This paper describes the development of a nanoscale optical pH probe based upon the surface-enhanced Raman scattering (SERS) properties of silica–gold core-shell nanoparticles. In this approach, a thin layer of gold is deposited onto a core of silica to form a metallic nanoshell with surface plasmon modes in the red-to-near-infrared spectral region. The surface of the nanoshell is functionalized with a pH-sensitive SERS reporter molecule, 4-mercaptopyridine (4-MPy). The SERS spectra of 4-MPy is shown to be sensitive to the pH of the surrounding media within the range of 3 to 7. In addition, it is shown that individual silica–gold core-shell nanoparticles yield more reliable SERS spectra than aggregates of core-shell nanoparticles.

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