Tuning of Förster Resonance Energy Transfer in Metal–Organic Frameworks: Toward Amplified Fluorescence Sensing

Open AccessCCS ChemistryRESEARCH ARTICLE1 Aug 2021Tuning of Förster Resonance Energy Transfer in Metal–Organic Frameworks: Toward Amplified Fluorescence Sensing Bo Gui, Xuefen Liu, Ge Yu, Weixuan Zeng, Arindam Mal, Shaolong Gong, Chuluo Yang and Cheng Wang Bo Gui Sauvage Center for Molecular Sciences and Hubei Provincial Key Laboratory on Organic and Polymeric Optoelectronic Materials, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072 , Xuefen Liu Sauvage Center for Molecular Sciences and Hubei Provincial Key Laboratory on Organic and Polymeric Optoelectronic Materials, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072 , Ge Yu Sauvage Center for Molecular Sciences and Hubei Provincial Key Laboratory on Organic and Polymeric Optoelectronic Materials, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072 , Weixuan Zeng Sauvage Center for Molecular Sciences and Hubei Provincial Key Laboratory on Organic and Polymeric Optoelectronic Materials, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072 , Arindam Mal Sauvage Center for Molecular Sciences and Hubei Provincial Key Laboratory on Organic and Polymeric Optoelectronic Materials, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072 , Shaolong Gong Sauvage Center for Molecular Sciences and Hubei Provincial Key Laboratory on Organic and Polymeric Optoelectronic Materials, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072 , Chuluo Yang Sauvage Center for Molecular Sciences and Hubei Provincial Key Laboratory on Organic and Polymeric Optoelectronic Materials, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072 and Cheng Wang *Corresponding author: E-mail Address: [email protected] Sauvage Center for Molecular Sciences and Hubei Provincial Key Laboratory on Organic and Polymeric Optoelectronic Materials, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072 https://doi.org/10.31635/ccschem.020.202000444 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail The assembly of Förster resonance energy transfer (FRET) donor and acceptor for amplified fluorescence sensing has been considered a big challenge. Herein, by using the multivariate approach, we report the design and synthesis of a series of FRET-based metal–organic frameworks (MOFs) with variable donor fluorophore-to-the-acceptor ratios. Owing to the efficient FRET process, these MOFs are almost nonfluorescent. Interestingly, the reduction of the acceptor leads to the prohibition of the FRET process and turns on the fluorescence of MOFs. More significantly, interesting amplification phenomena were observed when these MOFs were utilized as fluorescent turn-on sensors for a reductive analyte. For example, upon varying the ratio of donor fluorophores and acceptors from 0.7 to 11.4, the limit of detections exhibited 157 times decrease. We believe the present study not only provides a simple but general strategy for the efficient construction of amplified fluorescent sensors, but also will inspire us to design ultrasensitive MOF-based sensors in the future. Download figure Download PowerPoint Introduction The ultralow-level detection of target species has attracted significant interest from laboratory sensing/imaging to the urgent needs of human society.1–7 Amplified fluorescence sensing,8 in which a tiny molecular response can be converted to the optical signal of numerous fluorescent dyes, has proven to be a powerful technique in the fabrication of ultrasensitive sensors.9–12 To build a system with an amplified fluorescence property, the main concepts are based on either surface plasmon resonance13,14 or excited-state energy transfer.15,16 Förster resonance energy transfer (FRET),17 involving nonradiative energy transfer from the excited donor fluorophore to the acceptor, is of particular interest for constructing such systems.15 In principle, a single acceptor can ideally quench the fluorescence of plentiful donor fluorophores within the Förster sphere. Consequently, the stimuli-response of this single acceptor will restrict the FRET process, followed by turning on the fluorescence of surrounding donors with amplified behavior.18 Thanks to the efforts of many groups, FRET donors and acceptors have been successfully assembled into several systems,19–26 such as conjugated polymers,19–21 dendrimers,22,23 and nanoparticles,24–26 toward molecular sensing with fluorescence signal amplification. For example, Klymchenko26 reported FRET-based nanoparticles as an ultrabright platform for the amplified fluorescence detection of nucleic acids. However, the facile construction of such systems is still very challenging as most of the systems are involved in complicated synthetic processes. Therefore, it is crucial to develop a simple strategy that can efficiently assemble FRET donors and acceptors for amplified fluorescence sensing. Metal–organic frameworks (MOFs),27,28 a novel class of organic–inorganic hybrid crystalline porous materials, have shown interesting applications in gas adsorption and separation,29–34 catalysis,35–38 sensors,39–41 drug delivery,42–44 energy storage,45,46 and so on. A unique feature of MOFs is their ability to rationally introduce heterogeneity47,48 (i.e., more than one linker) into a single framework through the concept of reticular chemistry.49 Accordingly, MOFs can fundamentally provide an ideal platform to construct amplified fluorescence sensing systems (Scheme 1). First, the designed FRET donors and acceptors can be relatively easily incorporated into the same MOF crystal through the multivariate (MTV) approach.50 Second, the close arrangements of these donors and acceptors in the framework can favor the FRET process and thus quench the fluorescence of MOFs.51,52 Third, the porous nature of MOFs can enable the diffusion of analytes into the nanopores and then interact with the acceptors,53 which will prohibit the FRET process and turn on the fluorescence of MOFs. Finally, the amounts of donor fluorophores around an acceptor within the Förster sphere can be modulated by varying the feeding ratios in MTV-MOF synthesis, which can lead to analyte detection with possible amplification of fluorescence. Upon successful immobilization, the resulting FRET-based MOFs can be used as fluorescent turn-on sensors, and more importantly, a simple but general strategy for amplified fluorescence sensing will be adequately established. In this work, we report the design and synthesis of a series of FRET-based MOFs [UiO-68-DA(x), x = 0.7, 1.2, 3.0, 6.7, and 11.4], which have both donor and acceptor in a different ratio (Scheme 1). Due to the efficient FRET process in the framework, the obtained MOFs crystals showed very weak fluorescence. As expected, after treating these crystals with a stimulus, the acceptors were quantitatively transferred into the other state and thus prohibited the FRET process with fluorescence enhancement. More importantly, these designed FRET-based MOFs behaved as fluorescent turn-on sensors for analyte detection with interesting amplification phenomena. For example, when the ratio of donor fluorophores and acceptors varied from 0.7 to 11.4, the limit of detection (LOD) of the FRET-based MOFs was reduced by 157 times, strongly indicating the amplified fluorescence sensing. Scheme 1 | Amplified fluorescence sensing in MOFs by modulating the FRET donor and acceptor ratios in Förster sphere (the big red spheres). Download figure Download PowerPoint Experimental Section General methods All reagents and solvents were purchased from commercial sources and used without further purification. N,N-Dimethylformamide (DMF) and dichloromethane (DCM), when noted as anhydrous, were dried by the Innovative Technology (Baltimore, MD, USA) solvent purification system. Zirconium (IV) chloride (ZrCl4, 99.5%) and tetraethylene glycol were purchased from Alfa Aesar (Ward Hill, MA, USA). Iodobenzene diacetate (PIDA), ascorbic acid (VC), and tert-butyl hydroquinone (TBHQ) were purchased from Adamas-Beta® (Shanghai, China). H2D,54 H2A,55 and H2P55 were synthesized following previously reported procedures. 1H and 13C NMR spectra were measured on a Bruker AVANCE III (Fällanden, Switzerland) HD 400 MHz spectrometer. High-resolution mass spectra (HR-MS) were collected on Bruker Daltonics, Inc (Billerica, MA, USA). APEX II Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. Powder X-ray diffraction (PXRD) data were collected on Rigaku SmartLab (Tokyo, Japan) with Cu Kα1 (λ = 1.54056 Å) radiation operated at 45 kV and 200 mA, from 2θ = 2° up to 50° with 0.02° increment. The UV–vis absorption spectra were recorded on a Shimadzu UV-3600 spectrophotometer. Fluorescence measurements were carried out with a Hitachi F-4600 fluorescence spectrophotometer. Absolute photoluminescence quantum yields (PLQYs) were obtained using a Quantaurus-QY measurement system (C9920-02; Hamamatsu Photonics, Hamamatsu, Japan). Time-resolved photoluminescence (PL) decay curves were measured by monitoring the decay of the intensity at the PL peak wavelength using the time-correlated single-photon counting fluorescence lifetime system in FLS920 of Edinburgh Instruments with a picosecond pulsed UV-LASTER (LASTER377) as the excitation source. Field-emission scanning electron microscopy (SEM) images were performed on a Zeiss SIGMA operating at an accelerating voltage ranging from 0.1 to 20 kV. The nitrogen adsorption and desorption isotherms were measured at 77 K using a Quantachrome Autosorb-iQ2 automated gas sorption analyzer. Before measurement, approximately 50 mg of activated MOFs was degassed in a vacuum at room temperature for 24 h. The BET surface areas were calculated from selected isotherm points based upon the consistency criteria, detailed by Walton and Snurr.56 MOFs activation The MOF crystals were allowed to immerse in dry DCM for several times over 3 days to replace and remove DMF, which were then evacuated in an oil pump vacuum at room temperature. MOFs digestion and 1H NMR study In a typical procedure, approximately 5 mg of activated MOF samples was digested with sonication in approximately 0.6 mL DMSO and 40 μL CF3COOH. After that, water was added to the resulting solution until no further precipitate was detected. The precipitate was collected by filtration, washed with water, and dried in vacuum for HR-MS study. In addition, the activated samples were dissolved in DMSO-d6 with 40 μL CF3COOD for 1H NMR study. Synthesis of UiO-68-DP(x), x = 0.7, 1.2, 3.0, 6.7, and 11.4 Upon mixing H2D and H2P of various stoichiometry with ZrCl4, UiO-68-DP(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4) were synthesized under similar solvothermal conditions. The MOF (denoted as UiO-68-D) containing only H2D was also synthesized for energy transfer efficiency calculation. See "Section 2" in Supporting Information for the detailed synthetic procedures. Synthesis of UiO-68-DA(x), x = 0.7, 1.2, 3.0, 6.7, and 11.4 A fresh sample of the respective UiO-68-DP(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4) kept in the DMF was exchanged with dry DCM three times. About 10 mg of the respective UiO-68-DP(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4) was immersed in a 3 mL DCM solution of iodobenzene diacetate (25 mg mL−1), and the reaction mixture was repeatedly agitated by pumping with a pipette. After oxidation for 5 min at room temperature, the resulting crystals were isolated by centrifugation (4000 rpm for 5 min) and washed three times with dry DCM and ethanol, and then kept in ethanol for further use. In addition, samples not used for subsequent reduction were activated. Sensing study A fresh sample of UiO-68-DA(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4) was exchanged with ethanol several times. After that, the crystals (approximately0.1 mg) were immersed into ethanolic solutions of analytes for 15 min. Result and Discussion To construct a MOF with FRET donor and acceptor, a highly emissive 2′,5′-dimethoxy-[1,1′∶4′,1″-terphenyl]-4,4″-dicarboxylic acid linker ( H2D) was selected as donor fluorophore.54 For the acceptor, we chose the reported quinone-based linker H2A,55 because its absorption spectrum overlaps well with the emission spectrum of H2D while there will be minor overlap after reduction into H2P ( Supporting Information Figure S1). We then decided to build the Zr-MOFs through the MTV approach, due to their robust stability. Since H2A is unstable under the solvothermal condition to form a predictable MOF structure,55 we first took H2D and H2P to synthesize the Zr-MOFs, which could be further transformed to the designed FRET-based MOFs via postsynthetic modification (Figure 1). By mixing H2D and H2P of various stoichiometry with ZrCl4 in a preheated 120 °C oven, octahedral-shaped crystals were obtained after several hours ( Supporting Information Figure S2). According to the powder X-ray diffraction (PXRD) experiments ( Supporting Information Figure S3), the obtained crystals adopted typical UiO-68 structures, due to their identical PXRD patterns to the reported UiO-68-OH.55 In addition, the 1H NMR spectra of the digested samples suggested the linkers remained intact during the MOFs synthesis ( Supporting Information Figure S4), and the proportions of H2D and H2P were found to be 0.7, 1.2, 3.0, 6.7, and 11.4 ( Supporting Information Figure S5). Based on these results, a series of MTV-MOFs [UiO-68-DP(x), x = 0.7, 1.2, 3.0, 6.7, and 11.4] were obtained. We then synthesized the designed FRET-based MOFs with different ratios of donor and acceptor by reacting these MTV-MOFs with iodobenzene diacetate, since in this condition H2P can be oxidized to H2A. The PXRD and 1H NMR data indicated the retention of crystallinity (Figure 2a and Supporting Information Figure S7), and quantitative transformation of H2P into H2A while keeping H2D proportions unchanged (Figure 2b). Finally, the desired FRET-based MOFs [UiO-68-DA(x), x = 0.7, 1.2, 3.0, 6.7, and 11.4] were successfully obtained as octahedral-shaped crystals with high porosity (Figure 2a and Supporting Information Figures S8 and S9). Figure 1 | The synthesis of FRET-based MOFs [UiO-68-DA(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4)]. Download figure Download PowerPoint Figure 2 | Typical structure characterization of the FRET-based MOFs. (a) PXRD patterns of the representative UiO-68-DA(0.7) and UiO-68-DA(11.4). The insets show the corresponding SEM image of the samples. The scale bar is 5 μX. (b) 1H NMR (400 MHz, DMSO-d6 with 40 μwith3COOD) spectra of digested UiO-68-DA(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4). The arabic numerials in the figure show the ratios of the donor to acceptor from the integral of proton Hd and Ha. Download figure Download PowerPoint The fluorescent properties of UiO-68-DA(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4) were investigated by several techniques. Unlike the highly emissive UiO-68-D (synthesized from H2D and ZrCl4, see "Section 2" in Supporting Information) and digested FRET-based MOF solution, UiO-68-DA(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4) showed very weak fluorescence ( Supporting Information Figure S10) with PLQYs less than approximately 2% (Figure 3), indicating the efficient FRET process. We further studied the time-resolved decay profiles (inset of Figure 3), which were very close to the instrument response function (IRF) under 377 nm laser excitation. The semiquantitative analysis suggested that the energy transfer efficiency was higher than 77% ( Supporting Information Table S1), demonstrating again that efficient FRET can be achieved in the framework, even with a higher donor-to-acceptor ratio. The tunability of the FRET process in these FRET-based MOFs was then studied. After immersing the representative UiO-68-DA(0.7) and UiO-68-DA(11.4) into the ascorbic acid solution, the fluorescence of the resulting crystals exhibited more than 10 times enhancement ( Supporting Information Figures S12 and S13). Subsequent 1H NMR spectra analysis of the digested crystals ( Supporting Information Figure S14) indicated that H2A was quantitatively reduced to H2P while keeping H2D proportions unchanged. In addition, the PXRD patterns of the resulting MOFs showed that the crystallinity was retained ( Supporting Information Figure S15). Consequently, the FRET process can be efficiently tuned by the reduction of accessible acceptors in the frameworks. Figure 3 | The absolute PLQYs of UiO-68-D, UiO-68-DA(11.4), UiO-68-DA(6.7), UiO-68-DA(3.0), UiO-68-DA(1.2), and UiO-68-DA(0.7). The error bar represents the standard derivation of several measurements. The inset shows the corresponding fluorescence decay profiles of UiO-68-D, UiO-68-DA(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4), and IRF (λex = 377 nm) monitored at 428 nm in ethanol. Download figure Download PowerPoint The tunable FRET process in these UiO-68-DA(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4) implies their possible application in amplified fluorescence sensing. We chose TBHQ as the model reductive analyte (Figure 4a), which is extensively used as a food additive but shows potential genotoxicity when exceeding 0.02% of oil or fat content.57 Upon adding MOFs into the ethanolic solution of TBHQ, the photographs of resulting suspensions were recorded under UV light. Obviously, these FRET-based MOFs exhibited turned-on fluorescence but with different detection ability (Figure 4b). For example, UiO-68-DA(0.7) and UiO-68-DA(11.4) can detect TBHQ in a concentration of 5 × 10−4 and 5 × 10−6 mol/L, respectively. Accordingly, the designed FRET-based MOFs can be utilized as amplified fluorescence turn-on sensors. To figure out the detailed information of this amplified fluorescence sensing behavior, the PLQYs of TBHQ-treated FRET-based MOFs were further measured (Figure 5). Subsequently, the related LODs were calculated according to their IUPAC-recommended definition 3σ/s,58 where σ is the standard deviation of the PLQYs of these FRET-based MOFs and s is the slope of the linear calibration ( Supporting Information Figure S19 and Table S2). As shown in Figure 6, when the ratio of donor to acceptor increased in the frameworks, the values of LODs decreased with a remarkable amplification effect. For instance, from UiO-68-DA(0.7) to UiO-68-DA(11.4), the ratio of donor to acceptor increased 16 times, but the LOD became 157 times lower. Therefore, FRET-based MOFs with variable ratios of donor fluorophore to acceptor can be utilized to construct amplified fluorescence sensing systems. Figure 4 | (a) Schematic representation of fluorescence turn-on sensing of TBHQ by using UiO-68-DA(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4) as probe. (b) Photographs (under a lab UV lamp) of UiO-68-DA(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4) (blank) and after being immersed in the corresponding ethanolic solution of TBHQ for about 15 min. Download figure Download PowerPoint Conclusion We have synthesized a series of highly porous FRET-based MOFs, which showed very weak fluorescence due to the efficient FRET process in the framework. After the reduction of the acceptor, the FRET process can be prohibited, and consequently, the fluorescence of MOF crystals turned on. Interestingly, these FRET-based MOFs can be used as the fluorescent turn-on sensors for a reductive analyte. More importantly, by calculating the limit of detection, a significant amplified fluorescence sensing phenomenon was observed. Therefore, from this proof of principle study, the immobilization of FRET donor and acceptor into a single MOF can not only allow the resulting material to be used as a fluorescent turn-on sensor, but also provides a simple and general strategy to efficiently construct novel amplified fluorescence sensing platforms. Considering the broad interest in MOF- sensing,39–41 this result will definitely facilitate in designing ultrasensitive MOF-based sensors in the future. Figure 5 | Absolute PLQYs of UiO-68-DA(0.7) (a), UiO-68-DA(1.2) (b), UiO-68-DA(3.0) (c), UiO-68-DA(6.7) (d), and UiO-68-DA(11.4) (e) after being immersed in the corresponding ethanolic solution of TBHQ for 15 min. The error bar represents the standard derivation of several measurements. Download figure Download PowerPoint Figure 6 | The correlation of LODs and calculated fluorescence-sensing amplification factors [the specfic value of the LOD of UiO-68-DA(0.7) compared with UiO-68-DA(x) (x = 0.7, 1.2, 3.0, 6.7, and 11.4)] with the FRET donor and acceptor ratio in the framework. Download figure Download PowerPoint Supporting Information Supporting Information is available. Conflict of Interest The authors declare no competing financial interests. Acknowledgments The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (nos. 21975188, 21772149, and 21905211) and the China Postdoctoral Science Foundation (nos. 2019TQ0234 and 2019M652692). References 1. Holzmeister P.; Acuna G. P.; Grohmann D.; Tinnefeld P.Breaking the Concentration Limit of Optical Single-Molecule Detection.Chem. Soc. Rev.2014, 43, 1014–1028. Google Scholar 2. Sreejith S.; Joseph J.; Lin M.; Menon N. V.; Borah P.; Ng H. J.; Loong Y. X.; Kang Y.; Yu S. 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