uu.seUppsala universitets publikasjoner
Endre søk
Link to record
Permanent link

Direct link
BETA
Akhtar, Sultan
Alternativa namn
Publikasjoner (10 av 19) Visa alla publikasjoner
Rubino, S., Akhtar, S. & Leifer, K. (2016). A Simple Transmission Electron Microscopy Method for Fast Thickness Characterization of Suspended Graphene and Graphite Flakes. Microscopy and Microanalysis, 22(1), 250-256
Åpne denne publikasjonen i ny fane eller vindu >>A Simple Transmission Electron Microscopy Method for Fast Thickness Characterization of Suspended Graphene and Graphite Flakes
2016 (engelsk)Inngår i: Microscopy and Microanalysis, ISSN 1431-9276, E-ISSN 1435-8115, Vol. 22, nr 1, s. 250-256Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

We present a simple, fast method for thickness characterization of suspended graphene/graphite flakes that is based on transmission electron microscopy (TEM). We derive an analytical expression for the intensity of the transmitted electron beam I-0(t), as a function of the specimen thickness t (t < < lambda; where lambda is the absorption constant for graphite). We show that in thin graphite crystals the transmitted intensity is a linear function of t. Furthermore, high-resolution (HR) TEM simulations are performed to obtain lambda for a 001 zone axis orientation, in a two-beam case and in a low symmetry orientation. Subsequently, HR (used to determine t) and bright-field (to measure I-0(0) and I-0(t)) images were acquired to experimentally determine lambda. The experimental value measured in low symmetry orientation matches the calculated value (i.e., lambda = 225 +/- 9 nm). The simulations also show that the linear approximation is valid up to a sample thickness of 3-4 nm regardless of the orientation and up to several ten nanometers for a low symmetry orientation. When compared with standard techniques for thickness determination of graphene/graphite, the method we propose has the advantage of being simple and fast, requiring only the acquisition of bright-field images.

Emneord
graphene, TEM, bright field, thickness maps
HSV kategori
Identifikatorer
urn:nbn:se:uu:diva-299522 (URN)10.1017/S143192761501569X (DOI)000377142100026 ()26915000 (PubMedID)
Forskningsfinansiär
Göran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of Technology
Tilgjengelig fra: 2016-07-22 Laget: 2016-07-22 Sist oppdatert: 2019-04-24bibliografisk kontrollert
Yang, W., Akhtar, S., Leifer, K. & Grennberg, H. (2013). Noncovalent Functionalization of Graphene in Suspension. ISRN Organic Chemistry (Article ID 656185)
Åpne denne publikasjonen i ny fane eller vindu >>Noncovalent Functionalization of Graphene in Suspension
2013 (engelsk)Inngår i: ISRN Organic Chemistry, ISSN 2090-5149, E-ISSN 2090-5157, nr Article ID 656185Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

Suspensions of graphene, prepared from graphite foil by sonochemical exfoliation, have been treated with new nonpolar pyrenebutyric amides. The assemblies, in suspension and after deposition on solid supports, were characterized by NMR, absorption, and fluorescence spectroscopy and by transmission electron microscopy, where the well-defined shape and size of an appended [60]fulleropyrrolidine unit facilitates TEM detection of the nonstationary molecules. The accumulated evidence, also including direct comparisons of carbon nanotubes treated with pyrene amides under the same conditions, proves the successful noncovalent functionalization of graphene suspended in non-polar solvent with non-polar pyrene derivatives.

HSV kategori
Forskningsprogram
Kemi
Identifikatorer
urn:nbn:se:uu:diva-205627 (URN)10.1155/2013/656185 (DOI)
Tilgjengelig fra: 2013-08-21 Laget: 2013-08-21 Sist oppdatert: 2019-04-24bibliografisk kontrollert
Rubino, S., Akhtar, S., Melin, P., Searle, A., Spellward, P. & Leifer, K. (2012). A site-specific focused-ion-beam lift-out method for cryo Transmission Electron Microscopy. Journal of Structural Biology, 180(3), 572-576
Åpne denne publikasjonen i ny fane eller vindu >>A site-specific focused-ion-beam lift-out method for cryo Transmission Electron Microscopy
Vise andre…
2012 (engelsk)Inngår i: Journal of Structural Biology, ISSN 1047-8477, E-ISSN 1095-8657, Vol. 180, nr 3, s. 572-576Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

The focused-ion-beam (FIB) is the method of choice for site-specific sample preparation for Transmission Electron Microscopy (TEM) in material sciences. A lamella can be physically lifted out from a specific region of a bulk specimen with submicrometer precision and thinned to electron transparency for high-resolution imaging in the TEM. The possibility to use this tool in life sciences applications has been limited by the lack of lift-out capabilities at the cryogenic temperatures often needed for biological samples. Conventional cryo-TEM sample preparation is mostly based on ultramicrotomy, a procedure that is not site-specific and known to produce artifacts. Here we demonstrate how a cooled nanomanipulator and a custom-built transfer station can be used to achieve cryo-preparation of TEM samples with the FIB, enabling high-resolution investigation of frozen-hydrated specimens in the TEM.

Emneord
Cryo-EM, Cryo-FIB, Cryo-SEM, Cryo-TEM, FIB, Frozen-hydrated sample, Sample preparation, Specimen thinning, TEM
HSV kategori
Forskningsprogram
Teknisk fysik med inriktning mot materialvetenskap
Identifikatorer
urn:nbn:se:uu:diva-184896 (URN)10.1016/j.jsb.2012.08.012 (DOI)000311471000020 ()
Merknad

De två första författarna delar förstaförfattarskapet.

Tilgjengelig fra: 2012-11-20 Laget: 2012-11-15 Sist oppdatert: 2019-04-24bibliografisk kontrollert
Yang, X., Akhtar, S., Leifer, K., Hilborn, J. & Ossipov, D. (2012). Development of multifunctional hyaluronic acid-based carriers by 'click' chemistry. Journal of Tissue Engineering and Regenerative Medicine, 6(suppl 1), 191-192
Åpne denne publikasjonen i ny fane eller vindu >>Development of multifunctional hyaluronic acid-based carriers by 'click' chemistry
Vise andre…
2012 (engelsk)Inngår i: Journal of Tissue Engineering and Regenerative Medicine, ISSN 1932-6254, Vol. 6, nr suppl 1, s. 191-192Artikkel i tidsskrift, Meeting abstract (Fagfellevurdert) Published
Abstract [en]

Introduction: We have developed methods to simultaneously introduce chemoselective groups to hyaluronic acid (HA) that can be crosslinked and used for immobilization of various molecules. It is easy to switch the functionalities of obtained materials through orthogonal ‘click’ chemistries.

Methods: HA polymers were dually functionalized with hydrazide and thiol groups, or aldehyde groups. Different functional molecules such as bisphosphonate (BP) groups for mineralization or camptothecin (CPT) as anti-cancer drug can be linked to thiol groups. HA polymers were characterized by 1H, 31P NMR, and UV-Vis Spectroscopy. Characterizations of materials were performed using SEM, TEM, rheology, and colorimetric calcium assay.

Results and discussion: The functional polymers were successfully synthesized. They can form hydrogels within 1 min with HA-aldehyde polymer. BP groups showed high affinity to calcium ions and inorganic nanoparticle phases were observed within BP containing hybrid hydrogel. On the other hand, CPT-linked HA polymer via S-S bond can self-assembled into nanoparticles, and the release of the drug is triggered by the addition of reduce agents (e.g. dithiothreitol).

Conclusions: The use of orthogonal click modification opened a versatile way to prepare different kinds of HA based carriers with various functionalities by simple methods. The multifunctional HA polymers show high potentials to be used for drug delivery (pro-drug) or tissue regeneration (BP linked HA).

HSV kategori
Forskningsprogram
Teknisk fysik med inriktning mot materialvetenskap; Kemi med inriktning mot polymerkemi
Identifikatorer
urn:nbn:se:uu:diva-182411 (URN)10.1002/term.1586 (DOI)000308313001308 ()
Tilgjengelig fra: 2012-10-12 Laget: 2012-10-10 Sist oppdatert: 2019-04-24bibliografisk kontrollert
Yang, X., Akhtar, S., Rubino, S., Leifer, K., Hilborn, J. & Ossipov, D. (2012). Direct ”Click” Synthesis of Hybrid Bisphosphonate-Hyaluronic Acid Hydrogel in Aqueous Solution for Biomineralization. Chemistry of Materials, 24(9), 1690-1697
Åpne denne publikasjonen i ny fane eller vindu >>Direct ”Click” Synthesis of Hybrid Bisphosphonate-Hyaluronic Acid Hydrogel in Aqueous Solution for Biomineralization
Vise andre…
2012 (engelsk)Inngår i: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 24, nr 9, s. 1690-1697Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

We report the synthesis of injectable in situ forming hybrid hydrogel material and investigate its ability to support the mineralization process under mild conditions. To achieve this, we have prepared a hyaluronic acid (HA) derivative that is dually functionalized with cross-linkable hydrazide groups and bisphosphonate ligands (HA-hy-BP). The hybrid hydrogel can be formed by simple mixing of two solutions: the solution of HA-hy-BP and the Ca2+ ions containing solution of aldehyde-derivatized HA (HA-al). We found that the conjugation of BP, a P-C-P analogue of pyrophosphate, to the hydrogel matrix promotes an efficient and fast mineralization of the matrix. The mineralization is facilitated by the strong interaction between BP residues and Ca2+ ions that serve as nanometer-sized nucleation points for further calcium phosphate deposition within the HA hydrogel. Compared with previously reported hydrogel template-driven mineralization techniques, the present approach is maximally adapted for clinical settings since the formation of the hybrid takes place during quick mixing of the sterilized solutions. Moreover, the hybrid hydrogel is formed from in vivo degradable components of the extracellular matrix and therefore can be remodeled in vivo through concerted HA degradation and calcium phosphate mineralization.

Emneord
hybrid organic-inorganic materials, injectable materials, hydrogels, hyaluronic acid, mineralization
HSV kategori
Forskningsprogram
Teknisk fysik med inriktning mot materialvetenskap; Kemi med inriktning mot polymerkemi
Identifikatorer
urn:nbn:se:uu:diva-171989 (URN)10.1021/cm300298n (DOI)000303628100019 ()
Tilgjengelig fra: 2012-03-30 Laget: 2012-03-30 Sist oppdatert: 2019-04-24bibliografisk kontrollert
Widenkvist, E., Yang, W., Akhtar, S., Palmgren, P. l., Knut, R., Leifer, K. & Jansson, U. (2012). Graphene Formation by Sonochemical Exfoliation of Bromine-intercalated Graphite . Influence of Solvent Properties on Exfoliation Yield and Deposition Outcome. Chemistry - A European Journal,, 200, 1-8
Åpne denne publikasjonen i ny fane eller vindu >>Graphene Formation by Sonochemical Exfoliation of Bromine-intercalated Graphite . Influence of Solvent Properties on Exfoliation Yield and Deposition Outcome
Vise andre…
2012 (engelsk)Inngår i: Chemistry - A European Journal,, Vol. 200, s. 1-8Artikkel i tidsskrift (Fagfellevurdert) Published
Emneord
deposition, graphene preparation, intercalation-exfoliation, sonochemistry
HSV kategori
Identifikatorer
urn:nbn:se:uu:diva-223147 (URN)10.1002/chem.200)) (DOI)
Tilgjengelig fra: 2014-04-16 Laget: 2014-04-16 Sist oppdatert: 2019-04-24
Akhtar, S. (2012). Transmission Electron Microscopy of Graphene and Hydrated Biomaterial Nanostructures: Novel Techniques and Analysis. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Åpne denne publikasjonen i ny fane eller vindu >>Transmission Electron Microscopy of Graphene and Hydrated Biomaterial Nanostructures: Novel Techniques and Analysis
2012 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Transmission Electron Microscopy (TEM) on light element materials and soft matters is problematic due to electron irradiation damage and low contrast. In this doctoral thesis techniques were developed to address some of those issues and successfully characterize these materials at high resolution. These techniques were demonstrated on graphene flakes, DNA/magnetic beads and a number of water containing biomaterials. The details of these studies are given below.

A TEM based method was presented for thickness characterization of graphene flakes. For the thickness characterization, the dynamical theory of electron diffraction is used to obtain an analytical expression for the intensity of the transmitted electron beam as a function of thickness. From JEMS simulations (experiments) the absorption constant λ in a low symmetry orientation was found to be ~ 208 nm (225 ± 9 nm). When compared to standard techniques for thickness determination of graphene/graphite, the method has the advantage of being relatively simple, fast and requiring only the acquisition of bright-field (BF) images. Using the proposed method, it is possible to measure the thickness change due to one monolayer of graphene if the flake has uniform thickness over a larger area.

A real-space TEM study on magnetic bead-DNA coil interaction was conducted and a statistical analysis of the number of beads attached to the DNA-coils was performed. The average number of beads per DNA coil was calculated around 6 and slightly above 2 for samples with 40 nm and 130 nm beads, respectively. These results are in good agreement with magnetic measurements. In addition, the TEM analysis supported an earlier hypothesis that 40 nm beads are preferably attached interior of the DNA-coils while 130 nm beads closer to the exterior of the coils.

A focused ion-beam in-situ lift-out technique for hydrated biological specimens was developed for cryo-TEM. The technique was demonstrated on frozen Aspergillus niger spores which were frozen with liquid nitrogen to preserve their cellular structures. A thin lamella was prepared, lifted out and welded to a TEM grid. Once the lamella was thinned to electron transparency, the grid was cryogenically transferred to the TEM using a cryo-transfer bath. The structure of the cells was revealed by BF imaging. Also, a series of energy filtered images was acquired and C, N and Mn elemental maps were produced. Furthermore, 3 Å lattice fringes of the underlying Al support were successfully resolved by high resolution imaging, confirming that the technique has the potential to extract structural information down to the atomic scale. The experimental protocol is ready now to be employed on a large variety of samples e.g. soft/hard matter interfaces.

sted, utgiver, år, opplag, sider
Uppsala: Acta Universitatis Upsaliensis, 2012. s. 124
Serie
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 917
Emneord
Graphene flakes, magnetic beads/DNA coils, hydrated biomaterials, transmission electron microscopy, focused ion beam/scanning electron microscopy, bright-field/dark-field imaging, high resolution imaging, electron diffraction and cryogenic temperatures
HSV kategori
Forskningsprogram
Materialvetenskap
Identifikatorer
urn:nbn:se:uu:diva-171991 (URN)978-91-554-8333-3 (ISBN)
Disputas
2012-05-21, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2012-04-26 Laget: 2012-03-30 Sist oppdatert: 2012-08-01bibliografisk kontrollert
Talyzin, A. V., Luzan, S. M., Leifer, K., Akhtar, S., Fetzer, J., Cataldo, F., . . . Moons, E. (2011). Coronene Fusion by Heat Treatment: Road to Nanographenes. The Journal of Physical Chemistry C, 115(27), 13207-13214
Åpne denne publikasjonen i ny fane eller vindu >>Coronene Fusion by Heat Treatment: Road to Nanographenes
Vise andre…
2011 (engelsk)Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, nr 27, s. 13207-13214Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

The reactions of coronene dehydrogenation and fusion upon heat treatment in the temperature range of 500-700 degrees C were studied using XRD, TEM, Raman, IR, and NEXAFS spectroscopy. The formation of a coronene dimer (dicoronylene) was observed at temperatures 530-550 degrees C; dicoronylene can easily be separated using sublimation with a temperature gradient. An insoluble and not sublimable black precipitate was found to form at higher temperatures. Analysis of the data shows that dimerization of coronene is followed at 550-600 degrees C by oligomerization into larger molecules. Above 600 degrees C amorphization of the material and formation of graphitic nanoparticles was observed. Coronene fusion by annealing is proposed as a road to synthesis of larger polycyclic aromatic hydrocarbons and nanographenes.

HSV kategori
Forskningsprogram
Teknisk fysik med inriktning mot materialvetenskap
Identifikatorer
urn:nbn:se:uu:diva-156619 (URN)10.1021/jp2028627 (DOI)000292479700011 ()
Tilgjengelig fra: 2011-08-06 Laget: 2011-08-04 Sist oppdatert: 2019-04-24bibliografisk kontrollert
Strömberg, M., Akhtar, S., Gunnarsson, K., Zardán Gómez de la Torre, T., Russell, C., Herthnek, D., . . . Leifer, K. (2011). Immobilization of oligonucleotide-functionalized magnetic nanobeads in DNA-coils studied by electron microscopy and atomic force microscopy. In: L. Stanciu, S. Andreescu, T. Noguer, B. Liu (Ed.), MRS Online Proceedings Library: volume 1355. Paper presented at MRS Spring Meeting 2011, Symposium JJ – Biological Hybrid Materials for Life Sciences, April 26-29, 2011, San Francisco, California, USA.
Åpne denne publikasjonen i ny fane eller vindu >>Immobilization of oligonucleotide-functionalized magnetic nanobeads in DNA-coils studied by electron microscopy and atomic force microscopy
Vise andre…
2011 (engelsk)Inngår i: MRS Online Proceedings Library: volume 1355 / [ed] L. Stanciu, S. Andreescu, T. Noguer, B. Liu, 2011Konferansepaper, Publicerat paper (Fagfellevurdert)
Abstract [en]

Immobilization of oligonucleotide-functionalized magnetic nanobeads by hybridization in DNA-coils formed by rolling circle amplification has been investigated using transmission electron microscopy (TEM) and atomic force microscopy (AFM). The TEM results supported earlier made observations that small beads with low oligonucleotide surface coverage preferably immobilize in the interior of the DNA-coils and do not tend to link several DNA-coils together whereas large beads with high surface coverage to a larger extent connect several DNA-coils together to clusters of several DNA-coils with beads. AFM provided direct visualization of the DNA-coils as thread-like objects. DNA-coils with immobilized beads appeared as a collection of beads with thread-like features in their near vicinity.

Emneord
nanoscale; transmission electron microscopy (TEM); colloid
HSV kategori
Forskningsprogram
Teknisk fysik med inriktning mot nanoteknologi och funktionella material; Teknisk fysik med inriktning mot fasta tillståndets fysik; Teknisk fysik med inriktning mot materialvetenskap
Identifikatorer
urn:nbn:se:uu:diva-150876 (URN)10.1557/opl.2011.1135 (DOI)
Konferanse
MRS Spring Meeting 2011, Symposium JJ – Biological Hybrid Materials for Life Sciences, April 26-29, 2011, San Francisco, California, USA
Tilgjengelig fra: 2011-04-07 Laget: 2011-04-07 Sist oppdatert: 2019-04-24bibliografisk kontrollert
Wessman, P., Mahlin, D., Akhtar, S., Rubino, S., Leifer, K., Kessler, V. & Håkansson, S. (2011). Impact of matrix properties on survival of freeze-dried bacteria. Journal of the Science of Food and Agriculture, 91(14), 2518-2528
Åpne denne publikasjonen i ny fane eller vindu >>Impact of matrix properties on survival of freeze-dried bacteria
Vise andre…
2011 (engelsk)Inngår i: Journal of the Science of Food and Agriculture, ISSN 0022-5142, E-ISSN 1097-0010, Vol. 91, nr 14, s. 2518-2528Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

Background:

Disaccharides are in general first choice as formulation compounds when freezedrying microorganisms. Although polysaccharides and other biopolymers are considered too large to stabilize and interact with cell components in the same beneficial way as disaccharides, polymers have been reported to support cell survival. In the present study we compare the efficiency of sucrose, the polymers Ficoll, hydroxyethylcellulose, hydroxypropylmethylcellulose and polyvinylalcohol to support survival of three bacterial strains during freeze-drying. The initial osmotic conditions were adjusted to be similar for all formulations. Formulation characterization was used to interpret the impact that different compound properties had on cell survival.

Results:

Despite differences in molecular size, both sucrose and the sucrose based polymer Ficoll supported cell survival after freeze-drying equally well. All formulations became amorphous upon dehydration. Scanning electron microscopy and X-ray diffraction data showed that the discerned differences in structure of the dry formulations had little impact on the survival rates. The capability of the polymers to support cell survival correlated with the surface activity of the polymers in a similar way for all investigated bacterial strains.

Conclusion:

Polymer-based formulations can support cell survival as effectively as disaccharides if formulation properties of importance for maintaining cell viability are identified and controlled.

Emneord
formulation, freeze-drying, biopolymer, sucrose, bacteria, surface
HSV kategori
Forskningsprogram
Biologi med inriktning mot mikrobiologi; Teknisk fysik med inriktning mot materialvetenskap
Identifikatorer
urn:nbn:se:uu:diva-138967 (URN)10.1002/jsfa.4343 (DOI)000296385900004 ()
Tilgjengelig fra: 2010-12-22 Laget: 2010-12-21 Sist oppdatert: 2019-04-24bibliografisk kontrollert
Organisasjoner