2023, Göppert, Ann-Kathrin, Gonzalez-Rubio, Guillermo, Schnitzlein, Simon, Cölfen, Helmut

Heterogeneous nucleation processes are involved in many important phenomena in nature, including devastating human diseases caused by amyloid structures or the harmful frost formed on fruits. However, understanding them is challenging due to the difficulties of characterizing the initial stages of the process occurring at the interface between the nucleation medium and the substrate surfaces. This work implements a model system based on gold nanoparticles to investigate the effect of particle surface chemistry and substrate properties on heterogeneous nucleation processes. Using widely available techniques such as UV–vis–NIR spectroscopy and light microscopy, gold nanoparticle-based superstructure formation was studied in the presence of substrates with different hydrophilicity and electrostatic charges. The results were evaluated on grounds of classical nucleation theory (CNT) to reveal kinetic and thermodynamic contributions of the heterogeneous nucleation process. In contrast to nucleation from ions, the kinetic contributions toward nucleation turned out to be larger than the thermodynamic contributions for the nanoparticle building blocks. Electrostatic interactions between substrates and nanoparticles with opposite charges were crucial to enhancing the nucleation rates and decreasing the nucleation barrier of superstructure formation. Thereby, the described strategy is demonstrated advantageous for characterizing physicochemical aspects of heterogeneous nucleation processes in a simple and accessible manner, which could be potentially explored to study more complex nucleation phenomena.

2022-07-14, Göppert, Ann-Kathrin, Gonzalez-Rubio, Guillermo, Cölfen, Helmut

In this study, we analysed for the first time heterogeneous nucleation with anisotropic nanoparticles as a model system for non-spherical building units on the nanoscale. Gold nanorods were synthesised and assembled to investigate the phenomenon of heterogeneous nucleation. To determine the influence of the particle shape on heterogeneous nucleation, we utilised gold nanorods with varying aspect ratios, ranging from 3.00 and 2.25 to 1.75, while keeping the surface chemistry constant. First, the nucleation of the gold nanorod assemblies in solution and the process kinetics were analyzed with UV-vis-NIR spectroscopy followed by a microscopic examination of the gold nanorod-based superstructures formed heterogeneously on substrates. Here, positively charged cetyltrimethylammonium bromide (CTAB)-functionalized gold nanorods and negatively charged polystyrene sulfonate (PSS) functionalized substrates ensured the directed heterogeneous nucleation on the substrates. A combination of light microscopy with simultaneous UV-vis-NIR spectroscopy allowed us to observe the gold nanorod-based superstructure formation on the substrates in situ and to determine the nucleation rates of the process. We analysed the resulting data with the classical nucleation theory, which revealed a dominating kinetic term and a negligible thermodynamic term in contrast to ionic systems like calcium carbonate. Our studies consistently exhibit an influence of the aspect ratio on the nucleation behaviour resulting in faster nucleation of superstructures as the aspect ratio decreases. Hence our studies show unprecedented insight into the influence of particle anisotropy on the nucleation and growth of nanorod-based superstructures and reveal significant differences in the nucleation of nanoparticle building units compared to the nucleation of atoms or molecules as building units.

2020-07-09, Göppert, Ann-Kathrin, Gonzalez-Rubio, Guillermo, Cölfen, Helmut

Nucleation phenomena play an important role in our world, and understanding them is of major interest. However, we lack analytical methods with the sufficient temporal and spatial resolution to analyze nucleation processes. In this work we used CTAB-stabilized gold nanocubes as a model system for nucleation, meaning the nanoparticles act like ions or atoms and built up larger superstructures comparable to normal nucleation phenomena. Thereby we analyzed the heterogeneous nucleation of the gold nanocubes on hydrophobized and negatively charged mica surfaces with a combination of UV–vis–NIR spectroscopy and light microscopy. With the plasmon resonance of the gold nanocubes we gained valuable information about the early nucleation of the particles and their concentration in solution via UV–vis–NIR spectroscopy. The combination with a light microscope enabled the simultaneous detection of nucleated species on the surfaces and opened the possibility to analyze the kinetics of the heterogeneous nucleation process. With this, we were able to determine the nucleation rates. While the hydrophobized surfaces did not influence the nucleation of the gold nanocubes, the negatively charged surfaces greatly promoted the nucleation. Thereby, we could demonstrate that the combination of simple and commonly available light microscopy and optical spectroscopies in general is a suitable and easy strategy to analyze heterogeneous nucleation processes directly in solution on a relevant statistical basis.