Synthesis of PEI-CBA

 Dissolve 0.86 g of PEI in 10 mL 10% aqueous methanol
flask equipped with a condenser at 45 ◦ C under a nitrogen
atmosphere. To obtain the PEI-SS with a molar ratio of 1:2
between the reactive group in CBA and the primary amine in
PEI respectively, 0.325 g CBA was dissolved in 6 mL methanol
respectively and dropwise added to the PEI solution. The reaction
mixture was stirred in the dark under nitrogen atmosphere for at
least 24 h at 45 ◦ C. Subsequently, 10% excess PEI (0.086 g) in
2 mL methanol was added to consume any unreacted acrylamide
groups for another 6 h with stirring. After that the transparent
solution was diluted with water to 20 mL and acidified with 10 M
HCl to pH 4, and then dialyzed against distilled water (MWCO:
3500) for 2 days to remove the unreacted 800 Da PEI. Lastly, the
solution was lyophilized for 2 days to obtain the product

Synthesis of biotinylated PEI-SS
The biotinylated PEI-SS (BP-SS) was prepared according to the
literature. 6 Biotin (30 mg) was activated with NHS (14 mg) and
EDC (23 mg) in dimethyl sulfoxide (3 mL). The activated biotin
solution was added to PEI-SS (200 mg) in deionized water (12 mL).

The reaction mixture was stirred at 4 ◦ C for 24 h, purified by
dialyzing against water for 2 days (MWCO: 3500), lyophilized and
defined as BP-SS.