Organoclay-film modified electrode for the detection of ultra-traces of Hg2+ ions: Approach of one factor at a time by an experimental design

Efficient Electrochemical Lead Detection by a Histidine-Grafted Metal-Organic Framework MOF-808 Electrode Material

As the excessive presence of heavy metals in the environment significantly affects human health, it becomes necessary to develop efficient, selective, and sensitive methods for their detection. In this study, a novel electrochemical sensor for the detection of Pb2+ ions is described. The proposed sensor is based on a glassy carbon electrode (GCE) modified by a thin film of histidine-grafted metal-organic framework (MOF-808-His). The MOF-808 was obtained solvothermally, and then postsynthetically modified by substituting the coordinated acetate with histidinate. By electrochemistry, the MOF-808-His-modified GCE demonstrated high charge selectivity, while electrochemical impedance spectroscopy (EIS) and kinetic studies gave a lower charge transfer resistance (4196 Ω) and a better standard heterogeneous electron transfer rate constant (1.80 × 10-5 cm s-1) on MOF-808-modified GCE. These results indicated a swift and direct electron transfer rate from [Fe(CN)6]3-/4- to the electrode surface. Using square wave anodic stripping voltammetry (SWASV), the rapid and highly sensitive determination of Pb2+ was achieved on MOF-808-His-modified GCE. By optimizing the accumulation-detection parameters including pH of the detection medium, deposition time and potential, and concentration, a remarkable limit of detection (LoD, based on a signal-to-noise ratio of 3) of (1.12 × 10-10 ± 0.10 × 10-10) mol L-1 was obtained, with a sensitivity of (9.6 ± 0.1) μA L μmol-1. After interference and stability studies, the MOF-808-His-modified GCE was applied to the detection of Pb2+ in a tap water sample with a concentration of 10 μmol L-1 Pb2+.

Electrochemical detection of selected heavy metals in water: a case study of African experiences

The safety of water resources throughout the globe has been compromised by various human activities and climate change over the last decades. Consequently, the world is currently confronted with a severe shortage of water supply and a water safety crisis, amidst a growing population. With poor environmental regulations, indiscriminate budding of urban slums, poverty, and a lack of basic knowledge of hygiene and sanitation, the African water supply has been critically threatened by different organic and inorganic contaminants, which results in several health issues. Inorganic pollutants such as heavy metals are particularly of interest because they are mostly stable and non-biodegradable. Therefore, they are not easily removed from water. In different parts of the continent, the concentration of heavy metals in drinking water far exceeds the permissible level recommended by the World Health Organization (WHO). Worse still, this problem is expected to increase with growing population, industrialization, urbanization, and, of course, corruption of government and local officials. Most of the African population is ignorant of the standards of safe water. In addition, the populace lack access to affordable and reliable technologies and tools that could be used in the quantification of these pollutants. This problem is not only applicable to domestic, but also to commercial, communal, and industrial water sources. Hence, a global campaign has been launched to ensure constant assessment of the presence of these metals in the environment and to promote awareness of dangers associated with unsafe exposure to them. Various conventional spectroscopic heavy metal detection techniques have been used with great success across the world. However, such techniques suffer from some obvious setbacks, such as the cost of procurement and professionalism required to operate them, which have limited their applications. This paper, therefore, reviews the condition of African water sources, health implications of exposure to heavy metals, and the approaches explored by various indigenous electrochemists, to provide a fast, affordable, sensitive, selective, and stable electrochemical sensors for the quantification of the most significant heavy metals in our water bodies.

Enzymatic Preparation and Structure-activity Relationship of Sesaminol

As a valuable natural antioxidant, sesaminol can be used in food and medicine industries, but it is trace in sesame seeds and oil, and it is feasible to prepare sesaminol from sesaminol triglucoside (STG) which is abundant in defatted sesame cake. Therefore, in order to establish an effective enzymatic preparation method and elucidate the antioxidant structure-activity relationship of sesaminol, a suitable glycosidase for preparing sesaminol from STG were screened, enzymatic hydrolysis was optimized by single-factor test and response surface methodology, and finally, the structure-activity relationship of sesaminol was illustrated by comparative molecular field analysis (CoMFA). These results suggested that β-galactosidase was the optimal glycosidase for enzymatic hydrolysis of STG to prepare sesaminol. Under the optimal conditions of a reaction temperature of 50°C, reaction time of 4.0 h, pH of 5.5, substrate concentration of 1.0 mg/mL, and enzyme dosage of 20 mg/mL, the conversion rate of sesaminol was 98.88±0.67%. Sesaminol displayed excellent antioxidant ability in 2,2-diphenyl-1-picrylhydrazyl (DPPH, IC₅₀ = 0.0011 mg/mL), 2,2'-azinobis-(3-ethyl-benzothiazoline-6-sulfonate) (ABTS, IC₅₀ = 0.0021 mg/mL) radical scavenging activities and Ferric reducing antioxidant power (FRAP, 103.2998 mol/g) compared to other sesaminol derivatives. According to －log (IC₅₀ of DPPH) and －log (IC₅₀ of ABTS), CoMFA models were successfully established based on Q₂ >0.5 (Q_DPPH ² = 0.558, Q_ABTS ² = 0.534). The active site of sesaminol tended to be located on the hydroxyl group of the benzene ring (R₁ position). A positive correlation between the bulky and positively charged groups at the 1H, 3H-furo [3, 4-c] furan group, the small, negatively charged groups at the R₁ position and the antioxidant activity of sesaminol. This study provides an effective method to prepare sesaminol, reveals the structure-activity relationship of sesaminol and provides theoretical basis to design the novel compound.

In situ growth of CeO2 on g-C3N4 nanosheets toward a spherical g-C3N4/CeO2 nanozyme with enhanced peroxidase-like catalysis: a selective colorimetric analysis strategy for mercury(II )

Cerium dioxide (CeO2) nanocatalysts were initially grown in situ on 2D graphitic carbon nitride (g-C3N4) nanosheets to yield the nanocomposites g-C3N4/CeO2 with a spherical structure for the catalysis-based colorimetric analysis of Hg2+ ions in blood and wastewater. As the synergetic introduction of g-C3N4 nanosheets might promote the electron transfer in CeO2, the resulting g-C3N4/CeO2 nanozyme was found to present greatly enhanced catalytic activity, as demonstrated by the steady-state kinetic studies, which is nearly 4-fold higher than that of pure CeO2. Moreover, the g-C3N4/CeO2 nanozymes would aggregate in the presence of Hg2+ ions due to the strong interaction between Hg2+ and the nitrogen of g-C3N4, leading to a decrease of catalysis rationally depending on the Hg2+ ion concentration. A colorimetric analysis strategy is therefore developed for the selective detection of Hg2+ ions separately in the complex samples of blood and wastewater, showing a linear concentration range from 0.50 nM to 800 nM with the LOD of 0.23 nM as exemplified for Hg2+ ions in blood. Also, the recovery tests indicated that the developed colorimetric method can allow for the accurate analysis of Hg2+ ions in wastewater and blood. Such a route for the fabrication of composite nanozymes by growing catalytic nanomaterials on conductive 2D substrates may be extended to the design of other kinds of nanozymes with enhanced catalytic performances for developing catalysis-based detection platforms.