Voltammetric monitoring of gold nanoparticle formation facilitated by glycyl-L-tyrosine: relation to electronic spectra and transmission electron microscopy images.

Voltammetric techniques have been introduced to monitor the formation of gold nanoparticles produced via the reaction of the amino acid glycyl-L-tyrosine with Au(III) (bromoaurate) in 0.05 M KOH conditions. The alkaline conditions facilitate amino acid binding to Au(III), inhibit the rate of reduction to Au(0), and provide an excellent supporting electrolyte for voltammetric studies. Data obtained revealed that a range of time-dependent gold solution species are involved in gold nanoparticle formation and that the order in which reagents are mixed is critical to the outcome. Concomitantly with voltammetric measurements, the properties of gold nanoparticles formed are probed by examination of electronic spectra in order to understand how the solution environment present during nanoparticle growth affects the final distribution of the nanoparticles. Images obtained by the ex situ transmission electron microscopy (TEM) technique enable the physical properties of the nanoparticles isolated in the solid state to be assessed. Use of this combination of in situ and ex situ techniques provides a versatile framework for elucidating the details of nanoparticle formation.

Polarographic procedures without removal of oxygen, and other approaches to making the determinations more rapidly.

One disadvantage of conventional d.c. polarography as an analytical method has always been its relative slowness. The possibility of simplifying and speeding up analyses by avoiding the necessity for removal of oxygen is demonstrated under suitable conditions with current-sampled d.c., pulse and a.c. polarography. In particular it is shown that high-frequency phase-selective a.c. polarography gives considerable discrimination against the oxygen electrode process in some aqueous media. Under these conditions, the high-frequency a.c. technique can be combined with the method of short drop-time, fast scan-rate rapid a.c. polarography to provide a most attractive method of routine analysis. Polarographic analysis in non-aqueous media without removal of oxygen is also discussed.

Instrumental advantages obtained with short controlled drop-time a.c. polarography.

The performance of short drop-time a.c. polarography has been examined in detail, and found to be better than that for natural drop-times in almost every respect. Uncompensated resistance terms are smaller and potentiostat stability is improved. In addition, faster potential scanrates and more rapid data acquisition are possible. A variation of the drop-time over at least three orders of magnitude is possible and this, coupled with excellent instrumental performance, should offer considerable scope in studies of electrode kinetics.

Determination of tocopherols by reverse-phase liquid chromatography and electrochemical detection at a surface-oxide modified platinum microelectrode, without added electrolyte.

The reverse-phase separation and electrochemical detection of alpha-, gg-, and delta-tocopherol at a potential of +0.90 V vs. a gold pseudo-reference electrode is possible down to 10(-7)M concentrations, with surface-modified platinum microdisc electrodes in a methanol/water (95:5) solvent mixture. The use of microclectrodes with radii of 10-70 mum, rather than electrodes of conventional size, minimizes problems associated with iR drop and obviates the need for deliberately added electrolyte. These features simplify the analytical procedure. The background response of an untreated platinum microelectrode in the methanol/water (95:5) system at positive potentials is characterized by processes arising from adsorption/oxidation of methanol and formation of surface oxides. Amperometric detection is of little use under these conditions. However, preoxidation of the electrode surface in 2M nitric acid inhibits the methanol adsorption/oxidation reaction but not the tocopherol response and therefore allows highly sensitive amperometric detection.

Investigation by automated differential pulse anodic-stripping voltammetry of the problem of storage of dilute solutions.

The storage of dilute solutions of metal ions before their laboratory analysis presents a difficult problem in the examination of many environmental samples. By utilizing the solution container as an electrochemical cell and employing the method of differential pulse anodic-stripping voltammetry at a hanging mercury drop electrode, it is shown that an automated read-out system in an enclosed environment can be developed for monitoring the solution-container interactions that occur over short or extended periods of time. In the present work, interactions of dilute solutions (1-10 mug/l.) of cadmium(II), lead(II), copper(II), zinc(II), and thallium(I) in glass, polyethylene and Teflon containers have been investigated at various pH values and in different ionic environments. The results demonstrate the importance of factors other than pH.

Voltammetry in room temperature ionic liquids: comparisons and contrasts with conventional electrochemical solvents.

The recent literature is surveyed to explore the nature of voltammetry in room temperature ionic liquids. The extent of similarities with conventional electrochemical solvents is reported and some surprising differences are noted.

Reversed-phase high-performance liquid chromatography for the determination of steroid hormones in oral contraceptives.

Reversed-phase high-performance liquid chromatography with UV detection is studied for the determination of both progestogenic and oestrogenic components of oral contraceptive formulations. The applicability of the assay is demonstrated for a number of different progestogen-oestrogen combinations in both conventional tablet and novel "paper" formulations. The results show that the method developed is a versatile technique for the routine assay of these pharmaceutical formulations.

Voltammetric determination of the synthetic pyrethroid insecticide tetramethrin in acetonitrile.

The synthetic pyrethroid insecticide tetramethrin may be reduced reversibly (E°' = -1.650 V vs Ag/Ag(+)) in acetonitrile at hanging mercury drop electrodes (HMDE) and glassy carbon electrodes. On the voltammetric time scale, the initial electron-transfer process involves the reversible formation of a radical anion. Data obtained from electron paramagnetic resonance spectroscopy indicate that the unpaired electron of the radical is located within the phthalimide system of the molecule. The radical anion may be further reduced at very negative applied potentials with the number of processes being dependent on the nature of the voltammetric technique. The detection limit (3σ) for the determination of tetramethrin in acetonitrile at a glassy carbon electrode, using differential pulse voltammetry, was found to be 2.1 × 10(-6) M. At a HMDE the detection limit is lower, having a value of 9.6 × 10(-7) M. The limit of determination (10σ) at a glassy carbon electrode is 3.5 × 10(-6) M and at a HMDE is 3.0 × 10(-6) M. Tetramethrin was selectively determined in an insecticide formulation, at a glassy carbon electrode using differential pulse voltammetry, at a concentration (w/v) of 0.34 ± 0.02%. The determined concentration is in good agreement with the stated value of 0.350 ± 0.018% (w/v).

Comparison of the gold reduction and stripping processes at platinum, rhodium, iridium, gold and glassy carbon micro- and macrodisk electrodes.

The gold AuIII + 3e(-)-->Au0 reduction and Au0-->AuIII + 3e- oxidation stripping processes in dilute aqua regia electrolyte (0.1 M HCl + 0.32 M HNO3) were examined at platinum, rhodium, iridium, gold and glassy carbon disk electrodes. After ascertaining that the preferred material was platinum, the effect of electrode size was evaluated by using nine different platinum disk electrodes having diameters ranging from 2 to 2000 microns. The optimum analytical response was obtained with a 50 microns diameter platinum disk electrode. With this electrode diameter, a sharp symmetrical gold stripping peak was obtained and the deposition process occurred predominantly under conditions of radial diffusion so that stirring of the solution was not required. In contrast, larger sized platinum electrodes produced a broader, asymmetric stripping response for the gold oxidation peak, whereas electrodes of smaller diameter provided poorer signal-to-noise ratios. The limit of detection and limit of quantification were calculated to be 4.4 x 10(-7) M (86 ppb) and 13.1 x 10(-7) M (258 ppb), respectively, at the 50 microns diameter platinum disk electrode under conditions of linear sweep stripping voltammetry at a scan rate of 200 mV s-1 and a 140 s deposition time. The optimum electrode gave a very well defined gold oxidation signal with negligible background current when applied to the determination of gold in a gold ore sample.

Electrochemical behavior and determination of the insecticide synergist piperonyl butoxide.

Piperonyl butoxide may be reversibly oxidized in acetonitrile at a glassy carbon electrode to a cation radical under short time scale voltammetric conditions, e.g., cyclic voltammetry when the potential scan rate is above 500 mV s(-)(1). During longer time domain experiments, the cation radical decays in a rate-limiting heterolytic bond cleavage step and subsequent transfer of a second electron at the potential of the first process. Additionally, a second oxidation process develops at more positive potentials. One product isolated from the initial oxidation process in an almost quantitative yield, under controlled potential electrolysis conditions, is 6-n-propyl-1,3-benzodioxole-5-carboxaldehyde. This carboxaldehyde is oxidized at the same positive applied potential as the second oxidation process observed in long time domain voltammetric experiments with piperonyl butoxide. The limit of detection for piperonyl butoxide in acetonitrile, using differential pulse voltammetry at a glassy carbon electrode, is 1.6 × 10(-)(6) M (3σ), with a limit of determination of 4.1 × 10(-)(6) M (10σ). Piperonyl butoxide was selectively determined using differential pulse voltammetry with a concentration of 5.11 ± 0.02 g L(-)(1) in a commercial insecticide formulation containing pyrethrins. This result is in good agreement with the manufacturer's stated concentration of 5.07 g L(-)(1). The sample preparation requires only simple dilution of the formulation in an acetonitrile/dichloromethane (95:5) solvent mixture.

Voltammetric determination of platinum in inorganic complexes and in water, geological and biological matrices using laboratory- and field-based instrumentation.

The extremely sensitive catalytic hydrogen ion reduction wave observed after the formation of a platinum-formazone complex at a mercury electrode in a hydrazine-formaldehyde-H2SO4 medium has been utilised to determine platinum voltammetrically in well characterised platinum inorganic complexes (oxidation states O, II and IV) and in biological, geological and water samples. Experimental conditions have been optimised and sample-treatment procedures for various matrices have been critically evaluated for the quantification of platinum by the standard additions method. The determination of platinum in geological samples by this method has been compared with an inductively coupled plasma mass spectrometric method. Both conventional and portable field-based instrumentation have been used in the studies, and the possibility of developing a field-based method for the determination of platinum has been investigated. Despite the inherent sensitivity of the method, which enables concentrations down to 0.01 p.p.b. to be detected in simple matrices, natural levels in water and biological materials, where matrix effects suppress the voltammetric response, are often below the detection limit.

Reactions of M(CO)5X (M = Mn, Re; X = Cl, Br) with (Ph2PCH2)3CCH3 (P3) and (Ph2P(CH2)2)3P (P3P'): synthetic, spectroscopic, electrochemical, and electrospray mass spectrometric studies.

The reactions of M(CO)5X (M = Mn, Re; X = Cl, Br) with (Ph2PCH2)3CCH3 (P3) and (Ph2P(CH2)2)3P (P3P') are investigated, and the products are characterized by IR, NMR (31P and 13C), and electrospray mass spectrometric (ESMS) techniques. With P3, the major products are fac-M(CO)3(eta 2-P3)X (syn and anti isomers) and cis,fac-M(CO)2(eta 3-P3)X, and with P3P', the major product for each metal is cis,mer-M(CO)2(eta 3-P3P')X, but cis-[M(CO)2(eta 4-P3P')]X and fac-[Re(CO)3(eta 3-P3P')]X are also characterized. Addition of MeI to those complexes containing pendant phosphine groups produces the corresponding phosphonium cations without affecting the remainder of the molecule. On the voltammetric time scale, electrochemical oxidation of cis,fac-Mn(CO)2(eta 3-P3)X yields the corresponding 17e cation cis,fac-[Mn(CO)2(eta 3-P3)X]+, but on the longer time scale of exhaustive electrolysis or chemical oxidation, the product is fac-[Mn(CO)3(eta 3-P3)]+. In contrast, the rhenium cation cis,fac-[Re(CO)2(eta 3-P3)X]+ is stable on the synthetic time scale, but upon oxidation of cis,fac-Re(CO)2(eta 3-P3)X with NOBF4, the final product is the 18e [Re(CO)(NO)(eta 3-P3)X]+. cis,mer-Mn(CO)2(eta 3-P3P')X is reversibly oxidized to cis,mer-[Mn(CO)2(eta 3-P3P')X]+ on the voltammetric time scale, but on the longer synthetic time scale, the product isomerizes to trans-[Mn(CO)2(eta 3-P3P')X]+, which can be reduced to trans-Mn(CO)2(eta 3-P3P')X. Upon voltammetric oxidation, the corresponding rhenium complexes show an initial irreversible response associated with the pendant phosphine group prior to the reversible oxidation of the metal on the synthetic time scale; spectroscopic data indicate formation of cis,mer-Re(CO)2(eta 3-P3P'O)X. The complex cis,mer-[Re(CO)2(eta 3-P3P'Me)X]+ shows only the reversible metal oxidation response. ESMS data are obtained directly for the methylated cationic complexes, and neutral complexes are either oxidized or adducted with sodium ions to produce cationic species.

Copper determination in urine by flow injection analysis with electrochemical detection at platinum disk microelectrodes of various radii.

The incorporation of platinum disk microelectrodes of various radii (2.5-50 microns) in a well-jet flow cell offers reduced limits of detection for the determination of copper in urine by flow injection analysis compared with standard methods based on a conventional sized glassy carbon disk macroelectrode (radius 1.5 mm), in a thin-layer cell. The radius of the platinum disk microelectrode was found to be critical with respect to both the limit of detection and flow rate dependence. An optimal radius value of 28 microns was found with detection limits increasing with both larger and smaller electrode radii. In contrast, as theoretically expected, a diminished flow rate dependence was observed the smaller the radii of the platinum disk microelectrodes. Sample cleanup and preparation is conveniently achieved by the use of Sep-Pak cartridges and formation of a copper dithiocarbamate complex. The metal complex is easily oxidized at platinum disk microelectrodes in acetonitrile, which was the solvent used in the flow injection method of analysis.

An experimental evaluation of cyclic voltammetry of multicharged species at macrodisk electrodes in the absence of added supporting electrolyte.

The reversible reduction of [S2Mo18O62]4- to [S2Mo18O62]5- and [S2Mo18O62]6- at a glassy carbon macrodisk electrode has been studied by cyclic voltammetry in acetonitrile as a function of the concentration of [(C6H13)4N]4[S2Mo18O62] in the absence and presence of [(C6H13)4N]ClO4 as the added supporting electrolyte. Consideration is given to the influence of scan rate, reference-working electrode distance, [(C6H13)4N]4[S2Mo18O62], and electrolyte concentrations. Experimental data confirm theoretical predictions that cyclic voltammetry at a macrodisk electrode is a viable technique for studies of multiply charged electroactive species without added electrolyte, provided the influence of enhanced complexities associated with effects of increased solution resistance, the mass transport contribution from migration, and convection arising from enhanced density gradients are considered. Enhanced density gradients present in the absence of added supporting electrolyte give rise to a more marked dependence of voltammograms on the angle of the electrode and hence lead to significant distortion of wave shapes at low scan rates. The summation of all these obstacles implies that quantitative evaluation of cyclic voltammograms of multiply charged species requires significantly greater care in the absence than in the presence of added supporting electrolyte.

Synthesis and redox characterization of the polyoxo anion, gamma*-[S2W18O62]4-: a unique fast oxidation pathway determines the characteristic reversible electrochemical behavior of polyoxometalate anions in acidic media.

The synthesis and characterization of (Bu4N)4[S2W18O62].1.23MeCN.0.27H2O are reported. It crystallizes in the monoclinic space group C2/c with a = 22.389(6) A, b = 22.104(3) A, c = 25.505(5) A, beta = 95.690(15) degrees, V = 12560(5) A3, and Z = 4. The anion exists as the gamma* isomer, the second example of this isomer type to be characterized structurally. The equivalent molybdenum salt occurs as the alpha isomer. gamma*-[S2W18O62]4- in MeCN solution displayed four electrochemically reversible one-electron redox processes at E1/2 values of -0.24, -0.62, -1.18, and -1.57 V versus the Fc+/Fc couple. Upon addition of acid in MeCN/H2O (95/5 v/v), the two most cathodic processes converted to an overall two-electron process at -0.71 V. The total data suggested that this process actually comprises two one-electron transfer processes, occurring at different potentials, with associated proton-transfer reactions. The interpretation is supported by simulation of the effect of acid titration upon the cyclic voltammetry. While multiple pathways for correlated reduction and protonation are present in both the molybdenum and tungsten systems, only a single fast oxidation pathway is available. As the reduced forms of [S2W18O62]4- are much weaker bases than those of [S2Mo18O62]4-, the individual oxidation pathways are not the same. However, their existence determines the highly reversible electrochemical behavior that is characteristic of these anions, and that of polyoxometalate systems in general.

Electron self-exchange in the solid-state: cocrystals of hydroquinone and bipyridyl triazole.

Solid-state voltammetry, spectroscopy, and microscopy studies have been used to probe the proton and electron conductivity within a self-assembled cocrystal, HQBpt. This crystallographically defined material contains 3,5-bis(pyridin-2-yl)-1,2,4-triazole, HBpt, dimers that are pi-stacked and hydrogen bonded to 1,4-hydroquinone, H(2)Q, in a herringbone arrangement. When deposited onto platinum microelectrodes, the cocrystal exhibits a well-defined voltammetric response corresponding to oxidation of H(2)Q to the quinone, Q, across a wide range of voltammetric time scales, electrolyte compositions, and pH values. Scanning electron microscopy reveals that redox cycling in aqueous perchlorate solutions in which the pH is systematically varied from 1 to 7 triggers electrocrystallization and the extensive formation of rodlike crystals. Fast scan rate voltammetry reveals that the homogeneous charge transport diffusion coefficient, D(app), is independent of the perchlorate concentration for 0.1 < [ClO(4)(-)] < 1.0 M (pH 6.6) at 3.14 +/- 0.11 x 10(-)(9) cm(2) s(-)(1). Moreover, D(app) is independent of the perchloric acid concentration for concentrations greater than approximately 2.0 M, maintaining a value of 4.81 +/- 0.07 x 10(-)(8) cm(2) s(-)(1). The observation that D(app) is independent of the supporting electrolyte suggests that the rate-determining step for homogeneous charge transport is not the availability of charge-compensating counterions or protons, but the dynamics of electron self-exchange between H(2)Q and Q. We have used the Dahms-Ruff formalism to determine electron self-exchange rate constants which are 2.84 +/- 0.22 x 10(9) and 9.69 +/- 0.73 x 10(10) M(-)(1) s(-)(1) for pH values greater than approximately 2.0 and less than -0.3, respectively. Significantly, these values are more than 2 orders of magnitude larger that those found for benzoquinone self-exchange reactions in aqueous solution. These results indicate that hydrogen bonds play an important role in supporting rapid electron transfer. The increase in D(app) between pH 1.0 and -0.3 is associated with protonation of the HBpt moieties, which triggers a reversible change in the material's structure.

Coupled Electron- and Proton-Transfer Processes in the Reduction of α-[P(2)W(18)O(62)](6)(-) and α-[H(2)W(12)O(40)](6)(-) As Revealed by Simulation of Cyclic Voltammograms.

Quantitative analysis of the complex problem of coupled electron- and proton-transfer steps during reduction of the polyoxo anions α-[P(2)W(18)O(62)](6)(-) and α-[H(2)W(12)O(40)](6)(-) in aqueous NaCl (0.5 M) has been achieved by simulation of cyclic voltammograms (Rudolph, M.; Reddy, D. P.; Feldberg, S. W. Anal. Chem. 1994, 66, 589A) over wide ranges of anion concentration, pH, and scan rate. Since there are too many unknown parameters to attempt a one-step global form of simulation, a systematic, stepwise approach has been adopted by progressively accessing regimes of increasing voltammetric complexity. This protocol allows experimental behavior in each system over 5 orders of magnitude in proton concentration to be simulated by estimation of three protonation constants combined with experimentally determined reversible half-wave potentials for the two one-electron processes involved. Fast electron transfer and protonation kinetics are assumed. The importance of the values chosen for the diffusion coefficients of the proton and polyoxo anion species is considered. The simulations account for the fact that pairs of one-electron processes coalesce to give an apparent two-electron process in the pH range 1-6 for reduction of both anions.

Evidence for fast and discriminatory electron transfer of proteins at modified gold electrodes.

The electrochemistry of the redox proteins, cytochrome c, cytochrome b5, plastocyanin and ferredoxin at modified gold electrodes has been examined on the basis that electron transfer takes place at electroactive sites which are microscopic in size. Using this model, it is now proposed that electrochemistry of these proteins occurs at suitably modified sites with fast rates at potentials near the standard redox potential. The microscopic model implies that redox proteins and enzymes take part in fast electron transfer at specific sites on the electrode, other sites being completely ineffective. This form of molecular recognition, i.e. the ability to discriminate between the different sites on an electrode surface, mimics homogeneous redox reactions wherein redox active proteins 'recognize' their biological partners in a very specific sense. Previously, protein electrochemistry has been interpreted via use of a macroscopic model in which the proteins are transported to the electrode surface by linear diffusion followed by quasi-reversible or irreversible electron transfer to the electrode surface. The microscopic model, which assumes that the movement of the protein occurs predominantly by radial diffusion to very small sites, would appear to explain the data more satisfactorily and be consistent with biologically important, homogeneous redox reactions which are known to be fast.

Electrochemical synthesis and structural and physical characterization of one- and two-electron-reduced forms of [SMo12O40]2-.

Isolation of a soluble [NHex4]+ salt has allowed a detailed electrochemical study of the anion alpha-[SMo12O40]2- to be undertaken. Four reversible one-electron-reduction processes are observed in CH2Cl2 solution. Controlled potential electrolysis led to isolation of tetraalkylammonium salts of the one-electron-reduced anion alpha-[SMo12O40]3- and the two-electron-reduced anion alpha-[SMo12O40].4- [SMo12O40]3- is stable to disproportionation in dry solvents (Kdis = 10(-7.4). EPR and magnetic susceptibility data indicate that [SMo12O40]3- is a simple paramagnet (S = 1/2) while [SMo12O40]4- is paramagnetic with the mu eff values decreasing at low temperatures. Solutions of the two-electron-reduced species are EPR silent, but microcrystalline powders show very weak signals. The crystal structure of alpha-[NBu4]3[SMo12O40] has been determined (triclinic P1; a = 13.840(3) A; b = 15.587(4) A; c = 19.370(3) A; alpha = 94.82(2) degrees; beta = 93.10(1) degrees; gamma = 91.05(2) degrees; Z = 2). There is disorder around the C2 axis of the central SO4(2-) tetrahedron. In the presence of aqueous HClO4 (0.045 M) in thf/H2O or MeCN/H2O (98/2 v/v), [SMo12O40]2- exhibits five two-electron-reduction processes. Under these conditions, [SMo12O40]3- protonates and disproportionates into [SMo12O40]2- and the (2e-, 2H+)-reduced anion [H2SMo12O40]2-.

Malonaldehyde in cervical mucus associated with copper IUD.

PIP: Malonaldehyde was measured by a colorimetric assay in the cervical mucus specimens of 19 women wearing copper IUDs and 21 women wearing inert IUDs to determine the relationship between copper biochemistry and the potential production of this potential carcinogen. Malonaldehyde was detected in 8 of the 19 specimens from women fitted with copper IUDs but in none of the inert IUD group specimens. The concentrations of the component varied, but were generally in the range of 100 nmol/gm. Based on certain assumptions about the bioreactivity of malonaldehyde, it was estimated that the daily production rate of malonaldehyde associated with copper IUD is from .1-.9 mcmol (7-93 mcg) (this is the cervical rate measurement). Though malonaldehyde is associated with pelvic inflammatory exudates, it is concluded that since the epithelium of the uterus is shed monthly during menstruation, it is unlikely that malonaldehyde associated with the copper IUD is harmful; rather it may have something to do with the copper IUDs' mechanism of action.^ieng