Carbon-based quantum dots and their nanocomposites have sparked immense interest for researchers as sensors due to their attractive physico-chemical properties caused by edge effects and quantum confinement. In this review article, we have discussed the synthesis and application of nanocomposites of graphene quantum dots (GQDs) and carbon quantum dots (CQDs). Different synthetic strategies for CQDs, GQDs, and their nanocomposites, are categorized as top-down and bottom-up approaches which include laser ablation, arc-discharge, chemical oxidation, ultrasonication, oxidative cleavage, microwave synthesis, thermal decomposition, solvothermal or hydrothermal method, stepwise organic synthesis, carbonization from small molecules or polymers, and impregnation. A comparison of methodologies is presented. The environmental application of nanocomposites of CQDs/GQDs and pristine quantum dots as sensors are presented in detail. Their applications envisage important domains dealing with the sensing of pollutant molecules. Recent advances and future perspective in the use of CQDs, GQDs, and their nanocomposites as sensors are also explored.

Traditional wastewater treatment methods, such as reverse osmosis, adsorption, desalination, and others, are outweighed by the photocatalytic degradation of organic pollutants. Ferrites are prominent photocatalysts due to their tunable band gaps, surface areas, and magnetic properties, which render photodegradation economical. Ferrites and their nanocomposites have been reported as promising visible light active photocatalysts. The photocatalytic system is heavily reliant on a number of factors that influence the photodegradation of organic effluents. This review demonstrates various parameters such as substrate concentration, pH of solution, photocatalyst quantity, photocatalyst surface area, metal and non-metal ion doping, light intensity, irradiation time, quenchers, etc. affecting the photocatalytic degradation of organic effluents by ferrite nanoparticles and their nanocomposites in detail. The photodegradation efficiency of the ferrite nanoparticles alters with the change in the value of pH of the solution, which further depends upon the nature of the pollutant used. A dose of the substrate and the photocatalyst must be optimized so as to attain better photodegradation efficiency. Photocatalysts with different surface areas change the amount of active sites, which in turn affects the degradation of pollutant and render it a crucial factor. In addition, the mechanism of the action of photocatalysis is elaborated in this review. Future research perspectives for the advancement of ferrites and their nanocomposites are deliberated in order to improve their use as photocatalysts.

Magnesium ferrite nanoparticles (NPs) were synthesized by co-precipitation, sol-gel and solution combustion methods. Polyethylene glycol (PEG), urea and oxalyl dihydrazide (ODH) were used as fuels for the combustion. Various physicochemical techniques viz. X-ray diffraction (XRD), vibrating sample magnetometry (VSM), Fourier transform infrared spectroscopy (FT-IR), BET surface analysis and transmission electron microscopy (TEM) were utilized to study the effect of synthetic methodology on the properties of synthesized NPs. Differences in crystallinity, surface area, particle size and magnetic parameters of the ferrite NPs synthesized by different methods were observed. XRD pattern of NPs obtained by sol-gel and combustion methods confirmed phase purity where as in co-precipitation method ?-Fe2O3 was detected as impurity phase which also resulted in greater value of physical density and lowering of magnetic parameters of the final thermolysis product. TEM micrographs indicated that ferrite NPs are spherical with average diameter of 12-25 nm. Presence of rectangular shaped crystallites of ?-Fe2O3 was clearly evident in the TEM images of the NPs synthesized by co-precipitation method.