# The Detection of Long-Chain Bio-Markers Using Atomic Force Microscopy
## 1 Introduction  
Scanning probe microscopes are fundamental tools in microscopy and nanotechnology. The most widely utilized of the probe microscopes is the atomic force microscope (AFM) that was first described in 1986 by [PERSON], [PERSON] and [PERSON] [1]. With an AFM, a tip mounted on a micro-fabricated cantilever is scanned over the surface and the interaction between the tip and the substrate is detected by monitoring the deflection of the cantilever [2]. These microscopes are remarkable for their ability to image individual atoms or molecules. Related AFM technologies have been developed that broadly include molecular manipulation, force measurement, and tip-mediated spectroscopy. The small size, low power requirements and non-vacuum operation make the AFM a suitable instrument for in situ planetary science missions. Consequently, AFM instruments have been sent to Mars aboard the Phoenix lander mission and an AFM is currently on the Rosetta comet mission [3; 4]. The AFM cantilever-stylus mechanism is an inherently sensitive force sensor working down to the pico-newton force range. This force detection capability of the AFM has been harnessed to measure the interaction of the tip with surfaces and individual molecules. The force-distance signal of these AFM tip interactions is known as force spectroscopy (AFM-FS) and provides additional sensing capability to the AFM. This enables the AFM to measure localized hardness, friction and the study of molecular interactions with the AFM tip. In addition, the AFM tip-cantilever mechanism has been used as a localized photo-thermal detector for infrared spectroscopy and to mediate surface enhanced Raman spectroscopy. In this work, the AFM is applied to the detection of long chain molecules on mineral surfaces. Long-chain molecules have been suggested as a class of biomarkers [5; 6; 7]. This includes long chain polymers such as intra-cellular polymers (proteins, nucleotides, polysaccharides) and extra-cellular bio-films. Using this rationale, the detection of long chain molecules provides evidence for biotic, fossilized and possible pre-biotic molecules--particularly when used in context with other analytical methods and sample history. The basic concept for the detection of long chain molecules using AFM-FS is illustrated in Figure 1. The deflection of the AFM cantilever is monitored as a function of distance. As the tip approaches from a long distance, there is no force (deflection) on the cantilever. Once the tip is in proximity to the surface, there are short range van der Waal attractive (negative) forces on the tip and ultimately the cantilever is positively deflected when full contact is made with the surface. When in contact with the surface, molecules on the surface may adhere to the AFM tip [8]. As the tip is pulled away from the surface, molecular interactions produce adhesion forces on the tip until detachment of the molecule. The detachment is readily apparent in the force-distance plot. The presence of absorbed small molecules, such as water, produces comparatively short range forces and abrupt detachment in the force-distance curve. A crucial aspect of this work is that long chain molecules will produce highly characteristic force-distance plots. The tip detachment signal is at significantly greater distances when long chain molecules are present on a mineral surface. These mineral surfaces can be planetary rocks, meteorites or fossils. This approach is an extension of previous work, where AFM-FS has been used to measure the length of purified polymers that have one end chemically tethered to the surface [9]. The AFM-FS has also been used to detect the forces associated with the unraveling protein polymers [10; 11; 12]. Force spectroscopy has also been used to measure the molecular weight distribution of synthetic polymers chemically attached (tethered) to a surface [13; 14]. The use of AFM force spectroscopy has been expanded in this work to include non-tethered biomolecules on mineral surfaces without chemical surface preparation.