This mechanism was proposed by Jeanmarie and Van Duyne in Electromagnetic enhancement arises from the presence of surface plasmons on the substrate, Figure 4. Figure 4: Interaction of laser beam with the molecules on the nanoparticles. View Figure 4.
It results from an increase in molecular polarizability, due to the charge transfer between metal and sample molecule and due to specific interactions, forming charge-transfer complexes. When molecules are adsorbed to the surface, their electronic states can interact with the states in the metal and produce new transitions which cause enhancement of Raman signal.
It was proposed by Albrecht and Creighton in It involves charge transfer between the chemisorbed species and the metal surface. SERS methods are widely used for obtaining qualitative and quantitative information of different structures including pharmaceuticals. SERS line-widths are relatively narrow which allows for higher discrimination between samples with similar spectral profiles. Several pharmaceutical molecules show good Raman spectra even in diluted conditions. Commercial drugs are used in low doses and are formulated in an inert matrix or excipient to make them into a tablet form, or to modify the release rate into the patient's system.
Raman mapping and imaging of samples may provide data about the distribution and relative amounts of active agent, additives, and binders present. An example is depicted in Figure 5 which shows the distribution of ketoconazole in creams samples. The spectrum of the pure pharmaceutical agent can be obtained by subtracting the matrix spectrum from that of the commercial drug. Useful spectrum may sometimes be obtained without subtraction when the pharmaceuticals are strong Raman scatters and the fillers are weak Raman scatters [ 15 - 17 ].
Figure 5: Mapping and image of creams samples showing the distribution ketoconazole; selected band at cm View Figure 5. SERS allows the direct analysis of pharmaceuticals thus make its applications easy in quality control of manufacturing and formulation results in significant time and cost savings. For example, Raman spectroscopy system a Lab Ram HR Evolution Raman spectrometer- equipped with an internal He-Ne 17 mW laser at a nm excitation wavelength was utilized for methimazole determination with silver loaded graphene dendrimer as a substrate. SERS were obtained using a small cuvette by using a volume ratio of three portions aqueous methimazole solution to one portion of nanoparticles dispersion.
The data acquisition time was 20 seconds with one accumulation for collection with each SERS spectra.
Figure 6: SERS spectra of methimazole with concentrations; 0. View Figure 6. Eliasson and Matousek [ 18 ] demonstrated the use of spatially offset Raman spectroscopy SORS in the identification of counterfeit pharmaceutical tablets and capsules through different types of packaging. This method shows a higher sensitivity than that of conventional backscatter Raman spectroscopy and enables chemical information to be obtained from different depths within the sample.
Davies, et al. By applying metal nanoparticles in SERS drug analysis, Cunningham and coworkers [ 20 ] reported a design of an optical device to identify and measure the drug contents of the fluid in an intravenous line in real time. They incorporated into the tubing a nanostructured gold substrate to observe the SERS signals of the drugs. Reported results for drugs including morphine, methadone, phenobarbital, promethazine, and mitoxantrone found to be of trace concentrations. The system has proved its capability for the fast analysis of two drugs combination solutions.
The system could also be useful in urinary catheters, in hospital care, and in pharmaceutical manufacturing. Raman spectroscopy has been applied to the analysis of Chinese medicines. Feng, et al. Intense SERS bands were observed due to the strong interaction of the drugs with the silver colloid. Thus, SERS technique has a great potential for quick, effective, accurate, and nondestructive analysis. Current Journals. Archive Journals. All Journals. New Titles.
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