Experimental and theoretical insights into the electrooxidation pathway of azo-colorants on glassy carbon electrode

Three-dimensional dual-network magnetic conductive hydrogel for the highly sensitive electrochemical detection of ponceau 4R in foods

Synthetic pigment Ponceau 4 R is a commonly used additive in the process of various foods. Due to its potential toxicity to humans, realizing high sensitivity and rapid detection of Ponceau 4 R is extremely important. In this study, we synthesized a novel dual-network magnetic conductive hydrogel (MCHG) via a simple one-pot low temperature stirring method. In MCHG, cationic guar gum (CGG) and β-cyclodextrin (β-CD) formed a primary three-dimensional network cross-linked by N, N-methylene bisacrylamide. A second network was formed in MCHG by CGG, β-CD and magnetite@carboxylate-terminated carbon nanotubes (Fe3O4@COOH-MWCNTs) through hydrogen bonding and electrostatic interactions. Fe3O4@COOH-MWCNTs enhanced cross-linking in the MCHG hydrogel, whilst also boosting the equilibrium adsorption capacity of Ponceau 4 R (61.8 mg g-1), electrical conductivity and electrocatalytic performance. Application of MCHG to a glassy carbon electrode (GCE) created a highly sensitive electrochemical sensor for the detection of Ponceau 4 R. Under optimized testing conditions, the sensor offered a very wide linear range (0.01-200.0 μM) and a low limit of detection (1.8 nM) for Ponceau 4 R. When the sensor was applied to the detection of Ponceau 4 R in spiked honey and liqueur samples, excellent recoveries were achieved (88.2%-107.0%). Furthermore, analyses of commercial biscuit and candy samples using the MCHG/GCE sensor and a national standard ultraviolet spectrophotometry method afforded identical results. Results demonstrate that multifunctional hydrogels show great promise as signal amplification agents in electrochemical detection of target compounds in foods.

Applicability of a graphene oxide nanocomposite for fabrication of an electrochemical sensor for simultaneous detection of sunset yellow and rhodamine B in food samples

Developing a sensitive portable sensor for the determination of food additives is very important. Herein, a simple and sensitive electrochemical sensor has been constructed based on a carbon paste electrode (CPE) modified with graphene oxide (GO) nanosheets for the simultaneous determination of sunset yellow (SY) and rhodamine B (RhB) in phosphate buffer solution (pH = 4). The cyclic and differential pulse voltammetry (CV and DPV) results revealed two well-resolved anodic peaks for SY and RhB with a remarkable increase in oxidation signals of these colorants. Based on this, an electrochemical method was developed for the first time for the simultaneous detection of SY and RhB. Applicability of the sensor was confirmed in various concentrations of two analytes in two linear ranges of 1-20 μM for SY and 1-30 μM for RhB with a limit of detection (LOD) of 10.0 nM and 20.0 nM, respectively. Furthermore, the proposed sensor was successfully employed for the simultaneous detection of SY and RhB in food samples with recoveries of 93.1-106.0% indicating promising potential in practical application.

Unveiling the tartrazine binding mode with ds-DNA by UV-visible spectroscopy, electrochemical, and QM/MM methods

Here, we studied the interaction between the food colorant tartrazine (TZ) and double stranded DNA (dsDNA), using spectroscopic, electrochemical, and computational methods such as QM/MM combined with TD-DFT. Despite the UV-vis spectroscopy is widely used to study the interaction between molecules, for the case of TZ there are discrepancies in the analyses presented in the literature available, presenting both hyperchromic and hypochromic effects and consequently different rationalizations for their results. Herein we propose the combination of UV-vis experiments with the design of high-level computational models capable of reproducing the experimental behavior to finally define the proper binding mode at the molecular scale together with the rationalization of the experimental optical response due to the complex formation. To complement the UV-vis experiments, we propose the use of electrochemical measurements, to support the results obtained through UV-vis spectroscopy, as it has been successfully used for the determination of interaction modes between small molecules and biomolecules in any condition. Our UV-vis spectroscopy experiments showed only a hypochromic effect of the absorption spectra of TZ after interaction with DNA, indicative of TZ being deeply buried in the DNA structure. The effect of ionic strength in the experimental procedures led to the dissociation of TZ, thus indicating that the interaction mode was groove binding. On the other hand, the electrochemical studies showed an irreversible reduction peak of TZ, which after the interaction with DNA exhibited a positive shift in potential that can be attributed to groove binding. The binding constant for TZ-DNA was calculated as 4.45x10⁴M^-1 (UV-vis) and 5.75x10⁴M^-1 (electrochemistry), in line with other groove binder azo dyes. Finally, through the QM/MM calculations we found that the minor-groove binding mode interacting in zones rich in adenine and thymine was the model best suited to reproduce the experimental UV-vis response.

Analytical Applicability of Graphene-Modified Electrode in Sunset Yellow Electrochemical Assay

Due to the recent increase in average living standards, food safety has caught public attention. It is necessary to conduct a qualitative and quantitative rapid test of prohibited food additives since the inclusion of food additives or the improper usage of synthetic dyes can negatively impact on the human health. Herein, a highly sensitive method for Sunset Yellow detection based on a glassy carbon electrode modified with few-layer graphenes was proposed. The electrochemical behavior of SY at the GR-exf/GCE modified surface was investigated by Cyclic Voltammetry, Square Wave Voltammetry, Electrochemical Impedance Spectroscopy and Amperometry. The influences of pH, scan rate, and interfering species were studied. Under optimized conditions, the developed sensor shows good linearity over a broad SY concentration range, e.g., 0.028-30 µM, with a low limit of detection (LOD = 0.0085 µM) and quantification (LOQ = 0.028 µM) (data obtained by amperometric technique). Furthermore, the modified electrode shows good selectivity, precision and sensitivity and has been successfully applied for SY quantification from commercially available pharmaceutical formulation as well as from candy bars and orange juice.

Gradient Titanium Oxide Nanowire Film: a Multifunctional Solar Energy Utilization Platform for High-Salinity Organic Sewage Treatment

The treatment of high salt organic sewage is considered to be a high energy consumption process, and it is difficult to degrade organic matter and separate salt and water simultaneously. In this study, a gradient structure titanium oxide nanowire film is developed, which can realize the thorough treatment of sewage under sunlight. Among the film, part TiO_2-x has enhanced photocatalytic properties and can completely degrade 0.02 g·L^-1 methylene blue in 90 min under 2 sun. Part Ti_nO_2n-1 has excellent photothermal conversion efficiency and can achieve 1.833 kg·m^-2·h^-1 water evaporation rate at 1 sun. Through the special structure design, salt positioning crystallization can be realized to ensure the film's stable operation for a long time. The gradient hydrophilicity of the film ensures adequate and rapid water transfer, while the water flow can induce a significant hydrovoltaic effect. The measured V_OC is positively correlated with light intensity and photothermal area and corresponds to the water evaporation rate.

One-Step Fabrication of Nickel-Electrochemically Reduced Graphene Oxide Nanocomposites Modified Electrodes and Application to the Detection of Sunset Yellow in Drinks

This work describes a straightforward method using one-step preparation of graphene/nickel nanocomposite materials from low-cost materials including graphene oxide and nickel metal. Repetitive CVs lead to the simultaneous deposition of metallic nickel nanoparticles and reduced graphene oxide sheets onto glassy carbon electrode. The obtained nanocomposite-modified surfaces were characterised by cyclic voltammetry, differential pulse voltammetry and field emission scanning electron microscopy. The result demonstrated the ability to produce nickel nanoparticles with a small size of about 20 nm, uniformly dispersed on a graphene oxide matrix. The ERGO-NiNP nanocomposite could be used as a sensor material exhibiting high performance; it is used here in order to detect Sunset Yellow (SY) and for quantification in complex media. The sensor enables rapid quantification of SY with a good linearity (R2 = 0.996) in the range of 10–1000 nM, together with a low detection limit of 3.7 nM (equivalent to 1.7 µg L−1) and a high sensitivity up to 7 µA/µM. The sensor also displays high reliability with a RSD value = 1.08 (n = 10) and good reusability (signal response variation below 5% after 5 detection/cleaning cycles). Finally, we demonstrate how this GCE/ERGO-NiNP sensor can be used for the successful determination of SY in commercial soft drink samples with an acceptable deviation below 6.4% when compared to HPLC method.

Recent advances in Ponceau dyes monitoring as food colorant substances by electrochemical sensors and developed procedures for their removal from real samples

Ponceau dyes are one of the food coloring materials that are added to various pharmaceutical, health and food products and give them an appearance. These dyes contain contaminants such as Benzidine, 4-Aminobiphenyl, and 4-Aminoazobenzene that are safe in small amounts, but they are not approved by the US Food and Drug Administration (US-FDA) for human consumption. This study comprehensively was reviewed the properties, applications, chemistry, and toxicity of Ponceau dyes as food colorant substances. Electroanalysis of Ponceau dyes was discussed in detail, and the various electrochemical sensors used to detect and monitor these dyes as food colorant were examined. The applied methods of removing and degradation of these dyes in municipal and industrial wastes were also discussed. Conclusions and future perspectives to motivate future research were also explored.

Analysis of the tautomeric equilibrium of two red monoazo dyes by UV-Visible, Raman and SERS spectroscopies

Acid Red 26 and Acid Red 18 are two early synthetic dyes belonging to the monazo dye class. The molecular structure of this class of dyes is characterized by the chromophoric azo group (NN) generally attached to benzene or naphthalene derivatives containing electron withdrawing and/or donating groups as substituents. As both red dyes have an OH group in ortho- position respect to the azo group, they undergo an azo-hydrazone tautomerism. In this work, UV-Vis, Raman and SERS spectroscopic analysis of the red dye solutions were carried out at different pH conditions, in order to evaluate the preponderance of one tautomer over the other as a function of the pH. Different experimental conditions were tested in order to find the best ones for the detection of both dyes. Thus, Raman spectra of the powder and aqueous solutions of AR26 and AR18 were obtained at the natural pH of the solutions, and above and below that value. The SERS analysis of the dye solutions were carried out at various pH values between 2 and 10, and with excitation at 442, 532 and 633 nm. The molecular structure and the theoretical Raman spectra of the two tautomers of both red dyes were calculated by DFT methods. The obtained results were used for the assignment of the Acid Red 26 and Acid Red 18 vibrational modes. Finally, a textile sample dyed with AR18 was analyzed by SERS.

Synthetic Food Dyes – Some Aspects Of Use And Methods Of Determination

Color is one of the key ingredients for increasing the appetizing of food, so food dyes have become firmly established in food production technologies. However, with the acquisition of toxicity data of synthetic food dyes (SFD), there were restrictions and standards for their content in food have emerged. Numerous papers published in recent years demonstrate the importance of the problem of the use and definition of SFD. The review contains over 180 literary references in the field of usage and methods of determination of synthetic food dyes, among them regulatory documents (regulations), official internet resources of international and Ukrainian organizations, review articles and original works. Varieties of chromatography, enzyme-linked immunoassay, optical and electrochemical methods are used to identify and determine SFD. Special attention was paid to voltammetry (VA) as a method that is cheaper than chromatography and completely satisfies selectivity, sensitivity, reliability requirements and is compatible with the concept of green analytical chemistry, as it doesn't need organic solvents. Moreover, single sweep voltammetry can be considered as a screening method with low limits of determination and rapid respons

Highly sensitive and rapid determination of sunset yellow in drinks using a low-cost carbon material-based electrochemical sensor

Starting from simple graphite flakes, an electrochemical sensor for sunset yellow monitoring is developed by using a very simple and effective strategy. The direct electrochemical reduction of a suspension of exfoliated graphene oxide (GO) onto a glassy carbon electrode (GCE) surface leads to the electrodeposition of electrochemically reduced oxide at the surface, obtaining GCE/ERGO-modified electrodes. They are characterized by cyclic voltammetry (CV) measurements and field emission scanning electron spectroscopy (FE-SEM). The GCE/ERGO electrode has a high electrochemically active surface allowing efficient adsorption of SY. Using differential pulse voltammetry (DPV) technique with only 2 min accumulation, the GCE/ERGO sensor exhibits good performance to SY detection with a good linear calibration for concentration range varying 50-1000 nM (R² = 0.996) and limit of detection (LOD) estimated to 19.2 nM (equivalent to 8.9 μg L^-1). The developed sensor possesses a very high sensitivity of 9 μA/μM while fabricated with only one component. This electrochemical sensor also displays a good reliability with RSD value of 2.13% (n = 7) and excellent reusability (signal response change < 3.5% after 6 measuring/cleaning cycles). The GCE/ERGO demonstrates a successful practical application for determination of sunset yellow in commercial soft drinks. Graphical abstract.

Voltammetric, spectroscopic, and cellular characterization of redox functionality of eckol and phlorofucofuroeckol-A: A comparative study

Interest in phlorotannins has increased in recent years largely due to antioxidant capacity, however, the redox mechanism of phlorotannins is still obscure. In the present study, the electrochemical oxidation mechanisms of eckol (EL) and phlorofucofuroeckol-A (PFF-A), two representative phlorotannin compounds, were comparatively analyzed in a wide pH range using cyclic and differential pulse voltammetry as well as spectroscopic assay. The voltammetric study revealed that EL and PFF-A were successively oxidized in three pH-dependent steps. Moreover, it was found that the PFF-A presented a stronger proton and electron transferring activity as compared to EL since PFF-A exhibited lower acid-base dissociation constant (pKa ) value and higher heterogeneous rate constant (kbh ) value in the first oxidation step. These property were further evidenced by comparison of direct antioxidant activity (i.e., superoxide anion and peroxide radicals) as well as indirect antioxidant activity (i.e., mRNA expression of two phase II enzymes) between EL and PFF-A. PRACTICAL APPLICATIONS: Phlorotannins from edible algae have been regarded as novel antioxidants those presented high application potential in food industry. Even though antioxidant activity of phlorotannin compounds have been widely investigated in both in vitro and in vivo studies, very few reports focused on electron transferring functionality which is chemical basis for antioxidant process. Herein, the oxidative mechanisms of two representative phlorotannins were comparatively analyzed using multiple electrochemical methods. This is hopefully to give information on the chemical meaning behind the antioxidant activity of dietary phlorotannins.