4. Bottom-Up Nanomaterial Transformation
Further advantages of the VFD include its ability to create self-assembly structures such as micelles44, mesoporous nanomaterials45,46, hydrogels47, fullerene and its composites48,33,29 and vesicles49and the potential to control the shape and size of NPs during formation and for regulating the thickness and strength of coating at the nanoscale.
The VFD has also been utilized as an innovative strategy for preparing mesoporous silica prior to the required calcining to remove the polymer template44. Tong et al . reported that the processing time of the VFD was less than an hour, with 5 hours being the overall duration for the synthesis involving 60 mL of solution, contrasting to the 48 hours required for the traditional materials involving hydrothermal batch processing45. Moreover, the VFD process operated at ambient pressure and temperature, significantly reducing energy consumption, and remarkably the pore size can now be controlled to fine-tune the properties of mesoporous silica, Figure 6. This is under continuous flow VFD processing such that there is potential for scaling up the synthesis of the pre-calcined material with uniformity of the product along with controlled pore wall thickness and pore size. The latter relates to the micelles being stretched by the shear forces in the thin film within the VFD. Consequentially, the increase in pore wall thickness increased the thermal stability of the calcines material, which is essential for high-temperature applications.
Tong et al . also developed an effective method for the nucleation and growth of palladium NPs in situ in the above micelle templated mesoporous silica synthesis, also as a continuous flow VFD process, then for calcining under batch processing46. The Pd/SBA-15 composite calcined material is effective in a wastewater treatment process for removing nitrate-nitrogen, Figure 6, with the maximum adsorption value for nitrate-nitrogen at 9.5 mgL-1 after 16 hours, upwards of 41% removal efficiency, with 36% removal efficiency upon recycling. Because Pd/SBA-15 is easily separated from the effluent, the relatively low nitrate-nitrogen adsorption is significant. Of particular note is that the Pd NPs in the composite material are entirely embedded within the pores rather than on the surface of the mesoporous silica particles, which is important in avoiding leaching of the NPs and for taking advantage of any catalytic process being enhanced within the confined space within the pores.
Silica hydrogel is readily formed in the VFD in water under continuous flow as a benign process, without the need for organic solvents or any other acids or bases, with significantly reduced processing times relative to batch processing47. Curcumin nanoparticles were incorporated into the silica in situ through the same processing, with the silica hydrogel converting to an xerogel 3.5 hours post VFD processing. The functional composite material showed improved inhibition of bacterial growth compared to bulk curcumin, Figure 6. Here the xerogel silica network provides shielding for the curcumin particles for then slow leaching of the curcumin which becomes bioavailable. The xerogel silica has potential for drug delivery applications, noting that the overall process of incorporating drugs in situ is simple, without the need for auxiliary substances.