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Silica Physical Properties

Table of contents:


Our silica nanoparticles are produced via the condensation of silanes to form nanoparticles that consist of an amorphous network of silicon and oxygen.  The particles are monodisperse with narrow size distributions. The density of the particles depends on the degree of condensation but is approximately 2 g/cm^3.  The refractive index is 1.43.  The particles are readily suspended in polar solvents such as water and ethanol.  By binding different silanes to the particle surface, the particles can be rendered hydrophobic for suspension into non-polar matrices. 

Size Control

Spherical silica nanoparticles can be produced at large scale with sizes ranging from tens of nanometers to micrometers in diameter. Careful control over the growth conditions produces uniform particles with low coefficients of variation (CV). Colloidal silica particles are amorphous, or non-crystalline, meaning that the atoms do not have long-range order similar to the atomic structure of bulk glass.

TEM image of 20 nm  NanoXact silica nanospheres. TEM image of 100 nm NanoXact silica  nanospheres. TEM image of 1 µm NanoXact silica nanospheres.

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Silica Surface Chemistry and Stability

One of the most common questions that we receive is “What is on the surface of your nanoparticles?” NanoXact silica nanospheres are available with a negatively-charged bare silica (hydroxyl-terminated) surface and a positively charged amine-terminated surface. Non-functionalized silica is ideal for physadsorption of small molecules or for applications requiring high particle stability. Aminated silica is used for covalent binding to molecules such as dyes, proteins and antibodies.

At nanoComposix, we have experience fabricating particles with custom size and surface chemistry, or incorporating fluorophores, quantum dots, or dyes within the colloidal particles. For more information please visit our Custom Synthesis page or contact us.

Hydroxyl-Terminated Silica

Bare silica nanoparticles have very good colloidal stability in water and alcohols. The silica surface is very versatile and can be readily modified to attach specific functional groups. This can include creating hydrophobic, or fluorophilic, surfaces. The nanoporous structure of silica allows moderate molecular weight molecules, such as fluorophores or drug molecules, to be loaded into the silica shell.

The hydroxyl groups give silica colloids a large negative zeta potential at neutral and basic pH. Zeta potential measurements of 80 nm-diameter silica colloids as a function of pH show that the isoelectric point of silica nanoparticles is close to pH 2.

Amine-Terminated Silica

Amine-functionalized silica is useful for binding studies, conjugation with carboxyl-containing molecules through EDAC coupling, or binding to dyes and molecules with isothiocyanate (ITC) or amine-reactive esters. The amines at the colloid surface can be protonated at acidic pH to yield particles with a large positive zeta potential. Measurement of zeta potential versus pH for 120 nm amine-terminated silica particles indicates an isoelectric point near pH 7.5.

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, (for example, J. Nanosci. Nanotechnol. (2004) 4(5), 504-11), 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.

NanoComposix’s amine-functionalized silica is provided in ethanol in order to preserve the integrity of the amine functional groups on the surface, as silica has much higher solubility in water and under some solution conditions the silica network will partially dissolve, leading to a loss of some functional groups. While ethanol provides good material stability, it is also slightly basic with a pH of approximately ~7.0–7.5, which happens to be very close to the isoelectric point of the amine-functionalized silica. This can cause certain sizes of amine functionalized silica to flocculate and fall out of solution because of the low surface charge present under these solution conditions. This phenomenon is separate from large diameter nanoparticles settling over time due to gravity. This can be reversed be dispersing the silica in a low pH acidified buffer, such as acetate at pH 5. Sizes > 50 nm can be centrifuged down and redispersed in a low pH buffer. Contact us for details.

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Diameter, Surface Area and Volume

Nanoparticles have unique properties due to their small size. All nanoparticles regardless of their chemical constituents, have surface area:volume ratios that are extremely high (see the following table). Thus, many of the physical properties of the nanoparticles such as solubility and stability are dominated by the nature of the nanoparticle surface.

Size (nm) Surface area (nm2) Volume (nm3) Ratio of
Surface Area:Volume
20 1260 4190 0.30
50 7850 65500 0.12
80 20100 268000 0.08
100 31400 523600 0.06
120 45200 904800 0.05
140 61600 1437000 0.043
160 80400 2145000 0.037
180 101800 3054000 0.033
200 125700 4189000 0.030

For comparison, a regulation size baseball has a diameter of 73,000,000 nm, a surface area of 16,800,000,000,000,000 nm2, and a volume of 204,000,000,000,000,000,000,000 nm3. The surface area to volume ratio is 0.00000008, a factor of 7,500,000 less than 10 nm nanoparticles.

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Silica Nanoparticle Concentration

Different commercial vendors report concentration using various methods. When researching which nanoparticles to purchase and use it is critical to be able to compare formulations from different suppliers. There are a number of different ways that the concentrations are reported. Here we provide tables converting our silica nanoparticle formulations into other commonly reported concentration units.

Mass Concentration
Particle Concentration
SiO2 Mass Percent
Optical Density at λ350
20 10 1.1 × 1015 1.0 0.049
50 10 6.9 × 1013 1.0 0.79
80 10 1.7 × 1013 1.0 2.94
100 10 8.7 × 1012 1.0 4.24
120 10 5.0 × 1012 1.0 4.87
140 10 3.2 × 1012 1.0 6.74
160 10 2.1 × 1012 1.0 8.73
180 10 1.5 × 1012 1.0 10.55
200 10 1.1 × 1012 1.0 10.87


Solubility of Silica

Silica has a low but non-negligible solubility in water.  At pH's above 8 and below 3, the solubility of silica increases rapidly.  In dilute concentration, the silica can dissolve from the surface until equilibrium is reached.  This is especially noticeable for silica shells on metal nanoparticles.  At low concentrations, the silica shell can be completely removed in just a few hours.  The rate of dissolution can be modified by exposure to aluminum chloride, heating, or reduced temperature.   The advantage of the slow silica dissolution rate is that dried silica can often be deagglomerated by re-suspending in a slightly basic solution.  

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