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By WildCation - Own work, CC BY-SA 4.0, Link

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 particles 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 amine will readily displace citrate from the surface and strongly associated with gold or silver surfaces.
  • Small change to hydrodynamic diameter – TEM measured diameter is very close to hydrodynamic diameter as measured with DLS.
  • Negatively charged zeta potential with an isoelectric point of ~X.

Representative Source: Trisodium Citrate (Sigma Aldrich XXX, XXX)

Molecular Weight: Trisodium citrate Na3C6H5O7 is 258.07, citrate C6H5O7 is 189.10)

Isoelectric Point: ~3.5 (show entire chart) – (show for gold/silver/size)?

Comparison to Alternatives

  • Displaceable: Citrate is more displaceable than tannic acid but less displaceable than carbonate.
  • Negatively charged
  • Salt stability: destabilized in high salt concentrations above X mM.
  • Toxicity: Very low
  • Solvent compatibility: Water, weak buffers

Applications

  • SERS
  • Lateral flow
  • Color engineering