Research article Received: 4 August 2014,
Accepted: 3 January 2015
Published online in Wiley Online Library: 12 February 2015
(wileyonlinelibrary.com) DOI 10.1002/bio.2867
Sensitive detection of sodium cromoglycate with glutathione-capped CdTe quantum dots as a novel fluorescence probe Chenxia Hao,a Shaopu Liu,a Dan Li,a Jidong Yanga,b and Youqiu Hea* ABSTRACT: A sensitive and simple analytical strategy for the detection of sodium cromoglycate (SCG) has been established based on a readily detectable fluorescence quenching effect of SCG for glutathione-capped (GSH-capped) CdTe quantum dots (QDs). The fluorescence of GSH-capped CdTe QDs could be efficiently quenched by SCG through electron transfer from GSHcapped CdTe QDs to SCG. Under optimum conditions, the response was linearly proportional to the concentration of SCG between 0.6419 and 100 μg/mL, with a correlation coefficient (R) of 0.9964; the detection limit (3δ/K) was 0.1926 μg/mL. The optimum conditions and the influence of coexisting foreign substances on the reaction were also investigated. The very effective and simple method reported here has been successfully applied to the determination of SCG in synthetic and real samples. It is believed that the established approach could have good prospects for application in the fields of clinical diseases diagnosis and treatment. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: CdTe quantum dots; sodium cromoglycate; fluorescence; electron transfer
Introduction
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Semiconductor quantum dots (QDs), a particularly interesting class of emerging nanomaterials, exhibit unique optical and photophysical properties, such as a broad excitation spectrum, readily size-tunable narrow emission spectrum, high fluorescence quantum efficiency and long-term photostability, as well as resistance to photobleaching (1–5). In addition, the luminescence of QDs is extremely sensitive to the surface state. The interaction between different types of analytes and the surface of QDs can result in dramatic changes in their optical features (4,5). Based on these desirable characteristics, QDs have become ideal fluorescent semiconductor indicators for chemical and biological analysis. For example, Chen et al. used mercaptoacetic acid-modified ZnSe QDs as a fluorescent semiconductor probe for paeoniflorin determination based on a fluorescence quenching method (6). Liu et al. developed a near-infrared fluorescence probe for 2,4,6-trinitrophenol detection based on bovine serum albumin-coated CuInS2 QDs (7). 1,10-Phenanthroline-capped CdTe QDs were reported as a Cd2+ sensor by Hu et al., with a detection limit of 0.01 nM (8). In this paper, a typical GSH-capped CdTe QDs probe has been prepared and constructed for monitoring sodium cromoglycate. Sodium cromoglycate (SCG), also referred to as disodium cromoglycate or pharynx Thailand, is an important antiallergic, which is commonly used for the treatment of allergic asthma and has a good clinical efficacy in patients with mild or moderate persistent asthma (9–11). Furthermore, SCG has been employed in the prophylactic treatment of allergic rhinitis (12,13) and allergic conjunctivitis in spring (14). Nowadays, many analytical methods have been reported for the determination of SCG, such as liquid chromatographic-tandem mass spectrometry (LC-MS-MS) (15–17), capillary electrophoresis (CE) (18), thin-layer chromatographydensitometry (TLC-densitometry) (19) and cathodic striping voltammetry (CSV) (20). However, those methods tend to be expensive,
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time-consuming and technically complex and do not allow high throughput analysis. To better exploit and safely utilize SCG in the diagnosis and treatment of clinical diseases, it is necessary to develop a rapid, simple, high-sensitivity, low-cost and throughput method for the quantitative determination of SCG. To the best of our knowledge, to date, no endeavor has been made to use GSH-capped CdTe QDs as a fluorescence semiconductor probe for the quantitative determination of SCG. In this study, we investigated the interaction between GSH-capped CdTe QDs and SCG using fluorescence and UV/Vis absorption spectroscopy. The corresponding results revealed that the fluorescence of GSH-capped CdTe QDs could be efficiently quenched by SCG through electron transfer from GSH-capped CdTe QDs to SCG, and the quenching mechanism was dynamic quenching. Meanwhile, the optimum conditions and the interference of coexisting substances on the reaction were also investigated. The very effective and simple method reported here has been successfully applied to the detection of SCG in its synthetic samples and real samples. It is believed that the established approach could have good prospects for application in the fields of the clinical disease diagnosis and treatment.
* Correspondence to: Y. Q. He, Key Laboratory on Luminescence and RealTime Analysis, Ministry of Education, School of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, People’s Republic of China. E-mail: [email protected] a
Key Laboratory on Luminescence and Real-Time Analysis, Ministry of Education, School of Chemistry and Chemical Engineering, Southwest University, Chongqing400715, People’s Republic of China
b
School of Chemical and Environmental Engineering, Chongqing Three Gorges University, Chongqing, Wanzhou, 404000, People’s Republic of China
Copyright © 2015 John Wiley & Sons, Ltd.
Analytical strategy for the detection of sodium cromoglycate
Experimental
Results and discussion
Apparatus and reagents
Characterization of GSH-capped CdTe QDs
Fluorescence spectra were recorded using a Hitachi F-2500 spectrofluorophotometer (Hitachi Company, Tokyo, Japan) equipped with a 1.0 cm standard quartz cuvette and the slit width for excitation and emission was set at 10 nm. The UV/Vis absorption spectra were acquired by a UV-2450 spectrophotometer (Tianmei Corporation, Shanghai, China) with a 1.0 cm standard quartz cuvette. The morphology features and size of nanoparticles were carried out using a Hitachi-600 transmission electron microscopy (TEM), Hitachi Company, Tokyo, Japan. The pH measurements were conducted with a pHS-3C pH meter (Leici, Shanghai, China). Magnetic stirring was performed with a SZCL-A magnetic stirrer (Zhengzhou, China). Cadmium chloride (CdCl2·2.5H2O) and tellurium (Te) powder were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China)S Glutathione (GSH) and sodium borohydride (NaBH4) were obtained from Aladdin Chemistry Co. Ltd, (Shanghai, China) and Tianjin Huanwei Fine Chemical Co. Ltd, (Tianjin, China) respectively. Sodium cromoglycate (SCG) and sodium cromoglycate eye drops were purchased from Adamas Reagent Co. Ltd and Wuhan Wujing Medicine Co., Ltd, (Wuhan, China) respectively. Tris/HCl buffer solutions at different pH values were prepared by mixing 0.05 mol/L Tris and 0.1 mol/L HCl in suitable proportions. All reagents used in the experiments were of analytical grade or better. Ultrapure water was used to prepare all solutions.
The morphological features of the as-prepared CdTe QDs were studied using TEM. As shown in Fig. 1, the particle sizes were homogeneous and the average size was ~ 3 nm. In addition, the UV/Vis absorption spectrum and fluorescence spectrum of the GSH-capped CdTe QDs were obtained and are shown in Fig. 2. Curve b shows a strong UV absorption with a characteristic absorption peak at 542 nm. Meanwhile, curve a exhibits a symmetric and narrow fluorescence bandwidth, which further confirmed that the resulting CdTe QDs were homogeneous. The particle sizes of the resulting CdTe QDs could also be calculated from the first absorption maximum of the UV/Vis absorption spectrum by the following equation (23): D ¼ 9:8127107 λ3 1:7147103 λ2 þ ð1:0064Þλ 194:84 (1) where D is the diameter of resulting CdTe QDs and λ is the wavelength of the first excitonic absorption peak of resulting CdTe QDs. The outcome showed that the particle diameter of the GSH-capped CdTe QDs was ~ 3.1 nm (λ = 542 nm), which is in good agreement with the TEM observation results. The quantum yield (QY) of GSH-capped CdTe QDs was measured using Rhodamine 6G as a criterion (QY = 95%), which was used to calculate the QY of GSH-capped CdTe QDs as follows (24):
Synthesis of GSH-capped CdTe QDs Aqueous CdTe QDs modified by GSH were synthesized according to the previously reported method (21). Under an argon atmosphere and magnetic stirring, Te powder (0.0385 g) was placed in a 50.0 mL three-necked flask and reacted with excessive sodium borohydride in deionized water to produce a colorless solution of NaHTe. CdCl2·2.5H2O (0.1026 g) and GSH (0.1847 g) were dissolved in 150.0 mL of deionized water. Under an argon atmosphere and magnetic stirring, the pH of the mixture was adjusted to 10.96 by the dropwise addition of 1.0 mol/L NaOH solution. H2SO4 (0.5 mol/L) was then introduced to NaHTe to produce H2Te gas, which passed through the cadmium precursor with a slow argon flow for 30 min. CdTe precursors were formed at this stage. Subsequently, the resulting solution mixture was heated to 369 K and refluxed under argon for 1 h. A salmon pink GSHcapped CdTe solution was obtained. The concentration of GSH-capped CdTe QDs was 2.0 × 10-3 mol/L (determined by the concentration of Te2-) (22).
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3500
0.32
2800 2100
0.24
b 1400
0.16
a 700
0.08
400
500
Fluorescence intensity
0.40
600
/nm Figure 2. (a) UV/Vis absorption spectrum and (b) fluorescence spectrum (excited at 350 nm) of GSH-capped CdTe QDs.
Copyright © 2015 John Wiley & Sons, Ltd.
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In this step, to study the interaction between GSH-capped CdTe QDs and SCG, 1.2 mL of the above-synthesized GSHcapped CdTe QDs, 1.0 mL Tris/HCl solution, and a series of different concentrations of SCG were added to a 10.0 mL volumetric flask. The mixture was then diluted with deionized water to the mark, and mixed thoroughly by gentle shaking. After incubation for 20 min, the fluorescence and absorption spectra of the solution were measured at an excitation wavelength of 350 nm.
Absorption
General procedure for SCG detection
Figure 1. TEM image of GSH-capped CdTe QDs.
C. Hao et al. 2 Fu As n u2 Yu ¼ Y s Fs Au ns
(2)
where Yu is the QY of the sample solution to be measured and Ys is the reference solution. Fu and Fs are the integral intensity, Au and As are absorption values, nu and ns stand for the refractive indexes of the solvents, and the subscripts ‘u’ and ‘s’ refer to the samples and standard substance, respectively. The QY of GSH-capped CdTe QDs was 44.9%. Fluorescence quenching of GSH-capped CdTe QDs by SCG Under optimum conditions, the fluorescence spectra of GSHcapped CdTe QDs were recorded in the absence and presence of SCG, the results of which are shown in Fig. 3. The fluorescence intensity of GSH-capped CdTe QDs was efficiently quenched by SCG centered at 572 nm (excitation 350 nm), which was commonly attributed to the electron–hole pair separation. Simultaneously, no discernible shifts in the emission peak shape and location of the fluorescence spectra were found, even at relatively high concentrations (100 μg/mL) of SCG. On this basis, the possibility of establishing a sensitive method for SCG was evaluated. To evaluate the sensitivity of GSH-capped CdTe QDs for the detection of SCG, the fluorescence intensity of GSH-capped CdTe QDs at 572 nm was monitored by increasing the concentration of SCG over a certain range. As shown in Fig. 3(inset), a linear calibration plot of the fluorescence quenching intensity versus the concentration of SCG was observed over the range of 0.6419–100.00 μg/mL, with a correlation coefficient (R) of 0.9964 and a linear regression equation of F0 F = 20.84c + 80.77 (where c is the concentration of SCG in μg/mL). The detection limit of 0.1926 μg/mL for SCG was evaluated by using 3δ/K, where δ is the standard deviation of the fluorescence intensity of the blank samples from 11 replicate measurements and K is the slope of the calibration plot. Hence, it is quite obvious that the approach had a high sensitivity for detecting SCG in aqueous solution. Possible quenching mechanism Fluorescence quenching is generally classified as dynamic or static, and the two mechanisms can be distinguished by their differing dependence on temperature (25). If the quenching rate
2000
2500 2000
1500
276K 290K 303K
1.8
1000
1.6
0 0
20
40
C
60
80
100
( g/mL)
11
F0 /F
500
1500 1000
where F0 and F are the fluorescence intensity of GSH-capped CdTe QDs in the absence and presence of a quencher (SCG), respectively; [Q] is the concentration of quencher (SCG) and KSV shows that the quenching efficiency of the quencher was the Stern–Volmer quenching constant. Figure 4 shows the Stern– Volmer plots of F0/F versus [Q] at three different temperatures (296, 290 and 303 K). The quenching constant (KSV) was further calculated according to the Stern–Volmer equation and is given by Table 1. It was clearly shown that the corresponding values of KSV increased with the increase in temperature, which implied that the probable quenching process for the GSH-capped CdTe QDs-SCG solution system was dynamic quenching. In order to further confirm the reaction mechanism, UV/Vis absorption spectra of the GSH-capped CdTe QD–SCG solution system were explored Fig. 5. In the spectrum of pure GSH-capped CdTe QDs (curve a), there was strong absorption in the UV region, whereas the absorption in the visible area was relatively weak. In the spectrum of SCG with distilled water as the reference (curve b), there were two strong absorption peaks at 252 and 327 nm. Curve c is the absorption spectrum for a mixture of SCG and GSH-capped CdTe QDs. By comparing curves b and c, curve d, representing the absorption spectrum of SCG with GSH-capped CdTe QDs as the reference was obtained, implying that the absorption spectrum of the mixture was simply a superposition of the GSH-capped CdTe QDs and SCG. Namely, the quenching process was dynamic quenching. In dynamic quenching, the quencher collides with the excited fluorophore and returns to ground state via either electronic or energy transfer (27). Because there is no overlap integral between the emission spectra of GSH-capped CdTe QDs and the absorption spectra of SCG (Fig. 6), fluorescence resonance energy transfer (FRET) was excluded (27). Thus, electron transfer was the most probable mechanism for the interaction between GSH-capped CdTe QDs and SCG. However, the photoexcited QDs might participate in electron transfer via two general 2.0
1
3000 F -F
Fluorescence intensity
3500
constants increase with increasing temperature, the fluorescence quenching behavior is identified as dynamic quenching between quencher and fluorophore. In the case of the reverse effect, it is considered to be a static quenching. The well-known Stern–Volmer equation (26) could be utilized to analyze the florescence quenching mechanism. . F0 ¼ 1 þ K SV ½Q (3) F
1.4 1.2
500 1.0
540
600
660
720 0
/nm
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30
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CSCG ( g/mL)
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Figure 3. Fluorescence spectra of GSH-capped CdTe QD–SCG system. The concentrations of SCG added for spectra 1–11 were: 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 μg/mL, respectively. (Inset) Linear relationship between fluorescence quenching intensity and the concentrations of SCG.
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Figure 4. Stern–Volmer plots for the GSH-capped CdTe QD–SCG system at three different temperatures. The concentration of GSH-capped CdTe QDs was 4 2.4 × 10 mol/L, pH 7.8.
Copyright © 2015 John Wiley & Sons, Ltd.
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Analytical strategy for the detection of sodium cromoglycate Table 1. Stern–Volmer quenching constants for the interaction of GSH-capped CdTe QDs with SCG at different temperatures Temperature (K) 276 290 303
Stern–Volmer linear equation
KSV (L/mol)
Ra
SDb
F0/F = 0.9792 + 6.025 × 103 [Q] F0/F = 0.9768 + 7.946 × 103 [Q] F0/F = 1.0432 + 9.099 × 103 [Q]
6.025 × 103 7.946 × 103 9.099 × 103
0.9976 0.9997 0.9970
0.0150 0.0068 0.0245
a
R is the correlation coefficient. SD is the standard deviation for the Ksv values.
b
1.6 1.4
c
Absorption
1.2
a
1.0
d
0.8 0.6 0.4 0.2
b 300
400
500
600
/nm
1.0
3500
0.8
2800 2100
0.6
a 0.4
1400
b
700
0.2
300
400
500
Fluorescence intensity
Absorption
Figure 5. UV/Vis absorption spectra of (a) GSH-capped CdTe QDs, (b) SCG (with distilled water as the reference), (c) the mixture (GSH-capped CdTe QDs and SCG) and (d) SCG (with GSH-capped CdTe QDs as the reference). GSH-capped CdTe 4 QDs, 2.4 × 10 mol/L; SCG, 60 μg/mL).
600
/nm Figure 6. UV/Vis absorption spectrum of (a) SCG and the fluorescence spectrum of (b) GSH–CdTe QDs in Tris/HCl buffer solution at pH 7.8. GSH-capped CdTe 4 QDs, 2.4 × 10 mol/L; SCG, 60 μg/mL; Tris/HCl buffer solution, 1.0 mL.
pathways (27) because of the formation of a free electron in the conduction band and a positively charged hole in the valence band of QDs; (as outlined in Scheme 1). In the presence of an electron acceptor, electron transfer might proceed from the condution band of QDs to the acceptor units. However, in the presence of an electron donor, electron transfer might occur from the donation units to the valence band holes of QDs. The electron–hole pair separation leads to the luminescence quenching of QDs (28). In this system, SCG was most likely to serve as an efficient electron acceptor (29). On adding SCG to the solution of GSH-capped CdTe QDs, electron transfer proceeded from the condution band of GSH-capped CdTe QDs to SCG, which prevented the electron–hole recombination and caused the fluorescence quenching of GSH-capped CdTe QDs (Scheme 1).
Optimum reaction conditions Influence of the acidity. The influence of acidity of Britton– Robinson, phosphate, Hasting–Sendroy and Tris/HCl buffer solutions on the fluorescence intensity of GSH-capped CdTe QDsSCG system was investigated. The experiments showed that Tris/HCl buffer solution appeared to be optimal for the determination of SCG. Accordingly, Tris/HCl buffer solution was selected to control the acidity of analytical system. The effect of pH in a range between 7.0 and 8.0 was studied to obtain the optimal reaction pH for the determination of SCG with GSH-capped CdTe QDs and the results are shown in Fig. 7(a). It was found that the maximum fluorescence intensity of GSH-capped CdTe QD– SCG was obtained at pH 7.8. At the same time, the effect of the dosage of Tris/HCl buffer on the fluorescence intensity of GSH-capped CdTe QD–SCG was also discussed. As shown in Fig. 7(b), the optimal dosage of Tris/HCl was 1.0 mL. Influence of aqueous CdTe QDs concentration. The effect of GSH-capped CdTe QDs concentration on the fluorescence
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Scheme 1. Mechanism of interaction of GSH-capped CdTe QDs with SCG. A representative SCG.
C. Hao et al.
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F0 -F
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V/mL
Figure 7. Effect of acidity on the fluorescence intensity of GSH-capped CdTe QD–SCG system in the presence (F) and the absence (F0) of SCG. (a) Effect of pH of buffer -4 solution; (b) effect of dosage of buffer solution. GSH-capped CdTe QDs, 2.4 × 10 mol/L; SCG, 20 μg/mL.
Influence of incubation time. The impact of incubation time on the fluorescence intensity of the GSH-capped CdTe QD–SCG system was investigated at different time scales at room temperature and the results are shown in Fig. 9. The results implied that the reaction was finished completely in 20 min and the fluorescence intensity remained stable for at least 1 h. Therefore, a time scale of 20 min was adopted in subsequent experiments.
Selectivity and analytical applications
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Fluorescence intensity
intensity of the GSH-capped CdTe QD–SCG system was explored by keeping the SCG concentration and the pH constant and changing the GSH-capped CdTe QDs concentration. As shown in Fig. 8, the optimal concentration of GSH-capped CdTe QDs was 2.4 × 104 mol/L for the GSH-capped CdTe QD–SCG system, namely, 1.2 mL above prepared GSH-capped CdTe QDs.
3600 3400 3200 3000 2800 0
20
40
60
80
100
Time(min) Figure 9. Effect of incubation time on the fluorescence intensity of GSH-capped CdTe QD–SCG system in 1.0 mL Tris/HCl buffer solution at pH 7.8. GSH-capped -4 CdTe QDs, 2.4 × 10 mol/L; SCG, 20 μg/mL.
Interference of coexisting foreign substances In order to explore the selectivity of the proposed method, the interference of some coexisting substances on the determination of the GSH-capped CdTe QD–SCG system was tested under optimal conditions and the results are given in Table 2. When the concentration of SCG was 20 μg/mL, most of the common ions (Na+, K+, NH4+, Mg2+, Cl–, NO3–, CO32–, SO42–, HPO42–), amino acids, glucose, etc. had less effect on the determination of SCG, whereas, Zn2+, Ca2+, Al3+ and Fe3+ at relatively lower concentrations had no significant impact.
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F0-F
750 600 450 300 150 0.8
1.2
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C(10-4mol/L)
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Figure 8. Effect of GSH-capped CdTe QDs concentration on the fluorescence intensity of GSH-capped CdTe QD–SCG system in the presence (F) and the absence (F0) of SCG. SCG, 20 μg/mL; Tris/HCl buffer solution, 1.0 mL, pH 7.8.
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Analytical application The synthetic samples were analyzed according to the previously reported method (30). The composition of the synthetic samples is shown in Table 3. Under optimal conditions, 1.2 mL of the above-prepared GSH-capped CdTe QDs, 1.0 mL of Tris/HCl solution, an appropriate amount of synthetic samples and SCG were added to determine the SCG concentration. As shown in Table 3, the proposed method could be applied to detect SCG in synthetic samples with satisfactory results. The proposed method was also successfully employed for the determination of SCG in sodium cromoglycate eye drops. The sodium cromoglycate eye drops were diluted 100 times using ultrapure water to obtain detection samples. Under optimal conditions, 1.2 mL of the above-prepared GSH-capped CdTe QDs, 1.0 mL of Tris/HCl solution and an appropriate amount of pharmaceutical samples were added to determine the SCG concentration. As shown in Table 4, there was no significant difference between the determined values of SCG and the declared content, indicating the accuracy of the proposed method. The relative standard deviation (RSD) was < 4.75% and the recovery was between 99.80 and 105.3%, which indicated that the determination of SCG using GSH-capped CdTe QDs as fluorescence probe was sensitive and reproducible. The present method was able to meet the requirements of microanalysis in real samples.
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Analytical strategy for the detection of sodium cromoglycate Table 2. Effect of coexistent substances for GSH-capped CdTe QD–SCG system (GSH-capped CdTe QDs, 2.4 × 10-4 mol/L; SCG, 20 μg/mL) Coexistent material
Concentratin (μg/mL)
+
Na K+ NH4+ Mg2+ Ca2+ Zn2+ Al3+ Fe3+ Cl– NO3– SO42–
1500 600 800 800 4 2 10 2 1300 1500 800
Relative error (%) –3.06 –3.01 +2.97 –1.06 –3.88 –1.74 +1.81 +1.93 +2.16 –3.06 –1.06
Coexistentmaterial
Concentratin (μg/mL)
2–
HPO4 CO32– L-Alanine L-Threonine L-Leucine L-Proline L-Arginine Creatine Glucose Egg albumin Urea
Relative error (%)
800 600 700 600 600 600 500 400 600 400 600
+2.29 +3.91 –3.06 +1.20 +4.36 –3.27 –2.20 +2.36 –2.29 +3.20 +2.39
Table 3. Results for the determination of SCG in synthetic samples (n = 5) Sample
Added (μg/mL)
1 2 3
20.0 20.0 20.0
Foreign substances NaNO3, L-Alanine, Egg albumin MgSO4, L-Threonine, Glucose KCl, L-Leucine, Creatine
Found (n = 5, μg/mL)
RSD (n = 5, %)
20.541 19.534 20.052
Recovery (n = 5, %)
2.88 3.89 1.96
102.71 97.67 100.26
Concentrations (μg/mL): NaNO3, 40; L-alanine, 40; egg albumin, 40; MgSO4, 40; L-threonine, 40; glucose, 40; KCl, 40; L-leucine, 40; creatine, 40.
Table 4. Results for the determination of SCG in sodium cromoglycate eye drops (n = 5) Sample
Original found (μg/mL)
1 2 3
5 10 20
Determination found (μg/mL) 4.8254, 4.9003,5.0504, 5.0755, 5.1256 9.1794, 10.3968, 10.0616, 10.2567, 10.0060 20.7536, 20.7884, 21.0325, 21.2075, 21.5241
5.00 9.98 21.06
RSD (n = 5, %) 2.53 4.75 1.51
Recovery (n = 5, %) 99.91 99.80 105.30
References
Conclusions In summary, a novel and effective fluorescence semiconductor probe based on the fluorescence quenching effect of GSHcapped CdTe QDs was successfully established for the detection of SCG in aqueous medium. Under optimum conditions, the response was linearly proportional to the concentration of SCG between 0.6419 and 100 μg/mL, with a detection limit (3δ/K) of 0.1926 μg/mL. In additionally, the mechanism of the interaction of GSH-capped CdTe QDs with SCG was also discussed in detail. The proposed method had simplicity, rapidity and high sensitivity, and was applied to the quantitative analysis of SCG in synthetic samples and real samples with satisfactory results. Acknowledgements/Acknowledgments according to US/UK author
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This work was supported by Chongqing Municipal Key Laboratory on Luminescence and Real-Time Analysis, Southwest University (CSTC, 2006CA8006) and the National Natural Science Foundation of China (No. 21175015).
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Mean found (n = 5, μg/mL)
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20. Pereira FC, Fogg AG, Zanoni MVB. Regeneration of poly-L-lysine modified carbon electrodes in the accumulation and cathodic stripping voltammetric determination of the cromoglycate anion. Talanta 2003; 60: 1023–32. 21. Liu YF, Yu JS. Selective synthesis of CdTe and high luminescence CdTe/CdS quantum dots: the effect of ligands. J Colloid Interface Sci 2009; 333: 690–8. 22. Yu WW, Qu LH, Guo WZ, Peng XG. Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS nanocrystals. Chem Mater 2003; 15: 2854–60. 23. Zhang TL, Sun XG, Liu B. Synthesis of positively charged CdTe quantum dots and detection for uric acid. Spectrochim Acta A 2011; 79: 1566–72. 24. Li L, Qian HF, Fang NH, Ren JC. Significant enhancement of the quantum yield of CdTe nanocrystals synthesized in aqueous phase by controlling the pH and concentrations of precursor solutions. J Lumin 2006; 116: 59–66. 25. Lakowica JR. Principles of fluorescence spectroscopy. 2nd ed. New York: Plenum Press, 1999: 237. 26. Varlan A, Ionescu S, Hillebrand M. Study of the interaction between ofloxacin and human serum albumin by spectroscopic methods. Luminescence 2011; 26: 710–15. 27. Freeman R, Willner I. Optical molecular sensing with semiconductor quantum dots (QDs). Chem Soc Rev 2012; 41: 4067–85. 28. Jhonsi MA, Vaishnavi E, Suganya R, Kathiravan A, Renganathan R. Investigation on the photoinduced interaction between water soluble CdTe quantum dots and certain antioxidants. Adv Sci Lett 2011; 4: 3490–5. 29. Hu YJ, Liu Y, Sun TQ, Bai AM, Lü JQ, Pi ZB. Binding of antiinflammatory drug cromolyn sodium to bovine serum albumin. Int J Biol Macromol 2006; 39: 280–5. 30. Liang WJ, Liu ZQ, Liu SP, Yang JD, He YQ. A novel surface modification strategy of CdTe/CdS QDs and its application for sensitive detection of ct-DNA. Sens Actuator B 2014; 196: 336–44.
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Copyright © 2015 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 1112–1118
Accepted: 3 January 2015
Published online in Wiley Online Library: 12 February 2015
(wileyonlinelibrary.com) DOI 10.1002/bio.2867
Sensitive detection of sodium cromoglycate with glutathione-capped CdTe quantum dots as a novel fluorescence probe Chenxia Hao,a Shaopu Liu,a Dan Li,a Jidong Yanga,b and Youqiu Hea* ABSTRACT: A sensitive and simple analytical strategy for the detection of sodium cromoglycate (SCG) has been established based on a readily detectable fluorescence quenching effect of SCG for glutathione-capped (GSH-capped) CdTe quantum dots (QDs). The fluorescence of GSH-capped CdTe QDs could be efficiently quenched by SCG through electron transfer from GSHcapped CdTe QDs to SCG. Under optimum conditions, the response was linearly proportional to the concentration of SCG between 0.6419 and 100 μg/mL, with a correlation coefficient (R) of 0.9964; the detection limit (3δ/K) was 0.1926 μg/mL. The optimum conditions and the influence of coexisting foreign substances on the reaction were also investigated. The very effective and simple method reported here has been successfully applied to the determination of SCG in synthetic and real samples. It is believed that the established approach could have good prospects for application in the fields of clinical diseases diagnosis and treatment. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: CdTe quantum dots; sodium cromoglycate; fluorescence; electron transfer
Introduction
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Semiconductor quantum dots (QDs), a particularly interesting class of emerging nanomaterials, exhibit unique optical and photophysical properties, such as a broad excitation spectrum, readily size-tunable narrow emission spectrum, high fluorescence quantum efficiency and long-term photostability, as well as resistance to photobleaching (1–5). In addition, the luminescence of QDs is extremely sensitive to the surface state. The interaction between different types of analytes and the surface of QDs can result in dramatic changes in their optical features (4,5). Based on these desirable characteristics, QDs have become ideal fluorescent semiconductor indicators for chemical and biological analysis. For example, Chen et al. used mercaptoacetic acid-modified ZnSe QDs as a fluorescent semiconductor probe for paeoniflorin determination based on a fluorescence quenching method (6). Liu et al. developed a near-infrared fluorescence probe for 2,4,6-trinitrophenol detection based on bovine serum albumin-coated CuInS2 QDs (7). 1,10-Phenanthroline-capped CdTe QDs were reported as a Cd2+ sensor by Hu et al., with a detection limit of 0.01 nM (8). In this paper, a typical GSH-capped CdTe QDs probe has been prepared and constructed for monitoring sodium cromoglycate. Sodium cromoglycate (SCG), also referred to as disodium cromoglycate or pharynx Thailand, is an important antiallergic, which is commonly used for the treatment of allergic asthma and has a good clinical efficacy in patients with mild or moderate persistent asthma (9–11). Furthermore, SCG has been employed in the prophylactic treatment of allergic rhinitis (12,13) and allergic conjunctivitis in spring (14). Nowadays, many analytical methods have been reported for the determination of SCG, such as liquid chromatographic-tandem mass spectrometry (LC-MS-MS) (15–17), capillary electrophoresis (CE) (18), thin-layer chromatographydensitometry (TLC-densitometry) (19) and cathodic striping voltammetry (CSV) (20). However, those methods tend to be expensive,
Luminescence 2015; 30: 1112–1118
time-consuming and technically complex and do not allow high throughput analysis. To better exploit and safely utilize SCG in the diagnosis and treatment of clinical diseases, it is necessary to develop a rapid, simple, high-sensitivity, low-cost and throughput method for the quantitative determination of SCG. To the best of our knowledge, to date, no endeavor has been made to use GSH-capped CdTe QDs as a fluorescence semiconductor probe for the quantitative determination of SCG. In this study, we investigated the interaction between GSH-capped CdTe QDs and SCG using fluorescence and UV/Vis absorption spectroscopy. The corresponding results revealed that the fluorescence of GSH-capped CdTe QDs could be efficiently quenched by SCG through electron transfer from GSH-capped CdTe QDs to SCG, and the quenching mechanism was dynamic quenching. Meanwhile, the optimum conditions and the interference of coexisting substances on the reaction were also investigated. The very effective and simple method reported here has been successfully applied to the detection of SCG in its synthetic samples and real samples. It is believed that the established approach could have good prospects for application in the fields of the clinical disease diagnosis and treatment.
* Correspondence to: Y. Q. He, Key Laboratory on Luminescence and RealTime Analysis, Ministry of Education, School of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, People’s Republic of China. E-mail: [email protected] a
Key Laboratory on Luminescence and Real-Time Analysis, Ministry of Education, School of Chemistry and Chemical Engineering, Southwest University, Chongqing400715, People’s Republic of China
b
School of Chemical and Environmental Engineering, Chongqing Three Gorges University, Chongqing, Wanzhou, 404000, People’s Republic of China
Copyright © 2015 John Wiley & Sons, Ltd.
Analytical strategy for the detection of sodium cromoglycate
Experimental
Results and discussion
Apparatus and reagents
Characterization of GSH-capped CdTe QDs
Fluorescence spectra were recorded using a Hitachi F-2500 spectrofluorophotometer (Hitachi Company, Tokyo, Japan) equipped with a 1.0 cm standard quartz cuvette and the slit width for excitation and emission was set at 10 nm. The UV/Vis absorption spectra were acquired by a UV-2450 spectrophotometer (Tianmei Corporation, Shanghai, China) with a 1.0 cm standard quartz cuvette. The morphology features and size of nanoparticles were carried out using a Hitachi-600 transmission electron microscopy (TEM), Hitachi Company, Tokyo, Japan. The pH measurements were conducted with a pHS-3C pH meter (Leici, Shanghai, China). Magnetic stirring was performed with a SZCL-A magnetic stirrer (Zhengzhou, China). Cadmium chloride (CdCl2·2.5H2O) and tellurium (Te) powder were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China)S Glutathione (GSH) and sodium borohydride (NaBH4) were obtained from Aladdin Chemistry Co. Ltd, (Shanghai, China) and Tianjin Huanwei Fine Chemical Co. Ltd, (Tianjin, China) respectively. Sodium cromoglycate (SCG) and sodium cromoglycate eye drops were purchased from Adamas Reagent Co. Ltd and Wuhan Wujing Medicine Co., Ltd, (Wuhan, China) respectively. Tris/HCl buffer solutions at different pH values were prepared by mixing 0.05 mol/L Tris and 0.1 mol/L HCl in suitable proportions. All reagents used in the experiments were of analytical grade or better. Ultrapure water was used to prepare all solutions.
The morphological features of the as-prepared CdTe QDs were studied using TEM. As shown in Fig. 1, the particle sizes were homogeneous and the average size was ~ 3 nm. In addition, the UV/Vis absorption spectrum and fluorescence spectrum of the GSH-capped CdTe QDs were obtained and are shown in Fig. 2. Curve b shows a strong UV absorption with a characteristic absorption peak at 542 nm. Meanwhile, curve a exhibits a symmetric and narrow fluorescence bandwidth, which further confirmed that the resulting CdTe QDs were homogeneous. The particle sizes of the resulting CdTe QDs could also be calculated from the first absorption maximum of the UV/Vis absorption spectrum by the following equation (23): D ¼ 9:8127107 λ3 1:7147103 λ2 þ ð1:0064Þλ 194:84 (1) where D is the diameter of resulting CdTe QDs and λ is the wavelength of the first excitonic absorption peak of resulting CdTe QDs. The outcome showed that the particle diameter of the GSH-capped CdTe QDs was ~ 3.1 nm (λ = 542 nm), which is in good agreement with the TEM observation results. The quantum yield (QY) of GSH-capped CdTe QDs was measured using Rhodamine 6G as a criterion (QY = 95%), which was used to calculate the QY of GSH-capped CdTe QDs as follows (24):
Synthesis of GSH-capped CdTe QDs Aqueous CdTe QDs modified by GSH were synthesized according to the previously reported method (21). Under an argon atmosphere and magnetic stirring, Te powder (0.0385 g) was placed in a 50.0 mL three-necked flask and reacted with excessive sodium borohydride in deionized water to produce a colorless solution of NaHTe. CdCl2·2.5H2O (0.1026 g) and GSH (0.1847 g) were dissolved in 150.0 mL of deionized water. Under an argon atmosphere and magnetic stirring, the pH of the mixture was adjusted to 10.96 by the dropwise addition of 1.0 mol/L NaOH solution. H2SO4 (0.5 mol/L) was then introduced to NaHTe to produce H2Te gas, which passed through the cadmium precursor with a slow argon flow for 30 min. CdTe precursors were formed at this stage. Subsequently, the resulting solution mixture was heated to 369 K and refluxed under argon for 1 h. A salmon pink GSHcapped CdTe solution was obtained. The concentration of GSH-capped CdTe QDs was 2.0 × 10-3 mol/L (determined by the concentration of Te2-) (22).
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0.32
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0.24
b 1400
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/nm Figure 2. (a) UV/Vis absorption spectrum and (b) fluorescence spectrum (excited at 350 nm) of GSH-capped CdTe QDs.
Copyright © 2015 John Wiley & Sons, Ltd.
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In this step, to study the interaction between GSH-capped CdTe QDs and SCG, 1.2 mL of the above-synthesized GSHcapped CdTe QDs, 1.0 mL Tris/HCl solution, and a series of different concentrations of SCG were added to a 10.0 mL volumetric flask. The mixture was then diluted with deionized water to the mark, and mixed thoroughly by gentle shaking. After incubation for 20 min, the fluorescence and absorption spectra of the solution were measured at an excitation wavelength of 350 nm.
Absorption
General procedure for SCG detection
Figure 1. TEM image of GSH-capped CdTe QDs.
C. Hao et al. 2 Fu As n u2 Yu ¼ Y s Fs Au ns
(2)
where Yu is the QY of the sample solution to be measured and Ys is the reference solution. Fu and Fs are the integral intensity, Au and As are absorption values, nu and ns stand for the refractive indexes of the solvents, and the subscripts ‘u’ and ‘s’ refer to the samples and standard substance, respectively. The QY of GSH-capped CdTe QDs was 44.9%. Fluorescence quenching of GSH-capped CdTe QDs by SCG Under optimum conditions, the fluorescence spectra of GSHcapped CdTe QDs were recorded in the absence and presence of SCG, the results of which are shown in Fig. 3. The fluorescence intensity of GSH-capped CdTe QDs was efficiently quenched by SCG centered at 572 nm (excitation 350 nm), which was commonly attributed to the electron–hole pair separation. Simultaneously, no discernible shifts in the emission peak shape and location of the fluorescence spectra were found, even at relatively high concentrations (100 μg/mL) of SCG. On this basis, the possibility of establishing a sensitive method for SCG was evaluated. To evaluate the sensitivity of GSH-capped CdTe QDs for the detection of SCG, the fluorescence intensity of GSH-capped CdTe QDs at 572 nm was monitored by increasing the concentration of SCG over a certain range. As shown in Fig. 3(inset), a linear calibration plot of the fluorescence quenching intensity versus the concentration of SCG was observed over the range of 0.6419–100.00 μg/mL, with a correlation coefficient (R) of 0.9964 and a linear regression equation of F0 F = 20.84c + 80.77 (where c is the concentration of SCG in μg/mL). The detection limit of 0.1926 μg/mL for SCG was evaluated by using 3δ/K, where δ is the standard deviation of the fluorescence intensity of the blank samples from 11 replicate measurements and K is the slope of the calibration plot. Hence, it is quite obvious that the approach had a high sensitivity for detecting SCG in aqueous solution. Possible quenching mechanism Fluorescence quenching is generally classified as dynamic or static, and the two mechanisms can be distinguished by their differing dependence on temperature (25). If the quenching rate
2000
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1.8
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0 0
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40
C
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( g/mL)
11
F0 /F
500
1500 1000
where F0 and F are the fluorescence intensity of GSH-capped CdTe QDs in the absence and presence of a quencher (SCG), respectively; [Q] is the concentration of quencher (SCG) and KSV shows that the quenching efficiency of the quencher was the Stern–Volmer quenching constant. Figure 4 shows the Stern– Volmer plots of F0/F versus [Q] at three different temperatures (296, 290 and 303 K). The quenching constant (KSV) was further calculated according to the Stern–Volmer equation and is given by Table 1. It was clearly shown that the corresponding values of KSV increased with the increase in temperature, which implied that the probable quenching process for the GSH-capped CdTe QDs-SCG solution system was dynamic quenching. In order to further confirm the reaction mechanism, UV/Vis absorption spectra of the GSH-capped CdTe QD–SCG solution system were explored Fig. 5. In the spectrum of pure GSH-capped CdTe QDs (curve a), there was strong absorption in the UV region, whereas the absorption in the visible area was relatively weak. In the spectrum of SCG with distilled water as the reference (curve b), there were two strong absorption peaks at 252 and 327 nm. Curve c is the absorption spectrum for a mixture of SCG and GSH-capped CdTe QDs. By comparing curves b and c, curve d, representing the absorption spectrum of SCG with GSH-capped CdTe QDs as the reference was obtained, implying that the absorption spectrum of the mixture was simply a superposition of the GSH-capped CdTe QDs and SCG. Namely, the quenching process was dynamic quenching. In dynamic quenching, the quencher collides with the excited fluorophore and returns to ground state via either electronic or energy transfer (27). Because there is no overlap integral between the emission spectra of GSH-capped CdTe QDs and the absorption spectra of SCG (Fig. 6), fluorescence resonance energy transfer (FRET) was excluded (27). Thus, electron transfer was the most probable mechanism for the interaction between GSH-capped CdTe QDs and SCG. However, the photoexcited QDs might participate in electron transfer via two general 2.0
1
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Fluorescence intensity
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constants increase with increasing temperature, the fluorescence quenching behavior is identified as dynamic quenching between quencher and fluorophore. In the case of the reverse effect, it is considered to be a static quenching. The well-known Stern–Volmer equation (26) could be utilized to analyze the florescence quenching mechanism. . F0 ¼ 1 þ K SV ½Q (3) F
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/nm
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Figure 3. Fluorescence spectra of GSH-capped CdTe QD–SCG system. The concentrations of SCG added for spectra 1–11 were: 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 μg/mL, respectively. (Inset) Linear relationship between fluorescence quenching intensity and the concentrations of SCG.
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Figure 4. Stern–Volmer plots for the GSH-capped CdTe QD–SCG system at three different temperatures. The concentration of GSH-capped CdTe QDs was 4 2.4 × 10 mol/L, pH 7.8.
Copyright © 2015 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 1112–1118
Analytical strategy for the detection of sodium cromoglycate Table 1. Stern–Volmer quenching constants for the interaction of GSH-capped CdTe QDs with SCG at different temperatures Temperature (K) 276 290 303
Stern–Volmer linear equation
KSV (L/mol)
Ra
SDb
F0/F = 0.9792 + 6.025 × 103 [Q] F0/F = 0.9768 + 7.946 × 103 [Q] F0/F = 1.0432 + 9.099 × 103 [Q]
6.025 × 103 7.946 × 103 9.099 × 103
0.9976 0.9997 0.9970
0.0150 0.0068 0.0245
a
R is the correlation coefficient. SD is the standard deviation for the Ksv values.
b
1.6 1.4
c
Absorption
1.2
a
1.0
d
0.8 0.6 0.4 0.2
b 300
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/nm
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a 0.4
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b
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Absorption
Figure 5. UV/Vis absorption spectra of (a) GSH-capped CdTe QDs, (b) SCG (with distilled water as the reference), (c) the mixture (GSH-capped CdTe QDs and SCG) and (d) SCG (with GSH-capped CdTe QDs as the reference). GSH-capped CdTe 4 QDs, 2.4 × 10 mol/L; SCG, 60 μg/mL).
600
/nm Figure 6. UV/Vis absorption spectrum of (a) SCG and the fluorescence spectrum of (b) GSH–CdTe QDs in Tris/HCl buffer solution at pH 7.8. GSH-capped CdTe 4 QDs, 2.4 × 10 mol/L; SCG, 60 μg/mL; Tris/HCl buffer solution, 1.0 mL.
pathways (27) because of the formation of a free electron in the conduction band and a positively charged hole in the valence band of QDs; (as outlined in Scheme 1). In the presence of an electron acceptor, electron transfer might proceed from the condution band of QDs to the acceptor units. However, in the presence of an electron donor, electron transfer might occur from the donation units to the valence band holes of QDs. The electron–hole pair separation leads to the luminescence quenching of QDs (28). In this system, SCG was most likely to serve as an efficient electron acceptor (29). On adding SCG to the solution of GSH-capped CdTe QDs, electron transfer proceeded from the condution band of GSH-capped CdTe QDs to SCG, which prevented the electron–hole recombination and caused the fluorescence quenching of GSH-capped CdTe QDs (Scheme 1).
Optimum reaction conditions Influence of the acidity. The influence of acidity of Britton– Robinson, phosphate, Hasting–Sendroy and Tris/HCl buffer solutions on the fluorescence intensity of GSH-capped CdTe QDsSCG system was investigated. The experiments showed that Tris/HCl buffer solution appeared to be optimal for the determination of SCG. Accordingly, Tris/HCl buffer solution was selected to control the acidity of analytical system. The effect of pH in a range between 7.0 and 8.0 was studied to obtain the optimal reaction pH for the determination of SCG with GSH-capped CdTe QDs and the results are shown in Fig. 7(a). It was found that the maximum fluorescence intensity of GSH-capped CdTe QD– SCG was obtained at pH 7.8. At the same time, the effect of the dosage of Tris/HCl buffer on the fluorescence intensity of GSH-capped CdTe QD–SCG was also discussed. As shown in Fig. 7(b), the optimal dosage of Tris/HCl was 1.0 mL. Influence of aqueous CdTe QDs concentration. The effect of GSH-capped CdTe QDs concentration on the fluorescence
Luminescence 2015; 30: 1112–1118
Copyright © 2015 John Wiley & Sons, Ltd.
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Scheme 1. Mechanism of interaction of GSH-capped CdTe QDs with SCG. A representative SCG.
C. Hao et al.
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Figure 7. Effect of acidity on the fluorescence intensity of GSH-capped CdTe QD–SCG system in the presence (F) and the absence (F0) of SCG. (a) Effect of pH of buffer -4 solution; (b) effect of dosage of buffer solution. GSH-capped CdTe QDs, 2.4 × 10 mol/L; SCG, 20 μg/mL.
Influence of incubation time. The impact of incubation time on the fluorescence intensity of the GSH-capped CdTe QD–SCG system was investigated at different time scales at room temperature and the results are shown in Fig. 9. The results implied that the reaction was finished completely in 20 min and the fluorescence intensity remained stable for at least 1 h. Therefore, a time scale of 20 min was adopted in subsequent experiments.
Selectivity and analytical applications
3800
Fluorescence intensity
intensity of the GSH-capped CdTe QD–SCG system was explored by keeping the SCG concentration and the pH constant and changing the GSH-capped CdTe QDs concentration. As shown in Fig. 8, the optimal concentration of GSH-capped CdTe QDs was 2.4 × 104 mol/L for the GSH-capped CdTe QD–SCG system, namely, 1.2 mL above prepared GSH-capped CdTe QDs.
3600 3400 3200 3000 2800 0
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Time(min) Figure 9. Effect of incubation time on the fluorescence intensity of GSH-capped CdTe QD–SCG system in 1.0 mL Tris/HCl buffer solution at pH 7.8. GSH-capped -4 CdTe QDs, 2.4 × 10 mol/L; SCG, 20 μg/mL.
Interference of coexisting foreign substances In order to explore the selectivity of the proposed method, the interference of some coexisting substances on the determination of the GSH-capped CdTe QD–SCG system was tested under optimal conditions and the results are given in Table 2. When the concentration of SCG was 20 μg/mL, most of the common ions (Na+, K+, NH4+, Mg2+, Cl–, NO3–, CO32–, SO42–, HPO42–), amino acids, glucose, etc. had less effect on the determination of SCG, whereas, Zn2+, Ca2+, Al3+ and Fe3+ at relatively lower concentrations had no significant impact.
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Figure 8. Effect of GSH-capped CdTe QDs concentration on the fluorescence intensity of GSH-capped CdTe QD–SCG system in the presence (F) and the absence (F0) of SCG. SCG, 20 μg/mL; Tris/HCl buffer solution, 1.0 mL, pH 7.8.
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Analytical application The synthetic samples were analyzed according to the previously reported method (30). The composition of the synthetic samples is shown in Table 3. Under optimal conditions, 1.2 mL of the above-prepared GSH-capped CdTe QDs, 1.0 mL of Tris/HCl solution, an appropriate amount of synthetic samples and SCG were added to determine the SCG concentration. As shown in Table 3, the proposed method could be applied to detect SCG in synthetic samples with satisfactory results. The proposed method was also successfully employed for the determination of SCG in sodium cromoglycate eye drops. The sodium cromoglycate eye drops were diluted 100 times using ultrapure water to obtain detection samples. Under optimal conditions, 1.2 mL of the above-prepared GSH-capped CdTe QDs, 1.0 mL of Tris/HCl solution and an appropriate amount of pharmaceutical samples were added to determine the SCG concentration. As shown in Table 4, there was no significant difference between the determined values of SCG and the declared content, indicating the accuracy of the proposed method. The relative standard deviation (RSD) was < 4.75% and the recovery was between 99.80 and 105.3%, which indicated that the determination of SCG using GSH-capped CdTe QDs as fluorescence probe was sensitive and reproducible. The present method was able to meet the requirements of microanalysis in real samples.
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Luminescence 2015; 30: 1112–1118
Analytical strategy for the detection of sodium cromoglycate Table 2. Effect of coexistent substances for GSH-capped CdTe QD–SCG system (GSH-capped CdTe QDs, 2.4 × 10-4 mol/L; SCG, 20 μg/mL) Coexistent material
Concentratin (μg/mL)
+
Na K+ NH4+ Mg2+ Ca2+ Zn2+ Al3+ Fe3+ Cl– NO3– SO42–
1500 600 800 800 4 2 10 2 1300 1500 800
Relative error (%) –3.06 –3.01 +2.97 –1.06 –3.88 –1.74 +1.81 +1.93 +2.16 –3.06 –1.06
Coexistentmaterial
Concentratin (μg/mL)
2–
HPO4 CO32– L-Alanine L-Threonine L-Leucine L-Proline L-Arginine Creatine Glucose Egg albumin Urea
Relative error (%)
800 600 700 600 600 600 500 400 600 400 600
+2.29 +3.91 –3.06 +1.20 +4.36 –3.27 –2.20 +2.36 –2.29 +3.20 +2.39
Table 3. Results for the determination of SCG in synthetic samples (n = 5) Sample
Added (μg/mL)
1 2 3
20.0 20.0 20.0
Foreign substances NaNO3, L-Alanine, Egg albumin MgSO4, L-Threonine, Glucose KCl, L-Leucine, Creatine
Found (n = 5, μg/mL)
RSD (n = 5, %)
20.541 19.534 20.052
Recovery (n = 5, %)
2.88 3.89 1.96
102.71 97.67 100.26
Concentrations (μg/mL): NaNO3, 40; L-alanine, 40; egg albumin, 40; MgSO4, 40; L-threonine, 40; glucose, 40; KCl, 40; L-leucine, 40; creatine, 40.
Table 4. Results for the determination of SCG in sodium cromoglycate eye drops (n = 5) Sample
Original found (μg/mL)
1 2 3
5 10 20
Determination found (μg/mL) 4.8254, 4.9003,5.0504, 5.0755, 5.1256 9.1794, 10.3968, 10.0616, 10.2567, 10.0060 20.7536, 20.7884, 21.0325, 21.2075, 21.5241
5.00 9.98 21.06
RSD (n = 5, %) 2.53 4.75 1.51
Recovery (n = 5, %) 99.91 99.80 105.30
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Conclusions In summary, a novel and effective fluorescence semiconductor probe based on the fluorescence quenching effect of GSHcapped CdTe QDs was successfully established for the detection of SCG in aqueous medium. Under optimum conditions, the response was linearly proportional to the concentration of SCG between 0.6419 and 100 μg/mL, with a detection limit (3δ/K) of 0.1926 μg/mL. In additionally, the mechanism of the interaction of GSH-capped CdTe QDs with SCG was also discussed in detail. The proposed method had simplicity, rapidity and high sensitivity, and was applied to the quantitative analysis of SCG in synthetic samples and real samples with satisfactory results. Acknowledgements/Acknowledgments according to US/UK author
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This work was supported by Chongqing Municipal Key Laboratory on Luminescence and Real-Time Analysis, Southwest University (CSTC, 2006CA8006) and the National Natural Science Foundation of China (No. 21175015).
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Mean found (n = 5, μg/mL)
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