High-Sensitivity Detection of the Lung Cancer Biomarker CYFRA21-1 in Serum Samples Using a Carboxyl-MoS₂ Functional Film for SPR-Based Immunosensors

We constructed a novel surface plasmon resonance (SPR) detection assay using carboxyl-functionalized molybdenum disulfide (carboxyl-MoS₂) nanocomposites as a signal amplification sensing film for the ultrasensitive detection of the lung cancer-associated biomarker cytokeratin 19 fragment (CYFRA21-1). The experiment succeeded in MoS₂ reacted with chloroacetic acid giving carboxyl-MoS₂ as the reaction product. The additional shoulder in the C 1s and O 1s peaks of carboxyl-MoS₂, which were increased in X-ray photoelectron spectroscopy, confirmed the presence of O–C=O groups on the surface of the carboxyl-MoS₂. Compared to MoS₂, the experimental results confirmed that carboxyl-modified MoS₂ had improved low impedance and low refractive index. The carboxyl-MoS₂-based chip had a high affinity, with an SPR angle shift enhanced by 2.6-fold and affinity binding K_A enhanced by 15-fold compared to a traditional SPR sensor. The results revealed that the carboxyl-MoS₂-based chip had high sensitivity, specificity, and SPR signal affinity, while the CYFRA21-1 assay in spiked clinical serum showed a lower detection limit of 0.05 pg/mL and a wider quantitation range (0.05 pg/mL to 100 ng/mL). The carboxyl-MoS₂-based chip detection value was about 10⁴ times more sensitive than the limit of detection of an enzyme-linked immunosorbent assay (ELISA) (0.60 ng/mL). The results showed that the carboxyl-MoS₂-based chip had the potential to rapidly assay complex samples including bodily fluids, whole blood, serum, plasma, urine, and saliva in SPR-based immunosensors to diagnose diseases including cancer.