Analytical Spectroscopy

Minute Paper #8 – 11/06/09 – Michelle Henderson
Title: Enantioseparation and Amperometric Detection of Chiral Compounds by in Situ Molecular Imprinting on the Microchannel Wall
Authors: Ping Qu et. al.
Journal: Analytical Chemistry
The efficient separation of enantiomers will become increasingly more important as the FDA has increased restrictions for racemic mixtures in drugs. It is often the case that one enantiomer offers the desired physiological effect while the other causes unwanted side effects. A microfluidic device utilizing the molecular imprinting technique (MIT), analogous to enzyme-substrate recognition, was developed to separate the enantiomers tert-butoxycarbonyl-D-tryptophan (Boc-D-Trp) and Boc-L-Trp. First the optimum conditions for the synthesis of the Boc-L-Trp-imprinted capillary were determined (template to functional monomer ratio, functional monomer to cross-linker ratio, time, and temperature). These conditions were decided on by the number and quality of these sites. The separation conditions were then optimized (mobile phase composition, pH, salt concentration, separation voltage, and detection potential), which were based on retention times and resolution. An inverted fluorescence microscope was used to obtain the thickness of the MIT film, and atomic force microimaging and FTIR were used to confirm surface morphology.
The obvious advantage of this technique is a decreased analysis time, smaller sample size, and less solvents used compared to other chiral chromatography techniques. Also, a specialized column is needed for typical HPLC separation of chiral compounds. While the authors were able to implement a chiral separation on a microfluidic device, it was, in fact, a relatively poor separation. The peak shape of the unretained species, Boc-D-Trp is very good (tall and narrow), but the peak shape of Boc-L-Trp is miserable (short and wide). Also, the calibration curves for each enantiomer are linear over only one order of magnitude. In addition, there is a lot of preparatory work before the MIP-coated channel may be used each time.
The most simplistic next step would be to prepare a Boc-R-Trp-imprinted capillary and repeat the experiment. The results should be similar, but the separation may be better depending on if this enantiomer binds to the recognition site more strongly and the L-enantiomer. While this method employs a chiral selector in the stationary phase (on the walls of the capillary), there have also been studies in the literature in which the chiral selector is within the mobile phase. A hybrid technique could be tested in which a chiral selector within the mobile phase could be paired with the microfluidic device. In this case, AM (acrylamide) could still be used as the functional monomer template, but no cross-linker would be used. This would cause the AM to be in discrete units with recognition sites of choice. The mixture of enantiomers should then be combined with the mobile phase containing the chiral linker, allowing the enantiomer of choice to bind to the recognition sites. The mixture should then be separated via the microfluidic device, with the larger, bound enantiomers traveling at a different rate than the unbound enantiomers. If possible, size exclusion particles could also be implanted into the capillary. If the enantiomer bound to the chiral selector is unwanted (i.e. the enantiomer causing the side effect in a drug), it can simply be discarded. If the enantiomer is desired, some sort of washing technique will have to be used to rid it of the chiral selector. This hybrid technique combines the advantages of the microfluidic device (quicker separation, less reagents used) and being able to use the same device for a variety of enantiomers because the walls are not coated specifically to bind to only one.

Minute Paper #7 (11/6/2009) – Zhenpeng Qin
Title: Plasmon lasers at deep subwavelength scale
http://www.nature.com/nature/journal/v461/n7264/full/nature08364.html
By: Oulton et al.
Journal: Nature

The development of lasers at subwavelength scale is an interesting research area since this will break the diffraction limit of traditional macroscopic lasers and offer the platform to explore the light-matter interaction down to sub-100nm scale. The authors demonstrated experimentally a kind of plasmonic laser at deep subwavelength scale, by the use of a hybrid plasmonic waveguide, which circumvents the ohmic loss of using surface plasmons.

The structure of the nanoscale laser the authors built hybrids the dielectric waveguiding with plasmonics. A semiconductor nanowire is placed on top of a metallic surface (Ag), separated by a thin layer of insulating gap (MgF2). This structure enables the energy storage in the insulating gap by the coupling between the nanowire and metallic surface.

The authors compared the plasmonic laser, with the photonic laser generated by placing nanowire directly on top of a quartz substrate. The plasmonic laser has a weak dependence on the nanowire diameter, while the photonic laser strongly depends on the nanowire diameter. The use of nanowire with diameter (d) down to 52nm is only feasible for plasmonic laser. The authors claimed that the plasmonic laser does not experience mode cutoff and the rapid increase in the threshold pump intensity (at d= 52nm and smaller) is caused by the total gain volume. On the other hand, the photonic laser has sharp increase in the threshold pump intensity at d=150 nm and experience cutoff because of the strong leakage into the quartz substrate. The different polarization behaviors of plasmon laser and photonic laser are also compared. The scattered light of plasmonic laser is parallel to the direction of nanowire, while that of photonic laser is perpendicular to the nanowire.

The Purcell factor, usually defined as the enhanced radiation of an emitter within micro-cavities relative to its value in free space (Wikipedia), is observed for the insulating gap. Its value is more than 6 for a gap width of 5nm and nanowire diameter of 120nm. The spontaneous emission rate is dependent on the gap size. Reducing gap size will increase the emission rate. The authors also analyzed the efficiency of the laser. For an input power of 5 mW, an average output power of 10 nW was obtained. However, this includes the large amount of power that was not shined to the nanowire. Considering the fraction of power shined onto the nanowire, the typical laser efficiency of 10% was obtained.

While this is a very novel work, there are many unanswered questions. For example, is it possible and how to use other materials to tune the frequency of the plasmonic laser? Will other kind of nanowire or nanotube work as well to build the plasmonic laser? The plasmonic laser is emitting towards all directions, how to obtain a collimated light for this nanoscale laser?

One future experiment is to use this plasmonic laser for Raman spectroscopy to increase resolution. Current resolution of Raman spectroscopy is in the order of several hundred nanometers and is limited by the laser diffraction limit. If possible, the integration of the nanoscale plasmonic laser will bring this limit down to sub-100nm scale.

Minute Paper- 2009.11.06 – Antonio Campos
Title: Conjugated Polymers with Large Effective Stokes Shift: benzobisdioxole-Based Poly(phenylene ethynylene)s
By: Tanmoy Dutta et. al.
Journal: JACS
Link: http://pubs.acs.org/doi/abs/10.1021%2Fja9068134

Conjugated polymers such as poly(phenylene ethynyle) (PPE) are investigated and optimized to yield a Stokes shift varying from 0.15 – 0.95 eV. These polymers are commonly used in opto-electronic devices such as LEDs and solar cells. In this paper, the authors investigate co-polymers comprised of BDO (benzo[1,2-d:4,5-d’]bis[1,3]dioxole) and PPE, which showed intermolecular charge transfer and thus emission shifted to longer wavelengths as solvent polarity was increased. These Stokes shifts are preserved in the solid state polymer, and thus an ideal candidate for use in a laser. The monomers in this system yielded higher Stokes shifts (the highest is 1.45 eV), due to the steric effects of the functional groups present on the BDO (benzo[1,2-d:4,5-d’]bis[1,3]dioxole) backbone. As the steric bulk of the functional groups increased, so did the Stokes shifts. In general, the absorbance maximum for the polymers was around 400 nm and the emission occurred in the 520 nm (although the range was quite large for emission 442- 585 nm). The advantage of this system of co-polymers is the fact that they have a very large Stokes shift. The electronic structure of the co-polymers has not been observed before in the literature. The respective molecular orbitals in the BDO-PE co-polymer cannot mix due to the high lying HOMO on the BDO. A disadvantage in this system is that the energy transitions decrease as the length of the polymer chain increases.
Future applications of this system can be applied to lasers. One can envision using an LED light to pump the polymer transition for it to lase. One problem with this next experiment is that an LED light has to be focused onto the polymer, but in lieu of the inherent incoherence of LED lights one could use the polymer as the coating or use a waveguide made from the polymer to ensure lasing. Once again, this would require optimization of the polymer coating to ensure that a consistent emission wavelength can be obtained for a certain length polymer chain. Reproducing the correct length of polymer chains to ensure monochromatic emission could prove to be difficult and tedious. However, if the co-polymer system is tuned to give a variety of emission wavelengths, it would be possible to make a white light source such as a fluorescent light bulb using a mixture of the different polymer chain lengths. Another possible use of the polymer is in solar cells. The semiconductor arrays used in solar cells suffer from the inefficiencies of converting light to electricity because of the different band gap requirements not matching with solar light. If the light that impinges on the surface of the solar cell does not match with the band gap of the semiconducting material, the impinging photon cannot drive the electron-hole pair required for the production of electricity. The idea behind using these polymers as a coating for solar cells is that the polymer absorbs the incoming light and emits a photon that is the correct wavelength for the semiconducting material band gap, thus producing the electron-hole pairs and creating a current. Further optimization of the co-polymer system is needed. The different co-polymers and functional groups on this system is numerous, so many variables can be tuned in order to attain larger Stokes shifts, and a wider range of emission wavelengths.

Title: Conjugated Fluorenes Prepared From Azomethines Connections-II: The Effect of Alternating Fluorenones and Fluorenes on the Spectroscopic and Electrochemical Properties
Authors: St phane Dufresne et al.
Journal: the journal of physical chemistry B
Link: http://pubs.acs.org/doi/full/10.1021/jp907391y

In this paper, the authors try to understand what factors result in the quenched fluorescence of fluorene and fluorenone azomethine derivatives. A serial of fluorenone are synthesized varying in their functional group in benzene ring or the number and type of basic units. Fluorene’s singlet excited state is quenched with the addition of fluorenone and the fluorescence of fluorene covalently bonding to the fluorenone is also quenched which shows that intramolecular photoinduced electron transfer (PET) is responsible for the quench. High conjugation of azomethine facilitates the intersystem crossing which is also responsible for the reduced quenching. The fluorescence of the compound can be restored by protonating the ketone or amine group which further confirms PET process. This tunable on/off property is very promising for the fluorescence switch and probe.

However, despite the excellent design for the molecule structures and experiments, the authors still leave several question unanswered. First, DFT is only applied to calculate the HOMO and LUMO energy gap for compound 9 and 10. However, for compound 1, 1A and 14 which are most extensively discussed lack the theoretical evidence. This data is needed to confirm the energy mismatch between 1 and 14 as the authors claim. In addition, DFT calculations for 5 and 6 are also needed to prove that PET is also kinetically favored in these compounds. Second, the authors claim that ketone moieties have no effect on the fluorescence but there are only at most two ketone groups in the compound. So compound with more ketone groups should be designed and investigated in their fluorescence change. What’s more, the authors try to compare the quenching rate for 1A quenched by 1 and the rate for 1A quenched by 14 to demonstrate that the energy mismatch leads to decreasing rate but only the rate for 1 quenched by 14 is given which is quite confusing here. Finally, acid addition is used to protonate the ketone or amine group in order to investigate their effects on the quench. However, the compound may be in an equilibrium with its protonated counterpart and it’s probably that the counterpart is unfavorable product. So the effect may be due to the equilibrium but not the damage on the conjugation structure. What needs to be improved may be direct addition of H2 to the double bond and then investigating the effect.

Some work can be done based on the work in this paper. Compound with more units of fluorene and fluorenone azomethine can be synthesized. Each unit may alternatively appear in the molecular structure even in the polymer structure. Because fluorene is fluorescently active but fluorenone is the quench agent, we can see what the property is for this ABABAB structure. Total quenched? What will happen by changing the ratio of the two units? Another one is self-assembly Film. Because there is quench happening between the two units, so electron can transfer between fluorene and fluorenone. By assembling many molecules together, they may transfer electrons.

Minute Paper #8 – (11/06/2009) – Si Hoon(Sean) Lee
Title: Optical Anisotropy of Supported Lipid Structures Probed by Waveguide Spectroscopy and Its Application to Study of Supported Lipid Bilayer Formation Kinetics
By: Alireza Mashaghi et. al.
Journal: Analytical Chemistry http://pubs.acs.org/doi/pdf/10.1021/ac800027s

In this paper, Mashaghi et. al. attempt to verify the Dual Polarization Interferometry (DPI) system as a promising biosensing tool by measuring the thickness and the refractive index of various lipid bilayer. The conformational changes of the liposome during the rupturing onto the SiO2 layer are proved by the birefringence (n_TM – n_TE) measurement using DPI system and the results show good selectivity on the various conditions of the supported lipid bilayer (SLB). The sensing device has 4 layers dielectric materials stack (Silicon dioxide-Silicon Nitride). He-Ne laser (632nm) is illuminated the side of the dielectric materials. Top surface is used as a sensing waveguide and the other layer is used as a reference waveguide. The phase shifted electromagnetic waves between the sensing and the reference generate the interference fringes onto the far field detector (CCD Detector). To change the polarization, the polarizer controlled by the digital signal process (DSP) is placed between the light source and the sensing device.

One of the major efforts for the research related with Lipid is the formation of lipid bilayer: supported of suspended state. Many analytical tools like Quartz crystal microbalance with dissipation monitoring (QCM-D), surface Plasmon resonance (SPR), ellipsometry, atomic force microscopy (AFM) are used to prove the thickness, the formation of SLB and the critical coverage vesicles. However these analytical tools have some limitations: SPR can’t give the thickness information of the SLB while AFM only gives the thickness information and it is too laborious for optimizing the condition on the various types of liposomes and the medium. QCM-D has the power to distinguish the vesicles and the SLB on the surface but is not versatile for the kinetic measurements.

In this paper, for the verification of the DPI as a good sensing tool, the authors compare the birefringence data with the energy dissipation (ΔD) data of QCM-D first (Figure 2). Because this, as distinct from the change of damping (Δf), is solely related with the molecular conformation (ref 8, 23), the authors interprets the SLB formation and the critical coverage of various lipid, different ionic condition by using the birefringence data. In figure 3, the authors claim the steep drop of birefringence means the transformation of liposomes to SLB based on the theory that higher surface area per unit mass explains the rupturing of vesicles. The mixture of POPC:POPS (8:2) shows two distinct peak and POPC with Ca2+ shows slow transition which means slow rupturing at the first stage and sudden transition to SLB after the first change. In figure 4, the authors investigate the effect of Ca2+ on the birefringence and find out the charge interactions between liposomes lead to a more stable and homogeneous distribution of the cluster. Higher birefringence peak means more monodispersed cluster.

In this paper, the authors introduce the optical spectroscopic way to investigate the formation of SLB successfully. By utilizing both TM and TE mode which has different profile of evanescent field, the thickness and the refractive index of the isotropic SLB can be revealed in-situ. In case of anisotropic layers, the thickness of SLB can be calculated by fixing the refractive index (n=1.47 generally) and vice versa (d=5nm) by solving Maxwell equation. The authors show the thickness, the birefringence and the refractive index data of the various lipids with different ionic state in table 1. This information looks so useful and gives strong confidence on this DPI sensing method. However, Due to the interferometry configuration, the spatial SLB formation information seems hard to get this spectroscopic set-up. We can only get averaged information over the sensing area. If we want to form the SLB on the specific sensing spot and to use it as a barrier or a scaffold for receptor proteins in the real-time sensing method, this will show a limitation. I hope I can find the way to prove the lipid formation with the highly temporal and spatial resolution at the same time.