ОПТИКА И СПЕКТРОСКОПИЯ, 2013, том 115, № 4, с. 676-680
ГЕОМЕТРИЧЕСКАЯ И ПРИКЛАДНАЯ ОПТИКА
УДК 535.8
CdTe QUANTUM DOTS AS FLUORESCENCE SENSOR FOR THE DETERMINATION OF AMINOPHYLLINE IN AQUEOUS SOLUTION
© 2013 г. Rui-yong Wang, Jing Wu, Lv-jing Wang, Rui Wang, and Huan-jing Dou
Department of chemistry, Zhengzhou University, 450001 Zhengzhou, China E-mail: wangry@zzu.edu.cn Received January 9, 2013
A novel CdTe quantum dots (QDs) based technology platform was established in aqueous solution. It can perform accurate and simple determination of aminophylline concentration in pharmaceutical samples with satisfactory results. Under optimum conditions, the relative fluorescence intensity of CdTe quantum dots is linearly proportional to aminophylline concentration from 2.00 to 80.0 p.g ml-1 with a correlation coefficient of 0.9979 for aminophylline determination and a detection limit of 0.531 p.g ml-1.
DOI: 10.7868/S0030403413100140
INTRODUCTION
Semiconductor quantum dots (QDs) have generated a tremendous interests in the fields of physics, chemistry, biology and engineering. Because of quantum confinement effect, QDS have shown some unique optical and electronic properties [1—3], such as broad excitation spectra and narrow, symmetric and tinable emission spectra and so on [4—7]. Therefore quantum dots have found widespread application in chemical, material, biological and medical fields, such as versatile immunosensors, escherichia coli detection, light-emitting diodes, luminescent probes for labeling of cells and tissues, photosensitive films and detection of single DNA molecules.
Aminophylline is a bronchodilator. It is a compound of the bronchodilator theophylline with ethyl-enediamine in 2 : 1 ratio. The ethylenediamine improves solubility, and the aminophylline is usually found as a dehydrate. Aminophylline is a nonselective adenosine receptor antagonist and phosphodiesterase inhibitor capable of reversing ischemia-induced brad-yasystole [8]. It does this by improving the rate of return of spontaneous circulation (ROSC). After exhaustive conventional therapy, aminophylline is administered early in resuscitation efforts to reestablish cardiac rhythm. With aminophylline administration, the patient may have immediate resumption of cardiac electrical activity [9]. Aminophylline should be an intervention with the Advanced Cardiac Life Support (ACLS) treatment of atropine-resistant asystolic out-of-hospital cardiac arrest.
Up to now, a variety of methods have been developed for the determination of aminophylline, including chemiluminescence, FT-Raman spectroscopy, electrochemical oxidation, high performance liquid chromatography, spectrophotometry method [10—15]
and so on. In the present work, we report a new and simple method for fast detection of aminophylline based on fluorescence enhancement of TGA-capped CdTe QDs in aqueous media. Some influencing factors, such as pH, the concentration of QDs and foreign cations, were studied in detail. Under optimum conditions, the calibration graph was linear in the range of 2.00 and 80.0 ^g ml-1 of aminophylline concentration, with a correlation coefficient of0.9979 and a detection limit of 0.531 ^g ml-1. The proposed method was applied to the determination of amino-phylline in pharmaceutical samples, and the results were satisfactory.
EXPERIMENTAL
Reagents and Chemicals
All reagents used were of analytical grade. Cd(NO3)2 ■ 4H2O, thoglycolic acid (TGA), Te powder (Aladdin Chemicals Co, Shanghai, China), and KBH4 (Tianjin Kermel Chemicals Co, Tianjin, China) were used to prepare CdTe QDs. Aminophylline (98%) was purchased from Aladdin Chemical Reagents Factory, Shagghai, China. Pharmaceutical formulations of aminophylline (tablets and injection) were purchased from local drugstore. A set of buffer solution of different pH values was achieved by solution A (KH2PO4) and solution B (Na2HPO4). Aqueous solutions of K+, Na+, Ca2+, Mg2+, Mn2+, Ba2+, and NH+ were prepared from KCl, NaCl, CaCl2, MgCl2 ■ 6H2O, MnCl2 ■ 4H2O, Ba(NO3)2, and NH4Cl respectively. All reagents were analytical grade without further purification and ultrapure water was used throughout.
Wavelength, nm
Fig. 1. UV-Vis absorption spectrum (a) and fluorescence emission spectra (b) of TGA-capped CdTe QDs,
Apparatus
Synthesis of the Water-Soluible CdTe QDS
All fluorescence measurements were carried out with a 970CRT spectrofluorometer (Sanco, Shanghai) equipped with a plotter unit and a quartz cell (1 x x 1 cm). The fluorescent emission spectra were recorded in the wavelength range of 450—625 nm upon excitation at 330 nm. The slit width of excitation and emission was 5 and 5 nm respectively. The UV-Vis absorption spectra were recorded between 350 and 650 nm on a UV-1800PC with a 1.0 cm path-length quartz cuvette. A pHS-3C digital pH meter (Leici, Shanghai, China) was used to adjust the pH values of the solutions.
Luminescent CdTe QDs capped with TGA were synthesized according to the method reported by Li et al. [16] with some modifications. Briefly, in a three-necked flask with a condenser attached, TGA, as a stabilizer, was injected into N2-saturated Cd(NO3)2 aqueous solution, followed by adjustment of the pH to 10.0 with 1.0 mol L-1 NaOH. The solution was deaer-ated by N2 for at least 30 min. Then KHTe aqueous solution was injected into the flask while stirring vigorously. The Cd2+-Te-TGA molar ratio was fixed at 1 : 0.5 : 2.4. After mixing, the solution was heated to 100°C for 3 hours, and TGA-capped CdTe QDs could be obtained. The resulting products were precipitated by acetone, and superfluous TGA and Cd2+ that did not participate in the reaction were removed by cen-
Fig. 2. HR-TEM image of TGA-capped CdTe QDs solution (10-5 mol L-1).
Fig. 3. Effect of pH on the reaction between CdTe QDs and aminophylline. The concentrations of CdTe QDs and aminophylline were 10-5 mol L-1 and 15 ml-1 respectively.
AF
90 г
10-1-1-1-
0 6 12 18
CCdTe QDs x 106 mo1 L-1
Fig. 4. Effect of the concentration of CdTe QDs on the fluorescence intensity, aminophylline: 15 ml-1.
trifugation at 4000 rpm for 3.0 min. The process of purification had no obvious effect on the stability of the TGA-capped CdTe QDs. The fluorescence intensity, peak width at half-height of CdTe QDs, showed no ob-
Table 1. Effect of foreign substances (aminophylline 15 ^g mL-1)
Foreign substance Coexisting concentration, (^g mL-1) Change of fluorescence intensity, %
NaCl 1500 -4.0
KCl 1500 + 1.2
Glucose 800 + 1.2
Ascorbic acid 200 + 6.1
Cane sugar 800 +3.9
Starch 800 + 1.9
MgSO4 30 -4.9
MnCl2 80 +2.6
Vitamin B12 90 -3.8
CaCl2 40 +1.5
Ba(NO3>2 110 +5.1
Serine 80 +1.3
Leucinc 80 + 4.2
Valine 100 +2.5
Proline 150 +1.0
Glycine 65 + 4.3
Threonine 100 +2.7
NH4Cl 200 +5.7
Vitamin B1 100 +1.3
Threophylline 85 + 6.7
Diprophylline 310 +2.8
Caffeine 70 +2.1
Thocphylline-7-acctic 65 + 6.4
acid
vious changes after removing the redundant TGA. The resultant precipitate was redispersed in water, repre-cipitated with an amount of acetone more than twice, and then kept at 4°C in the dark for later use.
Sample Treatment
For pharmaceutical analysis, ten pieces of aminophylline tablets were taken and powered in a mortar. An accurately weighed portion of the powder was transferred into 50 ml calibrated flask and diluted to mark with distilled water. The solution was extracted in an ultrasonic bath for 20 min. Then, insoluble excipients were removed from the solution with centrif-ugation at 4000 rpm for 10 min. For aminophylline injection, transferring exactly 2.00 ml of it into a 100 mL volumetric flask and diluting to the mark with distilled water.
Procedures
TGA-capped CdTe QDs solution (10-5 mol L-1), 0.1 mL KH2PO4-Na2HPO4 buffer solution (pH 7.7) and specific amounts of aminophylline were added sequentially to a 5 mL calibrated test tube. The mixture was then diluted to the scheduled volume with double distilled water and mixed thoroughly. It should be mentioned that the fluorescence spectra were measured with the sample reacting for 40 min to obtain stable fluorescence intencity. The fluorescence intensity of the solution was recorded at 527 nm with excitation wavelength of 330 nm.
RESULTS AND DISCSSION
Characterization of CdTe QDs
Fluorescence spectra and absorption spectra are powerful tools to confirm quantum-confined property of semiconductor QDs, and are utilized to character the TGA-capped CdTe QDs [17]. The UV-Vis absorption spectrum of CdTe QDs showed a well-resolved absorption maximum of the first electronic transition, indicating a narrow size distribution of the CdTe QDs. The particle size is determined from the first absorption maximum (478 nm) by use of the empirical formula [18]:
D = (9.81278 x 10-7)X3 -- ( 1.7147 x 10-3)X2 + 1.0064X - 194.84.
Where D (nm) is the size of a given CdTe QD sample, and X (nm) is the wavelength of the first excitonic absorption peak of the corresponding sample.
Figure 1 shows the UV-Vis absorption and fluorescence spectra of TGA-capped CdTe QDs. The sizes were 1.6 nm CdTe QDs at the first excitonic absorption peak. With an excitation wavelength of 330 nm, TGA-capped CdTe QDs exhibited an obvious, symmetrical fluorescence emission spectrum with an emis-
ОПТИКА И СПЕКТРОСКОПИЯ том 115 № 4 2013
sion maximum at 527 nm, and indicating CdTe QDs have good fluorescence properties. The TGA-capped CdTe QDs are futher characterized by HR-TEM (Fig. 2). The results show that the CdTe QDs possess good crystalline structures, and the size of QDs is consistent with that calculated from the absorption spectra.
Effect of Reaction Time and Mixing Sequence
Under the optimum conditions, the effect of incubation time on fluorescence intensity was investigated. The results showed that the fluorescence intensity reached a maximum at 30 min after all the reagents had been added, and remained stable for over 1 h. Therefore in this study, the fluorescence emission spectrum were recorded after 40 min
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