A highly sensitive ascorbic acid sensor using a Ni–Pt electrode

An amperometric sensor that measured ascorbic acid by the oxidation of the ascorbic acid on a Ni–Pt electrode was fabricated. The Ni component of the Ni–Pt alloy played a crucial role as a modifier that developed an erinaceous surface, which enlarged the sensing area and increased the sensitivity of the electrode. The Pt₈₂Ni₁₈ electrode exhibited the best sensitivity of 333 μA cm⁻² mM⁻¹ for ascorbic acid sensing. This electrode was further tested for reproducibility of the sensitivity, endurance, and interference; it exhibited excellent performance compared with electrodes reported in the literature.     

Layer by layer assembly of glucose oxidase and thiourea onto glassy carbon electrode: Fabrication of glucose biosensor

For the first time a novel, simple and facile approach is described to construct highly stable glucose oxidase (GOx) multilayer onto glassy carbon (GC) electrode using thiourea (TU) as a covalent attachment cross-linker. The layer by layer (LBL) attachment process was confirmed by cyclic voltammetry, electrochemical impedance spectroscopy and Fourier transform infrared reflection spectroscopy (FT-IR-RS) techniques. Immobilized GOx shows excellent electrocatalytic activity toward glucose oxidation using ferrocenemethanol as artificial electron transfer mediator and biosensor response was directly correlated to the number of bilayers. The surface coverage of active GOx per bilayer, heterogeneous electron transfer rate constant (k_s) and Michaelis–Menten constant (K_M), of immobilized GOx were 1.50 × 10⁻¹² mol cm⁻², 9.2 ± 0.5 s⁻¹ and 3.42(±0.2) mM, respectively. The biosensor constructed with four-bilayers of TU/GOx showed good stability, high reproducibility, long life-time, fast amperometric response (5 s) with the high sensitivity of 5.73 μA mM⁻¹ cm⁻² and low detection limit of 6 μM at concentration range up to 5.5 mM.     

High-performance biosensors based on enzyme precipitate coating in gold nanoparticle-conjugated single-walled carbon nanotube network films

Single-walled carbon nanotube (SWCNT) network films with high network density were prepared by vacuum-filtering a suspension of SWCNTs, and used as a host of enzyme precipitate coating of glucose oxidase (EPC-GOx). EPC-GOx was fabricated into the SWCNT network films in a two-step process of enzyme precipitation and crosslinking. High GOx loading in a form of EPC expedited the generation of electrons while the good connectivity of conductive SWCNTs in the network structure increased the electron transfer rate. According to amperometric measurements, the sensitivities of GOx/SWCNT electrodes, governed by both generation and transfer of electrons, were significantly enhanced by filling up the open pores of SWCNT films with the EPC-GOx when compared to the approaches of covalent-attachment (CA) and enzyme coating (EC) with no step of enzyme precipitation. For example, the sensitivities of CA, EC and EPC-GOx were 0.039, 0.140, and 5.72 μA mM⁻¹, respectively. High sensitivity of EPC-GOx was maintained under iterative uses for 10 days. The deposition of gold nanoparticles into SWCNT films has resulted in high-performance glucose sensors with a remarkable sensitivity of 24.5 μA mM⁻¹, which can be explained by further expedited electron transfer due to deposited gold nanoparticles.     

Pyrene-adamantane-β-cyclodextrin: An efficient host–guest system for the biofunctionalization of SWCNT electrodes

The design of nanostructured biological architectures based on host–guest interactions between β-cyclodextrin and adamantane was investigated on SWCNT coatings using glucose oxidase (GOX) as biomolecule model. β-Cyclodextrin tagged GOX was immobilized on adamantane functionalized carbon nanotubes, deposited on platinum electrodes. Different functionalization techniques to attach “pyrene adamantane” on nanotubes were studied and compared in terms of the performances of the subsequently constructed glucose biosensors. The best results were obtained by dipping the nanotube deposit into a pyrene-adamantane solution followed by electropolymerization of the adsorbed pyrene monolayer. The constructed biosensor exhibited a good linear response toward glucose concentrations between 2 × 10⁻⁷ M and 1.6 × 10⁻³ M. The maximum current density and glucose sensitivity were 154.9 μA cm⁻² and 14.4 mA M⁻¹ cm⁻², respectively.     

An ultrasensitive electrochemical biosensor for glucose using CdTe-CdS core–shell quantum dot as ultrafast electron transfer relay between graphene-gold nanocomposite and gold nanoparticle

The paper reported an ultrasensitive electrochemical biosensor for glucose which was based on CdTe-CdS core–shell quantum dot as ultrafast electron transfer relay between graphene-gold nanocomposite and gold nanoparticle. Since efficient electron transfer between glucose oxidase and the electrode was achieved, the biosensor showed high sensitivity (5762.8 nA nM⁻¹ cm⁻²), low detection limit (S/N = 3) (3 × 10⁻¹² M), fast response time (0.045 s), wide calibration range (from 1 × 10⁻¹¹ M to 1 × 10⁻⁸ M) and good long-term stability (26 weeks). The apparent Michaelis–Menten constant of the glucose oxidase on the medium, 5.24 × 10⁻⁶ mM, indicates excellent bioelectrocatalytic activity of the immobilized enzyme towards glucose oxidation. Moreover, the effects of omitting graphene-gold nanocomposite, CdTe-CdS core–shell quantum dot and gold nanoparticle were also investigated. The result showed sensitivity of the biosensor is 7.67-fold better if graphene-gold nanocomposite, CdTe-CdS core–shell quantum dot and gold nanoparticle are used. This could be ascribed to improvement of the conductivity between graphene nanosheets due to introduction of gold nanoparticles, ultrafast charge transfer from CdTe-CdS core–shell quantum dot to graphene nanosheets and gold nanoparticle due to unique electrochemical properties of the CdTe-CdS core–shell quantum dot and good biocompatibility of gold nanoparticle for glucose oxidase. The biosensor is of best sensitivity in all glucose biosensors based on graphene nanomaterials up to now and has been satisfactorily applied to determination of the glucose in human saliva samples.     

Poly(phenosafranin)-functionalized single-walled carbon nanotube as nanocomposite electrocatalysts: Fabrication and electrocatalysis for NADH oxidation

We report a new type of amperometric dihydronicotinamide adenine dinucleotide (NADH) biosensor based on a poly(phenosafranin) functionalized single-walled carbon nanotube (PPS-SWCNT) nanocomposite. This nanocomposite, which was fabricated on an edge-plane pyrolytic graphite (EPPG) electrode surface, was formed by electropolymerizing a phenosafranin (PS) monomer onto SWCNTs. The PPS-SWCNT composites were characterized by surface analysis using atomic force microscopy (AFM), scanning electron microscopy (SEM) and UV–vis techniques and by electrochemical measurements using cyclic voltammetry (CV) and rotating disk electrode voltammetry (RDEV). Both the SEM and AFM studies revealed the successful immobilization of PPS on the SWCNT, whereas the UV–vis study suggested a strong charge–transfer interaction between them. The CV and RDEV results indicated that PPS-SWCNTs were able to mediate the oxidation of NADH effectively, with a remarkable decrease in overpotential by ca. 720 mV compared with the bare EPPG and with 10 times higher current than that at the PPS/EPPG electrode, (i.e., the catalytic reaction rate constant was estimated to be 3.36 × 10⁵ M⁻¹ s⁻¹). This nanocomposite electrode was found to possess excellent characteristics as an amperometric NADH sensor including a low detection potential (0.0 V vs. Ag|AgCl|KCl_(sat.)), rapid response (less than 2 s), low detection limit (10 nM), high sensitivity (576 μA cm⁻² mM⁻¹), high selectivity and reasonable stability. This new type of edge-plane-based electrochemical platform with a high surface area and catalytic activity offers great promise for creating a potentially alternative class of nanostructure electrodes for biosensing, biofuel cells and energy-conversion applications.     

Characterisation and corrosion studies of steel electrodes covered by polypyrrole/phosphotungstate using Electrochemical Impedance Spectroscopy

Polypyrrole/PW₁₂O₄₀³⁻ hybrid material was electrosynthesised on carbon steel electrodes in acetonitrile medium. The coatings obtained were characterised by Electrochemical Impedance Spectroscopy (EIS). On free-standing polypyrrole films the electrical response was mainly due to ion–ion charge transfer resistance, with a value of 175 Ω cm². A value of 2 × 10⁻⁵ S/cm was determined for the hybrid material conductivity. A charge transfer resistance about 7000 Ω cm² was obtained due to steel/oxide interface. Corrosion tests showed an important improvement in the protection against corrosion when the carbon steel electrodes were coated by these polymeric films.     

Development of highly sensitive non-enzymatic sensor for the selective determination of glucose and fabrication of a working model

Copper oxide (CuO)/copper oxalate (CuOx) modified non-enzymatic electrochemical sensor for the detection of glucose in alkaline medium was fabricated by electrochemical anodisation of copper electrodes in potassium oxalate solution. Morphology of the modified copper electrode was studied by Scanning Electron Microscopy (SEM) and its electrochemical behaviour by Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). The formation of CuOx on the copper electrode was confirmed by the Infra-red Reflection Absorption Spectrum (IRRAS). The modified electrodes were found to be microporous and rough. Linear Sweep Voltammetry (LSV) and amperometry were adopted to investigate the direct electrocatalytic oxidation of glucose on CuO/CuOx modified electrode in alkaline medium which showed excellent catalytic activity. The best performance of the sensor was obtained at 0.7 V and in 0.1 M sodium hydroxide (NaOH). At this optimum potential, the sensor was highly selective to glucose in the presence of ascorbic acid (AA) and uric acid (UA) which are common interfering species in biological fluids. The sensitivity was found to be very high (1890 μA mM⁻¹ cm⁻²) with excellent linearity (R = 0.9999) up to 15 mM having a low detection limit of 0.05 μM (S/N = 3). The modified electrode was tested for glucose level in blood serum. Based on the optimised conditions, a working model of the sensor was made and successfully tested for glucose.     

Vertically aligned carbon nanotube film electrodes for bioelectrocatalytic dioxygen reduction

We have prepared vertically aligned carbon nanotube (VACNT) film electrodes for bioelectrocatalytic dioxygen reduction. The electrodes were prepared by a CNT-transfer technique attaching the as-grown VACNTs to an ITO electrode with a conductive CNT/epoxy glue. The VACNTs greatly enhance the active electrode area as shown by the substantial current increase of the slow electroreduction of hydrogen peroxide as well as the increase of the efficiency of the oxidation and reduction of the water-insoluble redox probe t-butylferrocene. Several methods for immobilisation of the multicopper enzyme laccase, both with and without mediator, were evaluated. Very high non-mediated catalytic dioxygen reduction current was measured using VACNTs functionalised with 1-pyrenesulfonic acid. The VACNT electrodes were successfully applied as cathode in a zinc–oxygen battery, reaching a power density of 275 μW cm⁻² at 1.5 V with pyrene-functionalised VACNTs and 115 μW cm⁻² at 0.9 V with syringaldazine as a mediator in oxygen saturated buffer.     

A nano-structured Ni(II)–ACDA modified gold nanoparticle self-assembled electrode for electrocatalytic oxidation and determination of tryptophan

A nano-structured Ni (II)/ACDA (2-amino-1-cyclopentene-1-dithiocarboxylic acid) film was electrodeposited on a gold nanoparticle–cysteine–gold electrode. The formation of Ni (II)/ACDA film and electrocatalytic oxidation of tryptophan on the surface of the modified electrode were investigated with cyclic voltammetric and chronoamperometric techniques. The hydrodynamic amperometry at rotating modified electrode was used for determination of tryptophan in the range of 0.085–43.0 μmol l⁻¹. The detection limit was found to be 23 nmol l⁻¹. The rate constant, transfer coefficient for the catalytic reaction and the diffusion coefficient of tryptophan in the solution were found to be 9.1 × 10² M⁻¹ S⁻¹, 0.52 and 1.09 × 10⁻⁵ cm² s⁻¹ respectively. It is worth noting that the as formed matrix in our work possesses a 3D porous network structure with a large effective surface area and high catalytic activity and behaves like microelectrode ensembles. The modified electrode indicated reproducible behavior and a high level stability during the experiments, making it particularly suitable for the analytical purposes.     

Hybrid layered double hydroxides-polypyrrole composites for construction of glucose/O2 biofuel cell

In this work, two layered double hydroxides, Zn₂Cr–ABTS and Zn₂Al–Fe(CN)₆ LDH, have been synthesized and characterized by X ray diffraction and FTIR spectroscopy to confirm the intercalation of redox anions between inorganic layers. These redox active hybrid materials have been used to electrically connect laccase (Lac) and glucose oxidase (GOx) in biofuel cell devices. Co-immobilization of hybrid LDH and enzymes has been performed by entrapment in electropolymerized films of polypyrrole deposited on porous carbon tubular electrodes. Lac/Zn₂Cr–ABTS cathodic electrode allows the electro-enzymatic reduction of O₂, whereas the anodic electrode GOx/Zn₂Al–Fe^III(CN)₆was used for the electro-enzymatic oxidation of glucose. With a two compartment configuration, a maximum power density of 45 μW cm⁻²was obtained at 0.2 V.     

Fabrication of CuO nanoplatelets for highly sensitive enzyme-free determination of glucose

CuO nanoplatelets were grown on Cu foils by a one step, template free process. The structure and morphology of the CuO nanoplatelets were characterized by X-ray diffraction, scanning and transmission electron microscopy. The CuO nanoplatelets grown on Cu foil were integrated to be an electrode for glucose sensing. The electrocatalytic activity of the CuO nanoplatelets electrode for glucose in alkaline media was investigated by cyclic voltammetry and chronoamperometry. The electrode exhibits a sensitivity of 3490.7 μA mM⁻¹ cm⁻² to glucose which is much higher than that of most reported enzyme-free glucose sensors and the linear range was obtained over a concentration up to 0.80 mM with a detection limit of 0.50 μM (signal/noise = 3). Exhilaratingly, the electrode based on the CuO nanoplatelets is resistant against poisoning by chloride ion, and the interference from the oxidation of common interfering species, such as uric acid, ascorbic acid, dopamine and carbonhydrate compounds, can also be effectively avoided. Finally, the electrode was applied to analyze glucose concentration in human serum samples.     

A novel non-enzymatic hydrogen peroxide sensor based on single walled carbon nanotubes–manganese complex modified glassy carbon electrode

A simple procedure was developed to prepare a glassy carbon (GC) electrode modified with single wall carbon nanotubes (SWCNTs) and phenazine derivative of Mn-complex. With immersing the GC/CNTs modified electrode into Mn-complex solution for a short period of time 20–100 s, a stable thin layer of the complex was immobilized onto electrode surface. Modified electrode showed a well defined redox couples at wide pH range (1–12). The surface coverages and heterogeneous electron transfer rate constants (k_s) of immobilized Mn-complex were approximately 1.58 × 10⁻¹⁰ mole cm⁻² and 48.84 s⁻¹. The modified electrode showed excellent electrocatalytic activity toward H₂O₂ reduction. Detection limit, sensitivity, linear concentration range and k_cat for H₂O₂ were, 0.2 μM and 692 nA μM⁻¹ cm⁻², 1 μM to 1.5 mM and 7.96(±0.2) × 10³ M⁻¹ s⁻¹, respectively. Compared to other modified electrodes, this electrode has many advantageous such as remarkable catalytic activity, good reproducibility, simple preparation procedure and long term stability.     

Electroconductive coatings on porous ceramic supports

The preparation of porous conductive coatings on porous ceramic supports for potential use in electrosynthesis, anodic decomposition of organic compounds and electrosorption units is described. The prepared conductive coatings on porous ceramic supports consisted either of carbon, gold or nickel, or a combination of carbon and gold. Carbon coatings were obtained by pyrolytic decomposition of liquid petroleum gas (LPG), gold was sputter coated and nickel coatings were formed by electroless plating. The permeability for water and electrical resistance of each coated support were measured and compared. Pyrolytic carbon was deposited throughout the support whereas the nickel and gold coatings were formed on the outer surface of the support. The resistance of a carbon coating could be regulated between 0.5 and 2 cm^–1 of support while the permeability of the carbonized support was as high as 75% of the permeability of the unmodified support. The nickel and gold coatings had no significant effect on the permeability and could be prepared with a resistance of 0.25 and 1 cm^–1 of support, respectively.     

Flow-injection amperometric glucose biosensors based on graphene/Nafion hybrid electrodes

In this research, we demonstrated the fabrication of flow-injection amperometric glucose biosensors based on RGO/Nafion hybrids. The nanohybridization of the reduced graphene oxide (RGO) by Nafion provided the fast electron transfer (ET) for the sensitive amperometric biosensor platforms. The ET rate (k_s) and the charge transfer resistance (R_CT) of GOx-RGO/Nafion hybrids were evaluated to verify the accelerated ET. Moreover, hybrid biosensors revealed a quasi-reversible and surface controlled process, as confirmed by the low peak-to-peak (ΔE_p) and linear relations between I_p and scan rate (ν). Hybrid biosensors showed the fast response time of ∼3 s, the sensitivity of 3.8 μA mM⁻¹ cm⁻², the limit of detection of 170 μM, and the linear detection range of 2–20 mM for the flow-injection amperometric detection of glucose. Furthermore, interference effect of oxidizable species such as ascorbic acid (AA) and uric acid (UA) on the performance of hybrid biosensors was prevented at the operating potential of −0.20 V even under the flow injection mode. Therefore, the fast, sensitive, and stable amperometric responses of hybrid biosensors in the flow injection system make it highly suitable for automatically monitoring glucose.     

Carbon nanotube–carbon nanotube contacts as an alternative towards low resistance horizontal interconnects

We propose to use carbon nanotube (CNT)–CNT contacts instead of CNT–metal contacts as a route towards low resistance CNT horizontal interconnects. We characterize by electrical transport measurements and Kelvin probe force microscopy the contacts present in CNT horizontal interconnects grown by chemical vapor deposition between titanium nitride electrodes.When two CNTs with diameter around 30 nm grow from opposing electrodes and touch each other with their outermost shells, typical contact resistances around 16 kΩ are obtained. The corresponding contact resistivity of about 14 Ω μm² is one order of magnitude smaller than the contact resistivity of a CNT touching the TiN surface with its outermost shell. On the other hand, the contact resistance of a CNT grown from a nickel catalyst particle deposited on a TiN electrode is negligibly small when compared to the above mentioned contact resistances. Our results demonstrate that we are able to grow from opposing electrodes CNTs that touch each other with a corresponding high density (up to 10¹¹ cm⁻²) of CNT–CNT contacts. This may offer an effective solution towards the growth of low resistance CNT horizontal interconnects.     

A hydrogen peroxide sensor based on a horseradish peroxidase/polyaniline/carboxy-functionalized multiwalled carbon nanotube modified gold electrode

We have developed a polyaniline/carboxy-functionalized multiwalled carbon nanotube (PAn/MWCNTCOOH) nanocomposite by blending the emeraldine base form of polyaniline (PAn) and carboxy-functionalized multiwalled carbon nanotubes (MWCNT) in dried dimethyl sulfoxide (DMSO) at room temperature. The conductivity of the resulting PAn/MWCNTCOOH was 3.6 × 10⁻³ S cm⁻¹, mainly as a result of the protonation of the PAn with the carboxyl group and the radical cations of the MWCNT fragments. Horseradish peroxidase (HRP) was immobilized within the PAn/MWCNTCOOH nanocomposite modified Au (PAn/MWCNTCOOH/Au) electrode to form HRP/PAn/MWCNTCOOH/Au for use as a hydrogen peroxide (H₂O₂) sensor. The adsorption between the negatively charged PAn/MWCNTCOOH nanocomposite and the positively charged HRP resulted in a very good sensitivity to H₂O₂ and an increased electrochemically catalytical current during cyclic voltammetry. The HRP/PAn/MWCNTCOOH/Au electrode exhibited a broad linear response range for H₂O₂ concentrations (86 μM–10 mM). This sensor exhibited good sensitivity (194.9 μA mM⁻¹ cm⁻²), a fast response time (2.9 s), and good reproducibility and stability at an applied potential of −0.35 V. The construction of the enzymatic sensor demonstrated the potential application of PAn/MWCNTCOOH nanocomposites for the detection of H₂O₂ with high performance and excellent stability.     

A low-potential, H2O2-assisted electrodeposition of cobalt oxide/hydroxide nanostructures onto vertically-aligned multi-walled carbon nanotube arrays for glucose sensing

A novel nanocomposite was synthesized using a cathodic, low-potential, electrochemical reduction of H₂O₂ to homogeneously deposit cobalt oxide/hydroxide (denoted as CoOx·nH₂O) nanostructures onto vertically well-aligned multi-walled carbon nanotube arrays (MWCNTs), while the MWCNTs were prepared by catalytic chemical vapor deposition (CVD) on a tantalum (Ta) substrate. The CoOx·nH₂O–MWCNTs nanocomposite exhibits much higher electrocatalytic activity towards glucose (Glc) after modification with CoOx·nH₂O than before. This non-enzymatic Glc sensor has a high sensitivity (162.8 μA mM⁻¹ cm⁻²), fast response time (<4 s) and low detection limit (2.0 μM at signal/noise ratio = 3), and a linear dynamic range up to 4.5 mM. The sensor output is stable over 30 days and unaffected by common interferents that co-exist with Glc in analytical samples; it is also resistant to chloride poisoning. These features make the CoOx·nH₂O–MWCNTs nanocomposite a promising electrode material for non-enzymatic Glc sensing in routine analysis.     

Derivatization of SWCNTs with cobalt phthalocyanine residues and applications in screen printed electrodes for electrochemical detection of thiocholine

Single-walled carbon nanotubes (SWCNTs) derivatized with cobalt phthalocyanine (CoPh) were applied onto screen-printed graphite electrodes (SPEs) to be used for the low-potential electrochemical oxidation of thiocholine (TCh). Covalent attachment of CoPh to SWCNTs via stable sulfonamide bonds was confirmed by Raman/FT-IR spectroscopy and thermogravimetric analysis (TGA) coupled with FT-IR detection. The resulting modified SPE surfaces (CoPh-SWCNT-SPEs) were characterized by cyclic voltammetry and electrochemical impedance spectroscopy (EIS) with the redox probe [F₃(CN)₆]^3−/4−. Detection of TCh was accomplished using cyclic voltammetry and amperometry; a lower overpotential (100 mV vs. Ag/AgCl pseudoreference electrode) was obtained using CoPh-SWCNT-SPEs as compared to unmodified SPEs and SPEs modified with non-functionalized SWCNTs (SWCNT-SPEs). The linear range for TCh detection was 0.077–0.45 mM, with a sensitivity of 5.11 × 10⁻¹ μA mM⁻¹ and a limit of detection of 0.038 mM according to the 3 s/m definition.     

Pd/Ni/Si-microchannel-plate-based amperometric sensor for ethanol detection

A sensitive amperometric ethanol sensor composed of highly dispersed palladium nanoparticles on a vertically aligned nickel-coated silicon microchannel plate (MCP) has been constructed. The morphology of the palladium-modified nickel-coated silicon MCP electrode was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The performance of the Pd/Ni/Si MCP electrode for the electrochemical detection of ethanol was investigated by cyclic voltammetry and amperometry. The electrode with three-dimensional structure shows high-catalytic activity towards the oxidation of ethanol in a 0.10 M KOH solution. At an applied potential of −0.10 V, the Pd/Ni/Si MCP electrode presents a high sensitivity of 0.992 mA mM⁻¹ cm⁻², and the detection limit is 16.8 μM. The linear range is up to 60 mM with a linear correlation coefficient of 0.998. The Pd/Ni/Si MCP electrode also possesses excellent electrocatalytic properties, rapid response, and good stability and repeatability. This novel electrode has great potential in the accurate and effective detection of ethanol.