Carbonate terminated nanoparticles have a surface that is the closest to “bare” of all our particles. Similar to citrate, carbonate molecules provide a negative zeta potential across a wide range of pH. However, instead of three carboxylic acid groups like citrate (MW 189), carbonate has only a single carboxyl group and has a lower molecular weight (MW of CO32– is 60.0). The smaller size and reduced effective charge of the molecule makes it easier to displace, resulting in higher surface loadings of physisorbed protein. The ability to load higher amounts of protein onto particles with this surface makes them ideal starting materials for bioconjugation applications such as lateral flow where a robust, dense antibody layer increases assay sensitivity.
Advantages
- Highly displaceable surface group ideal for displacement with proteins or other ligands. Molecules with thiols or amines will strongly associate with gold or silver surfaces, readily displacing carbonate.
- Carbonate has a minor effect on hydrodynamic diameter with the TEM measured diameter very close to the hydrodynamic diameter as measured with DLS.
Representative Source: Potassium carbonate (Sigma Aldrich P5833)
Molecular Weight: K2CO3 (138.2), CO3 (60.0)
Property Highlights
- Isoelectric Point: < 2
- Displaceable: Carbonate is more displaceable than citrate and tannic acid; closest to “bare” surface of our particles
- Negatively charged
- Salt Stability: destabilized in salt concentrations above 25 mM
- Toxicity: Very low
- Solvent compatibility: Water, low osmolarity buffers
Applications
- SERS
- Lateral Flow
- Color engineering
Surface Charge
See above for a representative zeta potential-pH, or Isoelectric Point (IEP) curve for carbonate-capped 40 nm gold nanoparticles.This data was generated by manual titration using HCl and NaOH and subsequent measurement.
Carbonate capped nanoparticles have very low IEPs, which means that they remain negatively charged at all but the most acidic of pH ranges. The magnitude of the negative charge steadily increases as the pH becomes more basic until around pH 9, when electrical double layer suppression from high ionic content stops the trend.
- We have demonstrated that for 40 nm gold and silver bPEI and citrate-capped particles, the IEP curves are very similar. This should enable a reasonable basis for comparison of zeta potential for silver nanoparticles with the above data based on gold nanoparticles.
- For more information about zeta potential and IEP theory, click here.
Salt Stability
In the presence of a high enough salt concentration the surface charge of particles in solution can be shielded by the dissolved ions, leading to reduced colloidal stability. The ions in solution prevent the like charges from repelling one another as readily. For each particle type the salt concentration at which this destabilization occurs can be different.
The figure above provides UV-vis spectra of carbonate-capped 40 nm gold nanoparticles in varying concentrations of sodium chloride (NaCl) solution.The samples were prepared by spiking solutions of nanoparticles with NaCl at the listed concentrations and allowing the resulting solution to incubate for 10 minutes prior to UV-Vis measurement.
If the nanoparticles are stable at the given salt concentration, we would expect the spectrum to remain the same as the particles in a salt free solution with a 520 nm gold plasmon resonance optical absorption. If the particles have begun to aggregate, the intensity of the surface plasmon peak at 520 nm will decrease and an increase in absorption at the longer wavelengths at which aggregates absorb (700-1100 nm) will occur
Significant destabilization of the particles becomes apparent at 20–25 mM NaCl. At this concentration, a decrease in absorbance at 520 nm is observed, and a broad secondary peak at higher wavelengths arises due to the presence of aggregates (see arrow in above graph).
Silver nanoparticles (of a given surface) can generally be expected to have lower salt stability than their gold counterparts.