Nanofluidic Redox Cycling Amplification for the Selective Detection of Catechol

Electrochemistry at the Edge of a van der Waals Heterostructure

Artificial van der Waals heterostructures, obtained by stacking two-dimensional (2D) materials, represent a novel platform for investigating physicochemical phenomena and applications. Here, the electrochemistry at the one-dimensional (1D) edge of a graphene sheet, sandwiched between two hexagonal boron nitride (hBN) flakes, is reported. When such an hBN/graphene/hBN heterostructure is immersed in a solution, the basal plane of graphene is encapsulated by hBN, and the graphene edge is exclusively available in the solution. This forms an electrochemical nanoelectrode, enabling the investigation of electron transfer using several redox probes, e.g., ferrocene(di)methanol, hexaammineruthenium, methylene blue, dopamine and ferrocyanide. The low capacitance of the van der Waals edge electrode facilitates cyclic voltammetry at very high scan rates (up to 1000 V s-1), allowing voltammetric detection of redox species down to micromolar concentrations with sub-second time resolution. The nanoband nature of the edge electrode allows operation in water without added electrolyte. Finally, two adjacent edge electrodes are realized in a redox-cycling format. All the above-mentioned phenomena can be investigated at the edge, demonstrating that nanoscale electrochemistry is a new application avenue for van der Waals heterostructures. Such an edge electrode will be useful for studying electron transfer mechanisms and the detection of analyte species in ultralow sample volumes.

Hydrophobic Gating and Spatial Confinement in Hierarchically Organized Block Copolymer-Nanopore Electrode Arrays for Electrochemical Biosensing of 4-Ethyl Phenol

Hydrophobic gating in biological transport proteins is regulated by stimulus-specific switching between filled and empty nanocavities, endowing them with selective mass transport capabilities. Inspired by these, solid-state nanochannels have been integrated into functional materials for a broad range of applications, such as energy conversion, filtration, and nanoelectronics, and here we extend these to electrochemical biosensors coupled to mass transport control elements. Specifically, we report hierarchically organized structures with block copolymers on tyrosinase-modified two-electrode nanopore electrode arrays (BCP@NEAs) as stimulus-controlled electrochemical biosensors for alkylphenols. A polystyrene-b-poly(4-vinyl)pyridine (PS-b-P4VP) membrane placed atop the NEA endows the system with potential-responsive gating properties, where water transport is spatially and temporarily gated through hydrophobic P4VP nanochannels by the application of appropriate potentials. The reversibility of hydrophobic voltage-gating makes it possible to capture and confine analyte species in the attoliter-volume vestibule of cylindrical nanopore electrodes, enabling redox cycling and yielding enhanced currents with amplification factors >100× when operated in a generator-collector mode. The enzyme-coupled sensing capabilities are demonstrated using nonelectroactive 4-ethyl phenol, exploiting the tyrosinase-catalyzed turnover into reversibly redox-active quinones, then using the quinone-catechol redox reaction to achieve ultrasensitive cycling currents in confined BCP@NEA sensors giving a limit-of-detection of ∼120 nM. The mass transport controlled sensing platform described here is relevant to the development of enzyme-coupled multiplex biosensors for sensitive and selective detection of biomarkers and metabolites in next-generation point-of-care devices.

Integrated Glass Microfluidics with Electrochemical Nanogap Electrodes

We present a framework for the fabrication of chip-based electrochemical nanogap sensors integrated with microfluidics. Instead of polydimethylsiloxane (PDMS), SU-8 aided adhesive bonding of silicon and glass wafers is used to implement parallel flow control. The fabrication process permits wafer-scale production with high throughput and reproducibility. Additionally, the monolithic structures allow simple electrical and fluidic connections, alleviating the need for specialized equipment. We demonstrate the utility of these flow-incorporated nanogap sensors by performing redox cycling measurements under laminar flow conditions.

Rapid and sensitive detection of viral particles by coupling redox cycling and electrophoretic enrichment

The COVID-19 pandemic has highlighted the need for rapid, low-cost, and sensitive virus detection platforms to monitor and mitigate widespread outbreaks. Electrochemical sensors are a viable choice to fill this role but still require improvements to the signal magnitude, especially for early detection and low viral loads. Herein, finite element analysis of a novel biosensor concept for single virion counting using a generator-collector microelectrode design is presented. The proposed design combines a redox-cycling amplified electrochemical current with electrophoresis-driven electrode-particle collision for rapid virus detection. The effects of experimental (e.g. scan rate, collector bias) and geometric factors are studied to optimize the sensor design. Two generator-collector configurations are explored: a ring-disk configuration to analyze sessile droplets and an interdigitated electrode (IDE) design housed in a microchannel. For the ring-disk configuration, we calculate an amplification factor of ∼5 and collector efficiency of ∼0.8 for a generator-collector spacing of 600 nm. For the IDE, the collector efficiency is even larger, approaching unity. The dual-electrode mode is critical for increasing the current and electric field strength. As a result, the current steps upon virus capture are more than an order of magnitude larger compared to single-mode. Additionally, single virus capture times are reduced from over 700 s down to ∼20 s. Overall, the frequency of virus capture and magnitude of the electrochemical current steps depend on the virus properties and electrode configuration, with the IDE capable of single virus detection within seconds owing to better particle confinement in the microchannel.

Simple, rapid, and visual electrochemiluminescence sensor for on-site catechol analysis

Environmental pollution caused by aromatic compounds such as catechol (Cat) has become a major issue for human health. However, there is no simple, rapid, and low-cost method for on-site monitoring of Cat. Here, based on ECL quenching mechanism, we develop a simple, rapid and visual mesoporous silica (MSNs)-electrochemiluminescence (ECL) sensor for on-site monitoring of Cat. The mechanism of ECL quenching is due to the interaction between Cat and Ru(bpy)₃^2+* and the interactions between the oxidation products of Cat and DBAE. MSNs films with ordered perpendicular mesopore channels exhibit an amplification effect of ECL intensity due to the negatively charged pore channel. There is a good linear relationship between ECL intensity and Cat concentration in the range of 10 ∼ 1000 μM with the limit of detection (LOD) of 9.518 μM (R² = 0.99). The on-site sensor is promising to offer new opportunities for pharmaceuticals analysis, on-site monitoring, and exposure risk assessment.

A simple, rapid and visual mesoporous silica (MSNs)-electrochemiluminescence (ECL) sensor was developed for on-site monitoring of Cat.

Cobalt-imidazole metal-organic framework loaded with luminol for paper-based chemiluminescence detection of catechol with use of a smartphone

Chemiluminescence (CL) reagent luminol was loaded into the porous structure of cobalt-imidazole metal-organic framework (MOF) ZIF-67 to obtain luminol-functionalized ZIF-67 (luminol@ZIF-67) with CL property. The morphology, composition, CL property, and CL mechanism of luminol@ZIF-67 were carefully investigated. The obtained luminol@ZIF-67 exhibited strong, stable, and visible CL emission that reacted with H₂O₂, attributed to the strong catalytic effect of ZIF-67 combined with the shortened diffusion distance between luminol and the catalytic center. The CL intensity of luminol@ZIF-67 was more than 550 times higher than that of luminol. Catechol can effectively quench the CL emission of luminol@ZIF-67 that reacted with H₂O₂. Then, a simple paper-based CL imaging detection method was developed for the detection of catechol by using a smartphone as a portable detector. The linear calibration curve of the developed CL assay for catechol ranged from 5 to 100 mg/L with detection limit of 1.1 mg/L (S/N = 3δ). The strong CL emission of luminol@ZIF-67 combined with the effective quench ability of catechol guaranteed high sensitivity of the detection method. The practical application ability of the developed CL assay was tested by the determination of catechol in tea and tap water samples, resulting in acceptable results. This work provides an effective paper-based CL detection method for catechol and enriches the species of the chemiluminescent MOF material.

Ion beam etching redeposition for 3D multimaterial nanostructure manufacturing

A novel fabrication method based on the local sputtering of photoresist sidewalls during ion beam etching is presented. This method allows for the manufacture of three-dimensional multimaterial nanostructures at the wafer scale in only four process steps. Features of various shapes and profiles can be fabricated at sub-100-nm dimensions with unprecedented freedom in material choice. Complex nanostructures such as nanochannels, multimaterial nanowalls, and suspended networks were successfully fabricated using only standard microprocessing tools. This provides an alternative to traditional nanofabrication techniques, as well as new opportunities for biosensing, nanofluidics, nanophotonics, and nanoelectronics.

CMOS Interfaces for Internet-of-Wearables Electrochemical Sensors: Trends and Challenges

Smart wearables, among immediate future IoT devices, are creating a huge and fast growing market that will encompass all of the next decade by merging the user with the Cloud in a easy and natural way. Biological fluids, such as sweat, tears, saliva and urine offer the possibility to access molecular-level dynamics of the body in a non-invasive way and in real time, disclosing a wide range of applications: from sports tracking to military enhancement, from healthcare to safety at work, from body hacking to augmented social interactions. The term Internet of Wearables (IoW) is coined here to describe IoT devices composed by flexible smart transducers conformed around the human body and able to communicate wirelessly. In addition the biochemical transducer, an IoW-ready sensor must include a paired electronic interface, which should implement specific stimulation/acquisition cycles while being extremely compact and drain power in the microwatts range. Development of an effective readout interface is a key element for the success of an IoW device and application. This review focuses on the latest efforts in the field of Complementary Metal–Oxide–Semiconductor (CMOS) interfaces for electrochemical sensors, and analyses them under the light of the challenges of the IoW: cost, portability, integrability and connectivity.

Nanogap-Based Electrochemical Measurements at Double-Carbon-Fiber Ultramicroelectrodes

Electrochemical measurements with unprecedentedly high sensitivity, selectivity, and kinetic resolution have been enabled by a pair of electrodes separated by a nanometer-wide gap. The fabrication of nanogap electrodes, however, requires extensive nanolithography or nanoscale electrode positioning, thereby preventing the full exploration of this powerful method in electrode design and application. Herein, we report the simple fabrication of double-carbon-fiber ultramicroelectrodes (UMEs) with nanometer-wide gaps not only to facilitate nanogap-based electrochemical measurements but also to gain high time resolution, signal-to-background ratio, and kinetic selectivity for dopamine against ascorbic acid. Specifically, ∼7 μm-diameter carbon fibers are inserted into a double-bore glass capillary, heat-pulled, and milled by focused ion-beam technology to yield ∼50 μm-long double-cylinder UMEs. The redox cycling of the Ru(NH3)63+/2+ couple across a nanogap between voltammetric generator and amperometric collector electrodes reaches quasi-steady states at fast scan rates of 100 V/s as demonstrated experimentally and even 1000 V/s as predicted theoretically. The transient background of the amperometric collector response is suppressed ∼100 times in comparison with that of the voltammetric generator response. Nanogap voltammograms based on the collector response against the cycled generator potential are quantitatively analyzed without background subtraction to reproducibly yield nanogap widths of ∼0.18 μm and a standard electron-transfer rate constant of 0.9 cm/s. Moreover, nanogap-mediated redox cycling can be initiated by dopamine oxidation at the generator electrode to largely improve the dopamine selectivity of the collector response against ascorbic acid, which is also oxidized at the generator electrode to immediately and irreversibly produce a redox-inactive species.

Simulation of Fast-Scan Nanogap Voltammetry at Double-Cylinder Ultramicroelectrodes

High temporal resolution of fast-scan cyclic voltammetry (FSCV) is widely appreciated in fundamental and applied electrochemistry to quantitatively investigate rapid dynamics of electron transfer and neurotransmission using ultramicroelectrodes (UMEs). Faster potential scan, however, linearly increases the background current, which must be subtracted for quantitative FSCV. Herein, we numerically simulate fast-scan nanogap voltammetry (FSNV) for quantitative detection of diffusing redox species under quasi-steady states without the need of background subtraction while maintaining high temporal resolution of transient FSCV. These advantages of FSNV originate from the use of a parallel pair of cylindrical UMEs with nanometer-wide separation in contrast to FSCV with single UMEs. In FSNV, diffusional redox cycling across the nanogap is driven voltammetrically at the generator electrode and monitored amperometrically at the collector electrode without the transient background. We reveal that the cylindrical collector electrode can reach quasi-steady states ~10⁴ times faster than the generator electrode with identical sizes to allow for fast scan. Double-microcylinder and nanocylinder UMEs enable quasi-steady-state FSNV at hundreds volts per second as practiced for in-vivo FSCV and megavolts per second as achieved for ultra-FSCV, respectively. Rational design and simple fabrication of double-cylinder UMEs are proposed to broaden the application of nanogap voltammetry.

Field-Assisted Splitting of Pure Water Based on Deep-Sub-Debye-Length Nanogap Electrochemical Cells

Owing to the low conductivity of pure water, using an electrolyte is common for achieving efficient water electrolysis. In this paper, we have fundamentally broken through this common sense by using deep-sub-Debye-length nanogap electrochemical cells to achieve efficient electrolysis of pure water (without any added electrolyte) at room temperature. A field-assisted effect resulted from overlapped electrical double layers can greatly enhance water molecules ionization and mass transport, leading to electron-transfer limited reactions. We have named this process "virtual breakdown mechanism" (which is completely different from traditional mechanisms) that couples the two half-reactions together, greatly reducing the energy losses arising from ion transport. This fundamental discovery has been theoretically discussed in this paper and experimentally demonstrated in a group of electrochemical cells with nanogaps between two electrodes down to 37 nm. On the basis of our nanogap electrochemical cells, the electrolysis current density from pure water can be significantly larger than that from 1 mol/L sodium hydroxide solution, indicating the much better performance of pure water splitting as a potential for on-demand clean hydrogen production.

Nanostructures: Current uses and future applications in food science

Recent developments in nanoscience and nanotechnology intend novel and innovative applications in the food sector, which is rather recent compared with their use in biomedical and pharmaceutical applications. Nanostructured materials are having applications in various sectors of the food science comprising nanosensors, new packaging materials, and encapsulated food components. Nanostructured systems in food include polymeric nanoparticles, liposomes, nanoemulsions, and microemulsions. These materials enhance solubility, improve bioavailability, facilitate controlled release, and protect bioactive components during manufacture and storage. This review highlights the applications of nanostructured materials for their antimicrobial activity and possible mechanism of action against bacteria, including reactive oxygen species, membrane damage, and release of metal ions. In addition, an overview of nanostructured materials, and their current applications and future perspectives in food science are also presented.

Three-dimensional inkjet-printed redox cycling sensor

Electrochemical amplification through redox cycling in an all-inkjet-printed device utilizing four different functional inks.

Fabrication of a Horizontal and a Vertical Large Surface Area Nanogap Electrochemical Sensor

Nanogap sensors have a wide range of applications as they can provide accurate direct detection of biomolecules through impedimetric or amperometric signals. Signal response from nanogap sensors is dependent on both the electrode spacing and surface area. However, creating large surface area nanogap sensors presents several challenges during fabrication. We show two different approaches to achieve both horizontal and vertical coplanar nanogap geometries. In the first method we use electron-beam lithography (EBL) to pattern an 11 mm long serpentine nanogap (215 nm) between two electrodes. For the second method we use inductively-coupled plasma (ICP) reactive ion etching (RIE) to create a channel in a silicon substrate, optically pattern a buried 1.0 mm × 1.5 mm electrode before anodically bonding a second identical electrode, patterned on glass, directly above. The devices have a wide range of applicability in different sensing techniques with the large area nanogaps presenting advantages over other devices of the same family. As a case study we explore the detection of peptide nucleic acid (PNA)−DNA binding events using dielectric spectroscopy with the horizontal coplanar device.

Nanoscale Electrochemical Sensor Arrays: Redox Cycling Amplification in Dual-Electrode Systems

Micro- and nanofabriation technologies have a tremendous potential for the development of powerful sensor array platforms for electrochemical detection. The ability to integrate electrochemical sensor arrays with microfluidic devices nowadays provides possibilities for advanced lab-on-a-chip technology for the detection or quantification of multiple targets in a high-throughput approach. In particular, this is interesting for applications outside of analytical laboratories, such as point-of-care (POC) or on-site water screening where cost, measurement time, and the size of individual sensor devices are important factors to be considered. In addition, electrochemical sensor arrays can monitor biological processes in emerging cell-analysis platforms. Here, recent progress in the design of disease model systems and organ-on-a-chip technologies still needs to be matched by appropriate functionalities for application of external stimuli and read-out of cellular activity in long-term experiments. Preferably, data can be gathered not only at a singular location but at different spatial scales across a whole cell network, calling for new sensor array technologies. In this Account, we describe the evolution of chip-based nanoscale electrochemical sensor arrays, which have been developed and investigated in our group. Focusing on design and fabrication strategies that facilitate applications for the investigation of cellular networks, we emphasize the sensing of redox-active neurotransmitters on a chip. To this end, we address the impact of the device architecture on sensitivity, selectivity as well as on spatial and temporal resolution. Specifically, we highlight recent work on redox-cycling concepts using nanocavity sensor arrays, which provide an efficient amplification strategy for spatiotemporal detection of redox-active molecules. As redox-cycling electrochemistry critically depends on the ability to miniaturize and integrate closely spaced electrode systems, the fabrication of suitable nanoscale devices is of utmost importance for the development of this advanced sensor technology. Here, we address current challenges and limitations, which are associated with different redox cycling sensor array concepts and fabrication approaches. State-of-the-art micro- and nanofabrication technologies based on optical and electron-beam lithography allow precise control of the device layout and have led to a new generation of electrochemical sensor architectures for highly sensitive detection. Yet, these approaches are often expensive and limited to clean-room compatible materials. In consequence, they lack possibilities for upscaling to high-throughput fabrication at moderate costs. In this respect, self-assembly techniques can open new routes for electrochemical sensor design. This is true in particular for nanoporous redox cycling sensor arrays that have been developed in recent years and provide interesting alternatives to clean-room fabricated nanofluidic redox cycling devices. We conclude this Account with a discussion of emerging fabrication technologies based on printed electronics that we believe have the potential of transforming current redox cycling concepts from laboratory tools for fundamental studies and proof-of-principle analytical demonstrations into high-throughput devices for rapid screening applications.

Ion Accumulation and Migration Effects on Redox Cycling in Nanopore Electrode Arrays at Low Ionic Strength

Ion permselectivity can lead to accumulation in zero-dimensional nanopores, producing a significant increase in ion concentration, an effect which may be combined with unscreened ion migration to improve sensitivity in electrochemical measurements, as demonstrated by the enormous current amplification (∼2000-fold) previously observed in nanopore electrode arrays (NEA) in the absence of supporting electrolyte. Ionic strength is a key experimental factor that governs the magnitude of the additional current amplification (AFad) beyond simple redox cycling through both ion accumulation and ion migration effects. Separate contributions from ion accumulation and ion migration to the overall AFad were identified by studying NEAs with varying geometries, with larger AFad values being achieved in NEAs with smaller pores. In addition, larger AFad values were observed for Ru(NH3)6(3/2+) than for ferrocenium/ferrocene (Fc(+)/Fc) in aqueous solution, indicating that coupling efficiency in redox cycling can significantly affect AFad. While charged species are required to observe migration effects or ion accumulation, poising the top electrode at an oxidizing potential converts neutral species to cations, which can then exhibit current amplification similar to starting with the cation. The electrical double layer effect was also demonstrated for Fc/Fc(+) in acetonitrile and 1,2-dichloroethane, producing AFad up to 100× at low ionic strength. The pronounced AFad effects demonstrate the advantage of coupling redox cycling with ion accumulation and migration effects for ultrasensitive electrochemical measurements.

A non-enzymatic sensor based on Au@Ag nanoparticles with good stability for sensitive detection of H2O2

Synthesis of Au@Ag NPs by a seed-mediated growth procedure and fabrication of a non-enzymatic H₂O₂ sensor.

Preparation of Au@Ag nanoparticles at a gas/liquid interface and their application for sensitive detection of hydrogen peroxide

Preparation of Au@Ag NPs at a gas/liquid interface by a seed-mediated growth procedure and their application for sensitive detection of hydrogen peroxide.

A local redox cycling-based electrochemical chip device with nanocavities for multi-electrochemical evaluation of embryoid bodies

An electrochemical device, which consists of electrode arrays, nanocavities, and microwells, was developed for multi-electrochemical detection with high sensitivity. A local redox cycling-based electrochemical (LRC-EC) system was used for multi-electrochemical detection and signal amplification. The LRC-EC system consists of n(2) sensors with only 2n bonding pads for external connection. The nanocavities fabricated in the sensor microwells enable significant improvement of the signal amplification compared with the previous devices we have developed. The present device was successfully applied for evaluation of embryoid bodies (EBs) from embryonic stem (ES) cells via electrochemical measurements of alkaline phosphatase (ALP) activity in the EBs. In addition, the EBs were successfully trapped in the sensor microwells of the device using dielectrophoresis (DEP) manipulation, which led to high-throughput cell analysis. This device is considered to be useful for multi-electrochemical detection and imaging for bioassays including cell analysis.

Self-induced redox cycling coupled luminescence on nanopore recessed disk-multiscale bipolar electrodes

We present a new configuration for coupling fluorescence microscopy and voltammetry using self-induced redox cycling for ultrasensitive electrochemical measurements. An array of nanopores, each supporting a recessed disk electrode separated by 100 nm in depth from a planar multiscale bipolar top electrode, was fabricated using multilayer deposition, nanosphere lithography, and reactive-ion etching. Self-induced redox cycling was induced on the disk electrode producing ∼30× current amplification, which was independently confirmed by measuring induced electrogenerated chemiluminescence from Ru(bpy)32/3+/tri-n-propylamine on the floating bipolar electrode. In this design, redox cycling occurs between the recessed disk and the top planar portion of a macroscopic thin film bipolar electrode in each nanopore. Electron transfer also occurs on a remote (mm-distance) portion of the planar bipolar electrode to maintain electroneutrality. This couples the electrochemical reactions of the target redox pair in the nanopore array with a reporter, such as a potential-switchable fluorescent indicator, in the cell at the distal end of the bipolar electrode. Oxidation or reduction of reversible analytes on the disk electrodes were accompanied by reduction or oxidation, respectively, on the nanopore portion of the bipolar electrode and then monitored by the accompanying oxidation of dihydroresorufin or reduction of resorufin at the remote end of the bipolar electrode, respectively. In both cases, changes in fluorescence intensity were triggered by the reaction of the target couple on the disk electrode, while recovery was largely governed by diffusion of the fluorescent indicator. Reduction of 1 nM of Ru(NH3)63+ on the nanoelectrode array was detected by monitoring the fluorescence intensity of resorufin, demonstrating high sensitivity fluorescence-mediated electrochemical sensing coupled to self-induced redox cycling.

Large mesoporous carbons decorated with silver and gold nanoparticles by a self-assembly method: enhanced electrocatalytic activity for H2O2 electroreduction and sodium nitrite electrooxidation

The silver, gold nanoparticles were grown onto poly (diallyldimethyl ammoniumchloride, PDDA)-functionalized large mesoporous carbon (LMC) by simple self-assembly method. AuNPs or AgNPs/PDDA–LMC show superior electrocatalytic activity.

Redox cycling without reference electrodes

The reference electrode is a key component in electrochemical measurements, yet it remains a challenge to implement a reliable reference electrode in miniaturized electrochemical sensors. Here we explore experimentally and theoretically an alternative approach based on redox cycling which eliminates the reference electrode altogether. We show that shifts in the solution potential caused by the lack of reference can be understood quantitatively, and determine the requirements for accurate measurements in miniaturized systems in the absence of a reference electrode.

Sensing with nanopores--the influence of asymmetric blocking on electrochemical redox cycling current

Nanoporous redox cycling devices are highly efficient tools for the electrochemical sensing of redox-active molecules. By using a redox-active mediator, this concept can be exploited for the detection of molecular binding events via blocking of the redox cycling current within the nanopores. Here, we investigate the influence of different blocking scenarios inside a nanopore on the resulting redox cycling current. Our analysis is based on random walk simulations and finite element calculations. We distinguish between symmetric and asymmetric pore blocking and show that the current decrease is more pronounced in the case of asymmetric blocking reflecting the diffusion-driven pathway of the redox-active molecules. Using random walk simulations, we further study the impact of pore blocking in the frequency domain and identify relevant features of the power spectral density, which are of particular interest for sensing applications based on fluctuation analysis.

Electron-transfer kinetics and electric double layer effects in nanometer-wide thin-layer cells

Redox cycling in nanometer-wide thin-layer cells holds great promise in ultrasensitive voltammetric detection and in probing fast heterogeneous electron-transfer kinetics. Quantitative understanding of the influence of the nanometer gap distance on the redox processes in the thin-layer cells is of crucial importance for reliable data analysis. We present theoretical consideration on the voltammetric behaviors associated with redox cycling of electroactive molecules between two electrodes separated by nanometer widths. Emphasis is placed on the weakness of the commonly used Butler-Volmer theory and the classic Marcus-Hush theory in describing the electrochemical heterogeneous electron-transfer kinetics at potentials significantly removed from the formal potential of redox moieties and, in addition, the effect of the electric-double-layer on the electron-transfer kinetics and mass transport dynamics of charged redox species. The steady-state voltammetric responses, obtained by using the Butler-Volmer and Marcus-Hush models and that predicted by the more realistic electron-transfer kinetics formulism, which is based on the alignments of the density of states between the electrode continuum and the Gaussian distribution of redox agents, and by inclusion of the electric-double-layer effect, are compared through systematic finite element simulations. The effect of the gap width between the electrodes, the standard rate constant and reorganization energy for the electron-transfer reactions, and the charges of the redox moieties are considered. On the basis of the simulation results, the reliability of the conventional voltammetric analysis based on the Butler-Volmer kinetic model and diffusion transport equations is discussed for nanometer-wide thin-layer cells.

Integrated biodetection in a nanofluidic device

The sensing of enzymatic processes in volumes at or below the scale of single cells is challenging but highly desirable in the study of biochemical processes. Here we demonstrate a nanofluidic device that combines an enzymatic recognition element and electrochemical signal transduction within a six-femtoliter volume. Our approach is based on localized immobilization of the enzyme tyrosinase in a microfabricated nanogap electrochemical transducer. The enzymatic reaction product quinone is localized in the confined space of a nanochannel in which efficient redox cycling also takes place. Thus, the sensor allows the sensitive detection of minute amounts of product molecules generated by the enzyme in real time. This method is ideally suited for the study of ultra-small-volume systems such as the contents of individual biological cells or organelles.

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.

Electrochemical signal amplification for immunosensor based on 3D interdigitated array electrodes

We devised an electrochemical redox cycling based on three-dimensional interdigitated array (3D IDA) electrodes for signal amplification to enhance the sensitivity of chip-based immunosensors. The 3D IDA consists of two closely spaced parallel indium tin oxide (ITO) electrodes that are positioned not only on the bottom but also the ceiling, facing each other along a microfluidic channel. We investigated the signal intensities from various geometric configurations: Open-2D IDA, Closed-2D IDA, and 3D IDA through electrochemical experiments and finite-element simulations. The 3D IDA among the four different systems exhibited the greatest signal amplification resulting from efficient redox cycling of electroactive species confined in the microchannel so that the faradaic current was augmented by a factor of ∼100. We exploited the enhanced sensitivity of the 3D IDA to build up a chronocoulometric immunosensing platform based on the sandwich enzyme-linked immunosorbent assay (ELISA) protocol. The mouse IgGs on the 3D IDA showed much lower detection limits than on the Closed-2D IDA. The detection limit for mouse IgG measured using the 3D IDA was ∼10 fg/mL, while it was ∼100 fg/mL for the Closed-2D IDA. Moreover, the proposed immunosensor system with the 3D IDA successfully worked for clinical analysis as shown by the sensitive detection of cardiac troponin I in human serum down to 100 fg/mL.

Noise phenomena caused by reversible adsorption in nanoscale electrochemical devices

We theoretically investigate reversible adsorption in electrochemical devices on a molecular level. To this end, a computational framework is introduced, which is based on 3D random walks including probabilities for adsorption and desorption events at surfaces. We demonstrate that this approach can be used to investigate adsorption phenomena in electrochemical sensors by analyzing experimental noise spectra of a nanofluidic redox cycling device. The evaluation of simulated and experimental results reveals an upper limit for the average adsorption time of ferrocene dimethanol of ∼200 μs. We apply our model to predict current noise spectra of further electrochemical experiments based on interdigitated arrays and scanning electrochemical microscopy. Since the spectra strongly depend on the molecular adsorption characteristics of the detected analyte, we can suggest key indicators of adsorption phenomena in noise spectroscopy depending on the geometric aspect of the experimental setup.

Electroanalysis at the nanoscale

This article reviews the state of the art of silicon chip-based nanoelectrochemical devices for sensing applications. We first describe analyte mass transport to nanoscale electrodes and emphasize understanding the importance of mass transport for the design of nanoelectrode arrays. We then describe bottom-up and top-down approaches to nanoelectrode fabrication and integration at silicon substrates. Finally, we explore recent examples of on-chip nanoelectrodes employed as sensors and diagnostics, finishing with a brief look at future applications.

Redox cycling in nanoporous electrochemical devices

Nanoscale redox cycling is a powerful technique for detecting electrochemically active molecules, based on fast repetitive oxidation and reduction reactions. An ideal implementation of redox cycling sensors can be realized by nanoporous dual-electrode systems in easily accessible and scalable geometries. Here, we introduce a multi-electrode array device with highly efficient nanoporous redox cycling sensors. Each of the sensors holds up to 209,000 well defined nanopores with minimal pore radii of less than 40 nm and an electrode separation of ~100 nm. We demonstrate the efficiency of the nanopore array by screening a large concentration range over three orders of magnitude with area-specific sensitivities of up to 81.0 mA (cm(-2) mM(-1)) for the redox-active probe ferrocene dimethanol. Furthermore, due to the specific geometry of the material, reaction kinetics has a unique potential-dependent impact on the signal characteristics. As a result, redox cycling experiments in the nanoporous structure allow studies on heterogeneous electron transfer reactions revealing a surprisingly asymmetric transfer coefficient.

Nanoporous dual-electrodes with millimetre extensions: parallelized fabrication and area effects on redox cycling

Novel fabrication techniques lead to highly sensitive electrochemical sensors (left). The large-area characteristics of redox-cycling within the sensor's nanopores further cause potential-dependent variations of the overall analyte concentration (right).