Core selective etching

Core etching of metallic nanoparticles Here, the bigger metallic nanoparticles are etched into smaller clusters by selected molecules. 

The modification of the optical fiber involves two major steps, for chemical sensor application [19, 20] (1) removal of the passive cladding (fiber etching) and (2) application of active cladding (fiber coating). The etching and coating processes are explained next. An all-silica MM fiber with core/cladding/jacket dimension of 105/125/250 pm was selected for such application. 

There are a variety of materials that can be used as sacrificial cores. Inorganic sacrificial materials include Si02 and metals such as aluminum, " titanium, and nickel. Polymers such as PI, PMMA, PC, and photoresist have also been used as sacrificial materials. After deposition of the cover film, removal of the sacrificial layer can be achieved by dissolution, etching, or thermal degradation. These removal methods each have benefits and drawbacks selection of the optimal approach is specific to particular combinations of substrate, sacrificial layer, and cover film 73, 3 Recently Whitesides and coworkers " implemented a fabrication method using water-soluble sacrificial cores. Poly(acrylic acid) and dextran proved to be effective sacrificial layers that could be dissolved in water or aqueous NaCl, for making metallic microstructures by nickel electrodeposition. 

The assembly of such core-shell particles, both in two and three dimensions, further enables tailored complex structures to be created with new properties. The formation of even more complex structures can be accomplished by making use of the semipermeable nature of the silica shell. The slow diffusion of reagents through the shell allows selective core etching and chemical conversion to be effected. The kinetics of such reactions are controlled by the porosity and the thickness of the shell, so that the cores can be made resistant to almost any chemical reagent. 

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