Synthesis and optical properties of emission-tunable PbS/CdS core-shell quantum dots for in vivo fluorescence imaging in the second near-infrared window

Near-infrared (NIR) fluorescence imaging at wavelengths from 1000 to 1500 nm (2nd-NIR window) is a promising modality for in vivo fluorescence imaging because of the deeper tissue penetration with lower tissue scattering of the 2nd-NIR light. For such in vivo fluorescence imaging, highly fluorescent probes in the 2nd-NIR wavelength region are needed. Although single-walled carbon nanotubes and Ag2S quantum dots (QDs) have recently appeared as 2nd-NIR fluorescent probes, their fluorescence brightness is relatively low (quantum yields <6%). In this study, we developed a synthetic method for preparing highly fluorescent PbS/CdS core-shell QDs (quantum yields, 17% in water) with narrow band widths (<200 nm) that emit in the 2nd-NIR region. By overcoating of a CdS shell onto a PbS QD core, we could easily control the emission wavelengths of the PbS/CdS QDs at 1000 to 1500 nm. To use the QDs for in vivo imaging, we investigated the optical properties of QDs (penetration depth and blurring of fluorescence images in slices of skin, brain, and heart in mice) in the 2nd-NIR region. We found that the 2nd-NIR fluorescence imaging at ca.1300 nm using the PbS/CdS QDs results in the highest signal to background ratio with a low blurring for in vivo imaging. To confirm the capabilities of the PbS/CdS QDs for in vivo imaging, we conducted fluorescence angiography imaging of a mouse head.