Research article Received: 26 February 2014,
Revised: 25 April 2014,
Accepted: 25 April 2014
Published online in Wiley Online Library: 2 June 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2706
Study on the fluorescence resonance energy transfer between CdS quantum dots and Eosin Y Zhengyu Yan, Zhengwei Zhang, Yan Yu and Jianqiu Chen* ABSTRACT: Water-soluble CdS quantum dots (QDs) were prepared using mercaptoacetic acid (TGA) as the stabilizer in an aqueous system. A fluorescence resonance energy transfer (FRET) system was constructed between water-soluble CdS QDs (donor) and Eosin Y (acceptor). Several factors that impacted the fluorescence spectra of the FRET system, such as pH (3.05–10.10), concentration of Eosin Y (2–80 mg/L) and concentration of CdS QDs (2–80 mg/L), were investigated and refined. Donor-to-acceptor ratios, the energy transfer efficiency (E) and the distance (r) between CdS QDs and Eosin Y were obtained. The results showed that a FRET system could be established between water-soluble CdS QDs and Eosin Y at pH 5.0; donor-to-acceptor ratios demonstrated a 1: 8 proportion of complexes; the energy transfer efficiency (E) and the distance (r) between the QDs and Eosin Y were 20.07% and 4.36 nm,respectively. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: FRET; CdS quantum dots; Eosin Y
Introduction
Experimental
Fluorescence resonance energy transfer (FRET) occurs when the electronic excitation energy of a donor chromophore is transferred to an acceptor molecule nearby via a throughspace dipole–dipole interaction between the donor–acceptor pair (1,2). The FRET process is more efficient when there is an appreciable overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor (3,4). Semiconductor quantum dots (QDs) are inorganic nanoparticle fluorophores with unique optical and spectroscopic properties, such as a broad excitation spectrum, narrow emission spectrum, precise tunablity of their emission peaks, longer fluorescence lifetime and negligible photobleaching (5–7). QDs used as donors and acceptors overcome some of the limitations of pairs of organic dye molecules as donor–acceptor complexes, because organic dyes often have narrow excitation spectra and broad emission with red spectral tails, which makes it difficult to avoid the overlap between the donor and acceptor emission spectra (8). Several recent reports have confirmed that QDs, such as CdSe and CdTe, are able to participate in resonance energy transfer processes analogous to FRET (9–12). In this study, CdS QDs were prepared using mercaptoacetic acid (TGA) as a stabilizer in an aqueous system and a suitable size of water-soluble CdS QDs could be selected to maximize the extent of spectral overlap with Eosin Y. A FRET system was constructed between water-soluble CdS QDs (donor) and Eosin Y (acceptor). The unique spectral properties of CdS QDs were analyzed in FRET-based studies. Moreover, the influences of pH, Eosin Y concentration and the concentration of CdS QDs were investigated. Energy transfer parameters were also studied in some detail.
Instrumentation
Chemicals and materials CdCl2 · 2.5H2O, NaS, thioglycolic acid (TGA) and Eosin Y were purchased from Guangfu Fine Chemical Research Institute, China. All other chemicals were of analytical grade or the best grade commercially available. Doubly distilled water (DDW) was used throughout the study.
Preparation of water-soluble CdS QDs Water-soluble CdS QDs were prepared as reported in the literature (13). In brief, 0.2 mol/L of Na2S aqueous solution was added to 0.01 mol/L of CdCl2 aqueous solution, pH 7.0, in the presence of TGA as a stabilizing agent. The molar ratio of Cd2+: S2 was fixed at 2: 1. * Correspondence to: J. Chen, Department of Analytical Chemistry, China Pharmaceutical University, Nanjing 210009, China. Tel/Fax: +86-025-86185150. E-mail: [email protected] Department of Analytical Chemistry, China Pharmaceutical University, Nanjing, China
Copyright © 2014 John Wiley & Sons, Ltd.
155
Luminescence 2015; 30: 155–158
Fluorescence spectra and intensities were performed on a RF5301PC spectrofluorometer (Hitachi, Tokyo, Japan). UV absorption spectra were obtained using a UV-2100 UV/vis spectrophotometer (Shimadzu, Kyoto, Japan). All pH values were measured with a pHS-25 pH meter (Shanghai INESA Scientific Instrument Co., Shanghai, China). All optical measurements were performed at room temperature under ambient conditions.
Z. Yan et al. The mixed solution was treated ultrasonically for 15 min at room temperature. After being heated to 75ºC for 10 min, TGA-stabilized CdS QDs exhibiting strong fluorescence at 519 nm were obtained. For the preparation of size-selected CdS QDs, 0.7 mL of isopropanol was added to 1.0 mL of as-prepared CdS QDs aqueous solution, and the mixed solution was stirred thoroughly. It was then centrifuged at 12,000 r.p.m. for 10 min and the supernatant was retained. The same volume of isopropanol was added to the supernatant and the above steps were repeated. The precipitate obtained in the second stage was the size-selected CdS QDs, which were washed three times with ethanol. Size-selected CdS QDs, exhibiting strong fluorescence at 470 nm, were dispersed in 2 mL of DDW before subsequent use. Construction of the FRET system The FRET system was constructed by mixing a certain volume of 0.01 mg/mL Eosin Y with a certain volume of CdS QDs solution, and the mixed solution was adjusted at pH 5.0. The as-prepared solution was diluted to 2.0 mL with DDW, stirred thoroughly and incubated for 20 min at room temperature for the assay. All the fluorescence spectra were obtained with the excitation wavelength of 365 nm.
Figure 2. Construction of the FRET system between CdS QDs and Eosin Y.
quenching of the CdS QDs are observed. This confirms the FRET process in which the microenvironment around the donor and acceptor is altered. More obvious FRET phenomena are analyzed in full below.
Results and discussion Effects impacting the fluorescence spectra of the FRET system
Fluorescence and absorption spectra Figure 1 shows absorption and fluorescence spectra obtained from CdS QDs and Eosin Y, respectively. An appropriate size of CdS QDs was chosen to maximize the spectral overlap of the donor–acceptor emission and absorption spectra although still maintaining good spectral resolution of the donor and acceptor emission. It can be seen that the maximal emission peak of the CdS QDs is at 470 nm, whereas the maximal absorption and emission peaks of Eosin Y are at 518 and 539 nm. So there is appreciable overlap between the emission spectrum of CdS QDs (donor) and the absorption spectrum of Eosin Y (acceptor).
Effect of the pH. The pH influences the fluorescence intensity of QDs prepared using TGA as a stabilizer, and the FRET system was investigated at pH 3.05–10.10. As shown in Figure 3, Fa/Fd (where Fa is the fluorescence intensity of the acceptor and Fd is the fluorescence intensity of the donor) increased gradually with increasing pH from 3.05 to 5.00. After that, Fa/Fd decreased slowly. Therefore, a pH value of 5.0 was chosen in this study.
Figure 2 shows that the fluorescence spectra excited at 365 nm changes when CdS QDs interact with Eosin Y. Significant enhancement of the Eosin Y fluorescence intensity and the corresponding
Effect of the concentration of CdS QDs solution. In this work, the change in the fluorescent intensity of the FRET system with gradually increasing quantities of CdS QDs and a fixed amount of Eosin Y was studied. The effect of the quantity of CdS QDs solution was investigated first over a range of 100–500 μL (Figure 4). As shown in Figure 4, the enhancement in Eosin Y fluorescence observed for the increasing quantity of the CdS QDs solution from 100 to 500 μL indicated that a FRET process occurs between CdS QDs and Eosin Y. However, the fluorescence
Figure 1. Normalized absorption and emission spectra of CdS QDs and Eosin Y solutions.
Figure 3. The effect of pH on Fa/Fd of the FRET system.
Construction of the FRET system
156
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Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 155–158
Fluorescence resonance energy transfer Y reached a certain point. The progress was limited because the donor was saturated by the receptor. The coordinated saturation of CdS QDs and Eosin Y in the FRET system can be seen from the effect of their concentrations. Therefore, there was a relationship between the coordinated saturation and the donor-to-acceptor stoichiometry ratios. According to the method reported in the literature (14), assuming that 1 CdS QDs (donor) molecule and n Eosin Y (receptor) molecules produce effective fluorescence resonance energy transfer, then the relationship between them can be expressed as: D þ nA ¼ DAn
Figure 4. Effect of CdS QDs concentration on the fluorescence spectra of the FRET system.
decreased when the quantity of CdS QDs was much higher. The same was seen for the fluorescence intensity of CdS QDs. So, 500 μL of CdS QDs was chosen in this study. Effect of the concentration of Eosin Y. To investigate the process between CdS QDs and Eosin Y further, the change in fluorescence spectra was observed on gradually increasing the quantity of Eosin Y and a fixed amount of CdS QDs. The results are shown in Figure 5. On increasing the stock solution of Eosin Y added to the FRET system from 20 to 100 μL, the fluorescent intensity of CdS QDs decreased, while the fluorescent intensity of Eosin Y was gradually enhanced. In order to obtain a relative high FRET efficiency, 60 μL of Eosin Y was chosen in this study. Determination of the donor-to-acceptor stoichiometry ratios In the above-mentioned studies, the fluorescent intensity of CdS QDs decreased and the fluorescent intensity of Eosin Y was gradually enhanced when in the FRET system with a fixed amount of CdS QDs the amount of Eosin Y was slowly increased. However, the fluorescent intensity of CdS QDs and Eosin Y showed no significant change when the concentration of Eosin
Luminescence 2015; 30: 155–158
Calculation of the FRET parameters Based on the literature (15–17), FRET efficiency can be divided into quenching and enhancement. Energy transfer efficiency can be expressed as: E ¼ ΔF=F0
(1)
where F0 is the fluorescence intensity of the donor or acceptor alone and ΔF is the change in the fluorescence intensity of the donor or acceptor in the FRET system. According to the above-mentioned studies on the effect of the concentrations of CdS QDs and Eosin Y, 2 mL of the FRET system diluted with DDW has the largest ΔF value of the donor or acceptor when using 500 μL of CdS QDs solution and 100 μL of Eosin Y stock solution. Therefore, these conditions are selected to determine the FRET efficiency. Quenching efficiency is expressed as E ¼ 1–FDA =FD
(2)
Figure 6. Determination of donor-to-acceptor molar ratios.
Copyright © 2014 John Wiley & Sons, Ltd.
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Figure 5. Effect of the Eosin Y concentration on the fluorescence spectra of the FRET system.
where D and A are the donor and acceptor, respectively. As shown in Figure 6, where the vertical axis is the fluorescence intensity and the horizontal axis is the concentration ratio of CdS QDs and Eosin Y, using both sides of the tangent extrapolation, the point of intersection with the abscissa gives the value of n. Points A and B intersect with the abscissa at 8.5 and 8, 1 respectively, therefore the value of n is ~ 8. This suggests that energy transfer occurs between 1 CdS QDs molecule (donor) and 8 Eosin Y (receptor) molecules.
Z. Yan et al. where FDA is the fluorescence intensity of the donor in the presence of the acceptor and FD is the fluorescence intensity of the donor in the absence of the acceptor. Enhancement efficiency is expressed as Y ¼ IAD =IA –1
(3)
where IAD is the fluorescence intensity of the acceptor in the presence of the donor and IA is the fluorescence intensity of the acceptor in the absence of the donor. The quenching efficiency is calculated as 45.64% and the enhancement efficiency is 20.07%, indicating that there are other energy transfers leading to energy loss in the FRET system. According to Forster’s theory, the relationship between E (the energy transfer efficiency) and r (the distance between the donor–acceptor pair) is given as E¼
nR60 nR60 þ r 6
(4)
where n is the donor-to-acceptor molar ratio and R0 is the critical distance when the energy transfer efficiency is 50%, which is expressed as R60 ¼ 8:791025 K 2 n4 0 ϕ D J ðλÞ
(5)
where the spatial orientation factor K2 and the refraction index n0 are accepted as 2/3 and 1.336, respectively (18,19). ΦD is the quantum yield of CdS QDs, measured as 12.3% using Rhodamine 6G as the reference standard (20), and J (λ) is the spectral overlap integral which can be calculated from eqn (6): J ðλÞ ¼
∫F ðλÞεðλÞλ4 dλ ∫F ðλÞdλ
(6)
where F(λ) is the fluorescence intensity of the donor and ε(λ) is the molar absorptivity of the acceptor. The value of J(λ) is 9.6909 × 10-15 cm3/L/mol for this FRET system, and R0 is calculated to be 2.45 nm from eqn (5). E could reach 20.07% from eqn (3) for the enhancement efficiency in the FRET process. By fitting the data to eqn (4), r is found to be 4.36 nm, which is much smaller than the value of 7 nm required for energy transfer to occur (18,19), indicating that the energy transfer from CdS QDs to Eosin Y has high probability. These results also confirm that a FRET process occurs between CdS QDs and Eosin Y.
Conclusion
158
In this study, TGA-functionalized CdS QDs were prepared in aqueous solution using green and low-cost materials. The quenching of the fluorescence of CdS QDs and the enhancement of the fluorescence of Eosin Y confirmed the FRET process. The mechanism of the energy transfer was studied, and some parameters of this system were calculated. The FRET system between CdS QDs and Eosin Y has the advantages of low cost, simple operation and high luminescence intensity. Applying QDs with good fluorescence properties to FRET research and constructing more types of FRET system will expand the scope of QDs and satisfy the demands of multiple analytical applications.
wileyonlinelibrary.com/journal/luminescence
Acknowledgements This work was supported by The Natural Science Foundation of Jiangsu Province Youth Fund (BK20130646), Jiangsu Key Lab of Environmental Engineering Open Foundation (KF2012008), Doctor Scientific Research Foundation from China Pharmaceutical University (2012ZJ13002), National Undergraduate Training Programs for Innovation and Entrepreneurship (02640556), the Fundamental Research Funds for the Central Universities (JKQZ2013009, JKPZ2013017).
References 1. Van Der Meer BW, Coker G, Chen SYS. Resonance energy transfer: theory and data. New York: VCH, 1994. 2. Wu P, Brand L. Resonance energy transfer: methods and applications. Anal Biochem 1994;218:1–13. 3. Selvin PR. The renaissance of fluorescence resonance energy transfer. Nat Struct Biol 2000;7:730–4. 4. Kagan CR, Murray CB, Nirmal M, Bawend MG. Electronic energy transfer in CdSe quantum dot solids. Phys Rev Lett 1996;76:1517–20. 5. Li Z, Dong CQ, Tang LC, Zhu X, Chen HJ, Ren JC. Aqueous synthesis of CdTe/CdS/ZnS quantum dots and their optical and chemical properties. Luminescence 2011;26:439–48. 6. Gill R, Zayats M, Willner I. Semiconductor quantum dots for bioanalysis. Angew Chem Int Ed 2008;47:7602–25. 7. Wang YQ, Chen LX. Quantum dots, lighting up the research and development of nanomedicine. Nanomed NBM 2011;7:385–402. 8. Clapp AR, Medintz IL, Uyeda HT, Fisher BR, Goldman ER, Bawendi MG, et al. Quantum dot-based multiplexed fluorescence resonance energy transfer. J Am Chem Soc 2005;127:18212–21. 9. Marcin N, Krystyna P, Leszek K, Kazimiera AW, Wieslaw S, Marek S. Fluorescence resonance energy transfer in a non-conjugated system of CdSe quantum dots/zinc-phthalocyanine. J Luminesc 2010;130:2487–90. 10. Tang G, Du L, Su X. Detection of melamine based on the fluorescence resonance energy transfer between CdTe QDs and Rhodamine B. Food Chem 2013;141:4060–5. 11. Ge SG, Lu JJ, Yan M, Yu F, Yu JH, Sun XJ. Fluorescence resonance energy transfer sensor between quantum dot donors and neutral red acceptors and its detection of BSA in micelles. Dyes Pigments 2011;91:304–8. 12. Tao HL, Li SH, Li JP. Fluorescence resonance energy transfer between quantum dots of CdSe and CdTe and its application for determination of serum prostate-specific antigen. Chinese J Anal Chem 2012;40:224–9. 13. Xu ZJ, Liao SH, Hu YZ, Chen JQ, Yan ZY. Ultrasonic preparation and characterization of CdS quantum dots and interaction between CdS quantum dots and naphthalene. Chinese J Spectrosc Lab 2011;28:1271–6. 14. Wang YL, Lu JP, Tong ZF, Chen L. Fluorescence resonance energy transfer between CdTe quantum dots and rhodamine B. Acta Chim Sin 2009;67:2222–6. 15. Wargnier R, Baranov AV, Maslov VG, Stsiapura V, Artemyev M, Pluot M, et al. Energy transfer in aqueous solutions of oppositely charged CdSe/ZnS core/shell quantum dots and in quantum dot–nanogold assemblies. Nanoletters 2004;4:451–7. 16. Li J, Mei F, Li WY, He XW, Zhang YK. Study on the fluorescence resonance energy transfer between CdTe QDs and butylrhodamine B in the presence of CTMAB and its application on the detection of Hg(II). Spectrochim Acta A 2008;70:811–17. 17. Chen Q, Ma Q, Wan Y, Su X, Lin Z, Jin Q. Studies on fluorescence resonance energy transfer between dyes and water-soluble quantum dots. Luminescence 2005;20:251–5. 18. Yang P, Gao F. The principle of bioinorganic chemistry. Beijing: Science Press, 2002. 19. Wang C, Wu QH, Li C, Wang Z, Ma JJ, Zang XH, et al. Interaction of tetrandrine with human serum albumin: a fluorescence quenching study. Anal Sci 2007;23:429–33. 20. Grabolle M, Spieles M, Lesnyak V, Gaponik N, Eychmuller A, ReschGenger U. Determination of the fluorescence quantum yield of quantum dots: suitable procedures and achievable uncertainties. Anal Chem 2009;81:6285–94.
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 155–158
Revised: 25 April 2014,
Accepted: 25 April 2014
Published online in Wiley Online Library: 2 June 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2706
Study on the fluorescence resonance energy transfer between CdS quantum dots and Eosin Y Zhengyu Yan, Zhengwei Zhang, Yan Yu and Jianqiu Chen* ABSTRACT: Water-soluble CdS quantum dots (QDs) were prepared using mercaptoacetic acid (TGA) as the stabilizer in an aqueous system. A fluorescence resonance energy transfer (FRET) system was constructed between water-soluble CdS QDs (donor) and Eosin Y (acceptor). Several factors that impacted the fluorescence spectra of the FRET system, such as pH (3.05–10.10), concentration of Eosin Y (2–80 mg/L) and concentration of CdS QDs (2–80 mg/L), were investigated and refined. Donor-to-acceptor ratios, the energy transfer efficiency (E) and the distance (r) between CdS QDs and Eosin Y were obtained. The results showed that a FRET system could be established between water-soluble CdS QDs and Eosin Y at pH 5.0; donor-to-acceptor ratios demonstrated a 1: 8 proportion of complexes; the energy transfer efficiency (E) and the distance (r) between the QDs and Eosin Y were 20.07% and 4.36 nm,respectively. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: FRET; CdS quantum dots; Eosin Y
Introduction
Experimental
Fluorescence resonance energy transfer (FRET) occurs when the electronic excitation energy of a donor chromophore is transferred to an acceptor molecule nearby via a throughspace dipole–dipole interaction between the donor–acceptor pair (1,2). The FRET process is more efficient when there is an appreciable overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor (3,4). Semiconductor quantum dots (QDs) are inorganic nanoparticle fluorophores with unique optical and spectroscopic properties, such as a broad excitation spectrum, narrow emission spectrum, precise tunablity of their emission peaks, longer fluorescence lifetime and negligible photobleaching (5–7). QDs used as donors and acceptors overcome some of the limitations of pairs of organic dye molecules as donor–acceptor complexes, because organic dyes often have narrow excitation spectra and broad emission with red spectral tails, which makes it difficult to avoid the overlap between the donor and acceptor emission spectra (8). Several recent reports have confirmed that QDs, such as CdSe and CdTe, are able to participate in resonance energy transfer processes analogous to FRET (9–12). In this study, CdS QDs were prepared using mercaptoacetic acid (TGA) as a stabilizer in an aqueous system and a suitable size of water-soluble CdS QDs could be selected to maximize the extent of spectral overlap with Eosin Y. A FRET system was constructed between water-soluble CdS QDs (donor) and Eosin Y (acceptor). The unique spectral properties of CdS QDs were analyzed in FRET-based studies. Moreover, the influences of pH, Eosin Y concentration and the concentration of CdS QDs were investigated. Energy transfer parameters were also studied in some detail.
Instrumentation
Chemicals and materials CdCl2 · 2.5H2O, NaS, thioglycolic acid (TGA) and Eosin Y were purchased from Guangfu Fine Chemical Research Institute, China. All other chemicals were of analytical grade or the best grade commercially available. Doubly distilled water (DDW) was used throughout the study.
Preparation of water-soluble CdS QDs Water-soluble CdS QDs were prepared as reported in the literature (13). In brief, 0.2 mol/L of Na2S aqueous solution was added to 0.01 mol/L of CdCl2 aqueous solution, pH 7.0, in the presence of TGA as a stabilizing agent. The molar ratio of Cd2+: S2 was fixed at 2: 1. * Correspondence to: J. Chen, Department of Analytical Chemistry, China Pharmaceutical University, Nanjing 210009, China. Tel/Fax: +86-025-86185150. E-mail: [email protected] Department of Analytical Chemistry, China Pharmaceutical University, Nanjing, China
Copyright © 2014 John Wiley & Sons, Ltd.
155
Luminescence 2015; 30: 155–158
Fluorescence spectra and intensities were performed on a RF5301PC spectrofluorometer (Hitachi, Tokyo, Japan). UV absorption spectra were obtained using a UV-2100 UV/vis spectrophotometer (Shimadzu, Kyoto, Japan). All pH values were measured with a pHS-25 pH meter (Shanghai INESA Scientific Instrument Co., Shanghai, China). All optical measurements were performed at room temperature under ambient conditions.
Z. Yan et al. The mixed solution was treated ultrasonically for 15 min at room temperature. After being heated to 75ºC for 10 min, TGA-stabilized CdS QDs exhibiting strong fluorescence at 519 nm were obtained. For the preparation of size-selected CdS QDs, 0.7 mL of isopropanol was added to 1.0 mL of as-prepared CdS QDs aqueous solution, and the mixed solution was stirred thoroughly. It was then centrifuged at 12,000 r.p.m. for 10 min and the supernatant was retained. The same volume of isopropanol was added to the supernatant and the above steps were repeated. The precipitate obtained in the second stage was the size-selected CdS QDs, which were washed three times with ethanol. Size-selected CdS QDs, exhibiting strong fluorescence at 470 nm, were dispersed in 2 mL of DDW before subsequent use. Construction of the FRET system The FRET system was constructed by mixing a certain volume of 0.01 mg/mL Eosin Y with a certain volume of CdS QDs solution, and the mixed solution was adjusted at pH 5.0. The as-prepared solution was diluted to 2.0 mL with DDW, stirred thoroughly and incubated for 20 min at room temperature for the assay. All the fluorescence spectra were obtained with the excitation wavelength of 365 nm.
Figure 2. Construction of the FRET system between CdS QDs and Eosin Y.
quenching of the CdS QDs are observed. This confirms the FRET process in which the microenvironment around the donor and acceptor is altered. More obvious FRET phenomena are analyzed in full below.
Results and discussion Effects impacting the fluorescence spectra of the FRET system
Fluorescence and absorption spectra Figure 1 shows absorption and fluorescence spectra obtained from CdS QDs and Eosin Y, respectively. An appropriate size of CdS QDs was chosen to maximize the spectral overlap of the donor–acceptor emission and absorption spectra although still maintaining good spectral resolution of the donor and acceptor emission. It can be seen that the maximal emission peak of the CdS QDs is at 470 nm, whereas the maximal absorption and emission peaks of Eosin Y are at 518 and 539 nm. So there is appreciable overlap between the emission spectrum of CdS QDs (donor) and the absorption spectrum of Eosin Y (acceptor).
Effect of the pH. The pH influences the fluorescence intensity of QDs prepared using TGA as a stabilizer, and the FRET system was investigated at pH 3.05–10.10. As shown in Figure 3, Fa/Fd (where Fa is the fluorescence intensity of the acceptor and Fd is the fluorescence intensity of the donor) increased gradually with increasing pH from 3.05 to 5.00. After that, Fa/Fd decreased slowly. Therefore, a pH value of 5.0 was chosen in this study.
Figure 2 shows that the fluorescence spectra excited at 365 nm changes when CdS QDs interact with Eosin Y. Significant enhancement of the Eosin Y fluorescence intensity and the corresponding
Effect of the concentration of CdS QDs solution. In this work, the change in the fluorescent intensity of the FRET system with gradually increasing quantities of CdS QDs and a fixed amount of Eosin Y was studied. The effect of the quantity of CdS QDs solution was investigated first over a range of 100–500 μL (Figure 4). As shown in Figure 4, the enhancement in Eosin Y fluorescence observed for the increasing quantity of the CdS QDs solution from 100 to 500 μL indicated that a FRET process occurs between CdS QDs and Eosin Y. However, the fluorescence
Figure 1. Normalized absorption and emission spectra of CdS QDs and Eosin Y solutions.
Figure 3. The effect of pH on Fa/Fd of the FRET system.
Construction of the FRET system
156
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Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 155–158
Fluorescence resonance energy transfer Y reached a certain point. The progress was limited because the donor was saturated by the receptor. The coordinated saturation of CdS QDs and Eosin Y in the FRET system can be seen from the effect of their concentrations. Therefore, there was a relationship between the coordinated saturation and the donor-to-acceptor stoichiometry ratios. According to the method reported in the literature (14), assuming that 1 CdS QDs (donor) molecule and n Eosin Y (receptor) molecules produce effective fluorescence resonance energy transfer, then the relationship between them can be expressed as: D þ nA ¼ DAn
Figure 4. Effect of CdS QDs concentration on the fluorescence spectra of the FRET system.
decreased when the quantity of CdS QDs was much higher. The same was seen for the fluorescence intensity of CdS QDs. So, 500 μL of CdS QDs was chosen in this study. Effect of the concentration of Eosin Y. To investigate the process between CdS QDs and Eosin Y further, the change in fluorescence spectra was observed on gradually increasing the quantity of Eosin Y and a fixed amount of CdS QDs. The results are shown in Figure 5. On increasing the stock solution of Eosin Y added to the FRET system from 20 to 100 μL, the fluorescent intensity of CdS QDs decreased, while the fluorescent intensity of Eosin Y was gradually enhanced. In order to obtain a relative high FRET efficiency, 60 μL of Eosin Y was chosen in this study. Determination of the donor-to-acceptor stoichiometry ratios In the above-mentioned studies, the fluorescent intensity of CdS QDs decreased and the fluorescent intensity of Eosin Y was gradually enhanced when in the FRET system with a fixed amount of CdS QDs the amount of Eosin Y was slowly increased. However, the fluorescent intensity of CdS QDs and Eosin Y showed no significant change when the concentration of Eosin
Luminescence 2015; 30: 155–158
Calculation of the FRET parameters Based on the literature (15–17), FRET efficiency can be divided into quenching and enhancement. Energy transfer efficiency can be expressed as: E ¼ ΔF=F0
(1)
where F0 is the fluorescence intensity of the donor or acceptor alone and ΔF is the change in the fluorescence intensity of the donor or acceptor in the FRET system. According to the above-mentioned studies on the effect of the concentrations of CdS QDs and Eosin Y, 2 mL of the FRET system diluted with DDW has the largest ΔF value of the donor or acceptor when using 500 μL of CdS QDs solution and 100 μL of Eosin Y stock solution. Therefore, these conditions are selected to determine the FRET efficiency. Quenching efficiency is expressed as E ¼ 1–FDA =FD
(2)
Figure 6. Determination of donor-to-acceptor molar ratios.
Copyright © 2014 John Wiley & Sons, Ltd.
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Figure 5. Effect of the Eosin Y concentration on the fluorescence spectra of the FRET system.
where D and A are the donor and acceptor, respectively. As shown in Figure 6, where the vertical axis is the fluorescence intensity and the horizontal axis is the concentration ratio of CdS QDs and Eosin Y, using both sides of the tangent extrapolation, the point of intersection with the abscissa gives the value of n. Points A and B intersect with the abscissa at 8.5 and 8, 1 respectively, therefore the value of n is ~ 8. This suggests that energy transfer occurs between 1 CdS QDs molecule (donor) and 8 Eosin Y (receptor) molecules.
Z. Yan et al. where FDA is the fluorescence intensity of the donor in the presence of the acceptor and FD is the fluorescence intensity of the donor in the absence of the acceptor. Enhancement efficiency is expressed as Y ¼ IAD =IA –1
(3)
where IAD is the fluorescence intensity of the acceptor in the presence of the donor and IA is the fluorescence intensity of the acceptor in the absence of the donor. The quenching efficiency is calculated as 45.64% and the enhancement efficiency is 20.07%, indicating that there are other energy transfers leading to energy loss in the FRET system. According to Forster’s theory, the relationship between E (the energy transfer efficiency) and r (the distance between the donor–acceptor pair) is given as E¼
nR60 nR60 þ r 6
(4)
where n is the donor-to-acceptor molar ratio and R0 is the critical distance when the energy transfer efficiency is 50%, which is expressed as R60 ¼ 8:791025 K 2 n4 0 ϕ D J ðλÞ
(5)
where the spatial orientation factor K2 and the refraction index n0 are accepted as 2/3 and 1.336, respectively (18,19). ΦD is the quantum yield of CdS QDs, measured as 12.3% using Rhodamine 6G as the reference standard (20), and J (λ) is the spectral overlap integral which can be calculated from eqn (6): J ðλÞ ¼
∫F ðλÞεðλÞλ4 dλ ∫F ðλÞdλ
(6)
where F(λ) is the fluorescence intensity of the donor and ε(λ) is the molar absorptivity of the acceptor. The value of J(λ) is 9.6909 × 10-15 cm3/L/mol for this FRET system, and R0 is calculated to be 2.45 nm from eqn (5). E could reach 20.07% from eqn (3) for the enhancement efficiency in the FRET process. By fitting the data to eqn (4), r is found to be 4.36 nm, which is much smaller than the value of 7 nm required for energy transfer to occur (18,19), indicating that the energy transfer from CdS QDs to Eosin Y has high probability. These results also confirm that a FRET process occurs between CdS QDs and Eosin Y.
Conclusion
158
In this study, TGA-functionalized CdS QDs were prepared in aqueous solution using green and low-cost materials. The quenching of the fluorescence of CdS QDs and the enhancement of the fluorescence of Eosin Y confirmed the FRET process. The mechanism of the energy transfer was studied, and some parameters of this system were calculated. The FRET system between CdS QDs and Eosin Y has the advantages of low cost, simple operation and high luminescence intensity. Applying QDs with good fluorescence properties to FRET research and constructing more types of FRET system will expand the scope of QDs and satisfy the demands of multiple analytical applications.
wileyonlinelibrary.com/journal/luminescence
Acknowledgements This work was supported by The Natural Science Foundation of Jiangsu Province Youth Fund (BK20130646), Jiangsu Key Lab of Environmental Engineering Open Foundation (KF2012008), Doctor Scientific Research Foundation from China Pharmaceutical University (2012ZJ13002), National Undergraduate Training Programs for Innovation and Entrepreneurship (02640556), the Fundamental Research Funds for the Central Universities (JKQZ2013009, JKPZ2013017).
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Luminescence 2015; 30: 155–158
