Ultrahigh Vacuum Deposition of l-Cysteine on Au(110) Studied by High-Resolution X-ray Photoemission: From Early Stages of Adsorption to Molecular Organization

A cell-based electrochemical taste sensor for detection of Hydroxy-α-sanshool

The Transient Receptor Potential Vanilloid 1 (TRPV1) has been identified as a suitable candidate for a spicy taste (Zanthoxylum plant) sensor. In this study, we investigated the response of TRPV1 expressed on human HepG2 cell membranes following stimulation with Hydroxy-α-sanshool. A three-dimensional (3D) cell-based electrochemical sensor was fabricated by layering cells expressing hTRPV1. l-cysteine/AuNFs electrodes were functionalized on indium tin oxide-coated glass (ITO) to enhance the sensor's selectivity and sensitivity. HepG2 cells were encapsulated in sodium alginate/gelatin hydrogel to create a 3D cell cultivation system, which was immobilized on the l-cysteine/AuNFs/ITO to serve as biorecognition elements. Using differential pulse voltammetry (DPV), the developed biosensor was utilized to detect Hydroxy-α-sanshool, a representative substance in Zanthoxylum bungeanum Maxim. The result obtained from DPV was linear with Hydroxy-α-sanshool concentrations ranging from 0 to 70 μmol/L, with a detection limit of 2.23 μmol/L. This biosensor provides a sensitive and novel macroscopic approach for TRPV1 detection.

Investigating the electronic structure of high explosives with X-ray Raman spectroscopy

We investigate the sensitivity and potential of a synergistic experiment-theory X-ray Raman spectroscopy (XRS) methodology on revealing and following the static and dynamic electronic structure of high explosive molecular materials. We show that advanced ab-initio theoretical calculations accounting for the core-hole effect based on the Bethe-Salpeter Equation (BSE) approximation are critical for accurately predicting the shape and the energy position of the spectral features of C and N core-level spectra. Moreover, the incident X-ray dose typical XRS experiments require can induce, in certain unstable structures, a prominent radiation damage at room temperature. Upon developing a compatible cryostat module for enabling cryogenic temperatures ($$\approx$$ ≈ 10 K) we suppress the radiation damage and enable the acquisition of reliable experimental spectra in excellent agreement with the theory. Overall, we demonstrate the high sensitivity of the recently available state-of-the-art X-ray Raman spectroscopy capabilities in characterizing the electronic structure of high explosives. At the same time, the high accuracy of the theoretical approach may enable reliable identification of intermediate structures upon rapid chemical decomposition during detonation. Considering the increasing availability of X-ray free-electron lasers, such a combined experiment-theory approach paves the way for time-resolved dynamic studies of high explosives under detonation conditions.

Precise size-control and functionalization of gold nanoparticles synthesized by plasma-liquid interactions: using carboxylic, amino, and thiol ligands

Using gold nanoparticles (GNPs) in high-standard applications requires GNPs to be fabricated with high-quality size and surface properties. Plasma-liquid interactions (PLIs) have the unique ability to synthesize GNPs without using any reducing agents, and the GNP surface is free of stabilizing agents. It is an extreme advantage that ensures success for the subsequent functionalization processes for GNPs. However, fabricating GNPs via PLIs at the desired size has still been a challenge. Here, we present a simple approach to achieving the precise size-control of GNPs synthesized by PLIs. By adding suitable ligands to the precursor solution, the ligands wrap GNPs which interrupts and slows down the rapid growth of GNPs under PLIs. This way, the size of the GNPs can be precisely controlled by adjusting the ligand concentration. Our results showed that the size of the GNPs in the range of 10-60 nm can be fitted to reciprocal functions of the ligand concentration. The potency of the size-control depends on the type of ligands in the order of thiol > amine > carboxylate. The size-control has been well investigated with four common ligands: l-cysteine, glucosamine, salicylic acid, and terephthalic acid. XPS, FTIR, and zeta potential techniques confirmed the presence of these ligands on GNPs. The results indicated that functionalized ligands could be utilized to control the size and functionalize the GNP surface. Hence our approach could simultaneously achieve two goals: precise size-control and functionalization of GNPs without the ligand-exchange step.

Thermopower of Molecular Junction in Harsh Thermal Environments

Molecular junctions can be miniaturized devices for heat-to-electricity conversion application, yet these operate only in mild thermal environments (less than 323 K) because thiol, the most widely used anchor moiety for chemisorption of active molecules onto surface of electrode, easily undergoes thermal degradation. N-Heterocyclic carbene (NHC) can be an alternative to traditional thiol anchor for producing ultrastable thermoelectric molecular junctions. Our experiments showed that the NHC-based molecular junctions withstood remarkably high temperatures up to 573 K, exhibiting consistent Seebeck effect and thermovoltage up to approximately |1900 μV|. Our work advances our understanding of molecule-electrode contact in the Seebeck effect, providing a roadmap for constructing robust and efficient organic thermoelectric devices.

Amino-acid interactions with the Au(111) surface: adsorption, band alignment, and interfacial electronic coupling

The charge transport properties of biological molecules like peptides and proteins are intensively studied for the great flexibility, redox-state variability, long-range efficiency, and biocompatibility of potential bioelectronic applications. Yet, the electronic interactions of biomolecules with solid metal surfaces, determining the conductivities of the biomolecular junctions, are hard to predict and usually unavailable. Here, we present accurate adsorption structures and energies, electronic band alignment, and interfacial electronic coupling data for all 20 natural amino acids computed using the DFT+Σ scheme based on the vdW-DF and OT-RSH functionals. For comparison, data obtained using the popular PBE functional are provided as well. Tryptophan, compared to other amino acids, is shown to be distinctly exceptional in terms of the electronic properties related to charge transport. Its high adsorption energy, frontier-orbital levels aligned relatively close to the Fermi energy of gold and strong interfacial electronic coupling make it an ideal candidate for facilitating charge transfer on such heterogeneous interfaces. Although the amino acids in peptides and proteins are affected by the structural interactions hindering their contact with the surface, knowledge of the single-molecule surface interactions is necessary for a detailed understanding of such structural effects and tuning of potential applications.

Investigations into Spin- and Unpolarized Secondary Electron-Induced Reactions in Self-Assembled Monolayers of Cysteine

Cysteine is the simplest thiolated, chiral amino acid and is often used as the anchor for studies of self-assembled monolayers (SAMs) of complex biomolecules such as peptides. Understanding the interaction of SAMs of cysteine with low-energy secondary electrons (SEs) produced by X-rays can further our understanding of radiation damage in biomolecules. In particular, if the electrons are polarized, chiral-selective chemistry could have bearing on the origin of homochirality in nature. In the present paper, we use synchrotron radiation-based X-ray photoelectron spectroscopy to determine the changes that occur in the bonding of self-assembled layers of cysteine on gold as a result of soft X-ray irradiation. To investigate the possibility of chiral selectivity resulting from the interaction of low-energy, spin-polarized SEs (SPSEs), measurements were conducted on cysteine adsorbed on a 3 nm-thick gold layer deposited on a CoPt thin-film multilayer with perpendicular magnetic anisotropy. Time-dependent measurements of the C 1s, N 1s, O 1s, S 2p, and Au 4f core levels are used to follow the changes in surface chemistry and determine reaction cross-sections as a function of SE exposure. Analysis of the data results in cross-sections in the range of 5-7 Mb and suggests possible reaction pathways. Changing the magnetization direction of the CoPt multilayer produces SPSEs with opposite polarity. Some evidence of spin-dependent reactions is indicated but is inconclusive. Possible reasons for the discrepancy are posited.

N-Heterocyclic Carbenes for the Self-Assembly of Thin and Highly Insulating Monolayers with High Quality and Stability

As an organic nanostructure, self-assembled monolayers (SAMs) play a central role in many aspects of nanotechnology, including molecular electronics. In this work, we show that SAMs based on N-heterocyclic carbenes on a Au(111) substrate offer a high level of crystallinity and also exhibit the highest possible packing density. As a result of this structural optimization, defect concentrations were reduced by 2-3 orders of magnitude and thermal stability was ∼100 K higher than those of any other SAMs on Au. The conductivity of these SAMs is ∼4 orders of magnitude lower than that of standard alkanethiols of comparable length, which together with very low defect concentration and high thermal stability makes them a highly interesting material for potential application in organic thin film transistors. The self-assembly of such dense, highly crystalline, and notably stable structures is associated with strong C-Au bonding and the rational design of assembled molecules, resulting in the high mobility of both adsorbate and substrate atoms, as confirmed by the size of the molecular domains and the adsorbate-driven modification of the Au(111) substrate, respectively.

Predictions of Pattern Formation in Amino Acid Adlayers at the In Vacuo Graphene Interface: Influence of Termination State

Controlled self-assembly of biomolecules on graphene offers a pathway for realizing its full potential in biological applications. Microscopy has revealed the self-assembly of amino acid adlayers into dimer rows on nonreactive substrates. However, neither the spontaneous formation of these patterns, nor the influence of amino acid termination state on the formation of patterns has been directly resolved to date. Molecular dynamics simulations, with the ability to reveal atomic level details and exert full control over the termination state, are used here to model initially disordered adlayers of neutral, zwitterionic, and neutral-zwitterionic mixtures for two types of amino acids, tryptophan and methionine, adsorbed on graphene in vacuo. The simulations of the zwitterion-containing adlayers exhibit the spontaneous emergence of dimer row ordering, mediated by charge-driven intermolecular interactions. In contrast, adlayers containing only neutral species do not assemble into ordered patterns. It is also found that the presence of trace amounts of water reduces the interamino acid interactions in the adlayers, but does not induce or disrupt pattern formation. Overall, the findings reveal the balance between the lateral interamino acid interactions and amino acid-graphene interactions, providing foundational insights for ultimately realizing the predictable pattern formation of biomolecules adsorbed on unreactive surfaces.

Structural and Chemical Evolution of l-Cysteine Nanofilm on Si(111)-√3×√3-Ag: From Preferential Growth at Step Edges and Antiphase Boundaries at Room Temperature to Adsorbate-Mediated Metal Cluster Formation at Elevated Temperature

The interaction of cysteine molecules with the Si(111)-√3×√3-Ag surface has been investigated over the submonolayer to multilayer regime using X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations. With both upper step and lower step terraces, step edges, and antiphase boundaries, the √3×√3-Ag overlayer supported on Si(111) provides a rich two-dimensional template for studying site-specific biomolecular interactions. As an amino acid with three functional groups, cysteine is found to chemisorb through S-H bond cleavage and S-Ag bond linkage first at step edges and antiphase boundaries followed by island formation and expanded growth onto terraces. Intermolecular interactions are dominated by zwitterionic hydrogen bonding at higher coverages, producing a porous unordered interfacial layer composed of cysteine agglomerates at room temperature. Upon annealing, cysteine adsorbates induce structural transformation of the uniform √3×√3-Ag reconstructed surface lattice into metallic Ag clusters with a narrow size distribution and short-range ordering. Preferential nanoaggregate formation of cysteine at defect sites and cysteine-induced metal cluster formation promise a new approach to fabricating nanoclusters for potential applications in chemical sensing and catalysis.

Defects on a pyrite(100) surface produce chemical evolution of glycine under inert conditions: experimental and theoretical approaches

The presence of non-stoichiometric sites on the pyrite(100) surface makes it a suitable substrate for driving the chemical evolution of the amino acid glycine over time, even under inert conditions. Spectroscopic molecular fingerprints prove a transition process from a zwitterionic species to an anionic species over time on the monosulfide enriched surface. By combining experimental and theoretical approaches, we propose a surface mechanism where the interaction between the amino acid species and the surface will be driven by the quenching of the surface states at Fe sites and favoured by sulfur vacancies. This study demonstrates the potential capability of pyrite to act as a surface catalyst.

Pyrite-induced uv-photocatalytic abiotic nitrogen fixation: implications for early atmospheres and Life

The molecular form of nitrogen, N₂, is universally available but is biochemically inaccessible for life due to the strength of its triple bond. Prior to the emergence of life, there must have been an abiotic process that could fix nitrogen in a biochemically usable form. The UV photo-catalytic effects of minerals such as pyrite on nitrogen fixation have to date been overlooked. Here we show experimentally, using X-ray photoemission and infrared spectroscopies that, under a standard earth atmosphere containing nitrogen and water vapour at Earth or Martian pressures, nitrogen is fixed to pyrite as ammonium iron sulfate after merely two hours of exposure to 2,3 W/m 2 of ultraviolet irradiance in the 200-400 nm range. Our experiments show that this process exists also in the absence of UV, although about 50 times slower. The experiments also show that carbonates species are fixed on pyrite surface.

Ultraviolet Irradiation on a Pyrite Surface Improves Triglycine Adsorption

We characterized the adsorption of triglycine molecules on a pyrite surface under several simulated environmental conditions by X-ray photoemission spectroscopy. The triglycine molecular adsorption on a pyrite surface under vacuum conditions (absence of oxygen) shows the presence of two different states for the amine functional group (NH₂ and NH₃⁺), therefore two chemical species (anionic and zwitterionic). On the other hand, molecular adsorption from a solution discriminates the NH₂ as a unique molecular adsorption form, however, the amount adsorbed in this case is higher than under vacuum conditions. Furthermore, molecular adsorption on the mineral surface is even favored if the pyrite surface has been irradiated before the molecular adsorption occurs. Pyrite surface chemistry is highly sensitive to the chemical changes induced by UV irradiation, as XPS analysis shows the presence of Fe₂O₃ and Fe₂SO₄—like environments on the surface. Surface chemical changes induced by UV help to increase the probability of adsorption of molecular species and their subsequent concentration on the pyrite surface.

Adjusting the Linear Range of Au-MOF Fluorescent Probes for Real-Time Analyzing Intracellular GSH in Living Cells

A series of Au-loaded metal-organic frameworks (MOFs) were synthesized in this study and further employed for real-time quantitative analysis of intracellular glutathione (GSH) level. Different linear ranges can be acquired by altering the size of gold and MOF particles, or adjusting the proportion of 2-aminoterephthalic acid/1,4-benzenedicarboxylate linkers, which is also observed on fluorescein isothiocyanate-attached Au-MOFs. Further study reveals that the flexible molecular chain of GSH with the -COOH/-NH₂ and -SH terminals may readily tie on relevant gold nanoparticles through its -NH₂/-COOH groups, which then restricts the intramolecular motions of fluorescence probes and thus induces marked fluorescence enhancement. On the basis of these observations, the intracellular GSH levels of different cells including L02 cells, Hela, and U87 as well as HepG2 cancer cells can be rapidly evaluated by these Au-MOF probes.

Recognition of chiral zwitterionic interactions at nanoscale interfaces by chiroplasmonic nanosensors

The ability to detect chiral molecules renders plasmonic nanosensors as promising tools for the study of chirality phenomena in living systems. Using gold nanorod based plasmonic nanosensors, we investigated here typically chiral zwitterionic electrostatic (Zw-Es) and hydrogen-bonding (Hb) interactions occurring via amine and carboxylic groups at nanoscale interfaces in aqueous solutions. Our results reveal that the plasmonic circular dichroism responses of the nanosensors can have both conformational sensitivity and chiral selectivity to the interfacial molecular interactions. Such a dual function of the plasmonic nanosensors enables a new chiroptical way to differentiate between chiral Zw-Es and Hb interactions, to monitor the transformation between these two interaction forces, and particularly to recognize homochiral Zw-Es interactions in solution. Together with the surface enhanced Raman scattering (SERS) technique, this plasmonic CD based biosensing could have important values for the insightful understanding of chirality-dependent molecular recognition in biological and pharmaceutical systems.

Pyrite surface environment drives molecular adsorption: cystine on pyrite(100) investigated by X-ray photoemission spectroscopy and low energy electron diffraction

We have demonstrated that the annealing process for cleaning pyrite surfaces is a critical parameter in promoting ordering on the surface and driving surface reactivity. Furthermore, we describe a spectroscopic surface characterization of the presence or absence of the surface ordering, as indicated by the Low Energy Electron Diffraction (LEED) pattern, as a function of the surface annealing process. Complementary X-ray photoemission spectroscopy (XPS) results provide evidence that longer annealing processes of over 3 hours repair the sulfur vacancies in the pyrite, making FeS species partially disappear in favor of FeS₂ species. These features play an important role in molecular adsorption. We show that in the case of the cystine amino acid on the (100) pyrite surface, the substrate structure is responsible for the chemical adsorption form. The presence of an ordered structure on the surface, as indicated by the LEED pattern, favors the cystine NH₃⁺ chemical form, whereas the absence of the surface ordering promotes cystine NH₂ adsorption due to the sulfur-deficient surface. The cystine molecule could then act by changing its chemical functionalities to compensate for the iron surface coordination. The chemical molecular adsorption form can be selected by the surface annealing conditions, implying that environmental conditions could drive molecular adsorption on mineral surfaces. These findings are relevant in several surface processes, and they could play a possible role in prebiotic chemistry surface reactions and iron-sulfur scenarios.

Linkage, charge state and layer of L-Cysteine on copper surfaces

The control of linkage and charge state between biomolecules and metals represents a key issue for the architect of bioactive systems. In this paper, the linkage, charge state and layer of L-Cysteine (L-Cys) self-assembled films were handled on copper surfaces at pH=6.86. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed to measure the film quality and the details of self-assembled progress. X-ray photoelectron spectroscopy (XPS) and quantum chemical calculations of density functional theory (DFT) were used to characterize the linkage, charge state and layer of the L-Cys molecules on copper surfaces. The results indicate that, from 0s to 24h, the self-assembled process can be classified as three steps, fast adsorption at the beginning, and then rearrangement to form a monolayer, and then the formation of double layer. And L-Cys molecules link to the copper surface through CuS bond, not CuN bond. The thickness of monolayer is 10.5Å. Then the L-Cys molecules of second layer recline on the first layer. Finally, by the interaction of amine group and carboxylic acid group between the two layers, the second self-assembled film stands uprightly, and the -S^- group of the second layer point outward. The thickness of the double layer is 19.7Å. All the Cu/L-Cys films have negative charges because the pH (6.86) of the self-assembled solution is more than the isoelectric point of the L-Cys (5.05).

Theoretical Study of the Adsorption Mechanism of Cystine on Au(110) in Aqueous Solution

The adsorption and dynamics of cystine, which is the oxidized dimer of cysteine where the monomers are connected through a disulfide bond, on the Au(110) surface, in water solution, is characterized by means of classical molecular dynamics simulations based on a recently developed reactive force field (ReaxFF). The adopted computational procedure and the force field description are able to give a complete and reliable picture, in line with experiments, of the molecule behavior in solution and in close contact with the metal support. Many different aspects, which have never been explored computationally at this level of theory, are disclosed, namely, physisorption, chemisorption, disulfide bridge breaking/creation, and formation of staples. It is demonstrated that all these events are connected with the specific orientation and location of cystine on the substrate. Simulations in pure water reveal that the disulfide bridge is stable, whereas dissociation is observed on gold. This is favored at low coverage, whereas at high coverage both intact and dissociated forms can be observed depending on local arrangements. The computed photoemission spectra at different K-edges for the predicted adsorbate structures satisfactorily agree with the experimental measurements extracted from literature.

Properties of the gold-sulphur interface: from self-assembled monolayers to clusters

The gold-sulphur interface of self-assembled monolayers (SAMs) was extensively studied some time ago. More recently tremendous progress has been made in the preparation and characterization of thiolate-protected gold clusters. In this feature article we address different properties of the two systems such as their structure, the mobility of the thiolates on the surface and other dynamical aspects, the chirality of the structures and characteristics related to it and their vibrational properties. SAMs and clusters are in the focus of different communities that typically use different experimental approaches to study the respective systems. However, it seems that the nature of the Au-S interfaces in the two cases is quite similar. Recent single crystal X-ray structures of thiolate-protected gold clusters reveal staple motifs characterized by gold ad-atoms sandwiched between two sulphur atoms. This finding contradicts older work on SAMs. However, newer studies on SAMs also reveal ad-atoms. Whether this finding can be generalized remains to be shown. In any case, more and more studies highlight the dynamic nature of the Au-S interface, both on flat surfaces and in clusters. At temperatures slightly above ambient thiolates migrate on the gold surface and on clusters. Evidence for desorption of thiolates at room temperature, at least under certain conditions, has been demonstrated for both systems. The adsorbed thiolate can lead to chirality at different lengths scales, which has been shown both on surfaces and for clusters. Chirality emerges from the organization of the thiolates as well as locally at the molecular level. Chirality can also be transferred from a chiral surface to an adsorbate, as evidenced by vibrational spectroscopy.

Nonfouling property of zwitterionic cysteine surface

Applications of implantable bioelectronics for analytical and curative purposes are currently limited by their poor long-term biofunctionality in physiological media and nonspecific interactions with biomolecules. In an attempt to prolong in vivo functionality, recent advances in surface modifications have demonstrated that zwitterionic coatings can rival the performance of conventional poly(ethylene glycol) polymers in reducing nonspecific protein fouling. Herein, we report the fabrication of a very thin layer of nonfouling zwitterionic cysteine surface capable of protecting implantable bioelectronics from nonspecific adsorption of plasma proteins. This work is the first of its kind to fabricate, through solution chemistry, a cysteine surface exhibiting zwitterionic state as high as 88% and to demonstrate antibiofouling under the exposure of bovine serum albumin (BSA) and human serum. The fabricated surface utilized a minimal amount of gold substrate, approximately 10 nm, and an extremely thin antifouling layer at 1.14 nm verified by ellipsometry. X-ray photoelectron spectroscopy assessment of the nitrogen (N1s) and carbon (C1s) spectra conclude that 87.8% of the fabricated cysteine surface is zwitterionic, 2.5% is positively charged, and 9.6% is noncharged. Antibiofouling performance of the cysteine surface is quantitatively determined by bicinchoninic acid (BCA) protein assay as well as qualitatively confirmed using scanning electron spectroscopy. Cysteine surfaces demonstrated a BSA fouling of 3.9 ± 4.84% μg/cm(2), which is 93.6% and 98.5% lower than stainless steel and gold surfaces, respectively. Surface plasmon resonance imaging analysis returned similar results and suggest that a thinner cysteine coating will enhance performance. Scanning electron microscopy confirmed the results of BCA assay and suggested that the cysteine surface demonstrated a 69% reduction to serum fouling. The results reported in this paper demonstrate that it is possible to achieve a highly zwitterionic surface through solution chemistry on a macroscopic level that is capable of improving biocompatibility of long-term implantable bioelectronics.

Characterization of two-dimensional chiral self-assemblies L- and D-methionine on Au(111)

A combination of XPS, in situ RAIRS, LEED, and STM experiments together with ab initio DFT calculations were used to elucidate the self-assembly properties at the atomic level, and enabled the interpretation of the expression of surface chirality upon adsorption of both enantiomers of methionine on a clean Au(111) surface under UHV conditions. The combination of experimental results, in particular, LEED and STM data with quantum chemical calculations is shown to be a successful setup strategy for addressing this challenge. It was found that the methionine molecular self-assembly consists of the first molecule lying parallel to the gold surface and the second interacting with the first methionine through a 2D H-bond network. The interaction with the gold surface is weak. The stability of the assembly is mainly due to the presence of intermolecular H bonds, resulting in the formation of ziplike dimer rows on the Au(111) surface. The methionine molecules interact with each other via their amino acid functional groups. The assembly shows an asymmetric pattern due to a slightly different orientation of the methionine molecules with respect to the surface. Simulations of the STM image of methionine assemblies were consistent with the experimental STM image. The present study shows another example of Au(111) stabilizing a self-assembled biological layer, which is not chemically perturbed by the surface.

Confirmation of the coexistence of two tautomers of 2-mercaptothiazoline on the Ge(100) surface

We confirmed the coverage dependent variation of tautomers of 2-mercaptothiazoline (the thiolate and thione forms) adsorbed on the Ge(100) surface under UHV conditions by using HRXPS measurements in conjunction with the DFT calculation method, which was studied before only in aqueous systems. The C 1s, S 2p, and N 1s core-level spectra obtained using HRXPS revealed the simultaneous presence of two distinct adsorption structures in different proportions at both low (0.15 ML) and high (0.65 ML) coverages. Moreover, we modelled the adsorption structures and geometric configurations of the bond states of 2-mercaptothiazoline on the Ge(100) surface by using the DFT calculation method, and found that the S dative bonded structure is the most stable adsorption structure for the thione form of 2-mercaptothiazoline and that the S-H dissociated-N dative bonded structure is the most stable adsorption structure for the thiolate form.

Synthesis of uniform disk-shaped copper telluride nanocrystals and cation exchange to cadmium telluride quantum disks with stable red emission

We present the synthesis of novel disk-shaped hexagonal Cu2Te nanocrystals with a well-defined stoichiometric composition and tunable diameter and thickness. Subsequent cation exchange of Cu to Cd at high temperature (180 °C) results in highly fluorescent CdTe nanocrystals, with less than 1 mol % of residual Cu remaining in the lattice. The procedure preserves the overall disk shape, but is accompanied by a substantial reconstruction of the anion sublattice, resulting in a reorientation of the c-axis from the surface normal in Cu2Te into the disk plane in CdTe nanodisks. The synthesized CdTe nanodisks show a continuously tunable photoluminescence (PL) peak position, scaling with the thickness of the disks. The PL lifetime further confirms that the CdTe PL arises from band-edge exciton recombination; that is, no Cu-related emission is observed. On average, the recombination rate is about 25-45% faster with respect to their spherical quantum dots counterparts, opening up the possibility to enhance the emission rate at a given wavelength by controlling the nanocrystal shape. Finally, with a PL quantum efficiency of up to 36% and an enhanced PL stability under ambient conditions due to a monolayer of CdS formed on the nanocrystal surface during cation exchange, these flat quantum disks form an interesting enrichment to the current family of highly fluorescent, shape-controlled nanocrystals.

Adsorption sequence of multifunctional groups: a study on the reaction pathway and the adsorption structure of homocysteine on the Ge(100) surface

We investigated the adsorption mechanism of homocysteine (HS-CH2-CH2-CH(NH2)-COOH) on the Ge(100) surface along with its electronic structures and adsorption geometries to determine the sequence of adsorption of this amino acid's functional groups using core-level photoemission spectroscopy (CLPES) in conjunction with density functional theory (DFT) calculations. We found that the "SH-dissociated OH-dissociated N-dative-bonded structure" and the "SH-dissociated OH-dissociation-bonded structure" were preferred at a monolayer (ML) coverage of 0.30 (lower coverage) and 0.60 (higher coverage), respectively. The "SH-dissociated OH-dissociated N-dative-bonded structure" was the most stable structure. Moreover, we systematically confirmed the sequence of adsorption of the functional groups of the homocysteine molecule on the Ge(100) surface, which is thiol group (-SH), carboxyl group (-COOH), and amine group (-NH2).

DFT study of the adsorption of D-(L-)cysteine on flat and chiral stepped gold surfaces

The adsorption of cysteine onto the intrinsically chiral gold surface, Au(321)(R,S), was investigated by means of a periodic supercell density functional theory approach. The results are compared to those obtained at the same level of theory with a nonchiral surface having the same terrace orientation, the Au(111) surface. Neutral and zwitterionic cysteine forms of the L and D enantiomers are considered, as are surface coverage effects. It was found that at high coverage the zwitterionic forms of L- and D-cysteine are more stable on the Au(321)(R,S) faces of the stepped surface and also on the flat Au(111) surface, leading to highly organized cysteine monolayers. However, at low coverage the adsorption of cysteine dimers, with the pairs interacting through their carbonyl groups, is more favorable than or at least equally favorable to the adsorption of single cysteine molecules on both surfaces. A comparison between the cysteine adsorption on the two different surface structures shows that the adsorption on the stepped surface is clearly more favorable than on the flat surface, revealing the importance of the low-coordinated gold atoms in the adsorption of these species. Furthermore, non-negligible differences between the adsorption energy of the enantiomers of cysteine were found both at high and low coverage, thus showing the enantiospecificity of this intrinsically chiral surface regarding cysteine adsorption. This adsorption occurs with the cysteine binding the surface through only one contact point (by its sulfur atom), in contrast to previous work where the enantiospecific adsorption of cysteine has been related to two nonequivalent binding sites of the cysteine enantiomers with the surface.

Spectroscopic evidence for neutral and anionic adsorption of (S)-glutamic acid on Ag(111)

We report here on a combined photoemission and vibrational spectroscopy investigation of (S)-glutamic acid adsorption on Ag(111). We show that, in the temperature range 250 K ≤ T ≤ 400 K, non-zwitterionic adsorption takes place and the anionic form prevails at nonvanishing coverage. Significant conformational changes of the self-assembled layer must occur above 300 K, corresponding to a substantial reduction of the sticking probability and a modification of the vibrational spectrum. The similarity of behavior with respect to glutamic acid adsorption on the previously investigated Ag(100) and Ag(110) surfaces is also discussed.

Interaction of L-cysteine with naked gold nanoparticles supported on HOPG: a high resolution XPS investigation

We report the results of a synchrotron-based high-resolution XPS study of the interaction of L-cysteine (Cys) with well-characterized colloidal gold nanoparticles (NPs, typical size 3-4 nm), which were pre-deposited on highly oriented pyrolytic graphite and then brought into contact with the aqueous solution of Cys by drop-casting. By comparison with data previously obtained for Cys deposition on flat Au substrates (single crystals and high quality films), we demonstrate the formation of a strong Cys/NP thiolate bond. The analysis of the line shape and adsorbate-induced Au 4f core level shift, backed by simulations of the NP structure, reveals the interaction of Cys with low-coordinated Au atoms belonging to the NP edge and corners. The analysis of the N 1s core-level indicates that neutral molecules are the most abundant species. The small facet size limits the formation of extended networks of zwitterionic molecules, typical of single crystal surfaces. This study provides a spectroscopic insight into the intense poisoning effect caused by a limited amount of Cys on Au catalysts described in previous reports.

Study on the reversible changes of the surface properties of an L-cysteine self-assembled monolayer on gold as a function of pH

A stimuli-response biological surface of L-cysteine was prepared on a polycrystalline gold surface from aqueous solution. The effect of the pH value of the rinsing solution on the surface composition was studied with X-ray photoelectron spectroscopy (XPS). Qualitative and quantitative analysis of the amino, carboxyl, and thiol functional groups of these self-assembled monolayers indicate that L-cysteine molecules exist in the neutral and zwitterionic forms and that they are sensitive to the pH of the rinsing solution. In addition, the wetting properties of the functionalized surface were studied by contact angle (CA) analysis: they were also dependent on the pH of the rinsing solution. Furthermore, it was shown that this functionalization process was reversible.

Modification of the size of supported clusters by coadsorption of an organic compound: gold and L-cysteine on rutile TiO2(110)

Using X-ray photoelectron spectroscopy we studied the coadsorption of the amino acid L-cysteine and gold on a rutile TiO(2)(110) surface under ultrahigh vacuum conditions. Irrespective of the deposition order, i.e., irrespective of whether L-cysteine or gold is deposited first, the primary interaction between L-cysteine and the gold clusters formed at the surface takes place through the deprotonated thiol group of the molecule. The deposition order, however, has a profound influence on the size of the gold clusters as well as their location on the surface. If L-cysteine is deposited first the clusters are smaller by a factor two to three compared to gold deposited onto the pristine TiO(2)(110) surface and then covered by L-cysteine. Further, in the former case the clusters cover the molecules and thus form the outermost layer of the sample. We also find that above a minimum gold cluster size the gold cluster/L-cysteine bond is stronger than the L-cysteine/surface bridging oxygen vacancy bond, which, in turn, is stronger than the gold cluster/vacancy bond.

Optical properties of Yeast Cytochrome c monolayer on gold: an in situ spectroscopic ellipsometry investigation

The adsorption of Yeast Cytochrome c (YCC) on well defined, flat gold substrates has been studied by Spectroscopic Ellipsometry (SE) in the 245-1000 nm wavelength range. The investigation has been performed in aqueous ambient at room temperature, focusing on monolayer-thick films. In situ δΨ and δΔ difference spectra have shown reproducibly well-defined features related to molecular optical absorptions typical of the so-called heme group. The data have been reproduced quantitatively by a simple isotropic optical model, accounting for the molecular absorption spectrum and film-substrate interface effects. The simulations allowed a reliable estimate of the film thickness and the determination of the position and the shape of the so-called Soret absorption peak that, within the experimental uncertainty, is the same found for molecules in liquid. These findings suggest that YCC preserves its native structure upon adsorption. The same optical model was able to reproduce also ex situ results on rinsed and dried samples, dominated by the spectral features associated to the polypeptide chain that tend to overwhelm the heme absorption features.

Interaction of alkanethiols with nanoporous cluster-assembled Au films

This article presents a study of the interaction of octadecanethiol molecules (C(18)) with nanoporous cluster-assembled gold films under a liquid environment based on a combined spectroscopic ellipsometry and X-ray photoelectron spectroscopy investigation. By comparing the optical response, following the deposition of C(18), of cluster-assembled films with varying degrees of porosity with that of flat surfaces and by resolving the corresponding features of the molecule-Au bond, we have been able to define the conditions that either favor molecular in-depth diffusion into the pores or promote the formation of a molecular self-assembled monolayer (SAM) restricted to the film surface. In the presence of abundant open pores, C(18) molecules strongly diffuse within the film interior and bind to the pore walls, whereas in the presence of porous films with less abundant open pores we have observed that the molecules tend to remain confined to the surface region, adopting a SAM-like configuration.

UPS, XPS, and NEXAFS study of self-assembly of standing 1,4-benzenedimethanethiol SAMs on gold

We report a study of the self-assembly of 1,4-benzenedimethanethiol monolayers on gold formed in n-hexane solution held at 60 °C for 30 min and in dark conditions. The valence band characteristics, the thickness of the layer, and the orientation of the molecules were analyzed at a synchrotron using high resolution photoelectron spectroscopy and near edge X-ray adsorption spectroscopy. These measurements unambiguously attest the formation of a single layer with molecules arranged in the upright position and presenting a free -SH group at the outer interface. Near edge X-ray absorption fine structure (NEXAFS) measurements suggest that the molecular axis is oriented at 24° with respect to the surface normal. In addition, valence band features could be successfully associated to specific molecular orbital contributions thanks to the comparison with theoretically calculated density of states projected on the different molecular units.

(S)-glutamic acid on Ag(100): self-assembly in the nonzwitterionic form

The fundamental understanding of adsorption and self-organization of biological molecules at surfaces is of greatest importance for a huge variety of possible applications, ranging from molecular electronics to the study of biocompatible materials, hygiene, and biofouling. In spite of that, the characterization of the interactions of organic molecules of biological interest with surfaces is far from being complete. In the present paper we report on a combined microscopic (scanning tunneling microscopy (STM)) and spectroscopic (X-ray photoemission spectroscopy and high-resolution electron energy loss spectroscopy) study of glutamic acid (Glu) adsorption and self-assembly on Ag(100) at different temperature. STM allows one to determine the structures of the Glu layers, for which empirical models are proposed, while photoemission spectra exclude adsorption in the zwitterionic form, which is the most common especially for weakly interacting substrates.

Self-assembly of 1,4-benzenedimethanethiol self-assembled monolayers on gold

A study of the self-assembly of 1,4-benzenedimethanethiol (BDMT; HS-CH(2)-(C(6)H(4))-CH(2)-SH) monolayers on gold is presented. Self-assembled monolayers (SAMs) are characterized by reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE) measurements. The ensemble of measurements consistently shows that well-organized BDMT SAMs, with "standing-up" molecules, can be obtained on high quality gold films with incubation in n-hexane provided that N(2)-degassed solutions are used and all preparation steps are performed at 60 degrees C in the absence of ambient light. SE data indicate that the optical interface properties of the BDMT-Au system are different from those of simple alkanethiol SAMs. A possible mechanism for the formation of the "standing-up" phase from the lying-down phase via a hydrogen exchange reaction involving chemisorbed lying-down and free dithiol molecules is discussed.

Self-assembly of (S)-glutamic acid on Ag(100): a combined LT-STM and ab initio investigation

Self-assembly of organic molecules at metal surfaces is of greatest importance in nanoscience; in fact, it opens new perspectives in the field of molecular electronics and in the study of biocompatible materials. Combining an experimental low-temperature scanning tunneling microscopy investigation with ab initio calculations, we succeeded to describe in detail (S)-glutamic acid adsorption on Ag(100) at T = 350 K. We find that (S)-glutamic acid organizes in a squared structure and, at variance with the majority of cases reported in literature, it adsorbs in the neutral form, 4.6 A above the surface plane. The interaction with the poorly reactive Ag substrate is only due to weak van der Waals forces, while H-bonds between carboxyl groups and the formation of a OCOH-OCOH-OCOH-OCOH cycle at the vertex of the squares are the main responsible for the self-assembly.