Two-dimensional condensation of pyrimidine oligonucleotides during their self-assemblies at mercury based surfaces

For the first time it is shown that homopyrimidine oligodeoxynucleotides (ODNs) adsorbed at mercury or amalgam electrode surface can condensate upon applying negative potentials (around −1.35 V vs. Ag/AgCl/3M KCl). This 2D condensation resulted in formation of capacitance pits on the C-E curves resembling those observed earlier with monomeric nucleic acid bases, nucleosides and nucleotides. Differences in behavior of the condensed layers of dT₃₀ and dC₃₀ ODNs, reflecting different physico-chemical and electrochemical properties of thymine and cytosine, were observed. Formation of the ODN condensed film involved reorientation of the oligonucleotide molecules firmly adsorbed at the electrode and took place even in the absence of any ODN in the bulk of solution. Homopurine ODNs did not form these two-dimensional (2D) condensed monolayers under the same conditions. A preliminary thermodynamic analysis of the condensed ODN layers is presented.     

Nitrate reduction on single-crystal platinum electrodes

The structure sensitivity of the reduction of nitrate has been studied on a series of single-crystal platinum electrodes by cyclic voltammmetry and in situ FTIRAS in sulfuric and perchloric acid solutions. The nitrate reduction is a structure-sensitive reaction on single-crystal platinum electrodes. However, this structure sensitivity is essentially controlled by other species (hydrogen, sulfate) that interact strongly with the electrode surface rather than by a structure-sensitive nitrate adsorption, dissociation or reduction.Voltammetric and spectroscopic data point to adsorbed nitric oxide (NO) as the main stable intermediate of the nitrate reduction to ammonia. No evidence for the formation of N₂O was found. On surfaces with sufficiently wide (1 1 1) terraces in the absence of specifically adsorbed sulfate, an oxidizable nitrate reduction product is detected voltammetrically, which may tentatively be attributed to the formation of a small amount of hydroxylamine earlier during the voltammetric scan.     

Electrooxidation of ethanol at polycrystalline and platinum stepped single crystals: A study by differential electrochemical mass spectrometry

The electrooxidation of adsorbed and bulk solution of 10⁻² M ethanol and D₆-ethanol at polycrystalline platinum, smooth, roughened and Ru modified Pt(3 3 2), Pt(3 3 1) and Pt(1 1 1) electrodes was studied by on-line differential electrochemical mass spectroscopy (DEMS) using a dual thin layer flow through cell.On polycrystalline Pt, the main (or even single) product is acetaldehyde; due to the flow through conditions the amount of acetaldehyde further oxidized to acetic acid is negligible. At stepped single crystals with (1 1 1) terraces (Pt(s)[n(1 1 1) × (1 1 1)], acetic acid is produced at a lower potential than acetaldehyde. This demonstrates that in addition to the reaction path involving C-C bond splitting leading to CO₂ (via adsorbed CO and CH_x) and the reaction path leading to acetaldehyde there is a third, direct reaction path leading to the formation of acetic acid.Step decoration by Ru does not lead to an increased reactivity. This is different from the strong cocatalytic effect of Ru at step sites on the oxidation of CO. Furthermore, Ru does not influence the relative amount of acetaldehyde formed.     

Electrochemical characterization of irreversibly adsorbed germanium on platinum stepped surfaces vicinal to Pt(1 0 0)

The electrochemical behavior of germanium irreversibly adsorbed at stepped surfaces vicinal to the Pt(1 0 0) pole is reported. The process taking part on the (1 0 0) terraces is evaluated from charge density measurements and calibration lines versus the terrace dimension are plotted. On the series Pt(2n − 1,1,1) having (1 1 1) monoatomic steps, the charge involved in the redox process undergone by the irreversibly adsorbed germanium is able to account for (n − 0.5) terrace atoms, thus suggesting some steric difficulties in the growth of the adlayer on the (1 0 0) terraces. Conversely, no steric problems are apparent in the series Pt(n,1,0) in which more open (1 0 0) steps are present on the (1 0 0) terraces. In this latter case the charge density under the germanium redox peaks is proportional to the number of terrace atoms. Some comparison is made with other stepped surfaces to understand the behavior and stability of germanium irreversibly adsorbed on the different platinum surface sites.     

High saturation capacity of activated carbons prepared from mesophase pitch in the removal of volatile organic compounds

A series of binderless activated carbon monoliths (ACMs) have been prepared from petroleum pitch and using KOH as activating agent. Characterization shows that these activated carbons combine a large “apparent” surface area (up to S_BET ∼ 3000 m²/g) together with a well-developed narrow micropore size distribution. Dynamic column adsorption experiments using different volatile organic compounds (VOCs), ethanol and benzene, show that these activated carbons prepared from mesophase-based materials exhibit a superior saturation capacity compared to conventional carbon materials. The total amount adsorbed reaches values as high as 18 g/100 g AC and 40 g/100 g AC, for ethanol and benzene, respectively. These are the best results reported in the literature. The total amount adsorbed for both molecules correlates with the total volume of narrow micropores, thus confirming the pore size specificity required for the adsorption of VOC molecules. Regeneration studies show that ethanol can be easily desorbed at room temperature by flowing clean air through the adsorbent whereas benzene requires a further heating for complete desorption/regeneration.     

Electroreduction of nitrate ions on Pt(1 1 1) electrodes modified by copper adatoms

Kinetics and mechanism of nitrate ion reduction on Pt(1 1 1) and Cu-modified Pt(1 1 1) electrodes have been studied by means of cyclic voltammetry, potentiostatic current transient technique and in situ FTIRS in solutions of perchloric and sulphuric acids to elucidate the role of the background anion. Modification of platinum surface with copper adatoms or small amount of 3D-Cu crystallites was performed using potential cycling between 0.05 and 0.3 V in solutions with low concentration of copper ions, this allowed us to vary coverage θ_Cu smoothly. Following desorption of copper during the potential sweep from 0.3 to 1.0 V allowed us to estimate actual coverage of Pt surface with Cu adatoms. Another manner of the modification was also applied: copper was electrochemically deposited at several constant potentials in solutions containing 10⁻⁵ or 10⁻⁴ M Cu²⁺ and 5 mM NaNO₃ with registration of current transients of copper deposition and nitrate reduction.It has been found that nitrate reduction at the Pt(1 1 1) surface modified by copper adatoms in sulphuric acid solutions is hindered as compared to pure platinum due to induced sulphate adsorption at E < 0.3 V. Sulphate blocks the adsorption sites on the platinum surface and/or islands of epitaxial Cu(1 × 1) monolayer thus hindering the adsorption of nitrate anions and their reduction. The extent of inhibition weakly depends on the copper adatom coverage. Deposition of a small amount of bulk copper does not affect noticeably the rate of nitrate reduction.Nitrate reduction on copper-modified Pt(1 1 1) electrodes in perchloric acid solutions occurs much faster as compared to pure platinum. The steady-state currents are higher by 4 and 2 orders of magnitude at the potentials of 0.12 and 0.3 V, respectively. The catalytic effect of copper adatoms is largely caused by the facilitation of nitrate adsorption on the platinum surface near Cu_ad and/or on the islands of the Cu(1 × 1) monolayer (induced nitrate adsorption).Hydrogen adatoms block the adsorption sites on platinum for NO₃⁻ anion adsorption and inhibit reactions of nitrate reduction even at moderate surface coverage.The products of nitrate reduction in sulphuric and perchloric acids are essentially the same (NO and ammonia) irrespective of the presence or absence of Cu on the platinum surface.     

Electrochemistry behavior of adrenalin, serotonin and ascorbic acid at novel poly rutin modified paraffin-impregnated graphite electrode

A novel poly rutin (Ru) modified paraffin-impregnated graphite electrode (WGE) was fabricated by electrochemical method. The field emission scanning electron microscope (FE-SEM), infrared spectra (IR), in situ UV-spectroelectrochemical and electrochemical techniques proved the immobilization of rutin on WGE. Ru undergoes electrochemical oxidation in two ways related to the two catechol hydroxyl groups and the other two hydroxyl groups; the former not only carries out a two-electron two-proton reversible reaction, but also produces unstable phenoxy radicals which readily polymerize to strongly adhere to WGE surface companying Ru monomer embeded and adsorbed in the film (Ru/WGE). The Ru/WGE displayed strong catalytic function for the oxidation of adrenalin (EP), serotonin (5-HT), and ascorbic acid (AA) and resolved the overlap voltammetric response of EP and AA into two well-defined voltammetric peaks of about 172 mV with DPV. A linear response in the range of 3.0-90.0 μM with detection limit (s/n = 3) of 8.0 × 10⁻⁷ M for EP was obtained in coexistence of AA (0.01 mM).     

Adsorption and reduction of NO2 over activated carbon at low temperature

The reactive adsorption of NO₂ over activated carbon (AC) was investigated at 50 °C. Both the NO₂ adsorption and its reduction to NO were observed during the exposure of AC to NO₂. Temperature programmed desorption (TPD) was then performed to evaluate the nature and thermal stability of the adsorbed species. Adsorption and desorption processes have been proposed based on the nitrogen and oxygen balance data. The micropores in AC act as a nano-reactor for the formation of -C(ONO₂) complexes, which is composed by NO₂ adsorption on existing -C(O) complexes and the disproportionation of adsorbed NO₂. The generated -C(ONO₂) complexes are decomposed to NO and NO₂ in the desorption step. The remaining oxygen complexes can be desorbed as CO and CO₂ to recover the adsorptive and reductive capacity of AC.     

Adsorption of phosphate species on poly-oriented Pt and Pt(1 1 1) electrodes over a wide range of pH

The adsorption of phosphate anions from phosphate solutions at poly-oriented and single-crystal platinum electrodes, primarily Pt(1 1 1), was studied over a wide range of pH by cyclic voltammetry. The features observed at the poly-oriented Pt electrode in phosphate solution may be related to the different crystalline facets, the (1 1 1) orientation presenting the most significant behavior in terms of phosphate adsorption. On the reversible hydrogen electrode (RHE) scale, the phosphate adsorption strength decreases with increasing alkalinity of the solution. Qualitatively, three different pH regions can be distinguished. At pH < 6 only a broad reversible peak is observed, corresponding to the adsorption of H₂PO₄⁻ and further deprotonation to adsorbed HPO₄⁻. For 6 < pH < 11 a butterfly feature followed by one or two anodic peaks (depending on scan rate) is observed, ascribed to the adsorption of HPO₄⁻ followed by its subsequent deprotonation to adsorbed PO₄³⁻. The splitting into two or three voltammetric features, and the irreversibility of the two features at more positive potential, is ascribed to the deprotonation reaction leading to a surface species (i.e. phosphate) which needs to change its surface coordination. At pH > 11 a reversible pre-wave and a sharp spike are observed, ascribed to the co-adsorption of phosphate and hydroxide.     

Understanding CO-stripping mechanism from NiUPD/Pt(1 1 0) in view of the measured nickel formal partial charge number upon underpotential deposition on platinum surfaces in sulphate media

We recently showed nickel-underpotential deposition (Ni-UPD) occurs on polycrystalline or single crystal platinum electrodes in acidic media. Whereas the decoupling of the nickel and hydrogen adsorption/desorption peaks is difficult for low pH, these processes can be better separated for higher pH values, typically pH > 3. However, even for platinum single crystals, high pH solutions do not enable to sufficiently separate nickel from hydrogen phenomena. As a result, electrochemistry alone cannot yield important information about Ni-UPD, such as the formal partial charge number (valency of electrosorption) and the role of the sulphate or hydrogen sulphate anions.So, we decided to couple cyclic voltammetry to electrochemical quartz crystal microbalance (EQCM). EQCM measurements enable to decorrelate the simultaneous hydrogen and nickel adsorption/desorption peaks, which we could not attempt solely with electrochemistry. The coupling between gravimetric and electrochemical measurements allows us to detect the contribution of the anions and thus to isolate that of nickel: nickel coverage can then be determined. Nearly 4/5 Ni_UPD monolayer (θ_Ni ≈ 0.8) over platinum is reached at nickel equilibrium potential for high pH solutions (5.5). The QCM and electrochemistry coupling further allows the determination of nickel formal partial charge number: ι_Ni,EQCM = 1.3 ± 0.13. Direct electrochemistry measurements (Swathirajan and Bruckenstein method) yield: ι_Ni,Pt(poly) = 1.5 ± 0.17. These two values are close, which validates the electrochemical method for the nickel/platinum system. In consequence, we used Swathirajan and Bruckenstein method for Pt(1 1 0)-(1 × 2) crystal and found: ι_{Ni,Pt(1 1 0)} ≈ 1.4 ± 0.1. Whatever the system (Ni_UPD/Pt(poly) or Ni_UPD/Pt(1 1 0)-(1 × 2)) or the experimental technique, nickel formal partial charge number is lower than nickel cation charge: ι_Ni < z_Ni = 2. In consequence, upon underpotential deposition on platinum surfaces, nickel cations discharge and then undergo additional charge exchange processes, such as anion (or water) adsorption, resulting in apparent partial nickel cation discharge. Moreover, Ni_UPD/Pt(1 1 0) surface displays high activity towards CO_ad oxidation reaction. We explain such positive effect by the possible existence of a bifunctional mechanism in which oxygenated-species-covered Ni_UPD adatoms provide the oxygen atom to CO_ad?Pt species, enabling its facile oxidation.     

Preparation and electrochemical characterization of low-index rhodium single crystal electrodes in sulfuric acid

The electrochemical properties of low-index phase Rh(1 1 1), Rh(1 1 0) and Rh(1 0 0) single crystal bead electrodes, prepared by a novel technique combining electron beam heating with inductive annealing in a controlled atmosphere, have been characterized in 0.1 M H₂SO₄ by cyclic voltammetry and chronoamperometry. Hydrogen and sulfate adsorption as well as surface oxidation depend strongly on the crystallographic orientation of the surface. The potentials of zero total charge (E_pztc) of all three Rh electrodes in 0.1 M H₂SO₄ were determined by the combination of charge displacement and voltammetric experiments. The charge balance reveals unambiguousely that the (√3 × √7) adlayer on Rh(1 1 1) is composed of specifically adsorbed sulfate ions eventually coadsorbed with water molecules. Hydrogen-sulfate coadsorbed with hydronium ions could be excluded. The kinetics of sulfate ion desorption followed by the adsorption of hydrogen at less positive potentials could be represented by a nucleation and growth mechanism coupled with a parallel first order process.The electro-oxidation of irreversibly adsorbed carbon monoxide monolayers was also investigated and revealed distinct structure sensitivity. The reaction pathway on all three low-index phases of Rh proceeds according to a Langmuir-Hinshelwood mechanism and is controlled by nucleation of OH_ads at steps and other defect sites followed by a complex growth process on terrace sites. The low surface mobility of CO_ads leads to a slow and incomplete CO monolayer electro-oxidation on Rh(1 1 1). The high density of step sites on Rh(1 1 0) and the reversible formation of oxygenated species on Rh(1 0 0) at rather low potentials significantly enhance the electro-oxidation activity leading to the following reactivity sequence: Rh(1 1 1) ? Rh(1 1 0) ∼ Rh(1 0 0). The shape of the experimental transients and attempts to model them demonstrate the occurrence of at least two processes occurring in parallel. The long-term response represents clearly a process involving a slow surface-diffusion step.     

Electro-oxidation kinetics of adsorbed CO on platinum electrocatalysts

We describe the voltammetric measurement of the full oxidation of adsorbed CO on unsupported platinum electrocatalysts, with concomitant cyclic voltammetry of the hydrogen adsorption and desorption. The hydrogen region of platinum is used to parse the platinum surface into sites associated with weakly bound (WB) hydrogen and strongly bound (SB) hydrogen. By monitoring changes in the hydrogen region while following the two observed CO oxidation peaks, we are able to identify the WB sites as being the most active sites for CO_ads electro-oxidation. The full oxidation peak is fitted to a model based on a modified Butler-Volmer equation that includes the two families of sites. Excellent agreement with experimental results is obtained, and the resulting fits yield the kinetic parameters for the two families of sites. When combined with coulometry, these kinetic analyses also show the importance of linear- and bridged-CO_ads species in the electro-oxidation process. Limitations of the model and the role of CO_ads dynamics amongst the various surface sites are discussed.     

Thermal desorption of deuterium from modified carbon nanotubes and its correlation to the microstructure

The process of deuterium desorption from single-wall carbon nanotubes (SWNTs) modified by atomic (D) and molecular (D₂) deuterium treatment was investigated in an ultrahigh vacuum environment using thermal desorption mass spectroscopy (TDMS). Microstructural and chemical analyses of SWNT material, modified by this deuterium interaction, were performed by means of a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The results disclose characteristic features in the TDMS spectra of deuterium evolved from the SWNT material, which can be correlated to the microstructure of nanocarbon material modified by D-treatment. The TDMS spectra of deuterium originating from the large diameter rope type nanotube structures, resulting from a prolonged low-pressure (D + D₂) gas mixture treatment, exhibit three overlapping desorption peaks: a dominant one with a desorption activation energy (E_des) of approx. 2.86 eV and lower intensity peaks at E_des of ∼1.50 and 2.46 eV. On the other hand, the TDMS spectra of deuterium taken from the “coral reef”-like carbon nanostructures, obtained after prolonged treatment of SWNTs to a high-pressure (D + D₂) gas mixture produced at high temperature, reveal the coexistence of four superimposed desorption peaks with E_des ranging from 1.23 to 4.4 eV. A dominant desorption peak with E_des ≈ 4.4 eV, can be attributed to bulk diffusion of D trapped within this nanocapsule bulk structure.     

Electrochemical sensor for sulfite determination based on a nanostructured copper-salen film modified electrode

The electrochemical preparation described herein involved the electrocatalytic oxidation of sulfite on a platinum electrode modified with nanostructured copper salen (salen = N,N′-ethylenebis(salicylideneiminato)) polymer films. The complex was prepared and electropolymerized at a platinum electrode in a 0.1 mol L⁻¹ solution of tetrabutylammonium perchlorate in acetonitrile by cyclic voltammetry between 0 and 1.4 V vs. SCE. After cycling the modified electrode in a 0.50 mol L⁻¹ KCl solution, the estimated surface concentration was found to be equal to 2.2 × 10⁻⁹ mol cm⁻². This is a typical behavior of an electrode surface immobilized with a redox couple that can usually be considered as a reversible single-electron reduction/oxidation of the copper(II)/copper(III) couple. The potential peaks of the modified electrode in the electrolyte solution (aqueous) containing the different anions increase with the decrease of the ionic radius, demonstrating that the counter-ions influence the voltammetric behavior of the sensor. The potential peak was found to be linearly dependent upon the ratio [ionic charge]/[ionic radius]. The oxidation of the sulfite anion was performed at the platinum electrode at +0.9 V vs. SCE. However, a significant decrease in the overpotential (+0.45 V) was obtained while using the sensor, which minimized the effect of oxidizable interferences. A plot of the anodic current vs. the sulfite concentration for chronoamperometry (potential fixed = +0.45 V) at the sensor was linear in the 4.0 × 10⁻⁶ to 6.9 × 10⁻⁵ mol L⁻¹ concentration range and the concentration limit was 1.2 × 10⁻⁶ mol L⁻¹. The reaction order with respect to sulfite was determined by the slope of the logarithm of the current vs. the logarithm of the sulfite concentration.     

Adsorption of 1,6-hexanediol on Bi single crystal electrodes

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy have been employed for the quantitative study of 1,6-hexanediol (HD) adsorption at the Bi(0 0 1) and Bi(l 1 1)|0.05 M Na₂SO₄ aqueous solution interfaces. According to the experimental data the Bi(l 1 1) interface has more negative zero charge potential and the adsorption-desorption peaks appear at more negative potentials than in case of Bi(0 0 1) interface. However, the adsorption activity is practically similar for Bi(0 0 1) and Bi(l 1 1) planes. The surface area occupied by one adsorbed molecule S_max equal to 0.47 nm² at the maximum adsorption potential indicates that HD molecules have flat orientation on Bi(h k l) surfaces. For less concentrated solutions the capacitance decreases in the maximum adsorption area as HD concentration increases, but in case of HD concentrations above 0.5 M the capacitance starts to increase. Cyclic voltammetry results show the capacitive adsorption and desorption peaks at higher potential scan rates for solutions with addition of HD in the base electrolyte solution.     

Water vapor adsorption on chars and active carbons-oxygen sensors prepared from a tropical tree wood

Carbonizates and active carbons (AC) are useful as sensors of oxygen. Adsorption of water vapor on such materials, prepared from the wood of a tropical tree, Peltogyne from Surinam, was investigated. Reproducible and reversible water vapor isotherms were obtained of preliminarily unheated samples, as well as of heated samples after several adsorption-desorption cycles. Water vapor adsorption revealed very fine (ultramicropores) structure in the investigated materials. Such ultramicropores can not be detected by traditional adsorption of nitrogen at 77 K because of a molecular sieve blocking effect. Several carbonizates at high water vapor pressure exhibited large weight reduction. This weight loss is the result of expulsion of organic impurities from the porous structure under the influence of adsorbed water. These data were confirmed by mass-spectrometric thermodesorption analysis. X-ray data and water vapor adsorption isotherms indicated that the strongly bound water stabilizes the carbonizate structure. When heated at 623 K, the AC exhibited desorption of strongly bound water and modified AC structure.     

Separation and concentration effect of f-MWCNTs on electrocatalytic responses of ascorbic acid, dopamine and uric acid at f-MWCNTs incorporated with poly (neutral red) composite films

A novel conductive composite film containing functionalized multi-walled carbon nanotubes (f-MWCNTs) with poly (neutral red) (PNR) was synthesized on glassy carbon electrodes (GC) by potentiostatic method. The composite film exhibited promising electrocatalytic oxidation of mixture of biochemical compounds such as ascorbic acid (AA), dopamine (DA) and uric acid (UA) in pH 4.0 aqueous solutions. It was also produced on gold electrodes by using electrochemical quartz crystal microbalance technique, which revealed that the functional properties of composite film were enhanced because of the presence of both f-MWCNTs and PNR. The surface morphology of the polymer and composite film deposited on transparent semiconductor tin oxide electrodes were studied using scanning electron microscopy and atomic force microscopy. These two techniques showed that the PNR was fibrous and incorporated on f-MWCNTs. The electrocatalytic responses of neurotransmitters at composite films were measured using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV). These experiments revealed that the difference in f-MWCNTs loading present in the composite film affected the electrocatalysis in such a way, that higher the loading showed an enhanced electrocatalytic activity. From further electrocatalysis studies, well separated voltammetric peaks were obtained at the composite film modified GC for AA, DA and UA with the peak separation of 0.17 V between AA-DA and 0.15 V between DA-UA. The sensitivity of the composite film towards AA, DA and UA in DPV technique was found to be 0.028, 0.146 and 0.084 μA μM⁻¹, respectively.     

Comments on “Investigation of Ethylene Oxide on Clean and Oxygen-Covered Ag(110) Surfaces”

This letter is a re-examination of the ethylene oxide (EO) temperature programmed desorption (tpd) peak shapes (reported in references 1 and 2) which were obtained by tpd after having adsorbed EO at 250 K on to Ag(110) and Ag(111) surfaces. In these papers, the peaks were deemed to originate from a unimolecular rearrangement of an adsorbed oxametallacycle, producing EO, which desorbed on formation. Consequently, on the basis of this thesis, the activation energy of the composite process of internal rearrangement of the oxametallacycle and desorption of EO was determined by solution of the first order Redhead equation [3] at the peak maximum temperature, using an assumed value of 10¹³ s^?1 for the desorption pre-exponential term. The conclusions of this re-examination are that the m/z = 29 desorption peaks shown in Fig. 1 derive from a second order surface reaction of an adsorbed O atom and adsorbed ethylene molecule and that this is the rate determining step in the desorption of EO. An important corollary of these conclusions is that an oxametallacycle is not involved in the ethylene epoxidation reaction coordinate as the papers suggest.

Oligonucleotide synthesis onto poly(N‐acryloylmorpholine‐co‐N‐acryloxysuccinimide): Assessment of the resulting conjugates in a DNA sandwich hybridization test

A water‐soluble statistical poly(N‐acryloylmorpholine‐co‐N‐acryloxysuccinimide) [poly(NAM/NAS)] copolymer was studied for polymer–oligonucleotide (ODN) conjugate elaboration and for further use in diagnostic applications. Three different copolymers were first prepared by free‐radical solution polymerization with different N‐acryloylmorpholine (NAM) and N‐acryloxysuccinimide (NAS) molar ratios (80/20, 70/30, and 60/40). Their number‐average molecular weights ranged from 98,000 to 120,000 g/mol, as determined by aqueous size exclusion chromatography with an online light‐scattering detector. Then, polymer–ODN conjugates were obtained via a strategy consisting of the direct synthesis of ODNs onto polymer chains previously grafted onto a controlled pore glass support. Before the grafting of the polymer onto the solid support, a preliminary step was performed to bind a nucleotide starter along the polymer chain (via the reactive NAS units) to initiate automated DNA synthesis. To multiply the number of ODNs growing from starters, a branched phosphoramidite synthon [bearing two O‐dimethoxytrityl groups] was introduced at the first step of ODN elongation as a short sequence of four branched synthons alternated with three thymidine residues. Conjugates were assessed in a DNA sandwich hybridization test developed for hepatitis B virus detection. Sensitivity limits were evaluated and compared to those obtained with an other polymer, poly(maleic anhydride‐alt‐methyl vinyl ether) [poly(MA/MVE)]. A sensitivity limit of 2.6 × 10⁷ DNA copies/mL was reached with the poly(MA/MVE)–ODN conjugate at the capture phase and with the poly(NAM/NAS)–branched ODN conjugate at the detection phase of the test. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3784–3795, 2004     

DC and AC voltammetry of a free-base porphyrin adsorbed onto basal-plane graphite under acidic conditions: An example of a close to ideal reversible two-electron surface-confined redox process at sub-monolayer coverages

The free-base porphyrin, 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine (H₂TMPyP), adsorbs onto a basal-plane graphite electrode. Under DC cyclic voltammetric conditions, the fully protonated dication, [H₄TMPyP(0)]²⁺, undergoes an apparently close to ideal surface-confined two-electron reduction to the neutral [H₄TMPyP(-II)] species when the supporting electrolyte consists of aqueous 1 M HCl and 1 M NaCl and coverages are sub-monolayer. The reversible potential calculated from the average of the oxidation and reduction peak potentials is 0.138 ± 0.002 V (vs Ag/AgCl, 3 M NaCl) whilst their separation ΔE_p, approaches 0 mV at slow scan rates, as expected theoretically for an ideal surface-confined electron transfer process. Comparisons of simulated and experimental data imply that the increase in ΔE_p observed at scan rates above 10 V s⁻¹ is consistent with uncompensated Ohmic IR_u drop effects, and not limitations imposed by electron transfer kinetics. Analysis of fundamental and higher harmonic components derived from large-amplitude sine-wave AC voltammetry is consistent with a very fast electron transfer rate constant, k⁰, in excess of 10⁶ s⁻¹ for the overall two-electron process. However, careful comparison with AC theory highlights minor levels of non-ideality not attributable to purely capacitative background or uncompensated resistance effects. These are particularly evident when greater than monolayer surface coverages are employed. It is likely that subtle contributions from heterogeneity in the adsorbed layer and complexities in the reaction mechanism are present in this close to ideal surface-confined process, but they are more readily detected under conditions of large-amplitude Fourier transformed AC cyclic voltammetry than with the conventionally used DC cyclic format.