Plasmon filters and resonators in metal-insulator-metal waveguides.

We present the numerical and experimental demonstration of plasmonic Bragg filters and resonators inside metal-insulator-metal (MIM) waveguides. The presented filters and resonators are fabricated using standard top down lithography methods. The optical bandgap of the integrated Bragg filters is experimentally observed and its optical properties are investigated as a function of the grating pitch and the number of grating periods. Transmission filters based on a nanocavity resonance were measured, obtaining Q-factors above 30. Tuning of the cavity wavelength was experimentally achieved by varying the cavity length.

Shape-controlled synthesis of NIR absorbing branched gold nanoparticles and morphology stabilization with alkanethiols.

Gold nanoparticles are ideal candidates for clinical applications if their plasmon absorption band is situated in the near infrared region (NIR) of the electromagnetic spectrum. Various parameters, including the nanoparticle shape, strongly influence the position of this absorption band. The aim of this study is to produce stabilized NIR absorbing branched gold nanoparticles with potential for biomedical applications. Hereto, the synthesis procedure for branched gold nanoparticles is optimized varying the different synthesis parameters. By subsequent electroless gold plating the plasmon absorption band is shifted to 747.2 nm. The intrinsic unstable nature of the nanoparticles' morphology can be clearly observed by a spectral shift and limits their use in real applications. However, in this article we show how the stabilization of the branched structure can be successfully achieved by exchanging the initial capping agent for different alkanethiols and disulfides. Furthermore, when using alkanethiols/disulfides with poly(ethylene oxide) units incorporated, an increased stability of the gold nanoparticles is achieved in high salt concentrations up to 1 M and in a cell culture medium. These achievements open a plethora of opportunities for these stabilized branched gold nanoparticles in nanomedicine.

Impact of pre-concentration to covalently biofunctionalize suspended nanoparticles.

The effective biofunctionalization of nanoparticles is crucial for biomedical applications. In this study we investigated the covalent biofunctionalization of magnetic nanoparticles based on carbodiimide activation. An important aspect in the covalent biofunctionalization of nanoparticles has been neglected, namely pre-concentration. Exploiting the electrostatic attraction forces between a protein and the nanoparticle surface will favor the covalent immobilization. We showed that low ionic strength buffers with a pH slightly lower than the pI of the selected biomolecules is needed to increase the yield of covalent immobilization. Additionally, it is demonstrated that the covalently immobilized proteins are bioactive, relying on a sandwich assay using gold nanoparticles as reporter labels.

Increased stability of mercapto alkane functionalized Au nanoparticles towards DNA sensing.

The use of gold nanoparticles (GNPs) in bioassays is often hampered by their colloidal stability. In this study, gold nanoparticles coated with different mercapto alkanes were investigated towards their stability. Hereto, the effects of the alkane chain length (5-11 methylene groups), the type of functional end-group (-OH or -COOH) and the amount of incorporated poly-ethylene oxide units (none, 3 or 6) on the GNP stabilization was evaluated. Based on these results, an optimal mercapto alkane (HS(CH(2))(11)PEO(6)COOH) was selected to increase the colloidal stability up to 2 M NaCl. Furthermore, it was proved that this mercapto alkane is ideally suited to enhance the stability of DNA functionalized GNPs in high electrolytic hybridization buffers. The effectiveness of these DNA functionalized GNPs was demonstrated in a sandwich assay using a surface plasmon resonance biosensor. The superior stability of these nanoparticles during hybridization may lead to enhanced biosensor technologies.

In vitro and in vivo electrochemical characterization of a microfabricated neural probe.

The electrochemical behavior of neural implants with 50 microm-diameter platinum electrodes was tested during acute implantations in the motor cortex of anesthetized rats. Custom Ag|AgCl reference electrodes were prepared that could be co-implanted with the probes. The results obtained in vivo are compared with in vitro measurements performed in buffered saline solution (PBS) with and without the addition of bovine serum albumin (BSA). The presence of BSA clearly altered the performance of the electrodes which was studied by means of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), voltage transient measurements (VT) and monitoring of the open circuit potential (OCP). We found that hydrogen gas evolved at 1.22 A/cm(2) in BSA-free PBS whereas in BSA-containing PBS it occurred already at 0.51 A/cm(2).

Stroke diagnosis with lab-on-a-chip.

A magnetic-based sensing platform with increased sensitivity has been developed for integration into lab-on-a-chip systems. The methodology consists of moving the magnetic particles to the most sensitive spot of the sensor. To demonstrate its effectiveness, the protein S100betabeta, a diagnostic marker for stroke, was used as target protein.

Size dependence of microscopic Hall sensor detection limits.

In this paper the magnetic field detection limits of microscopic Hall sensors are investigated as a function of their lateral size. Hall sensors fabricated from GaAs/AlGaAs heterostructures and silicon are experimentally investigated at different temperatures using Hall effect and noise spectrum measurements. At room temperature a clear size dependence of the detection limit is observed, whereas at low temperatures this dependence is found to disappear. The results are explained using the theory of noise in semiconductors.

Chemical and biological characterization of thiol SAMs for neuronal cell attachment.

Cellular adhesion and growth on solid-state surfaces is the central theme in the development of cell-based biosensors and implantable medical devices. Suitable interface techniques must be applied to construct stable and well-organized thin films of biologically active molecules that would control the development of neuronal cells on chips. Peptides such as RGD fragments, poly-L-lysine (PLL), or basal lamina proteins, such as laminin or fibronectin, are often used in order to promote cellular adhesion on surfaces. In this paper we describe the characterization of several self-assembled monolayers (SAMs) for their ability to anchor a laminin-derived synthetic peptide, PA22-2, a peptide known to promote neuronal attachment and stimulate neurite outgrowth. We have evaluated the immobilization of PA22-2 onto 16-mercaptohexadecanoic acid, 4-maleimide-N-(11-undecyldithio)butanamide, and 2-(maleimide)ethyl-N-(11-hexaethylene oxide-undecyldithio)acetamide SAM functionalized Au substrates. The neuronal attachment and outgrowth have been evaluated in embryonic mouse hippocampal neuron cultures up to 14 days in vitro. Our results show that differences in the cell morphologies were observed on the surfaces modified with various SAMs, despite the minor differences in chemical composition identified using standard characterization tools. These different cell morphologies can most probably be explained when investigating the effect of a given SAM layer on the adsorption of proteins present in the culture medium. More likely, it is the ratio between the specific PA22-2 adsorption and nonspecific medium protein adsorption that controls the cellular morphology. Large amounts of adsorbed medium proteins could screen the PA22-2 sites required for cellular attachment.

Glutamate sensing with enzyme-modified floating-gate field effect transistors.

Neurotransmitter release is the key factor of chemical messaging in the brain. Fast, sensitive and in situ detection of single cell neurotransmitter release is essential for the investigation of synaptic transmission under physiological or pathophysiological conditions. Although various techniques have been developed for detecting neurotransmitter release both in vitro and in vivo, the sensing of such events still remains challenging. First of all, the amount of neurotransmitter released during synaptic transmission is unknown because of the limited number of molecules released and the fast diffusion and reuptake of these molecules after release. On the other hand, advances in microelectronic biosensor devices have made possible the fast detection of various analytes with high sensitivity and selectivity. Specifically, enzyme-modified field-effect (ENFET) devices are attractive for such applications due to their fast response, small dimensions and the possibility to integrate a large number of sensors on the same chip. In this paper, we present a floating-gate FET device coated with glutamate oxidase (GLOD) layer. The surface chemistry was optimized for maximal enzyme loading and long-term stability, and characterized by quartz crystal microbalance and colorimetric assays. Enzyme loading was largest on poly-L-lysin-based surfaces combined with glutaraldehyde. The surface chemistry showed excellent stability for at least one month in Tris buffers stored at 4 degrees C. A glutamate detection limit of 10(-7) M has been obtained with the GLOD-coated FET and our sensor proved to be selective to glutamate only. We show that this biosensor is a promising tool for the in vitro detection of glutamate and can be extended to other neurotransmitters.

Peptide-functionalized microfabricated structures for improved on-chip neuronal adhesion.

Extracellular, high signal-to-noise ratio recordings from electrogenic cells require a tight coupling between the cellular membrane and the recording electrode. Self assembled monolayers (SAMs) of alkanethiols functionalized with peptides were used in combination with micro- and nano-structured features on the sensor surface. This combination of surface chemistry and topography triggers a phagocytosis-like engulfment and ensures tight coupling. In this paper we report the results concerning usage of different SAMs and the influence of the peptide concentration towards cell adhesion and outgrowth. Later on, the optimized peptide functionalized SAMs were applied on micro- and nano-structured sensor surfaces. As a result, phagocytosis-like events could be shown using focused ion beam SEM and confocal fluorescence imaging.

Andreev reflection at high magnetic fields: evidence for electron and hole transport in edge states.

We have studied magnetotransport in arrays of niobium filled grooves in an InAs/Al(x)Ga(1-x)Sb heterostructure. The critical field of up to 2.6 T permits one to enter the quantum Hall regime. In the superconducting state, we observe strong magnetoresistance oscillations, whose amplitude exceeds the Shubnikov-de Haas oscillations by a factor of about 2, when normalized to the background. Additionally, we find that above a geometry-dependent magnetic field value the sample in the superconducting state has a higher longitudinal resistance than in the normal state. Both observations can be explained with edge channels populated with electrons and Andreev-reflected holes.

Experimental separation of Rashba and Dresselhaus spin splittings in semiconductor quantum wells.

The relative strengths of Rashba and Dresselhaus terms describing the spin-orbit coupling in semiconductor quantum well (QW) structures are extracted from photocurrent measurements on n-type InAs QWs containing a two-dimensional electron gas (2DEG). This novel technique makes use of the angular distribution of the spin-galvanic effect at certain directions of spin orientation in the plane of a QW. The ratio of the relevant Rashba and Dresselhaus coefficients can be deduced directly from experiment and does not relay on theoretically obtained quantities. Thus our experiments open a new way to determine the different contributions to spin-orbit coupling.

Local Observation of Field Polarity Dependent Flux Pinning by Magnetic Dipoles.

A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below T(c) at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.