Reductive Desorption Research Articles

Comparative electrochemical study of self-assembly of octanethiol from aqueous and aqueous ethanol solutions on a gold electrode

The self-assembly of octanethiol (OT) on the surface of a polycrystalline gold electrode in aqueous and aqueous ethanol thiol-containing (1 × 10–4 М) 0.1 М NaClO4 solutions was studied. The blocking properties and electrochemical stability of monolayer OT films were studied by chronopotentiometry during OT adsorption under the open circuit conditions (chronoamperometry at a fixed potential) combined with cyclic voltammetry for modified Au/OT electrodes. It was found from the change in the rate of electrochemical reactions in the range of monolayer stability potentials that in aqueous media, compact insulating OT monolayer films formed at a open circuit potential within ~100 s, and the shift of the adsorption potential toward negative values (to–0.6 V) allowed a considerable decrease in the monolayer self-assembly time. The potential shift toward higher negative values (–0.9 V) leads to a removal of OT from the electrode surface during the reductive desorption, with a multipeak current signal recorded on the voltammograms. A transition from aqueous to aqueous ethanol solutions accelerated the formation of an insulating OT monolayer (≈6 s) and led to a change in the shape of the desorption current peak, whose value was almost independent of the ОТ accumulation time and potential.

Nanostructuring of Molecular Assembly Using Electrochemical Reductive Desorption of Locally Stabilized Thiol Monolayers

Nanostructuring of molecular monolayers is indispensable for integration of multiple functionality within a monolayer. Here, a fabrication method of two-component molecular assembly with nanoscale structures is demonstrated by a combination of reductive desorption of thiol molecules from a Au(111) substrate with local stabilization of the molecular layers using adsorption of Au nanoparticles. The average sizes of the obtained nanoscale molecular features were controlled by changing the size of nanoparticles. A unique functionality of size-controlled nanostructures was presented using redox responses of ferrocene-terminated molecules as a collective response of ferrocene moieties in each nanostructured assembly.

Switchable Hydrophobic Valve for Controlled Microfluidic Processing.

A simple microfluidic valve, without any moving parts, is presented that can control solution flow on demand in microchannels of many different materials using a low-power electric signal. Many independently operating valves can easily be integrated into complex microfluidic systems. The valve consists of a self-assembled monolayer (SAM) formed on a platinum electrode that is incorporated directly in the microchannel. The normally-on valve stops the solution flow due to a hydrophobic SAM on the electrode surface. The solution is allowed to pass the valve by applying a potential to the electrode, which removes the SAM due to reductive desorption. The valve operation is highly stable and has switching times of the order of 1 s. The valve is ideal for controlled solution manipulation in integrated micro-analytical systems and autonomous microfluidic systems.

A pH-Sensitive Supramolecular Switch Based on Mixed Carboxylic Acid Terminated Self-Assembled Monolayers on Au(111).

We show that homogeneously mixed self-assembled monolayers (SAMs) of mercaptoalkanoic acids of different chain lengths can be used to build up a pH-sensitive supramolecular switch. The acids with short and long alkyl chains interact via the strong hydrogen bond between carboxylic acid groups. The pH acts as a trigger by breaking or restoring the hydrogen bond interaction in basic or acidic solutions, respectively. The corresponding changes in the monolayer structure were determined by ellipsometry, surface-enhanced Raman spectroscopy, and contact angle measurements. Density functional theory (DFT) calculations were performed to elucidate the structures of interacting molecules compatible with the surface coverage obtained from electrochemical reductive desorption experiments. The simplicity of the preparation procedure assures a high reproducibility whereas the stability of the homogeneous mixed SAM guarantees the reversibility of the switching process.

Sensitive Electrochemical Detection of Nereistoxin by Reductive Desorption from Au(111) and Au(100)

Sensitive Electrochemical Detection of Nereistoxin by Reductive Desorption from Au(111) and Au(100)

A First-Principles Surface Reaction Kinetic Model for Hydrogen Evolution under Cathodic and Anodic Conditions on Magnesium

A surface kinetic model is developed using a proposed mechanism for the anodic and cathodic branches of the hydrogen evolution observed over magnesium (i.e. the negative difference effect). The key element to the model is that hydrogen evolution requires the removal of adsorbed OH from the surface. This step is achieved in the cathodic branch by reductive desorption of OH to form OH− from the surface, and in the anodic branch by the dissolution of both adsorbed OH along with the Mg atom to which it is attached (thus combined desorption of OH− alongside dissolution of Mg2+). Steady state theory is applied to derive expressions for the hydrogen evolution rate as a function of potential. First-principles parameters obtained from the literature are used to compute the rate constants for the individual mechanistic steps, where available, and reasonable values based on cohesive energies are used to estimate the remaining parameters where first-principles information is not available. These rate constants are then applied to the surface kinetic model to show that the model provides reasonable agreement with the observed phenomenology for hydrogen evolution over magnesium, both in terms of the shape and order of magnitude of hydrogen evolved, and the transition potential (i.e. the open circuit potential) between the anodic and cathodic hydrogen evolution branches.

SERS of the Positive Charge Bearing Pyridinium Ring Terminated Self-Assembled Monolayers: Structure and Bonding Spectral Markers

Bifunctional self-assembled monolayers (SAMs) are widely used for construction of surfaces with desirable properties. To predict and control the function of SAMs, molecular level understanding of monolayer architecture is required. In this work, structure and bonding of positive charge bearing SAM of N-(6-mercapto)hexylpyridinium (MHP) on Au and Ag electrodes was probed by SERS, first-principles calculations, isotopic substitution, and reductive desorption voltammetry. Based on analysis of immersion time- and temperature-dependent SERS spectra as well as DFT calculations, the marker bands for MHP structure, bonding, and orientation have been established. The presence of a band around 1083 cm–1 was found to be a reliable SERS marker for the all-trans conformation of hydrocarbon chain. Soft C–H stretching mode near 2832 cm–1 was recognized as a marker for the probing of direct interaction of alkyl chain with the metal. Based on solvent-dependent Raman spectra the pyridinium ring ═C–H band (ν2) was proposed ...

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.

Multilevel hydrogeochemical monitoring of spatial distribution of arsenic: A case study at Datong Basin, northern China

To elaborate primary geochemical factors controlling the enrichment and spatial distribution of arsenic (As) species in groundwater systems, multilevel hydrogeochemical monitoring was conducted at an As-contaminated site in central part of Datong Basin, northern China. Aqueous As concentration was highly variable, ranging from 5.47 to 2690μg/L. High As groundwater was characterized by elevated HCO3−, Fe(II), HS−, NH4+ and dominated by As(III) species. The positive correlation between As and Fe contents was observed in easily-reducible Fe pool, indicating that poorly crystalline Fe minerals served as an important As sink via chemical adsorption and/or co-precipitation. Under moderately reducing and weakly alkaline conditions (Eh≥100mV and pH between 7.69 and 8.04), As mobilization could be controlled by pH-dependent desorption process. The relationships of aqueous As(V) and As(III) versus Fe(II) concentration confirm that reductive dissolution of As-bearing Fe oxides/hydroxides and reductive desorption of As(V) could be responsible for As enrichment and As(III) predominance in groundwater under more reducing conditions (Eh≤100mV). The co-presence of elevated As(III) and sulfide indicates that microbial sulfate reduction may promote As(V) transformation into thioarsenate and/or thioarsenite and further into As(III). However, the correlations of As(V) and As(III) concentration versus saturation indices of mackinawite suggest that decrease in aqueous As concentration may be due to re-sequestration of both As(V) and As(III) by Fe(II)-sulfide precipitates.

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).

Sensitive electrochemical aptamer cytosensor for highly specific detection of cancer cells based on the hybrid nanoelectrocatalysts and enzyme for signal amplification.

Human cancer is becoming a leading cause of death in the world and the development of a straightforward strategy for early detection of cancer is urgently required. Herein, a sandwich-type electrochemical aptamer cytosensor was developed for detection of human liver hepatocellular carcinoma cells (HepG2) based on the hybrid nanoelectrocatalysts and enzyme for signal amplification. The thiolated TLS11a aptamers were used as a selective bio-recognition element, attached to the gold nanoparticles (AuNPs) modified the glassy carbon electrode (GCE) surface. Meanwhile, the electrochemical nanoprobes were fabricated through the G-quadruplex/hemin/aptamer complexes and horseradish peroxidase (HRP) immobilized on the surfaces of Au@Pd core-shell nanoparticle-modified magnetic Fe3O4/MnO2 beads (Fe3O4/MnO2/Au@Pd). After the target cells were captured, the hybrid nanoprobes were further assembled to form an aptamer-cell-nanoprobes sandwich-like system on the electrode surface. Then, hybrid Fe3O4/MnO2/Au@Pd nanoelectrocatalysts, G-quadruplex/hemin HRP-mimicking DNAzymes and the natural HRP enzyme efficiently catalyzed the oxidation of hydroquinone (HQ) with H2O2, amplifying the electrochemical signals and improving the detection sensitivity. This electrochemical cytosensor delivered a wide detection range of 1×10(2)-1×10(7)cellsmL(-1), high sensitivity with a low detection limit of 15cellsmL(-1), good selectivity and repeatability. Finally, an electrochemical reductive desorption method was performed to break gold-thiol bond and desorb the components on the AuNPs/GCE for regenerating the cytosensor. These results have demonstrated that the electrochemical cytosensor has the potential to be a feasible tool for cost-effective cancer cell detection in early cancer diagnosis.

Self-Assembled Monolayer Experiments in Undergraduate Laboratories

This talk will describe a junior/senior level undergraduate laboratory for analytical or physical chemistry that can be used to demonstrate how self-assembled monolayers (SAM) form and pack onto a surface. The laboratory experiment will also introduce students to electrochemical techniques, instrumentation and surface analysis techniques. This experiment uses both chronocoulometry to determine the surface area of an electrode as well as using reductive desorption to determine the surface coverage and packing of the SAM. Discussion of how molecules interact with the monolayers will also be discussed.

In Situ Vibrational Spectroscopy of Electrode Interfaces

In situ sum frequency generation (SFG) vibrational spectroscopy is used to study the alkanethiolate monolayer on metal electrodes. The electrode potential is swept from the double layer region toward the cathodic limits in basic electrolyte solution and the reductive desorption process is observed. Correlations in the cathodic current and SFG spectra are made to help elucidate details of the monolayer film under the desorption process. SFG spectroscopy of the monolayer reveals feature that could not easily be deduced by ex situ analysis.

Photopatterning of stable, low-density, self-assembled monolayers on gold.

Photoinitiated thiol-yne chemistry is utilized as a click reaction for grafting of acid-terminated alkynes to thiol-terminated monolayers on a gold substrate to create stable, low-density monolayers. The resulting monolayers are compared with a well-packed 11-mercaptoundecanoic acid monolayer and the analogous low-density monolayers prepared through a solution phase synthetic approach. The overall structuring of the monolayer prepared by solid-phase grafting is characterized by contact angle goniometry and Fourier transform infrared spectroscopy. The results show that the product monolayer has an intermediate surface energy and a more disordered chemical structuring compared to a traditional well-packed self-assembled monolayer, showing a low-packing density of the chains at the monolayer surface. The monolayer's structure and electrochemical stability were studied by reductive desorption of the thiolates. The prepared low-density monolayers have a higher electrochemical stability than traditional well-packed monolayers, which results from the crystalline structure at the gold interface. This technique allows for simple, fast preparation of low-density monolayers of higher stability than well-packed monolayers. The use of a photomask to restrict light access to the substrate yielded these low-density monolayers in patterned regions defined by light exposure. This general thiol-yne approach is adaptable to a variety of analogous low-density monolayers with diverse chemical functionalities.

The influence of thiolate readsorption on the quality of mixed monolayers formed through an electrochemcial method.

Lateral Force Microscopy (LFM) was used to probe the quality of binary mixed monolayers formed on planar polycrystalline gold through an electrochemical method. In the approach, portions of a self-assembled monolayer (SAM) composed of 2-aminoethanethiol (AET) were removed from the Au(111) surface facets by selective reductive desorption which maintained undisrupted regions of AET elsewhere on the polycrystalline surface. Monolayer voids created by this method were backfilled with 11-mercaptoundecanoic acid (MUA) and the interface characterized with ex situ LFM. This produced images with domains of high and low friction corresponding to isolated zones of MUA and AET respectively. Reverse sequence mixed monolayers were also prepared with MUA as the starting layer and rendered LFM images that mirrored the AET based layers. This demonstrates flexibility of the electrochemical method to produce heterogeneous binary SAMs, and to further probe the quality of mixed monolayers, a number of experimental conditions including desorption time, electrode configuration, and initial incubation period were studied. AET/MUA layers that produced the most enhanced LFM images were formed on a planar electrode that was vertically submerged into the electrolyte while maintaining a selective desorption potential for 5 min before backfilling with MUA. This condition allowed for the effective diffusion of AET away from the interface and created well-defined monolayer voids for backfilling. At desorption times lower than 1 min, some of the AET molecules that remained near the interface would readsorb onto the surface and interfere with the backfilling process thereby creating lower contrast LFM images. Structural features of these layers were independent of initial incubation time (10 min and 16 h); however, the contrast between domains was improved when using AET layers formed over a longer incubation period. Interestingly, the contrast was significantly reduced when mixed layers were created on electrodes set in a hanging meniscus with the electrolyte. Here, electrochemical evidence pointed to prolonged readsorption of thiolates creating less well-defined voids for backfilling, and the event was most pronounced for MUA based layers.

In Situ Fluorescence Microscopy Study of the Interfacial Inhomogeneity of DNA Mixed Self‐Assembled Monolayers at Gold Electrodes

AbstractMixed self‐assembled monolayers composed of a fluorescently labeled DNA and a mercaptobutanol diluent immobilized on gold electrodes were characterized by electrochemical measurements coupled with in situ fluorescence microscopy. The reductive desorption of the self‐assembled monolayers was monitored in real time through variations in the capacitance and fluorescence intensity. Desorption occurred in several steps, which was related to substrate crystallinity. Fluorescence microscopy revealed the presence of spatial heterogeneities in the form of highly fluorescent aggregates that remained at the electrode surface even after a reductive desorption step. This in situ electrofluorescence microscopy technique is useful to optimize the formation of the mixed layer to obtain a homogeneous distribution of the probes, which thus improves the efficiency of the recognition process in the development of biosensors.

Effect of alkanethiol molecular structure on sensitivity of surface plasmon resonance sensor

This work aims to find the key factor determining the detection limit for an indirect competitive inhibition immunoassay using surface plasmon resonance (SPR) sensing. In our previous work, the thiol solution concentration used in a self-assembly process highly affected the alkanethiol monolayer structure on the sensor surface (Suherman et al., 2014). It was noticed that the monolayer structure determined the immunoassay sensitivity due to the orientation and the surface concentration of antigen in domain structure of monolayer. To study the effect of orientation of antigen, we examined here three types of alkanethiol compounds: dithiobis(succinimidyl undecanoate) (DSU; straight-chain alkanethiol), carboxy-EG6-undecanethiol (CEG6; flexible-chain alkanethiol), and 3,3′-dithiobis[N-(5-amino-5-carboxypentyl)propionamide-N,N′-diacetic acid] (C2-NTA; three-branched alkanethiol). By electrochemical reductive desorption, the surface concentration of DSU, CEG6, and C2-NTA were estimated to be 6.6×10−10, 8.5×10−10, 4.6×10−10molcm−2, respectively. Subsequently, SPR suggested that the ratios of immobilized antigen (clenbuterol) per thiol were 0.13, 0.16, and 0.15 for DSU, CEG6, and C2-NTA, respectively. This suggests that the amount of immobilized clenbuterol was defined by the molecular size of clenbuterol. Sensor surface structures on a molecular scale were evaluated using high-resolution scanning tunneling microscopy (STM), which showed tilted clenbuterol with θDSU>θCEG6>θC2-NTA. For SPR sensing of clenbuterol, C2-NTA showed the highest sensitivity (LOD=10ppt) among the examined alkanethiols, because clenbuterol formed a laid-on structure. Based on kinetics, the key factor for sensing performance is discussed.

A regenerating ultrasensitive electrochemical impedance immunosensor for the detection of adenovirus

We report on the development of a regenerable sensitive immunosensor based on electrochemical impedance spectroscopy for the detection of type 5 adenovirus. The multi-layered immunosensor fabrication involved successive modification steps on gold electrodes: (i) modification with self-assembled layer of 1,6-hexanedithiol to which gold nanoparticles were attached via the distal thiol groups, (ii) formation of self-assembled monolayer of 11-mercaptoundecanoic acid onto the gold nanoparticles, (iii) covalent immobilization of monoclonal anti-adenovirus 5 antibody, with EDC/NHS coupling reaction on the nanoparticles, completing the immunosensor. The immunosensor displayed a very good detection limit of 30 virus particles/ml and a wide linear dynamic range of 105. An electrochemical reductive desorption technique was employed to completely desorb the components of the immunosensor surface, then re-assemble the sensing layer and reuse the sensor. On a single electrode, the multi-layered immunosensor could be assembled and disassembled at least 30 times with 87% of the original signal intact. The changes of electrode behavior after each assembly and desorption processes were investigated by cyclic voltammetry, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy techniques.

Temporal variation of groundwater level and arsenic concentration at Jianghan Plain, central China

Elevated arsenic in groundwater affects some 60 thousand people in the Jianghan Plain, yet mitigation efforts are hindered by persistent uncertainty about the proximal source of As and mechanisms for its mobilization. Samples were collected every month from May 2012 to April 2014 in a set of 39 monitoring wells and two rivers in the field monitoring site, Jianghan Plain, to evaluate the temporal variation of groundwater level and chemical composition for As and other constituents. Results showed that groundwater tables fluctuated during the sampling period, with higher water levels during the rainy season and lower water levels during the dry season. Wells at 25m and 50m deep had lower water level than those of the wells at 10m deep. Water levels of the wells located in the south of the field monitoring site were always 1m higher than the wells in the north. Only five wells showed no significant changes in As concentrations during the monitoring period; 23 wells exhibited seasonal variations in groundwater As concentration that were consistent from year to year; the rest of the 11 wells not only showed seasonal variations but also exhibited increasing trends over the monitoring period. Seasonal As concentration changes in groundwater were positively correlated to groundwater level changes, with lower concentration corresponding to lower water level during dry season and vice versa during rainy season. The rise in As concentration during the rainy season could be attributed to enhanced reductive dissolution of iron oxyhydroxides and/or reductive desorption of As(V) as the conditions turn to be more reducing, while during the dry season more As could be scavenged onto fresh iron oxyhydroxides. Meanwhile, temporal variations showed compatible trends between As and Fe, Fe(II), S2−, indicating that As variation would be related to Fe and S cycling in the aquifer systems. Additionally, the availability of labile organic carbon as a driver of microbial reduction may play an important role in controlling the spatial and temporal variation of groundwater As concentration in the Jianghan Plain.

Succinimidyl ester surface chemistry: implications of the competition between aminolysis and hydrolysis on covalent protein immobilization.

N-Hydroxysuccinimide (NHS) ester terminal groups are commonly used to covalently couple amine-containing biomolecules (e.g., proteins and peptides) to surfaces via amide linkages. This one-step aminolysis is often performed in buffered aqueous solutions near physiological pH (pH 6 to pH 9). Under these conditions, the hydrolysis of the ester group competes with the amidization process, potentially degrading the efficiency of the coupling chemistry. The work herein examines the efficiency of covalent protein immobilization in borate buffer (50 mM, pH 8.50) using the thiolate monolayer formed by the chemisorption of dithiobis (succinimidyl propionate) (DSP) on gold films. The structure and reactivity of these adlayers are assessed via infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), electrochemical reductive desorption, and contact angle measurements. The hydrolysis of the DSP-based monolayer is proposed to follow a reaction mechanism with an initial nucleation step, in contrast to a simple pseudo first-order reaction rate law for the entire reaction, indicating a strong dependence of the interfacial reaction on the packing and presence of defects in the adlayer. This interpretation is used in the subsequent analysis of IR-ERS kinetic plots which give a heterogeneous aminolysis rate constant, ka, that is over 3 orders of magnitude lower than that of the heterogeneous hydrolysis rate constant, kh. More importantly, a projection of these heterogeneous kinetic rates to protein immobilization suggests that under coupling conditions in which low protein concentrations and buffers of near physiological pH are used, proteins are more likely physically adsorbed rather than covalently linked. This result is paramount for biosensors that use NHS chemistry for protein immobilization due to effects that may arise from noncovalently linked proteins.