Recent advances in the use of ionic liquids for electrochemical sensing

Ionic Liquids are salts that are liquid at (or just above) room temperature. They possess several advantageous properties (e.g. high intrinsic conductivity, wide electrochemical windows, low volatility, high thermal stability and good solvating ability), which make them ideal as non-volatile electrolytes in electrochemical sensors. This mini-review article describes the recent uses of ionic liquids in electrochemical sensing applications (covering the last 3 years) in the context of voltammetric sensing at solid/liquid, liquid/liquid interfaces and carbon paste electrodes, as well as their use in gas sensing, ion-selective electrodes, and for detecting biological molecules, explosives and chemical warfare agents. A comment on the future direction and challenges in this field is also presented.     

The Identification of Polynuclear Aromatic Hydrocarbon (PAH) Derivatives in Mutagenic Fractions of Diesel Particulate Extracts

Abstract

The soluble organic fraction (SOF) of particulate matter from diesel exhaust (from point sources, ambient air, etc.) contains hundreds of organic constituents. Normal-phase high pressure liquid chromatography (HPLC) has been used to separate the SOF into sub-fractions suitable for subsequent chemical analysis and bioassays. These fractions consist of non-polar(PAH), moderately polar (transition) and highly polar constituents. The non-polar fractions have been well characterized and consist of PAH and aliphatic hydrocarbons. The specific compounds present in the transition and polar fractions are for the most part unknown. This analytical information has been difficult to obtain since these compounds are highly labile, polar, of low volatility and in very low concentrations when compared to the bulk of material found in the SOF. Mutagenicity tests using the Ames Salmonella typhimurium assay indicate that the transition fraction accounts for most of the mutagenicity when compared to the non-polar (PAH) and polar fractions.

A variety of chromatographic and mass spectrometric techniques are described that have been used to determine the composition of the HPLC fractions. More than one hundred species have been identified in the transition fraction of diesel particulate matter using high resolution gas chromatography (HRGC)/high resolution mass spectrometry (HRMS), HPLC and direct-probe high resolution mass spectrometry. It has been found that the transition fraction contains mostly PAH derivatives consisting of hydroxy, ketone, quinone, carboxaldehyde, acid anhydride and dihydroxy derivatives of PAH. Three nitro-PAH species have been tentatively identified and 1-nitropyrene positively identified in the transition fraction. The 1-nitropyrene was found to account for approximately 45% and 30% of the direct-acting mutagenicity observed for the transition fraction and total extract, respectively. The HPLC separation procedure was shown to give better than 95% recovery of the mass and mutagenic activity. The problem of PAH oxidation during the analytical procedures and possible effect on bioassay results are discussed.     

Electrochemical oxidation of nitrite and the oxidation and reduction of NO2 in the room temperature ionic liquid [C2mim][NTf2

The electrochemical oxidation of potassium nitrite has been studied in the room temperature ionic liquid (RTIL) [C2mim][NTf2] by cyclic voltammetry at platinum electrodes. A chemically irreversible oxidation peak was observed, and a solubility of 7.5(+/-0.5) mM and diffusion coefficient of 2.0(+/-0.2)x10(-11) m2 s(-1) were calculated from potential step chronoamperometry on the microdisk electrode. A second, and sometimes third, oxidation peak was also observed when the anodic limit was extended, and these were provisionally assigned to the oxidation of nitrogen dioxide (NO2) and nitrate (NO3-), respectively. The electrochemical oxidation of nitrogen dioxide gas (NO2) was also studied by cyclic voltammetry in [C2mim][NTf2] on Pt electrodes of various size, giving a solubility of ca. 51(+/-0.2) mM and diffusion coefficient of 1.6(+/-0.05)x10(-10) m2 s(-1) (at 25 degrees C). It is likely that NO2 exists predominantly as its dimer, N2O4, at room temperature. The oxidation mechanism follows a CE process, which involves the initial dissociation of the dimer to the monomer, followed by a one-electron oxidation. A second, larger oxidation peak was observed at more positive potentials and is thought to be the direct oxidation of N2O4. In addition to understanding the mechanisms of NO2- and NO2 oxidations, this work has implications in the electrochemical detection of nitrite ions and of NO2 gas in RTIL media, the latter which may be of particular use in gas sensing.     

Electrochemical Ammonia Gas Sensing in Nonaqueous Systems: A Comparison of Propylene Carbonate with Room Temperature Ionic Liquids

First, the direct and indirect electrochemical oxidation of ammonia has been studied by cyclic voltammetry at glassy carbon electrodes in propylene carbonate. In the case of the indirect oxidation of ammonia, its analytical utility of indirect for ammonia sensing was examined in the range from 10 and 100 ppm by measuring the peak current of new wave resulting from reaction between ammonia and hydroquinone, as function of ammonia concentration, giving a sensitivity 1.29×10^?7 A ppm^?1 (r²=0.999) and limit‐of‐detection 5 ppm ammonia. Further, the direct oxidation of ammonia has been investigated in several room temperature ionic liquids (RTILs), namely 1‐butyl‐3‐methylimidazolium tetrafluoroborate ([C₄mim] [BF₄]), 1‐butyl‐3‐methylimidazolium trifluoromethylsulfonate ([C₄mim] [OTf]), 1‐Ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim] [NTf₂]), 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄mim] [NTf₂]) and 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([C₄mim] [PF₆]) on a 10 μm diameter Pt microdisk electrode. In four of the RTILs studied, the cyclic voltammetric analysis suggests that ammonia is initially oxidized to nitrogen, N₂, and protons, which are transferred to an ammonia molecule, forming NH via the protonation of the anion(s) (A^?). However, in [C₄mim] [PF₆], the protonated anion was formed first, followed by NH . In all five RTILs, both HA and NH are reduced at the electrode surface, forming hydrogen gas, which is then oxidized. The analytical ability of this work has also been explored further, giving a limit‐of‐detection close to 50 ppm in [C₂mim] [NTf₂], [C₄mim] [OTf], [C₄mim] [BF₄], with a sensitivity of ca. 6×10^?7 A ppm^?1 (r²=0.999) for all three ionic liquids, showing that the limit of detection was ca. ten times larger than that in propylene carbonate since ammonia in propylene carbonate might be more soluble in comparison with RTILs when considering the higher viscosity of RTILs.     

Development and validation of a high-performance thin-layer chromatography method for the simultaneous quantification of rosmarinic acid,quercetin, glycyrrhizin and betulinic acid in polyherbal immunostimulant formulation

JPC – Journal of Planar Chromatography – Modern TLC - Quantification of bioactive markers through modern analytical methods is very essential for establishing the authenticity and...     

Synthesis of phenazine derivatives for use as precursors to electrochemically generated bases

1,6-Disubstituted phenazine derivatives for use as precursors to electrochemically generated bases have been synthesized from readily available starting materials. Reaction of 1,6-dihydroxyphenazine with 1,10-diododecane, 1,11-diiodo-3,6,9-trioxaundecane or (R,R)-(-)-1,2-bis(3-iodopropoxy)cyclohexane gave planar chiral phenazinophanes containing ether-linked bridges; molecular structures of all these compounds have been determined by X-ray crystallography. Substituted 1,6-diaminophenazines were prepared by palladium-mediated amination of 1,6-dichlorophenazine and acylation of 1,6-diaminophenazine followed by reduction. Reaction of 1,6-bis(alkylamino)phenazines with sebacoyl chloride gave planar chiral phenazinophanes containing amide-linked bridges.     

A new method for the preparation of68Ga-generators following proton bombardment of gallium oxide targets

A method of preparing a⁶⁸Ge/⁶⁸Ga generator directly from an irradiated target of Ga₂O₃ is discussed, which utilises an ion exchange column containing hydrous zirconium oxide (HZO). A yield of 30–40% of⁶⁸Ga is reported with a breakthrough of⁶⁸Ge as low as one part in 10⁵. The leakage of stable zirconium from the column may be a problem for routine clinical use and this is being investigated further.     

Weighted complex network analysis of travel routes on the Singapore public transportation system

The structure and properties of public transportation networks have great implications for urban planning, public policies and infectious disease control. We contribute a complex weighted network analysis of travel routes on the Singapore rail and bus transportation systems. We study the two networks using both topological and dynamical analyses. Our results provide additional evidence that a dynamical study adds to the information gained by traditional topological analysis, providing a richer view of complex weighted networks. For example, while initial topological measures showed that the rail network is almost fully connected, dynamical measures highlighted hub nodes that experience disproportionately large traffic. The dynamical assortativity of the bus networks also differed from its topological counterpart. In addition, inspection of the weighted eigenvector centralities highlighted a significant difference in traffic flows for both networks during weekdays and weekends, suggesting the importance of adding a temporal perspective missing from many previous studies.     

On the Distances between Latin Squares and the Smallest Defining Set Size

In this note, we show that for each Latin square L of order , there exists a Latin square of order n such that L and differ in at most cells. Equivalently, each Latin square of order n contains a Latin trade of size at most . We also show that the size of the smallest defining set in a Latin square is .     

A Black-Box Multigrid Preconditioner for the Biharmonic Equation

We examine the convergence characteristics of a preconditioned Krylov subspace solver applied to the linear systems arising from low-order mixed finite element approximation of the biharmonic problem. The key feature of our approach is that the preconditioning can be realized using any “black-box” multigrid solver designed for the discrete Dirichlet Laplacian operator. This leads to preconditioned systems having an eigenvalue distribution consisting of a tightly clustered set together with a small number of outliers. Numerical results show that the performance of the methodology is competitive with that of specialized fast iteration methods that have been developed in the context of biharmonic problems. This revised version was published online in July 2006 with corrections to the Cover Date.