Covalent Organic Framework Nanosheet-Based Ultrasensitive and

May 2, 2019 - 10. Figure S9. XPS survey spectra of Tp-Bpy NSs and AuNPs@Tp-Bpy. nanocomposite. 11. Figure S10. Hg. 2+. -stimulated peroxides-like ...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 9408−9415

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Covalent Organic Framework Nanosheet-Based Ultrasensitive and Selective Colorimetric Sensor for Trace Hg2+ Detection Wei-Rong Cui,† Cheng-Rong Zhang,† Wei Jiang,† Ru-Ping Liang,† Shao-Hua Wen,† Dong Peng,† and Jian-Ding Qiu*,†,‡ †

College of Chemistry and Institute for Advanced Study, Nanchang University, Nanchang 330031, China Engineering Technology Research Center for Environmental Protection Materials and Equipment of Jiangxi Province, Pingxiang University, Pingxiang 337055, China

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ABSTRACT: Covalent organic frameworks (COFs) have shown extensive applications in energy storage, catalysis, and gas adsorption because of their regular pore structure, flexible topological connectivity, and excellent adjustable functionality. However, their potential applications in colorimetric sensing have not yet been explored. In this study, we synthesized bipyridine-containing covalent organic framework nanosheets (Tp-Bpy NSs) with a regular pore structure and abundant nitrogen-containing functional groups that function as active sites for the in situ generation of AuNPs to form AuNPs@Tp-Bpy. The anchoring of AuNPs onto Tp-Bpy NSs through coordination bonds can significantly enhance the dispersibility, stability, and catalytic activity of the AuNPs. We find that the synergistic effect of increased mimetic activity of gold amalgam and the higher access probability of Hg2+ provided by Tp-Bpy nanosheets makes the AuNPs@Tp-Bpy nanocomposite exhibit a high performance for the detection of Hg2+ with an ultralow detection limit of 0.33 nM. This sensing platform has been successfully used for the sensitive and stable detection of Hg2+ in various environmental samples. The present study extends the application of COFs and opens a new frontier for the design of novel nanocomposites for a variety of potential applications. KEYWORDS: Covalent organic framework, Au nanoparticles, Nanocomposite, Ultrasensitive, Colorimetric, Hg2+

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INTRODUCTION Mercury is a toxic heavy metal ion that can cause a series of very serious environmental and health problems.1,2 For example, it can accumulate continuously in the body through water and food, which may lead to disorders and irreversible damage to the endocrine,3 liver,4 kidney,5 brain,6 and nervous systems.7 Nowadays, numerous analytical techniques based on atomic spectroscopy,3 mass spectrometry,6 optical sensors,7 and electrochemical analyses8 have been developed to detect Hg2+. However, these analytical techniques typically require complex sample processing and precision instruments.6−8 Simultaneously, many detection methods based on oligonucleotides,9 organic fluorophores,10 proteins,11 and DNAase have been developed to detect Hg2+.12 However, most of these detection methods have drawbacks of complicated preparation processes, low sensitivity, poor water solubility, and very expensive costs.13−15 Therefore, there is an urgent need to develop portable, low-cost, fast, highly selective, and sensitive methods for the determination of Hg2+. Recently, colorimetric sensors have received great attention due to their signal visibility, fast response, sensitivity, and simple operation and are suitable for on-site analysis.16−18 Under the impetus of these advantages, many colorimetric sensors with peroxidase mimetic activity have been proposed and widely used in various environmental pollutant detectors and biosensors.19−22 In order to be universally available, the © 2019 American Chemical Society

detection sensitivity and stability of the existing materials still need to be addressed. In recent years, hybrid nanomaterials are particularly impressive based on the results of most nanoenzyme studies, which can significantly improve the catalytic performance and stability of the materials.23−25 Gold nanoparticles (AuNPs) have been widely used in various sensors owing to their excellent catalytic properties.25−27 However, in the preparation of AuNPs, due to the presence of van der Waals forces and electrostatic interaction, AuNPs aggregate, which results in inactivation. Thus, surfactants need to be added during the synthesis to avoid aggregation.28,29 The presence of the surfactants greatly limits the catalytic activity of AuNPs.28,29 As alternative approaches for the improvement of AuNPs dispersibility, supporting nanomaterials with larger surface areas and good conductivity, such as carbon nanotubes, graphitic carbon nitride, and graphene oxide, have been used.11,30,31 In addition, anchoring AuNPs onto two-dimensional nanomaterials through coordination bonds can significantly enhance the dispersibility, stability, and catalytic activity of the AuNPs in the catalytic reaction system. Covalent organic frameworks (COFs) and metal organic frameworks (MOFs) are classes of crystalline polymeric Received: January 30, 2019 Revised: April 22, 2019 Published: May 2, 2019 9408

DOI: 10.1021/acssuschemeng.9b00613 ACS Sustainable Chem. Eng. 2019, 7, 9408−9415

Research Article

ACS Sustainable Chemistry & Engineering

bath, evacuated, and flame-sealed. The tube was then placed in an oven at 120 °C and left undisturbed for 3 days to afford a reddishbrown product. The reddish brown product was isolated by filtration, washed with anhydrous tetrahydrofuran (THF) several times, and dried at 100 °C for 24 h to obtain Tp-Bpy (76.2 mg, 78%). Synthesis of Tp-Bpy NSs. Tp-Bpy COF (25 mg) was placed in a mortar and ground at room temperature for 30 min. The ground reddish-brown fine powder was dispersed in 50 mL of aqueous solution and then pulverized in the solution for 6 h. The dispersion was then centrifuged at 3000 rpm for 30 min and allowed to stand overnight to remove large-sized material. The resulting supernatant was then collected for further characterization and application. Synthesis of the AuNPs@Tp-Bpy Nanocomposite. Tp-Bpy NSs (20 mL, 100 mg/L) were mixed with HAuCl4 (2 mL, 10 mM) for 6 h with continuous stirring. The mixture was then heated to 100 °C, and sodium citrate (4 mL, 0.1 M) was added; the solution was stirred for an additional 1 h. The AuNPs@Tp-Bpy nanocomposite was then collected by centrifugation and washed with ethanol. Finally, the obtained AuNPs@Tp-Bpy nanocomposite was dispersed in 20 mL of aqueous solution. Preparation of Metal Ions Stock Solutions. The metal ion stock solutions (1 mM) were prepared by dissolving the nitrate salts, including Cr3+, Fe3+, Cu2+, Zn2+, Mg2+, Co2+, Ag+, Pb2+, Ni2+, Na+, K+, Ca2+, Al3+, Ba2+, Mn2+, NH4+, Cd2+, Fe2+, and Hg2+, in ultrapure water. Hg2+ Sensing. For Hg2+ sensing, the AuNPs@Tp-Bpy nanocomposite (5 μL, 2 mg/mL) was added to the buffer solutions (25 mM citric acid-phosphate, pH 4.0) which contained different amounts of Hg2+ (785 μL), and then H2O2 (200 μL, 1 M) and TMB (10 μL, 20 mM) were added to the mixture. The mixture reacted at 30 °C for 8 min prior to spectral measurements. The colorimetric detection of different concentrations of Hg2+ in actual samples was performed using a similar method.

materials that have shown promising applications in energy storage,32,33 metal ion separation and recovery,34−36 catalysis,37−39 and gas adsorption.40,41 Compared to MOFs or other polymeric materials,42−44 COF nanosheets have significant competitive advantages, such as a regular pore structure, flexible topological connectivity, and excellent adjustable functionality.45−48 Recently, various two-dimensional COF fluorescence sensors for detecting small molecules and ions have been developed.49−52 However, the potential applications of two-dimensional COF nanosheets or their nanocomposite in colorimetric sensing has not yet been explored. Herein, we synthesized bipyridine-containing covalent organic framework nanosheets (Tp-Bpy NSs) with a regular pore structure and abundant nitrogen-containing functional groups. AuNPs were then generated in situ on the Tp-Bpy NSs to obtain AuNPs@Tp-Bpy nanocomposite (Scheme 1). This Scheme 1. Synthesis Method of AuNPs@Tp-Bpy Nanocomposite

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RESULTS AND DISCUSSION Characterization of the Tp-Bpy COF and Tp-Bpy NSs. Tp-Bpy COF was synthesized by a solvothermal route through the Schiff base reaction of 2,4,6-triformylphloroglucinol (Tp) with 5,5′-diamino-2,2′-bipyridine (Bpy). As shown in Figure S1, the experimental powder X-ray diffraction (PXRD) patterns at 3.6° (2θ) are ascribed to the reflection from the (100) plane with a strong diffraction peak, indicating the highly crystalline phase of Tp-Bpy.37 The other peaks at 7.2° and 26.9° (2θ) are ascribed to the reflections of the (200) and (002) planes, respectively.37 The results demonstrate that the experimental PXRD pattern of Tp-Bpy COF matches well with the eclipsed model of the simulated Tp-Bpy COF structure (Table S1).37 As shown by the FT-IR spectra in Figure S2, the characteristic absorption peaks of Tp (−CHO at ∼2895 cm−1) and Bpy (−NH2 at ∼3280 cm−1) disappear, and a new peak at 1287 cm−1 (C−N) is observed, indicating the formation of TpBpy COF.37 The structure of Tp-Bpy was also confirmed by 13 C solid-state NMR spectra (Figure S3). The signal located at ∼134 ppm is attributed to the C−N bond, and the peaks from 100 to 120 ppm are attributed to phenyl carbons, which are consistent with the keto−enol tautomerized structure of TpBpy COF.37 The above results demonstrate that high crystallinity Tp-Bpy COF has been successfully prepared. Tp-Bpy NSs were prepared by ultrasonic-assisted exfoliation of Tp-Bpy COF. By comparing the PXRD patterns of Tp-Bpy COF and Tp-Bpy NSs (Figure S4), it can be seen that the preparation of NSs does not affect the crystalline phase and the structure of Tp-Bpy COF.37 The TEM image shows the presence of flat, thin Tp-Bpy NSs (Figure S5). The exfoliated Tp-Bpy NSs exhibit a flaky shape, indicating that the hydrophilic functional group can be sufficiently exposed to the environment which leads to the high stability of the Tp-

novel nanocomposite has an excellent water dispersibility, high stability, and enhanced peroxidase-like activity. The twodimensional regular pore structure of Tp-Bpy NSs exposes most of the anchored AuNPs to the environment. Therefore, based on the Hg2+-triggered enhanced catalytic activity of the highly dispersed AuNPs, the AuNPs@Tp-Bpy nanocomposite can be used for ultrasensitive detection of Hg2+ in aqueous solution.

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EXPERIMENTAL SECTION

Materials. 2,4,6-Triformylphloroglucinol (Tp), 5,5′-diamino-2,2′bipyridine (Bpy), and HAuCl4 were purchased from Saan Chemical Technology Co., Ltd. 3,3′,5,5′-Tetramethylbenzidine (TMB) and Hg(NO3)2 were purchased from Sigma-Aldrich. Acetic acid, 1,4dioxane, mesitylene, hydrogen peroxide (H2O2, 30%), citric acid (C6H8O7), ethanol, tetrahydrofuran (THF), Na2HPO4, and other nitrate salts (Cr3+, Fe3+, Cu2+, Zn2+, Mg2+, Co2+, Ag+, Pb2+, Ni2+, Na+, K+, Ca2+, Al3+, Ba2+, Mn2+, NH4+, Cd2+, and Fe2+) were purchased from Sinopharm Chemical Reagent Co., Ltd. Experimental water was prepared with a Millipore system (18.25 MΩ·cm). All purchased reagents were used without further purification. Synthesis of the Tp-Bpy COF. 5,5′-Diamino-2,2′-bipyridine (55.8 mg, 0.3 mmol) and 2,4,6-triformylphloroglucinol (42 mg, 0.20 mmol) were weighed into a glass tube (volume of ca. 25 mL, body length of 11 cm, outer diameter of 26 mm). 1,4-Dioxane (1.0 mL), mesitylene (1.0 mL), and 6 M acetic acid solution (0.2 mL) were then added to the mixture. The tube was flash frozen in a liquid nitrogen 9409

DOI: 10.1021/acssuschemeng.9b00613 ACS Sustainable Chem. Eng. 2019, 7, 9408−9415

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Figure 1. (A) N2 adsorption and desorption isotherms of Tp-Bpy NSs. (B) PXRD patterns of Tp-Bpy NSs, AuNPs, and the AuNPs@Tp-Bpy nanocomposite.

Figure 2. (A) TEM image of the AuNPs@Tp-Bpy nanocomposite (inset: the AuNPs size distribution at Tp-Bpy NSs). (B) HRTEM image and the selected area electron diffraction (SAED) pattern of the AuNPs@Tp-Bpy nanocomposite.

Figure 3. (A) Absorbance curve of Hg2+-triggered peroxide mimetic activity of the AuNPs@Tp-Bpy nanocomposite. (B) Absorbance changes at 652 nm of TMB reaction solutions reacted with (1) AuNPs@Tp-Bpy, Hg2+, and H2O2; (2) AuNPs, Hg2+, and H2O2; (3) Tp-Bpy NSs, Hg2+, and H2O2; (4) Hg2+ and H2O2; (5) AuNPs@Tp-Bpy and H2O2; (6) AuNPs and H2O2; (7) Tp-Bpy NSs and H2O2; and (8) H2O2. Experimental conditions: 0.20 mM TMB, 300 nM Hg2+, 200 mM H2O2, pH = 4.0, and AuNPs@Tp-Bpy (10 μg/mL).

Characterization of the AuNPs@Tp-Bpy Nanocomposite. We prepared the AuNPs@Tp-Bpy nanocomposite by loading AuNPs on Tp-Bpy NSs in situ. The TEM images of the AuNPs@Tp-Bpy nanocomposite at different magnifications (Figures 2A and 2B) demonstrate that the highly monodispersed AuNPs (an average size of 14 nm) are uniformly dispersed on the Tp-Bpy NSs surface, and the measured lattice fringe spacing matches well with the expected spacing of the Au (111) plane. Upon comparison of the PXRD patterns of Tp-Bpy NSs with the AuNPs@Tp-Bpy nanocomposite (Figure 1B), the additional peaks of AuNPs@TpBpy at higher (2θ) values of 38.2°, 44.4°, 64.7°, and 77.7° are attributed to the (111), (200), (220), and (311) planes reflections of Au (CCOD 9011613), respectively,27 indicating

Bpy NSs in water (Figure S6). Thermogravimetric analysis (TGA) was performed to analyze the thermal stability of TpBpy NSs. As shown in Figure S7, Tp-Bpy NSs are thermally stable up to 350 °C, indicating good thermal stability. The porosity and specific surface area were determined to be 755 m2 g−1 by N2 adsorption and desorption isotherm at 77 K (Figure 1A). As shown in Figure S8, the pore size distribution of Tp-Bpy NSs are measured based on the non-local density functional theory (NLDFT)34 and shows a narrow pore size distribution in the range of 1.69 nm. The above results indicate that Tp-Bpy NSs have a good thermal stability, excellent water dispersibility, well-ordered porous structure, and large specific surface area, showing enormous potential to become a carrier of metal nanoparticles. 9410

DOI: 10.1021/acssuschemeng.9b00613 ACS Sustainable Chem. Eng. 2019, 7, 9408−9415

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ACS Sustainable Chemistry & Engineering Scheme 2. Mechanism of Colorimetric Detection of Mercury Ions using AuNPs@Tp-Bpy

that AuNPs are successfully deposited on Tp-Bpy NSs. X-ray photoelectron spectroscopy (XPS) measurements were carried out for further analysis of the chemical composition of the AuNPs@Tp-Bpy nanocomposite. By comparing the survey XPS spectra of the Tp-Bpy NSs and AuNPs@Tp-Bpy nanocomposite, we can see that the main doublet peaks of Au 4f further confirm that AuNPs are firmly anchored on TpBpy NSs (Figure S9).30 Meanwhile, the C 1s and N 1s spectra of the AuNPs@Tp-Bpy nanocomposite demonstrate the existence of a large amount of nitrogen-containing functional groups.30 These results demonstrate the successful preparation of AuNPs@Tp-Bpy nanocomposite via in-situ formation of AuNPs that does not interfere with the structural integrity and overall crystallinity of the Tp-Bpy NSs. Mercury-Triggered Enhanced Peroxidase Mimetic Activity of AuNPs@Tp-Bpy. As shown in Figure 3A, the AuNPs@Tp-Bpy nanocomposite does not exhibit any peroxidase mimetic activity in the absence of Hg2+. However, addition of 1 μM Hg2+ significantly improves the catalytic activity of the AuNPs@Tp-Bpy nanocomposite as evidenced by the rapid oxidation of colorless TMB to bright blue oxidation state TMB (oxTMB). In order to clarify the key factors that may cause TMB to be oxidized, the timedependent absorbance changes were measured at 652 nm by UV−visible spectroscopy in the presence of a range of other chemicals. As shown in Figure 3B, only the AuNPs@Tp-Bpy nanocomposite shows great catalytic performance for TMB oxidation, accompanying a significant colorimetric response. Furthermore, we have investigated the other factors that may affect the Hg2+-triggered peroxides-like activity of AuNPs@TpBpy (Figure S10). After optimization, the solution temperature, pH, and concentrations of TMB and H2O2 were set at 30 °C, 4.0, and 0.20 and 200 mM, respectively. Detection Mechanism. The mercury-triggered enhanced catalytic reaction of AuNPs@Tp-Bpy was studied. As shown in Scheme 2, Hg2+ is reduced by citrate ions in the buffer solution to form Hg0 that deposits on the surface of AuNPs, forming gold amalgam. The newly formed gold amalgam layer greatly changes the physicochemical properties of the alloyed surface at which H2O2 molecules are decomposed efficiently to form highly reactive hydroxyl radicals that oxidize TMB to the bright blue product.28 The mercury-triggered enhanced catalysis mechanism proposed above is supported by the XPS data (Figure 4). After treatment of the AuNPs@Tp-Bpy nanocomposite with Hg2+ (Figure 4B and 4D), two binding energy peaks are observed at 100.8 eV (Hg 4f 7/2) and 104.9 eV (Hg 4f 5/2), indicating that Hg0 and Hg2+ coexist on the

Figure 4. XPS spectra of the Au 4f region of AuNPs@Tp-Bpy before (A) and after being treated with Hg2+ (C). XPS spectra of the Hg 4f region of AuNPs@Tp-Bpy before (B) and after being treated with Hg2+ (D).

surface of AuNPs.27 However, the binding energy of Au 4f remains unchanged after the Hg0 deposition on AuNPs (Figure 4A and 4B). As a result, the catalytic oxidation performance of the AuNPs@Tp-Bpy nanocomposite is significantly enhanced upon addition of Hg2+. Sensitivity and Selectivity for Hg2+ Detection. To assess the sensitivity for colorimetric detection of Hg2+ using AuNPs@Tp-Bpy, UV−vis absorption spectra were collected at various Hg2+ concentrations. As shown in Figure 5A, the absorbance increases with the Hg2+ concentration. The changes in absorbance at 1 nM Hg2+ can be easily detected, indicating that the present approach is ultrasensitive for detection of Hg2+. Figure 5B shows a calibration curve of absorbance at 652 nM versus different Hg2+ concentrations (1−400 nM). The detection limit for Hg2+ in water is 0.33 nM, well below the threshold level of drinking water (10 nM).27 In addition, the comparisons of AuNPs@Tp-Bpy with other mercury detection methods are shown in Table S2. The present AuNPs@Tp-Bpy sensor shows a higher sensitivity than the recently reported fluorescent or colorimetric assays.52−54 The AuNPs@Tp-Bpy nanocomposite exhibits higher mercurytriggered enhanced peroxidase mimetic activity than AuNPs (Figure S11) due to the synergy of gold amalgam formation and the bonding environment of AuNPs. The well-ordered 9411

DOI: 10.1021/acssuschemeng.9b00613 ACS Sustainable Chem. Eng. 2019, 7, 9408−9415

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ACS Sustainable Chemistry & Engineering

Figure 5. (A) UV−vis spectra after addition of Hg2+. Inset photos show the color changes at different concentrations of Hg2+. (B) The linear calibration plot for Hg2+ detection. Experimental conditions: 0.20 mM TMB, 200 mM H2O2, pH = 4.0, and AuNPs@Tp-Bpy nanocomposite (10 μg/mL). Error bars show the standard deviations of three experiments.

actual samples, indicating that the present sensing platform has a practical application value. Practical Application of AuNPs@Tp-Bpy for Detecting Hg2+. In order to verify the suitability of the AuNPs@Tp-Bpy nanocomposite for detection of Hg2+ in real samples, we tested tap water, Ganjiang river water, and Poyang lake water samples containing various concentrations of Hg2+. The collected samples were centrifuged for 10 min at 12500 rpm to remove the large particles and then were tested using our proposed method. In addition, to illustrate the feasibility of the AuNPs@ Tp-Bpy nanocomposite in practical applications, we designed a recycling experiment via spiked tap water, Ganjiang river water, and Poyang lake water samples (Table S3). The results show that for all the samples, the recovery of different concentrations of Hg2+ solution is close to 100% (from 96.58% to 103.72%). Furthermore, the morphology of the AuNPs@Tp-Bpy nanocomposite does not change after colorimetric detection of Hg2+ (Figure S13) and still exhibits a highly monodisperse state. The corresponding characteristics of the AuNPs@TpBpy nanocomposite with high stability in the catalytic reaction system are related to the reasonable selection of the stable support Tp-Bpy NSs having a well-ordered porous structure enriched with nitrogen-containing crystalline framework, which provides extra affinity to AuNPs. All of these results demonstrate that the sensing platform of AuNPs@Tp-Bpy is selective and sensitive for detecting Hg2+ under measurement conditions and has great application potential in actual water detection. To evaluate the reusability of AuNPs@Tp-Bpy, a regeneration experiment of Hg2+ detection in water was performed. It has been reported that Au can be recovered from Au−Hg amalgam using heat treatment due to the volatility of Hg0 increasing with temperature. Therefore, after detection of Hg2+, Hg0 adsorbed on AuNPs in AuNPs@Tp-Bpy was heated at 80 °C to regenerate AuNPs. After 6 cycles of repeated detection and mercury removal, the catalytic activity of AuNPs@Tp-Bpy was still greater than 95.5% (Figure S14). This result clearly shows that the developed AuNPs@Tp-Bpy nanocomposite can be repeatedly used for Hg2+ detection in aqueous solution.

porous structure, large specific surface area, and excellent water dispersion properties of Tp-Bpy NSs provide a much higher probability for Hg2+ to contact with the anchored AuNPs. The synergistic effect and the higher access probability of Hg2+ are the key factors of Tp-Bpy NSs in recognizing and sensing Hg2+. Due to the excellent affinity between Au and Hg, the AuNPs@Tp-Bpy nanocomposite shows excellent selectivity for Hg2+. The Hg2+ (300 nM) was added to the reaction solution, while using 10-fold that of other metal ions, including Cr3+, Fe3+, Cu2+, Zn2+, Mg2+, Co2+, Ag+, Pb2+, Ni2+, Na+, K+, Ca2+, Al3+, Ba2+, Mn2+, NH4+, Cd2+, and Fe2+, for comparison. As shown in Figure 6, when Hg2+ is added to the reaction

Figure 6. Catalytic activity of the AuNPs@Tp-Bpy nanocomposite stimulated by different metal ions (1, blank; 2, Cr3+; 3, Fe3+; 4, Cu2+; 5, Zn2+; 6, Mg2+; 7, Co2+; 8, Ag+; 9, Pb2+; 10, Ni2+; 11, Na+; 12, K+; 13, Ca2+; 14, Al3+; 15, Ba2+; 16, Mn2+; 17, NH4+; 18, Cd2+; 19, Fe2+; and 20, Hg2+). Concentrations of Hg2+ and other metal ions were 300 nM and 3 μM, respectively.

solution, the absorbance at 652 nM is much higher than that with the addition of other ions. Importantly, this excellent selectivity can be observed with the naked eye. These results indicate that only Hg2+ can significantly increase the catalytic activity of AuNPs@Tp-Bpy nanocomposite among the all tested metal ions. Furthermore, an interference experiment was performed to verify that this colorimetric method can be widely used. Under the optimized conditions, Hg2+ and 50-fold that of the other metal ions (Cr3+, Fe3+, Cu2+, Zn2+, Mg2+, Co2+, Ag+, Pb2+, Ni2+, Na+, K+, Ca2+, Al3+, Ba2+, Mn2+, NH4+, Cd2+, and Fe2+) were simultaneously added to the reaction solution. As shown in Figure S12, the other metal ions have little effect on the Hg2+ detection. These results confirm that the AuNPs@Tp-Bpy nanocomposite has excellent selectivity for the detection of Hg2+ and can be used for the detection of

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CONCLUSIONS In conclusion, we have synthesized Tp-Bpy NSs with regular pore structure and abundant nitrogen-containing functional groups that function as active sites for AuNPs in situ generation, forming AuNPs@Tp-Bpy. This AuNPs@Tp-Bpy nanocomposite can be used for high-performance colorimetric 9412

DOI: 10.1021/acssuschemeng.9b00613 ACS Sustainable Chem. Eng. 2019, 7, 9408−9415

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ACS Sustainable Chemistry & Engineering detection of Hg2+ in aqueous solution owing to the synergy of the Hg2+-triggered enhanced peroxidase mimetic activity and the higher access probability of Hg2+. This sensing method can achieve ultrasensitive and selective detection of Hg2+ and can be used for the detection of Hg2+ in various environmental samples. We expect that the metal NPs supported by COF NSs will open a new frontier for the design of novel nanocomposites for various potential applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00613. (Figure S1) Comparison of the PXRD patterns of experimental Tp-Bpy with simulated Tp-Bpy; (Table S1) fractional atomic coordinates for the unit cell of cis Tp-Bpy (eclipsed); (Figure S2) FT-IR spectra of TP, Bpy, and Tp-Bpy; (Figure S3) 13C solid-state NMR spectra of Tp-Bpy; (Figure S4) PXRD patterns of TpBpy and Tp-Bpy NSs; (Figure S5) TEM image of TpBpy NSs; (Figure S6) Tyndall tests for the water solutions of Tp-Bpy NSs; (Figure S7) TGA curve of TpBpy NSs; (Figure S8) experimental pore size distribution for Tp-Bpy; (Figure S9) XPS survey spectra of TpBpy NSs and AuNPs@Tp-Bpy nanocomposite; (Figure S10) Hg2+-stimulated peroxides-like catalytic activity of the AuNPs@Tp-Bpy nanocomposite; (Table S2) comparison of the presented work with other reported methods; (Figure S11) UV-vis spectra and different absorbance at 652 nm corresponding to various concentrations of Hg2+ for TMB + AuNPs + H2O2 and TMB + AuNPs@Tp-Bpy + H2O2; (Figure S12) mercury-stimulated peroxidase mimetic activity of the AuNPs@Tp-Bpy nanocomposite; (Table S3) recovery results of tap water, Ganjiang River water, and Poyang lake samples; (Figure S13) TEM image of AuNPs@TpBpy after colorimetric detection of Hg2+; (Figure S14) changes in catalytic activity of AuNPs@Tp-Bpy (10 μg/ mL) and 1 μM Hg2+ in recycling runs; and the principle of the limit of detection (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86-791-83969518. ORCID

Jian-Ding Qiu: 0000-0002-6793-9499 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the National Natural Science Foundation of China (21675078), the Jiangxi Province Natural Science Foundation (20165BCB18022), and the Research Innovation Program for College Graduates of Jiangxi Province (YC2016-B002 and YC2017-B006).

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