An in-situ strategy to construct uracil-conjugated covalent organic frameworks with tunable fluorescence/recognition characteristics for sensitive and selective Mercury(II) detection
Graphical abstract
An in-situ strategy is proposed to construct uracil-conjugated COFs and modulate their fluorescence properties for sensitive and selective mercury(II) detection. The π-conjugated framework serves as a signal reporter, the evenly and densely distributed uracil acts as a mercury(II) receptor, and the regular pores (channels) make the rapid and sensitive detection of the mercury(II) possible.
Introduction
Mercury is a heavy metal with bio-accumulative and high toxicity, responsible for environmental pollution and severe human diseases [1]. Moreover, the spreading mercury cycling brings more and more challenges to the environment worldwide [2,3]. In this context, detecting mercury in the environment is essential for protecting human health and ecological environments. Presently, the conventional assays to detect mercury(II) such as inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectrometry (AAS), must rely on large instruments that are not portable and limit the application. On the contrary, fluorescence measurement is a promising method for mercury(II) detection for its high sensitivity and good selectivity. In recent years, considerable efforts have been made to develop novel materials such as metal-organic frameworks (MOFs), mesoporous silica, DNAs, carbon dots, and hydrogel microparticles for sensitive detection and removal of mercury(II) [[4], [5], [6], [7], [8], [9], [10], [11]]. However, the specificity of these probes toward mercury(II) and the generality of the strategies designed for other targets need to be improved.
COFs are porous crystalline materials constructed by rigid building blocks, with a large specific surface and stable skeleton [12]. The various modified monomers can predefine the skeleton, pore size, and pore shape of the COFs [13,14]. Researches over the past decade have developed lots of COFs with different structures and functions for wide applications, including energy storage [15,16], photocatalytic hydrogen generation [17], catalysis [[18], [19], [20]], membrane separation [21], sensing [[22], [23], [24]], and metal-ion removing [[25], [26], [27]]. Especially, with designable skeleton/pore size and diverse functional groups, COFs are potential materials for heavy metal ion detection. Compared to the traditional chemical reaction methods to detect mercury(II) in aqueous solutions, COFs are more superior and sensitive owing to their porous structure and unique electronic properties, and some previous researches have achieved sensitive detection of mercury(II) by COFs [[28], [29], [30], [31]]. However, most of these COFs-based sensors need post-modification of functional groups or decoration with other nanomaterials to endow them with optical responses toward mercury(II) [25,[32], [33], [34], [35], [36], [37]]. Thus far, there have not been any reports that can simultaneously realize the preparation, functionalization, and modulation of fluorescent COFs for efficient mercury(II) sensing.
Inspired by the unique base-binding property of mercury(II) with thymine (T) to form T-Hg-T complexes at room temperature [38], herein we propose an in-situ strategy to construct a series of uracil-conjugated COFs (Py-U-COFs) with tunable fluorescence characteristics and adjustable recognition performance for mercury(II) sensing. Although poly T-DNA probes or T-modified fluorescent nanoparticles have been previously investigated for mercury(II) detection [[39], [40], [41]], DNAs suffer from instability under extreme chemical environments while the post-modification strategies usually involve complex/time-consuming processes and produce limited recognition sites. It is expected that the present in-situ synthesis method can effectively compensate for these shortcomings and retain the crystalline structure of pristine materials while endowing them with sufficient binding sites and excellent recognition performance.
Section snippets
Materials and chemicals
All reagents were obtained from commercial sources and used as received. 1,3,6,8-tetrakis(4-formylphenyl)pyrene (TFPPy) and 1,3,6,8-tetrakis(4-aminophenyl)pyrene (TAPPy) were purchased from Jilin Chinese Academy of Science-Yanshen Technology Co., Ltd. 5-Aminouracil (5-AU) was purchased from J&K SCIENTIFIC. The 1H NMR spectra of TFPPy, TAPPy, and 5-AU were shown in Fig. S1. CH2Cl2, tetrahydrofuran (THF), N,N-Dimethylformamide (DMF), acetic acid (HAc) and other common reagents were purchased from
Synthesis and characterization
The pyrene-based COFs (Py-COFs) were synthesized via Schiff base reaction between TFPPy and TAPPy by using 1:1 n-butylalcohol/1,2-dichlorobenzene as the solvent (Fig. 1a and b and Fig. S2). The uracil-functionalized Py-COFs (Py-U-COFs) were in-situ prepared under the same conditions, and the proportions of 5-AU were varied at 10%, 30%, 50%, 70%, and 90% to produce Py-U-COFs-1, Py-U-COFs-2, Py-U-COFs-3, Py-U-COFs-4, and Py-U-COFs-5, respectively (Fig. 1c). Py-COFs and Py-U-COFs were obtained as
Conclusions
In conclusion, an in-situ method has been proposed to simultaneously prepare, regulate, and functionalize COFs by using TFPPy, TAPPy, and different ratios of 5-AU as monomers. The crystallinity structure, the optical property as well as the fluorescence response to mercury(II) of the COFs can be modulated by simply varying the proportion of precursors. Specially, the Py-U-COFs-3 exhibits mercury(II) recognition performance with satisfactory sensitivity and selectivity, and the LOD is as low as
CRediT authorship contribution statement
Xi-Rui Deng: Conceptualization, Methodology, Formal analysis, Draft preparation. A-Wei Hu: Conceptualization, Methodology, Formal analysis, Draft preparation. Sheng-Qian Hu: Conceptualization. Wen-Li Yang: Conceptualization. Chen Sun: Revision, Discussion. Sai-Jin Xiao: Formal analysis, Discussion, Funding acquisition. Gui-Ping Yang: Formal analysis, Data curation. Qiong-Qing Zheng: Formal analysis, Data curation. Ru-Ping Liang: Formal analysis, Discussion, Funding acquisition. Li Zhang:
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 financially supported by the National Natural Science Foundation of China (21964011, 22166024, 22064001 and 22036003) and Jiangxi Provincial Natural Science Foundation (20212ACB203009 and 20212ACB203011).
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These authors contributed equally.