Cantilever systems for the next generation of biomechanical sensors

Ieder interactief systeem gebruikt apparaten om informatie over de omgeving te verkrijgen. Ook de mens gebruikt apparaten om zijn omgeving te onderzoeken; tast- en gehoor-apparatuur voor mechanische impulsen, zicht- voor elektromagnetische en smaak- en reuk- voor chemische eigenschappen. Het gaat om thermometers, microfoons, ccd camera’s, enzovoort: allemaal sensoren die onze waarnemingsmogelijkheden vergroten, prestaties verbeteren en soms zelfs de mens vervangen in autonome systemen. In de laatste decennia is door de opkomst van nano- en biotechnologie de ontwikkeling van chemische sensoren, in het bijzonder biosensoren, in een stroomversnelling geraakt. Biosensoren worden gekenmerkt door de aanwezigheid van een biologische component (bv. een antilichaam, enzym of DNA molecuul) die een interactie aangaat met het te detecteren chemische element. Deze interactie wordt omgezet in een macroscopisch signaal welke vervolgens kan worden uitgelezen door een mens of machine. In ons dagelijks leven zijn biosensoren al terug te vinden in de vorm van zwangerschapstesten en glucosemeters, maar ook in minder opvallende toepassingen zoals voedsel- en waterveiligheid. Er zijn echter nog vele gebieden, in het bijzonder in de geneeskunde, waarin biosensoren een belangrijke rol kunnen spelen. In geval van ziekte (eenvoudige griep of allergie tot levensbedreigende kanker) produceert ons lichaam biologische markers, eiwitten, die inzicht kunnen geven in wat er zich in ons lichaam afspeelt. Daardoor kan er een betere inschatting worden gemaakt van de prognose en kan de therapie mogelijk specifiek op de patiënt worden afgestemd. Helaas is vaak niet bekend welke markers een rol spelen, en als dit wel bekend is, is de detectie veelal zeer kostbaar of zelfs niet mogelijk. In my thesis work I investigated alternative geometries of nanomechanical oscillators to be employed as biomolecular sensors. Simple mechanical oscillators, such as cantilevers and double clamped beams have been deeply investigated in the last decade and single molecule sensitivity was demonstrated. However, beside few marginal exceptions, the proof of principle demonstrations did not yet evolve into commercial devices. Alternative geometries can, in principle, improve the simple micromechanical systems studied so far, with more complex transfer functions suitable to operate also in demanding environments. The thesis work was divided in two major sections. In the first section twin cantilevers are discussed. Couples of cantilevers facing each other and separated by a nanometer gap may change their resonance response when one or more molecules are absorbed in the gap. Two different geometries have been fabricated and tested. One, with identical cantilevers, takes advantage of the shift in resonance frequency occurring upon molecular detection; the second with asymmetrical cantilevers, uses the shortest one to actuate the motion of the longer one through a molecular link. In the second part the structure of the twin cantilevers is the starting point for creating a spatially confined chemical reaction in the gap between two cantilevers facing each other. This original process is extremely precise and represents an important milestone towards the future realization of complex micro- and nanomechanical systems for biomolecular detection.

Reflectionless tunneling in planar Nb/GaAs hybrid junctions

Reflectionless-tunneling was observed in Nb/GaAs superconductor/semiconductor junctions fabricated through a two-step procedure. First, periodic $\delta$-doped layers were grown by molecular beam epitaxy near the GaAs surface, followed by an As cap layer to protect the surface during {\it ex-situ} transfer. Second, Nb was deposited by dc-magnetron sputtering onto the GaAs(001) 2 $\times$ 4 surface {\it in-situ} after thermal desorption of the cap layer. The magnetotransport behavior of the resulting hybrid junctions was successfully analyzed within the random matrix theory of phase-coherent Andreev transport. The impact of junction morphology on reflectionless tunneling and the applicability of the fabrication technique to the realization of complex superconductor/semiconductor mesoscopic systems are discussed.Comment: 10 pages, 3 figures, to be published in Appl. Phys. Let

Andreev reflection in Si-engineered Al/InGaAs hybrid junctions

Andreev-reflection dominated transport is demonstrated in Al/n-In0.38Ga0.62As superconductor-semiconductor junctions grown by molecular beam epitaxy on GaAs(001). High junction transparency was achieved in low-doped devices by exploiting Si interface bilayers to suppress the native Schottky barrier. It is argued that this technique is ideally suited for the fabrication of ballistic transport hybrid microstructures.Comment: 9 REVTEX pages + 3 postscript figures, to be published in APL 73, (28dec98

Fluorescence Excitation by Enhanced Plasmon Upconversion under Continuous Wave Illumination

We demonstrate effective background-free continuous wave nonlinear optical excitation of molecules that are sandwiched between asymmetrically constructed plasmonic gold nanoparticle clusters. We observe that near infrared photons are converted to visible photons through efficient plasmonic second harmonic generation. Our theoretical model and simulations demonstrate that Fano resonances may be responsible for being able to observe nonlinear conversion using a continuous wave light source. We show that nonlinearity enhancement of plasmonic nanostructures via coupled quantum mechanical oscillators such as molecules can be several orders larger as compared to their classical counterparts.Comment: 11 pages, 8 figure

Atomic force spectroscopy-based essay to evaluate oocyte postovulatory aging

Postovulatory aging is a process occurring in the mature (MII) oocyte leading the unfertilized ones to apoptosis. The optimal time window of fertility for different mammalian species after oocytes maturation depends on its timeliness: the higher the time elapsed from the accomplishment of the MII stage, the lower are the chances of fertilization and of development of a viable embryo. In the in vitro fertilization, the selection of competent oocytes for intracytoplasmic sperm injection (ICSI) is mostly made by the visual inspection of the MII oocyte morphology, which does not allow to determine the oocyte postovulatory age. On the other hand, more specific tests usually involve some kind of staining, thus compromising the viability of the oocyte for reproductive purposes. Hence, the need of a noninvasive analysis of oocyte aging to improve the success rate of in vitro fertilization procedures. Here, we exploit atomic force microscopy to examine the evolution of the mechanical properties of mouse oocytes during in vitro postovulatory aging. Three hours before the occurrence of any visual morphological feature related to degradation, we observe a sudden change of the mechanical parameters: the elastic modulus doubles its initial value, while the viscosity decreases significantly. These mechanical variations are temporally correlated with the release of the cortical granules, investigated by fluorescence microscopy. Interestingly, the oocyte mechanics correlates as well with the yield of embryo formation, evaluated up to the blastocyst formation stage. These results demonstrate that minimally invasive mechanical measurements are very sensitive to the aging of the oocyte and can be used as a label-free method to detect the age of the postovulatory oocytes

AFM macro-probes to investigate whole 3D cardiac spheroids

In its many applications, the Atomic Force Microscope (AFM) is a promising tool in cardiac mechanobiology because it can unravel the viscoelastic and mechano-dynamic properties of individual cardiomyocytes. However, the biophysical investigation of more accurate 3D models is hampered by commercial probes, which typically operate at the cell sub-compartmental resolution. We have previously shown how flat macro-probes can overcome these limitations by extending the AFM mechanical measurements to multicellular aggregates. Such macro-probes are fabricated by standard micromachining and carry a flat polymeric wedge to offset the AFM mounting tilt. Therefore, the AFM is upgraded to a micro-parallel plate rheometer with unmatched force range and sensitivity. In this article, we show how these macro-probes can be applied to reveal the global rheology of primary cardiomyocytes spheroids, by performing stress-relaxation tests. More importantly, we demonstrate that these macro-probes can be used as passive sensors capable of monitoring the spheroid beating force and beating pattern, and to perform a “micro-CPR” on the spheroid itself

Corrigendum: Conformational rearrangements in the transmembrane domain of CNGA1 channels revealed by single-molecule force spectroscopy.

This corrects the article DOI: 10.1038/ncomms8093