Christoph Gerber

Multiple label-free biodetection and quantitative DNA-binding assays on a nanomechanical cantilever array

We report a microarray of cantilevers to detect multiple unlabeled biomolecules simultaneously at nanomolar concentrations within minutes. Ligand-receptor binding interactions such as DNA hybridization or protein recognition occurring on microfabricated silicon cantilevers generate nanomechanical bending, which is detected optically in situ. Differential measurements including reference cantilevers on an array of eight sensors can sequence-specifically detect unlabeled DNA targets in 80-fold excess of nonmatching DNA as a background and discriminate 3′ and 5′ overhangs. Our experiments suggest that the nanomechanical motion originates from predominantly steric hindrance effects and depends on the concentration of DNA molecules in solution. We show that cantilever arrays can be used to investigate the thermodynamics of biomolecular interactions mechanically, and we have found that the specificity of the reaction on a cantilever is consistent with solution data. Hence cantilever arrays permit multiple binding assays in parallel and can detect femtomoles of DNA on the cantilever at a DNA concentration in solution of 75 nM.

Label-free protein assay based on a nanomechanical cantilever array

We demonstrate continuous label-free detection of two cardiac biomarker proteins (creatin kinase and myoglobin) using an array of microfabricated cantilevers functionalized with covalently anchored anti-creatin kinase and anti-myoglobin antibodies. This method allows biomarker proteins to be detected via measurement of surface stress generated by antigen–antibody molecular recognition. Reference cantilevers are used to eliminate thermal drifts, undesired chemical reactions and turbulences from injections of liquids by calculating differential deflection signals with respect to sensor cantilevers. The sensitivity achieved for myoglobin detection is below 20 µg ml−1. Both myoglobin and creatin kinase could be detected independently using cantilevers functionalized with the corresponding antibodies, in unspecific protein background. This approach permits the use of up to seven different antigen–antibody reactions simultaneously, including an additional thermomechanical and chemical in situ reference. Applications lie in the field of early and rapid diagnosis of acute myocardial infarction.

Rapid functionalization of cantilever array sensors by inkjet printing

The controlled deposition of functional layers is the key to converting nanomechanical cantilevers into chemical or biochemical sensors. Here, we introduce inkjet printing as a rapid and general method to coat cantilever arrays efficiently with various sensor layers. Self-assembled monolayers of alkanethiols were deposited on selected Au-coated cantilevers and rendered them sensitive to ion concentrations or pH in liquids. The detection of gene fragments was achieved with cantilever sensors coated with thiol-linked single-stranded DNA oligomers on Au. A selective etch protocol proved the uniformity of the monolayer coatings at a microscopic level. A chemical gas sensor was fabricated by printing thin layers of different polymers from dilute solutions onto cantilevers. The inkjet method is easy to use, faster and more versatile than coating via microcapillaries or the use of pipettes. In addition, it is scalable to large arrays and can coat arbitrary structures in non-contact.

Quantitative time-resolved measurement of membrane protein- ligand interactions using microcantilever array sensors

Membrane proteins are central to many biological processes, and the interactions between transmembrane protein receptors and their ligands are of fundamental importance in medical research. However, measuring and characterizing these interactions is challenging. Here we report that sensors based on arrays of resonating microcantilevers can measure such interactions under physiological conditions. A protein receptor--the FhuA receptor of Escherichia coli--is crystallized in liposomes, and the proteoliposomes then immobilized on the chemically activated gold-coated surface of the sensor by ink-jet spotting in a humid environment, thus keeping the receptors functional. Quantitative mass-binding measurements of the bacterial virus T5 at subpicomolar concentrations are performed. These experiments demonstrate the potential of resonating microcantilevers for the specific, label-free and time-resolved detection of membrane protein-ligand interactions in a micro-array format.

How the doors to the nanoworld were opened.

The invention of the scanning tunnelling microscope 25 years ago, followed by the arrival of the atomic force microscope five years later, were crucial events in the history of nanoscience and nanotechnology. As the recent International Conference on Nanoscience and Technology in Basel made clear, scanning probe microscopes based on these discoveries are still having a tremendous impact on many areas of research.

Cantilever array sensors

Miniaturized microfabricated sensors have enormous potential in gas detection, biochemical analysis, medical applications, quality and process control, and product authenticity issues. Here, we highlight an ultrasensitive mechanical way of converting (bio-)chemical or physical processes into a recordable signal using microfabricated cantilever arrays.

Higher modes of vibration increase mass sensitivity in nanomechanical microcantilevers

We evaluated the potential and limitations of resonating nanomechanical microcantilevers for the detection of mass adsorption. As a test system we used mass addition of gold layers of varying thickness. Our main findings are: (1) A linear increase in mass sensitivity with the square of the mode number—a sensitivity increase of two orders of magnitude is obtained from mode 1 to mode 7 with a minimum sensitivity of 8.6 ag Hz −1 μm −2 and mass resolution of 0.43 pg at mode 7 for a 1 μm thick cantilever. (2) The quality factor increases with the mode number, thus helping to achieve a higher sensitivity. (3) The effective spring constant of the cantilever remains constant for deposition of gold layers up to at least 4% of the cantilever thickness. (Some figures in this article are in colour only in the electronic version)

Imaging modes of atomic force microscopy for application in molecular and cell biology

Atomic force microscopy (AFM) is a powerful, multifunctional imaging platform that allows biological samples, from single molecules to living cells, to be visualized and manipulated. Soon after the instrument was invented, it was recognized that in order to maximize the opportunities of AFM imaging in biology, various technological developments would be required to address certain limitations of the method. This has led to the creation of a range of new imaging modes, which continue to push the capabilities of the technique today. Here, we review the basic principles, advantages and limitations of the most common AFM bioimaging modes, including the popular contact and dynamic modes, as well as recently developed modes such as multiparametric, molecular recognition, multifrequency and high-speed imaging. For each of these modes, we discuss recent experiments that highlight their unique capabilities.

Resonating modes of vibrating microcantilevers in liquid

A study of nanomechanical cantilevers vibrating at various resonating modes in liquid is presented. Resonant frequency spectrum with 16 well resolved flexural modes is obtained. The quality factor increased from 1 at mode 1 to 30 at mode 16. The theoretical estimate of eigenfrequency using the Elmer–Dreier model [F.-J. Elmer and M. Dreier, J. Appl. Phys. 81, 12 (1997)] and Sader’s extended viscous model [C. A. Van Eysden and J. E. Sader, J. Appl. Phys. 101, 044908 (2007)] matched well with the experimental data. The apparent mass of the liquid comoved by the oscillating cantilevers decreased asymptotically with mode number.

Block‐by‐block deposition: A new growth method for complex oxide thin films

An alternative growth method for high‐Tc oxide thin films employing molecular beam deposition is proposed. Instead of an uncontrolled local nucleation followed by lateral growth and island coalescence, the new method provides substrate coverage by nonreacting constituents before nucleation is initiated, a controlled reaction path, and reduced lateral growth. DyBa2Cu3O6+δ films without precipitates, with a surface roughness of ±1 unit cell and showing finite size oscillations in the x‐ray diffraction spectrum, have been prepared. This method reveals that diffusion dominates the growth process at high substrate temperatures (≂700 °C).