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Doped carbon dots have attracted great attention from researchers across disciplines because of their unique characteristics, such as their low toxicity, physiochemical stability, photostability, and outstanding biocompatibility. Nitrogen is one of the most commonly used elements for doping because of its sizeable atomic radius, strong electronegativity, abundance, and availability of electrons. This distinguishes them from other atoms and allows them to perform distinctive roles in various applications. Here, we have reviewed the most current breakthroughs in nitrogen-doped CDs (N-CDs) for fluorescent sensor applications in the last five years. The first section of the article addresses several synthetic and sustainable ways of making N-CDs. Next, we briefly reviewed the fluorescent features of N-CDs and their sensing mechanism. Furthermore, we have thoroughly reviewed their fluorescent sensor applications as sensors for cations, anions, small molecules, enzymes, antibiotics, pathogens, explosives, and pesticides. Finally, we have discussed the N-CDs' potential future as primary research and how that may be used. We hope that this study will contribute to a better understanding of the principles of N-CDs and the sensory applications that they can serve. [Display omitted] • Extensive overview of fluorescence N-CDs for optical sensor applications. • The possible processes underlying the PL origins of N-CDs were outlined. • Discussed the effects of various dopants on the photophysical characteristics of CDs. • Use of N-CDs for detecting ions, small compounds, and pharmaceuticals was described. [ABSTRACT FROM AUTHOR]

Herein we reported the effect of doping and addition of surfactant on SnO 2 nanostructures for enhanced photocatalytic activity. Pristine SnO 2 , Zn–SnO 2 and SDS-(Zn–SnO 2) was prepared via simple co-precipitation method and the product was annealed at 600 °C to obtain a clear phase. The structural, optical, vibrational, morphological characteristics of the synthesized SnO 2 , Zn–SnO 2 and SDS-(Zn–SnO 2) product were investigated. SnO 2 , Zn–SnO 2 and SDS-(Zn–SnO 2) possess crystallite size of 20 nm, 19 nm and 18 nm correspondingly with tetragonal structure and high purity. The metal oxygen vibrations were present in FT-IR spectra. The obtained bandgap energies of SnO 2 , Zn–SnO 2 and SDS-(Zn–SnO 2) were 3.58 eV, 3.51 eV and 2.81 eV due to the effect of dopant and surfactant. This narrowing of bandgap helped in the photocatalytic activity. The morphology of the pristine sample showed poor growth of nanostructures with high level of agglomeration which was effectively reduced for other two samples. Product photocatalytic action was tested beneath visible light of 300 W. SDS-(Zn–SnO 2) nanostructure efficiency showed 90% degradation of RhB dye which is 2.5 times higher than pristine sample. Narrow bandgap, crystallite size, better growth of nanostructures paved the way for SDS-(Zn–SnO 2) to degrade the toxic pollutant. The superior performance and individuality of SDS-(Zn–SnO 2) will makes it a potential competitor on reducing toxic pollutants from wastewater in future research. • SnO 2 , Zn–SnO 2 and SDS-(Zn–SnO 2) product were investigated. • Bandgap of SnO 2 , Zn–SnO 2 and SDS-(Zn–SnO 2) were 3.58, 3.51, 2.81 eV. • Narrowing bandgap helped in photocatalytic activity. • SDS-(Zn–SnO 2) efficiency showed 90% RhB degradation. • SDS-(Zn–SnO 2) explored 2.5 times higher than pristine sample. [ABSTRACT FROM AUTHOR]