Recently, nanometer-size structures have been attracting researchers' interests. A living thing, however, has already realized many kinds of nanometer-size functional structures. For example, the bacterial flagellar motor whose diameter is only 30 nm rotates at 10,000-100,000 rpm. Muscle is a linear motor and its 'engine' is around 20 nm. These nano- structures are made by self-assembly manner. This self-assembly is the key characteristic for building molecular machines and indispensable for handling nanometer-size molecules not only in the cell but also in the nanotechnology.
Some protein has the ability to accommodate inorganic materials, which called biomineralization. Based on the protein's ability of self-assembly and biominearalization, we propose making inorganic nano-structures using protein supramolecules, which we name Bio-Nano-Process. As the first step, we fabricated two-dimensional array of quantum dots on a Si substrate using ferritin molecule.
Ferritin is a spherical protein supramolecule with a diameter of 12nm and composed of 24 hetero subunits. It has a 6 nm cavity which accommodates about 4,000 iron atoms in vivo. We employ a recombinant L-ferritin (1), composed of only L-type subunits from horse liver and its mutants. A two-dimensional crystal of iron-oxide loaded L-ferritin molecules which self-assembled at an air/water interface was transferred onto a hydrophobic Si surface (2). A well-ordered array of L-ferritin on the Si surface is observed by the high resolution scanning electron microscopy (SEM). L-ferritin shells were eliminated by heat-treatment under nitrogen or UV-ozone treatment, which left the array of nanometer size iron-oxide dots with little deformation. Fourier transform IR spectrophotometer (FTIR) analysis and XPS observation confirmed that the both treatments remove the protein shell completely. This well-ordered array of nanodot is suitable for quantum electronics key component.
We have also succeeded to lode inorganic materials other than iron into ferritin cavity. For example Co, Ni, Mn, and CdS, which have different work functions. As the ferritin surface could be modified by short DNA sequence, some specific combination of different kinds of ferritin with different material core would be realized. This gives a way to make groups of quantum wells with different energy levels and allows the design of more complex electronic circuits. The more precise and stronger interaction among ferritin molecules could be achieved modification of the ferritin surface by gene technology. With this method, we now try to make ferritin molecule dimmers and trimmers, each molecule of which has a different inorganic material core. This also leads to the complex nano-structures.
Our experimental results together with the idea of ferritin-ferritin interaction described above, it is expected that nanometer-size electronics circuits will be produced by this Bio-Nano-Process.
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