Branched polyethylenimine (BPEI) is an amine-rich polymer used to create a strongly positively charged nanoparticle surface. Its branched structure contains primary, secondary, and tertiary amines, which contribute to its cationic behavior across a broad pH range.
At nanoComposix, primary amines on BPEI can be covalently coupled to carboxyl-functionalized nanoparticles, producing a strongly bound BPEI coating. The resulting surface is useful when a high positive charge, electrostatic interaction with negatively charged materials, or strong salt tolerance is desired.
Looking for BPEI-coated nanoparticles?
BPEI is available as a surface option on select nanoComposix nanoparticles, with additional particle sizes, materials, and formulations available through custom development.
BPEI Surface Properties
- Surface charge: Strongly positive (cationic)
- Isoelectric point: Approximately pH 11
- Binding: Strongly bound to the nanoparticle surface and not readily displaced
- Salt stability: High salt tolerance relative to many electrostatically stabilized nanoparticle surfaces
- Solvent compatibility: Compatible with water, ethanol, chloroform, and other solvents depending on the nanoparticle system
- Biological considerations: BPEI has shown higher in vitro cytotoxicity than many other surface chemistries offered by nanoComposix, which should be considered when selecting materials for biological applications
Representative source material: Branched polyethylenimine, 25 kDa (Sigma-Aldrich, 408727).
Why Use BPEI on Nanoparticles?
The primary advantage of BPEI is its strong positive surface charge. The polymer contains multiple protonatable amine groups, allowing BPEI-coated nanoparticles to remain cationic across much of the commonly used pH range.
This surface chemistry can be useful for:
- Binding to negatively charged substrates, molecules, or larger particles
- Layer-by-layer assembly of nanoparticle coatings and structures
- Building nanoparticle systems based on controlled electrostatic interactions
- Color engineering and other particle-assembly applications
- Applications requiring greater salt tolerance than more weakly stabilized charged surfaces
BPEI Surface Charge & Isoelectric Point
The figure above shows representative zeta potential versus pH curves for BPEI-coated 20, 40, and 80 nm gold nanoparticles. These data were generated by manual titration with HCl and NaOH followed by zeta potential measurement. Although the magnitude varies somewhat with particle size, the overall pH-dependent behavior is similar.
BPEI-coated nanoparticles have a high isoelectric point (IEP), approximately pH 11 in this dataset. They therefore remain positively charged across most neutral and acidic conditions. As pH decreases, protonation of the BPEI amines generally increases the positive surface charge.
In these measurements, the magnitude of the positive zeta potential increases as conditions become more acidic until approximately pH 6–7, after which the measured magnitude begins to decrease. This behavior is likely influenced by the increasing ionic content introduced during titration, which compresses the electrical double layer and reduces the measured zeta potential.
nanoComposix has also observed similar overall IEP behavior when comparing BPEI-coated gold and silver nanoparticles of equivalent size. This provides a useful basis for interpreting the general pH-dependent surface-charge behavior of BPEI-coated silver using the gold nanoparticle data shown here.
Learn more about how pH, ionic strength, and surface chemistry influence these measurements in Zeta Potential Measurements.
Salt Stability of BPEI-Coated Nanoparticles
Dissolved ions can screen the electrostatic charge surrounding nanoparticles. As ionic strength increases, the electrical double layer becomes compressed and electrostatic repulsion between particles weakens. At sufficiently high salt concentrations, this can lead to particle aggregation.
The figure above shows the UV-Visible spectra of BPEI-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 alters the optical response, producing a decrease in the primary plasmon peak and increased extinction at longer wavelengths, approximately 700–1100 nm.
In this dataset, some decrease in optical density occurs at lower NaCl concentrations, but pronounced destabilization is not observed until the NaCl concentration exceeds approximately 600 mM. At this point, the 520 nm peak decreases substantially and a broad longer-wavelength feature develops, consistent with nanoparticle aggregation.
The salt concentration at which aggregation occurs depends on particle material, size, surface chemistry, concentration, and solution conditions. 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 BPEI and other nanoparticle surface chemistries.
Is BPEI the right surface for your application?
Talk with our technical team about nanoparticle surface charge, colloidal stability, particle interactions, custom surface functionalization, or selecting an alternative surface chemistry.
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