Practical control of SERRS enhancement

The demonstration that quantitative and sensitive analysis can be carried out using surface enhanced resonance Raman scattering (SERRS) prompted a discussion and investigation of the main variables which are within the control of the analyst using colloidal silver as the substrate. Previous papers have dealt with the crucial need to obtain good chemisorption of the analyte to the surface and have reported the use of specially designed dyes for SERRS. One of the most variable processes is the aggregation of the colloid. Here, we investigate the addition of controlled amounts of an organic aggregating agent, poly-L-lysine, at concentrations which reduce the zeta potential in a controlled manner, thus aiding aggregation control. The relationship between the excitation frequency, the surface plasmon resonance frequency of the silver colloid and the frequency of the maximum absorbance of the molecular chromophore is studied using low concentrations of dye and no aggregating agent. Under these conditions, little to no aggregation is expected. The magnitude of the enhancement is strongly dependent on the frequency of the molecular chromophore as well as the plasmon resonance frequency. However, when sodium chloride is used to aggregate the colloid, a larger enhancement is obtained and the strong dependence on the molecular chromophore largely disappears. A much broader enhancement profile is obtained which appears to be related more to the specific enhancement processes caused by aggregation than the frequency of the chromophore. However, the total enhancement for SERRS is higher than for SERS thus indicating that the chromophore is still important to the process.

Surface-enhanced resonance Raman scattering of black inkjet dyes in solution and in situ printed onto paper

Understanding the changes that occur when dyes are absorbed onto paper is crucial for the design of new inkjet dyes. This problem is particularly difficult for black dyes that have complex chromophores, and as a result, spectroscopic information on electronic and structural changes can be of importance. Surface-enhanced resonance Raman scattering (SERRS) and electronic structure calculations were used to probe in situ changes in the chromophore in black di-azo dyes printed onto paper. The data indicate that the low-energy chromophore is due mainly to the hydrazone group and the high-energy chromophore to both the azo and hydrazone groups. A comparison of SERRS from the dyes adsorbed onto silver particles in suspension and from the dyes on paper demonstrated a broadening of the chromophore into the red for both dyes and evidence of a structural change in one dye.