of a Raman Probe for In Situ Analysis of
Inclusive Dates: 05/01/02 - 10/01/02
Background - The most important goals for the future Mars missions are establishing the presence of water and identifying evidence of past or present life. In situ analyses using a suite of on-board instruments on a number of small scout missions to Mars are envisioned prior to more detailed sample return missions. There is a keen interest in Raman spectroscopy as an in situ analytical tool because of the possibility of obtaining molecular and mineralogical information. Raman spectroscopy is obtained by vibrational excitation of a molecule using an incident laser of a specific frequency and measuring the inelastically scattered laser frequencies. The main limitations of Raman spectroscopy are its inherent low intensity compared to the elastically scattered laser intensity and interference from fluorescence due to the electronic excitation of the molecule by the laser.
Analysis of biomarkers indicative of life on Mars will require increased sensitivity and improved signal to noise ratio. Surface Enhanced Raman Spectroscopy (SERS) is a phenomenon whereby the Raman intensity is increased by as much as a million fold due to the presence of nano-size silver particles on the surface to be analyzed. Therefore, SERS technique may be useful for analyzing small concentrations of biomarkers in the Mars soil or minerals. Many groups are developing Raman techniques to overcome the above limitations. However, the use of SERS for in-situ analysis of Mars surface has not been explored.
SERS is typically performed by depositing nanoparticles of silver, gold, or other suitable SERS metals on a substrate such as rough glass and then adding a solution of the molecule to be analyzed on the substrate. Adapting this technique to Mars requires depositing the silver particles directly on the material to be analyzed (i.e., an inverted SERS arrangement).
Approach - A small project to investigate the feasibility of SERS was initiated as part of the SwRI Initiative on Mars (SwIM) program. The objective of the project was to demonstrate the feasibility of producing SERS on mineral surfaces by depositing silver nanoparticles. The project team also explored the possibility of identifying small concentrations of organic molecules, analogous to biomarkers, mixed with rock material.
Accomplishments - The inverted SERS technique of depositing silver nanoparticles from a colloidal solution on top of the material to be analyzed was successfully demonstrated. The team used Benzotriazole as a probe molecule and showed that the detection limit was less than 1 femtomole (10-15 moles). SERS was effective in detecting small concentrations of organic molecules on naturally occurring minerals, such as quartz (see illustration below). Chlorophyll, used as an analog of a biomarker molecule that could be present in small concentrations on Martian soil or subsurface, yielded Raman spectra at concentrations as low as 78 femtomoles. Degradation of such molecules under laser light is significant. SERS is especially useful in this regard because of less exposure and laser power needed for analysis. Such a technique was also effective in detecting small concentrations of a powdered mineral mixed with a large concentration of another mineral (e.g., powdered pyrite mixed with silica powder). Minerals, such as zeolites, where ion-exchange reactions occur between sodium and silver, also showed enhanced Raman scattering.