Roberto Raiteri

Micromechanical cantilever-based biosensors

Abstract The merging of silicon microfabrication techniques with surface functionalization biochemistry offers new exciting opportunities in developing microscopic biomedical analysis devices with unique characteristics. Micro-mechanical transducers for chemical and biosensing applications represent one possibility. Microcantilevers can transduce a chemical signal into a mechanical motion with high sensitivity. In this review we summarize how cantilever-based sensors can be operated, and their working principle is presented in few selected biosensing experiments which have been performed recently in our groups in the study of biotin–streptavidin and antigen–antibody interactions, and specific surface charge development of organic molecules. We also discuss the advantages of this novel technique as well as its potentials.

Micromechanics senses biomolecules

Abstract How can microelectromechanical systems (MEMS) experts support molecular biologists in studying DNA hybridization? Cantilever-based devices are an example of how a ‘simple’ sensor can be tailored by microfabrication techniques and used to achieve an unprecedented performance. We review fascinating experiments, which use different mechanical transduction principles for detecting and analyzing small quantities of materials. The principles of these experiments allow biologists to study surface biochemistry on a nano-scale and offer new, exciting opportunities in developing microscopic biomedical analysis systems with unique characteristics. Cantilever sensors rely on relatively well known and simple transduction principles, and have attracted the interest of many researchers. This is, at least in part, because of the merging of silicon microfabrication techniques and surface functionalization biochemistry, together with the development of multi-cantilever sensing methods offering new opportunities in physical and (bio)chemical sensing.

Changes in surface stress at the liquid/solid interface measured with a microcantilever

Abstract The bending of microfabricated silicon nitride cantilevers was used to determine surface stress changes at solid–liquid interfaces. The radius of curvature of the bent cantilever is directly proportional to changes in the differential surface stress between its opposite sides. To demonstrate the possibilities and limitations of the technique, cantilevers coated on both sides with gold and densely packed monolayers of different thiols were put in a constant flow of aqueous electrolyte solution and the deflection was measured using a optical lever technique. Changes in the surface stress for the different thiol monolayers due to specific proton adsorption are presented. Possible applications and improvements of this technique are discussed.

PRELIMINARY RESULTS ON THE ELECTROSTATIC DOUBLE-LAYER FORCE BETWEEN TWO SURFACES WITH HIGH SURFACE POTENTIALS

Abstract The aim of this study is to measure interaction forces between surfaces with high electric potentials in aqueous electrolyte solutions. Therefore the force between a gold sample and a gold sphere attached to the end of an atomic force microscope cantilever was measured. Gold sample and sphere were electrically connected and served as the working electrode. A potential was applied via a platinized platinum electrode. Experimental results are compared to forces approximated with the Poisson-Boltzmann theory.

An automated microdrop delivery system for neuronal network patterning on microelectrode arrays

The aim of this work is to present a new technique for defining interconnected sub-populations of cultured neurons on microelectrode arrays (MEAs). An automated microdrop delivery technique allows to design and realize spatially distributed neuronal ensembles by depositing sub-nanoliter volumes of adhesion molecules in which neurons grow and develop. Electrophysiological tests demonstrate that functionally interconnected clusters are obtained and experimental results (both spontaneous and stimulus evoked activity recordings) attesting the feasibility of the proposed approach are presented. By means of the automated system, different and specific architectures can be easily designed and functionally studied. In the presented system the speed of drop deposition is about 30 drops/min; the mean diameter is 147 microm; typical cell survival time is 4-5 weeks. By changing drop size and spacing, investigations about how the network dynamics is related to the network structure can be systematically carried out.

Hydrophilic/hydrophobic nanostripes in lipopolymer monolayers.

The self-organization of macromolecules with a complex architecture is studied extensively in the last years, with the aim to nanostructure surfaces. Linear diblock copolymers with mutual antagonistic groups are a simple, much investigated system, since the respective block lengths determine the size of the nanostructure[1]. Yet little is known about the equilibrium structure when the forces from the two interfaces, and the intraand intermolecular interactions are all of comparable magnitude. An active control of the domain morphology as well as the possibility to manipulate the structures in situ would obviously improve our understanding.

pH-dependent charge density at the insulator-electrolyte interface probed by a scanning force microscope

Abstract This paper deals with the use of a scanning force microscope in the ‘force versus distance mode’, to detect the charge densities on the surfaces of insulators immersed in electrolyte solutions. The results of experiments on mica, Si3N4 and Al2O3, under various pH conditions, are compared with those of computer simulations performed to calculate the electrostatic forces acting between tip and insulator surface. The considered charge densities include a pH-dependent component, which is taken into account according to the site-binding theory. Simulation results are in good agreement with experimental ones, thus indicating the feasibility of the approach for the characterization of the surface charges of potentiometric microelectronic sensors.

Multiwell micromechanical cantilever array reader for biotechnology

We use a multiwell micromechanical cantilever sensor (MCS) device to measure surface stress changes induced by specific adsorption of molecules. A multiplexed assay format facilitates the monitoring of the bending of 16 MCSs in parallel. The 16 MCSs are grouped within four separate wells. Each well can be addressed independently by different analyte liquids. This enables functionalization of MCS separately by flowing different solutions through each well. In addition, each well contains a fixed reference mirror which allows measuring the absolute bending of MCS. In addition, the mirror can be used to follow refractive index changes upon mixing of different solutions. The effect of the flow rate on the MCS bending change was found to be dependent on the absolute bending value of MCS. Experiments and finite element simulations of solution exchange in wells were performed. Both revealed that one solution can be exchanged by another one after 200 microl volume has flown through. Using this device, the adsorption of thiolated DNA molecules and 6-mercapto-1-hexanol on gold surfaces was performed to test the nanomechanical response of MCS.

A new integrated system combining atomic force microscopy and micro-electrode array for measuring the mechanical properties of living cardiac myocytes

In this paper we present a new experimental set-up which combines the surface characterization capabilities of atomic force microscopy at the sub-micrometer scale with non-invasive electrophysiological measurements obtained by using planar micro-electrode arrays. In order to show the potential of the combined measurements we studied the changes in cell topography and elastic properties of cardiac muscle cells as during the contraction-relaxation cycle. The onset of each beating cycle was precisely identified by the use of the extracellular potential signal, allowing us to combine nanomechanical measurements from multiple cardiomyocyte contractions in order to analyze the time-dependent variation of cell morphology and elasticity. Moreover, by estimating the elastic modulus at different indentation depths in a single location on the cell membrane, we observed a dynamic mechanical behavior that could be related to the underlying myofibrillar structure dynamics.

Heterogeneous polymer-containing films: a comparison of macroscopic properties with microscopic properties determined by atomic force microscopy

The aim of the study was to evaluate the extent to which a quantitative and automatic analysis of the surface properties of heterogeneous polymer-containing films such as paints is possible by atomic force microscopy. Therefore, paints based on different dispersions were investigated. Force–distance curves were measured automatically at 64×64 points of the samples. From the analysis of these force curves, a distribution of properties such as local elasticity, adhesion and penetration depth were obtained. Peaks in these distributions could be correlated with specific regions in the film such as binder or pigment. Peaks corresponding to the binder correlated with macroscopic properties such as glass transition temperature, tack and ball pressure hardness. In addition, from topographic images the roughness was calculated. As the roughness increases, the gloss decreases.