A simple strategy for one-step construction of bienzyme biosensor by in-situ formation of biocomposite film through electrodeposition

Improving the sensitivity for hydrogen peroxide determination with active V2O5 nanocubes incorporated on mesoporous TiO2

The capacity to generate a constant signal response from an enzyme on an electrode surface has been a fascinating topic of research from the past three decades. To nourish the enzymatic activity during electrochemical reactions, the immobilization of dual enzymes on the electrode surface could prevent the enzymatic loss without denaturation and thus long-term stability can be achieved. For effective immobilization of dual enzymes, mesoporous materials are the ideal choice because of its numerous advantages such as 1. The presence of porous structure facilitates high loading of enzymes 2. The formation of protective environment can withstand the enzymatic activity even at acidic or basic pH values and even at elevated temperatures. Herein, we develop bienzymatic immobilization of horseradish peroxidase (HRP) and cholesterol oxidase (ChOx) on mesoporous V₂O₅-TiO₂ based binary nanocomposite for effective sensing of hydrogen peroxide (H₂O₂) in presence of redox mediator hydroquinone (HQ). The utilization of redox mediator in second-generation biosensing of H₂O₂ can eliminate the interference species and reduces the operating potential with higher current density for electrochemical reduction reaction. Using this mediator transfer process approach at HRP/ChOx/V₂O₅-TiO₂ modified GC, the H₂O₂ can be determined at operating potential (-0.2 V) with good linear range (0.05-3.5 mM) higher sensitivity (1040 μAμM^-1 cm^-2) and lower detection limit of about 20 μM can be attained, which is due to higher mediation of electrons were transferred to the enzyme cofactors. These interesting characteristics could be due to mesoporous structure of V₂O₅-TiO₂ can induce large immobilization and facilitate higher interaction with enzymes for wide range of biosensing applications.

One, two, many: Strategies to alter the number of carbohydrate binding sites of lectins

Carbohydrates are more than an energy-storage. They are ubiquitously found on cells and most proteins, where they encode biological information. Lectins bind these carbohydrates and are essential for translating the encoded information into biological functions and processes. Hundreds of lectins are known, and they are found in all domains of life. For half a century, researchers have been preparing variants of lectins in which the binding sites are varied. In this way, the traits of the lectins such as the affinity, avidity and specificity towards their ligands as well as their biological efficacy were changed. These efforts helped to unravel the biological importance of lectins and resulted in improved variants for biotechnological exploitation and potential medical applications. This review gives an overview on the methods for the preparation of artificial lectins and complexes thereof and how reducing or increasing the number of binding sites affects their function.

Dual-Signaling Electrochemical Ratiometric Method for Competitive Immunoassay of CYFRA21-1 Based on Urchin-like Fe3O4@PDA-Ag and Ni3Si2O5(OH)4-Au Absorbed Methylene Blue Nanotubes

A novel ratiometric electrochemical (EC) sensing platform was established for sensitive immunoassay of target cytokeratin 19 fragment 21-1 (CYFRA21-1) biomarker by combining competitive immunoreaction and multisignal output. This immunosensor utilized Ag nanoparticles (NPs)-functionalized urchin-like Fe₃O₄@polydopamine (u-Fe₃O₄@PDA-Ag) as a matrix to immobilize CYFRA21-1 antigens and methylene blue (MB)-absorbed Ni₃Si₂O₅(OH)₄-Au nanotubes (NTs) to label the anti-CYFRA21-1 (Ab). During the competitive immunoreaction, square wave voltammetric (SWV) current changes of Ag NPs from u-Fe₃O₄@PDA-Ag indicator and MB from Ni₃Si₂O₅(OH)₄-Au/MB indicator are relevant to the dosage of CYFRA21-1-acquired Ni₃Si₂O₅(OH)₄-Au/MB/Ab. More importantly, numerous CYFRA21-1 loaded stably on u-Fe₃O₄@PDA-Ag exhibited strong competitive capacity toward the target-CYFRA21-1 to combine Ni₃Si₂O₅(OH)₄-Au/MB/Ab, causing sensitive changes in the ratio of two measured SWV currents. Prominently, "ΔI = ΔI_MB + |ΔI_Ag NPs|" (ΔI_MB and |ΔI_Ag NPs| represents the change values of the oxidation peak currents of MB and Ag NPs, respectively) could be regarded as significantly amplifying the signal response and ultimately improving the sensitivity of CYFRA21-1 detection, from which we derived a wide dynamic range from 500 fg/mL to 50 ng/mL and a low detection limit of 0.39 pg/mL (S/N = 3). This work may exert a profound impact on monitoring other biomarkers in early diagnosis of diseases.

Electrophoretic deposition of polymers and proteins for biomedical applications

This review is focused on new electrophoretic deposition (EPD) mechanisms for deposition biomacromolecules, such as biopolymers, proteins and enzymes. Among the rich literature sources of EPD of biopolymers, proteins and enzymes for biomedical applications we selected papers describing new fundamental deposition mechanisms. Such deposition mechanisms are of critical importance for further development of EPD method and its emerging biomedical applications. Our goal is to emphasize innovative ideas which have enriched colloid and interface science of EPD during recent years. We describe various mechanisms of cathodic and anodic EPD of charged biopolymers. Special attention is focused on in-situ chemical modification of biopolymers and crosslinking techniques. Recent innovations in the development of natural and biocompatible charged surfactants and film forming agents are outlined. Among the important advances in this area are the applications of bile acids and salts for EPD of neutral polymers. Such innovations allowed for the successful EPD of various electrically neutral functional polymers for biomedical applications. Particularly important are biosurfactant-polymer interactions, which facilitate dissolution, dispersion, charging, electrophoretic transport and deposit formation. Recent advances in EPD mechanisms addressed the problem of EPD of proteins and enzymes related to their charge reversal at the electrode surface. Conceptually new methods are described, which are based on the use of biopolymer complexes with metal ions, proteins, enzymes and other biomolecules. This review describes new developments in co-deposition of biomacromolecules and future trends in the development of new EPD mechanisms and strategies for biomedical applications.

A dual-enzyme, micro-band array biosensor based on the electrodeposition of carbon nanotubes embedded in chitosan and nanostructured Au-foams on microfabricated gold band electrodes

We report the development of a dual-enzyme electrochemical biosensor based on microfabricated gold band array electrodes which were first modified by gold foam (Au-foam) in order to dramatically increase the active surface area. The resulting nanostructured Au-foam deposits then served as a highly porous 3D matrix for the electrodeposition of a nanocomposite film consisting of multi walled carbon nanotubes embedded in a chitosan matrix (CS:MWCNT) designed to provide a conducting, biocompatible and chemically versatile surface suitable for the attachment of a wide range of chemically or biologically active agents. Finally, a dual enzyme mixture of glucose oxidase (GOx) and horseradish peroxidase (HRP) was immobilised onto the CS:MWCNT nanocomposite film surface. It is shown that the resulting sensing platform developed demonstrates excellent analytical performance in terms of glucose detection with a sensitivity of 261.8 μA mM^-1 cm^-2 and a reproducibility standard deviation (RSD) of 3.30% as determined over 7 measurements. Furthermore, long term stability studies showed that the electrodes exhibited an effectively unchanged response to glucose detection after some 45 days. The example of glucose detection presented here illustrates the fact that the particular combination of nanostructured materials employed represents a very flexible platform for the attachment of enzymes or indeed any other bioactive agent and as such may form the basis of the fabrication of a wide range of biosensors. Finally, since the platform used is based on lithographically-deposited gold electrodes on silicon, we note that it is also very suitable for further miniaturisation, mass production and packaging- all of which would serve to reduce production costs.

Coupling Self-Assembly Mechanisms to Fabricate Molecularly and Electrically Responsive Films

Biomacromolecules often possess information to self-assemble through low energy competing interactions which can make self-assembly responsive to environmental cues and can also confer dynamic properties. Here, we coupled self-assembling systems to create biofunctional multilayer films that can be cued to disassemble through either molecular or electrical signals. To create functional multilayers, we: (i) electrodeposited the pH-responsive self-assembling aminopolysaccharide chitosan, (ii) allowed the lectin Concanavalin A (ConA) to bind to the chitosan-coated electrode (presumably through electrostatic interactions), (iii) performed layer-by-layer self-assembly by sequential contacting with glycogen and ConA, and (iv) conferred biological (i.e., enzymatic) function by assembling glycoprotein (i.e., enzymes) to the ConA-terminated multilayer. Because the ConA tetramer dissociates at low pH, this multilayer can be triggered to disassemble by acidification. We demonstrate two approaches to induce acidification: (i) glucose oxidase can induce multilayer disassembly in response to molecular cues, and (ii) anodic reactions can induce multilayer disassembly in response to electrical cues.

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.

Recent Progress in Lectin-Based Biosensors

This article reviews recent progress in the development of lectin-based biosensors used for the determination of glucose, pathogenic bacteria and toxins, cancer cells, and lectins. Lectin proteins have been widely used for the construction of optical and electrochemical biosensors by exploiting the specific binding affinity to carbohydrates. Among lectin proteins, concanavalin A (Con A) is most frequently used for this purpose as glucose- and mannose-selective lectin. Con A is useful for immobilizing enzymes including glucose oxidase (GOx) and horseradish peroxidase (HRP) on the surface of a solid support to construct glucose and hydrogen peroxide sensors, because these enzymes are covered with intrinsic hydrocarbon chains. Con A-modified electrodes can be used as biosensors sensitive to glucose, cancer cells, and pathogenic bacteria covered with hydrocarbon chains. The target substrates are selectively adsorbed to the surface of Con A-modified electrodes through strong affinity of Con A to hydrocarbon chains. A recent topic in the development of lectin-based biosensors is a successful use of nanomaterials, such as metal nanoparticles and carbon nanotubes, for amplifying output signals of the sensors. In addition, lectin-based biosensors are useful for studying glycan expression on living cells.

Fluorescence Ratiometric Assay Strategy for Chemical Transmitter of Living Cells Using H2O2-Sensitive Conjugated Polymers

A new water-soluble conjugated poly(fluorene-co-phenylene) derivative (PFP-FB) modified with boronate-protected fluorescein (peroxyfluor-1) via PEG linker has been designed and synthesized. In the presence of H2O2, the peroxyfluor-1 group can transform into green fluorescent fluorescein by deprotecting the boronate protecting groups. In this case, upon selective excitation of PFP-FB backbone at 380 nm, efficient fluorescence resonance energy transfer (FRET) from PFP-FB backbone to fluorescein occurs, and accordingly, the fluorescence color of PFP-FB changes from blue to green. Furthermore, the emission color of PFP-FB and the FRET ratio change in a concentration-dependent manner. By taking advantage of PFP-FB, ratiometric detection of choline and acetylcholine (ACh) through cascade enzymatic reactions and further dynamic monitoring of the choline consumption process of cancer cells have been successfully realized. Thus, this new polymer probe promotes the development of enzymatic biosensors and provides a simpler and more effective way for detecting the chemical transmitter of living cells.

Thermo- and sulfate-controllable bioelectrocatalysis of glucose based on horseradish peroxidase and glucose oxidase embedded in poly(N,N-diethylacrylamide) hydrogel films

Dual-responsive poly(N,N-diethylacrylamide) (PDEA) hydrogel films with entrapped horseradish peroxidase (HRP) and glucose oxidase (GOD) were successfully prepared on electrode surface with a simple one-step polymerization procedure under mild conditions, designated as PDEA-HRP-GOD. Cyclic voltammetric (CV) response of electroactive probe K3Fe(CN)6 at the film electrodes displayed reversible thermo- and sulfate-responsive switching behavior. For example, at 25 °C, the K3Fe(CN)6 demonstrated a well-defined CV peak pair with large peak currents for the films, showing the on state, while at 40 °C, the CV response was greatly suppressed and the system was at the off state. The influence of temperature and Na2SO4 concentration on the switching behavior of the film system was not independent or separated, but was synergetic. The responsive mechanism of the system was ascribed to the structure change of PDEA component in the films with temperature and sulfate concentration. This switching property of the PDEA-HRP-GOD films could be further used to realize dual-responsive catalytic oxidation of glucose sequentially by HRP and GOD entrapped in the films with Fe(CN)6 (3-) as the mediator through changing the surrounding temperature and Na2SO4 concentration. This system may establish a foundation for fabricating a new type of multi-switchable electrochemical biosensors based on bienzyme electrocatalysis.

Electrospun Porous ZnO Nanofibers for Glucose Biosensors

Porous 1 dimensional (1D) ZnO nanofibers were synthesized by electrospinning technology and sequent annealing process. The ZnO nanofibers have many nanopores in it and connect to each other to form a porous film. The diameters of the nanofibers are about 100 nm, and their lengths are so long as tens of millimeters. Owing to the porous nanofibers based matrix has 1D channels and high isoelectric point, the prepared ZnO porous film is wonderful platform to immobilize glucose oxidase enzyme for glucose biosensing. The as-fabricated biosensor exhibited high sensitivity (69 μA/mMcm-2), fast response (3 s) and low detection limit (10 μM), due to the nanomaterial with large surface-to-volume ratio for enzyme immobilization and excellent electrical properties. The biosensors can be fabricated controllably and repeatedly. These excellent results indicate the biosensors are attractive for application.

ZnO/Cu nanocomposite: a platform for direct electrochemistry of enzymes and biosensing applications

Unique structured nanomaterials can facilitate the direct electron transfer between redox proteins and the electrodes. Here, in situ directed growth on an electrode of a ZnO/Cu nanocomposite was prepared by a simple corrosion approach, which enables robust mechanical adhesion and electrical contact between the nanostructured ZnO and the electrodes. This is great help to realize the direct electron transfer between the electrode surface and the redox protein. SEM images demonstrate that the morphology of the ZnO/Cu nanocomposite has a large specific surface area, which is favorable to immobilize the biomolecules and construct biosensors. Using glucose oxidase (GOx) as a model, this ZnO/Cu nanocomposite is employed for immobilization of GOx and the construction of the glucose biosensor. Direct electron transfer of GOx is achieved at ZnO/Cu nanocomposite with a high heterogeneous electron transfer rate constant of 0.67 ± 0.06 s(-1). Such ZnO/Cu nanocomposite provides a good matrix for direct electrochemistry of enzymes and mediator-free enzymatic biosensors.

The inhibition of fluorescence resonance energy transfer between quantum dots for glucose assay

Fluorescence resonance energy transfer (FRET) between two quantum dots of different sizes causes fluorescence quenching. Hereby a binding site pre-blocking approach is proposed to avoid this effect. Pre-binding of glucose on the donor occupies the binding sites and thus blocks resonance energy transfer between the two quantum dots, protecting the fluorescence from being quenched. A glucose assay is developed based on this approach. The glucose content is correlated with the fluorescence difference in the absence and in the presence of glucose. In practice, Green QDs-Con A conjugates are used as donors and Red QDs-NH(2)-glu conjugates as acceptors to form FRET system. The inhibition of fluorescence quenching is then measured in the presence of glucose. A linear calibration graph is achieved within 0.1-2.0 mmolL(-1), along with a detection limit of 0.03 mmolL(-1) and a RSD of 2.1% (1.0 mmolL(-1)). 91-105% of glucose in serum and urine samples is recovered. It is worth mentioning that the present glucose assay approach also generates a fluorescence chromatic difference imaging, and the color display clearly identifies the glucose contents by visual detection with a distinguishing ability of ca. 0.5 mmolL(-1). The present approach can potentially be used for the clinical determination of glucose in biological samples which can be further developed into a glucose sensor.

A selective novel non-enzyme glucose amperometric biosensor based on lectin-sugar binding on thionine modified electrode

A novel non-enzyme glucose amperometric biosensor was fabricated based on biospecific binding affinity of concanavalin A (Con A) for D-glucose on thionine (TH) modified electrode. TH can be covalently immobilized on potentiostatically activated glassy carbon electrode through Schiff-base reaction. Subsequently, the surface-adherent polydopamine film formed by self-polymerization of dopamine attached to TH and afforded binding sites for the subsequent immobilization of Con A molecules via Michael addition and/or Schiff-base reaction with high stability. Thus, a sensing platform for specific detection towards D-glucose was established. The binding of Con A towards D-glucose can be monitored through the decrease of the electrode response of the TH moiety. Due to the high affinity of Con A for D-glucose and high stability of the resulting sensing platform, the fabricated biosensor exhibited high selectivity, good sensitivity, and wide linear range from 1.0×10(-6) to 1.0×10(-4) M with a low detection limit of 7.5×10(-7) M towards D-glucose.