Self-assembled monolayers of a disulphide-derivatised cobalt-porphyrin on gold

A self-assembled monolayer (SAM) of a novel cobalt(II)porphyrin disulphide derivative was prepared on flat gold(1 1 1) electrode. Evidence for surface modification was provided by electrochemical reductive desorption of the monolayer and ellipsometry, consistent with a coverage of 2.5 × 10⁻¹⁰ mol cm⁻² and a thickness of 13 Å, respectively. Both results support the presence of SAMs where the molecules share an intermediate position between perpendicular and flat orientation. Scanning tunnelling microscopy have also proven the formation of CoPSS SAMs, however high-resolution images could only be obtained when the CoPSS molecules were diluted in an hexanethiol SAM. The electrocatalytic activity of the surface confined Co-porphyrin was evaluated for the oxygen reduction. Voltammetric data indicate that reaction involves two electrons consistent with the formation of hydrogen peroxide. Under similar experimental conditions the data obtained for an iron-porphyrin analogue points for a full reduction of dioxygen to water.     

Potential-assisted deposition of mixed alkanethiol self-assembled monolayers

Preparation of self-assembled monolayers (SAMs) usually involves passive incubation. The recently developed potential-assisted deposition is indeed more selective as well as 100-fold faster than passive adsorption, thereby enhancing the reproducibility of the monolayer deposition.This article aims to identify the electrodeposition conditions necessary to prepare mixed alkanethiol SAMs on gold surface. Parameters such as concentrations in solution, electrode polarization and deposition time were examined for two chain lengths, C₃ (mercaptopropionic acid, MPA) and C₁₈ (octadecanethiol, ODT). The kinetics and composition of the SAMs were systematically characterized by reductive stripping analysis and by surface plasmon enhanced ellipsometry. Control of the surface concentration of the stable binary SAM was achieved by carefully adjusting the MPA fraction at between 70% and 95% in the deposition solution. Three theoretical models were developed. For MPA or ODT SAMs, whatever the adsorbate and the time period, the second order model gives the best fit. For the binary monolayer (MPA-ODT), a co-adsorption model was developed to describe the kinetics before 10 s.     

Insights into the surface and redox properties of single-walled carbon nanotube—cobalt(II) tetra-aminophthalocyanine self-assembled on gold electrode

This paper describes for the first time the electrochemical properties of redox-active self-assembled films of single-walled carbon nanotubes (SWCNTs) coordinated to cobalt(II)tetra-aminophthalocyanine (CoTAPc) by sequential self-assembly onto a preformed aminoethanethiol (AET) self-assembled monolayer (SAM) on a gold electrode. Both redox-active SAMs (Au-AET-SWCNT and Au-AET-SWCNT-CoTAPc) exhibited reversible electrochemistry in aqueous (phosphate buffer) solution. X-ray photoelectron spectroscopy (XPS) confirmed the appearance on the gold surface of the various elements found on the SAMs. Atomic force microscopy (AFM) images prove, corroborating the estimated electrochemical surface concentrations, that these SAMs lie normal to the gold surface. Electrochemical impedance spectroscopy (EIS) analyses in the presence of [Fe(CN)₆]^3−/4− as a redox probe revealed that the Au-AET-SWCNT-CoTAPc showed much lower (∼10 times) electron-transfer resistance (R_et) and much higher (∼10 times) apparent electron-transfer rate constant (k_app) compared to the Au-AET-SWCNT SAM. Interestingly, a preliminary electrocatalytic investigation showed that both SAMs exhibit comparable electrocatalytic responses towards the detection of dopamine in pH 7.4 phosphate buffer solutions (PBS). The electrochemical studies (cyclic voltammetry (CV) and EIS) prove that SWCNT greatly improves the electronic communication between CoTAPc and the Au electrode surface.     

Photolithography based on organosilane self-assembled monolayer resist

Micropatterning of organosilane self-assembled monolayers (SAMs) was demonstrated on the basis of photolithography using an excimer lamp radiating vacuum ultraviolet light of 172 nm wavelength. This lithography is generally applicable to micropatterning of organic thin films including alkylsilane SAMs, since its patterning mechanism involves cleavage of CC bonds in organic molecules and subsequent decomposition of the molecules. In this study, SAMs were prepared on Si substrates covered with native oxide by chemical vapor deposition in which an alkylsilane, that is, octadecyltrimethoxysilane [CH₃(CH₂)₁₇Si(OCH₃)₃, ODS] was used as a precursor. SAM was photoirradiated through a photomask placed on its surface so that it was decomposed and removed in the photoirradiated area while the masked areas remained undecomposed. Accordingly, the photomask image was printed on the SAM as a removed/unremoved pattern. The patterned SAM could be used as a mask for plasma etching or area-selective Ni electroless plating.     

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.     

Functional gold nanorod particles on conducting polymer poly(3-octylthiophene) as non-enzymatic glucose sensor

The immobilization of surface-functionalized self-assembled monolayer (SAM) gold nanoparticles onto poly(3-octylthiophene) (POT) was achieved by the cooperation of hydrophobic forces. SAMs were prepared by 11-mercaptoundecanoicacid (MUA), 4-mercaptophenyl boronic acid (MPB), and 1-decanethiol (DT) hydrophobic substrates. Nanoparticles-SAM-POT system was characterized by cyclic voltammetry, SEM, EDAX and contact angle measurements. SAMs (MUA) is closely packed providing effective blocking of the underlying platinum electrode and preventing a ferrocyanide molecule from penetrating. However, potential scanning was applied at SAMs (MUA) modified electrode on which electron penetrating holes or defects were occured. Since SAMs (MPB) is poorly packed according to SAMs (MUA), ferrocyanide molecules could penetrate to SAMs (MPB) modified electrode surface. POT-Au-SAM (MPB) electrode was used for glucose determination as potentiometric non-enzymatic glucose sensor. The analytical performance was evaluated and linear calibration graphs were obtained in the concentration range of 5-30 mM glucose including the level of human blood glucose.     

Rim instability in polymethylmethacrylate films on self-assembled monolayers with the hydrophilicity

The solvent-induced dewetting of polymethylmethacrylate (PMMA) films prepared on self-assembled monolayers (SAMs) supported by Si substrates was investigated with the hydrophilicity of the SAMs. Dewetting of the PMMA film was characterized as a function of the surface energy of the SAMs. At lower surface energies, the rim instability was found to be of type B. Type B instability created undulations in the rims of holes. As the holes merged, fingers and then droplets formed. At higher surface energies, the rim instability of the PMMA films was of type A. Type A instability did not form fingers. The formation of PMMA droplets was dominated by the total spreading coefficient (S) of the PMMA film on the SAMs, which can be expressed as the sum of the dispersion (S^d) and the polar components (S^p) of the spreading coefficient. S^d of the PMMA films decreased, whereas S^p increased as ultraviolet (UV) irradiation for periods of time between 0 s and 100 s, respectively. This result suggests that S^d destabilized but S^p stabilized the PMMA films.     

Formation kinetics and stability of phosphonate self-assembled monolayers on indium–tin oxide

Phosphonate self-assembled monolayers (SAMs) have been widely used for the surface modification of indium–tin oxide (ITO) electrodes; however, their formation kinetics and stability are not well understood. In this paper, we describe our electrochemical studies of the formation kinetics and stability of a series of phosphonate SAMs on ITO electrodes. In particular electrochemical impedance spectroscopic (EIS) and cyclic voltammetric (CV) measurements have been carried out on three carboxy-terminated phosphonate SAMs by using Fe(CN)₆^3−/4− as redox indicators. The dependence of the charge-transfer resistance (obtained from the EIS plots) on the incubation time allows an estimation of the apparent fractional surface coverage of phosphonate SAMs. The apparent formation rate constant (k_obs) was determined by fitting the experimental data to a Langmuir adsorption model. For 3-phosphonopropanoic acid (PPA), the k_obs value increases when the PPA concentration increases in the deposition solution, and is smaller than those of thiolate SAMs on Au. The stability of phosphonate SAMs was investigated in three different media (pure water, phosphate-buffered saline (PBS) solution, and ambient air condition). It has been shown that the phosphonate SAMs are rather stable in either PBS solution or ambient air condition.     

Electrochemical studies of self-assembled monolayers using impedance spectroscopy

Self-assembled monolayers (SAMs) using various organic molecules were constructed on thin thermal silicon dioxide on silicon structures. These structures have potential bio-applications. The monolayers are part of a capacitor sensor based on the electrolyte-insulator-silicon (EIS) structure. The main goal of this work is to investigate the ability of SAMs for acting as an intermediate layer between aqueous solutions and silicon based biosensors. This work presents monolayers based on trimethoxy silanes with various functional groups such as: (a) (3-aminopropyl)-trimethoxysilane, (b) octadecyltrimethoxysilane and (c) trimethoxy(3,3,3-trifluoropropyl)silane. The biosensor is a differential sensor with a built in reference based on the structure of electrolyte-insulator-silicon or in ion-sensitive-field-effect-transistors (ISFETs). While the methyl and fluorine groups can act as the reference part, which is passive to the bio-molecules detection, the capacitor containing the amine groups has the sensing capabilities to the biological molecules. The amine (NH₂) tail groups can be active for those attachments via glutaraldehyde.The film's study and characterization was made by spectroscopic ellipsometry, contact angle method (CA) and FT-IR spectroscopy. The electrical methods such as capacitance-voltage (C-V) and electrochemical impedance spectroscopy (IS) were used to investigate the whole electrolyte/SAM/SiO₂/Si structure. We will show the detection capabilities of these 2D structures, its electrical equivalent model through a simulation fitting and will discuss the application of those structures to bio-sensitive metal/oxide/silicon (MOS) devices.     

Immobilization of tetra-amine substituted metallophthalocyanines at gold surfaces modified with mercaptopropionic acid or DTSP-SAMs

This paper shows that amine substituted cobalt phthalocyanine (CoTAPc) can be deposited on gold surfaces by using an interconnecting layer of a self-assembled monolayer (SAM) of mercaptopropionic acid or Lomant's reagent (dithiobis(N-succinimidyl propionate) (DTSP)). In both cases the new bond formed is obtained by the creation of an amide. The layers were characterized by electrochemistry and showed high coverage fractions (near 100%). Reductive and oxidative desorption of the SAMs limit the useful potential window from −0.6 to +0.5 V versus Ag|AgCl. The SAM-CoTAPc layers show electrocatalytic activities towards oxygen reduction through the Co(I) central metal ion. The amount of CoTAPc molecules deposited (obtained from the Co central metal ion activity in nitrogen purged solutions) revealed that the CoTAPc molecules are bonded in a perpendicular manner at the surface. Taking into account a surface of 200 Å² for a flatly bonded MPc, this should result in a four times less amount of deposited CoTAPc compared to the experimental value obtained. Both methods showed good results and promising long-term stability and will be interesting tools for further research in surface modification and sensor development.     

Slip behavior in polymethylmethacrylate films in dependence of self-assembled monolayer wettability

The slip behavior of polymethylmethacrylate (PMMA) films prepared on self-assembled monolayers (SAM) supported by Si substrate was investigated according to the wettability of the SAM. To this end, the dewetting of thermally annealed PMMA films was measured varying the surface energies of underlyed SAM. Total surface energy (γ^t) of SAM increased with increasing UV exposure time. At surface energy of <31.3 mJ/m², the PMMA films showed slip behavior; radius of holes (R) ∝ t^0.68−0.72, where t is dewetting time. The slip behavior seems to be dominated by the viscous friction between the PMMA films and the SAM. As t increased, slip behavior disappeared, R ∝ t^0.34−0.39. At surface energy of >42.0 mJ/m², the PMMA films did not show slip behavior, R ∝ t^0.45. It means that interface effects between PMMA films and SAM layers increase with increasing of γ of SAM layers, and then the molecular interaction in the PMMA films increases. The spreading coefficient (S) of the PMMA films approached positive number for UV exposure times between 0 s and 300 s. We concluded that the stability of the PMMA films can be simply controlled by UV-ozone treatment on the SAM layers.     

Adsorption thermodynamics and kinetics study for the self-assembly of 1,8,15,22-tetraaminophthalocyanatocobalt(II) on glassy carbon surface

Spontaneous adsorption of 1,8,15,22-tetraaminophthalocyanatocobalt(II) (4α-Co^IITAPc) on glassy carbon (GC) electrode leads to the formation of a stable self-assembled monolayer (SAM). Since the SAM of 4α-Co^IITAPc is redox active, its adsorption on GC electrode was followed by cyclic voltammetry. SAM of 4α-Co^IITAPc on GC electrode shows two pairs of well-defined redox peaks corresponding to Co^III/Co^II and Co^IIIPc⁻¹/Co^IIIPc⁻². The surface coverage (Γ) value, calculated by integrating the charge under Co^II oxidation, was used to study the adsorption thermodynamics and kinetics of 4α-Co^IITAPc on GC surface. Cyclic voltammetric studies show that the adsorption of 4α-Co^IITAPc on GC electrode has reached the saturation coverage (Γ_s) within 3 h. The Γ_s value for the SAM of 4α-Co^IITAPc on GC electrode was found to be 2.37 × 10⁻¹⁰ mol cm⁻². Gibbs free energy (ΔG_ads) and adsorption rate constant (k_ad) for the adsorption of 4α-Co^IITAPc on GC surface were found to be −16.76 kJ mol⁻¹ and 7.1 M⁻¹ s⁻¹, respectively. The possible mechanism for the self-assembly of 4α-Co^IITAPc on GC surface is through the addition of nucleophilic amines to the olefinic bond on the GC surface in addition to a meager contribution from π stacking. The contribution of π stacking was confirmed from the adsorption of unsubstituted phthalocyanatocobalt(II) (CoPc) on GC electrode. Raman spectra for the SAM of 4α-Co^IITAPc on carbon surface shows strong stretching and breathing bands of Pc macrocycle, pyrrole ring and isoindole ring. Raman and CV studies suggest that 4α-Co^IITAPc is adopting nearly a flat orientation or little bit tilted orientation.     

Effect of chromate additive on the kinetics and mechanism of skeletal copper formation

The addition of chromate to the leach liquor for the preparation of skeletal copper increases and stabilizes the copper surface area and slows the leaching rate. The kinetics have been studied using a rotating disc electrode (RDE) at 269–293 K in 2–8 M NaOH and 0–0.1 M Na₂CrO₄. The rate of leaching was found to be constant with time, with an activation energy of 74 ± 7 kJ mol^–1. By monitoring the kinetics and free (mixed) corrosion potential, it was possible to elucidate the mechanistic effect of chromate causing the increased surface area. Chromate was found to deposit on the copper surface as chromium(III) oxide, hindering the leaching reaction as well as the dissolution/redeposition of copper, the main mechanism of structural formation/rearrangement for skeletal copper. This blocking of the surface resulted in a finer structure, with a corresponding larger surface area. It also stabilised the surface area by minimizing the rearrangement. The effect of chromate was found to reach a limit at around 0.03 M Na₂CrO₄.     

Clickable poly(ionic liquid)s for modification of glass and silicon surfaces

Covalent attachment of poly(ionic liquid)s (PILs) by click chemistry on glass or silicon (Si) surfaces was performed. Poly[1-(4-vinylbenzyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide] (polyVBBI⁺Tf₂N⁻), and copolymers of polyVBBI⁺Tf₂N⁻ with fluorescein O-methacrylate were synthesized by conducting an atom transfer radical polymerization (ATRP) from initiators containing azide or thioacetate groups. The azide- and thiol-terminated PILs were then successfully grafted onto alkyne and alkene modified glass/Si wafers by thermal azide–alkyne cycloaddition and photoinitiated thiol-ene click reactions, respectively. The modified surfaces were characterized by contact angle measurements and ellipsometry. The fluorescent PIL functionalized surfaces showed strong fluorescence under UV irradiation. This procedure of tethering PILs to substrates also provides an easy way to change the surface hydrophilicity by replacing the anions in the grafted PILs. The present approach could be readily applied for surface modifications with other types of PILs or their copolymers to achieve different functionalities on various surfaces.     

Effective hole‐injection characteristics of organic light‐emitting diodes due to fluorinated self‐assembled monolayer embedded as a buffer layer

The electrical and optical properties of organic light‐emitting diodes (OLEDs) are demonstrated by varying the length of the alkyl chain of a fluorinated self‐assembled monolayer (F‐nSAM). OLEDs containing F‐nSAMs that have a long alkyl chain length were found to exhibit excellent properties in terms of current density, luminance, turn‐on voltage, etc. The obtained current density at 6 V, which was the highest measurement voltage, was up to about 36 times higher for an OLED including an F‐12SAM thin film with the longest chain length than for an OLED including only an indium tin oxide (ITO) substrate. With regard to luminance characteristics depending on voltage, the luminance was about 13 times higher for the OLED including the F‐12SAM thin film than for the OLED including only the ITO substrate. Also, the turn‐on voltage of the OLED including the F‐12SAM thin film was decreased by approximately 1 V compared to that of the OLED including only the ITO substrate. Although F‐nSAMs with alkyl chains have insulating film properties, F‐nSAMs with long alkyl chains exhibited good electrical and optical properties because of an improvement in the hole‐injection barrier due to a large positive shift of the vacuum level and smooth carrier injection resulting from a high contact angle due to strong hydrophobic properties caused by the good alignment properties of F‐nSAMs resulting from strong van der Waals forces between the molecules due to the long alkyl chains. © 2019 Society of Chemical Industry     

Investigation of the Polyamide 6,6 dyeing process with Acid Blue 45 dye. Part I. Thermodynamics of Acid Blue 45 adsorption

Among the many synthetic fibres, polyamides are one of the most important. Improvement in the adsorption/ absorption strength of a dye to and inside the yarns is therefore very important for their practical applications. In this study, the kinetics and thermodynamics of the sorption process of Acid Blue 45 on Polyamide 6,6 (Nylon 6,6) were investigated. The sorption was conducted at 293, 303, and 313 ± 0.1 K for 48 h from 10^-5 -10^-4 M solutions in the presence of 10^-3 M NaCl. From the kinetic experiments it was found that this time was sufficient to attain the adsorption equilibrium. The first stage of the sorption (up to 160 min) can be well described by a first-order kinetic equation by its numerical fitting to the experimental results. It is a spontaneous process in which the rate of sorption and the sorbed amount increase with increasing temperature. However, it is a relatively slow endothermic process (positive enthalpy of sorption); thus, it must be entropy-driven to produce a negative free energy of sorption. From the sorption isotherms and calculated thermodynamic functions it can be concluded that at higher temperatures (303-313 K), more polar groups of Polyamide 6,6 are accessible for the dye molecules, which may be due to a glass transition. Data on the zeta potential changes and surface free energy components of Polyamide 6,6 will be presented in Part II of this study, which may help in a better understanding of the sorption process.     

Electrochemistry of conductive polymers: 41. Effects of self-assembled monolayers of aminothiophenols on polyaniline films

Effects of self-assembled monolayers (SAMs) of a few different aminothiophenols (ATPs) on growth, morphology, and electrical properties of polyaniline have been studied at gold electrodes employing current sensing atomic force microscopy. The 4-aminothiophenol (4-ATP) SAM showed the best capability of shuttling electrons from the gold electrode to the solution species, and the 4-ATP SAM covered electrode not only showed the best kinetics for the film growth but also produced the film of the highest electrical conductivity. The vertical conductivity of the film ranged from 58 S cm⁻¹ for a film prepared at a bare gold electrode to as high as 148 S cm⁻¹ for a film obtained at the 4-ATP SAM covered electrode. The conductivity was shown to depend on the thickness of the films as well as how they were prepared.     

A new cleaning methodology for efficient Au-SAM removal

The desorption of a self-assembled monolayer (SAM) of 11-amino-1-undecanethiol (AUT) formed on an Au polycrystalline electrode was investigated with the purpose of establishing the most efficient method for SAM removal prior to electrode re-use. The cleanliness of the surface was evaluated by assessing the characteristics (and their reproducibility) of a newly prepared SAM (on a freshly cleaned electrode) in the presence of Ru(NH₃)₆^3+/2+ probe species. The simple flame annealing of the modified electrodes showed poor reproducibility. Later, cleaning treatments based on previous reports about the electrochemical desorption of SAMs, were investigated in NaOH, HClO₄, and KCl solutions. The anodic removal in alkaline and chloride solutions was not efficient enough due to the effect of the Au surface oxidation and dissolution. The commonly used reductive desorption in alkaline solution did also not offer a high efficiency (in contrast with the behavior described in those reports) probably due to the low stability of the thiolate molecules at high-pH values. In addition, such procedures did not provide a way to visualize the evolution of the process. The oxidative desorption in perchloric acid showed a higher efficiency at each single desorption cycle, and allowed to monitor the extension of the removal by comparing the cyclic voltammograms obtained in that medium with the expected fingerprint for the bare Au. However the voltammetric characterization of new AUT films, prepared on such electrochemically treated surfaces by re-incubation in the AUT solution, showed that the conditions of substrate cleanliness and smoothness necessaries to reproduce a close-packed and compact AUT monolayer were not reached. A new cleaning strategy based on the coupled use of oxidative removal in HClO₄ 0.1 M and flame annealing showed to be efficient and reproducible, providing the proper substrate precursors for the formation of highly ordered AUT films. The results also showed that the methodology works well in the removal of other SAMs like the one formed by the neutral 1-dodecanethiol (1-DT).     

Molecular adsorption of n-alkyl amines,carboxylic acids,and amides onto well-defined,polar organic surfaces

This paper details the formation of oriented monolayer films by the solution-phase adsorption of n-alkyl-chained adsorbates (CH₃(CH₂)_n-1Y) onto the polar surfaces of terminally-substituted self-assembled monolayers (SAMs; Au/S(CH₂)_mX). The polar tail groups (X and Y) of the adsorbate and SAM included amine, carboxylic acid, and amide groups, and the formation of the adsorbed monomolecular films on the SAMs relied on non-covalent interactions between X and Y. Highly organized monomolecular adlayers could be produced on the SAM surface that were as densely packed as the underlying alkanethiolate SAMs comprising the first layer. The quality of the adsorbed monolayers was related to the identities of the tail groups X and Y.