A sensitive “off-on” carbon dots-Ag nanoparticles fluorescent probe for cysteamine detection via the inner filter effect
Graphical abstract
An “off-on” fluorescent probe based on carbon dots (CDs) and silver nanoparticles (AgNPs) for sensitive and selective detection of cysteamine.
Introduction
Cysteamine (2-aminoethaniol) is an aminothiol compound that is endogenously degraded from Coenzyme A and has important physiological regulation in animals. Nowadays, cysteamine is employed in increasing numbers of applications in livestock production and also as extremely safe and effective medicine for clinical treatments [1]. In livestock production, cysteamine is used as a feed additive to improve livestock growth by reducing the concentration of somatostatin [2]. Cysteamine contains free sulfhydryl groups that have antioxidant activity; thus, it can be used to treat cystine storage disease and radiation damage [[3], [4], [5]]. Besides, cysteamine not only has a therapeutic effect on neurodegenerative diseases, such as epilepsy, Parkinson’s disease, and Huntington’s disease caused by loss of neurons, it is also involved in the synthesis of hormones and neurotransmitters [6,7]. In addition, the potential use of cysteamine in adjunct cancer chemotherapy has been demonstrated [8]. When these diseases occur, abnormal levels of cysteamine can be detected in the patient's blood, biological tissues, and organelles. Therefore, developing methods for cysteamine analysis and determining cysteamine in biological tissues and cell samples have potential application prospects for the early diagnosis and treatment of diseases [9] (see Scheme 1).
Thus far, various analytical methods for the detection of cysteamine have been designed and developed, which can include fluorescence spectroscopy [10,11], chemical etching [12], electrochemistry [13,14], colorimetry [15], high-performance liquid chromatography [16], and Raman spectroscopy [17]. Among all methods, sensor-based fluorescence spectroscopy has received considerable attention owing to its high selectivity, low-cost equipments, and simple procedures. Common fluorescent sensors, such as organic small molecule [18], quantum dots [19], and carbon dots [20], have been widely used in detection technology. Since their first report in 2004 [21], fluorescent carbon dots (CDs; small carbon nanoparticles with sizes of less than 10 nm) have gained increasing attraction because of their high quantum yields [22], excellent water dispersibility [23], and low toxicity [24]. Carbon dots can be synthesized from a variety of inorganic, organic, and biological materials, as well as foods and their residues, these starting materials usually are non-fluorescent. Nano-sized fluorescent CDs can be synthesized by uncomplicated methods, such as hydrothermal treatment and microwave treatment. Owing to their unique properties, CDs are promising in numerous applications, including sensing, bioimaging [25], and drug delivery [26].
Although many studies on the detection of intracellular thiols (Cys, GSH, and Hcy) have been reported in the literature in recent years [[27], [28], [29]], only a few of these reports have demonstrated the detection of cysteamine. Recently, Konar et al. [10] have reported the use of one-pot microwave-assisted nitrogen-doped carbon dots (NCDs) as a fluorescent probe for the detection of cysteamine. They described that in the presence of cysteamine, the fluorescence intensity of NCDs is quenched due to the formation of ground-state non-fluorescent complexes. Compared with the “turn-off” sensors, the fluorescence “turn-on” sensors appear to be more preferable because they can reduce the chance of false positives. However, one of the requirements of these sensors is that the analytes must have a strong binding affinity for the quencher so that the interaction between the CDs and the quencher can be undermined [30,31]. Due to this advantage, it remains necessary to develop a simple, sensitive, and high specificity “turn-on” fluorescent sensor for cysteamine detection.
In this study, we synthesized an “off-on” CDs-AgNPs-based probe for cysteamine detection. The CDs were synthesized from citric acid and ethylenediamine using a simple hydrothermal route. The synthesized CDs had low toxicity, strong blue emission, and superior water solubility. The fluorescence of CDs was quenched in the presence of AgNPs, due to the inner filter effect (IFE), but was restored in the presence of cysteamine (the analyte). The developed fluorescent CDs-AgNPs probe not only is easy to use, but also has high selectivity and sensitivity towards cysteamine. It is also a promising probe that might be applied to detect various real samples in cell biology or disease diagnosis in the future.
Section snippets
Materials and reagents
Silver nitrate (AgNO3) and citric acid were purchased from J&K Chemical Company Ltd., China. Ethylenediamine, trisodium citrate dehydrate and sodium borohydride (NaBH4) were purchased from Sinopharm Chemical Reagent Company Ltd., China. Cysteamine and l-cysteine were purchased from Shanghai Yuanye Biotechnology Company Ltd., China. dl-Homocysteine (Hcy) was obtained from TCI (Shanghai) Development Co., Ltd., China. Glutathione (GSH), methionine (Met), and tryptophan (Trp) were purchased from
Characterization of CDs
The CDs were synthesized from the starting materials (citric acid and ethylenediamine) and were then characterized by spectroscopic methods, including TEM, FT-IR, and XPS. The TEM image depicted in Fig. 1a shows that CDs were spherical particles and monodisperse, and the inset shows that the CDs particles had a narrow diameter range (from 2.4 to 3.8 nm) with an average particle diameter of 3.1 nm. FT-IR spectroscopy was employed to investigate the functional group on the surface of CDs. As
Conclusion
In summary, we developed a simple and sensitive “off-on” fluorescent probe from CDs and AgNPs for detection of cysteamine. The detection had IFE-based mechanism caused by the overlap between the excitation and emission spectra of CDs and the absorption peak of AgNPs, which led to a significant decrease in fluorescence intensity of CDs. But upon the presence of cysteamine, which could interact and cause aggregation of AgNPs, the fluorescence of CDs was recovered. The developed fluorescent probe
Author contribution
Xiaowei Mu: Conceptualization, Methodology, Validation, Investigation. Minxing Wu: Software. Bo Zhang: Resources. Xin Liu: Writing - review & editing. Shaomei Xu: Data curation. Yibing Huang: Resources. Xinghua Wang: Supervision. Daqian Song: Supervision, Funding acquisition. Pinyi Ma: Supervision, Writing - review & editing. Ying Sun: 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.
Acknowledgments
This work was supported by the Science and Technology Developing Foundation of Jilin Province of China (Nos. 20200602047ZP, 20200404173YY, and 20180201050YY) and Graduate Innovation Fund of Jilin University (No. 101832018C172).
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