Dodecanethiol Monolayers Research Articles

Studies on the effect of electrode pretreatment on the coverage of self-assembled monolayers of dodecanethiol on gold by electrochemical reductive desorption determination

This paper examines the effect of five major pretreatments on the surface coverage Γ(m) of dodecanethiol self-assembled monolayer on polycrystalline gold electrode (C(12)SH-SAMs-Au). It is based on the electrochemical reductive desorption in the alkaline solution by cyclic voltammetry (CV). The five different pretreatment methods include: aqua regia pretreatment, reductive annealed pretreatment, UV/O(3) pretreatment, piranha reagents pretreatment and simple polishing pretreatment, and then all above pretreatments following the same procedure of electrochemistry cleaning. The experimental results show that the surface coverage Γ(m) for C(12)SH-SAMs-Au by the five pretreatment methods are: aqua regia pretreatment (8.0 × 10(-10) mol cm(-2)) ~ reductive annealed pretreatment (7.8 × 10(-10) mol cm(-2)) > UV/O(3) pretreatment (5.0 × 10(-10) mol cm(-2)) ~ piranha reagents pretreatment (4.1 × 10(-10) mol cm(-2)) ~ simple polishing pretreatment (4.0 × 10(-10) mol cm(-2)). This indicates that Au surfaces pretreated by aqua regia and reductive annealing can achieve the best results, and the Γ(m) values obtained are consistent with the theoretical coverage values (Γ(m) ≈ 8.0 × 10(-10) mol cm(-2)); however, the Γ(m) values for other three pretreatment methods (UV/O(3), piranha reagents and simple polishing) are not satisfactory, obtaining only almost half of the theoretical Γ(m) value. Thus, we recommend aqua regia and reductive annealed pretreatments as the best methods for self-assembling the alkyl thiol monolayer (C(n)SH-SAMs-Au), whereas UV/O(3), piranha reagents and simple polishing pretreatments are not recommended.

Irreversible Redox Reactions of Ferrocene/Ferrocenium on Self-Assembled Monolayer Modified Gold Electrode in Ionic Liquid BMIMBF4

Self-assembled monolayers of dodecanethiol on gold electrodes were characterized in an ionic liquid 1-butyl-4-methylimidazolium tetrafluoroborate containing redox probes ferrocene (Fc)/ferrocenium or others. An irreversible redox reaction of has been observed. The results may indicate a special double layer structure of an electrode/electrolyte interface and an interaction of imidazolium and Fc. Coverage of the monolayer has been determined by stripping voltammetry in .

Orientational Analysis of Dodecanethiol and p‐Nitrothiophenol SAMs on Metals with Polarisation‐Dependent SFG Spectroscopy

Polarisation-dependent sum frequency generation (SFG) spectroscopy is used to investigate the orientation of molecules on metallic surfaces. In particular, self-assembled monolayers (SAMs) of dodecanethiol (DDT) and of p-nitrothiophenol (p-NTP), grown on Pt and on Au, have been chosen as models to highlight the ability of combining ppp and ssp polarisations sets (representing the polarisation of the involved beams in the conventional order of SFG, Vis and IR beam) to infer orientational information at metallic interfaces. Indeed, using only the ppp set of data, as it is usually done for metallic surfaces, is not sufficient to determine the full molecular orientation. We show here that simply combining ppp and ssp polarisations enables both the tilt and rotation angles of methyl groups in DDT SAMs to be determined. Moreover, for p-NTP, while the SFG active vibrations detected with the ppp polarisation alone provide no orientational information, however, the combination with ssp spectra enables to retrieve the tilt angle of the p-NTP 1,4 axis. Though orientational information obtained by polarisation-dependent measurements has been extensively used at insulating interfaces, we report here their first application to metallic surfaces.

Quartz Crystal Microbalance Biosensor for Saxitoxin Based on Immobilized Sodium Channel Receptors

This work explored the possibility of coupling the toxin receptor-binding principle with the piezoelectric transduction principle. The sensing component of the saxitoxin biosensor involves a piezoelectric quartz crystal that was coated with sodium channel receptors. The sodium channel receptors were isolated from the electroplax organ of Electrophorus electricus. Binding of the sodium channel extracts to the quartz crystal was facilitated by pre-coating the gold electrode with a hydrophobic self-assembled monolayer of dodecanethiol. The instrumentation system consisted of a flow cell that held the quartz crystal, an oscillator circuit, an injection port, and a frequency counter that was connected to a personal computer. The various immobilization and measurement parameters were optimized. Binding of saxitoxin standards with the immobilized sodium channels was monitored through the decrease in the crystal oscillation frequency readings (ΔF) upon the introduction of saxitoxin into the flow cell. A calibration curve for saxitoxin was constructed by plotting the ΔF values vs. saxitoxin concentrations in the range from 0.1 to 2.0 μg/mL. A correlation coefficient of 0.9653 was obtained. The saxitoxin biosensor developed has the potential to be applied to the rapid screening of total paralytic shellfish poisoning toxins.

Mean-Field Theoretical Study of Bistability in Mixed Azobenzene-Alkylthiol Monolayers

Azobenzene derivatives exhibit a well-known cis−trans photoinduced isomerization. At the gold surface, scanning tunneling microscopy studies of the azobenzene derivative N-(2-mercaptoethyl)-4-phenylazobenzamide have revealed similar isomerization behavior under the influence of an electric field. The reversible conversion of this adsorbate between a low-current and a high-current isomer shows promise for applications in molecular switching. This paper employs a mean-field theoretical approach to study bistability at medium to high surface coverages of chemisorbed mixed monolayers of azobenzene and dodecanethiol on gold surfaces. We find that the trans isomer dominates the monolayer under most sets of conditions, stabilized both by interactions between its total dipole moment and the field, and by lateral interactions with matrix molecules. Our results also highlight two important effects, which are often overlooked in discussions of bistability in self-assembled monolayers. First, polarizability effects c...

Nanoporous structures on ZnO thin films

Porous structures were formed on ZnO thin films which were grown by an electrochemical deposition (ECD) method. The growth processes were carried out in a solution of dimethylsulfoxide (DMSO) zinc perchlorate, Zn(ClO4)2, at 120 ∘C on indium tin oxide (ITO) substrates. Optical and structural characterizations of electrochemically grown ZnO thin films have shown that the films possess high (0002) c-axis orientation, high nucleation, high intensity and low FWHM of UV emission at the band edge region and a sharp UV absorption edge. Nanoporous structures were formed via self-assembled monolayers (SAMs) of hexanethiol (C6SH) and dodecanethiol (C12SH). Scanning electron microscope (SEM) measurements showed that while a nanoporous structure (pore radius 20 nm) is formed on the ZnO thin films by hexanathiol solution, a macroporous structure (pore radius 360 nm) is formed by dodecanethiol solution. No significant variation is observed in X-ray diffraction (XRD) measurements on the ZnO thin films after pore formation. However, photoluminescence (PL) measurements showed that green emission is observed as the dominant emission for the macroporous structures, while no variation is observed for the thin film nanoporous ZnO sample.

Self-Assembled Film Organization in Fast Microcontact Printing Investigated by Sum Frequency Generation Spectroscopy

The ability of microcontact printing to build highly ordered alkanethiol self-assembled monolayers (SAMs) on Pt substrates within a short time is studied by sum frequency generation (SFG) spectroscopy and contact angle measurements. The deposition of ordered hexadecanethiol and dodecanethiol monolayers onto platinum substrates is achieved in less than 1 s. The film order and the alkane chain orientation are deduced from the SFG fingerprint acquired under different sets of laser polarization. Comparisons between the SAMs prepared by printing or by immersion demonstrate that both methods lead to the same high quality organization. Patterning effects within printed films are also investigated with respect to the layer conformation. Finally, wetting properties of printed layers are correlated with the printing duration, corroborating the spectroscopic results.

Single molecular switch based on thiol tethered iron(II)clathrochelate on gold

Molecular electronics has been associated with high density nano-electronic devices. Developments of molecular electronic devices were based on reversible switching of molecules between the two conductive states. In this paper, self-assembled monolayers of dodecanethiol (DDT) and thiol tethered iron(II)clathrochelate (IC) have been prepared on gold film. The electrochemical and electronic properties of IC molecules inserted into the dodecanethiol monolayer (IC-DDT SAM) were investigated using voltammetric, electrochemical impedance spectrpscopy (EIS), scanning tunneling microscopy (STM) and cross-wire tunneling measurements. The voltage triggered switching behaviour of IC molecules on mixed SAM was demonstrated. Deposition of polyaniline on the redox sites of IC-DDT SAM using electrochemical polymerization of aniline was performed in order to confirm that this monolayer acts as nano-patterned semiconducting electrode surface.

Electrolytic Gold Deposition on Dodecanethiol-Modified Gold Films

The electrochemical nucleation and growth of Au from a Au sulfite electrolyte onto dodecanethiol-modified Au surfaces is investigated by a combination of microscopy and chronoamperometry methods. The self-assembled dodecanethiol monolayers are continuous but exhibit defects in correspondence of the Au grain boundaries and on top of Au terraces. Nucleation of Au films occurs initially at these defect sites, but only a small fraction of these nuclei survive an initial competition process. The remaining nuclei expand through three-dimensional progressive nucleation followed by diffusion-limited growth. The resulting Au films exhibit microstructures which are widely different from those observed in the electrochemical growth of Au on Au and that depend on the applied potential: while at low overpotentials the film grows as an assembly of hemispherical clusters, at intermediate overvoltages the films are smooth and at high overvoltages become dendritic. Metal growth onto self-assembled monolayer-modified substrates can thus provide an alternative method for controlling film morphology for a wide range of applications.

Potassium deposition on a thiophene-terminated alkanethiol monolayer

Potassium deposition in ultrahigh vacuum on 12-(3-thienyl)dodecanethiol monolayers assembled on gold surfaces has been investigated using X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS). Angle-resolved XPS indicates that initially deposited potassium penetrates the self-assembled monolayer (SAM) and diffuses to the SAM/Au interface. Even after large metal doses, the presence of thiophene ring valence electronic states in the UPS spectra confirms that most of the thiophene rings (at the SAM/vacuum interface) are not covered by potassium. The binding energy shifts of the thiophene ring valence states and the C1s and thiophene S2p peaks, referenced to the Fermi level, are due to the work function changes of the gold substrate. This indicates that these electronic states are pinned to the vacuum level, in contrast to the thiolate S2p orbital, which is pinned to the Fermi level. For large potassium doses, the appearance of new features in the UPS spectrum indicates doping of the thiophene rings.

Determining Absolute Molecular Orientation at Interfaces: A Phase Retrieval Approach for Sum Frequency Generation Spectroscopy

We report a numerical algorithm, the maximum entropy method (MEM), to obtain the absolute phase of the sum frequency signal from vibrational sum frequency generation (VSFG) spectra, without the need for phase-sensitive measurements. From the phase of the VSFG susceptibility, we can determine the molecular orientation, i.e., whether molecular groups are pointing “up” or “down”, with respect to the interface. Furthermore, with previous knowledge of the nonresonant phase, the real and imaginary parts of second-order susceptibility can also be determined. The phase retrieval algorithm is successfully applied to spectra obtained from three distinct samples: (1) water vibrations of the SiO2−water interface, (2) methyl vibrations of a dodecanol monolayer on water, and (3) methyl vibrations of self-assembled dodecanethiol monolayers on a gold substrate. These results demonstrate that the approach is applicable to a wide range of spectra, with varying resonance widths and nonresonant background levels. For the SiO2−water interface at high pH, we find that the water molecules are oriented with their hydrogen atoms toward the surface, and we show that the procedure demonstrated here provides information on the interfacial vibrations that cannot be obtained from a multiresonance fit. For surfactant monolayers, we find, as expected, that the methyl groups point away from the substrate. Possible complications and limitations in determining the phase spectrum of the nonlinear susceptibility using MEM are also discussed.

Double-Scale Roughness and Superhydrophobicity on Metalized Toray Carbon Fiber Paper

A simple, inexpensive route for the fabrication of a superhydrophobic metal surface is described. Carbon-carbon composite paper (Toray TGP-H) is electroplated with copper. The copper layer is made hydrophobic by self-assembling a monolayer of dodecanethiol. The surface topography required to induce superhydrophobic behavior is achieved by varying the plating bath composition (Cl-, PEG, and SPS additives) and the time of deposition (effective thickness of the Cu layer). The surface morphology created by the original arrangement of the carbon fibers in the Toray paper (diameter 8 microm, spacing 30 microm) does not produce superhydrophobic behavior. This is true for both continuous and incomplete copper coatings. Truly superhydrophobic behavior (large contact angles, 160-165 degrees, and very small contact angle hysteresis, 2 to 3 degrees) is achieved when a continuous copper layer is deposited on the carbon fibers and also a second micrometer-range roughness is developed as a result of the formation of small copper crystallites (size approximately 1 microm).

Influence of an external magnetic field on the formation of self-assembled monolayers of dodecanethiol on polycrystalline gold electrode

This paper reports the preparation of dodecanethiol self-assembled monolayers (C 12SH-SAMs) on polycrystalline gold electrodes in a magnetic field. The qualities of C 12SH-SAMs were characterized by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results show that highly dense and well-ordered SAMs could form in a relatively short time (3 h), indicating that the rate of SAMs formation increased under an external magnetic field compared with the natural self-assembly process. Moreover, the results of CV and EIS measurements also suggested that the presence of a magnetic field had influenced the qualities of the SAMs; the stronger magnetic intensity can help to obtain much denser and well-ordered SAMs.

Iron (III) porphyrin bearing 2,6-di- tert-butylphenol pendants deposited onto gold electrodes for amperometric determination of l-histidine

A sensitive amperometric sensor for determination of l-histidine was developed using gold electrode modified with Fe(III)-porphyrin bearing three 2,6-di- tert-butylphenol groups and one palmitoyl chain. Two methods of electrode modification were applied: direct chemisorption and embedment into dodecanethiol monolayer. Both types of electrodes were used for detection of l-histidine using Osteryoung square-wave voltammetry. The sensitivity of sensors presented towards l-histidine depends on the method of electrode modification. The detection limits observed for the electrodes incorporating with Fe(III)-porphyrin host by embedment and chemisorption were in 1 and 100 nM ranges, respectively. In addition, the determination of l-histidine with electrode modified by embedment technique was more precise, in comparison to that obtained by the direct chemisorption. Applicability of gold electrodes modified with Fe(III)-porphyrin for the direct electrochemical determination of l-histidine was demonstrated using the artificial matrix mimicking human serum.

Effect of Electrode Roughness On the Capacitive Behavior of Self-Assembled Monolayers

Analytical gold electrodes were polished mechanically and electrochemically and the true area of the electrode surface was measured by quantitative oxidative/reductive cycling of the electrode. A roughness factor for each electrode was determined from the ratio of the true area to the geometric area. The roughness is fully described by a combination of microscopic roughness (up to tens of nanometers) and macroscopic roughness (on the order of hundreds of nanometers) terms. The electrodes were then derivatized with a self-assembled monolayer (SAM) of dodecanethiol or a thioalkane azacrown and characterized by impedance spectroscopy. The behavior of the electrodes was modeled with either a Helmholtz or Randles equivalent circuit (depending on the SAM used) in which the capacitance was replaced with a constant phase element. From the model, an effective capacitance and an alpha factor that quantifies the nonideality of the SAM capacitance was obtained. The effective capacitance divided by the roughness factor yields the capacitance per unit true area, which is only a function of microscopic roughness. The relationship between this capacitance and the alpha factor indicates that microscopic roughness predominantly affects the nonideality of the film while macroscopic roughness predominantly affects the magnitude of the film's capacitance. Understanding the contribution of the electrode topography to the magnitude and ideality of the SAM capacitance is important in the construction of SAM-based capacitive sensors because it predicts the importance of electrode-electrode variations.

Study of the Hyperthermal Proton Bombardment Effects on Self-Assembled Monolayers of Dodecanethiol on Au(111)

Proton bombardment on self-assembled monolayers (SAMs) of dodecanethiol formed on a gold surface is performed and its effect is studied by scanning tunneling microscope (STM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). STM gives clear evidence that a relatively uniform layer of organic nanoclusters with different sizes and densities can be created on the surface. Analysis of the mass spectra collected from the SAM samples reveals that large hydrocarbon fragments from the bombarded sample area show higher intensity than those from the virgin area, which is a strong indication that selective C-H bond cleavage and cross-linking are induced by the proton beam bombardment. We demonstrate that the size and density of the clusters can be harnessed by controlling the proton beam energy and fluence. The clusters grow bigger, at the expense of a decreasing number density, with increasing proton energy or fluence.

Two-Dimensional Crystal Structure of a Quaterthiophene−Alkanethiol Self-Assembled Monolayer on Gold

We investigated the fine structure of a self-assembled monolayer of dodecanethiol functionalized by alpha-quaterthiophene on gold (alpha-4TC 12H 24SH). The molecular orientation, quantified using polarization modulation infrared reflection-absorption spectroscopy, was studied as a function of the adsorption time. The alpha-4T moieties arrange in the upright position on the surface as the adsorption time increases, while the alkyl chain organization remains poor. Here we quantify the orientation of the self-assembled monolayer and, more significantly, reveal through surface X-ray diffraction that after a long incubation period (12 h) the alpha-4T on the gold surface adopts a 2D crystal structure.

Study of the solvent effect on the quality of dodecanethiol self-assembled monolayers on polycrystalline gold

This paper reports the study of the solvent (tetrachloromethane, toluene, tetrahydrofuran, acetone, N, N-dimethylformamide, acetonitrile, ethanol, ethanol–water mixture (1:1, v:v) and neat dodecanethiol liquid) effect on the quality of self-assembled monolayers of dodecanethiol (C 12SH-SAMs) on polycrystalline gold. The quality of the C 12SH-SAMs was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and the results show that highly dense and well-ordered C 12SH-SAMs can be prepared from ethanol–water mixture and neat dodecanethiol liquid. The property parameters of the solvents, such as polarity, solubility parameter, molecular diameter, octanol–water partition coefficients and viscosity were correlated with their ability to influence the quality of the C 12SH-SAMs.

Synthesis and Characterization of a Thiol-Tethered Tripyridyl Porphyrin on Au(111)

Porphyrins are actively studied for use in molecular and organic electronic components of devices because of their diverse tunable optical and electronic properties. In this study, mixed self-assembled monolayers (SAMs) of dodecanethiol and a tripyridyl porphyrin attached to a thiol tether via a perfluorinated phenyl ring (TPy3PF4−SC5SH) were prepared on Au(111) substrates. The synthetic strategy allows for rapid formation of derivatives with different tethers. The surface structural and electronic properties of mixed monolayer SAMs of the porphyrin inserted into the dodecanethiol matrix were investigated using scanning tunneling microscopy (STM), atomic force microscopy (AFM), Fourier transform infrared reflection−absorption spectroscopy (FT-IRAS), and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculations were also employed to evaluate the analytical vibrational frequencies of the TPy3PF4−SC5SH molecule as well as its electronic structure. For the mixed monolayers, the morphology of the porphyrin molecules was probed by STM where it was found that the molecules assembled into domains of ∼2 and 6 nm. AFM shows that the molecules protrude above the n-dodecanethiol layer by ∼0.9 nm, while by STM, apparent heights of only ∼0.5 nm were observed, suggesting limited tunneling efficiency. Stochastic switching of the porphyrin molecules was also observed during STM measurements in the mixed monolayer and is likely associated with conformational changes within the monolayer since these molecules tended to insert near defects within the SAM.

Extracellular biosynthesis of monodispersed gold nanoparticles by a SAM capping route

Monodispersed gold nanoparticles capped with a self-assembled monolayer of dodecanethiol were biosynthesized extracellularly by an efficient, simple, and environmental friendly procedure, which involved the use of Bacillus megatherium D01 as the reducing agent and the use of dodecanethiol as the capping ligand at 26 °C. The kinetics of gold nanoparticle formation was followed by transmission electron microscope (TEM) and UV-vis spectroscopy. It was shown that reaction time was an important parameter in controlling the morphology of gold nanoparticles. The effect of thiol on the shape, size, and dispersity of gold nanoparticles was also studied. The results showed that the presence of thiol during the biosynthesis could induce the formation of small size gold nanoparticles (<2.5 nm), hold the shape of spherical nanoparticles, and promote the monodispersity of nanoparticles. Through the modulation of reaction time and the use of thiol, monodispersed spherical gold nanoparticles capped with thiol of 1.9 ± 0.8 nm size were formed by using Bacillus megatherium D01.