Surface-enhanced Raman scattering (SERS) probes based on a charge transfer (CT) process with high stability and reproducibility are powerful tools under open-air conditions. However, the key problem ahead of practical usage of CT-based SERS technology is how to effectively improve sensitivity. Here, a novel ternary heterostructure SERS substrate, Fe3O4@GO@TiO2, with a significant enhancement factor of 8.08 × 106 was first synthesized. We found the remarkable enhanced effect of SERS signal to be attributed to the resonance effect of CuPc, CT between GO and TiO2, and enrichment from a porous TiO2 shell. In addition, we developed a robust SERS probe with good recyclability under visible light illumination on Fe3O4@GO@TiO2 nanocomposites toward ultrasensitive detection of cancer cells down to three cells. We have now successfully applied this probe for in situ quantification and imaging of programmed cell death receptor ligand 1 (PD-L1) on triple-negative breast cancer cell surface at the single-cell level and for monitoring the expression variation of PD-L1 during drug treatment.