Steffen Weidner | BAM Federal Institute for Materials Research and Testing (original) (raw)

Papers by Steffen Weidner

RATIONALE: The ionization of polystyrenes in matrix-assisted laser desorption/ionization mass spe... more RATIONALE: The ionization of polystyrenes in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is typically achieved by the use of silver salts. Since silver salts can cause severe problems, such as cluster formation, fragmentation of polymer chains and end group cleavage, their substitution by alkali salts is highly desirable. METHODS: The influence of various cations (Ag + , Cs + and Rb +) on the MALDI process of polystyrene (PS) mixtures and high mass polystyrenes was examined. The sample preparation was kept as straightforward as possible. Consequently, no recrystallization or other cleaning procedures were applied. RESULTS: The investigation of a polystyrene mixture showed that higher molecular polystyrenes could be more easily ionized using caesium, rather than rubidium or silver salts. In combination with the use of DCTB as matrix a high-mass polymer analysis could be achieved, which was demonstrated by the detection of a 1.1 MDa PS. CONCLUSIONS: A fast, simple and robust MALDI sample preparation method for the analysis of ultra-high molecular weight polystyrenes based on the use of DCTB and caesium salts has been presented. The suitability of the presented method has been validated by using different mass spectrometers and detectors. The detection of ultra-high-mass polymers by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) is an ongoing challenge and active research field. Current investigations are trying to push forward and (i) understand the role of the cationizing agent, (ii) improve the mass analysis of polydisperse polymers, and (iii) minimize intra-spot mass discrimination effects. In the analysis of synthetic polymers, gas-phase cationization by the attachment of alkali metal and other cations is known to be the dominant ionization mechanism. Polystyrene (PS) is a comparatively non-polar hydrocarbon with phenyl rings as the most likely site of cation attachment. [1] Initially, the suitability of alkali salts for the ionization of polystyrenes was a controversial issue: Belu et al. [2] reported that molecule-related ions of polystyrene are not observed when alkali metal salts are added to a dithranol matrix, whereas Montaudo et al. found sodium adduct formation of PS oligomers with m/z values up to 12,000. [3] Deery et al. [4] investigated different alkali metal ions (Li + , Na + , K + , Rb + and Cs +) and other metal ions (Ag + , Cu 2+ , Zn 2+ , Co 2+ , Au 3+ , Al 3+ , Fe 3+ , Ni 2+) regarding their suitability for the ionization of a low molecular mass PS 1700 using a dithranol matrix. [4] The decreasing solubility of rubidium and caesium salts in tetrahydrofuran (THF), and the inevitable use of water, resulted in inhomogeneous sample layers, which was supposed to be the reason for the failure to obtain [PS–Rb] + and [PS–Cs] + adduct ions in the mass spectra. The most favourable interaction between cation and polymer, for MALDI experiments involving dithranol as the matrix, occurs when either silver or copper ions are the dopant cations. Similar results were obtained by other groups using silver, copper, and palladium salts, whereas other transition metal salts tended to fail to provide adduct ions. [5–7] Another study of PS using silver salts for cationization suggested that the silver cation may interact with two phenyl rings. [8] The role of the matrix molecules has been the subject of different investigations. In addition to dithranol, it was shown that all-trans-retinoic acid seems to be a very good matrix for PS when silver or copper salts are used as the cationizing agent. [9] According to Guo et al., the suggested mechanism of cationization is based on a gas-phase metal attachment by a naked, singly charged metal cation. [10] These ions can be formed effectively when the bonding of the ligands to metals in the cationizing agent used is weak and, therefore, allows the effective replacement of the ligands by the matrix molecules.

Rationale: Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been sugg... more Rationale: Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been suggested as a promising tool for the investigation of pollen, but the usefulness of this approach for classification and identification of pollen species has to be proven by an application to samples of varying taxonomic relations. Method: MALDI-MS in combination with hierarchical cluster analysis (HCA) and principal component analysis (PCA) was used to delineate taxonomic relations between plants based on pollen biochemistry. To assess the robustness of the approach, pollen of 74 species of the plant orders Fagales and Coniferales were probed. Results: Discrimination at the levels of plant order and genus were achieved using the whole spectral range. In many cases, different species of the same genus could be distinguished. The sources of the spectral/chemical differences at the genus level can be understood using PCA. Specifically, typical mass regions for exact genus detection were identified. Conclusions: Our results indicate that the chemical information represented by MALDI-TOFMS data is useful for reconstructing taxonomic relationships and is complementary to other chemical information on pollen from other spectroscopic data.

Coating of carbon fibers with adhesion-promoting thin poly(acrylic acid) and poly(hydroxyethylmet... more Coating of carbon fibers with adhesion-promoting thin poly(acrylic acid) and poly(hydroxyethylmethacrylate) layers using electrospray ionization, Thin coatings of poly(acrylic acid) (PAA) and poly(hydroxyethylmethacrylate) (PHEMA) were deposited onto carbon fibers by means of the electrospray ionization (ESI) technique in ambient air. These high-molecular weight polymer layers were used as adhesion promoters in carbon fiber-epoxy resin composites. Within the ESI process, the carbon fibers were completely enwrapped with polymer in the upper 10 plies of a carbon fiber roving. As identified with scanning electron microscopy also shadowed fibers in a bundle as well as backsides of fiber rovings were pinhole-free coated with polymers ('electrophoretic effect'). Under the conditions used, the layers have a granular structure. Residual solvent was absent in the deposit. PAA and PHEMA films did not show any changes in composition and structure in comparison with the original polymers as analyzed by X-ray photo-electron spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Single-fiber pullout tests of coated fibers embedded in epoxy resin showed significantly increased interfacial shear strength. It is assumed that chemical bonds between carbon fiber poly(acrylic acid) and epoxy resin contribute significantly to the improved interactions.

Trimesoyl chloride is polycondensed with various α,ώ-alkanediols in dichloromethane at different ... more Trimesoyl chloride is polycondensed with various α,ώ-alkanediols in dichloromethane at different concentrations using equifunctional feed ratios. As evidenced by MALDI-TOF (matrix assisted laser desorption/ionization-time of fl ight) mass spectrometry the soluble reaction products mainly consist of perfect multicyclic oligomers and polymers. The solphase extracted from the gels also consists of perfect multicycles. SEC (size exclusion chromatography) measurements show that both soluble reaction products and extracted solphases also contain a high molar mass fraction of perfect and nonperfect multicycles extending up to masses beyond 10 5 g mol −1 . When the polycondensation is stopped after a few minutes perfect multicycles are already detectable in the reaction mixture along with functional (multi)cyclic oligomers. These results prove that at initial monomer concentrations < 0.2 mol L −1 networks and large multicyclic polymers are synthesized from functional cyclic oligomers formed in early stages of the polycondensation and not from hyperbranched polymers. This interpretation is presented as "egg-fi rst theory" and compared with the "hen-fi rst theory" of Stockmayer and Flory.

$Research paper thumbnail of Sample loss in asymmetric flow field-flow fractionation coupled to inductively coupled plasma-mass spectrometry of silver nanoparticles$

In this work, sample losses of silver nanoparticles (Ag NPs) in asymmetrical flow field-flow frac... more In this work, sample losses of silver nanoparticles (Ag NPs) in asymmetrical flow field-flow fractionation (AF4) have been systematically investigated with the main focus on instrumental conditions like focusing and cross-flow parameters as well as sample concentration and buffer composition. Special attention was drawn to the AF4 membrane. For monitoring possible silver depositions on the membrane, imaging laser ablation coupled to inductively coupled plasma mass spectrometry (LA-ICP-MS) was used. Our results show that the sample residue on the membrane was below 0.6% of the total injected amount and therefore could be almost completely avoided at low sample concentrations and optimized conditions. By investigation of the AF4 flows using inductively coupled plasma mass spectrometry (ICP-MS), we found the recovery rate in the detector flow under optimized conditions to be nearly 90%, while the cross-flow, slot-outlet flow and purge flow showed negligible amounts of under 0.5%. The analysis of an aqueous ionic Ag standard solution resulted in recovery rates of over 6% and the ionic Ag content in the sample was found to be nearly 8%. Therefore, we were able to indicate the ionic Ag content as the most important source of sample loss in this study.

Poly(alkylene isophthalate)s were prepared by different methods, either in solution or in bulk. T... more Poly(alkylene isophthalate)s were prepared by different methods, either in solution or in bulk. The SEC measurements were evaluated in such a way that all oligomers were included. In solution (monomer conc. 0.1-0.7 mol/L) large fractions of rings were formed and high dispersities (up to 12) were obtained, which disagree with theoretical predictions. Polycondensations in bulk did neither generate cyclics by "back-biting" nor by end-to-end cyclization, when the maximum temperature was limited to 210 8C. The dispersities of these perfectly linear polyesters were again higher than the theoretical values. Regardless of the synthetic method mono-meric cycles were never observed. Furthermore, SEC measurements performed in tetrahydrofuran and in chloroform and SEC measurements performed in three different institutes were compared. Finally, SEC measurements of five samples were performed with universal calibration and a correction factor of 0.71 6 0.02 was found for normal calibration with polystyrene.

The preparation and characterization of poly(ionic liquid)s (PILs) bearing a polystyrene backbone... more The preparation and characterization of poly(ionic liquid)s (PILs) bearing a polystyrene backbone via reversible addition fragmentation chain transfer (RAFT) polymerization and their photolithographic patterning on silicon wafers is reported. The controlled radical polymerization of the styrenic ionic liquid (IL) monomers ([BVBIM]X, X = Cl − or Tf 2 N − ) by RAFT polymerization is investigated in detail. We provide a general synthetic tool to access this class of PILs with controlled molecular weight and relatively narrow molecular weight distribution (2000 g mol −1 ≤ M n ≤ 10 000 g mol −1 with dispersities between 1.4 and 1.3 for p([BVBIM]Cl); 2100 g mol −1 ≤ M P ≤ 14 000 g mol −1 for p([BVBIM]Tf 2 N)). More importantly, we provide an in-depth characterization of the PILs and demonstrate a detailed mass spectrometric analysis via matrix-assisted laser desorption ionization (MALDI) as well asfor the first time for PILselectrospray ionization mass spectrometry (ESI-MS). Importantly, p([BVBIM]Cl) and p([DMVBIM]Tf 2 N) were photochemically patterned on silicon wafers. Therefore, a RAFT agent carrying a photoactive group based on ortho-quinodimethane chemistrymore precisely photoenol chemistrywas photochemically linked for subsequent controlled radical polymerization of [BVBIM]Cl and [DMVBIM]Tf 2 N. The successful spatially-resolved photografting is evidenced by surface-sensitive characterization methods such as X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The presented method allows for the functionalization of diverse surfaces with poly(ionic liquid)s. Scheme 1 Synthetic approach for the preparation of p([BVBIM]Cl) (3) and p([BVBIM]Tf 2 N) (4) from the monomers [BVBIM]Cl (1) and [BVBIM]Tf 2 N (2), respectively. Molar ratio [M] : [CTA] : [I] for (i) 200 : 1 : 0.2 and for (iii) 200 : 1 : 0.2 (i: RAFT polymerization; ii: Tf 2 N − vs. Cl − anion exchange; iii: RAFT polymerization; iv: anion exchange).

A reversed phase high performance liquid chromatography coupled to an inductively coupled plasma ... more A reversed phase high performance liquid chromatography coupled to an inductively coupled plasma mass spectrometer (HPLC-ICP-MS) approach in combination with isotope dilution analysis (IDA) for the separation and parallel quantification of nanostructured and ionic silver (Ag) is presented. The main focus of this work was the determination of the ionic Ag concentration. For a sufficient stabilization of the ions without dissolving the nanoparticles (NPs), the eluent had to be initially optimized. The determined Ag ion concentration was in a good agreement with results obtained using ultrafiltration. Further, the mechanism of the NP separation in the HPLC column was investigated. Typical size exclusion effects were found by comparing results from columns with different pore sizes. Since the recovery rates decreased with increasing Ag NP size and large Ag NPs did not elute from the column, additional interactions of the particles with the stationary phase were assumed. Our results reveal that the presented method is not only applicable to Ag NPs, but also to gold and polystyrene NPs. Finally, IDA-HPLC-ICP-MS experiments in single particle mode were performed to determine the particle cut-off size. The comparison with conventional spICP-MS experiments resulted in a similar diameter and particle size distribution.

Terpene alcohols (menthol, retinol, cholesterol, and betulin) together with the phosphazene base ... more Terpene alcohols (menthol, retinol, cholesterol, and betulin) together with the phosphazene base t-BuP 4 were used as initiating systems for anionic ring-opening polymerization of ethylene oxide. The polymerizations were conducted in a controlled manner with the initial molar ratio of t-BuP 4 to hydroxyl groups of 0.01-0.2, yielding a series of biohybrid polymers comprising terpene entities and poly(ethylene oxide) (PEO) chains with low polydispersities and tunable compositions (57-87 wt% of PEO). Samples were characterized by NMR and UV/visible spectroscopy, MALDI-TOF mass spectrometry, and size exclusion chromatography; thermal properties were studied by differential scanning calorimetry. The concept of this study opens a new toolbox of terpene-based biohybrid polymers with variable properties and functions.

are polymerized either at 120 °C where the polymerization process of l-lactide is accompanied by ... more are polymerized either at 120 °C where the polymerization process of l-lactide is accompanied by crystallization, or at 180 °C where poly(l-lactide) remains in the molten state. Polymerizations at 120 °C initially yield even-numbered chains (with respect to lactic acid units) having relatively low dispersity, but the fraction of odd-numbered chains increases with time and the entire molecular weight distribution changes. Traces of cyclics are only formed after 7 d. Polymerizations at 180 °C yield equilibrium of even and odd-numbered chains from the beginning, but at low monomer/initiator ratios and short reaction times (<4 h) cyclics are again not formed. They appear at longer reaction times and entail higher dispersities. The results are discussed in terms of five different transesterification mechanisms.

. A) TEM micrographs of pristine MCNP, B) P(2VP 58 -grad-AA 30 )@MCNP at pH 6.72, and C) P(2VP 58... more . A) TEM micrographs of pristine MCNP, B) P(2VP 58 -grad-AA 30 )@MCNP at pH 6.72, and C) P(2VP 58,S -grad-AA 30 )@MCNP at pH 6.72; D) A higher magnification where the polymeric shell can also be distinguished, highlighted by the white arrow.

Designing artificial macromolecules with absolute sequence order represents a considerable challe... more Designing artificial macromolecules with absolute sequence order represents a considerable challenge. Here we report an advanced light-induced avenue to monodisperse sequencedefined functional linear macromolecules up to decamers via a unique photochemical approach. The versatility of the synthetic strategy-combining sequential and modular concepts-enables the synthesis of perfect macromolecules varying in chemical constitution and topology. Specific functions are placed at arbitrary positions along the chain via the successive addition of monomer units and blocks, leading to a library of functional homopolymers, alternating copolymers and block copolymers. The in-depth characterization of each sequence-defined chain confirms the precision nature of the macromolecules. Decoding of the functional information contained in the molecular structure is achieved via tandem mass spectrometry without recourse to their synthetic history, showing that the sequence information can be read. We submit that the presented photochemical strategy is a viable and advanced concept for coding individual monomer units along a macromolecular chain. | www.nature.com/naturecommunications 1 (hexyl chain), 2 (lysine-methyl ester function), 3 (poly(ethylene glycol (PEG) chain), 4 (lysine-based monomer with alcohol function), 5 (lysine-based monomer with adamanthyl function), 6 (lysine-based monomer with fluorobenzyl function). (b) Sequential synthesis of symmetrical dimers 7a and 10a, tetramers 7b (homopolymer), 9a and 10b (alternating polymers), and hexamers 7c (homopolymer) and 8a, 9b and 10c (copolymers). (c) Synthesis of dimer blocks 7d, 9c and 10d with locked/unlocked function for modular ligation. (d) Synthesis of decamers as homopolymers 7 and copolymers 8-10 varying in composition and topology. (i) dry DCM, l ¼ 365 nm, 45 min; (ii) 115°C, vacuum, overnight, dark; (iii) p-TosOH, TMOF, 40°C, overnight, dry MeOH, dark. Overall yield for monomer 1 (23.1%), 2 (20.8%), 3 (48.2%), 4 (32.4%), 5 (43.7%) and 6 (42.5%).

The performance of a miniaturized channel for the separation of polymer and metal nanoparticles (... more The performance of a miniaturized channel for the separation of polymer and metal nanoparticles (NP) using Asymmetrical Flow Field-Flow Fractionation (AF4) was investigated and compared with a conventional AF4 system. To develop standard separation methods, experimental parameters like cross flow, gradient profile and injection time were varied and optimized. Corresponding chromatographic parameters were calculated and compared. Our results indicate that the chromatographic resolution in the miniaturized channel is lower, whereas significantly shorter analyses time and less solvent consumption were obtained. Moreover, the limit of detection (LOD) and limit of quantification (LOQ) obtained from hyphenation with a UV-detector are obviously lower than in a conventional channel, which makes the miniaturized channel interesting for trace analysis.

Molecular masses and end groups of polystyrene (PS) formed in a novel container-less polymerizati... more Molecular masses and end groups of polystyrene (PS) formed in a novel container-less polymerization strategy, based on levitated droplets in an acoustic trap, were determined by Gel Permeation Chromatography (GPC) and Matrix-assisted Laser Desorption/Ionization Time of Flight Mass spectrometry (MALDI-TOF MS).

L-Lactides were polymerized in bulk at 120 or 160°C with cyclic dibutyltin catalysts derived from... more L-Lactides were polymerized in bulk at 120 or 160°C with cyclic dibutyltin catalysts derived from 1,2dimercaptoethane or 2-mercaptoethanol. Only linear chains having one benzyl ester and one OH-end group were obtained when benzyl alcohol was added. When L-lactides were polymerized with neat dibutyl-2-stanna-1,3-dithiolane, exclusively cyclic polylactides were formed even at 120°C. The temperature, time and monomer/catalyst ratio (M/C) were varied. These results are best explained by a combination of ring-expansion polymerization and ring-extrusion of cyclic oligo-or polylactides with elimination of the cyclic catalyst. Neither syntheses of linear polylactides nor of cyclic lactides involved racemization up to 20 h at 160°C.

RATIONALE: Polymer sample spots for matrix-assisted laser desorption/ionization time-of-flight ma... more RATIONALE: Polymer sample spots for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) prepared by the dried-droplet method often reveal ring formation accompanied by possible segregation of matrix and sample molecules as well as of the polymer homologs itself. Since the majority of sample spots are prepared by this simple and fast method, a matrix or sample preparation method that excludes such segregation has to be found. METHODS: Three different ionic liquid matrices based on conventionally used aromatic compounds for MALDI-TOF MS were prepared. The formation of ionic liquids was proven by 1 H NMR spectroscopy. MALDI-Imaging mass spectrometry was applied to monitor the homogeneity. RESULTS: Our results show a superior sample spot homogeneity using ionic liquid matrices. Spots could be sampled several times without visible differences in the mass spectra. A frequently observed loss of matrix in the mass spectrometer vacuum was not observed. The necessary laser irradiance was reduced, which resulted in less polymer fragmentation. CONCLUSIONS: Ionic liquid matrices can be used to overcome segregation, a typical drawback of conventional MALDI dried-droplet preparations. Homogeneous sample spots are easy to prepare, stable in the MS vacuum and, thereby, improve the reproducibility of MALDI.

Polysarcosine (M n = 3650-20 000 g mol −1 , Đ ∼ 1.1) was synthesized from the air and moisture st... more Polysarcosine (M n = 3650-20 000 g mol −1 , Đ ∼ 1.1) was synthesized from the air and moisture stable N-phenoxycarbonyl-Nmethylglycine. Polymerization was achieved by in situ transformation of the urethane precursor into the corresponding N-methylglycine-N-carboxyanhydride, when in the presence of a nonnucleophilic tertiary amine base and a primary amine initiator.

Isosorbide, succinyl chloride and isophthaloyl chloride are polycondensed under various reaction ... more Isosorbide, succinyl chloride and isophthaloyl chloride are polycondensed under various reaction conditions. The heating in bulk with or without catalysts as well in an aromatic solvent without catalyst, and polycondensation with the addition of pyridine only yield low molar mass copolyesters. However, heating in chlorobenzene with addition of SnCl 2 or ZnCl 2 produces satisfactory molar masses. The number average molecular weights (M n ) of most copolyesters fall into the range of 7000-15,000 Da with polydispersities (PD) in the range of 3-9. The MALDI-TOF mass spectra almost exclusively displayed peaks of cyclics indicating that the chain growth was mainly limited by cyclization and not by side reactions, stoichiometric imbalance or incomplete conversion. The glass-transition temperatures increased with the content of isophthalic acid from 75 to 180 C and the thermo-stabilities also followed this trend.

Orally ingested nanoparticles may overcome the gastrointestinal barrier, reach the circulatory sy... more Orally ingested nanoparticles may overcome the gastrointestinal barrier, reach the circulatory system, be distributed in the organism and cause adverse health effects. However, ingested nanoparticles have to pass through different physicochemical environments, which may alter their properties before they reach the intestinal cells. In this study, silver nanoparticles are characterised physicochemically during the course of artificial digestion to simulate the biochemical processes occurring during digestion. Their cytotoxicity on intestinal cells was investigated using the Caco-2 cell model. Using field-flow fractionation combined with dynamic light scattering and small-angle X-ray scattering, the authors found that particles only partially aggregate as a result of the digestive process. Cell viabilities were determined by means of CellTiter-Blue Ò assay, 4¢,6-diamidino-2-phenylindole-staining and real-time impedance. These measurements reveal small differences between digested and undigested particles (1-100 mg/ml or 1-69 particles/cell). The findings suggest that silver nanoparticles may indeed overcome the gastrointestinal juices in their particulate form without forming large quantities of aggregates. Consequently, the authors presume that the particles can reach the intestinal epithelial cells after ingestion with only a slight reduction in their cytotoxic potential. The study indicates that it is important to determine the impact of body fluids on the nanoparticles of interest to provide a reliable interpretation of their nano-specific cytotoxicity testing in vivo and in vitro.

$Research paper thumbnail of Sample loss in asymmetric flow field-flow fractionation coupled to inductively coupled plasma-mass spectrometry of silver nanoparticles$