Advanced Characterization Approaches for Anisotropic Metal Patchy Particles

Plasmon resonant nanoparticles have garnered significant interest in recent decades due to their potential as sensing or marker particles in various applications. Particularly, anisotropic plasmonic particles offer the advantage of highly tunable optical properties within a particle system. However, the increased number of shape parameters compared to isotropic particles, challenges for thorough particle characterization. Precise descriptions of particle shape and product yield are, nevertheless, indispensable for product optimization, which requires, for example, particle growth models. Therefore, in this contribution we present a variety of characterization methods used to extract information from our anisotropic particle system, obtaining the desired parameters on different levels.<br/>We have developed several continuous flow processes for synthesizing gold and silver patchy particles whereby dielectric core particles are coated with thin islands of metal. The size, shape and morphology of the patches determine the optical resonance position which can be tuned over the visible and near-infrared spectral range. The synthesis is performed in a seed-mediated two-step process. Understanding the influence of seed particles on the growth of patches with different dimensions and morphologies is crucial. To this end, we will show how newly developed 4D-STEM analysis can resolve individual crystal grains’ orientations and probe the influence of seed particles on the crystal growth directions in the patch.<br/>On the whole patch scale, coating dimensions and morphology must be thoroughly characterized to obtain quantitative size parameters. These are essential for optical simulations and the development of patch growth models. STEM analysis and electron tomography are indispensable for revealing the patch structures where SEM imaging fails due to resolution limitations. Here we will show how these techniques can be applied to quenched samples, in which the growth has been “frozen” during patch formation, thus providing insight into the growth process. Additionally, patch shape degradation after synthesis, observed as changes in the optical spectra, can only be resolved through TEM imaging. Here, 3D data obtained from electron tomography can further be used as direct input for optical simulations to compare the obtained results with experimental spectra.<br/>At the overall particle level, it is important to note that not every particle is coated with a patch. Determining the so-called patch yield is crucial for optimizing process efficiency. In this regard, we demonstrate the use of different methods to identify the patch yield of the product. Firstly, single particle scattering and extinction, a novel technique derived from flow cytometry, can distinguish between bare and gold-coated polystyrene particles due to their different optical responses. Secondly, analytical ultracentrifugation, as an ensemble measurement, is used to determine product yield based on differences in particle density and sedimentation velocity.<br/>The diverse characterization methods we demonstrate for patchy particles enhance our understanding of their complex properties and synthesis. These methodologies, adaptable to other anisotropic systems, are expected to be pivotal for future advancements in plasmonic nanoparticle applications.