Synthesis, Transformation, and Utilization of Monodispersed Colloidal Spheres (2019)

Colloidal particles with a spherical shape and diameters in the range of 0.01-1 μm have been a subject of extensive research, with applications in areas such as photonics, electronics, catalysis, drug delivery, and medicine. For most of these applications, it is of critical importance to achieve monodispersity for the size while expanding the diversity in terms of structure and composition. The uniformity in size allows one to establish rigorous correlations between this parameter and the physicochemical properties of the colloidal particles while ensuring experimental repeatability and measurement accuracy. On the other hand, the diversity in structure and composition offers additional handles for tailoring the properties. By switching from the conventional plain, solid structure to a core-shell, hollow, porous, or Janus structure, it offers immediate advantages and creates new opportunities, especially in the context of self-assembly, encapsulation, and controlled release. As for composition, monodispersed colloidal spheres were traditionally limited to amorphous materials such as polystyrene and silica. For metals and semiconducting materials, which are more valuable to applications in photonics, electronics, and catalysis, they tend to crystallize and thus grow anisotropically into nonspherical shapes, especially when their sizes pass 0.1 μm. Taken together, it is no wonder why chemical synthesis of monodispersed colloidal spheres has been a constant theme of research in areas such as colloidal science, materials chemistry, materials science, and soft matter. In this Account, we summarize our efforts over the past two decades in developing solution-phase methods for the facile synthesis of colloidal spheres that are uniform in size, together with a broad range of compositions (including metals and semiconductors) and structures (e.g., solid, core-shell, hollow, porous, and Janus, among others). We start with the synthesis of monodispersed colloidal spheres made of semiconductors, metals with low melting points, and precious metals. Through chemical reactions, these colloidal spheres can be transformed into core-shell or hollow structures with new compositions and properties. Next, we discuss the synthesis of colloidal spheres with a Janus structure while taking a pseudospherical shape. Specifically, metal-polymer hybrid particles composed of one metal nanoparticle partially embedded in the surface of a polymer sphere can be produced through precipitation polymerization in the presence of metal seed. With these Janus particles serving as templates, other types of Janus structures such as hollow spheres with a single hole in the surface can be obtained via site-selected deposition. Alternatively, such hollow spheres can be fabricated through a physical transformation process that involves swelling of polymer spheres, followed by freeze-drying. All these synthesis and transformation processes are solution-based, offering flexibility and potential for large-scale production. At the end, we highlight some of the applications enabled by these colloidal spheres, including fabrication of photonic devices, encapsulation, and controlled release for nanomedicine.