Research article Received: 26 April 2014,
Revised: 5 September 2014,
Accepted: 17 September 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2802
Determination of ethanol using permanganate– CdS quantum dot chemiluminescence system Jafar Abolhasani* and Javad Hassanzadeh ABSTRACT: A novel and highly sensitive chemiluminescence (CL) method for the determination of ethanol was developed based on the CdS quantum dots (QDs)–permanganate system. It was found that KMnO4 could directly oxidize CdS QDs in acidic media resulting in relatively high CL emission. A possible mechanism was proposed for this reaction based on UV/Vis absorption, fluorescence and the generated CL emission spectra. However, it was observed that ethanol had a remarkable inhibition effect on this system. This effect was exploited in the determination of ethanol within the concentration range 12–300 μg/L, with detection at 4.3 μg/L. In order to evaluate the capability of presented method, it was satisfactorily utilized in the determination of alcohol in real samples. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: chemiluminescence; quantum dots; potassium permanganate; ethanol
Introduction When introduced into the human body, ethanol (EtOH), one of the most common toxic substances consumed by humans, is completely oxidized or can be partially eliminated via the skin, respiration or urine. Ethanol ingestion can affect the nervous and digestive systems, and at higher concentrations, can lead to coma or even death. Also, in industry, for example during fermentation and distillation, high ethanol vapour concentrations can cause inflammation of the nasal mucous membrane and conjunctiva, and irritation of the skin. The highest allowed level of ethanol vapour in the workplace, determined by the American Conference of Governmental Industrial Hygienists (ACGIH), is 1000 ppm (1). Therefore, the rapid, inexpensive, sensitive and selective determination of ethanol is of great importance in different areas. For example, it can help the police to recognize drink–driving offenders. The level of ethanol in a foodstuff might also be an indicator of quality in cases where ethanol is the product of food degradation (2). In alcoholic beverages, the level of ethanol indicates the quality of the product (3). Many analytical methods have been developed to measure ethanol in different real samples. In general, the working principle behind many alcohol biosensors is the enzymatic destruction of alcohol. The utility of such biosensors is restricted due to their low stability, short lifetime determined mainly by enzyme kinetics, the need to add the coenzyme to the measuring solution and temperature (4). Other methods used to determine ethanol include: gas chromatography (GC) (5), high-performance liquid chromatography (HPLC) (6) capillary electrophoresis (CE) (7), nuclear magnetic resonance spectroscopy (NMR) (8), infrared spectroscopy (IR) (9,10), Raman spectrometry (11), spectrophotometry (2,12), spectrofluorimetry (4,13), chemiluminescence (CL) (14), electrochemical methods (15,16) and methods based on distillation followed by density measurement (17). These methods have some disadvantages. For example, the GC, HPLC, CE and NMR techniques are slow and not easily portable, and they require trained personnel and expensive equipment. In addition, fabrication of an enzyme-
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based biosensor is rather complicated and expensive. Finally, methods based on distillation are time consuming. CL, the emission of light from an excited species produced by a chemical oxidation reaction, has frequently been used for analytical purposes due to its intrinsic advantages of a low detection limit, wide linear dynamic range and relatively simple and inexpensive instrumentation. Many reactions utilize CL generation, but few show sufficient intensity to be suitable for use in analysis. Potassium permanganate is the most common oxidant used in CL reactions and has recently attracted much attention because it is inexpensive. There have been several comprehensive reviews by Barnett concerning a wide range of analytical applications of permanganate in CL reactions (18,19). In recent years, nanomaterials have been introduced into CL reactions. The development of nanoparticle science provides new opportunities for the development of CL. In the meantime, metallic and semiconducting nanoparticles have been studied extensively because of their quantum confinement effects, which exhibit unique size-dependent electronic, magnetic, optical and electrochemical properties. In this respect, nanomaterials have usually been used as a catalyst or as energy acceptors in CL reactions (20,21). The CL properties of nanomaterials as emitting species have rarely been reported, and the existing reports are mostly for quantum dots (22–25). Wang et al. (22) and Poznyak et al. (23) described the CL property of CdTe QDs and CdSe/CdS core/shell nanostructures in aqueous solution. Li et al. investigated the CL properties of CdS QDs capped with mercaptoacetic acid as light emitters (24). By contrast, surface of QDs are in direct contact with the surrounding gaseous environment and thus their CL can be influenced by some compounds. * Correspondence to: Jafar Abolhasani, Department of Chemistry, College of Science, Tabriz Branch, Islamic Azad University, Tabriz, Iran. E-mail: [email protected] Department of Chemistry, College of Science, Tabriz Branch, Islamic Azad University, Tabriz, Iran
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J. Abolhasani and J. Hassanzadeh In this study, CL emission generated from a thioglycolic acid (TGA)-capped cadmium sulfide (CdS) quantum dots (QDs)– potassium permanganate reaction was investigated. The mechanism of this reaction was studied using UV/Vis absorption, fluorescence and generated CL emission spectra. Furthermore, it is observed that ethanol has an inhibiting effect on the CL signal of this system, and based on these observations, a new simple and sensitive CL method has been developed for the determination of trace amounts of ethanol in alcoholic beverages including beer. This method is simpler and less expensive than other analysis techniques; at the same time, it offers good accuracy and precision.
Experimental Instrumentation Fluorescence spectra were recorded using a Shimadzu RF-5301 spectrofluorophotometer (Japan) equipped with a quartz cell (1 cm × 1 cm). The CL intensities were recorded on the same spectrofluorimeter with a closed excitation slit. The CL spectra were measured on a spectrofluorimeter using the flow mode with the excitation light source being turned off. UV/Vis absorption spectra were obtained with a UV-1800 spectrophotometer (Shimadzu, Japan). Transmission electron microscopy (TEM) images of the CdS QDs were obtained using a Leo 906 transmission electron microscope (Germany).
Reagents All experiments were carried out using analytical grade chemicals and solvents. Doubly distilled deionized water was used in the preparation of all solutions. Cadmium chloride (CdCl2 · 5.H2O), sodium sulfide (Na2S · 9H2O), TGA and ethanol (99.5%) were all purchased from Sigma-Aldrich and used without further purification. Cetyl trimethyl ammonium bromide (CTAB), KMnO4 and H2SO4 were obtained from Merck (Darmstadt, Germany; www. merck-chemicals.com).
Synthesis of TGA-capped CdS quantum dots TGA-capped CdS QDs were synthesized in aqueous solution using a modified procedure (26). TGA (100 mL; 0.05 mol/L) solution and 100 mL of CdCl2 (0.02 mol/L) solution were mixed in a round-bottomed flask. To this, a solution of 1 mol/L NaOH was added dropwise to increase the pH to 8. A white precipitation appeared on first adding the NaOH solution, and then dissolved. Finally, a clear solution was obtained. This was due to the formation of Cd–thioglycolic complexes with different structures at different pH values because of the different dissociation of carboxylate and the sulfhydryl group. The reaction mixture was then heated to 90 °C in the presence of pure argon. After that, 50 mL of Na2S (0.02 mol/L) was added and the flask was submerged in an ice-water bath for 2 min until the temperature decreased to 37 °C. Purification of synthesized QDs was carried out using ethanol. Equal volumes of QDs solution and ethanol were mixed and then precipitated QDs were separated by centrifugation and dispersed in an equal volume of deionized water. The concentration of the obtained QDs was 0.002 mol/L.
General procedure for CL detection CL measurements were carried out in the batch condition. H2SO4 (250 μL; 0.001 mol/L), 250 μL of CTAB (0.01 mol/L) and 1250 μL of synthesized CdS QDs solution were added to a 3 mL quartz tube. An appropriate volume of sample or standard ethanol solution was added and the final volume was made up to 2.3 mL with distilled water. After injection of 200 μL of KMnO4 (0.005 mol/L) using an automatic injector, monitoring of CL signal versus time was started automatically. Maximum CL intensity was used as the analytical signal.
Results and discussion Characterization of QDs The absorption and photoluminescence (PL) spectra of the synthesized TGA-capped CdS QDs are shown in Fig. 1. According
Figure 1. (a) Absorption (b) excitation and (c) emission spectra of synthesized CdS QDs. (Inset) TEM image of QDs.
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Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Determination of ethanol by CdS quantum dot chemiluminescence system to Peng’s calculation method (27), the particle sizes of the CdS QDs were ~ 3.3 nm, corresponding to PL peaks of 555 nm (Fig. 1b): D ¼ 9:8127 10
7
λ3 – 1:7147 10
3
λ2 þ ð1:0064Þλ–194:84
where D (nm) is the size of a given nanocrystal and λ (nm) is the wavelength of the first excitonic absorption peak of the corresponding nanocrystal. Also, TEM images (Fig. 1: inset) show that the synthesized CdS QDs were spherical in shape with a size distribution within the range 3.5 ± 0.9 nm. The room temperature PL quantum efficiency (QE) of the assynthesized CdS QDs was estimated following the procedure of Demas and Crosby (28) by comparison with rhodamine 6G, assuming its PL QE to be 95%. The calculated PL QE of CdS was 22%. CL of TGA-capped CdS QDs–KMnO4 In primary experiments, it was found that CdS QDs could be directly oxidized by some oxidants to produce relatively strong CL radiation. Thus, various concentrations of KMnO4, Ce(IV), K3Fe(CN)6 and H2O2, were examined using known amounts of CdS QDs. It was found that KMnO4 resulted in the strongest CL light radiation. Therefore, KMnO4 was selected as the best oxidant for CL production. In addition, to further enhance the CdS QDs-induced CL intensity and improve its analytical application, we introduced some surfactants, including SDS (anionic surfactant), sodium bis(2-ethylhexyl)sulfosuccinate) (AOT, anionic surfactant), CTAB and didodecyl dimethyl ammonium bromide (DDABr, cationic surfactant) into the CdS QDs CL system. The results showed a remarkable enhancing effect of CTAB on this system (Fig. 2). Therefore, CTAB was used in later experiments as the enhancer species. The CL intensity–time profiles for this system are shown in Fig. 2. The profiles indicated that the CL reactions were very quick and the CL intensity reached a maximum value within < 1 s of initiating the reactions with KMnO4.
Optimization of reaction condition Parameters influencing the CL signals of the KMnO4–CdS QDs system were investigated to establish the optimal conditions for the CL reaction. The type and concentration of acid have a very significant influence on permanganate CL emission intensity (19). Therefore, to examine the effect of an acidic medium on the CL signal in this system, several acids (including HCl, H2SO4, HNO3 and H3PO4) were tested at different concentrations. As can be seen in Fig. 3(a), the maximum CL response was obtained for 0.1 mmol/L H2SO4. This is consistent with some other reports in which sulfuric acid was used as a suitable acidic medium (19). The effect of the concentration of CdS QDs solution on the CL response of the assay was examined. The results (Fig. 3b) indicated that the CL intensity increased linearly with CdS QDs concentration from 5.0 × 105 to 1.0 × 103 mol/L. This linear relationship may be important in analytical aims. Free functional groups on the surface of the CdS QDs were ready for covalent coupling to various molecules (24). Therefore, TGA-capped CdS QDs can react with these molecules, and a novel labelled CL method could be established to analyse molecules. A QDs solution concentration of 103 mol/L was used in the determinations. Furthermore, it is well known that some surfactants can affect CL intensity (29). Thus, the effect of different surfactants (CTAB, SDS, AOT and DDABr) at various concentrations on the CL intensity of the KMnO4–CdS QDs CL system was investigated. From Fig. 3(c), it can be seen that CTAB greatly enhanced the CL intensity with an optimum concentration of 8.0 × 104 mol/L. From the UV spectra (Fig. 4), it can be seen that the addition of CTAB affected the absorption spectrum of CdS QDs. However, the changes caused by the addition of other surfactants are small. This may be attributed to the interaction of active charge forces (24). In the presence of CTAB, CdS QSs tended to form aggregate nanoparticles through the charge interaction between CTAB and TGA-capped CdS QDs. The aggregation of QDs results in increasing overlap of the electron valve functions in neighbouring quantum dots and possibly led to the red shift in the spectra (24). However, the aggregation of CdS QDs can decrease the distance between the nanocrystals, resulting in the generation of chemical energy during the oxidized CL reaction. This energy can easily transfer between CdS QDs, leading to enhanced generation of excited state of CdS QDs. Therefore, the CL intensity improved remarkably. In order to investigate the effect of permanganate concentration, solutions with different concentrations of KMnO4 were prepared over the range 1 × 104 to 8 × 104 mol/L. As shown in Fig. 3(d), the CL signal increased to 4 × 104 mol/L KMnO4 and then remained steady at higher concentrations. At lower KMnO4 concentrations, the number of excited intermediates is decreased and the response is diminished. Considering the above discussion of CL intensity and the consumption of reagents, the optimized conditions for the KMnO4–CdS QDs CL system were as follows: 0.4 mmol/L KMnO4, 8 × 104 mol/L CTAB, 0.1 mmol/L H2SO4 and 3.3 nm CdS QDs at a concentration of 1.0 × 103 mol/L.
Possible mechanism for the CL reaction Figure 2. CL profiles of KMnO4–CdS QDs systems in the absence (a) and presence (b) of CTAB under optimum conditions.
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Usually, finding emitting species is the first step in describing the possible mechanism of CL generation. Therefore, the CL emission spectrum was recorded for the TGA-capped CdS QDs–acidic
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J. Abolhasani and J. Hassanzadeh
Figure 3. Effect of (a) type and concentration of acid (1 mmol/L CTAB, 0.6 mmol/L KMnO4 and 0.8 mmol/L CdS QDs); (b) concentration of CdS QDs (1 mmol/L CTAB, 0.6 mmol/L KMnO4 and 0.1 mmol/L H2SO4); (c) type and concentration of surfactants (1 mmol/L CdS QDs, 0.6 mmol/L KMnO4 and 0.1 mmol/L H2SO4); (d) concentration of KMnO4 (0.8 mmol/L CTAB, 0.1 mmol/L H2SO4 and 1 mmol/L CdS QDs) on the CL intensity.
KMnO4 system in the presence of CTAB, using a spectrofluorimeter with a closed exciting slit (Fig. 5). The results showed a characteristic peak around 555 nm, which is similar to the fluorescence spectrum of synthesized QDs. This reveals that QDs probably act as emitting species in this CL reaction. However, an interaction between KMnO4 and the applied QDs can be recognized in the fluorescence and absorption spectra of QDs (Fig. 6). It can be seen that the fluorescence and absorption intensities of QDs decrease strongly in the presence of KMnO4. Also, it was found that the reaction between TGA and KMnO4 results in a weak CL emission, which may be associated with QDs emission. It is observed that the CL emission of QDs is greatly affected by the type of capping agent. Thus, in the present system, it may be suggested that in the reaction with QDs, KMnO4 can oxidize the capping agent (TGA) resulting in the production of an excited intermediate. Transferring energy to CdS QDs creates the final CL emission (22,24):
KMnO4
TGA→
M
M þ CdS QDs → M þ ðCdS QDsÞ
ðCdS QDsÞ → CdS QDs þ hν
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Where, M* is oxidation product of TGA which is in excited state. Furthermore, ethanol has a remarkable inhibiting effect on this CL emission, and two different ideas can be put forward to explain this. First, ethanol can be oxidized by KMnO4 and compete with QDs for KMnO4 consumption. Investigation of this reaction showed that the KMnO4 absorption intensity decreased on addition of ethanol. Second, the ethanol–QDs interaction, for example, esterification of the carboxylic group of the capping agent on the QDs surface, may result in lower QDs activation in the presence of ethanol. It is observed experimentally that the fluorescence emission of QDs decreases on addition of ethanol. Therefore, it is obvious that ethanol can decrease the CL intensity of the represented system via both mechanisms, but it seems that the former interaction is efficient. Analytical application of the CL system It is found that the addition of ultratrace amounts of ethanol to the CL system under optimum conditions leads to a significant decrease in CL intensity that is proportional to the ethanol concentration (Fig. 7). Based on this, a sensitive and relatively selective method was developed for the determination of ethanol.
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Determination of ethanol by CdS quantum dot chemiluminescence system
Figure 3. (Continued)
3
Figure 4. Absorption spectra of CdS QDs (1.0 × 10 mol/L) in the presence of different surfactants (0.5 mmol/L). (1) No surfactant, (2) DDABr, (3) AOT, (4) SDS and (5) CTAB.
Figure 5. CL spectrum of KMnO4–CdS QDs–CTAB system in the absence (1) and presence (2) of ethanol.
Under the optimum conditions described, the linearity ranges of the calibration graphs (Fig. 7, insets) were 12–300 μg/L with limits of detection (3S) of 4.3 μg/L for ethanol. The equation for regression lines was ΔI = 149.4C + 838 (R2 = 0.9994), where
ΔI = I0 – I is the difference between the CL intensity in the absence (I0) and presence (I) of ethanol, and C is concentration of alcohol in μg/L. The relative standard deviation (RSD) was < 3.5% for five determinations of 30, 100 and 250 μg/L ethanol.
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J. Abolhasani and J. Hassanzadeh
Figure 6. (A) absorption and (b) emission spectra of 1 mmol/L CdS QDs in the absence (1) and presence (2) of 0.4 mmol/L KMnO4. Condition: H2SO4 0.1 mmol/L.
Figure 7. CL profiles in the presence of various concentrations of ethanol (μg/L) under optimum conditions and (inset) the corresponding calibration graph.
Table 1. Summary of published methods for the determination of ethanol Method Fluorescence sensor Gas chromatography Nano SrCO3-chemiluminescence Spectrophotometry Colorimetry using alcohol dehydrogenase Amperometry High-performance liquid chromatography Raman spectrometry Spectrophotometry Enzymatic-chemiluminescence Alcohol dehydrogenase-modified electrode Amperometry Density measurements Current method
Linear range
Sample
LOD
Ref.
1–75 vol% – 6–3750 mg/L 0.1–10 vol% 46–230 mg/L Up to 69 mg/L – – 1–6 vol% 2.5–25 vol% 0.46–18.4 mg/L 23–345 mg/L 0–40 vol% 12–300 μg/L
Wine samples Alcoholic beverages – – Alcoholic beverages Alcoholic beverages – Alcoholic beverages Distilled liquors Wines Beer samples Alcoholic drinks Alcoholic beverages Beverages
– 0.5 mg/L 2.1 mg/L 0.03 vol% – 0.6 mg/L 102 mg 1 vol% 0.09 vol% 0.3 vol% 0.23 mg/L – 0.11 vol% 4.2 μg/L
4 5 1 10 2 3 6 11 12 14 15 16 17 –
Multiwalled carbon nanotube alcohol dehydrogenase
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Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Determination of ethanol by CdS quantum dot chemiluminescence system The results indicate that this CL system has good linearity and high sensitivity and precision. Comparison between the presented procedure and some other reported analytical methods for the determination of ethanol is summarized in Table 1. The linear dynamic range and limit of detection of our method is better than most of other methods and comparable with those of GC methods.
Study of interferences In order to test the interference effect of some potentially coexisting substances, increasing amounts of these species were added to a standard solution of 50 μg/L ethanol. The tolerable concentration ratios for interferences with a relative error
of < 5% are shown in Table 2. The amounts of the most potentially interfering species are below their tolerable levels, so there would be no interferences from these species.
Analysis of real samples The procedure was easily applied to the determination of ethanol in some beverages. In order to validate the method, known quantities of ethanol were added to real samples before pretreatment step, and the samples were prepared and analysed according the general procedure described above. The obtained results are shown in Table 3. Statistical analysis of these results using the Student’s t-test showed that there are no significant differences between added and found values.
Table 2. Interference of different species on the CL intensity of CdS QDs–KMnO4–CTAB under optimum conditions with 50 μg/L ethanol Coexisting species 2+
+
+
Interference ratio
3 , CO2 3 , PO4 , Cl , 2+ 2+ 3+
+
NO 3
Ca , Na , K K , Cd2+, Co2+, Ni , Mg , Fe Glucose, sucrose, vitamin B1, B2 2+ 2+ SO2 4 , Cu , Zn Acetone Uric acid, acetic acid, ascorbic acid, cysteine, phenol Methanol Hg2+
2500 1000 1000 700 500 300 80 20
Table 3. Results for the determination of ethanol in real samples Sample Added Red wine (1)
Red wine (2)
White wine (1)
White wine (2)
Beer
Whisky
Delester (Iranian beer)
Recovery (%) a
Ethanol (mg/mL)
0 20 40 0 20 40 0 20 40 0 20 40 0 10 20 0 50 100 0 1 2
t-statistic
b
Found a 91.4 ± 1.3 111.6 ± 0.7 131.9 ± 0.9 86.4 ± 1.1 106.6 ± 0.6 127.4 ± 1.4 104.2 ± 2.0 124.9 ± 0.7 144.4 ± 0.7 102.1 ± 2.0 122.8 ± 0.6 141.3 ± 0.7 31.5 ± 0.7 42.0 ± 0.3 52.2 ± 0.5 324.5 ± 2.0 376.2 ± 2.1 427.6 ± 0.9 0.51 ± 0.03 (μg/mL) 1.01 ± 0.03 2.03 ± 0.02
– 100.8 ± 3.3 101.3 ± 2.3 – 101.0 ± 3.1 102.4 ± 3.5 – 103.7 ± 3.5 100.5 ± 1.9 – 103.5 ± 3.0 98.0 ± 1.6 – 104.2 ± 3.2 103.7 ± 2.5 – 99.0 ± 4.0 101.7 ± 0.9 – 101.0 ± 3.0 101.3 ± 0.8
– 0.4 1.0 – 0.6 1.2 – 1.8 0.5 – 2.0 2.1 – 2.5 2.6 – 0.4 2.1 – 0.6 3.0
a
Mean of three determinations ± SD t-critical = 4.3 for n = 2 and P = 0.05
b
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Conclusions We have described a relatively high CL emission in a TGAcapped CdS QDs–KMnO4 reaction, which can be increased greatly in the presence of CTAB. The CdS QDs were characterized as the emitting species. In addition, a sensitive CL method for the detection of ethanol was developed based on the inhibitory effect of ethanol on the CL system. This effect is related to oxidation of ethanol by KMnO4. Under optimum conditions, the decrease in CL showed a linear relationship with the concentration of ethanol. Acknowledgements The authors would like to thank Tabriz Branch, Islamic Azad University for the financial support of this research, which is based on a research project contract.
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12. Fletcher PJ, van Staden JF. Determination of ethanol in distilled liquors using sequential injection analysis with spectrophotometric detection. Anal Chim Acta 2003;499:123–8. 13. Zhang ZH, Lockwood R, Veinot JGC, Meldrum A. Detection of ethanol and water vapor with silicon quantum dots coupled to an optical fiber. Sensor Actuator B 2013;181:523–8. 14. Fernandes EN 1, Reis BF. Automatic flow procedure for the determination of ethanol in wine exploiting multicommutation and enzymatic reaction with detection by chemiluminescence. J AOAC Int 2004;87:920–6. 15. Alpat S, Telefoncu A. Development of an alcohol dehydrogenase biosensor for ethanol determination with toluidine blue covalently attached to a cellulose acetate modified electrode. Sensors 2010;10:748–64. 16. Akyılmaz E, Dinçkaya E. An amperometric microbial biosensor development based on Candida tropicalis yeast cells for sensitive determination of ethanol. Biosens Bioelectron 2005;20:1263–9. 17. González-Rodrı́guez J, Pérez-Juan P, Luque de Castro MD. Determination of ethanol in beverages by flow injection, pervaporation and density measurements. Talanta 2003;59:691–6. 18. Adcock JL, Barnett NW, Barrow CJ, Francis PS. Advances in the use of acidic potassium permanganate as a chemiluminescence reagent: a review. Anal Chim Acta 2014;807:9–28. 19. Adcock JL, Francis PS, Barnett NW. Acidic potassium permanganate as a chemiluminescence reagent – a review. Anal Chim Acta 2007;601:36–67. 20. Li Q, Zhang L, Li J, Lu C. Nanomaterial-amplified chemiluminescence systems and their applications in bioassays. Trends Anal Chem 2011;30:401–13. 21. Giokas DL, Vlessidis AG, Tsogas GZ, Evmiridis NP. Nanoparticle-assisted chemiluminescence and its applications in analytical chemistry. Trends Anal Chem 2010;29:1113–26. 22. Wang ZP, Li J, Liu B, Hu JQ, Yao X, Li JH. Chemiluminescence of CdTe nanocrystals induced by direct chemical oxidation and its sizedependent and surfactant-sensitized effect. J Phys Chem B 2005;109:23304–11. 23. Poznyak SK, Talapin DV, Shevchenko EV, Weller H. Quantum dot chemiluminescence. Nano Lett 2004;4:693–8. 24. Li YX, Yang P, Wang P, Huang X, Wang L. CdS nanocrystal induced chemiluminescence: reaction mechanism and applications. Nanotechnology 2007;18:225602–10. 25. Zhang LJ, Xu CL, Li BX, Chemiluminescence of CdTe quantum dots using K3Fe(CN)6 as oxidant and its capping ligand-dependent effect. Microchem J 2010;95:186–91. 26. Winter JO, Gomez N, Gatzert S, Schmidt CE, Korgel BA. Variation of cadmium sulfide nanoparticle size and photoluminescence intensity with altered aqueous synthesis conditions. Colloid Surf A 2005;254:147–57. 27. Yu WW, Qu L, Guo W, Peng X. Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS nanocrystals. Chem Mater 2003;15:2854–60. 28. Demas JN, Crosby GA. Measurement of photoluminescence quantum yields. Review. J Phys Chem 1971;75:991–1024. 29. Lin JM, Yamada M. Microheterogeneous systems of micelles and microemulsions as reaction media in chemiluminescent analysis. Trends Anal Chem 2003;22:99–105.
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Revised: 5 September 2014,
Accepted: 17 September 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2802
Determination of ethanol using permanganate– CdS quantum dot chemiluminescence system Jafar Abolhasani* and Javad Hassanzadeh ABSTRACT: A novel and highly sensitive chemiluminescence (CL) method for the determination of ethanol was developed based on the CdS quantum dots (QDs)–permanganate system. It was found that KMnO4 could directly oxidize CdS QDs in acidic media resulting in relatively high CL emission. A possible mechanism was proposed for this reaction based on UV/Vis absorption, fluorescence and the generated CL emission spectra. However, it was observed that ethanol had a remarkable inhibition effect on this system. This effect was exploited in the determination of ethanol within the concentration range 12–300 μg/L, with detection at 4.3 μg/L. In order to evaluate the capability of presented method, it was satisfactorily utilized in the determination of alcohol in real samples. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: chemiluminescence; quantum dots; potassium permanganate; ethanol
Introduction When introduced into the human body, ethanol (EtOH), one of the most common toxic substances consumed by humans, is completely oxidized or can be partially eliminated via the skin, respiration or urine. Ethanol ingestion can affect the nervous and digestive systems, and at higher concentrations, can lead to coma or even death. Also, in industry, for example during fermentation and distillation, high ethanol vapour concentrations can cause inflammation of the nasal mucous membrane and conjunctiva, and irritation of the skin. The highest allowed level of ethanol vapour in the workplace, determined by the American Conference of Governmental Industrial Hygienists (ACGIH), is 1000 ppm (1). Therefore, the rapid, inexpensive, sensitive and selective determination of ethanol is of great importance in different areas. For example, it can help the police to recognize drink–driving offenders. The level of ethanol in a foodstuff might also be an indicator of quality in cases where ethanol is the product of food degradation (2). In alcoholic beverages, the level of ethanol indicates the quality of the product (3). Many analytical methods have been developed to measure ethanol in different real samples. In general, the working principle behind many alcohol biosensors is the enzymatic destruction of alcohol. The utility of such biosensors is restricted due to their low stability, short lifetime determined mainly by enzyme kinetics, the need to add the coenzyme to the measuring solution and temperature (4). Other methods used to determine ethanol include: gas chromatography (GC) (5), high-performance liquid chromatography (HPLC) (6) capillary electrophoresis (CE) (7), nuclear magnetic resonance spectroscopy (NMR) (8), infrared spectroscopy (IR) (9,10), Raman spectrometry (11), spectrophotometry (2,12), spectrofluorimetry (4,13), chemiluminescence (CL) (14), electrochemical methods (15,16) and methods based on distillation followed by density measurement (17). These methods have some disadvantages. For example, the GC, HPLC, CE and NMR techniques are slow and not easily portable, and they require trained personnel and expensive equipment. In addition, fabrication of an enzyme-
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based biosensor is rather complicated and expensive. Finally, methods based on distillation are time consuming. CL, the emission of light from an excited species produced by a chemical oxidation reaction, has frequently been used for analytical purposes due to its intrinsic advantages of a low detection limit, wide linear dynamic range and relatively simple and inexpensive instrumentation. Many reactions utilize CL generation, but few show sufficient intensity to be suitable for use in analysis. Potassium permanganate is the most common oxidant used in CL reactions and has recently attracted much attention because it is inexpensive. There have been several comprehensive reviews by Barnett concerning a wide range of analytical applications of permanganate in CL reactions (18,19). In recent years, nanomaterials have been introduced into CL reactions. The development of nanoparticle science provides new opportunities for the development of CL. In the meantime, metallic and semiconducting nanoparticles have been studied extensively because of their quantum confinement effects, which exhibit unique size-dependent electronic, magnetic, optical and electrochemical properties. In this respect, nanomaterials have usually been used as a catalyst or as energy acceptors in CL reactions (20,21). The CL properties of nanomaterials as emitting species have rarely been reported, and the existing reports are mostly for quantum dots (22–25). Wang et al. (22) and Poznyak et al. (23) described the CL property of CdTe QDs and CdSe/CdS core/shell nanostructures in aqueous solution. Li et al. investigated the CL properties of CdS QDs capped with mercaptoacetic acid as light emitters (24). By contrast, surface of QDs are in direct contact with the surrounding gaseous environment and thus their CL can be influenced by some compounds. * Correspondence to: Jafar Abolhasani, Department of Chemistry, College of Science, Tabriz Branch, Islamic Azad University, Tabriz, Iran. E-mail: [email protected] Department of Chemistry, College of Science, Tabriz Branch, Islamic Azad University, Tabriz, Iran
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J. Abolhasani and J. Hassanzadeh In this study, CL emission generated from a thioglycolic acid (TGA)-capped cadmium sulfide (CdS) quantum dots (QDs)– potassium permanganate reaction was investigated. The mechanism of this reaction was studied using UV/Vis absorption, fluorescence and generated CL emission spectra. Furthermore, it is observed that ethanol has an inhibiting effect on the CL signal of this system, and based on these observations, a new simple and sensitive CL method has been developed for the determination of trace amounts of ethanol in alcoholic beverages including beer. This method is simpler and less expensive than other analysis techniques; at the same time, it offers good accuracy and precision.
Experimental Instrumentation Fluorescence spectra were recorded using a Shimadzu RF-5301 spectrofluorophotometer (Japan) equipped with a quartz cell (1 cm × 1 cm). The CL intensities were recorded on the same spectrofluorimeter with a closed excitation slit. The CL spectra were measured on a spectrofluorimeter using the flow mode with the excitation light source being turned off. UV/Vis absorption spectra were obtained with a UV-1800 spectrophotometer (Shimadzu, Japan). Transmission electron microscopy (TEM) images of the CdS QDs were obtained using a Leo 906 transmission electron microscope (Germany).
Reagents All experiments were carried out using analytical grade chemicals and solvents. Doubly distilled deionized water was used in the preparation of all solutions. Cadmium chloride (CdCl2 · 5.H2O), sodium sulfide (Na2S · 9H2O), TGA and ethanol (99.5%) were all purchased from Sigma-Aldrich and used without further purification. Cetyl trimethyl ammonium bromide (CTAB), KMnO4 and H2SO4 were obtained from Merck (Darmstadt, Germany; www. merck-chemicals.com).
Synthesis of TGA-capped CdS quantum dots TGA-capped CdS QDs were synthesized in aqueous solution using a modified procedure (26). TGA (100 mL; 0.05 mol/L) solution and 100 mL of CdCl2 (0.02 mol/L) solution were mixed in a round-bottomed flask. To this, a solution of 1 mol/L NaOH was added dropwise to increase the pH to 8. A white precipitation appeared on first adding the NaOH solution, and then dissolved. Finally, a clear solution was obtained. This was due to the formation of Cd–thioglycolic complexes with different structures at different pH values because of the different dissociation of carboxylate and the sulfhydryl group. The reaction mixture was then heated to 90 °C in the presence of pure argon. After that, 50 mL of Na2S (0.02 mol/L) was added and the flask was submerged in an ice-water bath for 2 min until the temperature decreased to 37 °C. Purification of synthesized QDs was carried out using ethanol. Equal volumes of QDs solution and ethanol were mixed and then precipitated QDs were separated by centrifugation and dispersed in an equal volume of deionized water. The concentration of the obtained QDs was 0.002 mol/L.
General procedure for CL detection CL measurements were carried out in the batch condition. H2SO4 (250 μL; 0.001 mol/L), 250 μL of CTAB (0.01 mol/L) and 1250 μL of synthesized CdS QDs solution were added to a 3 mL quartz tube. An appropriate volume of sample or standard ethanol solution was added and the final volume was made up to 2.3 mL with distilled water. After injection of 200 μL of KMnO4 (0.005 mol/L) using an automatic injector, monitoring of CL signal versus time was started automatically. Maximum CL intensity was used as the analytical signal.
Results and discussion Characterization of QDs The absorption and photoluminescence (PL) spectra of the synthesized TGA-capped CdS QDs are shown in Fig. 1. According
Figure 1. (a) Absorption (b) excitation and (c) emission spectra of synthesized CdS QDs. (Inset) TEM image of QDs.
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Determination of ethanol by CdS quantum dot chemiluminescence system to Peng’s calculation method (27), the particle sizes of the CdS QDs were ~ 3.3 nm, corresponding to PL peaks of 555 nm (Fig. 1b): D ¼ 9:8127 10
7
λ3 – 1:7147 10
3
λ2 þ ð1:0064Þλ–194:84
where D (nm) is the size of a given nanocrystal and λ (nm) is the wavelength of the first excitonic absorption peak of the corresponding nanocrystal. Also, TEM images (Fig. 1: inset) show that the synthesized CdS QDs were spherical in shape with a size distribution within the range 3.5 ± 0.9 nm. The room temperature PL quantum efficiency (QE) of the assynthesized CdS QDs was estimated following the procedure of Demas and Crosby (28) by comparison with rhodamine 6G, assuming its PL QE to be 95%. The calculated PL QE of CdS was 22%. CL of TGA-capped CdS QDs–KMnO4 In primary experiments, it was found that CdS QDs could be directly oxidized by some oxidants to produce relatively strong CL radiation. Thus, various concentrations of KMnO4, Ce(IV), K3Fe(CN)6 and H2O2, were examined using known amounts of CdS QDs. It was found that KMnO4 resulted in the strongest CL light radiation. Therefore, KMnO4 was selected as the best oxidant for CL production. In addition, to further enhance the CdS QDs-induced CL intensity and improve its analytical application, we introduced some surfactants, including SDS (anionic surfactant), sodium bis(2-ethylhexyl)sulfosuccinate) (AOT, anionic surfactant), CTAB and didodecyl dimethyl ammonium bromide (DDABr, cationic surfactant) into the CdS QDs CL system. The results showed a remarkable enhancing effect of CTAB on this system (Fig. 2). Therefore, CTAB was used in later experiments as the enhancer species. The CL intensity–time profiles for this system are shown in Fig. 2. The profiles indicated that the CL reactions were very quick and the CL intensity reached a maximum value within < 1 s of initiating the reactions with KMnO4.
Optimization of reaction condition Parameters influencing the CL signals of the KMnO4–CdS QDs system were investigated to establish the optimal conditions for the CL reaction. The type and concentration of acid have a very significant influence on permanganate CL emission intensity (19). Therefore, to examine the effect of an acidic medium on the CL signal in this system, several acids (including HCl, H2SO4, HNO3 and H3PO4) were tested at different concentrations. As can be seen in Fig. 3(a), the maximum CL response was obtained for 0.1 mmol/L H2SO4. This is consistent with some other reports in which sulfuric acid was used as a suitable acidic medium (19). The effect of the concentration of CdS QDs solution on the CL response of the assay was examined. The results (Fig. 3b) indicated that the CL intensity increased linearly with CdS QDs concentration from 5.0 × 105 to 1.0 × 103 mol/L. This linear relationship may be important in analytical aims. Free functional groups on the surface of the CdS QDs were ready for covalent coupling to various molecules (24). Therefore, TGA-capped CdS QDs can react with these molecules, and a novel labelled CL method could be established to analyse molecules. A QDs solution concentration of 103 mol/L was used in the determinations. Furthermore, it is well known that some surfactants can affect CL intensity (29). Thus, the effect of different surfactants (CTAB, SDS, AOT and DDABr) at various concentrations on the CL intensity of the KMnO4–CdS QDs CL system was investigated. From Fig. 3(c), it can be seen that CTAB greatly enhanced the CL intensity with an optimum concentration of 8.0 × 104 mol/L. From the UV spectra (Fig. 4), it can be seen that the addition of CTAB affected the absorption spectrum of CdS QDs. However, the changes caused by the addition of other surfactants are small. This may be attributed to the interaction of active charge forces (24). In the presence of CTAB, CdS QSs tended to form aggregate nanoparticles through the charge interaction between CTAB and TGA-capped CdS QDs. The aggregation of QDs results in increasing overlap of the electron valve functions in neighbouring quantum dots and possibly led to the red shift in the spectra (24). However, the aggregation of CdS QDs can decrease the distance between the nanocrystals, resulting in the generation of chemical energy during the oxidized CL reaction. This energy can easily transfer between CdS QDs, leading to enhanced generation of excited state of CdS QDs. Therefore, the CL intensity improved remarkably. In order to investigate the effect of permanganate concentration, solutions with different concentrations of KMnO4 were prepared over the range 1 × 104 to 8 × 104 mol/L. As shown in Fig. 3(d), the CL signal increased to 4 × 104 mol/L KMnO4 and then remained steady at higher concentrations. At lower KMnO4 concentrations, the number of excited intermediates is decreased and the response is diminished. Considering the above discussion of CL intensity and the consumption of reagents, the optimized conditions for the KMnO4–CdS QDs CL system were as follows: 0.4 mmol/L KMnO4, 8 × 104 mol/L CTAB, 0.1 mmol/L H2SO4 and 3.3 nm CdS QDs at a concentration of 1.0 × 103 mol/L.
Possible mechanism for the CL reaction Figure 2. CL profiles of KMnO4–CdS QDs systems in the absence (a) and presence (b) of CTAB under optimum conditions.
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Usually, finding emitting species is the first step in describing the possible mechanism of CL generation. Therefore, the CL emission spectrum was recorded for the TGA-capped CdS QDs–acidic
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Figure 3. Effect of (a) type and concentration of acid (1 mmol/L CTAB, 0.6 mmol/L KMnO4 and 0.8 mmol/L CdS QDs); (b) concentration of CdS QDs (1 mmol/L CTAB, 0.6 mmol/L KMnO4 and 0.1 mmol/L H2SO4); (c) type and concentration of surfactants (1 mmol/L CdS QDs, 0.6 mmol/L KMnO4 and 0.1 mmol/L H2SO4); (d) concentration of KMnO4 (0.8 mmol/L CTAB, 0.1 mmol/L H2SO4 and 1 mmol/L CdS QDs) on the CL intensity.
KMnO4 system in the presence of CTAB, using a spectrofluorimeter with a closed exciting slit (Fig. 5). The results showed a characteristic peak around 555 nm, which is similar to the fluorescence spectrum of synthesized QDs. This reveals that QDs probably act as emitting species in this CL reaction. However, an interaction between KMnO4 and the applied QDs can be recognized in the fluorescence and absorption spectra of QDs (Fig. 6). It can be seen that the fluorescence and absorption intensities of QDs decrease strongly in the presence of KMnO4. Also, it was found that the reaction between TGA and KMnO4 results in a weak CL emission, which may be associated with QDs emission. It is observed that the CL emission of QDs is greatly affected by the type of capping agent. Thus, in the present system, it may be suggested that in the reaction with QDs, KMnO4 can oxidize the capping agent (TGA) resulting in the production of an excited intermediate. Transferring energy to CdS QDs creates the final CL emission (22,24):
KMnO4
TGA→
M
M þ CdS QDs → M þ ðCdS QDsÞ
ðCdS QDsÞ → CdS QDs þ hν
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Where, M* is oxidation product of TGA which is in excited state. Furthermore, ethanol has a remarkable inhibiting effect on this CL emission, and two different ideas can be put forward to explain this. First, ethanol can be oxidized by KMnO4 and compete with QDs for KMnO4 consumption. Investigation of this reaction showed that the KMnO4 absorption intensity decreased on addition of ethanol. Second, the ethanol–QDs interaction, for example, esterification of the carboxylic group of the capping agent on the QDs surface, may result in lower QDs activation in the presence of ethanol. It is observed experimentally that the fluorescence emission of QDs decreases on addition of ethanol. Therefore, it is obvious that ethanol can decrease the CL intensity of the represented system via both mechanisms, but it seems that the former interaction is efficient. Analytical application of the CL system It is found that the addition of ultratrace amounts of ethanol to the CL system under optimum conditions leads to a significant decrease in CL intensity that is proportional to the ethanol concentration (Fig. 7). Based on this, a sensitive and relatively selective method was developed for the determination of ethanol.
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Determination of ethanol by CdS quantum dot chemiluminescence system
Figure 3. (Continued)
3
Figure 4. Absorption spectra of CdS QDs (1.0 × 10 mol/L) in the presence of different surfactants (0.5 mmol/L). (1) No surfactant, (2) DDABr, (3) AOT, (4) SDS and (5) CTAB.
Figure 5. CL spectrum of KMnO4–CdS QDs–CTAB system in the absence (1) and presence (2) of ethanol.
Under the optimum conditions described, the linearity ranges of the calibration graphs (Fig. 7, insets) were 12–300 μg/L with limits of detection (3S) of 4.3 μg/L for ethanol. The equation for regression lines was ΔI = 149.4C + 838 (R2 = 0.9994), where
ΔI = I0 – I is the difference between the CL intensity in the absence (I0) and presence (I) of ethanol, and C is concentration of alcohol in μg/L. The relative standard deviation (RSD) was < 3.5% for five determinations of 30, 100 and 250 μg/L ethanol.
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J. Abolhasani and J. Hassanzadeh
Figure 6. (A) absorption and (b) emission spectra of 1 mmol/L CdS QDs in the absence (1) and presence (2) of 0.4 mmol/L KMnO4. Condition: H2SO4 0.1 mmol/L.
Figure 7. CL profiles in the presence of various concentrations of ethanol (μg/L) under optimum conditions and (inset) the corresponding calibration graph.
Table 1. Summary of published methods for the determination of ethanol Method Fluorescence sensor Gas chromatography Nano SrCO3-chemiluminescence Spectrophotometry Colorimetry using alcohol dehydrogenase Amperometry High-performance liquid chromatography Raman spectrometry Spectrophotometry Enzymatic-chemiluminescence Alcohol dehydrogenase-modified electrode Amperometry Density measurements Current method
Linear range
Sample
LOD
Ref.
1–75 vol% – 6–3750 mg/L 0.1–10 vol% 46–230 mg/L Up to 69 mg/L – – 1–6 vol% 2.5–25 vol% 0.46–18.4 mg/L 23–345 mg/L 0–40 vol% 12–300 μg/L
Wine samples Alcoholic beverages – – Alcoholic beverages Alcoholic beverages – Alcoholic beverages Distilled liquors Wines Beer samples Alcoholic drinks Alcoholic beverages Beverages
– 0.5 mg/L 2.1 mg/L 0.03 vol% – 0.6 mg/L 102 mg 1 vol% 0.09 vol% 0.3 vol% 0.23 mg/L – 0.11 vol% 4.2 μg/L
4 5 1 10 2 3 6 11 12 14 15 16 17 –
Multiwalled carbon nanotube alcohol dehydrogenase
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Determination of ethanol by CdS quantum dot chemiluminescence system The results indicate that this CL system has good linearity and high sensitivity and precision. Comparison between the presented procedure and some other reported analytical methods for the determination of ethanol is summarized in Table 1. The linear dynamic range and limit of detection of our method is better than most of other methods and comparable with those of GC methods.
Study of interferences In order to test the interference effect of some potentially coexisting substances, increasing amounts of these species were added to a standard solution of 50 μg/L ethanol. The tolerable concentration ratios for interferences with a relative error
of < 5% are shown in Table 2. The amounts of the most potentially interfering species are below their tolerable levels, so there would be no interferences from these species.
Analysis of real samples The procedure was easily applied to the determination of ethanol in some beverages. In order to validate the method, known quantities of ethanol were added to real samples before pretreatment step, and the samples were prepared and analysed according the general procedure described above. The obtained results are shown in Table 3. Statistical analysis of these results using the Student’s t-test showed that there are no significant differences between added and found values.
Table 2. Interference of different species on the CL intensity of CdS QDs–KMnO4–CTAB under optimum conditions with 50 μg/L ethanol Coexisting species 2+
+
+
Interference ratio
3 , CO2 3 , PO4 , Cl , 2+ 2+ 3+
+
NO 3
Ca , Na , K K , Cd2+, Co2+, Ni , Mg , Fe Glucose, sucrose, vitamin B1, B2 2+ 2+ SO2 4 , Cu , Zn Acetone Uric acid, acetic acid, ascorbic acid, cysteine, phenol Methanol Hg2+
2500 1000 1000 700 500 300 80 20
Table 3. Results for the determination of ethanol in real samples Sample Added Red wine (1)
Red wine (2)
White wine (1)
White wine (2)
Beer
Whisky
Delester (Iranian beer)
Recovery (%) a
Ethanol (mg/mL)
0 20 40 0 20 40 0 20 40 0 20 40 0 10 20 0 50 100 0 1 2
t-statistic
b
Found a 91.4 ± 1.3 111.6 ± 0.7 131.9 ± 0.9 86.4 ± 1.1 106.6 ± 0.6 127.4 ± 1.4 104.2 ± 2.0 124.9 ± 0.7 144.4 ± 0.7 102.1 ± 2.0 122.8 ± 0.6 141.3 ± 0.7 31.5 ± 0.7 42.0 ± 0.3 52.2 ± 0.5 324.5 ± 2.0 376.2 ± 2.1 427.6 ± 0.9 0.51 ± 0.03 (μg/mL) 1.01 ± 0.03 2.03 ± 0.02
– 100.8 ± 3.3 101.3 ± 2.3 – 101.0 ± 3.1 102.4 ± 3.5 – 103.7 ± 3.5 100.5 ± 1.9 – 103.5 ± 3.0 98.0 ± 1.6 – 104.2 ± 3.2 103.7 ± 2.5 – 99.0 ± 4.0 101.7 ± 0.9 – 101.0 ± 3.0 101.3 ± 0.8
– 0.4 1.0 – 0.6 1.2 – 1.8 0.5 – 2.0 2.1 – 2.5 2.6 – 0.4 2.1 – 0.6 3.0
a
Mean of three determinations ± SD t-critical = 4.3 for n = 2 and P = 0.05
b
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Conclusions We have described a relatively high CL emission in a TGAcapped CdS QDs–KMnO4 reaction, which can be increased greatly in the presence of CTAB. The CdS QDs were characterized as the emitting species. In addition, a sensitive CL method for the detection of ethanol was developed based on the inhibitory effect of ethanol on the CL system. This effect is related to oxidation of ethanol by KMnO4. Under optimum conditions, the decrease in CL showed a linear relationship with the concentration of ethanol. Acknowledgements The authors would like to thank Tabriz Branch, Islamic Azad University for the financial support of this research, which is based on a research project contract.
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