Fluorescent labels for biomedical imaging should possess several qualities including high quantum yield (QY), good photochemical stability and excellent water solubility. Specifically, sufficient optical penetration in tissue requires the near infrared (NIR) emitting of fluorophore in the optical window of 650-900 nm [1-2]. Also the size of the fluorophore should be controlled, small enough to avoid possible accumulation in the body and enhance the transportation ability in cells.
In current work, CdTe/CdS coresmall/shellthick QDs were synthesized using lattice-mismatch strain tuning approach [3] in aqueous phase [4]. The small cores were effectively compressed by lattice strain owing to the growing thicker shell, and a transition of bandgap offset from type-I to type-II nanocrystals occurred during the well-controlled gradual growth of the shell, allowing giant spectral shift from visible to NIR spectral region (475–810 nm). These highly luminescent QDs with QY up to 65% were conjugated with folic acid (FA) for targeted biological in vivo imaging. The QD–FA probes exhibited low acute toxicity in mice in terms of the body weight and major organs histology (heart, liver, kidney, spleen, lung, and brain). The QD–FA probes were further injected into tumor bearing nude mice through the tail vein and the tumor was distinguishable from back ground signal (normal tissues) by the NIR light from QD–FA probes (700 nm) after 4 h injection (Fig. 1A).
Another approach to fabricate NIR emitting QDs is choosing the narrow bandgap semiconductor that enables longer emitting wavelength. In this case, I–III–VI2 CuInS2 (CIS) QDs are environmentally friendly and biocompatible, since they contain no heavy metal ions. Therefore, we prepared highly emissive ZnS passivated CIS QDs in 650-700 nm emitting region with quantum yield above 60% in the organic phase. To make water dispersible and brilliant CIS QDs ready for biomedical use, which is a main challenge in current researches; we developed an effective strategy using organosilane micellization [5] to incorporate multiple hydrophobic CIS/ZnS QDs directly into silica beads with a relative small diameter around 20 nm. The silica coating layer maintained the emission properties of QDs regarding the photoluminescent spectrum, quantum yield and the PL lifetime. The obtained CIS/ZnS@SiO2 nanoparticles exhibited advanced PL stability in aqueous phase which favored their further biomedical imaging applications.
Keywords: Near infrared emitting QDs, CdTe/CdS coresmall/shellthick, CIS/ZnS@SiO2, biomedical fluorescent imaging.
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