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Optimisation of microwave resonators
Microwave (MW) resonators are used in electron spin resonance (ESR) spectroscopy to concentrate the magnetic B1 field of microwaves at the sample location and to separate the electric field of the microwaves. This serves to ensure that the sample only interacts with the magnetic component of the MW and that the sample itself is not heated at excessively high MW powers. In ESR spectroscopy, there are different measurement methods (e.g. CW, transient ESR and pulse ESR) which all place different demands on MW resonators, e.g. high Q-factors, large MW conversion factors or homogeneous B1 fields. Commercially available spectrometers already offer good MW resonators for a wide range of applications. However, these spectrometers are already reaching the limits of sensitivity due to new research developments. Optimisation of MW resonators therefore offers a direct path to improved sensitivity.
Within the scope of the dissertation, three optimisation paths of MW resonators were investigated. The aim is to improve and optimise already available resonators without having to design a completely new resonator. The developed resonators were investigated using CW, transient and pulse ESR spectroscopy.
Publications:
- Wiedemann, H.T.A., S. Ruloff, R. Richter, C.W. Zollitsch, and C.W.M. Kay, Towards high performance dielectric microwave resonators for X-band EPR spectroscopy, Journal of Magnetic Resonance, 2023, p.107519.
- C.W. Zollitsch, S. Ruloff, S., Y. Fett, H.T.A. Wiedemann, R. Richter, J.D. Breeze, C.W.M. Kay, Maser threshold characterization by resonator Q-factor tuning, Communication Physics, 2023, 6, 295.
Spin-traps
Reactive oxygen species (ROS) refer to a large class of highly reactive oxygen-containing molecules. These molecules have various functions and properties ranging from biological functions through oxygen metabolism to important carrier roles in catalysis cycles. The detection of these ROS is difficult due to their short lifetimes, but spin-trapping offers a way out. In spin-trapping, a relatively stable radical is formed from the short-lived ROS radical and a spin trap (usually a nitroxyl compound) is formed which can be detected. Therefore, spin trapping has established itself as a valid method for the detection of ROS.
In this subproject, the radical reaction pathway of La2CoO4+δ nanoparticles is investigated as an effective catalyst for the activation of peroxymonosulphate (PMS). It is important to note that in the La2CoO4+δ system with PMS, different radicals play a role in the decay process of PMS. Thus, different ROS can also be distinguished by selecting appropriate spin-traps and the resulting ESR spectra. We have developed a radical scavenging protocol that could help to identify the most important ROS in the decay process of PMS for other heterogeneous catalysts. This could be useful in targeted development of nanoparticles for different ROS.
Publications:
- Hammad, M., Alkan, B., Al-kamal, A.K., Kim, C., Ali, M.Y., Angel, S., Wiedemann, H.T.A., Klippert, D., Schmidt, T.C., Kay, C.W. and Wiggers, H., 2022, Chemical Engineering Journal, 429, p.131447.
- Hammad, M., Angel, S., Al-kamal, A.K., Asghar, A., Amin, A.S., Kräenbring, M.A., Wiedemann, H.T.A., Vinayakumar, V., Ali, M.Y., Fortugno, P. and Kim, C., 2022, Chemical Engineering Journal, p.139900.
DEER spectroscopy
DEER (double electron electron resonance) or PELDOR (pulsed electron electron double resonance) spectroscopy is concerned with the observation of the interaction of usually two spins in the immediate vicinity of each other (1.8 - 10 nm). In most cases, two spin labels (nitroxide functionality) are introduced into the molecule or protein to be investigated. The spectroscopic investigation is then carried out at low temperatures (80 - 100 K, longer relaxation times) with the aid of irradiation at two different microwave frequencies. The exponential decay function obtained is converted into a probability distribution of the distance of the spin-spin interaction either via a Thikonov regularisation or other analytic methods. In this manner, very individual positions can be observed, which allows statements about the structure of a molecule/protein. For example, the movement of a protein can be observed when an inhibitor is embedded in the active centre. This allows conclusions about the mechanism of this movement, which provides further insight into how enzymatic catalysis takes place in the selected protein.
Publications:
- R. Dhiman, R. S. Perera, C. S. Poojari, H.T.A. Wiedemann, R. Kappl, C. W. M. Kay, J. S. Hub, and B. Schrul, Hairpin protein partitioning from the ER to lipid droplets involves major structural rearrangements, Nature Communications , 15, 1, 2024, 4504.
► Publications
R. Dhiman, R. S. Perera, C. S. Poojari, H.T.A. Wiedemann, R. Kappl, C. W. M. Kay, J. S. Hub, and B. Schrul, Hairpin protein partitioning from the ER to lipid droplets involves major structural rearrangements, Nature Communications , 15, 1, 2024, 4504.
C.W. Zollitsch, S. Ruloff, S., Y. Fett, H.T.A. Wiedemann, R. Richter, J.D. Breeze, C.W.M. Kay, Maser threshold characterization by resonator Q-factor tuning, Communication Physics, 2023, 6, 295.
H.T.A. Wiedemann, S. Ruloff, R. Richter, C.W. Zollitsch, and C.W.M. Kay, Towards high performance dielectric microwave resonators for X-band EPR spectroscopy, Journal of Magnetic Resonance, 2023, p.107519.
A. Carella, S. Ciuti, H.T.A. Wiedemann, C.W.M. Kay, A. van der Est, D. Carbonera, A. Barbon, P.K. Poddutoori, and M.D. Valentin, The electronic structure and dynamics of the excited triplet state of octaethylaluminum (III)-porphyrin investigated with advanced EPR methods, Journal of Magnetic Resonance, 2023, p.107515.
A. Michaely, O. Janka, E.C.J. Gießelmann, R. Haberkorn, H.T.A. Wiedemann, C.W.M. Kay and G. Kickelbick, Black Titania and Niobia within Ten Minutes–Mechanochemical Reduction of Metal Oxides with Alkali Metal Hydrides, Chemistry–A European Journal, 2023, e202300223.
D. Mandal, T.I. Demirer, T. Sergeieva, B. Morgenstern, H.T.A. Wiedemann, C.W.M. Kay and D.M. Andrada, Evidence of Al(II) Radical Addition to Benzene, Angewandte Chemie, 2023, e202217184.
M. Hammad, S. Angel, A.K. Al-Kamal, A. Asghar, A.S. Amin, M. Kräenbring, H.T.A. Wiedemann and C.W.M. Kay, Synthesis of novel LaCoO3/graphene catalysts as highly efficient peroxymonosulfate activator for the degradation of organic pollutants, Chemical Engineering Journal, 454, 2023, 139900.
M. Hammad, B. Alkan, A.K. Al-kamal, C. Kim, M.Y. Ali, S. Angel, H.T.A. Wiedemann and C.W.M. Kay, Enhanced heterogeneous activation of peroxymonosulfate by Ruddlesden-Popper-type La2CoO4+ δ nanoparticles for bisphenol A degradation, Chemical Engineering Journal, 429, 2022, 131447.