Dark field imaging of semicrystalline polymers by scanning transmission electron microscopy

Scanning transmission electron microscopy (STEM) has been suggested to have advantages over conventional transmission electron microscopy (CTEM) for the observation of diffraction contrast features and diffraction patterns from radiation sensitive crystalline polymers. Because of image intensification, control of illumination location and magnification independent focus, STEM operation for focusing, area selection and set up of optics permits a high yield of systematic data. Dark field (DF) imaging is most useful when employed in conjunction with scanning microarea diffraction. For convergent beam microdiffraction and efficient DF imaging of thin crystals the beam divergence should be less than 5×10^–3 radians. For single beam DF, the reflection of interest is selected by the intermediate lens aperture. Use of a STEM annular detector to collect more than one reflection results in increased DF image intensity and resolution. Use of the entire azimuthal range of a single powder pattern reflection permits examination of crystal texture — in particular, images produced by chain axis reflections show the detailed arrangements of lamellae.     

Scanning transmission electron microscopy (STEM)-transmission electron microscopy (TEM) analysis of nitrogen ion implanted austenitic 302 stainless steel

The microstructure of nitrogen implanted AISI 302 austenitic stainless steel and the effect of long-term room temperature ageing on it have been studied. Samples were implanted in 1992 with 2.5×10²¹ N₂⁺ m⁻² at 130 keV. The characteristics of the implanted layer and the depth profile have been investigated by scanning transmission electron microscope combined with energy dispersive X-ray spectrometry. Electron diffraction patterns recorded in the implanted layer using transmission electron microscopy confirm the formation of CrN along with the presence of Cr₂N. The identification of phases by glancing angle X-ray diffraction also indicates the formation of Cr₂N and nitrogen solid solutions. The effects of ageing on the microstructure are observed to be small.     

Morphology study of rubber based nanocomposites by transmission electron microscopy and atomic force microscopy

Rubber based nanocomposites were prepared using octadecyl amine modified Na-montmorillonite clay (OC) and Styrene Butadiene Rubber (SBR) having styrene content of 15, 23 and 40% respectively and Acrylonitrile Butadiene Rubber (NBR) having acrylonitrile content of 19, 34 and 50% respectively. The morphology of the nanocomposites was investigated using Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and X-ray Diffraction Technique (XRD). The TEM photographs of the unmodified clay loaded SBR nanocomposite showed agglomeration, while the modified clay loaded SBRs of all the grades revealed complete exfoliation. The NBRs, on the other hand, gave unexfoliated and intercalated clay structures both with the unmodified and the modified clays, except in the case of NBR having 19% of acrylonitrile and 4% of the unmodified clay. The AFM data were in good accord with the TEM results. The particle dimensions were within the range of 10–20 nm in the case of SBR sample having 4 parts of the modified clay. NBRs having 34 and 50% acrylonitrile contents and 4 parts of OC showed clay particles ranging from 50–70 nm and 70–100 nm respectively. On comparison of the rubbers having different nature and contents of functional groups and filler loadings, significant effect on the morphology of the composite was observed. The nature of solvent used to prepare the nanocomposites also affected the morphology. XRD data further corroborated the facts in all the above cases.     

Layer number and stacking sequence imaging of few-layer graphene by transmission electron microscopy

A method based on dark field transmission electron microscopy is developed to quantitively investigate the layer number and stacking order of multilayer graphene, demonstrated here on multilayer crystalline graphene synthesized by chemical vapor deposition. Our results show that the relative intensities of first- and second-order diffraction spots and contrast in corresponding dark field images are sufficient to identify the layer number and stacking order of graphene with layer number up to seven (7) or more with few-nanometer spatial resolution.     

A portable electrochemical sensor for caffeine and (-)epigallocatechin gallate based on molecularly imprinted poly(ethylene-co-vinyl alcohol) recognition element

The U.S. Food and Drug Administration allows a maximum of 72 mg of caffeine per 12 oz. serving (6 mg/oz). Consuming 400 mg of caffeine 3 times daily for 7 days may develop sleep disruption effects. However, it is still very hard for people to estimate how much caffeine is intake daily. Moreover, (-)epigallocatechin gallate (EGCG) is studied a potent antioxidant that may have therapeutic properties for anti-aging and cancer. Conventionally, both caffeine and EGCG could be measured by the protocols of high performance liquid chromatography; however, high precision instruments are required. In this work, the caffeine and EGCG are used as the template molecules and imprinted into poly(ethylene-co-vinyl alcohol), EVAL, via solvent evaporation. The EVAL membrane is then used as the sensing element for electrochemical analysis after templates removal. From the cyclic voltammetry measurement of the caffeine, the peak oxidation potential is shifted from 0.36 to 0.46 V when the final concentration of caffeine is from 0.01 to 1 mg/mL, and the highest current density is about 0.18 microA/cm2. The caffeine and EGCG concentrations measured in three real samples are about 0.10-0.13 mg/mL and 0.49-1.74 mg/mL, respectively. This molecularly imprinted polymeric coated electrode is potential employed as a home-care system.     

Uniformly sized molecularly imprinted polymer for (S)-nilvadipine. Comparison of chiral recognition ability with HPLC chiral stationary phases based on a protein

Uniformly sized molecularly imprinted polymers (MIPs) for (S)-nilvadipine have been prepared by a multistep swelling and polymerization method using methacrylic acid, 2-(trifluoromethyl)acrylic acid, 2-vinylpyridine, or 4-vinylpyridine (4-VPY) as a functional monomer and ethylene glycol dimethacrylate (EDMA) as a cross-linker. The chiral recognition abilities of the MIPs for nilvadipine and other dihydropyridine calcium antagonists were evaluated using a mixture of sodium phosphate buffer (or water) and acetonitrile or only acetonitrile as the mobile phase. The (S)-nilvadipine-imprinted 4-VPY-co-EDMA polymers gave the highest resolution for nilvadipine among the MIPs prepared. In addition, the enantioseparation of nilvadipine was attained using the (S)-nilvadipine-imprinted EDMA polymers, without use of a functional monomer. 1H NMR and molecular modeling studies suggested a one-to-one hydrogen-bonding-based complex formation of (S)-nilvadipine with 4-VPY in chloroform. These results reveal that the (S)-nilvadipine-imprinted EDMA polymers could recognize the template molecule by its molecular shape, and that in addition to this recognition, hydrophobic and hydrogen-bonding interactions seems to play important roles in the retention and chiral recognition of nilvadipine on the 4-VPY-co-EDMA polymers in hydroorganic mobile phases. By optimizing chromatographic conditions such as column temperature and flow rate, the baseline separation of nilvadipine enantiomers was attained with a short analysis time and with a column efficiency comparable to commercially available chiral stationary phases based on a protein, such as ovomucoid or alpha1-acid glycoprotein.     

Ultrafast electron microscopy (UEM): four-dimensional imaging and diffraction of nanostructures during phase transitions

Four-dimensional (4D) imaging during structural changes are reported here using ultrafast electron microscopy (UEM). For nanostructures, the phase transition in the strongly correlated material vanadium dioxide is our case study. The transition is initiated and probed in situ, in the microscope, by a femtosecond near-infrared and electron pulses (at 120 keV). Real-space imaging and Fourier-space diffraction patterns show that the transition from the monoclinic (P21/c) to tetragonal (P42/mnm) structure is induced in 3 +/- 1 ps, but there exists a nonequilibrium (metastable) structure whose nature is determined by electronic, carrier-induced, structural changes. For the particles studied, the subsequent recovery occurs in about 1 ns. Because of the selectivity of excitation from the 3d parallel-band, and the relatively low fluence used, these results show the critical role of carriers in weakening the V4+-V4+ bonding in the monoclinic phase and the origin of the nonequilibrium phase. A theoretical two-dimensional (2D) diffusion model for nanoscale materials is presented, and its results account for the observed behavior.     

Advantages of molecularly imprinted polymers LC-ESI-MS/MS for the selective extraction and quantification of chloramphenicol in milk-based matrixes. comparison with a classical sample preparation

A simple and fast selective extraction of the antibiotic chloramphenicol (CAP) from milk (raw milk, skimmed milk, and milk powder) using a molecularly imprinted polymer (MIP) sorbent is described. The method entails a single centrifugation step prior to loading the supernatant onto the MIP cartridge and subsequent elution with a mixture of solvents. CAP was further analyzed by isotope dilution liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) operating in negative ionization acquisition mode. The advantages of the MIP approach were assessed by comparing the data generated from a classical solid-phase and liquid-liquid extractions procedure, previously developed in our laboratory. A better recovery of CAP due to an enhanced selectivity and a faster turnaround time (18 samples processed within 3 h compared to 8 h with the classical approach) were evidenced when using the MIP cleanup. The analysis of CAP in raw milk was further validated according to the 2002/657/EC European Union criteria for the analysis of veterinary drug residues at the minimum required performance limit (MRPL) of 0.3 microg/kg, using CAP-d(5) as internal standard. Non-internal-standard corrected recovery values ranged between 50% and 87% over the range of concentrations considered. The decision limit (CCalpha) and detection capability (CCbeta) were calculated to be 0.06 and 0.10 microg/kg, respectively.     

Mapping of thermoplastic elastomeric nitrile rubber/poly(styrene-co-acrylonitrile) blends using tapping mode atomic force microscopy and transmission electron microscopy

Dynamically vulcanized thermoplastic elastomeric blends of nitrile rubber (NBR)/poly(styrene-co-acrylonitrile) (SAN) were mapped by tapping mode atomic force microscopy (TMAFM) and transmission electron microscopy (TEM). The morphology changes with the blend ratio and dynamic vulcanization. Roughness and surface analysis were used to study the effect of dynamic vulcanization and mixing sequence on the surface texture of the thermoplastic elastomeric blends. Surface geometry was quantified by power spectral density (PSD) and fractal analysis.     

Preparation and characterization of molecularly imprinted poly(hydroxyethyl methacrylate) microspheres for sustained release of gatifloxacin

Molecularly imprinted poly(hydroxyethyl methacrylate) microspheres (PHEMA MIPMs) were prepared via precipitation polymerization in this article, using gatifloxacin (GFLX), hydroxyethyl methacrylate (HEMA), and ethylene glycol dimethacrylate (EGDMA) as template molecule, functional monomer and cross-linker, respectively. The effects of reaction medium, initial total monomers, cross-linker and molecular imprinting on the polymerization were investigated systematically. The interaction between GFLX and HEMA in pre-solution was studied by UV–Visible spectrophotometer, both size and morphology of products were characterized by a scanning electron microscope. When the total initial monomer concentration was 1 vol%, EGDMA content was 70 mol%, a group of uniform PHEMA MIPMs were prepared at different GFLX/MAA molar ratios, with diameter range from 2.06 ± 0.07 to 2.82 ± 0.20 μm. The results of drug loading and in vitro release experiments demonstrated that PHEMA MIPMs could achieve a higher GFLX loading content and a more acceptable sustained release than non-imprinted ones.     

Selective electrochemical detection of dopamine based on molecularly imprinted poly(5-amino 8-hydroxy quinoline) immobilized reduced graphene oxide

The dopamine-imprinted conducting polymer film of 5-amino 8-hydroxy quinoline (AHQ) was electrodeposited on reduced graphene oxide (rGO)-modified glassy carbon (GC) electrode and was applied as a molecular recognition element for the selective determination of dopamine. The molecularly imprinted polymer (MIP)-modified electrode showed an excellent affinity towards dopamine due to the presence of imprinted site through hydrogen bonding interaction between dopamine and poly (AHQ) membrane. The molecular recognition ability of MIP-modified electrode was analyzed by cyclic voltammetric and differential pulse voltammetric techniques. The most stable geometry of the template–monomer complex in the pre-polymerization mixture was calculated by computational approaches. The rGO modification augmented both surface area and electron transfer kinetics of the bare electrode. The GC/rGO/MIP electrode possessed 2.83 fold current enhancements when compared to GC/MIP electrode, indicating the improvement in sensitivity due to rGO modification. The limit of detection and sensitivity of GC/rGO/MIP electrode was observed to be 32.7 nM and 13.3 AM^?1 cm^?2, respectively. The imprinting methodology provided an exceptional selectivity towards the detection of dopamine even in the presence of high concentration of possible physiological interferents. Moreover, the fabricated electrode was successfully employed for the detection of dopamine in human blood plasma samples proving the effectiveness of the sensor for the sensitive detection of dopamine from real samples.     

Sintering of silver and copper nanoparticles on (001) copper observed by in-situ ultrahigh vacuum transmission electron microscopy

The sintering of copper and silver nanoparticles with single crystal copper substrates has been studied using a novel in-situ ultrahigh vacuum transmission electron microscope (UHV TEM). The system is equipped with a UHV DC sputtering attachment enabling metal nanoparticles to be generated in-situ and transferred directly into the microscope in the gas phase. In both cases, we find the particles to be of initially random orientation on the substrate. Upon annealing, however, the particles reorient and assume the same orientation as the substrate. The process apparently occurs by a mechanism involving sintering and grain growth. In the case of silver on copper, grain growth cannot occur since the metals are immiscible. Our observations show that, upon annealing, the particles wet the substrate surface and form epitaxially oriented islands by surface diffusion and grain boundary migration. The post-anneal islands exhibit the orientation relationship (111)_Ag∥001)_Cu, [110]_Ag∥[110]_Cu.     

Structure determination of chitosan-stabilized Pt and Pd based bimetallic nanoparticles by X-ray photoelectron spectroscopy and transmission electron microscopy

Chitosan (CTS)-stabilized bimetallic nanoparticles were prepared at room temperature (rt.) in aqueous solution. Palladium (Pd) and platinum (Pt) were selected as the first metals while iron (Fe) and nickel (Ni) functioned as the second metals. In order to obtain the noble metal core-transition metal shell structures, bimetallic nanoparticles were prepared in a two-step process: the preparation of mono noble metallic (Pd or Pt) nanoparticles and the deposition of transition metals (Fe or Ni) on the surface of the monometallic nanoparticles. The structures of the nanoparticles were studied using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The XPS results show that Pd and Pt exist mainly in zero valences. The presence of Fe and Ni in the bimetallic nanoparticles affects the binding energy of Pd and Pt. Moreover, the studies of O 1s spectra indicate the presence of Fe or Ni shells. The analyses of TEM micrographs give the particle size and size distributions while the high-resolution TEM (HRTEM) micrographs show the existence of noble metal core lattices. The results confirm the formation of noble metal core-transition metal shell structures.     

High resolution transmission electron microscopy study of iron-silicide nanodot structures grown on faintly oxidized Si (111) surfaces

Epitaxial iron-silicide (α-FeSi₂, ε-FeSi and β-FeSi₂) nanodots were grown on Si(111) substrates with SiO₂ ultrathin films by Fe deposition on Si nanodots. The nanodots were characterized in-situ by reflection high-energy electron diffraction and scanning tunneling microscopy, and ex-situ high resolution transmission electron microscopy (HRTEM). For the β-FeSi₂ nanodots, the HRTEM images and the corresponding fast Fourier transform patterns analyses revealed that the coherent β-FeSi₂ nanodots with a relation of β-FeSi₂(110)/Si(111) had a compressive strain of ~ 0.8% in the [001]_β-FeSi2 direction and a tensile strain of ~ 2.6% in the direction normal to (110)_β-FeSi2 plane.