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Study of energy transfer mechanism from CdS quantum dots to Rhodamine 101 in reverse micelle medium
Synthetic Metals 245 (2018) 260–266
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Study of energy transfer mechanism from CdS quantum dots to Rhodamine 101 in reverse micelle medium
T
⁎
Ebru Bozkurta, , Yavuz Onganerb a b
Program of Occupational Health and Safety, Erzurum Vocational Training School, Ataturk University, 25240 Erzurum, Turkey Department of Chemistry, Faculty of Science, Ataturk University, TR-25240 Erzurum, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Quantum dots Rhodamine-101 Reverse micelle Fluorescence resonance energy transfer (FRET)
In this study, fluorescence resonance energy transfer (FRET) between CdS quantum dots (QDs) and Rhodamine 101 was investigated in reverse micelles. CdS QDs as donor were synthesized by using reverse micelle method. The particle sizes of CdS QDs were found as 1.18 ± 0.05 for QD1 and 2.11 ± 0.25 nm for QD2 with TEM measurements. It was determined that the fluorescence intensity of the QDs quenched as the concentration of Rh101 increased. The effect of the concentration of dye on the quenching of the fluorescence intensity of QDs was evaluated by the Stern-Volmer approach and KSV and kq values were calculated. It was observed that the quenching was not diffusion controlled. The FRET parameters were also calculated by using fluorescence spectroscopy. The distances between donor-acceptor (r) were also calculated as 4.74 nm for QD1 and 2.26 nm for QD2. The steady-state transfer efficiencies were calculated as 0.24 and 0.53 for QD1 and QD2, respectively. The time-resolved transfer efficiencies were also determined as 0.11 for QD1 and 0.48 for QD2. The results demonstrated that the novel donor-acceptor pairs may play an important role in the applications of many research areas such as FRET-based nanosensors and light harvesting devices.
1. Introduction Semiconductor quantum dots (QDs) are used in many technological applications such as photovoltaic cells, light emitting diodes (LEDs), photocatalysis, bioconjugates, and sensors due to their significantly changing optical properties depending on size [1–11]. But disadvantages such as uncontrollable size and aggregation formation of these structures create some difficulties for their use in technological applications. Highly uniform and size controllable QDs can be synthesized with reverse micelle method [12]. Reverse micelles are regular aggregates consisting of suitable surfactant dissolution in polar and nonpolar solvents. In these systems are often used water as the polar solvent and dioctyl sulfosuccinate sodium salt (AOT) as surfactant. The water pool size characterized by W0 (water/surfactant molar ratio) in reverse micelle systems [13]. Aggregates containing a small amount of water (W0 < 15) are referred to as reverse micelle, aggregates containing large amounts of water (W0 ≥ 15) are referred to as a microemulsion [14]. Reverse micelles have many applications such as nanoparticle synthesis [15,16], drug design [17], the textile industry [18] and purification of biomolecules [19]. When the quantum dots are photoexcited, the electron-hole pairs occur. The fluorescence light is emitted by this recombination due to ⁎
the small size quantum effect. Thus, quantum dots are considered an efficient donor in the visible region [5]. So, the studies on the fluorescence quenching mechanism of quantum dots are very important. The fluorescence quenching of quantum dots occurs due to resonance energy transfer or charge transfer [20]. Fluorescence resonance energy transfer (FRET) is a non-radiative intermolecular energy transfer from a fluorescent molecule (donor = D) to another molecule (acceptor = A) [21,22]. FRET is used quite often in recent years, has a unique capacity to detect molecular complexes over distances 10–100 Å and to make correct measurements. The mechanism of FRET can be explained by the Förster theory [5,23]. According to this theory, the occurrence of energy transfer depends on the spectral overlap between the donor emission and the acceptor absorption spectra, quantum yield of the donor, the relative orientation of the donor and acceptor transition dipoles, and the distance between donor and acceptor transition dipoles [21,24,25]. Furthermore, the energy transfer efficiency changes as proportional with the inverse sixth power of D-A distance [26–29]. This distance dependence supplies an advantage to measure changes in the angstrom scale [30]. FRET has many applications such as energy conversions like photosensitization and photosynthesis [31], elucidation the structure of DNA [32], the technology of ion sensors [33,34], investigation of supramolecular interactions [35].
Corresponding author. E-mail address: [email protected] (E. Bozkurt).
https://doi.org/10.1016/j.synthmet.2018.09.006 Received 4 May 2018; Received in revised form 11 September 2018; Accepted 18 September 2018 0379-6779/ © 2018 Elsevier B.V. All rights reserved.
Synthetic Metals 245 (2018) 260–266
E. Bozkurt, Y. Onganer
Rh101 in ethanol (1.0 × 10−3 M). To attain the desired dye concentration, a certain volume of Rh101 stock solution was transferred to the vial; the solvent of dye samples was evaporated by Argon gas purging. Afterwards, 5 mL of the prepared QD solution was added into the vial. 2.3. Apparatus Scanning electron microscope (SEM) and transmission electron microscope (TEM) images were obtained from Zeiss Sigma 300 and FEI Talos F200S 200 kV respectively. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra were obtained with PANalytical Empyrean and Bruker VERTEX 70v spectrophotometer, respectively. UV–vis. absorption and fluorescence spectra of the samples were recorded with Perkin Elmer Lambda 35 UV/VIS Spectrophotometer and Shimadzu RF-5301PC spectrofluorophotometer, respectively. The excitation source is 150 W Xenon lamp. For the steady-state fluorescence measurements, the sample solutions were excited at 350 nm and fluorescence intensities were recorded at 360–650 nm. The fluorescence lifetime measurements were carried out with a LaserStrobe Model TM3 spectrofluorophotometer from Photon Technology International (PTI). The excitation source is a combination of a pulsed nitrogen laser/tunable dye laser. The decay curves (λexc = 366 nm) were collected over 200 channels using a nonlinear time scale with the time increment increasing according to the arithmetic progression. The fluorescence decays were analyzed with the lifetime distribution analysis software from the instrument supplying company. The goodness of fits was assessed by the value of χ2 and weighed residuals [39]. Fluorescence quantum yields of donor molecules were calculated by using Parker-Rees equation. This equation;
Fig. 1. The molecular structure of Rhodamine 101.
In the present study, we aimed to investigate fluorescence resonance energy transfer (FRET) between CdS Quantum Dots (CdS QDs) synthesized by using reverse micelle and Rhodamine-101 (Rh101). For this purpose, UV–vis absorption, steady-state and time-resolved fluorescence measurements were taken to explain the interactions between CdS QDs with Rh101 molecules. In addition, energy transfer parameters were calculated. It was planned to acquire a new donor-acceptor pair that will play an important role in the literature with the results obtained from this study. 2. Experimental 2.1. Materials
2
η D 1−10−ODr ⎞ ∅s = ∅r ⎛ s ⎞ ⎜⎛ s2 ⎟⎞ ⎛ −OD D η r ⎝ ⎠ ⎝ r ⎠ ⎝ 1−10 s ⎠ ⎜
Rhodamine 101 (Fig. 1), dioctyl sulfosuccinate sodium salt (AOT) and other chemicals were obtained from Sigma, USA. CdCl2 and Na2S used for the synthesis of CdS QDs were purchased from Fluka, USA. nheptane was provided by Merck, Germany. The stock solution of Rh101 of 1.0 × 10−3 M was prepared in ethanol (EtOH).
The reverse micelle method used for the synthesis of CdS QDs. Two solutions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) of 0.09 M concentration were made separately in heptane. An aqueous solution of 0.3 M CdCl2 was added to one heptane solution of AOT, while an aqueous solution of 0.3 M Na2S.H2O was added to the other solution. Two reverse micelle solutions (1:1, v/v) were mixed to yield CdS quantum dots. The water content W0 defined by [H2O] / [AOT] was adjusted as 4 (QD1) and 8 (QD2). Since the water pool is a reaction field, the size of quantum dots can be controlled by changing the W0 values. [36]. All the experiments were performed at room temperature. The particle sizes of synthesized CdS QDs was examined by using Brus equation. This equation follows [37]: ⎜
⎟⎜
⎜
⎟
(2)
where D is the integrated area under the corrected fluorescence spectrum, n is the refractive index of the solution, and OD is the optical density at the excitation wavelength (λex= 350 nm). The subscripts s and r refer to the sample and reference solutions, respectively [38]. Quinine sulfate in 0.5 M H2SO4 solution was used as the reference. The fluorescence quantum yield of quinine sulfate is 0.55 in 0.5 M H2SO4 solution [40].
2.2. Synthesis of CdS quantum dots
ℏ2π 2 1 1 ⎞ e2 ΔE = ⎛ 2 ⎞ ⎛ + −1.8 mh ⎠ εCdS 4πε0 R ⎝ 2R ⎠ ⎝ me
⎟
2.4. Calculation of FRET parameters According to the Förster’s theory, the rate of fluorescence resonance energy transfer (kET) is given by
kET =
1 R0 6 ⎛ ⎞ τD ⎝ r ⎠
(3)
where τD is the lifetime of the donor in the absence of the acceptor, r is the distance between donor and acceptor and R0 is the Förster distance. R0 is a distance which energy transfer efficiency is 50%. This distance can be calculated by using the following equation:
R 0 = 0.211[κ 2η−4 QD J (λ )]1/6
(4)
⎟
(1)
where QD is the quantum yield of the donor in the absence of the acceptor, η is the refractive index of the medium, κ 2 is the orientation factor. This factor describes the relative orientation in space of the transition dipoles of the donor and acceptor and is usually assumed to be equal to 2/3, which is appropriate for dynamic random averaging of the donor and acceptor J (λ ) is the spectral overlap integral. This integral represents the degree of spectral overlap between the emission spectrum of the donor and the absorption spectrum of acceptor and
where ΔE is the band gap energy, h is the Planck’s constant, me ( = 0.3m0) and mh ( = 0.8m0) are the effective mass of electron and effective mass of hole, respectively, m0 ( = 9.109 × 10−31 kg) is the mass of electron, e( = 1.602 × 10-19 C) is the charge of electron, εCdS ( = 5.7) and ε0 ( = 8.854 × 10-12 F/m) are the dielectric constant of CdS and dielectric constant of the vacuum, respectively, R is the shell radius of CdS nanoparticle [38]. After that by using Eq. (1), the values of the particle size were found to be ∼1.52 nm and ∼2.17 nm for QD1 and QD2 respectively. The dye samples were freshly prepared by the stock solutions of
J (λ ) =
261
∞ ∫0 FD (λ ) εA (λ ) λ4dλ ∞ ∫0 FD (λ ) dλ
(5)
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E. Bozkurt, Y. Onganer
vibration due to the presence of n-heptane and surfactant. SEM images were taken to characterize the morphologic structure of QDs. The SEM images in the Figs. 4a and b indicates the presence of QDs with the spherical and uniform shape. The particle sizes of the QDs were determined by TEM images. Figs. 5a and b presents the TEM images of the obtained QD1 and QD2, respectively. According to TEM measurements, the particle sizes of CdS QDs were 1.18 ± 0.05 for QD1 and 2.11 ± 0.25 nm for QD2. The theoretical particle sizes of QDs were also calculated as ∼1.52 nm for QD1 and ∼2.17 nm for QD2 by using Eq. (1). It was observed that the theoretical values agree with the values obtained from TEM studies. It was also observed that QDs have the lattice structure by TEM images. Optical characterizations of CdS QDs synthesized by reverse micelle were also performed with UV–vis. absorption and steady-state fluorescence measurements. As shown in Fig. 6, CdS QDs have the maxima of absorption bands at 343 and 401 nm. The observed absorption bands were at a lower wavelength than CdS bulk crystal (∼515 nm) due to confined excitons. This indicates the formation of CdS QDs. Furthermore, the synthesized CdS QDs have a clear absorption band is an indication of their quite small sizes [46]. It was observed that QDs have broad emission bands at 512 nm (for QD1) and 546 nm (for QD2) on excitation at 350 nm (Fig. 7). The broad and low-energy fluorescence band was caused by defects in the QDs surfaces [47].
Fig. 2. X-ray diffraction patterns of QD1 and QD2.
is calculated by the Eq. (5). FD (λ ) is the normalized fluorescence intensity of the donor in the absence of acceptor and εA (λ ) is the extinction coefficient of the acceptor. The energy transfer efficiency is one of the important parameters were calculated using the relative fluorescence intensity of the donor, in the absence (FD) and presence (FDA) of acceptor or fluorescence lifetime of the donor, in the absence (τD) and presence (τDA) of acceptor by the following equation [41]:
E = 1−
FDA τ = 1− DA FD τD
3.2. Fluorescence resonance energy transfer from CdS QDs to Rhodamine101 Fluorescence resonance energy transfer between CdS QDs as the energy donor and Rhodamine-101 (Rh101) as acceptor was investigated. The spectral overlap between the emission spectrum of the donor (CdS QDs) and the absorption spectrum of the acceptor (Rh101) was determined and the overlap integral values were calculated as 1.33 × 1015 and 3.30 × 1014 M−1 cm−1 nm4 for QD1 and QD2, respectively (Fig. 8). These values indicate that resonance energy transfer between CdS quantum dots as a donor and Rh101 as an acceptor is possible. To prove the existence of an energy transfer between two molecules, the fluorescence measurements for CdS QDs, Rh101 and CdS QDsRh101 pair were taken at the excitation wavelength of the donor (λexc = 350 nm). The fluorescence spectrum of the CdS QDs-Rh101 had two peaks at 501 and 586 nm (Fig. 9). As can be seen in Fig. 9, the fluorescence peak at 501 nm had lower intensity compared to the pure CdS QDs and the fluorescence peak at 586 nm had higher intensity compared to the pure Rh101. This observation confirmed the presence of the energy transfer between CdS QDs and Rh101. After the energy transfer between CdS QDs and Rh101 was determined, absorption spectra of CdS QDs were recorded in the presence of Rh101 with different concentration (Fig. 10a and b). As the concentration of Rh101 increased, there were no significant changes in the absorption band of CdS QDs. The absence of any changes in the absorption spectrum of CdS QDs indicated that there was no formation a ground state complex between CdS QDs and Rh101. The variations of the fluorescence intensity of CdS QDs in the absence and the presence of Rh101 were investigated in reverse micelle (Fig. 11a and b). It was observed that the fluorescence intensity of CdS QDs decreased with the increase of Rh101 concentration and the fluorescence intensity of Rh101 increased. It was seen that the fluorescence intensities of CdS QDs quenched with increasing the concentration of Rh101. Quenching of the fluorescence intensities of CdS QDs exhibited that energy transfer from QDs to dye molecules takes place [48]. Also, the clearly visible isoemissive point observed at about 553 and 558 nm for QD1 and QD2, respectively (Fig. 11a and b) indicated that emission is due to two species, namely the presence of energy transfer from CdS QDs to Rh101 [5,49]. In order to correlate the energy transfer and the structures of QDs, the FTIR spectra of the samples were taken in the absence and presence
(6)
3. Results and discussion 3.1. Characterization of CdS quantum dots CdS quantum dots (QDs) were synthesized by using the reverse micelle method. The structural characterization of the synthesized QDs was made by XRD and FTIR measurements. Fig. 2 presented the XRD patterns of the CdS QDs. The three peaks correspond to the three crystal planes of (111), (220) and (311) of cubic-phase CdS, respectively. The broad peaks indicate that synthesized QDs have very small size [42,43]. FTIR spectra of CdS QDs synthesized by reverse micelle method were taken in the range of 400 cm−1 to 4000 cm−1 wavenumbers and under vacuum. As can be seen in Fig. 3, the peaks at 733 cm−1 which belongs to Cd-S stretching vibration proved the formation of QDs [44,45]. The peaks were observed at 2948 cm−1 which belong to CeH stretching vibration, at 1734 cm-1 which belong to CeC stretching vibration, and 1224 and 1210 cm−1 which belongs to phenolic CeO stretching
Fig. 3. FT-IR spectra of QD1 and QD2. 262
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Fig. 4. SEM images of (a) QD1 and (b) QD2 synthesized in reverse micelle.
Fig. 5. TEM images of (a) QD1 and (b) QD2 synthesized in reverse micelle.
Fig. 7. Fluorescence spectra of CdS QDs synthesized by the reverse micelle method.
Fig. 6. Absorption of CdS QDs synthesized by the reverse micelle method.
of Rh101. As shown in Fig. 12a and b, it was observed that there was no change in the functional groups in the FTIR spectra of QDs when Rh101 was added to the QD solution. It was suggested that energy transfer takes place with Rh101 molecules located in the lattice cavities of the QDs. The effect of the dye concentration on the fluorescence intensity quenching of QDs was evaluated with the Stern-Volmer equation as follows [38]:
F0 = 1 + kq τ0 [Q] = 1 + K SV [Q] F
(7)
where F0 is fluorescence intensity in the absence of quencher, F is fluorescence intensity in the presence of quencher, kq is bimolecular quenching constant, τ0 is fluorescence lifetime of the donor in the absence of quencher, [Q] is the quencher concentration, and KSV is the Stern-Volmer quenching constant. The KSV values obtained from the slopes of the Stern-Volmer plots given in Fig. 13 were found to be
Fig. 8. The spectral overlap between CdS QDs and Rh101.
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Fig. 9. Fluorescence spectra of CdS QDs, Rh101 and CdS QDs-Rh101 in reverse micelle (λex = 350 nm).
Fig. 11. Quenching of fluorescence emission of (a) QD1 and (b) QD2 at different concentration of Rh101 (λex = 350 nm).
QD2, respectively. The energy transfer efficiency depends on the distance between the donor-acceptor. The estimated Förster distance (R0) values were found as 3.81 and 2.28 nm using the Eq (4) for QD1 and QD2, respectively. These results were lower than those calculated in energy transfer studies where quantum dots were used as donors in recent years [28,51]. In addition, the distances between donor-acceptor (r) were also calculated as 4.74 and 2.26 nm for QD1 and QD2, respectively. The calculated values were smaller than the value required for energy transfer (7 nm) and this value indicated that the energy transfer from QDs to Rh101 occurs [52]. The energy transfer rate (kET) is a measure of the dipole-dipole coupling strength [53]. The energy transfer rates were determined as 4.22 × 10−7 s−1 and 0.13 × 10−7 s−1 for QD1-Rh101 and QD2-Rh101 systems (Table 1). It was also performed the lifetime measurements of CdS QDs in absence and presence of Rh101. The lifetime values of QDs in absence and presence of Rh101 are presented in Table 2 and the decay curves (λex = 366 nm) are presented in Fig. 14a and b. As listed in Table 2, the decay profile of the donor (QDs) in the absence and presence of the acceptor (Rh101) were analyzed by a tri-exponential function with the average lifetimes of donor QDs (< τ > = 7.24 ns for QD1 and < τ > = 8.95 ns for QD2) without the Rh101. In the presence of the acceptor (10 μM Rh101), the average lifetime values of QDs were found as 6.16 ns for QD1 and 4.35 ns for QD2 (Table 2). This reduction in lifetime values is due to the non-radiative energy transfer to Rh101 via FRET. The amplitude with the longest lifetime component for QDs is the effect on total fluorescence. This increases the probability that electrons and holes are on the surface of the QDs. That is, the surface of the QDs is definitely involved in the energy transfer process. Also, the time-resolved transfer efficiencies were calculated using Eq (6) and found as 0.11 for QD1 and 0.48 for QD2. These values were smaller than in the
Fig. 10. Absorption spectra of (a) QD1 and (b) QD2 at different concentration of Rh101.
73.0 × 104 M−1 and 1.22 × 105 M−1 for QD1 and QD2, respectively. The bimolecular quenching constant (kq) values which reflect the efficiency of quenching or the accessibility of the fluorophores to the quencher were calculated as 1.12 × 1013 M−1 s−1 and 1.51 × 1013 M−1 s−1 for QD1 and QD2, respectively. Since the obtained kq values are larger than the diffusion-controlled limit values (1.0 × 1010 M−1 s−1), this is not a diffusion controlled process [28,41,50]. It is very important to determine FRET parameters in energy transfer studies. The energy transfer parameters for FRET between CdS QDs-Rh101 are given in Table 1. The steady-state transfer efficiencies were calculated using Eq (6) and found as 0.24 and 0.53 for QD1 and 264
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Table 2 The fluorescence lifetime values of CdS QDs in absence and presence of Rh101. Sample
τ1 (ns)
τ2 (ns)
τ3 (ns)
A1
A2
A3
< τ > (ns)
QD1 QD1+Rh101 QD2 QD2+Rh101
0.55 9.94 9.49 5.56
10.72 0.81 4.41 3.80
11.5 0.90 10.39 1.80
0.36 0.58 0.39 0.50
0.39 0.01 0.18 0.34
0.25 0.41 0.43 0.16
7.24 6.16 8.95 4.35
Fig. 12. FT-IR spectra of (a) QD1 and (b) QD2 with and without Rh101 dye.
Fig. 14. Time-resolved fluorescence decays of (a) QD1 and (b) QD2 with and without Rh101 dye (λex = 366 nm).
steady-state studies. This result indicated that time-resolved data are not dependent on changes in the excitation source and concentration [50].
4. Conclusions In this study, the energy transfers between semiconductor CdS QDs and Rh101 were investigated in reverse micelle. The structural and morphological characterizations of the synthesized quantum dots were performed by XRD, FTIR and SEM analyses. The theoretical particle sizes of QDs were calculated as ∼1.52 nm and ∼2.17 nm for QD1 and QD2 by using Brus equation. The particle sizes of CdS QDs with TEM measurements were found as 1.18 ± 0.05 and 2.11 ± 0.25 nm for QD1 and QD2, respectively. Optical characterizations of CdS QDs synthesized with reverse micelle were also performed with UV–vis absorption and fluorescence measurements. The absorption band maxima of the QDs were observed at 343 and 401 nm for QD1 and QD2 and fluorescence band maxima of the QDs were observed at 512 and 546 nm for QD1 and QD2, respectively. When Rh101 with different concentrations was added to CdS QDs solutions, it was determined that the fluorescence intensities of the QDs quenched. The effect of the
Fig. 13. Stern-Volmer plots for CdS QDs.
Table 1 FRET parameters for CdS QDs and Rh101 pairs in reverse micelles. Media
ΦD
Jx1015 (M−1 cm−1 nm4)
R0 (nm)
r (nm)
kETx10−7 (s−1)
Ea
Eb
QD1 QD2
0.15 0.03
1.33 0.33
3.81 2.28
4.74 2.26
4.22 0.13
0.24 0.53
0.11 0.48
a b
From steady-state data. From time-resolved data.
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concentration of dye on the quenching of the fluorescence intensities of QDs was evaluated by the Stern-Volmer approach. KSV and kq values were calculated for QDs. KSV values were found to be 73.0 × 104 M−1 and 1.22 × 105 M−1 for QD1 and QD2, respectively. kq values were calculated as 1.12 × 1013 M−1 s−1 and 1.51 × 1013 M−1 s−1 for QD1 and QD2, respectively. The results showed that the quenching was not a diffusion controlled process. Also, the energy transfer parameters for FRET between CdS QDs-Rh101 were calculated. The steady-state transfer efficiencies were calculated as 0.24 and 0.53 for QD1 and QD2, respectively. The time-resolved transfer efficiencies were also determined and these values found as 0.11 for QD1 and 0.48 for QD2. R0 values were found as 3.81 and 2.28 nm for QD1 and QD2, respectively. r values were also calculated as 4.74 and 2.26 nm for QD1 and QD2, respectively. The energy transfer rates were determined as 4.22 × 10−7 s−1 and 0.13 × 10−7 s−1 for QD1-Rh101 and QD2-Rh101 systems. The results demonstrate that the novel donor-acceptor pairs may play an important role in applications in many research areas such as FRETbased nanosensors, light harvesting devices.
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Study of energy transfer mechanism from CdS quantum dots to Rhodamine 101 in reverse micelle medium
T
⁎
Ebru Bozkurta, , Yavuz Onganerb a b
Program of Occupational Health and Safety, Erzurum Vocational Training School, Ataturk University, 25240 Erzurum, Turkey Department of Chemistry, Faculty of Science, Ataturk University, TR-25240 Erzurum, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Quantum dots Rhodamine-101 Reverse micelle Fluorescence resonance energy transfer (FRET)
In this study, fluorescence resonance energy transfer (FRET) between CdS quantum dots (QDs) and Rhodamine 101 was investigated in reverse micelles. CdS QDs as donor were synthesized by using reverse micelle method. The particle sizes of CdS QDs were found as 1.18 ± 0.05 for QD1 and 2.11 ± 0.25 nm for QD2 with TEM measurements. It was determined that the fluorescence intensity of the QDs quenched as the concentration of Rh101 increased. The effect of the concentration of dye on the quenching of the fluorescence intensity of QDs was evaluated by the Stern-Volmer approach and KSV and kq values were calculated. It was observed that the quenching was not diffusion controlled. The FRET parameters were also calculated by using fluorescence spectroscopy. The distances between donor-acceptor (r) were also calculated as 4.74 nm for QD1 and 2.26 nm for QD2. The steady-state transfer efficiencies were calculated as 0.24 and 0.53 for QD1 and QD2, respectively. The time-resolved transfer efficiencies were also determined as 0.11 for QD1 and 0.48 for QD2. The results demonstrated that the novel donor-acceptor pairs may play an important role in the applications of many research areas such as FRET-based nanosensors and light harvesting devices.
1. Introduction Semiconductor quantum dots (QDs) are used in many technological applications such as photovoltaic cells, light emitting diodes (LEDs), photocatalysis, bioconjugates, and sensors due to their significantly changing optical properties depending on size [1–11]. But disadvantages such as uncontrollable size and aggregation formation of these structures create some difficulties for their use in technological applications. Highly uniform and size controllable QDs can be synthesized with reverse micelle method [12]. Reverse micelles are regular aggregates consisting of suitable surfactant dissolution in polar and nonpolar solvents. In these systems are often used water as the polar solvent and dioctyl sulfosuccinate sodium salt (AOT) as surfactant. The water pool size characterized by W0 (water/surfactant molar ratio) in reverse micelle systems [13]. Aggregates containing a small amount of water (W0 < 15) are referred to as reverse micelle, aggregates containing large amounts of water (W0 ≥ 15) are referred to as a microemulsion [14]. Reverse micelles have many applications such as nanoparticle synthesis [15,16], drug design [17], the textile industry [18] and purification of biomolecules [19]. When the quantum dots are photoexcited, the electron-hole pairs occur. The fluorescence light is emitted by this recombination due to ⁎
the small size quantum effect. Thus, quantum dots are considered an efficient donor in the visible region [5]. So, the studies on the fluorescence quenching mechanism of quantum dots are very important. The fluorescence quenching of quantum dots occurs due to resonance energy transfer or charge transfer [20]. Fluorescence resonance energy transfer (FRET) is a non-radiative intermolecular energy transfer from a fluorescent molecule (donor = D) to another molecule (acceptor = A) [21,22]. FRET is used quite often in recent years, has a unique capacity to detect molecular complexes over distances 10–100 Å and to make correct measurements. The mechanism of FRET can be explained by the Förster theory [5,23]. According to this theory, the occurrence of energy transfer depends on the spectral overlap between the donor emission and the acceptor absorption spectra, quantum yield of the donor, the relative orientation of the donor and acceptor transition dipoles, and the distance between donor and acceptor transition dipoles [21,24,25]. Furthermore, the energy transfer efficiency changes as proportional with the inverse sixth power of D-A distance [26–29]. This distance dependence supplies an advantage to measure changes in the angstrom scale [30]. FRET has many applications such as energy conversions like photosensitization and photosynthesis [31], elucidation the structure of DNA [32], the technology of ion sensors [33,34], investigation of supramolecular interactions [35].
Corresponding author. E-mail address: [email protected] (E. Bozkurt).
https://doi.org/10.1016/j.synthmet.2018.09.006 Received 4 May 2018; Received in revised form 11 September 2018; Accepted 18 September 2018 0379-6779/ © 2018 Elsevier B.V. All rights reserved.
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Rh101 in ethanol (1.0 × 10−3 M). To attain the desired dye concentration, a certain volume of Rh101 stock solution was transferred to the vial; the solvent of dye samples was evaporated by Argon gas purging. Afterwards, 5 mL of the prepared QD solution was added into the vial. 2.3. Apparatus Scanning electron microscope (SEM) and transmission electron microscope (TEM) images were obtained from Zeiss Sigma 300 and FEI Talos F200S 200 kV respectively. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra were obtained with PANalytical Empyrean and Bruker VERTEX 70v spectrophotometer, respectively. UV–vis. absorption and fluorescence spectra of the samples were recorded with Perkin Elmer Lambda 35 UV/VIS Spectrophotometer and Shimadzu RF-5301PC spectrofluorophotometer, respectively. The excitation source is 150 W Xenon lamp. For the steady-state fluorescence measurements, the sample solutions were excited at 350 nm and fluorescence intensities were recorded at 360–650 nm. The fluorescence lifetime measurements were carried out with a LaserStrobe Model TM3 spectrofluorophotometer from Photon Technology International (PTI). The excitation source is a combination of a pulsed nitrogen laser/tunable dye laser. The decay curves (λexc = 366 nm) were collected over 200 channels using a nonlinear time scale with the time increment increasing according to the arithmetic progression. The fluorescence decays were analyzed with the lifetime distribution analysis software from the instrument supplying company. The goodness of fits was assessed by the value of χ2 and weighed residuals [39]. Fluorescence quantum yields of donor molecules were calculated by using Parker-Rees equation. This equation;
Fig. 1. The molecular structure of Rhodamine 101.
In the present study, we aimed to investigate fluorescence resonance energy transfer (FRET) between CdS Quantum Dots (CdS QDs) synthesized by using reverse micelle and Rhodamine-101 (Rh101). For this purpose, UV–vis absorption, steady-state and time-resolved fluorescence measurements were taken to explain the interactions between CdS QDs with Rh101 molecules. In addition, energy transfer parameters were calculated. It was planned to acquire a new donor-acceptor pair that will play an important role in the literature with the results obtained from this study. 2. Experimental 2.1. Materials
2
η D 1−10−ODr ⎞ ∅s = ∅r ⎛ s ⎞ ⎜⎛ s2 ⎟⎞ ⎛ −OD D η r ⎝ ⎠ ⎝ r ⎠ ⎝ 1−10 s ⎠ ⎜
Rhodamine 101 (Fig. 1), dioctyl sulfosuccinate sodium salt (AOT) and other chemicals were obtained from Sigma, USA. CdCl2 and Na2S used for the synthesis of CdS QDs were purchased from Fluka, USA. nheptane was provided by Merck, Germany. The stock solution of Rh101 of 1.0 × 10−3 M was prepared in ethanol (EtOH).
The reverse micelle method used for the synthesis of CdS QDs. Two solutions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) of 0.09 M concentration were made separately in heptane. An aqueous solution of 0.3 M CdCl2 was added to one heptane solution of AOT, while an aqueous solution of 0.3 M Na2S.H2O was added to the other solution. Two reverse micelle solutions (1:1, v/v) were mixed to yield CdS quantum dots. The water content W0 defined by [H2O] / [AOT] was adjusted as 4 (QD1) and 8 (QD2). Since the water pool is a reaction field, the size of quantum dots can be controlled by changing the W0 values. [36]. All the experiments were performed at room temperature. The particle sizes of synthesized CdS QDs was examined by using Brus equation. This equation follows [37]: ⎜
⎟⎜
⎜
⎟
(2)
where D is the integrated area under the corrected fluorescence spectrum, n is the refractive index of the solution, and OD is the optical density at the excitation wavelength (λex= 350 nm). The subscripts s and r refer to the sample and reference solutions, respectively [38]. Quinine sulfate in 0.5 M H2SO4 solution was used as the reference. The fluorescence quantum yield of quinine sulfate is 0.55 in 0.5 M H2SO4 solution [40].
2.2. Synthesis of CdS quantum dots
ℏ2π 2 1 1 ⎞ e2 ΔE = ⎛ 2 ⎞ ⎛ + −1.8 mh ⎠ εCdS 4πε0 R ⎝ 2R ⎠ ⎝ me
⎟
2.4. Calculation of FRET parameters According to the Förster’s theory, the rate of fluorescence resonance energy transfer (kET) is given by
kET =
1 R0 6 ⎛ ⎞ τD ⎝ r ⎠
(3)
where τD is the lifetime of the donor in the absence of the acceptor, r is the distance between donor and acceptor and R0 is the Förster distance. R0 is a distance which energy transfer efficiency is 50%. This distance can be calculated by using the following equation:
R 0 = 0.211[κ 2η−4 QD J (λ )]1/6
(4)
⎟
(1)
where QD is the quantum yield of the donor in the absence of the acceptor, η is the refractive index of the medium, κ 2 is the orientation factor. This factor describes the relative orientation in space of the transition dipoles of the donor and acceptor and is usually assumed to be equal to 2/3, which is appropriate for dynamic random averaging of the donor and acceptor J (λ ) is the spectral overlap integral. This integral represents the degree of spectral overlap between the emission spectrum of the donor and the absorption spectrum of acceptor and
where ΔE is the band gap energy, h is the Planck’s constant, me ( = 0.3m0) and mh ( = 0.8m0) are the effective mass of electron and effective mass of hole, respectively, m0 ( = 9.109 × 10−31 kg) is the mass of electron, e( = 1.602 × 10-19 C) is the charge of electron, εCdS ( = 5.7) and ε0 ( = 8.854 × 10-12 F/m) are the dielectric constant of CdS and dielectric constant of the vacuum, respectively, R is the shell radius of CdS nanoparticle [38]. After that by using Eq. (1), the values of the particle size were found to be ∼1.52 nm and ∼2.17 nm for QD1 and QD2 respectively. The dye samples were freshly prepared by the stock solutions of
J (λ ) =
261
∞ ∫0 FD (λ ) εA (λ ) λ4dλ ∞ ∫0 FD (λ ) dλ
(5)
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vibration due to the presence of n-heptane and surfactant. SEM images were taken to characterize the morphologic structure of QDs. The SEM images in the Figs. 4a and b indicates the presence of QDs with the spherical and uniform shape. The particle sizes of the QDs were determined by TEM images. Figs. 5a and b presents the TEM images of the obtained QD1 and QD2, respectively. According to TEM measurements, the particle sizes of CdS QDs were 1.18 ± 0.05 for QD1 and 2.11 ± 0.25 nm for QD2. The theoretical particle sizes of QDs were also calculated as ∼1.52 nm for QD1 and ∼2.17 nm for QD2 by using Eq. (1). It was observed that the theoretical values agree with the values obtained from TEM studies. It was also observed that QDs have the lattice structure by TEM images. Optical characterizations of CdS QDs synthesized by reverse micelle were also performed with UV–vis. absorption and steady-state fluorescence measurements. As shown in Fig. 6, CdS QDs have the maxima of absorption bands at 343 and 401 nm. The observed absorption bands were at a lower wavelength than CdS bulk crystal (∼515 nm) due to confined excitons. This indicates the formation of CdS QDs. Furthermore, the synthesized CdS QDs have a clear absorption band is an indication of their quite small sizes [46]. It was observed that QDs have broad emission bands at 512 nm (for QD1) and 546 nm (for QD2) on excitation at 350 nm (Fig. 7). The broad and low-energy fluorescence band was caused by defects in the QDs surfaces [47].
Fig. 2. X-ray diffraction patterns of QD1 and QD2.
is calculated by the Eq. (5). FD (λ ) is the normalized fluorescence intensity of the donor in the absence of acceptor and εA (λ ) is the extinction coefficient of the acceptor. The energy transfer efficiency is one of the important parameters were calculated using the relative fluorescence intensity of the donor, in the absence (FD) and presence (FDA) of acceptor or fluorescence lifetime of the donor, in the absence (τD) and presence (τDA) of acceptor by the following equation [41]:
E = 1−
FDA τ = 1− DA FD τD
3.2. Fluorescence resonance energy transfer from CdS QDs to Rhodamine101 Fluorescence resonance energy transfer between CdS QDs as the energy donor and Rhodamine-101 (Rh101) as acceptor was investigated. The spectral overlap between the emission spectrum of the donor (CdS QDs) and the absorption spectrum of the acceptor (Rh101) was determined and the overlap integral values were calculated as 1.33 × 1015 and 3.30 × 1014 M−1 cm−1 nm4 for QD1 and QD2, respectively (Fig. 8). These values indicate that resonance energy transfer between CdS quantum dots as a donor and Rh101 as an acceptor is possible. To prove the existence of an energy transfer between two molecules, the fluorescence measurements for CdS QDs, Rh101 and CdS QDsRh101 pair were taken at the excitation wavelength of the donor (λexc = 350 nm). The fluorescence spectrum of the CdS QDs-Rh101 had two peaks at 501 and 586 nm (Fig. 9). As can be seen in Fig. 9, the fluorescence peak at 501 nm had lower intensity compared to the pure CdS QDs and the fluorescence peak at 586 nm had higher intensity compared to the pure Rh101. This observation confirmed the presence of the energy transfer between CdS QDs and Rh101. After the energy transfer between CdS QDs and Rh101 was determined, absorption spectra of CdS QDs were recorded in the presence of Rh101 with different concentration (Fig. 10a and b). As the concentration of Rh101 increased, there were no significant changes in the absorption band of CdS QDs. The absence of any changes in the absorption spectrum of CdS QDs indicated that there was no formation a ground state complex between CdS QDs and Rh101. The variations of the fluorescence intensity of CdS QDs in the absence and the presence of Rh101 were investigated in reverse micelle (Fig. 11a and b). It was observed that the fluorescence intensity of CdS QDs decreased with the increase of Rh101 concentration and the fluorescence intensity of Rh101 increased. It was seen that the fluorescence intensities of CdS QDs quenched with increasing the concentration of Rh101. Quenching of the fluorescence intensities of CdS QDs exhibited that energy transfer from QDs to dye molecules takes place [48]. Also, the clearly visible isoemissive point observed at about 553 and 558 nm for QD1 and QD2, respectively (Fig. 11a and b) indicated that emission is due to two species, namely the presence of energy transfer from CdS QDs to Rh101 [5,49]. In order to correlate the energy transfer and the structures of QDs, the FTIR spectra of the samples were taken in the absence and presence
(6)
3. Results and discussion 3.1. Characterization of CdS quantum dots CdS quantum dots (QDs) were synthesized by using the reverse micelle method. The structural characterization of the synthesized QDs was made by XRD and FTIR measurements. Fig. 2 presented the XRD patterns of the CdS QDs. The three peaks correspond to the three crystal planes of (111), (220) and (311) of cubic-phase CdS, respectively. The broad peaks indicate that synthesized QDs have very small size [42,43]. FTIR spectra of CdS QDs synthesized by reverse micelle method were taken in the range of 400 cm−1 to 4000 cm−1 wavenumbers and under vacuum. As can be seen in Fig. 3, the peaks at 733 cm−1 which belongs to Cd-S stretching vibration proved the formation of QDs [44,45]. The peaks were observed at 2948 cm−1 which belong to CeH stretching vibration, at 1734 cm-1 which belong to CeC stretching vibration, and 1224 and 1210 cm−1 which belongs to phenolic CeO stretching
Fig. 3. FT-IR spectra of QD1 and QD2. 262
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Fig. 4. SEM images of (a) QD1 and (b) QD2 synthesized in reverse micelle.
Fig. 5. TEM images of (a) QD1 and (b) QD2 synthesized in reverse micelle.
Fig. 7. Fluorescence spectra of CdS QDs synthesized by the reverse micelle method.
Fig. 6. Absorption of CdS QDs synthesized by the reverse micelle method.
of Rh101. As shown in Fig. 12a and b, it was observed that there was no change in the functional groups in the FTIR spectra of QDs when Rh101 was added to the QD solution. It was suggested that energy transfer takes place with Rh101 molecules located in the lattice cavities of the QDs. The effect of the dye concentration on the fluorescence intensity quenching of QDs was evaluated with the Stern-Volmer equation as follows [38]:
F0 = 1 + kq τ0 [Q] = 1 + K SV [Q] F
(7)
where F0 is fluorescence intensity in the absence of quencher, F is fluorescence intensity in the presence of quencher, kq is bimolecular quenching constant, τ0 is fluorescence lifetime of the donor in the absence of quencher, [Q] is the quencher concentration, and KSV is the Stern-Volmer quenching constant. The KSV values obtained from the slopes of the Stern-Volmer plots given in Fig. 13 were found to be
Fig. 8. The spectral overlap between CdS QDs and Rh101.
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Fig. 9. Fluorescence spectra of CdS QDs, Rh101 and CdS QDs-Rh101 in reverse micelle (λex = 350 nm).
Fig. 11. Quenching of fluorescence emission of (a) QD1 and (b) QD2 at different concentration of Rh101 (λex = 350 nm).
QD2, respectively. The energy transfer efficiency depends on the distance between the donor-acceptor. The estimated Förster distance (R0) values were found as 3.81 and 2.28 nm using the Eq (4) for QD1 and QD2, respectively. These results were lower than those calculated in energy transfer studies where quantum dots were used as donors in recent years [28,51]. In addition, the distances between donor-acceptor (r) were also calculated as 4.74 and 2.26 nm for QD1 and QD2, respectively. The calculated values were smaller than the value required for energy transfer (7 nm) and this value indicated that the energy transfer from QDs to Rh101 occurs [52]. The energy transfer rate (kET) is a measure of the dipole-dipole coupling strength [53]. The energy transfer rates were determined as 4.22 × 10−7 s−1 and 0.13 × 10−7 s−1 for QD1-Rh101 and QD2-Rh101 systems (Table 1). It was also performed the lifetime measurements of CdS QDs in absence and presence of Rh101. The lifetime values of QDs in absence and presence of Rh101 are presented in Table 2 and the decay curves (λex = 366 nm) are presented in Fig. 14a and b. As listed in Table 2, the decay profile of the donor (QDs) in the absence and presence of the acceptor (Rh101) were analyzed by a tri-exponential function with the average lifetimes of donor QDs (< τ > = 7.24 ns for QD1 and < τ > = 8.95 ns for QD2) without the Rh101. In the presence of the acceptor (10 μM Rh101), the average lifetime values of QDs were found as 6.16 ns for QD1 and 4.35 ns for QD2 (Table 2). This reduction in lifetime values is due to the non-radiative energy transfer to Rh101 via FRET. The amplitude with the longest lifetime component for QDs is the effect on total fluorescence. This increases the probability that electrons and holes are on the surface of the QDs. That is, the surface of the QDs is definitely involved in the energy transfer process. Also, the time-resolved transfer efficiencies were calculated using Eq (6) and found as 0.11 for QD1 and 0.48 for QD2. These values were smaller than in the
Fig. 10. Absorption spectra of (a) QD1 and (b) QD2 at different concentration of Rh101.
73.0 × 104 M−1 and 1.22 × 105 M−1 for QD1 and QD2, respectively. The bimolecular quenching constant (kq) values which reflect the efficiency of quenching or the accessibility of the fluorophores to the quencher were calculated as 1.12 × 1013 M−1 s−1 and 1.51 × 1013 M−1 s−1 for QD1 and QD2, respectively. Since the obtained kq values are larger than the diffusion-controlled limit values (1.0 × 1010 M−1 s−1), this is not a diffusion controlled process [28,41,50]. It is very important to determine FRET parameters in energy transfer studies. The energy transfer parameters for FRET between CdS QDs-Rh101 are given in Table 1. The steady-state transfer efficiencies were calculated using Eq (6) and found as 0.24 and 0.53 for QD1 and 264
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Table 2 The fluorescence lifetime values of CdS QDs in absence and presence of Rh101. Sample
τ1 (ns)
τ2 (ns)
τ3 (ns)
A1
A2
A3
< τ > (ns)
QD1 QD1+Rh101 QD2 QD2+Rh101
0.55 9.94 9.49 5.56
10.72 0.81 4.41 3.80
11.5 0.90 10.39 1.80
0.36 0.58 0.39 0.50
0.39 0.01 0.18 0.34
0.25 0.41 0.43 0.16
7.24 6.16 8.95 4.35
Fig. 12. FT-IR spectra of (a) QD1 and (b) QD2 with and without Rh101 dye.
Fig. 14. Time-resolved fluorescence decays of (a) QD1 and (b) QD2 with and without Rh101 dye (λex = 366 nm).
steady-state studies. This result indicated that time-resolved data are not dependent on changes in the excitation source and concentration [50].
4. Conclusions In this study, the energy transfers between semiconductor CdS QDs and Rh101 were investigated in reverse micelle. The structural and morphological characterizations of the synthesized quantum dots were performed by XRD, FTIR and SEM analyses. The theoretical particle sizes of QDs were calculated as ∼1.52 nm and ∼2.17 nm for QD1 and QD2 by using Brus equation. The particle sizes of CdS QDs with TEM measurements were found as 1.18 ± 0.05 and 2.11 ± 0.25 nm for QD1 and QD2, respectively. Optical characterizations of CdS QDs synthesized with reverse micelle were also performed with UV–vis absorption and fluorescence measurements. The absorption band maxima of the QDs were observed at 343 and 401 nm for QD1 and QD2 and fluorescence band maxima of the QDs were observed at 512 and 546 nm for QD1 and QD2, respectively. When Rh101 with different concentrations was added to CdS QDs solutions, it was determined that the fluorescence intensities of the QDs quenched. The effect of the
Fig. 13. Stern-Volmer plots for CdS QDs.
Table 1 FRET parameters for CdS QDs and Rh101 pairs in reverse micelles. Media
ΦD
Jx1015 (M−1 cm−1 nm4)
R0 (nm)
r (nm)
kETx10−7 (s−1)
Ea
Eb
QD1 QD2
0.15 0.03
1.33 0.33
3.81 2.28
4.74 2.26
4.22 0.13
0.24 0.53
0.11 0.48
a b
From steady-state data. From time-resolved data.
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concentration of dye on the quenching of the fluorescence intensities of QDs was evaluated by the Stern-Volmer approach. KSV and kq values were calculated for QDs. KSV values were found to be 73.0 × 104 M−1 and 1.22 × 105 M−1 for QD1 and QD2, respectively. kq values were calculated as 1.12 × 1013 M−1 s−1 and 1.51 × 1013 M−1 s−1 for QD1 and QD2, respectively. The results showed that the quenching was not a diffusion controlled process. Also, the energy transfer parameters for FRET between CdS QDs-Rh101 were calculated. The steady-state transfer efficiencies were calculated as 0.24 and 0.53 for QD1 and QD2, respectively. The time-resolved transfer efficiencies were also determined and these values found as 0.11 for QD1 and 0.48 for QD2. R0 values were found as 3.81 and 2.28 nm for QD1 and QD2, respectively. r values were also calculated as 4.74 and 2.26 nm for QD1 and QD2, respectively. The energy transfer rates were determined as 4.22 × 10−7 s−1 and 0.13 × 10−7 s−1 for QD1-Rh101 and QD2-Rh101 systems. The results demonstrate that the novel donor-acceptor pairs may play an important role in applications in many research areas such as FRETbased nanosensors, light harvesting devices.
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