Multifunctional-response Cd-LMOF based on triphenylamine with high selectivity for the detection of chromium ions

Abstract A luminescent Cd-MOF is synthesized based on a triphenylamine-core ligand tris(4-(1H-imidazol-1-yl)phenyl)amine (TIPA), [Cd2(TIPA)2(oba)2] n (LMOF-1, oba = 4,4′-oxybisbenzoic). Fluorescence analysis of LMOF-1 shows blue light which can be attributed to the double role of ligand-to-ligand charge transition (LLCT) and aggregation-induced emission (AIE) effect, indicating that it can be used as a potential LED phosphor. As a luminescent sensor, LMOF-1 has excellent performance in detecting Cr3+, CrO4 2−, and Cr2O7 2− in water. Significantly, the quenching coefficient (K SV) of Cr3+ is 1.64 × 104 M−1, which is greater than or comparable to those reported in the literature. Therefore, LMOF-1 may be an excellent multi-functional response and recyclable luminescent sensor. GRAPHICAL ABSTRACT

As a metal, chromium plays a crucial role in industrial production and in the human body.It mainly exists as Cr(III) and Cr(VI) ions.Cr(III) is an indispensable trace element in the human body, but extra Cr(III) ions will combine with DNA in the human body, resulting in pathological changes in cells [18,19].Cr(VI) ions have been listed as a serious pollutant by the United States Environmental Protection Agency [20,21].Longtime exposure to low concentrations of Cr(VI) ions may lead to allergic reactions, genetic deficiency, lung cancer, and other adverse effects [22][23][24][25][26]. Hence, it is important to exploit a high-speed and effective method to measure chromium ions in water [27][28][29][30][31].

Materials and instruments
The synthesis of TIPA was carried out following previously reported procedures [45,46], and other reagents were commercially acquired and used without purification.Infrared spectra were recorded on an FTIR-8400S spectrometer from 4000-500 cm À1 with samples in the form of potassium bromide pellets.Thermogravimetric analyses (TGA) were conducted on a Netszch TGA 209 F3 thermo gravimeter at a heating rate of 10 KÁmin À1 under nitrogen.The luminescence spectra of the samples were obtained on spectrofluorometer FS5.An Ultima IV was utilized for the collection of powder x-ray diffraction (PXRD) patterns with Cu-Ka (k ¼ 1.5406 Å) radiation field emission at 40 kV and 40 mA.

Single-crystal X-ray structure determination
Single-crystal diffraction data for LMOF-1 was collected on a Bruker APEX-II CCD diffractometer equipped with a graphite crystal and incident beam monochromator using Mo-Ka radiation (k ¼ 0.71073 Å).The structure was worked out by direct methods and refined anisotropically by full-matrix least-squares on F 2 using SHELXL and Olex2.Empirical absorption correction was applied using SADABS.Specific crystallographic data are shown in Table 1 (CCDC number: 2203565 (LMOF-1)).

Ion titration method
In order to study the influence of various ions on the luminescent properties of LMOF-1, 100 mg LMOF-1 was prepared as a suspension in 100 mL water.Then, 0.5 mL of 1.0 Â 10 À2 molÁL À1 ionic solution was added to 2 mL of suspension.The fluorescence meter records the change in its luminous intensity.After obtaining quenched ions, 2.0 Â 10 À3 molÁL À1 of the ion solution was added from low to high volume and the fluorescence intensity of the solution was recorded.

PXRD and TG analyses
Powder x-ray diffraction (PXRD) experiments were performed using an Ultima IV with Cu-Ka (k ¼ 1.5406 Å) radiation.The experimental results show that the pattern of the as-made sample is equivalent to the simulated spectrum, indicating that LMOF-1 is a pure phase and remains stable after heating (Figure S1).Thermogravimetric analyses (TGA) were recorded on LMOF-1 and TIPA from 15 C to 800 C under nitrogen (Figure S2); FT-IR of LMOF-1 is shown in Figure S3.In the thermogravimetric curve of LMOF-1, the decrease of weight ratio from room temperature to 330 C is 5.65%, which may be ascribed to the loss of solvent molecule.Then, the structure of LMOF-1 began to decompose gradually, and the weight loss ratio from 330 C to 400 C is 31.02%,from decomposition of free ligand molecules.The structure collapses completely from 400 C to 600 C with a weight loss of 42.66%, and the residue is mainly cadmium oxide.LMOF-1 has good thermal stability.

Fluorescence properties
The luminescence emission spectra of LMOF-1 and TIPA were studied at room temperature in their solid forms (Figure 2(a)).Compared with TIPA, the emission intensity

Sensing of metal ions
LMOF-1 can be considered as a potential fluorescence sensor of metal ions owing to its structural stability and fluorescence stability in various solvents (Figures S4 and S5).
To explore the influence of different metal ions on the fluorescence performance of LMOF-1, powder of LMOF-1 was suspended in water and different 1.0 ) were added.Figure 3(a) shows that various ions have effects on the fluorescence intensity of LMOF-1; Cr 3þ has obvious fluorescence quenching effect.Exploring the fluorescence sensitivity of LMOF-1 to Cr 3þ and titrating in an aqueous solution shows that the luminous intensity decreases gradually with an increase in Cr 3þ concentration (Figure 3b).Stern-Volmer (SV) equation (I0 [58,59] was used to evaluate the quenching efficiency, where I 0 and I represent the fluorescence intensity before and after addition of analyte, [Q] is the concentration of analyte, and K SV is the quenching constant used in quantitative analysis of sensing efficiency.As shown in the inset of Figure 3(c), the S-V curve of Cr 3þ shows a linear relationship at low concentrations.The fitting value of the linear relationship is 0.990, and the quenching constant is 1.64 Â 10 4 M À1 , comparable to or greater than previously reported compounds for sensing Cr 3þ (Table 2).The detection limit is LOD ¼ 3d/K SV , where d represents the standard deviation of fluorescence intensity of 10 blank samples [63].The calculated d is 0.0322, and the detection limit is 5.89 Â 10 À6 M. The sensitivity of LMOF-1 to Fe 3þ was studied by fluorescence titration experiment (Figure S6).The K SV and the detection limit are 1.20 Â 10 4 M À1 and 8.05 Â 10 À6 M, respectively.Distinct ion competition experiments prove that LMOF-1 selectively detects Cr 3þ and is not affected by other ions (Figure 3(d)).The result indicates that LMOF-1 has superiority for detecting Cr 3þ .c,d)).The K SV and the detection limit are 4.16 Â 10 4 M À1 and 2.32 Â 10 À6 M, respectively.In Figure S7, an anti-interference experiment showed that LMOF- on LMOF-1, the excitation spectra of LMOF-1 and the UV-vis spectra of the above three ionic aqueous solutions were analyzed (Figure S9).The UV-vis spectra show that the ultraviolet-visible absorption spectra of the three ions overlap with the excitation spectra of LMOF-1, so it can be considered that fluorescence quenching is caused by competition between the three ions and LMOF-1 in the aqueous solution to absorb excitation light [64][65][66][67][68].

Disclosure statement
No potential conflict of interest was reported by the authors.
According to Figure 1(b), two adjacent Cd(II) ions are linked together by N(2) and N(3) atoms of the TIPA ligands to form a 1-D chain along the b axis.Unexpectedly, a 2-Dlayered structure can also be formed by the attachment of Cd ions and N(1) atoms of the TIPA ligands along the b axis.Then, each 2-D-layered structure is connected by 4,4 0 -oba ligands and TIPA ligands into a 3-D network structure along the b axis (Figure 1(c)).Calculated by PLATON software, its porosity is 16.5%.The topological analysis indicates that LMOF-1 can be described as a two-nodal (2-c)(6-c) net with a topological symbol of {4^8.6^2.5^2.6^3}{4}(Figure 1(d)).

Figure 3 .
Figure 3. (a) The luminescence spectra of LMOF-1 suspension upon adding different metal cations.(b) The fluorescence intensity trend chart of LMOF-1 after adding Cr 3þ solution.(c) The SV curves after adding Cr 3þ solution.(d) Anti-interference experiment for the selective recognition of Cr 3þ .

A
new Cd-MOF based on a triphenylamine-core ligand tris(4-(1H-imidazol-1yl)phenyl)amine(TIPA) was synthesized and characterized.Thermogravimetry and PXRD show that LMOF-1 has good thermostability and solution stability.The fluorescence demonstrates LMOF-1 exhibits strong blue emission in the solid state due to the ligand and can be used as a new direction for the development of fluorescent materials.In addition, LMOF-1 has excellent selectivity and stabilization for the detection of Cr(III) and Cr(VI) ions in aqueous solution.LMOF-1 can be used as a multi-response luminescence sensor to detect Cr 3þ , CrO 4 2À , and Cr 2 O 7 2À ions in aqueous solution.

Figure 5 .
Figure 5. (a) The fluorescence intensity trend chart of LMOF-1 after adding CrO 4 2À solution.(b) The S-V curves after adding CrO 4 2À solution.(c) The fluorescence intensity trend chart of LMOF-1 after adding Cr 2 O 7 2À solution.(d) The S-V curves after adding Cr 2 O 7 2À solution.

Table 1 .
Crystallographic data and details of refinement for the LMOF-1.

Table 2 .
The quenching constants of the reported LMOFs for Cr 3þ in suspensions.