Potential-controlled Adsorption Research Articles

Design of 3D Carbon Nanotube Monoliths for Potential-Controlled Adsorption

The design of 3D monoliths provides a promising opportunity to scale the unique properties of singular carbon nanotubes to a macroscopic level. However, the synthesis of carbon nanotube monoliths is often characterized by complex procedures and additives impairing the later macroscopic properties. Here, we present a simple and efficient synthesis protocol leading to the formation of free-standing, stable, and highly conductive 3D carbon nanotube monoliths for later application in potential-controlled adsorption in aqueous systems. We synthesized monoliths displaying high tensile strength, excellent conductivity (up to 140 S m−1), and a large specific surface area (up to 177 m2 g−1). The resulting monoliths were studied as novel electrode materials for the reversible electrosorption of maleic acid. The process principle was investigated using chronoamperometry and cyclic voltammetry in a two-electrode setup. A stable electrochemical behavior was observed, and the synthesized monoliths displayed capacitive and faradaic current responses. At moderate applied overpotentials (± 500 mV vs. open circuit potential), the monolithic electrodes showed a high loading capacity (~20 µmol g−1) and reversible potential-triggered release of the analyte. Our results demonstrate that carbon nanotube monoliths can be used as novel electrode material to control the adsorption of small organic molecules onto charged surfaces.

Potential-Controlled Adsorption, Separation, and Detection of Redox Species in Nanofluidic Devices.

Nanoscale channels and electrodes for electrochemical measurements exhibit extreme surface-to-volume ratios and a correspondingly high sensitivity to even weak degrees of surface interactions. Here, we exploit the potential-dependent reversible adsorption of outer-sphere redox species to modulate in space and time their concentration in a nanochannel under advective flow conditions. Induced concentration variations propagate downstream at a species-dependent velocity. This allows one to amperometrically distinguish between attomole amounts of species based on their time-of-flight. On-demand concentration pulse generation, separation, and detection are all integrated in a miniaturized platform.

Voltammetric analysis of 5-(4-Azidophenyl)-2′-deoxycytidine nucleoside and azidophenyl-labelled single- and double-stranded DNAs

Voltammetric determination of a redox labeled nucleoside 5-(4-azidophenyl)-2′-deoxycytidine (dCAZP) and various polymerase-synthesized dCAZP-labeled DNAs in aqueous buffers is presented. Influence of: i) pH (2–12), ii) scan rates (0.02–10Vs−1), and iii) dCAZP concentration (0.02–10μmoll−1), on voltammograms of dCAZP were systematically studied for the first time using CV at a hanging mercury drop electrode. Electrode potential-controlled adsorption driven process allowed sensitive determination of dCAZP at nanomolar concentrations using adsorptive stripping voltammetry. Transfer stripping voltammetry (TSV) was used for the detection of dCAZP-labeled DNA in femtomole quantities. Precise sequence-specific incorporation of dCAZP into DNA by primer extension was used to demonstrate a perfect correlation between the number of incorporated AZP moieties and TSV responses. In addition, for the first time we used polymerase chain reaction to prepare an about 350-bp double-stranded DNA fragment globally modified with dCAZP, and of terminal deoxynucleotidyl transferase tailing reaction to generate end-labeled single stranded oligonucleotides. Effects of DNA structure on the AZP-modified DNA TSV responses are discussed.

In Situ SHINERS at Electrochemical Single-Crystal Electrode/Electrolyte Interfaces: Tuning Preparation Strategies and Selected Applications

We have studied Au(55 nm)@SiO2 nanoparticles (NPs) on two low-index phases of gold and platinum single crystal electrodes in ClO4(-) and SO4(2-) ion-containing electrolytes by both electrochemical methods and in-situ shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS). We showed the blocking of the electrode with surfactants originating from the synthesis of as-prepared SHINERS NPs. We introduce an efficient procedure to overcome this problem, which provides a fundamental platform for the application of SHINERS in surface electrochemistry and beyond. Our method is based on a hydrogen evolution treatment of the SHINERS-NP-modified single-crystal surfaces. The reliability of our preparation strategy is demonstrated in electrochemical SHINERS experiments on the potential-controlled adsorption and phase formation of pyridine on Au(hkl) and Pt(hkl). We obtained high-quality Raman spectra on these well-defined and structurally carefully characterized single-crystal surfaces. The analysis of the characteristic A1 vibrational modes revealed perfect agreement with the interpretation of single-crystal voltammetric and chronoamperometric experiments. Our study demonstrates that the SHINERS protocol developed in this work qualifies this Raman method as a pioneering approach with unique opportunities for in situ structure and reactivity studies at well-defined electrochemical solid/liquid interfaces.

Stripping voltammetric detection of insulin at liquid–liquid microinterfaces in the presence of bovine albumin

Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) provides a platform for label-free detection of biomolecules. In this study, adsorptive stripping voltammetry (AdSV) was implemented at an array of microscale ITIES for the detection of the antidiabetic hormone insulin. By exploiting the potential-controlled adsorption of insulin at the ITIES, insulin was detected at 10 nM via subsequent voltammetric desorption. This is the lowest detected concentration reported to-date for a protein by electrochemistry at the ITIES. Surface coverage calculations indicate that between 0.1 and 1 monolayer of insulin forms at the interface over the 10-1000 nM concentration range of the hormone. In a step toward assessment of selectivity, the optimum adsorption potentials for insulin and albumin were determined to be 0.900 V and 0.975 V, respectively. When present in an aqueous mixture with albumin, insulin was detected by tuning the adsorption potential to 0.9 V, albeit with reduced sensitivity. This provides the first example of selective detection of one protein in the presence of another by exploiting optimal adsorption potentials. The results presented here provide a route to the improvement of detection limits and achievement of selectivity for protein detection by electrochemistry at the ITIES.

In Situ Scanning Tunneling Microscopy Investigation of Subphthalocyanine and Subnaphthalocyanine Adlayers on a Au(111) Electrode

The adsorption behaviors of subphthalocyanine (SubPc) and subnaphthalocyanine (SubNc) on the Au(111) surface were investigated by electrochemical scanning tunneling microscopy (ECSTM). Two types of ordered adlayer structures of SubPc were observed at 550 mV versus the reversible hydrogen electrode (RHE). All of the SubPc molecules take the Cl-down adsorption configuration on Au(111) in both structures. The ordered adlayers exist in the potential range between 350 and 650 mV. The SubNc molecules adsorb on Au(111) in a less-ordered pattern than the SubPc molecules. The present work provides direct evidence for understanding the potential-controlled adsorption behaviors of SubPc and SubNc on the Au(111) surface.

EC-STM Study of Potential-Controlled Adsorption of Substituted Pyrimidinethiol on Au(111)

The ability of the pyrimidine derivatives to form numerous complexes and supramolecular assemblies makes them suitable for the construction of new functional surfaces. Therefore, in this paper, the adsorption behavior of 4-hydroxy-6-(trifluoromethyl)pyrimidine-2-thiol (HTPT) on a Au(111) surface has been investigated using electrochemical scanning tunneling microscopy (EC-STM). High-resolution imaging revealed that the HTPT molecules organize on a gold surface producing a highly ordered monolayer consistent with a (4 x radical3)R-30(0) superstructure. It has been observed that the arrangement of the molecules, as well as their orientation with respect to the substrate, remains stable over a relatively broad potential range from -0.40 to 0.55 V. It has been demonstrated that the presence of the functional groups attached to the aromatic ring affects the final structure of the HTPT adlayer, giving rise to the formation of the assembly with a uniform orientation of the molecules on the substrate.

Microcantilever-Based Sensors: Effect of Morphology, Adhesion, and Cleanliness of the Sensing Surface on Surface Stress

The surface stress response of micromechanical cantilever-based sensors was studied as a function of the morphology, adhesion, and cleanliness of the gold sensing surface. Two model systems were investigated: the adsorption of alkanethiol self-assembled monolayers at the gas-solid interface and the potential-controlled adsorption of anions at the liquid-solid interface. The potential-induced surface stress, on a smooth and continuous polycrystalline Au(111)-textured microcantilever in 0.1 M HClO4, is in excellent agreement with macroscopic Au(111) single-crystal electrode results. It is shown that ambient contaminants on the sensing surface dramatically alter the surface stress-potential response. This observation can be misinterpreted as evidence that for polycrystalline Au(111) microcantilever electrodes, surface stress is dominated by surface energy change. Results for anions adsorption on gold are in contrast to the gas-phase model system. We demonstrate that the average grain size of the gold sensing surface strongly influences the magnitude of the surface stress change induced by the adsorption of octanethiol. A 25-fold amplification of the change in surface stress is observed on increasing the average gold grain size of the sensing surface from 90 to 500 nm.

Dewetting phenomenon: Interfacial water structure at well-organized alkanethiol-modified gold–aqueous interface

The interfacial properties at well-ordered short-chain alkanethiol monolayer–aqueous interfaces are probed to understand the water structure near a hydrophobic surface. Monolayers of hexanethiol on highly oriented gold substrates have been prepared by various methods such as adsorption from alcoholic solution of the thiol, adsorption from neat thiol, and potential-controlled adsorption. The compactness and crystallinity of the monolayer have been probed using reflection–absorption infrared spectroscopy (RAIRS), atomic force microscopy (AFM), quartz crystal microbalance (QCM), and electrochemical techniques. The presence of a thin layer of solvent with reduced density/dielectric constant (termed “drying transition”) close to the methyl groups is identified. This is based on reduced interfacial capacitance observed in the presence of an aqueous electrolyte solution as compared to the expected value for a well-ordered monolayer–aqueous interface. Atomic force microscopy allows the determination of the variation in the dielectric constant of the solvent medium as a function of distance from the monolayer head group. The thickness of the transition layer (interphase) is found to be approximately 2 nm. The phenomenon of drying transition is not unique to water; preliminary studies indicate that formamide, which has a two-dimensional hydrogen-bonded network, shows similar characteristics.

Enhanced thermal stability and structural ordering in short chain n-alkanethiol monolayers on gold probed by vibrational spectroscopy and EQCM

Monolayers of alkanethiols with varied chain lengths, CH3(CH2)nSH where n=3, 5, and 7, on gold substrates have been prepared by adsorption from (1) neat thiol, (2) millimolar thiol solution in alcohol (conventional method), and (3) potential-controlled adsorption. Reflection absorption infrared spectroscopy (RAIRS) and electrochemical quartz crystal microbalance (EQCM) have been used to characterize the integrity of the monolayers. Methylene and methyl stretching modes along with C-S stretching modes have been used as benchmarks to follow the order-disorder transitions in the monolayer structure, in the temperature range from 25 to 175 degrees C. Monolayers adsorbed from neat thiol show superior quality in terms of stability and structural arrangement. Short chain thiols with n=3, 5, and 7 do show substantial stability. The possibility of multilayer formation is ruled out by EQCM studies comparing the frequency and mass change associated with the monolayer desorption. Self-assembled monolayers (SAMs) adsorbed under potential control behave very similarly to the monolayers adsorbed from neat thiol as far as stability and structural orientation are concerned, irrespective of the chain length. The adsorption from neat thiol gets rid of the solvent-induced defects and arrests the propagation of defects under temperature constraints.

Electrochemically controlled formation of a silicate membrane at a liquid ∣ liquid interface

A new method is described for the preparation of a silicate membrane. Its principal idea lies in the separation of the template and precursor ions participating in sol–gel processes, by the interface between two immiscible electrolyte solutions (ITIES). The sol–gel process is initiated by potential-controlled adsorption of the template ions at the interface and their transfer from the organic to the aqueous phase. Association of the positive template ions with the silicate ions at the aqueous side of the interface produces neutral species which condense and form a compact layer at the interface. The thickness of the layer can be controlled by the magnitude of the electrical charge passed.

Comparison of nanotube structures constructed from alpha-, beta-, and gamma-cyclodextrins by potential-controlled adsorption.

"Nanotube" structures of the alpha-, beta-, and gamma-cyclodextrins (CyD's), which are similar to that of CyD-polyrotaxane, were constructed by potential-controlled adsorption onto Au(111) surfaces in sodium perchlorate solution without a threaded polymer. CyD molecules adsorbed randomly on bare Au(111) surfaces without potential control and the desorption of CyD's from Au surfaces was observed at a negative potential of less than -0.60 V versus SCE. On the other hand, in the specific range between these potentials, ordered molecular arrays with "nanotube" structures of the CyD's (alpha-, beta-, and gamma-CyD) were observed on Au(111). The range of potentials for formation of the "nanotube" structures of alpha-, beta-, and gamma-CyD was from -0.15 to -0.20 V, from -0.25 to -0.45 V, and from -0.22 to -0.45 V, respectively. beta- and gamma-CyD require a more negative potential for adsorption-induced self-organization (AISO) than alpha-CyD in order to weaken adsorption and induce self-organization. Furthermore, we have succeeded in the visualization of the dynamic process in solution, such as the self-ordering, and the destruction of the nanotube structure. These results indicate that control of the electrode potential facilitates management of the delicate balance of various interactions, resulting in the formation of two-dimensional supramolecular structures on the substrates.

2D-supramolecular arrangements of dibenzo-18-crown-6-ether and its inclusion complex with potassium ion by potential controlled adsorption.

The construction of self-organized adlayers of dibenzo-18-crown-6 and its inclusion complex with potassium ion on a Au(111) surface was independently achieved by potential-controlled adsorption. Highly-ordered adlayers both for the metal-free state and the complex were visualized by in-situ STM with sub-molecular visualization. The potassium ions in the complex were also clearly visualized.

A two-dimensional molecular network structure of trimesic acid prepared by adsorption-induced self-organization.

A two-dimensional molecular network of trimesic acid on Au(111) was visualized by in situ scanning tunneling microscopy with submolecular resolution. The supramolecular structures including an 'order to order' phase transition were constructed by precise potential-controlled adsorption based on adsorption-induced self-organization.

Adsorption-Induced Self-Organization of Calix[4]arene on Au(111) Surfaces

Abstract The construction and sub-molecular visualization of a self-organized adlayer of calix[4]arene on a Au(111) surface were achieved by potential-controlled adsorption and in situ STM. In situ STM images revealed that the self-organized molecular array consisted of a calix[4]arene dimer. The two molecules in a cone conformation with a sideways orientation were connected in a “head to head” orientation between the hydroxy groups.

Formation of the ‘nanotube’ structure of β-cyclodextrin on Au(III) surfaces induced by potential controlled adsorption

The self-organization of β-cyclodextrin (β-CyD) into a ‘nanotube’ structure, similar to that of CyD-polyrotaxane, was found to be induced by potential controlled adsorption on Au(III) surfaces in sodium perchlorate solution without a threaded polymer. In-situ scanning tunneling microscopy (STM) revealed that the cavities of β-CyD faced sideward not upward in the tubes. This ordered structure can form only under conditions where the potential is controlled (−0.45 V to −0.25 V vs. SCE). β-CyD molecules were in a disordered state on bare Au(III) surfaces without potential control (+0.00 V vs. SCE). In addition, the desorption of β-CyD from Au surfaces was observed at a negative potential of less than −0.60 V. In the range −0.45 to −0.25 V, β-CyD molecules formed ordered arrays on Au(111) surfaces. Furthermore, the discontinuity of potential control led to disordered phases and the destruction of the ‘tube’ structure. This indicates that by controlling the electrode potential a delicate balance of various interactions can be achieved, resulting in the self-organization of molecules on the surface.

Self-organization of α-cyclodextrin on Au(111) surfaces induced by potential controlled adsorption

Abstract Potential-induced self-organization of α-cyclodextrin (CyD) on Au(111) surfaces in sodium perchlorate has been investigated by using in-situ scanning tunneling microscopy (STM). Near the open circuit potential (approximately +0.0 V vs. SCE), the CyD molecule adsorbs randomly on bare Au(111) surfaces and the desorption of CyD from Au surfaces is observed at negative potentials less than −0.40 V. On the other hand, it was found that the CyD molecules formed an ordered array with a cylindrical structure on the Au(111) surface in the potential region from −0.20 to −0.15 V. The cylindrical structure in which the cavities of CyD face sideways proved that the structure of the adlayer is formed not by simple epitaxy but by self-organization of the molecules.

Potential-Controlled Adsorption at Chemically Modified Graphite

Abstract The use of some chemically modified graphites for separation is described. Chemically bonded sulfonic acid groups can apparently be added by the use of fuming sulfuric acid.