Nitric oxide (NO) acts as a signal molecule in the nervous system, as a defense against infections, as a regulator of blood pressure, and as a gate keeper of blood flow to different organs. In vivo, it is thought to have a lifetime of a few seconds. Therefore, its direct detection at low concentrations is difficult. We report on a new type of hybrid, organic-semiconductor, electronic sensor that makes detection of nitric oxide in physiological solution possible. The mode of action of the device is described to explain how its electrical resistivity changes as a result of NO binding to a layer of native hemin molecules. These molecules are self-assembled on a GaAs surface to which they are attached through a carboxylate binding group. The new sensor provides a fast and simple method for directly detecting NO at concentrations down to 1 microM in physiological aqueous (pH=7.4) solution at room temperature.

Bifunctional conjugated molecules, consisting of electron donating or accepting groups that are connected, via a conjugated bridge, to a carboxylic acid group, were adsorbed as monomolecular carboxylate films on n-GaAs (100) and characterized by reflection FTIR, ellipsometry, and contact angle techniques. The way the donors and acceptors affected the electronic properties of the semiconductor was investigated. In agreement with theory, we find a linear relation between the calculated dipole moment of the molecules and the change in electron affinity of the moleculary modified surface, as well as between the barrier height of Au/molecule on n-GaAs junctions, extracted from their current-voltage characteristics and the dipole moment. The experimental results show little effect of the nature of the conjugated bridge in the molecules. Comparison with earlier work shows a clear decrease in the effect of the dipole of the free molecule on the semiconductor surface and interface behavior, notwithstanding the strongly conjugated link between the donor or acceptor groups of the molecule and the semiconductor surface. The simplest way to understand this is to consider the higher polarizability of the intervening bonds. Such effect needs to be considered in designing molecules for molecular control over devices.

Geringe Konzentrationen von NO in physiologischen Pufferlösungen (1 ppm, ca. 3 μM) lassen sich mit dem gezeigten maßgeschneiderten molekularen Zweikomponentensystem – ein pinzettenartig von einem organischen Liganden gehaltenes Eisen(III)‐porphyrin – nachweisen, das mit einer Seite an einen Detektor auf GaAs‐Basis gekoppelt ist. Bindet NO an das Eisen(III)‐porphyrin, ändert sich die Stromstärke.

Surface partitioning of 2,2,6,6-tetramethyl-1-piperidynyloxy radical (Tempo) to the air/water interface follows a Langmuir isotherm. The partition constant was obtained by the surface tension measurements in the concentration range of 1.0 x 10(-4)-2.4 x 10(-3) M yielding K = 640 +/- 99 M(-1). The lateral mobility of Tempo at the air/water interface was measured electrochemically in the surface concentration range of 2.0 x 10(-11)-1.4 x 10(-10) mol/cm2, corresponding to ca. 7.3-50% full monolayer coverage. The measurements employed cyclic voltammetry with line microelectrodes touching the air/water interface. The Tempo lateral diffusion constant of (1.5 +/- 0.7) x 10(-4) cm2/s is independent of surface concentration below 4.0 x 10(-11) mol/cm2. The extent of Tempo water interactions was assessed by the electronic structure calculations. These calculations showed that, at most, two water molecules can hydrogen bond with the oxygen atom of the nitroxyl group of Tempo, and that a single water molecule forms a hydrogen bond that is ca. 30% stronger than the H2O-H2O hydrogen bond. These calculations led to a postulate that Tempo diffuses along the interface largely unimmersed, and that it is coupled to the interfacial water via hydrogen bonding with H2O. In view of this postulate, the viscosity of the aqueous liquid/vapor interfacial region obtained by interpreting the Tempo diffusion constant in the low concentration region is as much as 4 times smaller than that of bulk liquid water.

Monolayer films of a water-soluble surfactant, 4-octaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (C8-TEMPO) were investigated at the air/water interface. An electrochemical, horizontal touch method was developed to measure the equilibrium surface concentrations (gamma) of C8TEMPO. The dependence of gamma on the solution concentration followed a Langmuir isotherm and yielded the partition constant K = (2.3+/-0.2) x 10(4) M(-1). These results were verified by surface tension measurements and Brewster angle microscopy. Within experimental error, the same K values were obtained. The lateral diffusion constants vs surface concentration of this molecule were measured by 2D voltammetry. In these experiments, the component of the oxidation current due to C8TEMPO in the bulk of the solution was subtracted from the total measured current to obtain the component due to the lateral surface diffusion. In the ange of mean molecular areas from 120 to 400 A2/molecule, the lateral diffusion constant of C8TEMPO increased from 1.0 x 10(-6) to 1.0 x 10(-5) cm2/s. The latter value is about 2.5 times larger than the C8TEMPO diffusion constant in bulk water. Comparison of the lateral mobilities of C8TEMPO and two longer alkane chain, water-insoluble homologues, C14TEMPO and C18TEMPO, showed no statistically significant differences.