Find answers to common questions about silica nanoparticle structure, concentration, storage, solvent transfer, fluorescent labeling, applications, and amine-functionalized surfaces. For deeper technical information, explore Silica Nanoparticle Physical Properties and Silica Surface Chemistry.
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General Silica Nanoparticle Questions
Are silica nanospheres amorphous or crystalline?
nanoComposix silica nanospheres are amorphous. Colloidal silica is non-crystalline, meaning the atoms do not exhibit long-range order, similar to the atomic structure of bulk glass.
Learn more about silica structure, density, porosity, and particle morphology in Silica Nanoparticle Physical Properties.
How can I concentrate a sample of small silica nanoparticles?
Small silica nanoparticles, particularly particles ≤50 nm, can be difficult to collect by centrifugation. Very high centrifugal forces may be required, and the process can sometimes result in irreversible particle aggregation.
For larger silica particles, solvent can often be removed through evaporation with mild heating. Small particles require more caution because heating can promote particle growth or aggregation. Rotary evaporation may be an option, but the dispersion should be monitored closely for changes in colloidal stability during concentration.
How can I calculate the concentration of silica nanoparticles after solvent transfer or concentration?
Begin by measuring the silica mass concentration gravimetrically. Dry a known volume of the nanoparticle dispersion in a tared container and determine the mass of silica per unit volume after drying.
Particle number concentration can then be estimated using:
where:
- nnp = particle number concentration
- ρnp = nanoparticle mass concentration
- mnp = average mass of an individual nanoparticle
For these calculations, nanoComposix uses a silica density of 2.2 g/cm3. The calculated particle number depends on the assumed particle diameter and density, so values should be treated as estimates when particles are porous, hollow, or otherwise differ from a dense solid sphere.
For additional calculation guidance, use the Nanoparticle Conversion Calculator.
How should I store silica nanoparticles?
Storage recommendations depend on the silica surface chemistry and formulation.
Non-Functionalized Silica
For long-term storage of non-functionalized silica in water, nanoComposix recommends maintaining a concentration of approximately 10 mg/mL, a near-neutral pH of approximately 7–8, and storage at room temperature.
These conditions help limit silica dissolution while maintaining colloidal stability.
Amine-Functionalized Silica
For amine-functionalized silica nanoparticles, long-term storage in an alcohol such as ethanol or isopropanol is recommended when preservation of the surface amine functionality is important. Silica has lower solubility in alcohol than in water, helping preserve the functionalized particle surface.
If the particles will not be used for subsequent chemistry through the surface amines, storage conditions may be less restrictive. Amine-functionalized silica can also be stored in an acidic aqueous environment such as acetate buffer at approximately pH 5. At this pH, the surface is sufficiently removed from its isoelectric region that the positively charged particles can remain colloidally stable.
Learn more about the relationship between pH, surface charge, and colloidal stability in Zeta Potential Measurements and Silica Nanoparticle Physical Properties.
Do you offer fluorescently labeled silica nanoparticles?
nanoComposix has experience fabricating fluorescently labeled silica nanoparticles with dyes incorporated into or associated with the silica particle structure. Fluorescent silica is not currently a standard catalog product, but custom particles can be developed around particle size, fluorophore, surface chemistry, and other application requirements.
See Custom Nanoparticle Development or contact our technical team to discuss a fluorescent silica project.
What are silica nanoparticles used for?
Silica nanoparticles support a broad range of applications because particle size, porosity, surface chemistry, and architecture can all be controlled. Applications include drug delivery, catalysis, optical and reference materials, coatings, pigments, composites, and other advanced-materials research.
See Silica Nanoparticle Applications for a more detailed overview.
Amine-Functionalized Silica
How can I exchange the solvent of amine-functionalized silica?
Silica particles larger than approximately 50 nm can typically be collected by centrifugation and redispersed in a compatible solvent.
For particles ≤50 nm, centrifugation can be more difficult and may increase the risk of aggregation. Dialysis for several hours or overnight can instead be used to exchange the original solvent for a compatible aqueous medium.
The appropriate approach depends on particle size, concentration, surface chemistry, and the compatibility of the desired final solvent.
How many amines are on the surface of amine-functionalized silica?
Based on the amount of reagent used during surface functionalization and the estimated surface area available for ligand attachment, nanoComposix calculates a maximum of approximately 2.5 amine groups/nm2 at the particle surface. This is consistent with literature reports estimating approximately two amine groups/nm2 for related functionalized silica systems.
The total calculated amine density should not be interpreted as the number of amines available for conjugation. Orientation, packing density, steric accessibility, and other factors can limit how many surface groups are accessible to a reactant or biomolecule.
Some amine groups may also become incorporated within the silica network below the outer particle surface. These embedded groups can contribute to properties such as zeta potential and may be detectable by analytical methods, but they are not accessible for surface conjugation.
For more information about silanol and amine-functionalized surfaces, see Silica Surface Chemistry.
Still have a question about a silica nanoparticle?
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