@article{MeilingSchuermannVogeletal.2018, author = {Meiling, Till Thomas and Sch{\"u}rmann, Robin Mathis and Vogel, Stefanie and Ebel, Kenny and Nicolas, Christophe and Milosavljevic, Aleksandar R. and Bald, Ilko}, title = {Photophysics and Chemistry of Nitrogen-Doped Carbon Nanodots with High Photoluminescence Quantum Yield}, series = {The journal of physical chemistry : C, Nanomaterials and interfaces}, volume = {122}, journal = {The journal of physical chemistry : C, Nanomaterials and interfaces}, number = {18}, publisher = {American Chemical Society}, address = {Washington}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.8b00748}, pages = {10217 -- 10230}, year = {2018}, abstract = {Fluorescent carbon nanodots (CNDs) are very promising nanomaterials for a broad range of applications because of their high photostability, presumed selective luminescence, and low cost at which they can be produced. In this respect, CNDs are superior to well-established semiconductor quantum dots and organic dyes. However, reported synthesis protocols for CNDs typically lead to low photoluminescence quantum yield (PLQY) and low reproducibility, resulting in a poor understanding of the CND chemistry and photophysics. Here, we report a one-step synthesis of nitrogen-doped carbon nanodots (N-CNDs) from various carboxylic acids, Tris, and ethylenediaminetetraacetic acid resulting in high PLQY of up to 90\%. The reaction conditions in terms of starting materials, temperature, and reaction time are carefully optimized and their influence on the photophysical properties is characterized. We find that citric acid-derived N-CNDs can result in a very high PLQY of 90\%, but they do not show selective luminescence. By contrast, acetic acid-derived N-CNDs show selective luminescence but a PLQY of 50\%. The chemical composition of the surface and core of these two selected N-CND types is characterized among others by high-resolution synchrotron X-ray photoelectron spectroscopy using single isolated N-CND clusters. The results indicate that photoexcitation occurs in the N-CND core, whereas the emission properties are determined by the N-CND surface groups.}, language = {en} } @article{SchuermannVogelEbeletal.2018, author = {Sch{\"u}rmann, Robin Mathis and Vogel, Stefanie and Ebel, Kenny and Bald, Ilko}, title = {The physico-chemical basis of DNA radiosensitization}, series = {Chemistry - a European journal}, volume = {24}, journal = {Chemistry - a European journal}, number = {41}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0947-6539}, doi = {10.1002/chem.201800804}, pages = {10271 -- 10279}, year = {2018}, abstract = {High-energy radiation is used in combination with radiosensitizing therapeutics to treat cancer. The most common radiosensitizers are halogenated nucleosides and cisplatin derivatives, and recently also metal nanoparticles have been suggested as potential radiosensitizing agents. The radiosensitizing action of these compounds can at least partly be ascribed to an enhanced reactivity towards secondary low-energy electrons generated along the radiation track of the high-energy primary radiation, or to an additional emission of secondary reactive electrons close to the tumor tissue. This is referred to as physico-chemical radiosensitization. In this Concept article we present current experimental methods used to study fundamental processes of physico-chemical radiosensitization and discuss the most relevant classes of radiosensitizers. Open questions in the current discussions are identified and future directions outlined, which can lead to optimized treatment protocols or even novel therapeutic concepts.}, language = {en} } @phdthesis{Vogel2018, author = {Vogel, Stefanie}, title = {Sequence dependency of photon and electron induced DNA strand breaks}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-419669}, school = {Universit{\"a}t Potsdam}, pages = {xii, 117}, year = {2018}, abstract = {Deoxyribonucleic acid (DNA) is the carrier of human genetic information and is exposed to environmental influences such as the ultraviolet (UV) fraction of sunlight every day. The photostability of the DNA against UV light is astonishing. Even if the DNA bases have a strong absorption maximum at around 260 nm/4.77 eV, their quantum yield of photoproducts remains very low 1. If the photon energies exceed the ionization energy (IE) of the nucleobases ( ̴ 8-9 eV) 2, the DNA can be severely damaged. Photoexcitation and -ionization reactions occur, which can induce strand breaks in the DNA. The efficiency of the excitation and ionization induced strand breaks in the target DNA sequences are represented by cross sections. If Si as a substrate material is used in the VUV irradiation experiments, secondary electrons with an energy below 3.6 eV are generated from the substrate. This low energy electrons (LEE) are known to induce dissociative electron attachment (DEA) in DNA and with it DNA strand breakage very efficiently. LEEs play an important role in cancer radiation therapy, since they are generated secondarily along the radiation track of ionizing radiation. In the framework of this thesis, different single stranded DNA sequences were irradiated with 8.44 eV vacuum UV (VUV) light and cross sections for single strand breaks (SSB) were determined. Several sequences were also exposed to secondary LEEs, which additionally contributed to the SSBs. First, the cross sections for SSBs depending on the type of nucleobases were determined. Both types of DNA sequences, mono-nucleobase and mixed sequences showed very similar results upon VUV radiation. The additional influence of secondarily generated LEEs resulted in contrast in a clear trend for the SSB cross sections. In this, the polythymine sequence had the highest cross section for SSBs, which can be explained by strong anionic resonances in this energy range. Furthermore, SSB cross sections were determined as a function of sequence length. This resulted in an increase in the strand breaks to the same extent as the increase in the geometrical cross section. The longest DNA sequence (20 nucleotides) investigated in this series, however, showed smaller cross section values for SSBs, which can be explained by conformational changes in the DNA. Moreover, several DNA sequences that included the radiosensitizers 5-Bromouracil (5BrU) and 8-Bromoadenine (8BrA) were investigated and the corresponding SSB cross sections were determined. It was shown that 5BrU reacts very strongly to VUV radiation leading to high strand break yields, which showed in turn a strong sequence-dependency. 8BrA, on the other hand, showed no sensitization to the applied VUV radiation, since almost no increase in strand breakage yield was observed in comparison to non-modified DNA sequences. In order to be able to identify the mechanisms of radiation damage by photons, the IEs of certain DNA sequences were further explored using photoionization tandem mass spectrometry. By varying the DNA sequence, both the IEs depending on the type of nucleobase as well as on the DNA strand length could be identified and correlated to the SSB cross sections. The influence of the IE on the photoinduced reaction in the brominated DNA sequences could be excluded.}, language = {en} } @article{VogelEbelHecketal.2019, author = {Vogel, Stefanie and Ebel, Kenny and Heck, Christian and Sch{\"u}rmann, Robin Mathis and Milosavljevic, Aleksandar R. and Giuliani, Alexandre and Bald, Ilko}, title = {Vacuum-UV induced DNA strand breaks}, series = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, volume = {21}, journal = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, number = {4}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1463-9076}, doi = {10.1039/c8cp06813e}, pages = {1972 -- 1979}, year = {2019}, abstract = {Radiation therapy is a basic part of cancer treatment. To increase the DNA damage in carcinogenic cells and preserve healthy tissue at the same time, radiosensitizing molecules such as halogenated nucleobase analogs can be incorporated into the DNA during the cell reproduction cycle. In the present study 8.44 eV photon irradiation induced single strand breaks (SSB) in DNA sequences modified with the radiosensitizer 5-bromouracil (U-5Br) and 8-bromoadenine ((8Br)A) are investigated. U-5Br was incorporated in the 13mer oligonucleotide flanked by different nucleobases. It was demonstrated that the highest SSB cross sections were reached, when cytosine and thymine were adjacent to U-5Br, whereas guanine as a neighboring nucleobase decreases the activity of U-5Br indicating that competing reaction mechanisms are active. This was further investigated with respect to the distance of guanine to U-5Br separated by an increasing number of adenine nucleotides. It was observed that the SSB cross sections were decreasing with an increasing number of adenine spacers between guanine and U-5Br until the SSB cross sections almost reached the level of a non-modified DNA sequence, which demonstrates the high sequence dependence of the sensitizing effect of U-5Br. (8Br)A was incorporated in a 13mer oligonucleotide as well and the strand breaks were quantified upon 8.44 eV photon irradiation in direct comparison to a non-modified DNA sequence of the same composition. No clear enhancement of the SSB yield of the modified in comparison to the non-modified DNA sequence could be observed. Additionally, secondary electrons with a maximum energy of 3.6 eV were generated when using Si as a substrate giving rise to further DNA damage. A clear enhancement in the SSB yield can be ascertained, but to the same degree for both the non-modified DNA sequence and the DNA sequence modified with (8Br)A.}, language = {en} } @article{VogelEbelSchuermannetal.2019, author = {Vogel, Stefanie and Ebel, Kenny and Sch{\"u}rmann, Robin Mathis and Heck, Christian and Meiling, Till and Milosavljevic, Aleksandar R. and Giuliani, Alexandre and Bald, Ilko}, title = {Vacuum-UV and Low-Energy Electron-Induced DNA Strand Breaks}, series = {ChemPhysChem : a European journal of chemical physics and physical chemistry}, volume = {20}, journal = {ChemPhysChem : a European journal of chemical physics and physical chemistry}, number = {6}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1439-4235}, doi = {10.1002/cphc.201801152}, pages = {823 -- 830}, year = {2019}, abstract = {DNA is effectively damaged by radiation, which can on the one hand lead to cancer and is on the other hand directly exploited in the treatment of tumor tissue. DNA strand breaks are already induced by photons having an energy below the ionization energy of DNA. At high photon energies, most of the DNA strand breaks are induced by low-energy secondary electrons. In the present study we quantified photon and electron induced DNA strand breaks in four different 12mer oligonucleotides. They are irradiated directly with 8.44 eV vacuum ultraviolet (VUV) photons and 8.8 eV low energy electrons (LEE). By using Si instead of VUV transparent CaF2 as a substrate the VUV exposure leads to an additional release of LEEs, which have a maximum energy of 3.6 eV and can significantly enhance strand break cross sections. Atomic force microscopy is used to visualize strand breaks on DNA origami platforms and to determine absolute values for the strand break cross sections. Upon irradiation with 8.44 eV photons all the investigated sequences show very similar strand break cross sections in the range of 1.7-2.3x10(-16) cm(2). The strand break cross sections for LEE irradiation at 8.8 eV are one to two orders of magnitude larger than the ones for VUV photons, and a slight sequence dependence is observed. The sequence dependence is even more pronounced for LEEs with energies <3.6 eV. The present results help to assess DNA damage by photons and electrons close to the ionization threshold.}, language = {en} } @article{VogelRackwitzSchuermanetal.2015, author = {Vogel, Stefanie and Rackwitz, Jenny and Schuerman, Robin and Prinz, Julia and Milosavljevic, Aleksandar R. and Refregiers, Matthieu and Giuliani, Alexandre and Bald, Ilko}, title = {Using DNA origami nanostructures to determine absolute cross sections for UV photon-induced DNA strand breakage}, series = {The journal of physical chemistry letters}, volume = {6}, journal = {The journal of physical chemistry letters}, number = {22}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.5b02238}, pages = {4589 -- 4593}, year = {2015}, abstract = {We have characterized ultraviolet (UV) photon-induced DNA strand break processes by determination of absolute cross sections for photoabsorption and for sequence-specific DNA single strand breakage induced by photons in an energy range from 6.50 to 8.94 eV. These represent the lowest-energy photons able to induce DNA strand breaks. Oligonudeotide targets are immobilized on a UV transparent substrate in controlled quantities through attachment to DNA origami templates. Photon-induced dissociation of single DNA strands is visualized and quantified using atomic force microscopy. The obtained quantum yields for strand breakage vary between 0.06 and 0.5, indicating highly efficient DNA strand breakage by UV photons, which is clearly dependent on the photon energy. Above the ionization threshold strand breakage becomes clearly the dominant form of DNA radiation damage, which is then also dependent on the nucleotide sequence.}, language = {en} }