Direct electrochemistry and electrocatalytic activity of catalase immobilized onto electrodeposited nano-scale islands of nickel oxide

Cyclic voltammetry was used for simultaneous formation and immobilization of nickel oxide nano-scale islands and catalase on glassy carbon electrode. Electrodeposited nickel oxide may be a promising material for enzyme immobilization owing to its high biocompatibility and large surface. The catalase films assembled on nickel oxide exhibited a pair of well defined, stable and nearly reversible CV peaks at about -0.05 V vs. SCE at pH 7, characteristic of the heme Fe (III)/Fe (II) redox couple. The formal potential of catalase in nickel oxide film were linearly varied in the range 1-12 with slope of 58.426 mV/pH, indicating that the electron transfer is accompanied by single proton transportation. The electron transfer between catalase and electrode surface, (k(s)) of 3.7(+/-0.1) s(-1) was greatly facilitated in the microenvironment of nickel oxide film. The electrocatalytic reduction of hydrogen peroxide at glassy carbon electrode modified with nickel oxide nano-scale islands and catalase enzyme has been studied. The embedded catalase in NiO nanoparticles showed excellent electrocatalytic activity toward hydrogen peroxide reduction. Also the modified rotating disk electrode shows good analytical performance for amperometric determination of hydrogen peroxide. The resultant catalase/nickel oxide modified glassy carbon electrodes exhibited fast amperometric response (within 2 s) to hydrogen peroxide reduction (with a linear range from 1 microM to 1 mM), excellent stability, long term life and good reproducibility. The apparent Michaelis-Menten constant is calculated to be 0.96(+/-0.05)mM, which shows a large catalytic activity of catalase in the nickel oxide film toward hydrogen peroxide. The excellent electrochemical reversibility of redox couple, high stability, technical simplicity, lake of need for mediators and short preparations times are advantages of this electrode. Finally the activity of biosensor for nitrite reduction was also investigated.     

Composite system based on chitosan and room-temperature ionic liquid: direct electrochemistry and electrocatalysis of hemoglobin

A novel polymer/room-temperature ionic liquid (RTIL) composite material based on chitosan (Chi) and 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIM.BF(4)) was explored. The composite system can be readily used as an immobilization matrix to entrap proteins and enzymes. Hemoglobin (Hb) was chosen as a model protein to investigate the composite system. A pair of well-defined quasireversible redox peaks of hemoglobin were obtained at the Chi-BMIM.BF(4)-Hb composite-film-modified glassy carbon (GC) electrode by direct electron transfer between the protein and the GC electrode. Dramatically enhanced biocatalytic activity was exemplified at the Chi-BMIM.BF(4)-Hb/GC electrode by the reduction of oxygen and trichloroacetic acid. Thermogravimetric analysis (TGA) suggests that the Chi-BMIM.BF(4)-Hb composite has higher thermal stability than Chi-Hb itself. The Chi-BMIM.BF(4)-Hb film was also characterized by UV-visible spectra, indicating excellent stability in solution and good biocompatibility for protein. The unique composite material based on polymer and ionic liquid can find wide potential applications in direct electrochemistry, biosensors, and biocatalysis.     

Glucose biosensor prepared by glucose oxidase encapsulated sol-gel and carbon-nanotube-modified basal plane pyrolytic graphite electrode

A new glucose biosensor has been fabricated by immobilizing glucose oxidase into a sol-gel composite at the surface of a basal plane pyrolytic graphite (bppg) electrode modified with multiwall carbon nanotube. First, the bppg electrode is subjected to abrasive immobilization of carbon nanotubes by gently rubbing the electrode surface on a filter paper supporting the carbon nanotubes. Second, the electrode surface is covered with a thin film of a sol-gel composite containing encapsulated glucose oxidase. The carbon nanotubes offer excellent electrocatalytic activity toward reduction and oxidation of hydrogen peroxide liberated in the enzymatic reaction between glucose oxidase and glucose, enabling sensitive determination of glucose. The amperometric detection of glucose is carried out at 0.3 V (vs saturated calomel electrode) in 0.05 M phosphate buffer solution (pH 7.4) with linear response range of 0.2-20 mM glucose, sensitivity of 196 nA/mM, and detection limit of 50 microM (S/N=3). The response time of the electrode is < 5s when it is stored dried at 4 degrees C, the sensor showed almost no change in the analytical performance after operation for 3 weeks. The present carbon nanotube sol-gel biocomposite glucose oxidase sensor showed excellent properties for the sensitive determination of glucose with good reproducibility, remarkable stability, and rapid response and in comparison to bulk modified composite biosensors the amounts of enzyme and carbon nanotube needed for electrode fabrication are dramatically decreased.     

Overoxidized polypyrrole film directed single-walled carbon nanotubes immobilization on glassy carbon electrode and its sensing applications

In this paper, the films of overoxidized polypyrrole (PPyox) directed single-walled carbon nanotubes (SWNTs) have been electrochemically coated onto glassy carbon electrode (GCE). Electroactive monomer pyrrole was added into the solution containing sodium dodecyl sulfate (SDS) and SWNTs. Then, electropolymerization was proceeded at the surface of GCE, and a novel kind of conducting polymer/carbon nanotubes (CNTs) composite film with the orientation of CNTs were obtained correspondingly. Finally, this obtained polypyrrole (PPy)/SWNTs film modified GCE was oxidized at a potential of +1.8 V. It can be found that this proposed PPyox/SWNTs composite film modified GCE exhibited excellent electrocatalytic properties for some species such as nitrite, ascorbic acid (AA), dopamine (DA) and uric acid (UA), and could be used as a new sensor for practical applications. Compared with previous CNTs modified electrodes, SWNTs were oriented towards the outside of modified layer by PPyox and SDS, which made the film easily conductive. Moreover, this proposed film modified electrode was more stable, selective and applicable.     

Glucose Biosensor Based on a Glassy Carbon Electrode Modified with Polythionine and Multiwalled Carbon Nanotubes

A novel glucose biosensor was fabricated. The first layer of the biosensor was polythionine, which was formed by the electrochemical polymerisation of the thionine monomer on a glassy carbon electrode. The remaining layers were coated with chitosan-MWCNTs, GOx, and the chitosan-PTFE film in sequence. The MWCNTs embedded in FAD were like “conductive wires” connecting FAD with electrode, reduced the distance between them and were propitious to fast direct electron transfer. Combining with good electrical conductivity of PTH and MWCNTs, the current response was enlarged. The sensor was a parallel multi-component reaction system (PMRS) and excellent electrocatalytic performance for glucose could be obtained without a mediator. The glucose sensor had a working voltage of −0.42 V, an optimum working temperature of 25°C, an optimum working pH of 7.0, and the best percentage of polytetrafluoroethylene emulsion (PTFE) in the outer composite film was 2%. Under the optimised conditions, the biosensor displayed a high sensitivity of 2.80 µA mM⁻¹ cm⁻² and a low detection limit of 5 µM (S/N = 3), with a response time of less than 15 s and a linear range of 0.04 mM to 2.5 mM. Furthermore, the fabricated biosensor had a good selectivity, reproducibility, and long-term stability, indicating that the novel CTS+PTFE/GOx/MWCNTs/PTH composite is a promising material for immobilization of biomolecules and fabrication of third generation biosensors.     

Nonenzymatic glucose sensor based on over-oxidized polypyrrole modified Pd/Si microchannel plate electrode

A silicon microchannel plate (MCP) array electrode modified with over-oxidized polypyrrole (OPPy) has been fabricated to detect glucose. The morphology and structure of the electrode are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The OPPy modified silicon MCP array electrode exhibits high electrocatalytic activity as well as excellent selectivity to the electrochemical oxidation of glucose. At a potential of +0.08 V, good sensitivity of 0.37 mA mM(-1) cm(-2) and detection limit of 2.06 μM are attained. The linear range is up to 24 mM with a linear correlation coefficient of 0.997. Furthermore, the electrode is highly resistant to interfering substances because the effects of common coexisting substances can be effectively eliminated by the OPPy film and the response in the current to interferences on the electrode surface is almost negligible. This novel electrode has high potential in nonenzymatic detection of glucose.     

Molecularly imprinted polyaniline film for ascorbic acid detection

Molecularly imprinted polyaniline (PANI) film (～ 100 nm thick) has been electrochemically fabricated onto indium-tin-oxide (ITO) coated glass plate using ascorbic acid (AA) as template molecule. Fourier transform infra-red spectroscopy, scanning electron microscopy, cyclic voltammetry and differential pulse voltammetry (DPV) studies indicate the presence of AA in PANI matrix, which also acts as a dopant for PANI. Further, the AA selective molecularly imprinted PANI electrode (AA-MI-PANI/ITO) has been developed via over-oxidation of AA doped PANI electrode which leads to the removal of AA moieties from PANI film. The response studies using DPV technique have revealed that this molecularly imprinted AA-MI-PANI/ITO electrode can detect AA in the range of 0.05-0.4 mM with detection limit of 0.018 mM and sensitivity of 1.2 × 10(-5) AmM(-1). Interestingly, this AA-MI-PANI/ITO electrode shows excellent reusability, selectivity and stability.     

Hybrid material based on chitosan and layered double hydroxides: characterization and application to the design of amperometric phenol biosensor

A new type of amperometric phenol biosensor based on chitosan/layered double hydroxides organic-inorganic composite film was described. This hybrid material combined the advantages of organic biopolymer, chitosan, and inorganic layered double hydroxides. Polyphenol oxidase (PPO) immobilized in the material maintained its activity well as the usage of glutaraldehyde was avoided. The composite films have been characterized by Fourier transform infrared. The results indicated that PPO retained the essential feature of its native structure in the composite film. The enzyme electrode provided a linear response to catechol over a concentration range of 3.6 x 10(-9) to 4 x 10(-5) M with a sensitivity of 2750 +/- 52 mA M(-1) cm(-2) and a detection limit of 0.36 nM based on S/N = 3. The apparent Michaelis-Menten constant K(app)(M) for the sensor was found to be 0.13 mM. The activation energy for enzymatic reaction was calculated to be 27.6 kJ mol(-1). Furthermore, the biosensor exhibited excellent long-term stability and satisfactory reproducibility.     

Differentiating the orientations of photosynthetic reaction centers on Au electrodes linked by different bifunctional reagents

The photosynthetic reaction center (RC) composite film was fabricated by self-assembled monolayers (SAMs) on the Au electrode with two different bifunctional reagents, 4-aminothiophenol (ATP) and 2-mercaptoethylamine (MEA), respectively. The square wave voltametry (SWV), bulk electrolysis and photocurrent test were employed for characterizing the composite film. The dramatic different electrochemical characteristics were observed for the two types of films, which strongly suggested an orientational difference for RC arising from the structural difference between the two bifunctional reagents. For RC-MEA film, three redox peaks which implying electron transfer (ET) between the primary donor (P) and the bacteriopheophytin (Bphe) were observed. While for RC-ATP film, two redox peaks implying ET between the nonheme iron and the primary quinone (Q(A)) were observed. The ET behavior driven by electric field also supported the result that the RC could be linked to the electrode at different sites. The site-specific immobilization approach reported here supplies a method to differentiate the protein orientation.     

Amperometric determination of sodium nitrite by a microbial sensor

Summary A microbial sensor was prepared to determine sodium nitrite. This microbial sensor consisted of immobilized Nitrobacter sp. and an oxygen electrode. When a sample solution containing sodium nitrite was tested, nitrite was changed to NO₂ gas in the buffer (pH 2.0) and the current of the electrode decreased with time until a steady state was reached. The steady state current was attained within 10 min and the maximum decrease in current was obtained at 30°C and pH 2.0. A linear relationship was observed between the current decrease and the sodium nitrite concentration below 0.59 mM, the minimum sodium nitrite concentration that could be determined was 0.01 mM. The current decrease was reproducible (5% relative error). The current output of the sensor was almost constant for more than 21 days and 400 assays.     

Immobilization of glucose oxidase on electrodeposited nickel oxide nanoparticles: direct electron transfer and electrocatalytic activity

For the first time glucose oxidase (GOx) was successfully co-deposited on nickel-oxide (NiO) nanoparticles at a glassy carbon electrode. In this paper we present a simple fabrication method of biosensor which can be easily operated without using any specific reagents. Cyclic voltammetry was used for electrodeposition of NiO nanoparticle and GOx immobilization. The direct electron transfer of immobilized GOx displays a pair of well defined and nearly reversible redox peaks with a formal potential (E(0')) of -0.420 V in pH 7 phosphate buffer solution and the response shows a surface controlled electrode process. The surface coverage and heterogeneous electron transfer rate constant (k(s)) of GOx immobilized on NiO film glassy carbon electrode are 9.45 x 10(-13)mol cm(-2) and 25.2+/-0.5s(-1), indicating the high enzyme loading ability of the NiO nanoparticles and great facilitation of the electron transfer between GOx and NiO nanoparticles. The biosensor shows excellent electrocatalytical response to the oxidation of glucose when ferrocenmethanol was used as an artificial redox mediator. Furthermore, the apparent Michaelis-Menten constant 2.7 mM, of GOx on the nickel oxide nanoparticles exhibits excellent bioelectrocatalytic activity of immobilized enzyme toward glucose oxidation. In addition, this glucose biosensor shows fast amperometric response (3s) with the sensitivity of 446.2nA/mM, detection limit of 24 microM and wide concentration range of 30 microM to 5mM. This biosensor also exhibits good stability, reproducibility and long life time.     

Direct electrochemistry of horseradish peroxidase bonded on a conducting polymer modified glassy carbon electrode

Direct electron transfer process of immobilized horseradish peroxidase (HRP) on a conducting polymer film, and its application as a biosensor for H2O2, were investigated by using electrochemical methods. The HRP was immobilized by covalent bonding between amino group of the HRP and carboxylic acid group of 5,2':5',2"-terthiophene-3'-carboxylic acid polymer (TCAP) which is present on a glassy carbon (GC). A pair of redox peaks attributed to the direct redox process of HRP immobilized on the biosensor electrode were observed at the HRPmid R:TCAPmid R:GC electrode in a 10 mM phosphate buffer solution (pH 7.4). The surface coverage of the HRP immobilized on TCAPmid R:GC was about 1.2 x 10(-12) mol cm(-2) and the electron transfer rate (ks) was determined to be 1.03 s(-1). The HRPmid R:TCAPmid R:GC electrode acted as a sensor and displayed an excellent specific electrocatalytic response to the reduction of H2O2 without the aid of an electron transfer mediator. The calibration range of H2O2 was determined from 0.3-1.5 mM with a good linear relation.     

A novel hydrogen peroxide biosensor based on the immobilization of hemoglobin on three-dimensionally ordered macroporous (3DOM) gold-nanoparticle-doped titanium dioxide (GTD) film

The three-dimensionally ordered macroporous gold-nanoparticle-doped titanium dioxide (3DOM GTD) film was modified on the indium-tin oxide (ITO) electrode surface. Hemoglobin (Hb) has been successfully immobilized on the 3DOM GTD film and the fabrication process was characterized by Raman and UV-vis spectra. The results indicated that the Hb immobilized on the film retained its biological activity and the secondary structure of Hb was not destroyed. The direct electrochemistry and electrocatalysis of Hb immobilized on this film have been investigated. The Hb/3DOM GTD/ITO electrode exhibited two couples of redox peaks corresponding to the Hb intercalated in the mesopores and adsorbed on the external surface of the film with the formal potential of -0.20 and -0.48 V in 0.1M PBS (pH7.0), respectively. The Hb/3DOM GTD/ITO electrode exhibits an excellent eletrocatalytic activity, a wide linear range for H(2)O(2) from 5.0 μM to 1.0mM with a limit of detection of 0.6μM, high sensitivity (144.5 μA mM(-1)), good stability and reproducibility. Compared with the TiO(2) nanoneedles modified electrode, the GTD modified electrode has higher sensitivity and response peak current. The 3DOM GTD provided a good matrix for bioactive molecules immobilization, suggesting it has the potential use in the fields of H(2)O(2) biosensors.     

A novel nitrite biosensor based on the direct electron transfer of hemoglobin immobilized on CdS hollow nanospheres

A novel nitrite biosensor based on the direct electron transfer of hemoglobin (Hb) immobilized on CdS hollow nanospheres (HS-CdS) modified glassy carbon electrode was constructed. The direct electron transfer of Hb showed a pair of redox peaks with a formal potential of -286 mV (vs. SCE) in 0.1M pH 7.0 phosphate buffer solution. It was a surface-controlled electrode process involving a single proton transfer coupled with a reversible one-electron transfer for each heme group of Hb. HS-CdS had a large specific surface area and good biocompatibility and had a better electrochemical response than that of solid spherical CdS. The immobilized Hb on HS-CdS displayed an excellent response to NO(2)(-) with one irreversible electrode process for NO reduction. Under optimal conditions, the biosensor could be used for the determination of NO(2)(-) with a linear range from 0.3 to 182 microM and a detection limit of 0.08 microM at 3 sigma based on the irreversible reduction of NO. HS-CdS provided a good matrix for protein immobilization and had a promising application in constructing sensors.     

Direct electrochemistry of myoglobin based on ionic liquid-clay composite films

The direct electron transfer of myoglobin (Mb) was realized by immobilizing Mb onto ionic liquid (1-butyl-3-methyl imidazolium tetrafluoraborate, [bmim][BF(4)])-clay composite film modified glassy carbon electrode. A pair of well-defined redox peaks of Mb with a formal potential (E(o)') of -0.297 V (vs. Ag/AgCl) was observed in 0.1M phosphate buffer solution (pH 6.0). The ionic liquid-clay composite film showed good biocompatibility and an obvious promotion capability for the direct electron transfer between Mb and electrode. The electron transfer rate constant (k(s)) of Mb was calculated to be (3.58+/-0.12)s(-1). UV-vis spectrum suggested that Mb retained its native conformation in the ionic liquid-clay system. Basal plane spacing of clay obtained by X-ray diffraction (XRD) indicated that there was an intercalation-exfoliation-restacking process, in ionic liquid and clay during the drying process of the modification, and the ionic liquid played the key role for promotion of the direct electron transfer between Mb and the ionic liquid-clay composite film modified electrode. The biocatalytic activity of Mb in the composite film was exemplified by the reduction of hydrogen peroxide. Under the optimal conditions, the reduction peak currents of Mb increased linearly with the concentration of H(2)O(2) in the range of 3.90 x 10(-6) to 2.59 x 10(-4)M, with a detection limit of 7.33 x 10(-7)M. The kinetic parameter I(max) and the apparent Michaelis constant (K(m)) for the electrocatalytic reactions were 3.87 x 10(-8)A and 17.6 microM, respectively. The proposed method would be valuable for the construction of a new third-generation H(2)O(2) sensor.     

Direct electron transfer of hemoglobin in layered alpha-zirconium phosphate with a high thermal stability

A heme protein hemoglobin (Hb) was reacted with preexfoliated layered alpha-zirconium phosphate (alpha-ZrP) platelets. An X-ray diffraction (XRD) pattern of small range showed that the exfoliated alpha-ZrP platelets reassembled after the addition of Hb molecules, with the protein intercalated between the layers. UV-Vis and Fourier transform infrared (FTIR) spectra analysis displayed that no significant denaturation occurred to the intercalated protein. The bioactivity of Hb was also investigated by testing the electrochemical properties of the Hb/alpha-ZrP composite. Results showed that the intercalation of Hb into the layered material not only improved the thermal stability of Hb but also enhanced the direct electron transfer ability between protein molecules and electrode. The protein still showed bioactivity after treatment at a temperature as high as 85 degrees C. A pair of well-defined redox peaks at approximately -0.37 and -0.32V was observed on the cyclic voltammograms (CVs) of the Hb/alpha-ZrP composite modified electrode, and the electrode reactions showed a surface-controlled process with a single proton transfer. The resultant biosensor constructed by the Hb/alpha-ZrP composite displayed an excellent response to the reduction of hydrogen peroxide (H(2)O(2)) with good reproducibility.     

Direct electrochemistry of glucose oxidase and biosensing for glucose based on boron-doped carbon nanotubes modified electrode

Due to their unique physicochemical properties, doped carbon nanotubes are now extremely attractive and important nanomaterials in bioanalytical applications. In this work, selecting glucose oxidase (GOD) as a model enzyme, we investigated the direct electrochemistry of GOD based on the B-doped carbon nanotubes/glassy carbon (BCNTs/GC) electrode with cyclic voltammetry. A pair of well-defined, quasi-reversible redox peaks of the immobilized GOD was observed at the BCNTs based enzyme electrode in 0.1M phosphate buffer solution (pH 6.98) by direct electron transfer between the protein and the electrode. As a new platform in glucose analysis, the new glucose biosensor based on the BCNTs/GC electrode has a sensitivity of 111.57 microA mM(-1)cm(-2), a linear range from 0.05 to 0.3mM and a detection limit of 0.01mM (S/N=3). Furthermore, the BCNTs modified electrode exhibits good stability and excellent anti-interferent ability to the commonly co-existed uric acid and ascorbic acid. These indicate that boron-doped carbon nanotubes are the good candidate material for the direct electrochemistry of the redox-active enzyme and the construction of the related enzyme biosensors.     

Amperometric third-generation hydrogen peroxide biosensor based on the immobilization of hemoglobin on multiwall carbon nanotubes and gold colloidal nanoparticles

A convenient and effective strategy for preparation nanohybrid film of multi-wall carbon nanotubes (MWNT) and gold colloidal nanoparticles (GNPs) by using proteins as linker is proposed. In such a strategy, hemoglobin (Hb) was selected as model protein to fabricate third-generation H2O2 biosensor based on MWNT and GNPs. Acid-pretreated, negatively charged MWNT was first modified on the surface of glassy carbon (GC) electrode, then, positively charged Hb was adsorbed onto MWNT films by electrostatic interaction. The {Hb/GNPs}n multilayer films were finally assembled onto Hb/MWNT film through layer-by-layer assembly technique. The assembly of Hb and GNPs was characterized with cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and transmission electron microscopy (TEM). The direct electron transfer of Hb is observed on Hb/GNPs/Hb/MWNT/GC electrode, which exhibits excellent electrocatalytic activity for the reduction of H2O2 to construct a third-generation mediator-free H2O2 biosensor. As compared to those H2O2 biosensors only based on carbon nanotubes, the proposed biosensor modified with MWNT and GNPs displays a broader linear range and a lower detection limit for H2O2 determination. The linear range is from 2.1x10(-7) to 3.0x10(-3) M with a detection limit of 8.0x10(-8) M at 3sigma. The Michaelies-Menten constant KMapp value is estimated to be 0.26 mM. Moreover, this biosensor displays rapid response to H2O2 and possesses good stability and reproducibility.     

Amperometric biosensor for hydrogen peroxide based on ferrocene-bovine serum albumin and multiwall carbon nanotube modified ormosil composite

A novel amperometric biosensor for hydrogen peroxide (H(2)O(2)) was developed by entrapping horseradish peroxidase (HRP) in a new ormosil composite doped with ferrocene monocarboxylic acid-bovine serum albumin conjugate and multiwall carbon nanotubes (MWNTs). The ormosil was prepared using 3-(aminopropyl)triethoxysilane and 2-(3,4 epoxycyclohexyl)-ethyltrimethoxy silane as monomers. The encapsulated conjugate showed excellent electrochemistry and acted as an electron transfer mediator. The presence of MWNTs improved the conductivity of the composite film. This matrix showed a biocompatible microenvironment for retaining the native activity of the entrapped HRP and a very low mass transport barrier to the substrate, which provided a fast amperometric response to H(2)O(2). The proposed H(2)O(2) biosensor exhibited a linear range of 0.02-4.0 mM with a detection limit of 5.0 microM (S/N = 3) and a K(M)(app) value of 2.0 mM. It could be used for flow injection analysis of hydrogen peroxide with a liner range from 0.02 to 4.5 mM, sensitivity of 0.042 microA/mM and analytical time of 20 s per sample. This biosensor possessed good analytical performance and storage stability.     

Amperometric detection of morphine at a Prussian blue-modified indium tin oxide electrode

In this work, the electrocatalytic oxidation of morphine (MO) at an optically transparent indium tin oxide (ITO) electrode modified by an electrodeposited Prussian blue (PB) thin film is first demonstrated, and the amperometric detection of MO was then investigated. Experimental results showed that the thin film on the ITO surface, confined to the PB/Berlin green (BG) redox pair, can serve as an excellent mediator which facilitates electron transfer and considerably lowers the overpotential required, as compared to a bare ITO electrode. Thus, PB can be regarded as a promising artificial peroxidase for MO. The rate of such an electrocatalytic reaction is pH dependent with the highest value at pH 5. By potential-step excitation from 0.55 to 0.70 V, a linear calibration curve, displaying the relationship between steady-state currents and MO concentrations (ranging from 0.09 to 1.0 mM), was obtained. The detection sensitivity is about 16.8 microA/cm2 mM. Most importantly, the method described herein can readily discriminate MO analogs lacking the phenolic -OH group, such as codeine, and can thus benefit the specific recognition of MO.