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Nehal I Abu-Lail

Publications and source records attributed to Nehal I Abu-Lail.

9 recordsLinked to original sources
PubMed2007

Microcantilever sensing and actuation with end-grafted stimulus-responsive elastin-like polypeptides.

Stimulus-responsive elastin-like polypeptides (ELPs) grafted onto surfaces are of significant technical interest because they can be exploited for force generation, in sensing applications, or as molecular switches with tunable properties. Changes in the conformational state of grafted ELPs, induced by a phase transition or changes in osmotic pressure, lead to significant changes in the surface stress in the ELP graft layer and translate into detectable changes in microcantilever deflection. In this study, we investigate the conformational mechanics of ELPs in response to changes in solution pH and ionic strength using atomic force microscopy (AFM) microcantilever deflection and quartz crystal microbalance (QCM) measurements. We show that the use of genetically encoded, surface-grafted ELPs is exciting for cantilever actuation and sensing because commonly available microfabricated cantilever springs offer a simple and nonintrusive way to detect changes in solvent type, temperature, and pH, promising great potential for sensing applications in microfluidic devices.

Biosensing Techniques↗
PubMed2006

Specific and nonspecific interaction forces between Escherichia coli and silicon nitride, determined by poisson statistical analysis.

The nature of the physical interactions between Escherichia coli JM109 and a model surface (silicon nitride) was investigated in water via atomic force microscopy (AFM). AFM force measurements on bacteria can represent the combined effects of van der Waals and electrostatic forces, hydrogen bonding, steric interactions, and perhaps ligand-receptor type bonds. It can be difficult to decouple these forces into their individual components since both specific (chemical or short-range forces such as hydrogen bonding) and nonspecific (long-range colloidal) forces may be present in the overall profiles. An analysis is presented based on the application of Poisson statistics to AFM adhesion data, to decouple the specific and nonspecific interactions. Comparisons with classical DLVO theory and a modified form of a van der Waals expression for rough surfaces were made in order to help explain the nature of the interactions. The only specific forces in the system were due to hydrogen bonding, which from the Poisson analysis were found to be -0.125 nN. The nonspecific forces of 0.155 nN represent an overall repulsive interaction. These nonspecific forces are comparable to the forces calculated from DLVO theory, in which electrostatic-double layer interactions are added to van der Waals attractions calculated at the distance of closest approach, as long as the van der Waals model for "rough" spherical surfaces is used. Calculated electrostatic-double layer and van der Waals interactions summed to 0.116 nN. In contrast, if the classic (i.e., smooth) sphere-sphere model was used to predict the van der Waals forces, the sum of electrostatic and van der Waals forces was -7.11 nN, which appears to be a large overprediction. The Poisson statistical analysis of adhesion forces may be very useful in applications of bacterial adhesion, because it represents an easy way to determine the magnitude of hydrogen bonding in a given system and it allows the fundamental forces to be easily broken into their components.

Bacterial Adhesion↗
PubMed2006

The effect of solvent polarity on the molecular surface properties and adhesion of Escherichia coli.

The elasticity and molecular surface characteristics of Escherichia coli JM109 were investigated via atomic force microscopy (AFM) in solvents expressing different polarities. The nature of bacterial adhesion and surface characteristics was probed in formamide, water, and methanol, with dielectric constants of 111, 80, and 33, respectively. Solvent polarity affected the elasticity of the bacterium, the conformation of the cell surface biopolymers, the height of the surface biopolymers, and measured adhesion forces between the bacterium and silicon nitride. By applying the Hertz model to force-indentation data, we determined that the Young's modulus was greatest in the least polar solvent, with values of 182 +/- 34.6, 12.8 +/- 0.1, and 0.8 +/- 0.3 MPa in methanol, water, and formamide, respectively. The thickness of the biopolymer brush layer on the bacterial surface was quantified using a steric model, and these values increased as polarity increased, with values of 27, 93, and 257 nm in methanol, water, and formamide, respectively. The latter results suggest that highly polar conditions favor extension of the biopolymer brush layer. Cross-sectional analysis performed on tapping mode images of the bacterial cells in methanol, water, and formamide further supported this hypothesis. The image height values are larger, since the image analysis measures the height of the bacterium and the polymer layer, but the trend with respect to solvent polarity was the same as was obtained from the steric model of the brush length. Measured adhesion forces scaled inversely with solvent polarity, with greatest adhesion observed in the least polar solvent, methanol. The combined conformational changes to the bacterial surface and biopolymer layer result in different presentations of macromolecules to a substrate surface, and therefore affect the adhesion forces between the bacterial molecules and the substrate. These results suggest that polarity of the solvent environment can be manipulated as a design parameter to control or modify the bacterial adhesion process.

Bacterial Adhesion↗
PubMed2005

Understanding the elasticity of fibronectin fibrils: unfolding strengths of FN-III and GFP domains measured by single molecule force spectroscopy.

While it is well established that fibronectin (FN) matrix fibrils are elastic, the mechanism of fibril elasticity during extension is still debated. To investigate the molecular origin of FN fibril elasticity, we used single molecule force spectroscopy (SMFS) to determine the unfolding behavior of a recombinant FN-III protein construct that contained eight FN-III domains ((1-8)FN-III) and two green fluorescent protein (GFP) domains. FN-III domains were distinguished from GFP domains by their shorter unfolding lengths. The unfolding strengths of both domains were determined for a wide range of pulling rates (50 to 1,745 nm/s). We found that the mechanical stabilities of FN-III and GFP domains were very similar to each other over the entire range of pulling speeds. FN fibrils containing GFP remain brightly fluorescent, even when stretched, meaning that GFP domains remain largely folded. Since GFP and FN-III have equal unfolding strengths, this suggests that FN-III domains are not extensively unraveled in stretched FN fibrils. Our results thus favor an alternative model, which invokes a conformational change from a compact to an extended conformation, as the basis for FN fibril elasticity.

Elasticity↗
PubMed2004

Dynamic post-treatment response of olive mill effluent wastewater using activated carbon.

Olive mill eflluent (OME) wastewater represents a serious environmental problem in the Mediterranean area. It has extremely high values of COD, BOD, and phenolic content. A new approach of treatment steps composed of settling, centrifugation, and filtration is suggested to be used to condition OME wastewater. The filtrate is then subjected to a post-treatment process, namely adsorption on activated carbon. The dynamic response of phenols concentration, pH, and COD, using different concentrations of activated carbon, shows a peak at which maximum adsorption capacity is achieved. The maximum adsorption capacity for the tested concentrations of activated carbon is reached in less than 4 h. The maximum removal of phenols is about 94%, while it reaches about 83% for organic matter.

Adsorption↗
PubMed2003

Role of lipopolysaccharides in the adhesion, retention, and transport of Escherichia coli JM109.

The role of lipopolysaccharides (LPS) in bacterial adhesion was investigated via atomic force microscopy (AFM). Adhesion between a silicon nitride tip and Escherichia coli JM109 was measured in water and 0.01 M phosphate-buffered saline (PBS) on untreated cells and on a sample of E. coli treated with 100 mM ethylenediaminetetraacetic acid (EDTA), which removes approximately 80% of the LPS molecules. LPS removal decreased the adhesion affinity between the bacterial cells and the AFM tip from -2.1 +/- 1.8 to -0.40 +/- 0.36 nN in water and from -0.74 +/- 0.44 to -0.46 +/- 0.23 nN in 0.01 M PBS (statistically different, Mann-Whitney rank sum test, P < 0.01). The distributions of adhesion affinities between E. coli LPS macromolecules and the AFM tip could be described by gamma distribution functions. Direct measurements of the adhesive force between E. coil and a surface were compared with adhesion in batch and column experiments, and agreement was observed between the influences of LPS on adhesion in each system. Bacterial batch retention to glass or in packed beds to quartz sand decreased after LPS removal. When interaction forces were measured during the approach of the AFM tip to a bacterium, steric repulsive forces were seen for both treated and untreated cells, but the repulsion was greater when the LPS was intact A model for steric repulsion predicted a reduction of the equilibrium length of the surface polymers from 242 to 64 nm in water and from 175 to 81 nm in buffer, after removal of a portion of the LPS. DLVO calculations based on conventional and soft-particle DLVO theories predicted higher energy barriers to adhesion for all surfaces after LPS removal, consistent with experimental findings. Adhesion forces between the AFM tip and bacterial polymers were correlated with bacterial attachment and retention, while measurements of interaction forces during the approach of the AFM tip to the bacterium did not correlate with subsequent adhesion behavior to glass or quartz sand.

Bacterial Adhesion↗
PubMedDate not supplied

Heterogeneity in bacterial surface polysaccharides, probed on a single-molecule basis.

The heterogeneity in bacterial surface macromolecules was probed by examining individual macromolecules on the surface of Pseudomonas putida KT2442 via single-molecule force spectroscopy (SMFS). Using an atomic force microscope (AFM), the silicon nitride tip was brought into contact with biopolymer molecules on bacterial cells and these macromolecules were stretched. Force-extension measurements on different bacterial cells showed a range of adhesion affinities and polymer lengths. However, substantial heterogeneity was also observed in the force-extension curves on a single bacterium. A given bacterium has biopolymers that range in size from tens to hundreds of nanometers, with adhesion affinities for the AFM tip from nearly zero to greater than 1 nN. A distribution of polymer sizes was confirmed by size-exclusion chromatography. The freely jointed chain (FJC) model for polymer elasticity was applied to individual force-extension curves in order to estimate the contour lengths and segment lengths of the polymer chains. A range of segment lengths was obtained using the FJC model, from 0.154-0.45 nm in water, 0.154-0.32 nm in 0.01 M KCl, and 0.154-0.65 nm in 0.1 M KCl. The modeling confirms that the heterogeneity in biopolymers is more than a matter of differences in molecular weights, since a range of stiffnesses (segment lengths) was also observed. The effect of salt concentration on biopolymer conformation and adhesion was also explored. While the biopolymers were flexible in all solvents, they were slightly more extended in water than in either of the salt solutions (0.01 and 0.1 M KCl). The adhesion of polysaccharides with the AFM tip was not dependent on salt concentration, because the polymers were not highly charged and heterogeneity overwhelmed any trends that could be observed in adhesion with respect to solution ionic strength. These experiments indicate that heterogeneity in biopolymer properties on an individual bacterium and within a population of bacterial cells may be much greater than previously believed and should be incorporated into models of bacterial adhesion.

Bacterial Adhesion↗
PubMedDate not supplied

Role of ionic strength on the relationship of biopolymer conformation, DLVO contributions, and steric interactions to bioadhesion of Pseudomonas putida KT2442.

Biopolymers produced extracellularly by Pseudomonas putida KT2442 were examined via atomic force microscopy (AFM) and single molecule force spectroscopy. Surface biopolymers were probed in solutions with added salt concentrations ranging from that of pure water to 1 M KCl. By studying the physicochemical properties of the polymers over this range of salt concentrations, we observed a transition in the steric and electrostatic properties and in the conformation of the biopolymers that were each directly related to bioadhesion. In low salt solutions, the electrophoretic mobility of the bacterium was negative, and large theoretical energy barriers to adhesion were predicted from soft-particle DLVO theory calculations. The brush layer in low salt solution was extended due to electrostatic repulsion, and therefore, steric repulsion was also high (polymers extended 440 nm from surface in pure water). The extended polymer brush layer was "soft", characterized by the slope of the compliance region of the AFM approach curves (-0.014 nN/nm). These properties resulted in low adhesion between biopolymers and the silicon nitride AFM tip. As the salt concentration increased to > or =0.01 M, a transition was observed toward a more rigid and compressed polymer brush layer, and the adhesion forces increased. In 1 M KCl, the polymer brush extended 120 nm from the surface and the rigidity of the outer cell surface was greater (slope of the compliance region = -0.114 nN/nm). A compressed and more rigid polymer layer, as well as a less negative electrophoretic mobility for the bacterium, resulted in higher adhesion forces between the biopolymers and the AFM tip. Scaling theories for polyelectrolyte brushes were also used to explain the behavior of the biopolymer brush layer as a function of salt concentration.

Bacterial Adhesion↗