Review
Amorphous silica nanohybrids: Synthesis, properties and applications

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Abstract

Hybridized nanomaterials have been extensively investigated due to their superior properties over individual nanomaterials and molecules. Amorphous silica nanoparticles are often employed as a matrix or carrier, along with a functional component, to form a silica-based nanohybrid. The functional component can be a molecule or another type of nanomaterial. These nanohybrids combine the advantages employing both silica and the functional component. So far, a variety of applications of such nanohybrids has been reported. In this review, we have covered several major types of silica nanohybrids. The functional components include regular fluorophores, chemiluminescent molecules, drug molecules, quantum dots, gold nanomaterials, magnetic nanoparticles and nanocatalysts. The synthesis strategies, properties and potential applications of each silica nanohybrid are discussed in detail. A conclusion is drawn based on the current progress and future perspectives of the silica nanohybrids.

Introduction

The rapid development of nanoscience and nanotechnology has produced a wide variety of novel nanomaterials. These nanomaterials overcome some of the limitations of bulk materials and have demonstrated great potential for various applications. However, each type of nanomaterial has its own drawbacks and limitations. Thus, direct use of the nanomaterials, without modification or functionalization, can be problematic. For example, most nanomaterials, including gold nanomaterials, magnetic nanoparticles and quantum dots, are difficult to directly and uniformly suspend in aqueous solutions. Furthermore, quantum dots and metal oxide nanomaterials become unstable in acidic environments. One option to overcome these limitations is to coat these nanomaterials with a more stable and physically adaptive material. Pure amorphous silica nanomaterials suffer from similar limitations and are also rarely employed alone for practical applications. However, when amorphous silica nanomaterials are hybridized with functional molecules or another nanomaterial, their applications extend broadly to bioimaging, biosensing, drug delivery, cancer therapy and catalysis.

Silica nanomaterials have several important properties that make them a unique matrix for incorporating functional components. First, the high porosity of amorphous silica nanoparticles provides the three dimensional space required for the doping of functional components, known as the dopants. The porosity is sufficiently adjustable to hold small molecules or large nanomaterials. The dopants can be easily embedded inside a silica shell or attached on a silica nanoparticle surface through chemical binding or physical adsorption. Second, silica nanomaterials are effectively “transparent”. They are unlikely to absorb light in the near-infrared, visible and ultraviolet regions or to interfere with magnetic fields, which allows the dopants inside silica matrix to keep their original optical and magnetic properties. Third, the silica matrices are nontoxic and biocompatible for biomedical research. Finally, the well-established silica chemistry facilitates the modification of silica-based nanohybrids.

Silica nanohybrids are formed by a variety of methods. Functional molecules, such as fluorophores, drug molecules and photosensitizers, are most often immobilized inside the silica during the synthesis of silica matrix. Less commonly, functional molecules are immobilized on the surface of silica nanoparticles. The decoration step, in which functional molecules are immobilized, follows the formation of the pure silica nanoparticles. The surfaces of silica nanoparticles are usually functionalized with amine or carboxyl groups prior to the decoration.

The introduction of functionalized materials into silica nanoparticles adds new properties to the hosts, such as fluorescence, magnetism, therapeutic ability and catalytic function. Furthermore, silica nanoparticles not only provide shelters for dopants, but also enhance or ameliorate their intrinsic properties. For example, entrapped fluorophores exhibit a higher quantum yield and a stronger photostability than free ones. The release rate of drug molecules can be regulated once loaded inside the silica matrix. The toxicities of quantum dots and metal nanoparticles are suppressed under the protection of silica shells. Moreover, silica nanomaterials are a good scaffold for designing multifunctional nanomaterials which can accomplish multiple tasks simultaneously.

Functionalized silica nanomaterials have been summarized in previous reviews [1], [2], [3], [4], [5]. The emphasis of this article is the hybridization of silica nanoparticles with other functional nanomaterials and molecules. In each section, the synthesis of the silica nanohybrids is summarized at the beginning, followed by their unique properties and applications. Finally, a perspective of likely future directions of silica nanohybrids is provided.

Section snippets

Molecule–silica nanohybrids

A wide variety of functional molecules have been doped into silica nanoparticles to form molecule–silica nanohybrids. The associations between molecules and silica matrices are varied. A common linking force is electrostatic interaction between the negatively charged silica matrix and positively charged functional molecules. Covalent binding is frequently employed as well, which provides more stable hybrids but requires additional chemical reactions. The properties of individual functional

Functional nanomaterial-silica nanohybrids

Silica nanoparticles have been widely used as a matrix in which other functional nanomaterials are doped. Hybridization of the two types of nanomaterials overcomes some limitations of the individual nanomaterials and ultimately helps to bring nanotechnology from laboratorial studies to practical applications. Up to this point, such silica nanohybrids have had some impact in the fields of bioimaging, cancer therapy and catalysis. In this section, several functional silica nanohybrids will be

Conclusions

In summary, amorphous silica nanoparticles are an excellent matrix for hybridization with functional molecules and nanomaterials. Commonly used fluorescent molecules and drugs can be doped inside the silica nanomatrix. The resultant nanohybrids exhibit enhanced abilities over the pure molecules. Commercially available functional nanomaterials including quantum dots, gold nanomaterials, magnetic nanoparticles and nanocatalysts, can also form nanohybrids with a silica matrix. These nanohybrids

Acknowledgments

The work was supported by the National Science Foundation under grant CHE-0616878 and REU grant CHE -0552762, the U.S. Department of Energy under grant DE-FG02-6ER46292), a University of North Dakota Doctoral Dissertation Assistantship and North Dakota Water Resource Research Institute Graduate Student Assistantship, and seed awards from the North Dakota EPSCoR. Neither the United States Government nor the agency thereof, nor any of the employees, makes any warranty, express or implied, or

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