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Minute Papers Due 09/21/2012

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.


Minute Paper #3 (09/19/2012) – Sarah Anciaux

Seminar Title: Nonlinear Spectroscopy of Organic Electrical Interfaces

By: Aaron Massari

In this seminar Massari discussed his groups’ attempts at developing a nonlinear spectroscopic method for determination of molecular structure and dynamics by use of vibrational sum frequency generation (VSFG) at electrical interfaces.

Massari has developed a method to explore the boundary areas of organic field effect transistor composed of different thin later materials. This is done by hitting the sample with two different wavelengths, one in the IR and one in the visible spectrum. When these two wavelengths hit the boundary layer and interact, the VSFG can be monitored and used to determine the accumulation and mobility of charge at an interface. At first they determined that the charge in the developed transistor was seen to have ambipolar accumulation and unipolar conduction. So whether a positive or negative voltage is applied they see accumulation and they only see conduction when a negative voltage is applied. After further experimentation was done, they discovered that when the device is switched on and off, without discharging, they can determine charge hoping and that the accumulation in the positive voltage setting was just slow in some devices, not impossible. The different materials explored for use in the organic field effect transistors also showed different effects on their carrier mobility when annealed or melted.

The group claims to have characterized the charge mobility and accumulation and the boundary region, but there are some areas that could likely be addressed a little further. Massari stated that the 100 μm spot size of the laser give a homogenous readout of the prepared device, I wonder if they might be able to decrease the sampling region and determine the individual contributions of the different conformation of molecules. Massari also mentioned that they don’t exactly know what region they are sampling in when they do the measurements. It might be possible that they think they are sampling the boundary region, when they are actually measuring just outside it. With boundary effects, it’s possible that a drastic change might be seen from the boundary region to the bulk region. I would think they have a thin enough film and can fine tune their sources enough to hit sufficiently close to the boundary layer they want to, but it would be hard to determine as the difference could be very small in a thin film. The group also investigated the effects of annealing temperature and found that it can drastically change the final structure and carrier mobility, but they only talked about one temperature program. So I wonder if changing the temperature ramp, max temp, etc. might also affect they final function and performance of the final material.

Nonlinear spectroscopy of organic electronic interfaces (UofM chemistry seminar 12.18.2012)
Author:Aaron Massari
People are always looking for better ways to improve circuits; by making them faster, lighter, or more flexible. Replacing the standard metal circuit with a carbon based circuit decreases mass and has the potential to be fairly inexpensive. The ideal view of the process of making these carbon based circuits can be visualized as being similar to printing paper according to Aaron Massari. Seeing these advantages and a market for carbon based circuits, the Massari group wanted to determine molecularly, how these carbon based circuits work by looking at what happens at the interface between the organic and SiO2 layers using nonlinear spectroscopy.
Before the Massari group picked this topic, there were previous groups which looked at how the charge moved through the organic material alone, but never at the interface. To analyze this problem, they used 2D IR spectroscopy which allowed them to recover all the individual Fourier transform infrared spectroscopy (FTIR) signals. This also allowed them to look at both the vibrations in the z and x axis in order to connect dynamics with mobility.
For their carbon layer they used carbon polymers because the dynamics of the polymer are unique and therefore it was not known how the charge could move across it. In order to eliminate some the hydrogen carbon translations signals, they exchanged the outside hydrogens with deuterium. After testing to see how changing the carrier mobility affected the dynamics they moved onto how de doping the polymer would affect the way the charge moved. They started out with polyaniline which was a conductive polymer and de doped it. This created a semi-metal. When they doped the sample to have it go back to the way it was, they found a surprising result in that the conductivness had changed. This lead to a new direction for the group to see why changing the polymer affects the dynamics of it and how it affects it.
They decided to focus on was why annealing made some devices better while making others worse. They created a setup where they could look at their sample while heating up and cooling down and measure the dynamics in real time using the signals and the mobility measurements. Initially the SiO2 signal was in the out of plane spectrum, but as the temp increased the signal went down along with the mobility. When it cooled, the signal went up, but part of it had changed to being in-phase with the plane. After the second heating, which was above the bp, they lost all signal and mobility. This experiment was also done with a material that worsens with annealing because the signal and mobility decreased throughout. One research area that should be looked at is the affects of time on the devices because even though they may be cheap to make, it could be a hassle to replace and we would also have to consider environmental effects.

Title: Controlling the Localization of Polymer-Functionalized Nanoparticles in Mixed Lipid/Polymer Membranes
Authors: A. Olubummo et al.
Journal: ACS Nano

Olubummo et al. studied the localization inside lipid/polymer membranes of nanoparticles (NPs) composed of an inorganic core with polymer tails of varying levels of hydrophobicity. CdSe quantum dots were used as the inorganic core because they were well-characterized in the literature. In order to access the entire spectrum of hydrophobicity, hydrophobic polyisobutylene (PIB), hydrophilic poly(ethylene oxide) (PEO), and amphiphilic block copolymer PI-b-PEO were used. The authors were motivated to do this study because the literature shows that the positioning of the NPs inside the membrane significantly affects several membrane properties, including bilayer permeability and biocompatibility. In order to study the interactions between these NPs and lipid/polymer membranes, the authors performed a multitude of tests on these systems, including atomic force microscopy (AFM), monolayer adsorption experiments, and confocal microscopy.

The authors studied the adsorption of the hydrophobic and amphiphilic NPs on a monolayer composed of natural lipids (1,2-dipalmitoyl-sn-glycero3-phophocholine, DPPC) and PIB-b-PEO by using AFM. The AFM of just the DPPC:PIB-b-PEO monolayer showed the formation of cylindrical towers of PIB with a height around 25.7 nm on the surface. The addition of the hydrophobic NPs showed that NPs formed a conical structure on top of the PIB domain, bringing the height of the tower to 127 nm. The addition of the amphiphilic NPs also resulted in the appearance conical structures on top of the PIB domain, but the height of the entire tower was only 52 nm. This indicates the amphiphilic NPs had a lower affinity for the PIB domain than the hydrophobic NPs.

The authors studied the adsorption properties of the hydrophilic NPs on DPPC, PIB-b-PEO, and DPPC:PIB-b-PEO monolayers by performing monolayer adsorption experiments. Based on their results, the hydrophilic NPs readily adsorb on all of the monolayers. However, as the concentration of PIB-b-PEO in the monolayer increased, the amount of adsorbed hydrophilic NPs on the surface decreased, indicating that the hydrophilic NPs have a higher affinity for the DPPC than the PIB-b-PEO block copolymer.

The authors also studied the NP localization in an actual membrane by injecting fluorescent-labeled hydrophobic NPs into vesicles composed of DPPC and PIB-b-PEO and analyzing the vesicles with confocal microscopy. The confocal microscopy images showed that the NPs localized into a single domain, which the authors assume is rich in the block copolymer.

Though this paper introduces a novel strategy for tuning the hydrophobicity of NPs, there are still several aspects of these systems that are not investigated. For example, the authors only used PIB as the hydrophobic tail. However, the authors could have also used polystyrene as the hydrophobic block to see if the NPs behave similar to the PIB NPs. If the results differ, this would show that the identity of the substituent groups plays an important role. The role of the polymer molecular weight could also be investigated. To corroborate the authors’ confocal microscopy images, the NP-loaded vesicles could be studied with transmission electron microscopy. The high resolution of this technique could identify the exact domain in which the NPs localize.

Title: Nonlinear Spectroscopy of Organic Electronic Interfaces
By: Dr. Aaron M. Massari
Seminar Paper #2 September 20, 2012

Organoelectronics are of high interest due to their cheap processing, flexibility, lightweight nature and being recyclable. In addition to the above mentioned positive attributes, they can also be cast into a variety of molds. The goal of these experiments is to understand the molecular parameters that influence electronic mobility using nonlinear spectroscopy. Nonlinear spectroscopy uses small amounts of induced polarization; however that polarization becomes larger when the light is resonant with molecular absorptions.

Two dimensional IR was used instead of Fourier transform infrared spectroscopy to extract correlation functions from solution dynamics. It was determined that the dynamics significantly change as the solvent changes; a solvent with higher oxidative addition had even more weighting present between the oxygenated and deoxygenated species. The molecule P3HT was also examined at annealed and unannealed phases. Results showed that the P3HT acted as a better mobility charge carrier when annealed due to the faster time scales of motion observed. These experiments as well as future endeavors in this area hope to relate charge mobility to molecular dynamics.

Organic field effect transistors (oFET) consist of a very thin (1-2nm) accumulation layer buried under a lot of inactive material. This has presented quite a challenge from an industry perspective due to the inefficiency of all charge mobility being conducted in 1-2nm out of approximately 700nm. IR and Raman active vibrations are also active for vibrational sum frequency generation (VSFG), in which only non centrosymmetric environments, or interfaces, produce VSFG signal. Examining P3HT at a negative gate bias showed conduction, or the accumulation of holes, whereas nothing happened when applying a positive gate bias. This detection indicated a unipolar device, but both electrons and holes were believed to be accumulating at the interface due to trapping. Measurements at a low frequency allowed for a visualization of these localized charges.

The instrument used in the laboratory experiments involved two HeNe lasers, one to determine the depth of the sample and another to show the exact position of the sample for in situ monitoring, such as the tilt and angle. These lasers were used to heat up samples of bare silicon dioxide and a fluorocarbon surface then cool them back to room temperature. Bare silicon dioxide had in plane and out of plane signal which indicates that the rings went from face to end on reorientation during the annealing process. Fluorocarbon surfaces on the other hand showed no change in orientation, however the electronic carrier mobility returned in a diminished state after the cooling phase ended. Future work in this field revolves around modeling multiple interfaces to better understand the electronic properties of the systems being investigated and further examining the interfacial dipoles at the interface of FETs, and how those change the orientation and electronic mobility of the compound. Another area I would experiment further in would be the trapping mechanism of the P3HT interface to see potentially if this could be overturned for use as a dipolar device.

Title: Atomically Resolved Site-Isolated Catalyst on MgO: Mononuclear Osmium Dicarbonyls formed from Os3(CO)12
By: Bruce C. Gates et al.
Journal: The journal of physical chemistry letters

In this paper, authors treated MgO-supported triosmium carbonyls clusters([Os3(CO)11]2−) in flowing He at 548 K for 2h and analyzed the treated sample using infrared (IR) spectroscopy, X-ray absorption near-edge spectroscopy (XANES), extended X-ray absorption fine structure spectroscopy (EXAFS), and STEM. They found that the treated osmium clusters were fragmented to mononuclear osmium dicarbonyls and dispersed well. They also modeled its surface structure using results from spectroscopies. This dispersed mononuclear metal is important because it would have uniformity, making easier to characterize. Usually, well dispersed metal catalysts show high catalytic activity. They picked osmium because oxide-supported osmium complexes catalyze many valuable reactions and osmium has high atomic number that makes STEM easier. They chose MgO as support because it is composed of light atoms and highly crystalline. Both points make STEM easier.
As I said, they figured out that the treated osmium-MgO sample formed mononuclear osmium dicarbonyls. They measured IR spectra and focused on frequency of carbonyl groups. They compared spectra of the original triosmium loaded sample and treated sample and found that frequencies for the triosmium cluster had disappeared and for the mononuclear osmium carbonyls had appeared. From XANES spectra, they could see oxidation of osmium by checking electron occupancies of orbitals. The oxidation would occur if osmium changed from a cluster to a mononuclear state. From EXAFS data, they could get coordination number of osmium with CO and MgO and distances. They got the surface species as Os(CO)2{Osupport}n, where n is 3 or 4. From STEM images, they could get visual evidence of mononuclear species on surface. Also, from image of MgO(110), they could specify the location of osmium, which is defect sites of Mg atoms.
For additional evidence of surface species change, they could measure mass spectroscopy and check carbon monoxide (CO) peak from treating unit. As cluster decomposes, carbonyl group would evaporate as CO. In addition, I think accuracy of EXAFS results is not good, ±20%. They might need to do more measurement. They could check their model with DFT calculation. They will be able to compare bond length and surface structure of Os. I am not sure about its uniformity. Checking the picture of the treated sample with eye would not be exact, so atomic scale microscopy might can check whether there is only one Os or not. The Os species need to be dispersed at equal distance for uniformity. They say it is uniform because there are only mononuclear Os carbonyls, but I think they need to control distance between Os species. They said they failed to measure Os−Osupport distance. It might be measurable when they use atomic scale microscopy or they can get this value from calculation as well.

Minute Paper #3 (9/21/12) – Matt Irwin
Title: Evolution of Small Molecule Content and Morphology with Dip-Coating Rate in Supramolecular PS–P4VP Thin Films
Authors: S. Roland et al.
Journal: Macromolecules

The development of controlled dip coating procedures for coating surfaces with self-organizing diblock copolymers are useful for the fabrication of new nanoreactors, semiconductors, and biosensors. For some applications, it is desirable to modify the structure of the block copolymer via the addition of a small molecule which preferentially interacts with one of the blocks. For dip coating applications, the concentration of this small molecule (SM) in the dip coating solution may not be the same as the concentration in the final coating. In this paper, the authors describe a model system in which they use attenuated total reflection infrared spectroscopy (ATR-IR) to determine how the amount of small molecules naphthol (NOH) and napthoic acid (NCOOH) to PS-b-P4VP present in dip-coated films compares to the bulk solution as a function of dip coating rate. The authors also investigate how the interactions between the small molecules and vinyl pyridine (VP) affect film morphology. Techniques used include ATR-IR, atomic force microscopy (AFM), and differential scanning calorimetry.

The authors prepared 5 mL solutions of PS-b-P4VP in THF at concentrations of 25 mg/mL with either NOH or NCOOH at ratios of 0.5 to 2 SM:VP. Cleaned silica substrates were then immersed into the solution at 5 mm/min, then withdrawn from the solution at rates from 0.5 and 5 mm/min. The uptake ratio of the films was calculated by normalizing the ATR-IR spectra to the block copolymer signature at 1493 cm-1 and comparing the peak height at 1510 cm-1 for NCOOH and 1387 cm-1 for NOH at various coating speeds. The uptake ratio of the small molecule was much smaller at lower dip coating rates, which the authors attribute to the THF solvent competing with vinyl pyridine for hydrogen boding with the small molecule. The film morphology for the NOH:VP films remained PS droplets in a P4VP matrix regardless of processing conditions or SM concentrations; the authors attribute this effect to the preferential interaction between NOH and PS which was observed as a large glass transition temperature (Tg) depression. The NCOOH:VP films demonstrated a transition from PS droplets in a P4VP matrix for uptake ratios less than 0.5 to in-plane cylindrical for 0.5 to 1 to in-plane lamellae for greater than 1; these changes in morphology were attributed to the hydrogen boding between NCOOH and VP which was seen as a shifting of both species’ bands to slightly higher wavenumbers.

The authors of this paper provide a great perspective on how processing conditions can affect film morphology, but additional work needs to be performed before this process could produce functional materials. Additional, faster dip coating rates should be tested to determine the effect on (1) the small molecule uptake ratio and (2) the film quality and morphology. The authors note that obtaining a homogenous, smooth film with the current methods is difficult, so post processing steps could also be investigated to produce more uniform films. For example, annealing the film would allow residual stresses in the film to relax, likely resulting in a more uniformly coated surface.

Thickness Determination of sub-nm Layers using Laser Ablation Inductively Coupled Plasma Mass Spectrometry

by: Hattendorf, et. al.
Journal: Analytical Chemistry

Laser ablation can be used to profile the depth of inorganic layer structures. Current lasers either melt near-surface regions or create conical crater shapes, reducing depth resolving power. The advantages of laser ablation include simpler sample handling, the ability to create three-dimensional distribution images. The authors propose that the high analyte sensitivity of Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) will allow the determination of very shallow features in layer structures. By completely removing the analyte layer of known composition and density in a defined region, the thickness of the layer can be determined.

Samples were made of neodymium layers of varying thickness in the nanometer range, sputtered onto silicon substrates and then covered by a layer of aluminum to prevent the oxidation of Nd upon exposure to air. Each sample was analyzed at 40 points across the plate, in different sessions on different days to test reproducibility.

Using 11 J/cm^2 laser pulses, the authors demonstrated the almost complete removal of Nd and Al from the sample with the first two pulses. Further pulses on the same ablation site showed residual amounts of Nd and Al, possibly from the lateral crater walls or the crater bottom, where melted Al and Nd may have mixed with the silicon substrate.

The authors established a linear relationship between the thickness of the Nd layer and the integrated counts of 146-Nd+. Extrapolation of this linear relationship showed a theoretical minimum detectable thickness to be 0.2 picometers, or about 0.1% of the thickness of a Nd monolayer.

The signal from the aluminum layer was used to normalize that of neodymium. A glass with a homogeneous distribution of Al and Nd was analyzed using the author's LA-ICPMS techniques to determine the relative sensitivity of the instrument to aluminum and neodymium. With this established, the authors were able to calculate the thickness of neodymium layers using the known thickness of the aluminum layer with +/-15% accuracy.

The authors also developed a technique for determining the thickness of neodymium layers using the absolute sensitivity of Nd in the samples and the diameter of the ablation crater. The absolute sensitivity of Nd was established by ablating metallic Nd samples. This technique proved to be much less reliable, especially for thinner Nd layers, producing deviations of up to 40% between the calculated and known Nd layer thicknesses.

The authors note the difficulty of determining absolute sensitivities by LA-ICPMS because this requires an accurate determination of the mass removed and transported to the detector. Because of this, it seems that the only way to accurately determine the thickness of these layers is through normalization, as the authors did with aluminum. Because the laser ablation causes some melting that can mix layers together slightly, the authors may see more success if they normalize with a pure metal of known relative sensitivity that is separated from the sample, rather than a metal that must be layered on top of the neodymium to protect it.

Minute Paper 3, Marzieh Ramezani

Title: 15N-Labeled Full-Length Apolipoprotein E4 as an Internal Standard for Mass Spectrometry Quantification of Apolipoprotein E Isoforms.
Author: Meiyao Wang.
Journal: Analytical Chemistry.

Apolipoprotein E (ApoE) is a category of proteins which can bind to lipids and has a critical role in catabolism of triglyceride-rich lipoprotein constituents. ApoE has been made of three isoforms (apoE2, apoE3, and apoE4) which affect Alzheimer’s disease. In this paper, Multiple Reaction Monitoring (MRM) Mass Spectrometry in conjunction with using 15N-labeled apoE4 (15N-apoE4) as an internal standard have been employed to measure concentrations of apoE4 and total apoE. The results of this study may help researches to elucidate the major effect of apoE on developing diseases.

In the first step, the 15N-labeled apoE4 was synthesized by the replacement of Cysteine-Arginine amino acid in 112th position, which generates a unique tryptic peptide. It should be considered that structure of apoE isoforms were different in just one amino acid which made it quite challenging to produce the labeled peptide. MALDI spectroscopy was used to investigate the efficiency of labeling process by measuring isotopic distribution of the peptide. The result revealed 99% incorporation of 15N, which means the labeling was complete.

In order to increase the accuracy of analysis, it was attempted to make the labeled peptide quite similar to the natural peptide. Thus, the effect of Methionine oxidation on apoE4 also has been investigated by fully oxidation of the Met with H2O2. Met oxidation is a common process to occur whenever a peptide in a protein has Methionine on its surface and it increases in aging. The fully oxidized and regular apoE4 were quantified by using several temporal cortex samples. Their concentrations were calculated by monitoring their transitions per individual peptide. The result revealed that the level of Met oxidation is low and subsequent measurements were done on the regular apoE4.

Finally, the concentrations of apoE4 in samples which were obtained from human frontal cortex were measured by using MRM method and their results were compared to the genotype information. It was found that apoE4 is present in all samples that have ɛ4 allele. 15N-apoE4 was also used as an internal standard and leads to accurate determination of total apoE and other isoforms (apoE2 and apoE3) concentrations for individual possessing ɛ2/ɛ4 or ɛ3/ɛ4 alleles.

The authors claimed that their method is a quantitative method for in vivo study and it is a good substituent for antibody-based techniques which cannot distinguish these three isoforms. They also mentioned that this is not the first report for applying MRM method along with isotope labeling and it has been used for other quantification studies on biological systems previously. I think other methods such as isoelectric focusing (IEF) which is rapid electrophoresis method for separation of molecules based on their isoelectric point might give a similar result.

Title: Single-Stranded DNA Curtains for Real-Time Single-Molecule Visualization of Protein–Nucleic Acid Interactions

Authors: Bryan Gibb, et al.

Journal: Analytical Chemistry

In this paper, the authors attempted to develop a new way for single-stranded DNA (ssDNA) imaging by using single-molecular techniques. Based on their previous work on fluid lipid bilayers coated microfluidic sample chambers, the authors tethered ssDNA to the edges of nanofabricated barriers in the lipid bilayer and visualized it by total internal reflection fluorescence microscopy (TIRFM). TIRFM selectively excites fluorophores in a restricted area of the sample and operates with an evanescent wave that occurs only when the incident light is totally internally reflected at the interface. However, this evanescent field decays quickly from the interface. As a result, only the thin area that is much closed to the interface can be excited, which avoids interference from the bulk sample. Therefore, TIRFM is widely used in biological systems, providing visualization of cell/substrate contact interface as well as fluorescence detection of single molecule.

The authors first prepared φ29 DNA polymerase, which is used to initiated the replication of ssDNA, and enhanced green fluorescent protein-tagged RPA (RPA-eGFP) by expressing their encoding genes in E. coli strain BL21. Single-stranded DNA substrates were obtained through in vitro rolling circle replication assay by annealing M13mp18 (NEB) to a biotinylated primer. As to the sample chamber, it was treated by electron-beam lithography to build barriers and then coated with lipid bilayers. After that, single-stranded DNA products, which possessed a biotin at the 5’ end, were coupled to the bilayer through tetravalent streptavidin linkage. Driven by the hydrodynamic force provided by the flowing buffer, ssDNA aligned at the linear barriers.

To visualize ssDNA, RPA-eGFP was then injected to the chamber and then bound to ssDNA, making it stretched to full contour length. As shown in figure 2C, after the injection, fluorescent ssDNA curtains showed up, indicating the formation of RPA-eGFP-ssDNA complex. However, since the single-tethered curtains could only be viewed in constant buffer flow, the authors designed double-tethered curtains by adding a pentagon-shaped anchor points to tether the other end of the complex. The results showed that double-tethered ssDNA curtains remained visible even in the absence of flowing buffer. As a preliminary test of the practical usage of this method, the authors visualized the binding of anti-HA quantum dots labeled Sgs1 protein to the double-tethered ssDNA. With two-color imaging, both the ssDNA and the bound Sgs1 could be seen.

As to the improvement of the experiments, since the ssDNA was generated by rolling circle replication, is it possible that the DNA ring at the downstream of the ssDNA would affect hydrodynamic force that is needed to stretch ssDNA? Also, would it influence the biochemical function of the ssDNA? If it does impact the function, we need to cut the DNA ring off. Then further experiments would be required to examine whether the new ssDNA can still be stretched to its full length.

Title: Metal Oxide Nanoparticle Mediated Enhanced Raman Scattering and Its Use in Direct Monitoring of Interfacial Chemical Reactions
Author: Li Li, et al.
Journal: Nano Letters

Li Li et al. focuses on enhancing a popular plasmonic application called Surface-enhanced Raman scattering (SERS). By utilizing electromagnetic enhancement through nanoscale modification of the surface via deposited metal oxide nanoparticles, surface Raman scattering is enhanced substantially for materials previously unavailable to this technique.

To date there has been some success in improving SERS via metallic nanoparticles deposited on the surface, which couple the propagating surface plasmon polaritons with the underlying metallic substrate or in certain cases dielectric substrates. The author claims that metal oxide nanoparticles (MONPs) deposited on a flat metallic surface are nonplasmonic and do not have a localized surface plasmon resonance (LSPR) in the wavelength range studied. Li Li et al. refers to their process as nanoparticle-enhanced Raman scattering (MONERS). The method of creating the nanoparticles is easy, cheap, and works with many different types of metal oxide materials. The MONPs allow for detection of biomaterial used for many current medical applications.

The author demonstrates MONERS by coating a monolayer of dithiol on flat Au substrates and then Fe3O4 NPs were adsorbed onto the monolayer. In Figure 1 you can see from SEM images where the particles lie on the surface, and in the MONERS image you can see the effect of the MONPs clearly, excited via 532 nm laser. Another substrate with processed surface roughness was made in order to rule out it’s influence. Without the attachment of Fe3O4 NPs, no Raman signal from the dithiol was detected. The author also states that although this publication observes molecules that have been adsorbed on the surfaces before NP attachment, the MONERS effect works equally well with molecules also adsorbed on NPs.

Five MONPs of different sizes and elements, as well as a reference Au NP were used in a test to determine the range of the MONERS effect. From figure 2 it is clear that the MONERS signal is visible for all of the MONPs. Some of these MONPs were irregularly shaped, and a few of them formed agglomerates but still detectable MONERS. Bulk MONP powders were used as a reference to analyze Raman modes.

Li Li et al. also used a simulation to confirm the optoelectrical effects and determined that the highest field enhancement is localized in the gap between the Fe3O4 NP and the Au substrate and influenced by the refractive index of the NP.

A good future experiment would be to figure out how dense the NPs can be distributed to increase maximum detection area before the MONERS effect is diminished. Another test could be performed coupling the performance of the MONPs with nanoscale surface topology enhancements which for example, could allow imaging of irregular surfaces previously not possible with SERS.

Seminar: Nonlinear Spectroscopy of Organic Electronic Interfaces

Presenter: Aaron Massari

Some of the challenges current electronic technologies must overcome are expensive processing, physical rigidity, a lack of tunable properties and poor recycling potential. In this seminar, Dr. Massari presents organic polymeric small molecule films as possible alternatives to conventional metal and semiconductor lattice technologies. In particular, he presents the characterization of organic electronic film interfaces via nonlinear spectroscopy methods.

Although previous researchers have investigated charge transfer of organic thin film materials, little work had been done to assess charge transfer along silica-organic interfaces, which are characteristic to organic field-effect transistors (oFETs). 2D infrared vibrational echo spectroscopy (2D-IR VES), by accessing discrete spectral features and fs resolution, provides considerably more information than 1D IR and was therefore utilized to analyze the charge transfer (mobility) rate and molecular dynamics along the silica-organic interface. Polyaniline films, when dedoped from high to low conductivity, suggested molecular dynamics upon charge transfer can change the morphology of the film even as it returns to its initial conductive state. Also, evidence suggested imperfections in the organic films reduce the rate of charge transfer.

Vibrational sum frequency generation (VSFG), a technique that irradiates a material with two different electromagnetic frequencies (one in the visible and the other in the IR spectrum) and releases one sum of the two frequencies, was also used to monitor molecular dynamics and charge transfer (mobility) along the silica-organic interfaces. Massari’s group found devices with carbon-carbon bonds in the plane of the interface (edge on) move charges more efficiently than materials with bonds against the plane of the interface (face on). In order to further understand interface interactions, mobility was analyzed via VSFG during annealing of self-assembled monolayers (SAMs), which is a currently essential process for the development of oFETs. Despite the challenges of assessing annealing, an irreversible high temperature process, Massari’s group constructed a creative device to assess charge transfer during annealing silica (hydrophilic) and fluorocarbon (hydrophobic) SAMs. Silica SAMs demonstrated face on to edge on orientation changes during cooling (post initial heat treatment), which improved mobility, but a loss in mobility by approximately three orders of magnitude upon heating during the second heating cycle. Similarly, fluorocarbon SAMs experience a loss in mobility upon finishing the annealing process, which is likely related to an elastic structural change. Despite mobility increases at high temperatures, which is likely due to more charge hopping, structural and phase transitions couple with mobility changes and therefore negate mobility of SAMs after the final cooling process.

Although these materials may be cheaper and more environmentally friendly than current lattice technology, a total negation in mobility upon their current annealing process significantly limits their potential for commercial use. Other organic films with resistance to mobility loss upon annealing should be experimented with. Alternative annealing procedures, possibly with lower temperature ramping, could be experimented with as well.

Title: Operando and Kinetic Study of Low-Temperature, Lean-Burn Methane Combustion over a Pd/γ-Al2O3 Catalyst

By. Jing Xu et al.

Journal: ACS Catalysis

Methane is commercially used as both a clean burning fuel and in the production of hydrogen via steam reformation. Methane, however, exhibits a strong greenhouse gas effect1, and effluents from these and other processes can still contain small amounts of unreacted methane. Traditional incineration of the remaining methane is not normally cost effective; thus, there is considerable interest in characterizing catalysts capable of combusting these lean streams at low temperatures. In this work, the authors investigate the kinetic mechanism of methane combustion over Pd catalysts using steady state kinetic measurements and X-ray photoelectron (XPS), operando Raman, and diffuse reflectance infrared Fourier transform (DRIFTS) spectroscopy.

The authors prepared a Pd catalyst supported onto a -Al2O3 substrate and performed steady state kinetic measurements at various temperatures and methane and oxygen partial pressures. They found that, as the temperature was increased from 473 K to 673 K, the oxygen pressure dependence of the rate of methane combustion dropped from a near first order dependence (0.86) to a near zero order dependence (0.16) while the methane pressure dependence remained relatively close to first order (0.95 and 0.81). This observation is inconsistent with both a simple Langmuir-Hinshelwood mechanism, in which both methane and oxygen adsorb onto the Pd surface and react, and a Mars−van Krevelen mechanism, in which palladium oxides (PdOx) are formed and the methane adsorbs and reacts with the lattice oxygen (the rate is expected to be invariant to oxygen partial pressure for this mechanism). XPS spectra were taken on a fresh catalyst sample and one which has undergone reaction at 673 K and a shift in the binding energy characteristic of the formation of PdOx species was observed between the catalyst samples (from 335.4 eV to 336.7 eV). In-situ DRIFTS of CO-exposed catalyst samples and operando Raman spectroscopy were both performed at various temperatures (from 298 K to 773K) to further investigate the formation of PdOx under reaction conditions. Peaks associated with PdOx species were observed to form with increasing temperature during the operando Raman measurements, while bands associated with CO adsorbed onto Pd metal were found to diminish with increasing temperature in the DRIFTS study. Based upon these observations, the authors concluded the mechanism for methane combustion is predominately a Langmuir-Hinshelwood mechanism over Pd metal at low temperatures, but shifts to a Mars−van-Krevelen mechanism as the temperature is increased due to the formation of PdOx.

The authors were successful in describing the observed change in oxygen pressure dependence with a shift in the mechanism of methane combustion as PdOx is formed. One would expect, however, that the activation energy of the reaction to change with this shift in mechanism, as was suggested by Cullis et al.2. A simultaneous investigation of the change in activation energy and the change in operando Raman band corresponding to PdOx formation with respect to temperature may allow for a valuable quantitative link between the relative importance of the two mechanisms and the PdOx surface density.

[1] Charlson, R. J. Nature 2005, 438, 165−166.
[2] Cullis, C. F.; Willatt, B. M. J. Catal. 1983, 83, 267−285.

Paper: Highly selective fluorescent probe for fast detection of hydrogen sulfide in aqueous solution and living cell
By: Qian et.al.
Link: http://pubs.rsc.org/en/content/articlelanding/2012/cc/c2cc36141h
Analytical Technique: Fluorescence

In this paper, the authors have discovered a new ratiometric fluorescence probe (E1) that is specific to H2S detection in living cells. H2S is an important signaling molecule in the gastrointestinal tract. It is an anti-inflammatory aid and is responsible for regulating the contracting forces in the digestive system.

Many chromatography approaches have been taken to measure the H2S concentration in cells, but the techniques (gas chromatography, electrochemical analysis) are limited to high cost and a lack of good resolving power for H2S. Because of this, fluorescence techniques are of particular interest. Current fluorescence probes, however, have slow response times and not very low limits of detection. This is not useful for H2S detection in vivo since it is metabolized so fast in cells.

The authors’ new molecule, E1, fluoresces at 356 nm without H2S present, but when NaHS is added there is a strong fluorescence shift to 483 nm. They theorize that a reaction occurs with H2S and E1 to create HMBT, a molecule that fluoresces at 483. However when E1 is exposed to a thiol group on some other molecule, that reaction doesn’t take place, and it still emits only at 356 nm. This shows the specificity of E1. Next, the authors worked on the detection limit of E1. They were able to estimate the detection limit to be around .12 µM quantitatively with a calibration curve of various concentrations of H2S. It is also important to note that above 30 µM, the fluorescence didn’t change much at all. Following that experiment, the authors then did a study of E1 in living hela cells. They were able to detect H2S in the hela cell without destroying it and the fluorescence of E1 was very fast, a 30 fold increase in signal in 2 minutes. They then concluded that E1 has the potential to visualize H2S level changes in a living cell.

I would love to see follow up on this project. I would want to run tests on an actual intestinal cell from a human. One question I wish the authors would have addressed was their future plans for E1. Yes, you can see this in a hela cell, but how would this work for actually seeing this in a gastrointestinal tract? It would also be important to know how selective it is for H2S versus other molecules seen in vivo, not just other molecules that the authors selected.

Minute Paper #2 – Tian Qiu

Title: Optofluidic Surface Enhanced Raman Spectroscopy Microsystem for Sensitive and Repeatable On-Site Detection of Chemical Contaminants

By Soroush H. Yazdi and Ian M. White

In this paper, the authors developed a portable and automated optofluidic surface enhanced Raman spectroscopy (SERS) microsystem. To demonstrate the performance of this microsystem, the authors tested it with two potential food/water contaminants: melamine, which is inhibited for food addition and may cause kidney stones, and thiram, a fungicide which is also a neurotoxin. The conclusion is that this optofluidic SERS device can detect trace amount of melamine and thiram at levels below the U.S. federal requirements.

This integrated microsystem consists a passive mixer, a porous microfluidic matrix and two fiber optic cables. The passive mixer mixes the analyte with silver nanoparticles and to promote adsorption of analyte molecules onto silver nanoparticles. Compared to active mixers which always uses electrodes, passive mixer does not require electric fields, thus not needing additional fabrication steps (e.g., metal electrodes) and active components (e.g., power supplies). By this way, passive mixers are more repeatable and robust. The porous microfluidic matrix is formed by silica microspheres to trap and concentrate the silver nanoparticles and analyte molecules. Silica has very little optical absorption, while modification can be done on the surface of the silica microsphere. For detection of the SERS signals, the authors used two fiber optic cables for the excitation of the sample and collection of the scattered photons. The two fiber cables are prealigned to the detection zone to avoid aligning and focusing the microfluidic device under a microscope objective. All these features above make the device suitable for sensitive, repeatable and automated on-site detections.

To demonstrate their device, first they use melamine. The results showed that as low as 125 ppb melamine is detected while the chip-to-chip repeatability is good. The experiment also shows their device improves the detection limit for melamine by a factor of approximately 100 and the result is repeatable. Furthermore, they detected the thiram in water. When the concentration is 1 ppb the highest thiram SERS peak is still easily visible, and other experiments also shows the repeatability and quantitative analysis.

I really love this device and hardly can find anything bad about this microsystem. The only thing I thought about is that this device might be not reusable because you can only load silica microsphere into the micro-column once.

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