Size-exclusion properties of nanoporous films derived from polystyrene-poly(methylmethacrylate) diblock copolymers assessed using direct electrochemistry of ferritin.

This paper reports the size-exclusion properties of nanoporous films derived from polystyrene-poly(methylmethacrylate) diblock copolymers (PS-b-PMMA) for biomacromolecules. These properties were assessed by measuring cyclic voltammetry of ferritin (12 nm in diameter) adsorbed onto recessed nanodisk-array gold electrodes (RNEs) fabricated from the nanoporous films having different effective pore diameters and surface functionalities. RNEs having 20-nm-diameter nanopores modified with a poly(ethylene glycol) (PEG) layer showed the redox currents of ferritin after their immersion in a ferritin solution (5 mg/mL) for longer than 2 h. The currents originated from the direct electron transfer reaction of ferritin molecules immobilized on the underlying gold surface as a result of their penetration through the 20-nm-diameter nanopores. The PEG modification of the nanopore surface was required for the penetration of ferritin, probably because it reduced the nonspecific adsorption of ferritin to the nanopore surface. In contrast, no redox current of ferritin was observed for RNEs having PEG-modified 15-nm-diameter nanopores after their immersion in the ferritin solution for 12 h, indicating the size-exclusion of ferritin from the 15-nm nanopores. The distinct size-exclusion properties of the PS-b-PMMA-derived nanoporous films reflect their uniform diameters and shapes and will provide a means for fabricating separation membranes for biomolecules with high size-based selectivity.