Controllable preparation of homogeneous nano-silica using 20 kHz ultrasonic microfluidic technology

Abstract

Uniform nano-silica, especially below 100 nm, has been widely used in the biomedical field due to its excellent physical and chemical properties. However, reducing particle size below 100 nm, reaction efficiency and optimizing monodispersity remain significant challenges for the traditional Stöber method, common 20 kHz ultrasonic process, as well as microchannel technology. A 20 kHz ultrasonic microreactor (USMR) was applied to improve the traditional Stöber method to meet the above challenges in this study. The effects of ammonia dosage, tetraethyl silicate (TEOS) dosage, USMR power, and reaction residence time on the hydrodynamic particle size (PS) and polydispersity index (PDI) of nano-silica were investigated. The morphology, elemental analysis, thermogravimetric analysis, and crystal structure of nano-silica were characterized by scanning electron microscope, energy dispersive X-ray spectrometer, thermogravimetric analyzer, and X-ray diffractometer, respectively. As a result, the prepared nano-silica particles are regularly spherical with uniform PS. The USMR power and residence time both affect the PS and morphology to some content. More importantly, the PS and the morphology of nano-silica were dominated by the dosage of ammonia and the dosage of TEOS, respectively. Under optimal conditions, the hydrodynamic PS of nano-silica prepared was 100 nm, while PDI reached 0.069, and the corresponding PS of dry silica was measured to be 55.1 ± 2.1 nm by using SEM. The results show that the PS and uniformity of spherical nano-silica can be controlled by reaction conditions using 20 kHz ultrasonic microfluidics technology.