1：Experimental Design and Method Selection:

The study employed a hydrothermal method to prepare carbon quantum dots using Citrus lemon pericarp. The formed carbon quantum dots were used in the reduction and stabilization of zinc acetate to synthesize carbon quantum dots-zinc oxide nanocomposite. A sandwich capping antibody-antigen-antibody immunosensing system was formed by immobilizing a non-conjugated monoclonal antibody BM 19.21 on the surface of carbon quantum dots-zinc oxide nanocomposite and another monoclonal antibody KS 19.1, which was coated on the microtiter well surface.

2：21 on the surface of carbon quantum dots-zinc oxide nanocomposite and another monoclonal antibody KS 1, which was coated on the microtiter well surface. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: Human serum samples were used to detect the tumor marker CYFRA 21-

3：List of Experimental Equipment and Materials:

Instruments included an Ultrospec 2100-Biochrom spectrophotometer, transmission electron microscope (TEM) JEOL 1200EX, scanning electron microscope (SEM) JSM-7610F, Biotek Synergy H1 multimode reader, Perkin Elmer FT-IR spectrophotometer, micro-Raman spectrometer, Kratos Axis Ultra X-ray spectroscopy system, and Siemens D-5000 diffractometer. Chemicals included CYFRA 21-1-non-conjugated monoclonal antibodies (mAb) BM 19.21 and KS 19.1, Citrus lemon fruits, phosphate-buffered saline (PBS), and carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS).

4：21 and KS 1, Citrus lemon fruits, phosphate-buffered saline (PBS), and carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS). Experimental Procedures and Operational Workflow:

4. Experimental Procedures and Operational Workflow: The green hydrothermal preparation of carbon quantum dots (CQDs) was conducted using Citrus lemon pericarp. The preparation of carbon quantum dots-zinc oxide nanocomposite involved a simple chemical reduction reaction using CQDs as a reducing and stabilizing agent. The immobilization process involved adding NHS and EDC to CQDs/ZnO nanocomposite solution under continuous stirring, followed by the addition of non-conjugated monoclonal BM 19.21 antibody.

5：21 antibody. Data Analysis Methods:

5. Data Analysis Methods: The fluorescence and Fourier transform infrared (FT-IR) spectra of the suggested immunosensing system were checked using a Biotek Synergy H1 multimode reader and Perkin Elmer FT-IR spectrophotometer, respectively. Raman spectra, X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD) pattern were measured using micro-Raman spectrometer, Kratos Axis Ultra X-ray spectroscopy system, and Siemens D-5000 diffractometer, respectively.