Structure Description of Zn-MOF
Zn-MOF crystallizes in a monoclinic space group P2(1)/c (Table S1, SI) and displays a 2D layered structure based on {ZnII2} clusters as SBUs. The asymmetric unit of Zn-MOF consists of two ZnII ions, one pmida4- ligand, one phen and one free H2O molecule. As shown in Fig. 1a, the two ZnII ions from {ZnII2} cluster SBU both adopt a five-coordinated mode but different coordinated details. The Zn1II coordinates with one O atom from -CH2-PO32- of one pmida4- ligand, one N and three other O atoms from the second chelating pmida4- ligand. The Zn2II links with three O atoms from one -CH2-PO32-, two -CH2CO2- of three unique pmida4- ligands, and two N atoms from one chelating phen ligand. The Zn-N and Zn-O bond lengths and angels of Zn-MOF are the same as the reported Zn-MOFs [10, 15, 25, 26, 32, 38, 39]. Moreover, all pmida4- ligands adopt the same chelating and bridging modes to link five ZnII ions belonging to three different {ZnII2} cluster SBUs using their one -CH2-PO32-, two -CH2CO2- and one N atom (Fig. 1b). Based on this coordination, the {ZnII2} cluster SBUs linked by pmida4- ligands to form a 1D chain (Fig. 1b,c). The {ZnII2} cluster SBUs occupy in the centre of the chains and phen decorating on the outside (Fig. 1c). The adjacent 1D chains are further connected by the -CH2-PO32- groups of pmida4- ligands to generate a 2D layer (Fig. 1d). The Fig. 1d shows the 2D layer contains square pores constructing by the {ZnII2} cluster SBUs. Between the 2D layers, it uses π-π stacking to form a 3D supramolecular structure (Fig. 1e), the distance of π-π stacking is 3.68 Å (Fig. 1f). It is noted that in this porous of 2D framework, the rich uncoordinated O atoms are hanging on surface of the channels to act as guest-accessible functional sites to fulfill sensing property.
Luminescence Sensing Cr2O72− of Zn-MOF in H2O Media and HEPES Solution
The solid state luminescence of Zn-MOF displays two main emission peaks of 369 and 387 nm excited at 281 nm (Fig. 2a). The CIE of Zn-MOF locates in purple light emitting area of (0.1672, 0.0086) (Fig. 2a, inset). In Zn-MOF, the main chromophore is 1,10-phen ligand, it also shows two main emission peaks of 363 and 380 nm excited at 281 nm (Fig. S2, SI). In comparison, the two emission peaks of Zn-MOF occur slight red shifts which is due to the π*-n or π*-π electron transfer of the 1,10-phen ligands excluded the d10 of ZnII [10, 15, 25, 26, 32, 38, 39].
The rich uncoordinated O atoms hanging on the pores of Zn-MOF are possibility act as functional sites to interact with analytes, which encourages us to investigate its luminescence sensing activities for various anions. Typically, a suspension mixture prepared by 2 mg Zn-MOF in 4 mL H2O aqueous or HEPES biological buffer solution containing 10− 3 M anions (e.g. SO42-, OH-, CO32-, C2O42-, NO3-, SO32-, Cl-, Br-, I-, F-, SCN-, PO43- and Cr2O72-) to execute the sensing performance. As shown in Fig. 2b, Zn-MOF has almost complete luminescence quenching response to Cr2O72- in H2O solution, the quenching ratio reaches to 88.41 times vs. blank of H2O solution. The other anions show only a very small quenching effect. This result shows a significant quenching effect only for Cr2O72-. In luminescence titration experiments of Zn-MOF for Cr2O72- in H2O and HEPES solution are shown in Figs. 2c,d and 3a ~ c. As Cr2O72- concentration increasing, the luminescence intensity decreases significantly. The Stern-Volmer (S-V) equation of I0/I = KSV[M] + 1 is applied to calculate the quenching constant Ksv ([M] is Cr2O72- molar concentration, I0 and I is the luminescence intensity before and after adding Cr2O72-) [24–30]. In H2O solution, the Ksv of Zn-MOF for detecting Cr2O72- is 4.837 × 104 M-1, and the corresponding Ksv of Zn-MOF for Cr2O72- is 4.436 × 103 M-1 in HEPES solution. The large Ksv values reveal that the Zn-MOF has good quenching effect for sensing Cr2O72- both in H2O and HEPES solution. Calculated using equation LOD = 3δ/k (δ is the standard deviation of ten blank experiments, and k is the slope of the linear relationship), the LOD of Zn-MOF for sensing Cr2O72- is 1.21 and 5.46 µM in H2O and HEPES solution (Fig. 2d,f), respectively [24–30]. The LOD (1.21 µM) in H2O solution is lower than the benchmark of drinking water recommended by USEPA (1.92 µM), and is also lower than many reported MOF sensors (Table S3, SI) [31–45]. The above analysis indicate that this Zn-MOF has good luminescence quenching effect to detect Cr2O72- both in H2O and HEPES solution.
Moreover, the Zn-MOF luminescence quenching for Cr2O72- also has good anti-interference ability. By adding Cr2O72- solution into other anion solutions, all of them display very obvious luminescence quenching situation (Fig. 2e), illustrating its marked anti-interference capability. At the same time, Zn-MOF has good cycling stability for detecting Cr2O72- in HEPES solution. After five cycles, the recovery rate in HEPES solution reaches above 65%, and the quenching efficiency is even exceeded the first cycle, indicating its good recyclable (Fig. 3d). In order to observe the luminescence sensing results more intuitively, the luminescence photographs of Zn-MOF in different concentrations of Cr2O72- H2O/HEPES solutions and in different anions solutions are captured under 365 nm UV lamp (Fig. 2f, and Fig. 3c inset). As can be seen from Fig. 2f (above) and Fig. 3c (inset), along with the concentration of Cr2O72− increases, the luminescence color gradually darken. Also, the obvious and vivid color quenching photo of Cr2O72− solution compared to other anions is observed by naked eyes (Fig. 2f, below). So the Zn-MOF is a good luminescent sensor for detecting Cr2O72− both in H2O and HEPES solution, illustrating the possible application in industrial wastewater monitoring.
The Luminescence Sensing Mechanism of Zn-MOF
The possible luminescence sensing mechanism of Zn-MOF for quenching Cr2O72− is proposed by experimental results [24–30]. The Zn-MOF structure collapse inducing luminescence quenching mechanism is firstly excluded due to the good match of the PXRD peak positions for as-synthesized Zn-MOF and Cr2O72− soaked Zn-MOF, illustrating the structural integrity of Zn-MOF after sensing Cr2O72− (Fig. 4a) [24–30]. Then, the luminescence excitation spectrum of Zn-MOF is overlapped with the UV-vis absorption spectra of Zn-MOF after sensing Cr2O72− (Fig. 4b), speculating the competitive energy absorption between Zn-MOF and Cr2O72− is possible luminescence quenching mechanism [24–30]. The energy transfer can cause weak interactions leading to luminescence quenching [24–30]. Due to the uncoordinated O atoms is possible to interact with Cr2O72−, so the O 1s XPS of Zn-MOF before and after sensing Cr2O72− was tested. The O 1s peak after sensing Cr2O72− occurs red-shift of 0.2 eV (Fig. 4c), further proving the existence of weak interactions between Zn-MOF and Cr2O72−. The fluorescence decay lifetime decreases from 3.3010 µs to 3.0309 µs for as-synthesized Zn-MOF to Cr2O72− treated Zn-MOF, also evidencing the occurrence of weak interactions (Fig. 4d) [24–30]. Combining the PXRD, UV-vis, XPS and fluorescence decay lifetimes proof that the luminescence sensing mechanism is possible the competitive energy absorption inducing the weak interaction between the uncoordinated O atoms of Zn-MOF with Cr2O72−.