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Strategic Design and Functionalization of an Amine-decorated Luminescent Metal Organic Framework for Selective Gas/Vapor Sorption and Nanomolar Sensing of 2,4,6-Trinitrophenol in Water Prasenjit Das, and Sanjay K. Mandal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06339 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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ACS Applied Materials & Interfaces

Strategic Design and Functionalization of an Amine-decorated Luminescent Metal Organic Framework for Selective Gas/Vapor Sorption and Nanomolar Sensing of 2,4,6-Trinitrophenol in Water Prasenjit Das and Sanjay K. Mandal* Department of Chemical Sciences, Indian Institute of Science Education and Research Mohali, Sector 81, Manauli PO, S.A.S. Nagar, Mohali (Punjab) 140306, INDIA Supporting Information Placeholder ABSTRACT: Based on the strategic design of a triazine-based dicarboxylate ligand with two primary amino groups and one secondary amino group, an amine-functionalized autofluorescent and polar three-dimensional MOF {[Cd(ATAIA)].4H2O}n (1), where H2ATAIA = 5-((4,6-diamino-1,3,5-triazin-2-yl)amino)isophthalic acid, has been synthesized under two different solvothermal conditions and structurally characterized. Single crystal X-ray analysis reveals that 1 crystallizes in the orthorhombic polar space group Fdd2, where each ATAIA ligand acts as a linear linker to connect four Cd(II) centers resulting in the formation of a 3D framework with a repeat of a double helical metal chain. It has been further characterized by elemental analysis, UV-vis and FT-IR spectroscopy, and thermogravimetric analysis. Its bulk phase purity and stability in aqueous acid and base solutions are confirmed by powder X-ray diffraction. Both FESEM and HRTEM images of 1 reflect the formation of micro-flowers by selfassembly of nano-petals. With the dehydrated framework of 1, sorption studies of different gases (N2, H2 and CO2) as well as polar and nonpolar solvents, such as water, benzene (Bz) and cyclohexane (Cy) have been investigated. The CO2 sorption isotherm depicts type I isotherm at 298 K and 273 K and type IV isotherm at 195 K. Furthermore, with an uptake of 129.2 cm3g-1 (25.62 wt%) at 195 K, sorption of CO2 is selective over N2 (77 K) and H2 (77 K) due to the strong adsorbate-adsorbent interaction as clearly evident from an isosteric heat of adsorption (Qst) at zero coverage of 37.5 kJmol-1, which is exceptionally higher than other functionalized MOFs. Using the Ideal Adsorption Solution Theory (IAST) calculation for a CO2/N2 (15:85) mixture, selectivity values are found to be 54.08 (298 K) and 46.96 (273 K) at 100 kPa. For a major application, activated 1 has been utilized for selective and ultrafast detection of TNP in water with a limit of 0.94 nM (0.2 ppb), which supersedes any previous reported value. Excellent recyclability and stability of 1 for sensing experiments have been established. Time-resolved fluorescence studies and DFT calculations have been used to establish its mechanism of action. Furthermore, a prototype experiment for the real time sensing of TNP in vapor phase by fluorescence microscopy provides an easy colorimetric monitoring. KEYWORDS: Luminescent Metal Organic Frameworks, amine-decorated multifunctional Cd-MOF, selective gas/vapor sorption, nanomolar and ultrafast TNP detection, fluorescence microscopy, vapor phase detection

INTRODUCTION

The study of highly selective and sensitive materials for the detection of nitro explosives (NEs) has become an emerging field for the concerns of national security, civilian safety and environmental protection.1,2 Out of these, 2,4,6-trinitrotoluene (TNT)3,4 and 2,4,6-trinitrophenol (TNP, also known as picric acid)5,6, are highly explosive and dangerous energy sources, and thus have been frequently used as common ingredients of explosive devices, such as grenades, bombs and mine fillings.7 For the use of TNP fireworks, leather and dye industries, it gets collected into the soil and aquatic system. As a consequence, this leads to serious health and environmental issues.8 Thus, the ultrafast, selective and judicious detection of even trace hidden explosive TNP in presence of other NEs like TNT is quite challenging because of their inherently similar high electron affinity.9 The current instrumental techniques for detection of NEs, such as surface-enhanced Raman spectroscopy (SERS)10, cyclic voltammetry11, nuclear quadrupole resonance (NQR)12, capillary electrophoresis

(CE)13 and gas chromatography-mass spectrometry (GCMS)14, are highly sophisticated and time consuming and therefore are not suitable for real time fast TNP detection in emergency situations. Thus, it is very urgent to come up with a forcible and reliable sensor for TNP that can be used with a simple but effective technique. Until now, a lot of efforts has been put with variety of materials including MOFs to showcase excellent performances for the detection of NEs based on the turn-off quenching response.15-18 However, few sensors are reported that work well in aqueous medium while most of the sensors have been studied in organic solvent systems.7,19 In order to use water as the medium, the material has to be highly stable in water for a huge demand in environmental application. In this context, functional Metal Organic Frameworks (FMOFs) are newly developed porous materials with high surface area, specific electronic and optical properties, and easily tailorable structures20,21 and therefore these are acting as promising candidates in various application fields, such as sensing22-28, gas storage/separation29,30, catalysis31,32 and drug

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delivery33. Moreover, using new fluorescent FMOFs - which retain their structures upon removal of lattice solvents and show high chemical and thermal stability - highly selective and ultrafast detection of TNP in water can become more feasible and accessible. Understanding the importance of amine-rich MOFs for such studies, we have strategically designed a dicarboxylate ligand with two primary amino groups and one secondary amino group to access an aminefunctionalized fluorescent MOF for maximizing the interactions with TNP molecules as represented in Scheme 1. Herein, we have demonstrated for the first time highly selective, ultrafast and nanomolar detection of TNP by a fluorescent FMOF {[Cd(ATAIA)].4H2O}n (1) prepared by two different solvothermal conditions (where H2ATAIA = 5-((4,6diamino-1,3,5-triazin-2-yl)amino)isophthalic acid). Scheme 1. Schematic Representation of Selective Detection of TNP by an Amine-decorated Nanospace of 1

EXPERIMENTAL SECTION Caution! TNT and TNP are highly explosive and should be used in small amounts. Their dilute aqueous solutions were used in sensing experiments with safety measures to avoid explosion. Materials. For synthesis, reagent grade starting materials and solvents were used. For sensing and sorption experiments, reagent grade analytes and HPLC grade solvents were used. All these materials were procured from commercial sources. The 5-((4,6-diamino-1,3,5-triazin-2-yl)amino)isophthalic acid (H2ATAIA) ligand was synthesized by following the procedure reported earlier.34 Physical Measurements. FTIR spectrum of 1 was recorded in the 4000-400 cm-1 range on a Perkin-Elmer Spectrum I spectrometer with samples prepared as KBr pellets. Thermogravimetric analysis was carried out from 25 to 500 ◦C (at a heating rate of 10 ◦C/min) under a dinitrogen atmosphere using an alumina pan as a sample holder on a Shimadzu DTG60H instrument. Microanalysis (C, H, N) of 1 was carried out in a Leco TruSpec Micro analyzer. UV-Vis-NIR and solid state reflectance spectra were recorded using Cary 5000 by Agilent Technology. Power X-ray diffraction (PXRD) measurements were done on a Rigaku Ultima IV diffractometer equipped with a 3 KW sealed tube Cu Kα Xray radiation and a DTex Ultra detector using Bragg-Brentano beam geometry as reported earlier35 over a 2-theta range 5° to 50° with a scanning speed of 1° per minute with 0.02° step. Field Emission Scanning Electron Microscopy (FESEM) experiments including energy dispersed X-ray (EDX) mapping of elements were performed on a JEOL instrument; samples were well dispersed in MeOH, drop casted in a silicon wafer, dried and coated with gold using a working distance of 4.5 to 15 mm and voltage 10 to 15 kV. High Resolution Transmission Electron Microscopy (HRTEM) was performed on FEI Tecnai G2 F20 equipped with a field emission gun operated at 200 Kv with 1 mg sample well dispersed in

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ethanol (10 mL) using a sonicator for 20 minutes and then put on the copper grid, which was allowed to dry using a lamp for 30 minutes. Photoluminescence experiments were performed using Horiba Scientific Fluorolog 3 Spectrophotometer with stirring mode; time resolved lifetime measurements were performed on Time Correlated Single Photon Counter with a 320 nm picosecond diode laser while the acquired data were analyzed by DAS analysis. Optical imaging were performed using fluorescence microscopy technique from Zeiss AXIO series, Scope.A1 with optimos camera. Sorption Studies. Following the procedure described earlier from our laboratory,35 for each measurement 100 mg of 1 was placed in an analysis tube to do pretreatment at 120 °C for 12-24 h (depending on the gas/vapor type). Data were collected in a BELSORP MAX instrument with warm and cold free-space (dead volume) correction measurements for all isotherms. Synthesis of {[Cd(ATAIA)].4H2O}n (1). Method I. A mixture of H2ATAIA (20 mg, 0.213 mmol) and Cd(NO3)2·4H2O (21 mg, 0.213 mmol) in 1 mL DMA/H2O (1:0.5) were sealed in a 7 mL Teflon lined stainless steel vessel and heated under autogenous pressure at 150 °C for 72 h and then cooled to room temperature in 24 h. The colorless block crystals were filtered, washed with DMA-H2O which were suitable for SCXRD. The compound is insoluble in water and common organic solvents such as MeOH, MeCN, toluene, DMF and DMSO. Yield: 24 mg (66%). Method II. A mixture of H2ATAIA (20 mg, 0.213 mmol) and Cd(NO3)2·4H2O (21 mg, 0.213 mmol) in 1 mL DMF/H2O (1:0.5) were sealed in a 7 mL Teflon lined stainless steel vessel and heated under autogenous pressure at 120 °C for 72 h and then cooled to room temperature in 24 h. The colorless block crystals were filtered, washed with DMA-H2O which were suitable for SCXRD. The compound is insoluble in water and common organic solvents such as MeOH, MeCN, toluene, DMF and DMSO. Yield: 16 mg (44%). Selected FTIR peaks (KBr pellet, cm-1): 3402s, 3315s, 3209s, 3116m, 1622s, 1564vs, 1526vs, 1428s, 1406s, 1367vs, 1318m, 1239m, 1138w, 1099w, 1061w, 1017w, 812m, 782m, 728m, 682m, 557w, 428w. Anal. Calcd for C11H16N6CdO8 (MW 497.052): C, 27.95; H, 3.41; N, 17.78. Found: C, 27.52; H, 3.84; N, 17.15. Reflectance: λmax, 323 nm. Single Crystal X-ray Data Collection and Refinements. Using an optical microscope, one suitable crystal of 1 was put inside a nylon loop attached to a goniometer head which was then placed under a cold stream of nitrogen gas for slow cooling to 100 K. Based on the crystal suitability from diffraction photographs and unit cell determination, data were collected on a Kappa APEX II diffractometer as described earlier from our laboratory.35 After integration of the data by the program SAINT36 to obtain values of F2 and σ(F2) for each reflection, data were further corrected for Lorentz and polarization effects followed by the application of an absorption correction (SADABS).36 Using Olex2,37 the structure was solved with the ShelXT structure solution program and refined with the ShelXL. Several full-matrix least-squares/difference Fourier cycles were performed to complete the refinement of the structure to convergence. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in ideal positions and refined as riding atoms. The number of lattice water molecules (four) for 1 was deduced from the TGA and elemental analysis. The structure could be finalized (R1= 0.0469, wR2 = 0.1350 and GOF =


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1.091) with four water molecules. However, partial occupancies over six sites with reasonable isotropic thermal parameters had to be considered and no hydrogen atoms could be added to the water molecules for the convergence of refinement. Thus, for these disordered water molecules we decided to report the refinement results with water molecules in 1 removed with the SQUEEZE procedure in PLATON.38 Its Flack parameter is 0.199(15), which is less meaningful compared to a chiral space group. In case of a polar space group, this indicates the orientation of the molecules with respect to the polar axis unlike enantio-purity of a chiral molecule. Final crystal data and refinement parameters are shown in Table 1. Selected bond lengths (Å) and bond angles (o) are listed in Table S1. Table 1. Crystallographic Data and Structure Refinement Parameters for 1

chemical formula formula weight (g/mol) temperature (K) wavelength (Å) crystal system

C11H8CdN6O8 440.63 100(2) 0.71073 orthorhombic

space group

Fdd2

a (Å)

15.2282(4)

b (Å)

40.8195(14)

c (Å)

13.2764(5)

α (°)

90

β (°)

90

γ (°)

90

Z V (Å3) density (g/cm3) µ (mm-1) F(000) θ (°) range for data coll. no. reflections collected no. of independent reflections

16 8252.7(5) 1.29 1.078 3136 2.00 to 25.04 11889 3367

no. of reflections with I >2σ(I)) Rint no. of parameters refined

3367 0.0301 199

GOF on F2

1.040

a

b

final R1 / wR2 (I >2σ(I)) 0.0217/0.0487 R1a / wR2b (all data) 0.0238/0.0492 largest diff. peak and hole (eÅ-3) 0.45/-0.27 a R1 = Σ||Fo| − |Fc||/Σ|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2, where, w = 1/[σ2(Fo2) + (aP)2 + bP], P = (Fo2 + 2Fc2)/3. Fluorescence Study. To carry out fluorescence experiments, 1 mg of finely ground activated 1 (guest free) was added to 2 mL of milli-Q contained in a quartz cuvette of path length 1 cm and stirred to form a uniform suspension. In this dispersible mode vicinal contact between the host framework activated 1 and nitro analytes occurs. Upon excitation at 310 nm, the fluorescence response of activated 1 and after incremental addition of freshly prepared aqueous analyte solutions (1 mM) to it was recorded in the 335-500 nm range

and the corresponding emission intensity was monitored at 376 nm for activated 1. During each experiment, the solution was stirred at a constant rate to maintain a well dispersed solution of activated 1. All experiments were conducted three times and consistent results were reported. Detection Limit Calculation. For calculating detection limits, each nitro-explosive (1 mM stock solution) was added to activated 1 (1 mg in 2 mL water) and its fluorescence intensity was recorded. By plotting fluorescence intensity with increasing concentration of NEs, a slope (m) was calculated from the graph. Standard deviation (σ) was calculated from four blank measurements of activated 1. Detection limit is calculated based on the formula: (3σ/m). Time-Resolved Emission Studies. Lifetime measurements of activated 1 excited at 310 nm were carried out using a picosecond time-correlated single photon counting system (TCSPC, model Horiba JobinYvon) equipped with a pulse diode laser. The repetition rate was kept constant at 1 MHz and time to amplitude converter was fixed in the range of 200 ns (0.54 ns per channel). The lifetime decay profiles of activated 1 were analysed and fitted with a tri-exponential term based on χ2 (1ାି 0.2) statistics by using the DAS 6.3 fluorescence decay analysis software. The average lifetime, τ(avg), was evaluated using the following equation:

where, τ is the average lifetime and α is the pre-exponential factor with subscripts 1, 2 and 3 representing various species. DFT Calculation. All calculations were carried out with Gaussian 09 suite of packages. The structure was optimized by hybrid functional, Becke’s three parameter exchange and the LYP Correlation Functional (B3LYP) at a split valance basis set 6-31G (d,p).39

RESULTS AND DISCUSSION

Synthesis and Structural Characterization. MOF 1 was synthesized under two different solvothermal conditions from a mixture of Cd(NO3)2.4H2O and H2ATAIA in a DMA/H2O (1:0.5) or DMF/H2O (1:0.5) mixture at 150 oC and 120 oC for 3 days, respectively (Scheme 2). In both cases, colorless block-shaped crystals that were isolated from the reactors were suitable for single crystal X-ray studies and found to be the same based on unit cell parameters. Scheme 2. Solvothermal Synthesis of 1

MOF 1 crystallizes in the orthorhombic noncentrosymmetric polar space group Fdd2 from achiral components. Each heptacoordinated Cd(II) center in 1 exhibits distorted pentagonal bipyramidal geometry with an N3O4 environment (Figures 1a-b and Figures S1-S2), where one nitrogen atom and one NH2 group from one triazine ring of the



ligand, one nitrogen atom from the triazine ring of the second ligand and two carboxylate groups (both in chelated mode) from two different ligands are involved. The bidentate chelating binding mode of the carboxylate groups to Cd(II) in its crystal structure is further confirmed by the difference in asymmetric and symmetric stretching bands of the carboxylate group in its FTIR spectrum that appear at 1564 cm−1 and 1367 cm−1 (∆ν = νasym − νsym = 197 cm−1), respectively (Figure S3).40 Each ATAIA ligand acts as a linear linker to connect four Cd(II) centers resulting in the formation of a three dimensional framework (Figure 1b and Figures S1a-c). The double helical metal chain that repeats in the framework is believed to be the source of its crystallization in the mm2 point group (Figure 1c and Figure S2a). 1 has a rare11-connected network with uninodal net topology bearing a point symbol {3^15.4^25.5^15} (Figure 1d and Figure S2b). The phase purity of bulk 1 was confirmed by powder X-ray diffraction (PXRD) by comparing the experimental pattern of the asprepared sample with the simulated pattern obtained from

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SCXRD (Figure S4). Upon removal of lattice water molecules by immersing 1 in MeOH followed by heating at 70 oC to obtain activated 1, there is no loss of its 3D structure (Figure S4c); furthermore, it is also stable in aqueous acid and base solutions based on the PXRD data (Figures S4d-e). The thermogravimetric analysis (TGA) of 1 shows a weight loss of 15.19% (calculated value: 15.23% for the loss of seven lattice water molecules) in the 40-172 °C range followed by its decomposition (Figure S5). 1D channels are formed along all three axes with pore dimensions (without van der Wall radii) of 11.305 X 5.923 Å2 (a-axis), 11.987 X 20.077 Å2 (b-axis) and 5.205 X 4.905 Å2 (c-axis) as shown in Figure S2c. Each of these pores decorated with free NH and NH2 groups of the ligand are occupied by water molecules. To comprehend the surface morphology of 1, field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) were performed. Both FESEM and HRTEM images reflect the formation of micro-flowers by self-assembly of nano-petals (Figure 2).

Figure 1. X-ray single crystal structure of 1: (a) Coordination geometry around the Cd(II) centers with an N3O4 environment, where one ATAIA ligand connects four Cd(II) as a linear linker. (b) Polyhedral representation showing the planarity of ATAIA ligand. (c) Formation of double helical metal chains that propagates in the 3D structure of 1. (d) Perspective structure of 1 viewed-down a-axis and its topological representation (free guests and hydrogen have been omitted for clarity) (Color code; carbon: grey, oxygen: red, nitrogen: blue, Cadmium: green). Figure 2. (a) FESEM image of 1. (b) EDX mapping of 1. (c) HRTEM image of 1.

Gas and Vapor Sorption Studies. The successful construction of dual-amine functionalized pore sites in 1 also influence to explore its potential gas sorption studies. By considering the stability based on TGA, PXRD and cognizable large solvent accessible void of 47.8% (8252.7 Å3 per unit cell) obtained using PLATON in the dehydrated framework of 1 (designated as activated 1), the sorption studies of different gases (N2 and H2 at 77 K; CO2 at 195, 273 and 298 K) as well as polar and nonpolar solvents water, benzene (Bz) and cyclohexane (Cy) have been performed. The N2 sorption isotherm at 77 K exhibits a mixture of type I and type IV reversible isotherm with the Brunauer-Emmet-Teller (BET) and Langmuir surface areas estimated to be 62 m2g-1 and 98 m2g-1, respectively (Figure S6a). Employing both the NonLocal Density Functional Theory (NLDFT) and microporous (MP) analysis, a narrow pore size distribution of 1 was found to be 0.6-2.2 nm and 0.4-1.7 nm, respectively. In both cases, the highest peak was observed at 1.0-1.1 nm (Figure S6b,c). The H2 sorption isotherm shows reversibility with an uptake of 20.12 cm3g-1 (Figure S7). The CO2 sorption isotherm depicts


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type I isotherm at 298 K and 273 K and type IV isotherm at 195 K. The uptake of CO2 at 298 K and 273 K was 17.1 and 24.3 cm3g-1, respectively (Figure S8). However, the CO2 sorption isotherm at 195 K follows a two-step gate opening adsorption isotherm, which sharply rises at lower pressure and then increases sharply from 0.27 bar to 1 bar with an uptake of 129.2 cm3g-1 (25.62 wt%) (Figure 3a). The desorption isotherm accompanied with a large hysteresis and incomplete branching indicates strong interactions between CO2 and activated 1. The gate-opening mechanism suggests that with an increase in pressure pore expansion takes place, followed by the affinity of polar groups towards CO2 is facilitated.41 Furthermore, sorption of CO2 (195 K) is selective over N2 (77 K) and H2 (77 K) due to the strong adsorbate-adsorbent interaction (Figure 3).42 Analysis of the isotherms at 273 and 298 K by the Clausius-Clapeyron equation and virial method, the isosteric heat of adsorption (Qst) was calculated to reveal the strength of CO2 adsorption in activated 1 (Figure S9). The Qst of activated 1 by the virial method at zero coverage is very high (37.5 kJmol-1), reflecting very strong interactions between amine functionalized pore of activated 1 and CO2, followed by a slow decrease to 30 kJmol-1 (Figure S10). Interestingly, its Qst value is higher than other functionalized MOFs, such as Cu-MOF (31.3 kJmol-1, ketone functionalized)43, Cu-TPBTM (26.3 kJmol-1, acrylamide functionalized)44 and IRMOF-3 (19 kJmol-1, amine functionalized).45 The separation of CO2/N2 mixture was evaluated by Ideal Adsorption Solution Theory (IAST) calculation at two different temperatures (298 K and 273 K). Dual-site Langmuir-Freundlich isotherm model has been utilized for fitting the unary isotherm of CO2 and N2. The experimental data of pure component (point) and Dual-site Langmuir-Freundlich fit (solid line) for CO2 and N2 adsorption isotherm at 298 K and 273 K, respectively are shown in Figures S11-S14. Dual-site Langmuir-Freundlich parameters are summarized in Table S2.

Figure 3. Gas adsorption-desorption curves for 1: CO2 at 195 K, N2 at 77 K, H2 at 77 K; Filled and open symbols indicate adsorption and desorption, respectively.

The selectivity values of a CO2/N2 mixture (15:85, flue gas composition) at 100 kPa are as follows: 54.08 at 298 K and 46.96 at 273 K (Figure 4). These values are significantly higher than many well-known MOFs: PCN-61 (CO2/N2: 15/298 K),46 PCN-61 (CO2/N2: 15/298 K),47 JUC-141 (CO2/N2: 21.6/273K and 27.6/298 K),48 Cu-BTTri (CO2/N2: 21/298 K), en-Cu-BTTri (CO2/N2: 25/298 K).49 The high selectivity of 1 can be attributed to the presence of amine-

functionalized Lewis basic sites, resulting in a significant selectivity of CO2 over N2. Furthermore, the water adsorptiondesorption shows a type IV sorption isotherm with an uptake of 285 cm3g-1 (corresponds to five H2O molecules per unit formula) at p/p0 = 0.9 accompanied with a large desorption hysteresis (Figure 5, top) attributing to strong interaction with activated 1. To ascertain the selective adsorption behavior of Bz/Cy, the single component vapor sorption experiment of Bz and Cy was measured at 298 K. Strikingly, the uptake amount of Bz was immensely higher than that of Cy (STP, 52 m2g-1 for Bz and 8 m2g-1 for Cy, at p/p0 ~ 1) (Figure 5, bottom). The aforementioned gate opening was also observed for Bz at p/p0 = 0.12, followed by higher uptake and hysteresis due to the presence of strong interaction between electron deficient triazine and electron rich Bz.50

(a)

(b)

Figure 4. Loading uptake and separation selectivity of 1 at 298 K (a) and 273 K (b) predicted by IAST for a mixture of CO2/N2 (15:85).

Photophysical Properties. MOFs based on d10 metal ion and amine functionalized π conjugated ligand are expected to be convenient candidates for luminescent materials.5 Its photoluminescence property was determined by solid state UV-vis and fluorescence spectroscopy (Figures S15-S16). The solid state absorption spectra of 1 shows the absorption maxima at 278 nm and 323 nm which corresponds to π-π* and n-π*, respectively (Figure S15). Finely ground guest-free crystals of activated 1 was used for the detection experiments.


5


Activated 1 was dispersed in milli-Q water, exhibiting a strong fluorescence-signature at 376 nm upon excitation at the wavelength of 310 nm (Figure S16). On comparing the emission maximum of activated 1 with that of the ligand, there is a slight red shift and spectral broadening due to strong binding between H2ATAIA and Cd(II) ion. Interestingly, the inherent auto-fluorescent nature shown by 1 is very rarely observed in non-biological systems and it was studied by optical image using UV illumination and fluorescence microscopy (Figure 6). Herein 1 shows fluorescence after

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Figure 5. Vapor adsorption-desorption curves for 1: (a) water at 298 K. and (b) Benzene (red) and Cyclohexane (blue) at 298 K. Filled and open symbols indicate adsorption and desorption, respectively.

(a)

Figure 6. (a) Auto-fluorescent nature of 1 excited in DAPI (λexc: 405 nm), FITC (λexc: 510 nm) and rhodamine B (λexc: 570 nm) region. (b) Fingerprint by 1: combination of UV illumination (λexc: 365 nm) and fluorescence microscopy imaging (Mixed color using ImageJ software).

excitation in three different regions DAPI (λexc: 405 nm), FITC (λexc: 510 nm) and rhodamine B (λexc: 570 nm) using fluorescence microscopy and ImageJ software has been used for color code (Figure 6a). In order to explore the scope of this auto-fluorescent property, the finger print detection of a thumb was carried out using UV-illuminator (λexc: 365 nm) and was scope of this auto-fluorescent property, the finger print further followed by fluorescence microscopy and Image J software (Figure 6b). This kind of facile finger print segmentation analysis is essential in many applicable areas, such as forensic. Solvent Effect on the Emission Spectrum of 1. Motivated by the above vapor sorption studies (hysteresis in adsorptiondesorption isotherms) the effect of various solvents such as water (H2O), ethanol (EtOH), methanol (CH3OH), acetonitrile (CH3CN), dioxane, tetrahydrofuran (THF), dimethylformamide (DMF), and dimethylsulfoxide (DMSO) on the emission spectrum of activated 1 was investigated (1 mg in 2 mL various solvents). Activated 1 shows maximum fluorescence intensity in water (H2O) followed by DMF and DMSO. The order of fluorescence intensity is H2O > DMF > DMSO > Dioxane > EtOH > Acetonitrile > THF > MeOH (Figure 7 and Figure S17).

(b)

Table 2. Solvent-dependent Fluorescence Properties of activated 1 Solvent parameters Solvent

Acetonitrile 1,4-Dioxane THF Water MeOH EtOH DMF DMSO

Reichardt’s parameters ‫்ܧ‬ே

Kamlet-Taft parameters

α

β

π∗

0.46 0.164 0.207 1 0.762 0.654 0.386 0.444

0.19 00 --0.98 0.86 00 00

0.40 0.37 --0.66 0.75 0.69 0.76

0.66 0.55 0.58 -0.60 0.54 0.88 1.00

Gutmann’s parameters AN DN 18.9 10.8 -54.8 41.5 37.9 13.6 19.3


14.1 14.8 20 16.4 19 19.2 26.6 29.8

Fluorescence Properties Emission Intensity maximum (107) λmax (nm) 370 379 375 376 374 374 383 387

2.41 2.67 2.38 3.12 2.26 2.45 2.86 2.81

6

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ACS Applied Materials & Interfaces α: Hydrogen-bond donor acidity, β: hydrogen-bond acceptor basicity, π∗: solvent dipolarity, AN: acceptor number and DN: Donor number.

Figure 8. Fluorescence intensity ratio histograms of activated 1 dispersed in different solvents and followed by the addition of nitrobenzene (1:1, v/v). Figure 7. Emission spectra of activated 1 in different solvents (1 mg in 2 mL).

In addition to this, the fluorescence emission maximum also undergoes a bathochromic shift from acetonitrile (370 nm) to DMSO (387 nm) (Figure S18). To gain better insight, the shift in emission intensities can be correlated with their solvent polarity parameters such as (i) Reichardt's polarity parameter, (ii) Kamlet-Taft parameter and (iii) Gutmann's parameter, respectively.51-54 For activated 1, the fluorescence property of solvents predominantly correlates with the Gutmann's donor parameter resulting in higher red shift for DMF (26.6), DMSO (29.8) and blue shift for acetonitrile (14.1), respectively (Table 2). This result indicates strong H-bonding interactions between the solvents and activated 1. Selective and Nanomolar Detection of TNP. To understand the sensing ability of activated 1 towards NEs, we first selected the simplest nitro-aromatic congener nitrobenzene (NB) for its effect on the emission intensity of activated 1 in presence of aforementioned solvents. As evident from Figure 8, the emission intensity of activated 1 has been totally quenched by NB and unperturbed by other solvents. Furthermore, the maximum fluorescence intensity of 1 is observed in water. This suggests that activated 1 is a potential and promising candidate for the detection of NB and other NEs in water. To investigate the sensing ability of activated 1 towards different NEs in water, the fluorescence quenching titration measurements were performed with the incremental addition of aqueous solutions (1 mM; 2.5−60 µL each) of different NEs (Figure 9 and Figures S19-S28). On the basis of the formula (I0 − I)/I0 x 100%, the quenching efficiency percentage for different NEs was evaluated, where I0 and I are the emission intensities of activated 1 before and after the addition of the NEs, respectively. It was observed that other NEs exhibit negligible to moderate quenching effect in comparison to TNP. The quenching efficiency of activated 1 in presence of other nitro analytes is found to be in the following order: TNP ˃> 2,4-DNP˃ 4-NP˃ 2-NP> 2,4-DNT > TNT > 2,6-DNT > DNB > 4-NT > 2-NT > NM > NB. It is noteworthy that the quenching efficiency of activated 1 is further visualized by fluorescence microscopy technique (by turn-off response in blue intensity of DAPI region) and UVirradiation, in which TNP is discriminated from all other NEs (Figure 9a, inset and 9b). In addition to this, for quick naked eye detection of NEs, a paper strip method was used. The pristine strip coated with activated 1 exhibits good fluorescence under UV light at 365 nm. It was

observed that a significant color change and quenching of fluorescence was observed only for TNP, blue to green (Figure 9c). To the best of our knowledge, this is the first reported MOF where the fluorescence microscopy technique has been used for detection of NEs. Focusing on TNP, an incremental addition of TNP (up to 60 µL) to the finely ground activated 1 produced an inconceivable and significant fluorescence quenching of 98% (Figure 10a). Interesting enough, the intensity decreased by 4% and 28% for activated 1 upon incremental addition of 6 nL (3 nM) and 5 µL of TNP (1 mM stock solution), respectively (Figure S19 and Figure 10a). For comparison, the intensity decreased by 26% and 97% upon addition of 5 µL and 60 µL of TNP (1 mM stock solution), respectively, to as-synthesized 1 (Figure S29). This clearly suggests that the sensing performance of 1 towards TNP is similar to activated 1. This is

Figure 9. (a) Percentage of fluorescence quenching obtained for different NEs at room temperature; (inset) Fluorescence Microscopy image: Response of activated 1 in presence of various NEs in DAPI region. (b) Response of activated 1 in presence of various NEs by UV-illuminator in solution. (c) Photograph of Whatman paper strips coated with activated 1 and 60 µL concentration of different NEs.


7


logical as the experiments are solid mediated sensing in water by 1, which contains water as guest molecules. The limit of detection of TNP is found to be as low as 0.2 ppb (parts per billion) or 0.94 nM (Figure S30, Table S3). These results vividly depict that activated 1 has high selectivity for TNP over other potentially interfering NEs. Moreover, using the Stern-Volmer (SV) equation: I0/I = 1 + KSV [A], KSV = Kqτ, where KSV is the quenching constant (M-1), with molar concentration [A] of the analytes, I0 and I are the luminescent intensities of activated 1, before and after addition of the NEs, respectively, such diversity in quenching efficiency can be clearly evaluated; Kq is the quenching rate constant and τ is the excited state lifetime without adding a quencher. The KSV and Kq for TNP are found to be 1.59 x 107 M-1 and 1.28 x 1016 M-1s-1, respectively, indicating stronger interaction between TNP and activated 1 (Figure 10b and Figures S31-S32). The Ksv value of activated 1 is surprisingly higher than organic polymers and other MOFs/COFs-based sensors including the best reported thus far in the literature, BUT-12 and BUT-13 in water (Table S4).55-60 Also, the Ksv value is 1000 times higher than the recently reported triazine based MOF (1.6 x 104 M1 55 ). The SV plot of TNP in activated 1 shows linearity at lower concentration, attributing to either static or dynamic quenching, and a deviation from linearity with bent upward curve at higher concentration is due to combination of static and dynamic

Figure 10. (a) Effect on the emission spectrum of activated 1 dispersed in water upon incremental addition of TNP. (b) SternVolmer plot of all nitro analytes in water showing high selectivity for TNP.

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quenching or self-absorption or an energy transfer process between TNP and activated 1.61,62 In case of other NEs, a linear curve with increase in concentration was observed in their corresponding SV plots (Figures S33-S40, Table S5). The quenching constant for TNP was 4035 times greater than that for TNT and other NEs, signifying the inimitable quenching ability of TNP towards luminescent activated 1 in water. This makes 1 an excellent candidate for environmental monitoring. Mechanistic Studies. To distinguish between the two types of quenching mechanisms, time-resolved fluorescence measurements were enumerated between activated 1 and after addition of different concentrations of TNP. The abate in fluorescence lifetime of luminescent material attributing to that quenching process is dynamic. It is due to the additional relaxation of the excited lifetime from collision with the quencher. Otherwise, the unchanged fluorescence lifetime of luminescent material attributing that quenching process is static in nature. The time-resolved fluorescence data gave the average lifetime of activated 1 as follows: 1.24 ns (before TNP addition), 1.24 ns (after 10 µL TNP addition) and 1.30 ns (after 50 µL TNP addition) (Figure S41, Table S6). The average lifetime value demonstrates that the static quenching predominates, while at higher concentration dynamic or photoinduced electron transfer (PET) takes place between electrondeficient TNP and activated 1.63 To gain better knowledge about TNP sensing ability of activated 1, density functional theory (DFT) calculations (B3LyP/6-31G(d,p) level) were carried out for the complexation between H2ATAIA ligand in activated 1 and

Figure 11. (a) Energy minimized structures of H2ATAIA and TNP complex. (b) HOMO-LUMO energy gaps of H2ATAIA, TNP and H2ATAIA@TNP.

8 ACS Paragon Plus Environment

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TNP.64 Usually, the conduction band of electron rich MOF is in higher energy compared to LUMO of the NEs. Thus, upon excitation an electron of the conduction band is transferred to the LUMO orbitals of the nitro analytes, resulting in the turnoff fluorescence quenching (Figure S42, Table S7).65,66 Based on the results described earlier, TNP shows an efficient intermolecular H-bond interactions between the ligand ATAIA in activated 1 and TNP, the LUMO energy is stabilised and localised on TNP which favor the electron transfer process (Figures 11a,b). The hydrogen bond distances are 2.13 Å (NH…O between second NH2 and OH of TNP) and 1.99 Å (NH…O between one NH2 and o-nitro of TNP) with a stabilization energy -4.0944 kcal/mol (Figure 11b).34 The aforementioned result attributing to the highest quenching efficiency of TNP is correlated to the strong electrostatic interactions and hydrogen bonding between two types of Lewis basic amine groups of the ATAIA ligand and the highly acidic hydroxyl group of TNP. To better understand strong Hbonding with highly acidic hydroxyl groups, the quenching experiment for different nitro-phenols was performed. Interestingly, it has been found that the quenching efficiency shows an order TNP >> 2,4-DNP > 4-NP > 2-NP, correlating well with the order of their acidity (Figures S43-S50). In order to clarify further, the spectral overlap between the emission spectra of activated 1 and absorbance spectra of different NEs was examined (Figure S51). The effective spectral overlap enhances the resonance energy transfer from the fluorophore of the sensor to the non-emissive analytes. The increase in the fluorescence quenching efficiency of the analyte suggests that more energy transfer takes place during the process. In the present study, it is observed that the maximum overlap takes place in presence of TNP with the emission band of (376 nm) activated 1, while other nitro-analytes have insignificant spectral overlap. In order to evaluate the extent of energy transfer, overlap integral J (λ) values were calculated by using the equation below67 where, FD(λ) is the corrected fluorescence intensity of donor of the range of λ to λ+dλ with the normalized intensity, and εA is the extinction coefficient of the NEs at λ in M-1cm-1. The height of spectral overlap for TNP was calculated as 4.1 x 1014 M-1cm-1nm4, which is almost four times greater than that of {[Cd(NDC)0.5(PCA)].Gx}n MOF.5 This spectral overlap value indicates that the resonance energy transfer phenomenon occurs for the quenching of emission intensity of activated 1 by TNP.68 Also, the red shift in emission intensity (9 nm) during the incremental addition of TNP to well-dispersed activated 1 further corroborates with this observation (vide supra). Thus, both resonance energy transfer and photoinduced electron transfer are contributing to the efficient fluorescence quenching of activated 1 by TNP.34 In order to gain insight into the miraculous TNP sensing ability of activated 1, FESEM is used to visualize the changes on surface due to interaction with all nitro analytes. Based on a closer look of the images, almost all the particles are covered with porous nanopetals like micro flower. Figure S52, shows that several porous nanopetals connect with each other forming 3D microflower by self-assembly. In case of TNP incorporated activated 1 (10-4 M, 3 days), it is clear that TNP interacts with the amine groups present in the framework. On

the other hand, other nitro analytes have insignificant change in surface morphology. Recyclability and Ultrafast TNP Sensing. To show stability and recyclability of activated 1, the material was regenerated after every quenching experiment of TNP by filtration, centrifugation and washing several times with water. Remarkably, after five repeated cycles, the material almost reclaimed its fluorescence intensity and quenching efficiency (Figure S53). Furthermore, activated 1 remains highly stable after five repeated cycles of fluorescence titration experiments with TNP and other NEs, which was confirmed by PXRD measurements (Figure S54). Also, the response time of activated 1 in presence of 20 µL TNP (1 mM stock solution) was examined by monitoring the fluorescence intensity before and after addition of TNP to well dispersed activated 1 in water. Figure 12 (top) shows that the response of activated 1 is almost fugacious, decreasing sharply from 7.55 X 107 to 2.33 X 107 a.u. (67%) within 10 seconds; however, up to 10 min there was negligible further fluorescence quenching (70%). In addition to this, for ultrafast naked eye detection of TNP, a handy paper strip method was used through instant spot. Rightly, after incremental addition of TNP (10-9 - 10-3 M, one drop) to it, a significant color change and quenching of fluorescence were observed (Figure 12, bottom). These results absolutely indicate that it can be used for reliable ultrafast and long-term in-field detection of NEs.

Figure 12. (Top) Ultrafast TNP sensing: Luminescence spectra of activated 1 before (0s) and after (10s) addition of TNP. (Bottom) photograph of Whatman filter paper strips coated with activated 1 at different concentration of TNP.

Competitive Nitro-Analyte Test. Motivated from the aforementioned results, the high selectivity and sensitivity of activated 1 towards TNP with concurrent NEs was extensively studied by competing nitro-analyte (CNA) test. It was noteworthy to observe that upon subsequent addition of TNT (20 µL each added 2 times, 1 mM aqueous solution) followed by the TNP addition (10 µL, 1 mM aqueous solution) it resulted in the pronounced and noticeable luminescence turn-



off response (Figure 13 and Figures S55-S62). Similar trend was obtained after the addition of aqueous solutions of other nitro-analyte congeners followed by TNP to activated 1. This result further justified the exclusive selectivity towards TNP, even in the presence of higher concentration of all other NEs. Vapor Phase Detection of TNP. It is noteworthy that the response of activated 1 was not only tested in water but also tested in vapor phase. A typical prototype experiment was carried out using fluorescence microscopy (Figure 14). In a conical flask TNP solution was taken where the top was sealed with an Eppendorf containing 5 mg of activated 1. As anticipated, a visual response of activated 1 was observed within 30 minutes using fluorescence microscopy technique in the DAPI region (Figure 14). A remarkable intensity drop was observed within 3 hours and the experiment was continued up to 7 hours. This prompt naked eye recognition of TNP under fluorescence microscopy enables activated 1 for its practical application to exhibit ultrafast and selective detection of TNP.

Figure 13. Fluorescence intensity percentage after the addition of aqueous solutions of different NEs followed by TNP; A = TNT (20+20 µL) + 10 µL TNP, B = 2,6-DNT (20+20 µL) + 10 µL TNP, C = 2,4-DNT (20+20 µL) + 10 µL TNP, D = 4-NT (20+20 µL) + 10 µL TNP, E = 2-NT (20+20 µL) + 10 µL TNP, F = DNB (20+20 µL) + 10 µL TNP, G = NB (20+20 µL) + 10 µL TNP, H = NM (20+20 µL) + 10 µL TNP.

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In summary, a new 3D MOF {[Cd(ATAIA)].4H2O}n (1) was synthesized under two different solvothermal conditions (both solvent and temperature were varied). It crystallized in the noncentrosymmetric polar space group Fdd2. Based on the single crystal X-ray structure, it has a double helical metal chain that repeats in the 3D framework. Its gas sorption data exhibited highly selective CO2 sorption over N2 and H2, with a very high Qst value for CO2 attributing to the strong interaction between the amine rich pore wall and the quadruple moment of the CO2. Based on the IAST calculation, high separation selectivity values of CO2 over N2 (for a 15:85 mixture, relevant to flue gas) were obtained at 100 kPa. The water sorption isotherm shows higher uptake with a hysteresis curve indicating strong polar-polar interaction. The selectivity of benzene over cyclohexane was observed due to strong interaction with the electron deficient triazine ring. The inherent auto-fluorescent nature shown by 1, after excitation in three different regions DAPI, FITC and rhodamine B using fluorescence microscopy, is very rarely observed in non-biological systems. 1 shows the fluorescence property of solvents that predominantly correlates with the Gutmann's donor parameter indicating strong H-bond interaction with the solvents. Its activated form (activated 1), which retains the 3D structure upon removal of lattice water molecules, was found to be highly selective and sensitive for TNP detection in water with a limit of 0.94 nM (0.2 ppb) compared to all other NEs. Its Ksv value (1.59 x 107 M-1) is the highest among MOF based sensors reported so far. With the help of SV plot, spectral overlap, time resolved experiments, surface study, DFT calculations and CNA test, the ultrafast and sub-nanomolar TNP detection by activated 1 was explained. Furthermore, activated 1 was used to develop (a) the vapor phase detection of TNP using fluorescence microscopy, and (b) a facile, portable and cost effective paper strip method, which is very suitable for practical environmental monitoring.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI:10.1021/acsami.xxxxx. Characterization of 1, gas sorption data, solvent dependent fluorescence spectra, selective and ultrafast detection of TNP, SV plot, life-time, DFT calculation, spectral overlap, recyclability, CNA test (PDF). Crystallographic data of 1 in CIF format (CCDC 1589458).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Figure 14. Fluorescence Microscopy image: A prototype experiment showing the response of activated 1 in presence of TNP vapor.

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS CONCLUSIONS

This work was funded by IISER Mohali. P. D. received a research fellowship from the MHRD of India. Authors acknowledge the use of NMR, X-ray and FESEM central


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facilities and other departmental facilities at IISER Mohali and the CHN and HRTEM analysis by NIPER Mohali.

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