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Minute Papers Due 11/02/2012

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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Title: Noninvasive Optical Imaging of Nanomedicine Biodistribution
Authors: Kunjachan, S., et al.
Journal: ACS Nano

Kunjachan et al. developed a novel noninvasive imaging technique for assessing the distribution of nanomedicines, vesicle or nanoparticle carrier materials loaded with drugs, throughout the body. Current noninvasive imaging techniques, such as 2D fluorescence reflectance imaging (FRI) and 3D fluorescence molecular tomography (FMT), cannot quantitatively assess the localization of the drug carriers due to low penetration depth or poor spatial resolution. To overcome these obstacles, the authors created a hybrid technique that combines microcomputed tomography (μCT) and FMT. In this CT-FMT technique, the carrier material biodistribution is assessed by 3D FMT and then overlaid onto a projection of tissues and organs that is determined by μCT. Using this technique, the authors were able to visualize and quantify the localization of the carrier materials in both healthy organs and tumors.

To display the improvement of CT-FMT over 2D FRI and 3D FMT, the authors used all three techniques to observe at various time points the biodistribution of N-(2-hydroxypropyl)methacrylamide (HPMA) nanocarriers (5-10 nm) labeled with Dy750, a near infrared fluorophore (NIRF). The nanocarrier was injected into rats that had colon tumors. 2D FRI and 3D FMT showed that the nanocarriers localized in the tumor after 72 hrs, but were unable to provide information about healthy organ localization because organ boundaries could not be resolved. With CT-FMT, high resolution scans determined the positions of the organs (i.e. the heart, kidney, etc.) and tumors. The biodistribution data were then overlaid onto the projection to show that the nanocarrier also localized in the lung, heart, and liver after 72 hours.

2D FRI was unable to quantify the efficiency of the tumor targeting (how much of the nanocarrier localized on the tumor vs. how much was injected). However, both 3D FMT and CT-FMT were able to quantitatively measure the nanocarrier concentration in the tumor. Additionally, with CT-FMT the authors were able to divide the 3D projection of the tumor into sections. Using this segmentation, the authors observed that the nanocarrier was more concentrated on the periphery of the tumor than the core, which corroborates the literature.

After the 72 hr time point, the healthy organs and tumors were removed from the rat and an ex vivo 2D FRI was performed. The ex vivo showed a similar biodistribution to that of the CT-FMT.

Though this technique shows great promise, this publication still leaves several unanswered questions. For instance, the authors did not discuss the feasibility of scaling this technique to work on humans. FMT, which uses lasers to excite the NIRFs, works well on small laboratory animals. However, for the human-scale, the technique would have to be tuned so that the laser is strong enough to penetrate through the human body, but mild enough so that it doesn’t harm the patient. Additionally, the authors only looked at the organic HPMA as the nanocarrier. The authors should test the robustness of the CT-FMT technique by looking at inorganics, such as gold nanoparticles, or polymers with different chemical structures, like hydroxypropyl methylcellulose acetate succinate.

Title: Structure and Dynamics of Interfacial Water Studied Using Femtosecond 2-Dimensional Surface Vibrational Spectroscopy
Speaker: Mischa Bonn
Seminar date : 11/01/12 Department of chemistry Bryce L. Crawford lectureship

Professor Bonn presented his study about structure and dynamics of the water-air interface. He said that many fields are dealing with the interface of water, such as electrochemistry and membrane biophysics, so understanding interface of water is important. He said water is special molecule. Water has dipole moment, so it can form hydrogen bond which makes water special. Because of hydrogen bond network in water, vibration of one O-H bond causes excitation through the water media. Reorientation of water molecule is explained by jump mechanism, not by continuous diffusion. He mentioned that this would not be same at the interface. Thus, he focused on O-H vibration of interfacial water molecule using ultrafast two-dimensional surface-specific sum-frequency generation vibrational spectroscopy (2D-SFG).

The SFG spectrum of water is different from bulk water and seems mixture of ice and bulk water and vapor-like free O-H. However, he said coupling of stretching and bending vibrational states happened at water interface. He proved this using isotopic substitution, using H-O-D, because deuterium is heavier and stretches slower than normal hydrogen. He investigated homogeneity of water using energy transfer at the water-air interface. He additionally pumped IR to normal SFG spectroscopy to get 2-D SFG. From the spectra, he said interfacial water molecules are not homogeneous, but it was not completely heterogeneous, either. Energy transfer at interface is slower than bulk water because many free O-H groups do not contribute to energy transfer. He also studied orientation of interfacial water. He used polarized spectrum to study this. He found that orientation change of interfacial water is faster than the change of bulk water. It means there is diffusion of motion with jump mechanism. He studied also differences between O-D and O-H.

Studying the interface seems very interesting. However, as someone asked during seminar, he did not study exact structure of interfacial water molecules. He usually explained phenomena by free O-H groups on air side of interface. He had no idea about how water molecules are connected at the surface of water. He said the sensitivity of spectroscopy is not enough for studying exact structure. If sensitivity is improved, I think he can see the vibrational frequency of hydrogen bond and study the connection and structure of interfacial water. In addition, to expand this idea, studying other interfacial system would be a great idea. For biological physics, protein solution or protein-water interface would be a great system to study. For first step, the interface of organic solution, such as ethanol, would be fine. They need to distinguish much more peaks in SFG spectrum, like C-H vibration. If someone focuses on various ionic solutions, it would be good for electrochemistry. There would be additional dipole moment, or charge caused by ions. It would change O-H vibration and one can also detect this using SFG technique.

Minute Paper #8 (10/26/12) – Matt Irwin
Title: Trifunctional Photoinitiators Based on a Triazine Skeleton for Visible Light Source and UV LED Induced Polymerizations
Authors: Tehfe, MA et al.
Journal: Macromolecules

The development of efficient polymerization photoinitiators is necessary for industrially relevant applications such as polymer film fabrication where monomer conversion and processing time are key parameters. The development of multifunctional photoinitiators is of particular interest for materials applications, as the larger size of the initiators leads to lower vapor pressures and reduced migration of the initiator in the final product. In this paper, the authors demonstrate that when the spacer between initiator sites is small, multifunctional photoinitiators also demonstrate improved extinction coefficients (ε) and red-shifted π to π* transitions relative to monofunctional photoinitiators due to coupling of the molecular orbitals of the photoinitiator units. These differences lead to relatively larger monomer conversion under soft irradiation conditions. Techniques used include radical polymerization; cationic polymerization; irradiation of samples via a halogen lamp, 365nm LED, and a Xe-Hg lamp; fluorescence spectroscopy, and Fourier transform infrared spectroscopy (FTIR).

The authors studied how well triazine scaffolds which have been trifunctionalized with benzophenone, pyrene, or anthracene moieties (denoted TzBP, TzPy, and TzAn, respectively) are able to photoinitiate polymerizations of 3,4-epoxycyclohexylcarboxylate (EPOX), blends of EPOX and triethylene glycol divinyl ether (DVE), and trimethylolpropane triacrylate (TMPTA). All three photoinitiators studied possessed higher extinction coefficients relative to the monofunctional photoinitiators; molecular orbital calculations suggested that this improvement was due to a strongly delocalized π to π* transition. The authors used FTIR to track the concentration of (1.) double bonds (1630 cm-1) during TMPTA radical polymerization and (2.) epoxy (790 cm-1) to polyether (1080 cm-1) during cationic polymerization. By studying the excited state behavior of TzPy in a solution containing tris(trimethylsilyl)silane ((TMS)3Si-H) diphenyliodonium hexfluorophosphate (Iod1), the authors deduced the following mechanism: TzPy absorbs an incident photon, then reacts with Iod1 to generate two radicals: TzPy*+ and Ph*. Ph* can then abstract a hydrogen from (TMS)3Si-H to generate (TMS)3Si*, which combines with TzPy*+ to form (TMS)3Si+, an effective initiator for epoxide ring opening polymerization. Analogously, solutions of TzPy with N-methyldiethanolamine (MDEA) produced MDEA* which can initiate polymerization of amines. The authors found in all cases that the trifunctional photoinitiators resulted in two to threefold higher monomer conversions, from 20 to 30% conversion for monofunctional initiators to 60 to 80% for trifunctional equivalents.

While the authors provide a method for improving the conversion of photoinitiated polymerizations, much more work needs to be done before this could be a useful pathway to materials development. Conversion to polymer was typically at most about 80%, meaning that the remaining monomer would need to be removed before the film could be used. Perhaps more troubling is that concentration is the only figure of merit used in this paper. We could expect that changing the photoinitiator could change the polymer’s molecular weight distribution, which would affect the film’s mechanical properties. To this end, a bulk synthesis of each polymer could be performed, and the molecular weight distribution for each polymer resulting from a monofunctional initiator versus a trifunctional photoinitiator could be compared. If the trifunctional photoinitiator results in high conversion but polymers with relatively low molecular weight and high polydispersity, the utility of the paper’s approach would have to be questioned.

Minute Paper #7 (Seminar #2)

Title: Structure and Dynamics of Interfacial Water Studied Using Femtosecond 2-Dimensional Surface Vibrational Spectroscopy

Lecturer: Professor Mischa Boon, the Max Planck Institute for Polymer Research in Germany

The lecturer talked about his work on using surface 2D surface vibrational spectroscopy to explore the structure and the dynamics, including energy transfer and reorientation, of the interfacial water. They measured the OH stretch vibration to characterize the structure and dynamics of interfacial water.

On determining the structure of interfacial water, sum frequency generation (SFG) spectroscopy was used to measure the frequency of OH stretch. Sum frequency generation is a non-linear optical process. Laser with the frequency of visible light and infrared light overlap at the surface of the water interface and an output beam is generated at a frequency which is the sum of the two input lights. By tuning the IR laser, the vibrational spectrum can be obtained. SFG was forbidden in bulk water, so it can be used to characterize the water interface.

They used SFG spectroscopy to test their vibrational coupling hypothesis, which supposes that the OH stretch would couple with the "dark state" , the bend overtone of OH group in the same water molecule. To "switch off coupling", they replaced one of the hydrogen in water molecule with deuterium. By the replacement, theoretically the degeneracy of the energy states of two hydrogen/deuterium atoms was eliminated so there were no splitting between the two energy states. The SFG spectroscopy verified the hypothesis by showing no splitting peaks on the spectrum.

Also, 2D-SFG was used to study the energy transfer on the water interface. An IR laser pump was used in the SFG spectroscopy and a frequency-frequency relationship function was obtained. There were two limiting cases. First, if the water molecules on the surface were completely inhomogeneous, the slope in the 2D spectrum should be 1. Another case was that if the water molecules were totally homogeneous, the spectrum would show a perfect round spot. So the slope should indicate the heterogeneity and the energy transfter dynamics on the water surface. By fitting the theoretical equation with the observation results, they confirmed that the energy transfer on water surface is 2 times slower than in bulky water.

Polarization-resolved SFG and IR pumping were also applied to study the reorientational dynamics on the interfacial water. A fs-IR spectroscopy with parallel and perpendicular inputs light was used to obtain the absorption at both inputs and an equation was applied to evaluate the polarization of the water molecules. They found that the motion on the surface was diffusive but not jump-like.

This technology has potential to study any surfaces. Now their study focuses on pure water interface. I think it might be a good idea to study some solutions, like polymer solution, especially those surfactants, to know more about there behavior on the interface. Also, can SFG be used in membrane protein studies? That might be a too complex system, though.

Sarah Anciaux – Minute Paper # 9 (11-02-12)

Seminar: Electroanalytical Applications of Scanning Ion Conductance Microscopy

Presenter: Dr. Lane Baker

In this seminar Dr. Baker presented a novel technique for topographic and potentiometric determination over cell monolayers, and specifically tight junctions between cells.

Previously, the patch clamp method was used to determine resistance through a cell channel. This method does not work well for tight junctions though, as there are many cells in a row and the detection needs to be done in-between the cells. In order to skirt this problem Dr. Baker described the fabrication of nano-pipettes that had 20-100 micron pores in the tip. By filling the pipette tip and covering the sample in 100 mM KCl, along with many well placed electrodes they were able to determine the topographical features of monolayer cells and also the electrochemical properties of the tight junctions. Through collaborating with the Hou group, they were able to compare a knocked-down cell sample (Claudin-2 knocked-down) to a wild-type. This demonstrated that the nano-pipettes could, though scanning ion conductance microscopy, determine species electrochemical variations in the tight junction of cells with relatively high spatial resolution.

Dr. Bakers group claims to have developed a new technique for topographical and electrochemical detection by use of a nano-pipette but there are some questions that could still be addressed. One area that would be interesting to investigate further would be the tip-to-sample distance and its affect on the spatial resolution for the topographical and potentiometric measurements. It would also be interesting to see how different solutions around the nano-pipette might affect the performance of the method or if it might even improve. Another thought on the surrounding solution would be how tolerant the probe is to harsher conditions, and if it would be able to hold up to changes in the environment that is being tested.

Title: Structure and Dynamics of Interfacial Water Studied Using Femtosecond 2-Dimensional Surface Vibrational Spectroscopy
Author: Mischa Bonn
From: Seminar

In the seminar, Bonn talked about structure characterization and dynamics study of interfacial water by using sum-frequency generation spectroscopy (SFG). In SFG, two laser beams are introduced to the material surface, with one in constant visible wavelength and the other one in tunable IR wavelength. The frequency of the output light is the sum frequency of the two input laser beam. As the IR laser beam scanning over a certain range of frequency, the vibrational spectrum of the surface is obtained. Also, since SFG is a nonlinear process, its output intensity is directly proportional to X2.

Bonn first studied the structure of the interfacial water. In fact, previous SFG study of the water molecules in the surface showed the existence of two broad bands in additional to the distinct free OH band, whose generation was attributed to the structure of ice-like water and liquid-like water. However, after repeating the experiment with HOD, these two bands became less obvious, which directly contradicted to the previous conclusion. Therefore, Bonn came up with a new suggestion that these two bands were the product of intramolecular coupling of HOH vibrations. In HOH, stretching vibration orbital coupled with the bending vibration orbital to form two new orbitals. But in HOD, since the O-D stretched more slowly than that of O-H, two stretching energy levels existed in HOD molecule, which could not couple with bending orbital any more. As a result, the bands became less distinctive. Therefore, no ice-like structure existed in interfacial water molecule.

After clarifying the structure, Bonn then studied the dynamic energy flow of the water molecule in the surface by utilizing two-dimensional SFG. In this case, D2O was investigated due to its relatively slower intermolecular energy transfer. A small portion of the D2O molecule in the surface was excited by an IR excitation laser beam, and then a pair of Vis and IR detection laser beams were pulsed to the surface and generated output beam. The SFG intensity was also affected by the delay time between the excitation pulse and the detection pulse. So the spectrum of 0fs delay was first evaluated. The slope of the spectrum, which was a measure of the heterogeneity of the surface, showed that the D2O surface was neither completely heterogeneous, with totally no interactions, nor completely homogeneous, like the bulk water. However, it also indicated that both strong hydrogen bonding and weak bonding existed in the surface D2O molecules. Moreover, the distinct cross-peaks revealed that vibrational energy transferred rapidly and efficiently between D2O molecules, whose correctness was further confirmed by the spectrum with various delay time.

Since water is a common solvent in chemical reactions, understanding its structure and dynamics at the interface would help us explore deeper into chemistry at the surface. Furthermore, using the same technique, we might be able to study the subtle structure and energy dynamics of cell membrane surface and thus pace new ways for cell targeting and drug delivery.

Author: Mischa Bonn
Seminar

Structure and Dynamics of Interfacial Water

The dynamics of resonant vibrational energy transfer and reorientation of H2O molecules in bulk water are well understood. The author sought to study these processes at the interface of water because these motions are relevant to surface chemistry. This ~one molecule thick interface was studied via the sum frequency generation caused by an IR laser pulse and a visible laser pulse targeted on the surface of the water. This sum frequency vibration gave spectra of the O-H stretch vibration only on the surface because the relevant IR and visible excitations give a resulting sum frequency relaxation that is forbidden in bulk water. The O-H stretch vibration was targeted because it is a marker of the water molecule’s local environment. These spectra showed three peaks: one high frequency vibration attributed to a ‘free’ O-H (one that is not hydrogen-bonded) and two lower peaks.

The author proposed a vibration coupling between the stretch and bend vibrations, splitting the vibrational transition into one symmetric and one antisymmetric state. This hypothesis was tested by isotopic substitution, where the bend-stretch coupling was eliminated by using HOD instead of H2O. Without the degeneracy caused by this coupling, no splitting was observed and the two lower peaks collapsed into one. This implied that the surface of water is relatively homogeneous.

To test how homogeneous the surface of water is, an IR pump pulse was added to the sum frequency vibration experiments. This pump pulse promotes some of the H2O molecules, decreasing the normal sum frequency generation sharply. The sum frequency generation then recovered over time after the pump. A two-dimensional spectrum showing the detection frequency vs. the excitation frequency was used to study the heterogeneity of the surface. A slope of unity indicated total heterogeneity, while a circular spectrum showing no correlation between excitation and response indicated total homogeneity. The experiment resulted in a slope of 0.23, indicating a small amount of homogeneity. Cross-peaks on the spectrum indicated a link between the ‘free’ O-H and the hydrogen bonded O-H, i.e., the excitation of one was felt by the other.

The slope of the 2-D spectrum was shown to decay to zero over the course of a few picoseconds, indicating that the heterogeneity was short-lived. This decay was explained by the transfer of vibrational energy from the excited water molecules to other molecules or other bonds within the same molecule. This decay was found to occur at half the rate as that in bulk water. This was explained by the fact that a bulk water molecule has an entire sphere of surrounding water molecules that can accept the energy, while surface water molecules only have a half-sphere.

Computer simulations were then used to study the reorientation of surface water. It was found that the in and out-of-plane diffusion rates were very similar because they are coupled, and that the free O-H rotates 3-4x faster than in bulk. The overall rotational motion is more diffusive, unlike the “jump-like” motion seen in bulk.

The author also studied the orientational distribution of free O-H and O-D groups in H2O, D2O and HOD. The difference between the angle at which the ‘free’ group projected out from the surface of the water was very similar in H2O and D2O, but these angles were very different between free O-H and free O-D in HOD. The stronger hydrogen bonding of O-D groups led to an excess of free O-H and a greater proportion of O-D that remained closer to the surface.

The author recognized a few problems with his methodology. First, the IR pump laser used to study surface homogeneity is not limited to only affecting surface molecules, and it is not known what effect the energy that this IR laser adds to the bulk water has upon the behavior of the surface water molecules.

The author could use this methodology to study the thin layer of liquid water present on solid ice, examining both the solid-liquid and liquid-vapor interfaces. It would also be interesting to use this IR pumped sum frequency vibrational spectroscopy to study the effect of surfactants or nanoparticles on the homo/heterogeneity of surface water.

Minute Paper #9 November 2, 2012
Title: Bioplasmonic Paper as a Platform for Detection of Kidney Cancer Biomarkers
Journal: Analytical Chemistry
Authors: Limei Tian, Jeremiah J. Morrissey, Ramesh Kattumenu, Naveen Gandra, Evan D. Kharasch and Srikanth Singamaneni

In regards to cancer-related deaths each year, renal cancer has been determined to be the sixth leading cause. Traditional assays to investigate biomarkers for this disease (such as enzyme-linked immunosorbent assay) are not able to be used routinely due to their cost and labor-intensive labeling process. The authors of this paper seek to use everyday laboratory filter paper as a backing for biofunctionalized plasmonic nanostructures. These nanostructures will provide quick detection for biomarkers.

The laboratory filter paper used in this experiment contained cellulose fibers of approximately 10 micrometers. High adsorption rates of the biomarkers onto the paper was observed and attributed to the rough surface yielding a large surface area. Capture biomarker was set as Goat Rabbit IgG while Goat anti-Rabbit IgG was set as the target biomarker and analyzation used localized surface plasmon resonance (LSPR) to examine these nanostructures. Results indicated that biomarkers were able to be detected successfully at a concentration of 10 ng/mL. A large dynamic range was observed and no labeling was necessary as is usually done in traditional approaches. The mechanical properties of the common filter paper itself yielded significant advantages such as being flexible and cheap as well as the ability to be paired with biochips and detectors for eventual multi-analyte analysis.

The experiments performed used only gold nanorods, as these were selected due to their high refractive index sensitivity and ease with LSPR pairing. However I am interested to determine whether this filter paper assay would still yield such desirable limits of detection if other nanorods were used which were not so specifically chosen for their known LSPR compatibility. If the goal of this research is to make the most efficient, accurate and cheapest biomarker detection system, then multiple nanorods should be examined instead of only selecting a specific one that is believed to produce the best results. Some measurements indicated a slight chance of molecule deformation during atomic force microscopy imaging; however this error was not explored any further. I would run additional experiments to determine if molecule deformation was in fact the cause of these obscure results, or if the abnormal results were acquired due to the fact these images were taken in a dry state. Further studies to explore efficient methods of pairing this filter paper LSPR biomarker analysis with microfluidic chip designs would be beneficial for the eventual studies of multi-analyte systems. All in all, this new technique provides a quick, efficient and cheap method for biomarker detection of renal cancer.

Seminar Title: Structure and Dynamics of Interfacial Water Studied Using Femtosecond 2-Dimensional Surface Vibrational Spectroscopy

Speaker: Mischa Bonn

Seminar Date: 11/01/12: Department of Chemistry Seminar Series

Water is the world’s most ubiquitous solvent. The unique hydrogen bonding nature of water lends itself to many of its unique and desirable properties. Little is understood, however, about the nature of water molecules and their dipole interactions at phase interfaces such as the water-air interface. The presenter discussed his group’s work on the analysis of the structure and dynamics of water molecules at the water-air interface using 1-D and polarized 2-D Sum Frequency Generation (SFG) vibrational spectroscopy.
SFD spectroscopy uses two light sources to excite a sample at the sum of the frequencies of the two sources. The advantage of SFD spectroscopy is that, because the intensity of the absorption is proportional to the second order susceptibility of the sample, which is zero for bulk water, it selectively excites water molecules on or near the liquid interface.
The 1-D SFD spectra of the water-air interface yields three identifiable O-H stretching bands: a band at high frequencies associated with free O-H bonds pointing out of the water interface and two bands at lower frequencies historically denoted to “ice-like” and “water-like” hydrogen bonding structures at the surface of water. The presenter argued that these two bands are a result of a coupling between the stretching and bending modes of water instead of attributing them to distinct H-bonding structures. This conclusion was verified by demonstrating that upon deuteration of one of the hydrogen atoms, which breaks the coupling between the vibration modes but will not change the structure of the interface, only one adsorption band was observed.
In bulk water, water reorients itself using a “jumping” mechanism where the orientation of water suddenly changes to another H-bonding orientation as opposed to random diffusion. The dynamics of free (pointing into the air) O-H groups at the water-air interface were analyzed using polarized 2-D SFG spectroscopy. In polarized 2-D SFG spectroscopy, the amount of decay time of an excited O-H vibration can be corresponded to the timeframe of either the transfer of energy associated with the O-H stretching vibration into the bulk liquid or the reorientation of the excited molecules to oppose the initial polarization. The measured decay time for the band associated with the free O-H groups was much faster than the decay time for hydrogen bonded O-H groups, demonstrating that the free O-H groups are capable of the faster diffusive orientation instead of the slower “jumping” mechanism associated with bulk water molecules.
The presenter clearly demonstrated the structural dynamics and energetic contribution of O-H groups at a water-air interface. It would be interesting, however, to see if the same conclusions apply for water interfaces with solids or nonpolar solvents. These interfaces are common for colloidal solutions and understanding their interface may prove helpful for improving common nanoparticle synthesis techniques which involve these systems.

Seminar: Structure and Dynamics of Interfacial Water

Speaker: Prof. Mischa Bonn

Bulk properties of water are well understood and known to academia. However, water acts very differently at the interface where air meets water, and one might ever say water is a “weird” molecule. It was this “weirdness” that drew Prof. Bonn to study the surface dynamics of water. His research focuses on using vibrational spectroscopies to study interfacial water. He accomplishes this by looking at the summation of frequencies given off when the surface of water is hit with an IR laser and a separate visible laser. This technique works at the surface of water, but is bulk forbidden. So when they look at the data given off, a lower IR frequency means that there is less movement in the stretching of an H-O bond. From this it can be inferred that there is a tight Hydrogen bond forming with that particular H causing it to stretch at a lower frequency. His initial findings show that there is not an ice-like structure at the surface as previously hypothesized.

By adding an IR pump beam to the above, more information can be obtained from the sample such as the heterogeneity of the sample and energy dynamics. He was able to find out that surface water relaxes from being excited about half as fast as bulk water. This can be attributed to the fact that there are less molecules around to transfer energy to at the surface. Another study they did was looking at how HOD acted at the surface. They were able to find that there is an induced orientation when a Deuterium is introduced in the molecule. The O-D bond points down and into the bulk, while the O-H bond points up out of the bulk and into the air.

For the next part of their research I would want to see how temperature effects all of the surface interactions. I would hypothesize that when cooler, but not quite as cold as freezing, the interfacial water will behave like bulk water. It would be interesting to find out at what specific temperature that would take place. Also, what happens to the dynamics when you work with warmer water? Did he encounter any problems with vaporizing his sample when he hit it with so many laser pulses? He never did mention what temperature they were working at. I should have asked!

Title: Structure and Dynamics of Interfacial Water Studied Using Femtosecond 2-Dimensional Surface Vibrational Spectroscopy
Author: Mischa Bonn
Seminar: Bryce L. Crawford Lectureship (11/1/12)

Professor Bonn claims that understanding water surface-air structure is useful to understanding how energy interacts with and disperses within the surface of water molecules relative to the bulk, and claims the surface dynamics are more simple than was previously thought.

He sets up a vibrational spectroscopy experiment where he uses IR plus visible light directed at the surface to excite the surface molecules. Ideally only a limited amount of surface molecules will take part in excitation, so bulk interactions can be excluded. He states that traditionally in bulk water, the normally detected 3 vibrational modes relate to one for the free hydrogen bond at higher energy, and two for the more closely bound hydrogen with one representing ice-type bond, and the other liquid-type bond. In contrast he proposes that the two smaller peaks are actually due to rotational vibration verses elongated vibration, and as a test uses deuterium because H-O-D will have no degeneracy, so no splitting of the modes.

In the results he shows in the case of surface interactions with Deuterium, the vibrational modes shifted such that their was still the free hydrogen bond at higher energy, but only one for the more closely bound deuterium (approximately centered between the previous two modes). He claims that this supports his theory of hydrogen bond vibration type variation at the surface.

He also talks about how it is traditionally theorized that energy disperses in bulk water, depending upon the distance between oxygen atoms (basically structure dependent). He says that this can better be represented in the bulk like a sphere and for the surface there is half of a sphere each molecule has influence with, so the rate at which energy disperses after excitation should be half as fast.

In the results he shows a time-decay comparison of bulk water IR excited signal strength vs. time, and also a surface water structure IR excited signal strength vs. time, and shows that there is a difference of around ½ or so picoseconds between decay time. He also shows that there is a difference between perpendicular and parallel light interaction with the surface molecules in terms of decay time.

He could repeat the first experiment with a heavier hydrogen such as 4H or another element. This may show a deeper orientation preference in the surface of the interface. It would be good if there could be a confirmation of this from another type of measurement. For example, if the molecules all have preferential surface structure, then maybe this could be observed by some sort of electrical measurement (capacitance maybe). It seemed like his results for time-decay at the surface from IR exited states could have been more complete. There was a trend in the data but it could be explained by other surface structure variations like a change in temperature at the surface.

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