Stochastic Electrical Detection of Single Ion-Gated Semiconducting Polymers.

Semiconducting polymer chains constitute the building blocks for a wide range of electronic materials and devices. However, most of their electrical characteristics at the single-molecule level have received little attention. Elucidating these properties can help understanding performance limits and enable new applications. Here, coupled ionic-electronic charge transport is exploited to measure the quasi-1D electrical current through long single conjugated polymer chains as they form transient contacts with electrodes separated by ≈10 nm. Fluctuations between internal conformations of the individual polymers are resolved as abrupt, multilevel switches in the electrical current. This behavior is consistent with the theoretical simulations based on the worm-like-chain (WLC) model for semiflexible polymers. In addition to probing the intrinsic properties of single semiconducting polymer chains, the results provide an unprecedented window into the dynamics of random-coil polymers and enable the use of semiconducting polymers as electrical labels for single-molecule (bio)sensing assays.

Utilizing the Oxygen Reduction Reaction in Particle Impact Electrochemistry: A Step toward Mediator-Free Digital Electrochemical Sensors.

The current blockade particle impact method opens a route toward highly parallelized single-entity electrochemical assays. An important limitation is, however, that a redox mediator must be present in the sample, which can detrimentally interfere with molecular recognition processes. Dissolved O2 that is naturally present in aqueous solutions under ambient conditions can in principle serve as a suitable mediator via the oxygen reduction reaction (ORR). Here, we demonstrate the validity of this concept by performing current blockade experiments to capture and detect individual microparticles at Pt microelectrodes using solely the ORR. The readout modality is independent of the absolute O2 concentration, allowing operation under varying conditions. We further determine how the trajectories of individual microparticles are influenced by the combination of electrophoresis and electroosmotic flows and how these can be utilized to provide continuous detection of cationic particles in water for environmental monitoring.

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.

Self-Assembly of Viral Capsid Proteins Driven by Compressible Nanobubbles.

Colloidal nanobubbles occur in gas-saturated aqueous solutions following high power water electrolysis. Here the influence of nanobubble solutions on the self-assembly properties of viral capsid proteins (CP) was investigated. Interestingly, we found that gas solutions were able to trigger the self-assembly of CP of cowpea chlorotic mottle virus (CCMV) in the absence of the viral genome, most likely by acting as a negatively charged template. The process was demonstrated by three distinct techniques, namely, dynamic light scattering (DLS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Furthermore, nanobubble-induced self-assembly of viral CP was found to depend on protein concentration. Low CP concentrations led to assembly of 18 nm virus-like particles (VLPs), comparable to T = 1 (Casper and Klug triangulation number) virus capsids, whereas high CP concentrations led to 28 nm VLPs (similar to T = 3 capsids). This paves a new route for self-assembly of VLPs.

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.

Electrochemistry in Micro- and Nanochannels Controlled by Streaming Potentials.

Fluid and charge transport in micro- and nanoscale fluidic systems are intrinsically coupled via electrokinetic phenomena. While electroosmotic flows and streaming potentials are well understood for externally imposed stimuli, charge injection at electrodes localized inside fluidic systems via electrochemical processes remains to a large degree unexplored. Here, we employ ultramicroelectrodes and nanogap electrodes to study the subtle interplay between ohmic drops, streaming currents, and faradaic processes in miniaturized channels at low concentrations of supporting electrolyte. We show that electroosmosis can, under favorable circumstances, counteract the effect of ohmic losses and shift the apparent formal potential of redox reactions. This interplay can be described by simple circuit models, such that the results described here can be adapted to other micro- and nanofluidic electrochemical systems.

Electrochemical Collisions of Individual Graphene Oxide Sheets: An Analytical and Fundamental Study.

We propose an analytical method based on electrochemical collisions to detect individual graphene oxide (GO) sheets in an aqueous suspension. The collision rate is found to exhibit a complex dependence on redox mediator and supporting electrolyte concentrations. The analysis of multiple collision events in conjunction with numerical simulations allows quantitative information to be extracted, such as the molar concentration of GO sheets in suspension and an estimate of the size of individual sheets. We also evidence by numerical simulation the existence of edge effects on a 2D blocking object.

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.

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.

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.

Noise as Data: Nucleation of Electrochemically Generated Nanobubbles.

Single-entity electrochemistry aims to expand the toolkit for probing matter at the nanometer scale. Originally focused largely on electrochemically active systems, these methods are increasingly turning into versatile probes complementary to optical, electrical, or mechanical methods. Recent studies of the nucleation, structure, and stability of gas nanobubbles, which exploit electrochemistry at nanoelectrodes as generation and stabilization mechanisms, are prototypical examples. These measurements illustrate the interplay between advances in electrochemical methods and strategies for extracting microscopic information from the results.