Physicists have predicted and confirmed that metal nanoparticles (NPs) with a diameter between 1 and 10 nm can display electronic structures that reflect the electronic band structure of the NPs and can exhibit extraordinary optical properties. These physicists have focused on developing silica nanotube (NT) membranes for the size-cut-off sieving of noble metal NPs and have proposed a design for the potential application of separated ultrafine NPs, for example in cancer diagnosis and treatment. In our current studies, we have been aiming to control the functionality of the mesofilter design for size-exclusion-cut-off separation. We have focused on successful and current designs for the development of silica (NT) membranes as promising filter candidates for the size-exclusive separation of ultrafine noble metal NPs. The schematic design of nanofilters is also highlighted in relation to engineering dense silica NTs into anodic alumina membrane (AAM) nanochannels. The multifunctional surface coating of AAM pore channels with organic coupling agents facilitates the production of extremely robust membrane sequences without air gaps between the NTs. This practical nanofilter design is capable of producing controlled homogeneous ultrafine noble metal NPs with preferred spherical morphology. Advancements in controlling the simple, scalable fabrication processes of ultrafine noble metal NPs are significant for designed applications in biotechnology, especially for in vivo diagnosis and treatment of cancer, because of their superior surface-to-volume ratio and smaller size (4.5 nm), which is less than the limiting hydrodynamic diameter (5.5 nm) for effective renal clearance from the human body.
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