Electrolyte-Gated Graphene Ambipolar Frequency Multipliers for Biochemical Sensing

In this Letter, the ambipolar properties of an electrolyte-gated graphene field-effect transistor (GFET) have been explored to fabricate frequency-doubling biochemical sensor devices. By biasing the ambipolar GFETs in a common-source configuration, an input sinusoidal voltage at frequency f applied to the electrolyte gate can be rectified to a sinusoidal wave at frequency 2f at the drain electrode. The extraordinary high carrier mobility of graphene and the strong electrolyte gate coupling provide the graphene ambipolar frequency doubler an unprecedented unity gain, as well as a detection limit of ∼4 pM for 11-mer single strand DNA molecules in 1 mM PBS buffer solution. Combined with an improved drift characteristics and an enhanced low-frequency 1/f noise performance by sampling at doubled frequency, this good detection limit suggests the graphene ambipolar frequency doubler a highly promising biochemical sensing platform.

Reconstitution of Fusion Proteins in Supported Lipid Bilayers for the Study of Cell Surface Receptor-Ligand Interactions in Cell-Cell Contact

Bioactive molecules such as adhesion ligands, growth factors, or enzymes play an important role in modulating cell behavior such as cell adhesion, spreading, and differentiation. Deciphering the mechanism of ligand-mediated cell adhesion and associated signaling is of great interest not only for fundamental biophysical investigations but also for applications in medicine and biotechnology. In the presented work, we developed a new biomimetic platform that enables culturing primary neurons and testing cell surface-receptor ligand interactions in cell-cell contacts as, e.g., in neuronal synapses. This platform consists of a supported lipid bilayer modified with incorporated neuronal adhesion proteins conjugated with the Fc-domain of IgG (ephrin A5 Fc-chimera). We extensively characterized properties of these protein containing bilayers using fluorescence recovery after photobleaching (FRAP), quartz crystal microbalance with dissipation (QCM-D), and immunostaining. We conclude that the Fc-domain is the part responsible for the incorporation of the protein into the bilayer. The biomimetic platform prepared by this new approach was able to promote neuronal cell adhesion and maintain growth as well as facilitate neuronal maturation as shown by electrophysiological measurements. We believe that our approach can be extended to insert other proteins to create a general culture platform for neurons and other cell types.

Effects of Morphology Constraint on Electrophysiological Properties of Cortical Neurons

There is growing interest in engineering nerve cells in vitro to control architecture and connectivity of cultured neuronal networks or to build neuronal networks with predictable computational function. Pattern technologies, such as micro-contact printing, have been developed to design ordered neuronal networks. However, electrophysiological characteristics of the single patterned neuron haven't been reported. Here, micro-contact printing, using polyolefine polymer (POP) stamps with high resolution, was employed to grow cortical neurons in a designed structure. The results demonstrated that the morphology of patterned neurons was well constrained, and the number of dendrites was decreased to be about 2. Our electrophysiological results showed that alterations of dendritic morphology affected firing patterns of neurons and neural excitability. When stimulated by current, though both patterned and un-patterned neurons presented regular spiking, the dynamics and strength of the response were different. The un-patterned neurons exhibited a monotonically increasing firing frequency in response to injected current, while the patterned neurons first exhibited frequency increase and then a slow decrease. Our findings indicate that the decrease in dendritic complexity of cortical neurons will influence their electrophysiological characteristics and alter their information processing activity, which could be considered when designing neuronal circuitries.

Influence of Self-Assembled Alkanethiol Monolayers on Stochastic Amperometric On-Chip Detection of Silver Nanoparticles

We investigate the influence of self-assembled alkanethiol monolayers at the surface of platinum microelectrode arrays on the stochastic amperometric detection of citrate-stabilized silver nanoparticles in aqueous solutions. The measurements were performed using a microelectrode array featuring 64 individually addressable electrodes that are recorded in parallel with a sampling rate of 10 kHz for each channel. We show that both the functional end group and the total length of the alkanethiol influence the charge transfer. Three different terminal groups, an amino, a hydroxyl, and a carboxyl, were investigated using two different molecule lengths of 6 and 11 carbon atoms. Finally, we show that a monolayer of alkanethiols with a length of 11 carbon atoms and a carboxyl terminal group can efficiently block the charge transfer of free nanoparticles in an aqueous solution.

HP-Xe to go: Storage and transportation of hyperpolarized (129)Xenon

Recently the spin-lattice relaxation time T1 of hyperpolarized (HP)-(129)Xe was significantly improved by using uncoated and Rb-free storage vessels of GE180 glass. For these cells, a simple procedure was established to obtain reproducible wall relaxation times of about 18 h. Then the limiting relaxation mechanism in pure Xe is due to the coupling between the nuclear spins and the angular momentum of the Xe-Xe van-der-Waals-molecules. This mechanism can be significantly reduced by using different buffer gases of which CO2 was discovered to be the most efficient so far. From these values, it was estimated that for a 1:1 mixture of HP-Xe with CO2 a longitudinal relaxation time of about 7 h can be expected, sufficient to transport HP-Xe from a production to a remote application site. This prediction was verified for such a mixture at a total pressure of about 1 bar in a 10 cm glass cell showing a storage time of T1≈9 h (for T1(wall)=(34±9) h) which was transported inside a magnetic box over a distance of about 200 km by car.

Toward Intraoperative Detection of Disseminated Tumor Cells in Lymph Nodes with Silicon Nanowire Field Effect Transistors

Within an hour, as little as one disseminated tumor cell (DTC) per lymph node can be quantitatively detected using an intraoperative biosensing platform based on silicon nanowire field-effect transistors (SiNW FET). It is also demonstrated that the integrated biosensing platform is able to detect the presence of circulating tumor cells (CTCs) in the blood of colorectal cancer patients. The presence of DTCs in lymph nodes and CTCs in peripheral blood is highly significant as it is strongly associated with poor patient prognosis. The SiNW FET sensing platform out-performed in both sensitivity and rapidity not only the current standard method based on pathological examination of tissue sections but also the emerging clinical gold standard based on molecular assays. The possibility to achieve accurate and highly sensitive analysis of the presence of DTCs in the lymphatics within the surgery time frame has the potential to spare cancer patients from an unnecessary secondary surgery, leading to reduced patient morbidity, improving their psychological wellbeing and reducing time spent in hospital. This study demonstrates the potential of nanoscale field-effect technology in clinical cancer diagnostics.

Ultra-thin resin embedding method for scanning electron microscopy of individual cells on high and low aspect ratio 3D nanostructures

The preparation of biological cells for either scanning or transmission electron microscopy requires a complex process of fixation, dehydration and drying. Critical point drying is commonly used for samples investigated with a scanning electron beam, whereas resin-infiltration is typically used for transmission electron microscopy. Critical point drying may cause cracks at the cellular surface and a sponge-like morphology of nondistinguishable intracellular compartments. Resin-infiltrated biological samples result in a solid block of resin, which can be further processed by mechanical sectioning, however that does not allow a top view examination of small cell-cell and cell-surface contacts. Here, we propose a method for removing resin excess on biological samples before effective polymerization. In this way the cells result to be embedded in an ultra-thin layer of epoxy resin. This novel method highlights in contrast to standard methods the imaging of individual cells not only on nanostructured planar surfaces but also on topologically challenging substrates with high aspect ratio three-dimensional features by scanning electron microscopy.

Inkjet printing of UV-curable adhesive and dielectric inks for microfluidic devices

Bonding of polymer-based microfluidics to polymer substrates still poses a challenge for Lab-On-a-Chip applications. Especially, when sensing elements are incorporated, patterned deposition of adhesives with curing at ambient conditions is required. Here, we demonstrate a fabrication method for fully printed microfluidic systems with sensing elements using inkjet and stereolithographic 3D-printing.

3D Au–SiO2 nanohybrids as a potential scaffold coating material for neuroengineering

We demonstrate 3D Au–SiO₂ hybrid nanoparticles render micro/nanotopography and provide a high density of stable adhesion cue domains facilitating strong adhesion, viability and guidance of the neurons.

Effect of magnetic field fluctuation on ultra-low field MRI measurements in the unshielded laboratory environment

Magnetic field fluctuations in our unshielded urban laboratory can reach hundreds of nT in the noisy daytime and is only a few nT in the quiet midnight. The field fluctuation causes the Larmor frequency fL to drift randomly for several Hz during the unshielded ultra-low field (ULF) nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) measurements, thus seriously spoiling the averaging effect and causing imaging artifacts. By using an active compensation (AC) technique based on the spatial correlation of the low-frequency magnetic field fluctuation, the field fluctuation can be suppressed to tens of nT, which is a moderate situation between the noisy daytime and the quiet midnight. In this paper, the effect of the field fluctuation on ULF MRI measurements was investigated. The 1D and 2D MRI signals of a water phantom were measured using a second-order low-Tc superconducting quantum interference device (SQUID) in three fluctuation cases: severe fluctuation (noisy daytime), moderate fluctuation (daytime with AC) and minute fluctuation (quiet midnight) when different gradient fields were applied. When the active compensation is applied or when the frequency encoding gradient field Gx reaches a sufficiently strong value in our measurements, the image artifacts become invisible in all three fluctuation cases. Therefore it is feasible to perform ULF-MRI measurements in unshielded urban environment without imaging artifacts originating from magnetic fluctuations by using the active compensation technique and/or strong gradient fields.

Multi-level logic gate operation based on amplified aptasensor performance

Conventional electronic circuits can perform multi-level logic operations; however, this capability is rarely realized by biological logic gates. In addition, the question of how to close the gap between biomolecular computation and silicon-based electrical circuitry is still a key issue in the bioelectronics field. Here we explore a novel split aptamer-based multi-level logic gate built from INHIBIT and AND gates that performs a net XOR analysis, with electrochemical signal as output. Based on the aptamer-target interaction and a novel concept of electrochemical rectification, a relayed charge transfer occurs upon target binding between aptamer-linked redox probes and solution-phase probes, which amplifies the sensor signal and facilitates a straightforward and reliable diagnosis. This work reveals a new route for the design of bioelectronic logic circuits that can realize multi-level logic operation, which has the potential to simplify an otherwise complex diagnosis to a "yes" or "no" decision.

Immobilization and surface functionalization of gold nanoparticles monitored via streaming current/potential measurements

A streaming current/potential method is optimized and used for the analysis of the variation of the surface potential upon chemical modifications of a complex interface consisting of different organic molecules and gold nanoparticles (AuNPs). The surfaces of Si/SiO2 substrates modified with 3-aminopropyltriethoxysilane (APTES), AuNPs, and 11-amino-1-undecanethiol (aminothiols) are analyzed via pH and time dependent ζ potential measurements that reveal the stability and modification of the surface and identify crucial parameters for each individual preparation step. For instance, surface activation and especially molecular adsorbate layers tend not to be stable in time, whereas the substrate and the AuNPs provide a stable surface potential as long as impurities are avoided. It is shown that the streaming potential/current technique represents an ideal tool to analyze and monitor the complex surfaces and their modification.

A novel bioelectrochemical interface based on in situ synthesis of gold nanostructures on electrode surfaces and surface activation by Meerwein's salt. A bioelectrochemical sensor for glucose determination

A novel effective bioelectrochemical sensor interface for enzyme biosensors is proposed. The method is based on in situ synthesis of gold nanostructures (5-15 nm) on the thin-film electrode surface using the oleylamine (OA) method, which provides a high-density, stable, electrode interface nanoarchitecture. New method to activate the surface of the OA-stabilized nanostructured electrochemical interface for further functionalization with biomolecules (glucose oxidase enzyme) using Meerwein's salt is proposed. Using this approach a new biosensor for glucose determination with improved analytical characteristics: wide working range of 0.06-18.5mM with a sensitivity of 22.6 ± 0.5 μAmM(-1)cm(-2), limit of detection 0.02 mM, high reproducibility, and long lifetime (60 d, 93%) was developed. The surface morphology of the electrodes was characterized by scanning electron microscopy (SEM). The electrochemical properties of the interface were studied by cyclic voltammetry and electrochemical impedance spectroscopy using a Fe(II/III) redox couple. The studies revealed an increase in the electroactive surface area and a decrease in the charge transfer resistance following surface activation with Meerwein's reagent. A remarkably enhanced stability and reproducibility of the sensor was achieved using in situ synthesis of gold nanostructures on the electrode surface, while surface activation with Meerwein's salt proved indispensable in achieving an efficient bioelectrochemical interface.

Nanostructured cavity devices for extracellular stimulation of HL-1 cells

Microelectrode arrays (MEAs) are state-of-the-art devices for extracellular recording and stimulation on biological tissue. Furthermore, they are a relevant tool for the development of biomedical applications like retina, cochlear and motor prostheses, cardiac pacemakers and drug screening. Hence, research on functional cell-sensor interfaces, as well as the development of new surface structures and modifications for improved electrode characteristics, is a vivid and well established field. However, combining single-cell resolution with sufficient signal coupling remains challenging due to poor cell-electrode sealing. Furthermore, electrodes with diameters below 20 µm often suffer from a high electrical impedance affecting the noise during voltage recordings. In this study, we report on a nanocavity sensor array for voltage-controlled stimulation and extracellular action potential recordings on cellular networks. Nanocavity devices combine the advantages of low-impedance electrodes with small cell-chip interfaces, preserving a high spatial resolution for recording and stimulation. A reservoir between opening aperture and electrode is provided, allowing the cell to access the structure for a tight cell-sensor sealing. We present the well-controlled fabrication process and the effect of cavity formation and electrode patterning on the sensor's impedance. Further, we demonstrate reliable voltage-controlled stimulation using nanostructured cavity devices by capturing the pacemaker of an HL-1 cell network.

Magnetic tweezers with high permeability electromagnets for fast actuation of magnetic beads

As a powerful and versatile scientific instrument, magnetic tweezers have been widely used in biophysical research areas, such as mechanical cell properties and single molecule manipulation. If one wants to steer bead position, the nonlinearity of magnetic properties and the strong position dependence of the magnetic field in most magnetic tweezers lead to quite a challenge in their control. In this article, we report multi-pole electromagnetic tweezers with high permeability cores yielding high force output, good maneuverability, and flexible design. For modeling, we adopted a piece-wise linear dependence of magnetization on field to characterize the magnetic beads. We implemented a bi-linear interpolation of magnetic field in the work space, based on a lookup table obtained from finite element simulation. The electronics and software were custom-made to achieve high performance. In addition, the effects of dimension and defect on structure of magnetic tips also were inspected. In a workspace with size of 0.1 × 0.1 mm(2), a force of up to 400 pN can be applied on a 2.8 μm superparamagnetic bead in any direction within the plane. Because the magnetic particle is always pulled towards a tip, the pulling forces from the pole tips have to be well balanced in order to achieve control of the particle's position. Active video tracking based feedback control is implemented, which is able to work at a speed of up to 1 kHz, yielding good maneuverability of the magnetic beads.

Complementary metal oxide semiconductor compatible silicon nanowires-on-a-chip: fabrication and preclinical validation for the detection of a cancer prognostic protein marker in serum

An integrated translational biosensing technology based on arrays of silicon nanowire field-effect transistors (SiNW FETs) is described and has been preclinically validated for the ultrasensitive detection of the cancer biomarker ALCAM in serum. High-quality SiNW arrays have been rationally designed toward their implementation as molecular biosensors. The FET sensing platform has been fabricated using a complementary metal oxide semiconductor (CMOS)-compatible process. Reliable and reproducible electrical performance has been demonstrated via electrical characterization using a custom-designed portable readout device. Using this platform, the cancer prognostic marker ALCAM could be detected in serum with a detection limit of 15.5 pg/mL. Importantly, the detection could be completed in less than 30 min and span a wide dynamic detection range (∼10(5)). The SiNW-on-a-chip biosensing technology paves the way to the translational clinical application of FET in the detection of cancer protein markers.

Inducing microscopic thermal lesions for the dissection of functional cell networks on a chip

We present a versatile chip-based method to inflict microscopic lesions on cellular networks or tissue models. Our approach relies on resistive heating of microstructured conductors to impose highly localized thermal stress on specific regions of a cell network. We show that networks can be precisely dissected into individual subnetworks using a microwire crossbar array. To this end, we pattern a network of actively beating cardiomyocyte-like cells into smaller subunits by inflicting thermal damage along selected wires of the array. We then investigate the activity and functional connectivity of the individual subnetworks using a Ca(2+) imaging-based signal propagation analysis. Our results demonstrate the efficient separation of functional activity between individual subnetworks on a microscopic level. We believe that the presented technique may become a powerful tool for investigating lesion and regeneration models in cellular networks.

Tuning neuron adhesion and neurite guiding using functionalized AuNPs and backfill chemistry

Gold nanoparticles are used to investigate the dependence of neuron adhesion on the density of cell binding sites and particle backfill. Neurons viability and neurite development depend differently on cell attractive and cell repellant surface cues.

Self-assembly of platinum nanoparticles and coordination-driven assembly with porphyrin

The easily accessible surfaces of Pt nanostructures were demonstrated by two kinds of assembly processes.

Label-free measurement of cell-electrode cleft gap distance with high spatial resolution surface plasmon microscopy

Understanding the interface between cells or tissues and artificial materials is of critical importance for a broad range of areas. For example, in neurotechnology, the interfaces between neurons and external devices create a link between technical and the nervous systems by stimulating or recording from neural tissue. Here, a more effective interface is required to enhance the electrical characteristics of neuronal recordings and stimulations. Up to now, the lack of a systematic characterization of cell-electrode interaction turns out to be the major bottleneck. In this work, we employed a recently developed surface plasmon microscope (SPM) to monitor in real-time the cell-metal interface and to measure in situ the gap distance of the cleft with the spatial resolution reaching to the optical diffraction limit. The SPM allowed determination of the distance of human embryonic kidney 293 cells cultured on gold surfaces coated with various peptides or proteins without any labeling. This method can dramatically simplify the interaction investigation at metal-living cell interface and should be incorporated into systematic characterization methods.

Defined patterns of neuronal networks on 3D thiol-functionalized microstructures

It is very challenging to study the behavior of neuronal cells in a network due to the multiple connections between the cells. Our idea is then to simplify such a network with a configuration where cells can have just a fixed number of connections in order to create a well-defined and ordered network. Here, we report about guiding primary cortical neurons with three-dimensional gold microspines selectively functionalized with an amino-terminated molecule.

Electrochemical current rectification-a novel signal amplification strategy for highly sensitive and selective aptamer-based biosensor

Electrochemical aptamer-based (E-AB) sensors represent an emerging class of recently developed sensors. However, numerous of these sensors are limited by a low surface density of electrode-bound redox-oligonucleotides which are used as probe. Here we propose to use the concept of electrochemical current rectification (ECR) for the enhancement of the redox signal of E-AB sensors. Commonly, the probe-DNA performs a change in conformation during target binding and enables a nonrecurring charge transfer between redox-tag and electrode. In our system, the redox-tag of the probe-DNA is continuously replenished by solution-phase redox molecules. A unidirectional electron transfer from electrode via surface-linked redox-tag to the solution-phase redox molecules arises that efficiently amplifies the current response. Using this robust and straight-forward strategy, the developed sensor showed a substantial signal amplification and consequently improved sensitivity with a calculated detection limit of 114nM for ATP, which was improved by one order of magnitude compared with the amplification-free detection and superior to other previous detection results using enzymes or nanomaterials-based signal amplification. To the best of our knowledge, this is the first demonstration of an aptamer-based electrochemical biosensor involving electrochemical rectification, which can be presumably transferred to other biomedical sensor systems.

Nanocavity crossbar arrays for parallel electrochemical sensing on a chip

We introduce a novel device for the mapping of redox-active compounds at high spatial resolution based on a crossbar electrode architecture. The sensor array is formed by two sets of 16 parallel band electrodes that are arranged perpendicular to each other on the wafer surface. At each intersection, the crossing bars are separated by a ca. 65 nm high nanocavity, which is stabilized by the surrounding passivation layer. During operation, perpendicular bar electrodes are biased to potentials above and below the redox potential of species under investigation, thus, enabling repeated subsequent reactions at the two electrodes. By this means, a redox cycling current is formed across the gap that can be measured externally. As the nanocavity devices feature a very high current amplification in redox cycling mode, individual sensing spots can be addressed in parallel, enabling high-throughput electrochemical imaging. This paper introduces the design of the device, discusses the fabrication process and demonstrates its capabilities in sequential and parallel data acquisition mode by using a hexacyanoferrate probe.