Tb

Article

Highly selective and sensitive detection of Cu2+ ions using Ce(III)/Tb(III)-doped SrF2 nanocrystals as fluorescent probe Shyam Sarkar, Manjunath Chatti, Venkata N K B Adusumalli, and Venkataramanan Mahalingam ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 03 Nov 2015 Downloaded from http://pubs.acs.org on November 3, 2015

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ACS Applied Materials & Interfaces

Highly selective and sensitive detection of Cu2+ ions using Ce(III)/Tb(III)-doped SrF2 nanocrystals as fluorescent probe

Shyam Sarkar, Manjunath Chatti, Venkata N. K. B. Adusumalli and Venkataramanan Mahalingam,*a

a

Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER),

Kolkata, Mohanpur, West Bengal, 741246, India. KEYWORDS:

Lanthanides, quenching, photoluminescence, nanocrystals, energy transfer,

detection. ABSTRACT We report a green synthetic approach to the synthesis of water dispersible Ce3+/Tb3+-doped SrF2 nanocrystals, carried out using environment friendly microwave irradiation with water as solvent. The nanocrystals display strong green emission due to energy transfer from Ce3+ to Tb3+ ions. This strong green emission from Tb3+ ions is selectively quenched upon addition of Cu2+ ions, thus making the nanocrystals a potential Cu2+ ions sensing material. There is barely any interference by other metal ions on the detection of Cu2+ ions and the detection limit is as low

as

2

nM. This

sensing

ability

is

highly

reversible

by

the

addition

of 1

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ethylenediaminetetraacetic acid (EDTA) with the recovery of almost 90% of the original luminescence. The luminescence quenching and recovery cycle was repeated multiple times without much effect on the sensitivity. The study was extended to real world water samples and obtained similar results. In addition to the sensing, we strongly predict the small size and high luminescence of the Ce3+/Tb3+-doped SrF2 nanocrystals can be used for bioimaging applications. INTRODUCTION The detection of heavy transition metal ions (viz. Cd2+, Hg2+,Cu2+) is of utmost importance because of their potential applications in biological systems.1-3 Particularly, Cu2+ ion is an essential trace element in the human body which plays vital role in organisms from bacteria to mammals.4-6 Presence of excess Cu2+ ions in neuronal cytoplasm may cause different neurodegenerative diseases like Alzheimer’s, Parkinson’s, Wilson’s, etc. and obstruct many biological activities.7-9 In addition, they are one of the abundant pollutant of drinking water. Therefore, detection of Cu2+ ions is essential and it is a challenging task to develop a robust material with high selectivity and sensitivity towards Cu2+ ions. There are quite a few fluorescent based sensors developed in the past decade for the selective detection of Cu2+ ions.10-12 Most of them are fluorescent organic moieties, which covalently bind to the Cu2+ ions. However, they are not water soluble due to the presence of hydrophobic moieties and thus are not appropriate for the detection of Cu2+ ions in biological systems. On the other hand, there is an increasing interest recently towards using nanoparticles for sensing of metal ions. The reason behind is due to their high selectivity, sensitivity, and rapid response time.13-16 Most of the nanoparticles based sensors are ligand functionalized silver, gold

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nanoparticles, quantum dots (viz. CdSe, ZnS) and few based on semiconducting polymer dots.1725

These nanoscale materials possess broad optical signals which changes with the size and shape

of the nanoparticles thus demanding high monodispersity in particle distribution.26-27 Alternatively, lanthanide (Ln3+) doped nanocrystals (NCs) are beneficial owing to their sharp luminescence and longer lifetimes (due to forbidden nature of intra 4f-4f transitions).28-36 Moreover, hosts for the Ln3+ ions are pretty robust and photo-chemically stable. Recently, Yb/Er-doped NaYF4 NCs showing upconversion have been used for the selective detection of Cu2+ ions via fluorescence resonance energy transfer (FRET) between rhodamine B hydrazide (acceptor) and nanocrystals (donor).37 In another study, Li et al. have shown in-depth Cu2+ ions detection using Yb/Er-doped NaYF4 NCs coated with mesoporous silica followed by incorporation of rhodamine B Hydrazide.38 The above two reports show selective and sensitive detection of Cu2+ ions via NIR excitation which is transparent to tissue. However, there are some disadvantages e.g. upconversion intensity is very low particularly in water, probably due to presence of more –OH groups. Furthermore, these methods involve the use of RhB-hydrazide dye, silica sphere, etc. and these are not reusable sensors. It is also worth to mention that the quantum efficiency of the Ln3+-based down-conversion luminescence is considerably higher than that of the upconversion process. Recently, we have reported selective and sensitive detection of Cu2+ ions using Eu3+-doped KZnF3 nanoparticles with a detection limit of ≈0.5 µM.39 The red photoluminescence from Eu3+ ions quenches selectively in the presence Cu2+ ions. The observed low detection limit is likely due to weak emission from Eu3+ ions. This motivates us to develop better material with suitable Ln3+ ions to extend the LOD to the nanomolar (nM) concentration range. Our idea is to use Ce3+/Tb3+ couple, as is well known that the Ce3+ has strong absorbance due to its spin and parity allowed 4f–5d transitions.40, 41 The Ce3+ ions possess broad emission



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band leading to strong overlap with the absorption levels of the Ln3+ ions. This leads to efficient energy transfer from Ce3+ to Ln3+ ions. Moreover, it is reasonable to assume that the relatively high surface area of the smaller nanocrystals (~ 5 nm) leads to better interaction with the Cu2+ ions near the surface of the nanocrystals. Herein, we report microwave assisted synthesis of ~ 5 nm Ce3+/Tb3+-doped SrF2 nanocrystals (NCs). The nanocrystals show strong green emission under UV excitation. Poly(acrylic) acid (PAA) attachment over the surface of NCs renders them water dispersible. The PAA molecules selectively bind to Cu2+ ions presumably through electrostatic interaction with free -COOgroups. The NCs selectively detect Cu2+ ions down to nM concentration via quenching of the photoluminescence intensity of the Tb3+ ions. Moreover, the photoluminescence intensity of the Tb3+ ions is almost 90% restored using EDTA. The entire detection and recovery process is schematically illustrated in Scheme 1.

Scheme 1. Schematic illustration of the detection of Cu2+ ions. (A) PAA capped Ce3+/Tb3+-doped SrF2 nanocrystals, (B) chelation and quenching of green emission from the nanocrystals upon Cu2+ ions addition, and (C) recovery of the PL by the addition of EDTA solution.


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EXPERIMENTAL SECTION Materials The chemicals used in this work, such as strontium nitrate [Sr(NO3)2], cerium nitrate [Ce(NO3)3.5H2O], terbium nitrate [Tb(NO3)3], copper nitrate [Cu(NO3)2], copper acetate [Cu(CH3COO)2], copper chloride [CuCl2], copper carbonate [CuCO3], copper sulfate [CuSO4. 5H2O], calcium nitrate [Ca(NO3)2], lead nitrate [Pb(NO3)2], ferric nitrate [Fe(NO3)3], cadmium acetate [Cd(CH3COO)2], cobalt nitrate [Co(NO3)2], manganese chloride [MnCl2], silver nitrate [AgNO3], poly (acrylic acid) [PAA, Mw=18000], sodium fluoride (NaF), ethylene glycol (EG) and absolute ethanol were all purchased from Sigma Aldrich used without further purification. Synthesis Ce3+(15%)/Tb3+(5%)-doped SrF2 nanocrystals were synthesized by microwave-assisted route. Briefly, 0.80 mmol of strontium nitrate, 0.15 mmol of cerium nitrate and 0.05 mmol terbium nitrate were dissolved in 10 ml of distilled ethylene glycol (EG). A 5 ml EG solution of sodium fluoride (2 mmol) was added to it. The entire mixture was stirred for 10 min. 100 mg of PAA was added to the mixture and stirred for another 10 min. The obtained reaction mixture was transferred to microwave reaction vessel. The reaction was carried out at 150°C temperature for 10 min and cooled to room temperature. The product was collected by centrifugation (rpm=7500) and washed three times with absolute ethanol. It should be noted that the microwave experiments were carried out in temperature control mode using a 30 mL Pyrex vessel and it was tightly sealed by teflon cap. Simultaneous gas jet cooling (3-5 bar) during microwave irradiation was performed by using compressed air (6 bar) and appropriate software option. All microwave experiments were carried out using magnetic stirring at a rate of 600 rpm internally. The Ce3+ and Tb3+ doping percentage was optimized to 15 % and 5% respectively, and all data were 5 ACS Paragon Plus Environment


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obtained using this composition. The same synthetic protocol was employed for the synthesis of PAA-coated SrF2:Ce3+(x%)/Tb3+(y%) (x= 5, 10, 20 and y= 1, 3, 8) nanocrystals except in the change of proportions of starting materials.

Characterization techniques Powder X-ray diffraction (PXRD) measurements were performed on a Rigaku-Smartlab diffractometer with Cu Kα operating at 200kV and 45mA at a scanning rate of 1º min-1 in the 2θ range from 20º to 90º. The samples were completely powdered and spread evenly on a quartz slide. TEM measurement was carried out using a high resolution FEG transmission electron microscope (JEOL, JEM 2100F) with a 200 KeV electron source. Briefly, a drop of the SrF2 nanocrystals in ethanol was spread on a carbon coated Cu grid (300 meshes) and dried in air. The FTIR spectra were obtained with a Perkin Elmer Spectrum RX1 spectrophotometer with the KBr disk technique in the range of 400-4000cm-1. Thermogravimetric analysis was performed using Mettler Toledo TGA 851 instrument under N2 atmosphere at a heating rate of 10º min-1. The photoluminescence measurements were done with the Horiba Jobin Yvon Fluorolog. All the emission spectra were recorded using steady state 450 W Xe lamp as the excitation source. The luminescence lifetime measurements were performed with the Horiba Jobin Yvon Fluoromax 4 machine with a pulsed Xe source of 150 W. The PL studies of excess EDTA addition, excess metal ions addition and detection analysis using lake, tap and mineral water were all performed using Photon Technology Instruments (PTI) limited fitted with 75 W Xe lamp.

Sample preparation for metal ion sensing


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Stock solutions of the metal ions with a concentration of 1 nM and a dilute dispersion of nanocrystals (5 mg in 25 mL of H2O) were prepared in deionized water. The PL experiments were performed by placing a dispersion of nanocrystals (1.9 mL) in a quartz cuvette of 1 cm optical path length, and then adding the Cu2+ salt solution (100 µL) by means of a micro-pipette. For selectivity experiments, the samples were prepared by adding 100 µL of individual metal ion stock solutions to 1.9 mL dispersion of nanocrystals. For recovery of photoluminescence, a solution of EDTA of concentration of 1 µM was prepared and added successively (100 µL) to the mixture of nanoparticles dispersion and metal ions and maintained for 10 min. We obtained maximum recovery (almost 90% of the initial PL) with the addition of 200 µL of EDTA solution.

RESULTS AND DISCUSSIONS Figure 1 displays the experimental XRD pattern of Ce3+/Tb3+-doped SrF2 nanocrystals along with standard pattern for SrF2 crystals (ICSD PDF Card No.-01-086-2418). The XRD patterns of the NCs matches well with that of the standard pattern suggesting that NCs crystallize in pure cubic phase. The size and morphology of the Ce3+/Tb3+-doped SrF2 nanocrystals was analyzed by transmission electron microscopy (TEM). The TEM image shown in Figure 2, shows the formation of spherical NCs with an average size of 5±1 nm. The HRTEM image of a single SrF2 NCs (inset of Figure 2) shows the lattice fringes with an inter fringe distance of about 0.33 nm, consistent with the lattice spacing of the (111) planes of SrF2.42 Dynamic light scattering (DLS) measurement shows the average hydrodynamic radius of nanocrystals is ≈8 nm (poly dispersity index (pdi) value is 0.032) which is shown in Figure S1 (see supporting information).



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The motives behind choosing PAA molecules as capping ligand are the binding of PAA molecules over the surface of the NCs would render them dispersible in aqueous medium and PAA has multiple binding sites and its orientation can be adjusted to reduce the surface defects. The surface functionalization of SrF2 NCs by PAA molecules is confirmed by FTIR and TGA analyses. The FTIR spectra for pure PAA and PAA capped NCs are shown in Figure S2 (see supporting information). The characteristic vibrational bands observed for free PAA molecules at 3448, 2952, 1720, 1456 and 1407 cm-1 are assigned to stretching vibrations of O-H, -CH2, -C=O, -C-O and –COOH groups respectively. For the PAA coated NCs the C=O peak shifted to 1561 cm-1. The presence of this peak along with the peak at 1720 cm-1 indicates that –COO- groups of PAA strongly attached to the surface of the nanocrystals.39,43 This is also supported by thermogravimetric analysis (TGA). The TGA plots of pure PAA molecules and PAA coated SrF2 NCs are shown in Figure S3 (see supporting information). The TGA curves clearly indicate that decomposition temperature for PAA coated SrF2 NCs is higher than that of the pure PAA molecules thus confirming the strong attachment of PAA molecules on to the surface of the NCs. The excitation (λemi=544 nm) and emission (λexi=290 nm) spectra of Ce3+/Tb3+-doped SrF2 NCs are shown in Figure 3. The excitation spectrum displayed a broad absorption band from 250 nm to 300 nm with a maximum at 290 nm which corresponds to the transition from ground state (2F5/2) to the excited 5d state of Ce3+ ions. Upon excitation at 290 nm the nanocrystals exhibit strong green emission. The emission spectrum shows four peaks at 488 nm, 544 nm, 584 nm and 620 nm which are characteristic for Tb3+ ions. These peaks are assigned respectively to the following transitions, 5D4 →7F6, 5D4 →7F5, 5D4 →7F4 and 5D4→7F3 (see Figure 4B (blank). The green emission at 544 nm (5D4 →7F5) is more intense than other transitions and is a magnetic


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dipole transition with ∆J= ±1. The luminescence quantum yield is measured using quinine sulfate as the reference standard. The quantum yield is calculated using the formula, Qsample = Qref (I/Iref) (Aref/A) (n2/n2ref) where, Qsample and Qref are the quantum yields of the nanocrystals and quinine-sulphate respectively, A is the absorbance, I is the integrated area of photoluminescence spectra, and n is the refractive index of the solution. The calculated quantum yield was 0.3762 (37.62%) for Ce3+/Tb3+-doped SrF2 nanocrystals. This value is relatively quite high for the lanthanide-doped nanoparticles which we believe due to efficient energy transfer from Ce3+ to Tb3+ ions. The corresponding excitation and emission spectra are shown in Figure S4. Upon the addition of the Cu2+ ions the photoluminescence intensity of Tb3+ ions quenched almost 95%. However, in the presence of other cations (such as Ca2+, Cd2+, Co2+, Pb2+, Ag+, Mn2+, and Fe3+) there is barely any effect on the photoluminescence intensity. This suggests the high selectivity of the NCs towards Cu2+ ions. The selective quenching of the photoluminescence of Tb3+ ions by Cu2+ ions along with other ions is shown in Figure 4A. For the practical applications in real water sample analysis or biological system it is important to check the interference of other physiologically important cations on the photoluminescence quenching of Tb3+ ions by Cu2+ ions. To achieve this, an experiment was carried out by taking Cu2+ ions along with other ions (1:1 mixture) followed by treatment with the aqueous dispersion of nanocrystals and subsequently photoluminescence of the resulting mixture were measured. It is clear from Figure 4B that addition of Cu2+ ions to the water dispersion of nanocrystals containing any of the interfering metal ions (i.e. Ca2+, Cd2+, Co2+, Pb2+, Ag+, Mn2+, and Fe3+) hardly affect the photoluminescence intensity of Tb3+ ions. In 9 ACS Paragon Plus Environment


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addition, for example increase in the Ca2+, Cd2+, Pb2+ and Co2+, ion concentration barely had any effect on the Cu2+ detection (see Figure S5 supporting information). Furthermore, we have also verified whether increase in the concentration of the metal ions would lead to any aggregation of nanocrystals due to the presence of PAA on the surface. The PL studies on the nanocrystals dispersion in the presence of 10-12 nM metal ion concentration were performed. The results clearly imply only a small decrease in the emission intensity of the Tb3+ ions. The PL spectra for four different metal ions are shown in Figure S6. To test the sensitivity of the Ce3+/Tb3+-doped SrF2 NCs, the photoluminescence spectra were measured as a function of Cu2+ ions concentration and it was observed that there is a gradual increase in the quenching of the photoluminescence intensity of Tb3+ ions with the increase in the Cu2+ ions concentration as shown in Figure 5. A Stern-Volmer equation i.e. I0/I=1+ KSV [Cu2+] was used to get a knowledge about the nature (dynamic/static) of the photoluminescence quenching, where, I0 and I are the photoluminescence intensity of Tb3+ ions from Ce3+/Tb3+-doped SrF2 NCs in the absence and presence of Cu2+ ions, respectively, KSV is the Stern-Volmer constant and [Cu2+] is the concentration of Cu2+ ions. A linear response is obtained with R2 value of 0.98708 from the plot of (I0/I) against concentration of Cu2+ ions (1 to 10 nM) which is shown in the inset of Figure 5, indicating the dynamic nature of the photoluminescence quenching.44 The limit of detection (LOD) is calculated using the formula 3σ/S where σ is the standard deviation of the blank experiment and S is the slope of the calibration plot. The calculated value is close to 2.2 nM. Please note this value is quite low (better sensitivity) compared to our previous result where the LOD is 0.5 µM.39 Furthermore, to understand the effect of counter anions on the sensing behavior of nanocrystals, copper salts with different anions such as copper acetate, chloride, carbonate, 10 ACS Paragon Plus Environment

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sulfate, and nitrate were used. We observed a similar quenching of the Tb3+ luminescence in the presence of different counter anions, thus ruling out any effect of anions on the luminescence. The results are shown in Figure S7 (see supporting information). In order to explain the photoluminescence quenching by transition metal ions several mechanisms have been proposed.45 Generally, photoluminescence quenching of quantum dots or metal nanoparticles occurs because of aggregation of the same in the presence of metal ions (referred as aggregation induced luminescence quenching). However, in our case DLS analysis shows there is barely any change in the particle size distribution for Ce3+/Tb3+-doped SrF2 nanocrystals before and after the addition of Cu2+ ions (see Figure S8, see supporting information) thus ruling out any such mechanism. UV-vis spectra shown in Figure 6, reveal the appearance of a strong absorption band with maxima at 300 nm for the Cu(II) ions. The maxima of this peak shifted 320 nm upon addition of the Cu(II) salt solution to the nanocrystals dispersion. This peak is assigned to a charge transfer band has a strong spectral overlap between the emission spectrum of Ce3+ and absorption spectrum of Cu2+ ions (see inset of Figure 6).46 We strongly believe this reduces the energy transfer efficiency between Ce3+ and Tb3+ ions thus leading to reduction in the green emission intensity . This is reasonable as the charge transfer band has higher molar absorption co-efficient of Cu2+ (ε = 3000 L mol-1cm-1) compared to Tb3+ ions. This is supported by the decrease in the PL intensity of Ce3+ in the presence of Cu2+ ions compared to the Ce/Tb-doped SrF2 nanocrystals alone (see Figure S9, see supporting information). We attribute the observed energy transfer to shorter separation between the Ce3+ and Cu2+ ions as well as to presence of more number of Ce3+ ions on the surface of nanocrystals. The absence of any reabsorption and possibility of excited state photoluminescence quenching is further verified through lifetime measurements. The photoluminescence decay curves shown in 11 ACS Paragon Plus Environment


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Figure. 7 reveal that the lifetime of PAA-encapsulated Ce3+/Tb3+-doped SrF2 nanocrystals decreases to 1.7 ms from 3.8 ms upon addition of Cu2+ ions thus ruling out the possibility of reabsorption process and supporting the energy transfer mechanism. The energy level diagram and plausible energy transfer mechanism from Ce3+ ions to Cu2+ ion is shown in Figure. 8. To check whether the detection of Cu2+ ions is reversible, appropriate amounts of EDTA solution (1 µM) was added to the Cu2+ quenched NCs solution. The emission intensity of the Ce3+/Tb3+-doped SrF2 NCs gradually recovered with the successive addition of EDTA solution. Almost upto 90% of the initial PL was recovered (with 200 µL of EDTA) and the whole process i.e. addition of Cu2+ ions followed by EDTA solution were repeated for many times with only a slight decrease in the emission intensity, suggesting the reusability of the material (see Figure 9). It is worth to mention that, the response time for both quenching and recovery of the photoluminescence intensity of Tb3+ ions was quite rapid (less than 1 minute) and the quenched emission intensity was unchanged even after 6 hours Figure S10 (see supporting information). The Cu(II) ions detection study was extended to real world water samples. In this regard we have collected three different water samples: Lake water, tap water and mineral water. Using these water samples we prepared similar nanocrystal dispersion (5 mg in 25 ml water). Indeed, we observed decrease in the emission intensity of Tb3+ ions upon addition of Cu2+ ions to the above nanocrystals mixed water samples. The results are shown in Figure S11 . The calculated LOD values are 1.87 nM, 1.75 nM and 1.98 nM respectively for lake, tap and mineral water. The details of σ and S are given in Table S1 (see supporting information).


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CONCLUSION In summary, we have developed a simple microwave irradiated route for the synthesis of ultrasmall size (≈5±1 nm) water dispersible Ce3+/Tb3+-doped SrF2 nanocrystals. The nanocrystals dispersion is very selective to the detection of Cu2+ ions in the nM concentration (LOD=2.2 nM) which falls in the physiological concentration range. Moreover, the sensing is reversible and can be performed several times without much decrease in the luminescence intensity. The study was extended to real world water samples like lake, tap and mineral waters and obtained similar results. We strongly believe the small size and high luminescence of the Ce3+/Tb3+-doped SrF2 nanocrystals can be used for bioimaging applications.

ASSOCIATED CONTENT Supporting Information. Experimental section, XRD pattern, DLS size distribution histogram, FTIR spectra, TGA curves, photoluminescence spectra, lifetime, energy transfer mechanism, time dependence of PL quenching curve “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author * Fax: 91-33-25873020; Tel: +91(0)9007603474; E-mail: [email protected] ACKNOWLEDGMENT VM thanks the Department of Science and Technology (DST) for the project EMR/2014/000204 and Indian Institute of Science Education and Research (IISER)-Kolkata for the funding. SS 13 ACS Paragon Plus Environment


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thanks UGC India for his scholarship. MC thanks KVPY for his scholarship. Authors thank Dr. Pradipta purkayastha, for helping with some of the photoluminescence studies. REFERENCES (1) Taki. M.; Desaki. M.; Ojida. A.; Iyoshi. S.; Hirayama. T.; Hamachi. I.; Yamamoto. Y. Fluorescence Imaging of Intracellular Cadmium Using a Dual-Excitation Ratiometric Chemosensor. J. Am. Chem. Soc. 2008, 130, 12564-12565. (2) Huang. C.; Yang. Z.;, Lee. K. H.; Chang. H. T. Synthesis of Highly Fluorescent Gold Nanoparticles for Sensing Mercury(II) Angew. Chem. Int. Ed. 2007, 46, 6824-6828. (3) Kramer. R. Fluorescent Chemosensors for Cu2+ Ions: Fast, Selective, and Highly Sensitive Angew. Chem. Int. Ed. 1998, 37, 772-773. (4) Barceloux. D. G. Copper J. Toxicol., Clin. Toxicol. 1999, 37, 217-230. (5) Zhang. X. B.; Peng. J.; He. C. L.; Shen. G. L.; Yu. R. Q. A Highly Selective Fluorescent Sensor for Cu2+ Based on 2-(2′-Hydroxyphenyl) Benzoxazole in a Poly(vinyl chloride) Matrix. Anal. Chim. Acta, 2006, 567, 189-195. (6) Sarkar, B. In Metal Ions in Biological Systems; Siegel, H., Siegel, A., Eds.; Marcel Dekker: New York, 1981; Vol. 12, p 233. (7) Barnham. K. J.; Masters. C. L.; Bush. A. I. Neurodegenerative Diseases and Oxidative Stress. Nat. Rev. Drug Discovery 2004, 3, 205-214. (8) Mare. S.; Penugonda. S.; Robinson. S. M.; Dohgu. S.; Banks. W. A.; Ercal. N. Copper Complexing Decreases the Ability of Amyloid Beta Peptide to Cross the BBB and Enter Brain Parenchyma. Peptides 2007, 28, 1424-1432. 14 ACS Paragon Plus Environment

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Figure 1. Powder XRD patterns of (a) Ce3+/Tb3+-doped SrF2 nanocrystals and (b) standard cubic SrF2 crystals (ICSD PDF Card No-01-086-2418).


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Figure 2. High resolution transmission electron microscopy (HRTEM) image of PAA-capped Ce3+(15 %)/Tb3+(5 %)-doped SrF2 nanocrystals. Inset shows the HRTEM image of a single nanocrystal with lattice fringe.



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Figure 3. Photoluminescence (PL) excitation and emission spectra of Ce3+(15%)/Tb3+(5%)doped SrF2 nanocrystals.


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Figure 4. (A) The effect of addition of various metal ions (100 µl of 10 nM) on the emission intensity of PAA capped Ce3+/Tb3+-doped SrF2 NCs. (B) The absence of any interference by other metal ions on the selective quenching of the PL intensity of Tb3+ ions by the Cu2+.



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Figure 5. The effect of concentration of Cu2+ ions on the PL intensity of Tb3+ ions (λex= 290 nm). Inset shows Stern-Volmer plot for PAA-capped Ce3+/Tb3+-doped SrF2 nanocrystals and Cu2+ ions mixture.


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Figure 6. UV-vis absorption spectra of aqueous solutions of (a) Cu2+ (b) PAA + Cu2+, (c) NCs + Cu2+ and (d) only NCs. Inset shows the overlap between emission spectrum of Ce3+ ions and absorbance of Cu2+ ions.



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Figure 7. Luminescence lifetime decay curves of Ce3+/ Tb3+ -doped SrF2 nanocrystals before and after the addition of Cu2+ ions.


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(B)

(A)

35

5d

5d 5

5

CT band

D1 D2 5D 3

30

D1 D2 5 D3

5

5

25 5

5

D4

D4

5 2F

0

2

Ce3+

7 F 7 3 F4 7 F5 7 F6

7/2

F5/2

Tb3+

2 2

584nm 620nm

488 nm 544nm

10

584nm 620nm

15

544nm

20 488 nm

EnergyX103 (cm-1)

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7 F 7 3 F4 7 F5 7F 6

F7/2

F5/2

Ce3+

Cu(II) (Quencher)

Tb3+

Figure 8. The plausible energy transfer mechanism between Ce3+ and Tb3+ ions in the absence (A) and presence (B) of the Cu2+ ions.



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Figure 9. Photoluminescence spectra of (1) only Ce3+/Tb3+-doped SrF2 nanocrystals (blank), (2) upon addition of Cu2+ in blank, (3) upon addition of 100 µl EDTA to Cu2+ coordinated Ce3+/Tb3+-doped SrF2, and (4) upon addition of 200 µl EDTA to Cu2+ coordinated Ce3+/Tb3+ doped SrF2. Inset shows the quenching and recovery of Tb3+ luminescence intensity upto five cycles.


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TOC Figure


Tb

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