Electrochemical single-molecule detection in aqueous solution using self-aligned nanogap transducers

Activationless Electron Transfer of Redox-DNA in Electrochemical Nanogaps

Our recent discovery of decreased reorganization energy in electrode-tethered redox-DNA systems prompts inquiries into the origin of this phenomenon and suggests its potential use to lower the activation energy of electrochemical reactions. Here, we show that the confinement of the DNA chain in a nanogap amplifies this effect to an extent to which it nearly abolishes the intrinsic activation energy of electron transfer. Employing electrochemical atomic force microscopy (AFM-SECM), we create sub-10 nm nanogaps between a planar electrode surface bearing end-anchored ferrocenylated DNA chains and an incoming microelectrode tip. The redox cycling of the DNA's ferrocenyl (Fc) moiety between the surface and the tip generates a measurable current at the scale of ∼10 molecules. Our experimental findings are rigorously interpreted through theoretical modeling and original molecular dynamics simulations (Q-Biol code). Several intriguing findings emerge from our investigation: (i) The electron transport resulting from DNA dynamics is many times faster than predicted by simple diffusion considerations. (ii) The current in the nanogap is solely governed by the electron transfer rate at the electrodes. (iii) This rate rapidly saturates as overpotentials applied to the nanogap electrodes increase, implying near-complete suppression of the reorganization energy for the oxidation/reduction of the Fc heads within confined DNA. Furthermore, evidence is presented that this may constitute a general, previously unforeseen, behavior of redox polymer chains in electrochemical nanogaps.

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.

Ultrasensitive Automatic Detection of Small Molecules by Membrane Imaging of Single Molecule Assays

Determination of trace amounts of targets or even a single molecule target has always been a challenge in the detection field. Digital measurement methods established for single molecule counting of proteins, such as single molecule arrays (Simoa) or dropcast single molecule assays (dSimoa), are not suitable for detecting small molecule, because of the limited category of small molecule antibodies and the weak signal that can be captured. To address this issue, we have developed a strategy for single molecule detection of small molecules, called small molecule detection with single molecule assays (smSimoa). In this strategy, an aptamer is used as a recognition element, and an addressable DNA Nanoflower (DNF) attached on the magnetic beads surface, which exhibit fluorescence imaging, is employed as the output signal. Accompanied by digital imaging and automated counting analysis, E2 at the attomolar level can be measured. The smSimoa breaks the barrier of small molecule detection concentration and provides a basis for high throughput detection of multiple substances with fluorescence encoded magnetic beads.

Integration of sequential analytical processes into sub-100 nm channels: volumetric sampling, chromatographic separation, and label-free molecule detection

The progress of nanotechnology has developed nanofluidic devices utilizing nanochannels with a width and/or depth of sub-100 nm (10¹ nm channels), and several experiments have been implemented in ultra-small spaces comparable to DNAs and proteins. However, current experiments utilizing 10¹ nm channels focus on a single function or operation; integration of multiple analytical operations into 10¹ nm channels using nanofluidic circuits and fluidic control has yet to be realized despite the advantage of nanochannels. Herein, we report the establishment of a label-free molecule detection method for 10¹ nm channels and demonstration of sequential analytical processes using integrated nanofluidic devices. Our absorption-based detection method called photothermal optical diffraction (POD) enables non-invasive label-free molecule detection in 10¹ nm channels for the first time, and the limit of detection (LOD) of 1.8 μM is achieved in 70 nm wide and deep nanochannels, which corresponds to 7.5 molecules in the detection volume of 7 aL. As a demonstration of sampling in 10¹ nm channels, aL-fL volumetric sampling is performed using 90 nm deep cross-shaped nanochannels and pressure-driven fluidic control from three directions. Finally, the POD and volumetric sampling are combined with nanochannel chromatography, and separation analysis in 10¹ nm channels is demonstrated. The experimental results reported in this paper will contribute to the advances in 10¹ nm fluidic devices which have the potential to provide a novel platform for chemical/biological analyses.

Stochasticity in Single-Entity Electrochemistry

Most electrochemical processes are stochastic and discrete in nature. Yet experimental observables, e.g., i vs E, are typically smooth and deterministic, due to many events/processes, e.g., electron transfers, being averaged together. However, when the number of entities measured approaches a few or even one, stochasticity frequently emerges. Yet all is not lost! Probabilistic and statistical interpretation can generate insights matching or superseding those from macroscale/ensemble measurements, revealing phenomena that were hitherto averaged over. Herein, we review recent literature examples of stochastic processes in single-entity electrochemistry, highlighting strategies for interpreting stochasticity, contrasting them with macroscale measurements, and describing the insights generated.

Single-cell omics analyses with single molecular detection: challenges and perspectives

The ultimate goal of single-cell analyses is to obtain the biomolecular content for each cell in unicellular and multicellular organisms at different points of their life cycle under variable environmental conditions. These require an assessment of: a) the total number of cells, b) the total number of cell types, and c) the complete and quantitative single molecular detection and identification for all classes of biopolymers, and organic and inorganic compounds, in each individual cell. For proteins, glycans, lipids, and metabolites, whose sequences cannot be amplified by copying as in the case of nucleic acids, the detection limit by mass spectrometry is about 10⁵ molecules. Therefore, proteomic, glycomic, lipidomic, and metabolomic analyses do not yet permit the assembly of the complete single-cell omes. The construction of novel nanoelectrophoretic arrays and nano in microarrays on a single 1-cm-diameter chip has shown proof of concept for a high throughput platform for parallel processing of thousands of individual cells. Combined with dynamic secondary ion mass spectrometry, with 3D scanning capability and lateral resolution of 50 nm, the sensitivity of single molecular quantification and identification for all classes of biomolecules could be reached. Further development and routine application of such technological and instrumentation solution would allow assembly of complete omes with a quantitative assessment of structural and functional cellular diversity at the molecular level.

Advances in Label-Free Detections for Nanofluidic Analytical Devices

Nanofluidics, a discipline of science and engineering of fluids confined to structures at the 1-1000 nm scale, has experienced significant growth over the past decade. Nanofluidics have offered fascinating platforms for chemical and biological analyses by exploiting the unique characteristics of liquids and molecules confined in nanospaces; however, the difficulty to detect molecules in extremely small spaces hampers the practical applications of nanofluidic devices. Laser-induced fluorescence microscopy with single-molecule sensitivity has been so far a major detection method in nanofluidics, but issues arising from labeling and photobleaching limit its application. Recently, numerous label-free detection methods have been developed to identify and determine the number of molecules, as well as provide chemical, conformational, and kinetic information of molecules. This review focuses on label-free detection techniques designed for nanofluidics; these techniques are divided into two groups: optical and electrical/electrochemical detection methods. In this review, we discuss on the developed nanofluidic device architectures, elucidate the mechanisms by which the utilization of nanofluidics in manipulating molecules and controlling light-matter interactions enhances the capabilities of biological and chemical analyses, and highlight new research directions in the field of detections in nanofluidics.

Revealing the sub-50 ms electrochemical conversion of silver halide nanocolloids by stochastic electrochemistry and optical microscopy

Silver based ionic crystal nanoparticles (NPs) are interesting nanomaterials for energy storage and conversion, e.g. their colloidal solutions could be used as a reversible redox nanofluid in semi-solid redox flow cells. In this context, the reductive transformation of Brownian silver halide, AgX, NPs into silver NPs is probed by single NP electrochemistry, complemented by operando high resolution monitoring. However, their light sensitivity and poor conductivity make the operando monitoring of their chemical activity challenging. The electrochemical collisions of single AgX NPs onto a negatively biased electrode evidence a full conversion through multiple reduction steps within 3-10 ms. This is further corroborated by simulation of the conversion process and operando through a high resolution optical microscopy technique (Backside Absorbing Layer Microscopy, BALM). Both techniques are interesting strategies to infer at the single NP level the intrinsic charge capacity and charging rate of redox active Brownian nanomaterials, demonstrating the interest of the fast and reversible AgX/Ag system as a redox nanofluid.

Nanoscale electrochemical kinetics & dynamics: the challenges and opportunities of single-entity measurements

The development of nanoscale electrochemistry since the mid-1980s has been predominately coupled with steady-state voltammetric (i-E) methods. This research has been driven by the desire to understand the mechanisms of very fast electrochemical reactions, by electroanalytical measurements in small volumes and unusual media, including in vivo measurements, and by research on correlating electrocatalytic activity, e.g., O2 reduction reaction, with nanoparticle size and structure. Exploration of the behavior of nanoelectrochemical structures (nanoelectrodes, nanoparticles, nanogap cells, etc.) of a characteristic dimension λ using steady-state i-E methods generally relies on the well-known relationship, λ2 ∼ Dt, which relates diffusional lengths to time, t, through the coefficient, D. Decreasing λ, by performing measurements at a nanometric length scales, results in a decrease in the effective timescale of the measurement, and provides a direct means to probe the kinetics of steps associated with very rapid electrochemical reactions. For instance, steady-state voltammetry using a nanogap twin-electrode cell of characteristic width, λ ∼ 10 nm, allows investigations of events occurring at timescales on the order of ∼100 ns. Among many other advantages, decreasing λ also increases spatial resolution in electrochemical imaging, e.g., in scanning electrochemical microscopy, and allows probing of the electric double layer. This Introductory Lecture traces the evolution and driving forces behind the "λ2 ∼ Dt" steady-state approach to nanoscale electrochemistry, beginning in the late 1950s with the introduction of the rotating ring-disk electrode and twin-electrode thin-layer cells, and evolving to current-day investigations using nanoelectrodes, scanning nanocells for imaging, nanopores, and nanoparticles. The recent focus on so-called "single-entity" electrochemistry, in which individual and very short redox events are probed, is a significant departure from the steady-state approach, but provides new opportunities to probe reaction dynamics. The stochastic nature of very fast single-entity events challenges current electrochemical methods and modern electronics, as illustrated using recent experiments from the authors' laboratory.

Converting Any Faradaic Current Generated at an Electrode under Potentiostatic Control into a Remote Fluorescence Signal

We describe the development of an original faradaic current-to-fluorescence conversion scheme. The proposed instrumental strategy consists of coupling the electrochemical reaction of any species at an electrode under potentiostatic control with the fluorescence emission of a species produced at the counter electrode. In order to experimentally validate this scheme, the fluorogenic species resazurin is chosen as a fluorescent reporter molecule, and its complex reduction mechanism is first studied in unprecedented detail. This kinetic study is carried out by recording simultaneous cyclic voltammograms and voltfluorograms at the same electrode. Numerical simulations are used to account for the experimental current and fluorescence signals, to analyze their degree of correlation, and to decipher their relation to resazurin reduction kinetics. It is then shown that, provided that the reduction of resazurin takes place at a micrometer-sized electrode, the fluorescence emission perfectly tracks the faradaic current. By implementing this ideal configuration at the counter electrode of a potentiostatic setup, it is finally demonstrated that the oxidation reaction of a nonfluorescent species at the working electrode can be quantitatively transduced into simultaneous emission of fluorescence.

A highly sensitive endotoxin sensor based on redox cycling in a nanocavity

A highly sensitive endotoxin sensor and novel analytical principle using diffusion coefficient difference was developed using a nanocavity device.

Enhanced annihilation electrochemiluminescence by nanofluidic confinement

The generation of stable enhanced light emission by electrochemiluminescence in microfabricated nanofluidic electrochemical devices is demonstrated for the first time by exploiting nanogap amplification.

Microfabricated nanofluidic electrochemical devices offer a highly controlled nanochannel geometry; they confine the volume of chemical reactions to the nanoscale and enable greatly amplified electrochemical detection. Here, the generation of stable light emission by electrochemiluminescence (ECL) in transparent nanofluidic devices is demonstrated for the first time by exploiting nanogap amplification. Through continuous oxidation and reduction of [Ru(bpy)₃]²⁺ luminophores at electrodes positioned at opposite walls of a 100 nm nanochannel, we compare classic redox cycling and ECL annihilation. Enhanced ECL light emission of attomole luminophore quantities is evidenced under ambient conditions due to the spatial confinement in a 10 femtoliter volume, resulting in a short diffusion timescale and highly efficient ECL reaction pathways at the nanoscale.

Asymmetric Nafion-Coated Nanopore Electrode Arrays as Redox-Cycling-Based Electrochemical Diodes

Inspired by the functioning of cellular ion channels, pore-based structures with nanoscale openings have been fabricated and integrated into ionic circuits, for example, ionic diodes and transistors, for signal processing and detection. In these systems, the nonlinear current responses arise either because asymmetric nanopore geometries break the symmetry of the ion distribution, creating unequal surface charge across the nanopore, or by coupling unidirectional electron transfer within a nanopore electrode. Here we develop a high-performance redox-cycling-based electrochemical diode by coating an asymmetric ion-exchange membrane, that is, Nafion, on the top surface of a nanopore electrode array (Nafion@NEA), in which each pore in the array exhibits one or more annular electrodes. Nafion@NEAs exhibit highly sensitive and charge-selective electroanalytical measurements due to efficient redox-cycling reaction, the permselectivity of Nafion, and the strong confinement of redox species in the nanopore array. In addition, the top electrode of dual-electrode Nafion@NEAs can serve as a voltage-controlled switch to gate ion transport within the nanopore. Thus Nafion@NEAs can be operated as a diode by switching voltages applied to the top and bottom electrodes of the NEA, leading to a large rectification ratio, fast response times, and simplified circuitry without the need for external electrodes. By taking advantage of closely spaced and individually addressable electrodes, the redox-cycling electrochemical diode has the potential for application to large-scale production and electrochemically controlled circuit operations, which go well beyond conventional electronic diodes or transistors.

Design and Application of Sensors for Chemical Cytometry

The bulk cell population response to a stimulus, be it a growth factor or a cytotoxic agent, neglects the cell-to-cell variability that can serve as a friend or as a foe in human biology. Biochemical variations among closely related cells furnish the basis for the adaptability of the immune system but also act as the root cause of resistance to chemotherapy by tumors. Consequently, the ability to probe for the presence of key biochemical variables at the single-cell level is now recognized to be of significant biological and biomedical impact. Chemical cytometry has emerged as an ultrasensitive single-cell platform with the flexibility to measure an array of cellular components, ranging from metabolite concentrations to enzyme activities. We briefly review the various chemical cytometry strategies, including recent advances in reporter design, probe and metabolite separation, and detection instrumentation. We also describe strategies for improving intracellular delivery, biochemical specificity, metabolic stability, and detection sensitivity of probes. Recent applications of these strategies to small molecules, lipids, proteins, and other analytes are discussed. Finally, we assess the current scope and limitations of chemical cytometry and discuss areas for future development to meet the needs of single-cell research.

Single-molecule electrochemistry in nanochannels: probing the time of first passage

The diffusive mass transport of individual redox molecules was probed experimentally in microfabricated nanogap electrodes. The residence times for molecules inside a well-defined detection volume were extracted and the resulting distribution was compared with quantitative analytical predictions from random-walk theory for the time of first passage. The results suggest that a small number of strongly adsorbing sites strongly influence mass transport at trace analyte levels.

Emerging tools for studying single entity electrochemistry

Electrochemistry studies charge transfer and related processes at various microscopic structures (atomic steps, islands, pits and kinks on electrodes), and mesoscopic materials (nanoparticles, nanowires, viruses, vesicles and cells) made by nature and humans, involving ions and molecules. The traditional approach measures averaged electrochemical quantities of a large ensemble of these individual entities, including the microstructures, mesoscopic materials, ions and molecules. There is a need to develop tools to study single entities because a real system is usually heterogeneous, e.g., containing nanoparticles with different sizes and shapes. Even in the case of "homogeneous" molecules, they bind to different microscopic structures of an electrode, assume different conformations and fluctuate over time, leading to heterogeneous reactions. Here we highlight some emerging tools for studying single entity electrochemistry, discuss their strengths and weaknesses, and provide personal views on the need for tools with new capabilities for further advancing single entity electrochemistry.

Chemical physics of electroactive materials - the oft-overlooked faces of electrochemistry

Electroactive materials and their applications are enjoying renewed attention, in no small part motivated by the advent of nanoscale tools for their preparation and study. While the fundamentals of charge and mass transport in electrolytes on this scale are by and large well understood, their interplay can have subtle manifestations in the more complex situations typical of, for example, integrated microfluidics-based applications. In particular, the role of faradaic processes is often overlooked or, at best, purposefully suppressed via experimental design. In this introductory article we discuss, using simple illustrations from our laboratories, some of the manifestations of electrochemistry in electroactive materials.

Single-molecule spectroelectrochemical cross-correlation during redox cycling in recessed dual ring electrode zero-mode waveguides

The ability of zero-mode waveguides (ZMW) to guide light into subwavelength-diameter nanoapertures has been exploited for studying electron transfer dynamics in zeptoliter-volume nanopores under single-molecule occupancy conditions. In this work, we report the spectroelectrochemical detection of individual molecules of the redox-active, fluorogenic molecule flavin mononucleotide (FMN) freely diffusing in solution. Our approach is based on an array of nanopore-confined recessed dual ring electrodes, wherein repeated reduction and oxidation of a single molecule at two closely spaced annular working electrodes yields amplified electrochemical signals. We have articulated these structures with an optically transparent bottom, so that the nanopores are bifunctional, exhibiting both nanophotonic and nanoelectrochemical behaviors allowing the coupling between electron transfer and fluorescence dynamics to be studied under redox cycling conditions. We also investigated the electric field intensity in electrochemical ZMWs (E-ZMW) through finite-element simulations, and the amplification of fluorescence by redox cycling agrees well with predictions based on optical confinement effects inside the E-ZMW. Proof-of-principle experiments are conducted showing that electrochemical and fluorescence signals may be correlated to reveal single molecule fluctuations in the array population. Cross-correlation of single molecule fluctuations in amperometric response and single photon emission provides unequivocal evidence of single molecule sensitivity.

Electrostatic Ion Enrichment in an Ultrathin-Layer Cell with a Critical Dimension between 5 and 20 nm

Electrostatic interactions play an essential role in many analytical applications including molecular sensing and transport studies using nanopores and separation of charged species. Here, we report the voltammetric quantification of electrostatic ion enrichment in a 5-20 nm thin electrochemical cell. A simple lithographic micro/nanofabrication process was used to create ultrathin-layer cells (UTLCs) with a critical dimension (i.e., cell thickness) as small as 5 nm. The voltammetric response of a UTLC was found to be largely dominated by the electrostatic interaction between charges on the cell walls and the redox species. We show that the ultrasmall cell dimension yielded a 100-300-fold enrichment for cationic redox species. An interesting surface adsorption effect was also demonstrated.

Single-Molecule Electrochemistry on a Porous Silica-Coated Electrode

Here we report the direct observation and quantitative analysis of single redox events on a modified indium-tin oxide (ITO) electrode. The key in the observation of single redox events are the use of a fluorogenic redox species and the nanoconfinement and hindered redox diffusion inside 3-nm-diameter silica nanochannels. A simple electrochemical process was used to grow an ultrathin silica film (∼100 nm) consisting of highly ordered parallel nanochannels exposing the electrode surface from the bottom. The electrode-supported 3-nm-diameter nanochannels temporally trap fluorescent resorufin molecules resulting in hindered molecular diffusion in the vicinity of the electrode surface. Adsorption, desorption, and heterogeneous redox events of individual resorufin molecules can be studied using total-internal reflection fluorescence (TIRF). The rate constants of adsorption and desorption processes of resorufin were characterized from single-molecule analysis to be (1.73 ± 0.08) × 10^-4 cm·s^-1 and 15.71 ± 0.76 s^-1, respectively. The redox events of resorufin to the non-fluorescent dihydroresorufin were investigated by analyzing the change in surface population of single resorufin molecules with applied potential. The scan-rate-dependent molecular counting results (single-molecule fluorescence voltammetry) indicated a surface-controlled electrochemical kinetics of the resorufin reduction on the modified ITO electrode. This study demonstrates the great potential of mesoporous silica as a useful modification scheme for studying single redox events on a variety of transparent substrates such as ITO electrodes and gold or carbon film coated glass electrodes. The ability to electrochemically grow and transfer mesoporous silica films onto other substrates makes them an attractive material for future studies in spatial heterogeneity of electrocatalytic surfaces.

Investigating Nanoscale Electrochemistry with Surface- and Tip-Enhanced Raman Spectroscopy

The chemical sensitivity of surface-enhanced Raman spectroscopy (SERS) methodologies allows for the investigation of heterogeneous chemical reactions with high sensitivity. Specifically, SERS methodologies are well-suited to study electron transfer (ET) reactions, which lie at the heart of numerous fundamental processes: electrocatalysis, solar energy conversion, energy storage in batteries, and biological events such as photosynthesis. Heterogeneous ET reactions are commonly monitored by electrochemical methods such as cyclic voltammetry, observing billions of electrochemical events per second. Since the first proof of detecting single molecules by redox cycling, there has been growing interest in examining electrochemistry at the nanoscale and single-molecule levels. Doing so unravels details that would otherwise be obscured by an ensemble experiment. The use of optical spectroscopies, such as SERS, to elucidate nanoscale electrochemical behavior is an attractive alternative to traditional approaches such as scanning electrochemical microscopy (SECM). While techniques such as single-molecule fluorescence or electrogenerated chemiluminescence have been used to optically monitor electrochemical events, SERS methodologies, in particular, have shown great promise for exploring electrochemistry at the nanoscale. SERS is ideally suited to study nanoscale electrochemistry because the Raman-enhancing metallic, nanoscale substrate duly serves as the working electrode material. Moreover, SERS has the ability to directly probe single molecules without redox cycling and can achieve nanoscale spatial resolution in combination with super-resolution or scanning probe microscopies. This Account summarizes the latest progress from the Van Duyne and Willets groups toward understanding nanoelectrochemistry using Raman spectroscopic methodologies. The first half of this Account highlights three techniques that have been recently used to probe few- or single-molecule electrochemical events: single-molecule SERS (SMSERS), superlocalization SERS imaging, and tip-enhanced Raman spectroscopy (TERS). While all of the studies we discuss probe model redox dye systems, the experiments described herein push the study of nanoscale electrochemistry toward the fundamental limit, in terms of both chemical sensitivity and spatial resolution. The second half of this Account discusses current experimental strategies for studying nanoelectrochemistry with SERS techniques, which includes relevant electrochemically and optically active molecules, substrates, and substrate functionalization methods. In particular, we highlight the wide variety of SERS-active substrates and optically active molecules that can be implemented for EC-SERS, as well as the need to carefully characterize both the electrochemistry and resultant EC-SERS response of each new redox-active molecule studied. Finally, we conclude this Account with our perspective on the future directions of studying nanoscale electrochemistry with SERS/TERS, which includes the integration of SECM with TERS and the use of theoretical methods to further describe the fundamental intricacies of single-molecule, single-site electrochemistry at the nanoscale.

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.

Recent advances in the development and application of nanoelectrodes

Nanoelectrodes have key advantages compared to electrodes of conventional size and are the tool of choice for numerous applications in both fundamental electrochemistry research and bioelectrochemical analysis. This Minireview summarizes recent advances in the development, characterization, and use of nanoelectrodes in nanoscale electroanalytical chemistry. Methods of nanoelectrode preparation include laser-pulled glass-sealed metal nanoelectrodes, mass-produced nanoelectrodes, carbon nanotube based and carbon-filled nanopipettes, and tunneling nanoelectrodes. Several new topics of their recent application are covered, which include the use of nanoelectrodes for electrochemical imaging at ultrahigh spatial resolution, imaging with nanoelectrodes and nanopipettes, electrochemical analysis of single cells, single enzymes, and single nanoparticles, and the use of nanoelectrodes to understand single nanobubbles.

Single molecular catalysis of a redox enzyme on nanoelectrodes

Due to a high turnover coefficient, redox enzymes can serve as current amplifiers which make it possible to explore their catalytic mechanism by electrochemistry at the level of single molecules. On modified nanoelectrodes, the voltammetric behavior of a horseradish peroxidase (HRP) catalyzed hydroperoxide reduction no longer presents a continuous current response, but a staircase current response. Furthermore, single catalytic incidents were captured through a collision mode at a constant potential, from which the turnover number of HRP can be figured out statistically. In addition, the catalytic behavior is dynamic which may be caused by the orientation status of HRP on the surface of the electrode. This modified nanoelectrode methodology provides an electrochemical approach to investigate the single-molecule catalysis of redox enzymes.

Single protein sensing with asymmetric plasmonic hexamer via Fano resonance enhanced two-photon luminescence

Fano resonances in plasmonic systems have been proved to facilitate various sensing applications in the nanoscale. In this work, we propose an experimental scheme to realize a single protein sensing by utilizing its two-photon luminescence enhanced by a plasmonic Fano resonance system. The asymmetric gold hexamer supporting polarization-dependent Fano resonances and plasmonic modes without in-plane rotational symmetry is used as a referenced spatial coordinate for bio-sensing. We demonstrate via the full-vectorial three-dimensional simulation that the moving direction and the spatial location of a protein can be detected via its two-photon luminescence, which benefits from the resonant near-field interaction with the electromagnetic hot-spots. The sensitivity to changes in position of our method is substantially better compared with the conventional linear sensing approach. Our strategy would facilitate the sensing, tracking and imaging of a single biomolecule in deep sub-wavelength scale and with a small optical extinction cross-section.

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.

Single-molecule bioelectronics

Experimental techniques that interface single biomolecules directly with microelectronic systems are increasingly being used in a wide range of powerful applications, from fundamental studies of biomolecules to ultra-sensitive assays. In this study, we review several technologies that can perform electronic measurements of single molecules in solution: ion channels, nanopore sensors, carbon nanotube field-effect transistors, electron tunneling gaps, and redox cycling. We discuss the shared features among these techniques that enable them to resolve individual molecules, and discuss their limitations. Recordings from each of these methods all rely on similar electronic instrumentation, and we discuss the relevant circuit implementations and potential for scaling these single-molecule bioelectronic interfaces to high-throughput arrayed sensing platforms.

Self-calibrating highly sensitive dynamic capacitance sensor: towards rapid sensing and counting of particles in laminar flow systems

In this report we propose a sensor architecture and a corresponding read-out technique on silicon for the detection of dynamic capacitance change. This approach can be applied to rapid particle counting and single particle sensing in a fluidic system. The sensing principle is based on capacitance variation of an interdigitated electrode (IDE) structure embedded in an oscillator circuit. The capacitance scaling of the IDE results in frequency modulation of the oscillator. A demodulator architecture is employed to provide a read-out of the frequency modulation caused by the capacitance change. A self-calibrating technique is employed at the read-out amplifier stage. The capacitance variation of the IDE due to particle flow causing frequency modulation and the corresponding demodulator read-out has been analytically modelled. Experimental verification of the established model and the functionality of the sensor chip were shown using a modulating capacitor independent of fluidic integration. The initial results show that the sensor is capable of detecting frequency changes of the order of 100 parts per million (PPM), which translates to a shift of 1.43 MHz at 14.3 GHz operating frequency. It is also shown that a capacitance change every 3 μs can be accurately detected.

Nanofluidics in point of care applications

Nanofluidics is generally described as the study of liquid flow in or around structures of 100 nm or smaller, and its use for lab on a chip devices has now been actively studied for two decades. Here a brief review is given of the impact that this nanofluidics research has had on point of care applications. Four areas are identified where nanofluidics has brought the largest contributions: single nanopores, nanoporous membranes, nanoconfinement and the use of concentration polarization. The sometimes revolutionary developments in these areas are briefly treated and finally challenges and future perspectives are described.

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.