Core-shell mesoporous silica nanospheres used as Zn²⁺ ratiometric fluorescent sensor and adsorbent

Abstract

Core-shell mesoporous silica nanospheres encapsulated with rhodamine 101 into the solid core and 8-aminoquinoline derivatives into the mesoporous shell were used as a ratiometric fluorescent sensor to detect metal ions, which shows a high selectivity and sensitivity to Zn²⁺. Concentrations as low as 5 × 10⁻⁸ M can be detected and the addition of other metal ions has a negligible influence on the fluorescence emission. Moreover, the dye-doped nanospheres show excellent photostability, no dye leaking and a good adsorption capacity to Zn²⁺, which make it repeatedly applicable for simultaneous heavy metal ion sensing and removal from polluted water.

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Introduction

Zn²⁺ is the second most abundant trace element in humans, with concentrations ranging from nM to mM,¹ and is actively involved in various biological processes.^2–6 However, unlike other metal ions such as Ni²⁺, Fe²⁺ and Cu²⁺, Zn²⁺ does not give any spectroscopic signal due to its 3d¹⁰4s⁰ electronic configuration. Therefore, most commonly used analytic techniques cannot be applied to detect this metal ion,⁷ which makes its detection more difficult than other metal ions. Fortunately, fluorescent methods are a good choice for the detection of Zn²⁺ since many fluorescent dyes are sensitive to tiny changes in the surrounding chemical environment, which is proven to be an effective way to well understand the biological function of Zn²⁺ in humans and has been receiving increasing attention.⁸ However, an effective and reliable Zn²⁺ fluorescent sensor needs elaborate design and preparation processes since it should have high fluorescent selectivity and sensitivity, chemical- and photostability, rapid sensitization and suitable solubility. Based on the urgent demand for highly efficient Zn²⁺ fluorescence sensors with a real-time detection ability at trace concentrations, more research still needs to be carried out.

Generally, a metal ion fluorescence sensor is composed of a fluorescent report unit and an appropriate recognition unit, where the covalent linkage pattern between these two units should be carefully designed to ensure that the coordination of metal ions with the recognition unit will trigger fluorescence changes of the fluorophore unit. The fluorescence responses include wavelength shift, intensity and lifetime changes. According to these changes, metal ion fluorescent sensors can be classified into two major types: 1) switch type based on “on-off” (fluorescence quench)^9,10 or “off-on” (fluorescence increasing)⁹ mechanisms and 2) ratiometric type¹⁰ based on a variation of intensity ratio between two emission wavelengths. The design for a ratiometric Zn²⁺ fluorescent sensor is more elaborate since the binding of Zn²⁺ with recognition units should cause a shift in wavelength, thus an intensity ratio between two wavelengths can be read out.^11–13

Recently, it has been reported that the ratiometric detection of an analyte can be achieved without elaborate molecular design by combing a traditional fluorescent sensor with nanomaterials such as polymers, QDs and silica nanoparticles.^14–17 The simplest method is arranging two kinds of dyes in different parts of a nanomaterial, which are inert and sensitive to analyte and are used as reference and report units, respectively. Among all nanomaterials, mesoporous silica nanoparticles seems to be a good alternative due to their tuneable pore diameter, high specific surface area and abundant Si–OH groups on the pore surface.¹⁸ Mesostructured materials doped with various fluorescent groups have attracted significant interest with respect to optical sensing applications.¹⁹ However, the difficulty in obtaining monodisperse and uniform mesoporous silica nanospheres has obstructed its application as a biosensor and adsorbent. Recently, some researchers have adopted novel surfactant or co-condensation synthesis methods to prepare monodisperse mesoporous silica particles or rods.²⁰ We previously reported a ratiometric pH sensor based on mesoprous silica nanoparticles with tuneable sensitivity and sensing-range by simply restricting the reference and report units for analytes in the nanometre-sized pore channel.²¹ This kind of pH sensor based on mesoporous silica nanoparticles is expected to be applied to both environmental and physiological systems.

Here, we report a novel kind of ratiometric fluorescence sensor for Zn²⁺ based on core-shell mesoporous silica nanoparticles (MSN). Rhodamine 101 and 8-aminoquinoline (8-AQ) derivatives were chosen as the reference and report units for Zn²⁺, and introduced into the solid core and mesoporous shell, respectively. The ratiometric sensing performance, photostability of dye molecules in the nanosphere by exposing it to UV irradiation and the removing efficiency of the nanosphere for Zn²⁺ were investigated.

Experimental

1. Materials

Chemicals including 8-aminoquinoline, rhodamine 101, N-hydroxysuccinimide, N,N′-dicyclohexyl carbodiimide and 3-aminopropyltriethoxysilane were purchased from Sigma-Aldrich and are of A.R. purity. NH₄OH (28 wt%) is directly used without further dilution. Solvents used in silica gel chromatography including ethyl acetate (EA), dichloromethane and petroleum ether (PE) were purchased from Sinopharm Chemical Reagent Co. Ltd. NMR solvent was purchased from J&K Chemical Ltd. All starting materials and solvents were used as received without further purification.

2. Preparation

Before being introduced into core-shell MSN, rhodamine 101 and 8-AQ were pre-modified with 3-aminopropyltriethoxysilane (APTS) to react with silica hydroxyls. Schemes 1 and 2 show the synthesis routes for the silylation of 8-AQ and rhodamine 101-succinimide, respectively. 8-AQ is chosen as the report unit for Zn²⁺ detection since it is a traditional “off-on” type Zn²⁺ sensor, which is non-fluorescent due to the intramolecular charge transfer from N of amine group to quinoline and tends to recover fluorescence after binding with Zn²⁺ since Zn²⁺ can get electron from N.²²

Scheme 1 Synthesis route for 8-AQ anchored mesoporous shell of MSN. (1) chloroacetyl chloride, N(Et)₃, CH₂Cl₂, 0 °C, and then warmed to room temperature, overnight, 93%; (2) K₂CO₃, APTS, CH₃CN, refluxed , N₂, 4 h, 16%; (3) core/shell MSN, toluene, 90 °C, N₂, 24 h.

Scheme 2 Synthesis route for rhodamine 101 embedded silica core. (1) N-hydroxysuccinimide, N,N′-dicyclohexyl carbodiimide, CH₂Cl₂, 45 °C, 48 h, 30%; (2) APTS, absolute EtOH, room temperature, N₂, 16 h; (3) NH₄OH (28 wt%), TEOS, absolute EtOH, room temperature, 24 h.

For the construction of ratiometric Zn²⁺ sensors based on core-shell mesoporous silica nanospheres (R-S-MSN), there are five main steps, as illustrated in Scheme 3: (1) Silylation of rhodamine 101 succinimide by its reaction with APTS; (2) Preparation of silica nanoparticle core by the hydrolysis of dye-APTS and its co-condensation with tetraethylorthosilicate (TEOS) in the stöber system; (3) Preparation of core-shell mesoporous silica nanoparticles using rhodamine 101 doped silica nanoparticles as the core and cetyltrimethylammonium bromide (CTAB) as porogen; (4) Extraction of CTAB template from mesoporous shell by refluxing in ammonium acetate EtOH solution; (5) Introduction of 8-AQ derivative into mesoporous shell by a mild esterification reaction with silica hydroxyls on the pore surface.

Scheme 3 Synthesis route for Zn²⁺ ratiometric sensor based on core-shell MSN. (1) absolute EtOH, room temperature, N₂, 16 h; (2) NH₄OH, TEOS, absolute EtOH, room temperature, 24 h; (3) NH₄OH, TEOS, CTAB, ethanol, water, room temperature, 16 h; (4) CH₃CO₂NH₄, EtOH, refluxed, 1 h; (5) N-(quinoline-8-yl)-2-(3-triethoxysilyl-propyl amino)-acetamide, toluene, 90 °C, N₂, 24 h.

Synthesis of 8-chloroacetylaminoquinoline. 288 mg of 8-AQ (2 mmol) and 202 mg (2.1 mmol) of N(Et)₃ in 10 mL of CH₂Cl₂ were mixed in a round flask for 20 min at 0 °C. Then 246 mg (2.2 mmol) of chloroacetyl chloride was added dropwise. The mixture was warmed to room temperature and allowed to react overnight. The solvent was evaporated in vacuum. The crude product was further purified by column chromatography (silica gel, PE/EA at 3 : 1) and a pale white solid was obtained (410 mg, 93% yield). ¹H NMR (400 MHz, CDCl₃, TMS): 4.32 (s, 2H, CH₂CO), 7.47–7.54 (q, 1H, quinoline), 7.56–7.59 (m, 2H, Quinoline), 8.18 (d, J = 8.4 Hz, 1H, quinoline), 8.76 (d, J = 9.2 Hz, 1H, quinoline), 8.87 (d, J = 5.6 Hz, 1H); 10.91 (s, 1H, NH); ¹³C NMR (100 MHz, CDCl₃): 43.36, 116.66, 121.81, 122.56, 127.97, 133.59, 136.30, 138.77, 148.70, 164.40; ESI found: 221.1 [M + H⁺]; calcd for C₁₁H₉ClN₂: 220.4.

Synthesis of N-(quinoline-8-yl)-2-(3-triethoxysilyl-propyl amino)-acetamide. A mixture of APTS (243 mg, 1.1 mmol), K₂CO₃ (274 mg, 2 mmol) in 10 mL of CH₃CN was added to 8-chloroacetylaminoquinoline, (220 mg, 1 mmol) in 20 mL of CH₃CN and refluxed for 4 h under N₂. Then the solvent was evaporated in vacuum. The crude product was purified by column chromatography (silica gel, EA) to give a yellow oil (64 mg, 16% yield). ¹H NMR (400 MHz, CDCl₃, TMS): δ = 0.79 (t, J = 8.40 Hz, 2H, SiCH₂), 1.20 (t, J = 7.80 Hz, 9H, CH₃), 1.70–1.79 (m, 2H, CH₂), 1.91 (s, 1H, NH), 2.77 (t, J = 7.6 Hz, 2H, CH₂NH), 3.54 (s, 2H), 3.80 (q, J = 7.20 Hz, 6H, CH₂CH₃), 7.41–7.44 (m, 1H, quinoline), 7.49–7.55 (m, 1H, quinoline), 8.13 (d, J = 8 Hz, 1H, quinoline), 8.81–8.87 (m, 2H, quinoline), 11.46 (s, 1H, NH); ¹³C NMR (100 MHz, CDCl₃): δ = 7.88, 18.30, 23.50, 52.94, 53.68, 58.35, 116.44, 121.45, 121.61, 127.26, 128.04, 134.40, 136.11, 139.00, 148.51, 170.93; ESI found: 406.1 [M + H⁺]; calcd for C₂₆H₃₆N₂O₄Si : 405.2.

Synthesis of rhodamine 101-succinimide. Rhodamine 101 (590 mg, 1 mmol), and N-hydroxysuccinimide (115 mg, 1 mmol) were dissolved in CH₂Cl₂ (40 ml) and N,N′-dicyclohexyl carbodiimide (206 mg, 1 mmol) was then added. The mixture was stirred at 45 °C for 48 h followed by cooling and filtering. The organic filtrate was evaporated and the residue was purified by column chromatography (silica gel, CH₂Cl₂: MeOH = 100 : 1) to afford R101-succinimide as a red solid (204 mg, 30% yield). ¹H NMR (400 MHz, CDCl₃, TMS): δ = 2.138–1.054 (m, 8H, CH₂), 2.772–2.640 (m, 8H, CH₂), 3.077–2.984 (m, 4H, CH₂), 3.559–3.484 (m, 8H, CH₂), 6.589 (s, 2H, ArH), 7.416 (d, J = 7.6 Hz, 1H, ArH), 7.797 (t, J = 7.8 Hz, 1H, ArH), 7.935 (t, J = 7.6 Hz, 1H, ArH), 8.414 (d, J = 8.0 Hz, 1H, ArH); ¹³C NMR (100 MHz, CDCl₃): δ = 19.651, 19.892, 20.554, 25.611, 27.583, 50.456, 50.937, 105.380, 112.592, 124.152, 125.587, 130.303, 131.392, 131.525, 134.790, 135.462, 151.258, 151.996, 152.640, 160.756, 168.957, ESI found: 589.3 [M + H⁺]; calcd for C₃₇H₃₅N₂O₅: 588.3.

Synthesis of R-S-MSN. (1) Silylation of rhodamine 101: APTS (136 μL, 0.58 mmol) and rhodamine 101-succinimide (20.6 mg, 3 μmol) were combined together in absolute EtOH (2 mL) and stirred magnetically overnight under the protection of N₂; (2) Preparation of silica nanoparticle core: Silylated rhodamine 101 and NH₄OH (28 wt%, 2.56 mL) were added into absolute ethanol (33 mL) and the mixture was stirred for 24 h at room temperature. Then TEOS (1.42 mL, 6.3 mmol) was added and the mixture stirred for another 24 h. Subsequently, the product mixture was centrifuged to collect the silica nanoparticles. The nanoparticles were further washed with ethanol and deionized water by centrifugation several times to remove the unreacted chemicals and then dried in vacuum; (3) Preparation of core-shell MSN: Silica nanoparticle core (100 mg) was suspended in a mixture of ethanol (18 mL) and water (41 mL). After ultrasonic treatment for 20 min, NH₄OH (0.7 mL) was added. Then a mixture of CTAB (0.24 g, 0.66 mmol), deionized water (4 mL) and absolute EtOH (2 mL) was added and the mixture stirred for a further 30 min. Subsequently, TEOS (0.44 mL, 1.97 mmol) was added and the whole mixture was stirred for 16 h. The core-shell MSN was collected by centrifugation and further washed with EtOH and deionized water by centrifugation and dried in vacuum; (4) Extraction of CTAB from the mesoporous shell: The synthesized core-shell MSN (50 mg) were suspended in EtOH (5 mL) and CH₃CO₂NH₄ (50 mg, 0.64 mmol) was added. The suspension was refluxed for 1 h, and then the materials were collected via vacuum filtration, washed extensively with EtOH and distilled water, and dried in vacuum; (5) Introduction of 8-AQ derivative into mesoporous shell: In a round bottomed flask connected to a Dean–Stark apparatus under nitrogen, 100 mg of dried core-shell MSN were suspended in 40 mL of anhydrous toluene. The mixture was heated at 140 °C to remove water by azeotropic distillation. After 20 mL of toluene was evaporated, the suspension was cooled to 90 °C and 35 mg (0.086 mmol) of N-(quinoline-8-yl)-2-(3-triethoxysilyl-propyl amino)-acetamide was added. The mixture was stirred for 24 h at 90 °C. The MSN was collected by filtration and repeatedly washed with anhydrous toluene, CH₂Cl₂, and then ethanol under ultrasonic condition. Unreacted organic material was completely removed by monitoring the fluorescence of the washing liquid. Modified MSN was dried in vacuum at 45 °C and called R-S-MSN.

3. Adsorption of Zn²⁺

The adsorption kinetics experiments were conducted as follows: 5 mg of R-S-MSN was added into 10 mL Zn²⁺ solution with an initial concentration of 60 mg L⁻¹. The adsorption isotherms experiments were conducted as follows: 5 mg of R-S-MSN was added into 10 mL Zn²⁺ solution with the concentration ranging from 13 to 60 mg L⁻¹ and collected until the adsorption equilibrium was reached. The adsorption experiments were carried out in conical flasks on a thermostat water bath shaker at 25 °C. The pH values of the above systems were maintained at 7.0. The suspensions were centrifuged at 12 000 rpm for 10 min to recover R-S-MSN and detect the adsorption efficiency by ICP/AES.

4. Characterization

X-ray powder diffraction (XRD) patterns of all samples were recorded on Rigaku D/MAX-2550 diffractometer using Cu-Kα radiation with a wavelength of 0.154 nm, operated at 40 kV and 100 mA. UV-visible absorbance spectra were obtained with a scan UV-visible spectrophotometer (Varian, Cary 500). The fluorescence spectra of all samples were recorded with a Cary Eclipse fluorescence spectrophotometer. Scanning electron microscopy (SEM) images were obtained with a JEOL JSM-6360LV microscope at an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) images were collected on a JEOL JEM 2010F, and the electron microscope was operated at an acceleration voltage of 200 kV. ¹H and ¹³C NMR spectra were obtained on a Bruker AVANCE DMX500 spectrometer in CDCl₃ with tetramethylsilane (TMS) as the internal standard. Electron impact mass spectra were recorded on an Agilent 5973N MSD instrument, and ESI mass spectra were recorded on Agilent 11100-Finnigan instrument. The Zn²⁺ concentrations were analyzed by ICP/AES data on a Varian-710-ES instrument.

Results and discussion

Fig. 1 shows the X-ray diffraction (XRD) pattern and high resolution transmission electron microscopy (HRTEM) images of core-shell R-S-MSN. The XRD pattern shown in Fig. 1a exhibits a broad peak in the low-angle region at approximately 2θ = 2° and no observable fine peak, which is often found from spherical mesoporous particles and can be ascribed to the distortion from hexagonal symmetry and indexed to (100) diffraction.²³ The HRTEM images shown in Fig. 1b and c reveal that R-S-MSN has pore channels diverging from center to fringes and the particle is highly dispersive with a uniform size of 200 nm by measuring over 100 particles, which is composed of a solid core with a radius of 65 nm and mesoporous shell with a radius of 35 nm, respectively.

Fig. 1 X-ray diffraction (XRD) pattern (a) and HRTEM images of core-shell R-S-MSN (b and c).

Fig. 2 shows the N₂ adsorption/desorption isotherms of samples before and after dye modification, which both exhibit a type IV isotherm typical for mesoporous material. As seen from the figure, the introduction of dye molecules decreases the N₂ adsorption ability of R-S-MSN and narrows the pore channel since the jump of the curve becomes moderate and shifts to the lower relative pressure. The BET surface area decreases from 384 to 232 m² g⁻¹ and the pore size calculated by BJH method decreases from 2.3 to 1.7 nm. According to the BET surface area and average pore size, the R-S-MSN particle remains open and the pore channels are accessible, although some of them are occupied by dye molecules.

Fig. 2 The nitrogen adsorption/desorption isotherms and pore size distributions (inset) of prepared core/shell R-S-MSN before and after being modified with dye.

The photostabilities of the reference unit (a, rhodamine 101) in the core and report unit (b, 8-AQ derivative) in the pore channel are investigated by being irradiated with a UV lamp for various periods, as shown in Fig. 3. No obvious fluorescence change of the reference or report units was observed after successive illumination experiments for 2 h with a UV source (300 W Mercury Lamp). The results indicate dye molecules doped into R-S-MSN are well protected by the silica matrix and not prone to photobleaching, which suggests that these core-shell nanospheres can be repeatedly applied to complex environments. Moreover, the dye leaking experiment was also performed by dissolving the nanospheres in 0.1 M phosphate buffer (pH = 7.4) and subsequently treating it with ultrasound and centrifugation. The result indicates that neither 8-AQ nor rhodamine 101 leaks from the core-shell nanospheres since the fluorescence intensity of the supernatant was close to that of the background signal. The above experiment was repeated 3 times with consistent results, which indicates that dye molecules covalently bound to the silica matrix are chemically stable and are not readily released from the solid core and mesoporous shell. Such an excellent performance is essential for the repeated application of this core-shell nanosphere as a sensor.

Fig. 3 Fluorescence variation of R-S-MSN irradiated with UV lamp for various periods, which last for 0, 0.5, 1, 1.5 and 2 h from the top to bottom: (a) Rhodamine 101 and (b) 8-AQ. The UV source is a 300 W mercury lamp.

Fig. 4 shows the fluorescence response of R-S-MSN to various pH values by plotting the normalized fluorescence intensity at different pH values and a fixed Zn²⁺ concentration of 10⁻⁴ M. The concentration of 8-AQ is 10⁻⁵ M and the pH value of the solution is adjusted using HCl and NaOH (2 M). It is obvious that the pH value of the solution has a great influence on the fluorescence intensity of R-S-MSN which shows two steep increases in the range of 5–6.5 and 8–10. These two increases can be ascribed to the protonation of 8-AQ in the strong acidic conditions and the dissociation between metal ions and 8-AQ in strong basic conditions, respectively. However, the fluorescence intensity remains unchanged in the pH range 6.5–8. Therefore, we fixed the pH value at 7 for the following studies. Moreover, dye leakage at different pH values is still not observed, which further proves the good chemical stabilities of 8-AQ and rhodamine encapsulated into R-S-MSN.

Fig. 4 Fluorescence response of R-S-MSN (10⁻⁵ M) to different pH values (3–12) in the presence of 10⁻⁴ M Zn²⁺ (EtOH-water solution with 30 vol% EtOH).

Fig. 5 shows the fluorescent intensity variation of 8-AQ introduced into the mesoporous shell (10⁻⁵ M) in the presence of various metal ions. It can be found that the fluorescence intensity of 8-AQ dramatically increases after the addition of Zn²⁺ (10⁻⁵ M). However, the addition of other metal ions such as Fe³⁺, Pb²⁺, Cd²⁺ and Ag⁺ results in no obvious change of fluorescence intensity. Generally, the detection efficiency of a sensor is determined by the coordinating and size matching state between metal ions and the recognition site of the sensor molecule. It is well known that ions such as Zn²⁺, Pb²⁺ and Ag⁺ have a high thermodynamic affinity for chelate ligands composed of N, O and S and fast metal-to-ligand binding kinetics. The failed detection of other metal ions such as Ag⁺ and Pb²⁺ should be attributed to the inefficient coordination between ions with large size and chelate groups. Furthermore, Zn²⁺ and Cd²⁺, both belonging to group IIB, often exhibit highly similar chemical properties and induce similar spectral changes while coordinating with fluorescent sensors, which are difficult to selectively detect. However, it is found that Cd²⁺ actually has no affect on the fluorescence intensity of R-S-MSN, while Zn²⁺ results in a dramatic increase in the fluorescence intensity. The above results well illustrate the high detection selectivity of R-S-MSN to Zn²⁺. Moreover, the interference study of different ions such as K⁺, Pb²⁺ and Ag⁺ to Zn²⁺ indicates that the presence of other metal ions almost has no interference effect on Zn²⁺.

Fig. 5 Bar graph of fluorescence intensity of 8-AQ in the pore channel of R-S-MSN (10⁻⁵ M) as I/I₀, where I and I₀ denote fluorescence intensities of 8-AQ in the presence and absence of various metal ions (10⁻⁴ M, EtOH–water solution of 0.05 M HEPES buffer, pH = 7.0 and λ_ex = 324 nm).

Fig. 6 shows the ratiometric performance of R-S-MSN for the detection of Zn²⁺ with various concentrations. As seen from Fig. 6a, the fluorescence intensity of 8-AQ as the report unit (I_P) gradually increases with increasing Zn²⁺ concentrations, while the fluorescence intensity of rhodamine 101 as the reference unit (I_R) hardly changes, which means the reference dye molecules encapsulated into the solid core part of R-S-MSN are well protected by the silica matrix, thus supplying a stable fluorescence as the reference signal. The intensity ratio between the report and reference units versus the concentration of Zn²⁺ is plotted and shown in Fig. 6b. The I_P/I_R value first shows a linear increase with the increasing Zn²⁺ concentration, then an inflection point when the Zn²⁺ concentration increases to 10⁻⁵ M, and ultimately remains constant for the higher Zn²⁺ concentration. The appearance of the inflection point can be attributed to the formation of a complex between Zn²⁺ and 8-AQ with a 1 : 1 mole ratio (Fig. 6c). Moreover, the association constant value of logK = 3.2 is obtained through calculating with a method developed by Descalzo et al.²⁴

Fig. 6 (a): Fluorescence spectra of report and reference dye molecules in the presence of Zn²⁺ with different concentrations, from bottom to top (0 M, 5 × 10⁻⁸ M, 4 × 10⁻⁷ M, 8 × 10⁻⁷ M, 1 × 10⁻⁶ M, 3 × 10⁻⁶ M 4 × 10⁻⁶ M, 5 × 10⁻⁶ M, 6 × 10⁻⁶ M, 7 × 10⁻⁶ M, 8 × 10⁻⁶ M, 1.2 × 10⁻⁵ M, 2 × 10⁻⁵ M, 3 × 10⁻⁵ M, 6 × 10⁻⁵ M and 1 × 10⁻⁴ M) in EtOH–water solution (30%) of 0.05 M HEPES buffer (pH = 7.0). The report and reference dyes are excited at 324 and 550 nm, and show emission wavelengths at 483 and 600 nm, respectively; (b): The fluorescence intensity ratio between report and reference dye molecules; (c) Illustration of turn-on fluorescence response of R-S-MSN to Zn²⁺.

To further evaluate the detection sensitivity of R-S-MSN to Zn²⁺, we adopt a 10% increase of the fluorescence intensity ratio in the presence of Zn²⁺, which is about 5 × 10⁻⁸ M, as found from the insert of Fig. 6b. Moreover, the detection limit of R-S-MSN to Zn²⁺ is calculated to be 2 × 10⁻⁸ M by using eqn (1), where x_min is the minimum analytical signal that can be detected, x₀ is the average fluorescence intensity of blank R-S-MSN solution in the same conditions, K is the confidence level (generally set as 3) and S₀ is the standard deviation of the blank.

x_min = x₀ + KS₀ (1)

The appealing detection sensitivity can be explained as follows: for the homogeneous system, unless the concentration of the ions or sensors is very high, significant increases in fluorescence requires fast diffusion to bring the sensor molecules and ions into contact. For an extreme situation, an effective fluorescence increase is expected if dozens of sensor molecules center around Zn²⁺ in a nanometre-sized circle region. However, it is known that high concentrations of dye molecules often lead to a significant fluorescence self-quench resulting from intermolecular collisions since all of molecules are highly free in the solution. For the R-S-MSN sensor, a large amount of 8-AQ can be introduced into mesoporous shell of R-S-MSN. The average dye molecule numbers doped into each R-S-MSN particle is calculated as follows: first, the standard curve of absorbance versus concentration for 8-AQ was plotted; then the loading amount of 8-AQ molecules per gram of R-S-MSN was calculated from the maximum absorption intensity of filtrated 8-AQ solution. Subsequently, the numbers per gram of R-S-MSN nanoparticles was determined by drying and weighing a certain volume of nanoparticle solution. As calculated, one R-S-MSN particle contains about 4300 quinoline molecules, which means the local concentration of 8-AQ in the mesoporous shell is high to 0.04 M. However, no significant fluorescent quench is found to be attributed to the generally observed intermolecular collisions in homogeneous solution. Different from the situation in homogeneous solution, quinoline is covalently fixed in the pore channel of R-S-MSN as a highly dispersive state since R-S-MSN has abundant pore channels and hydroxyl groups. Therefore, intermolecular collisions can be effectively avoided. Moreover, the negatively charged pore surface of mesoporous silica makes the sensor more accessible to Zn²⁺. When Zn²⁺ enters the pore channel, it is surrounded by quinoline enriched in a restricted nanosized space and the fluorescence can be more efficiently increased due to the high local concentration of quinoline in mesoporous pore channels, even though the total concentration of quinoline in solution is low.

Besides the sensing performance, we also investigated the adsorption capacity of R-S-MSN to Zn²⁺. The adsorption capacity was calculated according to eqn (2) by studying the mass balance of Zn²⁺, where q_e is adsorption capacity of R-S-MSN (mg g⁻¹), C₀ and C_e are the initial and equilibrium concentrations of Zn²⁺ (mg L⁻¹), M is the mass of R-S-MSN (g), and V is the volume of added solution (L).

Fig. 7a shows the adsorption kinetic curve of R-S-MSN to Zn²⁺. It can be found that the uptake of Zn²⁺ by the nanosphere is a very fast process, which finishes in about 1 min. This fast process is attributed to the porous structure of R-S-MSN and the strong affinity of the fluorescent receptor towards Zn²⁺. Afterwards, the adsorption capacity increases slowly, and the ultimate uptake of Zn²⁺ is 17.8 mg g⁻¹. The adsorption capacity of R-S-MSN for Zn²⁺ was also investigated at different initial metal concentrations (13–60 mg L⁻¹). The adsorption equilibrium data were fitted with the Langmuir adsorption isotherm model as shown in eqn (3), where C_e, q_eb and q_m represent the equilibrium solute concentration (mg L⁻¹), equilibrium adsorption capacity (mg g⁻¹), Langmuir adsorption equilibrium constant (L mg⁻¹) and maximum adsorption capacity (mg g⁻¹) of R-S-MSN, respectively.

Fig. 7 (a) Effect of time on adsorption and (b) Langmuir isotherm for the adsorption of Zn²⁺ by R-S-MSN.

The ultimate experimental adsorption data fitted with the Langmuir isotherm equation is shown in Fig. 6b . From Fig. 6b, it can be found that the values of b and q_m are 4.13 L mg⁻¹ and 17.85 mg g⁻¹, respectively. The correlation coefficient value, R, is calculated as 0.9994, which means that the adsorption process can be well described by the Langmuir adsorption isotherm. Such an isotherm is dependent on the amount of the available adsorption sites on the surface of the adsorbent. The high maximum adsorption capacity indicates that R-S-MSN is a good candidate for a metal ion adsorbent.

Conclusions

In conclusion, we have successfully prepared a novel ratiometric fluorescence sensor based on core-shell R-S-MSN with high sensitivity and selectivity to Zn²⁺. The ratiometric sensor obtained by simply arranging the reference and report units in the core and shell of R-S-MSN presents a convenient and effective method for the construction of a metal ion fluorescent sensor. Such a nanosphere also shows an excellent adsorption capacity to Zn²⁺, which is expected to be a robust alternative for simultaneous detection and removal of heavy metal ions.

Acknowledgements

This work has been supported by the National Nature Science Foundation of China (21007016, 20773039 and 20977030), National Basic Research Program of China (973 Program, 2007CB613301and 2010CB732306), the Science and Technology Commission of Shanghai Municipality (10520709900, 10JC1403900) and the Fundamental Research Funds for the Central Universities.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra01106a

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