We electrostatically placed a single ferritin molecule on a nanometric 3-aminopropyltriethoxysilane (APTES) pattern that was on an oxidized Si substrate. The numerical analysis of the total interaction free energy for ferritin predicted that a quadrilateral array of 15nm diameter APTES nanodisks placed at intervals of 100nm would accommodate a single molecule of ferritin in each disk under a Debye length of 14nm. The experiments we conducted conformed to theoretical predictions and we successfully placed a single ferritin molecule on each ATPES disk without ferritin adsorbing on the SiO2 substrate surface.
High-density cage-shaped proteins with inorganic cores were selectively adsorbed as a monolayer onto a 3-aminopropyltriethoxysilane (APTES) layer on a Si substrate. The electrostatic interaction between the protein and substrate surface was studied and it was proven that protein adsorption density depends on the quantitative balance of surface charge on the substrate and protein. The combination of a highly positive APTES layer and moderately negative ferritin, Fer-4, achieved an adsorption density of 7:6 Â 10 11 cm À2 and the combination of the APTES layer and Listeria ferritin (Lis-fer) reached an adsorption density of 1:3 Â 10 12 cm À2 . The adsorption process including the reduced charge of Lis-fer due to denaturation further enhanced the adsorption density up to 1:5 Â 10 12 cm À2 , whereas no Lis-fer was adsorbed onto the SiO 2 surface under the same conditions. This new technique makes it possible to produce a nanodot monolayer with a density higher than 1 Â 10 12 cm À2 , which can be applied to floating nanodot gate memories.
The cavity of the toroidal protein TRAP (trp RNA-binding attenuation protein) is modified to capture gold nanodots in solution. By engineering a titanium-binding peptide onto one surface of the ring it is also possible to bind it specifically and tightly to a TiO2 surface. TRAP bound in this way is then used to capture gold nanodots and attach them to prepared surfaces. Gold-protein complexes are observed using atomic force microscopy and transmission electron microscopy. The modified TRAP is used to build gold nanodots into the SiO2 layer of a metal oxide semiconductor. This is the first use of a ring protein, rather than the more commonly used spherical protein cages, to constrain nanodots to a surface. This method is an important addition to the current range of bionanotechnology tools and may be the basis for future, multicomponent electronic devices.
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