The quality of water is continuously under threat as increasing concentrations of pollutants escape into the aquatic environment. However, these issues can be alleviated by adsorbing pollutants onto adsorbents. Chitosan and its composites are attracting considerable interest as environmentally acceptable adsorbents and have the potential to remove many of these contaminants. In this review the development of chitosan-based adsorbents is described and discussed. Following a short introduction to the extraction of chitin from seafood wastes, followed by its conversion to chitosan, the properties of chitosan are described. Then, the emerging chitosan/carbon-based materials, including magnetic chitosan and chitosan combined with graphene oxide, carbon nanotubes, biochar, and activated carbon and also chitosan-silica composites are introduced. The applications of these materials in the removal of various heavy metal ions, including Cr(VI), Pb(II), Cd(II), Cu(II), and different cationic and anionic dyes, phenol and other organic molecules, such as antibiotics, are reviewed, compared and discussed. Adsorption isotherms and adsorption kinetics are then highlighted and followed by details on the mechanisms of adsorption and the role of the chitosan and the carbon or silica supports. Based on the reviewed papers, it is clear, that while some challenges remain, chitosan-based materials are emerging as promising adsorbents.

Supramolecular chemistry, although focused mainly on noncovalent intermolecular and intramolecular interactions, which are considerably weaker than covalent interactions, can be employed to fabricate sensors with a remarkable affinity for a target analyte. In this review the development of cyclodextrin-based electrochemical sensors is described and discussed. Following a short introduction to the general properties of cyclodextrins and their ability to form inclusion complexes, the cyclodextrin-based sensors are introduced. This includes the combination of cyclodextrins with reduced graphene oxide, carbon nanotubes, conducting polymers, enzymes and aptamers, and electropolymerized cyclodextrin films. The applications of these materials as chiral recognition agents and biosensors and in the electrochemical detection of environmental contaminants, biomolecules and amino acids, drugs and flavonoids are reviewed and compared. Based on the papers reviewed, it is clear that cyclodextrins are promising molecular recognition agents in the creation of electrochemical sensors, chiral sensors, and biosensors. Moreover, they have been combined with a host of materials to enhance the detection of the target analytes. Nevertheless, challenges remain, including the development of more robust methods for the integration of cyclodextrins into the sensing unit.

Graphene and its derivatives, especially graphene oxide (GO), are attracting considerable interest in the fabrication of new adsorbents that have the potential to remove various pollutants that have escaped into the aquatic environment. Herein, the development of GO/chitosan (GO/CS) composites as adsorbent materials is described and reviewed. This combination is interesting as the addition of graphene to chitosan enhances its mechanical properties, while the chitosan hydrogel serves as an immobilization matrix for graphene. Following a brief description of both graphene and chitosan as independent adsorbent materials, the emerging GO/CS composites are introduced. The additional materials that have been added to the GO/CS composites, including magnetic iron oxides, chelating agents, cyclodextrins, additional adsorbents and polymeric blends, are then described and discussed. The performance of these materials in the removal of heavy metal ions, dyes and other organic molecules are discussed followed by the introduction of strategies employed in the regeneration of the GO/CS adsorbents. It is clear that, while some challenges exist, including cost, regeneration and selectivity in the adsorption process, the GO/CS composites are emerging as promising adsorbent materials.

The layered double hydroxides (LDHs) have demonstrated significant potential as non-noble-metal electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Their unique compositional and structural properties contribute to their efficiency and stability as catalysts. In this study, CoCuFe-LDH composites were grown on graphene (G) via a cost-effective and straightforward one-step hydrothermal process. A 2-level full-factorial model was employed to determine the impact of Co (1.5, 3, and 4.5 mmol) and graphene (10, 30, and 50 mg) concentrations on the onset potential of OER and HER, which were the chosen response variables. OER and HER activity variabilities were assessed in triplicate using Co_[3]Cu_[3]Fe_[3]-LDH/G_[30] (central point), which were determined at 0.01% and 0.02%, respectively. Statistical analyses demonstrated that Co_[4.5]Cu_[3]Fe_[3]-LDH/G_[10] and Co_[1.5]Cu_[3]Fe_[3]-LDH/G_[10] showed the lowest onset potential at 1.52 V and -0.32 V (V vs RHE) for the OER and HER, respectively, suggesting that a high cobalt concentration enhances OER performance, while optimal HER catalysis was achieved with lower cobalt concentrations. Moreover, the trimetallic composites exhibited good stability with negligible loss of catalytic activity over 24 h.

Layered double hydroxides (LDH) have exhibited promising applications as electrocatalysts in oxygen evolution reactions (OER). In this work, trimetallic LDHs (CoNiFe-LDH) were designed and grown on graphene (G) through a one-step hydrothermal approach to obtain a structure that promotes efficient charge transfer. A 2-level full-factorial design was utilized to evaluate the effects of varying the concentrations of Co (1.5, 3, and 4.5 mmol) and graphene (10, 30, and 50 mg) on the OER activity. The potential needed to deliver 10 mA cm^-2 was chosen as the response parameter. The independent and dependent parameters were fitted to a linear model equation through ANOVA analysis. The computed p-values were below 0.05 signifying the statistical significance of the concentrations of cobalt and graphene and their interaction, suggesting a correlation with the OER activity. The OER experiments were conducted in triplicate using the Co_[3]Ni_[3]Fe_[3]-LDH/G_[30] (central point) to estimate variability (0.58%). Comparative analysis showed that Co_[1.5]Ni_[3]Fe_[3]-LDH/G_[10] achieved the lowest onset potential (1.54 V), potential at 10 mA cm^-2 (1.58 V), and Tafel slope (58.4 mV dec^-1), indicating that a low concentration of cobalt and graphene make an efficient electrocatalyst for OER. Furthermore, the optimized composite demonstrated favorable electronic properties, with a charge transfer resistance (R_CT) of 188.1 Ω, and exhibited good stability, maintaining its catalytic activity with no significant loss over a 24-h period.

As more emphasis is placed on renewable energy worldwide, the electrochemical splitting of water has attracted considerable attention. Herein, a simple and cost-effective electrocatalyst was studied for both the hydrogen and oxygen evolution reactions. The electrocatalyst consisted of an electrodeposited cobalt diselenide/cobalt oxide composite, which was fabricated from an aqueous solution containing cobalt and selenium salts on a glassy carbon substrate. The optimal deposition conditions were found to be a fixed potential at -1.2 V vs Ag/AgCl for 400 s. At this relatively low potential, the electrodeposition process is accompanied by the hydrogen evolution reaction, resulting in a hierarchical, flower-like morphology. The electrochemical active surface area was calculated to be 89 cm² in 1.0 M KOH and 72 cm² in 0.5 M H₂SO₄. This electrocatalyst facilitated the hydrogen evolution reaction in both KOH and H₂SO₄ solutions, with overpotentials of 305 and 205 mV at 10 mA cm^-2, respectively. Similarly, good oxygen evolution activity was achieved in KOH. The electrocatalyst demonstrated good stability over a 24 h period, with evidence of improved oxygen evolution activity following the stability studies.