Branched polyethylenimine (BPEI) is a polymer with repeating units composed of ethylene diamine groups. BPEIs contain primary, secondary and tertiary amino groups. Primary amines on the BPEI are used to covalently link BPEI to carboxyl functionalized nanoparticles to generate a robust BPEI surface that is highly positively charged.
Advantages
- Strongly positively charged surface (cationic)
Representative Source: Polyethylenimine, branched, 25 kDa (Sigma Aldrich, 408727)
Property Highlights
Applications
- Stable in combination with other positively charged particles
- Layer by layer construction of nanoparticle surfaces
- Binding to negatively charged substrates or larger particles
- Color engineering
Surface Charge
See above for a representative zeta potential-pH, or Isoelectric Point (IEP) curve for BPEI-capped 20, 40 and 80 nm gold nanoparticles. This data was generated by manual titration using HCl and NaOH and subsequent measurement of zeta potential. While there is some difference in magnitude between the sizes, the general behavior is consistent.
BPEI capped nanoparticles have very high IEPs, which means that they remain positively charged at all but the most basic of pH ranges. The magnitude of the positive charge steadily increases as the pH becomes more acidic until pH 6–7, when it starts to decrease, 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 destabilization occurs can be different.
The figure above provides the UV-vis spectrum of BPEI-capped 40 nm gold nanoparticles in varying concentrations of sodium chloride (NaCl) solution. The samples were prepared by spiking separate dispersions 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 the pure particle solution without salt, i.e. a 520 nm gold plasmon resonance absorption. 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 at the longer wavelengths at which aggregates absorb (700–1100 nm).
While some decrease in optical density is observed at lower NaCl concentrations, significant destabilization of the particles isn’t observed until the salt content increases over 600 mM. At this concentration, a dramatic decrease in absorbance at 520 nm is observed, and a broad secondary peak at higher wavelengths arises due to the presence of aggregates.
Silver nanoparticles (of a given surface) can generally be expected to have lower salt stability than their gold counterparts.