Functionalized plasmonic nanostructure arrays for direct and accurate mapping extracellular pH of living cells in complex media using SERS

The extracellular pH (pHe) of living cells is one of the major factors that influence cell behaviors including cycle progression, migration, and proliferation, as well as metastasis and invasion of tumor cells. Thus, accurate sensing and mapping of the pHe is still a critical yet challenging task in the study of pHe-dependent cell behaviors. In this work, we present a method to map pHe of living cells based on surface-enhanced Raman spectroscopy (SERS). We immobilized a pH probe molecule, 4-mercaptobenzoic acid (4-MBA), on a gold quasi three-dimensional plasmonic nanostructure array (Q3D-PNA) to enable an exceptionally sensitive and reproducible pH measurement. We prudentially investigated the influences of cations and complexity of detecting solutions on the responses of 4-MBA SERS spectra to pH variations to ensure the accuracy. Herein, a normal cell line (NIH/3T3) and a tumor cell line (HepG2) were cultured on the 4-MBA modified SERS substrates. Localized pHe was detected and mapped with good spatial resolution and pH sensitivity showing pHe domains on both cells. Moreover, the averaged pHe of tumor cells was shown to be more acidic compared with that of normal cells.