The hypothesis behind this research is that Atomic Force Microscopy (AFM) can be used to capture changes in the surface interaction force caused by sorption of ions, and thereby can be employed as a tool to study sorption at the nano-scale. The Derjaguin??dau??wey??rbeek (DLVO) theory was used to explain surface interactions and probe DLVO and other interparticle forces. In the first part of the work, sorption of copper ions onto a silica particle resulted in charge reversal detected by AFM. Transient measurements of the interaction force between the silica particle and a flat glass surface can be related to the kinetics of copper ion sorption. In the second part, the force-volume AFM mode was used to detect heterogeneously charged regions on quartz silica surfaces resulting from copper ion sorption. Conditions of copper ion concentration and pH were varied to confirm the sorption mechanism. The surprising result of this study was the observation of growing islands during sorption indicating a heterogeneous behavior of the surface. In the third part, AFM was used to measure interaction forces between a gold sample and the silicon nitride tip of the AFM in aqueous electrolyte solutions under various values of applied electrostatic potential. Along with the applied potential, pH conditions were also varied. Experimental results were compared to theoretical calculations using the non-linear Poisson-Boltzmann equation. This work showed a relationship between the surface potential and the externally applied potential. Finally, the total interaction force between a standard silicon nitride AFM tip and a gold-coated plate in the presence of cationic surfactant, cetyltrimethylammonium bromide (CTAB), was measured under various conditions of applied potential. The interaction force-distance profile between the surface and the tip can be related to the structure of surfactant molecules sorbed onto the surface, which is influenced by the magnitude of the applied potential. The results presented in this work are of importance in natural and engineered systems involving colloidal particles and charged species with implications in separations as well as naturally occurring facilitated transport.

The invention of the atomic force microscope (AFM) revolutionized the scientific world by providing researchers with the ability to make topographical maps of both conducting and non-conducting surfaces with nanometer resolution. As an alternative to optical AFM methods, thermal cantilevers have been investigated as a method to measure topography. This study reports the fabrication and testing of heated AFM cantilevers. This study transfers a fabrication process first developed at Stanford University to the Georgia Institute of Technology micro-fabrication facility and fabricates six different heated AFM cantilever designs. Selective impurity doping of a silicon cantilever allows it to become electrically conductive with a resistive element near the cantilever free end. Voltage applied across the cantilever legs induces current flow through the cantilever that generates heat in the resistive element. A deep understanding of the operational behavior and limits of the AFM cantilever is required to use the cantilever as an experimental tool. Characterization experiments determined the cantilever electrical resistance and temperature response. Experiments were conducted that electrically test heated AFM cantilevers at various system input voltages. Electrical and thermal responses of these cantilevers were compared against a theoretical model. The model utilizes heat transfer fundamentals and links the thermal response to the cantilever temperature-dependent electrical characteristics. Results of this study show that the fabricated heated AFM cantilevers have a tip with a radius of curvature as small as 20nm. Cantilever temperatures can exceed 700㠩n short pulses and, because the resistive heating element is also a temperature sensor, calibration of the cantilever temperature response is possible to within 1㮍

This research is dedicated to develop novel batch fabrication procedures for two distinct AFM (Atomic Force Microscope) probes integrated with electrodes enabling electrical sample characterization and electrochemical sample surface profiling respectively. These AFM probes allow for highly accurate control of the probe positioning, low contact force and sample surface imaging with high lateral resolution. As an electrical characterization tool, a nickel micro four-point probe integrated with solid nickel tips was developed. Low electrical resistance of the probe and contact resistance were achieved due to the solid nickel cantilever and tips. Low aspect ratio solid metal tips reduced contact resistance resulting in stable electrical measurement. Conductivity loss easily experienced while using metal coated AFM cantilevers was overcome by solid nickel tip integration to the electrically conductive AFM cantilevers. The fabrication method introduces selective conical nickel tip etching in silicon dioxide etching chambers. A novel batch fabrication method for SECM-AFM (Scanning Electrochemical Microscope-Atomic Force Microscope) tip integrated with a ring electrode was developed as a tool for electrochemical imaging as well as topological imaging. The electroactive area at an exactly defined distance above the apex of the AFM tip is fabricated using an inverse silicon mold technique. The electrode at a deliberately chosen distance from the end of a scanning probe tip allowing electrochemical sample imaging separated from sample topology imaging. The ring electrode coated with polymer entrapping enzymes enabled the probe to detect ATP from living epithelial cells.

The human body is an intricate network of many highly regulated biochemical processes and cell adhesion is one of them. Cell adhesion is mediated by specific interactions between molecules on apposing cell surfaces and is critical to many physiological and pathological processes like inflammation and cancer metastasis. During inflammation, blood-borne circulating leukocytes regularly stick to and roll on the vessel walls, which consist in part, adhesive contacts mediated by the selectin family of adhesion receptors (P-, E- and L-selectin). This is the beginning of a multi-step cascade that ultimately leads to leukocyte recruitment in areas of injury or infection. In vivo, selectin-mediated interactions take place in a hydrodynamic milieu and hence, it becomes imperative to study these interactions under very similar conditions in vitro. The goal of this project was to characterize the kinetic and mechanical properties of selectin interactions with different physiologically relevant ligands and selectin-specific monoclonal antibodies (mAbs) under a mechanically stressful milieu, using atomic force microscopy (AFM). Elasticity studies revealed that bulk of the complex compliance came from the selectins, with the ligands or mAbs acting as relatively stiffer components in the stretch experiments. Furthermore, molecular elasticity was inversely related to selectin length with the Consensus Repeats (CRs) behaving as Hookean springs in series. Besides, monomeric vs. dimeric interactions could be clearly distinguished from the elasticity measurements. L-selectin dissociation studies with P-selectin Glycoprotein Ligand 1 (PSGL-1) and Endoglycan revealed that catch bonds operated at low forces while slip bonds were observed at higher forces. These results were consistent with previous P-selectin studies and suggested that catch bonds could contribute to the shear threshold for L-selectin-mediated rolling By contrast, only slip bonds were observed for L-selectin-antibody interactions, suggesting that catch bonds could be a common characteristic of selectin-ligand interactions. Force History studies revealed that off-rates of L-selectin-sPSGL-1 (or 2-GSP-6) interactions were not just dependent on applied force, as has been widely accepted but in fact, depended on the entire history of force application, thus providing a new paradigm for how force could regulate bio-molecular interactions. Characterizing selectin-ligand interactions at the molecular level, devoid of cellular contributions, is essential in understanding the role played by molecular properties in leukocyte adhesion kinetics. In this aspect, data obtained from this project will not only add to the existing body of knowledge but also provide new insights into mechanisms by which selectins initiate leukocyte adhesion in shear.

Atomic force microscopy (AFM) is adapted to the measurement of adhesion forces between indoor-air-pollutant bioparticulates and synthetic carpet fiber materials. This novel technology is used to characterize the adhesion and release of a model bioparticulate, the bacterium E. coli on Nylon. This knowledge will lead to expanded studies of a wider range of biocontaminants, and ultimately to the ability to design carpet and rugs upholstery that reduce the spread of indoor air pollutants. Such an advance would improve life significantly for the 20+ million Americans who suffer from asthma, and countless others who are afflicted with allergies and illness spread via bioparticulates.

In this study, we developed the manufacturing pathways of carbon nanofibers (CNF) and activated carbon nanofibers (ACNF) via the ¡§melt-spinning¡¨ method. A novel route based on the solvent-free core/sheath melt-spinning of polypropylene/ (phenol formaldehyde-polyethylene) (PP/(PF-PE)) to prepare CNF. The approach consists of three main steps: co-extrusion of PP (core) and a polymer blend of PF and PE (sheath), followed by melt-spinning, to form the core/sheath fibers; stabilization of core/sheath fibers to form the carbon fiber precursors; and carbonization of carbon fiber precursors to form the final CNF. Both scanning electron microscopy and transmission electron microscopy images reveal long and winding CNF with diameter 100 - 600 nm and length greater than 80 £gm. With a yield of ~ 45 % based on its raw material PF, the CNF exhibits regularly oriented bundles which curl up to become rolls of wavy long fibers with clean and smooth surface. Results from X-ray diffractometry, energy dispersive X-ray, Raman spectroscopy, and selected area electron diffraction patterns further reveal that the CNF exhibits a mixed phase of carbon with graphitic particles embedded homogeneously in an amorphous carbon matrix. The carbon atoms in CNF are evenly distributed in a matrix having a composition of 90 % carbon element and 10 % in oxygen element. A series of ACNF have also been prepared based on the chemical activation on the thus-prepared CNF; their morphological and microstructure characteristics were analyzed by scanning electron microscopy, atomic force microscopy (AFM), Raman spectroscopy, and X-ray diffractometry, with particular emphasis on the qualitative and quantitative AFM analysis. The effect of activating agent, potassium hydroxide and phosphorous acid, is compared; factors affecting the surface morphology and microstructure of ACNF are analyzed. The ACNF also exhibits a mixed phase of carbon with graphitic particles embedded homogeneously in an amorphous carbon matrix. The resulting ACNF consists of 73 % C element and 27 % O element. The total pore volume of the all activated ACNF is larger than that of un-activated CNF. It can be inferred that chemical activation by KOH results in increased micropore volume in carbon nanofibers; while the micropores produced by the chemical activation of H3PO4 may further be activated and then enlarged to become the mesopores at the expense of micropore volume. For the concentration effect of KOH on ACNF, it can be inferred that high concentration KOH activation results in increased SBET and micropore volume in carbon nanofibers. The average pore diameter of ACNF gradually decreases as the KOH concentration increases.

The cytoskeleton is a diverse, multi-protein framework that plays a fundamental role in many cellular activities including mitosis, cell division, intracellular transport, cell motility, muscle contraction, and the regulation of cell polarity and organization. Furthermore, cytoskeletal filaments have been implicated in the pathogenesis of a wide variety of diseases including cancer, blood disease, cardiovascular disease, inflammatory disease, neurodegenerative disease, and problems with skin, nail, cornea, hair, liver and colon. Increasing evidence suggests that the distribution and organization of the cytoskeleton in living cells are affected by mechanical stresses and the cytoskeleton determines cell stiffness. We developed a fully nonlinear, constrained mixture model for adherent cells that allows one to account separately for the contributions of the primary structural constituents of the cytoskeleton and extended a prior solution from the finite elasticity literature for use in a sub-class of atomic force microscopy (AFM) studies of cell mechanics. The model showed that the degree of substrate stretch and the geometry of the AFM tip dramatically affect the measured cell stiffness. Consistent with previous studies, the model showed that disruption of the actin filaments can reduce the stiffness substantially, whereas there can be little contribution to the overall cell stiffness by the microtubules or intermediate filaments. To investigate the effect of mechanical stretching on cytoskeletal remodeling and cell stiffness, we developed a simple cell-stretching device that can be combined with an AFM and confocal microscopy. Results demonstrate that cyclic stretching significantly and rapidly alters both cell stiffness and focal adhesion associated vinculin and paxillin, suggesting that focal adhesion remodeling plays a critical role in cell stiffness by recruiting and anchoring F-actin. Finally, we estimated cytoskeletal remodeling by synthesizing data on stretch-induced dynamic changes in cell stiffness and focal adhesion area using constrained mixture approach. Results suggest that the acute increase in stiffness in response to an increased cyclic stretch was probably due to an increased stretch of the original filaments whereas the subsequent decrease back towards normalcy was consistent with a replacement of the highly stretched original filaments with less stretched new filaments.

In order to create informed predictive models that capture artery dependent responses during atherosclerosis progression and the long term response to hypertension, one needs to know the structural, biochemical and mechanical properties as a function of time in these diseased states. In the case of hypertension more is known about the mechanical changes; while, less is known about the structural changes over time. For atherosclerotic plaques, more is known about the structure and less about the mechanical properties. We established a congruent multi-scale model to predict the adapted salient arterial geometry, structure and biochemical response to an increase in pressure. Geometrical and structural responses to hypertension were then quantified in a hypertensive animal model. Eventually this type of model may be used to predict mechanical changes in complex disease such as atherosclerosis. Thus for future verification and implementation we experimentally tested atherosclerotic plaques and quantified composition, structure and mechanical properties. Using the theoretical models we can now predict arterial changes in biochemical concentrations as well as salient features such as geometry, mass of elastin, smooth muscle, and collagen, and circumferential stress, in response to hemodynamic loads. Using an aortic coarctation model of hypertension, we found structural arterial responses differ in the aorta, coronary and cerebral arteries. Effects of elevated pressure manifest first in the central arteries and later in distal muscular arteries. In the aorta, there is a loss and then increase of cytoskeleton actin fibers, production of fibrillar collagen and elastin, hyperplasia or hypertrophy with nuclear polypoid, and recruitment of hemopoeitic progenitor cells and monocytes. In the muscular coronary, we see similar changes albeit it appears actin fibers are recruited and collagen production is only increased slightly in order to maintain constant the overall ratio of ~55 percent. In the muscular cerebral artery, despite a temporary loss in actin fibers there is little structural change. Contrary to hypertensive arteries, characterizing regional stiffness in atherosclerotic plaques has not been done before. Therefore, experimental testing on atherosclerotic plaques of Apolipoprotein E Knockout mice was performed and revealed nearly homogenously lipidic plaques with a median axial compressive stiffness value of 1.5 kPa.

Atomic Force Microscopy (AFM) and Kelvin Probe Microscopy (KPM) are two surface characterization methods suitable for semiconductor nanocrystal applications. In this thesis work, an AFM system with KPM capability was developed and implemented. It was observed that, the effect of electrostatic interaction of the probe cantilever with the sample can be significantly reduced by using higher order resonant modes for Kelvin force detection. Germanium nanocrystals were grown on silicon substrate using different growth conditions. Both characterization methods were applied to the nanocrystal samples. Variation of nanocrystal sizes with varying annealing temperature were observed. Kelvin spectroscopy measurements made on nanocrystal samples using the KPM apparatus displayed charging effects.

In this study we use AFM-based nanolithography technique to produce nanofeatures of the single strand DNA and LNA probe molecules which are prepared via thiolated nucleic acid self-assembled monolayers (SAMs) on gold substrates. The goal is to observe the topographic changes of the DNA film structures resulting from the formation of rigid double strand DNA when the target and probe DNAs bind together. The so-called hybridization depends strongly on the probe density on the substrate surface. To find the proper probe density for hybridization, we vary the concentration of the probe DNA and search for the optimal conditions for measuring the height changes of the nanofeatures. We also monitor the topographic changes of the DNA nanofeatures in the different target DNA concentrations as a function of time, and the binding isotherms are fitted with the Langmuir adsorption model to derive the equilibrium dissociation constant and maximum hybridization efficiency. In addition, we extend the nanoscale hybridization reaction detection to mismatched DNA:DNA and LNA:DNA hybridization, and observe that topographic change of mismatched hybridization is inconspicuous and rapidly reach equilibrium. The results reveal the apparent difference between the perfect match and mismatch conditions, and validate that this approach can be applied to differentiate the situations for both perfect match and mismatch cases, demonstrating its potentials in the gene chip technology.