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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.
Posted by Christy Haynes on September 11, 2012 10:36 AM | Permalink
Minute Paper #2 (09/11/2012) – Sarah Anciaux
Title: Fluorescence Imaging of Cellular Metabolites with RNA
By: J. S. Paige et al.
Journal: Analytical Chemistry
In this paper the authors attempt to develop a new method of in vivo small molecule detection. The use of a small molecule binding aptamer integrated with a conformational induced fluorescent probe would mean the ability to bind a target without any fluorescent derivatization or the need to develop more complex genetically encoding sensors.
The authors developed a small molecule fluorescent detection method by combining Spinach, a fluorescent RNA molecule with three stem loops, with aptamers that are selective for specific small molecules. The inserted aptamer was chosen based on the desired small molecule for detection. In this paper they describe using aptamers that bind ADP, SAM, guanine and GTP. With the inserted aptamer the authors also created a critical stem that when bound results in a conformational change, inducing fluorescence. Through the careful selection of the base pairs in the stem, the molecule was devised such that the stem has very weak interactions and the only way the stem can form, and the molecule fluoresce, is upon the conformational change when the aptamer binds its intended small molecule target. The weaker interactions in the stem were made by either making the stem short, using weak base pairs of A-U and G-U, or else mismatching some of the base pairs in the stem.
The authors claim to have successfully developed and implemented the probe for the detection of the previously stated small molecules, but there are some areas that I think could be addressed further. The authors claim to selectively bind small molecule targets, but without experimentally determining the binding curve of the aptamer in the new molecule I question how selective it is. The aptamers they used might by themselves be highly selective but by fusing the previously determined aptamer with the Spinach molecule it is possible some conformational changes could occur in the aptamer, changing the selectivity for the intended target. Also, the authors state that the probes fluorescent increases were linear under physiological conditions but they say nothing about lower or higher concentrations than physiological. It would be interesting to see if the probes were useful for imaging outside of physiological concentrations. Another thing the authors do not fully address is the melting temperature of the critical stem between Spinach and the aptamer. They say they weakened the interactions by selecting their base pairs very carefully, but I think they need to determine at what temperature the stem actually melts, and if target-unbound fluorescence could be a problem if low temperature assays were desired. Finally, the authors also claim that the developed strategy for fusing the aptamer that binds the molecule of interest into Spinach should be able to in essence image any molecule of interest. While this might be true, there are some problems that could arise. Currently there are not developed aptamers for every molecule. It is also very difficult to select for aptamers, with difficulty varying greatly between molecules.
Sarah Anciaux |
September 11, 2012 5:49 PM
Title: Tunable Surface Properties from Sequence-Specific Polypeptoid-Polystyrene Block Copolymer Thin Films
Authors: W. van Zoelen et al.
Note: = is a double bond, =- is a triple bond.
van Zoelen et al. were able to tune the surface behavior of a block copolymer of polystyrene (PS) and polypeptoid, which contained separate hydrophilic and fluorinated side chains, by modifying the chemical composition and sequence of the peptoid block. By studying this material, the authors hope to use the block copolymer to make marine anti-fouling coatings. One of the main techniques used in this work was near edge X-ray absorption fine structure (NEXAFS).
The authors prepared thin films of the PS-peptoid block copolymer and then analyzed the composition of the film surfaces by using NEXAFS. They first analyzed pure peptoid films to record the bound state transitions in the C 1s spectrum. They discovered several peaks, including a C 1s --> pi*_C=-C at 285.5 eV and C 1s --> pi*_C=O at 288.6 eV. They then analyzed four PS-peptoid block copolymer films in which the peptoid side chain had zero, one, three, and five fluorines, respectively. The unfluorinated film had a pi*_C=C peak in the spectrum (which corresponds to the amount of PS at the surface) identical to that of the pure PS film, indicating that no peptoid assembled at the surface. As the number of fluorine groups increased, the pi*_C=C peak decreased and both the pi*_C=-C and pi*_C=O peaks increased, signifying the accumulation of peptoid at the surface. The three and five fluorine films were almost identical, indicating the peptoid surface aggregation maximized at three fluorines.
When the authors varied the length of the peptoid block, they observed the same trends in the NEXAFS spectrum as the original set of block copolymers. By altering the position of the fluorine groups of the peptoid so that they were closer to the PS block, the authors reduced the amount of peptoid at the surface of the film. This result was due to the peptoid block having to form a loop, which is entropically unfavorable, for the fluorines to reach the surface.
Though the NEXAFS data in the paper provides excellent support for the authors’ ability to tune the surface behavior of PS-peptoid, their argument could have been strengthened by the use of microscopy. Because the PS and peptoid blocks are composed of very different molecules, microscopy techniques should be able to differentiate between the two blocks. Using the same sort of film samples as in the NEXAFS experiments, the authors could analyze the samples using Confocal Raman microscopy to quantitatively measure not only the amount of peptoid at the surface, but also the distribution of the peptoid throughout the surface. To corroborate their experiments on the kinetics of the self-assembly, the authors could analyze the block copolymer after contact with water by using cryo-transmission electron microscopy to evaluate the composition of the surface at various time points. Additionally, the authors also suggest that the peptoid blocks with five fluorines show some crystallization. This hypothesis could be proven by analyzing the film samples with wide-angle x-ray scattering.
Ralm Ricarte |
September 12, 2012 5:43 PM
Title: Aminonaphthlanene 2‑Cyanoacrylate (ANCA) Probes Fluorescently
Discriminate between Amyloid‑β and Prion Plaques in Brain
By: Kevin Cao et al.
Journal: Journal of the American Chemical Society
Amyloids, improperly folded proteins named plaques, are the root of many neurodegenerative disorders such as Alzheimer’s and prion diseases. Current diagnostic techniques such as positron emission tomography (PET) fail to discriminate between amyloid proteins associated with different neurological disorders. In this work, using mice brain as a model, three different aminonaphthalenyl 2-cyanoacrylate (ANCA) probes were found to fluorescently differentiate between amyloid-β (Aβ) peptides and PrPSc proteins, which are associated with Alzheimer’s and prion diseases, respectively. The methods included organic synthesis, as well as fluorescence spectroscopy and microscopy.
As demonstrated in Figure S1, synthesized ANCA probes displayed notable increases in fluorescence emission intensity when bound to plaque over unbound probes in solution; ANCA, when bound to plaque, experience rotational restriction of single bonds between donor and acceptor groups, which therefore enhance the emission properties of the probes and classifies them within the molecular rotor family of fluorophores. According to Figure 1 and S3, when bound to Aβ and PrPSc, ANCA exhibited two characteristic spectra and individual λmax associated with each respective complex as well as a visual difference between the two complexes when analyzed via fluorescence microscopy.
To further understand ANCA plaque interactions, polarity of different organic solvents and its effect on fluorescence emission of each probe was studied; static relative permittivity (εo) was shown to override refractive index factors in terms of probe emission intensity, which led to εo estimations of each probe within the binding sites of Aβ and PrPSc. As shown in Figure 2D, εo estimations varied little between the three probes, but εo estimations were approximately twice as large when probes were bound to PrPSc over Aβ. Given the close relationship between εo and dielectric constants, Aβ and PrPSc binding sites were found to be hydrophobic. Since the three probes differ in nitrogen donor substituents, the probes’ emission properties were analyzed as a function of pH in order to determine if acid-base interactions were responsible for emission enhancement upon binding to Aβ and PrPSc. Although Figure 3 illustrates differences between the probes’ emission spectra at pH 3.8 and 7.8, all three probes demonstrated ~20 nm differences between emission λmax upon Aβ and PrPSc binding. This suggests acid-base interactions between probe and plaque are not the source of enhanced emission.
Despite the authors’ effective illustration that ANCA probes’ emission properties differentiate between diseases originating from Aβ and PrPSc, how practical is fluorescence microscopy for diagnosis of brain diseases within living patients? The authors should develop viable in vivo screening methods via Aβ and PrPSc analysis possibly within retinal tissue of the eye. Furthermore, clinical use of ANCA probes may be limited by narrow emission λmax differences between Aβ and PrPSc probe complexes; the minor spectral differences may be difficult to determine accurately via fluorescence microscopy in a clinical setting. To remedy this, ANCA probes with wide emission λmax differences upon binding to Aβ and PrPSc should be developed, as this would improve efficient screenings.
Sam Egger |
September 12, 2012 6:41 PM
Minute Paper #2 9/14/12
Article:Rapid and Simple Analysis of Pesticides Persisting on Green Pepper Surfaces Swabbing with Solvent-Moistened Cotton
Authors:Eiki Watanabe, Yuso Kobara, and Yasuhiro Yogo
Journal: Journal of Agriculture and Food Chemistry
A growing problem in the food industry is pesticide residue left on the plant. In order to prevent consumption of fruits and vegetables loaded with pesticides different testing procedures have popped up, mostly involving high performance liquid chromatography(HPLC) with mass spectrometry. The major problem this group of researchers saw with these procedures is that it did not take into affect the complex matrix of the fruit or vegetable that might suppress or enhance the signal. The other concern was the effectiveness of non destructive testing, where they wipe the food on the outside to detect the pesticides. For measure the amount of pesticide they used HPLC with a diode array detector (DAD).
To test the effectiveness of using a non destructive method they added a known concentration of pesticide on the green peppers. After letting it sit, they swabbed the green peppers using acetonitrile, acetone, or methanol. They used the amount detected with the DAD to figure out recovery and which extractant they should use to swab the green peppers for each pesticide. To see if the diode array detector would be susceptible to matrix effects, they looked at the area underneath the curve of a prepared sample with 0.5µg of the pesticide spiked into it and compared that to a pure sample of mobile phase with 0.5µg of pesticide in it.
These tests determined which extractant worked best for each pesticide and that this method only lead to a small matrix affect. Finally, they did time studies by letting the pesticide sit on the pepper for several days. The % recovery was found and they determined that the water solubility of the pesticide mattered because it affected the rate at which the pesticide moved into the pepper. In conclusion, the authors decided that their non destructive method worked for some pesticides depending on the water solubility and where the pesticide was sprayed.
One benefit of having the DAD that was not mentioned is that it is more rugged and less likely to break down except for the bulb. This means that almost anyone can run the samples and maintain the equipment. However, I think the most important fact that came out of this research is that swiping the food for pesticide is an acceptable form of measurement given the restrictions. This can be used to advance how fast results are received. If a way can be created to skip the HPLC component before going into a UV vis detector such as with pesticides aggregating nanoparticles (causing a color change) which is being worked on in the Haynes group or using an aptimer and SERS, this testing could be taken out into the fields giving faster results. One factor with these experiments that would have to be taken into account is limit of detection.
Sarah Gruba |
September 13, 2012 7:42 PM
Title: Fluorescence Imaging of Cellular Metabolites with RNA.
By: Jeremy S. Paige.
The main idea of this paper was to use fluorescent labels for imaging small molecules in living cells. Various small molecules bind to the genetic encoded sensors which have fluorescent properties. As a result of binding, conformational changes in target molecules occurred which are responsible for changes in fluorescence value and imaging the living cells of specific molecules. These structural reorientation leads to promote stabilization and gain the structure scientists are interested in.
In this paper, the authors showed the fluorescence imaging of Spinach as the target molecule. This species contain three stem loops which are short, contain mismatched base pairs, and composed of weak base pairs. RNA aptamers which are peptide or nucleic acid species that can be combined with specific target molecules, have been bound to one of the loops that specifically has an important role in Spinach fluorescence. The aptamers bind adenosine, adenosine 5-diphosphate (ADP), S-adenosylmethionine (SAM) and guanine. Then, a sequence of this stem-aptamer was designed that operated as a transducer which is thermodynamically unfavorable to the stem hybridization. Afterwards, they tried to alter transducers to approach the optimum ligand-induced fluorescence. The advantage of using these new RNA-based aptamers is that they can act more selective than previous protein-based sensors.
To investigate the effect of different metabolites on the fluorescence character of SAM, they used these RNAs to observe diverse metabolite dynamics. For example, they found out in the presence of methionine the fluorescence of SAM would increase and when there is no methionine in the environment SAM sensors show minimum fluorescence. Similar types of experiments were carried out on ADP implied the versatility of these RNA aptamers. It worth to know that the authors claimed the former technique, Forster Resonance Energy Transfer, (FRET) which typically have been used in imaging, revealed no observable approaches for examining these metabolites.
It is also a good idea to develop the application of using these new RNA-based aptamers by applying the same method to other small molecules rather than Spinach and explore the selectivity of these sensors. Moreover, I think they could have performed similar imaging method for quantitative tests in various physiological environments to show the impact of different metabolites on increasing or decreasing the fluorescence of small molecules and their binding aptamer in a numerical ratio. Likewise, it would be supportive to perform in vivo experimentation as another generalizable method to probe these isolated small molecules.
Marzieh Ramezani |
September 13, 2012 11:15 PM
A Simplified Sum-Frequency Vibrational Imaging Setup Used for Imaging Lipid Bilayer Arrays
By: Smith, K. A. and Conboy, J. C.
Journal: Analytical Chemistry
The authors present a sum-frequency vibrational imaging (SFVI) setup that they applied to imaging lipid asymmetry in lipid bilayers. Their SFVI setup is simpler than others presented in the literature, having only a single focusing lens and a combination of interference/notch filters where other SFVI setups include two collimating lenses, a microscope objective, grating, a tube lens and more interference/notch filters. This simpler SFVI setup afforded the authors a larger field of view.
The setup was tested on a lipid bilayer made of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and its deuterated analog. The vibrational selectivity was demonstrated by collecting images for three different pattern lines. The authors' simplified SFVI images were also used to probe the phase behavior of micropatterned lipid bilayer arrays.
The authors show that their technique can resolve vertical lines to 88um line-widths and horizontal lines to 39um line-widths using a single focusing lens. The authors do not explicitly discuss how these resolutions compare to less simplified SFVI techniques, but it is implied that these resolutions are relatively poor. However, the authors find this limitation acceptable because the goal of their study is to create a setup with a large field of view for the chemical imaging of lipid arrays.
A comparison of the simplified SFVI technique with sum-frequency vibrational spectroscopy (SFVS) showed that the author's technique produced similar vibrational sum frequency generation intensity spectra to the older SFVS technique, with the added benefit of having enough spatial resolution to generate images of lipid bilayers.
The simplified SFVI technique was then applied to study the change of a DSPC/DSPC-d70 asymmetric lipid bilayer to a symmetric bilayer as a function of temperature, again showing similar results to SFVS. The technique was then applied to several other lipids in micropatterned lipid bilayer arrays, consisting of DOPC:DSPC, DOPC:DPPC and DMPC:DSCP bilayer patches (all within the same array) at 20 degrees C. Significant sum-frequency intensity was observed in DOPC:DSPC and DOPC:DPPC, implying the presence of phase segregation. No significant intensity was detected in DMPC:DSCP, implying a single homogeneous phase. These results are consistent with the phase diagrams presented in the literature. Upon heating to 65 degrees C, no signal is detected from any of the bilayer patches, implying that all the lipid membranes are homogeneous. Upon cooling back to 20 degrees C, the two bilayers that showed sum-frequency intensity before (DOPC:DSPC and DOPC:DPPC) show signal again, but at slightly lower intensities. The authors suggest that this may be because the temperature sweeps disrupted membrane structure and caused defects in the membrane during cooling.
The authors demonstrate a potential application for their simplified SFVI technique, the analysis of binary lipid bilayer mixtures. Their technique offers the advantages of imaging over SFVS, while having a larger field of view than other SFVI techniques and maintaining sufficient resolution to study lipid bilayer mixtures.
Alex Schnepper |
September 14, 2012 8:59 AM
Title: Highly-Precise Measurements of Ambient Oxygen Using Near-Infrared Cavity-Enhanced Laser Absorption Spectrometry
By: Manish Gupta
From: Anal. Chem.
In this paper, the author used near-infrared off-axis integrated cavity output spectroscopy (off-axis ICOS) to measure ambient oxygen concentration. Compared to most of the previous methods that are used to quantify ambient oxygen, tunable diode laser absorption spectrometry (TDLAS) avoids the complicated process of condition optimization. With specific operating wavelength, it is also highly selective. In this work, the cavity-enhanced laser absorption spectrometry was utilized to increase the effective optical path length of the TDLAS so as to achieve higher precision.
To analyze the performance of the sensor, the author first determined the precision by measuring a constant air sample for 48 h. The result showed that for long time measurement, the data tend to be less precise due to the instrument drift. In order to reduce this effect, periodic calibration was adopted. Oxygen injected into the system was switched between two sources every 194 s for over 24 h. At this time, measurement precision was improved to be ±1 ppm in less than 7 h. With this achievement, the author then tested the linearity of the sensor. The plot of measured oxygen concentrations versus calculated oxygen concentrations showed a highly linear correlation (R2 > 0.9999) in the oxygen concentration range of 0-100%. From these results, the author suggested that the sensor could be employed to practical field-work detection. Therefore, he combined the sensor with a commercial Greenhouse Gas Analyzer to detect O2, CO2, CH4 and H2O in laboratory air and in urban air at different period of time. The testing results were shown in the plot of Measurement of δ(O2/N2), where the relative concentration of N2 was determined by 1- XO2- XCO2- XH2O -0.01, versus the concentrations of CO2. This plot indicated a good linear correlation between δ(O2/N2) and CO2 concentrations. Then the author calculated the oxidative ratio (OR) to identify possible CO2 source in the air by using the slop of the lines in the plot. For laboratory air, the calculated OR was 1.17, which meant CO2 was primarily from respiration process. For the urban air, OR was 1.54 and 1.64 respectively, indicating liquid fossil fuel and natural gas as the primary sources of CO2.
As mentioned by the author himself, the sensor can still be modified and improved. In my opinion, the most important one from what the author suggested is to develop sensors that can detect O2, CO2, CH4 and H2O with a single instrument. Since the field work data was obtained from two different instruments, it is not accurate to put them together for analysis. However, with a single instrument, it will be easier to operate and the systematic error can be reduced. Furthermore, as to this sensor, the ability to operate in a wide temperature range does not necessarily mean to operate with the same precise as it does in room temperature. Since the temperature in the open area can change greatly during 24 h, the author should also consider the influence of temperature to the measurement precision.
Yi Zhang |
September 14, 2012 9:49 AM
Minute Paper #2 Megan Weisenberger
Title: Chemical Sensors based on Electrochemistry with Fluorous Phases: From the Ultimate Limits of Low Polarity to extreme Selectivity’s and Detection Limits
Seminar by Dr. Philippe Buhlmann
Chemical sensors provide the unique opportunity to have in situ measurements of chemical species, produce immediate responses and require no sample preparation. For these reasons the Buhlmann group seeks to investigate the properties and optimize chemical sensors with long term goals ranging from permanent implementation into the human body to environmental measurements. Ionophore based Ion Selective Electrodes (ISE) were the main focus of the seminar presented.
Ionophore based ISE operate by use of a potentiometric measurement in which the potential difference is measured between an indicator and reference electrode. The ionophore aspect of each ISE acts as a receptor agent to selectively bind molecules of interest. Most ISE suffer drastic losses in selectivity; even those with an ionophore basis, therefore a washing cycle must be performed in between each measurement to attempt to lengthen the lifetime. The Buhlmann group began investigating fluorous membrane electrodes and discovered that the initial selectivity range had increased to a magnitude of eight times wider. Running continuous samples in human serum for a period of twenty four hours without washing cycles in between measurements demonstrated that the selectivity did not diminish over repeated use.
In order to measure certain anionic molecules of interest, a fluorophilic cation was needed, however the perfluoroalkylammonium salts only exit in patent literature. Initially the Buhlmann group designed a fluorinated triarylphosphine which yielded good solubility for use as an ion selective membrane, but it exhibited a limited lifetime and at low concentrations the surface of the sensor oxidized and then decomposed. To correct for these imperfections, Buhlmann created the bis(trialkylphophoranylidene) ammonium ion which still had eleven orders of selectivity and did not decompose. When comparing the sensors to Liquid Chromatography-Mass Spectrometry (LC-MS) using Ottawa Sand as an environmental sample, the results from each LC-MS and ISE were indistinguishable.
Potassium ions are a molecule of interest particular for physiological purposes. Crown ethers are known to have excellent potassium binding selectivity’s and therefore the Buhlmann group began investigating their use in an ISE membrane. Preliminary results showed an usual selectivity binding curve which was later attributed to multiple binding opportunities for the crown ethers. In a solution of potassium and cesium ions, the crown ethers initially bound singly to the cesium ion in the most abundance, followed by double crown ether binding to the cesium ion secondly. No potassium ions were initially bound. As the concentration of potassium ions increased, binding of crown ethers to cesium ions slowly decreased as the potassium binding went up. Buhlmann hopes with these results to give future researchers additional ideas on how to interpret unusual selectivity binding curves. There has been no work done at this time to investigate the ability to increase the selectivity of the crown ether for binding potassium ions. I would attempt experiments in which the steric hindrance around the crown ether was increased to see if potassium ions could be bound extremely selectively, thus negating the unusual selectivity binding curve entirely.
Megan Weisenberger |
September 14, 2012 10:17 AM
Title: Improving the sensitivity of Raman signal of ZnO thin films deposited on silicon substrate
By: A. Hammouda Et al.
Journal: Vibrational Spectroscopy
Raman spectroscopy is a useful method employed to analyze Transparent Conductive Oxide (TCO) films because it can reveal phase information about metal oxide films, as well as crystallographic information. However, many times these films are deposited in the sub micron thickness range due to the specific applications, but also because there can be special properties in thin films compared to bulk. A restriction of Raman spectroscopic analysis however is that it requires a thick film to prevent the thin film signal from being dominated by the underlying substrate signal. In this case the dominant Raman peak of Si is orders of magnitude stronger than the ZnO signal. In this paper a new method is shown whereby the weak signal of ZnO films on silicon substrates can be observed.
They use a combination of a small grating grooves edge filter and a device with a half-wave plate and polarizer aligned correctly within the scattered beam. They take advantage of the Raman tensor for crystalline silicon being of mode T2g and the polycrystalline nature of their ZnO film by using vertical-vertical VV polarization configuration. They rotate and orient the Si Wafer such that polarization of both excitation and scattering occurs along the [1 0 0] axis, and then the T2g mode does not contribute much to the overall signal.
ZnO films were sputtered onto glass and crystalline silicon (001) substrates at different temperatures and thicknesses between 40nm and 500nm. Laser power was varied across samples and spectra were recorded at different regions for consistency. In the results they show a comparison of Raman spectra for VV configuration oriented along the  and  axis. It is clear from figure 3 that with the  orientation the 520 cm−1 mode of Si is reduced by around 95%, and the rest of the spectrum is barely changed. Multiple graphs show the signature of the ZnO film now observable (dominant at ZnO mode at 583 cm−1) with a ZnO powder sample for reference.
Although the ZnO mode at 583 cm−1 is made clear by this, and reveals the influence of oxygen in the film, some portions of the ZnO signature still remain cloudy by the residual signature of the Si substrate. The new method is definitely useful and perhaps could be improved with a higher resolution grooves edge filter and polarizer. The other situation the author doesn’t address yet is dealing with a more single crystalline ZnO thin film on Si, which may result in a much diminished spectral response depending upon crystal orientation with respect to Si. However, since growing single crystal ZnO with a specific orientation is more understood now a workaround for Raman measurement may be feasible.
Forrest Johnson |
September 14, 2012 1:28 PM
Minute Paper #2 (9/14/12) – Matt Irwin
Title: Two-Way CO2-Switchable Triblock Copolymer Hydrogels
Authors: D. Han et al.
The development of reversible hydrogels has particular promise for the development of new technologies such as advanced sensors and drug delivery vehicles. In this article, the authors describe the tailored development of two different ABA triblock copolymers which exhibit either a sol-to-gel or gel-to-sol upon exposure to CO2 which can be reversed by ejecting the CO2 via argon gas. The observed behavior is attributed to the modulation of the polymer solution’s lower critical solution temperature (LCST) upon exposure to either gas. The authors demonstrate the usefulness of such a system via the controlled release of a protein in the solution. Techniques used in this study include reversible addition-fragmentation chain transfer (RAFT) polymerization, atom transfer radical polymerization (ATRP), 1H nuclear magnetic resonance, transmittance measurements, dynamic light scattering (DLS), and atomic force microscopy.
Two triblock copolymers were developed for this study. For the reversible gel-to-sol transition, a midblock polyethylene oxide (PEO) was capped on both sides by the random copolymer 2-(2-methoxyethoxy)ethyl methacrylate-co-N,N-dimethylaminoethyl methacrylate (P-(MEO2MA-co-DMAEMA)). The amount of DMAEMA was chosen so that the solution could undergo the gel-to-sol transition at physiological temperatures (~35 C). A 10 wt% polymer solution in water exhibited a LCST below physiological temperatures; the end blocks of the polymer formed a water insoluble core of a micelle. The LCST of this solution could be raised to above physiological temperatures via addition of CO2 to the solution, causing the solution to transition from a gel to a sol. The authors propose the reason for this transition is that the tertiary amines on the DMAEMA are protonated by CO2 which increases the water solubility of the end bocks. Their hypothesis is supported by DLS data which show that micelles dissociate from large clusters in the gel state to individual coils after exposure to CO2. Transmittance measurements show that the solution turns from cloudy to clear upon transitioning from a gel to a sol and that the process can be completely reversed via the addition of argon gas to the solution. For the reversible sol-to-gel transition, the same PEO midblock is capped by the random copolymer 2-(2-methoxyethoxy)ethyl methacrylate-co-methacrylic acid (MEO2MA-co-MAA). A 10 wt% solution of this polymer exhibits a LCST above physiological temperatures which can be lowered upon addition of CO2. The authors suggest that the reduction of the LCST is due to the CO2 protonating the acid groups, leading to a reduce water solubility of the end blocks. These results were similarly confirmed by DLS and transmittance measurements.
Although the authors demonstrate the usefulness of these hydrogels via a protein release study, additional work needs to be done before polymers could be used in practical applications. In particular, all work done in this paper discusses transitions that occur at temperatures just slightly above room temperature (~35 C). Therefore, additional triblock formulations should be considered that might allow the reversible transition to occur at room temperature. This would likely require a reduction in the size and a modulation of the overall formulation of the end blocks. Developing hydrogels which can transition at room temperature would allow the solutions to be used for additional applications such as room temperature sensors.
Matt Irwin |
September 14, 2012 2:24 PM
Title: Gold Nanoparticles Supported on Carbon Nitride: Influence of Surface Hydroxyls on Low Temperature Carbon Monoxide Oxidation
By: Joseph A. Singh et al.
Journal: ACS Catalysis
The controlled oxidation of hydrocarbons is an important step in a variety of chemical syntheses. The oxidation of carbon monoxide (CO) provides a valuable probe reaction to evaluate the activity of catalysts used for this class of reactions. Gold nanoparticles on oxygen containing supports such as TiO2 and MgO have been shown to catalyze the oxidation of CO1. The authors investigated the role these oxygen containing supports play in the oxidation of CO by preparing a catalyst consisting of gold nanoparticles supported on a non oxygen-containing catalyst carbon nitride support (C2N3). The atomic surface species on the catalyst was determined using X-ray photoelectron spectroscopy (XPS), while the presence of adsorbed CO species on the surface was monitored using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).
The activity of supported gold nanoparticles for the oxidation of CO was determined to increase with the basicity of the support2. For this reason, the authors prepared gold nanoparticles catalysts supported on both a normally synthesized C2N3 support and a C2N3 support synthesized under basic conditions (pH=10), as well as a TiO2 supported catalyst. The XPS spectra of the two C2N3 supported catalysts did not meaningfully differ. For both samples, the only band observed at the electron energy associated with oxygen on the surface was that corresponding to the presence of C=O moieties. This is differentiated from TiO2 supports which were previously shown to form surface hydroxyl groups when treated under basic conditions3. No significant oxidation of CO was observed at ambient temperature for both of the C2N3 supported catalysts, unlike the TiO2 supported gold nanoparticle catalyst which reacted readily. The DRIFTS spectra of the C2N3 supported catalysts before and after exposure to CO were found to have no significant difference, implying CO surface species do not form on either the gold nanoparticles or the catalyst support. An absorption band associated with adsorbed CO species (approximately 2120 cm-1) was observed, however, for the TiO2 supported catalyst exposed to CO. Based upon these observations the authors concluded that surface hydroxyl groups present on oxygen containing supports behave as the initial adsorption site for CO and play a vital role in oxidation reactions on supported gold nanoparticle catalysts.
While the authors demonstrated the need for surface hydroxyl groups on the support to act as adsorption sites for CO during oxidation, the exact interplay between the gold nanoparticle surface, oxygen molecules, and the CO molecules on the species is still not fully understood. One way to further investigate the mechanism of CO oxidation on these catalysts is to perform an in-situ DRIFTS analysis of the oxidation of the CO exposed TiO2 supported catalyst. The rate of disappearance of the band associated with adsorbed CO, the rate of appearance of bands associated with gas phase CO2, and the possible formation of other bands corresponding to the formation of intermediate surface species would provide valuable kinetic information which may help elucidate the mechanism for this reaction.
 Takei, T.; Okuda, I.; Bando, K. K.; Akita, T.; Haruta, M. Chem. Phys. Lett. 2010, 493, 207.
 Haruta, M. J. New Mater. Electrochem. Syst. 2004, 7, 163.
 Veith, G. M.; Lupini, A. R.; Dudney, N. J. J. Phys. Chem. C 2009, 113, 269.
Joseph DeWilde |
September 14, 2012 4:33 PM
Title: Multifunctionalized Cantilever Systems for Electronic Nose Applications
By: Yong Kyoung Yoo et al.
Journal: Analytical Chemistry
In this paper, the authors demonstrate a novel microcantilever array chip with four microreaction chamber. The microcantilevers are functionalized differently for the detection of different targets at the same time. The resonance of the microcantilever will change when the target binding to its surface due to the extra mass. The technique in this work includes: microcantilever fabrication, surface immobilization, direct electrical detection via piezoelectric thin film, etc.
The authors claim that they succeeded in using this device to demonstrate the ability of binding of 2,4-dinitrotoluene (DNT) onto four different surfaces. They designed the microcantilever chips with 4 microchambers. The microcantilever The microcantilever was composed of six multilayers, SiNx/Ta/Pt/PZT/Pt/SiO2, as piezoelectric thin films. Then they chose peptide receptor for DNT detection in surface immobilization: DNT specific peptide (HPNFSKYILHQRC), DNT nonspecific peptide (TSMLLMSPKHQAC), and self-assembled monolayer (SAM) and a bare cantilever as reference. After integrating the microcantilever chip into into a measurement system, they performed surface treatment for multiple target detection. They used a gas generator system and the reference cantilever to minimize mechanically induced resonant frequency shifts. After experiment, the differential signal of the DNT specific peptide and DNT nonspecific peptide gave out a 7.5 Hz resonant response, corresponding to 160 ppb DNT concentration, indicating that this is an effective way for real-time multiple target detection, which can apply to an artificial olfactory system.
There are still many things that can be explored in this work. First, because microcantilever is very sensitive, though the authors showed many ways to eliminate all other effects like thermal noice, vibration noice and humidity, they still didn’t show us if the experiment is repeatable. I would suggest them create a calibration curve using different concentration of DNT. Also, the next step could be using a mixture of analytes to see if the system can really achieve “multitarget” measurement, because in this article they only use DNT as demonstration. Last, it seems very tricky in fabrication, immobilization and surface treatment, it would be better if those steps can be simplified.
Tian Qiu |
September 14, 2012 4:59 PM
Title: In Situ Ambient Pressure XPS Study of CO Oxidation Reaction on Pd(111) Surfaces
By: H Kondoh et al.
Journal: The journal of physical chemistry C
In this paper, the authors researched CO oxidation reaction on Pd(111) surfaces under near real pressure conditions, rather than ultra high vacuum(UHV). They ran reaction using three different temperatures. They used ambient-pressure X-ray photoelectron spectroscopy(AP-XPS), differential pumping mass spectroscopy(MS), and deconvoluting technique for XPS peaks.
They say reactivity was low and Pd surface was poisoned by CO at 200˚C. They observed reaction at 300˚C. They say reactive site formed on surface at 300˚C, as a form of Pd5O4. However, they could not say there was no chemisorbed oxygen on surface. They say there are two different coordination numbers of oxygen species, three and four. They say only oxygen with three coordination number was reactive. As reaction proceeded, cluster palladium oxide was formed and it hindered reaction. At 400˚C, bulk oxide (PdO) was formed and it showed less reactivity even though its coordination number was three. Their conclusion was three coordinated oxygen is reactive and its chemical neighbor is important.
In my opinion, they can theoretically calculate activation energy and electrical conditions like orbitals for several oxygen species, such as three coordinated one, four coordinated one, and bulk oxide. They can give exact number and order of it and explain more systematically. They say cluster of palladium oxide clusters formed during reaction do not have reactivity. They can check this one with only palladium oxide clusters and see whether it was not suppressed by other species such as bulk oxide. Also, as they say, they can precisely measure activation energy of CO oxidation, which will be helpful to understand kinetics of CO oxidation. They can measure reaction rate at more temperatures and quantify amount of products using GC peaks.
Sorry for late.
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