Formation of holoferritin hexagonal arrays in secondary films due to Alder-type transition

Hexagonally packed holoferritin arrays were produced on a silicon wafer by mechanically expanding (using a scratching method) the holoferritin solution to form a wetting film on the surface. This film is then dried, during which the protein array forms. The packing state of holoferritin depended on the surface charge of the molecules in the wetting film and on the drying rate of the film (i.e., evaporation rate of the water from the film). We found that a secondary minimum was created and then annihilated near the air/water interface and had a potential profile during the evaporation of the water from the wetting film. Consequently, we introduced a concept called the lifetime of a secondary minimum. Since this minimum formed in two-dimensions in the wetting him (primary film), we called it a secondary film in contrast to the primary film that was primarily created in two-dimensions. Such a secondary film can provide a two-dimensional container for nanometer-sized particles, such as proteins, and bring about an Alder-type transition. This new formation technique has widespread application in industry, such as mass production of high-sensitivity bio-sensors.