**4. Importance of carbon dot in bio-imaging**

For clinical imaging application, good biocompatibility and low cytotoxicity of imaging probes are essential. The traditional quantum dots (QDs) such as CdSe, CdTe, CdS, and PbS were applied in *invitro* and *in vivo* optical bioimaging procedures [53]. But their bioimaging application is restricted due to heavy toxic metals which may cause toxicological and pathological problems for health and the environment. The silicon quantum dots (Si QDs) can be considered alternatives to heavy metal QDs in bioimaging [54]. But they undergo oxidative biodegradation in biological systems and are slightly toxic. The gold and silver nanoclusters were deemed to be alternative, non-toxic, photo luminescent nanomaterials, but suffering with poor water solubility and photostability [55]. So, there is a requirement for biocompatible imaging probes with low toxicity for bioimaging. Some CDs such as low toxicity, good biocompatibility, excellent photoluminescence and high photo stability makes them a novel nanoprobe for bioimaging.

#### **4.1 Fluorescence imaging**

FL imaging is a promising technique for observing and assessing various cells, tissues and organisms, by the high fluorescence emission with biocompatible

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

fluorescent agents [56, 57]. Currently, fluorescent CDs are considered as a significant alternative ideal contrast agent for fluorescence imaging. Several reports revealed that cells incubated with CDs emitted fluorescence mainly due to accumulation of CDs in the cells. Generally, unmodified/bare CDs are majorly identified in the cell membrane and cytoplasmic region without reaching the nuclei. To resolve this issue, excessive effort have been devoted to surface modify the CDs with targeting agents including antibodies, peptides and other biomolecules to enhance the specific targeted bioimaging. These target ligands modified CDs showed a significant capacity to bind to the overexpressed reprehensive receptor/biomarker on cells. In addition, to resolve the issues related to deep tissue fluorescence imaging, the NIR receptive CDs were designed with longer excitation/emission wavelengths that enhanced fluorescence imaging ability [58] for *in vivo* applications. Under suitable excitation wavelength the NIR emitting CDs can be well differentiated from the auto-fluorescence background (green) with good optical contrast.
