This dissertation aims to advance the current state of use of silicon atomic force microscope (AFM) cantilevers with integrated heaters. To this end, the research consists of two primary thrusts - demonstrating new applications for the cantilevers, and advancing the current state of understanding of their thermal and mechanical behavior to enable further applications. Among new applications, two are described. In the first application, the cantilevers are used for nanoscale material deposition, using heat to modulate the delivery of material from the nanoscale tip. In the second application, the cantilever performs thermal analysis with nanoscale spatial resolution, enabling thermal characterization of near surface and composite interphase regions that cannot be measured with bulk analysis techniques. The second thrust of the research seeks to address fundamental questions concerning the precision use of heated cantilevers. Efforts to this end include characterizing the mechanical, electrical, and thermal behavior of the cantilevers, and optimizing calibration methodology. A technique is developed for calibrating the cantilever spring constant while operating at elevated temperature. Finally, an analytical model is developed for the heat flow in the cantilever tip and relevant dimensionless numbers that govern the relative importance of the various components of the thermal environment are identified. The dimensionless numbers permit exploration of the sensitivity of the tip-substrate interface temperature to the environmental conditions.

This work describes two closed-loop atomic force microscopy methods that utilize the heated silicon probe to interrogate surfaces. The first method identifies the softening temperatures of a selected polymer and organic substrate as a function of contact force and surface hardness. Motivation partly stems from nanosampling, which requires knowledge of phase-specific transitions to identify and extract mass from multicomponent systems for chemical analysis. In the second method, the cantilever is implemented as a Kelvin probe to study the effect of temperature on the measured contact potential. The objective is to ascertain whether the probe functions as a capable electrode for scanning Kelvin probe microscopy (SKPM) applications. This was achieved by performing heated force-distance experiments on a biased gold film with the tip operating at various potentials. Both experiments examine the interaction between the tip and substrate and analyze sample effects both induced and sensed by the cantilever.

Surface modification has wide ranging implications in lubrication, microelectromechanical systems (MEMS), colloidal systems and biological membranes. Surface modification plays an important role in stabilizing gold nanoparticles, which have applications in targeted drug delivery and catalysis. A variety of surface modification techniques are used for controlling corrosion and wettability, as well as used extensively to understand the nature of interactions between surfaces. This thesis is mainly focused on understanding the kinetics, film growth and surface modification by long chain molecules physisorbed on a surface. The time evolution of film growth and domain formation of octadecylamine on a mica surface is studied using ex-situ AFM and reflectance FTIR. A novel technique of interface creation is developed to measure the height of the adsorbed film. The results show three distinct regions of film growth mechanism. Region I, corresponds to thin film and the interface height is in the monolayer regime. The transient regime (II) consists of a sharp increase in the film thickness, from 1.5 nm to 25 nm within a time span of 180 s. In the final stage of film growth the film thickness is invariant with time, during which domain coarsening is observed. Domain evolution reveals a non-monotonic variation in the domain size as a function of adsorption time. A three stage mechanism is proposed to explain the domain evolution on the surface. In order to explain the observed film thickness variation, we have developed and tested various models to explain the thin to thick film transition observed in the AFM experiments. A model based on adsorption kinetics is solved to obtain the evolution of the adsorbed film. The model with a two-step adsorption isotherm quantitatively captures the thin to thick film transition observed in the AFM experiments. The statistical thermodynamics of adsorption of long chain molecules on a surface has been studied using a lattice model. The molecules are characterized by backbone chain, either lying parallel or perpendicular to the surface. A square lattice with nearest neighbour interactions and a mean field approximation are used to generate the adsorption isotherms for different molecules as a function of chain length. The molecules change their orientation from a surface parallel to an upright configuration with an increase in chemical potential. A similar transition (with time) in the molecular orientation has been observed in the AFM experiments. The transition between these two orientations is accompanied by an entropy maximum The last part of the thesis is concerned with carbon-carbon interactions. More specifically, we are interested in the interactions between graphite surfaces and their modification in the presence of a lubricant or base oil. Diamond like carbon (DLC) AFM tips and highly oriented pyrolitic graphite (HOPG) have been used for this study. Experiments were carried out by treating HOPG graphite in hexadecane oil at different temperatures. It is observed that pull-off forces on bare graphite are smaller when compared to the treated surface. The magnitude of the pull-off forces increases with the temperature of the hexadecane oil bath. Presence of charged patches responsible for the higher adhesion have been confirmed using surface potential microscopy. Results also confirm the presence of a thin liquid-like hexadecane film at room temperature.

Polymer brushes provide the responsive smart surfaces which can be used for fabrication of various devices. In this thesis work, adhesion, friction, and wear of polystyrene (PS) - poly(2-vinyl pyridine) (P2VP) and polystyrene - poly(acrylic acid) (PAA) binary brushes and corresponding monobrushes were investigated in dried state under controlled environment. Spin-coated films were also investigated for comparison. The aim was to explore possibilities to control/tune adhesion, friction, and wear between inorganic or polymeric surfaces by use of polymer brushes. Atomic force microscopy (AFM) with sharp silicon nitride tip and colloidal probes was employed to investigate the nanoscale adhesion and friction forces between different inorganic and polymeric surfaces. Adhesion and friction on the polymer brushes were comparable to that on the spin-coated films. Adhesion and friction force values were correlated, and were in accordance with the wettability of the brush surfaces for most of the samples. Switching in the adhesion and friction forces was observed for the PS+P2VP and PS+PAA binary brushes on treatment with selective solvents. Maximum switching in adhesion force and friction coefficient was by a factor of 2.7 and 5.4, respectively. Furthermore, switching of friction for mixed brush surface was observed during macroscale friction measurements using nanoindenter. Friction coefficients at macroscale were higher than those at the nanoscale. Moreover, adhesion and friction forces between the surfaces were significantly influenced by the humidity, grafting density of polymer brushes, chemical composition of top of the binary brush surface, and tip scan velocity. Nanowear studies were carried out with AFM using sharp silicon nitride tip while macrowear studies were carried out using nanoindenter. Nanowear on the surfaces was affected by molecular entanglements, adhesion and friction forces as well as shape and status of the tip. It was observed that the typical wear mode for PS brushes (treated with toluene) was ripple formation. In case of P2VP brushes (treated with ethanol) and PAA brushes (treated with pH 10 water), wear occurred via removal of the polymeric material. Wear mechanism observed for the monobrushes was similar to that observed for the spin-coated thick films of the same polymeric material. However, extent of the wear on the brush surfaces significantly differed from that on the spin-coated films. In case of PS+P2VP and PS+PAA binary brush samples, change in the wear mode was observed on treatment with the different selective solvents. On treatment with toluene (PS on the top), both of these binary brushes showed the wear by formation of the ripples. On the other hand, when these binary brushes were treated with selective solvent for P2VP or PAA, wear occurred mainly via removal of the polymeric material. The amount of wear increased with the number of scans for all the polymer brush samples. Moreover, wear on the polymer brush surfaces was also increased on increase in the applied load and decrease in the scan speed. Wear behavior on macroscale was averaged due to contact between surfaces at large number of asperities. Our results show that adhesion, friction, and wear of polymer surfaces can be controlled/tuned by the use of binary polymer brushes.

Neste trabalho foram utilizadas técnicas de modelagem molecular computacional para descrever nanossuperfícies funcionalizadas com biomoléculas do sistema imunológico correlacionando resultados experimentais obtidos com o microscópio de força atômica, simulações de dinâmica molecular e dinâmica molecular direcionada. O objetivo principal proposto é avaliar as forças intermoleculares provenientes das interações antígeno-anticorpo (funcionalizados em nanossuperfícies) para aplicação no desenvolvimento de nanoimunossensores e detecção de doenças desmielinizantes, como a Neuromielite Óptica. A Neuromielite Óptica é uma doença inflamatória autoimune na qual o próprio sistema imunológico reage contra os nervos ópticos e a medula espinhal, causando lesão desmielinizante. Estudos na literatura estabeleceramo anticorpo anti-aquaporina4 como um importante biomarcador da doença. Neste contexto, um nanoimmunosensorvem sendo desenvolvido com a técnica de Microscopia de Força Atômica, o qual visa detectar o anticorpoanti-aquaporina4 no soro de portadores da doença. Tal estudo necessitou de uma nova abordagem computacional para a descrição de estruturas tridimensionais de anticorpos. Essa nova aproximação consistiu na aplicação de técnicas de computacionais de modelagem e engenharia molecular para a geração de modelos de anticorpos com base em sucessivas substituições dos resíduos componentes do sítio de interação com o antígeno. Testes realizados envolvendo modelos de anticorpos disponíveis em bancos de dados especializadosindicaram (48 ± 18) % e (65 ± 14) % de identidade das cadeias leve e pesada, respectivamente, entre os modelos gerados computacionalmente e as estruturas 3D reais de anticorpos. Por fim, para comprovar o funcionamento dos nanoimunossensores, foi desenvolvido um modelo estatístico para tratar e interpretar os dados experimentais. Este modelo foi eficiente para distinguir os pacientes soropositivos de sujeitos soronegativos para determinados biomarcadores relacionados à Neuromielite Óptica e a Esclerose Múltipla, fornecendo assim um novo e mais preciso processopara diagnóstico de doenças desmielinizantes. / Study of the biological coverage of nanosurfaces by computational modeling: application in the development of nanoimunoresensors. In this work, computational molecular modeling techniques were applied to describe nanosurfaces functionalized with immune system biomolecules, correlating data from atomic force microscope experiments, molecular dynamics, and steered molecular dynamics simulations. The main goal of this research was to evaluate intermolecular forces involved in the antigen-antibody interaction on the nanosurfaces during the development of nanoimmunosensors for demyelinating diseases detection, especially neuromyelitisoptica. The neuromyelitisoptica is an autoimmune inflammation in which components of the immune system respond against optical nerves and spinal cord, resulting in demyelinating lesions. In the literature, studies have established anti-aquaporin 4 as an important biomarker for neuromyelitisoptica. Then, a nanoimmunosensor for anti-aquaporin 4 antibodies detection in neuromyelitisoptica patients serum via Atomic Force Microscopy is in development. This study requested a computational approach for describing the tridimensional structure of antibodies. The novel approach consisted of computer molecular modeling and engineering to perform successive substitutions in residues of the antigen interaction site. Tests carried out using antibody structures available in specialized data banks demonstrated the similarity of (48 ± 18) % and (65 ± 14) % for light and heavy chains, respectively, of the computationally generated models and experimental 3D structures of antibodies. Additionally, a statistical model was developed to prove the nanoimmunosensor sensing activity, which was useful to treat and interpret the experimental data. This statistical model was efficient to distinguish seropositive patients from seronegative subjects considering specific biomarkers related to neuromyelitisoptica and multiple sclerosis, providing a novel and more precise process for demyelinating disease diagnosis.

"An apparatus for the delivery of radiation to the tip-sample interface of an Atomic Force Microscope (AFM) is demonstrated. The Pulsed Light Delivery System (PLDS) was fabricated to probe photoinduced conformational changes of molecules using an AFM. The PLDS is 67 mm long, 59 mm wide, and 21 mm high, leaving clearance to mount the PLDS and a microscope slide coated with a thin film of photoactive molecules beneath the cantilever tip of a stand-alone AFM. The PLDS is coupled into a fiber pigtailed Nd:Yag frequency doubled laser, operating at a wavelength of 532 nm. The radiation delivered to a sample through the PLDS can be configured for continuous or pulsed mode. The maximum continuous wave (CW) power delivered was 0.903 mW and the minimum pulse width was 12.3 ms (maximal 401 ms), corresponding to a minimal energy of 0.150 nJ (maximal 362 nJ), and had a cycle duration of 10.0 ms. The PLDS consists of micro-optical components 3.0 mm and smaller in diameter. The optical design was inspired by the three-beam pick-up method used in CD players, which could provide a method to focus the pulse of light onto the sample layer. In addition, the system can be easily modified for different operational parameters (pulse width, wavelength, and power). As proof that the prototype design works, we observed a photoinduced ‘bimetallic’ bending of the cantilever, as evidenced by observing no photoinduced bending when a reflective-coated cantilever was replaced by an uncoated cantilever. Using the apparatus will allow investigation of many different types of molecules exhibiting photoinduced isomerization."

The Atomic Force Microscope (AFM) is an instrument that is capable of measuring intermolecular forces between single molecules. Multi-Parameter Force Spectroscopy (MPFS) is a technique that uses the AFM. MPFS enables the acquisition of force curves and thermal resonance of the system under investigation. This technique can shed light on the mechanical behavior at the molecular level. Improvements described herein have enhanced the sensitivity of MPFS over background noise. This investigation focuses on the mechanical and interfacial properties of three carbon nanostructures: long nanotubes, nanocoils, and nanoloops. Different types of adhesion are encountered, measured and discussed: friction, rupture, and peeling. The elastic modulus of long carbon nanotubes is calculated from frequency shifts when the system is put into tension. An elastica model is applied to the post-buckled carbon nanotubes, which enables the estimation of the static coefficient of friction on chemically modified surfaces. The compression of a nanocoil at large contact angles reveals that changes in oscillation amplitude do not occur from damping, but from adding stiffness into the systems measured herein. This result is counter to the assumptions of dynamic force spectroscopy. Finally, carbon nanoloops are brought into and out of contact with several different surfaces. The force curve and frequency response of the system shows the difference between rupture and peeling. The results presented herein lead to a better understanding of the mechanical and tribological properties of the carbon nanostructures.

No description available.

This dissertation describes the development of temperature and pH-responsive interfaces, where the emphasis is placed on tuning the responsivities within a physiological temperature range. This tuning is achieved through the utilization of polymeric building blocks, where each component is specifically synthesized to have a unique responsivity. The assembly of these components onto surfaces permits the fabrication of stimuli-responsive interfaces. In addition, this dissertation explores the use of a self-assembling peptide as a modular building block to modify the interface of hydrogel microparticles, resulting in the formation of a new biosynthetic construct. Hydrogels are three-dimensional, crosslinked polymer networks that swell in water. Over the years, hydrogels have been extensively explored as biomaterials due to their high water content, tunable mechanics, and chemical versatility. Two areas where hydrogels have received considerable interest are drug delivery and extracellular matrices. Unfortunately, developing structurally and functionally complex hydrogels from the top down is challenging because many parameters cannot be independently tuned in a bulk material. An alternative route would be to develop a library of building blocks, where each is tailored for a given function, and assemble these components into composite structures. The building block synthesized and utilized in this dissertation is a microgel. Microgels are a colloidal dispersion of hydrogel microparticles, ranging in size from 100 to 1000 nm in diameter. The microgels were prepared from environmentally responsive polymers, sensitive to both temperature and pH. Microgels have been used in the fabrication of polyelectrolyte layer-by-layer films, where the microgel serves as the polyanion and a linear polycation is used to “stitch” the particles together. In Chapters 3 and 4, stimuli-responsive interfaces are prepared from environmentally responsive microgel building blocks. In particular, Chapter 3 demonstrates tuning of the film response temperature by preparing several different microgels with differing ratios of two thermoresponsive polymers. Chapter 4 evaluates the influence of the pH environment on the thermoresponsivity of microgel films. While the pH environment was found to substantially affect some films, it is possible to prepare microgel films that behave independently of pH. The swelling/de-swelling of the films was also investigated by atomic force microscopy (AFM) as a function of both pH and temperature. It was determined that the AFM imaging parameters can drastically affect the measured film thicknesses (Appendix A) due to the soft, deformable nature of microgel films. The studies in these chapters illustrate the advantages of preparing composite structures from discrete components, where the functionality of the composite is dictated by the constituent particles. In Chapter 5, attention is placed on modifying the surface of microgel particles. Many of the traditional routes used to modify microgels involve the incorporation of co-monomers into the network or the addition of polymer shells. However, a new core/shell construct is presented, where a microgel core is coated with a self-assembling peptide shell. In this scenario, the peptide shell serves as a modular scaffold, where surface-localized functional groups can participate in reactions. Although there are still a number of questions remaining in regard to the assembly process and stability of the construct, initial experiments suggests that this is an interesting and promising structure to study. Finally, a discussion of future directions and possible experiments is provided in Chapter 6. Hopefully, this will serve as a guide for further exploration of the research presented herein. Microgels remain a rich class of materials to study and employ. While their synthesis is rather straightforward, their use often results in complex behavior and interesting phenomena. Understanding their behavior is a crucial step in realizing their full potential.

<p>Virtually all biologically relevant functions and processes are mediated by non-covalent, molecular recognition events, demonstrating astonishingly diverse affinities and specificities. Despite extensive research, the origin of affinity and specificity in aqueous solution - specifically the relationship between ligand binding thermodynamics and structure - remains remarkably obscure and is further complicated in the context of multivalent interactions. Multivalency describes the combinatorial interaction of multiple discrete epitopes across multiple binding surfaces where the association is considered as the sum of contributions from each epitope and the consequences of multivalent ligand assembly. Gaining the insight necessary to predictably influence biological processes with novel therapeutics begins with an understanding of the molecular basis of solution-phase interactions, and the thermodynamic parameters that follow from those interactions. Here we continue our efforts to understand the basis of aqueous affinity and the nature of multivalent additivity.</p><p>Multivalent additivity is the foundation of fragment-based drug discovery, where small, low affinity ligands are covalently assembled into a single high affinity inhibitor. Such systems are ideally suited for investigating the thermodynamic consequences of multivalent ligand assembly. In the first part of this work, we report the design and synthesis of a fragment-based ligand series for the Grb2-SH2 protein and thermodynamic evaluation of the low affinity ligand fragments compared to the intact, high affinity inhibitor by single and double displacement isothermal titration calorimetry (ITC). Interestingly, our investigations reveal positively cooperative multivalent additivity - a binding free energy of the full ligand greater than the sum of its constituent fragments - that is largely enthalpic in origin. These results contradict the most common theory of multivalent affinity enhancement arising from a "savings" in translational and rotational entropy. The Grb2-SH2 system reported here is the third distinct molecular system in which we have observed enthalpically driven multivalent enhancement of affinity.</p><p>Previous research by our group into similar multivalent affinity enhancements in protein-carbohydrate systems - the so-called "cluster glycoside effect" - revealed that evaluation of multivalent interactions in the solution-phase is not straightforward due to the accessibility of two disparate binding motifs: intramolecular, chelate-type binding and intermolecular, aggregative binding. Although a number of powerful techniques for evaluation of solution-phase multivalent interactions have been reported, these bulk techniques are often unable to differentiate between binding modes, obscuring thermodynamic interpretation. In the second part of this work, we report a competitive equilibrium approach to Molecular Recognition Force Microscopy (MRFM) for evaluation of ligand binding at the single-molecule level with potential to preclude aggregative associations. We have optimized surface functionalization strategies and MRFM experimental protocols to evaluate the binding constant of surface- and tip-immobilized single stranded DNA epitopes. Surprisingly, the monovalent affinity of an immobilized species is in remarkable agreement with the solution-phase affinity, suggesting the competitive equilibrium MRFM approach presents a unique opportunity to investigate the nature of multivalent additivity at the single molecule level.</p> / Dissertation