A new covalent organic polymer used to highly selective detection of Fe 3+ ions

. Covalent organic polymers (COPs) received much attention due to their application in diverse fields, especially in sensing. A highly efficient fluorescence sensor in selective detection of Fe 3+ ions is necessary to develop. In this work, we successfully synthesized a new covalent organic polymers named COP-TP, with the excellent performance in selective detection of Fe 3+ ions, the quenching mechanism mainly due to the energy transfer process. The above results show COP-TP has the possibility to be a new candidate in selective detection as Fe 3+ ions sensor.


1.Introduction
Covalent organic polymers (COPs) [1][2], as a new class of organic materials have drawn lots of attention due to their attractive properties with pre-disigned structure [3], stable-band [4], and good dispersion [5] as well as the chemical and physical stability. And the selected monomers or building units make the COPs have the possible applications in sensing, gas storage, chemical separation, drug delivery, and catalysis [6][7][8][9][10]. Fe3+ ions are the important trace elements in the body, which influence the oxygen-uptake and electron transfer [11][12]. Morever, the concentration of Fe3+ ions should be moderated at a suitable range, the deficiency and the overload would cause the diverse diseases. Different ions will cause different environmental and health problems, and the materials with specific and selective selection of Fe3+ ions are worthy to attention. To date, there are many materials used to act as sensors to detect metal ions, such as metal-organic polymers (MOPs) [13][14][15][16] and small organic molecules [17][18][19][20]. However, covalent organic polymers act as sensors to detect Fe3+ ions are rarely reported, a COP with excellent selective ability to Fe3+ ions is promising to design.
In this work, we successful designed and synthesized a new covalent organic polymer named COP-TP through the simple-green Suzuki-coupling reaction, characterized by FT-IR, PXRD, fluorescence spectrum and SEM. The COP has the great performance as sensor to selective detection of Fe 3+ ions among the other trace elements ions and Mg 2+ ions. The COP may provide a new fast method to selectively detect and monitor the Fe 3+ in practice.

2.1.Materials and measurements
All reagents and solvents for the synthesis were purchased from commercial sources and used as received, unless otherwise indicated. Infrared spectra were obtained from KBr pellets in a wavelength ranging from 3300-400 cm-1 on a Nicolet 380 FT-IR spectrophotometer. PXRD pattern was recorded on a Siemens D5005 diffractometer with Cu Kα (λ = 1.5418 Å) radiation in the range of 2 -15°. SEM images were recorded on XL-30 ESEM-FEG Scanning Electron Microscope.Photoluminescence spectra were recorded on a FL-4600 FL spectrophotometer.

2.3.Chemical sensing of metal ions
Titration experiments of metal ions were carried out by adding same concentration of metal salt solutions (20 µL) into the dimethylacetamide (DMA) suspension (1 mL) containing COP-TP (0.1 mg/mL) at intervals of 5 min. Fluorescence spectra were recorded after the addition of metal salt solutions.
Fluorescence quenching % = (1 -I/I0) × 100 %, where I0 is the initial fluorescence intensity in the absence of metal ions, I is the fluorescence intensity in the presence of corresponding analyte [22].  Fig. 1 shows the FT-IR spectra of the COP-TP , DBTPA and TBB monomers as the reference units. The direct evidence of the Suzuki coupling reaction has already occurred is that the disappearance of characteristic peaks at 615 cm-1, belonging to the C-Br vibration of the building monomers (TBB). And peaks at 1180 cm-1 is also disappearance which belongs to the C-O vibration of the building monomers (PDABE). The B-O vibration peaks around 1350 cm-1 is a lot lower compare to the PDABE, suggesting the PDABE was excessive before the reaction and the TBB was completed reacted. The spectra of FT-IR proved that the COP-TP was successful synthesized.

3.Results and discussion
There is no obvious diffraction peak in the powder X-ray diffraction (PXRD) patterns (Fig. 2), indicating the amorphous nature of the COP-TP. The amorphous nature is similar to the reported materials that constructed by C-C coupling reaction [23]. The fluorescence spectrum (Fig. 3a) shows the COP-TP would emit the indigo color under the excitation wavelength at 300 nm, and the photographs of COP-TP dispersed in DMA have a obvious change under the 365 nm UV-lamp. The SEM image (Fig. 3b) exhibits the COP-TP is a irregularly layered structure.
Fe3+ ions play the very important roles in oxygen uptake, The quenching process of COP-TP after add Fe3+ ions mainly due to the energy transfer mechanism. As previous reported [24][25][26], the Fe3+ ions has the empty d orbital, which is the perfect receptor to electrons. And the fluorescence COP is a electrons donor material, when the UV-lamp excites the electrons from COP-TP, the electrons would transfer to the d orbital with the energy transfer, leading to the fluorescence quenching. This clear mechanism could provide a theoretical basis for the successful application of material.

4.Conclusion
We successful synthesized a new covalent organic polymers named COP-TP by Suzuki reaction, and confirmed by FR-IR, PXRD, Fluorescence spectrum, and SEM. The emission color change under the UV-Lamp shows the potential application in sensing, and the selective tests show the COP-TP has the great performance in selective detection of Fe 3+ ions. And The clear mechanism provides a theoretical basis for its application. The COP-TP may provide a new method to selective detection of Fe 3+ ions in aqueous.