Single Molecule Nanospectroscopy Visualizes Proton-Transfer Processes within a Zeolite Crystal

Visualizing proton-transfer processes at the nanoscale is essential for understanding the reactivity of zeolite-based catalyst materials. In this work, the Brønsted-acid-catalyzed oligomerization of styrene derivatives was used for the first time as a single molecule probe reaction to study the reactivity of individual zeolite H-ZSM-5 crystals in different zeolite framework, reactant and solvent environments. This was accomplished via the formation of distinct dimeric and trimeric fluorescent carbocations, characterized by their different photostability, as detected by single molecule fluorescence microscopy. The oligomerization kinetics turned out to be very sensitive to the reaction conditions and the presence of the local structural defects in zeolite H-ZSM-5 crystals. The remarkably photostable trimeric carbocations were found to be formed predominantly near defect-rich crystalline regions. This spectroscopic marker offers clear prospects for nanoscale quality control of zeolite-based materials. Interestingly, replacing <i>n</i>-heptane with 1-butanol as a solvent led to a reactivity decrease of several orders and shorter survival times of fluorescent products due to the strong chemisorption of 1-butanol onto the Brønsted acid sites. A similar effect was achieved by changing the electrophilic character of the <i>para</i>-substituent of the styrene moiety. Based on the measured turnover rates we have established a quantitative, single turnover approach to evaluate substituent and solvent effects on the reactivity of individual zeolite H-ZSM-5 crystals

Single Molecule Nanospectroscopy Visualizes Proton-Transfer Processes within a Zeolite Crystal

Visualizing proton-transfer processes at the nanoscale is essential for understanding the reactivity of zeolite-based catalyst materials. In this work, the Brønsted-acid-catalyzed oligomerization of styrene derivatives was used for the first time as a single molecule probe reaction to study the reactivity of individual zeolite H-ZSM-5 crystals in different zeolite framework, reactant and solvent environments. This was accomplished via the formation of distinct dimeric and trimeric fluorescent carbocations, characterized by their different photostability, as detected by single molecule fluorescence microscopy. The oligomerization kinetics turned out to be very sensitive to the reaction conditions and the presence of the local structural defects in zeolite H-ZSM-5 crystals. The remarkably photostable trimeric carbocations were found to be formed predominantly near defect-rich crystalline regions. This spectroscopic marker offers clear prospects for nanoscale quality control of zeolite-based materials. Interestingly, replacing <i>n</i>-heptane with 1-butanol as a solvent led to a reactivity decrease of several orders and shorter survival times of fluorescent products due to the strong chemisorption of 1-butanol onto the Brønsted acid sites. A similar effect was achieved by changing the electrophilic character of the <i>para</i>-substituent of the styrene moiety. Based on the measured turnover rates we have established a quantitative, single turnover approach to evaluate substituent and solvent effects on the reactivity of individual zeolite H-ZSM-5 crystals

Single Molecule Nanospectroscopy Visualizes Proton-Transfer Processes within a Zeolite Crystal

Visualizing proton-transfer processes at the nanoscale is essential for understanding the reactivity of zeolite-based catalyst materials. In this work, the Brønsted-acid-catalyzed oligomerization of styrene derivatives was used for the first time as a single molecule probe reaction to study the reactivity of individual zeolite H-ZSM-5 crystals in different zeolite framework, reactant and solvent environments. This was accomplished via the formation of distinct dimeric and trimeric fluorescent carbocations, characterized by their different photostability, as detected by single molecule fluorescence microscopy. The oligomerization kinetics turned out to be very sensitive to the reaction conditions and the presence of the local structural defects in zeolite H-ZSM-5 crystals. The remarkably photostable trimeric carbocations were found to be formed predominantly near defect-rich crystalline regions. This spectroscopic marker offers clear prospects for nanoscale quality control of zeolite-based materials. Interestingly, replacing <i>n</i>-heptane with 1-butanol as a solvent led to a reactivity decrease of several orders and shorter survival times of fluorescent products due to the strong chemisorption of 1-butanol onto the Brønsted acid sites. A similar effect was achieved by changing the electrophilic character of the <i>para</i>-substituent of the styrene moiety. Based on the measured turnover rates we have established a quantitative, single turnover approach to evaluate substituent and solvent effects on the reactivity of individual zeolite H-ZSM-5 crystals

Iron(III)-Based Metal–Organic Frameworks As Visible Light Photocatalysts

Herein, a new group of visible light photocatalysts is described. Iron­(III) oxides could be promising visible light photocatalysts because of their small band gap enabling visible light excitation. However, the high electron–hole recombination rate limits the yield of highly oxidizing species. This can be overcome by reducing the particle dimensions. In this study, metal–organic frameworks (MOFs), containing Fe₃-μ₃-oxo clusters, are proposed as visible light photocatalysts. Their photocatalytic performance is tested and proven via the degradation of Rhodamine 6G in aqueous solution. For the first time, the remarkable photocatalytic efficiency of such Fe­(III)-based MOFs under visible light illumination (350 up to 850 nm) is shown

Improving preservation state assessment of carbonate microfossils in paleontological research using label-free stimulated Raman imaging

<div><p>In micropaleontological and paleoclimatological studies based on microfossil morphology and geochemistry, assessing the preservation state of fossils is of the highest importance, as diagenetic alteration invalidates textural features and compromises the correct interpretation of stable isotope and trace elemental analysis. In this paper, we present a novel non-invasive and label-free tomographic approach to reconstruct the three-dimensional architecture of microfossils with submicron resolution based on stimulated Raman scattering (SRS). Furthermore, this technique allows deciphering the three-dimensional (3D) distribution of the minerals within these fossils in a chemically sensitive manner. Our method, therefore, allows to identify microfossils, to chemically map their internal structure and eventually to determine their preservation state. We demonstrate the effectiveness of this method by analyzing several benthic and planktonic foraminifera, obtaining full 3D distributions of carbonate, iron oxide and porosity for each specimen. Subsequently, the preservation state of each microfossil can be assessed using these 3D compositional maps. The technique is highly sensitive, non-destructive, time-efficient and avoids the need for sample pretreatment. Therefore, its predestined application is the final check of the state of microfossils before applying subsequent geochemical analyses.</p></div

Details of the walls of samples imaged with SRS (maximum projection, visualizing shell and also internal structures) (1) and SEM (2).

<p>(A) <i>Nuttallides truempyi</i> (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199695#pone.0199695.g004" target="_blank">Fig 4</a> row 5) with growth rings visible in A1 indicated by yellow arrows. (B) Detail of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199695#pone.0199695.g004" target="_blank">Fig 4</a> row 5 with spines close to aperture (C) <i>Anomalinoides midwayensis</i> with some iron oxide coating on surface. (D) Not well preserved <i>Nuttallides truempyi</i> (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199695#pone.0199695.g004" target="_blank">Fig 4</a> row 2). (E) <i>Heterohelix</i> test with oriented pustules (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199695#pone.0199695.g004" target="_blank">Fig 4</a> row 1). (F) <i>Nuttallides truempyi</i> from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199695#pone.0199695.g004" target="_blank">Fig 4</a> row 4 showing obstructed pores and high degree of calcite growth. (G) <i>Nuttallides truempyi</i> (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199695#pone.0199695.g004" target="_blank">Fig 4</a> row 3) with crystal overgrowth on inner wall test (G1) while the outer part (G2) is nicely preserved. (H) Dissolution marks along microfracture on outer shell part.</p

Illustration of advantages offered by SRS micro-imaging for preservation state assessment and volumetric calculations over other typically applied techniques.

<p>(A) Stereo microscopy image, (B) transmitted optical microscopy and (C) SEM image of <i>Anomalinoides midwayensis</i>. (D) Compositional image taken 35 μm deep inside the microfossil; turquoise color represents Mg calcite at 1092 cm^-1 and red displays the distribution of iron oxides and pyrite at the same depth by probing absorptive species ‘off resonance’. (E) Segmented image calculated from (D) (white is minerals, black is porosity) showing the porosity distribution 35 μm below the shell surface. (F) Maximum projection of the whole image stack containing a number of SRS images spaced each 0.5 um in depth. (See also Supporting Information for a movie showing the full 3D reconstruction).</p

Direct Laser Writing of δ- to α‑Phase Transformation in Formamidinium Lead Iodide

Organolead halide perovskites are increasingly considered for applications well beyond photovoltaics, for example, as the active regions within photonic devices. Herein, we report the direct laser writing (DLW: 458 nm cw-laser) of the formamidinium lead iodide (FAPbI₃) yellow δ-phase into its high-temperature luminescent black α-phase, a remarkably easy and scalable approach that takes advantage of the material’s susceptibility to transition under ambient conditions. Through the DLW of α-FAPbI₃ tracks on δ-FAPbI₃ single-crystal surfaces, the controlled and rapid microfabrication of highly luminescent structures exhibiting long-term phase stability is detailed, offering an avenue toward the prototyping of complex perovskite-based optical devices. The dynamics and kinetics of laser-induced δ- to α-phase transformations are investigated <i>in situ</i> by Raman microprobe analysis, as a function of irradiation power, time, temperature, and atmospheric conditions, revealing an interesting connection between oxygen intercalation at the surface and the δ- to α-phase transformation dynamics, an insight that will find application within the wider context of FAPbI₃ thermal phase relations

Principle of SRS microscopy (in paleoclimate research).

<p>(A) In spontaneous Raman scattering, the Pump photon (in blue) is scattered spontaneously and inelastically, and gets converted into a red-shifted Stokes photon. In SRS, Stokes photons are irradiated to stimulate the Raman transition of interest. The difference between Stokes and pump wavelength corresponds to the energy difference of the targeted vibrational levels. Dashed lines indicate virtual, short-lived molecular levels, solid lines denote real vibrational ones. Sensitive detection of the signal intensity relies on the rapid ON-OFF switching of the Stokes beam. When the Stokes beam is ON, Raman transitions will be stimulated leading to a decrease in pump laser intensity after the sample. When switching the Stokes beam OFF the Raman transition is no longer stimulated and the pump laser will again increase in intensity. Rapid modulation (10 MHz) of the Stokes laser leads to a modulated pump beam transmission which is detected and translated into a signal which is a quantitative measure of the number of Raman scatterers at that frequency (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0199695#pone.0199695.s001" target="_blank">S1 Fig</a>). (B) Pump-probe spectroscopy, based on transient absorption bleaching or ground state depletion, can be used to identify minerals with a strong light absorption i.e. colored such as iron oxides. Here a similar approach as in A is followed however now both lasers are in resonance with the electronic transition e.g. iron oxides etc. When the Stokes beam is ON, they will be absorbed by the colored species leading to a temporary depletion of the ground state. As a result, the pump photons of the second laser beam will no longer be absorbed. Switching the Stokes beam OFF results in the more efficient absorption of pump photons. Detection of these changes in pump beam intensity, based on the rapid modulation (10 MHz) of the Stokes laser, can be translated into a signal which is a quantitative measure of the presence of colored species at that location. As absorption bands (electronic resonances) are spectrally much broader than the corresponding vibrational resonances these can easily be discriminated from each other by hyperspectral SRS imaging. Note that the nomenclature of pump-probe is misleading in this context and adapted to our particular experiment which is primarily used for SRS where the Stokes laser is modulated. In a classical pump-probe experiment, the Stokes beam would be called the pump source. (C) In an actual experiment the tunable pump and Stokes lasers are tuned into a vibrational (SRS) or electronic (pump-probe) resonance of interest and coupled into a microscope for focusing onto the sample. Raster scanning the focus spot over the sample yields 2D (XY) images. Additional movement up and down of the objective with respect to the sample enables obtaining consecutive Z-sections. The detection module (more information in the supplementary information) is connected to a computer where tomographies are reconstructed. Additionally, a CCD camera in combination with a normal white-light source is used to record optical transmission images. (D) Flowchart of a paleoclimate data collection. After excavation and processing (e.g. sieving) of a sample, an initial pre-screening is performed. Microfossils found potentially interesting are thoroughly investigated using SRS chemical and spatial tomography. Samples classified as well preserved are being transferred to isotope analysis. To showcase that SRS is a well-suited technique in paleoclimate analysis we confirm the preservation state using SEM in this paper (dashed lines) after SRS measurements were done—which is not necessary.</p

SRS tomography for architecture assessments.

<p>Comparison between stereo (A), transmitted light (B), maximum projected SRS (at 1092 cm^-1) (C) and SEM images (D). Specimen 1 is a planktonic foraminifer of the <i>Heterohelix</i> genus with some infillings. Fossils 2 to 5 represent the benthic foraminiferal species <i>Nuttallides truempyi</i>. Severely diagenetically altered foraminifera are marked by a star.</p