3D Molecular ToF-SIMS Imaging of Artificial Lipid Membranes Using a Discriminant Analysis-Based Algorithm

Artificial lipid membranes play a growing role in technical applications such as biosensors in pharmacological research and as model systems in the investigation of biological lipid films. In the standard procedure for displaying the distribution of membrane components, fluorescence microscopy, the fluorophores used can influence the distribution of the components and usually not all substances can be displayed at the same time. The discriminant analysis-based algorithm used in combination with scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) enables marker-free, quantitative, simultaneous recording of all membrane components. These data are used for reconstruction of distribution patterns. In the model system used for this survey, a tear fluid lipid layer, the distribution patterns of all lipids correlate well in calculated ToF-SIMS images and epi-fluorescence microscopic images. All epi-fluorescence microscopically viewable structures are visible when using both positive and negative secondary ions and can be reproduced with high lateral resolution in the submicrometer range despite the very low signal intensity and a very low signal-to-noise ratio. In addition, three-dimensional images can be obtained with a subnanometer depth resolution. Furthermore, structures and the distribution of substances that cannot be made visible by epi-fluorescence microscopy can be displayed. This enables new insights that cannot be gained by epi-fluorescence microscopy alone.

Surface Functionalization with Carboxylic Acids by Photochemical Microcontact Printing and Tetrazole Chemistry

In this paper, we show that carboxylic acid-functionalized molecules can be patterned by photochemical microcontact printing on tetrazole-terminated self-assembled monolayers. Upon irradiation, tetrazoles eliminate nitrogen to form highly reactive nitrile imines, which can be ligated with several different nucleophiles, carboxylic acids being the most reactive. As a proof of concept, we immobilized trifluoroacetic acid to monitor the reaction with X-ray photoelectron spectroscopy. Moreover, we also immobilized peptides and fabricated carbohydrate-lectin as well as biotin-streptavidin microarrays using this method. Surface-patterning was demonstrated by fluorescence microscopy and time-of-flight secondary ion mass spectrometry.

Solid state electrochemiluminescence from homogeneous and patterned monolayers of bifunctional spirobifluorene

Electrochemiluminescence (ECL) from self-assembled monolayers of a spirobifluorene dye covalently linked to a transparent ITO surface is reported.

Patterning of Nanoclays on Positively Charged Self-Assembled Monolayers via Micromolding in Capillaries

Nanoclays are nanomaterials with versatile adsorptive properties. This contribution describes the generation of micropatterns of a nanoclay ("laponite") on ammonium-terminated, self-assembled monolayers (SAMs) on glass and silicon. Microstructured immobilization of the laponite was performed using micromolding in capillaries (MIMIC). The immobilization was verified using contact angle goniometry, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and fluorescence microscopy. Furthermore, laponite was modified with Nile red to generate a fluorescence enhancement-based surface sensor for the vitamin choline.

On surface O-glycosylation by catalytic microcontact printing

Carbohydrate microarrays are made by microcontact printing of glycosyl donors on hydroxyl terminated substrates.

ToF-SIMS and Laser-SNMS Imaging of Heterogeneous Topographically Complex Polymer Systems

Heterogeneous polymer coatings, such as those used in organic electronics and medical devices, are of increasing industrial importance. In order to advance the development of these types of systems, analytical techniques are required which are able to determine the elemental and molecular spatial distributions, on a nanometer scale, with very high detection efficiency and sensitivity. The goal of this study was to investigate the suitability of laser postionization secondary neutral mass spectrometry (Laser-SNMS) with a 157 nm postionization laser beam to image structured polymer mixtures and compare the results with time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements using Bi₃⁺ primary ions. The results showed that Laser-SNMS is better suited than ToF-SIMS for unambiguous detection and submicrometer imaging of the wide range of polymers investigated. The data also showed that Laser-SNMS has the advantage of being much more sensitive (in general higher by more than an order of magnitude and peaking at up to 3 orders of magnitude) than ToF-SIMS while also showing superior performance on topographically complex structured insulating surfaces, due to significantly reduced field effects and a higher dynamic range as compared to ToF-SIMS. It is concluded that Laser-SNMS is a powerful complementary technique to ToF-SIMS for the analysis of heterogeneous polymers and other complex structured organic mixtures, providing submicrometer resolution and high sensitivity.

Photochemical Microcontact Printing by Tetrazole Chemistry

We developed a simple method to pattern self-assembled monolayers of tetrazole triethoxylsilane with a variety of different molecules by photochemical microcontact printing. Under irradiation, tetrazoles form highly reactive nitrile imines, which react with alkenes, alkynes, and thiols. The covalent linkage to the surface could be unambiguously demonstrated by fluorescence microscopy, because the reaction product is fluorescent in contrast to tetrazole. The modified surfaces were further analyzed by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), atomic force microscopy (AFM), and contact angle goniometry. Protein-repellent micropatterns, a biotin-streptavidin array, and structured polymer brushes could be fabricated with this straightforward method for surface functionalization.

3D ToF-SIMS Analysis of Peptide Incorporation into MALDI Matrix Crystals with Sub-micrometer Resolution

The analytical sensitivity in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is largely affected by the specific analyte-matrix interaction, in particular by the possible incorporation of the analytes into crystalline MALDI matrices. Here we used time-of-flight secondary ion mass spectrometry (ToF-SIMS) to visualize the incorporation of three peptides with different hydrophobicities, bradykinin, Substance P, and vasopressin, into two classic MALDI matrices, 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (HCCA). For depth profiling, an Ar cluster ion beam was used to gradually sputter through the matrix crystals without causing significant degradation of matrix or biomolecules. A pulsed Bi3 ion cluster beam was used to image the lateral analyte distribution in the center of the sputter crater. Using this dual beam technique, the 3D distribution of the analytes and spatial segregation effects within the matrix crystals were imaged with sub-μm resolution. The technique could in the future enable matrix-enhanced (ME)-ToF-SIMS imaging of peptides in tissue slices at ultra-high resolution. Graphical Abstract ᅟ.

Calculation of Membrane Lipid Ratios Using Single-Pixel Time-of-Flight Secondary Ion Mass Spectrometry Analysis

Much evidence suggests that membrane domains, termed lipid rafts, which are enriched in sphingomyeline and cholesterol play important roles in the regulation of physiological and pathophysiological processes. A label-free quantitative imaging method for lipids is lacking at present. We report an algorithm which enables us to identify and calculate the percentages of the ingredients of lipid mixtures from single-pixel time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra in model systems. The algorithm is based on a linear mixing model. Discriminant analysis is used to reduce the dimension of the data space. Calculations were separately performed for positive and negative ion mass spectra. Phosphatidylcholine and sphingomyeline which have identical headgroups and cannot be easily distinguished from another by positive ion mass spectra were included in the analysis. The algorithm outlined may more generally be used to calculate the percentages of ingredients of mixtures from spectra acquired by quite different methods such as X-ray photoelectron spectroscopy.

Characterization of freeze-fractured epithelial plasma membranes on nanometer scale with ToF-SIMS

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to characterize the freeze-fracturing process of human epithelial PANC-1 and UROtsa cells. For this purpose, phosphatidylcholine, sphingomyelin, phosphatidylethanolamine, and phosphatidylserine standard samples were investigated to find specific signals with both high specificity and signal intensity. The results were used to investigate single cells of subconfluent cell layers prepared with a special silicon wafer sandwich preparation technique. This freeze-fracturing technique strips cell membranes off the cells, isolating them on opposing silicon wafer substrates. Criteria were found for defining regions with stripped off cell membranes and, on the opposing wafer, complementary regions with the remaining cells. Measured ethanolamine/choline and serine/choline ratios in these regions clearly showed that in the freeze-fracturing process, the lipid bilayer of the plasma membrane is split along its central zone. Accordingly, only the outer lipid monolayer is stripped off the cell, while the inner lipid monolayer remains attached to the cell on the opposing wafer, thus allowing detailed analysis of a single lipid monolayer. Furthermore, it could be shown that using different washing procedures did not influence the transmembrane lipid distribution. Under optimized preparation conditions, it became feasible to detect lipids with a lateral resolution of approximately 100 nm. The data indicate that ToF-SIMS would be a very useful technique to study with very high lateral resolution changes in lipid composition caused, for example, by lipid storage diseases or pharmaceuticals that interfere with the lipid metabolism.