Monolayer Surface Coverage Research Articles

Increased monolayer domain size and patterned growth of tungsten disulfide through controlling surface energy of substrates

We report a surface energy-controlled low-pressure chemical vapor deposition growth of WS2 monolayers on SiO2 using pre-growth oxygen plasma treatment of substrates, facilitating increased monolayer surface coverage and patterned growth without lithography. Oxygen plasma treatment of the substrate caused an increase in the average domain size of WS2 monolayers by 78% ± 2% while having a slight reduction in nucleation density, which translates to increased monolayer surface coverage. This substrate effect on growth was exploited to grow patterned WS2 monolayers by patterned plasma treatment on patterned substrates and by patterned source material with resolutions less than 10 µm. Contact angle-based surface energy measurements revealed a dramatic increase in polar surface energy. A growth model was proposed with lowered activation energies for growth and increased surface diffusion length consistent with the range of results observed. WS2 samples grown with and without oxygen plasma were similar high quality monolayers verified through transmission electron microscopy, selected area electron diffraction, atomic force microscopy, Raman, and photoluminescence measurements. This technique enables the production of large-grain size, patterned WS2 without a post-growth lithography process, thereby providing clean surfaces for device applications.

Preparation, Characterization, and Regeneration of Aluminum (Hydr)Oxide–Amended Molecular Sieves for Fluoride Removal from Drinking Water

AbstractMolecular sieves and zeolites showed increased fluoride adsorption capacities when amended with aluminum (hydr)oxide (AlOOH). When normalized by the AlOOH content, the adsorption capacities of most amended molecular sieves were higher than the maximum theoretical value expected for monolayer surface coverage, suggesting fluoride removal from processes beyond adsorption, such as precipitation. Although the mass-normalized adsorption capacities of most amended materials were less than that of an equivalent mass of pure AlOOH, several molecular sieves with pores of 1 to several nanometers showed mass-normalized adsorption capacities similar to pure AlOOH, possibly due to their larger pores, which may have facilitated fluoride adsorption after aluminum (hydr)oxide precipitation. Energy-dispersive X-ray spectroscopy detected elevated fluorine in a representative AlOOH-amended molecular sieve after repeated fluoride adsorption, also consistent with fluoride uptake by processes beyond only monolayer cove...

Self-healing gold mirrors and filters at liquid-liquid interfaces.

The optical and morphological properties of lustrous metal self-healing liquid-like nanofilms were systematically studied for different applications (e.g., optical mirrors or filters). These nanofilms were formed by a one-step self-assembly methodology of gold nanoparticles (AuNPs) at immiscible water-oil interfaces, previously reported by our group. We investigated a host of experimental variables and herein report their influence on the optical properties of nanofilms: AuNP mean diameter, interfacial AuNP surface coverage, nature of the organic solvent, and nature of the lipophilic organic molecule that caps the AuNPs in the interfacial nanofilm. To probe the interfacial gold nanofilms we used in situ (UV-vis-NIR spectroscopy and optical microscopy) as well as ex situ (SEM and TEM of interfacial gold nanofilms transferred to silicon substrates) techniques. The interfacial AuNP surface coverage strongly influenced the morphology of the interfacial nanofilms, and in turn their maximum reflectance and absorbance. We observed three distinct morphological regimes; (i) smooth 2D monolayers of "floating islands" of AuNPs at low surface coverages, (ii) a mixed 2D/3D regime with the beginnings of 3D nanostructures consisting of small piles of adsorbed AuNPs even under sub-full-monolayer conditions and, finally, (iii) a 3D regime characterised by the 2D full-monolayer being covered in significant piles of adsorbed AuNPs. A maximal value of reflectance reached 58% in comparison with a solid gold mirror, when 38 nm mean diameter AuNPs were used at a water-nitrobenzene interface. Meanwhile, interfacial gold nanofilms prepared with 12 nm mean diameter AuNPs exhibited the highest extinction intensities at ca. 690 nm and absorbance around 90% of the incident light, making them an attractive candidate for filtering applications. Furthermore, the interparticle spacing, and resulting interparticle plasmon coupling derived optical properties, varied significantly on replacing tetrathiafulvalene with neocuproine as the AuNP capping ligand in the nanofilm. These interfacial nanofilms formed with neocuproine and 38 nm mean diameter AuNPs, at monolayer surface coverages and above, were black due to aggregation and broadband absorbance.

Electrochemistry of the COMT Enzyme Inhibitor Entacapone and Its Determination in Pharmaceutical Formulation and Human Blood-Pharmacokinetic Study

Entacapone (ENT) is a potent inhibitor of catechol-O-methyltransferase enzyme. ENT is reduced via two irreversible cathodic waves over the pH range 2.0–11.5. It has strong interfacial adsorptive behavior onto Hg electrode surface. Langmuir isotherm indicated monolayer adsorption and the equilibrium parameter RL value (0.40 – 0.93) indicated that sorption process was favorable. A monolayer surface coverage of 6.06 × 10−10 mol cm−2 was estimated and each adsorbed ENT molecule occupied an area of 0.27 nm2. Taking advantage of the adsorption features of ENT, a sensitive and selective square-wave adsorptive cathodic stripping voltammetric (SW-AdCSV) method was developed for trace determination of ENT in pure form, commercial formulations (in Comtan tablets; as a single active drug and in Stalevo 200 tablets; as tertiary mixture of ENT, levodopa and carbidopa) and in human serum. Limits of quantification (LOQ) of 5.0 × 10−10 and 1.3 × 10−9 mol L−1 ENT were achieved in pure form and in spiked human serum, respectively. This analysis was carried out without addition of any surfactant, extraction of the drug and interference from some common excipients, co-formulated and co-administrated drugs. The proposed method has also been successfully applied for assay of ENT in real plasma samples and seems to be suitable for therapeutic drug monitoring purposes.

Nuclear Magnetic Resonance Investigation of the Fractionation of Water–Ethanol Mixtures with Cellulose and Its Cross-Linked Biopolymer Forms

Cellulose (CE) was cross-linked with epichlorohydrin (EPI) at variable compositions, and the fractionation properties were investigated in binary water–ethanol (W–E) solutions, including the pure solvent systems. The relative uptake of each solvent was measured using quantitative 1H nuclear magnetic resonance (qNMR) spectroscopy. This study highlights the utility of qNMR as a rapid screening method for estimation of solvent selective fractionation in binary mixtures. The uptake properties of CE–EPI cross-linked polymers with ethanol and water were well-described using the Sips isotherm model. Modeling shows that the monolayer surface coverage (Qm) of ethanol and water onto the polymer materials covers a range (1.13–2.44 g/g) of values with heterogeneous adsorption behavior, in agreement with the Sips exponential fitting parameter (ns ≠ 1). The CE–EPI adsorbents display unique fractionation with water and ethanol from binary solutions, as evidenced by the relative selectivity (Rselectivity) value in binary W–E solvent systems. The Rselectivity [Qm(W)/Qm(E)] values at saturative conditions varied (from 1.10 to 2.03) and further illustrate that CE materials display molecular selective solvent fractionation in binary W–E solutions. This study provides a greater molecular level understanding for the adsorptive uptake properties of CE that are relevant to developing CE-based adsorbent technology for the fractionation of biofuels and related chemical separations.

Wet formation and structural characterization of quasi-hexagonal monolayers

We have presented a simple and efficient method for producing dense particle monolayers with controlled surface coverage. The method is based on particle sedimentation, manipulation of the particle–substrate electrostatic interaction, and gentle mechanical vibration of the system. It allows for obtaining quasi-hexagonal structures under wet conditions. Using this method, we have produced a monolayer of 3μm silica particles on a glassy carbon substrate. By optical microscopy, we have determined the coordinates of the particles and surface coverage of the obtained structure to be 0.82. We have characterized the monolayer structure by means of the pair-correlation function and power spectrum. We have also compared the results with those for a 2D hexagonal monolayer and monolayer generated by random sequential adsorption at the coverage 0.50. We have found the surface fractal dimension to be 2.5, independently of the monolayer surface coverage.

Dual-Ligand Modified Polymer-Lipid Hybrid Nanoparticles for Docetaxel Targeting Delivery to Her2/neu Overexpressed Human Breast Cancer Cells.

In this study, a dual-ligand polymer-lipid hybrid nanoparticle drug delivery vehicle comprised of an anti-HER2/neu peptide (AHNP) mimic with a modified HIV-1 Tat (mTAT) was established for the targeted treatment of Her2/neu-overexpressing cells. The resultant dual-ligand hybrid nanoparticles (NPs) consisted of a poly(lactide-co-glycolide) core, a near 90% surface coverage of the lipid monolayer, and a 5.7 nm hydrated polyethylene glycol shell. Ligand density optimization study revealed that cellular uptake efficiency of the hybrid NPs could be manipulated by controlling the surface-ligand densities. Furthermore, the cell uptake kinetics and mechanism studies showed that the dual-ligand modifications of hybrid NPs altered the cellular uptake pathway from caveolae-mediated endocytosis (CvME) to the multiple endocytic pathways, which would significantly enhance the NP internalization. Upon the systemic investigation of the cellular uptake behavior of dual-ligand hybrid NPs, docetaxel (DTX), a hydrophobic anticancer drug, was successfully encapsulated into dual-ligand hybrid NPs with high drug loading for Her2/neu-overexpressing SK-BR-3 breast cancer cell treatment. The DTX-loaded dual-ligand hybrid NPs showed a decreased burst release and a more gradual sustained drug release property. Because of the synergistic effect of dual-ligand modification, DTX-loaded dual-ligand hybrid NPs exerted substantially better therapeutic potency against SK-BR-3 cancer cells than other NP formulations and free DTX drugs. These results demonstrate that the dual-ligand hybrid NPs could be a promising vehicle for targeted drug delivery to treat breast cancer.

Immobilization of cryptophane derivatives onto SiO2/Au and Au substrates.

The synthesis of a cryptophane molecule bearing five methoxy substituents and an alkanethiol chain, 4, as well as its subsequent grafting onto a gold surface, is reported. Immobilization of cryptophane derivatives onto silica (SiO2/Au) surfaces was also performed by reacting a cryptophane molecule bearing one or six acid functions, 5 or 6, respectively, with an amino-terminated self-assembled monolayer (SAM). Polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) was used to characterize the two types of cryptophane monolayers. Surface coverage of cryptophane monolayers was estimated by comparing the PM-IRRAS intensity of cryptophane bands with that calculated from the optical constants of pentamethoxy-cryptophane for a compact monolayer. A very efficient grafting of 4 onto a gold surface was found, with a surface coverage close to 100%. On the other hand, the reaction of mono-acid, 5, or hexa-acid, 6, cryptophanes with amino-terminated SAM was less efficient, since the surface coverage did not exceed 15%. Finally, a good surface coverage (75%) was also obtained by using a cysteamine coupling agent to modify 5 before its grafting onto a gold surface.

Spontaneous grafting of nitrophenyl groups on carbon: effect of radical scavenger on organic layer formation.

The effect of a radical scavenger (DPPH: 2,2-diphenyl-1-picrylhydrazyl) on the spontaneous covalent grafting of nitrophenyl functionalities on a vitreous carbon substrate using the 4-nitrobenzene diazonium cation has been studied by electrochemical measurements and X-ray photoelectron spectroscopy. The addition of micromolar concentrations of DPPH to the diazonium solution efficiently limits the multilayer formation and leads to monolayer surface coverage. Control of polyaryl layer formation via the capture of the reactive nitrophenyl radical was also found to increase the proportion of nitrophenyl groups grafted to the surface via azo bridges. This work validates the recently reported strategy using a radical scavenger to prevent the formation of a polyaryl layer without interfering with direct surface grafting.

Cyclic Voltammetry Characterization of Microparticle Monolayers

We have demonstrated the applicability of cyclic voltammetry and rotating disk electrode techniques for the characterization of microparticle monolayers. We have tested experimentally the recently developed theoretical model for the determination of surface coverage of spherical particle monolayer by means of diffusion current measurements. We have verified our electrochemical results by means of optical microscopy. We have found that the model applicability range increases with the decrease of the adsorbed particle size. For the 1.16μm particles we have found a good agreement between our theoretical and experimental results at the surface coverage larger than 0.45. Our results suggest that the voltammetric characterization of thin porous films may have practical applications for, e.g., detection and determination of surface coverage of bacteria, cells or other objects in the micron size range.

Effect of surface pretreatment on self-assembly of thiol-modified DNA monolayers on gold electrode

In this article we investigate the effect of gold pretreatment procedures on self-assembly of thiol-modified double-stranded DNA (ds-DNA) and single-stranded DNA (ss-DNA) on polycrystalline gold electrodes through cyclic voltammetry(CV), electrochemical impedance spectroscopy(EIS), chronocoulometry(CC) and scanning electron microscopy(SEM). The pretreatment procedures include M+E, M+C(piranha), M+C(dilute aqua regia), M+C+E(piranha), M+C+E(dilute aqua regia) and RM+C+E(piranha), where M is mechanical polishing, C is piranha or dilute aqua regia dipping, E is electrochemical polishing and RM is roughly mechanical polishing. Results indicate that electrochemical characteristics (ions penetration, charge transfer and surface coverage) of ds-DNA or ss-DNA self-assembled monolayers (SAMs) on gold pretreated by different procedures except M+C(dilute aqua regia) are almost the same, although surface roughness and configuration of the pretreated gold electrodes differ. This conclusion differs from literature reports that pretreatment procedures have a big influence on the compactness of simple alkanethiols SAMs. We consider that the disagreement might be due to the special structural characteristics of DNA molecule, as compared with simple alkanethiols. The bigger size of DNA molecule and the existence of electrostatic or hydration forces among adjacent DNA molecules (Van der Waals force for simple alkanethiols) lead to the formation of sparse DNA-SAMs, which make much bigger selectivity for thiols adsorption sites on gold surface, therefore electrochemical characteristics of DNA-SAMs is almost independent of the pretreatment procedures. This work might provide a useful reference for constructing DNA-SAMs on gold.

Structure and Dynamics of n-Alkanol Monolayers on a Mica Surface

All-atom molecular dynamics simulations are conducted to understand the structural and dynamical behavior of self-assembled monolayer of n-alkanols on a mica surface. In particular, we report the effect of increasing carbon chain length (C1–C10) on the self-assembly, surface diffusion, and preferential tilting of n-alkanol monolayer, for monolayer surface coverage ranging from 6 × 10–5 to 3.54 × 10–3 mol/m2. The adsorption phenomena typically follow the Langmuir adsorption isotherm. However, the maximum adsorption is observed for n-hexanol, and it drops with further increase in the chain length. The surface diffusion coefficient, Ds, within monolayer, is nonmonotonic in nature. The maximum value of Ds decreases with increasing carbon chain length, with an exception of methanol owing to its preferential attachment with the cage of mica due to the presence of K+. The behavior of Ds is clearly explained using instantaneous autocorrelation function of hydrogen bonds with the surface. Further, Ds, is found to ...

Analysis of the Surface Coverage of a Self-Assembled Monolayer of Octadecyl Silane on a Si(100) Surface by Infrared External-Reflection Spectroscopy

The coverage of a self-assembled monolayer (SAM) of octadecyl silane on a silicon surface is analyzed by infrared external-reflection spectrometry via molecular orientation analysis. A commercially available octadecyl trimethoxy silane (ODS) solution is employed, which readily reacts with an oxidized surface of Si(100) prepared by a UV/ozone treatment in an ambient condition to yield a highly reproducible SAM. With the present technique, a very flat surface with no holes is obtained in the entire area of the SAM. IR external-reflection spectra of the SAM apparently indicate that the alkyl chains are highly ordered having the all-trans zigzag conformation, and the molecular orientation agrees with that of a Langmuir-Blodgett (LB) monolayer film of octadecanoic acid. Since the orientation analysis is based on an assumption that the surface is fully covered by the SAM as the LB film, the agreement of the orientation confirms that the surface coverage of the present SAM on Si(100) is on the same level as the LB film.

Conductive Gold Nanoparticle Mirrors at Liquid/Liquid Interfaces

Gold nanoparticle (Au NP) mirrors, which exhibit both high reflectance and electrical conductance, were self-assembled at a [heptane + 1,2-dichloroethane]/water liquid/liquid interface. The highest reflectance, as observed experimentally and confirmed by finite difference time domain calculations, occurred for Au NP films consisting of 60 nm diameter NPs and approximate monolayer surface coverage. Scanning electrochemical microscopy approach curves over the interfacial metallic NP films revealed a transition from an insulating to a conducting electrical material on reaching a surface coverage at least equivalent to the formation of a single monolayer. Reflectance and conductance transitions were interpreted as critical junctures corresponding to a surface coverage that exceeded the percolation threshold of the Au NP films at the [heptane + 1,2-dichloroethane]/water interface.

Limiting diffusion current at rotating disk electrode with dense particle layer

Exploiting the concept of diffusion permeability of multilayer gel membrane and porous multilayer we have derived a simple analytical equation for the limiting diffusion current at rotating disk electrode (RDE) covered by a thin layer with variable tortuosity and porosity, under the assumption of negligible convection in the porous film. The variation of limiting diffusion current with the porosity and tortuosity of the film can be described in terms of the equivalent thickness of stagnant solution layer, i.e., the average ratio of squared tortuosity to porosity. In case of monolayer of monodisperse spherical particles, the equivalent layer thickness is an algebraic function of the surface coverage. Thus, by means of cyclic voltammetry of RDE with a deposited particle monolayer we can determine the monolayer surface coverage. The effect of particle layer adsorbed on the surface of RDE increases non-linearly with surface coverage. We have tested our theoretical results experimentally by means of cyclic voltammetry measurements of limiting diffusion current at the glassy carbon RDE covered with a monolayer of 3 μm silica particles. The theoretical and experimental results are in a good agreement at the surface coverage higher than 0.7. This result suggests that convection in a monolayer of 3 μm monodisperse spherical particles is negligibly small, in the context of the coverage determination, in the range of very dense particle layers.

Sorption of tylosin on clay minerals

The equilibrium sorption of tylosin (TYL) on kaolinite and montmorillonite was measured at different solution pH using batch reactor systems. The results showed that all the sorption isotherms were nonlinear and that the nonlinearity decreased as the solution pH increased for a given clay. At a specific aqueous concentration, the single-point sorption distribution coefficient (KD) of TYL decreased rapidly as the solution pH increased. A speciation-dependent sorption model that accounted for the contributions of the cationic and neutral forms of TYL fit the data well, suggesting that the sorption may be dominated by both ion exchange and hydrophobic interactions. The isotherm data also fit well to a dual mode model that quantifies the contributions of a site-limiting Langmuir component (ion exchange) and a non-specific linear partitioning component (hydrophobic interactions). X-ray diffraction analyses revealed that the interlayers of montmorillonite were expanded due to the uptake of TYL. TYL molecules likely form a monolayer surface coverage.

Transfer-free growth of graphene on SiO2 insulator substrate from sputtered carbon and nickel films

Here we demonstrate the growth of transfer-free graphene on SiO2 insulator substrates from sputtered carbon and metal layers with rapid thermal processing in the same evacuation. It was found that graphene always grows atop the stack and in close contact with the Ni. Raman spectra typical of high quality exfoliated monolayer graphene were obtained for samples under optimised conditions with monolayer surface coverage of up to 40% and overall graphene surface coverage of over 90%. Transfer-free graphene is produced on SiO2 substrates with the removal of Ni in acid when Ni thickness is below 100nm, which effectively eliminates the need to transfer graphene from metal to insulator substrates and paves the way to mass production of graphene directly on insulator substrates. The characteristics of Raman spectrum depend on the size of Ni grains, which in turn depend on the thickness of Ni, layer deposition sequence of the stack and RTP temperature. The mechanism of the transfer-free growth process was studied by AFM in combination with Raman. A model is proposed to depict the graphene growth process. Results also suggest a monolayer self-limiting growth for graphene on individual Ni grains.

Electrografting via Diazonium Chemistry: From Multilayer to Monolayer Using Radical Scavenger

A simple strategy to avoid the formation of polyaryl layer during the functionalization of carbon surface by diazonium electroreduction is presented. The approach proposes to directly act on the polymerization mechanism by the use of a radical scavenger. The kinetic gap between the surface coupling and the multilayer formation is exploited to prevent the growth of the layer without interfering with the grafting. The well-known 4-nitrobenzenediazonium electrografting was used to demonstrate the possibility of reaching a monolayer surface coverage with an excess of DPPH (2,2-diphenyl-1-picrylhydrazyl). Experimental conditions were varied to validate the efficiency of the grafting limitation and the radical capture was confirmed by isolation of the aryl radical/DPPH coupling product.

Conversion of diols by dehydrogenation and dehydration reactions on silica-supported copper catalysts

The gas-phase conversion of a 1,3-polyol (1,3-butanediol) containing primary and secondary OH functions was studied on a series of copper-silica catalysts, ZCuSiO2 (Z=1–25wt.% Cu), and thoroughly characterized by several techniques such as BET surface area, TPR, XRD, N2O chemisorption, and UV-vis-DRS. The physicochemical properties of the ZCuSiO2 catalysts depended on whether the metal loading was above or below the copper monolayer surface coverage (Z=13.5wt.% Cu). Copper species presenting different degrees of interaction with the silica support were detected. At low Z values Cu0 dispersion was high (D≈40%) due to a predominant contribution of nano-sized Cu species (3nm) which are difficult to reduce, but for Z>13.5wt.%, D abruptly dropped to 3–8% because of formation of larger tridimensional Cu clustered species (30nm) that reduced at lower temperatures because of a decreased copper-silica interaction.On ZCuSiO2 catalysts, dehydrogenation of the 1,3-butanediol secondary OH function prevailed over that of the primary one and therefore valuable ketones were the main reaction products. Consecutively to dehydrogenation, dehydration and hydrogenation reactions also took place. Products of the tandem reaction were the β-hydroxy ketone (4-hydroxy-2-butanone), the α,β-unsaturated ketone (methyl vinyl ketone) and the saturated ketone (methyl ethyl ketone). A direct 1,3-butanediol dehydration pathway toward methyl ethyl ketone was also found. Reaction pathways were strongly dependent on the Cu loading and therefore on the kind of Cu species (isolated or clustered). When compared at similar conversion levels, selectivity to the dehydrogenation product 4-hydroxy-2-butanone increased with Z suggesting that on large Cu0 particles 4-hydroxy-2-butanone was released to the gas phase before being converted in consecutive steps. On the contrary, on highly dispersed Cu0 crystals of low Cu loading catalysts, 1,3-butanediol was readily dehydrated giving the saturated ketone.Kinetically relevant reaction steps of 1,3-butanediol conversion by dehydrogenation and dehydration were promoted on Cu0 sites. Dehydration of the intermediate 4-hydroxy-2-butanone also occurred on Cu0 sites. Turnover rates were constant on Cu0 nano particles and slightly higher on clustered species.

Targeted delivery of 10-hydroxycamptothecin to human breast cancers by cyclic RGD-modified lipid–polymer hybrid nanoparticles

Lipid–polymer hybrid nanoparticles (NPs) combining the positive attributes of both liposomes and polymeric NPs are increasingly being considered as promising candidates to carry therapeutic agents safely and efficiently into targeted sites. Herein, a modified emulsification technique was developed and optimized for the targeting lipid–polymer hybrid NPs fabrication; the surface properties and stability of the hybrid NPs were systematically investigated, which confirmed that the hybrid NPs consisted of a poly (lactide-co-glycolide) core with ∼90% surface coverage of the lipid monolayer and a ∼4.4 nm hydrated polyethylene glycol (PEG) shell. Optimization results showed that the lipid:polymer mass ratio and the lipid-PEG:lipid molar ratio could affect the size, lipid association efficiency and stability of hybrid NPs. Furthermore, a model chemotherapy drug, 10-hydroxycamptothecin, was encapsulated into hybrid NPs with a higher drug loading compared to PLGA NPs. Surface modification of the lipid layer and the PEG conjugated targeting ligand did not affect their drug release kinetics. Finally, the cytotoxicity and cellular uptake studies indicated that the lipid coverage and the c(RGDyk) conjugation of the hybrid NPs gained a significantly enhanced ability of cell killing and endocytosis. Our results suggested that lipid–polymer hybrid NPs prepared by the modified emulsion technique have great potential to be utilized as an engineered drug delivery system with precise control ability of surface targeting modification.