Challenges of Biomolecular Detection at the Nanoscale: Nanopores and Microelectrodes

Integration of enzyme-encapsulated mesoporous silica between nanohole array electrode and hydrogel film for flow-type electrochemical biosensor

We herein propose a simple and sensitive electrochemical flow biosensor platform without an external flow device. The sensing unit comprises a platinum nanohole array electrode deposited on a nanoporous track-etched membrane (PtNH/NPM), a packed-layer of glucose oxidase-encapsulated mesoporous silica particles (GOD/MPS), and bovine serum albumin hydrogel film (BSA gel film). This sensing unit was fixed at the open window at the side of the plastic container with internal solution containing NaCl as osmotic reagent. When the sample glucose solution (0.10 mL) was pipetted at the sensing unit, a portion of the sample solution (5 μL) was spontaneously transferred into the BSA gel film. The concentration gradient of NaCl between the internal solution and the BSA gel film induced osmotic flow of water toward the internal solution. This osmotic flow assisted delivery of glucose to the GOD/MPS and enzymatically generated H₂O₂ to the PtNH/NPM. The proposed sensor could be used repeatedly and produced a linear current response for glucose, with a limit of detection of 16 μM. These sensor performances confirmed availability of the sensor design utilizing the osmotic flow.

Detecting Individual Proteins and Their Surface Charge Variations in Solution by the Potentiometric Nanoimpact Method

Label-free detection and analysis of proteins in their natural form and their dynamic interactions with substrates at the single-molecule level are important for both fundamental studies and various applications. Herein, we demonstrate a simple potentiometric method to achieve this goal by detecting the native charge of protein in solution by utilizing the principle of single-entity electrochemistry techniques. When a charged protein moves near the vicinity of a floating carbon nanoelectrode connected to a high-impedance voltage meter, the distinct local electrostatic potential changes induced by the transient collision event of protein, also called the "nanoimpact" event, can be captured by the nanoelectrode as a potential probe. This potentiometric method is highly sensitive for charged proteins, and low-molecular-weight proteins less than 10 kDa can be detected in low-salt-concentration electrolytes. By analyzing the shape and magnitude of the recorded time-resolved potential change and its time derivative, we can reveal the charge and motion of the protein in the nonspecific protein-surface interaction event. The charge polarity variations of the proteins at different pH values were also successfully probed. Compared with synthetic spherical nanoparticles, the statistical analysis of many single-molecule nanoimpact events revealed a large variation in the recorded transient potential signals, which may be attributed to the intrinsic protein dynamics and surface charge heterogeneity, as suggested by the finite element method and molecular dynamic simulations.

Chemically tailoring nanopores for single-molecule sensing and glycomics

A nanopore can be fairly-but uncharitably-described as simply a nanofluidic channel through a thin membrane. Even this simple structural description holds utility and underpins a range of applications. Yet significant excitement for nanopore science is more readily ignited by the role of nanopores as enabling tools for biomedical science. Nanopore techniques offer single-molecule sensing without the need for chemical labelling, since in most nanopore implementations, matter is its own label through its size, charge, and chemical functionality. Nanopores have achieved considerable prominence for single-molecule DNA sequencing. The predominance of this application, though, can overshadow their established use for nanoparticle characterization and burgeoning use for protein analysis, among other application areas. Analyte scope continues to be expanded, and with increasing analyte complexity, success will increasingly hinge on control over nanopore surface chemistry to tune the nanopore, itself, and to moderate analyte transport. Carbohydrates are emerging as the latest high-profile target of nanopore science. Their tremendous chemical and structural complexity means that they challenge conventional chemical analysis methods and thus present a compelling target for unique nanopore characterization capabilities. Furthermore, they offer molecular diversity for probing nanopore operation and sensing mechanisms. This article thus focuses on two roles of chemistry in nanopore science: its use to provide exquisite control over nanopore performance, and how analyte properties can place stringent demands on nanopore chemistry. Expanding the horizons of nanopore science requires increasing consideration of the role of chemistry and increasing sophistication in the realm of chemical control over this nanoscale milieu.

Single Entity Electrochemistry in Nanopore Electrode Arrays: Ion Transport Meets Electron Transfer in Confined Geometries

Electrochemical measurements conducted in confined volumes provide a powerful and direct means to address scientific questions at the nexus of nanoscience, biotechnology, and chemical analysis. How are electron transfer and ion transport coupled in confined volumes and how does understanding them require moving beyond macroscopic theories? Also, how do these coupled processes impact electrochemical detection and processing? We address these questions by studying a special type of confined-volume architecture, the nanopore electrode array, or NEA, which is designed to be commensurate in size with physical scaling lengths, such as the Debye length, a concordance that offers performance characteristics not available in larger scale structures.The experiments described here depend critically on carefully constructed nanoscale architectures that can usefully control molecular transport and electrochemical reactivity. We begin by considering the experimental constraints that guide the design and fabrication of zero-dimensional nanopore arrays with multiple embedded electrodes. These zero-dimensional structures are nearly ideal for exploring how permselectivity and unscreened ion migration can be combined to amplify signals and improve selectivity by enabling highly efficient redox cycling. Our studies also highlight the benefits of arrays, in that molecules escaping from a single nanopore are efficiently captured by neighboring pores and returned to the population of active redox species being measured, benefits that arise from coupling ion accumulation and migration. These tools for manipulating redox species are well-positioned to explore single molecule and single particle electron transfer events through spectroelectrochemistry, studies which are enabled by the electrochemical zero-mode waveguide (ZMW), a special hybrid nanophotonic/nanoelectronic architecture in which the lower ring electrode of an NEA nanopore functions both as a working electrode to initiate electron transfer reactions and as the optical cladding layer of a ZMW. While the work described here is largely exploratory and fundamental, we believe that the development of NEAs will enable important applications that emerge directly from the unique coupled transport and electron-transfer capabilities of NEAs, including in situ molecular separation and detection with external stimuli, redox-based electrochemical rectification in individually encapsulated nanopores, and coupled sorters and analyzers for nanoparticles.

Chemical sensing platforms for detecting trace-level Alzheimer's core biomarkers

This review provides an overview of recent advances in optical and electrical detection of Alzheimer's disease biomarkers in clinically relevant fluids.

Digital Single Virus Electrochemical Enzyme-Linked Immunoassay for Ultrasensitive H7N9 Avian Influenza Virus Counting

Electrochemistry has been widely used to explore fundamental properties of single molecules due to its fast response and high specificity. However, the lack of efficient signal amplification strategies and quantitative method limit its clinical application. Here, we proposed a digital single virus electrochemical enzyme-linked immunoassay (digital ELISA) for H7N9 avian influenza virus (H7N9 AIV) counting by integration of digital analysis, bifunctional fluorescence magnetic nanospheres (bi-FMNs) with monolayer gold nanoparticles (Au NPs) modified microelectrode array (MA). Bi-FMNs are fabricated by coimmobilizing polyclonal antibody (pAb) and alkaline phosphatase (ALP). At most, one target will be captured per bi-FMNs by controlling the proportion of bi-FMNs to target concentrations (≥5:1). The introduction of digital analysis can solve signal fluctuation and the reliability of single virus detection, enabling the digital ELISA to be sensitively and accurately applied for H7N9 AIV detection with a low detection limit of 7.8 fg/mL, which is greatly promising in single biomolecular detection, early diagnosis of disease, and practical application.

Through a Window, Brightly: A Review of Selected Nanofabricated Thin-Film Platforms for Spectroscopy, Imaging, and Detection

We present a review of the use of selected nanofabricated thin films to deliver a host of capabilities and insights spanning bioanalytical and biophysical chemistry, materials science, and fundamental molecular-level research. We discuss approaches where thin films have been vital, enabling experimental studies using a variety of optical spectroscopies across the visible and infrared spectral range, electron microscopies, and related techniques such as electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and single molecule sensing. We anchor this broad discussion by highlighting two particularly exciting exemplars: a thin-walled nanofluidic sample cell concept that has advanced the discovery horizons of ultrafast spectroscopy and of electron microscopy investigations of in-liquid samples; and a unique class of thin-film-based nanofluidic devices, designed around a nanopore, with expansive prospects for single molecule sensing. Free-standing, low-stress silicon nitride membranes are a canonical structural element for these applications, and we elucidate the fabrication and resulting features-including mechanical stability, optical properties, X-ray and electron scattering properties, and chemical nature-of this material in this format. We also outline design and performance principles and include a discussion of underlying material preparations and properties suitable for understanding the use of alternative thin-film materials such as graphene.

Electrochemical Amplification in Side-by-Side Attoliter Nanogap Transducers

We report a strategy for the fabrication of a new type of electrochemical nanogap transducer. These nanogap devices are based on signal amplification by redox cycling. Using two steps of electron-beam lithography, vertical gold electrodes are fabricated side by side at a 70 nm distance encompassing a 20 attoliter open nanogap volume. We demonstrate a current amplification factor of 2.5 as well as the possibility to detect the signal of only 60 analyte molecules occupying the detection volume. Experimental voltammetry results are compared to calculations from finite element analysis.

Mechanistic Challenges and Advantages of Biosensor Miniaturization into the Nanoscale

Over the past few decades, there has been tremendous interest in developing biosensing systems that combine high sensitivity and specificity with rapid sample-to-answer times, portability, low-cost operation, and ease-of-use. Miniaturizing the biosensor dimensions into the nanoscale has been identified as a strategy for addressing the functional requirements of point-of-care and wearable biosensors. However, it is important to consider that decreasing the critical dimensions of biosensing elements impacts the two most important performance metrics of biosensors: limit-of-detection and response time. Miniaturization into the nanoscale enhances signal-to-noise-ratio by increasing the signal density (signal/geometric surface area) and reducing background signals. However, there is a trade-off between the enhanced signal transduction efficiency and the longer time it takes to collect target analytes on sensor surfaces due to the increase in mass transport times. By carefully considering the signal transduction mechanisms and reaction-transport kinetics governing different classes of biosensors, it is possible to develop structure-level and device-level strategies for leveraging miniaturization toward creating biosensors that combine low limit-of-detection with rapid response times.

Rapid prototyping of all-solution-processed multi-lengthscale electrodes using polymer-induced thin film wrinkling

Three-dimensional electrodes that are controllable over multiple lengthscales are very important for use in bioanalytical systems that integrate solid-phase devices with solution-phase samples. Here we present a fabrication method based on all-solution-processing and thin film wrinkling using smart polymers that is ideal for rapid prototyping of tunable three-dimensional electrodes and is extendable to large volume manufacturing. Although all-solution-processing is an attractive alternative to vapor-based techniques for low-cost manufacturing of electrodes, it often results in films suffering from low conductivity and poor substrate adhesion. These limitations are addressed here by using a smart polymer to create a conformal layer of overlapping wrinkles on the substrate to shorten the current path and embed the conductor onto the polymer layer. The structural evolution of these wrinkled electrodes, deposited by electroless deposition onto a nanoparticle seed layer, is studied at varying deposition times to understand its effects on structural parameters such as porosity, wrinkle wavelength and height. Furthermore, the effect of structural parameters on functional properties such as electro-active surface area and surface-enhanced Raman scattering is investigated. It is found that wrinkling of electroless-deposited thin films can be used to reduce sheet resistance, increase surface area, and enhance the surface-enhanced Raman scattering signal.

Tracking motion trajectories of individual nanoparticles using time-resolved current traces

We report experiments and simulations demonstrating that multiple distinct motion trajectories of individual nanoparticles can be discerned from time-resolved current traces.

Single nanoparticle (NP) electrochemical measurements are widely described, both theoretically and experimentally, as they enable visualization of the electrochemical signal of a single NP that is masked in ensemble measurements. However, investigating the behavior of individual NPs using electrochemical signals remains a significant challenge. Here we report experiments and simulations demonstrating that multiple distinct motion trajectories could be discerned from time-resolved current traces by dynamic Monte Carlo simulations. We show that continuous monitoring and quantification of electrochemical oxidation of individual AgNPs using a low-noise electrochemical measurement platform produce significantly distinguished current traces due to the size-dependent motions of AgNPs. Our findings offer a view of the electrochemical signals of individual NPs that are largely different from that in the literature, and underscore the significance of motion behaviors in single NP electrochemistry.

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.

Covalent immobilization of glucose oxidase on amino MOFs via post-synthetic modification

Post Synthetic Modification (PSM) of amino-metal organic frameworks (NH₂-MOFs) with the enzyme Glucose Oxidase (GOx) is reported.