Citrate is one of the most common stabilizing molecules for metal nanoparticles. It provides a highly negatively charged surface that can be displaced by many other molecules or ligands such as those with terminal amines or mercapto groups. Citrate is a small molecule with multiple carboxylic acid groups that weakly associate with the particle surface. It has good stability in water and weakly-buffered solutions. However, citrate doesn’t provide steric stability and noble metal nanoparticles with citrate on their surface are susceptible to aggregation in high salt solutions and non-aqueous solvents.
Advantages
- Highly displaceable surface for performing ligand exchange with proteins or other ligands. Molecules with thiols or amines will readily displace citrate from the surface and strongly associate with gold or silver surfaces.
- Small change to hydrodynamic diameter – TEM measured diameter is very close to hydrodynamic diameter as measured with DLS.
Representative Source: Trisodium citrate dihydrate (Sigma Aldrich, 71402)
Molecular Weight: Trisodium citrate dihydrate Na3C6H5O7 · 2H2O is 294.10, citrate C6H5O7 is 189.10
Property Highlights
- Displaceable: Citrate is more displaceable than tannic acid but less displaceable than carbonate.
- Negatively charged
- Salt stability: Destabilized in low salt concentrations.
- Toxicity: Very low
- Solvent compatibility: Water, weak buffers
- Isoelectric Point: < 2
Applications
- SERS
- Lateral flow
- Color engineering
Surface Charge
See above for a representative zeta potential-pH, or Isoelectric Point (IEP) curve for citrate-capped 20, 40 and 80 nm gold nanoparticles. This data was generated by manual titration using HCl and NaOH and subsequent measurement of zeta potential. While there is some difference in magnitude between the sizes, the general behavior is consistent.
Citrate capped nanoparticles have very low IEP’s, 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 pH 8–9, when it starts to decrease likely due to electrical double layer suppression from high ionic content.
- 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 citrate-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)
While some decrease in optical density is observed at lower NaCl concentrations, significant destabilization of the particles isn’t observed until the salt content increases over 20 mM.
Silver nanoparticles (of a given surface) can generally be expected to have lower salt stability than their gold counterparts