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

Abstract

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.

Recent Advances in the Fabrication and Functionalization of Flexible Optical Biosensors: Toward Smart Life-Sciences Applications

“…( b ) Branched AuNPs on PS-P4VP nanofiber: uniform distribution of well-dispersed branched AuNP arrays, which allow for significantly higher SERS response than spherical AuNPs, and schematic representation of the fabrication procedure with application in the direct measure of the internal pH of cells and external pH gradients in cell culture environments. Adapted with permission from [ 107 ] Copyright (2020) American Chemical Society.…”

“…P-ATP was selected as a target analyte to demonstrate the high SERS sensitivity and reproducibility of the substrate, which showed an EF of about 9.0 × 10 3 and a LOD of 10 −14 mol/L (Figure 4a). Recently, Zhao et al [107], combined the low-invasiveness and high spatial resolution of nanofibers and branched AuNPs (nanostars and nanoribbons) arranged in a monolayered distribution into a reproducible SERS biosensor. The authors reported the spatial mapping of the pH gradient in a cellular environment with applications in cancer diagnosis and non-invasive cell monitoring.…”

“…(b) Branched AuNPs on PS-P4VP nanofiber: uniform distribution of well-dispersed branched AuNP arrays, which allow for significantly higher SERS response than spherical AuNPs, and schematic representation of the fabrication procedure with application in the direct measure of the internal pH of cells and external pH gradients in cell culture environments. Adapted with permission from[107] Copyright (2020) American Chemical Society.Moreover, Saravan et al[108] reported on the synthesis of electrospun nanofibers starting from a blend of modified thiol functionalized adenine (L) and PAN polymer. The AuNPs/PAN/L fibrous mats had an AuNPs concentration, which could be directly related to the percentage of the bioessential ligand available on the surface.…”

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

“…Among the three kinds of mainstream pH probe molecules, 4-mercaptobenzoic acid was often widely used as pH Raman report molecules by various research groups to construct pH SERS probes [35,36]. The main reasons why it can be used as a Raman report molecule for the construction of pH Raman probe are its simple structure, wide pH response range, high photochemical stability, mercaptan mediated metal surface affinity and its ability to form a single molecular layer on the metal surface through molecular self-assembly.…”

Design of Surface Enhanced Raman Scattering (SERS) Nanosensor Array

“…The colloidal approach has been applied to image the pH in living cells [ 13 ]. The literature has been summarized in reviews [ 10 , 14 ], while more recent work has been achieved on gold colloids to increase the number of hot spots in hyperbranched Au [ 15 ]. On the other hand, the plasmonic nanostructures have also been obtained from nanofabrication as an array of nanoneedles [ 16 ] on a polymeric substrate, which demonstrated pH sensing on a mechanically stable array.…”