Theoretical and pH dependent surface enhanced Raman spectroscopy study on caffeine

Area of Science:

• Analytical Chemistry
• Spectroscopy
• Physical Chemistry

Background:

• Understanding caffeine's molecular behavior is crucial for various applications.
• Surface-enhanced Raman spectroscopy (SERS) offers a sensitive method for molecular analysis.
• Previous assignments of caffeine's Raman spectrum require further validation.

Purpose of the Study:

• To investigate the vibrational behavior of caffeine on a silver colloid surface.
• To characterize caffeine's adsorption orientation and mechanism at different pH values.
• To refine spectral assignments using computational methods.

Main Methods:

• Recording SERS spectra of caffeine on a silver colloid across a range of pH values.
• Utilizing density functional theory (DFT) calculations for structural and vibrational analysis.
• Applying SERS surface selection rules for interpretation.

Main Results:

• DFT calculations accurately predicted caffeine's geometry and Raman spectra.
• At neutral and basic pH, caffeine likely adsorbs flat via pi electrons and nitrogen lone pairs.
• At acidic pH, caffeine adsorption is proposed to occur through oxygen atoms, particularly the carbonyl oxygen.

Conclusions:

• The study provides a detailed analysis of caffeine's SERS spectra and adsorption behavior.
• DFT calculations enhance the reliability of spectral assignments.
• The electromagnetic mechanism appears dominant in the observed SERS spectral changes.