Silica (SiO2) is a versatile material. Our drinking water contains dissolved silica. 5 to 25 mg/L is the range of silica in natural waters. In some areas of the world it can be over 100mg/L. Beer also contains silica. Pure silica can be found in crystalline or amorphous forms. Amorphous silica has found many important applications. Several industrial production processes exist to produce amorphous silica of different sizes (ranging from nano- to millimeter), porosities and surface functionalities. Typically, when the particle size of silica is in the range of few nanometers it is called nanosilica (with a further distinction of P-type and S-type according to their structure).
Just like silica, a wide variety of other nanomaterials have also been developed under the umbrella of nanotechnology. These nanomaterials have shown great potential for a number of applications. However, all types of nanomaterials have some limitations. In most cases, nanomaterials are difficult to use without any surface modification. For example, magnetic nanoparticles and gold nanoparticles are difficult to suspend in water uniformly. Furthermore, in acidic environments metal oxide nanomaterials and quantum dots get unstable. Approximately, same problems occur with nanosilica.
Hybridization of one nanomaterial with another material helps in overcoming the aforementioned problems. Hybridization of nanomaterials has also helped in expanding their application areas. Here we present a brief overview of how hybridization of silica and other nanomaterials have helped us in developing interesting and commercially important nanohybrids. It is important to mention that covering all different hybridization techniques and nanohybrid materials is beyond the scope of this blog post.
Why Amorphous (Nano)silica?
- Surface of silica particles ‘mainly’ have silanol (-Si-OH) groups. Silanol groups are reactive under suitable conditions with some other chemical compounds. These silanol groups act as fixation sites for other compounds (sometimes called as dopants). The fixation of dopants on/in silica particles can be achieved by a chemical reaction or by physical adsorption.
- Amorphous (nano)silica has porosity which can be tailored. This porous structure helps in diffusion and distribution of dopants through out the silica particles.
- Silica (nano)particles are unlikely to absorb light (near infrared (NIR), visible and UV) so they are considered as transparent to light. They do not interfere with the magnetic field. This enables the dopants supported on/in silica particles to remain safe from the light and magnetic field.
- Silica is biocompatible and non-toxic. Therefore, it is use able in biomedical applications.
How Silica Nanohybrids are Prepared?
There are several methods to produce silica nanohybrids. Most commonly, the dopants (e.g., drug molecules, flourophores, photosensitizers, catalysts etc.) are immobilized inside the silica particles during the synthesis of silica particles.
Immobilization of the dopant on the silica nanohybrids can be also be done by reacting the silica nanoparticles. Sometimes prior to the immobilization of actual species, an anchoring molecule (e.g., amine, carboxyl groups etc.) is first chemically attached to the silica. Afterwards, the actual dopant is fixed onto the silica using the already attached anchoring molecule. This method of producing silica nanohybrids is less common.
The Stöber method and the reverse microemulsion methods are common techniques to prepare such nanohybrids.
Win-Win for the Matrix and the Dopant
The process of silica hybridization with other molecules is beneficial for both i.e., the dopant molecule and the silica support (support is sometimes also called matrix). Silica particles can become magnetically active, catalyst and achieve therapeutic ability when doped with the corresponding molecule.
The dopant molecules also get improvement in their properties. For example, quantum yield and photostability of doped flourophores is higher than the free (or non-doped) ones. The release rate of drugs supported inside the silica matrix can be regulated. Toxicity of metallic nanoparticles and quantum dots is reduced when supported inside the silica particles.
Silica nanomaterials can also be used as scaffolds for designing multifunctional nanomaterials
Supporting a molecule inside or on the silica nanoparticles generates molecule-silica nanohybrids. This fixation of molecules on silica is achieved either by the electrostatic interactions between the (positively charged) molecule and (negatively charged) silica particles or by covalent bonding between the molecule and the silica nanoparticles. Covalent bonding involves more chemical reactions (steps) but generates more stable nanohybrids.
Usually, this supporting (or doping) process does not influence significantly the properties of the individual molecule (i.e., the dopant). Since we get more molecules in one silica nanoparticle, the properties of the dopant can be enhanced in molecule-silica nanohybrids.
As compared to the non-supported molecules, their nanosilica hybrids have a more extensive array of applications.
‘Some application areas of molecule-silica nanohybdrids include cancer therapy, drug delivery, biosensing and bioimaging.’
Fluorescent molecules (like fluorescein, rhodamine, cyanine, Alexa dyes etc.) have proven to be revolutionary for the biological world. They are used as tags to analyse different biological processes under fluorescence microscope and to estimate trace amounts of biological analytes in living organisms. Fluorescence intensity and photostability of individual fluorophore molecules is low. Therefore, in the bulk state they are not very attractive for bioimaging and biosensing. But once they are supported on silica nanoparticles, their photostability, fluorescence and biocompatibility increase significantly (because of numerous molecules present on one silica particle). Such fluorophore-silica nanohybrids have proven revolutionary for making novel fluorescent tags. They have been tested in the determination of various analytes of various sizes (i.e., from small ions and molecules to macromolecules and cellular samples). Multiple bacterial cells were determined employing multicolored FRET (fluorescence resonance energy transfer) silica nanoparticles.
In chemiluminescence (CL), silica nanoparticles have also become a major support material. CL-producing species is concentrated inside (or on the surface of) silica nanoparticles which helps in getting more intense CL signals during applications like trace analysis. In CL, energy released during a chemical reaction excites the species which eliminates the need for a stable, intense light source (unlike photoluminescence). The reaction can be controlled to manipulate the luminescent signals related to analyte concentration.
CL has two major attractive features; i) low background signal and ii) simple instrumentation. The major problem is the low intensity of most CL signals and this limits the application of these molecules directly into trace analysis. Therefore, doping (or supporting) CL molecules on silica nanoparticles provides efficient luminescent probes which are used in sophisticated trace analysis. Doping luminol and [Ru(bpy)3]2+ on silica nanoparticles to produce chemiluminescent particles, for uses like sensor platforms and labels, have been demonstrated in the open literature.
Molecule-Silica Nanohybrids for Drug Delivery:
The field of drug delivery research is also heavily investigating silica nanoparticles as the suitable supports for various drugs and photodynamic therapy. To date, amorphous (nano)silica has proven to be non-toxic which is one of the major requirements for a material to be tested/used as support for drugs. Surface functionality and ability to encapsulate and release drug molecules at the desired rate are additional properties of (nano)silica which attracted researchers to test this material extensively in drug delivery research.
Drugs doped onto (or into) silica particles are not chemically bound to the support as the drug needs to be released once inside the body and at the right place under correct conditions. Therefore, physical trapping is typically used to fix the drug on/in silica particles. Sometimes chemical modification of silica nanoparticles, prior to impregnating the drug, is done. This is mainly because mostly the drugs are hydrophobic and, thus, have low solubility in water. Organic (i.e., silane) modification of silica particles enables the fixation of drugs on them.
Functional Nanomaterial-Silica Nanohybrids
As the name indicates, these are hybrid materials of nanometer size produced by combining (or doping) silica and some other material. Silica nanoparticles may serve as a matrix in such nanohybrids. Limitations of the individual nanomaterials are alleviated by the hybridization technique. Different applications of otherwise difficult to use nanomaterials have been made possible by their hybridization with silica. Catalysis, cancer therapy and bioimaging fields have benefited from such nanohybrids.
In this section we provide a brief overview of some selected functional nanomaterial-silica nanohybrids. Some of the nanohybrid materials like silica-metal nanocatalysts are not discussed here.
Nanohybrids of Quantum Dots & Silica:
These nanohybrids are created by shielding the quantum dots with silica layers. Quantum dots are nanocrystals of semiconductor materials with diameters in the range of 10-50 atoms (i.e., 2-10 nm). Depending upon the size, these nanocrystals can emit any colour of light upon exposure to a UV source, as shown in the Figure-1.
The luminosity of quantum dots is high and resistant to photobleaching. Based upon these properties they can be considered ideal for biosensing and bioimaging but due to the presence of heavy metals (e.g., cadmium, lead or zinc) they can be toxic. As a result, they need to be hybridized with materials like silica.
Nanohybrids of Magnetic Materials & Silica:
These are hybrids of magnetic nanoparticles and amorphous silica. In medical treatments and separation of target molecules from a mixture, magnetic nanoparticles have many applications. However, magnetic nanoparticles have the tendency to aggregate and react with other molecules around them limiting their applications. These problems are avoided by preparing nanohybrids of such magnetic nanoparticles with silica. Moreover, magnetic nanoparticles hybridized with silica can be functionalized with other molecules.
Thick silica shells reduce the inter-particle interactions and leads to super-paramagnetism. Saturation magnetization is also reduced by the presence of silica shell around magnetic nanoparticles. Particle size and blocking temperature also influence the characteristics of silica-magnetic nanohybrids.
Use of these nanohybrids has been demonstrated in microwave-assisted protein digestion, thermal therapy and as enhancement agents for magnetic resonance imaging (MRI). Other applications of silica-magnetic nanohybrids include catalyst removal, solid phase extraction and bioseparation due to their field induced transport. Furthermore, there have been efforts to develop these nanohybrids with multiple functions like drug delivery, fluorescence imaging, MRI and identification of target cancer cells.
Three major advantages of modifying quantum dots with silica include reduction in their toxicity (also the primary aim of silica coating on quantum dots), reduced degradation and possibility to functionalize with other molecules or biomacromolecules. Studies have shown that the quantum dots coated with silica shell around them are non-toxic.
Nanohybrids of Gold & Silica:
They are prepared by either having a silica core and shell of gold nanoparticles or vice versa. The particle size, shape and structure of both materials (silica and gold) influence the properties of the final nanohybrid. Hybridization of gold with silica helps in the adjustment of optical properties of gold. Configuration of such nanohybrids (i.e., whether the gold is the shell or silica) can be varied to depending upon the application.
Gold-silica nanohybrid materials find applications mainly in the areas of labeling and optical sensing. These nanohybrids have been shown to be very promising in photothermic therapy for treating the tumor cells. Having silica shell around gold nanoparticles avoids the agglomeration of gold nanoparticles. Such stabilized silica-gold nanoparticles can find applications in surface enhanced Raman scattering (SERS) analysis but to date the application of silica-gold nanohybrids is limited in SERS analysis.
Finally, it can be concluded that hybridization extends the range of application of nanomaterials. The hybrid nanomaterials of amorphous silica with other molecules and nanomaterials makeup an important class of commercially attractive materials. Current research in the field of hybridization is partially focused on developing nanohybrids of amorphous silica and other nanomaterials suitable for multiple functions.