New electronic and magnetic properties emerging from adsorption of organized organic layers

New electronic and magnetic properties are induced by the adsorption of closed packed monolayers on solid substrates. For many thiolated molecules self-assembled on gold, a surprisingly large paramagnetism is observed. In the case where the layers are made from chiral molecules, in addition an unexpectedly large electronic dichroism is observed, which manifests itself as spin specific electron transmission. This dichroism was observed for monolayers made from polyalanine and from DNA. Self-assembled monolayers of double-stranded DNA oligomers on gold interact with polarized electrons similarly to a strong and oriented magnetic field. The direction of the field for right-handed DNA is away from the substrate. Moreover, the layer shows very high paramagnetic susceptibility. Interestingly, thiolated single-stranded DNA oligomers on gold do not show this effect. All the observations can be rationalized by assuming organization induced charge transfer between the substrate and the organic layer. The charge transfer results in spin alignment of the transferred electrons/holes. While for achiral molecules the spin alignment varies among the domains, in the case of monolayer made from chiral molecules the alignment is the same across the entire sample. When magnetic field is applied, large magnetic moment is observed that results from orbital magnetism.     

The Electron Spin as a Chiral Reagent

We show that enantioselective reactions can be induced by the electron spin itself and that it is possible to replace a conventional enantiopure chemical reagent by spin‐polarized electrons that provide the chiral bias for enantioselective reactions. Three examples of enantioselective chemistry resulting from electron‐spin polarization are presented. One demonstrates the enantioselective association of a chiral molecule with an achiral self‐assembled monolayer film that is spin‐polarized, while the other two show that the chiral bias provided by the electron helicity can drive both reduction and oxidation in enantiospecific electrochemical reactions. In each case, the enantioselectivity does not result from enantiospecific interactions of the molecule with the ferromagnetic electrode but from the polarized spin that crosses the interface between the substrate and the molecule. Furthermore, the direction of the electron‐spin polarization defines the handedness of the enantioselectivity. This work demonstrates a new mechanism for realizing enantioselective chemistry.     

Ab initio study of molecular oxygen adsorption on Pu (111) surface

Using the generalized gradient approximation to density functional theory (DFT), molecular and dissociative oxygen adsorptions on a Pu (111) surface has been studied in detail. Dissociative adsorption with a layer‐by‐layer alternate spin arrangement of the plutonium layer is found to be energetically more favorable, and adsorption of oxygen does not change this feature. Hor1 (O₂ is parallel to the surface and lattice vectors) approach on the center2 (center of the unit cell, where there is a Pu atom directly below on the third layer) site, both without and with spin polarization, was found to be the preferred chemisorbed site among all cases studied with chemisorption energies of 8.365 and 7.897 eV, respectively. The second‐highest chemisorption energy occurs at the Ver (O₂ is vertical to the surface) approach of the bridge site with chemisorption energies of 8.294 eV (non‐spin‐polarized) and 7.859 eV (spin‐polarized), respectively. We find that 5f electrons are more localized in the spin‐polarized case than the non‐spin‐polarized counterparts. Localization of the 5f electrons is higher in the oxygen‐adsorbed plutonium layers compared with the bare layers. The ionic part of O? Pu bonding plays a significant role in the chemisorption process, along with Pu 5f? O 2p hybridization. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Fabrication of Stable Chiral Polyaniline Nanocomposite‐Based Patterns

Summary: We report an artful method to form a stable pattern of chiral polyaniline nanocomposites (CPANs). It consists of the preparation of a diazoresin (DR)/poly(acrylic acid) (PAA) thin buffer layer on an Si substrate by self‐assembly, followed by the deposition of a multi‐layer film by spin‐assembly, leading to the formation of a (DR/PAA)₂DR/(CPAN/DR)_n film on the substrate. After selective exposure to UV light through a photomask and the development process, a defined pattern is formed.

Scanning electron microscopy image of the patterned (CPAN/DR)₅ thin film on Si wafer.     

Magnetless Device for Conducting Three‐Dimensional Spin‐Specific Electrochemistry

Electron spin states play an important role in many chemical processes. Most spin‐state studies require the application of a magnetic field. Recently it was found that the transport of electrons through chiral molecules also depends on their spin states and may also play a role in enantiorecognition. Electrochemistry is an important tool for studying spin‐specific processes and enantioseparation of chiral molecules. A new device is presented, which serves as the working electrode in electrochemical cells and is capable of providing information on the correlation of spin selectivity and the electrochemical process. The device is based on the Hall effect and it eliminates the need to apply an external magnetic field. Spin‐selective electron transfer through chiral molecules can be monitored and the relationship between the enantiorecognition process and the spin of electrons elucidated.     

Ligand Control over the Electronic Properties within the Metallic Core of Gold Nanoparticles

The behavior of electrons within the metallic core of gold nanoparticles (AuNPs) can be controlled by the nature of the surface chemistry of the AuNPs. Specifically, the conduction electron spin resonance (CESR) spectra of AuNPs of diameter 1.8–1.9 nm are sensitive to ligand exchange of hexanethiol for 4‐bromothiophenol on the surface of the nanoparticle. Chemisorption of the aromatic ligand leads to a shift in the metallic electron’s g‐factor toward the value expected for pure gold systems, suggesting an increase in metallic character for the electrons within the gold core. Analysis by UV/Vis absorption spectroscopy reveals a concomitant bathochromic shift of the surface plasmon resonance band of the AuNP, indicating that other electronic properties of AuNPs are also affected by the ligand exchange. In total, our results demonstrate that the chemical nature of the ligand controls the valence band structure of AuNPs.     

Topological and Electron‐Transfer Properties of Yeast Cytochrome c Adsorbed on Bare Gold Electrodes

The redox metalloprotein yeast cytochrome c was directly self‐chemisorbed on “bare” gold electrodes through the free sulfur‐containing group Cys102. Topological, spectroscopic, and electron transfer properties of the immobilised molecules were investigated by in situ scanning probe microscopy and cyclic voltammetry. Atomic force and scanning tunnelling microscopy revealed individual protein molecules adsorbed on the gold substrate, with no evidence of aggregates. The adsorbed proteins appear to be firmly bound to gold and display dimensions in good agreement with crystallographic data. Cyclic voltammetric analysis showed that up to 84 % of the electrode surface is functionalised with electroactive proteins whose measured redox midpoint potential is in good agreement with the formal potential. Our results clearly indicate that this variant of cytochrome c is adsorbed on bare gold electrodes with preservation of morphological properties and redox functionality.     

Stereoselective Interaction between DNA and Stable Chiral Surfaces Modified with 1,2‐Diphenylethylenediamine Enantiomers

This work described an interesting phenomenon of the stereoselective adsorption behaviors of DNA on stable chiral surfaces which were modified with 1,2‐diphenylethylenediamine enantiomers on gold electrodes. The modification process and electrochemical characterization of the chiral surfaces were measured by cyclic voltammetry (CV). The stereoselective adsorption behaviors of DNA on the two chiral surfaces were investigated via atomic force microscopy (AFM), CV, electrochemical impedance spectroscopy (EIS) and quartz crystal microbalance (QCM). All results confirmed that (1R,2R)‐1,2‐diphenylethylenediamine modified surface had stronger interaction with DNA molecules than (1S,2S)‐1,2‐diphenylethylenediamine modified surface, and the chirality of the surfaces created an different effect on the morphology and adsorption quantity of DNA.     

Gold‐Catalyzed Cascade Reactions for Synthesis of Carbo‐ and Heterocycles: Selectivity and Diversity

This account describes our recent efforts devoted to gold chemistry since 2009. Based on furyl–Au 1,3‐dipole analogues and related gold carbene intermediates, a rich variety of gold‐catalyzed cascade reactions have been developed, which provide facile access to a diverse range of novel carbo‐ and heterocycles. In these reactions, the selectivity can be well controlled by the catalyst (ligand and metal), substrate or reagent. In addition, we have also developed the corresponding enantioselective variants, which are guided by bis(phosphinegold) complexes derived from axially chiral scaffolds and asymmetric gold/chiral Brønsted acid relay catalysis.     

Determination of Norepinephrine Alone and in the Presence of Ascorbic and Uric Acids Using a Gold Electrode Modified with Gold Nanoparticles and Self‐Assembled Layers of meso‐2,3‐Dimercaptosuccinic Acid

This paper reports on the modification of gold electrodes with self‐assembled layers (SAMs) composed of meso‐2,3‐dimercaptosuccinic acid, cysteamine and gold nanoparticles, respectively and their application to quantitative determination of norepinephrine alone and in the presence of ascorbic and uric acids in solution at pH 7. The modification was carried out on two kinds of templates: a bare gold electrode (2D electrode) and a gold electrode coated in the first step with gold nanoparticles (3D electrode). Cyclic voltammograms reveal an enhancement of the norepinephrine electrooxidation in comparison to a bare, (non‐modified) gold electrode. The oxidation peaks for norepinephrine, ascorbic acid and uric acid have a peak‐to‐peak separation that enables their selective determination even in a complex mixture.     

Selective response of antigen‐antibody reactions on chiral surfaces modified with 1,2‐diphenylethylenediamine enantiomers

This work reported a comparative analysis of the amperometric responses of antigen‐antibody reactions on two stable chiral surfaces which were modified with 1,2‐diphenylethylenediamine enantiomers. Alpha‐fetoprotein antibody and antigen (anti‐AFP and AFP) were selected as model systems. First, (1R,2R)‐1,2‐diphenylethylenediamine or (1S,2S)‐1,2‐diphenylethylenediamine was modified on the gold surface of the electrode through amide linkage to construct chiral surfaces. Then, anti‐AFP was immobilized on the chiral electrode surface by electrostatic and hydrogen bonding interactions. The electrochemical characteristics of the modified electrodes were studied via cyclic voltammetry. The selective current responses of antigen‐antibody reactions on chiral electrode surfaces for different incubation time and varying AFP concentrations were monitored. The antigen‐antibody reactions were greatly influenced by the chirality of 1,2‐diphenylethylenediamine enantiomers, and the amperometric responses obtained from the (1S,2S)‐1,2‐diphenylethylenediamine modified electrode was obviously stronger than that from the (1R,2R)‐1,2‐diphenylethylenediamine modified electrode. Such work may not only offer valuable reference to the research of chiral drugs, but also help to comprehend the high selectivity of chiral molecular species in biosystems. Copyright © 2011 John Wiley & Sons, Ltd.     

Fabrication of a Sensitive Impedance Biosensor of DNA Hybridization Based on Gold Nanoparticles Modified Gold Electrode

In this work, we report on the preparation of a simple, sensitive DNA impedance sensor. Firstly gold nanoparticles were electrodeposited on the surface of a gold electrode, and then probe DNA was immobilized on the surface of gold nanoparticles through a 5′‐thiol‐linker. Electrochemical impedance spectroscopy (EIS) was used to investigate probe DNA immobilization and hybridization. Compared to the bare gold electrode, the gold nanoparticles modified electrode could improve the density of probe DNA attachment and the sensitivity of DNA sensor greatly. The difference of electron transfer resistance (ΔR_et) was linear with the logarithm of complementary oligonucleotides sequence concentrations in the range of 2.0×10^?12 to 9.0×10^?8 M, and the detection limit was 6.7×10^?13 M. In addition, the DNA sensor showed a fairly good reproducibility and stability during repeated regeneration and hybridization cycles.     

The profile of the 2140 cm(-1) solid CO band on different substrates

We have studied the profile of the 2140 cm(-1) fundamental band of solid carbon monoxide (CO) at low temperature (10-15 K) by infrared transmission spectroscopy and by reflection absorption infrared (RAIR) spectroscopy. In particular, transmission spectra have been taken after CO had been adsorbed on a bare crystalline silicon substrate and on pre-adsorbed solid N(2) layers of different thickness. RAIR spectra have been taken after CO had been adsorbed on a bare gold substrate and on pre-adsorbed solid N(2) layers of different thickness. Laboratory spectra show that the profile of the fundamental CO band at about 2140 cm(-1) is different in the different instances considered. In particular, we have found that the relative intensity of the LO and TO modes of the CO band depends on the thickness of the N(2) layer. Here we present the experimental results and show that these can be predicted by the elementary electromagnetic theory.     

Chiral Aggregates of Triphenylamine‐Based Dyes for Depleting the Production of Hydrogen Peroxide in the Photochemical Water‐Splitting Process

Recent studies on water‐splitting photoelectrochemical cells (PECs) have demonstrated the intriguing possibility of controlling the spin state in this chemical reaction to form H₂ and O₂ by exploiting the chirality of organic π‐conjugated supramolecular polymers. Although this fascinating phenomenon has been disclosed, the chiral supramolecular materials reported thus far are not optimized for acting as efficient photosensitizer for dye‐sensitized PECs. In this work we report on the design, synthesis, and characterization of chiral supramolecular aggregates based on C₃‐symmetric triphenylamine‐based dyes that are able to both absorb visible light and control the spin state of the process. Variable temperature‐dependent spectroscopic measurements reveal the assembly process of the dyes and confirm the formation of chiral aggregates, both in solution as well as on solid supports. Photoelectrochemical measurements on TiO₂‐based anodes validate the advantage of using chiral supramolecular aggregates as photosensitizer displaying higher photocurrent compared to achiral analogues. Moreover, fluorimetric tests for the quantification of the hydrogen peroxide produced, confirm the possibility of controlling the spin of the reaction exerting spin‐selection with chiral supramolecular polymers. These results represent a further step towards the next‐generation of organic‐based water‐splitting solar cells.     

Dramatic Fluorescence Enhancement of Bare Carbon Dots through Facile Reduction Chemistry

Reduction of bare carbon dots (CDs) in aqueous NaBH₄ solution is a facile and effective approach to enhance their fluorescence without any surface coverage. CDs are treated with dilute aqueous NaBH₄ solutions, enhancing their quantum yields (QYs) successfully from 1.6 % to 16 % which is comparable to semiconductive QDs in aqueous environments. If pristine CDs are treated hydrothermally prior to reduction by NaBH₄, QYs reach 40.5 %. This value is among the highest QYs reported for bare CDs in the literature. The approach to enhance fluorescence through chemical reduction is generally applicable to other kinds of CDs synthesized by various methods. Alteration of the chemical structure of the CDs by NaBH₄‐reduction is analyzed by ¹³C NMR, X‐ray photoelectron spectroscopy (XPS) and Raman spectroscopy, which demonstrate that the carbonyl group content is decreased after NaBH₄‐reduction, whereas the number of sp³‐type carbon defects is increased. The valence‐band maxima (VBM) near the surface related to the surface energy bands of the CDs are estimated by XPS. VBM data show a semiconducting layer on the surface of the CDs, and the VBM of the CDs decrease with increasing NaBH₄‐reduction time. The layered graphite structures in the cores of the CDs are clearly observed by transmission electron microscopy (TEM). CDs could perhaps be regarded as semiconductive surface defect layers formed by chemical erosion over conductive graphite cores. Chemical reduction by NaBH₄ changes the surface‐energy bands of the CDs, thus, enhances their fluorescence. The fluorescence properties of aqueous NaBH₄‐reduced CDs are also studied for possible biological applications.     

Adsorption behaviors of poly(amido amine) dendrimers with an azacrown core and long alkyl chain spacers on solid substrates

Adsorption behaviors of functional poly(amido amine) dendrimers with an azacrown core and long alkyl chain spacers were investigated on gold and self-assembled monolayer (SAM) by means of time course attenuated total reflection-surface enhanced infrared absorption and surface plasmon resonance spectroscopies. While 1.5th and 2.5th generation (G1.5 and G2.5) ester-terminated dendrimers were slightly adsorbed on all substrates examined, the adsorption of G2 amine-terminated dendrimer increased in the order dodecanethiol SAM相似文献   

Direct Heterogeneous Electron Transfer Reactions of Bacillus halodurans Bacterial Blue Multicopper Oxidase

Direct electron transfer reactions of Bacillus halodurans bacterial multicopper oxidase on bare spectrographic graphite, as well as bare and thiol‐modified gold electrodes were studied using cyclic voltammetry, potentiometry, amperometry, and spectroelectrochemistry. The redox potential of the T1 site of the enzyme was measured using mediatorless redox titration and found to be 325 mV±10 mV vs. NHE. From measurements with a mercaptopropionic acid‐modified gold electrode under aerobic conditions a midpoint potential of 360 mV vs. NHE for the T2/T3 copper cluster is deduced. Differing from most other characterized laccases of fungal and plant origins this bacterial enzyme exhibits bioelectrocatalytic activity at neutral pH and tolerates high chloride concentrations (200 mM), conditions that usually strongly inhibit catalysis. Moreover, it has the very high affinity towards molecular oxygen both in solution and in the adsorbed state (K_M≤50 μM).     

Electrochemical Characterization of (ZnO/dsDNA)n Layer-by-layer Films and Detection of Natural DNA Oxidative Damage

The positively charged nano‐ZnO and negatively charged natural DNA were alternately adsorbed on the surface of a gold electrode, forming (ZnO/dsDNA)_nlayer‐by‐layer films. Valuable dynamic information for controlling the formation and growth of the films was obtained by cyclic voltammetry and electrochemical impedance spectroscopy. Differential pulse voltammetric (DPV) measurements showed that the electroactive probe methylene blue (MB) could be loaded in the (ZnO/dsDNA)_nfilms from its solution, and then released from the films into Britton‐Robinson (B‐R) buffer. The complete reloading of MB in the films could be realized by immersing the films in MB solution again. However, after incubation in the solution of carcinogenic metal nickel, the damaged (ZnO/dsDNA)_n films could not return to their original and fully‐loaded state, and showed smaller DPV peak currents. The results demonstrated that the DNA damage induced by the hydroxyl radical could be achieved by electrochemistry.     

Double‐Layer Nanogold and Double‐Strand DNA‐Modified Electrode for Electrochemical Immunoassay of Cancer Antigen 15‐3

Various sensor‐based immunoassay methods have been extensively developed for the detection of cancer antigen 15‐3 (CA 15‐3), but most often exhibit low detection signals and low detection sensitivity, and are unsuitable for routine use. The aim of this work is to develop a simple and sensitive electrochemical immunoassay for CA 15‐3 in human serum by using nanogold and DNA‐modified immunosensors. Prussian blue (PB), as a good mediator, was initially electrodeposited on a gold electrode surface, then double‐layer nanogold particles and double‐strand DNA (dsDNA) with the sandwich‐type architecture were constructed on the PB‐modified surface in turn, and then anti‐CA 15‐3 antibodies were adsorbed onto the surface of nanogold particles. The double‐layer nanogold particles provided a good microenvironment for the immobilization of biomolecules. The presence of dsDNA enhanced the surface coverage of protein, and improved the sensitivity of the immunosensor. The performance and factors influencing the performance of the immunosensor were evaluated. Under optimal conditions, the proposed immunosensor exhibited a wide linear range from 1.0 to 240 ng/mL with a relatively low detection limit of 0.6 ng/mL (S/N=3) towards CA 15‐3. The stability, reproducibility and precision of the as‐prepared immunosensor were acceptable. 57 serum specimens were assayed by the developed immunosensor and standard enzyme‐linked immunosorbent assay (ELISA), respectively, and the results obtained were almost consistent. More importantly, the proposed methodology could be further developed for the immobilization of other proteins and biocompounds.     

Bare Clusters Derived from Protein Templates: Au25+, Au38+ and Au102+

A discrete sequence of bare gold clusters of well‐defined nuclearity, namely Au₂₅⁺, Au₃₈⁺ and Au₁₀₂⁺, formed in a process that starts from gold‐bound adducts of the protein lysozyme, were detected in the gas phase. It is proposed that subsequent to laser desorption ionization, gold clusters form in the gas phase, with the protein serving as a confining growth environment that provides an effective reservoir for dissipation of the cluster aggregation and stabilization energy. First‐principles calculations reveal that the growing gold clusters can be electronically stabilized in the protein environment, achieving electronic closed‐shell structures as a result of bonding interactions with the protein. Calculations for a cluster with 38 gold atoms reveal that gold interaction with the protein results in breaking of the disulfide bonds of the cystine units, and that the binding of the cysteine residues to the cluster depletes the number of delocalized electrons in the cluster, resulting in opening of a super‐atom electronic gap. This shell‐closure stabilization mechanism confers enhanced stability to the gold clusters. Once formed as stable magic number aggregates in the protein growth medium, the gold clusters become detached from the protein template and are observed as bare Au_n⁺ (n=25, 38, and 102) clusters.