关键词: 表面增强拉曼散射, 卟啉, 氧化银胶体

Abstract: The biological significance of porphyrins and metalloporphyrins has stimulated intensive investigations on these compounds for many years. On the other hand, porphyrins and metalloporphyrins also have extensive applications in many other fields such as chemical modified electrodes, organic photovoltaic devices and biomimetic catalysis. The adsorption behavior and chemical reactions of metalloporphyrins on surfaces or interfaces are very important for these applications. Surface?enhanced Raman spectroscopy has been used to investigate the physical and chemical processes of the porphyrins on surfaces in many scientific publications. Surface?reactions such as metalization, demetalization, aggregation, electrochemical redox and N?protonation of the porphyrins on colloids, electrodes or films have been revealed by SERS [1,2, 6] . Although most SERS experiments were carried out on noble?metals such Ag, Cu and Au, enhanced Raman signals were also obtained from molecules adsorbed on transition metals and semiconductors [3～7] . Recently, enhanced Raman spectra were observed for pyridine, cyanine dyes, azobenzene and several other molecules adsorbed on silver oxide (Ag 2O) [6,7] . The charge?transfer (CT) mechanism was thought to be the major origin for Raman enhancements and small Ag n + clusters on Ag 2O were believed to be active sites of SERS. It is known that silver oxide can catalyze a number of organic oxidation reactions. The oxidation and subsequent ring?openning of the porphyrins and their analog macrocycles are important for understanding the metabolism of relative prosthetic groups and cofactors in biological systems. Thus, the SERS studies of the porphyrins on Ag 2O may provide new insights to this natural process. Here we report Raman scattering of four metallotetraphenylporphyrins (MTPP; M = Ag, Cu, Pd, Mg) adsorbed on Ag 2O colloids. The Ag 2O colloids were prepared by mixing aqueous solutions of sodium hydroxide and silver nitrate. Raman spectra were measured on a Labram?010 Ra man spectrometer with the 488.0 nm line of an Ar + laser as the excitation source. Normal resonance Raman spectra (Figure 1) of MTPP solid powders with 488 nm excitation are characterized with strong bands of ν 2 ( 1 532 ～1 560 cm -1 ), ν 4 (1 339～1 360 cm -1 ), ν 8 (330～338 cm -1 ), etc. Frequencies of these three modes are known sensitively dependent on central metal ions. SERS of MTPP on Ag 2O colloids is dramatically different from the normal Raman spectra (NRS). As shown in Figure 2, the characteristic bands of AgTPP at 1 538 (ν 2), 1 339 (ν 4), 399 cm -1 (ν 8) almost completely vanished in the SERS of AgTPP on Ag 2O colloid, while new bands at 1 617, 1 530, 1 417 , 947, 674, and 297 cm -1 dominate the spectra. Similar SERS spectra were also measured for other metallo?TPP and the free base on Ag 2O colloid. These spectral observations reveal that the adsorbents on the colloids undergo surface?reactions, which was considered to be the irreducible oxidation of the adsorbed MTPP. UV?visible spectra manifest that the final product of the irreducible oxidation has a strong absorption at 460 nm. The Raman shifts in the SERS spectra of AgTPP, PdTPP and CuTPP on Ag colloid are very close to the corresponding NRS. For instance, structural sensitive bands (ν 2, ν 4, and ν 8) in SERS spectra were found well close to the corresponding NRS frequencies. Thus, SERS of AgTPP, PdTPP and CuTPP on Ag colloids (without OH - modification) reveals negligible structural alteration or surface reaction. Slight frequency differences between SESR and NRS spectra may be attributed to the environmental difference of the molecule in solid and on surface.