Fabrication of polyethyleneimine surface-functionalized fluorescent carbon dots and its applications towards highly sensitive and selective detection of glutathione in aqueous medium and in vitro cell imaging of HeLa cells

The present study aimed to synthesize polyethyleneimine (PEI) surface-functionalized fluorescent carbon dots (CDs)-based biosensor (GP-PEI-CDs) for highly sensitive and selective detection of glutathione (GSH). In brief, green pea (GP) shells were utilized for green synthesis of blue luminescent GP-CDs through hydrothermal method. The obtained GP-CDs were surface functionalized with PEI to improve surface defects and quantum confinement effects. The surface functionalization of GP-PEI-CDs was confirmed by different spectroscopic techniques, including FTIR, XPS, etc. Switch “on” of GP-PEI-CDs was quenched by Cu(II) ions (turn “off”), and the limit of detection (LOD) of Cu(II) was found to be 23 nM along with a linearity range as 0 µM to 50 µM. Then, turn “On” process enabled the restoration in fluorescence of surface-functionalized GP-PEI-CDs when different concentrations of GSH in phosphate buffer saline (PBS, pH 7.4) was added. This could be due to split up of Cu(II) from Cu(II)@GP-PEI-CDs complex by presenting selective affinity with thiol (–SH) group of GSH among the various biomolecules. The LOD of GSH was found to be 38 nM and linearity in the range of 0 to 25 µM. The cytotoxicity study confirmed the biocompatibility of surface-functionalized GP-PEI-CDs. Furthermore, a confocal analysis indicated exceptional penetrations of GP-PEI-CDs into the cell cytoplasm and nucleus, demonstrating the created probe's suitability for GSH sensing at the cellular level. The method was successfully applied to determine GSH in in human serum sample.