Guest-Induced Ultrasensitive Detection of Multiple Toxic Organics and

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Guest-induced Ultrasensitive Detection of Multiple Toxic Organics and Fe Ions in a Strategically Designed and Regenerative Smart Fluorescent MOF Ranadip Goswami, Shyama Charan Mandal, Biswarup Pathak, and Subhadip Neogi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20013 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

Guest-induced Ultrasensitive Detection of Multiple Toxic Organics and Fe3+ Ions in a Strategically Designed and Regenerative Smart Fluorescent MOF Ranadip Goswami,†‡ Shyama Charan Mandal,§ Biswarup Pathak,§ and Subhadip Neogi*,†‡ †Academy

of Scientific and Innovative Research (AcSIR), (CSIR-CSMCRI), Bhavnagar, 364002, Gujarat, India ‡Inorganic Materials & Catalysis Division, CSIR-CSMCRI Bhavnagar, Gujarat-364002, India §Discipline of Chemistry, Indian Institute of Technology (IIT) Indore, Indore, Madhya Pradesh, 453552, India *E-mail: [email protected] ABSTRACT: Luminescent metal-organic frameworks (LMOFs) are promising functional materials for sustainable applications, where analyte induced multiresponsive system with good recyclability is beneficial for detecting numerous lethal pollutants. We designedly built the dual functionalized, three-dimensional Zn(II)–framework [Zn3(bpg)1.5(azdc)3]∙(DMF)5.9∙(H2O)1.05 (CSMCRI-1) using –OH group integrated bpg linker, and –N═N– moiety containing H2azdc ligand, which functions as unique tetra-sensoric fluorescent probe. The activated CSMCRI-1 (1′) represents hitherto unreported pillar-layer framework for extremely selective fluorescence quenching by nitrofurazone (NZF) antibiotic as well as explosive nitro-aromatic 2,4,6-trinitrophenol (TNP), where ultrasensitive detection is achieved for both the electron lacking analytes. Impressively, 1′ represents the first ever MOF for significant fluorescence “turn-on” detection of toxic and electron rich 4aminophenol (4-AP) in the concurrent presence of isomeric analogues. Density functional theory calculations highlight the specific importance of pillar functionalization in “turn-on” or “turn-off” responses of 1′ by electronically divergent toxic organics, and provide further proof of supramolecular interactions between framework and analytes. The fluorescence intensity of 1′ dramatically quenches by trace amount of Fe3+ ions over other competing metal ions, alongside visible colorimetric change of the framework in solid and solution phase upon Fe3+ encapsulation. The sensing ability of 1′ remains unaltered for multiple cycles towards all the lethal pollutants. The sensing mechanism is attributed to both dynamic and static quenching as well as resonance energy transfer (RET), which strongly comply with the predictions of theoretical simulations. Considering long term and real time monitoring, AND as well as OR molecular logic gates are constructed based on the discriminative fluorescence response for each analyte that provides a platform to fabricate smart LMOFs with multimode logic operations. KEYWORDS: Pillar-layer structure; tetra-sensoric probe; colorimetric detection; regenerative MOF; molecular logic gate

fluorescence

turn-on;

INTRODUCTION With excessive economic growth caused by urbanization, an increasing number of chemicals are produced to meet the ever-increasing demand for various industrial products. This leads to severe environmental contamination, where various organic as well as inorganic pollutants pose serious threats to human health and environment.1,2 For example, antibiotics, though emerged as an important class of pharmaceuticals to cure bacterial infections, their extreme toxicity, degradation difficulty, and continuous release without thorough eradication ACS Paragon Plus Environment

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conferred them as serious organic pollutants.3,4 Particularly, nitro-containing antibiotics nitrofurazone (NZF), which increases resistance to bacterial infections in aquaculture, is in the "list of drugs prohibited in animal foods" since NZF contaminated foods causes hereditary genetic defects, immunity decline and cancers.5 Therefore, urgent need is to develop fast, selective, and sensitive detection technique of this antibiotic. On the other hand, nitro aromatic compounds (NACs) are class of utmost toxic and hazardous chemicals and can never be neglected. The most frequently applied NAC is 2,4,6-trinitrophenol (TNP) that is enormously released to environment, causing serious health problems like anaemia, respiratory organ injury, and gene mutation.6 Also, TNP has explosive power stronger than other common nitro compounds.7 Accordingly, it is of enduring importance to detect minute presence of TNP with regard to homeland security, military applications, and forensic science.8 Besides, the recent need is to establish reliable and sensitive chemical sensor for the recognition of phenolic compounds for controlling food quality, environmental safety, health and protection, as they are classified as “priority pollutants” by both the European Commission and the United States Environmental Protection Agency (USEPA).9,10 Among others, 4-amino phenol (4-AP) is highly carcinogenic and poses serious threat towards health and environment.11 Likewise, iron has experienced an ever-growing attention in environmental and biological systems, owing to its consequence and function in oxygen uptake and metabolic processes. While Fe3+ ions in moderate concentration is desirable in living systems, this metal ion is now considered as an industrial pollutant, and its overloading or deficiency causes numerous biological complications such as, endotoxemia or hepatic cirrhosis.12,13 Since luminescent based detection provides a simplistic method with high selectivity, sensitivity, and accuracy, construction of fluorescent based sensors has prompted the development of methods for monitoring of Fe3+ ions in solution.14,15 However, shaping an effective Fe3+ ion sensor is still limited by (i) its cross-sensitivity toward other metal ions, (ii) bad absorbing ability, and (iii) meticulous synthesis of probe materials.16 Above all, the current commercial methods and technique-based instruments for detecting all of the aforesaid pollutants are time consuming, expensive, and single analyte directed devices with complicated procedures that necessitates fabricating highly effective, multifunctional material to be applied easily for a broad range of lethal pollutants.17,18 In this milieu, metal–organic frameworks (MOFs) represent as flourishing subclass of porous crystalline materials, with potential applications in gas-storage, separation, and catalysis.19-21 Fluorescence sensing is another captivating area for MOFs, where ultrahigh surface areas, modifiable porosities, wide range of host–guest interactions, and functionalized pore walls are the major benefits.22,23 Thanks to the exceptional luminescent features of spectroscopically silent metal ions, in juxtaposition with diverse π-conjugated organic ligands that are used to construct efficient fluorescent MOFs.24,25 However, major pressing problems concerning the synthesis of luminescent MOFs involve: (i) flexible coordination geometry of metal ions, (ii) difficulty in attaching appropriate signal transmission groups to the MOF backbone, and (iii) lack in specific recognition of analytes at the molecular level for effective signal modulation with measurable change. In addition, energy or electron transfer mechanism between the host and guest molecules mostly results in “turn-off” fluorescence response,26,27 which urges the development of more favourable “turn-on” responsive MOFs for chemical sensing. In view of the above facts and our quest to develop single, multi-functionalized MOF with wide range of applications,28,29 the present research strategy evolved by selecting bpg linker, where methylene group bestows flexibility to the target framework, and pendent –OH functionalities should offer additional binding sites to the guest molecules. The unpredictability of the targeted MOF, using such a flexible linker is judiciously balanced by employing azo group functionalized H2azdc ligand, where formation of paddle-wheel secondary-building units (SBU) strengthens stability and dictates porosity of the resulting ACS Paragon Plus Environment

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pillar-layer framework CSMCRI-1 (CSMCRI = Central Salt & Marine Chemicals Research Institute). The activated framework demonstrates extremely selective quenching of NZF and TNP, with a very low limit of detection. Importantly, unique fluorescence “turn-on” detection of toxic 4-AP, among the isomeric analogues, is realized. Such “turn-off” or “turn-on” responses are exclusively governed by the individual electronic properties of these aromatics, where density functional theory calculations highlight the impact of pillar functionalization and provide proof of diverse supramolecular interactions between the framework and electronically dissimilar toxic organics. The multisensoric behaviour of the framework is certified by the exceptionally discriminative fluorescence quenching of Fe3+ ions with a visible color change. The quenching mechanism for all the analytes is ascribed to the presence of dynamic plus static quenching and resonance energy transfer (RET). The importance of selective and sensitive detection of noxious and hazardous compounds is further amplified when the spectral outputs can be transformed to real-time application. In this context, construction of molecular devices using basic building methodology as logic gates and formation of truth tables containing spectral inputs are practically desirable. Along this line, we constructed molecular logic gate from selective luminescent recognition of NZF, TNP, 4-AP, and Fe3+ ions by the framework. Although, the discriminative detection of different analytes has been reported based on logic operations,30,31 such reports on the construction of MOF-based logic gates rare.32-34 To the best of our knowledge, the dualfunctionalized CSMCRI-1 represents the first example of pillar-layer based, multiresponsive, ultrasensitive, and recyclable luminescent MOF-sensor that can be used to environments with a variety of pollutional sources. RESULTS AND DISCUSSION Crystal Structure of CSMCRI-1. The block-shaped, orange crystals of CSMCRI-1 were synthesized by the solvothermal reaction of Zn(NO3)2∙6H2O, H2azdc, and bpg (1:1:1) in DMF at 90 °C in 48 h. In spite of the poor crystallinity, we were able to collect single crystal X-ray diffraction on a selected crystal that revealed it crystallizes in the triclinic space group P-1 (Table S1). The asymmetric unit contains three Zn(II) ions, three deprotonated azdc ligands, one and a half bpg linkers (Figure S1). Additionally, we could locate N,N′-dimethylformamide (DMF) and water molecules as uncoordinated guest solvents. However, these guest molecules are highly disordered that is common in organic-inorganic hybrid porous structures. Close insight into the structure of CSMCRI-1 reveals that the equatorial positions of two Zn(II) ions are coordinated by eight oxygen atoms, originating from four azdc ligands, to construct paddle-wheel {Zn2(CO2)4} secondary building units (SBU; Figure 1a).35 These SBUs are connected through azdc ligands to form 2D square grids (Figure S1). The axial pillaring to the SBUs by the nitrogen atoms of bpg linkers afford an extended 3D pillar-layer framework with α-Po topology (Figure S2), which is typical for mixed ligand based metal-organic frameworks (MOFs).36 In the SBUs, each five coordinated Zn(II) ion adopts a square pyramidal geometry with Zn–N bond length of 2.033(3) Å, and average Zn–O bond lengths of 2.018 Å, which are in the normal range. A simplified version of this extended structure is depicted in Figure 1b that shows formation of a box shaped structure. The distances between the diagonal Zn(II) atoms approximates to 27.50 Å (distances excludes vander Walls radii), generating large pores. Structural analysis with TOPOS37 reveals a 6-c unimodal net with corresponding Schläfli symbol {412.63} (Figure S2).38 Importantly, the uncoordinated –OH groups are directed towards the pore, implementing electron rich and hydrophilic environment to the cavity. On the other hand,– N=N– moieties in azdc ligands are part of the MOF wall, manifesting electron deficient nature to the pore. Such dual functionalization in CSMCRI-1 should be advantageous for sensing applications. The introduction of extended dicarboxylate ligands and long dipyridyl linkers leads to the generation of huge pores that instigates three-fold interpenetration to the ACS Paragon Plus Environment


overall 3D structure (Figure 1c). As a result, the pore dimension is significantly reduced, where every large pore in the box shaped structure is divided into differently sized voids. When viewed through crystallographic a axis, the larger cavity shows a pore size of 8.8 × 8.9 Å2, while smaller sized pore has dimension of 4.4 × 8.9 Å2 (Figure 1d). Both these pores are filled with guest solvent molecules. Despite three-fold interpenetration, PLATON calculation39 suggests presence of large solvent accessible volumes of 48.0 %. a)

b)

c)

d)

Figure 1. (a) A simplified view of the paddle-wheel SBU, (b) The box-shaped structure with large pores in pillar-layer framework, (c) Representation of the 3-fold interpenetrated structure in CSMCRI-1, (d) Pore view along crystallographic a axis, showing two types of cavities. Establishment of Stability and Permanent Porosity. Thermogravimetric analysis (TGA) of CSMCRI-1 under N2 atmosphere shows a two-step weight loss (calcd. 25.32 wt %), that corresponds to the release of water and DMF guest molecules (Figure S3a). The desolvated CSMCRI-1 (hereafter 1′) does not reveal any plateau up to 370 °C and decomposes thereafter. The powder X-ray diffraction (PXRD) pattern of the as-synthesized crystals reveals complete agreement to that of simulated pattern, indicating high crystallinity and phase purity of the sample (Figure 2a). Desolvation leads to slight shift of the Bragg’s reflections (Figure 2a) that may result due to the release of solvent guests from the porous channel.40-42 Interestingly, the low angle peaks were restored upon exposure of 1′ with DMF (Figure 2a). It is worth mentioning that such a modification in PXRD patters upon desolvation and resolvation is not unique and has been realized in ACS Paragon Plus Environment

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interpenetrated pillar-layer frameworks.43-45 In addition, stability of CSMCRI-1 was checked in diverse organic solvents as well as in water and open air. As displayed in Figure 2b, the characteristic peaks in the PXRD remain unchanged in all the cases up to 24 hrs, while slight broadening of peaks occur upon exposure to water for prolonged time. Variable temperature PXRD (VTPXRD) up to 250 °C reveals sufficient stability of the framework (Figure S4). To assess the permanent porosity, nitrogen gas sorption isotherm was attempted. Prior to measurement, the guest solvents in the as-synthesized crystals were exchanged by dichloromethane, and degassed overnight under vacuum at 120 °C to generate 1′. The framework shows 86 cm3/g of N2 gas uptake at 77 K up to 1.0 relative pressure (P/P0) that corresponds to a BET surface area of 112.69 m2/g. The result corroborates presence of porosity (Figure S3b) and further indicates that −OH groups of the linker do not appreciably reduce the accessible space in 1′.46 a)

b)

Figure 2. (a) The simulated (black), experimental (red), desolvated (violet) and resolvated (green) powder X-ray diffraction (PXRD) patterns for CSMCRI-1 along with (b) PXRD curves of CSMCRI-1 in different organic solvents, water and open air. Photoluminescence Studies. The presence of electronically passive d10 transition metal ions, together with highly conjugated aromatic ligands allow the dual-functionalized CSMCRI-1 to act as potential fluorophore with venerable photoluminescence (PL) properties.47,48 To find the best suited solvent for fluorescence experiment, we dispersed 1′ in a series of solvents (1 mg in 20 mL of solvent) and individual suspensions were excited at the excitation wavelength. The maximum fluorescence emission intensity of 1′ was achieved in DMF (Figure S6c), which insisted us to perform all sensing experiments in this solvent. A homogeneous dispersion of 1′ (1 mg in 2 mL DMF) exhibits strong fluorescence emission at 388 nm with a shoulder near 427 nm, when excited at 350 nm at room temperature. The significant emission can be attributed to the mixed involvement of the intra-ligand charge transfer transition of π-electron-rich ligands (π–π* and n–π* transitions),49,50 ample π–π stacking interaction between the ligands in the interpenetrated structure, as well as photo-switching aptitude (trans → cis) of azdc ligand.28,51 Additionally, UV-vis spectra of 1′ displays red shifts of 17 nm and 81 nm compared to free H2azdc and bpg, respectively (Figure S6a), which stems from the perturbation effect due to their coordination with Zn(II) ions.52,53 Impressively, the framework exhibits large 88 % and 79 % intensity enhancement than that for free carboxylate ligand and pyridyl linker correspondingly, ascribing strong electronic communication between the neighbouring coordinated ligands through Zn(II) ions (Figure S6b). Such ligand-centred emission is highly anticipated, as their ACS Paragon Plus Environment


modification in bandgaps due to host–guest interactions may lead to different fluorescent responses, including changes of the luminescence intensity and position. Hence, luminescent performance of this MOF was investigated in detail towards diverse lethal pollutants. Fluorescence Detection of NZF Antibiotic. As already discussed, antibiotics are essential natural poisons, which urges their discriminative detection. Considering high porosity, excellent luminescence, and good stability in solution phase, 1′ was initially chosen for liquid phase fluorescence sensing of diverse classes of detrimental antibiotics such as nitrofuran (nitrofurazone, NZF; nitrofurantoin, NFT; furazolidone, FZD), nitroimidazole (metronidazole, MDZ; dimetridazole, DTZ; ornidazole, ODZ), sulfonamides (sulfadiazine, SDZ; sulfamethazine, SMZ), and chloramphenicols (thiamphenicol, THI). Fluorescence titrations were performed by incremental addition (20-160 µL) of 1 mM stock solution of each antibiotic into the suspension of 1′ (1mg in 2mL DMF) at room temperature. The consistency of individual results was cross-checked by performing each titration in triplicate fashion. A successive quenching in the fluorescence intensity of the MOF was observed depending on the presence specific organic groups and relative sizes in different classes of antibiotics (Figure S8-S15). Surprisingly, NZF shows remarkable 91 % quenching efficiency upon only 160 µL addition to 1′ (Figure 3a), while NFT and FZD follow sequential quenching efficiency of 87.4 % and 83.0 % (Figure S7), respectively. Notably, remaining antibiotics, which do not contain furan moiety as a part of their molecular structure, show trivial influence on the emission intensity of 1′ and furnished low quenching efficiencies (Figure 3b). The quenching competences of the framework for the entire classes of antibiotics was found in the order NZF > NFT > FZD >> MDZ > DTZ > ODZ > SMZ> SDZ > THI (Figure 3c and Figure S7). Given nitro-containing antibiotics, specifically NFZ, are prohibited to exist in food-producing; such a high level of detection of NZF is positively motivating. To explore more on such efficient sensing of NZF, and quantitatively assess the quenching phenomena, concentration-dependent decrease in fluorescence intensity of 1′ were plotted by using Stern-Volmer (SV) equation54 (I0/I) = KSV[A] + 1, where KSV is the quenching constant (M−1), [A] is the molar concentration of the analyte, I0 and I are the luminescence intensities before and after addition of the analyte, respectively. As depicted in Figure S16-S18, SV plots for NFs are nearly linear in the concentration range 0 to 0.05 mM, but bend upward as the concentrations rises. Such nonlinear SV plots might be a consequence of dynamic and static quenching processes and/or a resonance energy transfer (RET) mechanism.7,55 Quite in contrast, other classes of antibiotics revealed linear SV plots (Figure 3c) throughout the whole concentration range (0 mM - 0.08 mM). From the linear fitting of the plot, the quenching constant for NZF, NFT and FZD were calculated to be 8.75 × 104 M–1, 5.92 × 104 M–1, 4.8 × 104 M–1, respectively (Figure S19-S21). To the best of our insight, present quenching constant value for NZF is one of the highest among the reported MOFs for antibiotics sensing (Table S9). Further, limit of detection (LOD)56 was obtained from the plot of decrease in fluorescence intensity of 1′ upon incremental addition (20-160 µL) of 10 µM NZF solution (Figure S22). The LOD for NZF (Table S2) was found to be 0.8 ppm (0.66 µM), using equation 3σ/K, where σ denotes the standard deviations for five consecutive fluorescent measurements of blank MOF solution at fixed time intervals (2 mins), and K signifies slope of the aforesaid curve. Clearly, the LOD value is comparable or higher than handful examples of antibiotic sensory MOFs (Table S9), where 1′ stands as first pillar-layer MOF for detection of nitro-furan based NZF antibiotic, as far as we know.

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a)

b)

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Figure 3. Emission spectra of a homogeneous dispersion of 1′ upon incremental addition of 1 mM solution of (a) NZF and (b) THI, (c) S–V plots for different antibiotics (1 mM). Conditions: λmax for 1′ ca. 388 nm; 5 nm slit width, (d) The plot of interference test, showing decrease in fluorescence intensities upon addition of different antibiotics (1 mM, 160 µL), followed by NZF (1 mM, 160 µL) to 1′. Selectivity of NZF. Since anti-interference is one of the most important prerequisite for chemical sensors, we performed competitive anlaytes test of 1′ towards NZF in the presence of potentially interfering other classes of antibiotics, maintaining standard protocol (vide supra). As can be seen from Figure 3d, slight or negligible fluorescence change occurs upon addition of 160 µL of interfering analytes (1 mM). In contrast, simultaneous addition of 160 µL of NZF (1 mM) shows rapid and significant quenching of the emission intensity. The results reveal that noteworthy NZF selectivity of the framework is retained even in the presence of a higher concentration of other antibiotics, and interference from rest of the antibiotics can be solemnly neglected (Figure S23-S28). Such exceptionally specific and ultrasensitive detection of NZF in the presence of other analytes confers 1′ as a reliable sensor for NZF. Besides, recyclability as well as reusability of the framework was tested for long term and real- time monitoring. To this end, the sample was recovered via centrifugation after sensing studies, and washed with DMF for several times. The regenerated material was used again ACS Paragon Plus Environment


and it maintains quenching efficiency up to five sensing-recovery cycles with negligible variation in emission intensities (Figure S29). It should be also pointed that the structure remains intact, as confirmed by unaltered PXRD patterns of the framework after the sensing experiments (Figure S30). Fluorescence Detection of 2,4,6-trinitrophenol and 4-amino phenol. Inspired by the exciting photoluminescence behaviour and remarkable quenching of electron lacking antibiotic NZF, we further explored the potential of 1′ as a sensor for the detection of various hazardous nitro aromatic compounds (NACs) such as 2,4,6-trinitrophenol (TNP), 2,4dinitrophenol (2,4-DNP), 2,4-dinitrotoluene (2,4-DNT), nitro phenols (4-NP, 3-NP, 2-NP), nitro toluene (4-NT) and aliphatic nitro compound 2,3-dimethyl-2,3-dinitrobutane (DMDNB), which are also electron-deficient in nature. Following the typical procedure (vide supra), changes in the luminescence spectra was monitored by incremental addition of selected nitro analytes (1 mM in DMF) up to 200 µL to a dispersion of 1 mg guest free crystals of CSMCRI-1 (Figure S32-S37). As shown in Figure 4a, the fluorescence intensity of 1′ radically decreased with incremental addition of TNP solution. To our delight, 1′ showed quenching efficiency of 94 % and 86 % towards TNP, and 2,4 DNP, respectively. However, other NACs as well as nitro aliphatic DMDNB show minor quenching effect (Figure 4b and 4c). The order of quenching efficiencies was found in the order TNP > 2,4DNP >> 4-NP > 3-NP > 2-NP > 4-NT > 2,4-DNT > DMDNB (Figure S31), which firmly validate the selective recognition of TNP. To quantify the quenching constant (Ksv) and identify the nature of quenching process (static vs. dynamic), S-V plots were obtained by plotting relative fluorescence intensity [(I0/I)] with [A] molar concentration of TNP. The curve deviated from linearity at higher concentration (Figure S38), indicating the presence of dynamic and static quenching processes.7,55 However, S-V plots for all other NACs remain linear in nature. The quenching constant for TNP, calculated from the linear region of the above-mentioned curve at lower concentration range (0.0 mM-0.04 mM), was found to be 4.6 × 104 M–1 (Figure S39), which rank among the highest values to most of the TNP sensory MOFs and comparable to the famous MOFs for TNP detection (Table S10). Essentially, such an extraordinary quenching value further necessitates quantification of LOD, which was calculated by monitoring the quenching effect via incremental addition of 10 µM TNP solution to the dispersion of activated CSMCRI-1 (Figure S42). The LOD was found (Table S4) to be 0.53 ppm (0.4 µM), which is superior to most of the reported MOFs till date (Table S10). Encouraged by its extreme luminescence quenching, coupled with the utmost importance for selective detection of superiorly explosive TNP in presence of other nitro congeners, we next examined competitive analytes test of 1′ towards TNP. As shown in Figure 4d, the pristine emission intensity of the MOF was merely affected by other nitro compounds. In contrary, simultaneous addition of 200 µL of TNP solution leads to substantial fluorescence quenching (Figure S43-S48). The result clearly manifests the framework as an efficient material for selective and sensitive detection of TNP in the simultaneous presence of other nitro congeners. The reproducibility of emission intensity of the framework has been checked up to five sensing-recovery cycles, which showed that initial fluorescence intensity of 1′ was virtually restored (vide supra) (Figure S49). Besides, PXRD patterns (Figure S50) of the framework notify no alteration of peaks even after five repetitive cycles of fluorescence titration with 1 mM TNP solution, attesting to the exceptional reusability of the material for long-term in-field detection of noxious TNP.


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a)

b)

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Figure 4. Emission spectra of 1′ upon incremental addition of 1 mM solution of (a) TNP and (b) 2,4-DNT, (c) S–V plots for diverse nitro congeners (1mM) to a homogeneous suspension of 1′. Conditions: λmax for 1′ ca. 388 nm; 5 nm slit width, (d) The interference plot showing the decrease in fluorescence intensities upon the addition of various nitro analytes (1 mM , 200 µL ), followed by TNP (1 mM , 200 µL). In general, electron deficient NACs withdraw electrons from electron rich MOF by hostguest interaction that eventually results “turn-off” mode through a decrease in emission intensity. Motivated by this basic knowledge, we anticipated that electron-rich guests should in principle, result a more favourable “turn-on” response in the activated CSMCRI-1. Accordingly, we investigated fluorescence titration by successive addition (20-400 µL) of 1 mM stock solution of diverse amino phenolic compounds into the dispersion of 1′ (vide supra) at room temperature. Opposed to nitro explosives or nitrofuran based antibiotics, the fluorescence response of 1′ enhanced upon addition of amino phenol compounds (Figure 5a and Figure S51-S52). In particular, 4-aminophenol (4-AP) results most significant “turn-on” response, which is more than 3-fold of the pristine emission intensity of the framework. Although, 3-dimethyl amino phenol and 3-diethyl amino phenol moderately enhance the fluorescence intensity, negligible “turn-on” response of the framework was witnessed for 3amino phenol (3-AP) and 2-amino phenol (2-AP) (Figure 5b and 5c). This observation clearly manifests high selectivity of the activated CSMCRI-1 towards 4-AP over other isomeric analogues. To further verify this conclusion and analyse the discriminative identification of 4-AP in detail, we performed competitive fluorescence titration experiments (vide supra). The results are depicted in Figure 5d that divulge negligible effect on emission intensity of ACS Paragon Plus Environment


the framework upon 400 µL addition of either 2-AP or 3-AP. However, rapid enhancement of fluorescence emission was marked in the synchronous presence of 4-AP. The combined results conclude that 1′ is an outstanding material for selective identification of 4-AP, even when other isomeric amino phenols co-exist in the solution. The enhancement constant was obtained using the equation (I/I0) = KEC[A] + 1, where KEC stands for enhancement constant (M−1) and other terms bear their own significances (vide supra).57 The KEC was calculated to be 9.34 × 103 M–1, 5.14 × 103 M–1, 2.6 × 103 M–1 for 4-AP, 3-dimethyl amino phenol and 3diethyl amino phenol, respectively (Figure S53-S55). To the best of our knowledge, this is the first report of selective luminescent sensing for 4-AP among isomeric amino phenols by any MOF material. We calculated the limit of detection (LOD) of 1′ (Figure S56) from fluorescence enhancement via incremental addition of 4-AP (10 µM) to the well dispersed solution of 1ʹ (Table S6), which turned out to be 0.17 ppm (0.13 µM). Additionally, PXRD pattern of the recovered framework (by centrifugation, followed by washing with fresh DMF) exhibits well-matched diffraction peaks to that of CSMCRI-1, corroborating crystallinity and framework integrity are well maintained during sensing experiments (Figure S59). a)

b)

c)

d)

Figure 5. (a) Fluorescence spectra of 1′, showing large intensity enhancement upon incremental addition of 4-AP solution (1 mM), along with unaltered fluorescence spectra of 1′ in presence of 1 mM solution of (b) 3-AP and (c) 2-AP, (d) A plot showing selective “turnon” response for 4-AP in presence of isomeric amino phenols. Mechanism for Sensing of NZF, TNP and 4-AP. The quenching mechanism was investigated in detail to better understand such efficient luminescent sensing of the MOF through either “turn-on” or “turn-off” responses by three ACS Paragon Plus Environment

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different classes of toxic organic aromatics. Beforehand, encapsulation possibilities for any of the analytes into the framework channel can be ruled out based on the small aperture dimensions of pores in CSMCRI-1. Hence, interaction of 1′ with antibiotics, nitro explosives, and amino-phenols can only be associated to surface adsorption.58 The miraculous fluorescence quenching of NZF and TNP with pronounced non-linear S-V plots suggest the concurrent existence of static as well as dynamic quenching events. Given the electron-withdrawing nitro group is present in both these analytes, we provisionally justify the quenching response by a photoinduced electron transfer (PET) mechanism.59 Though MOFs entail extended network structures, they are envisaged as giant “molecules” due to highly localized electronic structures, and involve definite energy gap between valence-band (VB) and conduction-band (CB).60 Usually, the lowest unoccupied molecular orbitals (LUMOs) of electron lacking analytes are located at lower energy level than the CB of MOF. Upon excitation, quenching occurs by typical electron transfer from CB of MOF to the LUMOs of analytes. Accordingly, better quenching is anticipated if LUMO energy of a particular electron deficient compound is lower. However, the case is entirely opposite in case of electron rich amino-phenol, which develops operating “turn on” mechanism in the framework.61 In this milieu, electronic aspects of all the studied analytes, as well as –OH functionalized linker were studied in detail. The density functional theoretical (DFT) calculations were performed using the Gaussian 09 D.01 package.62 B3LYP functional63 and Pople diffuse basis set 6-311++G**64 have been used for all the calculations. In all calculations, we have included Grimme’s dispersion (DFT-D3) corrections to account the effect of weak intreactions.65

Figure 6. HOMO−LUMO energy levels of the framework constituent, individual analytes, and redistribution of the LUMO–HOMO energy gaps for linker in presence of (a) NZF, (b) TNP, ACS Paragon Plus Environment


demonstrating either fluorescence quenching, spectral overlap of the emission spectrum of 1′ with absorption spectra of the (c) antibiotics and (d) nitro congeners, corroborating RET mechanism. The HOMO and LUMO orbital energy plots of linker in the pristine MOF along with studied electron-deficient and electron rich analytes are depicted in Table S3, S5 and S7. Compared to rest of the electron deficient analytes, LUMOs of NZF (−3.14 eV) and TNP (−4.30 eV) lie in favourable energy levels that facilitates easy electron transfer from 1′ during fluorescence quenching. In contrast, LUMO of 4-AP (−0.51 eV) is located at a higher energy level for effective donation of electron to the CB of framework, leading to fluorescence enhancement effect (Figure 6a, 6b and 7a). For NZF and TNP, an obvious energy redistribution of the LUMO–HOMO orbitals of the MOF constituent, along with substantial reduction in energy gap (5.74 eV→ 3.28 eV for NZF; 5.74 eV→ 3.66 eV for TNP) was observed upon such interaction. As a result, the LUMO energy was stabilized and localized on individual electron deficient analytes for favourable electron transfer from an excited state of the probe to the LUMOs of NZF and/or TNP, causing highest fluorescence quenching.66 Similar rearrangement of the LUMO–HOMO orbitals and lowering of the energy gap for linker (5.74 eV→ 4.60 eV) is also evidenced when electron rich 4-AP interacts with the MOF constituent, validating easy electron transfer from CB of 4-AP to the LUMO of 1′ that in turn grounds “turn-on” response. In addition, geometry optimizations of bpg with NZF, TNP, and 4-AP molecules were carried out separately, and individual optimized structures revealed diverse supramolecular interactions between the pendent –OH functionality of the linker in CSMCRI-1 and electronically dissimilar toxic organo-aromatics (Figure 7b, 7c and 7d). Clearly, the fluorescence “turn-on” or “turn-off” responses of the framework are exclusively governed by the individual electronic properties of analytes. The estimated binding energy (BE) values for the interaction of NZF, TNP, and 4-AP with linker molecule are found to be 102.13, 73.85, and 59.50 kJ mol−1, respectively.

Figure 7. (a) HOMO−LUMO energy levels of the framework constituent, individual analytes, and redistribution of the LUMO–HOMO energy gaps for linker in presence of 4-AP, demonstrating either fluorescence enhancement effect, geometry optimizations of bpg with analyte (b) NZF, (c) TNP and (d) 4-AP molecules, showing efficient intermolecular Hbonding interactions with the pendent –OH functionality. However, the observed quenching efficiency order is not in perfect accordance with the LUMO energy trend of other analytes. This indicates resonance energy transfer (RET)7 might be another reason for PL quenching. Essentially, RET depends on the extent of overlap between emission band of the fluorophore and the absorption band of the non‐emissive analyte. As shown in Figure 6c, the UV–vis absorption spectra of NZF has largest degree of ACS Paragon Plus Environment

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overlapping with the 388 nm peak in emission spectra of 1′, followed by NFT, FZD, ODZ, DTZ, MDZ, RDZ, SAM, CAP, SDZ, PCL, and THI. Nevertheless, the peak at 470 nm has no overlap with the absorption spectrum of any of the antibiotics (Figure 6c).67 In case of nitro congeners, TNP (∼444 nm) has major spectral overlap with 388 nm peak of 1′, while rest of the NACs show minor overlap in the order: 2-NP (∼444 nm) > 4-NP (∼432 nm) > 3-NP (∼340 nm) > 4-NT (∼290 nm) > 2,4-DNT (∼280 nm) > DMDNB (∼270 nm) (Figure 6d). This in turn signifies that synchronous existence of electron transfer and energy transfer, with a prevalence of the latter mechanism, reasonably explains maximum photoluminescence quenching and observed nonlinearity in the S-V plots for both NZF and TNP. However, electron transfer process is the sole mechanism for rest of the antibiotics as well as remaining NACs. On the other hand, absorption band of 4-AP, though remains closest to the emission band of 1′ among isomeric amino-phenols, trivial overlap do not result much RET as compared to observed fluorescence enhancement in these classes of analytes (Figure S101). This observation essentially validates that PET is the only mechanism for observed fluorescence enhancement in case of electron rich amino phenolic compounds. Fluorescence Detection of Fe3+ ions. The pendant –OH functional groups in bpg linker can facilitate both electron transfer and binding to metal ions, leading to luminescent quenching induced detection of metal ions.18 Although, –OH functionalized organic chromophore molecules and graphene/graphene oxides are explored for highly efficient Fe3+ ion sensing,68 such functionalization for sensing of this particular metal ion are rare in MOF materials. These facts encouraged us to explore CSMCRI-1 as a prospective luminescent probe to detect metal ions in a rapid, selective and sensitive manner. At the onset, luminescence spectra of desolvated CSMCRI-1 was monitored by incremental addition (10 mM) of several metal ions namely Hg2+, Al3+, Mg2+, Ba2+, La3+, Ce3+, Ca2+, Cd2+, K+, Pd2+,Co2+, Mn2+ and Ni2+ that show negligible impact on the luminescence intensity (titration plots for individual metal ions are provided in the Supporting Information, Figure S62-S74). In sharp contrast, the biologically important Fe3+ ions displayed rapid quenching of the luminescence intensity of 1′ (Figure 8a). The quenching efficiency was calculated (vide supra) to be 86 % for Fe3+ ions (Figure S60). However, quenching efficiencies of other metal ions were either low or almost negligible. The order of quenching competence follows the order: Fe3+>> Cu2+>La3+> Ni2+> Al3+> Hg2+> Ce3+> Co2+> Pd2+> Cd2+> Mn2+> Ca2+> K+> Mg2+> Ba2+ (Figure 8c and Figure S60). This interesting behaviour motivated us to further investigate the effects of Fe3+ ion concentration on the fluorescence intensity of 1′ in detail. To determine the quenching constant (KSV), DMF solution of Fe3+ (5 mM) was added to the MOF dispersion and quenching phenomena was observed (Figure S61). The S-V plot deviates from linearity at higher concentration of Fe3+ ions, suggesting synchronous presence of static and dynamic quenching (Figure S75). The KSV value, from the linear region of the S-V plot at lower concentration ranges (0-0.04 mM), was calculated to be 2.54 × 104 M–1 (Figure S76), which is superior or comparable to the former MOF-based fluorescent sensors for Fe3+ ion detection (Table S11). Additionally, LOD for Fe3+ ions was obtained from the standard calculation method (Table S8) that turned out to be 1.71 ppm (1.29 µM). A comparison of detection limits for fluorescent based Fe3+ sensory MOFs is listed in Table S11. Clearly, 1′ highlights great potential for sensitive detection of Fe3+ ions, and exhibits one of the best performances with regard to detection limit among the formerly reported MOFs. Such efficient sensing could be a mutual consequence of strong Fe3+– framework interaction as well as proper accessibility of this particular metal ion inside the optimum sized cavity. A deeper insight of the structure of CSMCRI-1 reveals that the pendent oxygen atoms (from –OH groups) in the bpg linker are accurately projected through the pores of the channels, assisting the donation of electron lone pairs to the outer most shell of half field orbital in Fe3+ ions. This in turn reduces the energy-transfer efficiency from ligands to Zn(II) centres and decreases the ACS Paragon Plus Environment


luminescent intensity of the framework.17 To validate this logic further, 1′ was immersed in a Fe3+ solution to result Fe3+ ion incorporated framework. Remarkably, the pictures of Fe3+ ion incorporated framework showed a distinct colour change from orange to brown, confirming Fe3+–MOF interaction (Figure 8a inset). In addition, the pictures of Fe3+ ion incorporated 1′ in solid state showed a clear change in colour from orange to brown. To put more focus on Fe3+ ion encapsulation induced color change, we studied the variation in absorbance of 1′ with increasing Fe3+ ion concentration from solid state UV-Vis spectroscopy. The guest free CSMCRI-1 (10 mg) was dipped in varying concentration of Fe3+ ion containing solution (0 mM, 0.5 mM, 1 mM, 2.5 mM, 5 mM and 10 mM) for effective absorption, filtered after 24 hrs, and finally dried to check UV-Vis spectra. Compared to the pristine 1′, the absorbance embraced a sharp enhancement with increasing Fe3+ ion concentration (Figure S92). The plot of absorbance vs concentration for 1′@Fe3+ is linear at lower concentration (0 mM–2.5 mM). However, it becomes nonlinear at higher concentration, corroborating deviation from BeerLambert law (Figure S94-S95). On the contrary, solid state UV-Vis spectra of 1′ showed negligible variation in absorbance even in the presence of 10 mM of interfering metal ions (Figure S93). Given visual changes, apart from high luminescent sensitivity, are crucial standard for an ideal chemosensory material, the present results clearly manifest that 1′ changes colour upon specific metal encapsulation and can be considered as a naked-eye detector for Fe3+ ions. However, possibilities of the framework metal exchange by incoming Fe3+ metal ions as well as Fe3+ ion induced structural decomposition cannot be excluded during the quenching event. Inductively coupled plasma (ICP-OES) analysis resulted a ratio of 1:6.07 for Fe3+ and Zn2+ ions (Table S12), respectively, in Fe3+ ion incorporated 1′ that confirms Zn(II) ions are not exchanged while quenching process. Afterwards, long term reusability of the framework was checked. For this, the sample was recovered by centrifugation after sensing study with Fe3+ ions, and thoroughly washed with fresh DMF. The recovered material was air-dried and reused for fluorescence performance. Satisfactorily, 1′ retained most of its initial fluorescence emission intensity even after five successive cycles of Fe3+ ion sensing experiments (Figure S96) that validates high reusability of the framework for repeated use during real time monitoring. Interestingly, PXRD of the framework was maintained throughout these sensing cycles (Figure S97), which emphasizes high stability of the material. Additionally, discriminating identification of Fe3+ ions from a mixture of potentially interfering ions was monitored by quenching competence test. The emission intensity of welldispersed MOF solution was checked by addition of 100 µL of diverse interfering metal salt solution (10 mM), followed by 100 µL of Fe3+ solution (10 mM). As depicted in Figure 8d and Figure S78-S91, fluorescence intensity quenched remarkably in the presence of Fe3+ ions only, while negligible fall of intensity was observed for other metal ions. These cumulative results truly manifest the present pillar-layer framework as a proficient material for highly selective, sensitive and naked eye sensor of Fe3+ ions.


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a)

b)

c)

d)

Figure 8. Emission spectra of 1′ upon incremental addition of 10 mM solution of (a) Fe3+ ions (Inset shows distinct color change of solid as well as solution phase from 1′ to 1′@Fe3+ ) and (b) Mg2+, (c) S–V plots for different metal ions (10 mM) in a homogeneous suspension of 1′. Conditions: λmax for 1′ ca. 388 nm; 5 nm slit width, (d) The plot of competitive analyte test, showing small effect in fluorescence intensities upon addition of different metal ions (10 mM, 100 µL) and significant quenching by subsequent addition of Fe3+ ions (10 mM, 100 µL) to 1′. Construction of Molecular Logic Gate for Device Application. Numerous examples of fluorescence signalling based molecular logic gates have been described after the pioneering work by Silva and co-workers.69 Based on inputs, discriminative recognition of wide range of analytes have also been reported by various other groups.30,31 However, development of molecular logic gate by using responses of MOFs in presence of different analytes are still infrequent.32-34,70 Excellent selective detection of a wide range of analytes by CSMCRI-1 motivated us to construct logic gates that are made by combination of AND as well as OR gate. The spectral inputs were used as A and B, where A denotes to those analytes that show discriminative sensing behaviour of the framework, while B signifies other interfering analytes, showing no/ trivial change in fluorescence intensity. The presence or absence of the inputs is designated by ‘1’ or ‘0’, respectively, whereas altered / unaffected fluorescence signal is used as the notation for output. For example, inputs (0101) and (0011), given by A and B ports, respectively, are combined by AND gate methodological operation to produce (0001) as outputs. Together, the outcomes from AND gate and aforesaid inputs from A were used as combined inputs for OR gate to generate ACS Paragon Plus Environment


overall output (0101) for the coupled molecular logic gate. This term is designated as Y, which signifies the fluorescence outcome of the assembled circuit. Such combined output was formulated as A+AB. Desired fluorescence signal was detected by output ‘1’, while negligible luminescence effect was monitored by ‘0’ (Figure 9a). Along this line truth table for the individual logic gate was constructed based on two binary inputs. As displayed in Figure 9b, the resultant single output from AND gate and simultaneous input given to port A were used as combined binary inputs to obtain the main output of the complete circuit.

Figure 9. (a) Circuit for coupled AND-OR logic gates constructed from a four-input system, (b) Truth table for individual logic gate with possible inputs.

CONCLUSION In conclusion, we designedly built a dual functionalized, pillar-layer metal-organic framework (MOF) CSMCRI-1, from hydroxyl group integrated bpg linker and azo functionalized H2azdc ligand that shows presence of microporous channels in spite of three fold interpenetration. The high stability, admirable fluorescent property, as well as –OH and – N═N– functionality decorated porous channels render the activated framework (1′) as unique pillar-layer MOF for extremely selective quenching of both nitrofurazone (NZF) antibiotic, and explosive 2,4,6-trinitrophenol (TNP), with ultrasensitive detection. To our delight, 1′ demonstrates hitherto unreported, MOF based sensing of toxic 4-aminophenol among the isomeric analogues, through a significant fluorescence “turn-on” mechanism. Importantly, the fluorescence response of the framework is tuned depending upon the electronic properties of individual toxic organic aromatics. The importance of hydroxyl functionalization in bpg linker for such “turn-on” or “turn-off” responses is validated by density functional theory calculations, which also provide evidences of different supramolecular interactions between framework and analytes. The multiresponsive behaviour of the framework is certified by the exceptional quenching of fluorescence by Fe3+ ions over other potentially interfering metal ions. The detection of Fe3+ ions involve a visible colorimetric change in solid and solution phase upon Fe3+ ion encapsulation, which is important for practical applications. Comprehensive studies indicated that that concurrent presence of dynamic and static quenching as well as resonance energy transfer (RET) are responsible for such efficient sensing in 1ʹ, and the results are in harmony with the predictions from theoretical calculations. Besides, AND as well as OR molecular logic gates are constructed based on the discriminative luminescent recognition of each analyte. To our knowledge, the dualfunctionalized CSMCRI-1 represents exclusive example of a tetra-sensoric framework with good recyclability that opens up a platform to fabricate smart luminescent MOFs with ACS Paragon Plus Environment

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multimode logic operations for potential detection of diverse harmful pollutants using a single fluorescent material. EXPERIMENTAL SECTION Synthesis of [Zn3(bpg)1.5(azdc)3]∙(DMF)5.9∙(H2O)1.05 (CSMCRI-1) A mixture of Zn(NO3)2.6H2O (30 mg; 0.1mmol), Azobenzene-4,4'-dicarboxylic Acid (H2azdc) (27.2 mg; 0.1 mmol), meso-α,β-Di(4-pyridyl) Glycol (bpg) (21.6 mg; 0.1 mmol) were dissolved in 5ml DMF that placed in 15 ml tighly capped Teflon–lined glass vial and heated under autogenous pressure at 90 °C for 48 h. Block shaped orange crystals were isolated in 60 % yield. The crystals were washed with DMF, and finally dried in air. The guest solvents in the as-synthesized crystals were exchanged by dichloromethane, and degassed overnight under vacuum at 120 °C to generate solvent free framework [Zn3(bpg)1.5(azdc)3] (1′). Anal. calcd. for C60H42N9O15Zn3 (1′): C, 54.38; H, 3.19; N, 9.51 %. Found: C, 54.50; H, 3.23; N, 9.45 %. ASSOCIATED CONTENT Supporting Information Materials and physical measurements, additional structural figures, crystal data, PXRD patterns, TGA curves, FT-IR, fluorescence spectra, ICP data and tables (PDF). CCDC 1874413 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] ORCID Subhadip Neogi: 0000-0002-3838-4180 Biswarup Pathak: 0000-0002-9972-9947 Ranadip Goswami: 0000-0002-8522-3515 Shyama Charan Mandal: 0000-0002-4588-2874 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS S. N. and R. G. acknowledge DST-SERB (Grant No. ECR/2016/000156) and CSIR (Grant No. MLP-0028). The analytical support from ADCIF is greatly acknowledged. S. C. M. and B. P. acknowledge DST-SERB (Grant No. EMR/2015/002057) and CSIR [Grant No. 01(2886)/17/EMR(II)] for funding and IIT Indore for the lab and computing facilities. S.N. gratefully acknowledges Dr. E. Suresh and Dr. V. Smith for their help in crystallography. CSMCRI Communication No. 171/2018. REFERENCES (1) Song, X. Z.; Song, S. Y.; Zhao, S. N.; Hao, Z. M.; Zhu, M.; Meng, X.; Wu, L. L.; Zhang, H. J. Single-crystal-to-single-crystal Transformation of a Europium(III) Metal-organic Framework Producing a Multi-responsive Luminescent Sensor. Adv. Funct. Mater. 2014, 24, 4034– 4041. (2) Briggs, D. Environmental Pollution and the Global Burden of Disease. Br. Med. Bull. 2003, 68, 1–24. (3) Kümmerer, K. Antibiotics in the Aquatic Environment - A Review - Part I. Chemosphere 2009, 75 (4) 417– 434. ACS Paragon Plus Environment


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