A naked-eye pH-modulated ratiometric photoluminescence sensor based on dual-emission quantum dot@silica nanoparticles for Zn²⁺ and IO₃⁻

Abstract

In this study, we report development of a ratiometric photoluminescence (PL) sensor comprising dual-emission quantum dots (QDs)@silica nanoparticles for the detection of Zn²⁺ and IO₃⁻. The PL signal of the red-emitting QDs in the silica nanoparticles core worked as a reference, while the PL signal of the green-emitting QDs covalently linked onto the silica surface could be selectively quenched or restored by the analyte. After PL quenching of the green-emitting QDs by phenanthroline (Phen), Zn²⁺ can recover the PL under alkaline conditions via the formation of a Zn–Phen complex in the solution. Besides, IO₃⁻ can interact with the green-emitting QDs through oxidation–reduction reactions under acidic conditions, leading to further PL quenching. Under the optimized conditions, linear relationships between the PL intensity ratio of the ratiometric system and ion concentration were obtained from 5 to 100 μM for Zn²⁺ and from 5 to 150 μM for IO₃⁻. The limits of detection (LOD) for Zn²⁺ and IO₃⁻ were 1.15 and 1.76 μM, respectively. The proposed method was sensitive, highly selective and intuitional. It has been successfully applied towards the determination of Zn²⁺ and IO₃⁻ in serum samples and table salt with satisfactory results.

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1. Introduction

To date, a series of functional nanoparticles has been utilized to establish various types of nanosensor systems such as QDs, Au nanoparticles, Fe₃O₄ nanoparticles, and carbon dots.^1–3 Among them, QDs possess distinct optical properties, including broad excitation spectra, narrow emission peaks, high quantum yields and resistance to photobleaching.⁴ QDs with different sizes can emit different bands, affording the application of incorporating different-sized QDs in one nanosensor or nanoprobe. In addition, changes in the polychrome color provide convenience for detection by the naked eye. QD-based fluorescence methods have been widely applied to fluorescent labeling, biological imaging and sensing systems due to their simplicity, sensitivity, easy operation and low cost features. However, there are still two key questions in the development of QD-based PL sensors: the reliability and scope of their applications. On the one hand, most of the reported QD-based sensors are based on the quenching or enhancement of a single PL signal. These single lumophore-based sensors tend to be compromised by some other factors such as the instrumental efficiency and environmental fluctuations. On the other hand, the QD-based PL sensing systems modified using different chemical methods have been extensively exploited in the quantitative determination of cations, especially transition metal ions. However, there are few reports on their use for anion detection.⁵ Therefore, designing a variety of more sensitive and reliable QD-based PL sensors suitable for cations and anions is of great significance.

To improve the sensing reliability, ratiometric sensors have been proposed. Ratiometric PL sensors comprising two signal reporters have attracted increasing attention in recent years owing to their remarkable advantages.^6,7 In the ratiometric PL sensor systems, one signal transducer works as the analyte-sensor, whereas the other signal reporter maintains a stable signal as a reference. Two signal reporters provide a better measured response to the analyte by reducing the experimental fluctuations. Thus, the ratiometric PL sensors greatly improve the sensitivity and reliability at trace quantity levels via self-calibration of two or more different PL signals.^8–10 For example, a silica nanobead-based ratiometric Cu²⁺ sensor was reported.¹¹ RBITC (response dye) was covalently linked on the surface of silica nanoparticles, whereas FITC (reference dye) was doped in the interior. The RBITC emission band decreased through quenching in the presence of Cu²⁺, while the reference FITC was unaffected. In addition, we have designed a direct naked-eye colorimetric analysis of Hg²⁺ based on the bi-color QDs multilayer films.¹²

Zinc exists in the human body in the form of Zn²⁺ and is the second most abundant transition metal in the body.¹³ Zn²⁺ serves as a catalytic and structural co-factor, affecting gene expression, neural related signal transmission, and facilitates enzyme regulation. Disorder of Zn²⁺ metabolism can result in various diseases, such as Alzheimer's disease, epilepsy and infantile diarrhea.^14,15 Iodine is a necessary trace element during the growth of the human body. The lack of iodine can lead to thyromegaly, and can impact people's mental development at the same time. Long-term excessive iodine intake can also cause diseases and can affect the health of the human body.^16–18 Introducing potassium iodate in table salt is the most simple and effective method to prevent iodine deficiency. To date, many PL sensors have been designed for Zn²⁺ or IO₃⁻ detection based on proteins, nanoparticles, rhodamine, fluorescein, and quinoline derivatives.^19–22 In addition, a number of Zn²⁺ ratiometric sensors have been proposed such as polyethylenimine conjugated [Ru(bpy)₃]²⁺@SiNPs ratiometric fluorescent nanoprobe, squaraine based ratiometric PL probe, tetrazolylpyridine-based ratiometric fluorescent chemosensor, and phenylbenzimidazole derivative-based ratiometric PL probe.^23–26 Nevertheless, the ratiometric PL sensors based on QDs for Zn²⁺ and IO₃⁻ in one system have been rarely reported.

In this study, we have developed a novel type of dual-emission QD@silica nanoparticles-based ratiometric PL sensor (QSR) for the detection of Zn²⁺ and IO₃⁻. As shown in Scheme 1, the QSR consisted of two different CdTe QDs, namely, the red-emitting QDs were embedded in the core of silica nanoparticles and the green-emitting QDs were covalently linked onto the silica surface. The red-emitting QDs in the silica nanoparticles emitted stable red PL, avoiding the interference of the analyte, and their relatively constant PL intensity served as an internal reference to eliminate the instrument and surrounding environmental interference. In the presence of Phen, the PL intensity of the green-emitting QDs was quenched via the electronic interaction between the QDs and Phen, and the PL intensity ratio of the QSR was decreased accordingly. Under alkaline pH conditions, the PL intensity of the green-emitting QDs can be selectively recovered upon the addition of Zn²⁺. On the contrary, IO₃⁻ can continue to quench the PL intensity of the green-emitting QDs under acidic pH conditions due to the oxidation–reduction reactions between IO₃⁻ and CdTe QDs. Therefore, QSR can detect Zn²⁺ and IO₃⁻ with high sensitivity and selectivity via the modulation of the pH.

Scheme 1 The schematic illustration of the QSR structure and the detection principle for Zn²⁺ and IO₃⁻.

2. Experimental section

2.1 Materials and apparatus

All the chemical reagents were of analytical reagent grade and were used without further purification. Tellurium powder, CdCl₂, NaBH₄ and tetraethoxysilane (TEOS) were purchased from Aldrich Chemical Co. Mercaptosuccinic acid (MPA) was purchased from J&K Chemical Co. 1-Ethyl-3-[3-dimethylaminopropyl] carbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS), 3-mercaptopropyltrimethoxysilane (MPS) and 3-aminopropyltriethoxysilane (APTS) were purchased from Sigma-Aldrich Corporation. Phen, ammonium hydroxide, Zn(NO₃)₂·6H₂O, KIO₃, NaCl and other chemicals used were obtained from Beijing Dingguo Biotechnology Co. Ltd. 10 mM phosphate buffer solutions (PBS) of various pH values were prepared. The water used in all the experiments had a resistivity higher than 18 MΩ cm.

The fluorescence experiments were performed on a RF-5301 PC spectrofluorophotometer (Shimadzu Co., Kyoto, Japan) equipped with a xenon lamp at right-angle geometry. A 1 cm path-length quartz cuvette was used during the measurements of the PL spectrum. ICP (OPTIMA 3300DV) was used as a reference method for measuring serum zinc. All the pH measurements were made with a PHS-3C pH meter (Hangzhou, China).

2.2 Synthesis of CdTe QDs

CdTe QDs were synthesized by refluxing routes described in a previous method.²⁷ Briefly, the precursor sodium hydrogen telluride (NaHTe) was produced as an aqueous solution by the reaction of sodium borohydride (NaBH₄) with Te powder at a molar ratio of 2 : 1. Then, 1.25 mM CdCl₂ solution at pH 11.4 and the stabilizing agent MPA were placed into a 250 mL three-necked flask under an N₂ atmosphere. The molar ratio of Cd²⁺/MPA/HTe⁻ was 1 : 2.4 : 0.5. A fresh precursor solution was injected into the CdCl₂ solution, and the solution was heated at reflux (100 °C) with stirring under a condenser. Stable, water-compatible MPA-capped CdTe QDs were obtained. The PL emission wavelengths of the green and red emissive CdTe QDs used in the present experiments were 555 nm and 654 nm, respectively. The concentrations of both QDs were 1 μM.

2.3 Preparation of QD@silica nanoparticles and surface modification

QD@silica nanoparticles were synthesized using a modified process based on the Stöber method.^28,29 In a typical process, 15 mL ultrapure water, 40 mL ethanol, and 7 mL red-emitting CdTe QDs solution were mixed with stirring in a 100 mL flask for 10 min. The flask was coated with an aluminum foil, 20 μL of MPS was added, and the mixed solution was stirred for 12 h. Then, 0.5 mL of TEOS and ammonium hydroxide were added into the solution, and the solution was stirred for another 12 h. Then, 100 μL of APTS was introduced into the above mentioned mixture to modify the silica surface with amino groups and stirred for another 12 h. After the reaction, the resulting silica nanoparticles embedded with red-emitting QDs were obtained by centrifugation and washed with ethanol and ultrapure water for several times to remove the unreacted chemicals. Ultimately, the amino-modified QD@silica nanoparticles were redispersed in 10 mL ultrapure water and preserved for further experiments.

2.4 Preparation of dual-emission QD@silica nanoparticles-based ratiometric PL sensor (QSR)

0.5 mL of the green-emitting QDs solution was dispersed in 1 mL H₂O in a flask. 2 mL of EDC/NHS (2 mg mL⁻¹) was added and the solution was stirred for 15 min. Then, 3 mL of the amino-modified QD@silica nanoparticles was introduced, and the mixture was stirred for 4 h in the dark. The resulting dual-emission silica nanoparticles were centrifuged and washed three times with ultrapure water. The final product QSR was dispersed in 10 mL ultrapure water for further use.

2.5 Photoluminescence assays

In the experiments, all PL spectral measurements were performed under the same conditions: the slit widths of the excitation and emission were 5 nm and 10 nm, respectively, and the excitation wavelength was set at 400 nm. The emission was scanned from 420 to 780 nm. In the PL “turn off” step, 20 μL of QSR and different concentrations of Phen solution were placed into a 2.0 mL calibrated test tube. PL spectrum was obtained and the corresponding PL intensity ratio was calculated. In the procedure for detecting Zn²⁺ and IO₃⁻, different concentrations of Zn²⁺ and IO₃⁻ solution were added into the above mentioned Phen-QSR solution under alkaline or acidic conditions and the corresponding PL spectra were recorded. The relationships between Zn²⁺ or IO₃⁻ concentration and the PL intensity ratio of the QSR were plotted and the calibration curves were established.

2.6 Detection of Zn²⁺and IO₃⁻ in real samples

Fresh human blood samples were supplied from China-Japan Union Hospital, a hospital in Changchun. All the blood samples were mixed with acetonitrile (acetonitrile/blood was 1.5 : 1) and centrifuged at 10 000 rpm for 10 min. The supernatant serum samples were obtained by eliminating the large molecules and proteins. The serum samples were diluted by a factor of 2 with PBS buffer before analysis and different concentrations of Zn²⁺ were added to prepare the spiked samples. These serum samples were analyzed via the PL measurements under the optimal conditions. Table salt samples were purchased from local markets and dried at 120 °C for 6 h. Subsequently, a sample solution was prepared by dissolving 10 g of table salt in 50 mL of ultrapure water. The table salt samples were diluted by a factor of 2 with PBS buffer, and different concentrations of IO₃⁻ were added to prepare spiked samples.

3. Results and discussion

3.1 Characteristics of the QSR

In this study, ratiometric PL sensor QSR was prepared via the formation of red-emitting QD@silica nanoparticles and subsequent covalent linking of the green-emitting QDs on the silica surface, as shown in Scheme 1. The PL emission spectra of the green-emitting QDs, red-emitting QDs and the QSR are shown in Fig. 1. The green-emitting and red-emitting QDs have an emission maximum at 555 nm and 654 nm, respectively. The QSR exhibited a stable dual-emission PL spectrum (562 and 651 nm) under single wavelength excitation. The PL emission peak of the green-emitting QDs had a slight red shift from 555 nm to 562 nm after being linked on the silica surface. This indicated the surface state of the QDs was changed when chemically bonded on the silica surface. Moreover, the larger volume and surface area of the silica nanoparticles also led to a red shift of the PL emission peak.^30,31

Fig. 1 The PL spectra of green-emitting QDs (a), red-emitting QDs (b) and QSR (c).

3.2 PL recovery and quenching of Phen-QSR system with Zn²⁺ and IO₃⁻

The QSR exhibited a stable dual-emission PL peak in PBS buffer, as shown in Fig. 2 curve a. When Phen was introduced into the QSR system, there was a gradual attachment of Phen onto the QSR surface due to the affinity of the two nitrogen atoms of Phen with Cd atoms on the surface of the QDs. As a result, the PL intensity of the green-emitting QDs was quenched, while the red PL from the QDs in the silica nanoparticles remained stable. Phen quenched the PL of the green-emitting QDs primarily by an electronic interaction rather than a “surface chemistry” interaction. Upon excitation, the photogenerated holes on the QDs were preferentially transferred to the Phen ligands and trapped, preventing effective hole/electron recombination.³² The relatively constant red PL served as an internal reference to improve the sensitivity and reliability of the sensor. Fig. 2 curve b shows that the PL intensity ratio (I₅₆₂/I₆₅₁) of the QSR system decreased upon the addition of Phen. Accordingly, the PL colors of the QSR solution changed from yellow to pale red under a UV lamp, which can be seen with the naked eye. This reveals that the QSR gradually emitted red PL of the inner QDs in the silica nanoparticles with the quenching of green-emitting QDs. Upon the addition of Zn²⁺, the PL intensity ratio (I₅₆₂/I₆₅₁) of the Phen-QSR system increased (curve c in Fig. 2) and the PL color of the ratiometric solution turned orange to the naked eye under a UV lamp. This was due to the fact that Phen was dissociated from the surface of green-emitting QDs by Zn²⁺ and a soluble Zn–Phen chelate was formed in the solution. The electronic interaction between Phen and QDs was terminated, leading to the restoration of the PL intensity of the QDs. When the molar ratio of the added Zn²⁺ and Phen was higher than 1 : 2, the PL intensity of the green-emitting QDs cannot recover any more, suggesting that the chelating stoichiometric ratio of Zn²⁺ to Phen was 1 : 2. To verify the occurrence of the chelating reaction rather than the influence of Zn²⁺ itself on the system, the PL intensity of the QSR was recorded in the presence of Zn²⁺. As shown in Fig. S1,† Zn²⁺ itself did not affect the PL intensity ratio of QSR system in a wide range of concentrations, which demonstrated that the chelating reaction between Zn²⁺ and Phen indeed occurred. When IO₃⁻ was added to the Phen-QSR system under acidic conditions, IO₃⁻ reacted with the superficial green-emitting CdTe QDs, resulting in further quenching of its PL intensity. The PL intensity ratio of QSR system subsequently decreased, as shown in Fig. 2 curve d. In addition, the QSR solution changed from pale red to dark red under a UV lamp. The reaction mechanism is expressed by the following equations:³³

IO₃⁻ + 3CdTe QDs + 6H⁺ → I⁻ + 3Te + 3Cd²⁺ + 3H₂O (1)

5I⁻ + IO₃⁻ + 6H⁺ → 3I₂ + 3H₂O (2)

Fig. 2 The PL spectra of the QSR (a), QSR-200 μM Phen (b), QSR-200 μM Phen-100 μM Zn²⁺ (c), and QSR-200 μM Phen-40 μM IO₃⁻ (d).

3.3 Optimum conditions for the detection of Zn²⁺ and IO₃⁻

A series of experiments were performed to choose the optimal conditions for the detection of Zn²⁺ and IO₃⁻. Fig. 3 shows the effect of Phen concentration on the PL intensity ratio of the QSR. Upon the addition of Phen, the PL intensity at 562 nm of the green-emitting QDs in the QSR system was gradually decreased with increasing Phen concentration. Correspondingly, the PL intensity ratio of the QSR decreased gradually. When the Phen concentration was increased to 200 μM, the PL intensity ratio (I₅₆₂/I₆₅₁) reached its minimum and remained stable. Therefore, 200 μM of Phen was used in the subsequent experiments.

Fig. 3 The effect of Phen concentration on the PL intensity ratio I₅₆₂/I₆₅₁ of the QSR. (a) 0 μM, (b) 10 μM, (c) 20 μM, (d) 50 μM, (e) 100 μM, (f) 200 μM, (g) 300 μM and (h) 400 μM. Inset: the plot of the PL intensity ratio I₅₆₂/I₆₅₁ of the ratiometric sensor toward Phen.

The pH value was of significant importance for the sensitive detection of Zn²⁺ and IO₃⁻. Thus, we investigated the effect of pH on PL quenching and recovery of the QSR system in the pH range of 4–9. The QSR system was stable under weakly acidic or alkaline conditions. At low pH values (less than pH 4), the QSR was etched by the strong acid and the stability of the sensor was greatly reduced. The results in Fig. 4A show the PL intensity ratio of the Phen-QSR system with Zn²⁺ at different pH values. It can be seen that the maximum PL intensity ratio (I₅₆₂/I₆₅₁ = 3.11) of the ratiometric system was obtained at pH 8. This indicates that the chelation between Zn²⁺ and Phen can easily occur in a weak alkaline solution, and the PL intensity of the green-emitting QDs was recovered. However, a strong basic environment can lead to the hydrolysis of Zn²⁺, which affects the efficiency of the chelation reaction. Thus, we chose PBS buffer at pH 8 for the detection of Zn²⁺. The effect of pH on the PL intensity ratio of the Phen-QSR system with IO₃⁻ is displayed in Fig. 4B. It can be seen that under alkaline conditions (greater than pH 7.5), IO₃⁻ barely quenched the PL intensity of the green-emitting QDs on the surface of the silica nanoparticles. In addition, the quenching efficiency gradually increased with increasing acidity in the range of pH 5.0–7.5, which indicated that IO₃⁻ had a strong oxidizability under acidic conditions and the oxidation–reduction reaction between IO₃⁻ and CdTe QDs can be performed.³⁴ Therefore, pH 5 was chosen in the subsequent experiments for IO₃⁻ detection. In a word, based on the modulation of pH, the QSR system can effectively determine Zn²⁺ and IO₃⁻ under alkaline or acidic conditions, respectively.

Fig. 4 (A) The effect of pH on the PL intensity ratios I₅₆₂/I₆₅₁ for the QSR upon addition of 200 μM Phen and 100 μM Zn²⁺ and (B) the effect of pH on the PL intensity ratios I₅₆₂/I₆₅₁ for the QSR upon addition of 200 μM Phen and 120 μM IO₃⁻.

The effect of reaction time on the QSR sensing system was also studied. For this purpose, QSR and Phen were mixed according to the procedure described above for different time intervals. As shown in Fig. 5, the PL intensity ratio (I₅₆₂/I₆₅₁) rapidly decreased with an increase in the reaction time and reached a minimum at 7 min. This indicated that the PL quenching of the green-emitting QDs was completed. When Zn²⁺ was added to the above mentioned solution, the PL intensity ratio of the ratiometric system increased with increasing reaction time and then remained constant over 6 min. Therefore, 7 min and 6 min were chosen as the optimal reaction time for Phen quenching and Zn²⁺ detection. Fig. 6 shows the PL intensity ratio of the Phen-QSR system in the presence of IO₃⁻ as a function of incubation time. It can be seen that the PL of the green-emitting QDs continued to quench with increasing reaction time until 6 min, suggesting that the oxidation–reduction reactions between IO₃⁻ and the CdTe QDs was complete within 6 min. Based on the above mentioned results, we chose 6 min as the optimal reaction time for IO₃⁻ detection.

Fig. 5 The effect of incubation time on the PL intensity ratio I₅₆₂/I₆₅₁ for the QSR upon addition of 200 μM Phen (■) and subsequent 100 μM Zn²⁺ (●).

Fig. 6 The effect of incubation time on the PL intensity ratio I₅₆₂/I₆₅₁ for the QSR upon addition of 200 μM Phen (■) and subsequent 120 μM IO₃⁻ (●).

3.4 Detection of Zn²⁺and IO₃⁻

Under the optimized experimental conditions, the PL intensity ratio of the Phen-QSR system was closely related to the concentration of Zn²⁺ and IO₃⁻. Fig. 7A shows that the PL intensity of the QSR at 562 nm increased gradually with increasing concentration of Zn²⁺. In Fig. 7B, it can be noticed that the PL intensity ratio (I₅₆₂/I₆₅₁) exhibited a linear response to the Zn²⁺ concentration. The linear range was from 5 to 100 μM and a good linear correlation (R² = 0.998) was obtained. The linear regression equation was I₅₆₂/I₆₅₁ = 1.1526 + 0.0215 × C (Zn²⁺) (μM). According to the IUPAC 3s criterion, the limit of detection (LOD = 3σ/s) of Zn²⁺ was calculated to be 1.15 μM. Clearly, the distinguishable color changes of the QSR solution from pale red to pale yellow under a UV lamp were observed (Fig. 7C). As shown in Fig. 8, there was a linear relationship between the PL intensity ratio (I₅₆₂/I₆₅₁) of the Phen-QSR system and IO₃⁻ concentration in the range of 5–150 μM. The calibration curve can be expressed as I₅₆₂/I₆₅₁ = 1.0242 − 0.0076 × C (IO₃⁻), (μM) and I₅₆₂/I₆₅₁ = 0.7431 − 0.0018 × C (IO₃⁻), (μM). The correlation coefficients (R²) were 0.997 and 0.998, respectively. The limit of detection (LOD = 3σ/s) for IO₃⁻ was 1.76 μM. As shown in Fig. 8C, the color of the probe solution changed from pale red to red under a UV lamp. The reaction could be clearly distinguished by the naked eye. A comparison of the present method and other methods in the linear range and the limits of detection are shown in Table S1.† ^35–41 Compared with other sensors, our QSR system for Zn²⁺ and IO₃⁻ detection offered a comparable limit of detection and dynamic range.

Fig. 7 (A) The PL spectra of Phen-QSR with different Zn²⁺ concentrations from 0 to 100 μM. (a) 0 μM, (b) 5 μM, (c) 10 μM, (d) 20 μM, (e) 40 μM, (f) 60 μM and (g) 100 μM. (B) The linear plot of the PL intensity ratio I₅₆₂/I₆₅₁ for the QSR toward Zn²⁺. (C) The photographs of the mixed solution under a UV lamp.

Fig. 8 (A) The PL spectra of Phen-QSR with different IO₃⁻ concentrations from 0 to 150 μM. (a) 0 μM, (b) 5 μM, (c) 10 μM, (d) 20 μM, (e) 30 μM, (f) 50 μM, (g) 80 μM, (h) 100 μM, (i) 130 μM and (j) 150 μM. (B) The linear plot of the PL intensity ratio I₅₆₂/I₆₅₁ of the ratiometric sensor toward IO₃⁻. (C) The photographs of the mixed solution under a UV lamp.

3.5 Interference study

To evaluate the selectivity of the nanosensor for Zn²⁺ and IO₃⁻, PL changes in the QSR system in the presence of various ions were investigated. Tolerable concentrations, the concentrations of foreign species causing less than ±5% relative error, were examined. As shown in Table S2,† the tolerable concentration ratios of interference substances for Zn²⁺ detection was 500 fold for Na⁺, K⁺, Cl⁻, and NO₃⁻, 200 fold for Mg²⁺, SO₄²⁻, and Mn²⁺, 50 fold for Ni²⁺ and Ba²⁺, and 10 fold for Fe³⁺, Cd²⁺, and Cu²⁺. Most of the interfering substances had little effect on the detection of Zn²⁺. It can be seen from Table S3† that the tolerable concentration ratios of common interference ions and oxidant ions for IO₃⁻ detection was 500 fold for Na⁺, K⁺, and Cl⁻, 200 fold for Mg²⁺, Mn²⁺, NH₄⁺, NO₃⁻, and SO₄²⁻, 100 fold for CO₃²⁻, HCO₃⁻, CH₃COO⁻, F⁻, and I⁻, and 5 fold for SO₃²⁻, NO₂⁻, and O²⁻. This indicates that these interfering ions have little interference on IO₃⁻ detection. Thus, the proposed ratiometric PL sensor used for the detection of Zn²⁺ and IO₃⁻ has higher selectivity.

3.6 Detection of Zn²⁺ and IO₃⁻ in real samples

To evaluate the practicability of the QSR system, we detected Zn²⁺ and IO₃⁻ in human serum samples and table salt, respectively. All the samples were prepared by spiking with Zn²⁺ and IO₃⁻ at different concentration levels, respectively. During the determination of IO₃⁻ in table salt samples, to eliminate the influence of NaCl, 1 mL of 3.4 M NaCl solution was added to obtain the work curve. The analytical results obtained by the standard addition method are listed in Table 1 and 2. To evaluate the accuracy of the proposed method, ICP and UV spectrophotometry methods were applied as the reference standard methods for the detection of Zn²⁺ and IO₃⁻, respectively.^42,43 It can be seen that the concentrations observed with the proposed method were in good agreement with those obtained by the reference standard methods. The recoveries of the samples were in the range of 97%–103%. The relative standard deviation (RSD) for three repeated measurements was less than 5%. The iodine content of the salt samples was in accordance with a standard level (iodine content label of the sample is 2250 μg/100 g, and it is 17.7 μM before measurement). This indicated our QSR is feasible and reliable for real sample detection.

Table 1 Determination of Zn²⁺ in human serum samples. A certain amount of human serum sample with different Zn²⁺ concentrations: (1) 20 μM, (2) 40 μM and (3) 60 μM

Sample	Original (μM)	Added (μM)	Found (μM)		Recovery (%)	RSD (%, n = 3)
			The proposed method	ICP
1	6.82	20.00	26.40	26.21	97.90	3.73
2	7.35	40.00	47.81	48.42	101.15	2.28
3	7.54	60.00	67.16	67.59	99.37	1.65

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Table 2 Determination of IO₃⁻ in table salt. A certain amount of table salt sample solution with different IO₃⁻ concentrations: (1) 20 μM, (2) 40 μM and (3) 60 μM

Sample	Original (μM)	Added (μM)	Found (μM)		Recovery (%)	RSD (%, n = 3)
			The proposed method	UV spectrophotometry method
1	18.52	20.00	39.05	38.46	102.65	2.85
2	22.37	40.00	61.41	62.81	97.60	2.71
3	20.83	60.00	81.39	81.95	100.93	1.69

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4. Conclusions

In conclusion, a novel dual-emission QD@silica nanoparticles-based ratiometric PL sensor for the detection of Zn²⁺ and IO₃⁻ has been developed. The red-emitting QDs encapsulated in the silica nanoparticles are inert to the analyte and the green-emitting QDs attached to the surface of the silica nanoparticles are specifically sensitive to the analyte. Phen quenched the PL intensity of the green-emitting QDs. Then, Zn²⁺ can react with the Phen under alkaline conditions, resulting in the PL recovery of the green-emitting QDs. IO₃⁻ can continue to quench the PL of the green-emitting QDs under acidic conditions owing to the oxidation–reduction reaction between IO₃⁻ and the QDs. Under the optimized conditions, the pH-modulated sensor shows a limit of detection of 1.15 μM for Zn²⁺ and 1.76 μM for IO₃⁻. The proposed QSR system exhibits high sensitivity and selectivity to Zn²⁺ and IO₃⁻, and has been successfully used in the real sample analysis with a simple, convenient, and facile procedure.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (Nos 21275063 and 21005029) and the Youth Science Fund of Jilin Province (20140520081JH).

References

1. F. Wang, Z. Xie, H. Zhang, C. Y. Liu and Y. G. Zhang, Adv. Funct. Mater., 2011, 21, 1027–1031.
2. Y. Li, Q. Ma, Z. P. Liu, X. Y. Wang and X. G. Su, Anal. Chim. Acta, 2014, 840, 68–74.
3. Q. Gan, X. Y. Lu, Y. Yuan, J. C. Qian and H. J. Zhou, Biomaterials, 2011, 32, 1932–1942.
4. L. H. Jing, C. H. Yang and R. R. Qiao, Chem. Mater., 2010, 22, 420–427.
5. M. Xue, X. Wang, L. L. Duan, W. Gao, L. F. Ji and B. Tang, Biosens. Bioelectron., 2012, 36, 168–173.
6. Y. J. Gong, X. B. Zhang, C. C. Zhang, A. L. Luo, T. Fu, W. H. Tan, G. L. Shen and R. Q. Yu, Anal. Chem., 2012, 84, 10777–10784.
7. Z. X. Han, X. B. Zhang, L. Zhuo, Y. J. Gong, X. Y. Wu, J. Zhen and C. M. He, Anal. Chem., 2010, 82, 3108–3113.
8. C. F. Wu, B. Bull, K. Christensen and J. McNeill, Angew. Chem., Int. Ed., 2009, 48, 2741–2745.
9. D. W. Domaille, L. Zeng and C. J. Chang, J. Am. Chem. Soc., 2010, 132, 1194–1195.
10. J. Zhou, C. Fang, T. Chang, X. Liu and D. Shangguan, J. Mater. Chem. B, 2013, 1, 661–667.
11. C. Zong, K. Ai, G. Zhang, H. Li and L. Lu, Anal. Chem., 2011, 83, 3126–3132.
12. F. P. Yang, Q. Ma, Y. Wei and X. G. Su, Talanta, 2011, 84, 411–415.
13. P. J. Fraker and L. E. King, Annu. Rev. Nutr., 2004, 24, 277–298.
14. W. C. Fischer and R. E. Black, Annu. Rev. Nutr., 2004, 24, 255–275.
15. P. A. Adlard and A. I. Bush, J. Alzheimer's Dis., 2006, 10, 145–163.
16. A. M. H. Shabani, P. S. Ellis and I. D. McKelvie, Food Chem., 2011, 129, 704–707.
17. Y. X. Hou, L. P. Liu and Z. X. Du, Phys. Test. Chem. Anal., 2011, 47, 1262–3111.
18. T. L. Wang, S. Z. Zhao, C. H. Shen, J. Tang and D. Wang, Food Chem., 2009, 112, 215–220.
19. D. E. Benson, M. S. Wisz and H. W. Hellinga, Curr. Opin. Biotechnol., 1998, 9, 370–376.
20. M. J. Ruedas-Rama and E. A. H. Hall, Anal. Chem., 2008, 80, 8260–8268.
21. G. Crivat, K. Kikuchi, T. Nagano, T. Priel, M. Hershfinkel, I. Sekler, N. Rosenzweig and Z. Rosenzweig, Anal. Chem., 2006, 78, 5799–5804.
22. Y. Chen, K. Y. Han and Y. Liu, Bioorg. Med. Chem., 2007, 15, 4537–4542.
23. Y. P. Shi, Z. H. Chen, X. Cheng, Y. Pan and C. Q. Yi, Biosens. Bioelectron., 2014, 61, 397–403.
24. P. Kaleeswaran, I. A. Azath, V. Tharmaraj and K. Pitchumani, ChemPlusChem, 2014, 79, 1361–1366.
25. Y. W. Huang, Q. Lin, J. M. Wu and N. Y. Fu, Dyes Pigm., 2013, 99, 699–704.
26. L. J. Tang, M. J. Cai, P. Zhou, J. Zhao and Y. J. Bian, J. Lumin., 2014, 147, 179–183.
27. Q. Ma, E. Ha, F. Yang and X. G. Su, Anal. Chim. Acta, 2011, 701, 60–65.
28. K. Zhang, Q. S. Mei, G. J. Guan, B. H. Liu, S. H. Wang and Z. P. Zhang, Anal. Chem., 2010, 82, 9579–9586.
29. J. L. Yao, K. Zhang, H. J. Zhu, F. Ma, M. T. Sun and H. Yu, Anal. Chem., 2013, 85, 6461–6468.
30. Y. Q. Wang, Y. Y. Zhang, F. Zhang and W. Y. Li, J. Mater. Chem., 2011, 21, 6556–6562.
31. F. Vollmer, S. Arnold, D. Braun and I. Teraoka, Biophys. J., 2003, 85, 1974–1979.
32. S. Banerjee, S. Kar and S. Santra, Chem. Commun., 2008, 3037–3039.
33. M. Fan, L. Zhang and Q. L. Liu, J. Anal. Sci., 2014, 30, 16–20.
34. C. R. Tang, Z. H. Su, B. G. Lin, H. W. Huang and Y. L. Zeng, Anal. Chim. Acta, 2010, 678, 203–207.
35. Z. P. Liu, G. Y. Li, Q. Ma and X. G. Su, Microchim. Acta, 2014, 181, 1385–1391.
36. Q. Ma, Z. H. Lin, N. Yang, Y. Li and X. G. Su, Acta Biomater., 2013, 10, 868–874.
37. H. Xu, Z. P. Wang, Y. Li, S. J. Ma and P. Y. Hu, Analyst, 2013, 138, 2181–2191.
38. B. Haghighi, H. Hamidi and L. Gorton, Electrochim. Acta, 2010, 55, 4750–4757.
39. S. Abdollah, M. K. Hussein, H. Rahman and Z. Shiva, Electrochim. Acta, 2007, 52, 6097–6105.
40. M. A. Tabrizi and L. Ebrahimi, J. Electroanal. Chem., 2014, 724, 8–14.
41. J. Jakmunee and K. Grudpan, Anal. Chim. Acta, 2001, 438, 299–304.
42. Y. J. Li, W. T. Ma, J. H. Liu and E. R. Ma, Chin. J. Prev. Med., 1997, 31, 112–113.
43. G. N. P. Silva, A. Oliveira and E. A. Neves, J. Braz. Chem. Soc., 1998, 9, 171–174.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12910a

‡ These authors contributed equally to this work.

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