Direct electron transfer of cytochrome c and its biosensor based on gold nanoparticles/room temperature ionic liquid/carbon nanotubes composite film

A robust and effective composite film based on gold nanoparticles (GNPs)/room temperature ionic liquid (RTIL)/multi-wall carbon nanotubes (MWNTs) modified glassy carbon (GC) electrode was prepared by a layer-by-layer self-assembly technique. Cytochrome c (Cyt c) was successfully immobilized on the RTIL-nanohybrid film modified GC electrode by electrostatic adsorption. Direct electrochemistry and electrocatalysis of Cyt c were investigated. The results suggested that Cyt c could be tightly adsorbed on the modified electrode. A pair of well-defined quasi-reversible redox peaks of Cyt c was obtained in 0.10 M, pH 7.0 phosphate buffer solution (PBS). RTIL-nanohybrid film showed an obvious promotion for the direct electron transfer between Cyt c and the underlying electrode. The immobilized Cyt c exhibited an excellent electrocatalytic activity towards the reduction of H₂O₂. The catalysis currents increased linearly to the H₂O₂ concentration in a wide range of 5.0 × 10⁻⁵– 1.15 × 10⁻³ M. Based on the multilayer film, the third-generation biosensor could be constructed for the determination of H₂O₂.     

Direct electrochemistry and electrocatalysis of hemoglobin in sodium alginate film on a BMIMPF6 modified carbon paste electrode

A room temperature ionic liquid (RTIL) modified carbon paste electrode was constructed based on the substitute of paraffin with 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIMPF₆) as binder for carbon paste. Direct electrochemistry and electrocatalytic behaviors of hemoglobin (Hb) entrapped in the sodium alginate (SA) hydrogel film on the surface of this carbon ionic liquid electrode (CILE) were investigated. The presence of IL in the CILE increased the electron transfer rate and provided a biocompatible interface. Hb remained its bioactivity on the surface of CILE and the SA/Hb modified electrode showed a pair of well-defined, quasi-reversible cyclic voltammetric peaks with the apparent standard potential (E^0′) at about −0.344 V (vs. SCE) in pH 7.0 Britton–Robinson (B–R) buffer solution, which was attributed to the Hb Fe(III)/Fe(II) redox couple. UV–Vis absorption spectra indicated that heme microenvironment of Hb in SA film was similar to its native status. Hb showed a thin-layer electrochemical behavior in the SA film with the direct electron transfer achieved on CILE without the help of electron mediator. Electrochemical investigation indicated that Hb took place one proton with one electron electrode process and the average surface coverage of Hb in the SA film was 3.2 × 10⁻¹⁰ mol/cm². The immobilized Hb showed excellent electrocatalytic responses to the reduction of H₂O₂ and nitrite.     

A sensitivity-controlled hydrogen peroxide sensor based on self-assembled Prussian Blue modified electrode

In this communication, a hydrogen peroxide (H₂O₂) sensor based on self-assembled Prussian Blue (PB) modified electrode was reported. Thin film of PB was deposited on the electrode by self-assembly process including multiple sequential adsorption of ferric ions and hexacyanoferrate ions. The as-prepared PB modified electrode displayed sufficient stability for practical sensing application. At an applied potential of ?0.05 V vs. Ag/AgCl (sat. KCl), PB modified electrode with 30 layers exhibited a linear dependence on H₂O₂ concentration in the range of 1 × 10^?6–4 × 10^?4 M (r = 0.9998) with a sensitivity of 625 mA M^?1 cm^?2. It was found that the sensitivity of H₂O₂ sensors could be well controlled by adjusting the number of deposition cycles for PB preparation. This work demonstrates the feasibility of self-assembled PB modified electrode in sensing application, and provides an effective approach to control the sensitivity of PB-based amperometric biosensors.     

Stable free-standing aramid resin film containing vanadium pentoxide and new colour electrochromism of the film by electrodeposition of gold

Vanadium pentoxide (V₂O₅) was electrodeposited on a poly(p-phenylene terephtalamide) (PPTA)-film coated electrode. The cyclic voltammogram of the film had a reversible redox current peak. The film was dark green in the reduced state and yellow in the oxidized state. To obtain new colour, gold was further electrodeposited on the film. Not only the redox current peak but also a new redox current shoulder appeared in the cyclic voltammogram of the obtained film, and it exhibited a multicoloured electrochromism: blackish green ↔ dark green ↔ green ↔  bright red. The red colour in the oxidized state was first obtained for the V₂O₅ film. The new redox current shoulder and the colour were probably due to Au_yV₂O₅ partially formed during electrodeposition of the gold. The redox of the Au_yV₂O₅ was accompanied by egress and ingress of Li⁺ ions and the new colour change.     

Na4Co2.4Mn0.3Ni0.3(PO4)2P2O7: High potential and high capacity electrode material for sodium-ion batteries

Na₄Co_2.4Mn_0.3Ni_0.3(PO₄)₂P₂O₇ has been evaluated as a positive electrode for sodium-ion batteries. The novel material has two redox couples around 4.2 V and 4.6 V and can deliver the high capacity of ca. 103 mAh g^− 1 at the high current density of 850 mA g^− 1 (5 C). X-ray absorption spectroscopy (XAS) results show that the redox reactions of Co, Mn and Ni ions proceed simultaneously in the charge process and it is indicated the novel material provide high mixed potential by the redox reactions of Co, Mn and Ni ions. These findings suggest that the derivatives of Na₄Co₃(PO₄)₂P₂O₇ should be employed as high potential and high capacity electrode materials.     

Direct electrochemistry and electrocatalysis of hemoglobin immobilized in bimodal mesoporous silica and chitosan inorganic–organic hybrid film

Hemoglobin modified electrode was successfully fabricated to realize direct electrochemistry by immobilizing of Hemoglobin (Hb) in bimodal mesoporous silica (BMS) and chitosan (CS) inorganic–organic hybrid film. Here, BMS acted as a support to immobilize Hb due to its large pores and CS acted as a binder to increase film adherence and stabilizer to prevent the leakage of Hb. The resulting electrode (Hb/BMS/CS) gave a well-defined, reversible redox couple for HbFe(III)/Fe(II) with a formal potential of about −0.32 V (vs. Ag/AgCl) in pH 7.0 phosphate buffer solution. Hb/BMS/CS electrode showed a better electrocatalytial performance to H₂O₂ with wider linear detection range, lower detection limit, and higher sensitivity than that at electrode without BMS. The improved electrocatalytic performance for Hb/BMS/CS electrode was possibly contributed to BMS bimodal structure, whose large pores with 10–40 nm provide favorable conditions for protein immobilization and small pores with 2–3 nm avoid the mass-transfer limitations. In addition, UV–Vis and FTIR spectra indicated that Hb well maintained its native structure in the hybrid film.     

End-labeling of peptide nucleic acid with osmium complex. Voltammetry at carbon and mercury electrodes

Peptide nucleic acid (PNA), the DNA mimic with electrically neutral pseudopeptide backbone, is intensively used in biotechnologies and particularly in single-base mismatch detection in DNA hybridization sensors. We propose a simple method of covalent end-labeling of PNA with osmium tetroxide, 2,2′-bipyridine (Os,bipy). Os,bipy-modified PNA (PNA–Os,bipy) produces voltammetric stripping peaks at carbon and mercury electrodes. Peak potential (E_p) of one of the anodic peaks of PNA–Os,bipy at the pyrolytic graphite electrode (PGE) differs from E_p of the reagent, allowing PNA–Os,bipy analysis directly in the reaction mixture. At the hanging mercury drop electrode (HMDE) the PNA–Os,bipy yields a catalytic peak Cat_p, in addition to the redox couples. Using Cat_p it is possible to detect purified PNA–Os,bipy down to 1 pM concentration at accumulation time 60 s. To our knowledge this is the highest sensitivity of the electrochemical detection of PNA.     

Design of a bioelectrocatalytic electrode interface for oxygen reduction in biofuel cells based on a specifically adapted Os-complex containing redox polymer with entrapped Trametes hirsuta laccase

The design of the coordination shell of an Os-complex and its integration within an electrodeposition polymer enables fast electron transfer between an electrode and a polymer entrapped high-potential laccase from the basidiomycete Trametes hirsuta. The redox potential of the Os^3+/2+-centre tethered to the polymer backbone (+ 720 mV vs. NHE) is perfectly matching the potential of the enzyme (+ 780 mV vs. NHE at pH 6.5). The laccase and the Os-complex modified anodic electrodeposition polymer were simultaneously precipitated on the surface of a glassy carbon electrode by means of a pH-shift to 2.5. The modified electrode was investigated with respect to biocatalytic O₂ reduction to H₂O. The proposed modified electrode has potential applications as biofuel cell cathode.     

Direct electrochemistry of hemoglobin and glucose oxidase in electrodeposited sol–gel silica thin films on glassy carbon

This work points out that electrogeneration of silica gel (SG) films on glassy carbon electrodes (GCEs) can be applied to immobilize biomolecules – hemoglobin (Hb) or glucose oxidase (GOD) or both of them in mixture – without preventing their activity. These proteins were physically entrapped in the sol–gel material in the course of the electro-assisted deposition process applied to form the thin films onto the electrode surface. SG films were prepared from a precursor solution by applying a suitable cathodic potential likely to induce a local pH increase at the electrode/solution interface, accelerating thereby polycondensation of the silica precursors with concomitant film formation. Successful immobilization of proteins was checked by various physico-chemical techniques. Both Hb and GOD were found to undergo direct electron transfer, as demonstrated by cyclic voltammetry. GCE–SG–Hb gave rise to well-defined peaks at potentials E_c = −0.29 V and E_a = −0.17 V in acetate buffer, corresponding to the Fe^III/Fe^II redox system of heme group of the protein, while GCE–SG–GOD was characterized by the typical signals of FAD group at E_c = −0.41 V and E_a = −0.33 V in phosphate buffer. These two redox processes were also evidenced on a single voltammogram when both Hb and GOD were present together in the same SG film. Hb entrapped in the silica thin film displayed an electrocatalytic behavior towards O₂ and H₂O₂ in solution, respectively in the mM and μM concentration ranges. Immobilized GOD kept its biocatalytic properties towards glucose. Combined use of these two proteins in mixture has proven to be promising for detection of glucose in solution via the electrochemical monitoring of oxygen consumption (decrease of the oxygen electrocatalytic signal).     

Non-enzymatic detection of hydrogen peroxide using a functionalized three-dimensional graphene electrode

A novel three-dimensional (3D) electrochemical sensor was developed for highly sensitive detection of hydrogen peroxide (H₂O₂). Monolithic and macroporous graphene foam grown by chemical vapor deposition (CVD) served as the electrode scaffold. Using in-situ polymerized polydopamine as the linker, the 3D electrode was functionalized with thionine molecules which can efficiently mediate the reduction of H₂O₂ at close proximity to the electrode surface. Such stable non-enzymatic sensor is able to detect H₂O₂ with a wide linear range (0.4 to 660 μM), high sensitivity (169.7 μA mM^− 1), low detection limit (80 nM), and fast response (reaching 95% of the steady current within 3 s). Furthermore, this sensor was used for real-time detection of dynamic release of H₂O₂ from live cancer cells in response to a pro-inflammatory stimulant.     

The kinetics of the oxidation and reduction of H2O2 at a Pt electrode: A differential electrochemical mass spectrometric study

The kinetics of the oxidation and reduction of hydrogen peroxide (HPOOR, HPORR) at a Pt electrode in 0.1 M HClO₄ + 2 mM H₂O₂ are investigated by Differential Electrochemical Mass Spectrometry (DEMS) in a flow cell. The O₂ mass signal was recorded during cyclic voltammetry, and its potential dependence follows the shape of the cyclic voltammogram. Partial currents for HPOOR and HPORR are estimated based on the O₂ mass signal and the total Faradaic current. The onset overpotential for HPORR at the Pt electrode is above 0.6 V. It is limited by the thermodynamics of OH_ad desorption, as is also the case with ORR. The onset overpotential for HPOOR is below 0.1 V, due to the faster consumption of H₂O₂ through HPORR at these potentials and the small bulk H₂O₂ concentration used.     

Direct electrochemistry of myoglobin based on DNA accumulation on carbon ionic liquid electrode

Poly-anionic deoxyribonucleic acid (DNA) was accumulated on the positively charged surface of carbon ionic liquid electrode (CILE) with N-butylpyridinium hexafluorophosphate (BPPF₆) as binder, and then myoglobin (Mb) was immobilized onto the DNA film by electrostatic interaction to form Mb/DNA/CILE electrode. The direct electrochemistry of Mb was then investigated in detail. A pair of well-defined, quasi-reversible cyclic voltammetric peaks of Mb was obtained with the formal potentials (E⁰′) at ?0.304 V (vs. SCE) in phosphate buffer solution (PBS, pH 7.0). The Mb/DNA/CILE electrode showed excellent electrocatalytic activity to H₂O₂ and trichloroacetic acid (TCA) in the range of 1.0–160 μmol/L and 0.5–40.0 mmol/L, respectively. The apparent Michaelis–Menten constants (K_M) toward H₂O₂ and TCA were calculated as 0.42 and 0.82 mmol/L. So, the DNA/CILE had potential to study other proteins.     

Electrochemical synthesis of a ferrocenecarboxylic acid–C60 composite and its electrocatalysis to hydrogen peroxide

A new ferrocenecarboxylic acid–C₆₀ composite (Fc–C₆₀) has been synthesized by controlled potential electrolysis. A composite modified glassy carbon electrode has been prepared based on its good electrochemical activity. The modified electrode in 0.1 M NaClO₄ solution shows a reversible oxidation wave at E_1/2 = 0.32 V (vs. SCE) attributed to the oxidation of the ferrocene entity and a quasi-reversible reduction wave of C₆₀ entity at E_1/2 = ?0.54 V (vs. SCE). Electrocatalytic studies show that Fc–C₆₀ at the modified electrode can mediate the reduction of hydrogen peroxide (H₂O₂), and a broad linear range from 1.2 μM to 21.9 mM for H₂O₂ were obtained with a determination limit of 2.5 × 10^?7 M by amperometry.     

One-step fabrication of chitosan–hematite nanotubes composite film and its biosensing for hydrogen peroxide

This work reports on a novel chitosan–hematite nanotubes composite film on a gold foil by a simple one-step electrodeposition method. The hybrid chitosan–hematite nanotubes (Chi–HeNTs) film exhibits strong electrocatalytic reduction activity for H₂O₂. Interestingly, two electrocatalytic reduction peaks are observed at −0.24 and −0.56 V (vs SCE), respectively, one controlled by surface wave and the other controlled by diffusion process. The Chi–HeNTs/Au electrode shows a linear response to H₂O₂ concentration ranging from 1 × 10⁻⁶ to 1.6 × 10⁻⁵ mol L⁻¹ with a detection limit of 5 × 10⁻⁸ mol L⁻¹ and a sensitivity as high as 1859 μA μM⁻¹ cm⁻².     

Multilayer structured carbon nanotubes/poly-l-lysine/laccase composite cathode for glucose/O2 biofuel cell

Multilayer film of laccase, poly-l-lysine (PLL) and multi-walled carbon nanotubes (MWNTs) were prepared by a layer-by-layer self-assembly technique. The results of the UV–vis spectroscopy and scanning electron microscopy studies demonstrated a uniform growth of the multilayer. The catalytic behavior of the modified electrode was investigated. The (MWNTs/PLL/laccase)_n multilayer modified electrode catalyzed four-electron reduction of O₂ to water, without any mediator. The possible application of the laccase-catalyzed O₂ reduction at the (MWNTs/PLL/laccase)_n multilayer modified ITO electrode was illustrated by constructing a glucose/O₂ biofuel cell with the (MWNTs/thionine/AuNPs)₈ GDH film modified ITO electrode as a bioanode and the (MWNTs/PLL/laccase)₁₅ film modified ITO electrode as a biocathode. The open-circuit voltage reached to 700 mV, and the maximum power density achieved 329 μW cm⁻² at 470 mV of the cell voltage.     

Novel poly (neutral red) nanowires as a sensitive electrochemical biosensing platform for hydrogen peroxide determination

Poly (neutral red) nanowires (PNRNWs) have been synthesized for the first time by the method of cyclic voltammetric electrodeposition using porous anodic aluminum oxide (AAO) template and were examined by scanning electron microscopy (SEM) and transmission electron microscope (TEM). Moreover, horseradish peroxidase (HRP) was encapsulated in situ in PNRNWs (denoted as PNRNWs–HRP) by electrochemical copolymerization for potential biosensor applications. The PNRNWs showed excellent efficiency of electron transfer between the HRP and the glassy carbon (GC) electrode for the reduction of H₂O₂ and the PNRNWs–HRP modified GC electrode showed to be excellent amperometric sensors for H₂O₂ at −0.1 V with a linear response range of 1 μM to 8 mM with a correlation coefficient of 0.996. The detection limit (S/N = 3) and the response time were determined to be 1 μM and <5 s and the high sensitivity is up to 318 μA mM⁻¹ cm⁻².     

Facile end-labeling of RNA with electroactive Os(VI) complexes

Ribose at the 3′-end of oligonucleotides (oligos) selectively modified by Os(VI)2,2′-bipyridine (bipy) produced two CV redox couples at pyrolytic graphite electrode. Using square wave voltammetry (SWV) 22-mer oligos can be detected down to 250 nM. At mercury electrodes the Os(VI)bipy-oligo adducts produced an electrocatalytic peak at ~?1.2 V allowing their determination down to picomolar concentrations. High specificity of Os(VI)bipy for ribose in nucleic acids and high sensitivity of the determination at mercury and solid amalgam electrodes give promise for new efficient methods of microRNA determination.     

Iron (III) nanocomposites for enzyme-less biomimetic cathode: A promising material for use in biofuel cells

In this paper, we discuss the synthesis and electrochemical properties of a new material based on iron oxide nanoparticles stabilized with poly(diallyldimethylammonium chloride) (PDAC); this material can be used as a biomimetic cathode material for the reduction of H₂O₂ in biofuel cells. A metastable phase of iron oxide and iron hydroxide nanoparticles (PDAC–FeOOH/Fe₂O₃-NPs) was synthesized through a single procedure. On the basis of the Stokes–Einstein equation, colloidal particles (diameter: 20 nm) diffused at a considerably slow rate (D = 0.9 × 10^? 11 m s^? 1) as compared to conventional molecular redox systems. The quasi-reversible electrochemical process was attributed to the oxidation and reduction of Fe³⁺/Fe²⁺ from PDAC–FeOOH/Fe₂O₃-NPs; in a manner similar to redox enzymes, it acted as a pseudo-prosthetic group. Further, PDAC–FeOOH/Fe₂O₃-NPs was observed to have high electrocatalytic activity for H₂O₂ reduction along with a significant overpotential shift, ΔE = 0.68 V from ? 0.29 to 0.39 V, in the presence and absence of PDAC–FeOOH/Fe₂O₃-NPs. The abovementioned iron oxide nanoparticles are very promising as candidates for further research on biomimetic biofuel cells, suggesting two applications: the preparation of modified electrodes for direct use as cathodes and use as a supporting electrolyte together with H₂O₂.     

Structural and electrochemical properties of a porous nanostructured SnO2 film electrode for lithium-ion batteries

A B₂O₃-doped SnO₂ thin film was prepared by a novel experimental procedure combining the electrodeposition and the hydrothermal treatment, and its structure and electrochemical properties were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, energy dispersive X-ray (EDX) spectroscopy and galvanostatic charge–discharge tests. It was found that the as-prepared modified SnO₂ film shows a porous network structure with large specific surface area and high crystallinity. The results of electrochemical tests showed that the modified SnO₂ electrode presents the largest reversible capacity of 676 mAh g^?1 at the fourth cycle, close to the theoretical capacity of SnO₂ (790 mAh g^?1); and it still delivers a reversible Li storage capacity of 524 mAh g^?1 after 50 cycles. The reasons that the modified SnO₂ film electrode shows excellent electrochemical properties were also discussed.     

Application of carboxyl functionalized graphene oxide as mimetic peroxidase for sensitive voltammetric detection of H2O2 with 3,3′,5,5′-tetramethylbenzidine

A sensitive electrochemical method for H₂O₂ determination was proposed with carboxyl functionalized graphene oxide (GO-COOH) as mimetic peroxidase and 3,3′,5,5′-tetramethylbenzidine (TMB) as substrate. GO-COOH exhibited intrinsic peroxidase-like activity that could catalyze the oxidation of TMB with H₂O₂. The generated product exhibited a sensitive second order derivative linear sweep voltammetric reduction peak at − 0.93 V (vs. Ag/AgCl) in Britton–Robinson buffer. Under the optimal conditions the reduction peak current was proportional to H₂O₂ concentration in the linear range from 0.006 to 0.8 μmol L^− 1 with the detection limit of 1.0 nmol L^− 1 (3σ). This proposed method was further applied to determine H₂O₂ content in fresh milk samples with satisfactory results.