Semi-quantitative analysis of indigo by surface enhanced resonance Raman spectroscopy (SERRS) using silver colloids

Area of Science:

• Analytical Chemistry
• Spectroscopy
• Materials Science

Background:

• Indigo is a historically significant dye and pigment with low solubility, posing challenges for analysis.
• Traditional spectroscopic methods struggle with indigo due to its insolubility and fluorescence.
• Surface-enhanced Raman spectroscopy (SERS) and SERRS offer potential for analyzing insoluble molecules.

Purpose of the Study:

• To develop and validate semi-quantitative analysis of indigo using SERS and SERRS.
• To investigate the influence of experimental conditions, such as pH and laser excitation wavelength, on SERS/SERRS signal intensity.
• To determine the limit of detection (L.O.D.) for indigo using these spectroscopic techniques.

Main Methods:

• Utilized aggregated citrate-reduced silver colloids for SERS and SERRS measurements.
• Optimized experimental protocols to maximize SERRS signal intensity.
• Employed internal standards (silver sol band at 243 cm⁻¹) for band intensity normalization.

Main Results:

• Achieved good linear correlations between SERRS band intensity (1151 cm⁻¹) and indigo concentration (10⁻⁷–10⁻⁵ M at 514.5 nm, 10⁻⁸–10⁻⁵ M at 632.8 nm).
• Established limits of detection (L.O.D.) of 9 ppm for indigo using both SERS and SERRS at 514.5 nm and 632.8 nm.
• Observed significant pH dependence of SERS/SERRS signals, indicating protonation of indigo, and suggested monolayer coverage at concentrations > 7.85x10⁻⁶ M.

Conclusions:

• SERS and SERRS are effective methods for the semi-quantitative analysis of indigo, overcoming its solubility limitations.
• The study provides insights into indigo's behavior on silver colloid surfaces and its protonation states.
• These findings are valuable for the analysis of indigo in various applications, including textiles, historical artifacts, and art conservation.