Tannic acid offers an intermediate surface stability option when compared to the other non-covalently-bound surfaces of citrate and PVP we offer, all of which associate with the particle surface by Van der Waals attraction and metal ligand charge transfer. The tannic acid molecule is a naturally-occurring polyphenol that is a component of oak bark and leaves often used as a dye or stain. Tannic acid (~1.7 kDa, depending on source) is a polyphenol that associates with the particle surface more strongly than citrate (192 Da), but is smaller and more displaceable than PVP (40 kDa). Tannic acid is a common reagent in the fabrication of gold and silver nanoparticles and tannic acid or tannic acid/citrate surfaces are a common stability and surface coating for many colloidal solutions.
Advantages
- Intermediately displaceable as a surface for stabilizing particles in aqueous solutions. Molecules with thiols or amine will displace tannic acid from the surface.
- Useful in situations where the surface needs to be exchanged but a more stable surface than citrate is needed during the initial formulation.
- Only a moderate difference between the hydrodynamic and TEM measured diameter is observed.
- Negatively charged zeta potential
Representative Source: Tannic Acid (Sigma Aldrich, 16201)
Molecular Weight: 1700 g/mol; 1.7 kDa
Property Highlights
- Displaceable: Tannic acid is less displaceable than citrate or carbonate but more displaceable than PVP
- Negatively charged
- Isoelectric Point: < 2
- Salt stability: Moderately stable in low concentration salt solutions
- Toxicity: Very low
- Solvent compatibility: Water, weak buffers
Applications
- SERS
- Lateral Flow
- Color engineering
Surface Charge
See above for a representative zeta potential-pH, or Isoelectric Point (IEP) curve for tannic acid-capped 40 nm gold nanoparticles. This data was generated by manual titration using HCl and NaOH and subsequent zeta potential measurement.
Tannic acid capped nanoparticles have very low IEP’s, which means that they remain negatively charged at all but the most acidic of pH ranges (< 2).The magnitude of the negative charge steadily increases as the pH becomes more basic.
- 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 colloidal destabilization occurs can be different.
The chart above provides the UV-vis spectrum of tannic acid-capped 40 nm gold nanoparticles in varying concentrations of sodium chloride (NaCl) solution.The samples were prepared by spiking separate 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 for the nanoparticles in pure water, with a strong optical plasmon absorbance at 520 nm. If the particles have begun to aggregate, we would expect this the be reflected in the spectrum with a decrease in the surface plasmon peak at 520 nm and an increase in absorbance at the longer wavelengths at which aggregates absorb (700–1100 nm)
Significant destabilization of the particles becomes apparent at 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.
Silver nanoparticles (of a given surface) can generally be expected to have lower salt stability than their gold counterparts.