SIMS of organic anions adsorbed onto an aminoethanethiol self-assembled monolayer: an approach for enhanced secondary ion emission

Pattern-Selective Molecular Epitaxial Growth of Single-Crystalline Perovskite Arrays toward Ultrasensitive and Ultrafast Photodetector

The emergence of organic-inorganic perovskite has provided great flexibility for creating optoelectronic devices with unprecedented performance or unique functionality. However, the perovskite films explored so far have been difficult to be patterned to arrays owing to their poor solvent and moisture stability, which usually lead to serious structural damage of perovskites. The successful preparation of perovskite microarrays with uniform shape and size is more challenging. Here we report a straightforward approach to realize single-crystalline perovskite arrays through a relatively simple pattern-selective molecular epitaxial growth. This approach is applied to create diverse shaped perovskite arrays, such as hexagon, triangle, circle, square, and rectangle. A vertically aligned perovskite photodetector displays both an ultrasensitive and ultrafast photoresponse arising from the reduction in carrier diffusion paths and the high optical absorption. This work demonstrates a general approach to creating perovskite arrays with uniform shape, size, and morphology and provides a rich platform for producing high-performance photodetectors and photovoltage devices.

Methods for generating protein molecular ions in ToF-SIMS

One of the greatest challenges in mass spectrometry lies in the generation and detection of molecular ions that can be used to directly identify the protein from the molecular weight of the molecular ion. Typically, proteins are large (MW > 1000), nonvolatile, and/or thermally labile, but the vaporization process produced by many mass spectrometry techniques including time-of-flight secondary ion mass spectrometry (ToF-SIMS) is inherently limited to generating ions from smaller compounds or fragments of the parent molecule, making the identification of proteins complex. The application of specific molecules to aid in the generation of high molecular weight ions in ToF-SIMS has been recognized for some time. In this study we have developed a matrix-SAM substrate preparation technique based on the self-assembly of a matrix-like molecule, mercaptonicotinic acid (MNA), on gold. We then compare this substrate with two existing ToF-SIMS sample preparation techniques, cationized alkane thiol and matrix-enhanced SIMS (MESIMS). The results of this study illustrate that while there is a range of methods that can be used to improve the molecular ion yield of proteins in ToF-SIMS, their efficacy and reproducibility vary considerably and crucially are linked to the sample preparation and/or protein application methods used. Critically, the MNA modified substrate was able to simultaneously induce molecular ions for each protein present in a multicomponent solution, suggesting that this sample preparation technique may have future application in proteomics and DNA analysis.

Effect of water treatment on analyte and matrix ion yields in matrix-assisted time-of-flight secondary ion mass spectrometry: the case of insulin in and on hydroxycinnamic acid

A systematic study was performed to identify the origin of surprisingly high analyte-to-matrix yield ratios recently observed in time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis of oligo- and polypeptides mixed in matrices of alpha-cyano-4-hydroxycinnamic acid (4HCCA). Several sets of samples of porcine insulin in 4HCCA (1:3100 molar) were prepared from liquid solutions by a nebuliser technique, with more than one order of magnitude variation in sprayed material (substrate silicon). Following different periods of storage in air and/or vacuum as well as exposure to high-purity water, TOF-SIMS analysis was performed under oblique impact of 22 keV SF5+. Treatment with water involved either deposition of a droplet covering the whole sample for times between 1 and 20 min or spraying with water in droplet equivalent quantities. The analyte and matrix molecules were detected as protonated molecules (insulin also in doubly protonated form). Even the as-prepared samples usually showed insulin-to-4HCCA yield ratios exceeding the molar ratio of the mixed material. Upon ageing in vacuum the matrix ion yields remained constant but the analyte yields decreased, partly due to break-up of intrachain disulfide bonds. Water treatment resulted in a pronounced decrease in the 4HCCA yield, typically by a factor of five, in parallel with an increase of the insulin yield, by up to a factor of four. Evidence is provided that these changes occur concurrently with a partial dissolution of 4HCCA at the sample surface. The enhanced insulin yield was not correlated with the Na+ yield. The typically 20-fold increase in the insulin-to-4HCCA yield ratio, generated by water exposure of the samples, provides the explanation for the high yield ratios observed previously with water-treated samples. Spraying with water or repeated exposure to water droplets caused a pronounced degradation of the insulin parent yields in combination with an increasing appearance of signals due to the B- and A-chains of insulin. To clarify the issue of surface segregation, a few samples were prepared by spraying acetone-diluted solutions of insulin on previously deposited layers of 4HCCA. Whereas the insulin yields from as-prepared samples were rather low, the yields observed after water treatment were comparable with those observed with samples of insulin in 4HCCA. The results suggest that a large amount of insulin is present at the surface of samples prepared from liquid mixtures of insulin in 4HCCA. With both methods of sample preparation, however, high secondary ion yields of insulin were only obtained after exposure of the samples to water. The chemical changes responsible for this beneficial effect still need to be identified.

Characterization of photooxidized self-assembled monolayers and bilayers by spontaneous desorption mass spectrometry

We show that selected self-assembled monolayers (SAMs) and bilayers are readily characterized by the application of controlled photooxidation and spontaneous desorption mass spectrometry (SDMS) in the negative ion mode. Additionally, SDMS is used to characterize organic and inorganic anionic species adsorbed to the surface of a positively charged SAM surface, 2-aminoethanethiol (AET). Prominent peaks are observed that correspond both to the sulfonate form of each SAM and bilayer and to the anion form of each molecule adsorbed to AET. In addition, fragments of the oxidized thin films were also observed at m/z 80 (SO3-) and 97 (HSO4-). Other prominent fragment peaks more characteristic of the molecule are also seen in the mass spectra.