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At nanoComposix we use two different molecules to provide a surface with free chemically accessible carboxylic acids. The first is a discrete PEG molecule with a lipoic acid at one end and a carboxylic acid at the other:

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The lipoic-dPEG8-acid is used on particles with diameters less than 100 nm. For larger diameter particles (e.g. 150 nm gold nanoshells), we have obtained better results using lipoic acid:

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Carboxyl surfaces can be used to covalently bind molecules with free amines (e.g. antibodies) to the surface of the particles. An amide bond between the acid surface and the free amine is formed using EDC / NHS chemistry. We provide a detailed protocol to use EDC/NHS chemistry (provide link to BioReady) to covalently link molecules containing free amines to the carboxy surfaced nanoparticles.


  • Provides a stable particle surface in a variety of different solvents. Due to the strong binding of the lipoic acid to the surface of the particles, this ligand is not displaceable
  • Allows for covalent linkages to molecules with primary amines
  • Negative Zeta potential with an isoelectric point of ~X

Representative Source: Lipoic m-PEG (Quanta Biodesigns), Lipoic acid (Sigma Aldrich)

Molecular Weight: XX for m-PEG, 206.3 MW for lipoic acid

Comparison to Alternatives

  • Displaceable: Not displaceable
  • Negatively charged (Zeta ~-20 mV at pH 7.4)
  • Salt stability: stable in salt solutions below X mM
  • Toxicity: Generally regarded as safe, low toxicity
  • Solvent compatibility: Water, ethanol, chloroform, many other polar solvents
  • Applications

    • Lateral Flow