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Nanoscale pH Profile at a Solution/Solid Interface by Chemically Modified Tip-Enhanced Raman Scattering

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journal contribution
posted on 21.06.2016 by Prompong Pienpinijtham, Sanpon Vantasin, Yasutaka Kitahama, Sanong Ekgasit, Yukihiro Ozaki
A nanoscale pH profile on a 4 × 4 μm2 area of NH2-anchored glass slide in an aqueous solution is constructed using chemically modified tip-enhanced Raman scattering (TERS). p-Mercapto­benzoic acid (pMBA) and p-amino­thiophenol (pATP) are bonded to the tip surface. A pH change can be detected from a peak at 1422 cm–1 due to the −COO stretching vibration from pMBA and that at 1442 cm–1 due to the NN stretching vibration arising from the formation of 4,4′-dimercapto­azobenzene (DMAB) on the pATP-modified tip. The pMBA- and pATP-modified tip can be used to determine pH in the range of 7–9 and 1–2, respectively. The spatial resolution to differentiate pH of two areas can be considered as ∼400 nm. The measured pH becomes the pH of the bulk solution when the tip is far by ∼200 nm from the surface. This technique suggests a possibility for the pH sensing in wet biological samples. TERS tips could also be chemically modified with other molecules to determine other properties in a solution.