Resorcinol-formaldehyde-assisted dissolution-regrowth strategy for synthesis of hollow silica nanoparticles with tunable morphology

A surfactant-free, resorcinol-formaldehyde (RF)-assisted dissolution-regrowth strategy has been reported to prepare hollow silica nanoparticles with various morphologies. RF, as a pore-forming agent, is introduced into a typical stöber system (ethanol-water-ammonia) to achieve the purpose of pore-forming and as a nanoplatform. As the reaction progresses, the initially formed RF core gradually dissolves and then slowly deposits on the silica shell formed by the outer layer, resulting in the formation of a hollow structure. The RF deposited in the silica shell is then further removed by calcination in air, and a silica shell layer with a rough surface can be successfully obtained. More importantly, when additional phenolic is added to the reaction system again at the desired time, the hollow nanostructures with open pores can also be prepared. The formation of open channels is mainly attributed to the removal of RF in the SiO₂/RF composite shell by calcination. Other experimental parameters such as ethanol-to-water ratio, ammonia concentration, and reaction temperature are also investigated. The internal cavity can be precisely adjusted from 100 to 175 nm, and the thickness of the silica shell can be tuned from 25 to 60 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are employed to characterize the morphology, structure, and evolution process of the resulting hollow silica nanoparticles. A possible mechanism to account for the formation of hollow silica nanoparticles with different morphologies is also proposed. In addition, the adsorption of bovine serum albumin (BSA) by hollow silica nanoparticles with rough surface and mesoporous structure is also performed, and the results show that hollow mesoporous silica nanoparticles are more effective for the immobilization of BSA.