Next Article in Journal
Atomic Arrangements of Graphene-like ZnO
Next Article in Special Issue
Towards Red Emissive Systems Based on Carbon Dots
Previous Article in Journal
The Enhanced Hydrogen Storage Capacity of Carbon Fibers: The Effect of Hollow Porous Structure and Surface Modification
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Nanomaterials
Volume 11
Issue 7
10.3390/nano11071831
nanomaterials-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

One-Pot Hydrothermal Synthesis of Carbon Dots as Fluorescent Probes for the Determination of Mercuric and Hypochlorite Ions

by
Hsin Lee
1,
Yen-Chang Su
1,
Hsiang-Hao Tang
1,
Yu-Sheng Lee
1,
Jan-Yee Lee
2,
Cho-Chun Hu
1 and
Tai-Chia Chiu
1,*
1
Department of Applied Science, National Taitung University, Taitung 95092, Taiwan
2
Department of Environment Engineering, Kun Shan University, Tainan 710303, Taiwan
*
Author to whom correspondence should be addressed.
Nanomaterials 2021, 11(7), 1831; https://doi.org/10.3390/nano11071831
Submission received: 12 June 2021 / Revised: 6 July 2021 / Accepted: 13 July 2021 / Published: 14 July 2021
(This article belongs to the Special Issue Carbon Dots: Structure, Properties and Emerging Applications)
Browse Figures
Versions Notes

Abstract

:
Nitrogen and sulfur codoped carbon dots (NSCDs) were synthesized via a one-pot hydrothermal method, and citric acid, ethylenediamine, and methyl blue were used as precursors. The obtained NSCDs were spherical with an average size of 1.86 nm. The fluorescence emission spectra of the NSCDs were excitation independent and emitted blue fluorescence at 440 nm with an excitation wavelength at 350 nm. The quantum yield of the NSCDs was calculated to be 68.0%. The NSCDs could be constructed as fluorescent probes for highly selective and sensitive sensing mercuric (Hg2+) and hypochlorite (ClO−) ions. As the addition of Hg2+ or ClO− ions to the NSCDs, the fluorescence intensity was effectively quenched due to dynamic quenching. Under the optimal conditions, the linear response of the fluorescence intensity ranged from 0.7 μM to 15 μM with a detection limit of 0.54 μM and from 0.3 μM to 5.0 μM with a limit of detection of 0.29 μM for Hg2+ and ClO− ions, respectively. Finally, the proposed method was successfully used for quantifying Hg2+ and ClO− ions in spiked tap water samples.

Graphical Abstract

1. Introduction

Carbon dots (CDs), a new kind of zero-dimensional fluorescent nanomaterial with sizes under 10 nm, have drawn tremendous attention from researchers since they were first discovered in 2004 [1]. Owing to their chemical inertness, stable photoluminescence, good water dispersibility, excellent biocompatibility, and ease of preparation and functionalization, CDs have wide applications in bioimaging [2,3], fluorescent sensors [4,5], photocatalysis [6], drug delivery [7], optoelectronic devices (light-emitting diodes and solar cells) [8], and therapy [9]. At present, various synthetic methods, such as arc discharge, laser ablation, electrochemical carbonization, ultrasonication, pyrolysis, microwave irradiation, and hydrothermal treatment, have been adopted for the fabrication of CDs. These methods can be simply classified into top-down and bottom-up strategies. Among these synthetic methods, the hydrothermal route has been the most common because it is simple, cost-effective, and convenient. However, the fluorescence quantum yield (QY) of the obtained CDs is relatively low (<5%). Doping with heteroatoms (e.g., B, N, S, and P) is considered to be an effective way to enhance the fluorescence properties of CDs [10]. Up to now, many efforts have been devoted to the synthesis of heteroatom-doped CDs from numerous precursors [11]. For example, Dong et al. [12] reported the preparation of nitrogen and sulfur codoped CDs (NSCDs) using citric acid and L-cysteine as precursors. The fluorescence quantum yield (QY) of the NSCDs was 80.0%. Wang et al. [13] employed citric acid and thiourea to fabricate NSCDs through hydrothermal synthesis. Results showed that the CDs had a bright blue emission with a QY of 73.1%. Zhang et al. [14] chose citric acid and N-aminoethylpiperazine as precursors to prepare nitrogen-doped CDs with a QY as high as 56.0%. Although the heteroatom-doped CDs had achieved high QYs, a facile hydrothermal treatment is still needed for the synthesis of fluorescent CDs from organic compounds with heteroatoms and specific functional groups for trace analysis.
Rapid detection of hazardous metal ions or anions in diverse environmental samples is very important for the protection of environments and human health. Mercury ions (Hg2+) are one of the most toxic pollutants owing to their high toxicity, lack of biodegradability, and easy bioaccumulation in the human body [15]. As a result, they pose a serious threat to human health and the environment. Hypochlorite ions (ClO−) are one of the most efficient oxidizing agents and have been extensively used in water treatment processes for destroying microorganisms [16]. However, the excess of ClO− might react with organic compounds in aqueous solutions to produce undesirable trihalomethanes as byproducts. They have been reported to be harmful and carcinogenic [16]. Therefore, selective, sensitive, and efficient methods for Hg2+ or ClO− ion detection in water samples are highly desirable. Up to now, fluorescent probes or fluorescence-based sensors have drawn much more attention due to their simplicity, low cost, high sensitivity, and short analysis time. Various fluorescent sensory materials are currently being developed and designed such as semiconductor quantum dots, metal nanoclusters and organic dye molecules [17,18]. However, these materials are restricted from use by their toxicity, low sensitivity, and high cost. Therefore, CDs could be an excellent candidate for the determination of Hg2+ or ClO− ions [19,20,21,22,23,24]. For example, fluorescent CDs were synthesized from ascorbic acid and urea and used for the selective determination of Hg2+ and Cu2+ ions and bioimaging [19]. A fluorescence quenching sensor for Hg2+ based on CDs was designed by Yang et al. [20], which demonstrated improved selectivity based on the surface structural rational regulation. Wang et al. [21] reported multicenter-emitting CDs for the dynamic monitoring of wound-induced hypochlorous acid burst and oxidative stress in vivo. Nitrogen and phosphorus codoped CDs with long-wavelength emission have been synthesized and applied in sensing ClO− and cell imaging [22]. Bruno et al. compared the interaction pathways with Hg2+ ions of the CDs synthesized through the bottom-up and top-down approaches and provided a convenient view for utilizing bottom-up CDs as sensors [25]. Indeed, an alternative fluorescent sensor is still required for the selective and sensitive detection of Hg2+ or ClO− ions.
In this work, nitrogen and sulfur codoped carbon dots with strong blue fluorescence were synthesized through a hydrothermal treatment using citric acid, ethylenediamine, and methyl blue as carbon, nitrogen, and sulfur sources. NSCDs were used as a fluorescent sensor for Hg2+ and ClO− ions, and the assay principle is shown in Scheme 1. The fluorescence emission of NSCDs was found to be independent of the excitation wavelength and insensitive to environmental conditions such as pH and ionic strength. The fluorescence intensity could be quenched by adding Hg2+ or ClO− ions to the NSCDs. The selective and sensitive fluorescent probes were successfully developed for sensing Hg2+ or ClO− ions in actual samples. Finally, UV–vis absorption spectroscopy and fluorescence decay measurements were used to analyze the fluorescence sensing mechanism for Hg2+ or ClO− detection.

2. Materials and Methods

2.1. Chemicals and Reagents

Ascorbic acid, citric acid, disodium hydrogen phosphate, ethylenediamine, methyl blue, oxalic acid, phosphoric acid, quinine sulfate, sodium citrate, sodium dihydrogen phosphate, sodium hydroxide, trisodium phosphate, and the other metal salts used here were purchased from Sigma-Aldrich (St. Louis, MO, USA). Ultrapure water (≥18.2 MΩ cm) was used throughout all the experiments.

2.2. Characterization

The UV–vis absorption and fluorescence spectra were acquired by a U-2900 spectrophotometer (Hitachi, Tokyo, Japan) and an RF-6000 fluorescence spectrophotometer (Shimadzu, Kyoto, Japan). Fourier transform infrared (FT-IR) spectra were obtained on an FT-IR spectrometer (Perkin Elmer, Waltham, MA, USA) using the KBr pellet method. X-ray photoelectron spectroscopy (XPS) data were measured on a K-Alpha XPS system (Thermo Fisher Scientific, Waltham, MA, USA). Transmission electron microscopy (TEM) images were taken on a Philips CM-200 (Philips, Eindhoven, The Netherlands) with a working voltage of 200 kV.

2.3. Preparation of the NSCDs

The NSCDs were prepared through a one-pot hydrothermal approach. Briefly, citric acid (0.420 g) and methyl blue (0.003 g) were dissolved in ultrapure water (10 mL), and then ethylenediamine (536 μL) was added upon stirring for 10 min. The solution was subsequently transferred into a Teflon-lined stainless-steel autoclave and heated at 240 °C for 4 h. The resulting brown-color solution was purified by following the procedures in our previous research [26]. Finally, the purified NSCDs were stored at 4 °C for further characterization and applications.

2.4. Fluorescence Sensing of Hg2+ or ClO−

The NSCDs were applied for sensing Hg2+ or ClO− ions. For a typical process, ultrapure water (600 μL), NSCD solution (200 μL), phosphate buffer (200 μL, 100 mM, pH 5.0), and various concentrations of Hg2+ or ClO− (200 μL) were mixed well in 2 mL vials and reacted for 30 min. The fluorescence intensity of the NSCDs was recorded at 440 nm with an excitation wavelength of 350 nm. The working solutions of Hg2+ or ClO− ions were freshly prepared by diluting stock solution (1 mM) with ultrapure water to the desired concentrations. The selectivity and interference measurements were operated at the same conditions mentioned above.

2.5. Determination of Hg2+ or ClO− in Tap Water Samples

To evaluate the applicability of the NSCDs in practical applications for detecting of Hg2+ or ClO− ions, three concentrations of Hg2+ (5.0 μM, 8.0 μM, and 10.0 μM) or ClO− (0.5 μM, 2.0 μM, and 4.0 μM) were spiked in tap water samples. The water samples were centrifuged at 10,000 rpm for 10 min and then filtered through a 0.22 μm nitrocellulose filter to remove any suspended particles. The procedures for the quantitative analysis of Hg2+ or ClO− ions were the same as Section 2.4.

3. Results and Discussion

3.1. Preparation and Characterization of the NSCDs

The NSCDs were synthesized using citric acid, ethylenediamine, and methyl blue as precursors through a one-pot hydrothermal approach. To obtain the NSCDs with excellent fluorescence properties, the effects of methyl blue content, temperature, and duration time for hydrothermal treatment were studied at several synthetic conditions. Table S1 shows the fluorescence QYs of the NSCDs under different synthetic conditions. It was found that hydrothermal treatment of 0.420 g of citric acid, 536 μL of ethylenediamine, and 0.003 g of methyl blue in 10 mL of ultrapure water at 240 °C for 4 h constituted optimal conditions. The calculated fluorescence QY of the NSCDs was raised up to 68.0% by using quinine sulfate (QY = 54%) as a standard [26], and they were used for further experiments as fluorescent probes. When in the presence of Hg2+ or ClO− ions, the fluorescence intensity was quenched effectively and quickly. The morphology of the NSCDs was obtained by TEM that exhibited the NSCDs as spherical in shape and uniformly dispersed with an average diameter of 1.86 ± 0.04 nm (Figure S1).
The surface functional groups of the NSCDs were characterized by FT-IR spectroscopy (Figure 1). The broad peak at 3000–3500 cm–1 was assigned to the stretching vibration absorption peak of the N–H and O–H bonds [27]. The peak at 1652 cm–1 was related to the stretching vibration peak of the C=O bond [28]. The bending vibration of the C=C appears at 1557 cm–1 [28]. The peak at 1397 cm–1 could be identified as the stretching vibration of the C–C bond [29]. The wake peak at 1124 cm–1 was attributed to the stretching vibration of the C–N and C–S bonds [30]. All these results confirmed that sulfur-, nitrogen-, and oxygen-containing functional groups were successfully introduced on the surface of the NSCDs.
The XPS measurements were further characterized as the elemental composition and chemical bonds of the NSCDs (Figure 2). The full-scan XPS spectrum (Figure 2a) of the NSCDs presented four peaks at 168.08 eV, 286.08 eV, 400.08 eV, and 532.08 eV, which corresponded to sulfur (S2p), carbon (C1s), nitrogen (N1s), and oxygen (O1s) elements [31], and their atomic contents were 1.09%, 65.99%, 14.17%, and 18.75%, respectively. The low content of sulfur atoms may be owing to the lower mole addition ratio of methyl blue (3.8 μmole) compared with citric acid (2.2 mmole) and ethylenediamine (7.9 mmole). These observations indicated that the NSCDs were composed of S, C, N, and O elements. Specifically, the high-resolution C1s spectrum (Figure 2b) was deconvoluted into three peaks at 284.98 eV, 286.13 eV, and 288.43 eV, which were attributed to C–C/C=C, C–O, and C=O groups, respectively [23]. The high-resolution N1s spectrum (Figure 2c) of the NSCDs showed three peaks detected at 399.43 eV, 400.33 eV, and 401.78 eV, which were attributed to pyridinic N, pyrrolic N, and graphitic N, respectively [27]. The peak of the high-resolution O1s spectrum (Figure 2d) could be fitted into three peaks at 531.63 eV, 532.78 eV, and 533.88 eV, which were assigned to O=C, O–H, and O=C–O groups, respectively [31]. As for the S2p spectrum (Figure 2e), the peaks centered at 168.03 eV and 169.28 eV were related to the –C–SOx– (x = 3 and 4) species [31].

3.2. Optical Properties of the NSCDs

The UV–vis absorption and fluorescence spectra of the NSCDs were recorded to characterize their optical properties. Two obvious absorption peaks at 237 nm and 345 nm are shown in Figure 3, which could be attributed to the π–π* electronic transitions of the C=C bond and the n–π* electronic transitions of the C=O bond [32], respectively. The NSCDs showed a yellow color under daylight illumination and strong blue fluorescence excited by a UV lamp at 365 nm (inset of Figure 3). The excitation spectrum exhibited a characteristic peak at 350 nm, which corresponded to the UV–vis absorption spectrum, and the emission wavelength was observed at 440 nm. Moreover, the fluorescence spectra of the NSCDs were recorded under different excitation wavelengths from 320 nm to 380 nm. As a result, the emission wavelength almost remained at 440 nm with various excitation wavelengths (Figure S2). The excitation-independent fluorescence characteristic may be due to the narrow size distribution (Figure S1) and few surface defects on the NSCDs, which was similar to other reported CDs [12]. The NSCDs exhibited a fluorescence QY as high as 68.0%. The synergistic effect of the codoped sulfur and nitrogen atoms leads to the high-fluorescence QY [12].

3.3. Fluorescence Stability of the NSCDs

The effect of pH (2–12), ionic strength (0–1.0 M), UV irradiation time (0–60 min), and storage time (0–210 days) was employed to assess the fluorescence stability of the NSCDs. As shown in Figure S3, the NSCDs exhibited highly stable fluorescence intensity ranging from pH 4.0 to 10.0 (10 mM phosphate buffer) and at high ionic strength up to 1.0 M. The NSCDs also exhibited high photostability under a UV light irradiation for 60 min. Moreover, storage of the NSCDs for 7 months at 4 °C did not result in obvious aggregation, demonstrating the good stability of the NSCD solution. Therefore, these experimental results indicated potential applications of the NSCDs as effective fluorescent probes.

3.4. Fluorescence Quenching for Hg2+ or ClO− Detection

To determine the concentration of Hg2+ or ClO− ions in an aqueous solution, it was investigated by using the NSCDs as fluorescence probes. Figure 4 shows a gradually decreasing fluorescence intensity of the NSCDs at 440 nm with the increasing addition of Hg2+ or ClO− ions. The fluorescence quenching efficiency of the NSCDs was evaluated through fitting the Stern−Volmer equation [33,34]:
F0/F = 1 + Ksv [Q]
where F0 and F are the fluorescence intensity of the NSCDs at 440 nm in the absence and presence of the analytes (Hg2+ or ClO− ions), respectively; Ksv is the Stern−Volmer constant; and Q is the concentration of the analytes. It was found (Figure 4b) that the proposed method showed a good linear relationship with Hg2+ concentration in the range of 0.7 μM to 15 μM, and a linear regression coefficient (R2 = 0.9847) and Ksv value of 5.46 × 104 M–1 was obtained. Moreover, the calculated limit of detection (LOD) was 0.54 μM according to Equation 3σ/m (where σ is the standard deviation of the blank, m is the slope of the linear fit, and n = 10 independent measurements) [30]. Similarly, the fluorescence intensity was also obviously quenched by the addition of ClO− to the NSCDs (Figure 4d). The fluorescence intensity ratio (F0/F) was proportional to the ClO− concentration from 0.3 μM to 5 μM, and a linear regression coefficient (R2 = 0.9883) and Ksv value of 7.16 × 104 M–1 was obtained. The LOD was calculated to be 0.29 μM (3σ/m). The proposed fluorescent probes were compared with other CD-based sensors for sensing Hg2+ or ClO− ions, and the results are presented in Tables S2 and S3. As can be seen, the NSCDs had moderate sensitivity and a comparable linear range, thus indicating their potential applications for sensing Hg2+ or ClO− ions.
The selectivity experiments of the proposed method for Hg2+ or ClO− ions detection were examined by recording the fluorescence response of the NSCDs toward some interfering ions (100 μM) with Hg2+ or ClO− ions (100 μM). The fluorescence intensities of the NSCDs before and after the addition of the interfering ions were recorded. As shown in Figure 5a, the NSCDs showed a high affinity for the detection of Hg2+ under optimal conditions. Thus, Hg2+ was the only one that led to the obvious fluorescence quenching (85.9%) of the NSCDs, whereas the other metal ions caused slight or even no change. Similarly, among the possible interfering anions, they had almost no effect on the fluorescence intensity of the NSCDs, except with ClO− (fluorescence intensity quenched 98.4%) (Figure 5b). These results proved that the NSCDs could be utilized as highly selective fluorescent probes for sensing Hg2+ or ClO− ions.
The interference experiments revealed that the NSCDs were more selective toward Hg2+ and ClO− compared to other possible coexisting ions (Figure 5). The concentrations of the possible interfering ions were tenfold higher than the concentrations of Hg2+ and ClO− ions. It was clearly shown that the existence of other metal ions did not interfere with the detection of Hg2+. The interference experiment for other coexisting anions in the presence of ClO− also obtained similar results. These observations indicated that the NSCDs could be utilized for the determination of Hg2+ or ClO− ions selectively.

3.5. Sensing Mechanism

Generally, the fluorescence quenching of the NSCDs can be classified as a static or dynamic quenching mechanism [35]. These two mechanisms can be distinguished by the fluorescence lifetime and UV–vis absorption spectra of the NSCDs at various concentrations of the analytes. The fluorescence lifetime decay of the NSCDs in the absence and presence of Hg2+ or ClO− ions were investigated (Figure S4). It was found that the measured average fluorescence lifetime of the NSCDs was 13.10 ns. After the addition of Hg2+ or ClO−, the fluorescence lifetime of the NSCDs was reduced to 9.16 ns (0.1 mM), 6.67 ns (1.0 mM), and 5.33 ns (10 mM) for NSCDs-Hg2+ and 11.13 ns (0.1 mM), 4.13 ns (1.0 mM), and 1.38 ns (10 mM) for NSCDs-ClO−, respectively. The phenomenon was similar to the previous report [36]. These results confirmed that the existence of Hg2+ or ClO− ions alters the fluorescence lifetime of the NSCDs, indicating that the fluorescence quenching of the NSCDs by Hg2+ or ClO− ions followed the dynamic quenching mechanism [35]. Figure S5 showed that the absorption threshold and peak position did not change after the addition of various concentrations of Hg2+ or ClO− ions to the NSCDs in the wavelength range of 300 nm to 600 nm. The results indicated that Hg2+ or ClO− did not interact with the NSCDs to form stable complexes. Therefore, non-fluorescence electron transfer from the excited state of the NSCDs to Hg2+ or ClO− ions was the dominant fluorescence quenching mechanism [37].

3.6. Determination of Hg2+ or ClO− in Water Samples

The applicability of the NSCDs was evaluated using Hg2+ (5.0 μM, 8.0 μM, and 10.0 μM) or ClO− (0.5 μM, 2.0 μM, and 4.0 μM) spiked tap water samples by recording the fluorescence intensity change. The quenched fluorescence intensity was observed after the addition of Hg2+ or ClO− ions. The satisfactory recoveries between 97.53% and 102.01% with RSDs below 4.27% for Hg2+ and between 93.67% and 116.49% with RSDs below 6.12% for ClO− were obtained as shown in Table 1. The results clearly demonstrated that the NSCDs as fluorescence probes were applicable for the detection of Hg2+ and ClO− ions in water samples.

4. Conclusions

In this study, NSCDs were successfully prepared through a facile one-pot hydrothermal approach wherein citric acid, ethylenediamine, and methyl blue were used as precursors. The blue fluorescence NSCDs with a high QY of 68.0% are well dispersed in water solutions. In addition, the NSCDs show highly sensitive and selective sensing Hg2+ and ClO− ions through a fluorescence quenching process. The linear ranges and LODs are 0.7–15 μM and 0.54 μM for Hg2+ and 0.3–5 μM and 0.29 μM for ClO−, respectively. The results of UV–vis absorption spectra and fluorescence decay measurements indicate that the fluorescence quenching for sensing Hg2+ or ClO− ions is related to the dynamic quenching mechanism. Therefore, the NSCDs can be used as fluorescent probes with high sensitivity and selectivity for the determination of Hg2+ or ClO− ions in aqueous solutions.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/nano11071831/s1, Figure S1: TEM image and size distribution of the NSCDs, Figure S2: Fluorescence emission spectra of the NSCDs at different excitation wavelengths that change from 320 nm to 380 nm, Figure S3: Effect of various conditions on the fluorescence intensity of the NSCDs: (a) pH, (b) ionic strength, (c) UV light irradiation time, (d) storage time, Figure S4: Fluorescence decay curves of the NSCDs with (a) mercuric ions (0.1, 1.0, and 10 mM) and (b) hypochlorite ions (0.1, 1.0, and 10 mM), respectively, Figure S5: UV–vis absorption spectra of (a) Hg2+ and the NSCDs in the absence and presence of various concentrations of Hg2+, and (b) ClO– and the NSCDs in the absence and presence of various concentrations of ClO–, Table S1: QYs of the NSCDs under different synthetic conditions, Table S2: Comparison of different fluorescent CDs applied for Hg2+ detection, Table S3: Comparison of different fluorescent CDs applied for ClO– detection.

Author Contributions

Conceptualization, T.-C.C. and C.-C.H.; methodology, T.-C.C. and C.-C.H.; formal analysis, H.L., Y.-C.S., H.-H.T., J.-Y.L. and T.-C.C.; investigation, H.L., Y.-C.S., H.-H.T. and Y.-S.L.; data curation, H.L., Y.-C.S., H.-H.T. and J.-Y.L.; validation, H.L.; writing—original draft preparation, H.L. and T.-C.C.; writing—review and editing, T.-C.C.; supervision, T.-C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Ministry of Science and Technology of Taiwan under contract number MOST 109-2113-M-143-002.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors are thankful to the Joint Center of High Valued Instruments, National Sun Yat-sen University, for the TEM (EM001400) measurements.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Xu, X.; Ray, R.; Gu, Y.; Ploehn, H.J.; Gearheart, L.; Raker, K.; Scrivens, W.A. Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments. J. Am. Chem. Soc. 2004, 126, 12736–12737. [Google Scholar] [CrossRef]
  2. Chen, B.B.; Liu, M.L.; Huang, C.Z. Recent advances of carbon dots in imaging-guided theranostics. Trends Anal. Chem. 2021, 134, 116116. [Google Scholar] [CrossRef]
  3. Chung, Y.J.; Kim, J.; Park, C.B. Photonic carbon dots as an emerging nanoagent for biomedical and healthcare applications. ACS Nano 2020, 14, 6470–6497. [Google Scholar] [CrossRef] [PubMed]
  4. Ji, C.; Zhou, Y.; Leblanc, R.M.; Peng, Z. Recent developments of carbon dots in biosensing: A review. ACS Sens. 2020, 5, 2724–2741. [Google Scholar] [CrossRef] [PubMed]
  5. Li, X.; Zhao, S.; Li, B.; Yang, K.; Lan, M.; Zeng, L. Advances and perspectives in carbon dot-based fluorescent probes: Mechanism, and application. Coord. Chem. Rev. 2021, 431, 213686. [Google Scholar] [CrossRef]
  6. Rosso, C.; Filippini, G.; Prato, M. Carbon dots as nano-organocatalysts for synthetic applications. ACS Catal. 2020, 10, 8090–8105. [Google Scholar] [CrossRef]
  7. Devi, P.; Saini, S.; Kim, K.-H. The advanced role of carbon quantum dots in nanomedical applications. Biosens. Bioelectron. 2019, 141, 111158. [Google Scholar] [CrossRef] [PubMed]
  8. Ghosh, D.; Sarkar, K.; Devi, P.; Kim, K.-H.; Kumar, P. Current and future perspectives of carbon and graphene quantum dots: From synthesis to strategy for building optoelectronic and energy devices. Renew. Sust. Energ. Rev. 2021, 135, 110391. [Google Scholar] [CrossRef]
  9. Li, B.; Zhao, S.; Huang, L.; Wang, Q.; Xiao, J.; Lan, M. Recent advances and prospects of carbon dots in phototherapy. Chem. Eng. J. 2021, 408, 127245. [Google Scholar] [CrossRef]
  10. Kou, X.; Jiang, S.; Park, S.-J.; Meng, L.-Y. A review: Recent advances in preparations and applications of heteroatom-doped carbon quantum dots. Dalton Trans. 2020, 49, 6915–6938. [Google Scholar] [CrossRef]
  11. Miao, S.; Liang, K.; Zhu, J.; Yang, B.; Zhao, D.; Kong, B. Hetero-atom-doped carbon dots: Doping strategies, properties and applications. Nano Today 2020, 33, 100879. [Google Scholar] [CrossRef]
  12. Dong, Y.; Pang, H.; Yang, H.B.; Guo, C.; Shao, J.; Chi, Y.; Li, C.M.; Yu, T. Carbon-based dots co-doped with nitrogen and sulfur for high quantum yield and excitation-independent emission. Angew. Chem. Int. Ed. 2013, 52, 7800–7804. [Google Scholar] [CrossRef]
  13. Wang, H.; Lu, Q.; Hou, Y.; Liu, Y.; Zhang, Y. High fluorescence S, N co-doped carbon dots as an ultra-sensitive fluorescent probe for the determination of uric acid. Talanta 2016, 155, 62–69. [Google Scholar] [CrossRef] [PubMed]
  14. Zhang, H.; You, J.; Wang, J.; Dong, X.; Guan, R.; Cao, D. Highly luminescent carbon dots as temperature sensors and “off-on” sensing of Hg2+ and biothiols. Dyes Pigments 2020, 173, 107950. [Google Scholar] [CrossRef]
  15. Shahid, M.; Khalid, S.; Bibi, I.; Bundschuh, J.; Niazi, N.K.; Dumat, C. A critical review of mercury speciation, bioavailability, toxicity and detoxification in soil-plant environment: Ecotoxicology and health risk assessment. Sci. Total Environ. 2020, 711, 134749. [Google Scholar] [CrossRef]
  16. Dong, S.; Zhang, L.; Lin, Y.; Ding, C.; Lu, C. Luminescent probes for hypochlorous acid in vitro and in vivo. Analyst 2020, 145, 5068–5089. [Google Scholar] [CrossRef]
  17. Shuai, H.; Xiang, C.; Qian, L.; Bin, F.; Xiaohui, L.; Jipeng, D.; Chang, Z.; Jiahui, L.; Wenbin, Z. Fluorescent sensors for detection of mercury: From small molecules to nanoprobes. Dyes Pigments 2021, 187, 109125. [Google Scholar] [CrossRef]
  18. Zhang, R.; Song, B.; Yuan, J. Bioanalytical methods for hypochlorous acid detection: Recent advances and challenges. Trends Anal. Chem. 2018, 99, 1–33. [Google Scholar] [CrossRef]
  19. Bhatt, M.; Bhatt, S.; Vyas, G.; Raval, I.H.; Haldar, S.; Paul, P. Water-dispersible fluorescent carbon dots as bioimaging agents and probes for Hg2+ and Cu2+ ions. ACS Appl. Nano Mater. 2020, 3, 7096–7104. [Google Scholar] [CrossRef]
  20. Yang, M.; Liu, C.; Peng, Y.; Xiao, R.-Z.; Zhang, S.; Zhang, Z.-L.; Zhang, B.; Pang, D.-W. Surface chemistry tuning the selectivity of carbon nanodots towards Hg2+ recognition. Anal. Chim. Acta 2021, 1146, 33–40. [Google Scholar] [CrossRef]
  21. Wang, Y.; Guo, G.; Gao, J.; Li, Z.; Yin, X.; Zhu, C.; Xia, Y. Multicenter-emitting carbon dots: Color tunable fluorescence and dynamics monitoring oxidative stress in vivo. Chem. Mater. 2020, 32, 8146–8157. [Google Scholar] [CrossRef]
  22. Jiao, Y.; Meng, Y.; Lu, W.; Gao, Y.; Liu, Y.; Gong, X.; Liu, Y.; Shuang, S.; Dong, C. Design of long-wavelength emission carbon dots for hypochlorous detection and cellular imaging. Talanta 2020, 219, 121170. [Google Scholar] [CrossRef] [PubMed]
  23. Li, L.-S.; Jiao, X.-Y.; Zhang, Y.; Cheng, C.; Huang, K.; Xu, L. Green synthesis of fluorescent carbon dots from Hongcaitai for selective detection of hypochlorite and mercuric ions and cell imaging. Sens. Actuator B 2018, 263, 426–435. [Google Scholar] [CrossRef]
  24. Li, Y.-X.; Lee, J.-Y.; Lee, H.; Hu, C.-C.; Chiu, T.-C. Highly fluorescent nitrogen-doped carbon dots for selective and sensitive detection of Hg2+ and ClO− ions and fluorescent ink. J. Photochem. Photobiol. A 2021, 405, 112931. [Google Scholar] [CrossRef]
  25. Bruno, F.; Sciortino, A.; Buscarino, G.; Soriano, M.L.; Ríos, Á.; Cannas, M.; Gelardi, F.; Messina, F.; Agnello, S. A comparative study of top-down and bottom-up carbon nanodots and their interaction with mercury ions. Nanomaterials 2021, 11, 1265. [Google Scholar] [CrossRef] [PubMed]
  26. Huang, S.-W.; Lin, Y.-F.; Li, Y.-X.; Hu, C.-C.; Chiu, T.-C. Synthesis of fluorescent carbon dots as selective and sensitive probes for cupric ions and cell imaging. Molecules 2019, 24, 1785. [Google Scholar] [CrossRef] [Green Version]
  27. Li, L.; Yu, B.; You, T. Nitrogen and sulfur co-doped carbon dots for highly selective and sensitive detection of Hg(II) ions. Biosens. Bioelectron. 2015, 74, 263–269. [Google Scholar] [CrossRef]
  28. Wu, H.; Tong, C. Nitrogen- and sulfur-codoped carbon dots for highly selective and sensitive fluorescent detection of Hg2+ ions and sulfide in environmental water samples. J. Agric. Food Chem. 2019, 67, 2794–2800. [Google Scholar] [CrossRef]
  29. Shen, J.; Shang, S.; Chen, X.; Wang, D.; Cai, Y. Highly fluorescent N, S-co-doped carbon dots and their potential applications as antioxidants and sensitive probes for Cr(VI) detection. Sens. Actuator B 2017, 248, 92–100. [Google Scholar] [CrossRef]
  30. Kong, D.; Yan, F.; Luo, Y.; Ye, Q.; Zhou, S.; Chen, L. Amphiphilic carbon dots for sensitive detection, intracellular imaging of Al3+. Anal. Chim. Acta 2017, 953, 63–70. [Google Scholar] [CrossRef]
  31. Xu, S.; Liu, Y.; Yang, H.; Zhao, K.; Li, J.; Deng, A. Fluorescent nitrogen and sulfur co-doped carbon dots from casein and their applications for sensitive detection of Hg2+ and biothiols and cellular imaging. Anal. Chim. Acta 2017, 964, 150–160. [Google Scholar] [CrossRef] [PubMed]
  32. Li, Z.; Yu, H.; Bian, T.; Zhao, Y.; Zhou, C.; Shang, L.; Liu, Y.; Wu, L.-Z.; Tung, C.-H.; Zhang, T. Highly luminescent nitrogen-doped carbon quantum dots as effective fluorescent probes for mercuric and iodide ions. J. Mater. Chem. C 2015, 3, 1922–1928. [Google Scholar] [CrossRef]
  33. Louleb, M.; Latrous, L.; Ríos, Á.; Zougagh, M.; Rodríguez-Castellón, E.; Algarra, M.; Soto, J. Detection of dopamine in human fluids using N-doped carbon dots. ACS Appl. Nano Mater. 2020, 3, 8004–8011. [Google Scholar] [CrossRef]
  34. Yu, S.; Ding, L.; Lin, H.; Wu, W.; Huang, J. A novel optical fiber glucose biosensor based on carbon quantum dots-glucose oxidase/cellulose acetate complex sensitive film. Biosens. Bioelectron. 2019, 146, 111760. [Google Scholar] [CrossRef]
  35. Kamat, P.V. Photochemistry on nonreactive and reactive (semiconductor) surfaces. Chem. Rev. 1993, 93, 267–300. [Google Scholar] [CrossRef]
  36. Sciortino, A.; Madonia, A.; Gazzetto, M.; Sciortino, L.; Rohwer, E.J.; Feurer, T.; Gelardi, F.M.; Cannas, M.; Cannizzo, A.; Messina, F. The interaction of photoexcited carbon nanodots with metal ions disclosed down to the femtosecond scale. Nanoscale 2017, 9, 11902–11911. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  37. Yang, Y.; Xiao, X.; Xing, X.; Wang, Z.; Zou, T.; Wang, Z.; Zhao, R.; Wang, Y. Rhodamine B assisted graphene quantum dots flourescent sensor system for sensitive recognition of mercury ions. J. Lumin. 2019, 207, 273–281. [Google Scholar] [CrossRef]
Nanomaterials 11 01831 sch001 550
Scheme 1. Preparation of sulfur and nitrogen codoped carbon dots (NSCDs) and their application in sensing mercury or hypochlorite ions through fluorescence quenching.
Scheme 1. Preparation of sulfur and nitrogen codoped carbon dots (NSCDs) and their application in sensing mercury or hypochlorite ions through fluorescence quenching.
Nanomaterials 11 01831 sch001
Nanomaterials 11 01831 g001 550
Figure 1. FT-IR spectrum of the NSCDs.
Figure 1. FT-IR spectrum of the NSCDs.
Nanomaterials 11 01831 g001
Nanomaterials 11 01831 g002 550
Figure 2. (a) XPS full survey spectra of the NSCDs. High-resolution XPS spectra of (b) C1s, (c) N1s, (d) O1s, and (e) S2p of the NSCDs.
Figure 2. (a) XPS full survey spectra of the NSCDs. High-resolution XPS spectra of (b) C1s, (c) N1s, (d) O1s, and (e) S2p of the NSCDs.
Nanomaterials 11 01831 g002
Nanomaterials 11 01831 g003 550
Figure 3. UV–vis absorption (black) and the maximum fluorescence excitation (red) and emission (blue) spectra of the NSCDs. Inset: Photos of the above suspension under (a) daylight and (b) UV irradiation (365 nm).
Figure 3. UV–vis absorption (black) and the maximum fluorescence excitation (red) and emission (blue) spectra of the NSCDs. Inset: Photos of the above suspension under (a) daylight and (b) UV irradiation (365 nm).
Nanomaterials 11 01831 g003
Nanomaterials 11 01831 g004 550
Figure 4. (a) Fluorescence emission spectra for the NSCDs in the presence of different concentrations of Hg2+. (b) Linear relationships between fluorescence intensity and concentrations of Hg2+ in the range of 0.7–15 μM. (c) Fluorescence emission spectra for the NSCDs in the presence of different concentrations of ClO–. (d) Linear relationships between fluorescence intensity and concentrations of ClO– in the range of 0.3–5 μM.
Figure 4. (a) Fluorescence emission spectra for the NSCDs in the presence of different concentrations of Hg2+. (b) Linear relationships between fluorescence intensity and concentrations of Hg2+ in the range of 0.7–15 μM. (c) Fluorescence emission spectra for the NSCDs in the presence of different concentrations of ClO–. (d) Linear relationships between fluorescence intensity and concentrations of ClO– in the range of 0.3–5 μM.
Nanomaterials 11 01831 g004
Nanomaterials 11 01831 g005 550
Figure 5. (a) The corresponding fluorescence intensity before (yellow) and after (cyan) the treatment of Hg2+. (b) The corresponding fluorescence intensity before (pink) and after (blue) the treatment of ClO−.
Figure 5. (a) The corresponding fluorescence intensity before (yellow) and after (cyan) the treatment of Hg2+. (b) The corresponding fluorescence intensity before (pink) and after (blue) the treatment of ClO−.
Nanomaterials 11 01831 g005
Table
Table 1. Determination of Hg2+ and ClO− ions in tap water samples (n = 3).
Table 1. Determination of Hg2+ and ClO− ions in tap water samples (n = 3).
AnalyteAdded (μM)Found (μM)Recovery (%)RSD (%)
Hg2+5.05.1101.573.65
8.07.897.534.11
10.010.2102.014.27
ClO–0.50.5116.496.12
2.01.998.612.53
4.03.893.672.80
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Lee, H.; Su, Y.-C.; Tang, H.-H.; Lee, Y.-S.; Lee, J.-Y.; Hu, C.-C.; Chiu, T.-C. One-Pot Hydrothermal Synthesis of Carbon Dots as Fluorescent Probes for the Determination of Mercuric and Hypochlorite Ions. Nanomaterials 2021, 11, 1831. https://doi.org/10.3390/nano11071831

AMA Style

Lee H, Su Y-C, Tang H-H, Lee Y-S, Lee J-Y, Hu C-C, Chiu T-C. One-Pot Hydrothermal Synthesis of Carbon Dots as Fluorescent Probes for the Determination of Mercuric and Hypochlorite Ions. Nanomaterials. 2021; 11(7):1831. https://doi.org/10.3390/nano11071831

Chicago/Turabian Style

Lee, Hsin, Yen-Chang Su, Hsiang-Hao Tang, Yu-Sheng Lee, Jan-Yee Lee, Cho-Chun Hu, and Tai-Chia Chiu. 2021. "One-Pot Hydrothermal Synthesis of Carbon Dots as Fluorescent Probes for the Determination of Mercuric and Hypochlorite Ions" Nanomaterials 11, no. 7: 1831. https://doi.org/10.3390/nano11071831

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop