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In vitro study of the photocytotoxicity of bathochromically-shifted hypericin derivatives
Roelants, Mieke,Lackner, Bernd,Waser, Mario,Falk, Heinz,Agostinis, Patrizia,Van Poppeld, Hendrik,De Witte, Peter A.M. Korean Society of Photoscience 2009 Photochemical & photobiological sciences Vol.8 No.6
Hypericin has excellent photosensitizing properties and displays favorable tumouritropic characteristics, but at the same time exhibits minimal dark toxicity. As such, the compound is a promising photosensitizer in the context of clinical photodynamic therapy (PDT). The present study was undertaken to investigate whether a newly-synthesized series of hypericin derivatives with a bathochromic shift shows promise for future PDT applications. Potentially these structures offer an advantage over the parent compound by being photo-activated by red light, which penetrates deeper into tumour tissue. Our results show that 3 compounds (a dibenzoxazole, a pyridazinone, and especially a dibenzthiazole derivative of hypericin), designed to exhibit a bathochromic shift in their absorption spectrum, demonstrated an efficient singlet oxygen yield and intracellular uptake, and concomitantly a potent photocytotoxic effect under white-light conditions. These results indicate that it is possible to synthesize bathochromically-shifted compounds based on hypericin chemistry which maintain their PDT potential. However, the data also show that the present derivatives are only poor photosensitizers when used under red-light conditions.
Wang, Yue,Kang, Kyung-Mun,Kim, Minjae,Lee, Hong-Sub,Waser, Rainer,Wouters, Dirk,Dittmann, Regina,Yang, J. Joshua,Park, Hyung-Ho Elsevier 2019 Materials today Vol.28 No.-
<P><B>Abstract</B></P> <P>Resistance random-access memory (RRAM) is a promising candidate for both the next-generation non-volatile memory and the key element of neural networks. In this article, different types of Mott-transition (the transition between the Mott insulator and metallic states) mechanisms and Mott-transition-based RRAM are reviewed. Mott insulators and some related doped systems can undergo an insulator-to-metal transition or metal-to-insulator transition under various excitation methods, such as pressure, temperature, and voltage. A summary of these driving forces that induce Mott-transition is presented together with their specific transition mechanisms for different materials. This is followed by a dynamics study of oxygen vacancy migration in voltage-driven non-volatile Mott-transition and the related resistive switching performance. We distinguish between a filling-controlled Mott-transition, which corresponds to the conventional valence change memory effect in band-insulators, and a bandwidth-controlled Mott-transition, which is due to a change in the bandwidth in the Mott system. Last, different types of Mott-RRAM-based neural network concepts are also discussed. The results in this review provide guidelines for the understanding, and further study and design of Mott-transition-based RRAM materials and their correlated devices.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Zhang, Hehe,Yoo, Sijung,Menzel, Stephan,Funck, Carsten,Cü,ppers, Felix,Wouters, Dirk J.,Hwang, Cheol Seong,Waser, Rainer,Hoffmann-Eifert, Susanne American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.35
<P>Redox-type resistive random access memories based on transition-metal oxides are studied as adjustable two-terminal devices for integrated network applications beyond von Neumann computing. The prevailing, so-called, counter-eight-wise (c8w) polarity of the switching hysteresis in filamentary-type valence change mechanism devices originates from a temperature- and field-controlled drift-diffusion process of mobile ions, predominantly oxygen vacancies in the switching oxide. Recently, a bipolar resistive switching (BRS) process with opposite polarity, so-called, eight-wise (8w) switching, has been reported that, especially for TiO<SUB>2</SUB> cells, is still not completely understood. Here, we report on nanosized (<0.01 μm<SUP>2</SUP>) asymmetric memristive cells from 3 to 6 nm thick TiO<SUB>2</SUB> films by atomic layer deposition, which reveal a coexistence of c8w and 8w switching in the same cell. As important characteristics for the studied Pt/TiO<SUB>2</SUB>/Ti/Pt devices, the resistance states of both modes are nonvolatile and share one common state; i.e., the high-resistance state of the c8w mode equals the low-resistance state of the 8w-mode. A transition between the opposite hysteresis loops is possible by voltage control. Specifically, 8w BRS in the TiO<SUB>2</SUB> cells is a self-limited low-energy nonvolatile switching process. Additionally, the 8w reset process enables the programming of multilevel high-resistance states. Combining the experimental results with data from simulation studies allows to propose a model, which explains 8w BRS by an oxygen transfer process across the Pt/TiO<SUB>2</SUB> Schottky interface at the position of the c8w filament. Therefore, the coexistence of c8w and 8w BRS in the nanoscale asymmetric Pt/TiO<SUB>2</SUB>/Ti/Pt cells is understood from a competition between drift/diffusion of oxygen vacancies in the oxide layer and an oxygen exchange reaction across the Pt/TiO<SUB>2</SUB> interface.</P> [FIG OMISSION]</BR>