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Hexametaphosphate-capped quantum dots as fluorescent probes for detection of calcium ion and fluoride
Sensors and Actuators B 232 (2016) 306–312
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Hexametaphosphate-capped quantum dots as fluorescent probes for detection of calcium ion and fluoride Siyu Liu a , Hui Wang a , Zhen Cheng b,∗ , Hongguang Liu a,∗ a b
Institute of Molecular Medicine, College of Life and Health Sciences, Northeastern University, Shenyang 110000, China Molecular Imaging Program at Stanford, Stanford University, Palo Alto, CA 94305, USA
a r t i c l e
i n f o
Article history: Received 16 December 2015 Received in revised form 1 March 2016 Accepted 17 March 2016 Available online 19 March 2016 Keywords: Quantum dots Fluorescence Calcium ion Fluoride
a b s t r a c t In this paper, we prepared fluorescence-tunable water-soluble CdS quantum dots (QDs) with hexametaphosphate as stabilizers by a simple route. The prepared CdS QDs were utilized to detect calcium ion (Ca2+ ) based on an obvious fluorescence enhancement of CdS QDs induced by Ca2+ . The results demonstrated there was an excellent linear relationship between the fluorescence intensity of hexametaphosphate-capped CdS QDs and Ca2+ concentration in the range from 10 mol/L to 400 mol/L and the detection limit for Ca2+ was 4 mol/L. Furthermore, fluoride ions (F− ) can react with Ca2+ to generate the insoluble calcium fluoride that would inhibit the Ca2+ -induced fluorescence enhancement of CdS QDs. Therefore, the designed Ca2+ -induced fluorescence enhancement system was also able to effectively detect fluoride. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Calcium ion (Ca2+ ) as an intracellular signal transmitter plays a key role in the human body. It modulates numerous physiological and pathological procedures, including cell damage and death by changing Ca2+ concentrations to modulate different cellular functions [1–3]. Among various anions, fluoride ion (F− ) is highly relevant to health care because of its important roles in dental care and treatment of osteoporosis, which is widely used as an ingredient in toothpaste, pharmaceutical agents and even drinking water [4,5]. In several underdeveloped countries, excessive F− level in drinking water has been linked to the debilitating bone disease fluorosis. Furthermore, the high concentration of F− may lead to collagen break down, thyroid activity and even cancer [6–8]. Therefore, it is necessary to develop a simple and convenient method to assay Ca2+ and F− . Until now, many analytical techniques have been explored to detect Ca2+ and F− , including electrode method, NMR analysis, colorimetric (UV) and electrochemical methods [9–13]. However, some disadvantages have been found for these techniques such as fragile instrumentation and time consuming manipulations associated with electrochemical methods, the high detection limit with colorimetric techniques, and etc [14].
∗ Corresponding authors. E-mail addresses: [email protected] (Z. Cheng), [email protected] (H. Liu). http://dx.doi.org/10.1016/j.snb.2016.03.077 0925-4005/© 2016 Elsevier B.V. All rights reserved.
In recent years, fluorescence sensor systems for Ca2+ and F− detection have attracted great attentions owing to their operational simplicity and real-time detection, etc. Most of the reported sensors are based on organic dyes with special ion recognition unites containing rich amino, hydroxyl and carboxyl groups that can bind strongly to Ca2+ or F− [15–23]. For instance, Dong et al. studied an organic salt as a fluorescent probe with ratiometric emission at 490 and 594 nm based on intramolecular charge transfer for Ca2+ determination. And the probe displayed high selectivity for Ca2+ and a large Stokes shift of 189 nm [16]. Wang and co-workers synthesized benzoselenadiazole based diarylamine as a near-infrared optical sensor for F− , and the designed sensor showed a turn-on ratiometric fluorescence signalling with F− by inhibiting the excited state intramolecular proton transfer processes [21]. However, these reported dyes based probes for Ca2+ and F− are faced with the poor water solubility which severely restricts their application in water environment systems. Moreover, synthesis of many organic dyes is complex and expensive. Compared with traditional organic dyes and fluorescent proteins, quantum dots (QDs), as a kind of inorganic nanomaterials, have attracted more attentions owing to their unique optical properties including size tunable fluorescence, narrow and symmetric emission peak with a broad excited wavelength, and excellent photostability, [24,25]. Until now, QDs such as CdTe, CdS, etc. have been widely used in DNA and protein analysis, molecular recognition, and ion detection [26–33]. For example, Chen and Rosenzweig synthesized water-soluble CdS QDs to detect copper (II) and zinc (II)
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ions [29]. Batteas and colleagues used 16-mercaptohexadecanoic acid capped CdSe QDs as a probe for copper (II) detection [30]. Ribeiro and co-workers reported a fluorometric approach for the selective determination of calcium ions level in the range from 0.80 to 3.20 mg/L, which was based on the monitoring of the fluorescent recovery of the thioglycolic acid-capped CdTe QDs previously quenched by ethylene diamine tetraacetic acid (EDTA) in aqueous solution [31]. Tang et al. developed a simple method for the determination of iodate based on the fluorescence quenching of carboxymethyl cellulose-capped CdS QDs induced by the oxidation of iodate [32]. In fact, QDs as ion probes primarily concentrated on heavy ions detection. So far, QDs as fluorescence probe for selective detection Ca2+ and F− has been rarely reported. Herein, in order to obtain Ca2+ -sensitive QDs, we selected hexametaphosphate, that is a metal chelating agent, as coating material to synthesize hexametaphosphate-capped CdS QDs. The prepared hexametaphosphate-capped CdS QDs showed a significant fluorescent enhancement with the addition of Ca2+ , because of the formation of calcium polyphosphate shell. F− can react with Ca2+ to generate insoluble calcium fluoride, and the Ca2+ -induced fluorescence enhancement would be inhibited. Therefore, the proposed CdS QDs could be used to effectively detect both Ca2+ and F− . 2. Experimental section 2.1. Apparatus The fluorescence emission and UV–vis absorption spectra were obtained by using a TECAN infinite M100 PRO Biotek microplate reader. FT-IR spectra were recorded with a Bruker IFS66 V FT-IR spectrometer equipped with a DGTS detector (32 scans).The zeta potential and dynamic light scattering (DLS) analysis were finished on Malvern Nano ZS90.Transmission electron microscopy (TEM) experiments and Energy Dispersive Spectrum (EDS) analysis were performed on a Philips Tecnai F20 TEM operating at 200 KV acceleration voltage. TEM samples were prepared by dropping the aqueous CdS QDs or CdS QDs-Ca2+ mixed solution onto carbon-coated copper grids and allowing the excess solvent to evaporate.
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The synthesis of phosphate, pyrophosphate or triphosphatecapped CdS QDs was same with above process. Briefly, CdCl2 solution (0.2 mL of 1 mol/L) was added into 20 mL of Na3 PO4 , Na4 P2 O7 or Na5 P3 O10 aqueous solution (20 mmol/L) with vigorous stirring at room temperature. After 10 min, sodium sulphide solution (120 L of 1 mol/L) was injected slowly into the mixed solution. The reaction mixture was allowed to proceed for another 1 h under vigorous stirring. 2.4. Detection of Ca2+ based on hexametaphosphate-capped CdS QDs Tris-HCl buffer solution (20 L, pH 7.6, 0.1 mol/L), synthesized hexametaphosphate-capped CdS QDs solution (20 L) and varying amounts of CaCl2 were successively added into test tube. Then the solution was diluted to 200 L with deionized water followed by the thoroughly shaking and equilibrated for 2 min. The fluorescence spectra were recorded with the excitation wavelength of 420 nm. The slit widths of excitation and emission were both 10 nm. The fluorescence intensity of the maximum emission peak was used for the quantitative analysis of Ca2+ concentration. 2.5. The effect of metal ions on hexametaphosphate-capped CdS QDs Tris-HCl buffer solution (20 L, pH 7.6, 0.1 mol/L), synthesized hexametaphosphate-capped CdS QDs solution (20 L) and individual 40 L various metal ions (5 mmol/L K+ , Na+ , Mg2+ , Mn2+ , Zn2+ , Fe3+ , Fe2+ , Ni2+ , Cd2+ , Co2+ , Cu2+ , Pb2+ , Hg2+ , Ag+ and 2.5 mmol/L Ca2+ ) were successively added into test tube. Then the solution was diluted to 200 L with deionized water followed by the thoroughly shaking and equilibrated for 2 min. The fluorescence spectra were recorded with the excitation wavelength of 420 nm. The slit widths of excitation and emission were both 10 nm. The fluorescence intensity of the maximum emission peak was used for the quantitative analysis of Ca2+ concentration. 2.6. Detection of F− based on hexametaphosphate-capped CdS QDs
2.2. Reagents All reagents were of at least analytical grade. The water used in all experiments had a resistivity higher than 18 M cm−1 . Cadmium (II) chloride (CdCl2 ), calcium chloride (CaCl2 ), potassium fluoride (KF), sodium sulfide nonahydrate (Na2 S•9H2 O), tris, sodium phosphate (Na3 PO4 ), sodium pyrophosphate (Na4 P2 O7 ), sodium triphosphate (Na5 P3 O10 ), sodium hexametaphosphate (Na6 P6 O18 ) and other metal salts all were purchased from Sinopharm Chemical Reagent Co., Ltd.
Tris-HCl buffer solution (20 L, pH 7.6, 0.1 mol/L), 20 L of hexametaphosphate-capped CdS QDs solution, different concentration of potassium fluoride, and 20 L of CaCl2 (4 mmol/L) were successively added into 200 L test tube, and then diluted to the 200 L with deionized water followed by the thoroughly shaking and equilibrated for 2 min. The fluorescence intensity of the maximum emission peak was used for the quantitative analysis of F− concentration. 3. Results and discussion
2.3. Synthesis of polyphosphate-capped CdS QDs The CdS QDs were synthesized according to the previous reports with some modifications [29]. Hexametaphosphate-capped CdS QDs were prepared as following: CdCl2 solution (0.2 mL of 1 mol/L) was added into 20 mL of Na6 P6 O18 (20 mmol/L) aqueous solution with vigorous stirring at room temperature. After 10 min, sodium sulfide solution (120 L of 1 mol/L) was injected slowly into the mixed solution. The reaction mixture was allowed to proceed for another 1 h under vigorous stirring. The obtained hexametaphosphate-capped CdS QDs solution was dialyzed in membrane tubing with a molecular weight cut-off of 3 KDa against ultrapure water to remove small molecules and ions. The purified CdS QDs solution were freeze-dried into powder and measured, and then the CdS QDs powder was dissolved in aqueous solution again with 16.3 mg/mL mass concentration.
3.1. Synthesis and characterization of hexametaphosphate-capped CdS QDs In present work, we proposed a simple route to prepare fluorescent CdS QDs with hexametaphosphate as templates, and adopted the CdS QDs as a highly selective probe for the detection of Ca2+ and F− . As shown in Scheme 1, the fluorescent CdS QDs was generated in the presence of Cd2+ and S2− ions with hexametaphosphate as stabilizers. The hexametaphosphate molecule containing six phosphate groups could act as chelating groups for passivating the surface of QDs, rendering them water-soluble and stable against aggregation [29]. The protective coat of hexametaphosphate on the CdS QDs ensures the hydrophilicity of CdS QDs and favors the further
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Scheme 1. The synthetic process of heametaphosphate-capped CdS QDs and the detection of Ca2+ and F− based on the prepared CdS QDs.
Fig. 1. Fluorescence emission spectra of CdS QDs prepared with different hexametaphosphate concentrations as stabilizers (a: 1 mmol/L, b: 2 mmol/L, c: 5 mmol/L, d: 10 mmol/L, e: 20 mmol/L, f: 40 mmol/L). Reaction condition: 10 mmol/L Cd2+ cations and 6 mmol/L S2− ions.
biosensing application. Fig. 1 offered the fluorescence changes of CdS QDs prepared with different concentrations of hexametaphosphate as stabilizers. When the hexametaphosphate concentration just reached 1 mmol/L, the prepared CdS QDs solution showed a strong fluorescence emission signal around 510 nm with 420 nm excitation wavelength. With the hexametaphosphate concentration increasing from 1 to 40 mmol/L, the fluorescence emission intensity was gradually weakening. Moreover, as shown in Fig. 1Inset, these synthesized CdS QDs simutaneously showed a red shif emission wavelength from 510 nm to 682 nm, respectively named as QDs(510), QDs(560), QDs(596), QDs(620), QDs(664) and QDs(682). Herein, we successfully employed hexametaphosphate as stabilizers prepare fluorescence-tunable CdS QDs. Lakowicz et al. previously proposed that the fluorescence emission maximum of CdS QDs is shifted to longer wavelengths with increasing particle diameter [34]. As shown in Fig.S1, the generated hexametaphosphate-capped CdS QDs were near spherical, and the average size of QDs(510) and QDs(682) was respectively 3.7 nm and 5.6 nm. And the increased size of CdS QDs was consistent with red shift of their emission wavelength. Hexametaphosphate, as a chelating agent, can specifically combine with Ca2+ cation to generate soluble calcium polyphosphate complex. Therefore, these hexametaphosphate-capped CdS
QDs are potential probes to specially recognize Ca2+ cation. We respectively investigated the fluorescence response of these hexametaphosphate-capped CdS QDs with various fluorescence emission wavelengths to Ca2+ . As shown in Fig.S2, Ca2+ can induce the fluorescence enhancement of these hexametaphosphatecapped QDs, and among these CdS QDs, QDs(664) with the fluorescence emission peak around 664 nm had the most significant fluorescence enhancement with the addition of Ca2+ . Therefore, we adopted the QDs(664) as fluorescent probe for Ca2+ detection. Fig. S3 indicated that QDs(664) showed a well-shaped emission peak around 664 nm excited with 420 nm that was a characteristic trap state emission arising from the surface defect [33]. And there is an increasing absorption below 450 nm that is the result of 1Sh -1Se excitonic transition characteristic of CdS QDs [33–35]. The FT-IR spectra of CdS QDs prepared with hexametaphosphate as stabilizers were shown in Fig.S4. The characteristic absorption band of hexametaphosphate were clearly observed through the asymmetric stretching vibrations of the PO2 (1280 cm−1 ), the out of phase symmetrical stretches (1060 cm−1 ), and the P O stretches of the main chain (930 cm−1 ) in Fig.S4, which indicated the successful capping of hexametaphosphate on the surface of the CdS QDs. Furthermore, we tested the stability of QDs(664) under various pHs and ionic environments. Fig.S5 indicated the prepared hexametaphosphate-capped CdS QDs was sensitive to pH environment, which showed weak fluorescence emission intensity in acid environment and an obvious enhancement with the pH increase from 5.6 to 7.6. In alkaline environment, the phosphate groups of hexametaphosphate were easily deprotonated and offered more negative charges, which were expected to strengthen the coordination effect between cadmium on the surface of CdS QDs and hexametaphosphate. The hexametaphosphate provided well surface protection for aqueous CdS QDs and effectively reduced non-radiative recombination by minimizing the surface defects of QDs [36,37]. 3.2. The fluorescence enhancement induced by Ca2+ We further investigated the fluorescence response of various polyphosphate-capped CdS QDs to Ca2+ . The water-soluble CdS QDs capped by phosphate, pyrophosphate or triphosphate was respectively prepared in aqueous solution, which was similar to the synthesis of hexametaphosphate-capped CdS QDs. The emission maximum of phosphate, pyrophosphate, triphosphate or hexametaphosphate-capped CdS QDs was respectively observed at 490 nm, 480 nm, 520 nm and 664 nm (Fig.S6). The
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Fig. 2. (A) The zeta potential and (B) DLS measurements of hexametaphosphate-capped CdS QDs solution (a), hexametaphosphate-capped CdS QDs solution with 0.1 mmol/L Ca2+ ions (b), hexametaphosphate-capped CdS QDs solution with 0.5 mmol/L Ca2+ ions (c).
effect of various polyphosphate ligands on the fluorescent response of CdS QDs to 0.5 mmol/L Ca2+ were shown in Fig.S6. Only hexametaphosphate-modified CdS QDs provide a nearly 3-folds fluorescence enhancement in the presence of Ca2+ . And the fluorescence intensity of other polyphosphate-capped QDs just showed weak changes with addition of Ca2+ , comparing to the emission of these CdS QDs in an ion-free solution. Fig.S7 showed the temporal evolution of fluorescence intensity of hexametaphosphate-modified CdS QDs after the addition of Ca2+ ions. And it could be seen that the fluorescence rapidly increased and remained nearly constant after 1 min. As shown in Fig. 2A, the zeta potential of the hexametaphosphate-capped CdS QDs solution was determined to be −38.7 mV. And with the increase of Ca2+ ions concentration, the zeta potential of CdS QDs-Ca2+ mixture gradually increased, which indicated Ca2+ was adsorbed to the surface of CdS QDs to neutralize the negative charges of hexametaphosphate. Meanwhile the dynamic light scattering analysis demonstrated that the addition of Ca2+ ions leads to the obvious increase of the particle size of CdS QDs solution (Fig. 2B). And the morphology and size changes of CdS QDs solution after the addition of Ca2+ were further observed by transmission electron microscopy (Fig.S8). It could be found that the generated CdS QDs were near spherical with well-dispersion and the sizes distribution gave an average size of 4.7 nm with ±1.1 nm. After the addition of Ca2+ ion solution, the CdS QDs nanoparticles tend to agglomerate into larger particle that was coated by a new shell structure, which might be caused by the generation of calcium hexametaphosphate shell (Fig.S8B). In order to confirm the elemental composition of the large particle, energy dispersive spectrum analysis was performed. As shown in Fig.S9A, the element Cd, S and P were respectively probed in the energy dispersive spectrum of hexametaphosphatecapped CdS QDs, which revealed that the molar ratio of Cd/S/P was about 1/1/7.1. And the Cd, S, P and Ca element were all detected in the energy dispersive spectrum of CdS QDs-Ca2+ mixture with
the molar ratio 1/1/7.1/2.9, which proved the formation of calcium hexametaphosphate shell on the surface of QDs (Fig.S9B). The Ca2+ -induced fluorescence enhancement can be attributed to the formation of calcium hexametaphosphate shell on the surface of CdS QDs. The calcium hexametaphosphate, as an excellent coating material, is also a good matrix for hosting luminescence materials [38,39]. Therefore the enhanced transition energy transfer would give rise to the increase of population of inversion symmetry of the fluorescent CdS QDs. Moreover, the fluorescence lifetime of hexametaphosphate-capped CdS QDs was determined to be 15.1 ns, and the fluorescence lifetime of the CdS QDs-Ca2+ mixed system was 29.6 ns (Fig. S10). The Ca2+ -induced the extension of fluorescence lifetime of hexametaphosphate-capped CdS QDs was because that the overlap of electron and hole wave function decreased with the formation of calcium hexametaphosphate shell on the surface of CdS QDs. The effect of various concentration of Ca2+ on the fluorescence emission of hexametaphosphate-capped CdS QDs was provided in Fig. 3. The fluorescence emission intensity is significantly enhanced with the increase of Ca2+ concentration. The enhancement effect of Ca2+ ions on fluorescence of CdS QDs showed concentration dependence. The fluorescence enhancement is attributed to the formation of a calcium hexametaphosphate shell to passivate the surface of the CdS QDs that produce more new radiative centers at the CdS QDs-calcium hexametaphosphate complex and stimulate block nonradiactive electron/hole recombination defect sites on the surface of the CdS QDs [40,41]. Moreover, the fluorescence emission peak of CdS QDs showed a blue-shift accompanied by the increase of Ca2+ concentration. And it is because the adsorption of Ca2+ to the surface of CdS QDs neutralized the negative charges of hexametaphosphate, and reduced the polarizability of surrounding water molecules, which would result in the decrease of Stokes shift. Fig. 3Inset demonstrated there was a good linear relationship between the relative fluorescence intensity F/F0 (F0 is the original fluorescence intensity of CdS QDs, and F is the fluorescence intensity of CdS QDs with the addition of various concentration of Ca2+ )
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Fig. 3. The fluorescence emission spectra of hexametaphosphate-capped CdS QDs incubated with different concentration of Ca2+ (0, 10, 20, 40, 60, 80,120, 200, 250, 300, 350, 400, 500 mol/L) for 2 min. Inset: The linear plots of F/F0 versusthe Ca2+ concentration in the range from 10 to 400 mol/L.
Fig. 5. The fluorescence emission spectra of hexametaphosphate-capped CdS QDsCa2+ (0.4 mmol/L) mixed system incubated with different concentration of F− (0, 10, 25, 50, 75, 100, 150, 200, 250, 300, 400, 500, 700, 900 mol/L) for 2 min. Inset: The linear plots of F/F0 versus the F− concentration in the range from 10 to 300 mol/L.
and Ca2+ concentration in the range from 10 to 400 mol/L. And the relationship could be described as following regression equation:
of Ca2+ [12,15–18], our proposed method for Ca2+ detection based CdS QDs simplified the detection process and avoid the complex and high-cost probe synthesis.
F/F0 = 1.075 + 0.005[Ca2+ ], mol/L(1) The corresponding regression coefficient (R2 ) is 0.994 with 4 mol/L detection limit for Ca2+ calculated following the 3 IUPAC criteria. And we further investigated the effect of various metal ions including K+ , Na+ , Ca2+ , Mg2+ , Mn2+ , Zn2+ , Fe3+ , Fe2+ , Ni2+ , Cd2+ , Co2+ , Cu2+ , Pb2+ , Hg2+ and Ag+ on the fluorescence of CdS QDs (Fig. 4). It was found that the fluorescence of CdS QDs could be enhanced by 3fold with the addition of Ca2+ for 2 min, which might be attributed to the strong chelating ability of hexametaphosphate with Ca2+ ions. The heavy metal ions Cu2+ , Hg2+ and Ag+ could induce the significant fluorescence quenching of CdS QDs, and the interference of Cu2+ , Hg2+ and Ag+ on the fluorescence of CdS QDs could be eliminated by adding potassium iodide to generate insoluble substance. Therefore, the prepared hexametaphosphate-capped CdS QDs can be utilized for the development of a sensitive and selective sensor for Ca2+ ions. Compared with previous reports about detection
3.3. The inhibition of F− ions It is generally known that a common strategy for the design of fluoride ions sensors is based on the displacement approach, in which sensors bond to metal ions and then release them again in the presence of fluoride ions. Therefore, we could further utilize hexametaphosphate-capped CdS QDs and Ca2+ mixed system as a platform for detection of fluoride ions. As described in Scheme 1, the complexation with Ca2+ resulted in an obvious fluorescent enhancement of CdS QDs. F− ions preferentially react with Ca2+ ions to form insoluble calcium fluoride (CaF2 ) to exit the CdS QDs detection systems, and the fluorescence enhancement of CdS QDs would be inhibited [42]. In this work, we systematically investigated the influence of F− concentration on the fluorescence of CdS QDs-Ca2+ mixed system. From Fig. 5, it could be found that the fluorescence of CdS QDs-Ca2+ mixed system decreased accord-
Fig. 4. The fluorescence emission intensity of hexametaphosphate-capped CdS QDs respectively in the presence of 1 mmol/L K+ , Na+ , Mg2+ , Mn2+ , Zn2+ , Fe3+ , Fe2+ , Ni2+ , Cd2+ , Co2+ , Cu2+ , Pb2+ Hg2+ , Ag+ or 0.5 mmol/L Ca2+ .
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ingly with the increase of fluoride ions concentration. As shown in Fig. 5Inset, there was a good linear relationship between the fluorescence intensity ratio F/F0 (F is the fluorescence intensity of the CdS QDs enhanced by 0.4 mmol/L Ca2+ with various concentrations of F− ion, and F0 is the fluorescence intensity of the original CdS QDs) and the F− concentration in the range from 10 to 300 mol/L. The regression equation is: F/F0 = 2.939–0.003[F− ], mol/L, (2)
[4] [5]
[6]
[7]
(R2 )
The corresponding regression coefficient is 0.993, and the detection limit for F− was 6 mol/L calculated following the 3 IUPAC criteria. In order to investigate the fluorescence response of the obtained CdS QDs-Ca2+ mixed system against to some negatively-charged ions and molecules, Cl− , Br− , I− , SO4 2− , CO3 2− , S2 O8 2− , PO4 3− , NO3 − , acetate (Ac− ) and citrate (Cit3− ) was respectively added into the CdS QDs-Ca2+ mixed system. As shown in Fig.S11, only F− ions can induce an obvious fluorescence quenching among of these ions and molecules, which suggested that the proposed CdS QDs-Ca2+ mixed system also offered a good selectivity for F− ions detection. In order to evaluate the feasibility of the proposed method in real samples detection, the developed CdS QDs probe was applied to the determination of Ca2+ ions in fetal bovine serum samples. The commercial fetal calf serum solution was firstly centrifuged at 7500g for 30 min with a centrifugal filter unit (MWCO 3 KD) to remove some macromolecules. And then the obtained serum filtrate was diluted by 10 times with deionzed water. Different amount of Ca2+ were added to diluted serum samples prepare the spiked samples. The results obtained by standard addition method were shown in Table S1, and the accuracy of the proposed method was evaluated by determining the recoveries of Ca2+ in serum samples. It can be seen that the recoveries in the real samples was between 103 and 106% and the relative standard deviation (RSD) was no more than 3.6%. The above results demonstrated the potential applicability of the method for the detection of Ca2+ ions in serum samples. 4. Conclusions In summary, we have prepared the fluorescence-tunable hexametaphosphate-capped CdS QDs by a simple route and utilized it as a turn-on fluorescent probe for selective detection of Ca2+ ions. And the functionalized CdS QDs probe exhibited good selectivity for Ca2+ over other metal ions. Furthermore, the addition of fluoride can effectively inhibited the Ca2+ -induced fluorescence enhancements. Therefore, we developed a simple and low-cost probe system to respectively detect calcium ions and fluoride.
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[11]
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This work was supported by the Natural Science Foundation of China (81201141), the Fundamental Research Funds for Central Universities of China (N130520001;N142001001), and the Liaoning Provincial Educational Commission (L2014095). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.03.077.
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Biographies Siyu Liu received his doctor degree from Jilin University at 2014. Presently, he is a lecturer at College of Life and Health Sciences, Northeastern University, China. His research focuses on the fluorescence analysis and imaging based on nanomaterial. Hui Wang is currently pursuing her master’s degree under the guidance of Professor Hongguang Liu at Northeastern University, China. Her research focuses on the synthesis fluorescent nanomaterial and their application on biosensors. Zhen Cheng is an associate professor at Molecular Imaging Program, Stanford University. He received doctor degree from University of Missouri-Columbia at 2001. His research focuses on the development of multimodal nanosensor and molecular imaging techniques. Hongguang Liu is a professor at College of Life and Health Sciences, Northeastern University, China. He received doctor degree from Peking Union Medical College at 2010. His research focuses on the development of molecular imaging techniques.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Hexametaphosphate-capped quantum dots as fluorescent probes for detection of calcium ion and fluoride Siyu Liu a , Hui Wang a , Zhen Cheng b,∗ , Hongguang Liu a,∗ a b
Institute of Molecular Medicine, College of Life and Health Sciences, Northeastern University, Shenyang 110000, China Molecular Imaging Program at Stanford, Stanford University, Palo Alto, CA 94305, USA
a r t i c l e
i n f o
Article history: Received 16 December 2015 Received in revised form 1 March 2016 Accepted 17 March 2016 Available online 19 March 2016 Keywords: Quantum dots Fluorescence Calcium ion Fluoride
a b s t r a c t In this paper, we prepared fluorescence-tunable water-soluble CdS quantum dots (QDs) with hexametaphosphate as stabilizers by a simple route. The prepared CdS QDs were utilized to detect calcium ion (Ca2+ ) based on an obvious fluorescence enhancement of CdS QDs induced by Ca2+ . The results demonstrated there was an excellent linear relationship between the fluorescence intensity of hexametaphosphate-capped CdS QDs and Ca2+ concentration in the range from 10 mol/L to 400 mol/L and the detection limit for Ca2+ was 4 mol/L. Furthermore, fluoride ions (F− ) can react with Ca2+ to generate the insoluble calcium fluoride that would inhibit the Ca2+ -induced fluorescence enhancement of CdS QDs. Therefore, the designed Ca2+ -induced fluorescence enhancement system was also able to effectively detect fluoride. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Calcium ion (Ca2+ ) as an intracellular signal transmitter plays a key role in the human body. It modulates numerous physiological and pathological procedures, including cell damage and death by changing Ca2+ concentrations to modulate different cellular functions [1–3]. Among various anions, fluoride ion (F− ) is highly relevant to health care because of its important roles in dental care and treatment of osteoporosis, which is widely used as an ingredient in toothpaste, pharmaceutical agents and even drinking water [4,5]. In several underdeveloped countries, excessive F− level in drinking water has been linked to the debilitating bone disease fluorosis. Furthermore, the high concentration of F− may lead to collagen break down, thyroid activity and even cancer [6–8]. Therefore, it is necessary to develop a simple and convenient method to assay Ca2+ and F− . Until now, many analytical techniques have been explored to detect Ca2+ and F− , including electrode method, NMR analysis, colorimetric (UV) and electrochemical methods [9–13]. However, some disadvantages have been found for these techniques such as fragile instrumentation and time consuming manipulations associated with electrochemical methods, the high detection limit with colorimetric techniques, and etc [14].
∗ Corresponding authors. E-mail addresses: [email protected] (Z. Cheng), [email protected] (H. Liu). http://dx.doi.org/10.1016/j.snb.2016.03.077 0925-4005/© 2016 Elsevier B.V. All rights reserved.
In recent years, fluorescence sensor systems for Ca2+ and F− detection have attracted great attentions owing to their operational simplicity and real-time detection, etc. Most of the reported sensors are based on organic dyes with special ion recognition unites containing rich amino, hydroxyl and carboxyl groups that can bind strongly to Ca2+ or F− [15–23]. For instance, Dong et al. studied an organic salt as a fluorescent probe with ratiometric emission at 490 and 594 nm based on intramolecular charge transfer for Ca2+ determination. And the probe displayed high selectivity for Ca2+ and a large Stokes shift of 189 nm [16]. Wang and co-workers synthesized benzoselenadiazole based diarylamine as a near-infrared optical sensor for F− , and the designed sensor showed a turn-on ratiometric fluorescence signalling with F− by inhibiting the excited state intramolecular proton transfer processes [21]. However, these reported dyes based probes for Ca2+ and F− are faced with the poor water solubility which severely restricts their application in water environment systems. Moreover, synthesis of many organic dyes is complex and expensive. Compared with traditional organic dyes and fluorescent proteins, quantum dots (QDs), as a kind of inorganic nanomaterials, have attracted more attentions owing to their unique optical properties including size tunable fluorescence, narrow and symmetric emission peak with a broad excited wavelength, and excellent photostability, [24,25]. Until now, QDs such as CdTe, CdS, etc. have been widely used in DNA and protein analysis, molecular recognition, and ion detection [26–33]. For example, Chen and Rosenzweig synthesized water-soluble CdS QDs to detect copper (II) and zinc (II)
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ions [29]. Batteas and colleagues used 16-mercaptohexadecanoic acid capped CdSe QDs as a probe for copper (II) detection [30]. Ribeiro and co-workers reported a fluorometric approach for the selective determination of calcium ions level in the range from 0.80 to 3.20 mg/L, which was based on the monitoring of the fluorescent recovery of the thioglycolic acid-capped CdTe QDs previously quenched by ethylene diamine tetraacetic acid (EDTA) in aqueous solution [31]. Tang et al. developed a simple method for the determination of iodate based on the fluorescence quenching of carboxymethyl cellulose-capped CdS QDs induced by the oxidation of iodate [32]. In fact, QDs as ion probes primarily concentrated on heavy ions detection. So far, QDs as fluorescence probe for selective detection Ca2+ and F− has been rarely reported. Herein, in order to obtain Ca2+ -sensitive QDs, we selected hexametaphosphate, that is a metal chelating agent, as coating material to synthesize hexametaphosphate-capped CdS QDs. The prepared hexametaphosphate-capped CdS QDs showed a significant fluorescent enhancement with the addition of Ca2+ , because of the formation of calcium polyphosphate shell. F− can react with Ca2+ to generate insoluble calcium fluoride, and the Ca2+ -induced fluorescence enhancement would be inhibited. Therefore, the proposed CdS QDs could be used to effectively detect both Ca2+ and F− . 2. Experimental section 2.1. Apparatus The fluorescence emission and UV–vis absorption spectra were obtained by using a TECAN infinite M100 PRO Biotek microplate reader. FT-IR spectra were recorded with a Bruker IFS66 V FT-IR spectrometer equipped with a DGTS detector (32 scans).The zeta potential and dynamic light scattering (DLS) analysis were finished on Malvern Nano ZS90.Transmission electron microscopy (TEM) experiments and Energy Dispersive Spectrum (EDS) analysis were performed on a Philips Tecnai F20 TEM operating at 200 KV acceleration voltage. TEM samples were prepared by dropping the aqueous CdS QDs or CdS QDs-Ca2+ mixed solution onto carbon-coated copper grids and allowing the excess solvent to evaporate.
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The synthesis of phosphate, pyrophosphate or triphosphatecapped CdS QDs was same with above process. Briefly, CdCl2 solution (0.2 mL of 1 mol/L) was added into 20 mL of Na3 PO4 , Na4 P2 O7 or Na5 P3 O10 aqueous solution (20 mmol/L) with vigorous stirring at room temperature. After 10 min, sodium sulphide solution (120 L of 1 mol/L) was injected slowly into the mixed solution. The reaction mixture was allowed to proceed for another 1 h under vigorous stirring. 2.4. Detection of Ca2+ based on hexametaphosphate-capped CdS QDs Tris-HCl buffer solution (20 L, pH 7.6, 0.1 mol/L), synthesized hexametaphosphate-capped CdS QDs solution (20 L) and varying amounts of CaCl2 were successively added into test tube. Then the solution was diluted to 200 L with deionized water followed by the thoroughly shaking and equilibrated for 2 min. The fluorescence spectra were recorded with the excitation wavelength of 420 nm. The slit widths of excitation and emission were both 10 nm. The fluorescence intensity of the maximum emission peak was used for the quantitative analysis of Ca2+ concentration. 2.5. The effect of metal ions on hexametaphosphate-capped CdS QDs Tris-HCl buffer solution (20 L, pH 7.6, 0.1 mol/L), synthesized hexametaphosphate-capped CdS QDs solution (20 L) and individual 40 L various metal ions (5 mmol/L K+ , Na+ , Mg2+ , Mn2+ , Zn2+ , Fe3+ , Fe2+ , Ni2+ , Cd2+ , Co2+ , Cu2+ , Pb2+ , Hg2+ , Ag+ and 2.5 mmol/L Ca2+ ) were successively added into test tube. Then the solution was diluted to 200 L with deionized water followed by the thoroughly shaking and equilibrated for 2 min. The fluorescence spectra were recorded with the excitation wavelength of 420 nm. The slit widths of excitation and emission were both 10 nm. The fluorescence intensity of the maximum emission peak was used for the quantitative analysis of Ca2+ concentration. 2.6. Detection of F− based on hexametaphosphate-capped CdS QDs
2.2. Reagents All reagents were of at least analytical grade. The water used in all experiments had a resistivity higher than 18 M cm−1 . Cadmium (II) chloride (CdCl2 ), calcium chloride (CaCl2 ), potassium fluoride (KF), sodium sulfide nonahydrate (Na2 S•9H2 O), tris, sodium phosphate (Na3 PO4 ), sodium pyrophosphate (Na4 P2 O7 ), sodium triphosphate (Na5 P3 O10 ), sodium hexametaphosphate (Na6 P6 O18 ) and other metal salts all were purchased from Sinopharm Chemical Reagent Co., Ltd.
Tris-HCl buffer solution (20 L, pH 7.6, 0.1 mol/L), 20 L of hexametaphosphate-capped CdS QDs solution, different concentration of potassium fluoride, and 20 L of CaCl2 (4 mmol/L) were successively added into 200 L test tube, and then diluted to the 200 L with deionized water followed by the thoroughly shaking and equilibrated for 2 min. The fluorescence intensity of the maximum emission peak was used for the quantitative analysis of F− concentration. 3. Results and discussion
2.3. Synthesis of polyphosphate-capped CdS QDs The CdS QDs were synthesized according to the previous reports with some modifications [29]. Hexametaphosphate-capped CdS QDs were prepared as following: CdCl2 solution (0.2 mL of 1 mol/L) was added into 20 mL of Na6 P6 O18 (20 mmol/L) aqueous solution with vigorous stirring at room temperature. After 10 min, sodium sulfide solution (120 L of 1 mol/L) was injected slowly into the mixed solution. The reaction mixture was allowed to proceed for another 1 h under vigorous stirring. The obtained hexametaphosphate-capped CdS QDs solution was dialyzed in membrane tubing with a molecular weight cut-off of 3 KDa against ultrapure water to remove small molecules and ions. The purified CdS QDs solution were freeze-dried into powder and measured, and then the CdS QDs powder was dissolved in aqueous solution again with 16.3 mg/mL mass concentration.
3.1. Synthesis and characterization of hexametaphosphate-capped CdS QDs In present work, we proposed a simple route to prepare fluorescent CdS QDs with hexametaphosphate as templates, and adopted the CdS QDs as a highly selective probe for the detection of Ca2+ and F− . As shown in Scheme 1, the fluorescent CdS QDs was generated in the presence of Cd2+ and S2− ions with hexametaphosphate as stabilizers. The hexametaphosphate molecule containing six phosphate groups could act as chelating groups for passivating the surface of QDs, rendering them water-soluble and stable against aggregation [29]. The protective coat of hexametaphosphate on the CdS QDs ensures the hydrophilicity of CdS QDs and favors the further
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Scheme 1. The synthetic process of heametaphosphate-capped CdS QDs and the detection of Ca2+ and F− based on the prepared CdS QDs.
Fig. 1. Fluorescence emission spectra of CdS QDs prepared with different hexametaphosphate concentrations as stabilizers (a: 1 mmol/L, b: 2 mmol/L, c: 5 mmol/L, d: 10 mmol/L, e: 20 mmol/L, f: 40 mmol/L). Reaction condition: 10 mmol/L Cd2+ cations and 6 mmol/L S2− ions.
biosensing application. Fig. 1 offered the fluorescence changes of CdS QDs prepared with different concentrations of hexametaphosphate as stabilizers. When the hexametaphosphate concentration just reached 1 mmol/L, the prepared CdS QDs solution showed a strong fluorescence emission signal around 510 nm with 420 nm excitation wavelength. With the hexametaphosphate concentration increasing from 1 to 40 mmol/L, the fluorescence emission intensity was gradually weakening. Moreover, as shown in Fig. 1Inset, these synthesized CdS QDs simutaneously showed a red shif emission wavelength from 510 nm to 682 nm, respectively named as QDs(510), QDs(560), QDs(596), QDs(620), QDs(664) and QDs(682). Herein, we successfully employed hexametaphosphate as stabilizers prepare fluorescence-tunable CdS QDs. Lakowicz et al. previously proposed that the fluorescence emission maximum of CdS QDs is shifted to longer wavelengths with increasing particle diameter [34]. As shown in Fig.S1, the generated hexametaphosphate-capped CdS QDs were near spherical, and the average size of QDs(510) and QDs(682) was respectively 3.7 nm and 5.6 nm. And the increased size of CdS QDs was consistent with red shift of their emission wavelength. Hexametaphosphate, as a chelating agent, can specifically combine with Ca2+ cation to generate soluble calcium polyphosphate complex. Therefore, these hexametaphosphate-capped CdS
QDs are potential probes to specially recognize Ca2+ cation. We respectively investigated the fluorescence response of these hexametaphosphate-capped CdS QDs with various fluorescence emission wavelengths to Ca2+ . As shown in Fig.S2, Ca2+ can induce the fluorescence enhancement of these hexametaphosphatecapped QDs, and among these CdS QDs, QDs(664) with the fluorescence emission peak around 664 nm had the most significant fluorescence enhancement with the addition of Ca2+ . Therefore, we adopted the QDs(664) as fluorescent probe for Ca2+ detection. Fig. S3 indicated that QDs(664) showed a well-shaped emission peak around 664 nm excited with 420 nm that was a characteristic trap state emission arising from the surface defect [33]. And there is an increasing absorption below 450 nm that is the result of 1Sh -1Se excitonic transition characteristic of CdS QDs [33–35]. The FT-IR spectra of CdS QDs prepared with hexametaphosphate as stabilizers were shown in Fig.S4. The characteristic absorption band of hexametaphosphate were clearly observed through the asymmetric stretching vibrations of the PO2 (1280 cm−1 ), the out of phase symmetrical stretches (1060 cm−1 ), and the P O stretches of the main chain (930 cm−1 ) in Fig.S4, which indicated the successful capping of hexametaphosphate on the surface of the CdS QDs. Furthermore, we tested the stability of QDs(664) under various pHs and ionic environments. Fig.S5 indicated the prepared hexametaphosphate-capped CdS QDs was sensitive to pH environment, which showed weak fluorescence emission intensity in acid environment and an obvious enhancement with the pH increase from 5.6 to 7.6. In alkaline environment, the phosphate groups of hexametaphosphate were easily deprotonated and offered more negative charges, which were expected to strengthen the coordination effect between cadmium on the surface of CdS QDs and hexametaphosphate. The hexametaphosphate provided well surface protection for aqueous CdS QDs and effectively reduced non-radiative recombination by minimizing the surface defects of QDs [36,37]. 3.2. The fluorescence enhancement induced by Ca2+ We further investigated the fluorescence response of various polyphosphate-capped CdS QDs to Ca2+ . The water-soluble CdS QDs capped by phosphate, pyrophosphate or triphosphate was respectively prepared in aqueous solution, which was similar to the synthesis of hexametaphosphate-capped CdS QDs. The emission maximum of phosphate, pyrophosphate, triphosphate or hexametaphosphate-capped CdS QDs was respectively observed at 490 nm, 480 nm, 520 nm and 664 nm (Fig.S6). The
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Fig. 2. (A) The zeta potential and (B) DLS measurements of hexametaphosphate-capped CdS QDs solution (a), hexametaphosphate-capped CdS QDs solution with 0.1 mmol/L Ca2+ ions (b), hexametaphosphate-capped CdS QDs solution with 0.5 mmol/L Ca2+ ions (c).
effect of various polyphosphate ligands on the fluorescent response of CdS QDs to 0.5 mmol/L Ca2+ were shown in Fig.S6. Only hexametaphosphate-modified CdS QDs provide a nearly 3-folds fluorescence enhancement in the presence of Ca2+ . And the fluorescence intensity of other polyphosphate-capped QDs just showed weak changes with addition of Ca2+ , comparing to the emission of these CdS QDs in an ion-free solution. Fig.S7 showed the temporal evolution of fluorescence intensity of hexametaphosphate-modified CdS QDs after the addition of Ca2+ ions. And it could be seen that the fluorescence rapidly increased and remained nearly constant after 1 min. As shown in Fig. 2A, the zeta potential of the hexametaphosphate-capped CdS QDs solution was determined to be −38.7 mV. And with the increase of Ca2+ ions concentration, the zeta potential of CdS QDs-Ca2+ mixture gradually increased, which indicated Ca2+ was adsorbed to the surface of CdS QDs to neutralize the negative charges of hexametaphosphate. Meanwhile the dynamic light scattering analysis demonstrated that the addition of Ca2+ ions leads to the obvious increase of the particle size of CdS QDs solution (Fig. 2B). And the morphology and size changes of CdS QDs solution after the addition of Ca2+ were further observed by transmission electron microscopy (Fig.S8). It could be found that the generated CdS QDs were near spherical with well-dispersion and the sizes distribution gave an average size of 4.7 nm with ±1.1 nm. After the addition of Ca2+ ion solution, the CdS QDs nanoparticles tend to agglomerate into larger particle that was coated by a new shell structure, which might be caused by the generation of calcium hexametaphosphate shell (Fig.S8B). In order to confirm the elemental composition of the large particle, energy dispersive spectrum analysis was performed. As shown in Fig.S9A, the element Cd, S and P were respectively probed in the energy dispersive spectrum of hexametaphosphatecapped CdS QDs, which revealed that the molar ratio of Cd/S/P was about 1/1/7.1. And the Cd, S, P and Ca element were all detected in the energy dispersive spectrum of CdS QDs-Ca2+ mixture with
the molar ratio 1/1/7.1/2.9, which proved the formation of calcium hexametaphosphate shell on the surface of QDs (Fig.S9B). The Ca2+ -induced fluorescence enhancement can be attributed to the formation of calcium hexametaphosphate shell on the surface of CdS QDs. The calcium hexametaphosphate, as an excellent coating material, is also a good matrix for hosting luminescence materials [38,39]. Therefore the enhanced transition energy transfer would give rise to the increase of population of inversion symmetry of the fluorescent CdS QDs. Moreover, the fluorescence lifetime of hexametaphosphate-capped CdS QDs was determined to be 15.1 ns, and the fluorescence lifetime of the CdS QDs-Ca2+ mixed system was 29.6 ns (Fig. S10). The Ca2+ -induced the extension of fluorescence lifetime of hexametaphosphate-capped CdS QDs was because that the overlap of electron and hole wave function decreased with the formation of calcium hexametaphosphate shell on the surface of CdS QDs. The effect of various concentration of Ca2+ on the fluorescence emission of hexametaphosphate-capped CdS QDs was provided in Fig. 3. The fluorescence emission intensity is significantly enhanced with the increase of Ca2+ concentration. The enhancement effect of Ca2+ ions on fluorescence of CdS QDs showed concentration dependence. The fluorescence enhancement is attributed to the formation of a calcium hexametaphosphate shell to passivate the surface of the CdS QDs that produce more new radiative centers at the CdS QDs-calcium hexametaphosphate complex and stimulate block nonradiactive electron/hole recombination defect sites on the surface of the CdS QDs [40,41]. Moreover, the fluorescence emission peak of CdS QDs showed a blue-shift accompanied by the increase of Ca2+ concentration. And it is because the adsorption of Ca2+ to the surface of CdS QDs neutralized the negative charges of hexametaphosphate, and reduced the polarizability of surrounding water molecules, which would result in the decrease of Stokes shift. Fig. 3Inset demonstrated there was a good linear relationship between the relative fluorescence intensity F/F0 (F0 is the original fluorescence intensity of CdS QDs, and F is the fluorescence intensity of CdS QDs with the addition of various concentration of Ca2+ )
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Fig. 3. The fluorescence emission spectra of hexametaphosphate-capped CdS QDs incubated with different concentration of Ca2+ (0, 10, 20, 40, 60, 80,120, 200, 250, 300, 350, 400, 500 mol/L) for 2 min. Inset: The linear plots of F/F0 versusthe Ca2+ concentration in the range from 10 to 400 mol/L.
Fig. 5. The fluorescence emission spectra of hexametaphosphate-capped CdS QDsCa2+ (0.4 mmol/L) mixed system incubated with different concentration of F− (0, 10, 25, 50, 75, 100, 150, 200, 250, 300, 400, 500, 700, 900 mol/L) for 2 min. Inset: The linear plots of F/F0 versus the F− concentration in the range from 10 to 300 mol/L.
and Ca2+ concentration in the range from 10 to 400 mol/L. And the relationship could be described as following regression equation:
of Ca2+ [12,15–18], our proposed method for Ca2+ detection based CdS QDs simplified the detection process and avoid the complex and high-cost probe synthesis.
F/F0 = 1.075 + 0.005[Ca2+ ], mol/L(1) The corresponding regression coefficient (R2 ) is 0.994 with 4 mol/L detection limit for Ca2+ calculated following the 3 IUPAC criteria. And we further investigated the effect of various metal ions including K+ , Na+ , Ca2+ , Mg2+ , Mn2+ , Zn2+ , Fe3+ , Fe2+ , Ni2+ , Cd2+ , Co2+ , Cu2+ , Pb2+ , Hg2+ and Ag+ on the fluorescence of CdS QDs (Fig. 4). It was found that the fluorescence of CdS QDs could be enhanced by 3fold with the addition of Ca2+ for 2 min, which might be attributed to the strong chelating ability of hexametaphosphate with Ca2+ ions. The heavy metal ions Cu2+ , Hg2+ and Ag+ could induce the significant fluorescence quenching of CdS QDs, and the interference of Cu2+ , Hg2+ and Ag+ on the fluorescence of CdS QDs could be eliminated by adding potassium iodide to generate insoluble substance. Therefore, the prepared hexametaphosphate-capped CdS QDs can be utilized for the development of a sensitive and selective sensor for Ca2+ ions. Compared with previous reports about detection
3.3. The inhibition of F− ions It is generally known that a common strategy for the design of fluoride ions sensors is based on the displacement approach, in which sensors bond to metal ions and then release them again in the presence of fluoride ions. Therefore, we could further utilize hexametaphosphate-capped CdS QDs and Ca2+ mixed system as a platform for detection of fluoride ions. As described in Scheme 1, the complexation with Ca2+ resulted in an obvious fluorescent enhancement of CdS QDs. F− ions preferentially react with Ca2+ ions to form insoluble calcium fluoride (CaF2 ) to exit the CdS QDs detection systems, and the fluorescence enhancement of CdS QDs would be inhibited [42]. In this work, we systematically investigated the influence of F− concentration on the fluorescence of CdS QDs-Ca2+ mixed system. From Fig. 5, it could be found that the fluorescence of CdS QDs-Ca2+ mixed system decreased accord-
Fig. 4. The fluorescence emission intensity of hexametaphosphate-capped CdS QDs respectively in the presence of 1 mmol/L K+ , Na+ , Mg2+ , Mn2+ , Zn2+ , Fe3+ , Fe2+ , Ni2+ , Cd2+ , Co2+ , Cu2+ , Pb2+ Hg2+ , Ag+ or 0.5 mmol/L Ca2+ .
S. Liu et al. / Sensors and Actuators B 232 (2016) 306–312
ingly with the increase of fluoride ions concentration. As shown in Fig. 5Inset, there was a good linear relationship between the fluorescence intensity ratio F/F0 (F is the fluorescence intensity of the CdS QDs enhanced by 0.4 mmol/L Ca2+ with various concentrations of F− ion, and F0 is the fluorescence intensity of the original CdS QDs) and the F− concentration in the range from 10 to 300 mol/L. The regression equation is: F/F0 = 2.939–0.003[F− ], mol/L, (2)
[4] [5]
[6]
[7]
(R2 )
The corresponding regression coefficient is 0.993, and the detection limit for F− was 6 mol/L calculated following the 3 IUPAC criteria. In order to investigate the fluorescence response of the obtained CdS QDs-Ca2+ mixed system against to some negatively-charged ions and molecules, Cl− , Br− , I− , SO4 2− , CO3 2− , S2 O8 2− , PO4 3− , NO3 − , acetate (Ac− ) and citrate (Cit3− ) was respectively added into the CdS QDs-Ca2+ mixed system. As shown in Fig.S11, only F− ions can induce an obvious fluorescence quenching among of these ions and molecules, which suggested that the proposed CdS QDs-Ca2+ mixed system also offered a good selectivity for F− ions detection. In order to evaluate the feasibility of the proposed method in real samples detection, the developed CdS QDs probe was applied to the determination of Ca2+ ions in fetal bovine serum samples. The commercial fetal calf serum solution was firstly centrifuged at 7500g for 30 min with a centrifugal filter unit (MWCO 3 KD) to remove some macromolecules. And then the obtained serum filtrate was diluted by 10 times with deionzed water. Different amount of Ca2+ were added to diluted serum samples prepare the spiked samples. The results obtained by standard addition method were shown in Table S1, and the accuracy of the proposed method was evaluated by determining the recoveries of Ca2+ in serum samples. It can be seen that the recoveries in the real samples was between 103 and 106% and the relative standard deviation (RSD) was no more than 3.6%. The above results demonstrated the potential applicability of the method for the detection of Ca2+ ions in serum samples. 4. Conclusions In summary, we have prepared the fluorescence-tunable hexametaphosphate-capped CdS QDs by a simple route and utilized it as a turn-on fluorescent probe for selective detection of Ca2+ ions. And the functionalized CdS QDs probe exhibited good selectivity for Ca2+ over other metal ions. Furthermore, the addition of fluoride can effectively inhibited the Ca2+ -induced fluorescence enhancements. Therefore, we developed a simple and low-cost probe system to respectively detect calcium ions and fluoride.
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This work was supported by the Natural Science Foundation of China (81201141), the Fundamental Research Funds for Central Universities of China (N130520001;N142001001), and the Liaoning Provincial Educational Commission (L2014095). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.03.077.
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Biographies Siyu Liu received his doctor degree from Jilin University at 2014. Presently, he is a lecturer at College of Life and Health Sciences, Northeastern University, China. His research focuses on the fluorescence analysis and imaging based on nanomaterial. Hui Wang is currently pursuing her master’s degree under the guidance of Professor Hongguang Liu at Northeastern University, China. Her research focuses on the synthesis fluorescent nanomaterial and their application on biosensors. Zhen Cheng is an associate professor at Molecular Imaging Program, Stanford University. He received doctor degree from University of Missouri-Columbia at 2001. His research focuses on the development of multimodal nanosensor and molecular imaging techniques. Hongguang Liu is a professor at College of Life and Health Sciences, Northeastern University, China. He received doctor degree from Peking Union Medical College at 2010. His research focuses on the development of molecular imaging techniques.