Noise Characteristics of Nanoscaled Redox-Cycling Sensors: Investigations Based on Random Walks

Stochastic Electrochemical Measurement of a Biofouling Layer on Gold

Adsorption of a biofouling layer on the surface of biosensors decreases the electrochemical activity and hence shortens the service life of biosensors, particularly implantable and wearable biosensors. Real-time quantification of the loss of activity is important for in situ assessment of performance while presenting an opportunity to compensate for the loss of activity and recalibrate the sensor to extend the service life. Here, we introduce an electrochemical noise measurement technique as a tool for the quantification of the formation of a biofouling layer on the surface of gold. The technique uniquely affords thermodynamic and kinetic information without applying an external bias (potential and/or current), hence allowing the system to be appraised in its innate state. The technique relies on the analysis of non-faradaic current and potential fluctuations that are intrinsically generated by the interaction of charged species at the electrode surface, i.e., gold. An analytical model is extended to explain the significance of parameters drawn from statistical analysis of the noise signal. This concept is then examined in buffered media in the presence of albumin, a common protein in the blood and a known source of a fouling layer in biological systems. Results indicate that the statistical analysis of the noise signal can quantify the loss of electrochemical activity, which is also corroborated by impedance spectroscopy as a complementary technique.

Electrochemical Shot Noise of a Redox Monolayer

Redox monolayers are the base for a wide variety of devices including high-frequency molecular diodes or biomolecular sensors. We introduce a formalism to describe the electrochemical shot noise of such a monolayer, confirmed experimentally at room temperature in liquid. The proposed method, carried out at equilibrium, avoids parasitic capacitance, increases the sensitivity, and allows us to obtain quantitative information such as the electronic coupling (or standard electron transfer rates), its dispersion, and the number of molecules. Unlike in solid-state physics, the homogeneity in energy levels and transfer rates in the monolayer yields a Lorentzian spectrum. This first step for shot noise studies in molecular electrochemical systems opens perspectives for quantum transport studies in a liquid environment at room temperature as well as highly sensitive measurements for bioelectrochemical sensors.

Current Fluctuations in Nanopores Reveal the Polymer-Wall Adsorption Potential

Modification of surface properties by polymer adsorption is a widely used technique to tune interactions in molecular experiments such as nanopore sensing. Here, we investigate how the ionic current noise through solid-state nanopores reflects the adsorption of short, neutral polymers to the pore surface. The power spectral density of the noise shows a characteristic change upon adsorption of polymer, the magnitude of which is strongly dependent on both polymer length and salt concentration. In particular, for short polymers at low salt concentrations no change is observed, despite the verification of comparable adsorption in these systems using quartz crystal microbalance measurements. We propose that the characteristic noise is generated by the movement of polymers on and off the surface and perform simulations to assess the feasibility of this model. Excellent agreement with experimental data is obtained using physically motivated simulation parameters, providing deep insight into the shape of the adsorption potential and underlying processes. This paves the way toward using noise spectral analysis for in situ characterization of functionalized nanopores.

Zero-Mode Waveguide Nanophotonic Structures for Single Molecule Characterization

Single-molecule characterization has become a crucial research tool in the chemical and life sciences, but limitations, such as limited concentration range, inability to control molecular distributions in space, and intrinsic phenomena, such as photobleaching, present significant challenges. Recent developments in non-classical optics and nanophotonics offer promising routes to mitigating these restrictions, such that even low affinity (K D ~ mM) biomolecular interactions can be studied. Here we introduce and review specific nanophotonic devices used to support single molecule studies. Optical nanostructures, such as zero-mode waveguides (ZMWs), are usually fabricated in thin gold or aluminum films and serve to confine the observation volume of optical microspectroscopy to attoliter to zeptoliter volumes. These simple nanostructures allow individual molecules to be isolated for optical and electrochemical analysis, even when the molecules of interest are present at high concentration (μM - mM) in bulk solution. Arrays of ZMWs may be combined with optical probes such as single molecule fluorescence, single molecule fluorescence resonance energy transfer (smFRET), and fluorescence correlation spectroscopy (FCS) for distributed analysis of large numbers of single-molecule reactions or binding events in parallel. Furthermore, ZMWs may be used as multifunctional devices, for example by combining optical and electrochemical functions in a single discrete architecture to achieve electrochemical ZMWs (E-ZMW). In this review, we will describe the optical properties, fabrication, and applications of ZMWs for single-molecule studies, as well as the integration of ZMWs into systems for chemical and biochemical analysis.

Modulation of the Electrochemical Reactivity of Solubilized Redox Active Polymers via Polyelectrolyte Dynamics

Redox active polymers (RAPs) are electrochemically versatile materials that find key applications in energy storage, sensing, and surface modification. In spite of the ubiquity of RAP-modified electrodes, a critical knowledge gap exists in the understanding of the electrochemistry of soluble RAPs and their relation to polyelectrolyte dynamics. Here, we explore for the first time the intersection between polyelectrolyte behavior and the electrochemical response that highly soluble and highly substituted RAPs with viologen, ferrocene, and nitrostyrene moieties elicit at electrodes. This comprehensive study of RAP electrolytes over several orders of magnitude in concentration and ionic strength reveals distinct signatures consistent with surface confined, colloidal, and bulk-like electrochemical behavior. These differences emerge across polyelectrolyte packing regimes and are strongly modulated by changes in RAP coil size and electrostatic interactions with the electrode. We found that, unlike monomer motifs, simple changes in the ionic strength caused variations over 1 order of magnitude in the current measured at the electrode. In addition, the thermodynamics of adsorbed RAP films were also affected, giving rise to standard reduction potential shifts leading to redox kinetic effects as a result of the mediating nature of the RAP film in equilibrium with the solution. Full electrochemical characterization via transient and steady-state techniques, including the use of ultramicroelectrodes and the rotating disk electrode, were correlated to dynamic light scattering, ellipsometry, and viscometric analysis. These methods helped elucidate the relationship between electrochemical behavior and RAP coil size, film thickness, and polyelectrolyte packing regime. This study underscores the role of electrostatics in modulating the reactivity of redox polyelectrolytes.

Electrode-particle impacts: a users guide

We present a comprehensive guide to nano-impact experiments, in which we introduce newcomers to this rapidly-developing field of research. Central questions are answered regarding required experimental set-ups, categories of materials that can be detected, and the theoretical frameworks enabling the analysis of experimental data. Commonly-encountered issues are considered and presented alongside methods for their solutions.

Collision Dynamics during the Electrooxidation of Individual Silver Nanoparticles

Recent high-bandwidth recordings of the oxidation and dissolution of 35 nm radius Ag nanoparticles at a Au microelectrode show that these nanoparticles undergo multiple collisions with the electrode, generating multiple electrochemical current peaks. In the time interval between observed current peaks, the nanoparticles diffuse in the solution near the electrolyte/electrode interface. Here, we demonstrate that simulations of random nanoparticle motion, coupled with electrochemical kinetic parameters, quantitatively reproduce the experimentally observed multicurrent peak behavior. Simulations of particle diffusion are based on the nanoparticle-mass-based thermal nanoparticle velocity and the Einstein diffusion relations, while the electron-transfer rate is informed by the literature exchange current density for the Ag/Ag⁺ redox system. Simulations indicate that tens to thousands of particle-electrode collisions, each lasting ∼6 ns or less (currently unobservable on accessible experimental time scales), contribute to each experimentally observed current peak. The simulation provides a means to estimate the instantaneous current density during a collision (∼500-1000 A/cm²), from which we estimate a rate constant between ∼5 and 10 cm/s for the electron transfer between Ag nanoparticles and the Au electrode. This extracted rate constant is approximately equal to the thermal collisional velocity of the Ag nanoparticle (4.6 cm/s), the latter defining the theoretical upper limit of the electron-transfer rate constant. Our results suggest that only ∼1% of the surface atoms on the Ag nanoparticles are oxidized per instantaneous collision. The combined simulated and experimental results underscore the roles of Brownian motion and collision frequency in the interpretation of heterogeneous electron-transfer reactions involving nanoparticles.

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.

Influence of Self-Assembled Alkanethiol Monolayers on Stochastic Amperometric On-Chip Detection of Silver Nanoparticles

We investigate the influence of self-assembled alkanethiol monolayers at the surface of platinum microelectrode arrays on the stochastic amperometric detection of citrate-stabilized silver nanoparticles in aqueous solutions. The measurements were performed using a microelectrode array featuring 64 individually addressable electrodes that are recorded in parallel with a sampling rate of 10 kHz for each channel. We show that both the functional end group and the total length of the alkanethiol influence the charge transfer. Three different terminal groups, an amino, a hydroxyl, and a carboxyl, were investigated using two different molecule lengths of 6 and 11 carbon atoms. Finally, we show that a monolayer of alkanethiols with a length of 11 carbon atoms and a carboxyl terminal group can efficiently block the charge transfer of free nanoparticles in an aqueous solution.

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.

Recent Advances in Voltammetry

Recent progress in the theory and practice of voltammetry is surveyed and evaluated. The transformation over the last decade of the level of modelling and simulation of experiments has realised major advances such that electrochemical techniques can be fully developed and applied to real chemical problems of distinct complexity. This review focuses on the topic areas of: multistep electrochemical processes, voltammetry in ionic liquids, the development and interpretation of theories of electron transfer (Butler-Volmer and Marcus-Hush), advances in voltammetric pulse techniques, stochastic random walk models of diffusion, the influence of migration under conditions of low support, voltammetry at rough and porous electrodes, and nanoparticle electrochemistry. The review of the latter field encompasses both the study of nanoparticle-modified electrodes, including stripping voltammetry and the new technique of 'nano-impacts'.

Testing and validating electroanalytical simulations

Digital simulations are a commonly used tool in electrochemical and electroanalytical research. However, even though the employed computational methods often feature significant complexity, testing routines are rarely specified or discussed in literature. In this work, we address this topic and describe approaches towards testing electrochemical simulation software. While focussing on simple systems featuring Nernstian reactions in 1 : 1 stoichiometries, we guide through rigorous testing processes of one- and two dimensional simulations with regard to applications in cyclic voltammetry. To this end, we compile expressions for the calculation of key values as references, discuss the conduction of convergence studies, and suggest approaches to automated software testing.

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.

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.

How many molecules are required to obtain a steady faradaic current from mediated electron transfer at a single nanoparticle on a supporting surface?

We investigate the chronoamperometric noise characteristics of electron-transfer reactions occurring on single nanoparticles (NPs) and assemblies of well-separated NPs on a supporting surface. To this end, we combine a formerly described expression for the steady-state current of a single particle with the shot-noise model and derive an expression for the signal-to-noise ratio as a function of bulk concentration and particle radius. Our findings are supported by random-walk simulations, which closely match the analytical results.

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.