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Recently, many studies have been focused on the development of graphene-based biosensors. However, they rely on one type of signal and need to be calibrated by other techniques. In this study, a nonenzymatic graphene-based biosensor has been designed and constructed. Its ability to detect glucose and Escherichia coli by three different types of signals has been investigated. For its preparation, dopamine-functionalized polyethylene glycol and 2,5-thiophenediylbisboronic acid were conjugated onto the surface of graphene sheets by nitrene [2 + 1] cycloaddition and condensation reactions, respectively. Multivalent interactions between boronic acid segments and biosystems consequently increased the quantifiable fluorescence emission and UV absorption of dopamine segments. Additionally, changing the electrochemical behavior of the functionalized graphene sheets was possible and resulted in a measurable output signal. Conjugation of mannose onto the surface of the biosensor improved its magnitude of signals and specificity for sensing E. coli in a complex medium. The efficiency and accuracy of each signal was monitored by others, which resulted in a real-time self-calibrating biosensor. Taking advantage of the versatility of the three different indicators, including florescence, UV, and electrochemistry, the functionalized graphene sheets have been used as self-regulating biosensors to detect a variety of biosystems with a high accuracy and specificity in a short time.

Physisorbed self-assembled monolayers (SAMs) have been suggested as potential models for three-dimensional (3D) crystallization. This work studies the effect of altering the chain length of 5-alkoxyisophthalic acid (C nISA) on self-assembled morphology in both two-dimensional (2D) and 3D to explore the extent comparisons can be drawn between dimensions. Previous studies of 5-alkoxyisophthalic acid at solid-liquid interfaces (2D) reported different morphologies for C5ISA and C6ISA-alkoxy chains on the one hand and C10ISA and C18ISA on the other. Independently, also in 3D a dependence of morphology on chain length has been reported, including an unexpected inclusion of a solvent in the 3D morphology of C6ISA, while the previous reports of 2D self-assembly were driven only by molecule-molecule and molecule-substrate interactions. However, a complete set of data for comparison has been missing. Here, we report scanning tunneling microscopy (STM) and molecular dynamics simulations performed for C2ISA self-assembled monolayers (SAMs) and STM imaging of C6ISA-C9ISA SAMs, to further examine self-assembly behavior in 2D. In 3D, X-ray diffraction analysis of C2ISA single crystals was carried out to complete the data set. With a complete set of data, it was observed that regardless of the dimension, short-chain-length C nISAs formed H-bonding-dominated structures, mid-chain-length C nISAs exhibited solvent-dependent morphologies, and long-chain-length C nISAs displayed van der Waals-dominated solvent-independent structures. However, the transition point among morphologies occurred at different chain lengths in 2D and 3D regardless of the dominant interaction. The results of this study inform the design of 2D films and guide the application of knowledge from physisorbed SAMs to 3D systems, including mixed-dimensional (2D/3D) van der Waals heterostructures.

The structure of multicomponent fluids in confined geometries is a key to understanding their properties. However, it remains an experimental challenge to gain molecular-scale resolution information on this structure. Here we show that mono- and multilayers of graphene, conforming to heterogeneous monolayers of molecules in a flexible slit pore between a mica surface and the graphene layers, allow for mapping the phase separation of water and ethanol within such a slit pore. Employing scanning force microscopy, we readily distinguish clusters of ethanol and water molecules due their different sizes, and we show that the phase separated water-ethanol structures become coarser under thicker graphenes. Moreover, we obtain a lower bound for the two-dimensional diffusion coefficient of ethanol in water of D ≥ 2 × 10(-14) m(2) s(-1). Thus, the molecularly thin slit pore provides a powerful tool to control and to investigate mixed fluids in self-adjusting nanopores.

Synchrotron x-ray diffraction reciprocal space mapping was performed on perfluoropentacene (PFP) thin films on SiO2 in order to determine the crystal structure of a novel, substrate-induced thin film phase to be monoclinic with unit cell parameters of a = 15.76 +/- 0.02 A, b = 4.51 +/- 0.02 A, c = 11.48 +/- 0.02 A, and beta = 90.4 +/- 0.1 degrees . Moreover, layered and co-deposited heterostructures of PFP and pentacene (P) were investigated by specular and grazing-incidence x-ray diffraction, atomic force microscopy, and Fourier-transform infrared spectroscopy. For a ca. three-monolayers-thick PFP film grown on a P underlayer, slightly increased lattice spacing was found. In contrast, co-deposited P/PFP films form a new mixed-crystal structure with no detectable degree of phase separation. These results highlight the structural complexity of these technically relevant molecular heterojunctions for use in organic electronics.

We report on the self-assembly of several surfactant-encapsulated clusters (SECs) on the basal plane of graphite consisting of the doughnut-shaped tungstophosphate anion [Na(H2O)P5W30O110] covered by a hydrophobic shell of surfactants. Well-ordered rodlike structures are observed using scanning force microscopy. No such ordering is observed if the surfactant methyltrioctadecylammonium is used for encapsulation, suggesting that the density of alkyl chains around the polyoxometalate cluster is an important factor in determining the order of SEC assemblies on graphite. Coadsorption of tetratetracontane (n-C44H90) and (DODA)14[Na(H2O)P5W30O110] results in single, isolated SECs on a buffer layer of tetratetracontane, as determined by scanning tunneling microscopy.

The early stages of thermal oxidation of the basal plane of highly oriented graphite (HOPG) have been investigated in ozonized air. Scanning tunneling microscopy (STM) was employed to determine the surface topography on the nanometer scale. At temperatures within the range 300−500 °C and at O3 concentrations below 1 ppm, the abstraction of carbon atoms from the basal plane, that is, formation of pointlike defects, occurs. These defects upon further oxidation are enlarged to monolayer deep pits with diameters in the range 1−100 nm. Pit formation does not proceed continuously during oxidation and is strongly inhibited when a critical pit density is reached. The pits are not generated very close to the pit edges with the consequence of a quite uniform not purely statistical coverage of the HOPG surface with the pits. Oxidation time, temperature, and O3 concentration can be used to control the pit density and their distribution across the surface, the pit diameter, and the diameter distribution with unprecede...

Scanning tunneling microscopy is used to monitor the influence of a controlled nanometer scale surface roughness on structure and stability of physisorbed, crystallized monolayers at the interface between graphite and molecular solutions. The basal plane of graphite is modified by creating atomic scale defects, which are subsequently thermally oxidized to yield monolayer deep circular pits of controlled two-dimensional density and diameters. Alkanes and alkylated oligothiophenes are adsorbed as flat-lying lamellae from solution. Monolayer crystallization is inhibited when the mean distance between the pits becomes comparable to the size of a single molecule. On a time scale of 100 ms, the critical pit diameter for the formation of ordered domains within a pit is two to three lamella widths (6−8 nm). These domains are stabilized and decoupled from those on the terrace. When the pit diameter is increased by a factor of 8−10, the stability inside the pit can be increased by 4 orders of magnitude. This means ...

Thin spin-coated films of a smectic liquid crystalline main chain polymer with an azoxybenzene as mesogen were investigated as a function of temperature and thickness. Both lateral and vertical ord...

Langmuir films of tetracosanoic acid have been transferred at 10 mN/m from water to mica surfaces and were characterized by scanning force microscopy (SFM) in the tapping mode at first as a smooth monolayer. Then, upon repeated tapping cycles, many singularities appeared in form of 2.5 nm high pairs of peaks in a plane, which stretched only 1.8 nm above the mica subphase. These peaks are attributed to islands of upright-standing molecules in a layer of molecules tilted at an angle of 35°. Transferring the films thus results first in a nonequilibrated film on mica, which undergoes relaxation upon tapping to a nonhomogeneous equilibrium phase. The same phenomenon was observed in films made of rigid porphyrin and tetracosanoic acid domains at 20 mN/m. The formation of fluid fatty acid structures at pressures where strong ordering prevails in pure fatty acid films was related to a reorientation in the rigid porphyrin domains after the transfer to mica. SFM phase shift images were applied to different hard and...

We demonstrate the use of tapping mode atomic force microscopy (TMAFM) for the study of liquid crystal prewetting films spreading on silicon wafers. With AFM, we are able to image the edge of the prewetting film when it spreads on the solid and to obtain images on its surface with a resolution in the nanometer range. As an example, we study the roughness of the film surface: we have observed, in agreement with theoretical predictions, that the undulations of the liquid films follow those of the solid surface (at least when the wavelength of the undulations is large compared to the film thickness (∼5 nm)). Further, we show that when the drive amplitude or the set point is below a threshold, a liquid crystal micromeniscus forms between the tip and the sample, causing an instability of the tip. Finally, we have also observed that the apparent thickness of the film depends on the imaging parameters in a way that cannot be described by the models that are generally used to explain tapping mode operations.

In this paper we report on the effect that a single fluorine substitution in a hydrocarbon chain of stearic acid has on the molecular ordering of the material and, in addition, the effect that the fluorines have on the scanning tunneling microscopy (STM) contrast of an alkyl chain physisorbed on graphite. The area demand due to a single fluorine substitution is increased by only approximately 1%, demonstrating a very similar steric impact of hydrogen and fluorine. The required accuracy in the measurement is achieved by evaluating the moire pattern due to the adsorbate and substrate lattices. The predicted change in theSTM contrast caused by the fluorine atom was observed for the first time

Chemisorption of sulfides and alkanethiols on Ag(111) is studied by scanning tunneling microscopy (STM). Adsorption of sulfide from aqueous solutions of Na 2 S and H 2 S or from gas phase with H 2 S, respectively, leads to a highly ordered hexagonal adsorbate lattice (α 1 = 440 ± 10 pm) with a (√7 x √7)-R10.9° coincidence cell. A slightly denser packed hexagonal lattice (α 2 = 435 ± 10 pm) with a (3 x 3) coincidence cell is found only on a small fraction of the surface. An inherent Moire pattern which can be observed in both cases allows distinguishing the two very similar adsorbate lattices and evaluating adlayer models. Self-assembling of alkanethiols (H-(CH 2 ) n SH ; n = 1, 2, 4, 6, 8, 9, 10, 11) and dimethyl disulfide from organic solution results in a hexagonal adsorbate lattice (α 3 = 440 ± 15 pm). A (√7x√7)R10.9° coincidence cell, as observed for the sulfide adlayer can be proven only for adsorbed methanethiolate. For an increasing alkyl chain length a monotonous increase in the tunneling resistance is necessary to maintain molecular resolution. This can be regarded as a direct proof for the presence of the alkyl chains in the tunneling gap during imaging. In the case of hexanethiol the formation of the self-assembled monolayer has also been studied in situ.

Etude par ellipsometrie, des couches minces d'arachidate de cadmium deposees sur du silicium