A pH dependent Raman and surface enhanced Raman spectroscopic studies of citrazinic acid aided by theoretical calculations

作者：Sarkar Sougata; Chowdhury Joydeep; Dutta Soumen; Pal Tarasankar^*

来源：Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy , 2016, 169: 108-115.

DOI：10.1016/j.saa.2016.06.023

• Citrazinic acid
• pH dependent color change
• Surface-enhanced Raman scattering (SERS)
• Gold nanoparticles
• Theoretical calculations

A pH dependent normal Raman scattering (NRS) and surface enhanced Raman scattering (SEAS) spectral patterns of citrazinic acid (CZA), a biologically important molecule, have been investigated. The acid, with different pKa values (similar to 4 and similar to 11) for the two different functional groups (-COOH and-OH groups), shows interesting range of color changes (yellow at pH similar to 14 and brown at pH similar to 2) with the variation in solution pH. Thus, depending upon the pH of the medium, CZA molecule can exist in various protonated and/or deprotonated forms. Here we have prescribed the existence different possible forms of CZA at different pH (Forms "C", "H" and "D-prot" at pH similar to 14 and Forms "A", "D", and "P" at pH similar to 2 respectively). The NRS spectra of these solutions and their respective SERS spectra over gold nanoparticles were recorded. The spectra clearly differ in their spectral profiles. For example the SERS spectra recorded with the CZA solution at pH similar to 2 shows blue shift for different bands compared to its NRS window e.g. 406 to 450 cm(-1), 616 to 632 cm(-1), 1332 to 1343 cm(-1) etc. Again, the most enhanced peak at similar to 1548 cm(-1) in NRS while in the SERS window this appears at similar to 1580 cm(-1). Similar observation was also made for CZA at pH similar to 14. For example, the 423 cm(-1) band in the NRS profile experience a blue shift and appears at similar to 447 cm(-1) in the SERS spectrum as well as other bands at similar to 850, similar to 1067 and similar to 1214 cm(-1) in the SERS window are markedly enhanced. It is also worth noting that the SERS spectra at the different pH also differ from each other. These spectral differences indicate the existence of various adsorptive forms of the CZA molecule depending upon the pH of the solution. Therefore based on the experimental findings we propose different possible molecular forms of CZA at different pH (acidic and alkaline) conditions. For example forms 'A', 'D' and '13' existing in acidic pH (pH similar to 2) and three other deprotonated forms 'C', 'H' and 'D-prot' in alkaline pH (pH similar to 14). The DFT calculations for these prescribed model systems were also carried out to have a plausible understanding of their equilibrium geometries and the vibrational wavenumbers. An idea about the molecular orientation of the adsorbate over nanocolloidal gold substrate is also documented.

• 出版日期2016-12-5