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.