Distinct from conventional carbon nanostructures, such as fullerene, graphene, and carbon nanotubes, carbon nanodots (C-dots) exhibit unique properties such as strong fluorescence, high photostability, chemical inertness, low toxicity, and biocompatibility. Various synthesis routes for C-dots have been developed in the last few years, and now intense research efforts have been focused on improving their functionality. In this aspect, doping and surface functionalization are two major ways to control the chemical, optical, and electrical properties of C-dots. Doping introduces atomic impurities into C-dots to modulate their electronic structure, and surface functionalization modifies the C-dot surface with functional molecules or polymers. In this review, we summarize recent progress in doping and surface functionalization of C-dots for improving their functionality, and offer insight into controlling the properties of C-dots for a variety of applications ranging from biomedicine to optoelectronics to energy.

Improving the functionality of carbon nanodots: doping and surface functionalization

Rapid detection of Cr(VI) ions based on cobalt(II)-doped carbon dots

pH-mediated reversible fluorescence nanoswitch based on inner filter effect induced fluorescence quenching for selective and visual detection of 4-nitrophenol.

Highly selective detection of p-nitrophenol using fluorescence assay based on boron, nitrogen co-doped carbon dots

In this article, fresh tomatoes are explored as a low-cost source to prepare high-performance carbon dots by using microwave-assisted pyrolysis. Given that amino groups might act as nucleophiles for cleaving covalent bridging ester or ether in the crosslinked macromolecules in the biomass bulk, ethylenediamine (EDA) and urea with amino groups were applied as nucleophiles to modulate the chemical composites of the carbon nanoparticles in order to tune their fluorescence emission and enhance their quantum yields. Very interestingly, the carbon dots synthesized in the presence of urea had a highly crystalline nature, a low-degree amorphous surface and were smaller than 5 nm. Moreover, the doped N contributed to the formation of a cyclic form of core that resulted in a strong electron-withdrawing ability within the conjugated C plane. Therefore, this type of carbon dot exhibited marked quantum confinement, with the maximum fluorescence peak located in the UV region. Carbon nanoparticles greater than 20 nm in size, prepared using pristine fresh tomato and in the presence of EDA, emitted surface state controlled fluorescence. Additionally, carbon nanoparticles synthesized using fresh tomato pulp in the presence of EDA and urea were explored for bioimaging of plant pathogenic fungi and the detection of vanillin.

https://iopscience.iop.org/article/10.1088/1361-6528/aa900b/pdf

Highly crystalline carbon dots from fresh tomato: UV emission and quantum confinement.

Nitrogen-doped carbon quantum dots (N-CDs) have become a current focus of chemical research due to their superior photoluminescence (PL) properties and high fluorescence quantum yield (QY). In this paper, we reported a facile, simple and low-cost approach for the synthesis of highly fluorescent and stable N-CDs by hydrothermal treatment of maleic acid and ethylenediamine. The as-prepared N-CDs emitted strong blue fluorescence with an absolute QY of up to 45%, and exhibited pH-independent behavior in a wide pH range from 1 to 13 and good water solubility. In addition, the fluorescence of the N-CDs can be significantly quenched by p-nitrophenol (4-NP) through an electron-transfer-induced dynamic quenching mechanism, leading to 4-NP being sensed optically. The sensor presented a good linearity with 4-NP concentration in the range of 0.10–11 μg mL−1. The detection limit of 22 ng mL−1 4-NP could be obtained, which is lower than the 60 ng mL−1 limit allowed in drinking water by the U. S. Environmental Protection Agency. The sensor exhibited rapid recognition and distinguished selectivity to 4-NP over other structural analogues of phenols. Moreover, the sensing system was successfully applied for the determination of 4-NP in environmental water samples.

Suling Feng

Papers

Highly photoluminescent pH-independent nitrogen-doped carbon dots for sensitive and selective sensing of p-nitrophenol