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.

Ring Ultramicroelectrodes for Current-Blockade Particle-Impact Electrochemistry.

In current-blockade impact electrochemistry, insulating particles are detected amperometrically as they impinge upon a micro- or nanoelectrode via a decrease in the faradaic current caused by a redox mediator. A limit of the method is that analytes of a given size yield a broad distribution of response amplitudes due to the inhomogeneities of the mediator flux at the electrode surface. Here, we overcome this limitation by introducing microfabricated ring-shaped electrodes with a width that is significantly smaller than the size of the target particles. We show that the relative step size is somewhat larger and exhibits a narrower distribution than at a conventional ultramicroelectrode of equal diameter.

Electrochemical characterization of individual oil micro-droplets by high-frequency nanocapacitor array imaging.

CMOS-based nanocapacitor arrays allow local probing of the impedance of an electrolyte in real time and with sub-micron spatial resolution. Here we report on the physico-chemical characterization of individual microdroplets of oil in a continuous water phase using this new tool. We monitor the sedimentation and wetting dynamics of individual droplets, estimate their volume and infer their composition based on their dielectric constant. From measurements before and after wetting of the surface, we also attempt to estimate the contact angle of individual micron-sized droplets. These measurements illustrate the capabilities and versatility of nanocapacitor array technology.

Single-Entity Electrochemistry for Digital Biosensing at Ultralow Concentrations.

Quantifying ultralow analyte concentrations is a continuing challenge in the analytical sciences in general and in electrochemistry in particular. Typical hurdles for affinity sensors at low concentrations include achieving sufficiently efficient mass transport of the analyte, dealing with slow reaction kinetics, and detecting a small transducer signal against a background signal that itself fluctuates slowly in time. Recent decades have seen the advent of methods capable of detecting single analytes ranging from the nanoscale to individual molecules, representing the ultimate mass sensitivity to these analytes. However, single-entity detection does not automatically translate into a superior concentration sensitivity. This is largely because electrochemical transducers capable of such detection are themselves miniaturized, exacerbating mass transport and binding kinetic limitations. In this Perspective, we discuss how these challenges can be tackled through so-called digital sensing: large arrays of separately addressable single-entity detectors that provide real-time information on individual binding events. We discuss the advantages of this approach and the barriers to its implementation.

Electrochemical Detection of Tumor-Derived Extracellular Vesicles on Nanointerdigitated Electrodes.

Tumor-derived extracellular vesicles (tdEVs) are attracting much attention due to their essential function in intercellular communication and their potential as cancer biomarkers. Although tdEVs are significantly more abundant in blood than other cancer biomarkers, their concentration compared to other blood components remains relatively low. Moreover, the presence of particles in blood with a similar size as that of tdEVs makes their selective and sensitive detection further challenging. Therefore, highly sensitive and specific biosensors are required for unambiguous tdEV detection in complex biological environments, especially for decentralized point-of-care analysis. Here, we report an electrochemical sensing scheme for tdEV detection, with two-level selectivity provided by a sandwich immunoassay and two-level amplification through the combination of an enzymatic assay and redox cycling on nanointerdigitated electrodes to respectively enhance the specificity and sensitivity of the assay. Analysis of prostate cancer cell line tdEV samples at various concentrations revealed an estimated limit of detection for our assay as low as 5 tdEVs/µL, as well as an excellent linear sensor response spreading over 6 orders of magnitude (10-106 tdEVs/µL), which importantly covers the clinically relevant range for tdEV detection in blood. This novel nanosensor and associated sensing scheme opens new opportunities to detect tdEVs at clinically relevant concentrations from a single blood finger prick.

Self-Induced Convection at Microelectrodes via Electroosmosis and Its Influence on Impact Electrochemistry.

Faradaic reactions at low supporting electrolyte concentrations induce convection via electroosmotic flows. Here we combine finite-element simulations and electrochemical measurements on microparticles at ultramicroelectrodes to explore this effect. We show that convection becomes the dominant form of mass transport for experiments at low salt concentrations, violating the common assumption that convection can be neglected.

Quasi-One-Dimensional Generator-Collector Electrochemistry in Nanochannels.

Mass transport in fluidic channels under conditions of pressure-driven flow is controlled by a combination of convection and diffusion. For electrochemical measurements the height of a channel is typically of the same order of magnitude as the electrode dimensions, resulting in complex two- or three- dimensional concentration distributions. Electrochemical nanofluidic devices, however, can have such a low height-to-length ratio that they can effectively be considered as one-dimensional. This greatly simplifies the modeling and quantitative interpretation of analytical measurements. Here we study mass transport in nanochannels using electrodes in a generator-collector configuration. The flux of redox molecules is monitored amperometrically. We observe the transition from diffusion-dominated to convection-dominated transport by varying both the flow velocity and the distance between the electrodes. These results are described quantitatively by the one-dimensional Nernst-Planck equation for mass transport over the full range of experimentally accessible parameters.

Stochastic electrochemistry at ultralow concentrations: the case for digital sensors.

There is increasing demand, in particular from the medical field, for assays capable of detecting sub-pM macromolecular concentrations with high specificity. Methods for detecting single bio/macromolecules have already been developed based on a variety of transduction mechanisms, which represents the ultimate limit of mass sensitivity. Due to limitations imposed by mass transport and binding kinetics, however, achieving high concentration sensitivity additionally requires the massive parallelization of these single-molecule methods. This leads to a new sort of 'digital' assay based on large numbers of parallel, time-resolved measurements aimed at detecting, identifying and counting discrete macromolecular events instead of reading out an average response. In this Tutorial Review we first discuss the challenges inherent to trace-level detection and the motivations for developing digital assays. We then focus on the potential of recently developed single-entity impact electrochemistry methods for use in digital sensors. These have the inherent advantage of relying on purely electrical signals. They can thus in principle be implemented using integrated circuits to provide the parallelization, readout and analysis capabilities required for digital sensors.

Interaction of Anionic Bulk Nanobubbles with Cationic Liposomes: Evidence for Reentrant Condensation.

We investigated the interaction of bulk nanobubbles with cationic liposomes composed of 1,2-dioleoyl- sn-glycero-3-ethylphosphocholine and anionic liposomes assembled from 1-palmitoyl-2-oleoyl- sn-glycero-3-phospho-(1'- rac-glycerol). We employed dynamic light scattering and fluorescence microscopy to investigate both the hydrodynamic and electrophoretic properties of the nanobubble/liposome complexes. These optical techniques permit direct visualization of structural changes as a function of the bubble/liposome ratio. We observed reentrant condensation with cationic liposomes and gas nucleation with anionic liposomes. This is the first report of charge inversion and reentrant condensation of cationic liposomes induced by bulk nanobubbles.

Potential-Controlled Adsorption, Separation, and Detection of Redox Species in Nanofluidic Devices.

Nanoscale channels and electrodes for electrochemical measurements exhibit extreme surface-to-volume ratios and a correspondingly high sensitivity to even weak degrees of surface interactions. Here, we exploit the potential-dependent reversible adsorption of outer-sphere redox species to modulate in space and time their concentration in a nanochannel under advective flow conditions. Induced concentration variations propagate downstream at a species-dependent velocity. This allows one to amperometrically distinguish between attomole amounts of species based on their time-of-flight. On-demand concentration pulse generation, separation, and detection are all integrated in a miniaturized platform.

Electron Transfer Mediated by Surface-Tethered Redox Groups in Nanofluidic Devices.

Electrochemistry provides a powerful sensor transduction and amplification mechanism that is highly suited for use in integrated, massively parallelized assays. Here, the cyclic voltammetric detection of flexible, linear poly(ethylene glycol) polymers is demonstrated, which have been functionalized with redox-active ferrocene (Fc) moieties and surface-tethered inside a nanofluidic device consisting of two microscale electrodes separated by a gap of <100 nm. Diffusion of the surface-bound polymer chains in the aqueous electrolyte allows the redox groups to repeatedly shuttle electrons from one electrode to the other, resulting in a greatly amplified steady-state electrical current. Variation of the polymer length provides control over the current, as the activity per Fc moiety appears to depend on the extent to which the polymer layers of the opposing electrodes can interpenetrate each other and thus exchange electrons. These results outline the design rules for sensing devices that are based on changing the polymer length, flexibility, and/or diffusivity by binding an analyte to the polymer chain. Such a nanofluidic enabled configuration provides an amplified and highly sensitive alternative to other electrochemical detection mechanisms.

High-Frequency Nanocapacitor Arrays: Concept, Recent Developments, and Outlook.

We have developed a measurement platform for performing high-frequency AC detection at nanoelectrodes. The system consists of 65 536 electrodes (diameter 180 nm) arranged in a sub-micrometer rectangular array. The electrodes are actuated at frequencies up to 50 MHz, and the resulting AC current response at each separately addressable electrode is measured in real time. These capabilities are made possible by fabricating the electrodes on a complementary metal-oxide-semiconductor (CMOS) chip together with the associated control and readout electronics, thus minimizing parasitic capacitance and maximizing the signal-to-noise ratio. This combination of features offers several advantages for a broad range of experiments. First, in contrast to alternative CMOS-based electrical systems based on field-effect detection, high-frequency operation is sensitive beyond the electrical double layer and can probe entities at a range of micrometers in electrolytes with high ionic strength such as water at physiological salt concentrations. Far from being limited to single- or few-channel recordings like conventional electrochemical impedance spectroscopy, the massively parallel design of the array permits electrically imaging micrometer-scale entities with each electrode serving as a separate pixel. This allows observation of complex kinetics in heterogeneous environments, for example, the motion of living cells on the surface of the array. This imaging aspect is further strengthened by the ability to distinguish between analyte species based on the sign and magnitude of their AC response. Finally, we show here that sensitivity down to the attofarad level combined with the small electrode size permits detection of individual 28 nm diameter particles as they land on the sensor surface. Interestingly, using finite-element methods, it is also possible to calculate accurately the full three-dimensional electric field and current distributions during operation at the level of the Poisson-Nernst-Planck formalism. This makes it possible to validate the interpretation of measurements and to optimize the design of future experiments. Indeed, the complex frequency and spatial dependence of the data suggests that experiments to date have only scratched the surface of the method's capabilities. Future iterations of the hardware will take advantage of the higher frequencies, higher electrode packing densities and smaller electrode sizes made available by continuing advances in CMOS manufacturing. Combined with targeted immobilization of targets at the electrodes, we anticipate that it will soon be possible to realize complex biosensors based on spatial- and time-resolved nanoscale impedance detection.

The Influence of Supporting Ions on the Electrochemical Detection of Individual Silver Nanoparticles: Understanding the Shape and Frequency of Current Transients in Nano-impacts.

We report the influence of electrolyte composition and concentration on the stochastic amperometric detection of individual silver nanoparticles at microelectrode arrays and show that the sensor response at certain electrode potentials is dependent on both the conductivity of the electrolyte and the concentration of chloride ions. We further demonstrate that the chloride concentration in solution heavily influences the characteristic current spike shape of recorded nanoparticle impacts: While typically too short to be resolved in the measured current, the spike widths are significantly broadened at low chloride concentrations below 10 mm and range into the millisecond regime. The analysis of more than 25 000 spikes reveals that this effect can be explained by the diffusive mass transport of chloride ions to the nanoparticle, which limits the oxidation rate of individual silver nanoparticles to silver chloride at the chosen electrode potential.

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.