Multiresponsive Adenine-Based Luminescent Zn(II) Coordination

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Multiresponsive Adenine based Luminescent Zn(II) Coordination Polymer for Detection of Hg2+ and Trinitrophenol in Aqueous Media Yadagiri Rachuri, Bhavesh Parmar, Kamal Kumar Bisht, and Eringathodi Suresh Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01755 • Publication Date (Web): 30 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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Crystal Growth & Design

Multiresponsive Adenine based Luminescent Zn(II) Coordination Polymer for Detection of Hg2+ and Trinitrophenol in Aqueous Media Yadagiri Rachuri,a,b Bhavesh Parmar,a,b Kamal Kumar Bisht,c and Eringathodi Suresh a,b* a

Academy of Scientific and Innovative Research (AcSIR), CSIR-Central Salt and Marine

Chemicals Research Institute, G. B. Marg, Bhavnagar-364 002, Gujarat, India. E-mail: [email protected]; [email protected] b

Analytical Division and Centralized Instrument Facility, CSIR-Central Salt and Marine

Chemicals Research Institute, G. B. Marg, Bhavnagar-364 002, India. c

Department of Chemistry, RCU Government Post Graduate College, Uttarkashi-249193,

Uttarakhand, India.

ABSTRACT

The present work reports on synthesis, characterization and crystal structure of a neutral twodimensional adenine based luminescent coordination polymer (LCP) with Zn(II) metal node. Photoluminescence (PL) property of the d10 LCP [Zn(µ2-1H-ade)(µ2-SO4)] (1) has been exploited to use 1 as a dual detection probe for the selective sensing of Hg2+ as well as TNP in

ACS Paragon Plus Environment

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aqueous phase from variety of cations and a pool of nitro aromatic compounds. Competitive fluorometric experiments involving series of cation combinations or mixture of nitro compounds establish 1 as efficient and selective sensor for Hg2+ and TNP in aqueous solutions. Limit of detection (LOD) for sensing of Hg2+ and TNP in aqueous solutions using LCP 1 is 70 nM and 0.4 nM respectively. For in-filed sensing applications, LCP 1 coated test paper strip has been developed, which showed luminescence upon exposure to UV radiation. The luminescence intensity of test paper strips quenches upon adding the aqueous solutions of target analytes. The present investigation clearly demonstrates selective, recyclable detection of lethal environmental pollutants such as Hg2+ and TNP in aqueous media which has relevance in the context of environmental protection and homeland security.

INTRODUCTION Due to extensive industrialization, water contamination and environmental problems have become major issues as large volumes of effluents with high concentrations of heavy metal ions and toxic organic pollutants are being discharged in water bodies. The detection of toxic metal ions and organic pollutants such as nitroaromatics (NACs) has always been an important challenge regarding environmental protection, human health, homeland security and safety. Mercury(II) (Hg2+) is one of the significant metal pollutants since it is toxic to human health and poses threat to the environment. For potable water the maximum permitted level of 10 nM and 30 nM Hg2+ is regulated by the U.S. Environmental Protection Agency (EPA) and World Health Organization (WHO) respectively.1 The presence and accumulation of Hg2+ in the human body even in a low concentration can cause different diseases related to digestive, neurological and excretory systems.2-5 On the other hand, nitroaromatic compounds should not be neglected as a


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class of toxic and hazardous chemicals. Selective and sensitive detection of explosives or explosives-like substances has become one of the challenging issue in recent times. Chemical constituents of commercial explosives include nitroaromatic compounds (NACs) such as 2,4dinitrotoluene (2,4- DNT), 1,3-dinitrobenzene (1,3-DNB), 2,4,6-trinitrotoluene (TNT) and 2,4,6trinitrophenol (TNP). TNP is not only a strong explosive but also used extensively in dyes, fireworks, pharmaceutical and leather industries. Eventually alarming amounts of TNP are released to the environment by these industries as effluent. Metal organic frameworks/coordination polymers (MOFs/CPs) are emerging materials fabricated by metal nodes/clusters linked by organic ligands generating multidimensional framework.6 MOFs/CPs are not only identified for their aesthetic structural topologies but also for diverse applications in the area of heterogeneous catalysis, gas storage, gas separation, drug delivery and molecular sensing.7-11 In recent times design and synthesis of luminescent coordination polymers (LCPs) have gained enormous attention for the selective and sensitive detection of different toxic metal ions and harmful organic pollutants, which are literally discharged to the environment as industrial effluent.12,13 Detection and removal of the toxic heavy metal ions and harmful nitro aromatics (NACs) in waste water and drinking water distribution is a requisite considering health and security viewpoint.14 This has fortified researchers towards development of simple, userfriendly and cost effective methods to selectively detect and quantify them, preferably in aqueous phase. A convenient, effective, and rapid method for detection and quantification of Hg2+/nitroaromatics in water has thus become one of the top research priorities. It remains a challenge to develop LCPs based sensor for the selective detection of Hg2+ and TNP in aqueous phase by exploiting the fluorescence property. Lanthanide and d10 metal node based luminescence metal-organic frameworks (LMOFs) have been emerged as tunable luminescent


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sensors for chemical species.15-17 On the basis of the electron-deficient characteristic of nitroaromatic compounds and the photo-induced electron transfer (PET) quenching mechanism, LMOFs/LCPs have been designed as efficient fluorescence chemosensors for the detection of NACs in vapor/liquid phase because of their good sensitivity, easy operation, and visualization.18-29 Luminescence-based methods have been widely used for sensing and detection of TNP.30-35 Albeit a few luminescent sensors for the detection of Hg2+ have also been reported.36-41 However, aqueous phase detection of Hg2+/TNP by MOF based fluorescence sensors are limited in the literature may be because of the lack of chemical stability of LCPs in water or in the respective analyte solutions.42-59 In furtherance of our quest to devise new LCP sensor materials herein we are reporting an adenine based Zn(II) coordination polymer as a dual detection fluorosensor for selective and sensitive detection of Hg2+ and TNP molecule. Thus, neutral 2D coordination polymer [Zn(µ2-1H-ade)(µ2-SO4)]n (1) with good hydrolytic stability has been synthesized and characterized by various analytical methods including single crystal X-ray diffraction. We have chosen biomolecule adenine as a linker due to its multiple N-donor coordination sites and H-bonding capabilities together with the rigidity of its molecular structure which can generate topologically diverse MOFs/CPs.60-62 Intrinsic luminescent property of this d10 based LCP 1 has been utilized for competitive sensing of Hg2+ and TNP from variety of cations and NACs by fluorescence quenching. Further, 1 coated paper strip has been developed for the visual sensing applications towards Hg2+ and TNP upon exposure to UV light. EXPERIMENTAL Materials and General Methods 6-Aminopurine/adenine (HAde), metal salts and solvents were purchased from commercial sources. Deionized water was used for synthetic manipulations and the reagents and solvents


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were used as received without any further puriﬁcation. Elemental (CHNS) analyses were done using elementar vario MICRO CUBE analyzer. IR spectra were recorded using KBr pellet method on a Perkin Elmer, G-FTIR spectrometer. TGA analysis was carried out using Mettler Toledo and Netzsch. Powder X-ray diffraction (PXRD) data were collected using a PANalytical Empyrean (PIXcel 3D detector) system with CuKα radiation. Single crystal X-ray analysis were determined the crystal structures using BRUKER SMART APEX (CCD) diffractometer. Solid state UV–Vis spectra were recorded using Shimadzu UV-3101PC spectrometer using BaSO4 as a reference. Photoluminescence spectra were recorded at room temperature utilizing Fluorolog Horiba Jobin Yvon spectrophotometer. Field Emission-Scanning Electron Microscopy (FESEM) images were taken under a JEOL JSM-7100F instrument employing an 18kV accelerating voltage. Transmission electronic microscope (TEM) images were captured using a JEOL JEM 2100 microscope. The samples were prepared by mounting methanol dispersion on lacey carbon formvar coated Cu grids. Synthesis of [Zn(µ2-1H-ade)(µ2-SO4)]n (1) 15 mL aqueous solution of Zn(SO4)·7H2O (150 mg, 0.52 mmol) was slowly added to the 15 mL aqueous solution of HAde (60 mg, 0.4 mmol) in a 50 mL round bottom flask. The milky white turbid solution thus resulted was allowed to reflux at 393 K for 24 h and subsequently cool to room temperature. The colorless block shaped crystals suitable for single crystal X-ray analysis were obtained with good yield (∼79%). Elemental analysis (%), Cal. For C5H5N5O4SZn: C, 20.25; H, 1.70; N, 23.62; S, 10.81; found: C, 19.09; H, 2.24; N, 22.26; S, 10.19. IR cm-1 (KBr): 3424 (br), 3167 (w), 3108 (w), 2744 (m), 2369 (m), 1928 (m), 1869 (m), 1688 (s), 1612 (w), 1475 (s), 1400 (s), 1237 (w), 1214 (s), 1142 (s), 1298 (w), 1237 (w), 1213(m), 1142(s), 1041 (w), 1007 (m), 977 (m), 891 (m), 787 (s), 687 (m), 637 (s), 597 (w), 566 (m), 528 (w), 507 (w).


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Stability Experiments Aqueous and chemical stability of 1 was investigated by soaking the material in water, aqueous solutions of Hg2+ and TNP. Approximately 50 mg of the sample was dispersed to a 25 mL scintillation vial containing the respective aqueous solutions to make the emulsion. The solid material is filtered and removed from the emulsion after 7 days, washed thoroughly with water and acetone followed by drying at ambient temperatures. PXRD of the recovered solid samples were recorded to confirm the chemical stability. Fluorescence Experiments For the detection of Hg2+ and TNP, fluorescence experiments were investigated by 1 in aqueous medium with standard aqueous solutions of metal salt (1×10-3 M) and nitro aromatic compounds (2 mM). For detection and sensing studies , metal ions such as As3+, Zn2+, Cd2+, Cs+, Ba2+, Cu2+, Na+, K+, Pb2+, Co2+, Mn2+, Ca2+, Mg2+, Cr3+, Hg2+ and different nitro aromatic compounds like 2,4,6-trinitrophenol (TNP), 2,4-dinitrophenol (2,4-DNP), 2,4-dinitrotoluene (2,4-DNT), 4nitrotoluene (4-NT), 1,3-dinitrobenzene (1,3-DNB), 2-nitrophenol (2-NP) and nitrobenzene (NB) have been chosen. For the preliminary sensing studies, 2 mg of finely ground 1 was added to a cuvette of path length of 1 cm containing 2 mL of aqueous solutions of cations/nitroaromatics. The emulsion was stirred at constant rate in fluorescence instrument with stirring attachment during whole experiment to maintain homogeneity of solution. The emission spectra of each sample were recorded in the range of 330−560 nm upon excitation at 295 nm and fluorescence intensity at 395 nm was monitored. All titrations were carried in triplicate to establish the consistency of the results. The quenching efficiency was calculated by [(Io-I)/Io] x 100%, where Io and I are the fluorescence intensities before and after addition of aqueous solutions of analytes.


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X-ray crystallography Diffraction data collection of 1 was carried out on a Bruker Smart Apex CCD area-detector diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). The diffraction data integration as well as the intensity corrections for the Lorentz and polarization effects, was performed using the SAINT program.63 Semi empirical absorption correction was performed using SADABS program.64 The structure of 1 was solved by direct methods using SHELXS2014 and all the non-hydrogen atoms were refined anisotropically on F2 by the full matrix leastsquares technique with SHELXL-2014.65 The hydrogen atoms of the adenine moiety were generated geometrically and refined isotropically using the riding model. Details of crystal data and refinement parameters, selected bond lengths and angles are provided in Table S1 and Table S2. RESULTS AND DISCUSSION Crystal and molecular structure of [Zn(µ2-1H-ade)(µ2-SO4)]n (1) Single-crystal X-ray diffraction (SXRD) analysis reveals that 1 crystallizes in the monoclinic crystal system with the P21/c space group and features a two-dimensional (2D) coordination framework. The asymmetric unit of 1 contains crystallographically unique Zn2+ ions, one molecule of adenine and a sulfate anion. Conventionally accepted numbering scheme for chemical and biological purpose has been adopted for 1H-ade nucleobase. Adenine is a rigid linker with five potential coordination sites, viz., two pyrimidine nitrogen atoms (N1, N3), two imidazolate nitrogen atoms (N7, N9) and a -NH2 group (N10). Adenine can exist in a variety of neutral tautomeric66-67 and protonated forms68 to coordinate with the metal centers. In the structure of 1, ligand 1H-ade exists as a neutral entity with formal opposite charges on N9 and


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N1. Migration of N9 proton to N1 allows 1H-ade to bridge two Zn nodes through N9 and N7 atoms (Scheme S1). The distorted tetrahedral ZnN2O2 coordination environment around each zinc ion is provided µ−κN7:κN9 coordination from adenine moiety through imidazole nitrogen atoms N7 and N9 and µ−κO1:κO3 coordination from the sulfate anions (Figure S1).

Figure 1. (a) Two-dimensional network of [Zn(µ2-1H-ade)(µ2-SO4)]n oriented along bc-plane, (b) intermolecular H-bonding interactions (N-H···O) observed between adjacent 2D sheets in 1. As depicted in Figure 1a, both adenine moiety and sulfate units act as bridging ligands in the formation of 2D network oriented along bc-plane. Screw related Zn2+ ions are tethered by 1Hade and sulfate anions producing zig-zag chains along b-axis and c-axis respectively (Figure S2a & b). Zn···Zn separation through µ2 -bridged Zn–1H-ade–Zn and Zn–SO4–Zn is 6.024 and 4.973 Å respectively. With respect to the nucleobase ligand, the coordinated atoms are the imidazole nitrogen atoms with Zn1–N7 and Zn1–N9 bond lengths are 2.023(10) Å and 2.005(9) Å, longer than the Zn-O bond lengths from the sulfate unit, with Zn1-O1 and Zn1-O3 distances 1.961(8) Å and 1.994(9) Å respectively. Angles relating the distorted tetrahedral geometry around the central metal ion in the neutral two dimensional frame work ranges from 98.5(4) to 125.3(4)° (Table S2).


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In an attempt to understand the supramolecular interaction between the two dimensional sheets, packing and hydrogen bonding interactions are analysed in detail. Packing diagram viewed down c- and b-axes with hydrogen bonding interaction is depicted in Figure 1b & Figure S2c. The 2D nets oriented along ab-plane are involved in intermolecular N-H···O interaction with the adjacent sheets from either side. Thus the migrated amine hydrogen H1 of 1H-ade is making NH···O contact with sulfate oxygen O4 of the neighbouring sheets [N(1)-H(1)···O(4): H(1)···O(4) = 1.85 Å; N(1)···O(4) = 2.712(13) Å; ∠ N(1)-H(1)···O(4) = 177°; symmetry code = 1+x, y,1+z] from either side generating a layered hydrogen bonded 3D architecture. In addition to this intramolecular N-H···O (amino hydrogen H10), C-H···O contacts (pyrimidine hydrogen H2) with the sulfate oxygen atoms, as well as C-H···N contact between the imidazole hydrogen H8 with N3 are also observed in the stabilization of the molecule in the crystal lattice. Details of all the hydrogen bonding interactions with symmetry code is given in Table S3. Characterization and Stability Study [Zn(µ2-1H-ade)(µ2-SO4)]n (1) synthesized by reflux method was subjected to different analytical techniques such as FT-IR, PXRD, TGA and SEM for the characterization, assessment of thermal/chemical stability and phase purity. The FT-IR analysis of 1 revealed the characteristic peaks of adenine and sulfate. The bands in the range of 3429-3271 cm-1 can be assigned to the υ(N-H) modes of vibrations of -NH2 group. 3429 and 3333 cm-1 peaks belong to –NH2 and 3271 cm-1 band is assigned to N-H stretching of the heterocyclic ring. NH bending (δ) modes for NH2 and NH moieties of 1H-ade appeared at 1688 and 1612 cm-1, 1550 cm-1 respectively. The aryl CH stretching frequencies appeared in the range of 3161-3079 cm-1. The bidentate mode of sulfate coordination appeared in the range 1210-1120 cm-1 with medium peak at 1148 due to υ(S-O) vibrational modes (Figure S3).


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Figure 2. Comparison of PXRD data of bulk 1 with that simulated from SXRD data. Bulk phase purity of 1 has been confirmed by comparing the experimental PXRD with pattern simulated from single crystal X-ray diffraction (SXRD) data that shows very good agreement of the peak positions (Figure 2). Thermal stability of 1 was also established by TGA and variable temperature powder XRD (VT-PXRD) analysis. The TGA and DTG plots disclosed high thermal stability up to ~450 °C for 1 (Figure S4), which is also substantiated well with the VT-PXRD data (Figure S5). Chemical stability of 1 in water and aqueous solutions of Hg2+/TNP were also examined by soaking the LCP into respective media for 7 days. The samples of 1 were separated by filtration after 7 days and the PXRD data were compared with that simulated from SXRD (Figure S6). Recently, MOFs/CPs have also been exploited as precursors for the fabrication of nanomaterials (metal/metal oxide).69-70 Thermolysis of Zn(II) based MOFs in the presence or absence of air/N2 resulting various types of morphological ZnO nanostructures.71 ZnO is a wide bandgap (3.37 eV) n-type semiconductor having potential applications in optics, luminescent materials and


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catalysis.72-74 We synthesized spherical ZnO NPs by high temperature treatment of 1 at 600 °C for 6 h in a temperature controlled furnace. Scanning electron microscopy (SEM) (Figure S7a), and transmission electron microscopy (TEM) images of ZnO nanoparticles revealed spherical morphology and the size of the spherical nanoparticle is in the range ∼24 to 30 nm (Figure S8). Furthermore, the formation and phase purity of the ZnO NPs was also confirmed by corroborating the peak positions of the PXRD data with that of ZnO from the JCPDS database (JCPDS card no: 04-007-1614) (Figure S9) as well as EDX (Figure S7b). UV-Vis spectra of 1 and ZnO nanoparticles were recorded in BaSO4 at ambient temperatures in the solid state (Figure S10). Photoluminescence Properties Photoluminescence properties of MOFs/CPs comprising of d10 metal ions and electron rich multidentate ligands have been explored extensively due to strong luminescence properties and their potential applications in the field of photo luminescent chemical sensors.75-81 The solid-state luminescence property of 1 and free adenine was investigated at room temperature (Figure 3). The emission peak for free adenine appeared at 416 nm upon excitation at 295 nm.

Figure 3. Photoluminescence spectra of 1 and adenine in the solid state (λex = 295 nm) (Inset: Digital photographs of adenine and 1 under UV lamp at 365 nm).


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Compound 1 exhibited strong emission peak at 413 nm with slight blue-shift compared to the free ligand upon the same excitation. Both emission peaks of adenine and 1 can be attributed to π*–π or π*–n transitions. The emission of LCP 1 could be attributed to a ligand centered luminescent process based on almost identical emission peak at similar wavelength that observed in the case of adenine. We have examined the luminescence of 1 dispersed in commonly used solvent as depicted in Figure 4a-b. It is observed that not only the position of the emission bands of 1 but also their intensity changes on changing the dispersing solvents, with acetone as dispersion medium exhibiting weakest intensity.

Figure 4. (a) Fluorescence spectra of 1 in different solvents depicting selectivity for acetone and (b) emission intensity of all solvents revealed selective detection of acetone over other solvents. PL spectra of emulsions of 1 in different solvents exhibited broad peaks with emission maximum in the range 390-403 nm, similar to that of the solid-state sample. The luminescence intensity of 1 in case of acetonitrile dispersion was highest and that in case of acetone was smallest; i.e., acetone showed highest quenching. Such solvent-dependent luminescence phenomenon is of interest for detecting trace amounts of acetone from the above series of solvents. The interactions between the framework and small organic molecules may be the reason for observed


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fluorescence quenching effect. Upon excitation an energy transfer from the organic ligands to the acetone might be taking place that can result in the observed fluorescence quenching.82-83 Fluorescence Studies for Hg2+ Detection In light of the excellent photoluminescence and chemical stability of [Zn(µ2-1H-ade)(µ2-SO4)]n, the sensing and detection capability of 1 towards different metal ions in aqueous solution has been tested. For preliminary studies, 2 mg of 1 was dispersed in 2 mL of an aqueous solution of 1x10-3 M MClx and As2O3 (M = Zn2+, Cd2+, Cs+, Ba2+, Cu2+, Na+, K+, Pb2+, Co2+, Mn2+, Ca2+, Mg2+, Cr3+, Hg2+). The luminescence properties were studied by recording fluorescence spectra of the emulsion containing 1 and the respective metal ions upon excitation at 295 nm.

Figure 5. (a) The fluorescence quenching for different metal cation by water suspension of 1 (2.0 mg/2 mL) and (b) Luminescence responses of 1 (2 mg dispersed in 2 mL water) towards different concentrations of Hg2+ (0.05-1.0 mM) in water. As depicted in Figure 5a, the intensity of the emission band at 395 nm decreases considerably upon addition of Hg2+, while other metal ions do not show any radical change. Amongst different metal ions screened for detection in water by 1, As3+, Zn2+ and Cd2+ showed slight increase in the emission intensity while rest of the metal ions showed different degrees of quenching effects and Hg2+ almost completely quenched the emission band. These results clearly indicate that 1 is


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promising candidate for selective probing of Hg2+ in aqueous phase from the above mentioned cationic series. In order to evaluate the sensitivity and quenching efficiency of 1 towards Hg2+, the emissive responses in fluorometric titration of varying concentration of aqueous Hg2+ (0.051.0 mM) against 1 dispersed in water were recorded. As depicted in Figure 5b, the emission intensity obviously decreases with incremental addition of Hg2+ aqueous solution.

Figure 6. (a) Stern-Volmer plot of 1 for Hg2+, Concentration in the range of 0.0-1.0 mM and (b) Fluorescence responses of 1 to various ions. The blue bars represent the emission intensities of 1 in various metal ions (1x10-3 M) and red bars represent a change of the emission intensities upon subsequent addition of Hg2+ (1x10-3 M) to the above metal ion solutions.


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The quenching efficiency was calculated by fitting the fluorescence intensity ratio versus concentration of Hg2+ data into the Stern-Volmer equation (Io/I) = 1+ KSV [A]. Where, Io and I are the emission intensities of aqueous suspension of 1 before and after addition of aqueous solution of Hg2+ respectively, [A] is the molar concentration of Hg2+ and KSV is the quenching constant. As depicted in Figure 6a, the Stern-Volmer plot is nearly linear for the added Hg2+ concentration (0.0-1.0 mM) and the calculated quenching constant value is Ksv = 7.7x103 M-1 with linear fit coefficient value of 0.9975. In an attempt to acquire limit of detection for Hg2+ by 1, fluorescence intensity was monitored by addition of very low concentration of Hg2+ solution (0.1-0.5 µM) to 1 dispersed in water. The limit of detection (LOD) was calculated to be 70 nM based on 3σ/m (S1). In particular, aqueous phase detection of Hg2+ by LMOF/LCP as a fluorosensor is scantly reported in the literature. Important reports on aqueous phase detection of Hg2+ by LMOFs/LCPs are tabulated in Table 1. Evidently, present investigation is comparable to previous reports in the virtue of selective and sensitive aqueous phase sensing of Hg2+ ions. To determine 1 as a selective chemo sensor towards Hg2+, competitive experiments were conducted by addition of 1 mL of Hg2+ (1x10-3 M) to the 2 mL aqueous solutions of other metal ions (1x103

M). The sensing response of 1 towards Hg2+ was almost unaffected even in presence of added

metal ions, justifying its potential for selective Hg2+ recognition (Figure 6b). Several integrated factors may be responsible for the selectivity for Hg2+ amongst the group of metal ions investigated. Mercury ion (Hg2+) is well known for its versatile coordination geometry and high complexation affinity to free sulfur (S) and nitrogen (N) atoms.84-85 The free N atoms of the heterocyclic rings and amino functionality on adenine moiety in 1 are accessible to coordinate with the Hg2+ metal ions. Considering the free basic sites (N3 and N10) of adenine molecule which are not coordinated to the Zn2+ metal center in 1 can act as binding sites for the added


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Hg2+ ions possessing versatile coordination geometry. The overall reduction in luminescent intensity upon incremental addition of Hg2+ may be attributed to the coordination of free Ndonors sites from the adenine which may diminishing the energy-transfer efficiency from the π to π* transitions.

Table 1. Reported aqueous phase Hg2+ detection studies involving LCPs/LMOFs.

#

Material

LOD (nM)

Ref.

(LCP/LMOF)

Quenching Constant (KSV, M-1)

1

[Cd(2-NH2bdc)(tib)·4H2O·0.5DMA]n

13x105

42 nM

42

2

Ad/Tb/DPA

NA

0.2 nM

44

3

{[Cd1.5(C18H10O10)]·(H3O)(H2O)3}n

4.3x103

2 nM

47

4

Tb-CIP/AMP

NA

0.16 nM

48

5

TbL1.5(H2O)2]·H2O

7.4x103

NA

49

6

Zr6O4(OH)4(TCPP)1.5

6.4x105

6 nM

50

7

Eu/IPA-Im

NA

2 nM

51

8

[Zn(µ2-1H-ade)(µ2-SO4)]n (1)

7.7x103

70 nM

Present work

Fluorescence Studies for TNP Detection Strong emission of 1 in water suspension and considering its thermal, chemical and hydrolytic stability prompted us to check the sensing of nitroaromatic compounds in aqueous media which has relevance with environmental and security issues. The photoluminescence spectrum of 1 in water exhibits strong emission peak at 395 nm upon excitation at 295 nm at room temperature. In


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order to investigate the applicability of 1 as a promising detecting device for electron-deficient NACs, the fluorescence behavior of 1 in the presence of the NACs was examined. Therefore, we have chosen 2,4,6-trinitrophenol (TNP), 2,4-dinitrophenol (2,4-DNP), 2,4-dinitrotoluene (2,4DNT), 4-nitrotoluene(4-NT), 1,3-dinitro benzene (1,3-DNB), 2-nitrophenol (2-NP) and nitrobenzene (NB) for sensing experiments. In order to monitor the aqueous phase photoluminescence response, aqueous solution of different nitroaromatics were allowed to interact in incremental quantities (0.04 to 0.20 mM) with 2 mg of powdered sample of 1 dispersed in 2 mL of water in a cuvette with constant stirring.

Figure 7. (a) Emission spectra of aqueous dispersed 1 (2 mg/2 mL) after addition of 240 µL aqueous solution of NACs (2 mM), (b) The change in the fluorescence intensity of 1 (λex= 295, λem= 395 nm) in water upon incremental addition of aqueous 2 mM TNP solution, (c) Percentage


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of photoluminescence quenching of 1 upon addition of different nitro analytes and (d) S-V plots of 1 by gradual addition of NACs. As depicted in Figure 7a and 7c, all nitro analytes reduced the emission band of 1 to a certain extent and a significant quenching up to 88 % in the case of TNP and up to 33% for 2,4-DNP was observed. The quenching percentage were calculated using the formula (Io-I)/Io x 100%, which are in the following order: TNP > 2,4-DNP > 2,4-DNT > 4-NT > 1,3-DNB > 2-NP > NB and the corresponding quenching percentages are 88 > 33 > 21 > 16 > 11 > 9 > 3 respectively. As anticipated, addition of electron deficient TNP and 2,4-DNP resulted in fast and efficient fluorescence quenching of 88 % and 33 % respectively. Particularly, upon incremental addition of 0.04, 0.08, 0.12, 0.16, 0.20, 0.24 mM TNP to an aqueous suspension of 1, the observed fluorescence quenching were 30, 55, 69, 79, 85 & 88 % respectively (Figure 7b). Interestingly, incremental addition of equivalent amounts of other studied analytes resulted in relatively low or negligible quenching as depicted in the Figure S13. This result clearly validates the high sensitivity of 1 towards TNP compared to other nitro analyte. The photoluminescence quenching efficiency can be quantitatively explained by the Stern-Volmer (SV) equation: (Io/I) = 1+ KSV [A]. As shown in Figure 7d, TNP at low concentrations display nearly linear SV relationship. With increasing concentration of TNP, the SV plots follows a nonlinear pattern with upward bend. Except TNP, SV plots of remaining NACs under investigation showed nearly linear relationship upon incremental addition of the corresponding solution. This observed nonlinearity of the SV plot, particularly in the case of TNP can be attributable to the occurrence of an energy transfer phenomenon between 1 and TNP. The quenching constant (KSV) calculated from the SV plot for TNP was found to be 3.14 x 104 M−1 which is comparable to the highest values reported by Ghosh et al. for selective aqueous

phase detection of TNP using LMOF,


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[Zn4(DMF)(urotropine)2(L4)4].86 Present investigation is comparable to or better than those of previous reports on some of the recent LMOF/LCP based fluorescent sensors for aqueous phase detection of TNP (Table S4) which is analogues to the present investigation. So as to acquire limit of detection for TNP by 1, fluorescence quenching was monitored by incremental addition of very low concentration of TNP to 1 dispersed in water. The limits of detection (LOD) for TNP by 1 was found to be 0.4 nM which clearly validate the excellent potential of 1 as a highly sensitive sensor for TNP in aqueous media (S2). To explore the quenching mechanism, normalized absorption spectra of various analytes and emission spectra of water suspension of 1 were analyzed. Good spectral overlap is observed between absorption band of TNP as well as 2,4-DNP with emission profiles of 1, while almost no overlap was observed for other nitro analytes (Figure S14). The spectral overlap in the case of TNP/2,4-DNP advocates resonance energy transfer (RET) mechanism which is a long range process usually yielding considerably high fluorescence-quenching efficiencies. In event of contact or vicinity between excited state LCP 1 and TNP molecules, electron transfer from the conduction band of 1 to the LUMO of electron deficient TNP may occur, resulting in the quenching of fluorescence. In fact, TNP is recognized to interact favorably with Lewis basic sites owing to its acidic phenolic proton.86-87 Ghosh et al. and Zhang et al. demonstrated that free pyridine or pendent amino groups on the crystal surface in MOFs can lead to the sensitive detection of TNP by H-bonding interactions.53,55,88 The highest quenching efficiency of TNP may be related to the strong electrostatic interactions between the Lewis basic sites (N3 and N10) of the adenine and the highly acidic hydroxyl group of TNP. Thus, highly acidic TNP may selectively interact with the amine group via ionic and hydrogen-bonding interactions for good quenching response. On the whole, the concomitant effects of energy/electron transfer and electrostatic interactions may be


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attributed to the high selectivity of 1 for the detection of TNP. The competing experiments for selective detection of TNP by 1 over the other nitro analytes were performed by recording the luminescence spectra of aqueous suspensions of 1 in presence of TNP and other selected NACs. As depicted in Figure 8a, the results indicated that presence of other nitroanalytes did not make any significant change in the sensing efficiency of TNP by 1. Moreover, the detection ability of 1 can be restored and recycled after complete luminescence titration experiments.

Figure 8. (a) Fluorescence responses to different NACs. Blue bars represent emission intensity of 1 before addition TNP and red bars represent after addition TNP, and (b) bar diagram depicting the recyclability of 1 over 5 cycles of the luminescence quenching experiment with Hg2+/TNP (blue/green bar = initial intensity of 1; dark red = intensity after addition of 1 mL Hg2+ (1x10-3 M); orange bar = intensity after addition of 240 µL TNP (2 mM) solution). For recycling of 1, the aqueous dispersions containing 1 and TNP were centrifuged, collected by filtration and washed several times with water and dried at ambient temperature. Remarkably, the initial fluorescence intensity was almost retained even after five cycles suggesting a good recyclability of 1 for detection applications (Figure 8b). As confirmed by the PXRD data (Figure S15), 1 maintained its crystallinity and structural integrity after 5 repeated cycles of fluorescence


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titration experiments with TNP solution. High thermal/chemical stability and reusability of 1 can be potentially applied for in-field detection of explosive compounds such as TNP.

Visible Detection by LCP Coated Paper Strips for Practical Application Detection of Hg2+ ion and TNP by 1 coated paper strips were carried out for exploring their utility towards in-field practical applications. 1 coated test strips were prepared by dip-coating the LCP dispersed ethanolic solution of 1 on Whatman filter paper strips and subsequent drying in a desiccator. The test strips thus prepared emitted light blue color under 365 nm UV light. LCP 1 coated paper strips are partially dipped into dilute aqueous solutions of all metal ions and nitroaromatics.

Figure 9. (a) Digital images of 1 coated test strips after dipped in aqueous solution of metal ions (1x10-3 M) and (b) nitro aromatic compounds (2 mM). The digital photographs of 1 coated paper strips after partly treating with analyte solutions captured under 365 nm UV irradiation are shown in Figure 9. As evident from the figures, emission from the area of 1 coated paper strips where the Hg2+ and TNP came in contact with, quenched significantly (become dark), as could be observed by the naked eye under 365 nm UV


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light. The test strip experiments with adenine ligand did not show any appreciable quenching of emission intensity in presence of effective anlaytes (Cr3+, Hg2+, 2,4-DNP and TNP) (Figure S16). This experiment clearly demonstrates the utility of the newly developed LCP, [Zn(µ2-1Hade)(µ2-SO4)]n (1) as a dual fluorosensor towards practical applications. CONCLUSIONS In summary, Zn(II) LCP 1 with good thermal and hydrolytic stability, have been synthesized by reflux method from the self-assembly of zinc sulfate with adenine nucleobase. The neutral 2D framework is composed of both sulfate units as well as rigid adenine linkers via µ2 coordination with the Zn2+ metal nodes. LCP 1 is characterized by different analytical methods including SXRD as well as the thermal and chemical stability has been established by TGA and PXRD data. Selective and sensitive detection of Hg2+ and TNP from various cations and nitroaromatics has been studied by 1. Practically useful, 1 coated paper strips for detection of Hg2+ and TNP have been developed which offers visual sensing of Hg2+ and TNP under the UV light. Thus, the present investigation provides a rational strategy for design and synthesis of coordination polymer based dual fluorosensor for selective and sensitive aqueous phase detection of pollutants Hg2+ and TNP, which has significance in the domain of environmental protection and homeland security. ASSOCIATED CONTENT Supporting Information. ORTEP diagrams, Crystallographic figures, Bond length & bond angle table, FT-IR, PXRD, UV-Vis spectra, Fluorescence spectra, Crystallographic information files. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author Corresponding Author name: Dr. Eringathodi Suresh Corresponding Author e-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Authors acknowledge the CSIR, India (Project, Grant no. OLP0072) for financial support, Mr. Parthrajsinh Sodha for TGA data, Ms. Riddhi Laiya for PXRD data, Mr. Viral Vakani for elemental analysis, Ms. Megha Yadav for FT-IR data, Mr. Jayesh Chaudhari for FE-SEM images, Mr. Gopala Ram Bhadu

for TEM analysis and AD&CIF for all round analytical

support. YR and BP acknowledge UGC and CSIR (India) for SRF respectively. Publication Registration Number: CSIR-CSMCRI – 174/2016. REFERENCES (1) Drinking Water Criteria Document for Inorganic Mercury; Environmental Criteria and Assessment Office: Cincinnati, OH, 1988. (2) Hoyle, I.; Handy, R.D. Aquat. Toxicol. 2005, 72, 147-159. (3) Onyido, I.; Norris, A.R.; Buncel, E. Chem. Rev. 2004, 104, 5911-5930. (4) Quang, D. T.; Kim, J. S. Chem. Rev. 2010, 110, 6280-6301.


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For Table of Contents Use Only Multiresponsive Adenine based Luminescent Zn(II) Coordination Polymer for Detection of Hg2+ and Trinitrophenol in Aqueous Media Yadagiri Rachuri,a,b Bhavesh Parmar,a,b Kamal Kumar Bisht,c and Eringathodi Suresh a,b*

A 2D adenine based Zn(II) luminescent coordination polymer showing good thermal and chemical stability has been synthesized and characterized by various analytical methods including SXRD. Functional aspect of LCP 1 as a potential chemosensor for selective detection of Hg2+ and TNP in aqueous media as well as by paper strip method has been explored.


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Multiresponsive Adenine-Based Luminescent Zn(II) Coordination

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