Fig. 3. AuS-IR7p determines pH values via ratiometrically responsive Raman signals. (A) pH dependent Raman spectra of AuS-IR7p. The pH changed with an interval of 0.5 units. (B) Plot of integrated intensity ratios between Raman Peak 2 (450–595 cm−1) and Peak 1 (280–370 cm−1) as a function of pH. The inset shows measurements in the linear response region. (C) Proposed mechanism illustrating the pH ratiometric response of AuS-IR7p. Unprotonated IR7p shows parallel conformation relative to the metallic surface (left panel) but titled or even perpendicular conformation under acidic conditions (right panel) due to the intermolecular electrostatic repulsion triggered by the protonation of reporter molecule IR7p.
