**5. Black phosphorus QDs: synthesis methods, modification, cytotoxicity, and application in cancer cell imaging**

 Black phosphorus (BP) is another two-dimensional nanostructure that has attracted a lot of attention in less than a decade [ 52 , 61 ]. like graphene, BP is composed of only one element (phosphorus) and is widely used in bioimaging and drug delivery due to its high biocompatibility, biological activity, biodegradability [ 62 , 63 ]. Due to the high surface-to-volume ratio, BP nanosheets are highly efficient in loading anticancer drugs. In addition, with excellent photothermal conversion efficiency, BP nanosheets generate heat locally under laser irradiation in the NIR region, which can be used in photothermal therapy (PTT) of cancer [ 64 , 65 ]. Also, due to its unique electronic structure, BP can be used as a photosensitizer to produce singlet oxygen and be effective in photodynamic therapy (PDT) of cancer [ 66 , 67 ]. Moreover, BP nanosheets can also be used for photoacoustic imaging of tumors [ 68 ]. Black Phosphorus Quantum Dots (BPQDs) are a metal-free layered semiconductor derived from BP nanosheets that were first introduced in 2015 [ 69 , 70 ]. There are many ways for synthesis of BPQDs, including ultrasonic exfoliation, electrochemical exfoliation, solvothermal, milling crash, blender breaking and pulsed laser irradiation, which classified into two general categories: top-down and bottom-up methods ( **Figure 10** ). Since BPQDs are not very stable in aqueous environment and may react

 **Figure 10.**

 *The typical approaches for the synthesis of BPQDs [ 5 ].* 

#### **Figure 11.**

 *Application of BPQDs in cancer cell imaging. (A) fluorescence images of Hep G2 and 4 T1 cells incubated with RdB/PEG-BPQDs. reprinted with permission from [ 57 ]. (B) In vitro imaging of 293 T cells treated with FA-PEG@BPQD@DOX and PEG@BPQD@DOX. (C) In vivo thermal images of mice with 293 T tumors irradiated at 808 nm. reprinted with permission from [ 58 ].* 

with oxygen and oxidize, their surface modification should be done. Polymer modified black phosphorus quantum dot, assembly off quantum dot in device, quantum dot molecule complex, quantum dot doping film and quantum dot nanosheet hybrid are among the common modifications that can be performed on black phosphorus quantum dot [ 5 ].

 Several studies have shown that BPQDs have negligible cytotoxicity. However Gui et al. reported that BPQDs at a concentration up to 200 mg/mL had significant apoptotic effects on HeLa cells [ 69 ]. This type of QDs has many applications in fluorescence, photoacoustic, and thermal imaging of cancer cells [ 5 ]. In one study Li et al. reported that in the presence of RdB/PEG-BPQDs, the distinct fluorescence signals were observed in Hep G2 cells and 4 T1 cells ( **Figure 11A** ). Their observations also showed that the QDs were located almost exclusively in the cytoplasm, and no visible fluorescence was observed in cancer cells without the nanoprobes [ 71 ]. Wang and colleagues synthesized BPQDs modified with poly ethylene glycol and folic acid for cancer imaging. There in vitro experiment showed that the fluorescence was initially observed in the cytoplasm and was observed in the nucleus ( **Figure 11B** ). In addition, the thermal image indicated that after 4 h intravenous injection of FA-PEG@BPQD@ DOX, the temperature of tumor site was reached to 56.8°C, which is sufficient for killing tumor cells ( **Figure 11C** ) [ 72 ].
