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Supramolecular hybrid of ZnO nanoparticles with benzimidazole based organic ligand for the recognition of Zn2+ ions in semi-aqueous media
Accepted Manuscript Title: Supramolecular hybrid of ZnO nanoparticles with benzimidazole based organic ligand for the recognition of Zn2+ ions in semi-aqueous media Authors: Narinder Kaur, Pushap Raj, Navneet Kaur, Deuk Young Kim, Narinder Singh PII: DOI: Reference:
S1010-6030(17)30408-2 http://dx.doi.org/doi:10.1016/j.jphotochem.2017.07.009 JPC 10735
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
27-3-2017 17-6-2017 5-7-2017
Please cite this article as: Narinder Kaur, Pushap Raj, Navneet Kaur, Deuk Young Kim, Narinder Singh, Supramolecular hybrid of ZnO nanoparticles with benzimidazole based organic ligand for the recognition of Zn2+ ions in semi-aqueous media, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.07.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Supramolecular hybrid of ZnO nanoparticles with benzimidazole based organic ligand for the recognition of Zn2+ ions in semi-aqueous media Narinder Kaur1, Pushap Raj2, Navneet Kaur3, Deuk Young Kim1,*, Narinder Singh2,* 1
Semiconductor Materials and Device Laboratory, Department of Semiconductor Science,
Dongguk University-Seoul, Jung-gu, Seoul 04620, South Korea 2
Department of Chemistry, Indian Institute of Technology (IIT) Ropar, Rupnagar,
Panjab 140001, India 3
Department of Chemistry, Panjab University, Chandigarh, 160014, India
*
Corresponding Author: [email protected] & [email protected]
Graphical abstract
1
Highlights
ZnO nanoparticles decorated with organic ligand for the investigation of zinc ions by absorption spectroscopy.
Petal like morphology of coated nanoparticles is observed due to the surface directing nature of ligand.
The proposed sensor showed the selectivity of zinc ions by observing significant response at 442 nm in the absorption spectra.
Abstract Zinc oxide (ZnO) nanoparticles were decorated with benzimidazole based organic ligand (L) to form supramolecular hybrid for the recognition of Zn2+ in HEPES-buffered DMSO/H2O (7:3; v/v) solvent media using absorption spectroscopy. The resultant hybrid compound (N) showed the shifting of imine linkage bands (−CH=N−) in IR spectra due to the chemical attachment of organic ligand (L) on ZnO surface. The coating of organic ligand on the surface of ZnO was also confirmed with energy-dispersive x-ray (EDX) analysis. The resultant hybrid compound (N) exhibited high selectivity towards Zn2+ ions over other metal ions in HEPESbuffered DMSO/H2O (7:3; v/v) solution. The absorption peak intensity for Zn2+ ions was observed at 560 nm (1 nM), which was quenched by the addition of mole concentration and the peak shifted toward lower wavelength region at 442 nm (50 nM) resulted the development of hybrid chemosensor with a detection limit of 4.09 nM.
Keywords: ZnO nanoparticles; Organic ligand; Supramolecular hybrid; Absorption spectroscopy; Detection of Zn2+ ions.
1. Introduction
2
The importance of hybrid materials based chemosensors has been considerably raised due to its advantageous features such as ease of preparation, low detection limit, high selectivity and broad detection range [1, 2]. Considering the instability and non-selectivity issues of organic nanoparticles in aqueous media, the sensing performance of organic system can be improved by making the hybrid system in which small metal nanoparticles attached with organic nanoparticles in solid framework and gives better stability to organic system. Furthermore, these hybrid functional nanomaterials may provide the protection of sensor probe and the incorporation of nonselective receptor/ligand upon inorganic nanostructures changed binding affinity of organic ligands which offer steric organization to the pods of receptor for making selective towards a particular metal ion. In order to make the hybrid systems, the semiconductor nanoparticles like ZnO having remarkable properties (stable microstructure, high exciton binding energy ~60 meV and wide bandgap ~3.37 eV) can be decorated by the organic ligands for the better surface functionality and stability [3-5]. Besides, ZnO-organic hybrid material has several advantages such as cheap synthesis, biocompatibility, unique band gap and surface states that are used in numerous applications like photo catalysis, electrochemical catalysis and optoelectronic devices as compared to simple organic ligand and ZnO nanoparticle. Also, ZnO nanoparticles itself in the free state get aggregate in aqueous medium and decrease the properties like band gap, surface state and optical properties. The organic ligand coated ZnO have small size, high surface area and unique band gap which tune the photophysical properties of the nanoparticle that can be used for sensing of cations. The literature have shown the better detection limit of metallic ions when ZnO nanoparticles coated by organic ligands [6-8]. Benzimidazole based organic ligands are structurally heterocyclic ring compounds which have been introduced in the development of chemosensors based on the following considerations: (i) it changes the spatial effects within one molecule; (ii) realize the real-time detection; (iii) biocompatible nature; (iv) therapeutic nature (v) range of possible derivatives. Towards this approach, numerous work have been carried out on the designing of fluorescent, chromogenic and electrochemical sensors for in-vivo recognition of analytes [9, 10]. Owing to its advantageous sensing applications, benzimidazole based organic ligands have been explored for the construction of supramolecular assembly and functional hybrid materials. Recently, organic ligands containing 3
benzimidazole units have been used to decorate the surface of inorganic materials/metals such as CdSe, Au, Ag, ZnO and carbon nanoparticles or quantum dots and showed several important advantages such as biocompatibility, stability, solubility and enhanced emission spectra [11-13]. Due to interesting features of hybrid materials, it is highly imperative to put more efforts for the development of hybrid chemosensors which can determine the transition metal ions in the biologically and environmentally important samples. Among the metallic ions present in the human body, zinc (Zn2+) is the second most abundant transition metal ion next to iron, which plays an essential role in biological processes [14, 15]. The total concentrations of free zinc ions are loosely bound to proteins, which are low and tightly regulated. Although, the excessive amount of free zinc ions are involved in the regulation of programmed cell deaths and may cause to several severe diseases [16, 17]. The rapid increase of zinc concentrations in the soil reduced microbial activity [18]. Therefore, the detection of free Zn2+ ions is one of the crucial parameters for controlling its adverse effect on human health as well as biosphere. Recently, Rastogi et al. detected the Zn (II) ions from 8-Aminoquinoline functionalized silica nanoparticles with a limit of detection of 0.1 µM [19]. Carbon nanotubes modified with glycine-N-8-quinolylamide demonstrated the detection of Zn2+ ions in the micro molar (0.2 μM) range [20]. Lately, Sharma et al. detected Zn2+ ions from imine linked dipodal receptor decorated ZnO nanoparticles [21]. Despite the extensive studies for the detection of Zn2+ ions, reports demonstrated the nanomolar detection of zinc ions from the benzimidazole based organic ligand decorated ZnO nanoparticles are rarely available. In order to explore the methodology of hybrid sensors for the nanomolar detection of Zn2+ ions, the decoration of ZnO nanoparticles with benzimidazole based organic receptor has been performed. The resultant supramolecular hybrid can detect zinc ions in semi-aqueous solution (HEPES-buffered DMSO/H2O (7:3; v/v)) with a detection limit of 4.09 nM.
2. Experimental part 2.1 Materials and methods All solvents and reagents were obtained commercially and were used as received without further purification. The crystal structure of ZnO powders were examined by X-ray diffraction (XRD) 4
with a PANalytical X’PERT PRO diffractometer using Ni-filtered Cu Kα radiations with a scan speed of 10°/min for 2θ in a range from 10 to 75 degree. The UV−vis absorption spectra were measured with a Specord 250 plus Analytik Jena Spectrophotometer using quartz cells having a 1 cm path length. Solid state absorption spectra were analysed by using quartz plates coated with a fine coating of material. The morphology and elemental analysis (EDS) of ZnO samples were investigated with a Scanning Electron Microscope (SEM-JEOL JSM 6610 LV) with a voltage of 15 kV. The confirmation of organic ligand on ZnO nanoparticles was characterized by Transmission Electron Microscope (TEM) (Hitachi H-7500). The size distribution of the organic ligand coated ZnO particles (N) was confirmed on a Metrohm Microtrac Ultra Nanotrac particle size analyzer (dynamic light scattering). The Fourier transform infrared spectra (FTIR) of ligand L and compound N were recorded on Bruker Tensor 27 Spectrometer in the range of 600 to 4000 cm-1. The NMR spectra of L and N were recorded in DMSO-d6 solvent on a JEOL JNM−ECS400 Spectrophotometer, which operated at 400 MHz for 1H NMR spectra. In this study, experiments were performed using solvent system of DMSO/H2O in the ratio of 7:3.
2.2 Synthesis of ligand (L) The synthesis of organic ligand L was carried out by performing the condensation reactions between
2-(2-aminophenyl)
benzimidazole
and
2-hydroxy-5-((3-nitrophenyl)
diazenyl)
benzaldehyde in dry ethanol [22]. The prepared reaction mixture was stirred at room temperature for 5 h and obtained product in yellow color. Then, it was separated out and filtered as well as washed with cold ethanol. The structure elucidation data of L matched with the literature report. The imine linkages (−CH=N−) band was characterized at 1592 cm−1 using IR spectroscopy. The 1
H NMR spectrum exhibited 8.28 (s, 1H, CH=N), 8.26 (s, 1H, Ar-H ), 7.95 (s, 1H, Ar-H ), 7.82 -
7.78 (m, 4H, Ar-H), 7.64 - 7.60 (d, 1H, Ar-H), 7.38 - 7.30 (m, 4H, Ar-H), 7.14 - 7.07 (m, 6H, ArH), 6.85 - 6.75 (m, 2H, Ar-H). The 13C NMR spectrum indicated peaks at 159.9 corresponding to −CH=N− and the peaks at 110.5, 111.9, 115.2, 117.3, 118.3, 119.1, 122.5, 122.7, 125.5, 124.2, 124.9, 125.3, 127.7, 132.1, 133.3, 143.4, 144.2, 145.4, 147.6, 148.3 and 156.7 due to aromatic carbons.
5
2.3 Synthesis of Compound (N) The synthesis of Compound (N) (ligand L coated ZnO nanoparticles) was carried out by taking Zn(ClO4)2·6H2O along with ligand L in dry ethanol [7]. Then, NaOH was slowly added to the Zn solution by drop wise and constantly stirred at 25 °C for 2 h. Consequently, the dispersion of coated ZnO nanoparticles were formed in the prepared solution which are centrifugally filtered and washed several times with ethanol and distilled (DI) water. The resultant product was completely dried at 50 °C for 24 hours and characterized with spectroscopic techniques such as FTIR, 1H NMR and 13C NMR. IR (KBr): υ (in cm−1) 1651(−CH=N−); 1H NMR δ (ppm) 8.23 8.20 (d, 2H, CH=N), 7.94 - 7.92 (d, 1H, Ar-H), 7.72 - 7.62 (m, 3H, Ar-H), 7.24 - 7.05 (m, 5H, ArH), 6.85 - 6.77 (m, 3H, Ar-H). 13C NMR δ (ppm) 156.6, 147.8, 144.3, 143.9, 137.5, 133.3, 132.0, 128.1, 125.4, 125.1, 122.99, 122.94, 122.8, 122.6, 119.1, 118.4, 115.4, 112.1, 110.7. The presence of ligand L on ZnO nanoparticles was also confirmed from SEM and EDS analysis of compound N.
2.4 Recognition properties of ligand decorated ZnO (N) All the recognition studies were performed at room temperature (25±1°C). Before performing any experiment, the solution were well shaken to maintain its uniformity. In order to check the recognition properties of ligand decorated ZnO (N), we have used the mixed solvent of DMSO/H2O (7:3) as the ligand decorated ZnO particles were shown aggregation in other solvents. The aggregation of hybrid material in other solvents could be expected due to the presence of capping ligand. The cation binding affinity of N was determined by mixing the standard host solution (10 nM) with a fixed amounts of particular metal nitrate salt (50 nM) in HEPES-buffered DMSO/H2O (7:3; v/v) solvent. The cation binding test of N has been performed towards various cations such as Zn2+, Ca2+, Na+, Sr2+, Ba2+, K+, Mg2+, Mn2+and Cd2+. The recognition behaviour of metal was observed from the changes in the absorption spectra of sensor (N) during an addition of a particular metal salt. The titration experiment was performed by taking fixed amount of N (10 nM) along with successive addition of zinc nitrate salt (0-50 nM) in HEPESbuffered DMSO/H2O (7:3; v/v) solvent. To evaluate any possible interference of other metals for the selective detection of zinc ions, the solution was prepared by mixing the solutions of N (10 6
nM) and Zn2+ (50 nM) with and without other interfering metals ions (50 nM) in DMSO/H2O HEPES-buffered solution. The size distribution of N in DMSO/H2O solvent was estimated form the dynamic light scattering based particle size analyzer. Elemental analysis was monitored through a Fisons instrument (Model EA 1108 CHN). The recyclability studies of compound N have been carried out with UV − absorption spectra by alternate addition and decomplexation of N.Zn2+ by ultracentrifugation technique.
3. Results and Discussion The synthesis of organic ligand (L) is already reported in the previous literature [9]. A single step condensation reaction of 2-(2-aminophenyl) benzimidazole with 2-hydroxy-5-((3nitrophenyl) diazenyl) benzaldehyde has been conducted in dry ethanol and continuously stirred for 5 hours. Finally, yellow color yield was obtained after washing and filtration with cold ethanol. The chemical formula of L is shown in the Fig. 1 (a). The IR spectrum (Fig. S3) of L exhibits a band at 1592 cm-1 due to the stretching of imine linkage bond (−CH=N−). The formation of imine bond of L was confirmed through a signal at δ 8.28 and δ 159.9 in 1H NMR (Fig. S1) and
13
C
NMR (Fig. S2) spectra. Thereafter, organic ligand (L) was coated on the surface of ZnO through in-situ synthesis of ZnO along with ligand. ZnO crystals were synthesized by taking Zn (ClO4).6H2O along with organic ligand (L) in dry ethanol. The decoration of ZnO nanoparticles with benzimidazole based organic ligand (L) is represented in the Fig. 1 (b). To validate the decoration of ligand on ZnO surface, the compound N was characterized by IR, 1H NMR and 13C NMR spectroscopy. The FTIR spectra of pure ligand L compared with the ZnO coated organic ligand N is shown in Fig. S9. The investigation shows that the vibration frequency corresponding to imine group of ligand L at 1592 cm-1 was shifted to 1651 cm-1 in compound N. This large shift in the frequency of imine linkage confirms the interaction of ZnO nanoparticle with the imine group. The vibration frequency of OH and NH group at 3401 cm-1 in free ligand L was also shifted at 3331 cm-1 in compound N. These changes also exhibited the interaction of these functional group with the ZnO. Furthermore, the 1H NMR and 13C NMR spectra (Figs. S7 & S8) of ligand L and compound N were compared. The 1H NMR spectra of pure ligand L show imine linked proton at 8.28 ppm. However, in the case of ZnO coated ligand (N), this proton was shifted to up field 7
chemical shift value (8.23 ppm), as shown in Fig. S4. Similarly, the 13C NMR spectrum (Fig. S2) of free ligand showed imine linkage carbon peak at 159.91 ppm. In organic - inorganic hybrid ZnO material (N), these imine linkage signal was shifted to 156.61 ppm, which can be seen in the Fig. S5. The other aromatic proton and carbon signals in free ligand L were also show some distortion in their original chemical shift value when ZnO was coated with the ligand. These characteristics changes in spectroscopic behaviour of compound N makes the difference of ligand behaviour on the surface of ZnO, which results to tune its photophysical properties from the pure ligand and free ZnO nanoparticle. A comparison of solid state UV − absorption spectra of organic ligand (L) and compound (N) is shown in the Fig. 2. The absorption band of L was observed at 369 nm which can be attributed to the π-π* transitions. While the band was shifted towards higher wavelength (~460 nm) for N. The presence of organic moiety on the surface of ZnO was also quantified using elemental analysis. ZnO nanoparticles were consisted of only Zn and O elements, as it can be seen in the Fig. 3 (a). The EDS spectra of N (Fig. 3 (b)) showed the presence of ligand L peaks (nitrogen and carbon) along with peaks of ZnO. Furthermore, the compound N was characterized with CHN analysis and results are given in the Table S1, which clearly indicates the percentage of carbon, nitrogen and hydrogen elements observed in the compound N. Fig. 4 (a) shows the XRD-pattern of ZnO nanoparticles, presenting scattering angles (2θ) of 31.8°, 34.4°, 36.2°, 47.5°, 56.6° and 62.8° corresponding to the reflections from (100), (002), (101), (102), (110), and (103) planes. All peaks are matched with the hexagonal wurtzite structure, indicating the purity of ZnO nanoparticles. The average crystallite size of ZnO nanoparticles can be estimated from DebyeScherrer equation [23]:
D 0.89 / cos where, λ is the X-rays wavelength (0.154 nm), θ is the Bragg diffraction angle and β is the full width at half maximum (FWHM) of the diffraction peak. The average crystallite size of ZnO nanoparticles was calculated to be 43.76 nm. Fig. 4 (b) shows the FE-SEM image of ZnO nanoparticles. The surface morphology of ZnO particles were consisted of nanocrystals aggregates. The particle size evaluated from SEM image was observed to be ~50 nm, which is closely matched with the XRD analysis. Furthermore, morphology changes in the ligand decorated ZnO nanoparticles (N) were observed, as shown in the Fig. 4 (c). From FESEM images, the formation 8
of supramolecular self-assembled petal-like hybrid nanostructure was noticed which could be expected due to the non-covalent supramolecular interaction of ligand with ZnO particles. Previous literature survey indicate that the presence of surface directing agents such as organic ligands, surfactants, proteins, phospholipids, and other polymeric materials can control the shape of metal nanoparticles that results to use of these materials in the catalysis and sensing applications [24, 25]. In the present study, supramolecular self-assembled hybrid nanostructure was predicted because of the surface directing behaviour of organic ligand. Fig. 4 (d) shows the TEM image of N. The size of ligand coated ZnO nanoparticles N was observed to be about 50 nm. The large view of Fig. 4 (d) has been noticed in the bright field-TEM image (Fig. 4 (e)) of N. This shows that grain boundaries of compound N are of large size and also inclined with each other. These grain boundaries were not observed in the case of pure ZnO nanoparticles. Additionally, the size of pure ZnO nanoparticles was calculated to be 43.76 nm from XRD spectra. The size of pure ZnO particles (~43.76 nm) was smaller than ligand coated ZnO nanoparticles (N) (~50 nm), which indicated the presence of ligand L on the surface of ZnO nanoparticles. In order to check the crystallinity of ZnO nanoparticles, high resolution TEM image (HRTEM) has been taken, which is shown in the Fig. 4 (f). The lattice plane fringes were used to calculate the d-spacing value. The lattice spacing (d) of ligand coated ZnO nanoparticles (N) were observed to be 2.60 Å, which is in agreement with the interspacing of (002) planes of wurtzite ZnO, indicating the purity and crystalline behaviour of ZnO nanoparticles. The hydrodynamic size of coated ZnO nanoparticles was estimated from the size distribution plot (Fig. S10), which was measured with the particle size analyzer by dissolving the compound N in DMSO/H2O (7:3; v/v) solvent. The average particle size of N was observed to be in the range of 33-35 nm. A total of different nine metals ions were used for metal binding experiment with hybrid compound (N) to establish selectivity. The sensor selectivity of N with different metallic ions such as Zn2+, Ca2+, Na+, Sr2+, Ba2+, K+, Mg2+, Mn2+ and Cd2+ were tested by the absorption spectroscopy. The experiment was performed in HEPES-buffered DMSO/H2O (7:3; v/v) solvent system to understand the effect of ZnO nanoparticles for modulation in photophysical properties of organic ligand. In order to check the ligand L itself recognition behaviour with Zn2+ ions, the binding studies of pure ligand L were performed (not shown). However, no significant shift in the 9
absorption profile of ligand L was observed on the addition of Zn2+ ions. Saluja et al. [9] have synthesized similar ligand and tested their binding studies with the Zn2+ ions. Although there was no selectivity of ligand towards Zn2+ ions detected in absorption spectrum. This means that ligand L itself has no binding affinity for the zinc ions. Then, the compound N was selected to perform the sensor selectivity studies with different metallic ions through absorption spectroscopy. The changes in the absorption spectra by adding the three metal ions ((50 nM) (Zn2+, Ca2+, Na+) to the fixed solution (10 nM) of N were shown in the Fig. 5(a). The N showed different responses for Zn2+ ions by observing the absorption band near at 442 nm. The peak intensity at ~560 nm was quenched and shifted to lower wavelength at 442 nm, signifying the blue shift in the absorption spectra of N. While other two metal ions were displayed almost similar response in the absorption spectra (~560 nm) of N. From the Fig. 5 (a), it shows that compound N have tendency to selectively binding with zinc ions in semi-aqueous solution. Moreover, selectivity of metal ions such Sr2+, Ba2+, K+ has been checked from the absorption spectra given in Fig. 5 (b). The absorption peaks of N before and after the addition of metals ions to N appeared at almost similar wavelength and showed no remarkable response in the performed spectra, indicating no selectivity of given metals with prepared compound N. Furthermore, the solution of N was screened with metal ions like Mg2+, Mn2+ and Cd2+ and variations in the absorption spectra of N is displayed in the Fig. 5 (c). As we can see from the spectra, the intensity of absorption peak (~560 nm) for N was decreased as the metal ions (Mg2+, Mn2+ and Cd2+) were added to the solution of compound N. Besides, there were no significant new peaks noticed in the absorption spectra. Fig. 5 (d) shows the bar graph of absorption intensity (442 nm) versus different metals. It is clearly confirmed from the plot that the compound N has high selectivity with only Zn2+ ions. The selectivity of N with zinc ions can be predicted due to the interaction of ZnO with amine (Schiff base) and hydroxyl ions of L, which was clarified from spectroscopic studies. Due to this interaction, ZnO make complex with ligand L that prohibits the flexible rotation of L about its axis [7, 26]. Thus, L offers proper orientation to make the complex with zinc-ions. The complex system might be involved in the internal charge transfer process from ligand donor (amine) to Zn-acceptor following the electronic excitation [27, 28]. Due to these electronic excitations, a blue shift in absorption spectra could be expected. The selective binding of zinc ions with this complex system can be ascribed to the perfect fit cavity 10
size of N for the binding of zinc ions. To gain more insight on the properties of N as a sensor for Zn2+ ions, titration test (Fig. 6 (a)) was performed in DMSO/H2O (7:3; v/v) solvent by adding small aliquots of zinc ions. Upon addition of Zn2+ ions (0-50 nM) to the solution of N, the intensity of the absorption peak at 442 nm was continually increased and showed the decrease in intensity at 560 nm. The successive increase in intensity at 442 nm and decrease in intensity at 560 nm may indicate the estimation of Zn2+ ions by calibration plot (Fig. 6 (b)). The plot between absorption intensity of N and concentration of Zn2+ ions was increased linearly. The minimum detection limit of proposed sensor was observed to be ~ 4.09 nM, which can be calculated using IUPAC 3σ method (DL = 3σ/m), where σ is the standard deviation obtained from titration calibration plot and m is the slope from its corresponding calibration plot. In order to check the selectivity of compound (N) for Zn2+ ions, competitive experiments (Fig. 7) were performed in the presence of Zn2+ (50 nM) with one of following metals: Ba2+, Ca2+, Cd2+, K+, Mg2+, Mn2+, Na+ and Sr2+ (50 nM). Other metal ions exhibited no interference in the absorption profile of N. It is revealed that N have the high selectivity for Zn2+ ions, even in the presence of other ions, acted as interferent. The recyclability studies of compound N has been studied with UV − absorption spectra by alternate addition and removal (by ultracentrifugation) of Zn2+. The UV − absorption response were analysed at (λabs) 442 and 560 nm, as shown in Fig. S11 (A) - (B). The UV – VIS response of N showed hyperchromic on addition of Zn2+ and hypochromic effect in removal of Zn2+ at (λabs) 442 nm. Similarly, the opposite effect (hypochromic in presence of Zn2+ and hyperchromic in absence of Zn2+) was observed at 560 nm. The process was repeated for 3 times and little absorption efficiency loss (less than 1%) was observed. The results of proposed hybrid sensor were compared with previous reported results using hybrid materials for the detection of zinc ions. It is found that our sensor system has achieved a better detection limit using absorption spectroscopy (Table S2).
4. Conclusion ZnO nanoparticles decorated with organic ligand were used for the investigation of Zn2+ in semi-aqueous solution through the absorption spectroscopy. The microstructural analysis confirmed the coating of ligand on surface of ZnO nanoparticles. The titration studies revealed 11
that the quenching and enhancement in the absorption intensity at 560 nm and 442 nm leads to the development of hybrid sensor with a detection limit of 4.09 nM.
Acknowledgements This research was supported by Basic Science Research Programs (2016R1A6A1A03012877) funded by the Ministry of Education of the Korean government.
Appendix A. Supplementary data 1
H NMR,13C NMR, FTIR, size distribution plot, Recyclability test, CHN analysis (Table S1),
comparison of prepared sensor with previous reported sensors (Table S2).
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[24] M.S. Bakshi, S. Sachar, G. Kaur, P. Bhandari, G. Kaur, M.C. Biesinger, F. Possmayer, N.O. Petersen, Dependence of Crystal Growth of Gold Nanoparticles on the Capping Behavior of Surfactant at Ambient Conditions, Crystal Growth & Design, 8 (2008) 1713-1719. [25] P. Khullar, V. Singh, A. Mahal, P.N. Dave, S. Thakur, G. Kaur, J. Singh, S. Singh Kamboj, M. Singh Bakshi, Bovine Serum Albumin Bioconjugated Gold Nanoparticles: Synthesis, Hemolysis, and Cytotoxicity toward Cancer Cell Lines, The Journal of Physical Chemistry C, 116 (2012) 8834-8843. [26] H. Sharma, A. Singh, N. Kaur, N. Singh, ZnO-Based Imine-Linked Coupled Biocompatible Chemosensor for Nanomolar Detection of Co2+, ACS Sustainable Chemistry & Engineering, 1 (2013) 1600-1608. [27] Z. Xu, G.-H. Kim, S.J. Han, M.J. Jou, C. Lee, I. Shin, J. Yoon, An NBD-based colorimetric and fluorescent chemosensor for Zn 2+ and its use for detection of intracellular zinc ions, Tetrahedron, 65 (2009) 23072312. [28] M. Hosseini, Z. Vaezi, M.R. Ganjali, F. Faridbod, S.D. Abkenar, K. Alizadeh, M. Salavati-Niasari, Fluorescence “turn-on” chemosensor for the selective detection of zinc ion based on Schiff-base derivative, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 75 (2010) 978-982.
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Figures Caption Fig. 1 Schematic representation of (a) organic ligand (L), (b) synthesized compound (N). Fig. 2 Solid state UV-absorbance spectra of compound (N) and organic ligand (L). Fig. 3 EDS spectra of (a) ZnO nanoparticles, and (b) organic ligand (L) coated ZnO nanoparticles (N). Fig. 4 (a) XRD pattern of ZnO nanoparticles, (b) FE-SEM image of ZnO nanoparticles, (c) FESEM image of compound N, (d) TEM image of N, (e) Large view of bright field-TEM image (d) of N, and (f) HR-TEM image of N. Fig. 5 Changes in absorption intensity of N upon the addition of corresponding various metal ions : (a) Zn2+, Ca2+, Na+, (b) Sr2+, Ba2+, K+ (c) Mg2+, Mn2+, Cd2+, (d) Bar graph representing the absorption intensity of N (10 nM) at 442 nm upon the addition of 50 nM of a particular metal nitrate salt in HEPES-buffered DMSO/H2O (7:3,v/v) solvent. Fig. 6 (a) Changes in absorbance spectra of compound N by adding the small aliquots amounts of Zn2+ ions (0-50 nM) in HEPES-buffered DMSO/H2O (7:3,v/v), (b) Calibration plot of absorption intensity of N versus concentration of Zn2+ ions in HEPES-buffered DMSO/H2O (7:3, v/v) solvent. Fig. 7 Detection of Zn2+ in the presence of other metal ions (Ba2+, Ca2+, Cd2+, K+, Mg2+, Mn2+, Na+ and Sr2+) through competitive binding test.
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S1010-6030(17)30408-2 http://dx.doi.org/doi:10.1016/j.jphotochem.2017.07.009 JPC 10735
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
27-3-2017 17-6-2017 5-7-2017
Please cite this article as: Narinder Kaur, Pushap Raj, Navneet Kaur, Deuk Young Kim, Narinder Singh, Supramolecular hybrid of ZnO nanoparticles with benzimidazole based organic ligand for the recognition of Zn2+ ions in semi-aqueous media, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.07.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Supramolecular hybrid of ZnO nanoparticles with benzimidazole based organic ligand for the recognition of Zn2+ ions in semi-aqueous media Narinder Kaur1, Pushap Raj2, Navneet Kaur3, Deuk Young Kim1,*, Narinder Singh2,* 1
Semiconductor Materials and Device Laboratory, Department of Semiconductor Science,
Dongguk University-Seoul, Jung-gu, Seoul 04620, South Korea 2
Department of Chemistry, Indian Institute of Technology (IIT) Ropar, Rupnagar,
Panjab 140001, India 3
Department of Chemistry, Panjab University, Chandigarh, 160014, India
*
Corresponding Author: [email protected] & [email protected]
Graphical abstract
1
Highlights
ZnO nanoparticles decorated with organic ligand for the investigation of zinc ions by absorption spectroscopy.
Petal like morphology of coated nanoparticles is observed due to the surface directing nature of ligand.
The proposed sensor showed the selectivity of zinc ions by observing significant response at 442 nm in the absorption spectra.
Abstract Zinc oxide (ZnO) nanoparticles were decorated with benzimidazole based organic ligand (L) to form supramolecular hybrid for the recognition of Zn2+ in HEPES-buffered DMSO/H2O (7:3; v/v) solvent media using absorption spectroscopy. The resultant hybrid compound (N) showed the shifting of imine linkage bands (−CH=N−) in IR spectra due to the chemical attachment of organic ligand (L) on ZnO surface. The coating of organic ligand on the surface of ZnO was also confirmed with energy-dispersive x-ray (EDX) analysis. The resultant hybrid compound (N) exhibited high selectivity towards Zn2+ ions over other metal ions in HEPESbuffered DMSO/H2O (7:3; v/v) solution. The absorption peak intensity for Zn2+ ions was observed at 560 nm (1 nM), which was quenched by the addition of mole concentration and the peak shifted toward lower wavelength region at 442 nm (50 nM) resulted the development of hybrid chemosensor with a detection limit of 4.09 nM.
Keywords: ZnO nanoparticles; Organic ligand; Supramolecular hybrid; Absorption spectroscopy; Detection of Zn2+ ions.
1. Introduction
2
The importance of hybrid materials based chemosensors has been considerably raised due to its advantageous features such as ease of preparation, low detection limit, high selectivity and broad detection range [1, 2]. Considering the instability and non-selectivity issues of organic nanoparticles in aqueous media, the sensing performance of organic system can be improved by making the hybrid system in which small metal nanoparticles attached with organic nanoparticles in solid framework and gives better stability to organic system. Furthermore, these hybrid functional nanomaterials may provide the protection of sensor probe and the incorporation of nonselective receptor/ligand upon inorganic nanostructures changed binding affinity of organic ligands which offer steric organization to the pods of receptor for making selective towards a particular metal ion. In order to make the hybrid systems, the semiconductor nanoparticles like ZnO having remarkable properties (stable microstructure, high exciton binding energy ~60 meV and wide bandgap ~3.37 eV) can be decorated by the organic ligands for the better surface functionality and stability [3-5]. Besides, ZnO-organic hybrid material has several advantages such as cheap synthesis, biocompatibility, unique band gap and surface states that are used in numerous applications like photo catalysis, electrochemical catalysis and optoelectronic devices as compared to simple organic ligand and ZnO nanoparticle. Also, ZnO nanoparticles itself in the free state get aggregate in aqueous medium and decrease the properties like band gap, surface state and optical properties. The organic ligand coated ZnO have small size, high surface area and unique band gap which tune the photophysical properties of the nanoparticle that can be used for sensing of cations. The literature have shown the better detection limit of metallic ions when ZnO nanoparticles coated by organic ligands [6-8]. Benzimidazole based organic ligands are structurally heterocyclic ring compounds which have been introduced in the development of chemosensors based on the following considerations: (i) it changes the spatial effects within one molecule; (ii) realize the real-time detection; (iii) biocompatible nature; (iv) therapeutic nature (v) range of possible derivatives. Towards this approach, numerous work have been carried out on the designing of fluorescent, chromogenic and electrochemical sensors for in-vivo recognition of analytes [9, 10]. Owing to its advantageous sensing applications, benzimidazole based organic ligands have been explored for the construction of supramolecular assembly and functional hybrid materials. Recently, organic ligands containing 3
benzimidazole units have been used to decorate the surface of inorganic materials/metals such as CdSe, Au, Ag, ZnO and carbon nanoparticles or quantum dots and showed several important advantages such as biocompatibility, stability, solubility and enhanced emission spectra [11-13]. Due to interesting features of hybrid materials, it is highly imperative to put more efforts for the development of hybrid chemosensors which can determine the transition metal ions in the biologically and environmentally important samples. Among the metallic ions present in the human body, zinc (Zn2+) is the second most abundant transition metal ion next to iron, which plays an essential role in biological processes [14, 15]. The total concentrations of free zinc ions are loosely bound to proteins, which are low and tightly regulated. Although, the excessive amount of free zinc ions are involved in the regulation of programmed cell deaths and may cause to several severe diseases [16, 17]. The rapid increase of zinc concentrations in the soil reduced microbial activity [18]. Therefore, the detection of free Zn2+ ions is one of the crucial parameters for controlling its adverse effect on human health as well as biosphere. Recently, Rastogi et al. detected the Zn (II) ions from 8-Aminoquinoline functionalized silica nanoparticles with a limit of detection of 0.1 µM [19]. Carbon nanotubes modified with glycine-N-8-quinolylamide demonstrated the detection of Zn2+ ions in the micro molar (0.2 μM) range [20]. Lately, Sharma et al. detected Zn2+ ions from imine linked dipodal receptor decorated ZnO nanoparticles [21]. Despite the extensive studies for the detection of Zn2+ ions, reports demonstrated the nanomolar detection of zinc ions from the benzimidazole based organic ligand decorated ZnO nanoparticles are rarely available. In order to explore the methodology of hybrid sensors for the nanomolar detection of Zn2+ ions, the decoration of ZnO nanoparticles with benzimidazole based organic receptor has been performed. The resultant supramolecular hybrid can detect zinc ions in semi-aqueous solution (HEPES-buffered DMSO/H2O (7:3; v/v)) with a detection limit of 4.09 nM.
2. Experimental part 2.1 Materials and methods All solvents and reagents were obtained commercially and were used as received without further purification. The crystal structure of ZnO powders were examined by X-ray diffraction (XRD) 4
with a PANalytical X’PERT PRO diffractometer using Ni-filtered Cu Kα radiations with a scan speed of 10°/min for 2θ in a range from 10 to 75 degree. The UV−vis absorption spectra were measured with a Specord 250 plus Analytik Jena Spectrophotometer using quartz cells having a 1 cm path length. Solid state absorption spectra were analysed by using quartz plates coated with a fine coating of material. The morphology and elemental analysis (EDS) of ZnO samples were investigated with a Scanning Electron Microscope (SEM-JEOL JSM 6610 LV) with a voltage of 15 kV. The confirmation of organic ligand on ZnO nanoparticles was characterized by Transmission Electron Microscope (TEM) (Hitachi H-7500). The size distribution of the organic ligand coated ZnO particles (N) was confirmed on a Metrohm Microtrac Ultra Nanotrac particle size analyzer (dynamic light scattering). The Fourier transform infrared spectra (FTIR) of ligand L and compound N were recorded on Bruker Tensor 27 Spectrometer in the range of 600 to 4000 cm-1. The NMR spectra of L and N were recorded in DMSO-d6 solvent on a JEOL JNM−ECS400 Spectrophotometer, which operated at 400 MHz for 1H NMR spectra. In this study, experiments were performed using solvent system of DMSO/H2O in the ratio of 7:3.
2.2 Synthesis of ligand (L) The synthesis of organic ligand L was carried out by performing the condensation reactions between
2-(2-aminophenyl)
benzimidazole
and
2-hydroxy-5-((3-nitrophenyl)
diazenyl)
benzaldehyde in dry ethanol [22]. The prepared reaction mixture was stirred at room temperature for 5 h and obtained product in yellow color. Then, it was separated out and filtered as well as washed with cold ethanol. The structure elucidation data of L matched with the literature report. The imine linkages (−CH=N−) band was characterized at 1592 cm−1 using IR spectroscopy. The 1
H NMR spectrum exhibited 8.28 (s, 1H, CH=N), 8.26 (s, 1H, Ar-H ), 7.95 (s, 1H, Ar-H ), 7.82 -
7.78 (m, 4H, Ar-H), 7.64 - 7.60 (d, 1H, Ar-H), 7.38 - 7.30 (m, 4H, Ar-H), 7.14 - 7.07 (m, 6H, ArH), 6.85 - 6.75 (m, 2H, Ar-H). The 13C NMR spectrum indicated peaks at 159.9 corresponding to −CH=N− and the peaks at 110.5, 111.9, 115.2, 117.3, 118.3, 119.1, 122.5, 122.7, 125.5, 124.2, 124.9, 125.3, 127.7, 132.1, 133.3, 143.4, 144.2, 145.4, 147.6, 148.3 and 156.7 due to aromatic carbons.
5
2.3 Synthesis of Compound (N) The synthesis of Compound (N) (ligand L coated ZnO nanoparticles) was carried out by taking Zn(ClO4)2·6H2O along with ligand L in dry ethanol [7]. Then, NaOH was slowly added to the Zn solution by drop wise and constantly stirred at 25 °C for 2 h. Consequently, the dispersion of coated ZnO nanoparticles were formed in the prepared solution which are centrifugally filtered and washed several times with ethanol and distilled (DI) water. The resultant product was completely dried at 50 °C for 24 hours and characterized with spectroscopic techniques such as FTIR, 1H NMR and 13C NMR. IR (KBr): υ (in cm−1) 1651(−CH=N−); 1H NMR δ (ppm) 8.23 8.20 (d, 2H, CH=N), 7.94 - 7.92 (d, 1H, Ar-H), 7.72 - 7.62 (m, 3H, Ar-H), 7.24 - 7.05 (m, 5H, ArH), 6.85 - 6.77 (m, 3H, Ar-H). 13C NMR δ (ppm) 156.6, 147.8, 144.3, 143.9, 137.5, 133.3, 132.0, 128.1, 125.4, 125.1, 122.99, 122.94, 122.8, 122.6, 119.1, 118.4, 115.4, 112.1, 110.7. The presence of ligand L on ZnO nanoparticles was also confirmed from SEM and EDS analysis of compound N.
2.4 Recognition properties of ligand decorated ZnO (N) All the recognition studies were performed at room temperature (25±1°C). Before performing any experiment, the solution were well shaken to maintain its uniformity. In order to check the recognition properties of ligand decorated ZnO (N), we have used the mixed solvent of DMSO/H2O (7:3) as the ligand decorated ZnO particles were shown aggregation in other solvents. The aggregation of hybrid material in other solvents could be expected due to the presence of capping ligand. The cation binding affinity of N was determined by mixing the standard host solution (10 nM) with a fixed amounts of particular metal nitrate salt (50 nM) in HEPES-buffered DMSO/H2O (7:3; v/v) solvent. The cation binding test of N has been performed towards various cations such as Zn2+, Ca2+, Na+, Sr2+, Ba2+, K+, Mg2+, Mn2+and Cd2+. The recognition behaviour of metal was observed from the changes in the absorption spectra of sensor (N) during an addition of a particular metal salt. The titration experiment was performed by taking fixed amount of N (10 nM) along with successive addition of zinc nitrate salt (0-50 nM) in HEPESbuffered DMSO/H2O (7:3; v/v) solvent. To evaluate any possible interference of other metals for the selective detection of zinc ions, the solution was prepared by mixing the solutions of N (10 6
nM) and Zn2+ (50 nM) with and without other interfering metals ions (50 nM) in DMSO/H2O HEPES-buffered solution. The size distribution of N in DMSO/H2O solvent was estimated form the dynamic light scattering based particle size analyzer. Elemental analysis was monitored through a Fisons instrument (Model EA 1108 CHN). The recyclability studies of compound N have been carried out with UV − absorption spectra by alternate addition and decomplexation of N.Zn2+ by ultracentrifugation technique.
3. Results and Discussion The synthesis of organic ligand (L) is already reported in the previous literature [9]. A single step condensation reaction of 2-(2-aminophenyl) benzimidazole with 2-hydroxy-5-((3nitrophenyl) diazenyl) benzaldehyde has been conducted in dry ethanol and continuously stirred for 5 hours. Finally, yellow color yield was obtained after washing and filtration with cold ethanol. The chemical formula of L is shown in the Fig. 1 (a). The IR spectrum (Fig. S3) of L exhibits a band at 1592 cm-1 due to the stretching of imine linkage bond (−CH=N−). The formation of imine bond of L was confirmed through a signal at δ 8.28 and δ 159.9 in 1H NMR (Fig. S1) and
13
C
NMR (Fig. S2) spectra. Thereafter, organic ligand (L) was coated on the surface of ZnO through in-situ synthesis of ZnO along with ligand. ZnO crystals were synthesized by taking Zn (ClO4).6H2O along with organic ligand (L) in dry ethanol. The decoration of ZnO nanoparticles with benzimidazole based organic ligand (L) is represented in the Fig. 1 (b). To validate the decoration of ligand on ZnO surface, the compound N was characterized by IR, 1H NMR and 13C NMR spectroscopy. The FTIR spectra of pure ligand L compared with the ZnO coated organic ligand N is shown in Fig. S9. The investigation shows that the vibration frequency corresponding to imine group of ligand L at 1592 cm-1 was shifted to 1651 cm-1 in compound N. This large shift in the frequency of imine linkage confirms the interaction of ZnO nanoparticle with the imine group. The vibration frequency of OH and NH group at 3401 cm-1 in free ligand L was also shifted at 3331 cm-1 in compound N. These changes also exhibited the interaction of these functional group with the ZnO. Furthermore, the 1H NMR and 13C NMR spectra (Figs. S7 & S8) of ligand L and compound N were compared. The 1H NMR spectra of pure ligand L show imine linked proton at 8.28 ppm. However, in the case of ZnO coated ligand (N), this proton was shifted to up field 7
chemical shift value (8.23 ppm), as shown in Fig. S4. Similarly, the 13C NMR spectrum (Fig. S2) of free ligand showed imine linkage carbon peak at 159.91 ppm. In organic - inorganic hybrid ZnO material (N), these imine linkage signal was shifted to 156.61 ppm, which can be seen in the Fig. S5. The other aromatic proton and carbon signals in free ligand L were also show some distortion in their original chemical shift value when ZnO was coated with the ligand. These characteristics changes in spectroscopic behaviour of compound N makes the difference of ligand behaviour on the surface of ZnO, which results to tune its photophysical properties from the pure ligand and free ZnO nanoparticle. A comparison of solid state UV − absorption spectra of organic ligand (L) and compound (N) is shown in the Fig. 2. The absorption band of L was observed at 369 nm which can be attributed to the π-π* transitions. While the band was shifted towards higher wavelength (~460 nm) for N. The presence of organic moiety on the surface of ZnO was also quantified using elemental analysis. ZnO nanoparticles were consisted of only Zn and O elements, as it can be seen in the Fig. 3 (a). The EDS spectra of N (Fig. 3 (b)) showed the presence of ligand L peaks (nitrogen and carbon) along with peaks of ZnO. Furthermore, the compound N was characterized with CHN analysis and results are given in the Table S1, which clearly indicates the percentage of carbon, nitrogen and hydrogen elements observed in the compound N. Fig. 4 (a) shows the XRD-pattern of ZnO nanoparticles, presenting scattering angles (2θ) of 31.8°, 34.4°, 36.2°, 47.5°, 56.6° and 62.8° corresponding to the reflections from (100), (002), (101), (102), (110), and (103) planes. All peaks are matched with the hexagonal wurtzite structure, indicating the purity of ZnO nanoparticles. The average crystallite size of ZnO nanoparticles can be estimated from DebyeScherrer equation [23]:
D 0.89 / cos where, λ is the X-rays wavelength (0.154 nm), θ is the Bragg diffraction angle and β is the full width at half maximum (FWHM) of the diffraction peak. The average crystallite size of ZnO nanoparticles was calculated to be 43.76 nm. Fig. 4 (b) shows the FE-SEM image of ZnO nanoparticles. The surface morphology of ZnO particles were consisted of nanocrystals aggregates. The particle size evaluated from SEM image was observed to be ~50 nm, which is closely matched with the XRD analysis. Furthermore, morphology changes in the ligand decorated ZnO nanoparticles (N) were observed, as shown in the Fig. 4 (c). From FESEM images, the formation 8
of supramolecular self-assembled petal-like hybrid nanostructure was noticed which could be expected due to the non-covalent supramolecular interaction of ligand with ZnO particles. Previous literature survey indicate that the presence of surface directing agents such as organic ligands, surfactants, proteins, phospholipids, and other polymeric materials can control the shape of metal nanoparticles that results to use of these materials in the catalysis and sensing applications [24, 25]. In the present study, supramolecular self-assembled hybrid nanostructure was predicted because of the surface directing behaviour of organic ligand. Fig. 4 (d) shows the TEM image of N. The size of ligand coated ZnO nanoparticles N was observed to be about 50 nm. The large view of Fig. 4 (d) has been noticed in the bright field-TEM image (Fig. 4 (e)) of N. This shows that grain boundaries of compound N are of large size and also inclined with each other. These grain boundaries were not observed in the case of pure ZnO nanoparticles. Additionally, the size of pure ZnO nanoparticles was calculated to be 43.76 nm from XRD spectra. The size of pure ZnO particles (~43.76 nm) was smaller than ligand coated ZnO nanoparticles (N) (~50 nm), which indicated the presence of ligand L on the surface of ZnO nanoparticles. In order to check the crystallinity of ZnO nanoparticles, high resolution TEM image (HRTEM) has been taken, which is shown in the Fig. 4 (f). The lattice plane fringes were used to calculate the d-spacing value. The lattice spacing (d) of ligand coated ZnO nanoparticles (N) were observed to be 2.60 Å, which is in agreement with the interspacing of (002) planes of wurtzite ZnO, indicating the purity and crystalline behaviour of ZnO nanoparticles. The hydrodynamic size of coated ZnO nanoparticles was estimated from the size distribution plot (Fig. S10), which was measured with the particle size analyzer by dissolving the compound N in DMSO/H2O (7:3; v/v) solvent. The average particle size of N was observed to be in the range of 33-35 nm. A total of different nine metals ions were used for metal binding experiment with hybrid compound (N) to establish selectivity. The sensor selectivity of N with different metallic ions such as Zn2+, Ca2+, Na+, Sr2+, Ba2+, K+, Mg2+, Mn2+ and Cd2+ were tested by the absorption spectroscopy. The experiment was performed in HEPES-buffered DMSO/H2O (7:3; v/v) solvent system to understand the effect of ZnO nanoparticles for modulation in photophysical properties of organic ligand. In order to check the ligand L itself recognition behaviour with Zn2+ ions, the binding studies of pure ligand L were performed (not shown). However, no significant shift in the 9
absorption profile of ligand L was observed on the addition of Zn2+ ions. Saluja et al. [9] have synthesized similar ligand and tested their binding studies with the Zn2+ ions. Although there was no selectivity of ligand towards Zn2+ ions detected in absorption spectrum. This means that ligand L itself has no binding affinity for the zinc ions. Then, the compound N was selected to perform the sensor selectivity studies with different metallic ions through absorption spectroscopy. The changes in the absorption spectra by adding the three metal ions ((50 nM) (Zn2+, Ca2+, Na+) to the fixed solution (10 nM) of N were shown in the Fig. 5(a). The N showed different responses for Zn2+ ions by observing the absorption band near at 442 nm. The peak intensity at ~560 nm was quenched and shifted to lower wavelength at 442 nm, signifying the blue shift in the absorption spectra of N. While other two metal ions were displayed almost similar response in the absorption spectra (~560 nm) of N. From the Fig. 5 (a), it shows that compound N have tendency to selectively binding with zinc ions in semi-aqueous solution. Moreover, selectivity of metal ions such Sr2+, Ba2+, K+ has been checked from the absorption spectra given in Fig. 5 (b). The absorption peaks of N before and after the addition of metals ions to N appeared at almost similar wavelength and showed no remarkable response in the performed spectra, indicating no selectivity of given metals with prepared compound N. Furthermore, the solution of N was screened with metal ions like Mg2+, Mn2+ and Cd2+ and variations in the absorption spectra of N is displayed in the Fig. 5 (c). As we can see from the spectra, the intensity of absorption peak (~560 nm) for N was decreased as the metal ions (Mg2+, Mn2+ and Cd2+) were added to the solution of compound N. Besides, there were no significant new peaks noticed in the absorption spectra. Fig. 5 (d) shows the bar graph of absorption intensity (442 nm) versus different metals. It is clearly confirmed from the plot that the compound N has high selectivity with only Zn2+ ions. The selectivity of N with zinc ions can be predicted due to the interaction of ZnO with amine (Schiff base) and hydroxyl ions of L, which was clarified from spectroscopic studies. Due to this interaction, ZnO make complex with ligand L that prohibits the flexible rotation of L about its axis [7, 26]. Thus, L offers proper orientation to make the complex with zinc-ions. The complex system might be involved in the internal charge transfer process from ligand donor (amine) to Zn-acceptor following the electronic excitation [27, 28]. Due to these electronic excitations, a blue shift in absorption spectra could be expected. The selective binding of zinc ions with this complex system can be ascribed to the perfect fit cavity 10
size of N for the binding of zinc ions. To gain more insight on the properties of N as a sensor for Zn2+ ions, titration test (Fig. 6 (a)) was performed in DMSO/H2O (7:3; v/v) solvent by adding small aliquots of zinc ions. Upon addition of Zn2+ ions (0-50 nM) to the solution of N, the intensity of the absorption peak at 442 nm was continually increased and showed the decrease in intensity at 560 nm. The successive increase in intensity at 442 nm and decrease in intensity at 560 nm may indicate the estimation of Zn2+ ions by calibration plot (Fig. 6 (b)). The plot between absorption intensity of N and concentration of Zn2+ ions was increased linearly. The minimum detection limit of proposed sensor was observed to be ~ 4.09 nM, which can be calculated using IUPAC 3σ method (DL = 3σ/m), where σ is the standard deviation obtained from titration calibration plot and m is the slope from its corresponding calibration plot. In order to check the selectivity of compound (N) for Zn2+ ions, competitive experiments (Fig. 7) were performed in the presence of Zn2+ (50 nM) with one of following metals: Ba2+, Ca2+, Cd2+, K+, Mg2+, Mn2+, Na+ and Sr2+ (50 nM). Other metal ions exhibited no interference in the absorption profile of N. It is revealed that N have the high selectivity for Zn2+ ions, even in the presence of other ions, acted as interferent. The recyclability studies of compound N has been studied with UV − absorption spectra by alternate addition and removal (by ultracentrifugation) of Zn2+. The UV − absorption response were analysed at (λabs) 442 and 560 nm, as shown in Fig. S11 (A) - (B). The UV – VIS response of N showed hyperchromic on addition of Zn2+ and hypochromic effect in removal of Zn2+ at (λabs) 442 nm. Similarly, the opposite effect (hypochromic in presence of Zn2+ and hyperchromic in absence of Zn2+) was observed at 560 nm. The process was repeated for 3 times and little absorption efficiency loss (less than 1%) was observed. The results of proposed hybrid sensor were compared with previous reported results using hybrid materials for the detection of zinc ions. It is found that our sensor system has achieved a better detection limit using absorption spectroscopy (Table S2).
4. Conclusion ZnO nanoparticles decorated with organic ligand were used for the investigation of Zn2+ in semi-aqueous solution through the absorption spectroscopy. The microstructural analysis confirmed the coating of ligand on surface of ZnO nanoparticles. The titration studies revealed 11
that the quenching and enhancement in the absorption intensity at 560 nm and 442 nm leads to the development of hybrid sensor with a detection limit of 4.09 nM.
Acknowledgements This research was supported by Basic Science Research Programs (2016R1A6A1A03012877) funded by the Ministry of Education of the Korean government.
Appendix A. Supplementary data 1
H NMR,13C NMR, FTIR, size distribution plot, Recyclability test, CHN analysis (Table S1),
comparison of prepared sensor with previous reported sensors (Table S2).
12
References [1] L.-J. Fan, W.E. Jones, A Highly Selective and Sensitive Inorganic/Organic Hybrid Polymer Fluorescence “Turn-on” Chemosensory System for Iron Cations, Journal of the American Chemical Society, 128 (2006) 6784-6785. [2] S.J. Lee, D.R. Bae, W.S. Han, S.S. Lee, J.H. Jung, Different Morphological Organic–Inorganic Hybrid Nanomaterials as Fluorescent Chemosensors and Adsorbents for CuII Ions, European Journal of Inorganic Chemistry, 2008 (2008) 1559-1564. [3] H. Sharma, K. Narang, N. Singh, N. Kaur, Imine linked chemosensors coupled with ZnO: Fluorescent and chromogenic detection of Al3+, Materials Letters, 84 (2012) 104-106. [4] A. Asok, M.N. Gandhi, A. Kulkarni, Enhanced visible photoluminescence in ZnO quantum dots by promotion of oxygen vacancy formation, Nanoscale, 4 (2012) 4943-4946. [5] Z.-Y. Zhang, H.-M. Xiong, Photoluminescent ZnO Nanoparticles and Their Biological Applications, Materials, 8 (2015) 3101. [6] S.K. Sharma, N. Kaur, J. Singh, A. Singh, P. Raj, S. Sankar, D.Y. Kim, N. Singh, N. Kaur, H. Singh, Salen decorated nanostructured ZnO chemosensor for the detection of mercuric ions (Hg2+), Sensors and Actuators B: Chemical, 232 (2016) 712-721. [7] N. Kaur, J. Singh, P. Raj, N. Singh, H. Singh, S.K. Sharma, D.Y. Kim, N. Kaur, ZnO decorated with organic nanoparticles based sensor for the ratiometric selective determination of mercury ions, New Journal of Chemistry, 40 (2016) 1529-1534. [8] H. Sharma, N. Kaur, T. Pandiyan, N. Singh, Surface decoration of ZnO nanoparticles: A new strategy to fine tune the recognition properties of imine linked receptor, Sensors and Actuators B: Chemical, 166 (2012) 467-472. [9] P. Saluja, N. Kaur, J. Kang, N. Singh, D.O. Jang, Benzimidazole-based chromogenic chemosensor for the recognition of oxalic acid via counter ion displacement assay in semi-aqueous medium, Tetrahedron, 69 (2013) 9001-9006. [10] M.M. Henary, Y. Wu, C.J. Fahrni, Zinc(II)-selective ratiometric fluorescent sensors based on inhibition of excited-state intramolecular proton transfer, Chemistry (Weinheim an der Bergstrasse, Germany), 10 (2004) 3015-3025.
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[11] S.-L. Li, Y.-Q. Lan, J.-C. Ma, J.-F. Ma, Z.-M. Su, Metal−Organic Frameworks Based on Different Benzimidazole Derivatives: Effect of Length and Substituent Groups of the Ligands on the Structures, Crystal Growth & Design, 10 (2010) 1161-1170. [12] N. Yu, H. Peng, H. Xiong, X. Wu, X. Wang, Y. Li, L. Chen, Graphene quantum dots combined with copper(II) ions as a fluorescent probe for turn-on detection of sulfide ions, Microchimica Acta, 182 (2015) 2139-2146. [13] J. Han, X. Bu, D. Zhou, H. Zhang, B. Yang, Discriminating Cr(iii) and Cr(vi) using aqueous CdTe quantum dots with various surface ligands, RSC Advances, 4 (2014) 32946-32952. [14] H. Kim, J. Kang, K.B. Kim, E.J. Song, C. Kim, A highly selective quinoline-based fluorescent sensor for Zn(II), Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 118 (2014) 883-887. [15] Z. Xu, J. Yoon, D.R. Spring, Fluorescent chemosensors for Zn2+, Chemical Society Reviews, 39 (2010) 1996-2006. [16] P. Thirupathi, K.-H. Lee, A ratiometric fluorescent detection of Zn(II) in aqueous solutions using pyrene-appended histidine, Bioorganic & Medicinal Chemistry Letters, 23 (2013) 6811-6815. [17] C.J. Frederickson, A.I. Bush, Synaptically released zinc: Physiological functions and pathological effects, Biometals, 14 (2001) 353-366. [18] A. Voegelin, S. Pfister, A.C. Scheinost, M.A. Marcus, R. Kretzschmar, Changes in Zinc Speciation in Field Soil after Contamination with Zinc Oxide, Environmental Science & Technology, 39 (2005) 6616-6623. [19] S.K. Rastogi, P. Pal, D.E. Aston, T.E. Bitterwolf, A.L. Branen, 8-Aminoquinoline Functionalized Silica Nanoparticles: A Fluorescent Nanosensor for Detection of Divalent Zinc in Aqueous and in Yeast Cell Suspension, ACS Applied Materials & Interfaces, 3 (2011) 1731-1739. [20] Z. Dong, B. Yang, J. Jin, J. Li, H. Kang, X. Zhong, R. Li, J. Ma, Quinoline group modified carbon nanotubes for the detection of zinc ions, Nanoscale research letters, 4 (2009) 335. [21] H. Sharma, N. Singh, D.O. Jang, Imidazole and imine coated ZnO nanoparticles for nanomolar detection of Al(III) and Zn(II) in semi-aqueous media, Tetrahedron Letters, 55 (2014) 6623-6626. [22] P. Saluja, H. Sharma, N. Kaur, N. Singh, D.O. Jang, Benzimidazole-based imine-linked chemosensor: chromogenic sensor for Mg 2+ and fluorescent sensor for Cr 3+, Tetrahedron, 68 (2012) 2289-2293. [23] N. Kaur, S.K. Sharma, D.Y. Kim, N. Singh, Highly transparent and lower resistivity of yttrium doped ZnO thin films grown on quartz glass by sol–gel method, Physica B: Condensed Matter, 500 (2016) 179185. 14
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Figures Caption Fig. 1 Schematic representation of (a) organic ligand (L), (b) synthesized compound (N). Fig. 2 Solid state UV-absorbance spectra of compound (N) and organic ligand (L). Fig. 3 EDS spectra of (a) ZnO nanoparticles, and (b) organic ligand (L) coated ZnO nanoparticles (N). Fig. 4 (a) XRD pattern of ZnO nanoparticles, (b) FE-SEM image of ZnO nanoparticles, (c) FESEM image of compound N, (d) TEM image of N, (e) Large view of bright field-TEM image (d) of N, and (f) HR-TEM image of N. Fig. 5 Changes in absorption intensity of N upon the addition of corresponding various metal ions : (a) Zn2+, Ca2+, Na+, (b) Sr2+, Ba2+, K+ (c) Mg2+, Mn2+, Cd2+, (d) Bar graph representing the absorption intensity of N (10 nM) at 442 nm upon the addition of 50 nM of a particular metal nitrate salt in HEPES-buffered DMSO/H2O (7:3,v/v) solvent. Fig. 6 (a) Changes in absorbance spectra of compound N by adding the small aliquots amounts of Zn2+ ions (0-50 nM) in HEPES-buffered DMSO/H2O (7:3,v/v), (b) Calibration plot of absorption intensity of N versus concentration of Zn2+ ions in HEPES-buffered DMSO/H2O (7:3, v/v) solvent. Fig. 7 Detection of Zn2+ in the presence of other metal ions (Ba2+, Ca2+, Cd2+, K+, Mg2+, Mn2+, Na+ and Sr2+) through competitive binding test.
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