Carbonate is a small, highly displaceable surface species that provides a negatively charged interface on metal nanoparticles. Among the surface chemistries used by nanoComposix, carbonate provides one of the closest approximations to a minimally capped or “bare” metal surface while still providing electrostatic stabilization in low-ionic-strength aqueous media.
Compared with citrate, carbonate is much smaller and more readily displaced from the nanoparticle surface. The carbonate ion, CO32−, has a molecular weight of 60.0 g/mol, compared with approximately 189 g/mol for the citrate ion. This relatively accessible surface can support passive adsorption of proteins and displacement by other ligands, making carbonate useful for bioconjugation, lateral flow, SERS, and other applications where access to the underlying metal surface is important.
Interested in a carbonate surface for your application?
Talk with our team about carbonate and other surface chemistries for passive adsorption, ligand exchange, optical applications, or custom nanoparticle development.
Carbonate Surface Properties
- Surface charge: Negative
- Isoelectric point: < pH 2
- Displaceability: More readily displaced than citrate or tannic acid and one of the closest approximations to a minimally capped metal surface
- Salt stability: Relatively low; significant destabilization occurs around 20–25 mM NaCl in the 40 nm gold nanoparticle dataset shown below
- Hydrodynamic size: Carbonate contributes minimally to hydrodynamic diameter, so DLS diameter can remain close to the TEM-measured particle diameter
- Solvent compatibility: Water and low-ionic-strength aqueous buffers
Representative source material: Potassium carbonate (Sigma-Aldrich, P5833)
Molecular weight: K2CO3 = 138.2 g/mol; CO32− = 60.0 g/mol
Why Use a Carbonate Surface?
The primary advantage of carbonate is its highly displaceable, low-molecular-weight surface chemistry. Because carbonate interacts relatively weakly with the metal surface, proteins and other ligands can more readily access and replace the original stabilizer.
Thiols have particularly strong affinity for gold and silver surfaces and can readily displace carbonate. Amines and proteins can also interact with exposed metal surfaces, making carbonate useful as a starting surface for passive adsorption and ligand-exchange strategies.
Carbonate can support high passive protein loading because relatively little surface area is occupied by the original stabilizer. This has made carbonate useful in applications such as lateral flow conjugate development where protein adsorption to the nanoparticle surface is required.
Applications
- Passive protein adsorption and bioconjugation
- Lateral flow assay development
- Surface-enhanced Raman spectroscopy (SERS)
- Ligand exchange and surface modification
- Color engineering and controlled particle interactions
Carbonate Surface Charge & Isoelectric Point
The figure above shows a representative zeta potential versus pH curve for carbonate-capped 40 nm gold nanoparticles. These data were generated by manual titration with HCl and NaOH followed by zeta potential measurement.
Carbonate-capped nanoparticles have a very low isoelectric point, below approximately pH 2 in this dataset. They therefore remain negatively charged across most commonly used pH conditions.
The magnitude of the negative zeta potential increases as pH becomes more basic until approximately pH 9. At higher pH, the measured magnitude no longer follows the same trend, likely because the increasing ionic content introduced during titration compresses the electrical double layer.
In separate studies of 40 nm gold and silver nanoparticles coated with BPEI or citrate, nanoComposix observed similar overall pH-dependent zeta-potential behavior between the two metal systems. These observations can provide qualitative context when comparing gold and silver surfaces, although the carbonate dataset shown above was measured specifically using 40 nm gold nanoparticles.
Learn more about how pH, ionic strength, and surface chemistry influence these measurements in Zeta Potential Measurements.
Salt Stability of Carbonate-Capped Nanoparticles
Carbonate provides primarily electrostatic stabilization, so increasing ionic strength can reduce colloidal stability. Dissolved ions screen the charge surrounding the nanoparticles and compress the electrical double layer, reducing the electrostatic repulsion that helps keep particles dispersed.
The figure above shows UV-Visible spectra of carbonate-capped 40 nm gold nanoparticles exposed to increasing concentrations of sodium chloride (NaCl). Separate nanoparticle dispersions were spiked with the indicated NaCl concentrations and incubated for 10 minutes before UV-Vis measurement.
Stable 40 nm gold nanoparticles retain their characteristic plasmon resonance near 520 nm. When aggregation occurs, the 520 nm peak decreases and extinction increases at longer wavelengths, approximately 700–1100 nm, as a result of plasmon coupling between neighboring particles.
In this dataset, significant destabilization becomes apparent at approximately 20–25 mM NaCl. The 520 nm peak decreases and a broad longer-wavelength feature develops, consistent with nanoparticle aggregation.
The salt concentration at which destabilization occurs depends on particle material, size, surface chemistry, concentration, and solution conditions. nanoComposix has generally observed lower salt stability for silver nanoparticles than for comparable gold nanoparticles with the same surface chemistry.
See Salt Stability of Nanoparticles for comparisons among different nanoparticle surface chemistries.
Is carbonate the right surface for your application?
Talk with our technical team about passive adsorption, ligand exchange, surface charge, colloidal stability, or selecting a nanoparticle surface chemistry for your application.
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