Two-dimensional molecular imprinting approach for the electrochemical detection of trinitrotoluene

In this work, we demonstrated a sensitive and selective electrochemical sensing protocol for the detection of TNT prepared from alkanethiols self-assembled on AuNPs modified glassy carbon (GC) electrode with preadsorbed templates of TNT. It demonstrated that the 2D molecular imprinting monolayers (MIMs) can provide a better site accessibility and lower mass-transfer resistance, while the AuNPs can enhance electrode conductivity, facilitate the electron transfer and increase the amount of TNT-imprinted sites. The prepared sensor showed not only high selectivity toward TNT in comparison to other similar nitroaromatic compounds (NACs), but also a wide linear range over TNT concentration from 4.0 × 10⁻⁸ to 3.2 × 10⁻⁶ M with a detection limit of 1.3 × 10⁻⁸ M (S/N = 3). Moreover, the imprinted sensor has been applied to the determination of TNT in spiked environmental water samples and shows promise for fast and sensitive measurement of trace levels of TNT in real samples.     

Sensitive electrochemical determination of acetaminophen in pharmaceutical formulations at multiwalled carbon nanotube-alumina-coated silica nanocomposite modified electrode

A highly sensitive electrochemical sensor for the determination of acetaminophen at the multiwalled carbon nanotube-alumina-coated silica (MWCNT-ACS) nanocomposite modified glassy carbon electrode is reported. The morphology of the MWCNT-ACS nanocomposite was characterized by field emission scanning electron microscopy. The electrocatalytic properties of the MWCNT-ACS nanocomposite modified glassy carbon electrode were characterized by cyclic voltammetry and square-wave voltammetry in the presence of acetaminophen. The MWCNT-ACS nanocomposite modified glassy carbon electrode exhibited the abilities to raise the current response and to decrease the electrooxidation potential. In cyclic voltammetric responses, the oxidation peak current of acetaminophen obtained at the MWCNT-ACS modified glassy carbon electrode was 100 times greater than that of bare glassy carbon electrode. The MWCNT-ACS nanocomposite modified glassy carbon electrode for the determination of acetaminophen displayed a sensitivity of 376.5 A M⁻¹ cm⁻² and a detection limit of 0.05 μM using square-wave voltammetry. The analytical applicability of the developed method was achieved by analyzing the content of acetaminophen in five commercial drugs without pretreatment.     

Graphene-polyaniline composite film modified electrode for voltammetric determination of 4-aminophenol

An electrochemical sensor based on graphene-polyaniline (GR-PANI) nanocomposite for voltammetric determination of 4-aminophenol (4-AP) is presented. The electrochemical behavior of 4-AP at the GR-PANI composite film modified glassy carbon electrode (GCE) was investigated by cyclic voltammetry. 4-AP exhibits enhanced voltammetric response at GR-PANI modified GCE. This electrochemical sensor shows a favorable analytical performance for 4-AP detection with a detection limit of 6.5 × 10⁻⁸ M and high sensitivity of 604.2 μA mM⁻¹. Moreover, 4-AP and paracetamol can be detected simultaneously without interference of each other in a large dynamic range.     

Development of a dehydrogenase-based glucose anode using a molecular assembly composed of nile blue and functionalized SWCNTs and its applications to a glucose sensor and glucose/O2 biofuel cell

A simple and new way to immobilize glucose dehydrogenase (GDH) enzyme onto nile blue (NB) covalently assembled on the surface of functionalized single-walled carbon nanotubes (f-SWCNTs) modified glassy carbon (GC) electrode (GDH/NB/f-SWCNTs/GC electrode) was described. The GDH/NB/f-SWCNTs/GC electrode possesses promising characteristics as glucose sensor; a wide linear dynamic range of 100-1700 μM, low detection limit of 0.3 μM, fast response time (1-2 s), high sensitivity (14 μA cm⁻² mM⁻¹), anti-interference ability and anti-fouling. Moreover, the performance of the GDH/NB/f-SWCNTs/GC bioanode was successfully tested in a glucose/O₂ biofuel cell. The maximum power density delivered by the assembled glucose/O₂ biofuel cell could reach 32.0 μW cm⁻² at a cell voltage of 0.35 V with 40 mM glucose. The present procedure can be applied for preparing a potential platform to immobilize different enzymes for various bioelectrochemical applications.     

A sensitive hydrazine electrochemical sensor based on electrodeposition of gold nanoparticles on choline film modified glassy carbon electrode

A highly sensitive hydrazine sensor was developed based on the electrodeposition of gold nanoparticles onto the choline film modified glassy carbon electrode (GNPs/Ch/GCE). The electrochemical experiments showed that the GNPs/Ch film exhibited a distinctly higher activity for the electro-oxidation of hydrazine than GNPs with 3.4-fold enhancement of peak current. The kinetic parameters such as the electron transfer coefficient (α) and the rate of electron exchange (k) for the oxidation of hydrazine were determined. The diffusion coefficient (D) of hydrazine in solution was also calculated by chronoamperometry. The sensor exhibited two wide linear ranges of 5.0 × 10⁻⁷-5.0 × 10⁻⁴ and 5.0 × 10⁻⁴-9.3 × 10⁻³ M with the detection limit of 1.0 × 10⁻⁷ M (s/n = 3). The proposed electrode presented excellent operational and storage stability for the determination of hydrazine. Moreover, the sensor showed outstanding sensitivity, selectivity and reproducibility properties. All the results indicated a good potential application of this sensor in the detection of hydrazine.     

Electrocatalytic reduction of hydrogen peroxide and its determination in antiseptic and soft-glass cleaning solutions at phosphotungstate-doped-glutaraldehyde-cross-linked poly-l-lysine film electrodes

The present work describes the electrocatalytic behavior of phosphotungstate-doped glutaraldehyde-cross-linked poly-l-lysine (PLL-GA-PW) film electrode towards reduction of hydrogen peroxide (H₂O₂) in acidic medium. The modified electrode was prepared by means of electrostatically trapping the phosphotungstate anion into the cationic PLL-GA coating on glassy carbon electrode. The PLL-GA-PW film electrode showed excellent electrocatalytic activity towards H₂O₂ reduction in 0.1 M H₂SO₄. Under the optimized conditions, the electrochemical sensor exhibited a linear response for H₂O₂ concentration over the range 2.5 × 10⁻⁶ to 6.85 × 10⁻³ M with a sensitivity of 1.69 μA mM⁻¹. The curvature in the calibration curve at high concentration is explained in terms of Michaelis-Menten (MM) saturation kinetics, and the kinetics parameters calculated by three different methods were compared. The PLL-GA-PW film electrode did not respond to potential interferents such as dopamine, ascorbic acid and uric acid. This unique feature of PLL-GA-PW film electrode allowed selective determination of H₂O₂. Finally, the proposed electrochemical sensor was successfully applied to determine H₂O₂ in commercially available antiseptic solution and soft-contact lenses cleaning solution and the method has been validated using independent estimation by classical potassium permanganate titration method. Major advantages of the method are simple electrode fabrication, stability and high selectivity towards hydrogen peroxide.     

Electrochemical behavior of graphene doped carbon paste electrode and its application for sensitive determination of ascorbic acid

In present paper, the graphene doped carbon paste electrode (CPE) was firstly prepared with the addition of graphene into the carbon paste mixture. Compared with conventional CPE, an improved electrochemical response of graphene doped CPE toward the redox couple of Fe(CN)₆^3−/4− was demonstrated owing to the excellent electrical conductivity of graphene. The graphene doped CPE was further used for the successful determination of ascorbic acid (AA), and it showed an excellent electrocatalytic oxidation activity toward AA with a lower overvoltage, pronounced current response, and good sensitivity. Under the optimized experimental conditions, the proposed electrochemical AA sensor exhibited a rapid response to AA within 5 s and a linear calibration plot ranged from 1.0 × 10⁻⁷ to 1.06 × 10⁻⁴ M was obtained with a detection limit of 7.0 × 10⁻⁸ M.     

Electrocatalytic determination of hydrazine by a glassy carbon electrode modified with PEDOP/MWCNTs-Pd nanoparticles

The electrocatalysis of hydrazine oxidation by poly-ethylenedioxy pyrrole (PEDOP)-coated MWCNTs-palladium nanoparticles [PEDOP/MWCNTs-Pd] was investigated as an electrochemical sensor on the surface of glassy carbon electrode (GCE) in aqueous medium. Electrochemical oxidation of hydrazine in phosphate buffer (pH 7.4) was performed using cyclic voltammetry (CV) and chronoamperometry (CA) methods. Using the proposed electrode, the catalytic oxidation peak current of hydrazine was high and the overpotential of its oxidation decreased. Based on the obtained results, a mechanism for electrooxidation of hydrazine at [PEDOP/MWCNTs-Pd/GCE] demonstrated an irreversible diffusion-controlled electrode process and a four-electron transfer involved in the overall reaction. The experimental results showed that the mediated oxidation peak currents of the hydrazine were linearly dependent on the concentration of hydrazine in the range of 1.0 × 10⁻⁷ to 5.0 × 10⁻³ M. The detection limit (S/N = 3) was found to be 4 × 10⁻⁸ M with a fast response time of 10 s.     

High sensitive simultaneous determination of hydroquinone and catechol based on graphene/BMIMPF6 nanocomposite modified electrode

A novel nanocomposite, comprising of graphene sheet (GS) and ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆), was developed on the glassy carbon electrode (GCE) for the simultaneous determination of hydroquinone and catechol in 0.10 M acetate buffer solution (pH 5.0). At the GS/BMIMPF₆/GCE, both hydroquinone and catechol can cause a pair of quasi-reversible and well-defined redox peaks. In comparison with bare GCE and GS modified electrode, GS/BMIMPF₆/GCE showed larger peak currents, which was related to the higher specific surface area of graphene and high ionic conductivity of BMIMPF₆. Under the optimized condition, the cathodic peak current were linear over ranges from 5.0 × 10⁻⁷ M to 5.0 × 10⁻⁵ M for hydroquinone and from 5.0 × 10⁻⁷ M to 5.0 × 10⁻⁵ M for catechol, with the detection limits of 1.0 × 10⁻⁸ M and 2.0 × 10⁻⁸ M, respectively. The proposed method was successfully applied to the simultaneous determination of hydroquinone and catechol in artificial sample, and the results are satisfactory.     

An electrochemical biosensor for determination of hydrogen peroxide using nanocomposite of poly(methylene blue) and FAD hybrid film

An electrochemical biosensor for determination of hydrogen peroxide (H₂O₂) has been developed by the hybrid film of poly(methylene blue) and FAD (PMB/FAD). The PMB/FAD hybrid film was performed in PBS (pH 7) containing methylene blue and FAD by cyclic voltammetry. Repeatedly scanning potential range of −0.6-1.1 V, FAD was immobilized on the electrode surface by electrostatic interaction while methylene blue was electropolymerized on electrode surface. This modified electrode was found surface confined and pH dependence. It showed good electrocatalytic reduction for H₂O₂, KBrO₃, KIO₃, and NaClO as well as electrocatalytic oxidation for NADH. At an applied potential of −0.45 V vs. Ag/AgCl, the sensor showed a rapid and linear response to H₂O₂ over the range from 0.1 μM to 960 μM, with a detection limit of 0.1 μM and a significant sensitivity of 1109 μA mM⁻¹ cm⁻² (S/N = 3). It presented excellent stability at room temperature, with a variation of response current less than 5% over 30 days.     

Application of modified pyrolytic graphite electrode as a sensor in the simultaneous assay of adenine and adenosine monophosphate

A simple and sensitive method based on square wave voltammetry (SWV) at single-walled carbon nanotube (SWNT) modified edge plane pyrolytic graphite electrode (EPPGE) is proposed for the simultaneous determination of adenine and adenosine-5′-monophosphate (5′-AMP). The modified electrode exhibits remarkable electrocatalytic properties towards adenine and 5′-AMP oxidation with a peak potential of ∼850 and 1165 mV respectively. Linear calibration curves are obtained over the concentration range of 5-100 nM for adenine and 10-100 nM for 5′-AMP with sensitivity of 677 and 476 nA nM⁻¹ for adenine and 5′-AMP respectively. The limit of detection for adenine and 5′-AMP was found to be 37 × 10⁻¹⁰ M and 76 × 10⁻¹⁰ M, respectively. The effect of pH revealed that the oxidation of adenine and 5′-AMP at SWNT modified EPPGE involved equal number of electrons and protons. The modified electrode exhibited high stability and reproducibility.     

Electrochemical genosensor based on modified octadecanethiol self-assembled monolayer for Escherichia coli detection

An electrochemical genosensor based on 1-fluoro-2-nitro-4-azidobenzene (FNAB) modified octadecanethiol (ODT) self-assembled monolayer (SAM) has been fabricated for Escherichia coli detection. The results of electrochemical response measurements investigated using methylene blue (MB) as a redox indicator reveal that this nucleic acid sensor has 60 s of response time, high sensitivity (0.5 × 10⁻¹⁸ M) and linearity as 0.5 × 10⁻¹⁸-1 × 10⁻⁶ M. The sensor has been found to be stable for about four months and can be used about ten times. It is shown that water borne pathogens like Klebsiella pneumonia, Salmonella typhimurium and other gram-negative bacterial samples has no significant effects in the response of this sensor.     

A novel amperometric biosensor for oxalate determination using multi-walled carbon nanotube-gold nanoparticle composite

An amperometric oxalate biosensor using nanohybrid film of multi-walled carbon nanotubes (MWCNTs) and gold colloidal nanoparticles (GNPs) via carbodiimide chemistry by forming amide linkages between carboxylic acid groups on the CNTs and amine residues of cysteamine self-assembled monolayer (SAM) has been prepared. The c-MWCNTs were immobilized on the gold (Au) electrode and characterized by FTIR. The morphologies of the c-MWCNT/Au and GNPs/MWCNT/Au electrodes were investigated by scanning electron microscopy (SEM) and the electrochemical performance of the Au, c-MWCNT/Au and GNPs/c-MWCNT/Au electrodes were also studied amperometrically. The Cl⁻ and NO₃⁻ insensitive oxalate oxidase from grain sorghum was finally immobilized on this electrode. The influence of pH, temperature and oxalate concentration on electrode activity was studied. The electrode showed optimum response within 7 s. The electrocatalytic response showed a linear dependence on the oxalic acid concentration ranging from 1 to 800 μM with a detection limit of 1 μM. The K_m value for the oxalic acid sensor was 444.44 μM. The enzyme electrode retained 30% of its initial activity after 5 months, when stored at 4 °C. The electrode was employed for measurement of oxalic acid in serum, urine and foodstuffs.     

Direct electrochemistry and electrocatalysis of hemoglobin immobilized into poly (lactic-co-glycolic acid)/room temperature ionic liquid composite film

A promising material of poly(lactic-co-glycolic acid) (PLGA) and, room temperature ionic liquid (ILs) (1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF₄) was firstly used as an immobilization matrix to entrap proteins and its bioelectrochemical properties were studied. Direct electrochemistry and electrocatalytic behaviors of hemoglobin (Hb) entrapped in the PLGA/ILs composite film on the surface of glass carbon electrode were investigated. UV-vis spectroscopy, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the composite film. The obtained results demonstrated that the Hb molecule in the film kept its native structure and showed its good electrochemical behavior. A pair of well-defined redox peaks of Hb was obtained at the Hb/PLGA/ILs composite film-modified GC electrode through direct electron transfer between the protein and the underlying electrode. The proposed biosensor showed good reproducibility and high sensitivity to H₂O₂ with the detection limit of 2.37 × 10⁻⁷ M (S/N = 3). In the range of 5.0 × 10⁻⁶ to 8.05 × 10⁻³ M, the catalytic reduction current of H₂O₂ was proportional to its concentration. The apparent Michaelis-Menten constant of Hb in the PLGA/ILs composite film was estimated to be 0.069 mM, showing its high affinity.     

Development of molecularly imprinted electrochemical sensor with titanium oxide and gold nanomaterials enhanced technique for determination of 4-nonylphenol

4-Nonylphenol (4-NP) was reported to affect the health of wildlife and humans through altering endocrine function. A novel electrochemical sensor for sensitive and fast determination of 4-NP was developed. Titanium oxide (TiO₂) nanoparticles and gold nanoparticles (AuNPs) were introduced for the enhancement of electron conduction and sensitivity. 4-NP-imprinted functionalized AuNPs composites with specific binding sites for 4-NP was modified on electrode. The resulting electrodes were characterized by cyclic voltammetry (CV). Rebinding experiments were carried out to determine the specific binding capacity and selective recognition. The linear range was over the range from 4.80 × 10⁻⁴ to 9.50 × 10⁻⁷ mol L⁻¹, with the detection limit of 3.20 × 10⁻⁷ mol L⁻¹ (S/N = 3). The sensor was successfully employed to detect 4-NP in real samples.     

Potentiometric urea biosensor based on urease immobilized by an electrosynthesized poly(o-phenylenediamine) film with buffering capability

A simple and novel potentiometric biosensor for urea detection was prepared by employing an electrosynthesized polymer with buffering capability. It was obtained by deposition of a weighed amount of urease (Ur) at a glassy carbon (GC) electrode followed by immobilization by an electrosynthesized poly-o-phenylenediamine (PPD) film. An unconventional “upside-down” (UD) geometry was employed for the electrochemical cell. The response of GC/Ur/PPD sensor is linear with urea concentration in the range 10 μM to 1 mM (15 mV/mM, R² = 0.9999) due to buffering capability of PPD film, which represents a novel role of electrosynthesized polymers in their application to biosensors. At higher concentrations, the more common Nernstian response (28 mV/decade, R² = 0.9987) is observed. The sensor exhibits a sufficient sensitivity for practical determinations, rapid response and long term stability.     

Anthraquinone functionalized carbon composite electrode: Application to ammonia sensing

A simple strategy has been used to covalently modify the glassy carbon spheres with anthraquinone moieties through the diazonium salt reduction. The derivatized glassy carbon spheres were used to modify the basal plane pyrolytic graphite electrode by immobilizing them on its surface and examining its electrochemical behaviour. The composite electrode has been used to detect trace level ammonia in the concentration range 5 × 10⁻⁸ to 3 × 10⁻⁵ M and it was successfully applied to detect low levels of ammonia present in natural samples like urine and soil.     

Electrochemical studies of bovine serum albumin immobilization onto the poly-o-phenylenediamine and carbon-coated nickel composite film and its interaction with papaverine

A biosensor based on bovine serum albumin (BSA) and poly-o-phenylenediamine (PoPD)/carbon-coated nickel (C-Ni) nanobiocomposite film modified electrode has been developed to study the interaction of BSA with papaverine (PAP). The well-dispersed C-Ni nanoparticles were dripped onto the glassy carbon electrode (GCE) surface firstly, and PoPD films were subsequently electropolymerized by cyclic voltammetry (CV) to prepare PoPD/C-Ni/GCE. Finally, the BSA was easily immobilized on the PoPD films via electrostatic adsorption. The morphology and the electrochemical properties of the fabricated composite electrodes were examined by scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS), respectively. The interaction of PAP with BSA was monitored by differential pulse voltammetry (DPV), using PoPD as the electrochemical indicator. The binding constant (K), obtained by DPV, was 1.7 × 10⁴ L/mol, which was consistent with the fluorescence analysis. This constructed biosensor also exhibited a fine linear correlation with PAP concentration range of 2.5 × 10⁻⁹-4.5 × 10⁻⁵ mol/L and a detection limit of 8.3 × 10⁻¹⁰ mol/L was achieved by DPV.     

Lead (II) carbon paste electrode based on derivatized multi-walled carbon nanotubes: Application to lead content determination in environmental samples

For the first time a novel derivatized multi-walled carbon nanotubes-based Pb²⁺ carbon paste electrode is reported. The electrode with optimum composition, exhibits an excellent Nernstian response to Pb²⁺ ion ranging from 5.9 × 10⁻¹⁰ to 1.0 × 10⁻² M with a detection limit of 3.2 × 10⁻¹⁰ M and a slope of 29.5 ± 0.3 mV dec⁻¹ over a wide pH range (2.5-6.5) with a fast response time (25 s) at 25 °C. Moreover, it also shows a high selectivity and a long life time (more than 3 months). Importantly, the response mechanism of the proposed electrode was investigated using AC impedance technique. Finally, the electrode was successfully applied for the determination of Pb²⁺ ion concentration in environmental samples, e.g. soils, waste waters, lead accumulator waste and black tea, and for potentiometric titration of sulfate anion.     

Carbon felt-based bioelectrocatalytic flow-through detectors: Highly sensitive amperometric determination of H2O2 based on a direct electrochemistry of covalently modified horseradish peroxidase using cyanuric chloride as a linking agent

Horseradish peroxidase (HRP) was chemically modified using cyanuric chloride (CC) as a linking agent onto a carbon felt (CF), which is a microelectrode ensemble of micro carbon fiber (>7 μm, diameter) with a random three-dimensional structure. The resulting HRP-modified CF (HRP-ccCF) exhibited well-defined redox waves based on the HRP heme Fe^III/Fe^II redox couple at −0.23 V vs. Ag/AgCl (at pH 7.0), while the HRP-adsorbed CF (HRP-CF) showed no apparent redox couple in the same potential range, indicating that the chemical modification of HRP via CC facilitated the direct electron transfer (DET) between HRP and CF. The apparent heterogeneous electron transfer rate constant k_s was estimated to be 35 s⁻¹. Cyclic voltammetry and electrochemical impedance spectroscopy revealed that the interfacial properties (i.e., structure, morphology of enzyme-layer) of covalently modified HRP (HRP-ccCF) and physically adsorbed HRP (HRP-CF) are different, resulting in the difference in the electron transfer properties. The HRP-ccCF was successfully used as a working electrode unit in bioelectrocatalytic flow-through detector for highly sensitive amperometric determination of H₂O₂. Under the optimized conditions (i.e., applied potential, 0 V vs. Ag/AgCl; carrier flow rate, 3.25 ml/min; and carrier pH 7.0), the cathodic peak current of H₂O₂ linearly increased up to 3 μM (sensitivity, 1.94 μA/μM; the detection limit, 0.08 μM [S/N = 3]) with sample through-put of ca. 90 samples/h.