Nanoparticle technology or nanotechnology is the study of nanomaterials in the field of electronics, medicine and agriculture. Carbon is the most abundant material on the earth having various allotropes like diamond, fullerene and graphite. Nanodiamonds, fullerenes, carbon dots (CDs), graphenes and carbon nanofibers are the new class of carbon nanomaterials, and have attracted much attention in the field of nanotechnology. CDs are the newly discovered member among the family with size less than 10 nm . These nanomaterials are enriched with superior properties like excellent quantum yield, superior biocompatibility, good water solubility, higher degree of cell permeability and low cytotoxicity  which make them useful to be applied in the fields of biology and medicine . CDs can be synthesized using various chemical and natural precursors via both top down as well as bottom up approaches. However, most of the researchers are widely using chemical precursors to synthesize CDs [4, 5]; therefore, there is much need of using eco-friendly techniques and sources to synthesize CDs in order to avoid their adverse effects. Some of the researchers have started using natural precursors for the synthesis of CDs due to their high renewability, low cost, and better biocompatibility. Various sources like plant materials, beverages, fruits and vegetables have been used to synthesize carbon dots . Among those precursors, researchers are focusing more on the plant derived CDs.
Since ancient times, plants have not only been used as a food source but also been used to treat various diseases; therefore, several herbal medicines have been considered as the traditional methods of remedy [7, 8]. Sometimes, the plant extract shows more efficacy and low toxicity in comparison to the commercial drugs . Plants are enriched with different secondary metabolites such as flavonoids, glycosides, tannins, alkaloids, terpenoids etc which are responsible for showing their medicinal properties . Extraction of the biologically active compounds from plants depend on the solvent used in the extraction procedure . Different solvents such as alcohols, ether, ethyl acetate, chloroform and water are commonly used for the extraction purpose, and obviously water is the safest one among them . Therefore, researchers are now showing their efforts for synthesizing CDs from the aqueous extract of several medicinal herbs . Ginseng is one of the among medicinal herbs [14, 15]; therefore, we used the aqueous root extract of red Korean ginseng for the synthesis of CDs.
Panax ginseng Meyer, also called Korean ginseng is a plant which has been used as the traditional medicinal herb in the eastern Asian countries such as Korea, Japan and China [16, 17]. When the fresh Korean ginseng becomes red in color upon heating, it is called red Korean ginseng (RKG) . The main constituents of RKG are ginsenosides, flavones, amino acids, acetylenic compounds, pyran derivatives and peptide glycans . Ginsenosides have various medicinal properties like anticancer, antioxidant, antibacterial, anti-inflammatory, anti-diabetic, anti-stress, and anti-apoptotic . Besides, RKG has also been widely studied and used for the treatment or improving the conditions of the various human disease [21, 22]. However, most of the researchers have used RKG extract for the synthesis of some inorganic nanoparticles [23, 24] and only one report showed the use of ginsenosides for the preparation of CDs with anticancer potential .
Recently CDs have attracted the attention of researchers as a drug delivery system [26, 27] to increase the bioavailability of the drugs into the target organs while reducing its adverse effects to normal tissue. CDs have high surface area with sp2 core within it, and can conjugate various hydrophobic biomolecules via π−π* stacking or electrostatic interactions, thereby increasing their solubility in the aqueous media [2, 28]. Besides, drugs can also be conjugated via covalent bonding with the functional groups present on the surface of CDs. Furthermore, the drug delivery pathway can also be tracked due to the fluorescent nature of the CDs . However, there is only few studies showing the use of CDs as a drug delivery system as most of the studies have focused on its optical properties [2, 30, 31].
In the present study, for the first time, we have synthesized a novel green CDs from aqueous root extract of RKG using microwave assisted heating and checked their potentials to be used as an antioxidant, antibacterial and bioimaging agents. Besides, we have also tested whether those CDs could be used as a drug delivery system for a hydrophobic natural flavonoid, rutin. Rutin is an important part of our regular diet and has profound nutraceutical benefits. It is abundantly found in the several vegetables and fruits such as, asparagus, buckwheat, figs, apple, grapes, plum. orange, and also found in some beverages such as green tea, black tea and elderflower tea . Rutin exhibits multiple pharmacological actions and hence can be used for the treatment of several diseases like hypertension, cancer, hypercholesterolemia, diabetes, etc. Besides, it is also well-known for its cytoprotective, antioxidant, antibacterial, anticarccinogenic, neuroprotective, vasoprotective and cardioprotective activities . However, the pharmacological actions of rutin are greatly limited by their low bioavailability due to poor aqueous solubility and low cell permeability . In order to overcome those limitations, CDs were used as drug delivery vehicle to improve its pharmacological actions. Since both ginseng and rutin possess anti-oxidant & anti-bacterial activities; therefore, we hypothesized that the nanohybrids (CDs-RUT) would exhibit higher antioxidant and antibacterial activities compared with free rutin by increasing the hydrophilicity and pharmacological actions of the hydrophobic drug.