Ultra-thin two-dimensional nanosheets for in-situ NIR light-triggered fluorescence enhancement
Graphical abstract
Introduction
Stimuli-responsive fluorescent materials have attracted considerable attention for fundamental investigation and intelligent applications during the last few decades [1], [2], [3], [4], [5], [6]. Light [7], heat [8], pH [9] and mechanical force [10] are the typical stimuli utilized to tune the fluorescent behaviors of these materials. Stimuli-responsive fluorescent materials have the merits of simple synthesis, fast response speed and repeatable response, which play an important role in the fields of fluorescent sensors, anti-counterfeiting paper, optical coding, optical switches, data storage and biological imaging [11], [12]. Thus, searching for new types of stimuli-responsive fluorescent materials is of significant importance.
One promising approach to obtain multiple stimuli-responsive fluorescent materials is fluorescent variation via simple, noninvasive and effective stimuli under ambient conditions. Light is an attractive external stimuli for fluorescent materials, because it has the advantage of relatively high local precision and remote controllability [13], [14]. However most of the fluorescent materials were driven by high-energy ultraviolet (UV) or visible light, which suffers the drawbacks of limited penetration and invasiveness [15], [16]. Compared with UV, the non-toxic near infrared (NIR) light is more suitable for biological applications, especially for three-dimensional (3D) light scanning simulation and information storage applications. In light-control system for the biological field, NIR light plays an irreplaceable role [17], [18], [19], [20]. But the energy of NIR light is often too low to trigger most fluorescent variations. Lanthanide ions doped up-conversion nanoparticles (UCNPs) are capable of efficiently converting low energy photons in the NIR region to high-energy photons in the visible region. The transition between the energy levels of the rare earth ions is a forbidden f-f transition with a longer lifetime to achieve a two-photon or multiphoton effect [21], [22], [23], [24], [25]. Besides a deeper tissue penetration and lower biological tissue damage featured by the NIR light, the advantages of UCNPs include high photo-stability, adjustable luminescence spectra and relatively good biocompatibility. Thus, the combination of UCNPs with stimuli-responsive materials may represent an attractive target for NIR light-triggered application.
To demonstrate the above-mentioned promising aspects, the hybridization between ultra-thin two-dimensional (2D) metal–organic frameworks (MOFs) fluorescent nanomaterials with UCNPs, to achieve the NIR light-triggered fluorescent variations have been proposed. MOFs are classical porous materials that have been applicated in photoluminescence, sorption, magnetism. Molecular recognition, and so on [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. Ultra-thin 2D MOFs nanosheets possess the merits of large surface area and highly accessible active sites, which thus can act as a versatile nano-platform for various chemical-physical process [29], [30], [31]. Here in this work, we have prepared ultra-thin 2D fluorescent nanosheets of Zn-based MOF from the 3D bulk precursor [Zn(bpydc) (H2O)·H2O]n (bpydc = 2,2′-bipyridine-5,5′-dicarboxylate), by using a facile-operating ultrasonic force-assisted liquid exfoliation technology. Note that, this 2D fluorescent nanosheets can be well dispersed in various solvents, and have shown interesting solvent polarity-dependent fluorescent properties. More interestingly, by loading ytterbium (Yb3+) ion sensitized UCNPs (NaYF4: 20 mol% Yb3+, 1 mol% Er3+ [32]) on the 2D surfaces (Scheme 1), the obtained hybrid species Zn-MOF@UCNPs have shown NIR (980 nm) light triggered fluorescent enhancement, attributing to the energy match between up-conversion luminescence (UCL) for UCNPs and ligand-to-metal charge transfer (LMCT), from the pπ*-orbitals of ligands to 4sσ-orbitals of the Zn ions, for 2D nanosheets. These results are expected to provide new solutions for further application of in-situ NIR triggered molecular conversion [33], [34], [35], [36], [37], [38].
Section snippets
Preparation of ultra-thin 2D nanosheets
The 3D bulk precursor [Zn(bpydc)(H2O)·H2O]n, which was initially synthesized by Seong Huh et al., [33] exhibits a layered crystalline structure and shows potential to be exfoliated into ultrathin 2D nanosheets. Within the 2D layered structure, the coordination geometry around zinc ions is a distorted octahedral comprised by two nitrogen atoms from bipyridine, two oxygen atoms of a carboxylate, one oxygen atom of the other carboxylate, and one oxygen atom from the coordinated water molecular (
Conclusion
In summary, we have preparation ultra-thin 2D nanosheets of Zn-based MOF, and examined its luminescence properties, response characteristics to solvent polarity and fluorescence sensing mediated by UCNPs. These results show that the maximum emission wavelength of 2D nanosheets in polar solvents moves to the shorter wavelength, and the shift of the fluorescence emission spectrum of the 2D nanosheet dispersed in different solvents is also distinct, especially the difference between the polar
Material
All the reactions were carried out at room-temperature, the chemicals involved in the experiments were of analytical grade and used without further purification. Both the raw materials for Ultra-thin 2D nanosheets [Zn(bpydc)(H2O)·H2O]n-Zn(NO3)2·6H2O, bpydc; and the raw materials for UCNPs--NaOH, NH4F, ethanol, chloroform, toluene, cyclohexane, trifluoroacetic acid (CF3COOH), and HCl were all obtained commercially purchased from Sinopharm Chemical Reagent Co. Ltd (China), Beijing HWRK Chem Co., Ltd. (China),
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 research was supported by the Natural Science Foundation of China (Grant No. 21701023), Natural Science Foundation of Jiangsu Province (Grant No. BK20170660), Zhishan Youth Scholar Program of SEU and PAPD of Jiangsu Higher Education Institutions.
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