Determination of trace selenium on hanging copper amalgam drop electrode

Hanging copper amalgam drop electrode has been applied for trace determination of selenium by cathodic stripping analysis. Detection limit for Se(IV) as low as 0.25 nM (0.02 μg L⁻¹) at deposition time (120 s) could be obtained. For seven successive determinations of Se(IV) at concentration of 5 nM relative standard deviation was 2.3% (n = 7). Interferences from selected metals and surfactant substances were examined. Absence of copper ions in sample solution causes easier optimization and makes method less vulnerable on contamination. The developed method was validated by analysis of certified reference materials (BCRs) and applied to selenium determinations in natural water samples, snow, mushrooms and ox liver.     

Novel electrode for electrochemical stripping analysis based on carbon paste modified with bismuth powder

Bismuth-powder modified carbon paste electrode (Bi-CPE) is presented as an attractive “mercury-free” sensor applicable in electrochemical striping analysis of selected heavy metals. The electrode paste was prepared as a mixture of finely powdered metallic bismuth together with graphite powder and silicon oil. The Bi-CPE was characterized in nondeaerated solutions containing Cd(II) and Pb(II) at the μg/L level in conjunction with square-wave anodic stripping voltammetry. The electrode exhibited well-defined and separated stripping signals for both metals accompanied with a low background contribution, and a reproducibility of 5.6 and 6.0% (n = 12) for 20 μg/L Cd(II) and Pb(II), respectively. The Bi-CPE exhibited superior performance in comparison to the bare carbon paste electrode (CPE) and the bismuth paste electrode (BiPE) and surprisingly, yielded a higher response than the in situ prepared bismuth-film carbon paste electrode. The electrode displayed excellent linear behavior in the examined concentration range from 10 to 100 μg/L Cd(II) + Pb(II) (R² = 0.998 for both), with limits of detection of 1.2 μg/L for Cd(II) and 0.9 μg/L for Pb(II). The electroanalytical performance of Bi-CPE was successfully tested in a real sample of tap water spiked with Cd(II) and Pb(II).     

Insights into the simultaneous chronopotentiometric stripping measurement of indium(III), thallium(I) and zinc(II) in acidic medium at the in situ prepared antimony film carbon paste electrode

The simultaneous measurement of microgram per liter concentration levels of indium(III), thallium(I) and zinc(II) at the antimony film carbon paste electrode (SbF-CPE) is demonstrated. The antimony film was deposited in situ on a carbon paste substrate electrode and employed in chronopotentiometric stripping mode in deoxygenated solutions of 0.01 M hydrochloric acid (pH 2). The chronopotentiometric stripping performance of the SbF-CPE was studied and compared with constant current chronopotentiometric stripping and anodic stripping voltammetric operation. In comparison with its bismuth and mercury counterparts, the SbF-CPE exhibited advantageous electroanalytical performance; namely, at the bismuth film electrode, the measurement of zinc(II) was practically impossible due to hydrogen evolution, whereas the mercury film electrode exhibited a poorly developed signal for thallium(I). The SbF-CPE revealed favorable calculated LoDs (3σ) of 1.4 μg L⁻¹ for thallium(I) and 2.4 μg L⁻¹ for indium(III) along with good linear response in the examined concentration range from 10 to 100 μg L⁻¹ with correlations coefficients (R²) of 0.992 for thallium(I) and 0.994 for indium(III) associated with a 120 s deposition time. The chronopotentiometric stripping performance of the SbF-CPE was characterized also by satisfactory reproducibility of 1.62% for indium(III), 3.96% for thallium(I) and 2.11% for zinc(II) (c = 40 μg L⁻¹, n = 11).     

Determination of picomolar silver concentrations by differential pulse anodic stripping voltammetry at a carbon paste electrode modified with phenylthiourea-functionalized high ordered nanoporous silica gel

This study introduces the design of an anodic stripping voltammetric (ASV) method for the silver ion determination at a carbon paste electrode (CPE), chemically modified with phenylthiourea-nanoporous silica gel (Tu-SBA-15-CPE). The electroanalytical pro includes two steps: preconcentration of metal ions at an electrode surface, followed by quantification of the accumulated species by differential pulse anodic stripping voltammetric methods. Factors affecting the performance of the anodic stripping were investigated, including the modifier quantity in the paste, the electrolyte concentrations, the solution pH and the accumulation potential or time. The most sensitive and reliable electrode contained 10% Tu-SBA-15 and 90% carbon paste. The accumulation potential and time were set at, −200 mV and 300 s, respectively, and the scan rate at 50 mV s⁻¹ in the scan range of −200 to 700 mV. The resulting electrode demonstrated a linear response over range of silver ion concentration of 8.0-80 pmol/L with detection limit (S/N = 3) of 5 pmol/L. The prepared electrodes were used for the silver determination in sea and tap water samples and very good recovery results were obtained. The accuracy was assessed through recovery experiments and independent analysis by graphite furnace atomic absorption spectrometry.     

Study and determination of the herbicide cyclosulfamuron by square wave stripping voltammetry

The voltammeric behavior of the herbicide cyclosulfamuron has been studied by square wave stripping voltammetry (SWSV). Cyclosulfamuron was reduced on a static mercury drop electrode (SMDE) and gave a well-defined peak in the pH range of 3.0-7.0. The peak potential (E_p) shifts to a more negative potential with increasing pH. The ratio ΔE_p/ΔpH over the pH range studied was 59.5 mV/pH. A systematic study of the various experimental parameters that affect the stripping response was studied by SWV. The square wave parameters used were a frequency of 150 Hz, an amplitude of −60 mV and a staircase step of 6.0 mV. The quantifications were performed by the standard addition method, from the SW voltammetric peak obtained at −1348 mV. Calibration curves were linear in the range of 10-350 μg L⁻¹ with a detection limit of 3.5 μg L⁻¹ under the conditions used (pH 6.0 buffer solution, E_acc = −400 mV vs. Ag/AgCl, t_acc = 75 s). The validity of the developed methodology was assessed by recovery experiments at the 25-100 μg L⁻¹ level. The mean results for 3 determinations were 49.7 ± 3.3 μg L⁻¹, which is very close to the amount of cyclosulfamuron added to soil (50 μg L⁻¹), with a recovery of 99.4%. The sufficiently good recoveries and low relative standard deviation (RSD) data reflects the high accuracy and precision of the proposed SW voltammetric method. The possible influences of various inorganic species and other pesticides were also investigated.     

Anodic stripping voltammetric measurement of trace heavy metals at antimony film carbon paste electrode

The antimony film carbon paste electrode (SbF-CPE) was prepared in situ on the carbon paste substrate electrode as a “mercury-free” electrochemical sensor. Its aptitude for measuring some selected trace heavy metals has been demonstrated in combination with square-wave anodic stripping voltammetry in non-deaerated model solutions of 0.01 M hydrochloric acid with pH 2. Some important operational parameters, such as deposition potential, deposition time, and concentration of antimony ions were optimized, and the electroanalytical performance of the SbF-CPE was critically compared with both bismuth film carbon paste electrode (BiF-CPE) and mercury film carbon paste electrode (MF-CPE) using Cd(II) and Pb(II) as test metal ions. In comparison with BiF-CPE and MF-CPE, the SbF-CPE exhibited superior electroanalytical performance in more acidic medium (pH 2) associated with favorably low hydrogen evolution, improved stripping response for Cd(II), and moreover, stripping signals corresponding to Cd(II) and Pb(II) at the SbF-CPE were slightly narrower than those observed at bismuth and mercury counterparts. In addition, the comparison with antimony film electrode prepared at the glassy carbon substrate electrode displayed higher stripping current response recorded at the SbF-CPE. The newly developed sensor revealed highly linear behavior in the examined concentration range from 5 to 50 μg L⁻¹, with limits of detection (3σ) of 0.8 μg L⁻¹ for Cd(II), and 0.2 μg L⁻¹ for Pb(II) in connection with 120 s deposition step, offering good reproducibility of ±3.8% for Cd(II), and ±1.2% for Pb(II) (30 μg L⁻¹, n = 10). Preliminary experiments disclosed that SbF-CPE and MF-CPE exhibit comparable performance for measuring trace concentration levels of Zn(II) in acidic medium with pH 2, whereas its detection with BiF-CPE was practically impossible. Finally, the practical applicability of SbF-CPE was demonstrated via measuring Cd(II) and Pb(II) in a real water sample.     

Effect of Bi(III) concentration on the stripping voltammetric response of in situ bismuth-coated carbon paste and gold electrodes

The effect of Bi(III) concentration (over the wide concentration range of 10⁻⁷ to 10⁻⁴ M) on the determination of Pb and Cd metal ions (in the 10⁻⁸ to 10⁻⁵ M range), by means of anodic stripping voltammetry (ASV) at in situ bismuth-coated carbon paste (CPE) and gold electrodes, has been studied. It is shown that in square wave anodic stripping voltammetry (SWASV) experiments the sensitivity of the technique generally depends on the Bi(III)-to-metal ion concentration ratio. It was found that, unlike the usually recommended at least 10-fold Hg(II) excess in anodic stripping experiments at in situ prepared mercury film electrodes, Bi(III)-to-metal ion ratios less than 10 are either optimal or equally effective at CPE and Au electrode substrates. Detection limits down to 0.1 μg L⁻¹ for Pb(II) and 0.15 μg L⁻¹ for Cd(II) were estimated at CPEs under conditions of small or moderate Bi(III) excess. Depending on Bi(III) concentration and deposition time, multiple stripping peaks attributed to Bi were recorded (especially in the case of Au substrates), indicating various forms of Bi deposits.     

An electroanalytical application of 2-aminothiazole-modified silica gel after adsorption and separation of Hg(II) from heavy metals in aqueous solution

2-Aminothiazole covalently attached to a silica gel surface was prepared in order to obtain an adsorbent for Hg(II) ions having the following characteristics: good sorption capacity, chemical stability under conditions of use, and, especially, high selectivity. The accumulation voltammetry of mercury(II) was investigated at a carbon paste electrode chemically modified with silica gel functionalized with 2-aminothiazole (SIAMT-CPE). The repetitive cyclic voltammogram of mercury(II) solution in the potential range −0.2 to +0.6 V versus Ag/AgCl (0.02 mol L⁻¹ KNO₃; v=20 mV s−1) show two peaks one at about 0.1 V and other at 0.205 V. The anodic wave peak at 0.205 V is well defined and does not change during the cycles and it was therefore further investigated for analytical purposes using differential pulse anodic stripping voltammetry in differents supporting electrolytes. The mercury response was evaluated with respect to pH, electrode composition, preconcentration time, mercury concentration, “cleaning” solution, possible interferences and other variables. The precision for six determinations (n = 6) of 0.02 and 0.20 mg L⁻¹ Hg(II) was 4.1 and 3.5% (relative standard deviation), respectively. The detection limit was estimated as 0.10 μg L⁻¹ mercury(II) by means of 3:1 current-to-noise ratio in connection with the optimization of the various parameters involved and using the highest-possible analyser sensitivity.     

Determination of very toxic metal — Cadmium in natural water samples

A potentiometric stripping method for a direct measurement of cadmium in various natural water samples using a Stripping analyzer M1 (designed and manufactured in Serbia) unit has been developed. In order to ascertain optimum conditions for the determination of the effect of the mercury time electrodeposition, the electrolysis potential and the solution stirring rate of the cadmium analytical signal have been examined. Linearity of the cadmium analytical signal was achieved within the wide range of the mass concentrations, from 1 μg/L to 15 μg/L. A detection limit of 0.10 μg/L was obtained, with a reproducibility of 3.55% expressed as the coefficient of variation. The accuracy of the method was confirmed by parallel analyses by flameless atomic absorption spectrophotometry as the reference method.     

Electrochemical behavior and adsorptive stripping voltammetric determination of quercetin at multi-wall carbon nanotubes-modified paraffin-impregnated graphite disk electrode

Multi-wall carbon nanotubes-modified paraffin-impregnated graphite disk was fabricated by using choline bond and catalyzer (MWCNT/Ch/WGE), and the properties were, respectively, investigated by field emission scanning electron microscope (FE-SEM) and X-ray photoelectron spectroscopy (XPS). Electrochemical behavior of quercetin was studied in detail by FE-SEM, UV-spectroelectrochemical and various electrochemical methods, which related with the two catechol hydroxyl groups and the other three hydroxyl groups, the former is electron-donating group with a two electron two proton reversible reaction, and is pH dependent. A highly sensitive adsorptive stripping voltammetric measurement (AdSV) for quercetin was also shown at this electrode. On the optimum conditions, the adsorptive stripping response of the peak (E = 0.15 V, 3 min accumulation) was proportional to the concentration in a range of 9.0 × 10⁻⁹ to 4.0 × 10⁻⁵ M. A detection limit of 4.8 × 10⁻⁹ M was obtained with a signal-to-noise ratio (s/n) of 3 and a good precision (R.S.D.: 2.1%, n: 9). Such attractive ability of MWCNT/Ch/WGE suggests a great promise for a quercetin amperometric sensor.     

Determination of azinphos-methyl and parathion-methyl in honey by stripping voltammetry

An electrochemical method for the determination of azinphos-methyl and parathion-methyl in honey is presented. The determination is established by adsorptive stripping differential pulse voltammetry at hanging mercury working electrode.In contrast to the chromatographic methods for the determination of pesticide residues, the sample preparation of the proposed method is minimal; analytes were extracted from honey samples with a mixture of (acetone):(Britton-Robinson buffer) and then were analyzed without any additional pretreatment.The response of the analytes either individually or as a mixture was studied for a series of deposition time and molar ratio. Two quantitation protocols were compared, using either the external calibration or the standard addition method. Accuracy was tested with spiked honey samples obtaining good recovery values. The limit of detection for the honey sample (for deposition time of 10 s) was calculated 51.71 μg kg⁻¹ for MeP and 65.87 μg kg⁻¹ for AzMet.     

Polyvinylidene fluoride and polyetherimide hollow fiber membranes for CO2 stripping in membrane contactor

Porous polyvinylidene fluoride (PVDF) and polyetherimide (PEI) hollow fiber membranes incorporating polyethylene glycol (PEG) were prepared via spinning process for CO₂ membrane stripping. CO₂ loaded diethanolamine solution was used as liquid absorbent while N₂ was used as a strip gas. The characterization study of the fibers was carried out in terms of permeation test, contact angle measurement and liquid entry pressure (wetting pressure). Performance study via membrane contactor stripping was carried out at specific operating condition. The experimental results showed that PVDF membrane have high gas permeation, effective surface porosity and contact angle despite having lower liquid entry pressure in comparison with PEI membrane. PVDF-PEG membrane showed the highest stripping flux of 4.0 × 10⁻² mol m⁻² s⁻¹ at 0.7 ms⁻¹ compared to that of PEI membrane. Although the stripping flux for PEI-PEG membranes was slightly lower than PVDF membrane (e.g. 3.5 × 10⁻² mol m⁻² s⁻¹ at liquid velocity of 0.85 ms⁻¹), the membrane wetting pressure of PEI membrane is higher than hydrophobic PVDF membrane. Long term performance of both membranes showed severe flux reduction but started to level-off after 30 h of operation.     

An electrochemical biosensor based on Nafion-ionic liquid and a myoglobin-modified carbon paste electrode

An electrochemical biosensor was constructed based on the immobilization of myoglobin (Mb) in a composite film of Nafion and hydrophobic ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆) for a modified carbon paste electrode (CPE). Direct electrochemistry of Mb in the Nafion-BMIMPF₆/CPE was achieved, confirmed by the appearance of a pair of well-defined redox peaks. The results indicate that Nafion-BMIMPF₆ composite film provided a suitable microenvironment to realize direct electron transfer between Mb and the electrode. The cathodic and anodic peak potentials were located at −0.351 V and −0.263 V (vs. SCE), with the apparent formal potential (Ep) of −0.307 V, which was characteristic of Mb Fe(III)/Fe(II) redox couples. The electrochemical behavior of Mb in the composite film was a surface-controlled quasi-reversible electrode process with one electron transfer and one proton transportation when the scan rate was smaller than 200 mV/s. Mb-modified electrode showed excellent electrocatalytic activity towards the reduction of trichloroacetic acid (TCA) in a linear concentration range from 2.0 × 10⁻⁴ mol/L to 1.1 × 10⁻² mol/L and with a detection limit of 1.6 × 10⁻⁵ mol/L (3σ). The proposed method would be valuable for the construction of a third-generation biosensor with cheap reagents and a simple procedure.     

Synthesis and characterization of novel photoconducting carbazole derivatives in main-chain polymers for photorefractive applications

We have prepared hole-transporting polymers that have carbazole and 3,3′-dicarbazole in the main chain by polycondensation based on Fridel-Craft reaction via cations generated by proton of p-toluene sulfonic acid catalyst. In the device, they display high diffraction efficiency at low external electric field. The DiCz composite had a high diffraction efficiency of 82.4% at 52 V/μm and a photorefractive response time constant of 1.18 s at an electric field of 50 V/μm and a writing intensity of 60 mW/cm². The samples have excellent properties with respect to optical quality and shelf-lifetime.     

Micropatterning of active enzyme with a high-resolution by scanning electrochemical microscopy coupled with a nanometer-sized carbon fiber disk tip

We developed a new method for fabrication of nanometer-sized carbon fiber disk electrodes and applied them to micropattern active horseradish peroxidase (HRP) with a high-resolution by scanning electrochemical microscopy (SECM). In order to pattern active HRP, except for active HRP micropatterns predesigned other regions on a HRP-immobilized substrate was deactivated by a reactive species generated at the electrode as the tip of SECM held at 1.7 V through oxidation of Br⁻ in 0.20 mol/L phosphate buffer (PB) containing 2.5 × 10⁻² mol/L KBr and 2.0 × 10⁻³ mol/L BQ (pH 7.0). The micropatterns of active HRP were characterized using the feedback mode of SECM in PB containing 2.0 × 10⁻³ mol/L BQ and 2.0 × 10⁻³ mol/L H₂O₂, when the tip potential was held at −0.2 V.     

Development and application of a poly(2,2′-dithiodianiline) (PDTDA)-coated screen-printed carbon electrode in inorganic mercury determination

In this work, monomer solutions of aniline (ANI) and 2,2′-dithiodianiline (DTDA), an aniline derivative containing -S-S- links, were prepared and used in the electrochemical copolymerisation of ANI and DTDA by cyclic voltammetry on a screen-printed electrode (SPE) in 1 M HCl. Electropolymerisation of aniline on the surface of the screen-printed working electrode was performed by sweeping the potential between −500 and + 1100 mV (vs. Ag/AgCl) at a sweep rate of 100 mV/s. Electrocopolymerisation was performed with a mixture of ANI and DTDA by sweeping the potential between −200 and + 1100 mV (vs. Ag/AgCl) at a sweep rate of 100 mV/s [J.L. Hobman, J.R. Wilson, N.L. Brown, in: D.R. Lovley (Ed.), Environmental Microbe Metal Interactions, ASM Press, Herndon, Va, 2000, p. 177]. The cyclic voltammogram (CV) for each of the electrochemically deposited polyaniline (PANI) and the mixture of ANI and DTDA for the co-polymer polymerisation on SPCE were recorded for electrochemical analysis of the peak potential data for the mono and copolymer. Anodic stripping voltammetry (ASV) was used to evaluate a solution composed of (1 × 10⁻⁶ M HgCl₂, 0.1 M H₂SO₄, 0.5 M HCl), in the presence of the co-polymer sensor electrode. The Hg²⁺ ions were determined as follows: (i) pre-concentration and reduction on the modified electrode surface and (ii) subsequent stripping from the electrode surface during the positive potential sweep. The experimental conditions optimised for Hg²⁺ determination included the supporting electrolyte concentration and the accumulation time. The results of the study have shown the use of a conducting polymer modified SPCE as an alternative transducer for the voltammetric stripping and analysis of inorganic Hg²⁺ ions.     

Poly-o-phenylenediamine as redox mediator for laccase

Electrocatalytic activity towards oxygen reduction of fungal laccase entrapped in poly-o-phenylenediamine (POPDA) matrix on glassy carbon electrodes was studied. Cyclic voltammetry and amperometric responses to dissolved oxygen were investigated in pH range from 2 to 6. POPDA reveals a unique ability to serve as a redox mediator for laccase and immobilizing matrix at the same time. The entrapped enzyme efficiently catalyzes reduction of molecular oxygen without any additional mediators. The electrocatalytic current reaches 0.1 mA/cm⁻² per 1 μg of immobilized enzyme on cyclic voltammograms recorded at 1 mV/s in a not stirred electrolyte. Current density values are comparable with those revealed by dissolved laccase (60 μg/ml) mediated by hydroquinone and greatly higher than by the mediator less laccase/glassy carbon system. The potential of oxygen reduction is determined by the polymer redox couple. Consequently, the onset of the oxygen reduction shifts from −0.15 V versus Ag/AgCl in pH 6 to +0.05 V versus Ag/AgCl in pH 2. The laccase-POPDA layers immersed in the deaerated solution show fast amperometric responses to addition of the oxygen containing solution. The observed current values depend linearly on the oxygen concentration. Factors affecting the electrocatalytic activity of the laccase-POPDA system, including the layer thickness and the pH value, are studied. The electrodeposited laccase-POPDA films are characterized by infrared spectra. The results prove that the enzyme secondary structure remains unchanged during the entrapment procedure. POPDA matrix structure consists of the phenazine-type polymer according to infrared spectra.     

Microporous polyvinylidene fluoride hollow fiber membrane contactors for CO2 stripping: Effect of PEG-400 in spinning dope

In order to develop the structure of microporous PVDF membranes, PEG-400 was introduced into the polymer dope as a non-solvent additive. The hollow fiber membranes were prepared via a wet phase-inversion process and then used in the membrane contactor modules for CO₂ stripping from water. By addition of different amounts of PEG-400, cloud points of the polymer dope were obtained to examine phase-inversion behavior. From FESEM analysis, the membrane structure changed from a finger-like to an approximately sponge-like morphology with the addition of 4 wt.% of PEG-400. The prepared membranes presented smaller mean pore size (0.13 μm) and significantly higher wetting pressure (550 kPa) compared to the plain membrane. From CO₂ stripping test, at water velocity of 0.4 m/s, the PVDF membranes prepared by 4% PEG-400 demonstrated an approximate CO₂ stripping flux of 4.5 × 10⁻⁵ (mol/m² s) which is 125% higher than the flux of the plain membrane. It could be concluded that structurally developed hydrophobic PVDF hollow fiber membranes can be prepared by a controlled phase-inversion process to enhance the performance of gas–liquid membrane contactor.     

Towards the conception of an amperometric sensor of l-tyrosine based on Hemin/PAMAM/MWCNT modified glassy carbon electrode

A novel amperometric sensor was fabricated based on the immobilization of hemin onto the poly (amidoamine)/multi-walled carbon nanotube (PAMAM/MWCNT) nanocomposite film modified glassy carbon electrode (GCE). Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and ultraviolet visible (UV-vis) adsorption spectroscopy were used to investigate the possible state and electrochemical activity of the immobilized hemin. In the Hemin/PAMAM/MWCNT nanocomposite film, MWCNT layer possessed excellent inherent conductivity to enhance the electron transfer rate, while the layer of PAMAM greatly enlarged the surface average concentration of hemin (Γ) on the modified electrode. Therefore, the nanocomposite film showed enhanced electrocatalytical activity towards the oxidation of l-tyrosine. The kinetic parameters of the modified electrode were investigated. In pH 7.0 phosphate buffer solution (PBS), the sensor exhibits a wide linear range from 0.1 μM to 28.8 μM l-tyrosine with a detection limit of 0.01 μM and a high sensitivity of 0.31 μA μM⁻¹ cm⁻². In addition, the response time of the l-tyrosine sensor is less than 5 s. The excellent performance of the sensor is largely attributed to the electro-generated high reactive oxoiron (IV) porphyrin (O = Fe^IV-P) which effectively catalyzed the oxidation of l-tyrosine. A mechanism was herein proposed for the catalytic oxidation of l-tyrosine by oxoiron (IV) porphyrin complexes.     

Enhanced field emission of open-ended, thin-walled carbon nanotubes filled with ferromagnetic nanowires

A general strategy for in situ synthesis of open-ended, thin-walled carbon nanotubes (CNTs) filled with long ferromagnetic (FeNi, FeCo, FeCoNi, etc.) nanowires is proposed. The key feature of this strategy is the introduction of Cl-contained benzene (e.g. trichlorobenzene) as carbon precursor. Size-dependent etching effect of Cl radicals on the sidewalls of CNTs is proved by theoretical calculations. As-prepared thin-walled FeNi-filled CNTs (FeNi-CNTs) show much lower turn-on field of 0.30 V/μm (at 10 μA/cm²) and lower threshold field of 0.65 V/μm (at 1.0 mA/cm²) than those of thick-walled counterparts. Moreover, as-prepared thin-walled FeNi-CNTs show good field-emission stability at a low vacuum level (10⁻⁶ Torr). The enhanced field-emission performance can be attributed to a combined contribution of open-end tips, thinner sidewalls and metal nanowire fillings.