Ali Passian

Imaging nanoparticles in cells by nanomechanical holography

Nanomaterials have potential medical applications, for example in the area of drug delivery, and their possible adverse effects and cytotoxicity are curently receiving attention. Inhalation of nanoparticles is of great concern, because nanoparticles can be easily aerosolized. Imaging techniques that can visualize local populations of nanoparticles at nanometre resolution within the structures of cells are therefore important. Here we show that cells obtained from mice exposed to single-walled carbon nanohorns can be probed using a scanning probe microscopy technique called scanning near field ultrasonic holography. The nanohorns were observed inside the cells, and this was further confirmed using micro Raman spectroscopy. Scanning near field ultrasonic holography is a useful technique for probing the interactions of engineered nanomaterials in biological systems, which will greatly benefit areas in drug delivery and nanotoxicology.

New modes for subsurface atomic force microscopy through nanomechanical coupling

Non-destructive, nanoscale characterization techniques are needed to understand both synthetic and biological materials. The atomic force microscope uses a force-sensing cantilever with a sharp tip to measure the topography and other properties of surfaces. As the tip is scanned over the surface it experiences attractive and repulsive forces that depend on the chemical and mechanical properties of the sample. Here we show that an atomic force microscope can obtain a range of surface and subsurface information by making use of the nonlinear nanomechanical coupling between the probe and the sample. This technique, which is called mode-synthesizing atomic force microscopy, relies on multi-harmonic forcing of the sample and the probe. A rich spectrum of first- and higher-order couplings is discovered, providing a multitude of new operational modes for force microscopy, and the capabilities of the technique are demonstrated by examining nanofabricated samples and plant cells.

Measurement of Mechanical Properties of Cantilever Shaped Materials

Microcantilevers were first introduced as imaging probes in Atomic Force Microscopy (AFM) due to their extremely high sensitivity in measuring surface forces. The versatility of these probes, however, allows the sensing and measurement of a host of mechanical properties of various materials. Sensor parameters such as resonance frequency, quality factor, amplitude of vibration and bending due to a differential stress can all be simultaneously determined for a cantilever. When measuring the mechanical properties of materials, identifying and discerning the most influential parameters responsible for the observed changes in the cantilever response are important. We will, therefore, discuss the effects of various force fields such as those induced by mass loading, residual stress, internal friction of the material, and other changes in the mechanical properties of the microcantilevers. Methods to measure variations in temperature, pressure, or molecular adsorption of water molecules are also discussed. Often these effects occur simultaneously, increasing the number of parameters that need to be concurrently measured to ensure the reliability of the sensors. We therefore systematically investigate the geometric and environmental effects on cantilever measurements including the chemical nature of the underlying interactions. To address the geometric effects we have considered cantilevers with a rectangular or circular cross section. The chemical nature is addressed by using cantilevers fabricated with metals and/or dielectrics. Selective chemical etching, swelling or changes in Youngs modulus of the surface were investigated by means of polymeric and inorganic coatings. Finally to address the effect of the environment in which the cantilever operates, the Knudsen number was determined to characterize the molecule-cantilever collisions. Also bimaterial cantilevers with high thermal sensitivity were used to discern the effect of temperature variations. When appropriate, we use continuum mechanics, which is justified according to the ratio between the cantilever thickness and the grain size of the materials. We will also address other potential applications such as the ageing process of nuclear materials, building materials, and optical fibers, which can be investigated by monitoring their mechanical changes with time. In summary, by virtue of the dynamic response of a miniaturized cantilever shaped material, we present useful measurements of the associated elastic properties.

Critical Issues in Sensor Science To Aid Food and Water Safety

The stability of food and water supplies is widely recognized as a global issue of fundamental importance. Sensor development for food and water safety by nonconventional assays continues to overcome technological challenges. The delicate balance between attaining adequate limits of detection, chemical fingerprinting of the target species, dealing with the complex food matrix, and operating in difficult environments are still the focus of current efforts. While the traditional pursuit of robust recognition methods remains important, emerging engineered nanomaterials and nanotechnology promise better sensor performance but also bring about new challenges. Both advanced receptor-based sensors and emerging non-receptor-based physical sensors are evaluated for their critical challenges toward out-of-laboratory applications.

Microfluidic manipulation via Marangoni forces

A convective flow system is engendered when two liquid droplets, or a liquid droplet and a solid surface, are maintained at different temperatures. Such flows give rise to Marangoni forces which under proper conditions prevent droplet coalescence, cause fluid motion, and dewetting. We present a study of adsorbed and applied fluid movement on a solid surface driven by surface tension gradients created by thermal gradients. Flexible control over the silicone oil and 1,3,5-trinitrotoluene movement is accomplished with an array of individually controllable gold thin film thermal elements on a fused silica substrate surface. We thus demonstrate unlimited fluid movements in one dimension.

Optical modulation processes in thin films based on thermal effects of surface plasmons

Experimental results are presented for light-on-light modulation at low rates using coupling to nonradiative surface plasmons and their associated thermal effects in a thin gold foil. It is first shown that several modulated Gaussian beams simultaneously exciting surface plasmons in the same region of a thin gold film, will result in a coupling that is revealed in the reflected beams. The observed effects result in the reflected beams undergoing changes in both spatial distribution and intensity levels. A brief study is then presented of the coupling between surface plasmons and an electrical current in the excitation region to further support the role of the surface plasmon induced thermal processes in the gold foil.

Elastic phase response of silica nanoparticles buried in soft matter

Tracking the uptake of nanomaterials by living cells is an important component in assessing both potential toxicity and in designing future materials for use in vivo. We show that the difference in the local elasticity at the site of silica (SiO2) nanoparticles confined within a macrophage enables functional ultrasonic interactions. By elastically exciting the cell, a phase perturbation caused by the buried SiO2 nanoparticles was detected and used to map the subsurface populations of nanoparticles. Localization and mapping of stiff chemically synthesized silica nanoparticles within the cellular structures of a macrophage are important in basic as well as applied studies.

Gold island fiber optic sensor for refractive index sensing........

Abstract A fiber optic chemical sensor based on gold island surface plasmon excitation is presented. The sensing part of the fiber is the end of the fiber onto which a thin layer of gold has been deposited to form a particulate surface. Annealing the gold reshapes the particles and produces an optical absorbance near 535 nm with the fiber in air. The optical absorption resonance of the gold particles is shifted if the fiber is immersed in a medium other than air. These resonance shifts are examined by transmission spectroscopy through the fiber. Experimental results for the sensitivity and dynamic range in the measurement of liquid solutions are in agreement with a basic theoretical model that characterizes the surface plasmon using nonretarded electrodynamics.

Fiber optic sensor based on gold island plasmon resonance

Abstract A fiber optic chemical sensor based on gold-island surface plasmon excitation is presented. The sensing part of the fiber is a one inch portion on which cladding has been removed and onto which a thin layer of gold has been deposited to form a particulate surface. Annealing the gold reshapes the particles and produces an absorbance near 535 nm when the only medium residing outside the surface is air. A range of wavelengths provided by a white light source and monochromator is launched through the optical fiber. The transmitted spectra display shifts in the resonance absorption due to any changes in the medium surrounding, or adsorbed onto the fiber. Experimental results for the sensitivity and dynamic range in the measurement of liquid solutions are in agreement with a basic theoretical model which characterizes the surface plasmon using nonretarded electrodynamics. Furthermore, the model assumes the particles are isolated oblate spheroids with a distribution of eccentricities.

Environment effects on surface-plasmon spectra in gold-island films potential for sensing applications

The effects of the local dielectric environment on the surface-plasmon resonances of annealed gold-island films as a potential for sensing applications are studied experimentally and modeled theoretically. Gold-island films were annealed at 600 degrees C to produce spheroidal shape particles that exhibit well-resolved resonances in polarized, angle-resolved, absorption spectra. These resonances are shifted in different amounts by the depolarization effect of the surrounding medium (liquids with various refraction indices). Cross-section calculations based on nonretarded, single-particle, dielectric interaction for these various configurations are presented and are found to be in good agreement with the experimental observations. The results show an interesting potential for biosensing or environmental monitoring applications.