Reductive Desorption Peak Research Articles

Formation of long-range-ordered self-assembled monolayers of dodecyl thiocyanates on Au(111) via ambient-pressure vapor deposition

The surface structure, binding condition, and reductive desorption behavior of self-assembled monolayers (SAMs) of dodecyl thiocyanate (DDTC, C12–SCN) on Au(111) formed via solution and ambient-pressure vapor depositions at 50 °C for 24 h were examined by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). STM imaging clearly revealed that adsorption of DDTC molecules on Au(111) in a 1 mM ethanol solution led to the formation of short-range-ordered dodecanethiolate (C12S) SAMs with a domain size ranging from several nanometers to 20 nm, whereas the SAMs formed via vapor deposition had long-range-ordered C12S SAMs of size exceeding 60 nm, which were characterized as having a (4 × √3)rect packing structure. XPS measurements showed that the DDTC SAMs formed via vapor deposition on Au(111) contained approximately one-fourth the amount of unbound sulfurs and CN species compared to solution-deposited SAMs. CV measurements also showed that vapor-deposited SAMs had a sharp reductive desorption peak at −1.028 V, whereas solution-deposited SAMs had two broad desorption peaks at −0.671 and −0.946 V, implying that vapor-deposited SAMs are more uniform and electrochemically stable. The present results clearly demonstrate that DDTC SAMs on Au(111) with a high degree of structural order, homogeneous interface, and high electrochemical stability can be prepared by ambient-pressure vapor deposition.

Comparative Study of the Adsorption of Thiols and Selenols on Au(111) and Au(100).

The effect of the Au crystalline plane on the adsorption of different thiols and selenols is studied via reductive desorption (RD) and X-ray photoelectron spectroscopy (XPS) measurements. Self-assembled monolayers (SAMs) using aliphatic (ATs) and aromatic thiols (ArTs) on both Au(111) and Au(100) were prepared. The electrochemical stability of these SAMs on both surfaces is evaluated by comparing the position of the RD peaks. The longer the AT chain the more stable the SAM on Au(100) when compared to Au(111). By means of XPS measurements, we determine that the binding energy (BE) of the S 2p signal corresponding to the S atoms at the thiol/Au interface, commonly assigned at 162.0 eV, shifts 0.2 eV from Au(111) to Au(100) for SAMs prepared using thiols with the C* (C atom bonded to S) in sp3 hybridization, such as ATs. However, when the thiol presents the C* with an sp2 hybridization, such as in the case of ArTs, the BE remains at 162.0 eV regardless of the surface plane. Selenol-based SAMs were characterized comparatively on both Au(100) and Au(111). Our results show that selenol SAMs become even more electrochemically stable on Au(100) with respect to Au(111) than the analogue sulfur-based SAM. According to our results, we suggest that the electronic distribution around the Au-S/Se bond could be responsible for the different structural arrangements reported in the literature (gold adatoms, etc.), which should be dependent on the crystalline face (Au(hkl)-S) and the chemical nature of the environment of the adsorbates (sp3-C* vs sp2-C* and Au-SR vs Au-SeR).

Formation of 2-D Crystalline Intermixed Domains at the Molecular Level in Binary Self-Assembled Monolayers from a Lyotropic Mixture

The organization and intermolecular interactions of mixed self-assembled monolayers (SAMs) of 1-decanethiol (DT) and 11-mercaptoundecanoic acid (MUA) formed on Au(111) from a lyotropic medium have been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and infrared reflection–absorption spectroscopy (IRRAS) as a function of their surface composition. The presence of a single reductive desorption peak in the voltammograms and thiol surface coverages close to 7.6 × 10–10 mol/cm2 reveal the formation of homogeneous mixtures of both molecules in an upright configuration in the mixed SAMs. Despite this fact, two different distributions of the individual components have been found in these MUA-enriched SAMs according to their different electron transfer blocking behavior and conformational order shown by EIS and IRRAS results. At short modification times, MUA–MUA and DT–DT homomolecular interactions prevail during the assembly process, and nanodomains of like molecules are probabl...

Effects of Introducing 2-aminoethanethiol into 4-pyridineethanethiol Self-assembled Monolayer Applicable to Enhance Sensitivity of Hg(II) Electrochemical Analysis

In our work, enhancement of sensitivity of Hg(II) determination by introducing 2-aminoethanethiol (AET) from solutions into the self-assembled monolayers (SAMs) of 4-pyridineethanethiol (PET) on gold nanoparticles (Au-NPs) capped on glassy carbon electrode was studied. The formation of binary SAMs was indicated by the shift of reductive desorption peak to negative potentials in voltammograms recorded in 0.5 M KOH solution. Effects of introduction AET were monitored by FTIR that there was a rapid increase of intensities at bands of 1296 and 1189 cm-1 ascribed to (C-H) ring deformation and (C-N) ring stretching, respectively, with increasing immersion time. However, these bands became lower with prolonging more time as well as increasing the concentration of AET in solution. These effects were interpreted due to the conformation of pyridine ring and formation of intermolecular hydrogen bonding between PET and AET. The differential pulse voltammetry for reoxidation of Hg(0) showed the increase of peak current induced by the first effect, whereas the later caused the decrease. Since the optimized conditions, the detection limit for the binary SAMs could approach to 3.85 × 10-12 M Hg(II), approximately three times lower than that of PET SAM.

Self-assembly of a Ru(II)-deuteroporphyrin lipoic acid derivative on Au(111) surfaces

The synthesis of a new carbonyl ruthenium(II) deuteroporphyrin-IX lipoic acid derivative (Ru(PLip)(CO)) (4) and its self-assembly on Au(111) surfaces were accomplished. Ru(PLip)(CO) 4 was prepared by reduction of the ester groups of carbonyl-ruthenium(II) deuteroporphyrindimethylester 1 and further esterification with D,L-α-lipoic acid 3. The self-assembly of Ru(PLip)(CO) 4 was confirmed by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). The S2p XP spectrum of SAM formed by Ru(PLip)(CO) showed the S2p3/2 peak at 162.4 eV which corresponds to thiolated species bounded to gold. The influence of the interaction of porphyrin moieties on SAM stability was studied. The reductive desorption voltammogram of Ru(PLip)(CO) 4 self-assembled on gold showed an intense reduction peak at -1.02 V while when pyridine was coordinated to the central ruthenium(II) the reductive desorption peak was shifted to -0.85 V. The capacity of the modified Ru(PLip)(CO) gold electrode to detect nitric oxide (NO) was investigated by cyclic voltammetry. An irreversible reduction peak which increased with time on NO exposure was registered at -0.69 V indicating NO coordination to ruthenium(II).

Influence of Thiol Molecular Backbone Structure on the Formation and Reductive Desorption of Self-Assembled Aromatic and Alicyclic Thiol Monolayers on Au(111) Surface

The surface structure and electrochemical behavior of self-assembled monolayers (SAMs) prepared from benzenethiol (BT), cyclohexanethiol (CHT), and cyclopentanethiol (CPT) on Au(111) surface were examined by scanning tunneling microscopy (STM) and cyclic voltammetry (CV) to understand the influence of thiol molecular backbone structure on the formation and reductive desorption behavior of SAMs. STM imaging showed that BT and CPT SAMs on Au(111) surface formed at room temperature were mainly composed of disordered domains, whereas CHT SAMs were composed of well-ordered domains with three orientations. From these STM results, we suggest that molecule-substrate interaction is a key parameter for determining the structural order and disorder of simple aromatic and alicyclic thiol SAMs on Au(111). In addition, the reductive desorption peak potential for BT SAMs with aromatic rings was observed at a less negative potential of -566 mV compared to CHT SAMs (-779 mV) or CPT SAMs (-775 mV) with aliphatic cyclic rings. This reductive desorption behavior for BT SAMs is due to the presence of p-orbitals on the aromatic rings, which promote facile electron transfer from the Au electrode to BT as compared to CHT and CPT. We also confirmed that the reductive desorption behavior for simple alicyclic thiol SAMs such as CHT and CPT SAMs on Au electrodes was not significantly influenced by the degree of structural order.

Formation Process and Solvent-Dependent Structure of a Polyproline Self-Assembled Monolayer on a Gold Surface

The formation process and structure of a self-assembled monolayer (SAM) of lipoic-acid-terminated polyproline on a gold surface in aqueous solution were investigated by several techniques. The amount of polyproline molecules on the gold surface was determined from the area of the reductive desorption peak, and orientation and thickness of the polyproline SAM were determined in situ by attenuated total reflection infrared (ATR-IR) spectroscopy and ellipsometry. The kinetics of the polyproline SAM formation process were discussed on the basis of these results. The in situ IR study confirmed that the conformation of the polyproline SAM was changed by changing the solvent from water to methanol and methanol to water, as is the case for polyproline dissolved in solution.

Investigation of Streptococcus mutans biofilm growth on modified Au(1 1 1)-surfaces using AFM and electrochemistry

Biofilms of the bacterium Streptococcus mutans constitute perhaps the most important direct cause of human dental caries formation. We have studied S. mutans biofilm formation and properties on Au(1 1 1)-surfaces modified by self-assembled molecular monolayers (SAMs) of different thiol-based molecules based on a combination of atomic force microscopy (AFM) and electrochemistry using single-crystal Au(1 1 1)-surfaces. The thiols include both small, strongly hydrophilic molecules (cysteamine and the amino acid L-Cystein) and long straight-chain alkanethiol-based molecules with either hydrophobic (hexadecanethiol, MHD) or hydrophilic (mercapto-hexadecanoic acid, MHDA) end groups. The voltammetric reductive desorption (RD) peaks of the thiol-based SAMs in the absence and presence of biofilms and growth medium was in focus as a sensitive probe of the SAM local environment. AFM showed that S. mutans had grown to dense monolayers on all the four modified Au(1 1 1)-surfaces after 24 hour. The growth rates were slightly different and fastest for MHD-modified surfaces but the biofilms after 24 hour were indistinguishable. Reductive desorption signals of all the four compounds in phosphate buffer, pH 7.4 were very similar in the absence and presence of the biofilms and growth medium. RD in strongly alkaline solution where RD peak resolution is higher was also addressed. Most notably, the strong RD peaks of the long pure and functionalized MHD and MHDA in 0.1 M NaOH remained in the presence either of biofilm together with growth medium or of the growth medium alone. The RD peak potential of the hydrophilic MHDA surface remained, further largely unaffected but the RD peak of the hydrophobic MHD SAM is distinctly shifted compared to the MHD SAM alone. The shifts were further in different directions for the S. mutans biofilm plus growth medium (negative shift) and for the growth medium alone (positive shift). Both the AFM images and the electrochemical data suggest that the biofilms form more efficiently and interact more strongly with the hydrophobic surface than with the hydrophilic surfaces.

Properties of thiolate monolayers formed on different amalgam electrodes

Formation and properties of thiol (HS(CH2)10COOH or HS(CH2)10CH3) monolayers were investigated at stationary electrodes based on silver, copper, bismuth and cadmium solid amalgams covered by a mercury meniscus or by a mercury film (m-AgSAE, MF-AgSAE, m-CuSAE, m-BiAgSAE and m-CdSAE). For comparison, parallel experiments were performed at a classical hanging mercury drop electrode (HMDE). Thiol monolayers were prepared at an optimal deposition potential (e.g., for HS(CH2)10COOH Eac=−350mV for HMDE and silver amalgam electrodes, Eac=−750 to −800mV for m-CuSAE and m-BiAgSAE, and Eac=−1050mV for m-CdSAE) at which the formed layers exhibit high density and stability. It was found that the potential of a reductive desorption peak of a thiol monolayer strongly depends on the metal which forms the amalgam (for HS(CH2)10COOH: Ep=−864 to −859mV for HMDE and silver amalgam electrodes, Em-BiAgSAE=−1079mV, Em-CuSAE=−1136mV and Em-CdSAE=−1248mV). This allows to create a stable thiol layer in different potential regions according to its application. The influence of a thiol layer charge on the reduction potential was determined.

Influence of the crystallographic orientation on the reductive desorption of self-assembled monolayers on gold electrodes

The voltammetric behaviour of the self-assembled monolayers formed by distinct types of thiolated molecules, namely decanethiol, 2-mercaptobenzimidazole-5-sulphonate (MBIS) and a thiolated DNA sequence was examined at polycrystalline gold electrodes and at the low-index faces Au(1 1 1), Au(1 0 0) and Au(1 1 0) as well as at Au(2 1 0). For the three thiol compounds, the multiwave voltammetric responses at polycrystalline are discussed on the basis of the behaviours observed at well-defined single-crystal electrodes. The sequence of reductive desorption (RD) potentials follows qualitatively the sequence of the potential of zero charge ( pzc) of the uncovered electrodes in the case of MBIS. Such a sequence may be altered when attractive lateral interactions between adsorbed molecules, leading to an enhancement of the self-assembled monolayer stability, can be established. The RD peaks are therefore no longer simply directly related to the pzc of the substrate since the amplitude of lateral interactions depends on the atomic roughness of the substrate.

1,n-Alkanedithiol (n = 2, 4, 6, 8, 10) Self-Assembled Monolayers on Au(111): Electrochemical and Theoretical Approach

The structures of 1,n-alkanedithiol (n = 2, 4, 6, 8, 10) self-assembled monolayers (SAMs) on a Au(111) substrate were investigated by electrochemical measurements and theoretical calculations. The results of the experimental techniques indicated that the dithiols, except n = 2, showed an upright molecular structure in the SAMs, in which alkanedithiols were bound to the Au surface via only one thiol functionality and the other one faced up to the air. The results also suggested that the formed dithiol SAMs were densely packed and highly oriented. Except ethanedithiol, which was thought to form a bilayer, the reductive desorption peak potentials of 1,n-alkanedithiol (n = 4, 6, 8, 10) SAMs were more negative than those of the corresponding monothiol ones in 0.1 M KOH solutions. This illustrates that the dithiol SAMs had higher stability than the corresponding monothiol ones. The major part of the high stability may be attributed to the van der Waals interaction among the sulfur atoms on top of the dithiol SAMs. The molecular modeling calculation showed that the structures of dithiol SAMs were similar to those of the corresponding monothiol SAMs and that all the dithiol SAMs, except ethanedithiol, were more stable than the corresponding monothiol SAMs. The calculated energy differences between dithiol and monothiol SAMs decreased with the increment of alkyl-chain length.

Effects of concentration and temperature on the formation process of decanethiol self-assembled monolayer on Au(1 1 1) followed by electrochemical reductive desorption

The formation process of self-assembled monolayer (SAM) of decanethiol (C10SH) on Au(1 1 1) single crystal electrode has been investigated for wide range of C10SH concentration (0.1–500 μM) and temperature (253–298 K). The amount and quality of C10SH SAM were determined from area and position, respectively, of reductive desorption peak of the SAM modified Au(1 1 1) electrode measured in 0.5 M KOH aqueous solution. The kinetic analysis indicates that SAM formation proceeded by two steps, diffusion-limited physisorption followed by chemisorption.

Dissolution kinetics of octadecanethiolate monolayers electro-adsorbed on Au(1 1 1)

Monolayers of octadecanethiolate on Au(1 1 1) surface were formed under electrochemical control. The influence of the formation time on the reductive desorption process was studied by cyclic voltammetry and chronoamperometry. When the formation time is increased, the reductive desorption peak observed on the voltammograms is significantly shifted in the negative direction, while the cathodic charge is only slightly affected. This behaviour is attributed to a higher degree of organisation of the monolayers for longer formation times, highlighting the role of defect sites in promoting the dissolution. A good agreement was found between our experimental chronoamperograms and theoretical models describing the dissolution process by a shrinkage mechanism. It is demonstrated that a reorganisation process takes place, consisting in the merging of small condensed domains into larger ones. This annealing phenomenon is time and potential dependent, the largest condensed domains being obtained for the longest formation times and least negative potentials.

Conformational change in 4-pyridineethanethiolate self-assembled monolayers on Au(111) driven by protonation/deprotonation in electrolyte solutions

Self-assembled monolayers (SAMs) of 4-pyridineethanethiol (4-PyEtS) formed on a Au(111) surface were investigated in aqueous electrolyte solutions by cyclic voltammetry and in situ scanning tunneling microscopy (STM). On the basis of reductive desorption of 4-PyEtS on Au(111), the reduction peak potential was found to be -0.86 V vs. Ag/AgCl (sat. KCl). The surface excess was calculated from the reductive desorption peak area to be 5.4 x 10(-10) mol cm(-2). High-resolution STM images revealed that a p(5 x radical3 R-30 degrees ) lattice including two molecules was found in 0.1 M NaClO(4), whereas the 4-PyEtS-SAMs formed a p(10 x radical3 R-30 degrees ) lattice constituting four molecules in 0.05 M HClO(4). The transition from a p(10 x radical3 R-30 degrees ) to a p(5 x radical3 R-30 degrees ) in the 4-PyEtS-SAMs was directly observed by changing pH of the electrolyte solution, indicating that the conformational change in 4-PyEtS-SAMs is driven by protonation/deprotonation. The present results obtained by STM observation are also supported by the clear pH dependence of the pyridine moiety in the surface-enhanced infrared spectrum.

Building Layer-by-Layer a Bis(dithiocarbamato)copper(II) Complex on Au{111} Surfaces

A bis(dithiocarbamato)copper(II) complex (CuDTC2) was built on Au{111} surfaces (sheets and electrode beads) using different building blocks in a layer-by-layer (LbL) procedure. The process was followed by AFM and cyclic voltammetry. Initially 4-piperidinemethanethiol, which was synthesized here for the first time, was self-assembled on a gold surface and a highly organized array was obtained. The resulting monolayer was treated with CS2 and NH3 to transform the NH groups of piperidine into dithiocarbamate groups (DTC) with the formation of an amphiphilic ligand (DTCpipS) with thiolate and DTC terminal anionic groups. Two reductive desorption peaks were observed in the cyclic voltammogram of self-assembled DTCpipS, a more intense peak at -0.87 V (thiolate group) and a broader, less intense peak at -0.68 V, corresponding to the desorption of the DTC group bound to the gold surface after the ligand made a approximately 180 degrees flip on the surface. Copper(II) and the morpholyldithiocarbamate anion were associated with self-assembled DTCpipS in order to complete the formation of the CuDTC2 complex on the gold surface. In the voltammogram of the LbL self-assembled CuDTC2 complex the reductive desorption peak at -0.68 V disappeared and one single peak was observed at -0.85 V. This corresponds to the reorientation of all of the DTCpipS dianions in order to coordinate to copper(II) through the DTC groups, leaving the self-assembly only through the thiolate groups. The complete formation of the LbL self-assembled CuDTC2 complex was confirmed by XPS and ToF SIMS, with a detected fragment corresponding to the whole complex.

Structure of Mixed Carboxylic Acid Terminated Self-Assembled Monolayers: Experimental and Theoretical Investigation

The structure and stability of mixed self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) and 11-mercaptoundecanoic acid (MUA) prepared by immersion in ethanolic solutions were studied by cyclic voltammetry, electrochemical impedance spectroscopy, ellipsometry, and STM as a function of the thiol composition of the forming solution. The presence of a single reductive desorption peak in the voltammograms of the mixed SAMs and the lack of phase segregation observed by STM support the formation of homogeneous SAMs despite the large chain length difference between MPA and MUA. To explain the driving force leading to the formation of a homogeneous mixture, intermolecular interactions within the SAM were investigated using density functional theory. The carboxyl groups of adjacent MPA and MUA molecules in a compact monolayer can form a stable head to head cyclic dimer with a hydrogen bond strength of 16.2 kcal/mol. The flexibility of the alkyl chain of MUA allows the carboxyl groups of adjacent MPA and MUA molecules to be located on the same plane. However, the carboxyls of adjacent MPA−MPA and MUA−MUA pairs form much weaker hydrogen bonds because steric constraints avoid the formation of the stable cyclic dimer. Therefore, the prevalence of MPA−MUA interactions over MPA−MPA and MUA−MUA interactions explains the homogeneous mixing of MPA and MUA. The potential chemical switchability properties of mixed monolayers of mercaptoalcanoic acids of different chain lengths as a function of the solution pH are discussed.

Multicycle Desorption-Adsorption Voltammetry for Self-Assembled Mixed Monolayer Containing Ferrocenylthiol Molecules: A Discussion on Molecular Interaction in the Mixed Layer

Self-assembled mixed monolayers (mixed SAMs) composed of 11-ferrocenyl-1-undecanethiol (11-FUT) and hexanethiol (HT) molecules were prepared through an exchange method and characterized by multicycle electrochemical desorption-adsorption voltammetry. The first cycles of the voltammograms showed a single desorption peak although the SAMs have a domain structure composed of compact 11-FUT molecules, indicating that some residual HT molecules were present in 11-FUT domains and that complete phase separation did not take place in the mixed SAMs. The structure, however, altered toward a mildly phase-separated structure when the mixed SAMs experienced desorption-adsorption cycle(s), and two separate reductive desorption peaks were found in the second cycles of voltammograms, which clearly evidenced that all mixed SAMs (before voltammetric analysis), including those prepared by prolonged exchange reactions, really contained both 11-FUT and HT molecules. This demonstrates the importance of such multicycle voltammetry for mixed SAMs that are usually characterized and discussed only in terms of the first reductive, or negative-going, scan of voltammograms. Even after the desorption-adsorption cycle(s), ferrocenyl groups in mixed SAMs underwent FeII/FeIII redox, while the intensity of redox peaks became weakened.

Adsorption behavior of binary mixed alkanethiol molecules on Au: Scanning tunneling microscope and linear-scan voltammetry investigation

Adsorption behavior of binary mixed alkanethiol molecules of octanethiol (OT) and dodecanethiol (DDT) on Au substrate has been investigated with a variation of molar fraction in solution at room temperature. Linear-scan voltammograms (LSV) taken from the binary mixed self-assembled monolayer (SAM) on Au/Si showed a single reductive desorption peak, continuously shifting with a variation of mixing ratio in solution. Molecule-resolved scanning tunneling microscope (STM) images obtained from the binary SAM on Au/mica showed the phase separation with a few nanometer sized domains. The difference in the chain length between OT and DDT was not large enough to induce the phase separation with much larger domains, which could be well detected by LSV. There was no preferential adsorption of DDT to OT.

Diluting thiol-derivatized oligonucleotide monolayers on Au(1 1 1) by mercaptohexanol replacement reaction

Surface replacement reaction of thiol-derivatized, single-stranded oligonucleotide (HS-ssDNA) by mercaptohexanol (MCH) is investigated in order to reduce surface density of the HS-ssDNA adsorbed to Au(1 1 1) surface. Cyclic voltammograms (CVs) and scanning tunneling microscopy (STM) are employed to assess the composition and state of these mixed monolayers. It is found that each CV of mixed self-assembled monolayers (SAMs) only shows a single reductive desorption peak, which suggests that the resulted, mixed SAMs do not form discernable phase-separated domains. The peak potential gradually shifts to negative direction and the peak area increases step by step over the whole replacement process. By analyzing these peak areas, it is concluded that two MCH molecules will replace one HS-ssDNA molecule and relative coverage can also be estimated as a function of exposing time. The possible mechanism of the replacement reaction is also proposed. The DNA surface density exponentially reduces with the exposing time increasing, in other words, the replacement reaction is very fast in the first several hours and then gradually slows down. Moreover, the morphological change in the process is also followed by STM.

Lateral force microscope and linear-scan voltammetry studies on the replacement of adsorbed thiolates on Au with thiols in solution

Replacement of self-assembled octanethiolates (OT) on Au with carboxyl-terminated 11-mercaptoundecanoic acid (MUA) in solution was investigated. Lateral force microscope (LFM), which can distinguish the surface areas covered with molecules of different functional groups, was combined with the μ-contact printing technique to study the time dependent replacement reaction. The LFM image of the OT and MUA pattern was deteriorated with the replacement. As the replacement proceeded, the reductive desorption peak of OT continuously shifted towards the position of MUA in the linear-scan voltammogram. Within 50 h in MUA solution, more than 60% of OT molecules on Au were replaced by MUA while the reverse process, replacement of MUA with OT, occurred much more slowly. This confirms the higher stability of MUA self-assembled monolayer (SAM) than OT SAM.