Ag+ doped into azo-linked conjugated microporous polymer for volatile iodine capture and detection of heavy metal ions

We herein report the construction of a novel azo-linked conjugated microporous polymers (Ag@AzoTPE-CMP), which possesses permanent porous structure and Ag+ loading up of 7.62% in the skeleton as effective sorption sites. Ag@AzoTPE-CMP shows considerable adsorption capacity of iodine of 202 wt% in iodine vapor at 350 K. In addition, Ag@AzoTPE-CMP can effectively remove heavy ions from ethanol-water solution.

new iodine adsorption materials 32 . However, the poor stability of MOFs materials limits their practical applications. In this work, the integration of azo and phenolic -OH gave the ability of the CMP to chelate with metal silver ions, yielding Ag-doped CMP network with Ag + loading up of 7.62% (Ag@Azo TPE -CMP). Served as absorbents, the Ag + -coordinated CMP exhibits high-speed iodine capture both in vapor and solution, and excellent detection of heavy metal ion. Besides that, the resultant metal silver coordinated CMP showed good stability and recyclability.
The Azo TPE -CMP was extensively characterized using the solid-state cross-polarization magic angle spinning (CP/MAS) 13 C NMR, powder X-ray diffraction (PXRD), FT-IR spectroscopy, elemental analysis, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) (for details, see supporting information). Firstly, the structural integrity of polymers was verified by using CP-MAS 13 C NMR (Fig. S1). The broad chemical shifts appeared in the region between 120 and 170 ppm is associated with the aromatic carbon atoms of the framework. The peaks located at 108 ppm, which was attributed to the vinyl carbons of TAVA, also verified the successful synthesis of CMP. The broad featureless PXRD patterns of CMPs indicate that the two polymers have amorphous characters (Fig. S2). The structural integrity of CMPs was further verified using FT-IR analysis (Fig. S3). For Azo TPE -CMP, the bands around 1650 and 1400 cm −1 were attributed to the stretching of C=C and N=N bonds, and the broad peaks around 3430 cm −1 were assigned to the Ar-OH groups, confirmed the successful coupling reaction between the monomers. For Ag@Azo TPE -CMP, a new peak appeared at 1368 cm −1 , possibly assigning to the C-O vibration of Ar-OAg, which indicated Ag + was coordinated with phenolic -OH (Fig. S3). The scanning electron microscopy (SEM) analysis was performed in order to investigate the bulk scale morphology of the CMPs (Fig. 2a,b). From SEM images, we can see that the polymers are composed of irregular spherical solids from nanometers to microns. The two azo-linked CMPs were composed of agglomerated plate-shaped particles with a particle size more than 1 micron. Further analysis of the CMPs by TGA showed that the Azo TPE -CMP network was thermally stable up to 380 °C under N 2 atmosphere presumably due to well-spaced charged groups within three-dimensional network. The slight weight loss below 200 °C was mostly associated with the trapped moisture and solvent molecules in the pores (Fig. S4). Compare to Azo TPE -CMP, Ag@Azo TPE -CMP displayed much higher stability, and its value was as high as 500 °C. Through high-resolution transmission electron microscopy (HR-TEM), we can see that Ag particles have been included successfully for Ag@Azo TPE -CMP compared with non-Ag Azo TPE -CMP (Fig. S5). Notably, Azo TPE -CMP and Ag@Azo TPE -CMP polymers showed strong and broad  S6a). Compare to AgNO 3 /Azo TPE -CMP complex, the Ag@Azo TPE -CMP exhibited a broad absorbance band, also suggesting the coordination between Ag + and phenolic -OH (Fig. S6a). Then, we investigated the luminescence properties of the two CMPs. The Azo TPE -CMP was non-emissive, and the Ag@Azo TPE -CMP showed the emission band at 402 nm (Fig. S6b).
In addition, X-ray photoelectron spectroscopy (XPS) was performed to explore whether and how the silver coordinated with the porous polymer. Both survey scan and narrow scan (N1s) were performed. C1s (~282.50 ev), O1s (~530.53 ev) and N1s (~400.01 ev) peaks were observed in the XPS spectra of the two samples (Fig. S7).
The new peaks at 700-800 eV and 300-400 eV in the O1s XPS spectrum of Ag@Azo TPE -CMP may be assigned to -O-Ag, implying that Ag + might coordinate with the phenolic -OH, and AgNO 3 was not simply physically adsorbed. Moreover, the BE of N1s changed little, implying that -N=N-might not involve coordination with metal ions. Based on the above analysis, it can be deduced that AgNO 3 should be chelated with Ar-OH to form Ar-O-Ag moieties during the metallization process.
In order to characterize the porosity parameters of azo-linked CMPs, the nitrogen sorption isotherms were measured at 77 K. As shown in Fig. 2c, Azo TPE -CMP displays a combination of type-I and IV sorption profiles, according to the IUPAC classification. At low relative pressure, a sharp nitrogen gas uptake reflects the microporous nature of the Azo TPE -CMP network. And the nitrogen sorption in the high-pressure region (P/P 0 > 0.9) increases with increasing pressure, suggesting a large external surface owing to the loose packing of small particles. The apparent Brunauer-Emmett-Teller (BET) surface areas for the polymers were calculated over the relative pressure range P/P 0 = 0.015-0.1, which was found to give a positive value of C in the BET equation. Azo TPE -CMP exhibited the BET surface area of 366 m 2 g −1 . The total pore volume calculated with nitrogen gas adsorbed at P/P 0 = 0.99 was 1.072 cm 3 g −1 , and the pore size mainly centered at 1.7 nm, as obtained by the nonlocal density functional theory (NLDFT) (Fig. 2d). Compare to Azo TPE -CMP, Ag@Azo TPE -CMP showed the lower BET surface area and the total pore volume were 47 m 2 g −1 and 0.110 cm 3 g −1 , respectively. In addition, the pore size mainly centered at 0.5 nm. These results indicated that the metal silver ions have been loaded into the pores of the CMP skeleton.

Discussion
Iodine capture. The considerable porous characters, nitrogen-and silver-rich nature of Azo TPE -CMP prompted us to assessing their performances for iodine capture. The I 2 capture process was conducted at 350 K and ambient pressure, which are typical fuel reprocessing conditions. Due to the sample's color was deep-black, an apparent color change was not observed with time progressed. Figure 3a shows the weight of the CMPs at various time intervals during the iodine uptake. The iodine capture uptake increased significantly with extended contact time. The results suggested that the mass of iodine uptake increased significantly in the initial 10 h and reached a platform thereafter, implying the system basically is saturated after 36 h. The saturated I 2 loading of Azo TPE -CMP and Ag@Azo TPE -CMP were measured to be 108 and 202 wt.%, respectively. The thermogravimetric analysis (TGA) of the I 2 -loaded CMP samples reveal a significant weight loss from 90 to 300 °C (Fig. 3b), the calculated iodine mass loss were 122 and 194 wt.% for Azo TPE -CMP and Ag@Azo TPE -CMP, respectively, which are close to the saturated adsorption value. The I 2 uptake for Ag@Azo TPE -CMP is 1.87-times than that of Azo TPE -CMP, which may be attributed to the coordination interaction of silver ions with iodine molecules. X-ray photoelectron spectroscopy (XPS) of the Ag@Azo TPE -CMP indicated that the coexistence of elemental iodine and triiodide ions, which suggests a hybrid of physisorption and chemisorption (Fig. S8). However, the XPS of Azo TPE -CMP implied that the adsorption of iodine was mainly physisorption. Therefore, Ag@Azo TPE -CMP displayed a higher adsorption iodine value than that of Azo TPE -CMP. Furthermore, the two samples can be efficiently recycled and reused for five cycles without significant loss of iodine uptake (Fig. S9). At the same time, the addition of loading I 2 of azo-linked CMP in fresh ethanol could be easily remove the encapsulated iodine from the network. The color of the ethanol solution deepened from colorless to dark brown, which clearly indicates that I 2 guests are released from the azo-linked networks (Fig. S10).
In addition, the capability of trapping iodine of the two CMP polymers was also tested in solution at ambient conditions. The two samples (30 mg) were immersed in cyclohexane solution of I 2 in a small sealed flask at room temperature. The purple color of the iodine solution gradually changed from dark purple to light purple and finally to paler (Fig. S11). The UV/vis spectroscopy was used to characterize the adsorption kinetic of iodine (Fig. S12). The UV/Vis absorption intensity of the samples was decreased with the prolonged action time. The adsorption kinetics of iodine at 25 °C were presented, as illustrated in Fig. S13. Two stages of adsorption kinetics of the iodine were observed: the adsorption capacity for iodine increased quickly during the first 8 h, and after that a slowly increased iodine uptake until equilibrium. The adsorption performances of the two CMPs can be best fitted by Langmuir pseudo-first-order kinetic models (Fig. S14), which show the correlation coefficient R 2 values of 0.9232 and 0.9336 for Azo TPE -CMP and Ag@Azo TPE -CMP, respectively. Finally, the two polymers exhibited the removal efficiencies of up to 99.9% in the iodine solutions with a concentration of 4 mg mL −1 , which far exceeded that of functionalized MIL-53-NH 2 (60%) 33 , metalloporphyrin-based NiP-CMP (56%) 34 . Moreover, the adsorption isotherm is a significant factor in determining the saturated adsorption capacity (Fig. S15). The adsorption plot of equilibrium concentration versus adsorption capacity showed that the two adsorption stages. Firstly, the equilibrium absorption increased linearly with the increase of iodine concentration. Compared with the Freundlich equation, the fitting of Langmuir equation is more in line with the experimental curve, the calculation results suggested that a monolayer adsorption behavior for iodine on the surface of the two samples. The adsorption reached the maximum uptake without relation to the increasing iodine concentration. From the kinetic studies, Azo TPE -CMP and Ag@Azo TPE -CMP represent a high iodine uptake of 1991 and 2598 mg g −1 , respectively.
Detection of heavy metal. In recent years, with the rapid development of various industries in the world, the amount of industrial wastewater emissions also showed a sharp upward trend, which makes the water pollution become more and more serious, and the task of wastewater management needs to be carried out urgently. Heavy metal wastewater is considered to be one of the most serious industrial wastes endangering the environment and human health. In this work, the Ag@Azo TPE -CMP was found to be capable of efficiently detecting heavy metal ions (such as Cu 2+ , Hg 2+ , Cr 3+ , Ni 2+ ) (Figs 4a and S16). When the Ag@Azo TPE -CMP was treated in the ethanol-water solutions of corresponding metal salts with different concentrations, which can quenched the fluorescence of the Ag@Azo TPE -CMP. Cu 2+ , Cr 3+ , Hg 2+ , Ni 2+ can quench the degree of fluorescence up to 99%, 97%, 96%, and 93%, respectively. Remarkably, as the concentration of Cu 2+ decreased to 10 −6 and 10 −7 M, over 52% of the fluorescence was quenched. Even when the Cu 2+ concentration was decreased to 10 −9 M, the degree of fluorescence quenching was as high as 35% (Fig. 4b). The Ag@Azo TPE -CMP with such a high sensitivity for reporting Cu 2+ outperform other Cu 2+ chemo-sensors thus far reported [35][36][37][38] . Then, we further investigated the scope of ion species that can be sensed (Fig. 4a). Compared to Cu 2+ ions, the Ag@Azo TPE -CMP exhibited low sensitivity and torpid response to other metal ions, such as Zn 2+ , Na + , Ba 2+ , and Ca 2+ , etc. These results clearly demonstrate that the Ag@Azo TPE -CMP are responsive to transition metal ions, such as Cu 2+ , Hg 2+ , Cr 3+ , Ni 2+ (Fig. 4a). Besides the high sensitive of Ag@Azo TPE -CMP toward Cu 2+ , the anti-interference ability of the sensor is vitally important. Therefore, the emission spectra of Ag@Azo TPE -CMP dispersed in ethanol-water solutions containing three equal concentrations (10 −2 M) of each metal ion (1: Zn 2+ , Ba 2+ and Na + , Fig. 4c; 2: Al 3+ , Mn 2+ and Ca 2+ , Fig. S17a; 3: La 3+ , Mg 2+ , Co 2+ , Fig. S17b) and subsequent addition of Cu 2+ (10 −3 M) have been monitored. It is apparent that the effective fluorescence quenching could occur upon adding 10 −3 M Cu 2+ into the parallel tests (Figs 4c and  S17). These results indicate that Ag@Azo TPE -CMP possess outstanding anti-interference ability, sensitivity in the detection of Cu 2+ even in the complicated system.

Conclusions
In summary, two azo-linked conjugated microporous polymers have been successfully developed using a facile diazo-coupling reaction. Due to the metal silver effect, the Ag + loading CMP showed the I 2 uptake is 1.87-times than that of Azo TPE -CMP. In addition, Ag@Azo TPE -CMP displays outstanding performance for the detection of heavy ions such as Cu 2+ , Hg 2+ , Cr 3+ , Ni 2+ . Particularly, compared to other metal cations, Ag@AzoTPE-CMP shows more effective in the detection of Cu 2+ . These results clearly demonstrated that there is a wealth of opportunity for producing novel absorbent materials with enhanced iodine capture capacity, remove of heavy ions, and expanded the scope of applications.
Synthesis of Azo TPE -CMP. Taking the preparation of Azo TPE -CMP as an example, the diazo-coupling reaction was carried out in two steps. Firstly, 4-(1, 2, 2-tris(4-aminophenyl)vinyl)benzenamine (1.5 mmol) was loaded in a 250 mL flask charged with 100 ml of deionized water, and 0.7 mL of concentrated hydrochloric acid. After stirred for 15 min at 0-5 °C, the mixture was added with 30 mL of aqueous solution of sodium nitrite (3.1 mmol) and stirred for 25 min to make amino groups be completely converted to diazonium salts. Subsequently, the mixture was neutralized with dilute solution of Na 2 CO 3 , and then mixed with 30 mL of aqueous solution of m-trihydroxybenzene (2 mmol) and Na 2 CO 3 (3 mmol) at 0-5 °C. After 12 h solid sample was separated from the reaction solution by filtration, and washed by the solvents in the order: water, methanol, and THF, respectively. Followed by freeze drying, polymer sample was obtained in a high (78% yield) 39 .

Synthesis of Ag@AzoTPE-CMP.
Preparation of Ag nanoparticles in Azo TPE -CMP. A piece of Azo TPE -CMP (50 mg) was placed in 20 mL of aqueous solution 1 M NaOH to exchange the protons of -OH groups with Na + . After 3 h the Na + exchanged Azo TPE -CMP was collected through filtration and washed with H 2 O. The wet Na + @Azo TPE -CMP was placed in 20 mL of H 2 O resulting in pH = 10. In that system, 100 mg of AgNO 3 were added and allowed to react overnight. The collected monolithic piece was washed extensively with H 2 O, soaked in ethanol to exchange the H 2 O, and dried again with supercritical CO 2 . The final grey-white powder product was collected Ag@Azo TPE -CMP (200 mg) 40 .