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Cylinder-forming block copolymers have been used to prepare membranes containing cylindrical nanoscale pores with uniform diameters.  Such nanoporous membranes will make it possible to design novel membranes for efficient chemical catalysis and separations.  For these future applications, in-depth understanding of the chemical properties of the nanopore surface is essential.

In the last 12 months (September 2008 ~ August 2009), we have addressed the following issues related to the surface chemistry of nanoporous membranes derived from cylinder-forming polystyrene-poly(methylmethacrylate) diblock copolymers (PS-b-PMMA; the PMMA volume fraction is 0.3) based on our previous achievements (Langmuir 2007, 23, 12771.; 2008, 24, 8959.):

(1) We have found that the roughness of the underlying substrate induces the vertical orientation of cylindrical PMMA domains in a thin film of cylinder-forming PS-b-PMMA (Polymer 2009, 50, 2273.).  The domain orientation on a planar substrate was systematically assessed using atomic force microscopy (AFM) and cyclic voltammetry (CV).  AFM images of PS-b-PMMA films having thickness similar to the domain periodicity permitted us to study the effects of substrate roughness and block affinity on domain orientation.  PS-b-PMMA films on gold substrates showed metastable vertical domain orientation that was attained more slowly on rougher substrates.  In contrast, the domains were horizontally oriented on oxide-coated Si regardless of surface roughness and the annealing conditions examined.  In addition, CV data for PS-b-PMMA films on gold substrates whose PMMA domains were etched suggested that the metastable vertically-oriented domains reached the underlying substrates.  Understanding the effects of the substrate properties and annealing conditions on the domain orientation makes it possible for us to reproducibly prepare nanopores oriented vertically to the underlying electrode surface, which are essential to assess the surface chemistry of the nanopores via electrochemical and other approaches.

(2) We have investigated the effects of nanopore diameter and surface chemistry on the diffusion of ferritin, an iron-storage protein of 12 nm in diameter, through PS-b-PMMA-derived nanopores using CV (Anal. Chem. 2009, 81, 851.).  A gold substrate coated with a PS-b-PMMA-derived nanoporous film was immersed in a ferritin solution (5 mg/mL) in phosphate buffer and then washed with buffer several times.  Subsequently, CVs on the electrode were measured in a buffer solution containing no ferritin so that we could measure a redox current of ferritin molecules that could pass through the nanopores and were immobilized on the underlying gold surface.  To enhance the immobilization of ferritin onto the electrode surface and also to enhance the direct electron transfer between ferritin and the electrode, the gold surface was modified with a thiolate self-assembled monolayer with a terminal quaternary ammonium group.  Among PS-b-PMMA-derived nanopores examined, 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 for longer than 2 hours.  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 PEG-modified 15-nm-diameter nanopores after their immersion in the ferritin solution for 12-hours, 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.

(3) We have investigated the surface chemical properties of ca. 20 nm-wide domains on a UV-treated thin film of PS-b-PMMA (Langmuir submitted.).  UV irradiation and subsequent acetic acid (AcOH) treatment were used for selectively etching horizontally-aligned PMMA domains on a thin PS-b-PMMA film to obtain nanoscale trenches and ridges.  The surface charge and hydrophilicity of the trenches (etched PMMA domains) and ridges (PS domains) were investigated using three approaches based on scanning force microscopy.   Chemical force titration data with a COOH-terminated tip showed a prominent decrease in adhesion force from pH 3 to 5 due to electrostatic repulsion between negatively-charged functional groups on the tip and film surface, but could not clarify the difference in chemical properties between the two nanoscale domains.  Friction force images in n-dodecane showed higher friction over etched PMMA and PS domains with an OH-terminated tip and a CH_³-terminated tip, respectively, exhibiting higher hydrophilicity of the etched PMMA domains.  In an AFM image of a UV/AcOH-treated PS-b-PMMA film upon immersion in a ferritin solution, ~80% of the ferritin deposited on the film was found on the PS domains.  The preferential deposition of ferritin on the PS domains was probably due to the electrostatic repulsion between negatively-charged ferritin and negatively-charged etched PMMA surface in addition to the hydrophobic interaction between ferritin and the PS surface.  These results indicated that the etched PMMA domains were more hydrophilic than the PS domains due to the presence of acidic functional groups (e.g., -COOH groups) at a higher density.