Quantifying the force regime of pyrene adsorption on anatase

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are one type of meteoritic organic matter delivered to the surface of Mars. The further chemical evolution of meteoritic PAHs needs to be characterized in order to successfully differentiate between different possible sources of organic molecules detected by Mars rovers, including meteoritic, geological, and potential biological sources. In the ultraviolet regime of Mars, PAHs can be subject to metal-catalyzed degradation mechanisms in the presence of photocatalytic minerals. The efficacy of these mechanisms, however, will depend on the strength of the binding interaction between the PAH and the mineral surface. In this exploratory experimental study, we outline a protocol to characterize the force interactions between the PAH pyrene and an atomically flat mineral surface. The force regime of the adsorption was studied by functionalizing a probe and performing force curve measurements with an atomic force microscope (AFM). The distribution of pyrene on the functionalized probe was mapped with photo-induced force microscopy (PiFM). We produced atomically flat titanium dioxide pellets to obtain force information entirely attributable to the intentional modifications imposed on the probes. In addition we used density functional theory (DFT) to calculate the binding energies of pyrene and titanium dioxide to gain further insight into the nature of the chemical bonds formed in this system. Our method, though it could benefit from a more uniform distribution of pyrene at the surface of the AFM probe, revealed the binding energy between the pyrene molecules and the anatase surface in good agreement with that predicted by DFT calculations.