Portable surface-enhanced Raman scattering sensor for rapid detection of aniline and phenol derivatives by on-site electrostatic preconcentration

A portable surface-enhanced Raman scattering (SERS) sensor is developed and applied to simultaneous detection of aniline and phenol derivatives in a label-free way with an electrostatic preconcentration technique to amplify the signals. A SERS-active substrate, silver-electrodeposited screen-printed electrodes (Ag-SPEs), is used for qualification and quantification of polar organic pollutants. Observation of SERS spectra at different potentials indicates that polar pollutants are selectively adsorbed on the Ag-SPEs at a given potential, suggesting that Ag-SPEs could selectively attract polar pollutants to an oppositely charged electrode at different potentials. Optimum SERS-active substrate was obtained when a potential of -0.15 V vs Ag/AgCl was applied on the SPEs in 0.1 M AgNO(3) solution for 10 min. Moreover, the effects of experimental variables such as the electrodeposition time and potential of Ag and preconcentration time of polar molecules on the SERS signals are presented. Under optimum conditions and with a 785 nm laser, the method is effective over a wide range of concentration (1 nM to 1 μM) for aniline and phenol derivatives. The novel method described herein presents a new detection regime for environmental pollutant analysis and also demonstrates simultaneous multiplexed detection of polar organic pollutants using convenient Ag-SPEs.     

Electrodeposition of Cu from acidic sulphate solutions in the presence of polyethylene glycol and chloride ions

A surface-enhanced Raman scattering (SERS) spectroelectrochemical investigation has been carried out during Cu electrodeposition from an acidic sulphate solution in the presence of Polyethylene glycol (PEG) and chloride ions. PEG-related bands are clearly visible at the open circuit potential (OCP) and during the electroplating process, showing that the polymer is stably adsorbed on the Cu surface in a wide cathodic potential window. Our experimental range was limitated by the occurrence of fluorescence at potentials more cathodic then − 200 mV versus Ag/AgCl, indicating the formation of fluorescent species by electrodic reaction. A tentative reactivity scheme for PEG is provided. The co-adsorption of chloride and sulphate species from copper (II) sulphate and sulphuric acid was highlighted as well.     

Poly(L-cysteine) as an electrochemically modifiable ligand for trace metal chelation

The short-chain (n approximately equal to 50) homopolymer poly(L-cysteine) (PLC) has been previously studied for use as a novel metal chelator. PLC exhibits reversible oxidation-reduction chemistry involving the thiol groups of the cysteine (Cys) residues. Previously, chemical oxidation of the PLC immobilized on silica showed that metal binding capacity was minimal in the oxidized state. In this study, Cys and PLC are immobilized on a glassy carbon disk electrode (GCE) to study these redox processes and how they impact metal binding and release. Voltammetric and chronoamperometric methods were employed to demonstrate nearly monolayer coverage of both immobilized Cys monomer and immobilized PLC on GCE. The PLC-GCE exhibited a maximum metal binding capacity for Cd2+ of approximately 11 Cd2+ ions/chain. No detectable metal binding capacity was observed for oxidized PLC. The bound metals were capable of being efficiently released through disulfide bond formation and tertiary structure changes by means of repetitive oxidative pulses. The Cys-modified electrode exhibited a metal binding capacity for Cd2+ of approximately 1 Cd2+/Cys. Oxidized Cys did retain a significant capacity following oxidation, likely as a result of complexation with the terminal carboxylate site and unoxidized thiols. A glycine (Gly)-modified electrode was also evaluated as an amino acid control. Minimal Cd2+ binding was observed. Further metal binding studies were conducted using PLC-GCE with single metal solutions of Co2+, Cu2+, Ni2+, and Pb2+, as well as a multimetal solution composed of equal concentrations of all five target metals. The observed metal binding trend was as follows: Cu2+ > Cd2+ > Ni2+ > Pb2+ > Co2+. All metals were quantitatively released upon oxidation of PLC using the same anodic potential, 600 mV vs Ag/AgCl.     

Electrochemical hydrogen evolution by use of a glass carbon electrode sandwiched with clay,poly(butylviologen) and hydrogenase

Electrochemical hydrogen evolution was investigated by use of a hydrogenase-modified electrode, which was constructed by immobilizing hydrogenase between two layers of montmorillonite clay and poly(butylviologen) mixture. The optimum conditions for hydrogen evolution were found to be in a slightly acid buffer solution (pH 4–6) with the applied biases corresponding to the reduction potentials of viologens (−700 to −800 mV vs. Ag/AgCl) and a moderate temperature.     

Nonideal electrochemical behavior of biomimetic iron porphyrins: interfacial potential distribution across multilayer films

The electrochemical behavior of multilayer films formed by iron porphyrins deposited on an edge plane graphite electrode has been examined under anaerobic conditions. In the scan rate interval (1-250 mV/s) where the electrode reaction is reversible, CV diagrams of these films demonstrate substantial deviations from ideality in broadening and separation of the peaks. A model that describes the observed behavior is proposed by taking into account the potential distribution at the electrode/film interface and the concentration dependence of surface activity coefficients. The peak separation is described in terms of the electric double layer that affects the potential difference driving the electrode reaction. The effective potential difference deviates from the applied value due to the potential distribution across the film. The interfacial potential distribution depends on the ionic concentration inside the film. When different ionic concentrations are assumed for oxidation and reduction, different shifts from the applied potential lead to a hysteresis of the peaks. The peak broadening is modeled by using the lattice theory expression for the surface activity coefficients. The model shows that the midpoint potentials of the redox centers depend on the ionic concentration inside the film. At low ionic concentrations, they are remarkably close to the midpoints of the cytochrome c oxidase heme a3/CuB site.     

Kinetic aspects of Donnan membrane technique for measuring free trace cation concentration

Addition of ion complexation ligands in the acceptor solution in the Donnan membrane technique (DMT) can lower its detection limit for free metal ion concentration in natural samples. In this paper, the influence of added ligands on the transport behavior of trace ions in DMT was studied using numerical and analytical models and experimental tests. The results show that addition of ligands in the acceptor can significantly influence the time to reach the Donnan membrane equilibrium. Depending on several factors, the flux can be controlled by the diffusion in the stagnant solution film at the solution-membrane interface, by the diffusion in the membrane, or by both. The conditions under which the diffusion in the solution film or in the membrane becomes the rate-limiting step are discussed and approximate analytical solutions for some special cases are presented. Very low concentrations of free metal ion can be measured using the ligand complexation DMT. Depending on the degree of complexation in the sample, the measurement can be based on either the Donnan membrane equilibrium (when the complexation degree is low) or the kinetic interpretation of the ion transport (when the complexation degree is high).     

A prototype electrochemical chromatographic column for use with proteins

We developed electrochemical hardware and media targeted for protein chromatography. Two types of stationary phases were investigated. The first comprised gold-plated stainless 316L beads coated with a self-assembled monolayer of 6-mercaptohexan-1-ol and was expected to behave like an ion-exchange resin in the presence of an electric field. The secondary stationary phase comprised the first stationary phase with further functionalization with immobilized heme moieties and was expected to behave like immobilized metal affinity resin. We tested apparatus with both stationary phases using ribonuclease A as a model protein and applied potentials from -0.3 to +0.3 V versus the saturated calomel electrode. Despite low binding capacities, we demonstrated that protein retention on both stationary phases could be controlled with an applied potential. The greatest extent of electromodulation was achieved with the mercaptohexanol-based ion-exchange media.     

Electrochemically modulated separation, concentration, and detection of plutonium using an anodized glassy carbon electrode and inductively coupled plasma mass spectrometry

Plutonium is shown to be retained on anodized glassy carbon (GC) electrodes at potentials positive of +0.7 V (vs Ag/AgCl reference) and released upon potential shifts to values negative of +0.3 V. This phenomenon has been exploited for the separation, concentration, and detection of plutonium by the coupling an electrochemical flow cell on-line with an ICPMS system. The electrochemically controlled deposition and analysis of Pu improves detection limits by analyte preconcentration and by matrix and isobaric ion elimination. Information related to the parametric optimization of the technique and hypotheses regarding the mechanism of electrochemical accumulation of Pu are reported. The most likely accumulation scenario involves complexation of Pu(IV) species, produced under a controlled potential, with anions retained in the anodization film that develops during the activation of the GC electrode. The release mechanism is believed to result from the reduction of Pu(IV) in the anion complex to Pu(III), which has a lower tendency to form complexes.     

In situ raman spectroscopy studies of metal ion complexation by 8-hydroxyquinoline covalently bound to silica surfaces

Raman spectroscopy is applied to an investigation of the interfacial chemistry of silica-immobilized 8-hydroxyquinoline (8HQ) for binding of metal ions over a wide range of solution conditions. Since the derivatized silica has a high specific binding capacity, the mass of silica equilibrated with solution needs to be small for studies of reactions with trace-level (microM) metal ions; otherwise, the solution volume required to reach equilibrium becomes excessive. To address this problem, a small-volume flow cell is designed for this work using a fiber-optic Raman probe inserted directly into the packed end of a microcolumn, allowing excitation and collection of Raman scattering from less than 10 mg of derivatized silica. This cell is attached to a flow system that allows control of solution conditions while the response of the 8HQ-silica material is acquired by continuous monitoring of Raman scattering from the sample. Raman spectra of the deprotonated, neutral, protonated, and copper-complexed forms of the ligand can be distinguished, allowing proton-transfer and metal ion binding reactions of the ligand to be investigated. To account for the effects of changing surface potential on these reactions, zeta-potential measurements are made on the 8HQ-silica particles under the same solution conditions that are employed in the Raman scattering measurements. The observed pH dependence of metal ion binding was corrected for the effect of surface potential using the Boltzmann equation, and the resulting equilibrium constant for binding of Cu2+ was independent of metal ion concentration over a 100-fold range from 30 microM to 5 mM.     

Zr/ZrO2 sensors for in situ measurement of pH in high-temperature and -pressure aqueous solutions

The aim of this study is to develop new pH sensors that can be used to test and monitor hydrogen ion activity in hydrothermal conditions. A Zr/ZrO2 oxidation electrode is fabricated for in situ pH measurement of high-temperature aqueous solutions. This sensor responds rapidly and precisely to pH over a wide range of temperature and pressure. The Zr/ZrO2 electrode was made by oxidizing zirconium metal wire with Na2CO3 melt, which produced a thin film of ZrO2 on its surface. Thus, an oxidation-reduction electrode was produced. The Zr/ZrO2 electrode has a good electrochemical stability over a wide range of pH in high-temperature aqueous solutions when used with a Ag/AgCl reference electrode. Measurements of the Zr/ZrO2 sensor potential against a Ag/AgCl reference electrode is shown to vary linearly with pH between temperatures 20 and 200 degrees C. The slope of the potential versus pH at high temperature is slightly below the theoretical value indicated by the Nernst equation; such deviation is attributed to the fact that the sensor is not strictly at equilibrium with the solution to be tested in a short period of time. The Zr/ZrO2 sensor can be calibrated over the conditions that exist in the natural deep-seawater. Our studies showed that the Zr/ZrO2 electrode is a suitable pH sensor for the hydrothermal systems at midocean ridge or other geothermal systems with the high-temperature environment. Yttria-stabilized zirconia sensors have also been used to investigate the pH of hydrothermal fluids in hot springs vents at midocean ridge. These sensors, however, are not sensitive below 200 degrees C. Zr/ZrO2 sensors have wider temperature range and can be severed as good alternative sensors for measuring the pH of hydrothermal fluids.     

Immobilized pH gradient gel cell to study the pH dependence of drug lipophilicity

An experimental setup to study the pH dependence of standard ion-transfer potentials at the water/NPOE interface is presented. The system is composed of a microhole generated by laser photoablation in a 12-microm polyethylene terephthalate film, the aqueous phase consisting of a commercial immobilized pH gradient gel reswelled in electrolyte solution and a droplet of organic solution. Two electrodes are used, an Ag/AgCl aqueous electrode and an Ag/AgTPBCl organic electrode. This setup is applied to the study of two ionizable compounds (pyridine, 2,4-dinitrophenol). Thermodynamic parameters such as the standard transfer potential, the Gibbs energy of transfer, and the partition coefficients of the ionized forms as well as the neutral forms of these drug compounds are evaluated by differential pulse voltammetry. The data obtained are summarized in ionic partition diagrams, which are a useful tool for predicting and interpreting the transfer behavior of ionized drugs at the liquid/liquid interfaces mimicking the biological membranes.     

银/氯化银电极用于监测混凝土中氯离子含量的研究

采用干湿循环渗透法,通过实验室制备的银/氯化银电极来监测混凝土中的氯离子含量,测试了电极的响应性能和长期稳定性,研究了总氯离子含量与自由氯离子含量之间、总氯离子含量与电位之间的关系。结果表明:银/氯化银电极电位能比较好地监测到混凝土中氯离子含量变化的过程,具有良好的长期稳定性能。总氯离子含量随自由氯离子含量的增加而增加,两者之间呈现幂函数的规律。总氯离子含量随电位上升而下降,两者之间呈指数函数的规律。     

Analysis of the influence of elastic deformation on the electrode potential of a metal cylinder in a medium

We propose a new efficient procedure for the determination of the shift of the electrode potential on the boundary of an active medium with the surface of a stretched metal cylinder and determine the distributions of the electric potentials inside the cylinder, over its surface, and in the medium. We also establish the formulas for the evaluation of the electrode potential depending on the level of mechanical stresses and compute its values for copper and steel specimens. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 1, pp. 47–50, January–February, 2000.     

Peroxidase model electrodes: heme peptide modified electrodes as reagentless sensors for hydrogen peroxide

A peroxidase model electrode was devised for reagentless sensing of hydrogen peroxide (H2O2). A small model molecule, which mimics the vicinity of the reaction center of a redox enzyme, can communicate electrochemically with an electrode. Heme nonapeptide (MW congruent to 1600) having peroxidase activity was adopted as a peroxidase model compound and was covalently immobilized on a tin oxide (SnO2) electrode as a roughly monomolecular layer. The modified electrode thus obtained responded to H2O2 at concentrations down to 10(-6) M without electron mediator or promoter, at a mild potential of +150 or +300 mV vs Ag/AgCl. In a batch system, the response reached a steady state in a few seconds. Measurements were possible also in a flow system with an assay time of 0.5-1.0 min/sample. The steady-state response of the electrode was kinetically analyzed.     

尿素改性碳纤维电场电极制备及电化学性能研究

以尿素为氮源,对碳纤维表面进行改性,制备了一种新型海洋电极,并测试了其电化学性能和电场响应性能。结果表明,碳纤维表面的含氧和含氮基团对其电化学性能具有显著的影响,与尿素反应并在450℃条件下热处理的碳纤维具有最低的内阻和最大的比电容。电化学性能的变化也直接影响碳纤维电极对的电场性能,450℃条件下热处理的碳纤维电极也具有最佳的电场性能:电极对极差稳定性降到0.1 m V/24 h左右,自噪声为2.2 n V/rt Hz@1Hz,电极对能较好地响应出1 m Hz、1 m V电场信号。     

In situ surface-enhanced infrared study of hydrogen bond pairing of complementary nucleic acid bases at the electrochemical interface

Surface-enhanced infrared absorption spectroscopy with a Kretschmann-type attenuated total reflection configuration has been used to study hydrogen-bonded pairing between 6-amino-8-purinethiol, a thiol-derivatized adenine, immobilized on a gold electrode surface, and thymidine, a complimentary base derivative of adenine, in 0.1 M NaClO4 aqueous solution as a function of applied potential. 6-Amino-8-purinethiol is adsorbed on a gold surface via a sulfur atom to form a S-Au bond. Nearly half of the adsorbed molecules are protonated, and the long axis of the adenine moiety is tilted from the surface normal at open circuit potential. As the potential increases, the acid-base equilibrium is shifted toward the unprotonated form and the adenine moiety is reoriented toward a nearly perpendicular configuration. The hydrogen bond interaction between the adsorbed 6-amino-8-purinethiol with thymidine in solution is greatly affected by the protonation and orientation of the adenine moiety and is controllable by the applied potential. Due to steric hindrance, an adenine-thymine-type hydrogen bond pair is formed only at potentials more positive than 0.1 V (vs SCE) where the unprotonated adenine moiety is perpendicularly oriented.     

Adiabatic description of superfocusing of femtosecond plasmon polaritons

A surface plasmon polariton is a collective oscillation of free electrons at a metal–dielectric interface. As wave phenomena, surface plasmon polaritons can be focused with the use of an appropriate excitation geometry of metal structures. In the adiabatic approximation, we demonstrate a possibility to control nanoscale short pulse superfocusing based on generation of a radially polarized surface plasmon polariton mode of a conical metal needle in view of wave reflection. The results of numerical simulations of femtosecond pulse propagation along a nanoneedle are discussed. The space–time evolution of a pulse for the near field strongly depends on a linear chirp of an initial laser pulse, which can partially compensate wave dispersion. The field distribution is calculated for different metals, chirp parameters, cone opening angles and propagation distances. The electric field near a sharp tip is described as a field of a fictitious time-dependent electric dipole located at the tip apex.     

Surface-enhanced Raman spectroscopy of soft-landed polyatomic ions and molecules

Surface-enhanced Raman spectroscopy (SERS) was used to detect and characterize polyatomic cations and molecules that were electrosprayed into the gas phase and soft-landed in vacuum on plasma-treated silver substrates. Organic dyes such as crystal violet and Rhodamine B, the nucleobase cytosine, and nucleosides cytidine and 2'-deoxycytidine were immobilized by soft landing on plasma-treated metal surfaces at kinetic energies ranging from near thermal to 200 eV. While enhancing Raman scattering 10(5)-10(6)-fold, the metal surface effectively quenches the fluorescence that does not interfere with the Raman spectra. SERS spectra from submonolayer amounts of soft-landed compounds were sufficiently intense and reproducible to allow identification of Raman active vibrational modes for structure assignment. Soft-landed species appear to be microsolvated on the surface and bound via ion pairing or pi-complexation to the Ag atoms and ions in the surface oxide layer. Comparison of spectra from soft-landed and solution samples indicates that the molecules survive soft landing without significant chemical damage even when they strike the surface at hyperthermal collision energies.     

Resolution of intermediate adsorbate structures in the potential-dependent self-assembly of n-hexanethiolate on Silver by in situ surface-enhanced Raman spectroscopy

Resolution of the reaction steps and the associated component Raman spectra during the formation or desorption of self-assembled monolayers is challenging because intermediate adsorbate populations are present at low concentrations and their spectral bands overlap. By collecting Raman spectra versus applied potential into a two-dimensional data set, one can utilize multivariate statistical techniques to resolve the component concentration profiles along with their corresponding Raman spectra. In situ surface-enhanced Raman spectroscopy (SERS) spectra were collected during the potential-dependent formation and desorption (-1.50 to -0.70V versus Ag/AgCl) of n-hexanethiolate monolayer at a polycrystalline Ag electrode. Resolution of the pure component spectra from these components was accomplished by using self-modeling curve resolution (SMCR), which does not require a physical model. For monolayer adsorption, the potential-dependent Raman spectra could be described by three significant eigenvectors; the eigenvectors could be rotated into a set of pure component spectra and concentration profiles using a linear least-squares step to find a common plane in the space of the eigenvectors representing the linear combination of the real-component responses. The convex hull surrounding the data in the plane and positive amplitude criteria were utilized to identify the coordinates of the pure component responses. The C-S stretching vibrations of the resolved spectra show that the initial adsorbate is a gauche conformer, which allows the hydrocarbon chain to lie on the metal surface; a second phase arises at higher coverage with trans C-S conformation, where the hydrocarbon chains are oriented off the surface plane, and a final complete monolayer is formed with a well-ordered, all-trans C-S configuration. In contrast, desorption studies showed only two surface phases, the initial well-ordered monolayer and the low-density phase dominated by gauche conformations. The results illustrate the utility of self-modeling curve resolution to unravel interfacial reaction mechanisms and intermediate structures from two-dimensional SERS data, without requiring prior knowledge of a physical model for the process.