Surface Functionalization of Agglomerating Nanoparticles

　We propose a new method for surface chemistry of nanomaterials agglomerating in solvents. Crushing ability of powerful ultrasonication gets much stronger in the presence of hard ceramic beads than an ultrasonic treatment only. This bead-assisted sonic disintegration (BASD), which is likely caused by collisions between the beads propelled by ultrasonic cavitation, has a crushing capacity of equivalent or more than stirred media milling. The largest difference from stirred media milling is the simplicity of the device. Indeed, it can be easily integrated in the conventional glasswares for wet chemistry (right: Functionalization of agglomerating nanoparticles using BASD. The deagglomeration process exposes fresh surfaces of primary particles to the coupling agents), and allows both breaking up of persistent agglomerates and surface functionalisation at the same time. This new technique is expected to work for nanoparticles like as a magnetic stirring bar for soluble compounds.

Specific surface area of a particle becomes larger with reduction of size. This gives rise to the increasing influence of the surface character on the particle, and therefore, which turns prominent at nanoscale. On the usage of such a substance, surface properties are as important as the properties of the particle core. Full use of the synthetic techniques for functionalization of nanomaterials enables a variety of applications as nanofluids, nanomedicines, biocomposite materials, assembled devices, and so on.

  Reality, however, is not that simple. Unlike conventional compounds, nanoparticles are usually insoluble in any solvents. Homogeneous functionalization is limited to rare cases, where particles stay dispersed throughout reactions. This general behavior had been preventing surface chemistry of nanomaterials from remarkable development.

  Nanoparticles generally form agglomerates in solvents. Those particles sitting in agglomerates are kept free from reactions with any coupling agents. In order to attain a homogeneous chemical reaction, agglomerates must be broken apart to expose the interior surfaces. BASD enables it. In the present study, we succeeded to demonstrate deagglomeration and simultaneous functionalization of persistent nanodiamond agglomerates (left: Two different functionalization of primary particles performed on agglomerates of nanodiamonds. Condensation reaction gave silanization, whereas arylation was carried out using diazonium salts), which are unable to be deagglomerated in the absence of beads. Functionalization of deagglomerated particles was verified in two different reaction systems.

Left: Silylated nanodiamonds in THF. Nanodiamonds were sylilated using the commonly used magnetic stirring bar (left), ultra sonication (center), and BASD (right). Only the ones with BASD disperse well in THF, implying silylated surfaces of the primary particles.