CRYSTAL GROWTH & DESIGN
Genesis on Diamonds Andrei P. Sommer,*,† Dan Zhu,† and Hans-Joerg Fecht†,‡ Institute of Micro and Nanomaterials, UniVersity of Ulm, 89081 Ulm, Germany, and Institute for Nanotechnology, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany
2008 VOL. 8, NO. 8 2628–2629
ReceiVed May 14, 2008; ReVised Manuscript ReceiVed June 8, 2008
ABSTRACT: The scientific community paid little attention to a visionary paper, in which Szent-Gyo¨rgyi predicted that crystalline interfacial water layers would play a fundamental role in biology and evolution. It was 1971, and the proof of the existence of crystalline interfacial water layers at room temperature was virtually lacking in the literature. Recently, we provided experimental evidence for their existence on hydrogenated nanocrystalline diamond at room temperature. Crystallinity resulted from a decrease in conductance in response to an increase in humidity, associated with a decrease in the order of the interfacial water molecules implicated in proton conductivity. The correlation between conductance and humidity is not exclusive to synthetic diamond: It prevails on hydrogenated natural diamonds. Hydrogenation in nature is plausible: Volcanoes emit various hot gases including hydrogen. The capacity of interfacial water layers to impose order was exposed in the process of formation of supercubane carbon nanocrystals. It is important that the order imposed to molecules landing on hydrogenated diamond is more durable and superior to that realizable on any other origin of life platform, for instance, graphite. Hydrogenated diamond advances to the best of all possible origin of life platforms. The emergence of bioorganic molecules under primitive Earth conditions is one of the major unsolved origin of life questions. The principal problem is to identify physical and chemical conditions that are favorable for the formation of life precursor structures.1 Panspermia tries to circumvent the problem by assuming that primitive life forms, such as bacteria, have arrived on Earth from space, as star dust or via comets.2 Origin of life models starting with a primordial soup work from two assumptions, which do not exclude each other: chemistry, which could trigger the self-assembly of abiotic organic compounds, and transfer of order from preexistent orderings. Clearly, both organization processes must occur in a biorelevant environment, that is, a wet milieu, and both are energyconsuming. Interestingly, a hostile planetary environment such as the primitive Earth, subject to rapid changes, including but not limited to volcanic and hydrothermal activity,3 acidic atmosphere, and virtually planet-sterilizing meteorite impacts,4 favors precisely the conditions necessary for the formation of first ordered structures, regarded as possible starting points of biological evolution. In recent laboratory experiments we found evidence for the establishment of nanoscopic crystalline interfacial water layers on hydrogenated nanocrystalline diamond at room temperature.5 The circumstance that water adsorbed from the air becomes crystalline at room temperature attracted in fact more attention than the revelation of the implication of this water in the surface conductivity of hydrogenated diamond, a new feature in the 20-year-old puzzle.6 The report of our results triggered an intense discussion on the implications of water in the mechanism of surface conductivity on hydrogen-terminated diamond.7–9 For clarification, we repeated our experiments and used instead of synthetic diamond a natural one (Figure 1), which we hydrogenated according to the protocol described in our previous paper.5 Consistent with the tendency found by us using synthetic diamond, the conductivity decreased with increasing relative humidityschallenging both the findings of others and the surface transfer doping mechanism.8 We interpreted this tendency by assuming that the additional humidity destroyed the order in chains of polarized H2O molecules, spanned between the hydrogenated site and the electrode of the device employed to measure the conductivity. Presumably, such chains are highly conductive for protons that are pulled by the applied voltage from the hydrogenated diamond. Models of the conductivity * To whom correspondence should be addressed. E-mail: andrei.sommer@ uni-ulm.de. † University of Ulm. ‡ Forschungszentrum Karlsruhe.
Figure 1. Synthetic and natural diamond (inset) becomes conductive when hydrogenated. Films of non-hydrogenated nanocrystalline diamond, separated from the silicon wafer on which they were deposited by a CVD process, presented neither surface nor bulk conductance, excluding the possibility of grain boundary conductance due to sp2 carbon.
of hydrogenated diamond have never considered an active role of a crystalline water layer as a medium for rapidly transporting positive charge in the form of hydrated protons via the Grotthuss mechanism,10 responsible for the anomalously high proton mobilities in bulk water that exceed even those of fast solid-state proton conductors.11 In our study the conductivity on natural diamond was better by a factor of 10 than that on synthetic diamond. We conclude that current ideas of surface-conducting diamond should be expanded to include the crystalline water layer and, as a complementary mechanism, Grotthuss transport of the hydrated proton; doing so could enhance the predictive capabilities of emerging models. That life could have started with crystalline water layers inducing order to prebiotic molecules on solid surfaces was predicted by Albert Szent-Gyo¨rgyi.12 Mineral surfaces are catalytic platforms, regarded as necessary during the emergence of life on Earthsbecause the assembly of complex bioorganic molecules by random collisions in an aqueous environment is implausible. Carbon seems to represent even better platforms: Recently, adsorption isotherms for purine and pyrimidine bases were found at the graphite-water interface at 30 °C.13
10.1021/cg8005037 CCC: $40.75 2008 American Chemical Society Published on Web 07/10/2008
Communications
Crystal Growth & Design, Vol. 8, No. 8, 2008 2629
Graphite came into the focus of origin of life models by the discovery that graphite-liquid interfaces promoted the organization of supramolecular nanopatterns from complementary nucleobases (guanine and uracil).14 Diamond terminated with hydrogen provides not only an organic junction (C-H) but has one advantage when compared to graphite or inorganic surfaces: The crystalline interfacial water layer is stable, even under water.15 Hydrogen bonds stemming from the hydrogenation have a strong polarizing effect on interfacial water molecules, normal to the surface. Coincidently, by accentuating the hydrophobic character of diamond, the hydrogenation facilitates the mobility of the water molecules parallel to the surface. This interplay encourages the transfer of an unusually high order from the water layer on the diamond to molecules landing on its surfaceskey aspect and starting platform for the evolution of ordered monolayers. Diurnal temperature and light variation modulating nanoscopic water layers on diamond could further induce periodic flow and first energy conversion processes. Diamonds are older than the earliest forms of life on Earth.16,17 Their hydrogenation could occur naturally, that is, in, or in the vicinity of volcanoes, known to emit a variety of hot gases, including hydrogen.18 The hydrogenation is not altered by conventional contaminations5 and is resistant against hydrothermal activity (simulated in laboratory experiments by exposing hydrogenated diamond to sterilizing doses of water vapor). The paradigm that crystalline interfacial water layers played a fundamental role in the formation of early planetary life forms12 received indirect support from the capacity of interfacial water layers to induce order, even to metastable solids, as recently demonstrated in a process by which carbon nanoparticles were converted to diamond.19
Acknowledgment. We are grateful to the Landesstiftung BadenWu¨rttemberg Bionics Network for financial support.
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