Microfabricated Gold Electrodes Research Articles

Capture and Release of Cancer Cells Through Smart Bioelectronics.

Noninvasive collection of target cells such as circulating tumor cells (CTCs) is crucial for biology and medicine research. Conventional methods of cell collection are often complex, requiring either size-dependent sorting or invasive enzymatic reactions. Here, we show the development of a functional polymer film, which combines the thermoresponsive poly(N-isopropylacrylamide) and the conducting poly(3,4-ethylenedioxythiopene)/poly(styrene sulfonate), and its use for the capture and release of CTCs. When coated onto microfabricated gold electrodes, the proposed polymer films are capable of noninvasively capturing and controllably releasing cells while, at the same time, monitoring these processes with conventional electrical measurements.

Highly Sensitive Detection of Protein Biomarkers Using Graphene Field-Effect Transistor Integrated with a Microfluidic Platform

In this work, we present a highly sensitive biosensing platform for the detection of protein biomarkers by utilizing aptamer-modified graphene field-effect transistors (GFETs) contained within a microfluidic chip. We have chosen thrombin as a model biomarker and its corresponding DNA-based aptamer as a target receptor which is anchored to the graphene surface to modulate the drain current of the GFET. The proposed biosensing platform has the potential to be used in early-stage disease diagnosis and in applications related to lab-on-a-chip-based health monitoring technology. Aptamer-based biosensing offers many advantages including low-cost, chemical and thermal stability, mass producibility, long shelf-life and reusability. Among different target analytes, protein biomarker detection is critical in early screening and diagnostics of diseases. Thrombin is an important protein biomarker which is produced by the body and plays a significant role in blood clotting or coagulation process or regulation of tumor growth [1]. Therefore, detection of thrombin is key to monitoring a number of cardiovascular diseases. Graphene-based FET (GFET) has become a highly promising candidate as next-generation biosensors due to the various inherent advantages it offers such as facile integration with the existing planar technology due to its 2-dimensional structure, label-free sensing, and high sensitivity. However, one of the major challenges with GFET-based biosensing is the lack of reproducibility due to each graphene layer having different physical dimensions and therefore electronic properties. Moreover, since GFET is highly sensitive to the charge disturbances at the FET channel, the sample liquid that is in contact with the graphene surface must be controlled. Therefore, integration of GFET biosensor with a microfluidic device is expected to enhance the sensing performances in terms of sensitivity and reproducibility. In this work, we have implemented a microfluidic chip integrated with GFET device to build a miniaturized platform for aptamer-based detection of a protein biomarker. The GFET devices were fabricated by transferring CVD (chemical vapor deposition) grown graphene onto the microfabricated gold electrodes on the Si/SiO2substrate where the electrodes form the source and the drainas schematically depicted in Figure 1. The FET devices were then sealed with a PDMS block containing a microfluidic channel as shown in Figure 2. The graphene channel was then functionalized with amine reactive 1-pyrene butyric acid N-hydroxysuccinimide ester (PBASE) by delivering the PBASE dissolved in dimethyl formamide through the microfluidic channel. The PBASE was immobilized on graphene surface via the π-π stacking interaction between graphene and the pyrene group of PBASE. The graphene channel was then further functionalized with the aptamers by flowing the amine terminated thrombin-binding aptamer solution through the microfluidic chip. The aptamers were crosslinked with the PBASE on graphene surface via the covalent linkage between the amine-reactive succinimide group of PBASE and the amine group of the aptamers [2]. For the biomarker detection, various concentrations of thrombin solutions were introduced to the GFET device through the microfluidic flow system. For a given concentration of thrombin, the drain current (IDS) vs. gate voltage (VGS) characteristics were measured in order to observe the shift in the Dirac voltage (VDirac). Figure 3 shows the Raman spectra of graphene on Si/SiO2 substrate before and after PBASE functionalization. The additional peak at 1618 cm-1 in Figure 3(a) represents the pyrene group resonance peak due to π – π stacking interaction [3] between the aromatic pyrenyl group of PBASE and the basal plane of graphene, confirming the immobilization of PBASE on graphene. Figure 4 shows the UV-vis spectra of the device sample before and after the aptamer immobilization. The peak at 260 nm indicates the presence of aptamers on the device surface. The resulting measurements are presented in Figure 5. It can be seen that introduction of thrombin causes the Dirac voltage to shift to the right suggesting p-doping of the graphene surface [4]. This is consistent with the cationic nature of thrombin at neutral pH. The corresponding calibration curve is shown in Figure 6. The sensor shows a linear behavior over a wide range of concentrations from 1 nM to 1µM.

Spatiotemporal control of DNA-based chemical reaction network via electrochemical activation in microfluidics

In recent years, DNA computing frameworks have been developed to create dynamical systems which can be used for information processing. These emerging synthetic biochemistry tools can be leveraged to gain a better understanding of fundamental biology but can also be implemented in biosensors and unconventional computing. Most of the efforts so far have focused on changing the topologies of DNA molecular networks or scaling them up. Several issues have thus received little attention and remain to be solved to turn them into real life technologies. In particular, the ability to easily interact in real-time with them is a key requirement. The previous attempts to achieve this aim have used microfluidic approaches, such as valves, which are cumbersome. We show that electrochemical triggering using DNA-grafted micro-fabricated gold electrodes can be used to give instructions to these molecular systems. We demonstrate how this approach can be used to release at specific times and locations DNA- based instructions. In particular, we trigger reaction-diffusion autocatalytic fronts in microfluidic channels. While limited by the stability of the Au-S bond, this easy to implement, versatile and scalable technique can be used in any biology laboratory to provide new ways to interact with any DNA-based computing framework.

Systematic Approach to the Development of Microfabricated Biosensors: Relationship between Gold Surface Pretreatment and Thiolated Molecule Binding.

Despite the increasing popularity of microfabricated biosensors due to advances in technologic and surface functionalization strategies, their successful implementation is partially inhibited by the lack of consistency in their analytical characteristics. One of the main causes for the discrepancies is the absence of a systematic and comprehensive approach to surface functionalization. In this article microfabricated gold electrodes aimed at biosensor development have been systematically characterized in terms of surface pretreatment, thiolated molecule binding, and reproducibility by means of X-ray photoelectron scattering (XPS) and cyclic voltammetry (CV). It has been shown that after SU-8 photolithography gold surfaces were markedly contaminated, which decreased the effective surface area and surface coverage of a model molecule mercaptohexanol (MCH). Three surface pretreatment methods compatible with microfabricated devices were compared. The investigated methods were (i) cyclic voltammetry in dilute H2SO4, (ii) gentle basic piranha followed by linear sweep voltammetry in dilute KOH, and (iii) oxygen plasma treatment followed by incubation in ethanol. It was shown that all three methods significantly decreased the contamination and increased MCH surface coverage. Most importantly, it was also revealed that surface pretreatments may induce structural changes to the gold surfaces. Accordingly, these alterations influence the characteristics of MCH functionalization.

Template-Directed Electrodeposition of Iron-Palladium Nanowires and Their Electrical Transport and Sensing Properties

Template-directed electrodeposition was utilized to fabricate FexPd1-x nanowires where the dimension was controlled by template pore size (i.e., 30 and 200 nm) whereas the composition was tuned by varying the Fe+2/Pd+2 metal ion ratio in ammonium chloride baths at fixed deposition current density of 5 mA cm−2. A wide range of composition (i.e., Pd-rich, near equiatomic, and Fe-rich) were successfully fabricated and their magnetic properties were examined. Additionally, single nanowire was also integrated on to microfabricated gold electrodes by magnetic alignment, followed by post-annealing in a reducing environment to make good electrical contacts. Assembled single nanowire was utilized to determine their electrical transport properties including temperature-dependent resistance (TCR) and magnetoresistance (MR). Additionally, the feasibility of FexPd1-x nanowire based hydrogen sensor was examined at room temperature.

Circulating Disease Biomarker Detection in Complex Matrices: Real-Time, In Situ Measurements of DNA/miRNA Hybridization via Electrochemical Impedance Spectroscopy

The quantification of significant amounts of disease biomarkers circulating in the bloodstream represents one of the challenging frontiers in biomedicine. The complexity of blood composition has opened the quest for novel detection technologies, capable of discerning small amount of specific biomarkers from other blood proteins/oligonucleotides and of reliably measuring them. In this context, we have developed a label-free device based on differential double-layer capacitance readout at microfabricated gold electrodes and demonstrated its detection performance in real biosample volumes. By means of electrochemical impedance spectroscopy (EIS) measurements in a three electrodes setup, we first calibrated the system following the in situ hybridization of a self-assembled monolayer of single-stranded, short oligonucleotides on the gold microelectrode through the measurement of differential capacitance changes as a function of time, for different concentrations of complementary DNA in a saline buffer. Based on this calibration we used the device to quantify the presence of microRNAs (miRNAs) in human plasma. We demonstrated that our device is fast, sensitive, reusable, reproducible, and perfectly suited to detect biomarkers in complex matrices, as cell lysate, serum, and blood. We put forward the possibility to apply this platform to the bioaffinity detection of protein biomarkers as well as circulating drugs in blood, for therapeutic drug monitoring applications.

Bioaffinity Detection of Biomolecules in Complex Matrices: Real Time Detection of DNA/RNA-Hybridization in Cell Extract Via Electrochemical Impedance Spectroscopy Measurements

The quantification of significant amounts of disease biomarkers circulating in the bloodstream represents one of the challenging frontiers in biomedicine. The complexity of blood composition has opened the quest for novel detection technologies, capable of discerning small amount of specific biomarkers from other blood proteins/oligonucleotides and of reliably measuring them. In this context, we have developed a device based on differential double-layer capacitance readout at microfabricated gold electrodes and demonstrated its detection performance in real bio-sample volumes. By means of electrochemical impedance spectroscopy (EIS) measurements in a three electrodes setup, we first calibrated the system following the in situ hybridization of a self-assembled monolayer of single-stranded, short oligonucleotides on the gold microelectrode through the measurement of differential capacitance changes as a function of time, for different concentrations of complementary DNA in a saline buffer. Based on this calibration we used the device to quantify the presence of microRNAs (miRNAs) in cellular extract. We demonstrated that our device is fast, sensitive, reusable, reproducible and perfectly suited to measure binding affinities in complex matrices, as cell lysate, serum and blood. We put forward the possibility to apply this platform to the bioaffinity detection of protein biomarkers as well as circulating drugs in blood, for therapeutic drug monitoring applications.

Development of stable and reproducible biosensors based on electrochemical impedance spectroscopy: Three-electrode versus two-electrode setup

This work focuses on the development of electrochemical impedance biosensors based on capacitance readout, for the detection of biomolecules in small sample volumes. We performed electrochemical impedance spectroscopy (EIS) measurements of DNA hybridization in electrochemical cells with microfabricated gold electrodes. The time stability of the device was tested in two different configurations: two microelectrodes in a microfluidic channel; two microelectrodes plus a reference electrode in an electrochemical cell. Our results demonstrate that the three-electrode setup is more stable, more reproducible, and suitable for real-time measurements. In the last part of the work we perform a test study of DNA hybridization in real time, and we show that the three-electrode configuration can measure the process in situ and in real time.

Single-Walled Carbon Nanotubes Based Chemicapacitive Sensors

Carboxylated single-walled carbon nanotubes (SWNTs) based chemicapacitive gas sensors were fabricated by AC dielectrophoretically aligning SWNTs across microfabricated gold electrodes with controlled density/device resistance. Two different sensing configurations (i.e., horizontal/in-plane and vertical/out-of-plane) were utilized to compare their sensing performance. Upon exposure to water vapor at room temperature, the response (R = [(C--C0)/C0] x 100%) increased with an increase in water vapor concentration similar to that of resistance response. In horizontal configuration, the response was increased with an increase in device resistance which might be attributed to preferentially alignment of semiconducting SWNTs during initial phase of alignment. However, the response was independent of device resistance in vertical/out-of-plane configuration which indicated that the sensing mechanism is based on the change of dielectric constant of gate and atmosphere.

Selective and Rapid Room Temperature Detection of H2S Using Gold Nanoparticle Chain Arrays

AbstractA novel conductometric hydrogen sulfide (H2S) sensor was fabricated by AC dielectrophoretic assembly of amino acid mediated grown gold nanoparticles across microfabricated gold electrodes. Excellent sensing performance toward hydrogen sulfide, including a sensitivity of 29.84 % ppm−1, a lower detection limit in the sub‐ppm level, the upper detection limit of 2 ppm, and a response time of<20 s for 10 ppm H2S was achieved at room temperature. In addition, excellent selectivity was demonstrated toward CO and SO2 with minimum interference from NH3. Enhanced sensor performance may be attributed to the use of amino acid capped gold nanoparticles compared to thiolated gold nanoparticles and optimized assembly conditions to control the density of gold nanoparticle chains and electron transport properties.

Synthesis and characterization of electrodeposited permalloy (Ni 80Fe 20)/Cu multilayered nanowires

Permalloy (Ni 80Fe 20)/Cu multilayered nanowires (NWs) were electrodeposited using a template directed method from sulfate baths via pulse potential technique. Microstructures and compositions of the nanowires were characterized using various microscopy and spectroscopy techniques. To synthesize compositionally uniform nanowires with high efficiency, new sulfate baths with a high content of Ni 2+ were developed. The effects of deposition potential and concentration of metal ions were optimized to reduce composition inhomogeneity and incorporation of copper in the permalloy layers. Composition of the NiFe layers was found to be close to 20 at% Fe with a maximum of 5 at% Cu. TEM analysis indicated that individual nanowires exhibit distinct and coherent layering structure with rough and wavy interfaces. A synthesized single nanowire was also AC dielectrophoretically assembled across the microfabricated gold electrodes for subsequent magnetoresistance measurements.

Effect of Aspect Ratio (Length:Diameter) on a Single Polypyrrole Nanowire FET Device

Effect of different aspect ratio (length to diameter ratio, L:D) on single polypyrrole (Ppy) nanowire based field effect transistor (FET) sensor for real time pH monitoring was studied. Ppy nanowires with diameters of ~60, ~80 and ~200 nm were synthesized using electrochemical deposition inside anodized aluminium oxide (AAO) template and were assembled using AC dielectrophoretic alignment followed by maskless anchoring on a pair of gold electrodes separated with different gap lengths. Microfabricated gold electrode patterns with gap size between 1 - 4 μm were developed by means of MEMS technique (photolithography). Using field effect transistor geometry with pair of microfabricated gold contact electrodes serving as a source and a drain, and a platinum (Pt) mesh (anchored in a microfluidic channel) was used as a gate electrode. When effect of different aspect ratio of the nanowire were compared, higher sensitivity was recorded for higher aspect ratio. The sensitivity was further improved by modulating the gate potential. These FET sensors based on single polypyrrole nanowire exhibited excellent and tunable sensitivity towards pH variations.

Template Assisted Growth and Characterization of Electrodeposited Permalloy(Ni80Fe20)/Cu Multilayered Nanowires

Permalloy (Ni80Fe20)/Cu multilayered nanowires (NWs) were potentiostatically electrodeposited within the nanopores of anodized alumina from sulfate baths via pulse technique. Microstructures and compositions of the nanowires were characterized using various microscopy and spectroscopy techniques. The effects of deposition potential and concentration of metal ions were optimized to reduce composition inhomogeneity and incorporation of copper in the permalloy layers. Composition of the NiFe layers was found to be close to 20% Fe with maximum 5% Cu. Our experimental observations show that individual nanowires exhibit distinct and coherent layering structure with rough and wavy interfaces. Synthesized single nanowires was also AC dielectrophoretically assembled across microfabricated gold electrodes for subsequent magnetoresistance measurement.

Polyaniline nanowires-gold nanoparticles hybrid network based chemiresistive hydrogen sulfide sensor

We report a sensitive, selective, and fast responding room temperature chemiresistive sensor for hydrogen sulfide detection and quantification using polyaniline nanowires-gold nanoparticles hybrid network. The sensor was fabricated by facile electrochemical technique. Initially, polyaniline nanowires with a diameter of 250–320 nm bridging the gap between a pair of microfabricated gold electrodes were synthesized using templateless electrochemical polymerization using a two step galvanostatic technique. Polyaniline nanowires were then electrochemically functionalized with gold nanoparticles using cyclic voltammetry technique. These chemiresistive sensors show an excellent limit of detection (0.1 ppb), wide dynamic range (0.1–100 ppb), and very good selectivity and reproducibility.

Towards fully integrated Lab-on-Chip: design, assembly and experimental results

We present a direct-write CMOS-based platform for Lab-on-Chip (LoC) applications. A fully automated LoC consists of several active and passive components such as biosensors and microchannels integrated on a single chip for single purpose. In this paper, we describe a low-complexity platform consisting of capacitive biosensor and microfluidic structure. The capacitive biosensor is incorporated with nonolayers of polyelectrolyte molecules for creating sensing probes where the passive components such as microchannels are implemented through a direct-write assembly technique. We demonstrate the formation of Polyelectrolyte Sensing Layer (PSL) atop a Microfabricated Gold Electrode (MGE) and CMOS sensor using Chitosan and Alginate. This is an expanded version of a paper presented at the 3rd IEEE International Workshop on Medical Measurements and Applications, 9 10 May 2008, Ottawa, ON, Canada.

In Situ Fabrication of Single Poly(methyl pyrrole) Nanowire

AbstractOne hundred nm wide by four μm long poly(methyl pyrrole) (PMPy) nanowires without and with biomolecule, biotin‐quantum dot, were synthesized by electrochemical polymerization of 1‐methyl pyrrole with chloride and p‐toluene sulfonate dopants in nanochannels between microfabricated gold electrodes. Scanning electron microscopy examination revealed well‐confined dendrite free chloride‐doped PMPy nanowires with embedded biotin‐quantum dot. Current vs. voltage investigation demonstrated ohmic nature of contact for all nanowires. Chloride‐doped PMPy nanowires were more conductive than with p‐toluene sulfonate doped PMPy nanowires whereas entrapment of biotin‐quantum dot increased the conductivity.

Electro- and magneto-transport properties of a single CoNi nanowire

The temperature-dependent electro- and magneto-transport properties of a single nickel-rich CoNi alloynanowire (∼200 nm in diameter) bridging microfabricated gold electrodes were investigated. Thetemperature coefficient of resistivity (TCR) for a single nickel-rich CoNi nanowire is low incomparison with pure Ni nanowires, and this might be attributed to a higher residualresistivity. The magneto-resistance (MR) of a single nickel-rich CoNi nanowireshows typical anisotropic behaviour, where the magneto-resistance ratio decreaseswith increasing temperature and, as expected, the value is much greater than forpure Ni nanowires. The angular dependence of the magnetization switching field(HSW) indicated that the magnetization reversal mechanism for CoNi nanowiresat 300 K changed from curling to coherent rotation when the anglebetween the nanowire axis and the magnetic field direction is greater than75°.

Development of a novel, multi-analyte biosensor system for assaying cell division: Identification of cell proliferation/death precursor events

A novel, miniaturized biosensor system was created by combining the electrophysiological response of immobilized cells with superoxide-sensing technology, optical and fluorescence microscopy. Vero cells were immobilized in a calcium alginate matrix (at a density of 1.7 x 10(6) cells ml(-1)). A 0.5 cm x 0.5 cm piece of cell-containing gel matrix was aseptically adhered on a glass microscope slide with a microfabricated gold electrode array, sealed with a cover slip and provided with Dulbecco's medium +10% (v/v) fetal calf serum every day by means of a capillary feeding tube. During a culture period of 7 days, the membrane potential of immobilized cells was continuously monitored, while cell division was assayed with an optical microscope. In addition, daily measurements of immobilized cell membrane potential, viability, RNA and calcium concentration, radical oxygen species (ROS) and glutathione accumulation, were conducted by fluorescence microscopy after provision of an appropriate dye. Superoxide accumulation was assayed by covering the electrodes with superoxide dismutase (SOD). Maximum cell membrane potential values and superoxide production were observed upon initiation of cell division. Using the novel biosensor, we were able to correlate seven different cell physiological parameters to each other and formulate a model for ROS-mediated signaling function on cell division and death. In addition, we were able to predict cell proliferation or death by comparing the relative response of the electrophysiological and superoxide sensor during the culture period.

Controlled Growth of a Single Palladium Nanowire between Microfabricated Electrodes

We report a novel electrochemical method for dimensionally controlled growth of a single palladium nanowire between premicrofabricated electrodes. In this method, the dimensions of the nanowire are defined by the dimensions of the e-beam-patterned channel between the microfabricated gold electrodes. The formation of the nanowire is confirmed by both optical and scanning electron microscopy imaging in addition to the electrical measurement using the two terminal current−voltage (I−V) characteristics of the nanowire. Fabrication of a single localized palladium nanowire with controlled dimensions is demonstrated for 100 nm, 500 nm, and 1 μm wide and 2.5 μm long channels (length-to-diameter ratio ∼2.5−25). The growth profile, confinement, resistance, and morphology of the nanowires were strongly influenced by the deposition conditions. A current of −100 nA was found to provide well-confined, stable nanowires with ohmic contact with the gold electrodes.

High mobility of dithiophene-tetrathiafulvalene single-crystal organic field effect transistors.

Single-crystal field effect transistors of the organic semiconductor dithiophene-tetrathiafulvalene (DT-TTF) were prepared by drop casting. Long, thin crystals connected two microfabricated gold electrodes, and a silicon substrate was used as a back gate. The highest hole mobility observed was 1.4 cm2/Vs, which is the highest reported for an organic semiconductor not based on pentacene. A high ON/OFF ratio of at least 7 x 105 was obtained for this device.