Platinum–Dendrimer Nanocomposite Films on Gold Surfaces for Electrocatalysis

In this communication, we report a strategy for the preparation of Pt nanoparticles encapsulated in Generation 4.5 (Polyamido amine) PAMAM dendrimer and subsequent chemical linking of the nanocomposite to the gold electrode through a self assembled cystamine monolayer. The modification resulted in the formation of a robust electrochemically active thin film with very high surface area, reflected by the enhanced hydrogen adsorption coverage. Interestingly, TEM images revealed self-assembly of Pt nanoparticles and the SAED (Selected Area Electron Diffraction) patterns showed the presence of Pt single crystals (111). The Pt-dendrimer nanocomposite film obtained using the novel modification procedure exhibited high electrocatalytic activity for the oxidation of organic fuels like methanol, ethanol and ethylene glycol. The film did not suffer from degradation even after repeated use in solution-phase voltammetry. It is however observed that the intermediate SAM layer and the bulky PAMAM dendrimer (generation 4.5) have slowed down the electron transfer kinetics which is reflected by a relatively high overpotential for methanol oxidation. Nevertheless this shortcoming is more than compensated by the existence of Pt(111) planes, which alleviate CO poisoning.     

Preparation and Characterization of Covalently Binding of Rat Anti-human IgG Monolayer on Thiol-Modified Gold Surface

The 16-mercaptohexadecanoic acid (MHA) film and rat anti-human IgG protein monolayer were fabricated on gold substrates using self-assembled monolayer (SAM) method. The surface properties of the bare gold substrate, the MHA film and the protein monolayer were characterized by contact angle measurements, atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXRD) method and X-ray photoelectron spectroscopy, respectively. The contact angles of the MHA film and the protein monolayer were 18° and 12°, respectively, all being hydrophilic. AFM images show dissimilar topographic nanostructures between different surfaces, and the thickness of the MHA film and the protein monolayer was estimated to be 1.51 and 5.53 nm, respectively. The GIXRD 2θ degrees of the MHA film and the protein monolayer ranged from 0° to 15°, significantly smaller than that of the bare gold surface, but the MHA film and the protein monolayer displayed very different profiles and distributions of their diffraction peaks. Moreover, the spectra of binding energy measured from these different surfaces could be well fitted with either Au4f, S2p or N1s, respectively. Taken together, these results indicate that MHA film and protein monolayer were successfully formed with homogeneous surfaces, and thus demonstrate that the SAM method is a reliable technique for fabricating protein monolayer.     

Investigation of the electrochemical behavior of multilayers film assembled porphyrin/gold nanoparticles on gold electrode

Multilayers film of nanostructured gold nanoparticles (AuNPs) has been fabricated based on the layer-by-layer (LBL) technique using a self-assembled monolayer of 5,15-di-[p-(6-mercaptohexyl)-phenyl]-10,20-diphenylporphyrin (trans-PPS₂). AuNPs act as physical cross-link points in the multilayers. Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are applied to study the formation of the organic-inorganic multilayers film and have determined the electrochemical parameters, i.e., the heterogeneous electron transfer rate constant (K_et). The observed phenomena indicate that the electron transfer (ET) process is affected by material properties and the molecular structure of self-assembled monolayers (SAMs). Using the high sensitivity of ET of ferricyanide to the modification of the gold surface with multilayers film, we select this reaction as a probe to study the different modification stages at this modified electrode. ET is retarded on the trans-PPS₂ alternative deposition of layers on the electrode surface and is accelerated on the AuNPs’ layers. SECM images are used to collect surface information in the course of the successive modification process. SECM images obtained from bare and different modification stages show very high resolution with different topographies.     

Improved blocking properties of short-chain alkanethiol monolayers self-assembled on gold

Blocking of gold surfaces by self-assembled monolayers (SAMs) of n-octanethiol (OT) was investigated. Short-chain alkanethiols such as OT are known to form loosely-packed SAMs with abundant defects and pinholes, exhibiting poor blocking of the gold surface. Hence, gold substrates subjected to 20 h OT adsorption manifested excessive electrochemical penetrability, low contact angles with a large hysteresis, and extensive exchange of functional disulfide molecules into the OT SAM. It is shown that a single electrochemical cycle in H₂SO₄ solution after 1 h OT adsorption, followed by another 20 h adsorption, results in a dramatic improvement of the coverage and blocking properties of the SAM, showing considerably lower electrochemical penetrability, improved contact angles, and sluggish exchange of the functional molecules into the OT SAM. Insertion of the latter molecules into the monolayer was monitored by their ability to coordinatively attach ligand-substituted gold nanoparticles.     

The nature of high surface energy sites in graphene and graphite

Variations in the adsorption enthalpies of acetone to few-layer graphene and graphite nanopowders were analyzed as a function of surface coverage. The adsorption enthalpies were measured by inverse gas chromatography at low monolayer coverage levels (0.1–20%). The adsorption enthalpies increased from −13 kcal/mol at the lowest coverage to −7.5 kcal/mol. We fitted the measured adsorption enthalpies as a function of coverage using a two-state model and estimated the number of high-energy sites on both materials. The graphite powder had seven times more high-energy sites than the few-layer graphene, which explains why the adsorption enthalpies for graphite increased more slowly with increasing coverage. We also performed a theoretical study based on density functional theory calculations using a functional that accounts for dispersive interactions to elucidate the nature of the high-energy adsorption sites. The calculated adsorption enthalpies ranged from −16 to −1 kcal/mol while the adsorption enthalpy to a plain graphite surface was −9 kcal/mol. The high-energy adsorption sites were localized on surface steps and edge-cavities. The adsorption enthalpies at very low coverage therefore corresponded to adsorption on steps and edge cavities, while those measured at coverage levels of ∼4% or more reflected adsorption to the flat surface.     

Scanning electrochemical microscopy studies of micropatterned copper sulfide (Cu(x)S) thin films fabricated by a wet chemistry method

Patterned copper sulfide (Cu(x)S) microstructures on Si (1?1?1) wafers were successfully fabricated by a relatively simple solution growth method using copper sulfate, ethylenediaminetetraacetate and sodium thiosulfate aqueous solutions as precursors. The Cu(x)S particles were selectively deposited on a patterned self-assembled monolayer of 3-aminopropyltriethoxysilane regions created by photolithography. To obtain high quality Cu(x)S films, preparative conditions such as concentration, proportion, pH and temperature of the precursor solutions were optimized. Various techniques such as optical microscopy, atomic force microscopy (AFM), X-ray diffraction, optical absorption and scanning electrochemical microscopy (SECM) were employed to examine the topography and properties of the micro-patterned Cu(x)S films. Optical microscopy and AFM results indicated that the Cu(x)S micro-pattern possessed high selectivity and clear edge resolution. From combined X-ray diffraction analysis and optical band gap calculations we conclude that Cu(9)S(5) (digenite) was the main phase within the resultant Cu(x)S film. Both SECM image and cyclic voltammograms confirmed that the Cu(x)S film had good electrical conductivity. Moreover, from SECM approach curve analysis, the apparent electron-transfer rate constant (k) in the micro-pattern of Cu(x)S dominated surface was estimated as 0.04?cm/s. The SECM current map showed high edge acuity of the micro-patterned Cu(x)S.     

Self-assembled monolayers of Vitamin B12 disulphide derivatives on gold

Using the self-assembly technique, novel monolayers on gold have been prepared from new cobyrinate dialkyl disulphide derivatives. The successful formation is proved by cyclic voltammetry and by in situ ellipsometry. The electrochemical characterisation by reductive desorption allows to estimate the surface coverage and reveals that the presence of two cobyrinates introduces some disorganisation in the monolayer. More packed and organised monolayers have been observed in systems containing only one terminal redox centre. From ellipsometric measurements a possible orientation of the cobyrinate centre in the adsorbed monolayer is modelled. The modified electrodes display electrocatalytic activity for the reduction of dissolved oxygen.     

Thin,highly crosslinked polymer layer synthesized via photoinitiated graft copolymerization on a self‐assembled‐monolayer‐coated gold surface

A thin, highly crosslinked layer was grafted onto an alkyl thiol self‐assembled‐monolayer (SAM)‐coated gold surface with N,N′‐methylene bisacrylamide (MBAA), a widely used crosslinker with two polymerizable groups, as the monomer. Surface‐initiated photografting copolymerization was achieved through the immobilization of the hydrogen‐abstraction photoinitiator benzophenone on the hydrophobic alkyl surface via physical adsorption and subsequent UV irradiation in the presence of an MBAA solution. The growth of the grafted poly‐MBAA layers seemed to produce dendritic structures with low surface coverage. At a higher monomer concentration (15 g/L of water), full coverage of the gold surface with a thin layer was obtained and proved by scanning force microscopy and contact‐angle measurements. The evaluation of the gold, gold–SAM, and gold–SAM/grafted poly‐MBAA layers with a surface plasmon resonance sensor system revealed that the photografted, thin, highly crosslinked polyacrylamide layers had a very low affinity toward the adsorption of protein. Therefore, this provides a very promising approach to tailoring materials for sensors and other applications. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 158–164, 2005     

Capacitance and surface conductance study of the adsorption of pentan-1-ol on gold

The adsorption of pentan-1-ol on thin gold film electrodes was studied by capacitance and surface conductance, The pentan-1-old adsorption conforms to the Frumkin isotherm. The free energy of adsorption at the potential of maximum adsorption shows that the adsorption of pental-1-ol is weaker on gold than on mercury and very similar to that of the diethyl ether on (110) gold single crystals. The surface resistivity in the presence of the alcohol is lower than that of the base electrolyte, showing a minimum near the minimum of capacitance. The changes of resistance are tentatively explained in terms of the field effect and the scattering of surface electrons produced by the solvent molecules.     

Patterns of functional proteins formed by local electrochemical desorption of self-assembled monolayers

Patterned self-assembled monolayers (SAMs) were formed using the scanning electrochemical microscope (SECM). The procedures is based on the local electrochemical desorption of an alkanethiolate monolayer by applying a 5 kHz square-wave voltage of 2 V (peak-to-peak) to a two-electrode configuration consisting of an ultramicroelectrode (UME) of 10 μm diameter placed about 5 μm above a macroscopic SAM-covered gold electrode. Desorption occurs on well-defined regions with a diameter of (12.8±2.8) μm. These regions of bare gold are able to chemisorb a ω-functionalized thiol or disulfide such as cystamine to form patterns of amino-terminated surfaces. Functional proteins can be coupled to the amino groups present at the modified regions of the monolayer. This approach was demonstrated by imaging the activity of horseradish peroxidase bound to the patterned SAMs in the generation–collection mode of the SECM. A considerable improvement of the procedure could be achieved by performing the desorption in a solution containing a millimolar concentration of the ω-functionalized thiol/disulfide ensuring effective refilling of the monolayer by the desired molecules and hence high concentration of the immobilized proteins. The method is discussed with respect to prospective application in the field of chip-based bioanalytical assays.     

Statistical modeling of adsorption isotherm of potassium on aza[7]helicene-coated gold electrode attached to quartz crystal microbalance

ABSTRACT

A chemically coated piezoelectric sensor has been used for the determination in liquid phase of the adsorption isotherm of potassium. A thin film of aza[7]helicene was attached to the surface of a gold electrode of a quartz crystal microbalance using spin coating method.

Statistical physics formalism was applied to elucidate the adsorption mechanism of potassium onto coated electrode by aza[7]helicene. The simulation results showed that a monolayer adsorption occurs with the number of potassium per site about 1 and the adsorption energy is less than 30 kJ.mol^?1, characteristic of physical adsorption.

The statistical model was used to investigate three thermodynamic potentials such as the entropy, the free enthalpy, and the internal energy. It is found that the adsorption took place with releasing energy, such behavior confirms the exothermic character of the adsorption process.     

Plant derived cationic dye as an effective corrosion inhibitor for 7075 aluminum alloy in 3.5% NaCl solution

The inhibition of the corrosion of 7075 aluminum alloy in 3.5 wt.% NaCl solution by 5,6-dihydro-9,10-dimethoxybenzo[g]-1,3-benzodioxolo[5,6-a]quinolizinium (berberine) has been studied using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, scanning electrochemical microscopy (SECM) and scanning electron microscopy (SEM). Inhibition efficiency was found to increase with increasing concentration of berberine. The adsorption of the berberine on the 7075 aluminum alloy surface obeyed the Langmuir adsorption isotherm. The adsorbed film on 7075 aluminum alloy surface containing inhibitor was confirmed by the SEM, and SECM. The results obtained showed that the berberine could serve as an effective inhibitor of the corrosion of 7075 aluminum alloy in 3.5% NaCl.     

Using scanning tunneling microscopy to characterize adsorbates and reactive intermediates on transition metal oxide surfaces

Scanning tunneling microscopy (STM) is demonstrated to be a powerful tool to characterize adsorption and reaction on oxide surfaces by imaging molecular adsorbates and reactive intermediates. The molecules were used to probe surface structure and to study surface reactivity spatially at the atomic level. Results for three systems are presented: alcohol adsorption on WO₃(0 0 1), carboxylates on the anatase polymorph of TiO₂, and propene adsorption on a PdO monolayer on Pd(1 0 0). When the alcohols were exposed to the WO₃(0 0 1)-c(2×2) surface at room temperature the molecules could not be imaged. Heating the surface to temperatures above a water desorption peak associated with alcohol deprotonation, however, allowed 1-propoxide to be imaged. The images reveal that the alkoxide has no preference for defects, rather it binds to W⁶⁺ ions exposed on the fully oxidized c(2×2) surface. Temperature-programmed desorption revealed that alkoxides at these sites undergo only dehydration reactions. To probe the structure of the unusual (1×4) reconstruction on anatase (0 0 1), formic and acetic acid adsorption were used. Following dissociative adsorption, both formate and acetate adsorb solely centered atop the bright rows that define the surface reconstruction, and the molecules are always at least two lattice constants apart. This result may be attributed to carboxylates bridge-bonded to Ti atoms at the center of the bright rows. This finding eliminates several suggested models of the reconstruction and suggests that a recently proposed ad-molecule model is a good representation of the surface structure. Propene was observed to initially randomly adsorb on the PdO monolayer. At higher coverages, however, the adsorbates cluster, disrupting the surface structure and causing the adsorption rate adjacent to the clusters to increase. Temperature-programmed reaction revealed that once propene adsorbs, the oxide monolayer catalyzes its oxidation at lower temperatures than metallic Pd, but that the propene sticking coefficient on the ordered oxide layer is a factor of 5 lower.     

Amine‐functionalization of the nanotitanate ETS‐2

A series of nonporous, amine‐functionalized sodium titanates was prepared and the thermal and adsorptive behavior of the samples were characterized. Engelhard titanosilicate 2 was chosen as a substrate for its high surface area (～300 m²/g), native surface hydroxyl concentration, and lack of microporosity; eliminating the risk of fouling the adsorbent under certain process conditions. Aminosilanes containing a single (N1), two (N2), and three (N3) amine groups were chemically grafted to the surface of the substrate and the adsorption capacity for CO₂ measured through thermogravimetry‐mass spectroscopy (TG‐MS) desorption, volumetric adsorption, and gravimetric adsorption/desorption cycling. The N3 sample displayed complete monolayer coverage and was capable of adsorbing five times as much atmospheric CO₂ as the N1 sample. Testing under anhydrous conditions only engages the primary amine on the tether and the data consistently suggests a correlation between amine utilization and the proportion of monolayer coverage for these adsorbents. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4727–4734, 2013     

Adsorption properties of tetradecyl- and hexadecyl trimethylammonium bentonites

This study concerns the adsorption properties of organo-bentonites obtained through the modification performed at 25%, 50% and 100% of the cation exchange capacity (CEC) using tetradecyl trimethylammonium bromide (TDTAB) and hexadecyl trimethylammonium bromide (HDTAB). According to the XRD measurements, TDTAB and HDTAB form bilayers between the silicate layers when the montmorillonite surface is modified by 100% CEC. At 25% and 50% CEC, a random interstratification of monolayer and bilayer structures is observed. The surface area of bentonite decreases upon modification. The adsorption properties of organo-bentonites were tested using phenol as a model pollutant. Batch kinetic and isotherm studies were conducted over the concentration range from 50 to 1000 ppm at pH of 5.5. The adsorption excess isotherms of bentonites show different regions. A considerable increase was observed in the adsorption efficiency of montmorillonite modified with HDTAB at 100% CEC.     

Electron transport through alkanethiolate films decorated with monolayer protected gold clusters

Monolayers of 1,9-nonanedithiol or 1-decanethiol self-assembled directly on the gold substrate transfer charge to the K₄[Fe(CN)₆] molecule in solution more efficiently when they are decorated with 1-butanethiol protected gold nanoclusters. The clusters bound to the electrode by means of dithiol are more uniformly distributed on the monolayer and their efficiency in transferring electrons between the electrode and the redox couple is higher than when they are simply adsorbed through weak van der Waals interactions with the methyl groups of 1-decanethiol. With increasing adsorption time in the solution of clusters, the capacitance of the cluster decorated electrode significantly increases compared to the constant value of capacitance observed after prolonged immersion in pure toluene. This difference is explained assuming that gold clusters act as an array of capacitors on the monolayer modified electrode surface.     

Recovery of Pt (IV) from aqueous solutions using magnetic functionalized cellulose with quaternary amine

A new magnetic cellulose hybrids anchored with quaternary amine moieties were fabricated and used in recovering Pt(IV) ions from acidic solutions. Kinetic and thermodynamic parameters of adsorption process were reported through using batch experiments. An equilibrium uptake capacity of 178 mg g^?1 was achieved within 55 min. The adsorption process was found to follow the pseudo-second-order kinetics and fitted well Langmuir and D-R adsorption isotherms revealing monolayer surface coverage and physisorption mechanism, respectively. At low pH values and high chloride concentration ranges, ion exchange was proposed as the predominant mechanism for the adsorption of Pt(IV) ions on the sorbent. The obtained sorbent showed good durability, easy separation, and regeneration from the adsorption medium.     

Preparation and Characterization of Some NR and SBR Formulations Containing Different Modified Kaolinite

Kaolinite clay (KC) surface was modified with different surface modifiers such as methacrylic acid (MAA), polymethacrylic acid (PMAA1, mol wt 10,000) and polymethacrylic acid (PMAA2, mol wt 11,500). The adsorption isotherms of the above modifiers on the surface of KC were determined. The concentrations required for building up monolayer coverage of these modifiers on the surface of KC were determined from the adsorption isotherms. The optimum amount of modifier required for monolayer surface coverage on kaolinite was equal to 31 × 10^?5 mol/g for MAA, 35 × 10^?5 mol/gfor PMAA and 23.5 × 10^?5 mol/g for PMA. Different rubber mixes containing unmodified and modified KC were prepared. The rheometric characteristics of the rubber mixes and physico-mechanical properties of the rubber vulcanizates were measured. There was remarkable decrease in both the optimum cure time (tc₉₀) and scorch time (ts₂), following increase in the maximum torque of the SBR mixes by adding unmodified or modified KC. With the maximum torque increase, the values of both of the optimum cure time (tc₉₀) and scorch time (ts₂) decreases for the NR mixes by adding unmodified and modified KC. Also the mechanical properties of the investigated rubber vulcanizates as tensile strength and hardness were improved using unmodified KC and modified KC.     

Scanning electrochemical microscopy studies of micropatterned copper sulfide (CuxS) thin films fabricated by a wet chemistry method

Patterned copper sulfide (Cu_xS) microstructures on Si (1 1 1) wafers were successfully fabricated by a relatively simple solution growth method using copper sulfate, ethylenediaminetetraacetate and sodium thiosulfate aqueous solutions as precursors. The Cu_xS particles were selectively deposited on a patterned self-assembled monolayer of 3-aminopropyltriethoxysilane regions created by photolithography. To obtain high quality Cu_xS films, preparative conditions such as concentration, proportion, pH and temperature of the precursor solutions were optimized. Various techniques such as optical microscopy, atomic force microscopy (AFM), X-ray diffraction, optical absorption and scanning electrochemical microscopy (SECM) were employed to examine the topography and properties of the micro-patterned Cu_xS films. Optical microscopy and AFM results indicated that the Cu_xS micro-pattern possessed high selectivity and clear edge resolution. From combined X-ray diffraction analysis and optical band gap calculations we conclude that Cu₉S₅ (digenite) was the main phase within the resultant Cu_xS film. Both SECM image and cyclic voltammograms confirmed that the Cu_xS film had good electrical conductivity. Moreover, from SECM approach curve analysis, the apparent electron-transfer rate constant (k) in the micro-pattern of Cu_xS dominated surface was estimated as 0.04 cm/s. The SECM current map showed high edge acuity of the micro-patterned Cu_xS.