Highly fluorescent N,S,P tri-doped carbon dots for Cl− detection and their assistance of TiO2 as the catalyst in the degradation of methylene blue
Graphical abstract
Introduction
Carbon dots (CDs), emerging carbon-based fluorescent nanomaterials, are extensively applied in wide fields, owing to their diversified attractive properties, like small size, high water solubility, low toxicity, benign biocompatibility, photostability and ease of surface functionalization [[1], [2], [3], [4], [5], [6], [7], [8]]. Much effort has been devoted to developing the highly fluorescent CDs with desirable properties for multiple applications. Doping is an efficient approach to enhance the fluorescence performance of CDs, as different elements can adjust the surface states of CDs and provide diverse emissive sites [1,[9], [10], [11]]. Especially, the multi-heteroatom doping is in some cases more effective to tune the physical and chemical properties of CDs [12,13], extending the applicability of CDs. For examples, the fluorescence of S and N co-doped CDs prepared with water chestnut and onion was quenched by Cu(II) and then restored by coenzyme A, based on which an off-on fluorescence probe for high sensitivity determination of coenzyme A was constructed [14]. Wang et al. fabricated the N and Cl co-doped CDs using a choline chloride-glycerine deep eutectic solvent, showing pH-sensitive fluorescent emission and obvious fluorescence response to cytochrome c [15]. The fluorescent probes for manganese(VII) and l-ascorbic acid were established based on the N, S, P co-doped CDs fabricated with Saccharomyces cerevisiae [13].
Cl− is an essential electrolyte that maintains body fluid and ion homeostasis. The change of Cl− concentration levels in biological fluids may indicate various abnormal conditions and the occurrence of diseases [16]. Cystic fibrosis (CF) is a genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator gene presented in a variety of organs. Elevated Cl− concentration in swear (>60 mM) is the gold standard for diagnosis of CF [17,18]. However, most of the existing Cl− detection methods, like ion exchange chromatography, ion-selective electrodes and colorimetry, cannot meet the high-performance and low-cost clinical needs. Hence, it is of great importance to develop a selective, sensitive, rapid and low-cost method for Cl− determination. To the best of our knowledge, few studies about the fluorescence sensor for the determination of Cl− in sweat were reported.
Photocatalytic degradation is a commonly-adopted method for wastewater treatment [[19], [20], [21]]. Thanks to the advantages of low toxicity, chemical inertness, high catalytic efficiency and low cost, TiO2 nanoparticles were the most popular catalyst for photocatalytic degradation [22]. However, TiO2 had some disadvantages, such as electron-hole recombination, wide band gap energy, high concentration aggregation, low specific surface area, etc., which limited the degradation efficiency. Carbonaceous substance modification on the surface of TiO2 was documented as a solution to these issues [[23], [24], [25], [26]]. Since the introduced carbonaceous substance could act as the binding center of electron-hole pairs, the photoexcited electrons in TiO2 may be transferred to carbonaceous substance thus hindering the recombination of electrons and holes and enhancing the photocatalytic efficiency of TiO2 [[23], [24], [25], [26]]. As far as CDs are concerned, they are less than 10 nm, and more edges and defects are present on the surface. Additionally, different functional groups or chemical structures is beneficial to the adsorption and reduction of oxygen. Therefore, CDs is expected to enhance the catalytic ability of TiO2.
In the present study, the N, S, and P tri-doped CDs (NSP-CDs) were prepared by one-pot hydrothermal method with sulfonamide and phosphoric acid as precursors. The fluorescence of NSP-CDs was sensitive to the concentration of Cl−, which was adopted to determine Cl− in sweat. Meanwhile, NSP-CDs can also enhance the photocatalytic activity of TiO2 for the degradation of methylene blue under UV irradiation.
Section snippets
Reagents and instruments
Sulfanilamide, H3PO4, NaOH, H2SO4, H3BO3, K2SO4, Na2SO4, CaSO4, MgSO4, (NH4)2SO4, HAcO, NaAcO, NaH2PO4, Na2HPO4, Na3PO4, NaCl, KBr, KI, glucose, citric acid and methylene blue (MB) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ultrapure water was produced by a Heal Fore NW system (Shanghai, China). Aeroxide TiO2 P25 was received from Evonik Degussa Corporation (Frankfurt, Germany).
Transmission electron microscopic (TEM) images were captured by a JEM-2100 microscope
Preparation and characterization of NSP-CDs
In this study, sulfanilamide with both abundant nitrogen and sulfur was for the first time chosen as the carbon source to prepare carbon dots, in addition to use phosphoric acid as the phosphorus source. After the hydrothermal reaction, the pale brown solution was obtained, giving out bright blue fluorescence under UV irradiation. In order to achieve the high fluorescence quantum yield, the amount of sulfonamide, the hydrothermal temperature and the reaction time were optimized successively
Conclusion
The N,S,P tri-doped carbon dots (NSP-CDs) were prepared by one-pot hydrothermal method with sulfonamide and phosphoric acid as precursors. Under optimal conditions (0.10 g sulfanilamide dissolved in 5 mL phosphoric acid was heated at 220 °C for 10 h), the NSP-CDs achieved a high fluorescence quantum yield up to 33 %. The particularly multi-heteroatom-doping made the fluorescence intensity of NSP-CDs sensitive to pH and Cl−. The Cl− probe based on NSP-CDs was successively applied in sweat
CRediT authorship contribution statement
Lu-Shuang Li: Methodology, Writing - review & editing. Li Xu: Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors gratefully acknowledge the financial support of this research by the Fundamental Research Funds for the Central Universities (No. 2172019KFYRCPY112) and Starting Research Fund from the Hainan University (No. KYQD(ZR)19106).
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