An anchoring strategy for photoswitchable biosensor technology: azobenzene-modified SAMs on Si(111)

A mild and efficient procedure has been developed to obtain covalently attached self-assembled monolayers (SAMs) on Si(111) with photochromic azobenzene head-groups. Starting from neat or diluted carboxylic acid functionalized monolayers on-chip coupling reactions were applied to attach hydroxyl-functionalized azobenzene units to the SAMs by ester bond formation. The modified surfaces were characterized by high-resolution X-ray photoelectron spectroscopy (XPS), transmission Fourier transform infrared spectroscopy (FT-IR), and contact angle measurements. Reversible cis ↔ trans isomerizations of photoswitchable SAMs were monitored by wettability measurements. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users. Dedicated with respect and compliments to Professor Helmut Schwarz on the occasion of his 65th birthday.     

Silver nanoparticle supported on halloysite nanotubes catalyzed reduction of 4-nitrophenol (4-NP)

The silver nanoparticles with about 10 nm diameter were immobilized onto the halloysite nanotubes (HNTs) via the in situ reduction of AgNO₃ by polyol process. The silver nanoparticles supported halloysite nanotubes (Ag/HNTs), with Ag content of about 11%, were used for the catalyzed reduction of 4-nitrophenol (4-NP) with NaBH₄ in alkaline aqueous solutions. The effect of the reduction of 4-NP catalyzed by the catalysts in the presence of variable concentration NaBH₄ was investigated. It was found that the reduction rate increased with the increasing of the amounts of NaBH₄. And the larger amounts of NaBH₄ reduced the induction time.     

Quantitative Brewster angle microscopy measurements of chiral symmetry breaking and chiral boundaries in fatty acid Langmuir monolayers

Brewster angle microscopy (BAM) measurements of the low temperature region of the phase diagram of eicosanoic acid monolayers at an acidic subphase are performed. The existence of a new chiral I phase intervening between the L₂ and L ₂ ' phase recently discovered by Durbin et al. [M.K. Durbin, A. Malik, A.G. Richter, R. Ghastkadvi, T. Gog, P. Dutta, J. Chem. Phys. 106, 8216 (1997)] using grazing incidence X-ray diffraction (GIXD) is confirmed. Chiral symmetry breaking within the monolayers creates characteristic boundaries separating domains of opposite handedness. These disclination lines are associated with a jump in the tilt azimuth of the director, while the underlying hexatic orientation is continuous across the boundary. The disclination lines are observed with the Brewster angle microscope and analyzed as a function of surface pressure. The jump in tilt azimuth is determined. Agreement with an extended Landau theory proposed by Durbin is achieved. Received: 26 January 1998 / Revised: 3 August 1998 / Accepted: 7 August 1998     

Synthesis and Raman characterization of mono-sized single-wall carbon nanotubes in one-dimensional channels of AlPO4-5 crystals

Mono-sized single-wall carbon nanotubes were formed in one-dimensional channels of AlPO₄-5 single crystal (AFI) by pyrolysis of tripropylamine (TPA). Raman spectra have been measured for the TPA-AFI crystals thermally processed at different conditions. TPA molecules are carbonized at 400 °C, and carbon nanotubes were formed at 500 °C or above. The radial-breathing mode, which is special for carbon nanotube geometry, was observed. Three Raman-active modes with symmetry A _1g, E _1g, and E _2g were identified by detailed symmetrical analysis for the polarized-Raman spectra. Received: 29 October 1998 / Accepted: 29 March 1999 / Published online: 24 June 1999     

Specific features of the electronic structure of fluorinated multiwalled carbon nanotubes in the near-surface region

The C 1s and F 1s X-ray photoelectron spectra of fluorinated multiwalled carbon nanotubes with different fluorine contents have been measured using the equipment of the Russian-German beamline at the BESSY storage ring by varying the energy of exciting photons. It has been established that two fluorocarbon phases in which the chemical bonding is characterized by a different electron transfer from carbon atoms to fluorine atoms are formed in the near-surface region of nanotubes with fluorine concentrations of 10–39 wt %. The content of the dominant first phase with a large electron transfer in nanotubes remains unchanged with an increase in the probing depth. This phase is identified as a bulk phase formed as a result of the covalent attachment of fluorine atoms to graphene layers of the graphite skeleton without its destruction. The second phase with a small electron transfer is a near-surface phase, because it is predominantly located within two or three upper graphene monolayers and its contribution considerably decreases with an increase in the probing depth of fluorinated multiwalled carbon nanotubes.     

Mixed 2D molecular systems: Mechanic, thermodynamic and dielectric properties

Study of Langmuir monolayers consisting of stearic acid (SA) and dipalmitoylphosphatidylcholine (DPPC) molecules was done by surface pressure-area isotherms (π-A), the Maxwell displacement current (MDC) measurement, X-ray reflectivity (XRR) and atomic force microscopy (AFM) to investigate the selected mechanic, thermodynamic and dielectric properties based on orientational structure of monolayers. On the base of π-A isotherms analysis we explain the creation of stable structures and found optimal monolayer composition. The dielectric properties represented by MDC generated monolayers were analyzed in terms of excess dipole moment, proposing the effect of dipole-dipole interaction. XRR and AFM results illustrate deposited film structure and molecular ordering.     

有机分子超薄膜的结构对摩擦的影响

采用分子动力学方法, 模拟了由脂肪酸C_nH_2n+1COOH}和C₁₇H₃₁COOH (n=12,13,14,15,16,17)组成的混合单层Langmuir-Blodgett(LB)膜间的摩擦特性, 探究了膜结构的变化对超薄膜的摩擦的影响. 结果显示. 在滑动过程中, 随着n的增加, 膜内分子的运动受到邻近分子的约束逐渐增加, 膜结构的稳定性也逐渐增加, 其剪切压逐渐减小, n=17时的剪切压最小. 在两单层膜之间无氢键形成; 而混合膜内的分子之间形成的氢键是单层膜结构稳定的主要因素, 其中n=16时形成的氢键最稳定, 但全部由相同C₁₇H₃₁COOH分子组成的单层膜的滑动效果最好. 分子的弯曲形变能对剪切压影响非常小.     

Trianionic gold clusters

Using Penning-trap experiments and a shell-correction method incorporating ellipsoidal shape deformations, we investigate the formation and stability patterns of trianionic gold clusters. Theory and experiment are in remarkable agreement concerning appearance sizes and electronic shell effects. In contrast to multiply cationic clusters, decay of the trianionic gold clusters occurs primarily via electron autodetachment and tunneling through a Coulomb barrier, rather than via fission. Received 9 January 2001     

Orientation ordering of N<Subscript>2</Subscript> molecules in vertically aligned CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanotubes

Arrays of vertically aligned nitrogen-doped carbon (CN _x ) nanotubes have been synthesized by decomposition of aerosol mixture of acetonitrile and ferrocene at 850°C. Nitrogen concentration in the outer shells of the CN _x nanotubes was found from X-ray photoelectron spectroscopy (XPS) data to reach ∼6%. The XPS N 1s spectra and N 1s near-edge X-ray absorption fine structure (NEXAFS) spectra identified three chemical forms of nitrogen in the CN _x nanotube arrays: pyridine-like, graphitic, and molecular nitrogen. The π ^* resonance of molecular nitrogen showed clear polarization dependence that indicates predominant orientation of N₂ molecules along the nanotubes axis. The estimated range of the polar angle distribution of the N₂ molecules orientation in the CN _x nanotube array amounts to 15°.     

Entangling atoms in photonic crystals

We propose a method for entangling a system of two-level atoms in photonic crystals. The atoms are assumed to move in void regions of a photonic crystal. The interaction between the atoms is mediated either via a defect mode or via a resonant dipole-dipole interaction. We show that these interactions can produce pure entangled atomic states. We analyze the problem with parameters typical for currently existing photonic crystals and Rydberg atoms and we show that the atoms can emerge from photonic crystals in entangled states. Depending on the linear dimensions of the crystal we estimate that a pair of atoms entangled in a photonic crystal can be separated by tens of centimeters. Receive 11 June 1999 and Received in final form 4 October 1999     

Immobilization of galactose oxidase on self‐assembled monolayers of thiols on Au and Ag surfaces

Galactose oxidase (GalOD) was immobilized on self‐assembled monolayers of thiols on silver and gold surfaces using trans‐stilbene (4,4′‐diisothiocyanate)‐2,2′disulphonic acid (DIDS) as the bridging compound. DIDS is the symmetrical bifunctional reagent that reacted with the amine moiety of the thiol and with primary amino groups of enzyme. The Raman measurement revealed that onto cysteamine‐modified silver and gold electrodes, bands corresponding to the galactose oxidase (about 694, 1076, 1274 cm^—1 on Au and 762, 1058, 1274 cm^–1 on Ag ) appeared and clearly demonstrated its immobilization onto Au and Ag surfaces. Simultaneously, we have also observed changes in the ratio of trans–gauche conformers of adsorbed cysteamine molecules. Layers revealing high content of trans conformer are transformed into layers composed mainly of cysteamine molecule in gauche conformation after galactose oxidase adsorption. These observations deliver a strong support for enzyme immobilization on cysteamine‐modified gold and silver surfaces. The surface plasmon resonance experiment gave a surface coverage of ~8.4 × 10⁷ g/cm² for gold electrode modified cysteamine using DIDS chemistry and 1.1 × 10⁷ g/cm² for the cysteamine only modified gold substrate and demonstrated that galactose oxidase layers immobilized with DIDS coupling reagent are quite stable and cannot be easily removed from the surface by treatment with a buffer solution. The surface plasmon resonance results indicated that in this method, a multilayer of galactose oxidase have been immobilized. Our new method of covalent attachment of enzymes seems to be quite promising as a new way of manufacturing biosensors. Copyright © 2012 John Wiley & Sons, Ltd.     

One electron atoms in both a squeezed vacuum and a magnetic field via path integral methods

In the present paper we study the dynamics of one-electron atoms in the presence of both a linearly polarized squeezed vacuum and a magnetic field along the polarization vector of the photonic field. We adopt the dipole approximation and approach the problem via path integral methods. After integrating over the light variables for certain initial and final squeezed vacuum states we treat the path integral over the spatial variables via Monte-Carlo methods. As an application we calculate the survival probability of the ground state of a one-electron atom for various values of the magnetic field. Received 30 November 2000 and Received in final form 15 February 2001     

Band structure and CK α emission of ultrathin nanotubes

An attempt is made to calculate the energy bands and spectra of the characteristic CK _α emission of small-diameter carbon nanotubes. The calculated spectra for the nanotubes are compared with similar spectra for graphite monolayers used as a test object and with known experimental results for nanotubes. It is concluded that the x-ray emission spectra can be used to identify thin carbon nanotubes. A classification of solid-phase carbon is proposed which takes into account the position of carbon nanotubes in the family of allotropic carbon forms. The type of hybridization of the electrons in the carbon atom is used as the criterion for classification.     

Effect of relevant umklapp process on the two-leg Hubbard ladder with a half-filled band under pressure

By means of perturbative renormalization approach we study the effect of relevant umklapp process on dimensional crossover caused by interladder one particle hopping in weakly coupled two-leg Hubbard ladders with a half filled-band. We found that a crossover takes place at a finite value which increases as the amplitude of umklapp process increases. For the system undergoes a phase transition to the spin density wave phase (SDW) via the two particle hopping process, while for the system undergoes a crossover to the two dimensional Fermi liquid phase via one particle hopping process. Received 25 December 1998     

Spectroscopic studies of real space indirect symmetric GaAs/AlAs short period superlattices

The band structure and the optical transitions of symmetric ultra-short period (GaAs) _m /(AlAs) _n superlattices (n=m is the number of monolayers) have been studied using different spectroscopic techniques, namely photoacoustic spectroscopy, reflectivity and luminescence, photoluminescence excitation and high excitation intensity time-resolved luminescence. Direct observation of the transition from type I to type II energy band alignment is reported for superlattices whose configuration consists of more than 12 monolayers (i.e. m=n>12). The radiative recombination processes associated with the real space indirect transitions have been investigated as a function of the density of photogenerated electron-hole pairs, revealing an unusual density-dependent behaviour. A tentative interpretation in terms of a condensed electron-hole state at the indirect gap is given, which accounts for the long decay time of the type II luminescence at high excitation rate.     

Enhanced electrical and mass transfer characteristics of acid-treated carbon nanotubes for capacitive deionization

Capacitive deionization (CDI) has attracted significant attention for the next generation water treatment due to its low energy consumption and environment friendly properties in comparison to widely established methods. For CDI technology to move forward, however, the development of carbon electrodes having superb electrosorption behavior is essential. In this study, we demonstrate the functionalization of carbon nanotubes (CNTs) via acid treatment shows enhanced electrochemical characteristics and effectively improves the wettability of the acid-treated CNTs (a-CNTs) via the addition of oxygen functional groups, leading to a higher electric double layer capacitance. Furthermore, defect formation in a-CNTs increases the conductivity and decreases the mass transfer resistance during CDI operation. CDI measurements confirmed a 270% increase in performance of a-CNTs in contrast to pristine CNTs (p-CNTs), attributable to the improved characteristics outlined above.     

Modeling of recognition sites of nucleic acid bases aaand amide side chains of amino acids. Combination of experimental and theoretical approaches

A combined experimental-theoretical approach to modeling of building blocks of recognition complexes formed by nucleic acid bases and the amino-acids side-chain amino group is reviewed. The approach includes the temperature dependent field-ionization mass spectrometry and ab initio quantum chemical calculations. The mass spectrometric technique allows determination of interaction enthalpies of biomolecules in the gas phase, and the results it produces are directly comparable to the results obtained through theoretical modeling. In our works we have analyzed both thermodynamic and structural aspects of the recognition complexes of four canonical nucleic acid bases and acrylamide, which models the side chain of asparagine and glutamine. It has been shown that all bases can interact with amide group of the amino acids via their Watson-Crick sites when being incorporated into a single strand DNA or RNA. Stability of the complexes studied, expressed as - ΔH (kJ mole^-1) decreases as: m⁹Gua (- 59.5) > m ¹Cyt (- 57.0) > m ⁹Ade (- 52.0) ≫m ¹Ura (- 40.6). We have determined that in the double stranded DNA only purine bases can be recognized. Received 5 February 2002 and Received in final form 14 March 2002 Published online 13 September 2002     

Invariable-profile wavefunctions and Brittingham's focus wave modes

A family of invariable-profile wavefunctions is constructed. The relations found describe both transient and steady-state waves. The Gauss and Bessel-Gauss focused waves of order m can be obtained from these steady-state waves via Bateman's transformation. Received: 14 May 1998 / Accepted: 22 August 1998     

Synthesis, characterization and low field emission of CNx nanotubes

Aligned CN_x nanotubes were fabricated by pyrolyzing ethylenediamine on p-type Si(1 1 1) substrates using iron as the catalyst. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectrum (XPS) and Raman spectroscopy were used to characterize the CN_x nanotubes. The CN_x nanotubes with the average length of 20 μm and diameters in the range of 50–100 nm have the “bamboo-like” structure and worse crystalline order. The low-field emission measurements of the CN_x nanotubes indicated that 20 μA/cm² current densities were observed at an electric field of 1.4 V/μm and 1.280 mA/cm² were obtained at 2.54 V/μm. The CN_x nanotubes exhibit better field emission properties than the carbon nanotubes and the BCN nanotubes. The emission mechanism of CN_x nanotubes is also discussed.     

Investigation of auto-ionizing states in xenon by resonantly enhanced multi-photon ionization

We have examined the autoionization spectrum of xenon by resonantly enhanced three-photon ionization (2 + 1 REMPI) involving intermediate states 5p ⁵6p[J = 0, 2]. The properties of the observed autoionization resonances change significantly with the choice of the intermediate state. For ionization via an intermediate state with predominantly 5p ⁵(²P_3/2) core character, a strong continuum with embedded window-type 5p ⁵(²P _1/2)nd'-autoionization resonances is observed. For intermediate states, predominantly with 5p ⁵(²P_1/2) core character, both 5p ⁵(²P _1/2)nd'- as well as 5p ⁵(²P _1/2)ns'-resonances are present in the spectrum as overlapping, nearly symmetric peaks on a rather weak continuum. Calculations confirm the significant dependence of the spectral lineshapes upon the excitation pathway to the autoionizing state. The ionization data are compared with spectra obtained by monitoring third-harmonic generation via autoionizing states without resonant excitation of intermediate states. These spectra also exhibit the signature of both the nd'- and ns'-resonances. Received 30 September 2002 Published online 28 January 2003 RID="a" ID="a"Permanent address: Rostov State University of Transport Communication, 344038, Rostov-on-Don, Russia. RID="b" ID="b"e-mail: halfmann@physik.uni-kl.de