A straightforward and simpler use of an age-old technique was utilized for the fabrication of "red-emitting magnesium-nitrogen-embedded carbon dots" (r-Mg-N-CD) from the leaves extract of Bougainvillea plant as a natural source of carbon. This technique is similar to the solvent-based technique, which is used for the extraction of fragrances and essential oils from flowers and leaves. The as-derived leaves extract was further carbonized using a simple domestic microwave to obtain the small-sized red-emitting carbonaceous material as r-Mg-N-CD. The r-Mg-N-CD showed excitationindependent emissions at ∼678 nm with excellent photostability and a high quantum yield value (∼40%). Moreover, the important perspective of the present finding is to use this r-Mg-N-CD as a potential photocatalyst material for the degradation of pollutant dye (methylene blue) under the presence of sunlight. To infer the significant influence of using natural sunlight in the process of dye degradation, a comparative analysis was performed, demonstrating the higher rate of photodegradation (∼6 times faster) under the influence of sunlight compared to the artificial visible-light from a 100 W tungsten bulb.

Cr(VI) is a known toxic and non-biodegradable pollutant that results from multiple industrial processes, and can cause significant environmental damage if it is not removed from wastewater. However, it can be reduced to Cr(III), which is less toxic and can be readily precipitated out and removed. Here, a fast and facile single-step technique is reported for the synthesis of nitrogen-phosphorus doped fluorescent carbon dots (NP-CD) using a domestic microwave, as a potential photocatalytic material. Under natural sunlight, a simple photocatalytic experiment reveals that the NP-CD are highly efficient for the quantitative reduction of Cr(VI) to Cr(III) in synthetic contaminated water, in a linear range from 10 ppm (in approximately 10 min) to 2000 ppm (in approximately 320 min) by increasing the sunlight irradiation time followed by its removal by precipitation. NP-CD exhibit high recyclability of up to six cycles without any apparent loss in photocatalytic activity, demonstrating NP-CD as a potential photocatalyst material for Cr(VI) water treatment.

The present finding deals with a simple and low-cost fabrication of surface-passivated, brightly fluorescent zinc-oxide-decorated, red-emitting excitation-independent ultrafluorescent CDs, denoted as “C ZnO -Dots”. Surface doping of zinc oxide significantly improved the quantum yield by up to ∼72%, and these brightly fluorescent red-emitting C ZnO -Dots have been employed for the aqueous-phase photoreduction of 100 ppm hexavalent chromium(VI) to trivalent chromium(III) under the influence of sunlight irradiation. The overall utility of the prepared C ZnO -Dots can be ascertained by their recyclability over seven cycles.

The present finding describes an efficient facile approach for the fabrication of nitrogen-doped carbon dots (N-CDs) as a "fluorescent nanoswitch". Highly fluorescent blue-light-emitting N-CDs have been synthesized via a simpler hydrothermal method using 2,2′-(ethylenedioxy)-bis(ethylamine) and malic acid as the precursors. N-CDs showed excitation-dependent and pH-independent emission along with a quantum yield of ∼25%. The blue fluorescent emission of N-CDs has been selectively "turned off" (quenching of fluorescence (FL)) during the sensing of Cr(VI) with 0.02 μM limit of detection and further been selectively "turned on" (restoration of FL) on sensing ascorbic acid, compared with other metal cations and biomolecules tested. For testing the practical applicability of N-CDs, the switchable reversibility of the fluorescent nanoswitch has been tested for up to four cycles on the basis of FL "on−off−on". Furthermore, the toxicological test showed the antibacterial effect of the N-CDs on the tested Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli cells. Additionally, these N-CDs can also be used as a fluorescent ink for imaging purposes.

The same carbon-based nanomaterials have shown their potential photocatalytic applications toward environmental and energy-related problems based on their excellent light absorption abilities. Herein, we summarized the findings associated with visible light and sunlight-induced photocatalysis, specifically using carbon dots (CD), doped-CD, and their nanocomposite as an advanced photocatalytic material. Doped-CD and CD-based composite have improved light absorption abilities compared to bare CD, which is the essential parameter for photocatalysis applications. Various parameters, such as fabrication methodologies, efficiency, and stability, have also been discussed. The possible mechanistic insights involving the photocatalysis method applied in multiple chemical reactions have been discussed in detail, including degradation of organic dye, reduction of higher toxic oxidation states of metal ions, CO 2 conversion, water splitting, organic transformation reaction, and NOx removal, etc.