Current Analytical Chemistry - Volume 15, Issue 1, 2019

Objectives: To develop gold nanostructure on Pt substrate for detection of Hg(II) in environment by DPASV technique. Its structure was characterized by SEM and electrochemical performance was evaluated. The fabricated electrodes were used to measure Hg(II) samples in a concentration range from 2 to 100 ppb. Method: Gold nanoparticles (AuNPs) and AuNDs were deposited on platinum substrates by applying potential of +0.50 V for AuNPs deposition and galvanostatic of -50 mA for AuNDs/Pt in a solution 20 mM HAuCl4, 10 mM KI, 5 mM NH4Cl and 0.5 M H2SO4.The electrochemical behaviors of AuNPs/Pt and AuNDs/Pt were examined using CV in 5 mM K3[Fe(CN)6]/0.1 M PBS solution, pH = 7 and the detection of Hg(II) was performed by DPASV. Results: The SEM images show that largest surface area was obtained at 120 s depositing time. Effective surface areas (ESA) of AuNPs and AuNDs are about 1.39 and 5.19 times higher than electrode geometric area. Calibration curves achieved with R2= 0.9978; 0.9975; 0.9973 and LOD= 0.55; 0.105 and 0.042 ppb for Au disk, AuNPs/Pt and AuNDs/Pt respectively. Reproducibility with ten measurements of 10, 40 and 80 ppb of Hg(II), RSD (%) were 3.5, 2.8 and 1.5 respectively. No significant effect on Hg(II) signals was found except CCu(II) with 100 times higher than CHg(II). Comparison with AAS, data difference between the two techniques is acceptable, at only 4.34%. Conclusion: LOD for Hg(II) detection by AuNDs/Pt achieved 0.042 ppb with linear range of 2.0–100 ppb. Combining with a laboratory constructed galvano-potentiostat, it can be used in on-site measurement.

Background: The consumption of antioxidants, including phenolic compounds, is considered important for preventing the oxidative damage diseases and ageing. The total polyphenol content (TPC) is the parameter used to estimate the quality of plant-derived products. Methods: Phenol oxidase activity of green bean (Phaseolus vulgaris) crude extract (in the presence of hydrogen peroxide) and banana (Musa sp.) pulp crude extract has been studied spectrophotometrically using catechol, gallic acid, caffeic acid, ferulic acid, and quercetin as substrates. All studied compounds have been oxidized in the presence of green bean crude extract and hydrogen peroxide; all studied compounds except ferulic acid have been oxidized in the presence of banana pulp crude extract. Michaelis constants (Km) and maximum reaction rates (Vmax) have been determined for oxidation in the presence of green bean crude extract and hydrogen peroxide (Km are 3.80-4 M, 1.60-3 M, 2.20-4 M, 2.30-4 M, 1.40-4 M and Vmax are 0.046 min-1, 0.102 min-1, 0.185 min-1, 0.053 min-1, 0.041 min-1 for catechol, gallic acid, caffeic acid, ferulic acid, and quercetin, respectively) and for oxidation in the presence of banana pulp crude extract (Km are 1.60-3 M, 3.80-3 M, 2.20-3 M, 4.20-4 M and Vmax are 0.058 min-1, 0.025 min-1, 0.027 min-1, 0.015 min-1 for catechol, gallic acid, caffeic acid, and quercetin, respectively). The influence of 3-methyl-2-benzothiazolinone hydrazone (MBTH) on the oxidation reactions kinetics has been studied: Michaelis constants values decrease and maximum reaction rates increase, which contributes to the increase in sensitivity of the determination. Results: Kinetic procedures of Total Polyphenol Content (TPC) determination using crude plants extracts in the presence of MBTH have been proposed (time of analysis is 1 min). For gallic acid (used as a standard for TPC determination) detection limit is 5.30-5 M, quantitation limit is 1.80-4 M, and linear range is 1.80-4 - 1.30-3 M for green bean crude extract; detection limit is 2.90-5 M, quantitation limit is 9.50-5 M, and linear range is 9.50-5 - 2.40-3 M for banana pulp crude extract. Proposed procedures are characterized by higher interference thresholds for sulfites, ascorbic acid, and citric acid compared to pure enzymes (horseradish peroxidase and mushroom tyrosinase) in the same conditions. Compared with standard Folin-Ciocalteu (FC) method the procedures described in this work are also characterized by less interference and more rapid determination. Conclusion: The procedures have been applied to TPC determination in tea, coffee, and wine samples. The results agree with the FC method for tea and coffee samples and are lower for wine samples, probably, due to sulfites interference.

Background: Gentianella acuta (Michx.) Hulten is an important type of medicinal plant found in several Chinese provinces. It has been widely used in folk medicine to treat various illnesses. However, there is not enough detailed information about the chemical constituents of this plant or methods for their content determination. Objective: The focus of this work is the isolation and characterization of the major chemical constituents of Gentianella acuta, and developing an analytical method for their determination. Methods: The components of Gentianella acuta were isolated using (1) ethanol extraction and adsorption on macroporous resin. (2) and ethyl acetate extraction and high speed countercurrent chromatography. A HPLC-DAD method was developed using a C18 column and water-acetonitrile as the mobile phase. Based on compound polarities, both isocratic and gradient elution methods were developed. Results: A total of 29 compounds were isolated from this plant, of which 17 compounds were isolated from this genus for the first time. The main components in this plant were found to be xanthones. The HPLC-DAD method was developed and validated for their determination, and found to show good sensitivity and reliability. Conclusion: The results of this work add to the limited body of work available on this important medicinal plant. The findings will be useful for further investigation and development of Gentianella acuta for its valuable medicinal properties.

Background: Hydrophilic Interaction Liquid Chromatography (HILIC) orthogonal to conventional reversed phase High-Performance Liquid Chromatography (HPLC) mode allowing separation of polar compounds. HILIC has been reported to be an alternative to normal phase liquid chromatography, yet the separation mechanism reported in HILIC is much more complicated than that in normal phase liquid chromatography. Objective: To investigate the effect of water layer thickness on silica gel and the amount of ammonium ions present within the buffer on retention mechanism in hydrophilic interaction chromatography. Methodology: A test system was designed which used weak acids, neutrals and weak bases as probes with three different strengths (5, 10 and 20 mM) of ammonium acetate, ammonium formate and ammonium propionate as the counter-ions to compete with the test probes with ionised silanol groups and water present in the stationary phase. A Kromasil 60-5SIL column (150 mm×4.6 mm×4 μm, pore size 60Å) was used as the stationary phase to perform the study. Results: Retention times were examined for the test probes at 90% acetonitrile (ACN) with 10% of 5, 10 and 20 mM of ammonium acetate, ammonium formate and ammonium propionate. As the buffer strength increases, the thickness of the water layer on the surface of the silica gel increases and also the repulsion between ionized silanol groups and acidic test probes will decrease. On the other hand, such increase in buffer strength will increase the competition between the ammonium ions and basic test probes. In addition, the hydration energy of buffer’s counter ions and hydrophilicity may be important in retention mechanism in HILIC mode. Conclusion: At 20 mM buffer strength acidic probes with low log P values retain more due to reduced repulsion by silanol groups, while basic probes retention time will decrease due to increased competition from ammonium counter ions. However, in 5 mM buffer strength basic probes with low logP value will be retained longer, while acidic probes will be eluted earlier due to the repulsion between ionized acids and ionized silanol groups.

Background: Morin has many pharmacological functions including antioxidant, anticancer, anti-inflammatory, and antibacterial effects. It is commonly used in the treatment of antiviral infection, gastropathy, coronary heart disease and hepatitis B in clinic. However, researches have shown that morin is likely to show prooxidative effects on the cells when the amount of treatment is at high dose, leading to the decrease of intracellular ATP levels and the increase of necrosis process. Therefore, it is necessary to determine the concentration of morin in biologic samples. Method: Novel water-soluble and green nitrogen and sulfur co-doped carbon dots (NSCDs) were prepared by a microwave heating process with citric acid and L-cysteine. The fluorescence spectra were collected at an excitation wavelength of 350 nm when solutions of NSCDs were mixed with various concentrations of morin. Results: The as-prepared NSCDs were characterized by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The fluorescence intensity of NSCDs decreased significantly with the increase of morin concentration. The fluorescence intensity of NSCDs displayed a linear response to morin in the concentration 0.10-30 μM with a low detection limit of 56 nM. The proposed fluorescent probe was applied to analysis of morin in human body fluids with recoveries of 98.0-102%. Conclusion: NSCDs were prepared by a microwave heating process. The present analytical method is sensitive to morin. The quenching process between NSCDs and morin is attributed to the static quenching. In addition, the cellular toxicity on HeLa cells indicated that the as-prepared NSCDs fluorescent probe does not show obvious cytotoxicity in cell imaging. Our proposed method possibly opens up a rapid and nontoxic way for preparing heteroatom doped carbon dots with a broad application prospect.

Background: Nitrites can exert acute toxic effects in humans. It is widely used as a preservative in dairy and meat products. The nitrites form N-nitrosamines, which are potential carcinogens and cause detrimental health effects. Herein we report a disposable graphite screen-printed sensor developed using silver metal nano particle embedded chitosan composite in the quantification of nitrite at trace level. Methods: Conventional methods possess various limitations. Electrochemical methods provide an ideal platform for trace nitrite analysis. The prepared composite has been characterized by UV-Visible spectrometry, SEM, EDS and XRD techniques. The proposed sensor has been fabricated by using graphite screen-printed electrodes through drop coating of the composite material. The redox behavior and its application of the fabricated electrode have been studied using cyclic and anodic stripping voltammetric methods. Results: Graphite screen-printed electrodes after modification have been used to identify the electrocatalytic behavior of nitrite oxidation in an aqueous medium. All the parameters influencing the analytical signal have been optimized and incorporated in the recommended procedure. The proposed sensor has been used to measure the nitrite levels from commercially available milk powder samples and the results have been compared with the standard protocol. The results of the proposed sensor are in good agreement with the standard protocol. Conclusion: Ag metal nanoparticles have been embedded in chitosan matrix and used as a composite material in the chemical modification of graphite screen-printed electrodes. GSPEs are easy to fabricate. They provide wide linear working range i.e. 30 - 1140 μM of nitrite. The sensor is highly stable, reproducible and provides a very low detection limit of 1.84 μM. The method has been applied to measure trace level nitrite from milk powder samples.

Background: The main sources of antibiotic pollution are industries, hospitals, and urban effluents, as well as wastewater from farms that use antibiotics for veterinary purposes. Fluoroquinolones are very useful as antimicrobial agents and are probably among the most important classes of synthetic antibiotics in veterinary and human medicines worldwide. Despite this relevance, studies on the analysis of fluoroquinolones in wastewaters and alternative processes to degrade these compounds, and their effects on human health and environment are scarce. Here, we prepared different oxide electrodes (Ti/Ru0.3Ti0.7O2, Ti/Ru0.3Sn0.7O2, Ti/Ir0.3Ti0.7O2, and Ti/Ir0.3Sn0.7O2) and used them in the electrochemical oxidation of levofloxacin, an antibiotic belonging to the class of fluoroquinolones. Methods: The oxide electrodes with nominal compositions: Ti/Ru0.3Ti0.7O2, Ti/Ru0.3Sn0.7O2, Ti/Ir0.3Ti0.7O2, and Ti/Ir0.3Sn0.7O2 were prepared by the traditional method. Briefly, the precursor solution was dissolved in isopropanol and applied by brushing on both sides of the titanium substrate. The resulting material was thermally decomposed at 400°C for 5 min in a preheated oven, which was followed by cooling. This procedure was repeated until the desired oxide thickness was achieved (2 mm). Using the electrochemical cell, the electrolysis experiments were carried out by applying current densities of 25, 50, and 100 mA cm-2 on the oxide electrodes for 60 min. During this experiment, aliquots were removed at times: 5, 10, 15, 20, 30, 45 and 60 min for quantification. Levofloxacin was quantitatively determined by High-Performance Liquid Chromatography (HPLC). Results: The catalytic efficiency of different electrodes is measured as the yield of levofloxacin degradation, which in most cases reaches 50% within 1 h of electrolysis, regardless of the applied current. The electrodes bearing ruthenium afford the same % residual levofloxacin (18%) after 1 h of electrolysis under 100 mA cm-2. The electrodes that contain iridium provide similar results at all the applied currents, being less efficient as compared to the ruthenium-based electrodes. The electrode Ti/Ru0.3Sn0.7O2 presented the highest levofloxacin degradation value (levofloxacin residual is 8% at 50 mA cm-2) and levofloxacin removal rate was calculated considering order 1 kinetics (-lnC/Co=kt), for each of the applied current densities reaching 4.4, 4.9 and 4.5 mg L-1min-1 for the experiments at 25, 50, 100 mA cm-2. Therefore, the Ti/Ru0.3Sn0.7O2 electrode affords the highest yield and the best cost/benefit ratio. Conclusion: In this work, electrodes were prepared with different compositions to study the catalytic efficiency in the degradation of levofloxacin, an antibiotic belonging to the class of fluoroquinolones. The mixed oxide electrodes prepared herein have proven to be an efficient alternative to treat effluents contaminated with organic compounds. The electrode containing RuO2 and SnO2 oxidizes levofloxacin the most efficiently, reaching a removal efficiency of 92% (4.9 mg L-1 min-1) under 50 mA cm-2. Hence, the substitution of Ti for Sn generates better degradation efficiency.

Background: Rapid and easy technology which can mimic the tongue for the simultaneous perception of several tastes based on sensory analysis and mathematical statistics is sorely needed. Methods: Joint voltammetry technology was developed to qualitatively and quantitatively analyze four basic tastes namely sweetness, saltiness, sourness and bitterness with the multi-electrode array. Four taste stimuli were corresponded to four tastes. Cyclic Voltammetry (CV), Differential Pulse Voltammetry (DPV) and Square Wave Voltammetry (SWV) were employed. The original voltammetric signals were transformed by Continuous Wavelet Transform (CWT) in order to reveal more feature information for sensing taste stimuli. Joint voltammetry was applied via the combination of voltammetry. The data of feature points from the transformed signal as the input were used for neural network model. Results: Layer-Recurrent neural network (LRNN) could effectively identify the types of stimuli. The accuracies of the training set and test set by joint voltammetry were both higher than that of regular voltammetry, confirming that Back Propagation neural network (BPNN) could quantitatively predict single taste stimulus of the mixture. Conclusion: Joint voltammetry technology had a strong ability to sense basic tastes as human tongue.

Background: Boswellic acids (BAs) are extracted from oleo gum of Boswellia serrata and are utilized as potential anti-inflammatory, hypolipidemic, immunomodulatory and antitumor specialists. The present examination was meant to assess KBA and AKBA in Boswellia serrata separate by High-Performance Thin Layer Chromatography (HPTLC). Methods: The separation of bioactive compounds was performed utilizing mobile phase glacial acetic acid, n-hexane, ethyl acetate and toluene (0.3: 1: 8: 2) (v/v/v/v) and distinguished at wavelength 254 nm. The technique was approved for linearity, precision, accuracy, limit of detection (LOD), limit of quantification (LOQ), and so forth by International Conference on Harmonization guidelines. Results: The calibration range was observed to be 2- 14 μg/band for both the bioactive compounds. KBA was isolated with an Rf estimation of 0.39 ± 0.02 and AKBA with an Rf estimation of 0.42 ± 0.02. The accuracy was seen to be as high as 99.17% and 97.42 for KBA and KBA, respectively. The percentage RSD value for intra-day and between day varieties was under 2%. The system indicated high affectability and specificity. Conclusion: The developed HPTLC method was simple, precise, robust, specific, rapid, and costeffective and could be used for quality control analysis and quantification of KBA and AKBA in different herbal formulations containing the plant species.

Background: Pesticides residues in agricultural products have posed a serious threat to food safety and human health, so it is necessary to develop a rapid and accurate method to detect pesticide in the environment. N-OMC with excellent electroconductivity, high biocompatibility and the functional amino group that can be covalently attached to the enzyme can be applied to construct a sensitive and stable acetylcholinesterase biosensor for rapid and accurate detection of organophosphorus pesticides with the help of L-cysteine self-assembled monolayer and AuNPs. Methods: Transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and nitrogen adsorption measurements are used to characterize materials. Electrochemical impedance spectroscopy and cyclic voltammetry are used to study the surface features of modified electrodes. Differential pulse voltammetric is used to measure the peak current of modified electrodes. GC-MS is applied to verify the reliability of the prepared biosensor for organophosphorus pesticides detection. Results: N-OMC was synthesized and applied to constructed stable and sensitive acetylcholinesterase biosensors. The combination of N-OMC, L-cysteine self-assembled monolayer and AuNPs to modify the electrode surface has greatly improved the conductivity of biosensor and provided a stable platform for acetylcholinesterase immobilization. The linear detection range of paraoxon was from 3 to 24 nM with a lower detection limit of 0.02 nM. Conclusion: The biosensor exhibited satisfactory reproducibility, repeatability and stability, and was successfully employed to determine the paraoxon in vegetables as well as tap water samples, providing a promising tool for rapid and sensitive detection of organophosphorus pesticides in agricultural products.