Some Molecules and SERS Behavior

Two groups of molecules will be discussed in some detail (1) SERS of halides and their influence on SERS of other molecules and (2) SERS of molecules which are in resonance with the exciting light, dyes. The former will be discussed primarily because of the central role of halides in SERS and also from the point of view of the utilization of SERS. Dyes will be discussed because they show some special features which strongly bear on the SERS enhancement mechanism. They also have not been reviewed in the past. In addition to these two groups, a specific single molecule will be discussed—the water molecule. It is presumed that no explanation for that choice is needed. 

The importance of chloride ions for SERS was realized in the very first studies. In fact, Jeanmaire and Van Duyne have studied the dependence on chloride concentration and found that optimal signals were obtained for a halide/pyridine concentration ratio of about 2. This is dependent on the method of surface preparation. The role of chloride ions in the ORC, essentially in facilitating the formation of insoluble silver chloride in the oxidation stage, was already discussed in Section II.2(ii). 

However, there is the question of whether the chloride has any SERS activity of its own The only possibility is, of course, that a metal-chloride vibration can be detected. This is of some interest, because, if seen, it would mean that SERS can be extremely useful for surface studies where molecule-metal bonds are created. Van Duyne has noted a Raman signal at 239 cm at potentials positive of -0.4 V, which was replaced by a band at 216 cm at more cathodic potentials. He associated the first with an AgCl vibration and the last one to an Ag-N stretch vibration. Creighton et have made similar observations. Bunding et associated a 

Pettinger et have noted that the 240-cm band appears in SERS even in the absence of pyridine. Hexter et have 

The low-frequency modes associated with the halides were seen also in colloidal systems. Wetzel and Gerischer found on a silver sol Raman features at 235, 163, and 112 cm for chloride, bromide, and iodide. Garrel et monitored the replacement of chloride by bromide on a silver colloid. 

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