Microparticles and nanoparticles are a unique class of materials with enormous technological potential in energy, imaging, medical, and environmental applications. Nanoparticles are defined to have at least one physical dimension less than 100 nanometers. Microparticles have a physical dimension between approximately 1 and 1000 micrometers. Despite having the same composition as the corresponding bulk material, due to size effects, these particles display exceptional optical, electrical, thermal, and magnetic characteristics. Researchers have developed methods of synthesis to further control the properties, shape, composition, and size distribution to better suit specific applications.
Microparticle and nanoparticle synthesis is typically achieved by physical and chemical methods. In physical methods, particles are created by reducing the size of the source material, a so-called top-down approach to microfabrication and nanofabrication. Physical techniques include milling, gas condensation, electro-spraying, lithography, and thermal decomposition. In many chemical methods, particles are created by nucleating and growing particles from atomic or molecular precursors normally in the liquid or vapor phase of a chemical reaction, a so-called bottom-up approach. Chemical methods for synthesis of microparticles and nanoparticles include microemulsion, hydrothermal, microfluidic, chemical vapor, pyrolysis, and sol-gel processes. Chemical synthesis of nanoparticles produces nanostructures with less defects, provides access to more complex and homogeneous chemical compositions, and is easily scalable for low-cost and rapid fabrication.
Since these techniques are often labor-intensive and result in toxic byproducts, biological methods or green nanoparticle synthesis methods have emerged, such as biogenesis with microorganisms and plant extracts. These sustainable methods produce non-toxic, eco-friendly particles suitable for biomedical and environmental applications.