Discrimination between hydrogen bonding and protonation in the spectra of a surface-enhanced Raman sensor

Area of Science:

• Surface-enhanced Raman Spectroscopy (SERS)
• Computational Chemistry
• Physical Chemistry

Background:

• 4-Mercaptopyridine (4-MPY) is a known pH sensor.
• Its spectral properties change with variations in pH.
• Understanding molecular interactions in solution is crucial.

Purpose of the Study:

• To investigate the surface-enhanced Raman spectra of 4-mercaptopyridine on gold.
• To explore the sensitivity of the ring breathing mode to hydrogen bonding.
• To analyze the orientation of 4-mercaptopyridine in acidic environments.

Main Methods:

• Experimental surface-enhanced Raman spectroscopy (SERS).
• Density functional theory (DFT) calculations.
• Analysis of spectral contributions from free, protonated, and hydrogen-bonded species.

Main Results:

• The ring breathing mode of 4-mercaptopyridine is sensitive to both pH and hydrogen bonding.
• Three distinct spectral contributions were identified for free, protonated, and hydrogen-bonded 4-mercaptopyridine.
• Protonated adsorbed 4-mercaptopyridine exhibits a lower ring breathing frequency than the hydrogen-bonded species.

Conclusions:

• Adsorbed 4-mercaptopyridine can act as a sensor for complex solution environments beyond just pH.
• Cation/π interactions may influence the orientation of 4-mercaptopyridine in acidic solutions.
• The distinct spectral signatures enable detailed analysis of molecular states.