Live-Cell Surface-Enhanced Raman Spectroscopy Imaging of Intracellular pH: From Two Dimensions to Three Dimensions

Abstract

Visualization of intracellular pH (i-pH) using surface-enhanced Raman spectroscopy (SERS) plays an important role toward understanding cellular processes including their interactions with nanoparticles. However, conventional two-dimensional (2D) SERS imaging often fails to take into consideration changes occurring in the whole cell volume. We therefore aimed at obtaining a comprehensive i-pH profile of living cells by means of three-dimensional (3D) SERS imaging, thereby visualizing dynamic i-pH distribution changes in a single cell. We devised herefore a biocompatible and highly stable SERS pH probe, comprising plasmonic gold nanostars functionalized with a pH-sensitive Raman reporter tag-4-mercaptobenzoic acid (4MBA)and protected by a cationic biocompatible polymer, poly-L-arginine hydrochloride (PA). The positively-charged PA coating plays a double role of enhancing cell uptake and providing chemical and colloidal stability in cellular environments. The SERS-active pH probe allowed visualization of local changes in i-pH, such as acidification during NP endocytosis. We provide evidence of i-pH changes during NP endocytosis via high-resolution 3D SERS imaging, thereby opening new avenues toward the application of SERS to intracellular studies.

Recent Progress of SERS Nanoprobe for pH Detecting and Its Application in Biological Imaging

“…In addition, the phenomenon that flower-like and stellate-like nanoparticles have single-particle Raman signals has been widely reported. In this connection, Zhang [53] successfully obtained the visualized intracellular pH value (pHi) and the pHi curve of living cells through the functionalized nanostars, thus visualizing the dynamic distribution of pHi in a single cell (Figure 4c). Similarly, Zhang's research group [54] compared the pKa information behavior and pH SERS sensitivity of near-spherical isotropic and flower-like anisotropic gold coated silver nanoparticles (Figure 4d).…”

“…In addition, the phenomenon that flower-like and stellate-like nanoparticles have single-particle Raman signals has been widely reported. In this connection, Zhang [53] successfully obtained the visualized intracellular pH value (pHi) and the pHi curve of living cells through the func- Reprinted with permission from ref. [43].…”

Advances in Surface Enhanced Raman Spectroscopy for in Vivo Imaging in Oncology

“…Raman imaging with SERS nanoparticles has shown excellent sensitivity, high specificity, low background, and multiplexing capability. [30][31][32][33][34][35][36][37] Among the several plasmonic substrates capable of generating SERS enhancement, such as gold, silver, and copper nanomaterials, gold nanostructures stand out due to their biological inertness, which promises a greater potential for clinical translation. [38][39][40][41][42][43][44][45] In addition, of the different shapes of nanostructures, such as spheres, rods, cubes, prisms, and pyramids, the star-shaped nanoparticles (i.e., nanostars) (Fig.…”

Recent Progress of SERS Nanoprobe for pH Detecting and its Application in Biological Imaging

“…In addition, the phenomenon that flower-like and stellate-like nanoparticles have single-particle Raman signals has been widely reported. In this connection, Zhang [55] successfully obtained the visualized intracellular pH value (pHi) and the i-pH curve of living cells through the functionalized nanosar, thus visualizing the dynamic distribution of i-pH in a single cell (Figure . 3c).…”