Yi Qiao

Silica nanoparticle dispersion size measurement using dielectrophoresis on a microfabricated electrode array

In this article, we report a technique that uses dielectrophoresis to measure particle size distribution information of silica nanoparticle dispersions using a microfabricated periodic interdigitated electrode array. An AC voltage is applied to the electrode array, producing a non-uniform electric field. Depending on their relative permittivity with respect to the dispersion solution, nanoparticles aggregate at either electric field maxima or minima due to dielectrophoresis, forming a periodic density grating. We probe the nanoparticle density grating with a laser beam to generate a diffraction pattern, and then monitor how fast the nanoparticle density grating decays due to diffusion after the electric field is turned off. Particle size information is derived from the diffusion rate. The advantages of the technique include: a) able to operate over a wide range of concentrations and purity levels, b) relatively insensitive to outlier particles in the tail ends of the size distribution, and c) relatively fast (on the order of seconds) measurement response. These characteristics make the method suitable for industrial samples and real time process monitoring.

Silica nanoparticle inline size measurement using refractive index gradient in a microfluidic cell

In this article, we report a technique that can measure the size of silica nanoparticles in a microfluidic Y-Cell. In this technique, silica nanoparticle dispersion and a buffer solution are pumped through a micro-fabricated microfluidic Y-Cell, in which these two solutions form laminar flows and nanoparticles diffuse into the buffer due to the concentration gradient. The diffusion of nanoparticles into buffer causes a refractive index gradient across the boundary between nanoparticle dispersion and buffer. The refractive index gradients at different positions of the boundary can be measured by optical method and this information is used to calculate the nanoparticle diffusion coefficient, which is then used to calculate the nanoparticle size. We have demonstrated this technique with 5nm and 20nm silica nanoparticles. This technique is relatively fast and easy to implement, and proves to be an ideal candidate for inline process monitoring applications where fast and qualitative measurement is desired.