【Origin】
Generally, the chemical fluorescence sensor is based on the "turn off" mechanism , that is , the fluorescence chromophore undergoes the host-guest interaction or energy transfer, the electron migration of the fluorescent molecule to the guest molecule, thereby forming a "fluorescence quenching" effect, resulting in fluorescence The phenomenon of reduced or disappeared intensity. However, the detection line and sensitivity of chemical fluorescence sensors based on the "turn off" mechanism are generally difficult to improve! The chemical sensor based on the "turn on" mechanism is in principle easy to achieve high sensitivity, as an ideal fluorescent sensing mechanism needs to be developed!
The "turn on" sensor can be designed by combining a fluorophore and a recognition part . Based on the understanding of the designability of MOF structure, the author believes that MOFs can be a suitable platform for the design of fluorescent "on" sensors. This article reports a multi-component MOF “turn on” fluorescence sensor for cyano detection by linking anthracene-based fluorescent chromophores and hemicyanine-based CN-responsive molecules (Figure 1). The design of the "turn on" fluorescent sensor is based on the intramolecular charge transfer (ICT) mechanism. It should be noted that the two parts of the anthracene-based fluorescent chromophore and hemicyanine reported in this article are not covalently connected, but are connected to the Zr cluster through coordination bonds in the MOF.
Figure 1 The mechanism of MOF "turn on" fluorescent sensor for cyano detection
[MOF synthesis]
The author chose PCN-700 as a precursor for post-modification to obtain MOF fluorescent molecules. In order to successfully synthesize the required MOF fluorescent molecules, a step-by-step synthesis route was designed (Figure 2). First, in the DMF solution, by reacting the previously synthesized PCN-700 crystal with L1, L1 is inserted into the PCN-700 pocket (Figure 2c). Subsequently, a precursor of hemicyanine (L2CHO) containing aldehyde functional groups was introduced into PCN-700. Finally, a condensation reaction is carried out between 1,2,3,3-tetramethyl-3H-indole iodide and PCN-700-L1-L2CHO to obtain the desired product, PCN700-L1-L2Hcy (Figure 2e) . The author also tried other step-by-step synthetic routes (such as the sequence of monomer introduction, etc., see the original text for details), but only the three-step route successfully obtained the designed MOF. In addition, by adjusting the reaction concentration of L1 and L2CHO, the ratio of the two monomers introduced can be successfully controlled, which will facilitate the design adjustment of the fluorescence sensitivity later! More importantly, this method finally obtained the single crystal grade PCN700-L1-L2Hcy, which can be considered as a transformation of MOF to MOF, so the structure can be accurately resolved by single crystal diffraction !
Figure 2 Synthesis of MOF fluorescent molecules
【Fluorescence Test】
Next, the author conducted a test analysis of its fluorescence detection performance (Figure 3). The results showed high sensitivity, linearity and selectivity, and the fluorescence intensity was adjustable (by adjusting the ratio of the fluorescent chromophore to the binding motif , Figure 3b, c).
Figure 3 Fluorescence detection performance analysis
[Performance extension]
The particle size of MOF was controlled by adjusting the synthesis conditions, and a crystalline MOF fluorescence sensor with an average diameter of 500 nm was obtained . MOF‘s nanoparticle size enhances cell permeability, making it useful for in vitro CN-detection. The author placed them in Hela cells and found that when the fluorescently modified cells were placed in NaCN (1 μM) solution, they showed strong fluorescence (Figure 4)! These results prove the good cell membrane permeability of MOF nanoparticles, which can extend the application of these MOF sensors to the field of CN-detection and bio-imaging in living cells, and have practical application significance.
【to sum up】
In this paper, by introducing two organic functional groups in the MOF, the "turn on" fluorescence detection is achieved. The editor believes that this strategy can be fully introduced into the design and synthesis of functional COF, which will achieve better performance than MOF!
Original link: https://doi.org/10.1002/anie.202000702
Source of information: MOFs online
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