Novel fluorescent nano-sensor based on amino-functionalization of Eu3+:SrSnO3 for copper ion detection in food and real drink water samples

Lanthanide-doped nanoparticles exhibit unique optical properties and have been widely utilized for different sensing applications. Herein, the Eu3+:SrSnO3@APTS nanosensor was synthesized and its optical properties were analyzed using UV-Vis and photoluminescence spectroscopy. The TEM images of the synthesized nanophosphor Eu3+:SrSnO3@APTS exhibited peanut-like morphology, composed of two or more spherical nanoparticles with an average diameter ∼33 nm. Effects of environmental pH values and doping concentrations as well as amino functionalization on the structure of Eu3+:SrSnO3 were investigated. The as-synthesized optical nanosensor was used for determination of copper ions based on a fluorescence quenching approach. Red emission with a long lifetime was obtained in the case of the 0.06 mol Eu3+:SrSnO3@APTS sample. Under the optimal experimental conditions, a Stern–Volmer plot exhibited a good linearity for copper ions over the concentration (0.00–10.8) × 10−11 mol L−1 with a correlation efficient of 0.996 and a limit of detection 3.4 × 10−12 mol L−1. The fluorescent sensor was dynamically quenched via a coulombic interaction mechanism between the Eu3+ (5L6) and Cu2+. The Eu3+:SrSnO3@APTS nanosensor with the optimal Eu3+ dopant concentration of 0.06 mol was applied for copper determination in food and real drink water samples with high recovery values. We believe that the developed nanosensor probe can also be used for the detection of other toxic compounds, with high selectivity and sensitivity.


Introduction
Recently, the detection of copper ions in environmental and biological systems has attracted considerable attention because copper plays an important role in physiological processes of living organisms in certain doses. 1,2 However, the overuse of copper is highly toxic and leads to several diseases such as kidney failure, nervous system damage, and several others. 3,4 Moreover, at higher concentrations, Cu 2+ can react with molecular oxygen to create reactive oxygen species (ROS), leading to signicant damage to cell structures, due to its interaction with proteins, nucleic acids and lipids. 5,6 Additionally, excess levels of copper in the body for two weeks or more may lead to adverse health effects as nausea, vomiting, diarrhea and permanent organs damage. 7 Therefore, the detection of Cu 2+ in food samples and drinking water is a necessary precaution. Consequently, production of sensitive and low-cost sensors for measuring copper in different industrial elds remains an active area of research.
Many analytical methods had been reported for the detection of copper ion. Among these methods are atomic absorption spectroscopy (AAS), 8 inductively coupled plasma mass spectroscopy (ICP-MS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), 9 and electrochemical. 10,11 Although these analytical methods are efficient quantitatively, they need expensive instrumentation and are time intensive. They also demand high operating cost, which makes these methods unsuitable for eld monitoring.
In recent years, uorescence methods had been widely employed for sensing various heavy metal ions, 12,13 nanoparticles, 14 ngerprint 15,16 and biomarker, [17][18][19] due to their low cost and visualization. Moreover, it has robust, high reliability, reusability, fast response time and other properties. 20 Organic uorescent sensors are limited by their photo-bleaching, broad emission band, short lifetime and low sensitivity. 21 The simple, selective and sensitive nanopolar sensor based on a polyamine decorated b-cyclodextrin was used for the detection of copper(II) in real environmental samples. 22 However, inorganic uorescent sensors attracted much attention, as it avoiding the above-mentioned disadvantages. It was reported that functionalized silver nanoparticles, CdSe coated with 4-mercaptobenzoic acid 23 and ZnS quantum dots functionalize with L-cystine were used as a uorescent sensor for copper detection. Although these inorganic sensors are simple, high-selective and relatively cheap, their broad emission bands showed some aggregation affecting the sensing ability. Moreover, the sensing features of silver and Q-dot depends on their shape and size, therefore the use of monodispersed nanoparticles is essential. 24 Inorganic sensors based on lanthanide ions have been recently studied as an effective sensor for different applications. Lanthanide ions have distinct uorescent properties that include anti-Stokes shis, sharp emission peaks, 25 and long-live luminescence. 26,27 Lanthanide doped alkaline earth stannates MSnO 3 (M ¼ Sr, Ca, Ba) exhibit unique properties including lower chemical toxicity, lack of radioactive elements and greater thermal and chemical stability. 28 Note that the forbidden f-f transition of the Ln(III) ions, which induces signicantly low molar extinction coefficient (3 < 10 M À1 cm À1 ) limiting the practical applications of the lanthanides. 29 However, the presence of aromatic chromophores as linkers for lanthanides lead to minimize this effect and provide excellent sensing sensitivity toward the analyte. 29 Due to their unique luminescence properties, lanthanide-based probes have recently used for the detection of nitroaromatic compounds in water, 30 ions, tracewater, gas and temperature. 31 In the present study, we have developed nano-uorescent sensor based on SrSnO 3 :Eu 3+ @APTS as an efficient analytical tool with quick response, high selectivity and sensitivity for the detection of copper ion in food and drinking water samples.

Materials
All chemicals used in the present study were of analytical grade and used without further purication. with distilled water to remove excess salts. The product was nally dried at 80 C in air and annealed at 700 C. 32,33 2.2.2 Synthesis of SrSnO 3 :0.06 mol Eu 3+ @APTS. SrSnO 3 :-Eu 3+ (500 mg) was dispersed and sonicated in 600 mL ethanol/ water (5/1) for 20 minutes. Acetic acid was added dropwise to the solution until the solution pH equal to 4. Then 4 mL of APTS was added to the solution and the mixture stirred for 3 hours at room temperature. Finally, the nanoparticle was puried and washed several times with ethanol and then dried for 24 hours at 45 C in an drying oven. The obtained particles was characterized by FT-IR spectra.

Sensitivity of nanouorescent sensor
Stock solutions of metal salts were synthesized in distilled water. All dilute solutions were prepared from standard stock aqueous solutions of concentration 0.1 M of their respective metal ions. An aqueous solution of metal salt (5 mL) has been mixed with 0.005 g of nanosensor and stirred for 30 min. Then the nanopowder was centrifuged. The photoluminescence spectra of solutions were recorded at l ex ¼ 285 nm.

Analytical application in food and drinking water samples
A food sample including 2.0 g of green tea or 2.0 g of black tea or 5.2 of tomato sauce were put into porcelain crucible and burned with ame. Three powder samples were calcinated at 550 C for 4 h, then 10 mL of concentrated nitric acid was added, followed by adding 5 mL of hydrogen peroxide 30% and the mixture was heated for drying. Finally, 25 mL water were added. Aliquots of these solutions was subjected to the analytical determination. 34 The uorescent nano-sensor was immediately mixed with diluted 5 mL of the digested solution. Fluorescence intensity of the solution was measured at 610 nm and the total copper concentration was determined from the Stern-Volmer plot.
Drinking water samples were taken from Kafr ElSheikh governorate, Egypt. All water samples were ltered using 0.45 mm member lters before testing. 1 mL was taken and diluted to 10 mL distilled water and mixed with the uorescent sensor for copper ion determination.
Results obtained by developed uorescent sensor were compared to data obtained by standard method using Inductive Coupled Plasma Atomic Emission Spectrometry (ICP-AES) (Optima 5300DV, PerkinElmer type).

2.5
Characterization of x mol Eu 3+ :SrSnO 3 and 0.06 mol Eu 3+ :SrSnO 3 @APTS Nanouorescent sensor was characterized by high resolution Transmission Electron Microscope (TEM, JEM-2100, JEOL). The XRD patterns were recorded using Shimadzu X-ray diffractometer with CuKa 1 radiation (k ¼ 1.54056Å). The accelerating voltage of 40 kV and an emission current of 30 mA were used. The FT-IR spectra were recorded using JASCO-FT-IR 6800 spectrometer. The photoluminescence emission spectra were recorded at room temperature with a spectrouorometer shimaduz RF5301PC. The luminescent lifetime of the prepared compounds was recorded using a PerkinElmer LS 55 luminescence spectrometer (USA). Surface charge of nanoparticles were calculated by Brookhaven zeta potential/particle size analyser.  Table 1. It is readily seen that, the host matrix exhibit a peak shi to the higher angles aer addition of Eu 3+ ion and no new peak was obtained, suggesting a small decrease in the unit cell parameters, Table 1. This phenomenon is likely a result of the shrinkage of the unit cell when Sr 2+ ions were exchanged with smaller ionic radius Eu 3+ ion (Sr 2+ ¼ 1.44Å; Eu 3+ ¼ 1.12Å), 36 Fig. 1(C). However, we have found that the crystal size of the nanophosphor decreases as a function of Eu 3+ doping concentrations (pH ¼ 12.5), Table 1.   Fig. 1(B) shows the effect of pH of the synthesis solution on the crystal structure of SrSnO 3 :0.06 mol Eu 3+ system. It is apparent that the observed peak became weaker and broader by decreasing the pH values from 12.5 to 9.0 and by addition of APTS, indicating a decrease on the crystal sizes of the obtained samples by decreasing the pH values and by addition of APTS. It was also observed that the sample prepared at pH ¼ 9.0 showed an amorphous structure. In addition, the observed peak shi to  the lower angles reects a small increase in the unit cell parameters, Table 2. Fig. 2(a and b) shows transmission electron microscopy (TEM) images of the synthesized optimized nanophosphors Eu 3+ :SrSnO 3 @-APTS. The particles exhibited peanut owerlike morphology, and each peanut ower like particle composed of two or more spherical nanoparticles with an average particle size $33 nm, which is smaller than that ($45 nm) observed by C. Lee et al. 37 The high resolution transmission electron microscopy (HR-TEM) showed the lattice fringes with an interplanar d-spacing of 0.28-0.31 nm, Fig. 2(c). The elemental mapping demonstrating the homogeneous distribution of the elements in the investigated nano-phosphor samples, Fig. 3(a-f).

Microstructures and morphologies analysis.
3.1.3 Zeta potential. Fig. 4(A) shows that the initial SrSnO 3 particles exhibited zeta potential value of -33 mV. Aer doping with Eu 3+ ions, the zeta potential value decreased to À17 mV. This observation could be due to the ability of Eu 3+ ions to substitute for Sr 2+ in the host lattice upon lanthanide doping, which will make an unbalanced charge on the matrix surface. Consequently, it increases the surface polarity, positivity and chemical reactivity. However, the 0.06 mol Eu 3+ :SrSnO 3 functionalized with APTS exhibited much lower zeta potential of about À3.0 mV, Fig. 4. This observation is probably, due to an abundance of APTS amine groups on the high positive 0.06 mol Eu 3+ :SrSnO 3 surface. 38 This leads to a high activated surface with low surface potential, which highlight the potential sensing application.

Optical properties
3.2.1 FTIR. Fig. 4(B) shows the FT-IR spectra of samples SrSnO 3 , 0.06 mol Eu 3+ :SrSnO 3 and 0.06 mol Eu 3+ :SrSnO 3 @APTS. SrSnO 3 and 0.06 mol Eu 3+ . The SrSnO 3 show broad peaks at 3400 cm À1 and 1454 cm À1 related to stretching and bending vibration of hydroxyl group -OH, respectively. The observed peaks at 853 and 517 cm À1 are associated to stannate group (SnO 3 2À ) molecular vibrations. 39,40 For 0.06 mol Eu 3+ :SrSnO 3 @APTS, the FTIR spectra revealed deformation mode of the Si-CH 2 peak at 1415 cm À1 and the asymmetric stretching modes of the Si-O-Si bond at 1073 cm À1 . Also, the scissoring absorption mode of the Si-O-Si siloxane groups exhibited a broad band at 570 cm À1 . The observed peak at 1560 cm À1 indicates the presence of NH 2 deformation modes of the amine groups, which are very strongly bound to the silanol groups via hydrogen bonded to form cyclic structures. The observed broad band at 3420 cm À1 is related to the stretching modes of NH 2 and Si-OH groups. However, the weak peak at 2924 cm À1 is assigned to the stretching modes of CH 2 group. 41 These results clearly demonstrating the successful functionalization of in SrSnO 3 with APTS.
3.2.2 UV/Vis diffuse reectance. Generally, the UV-Vis diffuse reectance property depends on both doping concentration of Eu 3+ and pH values during preparation procedures. Fig. 4(C and D) shows the UV-Vis diffuse reectance spectra of Eu 3+ :SrSnO 3 as a function of Eu 3+ doping concentrations (0-0.1 mol) and pH values (12.5-9.0). The spectra of Eu 3+ :SrSnO 3 exhibited an intense band in the UV region, which may be attributed to the transition of electrons located in the valence band of O 2p states into Sn 5s states of the conduction band upon absorption of UV light. 42 This reectance band exhibited red shi as a Eu 3+ concentration increased. In addition, the SrSnO 3 sample doped with 0.06 mol% Eu 3+ also exhibit red shi as the pH value decreased. Moreover, a characteristic f-f transition bands of Eu 3+ appeared in visible region of the spectra (395 nm, 419 nm and 533 nm) especially at low pH values.
The optical band gap (E g ) of the samples can be calculated on the basis of the optical absorption spectra by using the following equation: where hn is the photon energy, A is the absorbance, b is related to the effective masses associated with the valence and conduction bands, and n is either equal to 2 for an indirect allowed transition or 1/2 for the direct allowed transition. The inset of Fig. 4(C and D) shows a good linear dependence of (ahn) 0.5 versus (hn) for all samples, suggesting that the samples have direct forbidden band gaps. The values of the band gap energy of the samples n were estimated using the linear extrapolation method and the estimated results are listed in Table 2.
It is readily seen that, the band gap value of SrSnO 3 decreases with incorporation of Eu 3+ ions and with decreasing the pH value during preparation procedures. However, the observed optical band gap value for optimized sample (0.06 mol Eu 3+ :SrSnO 3 @APTS) is 3.14 eV, which is relatively smaller than the reported value of bulk sample (4.3 eV). 12 This behavior is due to a difference in particle size of the sample by different treatments. These observed UV-Vis diffuse reectance data conrm the obtained results from XRD analysis.
3.2.3 Photoluminescence study. Fig. 5(B) displays the emission spectra (l ex ¼ 285 nm) of Eu 3+ :SrSnO 3 at different doping concentrations of Eu 3+ in the range 0.01-0.1 mol and keeping the pH xed at 12.5. It is apparent that an increase of Eu 3+ concentration to 0.06 mol% leads to an enhancement of the photoluminescence (PL) intensity and then decreases aer further increase in the concentration of Eu 3+ ions, due to the   concentration quenching. This observation suggesting that 0.06 mol Eu 3+ doped SrSnO 3 host gave the higher PL intensity. Therefore, the optimal Eu 3+ dopant concentration is 0.06 mol%. A similar effect have also been performed for excitation spectra (l em ¼ 615 nm) of Eu 3+ :SrSnO 3 at different doping concentrations of Eu 3+ as shown in Fig. 5(D). As can be seen from Fig. 5(A), the PL spectra exhibited four distinct peaks at about 593, 618, 656 and 710 nm. These observations are due to 4f transitions of Eu 3+ from the excited 5 D 0 level to ground 7 F J levels (J ¼ 0, 1, 2, 3 and 4), respectively, which are in close agreement to some reported literature values for Eu 3+ doped SrGd 2 O 4 phosphors. 43 The pH dependence of the PL intensity of the 0.06 mol Eu 3+ :SrSnO 3 are shown in Fig. 5(A). The results demonstrated that the PL intensity increases with decreasing pH value from 9 to 10.5, then begin to decrease upon more increase in the pH value to 12.5. An increase on the doping concentration and amorphous structure at relatively low pH value (9.0) increases the non-radiative deactivation process and consequently leading to decreasing the energy transfer probability. In addition, the sample 0.06 mol Eu 3+ :SrSnO 3 @APTS at pH 10.5 exhibited the relatively higher intense excitation band, due to increasing energy transfer probability, Fig. 5(C). This means that the presence of amino group of APTS on the Eu 3+ :SrSnO 3 surface act as another sensitizer for Eu 3+ in addition to the SrSnO 3 host. However, the high aggregated micro-morphology of Eu 3+ :SrSnO 3 prepared without APTS increases the nonradiative transitions compared to the sample prepared in the presence of APTS (SEM results). This behavior agree with the XRD data of the sample prepared by addition of APTS, which show a decrease on the crystal.
The excitation spectra of Eu 3+ :SrSnO 3 nano-phosphor were also characterized with sharp weak lines located at 364, 383, 395, 465 and 533 nm. They are related to the intra-congurational 4f-4f transitions of Eu 3+ ions doped in the host lattice. These peaks are assigned to 7 F 0 -5 D 4 , 7 F 0 -5 G 2 , 7 F 0 -5 L 6 , 7 F 0 -5 D 3 and 7 F 0 -5 D 2 transitions, respectively. 44 As shown in Fig. 6(B and D), the emission spectra obtained upon excitation of the phosphor at 285 nm are characterized with ve emission bands at 580, 593, 616, 655 and 701 nm, which attributed to 5 D 0 -7 F J (J ¼ 0-4), respectively. However, the emission spectrum is broad and less-resolved, especially in case of 0.06 mol Eu 3+ :SrSnO 3 @APTS at pH 10.5. These observations are attributed to the nanostructure property of this sample, which changes the strength of local electrostatic eld and site symmetry around lanthanide element due to structure disorder and surface defects. 45 This suggestion is supported by increasing of red emission line relative to orange one and the asymmetry ratio values by decreasing the crystal size as a result of different modications, Table 3.
We note that high asymmetry value was obtained in case of 0.06 mol Eu 3+ :SrSnO 3 @APTS at pH 10.5. This is attributed to the altering of the coordination sphere of Eu 3+ ions resulting from the further coordination of amino group of APTS to the Eu 3+ ions on the surface. Fig. 6(A and B) shows the photoluminescence decay curves of Eu 3+ :SrSnO 3 as a function of doping concentrations and pH values upon excitation at 285 nm. Table 3 presents the obtained data of the monoexponential ts of europium ion ( 5 D 0 / 7 F 2 ). It is apparent that the PL lifetime of Eu 3+ ions strongly depends on the doping concentration of Eu 3+ ions. It rst increases with increasing the doping concentration of Eu 3+ ions and reaches its maximum value at around 0.06 moles and then decreases as a result of the concentration quenching effect. Furthermore, the PL lifetime values increase with decreasing pH value and reaches to the maximum value at pH ¼ 10.5 and decrease at pH ¼ 9. This means that SrSnO 3 :Eu 3+ @APTS at pH ¼ 10.5 has a high energy transfer probability in which the amino group act as sensitizer in addition SrSnO 3 host. 46,47 The proposed energy transfer mechanism is shown in Scheme 1(A), where the Eu 3+ can be sensitized by two bath ways. The rst way is via host indirect excitation (IDE) (285 nm) and then the host transfers its energy to the Eu 3+ emitting state to emit its characteristic emission color. The second one is via direct f-f excitation (DE) at 395 nm. 48 The Commission international De I-Eclairage (CIE) coordinates of 0.06 mol Eu 3+ :SrSnO 3 @APTS was evaluated to be x ¼ 0.633 and y ¼ 0.367, which is found in pure red color region, Fig. 6(C). 49 The local structure, environmental surrounding the Eu 3+ ion inside host nanomaterial was investigated on the basies of Judd-Ofelt (J-O) theory. There parameters are then evaluated from emission spectra and lifetime and using intensity parameters U k (k ¼ 2, 4, 6) of Eu 3+ ion inside host nanomaterial according to the modied method based on Kodaira et al. approach. 50 Because the 5 D 0 / 7 F 6 was not identied in the emission spectrum (forbidden transition), the U 6 intensity parameter could not be determined. However, the intensity parameters U 2 and U 4 were calculated using the following relation: UtI \hjJkU l kj 00 J 00 i 2 : (2) where e is the charge of an electron, h is Planck's constant (6.63 Â 10 À27 erg s), n is the average transition energy (in cm À1 ), n is the refractive index of the medium and jh 5 D 0 kU k k 7 F J ij 2 is the squared reduced matrix. Most of the matrix elements for transitions starting from the 5 D 0 level are zero. 51 The 5 D 0 -7 F 2 transition (U (2) ¼ 0.0028), the 5 D 0 -7 F 4 transition (U (4) ¼ 0.002) and the 5 D 0 / 7 F 6 transition (U (6) ¼ 0.0002). The value of U 6 is very small (negligible). A 0Àl can be calculated by using the following relation: The sum of all A 0Àl provide the radiative decay rate (A rad ), whereas inverse lifetime gives sum nonradiative and radiative decay rate (A rad /A rad + A nrad ). The parameters U 2 and U 4 are complex, and can be determined using a suitable calculation program such as JEOS and LUMPAC. 52,53 The U 2 values are Scheme 1 Proposed guest-host energy transfer mechanism in the in Eu 3+ :SrSnO 3 nanophosphor (A) and quenching energy transfer process in the presence of Cu 2+ ion (B) as well as after addition of EDTA (C). related to the changing structure around Eu 3+ . For example, the very small value of U 2 suggesting a greatly symmetric environment around the rare earth ion. However, U 4 and U 6 are relatively more sensitive to the variation in the macroscopic properties, such as viscosity and rigidity of the matrix.
The theoretically calculated U 2 values are listed in Table 3. It should be noted that the theoretical values are in close agreement with the experimentally obtained values from PL spectra. The highest U 2 values are obtained for 0.06 mol Eu 3+ :SrSnO 3 prepared at pH ¼ 10.5 as well as in the presence of APTS. This is due to the high distortion environment around surface Eu 3+ cations polyhedra in the presence of SrSnO 3 and APTS units. 54 A similar effect has been observed for U 4 parameter, which corresponds to the electron density on of the surrounding sensitizers, and its value increase with increasing the doping concentration and the highest value was observed for 0.06 mol Eu 3+ :SrSnO 3 @APTS at pH ¼ 10.5, in the presence of APTS sensitizer, Table 3. The quantum efficiency values of nanophosphor increase with decreasing pH value and in the presence of APTS due to improved luminescence properties. These results are consistent with the obtained PL lifetime values.

Analytical performance and validation
Eu 3+ :SrSnO 3 coated with APTS nanophosphor (Eu 3+ :SrSnO 3 @-APTS) was tested as a uorescent sensor for copper ion in aqueous solution. The coupling agent APTSA have been used because it change the surface charge of NPs, which may improve their stability in aqueous media. Therefore, Eu 3+ :SrSnO 3 @APTSA exhibited a characteristic intense pure red emission color with long lifetime value in aqueous medium (pH ¼ 7) under UV illumination. We have studied the selectivity and sensitivity of Eu 3+ :SrSnO 3 @APTSA for different metal ions in aqueous medium (pH ¼ 7). Fig. 6(D) shows the effect of different studied metal ions (10 À11 M) on the uorescence intensity of the Eu 3+ :SrSnO 3 @APTS nanophosphor. The red emission of Eu 3+ :SrSnO 3 @APTSA nanophosphor was highly quenched with 85% in the presence of copper ion compared to the other studied metal ions. This phenomenon may be due to the negative charge of the silanes. 55 As a result, the amino group interacted with copper ion. Fig. 7(A) displays the change of emission spectra of Eu 3+ :SrSnO 3 @APTS nano-phosphor as a function of copper ion concentration in aqueous medium (pH ¼ 7). As the Cu 2+ ion concentration increases, the nanophosphor intensity decreases. The uorescence quenching of nanophosphor was analyzed by the Stern-Volmer quenching plot using the following relation: 56,57 where, F and F 0 are the uorescence intensities of the nanosensor in the absence and presence of quencher of concentration [Q], and K SV is the Stern-Volmer quenching rate constant. Fig. 7 R 2 ¼ 0.996. The slope of the straight line equal to K SV ¼ 8.216 Â 10 10 mol À1 L; standard deviation SD ¼ 0.09523. The detection limit (LOD ¼ from 3SD/K SV ) and quantication detection limit (LOQ ¼ 10SD/K SV ) values of Cu 2+ using the prepared nanomaterials was calculated to be equal to 3.4 Â 10 À12 mol L À1 and 1.16 Â 10 À12 mol L À1 , respectively, which is lower than that of 1.3 mg L À1 reported for copper(II) using atomic absorption spectrometry. 58 The half quenching concentration (C 1/2 ) was obtained from 1/K SV ¼ 1.217 Â 10 À11 mol L À1 . The critical energy transfer distance (R 0 ) between donor (Eu 3+ ) and acceptor (Cu 2+ ) was also calculated from the following relationship; R 0 ¼ 7.35/(C 1/2 ) 1/3 ¼ 43Å. Moreover, the luminescence decays of Eu 3+ :SrSnO 3 @APTS nanomaterials in the absence and presence of Cu 2+ were measured aer excitation at a wavelength of 285 nm for better understanding of quenching type. Lifetime values of Eu 3+ :SrSnO 3 @APTS before (s ¼ 1.25 ms) and aer addition of Cu 2+ (0.80 ms) was detected, Fig. 7(C). The luminescence lifetime and critical distance results demonstrated that the uorescent sensor was dynamically quenched via coulombic interaction mechanism between the Eu 3+ ( 5 L 6 ) and Cu 2+ in which the Cu 2+ excited state is well matched with an energy level 5 L 6 of Eu 3+ , Scheme 1(B). As a consequence, Cu 2+ ions absorb the energy from the excited state of Eu 3+ . Furthermore, the complexation of -NH 2 to Cu 2+ ions decrease the distance between Cu 2+ and Eu 3+ , which would signicantly improve the efficiency of energy transfer between the ions.
As revealed before, the interaction of Cu 2+ ions with NH 2 of APTS coated on the surface of as-prepared NPs proceeds via energy transfer between NPs and Cu 2+ ion. Support for this comes from adding EDTA as a complexing agent, and has stronger coordination ability to Cu 2+ ions. As shown in Fig. 7(D), almost 85% of the initial PL intensity was recovered by adding different concentrations of EDTA. This result suggesting that the detection of Cu 2+ ion by the as-prepared Eu 3+ :SrSnO 3 @APTS is reversible Scheme 1(C).

Interference with coexisting foreign substances
The anti-interference capacity of the new developed uorescent nanosensor was evaluated to detect the performance of this new sensor. Under the optimum conditions, the inuences of interference ions (Cl À , SO 4 2À , HCO 3 À , NO 3 À , C 6 H 5 O 7 3À , PO 4 2À , Cd 2+ , Ca 2+ , Mg 2+ , Fe 3+ , Co 2+ , Ni 2+ ) on the uorescence features were studied. As reported previously, any change in uorescent intensity $ (AE5%) the analytical signal value of Eu(III) was considered as an interference. 34 At xing concentration of Cu(II) at 10 ng L À1 , the change in the analytical signal before and aer addition of the interfering ion was determined and the calculated values for KCl, SO 4 2À , HCO 3À , NO 3À , citrate, PO 4 2À did not interfere even at 1000 fold of Cu(II); hence they were applied as a masking agent for Cd 2+ at 10-fold levels, Ca 2+ , Mg 2+ , Fe 3+ at 20fold levels, Co 2+ at 30 and Ni 2+ at 35-fold, respectively. It is worth mentioning that the uorescent nano-sensor exhibited high anti-interference of coexisting ions. The concentrations of these interfering ions were at least ten times higher than that of Cu 2+ ion in the sample solution.
3.5 Application of SrSnO 3 :Eu 3+ @APTS in the determination of Cu(II) in food samples The optimized uorescent nano-sensor (Eu 3+ :SrSnO 3 @APTS) was applied to copper sensing from food samples (tomato sauce, green and black tea) and wastewater samples were obtained from Kafr El Sheikh governorate, Egypt. These samples were measured in the absence and presence of Cu(II), Fig. 7(A) and then analysed. The uorescence spectra of nano-sensor were measured (l ex ¼ 285 nm) aer addition of all prepared food and drink water samples. The copper concentration was determined from the Stern-Volmer plot, Fig. 7(B), where three measurements were performed for each concentration and the results and the recoveries for samples are listed in Table 4. A good agreement was obtained between the added and found values of the analyte with high recovery values (average proposed/standard Â 100). Moreover, the analyte samples (without any addition of external known Cu 2+ concentration) were also analysed by ICP-OES for comparison and the results are summarised in Table 4. There results are in reasonable agreement and the t-test exhibited 95% condence limit. The recovery results for the proposed method conrm the sensitivity of the nano-sensor for copper detection.
To further check the sensitivity of the proposed approach, we compared the observed results with the other values obtained other published methods as shown in Table 5. It is obvious that, our results are of lowest detection among the reported values.