To date, various QD probes have been developed for NIR-fluorescence imaging [6, 9], while obtaining high-quality NIR-emitting QDs has remained a challenging research objective due to stability and biocompatibility. Consequently, it is essential to design an aqueous method to obtain NIR-emitting QDs for high-sensitivity targeted bioimaging [1]. Deng et al. had developed an aqueous method for the synthesis of stable and bright NIR-emitting water-soluble CdTe/CdS nanocrystals [5]. However, the approach was complicated and the nanocrystals were unstable in open air. Chen and colleagues reported an improved and easier aqueous synthesis of highly fluorescent and ultrasmall size CdTe/CdS [2]. The reaction they reported can be carried out with a single procedure in the open air because Na₂TeO₃ can be readily reduced by NaBH₄ to generate Te²⁻. After 6-h reflux, the research produced NIR-emitting QDs with a PL peak wavelength of 700 nm. In the present study, we used the one-pot aqueous approach outlined above to produce a high PL quantum yield in a QD with excellent physicochemical characteristics. We adjusted the molar ratio of Cd²⁺/TeO₃ ²⁻ to 30: 1 and pH to 11; as a result, we produced high-quality NIR-emitting QDs more quickly.

The RGD peptide can specifically recognize integrin, which is restrictively expressed on the cell surfaces of malignant glioma cells [10]. However, few studies have reported NIR-emitting QDs conjugated with RGD for both bioimaging and PDT in U251 cells. As can be seen distinctly in the present study, the QD-RGD could label U251 cells (Fig. 2), but not the fibroblast 3T3 negative control cells. Besides, neither the U251 nor the 3T3 cells could be labeled by QDs without the conjugation of RGD. This labeling process was achieved by the interaction between U251 and the RGD attached to the QD surface. Accordingly, the QD-RGD can recognize U251 cells specifically and sensitively.

Ion leakage from the core of QDs has been reported, especially the leakage of Cd²⁺, which has become a significant problem because the Cd²⁺ may kill cells [21]. However, one study reported that stable coating with a shell and capping may effectively prevent ion leakage and, additionally, protect the core from air oxidation [17]. Due to its stable structure, the QD we synthesized was verified to be safe for biomedical use. QDs offer great promise in PDT applications [13, 14]. And NIR QDs can be used to penetrate tissue to depths of several centimeters, thereby allowing access to deep-seated tumors, which makes them ideal agents for PDT applications. Researchers have found statistically significant ROS production from QDs. It appears that QDs with CdSe and CdTe are very efficient at ROS generation [4]. Here, irradiation was carried out at 632.8 nm with energy density of 30 mW/cm² for various times (30 s to 10 min) and at various concentrations (3–24 μg/ml). With the increasing of irradiation time and concentration, the survival rate of U251 cells decreased (Fig. 3). The survival rate of U251 cells was significantly related to the irradiation time and concentration (p < 0.01), which proved the PDT of QD-RGD can kill U251 cells effectively.

In PDT, the photosensitizing agent transfers its triplet state energy to nearby oxygen molecules to form reactive singlet oxygen (1O²) species, which cause cytotoxic reactions in the cells. The increasing popularity of PDT is largely due to its selectivity: photosensitizer, light, and oxygen are simultaneously necessary.