Tannic acid is a naturally occurring polyphenol used as a stabilizing and capping agent for metal nanoparticles. Its relatively large, multidentate structure associates with nanoparticle surfaces more strongly than small ligands such as citrate, while remaining more readily displaced than polymers such as PVP.
This intermediate surface binding makes tannic acid useful when a nanoparticle requires greater initial stabilization than a highly displaceable surface, but the surface may still need to be exchanged or modified downstream. Tannic acid and tannic acid/citrate systems have been widely used in the synthesis and stabilization of gold and silver colloids.
Need a tannic acid or other custom nanoparticle surface?
Our technical team can develop surface chemistries around ligand exchange, colloidal stability, particle accessibility, and application-specific formulation requirements.
Tannic Acid Surface Properties
- Surface charge: Negative
- Isoelectric point: < pH 2 in the 40 nm gold nanoparticle dataset shown below
- Displaceability: Less readily displaced than citrate or carbonate, but more readily displaced than PVP
- Salt stability: Moderate under low-ionic-strength conditions; significant destabilization occurs around 25 mM NaCl in the 40 nm gold dataset shown below
- Hydrodynamic size: Produces a moderate increase in hydrodynamic diameter relative to the TEM-measured particle diameter
- Solvent compatibility: Water and low-ionic-strength aqueous buffers
Representative source material: Tannic acid (Sigma-Aldrich, 16201)
Approximate molecular weight: 1700 g/mol (1.7 kDa), depending on source and composition
Why Use a Tannic Acid Surface?
The primary advantage of tannic acid is its position between highly displaceable small-molecule stabilizers and more persistent polymer coatings. It can provide a useful starting surface when stronger association with the nanoparticle is desired during synthesis or formulation, while retaining the possibility of subsequent ligand exchange.
Ligands with strong affinity for noble-metal surfaces, particularly thiol-containing molecules, can displace tannic acid under appropriate conditions. Other surface-active molecules and biomolecules may also replace or interact with the tannic acid coating depending on their chemistry and the surrounding formulation.
This makes tannic acid useful when the nanoparticle surface must balance initial colloidal stabilization with downstream surface accessibility.
Applications
- Ligand exchange and subsequent surface modification
- Lateral flow assay development
- Surface-enhanced Raman spectroscopy (SERS)
- Color engineering and controlled nanoparticle interactions
- Custom gold and silver nanoparticle synthesis
For applications where direct access to the metal surface is especially important, compare tannic acid with the more readily displaced citrate surface. For applications requiring greater steric stabilization, see PVP Surface Chemistry.
Tannic Acid Surface Charge & Isoelectric Point
The figure above shows a representative zeta potential versus pH curve for tannic acid-capped 40 nm gold nanoparticles. These data were generated by manual titration with HCl and NaOH followed by zeta potential measurement.
Tannic acid-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 the solution becomes more basic.
nanoComposix has observed similar overall pH-dependent zeta-potential behavior between comparable 40 nm gold and silver nanoparticles for other charged surfaces, including citrate and BPEI. These observations provide qualitative context when comparing metal nanoparticle systems, although the tannic acid dataset shown here was measured specifically using 40 nm gold nanoparticles.
Learn more about how pH, ionic strength, and particle surface chemistry affect these measurements in Zeta Potential Measurements.
Salt Stability of Tannic Acid-Capped Nanoparticles
Increasing ionic strength can screen nanoparticle surface charge and compress the electrical double layer, reducing electrostatic repulsion between particles. The salt concentration at which this results in colloidal destabilization depends on particle material, size, surface chemistry, concentration, and solution conditions.
The figure above shows UV-Visible spectra of tannic acid-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 primary plasmon peak decreases and extinction increases at longer wavelengths, approximately 700–1100 nm, due to plasmon coupling between neighboring particles.
In this dataset, significant destabilization becomes apparent at approximately 25 mM NaCl. The 520 nm peak decreases and a broad longer-wavelength feature develops, consistent with nanoparticle aggregation.
This threshold is specific to the 40 nm gold formulation and conditions tested. 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 tannic acid the right surface for your application?
Talk with our technical team about ligand exchange, surface accessibility, colloidal stability, or selecting a custom nanoparticle surface chemistry.
Related surface chemistry resources