Highly Selective and Sensitive Turn-Off-On Fluorescent Probes for

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Highly Selective and Sensitive Turn-Off-On Fluorescent Probes for Sensing Al3+ Ions Designed by Regulating the ESIPT Process in Metal-Organic Frameworks Yong-Peng Li, Xiao-Han Zhu, Shuni Li, Yucheng Jiang, Mancheng Hu, and Quanguo Zhai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20410 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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ACS Applied Materials & Interfaces

Highly Selective and Sensitive Turn-Off-On Fluorescent Probes for Sensing Al3+ Ions Designed by Regulating the ESIPT Process in Metal-Organic Frameworks Yong-Peng Li, Xiao-Han Zhu, Shu-Ni Li*, Yu-Cheng Jiang, Man-Cheng Hu, Quan-Guo Zhai* School of Chemistry & Chemical Engineering, Key Laboratory of Macromolecular Science of Shaanxi Province, Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, Shaanxi Normal University, Xi’an, Shaanxi, 710062, P. R. China.

ABSTRACT The concept of high-performance ESIPT-based (excited state intramolecular proton transfer) fluorescent MOF (metal-organic framework) probes for Al3+ is proposed in this work. By regulating the hydroxyl groups on the organic linker step-by-step, new fluorescent magnesiumorganic framework (Mg-MOF) probes for Al3+ ions were established based on excited state intramolecular proton transfer fluorescence mechanism. It is observed for the first time that the number of intramolecular hydrogen-bonds between adjacent hydroxyl and carboxyl groups can effectively adjust the ESIPT process and leads to tunable fluorescence sensing performance. Together with the well-designed porous and anionic framework, Mg-TPP-DHBDC probe decorating with a pair of intramolecular hydrogen-bonds exhibits extra-high quantitive fluorescence response to Al3+ through an unusual turn-off (0-1.2 µM) and turn-on (4.2-15 µM) luminescence sensing mechanism. Notably, the 28 nM LOD value represents the lowest record

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among all reported MOF-based Al3+ fluorescent sensors up to now. Benefited from the unique turn-off-on ESIPT fluorescence detection process, Mg-TPP-DHBDC MOF sensor exhibits single Al3+ detection compared with other sixteen common metal ions including Ga3+, In3+, Fe3+, Cr3+, Ca2+ and Mg2+. Impressively, such Al3+ selective sensing process even can be fulfilled by the reusable MOF test paper detected by naked eyes. Overall, the quantitive Al3+ detection, together with the extraordinary sensitivity, selectivity, fast response and good reusability strongly support our concept of ESIPT-based fluorescent MOF Al3+ probes and make Mg-TPPDHBDC one of the most powerful Al3+ fluorescent sensors.

KEYWORDS Fluorescent probes; Al3+ detection; Metal-organic frameworks (MOFs); Excited-state intramolecular proton transfer (ESIPT); Turn-off-on fluorescence

1. INTRODUCTION As the third most abundant metallic element, aluminum is extensively involved in many civil and industrial process.1 Specially, Al3+ ion existing in water and plants may enter the human body through foods and drinking water.2 The excessive accumulation of aluminum in body will lead to illnesses like Alzheimer’s disease, Parkinsonism dementia and so on.3-4 Notably, 3-10 mg/day and 7 mg/kg body weight for aluminum are the tolerable daily and weekly ingestion according to the WHO claim.5 Therefore, finding an effective analytical method to detect Al3+ has attracted increasing interest in the environmental and medical field.


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Among various detection methods, the fluorescent sensors have lately attracted significant interest because of their advantages of easy operation, high sensitivity, and real-time monitoring with rapid response.6-14 Different to other metal ions, Al3+ ion not only has strong hydration ability and poor coordination character, but also lacks spectroscopic characteristics, which severely hinder the development of suitable fluorescence sensors.15-17 Taking the hard acidic character into account, hard donor sites like N and O are usually selected as coordination atoms to design Al3+ sensors. Therefore, most common fluorescent probes for the detection of Al3+ ions are incorporated with nitrogen or oxygen donor sites such as Schiff bases, hydroxyflavone and 8-hydroxyquinoline.18-20 Recently, ESIPT (excited state intramolecular proton transfer) molecules are of great interesting for the design of new Al3+ probes21-22 because of their ultra-fast reaction rate and huge fluorescence Stokes shifts. In general, the ESIPT process requires adjacent proton donor (– OH or –NH2) and proton acceptor (–C=O or –N=) groups to generate the intramolecular hydrogen bond.23-26 The enol isomer in lower energy in the electronic ground state will undergo a proton transfer reaction upon excitation to the excited state.27-30 Upon irradiation, these molecules produce the keto forms as ESIPT tautomers, which show stronger fluorescence at longer

wavelength

compared

with

the

phenol

forms.

For

example,

2-(2-

hydroxyphenyl)benzothiazole (HBT)31, 2-(2’-hydroxyphenyl)benzoxazole (HBO)32, and 2hydroxybenzcarbaldehyde-(2-methylquinoline-4-formyl)hydrazone33 have been reported as fluorescent sensors for Al3+. However, most of these reported molecules are of similar Al3+ sensing performance due to their quite similar sensing mode, and enhanced selectivity and sensitivity is desired for ESIPT-based Al3+ probes.


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Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) built from metal ions or metal clusters and organic ligands represent a new class of porous materials with tunable architectures and pore sizes.34-36 Usually, the active functional groups on the pore surface can provide specific and unique molecular recognition between MOF frameworks and guest molecules, and thus promote the sensing performance.37-40 However, scarce metalorganic frameworks as chemosensors for selective detection of Al3+ at trace concentration level have been established although diverse MOFs display interesting luminescence properties.16 Searching for MOF-based Al3+ sensor exhibiting high selectivity and sensitivity, rapid response and recyclable performance is a very challenging issue. Out of the above considerations, we speculate that the incorporation of ESIPT mechanism into porous crystalline metal-organic framework materials would open a promising platform for the design of new high-performance ESIPT-based Al3+ MOF probes. The universal compatibility and modifiability of organic linkers in MOFs provide enormous opportunities to create the intramolecular hydrogen bond between proton donor and proton acceptor groups, which is a necessary condition for ESIPT process. Furthermore, the tunable porous characters and framework charge of MOFs may largely enhance the selectivity and sensitivity for ESIPT-based sensors. Also, the high stability of MOF materials will thus improve the reusability of sensors. To the best our knowledge, only AEMOFs41, LIFM-CL142, LIFM-2243, LIFM-2344, LIFM-42(Ln)45 and {Mg(DHT)(DMF)2}n46 exhibiting ESIPT-based luminescence have been reported more recently as fluorescence sensors for polar protic solvents. The ESIPT-based MOF fluorescence sensor for metal ions is rarely unexplored47-48 and no any Al3+ probes established to date.


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In this work, pacs-Mg-MOF platform49 constructed from [Mg3(OH)(CO2)6] cluster, linear ditopic 1,4-dicarboxybenzene (BDC) and triangular tripyridine linkers (TPP) is utilized to demonstrate our concept of ESIPT-based MOF probes for Al3+ (Figure 1). The following functions are supposed to be integrated into the Mg-TPP-BDC platform to achieve highperformance MOF sensors: (a) proton donor –OH or –NH2 groups can be easily introduced stepby-step (0, 1 or 2) through the BDC components to form the intramolecular hydrogen bonds with proton acceptor carboxylate groups; (b) the closed d subshell of Mg center has relatively low-lying HOMOs and high-lying LUMOs, which will preclude the participation in luminescence and make Mg-MOFs possess similar properties as the organic ligand chromophore, that is, Mg centers will not hinder the ESIPT process; (c) small and tunable pore size (about 4.5 Å) in pacsMOFs can effectively improve the selectivity for Al3+; (d) anionic character (-2) of whole MgMOF framework may highly promote the recognition between MOF framework and Al3+ ions; (e) 9-connected net of pacs-Mg-MOF can improve the framework stability and thus ensure the reusability of MOF probes. As a consequence, with the help of two intramolecular hydrogen-bonds in one linker (DHBDC), the Mg-TPP-DHBDC MOF sensor exhibits a unique turn-off (0-1.2 µM) and turn-on (4.2-15 µM) ESIPT-based fluorescence response to Al3+ with the lowest record LOD value (28 nM) among all reported MOF-based Al3+ fluorescent sensors up to now. Except for extraordinary sensitivity, the ESIPT-based fluorescent MOF probes for Al3+ show fast response, high selectivity, and good reusability. More importantly, the highly desirable naked eye detection of trace level Al3+ ions can also be realized by Mg-TPP-DHBDC MOF fluorescent probe herein.


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Figure 1. The schematic of step-by-step design for ESIPT-based fluorescent MOF probes in this work.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods All chemicals were purchased and directly used. The powder X-ray diffraction patterns (PXRD) were measured by Rigaku DMAX 2500 powder diffractometer using Cu-Kα (λ = 1.54056Å) operating at 40 kV and 100 mA with the scanning rate of 0.2 s/step at room temperature. Fourier-transform infrared spectroscopy (FT-IR) were tested in a Bruker Tensor 27 instrument and the region 400-4000 cm-1 were recorded with KBr pellets. The fluorescence spectra were measured with a Fluoromax-4 spectrophotometer at room temperature. 2.2. Preparation of Mg-MOFs


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[(CH3)2NH2][Mg3(OH)(TPP)(BDC)3] (Mg-TPP-BDC). Mg-TPP-BDC was synthesized according to our formerly reported method for pacs-MOFs with slightly modification.49 Briefly, a molar ratio of 4:2:1 mixture of Mg(NO3)2·6H2O, therephalic acid (BDC) and 2,4,6-tri(4-pyridyl)pyridine (TPP) was sonicated and dissolved in a 4:2:0.3 (v/v/v) mixture of dimethylacetamide (DMA), 1,3dimethyl-tetrahydropyrimidin-2(1H)-one (DMPU) and formic acid. The final mixture was heated to 130 oC for 5 days. Pure sample was obtained by filtering and washing the raw product with hot DMA/DMPU mixtures. The yield was about 42% based on Mg. IR data (KBr pellet, cm-1): 3071(w), 2946(w), 1647(s), 1401(s), 1341(s), 1237(m), 1058(w), 1014(w), 959(w), 828(m), 762(m), 632(w), 522(w). [(CH3)2NH2][Mg3(OH)(TPP)(OHBDC)3] (Mg-TPP-OHBDC). The synthetic steps for Mg-TPPOHBDC is similar to that for Mg-TPP-BDC. Except for equal proportion of 2-hydroxybenzene-1,4dicarboxylic acid (OHBDC) was used in the synthesis process. The yield was about 49% based on Mg. IR data (KBr pellet, cm-1): 3410(m), 2973(w), 2886(w), 1625(s), 1369(s), 1221(w), 1041(m), 878(w), 812(w), 746(m), 632(w), 528(w). [(CH3)2NH2][Mg3(OH)(TPP)(DHBDC)3] (Mg-TPP-DHBDC). The synthetic steps for Mg-TPPDHBDC is similar to that for Mg-TPP-BDC. Except for equal proportion of 2,5-dihydroxybenzene1,4-dicarboxylic acid (DHBDC) was used in the synthesis process. The yield was about 53% based on Mg. IR data (KBr pellet, cm-1): 3438(s), 3050(w), 2920(w), 1609(s), 1506(s), 1452(s), 1370(m), 1244(m), 1113(w), 1052(w), 900(w), 803(m), 627(w). 2.3. Fluorescence sensing experiments The fluorescence of BDC, OHBDC, DHBDC, Mg-TPP-BDC, Mg-TPP-OHBDC and Mg-TPP-DHBDC was systematically investigated in different solvents to verify their ESIPT-based luminescence


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character.50-54 Dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), ethanol (EtOH), acetonitrile (CH3CN), 1,4-dioxane (1,4-DO), cyclohexane (CYH) and dichloromethane (CH2Cl2) were selected in this work. Basically, 5 mg micrometer-size Mg-MOF sample was well-dispersed in 5 mL solvents in a 10 mL vial through ultrasonic agitation for about 30 minutes. The 3 mL dispersion was transferred into the quartz cuvette to conduct the photoluminescence experiment. The solid-state fluorescence spectra of Mg-MOFs were also measured (Figure S2). To exclude the mechano-responsive and TPP luminescent, the contrast experiments have also been conducted (Figures S3 and S4). In order to further evaluate the fluorescence sensing performance of corresponding ligands and Mg-MOFs towards Al3+, all solid samples (50.00 mg) were added into 50 mL DMA and ultrasonicated for 1 h to get the well-dispersed suspension of probes. Al3+ solution were prepared by dissolving Al(NO3)3 in DMA to obtain the desired concentrations (0.001 mol L-1). The Al3+ ion sensing experiments were performed by adding diverse amounts of above Al3+ solution to a quartz cuvette containing 3 mL of DMA suspension of probes and then detected by the PL spectra. The emission intensity was monitored after each addition. For all the fluorescence experiments, the emission spectra were recorded in the range of 380-690 nm with 362 nm excitation and excitation/emission slit widths was 3.0 nm. The selectivity of Mg-TPP-DHBDC probe between Al3+ and other metal ions was carefully investigated. Li(NO3), Mn(NO3)2, Zn(NO3)2, Cd(NO3)2, Co(NO3)2, Ni(NO3)2, Ca(NO3)2, Mg(NO3)2, Pb(NO3)2, Cr(NO3)3, Ga(NO3)3, In(NO3)3, Fe(NO3)3, Nd(NO3)3, Er(NO3)3 and Y(NO3)3 were dissolved in DMA to obtain the desired metal ion solution (0.001 mol L-1). The sensing


8

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experiments of the Mg-TPP-DHBDC probe to different metal ions were performed by adding 30 µL solutions to 3 mL Mg-TPP-DHBDC suspension and measured by the PL spectra. The reusability of Mg-TPP-DHBDC probe towards Al3+ ion was evaluated using the solution and solid phase sensing experiments. The original MOF crystal samples were dried by flowing nitrogen gas and grinded with Al3+ analyst. The luminescent colors of the crystal changed from yellow green to blue under the irradiation of UV light of 365 nm. After response Mg-TPP-DHBDC were subjected to solvent exchange by soaking into fresh DMA for 2 days and exchanged for 5 times, and then the sample was N2-dried again. The sample can return original luminescent color upon the UV light irradiation (365 nm), which also can response to Al3+ solution similar the first time by our naked eyes through the distinguishing colors. To investigate the practical application of proposed Mg-TPP-DHBDC sensor, the paper test strips for rapid on-site naked eye detection of Al3+ were developed. DMA solution of micrometer-size Mg-TPP-DHBDC (3 mg/mL) was firstly prepared by the grounded MOF sample dispersed in DMA by ultrasonic agitation. Then the filter paper is placed on a Buchner funnel to filter the Mg-TPP-DHBDC solution, and the resulting filter paper was dried and divided as paper test strips for Al3+.

3. RESULTS AND DISCUSSION 3.1. Synthesis and characterizations of Mg-MOF probes By a solvothermal method, Mg(NO3)2·6H2O, ditopic carboxylate linkers (BDC, OHBDC, DHBDC) and TPP produced three-dimensional Mg-MOF crystal probes. The structure of these MOFs was affirmed by the powder X-ray diffraction (PXRD) results. As shown in Figure 2 and S1, the


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diffraction peaks of the as-synthesized Mg-TPP-BDC, Mg-TPP-OHBDC and Mg-TPP-DHBDC samples matched well with that simulated patterns from the single-crystal data, indicating their structural type of pacs-MOFs49 and high phase purity.

Figure 2. Simulated and experimental PXRD patterns for Mg-MOF probes in this work. In these Mg-MOFs, three magnesium atoms, one µ3-OH and six bridging carboxylate groups firstly generate a [Mg3(OH)(CO2)6] trinuclear building blocks, which are linked by six ditopic carboxylate linkers to give a MIL-88 framework with 1D hexagonal channel along the c-axis direction. TPP ligands then insert into the channel to form anionic pacs-MOF framework with 1D channel partitioned into cylindrical cage of about 10.4 Å × 4.5 Å (Figure 1). The most characteristic feature of these Mg-MOFs is the easy functionalizability on the windows of the cages, which provide ideal molecular recognition sites for the sensing function of MOFs. As stated above, the intramolecular hydrogen bond between proton acceptor and proton donor groups is generally necessary for the ESIPT process. In these Mg-MOFs, dicarboxylate linkers provide proton acceptor (-C=O), and the utilization of OHBDC and DHBDC can introduce one and two -OH as proton donor groups. So, by regulating the hydroxyl groups on the organic


10

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linker step-by-step, the intramolecular hydrogen bonds can be tuned and therefore may evoke excited state intramolecular proton transfer (ESIPT) fluorescence for these Mg-MOFs. Together with the small window sizes and anionic framework, these Mg-MOFs should have good ESIPTbased fluorescence performance for Al3+ ions. 3.2. Verification of the ESIPT-based luminescence character of Mg-MOF probes

(a)

(b)

(c)

(d)

(e)

Figure 3. Photographs of the organic linkers and Mg-MOF probes in different solvents under UV-light irradiation: (a) BDC; (b) OHBDC; (c) Mg-TPP-OHBDC; (d) DHBDC; (e) Mg-TPP-DHBDC. In order to verify the our above supposed ESIPT-based luminescence character of Mg-MOF probes, the fluorescence of Mg-TPP-BDC, Mg-TPP-OHBDC and Mg-TPP-DHBDC was systematically investigated in a series of polar protic solvents. For comparison, the corresponding organic linkers, BDC, OHBDC and DHBDC were also investigated.


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As depicted in Figure 3a, BDC linker without -OH group did not show emission changes when dissolved in different solvents and the fluorescence is very weak. Once the -OH group is introduced, the organic linkers and corresponding Mg-MOFs all expressed emission shifts with

(a)

(b)

(c)

(d)

(e) Figure 4. Emission spectra of the organic linkers and Mg-MOF probes in different solvents: (a) OHBDC; (b) Mg-TPP-OHBDC; (c) DHBDC; (d) Mg-TPP-DHBDC, and CIE chromaticity diagram (e) showing the color coordinates of Mg-TPP-DHBDC.


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the change of solvent types, which can be detected by naked eyes (Figures 3b-3e). Furthermore, the solvent-dependent emissions of these compounds were further verified by PL spectra. Compared with OHBDC and resulting Mg-MOF with one -OH group, DHBDC and Mg-TPP-DHBDC with a pair -OH proton donors exhibit more prominent solvent-dependent fluorescence. This clearly verify that the number of intramolecular hydrogen-bonds between adjacent hydroxyl and carboxyl groups can effectively adjust the ESIPT process and leads to tunable fluorescence emissions in different solvents. Moreover, the emissions of DHBDC and Mg-TPP-DHBDC demonstrate that the incorporation of ESIPT-ligands into MOF can effectively improve their fluorescence performance. This provide an important route for the further design of new ESIPTbased fluorescent materials in the future. In detail, Mg-TPP-DHBDC exhibits green emission at 518 nm in polar solvent DMSO, emission at 497 nm in protic solvent ethanol, while emission around 430 nm for those aprotic solvents such as acetonitrile, 1,4-dioxane, cyclohexane and CH2Cl2 due to the excellent excited state intramolecular proton transfer process. Further, after immersed in DMA, the main solvent used for the MOF synthesis, Mg-TPP-DHBDC showed strong green fluorescence with an emission peak at 505 nm. These fluorescence emission changes match well with the CIE chromaticity diagram (Figure 4e) and above clearly and directly observed colors by naked eyes (Figure 3e). 3.3. Fluorescence sensing performance of Mg-MOF probes for Al3+ ions Above solvent-dependent ESIPT-based luminescence character of Mg-MOFs further encourage us to study their potential application as Al3+ ion fluorescence sensors. As described in Figure 5, two ESIPT-ligands and corresponding Mg-MOFs all exhibit interesting turn-off-on


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(a)

(b)

(c)

(d)

(e)

(f)

(g) (h) Figure 5. Fluorescence sensing performance of the organic linkers and Mg-MOF probes towards Al3+ (a, c, e g) and corresponding CIE chromaticity diagram showing the color coordinates (b, d, f, h): (a and b) OHBDC; (c and d) DHBDC; (e and f) Mg-TPP-OHBDC; (g and h) Mg-TPP-DHBDC.


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fluorescence sensing performance with the addition of Al3+ ions. Specially, Mg-TPP-DHBDC showed extremely high sensitivity compared to the reported Al3+ probes. Figure 5g shows that Mg-TPP-DHBDC in pure DMA exhibits a maximum emission band around 505 nm upon excitation at 367 nm. Upon the addition of Al3+ ions from 0 to 1.2 µM, the fluorescence intensity continuously decreased. When the Al3+ ion was further enhanced, a clear blue-shift was observed. Notably, the position and intensity of emission are unstable with the concentrations of Al3+ varying from 1.2 to 4.2 µM. Finally, the emission peak keeps at 460 nm and the intensity increases with Al3+ concentration reaches 15 µM (Figure 5g). Compared to the original fluorescent emission of Mg-TPP-DHBDC in DMA, the fluorescence turns off firstly and then turns on by trace of Al3+ additive, and apparent blue shifts of 45 nm was observed accompanied with enhanced fluorescence intensity. We attribute the significant luminescence variation of Mg-TPP-DHBDC to the strong bonding of Al3+ ions toward -OH groups of DHBDC ligand with inhibition ESIPT process. When the sample of Mg-TPP-DHBDC (5 mg) in a DMA (5 mL) solution was immersed of Al3+ (0.001 mol L-1), the immediately color change from green to blue was able to identify by human eye under 356 nm irradiation (Figure 6c inset). Also, according to the CIE chromaticity diagram, the CIE coordinates change from the green region (0.21, 0.40) to the blue region (0.17, 0.22) (Figure 5h) with the addition of Al3+ ions. Furthermore, we explored the quantitative detection ability of Mg-TPP-DHBDC towards Al3+. A quantitative experiment was carried out with the gradual addition (0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 3, 4.2, 5.4, 6.6, 7.8, 9, 10.2, 11.4, 12.6, 13.8 and 15 µM) of Al3+ ion solution to the Mg-TPPDHBDC suspension. The fluorescence intensity has a clear linear relationship with the Al3+


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concentration for Mg-TPP-DHBDC sensor apart from the negligible unstable emission stage (Figure 6). Further, the Stern-Volmer equation: I/I0 = 1 + Ksv[Al3+] was used to analyze and the results show that the fluorescence intensity was closely related to the concentration of Al3+. During the fluorescence quenching stage, I0 is defined as the intensity in pure DMA and [Al3+] ranges from 0 to 1.5 uM. But for the fluorescence enhancing stage, I0 is defined as I1.5uM, which represents the intensity with [Al3+] of 1.5 µM.

Figure 6. Linear relation between the fluorescence quenching (0-1.2 µM) and enhancing (4.2-15 µM) efficiency and the Al3+ concentration for Mg-TPP-DHBDC probe (inserted: the photos showing the turn-off and turn-on fluorescence triggered by Al3+). Both calibration curves show perfect linear relationship with the correlation coefficient R2 greater than 0.99. As shown in Figure 6, the emission position and intensity both are unstable when the [Al3+] varies from 1.5 to 4.2 µM. The Ksv values of Stern-Volmer constants are of 2.422 × 105 and 3.6685 × 105 M-1, which indicate ultra-high quenching and enhancing efficiencies for Mg-TPP-DHBDC probe. Notably, these Ksv values are much higher than nearly all


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reported fluorescent Al3+ MOF sensors up to now (Table 1), demonstrating an extraordinary sensitivity of Mg-TPP-DHBDC for the detection of Al3+ ion. In our opinion, the enhanced sensitivity of MOF probe should be achieved by preconcentration of analyte (Al3+) through the inherent porosity and anionic framework of our MOF probe. Meanwhile, unlike the regular organic species, the coordination geometry and pore confinement of MOFs will prohibit the aggregation of functional groups. Also, MOFs can retain the intrinsic properties of each functional group. For Mg-TPP-DHBDC, the -OH groups were designed as the only residue active sites to act with Al3+. But for the free DHBDC ligand, -OH and -COOH groups both can coordinate with Al3+ and thus lead to uncontrollable and unpredictable results. Table 1. Sensing performance comparison of reported fluorescent MOF sensors for Al3+ ion. Fluorescence probe

Detection process

Detection

Linear range (µM)

Ksv (M-1)

Ref

0-1.2 and 1.8-15

2.42×105

This work

0-8

1.127×105

[57] [58]

limits (µM) Mg-TPP-DHBDC [Zn2(HL)3]+@MOF-5

TURN-OFF-ON TURN-ON

0.028 -

[Eu(BTB)(phen)]

TURN-OFF

0.05

0-500

1.59×104

[Ba3La0.5(µ3-L)2.5(H2O)3(DMF)]

TURN-OFF

0.19

5-60

1.445×104

[59]

[Cd(PAM)(4-bpdb)1.5]

TURN-OFF

0.56

0-50

2.3×104

[60]

[Co2(dmimpym)(nda)2]n

TURN-ON

0.7

-

-

[61]

[Tb3(TCA)2(DMA)0.5(OH)3(H2O)0.5]

TURN-ON

0.7

1-1000

-

[62]

5-45

1.33×104

[63]

-

[56]

[Zn(DMA)(TBA)]

TURN-ON

1.97

[Co(OBA)(DATZ)0.5(H2O)]

TURN-ON

2.13

NUM-2

TURN-ON

3.7

10-1000

-

[64]

UiO-66-NH2-SA

TURN-ON

6.98

10-500

-

[55]

[Eu2.5(BTB)3(OAc)0.5(H2O)3]

TURN-OFF

100

100-700

-

[65]

[Eu(H2O)2(BTMIPA)]

TURN-OFF

1000

0-10000

3.79×104

[66]

[Zn2(1,4-ndc)2(3-abpt)]

TURN-ON

-

-

6.98×104

[67]

[Cd(1,4-ndc)(3-abit)]

TURN-ON

-

-

3.84×104

[67]

[Tb(OBA)2]·(Hatz)·(H2O)1.5

TURN-ON

-

500 to 10000

-

[68]

Based on 3σ/slope, Mg-TPP-DHBDC exhibits superior sensitivity for Al3+ with the detection


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limit as low as 28 nM (0.76 ppb) determined from the quenching fluorescence profile of 0-1.2 µM, which is clearly lower than the limit of the U.S. Environmental Protection Agency (EPA) (200 ppb). To the best of our knowledge, this limit of detection (LOD) is the lowest value among the reported MOF-based fluorescent Al3+ sensors (Table 1). LOD value manifests sensitivity of the probe towards the analyte and the lower the value, the better is the sensitivity. Although some MOF sensors for Al3+ ion via luminescence turn-on or turn-off process have been reported, to the best of our knowledge, Mg-TPP-DHBDC is the first combination of negative and positive fluorescent response MOF-based probe which allowed the naked eye to see a broad range of Al3+.55-56 In addition, the clear change of fluorescence intensity is observed within 35s, which is a very short response time (Figure S7). The reusability of Mg-TPP-DHBDC in DMA were also evaluated in solution detection, after more than three continuous response and recovery experiments, Mg-TPP-DHBDC probes can keep the same Al3+ sensing performance distinguished by naked eyes (Figure S5). On the other hand, PXRD patterns have unambiguously demonstrated that the crystallinity and framework integrity of Mg-TPP-DHBDC can be well retained after the detection of Al3+ ions (Figure 2). 3.4. Selectivity of Mg-TPP-DHBDC probe for Al3+ from other metal ions The selectivity of fluorescence sensors is an important indicator for their practical applications. We further evaluated the selectivity of Mg-TPP-DHBDC probe for Al3+ from other metal ions. The examined alkali and d10 transition metal of Li+, Mg2+, Ca2+, Zn2+, Cd2+ and the group IIIA metal of Y3+, Ga3+ exhibited slight changes relative to Mg-TPP-DHBDC, whereas other metal ions Mn2+, Pb2+, Co2+, Ni2+, Cr3+, Fe3+, Nd3+ or Er3+ quench the emission intensity (Figure 7). Interestingly, Al3+ and In3+ showed distinct behavior with a new peak appeared at 460 nm. To


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this blue shift, Al3+ was also accompanied by enhanced fluorescence intensity. Overall, Mg-TPPDHBDC probe shows extra-high selectivity for Al3+ ions over other metal ions. The color change of a chemosensor in the presence of an analyte is significantly useful performance for the naked eye detection. We further checked the color change of solution containing the Mg-TPP-DHBDC chemosensor in the presence of different metal ions under 365 nm of UV light. As shown in Figure 7c, Mg-TPP-DHBDC in the presence of Al3+ could produce blue coloration under UV light, which is significantly different from the light produced by other metal ions. Clearly, Mg-TPP-DHBDC could be used for the naked eye detection of Al3+.

(a)

(b)

(c) Figure 7. Fluorescence emission spectra (a) and comparison of the luminescence intensities (b) for Mg-TPP-DHBDC probe towards different metal ions (λex = 367 nm) and the corresponding images (c) under UV-light irradiation at 365 nm.


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The competitive experiments further demonstrate the selectivity of Mg-TPP-DHBDC to Al3+. The Mg-TPP-DHBDC in DMA system was firstly added with other sixteen common metal ions (Cr3+, Fe3+, Er3+, Nd3+, Co2+, Ni2+, Pb2+, Mn2+, Li+, Zn2+, Y3+, Ga3+, Cd2+, In3+, Ca2+ and Mg2+) and then Al3+ was added. As depicted in Figure 8, the competition metal ions produce little change for the detection of Al3+. In our opinion, the high selectivity of Mg-PP-DHBDC to Al3+ might attribute to the fact that Al3+ possesses the smaller ionic radii (0.54 Å), greater charge density (r = 4.81), larger ionic potential (ф = 0.06) and the bigger hardness parameters (Ƞ = 45.8) compared to other investigated metal ions. Taking the hard sites of -OH groups on the pore surface and the small cage window into account, the smaller ionic radii and hardness of Al3+ have been enabled the suitable coordination mode and strong coordination between -OH and Al3+.

Figure 8. Luminescence intensities of Mg-TPP-DHBDC upon addition of Al3+ in the existence of other metal ions in DMA.


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3.5. Mg-TPP-DHBDC paper test strips for rapid on-site detection of Al3+ ions To further explore the practical application of proposed sensor Mg-TPP-DHBDC, we tried to develop paper test strips for rapid on-site detection of Al3+. The solid-state emission properties of Mg-TPP-DHBDC to Al3+ is firstly investigated. As shown in Figure 9a, Mg-TPP-DHBDC could detect Al3+ in solid state via color change from yellow to blue under UV light. What is more, this is a reversible sensing process for Mg-TPP-DHBDC. After detection of Al3+, the sample of MgTPP-DHBDC was immersed in fresh DMA and it can restore to its original color under 365 nm UV light. Mg-TPP-DHBDC probe can maintain the original sensing performance distinguished by naked eyes even more than three times of successive repeating experiments. These results encourage us to further design paper test strips. The filter paper was firstly utilized to filter a 3 mg/mL DMA solution of grounded Mg-TPP-DHBDC MOF probe, which was then dried and made as. Under UV light, the color of paper test strips was yellow green, which exhibited a significant visible color change immediately when dipped into the Al3+ ion solution (0.001 mol/L). This is obviously different from other metal ions and indicate that Mg-TPP-DHBDC paper test strips can be used as rapid on-site probe for Al3+ ions.

(a)


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(b) Figure 9. Repeated solid-state detection of Al3+ (a) and the corresponding luminescence picture of Mg-TPP-DHBDC test paper after dropping different metal ions (b). 3.6. Fluorescence sensing mechanism of Mg-MOF probes The overall sensing mechanism in this work should be excited state intramolecular proton transfer (ESIPT) process, which is originated from the adjacent proton donor (–OH) and proton acceptor (–COOH) groups, and verified by the solvent-dependent luminescence character. The addition of Al3+ will destroy the intramolecular hydrogen-bonds and leads to coordination between -OH and Al3+. The initial quenching of the emission intensity around 505 nm due to binding of Al3+ to uncoordinated -OH of DHBDC, intramolecular hydrogen bond has been replaced by coordination to Al3+ thus resulting in inhibition of the ESIPT process. So, the large spectral blue shift indicated the deprotonation, as a consequence of Al3+ binding to phenol (Figure 10). The band of -OH groups was largely weakened in FT-IR spectra after contacting with Al3+, which verify the role of hydroxyl groups in Al3+ sensing (Figure S8). Moreover, Al3+ has negligible influence on the fluorescence emission of Mg-TPP-BDC, while Mg-TPP-OHBDC also show a new peak at 400 nm after attaching Al3+, demonstrating the key role of hydroxyl groups in Al3+ sensing. Furthermore, Mg-TPP-DHBDC MOF probe with two -OH


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groups exhibited distinct fluorescence quenching of the emission at 505 nm upon the addition of Al3+and a new peak at 460 nm appeared. Notably, this blue shift was accompanied by the enhanced intensity. A compared experiment with DHBDC ligand as fluorescence sensor show that the fluorescence intensity change is chaotic (Figure 5e). Energy Dispersive Spectrometer (EDS) were also used to demonstrate the sensing of Al3+ by Mg-TPP-DHBDC (Figure S9). Elemental analysis of the original Mg-TPP-DHBDC indicates no Al element. Nevertheless, after the sensing of Al3+, the final MOF probe are observed with Al element. This demonstrated that the Al3+ was captured into the functionalized pore by the coordination with -OH. The time-resolved experiments for Mg-TPP-DHBDC probe in DMA with and without Al3+ were also conducted. The decay curves nicely fit with double-exponential decay (Figure S10). The average lifetime of Mg-TPP-DHBDC in DMA (λem = 505 nm) is 3.0 ns and 9.2 ns. After addition of Al3+ to the Mg-TPP-DHBDC - DMA solution (λem = 460 nm), the average time increase to 5.1 ns and 10.3 ns. The fluorescence intensity was enhanced and the postpone in the decay process was observed with the addition of Al3+. These results further confirm that Al3+ can interact with the -OH groups from the Mg-TPP-DHBDC MOF probe, which is in according with our proposed ESIPT process (Figure 10). Anyway, all these results clearly illustrate that the step-by-step functionalization of -OH groups in Mg-MOFs has significant influence on the response to Al3+ ion because of binding of Al3+ to uncoordinated -OH, ESIPT process has been inhabited due to intramolecular hydrogen bond has been replaced by coordination to the metal cation, which thus resulting in the blue shift in emission spectra.


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Figure 10. Proposed fluorescence sensing mechanism of Mg-TPP-DHBDC probe on the base of ESIPT process.

4. CONCLUSION In this work, for the first time, we achieve a new way of turn-off-on process for a MOF-based luminescent probe for Al3+ ions along with the blue shifting of the maximum emission peak. The excellent luminescent sensing ability of Mg-TPP-DHBDC mainly comes from the free -OH functional groups pointing towards the pores, which facilitates both electron transfer and binding interaction between Mg-TPP-DHBDC and Al3+. In addition, Mg-TPP-DHBDC could detect Al3+ with high selectivity and high sensitivity with the lowest detection limit (28 nM) among reported MOF-based Al3+ sensors. What is more, the Mg-TPP-DHBDC probe recovers easily and faster after detection, which can be made into paper test strips for rapid on-site naked-eye detection of Al3+ ions. The combination of non-toxic Mg centers and cheaper organic linkers with the ESIPT process provides a greater possibility for detection of metal ions in environmental and biomedical.

ASSOCIATED CONTENT Supporting Information


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PXRD patterns for Mg-MOFs. Fluorescence spectra of solid Mg-MOFs. The contrast experiments to exclude the mechano-responsive and TPP luminescent. Repeated detection of Al3+ in DMA. Absorption spectra of Mg-TPP-DHBDC probe in DMA with and without Al3+. Time course of the luminescence intensity of Mg-TPP-DHBDC in DMA before and after the addition of Al3+. FT-IR spectra and EDS results. Florescence decays of Mg-TPP-DHBDC probe in DMA before and after the addition of Al3+.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (S. N. Li); [email protected] (Q.-G. Zhai) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21671126), the Natural Science Foundation of Shaanxi Province (2018JC-019), the Fundamental Research Funds for the Central Universities (GK201701003 and GK2017TS017).


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Highly Selective and Sensitive Turn-Off-On Fluorescent Probes for

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