A New Bio-metal-organic Framework: Synthesis, Crystal Structure and Selectively Sensing of Fe(III) Ion in Aqueous Medium①

YIN Yu-Jie FANG Wang-Jian LIU Shu-QinCHEN Jun ZHANG Jian-Jun NI Ai-Yun

(School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China)

ABSTRACT A new two-dimensional (2D) bio-metal-organic framework based on trinuclear cluster units,[Zn3(adeninate)2(CH3COO)4]?DMF (Zn–A), was synthesized by the self-assembly of biomolecule adenines with zinc ions. Topological analysis reveals its 44 connected layer structure. Remarkably, Zn–A bears high sensitivity and selectivity detection ability of iron ion with the limit of detection being 4.0 × 10-6 mol/L.

Keywords: supramolecule, crystal structure, bio-metal-organic framework, Fe3+ ion, sensing;

1 INTRODUCTION

Iron, the fourth most abundant element of the earth, is widely found in nature ecosystem. Effective and rapid detection of Fe3+content is of great significance for environmental protection and disease monitoring. Although traditional analytical methods, such as inductively coupled plasma mass spectrometry (ICP-MS) and chromatography,have high precision characteristics, they still suffer from the demerits of high costs and non-portability[1]. Therefore, it is particularly important and necessary to develop a faster, more convenient and low-cost approach to detect Fe3+.

Luminescent metal-organic frameworks (LMOFs), a new kind of functional materials[2-5], are composed of metal ion/cluster nodes and organic ligands[6]. Their structures can be effectively regulated and optimized, enabling the effective contact with guest species to have different luminescence responses[7]. So far, many LMOFs show excellent ability in the detection of numerous guest species including Fe3+ion.However, the drawback is that due to toxic aromatic ligands or metal ions, many LMOFs are bio-unfriendly, which greatly limits the practical application of LMOFs. Hence, the struggling for non-toxic LMOFs for detection applications is extensively needed to be explored and is still on the way.

Herein we selected adenine[8], a biomolecule with strong coordination ability as the ligand, and zinc ion, an element required by human body as the metal ion, and successfully isolated a new two-dimensional (2D) bio-MOF [Zn3(adeninate)2(CH3COO)4]?DMF (Zn–A). The luminescence sensing behaviors of Zn–A were investigated. The results show that it has a high sensitivity and selectivity detection ability of iron ion with the limit of detection (LOD) of 4.0 ×10-6mol/L. Remarkably, the probe is regenerable. These results reveal the great potential of Zn–A in detecting of Fe3+.

2 EXPERIMENTAL

2. 1 Materials and methods

All the reagents and solvents were commercially purchased,and used as received without further purification. The IR spectra were recorded (400～4000 cm-1) from a Nicolet-20DXB spectrometer using KBr pellets. Thermogravimetric analyses (TGA) were carried out on a TA-Q50 thermogravimetric analyzer under N2atmosphere with the heating rate of 10 ℃/min. Elemental analyses (C, H and N) were performed on a Vario EL III elemental analyzer, while Zn was determined on a Perkinelmer OPTIMA 2000DV inductively coupled plasma emission spectrometer. Powder X-ray diffraction patterns (PXRD) data were collected on a D/MAX-2400 X-ray diffractometer with Cu-Kαradiation (λ=1.54060 ?) at a scan rate of 5 °/min. The steady state emission spectra were collected on a Hitachi F-7000 FL spectrophotometer.

2. 2 Synthesis of [Zn3(adeninate)2(CH3COO)4]?DMF (Zn–A)

Adenine (20 mg, 0.15 mmol) and Zn(NO3)2?6H2O (60 mg,0.20 mmol) were dissolved in a solvent mixture of DMF and acetic acid (5mL/1mL). Then the obtained mixture was sealed in a Teflon-lined stainless-steel vessel, and heated at 130 ℃ for 3 days under autogenous pressure. After gradually cooling to room temperature, colorless blockshaped crystals of Zn–A were collected by filtration, washed with DMF, and dried in air (70.28% yield based on adenine).Element analysis (%) calcd. for C21H27N11O9Zn3: C, 32.59; H,3.52; N, 19.91. Found: C, 32.61; H, 3.35; N, 19.92.

2. 3 Structure determination

Intensity data from single crystals of Zn–A were measured at 295 K on a Bruker SMART APEX II CCD area detector system with graphite-monochromated Mo-Kα(λ= 0.71073 ?)radiation. Data reduction and unit cell refinement were performed with Smart-CCD software. The structures were solved by direct methods using SHELXS-2014 and refined by full-matrix least-squares methods using SHELXL-2014[9].All non-hydrogen atoms were refined anisotropically. The hydrogen atoms related to C and N atoms were generated geometrically. Crystal data: monoclinic, space groupP21/cwithMr= 773.64,a= 8.9598(4),b= 12.0398(5),c=13.3235(6) ?,β= 94.77(3)o,V= 1433.53(11) ?3,Z= 2,Dc=1.792 g/cm3,F(000) = 784,μ(MoKα) = 2.560 mm–1, the finalR= 0.0279 andwR= 0.0775 (w= 1/[σ2(Fo2) + (0.0482P)2+2.9522P], whereP= (Fo2+ 2Fc2)/3),S= 1.117, (Δ/σ)max=0.001, (Δρ)max= 0.79 and (Δρ)min= –0.74 e/?3. Selected bond lengths and bond angles of Zn–A are given in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for the Compound

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of Zn–A

Single-crystal X-ray diffraction analysis indicates that Zn–A crystallizes in the monoclinic space groupP21/cwith the asymmetric unit consisting of one and a half Zn2+ions,one adeninate, two coordinated acetate ions, and 1/2 lattice disordered DMF molecule. As shown in Figs. 1a, S1 and S2,Zn(1) is four-coordinated and displays a distorted tetrahedral geometry composed of two N atoms from two adeninate ligands and two O atoms from two acetate ions. Zn(2) is six-coordinated, showing a distorted octahedral configuration built by four O atoms from four acetate ions and two N atoms from two adeninate ligands. The Zn–O bonds range from 1.976(3) to 2.170(2) ? and Zn–N bond lengths fall within the range of 2.025(3)～2.114(3) ?. Each Zn(2) ion is connected to two neighbouring Zn(1) by four acetate auxiliary ligands to form the linear Zn3O6N6secondary building unit (SBU).

The trinuclear SBUs are connected by the adeninate linkers to form a 2Dneutral layer that extends along thebcplane(Fig. 1b). In this case, the network features a 4-connected 2Dnetwork with 44topology (Fig. 1c). Interestingly, adjacent 2Dsheets are further connected via hydrogen bonds between the free amino groups and the uncoordinated pyrimidine nitrogen atoms with N···N separation and N–H···N angle of 2.956(4)? and 173.6°, respectively. The existence of hydrogen bond interactions in Zn–A not only increases the stability of the structure, but also extends the structure into a 3Dsupramolecular framework. The solvent accessible volume of 1 without guest molecules, calculated by PLATON, is about 17.8% of the unit cell volume (255.8 ?3out of the 1433.5 ?3)[10].

3. 2 Characterization of the compound

As shown in Fig. S3, the experimental pattern of Zn–A is highly consistent with its corresponding simulated pattern,confirming the phase purity and the crystallinity. TGA analysis reveals that it is stable below ～280 ℃ (Fig. S4).Above this temperature, it shows a striking weight loss,indicating the complete decomposition of the framework.

As shown in Fig. 2, the solid-state luminescent spectrum reveals that Zn–A bears a strong blue emission with maximum peaks at 438 nm. Under the same excitation wavelength (365 nm), the adenine ligand presents an emission band centered at 416 nm. The emission of Zn–A can be ascribed to the adenine based emission. The red shift of Zn–A compared with adenine can be due to the deprotonation of adenine and the coordination interactions between adeninate and Zn2+ions[11,12]. The emission spectrum of Zn–A in H2O was measured. Strong emission at 412 nm was observed. Both strong emission and excellent stability make Zn–A a desirable candidate for luminescent sensing.

Fig. 1. Structure of Zn–A. (a) Coordination environment of the ZnII atoms. Hydrogen atoms are omitted for clarity.Symmetry codes: A: 2 – x, –y, 1 – z; B: x, 0.5 – y, 0.5 + z; C: 2 – x, –0.5 + y, 0.5 – z. (b) 2D layer structure.(c) Representation of 44 topological net. (d) A view along the b axis of the stacking structure

Fig. 2. Luminescence spectra of solid adenine and Zn–A and H2O suspension of Zn–A. Excitation wavelength is 365 nm

3. 3 Luminescence sensing of Fe3+

As shown in Fig. 3, the luminescence intensities of suspensions of Zn–A in H2O towards different metal ions(1.0 × 10-3mol/L) are not exactly the same. The introduction of different ions affects the luminescence intensity of Zn–A to a certain extent. For instance, Cd2+slightly enhances the luminescence intensity, whereas the luminescence intensities are kept still in the cases of Co2+, Ca2+and Li+(Fig. S5).However, among the rest metal ions, only Fe3+bears a remarkable quenching effect, which can also be observed by naked eyes (Fig. 4) and the quenching efficiency (> 0.97) is much higher than those of other metal ions (Fig. S6).

Fig. 3. Luminescence intensities of different metal cations(1.0 × 10-3 mol/L) towards the water dispersion of Zn–A

To further examine the detection sensitivity toward Fe3+,Zn–A was well dispersed in aqueous solutions with gradually increasing the concentration of added Fe3+from 0.06 mM to 1 mM. The corresponding emission spectra were measured.As shown in Fig. 4, the luminescence intensity decreased gradually with the increase of Fe3+concentration. As shown in Fig. 5, in the low concentration range (0～3 × 10-4mol/L),a linear relationship was observed between the suspension’s luminescence intensity and the concentration of Fe3+. The corresponding correlation coefficient (R2) is 0.987. The limit of detection (LOD) was determined as LOD = 3σ/K(whereKis the slope of the linear curve andσis the standard deviation of the luminescence intensities). The calculated LOD value was determined to be 4.0 × 10-6mol/L.

Fig. 4. Luminescence intensity of the water dispersion of Zn–A after incremental additionof Fe3+ solution. Excitation wavelength is 365 nm

Fig. 5. Variation in the luminescence intensity of the water suspension of Zn–A as a function of the concentration of Fe3+ solution

The interference experiments for Fe3+ion sensing were also conducted in the presence of other metal cations. The solutions of various metal cations were added individually accompanied with the addition of the solution of Fe3+ion.The emission spectra of the resulting mixtures revealed that the quenching efficiency of Fe3+ion did not change considerably even when other metal ions present in the sensing medium (Figs. S7 and S8). These results demonstrate that Zn–A bears highly anti-interference ability in sensing of Fe3+ions.

It is advantageous for sensing applications if a probe can be reutilized for several times. The recycling performance was evaluated by centrifuging Fe3+@ Zn–A from the Fe3+aqueous solution, and then washed with H2O for several times. The results reveal that the luminescence intensity can be restored at least 3 times (Fig. S9). Remarkably, PXRD analysis shows the framework is still maintained after three cycles (Fig. S10). These results render the probe more potential in practical Fe3+detection application.

The sensing mechanism was also studied. Firstly, after the cycle experiment of Fe3+detection, the MOF still maintains good crystallinity, so the luminescence quenching is not caused by the collapse of the framework. To clarify the pH interference in the sensing experiments, we also test the luminescence intensities of Zn–A water suspensions in different pH values. The results (Fig. S11) indicate that the luminescence intensity only slightly decreases below pH = 3,confirming the quenching of Zn–A upon the addition of Fe3+is not caused by pH but by Fe3+itself. UV-Vis absorption spectrum of Fe3+solution was tested (Fig. S12). The result shows the solution has a strong and wide UV-Vis adsorption band in the 300～410 nm region. Since the excitation wavelength of the above sensing experiments is 365 nm,obviously there is a competition absorption of the light source energy between the added Fe3+ions and Zn–A. In other words, Fe3+filters the light absorbed by Zn–A and quenches its luminescence, as also observed in the literature[13].

4 CONCLUSION

In summary, a new 2Dbio-MOF (Zn–A) was synthesized by the reaction of adenine ligand and Zn2+ion. The compound can be employed as probe for the selective and sensitive detection of Fe3+ion in aqueous solution with LOD of 4.0×10-6mol/L. This work provides a good example for the design and synthesis of CP-based probe for metal ion detection.