Chemical derivatization technique in ToF-SIMS for quantification analysis of surface amine groups

A chemical derivatization technique in ToF-SIMS along with principal component analysis (PCA) were used to perform a quantitative study of the surface amine density of the plasma-polymerized ethylenediamine (PPEDA) thin film. We used the scores on principal component (PC) 1 from a PCA of ToF-SIMS data for the PPEDA films and their chemical-derivatized surfaces for comparison with the surface amine densities. These surface amine densities were independently determined by UV-visible spectroscopy. Our work found a good linear relationship between the surface amine densities and the scores on PC 1 from a PCA of the ToF-SIMS data for the chemical-derivatized PPEDA surfaces, but not for the PPEDA thin films themselves. In addition to quantification, our PCA results provided insights into the surface chemical composition of each surface.     

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.     

Adsorption and electrically stimulated desorption of the triblock copolymer poly(propylene sulfide-bl-ethylene glycol) (PPS-PEG) from indium tin oxide (ITO) surfaces

Protein-resistant triblock copolymers, poly(propylene sulfide-bl-ethylene glycol) (PPS-PEG) have been previously demonstrated to chemisorb onto gold surfaces forming monolayers that resist non-specific protein adsorption and are stable against oxidation. In this paper, we report on the adsorption of PPS-PEG onto a transparent and electrically conductive substrate, indium tin oxide (ITO). In addition, we demonstrate the controlled desorption of PPS-PEG by applying an electrical stimulus. We have used three complementary surface characterization techniques: variable angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to analyze the adsorption and electro-desorption of PPS-PEG from an ITO surface. All three methods confirmed the formation of PPS-PEG adlayers on the ITO surfaces. Based on our experimental XPS and ToF-SIMS results as well as former publications, we postulate that the chemisorption of the PPS-PEG on ITO involves direct sulfide-indium (or tin) interactions. When an ascending anodic electrical stimulus was applied to the surface of the modified samples, a gradual and steady polymer removal was observed, with complete loss of the polymeric monolayer at a potential of 2000 mV (referenced to Ag electrode). Anodic polarization did not result in oxidation of the thioether function of the PPS-PEG adlayers, indicating excellent oxidation resistance of PPS-PEG on ITO surfaces. This work is focused on exploiting electrical stimuli for the in situ surface modification under dynamic control.     

ToF-SIMS study on the cleaning methods of Au surface and their effects on the reproducibility of self-assembled monolayers

The production of reproducible self-assembled monolayers (SAMs) is essential to many nano(bio)technology applications. To check the effects of different cleaning methods on a reproducible SAMs formation, the cleaning methods were varied and then used for preparing each SAM. The reproducibility of each SAM was examined by ToF-SIMS analysis along with principal component analysis (PCA). Using what we found to be a superior method of cleaning gold surfaces, alkanethiol SAMs with different terminal groups such as 1-dodecanethiol (DDT), 11-mercaptoundecanoic acid (MUA), 11-mercapto-1-undecanol (MUD) were reproduced. Our statistical results show that reproducible alkanethiol SAMs on a well-cleaned gold surface can be produced within only a few standard deviation percentages obtained from point-to-point and sample-to-sample spectra.     

Multifunctional ToF-SIMS: combinatorial mapping of gradient energy substrates

We present a simple method for chemical modification of chlorosilane self-assembled monolayers (SAMs) on Si surfaces by exposure to a gradient of UV-ozone radiation to create stable substrates with a range of contact angles (θ_H2O≈5–95°) and surface energies on a single substrate. These gradient energy substrates are developed to potentially generate libraries for combinatorial studies of thin film phenomenology, where a systematic variation of interfacial surface energy represents one of the significant parameters along one axis. The graded oxidation process presents a systematic variation of surface chemical composition. We have utilized contact angle measurements and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to investigate this variation for a series of ions, among which are SiCH₃⁺, SiOH⁺ and COOH⁻. We show that the macroscopic measurements of surface free energy/contact angle correlate with the detailed analysis of surface chemistry (as assessed by ToF-SIMS) on these test substrates.     

Self-assembled monolayer and multilayer formation using redox-active Ru complex with phosphonic acids on silicon oxide surface

The formation of self-assembled monolayer and multilayer using redox-active Ru complex molecules with phosphonic acids on SiO₂ surface has been examined using X-ray photoelectron spectroscopy (XPS), ellipsometry, and time of flight secondary mass-ion spectroscopy (TOF-SIMS). We found that an introduction of a Zr adlayer leads to higher surface molecular density of Ru complex SAMs on the SiO₂ surface, compared to that of obtained from the direct adsorption of Ru complex monolayer on the SiO₂ surface. We further tried to fabricate a multilayer film using this molecule with Zr(IV) ion acting as a chemical glue by a successive immersion process. The XPS data revealed that the molecular densities of the multilayers were also higher for the immobilization with Zr adlayer between Ru complex and SiO₂ surface than those without the Zr adlayer, suggesting that Zr adlayer is effective in forming highly packed molecular layer of phosphonic acids on SiO₂ surface. We found the film growth reached a saturation point after 6 layers on the SiO₂ surface. The film growth saturation can be explained by a molecular domain boundary effect encountered due to the large tilt angle of the molecular layer.     

Chemistry of metal atoms reacting with alkanethiol self-assembled monolayers

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is utilized to investigate the behavior of vapor-deposited K, Au and Ti atoms on several alkanethiol self-assembled monolayers (SAM). The goals are to acquire information about chemical reactions between metal atoms and surface organic functional groups, penetration of metal atoms through the SAMs, growth modes of metal overlayers on top of the SAMs and damage of organic molecules. It is found that appearance of new characteristic peaks and disappearance of initial peaks may indicate chemical reactions or decomposition of organic molecules. The relationship between metal dose and intensity of surface organic functional group-related peaks provides information about penetration or cluster-formation of metal atoms. In addition, removing the metal overlayers by chemical etching and then characterizing samples again is a complementary approach that can reveal valuable information about the location of the metal atoms.     

Characterization of fluoroalkylsilane monolayer on polystyrene sphere arrays after plasma treatment

Polystyrene (PS) colloidal crystal films with well-ordered arrays of PS spheres treated with argon plasma and coated with fluoroalkylsilane (FAS) were characterized by means of spectroscopy ellipsometry, X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). The XPS analysis indicated that the FAS film on the plasma treated PS surface was a monolayer with an orderly packed CF₃ group pointing outwards from the surface. The chemical composition of the PS surface changed immediately after a very short period of argon plasma treatment, while the subsequent coating of FAS on the plasma treated PS surface further modified the surface chemistry. The untreated PS surface exhibited poor interaction with FAS molecules. XPS and ToF-SIMS analyses showed the plasma treatment involved the oxidation of PS surface, where oxygen functional groups -O and O were generated, promoting FAS deposition on the plasma treated surface with strong secondary ion fragments originating from the FAS. The overall results indicated that plasma treatment was beneficial to the deposition of the FAS monolayer.     

Quantitative analysis of mixed self-assembled monolayers using ToF-SIMS

The spacing of chemical functional groups on self-assembled monolayers (SAMs) plays an important role in controlling the density of biomolecules in biochips and biosensors. In this sense, a mixed SAM made of two different terminal groups is a useful organic surface since spacing can be easily controlled by changing a relative mole fraction in a mixture solution. In this study, an acetylene-OCH₂O(EG)₃(CH₂)₁₁S-S(CH₂)₁₁(EG)₃OCH₂O-propene (Eneyne) SAM and mixed SAMs made by a mixture of (S(CH₂)₁₁(EG)₃OCH₂O-acetylene)₂ (Diyne) and (S(CH₂)₁₁(EG)₃OCH₂O-propene)₂ (Diene) were produced on gold substrates and measured by using ToF-SIMS. The secondary ion yield ratio of [Au·S(CH₂)₁₁(EG)₃OCH₂O-acetylene]⁻ to [Au·S(CH₂)₁₁(EG)₃OCH₂O-propene]⁻ was measured for each mixed SAM and plotted as a function of the mole fraction of Diyne to Diene in a SAM solution. The ion yield ratio of a mixed SAM produced from a solution with a mole fraction of 0.5 (i.e., 1:1 mixture) was 0.3, which corresponded well to the ion yield ratio measured from an Eneyne SAM. A time-dependent experiment of Eneyne SAM formation and immersion experiment of Eneyne SAM into Diyne solution or into Diene solution were performed. The relative ion yield ratio of 0.3 was due to a different secondary ion formation and not due to the difference in the amount of adsorbates on the surface, nor to the different binding strengths onto the gold surface. Our study shows that a mixed SAM with well-controlled spacing can be produced and quantified by using the ToF-SIMS technique.     

Lamellar orientation on the surface of a polymer determined by ToF-SIMS and AFM

The fold and lateral surfaces of chain-folded lamellae of poly(bisphenol-A-co-etheroctane), containing both aliphatic CH₂ and aromatic segments, were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). At low and high crystallization temperatures, the surfaces of the polymer films were shown to consist mainly of edge-on and flat-on lamellae, respectively. Surfaces with a mixture of edge-on and flat-on lamellae were produced at intermediate temperatures. The edge-on and flat-on lamellae were identified by using ions that recognize the flexible and rigid segments of the polymer. Ion images produced using selected ions that are related to the edge-on or flat-on orientation can be used to identify the location of these lamellae on the polymer surface. Our results indicate that ToF-SIMS can be used to detect different lamellar orientations at the surfaces of semi-crystalline polymers.     

Laser activation-modification of semiconductor surfaces (LAMSS) of 1-alkenes on silicon: A ToF-SIMS, chemometrics, and AFM analysis

Laser-activation-modification of semiconductor surfaces (LAMSS) was carried out on silicon with a series of 1-alkenes. These laser spots were studied by time of flight secondary ion mass spectrometry (ToF-SIMS). The resulting spectra were analyzed using the multivariate curve resolution (MCR) method within the Automated eXpert Spectral Image Analysis (AXSIA) toolkit, and also by MCR and cluster analysis using commercially available toolboxes for Matlab: the PLS_Toolbox and the MIA_Toolbox, respectively. AXSIA based MCR generally finds three components for the spectral images: one for the background and two for the laser-activated spots, for both the positive and negative ion images. The negative ion component spectra from the spots show increased carbon and hydrogen signals compared to oxygen. They also show reduced chlorine and fluorine (contamination) peaks. In order to compare AXSIA-MCR results from different images, the AXSIA component spectra of different spots were further analyzed by principal components analysis (PCA). PCA of all of the negative ion components shows that component 1 is chemically distinct from components 2 and 3. PCA of all of the positive ion components yields the same result. The loadings plots of this PCA analysis confirm that component 1 generally contains fragments expected from the substrate, while components 2 and 3 contain fragments expected from an overlayer composed of alkyl chains in the spots. A comparison of the two MCR analyses suggests that roughly the same information can be obtained from AXSIA, which is not commercially available, and the PLS_Toolbox. Cluster analysis of the data also clearly separates the spots from the backgrounds. A key finding from these analyses is that the degree of surface functionalization in a LAMSS spot appears to decrease radially from the center of the spot. Finally, a comparison of atomic force microscopy (AFM) of the spots versus the AXSIA analysis of the ToF-SIMS data produced another important result, which is that the surface morphology is only weakly correlated with the LAMSS chemistry.     

ToF-SIMS imaging of gradient polyethylene and its amine-functionalized surfaces

A ToF-SIMS imaging technique has been used to obtain dye label-free chemical images of gradient polyethylene (PE) and its amine-functionalized surfaces. The gradient PE surface was prepared by a corona treatment with a power increase along the PE length and confirmed by ToF-SIMS. Amine reagents with varying molecular weights (MW 158, 600, 1200, 10,000 and 60,000) were successively reacted with a gradient PE surface and examined using ToF-SIMS. Our work found that the lower the molecular weight of amine reagent, the more gradient surface of amine surface density was generated. The amine-functionalized gradient surface can be used as a template surface for evaluating bioactive molecules on the gradient surface in just one experiment.     

Differentiation between human normal colon mucosa and colon cancer tissue using ToF-SIMS imaging technique and principal component analysis

Human normal colon mucosa and colon cancer tissue were studied using the time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and principal component analysis (PCA) techniques. The surfaces of the tissues were successfully cleaned by C₆₀²⁺ cluster-ion beams before the ToF-SIMS images were obtained. A PCA on the spectra and images were performed to compare differences in the peaks and images of normal and cancer tissues. Significant differences in principal component 1 (PC 1) score values for normal and cancer tissues were observed, and each PC 1 loadings had a specific peak profile of proteins. In addition, the PC images obtained from the ToF-SIMS images for normal and cancer tissues were clearly distinguishable, and the amino acid fragments associated with normal and cancer tissues were found to have originated from the lamina propria region and the epithelium cells, respectively. Based on the PCA results, structural distortion of the crypts in the cancer colon tissue could be attributed to the proliferation of the cancerous epithelium cells. This work shows that the application of the ToF-SIMS imaging technique with PCA could be a useful method of obtaining valuable information for cancer analysis.     

Preparation and tribological tests of thin fluoroorganic films

Adhesion, friction and consequent wear of sliding surfaces are the basic problems that limit the performance and reliability of microelectromechanical devices. Lubrication of these nano- and microscale contacts is different from traditional lubricants. Self-assembled monolayers (SAMs) chemically bonded to the substrate are considered to be the best solution of lubrication. The majority of these monolayers are hydrophobic providing low friction, adhesion and wear.Chemical vapor deposition was used to grow a fluorosilane film on silicon Si(1 0 0) and a condensed monolayer of 3-mercaptopropyltrimethoxysilane (MPTMS) on Au(1 1 1). The films were characterized by means of a contact angle analyzer for hydrophobicity, and time-of-flight secondary ion mass spectrometry (ToF-SIMS) for identification of thin fluoroorganic monolayers deposited on silica surfaces and condensed monolayer MPTMS. Adhesion and friction measurements were performed using atomic force microscopy (AFM) and compared with measurements performed using a microtribometer operating in millinewton (mN) normal load range. Nanotribological measurements indicated that silica and MPTMS modified by fluorosilanes have the lowest friction coefficient and indicated a decrease friction coefficient with increasing fluoric alkyl chain length.     

Quantitative analysis of surface amine groups on plasma-polymerized ethylenediamine films using UV-visible spectroscopy compared to chemical derivatization with FT-IR spectroscopy, XPS and TOF-SIMS

A quantitative analysis of the surface density of amine groups on a plasma-polymerized ethylenediamine thin film deposited on a platinum surface using inductively coupled plasma chemical vapor deposition method is described. UV-visible spectroscopy together with a chemical derivatization technique using Fourier transform infrared (FT-IR) spectroscopy was used to obtain the quantitative information. Chemical tags of pentafluorobenzaldehyde were hybridized with the surface amine groups and were easily detected due to the characteristic absorption bands of C-F stretching, aromatic ring and CN stretching vibrations in the reflection-absorption FT-IR spectra. The surface amine density was reproducibly controlled as a function of deposition plasma power and quantified using UV-visible spectroscopy. A good linear correlation was observed between the FT-IR intensities of the characteristic absorption bands and the surface amine densities, suggesting the possibility of using this chemical derivatization technique to quantify the surface densities of specific functional groups on an organic surface. Chemical derivatization was also used with X-ray photoelectron spectroscopy on the same samples, and the results were compared with those obtained from FT-IR and time-of-flight secondary ion mass spectrometry. Although each analysis technique has different probing depths from the surface, the three different data sets obtained from the chemical tags correlated well with each other since each analysis technique measured the chemical tags on the sample surface.     

SIMS depth profiling of polymer blends with protein based drugs

We report the results of the surface and in-depth characterization of two component blend films of poly(l-lactic acid) (PLLA) and Pluronic surfactant [poly(ethylene oxide) (A) poly(propylene oxide) (B) ABA block copolymer]. These blend systems are of particular importance for protein drug delivery, where it is expected that the Pluronic surfactant will retain the activity of the protein drug and enhance the biocompatibility of the device. Angle dependant X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) employing an SF₅⁺ polyatomic primary ion source were both used for monitoring the surfactant's concentration as a function of depth. The results show an increased concentration of surfactant at the surface, where the surface segregation initially increases with increasing bulk concentration and then remains constant above 5% (w/w) Pluronic. This surface segregated region is immediately followed by a depletion region with a homogeneous mixture in the bulk of the film. These results suggest the selection of the surfactant bulk concentration of the thin film matrices for drugs/proteins delivery should achieve a relatively homogeneous distribution of stabilizer/protein in the PLLA matrix. Analysis of three component blends of PLLA, Pluronic and insulin are also investigated. In the three component blends, ToF-SIMS imaging shows the spatial distribution of surfactant/protein mixtures. These data are reported also as depth profiles.     

Adsorption and film growth of N-methylamino substituted triazoles on copper surfaces in hydrocarbon media

The adsorption of benzotriazole (BTA), tolyltriazole (TTA) and two different N-methylaminosubstituted triazoles on copper surfaces in hydrocarbon media has been examined by in situ ellipsometry and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). All four triazoles were found to form films and from the ellipsometric study were the film thicknesses estimated to be in the range of 0.5-2 nm after 1000 min exposure time. The layers formed from BTA and TTA were thicker (up to 2 nm) than the layers from N-aminomethyl substituted triazoles (roughly 0.5 nm). Desorption was studied qualitatively and 20% or less of the adsorbed material were found to desorb. The ToF-SIMS study showed that while BTA and TTA adsorbed intact did the N-aminomethyl substituted triazoles appear to loose their aminomethyl tails on binding since only signals corresponding to triazole moieties of the compounds were detected.     

PLA-PMMA blends: A study by XPS and ToF-SIMS

This paper reports which are the possibilities of quantification by time of flight secondary ion mass spectrometry (ToF-SIMS) for some polymer blends. In order to assess the composition of the mixtures, we studied first different poly(l-lactide)/polymethylmethacrylate (PLA/PMMA) blends by X-ray photoelectron spectroscopy (XPS), this technique being quantitative. By XPS fitting of the C 1s level, we found a very good agreement of the measured concentrations with the initial compositions. Concerning ToF-SIMS data treatment, we used principal component analysis (PCA) on negative spectra allowing to discriminate one polymer from the other one. By partial least square regression (PLS), we found also a good agreement between the ToF-SIMS predicted and initial compositions. This shows that ToF-SIMS, in a similar way to XPS, can lead to quantitative results. In addition, the observed agreement between XPS (60-100 Å depth analyzed) and ToF-SIMS (10 Å depth analyzed) measurements show that there is no segregation of one of the two polymers onto the surface.     

Surface segregational behaviour studied as an effect of thickness by SIMS and AFM in annealed PS-PMMA blend and block copolymer thin films

The surface behaviour of a two-phase polymer mixture depends on the chemical structure of the polymer components, the interaction between the two polymers and the processing conditions. The microscopic morphology and the surface composition need to be known in order to fully utilize the thin film properties. The technique of static time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to obtain the molecular surface composition of thin films of blends and block copolymers. The depth profiling tool of Nano-SIMS, a dynamic SIMS technique, helps to provide the chemical mapping of the surface in 2D and 3D. The surface morphology is investigated using AFM. Thin films of PS and PMMA diblock copolymers with molecular weight of 12K-12K and 10K-10K and blends of PS/PMMA (10K/10K) for thicknesses ranging from 5 nm to 50 nm are examined. For the blends, the ToF-SIMS spectra for all the thicknesses show the same behaviour of a high increase of PMMA on the surface after annealing. Nano-SIMS images reveal the formation of nanostructures on the annealed surfaces and AFM studies show these nanostructures to be droplets having distinct phase shift from the surrounding matrix. The droplet dimensions increase with the increase of the thickness of the film but the absolute intensity from the ToF-SIMS spectra for all the annealed films remains almost the same. For the copolymers, the ToF-SIMS spectra show that there is a decrease of PMMA on the surface for the annealed films when compared to the as-cast ones. AFM morphology reveals that, for different thicknesses, annealing induces different topographical features like droplets, holes, spinodal patterns, etc. but with no distinct phase shift between the patterns and the surrounding matrix. The two different copolymers of comparable molecular weight are found to exhibit very different topography even when the thickness of the films remained the same. The surface composition from the ToF-SIMS data, however, was not found to vary even when the topography was completely different.     

Flame treatment of low-density polyethylene: Surface chemistry across the length scales

The relationship between surface chemistry and morphology of flame treated low-density polyethylene (LDPE) was studied by various characterization techniques across different length scales. The chemical composition of the surface was determined on the micrometer scale by X-ray photoelectron spectroscopy (XPS) as well as with time of flight secondary ion mass spectrometry (ToF-SIMS), while surface wettability was obtained through contact angle (CA) measurements on the millimeter scale. The surface concentration of hydroxyl, carbonyl and carboxyl groups, as a function of the “number” of the flame treatment passes (which is proportional to the treatment time) was obtained. Moreover, a correlation was found with chemical composition and polarity, emphasizing the role of oxygen-containing functional groups introduced during the treatment. Carboxyl functional groups were specifically identified by fluorescent labeling and the results were compared with the ToF-SIMS data. In addition, atomic force microscopy (AFM) was used to evaluate changes in surface topography and roughness on the nanometer to micrometer length scales. After flame treatment, water-soluble low molecular weight oxidized materials (LMWOM), which were generated as products of oxidation and chain scission of the LDPE surface, agglomerated into small topographical mounds that were visible in the AFM micrographs. After rinsing the flame treated samples with water and ethanol, bead-like nodular surface structures were observed. The ionization state of flame treated LDPE surfaces was monitored by chemical force microscopy (CFM). The effective surface pK_a values of carboxylic acid (-COOH) obtained by AFM were revealed by chemical force titration curves and the effective surface pK_a values were found to be around 6.