Electrochemical formation of Se atomic layers on Au(111) surfaces: the role of adsorbed selenate and selenite

Materials interacting with inorganic selenium from the perspective of electrochemical sensing

Inorganic selenium, the most common form of harmful selenium in the environment, can be determined using electrochemical sensors, which are compact, fast, reliable and easy-to-operate devices. Despite progress in this area, there is still significant room for developing high-performance selenium electrochemical sensors. To achieve this, one should take into account (i) the electrochemical process that selenium undergoes on the electrode; (ii) the valence state of selenium species in the sample and (iii) modification of the sensor surface by a material with high affinity to selenium. The goal of this review is to provide a knowledge base for these issues. After the Introduction section, mechanisms and principles of the electrochemical reduction of selenium are introduced, followed by a section introducing the modification of electrodes with materials interacting with selenium and a section dedicated to speciation methods, including the reduction of non-detectable Se(VI) to detectable Se(IV). In the following sections, the main types of materials (metallic, polymers, hybrid (nano)materials…) interacting with inorganic selenium (mostly absorbents) are reviewed to show the diversity of properties that may be endowed to sensors if the materials were to be used for the modification of electrodes. These features for the main material categories are outlined in the conclusion section, where it is stated that the engineered polymers may be the most promising modifiers.

Template-Free Electrochemical Deposition of t-Se Nano- and Sub-micro Structures With Controlled Morphology and Dimensions

Selenium, depending on its crystal structure, can exhibit various properties and, as a result, be used in a wide range of applications. However, its exploitation has been limited due to the lack of understanding of its complex growth mechanism. In this work, template-free electrodeposition has been utilized for the first time to synthesize hexagonal-selenium (t-Se) microstructures of various morphologies at 80°C. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) revealed 5 reduction peaks, which were correlated with possible electrochemical or chemical reaction related to the formation of selenium. Potentiostatic electrodeposition using 100 mM SeO₂ showed selenium nanorods formed at−0.389 V then increased in diameter up to −0.490 V, while more negative potentials (-0.594 V) induced formation of sub-micron wires with average diameter of 708 ± 116 nm. Submicron tubes of average diameter 744 ± 130 nm were deposited at −0.696 V. Finally, a mixture of tubes, wires, and particles was observed at more cathodic potential due to a combination of nucleation, growth, dissolution of structures as well as formation of amorphous selenium via comproportionation reaction. Texture coefficient as a function of applied potential described the preferred orientation of the sub-microstructures changed from (100) direction to more randomly oriented as more cathodic potentials were applied. Lower selenium precursor concentration lead to formation of nanowires only with smaller average diameters (124 ± 42 nm using 1 mM, 153 ± 46 nm using 10 mM SeO₂ at −0.389 V). Time-dependent electrodeposition using 100 mM selenium precursor at −0.696 V explained selenium was formed first as amorphous, on top of which nucleation continued to form rods and wires, followed by preferential dissolution of the wire core to form tubes.

Surface morphology and adlayer structure of Se on Rh(111)

The deposition of Se from SeO₃^2- solutions was examined in the submonolayer regime by cyclic voltammetry, scanning tunneling microscopy and atomic force microscopy. Up to a coverage of ca. 0.5 (Se atoms to substrate atoms) a smooth adlayer is obtained with a 2 × √3 structure. When the coverage is increased, at around 0.55 V further deposition is paralleled by a roughening starting at the monoatomic steps. The adsorbed Se is stable in the SeO₃^2- free solution. For coverages below 0.25, separate domains for the Se covered regions and Se free regions were observed for potentials above 0.6 V. Since this is the potential of the spike corresponding to adsorption of OH at the clean Rh(111) surface in HClO₄, we have to assume that Se and OH adsorb in separate domains. At lower potentials, where OH is desorbed, Se spreads over the complete surface which then appears completely smooth in the STM images. When the coverage is about 0.25 or above, the roughening is also observed in SeO₃^2- free solution, demonstrating that the rough structures are not due to disordered deposition, but really due to a roughening by place exchange.

Fabrication of free-standing graphene paper decorated with flower-like PbSe0.5S0.5 structures

Free-standing graphene/PbSe_0.5S_0.5 paper was fabricated by one-pot electrodeposition on an rGO paper electrode from a solution containing saturated PbS and PbSe.

Formation, characterization, and stability of methaneselenolate monolayers on Au(111): an electrochemical high-resolution photoemission spectroscopy and DFT study

We investigated the mechanism of formation and stability of self-assembled monolayers (SAMs) of methaneselenolate on Au(111) prepared by the immersion method in ethanolic solutions of dimethyl diselenide (DMDSe). The adsorbed species were characterized by electrochemical measurements and high-resolution photoelectron spectroscopy (HR-XPS). The importance of the headgroup on formation mechanism and the stability of the SAMs was addressed by comparatively studying methaneselenolate (MSe) and methanethiolate (MT) monolayers. Density Functional Theory (DFT) calculations were performed to identify the elementary reaction steps in the mechanisms of formation and decomposition of the monolayers. Reductive desorption and HR-XPS measurements indicated that a MSe monolayer is formed at short immersion times by the cleavage of the Se-Se bond of DMDSe. However, the monolayer decomposes at long immersion times at room temperature, as evidenced by the appearance of atomic Se on the surface. The decomposition is more pronounced for MSe than for MT monolayers. The MSe monolayer stability can be greatly improved by two modifications in the preparation method: immersion at low temperatures (-20 °C) and the addition of a reducing agent to the forming solution.

Investigations of the pore formation in the lead selenide films using glacial acetic acid- and nitric acid-based electrolyte

We report a novel synthesis of porous PbSe layers on Si substrates using anodic electrochemical treatment of PbSe/CaF2/Si(111) epitaxial structures in an electrolyte solution based on glacial acetic acid and nitric acid. Electron microscopy, x-ray diffractometry, and local chemical microanalysis investigations results for the porous layers are presented. Average size of the synthesized mesopores with ~1010 cm-2 surface density was determined to be 22 nm. The observed phenomenon of the active selenium redeposition on the mesopore walls during anodic treatment is discussed.

Investigation into the voltammetric behaviour and detection of selenium(IV) at metal electrodes in diverse electrolyte media

The voltammetric behaviour of selenium(IV) was studied at platinum and gold electrodes in sulphuric acid, perchloric acid and potassium chloride media as a basis for its voltammetric detection. The best voltammetric behaviour was recorded at gold electrodes with perchloric acid as the supporting electrolyte. The concomitant presence of metals, such as copper or lead, and of model biomolecules, such as bovine serum albumin, in the solution resulted in a deterioration of the electrochemical response for selenium(IV). Quantitative detection of selenium(IV) by square wave anodic stripping voltammetry at both a millimetre-sized gold disc electrode and a microband electrode array revealed linear responses to selenium concentration in the ranges 5-15 μM and 0.1-10 μM, respectively, with 60s preconcentration. The sensitivities were 6.4 μA μM(-1) cm(-2) and 100 μA μM(-1) cm(-2) at the disc and the microband array, respectively. The detection limit at the microband electrode array was 25 nM, illustrating the potentiality of such microelectrodes for the development of mercury-free analytical methods for the trace detection of selenium(IV).

Ultratrace Determination of Inorganic Selenium without Signal Calibration

To more readily evaluate the complex biogeochemistry of selenium, a flow-through electrochemical method was developed that can accurately determine Se(IV) concentrations in aqueous samples to part-per-trillion levels without signal calibration. Stripping methods were used in conjunction with a high-efficiency, flow-through cell. The cell was designed with a novel gold working electrode that was separated from a porous counter electrode by a Nafion membrane. Because this design permitted exhaustive deposition of selenium from the sample stream as well as efficient coulometric stripping, determinations obeyed Faraday's law over a reasonably wide range of operating conditions. The method had a minimum quantitation limit of approximately 8 ng and a maximum limit of 800 ng for Se(IV). It was reliable for sample volumes as small as 0.5 mL and as high as 20 mL, thereby allowing determinations from part-per-million to just below part-per-billion levels. Interferences from Cu(II) and arsenate were evident, but only when these species were present at concentrations exceeding 10 mg.L-1. Overall, the method had a performance comparable to hydride-generation atomic absorption spectrometry but with less cumbersome equipment and freedom from calibration.

The electrochemical reaction mechanism of selenocystine on selenium-gold film modified glassy carbon electrode

A simple and selective voltammetric method based on selenium-gold film modified glassy carbon electrode has been developed for investigating electrochemical reaction mechanism of selenocystine. With N2 saturated, redox reactions between selenocystine (SeC) and selenocysteine (SeCys) were judged to be two simple electron-transfer processes. With air saturated, the reduction reaction was diagnosed to be EC catalytic reaction (the chemical oxidation reaction of the SeCys by O2 (C) following the electron-transfer reaction (E)) and oxidation reaction is a simple electron-transfer process. With pure O2 saturated, only reduction peak was observed and the reaction was judged to be EC catalytic reaction. The electron-transfer numbers of redox reaction were calculated to be 2 by chronocoulometry and rotating disk electrode.

O1-[6-(Methylselanyl)hexanoyl]glycerol as an Anchor for Self-Assembly of Biological Compounds at the Gold Surface

Adsorption ofO1-[6-(methylselanyl)hexanoyl]glycerol (SeOG) on the gold surface was investigated by cyclic voltammetry, phase-sensitive AC voltammetry, electrochemical impedance spectroscopy and piezoelectric microgravimetry. SeOG adsorption results in a stable and compact surface layer with the coverage degree close to unity for an adsorption time of 30 to 80 min and 4.6 mM SeOG acetonitrile solution. Such a layer displays minute defects (pinholes) with the radius of ca. 1-3 μm, separated by 6-50 μm intervals (depending on the adsorption time). The adsorbed compound undergoes anodic desorption in the gold oxide region and also undergoes a cathodic process leading to the removal of the surface layer. Both these processes are similar to those demonstrated by short-chain alkanethiols and have been interpreted as a indication for the conversion of the selena to selenol function as a result of a dissociative adsorption process. Apparently, the main component of the surface layer isO1-(6-selanylhexanoyl)glycerol that results by the cleavage of the C6-Se bond in SeOG. The two free hydroxy groups in SeOG allow to use it as a bridge for binding other compounds to the gold surface. This possibility was illustrated by building up surface layers of a carotenoid derivative (O1-(8'-apo-β-apo-caroten-8'-oyl)-O2-[6-(methylselanyl)hexanoyl]glycerol,II) or carotenoid- and phosphocholine-derivatized SeOG (O1-(8'-apo-β-caroten-8'-oyl)-O2-[6-(methylselanyl)hexanoyl]-O3-{[2-(trimethylammonio)ethoxy]phosphoryl}glycerol,III). The compoundIIIgenerates a less densely packed layer due to the constraints induced by the phosphocholine substituent. Each of these compounds undergoes anodic reactions that are typical of carotenoids in the adsorbed state. However, the polar and hydrophilic phosphocholine residue inIIIshifts the anodic peak to a less pozitive potential. SeOG allows therefore to tune the molecular environment of a surface attached compound by means of a suitable co-substituent.