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.     

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.     

A glucose/O2 biofuel cell base on nanographene platelet-modified electrodes

This study demonstrated a novel nanographene platelets (NGPs)-based glucose/O₂ biofuel cell (BFC) with the glucose oxidase (GOD) as the anodic biocatalysts and the laccase as the cathodic biocatalysts. The GOD/NGPs-modified electrode exhibited good catalytic activity towards glucose oxidation and the laccase/NGPs-modified electrode exhibited good catalytic activity towards O₂ electroreduction. The maximum power density was ca. 57.8 μW cm^? 2 for the assembled glucose/O₂ NGPs-based BFC. These results indicated that the NGPs were very useful for the future development of novel carbon-based nanomaterials BFC device.     

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.     

Quasi-reversible two-electron reduction of oxygen at gold electrodes modified with a self-assembled submonolayer of cysteine

The electrochemical reduction of molecular oxygen (O₂) has been performed at gold electrodes modified with a submonolayer of a self-assembly (sub-SAM/Au) of a thiol compound (typically cysteine (CYST)) in O₂-saturated 0.5 M KOH. At bare gold electrode O₂ reduction reaction proceeds irreversibly, while this reaction is totally hindered at gold electrodes with a compact structure of CYST over its surface. The partial reductive desorption of the compact CYST monolayer was achieved by controlling the potential of the CYST/Au electrode, leading to the formation of a submonolayer coverage of the thiol compound over the Au electrode surface (sub-SAM/Au), at which the CYST molecules selectively block the Au(1 0 0) and Au(1 1 0) fractions (the so-called rough domains) of the polycrystalline Au while the Au(1 1 1) component (the so-called smooth domains) remains bare (i.e., uncovered with CYST). This sub-SAM/Au electrode extraordinarily exhibits a quasi-reversible two-electron reduction of molecular oxygen (O₂) in alkaline medium with a peak separation (ΔE_p) between the cathodic and anodic peak potentials (E_p^c,E_p^a) of about 60 mV. The ratio of the anodic current to the cathodic one is close to unity. The formal potential (E^o′) of this reaction is found to equal −150 mV vs. Ag/AgCl/KCl(sat.).     

A solid-state pH sensor based on WO3-modified vertically aligned multiwalled carbon nanotubes

In this communication, a novel solid-state pH sensor based on WO₃/MWNTs nanocomposite electrode will be reported. WO₃ nanoparticles were homogeneously coated on vertically aligned MWNTs by magnetron sputtering. Potentiometric pH response of the WO₃/MWNTs electrode in Britton–Robinson buffers revealed a linear working range from pH 2 to12 with a slope of about ?41 mV pH^?1 and a response time less than 90 s. The stability of the electrode remained over a month. Moreover, the WO₃/MWNTs electrode displayed excellent anti-interference property. Compared to conventional pH sensors, the pH sensor based on WO₃/MWNTs nanocomposite electrode also showed excellent reproducibility, high stability and superb selectivity.     

Simultaneous determination of ascorbic acid,dopamine and serotonin at poly(phenosafranine) modified electrode

The oxidation of phenosafranine at glassy carbon electrode gives rise to stable redox active electropolymerized film containing a polyazine moiety (poly(phenosafranine)). The redox response of the poly(phenosafranine) film was observed at the modified electrode at different pH and the pH dependence of the peak potential is 60 mV/pH, which is very close to the expected Nernstian behavior. The apparent diffusion coefficient (D_app) of poly(phenosafranine) film was measured as 2.51 × 10⁻⁹ cm²/s. This film exhibits potent and persistent electron-mediating behavior followed by well-separated oxidation peaks towards ascorbic acid (AA), dopamine (DA) and serotonin with activation overpotential, which is 200 mV lower than that of the bare electrode for AA oxidation. Using differential pulse voltammetry (DPV) studies, the limit of detection of DA in the presence of AA is estimated to be in the submicromolar regime. This method has been used for determining DA and AA concentrations in real samples with satisfactory results.     

Potentiometric coulometry based on charge accumulation with a peroxidase/osmium polymer-immobilized electrode for sensitive determination of hydrogen peroxide

The charge accumulation due to peroxidase (POD)-catalyzed reduction of H₂O₂ in a test solution (4 μL) by Os(II) in a POD/PVI[Os(dmebpy)₂Cl]-immobilized layer on an electrode (PVI = poly(1-vinylimidazole), dmebpy = 4,4′-dimethyl-2,2′-bipyridine) was monitored potentiometrically for the detection of H₂O₂. Before potentiometry, the Os(II)/Os(III) ratio of the modified electrode was controlled by pre-electrolysis at a given potential in a separated electrolysis cell. The redox potential of the Os polymer film in the test solution shifted to the positive side on the addition of H₂O₂ and reached a constant value due to the accumulation of Os(III) in the film. The total amount of the accumulated charge was determined from the area of the portion corresponding to the redox potential shift on a reversible cyclic voltammogram recorded separately. The low detection limit (5 pmol H₂O₂) was realized with 82–90% of the recovery percentage.     

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.     

Functional multiwalled carbon nanotube nanocomposite with iron picket-fence porphyrin and its electrocatalytic behavior

A kind of nanocomposite with good dispersion in water was prepared through noncovalent adsorption of iron picket-fence porphyrin (FeTMAPP), iron-5,10,15,20-tetrakis[αααα-2-trismethylammoniomethyl-phenyl]porphyrin, on multiwalled carbon nanotubes (MWNTs). UV–visible spectroscopic and electrochemical methods were used to characterize the nanocomposite. A gold nanoparticles/nanocomposite self-assembled monolayer was formed on gold electrode and showed highly synergetic behavior towards the electrocatalytic reduction of O₂ with a decrease of overpotential of 200 mV. FeTMAPP acted as the catalytic active center, and MWNTs increased the amount of FeTMAPP adsorbed and accelerated the electron transfer between FeTMAPP and electrode. The resulting biosensor exhibited good response to oxygen with a linear range from 0.52 to 180 μM and a detection limit of 0.38 μM, without the interference of ascorbic acid and uric acid, which showed an application potential of the proposed nanocomposite and monolayer in detection of dissolved oxygen and oxidase substrates.     

Amperometric enzyme biosensor for hydrogen peroxide via Ugi multicomponent reaction

A novel strategy based on the Ugi multicomponent reaction was employed for immobilizing horseradish peroxidase on sodium alginate-coated gold electrode. The electrode was employed for constructing an amperometric biosensor device using 1 mM hydroquinone as electrochemical mediator. The electrode showed linear response (poised at −300 mV vs Ag/AgCl) toward H₂O₂ concentration between 70 μM and 8.8 mM at pH 7.0. The biosensor reached 95% of steady-state current in about 12 s and its sensitivity was 33.8 mA/M cm². The electrode retained full initial activity after 30 days of storage at 4 °C in 50 mM sodium phosphate buffer, pH 7.0.     

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.     

Amperometric sensor for hydrogen peroxide based on direct electron transfer of spinach ferredoxin on Au electrode

A protein-based electrochemical sensor for hydrogen peroxide (H₂O₂) was developed by an easy and effective film fabrication method where spinach ferredoxin (Fdx) containing [2Fe–2S] metal center was cross linked with 11-mercaptoundecanoic acid (MUA) on a gold (Au) surface. The surface morphology of Fdx molecules on Au electrodes was investigated by atomic force microscopy (AFM). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were employed to study the electrochemical behavior of adsorbed Fdx on Au. The interfacial properties of the modified electrode were evaluated in the presence of Fe(CN)₆^3?/4? redox couple as a probe. From CV, a pair of well-defined and quasi-reversible redox peaks of Fdx was obtained in 10 mM, pH 7.0 Tris–HCl buffer solution at ?170 and ?120 mV respectively. One electron reduction of the [2Fe-2S]²⁺ cluster occurs at one of the iron atoms to give the reduced [2Fe-2S]⁺. The formal reduction potential of Fdx ca. ?150 mV (vs. Ag/AgCl electrode) at pH 7.0. The electron-transfer rate constant, k_s, for electron transfer between the Au electrode and Fdx was estimated to be 0.12 s^?1. From the electrochemical experiments, it is observed that Fdx/MUA/Au promoted direct electron transfer between Fdx and electrode and it catalyzes the reduction of H₂O₂. The Fdx/MUA/Au electrode displays a linear increase in amperometric current for increasing concentration of H₂O₂.The sensor calibration plot was linear with r² = 0.998 with sensitivity approximately 68.24 μAm M^?1 cm^?2. Further, the effect of nitrite on the developed sensor was examined which does not interfere with the detection of H₂O₂. Finally, the addition of H₂O₂ on MUA/Au electrode was observed which has no effect on amperometric current.     

Polyaniline nanotubes for impedimetric triglyceride detection

Polyaniline nanotubes (PANI-NT) based film electrophoretically deposited onto indium–tin–oxide (ITO) coated glass plate has been utilized for covalent immobilization of lipase (LIP), via glutaraldehyde (Glu), for triglyceride detection using impedimetric technique. It is shown that fatty acid molecules produced during triglyceride hydrolysis result in change in charge transfer resistance (R_CT) of PANI-NT film with varying triglyceride concentration. LIP/Glu/PANI-NT/ITO bioelectrode has linearity as 25–300 mg dL^?1, sensitivity as 2.59 × 10^?3 KΩ^?1 mg^?1 dL, response time as 20 s and regression coefficient as 0.99. A low value of apparent Michaelis–Menten constant (～0.62 mM) indicates high enzyme affinity to tributyrin. The LIP/Glu/PANI-NT/ITO bioelectrode has been utilized to estimate triglyceride in serum samples.     

In situ scanning tunneling microscopy imaging of mercury film electrodeposited on Ir(1 1 1)

In situ scanning tunneling microscopy (STM) was used to examine multilayer Hg film electrodeposited on a well-ordered Ir(1 1 1) single crystal electrode in 0.1 M HClO₄ + 1 mM Hg(ClO₄)₂. Topography STM scans showed that the Ir(1 1 1) – supported Hg film electrode contained well-defined terraces separated by monatomic steps (Δz = 2.3 Å). The STM’s tip could be used to induce local dissolution of the Hg deposit under proper operating conditions and the depth of the etched pit informed directly the thickness of Hg deposit. Although in situ STM imaging with a tungsten tip could not result in atomic structure of bare Hg film in 0.1 M HClO₄, it discerned highly ordered iodine adlayers, represented by a (2 × 8√3)rect – I structure, on the Hg film in solution containing potassium iodide. These STM results suggested that the Hg substrate could have an ordered atomic structure.     

Detection of hypoxanthine based on the electrochemiluminescent of 6-(4-methoxyphenyl)-2-methylimidazo[1,2-a]pyrazin-3(7H)-one on the electrically heated indium–tin-oxide electrode

An electrochemiluminescent (ECL) enzyme biosensor was developed for detection of hypoxanthine based on ECL of 6-(4-methoxyphenyl)-2-methylimidazo[1,2-a]pyrazin-3(7H)-one (MCLA). The xanthine oxidase (XOD) was modified on the heated indium–tin-oxide (ITO) electrode, and the ECL enzyme biosensor was found to be electrode temperatures depend, this was because that both the diffusion and convection of compounds near the electrode surface and the catalytic reaction of immobilized enzyme were affected by the temperature of electrode (T_e). In this work, the highest sensitivity was gained when T_e was 36 °C. Under the optimum condition, the linear response range of hypoxanthine (HX) was found to be 0.3–27 μmol/L, and the detection limit (defined as the concentration that could be detected at the signal-to-noise ration of 3) was 0.15 μmol/L.     

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.     

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.     

Selective determination of uric acid with DNA doped polymers modified carbon fiber microelectrode

New biocomposite materials, based on the incorporation of DNA doped p-aminobenzensulfonic acid, was fabricated by electrochemical method. A carbon fiber microelectrode modified by this thin film was fabricated for selective determination of uric acid (UA) in the presence of a larger amount of ascorbic acid (AA). It was found that the voltammetric oxidation peak separation between UA and AA is about 260 mV at the modified electrode. A linear response of the peak current versus the concentration was found in the range of 8 × 10⁻⁷–6 × 10⁻⁴ M with correlation coefficient of 0.9991 and the detection limit was 5 × 10⁻⁷ M (s/n = 3) at the 5 × 10⁻⁴ M AA. The presence of high concentration AA did not interference the determination. The electropolymerized film was characterized by SEM techniques. The modified electrode shows good sensitivity, selectivity and stability.     

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.