Cosmetics applications
Analysis of the chemical composition of samples using vibrational molecular imaging
Background
Biomolecules can be visualized directly using optical microscopy by exploiting the fact that they possess specific frequencies at which they rotate or vibrate in correspondence with discrete energy levels called vibrational modes. These vibratory modes absorb in the near- and far-infrared range and are used as a true molecular signature, even when the molecules of interest are embedded in complex environments such as cells and tissues.
Nevertheless, the best-known vibrational spectroscopy techniques (infrared spectroscopy, Raman scattering) suffer from significant drawbacks when applied to life sciences: the high absorption of water for IR spectroscopy, or the very low Raman cross-section, limit the real interest of these techniques in biological imaging. It is against this backdrop that researchers in Institut Fresnel's MOSAIC team are developing ways of imaging complex samples in which the chemical targets of interest produce specific signals.
In the case of coherent vibrational processes, the vibrational modes of all the molecules present in the light-matter interaction zone are made to vibrate in concert, synchronously. As a result, these processes will benefit from the coherent addition of scattered waves, increasing the sensitivity of coherent Raman by a factor of a million compared with spontaneous Raman. The price to be paid is that the sample must then be subjected to very short laser pulses (10-12 s or less), so as to generate sufficient peak powers while keeping average powers non-destructive of the sample, in order to trigger coherent processes with sufficient efficiency.
The project in brief
At present, the Institut Fresnel project is producing images from several modalities using coherent vibrational processes (Coherent Anti-Stokes Raman Scattering - CARS; Stimulated Raman Scattering - SRS), on prototype devices that can also produce images based on so-called non-linear contrasts (Second Harmonic Generation - SHG; two-photon Emission Fluorescence - TPEF), also using short pulse laser sources.
The Institut Fresnel team uses this equipment in particular to monitor the penetration dynamics of cosmetic products into the skin or hair, and to determine the depths reached by molecular active ingredients in these tissues. In certain cases, it is possible to improve the contrasts obtained by using an isotopic marker such as deuterium, which does not alter the structure and functionality of bio-molecules. The carbon/deuterium bond is particularly interesting as it presents a Raman signature over a spectral range that is virtually devoid of vibrational modes for the endogenous molecules found in living systems. This strategy has been used at the Institut Fresnel to image pharmacologically and cosmetically active molecular compounds penetrating human skin (see document below, which presents a study of the LR2412 molecule used in the Visionnaire anti-aging cream marketed by Lancôme).
Future prospects
In addition to cosmetics applications, the technique developed has been demonstrated to image various biological samples (mainly tissues), for applications mainly related to health and histological imaging. The team is currently developing new methodologies based on the use of Deuterium, but above all Alkyne labeling, to monitor DNA and RNA synthesis, as well as the metabolism of drug molecules (see document below).