Polyethylene glycol (PEG) is a polyether compound that consists of repeating units of ethylene oxide. It is a safe compound that is used in a wide variety of application including food additives, as an excipient in pharmaceutical products, and as a stealth coating in biomedical applications to reduce non-specific binding and to evade the body’s immune system. The PEG surface is the most stable of our surfaces in buffers containing high salt concentrations found in culture media or other high ionic buffers. The PEG surface functionality disperses very well in water and protic solvents, increasing compatibility in biological systems. The methoxy PEG surface has a slightly negative to neutral surface charge which is advantageous for in-vivo or in-vitro applications.
Methoxy PEG sulfhydryl
nanoComposix provides two different PEG functionalizations, either a lipoic acid derivative of PEG with a terminal acid group (see Carboxyl functionalization) and a 5 kDa methoxy polyethylene glycol sulfhydryl (m-PEG-SH). The methoxy terminated PEG is available on both gold and silver nanoparticles increasing particle stability in a wide range of solvents. Our m-PEG-SH functionalized metal particles have the highest salt stability of any of our surfaces.
Advantages
- High salt stability
- Sterically induced stability
- Biologically compatible
Representative Sources: mPEG-Thiol MW 5000 (Laysan Bio, #MPEG-SH-5000-1g); methoxy PEG Thiol MW 6000 (Sigma Aldrich, 729159)
Property Highlights
- Displaceable: Not displaceable – strong binding affinity to the particle surface via the thiol group
- Neutrally charged
- Isoelectric Point: ~2.5
- Excellent salt stability: stable in highly saline solutions
- Toxicity: low toxicity and high biocompatibility
- Solvent compatibility: Water, ethanol, chloroform, many other polar solvents
Applications
- Biodiagnostic and nanomedicine applications
- Photothermal applications
- In-vitro and in-vivo toxicology experiments
Surface Charge
See above for a representative zeta potential-pH, or Isoelectric Point (IEP) curve for PEG-capped 40 nm gold nanoparticles.This data was generated by manual titration using HCl and NaOH and subsequent zeta potential measurement.
PEG capped nanoparticles have low IEP’s, which means that they remain negatively charged at all but the most acidic of pH ranges (< 2). The magnitude of the negative charge steadily increases as the pH becomes more basic until around pH 7, 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 PEG 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.
- For a live demonstration of the comparison of PEG and citrate capped silver nanoparticles in saturated NaCl solution, see our Intro to Zeta Potential video.
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 chart provides UV-vis spectra of PEG-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 solutions 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 without salt with a strong plasmon resonance optical absorption at 520 nm. If the particles have begun to aggregate, we would expect this the 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 are stable in saturated salt solution and feature the best salt stability of any surface offered at nanoComposix. We observe no significant baseline elevation or secondary peak formation from aggregated particles.
Silver nanoparticles (of a given surface) can generally be expected to have lower salt stability than their gold counterparts