The following chemicals were used for preparation of core-shell Ag@SiO₂ nanocomposites: silver nitrate (AgNO₃, AR), sodium citrate (Na-cit, AR), tetraethylorthosilicate (TEOS, AR), sodium benzoate (C₆H₅COONa, AR), anhydrous ethanol (EtOH, AR), ammonia (NH₃· H₂O, AR).

Dried Flos Sophorae powder (18 g) were exhaustively extracted according to relevant paper^[20]. The product was recrystallized from ethanol to afford yellow crystals (1.018 2 g, yield 6.0%). M. p.> 300 C. ¹H NMR (DMSO-d₆, 600 MHz): 12.50 (s, 1H, 5-OH), 10.78 (s, 1H, 7-OH), 9.59 (s, 1H, 3-OH), 9.37 (s, 1H, 4’ -OH), 9.31 (s, 1H, 3’ -OH), 7.68 (d, J=1.8 Hz, 1H, 2’ -H), 7.54 (d× d, J1 1410.8 Hz, J2 141.8 Hz, 1H, 6’ -H), 6.89 (d, J=8.4 Hz, 1H, 5’ -H), 6.41 (d, J 142.4 Hz, 1H, 8-H), 6.19 (d, J 141.8 Hz, 1H, 6-H) (Fig.S).

The Ag@SiO₂ was prepared by slightly improved method according to literature^[21]. The synthetic route for Ag@SiO₂ is presented in Scheme 2.

Scheme 2

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	Scheme 2 The synthetic route for Ag@SiO2 nanoparticles and Ag@SiO2@Quercetin

After the mixture of 2 mL 0.129 mol· L^-1 AgNO₃, 1 mL 0.200 mol· L^-1 sodium citrate and 250 mL redistilled water was stirred vigorously and refluxed for 20 min. It was centrifuged for 0.5 h at 3 000 r· min^-1 to remove the larger silver nanoparticles and then obtained silver colloid.

In order to synthesize the Ag@SiO₂ nanoparticles, 10 mL silver colloid, 30 mL anhydrous ethanol, and 1 mL sodium citrate solution were stirred vigorously under pH 8. Then TEOS were added, and the reaction was maintained for 20 h.

In order to synthesize Ag@SiO₂@Quercetin, 3.00 mL 1.0× 10^-5 mol· L^-1 of Quercetin was added into 0.1 mL Ag@SiO₂ solution. The mixture was stirred for 1 h and then allowed to stand for another 10 min to equilibrium, after which the fluorescence spectra were detected.

Transmission electron microscopy (TEM, JEM-1400) was used to investigate themorphology and structure of the core-shell Ag@SiO₂ nanoparticles. The particle size of Ag@SiO₂ was investigated by means of laser particle analyzer (Mastersizer 2000). Fluorescence spectra were recorded with a Perkin-Elmer LS55 spectrophotometer, and the excitation wavelength was 390 nm for all measurements. The content of copper ions were measured on AA900T AAS

In order to check the fluorescence response of Ag@SiO₂@Quercetin to different metal ions, stock solutions(1× 10^-3 mol· L^-1) of acetate/nitrate salts of different metal ions (Pb²⁺, Zn²⁺, Co²⁺, Tb³⁺, Nd³⁺, Ag⁺, K⁺, Sb³⁺, Cd²⁺, Mn²⁺, Hg²⁺, Ni²⁺, Fe³⁺, Fe²⁺, Ho³⁺, Cu²⁺). The test solutions were prepared by 3 mL of the sensor stock solutions and a small quantity of certain ion stock solutions. In the test solutions, [Cu²⁺]=1.0× 10^-5 mol· L^-1, the concentration of other metal cations was 1.0× 10^-5 mol· L^-1.

In order to check the influence of other metal ions on fluorescence responses of Ag@SiO₂@Quercetin to Cu²⁺, test solutions were prepared by placing 3 mL of the sensor stock solution and 5μ L of Cu²⁺ stock solution into a test tube, then adding a small quantity of certain metal ions stock solution to it. In the test solutions, [Cu²⁺] = 1× 10^-5mol· L^-1and the concentration of other metal cations was 1× 10^-4 mol· L^-1.

We designed an experiment to detect Cu²⁺ in aqueous solution using Ag@SiO₂@Quercetin as fluorescence sensor. Water samples were collected from suburban and urban areas in Nanjing and Taizhou. Water samples were filtrated through a 0.22-μ m-pore-size membrane. Nitric acid digestion was performed before metal analysis by ASS and Ag@SiO₂@Quercetin fluorescent sensor.

TEM images of silver nanoparticles and Ag@SiO₂ nanoparticles are shown in Fig.1. Fig.1(a) showed that most of the silver nanoparticles are nearly spherical and their average diameter is about 40 nm. Fig.1(b) exhibits that the silver nanoparticles are coated with SiO₂ shell. XPS results suggest there are Ag and Si elements in Ag@SiO₂ nanoparticles (Table 1).

Fig.1

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	Fig.1 TEM images of silver nanoparticles (a) and core-shell Ag@SiO2 nanoparticles (b)

Table 1

Table 1

Table 1 X-Ray Photoelectron-Spectroscopy of Ag@SiO2 nanoparticles Peak Type Position Be/eV FWHM /eV Raw Area (cps eV) RSF Atomic Mass Atomic Conc/% Mass Conc/% Ag 3d Reg 369.626 0.738 1 236.2 5.987 107.878 1.57 5.77 Si 2p Reg 104.926 1.964 3 973.9 0.328 28.086 98.43 94.23

Table 1 X-Ray Photoelectron-Spectroscopy of Ag@SiO2 nanoparticles

Because the concentration of TEOS will affect the rate of hydrolysis and polymerization, the SiO₂ shell thickness of the corn-shell Ag@SiO₂ could be controlled by the concenuration of TEOS. Fig.2 shows the sizes of four kinds of core-shell Ag@SiO₂ nanoparticles via controlling the concentration of TEOS on 40 nm silver core. And sizes of 1^#, 2^#, 3^# and 4^# are 50, 60, 95 and 120 nm, respectively, in which the SiO₂ shell thickness of 1^#, 2^#, 3^# and 4^# are 10, 20, 55, 80 nm respectively (shown in Table 2).

Table 2

Table 2

Table 2 The sizes of Ag@SiO2 nanoparticles and the thickness of SiO2 shell of 1#, 2#, 3#, 4# Sample Ag-core/nm Ag@SiO2/nm SiO2/nm 1# 40 50 10 2# 40 60 20 3# 40 95 55 4# 40 120 80

Table 2 The sizes of Ag@SiO2 nanoparticles and the thickness of SiO2 shell of 1#, 2#, 3#, 4#

Fig.2

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	Fig.2 The sizes of Ag@SiO2 nanoparticles 1#, 2#, 3# and 4#

Ag@SiO₂@Quercetin Q1^#, Q2^#, Q3^#, Q4^# was obtained by adding Ag@SiO₂ 1^#, 2^#, 3^#, 4^# into Quercetin solution, respectively. The fluorescence intensities of Q1^#, Q2^#, Q3^#, Q4^# was investigated, shown in Fig.3. The results indicated that Ag@SiO₂@Quercetin nanocomposites with different SiO₂ shell thickness all showed stronger fluorescence intensities than quercetin, and the emission intensity first increased then decreased with the increase of the thickness of SiO₂ shell. When the thickness of SiO₂ shell was 10 nm (in Q1^#), the distance between quercetin and Ag core was too close that it may lead to non-radiative energy transfer between quercetin and Ag@SiO₂ nanoparticles. While the thickness of the SiO₂ shell was 80 nm (in Q4^#), the distance between the quercetin and Ag nanoparticles was too far and the electromagnetic field around the quercetin was weak, so the emission intensity showed weaker than others. In Q2^#, the SiO₂ shell was 20nm, which can not only avoid molecular fluorescence quench but also make sure that the electromagnetic field around the quercetin was stronger^{[22, 23, 24, 25]}, so Q2^# showed the strongest emission intensities. Therefore, Ag@SiO₂@Quercetin Q2^# was investigated as sensor in the detection of copper ions in later assay.

Fig.3

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	Fig.3 The fluorescence intensity of Ag@SiO2@Quercetin Q1#, Q2#, Q3# and Q4#

To evaluate the selectivity of the Ag@SiO₂@Quercertin system, many metal ions (Pb²⁺, Zn²⁺, Co²⁺, Tb³⁺, Nd³⁺, Ag⁺, K⁺, Sb³⁺, Mg²⁺, Cd²⁺, Mn²⁺, Hg²⁺, Ni²⁺, Fe³⁺, Ho³⁺, Cu²⁺) were used for the nanosensor study. Each of the ions was added to the prepared detection system individually, and the fluorescence intensity variation of Ag@SiO₂@Quercertin was psesented in Fig.4, which indicated that only Cu²⁺ and Fe²⁺ induced obvious fluorescence quenching at their corresponding peaks (546.5 nm). However, Fe²⁺ is not stable and oxidizes to Fe³⁺ easily under natural conditions, which cannot affect the sensing process of Cu²⁺ in lakes and rivers. So, Fe²⁺is not involved in the discussion. And none of the other metal ions (Pb²⁺, Zn²⁺, Co²⁺, Tb³⁺, Nd³⁺, Ag⁺, K⁺, Sb³⁺, Mg²⁺, Cd²⁺, Mn²⁺, Hg²⁺, Ni²⁺, Fe³⁺, Ho³⁺)generated a significant influence on fluorescence signals. Thus, the sensing process can happened in the presence of Cu²⁺ with fluorescence quenching.

Fig.4

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	Fig.4 Fluorescence responses of Ag@SiO2@Quercetin (unlabeled bars are Pb2+, Zn2+, Co2+, Tb3+, Nd3+, Ag+, K+, Sb3+, Mg2+, Cd2+, Mn2+, Hg2+, Ni2+, Fe3+, Fe2+, Ho2+, Cu2+, respectively; λ ex=390 nm; [Q]=1× 10-5 mol· L-1, [Cu2+]=1× 10-5 mol· L-1, the concentration of other transition metal cations was 10-5mol· L-1)

For Cu²⁺ detection, 3 mL of the as-prepared fluorescent nanosensor (Q2^#) was added into the spectrophotometer quartz cuvette, and the emission spectra of the nanosensor were recorded from 450 to 650 nm. Then 0.3, 0.6, 1.0, 1.4, 1.8, 2.2, 2.6, 3.0, 3.4, 3.8, 4.0, 4.2, 4.4, 4.6, 4.8 μ mol· L^-1 of Cu²⁺ solution were added into the sensor solution, and the fluorescence intensity (546.5 nm) of Q2^# gradually decreased as the Cu²⁺ concentrations increased (Fig.5). The process of fluorescence quenching was mostly related to static quenching that originated from the formation of non-fluorescent Ag@SiO₂@Quercetin-Cu(Ⅱ ) complex^[13].

Fig.5

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	Fig.5 Fluorescence titration of Ag@SiO2@Quercetin and standard curve of fluorescence titration spectra with varying concentrations of Cu2+ in CH3OH-PBS buffer solution (1:99, V/V, pH=7.40; [Q]=1× 10-5 mol· L-1; λ ex=390 nm; [Cu2+]=(0, 0.3, 0.6, 1.0, 1.4, 1.8, 2.2, 2.6, 3.0, 3.4, 3.8, 4.0, 4.2, 4.4, 4.6, 4.8)× 10-6 mol· L-1)

The fluorescence titration experiments showed that the fluorescence intensity was inversely proportional to copper ion concentrations, and gave a good linear change in fluorescence emission intensity in response to the concentration of Cu²⁺ ranging from 3.0× 10^-7 to 4.8× 10^-6 mol· L^-1. The calibration curve was y=-32.864x+587.59(R²=0.998). The detection limit of Cu²⁺ was as low as 1.0× 10^-7 mol· L^-1. The standard curve with good linearity can be used for quantification.

Furthermore, fluorescence responses of Ag@SiO₂@Quercetin to Cu²⁺ in the presence of certain other metal ions were also tested (shown in Fig.6), and the result showed that these competitive cations did not lead to any significant change of fluorescence intensities even in a high concentration. Therefore, the presence of other metal ions hardly interfered with the fluorescence response of the Ag@SiO₂@Quercetin-Cu(Ⅱ ) system, so Ag@SiO₂@Quercetin showed high selectivity for sensing of Cu²⁺.

Fig.6

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	Fig.6 The influence of other metal ions on fluorescence intensity Ag@SiO2@Quercetin-Cu(Ⅱ ) system (unlabeled bars are Ag@SiO2@Q-Cu(Ⅱ ) system and other metal ions (Pb2+, Zn2+, Co2+, Tb3+, Nd3+, Ag+, K+, Sb3+, Mg2+, Cd2+, Mn2+, Hg2+, Ni2+, Fe3+, Ho3+) added to Ag@SiO2@Q-Cu(Ⅱ )system

Fig.7

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	Fig.7 UV/Vis absorption spectra of Quercetin and Ag@SiO2@Quercetin in CH3OH-PBS buffer solution (1:99, V/V, pH=7.40)

We speculate that the fluorescence quenching sensing mechanism could result from the intramolecular charge transfer (ICT) according to previously reported paper^[20]. The UV-visible spectra of Ag@SiO₂@Quercetin in the presence of Cu²⁺ and Co²⁺have been depicted in Fig.7. It clearly expresses that there is an obviously change in the characteristics of the Ag@SiO₂@Quercetin when addition of Cu²⁺ into the Ag@SiO₂@Quercetin solution while there is no change in addition of Co²⁺. The UV-visible spectrum of Ag@SiO₂@Quercetin in buffer solution shows an absorption band at 375 nm which are related to the π — π ^* transitions within the aromatic rings, similar to Quercetin solution. It also shows that the absorbance of Ag@SiO₂@Quercetin is stronger than quercetin in the same concentration. Then the addition of Cu²⁺ into Ag@SiO₂@Quercetin solution shows a distinguishable change in the absorption band with shifting to the long-wavelength region (430 nm). Such bathochromic shift can be explained by the extension of the conjugated system with the complexation. Scheme 3 shows the most probable structure of Ag@SiO₂@Quercetin-Cu(Ⅱ ) complex. As the 3-hydroxy group of quercetin has a more acidic proton, therefore the 3-OH and 4-carbonyl groups are the most probable coordination sites to be involved in the complexation process between Ag@SiO₂@Quercetin and Cu²⁺ which leads to a change of the UV absorption band. When Cu²⁺ reacted with Ag@SiO₂@Quercetin system, Ag@SiO₂@Quercetin-Cu(Ⅱ ) complex obtained, and intramolecular charge transfer (ICT) took place due to the extension of the conjugated system, subsequently, fluorescence quenching of Ag@SiO₂@Quercetin system occurred (scheme 4).

Scheme 1

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	Scheme 3 The most probable structure of Ag@SiO2@Quercetin-Cu(Ⅱ ) complex

Scheme 1

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	Scheme 4 The proposed mechanism of detection of Cu2+ with Ag@SiO2@Quercetin

To demonstrate the feasibility of the sensor Ag@SiO₂@Quercetin, we applied this method for detecting Cu²⁺ in several rivers and lakes with Ag@SiO₂@Quercetin as fluorescent sensor. The results were checked by ASS (shown in Table 3). Compared with ASS method, the results obtained by the sensor were also accurate and had good recovery, which proved that Cu²⁺ can be detected quantitatively by using Ag@SiO₂@Quercetin as sensor with this method.

Table 3

Table 3

Table 3 Determination of Cu2+ ions in water samples using the Ag@SiO2@Quercetin fluorescent sensor Sample AAS/(10-6 mol· L-1) Found/(10-6 mol· L-1) RSD /% Recovery /% Xuanwu lake 0.749 0.753 0.37 100.53 Dongcheng river 0.761 0.766 0.46 100.66 Qinhuai river 0.801 0.805 0.35 100.50 Moats of Nanjing 0.943 0.954 0.82 101.17 The Yangtze river 0.836 0.854 1.50 102.15 Luting river 0.746 0.754 0.76 101.07 Qili river 1.235 1.256 1.19 101.70 Wuli river 1.267 1.283 0.89 101.26 Heishui river 2.744 2.725 0.49 99.31 Zhoushan river 2.893 2.902 0.22 100.31

Table 3 Determination of Cu2+ ions in water samples using the Ag@SiO2@Quercetin fluorescent sensor