Fluorescent Silkworm Silk Prepared via Incorporation of Green, Yellow, Red, and Near-Infrared Fluorescent Quantum Dots

Novel fluorescent silkworm silks incorporated with CdTe quantum dots (QDs) have been prepared via electrostatic assembly using a polyelectrolyte (PE) as a fixer. The bright fluorescence of the silks/QDs obtained could be tuned from green to near-infrared (NIR) by changing the QD size, thereby enabling the fluorescence to penetrate through an opaque pig skin of $\sim$3.5 mm in thickness. A large amount of the QDs were not only deposited on the surface of the silk, but also found permeated into the silk. The silk incorporated with just one PE/QDs/PE layer exhibited a smooth surface. However, the presence of more PE/QDs/PE layers in the silk resulted in an increased surface roughness. The mechanical properties of the silks incorporated with one PE/QDs/PE layer were close to those of pure silks, but the maximum pull force and elastic limit elongation at break of a single silk thread decreased with increasing number of PE/QDs/PE layers. Although a very small amount of ${{\rm Cd}^{2+}}$ and ${{\rm Te}^{2-}}$ ions were released from the silks/QDs in neutral and alkaline aqueous solutions, nearly no QD were released from these fluorescent silks. The fluorescence intensity of the silk/QDs was stable in neutral and alkaline aqueous solutions, but significantly decreased in acidic solution. The simple preparation method of the silks/QDs reported in this paper is both highly efficient and low cost, and the fluorescent silks obtained are anticipated to be of great interest for fabricating multicolored fluorescent cloths and anticounterfeiting labels, as well as being useful for other bio- or physico-related applications.