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.     

An All-solid-state Cd-selective electrode with a low detection limit

A new all-solid-state Cd²⁺-selective electrode with a low detection limit was prepared by using conjugated thiophene oligomer α-sexithiophene (α-6T) as solid contact deposited between an ionophore-doped poly(vinyl chloride) membrane and a gold disc substrate. The electrode exhibited a Nernstian response for Cd²⁺ ions over a wide concentration range of 10⁻³-10⁻⁷ M with a detection limit as low as 1.3 × 10⁻⁸ M. Results showed that the fabricated potentiometric sensor was suitable for use within the pH range of 2.0-9.0 and exhibited good reproducibility for long-term measurements.     

Aptamer-based colorimetric biosensing of dopamine using unmodified gold nanoparticles

A simple, sensitive and selective colorimetric biosensor for the detection of dopamine (DA) was demonstrated with a 58-mer dopamine-binding aptamer (DBA) as recognition element and unmodified gold nanoparticles (AuNPs) as probes. Upon the addition of DA, the conformation of DBA would change from a random coil structure to a rigid tertiary structure like a pocket and this change has been demonstrated by circular dichroism spectroscopic experiments. Besides, the conformational change of DBA could facilitate salt-induced AuNP aggregation and lead to the color change of AuNPs from red to blue. The calibration modeling showed that the analytical linear range covered from 5.4 × 10⁻⁷ M to 5.4 × 10⁻⁶ M and the corresponding limit of detection (LOD) was 3.6 × 10⁻⁷ M. Some common interferents such as 3,4-dihydroxyphenylalanine (DOPA), catechol, epinephrine (EP), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and ascorbic acid (AA) showed no or just a little interference in the determination of DA.     

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.     

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.     

Electrochemical behaviors and determination of isoniazid at ordered mesoporous carbon modified electrode

In this work, an electrochemical sensor based on ordered mesoporous carbon (OMC) for the amperometric detection of isoniazid was developed. OMC was dispersed in a solution of Nafion, and the suspension was modified onto the surface of glassy carbon (GC) electrode. Cyclic voltammetry and amperometry were used to investigate the electrochemical behaviors of isoniazid on Nafion-OMC modified electrode (Nafion-OMC/GC). The results indicate that OMC can facilitate the electrochemical oxidation of isoniazid with a great decrease of overpotential in pH 7.0 phosphate buffer solution. The proposed biosensor provides excellent performance towards the determination of isoniazid with a high sensitivity of 0.031 μA/μM, a low detection limit of 8.4 × 10⁻⁸ M and wide linear range from 1.0 × 10⁻⁷ M to 3.7 × 10⁻⁴ M at +0.20 V vs. Ag/AgCl. The method was successfully applied to the determination of isoniazid tablets with satisfying results. All the results suggest that Nafion-OMC/GC electrode is a potential candidate for a stable and efficient electrochemical sensor to detect isoniazid.     

Development of a new fluorescent sensor based on a triazolo-thiadiazin derivative immobilized in polyvinyl chloride membrane for sensitive detection of lead(II) ions

A new sensor membrane based on a novel triazolo-thiadiazin derivative immobilized in polyvinyl chloride has been developed for the determination of Pb(II) ions that displays excellent performance. The parameters involved in the preparation of the optode and determination of Pb(II) were optimized. Under the optimal conditions, the proposed sensor displays a calibration response for Pb(II) over a wide concentration range of 5.0 × 10⁻⁸ to 3.8 × 10⁻⁴ M with the detection limit of 2.2 × 10⁻⁸ M. In addition to high reproducibility and reversibility of the fluorescence signal, the sensor also exhibits good selectivity over common metal ions. The optode membrane developed is easily prepared, stable, rapid, and simple for the determination of Pb(II). The accuracy of the proposed sensor was confirmed by analyzing standard reference materials of natural water and surface water. The sensor was successfully used for the determination of Pb(II) ions in water samples with satisfactory results.     

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.     

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.     

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.     

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.     

DC humidity sensing properties of BaTiO3 nanofiber sensors with different electrode materials

Barium titanate (BaTiO₃) nanofibers were synthesized by electrospinning and calcination techniques. Two direct current (DC) humidity sensors with different electrodes (Al and Ag) were fabricated by loading BaTiO₃ nanofibers as the sensing material. Compared with the Al electrode sensor, the Ag electrode sensor exhibits larger sensitivity and quicker response/recovery. The current of Al electrode sensor increases from 4.08 × 10⁻⁹ to 1.68 × 10⁻⁷ A when the sensor is switched from 11% to 95% relative humidity (RH), while the values are 2.19 × 10⁻⁹ and 3.29 × 10⁻⁷ A for the Ag electrode sensor, respectively. The corresponding response and recovery times are 30 and 9 s for Al electrode sensor, and 20 and 3 s for Ag electrode sensor, respectively. These results make BaTiO₃ nanofiber-based DC humidity sensors good candidates for practical application. Simultaneously, the comparison of sensors with different electrode materials may offer an effective route for designing and optimizing humidity sensors.     

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.     

Screen-printed carbon electrode for choline based on MnO2 nanoparticles and choline oxidase/polyelectrolyte layers

This paper presents the amperometric biosensor that determines choline and cholinesterase activity using a screen printed graphite electrode. In order to detect H₂O₂ we have blanket modified the electrode material with manganese dioxide nanoparticles layer. Using layer-by-layer technique on the developed hydrogen peroxide sensitive electrode surface choline oxidase was incorporated into the interpolyelectrolyte nanofilm. Its ability to serve as a detector of choline in bulk analysis and cholinesterase assay was investigated. We examined the interferences from red-ox species and heavy metals in the blood and in the environmental sample matrixes. The sensor exhibited a linear increase of the amperometric signal at the concentration of choline ranging from 1.3 × 10⁻⁷ to 1.0 × 10⁻⁴ M, with a detection limit (evaluated as 3σ) of 130 nM and a sensitivity of 103 mA M⁻¹ cm⁻² under optimized potential applied (480 mV vs. Ag/AgCl). The biosensor retained its activity for more than 10 consecutive measurements and kept 75% of initial activity for three weeks of storage at 4 °C. The R.S.D. was determined as 1.9% for a choline concentration of 10⁻⁴ M (n = 10) with a typical response time of about 10 s. The developed choline biosensor was applied for butyrylcholinesterase assay showing a detection limit of 5 pM (3σ). We used the biosensor to develop the cholinesterase inhibitor assay. Detection limit for chlorpyrifos was estimated as 50 pM.     

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.     

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.     

Development of an optical sensor for determination of zinc by application of PC-ANN

A very sensitive and reversible optical chemical sensor based on dithizone as chromoionophore immobilized within a plasticized carboxylated PVC film for Zn²⁺ determination is described. At optimum conditions (i.e. pH 5.0), the proposed sensor displays a linear response to Zn²⁺ over 5.0 × 10⁻⁸-5.0 × 10⁻⁶ mol L⁻¹ range. This range was improved to 2.5 × 10⁻⁸-5.8 × 10⁻⁵ mol L⁻¹ range by applying principle component-feed forward artificial neural network with back-propagation training algorithm (PC-ANNB). Detection limit of 8.0 × 10⁻⁹ mol L⁻¹ was obtained. The sensor is fully reversible within the dynamic range and the response time (t_95%) is approximately 4 min under batch conditions. In addition to its high stability and reproducibility, the sensor shows good selectivity towards Zn²⁺ ion with respect to common metal cations. The sensor was successfully applied for determination of Zn²⁺ ion in hair sample.     

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.     

Rapid parallelized and quantitative analysis of five pathogenic bacteria by ITS hybridization using QCM biosensor

We developed a 2 × 5 model quartz crystal microbalance (QCM) DNA biosensor array for detection of five bacteria, which based on hybridization analysis of bacterial 16S-23S rDNA internal transcribed spacer (ITS) region. A pair of universal primers was designed for PCR amplification of the ITSs. The PCR products were analyzed by the biosensor. We used gold nanoparticles to amplify the frequency shift signals. Fifty clinical samples were detected by both the biosensor and conventional bacteria culture method. We found a linear quantitative relationship between frequency shift and logarithmic concentration of synthesized oligonucleotides or bacteria cells. The measurable concentration ranged from 10⁻¹² to 10⁻⁸ M for synthesized oligonucleotides and 1.5 × 10² to 1.5 × 10⁸ CFU/mL for bacteria. The 10⁻¹² M of synthesized oligonucleotides or 1.5 × 10² CFU/mL of Pseudomonas aeruginosa could be detected by the biosensor system. The detection could be completed within 5 h including the PCR amplification procedure. Compared with bacteria culture method, the detection sensitivity and specificity of the biosensor system were 94.12% and 90.91%, respectively. There was no significant difference between these two methods (P = 0.625 > 0.05). The biosensor system provides a rapid and sensitive method for parallelized and quantitative analysis of multiple pathogenic bacteria in clinical diagnosis.     

A nano-copper electrochemical sensor for sensitive detection of chemical oxygen demand

Copper nanoparticle (nano-Cu) was electrodeposited on the surface of Cu disk electrode under −1 V for 60 s, and then used to construct an electrochemical sensor for chemical oxygen demand (COD). The electrochemical oxidation behavior of glycine, a standard compound for evaluating the COD, was investigated. The potential shifts negatively, and the current increases greatly at the surface of nano-Cu, indicating remarkable enhancement effect on the detection of COD. The analytical conditions such as electrolyte, deposition potential, deposition time and detected potential were studied. As a result, a sensitive, simple and rapid electroanalytical method was developed for COD using amperometric detection. The linear range is from 4.8 to 600 mg L⁻¹, and the limit of detection is as low as 3.6 mg L⁻¹. Moreover, this method exhibits high tolerance level to chloride ion, and 0.02 M chloride ion has no influence. Finally, the sensor was used to detect the COD values of different water samples, and the results were testified by the standard dichromate method.