On the mechanism of laser-induced desorption–ionization of organic compounds from etched silicon and carbon surfaces

Alimpiev, S. S.; Никифоров, С. М.; Karavanskiĭ, V. A.; Minton, Tim; Sunner, Jan

doi:10.1063/1.1381531

Cited by 131 publications

(178 citation statements)

References 29 publications

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“…In MALDI, the [M ϩ 1]/M peak ratio was shown to vary from 34% to 123%, depending on matrix-to-analyte ratios [17]. Such fragment intensities may also depend on the micro-structure of the fractured surface [18] and the primary ion properties [19,20]. The cationic dye fragment possesses a nonlocalized charge due to resonance forms of methylene blue with a positively charged S and a positively charged quaternary ammonium group.…”

Section: Resultsmentioning

confidence: 99%

Use of TOF-SIMS to study adsorption and loading behavior of methylene blue and papain in a nano-porous silicon layer

Kempson

Barnes

Prestidge

2010

J. Am. Soc. Mass Spectrom.

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TOF-SIMS was applied to study the cross-sectional distribution of methylene blue and papain in porous silicon layers. Elemental and molecular information were used to study their distributions in the porous region and the chemistry of their adsorption. Methylene blue (MW ϭ 284 Da) [4], and the delivery of active pharmaceutical ingredients [5,6]. The ongoing interest in pSi for use as a therapeutic delivery system is due to the large internal surface area available, the readily modified surface chemistry [7] and, in particular, the excellent in vivo biodegradation and biocompatibility (low toxicity) of the silicon substrate, hydrolysing to form orthosilicic acid, which is readily excreted [8]. The loading dynamics of molecules into a pSi matrix is dependent on wetting and the size of the pores. Likewise, the interaction between a loaded molecule and the pSi substrate is of interest, particularly in the case of protein based therapeutics, whose bioactivity is dependent on remaining in a specific 3D conformation.TOF-SIMS analysis provides information regarding elemental and molecular species present in a sample surface (i.e., within 1-1.5 nm) with high mass resolution (up to ϳ10,000 m/⌬m) and spatial resolution for imaging (ϳ1 micron). Molecular and elemental information is obtained for organic and inorganic materials providing a powerful technique to image organic compounds associated with inorganic substrates or vice versa [9,10]. This has been demonstrated by imaging solid-state pharmaceutical formulations [11][12][13][14], however, to date this has not been extended to porous silicon substrates.In this study, TOF-SIMS was used to map the distribution of methylene blue and papain across porous silicon layers after loading from aqueous solution. Methylene blue (C 16 H 18 N 3 S ϩ , MW ϭ 284.12 g/mol) is a relatively small organic dye molecule commonly used in adsorption studies and can be considered analogous to therapeutic molecules. A molecular mass of 284 g/mol is within a reasonable mass range for a TOF-SIMS' Ga ϩ primary ion beam and can be easily and uniquely identified without interference or confusion from spectral peaks originating from the silicon substrate or in the instance of organic contamination. This molecule was chosen for its convenience, and to assess the ability to use smaller organic fragments to reflect the distribution of the molecule of interest. This is important to be able to extend this work to studying larger molecules, such as proteins and enzymes of pharmaceutical interest, which will not provide a molecular ion in TOF-SIMS. In contrast to methylene blue, papain is a much larger molecule, detailed elsewhere [15], comprising 212 residue units in two domains, giving a molecular weight of 23,406 g/mol. Papain is a cysteine protease hydrolase enzyme present in papaya. This molecular weight is far greater than can be detected with TOF-SIMS, however, fragmentation products can be detected. Smaller organic fragments can be compared with the distribution of methylene blue to investigate si...

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Section: Resultsmentioning

confidence: 99%

Use of TOF-SIMS to study adsorption and loading behavior of methylene blue and papain in a nano-porous silicon layer

Kempson

Barnes

Prestidge

2010

J. Am. Soc. Mass Spectrom.

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“…The previous findings on the aqueous-phase basicity of an analyte contributing more to ionization than the gasphase basicity of the same analyte suggested that proton transferring probably occurred in the condensed phase [6]. Consequently, this investigation focused on protonation between the analyte and its surrounding molecules in the condensed phase, including proton exchange with (1) the solvent where the analytes were dissolved in, (2) the solid surface where they were deposited on, and (3) other chemical species coadsorbed on the surface.…”

Section: Resultsmentioning

confidence: 86%

Quantitative study of solvent and surface effects on analyte ionization in desorption ionization on silicon (DIOS) mass spectrometry

Liu

2008

J. Am. Soc. Mass Spectrom.

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Deuterated solvents and DIOS surfaces derivatized with different functional groups are used to investigate impacts of local chemical environment on analyte ionization. Both solvent molecules and surface functional groups are found to directly participate in analyte protonation in the condensed phase. The corresponding protonation effectiveness is quantitatively estimated based on the relative MS peak intensities of [M ϩ 2] ϩ /[M ϩ 1] ϩ . A direct correlation between ionization of triethylamine and the relative acidities of the surface and the solvent is evident. In addition, the proton donating effectiveness of a solvent is found to be related to its vapor pressure. Improved MS detection of small molecules via proper surface treatment and solvent selection is demonstrated. (J Am Soc Mass Spectrom 2008, 19, 8 -13) © 2008 American Society for Mass Spectrometry E xtensive research in metabolite profiling in recent years has revived interest in desorption ionization on porous silicon mass spectrometry (DIOS-MS). Elimination of matrix molecules in DIOS-MS reduces background noise in the low-mass range and enables direct detection of biologically significant small molecules [1][2][3][4]. It also overcomes matrix-induced analyte redistribution on a sample surface, which allows two-dimensional imaging of tissue samples with good spatial accuracy [5]. Recent studies have shown that the local chemical properties of a DIOS substrate, including the residual solvent on the surface and the surface functional groups, are critical in analyte ionization [6 -9]. Quantitative investigation of these factors, however, is lacking. In this report, we semiquantitatively assessed contributions of various proton sources in DIOS-MS and the feasibility of improving MS detection of lipids and peptides by proper sample treatment. Experimental DIOS Substrate PreparationDIOS substrates were prepared by dipping the wafer into a solution of 5% HF/EtOH for 1 min to remove the oxidized layer, followed by a 1.5-min anodic etching in 25% HF/EtOH at 5 mA/cm 2 [10,11]. Before MS experiments, the DIOS substrates were dipped in 5% HF/ EtOH to regenerate the H-terminated surface. For OHterminated surfaces, the DIOS substrates were dipped in a solution of 15% H 2 O 2 /MeOH (or 15% D 2 O 2 / MeOH-d 4 ) for 30 min. For RNH 2 -terminated surfaces, the same OH-terminated surfaces were refluxed in a solution of 10% APTMS/toluene under N 2 for 4 h. To remove residual solvent molecules, the DIOS or chemically modified DIOS substrates were dried under high vacuum overnight, unless specified otherwise.

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“…Sunner et al suggested that the ionization mechanism of DIOS is similar to surface-assisted laser desorption/ionization (SALDI), and the formation of gas-phase ions was initiated on the surface of sharp crystal tips and edges that protrude out of the sample surface. 43,44 On the other hand, many researchers suggested that the secondary ion-molecule reactions in the plume are essential to understanding the MALDI ionization process. 7,9,11 The ionization mechanism of DIOS may differ much from that of MALDI.…”

Section: Resultsmentioning

confidence: 99%

Oxidation of Ferrocene Derivatives in Desorption/Ionization on Porous Silicon

2005

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Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is currently an indispensable method for analyzing biomolecules and synthetic polymers. [1][2][3][4][5][6] While the mechanism of ionization is largely unknown, physical data on the ionization potentials and the gas-phase acidity or basicity have been accumulating, and the roles of the matrix-matrix, matrix-analyte and analyte-analyte reactions in both primary and secondary ionization events have been evaluated in some detail. [7][8][9][10][11][12][13][14][15][16] These studies have suggested the secondary ionmolecule reactions in the plume to be essential to understanding the process of ionization and the mass spectrum. 7,9,11 Some analytes are reduced by these reactions as follows: Copper(II) ions are reduced by laser desorption/ionization (LDI) with or without a chemical matrix. 16Flavin-containing compounds, such as riboflavin, riboflavin 5′-phosphate and flavin-adenine dinucleotide, are reduced in MALDI and fast atom bombardment (FAB) ionization, but not in electrospray ionization, as reported by Itoh et al., who also found that the reduction levels depend on the matrix molecule types. 17 In the case of nitrotyrosine, the NO2 moiety is partially converted into NH2, probably due to reduction after photo-induced dissociation by UV laser irradiation. 18,19 Similarly, the S-nitrosylated cysteine, Cys(SNO), in peptides is completely converted to cysteine, Cys(SH), in MALDI. 20,21 Different mechanisms, such as electron capture and charge exchange with the chemical matrix, have been proposed to account for the reductions in MALDI. 8,9 A line of studies has indicated that free electrons are formed by photoelectric emission from the metal/dielectric-substance interface. [22][23][24][25] The metal sample target is not the photoelectron source, because the work function of metals is greater than the photon energy. 23 The presence of matrix molecules or analytes placed on a metal target enhances electron emission via band bending and the associated reduction in work function. 22,23 Thus, electron transfer from the sample target would be essential for understanding the analyte reduction in LDI.In desorption/ionization on porous silicon, termed DIOS, 26 analytes are deposited on a silicon chip, which is prepared by electrochemical anodization or chemical etching of crystalline silicon, for LDI without the addition of a chemical matrix. The DIOS mass spectra are thus free of cluster ions of the matrix, facilitating identification of the ions of small analyte molecules. DIOS-MS has been applied to various kinds of compounds, such as peptides, natural products, small organic molecules and synthetic polymers. [26][27][28][29][30][31][32][33][34][35][36] The ionization efficiency of DIOS relies on a porous scaffold providing a sufficient surface area to retain analytes, and on the UV-absorptive property, which affords the transfer of laser energy to analytes. 26,27 In contrast to MALDI, chemical reactions in a plume composed of analytes and the sample ma...

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On the mechanism of laser-induced desorption–ionization of organic compounds from etched silicon and carbon surfaces

Cited by 131 publications

References 29 publications

Use of TOF-SIMS to study adsorption and loading behavior of methylene blue and papain in a nano-porous silicon layer

Use of TOF-SIMS to study adsorption and loading behavior of methylene blue and papain in a nano-porous silicon layer

Quantitative study of solvent and surface effects on analyte ionization in desorption ionization on silicon (DIOS) mass spectrometry

Oxidation of Ferrocene Derivatives in Desorption/Ionization on Porous Silicon

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