Direct electron transfer and bioelectrocatalysis of hemoglobin at a carbon nanotube electrode

On the intersection of molecular bioelectronics and biosensors: 20 Years of C3B

Formed in 2000 at Virginia Commonwealth University, the Center for Bioelectronics, Biosensors and Biochips (C3B®) has subsequently been located at Clemson University and at Texas A&M University. Established as an industry-university collaborative center of excellence, the C3B has contributed new knowledge and technology in the areas of i) molecular bioelectronics, ii) responsive polymers, iii) multiplexed biosensor systems, and iv) bioelectronic biosensors. Noteworthy contributions in these areas include i) being the first to report direct electron transfer of oxidoreductase enzymes enabled by single walled carbon nanotubes and colloidal clays, ii) the molecular level integration of inherently conductive polymers with bioactive hydrogels using bi-functional monomers such as poly(pyrrole-co-3-pyrrolylbutyrate-conj-aminoethylmethacrylate) [PyBA-conj-AEMA] and 3-(1-ethyl methacryloylate)aniline to yield hetero-ladder electroconductive hydrogels, iii) the development of a multi-analyte physiological status monitoring biochip, and iv) the development of a bioanalytical Wien-bridge oscillator for the fused measurement to lactate and glucose. The present review takes a critical look of these contributions over the past 20 years and offers some perspective on the future of bioelectronics-based biosensors and systems. Particular attention is given to multiplexed biosensor systems and data fusion for rapid decision making.

Bioelectrocatalysis at carbon nanotubes

This paper summarizes several examples of enzyme immobilization and bioelectrocatalysis at carbon nanotubes (CNTs). CNTs offer substantial improvements on the overall performance of amperometric enzyme electrodes mainly due to their unique structural, mechanical and electronic properties such as metallic, semi-conducting and superconducting electron transport. Unfortunately, their water insolubility restrains the kick-off in some particular fields. However, the chemical functionalization of CNTs, non-covalent and covalent, attracted a remarkable interest over the past several decades boosting the development of electrochemical biosensors and enzymatic fuel cells (EFCs) based on two different types of communications: mediated electron transfer (MET)-type, where the use of redox mediators, small electroactive molecules (freely diffusing or bound to side chains of flexible redox polymers), which are able to shuttle the electrons between the enzyme active site and the electrode (second electron transfer generation system); direct electron transfer (DET)-type between the redox group of the enzyme and the electrode surface (third electron transfer generation system).

Electrochemical Enzyme Biosensor Bearing Biochar Nanoparticle as Signal Enhancer for Bisphenol A Detection in Water

An electrochemical tyrosinase enzyme (Tyr) biosensor using a highly conductive sugarcane derived biochar nanoparticle (BCNP) as a transducer and signal enhancer (BCNPs/Tyr/Nafion/GCE) was developed for the sensitive detection of bisphenol A (BPA). The BCNPs/Tyr/Nafion/GCE biosensor exhibited improved amperometric current responses such as higher sensing signal, decreased impedance and lowered reduction potential compared with the Tyr/Nafion/GCE due to high conductivity property of the biochar nanoparticle. Under the optimized conditions, it could detect BPA in good sensitivity with linear range from 0.02 to 10 μM, and a lowest detection limit of 3.18 nM. Moreover, it showed a low K_m value, high reproducibility and good selectivity over other reagents, and the BCNPs/Tyr complex solution also showed good stability with 86.9% of sensing signal maintained after one month storage. The biosensor was also successfully utilized for real water detection with high accuracy as validated by high performance liquid chromatography. Therefore, the biochar nanoparticle based enzyme biosensor proved to be a potential and reliable method for high performance detection of pollutants in the environment.

Biocatalyst and Colorimetric/Fluorescent Dual Biosensors of H2O2 Constructed via Hemoglobin-Cu3(PO4)2 Organic/Inorganic Hybrid Nanoflowers

In this article, the three-dimensional hemoglobin (Hb)-Cu₃(PO₄)₂ organic/inorganic hybrid nanoflowers (Hb-Cu₃(PO₄)₂ HNFs) self-assembled by nanopetals were synthesized via a facile one-pot green synthetic method. The compositions and microstructure of the Hb-Cu₃(PO₄)₂ HNFs were well-characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and UV-vis spectrometry, respectively. The as-prepared Hb-Cu₃(PO₄)₂ HNFs were to be used as a biocatalyst to construct colorimetric/fluorescent dual biosensors. The experimental results show that the colorimetric/fluorescent dual biosensors exhibited two linear responses in the range of 2-10 ppb and 20-100 ppb for H₂O₂. The colorimetric and fluorescent detection limits were 0.1 and 0.01 ppb, respectively. Compared with the free Hb, the biocatalytic activity of the Hb-Cu₃(PO₄)₂ HNFs can be improved for 3-4 times under optimal conditions. The sensing performance of these Hb-Cu₃(PO₄)₂ HNF-based dual biosensors can be contributed such that the active sites of Hb molecules were more exposed on the surface of the Cu₃(PO₄)₂ nanopetals. Second, the unique nanopetal-assembled hybrid flowerlike structure was favorable to contact the detected substance with the biosensors. The dual biosensors were successfully applied for the determination of H₂O₂ in rainwater, tap water, and waste water samples. These results show that the dual biosensors had a potential application in the field of medical analysis, environmental monitoring, and food engineering.

A blood-serum sulfide selective electrochemical sensor based on a 9,10-phenanthrenequinone-tethered graphene oxide modified electrode

The sulfide ion and its associated species (H₂S and HS^-) are widely referred to as toxic chemicals. However, at concentrations of ∼10-100 μM, it serves as a neurotransmitter and signaling agent in biological systems. Abnormalities in blood serum sulfide can be an indication of several diseases, including diabetes, wherein there is a significant reduction in the sulfide ion concentration (<10 μM). Herein, we wish report a 9,10-phenanthrenequinone (PQn) tethered graphene oxide (GO) modified glassy carbon electrode (GCE/GO@PQn) for the highly selective and stable electrocatalytic oxidation and flow injection analysis (FIA) of sulfide ions. The electrode exhibits a detection range of 1-100 μM, and is suitable for the common biochemical interference-free detection of blood serum sulfide in pH 7 phosphate buffer solution. The modified electrode was found to be tolerant of interfering chemicals such as cysteine, uric acid, ascorbic acid, nitrate, glucose, hydrogen peroxide, nitrate, nitrite and dissolved oxygen. This is unlike conventional redox mediator modified electrodes, which all show marked interference with the above-mentioned chemicals during sulfide detection. A constructed FIA calibration plot (applied potential, E_app = 0.15 V vs. Ag/AgCl) was linear in the sulfide concentration ranges of 1-100 μM (1^st region) and 300 μM-5 mM (2^nd region) with a detection limit value of 700 nM. The selective and quick FIA of sulfide ions in three diabetic patient blood samples along with a control was demonstrated as a proof of concept.

Axial Coordination Site-Turned Surface Confinement, Electron Transfer, and Bio-Electrocatalytic Applications of a Hemin Complex on Graphitic Carbon Nanomaterial-Modified Electrodes

Understanding the relation between the chemical bonding and the electron-transfer (ET) reaction of surface-confined hemin (a five-coordinated Fe-porphyrin-with-chlorine complex) is a special interest in the biomimicking studies of heme proteins. Owing to the difficulty in ET function, scanty electrochemical reports of hemin in aqueous solution were reported. It has been noticed that in most of the reported procedures, the sixth axial coordination position of the hemin complex has been unknowingly turned by attaching with water molecules (potential cycling in alkaline conditions or heating), solvents such as ethanol and dimethyl sulfoxide, and nitrogen-donating compounds that have helped for the heme ET reaction. In this work, a systematic effort has been taken to find out the contribution of hemin and its axial bond coordination with π-π interaction, hydrogen bonding, and hydrophobic binding systems toward the ET reaction. Various graphitic carbons such as graphitized mesoporous carbon (GMC), mesoporous carbon-hydrophilic and hydrophobic units, graphite nanopowder, graphene oxide, single-walled carbon, multiwalled carbon nanotube (MWCNT), and carboxylic acid-functionalized MWCNT (as a source for π-π interaction, hydrogen bonding, and hydrophobic environment) along with the amino functional group of chitosan (Chit; as an axial site coordinating system) have been tested by modifying them as a hemin hybrid on a glassy carbon electrode (GCE). In addition, a gold nanoparticle (Aunano) system was combined with the above matrix as a molecular wiring agent, and its role was examined. A highly stable and well-defined redox peak at an apparent formal potential (Eo') of -320 mV versus Ag/AgCl with the highest surface excess of 120 × 10-10 mol cm-2 was noticed with the GCE/Aunano-GMC@hemin-Chit hybrid system, wherein all interactive features have been utilized. Omitting any of the individual interactions resulted in either decreased (with Aunano) or nil current response. As applications, efficient bio-electrocatalytic reduction and sensing of dissolved oxygen and hydrogen peroxide have been demonstrated.

Mechanisms of Enhanced Hemoglobin Electroactivity on Carbon Electrodes upon Exposure to a Water-Miscible Primary Alcohol

Exposing a carbon electrode to hemoglobin (Hb) and alcoholic solvents, such as methanol, ethanol or 1-propanol, drastically changes Hb electroactivity, but until this work, the important underlying mechanisms were unclear. For the first time, we show that these alcohols impact Hb electroactivity via three mechanisms: modification of the carbon surface oxides on the glassy carbon (GC) electrode, Hb film formation, and structural changes to Hb. C_1s X-ray photoelectron spectroscopy provided evidence for significant alcohol-induced modification of the carbon surface oxides, and differential pulse voltammetry showed links between these modifications and Hb electroactivity. Spectroscopic ellipsometry showed that Hb films formed during exposure to Hb- and alcohol-containing electrolytes increased in thickness with increasing alcohol content, although film thickness played only a minor role in Hb electroactivity. Alcohol-induced structural changes in Hb are confirmed with UV-visible absorption and fluorescence data, showing that Hb denaturation also was a significant factor in increasing Hb electroactivity. Carbon-surface-oxide modification and Hb denaturation worked in tandem to maximally increase the Hb electroactivity in 60% methanol. While in ethanol and 1-propanol, the significant increases in Hb electroactivity caused by Hb denaturation were offset by an increase in Hb-inhibiting carbon surface oxides. Knowledge of these mechanisms shows the impact of alcohols on both Hb and carbon electrodes, allows for thoughtful design of the Hb-sensing system, is vital for proper analysis of Hb electroactivity in the presence of these alcohols (e.g., when used as binder solvents for immobilizing Hb into films), and provides fundamental understanding of the Hb-carbon interactions.

Electrochemical redox signaling of hemoglobin in human whole blood and its relevance to anemia and thalassemia diagnosis

A highly redox active human whole blood-carbon nanomaterial modified electrode has been developed, which showed a redox peak (vs. Ag/AgCl) similar to that of hemoglobin (vs. Ag/AgCl) in red blood cells. Clinical relevance of this for direct electrochemical analysis of blood hemoglobin content (anemia) and thalassemia disease diagnosis was demonstrated.

Direct Electron Transfer Kinetics of Peroxidase at Edge Plane Sites of Cup-Stacked Carbon Nanofibers and Their Comparison with Single-Walled Carbon Nanotubes

Electron transfer kinetics at the graphene edge site is of great interest from the viewpoints of application to sensing and energy conversion and storage. Here we analyzed kinetics of direct electron transfer of horseradish peroxidase (HRP) adsorbed through surfactant sodium dodecyl sulfate at cup-stacked carbon nanofibers (CSCNFs), which provide highly ordered graphene edges, and compared it with that at single-walled carbon nanotubes (SWCNTs), which consist of a rolled-up basal plane graphene. The heterogeneous electron transfer rate constant of the Fe(2+/3+) couple of the HRP reaction center at CSCNFs (ca. 34.8 s(-1)) was an order of magnitude larger than that at SWCNTs (ca. 4.7 s(-1)). In addition, the overall rate constant of the electron transfer reaction from CSCNFs to HRP oxidized by H2O2 was higher than that from SWCNTs by a factor of 3. CSCNFs also allowed enhancement of the complex-formation reaction rate of HRP with H2O2, in comparison with that at SWCNTs. CSCNFs would therefore be applied to not only biosensors but also biofuel cells with enhanced performance.

Control of electrochemical signals from quantum dots conjugated to organic materials by using DNA structure in an analog logic gate

Various bio-logic gates have been studied intensively to overcome the rigidity of single-function silicon-based logic devices arising from combinations of various gates. Here, a simple control tool using electrochemical signals from quantum dots (QDs) was constructed using DNA and organic materials for multiple logic functions. The electrochemical redox current generated from QDs was controlled by the DNA structure. DNA structure, in turn, was dependent on the components (organic materials) and the input signal (pH). Independent electrochemical signals from two different logic units containing QDs were merged into a single analog-type logic gate, which was controlled by two inputs. We applied this electrochemical biodevice to a simple logic system and achieved various logic functions from the controlled pH input sets. This could be further improved by choosing QDs, ionic conditions, or DNA sequences. This research provides a feasible method for fabricating an artificial intelligence system.

Adsorption Properties of Arc Produced Multi Walled Carbon Nanotubes for Bovine Serum Albumin

In this study, adsorption properties of arc produced Multi Walled Carbon Nanotubes (MWNT) were investigated for Bovine Serum Albumin (BSA) in aqueous phase. Solution pH, adsorbent amount and temperature effects were examined on protein adsorption. The results show that, the temperature and the adsorbent amount both increase the BSA adsorption, whereas the solution pH has a decreasing effect. The equilibrium behavior of protein adsorption was examined by Langmuir and Freundlich isotherms. The monolayer adsorption capacities at 40 °C for solution pH 4 and 5 were determined as 139.5 and 127.2 mg g−1, respectively, which were much higher than the BSA adsorption capacities of various metal oxides investigated in our previous studies. The adsorption rate data were compared by the pseudo-first and the second-order kinetics equations. Evaluation of the experimental kinetics data have shown that the adsorption of BSA by MWNT followed the pseudo-first-order kinetics. The pseudo-first order adsorption rate constants at pH 4 and 5 decreased with an increase in temperature which results in a decrease in diffusion rate of BSA molecules across the external boundary layer, and favors the sorption process. The adsorption behavior of protein by carbon nanotubes was explained also using the zeta potential measurements. The adsorption capacity decreased with increasing pH due to the electrostatic repulsions. The thermodynamic parameters evaluated to predict the nature of adsorption confirmed the non-spontaneous and endothermic behaviour of the BSA/MWNT adsorption process. Adsorption standard enthalpy values were found as ∆H0=59.5 kJ mol−1 and ∆H0=14.3 kJ mol−1 for pH 4 and 5, respectively indicating that the protein molecules are adsorbed electrostatically on the carbon nanotubes.

Hemoglobin-carbon nanotube derived noble-metal-free Fe5C2-based catalyst for highly efficient oxygen reduction reaction

High performance non-precious cathodic catalysts for oxygen reduction reaction (ORR) are vital for the development of energy materials and devices. Here, we report an noble metal free, Fe₅C₂ nanoparticles-studded sp² carbon supported mesoporous material (CNTHb-700) as cathodic catalyst for ORR, which was prepared by pyrolizing the hybrid adduct of single walled carbon nanotubes (CNT) and lyophilized hemoglobin (Hb) at 700 °C. The catalyst shows onset potentials of 0.92 V in 0.1 M HClO₄ and in 0.1 M KOH which are as good as commercial Pt/C catalyst, giving very high current density of 6.34 and 6.69 mA cm⁻² at 0.55 V vs. reversible hydrogen electrode (RHE), respectively. This catalyst has been confirmed to follow 4-electron mechanism for ORR and shows high electrochemical stability in both acidic and basic media. Catalyst CNTHb-700 possesses much higher tolerance towards methanol than the commercial Pt/C catalyst. Highly efficient catalytic properties of CNTHb-700 could lead to fundamental understanding of utilization of biomolecules in ORR and materialization of proton exchange membrane fuel cells for clean energy production.

A biomemory chip composed of a myoglobin/CNT heterolayer fabricated by the protein-adsorption-precipitation-crosslinking (PAPC) technique

In this study, a biomemory chip consisting of a myoglobin/carbon nanotube (CNT) heterolayer is fabricated via the protein-adsorption-precipitation-crosslinking (PAPC) technique for electrochemical signal enhancement, long-term stability, and improved memory function. The PAPC technique is used to fabricate a myoglobin/CNT heterolayer with a CNT core and a high-density myoglobin-shell structure to achieve efficient heterolayer formation and improved performance of the heterolayer. The fabricated myoglobin/CNT heterolayer is immobilized onto a Au substrate through a chemical linker. The surface morphology of the deposited heterolayer is investigated via transmission electron microscopy and atomic force microscopy. The redox properties of the myoglobin/CNT heterolayer are investigated by cyclic voltammetry, and the memory function of the heterolayer, including the "write step" and "erase step," is measured by chronoamperometry. Compared with the myoglobin monolayer without CNT, the myoglobin/CNT heterolayer fabricated by the PAPC technique exhibits greater electrochemical signal enhancement, long-term stability at room temperature, and improved memory function. The results suggest that the proposed myoglobin/CNT heterolayer produced via the PAPC technique can be applied as a platform for bioelectronic devices to achieve improved signal intensity and durability.

Sub-5 nm porous nanocrystals: interfacial site-directed growth on graphene for efficient biocatalysis

An interfacial site-directed, capping-agent-free growth method for direct production of macromolecular scale (sub-5 nm) porous nanocrystals that are fully crystalline with a high surface area were developed for efficient biocatalysis.

The direct production of macromolecular scale (sub-5 nm) porous nanocrystals with high surface area has been a considerable challenge over the past two decades. Here we report an interfacial site-directed capping agent-free growth method to directly produce porous ultrasmall (sub-5 nm), fully crystalline, macromolecular scale nanocrystals. The porous sub-5 nm Prussian blue nanocrystals exhibit uniform sizes (∼4 ± 1 nm), high surface area (∼855 m² g^–1), fast electron transfer (rate constant of ∼9.73 s^–1), and outstanding sustained catalytic activity (more than 450 days). The nanocrystal-based biointerfaces enable unprecedented sub-nanomolar level recognition of hydrogen peroxide (∼0.5 nM limit of detection). This method also paves the way towards the creation of ultrasmall porous nanocrystals for efficient biocatalysis.

Recent progress in oxygen-reducing laccase biocathodes for enzymatic biofuel cells

This review summarizes different approaches and breakthroughs in the development of laccase-based biocathodes for bioelectrocatalytic oxygen reduction. The use of advanced electrode materials, such as nanoparticles and nanowires is underlined. The applications of recently developed laccase electrodes for enzymatic biofuel cells are reviewed with an emphasis on in vivo application of biofuel cells.

Engineered proteins for bioelectrochemistry

It is only in the past two decades that excellent protein engineering tools have begun to meet parallel advances in materials chemistry, nanofabrication, and electronics. This is revealing scenarios from which synthetic enzymes can emerge, which were previously impossible, as well as interfaces with novel electrode materials. That means the control of the protein structure, electron transport pathway, and electrode surface can usher us into a new era of bioelectrochemistry. This article reviews the principle of electron transfer (ET) and considers how its application at the electrode, within the protein, and at a redox group is directing key advances in the understanding of protein structure to create systems that exhibit better efficiency and unique bioelectrochemistry.

Biomimetic sensor based on hemin/carbon nanotubes/chitosan modified microelectrode for nitric oxide measurement in the brain

A novel biomimetic microsensor for measuring nitric oxide (NO) in the brain in vivo was developed. The sensor consists of hemin and functionalized multi-wall carbon nanotubes covalently attached to chitosan via the carbodiimide crosslinker EDC followed by chitosan electrodeposition on the surface of carbon fiber microelectrodes. Cyclic voltammetry supported direct electron transfer from the Fe(III)/Fe(II) couple of hemin to the carbon surface at -0.370 V and -0.305 V vs. Ag/AgCl for cathodic and anodic peaks, respectively. Square wave voltammetry revealed a NO reduction peak at -0.762 V vs. Ag/AgCl that increased linearly with NO concentration between 0.25 and 1 μM. The average sensitivity of the microsensors was 1.72 nA/μM and the limit of detection was 25 nM. Oxygen and hydrogen peroxide reduction peaks were observed at -0.269 V and -0.332 V vs. Ag/AgCl, respectively and no response was observed for other relevant interferents, namely ascorbate, nitrite and dopamine. The microsensor was successfully applied to the measurement of exogenously applied NO in the rat brain in vivo.

Poly(lactic acid)/Carbon Nanotube Fibers as Novel Platforms for Glucose Biosensors

The focus of this paper is the development and investigation of properties of new nanostructured architecture for biosensors applications. Highly porous nanocomposite fibers were developed for use as active materials in biosensors. The nanocomposites comprised poly(lactic acid)(PLA)/multi-walled carbon nanotube (MWCNT) fibers obtained via solution-blow spinning onto indium tin oxide (ITO) electrodes. The electrocatalytic properties of nanocomposite-modified ITO electrodes were investigated toward hydrogen peroxide (H₂O₂) detection. We investigated the effect of carbon nanotube concentration and the time deposition of fibers on the sensors properties, viz., sensitivity and limit of detection. Cyclic voltammetry experiments revealed that the nanocomposite-modified electrodes displayed enhanced activity in the electrochemical reduction of H₂O₂, which offers a number of attractive features to be explored in development of an amperometric biosensor. Glucose oxidase (GOD) was further immobilized by drop coating on an optimized ITO electrode covered by poly(lactic acid)/carbon nanotube nanofibrous mats. The optimum biosensor response was linear up to 800 mM of glucose with a sensitivity of 358 nA·mM⁻¹ and a Michaelis-Menten constant (K_M) of 4.3 mM. These results demonstrate that the solution blow spun nanocomposite fibers have great potential for application as amperometric biosensors due to their high surface to volume ratio, high porosity and permeability of the substrate. The latter features may significantly enhance the field of glucose biosensors.