Polyethylene glycol (PEG) is a hydrophilic polymer composed of repeating ethylene oxide units. When attached to a nanoparticle surface, PEG creates a hydrated steric barrier that can improve colloidal stability, reduce nonspecific interactions, and support dispersion in water, biological buffers, and other compatible solvents.
nanoComposix uses methoxy-terminated PEG thiol (mPEG-SH) to create a nonreactive PEG surface on noble-metal nanoparticles. The thiol group binds strongly to the metal surface while the PEG chain extends into solution. Compared with surfaces that rely primarily on electrostatic repulsion, PEG provides steric stabilization and can maintain colloidal stability under much higher ionic-strength conditions.
Methoxy PEG sulfhydryl (mPEG-SH)
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Explore standard PEG-coated gold nanoparticles or work with our technical team on custom PEG surfaces, particle materials, sizes, and formulations.
Need a reactive terminal carboxyl group rather than a nonreactive methoxy terminus? See PEG-Carboxyl Surface Chemistry.
PEG Surface Properties
- Surface character: Hydrophilic and sterically stabilizing
- Surface charge: Slightly negative to near-neutral depending on pH and solution conditions
- Binding: Strongly bound to noble-metal surfaces through the thiol group and not readily displaced under typical handling conditions
- Salt stability: Very high due to steric stabilization by the PEG layer
- Nonspecific interactions: PEG can reduce adsorption of proteins and other molecules to the nanoparticle surface
- Solvent compatibility: Water and a range of compatible aqueous and polar organic solvents
Representative source materials: mPEG-Thiol, MW 5000 (Laysan Bio, MPEG-SH-5000-1g); methoxy PEG thiol, MW 6000 (Sigma-Aldrich, 729159)
Why Use a PEG Surface?
The primary advantage of PEG is its combination of steric stabilization and a hydrophilic, relatively noninteracting particle interface. Hydrated PEG chains extend from the nanoparticle surface and create a physical barrier that helps prevent close particle-particle contact.
Because this stabilization does not depend exclusively on surface charge, PEG-coated nanoparticles can remain dispersed under ionic-strength conditions that destabilize more weakly protected surfaces such as citrate.
PEG coatings are also widely used in biological research because they can reduce nonspecific adsorption of proteins and other biomolecules to the nanoparticle surface. The resulting behavior depends on PEG molecular weight, surface coverage, nanoparticle properties, and the surrounding biological environment.
Methoxy PEG provides a relatively nonreactive terminal group. When downstream covalent conjugation is required, PEG-carboxyl provides a PEG spacer terminated with a chemically accessible carboxyl group.
Applications
- Nanomedicine and drug-delivery research
- Biological and cell-based nanoparticle studies
- Applications requiring stability in higher-ionic-strength buffers
- Photothermal research
- In vitro and in vivo nanoparticle research
- Optical and sensing applications requiring a stable hydrophilic surface
For broader biomedical development applications, see Nanomedicine CDMO Services and Photothermal Applications of Nanoparticles.
PEG Surface Charge
The figure above shows representative zeta potential versus pH data for PEG-coated 40 nm gold nanoparticles. These data were generated by manual titration with HCl and NaOH followed by zeta potential measurement.
Although methoxy PEG itself is nonionic, PEG-coated metal nanoparticles can exhibit a slightly negative measured zeta potential because the measurement reflects the complete particle interface rather than the PEG terminal group alone. In this dataset, the zeta potential approaches zero only under strongly acidic conditions, with an apparent crossover in approximately the pH 2–3 range.
The magnitude of the negative zeta potential increases as conditions become less acidic before beginning to decrease again around neutral pH in this titration series. Increasing ionic content during titration can compress the electrical double layer and reduce the magnitude of the measured zeta potential.
nanoComposix has observed similar overall pH-dependent behavior for PEG-coated 40 nm gold and silver nanoparticles. The magnitude of the measured zeta potential can still vary with particle material, PEG coverage, pH, ionic strength, and other formulation conditions.
Learn more about interpreting these measurements in Zeta Potential Measurements.
Salt Stability of PEG-Coated Nanoparticles
PEG differs from surfaces such as citrate because its colloidal stabilization is primarily steric rather than purely electrostatic. As salt concentration increases and electrostatic interactions become screened, the hydrated PEG layer can continue to provide a physical barrier between neighboring particles.
The figure above shows UV-Visible spectra of PEG-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.
Aggregation of 40 nm gold nanoparticles would typically produce a decrease in the primary plasmon resonance near 520 nm together with increased extinction at longer wavelengths, approximately 700–1100 nm.
In this dataset, the PEG-coated particles remain stable even in saturated NaCl solution, with no significant baseline elevation or secondary long-wavelength peak associated with aggregation. This demonstrates the very high salt tolerance that a dense PEG coating can provide.
The result is specific to the 40 nm gold formulation and conditions tested. Salt stability can vary with particle material, size, PEG molecular weight, ligand coverage, concentration, and formulation. nanoComposix has generally observed lower salt stability for silver nanoparticles than for comparable gold nanoparticles with the same surface chemistry.
For a visual comparison of PEG- and citrate-coated nanoparticles under high-salt conditions, see our Zeta Potential video series. You can also compare surface chemistries in Salt Stability of Nanoparticles.
Is PEG the right surface for your nanoparticle system?
Talk with our technical team about high-salt stability, biological compatibility, particle size, methoxy versus reactive PEG surfaces, or a custom PEG formulation.
Related surface chemistry resources