Electrochemical AFM and STM Studies of Redox Active Oligomers, Polymers and Drugs at Graphitic Materials

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Title: Electrochemical AFM and STM Studies of Redox Active Oligomers, Polymers and Drugs at Graphitic Materials
Authors: Sadaf, Shamaila
Thesis advisor: Prof. Dr. Lorenz Walder
Thesis referee: Prof. Dr. Markus Haase
Abstract: Carbon nanotubes (CNTs) and graphene (G) are the two (semi-)conducting allotropes of carbon offering a very high surface to bulk ratio. When combined with electrochemistry and using the principles of molecular self-assembling, many new applications of CNTs and graphene have become feasible, e.g. in the field of molecular electronics, as energy storage materials and for drug delivery. In this work scanning probe microscopy techniques (STM, AFM and EC-AFM) are used to understand the structure of self-assembled organic and electroactive molecules, oligomers and polymers on the surface of CNTs and graphene, and to rationalize their function as supramolecular system in the macroscopic world. It was found that there is a strong tendency for self-assembling in solution of aromatic electrophores with HOMO/LUMO levels close to the semiconducting CNT frontier orbitals. The tunneling currents through the aromatic guest molecules on CNT are much higher than for the same molecule on highly oriented pyrolytic graphite (HOPG). In the 1st chapter of this thesis the different microscopic techniques used with special emphasis on the electrochemical atomic force microscopy (EC-AFM), a relatively new combination technique which plays an important role in this work, are presented. It is followed by the structural analysis of stiff, semi-flexible and flexible oligo-viologen on CNTs. Using a large set of molecules with well defined small structural differences allowed to study how the superstructure (guests@CNT) is determined by the tiny changes in the guest structure. Mostly helical super structures of guests around the CNT host were observed. The guest’s rod length, its side chain length and its flexibility translate unambiguously into the corresponding STM images. A non-linear, star shaped oligo-viologen cannot wrap CNT without overlapping star branches, as expected from model considerations. In collaboration with a Korean research group we were able to build an n-doped FET using a reduced rigid oligo-viologen@CNT. Along with the oligomer the formation of stiff poly-viologens@CNT and poy-imides@CNT are studied. As compared to the oligomers@CNT, the polymers@CNT have less conformational freedom when wrapping a CNT. Thus, exclusively double to multi stranded helical wrapping was observed. An interesting new phenomenon was discovered with stiff poly-viologen or poly-naphthaline tetracarboxylic acid diimide with purely sp2 configured atoms, i.e. an outer, large diameter helical structure of “the guest” polymer and a CNT “host” sitting inside the spiral. The spiral diameter was simulated using PM7 calculations. The CNT can be moved within the large spiral by voltage pulse application. Subsequently, the conformation of flexible poly-viologens and poly-TEMPO on host materials such as CNTs and vapor grown carbon fibers (VGCF) was studies. Again, helical wrapping is observed, but the diameter adapts here to the CNT diameter. Monomer subunit resolution was achieved in case of polyTEMPO. The practical importance in energy storage is discussed in the corresponding original paper. The next chapter of thesis focuses on AFM imaging of a new battery material, i.e. poly-ferrocene on graphene oxide (PVF@GO) and on reduced graphene oxide (PVF@rGO), as well as poly-viologen as PV@GO and PV@rGO. A highlight is definitely the visualization of the so-called ion-breathing, i.e. the reversible counter ion movement from solution into the battery material composite upon electrochemical reduction/oxidation. To the best of my knowledge, this phenomenon is for the first time visualized here by a combination of electrochemistry and AFM. STM analysis of electrically conductive rGO allows for subunit resolution of polyviologen@rGO sitting in partially 2D crystalline structure on rGO. In the last chapter, my publication on the drug delivery system doxorubicin at carbon nanotubes is described. Dox@CNT is already used as drug delivery system in animal tests, but little is known on the structure of the drug on the carrier, and a reductive release trigger has so far not been identified. Rich structural variations of the drug on the CNT (helical strands of monomers and dimers) were found. It is possible to get the drug loading efficiency from STM image analysis. Reductive release of Dox@CNT was also unknown so far. Electron injection into Dox@CNT from an electrode or from the biological reducing agent glutathione (GSH) leads to irreversible release of Dox. Experimental results are in excellent agreement with semi-empirical simulations.
URL: https://repositorium.ub.uni-osnabrueck.de/handle/urn:nbn:de:gbv:700-2017113016342
Subject Keywords: AFM; STM
Issue Date: 30-Nov-2017
Type of publication: Dissertation oder Habilitation [doctoralThesis]
Appears in Collections:FB05 - E-Dissertationen

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