Extending Photocatalyst Activity through Choice of Electron Donor.

Sacrificial additives are commonly employed in photoredox catalysis as a convenient source of electrons, but what occurs after electron transfer is often overlooked. Tertiary alkylamines initially form radical cations following electron transfer, which readily deprotonate to form strongly reducing, neutral α-amino radicals. Similarly, the oxalate radical anion (C2O4•-) rapidly decomposes to form CO2•- (E0 ≈ -2.2 V vs SCE). We show that not only are these reactive intermediates formed under photoredox conditions, but they can also impact the desired photochemistry, both positively and negatively. Photoredox systems using oxalate as an electron donor are able to engage substrates with greater energy demands, extending reactivity past the energy limits of single and multiphoton transition metal catalysts. Furthermore, oxalate offers better chemoselectivity than the commonly employed triethylamine when reducing substrates with moderate energy requirements.

Using Nitroxides to Enhance Carbon Fiber Interfacial Adhesion and as an Anchor for "Graft to" Surface Modification Strategies.

Nitroxide groups covalently grafted to carbon fibers are used as anchoring sites for TEMPO-terminated polymers (poly-n-butylacrylate and polystyrene) in a "graft to" surface modification strategy. All surface-modified fibers are evaluated for their physical properties, showing that several treatments have enhanced the tensile strength and Young's modulus compared to the control fibers. Up to an 18% increase in tensile strength and 12% in Young's modulus are observed. Similarly, the evaluation of interfacial shear strength in an epoxy polymer shows improvements of up to 144% relative to the control sample. Interestingly, the polymer-grafted surfaces show smaller increases in interfacial shear strength compared to surfaces modified with a small molecule only. This counterintuitive result is attributed to the incompatibility, both chemical and physical, of the grafted polymers to the surrounding epoxy matrix. Molecular dynamics simulations of the interface suggest that the diminished increase in mechanical shear strength observed for the polymer grafted surfaces may be due to the lack of exposed chain ends, whereas the small molecule grafted interface exclusively presents chain ends to the resin interface, resulting in good improvements in mechanical properties.

The Tandem Photoredox Catalysis Mechanism of [Ir(ppy)2(dtb-bpy)]+ Enabling Access to Energy Demanding Organic Substrates.

We report the discovery of a tandem catalytic process to reduce energy demanding substrates, using the [Ir(ppy)2(dtb-bpy)]+ (1+) photocatalyst. The immediate products of photoinitiated electron transfer (PET) between 1+ and triethylamine (TEA) undergo subsequent reactions to generate a previously unknown, highly reducing species (2). Formation of 2 occurs via reduction and semisaturation of the ancillary dtb-bpy ligand, where the TEA radical cation serves as an effective hydrogen atom donor, confirmed by nuclear magnetic resonance, mass spectrometry, and deuterium labeling experiments. Steady-state and time-resolved luminescence and absorption studies reveal that upon irradiation, 2 undergoes electron transfer or proton-coupled electron transfer (PCET) with a representative acceptor (N-(diphenylmethylene)-1-phenylmethanamine; S). Turnover of this new photocatalytic cycle occurs along with the reformation of 1+. We rationalize our observations by proposing the first example of a mechanistic pathway where two distinct yet interconnected photoredox cycles provide access to an extended reduction potential window capable of engaging a wide range of energy demanding and synthetically relevant organic substrates including aryl halides.

Mixed annihilation electrogenerated chemiluminescence of iridium(iii) complexes.

Previously reported annihilation ECL of mixtures of metal complexes have generally comprised Ir(ppy)3 or a close analogue as a higher energy donor/emitter (green/blue light) and [Ru(bpy)3]2+ or its derivative as a lower energy acceptor/emitter (red light). In contrast, here we examine Ir(ppy)3 as the lower energy acceptor/emitter, by combining it with a second Ir(iii) complex: [Ir(df-ppy)2(ptb)]+ (where ptb = 1-benzyl-1,2,3-triazol-4-ylpyridine). The application of potentials sufficient to attain the first single-electron oxidation and reduction products can be exploited to detect Ir(ppy)3 at orders of magnitude lower concentration, or enhance its maximum emission intensity at high concentration far beyond that achievable through conventional annihilation ECL of Ir(ppy)3 involving comproportionation. Moreover, under certain conditions, the colour of the emission can be selected through the applied electrochemical potentials. We have also prepared a novel Ir(iii) complex with a sufficiently low reduction potential that the reaction between its reduced form and Ir(ppy)3+ cannot populate the excited state of either luminophore. This enabled, for the first time, the exclusive formation of either excited state through the application of higher cathodic or anodic potentials, but in both cases, the ECL was greatly diminished by parasitic dark reactions.

Detection of biogenic amines in pet food ingredients by RP-HPLC with automated dansyl chloride derivatization.

The detection of biogenic amines is of significant interest to the food industry, as they can be used as indicators of food spoilage and they are potentially toxic. Because of their importance, there is a need for automated methods suitable for industry use that can detect a wide range of biogenic amines at sufficient levels for food analysis. In this work, optimized conditions for the automated determination of biogenic amines (histamine, putrescine, cadaverine, spermine, spermidine, tyramine, and tryptamine) derivatized with dansyl chloride are presented. Limits of detection below 0.2 ppm were achieved for seven biogenic amines and percentage recoveries were between 80 and 109% for the seven analytes spiked into meat meal samples. The method is simple and compared well to an existing method for the detection of biogenic amines in pet food ingredients.

Determination of neurotransmitters and their metabolites using one- and two-dimensional liquid chromatography with acidic potassium permanganate chemiluminescence detection.

High-performance liquid chromatography with chemiluminescence detection based on the reaction with acidic potassium permanganate and formaldehyde was explored for the determination of neurotransmitters and their metabolites. The neurotransmitters norepinephrine and dopamine were quantified in the left and right hemispheres of rat hippocampus, nucleus accumbens and prefrontal cortex, and the metabolites vanillylmandelic acid, 3,4-dihydrophenylacetic acid, 5-hydroxyindole-3-acetic acid and homovanillic acid were identified in human urine. Under optimised chemiluminescence reagent conditions, the limits of detection for these analytes ranged from 2.5 × 10(-8) to 2.5 × 10(-7) M. For the determination of neurotransmitter metabolites in urine, a two-dimensional high-performance liquid chromatography (2D-HPLC) separation operated in heart-cutting mode was developed to overcome the peak capacity limitations of the one-dimensional separation. This approach provided the greater separation power of 2D-HPLC with analysis times comparable to conventional one-dimensional separations.

Chemiluminescence and electrochemiluminescence of water-soluble iridium(III) complexes containing a tetraethylene-glycol functionalised triazolylpyridine ligand.

BACKGROUND: Iridium(III) complexes, exhibiting high luminescence quantum yields and a wide range of emission colours, are promising alternatives to tris(2,2'-bipyridine)ruthenium(II) for chemiluminescence (CL) and electrochemiluminescence (ECL) detection. This emerging class of reagent, however, is limited by the poor solubility of many iridium(III) complexes in aqueous solution, and lack of understanding of their remarkably variable selectivities towards different analytes. RESULTS: Seven [Ir(C^N)2(pt-TEG)]+ complexes, exhibiting a wide range of reduction potentials and emission energies, were examined with six model analytes. For CL, cerium(IV) was used as the oxidant. The alkylamine analytes generally produced greater CL and ECL with the more readily oxidised Ir(III) complexes (C^N = piq, bt, ppy), predominantly through the 'direct' pathway requiring oxidation of both metal complex and analyte. Aniline derivatives that did not also contain secondary or tertiary alkylamines elicited CL from the less readily oxidised complexes (C^N = df-ppy-CF3, df-ppy) via energy transfer. The most difficult to oxidise complexes (C^N = df(CF3)-ppy-Me, df(CN)-ppy) gave poor responses due to the limited potential window of the solvent and inefficiency of energy transfer to their high energy excited states. Greater CL and/or ECL intensities were generally obtained for each analyte with at least one Ir(III) complex than with [Ru(bpy)3]2+; superior limits of detection for two analytes were demonstrated. SIGNIFICANCE: This exploration of CL/ECL in which the properties of luminophore, analyte and oxidant are all varied provides a new understanding of the influence of the metal-complex potentials and excited state energy on the light-producing and quenching pathways, and consequently, their distinct selectivity towards different analytes. These findings will guide the development of water-soluble Ir(III) complexes as CL and ECL reagents.

Controlled formation of mono- and dihydroxy-resolvins from EPA and DHA using soybean 15-lipoxygenase.

Resolvins and protectins are important anti-inflammatory and pro-resolution compounds derived from the enzymatic oxidation of omega-3 fatty acids all-cis-5,8,11,14,17-eicosapentaenoic acid (EPA) and all-cis-4,7,10,13,16,19-docosahexaenoic acid (DHA). We have developed a simple, controlled method to synthesize an array of resolvin and protectin analogs from fatty acid starting materials using soybean 15-lipoxygenase. The conditions were optimized for the production of both mono- and dihydroxy derivatives, with enzyme concentration and pH found to have a significant effect on the reaction products. The methods were applied to five biologically important omega-3 and omega-6 fatty acid substrates. Mono- and dihydroxy compounds were successfully synthesized from all substrates and the products were characterized by normal phase (NP) HPLC, GC-MS, TOF-MS, UV-visible (UV-vis) spectroscopy, and NMR spectroscopy. The methods could be further applied to any polyunsaturated fatty acids containing the cis-1,4,7,10-undecatetraene moiety to produce a range of novel compounds with potential biological activity.

Chemiluminescence evidence supporting the selective role of ligands in the permanganate oxidation of micropollutants.

The selective increase in the oxidation rate of certain organic compounds with permanganate in the presence of environmental "ligands" and reduced species has been ascribed to the different reactivity of the target compounds toward Mn(III), which bears striking similarities to recent independent investigations into the use of permanganate as a chemiluminescence reagent. In spite of the importance of Mn(III) in the light-producing pathway, the dependence of the oxidation mechanism for any given compound on this intermediate could not be determined solely through the emission intensity. However, target compounds susceptible to single-electron oxidation by Mn(III) (such as bisphenol A and triclosan) can be easily distinguished by the dramatic increase in chemiluminescence intensity when a permanganate reagent containing high, stable concentrations of Mn(III) is used. The differences are accentuated under the low pH conditions that favor the chemiluminescence emission due to the greater reactivity of Mn(III) and the greater influence of complexing agents. This study supports the previously postulated selective role of ligands and reducing agents in permanganate oxidations and demonstrates a new approach to explore the chemistry of environmental manganese redox processes.

Pollen baiting facilitates the isolation of marine thraustochytrids with potential in omega-3 and biodiesel production.

Marine heterotrophic microbes are capable of accumulating large amounts of lipids, omega-3 fatty acids, carotenoids, and have potential for biodiesel production. Pollen baiting using Pinus radiata pollen grain along with direct plating techniques were used in this study as techniques for the isolation of oil-producing marine thraustochytrid species from Queenscliff, Victoria, Australia. Thirteen isolates were obtained using either direct plating or using pine pollen, with pine pollen acting as a specific substrate for the surface attachment of thraustochytrids. The isolates obtained from the pollen baiting technique showed a wide range of docosahexaenoic acid (DHA) accumulation, from 11 to 41 % of total fatty acid content (TFA). Direct plating isolates showed a moderate range of DHA accumulation, from 19 to 25 % of TFA. Seven isolates were identified on the basis of 18S rRNA sequencing technique as Thraustochytrium species, Schizochytrium species, and Ulkenia species. Although both methods appear to result in the isolation of similar strains, pollen baiting proved to be a simpler method for the isolation of these relatively slow-growing organisms.

Enhanced permanganate chemiluminescence.

The significant enhancement of acidic potassium permanganate chemiluminescence by Mn(II) results from the concomitant presence of permanganate and Mn(III) in the reagent solution, which enables rapid production of the excited Mn(II) emitter with a wide range of analytes. Furthermore, the key Mn(III) co-reactant can be quickly generated by reducing permanganate with sodium thiosulfate, instead of the slow (~24 h) equilibration required when Mn(ii) is used. The emission from reactions with analytes such as tyrosine and fenoterol was over two orders of magnitude more intense than with the traditional permanganate reagent.

Mechanism of permanganate chemiluminescence.

Spectroscopic and synthetic methods have been exploited to deduce the mechanism for acidic potassium permanganate chemiluminescence. We have employed electron paramagnetic resonance (EPR) spectroscopy with a continuous flow assembly to monitor the formation of radical intermediates in real time generated from substrate oxidation by manganese(VII). These transient species react with manganese(III) in solution to produce the previously characterized manganese(II)* emission source. Using UV-vis, EPR, attenuated total reflection (ATR)-FT-IR, and chemiluminescence spectroscopies, we have established that there are two distinct enhancement mechanisms that in combination afford a 50-fold increase in emission intensity when the reaction is conducted in the presence of phosphate oligomers. In addition to preventing disproportionation of the manganese(III) precursor, the phosphate oligomers form protective "cagelike" structures around the manganese(II)* emitter, thus preventing nonradiative relaxation pathways.

Peak modeling approach to accurate assignment of first-dimension retention times in comprehensive two-dimensional chromatography.

Modeling of first-dimension retention of peaks based on modulation phase and period allows reliable prediction of the modulated peak distributions generated in the comprehensive two-dimensional chromatography experiment. By application of the inverse process, it is also possible to use the profile of the modulated peaks (their heights or areas) to predict the shape and parameters of the original input chromatographic band (retention time, standard deviation, area) for the primary column dimension. This allows an accurate derivation of the first-dimension retention time (RSD 0.02%) which is equal to that for the non-modulated experiment, rather than relying upon the retention time of the major modulated peak generated by the modulation process (RSD 0.16%). The latter metric can produce a retention time that differs by at least the modulation period employed in the experiment, which displays a discontinuity in the retention time vs modulation phase plot at the point of the 180 degrees out-of-phase modulation. In contrast, the new procedure proposed here gives a result that is essentially independent of modulation phase and period. This permits an accurate value to be assigned to the first-dimension retention. The proposed metric accounts for the time on the second-dimension, the phase of the distribution, and the hold-up time that the sampled solute is retained in the modulating interface. The approach may also be based on the largest three modulated peaks, rather than all modulated peaks. This simplifies the task of assigning the retention time with little loss of precision in band standard deviation or retention time, provided that these peaks are not all overloaded in the first or second dimension.

Precision milled flow-cells for chemiluminescence detection.

Novel flow-cells with integrated confluence points and reaction channels designed for efficient mixing of fast chemiluminescence systems were constructed by machining opposing sides of a polymer chip and sealing the channels with transparent epoxy-acetate films. A hole drilled through the chip provided the conduit from the confluence point on one side to the centre of the reaction zone on the other side, allowing rapid presentation of the reacting mixture to the photodetector. The effectiveness of each flow-cell was evaluated by comparing the chemiluminescence intensity using flow injection analysis methodology, and examining the distribution of light emanating from the reaction zone (captured by photography in a dark room) when the reactants were continuously merged. Although previously reported chemiluminescence detectors constructed by machining channels into polymers have almost exclusively been prepared using transparent materials, we obtained far greater emission intensities using an opaque white chip with a thin transparent seal, which minimised the loss of light through surfaces not exposed to the photomultiplier tube. Furthermore, this approach enabled the exploration of reactor designs that could not be incorporated in traditional coiled-tubing flow-cells.

Emitting species in chemiluminescence reactions with acidic potassium permanganate: a re-evaluation based on new spectroscopic evidence.

The reaction of acidic potassium permanganate with a wide range of compounds is known to produce a broad red emission, and there is strong evidence for an excited manganese(II) emitting species. Nevertheless, numerous researchers have proposed other emitters for reactions with acidic potassium permanganate, particularly for systems where fluorescent compounds were present, either as enhancers or reaction products. We have examined many reactions of this type and found that, in most cases, the same red emission was produced. There were, however, some exceptions, including the oxidation of dihydralazine, certain thiols and sulphite (each in the presence of an enhancer).

Effects of variation in modulator temperature during cryogenic modulation in comprehensive two-dimensional gas chromatography.

Many modulation systems in comprehensive 2D GC (GC x GC) are based on cryogenic methods. High trapping temperatures in these systems can result in ineffective trapping of the more volatile compounds, whilst temperatures that are too low can prevent efficient remobilisation of some compounds. To better understand the trapping and release of compounds over a wide range of volatilities, we have investigated a number of different constant temperature modulator settings, and have also examined a constant temperature differential between the cryo-trap and the chromatographic oven. These investigations have led us to modify the temperature regulation capabilities of the longitudinally modulated cryogenic system (LMCS). In contrast to the current system, where the user sets a constant temperature for the cooling chamber, the user now sets the temperature difference between the cryo-trap and the chromatographic oven. In this configuration, the cooling chamber temperature increases during the chromatographic run, tracking the oven temperature ramp. This produces more efficient, volatility-dependent modulation, and increases the range of volatile compounds that can be analysed under optimal trap-and-release conditions within a single analytical run. This system also reduces cryogenic fluid consumption.

Chemiluminescence detector with a serpentine flow cell.

We present a new chemiluminescence detector, with solution channels that have been machined into a Teflon disk and sealed with a sapphire window. The configuration of the flow cell can be conveniently modified by replacing the Teflon disk. A comparison of some existing and novel designs, using the chemiluminescence reaction of morphine with acidic potassium permanganate and the bioluminescence reaction of ATP with the commercially available "BacTiter-Glo" reagent, has revealed that a serpentine channel allows greater quantities of light to be captured than a spiral channel, due to more efficient mixing of the analyte and reagent solutions within the cell.

Chemiluminescence detection of opium poppy (Papaver somniferum) alkaloids.

A review with 98 references. The determination of the opium poppy (Papaver somniferum) alkaloids and their semi-synthetic derivatives has important applications in industrial process monitoring, clinical analysis and forensic science. Liquid-phase chemiluminescence reagents such as tris(2,2'-bipyridyl)ruthenium(II) and acidic potassium permanganate exhibit remarkable sensitivity and complementary selectivity for many P. somniferum alkaloids, which has been exploited in the development of a range of analytical procedures using flow analysis, high-performance liquid chromatography, capillary electrophoresis and microfluidic instrumentation.

Soy flour as an alternative to purified lipoxygenase for the enzymatic synthesis of resolvin analogues.

Specialized pro-resolving mediators are lipid signaling molecules synthesized from omega-3 and -6 polyunsaturated fatty acids, which promote the resolution of the inflammatory response. They are potential drug targets for the treatment of numerous conditions linked with uncontrolled inflammation. Many of these mediators can be effectively synthesized using enzymes, such as lipoxygenases. However, these enzymes are expensive to purchase and can be difficult to isolate. In this work, we show that commercial soy flour can be used directly as a source of lipoxygenase for the biosynthesis of specialized pro-resolving mediators from DHA and other biologically important fatty acids. The reaction was optimized and the products characterized. We found that the reaction yield and products were comparable to those synthesized using a commercial 15-lipoxygenase preparation.