Confocal Raman spectroscopy applied to microemulsions and nanoemulsions

Abstract

Raman spectroscopy is a light scattering technique in which a molecule scatters incident light at different wavelengths depending on the chemical structure of the analyte, from a high-intensity laser light source. The Raman scattered light is detected to create the Raman spectrum, which is unique to the chemical compounds present in the sample, acting as chemical fingerprints. Since Raman spectrometers can be coupled to a confocal microscope, the selection of regions of interest and a high magnification of the image of the sample can be achieved. Furthermore, 2D and 3D mapping can be generated from thousands of Raman spectra acquired from different positions on the sample, showing the distribution of individual chemical components in the sample. This review summarizes recent advances in the application of confocal Raman spectroscopy to the physicochemical and analytical characterization of microemulsions and nanoemulsions, focusing primarily on topical formulations and biological studies (in-vitro and ex-vivo), and including representative in-vivo and clinical investigations. Identification, quantification, and distribution of components (surfactants, oil phase, active ingredients, etc.) in the colloidal systems, as well as to biological studies related to dynamic processes of formulations in different tissues, even in real-time, were performed. The methodological aspects of confocal Raman spectroscopy in recent works were adequately described, discussed, and compared. Also, we analyzed the limitations and advantages of the spectroscopic technique, including its contribution to green chemistry. Finally, we discuss future perspectives considering its combination with chemometric and artificial intelligence algorithms to improve the quality of the results.