Branched Au Nanoparticles on Nanofibers for Surface-Enhanced Raman Scattering Sensing of Intracellular pH and Extracellular pH Gradients.

The development of plasmonic-active nanosensors for surface-enhanced Raman scattering (SERS) sensing is important for gaining knowledge on intracellular and extracellular chemical processes, hypoxia detection, and label-free detection of neurotransmitters and metabolites, among other applications in cell biology. The fabrication of SERS nanosensors for optophysiology measurements using substrates such as nanofibers with a uniform distribution of plasmonic nanoparticles (NPs) remains a critical hurdle. We report here on a strategy using block copolymer brush-layer templating and ligand exchange for fabricating highly reproducible and stable SERS-active nanofibers with tip diameters down to 60 nm and covered with well-dispersed and uniformly distributed branched AuNPs, which have intrinsic hotspots favoring inherently high plasmonic sensitivity. Among the SERS sensors investigated, those with Au nanostars with short branches [AuNS(S)s] exhibit the greatest SERS sensitivity, as verified also by COMSOL Multiphysics simulations. Functionalization of the AuNS(S)s with the pH-sensitive molecule, 4-mercaptobenzoic acid, led to SERS nanosensors capable of quantifying pH over a linear range of 6.5-9.5, covering the physiological range. These pH nanosensors were shown to be able to detect the intracellular pH as well as extracellular pH gradients of in vitro breast cancer cells with minimal invasiveness and improved SERS sensitivity, along with a high spatial resolution capability.