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Current and prospective research topics

1. Depth distributions of organics within nanostructures

Nanostructured oxides are coated with self-assembled monolayers (SAMs) of functional organic molecules for various applications. However, we still lack control and understanding about the depth distribution of the SAMs within the nanostructures. We apply ToF-SIMS in combination with focused ion beam cuts to reliably evaluate the depth distribution within nanostructures. We furthermore work on improving the filling of the nanostructures for a more homogeneous self-assembly. Selective modification of the nanostructures at defined depths will allow us to produce novel materials for biomedical and photoelectric applications. 


2. Nanobiotechnology: Protein modified oxide nanostructures

The attachment of biomolecules to implant surfaces can be used to produce functional materials that either improve acceptance of the implant (e.g., dental or hip implants) or prevent the ingrowth of tissue (e.g., stents). Protein coatings of TiO2 nanotubes with BMP-2 were shown to direct the cell differentiation of mesenchymal stem cells to cartilage or bone in dependence of the diameter of the underlying TiO2 nanotubes [1]. The protein adsorption was investigated in dependence of the diameter of TiO2 nanotubes [2]. We could show that from the linkers commonly used in literature, carbonyldiimidazole and 11-hydroxyundecylphosphonic acid lead to the highest surface coverage with active protein [3]. In ToF-SIMS, we could observe heat induced denaturation by disulfide bond cleavage [4]. The denaturation of adsorbed protein is significantly faster on compact oxide surfaces than in nanotubes, presumably due to the greater exposure to air. Protective coatings with sugar molecules and compatible solutes from extremophilic bacteria were able to stabilize protein activity in dry coatings for 24h [5].


3. Selective self-assembly of organic molecules on metal oxides

The surfaces of oxides generally are terminated with hydroxyl groups, which means that an inherent selectivity as it is observed for composite materials of noble and non-noble metals is not to be expected. The selective modification of materials consisting of ZrO2-TiO2, ZrO2-MgO, TiO2-MgO, TiO2-Co3O4 and TiO2-NiO is investigated. Some of these composite materials show differences in the affinity towards carboxylic acids, oxides with higher isoelectric points showed higher coverage [6]. Selective adsorption furthermore may be achieved via the detour over selective decomposition. We aim at developing general concepts for oxide-induced selective molecular adsorption and at the construction of complex multifunctional oxide nanostructures. Potential applications include delayed drug release systems, nanostructured sensor arrays and tandem dye-sensitized solar cells.


4. NiO nanostructures for application in p-type photocathodes

Dye-sensitized p-type NiO based cathodes have the potential to improve the potential of dye-sensitized solar cells (DSSCs) and water splitting devices. We aim at improving the performance of the photocathode by producing suitable NiO nanostructures that facilitate fast charge separation and a directional charge transport by anodic growth. We were able to produce non-thicknnes-limited NiO in an anodic process. The novel material shows improved performance in p-type DSSCs compared to other NiO morphologies [7]. The NiO-photocathodes will be sensitized with novel dyes of appropriate energetic levels and anchoring stability synthesized by our collaboration partners.


5. Phosphate interlayers for metal oxide bilayers

Phosphate termination of oxide surfaces allows to adsorb a monolayer of a second metal oxide on the material's surface. This can be useful for improving the adsorption properties of SAMs and potentially tune the (photo-)electronic properties of the surface. We were able to introduce both NiO- and Co3O4-termination to TiO2 surfaces, sigificantly improving the efficiency of DSSCs [6].


6. Surface modification and surface analysis of building materials

The modification of building materials (concrete, gypsum), which basically are oxides in a wider sense, with organic molecules can open a wide field of application (tuning hydrophobicity, green fassades...). To produce such property tailored materials, a deep knowledge of the building material's surface is a prerequisite. We will use ToF-SIMS and XPS in combination with SEM and XRD to obtain a complete picture of the building material's surface properties, e.g. preferred adsorption to certain crystallographic axes will be investigated.



[1] J. Park, S. Bauer, A. Pittrof, M.S. Killian, P. Schmuki, K. von der Mark "Synergistic control of mesenchymal stem cell differentiation by nanoscale surface geometry and immobilized growth factors on TiO2 nanotubes" Small 2012, 8, 98.

[2] M. Kulkarni, A. Mazare, J. Park, E. Gongadze, M.S. Killian, S. Kralij, K. von der Mark, A. Iglic, P. Schmuki "Protein interactions with layers of TiO2 nanotube and nanopore arrays: Morphology and surface charge influence" Acta Biomaterialia 2016, 45, 357.

[3] M.S. Killian, P. Schmuki "Influence of bioactive linker molecules on protein adsorption" Surf. Interf. Analysis 2014, 46, 193.

[4] M.S. Killian, H. Krebs, P. Schmuki "Protein denaturation detected by time-of-flight secondary ion mass spectrometry" Langmuir 2011, 27, 7510.

[5] M.S.Killian, A.J. Taylor, D.G. Castner "Stabilization of dry protein coatings with compatible solutes", Biointerphases 2018, 13, 06E401 - featured article.

[6] M. C.O. Monteiro, G. Cha, P. Schmuki, M.S. Killian "Metal-Phosphate Bilayers for Anatase Surface Modification" ACS Appl. Mat. Interf. 2018, 10, 6661.

[7] U. Sultan, F. Ahmadloo, G. Cha, B. Gökcan, S. Hejazi, J.-E. Yoo, N.T. Nguyen, M. Altomare, P. Schmuki, M.S. Killian "A high-field anodic NiO nanosponge with tuneable thickness for application in p-type dye sensitized solar cells" ACS Appl. Energy Mater. 2020, 3, 8, 7865.