Polyethylenimine (PEI), linear (1 mg/mL)

MATERIALS

12 M HCl

10 M sodium hydroxide (NaOH)

Polyethylenimine (PEI), 25 kDa, linear, powder (Polysciences Inc.)

H₂O, Milli-Q-filtered

Rinse all glassware three times with high-purity H₂O.

Beaker, 500-mL

Cylinder, glass, graduated, 500-mL

pH meter

Tube, conical, 50-mL

Vacuum filtration unit, 0.22-μm

1. Pour ~450 mL of Milli-Q H₂O into a 500-mL glass beaker.
2. Add 500 mg of PEI to the beaker with stirring.
3. Add concentrated HCl drop-wise to the solution to pH <2.0.
4. Stir until the PEI is dissolved (~2-3 h). Maintain the pH <2.0 throughout.

Approximately 800 μL of 12 M HCl will be required for full PEI dissolution. There may still be some small fiber particles that will not dissolve.
5. Add concentrated NaOH drop-wise to the solution to pH 7.0.

Approximately 500 μL of 10 M NaOH will be required to neutralize the PEI solution.
6. Pour the solution into a 500-mL glass cylinder. Adjust the final volume to 500 mL with Milli-Q H₂O.
7. Filter-sterilize the solution through a 0.22-μm membrane.
8. Store aliquots of the desired volume at -20°C.