Cölfen, Helmut

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Cölfen
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Helmut
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Silica‐Functionalized Nanolimes for the Conservation of Stone Heritage

2023-04-18, Burgos‐Ruiz, Miguel, Elert, Kerstin, Ruiz‐Agudo, Encarnacion, Cölfen, Helmut, Rodriguez‐Navarro, Carlos

The relatively recent development of nanolimes (i.e., alcoholic dispersions of Ca(OH)2 nanoparticles) has paved the way for new approaches to the conservation of important art works. Despite their many benefits, nanolimes have shown limited reactivity, back-migration, poor penetration, and lack of proper bonding to silicate substrates. In this work a novel solvothermal synthesis process is presented by which extremely reactive nanostructured Ca(OH)2 particles are obtained using calcium ethoxide as the main precursor species. Moreover, it is demonstrated that this material can be easily functionalized with silica-gel derivatives under mild synthesis conditions, thereby preventing particle growth, increasing total specific surface area, enhancing reactivity, modifying colloidal behavior, and functioning as self-integrated coupling agents. Additionally, the formation of calcium silicate hydrate (CSH) nanocement is promoted by the presence of water, resulting in optimal bonding when applied to silicate substrates, as evidenced by the higher reinforcement effect produced on treated Prague sandstone specimens as compared to those consolidated with nonfunctionalized commercial nanolime. The functionalization of nanolimes is not only a promising strategy for the design of optimized consolidation treatments for the cultural heritage, but may also have important implications for the development of advanced nanomaterials for building, environmental, or biomedical applications.

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Controlling Oriented Attachment of Gold Nanoparticles by Size and Shape

2021, Schupp, David Joshua, Angst, Jonathan, Schaefer, Eva A., Schupp, Stefan, Cölfen, Helmut

The concept of nanoparticles (NPs) as building blocks offers new possibilities to produce complex and tailored structures from the nano- to the mesoscale. In order to control a “polymerization” of particles, knowledge of the mechanism and kinetics of the reaction are necessary. We show that controlled assembly of cetylpyridinium chloride-stabilized gold NPs utilizing induced dipole–dipole interactions can lead to the formation of defined one-dimensional structures in solution. Three different shaped NPs (cubes, octahedra, and truncated cuboctahedra) were investigated individually. The assembly process is analogous to a step growth polymerization and is quantitatively describable with kinetics of a polyesterification. In situ kinetic studies reveal that there is an ideal particle size and shape for the induced dipole-driven assembly. Even small changes in size have remarkable effects on the assembly behavior. We further demonstrate that the transition from oriented assembly to oriented attachment requires a critical particle size (critical interface area) resulting from a size-dependent energy barrier for the crystallographic fusion. A combination of ideal size, shape, and degree of destabilization enables controlled oriented attachment of gold NPs in solution to chainlike structures under ambient conditions.