Browsing by Subject "Dichroism"

Now showing 1 - 5 of 5

Open Access
Chiral ceramic nanoparticles and peptide catalysis
(American Chemical Society, 2017) Jiang S.; Chekini, M.; Qu, Z.-B.; Wang Y.; Yeltik A.; Liu, Y.; Kotlyar, A.; Zhang, T.; Li, B.; Demir, Hilmi Volkan; Kotov, N. A.
The chirality of nanoparticles (NPs) and their assemblies has been investigated predominantly for noble metals and II-VI semiconductors. However, ceramic NPs represent the majority of nanoscale materials in nature. The robustness and other innate properties of ceramics offer technological opportunities in catalysis, biomedical sciences, and optics. Here we report the preparation of chiral ceramic NPs, as represented by tungsten oxide hydrate, WO3-x·H2O, dispersed in ethanol. The chirality of the metal oxide core, with an average size of ca. 1.6 nm, is imparted by proline (Pro) and aspartic acid (Asp) ligands via bio-to-nano chirality transfer. The amino acids are attached to the NP surface through C-O-W linkages formed from dissociated carboxyl groups and through amino groups weakly coordinated to the NP surface. Surprisingly, the dominant circular dichroism bands for NPs coated by Pro and Asp are different despite the similarity in the geometry of the NPs; they are positioned at 400-700 nm and 500-1100 nm for Pro- and Asp-modified NPs, respectively. The differences in the spectral positions of the main chiroptical band for the two types of NPs are associated with the molecular binding of the two amino acids to the NP surface; Asp has one additional C-O-W linkage compared to Pro, resulting in stronger distortion of the inorganic crystal lattice and greater intensity of CD bands associated with the chirality of the inorganic core. The chirality of WO3-x·H2O atomic structure is confirmed by atomistic molecular dynamics simulations. The proximity of the amino acids to the mineral surface is associated with the catalytic abilities of WO3-x·H2O NPs. We found that NPs facilitate formation of peptide bonds, leading to Asp-Asp and Asp-Pro dipeptides. The chiroptical activity, chemical reactivity, and biocompatibility of tungsten oxide create a unique combination of properties relevant to chiral optics, chemical technologies, and biomedicine.
Open Access
Chiral metamaterials with negative refractive index based on four "U" split ring resonators
(American Institute of Physics, 2010-08-23) Li, Z.; Zhao, R.; Koschny, T.; Kafesaki, M.; Alici, K. B.; Colak, E.; Caglayan, H.; Özbay, Ekmel; Soukoulis, C. M.
A uniaxial chiral metamaterial is constructed by double-layered four "U" split ring resonators mutually twisted by 90°. It shows a giant optical activity and circular dichroism. The retrieval results reveal that a negative refractive index is realized for circularly polarized waves due to the large chirality. The experimental results are in good agreement with the numerical results.
Open Access
Chiral metamaterials: From negative index to asymmetric transmission
(IEEE, 2013) Mutlu, Mehmet; Li, Zhaofeng; Özbay, Ekmel
Chiral metamaterials are attractive for their intriguing properties such as negative refractive index, optical activity and circular dichroism, and asymmetric transmission. In this paper, we review the research we have conducted for the purpose of investigating these exciting properties. © 2013 EurAAP.
Open Access
Electrostatic effects on nanofiber formation of self-assembling peptide amphiphiles
(Elsevier, 2011) Toksoz, S.; Mammadov R.; Tekinay, A. B.; Güler, Mustafa O.
Self-assembling peptide amphiphile molecules have been of interest to various tissue engineering studies. These molecules self-assemble into nanofibers which organize into three-dimensional networks to form hydrocolloid systems mimicking the extracellular matrix. The formation of nanofibers is affected by the electrostatic interactions among the peptides. In this work, we studied the effect of charged groups on the peptides on nanofiber formation. The self-assembly process was studied by pH and zeta potential measurements, FT-IR, circular dichroism, rheology, atomic force microscopy, scanning electron microscopy and transmission electron microscopy. The aggregation of the peptides was triggered upon neutralization of the charged residues by pH change or addition of electrolyte or biomacromolecules. Understanding the controlled formation of the hydrocolloid gels composed of peptide amphiphile nanofibers can lead us to develop in situ gel forming bioactive collagen mimetic nanofibers for various tissue engineering studies including bioactive surface coatings. © 2010 Elsevier Inc.
Open Access
Investigation of the crystallinity and orientation of polypropylene with respect to temperature changes using FT-IR, WRD, and Raman techniques
(2004) Türkçü, Harun Nezih
Having good mechanical and barrier properties, polypropylene films are widely used in food-packing industry. The relation of crystallinity and orientation of polypropylene with these properties are well known. Temperature increase has an important effect on the crystallinity and orientation of polypropylene. The purpose of this study is to understand better crystallinity and orientation changes of the cast and bi-axially oriented polypropylene films with respect to temperature effect. These changes were investigated using especially in-situ FT-IR and dichroism as well as XRD and Raman spectroscopy. For this purpose, an in-situ variable temperature set-up is developed and adapted for FT-IR studies. Accordingly, the changes in the peaks related to the amorphous and crystalline contents of the films as well as the peaks reflecting the orientation of the films were investigated over the temperature range from 120-170o C. Temperature cycle was also applied. Moreover, the films were investigated after application of additional stress at 120o C for various durations. The crystallinity and orientation of the films are affected significantly when the films were analyzed at higher temperature (>150o C). The crystallinity content of the film recovers after returning back to room temperature whereas orientational loss are permenant.