Novel and Highly Sensitive Electrochemical Sensor for the Determination of Oxytetracycline Based on Fluorine-Doped Activated Carbon and Hydrophobic Deep Eutectic Solvents.

Residues of oxytetracycline (OTC), a veterinary antibiotic and growth promoter, can be present in animal-derived foods; their consumption is harmful to human health and their presence must therefore be detected and regulated. However, the maximum residue limit is low, and consequently highly sensitive and accurate detectors are required to detect the residues. In this study, a novel highly sensitive electrochemical sensor for the detection of OTC was developed using a screen-printed electrode modified with fluorine-doped activated carbon (F-AC/SPE) combined with a novel deep eutectic solvent (DES). The modification of activated carbon by doping with fluorine atoms (F-AC) enhanced the adsorption and electrical activity of the activated carbon. The novel hydrophobic DES was prepared from tetrabutylammonium bromide (TBABr) and a fatty acid (malonic acid) using a green synthesis method. The addition of the DES increased the electrochemical response of F-AC for OTC detection; furthermore, it induced preconcentration of OTC, which increased its detectability. The electrostatic interactions between DES and OTC as well as the adsorption of OTC on the surface of the modified electrode through H-bonding and π-π interactions helped in OTC detection, which was quantified based on the decrease in the anodic peak potential (E pa = 0.3 V) of AC. The electrochemical behavior of the modified electrode was investigated by cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy. Under optimum conditions, the calibration plot of OTC exhibited a linear response in the range 5-1500 µg L-1, with a detection limit of 1.74 µg L-1. The fabricated electrochemical sensor was successfully applied to determine the OTC in shrimp pond and shrimp samples with recoveries of 83.8-100.5% and 93.3-104.5%, respectively. In addition to the high sensitivity of OTC detection, the proposed electrochemical sensor is simple, cost-effective, and environmentally friendly.

Electrochemical detection of methyl parathion using calix[6]arene/bismuth ferrite/multiwall carbon nanotube-modified fluorine-doped tin oxide electrode.

A highly sensitive electrochemical sensor using a calix[6]arene/bismuth ferrite/multiwall carbon nanotube-modified fluorine-doped tin oxide electrode (CA6/BFO/MWCNTs/FTO) was fabricated for the detection of methyl parathion. The MWCNTs, BFO, and CA6 were consecutively cast onto the FTO electrode surface to enhance the surface area, electron transfer, and selectivity of sensors. The electrochemical behavior of CA6/BFO/MWCNTs/FTO was studied via cyclic voltammetry and electrochemical impedance spectroscopy. MP was detected via cyclic voltammetry in a phosphate buffer solution at pH 7.0. The working principle of the sensor involves a linear decrease in the anodic peak current of BFO with increasing MP concentration. The linear working ranges are 0.005-0.05 nM and 0.07-1.5 nM. The CA6/BFO/MWCNTs/FTO sensor provides a low detection limit (S/N = 3) of 5 pM and a high electrochemical sensitivity of 1.23 A µM-1 cm-2. The fabricated sensor was successfully applied to assess the presence and amount of MP in vegetables and fruits (recoveries of 82.0-106.8%), with results comparable to high-performance liquid chromatography.

Facile liquid colorimetric sensor using high-density deep eutectic solvent for trace detection and speciation of iron in milk.

An efficient colorimetric sensor was developed using a high-density deep eutectic solvent (HD-DES) for the trace detection and speciation of iron in various milk samples. A liquid colorimetric probe was fabricated by dissolving ferrozine (FZ) in HD-DES prepared from TBABr and PBA. The prederivatization of Fe2+ via complexation with FZ on the HD-DES/FZ probe provided the [Fe(FZ)3]4- complex, which led to a color change from pale yellow to purple before it was simultaneously extracted by HD-DES. The Fe3+ content was calculated by subtracting the amount of Fe2+ from the total Fe content following the reduction of Fe3+ to Fe2+ by L-ascorbic acid in an acid buffer. Under the optimized conditions, the proposed colorimetric sensor exhibited appreciable linearity in the concentration range of 0.003-0.04 mg L-1, a low limit of detection (0.95 µg L-1), high enrichment factor (50), and outstanding repeatability. The liquid colorimetric probe was successfully applied for the determination and speciation of iron in milk samples, and the results were compared with those obtained using the standard atomic absorption spectrometry method. Moreover, quantitative analysis was performed on a smartphone using the Image J application to estimate the color intensity change, which eliminated the requirement of sophisticated scientific instruments.

A novel liquid colorimetric probe for highly selective and sensitive detection of lead (II).

A novel liquid colorimetric sensor based on deep eutectic solvent (DES) was developed for the preconcentration and detection of Pb2+ in fruit juice, milk and cereal samples. The colorimetric probe was simply fabricated by adding dithizone (DZ) into DES, prepared from choline chloride and phenol. Pb2+ was formed complex with DZ in the probe, providing hydrophobic complex of [Pb-DZ] which was simultaneously extracted into DES and the color was changed from light orange to carmine red. The enriched [Pb-DZ] in DES was detected using spectrophotometer at 520 nm and naked-eyes. In addition, a smartphone in combination with an Image J program was used as an alternative detection device. Under optimal conditions, the enrichment factor was 92 with LOD of 2.1 µg L-1 and the linear range was 0.007-0.075 mg L-1. The proposed liquid colorimetric sensor was successfully applied for Pb2+ detection in various food samples and the results were in good agreement with those obtained by FAAS. The advantages of this method are simple, rapid, environmental friendly and low cost.

An electrochemical sensor for voltammetric detection of ciprofloxacin using a glassy carbon electrode modified with activated carbon, gold nanoparticles and supramolecular solvent.

A highly sensitive and novel electrochemical sensor for ciprofloxacin (CIP) has been developed using gold nanoparticles deposited with waste coffee ground activated carbon on glassy carbon electrode (AuNPs/AC/GCE) and combined with supramolecular solvent (SUPRAS). The fabricated AuNPs/AC/GCE displayed good electrocatalytic activity for AuNPs. The addition of SUPRAS, prepared from cationic surfactants namely didodecyldimethylammonium bromide (DDAB) and dodecyltrimethylammonium bromide (DTAB), increased the electrochemical response of AuNPs. The detection of CIP was based on the decrease of the cathodic current of AuNPs. The electrochemical behavior of the modified electrode was investigated using cyclic voltammetry, differential pulse voltammetry and electrochemical impedance spectroscopy. Under optimum conditions, the calibration plot of CIP exhibited a linear response in the range 0.5-25 nM with a detection limit of 0.20 nM. The fabricated electrochemical sensor was successfully applied to determine CIP in milk samples with achieved recoveries of 78.6-110.2% and relative standard deviations of <8.4%. The developed method was also applied to the analysis of pharmaceutical formulation and the results were compared with high-performance liquid chromatography.Graphical abstract.

A rapid and simple method for the removal of dyes and organophosphorus pesticides from water and soil samples using deep eutectic solvent embedded sponge.

A new treatment method using a deep eutectic solvent embedded melamine sponge (DES-MS) was studied for the removal of organic pollutants from water and soil samples. Five organophosphorus pesticides (OPPs) consisting of azinphos-methyl (AZP), parathion-methyl (PRT), fenitrothion (FNT), diazinon (DIZ) and chlorpyrifos (CPF), and two dyes including acid blue 29 (AB29) and malachite green (MG) were used as the model pollutants. DESs were easily prepared from tetrabutylammonium bromide (TBABr) and various fatty acids. The synthesised DESs were loaded into the sponge before being utilized for the removal of the studied pollutants. After the removal, the residual OPPs or dyes in the supernatant was quantified by high performance liquid chromatography or derivative spectrophotometry, respectively. The proposed method was simple, rapid, environmentally friendly and effective with the removal efficiency higher than 70% for various samples. Moreover, the removal of various dyes was successfully achieved with the efficiency greater than 65% under the optimum condition.

Supramolecular Electrochemical Sensor for Dopamine Detection Based on Self-Assembled Mixed Surfactants on Gold Nanoparticles Deposited Graphene Oxide.

A new supramolecular electrochemical sensor for highly sensitive detection of dopamine (DA) was fabricated based on supramolecular assemblies of mixed two surfactants, tetra-butylammonium bromide (TBABr) and sodium dodecyl sulphate (SDS), on the electrodeposition of gold nanoparticles on graphene oxide modified on glassy carbon electrode (AuNPs/GO/GCE). Self-assembled mixed surfactants (TBABr/SDS) were added into the solution to increase the sensitivity for the detection of DA. All electrodes were characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The supramolecular electrochemical sensor (TBABr/SDS⋅⋅⋅AuNPs/GO/GCE) showed excellent electrocatalytic activity toward the oxidation of DA. Under the optimum conditions, the concentration of DA was obtained in the range from 0.02 µM to 1.00 µM, with a detection limit of 0.01 µM (3s/b). The results displayed that TBABr/SDS⋅⋅⋅AuNPs/GO/GCE exhibited excellent performance, good sensitivity, and reproducibility. In addition, the proposed supramolecular electrochemical sensor was successfully applied to determine DA in human serum samples with satisfactory recoveries (97.26% to 104.21%).

A Simple in Syringe Low Density Solvent-Dispersive Liquid Liquid Microextraction for Enrichment of Some Metal Ions Prior to Their Determination by High Performance Liquid Chromatography in Food Samples.

A simple and highly sensitive method is developed for the simultaneous determination of Ni2+, Cr2O72-, Co2+, and Hg2+ by using in syringe low density solvent-dispersive liquid liquid microextraction (ISLD-DLLME), followed by high performance liquid chromatography with a UV detector. The four metal ions were derivatized with pyrrolidine dithiocarbamate (PDC) based on complexation before their enrichment by ISLD-DLLME in which 1-octanol and methanol were used as the extraction solvent and the dispersive solvent, respectively. The extraction was performed in a commercially available syringe under vortex agitation. Phase separation was achieved without centrifugation, and the extraction phase was easily collected by moving the syringe plunger. Parameters affecting the extraction efficiency were studied and optimized. Under the optimum conditions, the four metal-PDC complexes were detected within 18 min, and ISLD-DLLME could increase the detection sensitivity in the range of 64-230 times compared to the direct HPLC analysis. The obtained limits of detection (LODs) were found to be in the range of 0.011-2.0 µg L-1. The applicability of the method is demonstrated for freshwater fish, shrimp, and shellfish samples. In addition, the results are in good agreement with those obtained by inductively-coupled plasma-optical emission spectrometry (ICP-OES).

AuNPs-LISA, an efficient detection assay for Opisthorchis viverrini (Ov) antigen in urine.

The enzyme-linked immunosorbent assay (ELISA) is currently a powerful technique for the detection of Opisthorchis viverrini antigen (OvAg) in urine samples. However, its sensitivity and analysis time need to be improved. In the present study, we aimed to improve the signal enhancing system of traditional ELISA by using gold nanoparticles (AuNPs) with peroxidase-like activity on its surface instead of the horseradish peroxidase (HRP) system. The catalytic activity of the AuNPs probe can be boosted by the gold enhancing solution and the addition of ATP. The catalytic ability of the AuNPs probe depended on the probe and the H2O2 concentration. The proposed approach can reduce the number of the traditional ELISA steps with better detection sensitivity. Interestingly, the limit of detection (LOD) of the test was 23.4 ng mL-1, substantially lower than the 93.8 ng mL-1 for the traditional ELISA. The AuNPs-LISA assay showed higher sensitivity and specificity, 93.81% and 91.34%, respectively, compared to the traditional ELISA. The proposed assay was successfully applied for the detection of OvAg in urine samples. This will provide an effective tool for the detection, control and elimination of human opisthorchiasis.

Vesicular supramolecular solvent-based microextraction followed by high performance liquid chromatographic analysis of tetracyclines.

A simple liquid phase microextraction (LPME) using a vesicular supramolecular solvent (SUPRAS) as the green extraction solvent has been developed for the enrichment of tetracycline antibiotics (TCs) prior to their analysis by reversed phase high performance liquid chromatography. SUPRAS was prepared from a mixture of cationic surfactants: didodecyldimethylammonium bromide (DDAB) and dodecyltrimethylammonium bromide (DTAB) under salt (NaCl) addition for coacervation. The LPME is based on electrostatic interaction between TCs and SUPRAS under salt induced phase separation. Under the optimum conditions, high enrichment factors (EFs) of 48-198, low limits of detection (LODs) in the range of 0.7-3.4 µg L-1 and good precisions with relative standard deviations (RSDs) less than 7.85% were obtained. The applicability of the proposed method was demonstrated for the determination of TCs in milk, egg and honey samples. In addition, morphology, particle sizes and zeta potential of SUPRAS were also investigated.

Zinc/Aluminium⁻Quinclorac Layered Nanocomposite Modified Multi-Walled Carbon Nanotube Paste Electrode for Electrochemical Determination of Bisphenol A.

This paper presents the application of zinc/aluminium-layered double hydroxide-quinclorac (Zn/Al-LDH-QC) as a modifier of multiwalled carbon nanotubes (MWCNT) paste electrode for the determination of bisphenol A (BPA). The Zn/Al-LDH-QC/MWCNT morphology was examined by a transmission electron microscope and a scanning electron microscope. Electrochemical impedance spectroscopy was utilized to investigate the electrode interfacial properties. The electrochemical responses of the modified electrode towards BPA were thoroughly evaluated by using square-wave voltammetry technique. The electrode demonstrated three linear plots of BPA concentrations from 3.0 × 10-8⁻7.0 × 10-7 M (R² = 0.9876), 1.0 × 10-6⁻1.0 × 10-5 M (R² = 0.9836) and 3.0 × 10-5⁻3.0 × 10-4 M (R² = 0.9827) with a limit of detection of 4.4 × 10-9 M. The electrode also demonstrated good reproducibility and stability up to one month. The presence of several metal ions and organic did not affect the electrochemical response of BPA. The electrode is also applicable for BPA determination in baby bottle and mineral water samples with a range of recovery between 98.22% and 101.02%.

A novel colorimetric paper sensor based on the layer-by-layer assembled multilayers of surfactants for the sensitive and selective determination of total antioxidant capacity.

Herein, a new colorimetric paper sensor based on the layer-by-layer assembled multilayers of a cationic surfactant, tetrabutylammonium bromide (TBABr), and an anionic surfactant, sodium dodecyl sulfate (SDS), modified on filter paper was developed for the determination of total antioxidant capacity (TAC). In this study, gallic acid (GA) was used as the antioxidant standard. The fabricated (TBABr/SDS)3/PAD was loaded with Fe3+ ions to obtain Fe3+/(TBABr/SDS)3/PAD, exhibiting high selectivity for the detection of GA when compared with the case of other metal ions. The interaction between GA and the Fe3+/(TBABr/SDS)3/PAD sensor occurred rapidly, and the colorimetric paper sensor changed from yellow to purple immediately. The quantitative detection of GA was enabled by taking an image using an ordinary smartphone and applying the ImageJ software based on the change in color. Under optimum conditions, a linear response was obtained between the change in the color of the sensor and the TAC value expressed in terms of gallic acid equivalents. The linear range was from 0.50 µM to 6.50 mM with the detection limit of 0.35 µM. The colorimetric paper sensor was applied to detect the TAC in three kinds of green tea and vegetable samples, which provided the good recoveries of 86.0-109.9%. The proposed sensor is simple, cheap, equipment-free, rapid and environmentally friendly. In addition, the colorimetric sensor Fe3+/(TBABr/SDS)3/PAD has potential applicability for TAC detection in real food samples.

Two-step microextraction combined with high performance liquid chromatographic analysis of pyrethroids in water and vegetable samples.

Dispersive liquid microextraction (DLME) combined with dispersive µ-solid phase extraction (D-µ-SPE) has been developed as a new approach for the extraction of four pyrethroids (tetramethrin, fenpropathrin, deltamethrin and permethrin) prior to the analysis by high performance liquid chromatography (HPLC) with UV detection. 1-Octanol was used as the extraction solvent in DLME. Magnetic nanoparticles (Fe3O4) functionalized with 3-aminopropyl triethoxysilane (APTS) were used as the dispersive in DLME and as the adsorbent in D-µ-SPE. The extracted pyrethroids were separated within 30 min using isocratic elution with acetonitrile:water (72:28). The factors affecting the extraction efficiency were investigated. Under the optimum conditions, the enrichment factors were in the range of 51-108. Linearity was obtained in the range 0.5-400 ng mL(-1) (tetramethrin) and 5-400 ng mL(-1) (fenpropathrin, deltamethrin and permethrin) with the correlation coefficients (R(2)) greater than 0.995. Detection limits were 0.05-2 ng mL(-1) (water samples) and 0.02-2.0 ng g(-1) (vegetable samples). The relative standard deviations of peak area varied from 1.8 to 2.5% (n=10). The extraction recoveries of the four pyrethroids in field water and vegetable samples were 91.7-104.5%. The proposed method has high potential for use as a sensitive method for determination of pyrethroid residues in water and vegetable samples.