Electrical conductivity of carbon nanofiber reinforced resins: potentiality of Tunneling Atomic Force Microscopy (TUNA) technique

Epoxy nanocomposites able to meet pressing industrial requirements in the field of structural material have been developed and characterized. Tunneling Atomic Force Microscopy (TUNA), which is able to detect ultra-low currents ranging from 80 fA to 120 pA, was used to correlate the local topography with electrical properties of tetraglycidyl methylene dianiline (TGMDA) epoxy nanocomposites at low concentration of carbon nanofibers (CNFs) ranging from 0.05% up to 2% by wt. The results show the unique capability of TUNA technique in identifying conductive pathways in CNF/resins even without modifying the morphology with usual treatments employed to create electrical contacts to the ground

Intentional nonlinearity in the small scale with applications to atomic force microscopy (AFM) and mass sensing

Over the past several decades, the development of ultra-sensitive nano/micromechanical sensor technology has had a transformative effect on the field of nanoscience. These devices are currently used in many different applications including biological, chemical and inertial sensing; atomic force microscopy and infrared spectroscopy; and precise time keeping and synchronization. Traditionally, these systems were studied within the framework of linear dynamics, and incidental nonlinearity was suppressed by design. More recently, researchers have intentionally incorporated nonlinearity in the design of such devices in order to exploit the rich nonlinear behavior. Some of the nonlinear phenomena that researchers aim to utilize include internal resonance, resonant bandwidth expansion, ultra-sensitive bifurcation frequencies associated with sudden jumps in the response, coexistence of multiple solution branches and higher harmonic generation. In this dissertation, I investigate further ways in which intentional nonlinearity can be leveraged to enhance micromechanical resonant sensing techniques. In particular, I focus on applications to AFM and mass sensing. Within the area of AFM, the performance of a new cantilever design during multi-frequency tapping mode AFM is studied. The system consists of a base cantilever with an inner paddle which, under harmonic excitation, vibrates like a system of linearly coupled oscillators engaging simultaneously a lower, in-phase and a higher, out-of phase resonant mode. The cantilever is designed so that the 2nd mode frequency (i.e., the out-of-phase eigenfrequency) coincides with an integer multiple of the fundamental mode frequency, providing the necessary conditions for realization of internal resonance. During tapping mode, the nonlinear tip-sample force activates the internal resonance and thereby amplifies the out-of-phase resonant mode. In contrast to other multi-frequency AFM techniques, the advantage of this approach is that multiple harmonics with strong signal-to-noise ratios (SNR) are excited while maintaining the simplicity of a single excitation frequency. The ability of this inner-paddled cantilever to measure compositional properties of polymers and bacteria was studied, and it was found that the internal resonance-based design results in enhanced sensitivity to Young’s modulus. In another study, a new micromechanical resonant mass sensor design is introduced consisting of a doubly clamped beam having a concentrated mass at its center, subjected to harmonic base excitation. The resonator is specifically designed to exhibit geometric nonlinearity due to midplane stretching. The reduced order model of the system’s fundamental bending mode is that of a Duffing oscillator (i.e., an oscillator having cubic stiffness in addition to linear stiffness) under harmonic base excitation. For positive cubic stiffness, it is well known that the Duffing oscillator exhibits hardening in the frequency response curve resulting in a broadband resonance. The bandwidth of the resonator is determined by the linear resonant frequency (lower bound) and the jump-down bifurcation frequency (upper bound). Under harmonic excitation at a fixed forcing level, the jump down bifurcation frequency is proportional to the forcing level, and at each forcing level there indeed exists a jump down bifurcation. In the proposed system, the forcing level is not fixed; rather, it is proportional to the square of the driving frequency of the base excitation. Interestingly, analytical and computational analyses predict the existence of a critical excitation amplitude above which there is no theoretically predicted jump down bifurcation. It is shown that the effect of the concentrated mass is to lower the threshold of the critical excitation amplitude to a realizable level. In practice, there must inevitably be a jump down bifurcation and this bifurcation may be triggered by the excitation of internal resonances, shrinking domain of attraction of the upper solution branch, variations in the initial state due to noise and/or the presence of nonlinear damping. However, the critical excitation amplitude appears to correspond to sudden and significant bandwidth expansion. Experimental results from a Duffing-like oscillator provide some verification of the powerful theoretical predictions. Ultimately, by operating at an excitation amplitude above the critical level, the ultra-wide resonant bandwidth can be exploited in a mass detection scheme based on amplitude tracking. In comparison to other micromechanical mass sensors, this technique and design offers a wide range of operational frequencies and amplitudes with strong SNR, eliminates the need for frequency sweeping and sophisticated feedback control, and requires relatively simple actuation and microfabrication methods

Force sensing with nanowires

‘Bottom-up’ fabricated nano-resonators have emerged as particularly promising mechanical transducers, over the last decade. In fact, the exceptional force sensitivity exhibited by such nearly defect-free structures, is ensured by their small motional mass, low dissipation and high resonant frequency. In this dissertation, we aim to explore the potential of as-grown nanowires (NWs) as scanning force sensors. Their singly clamped structure makes them suitable to scan over a sample in the pendulum geometry, enabling the measurement of very weak lateral force gradients. Furthermore, by virtue of slight cross-sectional asymmetries, the flexural modes of a NW are split into doublets oscillating along two orthogonal directions. These characteristics enable the peculiar vectorial sensing nature of such devices. We developed a custom built scanning probe microscope, operating at cryogenic temperatures in a liquid helium bath cryostat. The microscope features an integrated fiber-based interferometer setup for the optical detection of individual NWs' motion. To demonstrate their vectorial scanning capabilities, we scan them over a sample with gold patterned electrodes. By monitoring the frequency shift and direction of oscillation of both fundamental modes, we construct a map of all spatial tip–sample force derivatives in the plane. Moreover, using an electric field to resonantly drive the mechanical modes, we are able to spatially probe forces of distinct origins, arising from the NW's residual charge and its polarizability, respectively. In addition, we show quantitative control over the coupling between two orthogonal mechanical modes, obtained by measuring avoided crossings as a function of position and applied electric field, which allowed to record Rabi oscillations between the two modes in the strong-coupling regime. In general, such universally applicable scanning technique enables a form of atomic force microscopy particularly suited to mapping the size and direction of weak tip-sample forces. NWs produced by molecular beam epitaxy also offer the possibility of ‘in-situ’ functionalization of the mechanical resonator during the growth process. In particular, we studied a scanning magnetic force sensor based on an individual magnet-tipped GaAs NW. Its magnetic tip consists of a final segment of single-crystal MnAs formed by sequential crystallization of the liquid Ga catalyst droplet. We characterize the mechanical and magnetic properties of such NWs by measuring their flexural mechanical response in an applied magnetic field. Taking advantage of the excellent force sensitivity, the magnetic properties of such tips are studied via dynamic torque magnetometry and precisely fitted by micro-magnetic simulations, showing vortex and dipole-like configurations. To determine a NW’s performance as a magnetic scanning probe, we measure its response to the field profile produced by a current-carrying micro-wire, characterizing its behavior as current sensor and its high sensitivity. The ability of a NW sensor to map all in-plane spatial force derivatives can provide fine detail of stray field profiles above magnetic and current carrying samples, in turn revealing information on the underlying phenomena and anisotropies. Directional measurements of dissipation may also prove useful for visualizing domain walls and other regions of inhomogeneous magnetization

A high resolution microscopy study of biological components for the incorporation in opto-electronic hybrid devices.

Optical microscopy and scanning probe microscopy techniques have been utilised to acquire high resolution topography and fluorescence images of several biological samples. Applying these techniques to patterned samples and single molecules allow the optical properties of a sample to be investigated near to and below the diffraction limit, allowing emission properties to be correlated with those of topography. Optically active biological samples outside of their cellular environment are prone to photo-degredation and in measuring them a challenge is to ensure that optical measurements can be made before the onset of damage to the fluorophore. In this study two forms of fluorescence microscope have been utilised with scanning probe techniques of AFM and SNOM. These techniques have been used alongside microcontact printed arrays of fluorescent proteins and photosynthetic light harvesting complexes to address the accuracy of the printing technique and it's applicablity to biological components for future bionanotechnological applications. Furthermore, the periodicity associated with the arrays has been applied to the techniques to address the relative resolutions of the microscopes as well as the samples being a drive behind implimenting biologically friendly components/techniques to the microscopes (such as liquid cells). Larger structures from photosynthetic bacteria have also been addressed in this study in the form of chlorosomes which are model structures for light harvesting in low light conditions. Studies on the spectral properties of populations of 3 species have been conducted in this work with fluorescence microscopy and it has been shown that populations show small local variations in fluorescence. Furthermore it has been shown that the developed scanning fluorescence technique can be applied to photo senstitive samples successfully with only a small number of cases where spectral properties were affected by the measurement technique. Using high resolution microscopy techniques this research shows the surface patterning techniques in conjunction with biological samples to have mixed success depending on the sample. It also shows spectral measurements on newly discovered chlorosomes with little photo degredation. It further shows the role that the microscopy techniques have in analysing biological systems in different configurations on substrates

Investigación mediante AFM de estructuras onduladas en la fricción, el desgaste y la adhesión en la nanoescala

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 24-03-2017Esta tesis tiene embargado el acceso al texto completo hasta el 24-09-201

INTERFACIAL PROPERTIES OF IONIC LIQUIDS:ELECTRIC PROPERTIES OF THIN FILMS AND INTERACTION WITH MODEL MEMBRANES AND LIVING CELLS

Room-Temperature Ionic Liquids (ILs) have attracted considerable interest in recent years. This interest is motivated by the physico-chemical properties of these systems, tunable modifying the chemical structure of ions. Generally, ILs show chemical and thermal stability, i.e. they do not easily decompose or react. Furthermore, these compounds remain liquid over an extended range of temperatures, in which they show also a remarkably low volatility. The low vapor pressure of ILs, promote them as good solvents for the growing field of the \u201dGreen Chemistry\u201d, in substitution of the volatile organic compounds. The fact that ILs are composed solely by ions, and can have a quite wide electrochemical window, make them very interesting as electrolytes. For these purposes, this PhD thesis is devoted to the investigation of ILs in contact with solid interfaces, primary targets of interaction. To deepen the analysis of electric properties at the solid interface, thin layers of ILs deposited on conductive substrates were investigated by means of AFM. The \u201dGreen\u201d character of these compounds was investigated studying their interaction with biomembrane models and external membranes of living cells, by means of AFM and electrochemical methods. Because of their ionic nature, ILs can be used as electrolytes in several devices aimed at conversion and storage of energy, such as electrochemical supercapacitors, Graetzel solar cells and batteries. In these devices a key role is played by the interface between the surface of the electrodes and the electrolyte; in particular, structural-morphological and electrical properties of the first few nanometers of IL interacting with the solid electrode surface are expected to have the strongest impact on device performance. AFM morphological analysis of small quantity of [C 4 MIM] [NTf 2 ] IL, deposited on various insulating surfaces revealed a population of nanodroplets and new structures. Remarkably, the solid surfaces induce the organization of the ionic liquid into regular, lamellar solid-like nanostructures presenting a high degree of vertical structural order and high mechanical resistance to normal compressive stresses. Nanomechanical investigation reveals that the structures resist to normal compressive loads up to 1.5 MPa; beyond that limit, indentation, in discrete steps, occurs. Furthermore, lamellar [C 4 MIM] [NTf 2 ] islands are not affected when scanned by a biased AFM tip under the influence of an electric field as intense as 10 8 V/m, while the liquid nano-and micro-droplets are easily swept away. These results confirm the solid-like character of the ordered lamellar nanostructures observed when thin films of [C 4 MIM] [NTf 2 ] are deposited on solid surfaces, and suggest that these films may possess an insulating, dielectric behavior, at odd with the case of the bulk ionic liquids. Nanoscale impedance measurements (capacitance vs. distance) and electrostatic force spectroscopy (electric force vs. distance) between a conductive AFM tip and the IL structures confirmed that values of the dielectric constant (\u3b5 r = 3-5) are significantly smaller than those measured in the bulk liquid (\u3b5 r = 9-15). These results support the picture of solid-like ordered domains where the ion mobility is significantly inhibited with respect to the bulk liquid phase. These findings also highlight the potentialities of scanning probe techniques for the quantitative investigation of the interfacial electrical properties of thin ionic liquid films, suggesting that ILs at electrified solid surfaces may possess unexpected electrical and structural properties, thus influencing the behavior of photo-electrochemical devices. The \u201dgreen\u201d character of ionic liquids (ILs) is dependent on their negligible vapor pressure but in contrast to their environmental behavior their intrinsic toxicity is not at present completely understood. Accordingly, although ILs will not evaporate which alleviates air pollution problems, a potential hazard of Ils to living organisms via aqueous media cannot be ruled out. A rigorous investigation on the interaction of ILs with biomaterials is required to provide information about their intrinsic toxicity. In order to test the biocompatible character of ILs, as a first objective, the interaction of various ILs with biological membrane (biomembrane) models was studied using electrochemical methods. A series of imidazolium based ILs were investigated whose interactions highlighted the role of anion and lateral side chain of cation during the interaction with dioleoyl phosphatidylcholine (DOPC) monolayer. It was shown that the hydrophobic and lipophilic character of the IL cations is a primary factor responsible for this interaction. The modifications of the Hg supported monolayer caused by ILs were simultaneously monitored electrochemically in a well controlled manner using rapid cyclic voltammetry (RCV), alternating current voltammetry (ACV), and electrochemical impedance spectroscopy (EIS). Hg supported monolayers provide an accurate analysis of the behavior of ILs at the interface of a biomembrane leading to a comprehensive understanding of the interaction mechanisms involved. At the same time, these experiments show that the Hg-phospholipid model is an effective toxicity sensing technique as shown by the correlation between literature in vivo toxicity data and the data from this study. Cell membrane is the main target of ILs interaction, depending on the lipophilicity of hydrophobic lateral chain of cation. In order to test the biocompatible character of ILs, the interaction of various imidazolium-based ILs with supported DOPC phospholipid bilayers (as models of the cell membrane) and living MDA-MB-231 cells (@37 \u25e6C) was investigated. Atomic Force Microscopy (AFM) was used to carry on a combined topographic and mechanical analysis of supported DOPC bilayers as well as of living cells. During the analysis of DOPC bilayers we have observed modifications in breakthrough force and membrane elasticity related to the ingress of lateral chains of cations in the bilayer, demonstrating agreement with electrochemical results. The parallel nanomechanical analysis performed on living cells interacting with ILs at various concentrations showed modifications of elasticity (effective Young\u2019s modulus) and morphology of cells after exposure to ILs dispersed in their culture medium. The measurements confirmed the primary action of ILs on membrane and actin cytoskeleton, highlighting a subtoxic/toxic effect dependent on ILs concentration and chemical nature of cation. Our results may be helpful for filling existing gaps of knowledge about ionic liquids toxicity and their impact on living organisms. From these evidences, interaction of ILs with micro-organisms and single cells is an important step to assess the environmental sustainability of this novel and promising class of solvents and to attribute a \u201dgreen\u201d label to it. Studying the interaction of ionic liquids, it has been recognized that the interface is a vital component. When the bulk symmetry of IL is broken by surfaces, the electrical properties are greatly affected, leading from a ion conductor to an insulator behavior. Also the interaction with biological entity is driven, in first instance, by surface interaction. Biomembrane models and cell membranes are affected by ILs that accumulate/penetrate the surface interface, leading to structural reorganization/damage of external membrane

Enzymatic degradation of the cornea to develop an experimental model for keratoconus: Biomechanical and optical characterisation

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. It is composed of five layers, in which the stroma is the thickest layer (approximately 90% of corneal thickness) that consists mainly of laminated collagen fibrils associated with proteoglycans. The cornea acts as the eye’s outermost lens that is accounted for approximately two-thirds of the eye's total optical power. Like other lenses, the cornea’s geometrical characteristics, such as the curvature, are important to maintain its functions for clear and stable vision. These geometrical characteristics are highly affected by the biomechanical properties of the cornea. For example, in keratoconus, the cornea is characterised by a progressive and localised thinning in corneal thickness, which is associated with a reduction in stiffness and other biomechanical properties. These alterations happen at the collagenous network, which is mainly responsible for the biomechanical features of the cornea, and are mostly attributed to genetic factors and abnormal enzymatic activity. Histological and biochemical studies suggested the role of amylase and collagenase activities in degradation of collagenous network and progression of keratoconus. However, the role of amylase and collagenase on biomechanical and optical properties have not been investigated. In this study, in vitro enzymatic degradation of porcine corneas was conducted with varying concentrations of α-amylase and collagenase (crude and purified) enzymes for different incubation periods. Several techniques, including atomic force microscopy, nanoindentation and optical coherence tomography, were utilised to assess the effect of the enzymes on biomechanical of corneal tissue at macroscale, microscale and nanoscale levels. Corneal transparency and absorption following enzymatic incubation were also measured using spectrophotometry. The biomechanical techniques that were utilised indicated that amylase and collagenase decrease corneal stiffness and thickness following incubation the corneas with amylase and collagenase. Further reduction in biomechanical properties and thickness of the corneas was found with increased enzymes concentrations and incubation periods. Corneal transparency was increased following incubation with the enzymes. The results suggest depletion of proteoglycans by amylase and digestion of collagen fibrils by collagenase. These results were used to propose an animal biomechanical model for keratoconus