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UiO-66-NDC (1,4-naphthalenedicarboxilic acid) as a novel fluorescent probe for the selective detection of Fe3+
Journal Pre-proof UiO-66-NDC (1,4-naphthalenedicarboxilic acid) as a novel fluorescent probe for the 3+ selective detection of Fe Yuchun He, Laixiang Shi, Ji Wang, Jun Yan, Yunlin Chen, Xiangqian Wang, Yuzhe Song, Genliang Han PII:
S0022-4596(20)30036-0
DOI:
https://doi.org/10.1016/j.jssc.2020.121206
Reference:
YJSSC 121206
To appear in:
Journal of Solid State Chemistry
Received Date: 18 December 2019 Revised Date:
8 January 2020
Accepted Date: 16 January 2020
Please cite this article as: Y. He, L. Shi, J. Wang, J. Yan, Y. Chen, X. Wang, Y. Song, G. Han, UiO-663+ NDC (1,4-naphthalenedicarboxilic acid) as a novel fluorescent probe for the selective detection of Fe , Journal of Solid State Chemistry (2020), doi: https://doi.org/10.1016/j.jssc.2020.121206. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Inc.
Credit Author Statement Yuchun He: Conceptualization, Investigation, Writing-Original Draft Laixiang Shi: Validation, Investigation Ji Wang: Validation Jun Yan: Resources Yunlin Chen: Supervision, Project administration Xiangqian Wang: Supervision Yuzhe Song: Project administration Genliang Han: Project administration
Graphical abstract
UiO-66-NDC (1,4-naphthalenedicarboxilic acid) as a novel fluorescent probe for the selective detection of Fe3+ Yuchun Hea, Laixiang Shia, Ji Wanga, Jun Yana, Yunlin Chena,*, Xiangqian Wangb, Yuzhe Songb,**, Genliang Hanb a
Institute of Applied Micro-Nano Materials, School of Science, Beijing Jiaotong University,
Beijing 100044, PR China. b
Institute of Sensor Technology, Gansu Academy of Science, Lanzhou 730000, China
Corresponding authors E-mail: [email protected] (Y. Chen), [email protected]
Abstract: The development of a simple fluorescent sensor for detecting metal ions is fundamentally essential. Some metal-organic frameworks exhibit excellent photoluminescence properties associated with metal ions. Herein, we synthesized a new naphthalene-substituted UiO-66 analog (UiO-66-NDC) by the solvothermal method using 1,4-naphthalenedicarboxilic acid as the organic linker. Due to UiO-66-NDC has excellent photoluminescence property and good dispersibility, we explored the potential application of UiO-66-NDC in fluorescence sensing. The results show that UiO-66-NDC not only featured different fluorescent response ability for some small-molecule solvents but also exhibited significant selectivity to several different metal ions. In particular, Fe3+ ions could cause serious fluorescence quenching of UiO-66-NDC in N,N-dimethylformamide (DMF). The sensing detection for different Fe3+ ions concentrations was performed at room temperature, and the results indicated that UiO-66-NDC can be utilized as an excellent fluorescent probe for sensing Fe3+ ions. Keywords:
UiO-66-NDC,
Fluorescence quenching
Fluorescent
probes,
Selective
sensing
detection,
Fe3+ions,
1. Introduction As one of the most important trace elements, Iron (III) is involved in the processes of electron transfer enzyme catalysis, oxygen transport, cell metabolism, and tissue respiration to maintain the normal hematopoietic function of the human body [1]. What matters is that excess or deficiency of Fe3+ ions can cause a variety of tissue changes and dysfunction, such as the cell component damage, the organization inflammation, multi-organ fibrosis arthritis, mental decline, anemia, and metabolic disorder for humanity [2,3]. Therefore, it is especially significant to find the material with high selectivity and sensitivity to detect Fe3+ ions. Because the fluorescence detection method is simple, convenient, accurate and economical, and suitable for online monitoring, so it is an excellent strategy to design the fluorescence probe [4-6]. Metal-organic framework materials (MOFs) are built through the coordination between metal ions/clusters and organic ligands, providing high porosity, large surface area, diverse structures and functional tunability [7], which have good application prospects in gas separation/adsorption, catalysis, fluorescence sensing, biomedicine, white light emission, and up-conversion luminescence [8-16]. It has become one of the research focus to assemble light-emitting MOFs and explore the relationship between structure and performance by introducing light-emitting metal ions, conjugated organic ligands or light-emitting objects. Recently, MOFs have been widely applied in recognition of small molecules, metal ions, anions, and nitro explosive complexes [17-20]. In 2008, Lillerud et al. [21] first reported a zirconium (IV) dicarboxylate porous material, which was denominated as UiO-66. UiO-66 is composed of Zr cluster secondary building units (SBUs) and H2BDC (1,4-benzenedicarboxylicacid) as the linkers. Due to the high oxygen affinity
of Zr, UiO-66 exhibits exceptional chemical stability, hydrothermal stability, high BET surface area, and is predicted to be one of the most promising MOFs for practical applications [22-24]. Besides, the functional Zr-based MOFs can be easily prepared by modifying the ligand H2BDC with other functional groups to coordinate with Zr [25]. For example, Yang et al. [26] prepared UiO-66-NH2 for the sensitive and selective phosphate anion detection in aqueous medium. Dong et al. [27] fabricated UiO-66-BI via microwave synthesis for sensing of Fe3+ ions in water. Zhang et al. [28] used 2,6-naphthalenedicarboxylic acid as the ligand to synthesize Zr-based MOF with high stability and long-term reusability for sensing solvents, etc. Recently, Wang et al. [29] reported that UiO-66-NDC be used to optical transmittance measured. However, to our best knowledge, the relevant researches to design a UiO-66-NDC fluorescent probe for the selective detection of metal ions have not been reported. In this paper, we fabricated a new naphthalene-substituted UiO-66 nanocrystal (UiO-66-NDC) via the solvothermal method using 1,4-naphthalenedicarboxilic acid as the organic linker. The UiO-66-NDC shows excellent luminescent properties and exhibits high selectivity and sensitivity for the detection of Fe3+ ions. The mechanism of the fluorescence quenching behavior of UiO-66-NDC for Fe3+ ions was also discussed.
2. Experimental Section 2.1 Materials All reagents, such as zirconium tetrachloride (ZrCl4, 98%), acetic acid (99.5%), 1,4-benzene-dicarboxylate (H2BDC, 98%), 1,4-naphthalenedicarboxilic acid (1,4-NDC, 98%), N,N-dimethylformamide (DMF, 99%), are analytical grade and used without further purification.
2.2 Synthesis 2.2.1 Synthesis of UiO-66-NDC 0.466 g (2 mmol) ZrCl4 and 8mL acetic acid were dissolved in 30 mL DMF with stir for 15 min, 0.432 g (2 mmol) 1,4-NDC was dissolved in 30 mL DMF with stir for 15 min. After the complete dissolution of the reagents, the DMF solution of 1,4-NDC was slowly dropped into the DMF solution of ZrCl4 by stirring treatment to obtain a clear solution. Then, the mixture was placed in a 100 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 24 h. After cooled to room temperature, the precipitate was separated from the liquid by centrifugation, and washed several times with DMF and CH2Cl2. Finally, the product was dried overnight at room temperature. 2.2.2 Synthesis of UiO-66 According to the above method, the only difference is to replace the ligand 1,4-NDC with 0.332 g (2 mmol) of H2BDC. 2.3 Characterization Powder X-ray diffraction (PXRD) data were recorded on a Purkinje General XD-3 X-ray diffractometer with a Cu Kα radiation source (λ = 1.5418 Å). Scanning electron microscope (SEM) pictures of carbon spraying samples were taken by using a Merlin Compact scanning electron microscope. The UV–Vis absorption spectra were measured on a TU-1901 spectrometer equipped with an integrating sphere. The photoluminescence (PL) spectra were taken on an SP-2500i photoluminescence system, and the N2 sorption data were collected on a JW-BK122W static nitrogen adsorption instrument at 77 K in a liquid nitrogen bath. 2.4 Fluorescent sensing detection
The fluorescent responses of UiO-66-NDC towards various small-molecule solvents were measured by PL spectra. Before PL spectra measurement, 3 mg UiO-66-NDC were dispersed into different solvents of 3 mL, including CH2Cl2, DMF, acetonitrile (MeCN), methanol (MeOH), ethanol (EtOH), isopropanol (IPA) and H2O, and then placed under the ultrasonic condition for 30 min to form stable suspensions. To investigate the fluorescence quenching of Fe3+ ions, 3 mg UiO-66-NDC were dispersed into 3 mL DMF solution with different concentrations (0-1 mM) of Fe3+ ions, and the fluorescence intensities were also measured. 3. Results and discussion 3.1 Preparation and characterization
Fig. 1. (a) Synthesis of UiO-66-NDC by a solvothermal method. The UiO-66-NDC framework with its Zr6O6 cuboctahedron is schematically represented as an octahedron; (b) the photographs of UiO-66-NDC under ambient and 365 nm UV light.
Fig. 2. (a) PXRD patterns of the simulated UiO-66, UiO-66, and UiO-66-NDC; (b) N2 adsorption isotherms of UiO-66 and UiO-66-NDC. The inset is the diagram of the pore size distribution of UiO-66 and UiO-66-NDC. A schematic illustration of the solvothermal method synthesis of UiO-66-NDC is shown in Fig. 1(a). The obtained UiO-66-NDC powder appeared white color and turned to be bright blue under 365 nm UV light, as shown in Fig. 1(b). Fig. 2(a) shows the PXRD patterns of UiO-66-NDC and UiO-66 remarkably consistent with the simulated pattern of UiO-66, indicating that this naphthalene-substituted UiO-66 analog and UiO-66 have the same structure. Fig. 2(b) shows the N2 sorption isotherms and the diagram of pore size distribution of UiO-66 and UiO-66-NDC. Comparing with UiO-66, the calculated Brunauer-Emmett-Teller (BET) specific surface area and pore size of UiO-66-NDC decreased from 1165 m2/g to 648 m2/g, 0.664 nm to 0.639 nm, respectively. This result makes sense, as the larger functional group will reduce the free space in the UiO-66 structure, leading to diminished porosities.
Fig. 3. Low-magnifcation and high-magnifcation SEM images of the samples. (a,b): UiO-66; (c,d): UiO-66-NDC.
The SEM images of UiO-66 and UiO-66-NDC are shown in Fig. 3 to reveal the corresponding morphology and structure, respectively. UiO-66-NDC and UiO-66 demonstrated the same well-proportioned octahedral crystallite morphology, according to the XRD analysis. UiO-66-NDC has significantly smaller dimensions than UiO-66 crystallites, the average edge length about 100 nm, and it is possible to facilitate UiO-66-NDC uniform dispersion in different solutions.
Fig. 4. (a) The solid state UV–Vis absorption spectra of UiO-66 and UiO-66-NDC. The inset shows the KubelkaMunk-transformed diffuse reflectance spectra of UiO-66 and UiO-66-NDC; (b) PL spectra of UiO-66 and UiO-66-NDC. The inset shows the photographs of samples under ambient and 365 nm UV light. The solid state UV–Vis absorption spectra in Fig. 4(a) shows that UiO-66-NDC powders exhibited a strong absorption band at 300-380 nm, which was attributed from the π-π* transition of naphthalene rings. In comparison, the absorption of UiO-66 in the visible region is very weak, and the absorption band edge is about 308 nm. It follows that the introduction of the naphthalene group in UiO-66 can cause the absorption band edge red-shift and strengthen the light absorption capacity in the visible region. In addition, the optical band gap (Eg) is the crux to assess optical
absorption property. Generally, the band gap should be reduced to generate excellent luminescent performance. From Kubelka-Munk (K-M) function, F(R) is calculated which is related to the absorption coefficient (α) given by, F R =
=
(1)
where R is the diffuse reflectance, S is the scattering coefficient. In order to obtain the band gap from the (αhv)2 ~ hv curve, the intersection of linear absorption edge tangent with y = 0 was used. As shown in the inset in Fig. 4(a), the values of the band gap for UiO-66-NDC and UiO-66 are 3.37 eV and 4.05 eV, respectively, indicating that introducing the naphthalene group can reduce the band gap, and UiO-66-NDC possesses a higher electron-donating ability than UiO-66. The PL spectra in Fig. 4(b) show that with excitation at 365 nm, solid-state UiO-66-NDC exhibited a strong emission peak at 408 nm, but UiO-66 has almost no PL emission. The UiO-66-NDC powders emit bright blue light under 365 nm UV light in coincidence with PL spectra, indicating UiO-66-NDC could be a good candidate for the potential fluorescent sensing probe. 3.2 Sensing properties 3.2.1 Small-molecule solvents sensing properties
Fig. 5. (a) PL emission spectra of UiO-66-NDC immersed in different solvents under 365 nm UV excitation. The inset shows the photographs of samples under illumination by a 365nm UV lamp;
solvents: (A) H2O (B) MeOH (C) EtOH (D) IPA (E) CH2Cl2 (F) DMF (G) MeCN; (b) PL intensity of samples against the Reichardt’s solvent polarity parameters. As shown in Fig. 5, the sensing properties of UiO-66-NDC was studied in different solvents, including H2O, methanol (MeOH), ethanol (EtOH), isopropyl alcohol (IPA), CH2Cl2, DMF, and acetonitrile (MeCN). UiO-66-NDC exhibits different fluorescent response toward various solvents, it is evident that UiO-66-NDC in CH2Cl2 emits bright blue fluorescence, but hardly in H2O. By the PL spectrum measurement analysis, the fluorescence intensity of these samples depends mainly on the type and polarity of solvents. According to the presence or absence of hydroxyl groups, solvents are divided into two categories, and the PL intensity decreases linearly with increasing the polarity of solvents. The different dependence of UiO-66-NDC on these two kinds of solvents may be due to their different hydrogen bond donor capacities [30]. 3.2.2 Fe3+ ions sensing properties
Fig. 6. Photographs of UiO-66-NDC immersed in DMF with different Fe3+ concentrations under illumination by a 365nm UV lamp; Fe3+ concentration: (A) 0 M (B) 0.01 mM (C) 0.025 mM (D) 0.05 mM (E) 0.075 mM (F) 0.10 mM (G) 0.25 mM (H) 0.50 mM (I) 0.75 mM (J) 1.00 mM. In consideration of the solubility of metal salts in solvents, DMF was used as the solvent
carrier. To evaluate the quenching properties of the fluorescent probe for Fe3+ ions, the luminescent properties of UiO-66-NDC to different Fe3+ ions concentrations were investigated. 3 mg UiO-66-NDC powder was dispersed in DMF solution with Fe3+ ions. All solutions were 3 mL, and the Fe3+ ions concentration gradually increased from 0 to 1 mM. Fig. 6 shows the photographs of UiO-66-NDC in different concentrations of Fe3+ ions solutions under a 365 nm UV lamp in a dark room, and these solutions emit blue fluorescence. As the Fe3+ ions concentration increased from 0 to 1 mM, the luminescent brightness perceived by human eyes became weaker significantly. When the concentration of Fe3+ ions is 0.5 to 1 mM, it is observed that these solutions are almost no luminescent.
Fig. 7. PL emission spectra of UiO-66-NDC immersed in DMF with different Fe3+ ions concentrations under 365 nm UV excitation. Under excitation at 365 nm, the dependence of PL spectra of UiO-66-NDC solutions on Fe3+ ions concentrations is shown in Fig. 7, which illustrates that the PL intensity decreases remarkably with increasing the concentrations of Fe3+ ions, and the maximum emission wavelength exhibits red-shifted. When the concentration of Fe3+ ions reaches 1 mM, the fluorescence is almost
quenched completely. The above results suggest that UiO-66-NDC may be appreciated for sensing Fe3+ ions.
Fig. 8. (a) Relationship between PL intensity and Fe3+ concentrations, and the inset shows the PL intensity values and Fe3+ ions at low concentrations; (b) Relationship between I0/I and different Fe3+ concentrations, and the inset shows the Stern-Volmer plot for the I0/I values and Fe3+ ions at low concentrations. To obtain a quantitative relationship, we fitted the PL intensity of the solutions as a function of Fe3+ ions concentration. Depending on Fe3+ ions concentrations in the range from 0 to 1 mM, the PL intensity satisfies function y = 1997.36 exp (-x/0.01) + 3499.39 exp (-x/0.16) + 220.73, as shown in Fig. 8 (a). The inset in Fig. 8 (a) shows that the PL intensity has a good linear relationship in the range of 0.01 to 0.1 mM Fe3+ ions concentrations with the correlation coefficient R2 of 0.986. In addition, the limit of detection (LOD) of UiO-66-NDC for Fe3+ ions can be calculated as 0.65 μM by 3σ/κ (σ is the standard deviation of the blank measurement, κ is the slope of the calibration curve). The luminescence quenching effect was further analyzed by the Stern-Volmer equation [31]: I0/I=1+Ksv[C] where I and I0 denote the PL intensities of the solution in the presence and absence of Fe3+ ions,
respectively. [C] represents the Fe3+ ions concentration, and Ksv is quenching constant (slope). Fig. 8(b) illustrates that the fitting effect of I0/I-Fe3+ ions concentration curve, it presents an exponential relationship among the concentration range of 0 to 1 mM, and the equation is I0/I=5.34exp(2.17[C])-3.89 with the correlation coefficient R2 of 0.997. For the low concentrations of 0.01 to 0.1 mM, the I0/I can be fitted to I0/I=16.01[C]+1.29 (Ksv=1.6×104 M-1) with the correlation coefficient R2 of 0.992. The curve gradually variation from linear to nonlinear with the continues increase of Fe3+ ions concentration, which is similar to the reports of some MOFs-based fluorescence quenching sensors [32]. The sensing ability of UiO-66-NDC in the present investigation is compared with previously reported excellent MOF sensors for detection of Fe3+ is shown in Table 1. Table 1. Comparison among various MOF sensors for detection of Fe3+. Material
Ksv (M-1)
LOD (µM)
Media
Refs.
[Cd(Hcbic)]n
1.8×105
0.31
H 2O
[33]
PNT-2
2.0×104
0.31
ethanol
[34]
PNT-3
2.4×104
0.50
ethanol
[34]
ZnL
1.1×104
2.42
H 2O
[35]
67.7
H 2O
[36]
0.65
DMF
This work
SUF-1 UiO-66-NDC
1.6×104
Fig. 9. PXRD patterns of UiO-66-NDC. The stability of UiO-66-NDC in the presence of Fe3+ ions has also been measured by collecting PXRD patterns after the UiO-66-NDC powders immersed in DMF with 1mM Fe3+ ions for 24 hours and 30 days, respectively. Before PXRD measurement, the samples obtained by centrifugation was washed several times with DMF and CH2Cl2, and dried at room temperature. As shown in Fig. 9, the result shows the PXRD patterns are completely identical, indicating the samples are stable after sensing experiment. 3.2 Selectivity sensing properties
Fig. 10. PL response of UiO-66-NDC to different metal ions (1 mM). To evaluate the selective detection ability of UiO-66-NDC for Fe3+ ions, the effect of
different metal ions on the fluorescence emission intensities of UiO-66-NDC was investigated, as shown in Fig. 10. 3 mg UiO-66-NDC were dispersed into 3 mL DMF with different metal ions of the same concentration (1 mM), including Co2+, Mg2+, Zn2+, Al3+, Ni2+, Ga2+, K+, Cu2+ and Fe3+ ions. Before fluorescence intensities were measured, all suspensions were sonicated for 30 min to ensure dispersion. The spectroscopy response shows that the fluorescence intensity of the sample is closely related to the types of metal ions, and only Fe3+ ions gave rise to a clear fluorescence quenching effect for UiO-66-NDC solutions, the quenching percentages over 90%, indicating that the sensing probe has good selectivity for Fe3+ ions. 3.3. Quenching mechanism Most of the MOFs sensing ions by fluorescence quenching is due to atoms in their ligands which will cooperate with foreign metal ions and lead to fluorescence quenching by hydrogen bonding[18], but the quenching process of UiO-66-NDC is different. The dependence of the relative fluorescence intensity of UiO-66-NDC for Fe3+ ions was discussed.
Fig. 11. The UV–Vis absorption spectra of various metal ions (1 mM) and UiO-66-NDC in DMF. According to absorption competition quenching mechanism [34], if the absorption spectrum
of the detected substance is effectively overlapped with the absorption spectrum of the fluorescent material, the competition absorption of the irradiated light between the detected substance and the fluorescent material can reduce the light or energy absorbed by the fluorescent material, and the electrons in the ground state of the fluorescent material will little or no transition to the excited state, which will eventually lead to fluorescence quenching. So it is necessary to measure the UV-Vis absorption spectra of UiO-66-NDC and the metal ions in DMF. As shown in Fig. 11, the absorption spectrum of Fe3+ ions solution seriously overlap with the absorption spectrum of UiO-66-NDC at 280-425 nm, but other metal ions do not. That means Fe3+ ions dramatically filter the light absorbed by UiO-66-NDC, which therefore leads to fluorescence quenching. Hence, the results indicate that the absorption competition quenching mechanism can be responsible for UiO-66-NDC sensing Fe3+ ions, and it is also a strategy to design the ions fluorescence sensor. 4. Conclusion In summary, we successfully fabricated a naphthalene-substituted UiO-66 analog (UiO-66-NDC) with high photoluminescence performance via the solvothermal approach. It was demonstrated that UiO-66-NDC exhibited significant selectivity and sensitivity to Fe3+ ions by the fluorescence quenching effect. The PL intensity of the UiO-66-NDC solution decreases with increasing Fe3+ ions concentration, exhibiting a good linear relationship in the low concentrations range of 0.01 to 0.1 mM with the value of Ksv is 1.6×104 M-1. The quenching performance of UiO-66-NDC for Fe3+ ions can be ascribed to the absorption competition quenching mechanism, that is, the serious overlap of the absorption spectra of Fe3+ ions solution and UiO-66-NDC leads to fluorescence quenching. Based on the absorption competition quenching mechanism, it is feasible to design other novel MOFs for metal ions detection in the future.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No.61875235); Gansu Academy of Science Project (No.2019HZ-04).
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Graphical abstract Legend The UiO-66-NDC has been synthesized via the solvothermal method with excellent photoluminescence property, which exhibits significant selectivity and sensitivety to Fe3+ ions.
Highlights: A naphthalene-substituted UiO-66 nanocrystal (UiO-66-NDC) was synthesized via the solvothermal method using 1,4-naphthalenedicarboxilic acid as the organic linker. The UiO-66-NDC showed excellent light absorption capacity and photoluminescence property. The UiO-66-NDC exhibited significant selectivity and sensitivity to Fe3+ ions.
Declaration of interests 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.
S0022-4596(20)30036-0
DOI:
https://doi.org/10.1016/j.jssc.2020.121206
Reference:
YJSSC 121206
To appear in:
Journal of Solid State Chemistry
Received Date: 18 December 2019 Revised Date:
8 January 2020
Accepted Date: 16 January 2020
Please cite this article as: Y. He, L. Shi, J. Wang, J. Yan, Y. Chen, X. Wang, Y. Song, G. Han, UiO-663+ NDC (1,4-naphthalenedicarboxilic acid) as a novel fluorescent probe for the selective detection of Fe , Journal of Solid State Chemistry (2020), doi: https://doi.org/10.1016/j.jssc.2020.121206. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Inc.
Credit Author Statement Yuchun He: Conceptualization, Investigation, Writing-Original Draft Laixiang Shi: Validation, Investigation Ji Wang: Validation Jun Yan: Resources Yunlin Chen: Supervision, Project administration Xiangqian Wang: Supervision Yuzhe Song: Project administration Genliang Han: Project administration
Graphical abstract
UiO-66-NDC (1,4-naphthalenedicarboxilic acid) as a novel fluorescent probe for the selective detection of Fe3+ Yuchun Hea, Laixiang Shia, Ji Wanga, Jun Yana, Yunlin Chena,*, Xiangqian Wangb, Yuzhe Songb,**, Genliang Hanb a
Institute of Applied Micro-Nano Materials, School of Science, Beijing Jiaotong University,
Beijing 100044, PR China. b
Institute of Sensor Technology, Gansu Academy of Science, Lanzhou 730000, China
Corresponding authors E-mail: [email protected] (Y. Chen), [email protected]
Abstract: The development of a simple fluorescent sensor for detecting metal ions is fundamentally essential. Some metal-organic frameworks exhibit excellent photoluminescence properties associated with metal ions. Herein, we synthesized a new naphthalene-substituted UiO-66 analog (UiO-66-NDC) by the solvothermal method using 1,4-naphthalenedicarboxilic acid as the organic linker. Due to UiO-66-NDC has excellent photoluminescence property and good dispersibility, we explored the potential application of UiO-66-NDC in fluorescence sensing. The results show that UiO-66-NDC not only featured different fluorescent response ability for some small-molecule solvents but also exhibited significant selectivity to several different metal ions. In particular, Fe3+ ions could cause serious fluorescence quenching of UiO-66-NDC in N,N-dimethylformamide (DMF). The sensing detection for different Fe3+ ions concentrations was performed at room temperature, and the results indicated that UiO-66-NDC can be utilized as an excellent fluorescent probe for sensing Fe3+ ions. Keywords:
UiO-66-NDC,
Fluorescence quenching
Fluorescent
probes,
Selective
sensing
detection,
Fe3+ions,
1. Introduction As one of the most important trace elements, Iron (III) is involved in the processes of electron transfer enzyme catalysis, oxygen transport, cell metabolism, and tissue respiration to maintain the normal hematopoietic function of the human body [1]. What matters is that excess or deficiency of Fe3+ ions can cause a variety of tissue changes and dysfunction, such as the cell component damage, the organization inflammation, multi-organ fibrosis arthritis, mental decline, anemia, and metabolic disorder for humanity [2,3]. Therefore, it is especially significant to find the material with high selectivity and sensitivity to detect Fe3+ ions. Because the fluorescence detection method is simple, convenient, accurate and economical, and suitable for online monitoring, so it is an excellent strategy to design the fluorescence probe [4-6]. Metal-organic framework materials (MOFs) are built through the coordination between metal ions/clusters and organic ligands, providing high porosity, large surface area, diverse structures and functional tunability [7], which have good application prospects in gas separation/adsorption, catalysis, fluorescence sensing, biomedicine, white light emission, and up-conversion luminescence [8-16]. It has become one of the research focus to assemble light-emitting MOFs and explore the relationship between structure and performance by introducing light-emitting metal ions, conjugated organic ligands or light-emitting objects. Recently, MOFs have been widely applied in recognition of small molecules, metal ions, anions, and nitro explosive complexes [17-20]. In 2008, Lillerud et al. [21] first reported a zirconium (IV) dicarboxylate porous material, which was denominated as UiO-66. UiO-66 is composed of Zr cluster secondary building units (SBUs) and H2BDC (1,4-benzenedicarboxylicacid) as the linkers. Due to the high oxygen affinity
of Zr, UiO-66 exhibits exceptional chemical stability, hydrothermal stability, high BET surface area, and is predicted to be one of the most promising MOFs for practical applications [22-24]. Besides, the functional Zr-based MOFs can be easily prepared by modifying the ligand H2BDC with other functional groups to coordinate with Zr [25]. For example, Yang et al. [26] prepared UiO-66-NH2 for the sensitive and selective phosphate anion detection in aqueous medium. Dong et al. [27] fabricated UiO-66-BI via microwave synthesis for sensing of Fe3+ ions in water. Zhang et al. [28] used 2,6-naphthalenedicarboxylic acid as the ligand to synthesize Zr-based MOF with high stability and long-term reusability for sensing solvents, etc. Recently, Wang et al. [29] reported that UiO-66-NDC be used to optical transmittance measured. However, to our best knowledge, the relevant researches to design a UiO-66-NDC fluorescent probe for the selective detection of metal ions have not been reported. In this paper, we fabricated a new naphthalene-substituted UiO-66 nanocrystal (UiO-66-NDC) via the solvothermal method using 1,4-naphthalenedicarboxilic acid as the organic linker. The UiO-66-NDC shows excellent luminescent properties and exhibits high selectivity and sensitivity for the detection of Fe3+ ions. The mechanism of the fluorescence quenching behavior of UiO-66-NDC for Fe3+ ions was also discussed.
2. Experimental Section 2.1 Materials All reagents, such as zirconium tetrachloride (ZrCl4, 98%), acetic acid (99.5%), 1,4-benzene-dicarboxylate (H2BDC, 98%), 1,4-naphthalenedicarboxilic acid (1,4-NDC, 98%), N,N-dimethylformamide (DMF, 99%), are analytical grade and used without further purification.
2.2 Synthesis 2.2.1 Synthesis of UiO-66-NDC 0.466 g (2 mmol) ZrCl4 and 8mL acetic acid were dissolved in 30 mL DMF with stir for 15 min, 0.432 g (2 mmol) 1,4-NDC was dissolved in 30 mL DMF with stir for 15 min. After the complete dissolution of the reagents, the DMF solution of 1,4-NDC was slowly dropped into the DMF solution of ZrCl4 by stirring treatment to obtain a clear solution. Then, the mixture was placed in a 100 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 24 h. After cooled to room temperature, the precipitate was separated from the liquid by centrifugation, and washed several times with DMF and CH2Cl2. Finally, the product was dried overnight at room temperature. 2.2.2 Synthesis of UiO-66 According to the above method, the only difference is to replace the ligand 1,4-NDC with 0.332 g (2 mmol) of H2BDC. 2.3 Characterization Powder X-ray diffraction (PXRD) data were recorded on a Purkinje General XD-3 X-ray diffractometer with a Cu Kα radiation source (λ = 1.5418 Å). Scanning electron microscope (SEM) pictures of carbon spraying samples were taken by using a Merlin Compact scanning electron microscope. The UV–Vis absorption spectra were measured on a TU-1901 spectrometer equipped with an integrating sphere. The photoluminescence (PL) spectra were taken on an SP-2500i photoluminescence system, and the N2 sorption data were collected on a JW-BK122W static nitrogen adsorption instrument at 77 K in a liquid nitrogen bath. 2.4 Fluorescent sensing detection
The fluorescent responses of UiO-66-NDC towards various small-molecule solvents were measured by PL spectra. Before PL spectra measurement, 3 mg UiO-66-NDC were dispersed into different solvents of 3 mL, including CH2Cl2, DMF, acetonitrile (MeCN), methanol (MeOH), ethanol (EtOH), isopropanol (IPA) and H2O, and then placed under the ultrasonic condition for 30 min to form stable suspensions. To investigate the fluorescence quenching of Fe3+ ions, 3 mg UiO-66-NDC were dispersed into 3 mL DMF solution with different concentrations (0-1 mM) of Fe3+ ions, and the fluorescence intensities were also measured. 3. Results and discussion 3.1 Preparation and characterization
Fig. 1. (a) Synthesis of UiO-66-NDC by a solvothermal method. The UiO-66-NDC framework with its Zr6O6 cuboctahedron is schematically represented as an octahedron; (b) the photographs of UiO-66-NDC under ambient and 365 nm UV light.
Fig. 2. (a) PXRD patterns of the simulated UiO-66, UiO-66, and UiO-66-NDC; (b) N2 adsorption isotherms of UiO-66 and UiO-66-NDC. The inset is the diagram of the pore size distribution of UiO-66 and UiO-66-NDC. A schematic illustration of the solvothermal method synthesis of UiO-66-NDC is shown in Fig. 1(a). The obtained UiO-66-NDC powder appeared white color and turned to be bright blue under 365 nm UV light, as shown in Fig. 1(b). Fig. 2(a) shows the PXRD patterns of UiO-66-NDC and UiO-66 remarkably consistent with the simulated pattern of UiO-66, indicating that this naphthalene-substituted UiO-66 analog and UiO-66 have the same structure. Fig. 2(b) shows the N2 sorption isotherms and the diagram of pore size distribution of UiO-66 and UiO-66-NDC. Comparing with UiO-66, the calculated Brunauer-Emmett-Teller (BET) specific surface area and pore size of UiO-66-NDC decreased from 1165 m2/g to 648 m2/g, 0.664 nm to 0.639 nm, respectively. This result makes sense, as the larger functional group will reduce the free space in the UiO-66 structure, leading to diminished porosities.
Fig. 3. Low-magnifcation and high-magnifcation SEM images of the samples. (a,b): UiO-66; (c,d): UiO-66-NDC.
The SEM images of UiO-66 and UiO-66-NDC are shown in Fig. 3 to reveal the corresponding morphology and structure, respectively. UiO-66-NDC and UiO-66 demonstrated the same well-proportioned octahedral crystallite morphology, according to the XRD analysis. UiO-66-NDC has significantly smaller dimensions than UiO-66 crystallites, the average edge length about 100 nm, and it is possible to facilitate UiO-66-NDC uniform dispersion in different solutions.
Fig. 4. (a) The solid state UV–Vis absorption spectra of UiO-66 and UiO-66-NDC. The inset shows the KubelkaMunk-transformed diffuse reflectance spectra of UiO-66 and UiO-66-NDC; (b) PL spectra of UiO-66 and UiO-66-NDC. The inset shows the photographs of samples under ambient and 365 nm UV light. The solid state UV–Vis absorption spectra in Fig. 4(a) shows that UiO-66-NDC powders exhibited a strong absorption band at 300-380 nm, which was attributed from the π-π* transition of naphthalene rings. In comparison, the absorption of UiO-66 in the visible region is very weak, and the absorption band edge is about 308 nm. It follows that the introduction of the naphthalene group in UiO-66 can cause the absorption band edge red-shift and strengthen the light absorption capacity in the visible region. In addition, the optical band gap (Eg) is the crux to assess optical
absorption property. Generally, the band gap should be reduced to generate excellent luminescent performance. From Kubelka-Munk (K-M) function, F(R) is calculated which is related to the absorption coefficient (α) given by, F R =
=
(1)
where R is the diffuse reflectance, S is the scattering coefficient. In order to obtain the band gap from the (αhv)2 ~ hv curve, the intersection of linear absorption edge tangent with y = 0 was used. As shown in the inset in Fig. 4(a), the values of the band gap for UiO-66-NDC and UiO-66 are 3.37 eV and 4.05 eV, respectively, indicating that introducing the naphthalene group can reduce the band gap, and UiO-66-NDC possesses a higher electron-donating ability than UiO-66. The PL spectra in Fig. 4(b) show that with excitation at 365 nm, solid-state UiO-66-NDC exhibited a strong emission peak at 408 nm, but UiO-66 has almost no PL emission. The UiO-66-NDC powders emit bright blue light under 365 nm UV light in coincidence with PL spectra, indicating UiO-66-NDC could be a good candidate for the potential fluorescent sensing probe. 3.2 Sensing properties 3.2.1 Small-molecule solvents sensing properties
Fig. 5. (a) PL emission spectra of UiO-66-NDC immersed in different solvents under 365 nm UV excitation. The inset shows the photographs of samples under illumination by a 365nm UV lamp;
solvents: (A) H2O (B) MeOH (C) EtOH (D) IPA (E) CH2Cl2 (F) DMF (G) MeCN; (b) PL intensity of samples against the Reichardt’s solvent polarity parameters. As shown in Fig. 5, the sensing properties of UiO-66-NDC was studied in different solvents, including H2O, methanol (MeOH), ethanol (EtOH), isopropyl alcohol (IPA), CH2Cl2, DMF, and acetonitrile (MeCN). UiO-66-NDC exhibits different fluorescent response toward various solvents, it is evident that UiO-66-NDC in CH2Cl2 emits bright blue fluorescence, but hardly in H2O. By the PL spectrum measurement analysis, the fluorescence intensity of these samples depends mainly on the type and polarity of solvents. According to the presence or absence of hydroxyl groups, solvents are divided into two categories, and the PL intensity decreases linearly with increasing the polarity of solvents. The different dependence of UiO-66-NDC on these two kinds of solvents may be due to their different hydrogen bond donor capacities [30]. 3.2.2 Fe3+ ions sensing properties
Fig. 6. Photographs of UiO-66-NDC immersed in DMF with different Fe3+ concentrations under illumination by a 365nm UV lamp; Fe3+ concentration: (A) 0 M (B) 0.01 mM (C) 0.025 mM (D) 0.05 mM (E) 0.075 mM (F) 0.10 mM (G) 0.25 mM (H) 0.50 mM (I) 0.75 mM (J) 1.00 mM. In consideration of the solubility of metal salts in solvents, DMF was used as the solvent
carrier. To evaluate the quenching properties of the fluorescent probe for Fe3+ ions, the luminescent properties of UiO-66-NDC to different Fe3+ ions concentrations were investigated. 3 mg UiO-66-NDC powder was dispersed in DMF solution with Fe3+ ions. All solutions were 3 mL, and the Fe3+ ions concentration gradually increased from 0 to 1 mM. Fig. 6 shows the photographs of UiO-66-NDC in different concentrations of Fe3+ ions solutions under a 365 nm UV lamp in a dark room, and these solutions emit blue fluorescence. As the Fe3+ ions concentration increased from 0 to 1 mM, the luminescent brightness perceived by human eyes became weaker significantly. When the concentration of Fe3+ ions is 0.5 to 1 mM, it is observed that these solutions are almost no luminescent.
Fig. 7. PL emission spectra of UiO-66-NDC immersed in DMF with different Fe3+ ions concentrations under 365 nm UV excitation. Under excitation at 365 nm, the dependence of PL spectra of UiO-66-NDC solutions on Fe3+ ions concentrations is shown in Fig. 7, which illustrates that the PL intensity decreases remarkably with increasing the concentrations of Fe3+ ions, and the maximum emission wavelength exhibits red-shifted. When the concentration of Fe3+ ions reaches 1 mM, the fluorescence is almost
quenched completely. The above results suggest that UiO-66-NDC may be appreciated for sensing Fe3+ ions.
Fig. 8. (a) Relationship between PL intensity and Fe3+ concentrations, and the inset shows the PL intensity values and Fe3+ ions at low concentrations; (b) Relationship between I0/I and different Fe3+ concentrations, and the inset shows the Stern-Volmer plot for the I0/I values and Fe3+ ions at low concentrations. To obtain a quantitative relationship, we fitted the PL intensity of the solutions as a function of Fe3+ ions concentration. Depending on Fe3+ ions concentrations in the range from 0 to 1 mM, the PL intensity satisfies function y = 1997.36 exp (-x/0.01) + 3499.39 exp (-x/0.16) + 220.73, as shown in Fig. 8 (a). The inset in Fig. 8 (a) shows that the PL intensity has a good linear relationship in the range of 0.01 to 0.1 mM Fe3+ ions concentrations with the correlation coefficient R2 of 0.986. In addition, the limit of detection (LOD) of UiO-66-NDC for Fe3+ ions can be calculated as 0.65 μM by 3σ/κ (σ is the standard deviation of the blank measurement, κ is the slope of the calibration curve). The luminescence quenching effect was further analyzed by the Stern-Volmer equation [31]: I0/I=1+Ksv[C] where I and I0 denote the PL intensities of the solution in the presence and absence of Fe3+ ions,
respectively. [C] represents the Fe3+ ions concentration, and Ksv is quenching constant (slope). Fig. 8(b) illustrates that the fitting effect of I0/I-Fe3+ ions concentration curve, it presents an exponential relationship among the concentration range of 0 to 1 mM, and the equation is I0/I=5.34exp(2.17[C])-3.89 with the correlation coefficient R2 of 0.997. For the low concentrations of 0.01 to 0.1 mM, the I0/I can be fitted to I0/I=16.01[C]+1.29 (Ksv=1.6×104 M-1) with the correlation coefficient R2 of 0.992. The curve gradually variation from linear to nonlinear with the continues increase of Fe3+ ions concentration, which is similar to the reports of some MOFs-based fluorescence quenching sensors [32]. The sensing ability of UiO-66-NDC in the present investigation is compared with previously reported excellent MOF sensors for detection of Fe3+ is shown in Table 1. Table 1. Comparison among various MOF sensors for detection of Fe3+. Material
Ksv (M-1)
LOD (µM)
Media
Refs.
[Cd(Hcbic)]n
1.8×105
0.31
H 2O
[33]
PNT-2
2.0×104
0.31
ethanol
[34]
PNT-3
2.4×104
0.50
ethanol
[34]
ZnL
1.1×104
2.42
H 2O
[35]
67.7
H 2O
[36]
0.65
DMF
This work
SUF-1 UiO-66-NDC
1.6×104
Fig. 9. PXRD patterns of UiO-66-NDC. The stability of UiO-66-NDC in the presence of Fe3+ ions has also been measured by collecting PXRD patterns after the UiO-66-NDC powders immersed in DMF with 1mM Fe3+ ions for 24 hours and 30 days, respectively. Before PXRD measurement, the samples obtained by centrifugation was washed several times with DMF and CH2Cl2, and dried at room temperature. As shown in Fig. 9, the result shows the PXRD patterns are completely identical, indicating the samples are stable after sensing experiment. 3.2 Selectivity sensing properties
Fig. 10. PL response of UiO-66-NDC to different metal ions (1 mM). To evaluate the selective detection ability of UiO-66-NDC for Fe3+ ions, the effect of
different metal ions on the fluorescence emission intensities of UiO-66-NDC was investigated, as shown in Fig. 10. 3 mg UiO-66-NDC were dispersed into 3 mL DMF with different metal ions of the same concentration (1 mM), including Co2+, Mg2+, Zn2+, Al3+, Ni2+, Ga2+, K+, Cu2+ and Fe3+ ions. Before fluorescence intensities were measured, all suspensions were sonicated for 30 min to ensure dispersion. The spectroscopy response shows that the fluorescence intensity of the sample is closely related to the types of metal ions, and only Fe3+ ions gave rise to a clear fluorescence quenching effect for UiO-66-NDC solutions, the quenching percentages over 90%, indicating that the sensing probe has good selectivity for Fe3+ ions. 3.3. Quenching mechanism Most of the MOFs sensing ions by fluorescence quenching is due to atoms in their ligands which will cooperate with foreign metal ions and lead to fluorescence quenching by hydrogen bonding[18], but the quenching process of UiO-66-NDC is different. The dependence of the relative fluorescence intensity of UiO-66-NDC for Fe3+ ions was discussed.
Fig. 11. The UV–Vis absorption spectra of various metal ions (1 mM) and UiO-66-NDC in DMF. According to absorption competition quenching mechanism [34], if the absorption spectrum
of the detected substance is effectively overlapped with the absorption spectrum of the fluorescent material, the competition absorption of the irradiated light between the detected substance and the fluorescent material can reduce the light or energy absorbed by the fluorescent material, and the electrons in the ground state of the fluorescent material will little or no transition to the excited state, which will eventually lead to fluorescence quenching. So it is necessary to measure the UV-Vis absorption spectra of UiO-66-NDC and the metal ions in DMF. As shown in Fig. 11, the absorption spectrum of Fe3+ ions solution seriously overlap with the absorption spectrum of UiO-66-NDC at 280-425 nm, but other metal ions do not. That means Fe3+ ions dramatically filter the light absorbed by UiO-66-NDC, which therefore leads to fluorescence quenching. Hence, the results indicate that the absorption competition quenching mechanism can be responsible for UiO-66-NDC sensing Fe3+ ions, and it is also a strategy to design the ions fluorescence sensor. 4. Conclusion In summary, we successfully fabricated a naphthalene-substituted UiO-66 analog (UiO-66-NDC) with high photoluminescence performance via the solvothermal approach. It was demonstrated that UiO-66-NDC exhibited significant selectivity and sensitivity to Fe3+ ions by the fluorescence quenching effect. The PL intensity of the UiO-66-NDC solution decreases with increasing Fe3+ ions concentration, exhibiting a good linear relationship in the low concentrations range of 0.01 to 0.1 mM with the value of Ksv is 1.6×104 M-1. The quenching performance of UiO-66-NDC for Fe3+ ions can be ascribed to the absorption competition quenching mechanism, that is, the serious overlap of the absorption spectra of Fe3+ ions solution and UiO-66-NDC leads to fluorescence quenching. Based on the absorption competition quenching mechanism, it is feasible to design other novel MOFs for metal ions detection in the future.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No.61875235); Gansu Academy of Science Project (No.2019HZ-04).
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Graphical abstract Legend The UiO-66-NDC has been synthesized via the solvothermal method with excellent photoluminescence property, which exhibits significant selectivity and sensitivety to Fe3+ ions.
Highlights: A naphthalene-substituted UiO-66 nanocrystal (UiO-66-NDC) was synthesized via the solvothermal method using 1,4-naphthalenedicarboxilic acid as the organic linker. The UiO-66-NDC showed excellent light absorption capacity and photoluminescence property. The UiO-66-NDC exhibited significant selectivity and sensitivity to Fe3+ ions.
Declaration of interests 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.