*4.2.2 Fluorescence/magnetic resonance imaging*

Magnetic resonance imaging (MRI), is a radiation-free and non-invasive imaging technique widely used to detect various diseases including clinical cancer diagnosis and therapeutic response assessment [63]. With an external radiofrequency pulse magnetic field applied on the body, it simultaneously obtains anatomical and physiological information of regions of interests with a high spatial resolution by manipulating magnetic nucleus's resonance (1 H). Further, the integration of FL imaging with MR imaging considered as most effective non-invasive imaging tool in diagnosis and clinical research due its excellent spatial resolution, high temporal and sensitivity. Hence, the CDs assisted FL/MR dual modality potentiality can take benefits of the spatial resolution, outstanding soft-tissue contrast with MR imaging as well as superior sensitivity and the rapid data acquiring with FL imaging. This dual imaging modality is facilitated precise diagnosis with effective treatment based on corresponding imaging evidence. The CDs-relayed FL/MR dual-mode imaging modality probe can be obtained by the doping/ conjugation of magnetic elements. Specifically, extremely paramagnetic ions including Gd3+, Mn2+, and Fe3+ employed as dopant for the preparation of FL/MR bimodal contrast agent such as Gd or Mn elements into CDs. For example, Gd3+ doped CDs were synthesized from Gd3+ containing precursors and sucrose as carbon precursors via microwave assisted method polyol by Gong Ningqiang, *et al.* The attained Gd-CDs showed green fluorescence emission, low cytotoxicity and optically label cells. Meanwhile, the r1 relaxivity of Gd-CDs was measured to be 11.356 mM−1 s−1. This high r1 value together with the r2/r1 ratio approximately 1. These results indicating that Gd-CDs is not only significant fluorescent imaging agent but also remarkable T1 contrast agent for MR imaging [64].

Further, Jia Qingyan, *et al.* demonstrated the magneto-fluorescent Mn-doped CDs for bimodal FL/MR imaging in a single probe. The study reported the development of ultrafine Mn-doped CDs with a concurrent bimodal imaging ability through the solvothermal procedure of the precursor manganese (II) phthalocyanine [65]. The Mn-doped CDs showed strong T1-weighted MRI signals and low cytotoxicity. The MRI signal intensities increased with the concentration, exhibiting a clear difference in brightness with a measured relaxation (r1) value of ≈6.97 mM−1 s −1. Furthermore, the *in vivo* T1-weighted results fortified the high retention rate of the Mn-doped CDs in tumors. The MRI signal intensity at the tumor site increased quantitatively by ≈320%, after 6 hrs injection while the MRI signal remained nearly unchanged for the analogous CDs without manganese (II) doping. CDs doped with dysprosium for a magneto-fluorescent bimodal imaging agent showed strong blue-green fluorescence at 452 nm. The excellent transverse relaxivity r2 makes them also suitable for T2 weighted imaging of live cells [66].

### *4.2.3 Fluorescence/X-ray computed tomography*

X-ray computed tomography (CT) is a non-invasive medical imaging technique for disease diagnosis. The CT has intrinsic advantages such as high spatial resolution and good density; it still has inherent drawbacks of low sensitivity. On the other hand, the fluorescence imaging has high sensitivity, facile operation and low cost, but its application was hampered due to the low spatial resolution and limited penetration depth. To improve clinical diagnostic accuracy and sensitivity, the FL and CT imaging are combined for a synergistic effect. The CDs doped with Hafnium (Hf) was used for diagnostic imaging of the orthotopic liver cancer preclinical model [67]. Rapid imaging was achieved with Hafnium doped CDs due to preferential tumor accumulation within 1 min. These imaging nanoprobes with efficient renal clearance offers good biocompatibility. Iodine doped CDs conjugated with a chemotherapeutic agent like cetuximab simultaneously rendered the cancer diagnosis and targeted anti-cancer therapeutic potential in lung cancer cells [68].

### **5. Imaging guided therapeutic application**

Nanotechnology provides the possibility of developing non-toxic CDs nanoprobes with enhanced sensitivity, accuracy and advanced functionalities for imaging-guided synergistic therapy [69, 70]. The unique advantages of CDs include high relaxivity, prolonged blood circulation time, multiple functionalities for accurate accumulation in the target site, good biocompatibility and renal clearance. The inherent radio resistance of tumors and inaccurate positioning of the radiotherapeutic equipment leads to decreased radiotherapy effectiveness. Du Fengyi, *et al.* reported the theragnostic Gd-CDs with stable photoluminescence at the visible region, relatively long circulation time, efficient passive tumor targeting ability and renal clearance for MRI-guided radiotherapy a tumor [71]. Changhong Zhao *et al.* developed red-emitting wavelength multifunctional CDs for cancer theragnostic with *in vivo* bimodal imaging of tumor tissues and anti-cancer chemo-dynamic treatment (CDT). The functionalization of red CDs was done with Ethylene di amine tetra-acetic acid, Fe2+and Gd3+exhibited strong T1 weighted MR imaging and excellent bright and stable fluorescence. The anticancer CDT effect was based on Fenton reaction, by releasing Fe2+ into the tumor both *invitro* and *in vivo* [72].

### **6. Conclusions and outlook**

Naturally, renewable sources derived CDs are kind of newly born luminescent carbon-based nanomaterials in this decade. They gained great potential in bio-imaging not only because of their cost-effective and eco-friendly green synthetic approaches but also their physical, chemical and biological properties. We have elaborately discussed various synthesis methods, significant properties. Furthermore, the recent development of CD in multimodal bio-imaging. Their strong fluorescence emission, high fluorescent quantum yield, and good absorbance are widely used for fluorescence imaging. Specific CDs also allow for multicolour bioimaging due to their multicolour emission capability. Further, numerous surface functional groups provide an opportunity to conjugate with targeting moieties such folic acid for targeting imaging. Accordingly, CDs conjugation with targeted moieties can precisely transport imaging contrast agents to internal organelles or cell membranes to attain the goal of targeted bio-imaging. In the meantime, the large surface area of CDs permits them to have a more quantity of hetero atom loading ability, consequently showing remarkable multimodal imaging ability. Finally, the nano size of CDs (typically >10 nm) facilitates their navigation in tissues, endocytosis, and intracellular trafficking. Even though significant efforts have devoted to improving the multimodal imaging effect of CDs, several limitations hinder the

*Naturally Derived Carbon Dots as Bioimaging Agents DOI: http://dx.doi.org/10.5772/intechopen.96912*

application of CDs in bio-imaging. Primarily, the emission from most of the natural sources derived CDs showed blue or green, thus developing the methods and finding suitable natural precursor for yellow or red emissive CDs is highly desired. CDs exhibit excellent biocompatibility; however, majority studies are confined to cellular and preclinical experiments, but translation into clinical investigations is still unclear. In summary, more research still needs to be made for the effective and real-time clinical application of CDs in multimodal imaging.
