Rhodamine 6G loaded zeolitic imidazolate framework-8 (ZIF-8) nanocomposites for highly selective luminescent sensing of Fe3+, Cr6+ and aniline

Accepted Manuscript Rhodamine 6G loaded zeolitic imidazolate framework-8 (ZIF-8) nanocomposites for 3+ 6+ highly selective luminescent sensing of Fe , Cr and aniline Ting-Ting Han, Jin Yang, Ying-Ying Liu, Jian-Fang Ma PII:

S1387-1811(16)30085-3

DOI:

10.1016/j.micromeso.2016.04.005

Reference:

MICMAT 7667

To appear in:

Microporous and Mesoporous Materials

Received Date: 13 January 2016 Revised Date:

3 April 2016

Accepted Date: 6 April 2016

Please cite this article as: T.-T. Han, J. Yang, Y.-Y. Liu, J.-F. Ma, Rhodamine 6G loaded zeolitic 3+ 6+ imidazolate framework-8 (ZIF-8) nanocomposites for highly selective luminescent sensing of Fe , Cr and aniline, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2016.04.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

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In this work, the organic dye Rhodamine 6G (R6G) loaded R6G@ZIF-8 nanocomposites were prepared and well characterized by transmission electron

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microscope (TEM), powder X-ray diffraction (PXRD) and N2 adsorption, where they exhibit highly selective and sensitive luminescent detections of metal ions, anions, organic small molecules and polyoxometalates (POMs).

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Rhodamine

6G

loaded

zeolitic

imidazolate

framework-8

(ZIF-8)

nanocomposites for highly selective luminescent sensing of Fe3+, Cr6+ and

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aniline

Ting-Ting Han,[a,b] Jin Yang, [a] Ying-Ying, Liu*,[a] and Jian-Fang Ma*,[a]

Key Laboratory of Polyoxometalate Science, Department of Chemistry, Northeast Normal

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[a]

[b]

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University, Changchun 130024, P.R. China

Department of Chemistry, Changchun Normal University, Changchun 130032, P.R. China

* Correspondence authors

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E-mail: [email protected] (Y.-Y. Liu) E-mail: [email protected] (J.-F. Ma)

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Fax: +86 431 85098620

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ACCEPTED MANUSCRIPT Abstract. Fluorescence-based probes exhibit apparent superiority in high sensitivity, selectivity and easy operation. As a typical representative of zeolitic imidazolate frameworks (ZIFs), highly porous ZIF-8 has attracted intense interests because of its potential applications.

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In this work, the organic dye Rhodamine 6G (R6G) loaded ZIF-8 nanocomposites were prepared and well characterized by transmission electron microscope (TEM), powder X-ray diffraction (PXRD) and N2 adsorption. The R6G loaded ZIF-8 (R6G@ZIF-8) nanocomposites

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exhibit intense luminescence with wide wavelength band in the visible light region. The

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luminescent detections of metal ions, anions, organic small molecules and polyoxometalates were fully studied using the as-synthesized R6G@ZIF-8 nanocomposites as sensory materials. Strikingly, the luminescent detections indicate that R6G@ZIF-8 nanocomposites are capable 6+

of highly selective sensing of Fe3+, highly toxic Cr

and organic aniline with a low detection

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limit of 5 µM, 50 µM and 5 mM, respectively. More significantly, luminescence quenching and recovery tests demonstrate that R6G@ZIF-8 nanomaterials are reusable for the detections 6+

and organic aniline.

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of Fe3+, Cr

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Keywords: Luminescent sensing, Rhodamine 6G, ZIF-8, Quenching effect

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ACCEPTED MANUSCRIPT 1. Introduction Over the past few years, there has been an extensive interest in recognition and sensing of metal ions, toxic anions and small-molecule pollutants because of their important roles in

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environment and human beings [1-8]. Among these, detection of metal ions has become of particular significance owing to their relationship with human health and environment [9-13]. In this regard, Fe3+, as an indispensable metal ion for most organisms, plays a significant role

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in biochemical processes at the cellular level such as structure and activity retention for enzymes and proteins, nitrogen fixation, and oxygen transportation [14-16]. Thereby, it is very

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important to develop highly efficient and convenient methods to probe Fe3+ [8]. It needs to be emphasized that hexavalent chromium CrVI, as one of the most common heavy metal ions in environments, exhibits a high toxicity and carcinogenicity, and is proven

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to be one of the most severe environmental non-biodegradable pollutants [17, 18]. Generally, the CrVI contaminant widely arises from industry regions such as leather tanning, textile manufacturing, steel fabrication and so on [19]. The fast, facile, visual, and quantitative

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detection of trace aqueous CrVI is highly necessary and desirable from the viewpoint of

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environmental monitoring and protection [20]. Fluorescence-based probes exhibit apparent superiority in high sensitivity, selectivity and easy operation, making them excellent alternatives for the detection of metal ions [21-23]. Zeolitic imidazolate frameworks (ZIFs) as a class of metal-organic frameworks (MOFs) are crystalline three-dimensional (3D) frameworks composed of metal ions and tetrahedral imidazolate linker [24, 25]. In this regard, highly porous ZIF-8 nanocomposites, as a typical representative of ZIFs possessing exceptional thermal and chemical stabilities, have attracted

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ACCEPTED MANUSCRIPT intense interests owing to their potential applications in matrix membranes, gas storage, sensing, heterogeneous catalysis, and drug delivery [26-28]. Markedly, encapsulating guest molecules into ZIF-8 provides an outstanding strategy for constructing functionalized ZIF-8

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nanocomposites [29]. For example, highly luminescent GaN@ZIF-8 nanocrystals were achieved by encapsulating GaN semiconductor quantum dots into the ZIF-8 [30]. Encapsulating BPEI-CQDs (branched poly(ethylenimine)-capped carbon quantum dots) into

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ZIF-8 composites gives the highly luminescent BPEI-CQDs@ZIF-8 nanocrystals with strong

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fluorescence quantum yield, which were used to develop an ultrasensitive and highly selective sensor for Cu2+ ion [31]. Although much success has been achieved in developing functional ZIF-8 nanomaterials, the fluorescent organic dyes hosted in the ZIF-8 nanocomposites as luminescent probes for basic molecules have received much less attention to date.

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On the basis of above consideration, the typical fluorescent organic dye Rhodamine 6G (R6G) hosted in the ZIF-8 nanocomposites were prepared and well characterized by transmission electron microscope (TEM), powder X-ray diffaction (PXRD) and N2 adsorption.

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Notably, the as-synthesized R6G@ZIF-8 nanocomposites were used as sensory materials for

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the sensing of metal ions, anions, POMs ions and organic small molecules with high selectivity and rapid response. Particularly, these materials are capable of highly selective detections of Fe3+, toxic Cr6+ and organic aniline with very low detection limits.

2. Experimental 2.1. Materials and measurements All the chemicals were analytical grade and without further purification. The POMs

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ACCEPTED MANUSCRIPT H4[SiW12O40]·26H2O,

H3[PW12O40]·12H2O,

Na9[PW9O34]·7H2O,

K6[P2W18O62]·15H2O,

K14[NaP5W30O110]·15H2O,

K6[P2Mo18O62],

(NH4)6[Mo7O24]·4H2O

and

K[SiW11CoO40H3]·3H2O were synthesized according to the literature [32]. Transmission

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electron microscope (TEM) images were performed on JEOL-2100F operated at 200 kV. Powder X-ray diffraction (PXRD) data were recorded on a Rigaku Dmax 2000 X-ray diffractometer with graphite monochromatized CuKα radiation (λ = 0.154nm) at 2θ ranging

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from 5 to 45°. The gas adsorption-desorption experiments were conducted on automatic

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volumetric adsorption equipment (V-Sorb 2800S). The emission spectra were carried out on a Perkin-Elmer FLSP920 Edinburgh Fluorescence spectrometer using a 450 W xenon lamp as excitation source at room temperature. Diffuse reflectivity spectra were collected on a finely ground sample with a Cary 500 spectrophotometer.

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2.2. Synthesis of ZIF-8, ZR-1, ZR-2 and ZR-3

ZIF-8 and R6G@ZIF-8 nanocomposites with different amounts of encapsulated R6G denoted as ZR-1, ZR-2 and ZR-3 were prepared by following the literature method [33, 34].

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Typically, 2-methylimidazole (Hmim) (660 mg, 8.0 mmol) and Zn(NO3)2·6H2O (300 mg, 1.0

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mmol) were dissolved in 15 mL methanol, respectively. Subsequently, the two methanol solutions were mixed, and further stirred for 6 hours. The resulting white precipitates of ZIF-8 were achieved by centrifugation, and washed with methanol for several times. ZIF-8 (75 mg, 0.33 mmol), Hmim (181 mg, 2.2 mmol), and variable amounts of R6G (9.3 mg, 0.02 mmol; 29.2 mg, 0.06 mmol; 292.4 mg, 0.61 mmol) were placed in a glass vial. Then n-butanol (15 mL) was added to the vial and the mixture was ultrasonicated for 10 minutes. The glass vial with cap was placed in an isothermal oven at 100 °C for 7 days. Finally, the R6G-loaded ZR-1,

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ACCEPTED MANUSCRIPT ZR-2 and ZR-3 were collected by centrifugation and washed with methanol until the supernatant was completely transparent. 2.3. Luminescent measurements

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The cyanoacrylate adhesive (5 mg) was dropped onto a quartz slide (12 × 45 mm), and 3 mg of ZR-1, ZR-2 or ZR-3 was laid gently on the surface of the slide and dried in air. Then each slide was put into a suprasil cuvette. Subsequently, various aqueous solutions (2 mL) of

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metal cations (1×10−2 mol·L−1), anions (1×10−2 mol·L−1), and POMs (1×10−2 mol·L−1) were

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respectively dropped into the cuvettes to give metal cations, anions and POMs loaded R6G@ZIF-8 species. After keeping the solution at room temperature for 5 min, the measurement of photoluminescence was conducted [35]. During the sensing of organic small molecules, the finely ground samples of ZR-1, ZR-2 and ZR-3 (3 mg) were dispersed in

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different organic solvents (3 mL) and ultrasonicated at room temperature for 30 min, then aged to form stable suspensions before fluorescence measurements [36]. The experimental details of the quenching and recovery cycles of ZR-1 were performed

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by the literature method [37]. Typically, the slide adhered ZR-1 was dried in air, and then

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immersed into a suprasil cuvette containing water (2 mL). The luminescent spectrum of ZR-1 on the slide was measured in water. Subsequently, the slides with adhered ZR-1 were put into aqueous solutions of Fe3+ (1 mM) and Cr2O72- (1 mM), respectively, and their emission spectra were recorded. Next, the slides were repeatedly washed with deionized water and air dried for another cyclic test. The quenching and recovery cycles of ZR-1 for detection of aniline were performed as follows. The dried ZR-1 (3 mg) was added to a solution of acetone (3 mL) and the emission spectrum without the addition of aniline was measured. Then, the samples of

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ACCEPTED MANUSCRIPT ZR-1 were separated from acetone by centrifuging and dispersed into a solution of aniline in acetone (3.0 mL). The achieved mixture was ultrasonicated for 30 min to give stable suspension for luminescent quenching study. After that, the resulting suspension was

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transferred to a centrifuge tube. Finally, the samples of ZR-1 were air dried for another cyclic test after they were repeatedly washed with acetone and centrifuged for several times. 3. Results and discussion

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3.1. TEM and PXRD

As shown in Fig. 1, the transmission electron microscope (TEM) images of ZIF-8, ZR-1,

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ZR-2 and ZR-3 reveal the uniform sodalite zeolite-type structure with an average size of 100 nm. The consistency in shape and size between the pure ZIF-8, ZR-1, ZR-2 and ZR-3 indicates that the encapsulation of R6G does not influence the morphology of ZIF-8 [38]. To further confirm the structural integrity after the successfully encapsulated R6G, the PXRD

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measurements of ZIF-8, ZR-1, ZR-2 and ZR-3 were carried out. As illustrated in Fig. 2, the PXRD patterns of ZIF-8, ZR-1, ZR-2 and ZR-3 are identical to the peaks from a standard ZIF-8 crystal structure [34]. The TEM images and PXRD patterns suggest that ZIF-8 scaffold

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did not collapse after the R6G encapsulation.

Fig. 1. TEM images of ZIF-8 (a), ZR-1 (b), ZR-2 (c) and ZR-3 (d). 7

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Fig. 2. PXRD patterns of ZIF-8 (a), ZR-1 (b), ZR-2 (c) and ZR-3 (d).

3.2. N2 adsorption

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To clarify that the R6G was incorporated into the cavities of ZIF-8, N2 adsorption for the as-synthesized ZIF-8, ZR-1, ZR-2 and ZR-3 were measured at 77 K. As depicted in Fig. 3, ZIF-8, ZR-1, ZR-2 and ZR-3 exhibit type I isotherms based on N2-sorption measurement

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results. The drastic rise and high N2 uptake at the beginning of the isotherm indicate the

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presence of a large proportion of micropores [24]. The saturation adsorption volumes with N2 are 361, 339, 222 and 204 cm3/g for ZIF-8, ZR-1, ZR-2 and ZR-3, respectively. Their corresponding size distributions are shown in Fig. S1. The adsorption values of ZR-1, ZR-2 and ZR-3 are reduced by 6.0%, 38.5% and 43.5% in respect to ZIF-8. Apparently, the surface areas and pore volumes of the as-synthesized R6G@ZIF-8 nancomposites can be tunable by varying the proportion of loaded R6G [11]. Moreover, the saturation adsorption results also reveal that the R6G molecules were successfully incorporated into the cavities of ZIF-8 [38].

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Fig. 3. N2 adsorption (●) and desorption (○) isotherms of ZIF-8 (black), ZR-1 (green), ZR-2 (pink) and ZR-3 (blue).

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3.3. UV-vis spectra

The R6G@ZIF-8 frameworks (3 mg) were digested by 1 wt% hydrochloric

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acid/methanol solution (3 mL). After stirring for 2 minutes, the resulting solutions were transferred to quartzose cuvettes to measure the absorption spectra. The encapsulated amounts

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of the R6G in different R6G@ZIF-8 nanocomposites were determined by calibration curve between R6G concentration prepared by varying amounts of the R6G in 1 wt% hydrochloric acid/methanol and absorbance of light at 525 nm (Fig. 4). The encapsulated contents of R6G by calculation are 0.02 wt%, 0.03 wt% and 0.12 wt% for ZR-1, ZR-2 and ZR-3, respectively.

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Fig. 4. UV-vis spectra of R6G@ZIF-8 and various amounts of R6G in 1 wt% hydrochloric

and absorbance (right).

3.4. Luminescent properties

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acid/methanol at room temperature (left) and calibration curve between R6G concentration

For the sake of removing the possible R6G attaching to the surface of R6G@ZIF-8, all

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the samples were washed with n-butanol for several times until the supernatant does not exhibit luminescence under UV light (λ = 365 nm). Under natural light, the pure solid-state

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ZIF-8 and R6G are white and purple, respectively. The solid-state R6G@ZIF-8 samples (ZR-1, ZR-2 and ZR-3) exhibit variable pink colors, as shown in Fig. 5. Under UV light,

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ZIF-8 displays pale blue color, while R6G@ZIF-8 exhibits white color. The luminescent spectra of ZIF-8 (λex = 380 nm) and R6G@ZIF-8 nanocomposits (λex = 350 nm) in the solid state were measured at room temperature (Fig. 6). The solid ZIF-8 displays a broad emission band centered at 448 nm, and the solid ZR-1, ZR-2 and ZR-3 exhibit the same strong emission band at 560 nm. The emission of the R6G in R6G@ZIF-8 is mainly sensitized by the mim moiety within the same framework [6, 39, 40]. To confirm the energy transfer behavior from mim to R6G, the emission spectra for ZR-1 in ethanol 10

ACCEPTED MANUSCRIPT suspension excited at different wavelengths were studied. As shown in Fig. S2, upon increasing the excitation wavelength, the intensities of emission peaks at about 430 and 550 nm gradually changed. The results clearly demonstrate that the emission of the R6G in

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R6G@ZIF-8 is mainly sensitized by the mim moiety within the same framework [6]. Thereby,

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the emission peak height of R6G in R6G@ZIF-8 can be utilized for luminescent probing.

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Fig. 5. Photographs of the samples under natural light (above) and UV light (below).

Fig. 6. Emission spectra of ZIF-8, R6G, ZR-1, ZR-2 and ZR-3 in the solid-state at room temperature.

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chemistry research and environment [16]. In light of the excellent fluorescence and good water stability of R6G@ZIF-8 nanocomposites, their potential application for detecting metal cations was examined in water. Typically, the slides with loaded samples of ZR-1, ZR-2 and

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ZR-3 were simply treated by aqueous solution of 1×10−2 mol·L−1 MCln (M = Na+, Al3+, Ca2+,

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Cr3+, Mn2+, Fe2+, Fe3+, Ni2+, Cu2+ and Zn2+), AgNO3 and Hg(NO3)2 to give the metal cations loaded ZR-1@M, ZR-2@M and ZR-3@M solids, respectively.

As shown in Fig. 7, the luminescent intensities of the ZR-1@M, ZR-2@M and ZR-3@M are highly dependent on the loaded metal species. For ZR-1@M, the luminescent intensities (λem = 560 nm) are enhanced when the Al3+, Na+ and Zn2+ cations are loaded, but the

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luminescent intensities drastically decreased when the Fe2+, Mn2+, Hg2+, Cr3+, and Fe3+ cations are involved. Particularly, the Fe3+ cations exhibit the most significant quenching effect on the luminescent ZR-1 (Fig. 7). The ZR-2@M and ZR-3@M also exhibit the similar fluorescence

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performance to ZR-1@M. In other words, in contact with aqueous solution of Fe3+, the

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luminescent ZR-1, ZR-2 and ZR-3 are completely quenched. The result indicates the high selectivity of ZR-1, ZR-2 and ZR-3 for the sensing and specific recognition of Fe3+ in the aqueous environment.

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Fig. 7. Emission spectra and intensities of ZR-1 (a and b), ZR-2 (c and d) and ZR-3 (e and f) in water and various aqueous solution of metal cations (K = 1000).

To understand the selectivity of the probes for detection of Fe3+, the anti-interference sensing ability of ZR-1 as a representative of R6G@ZIF-8 was further investigated by competing experiments. The slides with adhered ZR-1 were put into aqueous solution of Fe3+ (0.01 mol·L-1) and equal amounts of mixed Fe3+ and other metal cation, respectively. As illustrated in Fig. 8, the fluorescence intensities of ZR-1 in aqueous solutions of mixed metal 13

ACCEPTED MANUSCRIPT cations are also significantly quenched, and the Fe3-induced fluorescent quenching was nearly unaffected in the background of 1 equiv of other metal cations (Fig. S3-S13). The competition experiments demonstrate the high selectivity of ZR-1 for sensing of Fe3+ in the presence of

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other metal cations.

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Fig. 8. Emission intensities of ZR-1 in aqueous solutions of mixed metal cations. The concentrations of Fe3+ and other metal cations were 0.01 mol·L-1, respectively.

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For better understanding the fluorescence responses of ZR-1, ZR-2 and ZR-3 to Fe3+

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cations, the concentration-dependent experiments were conducted in the presence of Fe3+ with different concentrations. As illustrated in Fig. 9, the emission intensities of ZR-1, ZR-2 and ZR-3 are reduced with the increasing Fe3+ concentrations from 0 to 5×10−3 mol·L−1. Drastically, the luminescent intensities of ZR-1@Fe3+, ZR-2@Fe3+ and ZR-3@Fe3+ almost disappeared when the Fe3+ concentration reached to 5 mM. Notably, the detection limit of Fe3+ is determined as 5 µM using R6G@ZIF-8 sensors, which is comparable to or better than some previously reported fluorescence sensors for Fe3+ [41-43]. The emitting color changes

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demonstrated by Fig. 9 (insets).

Fig. 9. Luminescent intensities for ZR-1 (a and b), ZR-2 (c and d) and ZR-3 (e and f) in various concentrations of Fe3+ (K =1000). The insets in a, c and e are the emission images of ZR-1, ZR-2 and ZR-3 in water and in the presence of 5×10−3 mol·L−1 of Fe3+ ions under UV light irradiation. 15

ACCEPTED MANUSCRIPT To well understand the detailed quenching effect of Fe3+ on the R6G@ZIF-8, the quenching effect was quantitatively rationalized by the Stern-Volmer equation I0/I = 1 + Ksv[M] [44]. The values I0 and I represent the emission intensities of ZR-1 without and with addition

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of Fe3+, [M] is the concentration of Fe3+, and Ksv is the quenching constant. On the basis of the quenching experimental data, the linear correlation coefficient (R) in the Ksv curve of ZR-1 with addition of Fe3+ is calculated to be 0.98069, as shown in Fig. S14, indicating that the

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quenching effect of Fe3+ on the luminescence of ZR-1 fits the Stern-Volmer model. The Ksv

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value is calculated to be 4.01×103, suggesting a strong quenching effect on the luminescence of ZR-1 [45].

The sensing sensitivities of ZR-1, ZR-2 and ZR-3 on the Fe3+ ions were compared, and the quenching effect of the identical concentration of Fe3+ on the luminescent intensities of ZR-1, ZR-2 and ZR-3 were plotted. As depicted in Fig. 10, the emission intensities decrease in

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the order from ZR-1 to ZR-3 in each concentration of Fe3+ from 0 to 1×10-3 mol·L-1. The

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result demonstrates that ZR-1 exhibits much higher sensitivity for the sensing of Fe3+.

Fig. 10. Emission intensities of ZR-1, ZR-2 and ZR-3 in various aqueous solutions of Fe3+ (K = 1000). 16

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quite important to establish rapid, accurate and efficient methods for determination of trace chromate ions in water. To uncover the potential application of ZR-1, ZR-2 and ZR-3 for detecting anions, the luminescent responses of ZR-1, ZR-2 and ZR-3 toward the aqueous

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solutions of NaXn (1×10−2 mol·L−1, X = F−, Cl−, Br−, I−, SiO32−, SO42−, ClO4−, CO32−, NO2−

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NO3− and CrO42-), K2Cr2O7 and KMnO4 were investigated. As depicted in Fig. 11 (a and b), for the anions loaded ZR-1@X solids, the luminescent intensities at 560 nm are enhanced when the F−, Br−, Cl−, ClO4− and NO3− anions are loaded, and reduced when the anions CO32−, SO42−, SiO32−, I−,

NO2− are involved. Strikingly, the MnO4−, CrO42- and Cr2O72− anions

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exhibit the most significant quenching effect on the luminescence of ZR-1. The anions loaded ZR-2@X and ZR-3@X also show the similar fluorescence performance to ZR-1@X (Fig. 11). The result demonstrates the high sensitivity of R6G@ZIF-8 for the detection and recognition

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of MnO4−, CrO42− and Cr2O72− in aqueous solution.

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Fig. 11. Emission spectra and intensities of ZR-1 (a and b), ZR-2 (c and d) and ZR-3 (e and f) in water and various aqueous solutions of anions (K = 1000).

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To further clarify the luminescent sensitivity of R6G@ZIF-8 to trace amounts of MnO4−,

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CrO42- and Cr2O72- in water, the concentration-dependent luminescent studies were conducted in the presence of different concentrations of MnO4−, CrO42- and Cr2O72-. As shown in Fig. 12 and 13, the luminescent intensities of ZR-1 gradually decrease with the increasing concentrations of MnO4−, CrO42- and Cr2O72- from 0 to 5×10−3 mol·L−1. Remarkably, the luminescent intensities of MnO4−, CrO42- and Cr2O72- loaded ZR-1@MnO4−, ZR-1@CrO42and ZR-1@Cr2O72- almost disappeared when the content of MnO4−, CrO42- or Cr2O72- is near to 10−3 mol·L−1. The quenching effect of MnO4−, CrO42- and Cr2O72- on ZR-1 can be 18

ACCEPTED MANUSCRIPT rationalized by the Stern-Volmer equation I0/I = 1 + Ksv[M], and the calculated Ksv values are 6.7×103 (Fig. 12b), 1.11×104 (Fig. 12d) and 8.94×103 (Fig. S15), respectively. Obviously, the Ksv value toward MnO4− is slightly smaller than the ones of CrO42- and Cr2O72-, indicating that

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the luminescence quenching effect of ZR-1 on both CrO42− and Cr2O72− is more sensitive than MnO4− [42]. Emission spectra and intensities of ZR-1, ZR-2 and ZR-3 in aqueous solutions of different concentrations of Cr2O72- ions were shown in Fig. 13. Remarkably, the detection limit

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previously reported fluorescence sensor for Cr(VI) [43].

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of Cr(VI) for R6G@ZIF-8 is determined as 50 µM (Fig. 13), which is comparable to the

Fig. 12. Emission spectra and linear relationships for ZR-1 in aqueous solutions of different concentrations of MnO4− (a and b) and CrO42- (c and d).

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Fig. 13. Emission spectra and intensities of ZR-1 (a and b), ZR-2 (c and d) and ZR-3 (e and f) in aqueous solutions of different concentrations of Cr2O72- ions. Insets in (a, c and e): photographs of ZR-1, ZR-2 and ZR-3 were placed into water and aqueous solutions of Cr2O72ions (5×10−3 mol·L−1) under UV light. 20

ACCEPTED MANUSCRIPT To determine whether ZR-1 acts as a highly selective luminescent sensor for Cr2O72-, the interference of other anions (F−, Cl−, Br−, I−, SiO32−, SO42−, ClO4−, CO32−, NO2− and NO3−) on the detection of Cr2O72- was also studied. The slides with adhered ZR-1 were socked into

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aqueous solutions of Cr2O72- and equal amounts of mixed anions, respectively. As shown in Fig. 14, the relatively low interference was observed for the detection of Cr2O72- using ZR-1 in the presence of aforesaid other potential interfering anions (Fig. S16-S25). This observation

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demonstrates the high selectivity of R6G@ZIF-8 for the recognition of Cr2O72- in aqueous

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solution.

Fig. 14. Emission intensities of ZR-1 in aqueous solutions of mixed anions. The concentrations of Cr2O72- and other anions were 0.01 mol·L-1, respectively.

To evaluate the sensing sensitivity of ZR-1, ZR-2 and ZR-3 on Cr(VI), the quenching effect of the identical concentration of Cr2O72- ions on the luminescent intensities are plotted.

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ACCEPTED MANUSCRIPT As illustrated in Fig. 15, the emission intensities decrease from ZR-1 to ZR-3 in aqueous solutions of each concentration of Cr2O72- ions from 0 to 1×10-3 mol·L-1. The result

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demonstrates the higher sensitivity of ZR-1 for the sensing of Cr(VI).

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Fig. 15. Emission intensities of ZR-1, ZR-2 and ZR-3 in aqueous solutions of Cr2O72- ions (K

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= 1000).

3.7. Sensing of POMs

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Polyoxometalates (POMs), a class of inorganic metal-oxide clusters, have received considerable attention [46]. Thus far, only several examples that the luminescent MOFs are used to probe POMs in aqueous solution have been reported [40, 47]. In this regard, the luminescent R6G@ZIF-8 was utilized to sense POMs. Several POMs aqueous solutions including H4[SiW12O40]·26H2O, H3[PW12O40]·12H2O, K6[P2W18O62]·15H2O, K6[P2Mo18O62], Na9[PW9O34]·7H2O,

K14[NaP5W30-O110]·15H2O,

(NH4)6[Mo7O24]·4H2O

and

K[SiW11CoO40H3]·3H2O were selected to quench the luminescence of R6G@ZIF-8. Typically, 22

ACCEPTED MANUSCRIPT the ZR-1, ZR-2 and ZR-3 were simply treated by aqueous solution of 1×10−2 mol·L−1 POMs to give the POMs loaded ZR-1@POM, ZR-2@POM and ZR-3@POM solids. As illustrated in Fig. 16, the luminescent intensities of ZR-1, ZR-2 and ZR-3 decrease in moderate degrees they

were

immersed

in

aqueous

solutions

of

Na9[PW9O34]·7H2O

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when

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(NH4)6[Mo7O24]·4H2O. Nevertheless, other POMs solutions exhibit apparently quenching effect on ZR-1, ZR-2 and ZR-3 solids. The result indicates that the ZR-1, ZR-2 and ZR-3 can

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be considered as potential fluorescent probes for POMs in aqueous solution.

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ACCEPTED MANUSCRIPT Fig. 16. Emission spectra of ZR-1 (a and b), ZR-2 (c and d) and ZR-3 (e and f) with the addition of 1×10-2 mol·L-1 of various POMs in aqueous solutions (K = 1000).

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3.8. Sensing of volatile organic molecules (VOMs) Currently, the VOMs are increasingly concerned because of their environmental hazards [48, 49]. Thus, the luminescent R6G@ZIF-8 materials are expected as luminescent platform for probing VOMs. To investigate the luminescent recognition of VOMs, the luminescent

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experiments of the suspensions of R6G@ZIF-8 are performed. Typically, the fine ground

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samples of ZR-1, ZR-2 and ZR-3 (3 mg) were dispersed in different organic solvents (3 mL), including methanol, ethanol, acetone, acetonitrile, propanol, THF, chloroform and aniline. Subsequently, the suspensions were ultrasonicated for 30 min for fluorescence studies. As depicted in Fig. 17, the luminescent intensities of ZR-1, ZR-2 and ZR-3 are highly dependent on solvent molecules. The organic solvents exhibit different degrees of quenching effects on

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the luminescent intensities of ZR-1, ZR-2 and ZR-3, particularly with regard to aniline, which shows the most serious quenching effect. Thereby, the R6G@ZIF-8 could serve as potential

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luminescent sensors for detecting aniline.

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Fig. 17. Luminescent spectra and intensities of ZR-1 (a and b), ZR-2 (c and d) and ZR-3 (e

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and f) in different organic solvents (K = 1000).

To further evaluate the ability of R6G@ZIF-8 materials for sensing a trace quantity of

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aniline, a series of suspensions were prepared via dispersing 3 mg of ZR-1, ZR-2 and ZR-3 into 3 mL of acetone with the aniline concentrations from 0 to 2000 ppm (0.02 M). As shown in Fig. 18, the luminescent intensities of the ZR-1, ZR-2 and ZR-3 suspensions decreased gradually with the increasing quantity of aniline. The emissions of the ZR-1, ZR-2 and ZR-3 suspensions almost disappeared when the concentration of aniline reached to 2000 ppm. Strikingly, the detection limit of aniline is determined as 500 ppm (5 mM) for R6G@ZIF-8, suggesting that R6G@ZIF-8 is a potential luminescent probe for the aniline. As shown in Fig. 25

ACCEPTED MANUSCRIPT S26, the linear correlation coefficient R in the Ksv curve of ZR-1 with addition of aniline is 0.95297, suggesting that the quenching effect of aniline on the fluorescence of ZR-1 fits the Stern-Volmer model. The Ksv value is calculated to be 0.73×103, demonstrating a strong

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quenching effect on the luminescence of R6G@ZIF-8.

Fig. 18. Luminescent spectra and intensities of ZR-1 (a and b), ZR-2 (c and d) and ZR-3 (e and f) in acetone with various aniline concentrations from 0 to 2000 ppm (K = 1000). Inset in (a, c and e): photographs of ZR-1, ZR-2 and ZR-3 in acetone and acetone solution of aniline

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To confirm the effective aniline detection for R6G@ZIF-8, the competition experiments were further conducted in the presence of other solvents. The suspensions of ZR-1 were

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achieved by soaking the finely ground samples of ZR-1 (3 mg) into equal amounts of mixed solvents (3 mL), respectively. There are no apparent emission intensity changes for ZR-1 upon mixing aniline with the equal amounts of other solvents, as illustrated in Fig. 19. In other

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words, the quenching selectivity towards aniline is nearly not interfered by the addition of

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other solvents (Fig. S27-S33). The result suggests that R6G@ZIF-8 can be used as a highly

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selective sensor for aniline over other organic solvents.

Fig. 19. Emission intensities of ZR-1 in various equal amounts of organic solvents.

To understand the sensing sensitivity of ZR-1, ZR-2 and ZR-3 on the aniline in acetone, the quenching effects of the identical concentration of aniline on the luminescent intensities are plotted. As depicted in Fig. 20, the emission intensities gradually reduce from ZR-3 to

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ZR-2.

Fig. 20. Emission intensities of ZR-1, ZR-2 and ZR-3 in acetone solution of aniline (K =

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1000).

3.9. Recovery cycles for the luminescent sensing

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It is noticeable that the sensing ability of ZR-1 could be regenerated and reused even after several cycles. Emission spectra of ZR-1 in quenching and recovery tests of Fe3+, Cr2O72- or

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aniline were shown in Fig. 21. The luminescent intensities of ZR-1 in aqueous solutions of Fe3+, Cr2O72- or acetone solvent of aniline after five cycles of quenching and recovery does not decrease compared with their initial states. The result demonstrates that R6G@ZIF-8 has high stability for repeated usage in luminescent detection of Fe3+, Cr2O72- or aniline.

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Fig. 21. Quenching and recovery test of ZR-1 in Fe3+ (a and b), Cr2O72- (c and d) and aniline in acetone solution (e and f). The upper symbols in (b), (d) and (f) represent the initial luminescent intensity and the lower symbols represent the intensity on the addition of Fe3+, Cr2O72- or aniline.

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ACCEPTED MANUSCRIPT 3.10. Luminescent sensing process The possible sensing process for the luminescence quenching by the metal ions, anions and organic small molecules have been further speculated. The UV-vis absorption spectra for

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ZIF-8, R6G@ZIF-8, metal ions, anions and organic small molecules were recorded to identify the fluorescence quenching process. For R6G@ZIF-8, the strong absorption bands range from 200 to 250 nm and the weak absorption bands are in the range of 450-580 nm (absorption band

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of R6G), as illustrated in Fig. S34. The Fe3+ ions in aqueous solution exhibit a strong

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absorption band from 200 to 400 nm, but relatively weak absorption bands from 200 to 275 nm were found for the remaining metal ions in aqueous solutions (Fig S35). Clearly, the absorption bands of R6G@ZIF-8 are completely mantled by the Fe3+ ions in aqueous solution, which leads to competition with mim ligands for the absorption of light energy and results in a

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little energy transfer from mim to R6G [41]. Finally, a drastic decrease in luminescence intensities or even quenching occurs. Nevertheless, the rest of metal ions only partially overlap the absorption band of R6G@ZIF-8, and exhibit a little influence for the energy transfer and

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luminescent intensity of R6G@ZIF-8 [50]. Similarly, the luminescent responses induced by

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anions and organic small molecules were also demonstrated by the UV-vis spectra (Fig. S36 and S37).

4. Conclusion

In summary, the R6G-loaded R6G@ZIF-8 nanomaterials (ZR-1, ZR-2 and ZR-3) exhibit strong luminescence with broad emission band in the visible light region. The systematic luminescent investigation demonstrates that R6G@ZIF-8 nanomaterials are excellent

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ACCEPTED MANUSCRIPT candidates as potential multifunctional luminescent probes for sensing of metal cations, anions, POMs ions and VOMs, especially for Fe3+, Cr(VI) and aniline through fluorescence quenching. Moreover, the R6G@ZIF-8 nanomaterials as luminescent probe exhibit simple preparation

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procedure, fast detection time, excellent selectivity, and high sensitivity with a low detection limit of 5 µM, 50 µM and 5 mM for Fe3+, Cr6+ and organic aniline, respectively. Remarkably, the luminescence quenching and recovery tests show that the R6G@ZIF-8 nanomaterials are

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reusable for the sensing of Fe3+, Cr6+ and organic aniline. The present study provides a facile

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route for the design of luminescent hybrid nanocomposites as luminescent probing materials to develop practical applications, and further studies are currently under way in our group.

Acknowledgements

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We thank the Natural Science Foundation of China (Grant No. 21471029, 21301026, 21277022 and 21371030) and the Open Funds for Key Lab of Polyoxometalate Science of

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ACCEPTED MANUSCRIPT Highlights ► The Rhodamine 6G (R6G) loaded zeolitic imidazolate framework-8 (ZIF-8) nanocomposites were prepared and well characterized. ► The R6G@ZIF-8 nanocomposites exhibit highly selective and sensitive luminescent sensing of Fe3+,

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CrVI and aniline. ► The R6G@ZIF-8 nanomaterials are reusable for luminescent

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detection of Fe3+ and Cr(VI).