Highly fluorescent nitrogen-doped carbon dots for selective and sensitive detection of Hg2+ and ClO− ions and fluorescent ink

doi:10.1016/j.jphotochem.2020.112931

Journal of Photochemistry and Photobiology A: Chemistry

Volume 405, 15 January 2021, 112931

https://doi.org/10.1016/j.jphotochem.2020.112931 Get rights and content

Highlights

•
Nitrogen-doped carbon dots (NCDs) were synthesized by one-pot hydrothermal treatment.
•
NCDs exhibited strong blue fluorescence with a high fluorescence quantum yield.
•
A highly sensitive and selective NCDs fluorescent probe for detection of Hg²⁺ and ClO⁻.
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NCDs could be used as a fluorescent ink.
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The sensor was successfully applied in water samples with satisfactory recovery.

Abstract

A facile one-step hydrothermal strategy using citric acid, ethylenediamine, and fluorescein as the precursors for the synthesis of fluorescent nitrogen-doped carbon dots (NCDs). The obtained NCDs exhibited blue fluorescence with maximum excitation/emission wavelengths at 350/447 nm and fluorescence quantum yield was up to 61%. The blue emission of the NCDs was independent of the excitation wavelength from 330 to 390 nm, while they possessed spherical morphology. The NCDs were also very stable in a wide pH range and in solutions with high ionic strength. More specifically, the fluorescence of the NCDs was effectively quenched by mercuric (Hg²⁺) and hypochlorite (ClO⁻) ions, suggesting that the NCDs can be utilized for detecting Hg²⁺ and ClO⁻ with high selectivity and sensitivity. The linear ranges and limits of detection were 0.05–30 μM and 19 nM for Hg²⁺, and 0.1–70 μM and 90 nM for ClO⁻, respectively. Moreover, the NCDs were applied for detecting Hg²⁺ and ClO⁻ in water samples with satisfactory results, indicating its sensing potential in environmental monitoring. Additionally, it was proven that the synthesized NCDs could be utilized as a 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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Citation Excerpt :
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Highly fluorescent nitrogen-doped carbon dots for selective and sensitive detection of Hg2+ and ClO− ions and fluorescent ink

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Synthesis and characterization of the NCDs

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgments

Trends Analyt. Chem.

Trends Anal. Chem.

J. Environ. Sci.

Reprod. Toxicol.

J. Trace Elem. Med. Biol.

Chem. Eng. J.

Anal. Chim. Acta

Trends Anal. Chem.

Sci. Total Environ.

Sens. Actuator B Chem.

Adv. Colloid Interface Sci.

Mater. Sci. Eng. C

J. Alloy. Compd.

Carbon

Sens. Actuator B Chem.

Talanta

Sens. Actuator B Chem.

Biosens. Bioelectron.

J. Mol. Liq.

Sustainable technologies for water purification from heavy metals: review and analysis

Chem. Soc. Rev.

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Analyst

Highly fluorescent nitrogen-doped carbon dots for selective and sensitive detection of Hg²⁺ and ClO⁻ ions and fluorescent ink