Öffentliche Seminare

Raum: Digital see link

Development of a coarse-grained model for liquid-like protein assemblies

In this talk, I will describe ongoing efforts in my group aimed at developing an accurate simulation model to study the thermodynamics and kinetics of multiprotein assembly. We use a "top-down" approach for constructing a Cα-based (one interaction site per amino acid) protein model. Development of the proposed model involves comparisons with experimental data available from the recent literature as well as comparisons with atomistic simulations of a single protein chain. The usefulness of our approach will be demonstrated by discussing results on multiple biological systems of interest. Of particular interest to us is the formation of liquid-like assemblies of disordered proteins that have been found to be important for the physiological function of membraneless compartments in living cells including the nucleolus and ribonucleoprotein (RNP) granules as well as many organelles in prokaryotic cells. [mehr]

Conducting polymers and bioinspired materials for organic bioelectronics

Organic electrochemical transistors (OECTs) have rapidly surged as amplifying transducers forbiosensing or diagnostic devices and for cells/nerves stimulation. 1 OECTs translate ionic signalsinto electric current using an electrolyte in direct contact with a conducting polymer channel.Unlike in organic field effect transistors where charge transport involves only a small layer, ionsin OECTs permeate the entire volume of the active material. This results in devices having highersignal amplification and lower operating voltage. 1 At the heart of OECTs working principle is theorganic mixed ionic-electronic conductor, usually a π- conjugated polymer (or polymer blend)able to host (or chemically linked to) charged groups.The greatest challenge of developing high-performing mixed conductors is optimizing theseemingly conflicting processes of electronic and ionic charge transport. 2 The chemical featuresof mixed conductors make them hydrophilic; however, little is known about how water and ionsaffect their microstructure and charge transport, as well as their long-term operational stabilityand biocompatibility.A successful strategy towards suitable materials for OECTs is to modify conducting polymerssuch as polythiophenes to make them hydrophilic, using glycolated substituents. 3 I am currentlyinvestigating the role of glycolated side chains in ion coordination and polymer morphology, 4,5and developing a methodology to describe swelling and morphology changes upon ionpenetration and doping in these materials.A different path towards new bioelectronic materials is to instead use natural mixed conductingpolymers that are intrinsically biocompatible, either alone or in composites. Synthetic polymersderiving from eumelanin (the black pigment in our skin, hair and eyes) are promisingbiocompatible, non-cytotoxic components in OECTs or other optoelectronic devices: 6-8 theyfeature both electronic and protonic charge carriers, can be prepared in large batches undercontrolled conditions, and easily incorporated into hybrid materials.I am currently studying the structure, self-assembly and electronic properties of eumelanin-derived materials. My model takes into account the chemical disorder of eumelanin(tautomerisation, oxidation and proton exchange sites) and aims to elucidate its effect on theelectronic structure and charge transport characteristics of this material. In particular, I aminvestigating the peculiar dipole-dependent properties of DHICA melanin 9 and itssupramolecular organisation; this rigid polymer can be spun into fibers with promisingmechanical and charge transport properties. [mehr]
The invention of new materials combined with an improved knowledge of structure-property relationships of organic donor-acceptor blends led to an impressive improvement in their energy conversion efficiency to 17 % [1]. This success seems to contradict the simple view that the long range Coulomb interaction between electrons and holes in organic semiconductors causes inefficient formation but efficient recombination of free charge. An important characteristic of organic solar cells is that they comprise at least two organic components of different chemical structure, introducing a large complexity of the morphology and electron landscape of the active layer. In this talk, I will present results regarding the generation and recombination of free charges in selected bulk heterojunction solar cells, with particular focus on the role of the interfacial CT state. We show that free charge formation proceeds predominately through low energy CT states, ruling out the predominance of a hot CT dissociation pathway, and that the same states dominate the subsequent recombination [2]. For fullerene-based solar cells with a low donor content, we find that the efficiency of charge generation is limited by the same mechanism that limits the VOC, namely non-radiative recombination of the CT states via vibronic coupling [3]. Notably, the rate of this recombination process obeys the classical energy gap law, implying that donor-acceptor blends benefit from a higher CT energy through longer CT lifetimes and more efficient photocurrent generation [4]. Consistent with this result, we observe that devices suffer from inefficient CT dissociation also through a higher rate of non-geminate recombination [5]. As a consequence, it’s only the systems with very efficient charge generation and very fast CT dissociation that the free carrier recombination is strongly suppressed, irrespective of the details of the spin statistics. We, finally, present recent results on a highly efficient polymer:NFA blend, where we find a surprisingly low activation energy for free charge generation, despite a low energy offset at the heterojunction [6]. These results highlight the importance of a comprehensive understanding of the energy landscape, and how it affects the pathway from the bound CT exciton to the spatially separated electron-hole pair. [mehr]
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