*6.2. Composites of Nanocellulose and Carbon Quantum Dots*

Carbon quantum dots (CQDs) are quasispherical carbon nanoparticles (less than 10 nm in diameter) with a chemical structure and physical properties similar to those of graphene oxide. These nanoparticles emit a strong wavelength-dependent fluorescence. By changing the CQD size, the color of the emitted light can be tuned from deep ultraviolet to visible and near-infrared light. In addition, the fluorescence of CQDs, and also their water solubility, can be further modulated by functionalizing their surface with various atoms, chemical functional groups and molecules, such as metals, carboxyl groups, organic dyes and polymers. CQDs present good photostability, low photobleaching and relatively low cytotoxicity, and they are therefore considered to be suitable for biomedical applications such as bioimaging, biosensing, photodynamic and photothermal therapy of cancer, and drug delivery [165].

In hybrid materials with nanocellulose, CQDs were applied for constructing biosensors and drug delivery systems, and also for water purification. An optical sensor for visual discrimination of biothiols was based on a bacterial cellulose nanopaper substrate with ratiometric fluorescent sensing elements. These elements included N-acetyl l-cysteine capped green cadmium telluride (CdTe) quantum dots-rhodamine B and red CdTe quantum dots-carbon dots [26]. Hybrid materials containing carbon quantum dots and cellulose are also promising carriers for drug delivery. Composite core/shell chitosan-poly (ethylene oxide)-carbon quantum dots/carboxymethyl cellulose-poly(vinyl alcohol) nanofibers were prepared through coaxial electrospinning as a biodegradable implant for local delivery of temozolomide (TMZ), an anticancer drug. When tested in vitro, the antitumor activity of TMZ conjugated with carbon quantum dots against the tumor U251 cell lines was higher than the activity of the free drug [28]. Last but not least, carbon quantum dots, homogeneously dispersed together with magnetic Fe3O4 nanoparticles in electrospun cellulose nanofibers, were promising for the removal of Hg(II) ions from water [27].
