the usefulness of this important new technique by describing its capabilities and by reviewing data obtained from two separate studies involving stainless steel.

Significant findings obtained in these mapping experiments were:

• . Stainless steel 316 tubing, which had been found previously to be catalytically active, showed remarkable differences between the used and unused samples. Sodium, potassium, and hydrocarbons were found to exist in agglomerations (islands) in the unused tubing, but were very well distributed in the used tubing.

• . A used stainless steel impeller was found to be inhomogeneous with respect to iron and chromium. The surface was entirely covered by chlorine, which is believed to be responsible for the corrosion. Water and elevated levels of chloride ion were found inside numerous scratches or fissures covering the surface. The unused stainless steel showed no signs of streaks or segregated metal regions, and was free from fissures.

     

Comparison of secondary ion intensity enhancement from polymers on silicon and silver substrates by using Au-TOF-SIMS

The usefulness of the usage of cluster primary ion source together with an Ag substrate and detection of Ag cationized molecular ions was studied from the standpoint to realize high sensitivity TOF-SIMS analysis of organic materials. Although secondary ions from polymer thin films on a Si substrate can be detected in a higher sensitivity with Au₃⁺ cluster primary ion compared with Ga⁺ ion bombardment, it was clearly observed that the secondary ion intensities from samples on an Ag substrate showed quite a different tendency from that on Si. When monoatomic primary ions, e.g., Au⁺ and Ga⁺, were used for the measurement of the sample on an Ag substrate, [M+Ag]⁺ ions (M corresponds to polyethylene glycol molecule) were detected in a high sensitivity. On the contrary, when Au₃⁺ was used, no intensity enhancement of [M+Ag]⁺ ions was observed. The acceleration energy dependence of the detected secondary ions implies the different ionization mechanisms on the different substrates.     

Static secondary ion mass spectrometry (S-SIMS) analysis of atmospheric plasma treated polypropylene films

Time-of-Flight (TOF) static secondary ion mass spectrometry (S-SIMS) was used to gain molecular information on the surface modifications introduced by plasma treatment of polypropylene (PP) films. A procedure using slotted electron microscopy grids was developed to deal with the charge build-up of samples with a thickness of about 30 μm. The surface composition was studied as a function of the plasma treatment time. A comparison of the mass spectra from untreated and treated PP showed significant differences of signal intensities of ions that could be specifically related to the presence of oxygen-containing species.     

Analysis of solids by secondary ion and sputtered neutral mass spectrometry

A mass spectrometer is described, which allows the analysis of sputtered neutral and charged particles as well as of residual gas composition. This combined SIMS, SNMS, and RGA instrument consists of a scanning primary ion beam column, an electron impact ionizer, an electrostatic energy filter and an rf quadrupole mass analyzer.Various examples of surface and bulk analysis are presented which demonstrate the beneficial complementary features of these techniques. These are, in particular: a substantial reduction of the matrix effect and fewer complications with samples of low electrical conductivity in SNMS, and the possibility of measuring the depth distribution of gases included in small cavities in the solid in the SNMS/RGA mode. SIMS, on the other hand, allows in many cases higher detection sensitivities.EURATOM Association     

Analysis of intensities of positive and negative ion species from silicon dioxide films using time-of-flight secondary ion mass spectrometry and electronegativity of fragments

Intensities of positive and negative ion species emitted from thermally oxidized and plasma-enhanced chemical vapor deposited (PECVD) SiO₂ films were analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and the Saha-Boltzmann equation. Intensities of positive and negative secondary ion species were normalized to those of ²⁸Si⁺ and ²⁸Si⁻ ions, respectively, and an effective temperature of approximately (7.2 ± 0.1) × 10³ K of the sputtered region bombarded with pulsed 22 kV Au₃⁺ primary ions was determined. Intensity spectra showed polarity dependence on both n and m values of Si_nO_m fragments, and a slight shift to negative polarity for PECVD SiO₂ compared to thermally oxidized SiO₂ films. By dividing the intensity ratios of negative-to-positive ions for PECVD SiO₂ by those for thermally oxidized SiO₂ films to cancel statistical factors, the difference in absolute electronegativity (half the sum of ionization potential and electron affinity of fragments) between both films was obtained. An increase in electronegativity for SiO_m (m = 1, 2) and Si₂O_m (m = 1-4) fragments for PECVD SiO₂ films compared to thermally oxidized films was obtained to be 0.1-0.2 Pauling units, indicating a more covalent nature of Si-O bonds for PECVD SiO₂ films compared to the thermally oxidized SiO₂ films.     

Model study of electron beam charge compensation for positive secondary ion mass spectrometry using a positive primary ion beam

A new modeling approach has been developed to assist in the SIMS analysis of insulating samples. This approach provides information on the charging phenomena occurring when electron and positive primary ion beams impact a low conductivity material held at a high positive potential. The concept of effective leakage resistance aids in the understanding of the dynamic electrical properties of an insulating sample under dynamic analysis conditions. Modeling of steady state electron beam charge compensation involves investigation of electron injection and charge drift. Using a Monte Carlo program to simulate electron injection and dc conduction calculations to predict charge drift, detailed information regarding charging phenomena can be determined.     

Distribution of boron within the microstructure of a ferritic steel determined using secondary ion mass spectrometry

At low concentrations, boron in ferritic steels can potentially have a significant effect on the mechanical properties. High spatial resolution microanalyses have been undertaken on a normalised C–Mn ferritic steel containing less than 1?wt. ppm boron. Secondary ion mass spectrometry on bulk samples has been used to map the distribution of boron with the overall microstructure. Image analysis has been used to determine the location and concentration of boron within the overall microstructure of the steel. The results are discussed in terms of the microstructure, distribution, detection capability and overall audit of boron.     

A differential technique to measure small product yield increments in a time-of-flight mass spectrometer for investigating IR spectra of highly excited polyatomics

We present an experimental technique for measuring small product yield increments in a specially designed time-of-flight mass spectrometer with a supersonic jet. Advantages of this technique are demonstrated in measuring the photodissociation IR spectrum (990–1050 cm^–1) of CF₃I molecules with vibrational energy at the threshold value. The differential technique has enabled us to reveal an anomalously narrow width of the spike of thev ₁ band of CF₃I. We discuss this anomaly.     

Quantification routines for adsorption studies in static secondary ion mass spectrometry and the effect of ionisation probability

It is shown that the usual method of quantification of surface composition in static secondary ion mass spectrometry (SSIMS), which is purely comparative in nature, is unsuitable for adsorption studies by SSIMS. This is because of the effect of the ionisation efficiency and ion stability of a particular ion produced from a molecule adsorbed on the surface of a substrate. The established routine results in a non-linear relationship between calculated relative surface coverage and the ion selected to characterise the adsorbate. The application of a new normalisation routine to time-of-flight secondary ion mass spectrometry (ToF-SIMS) data has been used to account for this discrepancy, and also takes into account the effect of a possible contribution from the clean substrate to the ion selected to characterise the adsorbate molecule. This routine is suggested only for use with organic secondary ions, where the ionisation potential of such ions is of a comparable magnitude, and should prove particularly useful in the application of surface analysis techniques to adsorption studies.     

Empirical evaluation of metal deposition for the analysis of organic compounds with static secondary ion mass spectrometry (S-SIMS)

Metal-assisted (MetA) SIMS using the deposition of a thin Au or Ag layer on non-conducting samples prior to analysis has been advocated as a means to improve the secondary ion (S.I.) yields of organic analytes. This study focuses on the influence of time and temperature on the yield enhancement in MetA-SIMS using thick layers of poly(vinylbutyral-co-vinylalcohol-co-vinylacetate) (PVB) containing dihydroxybenzophenone (DHBPh) or a cationic carbocyanine dye (CBC) and spin-coated layers of the cationic dye on Si. Pristine samples as well as Au- and Ag-coated ones were kept between −8 °C and 80 °C and analysed with S-SIMS at intervals of a few days over a period of 1 month. The yield enhancement was found to depend strongly on the kind of evaporated metal, the storage temperature and time between coating and analysis.     

Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection

The development of a compact tunable mid-IR laser system at 3.5 μm for quantitative airborne spectroscopic trace gas absorption measurements is reported. The mid-IR laser system is based on difference frequency generation (DFG) in periodically poled LiNbO₃ and utilizes optical fiber amplified near-IR diode and fiber lasers as pump sources operating at 1083 nm and 1562 nm, respectively. This paper describes the optical sensor architecture, performance characteristics of individual pump lasers and DFG, as well as its application to wavelength modulation spectroscopy employing an astigmatic Herriott multi-pass gas absorption cell. This compact system permits detection of formaldehyde with a minimal detectable concentration (1σ replicate precision) of 74 parts-per-trillion by volume (pptv) for 1 min of averaging time and was achieved using calibrated gas standards, zero air background and rapid dual-beam subtraction. This corresponds to a pathlength-normalized replicate fractional absorption sensitivity of 2.5×10^-10 cm^-1. Received: 29 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-1492, E-mail: dr@ucar.edu     

Development of accelerator mass spectrometry assisted by isotope-selective laser photodetachment for monitoring 129I

A prototype system was developed to investigate an isotope-selective laser photodetachment technique for a ¹²⁹I accelerator mass spectrometer. A pulsed ion source for negative iodine ions (I⁻) that employed laser ablation proved to be suitable for isotope-selective laser photodetachment. Laser photodetachment of pulsed negative iodine ions was estimated to have an efficiency of about 81% based on a model calculation for the prototype system with a tunable pulsed Ti:sapphire laser.