In Situ Imaging of Live-Cell Extracellular pH during Cell Apoptosis with Surface-Enhanced Raman Spectroscopy.

Extracellular pH (pHe) is an important regulating factor that determines many cellular processes, including proliferation, differentiation, and apoptosis. In our previous work, we developed 4-MPy (4-mercaptopyridine) modified Au nanoparticles as intracellular pH sensors based on surface-enhanced Raman spectroscopy (SERS). We herein modified a Au-nanoparticle-assembled solid SERS substrate with 4-MPy molecules for in situ pHe sensing during apoptosis. We found a more acidic extracellular environment of cancer cells than that of normal cells from the pH imaging. We then in situ investigated the temporal and spatial evolution of pHe of cancer cells after addition of transforming growth factor-β (TGF-β). The pHe showed a fast decrease at the beginning, followed by a slow decrease until the complete loss of cellular functions, and the pH values in and out of the cells became similar. This work shows that our SERS substrate combined with an in situ cell culture system is well suitable for in situ pHe sensing during cell processes and will be a promising technique for understanding more pHe-related biological and pathological issues.