Silica Nanoparticles

Silica nanoparticles are mesopores (2- to 50-nm pores) of silica that display unique physicochemical properties [17]. These nanocarriers can be prepared in a variety of sizes and shapes including nanohelices, nanotubes, nanozigzags, and nanoribbons. With tunable optical, electrical, and mechanical properties, these particles are applied in catalysis, PDT, adsorption, separation, diagnostic sensors, and therapeutic drug delivery [18].

Silica nanoparticles are synthesized with surfactants like cetyltrimethylammonium bromide as templates (or structure-directing agents) and tetraethyl orthosilicate or sodium metasilicate (Na2SiOg) as the silica precursors. With the deep understanding of the sol—gel techniques, a variety of nanoparticles with precise shapes and sizes were developed to meet the growing need for multifunctional nanosystems [19]. Although by nature silica particles are hydrophilic in nature, this material can be surface modified with silanol groups (~SiOH) to add required functionalization with basic silane chemistry [17].

Organic silica is widely recommended as a supplement and a therapeutic agent in amelioration of bone- and skin-related conditions. Small size and positive surface charge of organic silica-based nanoparticles render them an ideal platform for drug and gene therapy where negatively charged membrane enhances cellular uptake. These materials are also biocompatible with low toxicity. The mesoporous particle surface enables controlled delivery of the therapeutic agent. Fluorophore-doped organic silica nanoparticles are well known as optical sensors [20]. Photoluminescence properties along with the ability to entrap drug (either hydrophilic or hydrophobic) can be tuned by changing the drug/dye type. Although fluorophore-doped organic silica nanoparticles are attractive as theranostic agents, functionalization of the surface for tumor cell targeting has widened their application as biosensors with very low cytotoxicity [3,21].

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