(103d) Synthesis of Porous Silica with Unique Morphology in Narrow Passages Using the Ice Templating Method

Recently, the authors have introduced the ?Ice Templating? method, a method which allows the synthesis of nano-structured porous materials with unique morphology, such as microfibers and monolithic microhoneycombs. In this method, materials are synthesized by freezing their parent hydrosol or hydrogel unidirectionally. Ice crystals which grow within the precursor during freezing act as the template, therefore unlike other methods which use sacrificial templates to mold materials to have a desired morphology, the template is extremely inexpensive. Moreover, the template can be easily removed just by simply thawing and drying the material.

In this work, the authors attempted to synthesize porous silica with unique morphology in narrow passages using the ice templating method. It was found that microfibers and monolithic microhoneycombs similar to those obtained in larger passages can be synthesized. This indicates that porous materials which diffusion paths within them are extremely short, and which do not cause severe pressure drops when fluids are passed through them, can be introduced into narrow passages using the ice templating method. Such materials are beneficial to microdevices as when conventional porous materials are used in narrow passages, short diffusion paths and low pressure drops are usually not compatible. However, it was found that in certain cases, the morphology of the resulting material was significantly different from those obtained in larger passages. The high surface area to volume ratio of narrow passages is thought to affect the manner of heat removal during freezing. This difference is thought to be significant when the walls of the passages easily conduct heat.

The ice templating method can be easily applied to the synthesis of a microdevice for Solid Phase Dynamic Extraction (SPDE). SPDE is used to analyze trace level amounts of analytes included in various fluids. In this method, first, the target fluid is passed through the needle type microdevice. In this process, the analytes included in the target fluid are concentrated in the microdevice. Next, the microdevice is set to a syringe filled with a small amount of gas for desorption, and then this gas is injected via the microdevice into a gas chromatograph (GC). The concentrated analytes desorb by the heat from the injection port, and enter the column of the GC along with the desorption gas. Therefore, accurate analysis of the analytes can be conducted quite simply using this needle device.

The microdevice used in this method is basically a syringe needle including porous materials. Fast adsorption/desorption rates and a high adsorption capacity is required in the porous materials used in such devices. Moreover, the porous material should not cause severe pressure drops upon usage, therefore porous material obtained through ice templating are thought to be ideal for this purpose. It was found that microdevices synthesized using the ice templating method efficiently concentrate trace level substances although the sampling time is generally extremely short.