Direct surface-enhanced Raman detection and imaging

Complex carbon compounds are the basis for life on Earth. The search for organics on Mars remains a key objective for future space exploration. The Mars Science Laboratory mission is scheduled to land on the red planet in 2012. Samples will be chosen based solely on imaging and elemental composition as determined remotely by ChemCam, which uses laser-induced breakdown spectroscopy but provides no information on organic content. Therefore, rough organic detection (similar to ChemCam elemental detection) would be useful on a future Mars mission. If functional organic chemical groups associated with life—such as DNA, beta-carotene, or protein—can be identified, then a claim for life can be made.¹ This is the case for young samples when the remains of cellular components have not completely decayed. For ancient samples that have undergone geologic processing, these features have decayed, preventing clear identifications.² However, decayed products of ancient cellular life are recognizable as disordered carbonaceous materials. These include kerogens,³ which are ideally suited for Raman spectroscopy chemical identification and imaging.^4,5

Raman spectroscopy is a good onboard instrument for a sample-return mission.^6,7 The non-destructive technology is ideal for selecting samples with potential biogenic origins. The presence of graphitic and disordered carbon can be easily recognized by Raman spectroscopy and distinguished from clearly non-biogenic minerals. In addition, Raman imaging based on chemical composition distribution in heterogeneous samples can offer significant advantages in the field. However, since commercially available Raman imaging typically accumulates thousands of spectra and then generates Raman mapping by mathematical data-processing, such off-the-shelf detection is time consuming. With the aim of deploying Raman global chemical imaging capability for space and planetary exploration, we have retrofitted a commercial Raman spectrometer with two sets of narrow dielectric band-pass filters. These cover the whole spectral window of chemical characteristics for organic species of astrobiological interest. We examined a variety of complex samples including nanostructured materials, 1.5 billion-year-old stromatolites, and pigments in biological cells.

In order to further Raman imaging as a high-throughput chemical-imaging tool for field-sample analysis and high-sensitivity detection, we focused on surface-enhanced Raman scattering (SERS) substrates directly coated onto the sampling probes. We applied the Langmuir-Blodgett (LB) technique to arrange silver nanostructure materials in desirable packing orientations and densities (see Figure 1). This technique enabled us to deposit ordered arrays of nanostructures in a simple, reproducible manner.⁷ For example, the silver nanowire thin films can be tuned with monolayer precision and nanometer scale alignment. They have a very stable shelf life and are capable of detecting a benchmark Rhodamine 6G (R6G) solution having a concentration as low as 10⁻¹⁰M. This is the highest reported enhancement from a solid SERS film, and it is the most reproducible and tunable thin-film fabrication.

Figure 1. Optical and scanning transmission microscope (SEM) images of silver nanowires, averaging 70nm and up to 5μm long.

Using the LB technique, we can precisely tune the density and orientation of nanostructures, or the ‘hot spots,’ on the nanowire bundles. Figure 2 shows the R6G imaged at the 30×40μm² substrate area for 30 seconds. A similar method can be adapted to coat-sampling probes. SERS enhancement with polarization (see Figure 3) indicates that aligned substrates have the potential to be very powerful SERS detectors for low concentrations of constituents in heterogamous field samples.

Figure 2. Optical and Raman images of (top) a nanoparticle substrate and (bottom) a nanowire substrate. Raman image for nanowires shows higher enhancement with higher percentage of ‘hot spots,’ as well as higher order of alignment.

Figure 3. Raman spectra of Rhodamine 6G adsorbed on Ag nanowire (NW) substrates. Inset shows silver orientations at which Raman spectra were taken (514nm, 10s exposure).

In addition to conducting ongoing studies of biomarkers of the protein, P53, and the amino acid, LPA, we have been working on integrating a miniaturized Raman spectrometer onto a Rover vehicle to be deployed to field sites, such as the Mojave and Atacama deserts. We expect these high-throughput and high spatial resolution chemical-imaging techniques to play unique roles in planetary field explorations.

We acknowledge the following NASA support and fellowships: the Planetary Instrument Definition and Development program, the Astrobiology Science and Technology for Exploring Planets program, the Graduate Student Researchers Program, and the Undergraduate Student Researchers Program.