Electronic biosensors based on graphene FETs.

Olfaction is capable of accomplishing incredible tasks: it starts with capturing an odor molecule, delivering it to the odorant receptors, converting it into an electrical stimulus and transmitting the data to the brain. And all of this in milliseconds. The sense of smell is not yet fully decoded and is far from being replicated by modern sensor technologies. One approach to convert biological recognition- and binding events in real-time and in a label-free manner to electrical signals is emulated in a "biomimetic electronic smell sensor". It is based on a transistor, in many cases realized as a field-effect transistor (FET) with a biorecognition element, e.g., an odorant binding protein (OBP) converting the binding event of one of its typically many ligands directly into a measurable electrical signal. OBPs are immobilized on these FETs and modulate the current in the presence of smell molecules due to the charge redistribution in the gated channel. Graphene is an elegant candidate to realize such a sensor device because an atomic monolayer of a semiconducting material leads to increased sensitivity. Beside the direct molecule interaction with the substrate upon binding and its excellent biocompatible character, graphene has the advantage of a biological-friendly working point in the sub-Volt regime. Different approaches of preparation and functionalization of graphene field-effect transistors (gFETs) are utilized to tune the performance for odorant sensing. The evaluation of kinetic binding parameters like association and dissociation rate constants and the equilibrium affinity constants of protein-ligand interactions can be derived from the direct electrical read-out of such miniaturized sensor systems. In this article, the state of the art of gFET preparation, functionalization, and operation for odorant sensing will be discussed.

The structure of a valinomycin-hexaaquamagnesium trifluoromethanesulfonate compound.

Valinomycin is a naturally occurring cyclic dodecadepsipeptide with the formula cyclo-[D-HiVA→L-Val →L-LA→L-Val]3 (D-HiVA is D-α-hydroxyisovaleic acid, Val is valine and LA is lactic acid), which binds a K(+) ion with high selectively. In the past, several cation-binding modes have been revealed by X-ray crystallography. In the K(+), Rb(+) and Cs(+) complexes, the ester O atoms coordinate the cation with a trigonal antiprismatic geometry, while the six amide groups form intramolecular hydrogen bonds and the network that is formed has a bracelet-like conformation (Type 1 binding). Type 2 binding is seen with the Na(+) cation, in which the valinomycin molecule retains the bracelet conformation but the cations are coordinated by only three ester carbonyl groups and are not centrally located. In addition, a picrate counter-ion and a water molecule is found at the center of the valinomycin bracelet. Type 3 binding is observed with divalent Ba(2+), in which two cations are incorporated, bridged by two anions, and coordinated by amide carbonyl groups, and there are no intramolecular amide hydrogen bonds. In this paper, we present a new Type 4 cation-binding mode, observed in valinomycin hexaaquamagnesium bis(trifluoromethanesulfonate) trihydrate, C54H90N6O18·[Mg(H2O)6](CF3SO3)2·3H2O, in which the valinomycin molecule incorporates a whole hexaaquamagnesium ion, [Mg(H2O)6](2+), via hydrogen bonding between the amide carbonyl groups and the hydrate water H atoms. In this complex, valinomycin retains the threefold symmetry observed in Type 1 binding, but the amide hydrogen-bond network is lost; the hexaaquamagnesium cation is hydrogen bonded by six amide carbonyl groups. (1)H NMR titration data is consistent with the 1:1 binding stoichiometry in acetonitrile solution. This new cation-binding mode of binding a whole hexaaquamagnesium ion by a cyclic polypeptide is likely to have important implications for the study of metal binding with biological models under physiological conditions.