« Paper to Read for 11/14/2012 |
| Lecture 19 Notes »
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 November 15, 2012 5:26 AM | Permalink
Title: Convenient Quantification of Accessible Surface-Attached ATRP Initiators and RAFT Chain Transfer Agents on Cross-Linked Polystyrene Nanoparticles
Authors: Mazurowski, G., et al.
Journal: ACS Macro Letters
Mazurowski et al. developed a novel technique to measure the amount of atom transfer radical polymerization (ATRP) initiators that are grafted onto the surface of a nanoparticle (NP). There is significant interest in growing stimuli-responsive polymers from the surface of both organic and inorganic NPs in order to create particles with so-called “smart surfaces”. However, to fully understand the polymerization mechanism of this system, the amount of surface-attached initiator must be determined. In this publication the authors address this issue by converting the ATRP initiator into a reversible addition-fragmentation chain-transfer (RAFT) agent, which is a chromophoric molecule that can be detected by UV/vis spectroscopy.
The authors first synthesized cross-linked polystyrene (PS) NPs and then grafted an ATRP-inimer to the particle surface. They tuned the graft-density of the initiator to study different systems of graft polymer precursors, i.e. at low initiator concentrations mushroom-like polymers are grown, while at high concentrations polymer brushes are obtained. Each sample was then reacted with equimolar amounts of bis(thiobenzoyl) disulfide (BTBD) to convert the ATRP-inimers to RAFT chain-transfer agents (CTA).
The RAFT-functionalized NPs were then dispersed into 2-ethylnaphthalene, which is isorefractive with the PS cores. The authors then measured the UV/vis absorption of each NP sample. By comparing the spectra of the NP samples to that of a solution of just the RAFT CTA in solution, the authors calculated the surface concentration of the initiators. The experimental results of the low and medium grafting-density samples were consistent with theoretical models, which assumed 100% of the initiators were located on the surface of the particle. However, the surface concentration of the high grafting density sample was significantly lower than the theoretical prediction. The authors hypothesized that this overestimation was due to the fact that some of the initiators were actually buried under the surface of the NP and were inaccessible to conversion.
Though the authors’ novel technique is a very clever use of RAFT CTAs, the conclusions from their experiments do not fully prove that their method is robust. Because the authors only studied systems of PS NPs, the authors should extend this technique to other organic and inorganic NPs to see if the ATRP-to-RAFT conversion works for these systems. Additionally, the authors’ hypothesis that some of the initiator is buried within the surface is unconvincing. The overestimation of the theoretical model could possibly be the result of diffusion limitations of the BTBD in high grafting density systems. For instance, as the conversion of the ATRP to RAFT increases in the high grafting density system, it may be harder for the BTBD to diffuse to the initiator due to steric hindrance caused by the CTA. To avoid these mass transfer limitations, the authors should run a reaction that cleaves the ATRP-initiator from the surface of the NPs. The free-floating ATRP initiator could then be converted to the RAFT CTA and the UV/vis absorbance could then be measured.
Ralm Ricarte |
November 15, 2012 4:04 PM
Minute Paper 10
Title: Self-cleaning porous TiO2-Ag core-shell nanocomposite material for surface-enhanced Raman scattering
By: Asefa et. al.
Journal: Chem Comm
Surface-enhanced Raman scattering (SERS) is an important non-invasive analytical technique for structure determination and single molecule detection. The technique involves molecules adsorbed on a rough metal surface. This increases signal by as much as 10 orders of magnitude, making single molecule detection possible. In this paper, the authors synthesize a porous TiO2-Ag material in a quick 2-step method.
They adsorbed 4-Mpy to it and compared it to other nanoparticles used in SERS. They found that their nanoparticle increased signal, and could be cleaned by shining UV light on it. This makes it an easy to use, recyclable nanomaterial for SERS. This is very helpful for researchers who use SERS. I would like to see how many times these nanoparticles can be “cleaned.” If it is not very many cycles that these survive, then they are not very useful. I would also like to see its limits of detection tested. Just how low of concentration can this nanoparticle help detect? Also, I would like to see how this can be used for detection of biological species. In the communication, the authors just chose an arbitrary molecule to attach and detect, but they don’t have any reason for choosing it. They just chose it because it worked. I would like to use this for a biological application, so I would like to see how well it detects biological molecules.
Alex Johnson |
November 15, 2012 8:17 PM
Minute Paper 10
Confocal Raman Microscopy Probing of Temperature-Controlled Release from Individual, Optically-Trapped Phospholipid Vesicles
Authors: Jonathan J. Schaefer, Chaoxiong Ma, and Joel M. Harris
Journal: Analytical Chemistry
In recent years phospholipid vesicles have found several uses in the scientific world that involve incapsulating different molecules and then monitoring their release and reactions or as sensing structures. One big use for them is in the medical field for drug therapy. These vessels allow the cytoxic drug to travel through the body without interacting with living tissues until it reaches its destination. Initially the drugs were released into the system and would eventually diffuse out, but it was found that as you increase temperature to near the melting transition of the vesicle, your membrane becomes more permeable. In order to create a specific membrane that was sensitive to temperature; single acyl-chain lysolipids were added to the membrane resulting in more of the drug being released at a faster rate. Currently to study this process the vesicles were looked at in bulk giving only a fraction of information about the structure and release mechanism. Using optical trapping confocal Raman microscopy these authors were able to look at vesicles on a single cell level determining the phase in which the lipid bilayer is in and the amount of 3-NBS released compared to temperature. In order to compare their results they looked at the release for vesicles with and without the single acylchain lysolipid.
Their experiment consisted of making each type of vesicle and then optically trapping one for up to an hour as they increased the temperature and looked at the change in Raman peaks. The change in Raman peaks helped indicate when the drug was passing into the membrane and also the phase the membrane was in. The results showed that the pure membranes without the lysolipids in them had to start going through the phase change and then slowly released the 3-NBS. The ones with the lysolipids went 4X faster when they started to release around the phase change, but also stayed contained within an aqueous solution suggesting that the mixed membrane creates water filled pores in which the molecules can diffuse through.
I think the next step in this, is to try switching up the “drug” they encapsulated. They used 3-NBS because it had a sulfonate group that is negatively charged in most conditions to ensure it wouldn’t escape before the transition temperature. However, I think just looking at this defeats the purpose of seeing how the vesicles actually work with real drugs. First off not all drugs are negatively charged, so they could escape at a lower temperature and how they do it could be a completely different mechanism since you are no longer seeing the phase change and then release. Another way they could look at single vesicle release is fluorescently tagging the membranes one color and tagging the drug another color then using TIRFM to watch the drug flow out.
Sarah Gruba |
November 15, 2012 8:17 PM
Title: Site-Specific Coupling of Hydration Water and Protein Flexibility Studied in Solution with Ultrafast 2D-IR Spectroscopy
Author: John T. King, et al.
Journal: J. A. C. S.
In this paper, the authors studied the ultrafast hydration and protein dynamics of hen egg white lysozyme (HEWL) in D2O/glycerol solutions. A metal carbonyl probe was labeled to the surface of HEWL and protein-water interface dynamics was achieved by 2D-IR spectroscopy detection of the vibrational probes’ frequency. Compared to the linear FTIR, 2D-IR utilizes two pump pulses to excite the sample. The delay time between these two pulses is scanned to study dynamics of the system. Then a third pulse acts as the detection beam and generates emitted light.
As to the experiment, bulk water dynamics was first accessed with two small metal carbonyl molecules, CORM-2 [RuCl2(CO3)]2 and CORM ([CO]Fe[N5C22H21]). Extracted from 2D-IR spectrum, Frequency-frequency correlation function (FFCF) was plotted versus waiting time to measure the spectra diffusion. As can be seen in figure 2, the two molecules showed almost the same FFCF decaying time constant, which also corresponded to the bulk D2O hydrogen-bonding dynamics. Thus the introduction of small hydrophobic probe would not significantly affect the dynamics of the bulk water. When ruthenium dicarbonyl labeled HEWL (HEWL-RC) was added to D2O, the decay time of FFCF was slightly longer than bulk-like water, with a slowdown factor of approximately 1.8. This factor also agreed with the result (a factor of 2) from molecular dynamic simulations. However, different from the small molecule probes, the FFCF of HEWL-CR first showed a rapid initial decay, which was caused by hydration dynamics and then a static offset, which was attributed to the protein dynamics, since the time scales for protein dynamics was longer than the experimental detection window.
To further study the hydration dynamics of the protein-water interface, glycerol was added to the solution gradually. Diffusion spectra were measured. It was calculated that a 100-fold in solution viscosity (0-80% glycerol) only resulted in a 3-fold slowdown in hydration dynamics, which indicated that the added glycerol didn’t destroy the hydration of protein surface. The result also demonstrated a weak interaction between the hydration water and the bulk water and a relatively stronger coupling between the hydration water and the protein.
2D-IR spectroscopy is indeed a very powerful technique for dynamics study. However, one limitation of this work is that it is not suitable for vivo detection. Site-specific labeling is also hard to obtain. In this paper, the probe is covalently bound to the histine 15 residue. But how can we ensure only one probe is labeled if multiple histine residues exist in the protein. If more than one probe is bound to the protein, how would it affect the results? Also, from the crystal structure of HEWL-RC, it can be seen that the probe is buried inside the protein, which brings out another question: does the location of the probe affect the results? A good control is to label the probe on the outer surface of the protein. Considering the possibility that no histine exists on the outer surface, different probes can be chosen according to different existed residues.
Yi Zhang |
November 16, 2012 12:54 AM
Minute Paper #9: Joseph DeWilde
Title: High-Pressure XPS of Crotyl Alcohol Selective Oxidation over Metallic and Oxidized Pd(111)
Authors: Lee, A. et al.
Journal: ACS Catalysis
Catalysts capable of the partial and selective oxidation of common alcohols and ethers into aldehydes, acids, and ketones are of extreme interest to a variety of chemical industries. In particular, the partial oxidation of crotyl alcohol (CrOH) into crotonaldehyde over noble metal catalysts such as palladium is of particular interest for the production agrochemicals and food preservatives. Previous research has suggested that surface metal oxides play an important role in partial oxidation over palladium1. In this work, the authors use high pressure X-ray photoelectron spectroscopy (HP-XPS) to elucidate the surface species present during CrOH oxidation Pd catalysts and gain a better understanding of the mechanism of this chemistry.
To investigate the role of surface metal oxides in CrOH oxidation, the authors prepared catalyst samples consisting of clean Pd, Pd with a Pd5O4 surface layer, and Pd with a PdO surface layer and determined the identity and relative abundance of the surface species present during CrOH oxidation on these samples using HP-XPS. The higher pressures used in HP-XPS than more traditional X-ray photoelectron spectroscopic techniques allows for measurement of the catalyst surface under conditions relevant for catalytic reactions.
Peaks associated with adsorbed CrOH species (285 and 286.7 eV) were observed in the C 1s HP-XPS spectra for the clean Pd catalyst sample doused with CrOH at 40 °C. Upon heating, these peaks disappeared and a band associated with adsorbed alkylidyne fragments (285.5 eV) formed, indicating that the clean Pd catalyst is unable to dehydrogenate CrOH into crotonaldehyde and instead merely decomposes the alcohol into propene and CO. Conversely, a peak associated with the formation of adsorbed crotonaldehyde species (288.2 eV) was observed in the C 1s HP-XPS spectra of both the Pd5O4 and PdO catalyst samples upon exposure to CrOH. Upon heating in an inert atmosphere, the intensities of both the crotonaldehyde and of the surface O 1s peaks dropped and the alkylidyne peak formed, causing the authors to propose surface oxygen atoms are consumed in the formation crotonaldehyde. This hypothesis is supported by the observation that the disappearance of these peaks is retarded when the samples are heated in an oxygen-containing atmosphere. Based on these observations, the authors concluded that CrOH partial oxidation occurs through a Mars−van Krevelen, in which surface oxygen is consumed during reaction and must be replaced with an oxygen co-feed to restore the oxide on the catalyst surface.
The authors presented compelling evidence for a Mars−van Krevelen mechanism for the partial oxidation of CrOH into crotonaldehyde. Additional kinetic information could be gained, however, by attaching a mass spectrometer to the sample chamber and analyzing the composition of the gas above the catalyst surface as a function of reaction time. Correlations between the HP-XPS peak intensity for the surface species and the product gas phase concentrations as a function of reaction time would help elucidate not only the mechanism but also the kinetics of this important reaction.
 Parlett, C. M. A.; Bruce, D. W.; Hondow, N. S.; Lee, A. F.; Wilson, K. ACS Catal. 2011, 1 (6), 636−640.
Joseph DeWilde |
November 16, 2012 2:23 PM
Title: Oxidation kinetics of nanoscale copper films studied by terahertz
Author: Gopika K. P. Ramanandan
Journal: JOURNAL OF APPLIED PHYSICS
The study of copper thin films is important because they are uses in a vast amount of electronic applications however it easily oxides and does not form a barrier to further oxidation like aluminum. Surface oxidized copper presents many problems for these applications. In this study Ramanandan et al. focuses on the surface oxidation kinetics in copper through the use of non-contact, in situ transmission measurements of broadband terahertz pulses, called Terahertz Transmission Spectroscopy (THS).
Thin films of copper were evaporated via electron-beam method onto high-resistivity silicon substrates. Heating of the films was done by sandwiching the substrate between two copper plates with thermal compound in between. 3mm diameter holes were drilled into the copper plates to allow transmission of the THz beam through the sample. For the THS measurements a 50fs pulsed laser of 2.5W at 5.2MHz is passed through a pump beam of GaP crystal to generate the THz pulse by optical rectification. Mirrors are used to focus the beam onto the sample and mirrors on the opposite side of the sample focus the transmitted beam onto another GaP crystal which acts as the detector.
In the results Ramanandan et al. shows a FTIR plot of the time-referenced transmission signal through 21nm thick copper films heated at temperatures 120 to 150oC. From the graph it is obvious that the unheated film has very low transmission, which increases with (heated) oxidation time until it saturates. A higher temperature gives a much higher rate of oxidation increase.
SEM topography images are taken which show a change in grain size (I’m confused as to what films these are associated with), which the author theorizes is due to the percolation threshold of the electrons in the film. He claims that this directly limits the amount of diffusion of copper atoms to the surface to continue oxidation (and thus saturation of oxide thickness is reached). He proposes a modified Arrhenius-based diffusion equation for these films and gives activation energy of 0.55eV. Electrical conductivity measurements are used to verify remaining metallic copper layer thickness for all samples.
The author did not explain the conditions for which the sample was kept after copper film deposition before characterization began. If this period was relatively short than it shouldn’t affect the results, but considering the study is about the oxidation of copper (which usually occurs in normal atmospheric conditions) this should have been stated.
Compositional/structural characterization was not performed of the copper/substrate interface before & after the oxidation process. This is a mistake because it is well known that copper diffuses aggressively into silicon and other materials to a lesser degree (especially at elevated temperatures) and this would influence oxidation rate of the surface as well as their electrical measurements. He even sites diffusion towards the surface for his theory without examining diffusion into the bulk.
The SEM measurements were a helpful but AFM would have been even better, potentially also giving work function variation over the surface –pointing to grain conductivity associated with the associated oxidation temperature process (grains promote Cu diffusion more aggressively than volume diffusion).
Forrest Johnson |
November 16, 2012 3:09 PM
Title: Highly Efficient Binding of Paramagnetic Beads Bioconjugated with 100,000 or More Antibodies to Protein-Coated Surfaces
By: Mani et al.
Journal: Analytical Chemistry
Sensitive, multifunctional protein measurement techniques are critical for biomedical research, as they may serve as a means for detection of diseases and development of new therapeutics. In this work, antigen-antibody binding kinetics were studied via surface functionalized superparamagnetic particles (MP) and gold films. Antibody-antigen binding kinetics were analyzed by surface plasmon resonance (SPR).
Surfaces of MP Dynabeads (1 μm) were decorated with over a 100,000 antibodies via surface functionalization methods. Gold films were surface modified with self-assembled monolayers (SAMs), which incorporated two forms of cancer antigen biomarkers. These two surface modified materials served as an analytical tool for evaluating antigen-antibody binding kinetics, as the antibody decorated MP, upon interaction with the conjugate antigen functionalized gold surface, initiated a
significant SPR signal.
SPR data suggested antibody MP-antigen gold surface interactions exhibited irreversible binding with two orders of magnitude increase in association rate over free antibodies. This may be explained by the number of possible antibody-antigen interactions, as the MPs have a host of linkage sites; by the time an antibody-antigen interaction disassociates, more linkages are formed. Using antibody surface functionalized MPs, limits of detection were in the fM to aM ranges by SPR, which is a significant increase in detection limits (nM) over free antibodies. The superparamagnetic properties of the MPs may be attributed to the significant increase in SPR signal enhancement, as MPs were found to aggregate on the gold surface, which thereby offered more opportunity for antibody-antigen interactions.
Although the authors demonstrated significant increases in signal detection, I failed to understand the purpose of using MPs surface modified by 100,000 different antibodies. Furthermore, the authors only used only two different antigens associated with cancer cells. More work concerning this approach should include analysis of antibody-MP and antigen-gold selectivity, as this would provide more meaningful information towards understanding specific interactions. Careful choice of antibodies and antigens as surface modification schemes could provide specific association constants for conjugate pairs. Also, it was suggested the superparamagnetic characteristic of MP was attributed to high association constants. I feel this unique trait could be enough to elucidate more specific information concerning antibody-antigen interactions.
Lastly, the MP particle size (1 μm) was very large for a probe ultimately designed for in vivo use. Smaller MP diameters, possibly somewhere in the nm range, would provide a more suitable platform for in vivo detection.
Sam Egger |
November 16, 2012 4:00 PM
Chemical Gradients within Brain Extracellular Space Measured using Low Flow Push-Pull Perfusion Sampling in Vivo
Authors: Thomas Slaney, Omar Mabrouk, Kirsten Porter-Stranksy, Brandon Aragona, Robert Kennedy
ACS Chemical Neuroscience
Measuring neurotransmitters and other molecules known to aide in neurological functions is a top priority in the research objectives for both neuroscience and pharmaceutical development. In vivo experiments with these objectives employ techniques of either microdialysis or microelectrodes however the authors in this paper used low-flow push-pull perfusion in order to record measurements. Tradition push-pull perfusion operates by diffusing a physiological buffer while withdrawing a sample through a small capillary via capillary action. Low-flow push-pull perfusion uses smaller capillaries and low flow rates, which enhance the resolution. Basal concentrations were examined to determine if chemical gradients existed at these levels as compared to the gradients known to exist at the micrometer level.
Large changes, as little as 200 micrometers apart were measured in the midbrain regions for dopamine and glutamate. However dopamine and serotonin did not show any measurable differences in the core and shell regions of the brain. Chemical gradients were determined to be dependent on biological mechanisms since they were observed in certain areas/with specific molecules more than others. Cortical neurons appeared to have a higher release with a decreased lifetime while obvious chemical gradients for glutamate point to this neurotransmitter having “hot spots” – something of keen interest for any pharmaceutical targeting.
Although the concentrations of neurotransmitters measured from perfusion are believed to correctly approximate the true extracellular concentration, there are a lot of inherent problems with this method. First, the sampling technique appears to directly interfere with the data the authors are trying to infer. By actually removing fluid from the extracellular location, they are depleting the region of whatever concentration of neurotransmitter they are measuring. By removing this fluid, even if in a small amount, they are disturbing the concentration of the fluid and therefore their future measurements, if taken consecutively, may indicate a chemical gradient that is in fact due to their sampling protocols, not a function of biological mechanisms. Secondly, the authors reported severe tissue disruption along the track of the probe, however since no damage was done near the sampling region, these results were still considered valid. Even though the fluid directly surrounding the probe tip and healthy tissue may still be accurate, one could argue that damaging the tissue up by the probe track could affect the surrounding fluid. Damage to the surrounding fluid might not directly affect the measured concentration, but it could influence the determination of chemical gradients. Finally, glutamate concentration had a dependence on probe depth, however only one probe was inserted at a time. Multiple probe insertion at varying depths would allow for simultaneous measurements of glutamate concentration, in order to determine if differing concentrations exist at the same time with depth dependence. I would also suggest running experiments with a microdialysis probe, as this sampling protocol does not remove any extracellular fluid, and therefore these results might be more informative in chemical gradient determination.
Megan Weisenberger |
November 16, 2012 4:20 PM
Minute Paper #10 (11/16/12) – Matt Irwin
Title: Imaging the Molecular Structure of Polyethylene Blends with Broadband Coherent Raman Microscopy
Authors: Lee, Y. et al.
Journal: ACS Macro Letters
Polyethylene is one of the biggest consumer products in the world, with over 180 billion pounds produced per year for applications ranging from plastic bags for groceries to proton exchange membranes for fuel cells. For many applications, blends of hard, tough high density polyethylene (HDPE) and soft, stretchable linear low density polyethylene (LLDPE) are made to create affordable products that have long lifetimes, high durability, and exceptional durability. Although much work has been done to relate how the macroscopic properties of the blends affect their mechanical performance, determining the microstructure of such blends has proven difficult due to the low inherent electronic, nuclear, and optical contrast of the chemically similar blend components. Understanding how the microstructure is affected by relative crystallinity and phase separation is vital for intelligent materials design. In this paper, the authors use broadband coherent anti-Stokes Raman scattering (CARS) to assess the chemical composition and molecular orientation of a blend of low molecular weight, deuterated HDPE (D-HDPE) and high molecular weight, hydrogenated LLDPE (H-LLDPE). Additional techniques used include atomic force microcopy and differential scanning calorimetry (DSC).
One of the authors’ goals for this publication was to demonstrate the ability of CARS to simultaneously image the composition and orientation of a system by measuring well-separated peaks for C-H (or C-D) bending and twisting and C-C stretching. In their analysis, the authors color code and combine Raman intensity spectra for the H-LLDPE and the D-HDPE to analyze the relative intensity from each component of the blend. First, the authors used linear polarized excitation compared the relative effect of polarization on each of the two polymers. The authors found that when the polarization was rotated 90°, a bow-tie pattern was also rotated by 90° but that there were no radial variations in the concentric undulations. This phenomenon has previously been observed for crystalline polyethylene and is best described via a collective twisting lamellae model where the C-H and C-D bonds go out of the polarized plane periodically. Their compound images are able to provide new insight into the mechanism by which the crystalline domains form: the high molecular weight H-LLDPE acts as spherulite nucleation sites, and as the spherulites grow, they exclude the low molecular weight both at the spherulite boundaries and at radial lines which constitute amporphous inter-stack regions. Finally, the authors compare the undulation amplitude of both polymers to estimate crystallinities of 43% and 58% for H-LLDPE and D-HDPE, which are in good agreement with DSC results.
The authors provide an excellent method for characterizing blends with inherently low chemical contrast, but the analysis performed seems incomplete. The authors’ use of deuterated HDPE was necessary to distinguish the Raman scattering peaks from LLDPE, but previous work has demonstrated that the segmental interaction parameter χ between two hydrogenated polymers differs from the parameter between a hydrogenated and a deuterated polymer. Given the relatively high (~100 kDa) molecular weight studied here, and hence relatively large segregation strength χN, the phase behavior observed may differ from that of the fully hydrogenated blends which would be used for real applications. One first step to determining what, if any, effect the deuteration has on morphology would be to deuterate the LLDPE and hydrogenate the HDPE. If the phase behavior is the same as is reported in this paper, then perhaps blends with deuterated HDPE and hydrogenated LLDPE exhibit the same morphology as blends in which both components are hydrogenated.
Matt Irwin |
November 16, 2012 4:44 PM
Title: Characterizing the Photoinduced Switching Process of a Nitrospiropyran Self-Assembled Monolayer Using In Situ Sum Frequency Generation Spectroscopy
Authors: Tamim A. Darwish, Yujin Tong, Michael James, Tracey L. Hanley, Qiling Peng, and Shen Ye
This article employed sum frequency generation (SFG) vibrational spectroscopy to investigate the photoinduced switching process of a nitrospiropyran self-assembled monolayer (SAM). Sum frequence generation vibrational spectroscopy is a second order nonlinear optical technique. An infrared and a visible laser are overlapped spatially and temporally at the interface. The frequency of the emission light is the sum of to incident lights. SFG is very suitable to study various kinds of surfaces and interfaces.
The photochromic couple spiropyran (SP) and merocyanine (MC) undergo a reversible isomerization between each other under the condition of ultraviolet light or visible light excitation. The SP-forms are threedimensional, inert and colorless while the MC-forms are planar, colored and with a large dipole moment. While most of the studies on this photochromic pair have been performed in solution, this article studied on their photoswitching behaviors in self-assembled monolayer (SAM) at a molecular level.
To prepare SAM, they used 1,2-dithiolane [S(CH2)3S] as "alligator clips" to attach 2-(3′,3′-dimethyl-6-nitro-3′H-spiro[chromene-2,2′-indol]-1′-yl)ethyl (1,2-dithiolane-3)-pentanoate (SP-LA) onto a thin gold film with a thickness of ~200nm. UV and visible light exposure were applied to the immoblized SP-LA SAM. To carry out SFG, p-polarized visible light with a wavelength of 800nm and infrared beams were used and their wavelengths were out of the range required to trigger the SP/MC isomerization, thus no significant effects being found by the laser surface irradation.
First a SFG spectrum of SP-LA monolayer and an FTIR transmission spectrum of bulk SP-LA were obtained and compared. Appearance of the peaks in SFG spectrum showed that SP-LA formed a well-ordered monolayer at the surface. The conformation of SP-LA at interface were represented by the symmetric and antisymmetric stretching modes of NO2 group. It was appropriate because NO2 modes showed the most intense peak in the SFG spectrum due to its strong dipole moment.
The characterization of initial state of the closed form of SP-LA indicated that the initially assembled nitrophenylpyran rings had a direction of the antisymmetric vibrational dipole moment of NO2 almost parallel to the substrate. After applying UV irradiation to the closed-form SP-LA, a decrease in the amplitude of the symmetric vibrational mode and the increase of the antisymmetric vibrational mode of NO2 were observed, which implied an increase in the tilt angle θ and an increase in the twist angle ψ. However, when visible light was applied to the open form to achieve a reverse reaction, observations suggested that this process is dominated by a decrease in the twist angle ψ while the tilt angle θ remains fairly constant.
An SFG simulation of the three states of SP-LA on the gold surface was carried out which depicted the possible conformation of SP-LA during photoirradiation and the orientation of the nitrophenyl group and gave out the intensity of both symmetric and antisymmetric vibrational mode of NO2 group. Furthermore, the reaction kinetics of the UV and visible irradiation process of the SP-LA SAM were studied.
As this technique could be used to study the orientation of the functional groups on surface, I'm thinking of applying it into sensor development. For example, if the target can change the orientation of the functional groups on the surface or change the functional groups, it could be sensitively detected by SFG. That might be a sensitive potential sensor.
Tian Qiu |
November 16, 2012 4:45 PM
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