W. Van Roy

imec Belgium, Louvain, Flemish, Belgium

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Publications (150)426.74 Total impact

  • Yan Li · Willem Van Roy · Philippe M. Vereecken · Liesbet Lagae
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    ABSTRACT: We present an amperometric biosensor enhanced by redox cycling under laminar flow at microelectrodes. The device integrates the enzyme-linked immunosorbent assay and the interdigitated electrode array within a photo-patternable adhesive microfluidic channel. The adhesive enables us to perform high quality bioassay within a microfluidic channel. Laminar flow enhances the mass transport not only from enzymatic region to the redox cycling electrodes, but also between the redox cycling electrodes. As a result, the performance of biosensors is enhanced.
    No preview · Article · Jul 2015
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    ABSTRACT: Previous reports on Poisson-Nernst-Planck (PNP) simulations of solid-state nanopores have focused on steady state behaviour under simplified boundary conditions. These are Neumann boundary conditions for the voltage at the pore walls, and in some cases also Donnan equilibrium boundary conditions for concentrations and voltages at both entrances of the nanopore. In this paper, we report time-dependent and steady state PNP simulations under less restrictive boundary conditions, including Neumann boundary conditions applied throughout the membrane relatively far away from the nanopore. We simulated ion currents through cylindrical and conical nanopores with several surface charge configurations, studying the spatial and temporal dependence of the currents contributed by each ion species. This revealed that, due to slow co-diffusion of oppositely charged ions, steady state is generally not reached in simulations or in practice. Furthermore, it is shown that ion concentration polarization is responsible for the observed limiting conductances and ion current rectification in nanopores with asymmetric surface charges or shapes. Hence, after more than a decade of collective research attempting to understand the nature of ion current recti-fication in solid-state nanopores, a relatively intuitive model is retrieved. Moreover, we measured and simulated current-voltage characteristics of rectifying silicon nitride nanopores presenting overlimiting conductances. The similarity between measurement and simulation shows that overlimiting conductances can result from the increased conductance of the electric double-layer at the membrane surface at the depletion side due to voltage-induced polarization charges. The MATLAB source code of the simulation software is available via the website http://micr.vub.ac.be.
    Full-text · Article · May 2015 · PLoS ONE
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    ABSTRACT: Surface Plasmon Resonance (SPR) is a well-established technique for studying binding kinetics and is extensively used in assay development as well as in drug discovery. Many biosensors contain an oxide surface instead of the conventional Au surface typically used in SPR sensing, which may introduce additional variables when using established protocols. Therefore, SiOx-covered SPR substrates are of great interest as a benchmarking tool for silicon-based biosensors. Moreover, SiOx has multiple advantages over Au, for instance with respect to the thermal stability of commonly used surface coupling strategies. In this paper, the bulk sensitivity of SiO2-covered Au substrates was evaluated for use in SPR. Both theoretical simulations and experimental results showed that the presence of ∼10 nm of SiO2 resulted in minimal loss of bulk sensitivity compared to Au substrates. This was proven for a prostate specific antigen (PSA) recognition immuno-assay. Thus we clearly demonstrated that SiO2-covered Au substrates can be used for biosensing applications and do not generate significant differences compared to the original Au substrates.
    Full-text · Article · Sep 2014 · Sensors and Actuators B Chemical
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    ABSTRACT: Spin-dependent quantum transport experiments on InSb and InAs heterostructures and Bi thin films are discussed, focusing on mesoscopic geometries where spin-orbit interaction and quantum coherence determine the properties. The narrow-bandgap semiconductors InSb and InAs, and the semimetal Bi have substantial spin-orbit interaction. The experiments use antilocalization to study spin-orbit interaction and spin coherence lengths in nanolithographic wires fabricated on the materials. In the three systems the spin coherence lengths increase with decreasing wire widths if other parameters stay constant, of technological importance for spin-based devices. The experiments also indicate that Bi has surface states with Rashba-like spin-orbit interaction. A quasi-one-dimensional model of antilocalization, as fitted to the data, is explained and its consequences for quantum coherence in mesoscopic structures is explored. A united understanding of the experiments is presented relying on the duality between the Aharonov-Bohm and the Aharonov-Casher phases, the latter resulting from spin-orbit interaction. The duality strengthens the analogy between phenomena under magnetic fields and under spin-orbit interaction.
    No preview · Conference Paper · Aug 2014
  • Yan Li · Willem Van Roy · Liesbet Lagae · Philippe M. Vereecken
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    ABSTRACT: In this paper we demonstrated a functional on-chip multi-microelectrode array within microfluidic channels as an fully on-chip electrochemical cell. The response of Pt electrodes within the multi-microelectrode array in the microfluidic channel filled with stagnant Fe(III) solution was characterized using cyclic voltammetry. We found that the mass transport is confined by the small channel dimensions. Furthermore, the interaction between closely-spaced working electrode (WE)-counter electrode (CE) pairs induced the so-called redox cycling, as a result of the overlap of their diffusion regions. The redox cycling modified the electrochemical response of the micro system, resulting in a larger peak separation and higher steady-state current plateaus compared to a bulk system. These experimental findings were validated by using the COMSOL simulations, which also visualize the underlying concentration profiles: thin-layer behavior in the absence of redox cycling and a steep concentration profile in the presence redox cycling, where the depletion-layer width is exactly identical to the WE-CE distance. This analysis from a particular geometry provides an example for the analysis of general micro systems. This work indicates that redox cycling at the WE-CE mode can enhance the current response of amperometric biosensors.
    No preview · Article · Jun 2014 · Journal of The Electrochemical Society
  • Nima Arjmandi · Willem Van Roy · Liesbet Lagae
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    ABSTRACT: Nanopores have recently been developed for the detection and physical characterization of nanoparticles, viruses, proteins, nucleic acids and other macromolecules in liquids. The method provides the ability to rapidly estimate the size and electrical charge of analytes over a wide range of concentration, potentially with small sample volumes and low cost. Here, we use the technique to measure the mass of nanoparticles and viruses, and their sedimentation. The analyte sedimentation-time measurement provides an estimate for the nanoparticle mass-density. We also show that the method can be used with samples at low concentration and in small volumes.
    No preview · Article · Apr 2014 · Analytical Chemistry
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    ABSTRACT: We present the top-down synthesis of a novel type of MRI T2 contrast agent with great control over size and shape using a colloidal lithography technique. The resulting synthetic antiferromagnetic nanoparticles (SAF-NPs) yield improved relaxivities compared to superparamagnetic iron oxide alternatives (SPIONs). For T2 weighted imaging, the outer sphere relaxation theory has shown that the sensitivity of a T2 contrast agent is dependent on the particle size with an optimal size that exceeds the superparamagnetic limit of SPIONs. Using the interlayer exchange coupling effect, the SAF-NPs presented here do not suffer from this limit. Adjusting the outer sphere relaxation theory for spherical particles to SAF-NPs, we show both theoretically and experimentally that the SAF-NP size can be optimized to reach the r2 maximum. With measured r2 values up to 355 s-1mM-1 our SAF-NPs show better performance than commercial alternatives and are competitive with the state-of-the-art. This performance is confirmed in an in vitro MRI study on SKOV3 cells.
    No preview · Article · Feb 2014 · ACS Nano
  • C. Liu · J. Hoet · W. Van Roy · T. Piessens · L. Lagae
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    ABSTRACT: In this paper we present a microelectronic device for high throughput single cell capture and impedance analysis for rare cell detection such as circulating tumor cells. With every pair of electrodes among the nearly 10,000 electrode pairs, three functions were enabled for single cell analysis: (a) single cell capture by dielectrophoresis (DEP), (b) single cell electrical impedance spectroscopy and (c) electrical cell lysis. In this report, single cell DEP capture, fF cell membrane capacitance measurement as well as single cell impedance spectroscopy was demonstrated.
    No preview · Article · Jan 2014
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    ABSTRACT: We propose a write scheme for perpendicular spin-transfer torque magnetoresistive random-access memory that significantly reduces the required tunnel current density and write energy. A sub-nanosecond in-plane polarized spin current pulse is generated using the spin-Hall effect, disturbing the stable magnetic state. Subsequent switching using out-of-plane polarized spin current becomes highly efficient. Through evaluation of the Landau-Lifshitz-Gilbert equation, we quantitatively assess the viability of this write scheme for a wide range of system parameters. A typical example shows an eight-fold reduction in tunnel current density, corresponding to a fifty-fold reduction in write energy, while maintaining a 1 ns write time.
    Full-text · Article · Dec 2013 · Applied Physics Letters
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    ABSTRACT: Magnetic vortices in thin films are in-plane spiral spin configurations with a core in which the magnetization twists out of the film plane. Vortices result from the competition between atomic-scale exchange forces and long-range dipolar interactions. They are often the ground state of magnetic dots, and have applications in medicine, microwave generation and information storage. The compact nature of the vortex core, which is 10-20 nm wide, makes it a suitable probe of magnetism at the nanoscale. However, thus far the positioning of a vortex has been possible only in confined structures, which prevents its transport over large distances. Here we show that vortices can be propagated in an unconstrained system that comprises electrical nanocontacts (NCs). The NCs are used as tunable vortex attractors in a manner that resembles the propelling of space craft with gravitational slingshots. By passing current from the NCs to a ferromagnetic film, circulating magnetic fields are generated, which nucleate the vortex and create a potential well for it. The current becomes spin polarized in the film, and thereby drives the vortex into gyration through spin-transfer torques. The vortex can be guided from one NC to another by tuning attractive strengths of the NCs. We anticipate that NC networks may be used as multiterminal sources of vortices and spin waves (as well as heat, spin and charge flows) to sense the fundamental interactions between physical objects and fluxes of the next-generation spintronic devices.
    Full-text · Article · Dec 2013 · Nature Nanotechnology
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    ABSTRACT: Magnetic nanoparticle (MNP) enabled cell visualization with magnetic resonance imaging (MRI) is currently an intensively studied area of research. In the present study, we have synthesized polyethylene glycolated (PEG) MNPs and validated their suitability as MR cell labeling agents in in vitro and in vivo experiments. The labeling of therapeutic potent mesenchymal stem cells (MSCs) with small core and large core MNPs was evaluated. Both MNPs were, in combination with a transfection agent, stably internalized into the MSCs and didn’t show an effect on cell metabolism. The labeled cells showed high contrast in MRI phantom studies. For quantification purposes, the MRI contrast generating properties of cells labeled with small core MNPs were compared with large core MNPs and with the commercial contrast agent Endorem. MSCs labeled with the large core MNPs showed the highest contrast generating properties in in vitro phantom studies and in in vivo intracranial stereotactic injection experiments, confirming the sizeerelaxivity relationship in biological systems. Finally, the distribution of MSCs prelabeled with large core PEGylated MNPs was visualized non-invasively with MRI in a glioma model.
    Full-text · Article · Nov 2013 · Biomaterials
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    ABSTRACT: Measurements of low-temperature magnetotransport in lithographic wires of submicron widths fabricated from high-mobility AlGaSb/InAs/AlGaSb two-dimensional electron system heterostructures are presented. The dependence of the spin and phase coherence lengths on wire width and diffusion constant is investigated by analyzing the conductance in low applied magnetic fields with antilocalization models. Predominantly diffusive boundary scattering is deduced from the magnitude and wire width dependence of the conductance. Diffusive boundary scattering leads to a diffusion constant decreasing with wire width and hence allows the dependence of spin coherence on wire width and diffusion constant to be investigated concurrently. The spin coherence lengths are experimentally found to be proportional to the ratio of the diffusion constant to wire width. The phase coherence lengths follow Nyquist decoherence for low-dimensional wires.
    No preview · Article · Nov 2013 · Physical Review B
  • Yan Li · Willem Van Roy · Philippe M. Vereecken · Liesbet Lagae

    No preview · Conference Paper · May 2013
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    ABSTRACT: Tailoring the properties of superparamagnetic nanoparticles (MNPs) is essential for various nano-based biological applications. Having control over the properties of the MNPs permits a maximum flexibility. Starting from monodisperse iron oxide MNPs produced by thermal decomposition, we report on the optimization and characterization of a first and second seed mediated growth step by varying the surfactant amount and by optimizing the heating steps. We demonstrate the ability to gradually increase the size of crystalline MNPs from 6 over 9 to 12 nm with an improving monodispersity as demonstrated by Transmission Electron Microscopy, Dynamic Light Scattering and X-ray diffraction. The magnetic properties of the MNPs, studied by Vibrating Sample Magnetometry, were in concert with their size increase. We also show the functionalization of these particles with polyethylene glycolated silanes, to render the MNPs stable in water. Different characterization techniques, namely Transmission Electron Microscopy, Dynamic Light Scattering, Fourier-transform InfraRed, Thermo gravimetric analysis and X-ray Photoelectron Spectroscopy, confirmed the successful engraftment of the silanes on the MNP's surface. In conclusion, the proposed route of step-wise synthesis in combination with silane functionalization allows fine tuning the physical properties of iron oxide MNPs for applications in an aqueous environment.
    No preview · Article · Feb 2013 · IEEE Transactions on Magnetics
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    ABSTRACT: We have determined the temperature profile in magnetic nanocontacts under applied current densities typical of spin-torque oscillators (∼10^{8} A/cm^{2}). The study combines experimental measurements of the electrical and magnetic properties of the nanocontacts and full three-dimensional simulations of the heat and current flow in these systems. It is found that the quadratic current-induced increase of the resistance due to Joule heating is independent of the applied temperature from 6 to 300 K. In terms of magnetization dynamics, the measured current-induced vortex nucleation, a thermally activated process, is found to be consistent with local temperatures increases of between 147 and 225 K. Simulations reproduce the experimental findings and show that significant thermal gradients exist out to 450 nm from the nanocontact.
    Full-text · Article · Dec 2012 · Physical Review Letters
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    Nima Arjmandi · Willem Van Roy · Liesbet Lagae · Gustaaf Borghs

    Full-text · Dataset · Dec 2012
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    ABSTRACT: Localized and propagating surface plasmon resonances are known to show very pronounced interactions if they are simultaneously excited in the same nanostructure. Here we study the fano interference that occurs between localized (LSPR) and propagating (SPP) modes by means of phase sensitive spectroscopic ellipsometry. The sample structures consist of periodic gratings of gold nanodisks on top of a continuous gold layer and a thin dielectric spacer, in which the structural dimensions were tuned in such a way that the dipolar LSPR mode and the propagating SPP modes are excited in the same spectral region. We observe pronounced anti-crossing and strongly asymmetric line shapes when both modes move to each others vicinity, accompagnied of largely increased phase differences between the respective plasmon resonances. Moreover we show that the anti-crossing can be exploited to increase the refractive index sensitivity of the localized modes dramatically, which result in largely increased values for the Figure-Of-Merit which reaches values between 24 and 58 for the respective plasmon modes.
    Full-text · Article · Nov 2012 · Plasmonics
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    Full-text · Dataset · Nov 2012
  • Nima Arjmandi · Willem Van Roy · Liesbet Lagae · Gustaaf Borghs
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    ABSTRACT: Nanometer-scale pores are capable of detecting the size and concentration of nanometer-sized analytes at low concentrations upon analyzing their translocation through the pore, in small volumes and over a short time without labeling. Here, we present a simple, widely applicable, robust, and precise method to measure the zeta-potential of different nano-objects using nanopores. Zeta-potential i.e., a quantity that represents electrical charge in nanocolloids, is an important property in manufacturing of pharmaceuticals, inks, foams, cosmetics, and food. Its use is also imperative in understanding basic properties of complex dispersions including blood, living organisms, and their interaction with the environment. The characterization methods for zeta-potential are limited. Using the nanopore technique, the zeta-potential and the charge of nanoparticles can be measured independently of other parameters, such as particle size. This simple method is based on measuring the duration of the translocation of analytes through a nanopore as a function of applied voltage. A simple analytical model has been developed to extract the zeta-potential. This method is able to detect and differentiate nanometer-sized objects of similar size; it also enables the direct and precise quantitative measurement of their zeta-potential. We have applied this method to a wide range of different nanometer-sized particles and compared the results with values measured by commercially available tools. Furthermore, potential capability of this method in detection and characterization of virions is shown by measuring the low zeta-potential of HIV and EBV viruses.
    No preview · Article · Aug 2012 · Analytical Chemistry
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    ABSTRACT: We have developed and characterized two novel micro flow sensors based on measuring the electrical impedance of the interface between the flowing liquid and metallic electrodes embedded on the channel walls. These flow sensors are very simple to fabricate and use, are extremely compact and can easily be integrated into most microfluidic systems. One of these devices is a micropore with two tantalum/platinum electrodes on its edges; the other is a micro channel with two tantalum /platinum electrodes placed perpendicular to the channel on its walls. In both sensors the flow rate is measured via the electrical impedance between the two metallic electrodes, which is the impedance of two metal-liquid junctions in series. The dependency of the metal-liquid junction impedance on the flow rate of the liquid has been studied. The effects of different parameters on the sensor's outputs and its noise behavior are investigated. Design guidelines are extracted and applied to achieve highly sensitive micro flow sensors with low noise.
    Full-text · Article · Jul 2012 · Microfluidics and Nanofluidics

Publication Stats

2k Citations
426.74 Total Impact Points

Institutions

  • 1994-2014
    • imec Belgium
      • Smart Systems and Energy Technology
      Louvain, Flemish, Belgium
  • 2007
    • University of Oxford
      • Department of Materials
      Oxford, England, United Kingdom
  • 1995
    • Radboud University Nijmegen
      Nymegen, Gelderland, Netherlands