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Minute Papers Due 10/19/2012

Please post this week's minute papers as "comments" to this post. Minute papers should be posted by 5 pm on Friday. Feel free to read your classmate's posts.

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Minute Paper #7 (10/17/2012) – Sarah Anciaux

Title: Nano-LC FTICR Tandem Mass Spectrometry for Top-Down Proteomics: Routine Baseline Unit Mass Resolution of Whole Cell Lysate Proteins up to 72 kDa

Author: Marshall et al.

Journal: Analytical Chemistry

In this paper the authors attempt to develop a high throughput method for higher molecular weight proteins. By use of a separation platform coupled to a high resolution Fourier transform ion cyclotron resonance (FTICR) mass analyzer, proteins up to 72 kDa are baseline mass resolved and identified.

The authors developed a platform capable of handling these high molecular weight separations and protein identifications by coupling a powerful 2D separation, solution electrophoresis and nano-liquid chromatography (nLC), with FTICR. Routine identifications for a HeLa cell lysate were made by this platform with accurate mass measurements down to 10 ppm. The use of nLC in the 2D separation allowed for baseline resolution of the proteins before introduction into the FTICR and reduced the analysis time from 10 minutes for an off-line direct infusion to 20 seconds for the coupled platform.

The authors claim to have developed a new platform for high throughput identification of higher molecular weight proteins, but there are some areas that I think could be addressed further. For instance, the authors only investigated one separation platform, solution electrophoresis and nLC. It’s possible that with even better resolution more proteins might be separated, resulting in more protein identifications. Improved separation resolution could also allow the FTICR more time to run analysis before a new peak is introduced. Furthermore, the authors performed one separation off-line and then did fraction collection and introduced those fractions into the nLC. It would be interesting to see if the 2D separation could all be done in-line to further reduce the total analysis time. For instance in-line nLC columns of different phases could be tried. Selection of the phases would be very important to ensure a well resolved separation. Finally, in top-down proteomics collision-induced dissociation (CID) is typically not the fragmentation method of choice due to its poor sequence coverage. Electron capture dissociation and electron transfer dissociation produce better sequence coverage than CID but also reduce the signal. So this implies that this method would not be able to determine protein sequences and PTMs nearly as well as using ECD or ETD. It would be interesting to see if similar protein coverage and resolution could be seen with ECD or ETD instead of CID.

Seminar Title: Unraveling Cellulose for Renewable Chemicals and Biofuels
Speaker: Paul J. Dauenhauer, Assistant Professor of Chemical Engineering, University of Massachusetts Amherst
Date: October 16th, 2012 CEMS Seminar Series

Paul Dauenhauer spoke about his group’s research on the use of cellulose for the production of commodity chemicals, such as p-xylene, and biofuels. The group is motivated by the fact that the U.S. imports most of its oil, coal, and gas. To alleviate this foreign reliance, lignocellulosic biomass – a source of cellulose which comes from trees and grass – has garnered interest as a renewable energy source because it doesn’t compete with the food supply chain. Because of this interest, the Dauenhauer group focuses on studying the reaction mechanisms and pathways of cellulosic derivatives.

During the first half of his talk, Dauenhauer discussed his group’s study of making biofuels from cellulose using pyrolysis, which involves the heating of the cellulose to 500 C. At this elevated temperature, the cellulose becomes activated and reacts to form inactive char-gases and the biofuel vapors. This activated cellulose, which exists in a molten form, dictates the chemical distribution of the final product. To gain insight into this activated phase, the group studied both thin film and droplet systems. In the thin films the glucose formed mostly furans and levoglucosan. However, in the droplets the glucose mostly formed furans, light oxygenates, and pyrans. By using a novel technique in which isotopically labeled carbohydrates were added to the droplet system, the group discovered that the levoglucosan underwent a secondary reaction pathway to form the oxygenates and pyrans. Additionally, using a novel high-speed camera technique, the group was able to observe the ejection aerosols from the molten droplet. The group determined that these aerosols contain a high amount of inert products which hinder the formation of the biofuel vapors. Using this newfound knowledge, the group is trying to develop a way to suppress this aerosol formation.

During the second half of his talk, Dauenhauer spoke about his group’s efforts in using cellulose to produce p-xylene. To catalyze this reaction, the group used three different zeolites: one with a caged structure, one with an active aluminum site, and one with both a caged structure and active sites (a Fauja site zeolite). While the Fauja zeolite performed the best, by studying all three zeolites the group determined that the presence of the active sites makes the structure of the catalyst the rate-limiting factor. Using this knowledge, the group was able to get a 75% yield of p-xylene, which is more than four times greater than the highest yield in the literature.

Dauenhauer presented some very excellent work that could serve as the basis for several other studies. For instance, to suppress the aerosol ejection, the group could look into adding inert agents to the starting feed to increase the viscosity of the droplet. Furthermore, Dauenhauer didn’t go into detail about how the structure of the zeolite affects the reaction mechanism. Tying into the seminar by Aditya Bhan that I attended a few weeks ago, the charge of the zeolite and the steric interaction between the zeolite and the cellulose may greatly influence the kinetics.

Minute Paper #5
Tian Qiu 4651092

Title: A Mild and Reliable Method to Label Enveloped Virus with Quantum Dots by Copper-Free Click Chemistry
Authors: Jian Hao, Li-Li Huang, Rui Zhang, Han-Zhong Wang, and Hai-Yan Xie
Journal: Analytical Chemistry

In this article, the authors demonstrated a mild and reliable method to label enveloped virus using quantum dots (QDs) with copper-free click chemistry to study the process of virus infection. Quantum dots (QDs) are semiconductor crystals with nano-scaled sizes and are great alternatives to the organic fluorophores or fluorescent proteins in fluorescent assay. Click chemistry described a category of chemical reactions that can join small units together quickly and reliably. Here copper-free click chemistry is used to label the virus.

First, they modified the QDs with 4-dibenzocyclooctynol (DBCO), which will react with azide. QDs with PEG-NH2 terminal groups were used and the NH2 group reacted with DBCO-PEG4-NHS ester. NHS referred to N-hydroxysuccinimide, which activated the carboxyl group. By doing this DBCO-QDs were prepared. And they confirmed there was no size increase of the QDs when they were conjugated with DBCO. (but didn’t check the recognition ability and infectivity)

Secondly, they attached the azide group to the virus. Similar to the modification of QDs, the azide-PEG4-NHS was used to react with the amino acid groups with terminal NH2 on the virus surface. And the recombinant vaccinia virus (VACV) carrying a green fluorescent protein (GFP) reporter was used to show that no obvious difference between the modified VACV and the control.

The next step was labeling azide-virus with DBCO-QDs by copper-free click chemistry and then incubating with cells. And fluorescence microscopic colocalization assay is performed to evaluate the ligation efficiency, confirming that the reaction is highly efficient and specific. Also they carried out quantitative analysis to show that the click chemistry less influenced the fluorescence of the QDs. Virus titer assay and flow cytometry were performed, indicating that the infectivity was maintained.

I think it is really a good idea to use copper-free click chemistry in fluorescent labeling, but I’m still wondering why they don’t use other fluorescent species like organic fluorophores or fluorescent proteins in click chemistry. They are smaller and less toxic to the cell. Also, for dynamic tracking of the virus, I’m thinking about whether we can label the host cell membrane with fluorescent proteins and then film the process of endocytosis of the virus. That could be interesting.

Minute Paper #7 October 19, 2012

Title: A High-Throughput Diagnostic Method for Measuring Human Exposure to Organophosphorous Nerve Agents

Authors: Jennifer S. Knaack, Yingtao Zhou, Carter W. Abney and Justin T. Jacob

Journal: Analytical Chemistry

Current sample processing for exposure to organophosphorous nerve agents (OPNA’s) has a limited capacity due to the requirement of a manual extraction method during sample preparation. In addition, an existing baseline measurement of butyrylcholinesterase (BuChE), a molecule which forms adducts with OPNA’s, is necessary to establish a comparison bias for each individual being studied in order to understand the severity of the levels being measured. For these reasons, the authors of this paper sought to develop an automated immunomagnetic separation method with a high throughout that would better, and more efficiently, analyze OPNA exposure in serum samples.

For the sampling protocol, magnetic beads were introduced to serum samples and then underwent three separate washing cycles. After three washings, the beads were transferred to a digestion plate which contained the extracted sample with an enzymatic digest. The magnetic beads were removed and the extracted sample was analyzed via liquid-chromatography/mass-spectrometry/mass-spectrometry (LC/MS/MS). Ninety-six wells were analyzed at once, allowing for eighty six serum samples of interest plus comparison tests to be run. The serum samples contained OPNA’s such as sarin, cyclohexylsarin, VX and Russian VX.

The limits of detection for this protocol were determined using synthetic peptides, and found to be approximately 1 ng/mL. Percent of adducted samples that varied during the measurement was less than 12%, as determined by quality control samples already vigorously characterized. Unexposed BuChE concentration, for determination of OPNA exposure, was found to be between 2.8 and 10.6 μg/mL, with an average concentration approximately 6.4 μg/mL. Analysis showed this new automated protocol yielded results that allowed for calculation of BuChE adducts without the need for a previous baseline measurement. This is of importance for individual clinical assessment, especially in emergency situations. Automated protocols removed the need for manual extraction, thereby increasing the efficiency and sampling size for these measurements. Although this approach yields a result of the percent adducted BuChE, which can further be correlated to OPNA exposure, no studies have been done at this time to effectively quantitate at what percent exposure, or OPNA level, are negative effects expected to appear in human patients. Being able to quantitatively determine appropriate OPNA levels and the corresponding course of treatment is essential when attempting to translate this technology into emergency-response situations. Since no baseline is required for this technique, it would be beneficial to equip all read-out detectors with a known average BuChE concentration that the instrument can compare the detected levels to, thereby giving a definitive readout as to whether the calculated OPNA levels are of concern to emergency response personnel.

Minute Paper #7, Marzieh Ramezani

Title: Characterization of a Saturated and Flexible Aliphatic Polyol Anion Receptor

By: Steven R. Kass
Journal: JACS Communication

Anion transport channels play a critical role in biological processes. A lot of synthetic anion receptors have been synthesized that use N-H group as a hydrogen bond donor in a rigid framework to associate anions. In this regard, not much attention has been paid to hydroxyl group as a donor group, and there is no report for employing a flexible structure for recognition of anions. In this work, a simple flexible aliphatic alcohol with seven hydroxyl group (heptaol) has been found that can bind to chloride anion and resulting complex has been fully characterized by photoelectron spectroscopy (PES), infrared multiphoton dissociation (IRMPD) in gas phase, and H NMR spectroscopy to measure the binding constants in solution. Also, computational calculation was employed to support the experimental observations.

After obtaining electron ionization mass spectrum of heptaol, the authors found that it can bind to one chloride in gas phase without adding any chloride salt to its solution. It was a sign that it has good affinity for chloride in gas phase. IRMPD which is a technique in mass spectroscopy to fragment ions in gas phase by multiphoton to obtain structural information about the ions was used to record gas phase IR spectrum of the ion (heptaol . Cl-). Four different bands were observed in the range of 2800-3600 cm-1. Recorded IR spectrum of deuterium labeled derivate of the cluster revealed that the first band at 2945 cm-1 arouse from C-H vibrations, and the next two bands (3260 and 3395 cm-1) are correspond to the OH…OH and OH…Cl- vibrations and the last peak was due to the free hydroxyl group. These results confirmed by obtaining the calculated structure of heptaol . Cl- which showed formation of four hydrogen bonds with chloride in the cluster.

To obtain the energy of complexation in gas phase, negative ion photoelectron spectra were used. In photodetachment process, a photon can knock out an electron from a molecule if the photon has a greater energy than the binding energy of the molecule. Adiabatic detachment energy can be obtained from the difference between initial photon energy and kinetic energy of photoelectron. The result revealed that the dissociation of heptaol . Cl- is endothermic by 48.8 kcal/mol. Finally, binding constants of heptaol . Cl- were obtained in acetonitrile at three different temperatures and thermodynamic parameters of the complexation were measured. It was found that this flexible alcohol is a better receptor than a lot of rigid receptors and cooperation of hydrogen bonds can be used to make more efficient anion receptors.

I think authors can measure the IR spectrum of the complex in solution as well to find out the differences between the gas-phase structure and solution-phase structure. Also, binding with other anions can be done to show the generality of their work. The UV/VIS spectroscopy could also be useful to study dissociation of salts which have chromophore cations to measure the binding constants because it is a common tool for that purpose.

Sarah Gruba
Minute Paper 7
Surface Plasmon resonance sensor for dissolved and gaseous carbon dioxide
Authors, Lang, T. et. Al
http://pubs.acs.org/doi/abs/10.1021/ac301673n?mi=10mth2o&af=R&pageSize=20&publication=40025958&searchText=MS

Detection of CO2 has become an important part of monitoring the atmosphere and blood levels of CO2. Currently most detection systems cannot continuously monitor changes. However, recently it was discovered that using NNN tributylpentan amidine helps promote both the binding and release of CO2. This means there is the potential to use this molecule for a continuous sensing chip since the hydrophilicity changes as CO2 is bound or unbound causing a change in the refractive index. By using surface plasmon resonance the authors were able to detect these small differences for both gaseous CO2 and dissolved CO2.
The high refractive index sensing chip was attached to a flow cell that separated the chip into two parts; the control where only nitrogen was passed through and the part for the sample. For the dissolved CO2, they put different concentrations into an aqueous bicarbonate solution and for the CO2 gas solution they mixed it with air. To insure that the only thing that was changing in the solutions was the CO2 binding or releasing they kept the ionic strength of all the solutions constant. After running the dissolved CO2 samples they found that they were able to detect the difference in CO2 concentrations due the greater refractive index as you increased concentrations. The gaseous CO2 experiment was found to only work if it was bubbled through an aqueous solution and was in a very humid environment. Both experiments were able to return to base line levels which again showed that the CO2 can unbind.
This procedure makes preparation in some CO2 sensing experiments in laboratories easier. However taking them out into the real world doesn’t seem applicable due to restraints put on the samples. For the diluted sample, they needed to have a constant solution that only varied in the concentration of CO2 whereas real matrixes are complex. You would have to worry about samples degrading the surface, which they never discussed how long their chip lasts. I think it would be an interesting study to see how long the molecules on the chip can rebind and release the CO2 in different matrixes. Another thing they might want to consider is keeping their chip at a constant temperature by actively cooling it. In their data they saw a rise in the baseline due to the heat being added from running the experiment. This rise in baseline by the end was higher than the lowest concentration of CO2 at the beginning. Without cooling the chip they wouldn’t be able to create a calibration curve where they determine the refractive index by concentration since it is constantly going up. They could try seeing how much the RI changes between the baseline and the sample, but then this would mean they would have to flow a control between each sample, which is harder to do in a non laboratory setup.

Title: Operando Studies of the Catalytic Hydrogenation of Ethylene on Pt(111) Single Crystal Surfaces

By: Tilekaratne et al.

Journal: ACS Catalysis

Despite being the area of heavy research, a complete kinetic understanding of the surface species present during the hydrogenation of light olefins over noble metal catalysts such as platinum has not yet been obtained. Linear olefins have been previously shown to form alkylidyne species on the (111) surface of Pt (where a terminal carbon is bonded to three Pt atoms on the surface) at room temperature1. To gain a better understanding the kinetics of these alkylidyne species during olefin hydrogenation, the authors investigated the kinetics of and relevant surface species present during the hydrogenation of ethylene over the (111) face of Pt using mass spectrometry and operando reflection absorption infrared spectroscopy (RAIRS).
The authors performed ethylene hydrogenation at 300 K over both a clean Pt (111) catalyst sample and a Pt (111) sample which was been saturated with either ethylidyne or propylidyne species prior to reaction at various pressures of hydrogen. Bands associated with the formation of ethylidyne species on the catalyst surface (1340, 1120, and 2885 cm-1) appeared almost immediately in the operando RAIRS spectra upon feeding ethylene and hydrogen over the clean catalyst sample, demonstrating that the formation of catalytically inactive alkylidynes is favored on clean catalyst samples during reaction conditions. The alkylidyne bands of the RAIRS spectra of both the catalyst samples saturated with ethylidyne and propylidyne were found to disappear upon exposure to hydrogen, demonstrating alkylidyne species can be hydrogenated and removed from the surface. The rate of alkylidyne hydrogenation, however, was observed to be much slower than the observed rate of ethylene hydrogenation on all catalyst samples, causing the authors to conclude that alkylidyne formation plays no catalytic role in olefin hydrogenation other than to block potential active sites on the Pt surface. Additionally, the rate of alkylidyne hydrogenation was inversely proportional to hydrogen partial pressure. The authors explained this observation by suggesting that alkylidyne hydrogenation requires empty catalytic sites on the surface, and that these empty sites become saturated with hydrogen at increased gas phase hydrogen partial pressure.
The authors demonstrated that alkylidyne species readily form during olefin hydrogenation at 300 K, but the suggestions that they play no catalytic role and that their hydrogenation requires empty surface sites seem premature. If one expects that the alkylidyne species play no role in olefin hydrogenation, then the initial rate of olefin hydrogenation, determined from transient mass spectrometric measurements, on a catalyst surface completely saturated with hydrogen species prior to reaction would be unaffected aside from the increased number of surface sites. Additionally, one would expect the rate of formation of alkylidyne species, determined from the transient behavior of the operando RAIRS spectra, to be much slower on a hydrogen saturated catalyst surface than on a clean surface due to the lack of available empty sites.

[1] Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. Chem. Phys. Lett. 1978, 56, 267.

Title: Tunable Photoluminescence from Graphene Oxide
Author: Chih-Tao Chien
Journal: Angewandte Chemie International Edition

In this publication Chih-Tao Chien et al. describes a method of adjusting the photoluminescence (PL) properties of Graphene oxide (GO: graphene sheet modified with oxygen functional groups) by steady-state photothermal reduction. This technique provides a controlled, smooth transition from GO to reduced GO (rGO), and allows characterization of the emission mechanism. They deduce that two different types of electronically excited states are responsible for the observed emission characteristics by analysis of the steady-state and transient PL data along with photoluminescence excitation (PLE) spectroscopy measurements.

GO was synthesized from aqueous GO solutions and subjected to steady-state Xe lamp irradiation (500 W) with different exposure times of up to three hours. XPS results show an increase in sp2 carbon bonding with increased reduction time that can be observed with decreasing peak intensities of the oxygen functional groups. The initial fraction of carbon atoms that were sp2 bonded in the synthesized GO was about 25% with a maximum of 69 % for the sample with 3hrs reduction exposure.

The PL spectra were obtained by exciting the samples with a laser at 5mW excitation power, with measurement time for each spectrum less than 1 minute. As a control, PL acquisition was done for 30min without photothermal reduction which resulted in no spectral shift. It is shown that the as-prepared GO samples had broad PL (400 to 800 nm). The PL peaks slowly shifted towards shorter wavelengths and narrower bandwidths with reduction, as a function of exposure time. The colors of PL emission can be gradually tuned from the original yellow-red in the as-synthesized GO solution to the blue in the rGO solution after reduction for 3 hrs.

Chih-Tao Chien et al. also performed PLE and time-resolved PL spectroscopy to examine the electron–hole recombination processes responsible for the two primary emissions. For the as-prepared GO sample, the 605 nm emission shows a multi-exponential PL decay, with an average PL decay time of about 600 ps. For the rGO sample, the PL decay times for the 605 nm emission decreased while the curves of the 455 nm emission was observed to increase with reduction time (1.5ns at 3hr reduction). The very different PLE and TRPL results suggest that the luminescence of 605nm and 455nm emissions arises from two different types of electronically excited states in the heterogeneous electronic structures of GO and rGO. The author then describes how the optical transitions in GO between disorder-induced localized states may cause a broad absorption or emission band, and that during deoxygenation by reduction, the number of these disorder-induced states decreases so that the intensity of the 605nm emission is diminished.

One important piece of information the author did not mention was the influence of underlying substrate morphology and stress/strain on reducing disordered states during the photothermal reduction process due to surface interactions. The XPS data showing an apparent increase in sp2 carbon bonding may actually be correlated to film/substrate bond formations.

Title: Concentration-Independent pH Detection with a Luminescent Dimetallic Eu(III)-Based Probe
Author: Jeremiah D. Moore, et al.
Journal: J.A.C.S.

In this paper, the authors propose a new strategy to detect biological relevant pH with a luminescent dimetallic Eu(III)-containing complex. Different from the conventional luminescence methods, which take advantages of the ratio of two emission peaks, the method introduced here relies on the luminescence-decay rate of the Eu(III)-containing complex. Since the luminescence-decay rate would not be affected by the concentration of the metal ions, the pH detection would also be independent of the concentration.

When designing the Eu(III)-containing complex, two factors are taken into account. For one thing, luminescence-decay rate of Eu(III)-containing complex is proportional to the number of hydroxyl oscillators coordinated to the metal ion. More hydroxyl oscillators would make the decay reaction faster. For another, since Lanthanide-based probes often have low sensitivity, multimetallic complexes are designed to address this problem. As a result, Eu(III)-containing complex, 1, is synthesized, with two Eu(III) ions bridged by an alcohol. The bridging oxygen exhibits different acid-base equilibrium corresponding to the change of pH levels, leading to the different luminescence-decay rate of the complex. Then pH detection is achieved.

As to the experiment, complex 1 was synthesized and characterized by 1H and 13C NMR spectra. For the luminescence-decay rate measuring, spectrofluorometer with the phosphorescence lifetime setting was utilized. The slope of the plot with natural log of the intensity against time was used as the decay rate. The authors first tested the coordination environment of complex 1 by determining q value, which represented the number of water molecules coordinated to the Eu(III) ion. Dissolved in H2O and D2O respectively, the luminescence-decay rate of complex 1 was tested and q was calculated to be 0. This meant that no water molecules were bound to the Eu(III) ions so that complex 1 would not be easily affected by surrounding environment. Then to detect pH and confirm the concentration-independent characteristic, 0.5 mM and 0.1 mM solutions of complex 1 were prepared and tested in the pH range of 4-8 separately. The results of these experiments showed identical luminescence-decay rate changes in the same pH levels, thus demonstrating that the detection was indeed concentration independent.

By utilizing luminescence-decay rate, the detection method is relatively new and interesting. Further experiments can be conducted to explore the minimum required complex 1 concentration so as to achieve the best detection efficiency. In terms of selectivity, control experiments with biomolecules containing medium are also necessary. As a potential in vivo probe for pH detection, toxicity tests should be performed in cells to investigate the influence of the bioactivity of the living cells. Moreover, real sample tissue extraction can be tested to determine the accuracy of this method by comparing to the known values. Finally, when evaluating the development of this probe, economic factors should also be considered. As a rare earth metal, Europium storage is not abundant and the extraction and purification process of Europium from the minerals are complicated. Therefore, to develop the new strategy, it would be better if other complexes containing other common metals can be used.

Title: Selective detection of ATP and ADP in aqueous solution by using a
fluorescent zinc receptor

By: Pellecchia et. al.

Journal: Chem Comm

Link: http://pubs.rsc.org/en/content/articlepdf/2012/cc/c2cc35730e


In this communication, the authors report a fluorescent zinc receptor capable of selectively detecting ATP and ADP in water. Fluorescent detection of ATP and ADP is particularly relevant since it can then be watched in real time in cells. Using fluorescence to detect ATP and ADP remains a difficult task. Before, phosphate sensors have been made using lanthanoid metal complexes as these have a large Stokes’ shift, and a long fluorescence lifetime. However, using a transition metal, such as zinc, gives the advantage of inducing fluorescence than having to quench it.

The authors then went ahead and synthesized this Zn coordinated compound and studied its ability to bind to ATP and ADP via 31P-NMR. The spectra showed a shift when bound to ADP. They believe that the ADP binds to the Zn complex via the OH group in the complex (see figure 3 in the paper). They then measure the fluorescence signal given when they add increasing amounts of ADP in water. What they saw was a nonlinear increase in signal and a blue shift in the λmax.

This raises some questions. It seems like their work is not complete. While it is great that the Zn complex does actually detect ADP and ATP in aqueous solution, it does not necessarily do it quantitatively. I would like to see how quantitative this sensor can be. Also, can one tell the difference between a signal from bound ATP versus one from ADP? While it is useful to see both, it would be more useful to look at each independently in cells. This was not clear from the communication, the authors just stated that it is responsive to both. Also, does this mean it will respond to any phosphate group? I would think it would. This raises another question, is this compound toxic to cells? If it is toxic to cells, this Zn complex is not of particular interest to me. I would like to test it for its toxicity, and to know if it can be used for real time imaging of ATP and ADP molecules in cells.

Minute Paper #6 (10/19/12) – Matt Irwin
Title: Solvent Selective Hydrogen–Deuterium Exchange on Saturated Polyolefins
Authors: B. Habersberger, T. Lodge, F. Bates
Journal: Macromolecules

Small angle neutron scattering (SANS) is an important technique for the study of the thermodynamics and kinetics of polymeric systems. For example, the technique is essential for determining the phase behavior of block copolymers and for understanding micelle formation and equilibration. Frequently, for SANS sample preparation, it is desirable to selectively deuterate one component of the system in order to increase the neutron scattering contrast. This is because the neutron scattering intensity increases with the square of contrast. In this paper, the authors describe an effective method for exchanging hydrogen with deuterium on saturated polyolefins via high pressure, high temperature deuterium gas. Techniques used include 1H and 2H nuclear magnetic resonance (NMR) spectroscopy, density measurements, SANS, Fourier transform infrared spectroscopy (FTIR), and size exclusion chromatography.

In a typical experiment, 5 g of saturated polyolefin, 2 g of a Pt-Re/SiO2 catalyst, and 450 mL of solvent were placed in a stirred reactor with 600 psi of deuterium gas heated to 170 °C for 16 hours. The polymers studied in this paper were high density polyethylene (HDPE), isotactic polypropylene (iPP), and poly(ethylene-¬alt-propylene) (PEP). The solvents used were decalin, decane, heptane, and isooctane for HDPE; decane and isooctane for iPP; and heptane for PEP. It has previously been established that hydrogen-deuterium exchange occurs via the solvent or polymer molecule coordinating with the heterogeneous catalyst surface and then exchanging hydrogens with the bulk dissolved deuterium. The authors found that the cyclic decalin outcompeted HDPE for coordination with the catalyst, resulting in all of the deuterium in the reactor being substituted onto the solvent. For decane, heptane, and isooctane, the relative extent of deuteration of HDPE increased as both the solvent’s chain length decreased and the solvent’s degree of branching increased. Analogously, the authors found that the extent of deuteration decreased from HDPE to iPP to PEP; the authors conclude that the bulky methyl side groups on the iPP and the PEP effectively reduced the ability of the polymer to coordinate with the catalyst, resulting in less deuteration. Finally, by comparing a random exchange model to SANS and FTIR data, the authors found that SANS results indicated that the HDPE backbone is deuterated evenly over long length scales, while FTIR spectroscopy showed that there are small “patches” of deuterated areas locally on the backbone via peaks at 590 cm-1 and 525 cm-1.

This paper provides a good mechanism for the deuteration of already synthesized polyolefins, but more work could be done to make the results more conclusive. In particular, the authors conclude that the primary reason that iPP and PEP are less readily deuterated than HDPE is simply due to steric effects. However, this conclusion was not drawn from well controlled experiments, as the same solvent was not used for each of the three polymers. A more conclusive method for determining the effect of steric hindrance would be to attempt to deuterate a very bulky polymer such as polyvinylcyclohexane in each of the solvents studied in this paper. If this polymer is unable to be deuterated relative to the solvent, then the paper’s argument of steric hindrance would be supported. On the other hand, if the polymer can be deuterated, this would suggest that other effects such as tacticity ultimately determine how effectively a polymer can be deuterated.

Article: Chemiluminescence Switching on Peroxidase-Like Fe3O4 Nanoparticles for Selective Detection and Simultaneous Determination of Various Pesticides

By: Guan et al.

Journal: Analytical Chemistry

Chemiluminescence (CL) has great promise for chemical analysis, as it exemplifies great sensitivity and cost-effectiveness. A current disadvantage of CL technology includes a lack of differentiation between specific analyte species. This limits CL as a practical means for chemical analyses in scenarios where rapid identification would be highly valued. Herein, CL via peroxidase-like Fe3O4 nanoparticles (FNP) permitted quantitative analysis of pesticides.

The FNP coprecipitation synthesis utilized in this work yielded monodisperse 10 nm size nanoparticles with superparamagnetic characteristics. Control CL detection experiments were done by comparisons of FNP treated with pesticides and luminol versus FNP treated with luminol only. Real CL detection scenarios were conducted by treating green tea and grape juice samples with several different pesticides, FNP and luminol. CL signals were measured via a polypropylene microtiter microplate luminometer.

Comparisons of luminol CL controls in various media, which included Fe2O3 suspensions and nitrogen-saturated FNP solutions, suggested CL is attributed to Fe2+ ions on FNP surfaces and dissolved oxygen. Furthermore, optimal light emission conditions required acidic FNP suspensions and basic luminol solutions prior to their combination. These results proposed a mechanism for luminol-FNP interactions, which is summarized by a decomposition of dissolved oxygen into superoxide anions at the FNP interface. This mechanism is indicative of peroxidase-like catalytic activity of FNP in acidic conditions, which gives rise to luminol oxidation in basic conditions and therefore produces strong CL emission. Interestingly, ethoprophos (EP), a commonly used organophosphorus pesticide, did not show CL in the presence of FNP-luminol aqueous solutions, but demonstrated significantly enhanced CL responses with the addition of ethanol. Ethanol is a radical scavenger and typically quenches CL emission in the presence of luminol, but FNP-pesticide complexes inhibit ethanol scavenging of FNP oxide surface radicals and therefore serve as a CL “on switch.” The authors used ratios of CL quenched (Io) and CL “on switch” (I) as a means to provide quantitative measurements of pesticides. Lastly, when testing pesticide spiked green tea and grape juice samples, the superparamagnetic behavior of FNP was utilized to magnetically separate FNP-pesticide complexes from the supernatant. This served as a simple method for purifying samples prior to CL detection.

Although this work demonstrated methodology for CL detection of EP via FNP, the authors failed to enhance sensitivity of pesticides without phosphorus-oxygen and phosphorus-sulfur bonds. Their claims of analyte selectivity are exaggerated, as they required specific FNP surface modifications to enhance CL detection of other pesticides. This negates any potential for a single FNP platform capable of analyzing many different pesticides. I wonder if functionalized Au nanoparticles (ANP) for SERS could provide better universal pesticide selectivity, because pesticide binding via both thiol and amine substituents to ANP is possible. This may provide a platform more capable of assessing varieties of pesticides than FNP.

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