Multivariate spectral analysis of pH SERS probes for improved sensing capabilities

Abstract

Adsorption of functional groups to the surface of plasmonic nanoparticles provides a platform for localised optical sensing. Over the past decade, nanoscale sensors for intracellular pH measurement based on surface-enhanced Raman spectroscopy (SERS) have been developed. However, the approaches by which pH-SERS measurements are made and analysed can greatly impact the precision and accuracy of pH calibration. To improve pH nanosensors, the sources of experimental variation must be determined and the data must be optimally analysed. Here we report the plasmon-induced decarboxylation of para-mercaptobenzoic acid (pMBA) pH reporters attached to gold nanoparticles and conclude a strong association with laser power. The detrimental decarboxylation of pMBA has profound implications on the sensitivity and reliability of the pH sensor. Decarboxylation spectral signatures map directly onto those that are typically used to record pH changes, and, hence, the greatest implication of decarboxylation of pH sensors is inaccurate or false pH reporting. Here a more robust spectral analysis for pH sensing based upon an optimal spectral region for pH calibration is presented together with a unique application of the multivariate statistical technique, principal component analysis (PCA). PCA interprets complex spectral dynamics, and by direct comparisons with the typically employed ratiometric analysis, a significant improvement in generating accurate pH sensing is demonstrated. An application of these methods in determining the pH of internalised nanosensors in macrophage cells further promotes these step changes in pH measurement methodology via the avoidance of disruptive spectral signatures that arise in real applications.

Exploring the Potentiality of a SERS-Active pH Nano-Biosensor

“…In this context, colloidal noble metal nanoparticles play a central role thanks to their high surface-to-volume ratio and to their ease of synthesis and functionalization in different environmental conditions, which make them a versatile platform to develop SERS-active chemical sensors (Yeh et al, 2012). In particular, by employing weak acids as reporter molecule and gold or silver nanoparticles as enhancing scaffold, it has been possible to develop pH sensors capable to convey the SERS potential in providing localized and molecular specific information (Bishnoi et al, 2006; Lawson et al, 2014; Gühlke et al, 2015; Williams et al, 2016). The development of tools for the evaluation of pH at the nanoscale enables to obtain information on small sample volumes, e.g., microfluidic devices or even single cells (Jamieson et al, 2015; Puppulin et al, 2018).…”

Simulating pH‐dependent surface‐enhanced Raman spectra by density functional theory calculations

“…Thiophenol derivatives can strongly adsorb on metal surfaces through the formation of S‐metal bond and thus be used as surface probe molecules in SERS detection. p ‐mercaptobenzoic acid (PMBA) and p ‐mercaptopyridine (PMPY) can be transformed to their corresponding conjugated base and acid during deprotonation and protonation processes. Thus, these two molecules are widely applied as pH sensors to determine the surface pK a by measuring the pH‐dependent SERS .…”

Direct monitoring of light mediated hyperthermia induced within mammalian tissues using surface enhanced spatially offset Raman spectroscopy (T-SESORS)

“…porcine tissue signal, and being generally considered invariant to temperature. 27 Initially, the temperature probe was tested alone. As is to be expected ( Fig.…”