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An ultra-robust luminescent CAU-10 MOF acting as a fluorescent “turn-off” sensor for Cr2O72− in aqueous medium
Inorganica Chimica Acta 497 (2019) 119078
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Research paper
An ultra-robust luminescent CAU-10 MOF acting as a fluorescent “turn-off” sensor for Cr2O72− in aqueous medium Soutick Nandia, Ankita Mondala, Helge Reinschb, Shyam Biswasa, a b
T
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Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India Institut für Anorganische Chemie, Christian-Albrechts-Universität, Max-Eyth-Strasse 2, 24118 Kiel, Germany
ARTICLE INFO
ABSTRACT
Keywords: Metal-organic framework Aluminum CAU-10 Cr2O72− sensing Fluorescence turn-off
For public health and environmental safety, the detection of heavily poisonous Cr2O72− ion from aqueous medium is a highly demanding task. A highly robust and reusable porous material can address this job. In this work, an aldehyde functionalized Al(III)-based luminescent metal-organic framework (MOF) denoted as CAU10-CHO (1, CAU stands for Christian-Albrechts-University) was employed for the detection of toxic Cr2O72− ion in aqueous medium. The compound was synthesized solvothermally and thoroughly characterized. The desolvated compound showed ultra-fast, specific and sensitive detection behavior towards Cr2O72− in HEPES buffer (10 mM, pH = 7.4). Moreover, the material could retain its specificity for Cr2O72− even in co-existence of possibly disturbing anionic species. In addition, the material showed efficient sensing properties in wide pH range (2–12) without losing its structural integrity. Moreover, the material could be reusable up to 5 cycles. The material has the ability to detect Cr2O72− in real water specimens. Furthermore, MOF-coated paper test strips were utilized for the on-site fluorimetric detection of Cr2O72−. The limit of detection of the material for Cr2O72− was found to be 0.69 μM. The mechanism of the detection event was also investigated in detail.
1. Introduction Anions play vital roles in several biological and environmental processes such as maintaining osmotic pressure, controlling cell volume, etc. [1]. With rapid growth of modern industries, toxic anion contamination has become a global threat for living beings. Out of several anionic species, a number of oxo-anions such as TcO4−, AsO43−, SeO32− and Cr2O72− have been listed as highly hazardous anionic contaminants by U. S. Environmental Protection Agency [2,3]. Out of various oxo-anions, due to carcinogenic effect, Cr2O72− ion is considered as the most harmful oxo-anion [4]. Dichromate anion is widely employed in chromium electroplating, pigment production, leather tanning, metallurgy, etc. [5]. The waste water from these industries contains huge amounts of Cr2O72− which produce very harmful impacts on ecosystem [6]. Due to toxic nature of Cr2O72−, it may cause carcinoma, kidney damage, gene mutation, etc. [7,8]. Moreover it is also hazardous to DNA and cellular structure [9,10]. In addition, Cr(VI) permeate through biological membrane and cause allergic reaction such as dermatitis, irritant dermatitis and chrome ulcer in human body [11]. Hence, direct and efficient tracking of Cr2O72− is a serious job to control the environmental pollution. Till date, different types of methods have been employed for the recognition and removal ⁎
purpose of Cr2O72− such as ion exchange [12], adsorption [13], resins [14], photo-catalytic degradation [15], and membrane separation [16]. Based on the above methods, several materials like zeolites, activated charcoals, π-electron containing organic molecules, organic polymer resins, metal complexes etc. have been utilized for Cr2O72− detection and capture [17–19]. But, most of the above-mentioned materials and methods are restricted in practical application purpose due to their poor thermal and chemical stability, poor selectivity and sensitivity and nonrecycling nature of sensing. Therefore, design and synthesis of highly stable materials having specific and sensitive detection nature towards Cr2O72− are extremely desirable for environmental waste water monitoring. Metal-organic frameworks (MOFs) are a new class of crystalline compounds having metal ions and clusters coordinated to organic linkers to construct one, two or three dimensional structures [20,21]. These porous materials have been extensively used in different purposes such as gas adsorption/storage, sensing, catalysis, drug delivery, etc. [22–26]. Recently, detection and removal of contaminants are conducted by applying MOF materials for their high surface areas, welldefined pores and functional pore surfaces [27,28]. Some Zr based MOFs (NU-1000, PCN-134, etc.) are employed for the detection and elimination of oxo-anions [29,30]. In addition, several cationic MOFs
Corresponding author. E-mail address: [email protected] (S. Biswas).
https://doi.org/10.1016/j.ica.2019.119078 Received 11 June 2019; Received in revised form 26 July 2019; Accepted 14 August 2019 Available online 16 August 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Schematic representation showing detection behavior of 1′ towards Cr2O72− in HEPES buffer medium.
such as 1·SO4, ZJU-101, ABT·2ClO4, FIR-53, FIR-54, NUM-4, etc. have been developed and used for capturing Cr2O72− ion and anionic dyes [31–35]. However, cationic MOFs suffer from several problems such as poor recyclability and slow capture response towards Cr2O72−. Some Zn and lanthanide MOFs are also investigated for the detection of Cr2O72− ion [36–41]. Unfortunately, most of them are devoid of practical application purpose for their slow response and poor reusability. Hence, MOFs containing luminescent organic ligands having high chemical and thermal stability with selective, sensitive and reusable detection property towards Cr2O72− ion can serve as promising sensor materials for real-time tracking of Cr2O72−. In this regard, we have synthesized a new aluminium based MOF called CAU-10-CHO (1) by incorporating 5-formyl isophthalic acid as a linker molecule. The activated material shows selective detection of Cr2O72− in HEPES buffer medium by quenching its emission intensity with high recycling property (Scheme 1). At wide pH range (2–12), the material exhibits structural robustness. Moreover, detection behavior towards Cr2O72− is also retained at the above-mentioned pH range. The limit of detection of this material towards Cr2O72− is 0.69 μM (0.21 mg/L) which is below the WHO recommended long term exposure limit of hexavalent chromium for vertebrates (0.26–2.0 mg/L) [42]. Moreover, the material is successfully employed to monitor the Cr2O72− concentration in environmental water samples.
the mixture became 5.60. It is well-established that DMF molecules decarbonylate under heating conditions to form dimethylamine [44,45]. Due to formation of dimethylamine, pH value of the mixture enhanced slightly after the solvothermal reaction. In order to eliminate occluded solvent molecule from the pore of the MOF material, 50 mg of as-prepared sample was stirred in 100 mL methanol at room temperature for 12 h. Then, the material was filtered. Afterwards, the compound was heated under vacuum at 120 °C for 12 h. The obtained desolvated compound is hereafter denoted as 1′. FE-SEM images of the material have been shown in Fig. S3. 2.2. Preparation of the HEPES buffer suspension of 1′ Compound 1′ (50 mg) was finely ground and 100 mL of 10 mM HEPES buffer (pH = 7.4) solution was poured into it. Thereafter, the resulting suspension was sonicated for an hour and kept at undisturbed conditions for 1 day to get a stable suspension of the material. The above-mentioned suspension was used for detection purpose. It is worth to mention here that by following previously reported protocol, 10 mM HEPES buffer solution was prepared [46]. 2.3. Fluorescence detection experiments of Cr2O72− HEPES buffer suspension of 1′ (3 mL) was utilized for each fluorescence titration experiment. During fluorescence experiment, the suspension of 1′ was excited at 320 nm and all fluorescence spectra were collected within 335–620 nm range. For time dependent fluorescence experiment, 200 µL of 10 mM Cr2O72− was included to the suspension of 1′ and after each minute, emission spectra were recorded. To perform concentration-dependent detection experiment, 0–200 µL of Cr2O72− (10 mM) solution was introduced stepwise to the HEPES buffer suspension of 1′. After each addition, the emission response of the compound was monitored. To examine the influence of other anions during the detection process of Cr2O72−, 10 mM solutions of different anions were prepared. Sodium salts of different anions i.e. NaBr, NaCl, NaI, NaNO3, Na3PO4, Na2CO3, NaHCO3, NaCN, NaSCN, Na2SO4, NaOAc, NaClO4, NaN3 and NaF were utilized to make the solutions.
2. Experimental section 2.1. Synthesis and activation of [Al(OH)(IPA-CHO)]∙0.5H2O∙0.4DMF (CAU-10-CHO, 1) Reported synthetic procedures of other CAU-10 MOFs were followed for the synthesis of 1 [43]. In brief, 2(M) Al3+ solution was prepared by dissolving required amount of AlCl3·6H2O in water. Afterward, a mixture of H2IPA-CHO ligand (93.71 mg, 0.48 mmol), 2(M) Al3+ solution (236 µL, 0.48 mmol), DMF (277 µL) and H2O (875 µL) was added in a Teflon-lined hydrothermal autoclave reactor and it was heated for 12 h at 120 °C in conventional oven. The white precipitate was collected by filtration after cooling down to ambient temperature. The obtained material was re-dispersed in water and sonicated for half an hour for the removal of unreacted Al-salts. The suspension was collected by filtration and the white crystalline solid was dried in a conventional oven at 140 °C for 12 h. The yield was 85 mg (0.31 mmol, 64%) based on the used Al salt. FT-IR (KBr, cm−1): 3433 (br), 2966 (sh), 2924 (w), 2854 (w), 1699 (s), 1657 (sh), 1622 (vs), 1567 (s), 1462 (s), 1413 (s), 1385 (sh), 1253 (m), 1134 (m), 1016 (w), 772 (m), 724 (m), 696 (w), 647 (w),598 (w), 514 (m). It is worth to mention that the pH value of the reaction mixture before solvothermal reaction was 3.95. After reaction, the pH value of
2.4. Cr2O72− detection experiments in real water samples For examining the detection ability of Cr2O72− by 1′ in real water specimens, different kinds of water specimens were collected such as tap water, mineral water and lake water (taken from the IITG lake, Guwahati, India). In nine different glass vials, each of the water samples was spiked with known amount of Cr2O72− (final concentrations = 5, 25 and 50 µM). The Cr2O72−-spiked water specimens (100 µL each) having three different concentrations were added separately to HEPES 2
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buffer suspension of 1′. After the inclusion, their emission spectra were collected (λex = 320 nm, λem = 427 nm). 2.5. Cr2O72− detection experiments in portable paper strips Commercially available Whatman filter paper was immersed in the HEPES buffer suspension of the MOF material for 30 min. Thereafter, the filter paper was dried at 75 °C in conventional oven and cut into 2 cm × 3 cm size to obtain fluorescent paper strips. These paper strips were further used for Cr2O72− detection. 3. Results and discussion 3.1. FT-IR study Fig. 2. Framework topology of CAU-10-CHO (1) in ball-and-stick representation. Color codes: Al, blue polyhedra; C, grey; O, red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
To ensure the inclusion of the ligand in the framework of the material and removal of solvent molecules from the pores after activation, FT-IR spectra of as-synthesized and desolvated compound were recorded. The FT-IR spectra (Fig. S4) of as-synthesized 1 and thermally desolvated sample 1′ show strong absorption bands at around 1622 and 1567 cm−1, which can be assigned to the asymmetrical and symmetrical –CO2 stretching vibrations of the metal (Al) coordinated H2IPACHO ligands, respectively. The strong peak at around 1699 cm−1 denotes the carbonyl stretching frequency of aldehyde functionality grafted with the ligand [47]. In the spectrum of as-synthesized compound, a shoulder band near around 1657 cm−1 appears due to the carbonyl stretching frequency of the trapped DMF molecule [48]. This band is totally vanished in the activated sample 1′, which suggests the completion of de-solvation process.
Depending on the functional group present in the ligand, the space group symmetry of the framework may change slightly while the main topology of the framework remains intact. In brief, we can say the framework of CAU-10-CHO is formed by cis-connected corner sharing [AlO6] polyhedra, which form helical chains. Four oxygen atoms out of six oxygen atoms of [AlO6] octahedron originate from four carboxylate groups of formyl functionalized isophthalate and bridging OH− ions provide the remaining two oxygen atoms. Each of the helices is connected with four nearby inorganic building units which have alternating rotational arrangement through the coordinated 5-formyl isophthalate (IPA) ligands. This specific arrangement of the inorganic building units and ligand molecules results in formation of squareshaped one-dimensional channels. The interior side of the channel is decorated by the formyl functionality of the co-ordinated IPA-CHO ligands (Fig. 2). The structural formula of the ligand and the coordination mode of the ligand with Al3+ ion is shown in the Scheme 2.
3.2. Structure description The PXRD pattern of CAU-10-CHO (1) was successfully indexed in a tetragonal unit cell with extinction conditions corresponding to the space group I41md. For the Rietveld refinement, the structure of isotypic compound CAU-10-F0.29 was utilized as the starting model (for details, see SI). The final plot (Fig. 1) and the structural parameters obtained from this refinement (Table S1) depict that the present material bears identical topology with the pristine and functionalized CAU-10 frameworks. The asymmetric unit of 1 with numbering scheme is displayed in Fig. S5.
3.3. Thermal stability The thermal stability of the activated (1′) and as-synthesized (1) material was examined by conducting thermogravimetric (TG) analyses under air in the temperature range of 25–800 °C. From the TG data (Fig. S6), it becomes obvious that the MOF material shows robust nature of its structure up to 410 °C. Three weight loss steps in the TG curve of the as-prepared compound are found. The first weight loss of 3.2 wt% in the temperature range 0–130 °C shows good agreement with the removal of 0.5 guest water molecule per formula unit (calcd.: 3.3 wt%). The second weight loss of 7.2 wt% in the temperature range 130–310 °C can be attributed to the removal of 0.4 DMF molecule per formula unit (calcd.: 7.3 wt%). After 410 °C, the sudden break in the TG trace indicates the loss of coordinated ligand molecules from the framework. The high decomposition temperature (410 °C) unveils the ultra-robust nature of the material.
Fig. 1. Plot for the Rietveld refinement. The black curve refers the measured data, red curve indicates the theoretical data and blue curve corresponds to their difference. Vertical black bars indicate the allowed Bragg reflection positions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Scheme 2. (a) Structure of H2IPA-CHO ligand. (b) Coordination mode of the ligand with Al3+ ion. 3
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I−, F−, PO43−, CO32−, NO3−, HCO3−, SO42−, CN−, SCN−, OAc−, ClO4− and N3−) were added into the HEPES buffer suspension of 1′ and corresponding emission spectra were recorded. Compound 1′ exhibited the highest fluorescence quenching efficiency in presence of Cr2O72− (around 94%) whereas the other 15 anions could show negligible quenching efficiency (around 3–24%) except MnO4− and CrO42− (Fig. 4 and Figs. S11–S27). In presence of MnO4− and CrO42− (200 μL, 10 mM), around 80 and 70% quenching efficiencies have been observed for the material, respectively. Nevertheless, the quenching efficiency of the material in presence of Cr2O72− is higher (~94%) than the quenching efficiency observed for MnO4− and CrO42−. From these observations, we can conclude that 1′ showed sufficient selective sensing nature towards Cr2O72−. In order to serve as a widely applicable sensor material, 1′ should exhibit specific response towards Cr2O72− ion in the co-existence of disturbing other anions. To investigate it, 200 µL of 10 mM aqueous Cr2O72− solution was introduced in the HEPES buffer suspension of 1′ which already contained interfering anions. The obtained bar plot (Fig. 5) and Figs. S28–S44 unveil that Cr2O72− could be efficiently recognized by the present material even in the co-existence of several congeners. Material 1′ should show distinct quenching performance towards Cr2O72− at specific concentration. Fig. S45 reveals the correlation between quenching efficiency of 1′ and the concentration of anions used during sensing event. It becomes apparent from the figure that the quenching efficiency of 1′ is gradually enhanced with incremental addition of Cr2O72− solution. On the other hand, other anions are unable to produce any considerable change in the fluorescence quenching efficiency even at higher concentration. During fluorescence quenching event, Stern-Volmer (S-V) plot can provide important information regarding the nature of quenching process. As the detection process of Cr2O72− ion by the present material occurred via quenching of its fluorescence, S-V quenching constant (Ksv) was evaluated by employing the below mentioned equation:
3.4. Chemical stability In order to investigate chemical stability, the activated 1′ was stirred for 24 h in variety of liquids (MeOH, 1 M HCl, DMF and acetic acid). Then, the compounds were filtered off and PXRD measurements were conducted (Fig. S7). The PXRD patterns of the compound remained almost same after treatment with the above mentioned liquids, which discloses that structural integrity is maintained. During detection of Cr2O72− in industrial waste water, MOF material should withstand at different pH media. Specially, in tannery and metal plating industries, wastewater is highly acidic. Hence, 1′ was soaked in different pH media (2–12) for overnight and PXRD patterns were collected after filtration. Fig. S8 reveals that structural robustness is well maintained in the above mentioned pH media. Hence, presented material is a useful candidate for the detection of Cr2O72− in industrial waste water. 3.5. N2 Sorption analysis N2 adsorption measurements were conducted with the activated compound to examine the permanent porosity of the micro-porous material (Fig. S9). It has been well documented that functional group of the ligand molecules have great impact on the porosity of the material [43]. In case of CAU-10 series of MOFs, the functional groups can increase the narrowness of the channels and slow down the diffusion process of the N2 gas. Hence, accessible pores towards N2 gas molecule decrease. The BET surface area and micropore volume of the present material are 55 m2 g−1 and 0.03 cm3 g−1 (at p/p0 = 0.50), respectively. Hence, the material is almost non-porous like most of the functionalized CAU-10-X MOFs (X = -OH, -OCH3, -NH2, -N3, -N2H3) [43,46,49]. 3.6. Detection behavior for Cr2O72− in HEPES buffer For toxic impacts of Cr2O72− on living beings and environment, realtime tracking of Cr2O72− is a serious task. To address this purpose, a number of sensor materials such as zeolites, activated charcoals, πelectron-rich organic molecules, organic polymer resins and MOFs have been investigated for Cr2O72− detection [17–19,31–34,36,37,39,50–52]. But, most of them cannot be employed for practical application purpose as they do not meet the criteria of supreme sensor compounds like fast, selective and sensitive response, high physicochemical stability, nontoxic nature, reusable property, etc. All these facts motivated us to examine the sensing behavior towards Cr2O72− by the newly designed 5formyl isophthalic acid containing luminescent Al(III)-based MOF called CAU-10-CHO. It is worth to mention that aluminium based MOFs are more aqua-stable and bio-friendly than other MOFs containing other metal ions. The detection capability of 1′ for Cr2O72− was examined by introduction of 10 mM Cr2O72− solution into 3 mL HEPES buffer suspension of 1′. The emission spectra were measured after each minute up to 10 min. Before addition of Cr2O72−, 1′ showed strong fluorescence due to the presence of auxochromic aldehyde group. As soon as the material came into contact with Cr2O72− ions, the emission property of the material is nearly vanished (Fig. 3). Moreover, Fig. S10 suggests that under day light, colorless HEPES buffer suspension of the MOF material is converted to light yellow solution in presence of Cr2O72− solution (10 mM, 200 µL). This color transformation is due to the addition of orange colored Cr2O72− solution. Under UV lamp, the HEPES buffer suspension of 1′ showed pale blue fluorescence but the blue emission was completely quenched with the addition of Cr2O72− solution. A potential fluorescent chemo-sensor should have great specificity for the target analyte over other competitive analytes. To examine the selective nature of 1′ towards Cr2O72− ion, solutions (200 μL, 10 mM) of possibly competitive anions (e.g., MnO4−, CrO42−, ClO−, Cl−, Br−,
(I0/ I ) = Ksv [Q] + 1 where, I0 and I represent intensity of 1′ in absence and presence of the analyte, respectively. [Q] denotes the molar concentration of the used analyte. The estimated Ksv value towards Cr2O72− was found to be 1.43 × 104 M−1 (Fig. S46). Fig. 6 depicts the S-V plots for different anions along with Cr2O72− ion. The figure shows that at lower concentration of Cr2O72−, S-V plot is almost linear whereas at higher concentration of Cr2O72−, the plot deviates from its linear nature. This event is attributed to the self-absorption or energy transfer-process [53,54]. Fluorescence titration experiments were carried out for quantification of the detection process of Cr2O72− ion by 1′. For this aim, the fluorescence emission spectra of the HEPES buffer suspension of 1′ were collected with the progressive addition (20 μL in each addition) of 10 mM Cr2O72− solution. Fig. 7 suggests that stepwise addition of Cr2O72− solution brings gradual suppression of emission intensity of 1′. There was ~ 94% quenching of the emission intensity after addition of 200 μL of Cr2O72− solution. To check the sensitivity of the material towards Cr2O72− ion, limit of detection (LOD) was evaluated by including very low concentrated solution of Cr2O72− ion. By plotting the emission intensity of the material against the employed concentration of Cr2O72− (Fig. S47), a straight line was obtained. By employing the formula 3σ/k (k = slope obtained from the line, σ = standard deviation of repetitive blank measurements), the estimated LOD value is 0.69 µM, which is comparable with the LOD values of formerly reported MOF materials (Table S3) [1,32,34,36,55]. The recyclability experiments of compound 1′ were examined up to five cycles of detection experiments to prove the fluorescent restoration property of the material. After detection experiment, 1′ was filtered and washed thoroughly with distilled water repeatedly. Then, it was further 4
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Fig. 3. (a) Fluorescence “turn-off” response of HEPES buffer suspension of 1′ towards the addition of Cr2O72− solution up to 10 min. (b) Time-dependency of the emission intensity monitored at 427 nm.
Fig. 4. Quenching efficiencies of 1′ towards different anions in HEPES buffer (monitored at 427 nm).
Fig. 5. Fluorescence quenching efficacy of Cr2O72− in presence of various disturbing anions.
utilized for the next detection experiment. This process was repeated up to 5 cycles. Fig. S48 depicts that even after five cycles of repeated fluorescence detection experiments, the probe showed similar emission
intensity as the initial one and the quenching efficiency of 1′ in presence of Cr2O72− ion is retained at similar level. PXRD study concludes that the structure of the MOF material remains intact after five cycles of 5
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Fig. 6. Stern-Volmer plots for the fluorescence quenching efficiency of 1′ with incremental addition of various analytes.
medium whereas, HCrO4− is the dominant species in acidic medium. It is noticeable that though different species of chromium oxo-anions (CrO42−, Cr2O72− and HCrO4−) are present at different pH values, all of them have Cr in + 6 oxidation state. Hence, in broad sense, we can say that the present probe can detect all Cr(VI) oxoanions. To investigate the influence of formyl functionality during the detection process of Cr2O72−, CAU-10 MOF without any functionalization was synthesized and emission spectra of the HEPES buffer suspension of the material were collected before and after addition of Cr2O72− solution. From Fig. S51, it is quite evident that without formyl functionality the material is almost non-fluorescent with respect to 1′ and no observable change in emission intensity is visualized after the introduction of Cr2O72− solution, but 1′ is showing drastic quenching of its emission property in presence of Cr2O72−. Thus, without formyl functionalization, the material fails to detect Cr2O72−. Hence, formyl functionality plays a significant role to enhance the luminescent property of the material and detection of Cr2O72−. Fig. 7. Quenching of the fluorescence intensity of HEPES buffer suspension of activated compound upon progressive inclusion of 10 mM Cr2O72− solution.
3.7. Detection of Cr2O72− in real water samples For examining the Cr2O72− sensing behavior of the MOF material in real world samples, total nine water specimens were prepared. They contain three different concentrations of Cr2O72− for each type of water specimen (mineral, tap and lake water). The concentrations found for Cr2O72− and the recovery percentages for the environmental water specimens are tabulated in Table 1. The recovery percentages of Cr2O72− were observed to be in the range of 97–102% for all the samples and the RSD values were in between 1.06 and 5.39 (RSD = (SD/mean value) × 100). Low RSD values and good recovery percentages suggest that the probe can provide suitable recoveries as well as good analytical precisions for real water specimens.
detection experiments (Fig. S49). For wide applicability purpose, 1′ should show quenching behavior at wide pH range because pH of industrial waste water varies widely (For example, tannery and metal plating waste water are acidic in nature). To address this purpose, quenching behavior of 1′ for Cr2O72− ion was studied in a wide pH range (2–12). Fig. S50 signifies that though the initial intensity of 1′ varies with different pH values of the media, at any pH value, emission intensity of 1′ is suppressed in presence of 200 µL of 10 mM Cr2O72−. All discussions suggest that the present probe is highly photo-stable and reusable. Hence, the probe could be applied for long term detection of Cr2O72− ion in real world. It is worth to mention that the presence of chromate species is influenced by the pH and concentration of the medium [56]. Above pH 6, the dominant species is yellow chromate (CrO42−) ion. Between pH 2 and 6, HCrO4− and orange-red dichromate (Cr2O72−) ion are in equilibrium [57,58]. Hence, in dilute solution, Cr2O72− exists as CrO42− in basic
3.8. Detection of Cr2O72− in portable paper strips Recently, due to rapid and low-cost detection, portable devices based on paper strips have drawn great interest. Here, MOF-coated paper strips were prepared by dip coating method, which were utilized 6
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Table 1 Detection performance of the MOF material for Cr2O72− in real water samples. Cr2O72− Spiked (µM)
Mineral Water Cr2O72−
5 25 50 a
Tap Water a
Found (µM)
5.01 ± 2.03 25.13 ± 1.49 48.72 ± 1.96
Lake Water a
Recovery (%)
Cr2O72−
100.20 100.52 97.44
4.85 ± 4.84 24.97 ± 1.71 49.83 ± 1.73
Found (µM)
Recovery (%)
Cr2O72− Founda (µM)
Recovery (%)
97.00 99.88 99.66
5.14. ± 5.39 24.44 ± 3.23 49.93 ± 1.06
102.80 97.76 99.86
Cr2O72− found ± RSD (n = 3).
transfer process, ground state complex formation, excited-state reaction or through inner filter effect (IFE). All these events can be categorized in two types of fluorescence quenching mechanisms: one is dynamic quenching and another one is static quenching. Dynamic quenching mainly arises due to collision process or diffusion of quencher to the excited state fluorophore. On the other side, close association between quencher and fluorophore reduces the emission intensity in case of static quenching [59]. Fluorescence lifetime measurement technique could be employed to differentiate between dynamic and static quenching process. Lowering of lifetime of excited fluorophore in presence of quencher molecule occurs for dynamic quenching process but in case of static quenching, lifetime of the fluorophore remains same even in the presence of quencher molecule [60]. Time-resolved fluorescence lifetime decay of HEPES buffer suspension of 1′ in absence and presence of Cr2O72− was investigated. The obtained fluorescence decay profile (Fig. S52) and the fitted results (Table S2) unveil that Cr2O72− solution brings a dramatic lowering in the lifetime (12.61 ns to 7.04 ns). This result assured that the quenching process occurred through dynamic quenching rather than static quenching process and IFE process. In IFE and static quenching processes, the lifetime of fluorophore moiety would not change in presence of quencher [61]. To deeply examine the quenching process, UV–vis spectra of all competitive anions including Cr2O72− ion were collected (Fig. 9a). It becomes obvious from Fig. 9b that partial overlap occurs between the emission spectrum of 1′ and the absorption bands of Cr2O72−, MnO4− and CrO42−, whereas other anions do not absorb in 300–450 nm range. Hence, we can conclude that resonance energy transfer from 1′ to Cr2O72−, MnO4− and CrO42− is the possible reason for the emission reduction of 1′ in presence of the mentioned anions [55]. In addition, due to high oxidizing nature of the anions (Cr2O72−, MnO4− and CrO42−), electron transfer from ligand (H2IPA-CHO) present in the framework to the oxidant is also highly possible, which restricts the ππ* electron transfer [52]. As a result, quenching of luminescence property of the material is observed. It is worth to mention that though MnO4− and CrO42− can reduce the emission intensity of 1′ (quenching efficiencies are 80.3 and 70.7%, respectively), 1′ showed the highest
Fig. 8. Digital images of 1′-coated paper test strips (under UV lamp) after the introduction of 10 µL of Cr2O72− solution of various concentrations. A dark spot can be seen only in presence of Cr2O72− solution.
for Cr2O72− detection. The MOF-coated paper strips were treated with various concentrations of Cr2O72− solution (10 µL volume in each paper strip) by using a micropipette. From Fig. 8, it can be observed that the MOF-coated paper strip showed blue fluorescent under UV lamp before treatment with Cr2O72− solution and dark spots appeared after the addition of Cr2O72− solution. The darkness of the spot was low at lower concentrations of Cr2O72− solution. The dark spot appeared due to quenching of the blue fluorescence of the MOF-coated paper strip in presence of Cr2O72− solution. Hence, this is a useful method for the on-site detection of Cr2O72−. 3.9. Plausible mechanism for detection of Cr2O72− Fluorescence quenching of 1′ by Cr2O72− ion may occur due to various phenomena such as collision between molecules, energy
Fig. 9. (a) UV–vis spectra of different anions in HEPES buffer solution. (b) Spectral overlap between emission spectrum of 1′ and absorption spectrum of Cr2O72−. 7
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quenching efficiency (94%) in presence of Cr2O72−. Moreover, reduction in life time value of the material in presence of Cr2O72− ion strongly supports the above discussed electron and energy transfer processes. Hence, we can conclude that the combined effect of resonance energy transfer and electron transfer is the principal cause for the emission quenching of 1′ in presence of Cr2O72−. The above discussed mechanistic processes are in good agreement with the previous reports [50,62].
[14] [15] [16] [17]
4. Conclusions
[18]
In summary, an aldehyde functionalized Al(III) based MOF called CAU-10-CHO has been hydrothermally prepared by employing 5formyl isophthalic acid as a linker molecule. TG study shows that the compound displays its robust nature up to 410 °C. The desolvated compound in HEPES buffer medium shows great quenching performance towards Cr2O72− over other possible intrusive anions. Moreover, the material has excellent sensitivity for Cr2O72− (LOD = 0.69 µm). Cr2O72− detection ability of the material in real water samples is also tested. In addition, the material retains its detection ability towards Cr2O72− at any pH media (2–12). Furthermore, high structural robustness and reusability have been displayed by 1′ for the sensing of Cr2O72−. Out of several MOF materials which were reported for the detection of Cr2O72−, the present material showed quite better sensing performance in-terms of response time, detection limit, stability and reusability for real time detection of Cr2O72−. Therefore, the MOF material is a potential probe towards selective and quantitative detection of Cr2O72− in aqueous media by quenching emission intensity.
[19] [20] [21] [22] [23] [24]
[25]
[26] [27]
Acknowledgments
[28]
We are highly thankful to the Science and Engineering Research Board (SERB; grant no. EEQ/2016/000012), New Delhi for generous financial support.
[29] [30]
Appendix A. Supplementary data
[31]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ica.2019.119078.
[32]
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Research paper
An ultra-robust luminescent CAU-10 MOF acting as a fluorescent “turn-off” sensor for Cr2O72− in aqueous medium Soutick Nandia, Ankita Mondala, Helge Reinschb, Shyam Biswasa, a b
T
⁎
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India Institut für Anorganische Chemie, Christian-Albrechts-Universität, Max-Eyth-Strasse 2, 24118 Kiel, Germany
ARTICLE INFO
ABSTRACT
Keywords: Metal-organic framework Aluminum CAU-10 Cr2O72− sensing Fluorescence turn-off
For public health and environmental safety, the detection of heavily poisonous Cr2O72− ion from aqueous medium is a highly demanding task. A highly robust and reusable porous material can address this job. In this work, an aldehyde functionalized Al(III)-based luminescent metal-organic framework (MOF) denoted as CAU10-CHO (1, CAU stands for Christian-Albrechts-University) was employed for the detection of toxic Cr2O72− ion in aqueous medium. The compound was synthesized solvothermally and thoroughly characterized. The desolvated compound showed ultra-fast, specific and sensitive detection behavior towards Cr2O72− in HEPES buffer (10 mM, pH = 7.4). Moreover, the material could retain its specificity for Cr2O72− even in co-existence of possibly disturbing anionic species. In addition, the material showed efficient sensing properties in wide pH range (2–12) without losing its structural integrity. Moreover, the material could be reusable up to 5 cycles. The material has the ability to detect Cr2O72− in real water specimens. Furthermore, MOF-coated paper test strips were utilized for the on-site fluorimetric detection of Cr2O72−. The limit of detection of the material for Cr2O72− was found to be 0.69 μM. The mechanism of the detection event was also investigated in detail.
1. Introduction Anions play vital roles in several biological and environmental processes such as maintaining osmotic pressure, controlling cell volume, etc. [1]. With rapid growth of modern industries, toxic anion contamination has become a global threat for living beings. Out of several anionic species, a number of oxo-anions such as TcO4−, AsO43−, SeO32− and Cr2O72− have been listed as highly hazardous anionic contaminants by U. S. Environmental Protection Agency [2,3]. Out of various oxo-anions, due to carcinogenic effect, Cr2O72− ion is considered as the most harmful oxo-anion [4]. Dichromate anion is widely employed in chromium electroplating, pigment production, leather tanning, metallurgy, etc. [5]. The waste water from these industries contains huge amounts of Cr2O72− which produce very harmful impacts on ecosystem [6]. Due to toxic nature of Cr2O72−, it may cause carcinoma, kidney damage, gene mutation, etc. [7,8]. Moreover it is also hazardous to DNA and cellular structure [9,10]. In addition, Cr(VI) permeate through biological membrane and cause allergic reaction such as dermatitis, irritant dermatitis and chrome ulcer in human body [11]. Hence, direct and efficient tracking of Cr2O72− is a serious job to control the environmental pollution. Till date, different types of methods have been employed for the recognition and removal ⁎
purpose of Cr2O72− such as ion exchange [12], adsorption [13], resins [14], photo-catalytic degradation [15], and membrane separation [16]. Based on the above methods, several materials like zeolites, activated charcoals, π-electron containing organic molecules, organic polymer resins, metal complexes etc. have been utilized for Cr2O72− detection and capture [17–19]. But, most of the above-mentioned materials and methods are restricted in practical application purpose due to their poor thermal and chemical stability, poor selectivity and sensitivity and nonrecycling nature of sensing. Therefore, design and synthesis of highly stable materials having specific and sensitive detection nature towards Cr2O72− are extremely desirable for environmental waste water monitoring. Metal-organic frameworks (MOFs) are a new class of crystalline compounds having metal ions and clusters coordinated to organic linkers to construct one, two or three dimensional structures [20,21]. These porous materials have been extensively used in different purposes such as gas adsorption/storage, sensing, catalysis, drug delivery, etc. [22–26]. Recently, detection and removal of contaminants are conducted by applying MOF materials for their high surface areas, welldefined pores and functional pore surfaces [27,28]. Some Zr based MOFs (NU-1000, PCN-134, etc.) are employed for the detection and elimination of oxo-anions [29,30]. In addition, several cationic MOFs
Corresponding author. E-mail address: [email protected] (S. Biswas).
https://doi.org/10.1016/j.ica.2019.119078 Received 11 June 2019; Received in revised form 26 July 2019; Accepted 14 August 2019 Available online 16 August 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Schematic representation showing detection behavior of 1′ towards Cr2O72− in HEPES buffer medium.
such as 1·SO4, ZJU-101, ABT·2ClO4, FIR-53, FIR-54, NUM-4, etc. have been developed and used for capturing Cr2O72− ion and anionic dyes [31–35]. However, cationic MOFs suffer from several problems such as poor recyclability and slow capture response towards Cr2O72−. Some Zn and lanthanide MOFs are also investigated for the detection of Cr2O72− ion [36–41]. Unfortunately, most of them are devoid of practical application purpose for their slow response and poor reusability. Hence, MOFs containing luminescent organic ligands having high chemical and thermal stability with selective, sensitive and reusable detection property towards Cr2O72− ion can serve as promising sensor materials for real-time tracking of Cr2O72−. In this regard, we have synthesized a new aluminium based MOF called CAU-10-CHO (1) by incorporating 5-formyl isophthalic acid as a linker molecule. The activated material shows selective detection of Cr2O72− in HEPES buffer medium by quenching its emission intensity with high recycling property (Scheme 1). At wide pH range (2–12), the material exhibits structural robustness. Moreover, detection behavior towards Cr2O72− is also retained at the above-mentioned pH range. The limit of detection of this material towards Cr2O72− is 0.69 μM (0.21 mg/L) which is below the WHO recommended long term exposure limit of hexavalent chromium for vertebrates (0.26–2.0 mg/L) [42]. Moreover, the material is successfully employed to monitor the Cr2O72− concentration in environmental water samples.
the mixture became 5.60. It is well-established that DMF molecules decarbonylate under heating conditions to form dimethylamine [44,45]. Due to formation of dimethylamine, pH value of the mixture enhanced slightly after the solvothermal reaction. In order to eliminate occluded solvent molecule from the pore of the MOF material, 50 mg of as-prepared sample was stirred in 100 mL methanol at room temperature for 12 h. Then, the material was filtered. Afterwards, the compound was heated under vacuum at 120 °C for 12 h. The obtained desolvated compound is hereafter denoted as 1′. FE-SEM images of the material have been shown in Fig. S3. 2.2. Preparation of the HEPES buffer suspension of 1′ Compound 1′ (50 mg) was finely ground and 100 mL of 10 mM HEPES buffer (pH = 7.4) solution was poured into it. Thereafter, the resulting suspension was sonicated for an hour and kept at undisturbed conditions for 1 day to get a stable suspension of the material. The above-mentioned suspension was used for detection purpose. It is worth to mention here that by following previously reported protocol, 10 mM HEPES buffer solution was prepared [46]. 2.3. Fluorescence detection experiments of Cr2O72− HEPES buffer suspension of 1′ (3 mL) was utilized for each fluorescence titration experiment. During fluorescence experiment, the suspension of 1′ was excited at 320 nm and all fluorescence spectra were collected within 335–620 nm range. For time dependent fluorescence experiment, 200 µL of 10 mM Cr2O72− was included to the suspension of 1′ and after each minute, emission spectra were recorded. To perform concentration-dependent detection experiment, 0–200 µL of Cr2O72− (10 mM) solution was introduced stepwise to the HEPES buffer suspension of 1′. After each addition, the emission response of the compound was monitored. To examine the influence of other anions during the detection process of Cr2O72−, 10 mM solutions of different anions were prepared. Sodium salts of different anions i.e. NaBr, NaCl, NaI, NaNO3, Na3PO4, Na2CO3, NaHCO3, NaCN, NaSCN, Na2SO4, NaOAc, NaClO4, NaN3 and NaF were utilized to make the solutions.
2. Experimental section 2.1. Synthesis and activation of [Al(OH)(IPA-CHO)]∙0.5H2O∙0.4DMF (CAU-10-CHO, 1) Reported synthetic procedures of other CAU-10 MOFs were followed for the synthesis of 1 [43]. In brief, 2(M) Al3+ solution was prepared by dissolving required amount of AlCl3·6H2O in water. Afterward, a mixture of H2IPA-CHO ligand (93.71 mg, 0.48 mmol), 2(M) Al3+ solution (236 µL, 0.48 mmol), DMF (277 µL) and H2O (875 µL) was added in a Teflon-lined hydrothermal autoclave reactor and it was heated for 12 h at 120 °C in conventional oven. The white precipitate was collected by filtration after cooling down to ambient temperature. The obtained material was re-dispersed in water and sonicated for half an hour for the removal of unreacted Al-salts. The suspension was collected by filtration and the white crystalline solid was dried in a conventional oven at 140 °C for 12 h. The yield was 85 mg (0.31 mmol, 64%) based on the used Al salt. FT-IR (KBr, cm−1): 3433 (br), 2966 (sh), 2924 (w), 2854 (w), 1699 (s), 1657 (sh), 1622 (vs), 1567 (s), 1462 (s), 1413 (s), 1385 (sh), 1253 (m), 1134 (m), 1016 (w), 772 (m), 724 (m), 696 (w), 647 (w),598 (w), 514 (m). It is worth to mention that the pH value of the reaction mixture before solvothermal reaction was 3.95. After reaction, the pH value of
2.4. Cr2O72− detection experiments in real water samples For examining the detection ability of Cr2O72− by 1′ in real water specimens, different kinds of water specimens were collected such as tap water, mineral water and lake water (taken from the IITG lake, Guwahati, India). In nine different glass vials, each of the water samples was spiked with known amount of Cr2O72− (final concentrations = 5, 25 and 50 µM). The Cr2O72−-spiked water specimens (100 µL each) having three different concentrations were added separately to HEPES 2
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buffer suspension of 1′. After the inclusion, their emission spectra were collected (λex = 320 nm, λem = 427 nm). 2.5. Cr2O72− detection experiments in portable paper strips Commercially available Whatman filter paper was immersed in the HEPES buffer suspension of the MOF material for 30 min. Thereafter, the filter paper was dried at 75 °C in conventional oven and cut into 2 cm × 3 cm size to obtain fluorescent paper strips. These paper strips were further used for Cr2O72− detection. 3. Results and discussion 3.1. FT-IR study Fig. 2. Framework topology of CAU-10-CHO (1) in ball-and-stick representation. Color codes: Al, blue polyhedra; C, grey; O, red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
To ensure the inclusion of the ligand in the framework of the material and removal of solvent molecules from the pores after activation, FT-IR spectra of as-synthesized and desolvated compound were recorded. The FT-IR spectra (Fig. S4) of as-synthesized 1 and thermally desolvated sample 1′ show strong absorption bands at around 1622 and 1567 cm−1, which can be assigned to the asymmetrical and symmetrical –CO2 stretching vibrations of the metal (Al) coordinated H2IPACHO ligands, respectively. The strong peak at around 1699 cm−1 denotes the carbonyl stretching frequency of aldehyde functionality grafted with the ligand [47]. In the spectrum of as-synthesized compound, a shoulder band near around 1657 cm−1 appears due to the carbonyl stretching frequency of the trapped DMF molecule [48]. This band is totally vanished in the activated sample 1′, which suggests the completion of de-solvation process.
Depending on the functional group present in the ligand, the space group symmetry of the framework may change slightly while the main topology of the framework remains intact. In brief, we can say the framework of CAU-10-CHO is formed by cis-connected corner sharing [AlO6] polyhedra, which form helical chains. Four oxygen atoms out of six oxygen atoms of [AlO6] octahedron originate from four carboxylate groups of formyl functionalized isophthalate and bridging OH− ions provide the remaining two oxygen atoms. Each of the helices is connected with four nearby inorganic building units which have alternating rotational arrangement through the coordinated 5-formyl isophthalate (IPA) ligands. This specific arrangement of the inorganic building units and ligand molecules results in formation of squareshaped one-dimensional channels. The interior side of the channel is decorated by the formyl functionality of the co-ordinated IPA-CHO ligands (Fig. 2). The structural formula of the ligand and the coordination mode of the ligand with Al3+ ion is shown in the Scheme 2.
3.2. Structure description The PXRD pattern of CAU-10-CHO (1) was successfully indexed in a tetragonal unit cell with extinction conditions corresponding to the space group I41md. For the Rietveld refinement, the structure of isotypic compound CAU-10-F0.29 was utilized as the starting model (for details, see SI). The final plot (Fig. 1) and the structural parameters obtained from this refinement (Table S1) depict that the present material bears identical topology with the pristine and functionalized CAU-10 frameworks. The asymmetric unit of 1 with numbering scheme is displayed in Fig. S5.
3.3. Thermal stability The thermal stability of the activated (1′) and as-synthesized (1) material was examined by conducting thermogravimetric (TG) analyses under air in the temperature range of 25–800 °C. From the TG data (Fig. S6), it becomes obvious that the MOF material shows robust nature of its structure up to 410 °C. Three weight loss steps in the TG curve of the as-prepared compound are found. The first weight loss of 3.2 wt% in the temperature range 0–130 °C shows good agreement with the removal of 0.5 guest water molecule per formula unit (calcd.: 3.3 wt%). The second weight loss of 7.2 wt% in the temperature range 130–310 °C can be attributed to the removal of 0.4 DMF molecule per formula unit (calcd.: 7.3 wt%). After 410 °C, the sudden break in the TG trace indicates the loss of coordinated ligand molecules from the framework. The high decomposition temperature (410 °C) unveils the ultra-robust nature of the material.
Fig. 1. Plot for the Rietveld refinement. The black curve refers the measured data, red curve indicates the theoretical data and blue curve corresponds to their difference. Vertical black bars indicate the allowed Bragg reflection positions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Scheme 2. (a) Structure of H2IPA-CHO ligand. (b) Coordination mode of the ligand with Al3+ ion. 3
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I−, F−, PO43−, CO32−, NO3−, HCO3−, SO42−, CN−, SCN−, OAc−, ClO4− and N3−) were added into the HEPES buffer suspension of 1′ and corresponding emission spectra were recorded. Compound 1′ exhibited the highest fluorescence quenching efficiency in presence of Cr2O72− (around 94%) whereas the other 15 anions could show negligible quenching efficiency (around 3–24%) except MnO4− and CrO42− (Fig. 4 and Figs. S11–S27). In presence of MnO4− and CrO42− (200 μL, 10 mM), around 80 and 70% quenching efficiencies have been observed for the material, respectively. Nevertheless, the quenching efficiency of the material in presence of Cr2O72− is higher (~94%) than the quenching efficiency observed for MnO4− and CrO42−. From these observations, we can conclude that 1′ showed sufficient selective sensing nature towards Cr2O72−. In order to serve as a widely applicable sensor material, 1′ should exhibit specific response towards Cr2O72− ion in the co-existence of disturbing other anions. To investigate it, 200 µL of 10 mM aqueous Cr2O72− solution was introduced in the HEPES buffer suspension of 1′ which already contained interfering anions. The obtained bar plot (Fig. 5) and Figs. S28–S44 unveil that Cr2O72− could be efficiently recognized by the present material even in the co-existence of several congeners. Material 1′ should show distinct quenching performance towards Cr2O72− at specific concentration. Fig. S45 reveals the correlation between quenching efficiency of 1′ and the concentration of anions used during sensing event. It becomes apparent from the figure that the quenching efficiency of 1′ is gradually enhanced with incremental addition of Cr2O72− solution. On the other hand, other anions are unable to produce any considerable change in the fluorescence quenching efficiency even at higher concentration. During fluorescence quenching event, Stern-Volmer (S-V) plot can provide important information regarding the nature of quenching process. As the detection process of Cr2O72− ion by the present material occurred via quenching of its fluorescence, S-V quenching constant (Ksv) was evaluated by employing the below mentioned equation:
3.4. Chemical stability In order to investigate chemical stability, the activated 1′ was stirred for 24 h in variety of liquids (MeOH, 1 M HCl, DMF and acetic acid). Then, the compounds were filtered off and PXRD measurements were conducted (Fig. S7). The PXRD patterns of the compound remained almost same after treatment with the above mentioned liquids, which discloses that structural integrity is maintained. During detection of Cr2O72− in industrial waste water, MOF material should withstand at different pH media. Specially, in tannery and metal plating industries, wastewater is highly acidic. Hence, 1′ was soaked in different pH media (2–12) for overnight and PXRD patterns were collected after filtration. Fig. S8 reveals that structural robustness is well maintained in the above mentioned pH media. Hence, presented material is a useful candidate for the detection of Cr2O72− in industrial waste water. 3.5. N2 Sorption analysis N2 adsorption measurements were conducted with the activated compound to examine the permanent porosity of the micro-porous material (Fig. S9). It has been well documented that functional group of the ligand molecules have great impact on the porosity of the material [43]. In case of CAU-10 series of MOFs, the functional groups can increase the narrowness of the channels and slow down the diffusion process of the N2 gas. Hence, accessible pores towards N2 gas molecule decrease. The BET surface area and micropore volume of the present material are 55 m2 g−1 and 0.03 cm3 g−1 (at p/p0 = 0.50), respectively. Hence, the material is almost non-porous like most of the functionalized CAU-10-X MOFs (X = -OH, -OCH3, -NH2, -N3, -N2H3) [43,46,49]. 3.6. Detection behavior for Cr2O72− in HEPES buffer For toxic impacts of Cr2O72− on living beings and environment, realtime tracking of Cr2O72− is a serious task. To address this purpose, a number of sensor materials such as zeolites, activated charcoals, πelectron-rich organic molecules, organic polymer resins and MOFs have been investigated for Cr2O72− detection [17–19,31–34,36,37,39,50–52]. But, most of them cannot be employed for practical application purpose as they do not meet the criteria of supreme sensor compounds like fast, selective and sensitive response, high physicochemical stability, nontoxic nature, reusable property, etc. All these facts motivated us to examine the sensing behavior towards Cr2O72− by the newly designed 5formyl isophthalic acid containing luminescent Al(III)-based MOF called CAU-10-CHO. It is worth to mention that aluminium based MOFs are more aqua-stable and bio-friendly than other MOFs containing other metal ions. The detection capability of 1′ for Cr2O72− was examined by introduction of 10 mM Cr2O72− solution into 3 mL HEPES buffer suspension of 1′. The emission spectra were measured after each minute up to 10 min. Before addition of Cr2O72−, 1′ showed strong fluorescence due to the presence of auxochromic aldehyde group. As soon as the material came into contact with Cr2O72− ions, the emission property of the material is nearly vanished (Fig. 3). Moreover, Fig. S10 suggests that under day light, colorless HEPES buffer suspension of the MOF material is converted to light yellow solution in presence of Cr2O72− solution (10 mM, 200 µL). This color transformation is due to the addition of orange colored Cr2O72− solution. Under UV lamp, the HEPES buffer suspension of 1′ showed pale blue fluorescence but the blue emission was completely quenched with the addition of Cr2O72− solution. A potential fluorescent chemo-sensor should have great specificity for the target analyte over other competitive analytes. To examine the selective nature of 1′ towards Cr2O72− ion, solutions (200 μL, 10 mM) of possibly competitive anions (e.g., MnO4−, CrO42−, ClO−, Cl−, Br−,
(I0/ I ) = Ksv [Q] + 1 where, I0 and I represent intensity of 1′ in absence and presence of the analyte, respectively. [Q] denotes the molar concentration of the used analyte. The estimated Ksv value towards Cr2O72− was found to be 1.43 × 104 M−1 (Fig. S46). Fig. 6 depicts the S-V plots for different anions along with Cr2O72− ion. The figure shows that at lower concentration of Cr2O72−, S-V plot is almost linear whereas at higher concentration of Cr2O72−, the plot deviates from its linear nature. This event is attributed to the self-absorption or energy transfer-process [53,54]. Fluorescence titration experiments were carried out for quantification of the detection process of Cr2O72− ion by 1′. For this aim, the fluorescence emission spectra of the HEPES buffer suspension of 1′ were collected with the progressive addition (20 μL in each addition) of 10 mM Cr2O72− solution. Fig. 7 suggests that stepwise addition of Cr2O72− solution brings gradual suppression of emission intensity of 1′. There was ~ 94% quenching of the emission intensity after addition of 200 μL of Cr2O72− solution. To check the sensitivity of the material towards Cr2O72− ion, limit of detection (LOD) was evaluated by including very low concentrated solution of Cr2O72− ion. By plotting the emission intensity of the material against the employed concentration of Cr2O72− (Fig. S47), a straight line was obtained. By employing the formula 3σ/k (k = slope obtained from the line, σ = standard deviation of repetitive blank measurements), the estimated LOD value is 0.69 µM, which is comparable with the LOD values of formerly reported MOF materials (Table S3) [1,32,34,36,55]. The recyclability experiments of compound 1′ were examined up to five cycles of detection experiments to prove the fluorescent restoration property of the material. After detection experiment, 1′ was filtered and washed thoroughly with distilled water repeatedly. Then, it was further 4
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Fig. 3. (a) Fluorescence “turn-off” response of HEPES buffer suspension of 1′ towards the addition of Cr2O72− solution up to 10 min. (b) Time-dependency of the emission intensity monitored at 427 nm.
Fig. 4. Quenching efficiencies of 1′ towards different anions in HEPES buffer (monitored at 427 nm).
Fig. 5. Fluorescence quenching efficacy of Cr2O72− in presence of various disturbing anions.
utilized for the next detection experiment. This process was repeated up to 5 cycles. Fig. S48 depicts that even after five cycles of repeated fluorescence detection experiments, the probe showed similar emission
intensity as the initial one and the quenching efficiency of 1′ in presence of Cr2O72− ion is retained at similar level. PXRD study concludes that the structure of the MOF material remains intact after five cycles of 5
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Fig. 6. Stern-Volmer plots for the fluorescence quenching efficiency of 1′ with incremental addition of various analytes.
medium whereas, HCrO4− is the dominant species in acidic medium. It is noticeable that though different species of chromium oxo-anions (CrO42−, Cr2O72− and HCrO4−) are present at different pH values, all of them have Cr in + 6 oxidation state. Hence, in broad sense, we can say that the present probe can detect all Cr(VI) oxoanions. To investigate the influence of formyl functionality during the detection process of Cr2O72−, CAU-10 MOF without any functionalization was synthesized and emission spectra of the HEPES buffer suspension of the material were collected before and after addition of Cr2O72− solution. From Fig. S51, it is quite evident that without formyl functionality the material is almost non-fluorescent with respect to 1′ and no observable change in emission intensity is visualized after the introduction of Cr2O72− solution, but 1′ is showing drastic quenching of its emission property in presence of Cr2O72−. Thus, without formyl functionalization, the material fails to detect Cr2O72−. Hence, formyl functionality plays a significant role to enhance the luminescent property of the material and detection of Cr2O72−. Fig. 7. Quenching of the fluorescence intensity of HEPES buffer suspension of activated compound upon progressive inclusion of 10 mM Cr2O72− solution.
3.7. Detection of Cr2O72− in real water samples For examining the Cr2O72− sensing behavior of the MOF material in real world samples, total nine water specimens were prepared. They contain three different concentrations of Cr2O72− for each type of water specimen (mineral, tap and lake water). The concentrations found for Cr2O72− and the recovery percentages for the environmental water specimens are tabulated in Table 1. The recovery percentages of Cr2O72− were observed to be in the range of 97–102% for all the samples and the RSD values were in between 1.06 and 5.39 (RSD = (SD/mean value) × 100). Low RSD values and good recovery percentages suggest that the probe can provide suitable recoveries as well as good analytical precisions for real water specimens.
detection experiments (Fig. S49). For wide applicability purpose, 1′ should show quenching behavior at wide pH range because pH of industrial waste water varies widely (For example, tannery and metal plating waste water are acidic in nature). To address this purpose, quenching behavior of 1′ for Cr2O72− ion was studied in a wide pH range (2–12). Fig. S50 signifies that though the initial intensity of 1′ varies with different pH values of the media, at any pH value, emission intensity of 1′ is suppressed in presence of 200 µL of 10 mM Cr2O72−. All discussions suggest that the present probe is highly photo-stable and reusable. Hence, the probe could be applied for long term detection of Cr2O72− ion in real world. It is worth to mention that the presence of chromate species is influenced by the pH and concentration of the medium [56]. Above pH 6, the dominant species is yellow chromate (CrO42−) ion. Between pH 2 and 6, HCrO4− and orange-red dichromate (Cr2O72−) ion are in equilibrium [57,58]. Hence, in dilute solution, Cr2O72− exists as CrO42− in basic
3.8. Detection of Cr2O72− in portable paper strips Recently, due to rapid and low-cost detection, portable devices based on paper strips have drawn great interest. Here, MOF-coated paper strips were prepared by dip coating method, which were utilized 6
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Table 1 Detection performance of the MOF material for Cr2O72− in real water samples. Cr2O72− Spiked (µM)
Mineral Water Cr2O72−
5 25 50 a
Tap Water a
Found (µM)
5.01 ± 2.03 25.13 ± 1.49 48.72 ± 1.96
Lake Water a
Recovery (%)
Cr2O72−
100.20 100.52 97.44
4.85 ± 4.84 24.97 ± 1.71 49.83 ± 1.73
Found (µM)
Recovery (%)
Cr2O72− Founda (µM)
Recovery (%)
97.00 99.88 99.66
5.14. ± 5.39 24.44 ± 3.23 49.93 ± 1.06
102.80 97.76 99.86
Cr2O72− found ± RSD (n = 3).
transfer process, ground state complex formation, excited-state reaction or through inner filter effect (IFE). All these events can be categorized in two types of fluorescence quenching mechanisms: one is dynamic quenching and another one is static quenching. Dynamic quenching mainly arises due to collision process or diffusion of quencher to the excited state fluorophore. On the other side, close association between quencher and fluorophore reduces the emission intensity in case of static quenching [59]. Fluorescence lifetime measurement technique could be employed to differentiate between dynamic and static quenching process. Lowering of lifetime of excited fluorophore in presence of quencher molecule occurs for dynamic quenching process but in case of static quenching, lifetime of the fluorophore remains same even in the presence of quencher molecule [60]. Time-resolved fluorescence lifetime decay of HEPES buffer suspension of 1′ in absence and presence of Cr2O72− was investigated. The obtained fluorescence decay profile (Fig. S52) and the fitted results (Table S2) unveil that Cr2O72− solution brings a dramatic lowering in the lifetime (12.61 ns to 7.04 ns). This result assured that the quenching process occurred through dynamic quenching rather than static quenching process and IFE process. In IFE and static quenching processes, the lifetime of fluorophore moiety would not change in presence of quencher [61]. To deeply examine the quenching process, UV–vis spectra of all competitive anions including Cr2O72− ion were collected (Fig. 9a). It becomes obvious from Fig. 9b that partial overlap occurs between the emission spectrum of 1′ and the absorption bands of Cr2O72−, MnO4− and CrO42−, whereas other anions do not absorb in 300–450 nm range. Hence, we can conclude that resonance energy transfer from 1′ to Cr2O72−, MnO4− and CrO42− is the possible reason for the emission reduction of 1′ in presence of the mentioned anions [55]. In addition, due to high oxidizing nature of the anions (Cr2O72−, MnO4− and CrO42−), electron transfer from ligand (H2IPA-CHO) present in the framework to the oxidant is also highly possible, which restricts the ππ* electron transfer [52]. As a result, quenching of luminescence property of the material is observed. It is worth to mention that though MnO4− and CrO42− can reduce the emission intensity of 1′ (quenching efficiencies are 80.3 and 70.7%, respectively), 1′ showed the highest
Fig. 8. Digital images of 1′-coated paper test strips (under UV lamp) after the introduction of 10 µL of Cr2O72− solution of various concentrations. A dark spot can be seen only in presence of Cr2O72− solution.
for Cr2O72− detection. The MOF-coated paper strips were treated with various concentrations of Cr2O72− solution (10 µL volume in each paper strip) by using a micropipette. From Fig. 8, it can be observed that the MOF-coated paper strip showed blue fluorescent under UV lamp before treatment with Cr2O72− solution and dark spots appeared after the addition of Cr2O72− solution. The darkness of the spot was low at lower concentrations of Cr2O72− solution. The dark spot appeared due to quenching of the blue fluorescence of the MOF-coated paper strip in presence of Cr2O72− solution. Hence, this is a useful method for the on-site detection of Cr2O72−. 3.9. Plausible mechanism for detection of Cr2O72− Fluorescence quenching of 1′ by Cr2O72− ion may occur due to various phenomena such as collision between molecules, energy
Fig. 9. (a) UV–vis spectra of different anions in HEPES buffer solution. (b) Spectral overlap between emission spectrum of 1′ and absorption spectrum of Cr2O72−. 7
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quenching efficiency (94%) in presence of Cr2O72−. Moreover, reduction in life time value of the material in presence of Cr2O72− ion strongly supports the above discussed electron and energy transfer processes. Hence, we can conclude that the combined effect of resonance energy transfer and electron transfer is the principal cause for the emission quenching of 1′ in presence of Cr2O72−. The above discussed mechanistic processes are in good agreement with the previous reports [50,62].
[14] [15] [16] [17]
4. Conclusions
[18]
In summary, an aldehyde functionalized Al(III) based MOF called CAU-10-CHO has been hydrothermally prepared by employing 5formyl isophthalic acid as a linker molecule. TG study shows that the compound displays its robust nature up to 410 °C. The desolvated compound in HEPES buffer medium shows great quenching performance towards Cr2O72− over other possible intrusive anions. Moreover, the material has excellent sensitivity for Cr2O72− (LOD = 0.69 µm). Cr2O72− detection ability of the material in real water samples is also tested. In addition, the material retains its detection ability towards Cr2O72− at any pH media (2–12). Furthermore, high structural robustness and reusability have been displayed by 1′ for the sensing of Cr2O72−. Out of several MOF materials which were reported for the detection of Cr2O72−, the present material showed quite better sensing performance in-terms of response time, detection limit, stability and reusability for real time detection of Cr2O72−. Therefore, the MOF material is a potential probe towards selective and quantitative detection of Cr2O72− in aqueous media by quenching emission intensity.
[19] [20] [21] [22] [23] [24]
[25]
[26] [27]
Acknowledgments
[28]
We are highly thankful to the Science and Engineering Research Board (SERB; grant no. EEQ/2016/000012), New Delhi for generous financial support.
[29] [30]
Appendix A. Supplementary data
[31]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ica.2019.119078.
[32]
References
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