In situ scanning electrochemical microscopy (SECM) detection of metal dissolution during zinc corrosion by means of mercury sphere-cap microelectrode tips

This work presents a scanning electrochemical microscopy (SECM)-based in situ corrosion probing methodology that is capable of monitoring the release of zinc species in corrosion processes. It is based on the use of Hg-coated Pt microelectrodes as SECM tips, which offer a wider negative potential range than bare platinum or other noble-metal tips. This allows for the reduction of zinc ions at the tip to be investigated with low interference from hydrogen evolution and oxygen reduction from aqueous solutions. The processes involved in the corrosion of zinc during its immersion in chloride-containing solutions were successfully monitored by scanning the SECM tip, set at an adequate potential, across the sample either in one direction or in the X-Y plane parallel to its surface. In this way, it was possible to detect the anodic and cathodic sites at which the dissolution of zinc and the reduction of oxygen occurred, respectively. Additionally, cyclic voltammetry (CV) or constant potential measurements were used to monitor the release of zinc species collected at the tip during an SECM scan.     

Short-term influence of interfering ion activity change on ion-selective micropipette electrode potential; another factor that can affect the time needed for imaging in potentiometric SECM

The applicability of ion-selective microelectrodes (ISME) in scanning electrochemical microscopy (SECM) in the presence of potentially interfering ions is investigated. Decreasing the time needed for potentiometric SECM imaging is a key aim. It is generally accepted that the long response time of potentiometric cells with high impedance microelectrodes limits the imaging speed that can be obtained without distortion. In this study we show that a change in the activity of interfering ions can result in a short-term electrode potential transient that should be taken into consideration in parameter setting for SECM measurements. The activity step method was employed, using NH₄⁺-ion-measuring micropipettes. Solutions containing equal concentrations of NH₄⁺ ions with or without K⁺ ions were rapidly introduced and the short-term change in electrode potential was recorded. Rapid transient potential signals appeared following the K⁺ activity step in the range where the interfering K⁺ did not affect the long-term electrode potential. The possible influence of this on SECM applications is discussed.     

Constant-distance mode AC-SECM for the visualisation of corrosion pits

The local visualisation of corrosion pits has so far failed to convincingly prove the advantage of scanning microscopy in overcoming Abbey’s Limit. Thus, for imaging pit initiation and instant pit growth the resolution of the methods used for detecting the related changes in local electrochemical activity has to be significantly improved. Progress into this direction has been achieved by combining shear force based constant-distance mode scanning electrochemical microscopy (SECM) with the alternating current mode of this technique (AC-SECM). A very small corrosion pit was generated by inducing active corrosion underneath an accurately positioned 5 μm diameter Pt disk SECM tip that served as counter electrode in the direct mode of SECM. Before and after pit initiation, constant-distance mode AC-SECM images were recorded and subsequently used to identify the area of broken passivity. The results demonstrate the feasibility of the suggested approach to image actively corroding sites with overall dimensions of only a few micrometers. Combination of constant-distance scanning with AC-SECM is an important prerequisite for further improving spatial resolution by employing smaller-diameter micro- or even nano-electrodes as SECM tips.     

Enhanced zinc ion transport in gel polymer electrolyte: effect of nano-sized ZnO dispersion

The effect of the dispersion of zinc oxide (ZnO) nanoparticles in the zinc ion conducting gel polymer electrolyte is studied. Changes in the morphology/structure of the gel polymer electrolyte with the introduction of ZnO particles are distinctly observed using X-ray diffraction and scanning electron microscopy. The nanocomposites offer ionic conductivity values of >10^?3 S cm^?1 with good thermal and electrochemical stabilities. The variation of ionic conductivity with temperature follows the Vogel–Tamman–Fulcher behavior. AC impedance spectroscopy, cyclic voltammetry, and transport number measurements have confirmed Zn²⁺ ion conduction in the gel nanocomposites. An electrochemical stability window from ?2.25 to 2.25 V was obtained from voltammetric studies of nanocomposite films. The cationic (i.e., Zn²⁺ ion) transport number (t ₊) has been found to be significantly enhanced up to a maximum of 0.55 for the dispersion of 10 wt.% ZnO nanoparticles, indicating substantial enhancement in Zn²⁺ ion conductivity. The gel polymer electrolyte nanocomposite films with enhanced Zn²⁺ ion conductivity are useful as separators and electrolytes in Zn rechargeable batteries and other electrochemical applications.     

Micropotentiometric mapping of local distributions of Zn2+ relevant to corrosion studies

A Zn²⁺-selective microelectrode is developed and adopted for determination of Zn²⁺ in the course of corrosion processes. Details of construction are given, together with a preliminary characterization of the electrode’s properties. Successful application to examples of zinc dissolution, zinc electroplating and corrosion in defects of coated galvanised steel shows the suitability of this microelectrode for materials science and corrosion research.     

Probing TCNQ‐mediated Metal Reduction Reactions at Liquid‐liquid Interface with SECM

Metal reduction at the interface between two immiscible electrolyte solutions (ITIES) has been studied with scanning electrochemical microscopy (SECM). Metal cations in the aqueous phase are reduced by 7,7,8,8‐tetracyanoquinodimethane anion (TCNQ^?) residing in the oil phase, methyl isobutyl ketone (MIBK). TCNQ^? is formed at the SECM tip by reducing TCNQ, which results in a positive feedback loop between the tip and the ITIES when an electron is donated to a metal cation. The effect of the Galvani potential difference on the rate of the interfacial electron transfer was investigated, establishing the potential difference either by an additional substrate electrode in the aqueous phase or by an a common ion in both phases. It is shown that the Galvani potential difference as a driving force does enable TCNQ^? mediated Cu²⁺ reduction. Finite element method (FEM) simulations were run to provide information on the reaction kinetics and stoichiometry.     

Binding of the Zn2+ ion to ferric uptake regulation protein from E. coli and the competition with Fe2+ binding: a molecular modeling study of the effect on DNA binding and conformational changes of Fur

The three dimensional structure of Ferric uptake regulation protein dimer from E. coli, determined by molecular modeling, was docked on a DNA fragment (iron box) and Zn²⁺ ions were added in two steps. The first step involved the binding of one Zn²⁺ ion to what is known as the zinc site which consists of the residues Cys 92, Cys 95, Asp 137, Asp141, Arg139, Glu 140, His 145 and His 143 with an average metal-Nitrogen distance of 2.5 Å and metal-oxygen distance of 3.1–3.2 Å. The second Zn²⁺ ion is bound to the iron activating site formed from the residues Ile 50, His 71, Asn 72, Gly 97, Asp 105 and Ala 109. The binding of the second Zn²⁺ ion strengthened the binding of the first ion as indicated by the shortening of the zinc-residue distances. Fe²⁺, when added to the complex consisting of 2Zn²⁺/Fur dimer/DNA, replaced the Zn²⁺ ion in the zinc site and when a second Fe²⁺ was added, it replaced the second zinc ion in the iron activating site. The binding of both zinc and iron ions induced a similar change in Fur conformations, but shifted residues closer to DNA in a different manner. This is discussed along with a possible role for the Zn²⁺ ion in the Fur dimer binding of DNA in its repressor activity. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ex Situ Scanning Electrochemical Microscopy (SECM) Investigation of Bismuth‐ and Bismuth/Lead Alloy Film‐Modified Gold Electrodes in Alkaline Medium

Scanning electrochemical microscopy (SECM) in feedback mode was employed to characterise the reactivity and microscopic peculiarities of bismuth and bismuth/lead alloys plated onto gold disk substrates in 0.1 mol L^?1 NaOH solutions. Methyl viologen was used as redox mediator, while a platinum microelectrode was employed as the SECM tip. The metal films were electrodeposited ex situ from NaOH solutions containing either bismuth ions only or both bismuth and lead ions. Approach curves and SECM images indicated that the metal films were conductive and locally reactive with oxygen to provide Bi³⁺ and Pb²⁺ ions. The occurrence of the latter chemical reactions was verified by local anodic stripping voltammetry (ASV) at the substrate solution interface by using a mercury‐coated platinum SECM tip. The latter types of measurements allowed also verifying that lead was not uniformly distributed onto the bismuth film electrode substrate. These findings were confirmed by scanning electron microscopy images. The surface heterogeneity produced during the metal deposition process, however, did not affect the analytical performance of the bismuth coated gold electrode in anodic stripping voltammetry for the determination of lead in alkaline media, even in aerated aqueous solutions. Under the latter conditions, stripping peak currents proportional to lead concentration with a satisfactory reproducibility (within 5 % RSD) were obtained.     

Effect of metal cations on corrosion behavior and surface film structure of the A3003 aluminum alloy in model tap waters

The effect of the metal cations, Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, and Ni²⁺, on the oxide film structure and morphology changes during long-term immersion corrosion tests of aluminum alloy (A3003) in model tap waters was investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy. The effect of the metal cations on the corrosion behavior was also investigated with mass change and electrochemical tests. The hardness of the metal cations, X, based on the hard and soft acids and bases (HSAB) concept was applied to explain the effect of metal cations on the passive oxide film structure and corrosion resistance. The mass change rate and corrosion current density decreased with increasing metal cation hardness. The XPS results showed that hard metal cations like Zn²⁺ and Ni²⁺ were incorporated in the oxide films, while the four soft metal cations were not incorporated in the oxide films. The results are in good agreement with those which could be expected from the HSAB hardness of the metal cations.     

Impacts of Forced Convection Generated via High Precision Stirring on Scanning Electrochemical Microscopy Experiments in Feedback Mode

In this study, the effects of forced convection on scanning electrochemical microscopy (SECM) experiments in feedback mode using ferrocenemethanol as redox mediator are presented. Forced convection, which enhances the mass transfer inside the system, was generated via an electrical high precision stirrer integrated into the SECM setup. A thin‐film interdigitated array electrode serving as model substrate was investigated with probe scan curves in z‐direction and SECM imaging in constant height mode utilizing ultramicroelectrodes (UME) with diameters (d_probe) of 25 μm and 12.5 μm. It was found that forced convection increased the overall current during SECM imaging without distorting distinctive features of the imaged structure when working with a 25 μm UME at substrate‐to‐tip distances of 14 μm and 11 μm. Furthermore, the electrochemical contrast was improved under hydrodynamic conditions for a substrate‐to‐tip distance of 11 μm and scan rates of 5 μm s^?1, 10 μm s^?1, 20 μm s^?1 and 40 μm s^?1. When further decreasing the gap between the UME and the substrate to 9 μm almost no effects of the forced convection were observed. Consequently, for a 25 μm UME, forced convection led to higher currents and improved performance during SECM experiments in feedback mode at substrate‐to‐tip distances of 14 μm and 11 μm, whereas no effects were observed for a 12.5 μm UME at a distance of 8 μm.     

Auger electron spectroscopic analysis of corrosion products formed on A3003 aluminum alloy in model fresh water with different Zn2+ concentration

The corrosion morphology and composition of corrosion products of A3003 formed in model fresh water with different Zn²⁺ concentrations were investigated by immersion tests combined with surface observations and analysis using an Auger electron spectroscope (AES). The cross-sectional AES observations showed that the thickness of the corrosion product layer formed on A3003 decreases with increases in the Zn²⁺ concentration of the model fresh water. A cross-sectional AES point analysis suggested that the corrosion products formed on the A3003 in the Zn²⁺ containing model fresh water (Zn²⁺ > 0.1 mM) have a multilayered structure and that the inner of Zn-rich layer would have high corrosion protective properties.     

Zinc-induced conformational changes in the DNA-binding domain of the vitamin D receptor determined by electrospray ionization mass spectrometry

Electrospray ionization mass Spectrometry (ESI-MS) was used to measure conformational changes within the DNA-binding domain of the vitamin D receptor (VDR DBD) upon binding zinc (Zn²⁺). As increasing concentrations of Zn²⁺ were added to the VDR DBD, a gradual shift in the mass envelope to lower charge states was observed in the multiply charged spectrum. The shift in the charge states was correlated to changes observed in the far-ultraviolet circular dichroic (far-UV CD) spectrum of the protein as it was titrated with Zn²⁺. Both the multiply charged ESI and far-UV CD spectra of the Zn²⁺-titrated protein show that the binding of the first Zn²⁺ ion to the protein results in very little conformational change in the protein. The binding of a second Zn²⁺ ion resulted in a significant alteration in the structure of the protein as indicated by changes in both the multiply charged ESI and far-UV CD spectra. Much smaller changes were seen within the multiply charged ESI or far-UV CD spectra upon increasing the Zn²⁺ concentration beyond 2 mol/mol of protein. The results presented indicate that ESI-MS in combination with CD is a powerful method to measure gross conformational changes induced by the binding of metals to metalloproteins.     

Convenient extraction method for quantification of thin zinc patina layers

Synthetic zinc patina was grown on galvanized steel sheets in supercritical carbon dioxide atmosphere. Different patina compounds were dissolved and quantified using a stepwise immersion and dissolution procedure. The distinct patina components, namely anhydrous zinc carbonate (a dense layer adjacent to metallic zinc) and zinc hydroxy carbonate (nanowires on the surface), were dissolved in glycine solutions, followed by quantification of Zn²⁺ in the solutes by X‐ray fluorescence. The zinc hydroxy carbonate nanowires were readily glycine soluble, and the anhydrous zinc carbonate showed scarce glycine solubility, which enabled their selective quantification. The amount of the remaining (anhydrous) zinc carbonate after glycine extraction was determined from the glycine‐soluble zinc oxide after calcination (heat treatment for 10 minutes at 350°C). The results were verified by scanning electron microscopy imaging and Fourier transform infrared spectroscopy measurements.     

Salicylaldehyde hydrazones as fluorescent probes for zinc ion in aqueous solution of physiological pH

Salicylaldehyde hydrazones of 1 and 2 were synthesized and their potential as fluorescent probes for zinc ion was investigated in this paper. Both of the probes were found to show fluorescence change upon binding with Zn²⁺ in aqueous solutions, with good selectivity to Zn²⁺ over other metal ions such as alkali/alkali earth metal ions and heavy metal ions of Pb²⁺, Cd²⁺ and Hg²⁺. They showed 1:2 metal-to-ligand ratio when their Zn²⁺ complex was formed. By introducing pyrene as fluorophore, 2 showed interesting ratiometric response to Zn²⁺. Under optimal condition, 2 exhibited a linear range of 0-5.0 μM and detection limit of 0.08 μM Zn²⁺ in aqueous buffer, respectively. The detection of Zn²⁺ in drinking water samples using 2 as fluorescent probe was successful.     

A highly sensitive fluorescent probe for tracking intracellular zinc ions and direct imaging of prostatic tissue in mice

A highly sensitive fluorescent sensor ZnDN was designed, synthesized and used for tracking intracellular zinc ions in various living cells and direct imaging of prostatic tissue in mice. ZnDN was prepared from the heterocyclic-fused naphthalimide fluorophore, and the zinc receptor, N,N-bis(2-pyridylmethyl)ethylenediamine (BPEN). Upon addition of Zn²⁺ to the solutions of ZnDN, a remarkable fluorescence enhancement was observed, which could be attributed to the photo-induced electron transfer (PET) mechanism. Since ZnDN exhibited high sensitivity toward Zn²⁺ in phosphate buffer solution, with a limit of detection of 4.0 × 10⁻⁹ mol/L, it was further applied for the imaging of exogenous and endogenous Zn²⁺ in different living cells. Living cells imaging experiments suggested that ZnDN could image the changes of intracellular free zinc ions, and could be used for two-photon imaging. Moreover, flow cytometry suggested that ZnDN could distinguish cancerous prostate cells from normal cells. Animal experiments indicated that ZnDN had the potential in imaging prostate tissue in vivo.     

Study of some metallic silicates as ion-exchangers

The extraction of ammine complexes of Cu²⁺, Cd²⁺, Ni²⁺, Co²⁺ and Zn²⁺ by synthesised manganese, nickel and zinc silicates has been studied at different pH and ionic concentrations in the external solution. It has been found that the uptake of the metal ion Co²⁺, Cu²⁺ and Zn²⁺ increases with increase in pH of the external solution, attains a maximum and then decreases. However, the uptake for Cd²⁺ and Ni²⁺ increases continuously. The q_A values of all the silicates increases with the increase in the concentration of the exchanging ion and its order for the investigated metal ions is Ni²⁺ < Co²⁺ < Cd²⁺ < Zn²⁺ < Cu²⁺.     

A ratiometric fluorescent probe for zinc ions based on the quinoline fluorophore

A ratiometric fluorescent zinc probe 1 of carboxamidoquinoline with a carboxylic acid group was designed and synthesised. Probe 1 exhibits high selectivity for sensing Zn²⁺; about a 13-fold increase in fluorescence emission intensity and an 82?nm red-shift of fluorescence emission are observed upon binding Zn²⁺ in EtOH/H₂O (1?:?1, V/V) solution. The ratiometric fluorescence response is attributed to the 1?:?1 complex formation between probe 1 and Zn²⁺ which has been utilised as the basis for the selective detection of Zn²⁺. The analytical performance characteristics of the proposed Zn²⁺-sensitive probe were investigated. The linear response range covers a concentration range of Zn²⁺ from 2.0?×?10^?6 to 5.0?×?10^?5?mol?L^?1 and the detection limit is 2.7?×?10^?7?mol?L^?1. The determination of Zn²⁺ in both tap and river water samples shows satisfactory results.     

Photophysical Properties of Metal Ion Functionalized NaY Zeolite

A series of luminescent ion exchanged zeolite are synthesized by introducing various ions into NaY zeolite. Monometal ion (Eu³⁺, Tb³⁺, Ce³⁺, Y³⁺, Zn²⁺, Cd²⁺, Cu²⁺) exchanged zeolite, rare‐earth ion (Eu³⁺, Tb³⁺, Ce³⁺) exchanged zeolite modified with Y³⁺ and rare‐earth ion (Eu³⁺, Tb³⁺, Ce³⁺) exchanged zeolite modified with Zn²⁺ are discussed here. The resulting materials are characterized by Fourier transform infrared spectrum radiometer (FTIR), XRD, scanning electronic microscope (SEM), PLE, PL and luminescence lifetime measurements. The photoluminescence spectrum of NaY indicates that emission band of host matrix exhibits a blueshift of about 70 nm after monometal ion exchange process. The results show that transition metal ion exchanged zeolites possess a similar emission band due to dominant host luminescence. A variety of luminescence phenomenon of rare‐earth ion broadens the application of zeolite as a luminescent host. The Eu³⁺ ion exchanged zeolite shows white light luminescence with a great application value and Ce³⁺ exchanged zeolite steadily exhibits its characteristic luminescence in ultraviolet region no matter in monometal ion exchanged zeolite or bimetal ions exchanged zeolite.     

Study on Electrocatalytic Activity of Platinum for Hydrogen Oxidation in Acidic Solution by Scanning Electrochemical Microscopy

The electrocatalytic activity of platinum for hydrogen oxidation in 0.01 M H₂SO₄ + 0.1 MNa₂SO₄ solution has been investigated by scanning electrochemical microscopy (SECM) technique. The cyclic voltammogram (CV), approach curve, area scan imaging and chronoamperometric methods have been used. The results indicate that the imaging capability of the SECM feedback mode can be used more efficiently to visually identify materials' electrocatalytic activity, compared with the approach curve method for identification of the conductive or insulating nature of a surface. The SECM imaging method has demonstrated the effects of Pt substrate potential on the electrocatalytic oxidation of hydrogen under a constant tip potential. It is found that the more positive the Pt substrate potential, the lower the electrocatalytic activity of the Pt. Furthermore, the chronoamperometric results support the variation of the electrocatalytic activity with the Pt substrate potential as well.     

Fabrication of Active Horseradish Peroxidase Micropatterns with a High Resolution by Scanning Electrochemical Microscopy

We used a new reactive species OH^? to fabricate active horseradish peroxidase (HRP) micropatterns with a high resolution by scanning electrochemical microscopy (SECM) coupled with a carbon fiber disk electrode as the SECM tip. In this method, except for active HRP micropatterns predesigned other regions on a HRP‐immobilized substrate were deactivated by OH^? generated at the tip held at ?1.7 V in 1.0 mol/L KCl containing 2.0×10^?3 mol/L benzoquinone (BQ) (pH 8.0). The feedback mode of SECM with a tip potential of ?0.2 V was used to characterize the active HRP micropatterns in 1.0 mol/L KCl containing 2.0×10^?3 mol/L BQ and 2.0×10^?3 mol/L H₂O₂.