Fluorescence enhancement of europium nitrobenzoates by Ag@SiO2 nanoparticles in solution

Journal of Luminescence 186 (2017) 255–261

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Fluorescence enhancement of europium nitrobenzoates by Ag@SiO2 nanoparticles in solution Ling-juan Kong a, Yan-fang Zhao a,b, Kai Kong a, Yong-liang Zhao a, Hai-bin Chu a,n a b

College of Chemistry and Chemical Engineering, Inner Mongolia University, Huhhot 010021, China Inner Mongolia Vocational College of Chemical Engineering, Huhhot 010070, China

art ic l e i nf o

a b s t r a c t

Article history: Received 4 November 2016 Received in revised form 20 February 2017 Accepted 21 February 2017 Available online 24 February 2017

Europium complexes with o-nitrobenzoic acid (o-NBA), m-nitrobenzoic acid (m-NBA) and p-nitrobenzoic acid (p-NBA) were synthesized. Elemental analysis, IR spectroscopy and UV–vis absorption spectroscopy show that the compositions of the complexes are Eu(o-NBA)3
4H2O, Eu(m-NBA)3
4H2O and Eu(pNBA)3
4H2O, respectively, and the europium ions coordinate with the carboxyl oxygen atoms of nitrobenzoates. Ag@SiO2 nanoparticles with different core size (32 nm, 49 nm and 72 nm) and varied silica shell thickness (5 nm, 15 nm, 19 nm, 25 nm and 53 nm) were prepared. The influence of these nanoparticles with different addition amounts (50 μL, 100 μL, 150 μL, 200 μL and 250 μL) on the luminescence properties of the europium complexes in solution were investigated, respectively. The results show that the luminescence intensities of the europium complexes are enhanced remarkably after the introduction of Ag@SiO2 nanoparticles. The fluorescence enhancement factors depend on the addition amount, shell thickness and core size of the Ag@SiO2 nanoparticles. Moreover, the fluorescence enhancement factors vary for the europium complexes with different ligands. The enhancement factors for the complex Eu(o-NBA)3
4H2O can reach as high as 3.1. The investigations indicate that the enhancement effect on the europium complexes mainly results from the surface plasmon resonance of the Ag@SiO2 nanoparticles. Besides, the introduction of Ag@SiO2 nanoparticles is also able to elongate the fluorescence lifetimes and increase the quantum efficiencies of the europium complexes. & 2017 Elsevier B.V. All rights reserved.

Keywords: Europium nitrobenzoate Ag@SiO2 core-shell nanoparticles Metal-enhanced fluorescence Surface plasmon resonance

1. Introduction Lanthanide complexes have attracted considerable interests in the fields of analytical sensors [1–3], biomedical imaging, fluoroimmuno-assays, liquid crystal displays [4], biological labels and luminescent materials [5–9]. These interests mainly attribute to the unique narrow emission, large Stokes shifts and long luminescent lifetimes of the complexes [10,11]. As f–f transitions are electric-dipole forbidden, hence the suitable organic ligands are chosen to sensitize the luminescence of lanthanide ions via transferring energy to the lanthanide ions effectively. The ligands include organic carboxylic acids which give priority to aromatic carboxylic acid, such as phthalic acid, benzoic acid and salicylic acid, and nitrogen heterocyclic ligands, such as 2,4,6-tris-(2-pyridyl)-s-triazine, 1,10-phenanthroline and 2,2, -bipyridine [12–16]. In addition, bridging ligands have also been studied [17]. Metal-enhanced fluorescence (MEF) is another way to increase fluorescence emissions of lanthanide complexes. MEF is based on the n

Corresponding author. E-mail address: [email protected] (H.-b. Chu).

http://dx.doi.org/10.1016/j.jlumin.2017.02.044 0022-2313/& 2017 Elsevier B.V. All rights reserved.

utilization of plasmonic nano structures of the metals, including gold and silver [18,19]. The interaction between surface plasmon resonance (SPR) and the fluorescent molecule is important for MEF [20,21]. The MEF effect is suggested to relate with the modification in the fluorophore’s intrinsic radiative decay rate [22], as well as the radiation of coupled quanta from the metal nanoparticles [23]. Although the actual mechanism of MEF is still debated at present [24], the applications of MEF are constantly growing [25–27]. Core-shell Ag@SiO2 nanoparticles have been demonstrated for MEF of lanthanide complexes [28,29]. The Ag@SiO2 nanoparticle plasmon resonance scattering bands may overlap with the excitation spectra of the lanthanide complexes, contributing to the fluorescence enhancement of the complexes. Recently, we have reported the fluorescence enhancement effect on lanthanide complexes by Ag@SiO2 nanoparticles with different silica shell thickness [30–33]. However, the effects of core size and the addition amount of Ag@SiO2 nanoparticles have not been investigated. Meanwhile, because the energy difference between triplet state of the nitrobenzoates and Eu3þ excited state is approximately 8000 cm  1, which is too large for effectively sensitizing luminescence, the fluorescent intensities of the europium nitrobenzoates are relatively weak [4]. Henceforth, we chose the europium complexes with o-, mand p-nitrobenzoates as model compounds, and investigated the

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fluorescence enhancement effect of the Ag@SiO2 nanoparticles with different core size and various silica shell thickness on the europium nitro-benzoates. In particular, the influence of the addition amount of Ag@SiO2 nanoparticles on the fluorescence intensities, fluorescence lifetimes and quantum efficiencies of the europium nitrobenzoates has also been investigated.

2. Experimental section 2.1. Chemical reagents and instruments The purity of europium oxide (Eu2O3) was 99.99%. o-Nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid and other reagents were all of analytical grade. The elemental analysis (C, H and N) of the complexes was performed on a Vario EL Cube elemental analyzer, and the contents of europium ions were analyzed by the EDTA complexometric titration method with xylenol orange as indicator. The UV-visible spectra were determined by the TU-1901 spectrophotometer. The infrared spectra were determined by a Nicolet Nexus 670 FT-IR spectrometer. Transmission electron microscopic (TEM) images of the Ag@SiO2 nanoparticles were obtained on a FEI Tecnai F20 transmission electron microscope using an accelerating voltage of 200 kV. The FLS-920 fluorescence spectra-photometer was used to obtain fluorescence spectra and fluorescence lifetimes. Both excitation and emission slit widths were 0.5 nm. 2.2. Synthesis of europium complexes 1.76 g Eu2O3 was added into a solution of 10 ml ethanol with a certain amount of hydrochloric acid with stirring. The solution was heated until crystallized film appeared above the solution, after which the solution was cooled to room temperature to get the white powder. The powder was dissolved in anhydrous ethanol to obtain 0.1 mol L-1 EuCl3 ethanol solution. p-Nitrobenzoic acid (0.9 mmol) was dissolved in the anhydrous ethanol with vigorously stirring. 0.3 mmol EuCl3 was added into the above solution. The pH value of the solution was adjusted to be between 6.2 and 6.7 with NH3
H2O. White precipitate appeared, and the mixture was continued to heat at 50 °C for 3 h under vigorous stirring. Then the solution was left at room temperature overnight. The precipitate was filtrated, washed with ethanol for several times. After dried to a constant weight, the powder of europium complex Eu(p-NBA)3
4H2O was obtained. Anal. Calcd (%) for Eu(p-NBA)3
4H2O: C 34.95; H 2.79; N 5.82; Eu 21.04; found (%):C 34.83; H 3.01; N 5.82; Eu 20.98. The synthetic procedures of Eu(m-NBA)3
4H2O and Eu(oNBA)3
4H2O complexes are similar to that of Eu(p-NBA)3
4H2O except that p-NBA was replaced by m-NBA and o-NBA, respectively. Anal. Calcd (%) for Eu(m-NBA)3
4H2O: C 34.95; H 2.79; N 5.82; Eu 21.04; found (%):C 34.98; H 2.94; N 5.85; Eu 21.09. Anal. Calcd (%) for Eu(o-NBA)3
4H2O: C 34.95; H 2.79; N 5.82; Eu 21.04; found (%):C 34.81; H 3.04; N 5.81; Eu 20.96. 2.3. Measurement of the fluorescence spectra of the europium complexes before and after the adding of Ag@SiO2 nanoparticles Core-shell Ag@SiO2 nanoparticles with different core size

(32 nm, 49 nm and 72 nm) and varied shell thickness (5 nm, 15 nm, 19 nm, 25 nm and 53 nm) were prepared by the methods described in our previous report [30,31]. As shown in Fig. 1, the silver nanoparticles were prepared by the reduction of AgNO3 with sodium citrate at 100 °C. Then they were coated with SiO2 shell with controlled thickness by modified-Stöber method. The concentrations of silver in the solution of Ag@SiO2 nanoparticles were about 8.0  10-5 mol L-1. The detailed preparation procedures can be found in Supplementary Material. 2.00 mL of the europium complex in ethanol (1.0  10-4 mol L-1) was transferred into a quartz cell and the fluorescence spectra were measured. Then a certain volume of Ag@SiO2 solution was added. The solution was allowed to stand for 3 min to equilibrium, after which the fluorescence spectra of the solution were detected.

3. Results and discussions 3.1. Characterization of the europium complexes 3.1.1. UV absorption spectra The UV absorption spectra of the ligands and complexes were measured in the range of 195–375 nm. The ethanol solution was used as the solvent and reference. As shown in Fig. 2, p-nitrobenzoic acid exhibited two ultraviolet absorption peaks at 202 nm and 259 nm. These two peaks red-shifted to 212 nm and 269 nm, respectively, in the complex Eu(p-NBA)3
4H2O. For o-nitrobenzoic acid and its complex, the absorption peaks red-shifted from 204 nm and 261 nm to 216 nm and 273 nm, respectively. For m-nitrobenzoic acid and its corresponding complex, the UV absorption peaks of the ligand redshifted from 201 nm and 258 nm to 214 nm and 271 nm, respectively. The changes suggested that the coordination bonds formed between europium ions and the ligands [14,30,32]. 3.1.2. Infrared spectra As shown in Fig. 3, IR spectrum of p-nitrobenzoic acid show the carbonyl stretching vibration band around 1692.9 cm-1. This band disappeared in the complex Eu(p-NBA)3
4H2O, and new bands attributed to carboxyl asymmetric stretching vibration and carboxyl symmetric stretching vibration appeared around 1565.9 cm-1 and 1429.7 cm-1, respectively. For m-nitrobenzoic acid and o-nitrobenzoic acid, their carbonyl stretching vibration bands appeared around 1692.6 and 1686.4 cm-1. In their complexes, the carbonyl stretching vibration peaks also disappeared, and the new absorption peaks of carboxyl asymmetric stretching vibration and carboxyl symmetric stretching vibration at 1530.7 and 1410.8 cm-1 for Eu(m-NBA)3
4H2O, and 1531.2 and 1414.3 cm-1 for Eu(o-NBA)3
4H2O appeared. These changes indicated that the europium ions had coordinated to the ligands [30,32]. 3.2. Characterization of the Ag@SiO2 nanoparticles Fig. 1S and Fig. 4 show the typical TEM images of silver nanoparticles and Ag@SiO2 nanoparticles. By measuring 100 particles for each sample, the average size of the nanoparticles were obtained. The mean diameters of the three kinds of silver nanoparticles are 32.073.5 nm, 48.574.2 nm and 72.377.2 nm, which are denoted as 32 nm, 49 nm and 72 nm, respectively. And the silica shell thickness of the five kinds of Ag@SiO2 nanoparticles with core size of

Fig. 1. Scheme for the preparation of Ag@SiO2 nanoparticles and their composites with europium complexes.

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3.3. Fluorescent properties of the europium nitro-benzoates before and after the addition of Ag@SiO2 nanoparticles

Fig. 2. UV absorption spectra of the ligands and corresponding europium complexes.

Fig. 3. IR spectra of the ligands and the corresponding europium complexes.

32 nm are 4.770.6 nm, 14.870.8 nm, 18.571.1 nm, 24.871.1 nm and 52.5 7 3.1 nm, which are denoted as 5 nm, 15 nm, 19 nm, 25 nm and 53 nm, respectively.

The concentration of europium complexes in ethanol was 1.0  10-4 mol L-1. The concentrations of silver atoms in all kinds of Ag@SiO2 nanoparticles were about 8.0  10-5 mol L-1. The effect of addition volume, silica shell thickness and particles size of silver core of Ag@SiO2 nanoparticles on the fluorescent intensities, fluorescence lifetimes and quantum efficiencies of the europium complexes was investigated. As shown in Fig. 5A, the luminescent emission spectrum of the Eu(p-NBA)3
4H2O in ethanol solution shows three typical bands at about 590, 615 and 697 nm, which correspond to the transitions of the excited state 5D0 to the ground states 7FJ (J ¼ 1, 2, 4) of the Eu (III) ions, respectively. Among the three bands, the emission band around 615 nm (5D0-7F2) is the strongest. Therefore, we focus on the intensity enhancement of the peak around 615 nm. Different volumes (50 μL, 100 μL, 150 μL, 200 μL and 250 μL) of Ag@SiO2 solution with the core diameter about 32 nm and shell thickness about 19 nm were added to 2.0 mL solution of the europium complex Eu(p-NBA)3
4H2O, respectively. The fluorescence spectra of the solution were detected after standing for 3 min at room temperature. As shown in Fig. 5A, the emission intensities of the complex are enhanced after the addition of Ag@SiO2 solution. When the adding volume of Ag@SiO2 increases from 50 μL to 200 μL, the intensities of Eu(p-NBA)3
4H2O increase with the increasement of the adding volumes of Ag@SiO2 solution (Fig. 5B). The maximum enhancement factor of 1.7 for the emission intensity at 615 nm is reached with an adding volume of 200 μL. However, the enhancement factor decreases to 1.5 when 250 μL of Ag@SiO2 solution is added. The fluorescence intensities of complexes Eu(o-NBA)3
4H2O and Eu(m-NBA)3
4H2O (Figs. S2 and 5B) exhibit the similar trend as that of Eu(p-NBA)3
4H2O when different amount of Ag@SiO2 solution is added. The maximum enhancement factors are 3.1 and 1.6 for Eu(o-NBA)3
4H2O and Eu(mNBA)3
4H2O, respectively, when 200 μL of Ag@SiO2 solution is added. One of the most important origins of the fluorescence enhancement of europium complexes is the surface plasmon resonance of silver nanoparticles. Therefore, more Ag@SiO2

Fig. 4. TEM images of Ag@SiO2 nanoparticles nanoparticles with various shell thickness. (A) 5 nm, (B) 15 nm, (C) 19 nm, (D) 25 nm, (E) 53 nm. The size of the silver core is about 32 nm.

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Fig. 5. (A) Luminescent emission spectra of Eu(p-NBA)3
4H2O before and after the addition of Ag@SiO2 nanoparticle solution with different volumes . (B) Fluorescence enhancement factors for the complexes (a) Eu(o-NBA)3
4H2O, (b) Eu(m-NBA)3
4H2O and (c) Eu(p-NBA)3
4H2O by adding Ag@SiO2 nanoparticle solution of different volumes. The size of silver core is about 32 nm and the silica shell thickness is about 19 nm.

nanoparticles in the solution result in stronger emission intensities of the complexes. At the adding volume around 200 μL, the europium complexes around Ag@SiO2 nanoparticles may get saturation. Thus, when more Ag@SiO2 nanoparticles are added, the emission intensities of the complexes could not be enhanced any more. Besides, the extra Ag@SiO2 nanoparticles may scatter the fluorescence emissions from the europium complexes, resulting in the decrease of the emission intensities. The silica shell thickness of Ag@SiO2 nanoparticle is the determinant parameter for the distance between the europium complex molecules and plasmonic silver nanoparticles. Thus, we prepared five kinds of Ag@SiO2 nanoparticles with the identical silver core size of about 32 nm but with distinct silica shell thickness at about 5 nm, 15 nm, 19 nm, 25 nm and 53 nm, respectively (Fig. 4). The added volumes of Ag@SiO2 nanoparticle solutions are all 200 μL. As shown in Fig. 6A, the europium complex solutions added with Ag@SiO2 nanoparticles of various shell thickness all exhibit stronger emission intensities than the europium complex solution itself. The fluorescent intensities of Eu(oNBA)3
4H2O increase with the increasement of the silica shell thickness in the range of 5 nm to 25 nm (Fig. 6B). However, they become weak when Ag@SiO2 nanoparticle with a coating of 53 nm is added. Among the five kinds of Ag@SiO2 nanoparticles, the kind with the shell thickness of 25 nm possesses the largest enhancement effect of fluorescent intensities for Eu(o-NBA)3
4H2O. The orders of fluorescent enhancement for the other two complexes Eu (m-NBA)3
4H2O and Eu(p-NBA)3
4H2O are similar (Figs. S3 and 6B). And the addition of Ag@SiO2 nanoparticles with a shell

thickness of 25 nm results in the largest enhancement effect in all the three complexes. The largest enhancement factors can reach 3.1, 1.6 and 1.5 times (Fig. 6, Fig.S3) at the emission peak of 615 nm for Eu(o-NBA)3
4H2O, Eu(m-NBA)3
4H2O and Eu(p-NBA)3
4H2O, respectively. These results reflect that the fluorescence enhancement by the surface plasmon resonance of silver nanoparticles requires a reasonable distance. If the distance is too close between metal surface and europium complexes, the fluorescence emission of the europium complexes may be quenched by the metal nanopartciles, which will result in poor fluorescence enhancement. However, because of the fast decay of surface plasmon resonance absorbance of light [22,34], too far distance means the weak effect of surface plasmon resonance on the europium complexes. Therefore, the fluorescence enhancement of the europium complex becomes weaker in far distance such as 53 nm. The shell thickness of about 25 nm may be the optimal distance between europium complex molecules and silver nanoparticles. Consequently, the largest fluorescence enhancement for the three kinds of europium complexes is reached when the Ag@SiO2 nanoparticles with a silica thickness of 25 nm are added, which is similar with previous report [30–33]. We also prepared three kinds of Ag@SiO2 nanoparticles with different core diameters (32 nm, 49 nm and 72 nm). Their shell thicknesses are all about 19 nm (Fig. S1), and the addition volumes of Ag@SiO2 nanoparticles are 200 μL. The fluorescence measurement showed that the fluorescent intensities of the complex Eu(pNBA)3
4H2O were enhanced to be a different extent by the three kinds of nanoparticles (Fig.7). The order of fluorescent intensities is 32 nm 4

Fig. 6. (A) Luminescent emission spectra of Eu(o-NBA)3
4H2O before and after the addition of Ag@SiO2 nanoparticles with varied thickness of silica shell. (B) Fluorescence enhancement factors of Ag@SiO2 nanoparticles with different shell thickness on the complexes (a) Eu(o-NBA)3
4H2O, (b) Eu(m-NBA)3
4H2O and (c) Eu(p-NBA)3
4H2O. The sizes of silver core are about 32 nm and the addition volumes of the Ag@SiO2 nanoparticle solutions are 200 μL.

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259

Fig. 7. (A) Luminescent emission spectra of Eu(p-NBA)3
4H2O before and after the addition of Ag@SiO2 nanoparticles with different size of silver core. (B) Fluorescence enhancement factors of Ag@SiO2 nanoparticles of varied core size on the complexes (a) Eu(o-NBA)3
4H2O, (b) Eu(m-NBA)3
4H2O and (c) Eu(p-NBA)3
4H2O. The shell thicknesses of the nanoparticles are about 19 nm and the volumes of the Ag@SiO2 nanoparticle solutions are all 200 μL.

49 nm 4 72 nm. The enhancement factor at the emission peak of 617 nm is 1.6 when Ag@SiO2 nanoparticles with silver core diameter about 32 nm are added. The silver core diameters exhibit the similar effect on the fluorescent intensities of Eu(o-NBA)3
4H2O and Eu(mNBA)3
4H2O (Fig. S4). The maximum enhancement factors are also reached (2.9 and 1.4 for the two complexes) by the Ag@SiO2 nanoparticles with silver core diameter about 32 nm. Because the concentrations of the silver atoms are almost identical in the three kinds of Ag@SiO2 nanoparticles, the smaller diameter results in more particles. Therefore, more europium complex molecules situate around the Ag@SiO2 nanoparticles with core diameter about 32 nm than those around nanoparticles with core diameters of 49 nm and 72 nm. Consequently, the maximum fluorescence enhancement factor is reached by the Ag@SiO2 nanoparticles with core diameter about 32 nm. 3.4. The effect of Ag@SiO2 nanoparticles on the fluorescence lifetimes and quantum efficiencies of the europium complexes The FLS-920 fluorescence spectrometer was used to determine fluorescence decay curves of the europium complexes. As shown in Figs. S5 and S6, the decay profile is well expressed by the equation of ln(I(t))/I0 ¼  k/t ¼  t/τ, where I0 is the initial luminescence intensity and τ is the luminescence lifetime. It shows that the luminescence lifetime τ equals to the negative reciprocal of slope and the Eu3 þ ions occupy the same average coordination environment [14,35]. The calculated τ and the goodness of fit parameters χ2 are shown in Table 1. It can be seen that the Ag@SiO2 nanoparticles can elongate the fluorescence lifetimes of the both complexes Eu(p-NBA)3
4H2O and Eu(o-NBA)3
4H2O. When the addition amount of Ag@SiO2 solution in 2 mL solution of the europium complex increases from 0 to 200 μL, the fluorescence lifetime of Eu(p-NBA)3
4H2O increases from 684.1 μs to 864.3 μs with a largest interval of 180.2 μs, then decreased to 820.6 μs when 250 μL of Ag@SiO2 solution is added. The fluorescence lifetime of Eu(o-NBA)3
4H2O increases slightly from 448.7 μs to 476.3 μs (interval of 27.6 μs) when 200 μL of Ag@SiO2 solution is added, and decreases to 467.1 μs when 250 μL of Ag@SiO2 solution is added. Assuming that only radiative and nonradiative transitions are involved in the depopulation of the first excited state of Eu3 þ , the quantum efficiency (η) can be achieved by the following formulas [36–38]:

η=

Ar A r + A nr

(1)

Table 1 The effect of adding volume (μL) of Ag@SiO2 nanoparticle solution on the fluorescence lifetimes of Eu(p-NBA)3
4H2O and Eu(o-NBA)3
4H2O. Complexes Eu(p-NBA)3
4H2O Eu(p-NBA)3
4H2O Eu(p-NBA)3
4H2O Eu(p-NBA)3
4H2O Eu(p-NBA)3
4H2O Eu(p-NBA)3
4H2O Eu(o-NBA)3
4H2O Eu(o-NBA)3
4H2O Eu(o-NBA)3
4H2O Eu(o-NBA)3
4H2O Eu(o-NBA)3
4H2O Eu(o-NBA)3
4H2O

Ar =

þ þ þ þ þ

50 100 150 200 250

þ þ þ þ þ

50 100 150 200 250

fluorescence lifetime (μs)

fit parameters χ2

684.1 711.6 732.6 820.6 864.3 820.6 448.7 452.3 461.7 474.7 476.3 467.1

0.99801 0.99800 0.99898 0.99753 0.99792 0.99752 0.99916 0.99808 0.99859 0.99917 0.99790 0.99826

∑ A 0J = A 00 + A 01 + A 02 + A 03 + A 04

A 0J = A 01(I0J /I01)(ν01/ν0J )

(2) (3)

Ar and Anr refer to the radiative and nonradiative transition rates, respectively. A0J is the spontaneous emission coefficient of 5D0-7FJ( J ¼ 0-4) transitions for Eu3 þ , and A01 (50 s-1) is Einstein coefficient of spontaneous radiation 5D0-7F1 in vacuum [14,39]. I01 and I0J are the fluorescence emission intensities of 5D0-7F1 and 5D0-7FJ (J ¼ 0-4), respectively. Eventually, we can calculate the η value by combining the above three equations and the following equation:

A tot = 1/τ = A r + A nr

(4)

The data of fluorescence quantum efficiency of the complexes Eu(p-NBA)3
4H2O and Eu(o-NBA)3
4H2O with the addition of different volumes of Ag@SiO2 solution are given in Tables 2 and 3. The results show that the quantum efficiencies of europium complexes are higher when Ag@SiO2 nanoparticles are added. On the basis of the above discussion, we can see that the quantum efficiency mainly depends on two aspects. One is lifetime and the other is I02/I01. As shown in Tables 1–3, Ag@SiO2 nanoparticles not only can elongate the luminescence lifetimes of complexes, but are also able to increase the radiative transition rates (Ar) and reduce the nonradiative transition rates (Anr) of the europium complexes. For the complex Eu(p-NBA)3
4H2O, when the volumes of Ag@SiO2 nanoparticles increase from 0 to 200 μL, the radiative transition rate increases from 329.5 s-1 to 390.1 s-1. Meanwhile, the nonradiative transition rate decreases from 1132.3 s-1 to 791.6 s-1. As a result, the quantum efficiency of the

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Table 2 Quantum efficiencies of Eu(p-NBA)3
4H2O before and after the addition of different volumes (μL) of Ag@SiO2 nanoparticles.

ν00 (cm-1) ν01 (cm-1) ν02 (cm-1) ν03 (cm-1) I01 (a.u.) I02 (a.u.) I02/I01 τ (μs) 1/τ Ar (s-1) Anr (s-1) η%

Eu(p-NBA)3
4H2O

Eu(p-NBA)3
4H2O þ 50

Eu(p-NBA)3
4H2O þ 100

Eu(p-NBA)3
4H2O þ 150

Eu(p-NBA)3
4H2O þ 200

Eu(p-NBA)3
4H2O þ 250

17,271 16,920 16,234 14,368 293 1390 4.74 684.0 1.46 329.5 1132.3 22.5

17,271 16,920 16,234 14,368 334 1540 4.61 711.6 1.41 319.3 1086.0 22.7

17,271 16,920 16,234 14,368 314 1750 5.57 732.6 1.37 375.5 989.6 27.5

17,271 16,920 16,234 14,368 330 2020 6.12 820.6 1.22 403.4 815.2 33.1

17,271 16,920 16,234 14,368 403 2410 5.59 864.3 1.18 390.1 791.6 33.0

17,271 16,920 16,234 14,368 387 2200 5.68 820.6 1.22 375.6 843.0 30.8

Table 3 Quantum efficiencies of Eu(o-NBA)3
4H2O before and after the addition of different volumes (μL) of Ag@SiO2 nanoparticles.

-1

ν00 (cm ) ν01 (cm-1) ν02 (cm-1) ν03 (cm-1) I01 (a.u.) I02 (a.u.) I02/I01 τ (μs) 1/τ Ar (s-1) Anr (s-1) η%

Eu(o-NBA)3
4H2O

Eu(o-NBA)3
4H2O þ 50

Eu(o-NBA)3
4H2O þ 100

Eu(o-NBA)3
4H2O þ 150

Eu(o-NBA)3
4H2O þ 200

Eu(o-NBA)3
4H2O þ 250

17,271 16,920 16,367 14,368 210 627 2.99 448.7 2.229 262.6 1966 11.8

17,271 16,920 16,367 14,368 210 688 3.28 452.3 2.211 265.6 1945.3 12.0

17,271 16,920 16,367 14,368 239 933 3.18 461.7 2.166 282.7 1883 13.1

17,271 16,920 16,367 14,368 296 1140 3.85 474.7 2.106 288.8 1817.7 13.7

17,271 16,920 16,367 14,368 544 1920 3.53 476.3 2.100 287.9 1812 13.7

17,271 16,920 16,367 14,368 404 1380 3.42 467.1 2.141 289.8 1851 13.5

europium complexes increases obviously, from 22.5% to 33.0%. The larger change in the nonradiative transition rate (340.7 s-1) than that of the radiative transition rate (60.6 s-1) indicates that the nonradiative transition rate plays a more important role in the enhancement of the quantum efficiency. However, the quantum efficiency decreases to 30.8% when the volume of Ag@SiO2 solution further increases to 250 μL, which may result from the increase of the nonradiative transition rates (791.6 to 843.0 s-1) and the decrease of the radiative transition rate (390.1 s-1 to 375.6 s-1). The quantum efficiencies of complex Eu(o-NBA)3
4H2O show the similar changes with the addition of Ag@SiO2 nanoparticles (Table 3), increasing from 11.8% to 13.7% when 200 μL of Ag@SiO2 solution is added, and decreasing when more solution is added. The fluorescence enhancement effect of the europium complexes is determined by three aspects: (1) excitation enhancement, (2) emission enhancement (increased radiative transition) and (3) quenching (increased nonradiative transition) [29,40]. The introduction of Ag@SiO2 nanoparticles in the solution of europium complexes may lead to both excitation and emission enhancement (Figs.S7–9 and 5). Meanwhile, the nonradiative transition rates of the complexes decrease dramatically by the introduction of the nanoparticles, which may be due to the reduction of solvent quench by bounding the complexes to the surface of Ag@SiO2 nanoparticles. Thus, the energy loss by the nonradiative transition may be greatly reduced by the nanoparticles [41]. Therefore, the fluorescence intensity, lifetime and quantum efficiency of the europium complexes increase with the increased amount of adding nanoparticles when the volume of nanoparticle solution is less than 200 μL. However, too many Ag@SiO2 particles may induce the decrease of the radiative transition rates and the increase of nonradiative transition rates, which results in the drop in the fluorescence intensity, lifetime and quantum efficiency of the europium complexes.

4. Conclusions Three kinds of europium complexes (Eu(p-NBA)3
4H2O, Eu(oNBA)3
4H2O and Eu(m-NBA)3
4H2O) with weak fluorescent emissions were synthesized. Different volumes of the core-shell Ag@SiO2 nanoparticles with distinct core sizes and varied shell thickness were added into these europium complex solutions. The fluorescent property investigations prove that the introduction of Ag@SiO2 nanoparticles is one of the effective methods to enhance the fluorescent intensities of those complexes. Their intensities increase as high as 1.6–3.1 times. Furthermore, Ag@SiO2 nanoparticles can also elongate the luminescence lifetimes and increase the quantum efficiencies of the europium complexes. The enhancement effect is found to be related with the size of silver core, silica shell thickness and the addition amount of Ag@SiO2 nanoparticles. The best fluorescence enhancement effect for the three europium complexes is achieved when 200 μL of Ag@SiO2 nanoparticles with core diameter of about 32 nm and shell thickness of about 25 nm is added. Besides, the increase in the quantum efficiencies of the europium complexes may result from the increase of the radiation transition rates and the decrease of nonradiative transition rates of the complexes after the introduction of Ag@SiO2 nanoparticles.

Acknowledgements The research work is supported by the National Natural Science Foundation of China (21561023, 51501094, 21161013), the Opening Foundation for Significant Fundamental Research of Inner Mongolia Autonomous Region of China (2010KF03), and the Program of High-level Talents of Inner Mongolia University (21300-5155104).

L.-j. Kong et al. / Journal of Luminescence 186 (2017) 255–261

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.02.044.

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