Methods: We synthesized PbS cores via hot injection precipitation followed by cadmium sulfide shelling through cation exchange with cadmium oleate to create PbS/CdS QDs. To generate the core/shell/shell structure, we employed zinc diethyldithiocarbamate (Zn(DDTC)₂) as a single-source precursor in a low-temperature, one-pot shelling reaction to produce PbS/CdS/ZnS QDs. Both QD compositions were thoroughly characterized using optical spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). For in vivo studies, we encapsulated both QD types in 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino(polyethylene glycol)-2000] (DSPE-PEG) lipid-PEG micelles and administered them to mice via retro-orbital injection at a dose of 5 mg/kg (combined mass of Pb and Cd). We conducted longitudinal SWIR fluorescence imaging and pharmacokinetic analysis by measuring tail artery signal intensities at multiple timepoints and optically tracking whole-body biodistribution for one week. Ex vivo organ imaging and elemental analysis provided ground truth data for validating in vivo findings.
Results: Spectroscopic analysis revealed that PbS/CdS QDs exhibited first excitonic and emission peaks at 1165 nm and 1275 nm, while PbS/CdS/ZnS QDs showed peaks at 1115 nm and 1245 nm. XRD measurements confirmed the ZnS shell presence through characteristic diffraction peaks at 28.5, 47.3, and 56 degrees. TEM imaging determined average diameters of 5.21 ± 0.40 nm for PbS/CdS QDs and 6.09 ± 0.51 nm for PbS/CdS/ZnS QDs, indicating an average ZnS shell thickness of 0.44 nm.
Pharmacokinetic analysis showed similar circulation half-lives for both particle types (PbS/CdS: 95.4 min, PbS/CdS/ZnS: 76.0 min). However, longitudinal whole-mouse imaging revealed significant differences in tissue accumulation patterns. PbS/CdS/ZnS QDs exhibited sustained fluorescence signal in the liver with an initial increase peaking around 360 min post-injection, followed by gradual decline over one week. In contrast, PbS/CdS signal in the liver diminished rapidly across all timepoints.
Ex vivo organ analysis confirmed these findings and provided crucial insights into particle degradation. The elemental analysis showed higher accumulation of PbS/CdS/ZnS QDs in liver and spleen compared to PbS/CdS QDs, while PbS/CdS unexpectedly showed higher kidney accumulation. Analysis of the Pb/Cd molar ratio revealed dramatic changes in Pb/Cd content 4 hours after PbS/CdS injection in most organs, indicating differential retention following particle degradation. PbS/CdS/ZnS maintained more consistent elemental ratios with only slight deviations at the 7-day timepoint. When plotting imaging intensities against Pb content, PbS/CdS/ZnS showed a consistent correlation between optical signal and elemental composition, while PbS/CdS exhibited poor correlation.
Discussion: Our findings demonstrate that the ZnS shell significantly enhances the biostability of PbS/CdS QDs in physiological environments, enabling more accurate longitudinal imaging for pharmacokinetic and biodistribution studies. The strong correlation between in vivo SWIR imaging and ex vivo elemental analysis for PbS/CdS/ZnS QDs validates the quantitative potential of this optical approach for tracking nanomaterials. This has substantial implications for preclinical experimental design, as PbS/CdS/ZnS QDs can serve as reliable contrast agents for longitudinal imaging of nanomedicines, enabling advanced drug delivery investigations with reduced animal numbers. The distinct degradation profiles of the two QD compositions suggest different optimal applications: PbS/CdS for applications where rapid signal reduction is beneficial (e.g., tumor imaging, guided surgeries) and PbS/CdS/ZnS for quantitative longitudinal tracking.
Conclusion: We have successfully developed and characterized PbS/CdS/ZnS quantum dots that demonstrate significantly enhanced biostability compared to PbS/CdS QDs, enabling reliable longitudinal SWIR fluorescence imaging for non-invasive pharmacokinetic and biodistribution analyses. This work advances both nanoparticle design strategies for improved in vivo stability and provides a powerful optical approach for investigating nano-bio interactions while reducing the number of study animals required.