Incorporation of quantum dots into silk biomaterials for fluorescence imaging

Tracking the distribution and degradation of biomaterials after <i>in vivo</i> implantation or injection is important for tissue engineering and drug delivery. Intrinsic and externally labeled fluorescence has been widely used for these purposes. In the present study, 3-mercaptopropionic acid (MPA)-coated CdTe quantum dots (QDs) were incorporated into silk materials via strong interactions between QDs and silk, likely involving the hydrophobic beta-sheet structures in silk. MPA-QDs were pre-mixed with silk solution, followed by ultrasonication to induce silk gelation or by blending with polyvinyl alcohol (PVA) to generate silk microspheres. Silk structural changes and hydrogel/microsphere morphologies were examined by ATR-FTIR and SEM, respectively. The fluorescence of QDs-incorporated silk hydrogels and microspheres remained stable in PBS pH 7.4 for more than 4 days. The amount of QDs released from the materials during the incubation was dependent on loading; no QDs were released when loading was below 0.026 nmol/mg silk. After subcutaneous injection in mice, the fluorescence of QDs-incorporated silk microspheres was quenched within 24 h, similar to that of free QDs. In contrast, the QDs-incorporated silk hydrogels fluoresced for more than 4 days <i>in vivo</i>.