PEG Carboxyl Acid Surface

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nanoComposix PEG-Carboxyl products contain a lipoic-dPEG12-COOH ligand that’s covalently bound to the metal nanoparticle with a dithiol which forms a strong, stable bond to the metal surface. The acid group provides a highly negatively charged surface and a chemical handle for further functionalization.

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. Our BioReady materials are provided with a detailed protocol to use EDC/NHS chemistry to covalently link molecules containing free amines to the carboxy surfaced nanoparticles.

At nanoComposix we use lipoic-dPEG12-COOH, which is similar to lipoic acid but contains a PEG spacer between the thiol and carboxylic acid functional groups.

For BioReady gold nanoparticles with diameters less than 100 nm, the lipoic-dPEG12-COOH is used. For larger diameter BioReady nanoparticles (e.g. 150 nm gold nanoshells), lipoic acid is used. For more information on Lipoic Acid capped gold nanoparticles, click here


  • Provides a stable particle surface in a variety of different solvents. Due to the strong binding affinity of the thiols present in the PEG-carboxyl ligand to the surface of the particles, this ligand is not displaceable
  • Allows for covalent linkages to molecules with primary amines
  • Representative Source: Lipoic m-PEG (Quanta Biodesign, 10808)
  • Molecular Weight: 806.03

Property Highlights

  • Displaceable: Not displaceable – strong binding affinity to the particle surface via the thiol groups
  • Negatively charged
  • Isoelectric Point: ~3
  • Salt stability: Very stable in highly saline solutions
  • Toxicity: Generally regarded as safe, low toxicity
  • Solvent compatibility: Water, ethanol, chloroform, many other polar solvents


  • Lateral Flow
  • Bioconjugation

Surface Charge

See above for a representative zeta potential-pH, or Isoelectric Point (IEP) curve for PEG carboxyl-capped 40 nm gold nanoparticles. This data was generated by manual titration using HCl and NaOH and subsequent zeta potential measurement.

PEG carboxyl capped nanoparticles have low IEP’s, which means that they remain negatively charged at all but the most acidic of pH ranges (< 3).The magnitude of the negative charge steadily increases as the pH becomes more basic until around pH 8, when it starts to become more neutral 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 above figure provides UV-vis spectra of PEG carboxyl-capped 40 nm gold nanoparticles in varying concentrations of sodium chloride (NaCl) solution. The samples were prepared by spiking 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 that of the particles in pure water, with a strong plasmon resonance optical absorbance at 520 nm. If the particles have begun to aggregate, we would expect this to 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).

The particles remain stable up to 2.5 M NaCl. PEG carboxyl is one of the most salt stable surface capping agents offered at nanoComposix.

Silver nanoparticles (of a given surface) can generally be expected to have lower salt stability than their gold counterparts.

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