DEVELOPMENT OF A SURFACE ENHANCED RAMAN SPECTROSCOPY TECHNIQUE FOR IDENTIFICATION OF BIOMARKERS ON MARS

In-situ analyses using a suite of on-board instruments on a number of small, scout missions to Mars are envisaged prior to more detailed sample return missions. Raman spectroscopy is a possible in-situ analytical tool because of the possibility of obtaining molecular and mineralogical information. 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-sized silver or other appropriate metal particles on the surface to be analyzed. Using SERS may overcome some of the limitations of Raman spectroscopy including the inherently low intensity compared to the elastically scattered laser intensity and interference from fluorescence due to the electronic excitation of the molecule by the laser and may be useful for analyzing small concentrations of biomarkers in the Mars soil or minerals. SERS requires extremely close proximity of the molecule to be analyzed to the metal particles that form the SERS substrate. Such close proximity is difficult to achieve for solid phase. An “inverted” SERS technique is demonstrated where SERS-sensitive metal nanoparticles are deposited on the material to be analyzed.

We 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. In this method, the colloidal silver was deposited from an aqueous phase. Chlorophyll, used as an analog of a biomarker molecule that could be present in small concentrations on Martian soil or subsurface, yielded Raman spectrum at concentrations as low as 78 femtomoles. Degradation of such molecules under 532 nm laser light was significant. SERS is especially useful in this regard because the time exposure and laser power needed for analysis could be reduced. Other biomarker analogs including phthalic acid and benzoic acid were also observed using SERS. 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). A comparison of different methods to obtain SERS of biomarkers that may be present on the Martian surface is described.