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Minute Papers Due 11/30/2012

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.


Marzieh Ramezani, Minute paper #10

Title: Quantitative Imaging of Gold and Silver Nanoparticles in Single Eukaryotic Cells by Laser Ablation ICP-MS

By: Norbert Jakubowski

Journal: analytical chemistry

In this paper, the authors have investigated the interaction of silver and gold nanoparticles with animal cells which are found to be important in many aspects like therapeutic applications and various analytical methods. Previously, different optical approaches such as surface enhanced Raman scattering, photothermal microscopy and X-ray microscopy have been used to study these interactions. However, these methods are time consuming and need complicated sample preparation. Also, they are not able to perform quantitative analysis. In this approach, to measure molecular and elemental distributions of nanoparticles, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been used which has high sensitivity, high spatial resolution and is easy in calibration for elemental quantification.

For this purpose, Swiss albino mouse fibroblast cells were incubated with Au and Ag nanoparticles and grown as a monolayer, fixed, and dried under standard conditions for ICP-MS analysis. To get a high resolution image with less background, they tried to get a repetition rate higher than scan speed and also high laser energy for complete ablation of the material. After optimizing the laser energy, repetition rate, scan speed and spot diameter they analyzed fibroblast cells individually under different conditions with respect to the incubation time and concentration. The results have shown that there are differences in both intensity and the distribution of nanoparticles after longer exposure or higher concentrations. Then, by observing the LA-ICP-MS images and the intensity plots as a function of Au and Ag distribution, they found out the aggregation of Au and Ag nanoparticles within a substrate which can be useful to study the nanoparticles uptake.

As I mentioned earlier, quantification of silver and gold nanoparticles also has been examined by using nitrocellulose membranes fixed with nanoparticles suspension. The membrane then were ablated and analyzed by ICP-MS and the results revealed that the nanoparticle's uptake was strongly dependent on the particle concentration and the incubation time. Considering all aspects, this method can be generalized for nanotoxicity investigations and other quantitative measurements.

Title: Composition and Properties of Complexes between Spherical and Polycationic Brushes and Anionic Liposomes
Authors: Sybachin, A.V., et al.
Journal: Langmuir

Sybachin et al. studied the interactions between spherical polycationic brushes (SPB) and anionic liposomes. There is significant interest in using liposomes to deliver bioactive compounds due to their ability to carry both hydrophilic and hydrophobic molecules. However, efforts to commercialize this strategy have failed due to the thermodynamic instability of liposomes. Recently, methods have been developed to stabilize liposomes by adsorbing them on the surface of a solid carrier. In this publication, the authors studied the adsorption of a fluorescent liposome onto a SPB by employing UV/vis spectroscopy.

The SPBs were synthesized by polymerizing cationic (2-methylpropenoyloxyethyl)trimethylammonium chloride on the surface of polystyerene beads. The liposomes were partly composed of the anionic cardiolipin (CL2-) and N-fluorescein-isothiocyanyldipalmitoyl-phosphatidylethanolamine, a fluorophore. Liposomes with varying mole fractions of CL2- were used in the study. The liposomes were mixed with the SPBs. Because the adsorbed liposomes don’t fluoresce, the concentration of adsorbed liposomes was determined by measuring the fluorescence of the liposome-SPB solution and calculating the concentration of unbound liposomes.

The authors determined that as the mole fraction of CL2- increased in the liposome, the maximum adsorbed liposome concentration decreased in a nonlinear fashion. The authors postulated that this nonlinear relationship was due to the localization of the CL2- in the adsorbed lipsome. At low CL2- mole fractions, most of the CL2- is in direct contact with the polycationic chain. At higher CL2- mole fractions, a significant portion of CL2- is localized on the opposite end of the liposome, away from the polymer chain. This excess CL2- then repels other liposomes from adsorbing onto the SPB surface.

To probe the stability of the liposome-SPB complexes, the authors studied the behavior of the complexes in NaCl solutions. As the mole fraction of CL2- increased, the amount of NaCl needed to cause the liposome to de-adsorb from the SPB increased. At very high CL2- mole fractions, the liposome was irreversibly bound to the SPB. The authors concluded that this phenomenon was due to the fact that at high CL2- mole fractions the polycationic chains became embedded within the liposome membrane.

Because embedded polycationic chains could possibly compromise the integrity of the liposome membrane, the authors performed release measurements with liposome-SPB complexes in which the liposome was loaded with a NaCl solution. For the high CL2- mole fraction liposome, the NaCl release behavior was similar to that of a liposome that was completely destroyed by a surfactant. Because large defects weren’t visible through transmission electron microscopy, the authors hypothesized that the membranes had several “point” defects that led to the increase in NaCl release.

With this publication, the authors did a good job of laying the groundwork for future studies. For instance, if the authors claim that at low CL2- mole fraction the CL2- of the liposome is in contact with the polycationic chain is correct, there may be poly-layer adsorption of the liposome. The authors could easily study this by measuring the adsorbed liposome concentration and comparing it to poly-layer adsorption models. Additionally, the authors could calculate the number of “point” defects by comparing the salt flux through the irreversibly adsorbed liposome to that of the reversibly adsorbed liposome.

Title: Sensitive Cylindrical SERS substrate Array for Rapid Microanalysis of Nucleobases
Author: Panneerselvam Rajapandiyan and Jyisy Yang
From: Analytical Chemistry

In this paper, the authors presented a new cylindrical SERS substrate to detect nucleobases. The substrate was made of PMMA fiber and coated with PVDF on the tip aiming to increase the roughness of the surface. AgNPs were then deposited on the PVDF coating layer through silver mirror reaction. Due to the hydrophobicity of PVDF layer, aqueous sample would form a small droplet on the substrate, which eliminated the uncertainties in spreading of aqueous samples found on conventional flat SERS substrate. As a result, both reproducibility and sensitivity were increased and the enhancement factor reached 7 orders in magnitude for nucleobases.

As to the experiment, cylindrical SERS substrates were first prepared. Several parameters were optimized, including diameter of the substrate, concentration of coating PVDF solution, concentration of AgNO3 and glucose solution for silver mirror reaction and the reaction time. The diameter of the PMMA fiber was chosen to be 2 mm according to the results of the performance experiment. For PVDF solution, due to the hydrophobicity, AgNPS couldn’t easily deposit on the surface. To address this problem, DMF which had been used as a reducing agent for AgNP formation was introduced. It was found that increased concentration of DMF resulted in increased SERS intensity of pATP, indicating that DMF helped to reduce or absorb AgNPs onto the substrate. Limited by the solubility of PVDF in DMF, the optimal concentration for PVDF solution was 15%. In terms of AgNO3 concentration, experimental results showed that SERS intensity for pATP went up as the concentration of AgNO3 increased. However, when it reached a certain point, the signal intensity began to go down, probably owing to the formation of large particles that had smaller SERS intensities. In the end, the optimal concentration was 20 mM. The concentration of glucose solution and silver mirror reaction time were both optimized in the same way and set to be 0.5 M and 20 min respectively. SEM analysis was also carried out to characterize the surface of the substrate, which confirmed that PVDF coating increased the roughness of PMMA surface dramatically.

After the preparation of the SERS substrate, four nucleobases solution samples with a volume of 4uL were tested without drying. The analyte included adenine, cytosine, thymine and uracil. All characteristic bands were observed. For adenine in particular, the results of linearity analysis show a strong linear relationship between SERS intensities and the concentration lower than 100 uM. The detection limit was determined to be 215 nM. And the enhancement factor was calculated to be 1.21 x 107.

The new substrate presented in this paper shows great properties for SERS detection. However, as a complete experiment for nucleobases analysis, guanine should also be examined. Also for the linearity analysis, it is unclear whether this linear range fit the requirement for nucleobases detection, which might differ for various samples. Further study includes how to change the detection window to meet different requirement can also be conducted. This may be achieved by enlarge the diameter to allow more AgNPs deposit.

Title: Modifying the Atomic and Electronic Structures of Gold Nanocrystals via Changing the Chain Length of n-Alkanethiol Ligands

Author: Jiang, Y.; Yin, P.; Li, Y.; Sun, Z.; Liu, Q.; Yao, T.; Cheng, H.; Hu, F.; Xie, Z.; He, B.; Pan, G.; Wei, S.

Journal: J. Phys. Chem. C

Gold nanoparticles are often coated with alkanethiols to modify their surface chemistry properties; however, it is not well understood why the alkyl chain lengths influence the Au-S interface structure and electronic properties. The authors prepared Au nanocrystals capped by n-alkanethiols of varying length to study the Au-S interface structures and density states. It was found that longer alkane chains led to stronger Au-S interactions and the largest charge transfer from the Au surface to the S atoms.

The authors used X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) as sensitive local-structure probes of electronic and atomic structure of the Au nanocrystals. The XANES experiment showed that the electronic properties of Au atoms near Au-S interfaces are modified by variations in the alkanethiol chain length. The EXAFS study showed that the head S atom in long n-alkanethiols coordinated more strongly with surface Au atoms than the S atom in shorter chain n-alkanethiols.

To confirm that these results were not caused by variations in the size of the Au nanocrystals, the authors performed UV-Vis measurements and collected transmission electron microscopy (TEM) images. The TEM study showed that nearly all particles had a mean diameter of ~3nm and were spherical in shape. The UV-Vis measurements showed similar absorption bands at 507nm from the plasmon resonance absorbance of spherical Au nanocrystals.

Curve fitting of the EXASF FT peaks revealed that the surface Au-Au bond lengths contracted due to the near-surface stress of thiol adsorption. The Au-S bond length was correlated with the Au-S coordination numbers. Three different coordination numbers were calculated, corresponding with different Au-S bonding sites: on-top, bridge and hollow site, with on-top having the lowest coordination number and longest bond length, and hollow having the highest coordination number and lowest bond length. It was proposed that long-chain alkanethiols assembled in order, standing-up conformations, while short-chain alkanethiols formed disorder, lying-down states. Longer alkanethiols introduced stronger steric effects, leading the headgroup S atom to adopt the more energetically stable 'hollow site' bonding mode.

A peak from the XANES study provided probe into the unoccupied densities of d state. The magnitude of this peak increased with longer alkyl chains, indicating an increase in the d-hole population (d charge depletion in the Au nanocrystal), suggesting charge transfer from the Au nanocrystal to the S atom. This charge transfer was more significant for higher-coordination sites. In summary, longer alkyl chains forced the headgroup S atoms to adopt higher-coordination bond modes, which facilitated more charge transfer from the Au particle to the S atom.

Title: Carbon Dioxide Capture in Metal-Organic Frameworks
Author: Professor Jeffrey Long
UMN CHEMS seminar

The increasing level of CO2 in the atmosphere has been identified as a problem for some time and a number of possible solutions have been presented over the years for its reduction. Professor Long identifies coal power plants as the primary source of reduction for achieving the targeted levels. In this talk he gives three main approaches to solving the problem, of which he focuses on the first. These are Post-combustion CO2 capture, pre-combustion CO2 capture, and oxy-fuel combustion.

Normally burning coal in traditional power plants produces ~77% N2 and 15% CO2 with minor other gasses. His group focuses on using metal-organic framework (MOF) materials to absorb the CO2 in the first part of the cycle under high pressure, and then release it under N2 at low pressure in the last part of the cycle. They try a number of metal-organic frameworks and results are shown for Zn4O(1,4-benzenedicarboxylate)3 MOF-5 metal-organic framework material with large surface areas (6200 m2/g), low density at .22 g/cm3, tunable pore sizes up to 5nm, and internal functionalized surface.

All experiments were performed using gas pressure measurements to determine amount of CO2 remaining in the test chamber over time. Results for Zn4O(1,4-benzenedicarboxylate)3 show this material can store about 9 times more CO2 for a given storage pressure compared to normal cylinder. This compares against aqueous amine-based CO2 capture, which gives ~30% energy penalty for desorption at 120oC.

To get a higher selectivity, Long et al. try a framework with exposed Mn2+ coordination sites, with BET surface area of 2100 m2/g. He claims that selectivity can be enhanced by increasing charge density at the metal ion site. CO2 capture results show plot of gas absorption vs. pressure in chamber, with N2 slowly increasing but CO2 increasing logarithmically, giving a selectivity around 5.5 at most pressures.

Repeating the experiments with a new type of Mg2+ coordination site material (much more stable with H2O) showed selectivity of 175. With an expanded form of Mg2(dobdc)(Mg-MOF-74), a selectivity of 200 is achieved. The final energy requirement for this process is 2.3MJ to regenerate 1kg of CO2 absorbed on mmen-Mg2(dobpdc), compared to the current MEA of 3.5-4.5MJ.

Using Mass spectroscopy for verification they monitor N2, CO2, and H2O in real time and results shows N2 mostly at outlet until around 50min of flow, and then CO2 starts to increase substantially, which means the porous structure is full and must be CO2 purged.

The results seemed impressive to me, although I couldn’t follow half the chemistry. For future work, I would suggest further examination on the influence of temperature on the MOF’s tested, because these will likely be used very different climates and data for existing power plant CO2 capture is temperature dependent. Probably test a few different temperatures spanning the expected environment range variation. The cycle endurance of the material, which Long et al. only studied briefly, is also important to look at. A future issue would be to focus on the production efficiency of MOF in large quantities.

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