Applications of Surface-Enhanced Raman Spectroscopy (SERS) for improved detection of astrobiologically relevant organics

Laser Raman spectrometers are ideal instruments for Ocean Worlds missions as they can identify minerals and organic compounds tied to detecting potential biosignatures as evidence of extant and/or prior evidence of extraterrestrial life. While suitable for Mars, the current Raman instruments do not meet the requirements outlined in the Europa Lander Science Definition Team report (Hand et al., 2017). For example, the detection limit requirement for organic compounds on Europa is 1 ppb, several orders of magnitude lower than on Mars. Hence, the sensitivity of future spaceflight Raman spectrometers must increase accordingly. Surface Enhanced Raman Spectroscopy (SERS) is a technique that amplifies originally weak Raman signals to improve the detection of trace compounds. Here we demonstrate the benefits of SERS by comparing traditional Raman measurements of astrobiologically-relevant organics with and without SERS and test the detection limits of both techniques. For SERS measurements, we use a TRL 4, in-house 2D Raman mapping instrument, Mapping Exobiology Raman Laser Investigation (MERLIN). It uses a 532 nm CW laser to perform high-resolution mapping of the spatial distribution of the sample of interest. We compare SERS measurements to those done with spontaneous (non-enhanced) Raman spectroscopy in which we deposit identical samples on non SERS type substrates (sapphire) that then do not give the SERS type enhancement of the Raman signal. To assess the capabilities of SERS in the context of life detection applications, Europa organic and Venus atmospheric analogue samples were measured and compared with non SERS Raman measurements. Analyzed samples include: (1) Venus atmospheric analogues (mixtures of sulfuric acid, propanol, and glyoxal) with and without spiked trace organics (-carotene and peptidoglycan), and (2) Europa relevant analogue organics (PAHs, alkanes, fatty acids, amino acids, and -carotene). Preliminary results indicate the benefits of using SERS for enhancing detection of astrobiologically-relevant organics and highlight that this advanced Raman technique should be considered as part of an instrument payload for future in-situ exploration of Ocean Worlds. References: Hand, K.P. et al. (2017): Report of the Europa Lander Science Definition Team. Posted February, 2017.