SERS-Based Immunoassay on Ag/ZnO Nanorod Substrates for Detection of CA125 Antigen

Several reports have been published on the detection of the carbohydrate antigen 125 (CA125) cancer biomarker, where the immunoassay is completed with a molecule tag that is detected via surface-enhanced Raman scattering (SERS); however, it is still challenging to detect protein biomarkers without a Raman reporter. In this study, a SERS substrate based on zinc oxide nanorods (ZnO NRs) decorated with silver nanoparticles was fabricated, functionalized, and bioconjugated to detect CA125. Functionalization was performed by using an MPA self-assembled monolayer, which was subsequently surface-activated with an EDC/NHS solution. This process was optimized by using Raman measurements to determine the surface protonation of the substrate. The effect of the concentration and incubation time of the CA125 antibodies on the bioconjugation of the substrate were evaluated. SERS detection of CA125 was successfully achieved in a concentration range of 15-1000 U/mL, demonstrating performance comparable to the ELISA approach. A vibration mode at 829 cm^-1 associated with proline and tyrosine was identified and exhibited excellent linearity with CA125 concentration. A limit of detection (LoD) of 14 U/mL was estimated. This report confirms the potential of Ag/ZnO NR substrates for developing SERS assays for tumor marker detection using a portable Raman spectrometer.