Detecting tumor biomarkers plays a vital role in early identification and treatment of cancer. However, an effective diagnostic method must be cost-efficient and highly sensitive to meet the essential requirements for accurate diagnosis. Present work reports a novel fluorescence sensing approach for the selective detection of kallikrein 7 (KLK7) using poly(L-glutamic acid) (PLGA) functionalized sulfur-nitrogen doped graphene quantum dots (NSGQDs). The NSGQDs were synthesized through an eco-friendly method using Custard apple (Annona squamosa) seeds (NSGQDsCS), and their functionalization was achieved with PLGA (PLGA-NSGQDsCS). The PLGA-GQDs, with their rich surface functionality, exhibited selective binding with the activated KLK7 antibody (KLK7-Ab), inducing fluorescence quenching. The affinity between KLK7 and the KLK7-Ab triggered the unwrapping of the antibody, forming an antibody-antigen complex with fluorescence recovery. Based on this interaction, a "turn on-off-on" mechanism of sensing was established, exhibiting a robust response to KLK7 (0.1 ng/mL to 100 μg/mL) with a LOD of 0.1 ng/mL and LOQ of 0.411 ng/mL. In addition, the developed KLK7 immunosensor (KLK7-Ab@PLGA-NSGQDsCS) demonstrated excellent performance in the analysis of spiked samples, achieving an average recovery rate of 97.52%. This study highlights a highly sensitive immunosensor, offering potential for early cancer detection, with promising avenues for broader biomarker applications in clinical diagnostics.