Flexible ultrathin free-standing fluorescent films of CdSexS1−x/ZnS nanocrystalline and protein

Abstract

The integration of fluorescent inorganic nanocrystals into hybrid thin films with desirable optoelectronic properties is strongly demanded for their applications. We prepared flexible free-standing ultrathin protein films containing fluorescent nanocrystals by means of a filtration technique assisted by metal hydroxide nanostrands with high aspect ratio and extremely high positive charges on the surfaces. First, negatively charged proteins (apoferritin), CdS_xSe_1−x/ZnS nanocrystals (and/or fluorescent dye) were assembled on the surfaces of cadmium hydroxide nanostrands in water through electrostatic interaction. The resultant dispersion of nanocomposite fibers was filtered on a porous membrane to give a thin filter cake. After cross-linking apoferritin, the nanostrands were removed, and the remaining protein/nanocrystal composite layer was peeled off from the substrate in ethanol as a free-standing fluorescent film. The film thickness could be as thin as 40 nm, without limitation in the diameter. The free-standing fluorescent films are robust enough to be transferred onto any solid substrate without rupturing. The fluorescent films with different colors were obtainable by choosing fluorescent nanocrystals (and/or fluorescent dye) to be immobilized into the protein matrix. The optical properties of these films were changed with the combination of the nanocrystals and fluorescent dye, and the fluorescent resonance energy transfer quenching (or enhancement) was clearly observed in certain conditions. The amount of CdS_xSe_1−x/ZnS nanocrystals in the film was as high as 41% in weight, and 13% in volume. The fluorescent film was transparent and without obvious segregation of the nanocrystals, as confirmed by high-resolution transmission electron microscope (HR-TEM). The fluorescent emission was bright, uniform, and as narrow as that in the original solution of the nanocrystals. This is remarkable for the application of fluorescent inorganic nanocrystals to flexible solid optical devices.