PEG-carboxyl provides a strongly anchored nanoparticle surface with an accessible terminal carboxyl group for covalent conjugation. nanoComposix uses lipoic-dPEG12-COOH, which combines a lipoic acid-derived anchor, a PEG spacer, and a terminal carboxyl group.
The sulfur-containing anchor binds strongly to noble-metal surfaces, while the PEG spacer extends the reactive carboxyl group away from the nanoparticle. The terminal carboxyl group can be activated using EDC/NHS chemistry and coupled to molecules containing primary amines, including antibodies and other proteins.
Looking for PEG-carboxyl nanoparticles for covalent conjugation?
Explore carboxyl gold nanospheres, highly uniform PEG-carboxyl gold nanoparticles, and conjugation kits for EDC/NHS coupling.
Gold Nanospheres • Ultra Uniform Gold • Covalent Conjugation Kits
Need a carboxyl surface without the PEG spacer? See Lipoic Acid Surface Chemistry. Lipoic acid is used on larger structures such as 150 nm carboxyl gold nanoshells.
PEG-Carboxyl Surface Properties
- Surface charge: Negative when the terminal carboxyl groups are deprotonated
- Apparent isoelectric region: Approximately pH 3 in the 40 nm gold nanoparticle dataset shown below
- Binding: Strongly anchored to the metal surface through sulfur groups and not readily displaced under typical handling conditions
- Functional group: Terminal carboxyl group available for subsequent chemical modification
- Conjugation: Supports covalent coupling to primary amines using EDC/NHS chemistry
- Salt stability: Very high in the 40 nm gold nanoparticle formulation tested below
- Solvent compatibility: Water and a range of compatible aqueous and polar organic solvents
Representative source material: Lipoic m-PEG (Quanta BioDesign, 10808)
Molecular weight: 806.03 g/mol
Why Use a PEG-Carboxyl Surface?
PEG-carboxyl combines covalent conjugation chemistry with the steric and hydrophilic properties of a PEG spacer. This makes it useful when a nanoparticle requires a strongly anchored coating and a chemically accessible functional group for attachment of biomolecules.
Activating the terminal carboxyl groups with EDC and NHS or Sulfo-NHS enables reaction with primary amines to form stable amide bonds. This approach is commonly used for covalent attachment of antibodies and proteins to nanoparticles.
The PEG spacer also separates the terminal carboxyl group from the particle surface and contributes steric stabilization. Compared with a lipoic acid surface without the spacer, PEG-carboxyl can provide greater separation between the nanoparticle and the subsequently attached biomolecule.
Applications
- Covalent antibody and protein conjugation
- Lateral flow assay development
- Biosensors and other diagnostic research
- Attachment of amine-containing biomolecules using EDC/NHS chemistry
- Further functionalization through terminal carboxyl chemistry
For practical coupling guidance, see Covalent Conjugation of Antibodies to Gold Nanoparticles and our Nanoparticle Conjugation Protocols.
PEG-Carboxyl Surface Charge
The figure above shows representative zeta potential versus pH data for PEG-carboxyl-coated 40 nm gold nanoparticles. These data were generated by manual titration with HCl and NaOH followed by zeta potential measurement.
In this dataset, PEG-carboxyl-coated nanoparticles approach neutral zeta potential under strongly acidic conditions, with an apparent crossover around pH 3. At higher pH, deprotonation of the terminal carboxyl groups contributes to a negatively charged particle interface.
The magnitude of the negative zeta potential increases as pH becomes more basic until approximately pH 8. At higher pH in this titration series, the magnitude begins to decrease, likely because increasing ionic content compresses the electrical double layer.
nanoComposix has observed similar overall pH-dependent zeta-potential behavior between comparable gold and silver nanoparticles for other charged surfaces, including citrate and BPEI. These observations provide qualitative context when comparing metal nanoparticle systems, although the PEG-carboxyl dataset shown here was measured specifically using 40 nm gold nanoparticles.
Learn more about how pH, ionic strength, and particle surface chemistry affect these measurements in Zeta Potential Measurements.
Salt Stability of PEG-Carboxyl Nanoparticles
PEG-carboxyl provides both a charged terminal group and steric stabilization from the PEG spacer. This can provide substantially greater resistance to salt-induced aggregation than surfaces that depend primarily on electrostatic stabilization.
The figure above shows UV-Visible spectra of PEG-carboxyl-coated 40 nm gold nanoparticles exposed to increasing concentrations of sodium chloride (NaCl). Separate nanoparticle dispersions were spiked with the indicated NaCl concentrations and incubated for 10 minutes before UV-Vis measurement.
Stable 40 nm gold nanoparticles retain their characteristic plasmon resonance near 520 nm. Aggregation would typically produce a decrease in the primary plasmon peak and increased extinction at longer wavelengths, approximately 700–1100 nm, due to plasmon coupling between neighboring particles.
In this dataset, the PEG-carboxyl-coated particles remain stable at NaCl concentrations up to 2.5 M, demonstrating the high salt tolerance provided by this surface chemistry under the conditions tested.
This result is specific to the 40 nm gold formulation and experimental conditions. Salt stability can vary with particle material, size, ligand coverage, concentration, buffer composition, and other formulation variables.
nanoComposix has generally observed lower salt stability for silver nanoparticles than for comparable gold nanoparticles with the same surface chemistry.
See Salt Stability of Nanoparticles for comparisons among different nanoparticle surface chemistries.
Is PEG-carboxyl the right surface for your conjugate?
Talk with our technical team about EDC/NHS coupling, antibody conjugation, PEG spacer chemistry, particle size, salt stability, or selecting the right carboxyl surface for your application.
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