SERS-active sites

For the outer-sphere Co(NH3)63+ reduction, the SERS and current-potential data are closely compatible in that the SERS intensities drop sharply at potentials towards the top of the voltammetric wave where the overall interfacial reactant concentration must decrease to zero. Some discrepancies between the SERS and electrochemical data were seen for the inner-sphere Cr(NH3)sBr2 and Cr(NH3)sNCS2+ reductions, in that the SERS intensities decrease sharply to zero at potentials closer to the foot of the voltammetric wave. This indicates that the inner-sphere reactant bound to SERS-active sites is reduced at significantly lower overpotentials than is the preponderant adsorbate. (15)This suggests that SERS-active surface sites might display unusual electrocatalytic activity in some cases. 

Instead of creating SERS-active sites in an optical trap, Raman enhancement can also be obtained by moving Raman active probes to the vicinity of a SERS-active substrate using an optical tweezers [50]. As schemed in Eig. 18.4a, gold nanoparticles were pre-immobUized on the surface of a glass slide facing the solution where biological spores were trapped. When the spore is far from the... 

A word of caution in addition to the uncertainties and problems discussed above, one should take note that all the enhancement factors quoted in the literature are merely average values. The average is over the total amount of adsorbed material, be it at submonolayer, monolayer, or multilayer coverages. If there are special SERS-active sites which are a small fraction of the surface, then the true intrinsic enhancement factor per molecule may be much larger than the estimated one. We should be aware of this fact when interpretation of results are considered in Section III. 

SERS-active sites are those locations on the surface that have atomic-scale roughness features. Thus the apparent enhancement will consist of an average over the (small number of) enhanced scatterers at the active sites and the (large number of) molecules adsorbed at smoother parts of the surface, and which do not contribute much to the total signal. The net result should be apparent enhancements that are much smaller than the enhancement per site. This remark is valid for any other model which calls for a limited number of active sites. 

The targets of present SERS on-chip are (1) to solve the problem of poor detection sensitivity caused by the small sample volume and (2) to develop robust methods to obtain a reliable and reproducible SERS signal. Aggregation of metal nanoparticles and mixing of enhancer with analytes are the straightforward ways to increase the SERS signal intensity and the uniformity of the SERS active site. 

Gold electrodes have come to look particularly interesting in recent times. Not only are they more inert than silver, but they support a much wider range of potentials in aqueous solution, their derived salts are less photochemically labile, and their SERS-active sites appear to be more stable than those of silver. ORC-roughened gold electrodes remain fully SERS active when transferred from one electrochemical cell to another [18, 24],... 

Surface roughness with a scale of 10 200 nm is required to produce huge SERS. Rough surfaces with these submicroscopic dimensions are also related inevitably to nanoscopic features, such as surface complexes, ad-atoms, adSERS effects are named as SERS-active sites and have proved to contribute to the enhancement in the range of 10 to 10 fold. 

It should be emphasized that the SERS-active sites include various microscopic... 

A unique method to overcome the problem of gradual change in the SERS activity during the measurement is to use the PARS method, which has been described in detail earlier. The essentials of the tactics are (1) to shorten the time for the electrode at potentials that are unfavorable for the SERS-active site and (2) to ensure the SERS activities at the two potentials are exactly the same. 

The special Funtikov s oxidation/reduction cycle method was used to maintain stable SERS-active sites with no adsorbed (pseudo-)halide ions. 

Although the SERS of benzene has been reported at an Ag electrode, it has been difficult to duplicate, and absorbed hydrocarbons at Ag electrodes appear to give very weak SERS. Nonpolar unsaturated hydrocarbon molecules can chemisorb to metal surfaces via 7r-bonding interactions, and these interactions seem to be stronger on Au than on Ag. In addition, the SERS-active sites seem to be more stable at lower adsorbate coverage on Au than on Ag. Alkenes and alkynes adsorb on pretreated Au electrodes from saturated aqueous solutions primarily by donation of electron density from TT-orbitals to vacant Au orbitals. This process leads to lower-frequency bands for the C=C stretching modes. 

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