Redox-amplified biosensors based on selective modification of nanopore electrode structures with enzymes entrapped within electrodeposition paints

Automated System for Attomolar-Level Detection of MiRNA as a Biomarker for Influenza A Virus

We have developed an automated sensing system for the repeated detection of a specific microRNA (miRNA) of the influenza A (H1N1) virus. In this work, magnetic particles functionalized with DNAs, target miRNAs, and alkaline phosphate (ALP) enzymes formed sandwich structures. These particles were trapped on nickel (Ni) patterns of our sensor chip by an external magnetic field. Then, additional electrical signals from electrochemical markers generated by ALP enzymes were measured using the sensor, enabling the highly sensitive detection of target miRNA. The magnetic particles used on the sensor were easily removed by applying the opposite direction of external magnetic fields, which allowed us to repeat sensing measurements. As a proof of concept, we demonstrated the detection of miRNA-1254, one of the biomarkers for the H1N1 virus, with a high sensitivity down to 1 aM in real time. Moreover, our sensor could selectively detect the target from other miRNA samples. Importantly, our sensor chip showed reliable electrical signals even after six repeated miRNA sensing measurements. Furthermore, we achieved technical advances to utilize our sensor platform as part of an automated sensing system. In this regard, our reusable sensing platform could be utilized for versatile applications in the field of miRNA detection and basic research.

Overview on the Development of Electrochemical Immunosensors by the Signal Amplification of Enzyme- or Nanozyme-Based Catalysis Plus Redox Cycling

Enzyme-linked electrochemical immunosensors have attracted considerable attention for the sensitive and selective detection of various targets in clinical diagnosis, food quality control, and environmental analysis. In order to improve the performances of conventional immunoassays, significant efforts have been made to couple enzyme-linked or nanozyme-based catalysis and redox cycling for signal amplification. The current review summarizes the recent advances in the development of enzyme- or nanozyme-based electrochemical immunosensors with redox cycling for signal amplification. The special features of redox cycling reactions and their synergistic functions in signal amplification are discussed. Additionally, the current challenges and future directions of enzyme- or nanozyme-based electrochemical immunosensors with redox cycling are addressed.

Electrochemical Assembly Strategies of Polymer and Hybrid Thin Films for (Bio)sensors, Charge Storage, and Triggered Release

In the context of functional and hierarchical materials, electrode reactions coupled with one or more chemical reactions constitute the most powerful bottom-up process for the electrosynthesis of film components and their electrodeposition, enabling the localized functionalization of conductive surfaces using an electrical stimulus. In analogy with developmental biological processes, our group introduced the concept of morphogen-driven film buildup. In this approach, the gradient of a diffusing reactive molecule or ion (called a morphogen) is controlled by an electrical stimulus to locally induce a chemical process (solubility change, hydrolysis, complexation, and covalent reaction) that induces a film assembly. One of the prominent advantages of this technique is the conformal nature of the deposits toward the electrode. This Feature Article presents the contributions made by our group and other researchers to develop strategies for the assembly of different polymer and nanoparticle/polymer hybrid films by using electrochemically generated reagents and/or catalysts. The main electrochemical-chemical approaches for conformal films are described in the case where (i) the products are noncovalent aggregates that spontaneously precipitate on the electrode (film electrodeposition) or (ii) new chemical compounds are generated, which do not necessarily spontaneously precipitate and enable the formation of covalent or noncovalent films (film electrosynthesis). The applications of those electrogenerated films will be described with a focus on charge storage/transport, (bio)sensing, and stimuli-responsive cargo delivery systems.

Reusable surface amplified nanobiosensor for the sub PFU/mL level detection of airborne virus

We developed a reusable surface-amplified nanobiosensor for monitoring airborne viruses with a sub-PFU/mL level detection limit. Here, sandwich structures consisted of magnetic particles functionalized with antibodies, target viruses, and alkaline phosphatases (ALPs) were formed, and they were magnetically concentrated on Ni patterns near an electrochemical sensor transducer. Then, the electrical signals from electrochemical markers generated by ALPs were measured with the sensor transducer, enabling highly-sensitive virus detection. The sandwich structures in the used sensor chip could be removed by applying an external magnetic field, and we could reuse the sensor transducer chip. As a proof of concepts, the repeated detection of airborne influenza virus using a single sensor chip was demonstrated with a detection limit down to a sub-PFU/mL level. Using a single reusable sensor transducer chip, the hemagglutinin (HA) of influenza A (H1N1) virus with different concentrations were measured down to 10 aM level. Importantly, our sensor chip exhibited reliable sensing signals even after more than 18 times of the repeated HA sensing measurements. Furthermore, airborne influenza viruses collected from the air could be measured down to 0.01 PFU/mL level. Interestingly, the detailed quantitative analysis of the measurement results revealed the degradation of HA proteins on the viruses after the air exposure. Considering the ultrasensitivity and reusability of our sensors, it can provide a powerful tool to help preventing epidemics by airborne pathogens in the future.

Review of Electrochemically Triggered Macromolecular Film Buildup Processes and Their Biomedical Applications

Macromolecular coatings play an important role in many technological areas, ranging from the car industry to biosensors. Among the different coating technologies, electrochemically triggered processes are extremely powerful because they allow in particular spatial confinement of the film buildup up to the micrometer scale on microelectrodes. Here, we review the latest advances in the field of electrochemically triggered macromolecular film buildup processes performed in aqueous solutions. All these processes will be discussed and related to their several applications such as corrosion prevention, biosensors, antimicrobial coatings, drug-release, barrier properties and cell encapsulation. Special emphasis will be put on applications in the rapidly growing field of biosensors. Using polymers or proteins, the electrochemical buildup of the films can result from a local change of macromolecules solubility, self-assembly of polyelectrolytes through electrostatic/ionic interactions or covalent cross-linking between different macromolecules. The assembly process can be in one step or performed step-by-step based on an electrical trigger affecting directly the interacting macromolecules or generating ionic species.

Nanoporous Gold for Enzyme Immobilization

Nanoporous gold (NPG) is a material of emerging interest for immobilization of biomolecules, especially enzymes. The material provides a high surface area form of gold that is suitable for physisorption or for covalent modification by self-assembled monolayers. The material can be used as a high surface area electrode and with immobilized enzymes can be used for amperometric detection schemes. NPG can be prepared in a variety of formats from alloys containing between 20 and 50 % atomic composition of gold and less noble element(s) by dealloying procedures. Materials resembling NPG can be prepared by hydrothermal and electrodeposition methods. Related high surface area gold structures have been prepared using templating approaches. Covalent enzyme immobilization can be achieved by first forming a self-assembled monolayer on NPG bearing a terminal reactive functional group followed by conjugation to the enzyme through amide linkages to lysine residues. Enzymes can also be entrapped by physisorption or immobilized by electrostatic interactions.

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.

Nanoporous dual-electrodes with millimetre extensions: parallelized fabrication and area effects on redox cycling

Novel fabrication techniques lead to highly sensitive electrochemical sensors (left). The large-area characteristics of redox-cycling within the sensor's nanopores further cause potential-dependent variations of the overall analyte concentration (right).

Multiscale-tailored bioelectrode surfaces for optimized catalytic conversion efficiency

We describe the elaboration of a multiscale-tailored bioelectrocatalytic system. The combination of two enzymes, D-sorbitol dehydrogenase and diaphorase, is studied with respect to the oxidation of D-sorbitol as a model system. The biomolecules are immobilized in an electrodeposited paint (EDP) layer. Reproducible and efficient catalysis of D-sorbitol oxidation is recorded when this system is immobilized on a gold electrode modified by a self-assembled monolayer of 4-carboxy-(2,5,7-trinitro-9-fluorenylidene)malonitrile used as a mediator. The insertion of mediator-modified gold nanoparticles into the EDP film increases significantly the active surface area for the catalytic reaction, which can be further enhanced when the whole system is immobilized in macroporous gold electrodes. This multiscale architecture finally leads to a catalytic device with optimized efficiency for potential use in biosensors, bioelectrosynthesis, and biofuel cells.

Light triggered detection of aminophenyl phosphate with a quantum dot based enzyme electrode

An electrochemical sensor for p-aminophenyl phosphate (pAPP) is reported. It is based on the electrochemical conversion of 4-aminophenol (4AP) at a quantum dot (QD) modified electrode under illumination. Without illumination no electron transfer and thus no oxidation of 4AP can occur. pAPP as substrate is converted by the enzyme alkaline phosphatase (ALP) to generate 4AP as a product. The QDs are coupled via 1,4-benzenedithiol (BDT) linkage to the surface of a gold electrode and thus allow potential-controlled photocurrent generation. The photocurrent is modified by the enzyme reaction providing access to the substrate detection. In order to develop a photobioelectrochemical sensor the enzyme is immobilized on top of the photo-switchable layer of the QDs. Immobilization of ALP is required for the potential possibility of spatially resolved measurements. Geometries with immobilized ALP are compared versus having the ALP in solution. Data indicate that functional immobilization with layer-by-layer assembly is possible. Enzymatic activity of ALP and thus the photocurrent can be described by Michaelis- Menten kinetics. pAPP is detected as proof of principle investigation within the range of 25 μM-1 mM.

Nanoporous gold for enzyme immobilization

Nanoporous gold (NPG) is a material of emerging interest for immobilization of biomolecules and -especially enzymes. NPG materials provide a high gold surface area onto which biomolecules can either be directly physisorbed or covalently linked after first modifying the NPG with a self-assembled monolayer. The material can be used as a high surface area electrode and with immobilized enzymes can be used for amperometric detection schemes. NPG can be prepared in a variety of formats from alloys containing less than 50 atomic% gold by dealloying procedures. Related high surface area gold structures have been prepared using templating approaches. Covalent enzyme immobilization can be achieved by first forming a self-assembled monolayer on NPG bearing a terminal reactive functional group followed by conjugation to the enzyme through amide linkages to lysine residues.