Table of contents:
Silica: General Questions
Silica: General Questions
Are your silica nanosphere amorphous or crystalline?
Amorphous. Colloidal silica particles are non-crystalline, meaning that the atoms do not have long-range order, resembling the structure of bulk glass.
How can I concentrate a sample of small silica nanoparticles?
Unfortunately, small silica (≤ 50 nm) can be difficult to collect via centrifugation. This process typically requires very high spin speeds for small particles, and often results in irreversible particle aggregation.
For larger silica particles, we are able to remove solvent through evaporation with mild heating, but for small particles, applying heat can induce particle growth or aggregation. You may try removing solvent by rotary evaporation, but it is essential to keep an eye on the particles and look for any signs of instability throughout the process. Under no circumstances should the particles be dried completely, as it is highly unlikely that they be colloidally stable thereafter without some degree of irreversible aggregation.
How can I calculate the concentration of my silica nanoparticles after solvent transfer or particle concentration?
Begin by measuring the mass concentration gravimetrically, by drying down a known volume of the dispersion into a tared container and recording the mass per unit volume after drying. Then use the following equation: nnp = ρnp / mnp where nnp is the particle number concentration, ρnp is the material density, and mnp is the average nanoparticle mass to calculate the particle concentration. We use 2.2 g/cm3 for the density of silica (ρnp). For more information on using this equation, please click here.
How should I store my silica nanoparticles?
Due to considerations around the solubility of silica, we recommend that non-functionalized silica stored in water should be kept at a concentration about ~10 mg/mL, at a neutral pH (7-8), and at room temperature for long term storage. The solubility of silica in aqueous solution is relatively low and the particles are typically stable in water for days. When you are ready to use the silica in water-based experiments, simply centrifuge the particles into water before use.
For aminated silica nanoparticles, we recommend long term storage in alcohol such as ethanol or isopropanol. Since the silica has low solubility and high stability in alcohols, this helps preserve the amine groups on the surface. If the aminated silica is not being used for subsequent surface chemistry via the amine, the storage conditions are not as critical and the particles can be stored in a low pH aqueous solution. Generally, we recommend a buffer such as acetate (pH=5). At this pH, the aminated silica is far enough away from the isoelectronic point that the positively charged particles remain stable.
For more information about particle stability, visit our page about Zeta Potential Measurements.
Do you offer fluorescently labelled silica?
We have considerable experience fabricating fluorescently labelled silica nanoparticles. Though we do not offer them as a standard product, we are happy to make them on a custom basis to suit your individual requirements. For more information about this, please visit our Custom Synthesis page.
What are silica nanoparticles used for?
Due to the versatility of silica in terms of porosity, surface chemistry, and nanoparticle size, silica has a wide range of applications, ranging from drug delivery and catalysis to its use as an ingredient in paint and cement. Visit our page about Silica Nanoparticle Applications to learn more.
How can I exchange the solvent of amine-terminated silica?
Silica particles > 50 nm can typically be centrifuged down for redispersion in the compatible solvent of your choice. For sizes ≤ 50 nm, we recommend performing dialysis overnight (or for several hours) to diffuse out the original solvent and enable suspension in the compatible solvent of your choosing.
How many amines are on the surface of your aminated silica?
Based on the amount of reagent used during the surface functionalization step and the surface area available for the ligand to bind, we calculate a maximum of ~2.5 amine groups/nm2 at the particle surface. This is consistent with literature reports, which estimate approximately two amine groups/nm2. Depending on orientation, packing density, and other factors, only a portion of the amines may be accessible for conjugation. Further, in some cases there are also amine groups that are incorporated into the silica network below the particle surface, and which contribute to the zeta potential of the particle and can be detected using different characterization methods. Because they are embedded within the silica shell, however, these amines are not accessible for conjugation.