Molecular reactors and machines: applications, potential, and limitations

Molecular reactors are miniature vessels for the assembly of reactants at the molecular level, in order to change the nature of chemical transformations. It seems probable that those that will find most immediate applications are those that change product ratios or give products which would not readily form in the absence of the reactors, and thereby afford easy access to materials that are otherwise difficult to obtain. Molecular machines consist of interrelated parts with separate functions and perform some kind of work, at the molecular level. Practical examples are likely to be relatively uncomplicated and not based on individual functions of single-molecule devices. Instead they will probably rely on extensive redundancy of the molecular components and their interactions and reactions, as well as of the machines themselves.     

Advantages and limitations of combining separation techniques with voltammetry

In this review the combination of voltammetric methods of analysis with separation techniques, such as electrochemical separations, extractive separations, chromatography, electrochromatography, electrophoresis, volatilization and coprecipitation is critically discussed. Based on available publications and their own experience, the authors try to evaluate different separation techniques in view of future developments, paying special attention to extraction methods. For organic analysis, chromatography and electrophoresis with voltammetric detection are considered to be especially attractive while for stripping analysis, major progress is expected to be made in the development of electrodes and electronic equipment. Extraction, coprecipitation and volatilization will be of minor significance.     

Differential pulse adsorptive stripping voltammetry of osmium-modified peptides

Complexes of osmium tetroxide with nitrogen ligands were developed and used in our laboratory as probes of the DNA structure. Here, we show that the complex of osmium tetroxide with 2,2'-bipyridine (Os,bipy) can be used for modification and electrochemical detection of proteins at neutral pH. Salmon luteinizing hormone (SLH) containing two tryptophan (Trp) residues and human luteinizing hormone (HLH) containing one Trp were modified by Os,bipy and measured by differential pulse adsorptive stripping voltammetry (DPAdSV) at a hanging mercury drop electrode (HMDE). The intensity of the DPAdSV catalytic signals corresponded to the number of Trp residues in the peptide molecule. Decreasing pH of the background electrolyte from 6.6 to 3.8 led to the increase of DPAdSV signals, suggesting that at pH 3.8, the DPAdSV detection limit might be well below 1 ng/ml. Our results suggest that Os,bipy is potentially useful for chemical modification of proteins.     

Rapid stopped-flow voltammetry with potential scanning

The stopped-flow voltammetric technique described is based on measurements of the differences between currents with the solution flow on and off while the applied potential is scanned linearly. A flow cell with an impinging jet of solution on a stationary planar or porous carbon disk electrode is employed. A nonsteady-state modulation approach is utilized as a compromise between sensitivity and speed. Charging and surface-transient background currents are eliminated, allowing measurements at micromolar concentrations. Well-defined current—potential curves are obtained and compared with those obtained by differential pulse voltammetry. Ascorbic acid, hexacyanoferrate(II), NADH, and dopamine were used as test systems. When 3 s on and 3 s off flow periods were used, detection limits of 0.1 μM dopamine and 0.2 μM ascorbic acid or hexacyanoferrate(II) were obtained.     

Ultrafast cyclic voltammetry with asymmetrical potential scan

Based on the perfect ohmic drop compensation by online electronic positive feedback, ultrafast cyclic voltammetry with asymmetrical potential scan is achieved for the first time, with the reduction of anthracene acting as the test system. Compared with the traditional cyclic voltammetry utilizing symmetrical triangular waveform as the excitation one, the new method allows a simpler approach to mechanistic analysis of ultrafast chemical reactions coupled with a charge transfer. And perhaps more important, it also provides a way to eliminate the interference of the adsorbed product in dynamic monitoring. 2007 Zhi Yong Guo. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.     

Stir-bar sorptive extraction: A view on method optimisation,novel applications,limitations and potential solutions

Introduced in 1999 as a novel solventless sample preparation method, stir-bar sorptive extraction (SBSE) has become a popular analytical technique for the pre-concentration of organic compounds into a polydimethylsiloxane (PDMS)-coated stir-bar. In the last 10 years, hundreds of applications in the environmental, food and biomedical fields can be found in the literature. However, only PDMS-coated stir-bars are commercially available, which reduces the applicability of SBSE to the extraction of the non-polar compounds due to the poor extractability of more polar analytes. In this review, a view on method optimisation, limitations, potential solutions such as in-house coatings and derivatisation and novel applications in multi-residue analysis and passive sampling are revised.     

The dual reciprocity method: simulation of potential step voltammetry at microelectrodes

The dual reciprocity method (DRM) is presented as an efficient and powerful method for the analysis of electrochemical processes occurring under diffusional transport control. This article outlines the theory and numerical details required to apply the DRM for modelling chronoamperometry at microband and microcylinder electrodes. The results obtained are found to be in good agreement with analytical approximations for the two geometries. The benefits of the DRM approach are discussed, including the reduction in dimensionality brought about by the formulation procedure which permits a two dimensional diffusion problem to be modelled using a one dimensional boundary mesh. The flexibility of the method for tackling geometries of complex topography are noted.     

Convolution potential sweep voltammetry: Part IV. Homogeneous follow-up chemical reactions

The application of Convolution Potential Sweep Voltammetry to mechanism analysis and rate determination in electrochemical processes involving homogeneous chemical reaction is discussed. The formal analysis of the transition between pure diffusion control and pure kinetic condition is treated in the case of a first order follow-up reaction. The practical applicability of the method is then tested on the reductive pinacolization of acetophenone in acetonitrile using as operational parameters the sweep rate, the initial concentration and the water content of the medium.     

A straightforward approach towards cyclic peptides via ring-closing metathesis--scope and limitations

N- and C-terminal diallylated peptides are obtained by several approaches, such as peptide Claisen rearrangement, N- and O- allylation, and the Ugi reaction of allyl-protected components. These diallylated peptides are suitable substrates for ring-closing metathesis and the success of this cyclisation was investigated with respect to the ring size, the position of the allyl moieties and the reaction parameters. In general, excellent yields are obtained for cyclisation of allyl glycine subunits and N-allylated amides, while allyl esters and allyl carbamates often presented serious problems. However, yields of up to 73% were obtained under optimised conditions, and the new generated double bond is formed with excellent trans-selectivity.     

Oligonuclear ferrocene amides: mixed-valent peptides and potential redox-switchable foldamers

Trinuclear ferrocene tris-amides were synthesized from an Fmoc- or Boc-protected ferrocene amino acid, and hydrogen-bonded zigzag conformations were determined by NMR spectroscopy, molecular modelling, and X-ray diffraction. In these ordered secondary structures orientation of the individual amide dipole moments approximately in the same direction results in a macrodipole moment similar to that of α-helices composed of α-amino acids. Unlike ordinary α-amino acids, the building blocks in these ferrocene amides with defined secondary structure can be sequentially oxidized to mono-, di-, and trications. Singly and doubly charged mixed-valent cations were probed experimentally by Vis/NIR, paramagnetic 1H NMR and M?ssbauer spectroscopy and investigated theoretically by DFT calculations. According to the appearance of intervalence charge transfer (IVCT) bands in solution, the ferrocene/ferrocenium amides are described as Robin-Day class II mixed-valent systems. M?ssbauer spectroscopy indicates trapped valences in the solid state. The secondary structure of trinuclear ferrocene tris-amides remains intact (coiled form) upon oxidation to mono- and dications according to DFT calculations, while oxidation to the trication should break the intramolecular hydrogen bonding and unfold the ferrocene peptide (uncoiled form).     

Laser microprobe mass spectrometry: potential and limitations for inorganic and organic micro-analysis

Summary Laser microprobe mass spectrometry (LMMS) employs a highly focused UV laser beam to ionise a microvolume in the order of 1 m³. The ions produced are then mass-separated in a time-of-flight (TOF) or a Fourier Transform (FT) mass spectrometer. TOF LMMS allows element localisation, detailed speciation of inorganic substances and structural information of organic molecules. Quantitation is difficult. This paper focuses on instrumental aspects and inorganic analysis. Organic applications are treated in part II of this series. Selected examples illustrate that TOF LMMS is a valuable tool for the qualitative characterisation of micro-samples. Also, the applicability to the analysis with high spatial resolution is shown. The current technology and the prospects from the recent FTMS development are discussed.     

Voltammetry at partially covered electrodes: Part II. Linear potential sweep and cyclic voltammetry

Effect of inhomogeneity of the electrode surface on the linear potential sweep and cyclic voltammograms is investigated theoretically and experimentally using model electrodes partially covered with photoresist layer. Good agreement between the theoretical and experimental results is obtained.     

Microcells for voltammetry and stripping voltammetry

The designs and applications of "non-flow" microcells in voltammetry and stripping voltammetry for samples with volumes from sub-microliters to 5 ml are reviewed. The analysis of microcell designs was carried out on the basis of their classification: (i) microcells with a static sample, including thin-layer microcells with a static sample; (ii) microcells with forced convection of the sample; and (iii) microcells for batch injection of samples. Two working electrode types (the usual state and inverted state) are discussed. The use of working micro- and mercury-film electrodes and of the accelerated removal of dissolved oxygen are also considered in detail.     

Some observations on the accumulation potential in adsorptive stripping voltammetry

A newly named parameter, “initial scan potential”, the potential from which the voltammetric scan starts rather than the widely accepted accumulation potential, has been found to be responsible for some of the important features of voltammograms obtained in the adsorptive stripping voltammetry (AdSV) of some organochlorine compounds (DDT, Dieldrin, 2,4- dichlorophenoxyacetic acid, 2,4,5-trichlorophenoxyacetic acid). The experimental evidence reported strongly confirms the nonfaradaic nature of the preconcentration step in adsorptive stripping voltammetry.     

Additive differential pulse voltammetry,instead of double differential pulse voltammetry

A new electrochemical double pulse potential technique called additive differential pulse voltammetry (ADPV) is proposed. This technique is inspired by the original idea of Birke et al. [Anal. Chem. 53 (1981) 852] of recording two differential pulse (DP) voltammograms and it consists of plotting the sum of these two signals versus the first pulse potential, although in this paper the proposal is to obtain the ADPV signal through just one experiment. ADPV behaves in an identical way to the triple-pulse technique double differential pulse voltammetry (DDPV) for reversible processes when diffusion coefficients are equal for spherical electrodes and for any value of diffusion coefficients in planar electrodes. In the case of reversible electrode processes with amalgamation of reaction product or other more complex processes, ADPV is more advantageous than DDPV. This is due, among other reasons, to the fact that, under these conditions, a double potential step is much simpler to analyse than a triple potential step.     

Prospect of determining copper sulfide nanoparticles by voltammetry: A potential artifact in supersaturated solutions

Certain natural waters appear to contain copper sulfide (Cu_xS) nanoparticles in nanomolar concentrations (as Cu). These nanoparticles have been tentatively identified by the characteristic pH below which they deposit sulfide onto Hg electrodes. A proposed alternate approach to studying Cu_xS nanoparticles relies on their hydrophobicity, which causes them to sorb to Hg electrodes; there they can undergo reduction at −0.9 to −1.1 V vs. Ag/AgCl. However, solutions supersaturated with respect to Cu sulfide phases also form Cu_xS directly at Hg electrode surfaces. The voltammetric reduction peaks obtained from these deposits are not clearly distinguishable from those obtained from sorbed nanoparticles. Surface formation of Cu_xS, which appears to be limited to approximately two layers, involves a reaction between Cu amalgam and electrodeposited HgS. Surface-formed Cu_xS could be problematic in studies of Cu_xS nanoparticles, but this obstacle can be avoided by conducting voltammetric accumulations at potentials too negative for HgS electrodeposition (e.g. −0.85 V). Electroreduction of surface-formed Cu_xS occurs by a two-dimensional instantaneous hole nucleation and growth process.