Eric Finot

Imaging of the Surface of Living Cells by Low-Force Contact-Mode Atomic Force Microscopy

The membrane surface of living CV-1 kidney cells in culture was imaged by contact-mode atomic force microscopy using scanning forces in the piconewton range. A simple procedure was developed for imaging of the cell surface with forces as low as 20-50 pN, i.e., two orders of magnitude below those commonly used for cell imaging. Under these conditions, the indentation of the cells by the tip could be reduced to less than l0 nm, even at the cell center, which gave access to the topographic image of the cell surface. This surface appeared heterogeneous with very few villosities and revealed, only in distinct areas, the submembrane cytoskeleton. At intermediate magnifications, corresponding to 20-5 microm scan sizes, the surface topography likely reflected the organization of submembrane and intracellular structures on which the plasma membrane lay. By decreasing the scan size, a lateral resolution better than 20 nm was routinely obtained for the cell surface, and a lateral resolution better than 10 nm was obtained occasionally. The cell surface appeared granular, with packed particles, likely corresponding to proteins or protein-lipid complexes, between approximately 5 and 30 nm xy size.

Effects of temperature and pressure on microcantilever resonance response

The variation in resonance response of microcantilevers was investigated as a function of pressure (10(-2)-10(6)Pa) and temperature (290-390K) in atmospheres of helium (He) and dry nitrogen (N(2)). Our results for a silicon cantilever under vacuum show that the frequency varies in direct proportion to the temperature. The linear response is explained by the decrease in Youngs modulus with increasing the temperature. However, when the cantilever is bimaterial, the response is nonlinear due to differential thermal expansion. Resonance response as a function of pressure shows three different regions, which correspond to molecular flow regime, transition regime, and viscous regime. The deflection in flow transition regime resulting from thermal variation has minimal effect on frequency. The frequency variation of the cantilever is caused mainly by changes in the mean free path of gas molecules.

Tapping-mode atomic force microscopy on intact cells: optimal adjustment of tapping conditions by using the deflection signal.

Difficulties in the proper adjustment of the scanning parameters are often encountered when using tapping-mode atomic force microscopy (TMAFM) for imaging thick and soft material, and particularly living cells, in aqueous buffer. A simple procedure that drastically enhances the successful imaging of the surface of intact cells by TMAFM is described. It is based on the observation, in liquid, of a deflection signal, concomitant with the damping of the amplitude that can be followed by amplitude-distance curves. For intact cells, the evolution of the deflection signal, steeper than the amplitude damping allows a precise adjustment of the feedback value. Besides its use in finding the appropriate tapping conditions, the deflection signal provides images of living cells that essentially reveal the organization of the membrane cytoskeleton. This allows to show that changes in the membrane surface topography are associated with a reorganization of the membrane skeleton. Studies on the relationships between the cell surface topography and membrane skeleton organization in living cells open a new field of applications for the atomic force microscope.

Tuning of an Optical Dimer Nanoantenna by Electrically Controlling Its Load Impedance

Optical antennas are elementary units used to direct optical radiation to the nanoscale. Here we demonstrate an active control over individual antenna performances by an external electrical trigger. We find that by an in-plane command of an anisotropic load medium, the electromagnetic interaction between individual elements constituting an optical antenna can be controlled, resulting in a strong polarization and tuning response. An active command of the antenna is a prerequisite for directing light wave through the utilization of such a device.

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.

Trace Explosive Detection using Photothermal Deflection Spectroscopy

Satisfying the conditions of high sensitivity and high selectivity using portable sensors that are also reversible is a challenge. Miniature sensors such as microcantilevers offer high sensitivity but suffer from poor selectivity due to the lack of sufficiently selective receptors. Although many of the mass deployable spectroscopic techniques provide high selectivity, they do not have high sensitivity. Here, we show that this challenge can be overcome by combining photothermal spectroscopy on a bimaterial microcantilever with the mass induced change in the cantilever’s resonance frequency. Detection using adsorption-induced resonant frequency shift together with photothermal deflection spectroscopy shows extremely high selectivity with a subnanogram limit of detection for vapor phase adsorbed explosives, such as pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX), and trinitrotoluene (TNT).

Fluorescence relaxation in the near-field of a mesoscopic metallic particle: distance dependence and role of plasmon modes

We analytically and numerically analyze the fluorescence decay rate of a quantum emitter placed in the vicinity of a spherical metallic particle of mesoscopic size (i.e with dimensions comparable to the emission wavelength). We discuss the efficiency of the radiative decay rate and non-radiative coupling to the particle as well as their distance dependence. The electromagnetic coupling mechanisms between the emitter and the particle are investigated by analyzing the role of the plasmon modes and their nature (dipole, multipole or interface mode). We demonstrate that near-field coupling can be expressed in a simple form verifying the optical theorem for each particle modes.

A novel self-assembled monolayer (SAM) coated microcantilever for low level caesium detection

We report a new sensor concept based on an ion-selective SAM modified microcantilever which can detect caesium ion concentrations in situ in the range 10−11–10−7 M and shows potential for use in developing a new family of real time in situ metal ion sensors with high sensitivity/selectivity and low cost, for chemical and biological applications.

Performance of interdigitated nanoelectrodes for electrochemical DNA biosensor

An electrochemical methodology for bio-molecule sensing using an array of well-defined nanostructures is presented. We describe the fabrication by e-beam lithography of nanoelectrodes consisting of a 100 micro m x 50 micro m area containing interdigitated electrodes of 100 nm in width and interelectrode distance of 200 nm. Sensitivity and response time of the nanoelectrodes are compared to the responses of macro- and microelectrodes. The specificity of the sensor is studied by modifying the gold electrodes with DNA. The technique enables to characterize both single and double-stranded DNA of 15 nucleotides. A special electrochemical cell is adapted to control the temperature and measure the DNA concentration by UV analysis. The electrochemical method requires no label on the DNA, only redox mediators were used.

Chemical sensing in Fourier space

Chemical sensing using optical diffraction from an array of microcantilevers is demonstrated. Properly fashioned arrays of micromachined silicon-nitride cantilevers containing embedded deformable diffraction gratings are functionalized with chemically selective coatings. Adsorption of specific molecules on the cantilever leads to bending, which changes the diffraction pattern of a laser beam reflecting off the array. Quantitative chemical information can be obtained by monitoring the displacement of diffraction peaks as a function of analyte exposure.