## 4 Applications of CDs in Cancer Theranosis
### Cancer Therapy via Drug Delivery, PTT, PDT, PTT+PDT, siRNA Delivery, SDT, and Gas Therapy
#### 4.2.2 PTT  
The PTT has emerged as a promising approach for cancer treatments due to its high therapeutic efficacy and low side effects [159; 160]. The PTT utilizes light-absorbing photothermal agents (PTAs), which absorb NIR light and convert it into heat [159]. The elevated heat induces cancer cell death, as cancer cells are more heat-sensitive than normal cells. The mechanism of PTT involves the delivery of PTAs to the cancer site. Upon NIR irradiation, the PTAs generate localized heat, leading to the denaturation of proteins, disruption of cancer cell membranes, and subsequent thermal ablation of cancer cells. Among the various PTAs, CDs have shown great potential due to their strong NIR absorption and high photothermal conversion efficiency (PCE). CDs possess \(\pi\)-conjugated electrons in the sp\({}^{2}\) domains, which enable efficient absorption and transformation of NIR light into heat [161]. In addition, heteroatom (N or S)-doped CDs exhibited enhanced NIR absorption and higher PCE, making them highly effective PTAs for cancer therapy.  
For example, [PERSON] et al. used cycph and poly(ethylene glycol) (PEG800) as the carbon sources to synthesize CDs (CyCDs) in a solvothermal method and investigated the in vivo therapeutic effects of multifunctional CDs (CyCD) using CT26 cancer-bearing BALB/c mice [162]. These CDs exhibited NIR absorption and emission in the range of 600-900 nm with a high PCE of 38.7%. The incorporation of PEG800 enhanced the hydrophilicity and photostability of the CDs, making them ideal candidates for cancer-targeted imaging and PTT. The treatment involved intracancerously injected CyCDs at a dosage of 4.0 mg CyCDs/kg body weight, followed by exposure to an 808 nm laser for 5 min to maintain the cancer temperature of approximately 45 \({}^{\circ}\)C. The treatment was administered four times at a two-day interval. After 11 days, the mice were sacrificed, and cancer weights were measured. The results indicated a significant cancer inhibitory rate of 91% in the CyCD + irradiation group as shown in Figure 11a(i,ii),b, whereas cancers in the saline and CyCD only groups exhibited a rapid growth with cancer volumes increased by 8- and 13-fold the initial volumes, respectively, demonstrating the potential of CyCDs to serve as a promising candidate in cancer theranosis. [PERSON] et al. investigated the PTT of HeLa cancer-bearing nude mice using CDs that were synthesized using polythiophene phenylpropionic acid via the hydrothermal method [163]. The treatment group received CDs via IV administration, followed by irradiation with a 671 nm laser for 10 min, while one control group was treated with CDs only without laser irradiation (labeled as C1), and the other control group received saline with laser treatment (labeled as C2) (Figure 11c,d). Results indicated that cancer tissues of the two control groups (C1 and C2) continued to grow without any significant therapeutic effect, whereas the mice that received both CDs and laser irradiation exhibited substantial empyrosis at the cancer sites, leading to a significant suppression of cancer growth. Moreover, the dark burn scab that formed on the skin of the mice was detached on day 16 after treatment, showing a complete recovery and demonstrating that CDs should be a prominent candidate for cancer theranosis. [PERSON] et al. investigated the PTT of CDs using 4T1 subcutaneous xenograft murine models [164]. These CDs exhibited broad absorption with a high PCE of 54.7% under 808 nm laser irradiation for 10 min. The study revealed that the in vivo PTT performance of the CDs was significantly effective in inhibiting cancer growth compared to control groups; neither the CDs nor the laser treatment alone effectively destroyed cancer cells or reduced cancer growth (Figure 11e,f).