Mitoxantrone is an anticancer agent for which it is important to know the concentration in blood during therapy. Current methods of analysis are cumbersome, requiring a pretreatment stage. A method based on surface-enhanced resonance Raman scattering (SERRS) has been developed using a flow cell and silver colloid as the SERRS substrate. It is simple, sensitive, fast, and reliable. Both blood plasma and serum can be analyzed directly, but fresh serum is preferred here due to reduced fluorescence in the clinical samples available. Fluorescence is reduced further by the dilution of the serum in the flow cell and by quenching by the silver of surface-adsorbed material. The effectiveness of the latter process is dependent on the contact time between the serum and the silver. The linear range encompasses the range of concentrations detected previously in patient samples using HPLC methods. In a comparative study of a series of samples taken from a patient at different times, there is good agreement between the results obtained by HPLC and SERRS with no significant difference between them at the 95% limit. The limit of detection in serum using the final optimized procedure for SERRS was 4.0 x 10(-11) M (0.02 ng/mL) mitoxantrone. The ease with which the SERRS analysis can be carried out makes it the preferred choice of technique for mitoxantrone analysis.
The synthesis of seven monoazo benzotriazole dyes for use in surface enhanced resonance Raman scattering, SERRS, is reported. The dyes are all capable of complexing to the silver surface used to provide the surface enhancement found in SERRS and hence act as 'model' analytes. One dye was examined in detail and showed a quantitative relationship between concentration and signal intensity.
Factors that affect quantitative analysis by surface-enhanced resonance Raman scattering (SERRS) have been investigated using azobenzotriazol and reactive dyes. Preaggregation of the silver colloid was the most effective method to obtain repeatable and reproducible scattering. Aggregation by poly(l-lysine) or spermine provided better precision than aggregation by sodium chloride or nitric acid. Repeatable quantitative analysis was achieved with the azobenzotriazol dyes. A linear calibration graph was obtained over different concentration ranges below 10(-)(8) M, depending on the nature of the colloid. Calculations estimate that 10(-)(8) M is the concentration at which monolayer coverage of the dye on the silver colloid is achieved. Above 10(-)(8) M, there was only a minor increase in the scattering intensity from the azobenzotriazol dyes. In contrast, the reactive dyes did not give a response proportional to concentration over the range studied. The different responses obtained for the two types of dye are believed to be caused by differences in the nature of the interaction of the molecules with the silver surface. The conclusion reached is that control of the colloid preparation, aggregation process, and surface chemistry are essential for successful quantitative analysis of dyes on colloidal silver by SERRS.
Surface enhanced Raman scattering (SERS) and surface enhanced resonance Raman scattering (SERRS) from
a silver colloid suspension are compared using a dye designed to bond strongly to a silver surface. Titration
of the dye into a silver colloid suspension caused aggregation in a controlled manner without an aggregating
agent being added. Concentrations of dye equivalent to between 5 × 10-9 and 10-3 M in the final dilution
before adsorption to the silver were used. The results suggest that monolayer coverage of the surface occurs
at approximately 10-6 M. Above this concentration, the suspensions are less stable, and the relationship between
intensity and concentration is complex. Below this concentration, three main regions can be identified by
electronic absorption spectroscopy. At dye concentrations up to 7.5 × 10-8 M, there is little evidence of
aggregation, although there are changes in the spectra ascribed here to surface changes caused by dye adsorption.
Between 7.5 × 10-8 M and 2.5 × 10-7 M, a well-defined small aggregate appears to occur and above 2.5 ×
10-7 M larger less well-defined aggregates form. SERRS gave linear concentration dependence below 7.5 ×
10-8 M suggesting scattering from single particles. A changeover region occurs close to where the first evidence
of aggregation is detected by electronic spectroscopy, and at higher concentrations up to about monolayer
coverage a second linear region was obtained. SERS below the concentration at which aggregation is detected
by electronic spectroscopy was weak and difficult to obtain. At higher concentrations, the SERS gradients
are steeper and the maximum enhancement observed is within a factor of 4 of that obtained in SERRS. The
study shows that there is a different mechanism in SERRS compared to SERS with single particle enhancement
being much greater in SERRS.
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