Nanostructured Microelectrodes Research Articles

Effect of microelectrode structure on electrocatalysis at nucleic acid-modified sensors.

The electrochemical detection of nucleic acids using an electrocatalytic reporter system and nanostructured microelectrodes is a powerful approach to ultrasensitive biosensing. In this report we systematically study for the first time the behavior of an electrocatalytic reporter system at nucleic acid-modified electrodes with varying structures and sizes. [Ru(NH3)6](3+) is used as a primary electron acceptor that is electrostatically attracted to nucleic acid-modified electrodes, and [Fe(CN)6](3-) is introduced into the redox system as a secondary electron acceptor to regenerate Ru(3+) after electrochemical reduction. We found that the electrode structure has strong impact on mass transport and electron-transfer kinetics, with structures of specific dimensions yielding much higher electrochemical signals and catalytic efficiencies. The electrocatalytic signals obtained when gold sensors were electrodeposited in both circular and linear apertures were studied, and the smallest structures plated in linear apertures were found to exhibit the best performance with high current densities and turnover rates. This study provides important information for optimal assay performance and insights for the future design and fabrication of high performance biomolecular assays.

Ultrasensitive electrochemical biomolecular detection using nanostructured microelectrodes.

Electrochemical sensors have the potential to achieve sensitive, specific, and low-cost detection of biomolecules--a capability that is ever more relevant to the diagnosis and monitored treatment of disease. The development of devices for clinical diagnostics based on electrochemical detection could provide a powerful solution for the routine use of biomarkers in patient treatment and monitoring and may overcome the many issues created by current methods, including the long sample-to-answer times, high cost, and limited prospects for lab-free use of traditional polymerase chain reaction, microarrays, and gene-sequencing technologies. In this Account, we summarize the advances in electrochemical biomolecular detection, focusing on a new and integrated platform that exploits the bottom-up fabrication of multiplexed electrochemical sensors composed of electrodeposited noble metals. We trace the evolution of these sensors from gold nanoelectrode ensembles to nanostructured microelectrodes (NMEs) and discuss the effects of surface morphology and size on assay performance. The development of a novel electrocatalytic assay based on Ru(3+) adsorption and Fe(3+) amplification at the electrode surface as a means to enable ultrasensitive analyte detection is discussed. Electrochemical measurements of changes in hybridization events at the electrode surface are performed using a simple potentiostat, which enables integration into a portable, cost-effective device. We summarize the strategies for proximal sample processing and detection in addition to those that enable high degrees of sensor multiplexing capable of measuring 100 different analytes on a single chip. By evaluating the cost and performance of various sensor substrates, we explore the development of practical lab-on-a-chip prototype devices. By functionalizing the NMEs with capture probes specific to nucleic acid, small molecule, and protein targets, we can successfully detect a wide variety of analytes at clinically relevant concentrations and speeds. Using this platform, we have achieved attomolar detection levels of nucleic acids with overall assay times as short as 2 min. We also describe the adaptation of the sensing platform to allow for the measurement of uncharged analytes--a challenge for reporter systems that rely on the charge of an analyte. Furthermore, the capabilities of this system have been applied to address the many current and important clinical challenges involving the detection of pathogenic species, including both bacterial and viral infections and cancer biomarkers. This novel electrochemical platform, which achieves large molecular-to-electrical amplification by means of its unique redox-cycling readout strategy combined with rapid and efficient analyte capture that is aided by nanostructured microelectrodes, achieves excellent specificity and sensitivity in clinical samples in which analytes are present at low concentrations in complex matrices.

Nanostructured Pd hydride microelectrodes: in situ monitoring of pH variations in a porous medium.

In this study we report the exceptional potentiometric properties of pH microprobes made with nanostructured palladium hydride microelectrodes and demonstrate their application by monitoring pH variations resulting from a reaction confined in a porous medium. Their potentiometric response was found to be reproducible and stable over several hours but primarily Nernstian over a remarkably wide pH range, including alkaline conditions up to pH 14. Continuous operation was demonstrated by reloading hydrogen at regular intervals to maintain the correct hydride composition thereby alleviating the need for calibration. These properties were validated by detecting pH transients during the carbonation of Ca(OH)2 within a fibrous mesh. Experimental pHs recorded in situ were in excellent agreement with theoretical calculations for the CO2 partial pressures considered. Results also showed that the electrodes were sufficiently sensitive to differentiate between the formation of vaterite and calcite, two polymorphs of CaCO3. These nanostructured microelectrodes are uniquely suited to the determination of pH in highly alkaline solutions, particularly those arising from interfacial reactions at solid and porous surfaces and are thus highly appropriate as pH sensing tips in scanning electrochemical microscopy.

Electrical coupling of mammalian neurons to microelectrodes with 3D nanoprotrusions

Ion Beam Induced Deposition (IBID) is employed to fabricate three-dimensional nanoprotrusions on top of the recording pads of an active pixel sensor array (APS-MEA) featuring 4096 microelectrodes. Modified APS-MEAs are envisioned as enhanced tools to achieve real-time “in-cell” recordings from thousands of sensing elements, thus aiming to large-scale in-vitro registrations with unprecedented signal quality. A generalized electric model is proposed to address the revealed complexity of the neuron/electrode interface, and simulations have been conducted revealing the most advantageous cell/electrode coupling conditions. Preliminary results on the recording of spontaneous activity in cultured neuronal networks by means of nanostructured microelectrodes demonstrate the compatibility of IBID technology and APS-MEA infrastructure. The interface between cultured mammalian neurons and modified microelectrodes is revealed by FIB/SEM analysis, fostering the employment of the proposed electrical model for interpretation of electrical recordings from nanostructured microelectrodes.

Chip-Based Nanostructured Sensors Enable Accurate Identification and Classification of Circulating Tumor Cells in Prostate Cancer Patient Blood Samples

The identification and analysis of circulating tumor cells (CTCs) is an important goal for the development of noninvasive cancer diagnosis. Here we describe a chip-based method using nanostructured microelectrodes and electrochemical readout that confirms the identity of isolated CTCs and successfully interrogates them for specific biomarkers. We successfully analyze and classify prostate tumor cells, first in cultured cells, and ultimately in a pilot study involving blood samples from 16 prostate cancer patients as well as additional healthy controls. In all cases, and for all biomarkers investigated, the novel chip-based assay produced results that agreed with polymerase chain reaction (PCR). The approach developed has a simple workflow and scalable multiplexing, which makes it ideal for further studies of CTC biomarkers.

Kinetics of thiocyanate orientation conversion on Pt surface studied by in situ step-scan time-resolved microscope FTIR spectroscopy

Fast surface dynamic processes of thiocyanate adsorbed on a nanostructured Pt microelectrode were studied by in situ step-scan time-resolved microscope FTIR spectroscopy (in situ SSTR-MFTIRS) at a time resolution of 100 μs. It was found that SCN− adsorbs preferentially on Pt surface through N atom at low potential, while through S atom at high potential. The potential-induced orientation conversion between N- and S-bounded forms is very reversible, and occurs only within 2 ms after potential alternation. However, the rate constant (4×103 s−1) of conversion from N- to S-bounded adsorption is nearly double to that of the reverse process (2×103 s−1). The difference in kinetics of two processes may be explained through the hard-soft acid-base principle.

Fabrication of Nanoporous 1- 3 nm Thick Membranes on Nanostructured Microelectrodes for Low nM Detection

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Biofuel cell as a power source for electronic contact lenses

Here we present unequivocal experimental proof that microscale cofactor- and membrane-less, direct electron transfer based enzymatic fuel cells do produce significant amounts of electrical energy in human lachrymal liquid (tears). 100μm diameter gold wires, covered with 17nm gold nanoparticles, were used to fashion three-dimensional nanostructured microelectrodes, which were biomodified with Corynascus thermophilus cellobiose dehydrogenase and Myrothecium verrucaria bilirubin oxidase as anodic and cathodic bioelements, respectively. The following characteristics of miniature glucose/oxygen biodevices operating in human tears were registered: 0.57V open-circuit voltage, about 1μWcm−2 maximum power density at a cell voltage of 0.5V, and more than 20h operational half-life. Theoretical calculations regarding the maximum recoverable electrical energy can be extracted from the biofuel and the biooxidant, glucose and molecular oxygen, each readily available in human lachrymal liquid, fully support our belief that biofuel cells can be used as electrical power sources for so called smart contact lenses.

Nanomolar detection of p-nitrophenol via in situ generation of p-aminophenol at nanostructured microelectrodes

A new method and a new buffer medium, expected to be practical for miniaturized electrochemical immunoassays, were developed for rapid detection of nanomolar levels of p-nitrophenol (p-NP). The method exploits rapid reduction of p-NP to p-aminophenol (p-AP) by fast scan cyclic voltammetry at 500 V s(-1) at nanostructured carbon fiber microdisk electrodes (~7 μm dia.) fabricated from polyacrylonitrile (PAN) fibers. Large surfaces of the nanostructured microdisks facilitate the rapid reduction of p-NP to p-aminophenol (p-AP), as confirmed by the overlap with the analytical signals of the standards, which is then rapidly detected by fast scan cyclic voltammetry. A new 2-amino-2-(hydroxymethyl)propane-1,3-diol (Tris)-HAc buffer medium was developed in order to allow adaptation of this detection strategy to alkaline phosphatase (ALP)-based immunoassays. Tris-HAc is a stable medium, while the traditional Tris-HCl buffer medium produces large residual faradaic currents attributed to chloride oxidation. Addition of sodium acetate to a Tris-HAc buffer medium allows sensitivity enhancement by a factor of 2 to 0.85 nA μM(-1), similar to the best sensitivity reported at the nanostructured PAN carbon fiber microdisk sensors.

Nanostructured gold microelectrodes for extracellular recording from electrogeniccells

We present a new biocompatible nanostructured microelectrode array for extracellularsignal recording from electrogenic cells. Microfabrication techniques were combined with atemplate-assisted approach using nanoporous aluminum oxide to develop gold nanopillarelectrodes. The nanopillars were approximately 300–400 nm high and had a diameter of60 nm. Thus, they yielded a higher surface area of the electrodes resulting in a decreasedimpedance compared to planar electrodes. The interaction between the large-scale goldnanopillar arrays and cardiac muscle cells (HL-1) was investigated via focusedion beam milling. In the resulting cross-sections we observed a tight couplingbetween the HL-1 cells and the gold nanostructures. However, the cell membranesdid not bend into the cleft between adjacent nanopillars due to the high pillardensity. We performed extracellular potential recordings from HL-1 cells withthe nanostructured microelectrode arrays. The maximal amplitudes recordedwith the nanopillar electrodes were up to 100% higher than those recorded withplanar gold electrodes. Increasing the aspect ratio of the gold nanopillars andchanging the geometrical layout can further enhance the signal quality in the future.

The Contribution of Microelectrodes to Electroanalytical Chemistry: From Reaction Mechanisms and Scanning Electrochemical Microscopy to Ocean Sensors

AbstractThe advent of microelectrodes has opened new areas of study and significantly improved the quality of electroanalytical data. Using examples from the author’s work, this brief review describes contributions made by microelectrodes to the field of electroanalytical chemistry. Several fundamental studies, ranging from the development of theoretical approaches for the scanning electrochemical microscope to the determination of the mechanism and kinetics of coupled chemical reactions, are recalled. Most involve a serious dose of numerical simulations. Several applied studies illustrate the development of experimental approaches, in particular the development of nanostructured microelectrodes, to improve the reliability and quality of the microelectrode response for analytical measurements and sensing.

Direct, Electronic MicroRNA Detection for the Rapid Determination of Differential Expression Profiles

In the time it takes for lunch: An electronic chip featuring nanostructured microelectrodes (NMEs) enables the analysis of microRNA expression profiles in just 30 min in small RNA samples without enzymatic amplification or sequence labeling. The multiplexed chip detects the hybridization of microRNA targets to NME surfaces and provides large electrocatalytic gain through the use of an ultrasensitive redox reporter system (see picture).

Direct Profiling of Cancer Biomarkers in Tumor Tissue Using a Multiplexed Nanostructured Microelectrode Integrated Circuit

The analysis of panels of nucleic acid biomarkers offers valuable diagnostic and prognostic information for cancer management. A cost-effective, highly sensitive electronic chip would offer an ideal platform for clinical biomarker readout and would have maximal utility if it was (i) multiplexed, enabling on-chip assays of multiple biomarkers, and (ii) able to perform direct (PCR-free) readout of disease-related genes. Here we report a chip onto which we integrate novel nanostructured microelectrodes and with which we directly detect cancer biomarkers in heterogeneous biological samples-both cell extracts and tumor tissues. Coarse photolithographic microfabrication defines a multiplexed sensing array; bottom-up fabrication of nanostructured microelectrodes then provides sensing elements. We analyzed a panel of mRNA samples for prostate cancer related gene fusions using the chip. We accurately identified gene fusions that correlate with aggressive prostate cancer and distinguished these from fusions associated with slower-progressing forms of the disease. The multiplexed nanostructured microelectrode integrated circuit reported herein provides direct, amplification-free, sample-to-answer in under 1 h using the 10 ng of mRNA readily available in biopsy samples.

Probing anodic reaction kinetics and interfacial mass transport of a direct formic acid fuel cell using a nanostructured palladium–gold alloy microelectrode

The anodic reaction kinetics and interfacial mass transport of a direct polymer electrolyte membrane formic acid fuel cell have been investigated in an all solid-state electrochemical cell using a highly active nanostructured palladium–gold alloy microelectrode as an in situ probe. Well-defined “S-shaped” steady-state cyclic voltammograms exhibiting current-rising region at lower overpotentials and limiting current region at higher overpotentials have been first obtained for the electrochemical oxidation of formic acid at varying temperature. The “S-shaped” steady state polarization curves and chronoamperometric curves enable convenient measurements of the anodic reaction kinetics and interfacial mass transport of formic acid under real polymer electrolyte membrane conditions. It is encouragingly found that formic acid can be directly oxidized to CO 2 with the first electron transfer being the likely rate-determining step and the formation of surface poison can be neglected. The exchange current density for the electrooxidation of formic acid is on the order of magnitude of 10 −7 A cm −2 in the temperature range of 20–60 °C. The permeability and diffusion coefficient of formic acid through a Nafion ® 117 membrane are of the order of magnitude of 10 −9 mol cm −1 s −1 and 10 −6 cm 2 s −1, respectively. The combination of a nanostructured microelectrode and an all solid-state electrochemical cell offers a versatile approach to evaluate potential electrocatalysts for fuel cells and electrochemical sensors employing polymer electrolyte membranes.

Integration of Au Nanorods With Flexible Thin-Film Microelectrode Arrays for Improved Neural Interfaces

In neural prosthetic systems, a low-impedance electrode-tissue interface is important for maintaining signal quality for recording, as well as effective charge transfer for stimulation. However, neural microelectrodes often have high impedance due to their small surface areas. In this paper, we present a simple method of increasing their effective surface areas by introducing nanostructures on the electrode sites. The method combines photolithography and electron-beam evaporation with a locally patterned anodized porous alumina template to integrate Au-nanorod arrays on the flexible thin-film microelectrode, which can significantly increase the effective surface area and decrease impedance due to its 3-D and compact nanostructures. Moreover, the geometrical and electrical properties of Au-nanorod electrodes were demonstrated and compared with conventional planar microelectrodes by using a scanning electron microscope and an impedance spectroscopy system. Experimental results showed that approximately 25 times lower interface impedance was achieved for this nanostructured microelectrode. Such Au-nanorod integrated microelectrode arrays will be a promising tool for neural engineering.

In situ step-scan time-resolved microscope FTIR spectroscopy applied in irreversible electrochemical reactions

We demonstrated in the present paper, for the first time, the application of step-scan time-resolved FTIR external reflection spectroscopy in studies of irreversible electrochemical reactions. Through combination of a particularly designed flow thin layer IR cell and a nanostructured microelectrode, the solution in the thin layer between IR window and microelectrode as well as adsorbates on electrode surface can be rapidly renewed, which allows irreversible reactions to be repeated quickly. Electrooxidation of CO adsorbed on a nanostructured platinum microelectrode (nm-PtME) in H 2SO 4 solutions was employed as a test reaction to illustrate the performance of the newly developed technology. It has confirmed that to regenerate a CO adlayer on the nm-PtME after stripping completely CO adsorbates may take about 6 s in CO-saturated solution (CO concentration ∼1.0 mM) at a flow rate of 20 ml min −1, which satisfies the requirement of the repeatability in step-scan FTIR data collection. Time-resolved FTIR spectra for CO irreversible oxidation have been recorded at a time resolution of 250 μs. It has been revealed that the IR band center of CO slightly decreases with time within initial 4 ms and then maintains a constant value in the evolvement of the oxidation. Such variation demonstrates that CO oxidation on nm-PtME is mainly occurred in through a nucleation and growth mechanism. The present study has thrown light on investigation of fast dynamic processes and kinetics of irreversible electrochemical reactions by using in situ step-scan time-resolved FTIR spectroscopy.

Scanning electrochemical microscopy (SECM): localized glucose oxidase immobilization via the direct electrochemical microspotting of polypyrrole–biotin films

This paper describes the successful combination of nanostructured microelectrodes, biotin–avidin chemistry and the direct and generation–collection modes of the scanning electrochemical microscope (SECM) to enable the fabrication, functionalization and characterization of biologically active microspots. The SECM tip was used as an electrochemical “pen” to drive the deposition of a micrometre size spot of biotinylated polypyrrole. Subsequent reaction with avidin and a biotinylated enzyme enabled the construction of a “molecular sandwich” capable of producing H 2O 2. The SECM tip was then used to “read” the activity of the microspot. A major contribution to this work was the use of mesoporous platinum tips to reliably detect the localized production of H 2O 2. In contrast to previous approaches this combination of localized deposition, high selectivity of the biotin–avidin binding and reliable imaging only requires one instrument and offers a valuable alternative for the fabrication and characterization of novel multi-analyte biosensor arrays.