Fabrication of Poly-l-lysine-Functionalized Graphene Quantum Dots for the Label-Free Fluorescent-Based Detection of Carcinoembryonic Antigen.

The diagnosis of tumor biomarkers is an attentive approach for the early detection and treatment of cancer. However, a cost-effective, simple, rapid, selective, and sensitive method is a basic prerequisite for diagnostic research. Herein, we present a novel fluorescence-based label-free sensing strategy for the sensitive and selective detection of carcinoembryonic antigen (CEA) using poly-l-lysine (PLL)-functionalized graphene quantum dots (GQDs). The GQDs were synthesized using a greener method by employing carbonized peanut shell (PNS) waste as a carbon source, and functionalization was accomplished using PLL (PLL-GQDs). The fluorescence stability of the PLL-GQDs was tested in a variety of solvent systems and pH solutions. When compared to nonfunctionalized GQDs (PNS-GQDs), prepared PLL-GQDs demonstrated increased fluorescence lifetime, high quantum yield, excellent photostability, biocompatibility, and greater cellular uptake. The PLL-GQDs with abundant surface amine and carboxylic groups showed selective interactions with an activated CEA antibody (CEA-Ab), resulting in the quenching of fluorescence signals. Because of the strong bioaffinity of CEA to the CEA-Ab, the antibody was unwrapped, resulting in the formation of an antibody-antigen complex and the recovery of fluorescence. As a result of this relationship, a turn "on-off-on" sensing mechanism with a strong response to CEA concentration (0.01 ng mL to 100 μg mL) and a detection limit of 1.19 pg mL was demonstrated. Furthermore, the fabricated CEA immunosensor (CEA-Ab@PLL-GQDs) performed admirably in real sample analysis, with an average recovery of 98.32%. The cellular uptake performance of PLL-GQDs was also demonstrated in the A427 cell lines, exhibiting a greater cellular uptake potential than PNS-GQDs. The cellular bioimaging study demonstrates that PLL-GQDs can be used for additional therapeutic and biological applications.