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Multivariate spectral analysis of pH SERS probes for improved sensing capabilities
Journal of Raman Spectroscopy
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DOI (Published version): 10.1002/jrs.4910
Appropriate functional groups adsorbed to the surface of plasmonic nanoparticles provides a platform for localised optical sensing. For example, intracellular pH nanosensors based on surface enhanced Raman spectroscopy (SERS) have been developed. However, the measurement methods and analysis of pH-S...
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Appropriate functional groups adsorbed to the surface of plasmonic nanoparticles provides a platform for localised optical sensing. For example, intracellular pH nanosensors based on surface enhanced Raman spectroscopy (SERS) have been developed. However, the measurement methods and analysis of pH-SERS can greatly impact the precision and accuracy of pH calibration. This paper provides several key improvements to the performance and analysis pH nanosensors which thus transforms the performance into a useable intracellular pH sensor. We report the plasmon-induced decarboxylation of para-mercaptobenzoic acid (pMBA) pH-reporters which are bound to the gold nanoparticles, and attribute this to the laser power. This detrimental decarboxylation of pMBA has significant implications for accurate reporting and analysis due to the sensitivity and reliability of the pH sensor. The greatest implication of decarboxylation of pH sensors is inaccurate or false pH reporting, because the decarboxylation spectral signatures map directly onto those that are typically used to record pH changes. Here a unique application of the multivariate statistical technique, principal components analysis (PCA) is presented along with an optimal spectral region for pH calibration. By direct comparisons between the PCA method with the typically employed ratio-metric analysis a significant improvement in generating accurate pH sensing is demonstrated. An application of intracellular pH sensing in macrophage cells using these nanosensors promotes these step-changes in pH measurement methodology.
The paper represents a significant enhancement on the analysis of spectral properties relating to producing reliable nano-reporter of pH, which can be used for in-vivo studies. The paper describes an adapted analysis methodology which focussed on key spectral markers that enable better discrimination when the reporter is in a biological host. The use of pH intracellular probes has been limited but with this newly developed analysis and extended protocols, the practical use of pH nano-reporters has the opportunity to be fully exploited in biological systems. Indeed the paper then describes an application in a human macrophage cell. The work adds significantly to the field due to the improvements made at reliably identifying pH.
nanosensors, pMBA, Raman Spectroscopy, PCA
College of Science