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Selver Ahmed

Postdoctoral researcher - Colloid and Soft Matter Lab

Philadelphia, PA

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Work Experience

Postdoctoral researcher

Colloid and Soft Matter Lab, Department of Chemical and Biomolecular Engineering, North Carolina State University, with Prof. Orlin D. Velev, Feb. 2012- present
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Raleigh, NC
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February 2012 to Present

• Developed a new (patent pending) technology for large scale synthesis of bio/polymeric sheet-like particles with anisotropic shape and unusual structuring (radical viscosity modifying) and stabilizing properties from liquid-liquid dispersions that is simple, sustainable and inexpensive process. 
• Developed novel formulations of ultra-stable and reversible (on-demand) foams and stable emulsions by using polymeric sheet particles. 
• Investigated the rheology of gels structured by modified biopolymeric sheets and found that sheets thicken oils much more than fibers and rods for same wt. % polymer. 
• Fabricated ultra-light, porous polymer sheets based sponge-like materials (aerogel) that can be used for various applications ranging from superabsorbent and separation for oil/water mixture (emulsion) to nanoparticle synthesis and delivery matrix 
• Synthesized and characterized various shaped colloidal particles from biopolymers and modified biopolymers by liquid-liquid dispersion method to be used as thickeners, stabilizers or active material delivery systems 
• Worked in close collaboration with an industrial research & development department (Unilever R&D, Vlaardingen, The Netherlands) 
• Supervised a project on understanding and characterization of cell-surfactant-bubble interactions in bioreactor operation with an industrial collaborator (Biogen Idec)

Research Assistant

Polymer, Colloids & Materials Lab, Department of Chemistry, Temple University with Prof. Stephanie L. Wunder
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Philadelphia, PA
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May 2004 to May 2011

Studied and characterized lipid vesicles and SiO2 colloidal systems and investigated their interactions 
• Designed, formed and characterized self-assembled lipid bilayer on colloidal particles called supported lipid bilayers (SLBs), a novel liposomal technology as a nanocariers/delivery matrix as well as a model for cell membrane 
• Formed and characterized lipid sheaths/sacs that can encapsulate and release nanoparticle supported lipid bilayers as a response to temperature and can be used as novel delivery vehicle for drug or combinations of drugs or other active materials/molecules 
• Expertise in colloidal stability of nanoparicles, vesicles and nanoparticle-encapsulated vesicles (SLBs) 
• Investigated conformational order and alkyl chain packing of lipid molecules in vesicles and supported lipid bilayers on SiO2 nanoparticles as well as effect of the curvature to bilayer morphology using ATR-FTIR and Raman spectroscopy 
• Studied kinetics of symmetric lipid molecule transfer of vesicles and supported lipid bilayers as a function of the curvature 
• Investigated effect of silanol density on the adsorption of lipid molecules on the surface of the colloidal particles and formation of supported lipid bilayers 
• Studied the influence of nanoscale curvature of silica particles on supported lipid bilayer morphology and properties 
• Prepared self-assembled monolayers on silica 
• Synthesized inorganic-organic hybrid materials 
• Initiated collaborations with external academic groups (Prof. G. Bothun and Dr. Y. Chen, Rhode Island University) and characterization centers (Dr. Z. Nikolov, Drexel University)

Teaching Assistant

Department of Chemistry, Temple University
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Philadelphia, PA
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September 2003 to May 2011

Responsibilities 
 
• Taught upper and freshmen level laboratories, recitations/discussion, led tutoring and mentoring of undergraduate and graduate students 
• Undergraduate students mentored 
- Robert Burnette – North Carolina State University, 2013  
- Andrew Talarico – North Carolina State University, 2013 
- Esther Lee – North Carolina State University, 2012-2013  
- Janine M Villano – Temple University, 2009 
- Tien Dinh – Temple University, 2008 
• Teaching Assistant: Department of Chemistry, Temple University, Sep. 2003-May 2011 
- (CHEM 4503) Introduction to Polymer Chemistry/Lab (Fall 2007) 
- (CHEM 4108) Investigative Chemistry/Lab (Spring 2007 & 2008) 
- (CHEM 4107) Drug Analysis/Lab (Spring 2005) 
- (CHEM 1032) General Chemistry II/Recitation (Fall 2006 & 2008, Spring 2004 & 2007, Summer 1 2007),  
- (CHEM1031) General Chemistry I/Recitation (Fall 2003 & 2005) 
- (CHEM 1034) General Chemistry II/Lab (Spring 2009, 2010 & 2011 and Summer 1, 2007, 2008 & 2010), 
- (CHEM 1033) General Chemistry I/Lab (Fall 2009)

Research Assistant

Laboratory of Thermodynamics and Physicochemical Hydrodynamics, Department
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December 1997 to September 2000

of Chemistry, Sofia University, Sofia, Bulgaria with Prof. Nikolai D. Denkov and Rossitsa Alargova, Ph.D, Dec. 1997 - Sep. 2000 
• Studied the hydrodynamic radius of micelles as a function of the mol fraction of SDP2S in a mixture of ionic and nonionic surfactants 
• Investigated ζ-potential of drops of soybean oil as a function of the mol fraction of SDP2S

Education

Ph.D. in Chemistry

Temple University -
Philadelphia, PA

October 2011

M.S. in Chemistry

Sofia University

December 1999

Skills

Calorimetric analysis (DSC, Nano-DSC), Thermogravimetric analysis (TGA), Light scattering (DLS, Zetasizer), FT-IR, ATR-FT-IR, UV-Vis, Raman Spectroscopy, NMR, HPLC, GC, GPC, Surface area analysis (BET), Optical, Polarized light, Fluorescence and Confocal microscopies, SEM, TEM, Rheological analysis (AR-2000), Contact angle measurement, IKA-Colloid Mill, Lyophilizer

Awards

• CST Graduate Conference Travel Award, Temple University to attend ACS Fall National Meeting, Boston, MA, 2007

2007

• CST Graduate Conference Travel Award, Temple University to attend 82nd ACS Colloid & Surface Science Symposium, Raleigh, NC, 2008

• Outstanding Teaching Graduate Student Award by College of Science and Technology, Temple University, 2009 (annually awarded to graduate student nominated for exceptional work as a Teaching Assistant)

• PASI fellowship “Scalable, Functional Nanomaterials”, Costa Rica, August 4th-14th, 2011

Groups

Member of American Chemical Society (ACS), Colloid and Surface Science Division (Coll)

Patents

Sheet-like particulates comprising an alkylated cellulose ether and method for making these(#WO2014001033 A1)

http://www.google.com/patents/WO2014001033A1?cl=en

January 2014

The present invention relates to a sheet-like particulate comprising a lipophilic cellulose-based polymer, for example ethylcellulose. The invention also relates to a method for production of these sheet-like particulates. Moreover the invention relates to the use of these sheets for structuring of non-aqueous liquids (for example a vegetable oil), or as foam stabiliser. The present invention relates to a composition comprising a non-aqueous liquid phase, for example a vegetable oil, that is structured by a sheet-like particulate comprising a lipophilic cellulose-based polymer, for example ethylcellulose

“Shear-driven Fabrication of Ethyl Cellulose Sheet Particulates with Application as Foam Stabilizers and Rheology Modifiers”, Selver Ahmed, J. Peters, S. Smoukov, S. Stoyanov, E. Pelan, O. D. Velev, Invention disclosure, May 29th , 2012, NCSU(#case # 12274, May 29th , 2012, NCSU)

http://execportal.ncsu.edu/UREF152711221717771985/RP_Detail/K2AA4084163919326971/33/KitsResultsR1T1/A11117772957

The invention is a new process for scalable formation of polymer sheet-like anisotropic particles. Such sheet-like particulates with anisotropic shape and characteristic sizes in the micrometer and sub-micrometer range can find numerous applications in medicine and technology. The process utilizes simple equipment that is readily available and the procedure is inexpensive, environmentally friendly, and easy to implement. The novel method can be easily scaled up to achieve production of large amounts of polymer sheets. It allows control over the characteristic sizes and aspect ratios. Some possible applications of the sheet-like particles are in multimillion dollar industries, such as in structuring and texturing of food products (making it healthier by reducing the amount of saturated fat), cosmetics, personal care products, dishwasher liquids, and developing new polymer Ѭatex-sheetѠpaints with higher viscosity and better optical properties

“Oleophilic aerogels from modified ethyl cellulose sheets”, Selver Ahmed, S. Stoyanov, E. Pelan, O. D. Velev, Invention disclosure, July 29th, 2014, NCSU(#case # 15034)

http://execportal.ncsu.edu/UREF152711221717771985/RP_Detail/K2AA4858183516353236/33/KitsResultsR1T1/A11117782175

2014

The invention is a method for large-scale fabrication of a new class of aerogels, an ultralight and highly porous, sponge-like materials from naturally hydrophobic polymers such as modified cellulose. The aerogel is prepared by freeze-drying of aqueous foam-like solution of ethyl cellulose giant sheets, new building blocks for such materials, macroscopically self-assembled in 3D interconnected network that forms the aerogel. These sheet-aerogels offer outstanding properties of low density, hierarchical porosity, high structural flexibility and excellent selective absorbtivity toward oils originating from the synergistic effect of soft ethyl cellulose polymer and sheet morphology. The aerogels are hydrophobic with density 0.08 -11 mg cm-3 and porosity of 99.4-99-7%. The ethyl cellulose sheet aerogels demonstrate high absorption capacity Q (defined by the ratio between the final and initial weight) of 24 to 86 times their own weight for a range of solvents and oils, with larger absorption capacity for nonpolar and higher density liquids. The absorption capacity for sunflower oil with density 0.9 g cm-3 is ~ 7 times larger than the commercial available absorbents (as shown in the supplementary material). Moreover, they have the ability to selectively absorb oil from two-phase systems such as oil/water, including mixture of oil/water as well as surfactant free and surfactant stabilized emulsions. Only certain carbon nanotube or graphene based aerogels show higher absorption capacity for oil compared to the aerogel described in this invention disclosure. However, the synthesis of these few alternative materials is complex and expensive and their production in bulk quantities poses challenges. The synthesis of the building blocks of the aerogel disclosed here, ethyl cellulose sheets, is easy, inexpensive and easily scalable to large quantities. It was described in detail in our earlier invention disclosure (12274) as well as in our manuscript in preparation. Therefore, the combination of the previous and the present invention disclosure provides a simple, efficient and inexpensive way to prepare large quantities of polymer sheets aerogels.

Publications

Effect of High Surface Curvature on the Main Phase Transition of the Supported Phospholipid Bilayers on SiO2 Nanoparticles

https://apps.webofknowledge.com/full_record.do?product=WOS&search_mode=GeneralSearch&qid=1&SID=1CmVJD3AHXF6jDc2iPH&page=1&doc=1

2009

Investigation of the physical properties of highly curved membranes is important in biology, for example, in fusion intermediates, and in pharmaceutical or chromatographic applications, where nanoscale features may affect substrate binding. However, vesicle fusion below 40 nm precludes study of this size regime. In this investigation, the effect of high surface curvature on the adsorption and morphology of phosphotidylcholine lipids with alkyl chain lengths of 14 (DMPC), 16 (DPPC), and 18 (DSPC) onto silica (SiO2) nanobeads was investigated by thermogravimetric analysis (TGA), high sensitivity nanocalorimetry, and vibrational spectroscopy. The SiO2 beads ranged in size from 5 to 100 nm. Stable supported bilayers were formed on all bead sizes by vesicle fusion of the parent MLVs at temperatures above the main phase transition temperature (T-m) of the lipids. A downward shift in T-m, and a broadening (Delta T-1/2) of the transition with respect to the parent MLVs, was observed for the 100 nm beads. With decreasing bead size, T-m first decreased, but then increased. On the smallest bead size, whose dimensions were comparable to those of the adsorbed lipids, T-m's were higher than those of the parent MLVs. The increase in T-m indicated a stiffening of the supported bilayer, which was confirmed by Raman spectroscopic data. Narrowing of the phase transition or the appearance of peak doublets occurred at the smaller bead sizes. The results were consistent with a model in which the high free volume and increased outer headgroup spacing of lipids on highly curved surfaces induced interdigitation in the supported lipids.

“Formation and Colloidal Stability of DMPC Supported Lipid Bilayers on SiO2”,

https://apps.webofknowledge.com/full_record.do?product=WOS&search_mode=GeneralSearch&qid=4&SID=1CmVJD3AHXF6jDc2iPH&page=1&doc=1

2010

Supported lipid bilayers (SLBs) of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were formed on 20-100 nm silica (SiO2) nanobeads, and the formation was accompanied by an 8 nm increase in diameter of the SiO2, consistent with single nanobeads surrounded by a DMPC bilayer. Complete SLBs were formed when the nominal surface areas of the DMPC matched that of the silica, SA(DMPC)/SA(SiO2) = 1, and required increasing ionic strength and time to form on smaller size nanobeads, as shown by a combination of nano-differential scanning calorimetry (nano-DSC), dynamic light scattering (DLS), and zeta potential (zeta) measurements. For 5 nm SiO2, where the nanoparticle and DMPC dimensions were comparable, DMPC fused and formed SLBs on the nanobeads, but it did not form single bilayers around them. Instead, stable agglomerates of 150-1000 nm were formed over a wide surface ratio range (0.25 <= SA(DMPC)/SA(SiO2) < 2) in 0.75 mM NaCl. At ionic strengths > 1 mM NaCl, charge shielding, as measured by zeta potential measurements (zeta --> 0), resulted in precipitation of the SLBs.

Hydration Repulsion Effects on the Formation of Supported Lipid Bilayers”

https://apps.webofknowledge.com/full_record.do?product=WOS&search_mode=GeneralSearch&qid=8&SID=1CmVJD3AHXF6jDc2iPH&page=1&doc=1

2011

When zwitterionic lipids fuse onto substrates such as silica (SiO(2)), the water of hydration between the two approaching surfaces must be removed, giving rise to an effective hydration repulsion. Removal of water around the polar headgroups of the lipid and the silanols (SiOH) of SiO(2) allows supported lipid bilayer (SLB) formation, although an interstitial water layer remains between the lipid and surface. The importance of hydration repulsion in SLB formation is demonstrated by monitoring fusion of zwitterionic lipids onto silica (SiO(2)) nanoparticles heat treated to control the silanol group (SiOH) density and thus the amount of bound water. SLB formation, observed by cryo-TEM and nano-differential scanning calorimetry, was found to be slower for the more hydrated surfaces. Although the SiOH density decreased with increasing heat treatment temperature, zeta-potentials were the same for all the SiO(2). This arose since at the pH 8 of the experiments, only isolated silanols, with a pK(a) = 4.9, and not hydrogen bonded silanols, with a pK(a) = 8.5, were dissociated/charged.(1) Since there were no differences in double layer forces between the SUVs and SiO(2), which are the largest and most important interactions determining lipid fusion onto surfaces,(2,3) the slower rate of SLB formation of DMPC onto SiO(2) nanoparticles with higher silanol densities and more bound water was therefore attributed to greater hydration repulsion of the more hydrated nanoparticles. For SiO(2) heated to 1000 degrees C, with only a few isolated silanols, little adsorbed water and many hydrophobic Si-O-Si groups, particle aggregation occurred and lipid sheaths formed around the nanoparticle aggregates

Stabilization of Soft Lipid Colloids: Competing Effects of Nanoparticle Decoration and Supported Lipid Bilayer Formation

https://apps.webofknowledge.com/full_record.do?product=WOS&search_mode=GeneralSearch&qid=10&SID=1CmVJD3AHXF6jDc2iPH&page=1&doc=1

Stabilization against fusion of zwitterionic lipid small unilamellar vesicles (SUVs) by charged nanoparticles is essential to prevent premature inactivation and cargo unloading. In the present work, we examined the stabilization of DMPC and DPPC SUVs by monolithic silica (SiO2) nanoparticle envelopment, for SiO2 with 4-6, 10-20, 20-30, and 40-50 nm nominal diameter. We found that for these soft colloids stabilization is critically dependent on whether fusion occurs between the charged nanoparticles and neutral SUVs to form supported lipid bilayers (SLBs), or whether the reverse occurs, namely, nanoparticle decoration of the SUVs. While SIB formation is accompanied by precipitation, nanoparticle decoration results In long-term stabilization of the SUVs. The fate of the nanosystem depends on the size of the nanoparticles and on the Ionic strength Of the medium. We found that, in the case of highly charged SiO2 nanoparticles in water, there is no SUV fusion to SiO2 for a specific range,of nanoparticle,sizes. Instead, the negatively charged SiO2 nanoparticles surround the uncharged SUVS, resulting In electrostatic repulsion between the decorated SUVs, thus preventing their aggregation and precipitation. Addition of millimolar amounts of NaCl results in rapid SLB formation and precipitation. This study has great potential impact toward better understanding the interaction of nanoparticles with biological membranes and the factors affecting their use as drug carriers or sensors.

Effect of Curvature on Nanoparticle Supported Lipid Bilayers Investigated by Raman Spectroscopy

https://apps.webofknowledge.com/full_record.do?product=WOS&search_mode=GeneralSearch&qid=12&SID=1CmVJD3AHXF6jDc2iPH&page=1&doc=1

2011

The packing of lipids on silica (SiO(2)) nanoparticles (NPs) was investigated by Raman spectroscopy for 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) as a function of their size, for SiO(2) NPs of 5, 15, 25, 45, and 100 nm nominal diameter. Raman spectral indicators in the C-C and C-H stretching regions were used to determine conformational order and alkyl chain packing for these systems. As the ratio of NP to lipid size decreases, packing in a normal bilayer configuration increases free volume and decreases hydrophobic interaction between the chains. For the 15, 25, 45, and 100 nm SiO(2), for which single supported lipid bilayers (SLBs) are formed around the NPs, the Raman data indicate that there is increased interdigitation and increased lateral packing order between the chains with decreasing NP size, which improves hydrophobic association and decreases the voids that would occur for normal bilayers. For the same size NP, there is increased interdigitation and lateral packing for the DSPC compared with DPPC lipids, as expected based on the greater void volume that would be created for the longer alkyl chain lengths. Another mechanism for filling this void space is the formation of gauche kinks for the terminal methyl groups at the center of the bilayer, which can be monitored by a Raman band at 1122 cm(-1). These gauche defects are most prevalent for the largest size (100 nm) NPs but are observed for all NP sizes. For the 5 nm SLBs, which form aggregates, we hypothesize that bilayer bridging can occur between the NPs. Compared with the 15 nm NPs, the order parameter increases but there are fewer trans conformers, possibly due to chains that are loosely packed or isolated in the interstitial regions

Formation of Lipid Sheaths around Nanoparticle Supported Lipid Bilayers

https://apps.webofknowledge.com/full_record.do?product=WOS&search_mode=GeneralSearch&qid=14&SID=1CmVJD3AHXF6jDc2iPH&page=1&doc=2

2012

High-surface-area nanoparticles often cluster, with unknown effects on their cellular uptake and environmental impact. In the presence of vesicles or cell membranes, lipid adsorption can occur on the nanoparticles, resulting in the formation of supported lipid bilayers (SLBs), which tend to resist cellular uptake. When the amount of lipid available is in excess compared with that required to form a single-SLB, large aggregates of SLBs enclosed by a close-fitting lipid bilayer sheath are shown to form. The proposed mechanism for this process is one where small unilamellar vesicles (SUVs) adsorb to aggregates of SLBs just above the gel-to-liquid phase transition temperature, Tm, of the lipids (as observed by dynamic light scattering), and then fuse with each other (rather than to the underlying SLBs) upon cooling below Tm. The sacks of SLB nanoparticles that are formed are encapsulated by the contiguous close-fitting lipid sheath, and precipitate below Tm, due to reduced hydration repulsion and the absence of undulation/protrusion forces for the lipids attached to the solid support. The single-SLBs can be released above Tm, where these forces are restored by the free lipid vesicles. This mechanism may be useful for encapsulation/release of drugs/DNA, and has implications for the toxic effects of nanoparticles, which may be mitigated by lipid sequestration

Raman Spectroscopy of Supported Lipid Bilayer Nanoparticles” invited paper to Spectroscopy, vol. p.40, June 2011, Special Issue: Raman Technology for Today’s Spectroscopist (Invited)

2011

Additional Information

Experimental Skills 
Calorimetric analysis (DSC, Nano-DSC), Thermogravimetric analysis (TGA), Light scattering (DLS, Zetasizer), FT-IR, ATR-FT-IR, UV-Vis, Raman Spectroscopy, NMR, HPLC, GC, GPC, Surface area analysis (BET), Optical, Polarized light, Fluorescence and Confocal microscopies, SEM, TEM, Rheological analysis (AR-2000), Contact angle measurement, IKA-Colloid Mill, Lyophilizer,