High quantum-yield carbon dots embedded metal-organic frameworks for selective and sensitive detection of dopamine

https://doi.org/10.1016/j.microc.2020.105718Get rights and content

Highlights

  • The CDs@ZIF-8 composites have been fabricated via an in-situ embedding strategy.

  • The CDs@ZIF-8 demonstrates uniform size and excellent FL efficiency.

  • A fluorescent sensor for DA detection was favorably developed based on CDs@ZIF-8.

  • By virtue of synergy of ZIF-8 and CDs properties, sensor exhibits good performance.

Abstract

By embedding high quantum yield blue-emitting carbon dots (CDs) into zeolitic imidazolate framework-8 (ZIF-8), the luminescent metal-organic frameworks (LMOFs) - CDs@ZIF-8 composites were fabricated facilely and served as the fluorescent probe for selective and sensitive detection of dopamine (DA). In which the sustainable pores within ZIF-8 would not only provide a natural habitat for analyte but also can selectively capture and concentrate the DA molecules via the interaction between the analyte DA and the basic site of the frame. Meanwhile, the CDs can be served as the signal probes to transform chemical signals of the interaction between CDs and analytes into fluorescent signals. Thus, the as-synthesized CDs@ZIF-8 composites generate new sensing performance when compared with CDs. As a consequence, a wide concentration dynamic range of 0.1–200 μM with a low detection limit (LOD) 16.64 nM was favorably obtained for the CDs@ZIF-8 composites based detection method for DA. Besides, the practicality and viability of the fluorescent composites were also successfully verified via challenging the CDs@ZIF-8 based sensing method to detect DA in complicated serum and urine.

Introduction

Metal-organic frameworks (MOFs) are one of fascinating material classes for preparing functional composites due to their adjustable surface function, permanent porosity and hybrid structural composition [1], and by combining specific metal nodes with functional organic ligands, unique functional MOFs can also be specially designed. The diversified physicochemical properties, tunable size and structural tunability of MOFs make them widely applied in gas storage and separation [2], [3], catalysis [4], [5], [6], chemical and physical sensing [7], [8], [9], [10], [11], medical delivery [12], [13] as well as ion-exchange [14], [15]. In particular, the metal nodes with unsaturated coordination, different basic/acidic functions and luminescent structural units can serve as functional sites for selective sensing [16], [17], [18].

As an important sub-category of MOFs, luminescent metal–organic frameworks (LMOFs) are by far the most widely explored type of MOFs sensor to date. On the one hand, the sustainable pores in MOFs can offer a natural habitat for guest molecules to improve the possibility for guest–host interaction, thus the sensing sensitivity would be improved [19]. On the other hand, the non-radiative relaxation of the organic ligands fixed in the rigid framework can potentially be reduced, thus resulting in stronger fluorescence (FL) emission [20]. Also, the rigid framework can endow MOFs with high thermal stability and the crystal structure can sustain temperatures of hundreds of degrees [21], providing structural support for fluorescent guests even in high temperature settings. Those features promise luminescent frameworks distinct advantages over other candidate classes of sensory materials. And rational design and build-up of these architectures become the primary tasks in exploration of MOFs as sensing material.

Luckily, introduction of guest luminescent nanoparticles with high luminous quantum yield (QY) into the non-luminescent MOFs lights an ideal method for LMOFs fabrication for chemical sensing. It is recently reported that the LMOFs with excellent luminescence property would be favorably obtained by encapsulating guest molecules in the pores of MOFs [22], [23]. And in our recent report, by conjugating aggregation-induced emission luminogens (AIEgens) with ZIF-8, a novel kind of LMOFs has been facilely synthesized for ultrasensitive copper ion sensing and visual glucose detection [8]. Besides, as an emerging nano-carbon material [24], CDs have stimulated researchers’ extremely great interest and been widely used in manufacturing a variety of fluorescent probes and sensors due to their excellent properties including high photoluminescence stability [25], environmental friendliness [25], easy preparation [26], [27], low toxicity [28] as well as excellent biocompatibility [29], [30], [31] and water solubility [32], [33]. Meanwhile, among various types of MOFs, ZIF-8 is an ideal substrate of preparing functional composites due to its unique advantages such as porous appearance, large pores, high stability, easy synthesis, and benign nature [34], [35]. Their porous substrate can accommodate smaller nanoparticles, resulting in the composites with better functions. We are thus inspired to attempt to prepare and synthesize LMOFs in a simple but effective way by adopting CDs with high QY as guest fluorescent nanoparticles.

Thanks to the excellent properties of CDs and ZIF-8, by embedding CDs into ZIF-8 crystal at room temperature, the composites CDs@ZIF-8 were fabricated via a simple one-pot method, and employed to build a LMOFs sensor for sensitive and selective detection of dopamine (DA). DA is a catecholamine neurotransmitter of human central nervous system in the brain, and participates in many biological processes in hormonal, central nervous and cardiovascular systems [36], [37]. The relative lack of DA might lead to lose control of muscle movement and result in distraction and even Parkinson's disease. Besides, the abnormal concentration of DA could be a potential signal of several diseases. Thus, accurate determination and quantification of DA is a significant and challenging issue. Up to now, numerous methods including electrochemical analysis [38], [39], ultraviolet visible spectrophotometer [40], high performance liquid chromatography [41] and fluorescent probes [42], [43] have been developed to determine DA. Nevertheless, some limitations of the reported methods, such as burdensome instrument procedure, intricate sample pre-treatment process, toxic substances and organic solvent would restrict their applications. To conquer these shortcomings, it is highly desirable to develop facile, eco-friendly, non-toxic and economic methods to determine DA. And in this work, by virtue of the synergistic effect of CDs and ZIF-8, the analyte molecules DA can be pre-concentrated in the sustainable pores within the ZIF-8 frame and independently oxidized to its produce dopamine-quinone under the action of the basic site on ZIF-8. Then the dopamine-quinone can be absorbed on the ZIF-8 framework by π-π interaction with the aromatic ring of ZIF-8. Besides, the electrostatic interaction between dopamine-quinone and CDs would cause FL quenching of CDs, thus a novel detection method is developed by virtue of synergy of properties of ZIF-8 and CDs. In addition, the as-prepared LMOFs (CDs@ZIF-8) not only exhibit an outstanding FL activity but also can selectively concentrate DA to afford sensitive detection of DA. Besides, the as-developed FL sensor showed high specificity and selectivity when we challenged the detection by using potential interferents or in complicated serum and urine.

Section snippets

Synthesis of CDs

The strategy that thermal treatment of molecular organic salts was adopted to prepare CDs [44]. In the preparation procedure, glutathione (GSH, 153 mg) and citric acid (CA) monohydrate (105 mg) were dissolved in 2 mL of water at flask and ultrasound to make its dispersed, and then dried in a vacuum for 12 h at 70 °C. The evaporated thick syrup was heated in oil bath for 10 min at 200 °C. The brown syrup product was then diluted to 2 mL using ultrapure water. After full reaction, the product was

Synthesis and characterization of CDs@ZIF-8

As the synthesis schematic diagram of CDs@ZIF-8 shown in Fig. 1, we firstly synthesized the blue fluorescent CDs by referring to the previous report [44], and its morphology was confirmed by TEM. As the results shown in Fig. S1, the CDs feature clearly revealed their spherical morphology and the average diameter was 2.6 nm. In addition, the FL QY of CDs was determined using quinine sulphate as the standard (measured at 350 nm excitation wavelength, QY = 54%), the average QY of CDs in aqueous

Conclusions

In summary, by introducing a simple one-pot method, the CDs@ZIF-8 composites have been facially fabricated via an in-situ embedding strategy. Based on the as-prepared CDs@ZIF-8 composites, a simple and cost-effective DA detection method has been favorably developed by employing ZIF-8 as the target recognition unit while CDs in the pores of ZIF-8 frame as the signal transduction unit. The as-prepared CDs@ZIF-8 composites not only have uniform size, but also demonstrate excellent FL efficiency.

CRediT authorship contribution statement

Siqi Xie: Conceptualization, Methodology, Writing - original draft. Xujie Li: Validation, Data curation. Lumin Wang: Software, Validation. Fawei Zhu: Visualization, Investigation. Xinyi Zhao: Validation. Tianqi Yuan: Data curation. Qi Liu: Writing - review & editing, Funding acquisition. Xiaoqing Chen: Resources, Supervision, Project administration, 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

This work was financially supported by the National Natural Science Foundation of China (No. 21878339, 21904141 and 22078369), the Hunan Provincial Natural Science Foundation of China (No. 2019JJ50791), the Hunan Provincial Key Laboratory of Food Safety Monitoring and Early Warning (No. 2020KFJJ06), and the Key R&D Project in the industrial field funded by Hunan Provincial Science &Technology Department, China (2019GK2131).

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      Moreover, similar diffraction peaks are observed in the XRD patterns of the original ZIF-8 and ZIF-based nanocomposite, corroborating that the distribution of guest molecules does not disorganize the crystalline structure of the ZIF-8. However, it is worth mentioning that the peaks of Ni,N-carbon dots and Fe3O4 nanoparticles are not seen in the XRD pattern of ZIF-based nanocomposite, which is presumably due to the low concentrations of these particles and the weak intensity of their diffraction peaks compared to strong peaks of ZIF-8 (Asadi et al., 2019; Xie et al., 2021). The result of EDX analysis is depicted in Fig. S2, illustrating the existence of C, O, Zn, Fe, N, and Ni elements in the structure of synthesized ZIF-based nanocomposite.

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