Search

Fast scan cyclic voltammetry is a powerful technique for selective detection of biogenic amines in vivo and in vitro. Here we investigate this technique for multichannel spatiotemporal detection of neurotransmitter concentrations on a chip, using metal microelectrode arrays. The array-based approach allows monitoring concentration gradients of redox-active species at multiple highly localized (3 µm) positions simultaneously, while maintaining selectivity from the characteristic voltammogram shapes. We demonstrate that concentration fluctuations at individual electrode locations are resolved after spatially confined neurotransmitter injections. Furthermore, we discuss effects of microelectrode degradation caused by electrode sweeping during long-term experiments.

We simulate the amperometric response time of an interdigitated electrode array with respect to reversible adsorption of redox active molecules at the electrode surface. When neighboring electrodes are biased to potentials below and above the redox potential of the species under investigation, molecules can participate in repetitive reactions leading to an amplification of the Faradaic current. During collisions between molecules and electrodes, the molecules may undergo reversible adsorption. We simulate the behavior of this redox cycling process by using a discrete random walk model, which mimics the diffusive pathway of the molecules. Adsorption probabilities are implemented via the boundary conditions at the electrodes. Using this simulation, we investigate the effect of reversible adsorption on the response time of the interdigitated electrode array.

Nanocavity sensor arrays are powerful devices for parallel extracellular recordings of action potentials from cell networks. The sensors combine a high spatial resolution and seal resistance between cell and electrode with low electrode impedances, allowing low-noise electrophysiological measurements of individual cells in a network without the use of CMOS technology. In this paper, we present a new protocol for the simple fabrication of nanocavity sensors, which enables easy modification of standard microelectrode arrays. Our approach enables processing of devices with improved sensor characteristics and flexibility regarding electrode area and aperture size with minimal effort. We characterize the sensors by impedance spectroscopy and demonstrate their applicability by recording action potentials of individual cardiomyocyte-like cells growing in a network on the chip surface.

We describe the fabrication process and characterization of polymer coated multielectrode array chips featuring 64 planar Ag/AgCl electrodes for electrophysiological and electrochemical measurements of ionic currents. The multielectrode chip consists of platinum feedlines and microelectrodes fabricated on an oxidized silicon substrate. The feed lines are passivated with a polyimide layer. Silver is deposited electrochemically on the platinum electrodes and chlorinated afterwards resulting in stable micro Ag/AgCl reference electrodes that are recessed within picoliter cavities. We envisage the chip as a platform for high-throughput investigation of lipid bilayers. Schematics of a planar reference electrode. The chips are processed on oxidized silicon (blue) and consist of Pt feedlines (yellow), the Ag/AgCl electrode (black) passivated with polyimide (light blue).

Molecular recognition is the key feature of bioinspired sensor applications. A profound understanding of recognitive binding events is crucial for the design of highly specific and effective binding molecules. In this paper a series of simple to complex oligopeptides, derived from the Ca2+ chelator ethylenediaminetetraacetic acid (EDTA) and the Ca2+ binding protein calmodulin, were investigated. It was shown, that specifically designed oligopeptides can fulfil protein-like recognition functions but are much easier to immobilize and enable high functional integration. Impedance spectroscopy (IS) could be used to detect binding of Ca2+ to different peptides because the double layer capacitance is highly sensitive to changes in the morphology and dielectric properties of adsorbate layers. Finally, the designed functional oligopeptides were implemented in silicon nanowire field effect transistors (SiNW FETs) as a proof of concept for future problem-optimized molecules in sensor devices. The graph shows frequency-normalized admittance plots and corresponding schemes of a peptide monolayers layer before and after binding of Ca2+ ions.

We report the fabrication of a silicon nanowire (SiNW) transistor array realized in a top-down process on 4" silicon-on-insulator (SOI) wafers. This process is a second, optimized version combining thermal nanoimprint lithography and wet chemical etching to define planar nanowires and contact lines out of the top silicon layer in one etching step. We used ion implantation to reduce series resistance of source and drain contact lines. In total 56 individually addressable SiNWs were aligned in two lines. Wires dimensions were down to 60 nm in width and 40 nm in height having lengths up to 40 μm. Due to an improved processing we obtained almost identical wire structures and electronic properties throughout the 4" wafer enabling direct comparison of signals from one chip. The wires were capped by a microfuidic structure, which was used to administer different analyte solutions. First pH characterization measurements and molecular detection experiments using charged polyelectrolytes (PEs) and poly(D)lysine were successful. In future these devices could be used to observe diffusion of analytes in a time-dependent, spatially- and quantitatively-resolved readout.

Mecular bridged nanocontacts allow direct electrical addressing of electroactive molecules, which is of interest for the development of molecular based electronic devices. In the present paper, the electroactive molecule 6-ferrocenyl-1-hexanethiol (Fc-HT) was integrated into metal―bridge―metal (MBM) junctions assembled in a scanning tunneling microscope (STM) setup. A diode-like behavior was observed from the current/bias (I t /V b ) signal through Au (substrate)/Fc-HT/Au (tip) junction, which presented an asymmetric current response due to the resonant tunneling between metal electrode and ferrocenylthiol molecules. With gate electrode modulation, the enhancement of the tunneling current can be controlled, which allows to tune the direction of the current rectification. Our investigations demonstrated that ferrocenylthiol bridged MBM nanostructure has potential applications in the future design of higher-order heterojunctions components in combination with electrochemical logic gates.

We describe the fabrication, electrical and electrochemical characterization of silicon nanowire arrays, which were processed in a top-down approach using combined nanoimprint lithography and wet chemical etching. We used the top silicon layer as contact line and observed an influence of implantation and subsequent annealing of these lines to the device performance. In addition we found a subthreshold slope dependence on wire size. When operated in a liquid environment, wires can be utilized as pH sensors. We characterized the pH sensitivity in the linear range and in the subthreshold operation regime. As a first proof-of-principle experiment for the later use of the sensors in bioassays, we monitored the buildup ofpolyelectrolyte multilayers on the wire surface.

In this work cytochrome c (cyt c), a heme-containing redox protein, was used as a model system for studying general aspects of wiring proteins into conceptual metal–biomolecule–metal nanoparticle junctions. Electron transfer kinetics of cyt c immobilized on Au(111) via MUA spacer has been investigated in the absence and presence of surface-bound gold nanoparticles. The combined effect of size and immobilization time of AuNPs as well as their binding site to cyt c on the tunnelling rate of the protein has been explored. It has been observed that electrostatic binding of 5 nm-in-diameter gold nanoparticles obviously accelerates the electron transfer rate of cyt c. Potential applications of gold nanoparticles as electric contacts in nanodevices as well as their beneficial or adverse health effects have been suggested. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A method for impedimetric detection of biomolecules with field-effect transistor (FET) arrays is presented. For biomolecular detection, the corresponding molecules such as single-stranded DNA or bovine serum albumine (BSA) were covalently immobilized to the gate structure of 16-channel open-gate FET devices. A micro spotter system was used to site-selectively attach biomolecules to selected channels out of the array. The differential transistor transfer function (TTF) of the FETs with attached biomolecules compared to free FETs was recorded. With this impedimetric technique it was possible to reliably detect biomolecules, because the recordings were not disturbed by the typical long-term drift of the sensors like in potentiometric readout mode. For transistor gates, where DNA sequences of different length were attached, small differences in the TTF spectra were detected. When BSA was covalently immobilized to the FETs clear differences in the TTF spectra were detected, which were independent on buffer pH variations around the isoelectric point of the protein. Based on the results presented in this article it can be concluded, that the TTF method detects passive components of the biomolecules like resistance and capacitance rather than surface charge effects.

Electronic DNA sensors based on field-effect transistor arrays operating in liquid environment, offer an alternative method for the detection of biomolecular binding events, without the requirement to label the target molecules. These semiconductor devices are sensitive to electrical charge variations that occur at the surface/electrolyte interface. Using such devices, the hybridization reaction of oligonucleotides with complementary single-stranded oligonucleotides, which are immobilized on the oxide surface of the transistor gate, can be detected. The detection principle is based on the intrinsic charge of the nucleic acid molecules in liquid environment. In this article we present a summary of the ‘DNA BioFET’ project, which has been conducted in our group in recent years. We compare an electrostatic and a covalent immobilization protocol for the probe DNA. Most important for reliable signal readout is the use of a differential approach, for which a DNA microarray represents an ideal bioassay. With our FET setup we can either use a chip-to-chip reference with two electrically identical chips, which are coated with different probe sequences. A more sophisticated method, which makes use of the DNA microarray technology, was established with an aligned microspotting system enabling the coating of different channels out of the microarray with different probe sequences. So far our current chip design uses 16 channels enabling first proof-of-principle experiments. The detection method itself, however, offers the possibility to be up scaled to many thousands of sensor spots like currently used in fluorescence based DNA microarray chips. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Microwire arrays are a powerful tool for the exertion of localized thermal stress on cellular networks. Combining microwire arrays with a set of orthogonal axon-guiding microchannels on-chip allows for the positioning of neurites, as well as control over their polarity. In this paper, we present a new fabrication approach, based on standard clean room fabrication and sacrificial layer etching, for the integration of microwire arrays into neurite guiding structures. The system permits the application of strong temperature gradients, enabling localized thermal stimulation inside microchannels.