Carbon dots (CDs) are nanosized carbon particles that have attracted the attention of researchers from different fields for their potential applications due to their high photostability, tunable excitation and emission, low toxicity and high biocompatibility. Reports on CDs that are soluble in non-aqueous solutions are still scarce, despite being recognized as promising materials and their compatibility with biological membranes facilitates their traversing. This feature is attractive for biomedical applications, in particular in cancer, where drug resistance and low specificity (i.e. side effects) urge the development of new drugs whose fate may be compromised by solubility, stability and clearance rate.
Recently the group of Prof. Santoyo and collaborators at Universidad of Granada (Spain) have demonstrated that the thermolysis of citric acid in DMSO and then reaction with alkyl amines yields CDs (LCDs) that bear both hydrophobic alkyl chains and carboxylic groups, and that the former make them suitable for hosting hydrophobic guests and the latter allow the modulation of their hydrosolubility. As a proof of concept the hydrophobic drug camptothecin (CPT) and the NIR fluorescent hydrophobic dye IR780 were assayed. The clinical implementation of both molecules is limited by their poor solubility, although CPT is a potent chemotherapeutic agent and IR786, besides the emission in the 807–823 nm wavelength range that makes it suitable for bioimaging, shows an absorption peak at 792 nm that yields a temperature increase and a production of ROS upon illumination with NIR light, enabling its use in photothermal and photodynamic therapies.
When LCD-2Na was loaded with CPT to yield LCD-2Na@CPT and the toxicity on a battery of cell lines was compared with an equivalent amount of free fresh CPT (Fig. 1), results demonstrated that the interaction between LCD-2Na and CPT is reversible and that the released drug is functional, despite it underwent a processing incompatible with the stability of free CPT.
Fig.1 Comparison of the cytotoxicity of LCD-2Na@CPT (green) and free CPT (red) on different cancer cell lines at different equivalent concentrations of CPT. Cell lines were incubated for 24 h with suitable amounts of either free CPT or LCD-2Na@CPT and cytotoxicity was assayed by the MTT method. Results are means ±1 standard deviation.
Additionally, the system LCD-2Na@IR780 was found to provoke an increase in the temperature of the solution up to 68℃ upon illumination with a 808 nm laser and yielded the formation of oxygen singlet with the concomitant destruction of IR780. At this point, it is important to recall that temperatures above 48℃ for minutes provoke irreversible injury that is enhanced by the reactive species produced during the destruction of IR780 by the laser.
Fig. 2 Temperature increase as a function of time of 200 μL of a water solutions containing 133 μM IR783 (red), 72 μM LCD-2Na@IR780 (blue) and 285 μM LCD-2Na@IR780 (magenta) irradiated with a 808 nm NIR laser at a power of 1.2 W cm−2 for 5 min (A) or 10 min (B). Inset B: Thermal image of the epperdorf containing the LCD-2Na@IR780 solution and the sample holder during the illumination. The temperature was recorded in real time with a high-resolution infrared camera.
It is important to highlight that LCDs are well tolerated by cells and, using a suitable length of the alkyl chains, they form inclusion complexes with hydrophobic guests of complementary size. The values of log P and the results obtained from the model molecules CPT and IR780 support the biotechnological potential of LCDs as drug carriers and in photothermal therapy.
Francisco Santoyo-Gonzalez is a Full Professor in Organic Chemistry at Universidad de Granada (Spain), full member of the Academy of Mathematics, Physical-Chemistry and Natural Sciences and founder and leader of the group Glycochemistry & Bioconjugation. His research focuses on the development of new synthetic methodologies, with a particular emphasis in those related with the click-chemistry concept, and their application in a variety of diverse (bio)fields including cyclodextrins, glycochemistry, bioconjugation, non catalytic and catalytic hybrid-materials, nano-materials, targeted drug delivery, gene-transfection and (bio)sensors.
