Synthesis of carbon dot based Schiff bases and selective anticancer activity in glioma cells

Abstract

Schiff bases have remarkable anticancer activity and are used for glioma therapy. However, the poor water solubility/dispersibility limits their therapeutic potential in biological systems. To address this issue, carbon dots (CDs) have been utilized to enhance the dispersibility in water and biological efficacy of Schiff bases. The amino groups on the surface of CDs were conjugated effectively with the aldehyde group of terephthalaldehyde to form novel CD-based Schiff bases (CDSBs). The results of the MTT assays demonstrate that CDSBs have significant anticancer activity in glioma GL261 cells and U251 cells, with IC50 values of 17.9 μg mL^-1 and 14.9 μg mL^-1, respectively. CDSBs have also been found to have good biocompatibility with normal glial cells. The production of reactive oxygen species (ROS) in GL261 glioma cells showed that CDSBs, at a concentration of 44 μg mL^-1, resulted in approximately 13 times higher intracellular ROS production than in the control group. These experiments offer evidence that CDSBs induce mitochondrial damage, leading to a reduction in mitochondrial membrane potential in GL261 cells. In particular, in this work, CDs serve not as carriers, but as an integral part of the anticancer drugs, which can expand the role of CDs in cancer treatment.

Fig. 1. TEM images of (A) CDs and (B) CDSBs, AFM images of (C) CDs and (D) CDSBs. The insets show HRTEM images, the size distributions, and FFT images.

Fig. 2. (A) UV-Vis absorption spectra, (B) fluorescence spectra, (C) FTIR of CDs, CDSBs and TPA.

Fig. 3. Full XPS spectra of (A) CDs and (B) CDSBs, C 1s XPS spectra of (C) CDs and (D) CDSBs, N 1s XPS spectra of (E) CDs and (F) CDSBs, O 1s XPS spectra of (G) CDs and (H) CDSBs.

Fig. 4. (A) Results of the MTT test for CDSBs in GL261 cells and U251 cells. (B) Results of the CDs MTT test in GL261 cells. (C) MTT results of CDSBs in BV-2 cells. (D) ROS levels in GL261 cells with DCF as an indicator. (E) The fluorescence intensity of DCF. (F) The rate of quantitative cell apoptosis. (G) Cell apoptosis analyzed by flow cytometry. (H) The expression of apoptosis-related proteins in GL261 cells. Data were presented as mean ± standard deviation (SD). *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001 versus control (without treated CDSBs).

Fig. 5. CDSBs-induced mitochondrial damage. (A) Mitochondrial membrane potential detected by flow cytometry, which was stained with JC-1. (B) Mitochondrial membrane potential detected by confocal laser microscopy. Red fluorescence indicated aggregates, and green fluorescence indicated monomers. (C) The percentage of JC-1 monomer-positive cells. (D) The red-to-green fluorescence ratio of cells was quantified. (E) Representative TEM images showing morphological changes in mitochondria in GL261 cells. Green and red arrows indicated normal and damaged mitochondria, respectively. The scale bars were 2 μm (upper panel) and 500 nm (lower panel). Data were presented as mean ± standard deviation (SD). *p < 0.05, **p < 0.01 versus control.

Fig. 6. The apoptosis mechanism of GL261 cells.