Fabrication of β‐Cyclodextrin/Glycine/Carbon Nanotubes Electrochemical Neurotransmitters Sensor – Application in Ultra‐sensitive Determination of DOPAC in Human Serum

In this work, an electrochemical sensor was constructed by applying two successive thin layers of glycine‐carbon nanotubes mixture and β‐cyclodextrin (CNTs‐Gly)/CD over glassy carbon electrode surface for some neurotransmitters determination. A host‐guest interaction between CD and neurotransmitters molecules is expected and resulted in enhanced sensitivity, selectivity and stability of sensor response. Other components of the sensor are crucial for the unique electrochemical response. Carbon nanotubes allowed large surface area for glycine distribution that provided hydrogen bonding to CD moieties and contributed to facilitated charge transfer. It was possible to determine 3,4‐dihydroxy phenyl acetic acid (DOPAC) in the linear range of 0.1 μmol L⁻¹ to 80 μmol L⁻¹ with detection limit of 9.40 nmol L⁻¹, quantification limit of 31.5 nmol L⁻¹ and sensitivity of 4.16 μA/μmol L⁻¹. The proposed sensor was applied in synthetic cerebrospinal fluids samples using random standard addition method. Also, the proposed sensor was used to determine DOPAC in presence of common interferences and acceptable recovery results were achieved for its analysis in real blood serum. Figures of merit for (CNTs‐Gly)/CD composite in terms of precision, robustness, repeatability and reproducibility were reported.     

Development of an Amperometric Biosensor for β‐N‐Oxalyl‐L‐α,β‐Diaminopropionic Acid (β‐ODAP)

An amperometric method for the determination of the neurotoxic amino acid β‐N‐oxalyl‐L ‐α,β‐diaminopropionic acid (β‐ODAP) using a screen printed carbon electrode (SPCE) is reported. The electrode material was bulk‐modified with manganese dioxide and used as a detector in flow injection analysis (FIA). The enzyme glutamate oxidase (GlOx) was immobilized in a Nafion‐film on the electrode surface. The performance of the biosensor was optimized using glutamate as an analyte. Optimum parameters were found as: operational potential 440 mV (vs. Ag/AgCl), flow rate 0.2 mL min^?1, and carrier composition 0.1 mol L^?1 phosphate buffer (pH 7.75). The same conditions were used for the determination of β‐ODAP. The signal was linear within the concentration range 53–855 μmol L^?1 glutamate and 195–1950 μmol L^?1 β‐ODAP. Detection limits (as 3σ value) for both analytes were 9.12 and 111.0 μmol L^?1, respectively, with corresponding relative standard deviations of 3.3 and 4.5%. The biosensor retained more than 73% of its activity after 40 days of on‐line use.     

Development of Electrochemical Biosensor Based on Nanostructured Carbon Materials for Paracetamol Determination

This work describes the development of a biosensor for paracetamol (PAR) determination based on a glassy carbon electrode (GCE) modified with multiwalled carbon nanotubes (MWCNT) and laccase enzyme (LAC), which was immobilized by means of covalent crosslinking using glutaraldehyde. Voltammetric investigations were carried out by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and square wave voltammetry (SWV). The biosensor was characterized by Scanning Electron Microscope (SEM) and Fourier Transform Infrared Spectroscopy (FT‐IR). The results showed that the use of MWCNT/LAC composite increased the sensor sensitivity, compared to bare glassy carbon electrode. Factors affecting the voltammetric signals such as pH, ionic strength, scan rate and interferents were assessed. Linear range, limit of detection (LOD) and limit of quantitation (LOQ) obtained were 10–320 μmol L^?1, 7 μmol L^?1 and 10 μmol L^{? 1}, respectively. The developed biosensor was successfully applied to PAR determination in urine and pharmaceutical formulations samples, with recovery varying from 99.96 to 106.20 % in urine samples and a relative standard deviation less than 1.04 % for PAR determination in pharmaceutical formulations. Therefore, the MWCNT‐LAC/GCE exhibits excellent sensitivity and can be used to PAR determination as a viable alternative in clinical analyzes and quality control of pharmaceutical formulations, through a simple, fast and inexpensive methodology.     

Bioelectroanalytical Determination of Rutin Based on bi‐Enzymatic Sensor Containing Iridium Nanoparticles in Ionic Liquid Phase Supported in Clay

A matrix comprising iridium nanoparticles and 1‐butyl‐3‐methylimidazolium tetrafluoroborate ionic liquid (Ir‐BMI.BF₄) supported in montmorillonite (MMT) was obtained through an efficient incorporation process. This modified clay matrix (Ir‐BMI.BF₄‐MMT) was used for the immobilization of the enzymes laccase (LAC) and polyphenol oxidase (PPO) and employed in the construction of a bi‐enzymatic biosensor for determination of rutin by square‐wave voltammetry. Under optimized conditions, the analytical curve showed a linear range for rutin concentrations from 9.17×10^?8 to 3.10×10^?6 mol L^?1 with a detection limit of 3.09×10^?8 mol L^?1. The method was successfully applied to the determination of rutin content in pharmaceutical samples.     

Electrochemical Detection of Rutin on Mg2Al‐Cl Layered Double Hydroxide Modified Carbon Ionic Liquid Electrode

In this paper a Mg₂Al‐Cl layered double hydroxide (Mg₂Al‐LDH) modified carbon ionic liquid electrode (CILE) was prepared and further used for the electrochemical detection of rutin. Cyclic voltammograms of rutin on Mg₂Al‐LDH/CILE were recorded with a pair of well‐defined redox peaks appeared in pH 2.5 phosphate buffer solution, which was ascribed to the electrochemical reaction of rutin. Due to the presence of Mg₂Al‐LDH on the electrode surface, the redox peak currents increased greatly and the electrochemical parameters were calculated. Under the optimal conditions the oxidation peak current was proportional to rutin concentration in the range from 0.08 μmol L^‐1 to 800.0 μmol L^‐1 with the detection limit on 0.0255 μmol L^‐1 (3σ). The fabricated electrode showed good reproducibility and stability, which was successfully applied to rutin tablet samples determination.     

Selective Determination of Verapamil in Pharmaceutics and Urine Using a Boron‐doped Diamond Electrode Coupled to Flow Injection Analysis with Multiple‐pulse Amperometric Detection

This work presents a simple and low‐cost method for fast and selective determination of Verapamil (VP) in tablets and human urine samples using a boron‐doped diamond working electrode (BDD) coupled to a flow injection analysis system with multiple pulse amperometric detection (FIA‐MPA). The electrochemical behaviour of VP in 0.1 mol L⁻¹ sulfuric acid showed three merged oxidation peaks at around +1.4 V and upon reverse scan, one reduction peak at 0.0 V (vs. Ag/AgCl). The MPA detection was performed applying a sequence of three potential pulses on BDD electrode: (1) at +1.6 V for VP oxidation, (2) at +0.2 V for reduction of the oxidized product and (3) at +0.1 V for cleaning of the working electrode surface. The FIA system was optimized with injection volume of 150 μL and flow rate of 3.5 mL min⁻¹. The method showed a linear range from 0.8 to 40.0 μmol L⁻¹ (R>0.99) with a low limit of detection of 0.16 μmol L⁻¹, good repeatability (RSD<2.2 %; n=10) and sample throughput (45 h⁻¹). Selective determination of VP in urine was performed at+0.2 V due to absence of interference from ascorbic and uric acids in this potential. The addition‐recovery tests in both samples were close to 100 % and the results were similar to an official method.     

Electrochemical Sensor Based on Gold Nanoparticles Stabilized in Poly(Allylamine hydrochloride) for Determination of Vanillin

Gold nanoparticles stabilized in poly(allylamine hydrochloride) (AuNP‐PAH) were synthesized, characterized and applied in the development of a new sensor for the determination of vanillin by square‐wave voltammetry. Under optimized conditions, the calibration curve showed a linear range for vanillin of 0.90 to 15.0 µmol L^?1, with a limit of detection of 55 nmol L^?1. The sensor demonstrated acceptable selectivity and stability, as well as good intra‐day and inter‐day repeatability and electrode‐to‐electrode repeatability (with relative standard deviations of 3.5, 4.5 and 3.9 %, respectively). The sensor was successfully applied in the determination of vanillin in different commercial samples.     

Quantitative Biodetection of Anticancer Drug Rituxan with DNA Biosensor Modified PAMAM Dendrimer/Reduced Graphene Oxide Nanocomposite

PAMAM dendrimer/reduced graphene oxide nanocomposite modified pencil graphite electrode (PAMAM/RGO/PGE) was used to fabricate an electrochemical DNA biosensor for determination of Rituxan (RTX) at low concentrations, for the first time. The fabricated biosensor was characterized with FE‐SEM, EIS, and CV techniques. The ds‐DNA/PAMAM/RGO/PGE was used as a working electrode to study the interaction between the RTX and salmon sperm ds‐DNA by DPV technique. Because of the interaction between the drug and DNA leads to a decrease in the guanine oxidation peak current, it was used as an indicator for the determination of the RTX. Under the optimized experimental conditions, a wide linear relationship between RTX concentration and guanine signal was obtained within the range of 7.0 to 60.0 μmol L⁻¹ and 60.0 to 300.0 μmol L⁻¹ with a low detection limit (0.56 μmol L⁻¹). To clarify the interaction mechanism between the RTX and the ds‐DNA, DPV and UV‐Vis measurements were used. The reproducibility, stability, and performance of the constructed biosensor was examined by quantitative measuring RTX in pharmaceutical and human serum samples with good precision (RSD; 2.0–6.0 %) and acceptable recoveries (100.04–101.95 %).     

Magnetite Nanoparticles Bonded Carbon Quantum Dots Magnetically Confined onto Screen Printed Carbon Electrodes and their Performance as Electrochemical Sensor for NADH

Hybrid magnetite/carbon quantum dots (MagNP/C‐dots) were prepared and their characterization performed by high resolution transmission electron microscopy (HR‐TEM), X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS). Because of their suitable magnetization and electrochemical properties, they were used as versatile electrode modifiers after magnetically confining onto screen printed carbon electrodes (SPE), with the aid of a miniature external magnet. The reported strategy introduces a convenient procedure for assembling modified electrodes, since the nanoparticles can be easily released by removing the magnet. The non‐enzymatic magnetic biosensor showed excellent performance in the determination of NADH at the concentration range 2×10⁻⁷ to 5×10⁻⁶ mol L⁻¹, exhibiting a sensitivity of 0.15 μmol L⁻¹ and detection limit of 20 nmol L⁻¹. The MagNP/C‐dots/SPE sensor was also successfully applied for the determination of NADH in serum samples. The interference of typical biological molecules has also been investigated.     

Carbon‐nanotube Modified Screen‐printed Electrode for the Simultaneous Determination of Nitrite and Uric Acid in Biological Fluids Using Batch‐injection Amperometric Detection

A portable electroanalytical system applied for rapid and simultaneous determination of uric acid (UA) and nitrite (NIT) in human biological fluids (urine, saliva and blood) is reported. The system is based on batch‐injection analysis with multiple‐pulse amperometric (BIA‐MPA) detection using screen‐printed electrodes (SPEs) modified with multi‐walled carbon nanotubes. Sample dilution in optimized electrolyte (0.1 mol L⁻¹ Britton‐Robinson buffer pH 2) followed by injection of 100 μL on the electrode surface using an electronic micropipette is performed. UA is detected at +0.45 V and both UA+NIT at +0.70 V. Linear calibration plots for UA and NIT were obtained over the range of 1–500 μmol L⁻¹ with detection limits of 0.05 and 0.06 μmol L⁻¹, respectively. For comparison, a differential‐pulse voltammetric (DPV) method was optimized, and linear calibration plots for UA and NIT were obtained over range of 1–30 μmol L⁻¹ and 1–40 μmol L⁻¹ with detection limits of 0.1 and 0.3 μmol L⁻¹, respectively. BIA‐MPA is highly precise (RSD<1.3 %), fast (160 h⁻¹) and free from sample‐matrix interferences as recovery values ranged from 77 to 121 % for spiked samples (short contact time of sample aliquot with SPE). Contrarily, recovery tests conducted using DPV did not provide adequate recovery values (>150 %), probably due to the longer contact time of the SPE with the biological samples during analysis leading to a severe interference of sample matrices.     

Amperometric Biosensor for Detection of Phenolic Compounds Based on Tyrosinase,N‐Acetyl‐L‐cysteine‐capped Gold Nanoparticles and Chitosan Nanocomposite

A novel biosensor was fabricated based on the immobilization of tyrosinase and N ‐acetyl‐L ‐cysteine‐capped gold nanoparticles onto the surface of the glassy carbon electrode via the film forming by chitosan. The NAC‐AuNPs (N ‐acetyl‐L ‐cysteine‐capped gold nanoparticles) with the average size of 3.4 nm had much higher specific surface area and good biocompatibility, which were favorable for increasing the immobilization amount of enzyme, retaining the catalytic activity of enzyme and facilitating the fast electron transfer. The prepared biosensor exhibited suitable amperometric responses at −0.2 V for phenolic compounds vs. saturated calomel electrode. The parameters of influencing on the working electrode such as pH , temperature, working potential were investigated. Under optimum conditions, the biosensor was applied to detect catechol with a linear range of 1.0 × 10⁻⁷ to 6.0 × 10⁻⁵ mol•L⁻¹ , and the detection limit of 5.0 × 10⁻⁸ mol•L⁻¹ (S /N =3). The stability and selectivity of the proposed biosensor were also evaluated.     

Development of a Sensor Based on Modified Carbon Paste with Com Iron(III) Protoporphyrin Immobilized on SiNbZn Silica Matrix for L‐tryptophan Determination

In this work, SiO₂/Nb₂O₅/ZnO prepared by the sol‐gel processing method was used as substrate base for immobilization of the protoporphyrin‐IX ion. Iron(III) ion was inserted into the porphyrin ring (SiNbZn‐PPFe). A simple square wave voltammetry method based on a composite sensor carbon paste electrode of this material,designed as EPC‐SiNbZn‐PPFe, was developed and validated successfully for the determination of L‐tryptophan (Trp). The optimum conditions were obtained by using sensor modified with 18.00 mg SiNbZn‐PPFe material, 12.00 mg graphite powder and 6.0 μL mineral oil and phosphate buffer 0.3 mol L⁻¹ pH 7.0. The sensitivity of the sensor was found to be 0.523 AL mol ⁻¹, linear range from 10 to 70 μmol L⁻¹ and limit of detection of 3.28 μmol L⁻¹. Therefore, the developed method was successfully applied for the Trp determination in real samples of pharmaceutical formulation and can be used for routine quality control pharmaceutical formulations containing Trp.     

Bi‐enzyme Electrochemical Sensor for Selective Determination of Organophosphorus Pesticides with Phenolic Leaving Groups

An amperometric bi‐enzyme sensor for detection of organophosphorus pesticides (OPs) with phenolic leaving groups, which are not electroactive, is presented in this work. The biosensing platform was created by a simple, controllable, and reproducible one‐step electrodeposition onto the surface of a glassy carbon electrode of a chitosan bionanocomposite with entrapped carboxylated multi walled carbon nanotubes, organophosphorus hydrolase (OPH), and horseradish peroxidase (HRP). The OPs determination involved a sequence of OPH and HRP‐catalyzed reactions resulting in phenolic radicals production, which were quantified by registering the current of their reduction at a potential of −50 mV vs. Ag, AgCl/KCl_sat.The developed sensor was applied for the determination of prothiofos, as an example. At optimized conditions (pH 7.25 and H₂O₂ concentration 200 μmol L⁻¹), a LOD as low as 0.8 μmol L⁻¹ was attained, while the linear concentration range was extended from 2.64 μmol L⁻¹ up to 35 μmol L⁻¹. The main advantage of the proposed bi‐enzyme sensor is its selectivity toward the OPs with phenolic leaving groups, excluding the interference of the nitrophenyl‐substituted OPs.     

Differential Pulse Voltammetric Determination of 2‐Methyl‐4,6‐Dinitrophenol using Bismuth Bulk Electrode

This work reports the application of bismuth bulk electrode (BiBE) for the determination of 2‐methyl‐4,6‐dinitrophenol (MDNP) by differential pulse voltammetry (DPV) in Britton‐Robinson buffer of pH 12.0 as an optimal medium. BiBE was prepared by transferring molten bismuth into a glass tube under constant stream of nitrogen. The linear concentration dependences were measured from 1 to 10 μmol ? L^?1 and from 10 to 100 μmol ? L^?1 by using optimum accumulation potential of ?0.7 V and optimum accumulation time 30 s. Under these conditions limit of determination and limit of quantification was 0.45 and 1.5 μmol ? L^?1, respectively. The developed method was successfully applied for the analysis of tap water as a model sample.     

Fabrication of Co3O4 particles-modified multi-walled carbon nanotube and poly(phenosafranine) composite electrode for selective and sensitive rutin detection

The preparation and characterisation of a new composite electrode with Co₃O₄ particles-modified multi-walled carbon nanotube (MWCNT) and poly(phenosafranine), as well as its novel application for the voltammetric detection of rutin was described. The resulting composite electrode was characterised using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). In the optimised experimental conditions, the oxidation peak current (I_pa) of rutin showed a linear increase in concentration, between 0.008–0.6 and 0.80–6.0 μmol L⁻¹, with a detection limit of 0.00379 μmol L⁻¹. Due to its good selectivity and stability, the composite electrode was successfully applied in detecting rutin in pharmaceutical formulations.     

Simultaneous Electrochemical Determination of Hydroquinone and Bisphenol A using a Carbon Paste Electrode Modified with Silver Nanoparticles

In this paper, a rapid and sensitive modified electrode for the simultaneous determination of hydroquinone (HQ) and bisphenol A (BPA) is proposed. The simultaneous determination of these two compounds is extremely important since they can coexist in the same sample and are very harmful to plants, animals and the environment in general. A carbon paste electrode (CPE) was modified with silver nanoparticles (nAg) and polyvinylpyrrolidone (PVP). The PVP was used as a reducing and stabilizing agent of nAg from silver nitrate in aqueous media. The nAg‐PVP composite obtained was characterized by transmission electron microscopy and UV‐vis spectroscopy. The electrochemical behavior of HQ and BPA at the nAg‐PVP/CPE was investigated in 0.1 mol L⁻¹ B−R buffer (pH 6.0) using cyclic voltammetry (CV) and square wave voltammetry (SWV). The results indicate that the electrochemical responses are improved significantly with the use of the modified electrode. The calibration curves obtained by SWV, under the optimized conditions, showed linear ranges of 0.09–2.00 μmol L⁻¹ for HQ (limit of detection 0.088 μmol L⁻¹) and 0.04–1.00 μmol L⁻¹ for BPA (limit of detection 0.025 μmol L⁻¹). The modified electrode was successfully applied in the analysis of water samples and the results were comparable to those obtained using UV‐vis spectroscopy.     

Rapid Determination of Hydrogen Peroxide in Milk with Non-enzymatic Amperometric Sensor Based on Porous Gold Modified Screen-printed Electrode in Online Dialysis System

A rapid method for determination of hydrogen peroxide based on a non-enzymatic amperometric sensor with an online dialysis unit was developed for analysis of sample with complicated matrices. A porous gold modified gold screen-printed electrode was prepared by a simple method using 9 V-battery electrodeposition. Based on data from amperometry, a linear dynamic range can be obtained in the range of 10 to 1000 μmol L⁻¹, with a limit of detection of 0.37 μmol L⁻¹. The proposed system was successfully applied to determine H₂O₂ in various milk samples with rapid analysis, high electrode stability, and excellent repeatability (1.95 % RSD with 100 replications).     

Voltammetric Method for the Simultaneous Determination of Melatonin and Pyridoxine in Dietary Supplements Using a Cathodically Pretreated Boron‐doped Diamond Electrode

The simultaneous voltammetric determination of melatonin (MT) and pyridoxine (PY) has been carried out at a cathodically pretreated boron‐doped diamond electrode. By using cyclic voltammetry, a separation of the oxidation peak potentials of both compounds present in mixture was about 0.47 V in Britton‐Robinson buffer, pH 2. The results obtained by square‐wave voltammetry allowed a method to be developed for determination of MT and PY simultaneously in the ranges 1–100 μg mL⁻¹ (4.3×10⁻⁶–4.3×10⁻⁴ mol L⁻¹) and 10–175 μg mL⁻¹ (4.9×10⁻⁵–8.5×10⁻⁴ mol L⁻¹), with detection limits of 0.14 μg mL⁻¹ (6.0×10⁻⁷ mol L⁻¹) and 1.35 μg mL⁻¹ (6.6×10⁻⁶ mol L⁻¹), respectively. The proposed method was successfully to the dietary supplements samples containing these compounds for health‐caring purposes.     

Simultaneous Determination of Hydroquinone and Catechol by Differential Alternative Pulses Voltammetry

The second order voltammetric technique of high resolution, Differential Alternative Pulses Voltammetry (DAPV), was applied for the simultaneous determination of hydroquinone (HQ) and catechol (CC) on bare spectroscopic graphite electrode. Well resolved anodic and cathodic peaks situated on both sides of the zero line were obtained, while the differential pulse voltammograms were overlapped. The linear concentration range for HQ and CC quantification by DAPV was extended up to 20 μmol L⁻¹ for both the isomers. The sensitivity of the determination was found to be 6.00 μA L μmol⁻¹ and 3.61 μA L μmol⁻¹, while the limit of detection reached was 0.2 μmol L⁻¹ and 0.5 μmol L⁻¹ for HQ and CC, respectively. No interference was observed from the commonly coexisting organic species such as resorcinol, phenol and p‐benzoquinone. The great resolution power of DAPV permitted obtaining excellent results without any electrode modification and any mathematical data processing.     

A MnOOH‐Polyaniline Nanocomposite Modified Gold Electrode for Electrochemical Sensing of Nitrite

A novel non‐enzymatic nitrite sensor was fabricated by immobilizing MnOOH‐PANI nanocomposites on a gold electrode (Au electrode). The morphology and composition of the nanocomposites were investigated by transmission electron microscopy (TEM ) and Fourier transform infrared spectrum (FTIR ). The electrochemical results showed that the sensor possessed excellent electrocatalytic ability for NO₂⁻ oxidation. The sensor displayed a linear range from 3.0 μmol•L⁻¹ to 76.0 mmol•L⁻¹ with a detection limit of 0.9 μmol•L⁻¹ (S/N = 3), a sensitivity of 132.2 μA •L•mol⁻¹•cm⁻² and a response time of 3 s. Furthermore, the sensor showed good reproducibility and long‐term stability. It is expected that the MnOOH‐PANI nanocomposites could be applied for more active sensors and used in practice for nitrite sensing.