Wide Linear Calibration Range Research Articles

An ultra-selective and non-enzymatic colorimetric sensor based on imprinted polymer for ephedrine assay in urine samples

A novel non-enzymatic, rapid, and portable molecularly imprinted polymer (MIP)-based colorimetric sensor was developed for ephedrine (EPD) assay in urine samples. The EPD-imprinted polymer was provided via the precipitation polymerization method and applied to fabricate an EPD colorimetric sensor utilizing the carbon disulfide and Cu2+ reagents. The calibration curve for different concentrations of EPD was obtained using the captured images on a cell smartphone and associated RGB data. Some effective parameters including pH of the EPD extraction, time of data collection after adding the reagents, and reagents sequence were investigated and optimized. The selectivity of the sensor was studied using different similar structural compounds, which proved the favorable capability of this method for the selective determination of EPD. This sensor platform with a relatively wide linear calibration range (1-100μM), and the detection limit of 0.6μM represents the efficient, reliable, and easy approach for EPD assay in urine samples.

Superhydrophobic eggshell for fabrication of hydrophobic barrier of paper-based analytical device for colorimetric determination of ammonium ion in water

This work demonstrates the first application of the superhydrophobic eggshell as an environmental-friendly material for fabricating the hydrophobic barrier of a paper-based analytical device (PAD). The eggshell surface was modified by mixing the pulverized chicken eggshell biowaste with stearic acid and a polystyrene binder to accomplish superhydrophobicity. The PAD (10 × 10 mm2) is comprised of the circular-shaped hydrophilic reservoir (Ø 5 mm). The liquid barrier was fabricated by painting both the topside and the underside of the filter paper with the dispersed superhydrophobic eggshell solution. The SEM images revealed that the microsized superhydrophobic eggshell particles absorbed onto the porous surface of the cellulose fibril. A coated surface with a water contact angle of 155.39° ± 1.02° (n = 10) on the paper substrate was achieved, and this indicated superhydrophobicity. The eggshell barrier enabled excellent water and chemical resistance. The fabricated PAD was applied for the colorimetric determination of ammonium cations (NH4+), based on the modified Berthelot reaction. Samples and chromogenic solutions were aliquoted onto the hydrophilic reservoir, with subsequent capture of the optical image of the stable, green-colored product by a smart phone under a light-controlled studio. The green color intensities were evaluated by ImageJ™ and plotted against the standard NH4+ concentrations. A wide linear calibration range 5.0 to 100 mg N/L was achieved with good linearity (r2 > 0.99). The PAD confirmed satisfied analytical recovery (Mean ± SD: 100.6 % ± 0.97, n = 11 samples) and high precision (RSD = 0.85 %: 10-PAD replicate measurements of 1.0 mg N/L). The limit of detection (3 SD of blank/slope) of 1.05 mg N/L was found sensitive enough to monitor the NH4+ contents in freshwater. The results, determined by the developed PAD and ion chromatography, showed no significant difference under the statistical paired t-test at 95 % confidence (tstat = -1.80, tcri = 2.57, n = 6 samples).

Improved LC/MS/MS Quantification Using Dual Deuterated Isomers as the Surrogates: A Case Analysis of Enrofloxacin Residue in Aquatic Products.

Extensive and high residue variations in enrofloxacin (ENR) exist in different aquatic products. A novel quantitative method for measuring ENR using high-performance liquid chromatography-tandem mass spectrometry was developed employing enrofloxacin-d5 (ENR-d5) and enrofloxacin-d3 (ENR-d3) as isotope surrogates. This reduced the deviation of detected values, which results from the overpass of the linear range and/or the large difference in the residue between the isotope standard and ENR, from the actual content. Furthermore, high residue levels of ENR can be directly diluted and re-calibrated by the corresponding curve with the addition of high levels of another internal surrogate without repeated sample preparation, avoiding the overflow of the instrument response. The validation results demonstrated that the method can simultaneously determine ENR residues from MQL (2 µg/kg) to 5000 × MQL (method quantification limit) with recoveries between 97.1 and 106%, and intra-precision of no more than 2.14%. This method realized a wide linear calibration range with dual deuterated isomers, which has not been previously reported in the literature. The developed method was successfully applied to the analysis of ENR in different aquatic products, with ENR residue levels varying from 108 to 4340 μg/kg and an interval of precision in the range of 0.175~6.72%. These results demonstrate that batch samples with a high variation in ENR residues (over the linear range with a single isotope standard) can be detected by the dual isotope surrogates method in a single sample preparation process.

An electrochemical biosensor for the rapid genetic identification of Musang King durian

More than 200 different cultivars of durian exist worldwide but Durio zibethinus or Musang King (MK) is the most premium and prized durian fruit among the recommended varieties. Early identification of this premium variety is critical to protect from non-authentic MK durian cultivars. However, the MK variety's morphological traits are nearly identical to other varieties. Currently, the identification of durian varieties is mostly performed via evaluation of leaf shape, fruit shape, aroma, taste and seed shape and this requires trained personnel for the morphology observation. To enable the rapid identification of the MK variety, PCR amplification of ten durian varieties using six gene candidates from the chloroplast genome was first performed to obtain DNA probes that were specific to the MK durian variety. PCR amplification of ten durian varieties using primers designed confirmed that the nadhA gene sequence showed an obvious difference in the MK variety from other durian varieties. The unique sequence of MK was used as a DNA probe to develop an electrochemical biosensor for the direct identification of the MK durian variety. The electrochemical biosensor was based on the hybridization response of the immobilized DNA probe with the target DNA from the MK variety and was monitored via differential pulse voltammetry technique. Under optimal conditions, the DNA electrochemical biosensor showed a low detection limit at 10% of MK genomic DNA concentration with a wide linear calibration range of 0.05–1.5 µM (R2 = 0.9891) and RSD value of 3.77% (n = 3). The results of the developed DNA biosensor provide high promise for the development of portable sensors employed in the determination of MK variety in the field.

Fluorescent and electrochemical bimodal bioplatform for femtomolar detection of microRNAs in blood sera

Noncoding RNAs such as miRNAs (miRs) are extensively researched as a novel class of cancer biomarkers. Herein, we describe a new method to sensitively determine the levels miRNAs via dual signal readout involving competitive hybridization between the miR-21 target and its biotinylated analog towards the same thiolated DNA probe attached onto the surface of gold nanoparticles. Hybridization of the DNA probes by the biotinylated miRs followed by conjugation with streptavidin-horseradish peroxidase, which catalyzes the oxidation of o-phenylenediamine into 2,3-diaminophenazine, allows detecting the target using fluorescence and electrochemistry. The two signals varied in a miRNA concentration-dependent manner. The bioplatform has limits of detection of 15 fmol/L and 19 fmol/L (0.15 and 0.19 attomol in 20 μL, respectively). Furthermore, it displays a wide linear calibration range varying from 19 fmol/L to 100 pmol/L and 15 fmol/L to 100 pmol/L using voltamperometry and fluorescence, respectively. The bioplatform is able to provide very low detection limits without any DNA/RNA amplification step, an excellent selectivity toward non-complementary miRs along with an acceptable distinction with a single nucleotide mismatched sequence. Finally, the method was used to determine miR-21 expression levels in blood sera from patients diagnosed with breast cancer.

Amperometric assay of hydrazine utilizing electro-deposited cobalt hexacyanoferrate nanocrystals on graphene oxide sheets

In-situ electrochemical deposition of cobalt hexacyanoferrate (CoHCF) on graphene oxide (GO) and its application for the electrocatalytic hydrazine determination in real samples are described in this research study. Co2+ is immobilized on GO and the resulting material, GO-Co2+ is coated on the surface of glassy carbon (GC) electrode. The fabricated electrode (GC/GO-Co2+) is subjected to a continuous potential cycling in the range of 0.0–1.0 V which results in the formation of a thin CoHCF film on the surface of GO coated on the GC electrode (abbreviated as GC/GO-CoHCF). The synthesized GO-CoHCF composite material is characterized by Fourier transform infrared and scanning electron microscopy. GC/GO-CoHCF electrode electrocatalytically oxidizes hydrazine at low overpotential (0.63 V) and this phenomenon is subsequently utilized for the sensitive determination of hydrazine in aqueous solutions. It exhibits a wide linear calibration range (0.1–400 µM), high sensitivity (0.93 µA µM−1 cm−2) and low limit of detection (17.5 nM) for the determination of hydrazine. Further, this electrode is employed for hydrazine determination in real samples.

Assessment and Measurement of Atmospheric Pollution (Particulate Matter) Around Zawiya City-Libya

Total air suspended particulates (TSP) samples were collected over glass fiber filter and determined gravimetrically. The contents on the glass filter was digested in HF-HCl-HNO3 mixture and the trace elemental analysis of Al, Fe, Mn, and Pb were performed using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) technique, which offers many advantages such as low detection limits, wide linear calibration range and no matrix interference in addition to simultaneous multi-element determination. The filter with TSP was weighted, dissolved in HF-HCl-HNO3 acid mixture and the trace element such as Al, Fe, Mn and Pb were determined using (ICP-OES) technique. This work was carried out at four sites around Zawia city. Three sampling location were selected on sites of heavy traffics and industrial activities. These sites were (A1) at the center of Zawia city; (A2) near the power station and the refinery; (A3) nearby the main coastal road and (A4 control) at zone where there are no industrial activities or any obvious source of pollution. These samples were collected during Summer, Autumn, Spring, and Winter. The daily variation of TSP during this period showed the lowest value 70 µg/m3 in January (Winter), and the highest value 587.121 µg/m3. This value is relatively high compared with EU limit values (300 µg/m3), USEPA (150 µg/m3), and WHO (150-230 µg/ m3). The mean monthly variation of TSP showed the lowest measured result of 78.28 ± 0.08 µg/m3 in January and the highest value 306 µg/m3 in May. Seasonal variation of TSP showed high results during Spring season which is 239.44 µg/m3 and all other seasons showed lower values of 131.19 µg/m3, 198.28 µg/m3 and 159.85 µg/m3 for Winter, Autumn, and Summer, respectively compared with Spring season. The trace elements levels in TSP were Al>Fe>Pb>Mn. The concentration of Al, Fe, Mn and Pb are lower in site A4 and Al and higher in sites A3 and A2 due to heavy traffic and industrial activities. The meteorological parameters over the study area showed some factors such as wind speed, humidity and temperature that may affect TSP concentrations.

Low-Cost Paper Electrode Fabricated by Direct Writing with Silver Nanoparticle-Based Ink for Detection of Hydrogen Peroxide in Wastewater.

A simple, low cost and user-friendly method for the fabrication of paper electrode (PE) using silver nanoparticles capped with octylamine (AgNPs-OA) is reported for detection of hydrogen peroxide (H2O2) in wastewater samples. The PE was prepared by direct writing onto the photo paper using a ball-point pen filled with nanoink (10 wt % of AgNPs-OA in chloroform). The prepared electrode was sintered at 100 °C for 1 h to make it conductive. The PE/AgNPs-OA was used as a working electrode in cyclic voltammetry (CV) for the detection of H2O2. The PE/AgNPs-OA exhibited a wide linear calibration range from 1.7 μM to 30 mM for the determination of H2O2 with a low limit of detection, 0.5 μM. The good recovery percentage (95.2-96.2%) and interference study for determination of H2O2 in wastewater samples demonstrated the selectivity of the method from the complex sample matrices. The PE/AgNPs-OA electrode is found to be economic, facile and user-friendly for multiple analyses (n = 60) of H2O2 in CV compared to other commercially available electrodes and custom-made modified electrodes.

Cadmium oxide based efficient electrocatalyst for hydrogen peroxide sensing and water oxidation

In this study, plant-mediated synthesis of cadmium oxide nanoparticles (CdO NPs) have been reported using hydrazine as reducing agent and aqueous extract of Convolvulus arvensis as capping ligand. The synthesized nanomaterial was characterized by using different techniques like UV–visible, FT-IR, FESEM, XRD, EDS and TGA. FESEM image showed the average crystallite size to be 23 ± 2 nm. TGA was carried out for thermal stability which showed that the synthesized nanoparticles were stable up to 365 °C. The electro-catalytic properties of synthesized nanomaterials were investigated for the oxidation of H2O2 and H2O. The response of the physically modified electrode was examined using cyclic voltammetry and DC potential amperometry (DCPA vs. Ag/AgCl reference electrode) in a phosphate buffer (pH = 7.0). The designed H2O2 biosensor illustrated high sensitivity 4.9757 μA mM−1 cm−2, wide linear calibration range from 1 to 30 mM and detection limit of 1.6172 mM. The nanoparticles also displayed high electrocatalytic properties for the oxidation of water using linear sweep voltammetry.

Polyelectrolyte-free layer by layer self-assembled multilayer films of cationic phthalocyanine cobalt(II) and carbon nanotube for the efficient detection of 4-nitrophenol

4-Nitrophenol (4-NP) sensors with high sensitivity and reproducibility are desirable for biosafety and environmental monitoring. Herein, cationic tetra-β-(N,N,N-trimethylaminoethoxy) phthalocyanine cobalt(II)/acid-treated carbon nanotubes multilayer films, marked as (tmaePcCo/CNT)n films (n represents the bilayer number), were successfully constructed on glassy carbon electrode (GCE) by a facile, effective layer by layer (LBL) electrostatic assembly method without the assistant of polyelectrolyte. Because CNTs within the multilayer films are beneficial to collect and transfer charges, and polyelectrolyte-free assembly ensures active sites exposing, the obtained (tmaePcCo/CNT)10/GCE exhibits excellent 4-NP electrocatalytic performance with good reproducibility and acceptable stability, a wide linear calibration range over 3 orders of magnitude of 4-NP concentrations (1μM–1.8mM) and a low detection limit of 0.2μM (S/N=3), and it can be applied to the quantification analysis of 4-NP under optimum conditions. This work shows that the LBL multilayer films fabricated without polyelectrolyte can provide a valid way to improve the sensitivity and reproducibility of 4-NP sensors.

Synthesis and characterization of gold nanoparticles incorporated bentonite clay for electrocatalytic sensing of arsenic(III)

In the present manuscript, a simple and easy route to synthesize bentonite (bt) clay-supported gold nanoparticles (Au NPs) is reported (represented as Au-bt). Application of this new environmentally benign material in electrocatalytic determination of arsenite (As(III)) was studied. The successful synthesis and incorporation of Au NPs into the bt clay is supported by spectroscopic, microscopic and electrochemical methods. The synthesized Au-bt material was used to modify glassy carbon electrode (GC) by the evaporation of Au-bt aqueous suspension dropped on the surface of the GC (GC/Au-bt). Cyclic voltammetry and chronoamperometry studies of As(III) solutions were performed with this GC/Au-bt electrode which act as efficient platform for the electro-oxidation of As(III) to As(V) at a very low overpotential. Kinetic parameters were evaluated for the oxidation of As(III) at the GC/Au-bt platforms. A wide linear calibration range for the determination of As(III) from 1 to 1700 μM was obtained with high reproducibility and stability. A limit of detection, 0.1 μM was achieved with high sensitivity. Additionally, it showed a good selectivity for the determination of As(III) in the presence of copper(II) and other interfering ions suggesting a promising new route for trace level determination of As(III) in neutral conditions.

A novel electrochemical immunosensor using β-cyclodextrins functionalized silver supported adamantine-modified glucose oxidase as labels for ultrasensitive detection of alpha-fetoprotein

In this work, a novel sandwich-type electrochemical immunosensor based on host-guest interaction was fabricated for the detection of alpha-fetoprotein (AFP). Due to the large specific surface area of multiwalled carbon nanotubes and the unique supramolecular recognition ability of β-cyclodextrins, ferrocenecarboxylic acid (Fc) was incorporated into this sensor platform by host-guest interaction to generate an electrochemical signal. And β-cyclodextrins functionalized silver supported adamantine-modified glucose oxidase (GOD-CD-Ag), was used as a label to improve the analytical performance of the immunosensor by the dual amplification strategy. The obtained GOD-CD-Ag conjugates could convert glucose into gluconic acid with the formation of hydrogen peroxide (H2O2). And then silver nanoparticles could in situ catalyze the reduction of the generated H2O2, dramatically improving the oxidation reaction of Fc. The developed immunosensor shows a wide linear calibration range from 0.001 to 5.0 ng/mL with a low detection limit (0.2 pg/mL) for the detection of AFP. The method, with ideal reproducibility and selectivity, has a wide application prospect in clinical research.

Paper-based electrochemical cyto-device for sensitive detection of cancer cells and in situ anticancer drug screening

In this work, using human acute promyelocytic leukemia cells (HL-60) as a model, a novel microfluidic paper-based electrochemical cyto-device (μ-PECD) was fabricated to demonstrate a facile, portable, and disposable approach for cancer cell detection and in situ screening of anticancer drugs in a high-throughput manner. In this μ-PECD, aptamers modified three-dimensional macroporous Au-paper electrode (Au-PE) was fabricated and employed as the working electrode for specific and efficient cancer cell capture as well as for sequential in-electrode 3D cell culture. This Au-PE showed enhanced capture capacity for cancer cells and good biocompatibility for preserving the activity of captured living cells. Sensitive cancer cell detection was achieved in this μ-PECD, which could respond down to four HL-60 cells in 10μL volume with a wide linear calibration range from 5.0×102 to 7.5×107 cellsmL−1 and exhibited good stability and reproducibility. Then, in situ anticancer drug screening was successfully implemented in this μ-PECD through monitoring of the apoptotic cancer cells after the in-electrode 3D cell culture with drug-containing culture medium, demonstrating its wide range of potential applications to facilitate effective clinical cancer diagnosis and treatment.

Cyto-sensing in electrochemical lab-on-paper cyto-device for in-situ evaluation of multi-glycan expressions on cancer cells

A novel electrochemical lab-on-paper cyto-device (ELPCD) was fabricated to demonstrate sensitive and specific cancer cell detection as well as in-situ monitoring of multi-glycans on living cancer cells. In this ELPCD, aptamers modified three-dimensional macroporous Au-paper electrode (Au-PE) was employed as the working electrode for specific and efficient cancer cell capture. Using a sandwich format, sensitive and reproducible cell detection was achieved in this ELPCD on the basis of the electrochemical signal amplification of the Au-PE and the horseradish peroxidase-lectin electrochemical probe. The ELPCD displayed excellent analytical performance for the detection of four K562 cells with a wide linear calibration range from 550 to 2.0×107cellsmL−1. Then, this ELPCD was successfully applied to determine cell-surface multi-glycans in parallel and in-situ monitor multi-glycans expression on living cells in response to drug treatment through in-electrode 3D cell culture. The proposed method provides promising application in decipherment of the glycomic codes as well as clinical diagnosis and treatment in early process of cancer.

A New Amperometric Glucose Biosensor Based on One-Step Electrospun Poly(Vinyl Alcohol)/Chitosan Nanofibers

In this work, a new glucose amperometric biosensor was developed by directly electrospinning poly(vinyl alcohol)/chitostan nanofibers on the surface of the platinum electrode, in which glucose oxidase (GOD) was effectively immobilized in nanofibers by encapsulation. After been cross-linked in glutaraldehyde vapor and modified with a thin nafion film, the nanofibers (PVA/chitosan/GOD)/nafion electrode was used for glucose amperometric measurements. The electrospun nanofibrous enzyme membrane served as a better sensing element than the casing one due to the unique properties of nanofibers such as the special three-dimensional network structure, large pores, high porosity, and large surface to volume ratios. The as-prepared biosensor showed a wide linear calibration range, low detection limit, and low apparent Michaelis-Menten constant in the glucose determination. The stability, reproducibility and anti-interference capability of biosensor were also presented. Furthermore, the new biosensor was successfully applied to detect glucose in human serum samples.

A nanostructured AuCu3 alloy electrode for highly sensitive detection of hydrazine at low potential in neutral medium

We recently reported in this journal the preparation and characterization of a nanostructured AuCu3 alloy electrode for use as chemical sensors. This alloy electrode was prepared from a barrel-plated gold electrode (AuBPE) consisting of copper main body with successive plating of nickel and gold by manipulating the grain boundary diffusion through thermal annealing treatment and electrochemical activation in alkaline solution. In this study, based on a more detailed XRD study, we tend to explain how the nanostructured film is attained for this AuCu3 alloy electrode. The structural characterization and electrochemical behavior of the AuCu3 alloy were performed by XRD, EDS, and cyclic voltammetry. We further demonstrated its application as an electrochemical sensor towards non-enzymatic detection of hydrazine with good selectivity. Compared to that of AuBPE, the sensitivity for hydrazine detection using cyclic voltammetry was found to increase by ∼24.8 times with a low peak potential of ∼0.0V vs. Ag/AgCl in neutral medium. Flow injection analysis of hydrazine shows a wide linear calibration range of 0.1μM to 1mM and further increases the sensitivity with a detection limit down to 0.04μM (S/N=3). Real sample analysis was successfully demonstrated for the detection of hydrazine in environmental water samples.

Determination of aliskiren in human serum quantities by HPLC–tandem mass spectrometry appropriate for pediatric trials

The orally active direct renin inhibitor aliskiren is approved for the treatment of essential hypertension in adults. Analytical methods utilized in clinical studies on efficacy and safety have not been fully described in the literature but need a large sample volume ranging from 200 to 700 μL, rendering them unsuitable particularly for pediatric applications. In the assay presented only 100 μL of serum is needed for mixed-mode solid-phase extraction. The chromatographic separation was performed on Xselect(TM) C18 CSH columns with mobile phase consisting of methanol-water-formic acid (75:25:0.005, v/v/v) and a flow rate of 0.4 mL/min. Running in positive electrospray ionization and multiple reaction monitoring the mass spectrometer was set to analyze precursor ion 552.2 m/z [M + H](+) to product ion 436.2 m/z during a total run time of 5 min. The method covers a linear calibration range of 0.146-1200 ng/mL. Intra-run and inter-run precisions were 0.4-7.2 and 0.6-12.9%. Mean recovery was at least 89%. Selectivity, accuracy and stability results comply with current European Medicines Agency and Food and Drug Administration guidelines. This successfully validated LC-MS/MS method with a wide linear calibration range requiring small serum amounts is suitable for pharmacokinetic investigations of aliskiren in pediatrics, adults and the elderly.

Gas-phase chemiluminescent determination of inorganic selenium in water samples following flow injection hydride generation and cryotrapping

We describe a method for the determination of inorganic selenium in water samples via gas-phase chemiluminescence (GPCL). Se(IV) was first derivatized with 4-nitro-o-phenylenediamine to form 5-nitropiazselenol. The latter was decomposed by persulfate through photocatalytic oxidation to give Se(VI), which was reduced to Se(IV). Selenium hydride was generated from Se(IV) through reduction with sodium borohydride and then preconcentrated using cryotrapping. The cryotrapped hydride was evaporated and carried to a reaction chamber by a stream of helium, where it produced GPCL as a result of ozonation. The method exhibits a wide linear calibration range (from 0.5 μg L−1 to 1.0 mg L−1) with a detection limit of 0.12 μg L−1 (for n = 11), and a relative standard deviation of 3.90 % (at n = 11) at 5.0 μg L−1 level of selenium. The method was applied to the determination of inorganic selenium in water samples and gave satisfactory results.

Efficient electrocatalytic oxidation and selective determination of isoniazid by Fe(tmphen) 3 2+ -exchanged Nafion®-modified electrode

Cyclic voltammetry and electrochemical impedance studies of Fe(tmphen)32+ (where tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline)-immobilized Nafion®-modified glassy carbon electrode (GC/Nf/Fe(tmphen)32+) are carried out in 0.1 M Na2SO4 solution. Nafion–Fe(tmphen)32+ complex exhibits efficient electrocatalytic oxidation of isoniazid. The linear double reciprocal plot of current and concentration of isoniazid shows a Michaelis–Menten-type catalytic process. The catalytic oxidation currents are proportional to the concentration of isoniazid and show a wide linear calibration range for the quantitative determination of isoniazid. Detection limit and sensitivity are found to be 13 μM and 2.5 μA mM−1, respectively.

Determination of hydrazine by polyNi(II) complex modified electrodes with a wide linear calibration range

Polymeric [Ni II(teta)] 2+ (poly(Ni); teta = C-meso-(5,5,7,12,12,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecane)) modified glassy carbon and Nafion (Nf)-coated glassy carbon electrodes (GC/poly(Ni) and GC/Nf/poly(Ni), respectively) were used for the efficient electrocatalytic oxidation of hydrazine (HZ) in 0.1 M NaOH. The catalytic currents were proportional to the concentration of HZ with a wide linear calibration range (LCR). GC/poly(Ni) had a LCR from 1.0 μM to 10.0 mM, whereas that of GC/Nf/poly(Ni) ranged from 1.0 μM to 100.0 mM. The latter is the widest LCR reported to date. The presence of the Nf film (i.e., in GC/Nf/poly(Ni)) not only increased the stability of the poly(Ni) film but also widened the LCR. The electrocatalytic oxidation of HZ obeyed first-order kinetics with respect to HZ concentration. The kinetic parameters were investigated by cyclic voltammetry and chronoamperometry.