It is estimated that over 2,000 women globally are impacted by ovarian cancer (OC). Unfortunately, every year, over half of the cases end in death. This underscores the critical importance of better early detection and diagnosis of OC. Research into improving a sensitive, dependable, and rapid cancer serum antigen (CA-125) detector is essential for medical diagnosis. This antigen may signal the earliest stages of ovarian cancer. The typical crystalline size of the Cadmium selenide (CdSe) quantum dots (QDs) that we synthesized and coated is adjusted in this study to be 8 ± 0.2 nm. Multiple characterization methods, including X-ray diffraction, Z-scan, and transmission electron microscopy, were utilized to verify their properties. We used these QDs as optical tags for a sandwich assay, which was used to test a high-binding surface where the capture antibody was immobilized. Popular optical tags (QDs), semiconductor nanoparticles with size-dependent fluorescence spectra, brightness, and photostability, were used and can be detected by a microplate spectrophotometer. A linear curve was observed within a range of 5–1000 U/ml CA-125, which is suitable for quantitative work, and the detection limit was 4.5 U/ml. The interference with the detection of CA-125 was tested in human serum, and the results showed no significant difference in the fluorescence spectra. Third-order nonlinear optical parameter analysis of conjugated CdSe QDs under an excitation beam power level of approximately 120 W and a wavelength of 532 nm was determined in detail. The third-order optical susceptibility (χ3) was 2.746×10^(-7) esu with a sound absorption coefficient of 4.68556×10^(-5) cm/W and nonlinear refractive index -5.6132×10^(-9) 〖cm〗^2/W. Finally, the resulting CdSe QDs have been effectively employed in the labeling and, hence, detection of ovarian cancer antigen (CA-125).