Carbon dots derived from flax straw for highly sensitive and selective detections of cobalt, chromium, and ascorbic acid
Graphical abstract
Introduction
Carbon dots (CDs) are zero-dimensional carbon nanomaterials and have drawn a lot of attention owing to their distinctive photoluminescence, excellent water dispersion, easy functionalization, good biocompatibility, and high photobleaching resistance [1], [2], [3]. CDs have shown promising application potential in chemical probes [4], biological imaging [5], and optoelectronic devices [6] due to their excellent optical properties. In recent years, different types of wastes and natural biomass such as waste polyethylene terephthalate [7], orange juice [8], sugarcane bagasse pulp [9], wheat straw [10], coffee grounds [11], egg [12], waste expanded polystyrene [13], and peach gum polysaccharide [14] (see Table S1 in Supplementary Material) were used as the green carbon source to prepare CDs because biomass itself can not only provide abundant carbon sources but also minimize the use of expensive and toxic chemicals. However, quantum yield (QY) of the obtained CDs is too low (mostly below 10%) and should be enhanced. The QY is a critical property of CDs and determines the down-to-earth applicability. Doping the CDs with a diversity of heteroatoms such as nitrogen, sulfur, and phosphorus is an effective approach to enhance their QY. Among them, N doping is the most popular because N can change the electronic structure and create active sites in the CDs to broaden their potential applications [13], [14]. A variety of methods such as arc discharge [15], microwave heating [16], electrochemical exfoliation [17], acidic oxidation [18], high-energy ball milling [19], and ultrasonic synthesis [20] were employed to prepare CDs. However, all these approaches involve more or fewer disadvantages including high cost, complicated purification procedures, expensive carbon precursors, harsh reaction conditions, and low yield. Therefore, facile, low cost, and green processes for preparing CDs are desired. Yuan et al. [10] synthesized CDs using the wheat straw as the carbon source through the hydrothermal method; their QY is about 9.2%. Liu et al. [21] reported the synthesis of CDs via the hydrothermal treatment of rice straw and ionic liquid. However, ionic liquids are expensive and not suitable for the large-scale production of CDs. Flax straw is waste left in the field after the flax seeds are harvested. 14,000 tons of flax straw was produced annually in the prairie of western Canada [22]. The problem with discarded flax straw for farmers is that the stalks are tough and difficult to be incorporated into the soil for recycling [23]. Flax straw contain cellulose, hemicellulose (~17%), and lignin and can be used to prepare CDs greenly. Thus, it is desired to utilize flax straw to synthesize CDs.
Cobalt ions (Co2+) are one of the important trace metal ions in the human body. A small amount of Co2+ ions can promote the production of red blood cells in the blood [24]. However, excessive content of Co2+ ions can cause Co poisoning and have negative effects on the human body such as emesis, paralysis, diarrhea, and hypotension [25], [26]. Cr mainly exists in two oxidation states, trivalent and hexavalent, but the physical and chemical properties and toxicity of trivalent and hexavalent Cr are totally different. Cr3+ is found to be an indispensable trace metal ion in human body while Cr6+ compounds such as dichromate (Cr2O72−) and chromate (CrO42−) have higher mobility and carcinogenesis and is considered to be toxic to the human body [27]. Thus, the detections of Co2+ and Cr6+ are extremely important for the beforehand diagnosis of these diseases. Sundry methods such as atomic absorption spectrometry, chemiluminescence, and fluorescence spectroscopy were applied to detect Co2+ and Cr6+ [28], [29], [30]. But most methods suffer from high cost and tedious processing of samples before detection which limit their wider applications. Meanwhile, fluorescent probes are new types of detection methods having the advantages of facile operation and excellent sensitivity and are turning into promising analytical methods for detecting Co2+ and Cr6+. Ascorbic acid (AA) is one of the most important brain system neurochemicals that have antioxidant effects not only on cells but also on neuroprotection [31]. In addition, AA can support the formation of interstitial cells and maintain the normal function of teeth, bones, blood vessels, and muscles and enhance the immunity of human body [32]. Too low or too high level of AA is closely related to some diseases. Lack of AA in the human body may give rise to skin bruising, bleeding gums, muscle weakness, joint swelling, and edema whereas excessive AA intake can bring about urinary stones, diarrhea, and stomach convulsions [33]. Therefore, facile, straightforward, low-cost, and high-sensitivity probes for the detection of AA are essential for the early precaution of these diseases.
In our present work, highly fluorescent CDs were prepared via a one-step hydrothermal treatment of flax straw. The non-fluorescent compounds form by the strong chelation or coordination interactions between the rich surface functional groups of the CDs and Co2+ or Cr6+. The non-fluorescent compounds further absorb the excitation light, resulting in an obvious fluorescence quenching. Meanwhile, AA can recover the fluorescence quenched by Cr6+ through reduction of Cr6+ to Cr3+. Thus an efficacious fluorescence “off-on” nanoprobe for AA can be achieved (Fig. 1). Furthermore, the “on-off” and “on-off-on” fluorescent sensors have distinct features of convenience, simplicity, time-saving, high sensitivity, and high selectivity.
Section snippets
Materials
Flax straw was obtained from local farmers (Lanzhou, China). The dialysis bags (MWCO = 3500 Da) and 2-amino-2-hydroxymethyl-1,3-propanediol (Tris) were from Beijing Solarbio S&T Co. Ltd (Beijing, China). Fe(NO3)3, AgNO3, Al(NO3)3, Cd(NO3)2, CuSO4, Zn(NO3)2, Mg(NO3)2, FeCl2, MnCl2, Ni(NO3)2, Pb(NO3)2, Co(NO3)2, Na2CrO4, Na3PO4, NaClO4, Na2HPO4, glucose (Glc), citric acid (CA), ethylenediamine (EDA), and AA were from Kermel Chemical Reagent Co. Ltd (Tianjin, China). Glycine (Gly), glutamic acid
Characterizations of CDs
The TEM image of the CDs synthesized from flax straw in Fig. 2a shows that the CDs are nearly spherical in shape, fine in size, and fully dispersed. The size distribution histogram statistically determined from more than 700 CDs observed in the different areas of the sample by TEM in Fig. 2b indicates that the CDs have a narrow size distribution of 1.1–4.0 nm and an average diameter of 2.2 nm. The SAED pattern in the inset of Fig. 2a exhibits broad diffraction halos and a faint larger one,
Conclusions
We developed a facile, green, and environment-friendly route for the synthesis of CDs from flax straw. The as-prepared CDs with abundant surface functional groups exhibit down-conversion fluorescence and the optimum emission wavelength of 449 nm. The QY under the optimum excitation of 360 nm is 20.7%. The fluorescence of the CDs can be quenched by Co2+ or Cr6+. The observed fluorescence quenching may be due to static quenching and IFE effect induced fluorescence quenching mechanism. Meanwhile,
CRediT authorship contribution statement
Guangkuo Hu: Conceptualization, Methodology, Software, Formal analysis, Data curation, Writing - original draft, Investigation. Lin Ge: Software, Validation. Yuanyuan Li: Resources. Masood Mukhtar: Writing - review & editing. Bing Shen: Resources. Desheng Yang: Resources. Jiangong Li: Conceptualization, Writing - review & editing, Supervision, Project administration.
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.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (51551201 and 51772137) and the Fundamental Research Funds for the Central Universities (lzujbky-2019-sp03).
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