Abstract: There is provided a pH responsive optical nanoprobe comprising metallic SWCNTs or graphene coated with a transition metal M. The coated metallic SWCNTs or graphene have an absorption spectrum comprising an optical resonance, and have a Raman scattering spectrum responsive to optical excitation at said optical resonance comprising at least one pH-dependent peak having at least one of a Raman shift value and an intensity that is function of a solution pH, when the nanoprobe is in contact with a solution at said solution pH. There is also provided a method to measure the pH of a solution, by contacting the solution with the nanoprobe; illuminating the nanoprobe with an excitation light beam having a wavelength at said optical resonance, thereby generating a Raman signal from the nanoprobe according to said Raman scattering spectrum; measuring a spectral distribution of the Raman signal; and determining the pH of the solution from the spectral distribution.