Sol-gel Silica Nanoparticles in Medicine: A Natural Choice. Design, Synthesis and Products

Abstract

Silica is one of the most abundant minerals in the Earth's crust, and over time it has been introduced first into human life and later into engineering. Silica is present in the food chain and in the human body. As a biomaterial, silica is widely used in dentistry, orthopedics, and dermatology. Recently amorphous sol-gel SiO₂ nanoparticles (NPs) have appeared as nanocarriers in a wide range of medical applications, namely in drug/gene target delivery and imaging diagnosis, where they stand out for their high biocompatibility, hydrophilicity, enormous flexibility for surface modification with a high payload capacity, and prolonged blood circulation time. The sol-gel process is an extremely versatile bottom-up methodology used in the synthesis of silica NPs, offering a great variety of chemical possibilities, such as high homogeneity and purity, along with full scale pH processing. By introducing organic functional groups or surfactants during the sol-gel process, ORMOSIL NPs or mesoporous NPs are produced. Colloidal route, biomimetic synthesis, solution route and template synthesis (the main sol-gel methods to produce monosized silica nanoparticles) are compared and discussed. This short review goes over some of the emerging approaches in the field of non-porous sol-gel silica NPs aiming at medical applications, centered on the syntheses processes used.

Keywords: LaMer; Stöber; biogeochemical cycle; nanoparticles; reverse emulsion; silica; sol-gel.

Figure 1

Engineered multifunctional NP.

Figure 2

The diatomite deposition in the Mediterranean region. Reprint with permission from [51].

Figure 3

Leaf blast symptoms in rice after inoculated with Magnaporthe grisea for 10 days. Rice plants were continuously treated with (+Si) or without silicon (−Si) (A). Silica layer was formed in the cell wall of Si-treated plants and enhanced plant resistance to fungi infection by physical barriers (B). Reprint with permission from [57].

Figure 4

Major organ sites of NPs localization.

Figure 5

Sol-gel silica NPs growth through Ostwald ripening: (a) initial and (b) final stages.

Figure 6

Photomicrographs of diatoms through Scanning Electron Microscope. Reproduce with permission from [101].

Figure 7

Sol-gel products diversity: the right choice of catalyst, pH value, water to silica precursor’s ratio, type of solvent and solvent to water ratio, type of silicon precursor, presence of chelating agent, and temperature, allow the control of SiO₂ topology.

Figure 8

NPs sol-gel Stöber synthesis method.

Figure 9

NPs sol-gel reverse emulsion synthesis method.

Figure 10

Examples of sol-gel NPs core-shells possibilities (from left-to-right, up-to-bottom): hollow-spheres (core, core-shell and hollow sphere); nanostructure mesoporous spheres; LIPOSIL structure; SPION-core-silica-shell and ORMOSIL NPs.

Figure 11

Sol-gel mesoporous silica NPs from in situ replica of self-assembled molecular aggregates.