Highly fluorescent nitrogen-doped carbon dots for selective and sensitive detection of Hg2+ and ClO− ions and fluorescent ink
Graphical abstract
Introduction
The pollution from heavy metal ions has become an urgent concern because of their bioaccumulation in the environment and the potential risks to human health [[1], [2], [3]]. Due to its powerful affinity with the thiol groups in intracellular proteins and enzymes, the mercuric ion (Hg2+) is one of the most hazardous species. The excessive accumulation of Hg in the body can cause serious damage to DNA, central nervous system, liver, kidney, and intestinal and even lead to death [[4], [5], [6]]. In addition, the hypochlorite ions (ClO−) have been widely used as powerful oxidants and disinfectants in our daily lives as e.g., water disinfection, sewage purification, and cooling-water treatment agents [7,8]. However, the excessive ClO− concentration may lead to pathological disorders including atherosclerosis, lung and kidney injury, and various cancer types [9]. Therefore, it is important to develop reliable, rapid, sensitive, and selective methods for the determination of low Hg2+ and ClO− concentrations in complex matrices.
To date, several analytical methods have been developed for the determination of Hg2+ and ClO− with high accuracy and sensitivity [[10], [11], [12]]. However, they require sophisticated and expensive instrument and complicated sample preparation procedures, which limit their widespread application. Therefore, the development of various approaches for the detection of Hg2+ and ClO− with high selectivity and sensitivity is still an imperative need. Recently, fluorescence sensing approaches have attracted increasing interest due to the excellent sensitivity, fast response, and simple operation [13,14].
Fluorescent carbon dots (CDs) are a new type of carbon-based nanoparticles with a size less than 10 nm. Compared to the conventional fluorescent probes, such as organic dyes, quantum dots, and metal nanoclusters, CDs outperform due to their low cost, simple synthesis, chemical inertness, easy functionalization, excellent optical properties, good biocompatibility, great water solubility, and lower toxicity [15,16]. Thus, CDs have been used in many practical applications including chemical sensing, photocatalysts, and bioimaging/biosensing [[17], [18], [19], [20]], while various strategies, such as pyrolysis, hydrothermal, microwave, combustion, and electrochemical approaches, have been adopted for their synthesis [[21], [22], [23], [24], [25]]. Nevertheless, most reported CDs exhibit relatively weak fluorescence intensity and low quantum yield (QY). Therefore, several strategies, such as heteroatom doping, surface functionalization, and passivation of the CDs, have been applied to effectively tune their energy levels, electronic density, and local chemical properties, further improving the CDs-based probe performance [16]. Recently, nitrogen-doped CDs (NCDs) were synthesized using ethanolamine and 1-carboxyethyl-3-methyl imidazole chloride as the precursor [26], while nitrogen- and sulfur-co-doped CDs with a fluorescence QY of 16% were prepared by an one-pot hydrothermal approaches [27].
In this work, a new approach was developed for the facile synthesis of highly fluorescent NCDs by the hydrothermal of citric acid, ethylenediamine, and fluorescein in one step. The obtained NCDs were used to detect Hg2+ and ClO− ions (Scheme 1) and as a fluorescent ink. Moreover, the synthesized NCDs exhibited strong blue fluorescence with a high relative QY (61%), good aqueous solubility and photostability, and narrow size distribution. Interestingly, both Hg2+ and ClO− significantly quenched the fluorescence of the NCDs with high efficiency. Furthermore, this sensing system has been successfully applied to the detection of Hg2+ and ClO− in water samples with high selectivity and sensitivity.
Section snippets
Materials
Citric acid, ethylenediamine, fluorescein, quinine sulfate, sodium hydroxide, trisodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, phosphoric acid, anhydrous metal salts LiNO3, NaNO3, KNO3, Mg(NO3)2, Ca(NO3)2, Ba(NO3)2, Al(NO3)3, Pb(NO3)2, FeCl3, Co(NO3)2, Ni(NO3)2, Cu(NO3)2, ZnCl2, HgCl2, HAuCl4, NaCl, NaBr, KI, Na2SO4, H2C2O4, KCl, NaNO2, NaBH4, ascorbic acid, Na2SO3, Na3C6H5O7, and NaClO were obtained from Sigma-Aldrich (St. Louis, MO, USA). The chemicals and
Synthesis and characterization of the NCDs
The experimental parameters, i.e., the fluorescein weight, duration time, and temperature, were investigated to achieve the high fluorescence QY. The effect of the fluorescein weight was first examined by selecting the reaction temperature of 240 °C and a duration of treatment of 4 h. As presented in Table 1, all the NCDs were formed at various weights of fluorescein, while the highest fluorescence QY of 61% was achieved. Hence, based on their further application in fluorescence sensing, the
Conclusions
In conclusion, we have prepared highly fluorescent nitrogen-doped CDs via a facile one-pot hydrothermal approach using citric acid, ethylenediamine, and fluorescein as precursors. The obtained NCDs exhibited a uniform spherical morphology, a high fluorescent QY of 61%, excellent fluorescence properties, good water solubility, and high stability. In particular, the NCDs could successfully detect Hg2+ and ClO− ions with high sensitivity and selectivity, while their estimated LODs were 19 and
CRediT authorship contribution statement
Yu-Xuan Li: Methodology, Software, Data curation, Visualization, Investigation, Validation. Jan-Yee Lee: Software, Data curation. Hsin Lee: Data curation. Cho-Chun Hu: Conceptualization, Methodology. Tai-Chia Chiu: Conceptualization, Data curation, Visualization, Writing - original draft, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that there are no conflicts of interest.
Acknowledgments
This work was financially supported by the Ministry of Science and Technology of Taiwan under contract number MOST 109-2113-M-143-002.
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2022, Applied Surface ScienceCitation Excerpt :During the quenching process, the ground state electrons are excited and entered the excited state, and the ground state orbital generates holes to form electron-hole pairs. Cu2+ ions can serve as an electron acceptor and block electrons from interacting with the holes, which leads to the fluorescence quenching [58,59]. In order to restore the fluorescence of B-CDs quenched by Cu2+ ions, DL-serine is selected to restore the fluorescence of Cu2+-B-CDs solution, while other substances such as L-Threonine, Tyrosine, EDTA, etc. have no ideal effect like DL-serine (Figure S13).