Metal-enhanced fluorescence-based multilayer core–shell Ag-nanocube@SiO₂@PMOs nanocomposite sensor for Cu²⁺ detection

Abstract

A silver nanocube (Ag NC) nanoparticle-enhanced fluorescent nanocomposite was developed. The designed nanocomposite contained an Ag NC core of 50 nm in size as enhancer, the 10 nm silica interlayer, and outer shell-periodic mesoporous organosilicas (PMOs) which incorporated the bis(rhodamine Schiff-base derivative) siloxane groups. Little fluorescence emission of nanocomposites was observed due to the existence of spirolactam in the rhodamine derivative, however, upon the addition of Cu²⁺, the fluorescence emission intensity increased dramatically, which resulted from ring-opening of the spirolactam in the rhodamine moieties. We showed the effect of using an Ag NC core on the fluorescence emission intensity of the novel system. The emission intensity was approximately 3-fold higher for Ag-nanocube@SiO₂@PMOs than the control sample prepared by etching the silver core away with iodide. Moreover, the metal-enhanced fluorescence (MEF) mechanism in this system was investigated through measuring the fluorescence lifetime. In addition, the designed core–shell Ag-nanocube@SiO₂@PMOs nanocomposite could act as a sensitive and selective sensor to detect the Cu²⁺, and we have been expecting that the detection limit would be reduced by reaping benefits from the plasmon resonance of Ag NC core in this nanocomposite system.

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1. Introduction

Fluorescence has become one of the most popular methods for sensing and imaging in chemical biology fields.^1–4 A fluorescence detection strategy is not ideal in many systems due to low fluorescence quantum yield, background signals and autofluorescence from the samples,⁵ which need to be overcome. During the past decade, it has been found that metal nanoparticles affect fluorescence quenching or enhancement.⁶ Metal-enhanced fluorescence (MEF) arises from the interaction between fluorophore and plasmon resonance of metal nanoparticle (Ag or Au).^7–9 Specifically, this resonant interaction results in the enhancement of excitation rates through enhanced local field experienced by fluorophore and the increase of emission efficiency by enhanced radiative decay rates, both of which contribute to an overall increase in fluorescent emission intensities.^10–14

It has been demonstrated by experiment and theoretical calculation that the degree of fluorescence enhancement depends on many factors strongly, such as the metal type, size, shape, particle resonance, metal-fluorophore distance, the radiative decay rate and quantum yield of the fluorophore, the overlap of the absorption and emission bands of the fluorophore with the plasmon band of the metal.^15–18 The interaction between fluorophore and metal particle is through-space with maximum fluorescence enhancement efficiency at metal-fluorophore distance of about 7–10 nm.¹⁹ However, if the fluorophore is located at a very short distance (<5 nm) from the metal surface, the emission intensity of fluorophore is even lower than that of the control sample, in other words, fluorescence quenching will be observed, which is due to the increased nonradiative decay caused by metal core dramatically.²⁰ Several materials as space layers e.g. biomolecules,^21–23 polyelectrolytes,^24,25 and silica^26–28 are used to adjust the distance between metal and fluorophore. In addition, the space layer could increase the versatility in basic and applied research.^29,30

Inspired by metal-enhanced fluorescence mentioned above, we are reminded that the sensor based on OFF–ON model such as rhodamine derivative located around metal nanoparticle could lower the limit of the detection (LOD) of metal ions governed by fluorescence intensity. Here, we have fabricated a novel multilayer core–shell Ag-nanocube@SiO₂@PMOs nanocomposite to detect Cu²⁺. As illustrated in Fig. 1, the preparation of Ag-nanocube@SiO₂@PMOs was achieved in several steps. Firstly, Ag nanocubes (Ag NCs) were prepared via the reduction of silver nitrate by ethylene glycol as reductant in the presence of polyvinylpyrrolidone (PVP) at elevated temperature.³¹ Then, the rigid silica spacer shell was coated onto the surface of Ag NC by the hydrolysis and condensation of tetraethyl orthosilicate (TEOS). Thirdly, the outer periodic mesoporous organosilicas (PMOs), in which was located bis(rhodamine Schiff-base derivative) siloxane precursors, were constructed via a surfactant-templating sol–gel approach by using hexadecyltrimethylammonium bromide (CTAB) surfactant as template.

Fig. 1 Schematic illustration of multilayer core–shell Ag-nanocube@SiO₂@PMOs nanocomposite sensor for Cu²⁺ detection. The green parts present the CTAB, and yellow parts present bis(rhodamine Schiff-base derivative) siloxane groups in spiro-ring form without fluorescence emission.

Upon the addition of Cu²⁺ to the nanocomposites material suspension, long-wavelength absorption at 526 nm and the strong fluorescence emission at about 550 nm from rhodamine derivative were observed, which was due to the ring-opening of the spirolactam in the bis(rhodamine Schiff-base derivative) groups driven by complexation with Cu²⁺, meanwhile, a yellowish-brown solution in ethanol turned plum red. The designed core–shell Ag-nanocube@SiO₂@PMOs nanocomposite showed high selectivity and sensitivity towards Cu²⁺ over other metal ions.

As usual, several methods could be used for fastening the fluorophores for investigating the MEF effect, such as aptamer labeling,³² electrostatic interaction,³³ tailored polymer,²⁷ modification on the surface of Ag@SiO₂,³⁰ and adsorption function in the open mesopores channels.³⁴ The MEF system designed here was different from the means mentioned above. The rhodamine derivatives were anchored in the silica pore wall on the outer shell of Ag-nanocube@SiO₂@PMOs nanocomposite through covalent bonding, which could avoid leaching or releasing of the fluorophores in the solution.

As expected, compared with the control sample, the Ag-nanocube@SiO₂@PMOs nanocomposite showed a pronounced raise of absorption and fluorescence intensity simultaneously, which demonstrated significant benefits from the Ag NC core for lowering the LOD for Cu²⁺.

2. Experimental section

2.1 Chemicals

All chemicals were analytical grade and used without further purification. Solvents were dried by standard procedures. Rhodamine 6G (R6G) was purchased from TCI Co. (Japan). Polyvinylpyrrolidone (PVP, M_w = 55 000) and tetraethyl orthosilicate (TEOS) were purchased from Sinopharm Chemical Reagent Co. (China). 2,5-Thiophenedicarboxaldehyde, (3-aminopropyl) triethoxysilane (APTES), ethylene glycol (EG), hexadecyltrimethylammonium bromide (CTAB) and sodium iodide (NaI) were obtained from Aladdin. Deionized water was used in all experiments. Other chemicals and solvents were obtained from commercial suppliers.

2.2 Synthesis bis(rhodamine Schiff-base derivative) bridged precursor

A bis(rhodamine Schiff-base derivative) bridged siloxane precursor (compound 3) was synthesized according to Scheme 1.

Scheme 1 Synthesis of bis(rhodamine Schiff-base derivative) bridged siloxane precursor (compound 3).

2.2.1 Synthesis of compound 1. Rhodamine-6G hydrazide (compound 1) was prepared according to the reported procedure.³⁵

2.2.2 Synthesis of compound 2. Rhodamine-6G hydrazide (5 mmol, 2.14 g) and 2,5-thiophene dicarboxaldehyde (2 mmol, 0.28 g) were mixed in boiling methanol (40 mL) with 5 drops of glacial acetic acid. The reaction mixture was refluxed for 5 hours.³⁶ After cooling to room temperature, the resulting yellow solid was recrystallized by CH₂Cl₂ to give 1.35 g of compound 2 in 55.8% yield. ¹H NMR (300 MHz, pyridine-d₅): δ = 0.82 (t, 6H), 1.84 (d, 6H), 3.26 (t, 4H), 5.00 (d, 2H), 6.75–6.81 (d, 4H), 7.31 (d, 2H), 7.45 (m, 2H), 8.17 (d, 1H), 9.11 ppm (s, 1H) (Fig. S1†).

2.2.3 Synthesis of compound 3. Compound 2 (0.48 g) and NaH (0.065 g) were dissolved in 120 mL THF and stirred under an atmosphere of dry nitrogen at 0 °C. After 1 h, 3-iodomethyltriethoxysilane (0.73 g, 2.41 mmol) in 20 mL dry THF and trace amount of 15-crown-5 were added to the above solution dropwise. The resulting mixture was stirred at room temperature under dry nitrogen atmosphere for another 20 h. The THF was evaporated from the filtrate and the residue was washed thoroughly with anhydrous n-hexane to give the yellow product as compound 3 (yield 0.62 g, 75%). ¹H NMR (300 MHz, CDCl₃): δ = 0.88 (t, 6H), 1.29 (m, 18H), 1.88 (t, 6H), 2.15 (d, 4H), 3.20 (m, 4H), 3.74 (d, 12H), 6.28 (s, 2H), 6.38 (s, 2H), 6.85 (s, 2H), 7.02 (m, 2H), 7.97 (t, 1H), 8.44 ppm (s, 1H) (Fig. S2†).

2.3 Synthesis of Ag NCs

Ag NCs with the diameter of about 50 nm were synthesized with slight modification reported by Xia et al.³¹ EG (60 mL) was heated under vigorous magnetic stirring in an oil bath to 150 °C in a 250 mL three-neck round-bottomed flask. After 50 min of preheating under uncapped conditions, a flow of nitrogen was introduced via a glass pipet with a small opening to allow the gaseous species to escape from the flask. After 10 min, NaHS (0.7 mL, 3 mM) in EG was injected into the preheated EG solution, followed by injection of PVP (15 mL, 0.18 M) in EG. 8 min later, AgNO₃ (5 mL, 0.28 M) in EG was added dropwise into the above solution in 10 s. Shortly after the addition of AgNO₃, the reaction solution went through four distinct stages of color change from golden yellow to deep red, reddish gray, and then green ocher within 15 min. The reaction solution was then quenched by placing the reaction flask in a water bath at room temperature with stirring. The Ag NCs were isolated by precipitating the solution with acetone (400 mL), followed by centrifugation at 12 000 rpm for 10 min, and then washed with ethanol and water to remove the remaining precursor. Finally, the Ag NCs were collected and redispersed in ethanol (20 mL) for further use.

In addition, the size of Ag NC could be changed with altering the reaction time. For example, Ag NC with the size of 70 nm was obtained by extending the reaction time to 23 min using the same procedure.

2.4 Synthesis of Ag-nanocube@SiO₂ core–shell nanoparticles

The Ag-nanocube@SiO₂ nanoparticles with 10 nm silica shell were prepared according to Stöber method with slight modification.³⁷ Briefly, Ag NCs (half of the total) obtained above were dispersed in the mixture of ethanol (40 mL) and water (10 mL). Then, a mixture of TEOS (36 μL) and APTES (3.6 μL) was added slowly with continuous stirring. After 30 min of stirring, ammonia aqueous (0.5 mL, 28 wt%) was added into the solution. The reaction was continued for 24 h at room temperature. The Ag-nanocube@SiO₂ nanoparticles were separated by centrifugation and washed by ethanol and water for several times, and then redispersed in water for further use.

2.5 Synthesis of Ag-nanocube@SiO₂@PMOs core–shell nanocomposites

The Ag-nanocube@SiO₂@PMOs core–shell nanocomposites were prepared according to a modified sol–gel process using CTAB as a template.³⁴ Briefly, the Ag-nanocubes@SiO₂ particles synthesized above were dispersed in a mixed solution containing H₂O (25 mL), ethanol (15 mL) and CTAB (75 mg). After stirring for 30 min at room temperature, the mixture of TEOS (40 μL), APTES (4 μL) and compound 3 (0.05 g) was added dropwise to the solution with vigorous stirring. The mixed solution was homogenized for 0.5 h at room temperature with continuous stirring. Then, ammonia aqueous (0.275 mL, 28 wt%) was added and the solution was stirred for 24 h. The Ag-nanocube@SiO₂@PMOs nanocomposites were collected by centrifugation and washed with ethanol and water for several times, respectively. Subsequently, to extract CTAB from the nanocomposites, the products were added to NH₄NO₃/ethanol solution (60 mL, 6 g L⁻¹) with stirring at 50 °C for 3 h, and this process was repeated three times. Finally, the Ag-nanocube@SiO₂@PMOs nanocomposites were obtained after centrifugation and washing with ethanol and water.

2.6 Preparation of control sample by dissolving Ag core from Ag-nanocubes@SiO₂@PMOs

Sodium iodide was employed as etching agent to dissolve the Ag core from Ag-nanocubes@SiO₂@PMOs. Briefly, Ag-nanocubes@SiO₂@PMOs nanocomposites suspension in ethanol (600 μL, 0.25 mg mL⁻¹) was dispersed into NaI (4 mL, 1 M) aqueous solution and kept stirring two days with protection from light at ambient temperature. After two days, the yellowish-brown suspension became almost colorless due to the dissolution of Ag core, which could be further confirmed by disappearance of plasmon resonance band at about 450 nm in the UV-vis spectra. Finally, the obtained control samples, followed by centrifugation at 12 000 rpm for 10 min and then washed with ethanol and water, were redispersed in ethanol.

2.7 Characterization

Transmission electron microscopy (TEM) measurements were performed on a JEM 2011 microscope (Japan) operated at 100 kV. ¹H NMR spectra were recorded using a BRUKER 300M spectrometer. ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS NMR) spectrum was acquired on a Bruker AVANCE III 400WB spectrometer (5000 transients, spin speed 6 kHz, acquisition time 0.02 s, pulse delay 3 s). FT-IR spectra were recorded on a Nicolet 700 FT-IR spectrometer (Thermo-Fisher Scientific, Inc., Waltham, MA) with KBr wafer. Nitrogen adsorption–desorption isotherms were measured using a Micromeritics ASAP2020 Surface Area and Porosity Analyzer at 77 K. Prior to analysis, the sample was evacuated at 80 °C for 4 h. Surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. Pore size distributions were calculated according to the adsorption branch using the Barret–Joyner–Halenda (BJH) model, and the pore volume was also derived from the adsorption isotherm. Time-resolved fluorescence spectra were determined on a FLS920-stm spectrometer equipped with picosecond pulsed diode lasers. UV-vis spectra were measured with an Aglient HP8453E spectrometer. Fluorescence measurements of the nanocomposite suspension were carried out using a Perkin-Elmer LS-55 spectrometer.

3. Results and discussion

3.1 Design of the nanocomposite

Ag nanoparticle was selected as fluorescence enhancer to build the core for its high scattering and enhancement efficiency among the noble metals.^38,39 Ag NC has four plasmonic absorption peaks, exhibiting more peaks than the sphere because the cube has several distinct symmetries for dipole resonance compared with only one for the sphere.⁴⁰ Significantly, the electric field at the sharp corners of a nanocube is much stronger than other regions of the particle(s), which is therefore referred to as “hot spots”. For lots of other morphologies with sharp corners, like nanorod and nanoplate, the positions of the plasmonic absorption peaks are generally red-shifted to the near-IR region, which is far away from the absorption of fluorophore (rhodamine 6G derivative) at 520 nm.^41,42 Therefore, here, we selected Ag nanocube as fluorescence enhancer and invesgated its MEF effect.

Ag NCs were synthesized successfully via reduction of AgNO₃ by EG with PVP as a capping agent based on Xia's group with some modifications.⁴³ We investigated the vital role of the NaHS in the synthesis process that created Ag₂S nanocrystallites as nuclei to help induce and maintain single-crystal structure for the seeds, which could catalyze the reduction of AgNO₃.⁴⁴ In the absence of NaHS, when the molar ratio between repeating unit of PVP and AgNO₃ was 1.5, there appeared some unexpected spheroid, right bipyramid, rodlike nanoparticles, even the nanowires (Fig. 2A–C). In addition, when the concentration of PVP was increased by a factor of 4, Ag nanoparticles with a quasi-spherical shape were obtained, and it was worth noting that other shapes, like nanorod and nanowire, disappeared (Fig. 2D).

Fig. 2 TEM images of the prepared Ag nanoparticles in different reaction conditions. The ratios of PVP to AgNO₃ were 1.5 (A, B, C) and 4 (D).

Interestingly, by adding a trace amount of NaHS to the EG, monodispersed Ag NCs with sharp corners of 28–70 nm in edge length were routinely and rapidly obtained. When the reaction time was 9 min, Ag NCs of 28 nm with slightly truncated corners in shape were prepared, meanwhile, the color of reaction solution was blackish green (Fig. 3A and B). After another 2 min, Ag NCs of 35 nm were obtained, and it was observed that the sharp corner appeared (Fig. 3C and D). By reaction time reaching to 15 min, the solution turned green-ochre in color, the synthesized Ag NCs were perfect in shape and approximately 50 nm (Fig. 3E and F). Allowing the reaction to proceed for a longer time, up to 23 min, would yield larger nanocubes with a size of 70 nm, and the solution was khaki (Fig. 3G and H). Therefore, the monodispersed Ag NCs with well-controlled sizes and shapes can be produced by introducing a small amount of NaHS with different reaction time.

Fig. 3 TEM images of the prepared Ag nanocubes with different reaction time: 9 min (A, B), 11 min (C, D), 15 min (E, F) and 23 min (G, H). A trace amount of NaHS was introduced.

Based on previous reports, the size of Ag nanoparticle was optimal in the range of 40–70 nm with suitable plasmon absorption for MEF,¹² therefore, 50 nm Ag NC was chosen as the core of the nanocomposite system for fluorescence enhancement.

As shown in Fig. 4, the pore size of Ag NCs was ca. 50 nm, and the SiO₂ spacer shell coating was realized by Stöber method via the hydrolysis and condensation with a thickness of 10 nm. The absorption spectra of 50 nm Ag NCs exhibit the plasmonic absorption band at 454 nm (Fig. 5, curve a). However, for the Ag-nanocube@SiO₂, the coating of silica shell leads to the redshift of plasmon resonance band from 454 nm to 465 nm because of the increase of the local refraction index and decrease of the plasmon oscillation energy (Fig. 5, curve b).⁴⁵

Fig. 4 TEM images of Ag-nanocube@SiO₂ nanoparticles (A, B) and Ag-nanocube@SiO₂@PMOs nanocomposites (C, D).

Fig. 5 Absorption spectra of Ag NCs (curve a) and Ag-nanocube@SiO₂ nanoparticles (curve b) with 10 nm shell thickness.

Bis(rhodamine Schiff-base derivative) bridged precursor that acted as not only the target for measuring the MEF factor but also the fluorescence sensor for detecting Cu²⁺, was synthesized successfully by a series of steps (Scheme 1). Using CTAB as a template, the resultant Ag-nanocube@SiO₂ nanoparticles were further coated by a uniform layer of PMOs with thickness of about 8 nm via a sol–gel approach, and Ag-nanocube@SiO₂@PMOs core–shell nanocomposite was obtained (Fig. 4C and D).

The FT-IR spectra (Fig. 6) of the Ag-nanocube@SiO₂@PMOs nanocomposite show new bands in the range of 1383–1625 cm⁻¹,⁴⁶ which are consistent with the characteristic bands of rhodamine xanthene ring, that is, the outer shell (PMOs) was successfully coated onto the surface of Ag-nanocube@SiO₂. Further, a band at around 1082 cm⁻¹ is associated with the typical Si–O–Si bands in the spectra of Ag-nanocube@SiO₂ and Ag-nanocube@SiO₂@PMOs,⁴⁷ demonstrating the formation of the silica network.

Fig. 6 FT-IR spectra of compound 3, Ag-nanocube@SiO₂ and Ag-nanocube@SiO₂@PMOs.

A ²⁹Si MAS NMR spectrum of the Ag-nanocube@SiO₂@PMOs was investigated to study the covalent bond of organosiloxanes during the second coating process (Fig. 7). The peaks at −90.3, −100.0, −109.0 ppm are assigned to Q₂ [Si(OSi)₂(OH)₂], Q₃ [Si(OSi)₃(OH)], and Q₄ [Si(OSi)₄] sites, respectively, indicating occurrence of hydrolyzation of TEOS. Furthermore, the signals at −58.4 and −66.3 ppm could be attributed to T_n-peaks: T₂ [R–Si(OH)(OSi)₂] and T₃[R–Si(OSi)₃] sites (R represents the bridged organic groups in PMOs), suggesting that the functional organic groups were covalently embedded in the silica framework in the outer shell of nanocomposite.

Fig. 7 Solid-state ²⁹Si MAS NMR spectrum of Ag-nanocube@SiO₂@PMOs.

N₂ sorption–desorption isotherms of Ag-nanocube@SiO₂@PMOs exhibit the typical type-IV curves, with the average pore size of 2.3 nm. The hysteresis loop is type H4, corresponding to uniform slit-shape channels. BET surface area and the total pore volume are calculated to be 431 m² g⁻¹ and 0.29 cm³ g⁻¹, respectively (Fig. 8).

Fig. 8 N₂ adsorption–desorption isotherms and pore size distribution (inset) curves of the Ag-nanocube@SiO₂@PMOs.

3.2 Optical properties and MEF mechanism of Ag-nanocube@SiO₂@PMOs nanocomposite

In order to evaluate the effect of the Ag core on fluorescence emission, rather than doping the bis(rhodamine Schiff-base derivative) bridged organosilanes directly onto silica layer without a Ag core, as many other groups have done, the control sample without Ag core was prepared via a new etching agent. NaI was selected as to dissolve the Ag cores in the Ag-nanocube@SiO₂@PMOs nanocomposites. As shown in Fig. 9A, the Ag cores were completely dissolved with the use of NaI as etching agents, meanwhile, the plasmonic peak of the Ag at about 450 nm disappeared (Fig. 9B, curves a and d).

Fig. 9 TEM image (A) of the control samples prepared from Ag-nanocube@SiO₂@PMOs with NaI as etching agent. Absorption spectra (B) of Ag-nanocube@SiO₂@PMOs (curve a) and control sample after dissolving the silver core (curve d). Curves b and c are the corresponding Ag-nanocube@SiO₂@PMOs and control sample in the presence of 1 × 10⁻⁵ M Cu²⁺, respectively.

In the absence of metal cations, Ag-nanocube@SiO₂@PMOs nanocomposites material suspension in ethanol was yellowish-brown and nonfluorescent due to the spirolactam ring structure of rhodamine derivatives embed in them. Upon the addition of Cu²⁺ (1 × 10⁻⁵ M), a new peak at 520 nm appeared in the absorption spectra (Fig. 9B, curve b), accompanied by the color change from yellowish-brown to plum red, which resulted from ring-opening of spirolactam in the bis(rhodamine Schiff-base derivative) groups. In addition, it is obvious that the absorption intensity of the Ag-naocube@SiO₂@PMOs (Fig. 9B, curve b) is higher than that of the control sample (Fig. 9B, curve c) after counteracting the plasmonic absorption from the Ag NCs. Meanwhile, the fluorescence intensity underwent a significant increase at 545 nm following excitation at 500 nm for these two samples.

As shown in Fig. 10, the fluorescence intensity of Ag-nanocube@SiO₂@PMOs is 3-fold higher than that of control sample without Ag NC cores, which is ascribed to the MEF. There are two dominated way for MEF process: firstly, the excitation rate of the fluorophore can be increased by the enhanced local electric field around the metal NPs.⁴⁸ Secondly, the quantum yield is increased and fluorescence lifetime is decreased, which is due to the acceleration of radiative decay by the excitation–plasmon interaction.⁴⁹

Fig. 10 Fluorescence spectra of Ag-nanocube@SiO₂@PMOs (curve a) and control sample (curve b) upon addition of Cu²⁺ (1 × 10⁻⁵ M) in C₂H₅OH, respectively.

Metal nanoparticle influences the lifetime of adjacent fluorophores in a manner dependent on the properties of the nanostructure.⁹ To explore the origin of MEF in this system, the fluorescence lifetime was investigated. The emission decay curves were obtained for the Ag-nanocube@SiO₂@PMOs and the control sample, which were ascribed to both biexponential kinetics (Fig. 11 and Table 1). However, the Ag-nanocube@SiO₂@PMOs have the faster average lifetime (2.273 ns) than that of the control sample (3.327 ns), which is in accordance with the previous reports.⁵⁰ It is worth noting that the plasmon resonance band of Ag-nanocube@SiO₂@PMOs (Fig. 9A, curve a) at 450 nm decreases slightly after the addition of Cu²⁺ (Fig. 9A, curve b) because of the resonance energy transfer from Ag core to the fluorophore. Moreover, the enhancing excitation induced by electromagnetic-field enhancement due to the surface plasmon contributes to the emission, which is the another fundamental element for the 3-fold fluorescence enhancement. A combination of enhanced excitation and increased radiative decay rate, leads to the large enhancement of the quantum efficiency.⁵¹

Fig. 11 Fluorescence decay of Ag-nanocube@SiO₂@PMOs (curve a) and control sample (curve b).

Table 1 Fluorescence lifetime of Ag-nanocube@SiO₂@PMOs and the control sample

	τ1 (ns)	A1%	τ2 (ns)	A2%	τAv (ns)	χ^2
Ag-nanocube@SiO2@PMOs	1.0847	45.52	3.2660	54.48	2.273	0.984
Control sample	0.9143	13.28	3.6970	86.72	3.327	1.040

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3.3 Sensitivity and selectivity of the “Off–on” sensor for Cu²⁺ detection and the proposed binding mode

As mentioned above, the addition of Cu²⁺ towards the nanocomposites suspension in ethanol would induce the ring-opening of rhodamine moieties from spirocyclic structure, followed by the emergence of fluorescent signals. We estimated the selectivity of this sensor toward Cu²⁺. As depicted in Fig. 12, only Cu²⁺ has a remarkable effect on the fluorescence intensity of the materials at 550 nm. Upon the addition of other competitive metal ions, such as Ag⁺, K⁺, Na⁺, Ba²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Fe²⁺, Hg²⁺, Mn²⁺, Mg²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ ions, the fluorescence intensities show no discernible changes, except the weak influence of Pb²⁺.

Fig. 12 Fluorescence spectra of Ag-nanocube@SiO₂@PMOs (0.05 mg mL⁻¹) upon addition of various metal ions (10⁻⁴ M) in C₂H₅OH.

In addition, the competition experiment was conducted. It is noted that the addition of the mixture of Cu²⁺ and other metal ions mentioned above respectively show almost identical fluorescence intensity compared with the sample with Cu²⁺ only (Fig. 13). All of these results indicate that the designed Ag-nanocube@SiO₂@PMOs nanocomposite is able to be used as a qualified sensor toward Cu²⁺ detection.

Fig. 13 Fluorescence emission intensity of Ag-nanocube@SiO₂@PMOs (0.05 mg mL⁻¹) upon the addition of various metal ions (10⁻⁴ mol L⁻¹) in C₂H₅OH. Red bars represent the fluorescence response of Ag-nanocube@SiO₂@PMOs to selected metal ions. Black bars represent the emission intensity after the subsequent addition of Cu²⁺ (10⁻⁴ mol L⁻¹) to the above suspensions.

A linear relation between the fluorescence intensity and the Cu²⁺ concentrations in the range of 1 × 10⁻⁶ M to 5 × 10⁻⁶ M is shown in Fig. 14. The LOD for Cu²⁺ is 3 × 10⁻⁷ M, which is calculated by the equation LOD = 3S₀/k, where S₀ is the standard deviation of the blank and k is the slope of the calibration curve. It is conceivable that the MEF-based method might bring out the lower LOD than the control sample without Ag core.⁵²

Fig. 14 Fluorescence spectra of Ag-nanocube@SiO₂@PMOs in C₂H₅OH with different concentrations of Cu²⁺ (1 × 10⁻⁶ M to 5 × 10⁻⁶ M). Inset: linear calibration plot for Cu²⁺ in low concentration.

In this study, synchrotron-based scanning transmission X-ray microscope (STXM) provided further proof to demonstrate the integration between the nanocomposites and Cu²⁺ toward other metal ion (Fig. 15). Zn²⁺ was selected as contrast metal ion since its complexation property was similar to Cu²⁺. Saturated Cu²⁺ and Zn²⁺ solutions were used in this process. Compared with Zn²⁺, it was easy to observe that Cu²⁺ distribution was abroad due to the complexation, which implied that the nanocomposites provided a higher binding sites toward Cu²⁺ rather Zn²⁺. For the residual Zn²⁺, we reason the adsorption effect of mesoporous for Zn²⁺.

Fig. 15 Distribution of Cu and Zn elements in the Ag-nanocube@SiO₂@PMOs nanocomposite determined by STXM. (A) Cu element; (B) Zn element; (C) merged image of Cu and Zn elements.

The titration curve revealed that one bis(rhodamine Schiff-base derivative) bridged siloxane molecule could only coordinate with one Cu²⁺, and the association constant (K_a) determined by the Benesi Hildebrand equation was about 9193 M⁻¹ (Fig. S3†).⁵³

Furthermore, X-ray absorption near-edge spectroscopy (XANES) was used to provide information on coordination structure of Cu²⁺-chelated Ag-nanocube@SiO₂@PMOs nanocomposites.⁵⁴ The energy of the K-edge of Cu²⁺ in nanocomposites was 9001 eV (Fig. S4†), which was distinctly different from that of copper metal (8976 eV). First derivative spectrum and corresponding Fourier transforms were shown in Fig. S4.† The proposed probable coordination structure of the bis(rhodamine Schiff-base derivative) in the Ag-nanocube@SiO₂@PMOs to Cu²⁺ has been shown in Fig. 1. The N atoms in the Schiff base of the bis(rhodamine Schiff-base derivative) take an important role in the selectivity to Cu²⁺ for the nanocomposites. The electron transfer would take place from the rhodamine units to copper ions after complexation with copper ions, leading to ring-opening of the spiro-structure in the rhodamine units, and thus the strong fluorescence emerged.

4. Conclusion

In conclusion, we have designed a novel core–shell Ag-nanocube@SiO₂@PMOs. In this multilayer nanocomposite, 50 nm Ag NC acted as enhancer core, 10 nm SiO₂ interlayer was utilized to separate the metal core and the rhodamine derivative fluorophore that was embedded in the outer mesoporous wall. Compared with the control samples prepared by dissolving the Ag core with iodide, the fluorescence emission yielded up to 3-fold enhancement, which resulted from the acceleration of radiative decay by the excitation–plasmon interaction and the dominant electromagnetic-field enhancement around the mental core. In addition, the elaborate nanocomposite could be a sensitive and selective sensor to detect the Cu²⁺. It is conceivable that the fluorescence enhancement here could reduce the relevant detection limit. Finally, it is envisioned that this plasmon core–shell system would be applied widely in biological and environmental fields.

Acknowledgements

This research was financially supported by the Natural Science Foundation of China (No. 50572057). Soft X-ray absorption experiment was conducted at Beamline BL08U of Shanghai Synchrotron Radiation Facility. Hard X-ray absorption experiment was conducted at the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11598e

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