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Silicon dioxide nanoparticles, also known as silica nanoparticles, are promising for biological applications owing to their excellent biocompatibility, low toxicity, thermal stability, facile synthetic route, and large-scale synthetic availability. The particle size, crystallinity, porosity, and shape can be precisely manipulated, enabling the silica nanoparticles for various applications. Moreover, numerous available surface modifications of silica nanoparticles permit their control of surface chemistry to achieve drug loading, good dispensability, and site-specific targeting. These properties, if combined and developed appropriately, make silica nanoparticles a platform for biomedical imaging, detecting, therapeutic delivery, monitoring, and ablative therapies. With the design of diverse dopants, surface functional groups, and assembly techniques, multifunctional nanoparticles can be developed with theranostic applications. Silica nanoparticles have also widely applied in other areas such as energy source, electronic, sensor, and catalysis purposes.
Silica nanoparticles are divided into P-type and S-type based on their structure. The former nanoparticles are characterized by numerous nanopores featuring a pore rate of 0.61 ml/g. The latter nanoparticles have a relatively smaller surface area. In contrast to the S-type, the P-type silica nanoparticles manifest a higher ultraviolet reflectivity.
The silica nanoparticles are fabricated via the condensation of silanes to form nanoparticles composed of an amorphous network of silicon and oxygen. The nanoparticles are monodisperse with high stability, and the nanoparticles have narrow size distributions. The density of the nanoparticles is approximately 2 g/cm-3 slightly affected by the degree of condensation. The refractive index is determined to be 1.43. The nanoparticles are well-dispersed in polar solvents like water and ethanol. The nanoparticles can be converted into hydrophobic ones through coupling different silanes to the nanoparticle surface.
1. Protein adsorption and separation
Owing to the facile preparation, low cost, high specific surface area, and availability of diverse surface functionalization, silica nanoparticles are ideal for specific protein adsorption and separation. For example, Ester-functionalized polypyrrole silica nanoparticles have the ability to bind covalently with human serum albumin (HSA) protein. The resulting HAS functionalized silica nanoparticle can form flocculation when incubating with anti-HAS, and thus diagnostic assays and biosensors can be developed based on the silica nanoparticles.
Recent techniques enable the silica nanoparticle to serve as a solid medium for protein immobilization. On the other hand, silica-coated iron oxide nanoparticles have been utilized for protein binding and separation, taking the advantage of their magnetic properties which afford a simple and fast approach for separation. In this case, the iron oxide magnetic core can respond to external magnetic field, and used for fast particle separation, while the silica shell offers biocompatibility, stability, and a platform for protein entrapment. Such well-designed nanostructure system is very useful for enzyme immobilization, bioseparation, biosensors, and immunoassays.
2. Nucleic acid detection and purification
Silica nanoparticles have also been used for DNA detection, separation and purification. The adsorption of DNA onto the surface of silica nanoparticles is mainly influenced by three effects: weak electrostatic repulsion forces, dehydration, and hydrogen bond formation. Silica nanoparticles are also designed as DNA biosensors by means of their functionalization with oligonucleotides by hybridization with target complementary DNA or RNA probes to obtain variable fluorescent intensity. Just similar to protein separation, silica-coated magnetic nanoparticles are also employed to extract DNA from biological samples.
3. Drug and gene delivery
Mesoporous nanoparticles and surface functionalized nanoparticles are two different types of silica nanoparticles applied as a carrier for drugs and genes delivery. The pores in the nanoparticles provide sites to keep drug molecules, and addition agents like gold nanoparticles, are required as caps to close the pores. To release the drugs, certain molecules that can break the covalent and open the pores are needed. Functionalized silica nanoparticles require good affinity to conjugate with genes. As DNA molecules are negatively charged, thus the silica nanoparticles have to be prepared with positively charged prosperity. Weak interactions are preferable between the functionalized silica nanoparticles and the genes ensure the release of genes.
4. Imaging contrast agents construction
Silica nanoparticles play an important auxiliary role in medical imaging, and they are utilized to encapsulate contrast agents particles, such as organic dyes, quantum dots, gold nanoparticles, iron oxide. Silica nanoparticles are capable of offering outstanding biocompatibility, low cytotoxicity, controllable size with a narrow distribution, and considerable options of surface functionalization. In addition, the silica nanoparticles can be developed as multifunctional tools featured with contrast agents, and drug/gene/protein delivery.