FÃ¶rster Resonance Energy Transfer from Quantum Dots to Rhodamine

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Förster Resonance Energy Transfer from Quantum Dots to Rhodamine B as Mediated by a Cationic Surfactant: A Thermodynamic Perspective Ying-Ying Wang, Xun Xiang, Ren Yan, Yi Liu, and Feng-Lei Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08236 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Förster Resonance Energy Transfer from Quantum Dots to Rhodamine B as Mediated by a Cationic Surfactant: A Thermodynamic Perspective

Ying-Ying Wang, Xun Xiang, Ren Yan, Yi Liu, Feng-Lei Jiang*

State Key Laboratory of Virology & Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China. * Corresponding author. Email: [email protected] (F.-L. Jiang). Tel: +86 - 27 68756667.

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Förster resonance energy transfer (FRET) has attracted much attention for its wide applications in the fields of bioimaging, bioanalysis, etc. One of the critical problems in FRET is the construction of suitable donor-acceptor pair. The fluorescent quantum dots (QDs) can well meet the requirements both for a donor and an acceptor, owing to their tunable emission and broad absorption. Besides, the QDs possess high quantum yield, which highly benefits the FRET efficiency. In this work, glutathione capped CdTe QDs (GSH-CdTe QDs) was chosen as the energy donor (D) and Rhodamine B (RhB) as the energy acceptor (A). However, no FRET occurred when there were only QDs and RhB, even though there was much overlap between the absorption of RhB and the emission of QDs. Interestingly, after the addition of a cationic surfactant, cetyltrimethyl ammonium bromide (CTAB), FRET was induced favorably. Further understanding of this phenomenon was studied by fluorescence spectroscopy, dynamic light scattering and zeta potential. The results indicated that QDs aggregated and were cross-linked by CTAB due to electrostatic interactions. Then, RhB was trapped in the aggregates. Therefore, QDs and RhB were pulled closer to a reasonable distance and FRET happened prosperously. Notably, thermodynamics in this process was well studied for an in-depth understanding. This work will render the better design of donor-acceptor pairs to overcome the long distances, as well as the deep understanding of FRET with spreading applications.

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1 Introduction Förster resonance energy transfer (FRET) is a process of energy transfer, which means the energy is transferred from an energy donor (D) to an energy acceptor (A) through dipole-dipole interaction mechanism. This mechanism was proposed a hundred years ago and developed by Förster.1,2 Once an appropriate condition of D-A pair is established, the energy transfer will result in the quenching of the fluorescence of the donor, and meanwhile the enhancement of the fluorescence of the acceptor. It is widely used in bioimaging,3,4 detection,5-7 sensing8 and many other applications.9,10 However, the requirements for FRET are very strict. First, the absorption spectrum of the acceptor and emission spectrum of the donor should be overlapped. Second, the distance between the D-A pair needs to be close (1~10 nm). Third, the orientation of fluorescent chromophores should be appropriate. FRET may occur only when the above three conditions are satisfied simultaneously.11,12 Besides, the excitation wavelength should excite the donor more, while avoid the excitation of the acceptor as much as possible. Therefore, the D-A pair needs to be designed and considered seriously before application. However, even D-A pair is designed carefully, the efficiency of FRET could also be regulated by tuning the distance between the donor and

acceptor.

For

example,

Chowdhury

et

al.

used

7-diethylamino-3-(4-maleimidophenyl)-4-methylcoumarin (CPM) as a donor, Alexa Fluor 488 as an acceptor and HSA as a bridge connecting the D-A pair. FRET could happen in this D-A pair once the distance was appropriate, and the energy transfer efficiency between this D-A pair could be regulated by altering the conformation of 3



HSA. When the domain was folded, the D-A pair would be pulled together, thus the efficiency was enhanced, and vice versa. By this way, the unfolding stepwise of different domains of HSA could be observed.13 Semiconductor quantum dots (QDs) are a kind of special luminescent nanomaterials, whose florescence is caused by the recombination of the electrons and holes.14 In addition to the broad absorption, sharp emission peak and long lifetime, one of the advantages of QDs is that their fluorescence could be regulated in a wide range by tuning their size during synthesis. Their applications for imaging,15-17 diagnostics,18 drug delivery19,20 and even latent fingerprints21 have attracted much attention since they appeared. Meanwhile, these unique advantages mentioned above make it a more suitable choice as donor for FRET systems compared with organic dyes. Broad absorption enables that excitation wavelength could be far away from the absorption of the acceptor, so that the interference arose from the absorption of acceptor could be reduced to the lowest. The narrow emission helps the emission spectrum of acceptor separate from the emission spectrum of QDs to avoid the interference of QDs’ emission. Furthermore, long lifetime and high quantum yield of QDs can enhance the efficiency of energy transfer. Although there are numbers of literatures about the applications of FRET where QDs are used as acceptors or donors,22 relatively little is known about the thermodynamics of the association between D-A pairs in the FRET process. Herein, we performed a model system of FRET between water-soluble CdTe QDs capped with glutathione (GSH) and Rhodamine B (RhB) to study the thermodynamics of the 4


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association between D-A pairs in the FRET process. Although there was a large overlap between the emission spectrum of donor and the absorption spectrum of acceptor, FRET couldn’t happen. Interestingly, it can be induced by a cationic surfactant, cetyltrimethyl ammonium bromide (CTAB). This phenomenon was studied through fluorescence spectroscopy, UV-vis absorption spectroscopy, lifetime, dynamic light scattering (DLS), and zeta potential. Then, the mechanism of the FRET process mediated by CTAB was proposed. Finally, thermodynamic parameters of the association between the D-A pair in the FRET process were also obtained. 2 Experimental section 2.1 Materials Chromium chloride (CdCl2, 99.99%), sodium borohydride (NaBH4, 99%), tellurium powder (99.999%, about 200 mesh), cetyltrimethyl ammonium bromide (CTAB, 99%), sodium dodecyl sulfate (SDS), Rhodamine B, hydrochloric acid (HCl) and sodium hydrate (NaOH) were purchased from Sinopharm Chemical Reagent Co. (China). Dodecyl trimethyl ammonium bromide (DTAB) was purchased from Shanghai Macklin Biochemical Co. Glutathione (98%) and Rhodamine 123 were obtained from Aladdin. Ultrapure water with 18.2 MΩ cm-1 (Millipore Simplicity) was used in all experiments. All reagents were used without further purification. 2.2 The synthesis of CdTe QDs The synthesis of CdTe QDs was according to the literature with minor modifications.23 Te precursor and Cd precursor were synthesized firstly. Te powder and NaBH4 (50 mg, 0.39 mmol and 50 mg, 1.3 mmol) were added into 4 mL ultrapure 5



water under N2 atmosphere. And the mixture was stirred at room temperature before the mixture was turned to light red. Cd precursor was synthesized at the same time. GSH (0.0368 mg, 0.12 mmol) and CdCl2 (0.0183 mg, 0.069 mmol) were put in 50 mL water. Then the solution of Cd precursor was pumped and inflated 3 times by Schlenk technology to exhaust O2, which may result in the oxidation of Te powder. After the precursors of Cd and Te were prepared, 0.2 mL Te precursor was injected into the solution of Cd precursor to nucleate under N2 atmosphere and room temperature for 15 minutes. Finally, the mixture was stirred vigorously and refluxed at 120 oC for 2 h. Isopropanol was then mixed with the crude production at 3:1 (v:v) and centrifuged at 6500 rpm for further purification. Afterwards, the precipitate was dissolved in a small amount of ultrapure water and dialyzed. The final product was kept at 4 oC in dark for further use. The size and concentration of CdTe QDs were calculated by the equation proposed by Peng.24 The equations are as follows: D = (9.8127×10-7 )λ3 - (1.7147×10-3 )λ2 + (1.0064)λ - (194.84) (1) A = ϵbc

(2)

ϵ = 10043(D)2.12

(3)

where D represents the size of QDs, λ is the wavelength of the first excitonic absorption peak of QDs, A is the absorbance, ϵ is the extinction coefficient of QDs, b is the path length, and c represents the concentration of the given QDs system. 2.3 Transmission electron microscopy (TEM) TEM mapping were recorded on a JEM-2100 (HR) equipped with an energy dispersive X-ray (EDX) spectroscope, operating at 200 kV. Samples were prepared by 6


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dispersing a suitable amount of QDs and CTAB in ultrapure water. A drop of the suspension was then dropped onto a copper network attached with an ultrathin carbon film. 2.4 Fluorescence spectroscopy The fluorescence spectroscopy was performed by a Cary Eclipse Fluorescence Spectrophotometer (Agilent Technologies). The excitation wavelength was set at 330 nm for all experiments, and the emission maxima of QDs and RhB were at 530 nm and 576 nm, respectively. 2.4 UV-visible absorption spectroscopy The absorption spectra of CdTe QDs were recorded by a Cary series UV-vis spectrophotometer with a 1 cm quartz cell. The spectra were measured between 200 nm and 800 nm. 2.5 Dynamic light scattering (DLS) and zeta potential Malvern Zetasizer NanoZS was used to observe the changes of hydrodynamic diameters and zeta potentials of QDs with the titration of CTAB and SDS. The concentration of QDs was kept at 1.5 µM. 2.6 Fluorescence lifetime Fluorescence lifetime measurements were taken on a FLS920 (Edinburgh Instruments Co.) through time-correlated single photo counting (TCSPC) method. The samples were excited at 330 nm by a picosecond diode laser. 2.7 Isothermal titration calorimetry (ITC) Isothermal titration calorimetry (ITC) was performed on a nano-ITC2G (TA 7



Instruments, USA) to study the interactions in the FRET process. The concentrations of RhB, QDs and CTAB were 2.5 mM, 0.5 µM and 0.04 mM, respectively. 250 µL of RhB was titrated into the cell that contained QDs and CTAB, the drop interval was 600 s, the volume of each drop was 10 µL, and the stirring rate was 150 rpm. 2.8 Determination of critical micelle concentration (CMC) of CTAB and SDS The CMC values of CTAB and SDS in the absence and presence of QDs were determined by the surface tension method with a surface tension meter (K100-KRUSS, Germany). The concentrations of QDs were 0 µM, 0.5 µM, 1 µM and 1.5 µM. The volumes of solution were set as 40 mL for CTAB, and 20 mL for SDS.

3 Results and discussion 3.1 Characterization The morphology and mean size of the synthesized CdTe QDs were characterized by TEM (Figure 1A). A particle count, obtained from different regions of the sample, confirmed the presence of essentially monodispersed CdTe QDs of mean diameter 2.16 ± 0.21 nm (Figure 1B). It was close to the particle size (2.38 nm) calculated from eq. (1) when the first excitonic absorption peak of QDs was ~ 500 nm (Figure 1C). The emission maximum of QDs was 530 nm (Figure 1C). Besides, the absorption spectrum of RhB was also measured (Figure 1D). The large overlap of the fluorescence emission spectrum of CdTe QDs (Figure 1D, black line) and the absorption spectrum of RhB (Figure 1D, red line) indicated FRET could occur between the chosen D-A pair. 8


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Count

18

2.16 ± 0.21 nm

(B)

12

6

0

1.8

2.0

2.2 D (nm)

2.4

2.6

0.4

0.8 0.6

0.2 0.1 0.0 300

0.4

500 nm

0.2 400 500 600 Wavelength (nm)

1.0

1.0

0.0 700

Normalized

530 nm

0.3

FL Intensity (a.u.)

(C)

Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


i ii iii

(D)

0.8 0.6 0.4 0.2 0.0

480

520 560 600 Wavelength (nm)

Figure 1. (A) TEM images of synthesized CdTe QDs (the scale bar of the inset is 5 nm). (B) The size distribution histogram with the mean diameter. (C) The absorption and fluorescence emission spectra of synthesized CdTe QDs. (D) The normalized emission spectra of QDs (i) and RhB (iii) and the absorption spectrum of RhB (ii).

3.2 FRET between CdTe QDs and RhB induced by CTAB Although there was a great overlap between the fluorescence spectrum of CdTe QDs and the absorption spectrum of RhB (Figure 1D), the fluorescence intensity of RhB wasn’t enhanced after the addition of RhB (Figure 2A), indicating the energy wasn’t transferred to RhB successfully. However, if RhB was added into the system that contained both CTAB and QDs, the fluorescence of RhB would be greatly enhanced (Figure 2B). This fact indicated that FRET happened between QDs and RhB in the presence of CTAB. Considering the tendency of dimerization of RhB in aqueous solution,25 it must 9



be figured out whether the enhancement of RhB was attributed to the return of RhB monomers from its dimerization form, which might be promoted by the addition of CTAB. According to the reported literatures, the dimerization of RhB could be negligible below 5 µΜ,26-30 a concentration higher than that used in this work. To further rule out the possibilities of dimerization of RhB, the fluorescence spectra of different concentrations of RhB were recorded. As expected, the fluorescence integrated area showed good linear relationship with the increasing concentration of RhB (0-5 µM) (Figure S1A). These facts mentioned above indicated that enhancement of the fluorescence of RhB was mainly attributed to the FRET process rather than the de-dimerization of RhB. Besides, when CTAB was added into the solution that contained both QDs and RhB, the fluorescence intensity of RhB was also enhanced (Figure 2C and D). The fluorescence intensity reached a maximum in the presence of 40 µM CTAB but then tended to decrease slightly after this concentration, which might be attributed to the instability of QDs.

10


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400 300

QDs QDs+RhB RhB

200 100 0 450

QDs QDs+RhB RhB

(B)

250 FL Intensity (a.u.)

FL Intensity (a.u.)

(A)

200 150 100 50 0


500

550 600 Wavelength (nm)

650

160 (C)

60

120

FL Intensity (a.u.)

FL Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 40 0

56 52 48

450


0

650

10

20

30

40 50 (µM)

60

70

[CTAB]

CTAB 0 µM

Figure 2. (A) The fluorescence emission spectra of QDs, RhB and the system that contains both in the absence of CTAB, respectively. [QDs] = 0.2 µM, [RhB] = 1.5 µM. (B) The fluorescence emission spectra of QDs, RhB and the system that contains both in the presence of CTAB, respectively. [QDs] = 0.4 µM, [RhB] = 1.5 µM, [CTAB] = 0.04 mM. (C) The emission spectra of QDs and RhB with different concentrations of CTAB. [QDs] = 0.5 µM, [RhB] = 1.5 µM, [CTAB] = 0-70 µM. (D) The scatter plots of fluorescence intensity of RhB with the titration of CTAB in the presence of QDs.

3.3 Interaction of CdTe QDs with CTAB To further understand why CTAB could promote FRET between RhB and QDs, a group of study has been done. First, CTAB was added into a solution that contained only RhB so as to examine the influence of CTAB on the fluorescence of RhB. It was found that the fluorescence intensity of RhB remained nearly unchanged with the 11



titration of CTAB (Figure S2), which suggested CTAB wouldn’t influence the fluorescence of RhB. In contrast, the addition of CTAB would result in the fluorescence quenching of CdTe QDs. The fluorescence of CdTe QDs was quenched by the titration of CTAB until it reached a plateau (Figure 3A), meanwhile turbid suspension was observed, which suggested that apparent aggregation happened with the addition of higher concentrations of CTAB (>80 µM). There were many reported reasons responsible for the quenching of fluorescence, including aggregation, electron transfer and trap state, etc. According to the aggregation occurred in this work, the fluorescence quenching of CdTe QDs was attributed to the aggregation of QDs caused by CTAB, which was consistent with reported literatures.31 Besides, given the positive charge carried by CTAB and the negative charge carried by GSH-CdTe QDs, CTAB might be adsorbed onto the surface of QDs due to electrostatic adsorption. Based on these speculations, the interaction between QDs and CTAB was studied to find out the mechanisms of the fluorescence quenching and aggregation of QDs induced by CTAB. Hill equation is often used to analyze the binding constants of associative interactions.32-35 Since QDs can be expected to have multiple associative interactions with CTAB,35 we can expect the binding equilibrium to exhibit cooperativity, as in the classic example of binding of multiple O2 molecules to hemoglobin.36-37 The functional dependence in this research can be described quantitatively by the Hill equation, which is shown as equation (4):35

I0 -I=

I0 - Imin 1+ KD ⁄QDsn

(4)

12


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Here, I0 and I are the fluorescence intensities of QDs in the absence and presence of CTAB, Imin represents the minimum fluorescence intensity of QDs in the presence of CTAB, KD refers to the dissociation constant, and n is the Hill coefficient. This equation provides excellent fit to the data recorded at four temperatures, namely 298, 302, 306 and 310 K. Ka, which is considered as the binding constant, is defined as the reciprocal of KD. The binding constant between QDs and CTAB decreased with increasing temperatures (Table 1), indicating the interaction process followed a static quenching mechanism. Due to the positive charge of CTAB and the negative charge of QDs, electrostatic interaction could take place. To elucidate the driving forces between QDs and CTAB, the related thermodynamic parameters were calculated by the van’t Hoff equation:38-40 ln Ka = -

∆H ∆S + RT R

(5)

where R is the gas constant, T represents the temperature, ∆H is the enthalpy change, and ∆S is the entropy change. Finally, the free energy change (∆G) was estimated by the following equation: ∆G = ∆H - T∆S

(6)

The result showed a negative ∆H and a positive ∆S. According to the literature,41 electrostatic interaction might be the driving force for the interactions with negative enthalpy change and positive entropy change. In consideration of the positive charge carried by CTAB and negative charge carried by GSH-CdTe QDs, this result and the speculation mentioned above were reasonable. Moreover, the positive ∆S 13



demonstrated the adsorption of CTAB on the surface of QDs might remove the water molecules on the surface of QDs, therefore causing the increase of entropy. Besides, the fluorescence of QDs in the presence of an anionic surfactant SDS was also recorded for comparison. The results showed that the fluorescence of QDs was almost not influenced by SDS (Figure 3D). But it’s still hard to understand the unobvious effect of SDS on the QDs’ fluorescence was due to the negative charges or the weaker hydrophobic interaction. Therefore, the effect of dodecyl trimethyl ammonium bromide (DTAB) on the fluorescence of QDs, which possessed the same number of carbon atoms with SDS and the same charge with CTAB, was studied. As the result showed (Figure S3), the fluorescence of QDs was also quenched by DTAB, which further confirmed the electrostatic interactions between CTAB and GSH-CdTe QDs. Thus, the interaction between CTAB and QDs could be described as follows. First, CTAB was attracted onto the surface of QDs due to the electrostatic adsorption. Then the negative charges of QDs, which were supposed to sustain QDs stable, would be neutralized. As a result, the stability of QDs was reduced and aggregation occurred. But on the other hand, since the hydrophilic head group of CTAB was interacted with QDs, the hydrophobic tail would be exposed to the aqueous phase, which would increase the thermodynamic instability of the system. Thus, it was highly possible that the hydrophobic tail on the surface of aggregated QDs would associate with each other to decrease the thermodynamic instability. By this way, the aggregated QDs were cross-linked. Further, the aggregation would induce exciton energy transfer from 14


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smaller QDs of larger bandgap to larger QDs of smaller bandgap, and cause the quenching and redshift of the emission of QDs.42-44

(A)

200

310 K 306 K 302 K 298 K

240

F0-F

300

(B)

320

10 µM [CTAB]

FL Intensity (a.u.)

400

160

140 µM 100

80

0 450

0


0

40

80 [CTAB] (µM)

120

900

(D)

10.04

FL Intesity (a.u.)

(C)

ln(Ka)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ln(Ka)=8.92+336.26/T R2=0.9982

10.02

800 700 600 500

10.00

400 3.24

3.28 3.32 1000/T (K-1)

3.36

0

10 20 [SDS] (µM)

30

Figure 3. (A) Fluorescence spectra of QDs in the presence of different concentration of CTAB at 298 K. [QDs]=1.5 µM. (B) Nonlinear fitting of (F0-F) as function of the concentration of CTAB with Hill equation at different temperatures. (C) Van’t Hoff plot of ln(Ka) as function of 1/T. (D) Emission intensities of QDs with different concentrations of SDS.

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Table 1. Binding constants and thermodynamic parameters of CTAB-QDs interaction T

KD

∆H

∆S

∆G

(kJ.mol-1)

(J.mol-1.K-1)

(kJ.mol-1)

Ka n

R 4

2

-1

(K)

(µM)

298

43.28±0.67

4.23

2.31±0.04

0.9947

-24.90±0.07

302

43.96±0.58

4.12

2.28±0.03

0.9964

-25.19±0.07

(10 L.mol )

-2.80±0.07

74.16±0.22

306

44.54±0.47

3.99

2.25±0.03

0.9976

-25.49±0.07

310

45.23±0.57

4.27

2.21±0.03

0.9965

-25.79±0.07

As for the surfactants, critical micelle concentration (CMC) often has a great effect on their behavior. According to the literatures, the CMC of CTAB is ~ 1 mM, and that of SDS is ~ 8 mM.45,46 But considering that the presence of QDs might alter the CMC of surfactant, the CMC values of CTAB and SDS in the absence and presence of QDs were determined by surface tension method (Figure S4). It turned out that the CMC of both CTAB and SDS with QDs increased slightly but then decreased with higher concentrations of QDs. Besides, the surface tension of CTAB in the presence of QDs showed two inflection points, which meant a complex was formed (Figure S4B-D).47 The CMC values of CTAB and SDS with 1.5 µΜ QDs (the largest concentration used in this work) were calculated as 0.81 mM and 4.83 mM, respectively (Figure S4D and H). In general, the CMC values of CTAB and SDS were not remarkably changed in the presence of QDs in this work. Subsequently, the fluorescence intensities of QDs with different concentrations of CTAB and SDS were 16


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studied respectively to clearly understand the possible effects of CMC on QDs (Figure S5). It was found that the fluorescence intensity of QDs decreased sharply first with the addition of CTAB, then the quenching tendency slowed down and nearly reached a plateau around the CMC of CTAB (Figure S5A). On the contrary, the impact of SDS on the fluorescence of QDs was small even around the CMC (Figure S5C). And the scatter plots were also exhibited to clearly express the changes (Figure S5B and D).

3.4 UV-visible absorption spectroscopy In general, there are many methods to study the interaction mechanism between two molecules.48 Among them, the absorption spectroscopy can provide evidences as supplementary. If there is an electrostatic interaction between QDs and CTAB, the surface state of QDs (zeta potential for instance) will be altered, so the absorption spectrum of QDs will be changed. On the contrary, if the quenching of QDs is caused by dynamic quenching, i.e., there is no electrostatic interactions between QDs and CTAB, the surface state of QDs will remain unchanged and the absorption spectrum will never change. To this end, the absorption spectra of QDs with the different concentrations of CTAB and SDS were recorded, respectively (Figure 4). Although it was hard to recognize whether the shape of the absorption spectrum was changed, the spectrum of QDs with CTAB reflected a significant light scattering background (Figure 4A, red line). This indicated that a larger size of the aggregates occurred, which was consistent with the evidences and the hypothesis mentioned above. However, when QDs encountered SDS, the shape of absorption spectrum remained 17



unchanged, and the absorbance at the first excitonic absorption peak exhibited negligible change. These evidences could further confirm the electrostatic interactions between CTAB and QDs.

0.2 (A)

(B)

QDs QDs+CTAB

Abs.

0.2

Abs.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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QDs QDs+SDS

0.1

0.1

0.0

400

500 Wavelength (nm)

0.0

600

400

500 Wavelength (nm)

600

Figure 4. Absorption spectra of QDs in the absence and presence of CTAB (A) and SDS (B).

3.5 DLS and zeta potential As we presumed, CTAB would be attached onto the surface of QDs through electrostatic interaction according to the results revealed above. Given this, once QDs interacted with CTAB, the surface charge of QDs would be partly neutralized by CTAB. Therefore, the zeta potential moved toward positive from -32 mV to -5 mV (Figure 5A). Then, reduction of the surface charge would cause the decrease of electrostatic repulsion, which was responsible for the stability of QDs, and the aggregation of QDs was induced. This process was reflected as the increase of the hydrodynamic size from 12 nm to 120 nm (Figure 5C). This result was consistent with the large scattering background plus the absorptive component of QDs with

18


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CTAB (Figure 4A, red line). Besides, high concentrations of CTAB (> 80 µM) caused the data for zeta potential and hydrodynamic size of QDs to be fluctuating. No reliable data could be obtained and visible turbidity would be observed. In contrast, the zeta potential and hydrodynamic size of QDs were prone to keep unchanged in the presence of negative SDS (Figure 5B and D). It was reasonable because the negatively charged SDS and QDs would repel each other. So, QDs tended to be more stable with SDS. Based on the apparent increase of the hydrodynamic diameter of QDs, the aggregation was formed significantly, as confirmed by TEM (Figure S6). In the absence of CTAB, the QDs were well dispersed (Figure S6A). Upon the gradual titration of CTAB, aggregated QDs appeared. For instance, some reached 10-30 nm (Figure S6B, C and D), 50-90 nm (Figure S6E and F), and then 90-120 nm (Figure S6G and H). These data demonstrated that the addition of positive CTAB would certainly make QDs aggregate, and the aggregated QDs became larger with higher concentrations of CTAB. This result was not only consistent with the DLS data (Figure 5C), but also met the presumption of adsorption and aggregation.

19



0

0

(B)

-8

zeta potential (mV)

Zeta potential (mV)

(A)

-16 -24 -32 -40

0

20

40 60 [CTAB] (µM)

-8 -16 -24 -32 -40

80

0

20

40 60 [SDS] (µM)

25

50 [SDS] (µM)

80

24 150

(D)

(C)

D (nm)

16 D (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 36

100

8

50

0

0 0

20 40 [CTAB] (µM)

60

0

75

100

Figure 5. The change of zeta potential (A: CTAB, B: SDS) and hydrodynamic size (C: CTAB, D: SDS) of QDs in the presence of CTAB and SDS, respectively.

3.6 Fluorescence lifetime The fluorescence lifetimes of QDs in the absence and presence of CTAB were performed to explore whether CTAB was adsorbed onto the surface of QDs. Figure 6A exhibited the fluorescence decay curves of QDs before and after the addition of CTAB. The lifetimes (τ) were calculated by fitting the curves with multiexponential equations described as follows:

I(t)=A1 e-t⁄τ1 +A2 e-t⁄τ2 (7) τ=

A1 τ21 +A2 τ22 A1 τ1 +A2 τ2

(8)

where τ1, τ2 are time constants, A1, A2 are the amplitudes of the components, and τ is

20


Page 21 of 36

the average fluorescence decay time. Before the addition of CTAB, the fluorescence lifetime was calculated as 28.52 ns, with life component of 38.64 ns (0.62) and 12.29 ns (0.38). However, the lifetime was decreased to 4.62 ns after the addition of CTAB, with the component of 21.63 ns (0.07) and 3.34 ns (0.93). The reduced average lifetime and increased non-radiative lifetime proportion might be ascribed to the aggregation of QDs. In the aggregates, energy would be preferentially transferred from the smaller QDs of larger bandgap to the larger QDs of smaller bandgap, which possessed lower quantum yields from faster non-radiative decay rates than the ensemble average, resulting in a decrease in the ensemble averaged lifetime.42 This result was consistent with the speculation of adsorption and aggregation. However, the decay curves of QDs in the absence and presence of SDS were overlapped perfectly. The calculated average lifetime of QDs with SDS was slightly increased to 30.61 ns, with the component of 43.05 ns (0.54) and 15.93 ns (0.46), which might be an artifact of mathematical fitting.

1000

(A)

1000

QDs QDs+CTAB

600 400 200 0

(B)

QDs QDs+SDS

800 Counts

800 Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600 400 200

25

50

0

75 100 125 150 175 200 Time (ns)

25

50

75 100 125 150 175 200 Time (ns)

Figure 6. The fluorescence decay curves of QDs in the absence and presence of CTAB (A) and SDS (B). 21



Page 22 of 36

3.7 The mechanism of FRET between QDs and RhB induced by CTAB According to all the evidences observed above, the highly probable mechanism of FRET between QDs and RhB induced by CTAB was proposed (Scheme 1). The capping molecules of GSH made the surface of QDs negatively charged, which was proved by the zeta potential. There was no appropriate condition to render QDs and RhB closer enough, therefore no FRET occurred between this D-A pair due to the long distance. Nevertheless, the cationic surfactant, CTAB, possessed positive charge, which could interact with the negative charge of QDs. After the addition of CTAB, the headgroup of CTAB would be inclined to be adsorbed onto the surface of QDs owing to

the

electrostatic

attraction.

However,

this

process

would

reduce

the

electronegativity of QDs and result in the exposure of the hydrophobic tail of CTAB in the aqueous phase, thus the QDs lost colloidal stability. Therefore, aggregation took place. In addition, the exposed hydrophobic tails of CTAB in the aqueous phase tended to associate with each other so as to decrease the thermodynamic instability. Finally, the aggregated QDs was cross-linked by CTAB. The cross-linking process was similar with a previous study.49 On the other hand, the aggregation and cross-linking of QDs would increase the apparent size of QDs, which was proved by the DLS data and TEM images. Then, the RhB might be located inside the cross-linking area. Consequently, QDs and RhB were pulled in an appropriate distance, and energy could be transferred from QDs to RhB due to FRET.

22


Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Schematic illustration of CTAB-induced FRET between GSH-CdTe QDs and RhB.

3.8 FRET between QDs and RhB induced by CTAB After the interpretation of why FRET between QDs and RhB could be induced by CTAB, basic parameters about FRET process were calculated by Förster theory, which was explained by the following equations.50-51 ∞

J=

0 FD (λ)ϵλ4 d λ ∞

0 FD(λ) d λ

9000ln(10) κ2 ϕD R0 = J 128π5 NAV n4

E =1-

NR0 6 = F0 NR0 6 +r6

(9)

(10)

(11)

Here, J is the spectra overlap of the emission spectrum of donor (QDs) and absorption spectrum of acceptor (RhB), FD is the fluorescence spectrum of QDs in the absence of RhB, ϵ refers to the extinction coefficient of acceptor, which is viable at different wavelength, and λ represents wavelength. Besides, κ2 is the spatial orientation factor and is regularly reckoned as 2/3, ΦD is the quantum yield of donor (QDs), n is the refraction coefficient, NAV refers to Avogadro's number, E is the energy transfer efficiency, R0 is the critical distance between QDs and RhB at which distance the

23



FRET efficiency is 50%, N represents the average number of acceptors interacting with one donor, and r is the distance between QDs and RhB. The spectra overlap is shown as Figure 1D. According to the results, the spectra overlap J of this D-A pair was calculated as 4.63×10-13 L.mol-1.cm3, and the critical distance was 5.2 nm, which was in the normal range based on Förster theory. According to the proposed mechanism, there were multiple donors and acceptors inside these aggregates. Therefore, equation (11) was used as a crude approximation. This equation was commonly used for the systems where one donor was surrounded by multiple acceptors at equal distance. The stoichiometric ratio between RhB and QDs in this work was 0.5 - 4.5, based on which the distance between the D-A pair was calculated to be 5.4 - 5.6 nm. Finally, the thermodynamics in the process was studied by fluorescence titration at different temperatures. With the addition of RhB, the fluorescence of QDs was quenched, while the fluorescence of RhB was enhanced gradually (Figure 7A). Since the fluorescence of QDs would be quenched by RhB, Stern-Volmer equation could be used to study the process of FRET. The equation describes the quenching of donor’s emission is a function of varying concentration of acceptor, as shown by equation (12). F0 =1+KSV Q F

(12)

Where F0 and F are the fluorescence intensities of QDs in the absence and presence of RhB, respectively, and KSV represents the Stern-Volmer quenching constant. The KSV could be obtained at different temperatures. The results showed that the quenching 24


Page 24 of 36

Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


constant was reduced with the increasing temperature (Table 2). This was consistent with the regular pattern of static quenching. Therefore, the thermodynamics could be further studied by the modified Stern-Volmer equation. The modified Stern-Volmer equation is usually applied to calculate the binding constant, as described by equation (13). F0 1 1 1 = + (13) ∆F fa Ka [Q] fa Herein, F0 is the fluorescence intensity of QDs without RhB, ∆F is the change of fluorescence intensity with the addition of RhB, fa represents the fraction of accessible fluorescence, [Q] refers to the concentration of RhB, and Ka is the binding constant. Results showed that the binding constants were around 1 × 106 L.mol-1 at 298 ~ 310 K (Figure 7C, Table 2). And the related thermodynamic parameters were calculated through equation (5) and (6) (Figure 7D). The results showed a negative ∆H and a positive ∆S, indicating that the association between the QDs and RhB was exothermic, and the entropy of the system would increase during this process (Table 2). The positive ∆S might result from the disorder of the cross-linking formed by CTAB, as well as the exclusion of water molecules due to the interactions with RhB. The negative ∆G demonstrated the interactions between RhB and QDs induced by CTAB was a spontaneous process from a physicochemical view. Isothermal titration calorimetry (ITC) was also employed to confirm the thermodynamic mechanism (Figure S7). ITC analysis showed that ∆H and ∆S were -4.501 kJ.mol-1 and 80.73 J.K-1.mol-1, respectively. These data could well support that the association process

25



was driven by both enthalpy and entropy.

600

4

(A)

(B)

RhB 0 µM

400

3

300

298 K 302 K 306 K 310 K

F0/F

FL Intensity (a.u.)

500

RhB 3 µM

200

2

100

1 0.0

0 500

5

550 600 Wavelength (nm)

14.08

(C)

4

14.00

298 K 302 K 306 K 310 K

3 2 1

0

1

2 3 1/[Q] (µM)

0.5

1.0 1.5 [Q] (µM)

2.0

2.5

(D) ln(Ka)=7.25+2032.87/T R2=0.9938

ln(Ka)

F0/(F0-F)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 36

13.92 13.84

4

3.24

3.28 3.32 1000/T (K-1)

3.36

Figure 7. (A) The fluorescence spectrum of QDs with the addition of RhB. (B) Stern-Volmer plots of QDs at different temperatures. (C) Modified Stern-Volmer plots at four different temperatures. (D) Van’t Hoff plot of the titration process.

26


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Table 2. Binding constants and thermodynamic parameters of FRET T

KSV

Ka R

6

∆H

2

–1

R 6

∆S

∆G

2

-1

(kJ.mol-1) (J.mol-1.K-1) (kJ.mol-1)

(K)

(10 L.mol )

(10 L.mol )

298

0.95±0.02

0.9988

1.29±0.01

0.9977

-34.84±0.76

302

0.91±0.04

0.9958

1.20±0.01

0.9963

-35.11±0.76 -16.90±0.76 60.27±2.52

306

0.90±0.04

0.9951

1.09±0.01

0.9971

-35.35±0.76

310

0.88±0.04

0.9950

0.99±0.01

0.9962

-35.59±0.76

4 Conclusions In this work, a model of FRET was established and studied by spectroscopic and other methods. It turned out that no energy was transferred from the synthesized GSH-CdTe QDs to RhB if the system was only constituted by these two substances. However, FRET could occur when CTAB was added. The mechanism could be explained as follows. CTAB was adsorbed onto the surface of QDs due to electrostatic interaction, and then induced the aggregation and cross-linking among QDs. RhB might be located inside the aggregates. Therefore, QDs and RhB was pulled in an appropriate distance and FRET could occur significantly. Thermodynamic results showed that the association between QDs and RhB as mediated by CTAB was exothermic and the entropy of the system increased. The ITC analysis also confirmed that this process was driven by both enthalpy and entropy. The critical distance of this energy transfer model was 5.2 nm. Technically, lots of surfactants could be adsorbed 27



onto the surface of QDs because of electrostatic interactions and other forces. Therefore, this model can also be applied in the future construction of the FRET systems with long distances between the D-A pairs. For better applications in bioanalysis, those not only can induce FRET process, but also sustain the superior dispersion instead of aggregation, will be developed in the future.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Integrated emission of different concentrations of RhB (Figure S1); Emission of RhB with different concentrations of CTAB (Figure S2); Emission of QDs with different concentrations of DTAB (Figure S3); Determination of CMC of CTAB and SDS in the presence of QDs (Figure S4); Emission of QDs with different concentrations of CTAB and SDS (Figure S5); TEM morphologies of QDs with different concentrations of CTAB (Figure S6); and ITC analysis (Figure S7).

Acknowledgments The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (21773178, 21573168, 21473125). The authors sincerely thank Dr. Cai-Fen Xia at School of Chemistry and Materials Science, Hubei Engineering University for her help in testing fluorescence lifetime.

28


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TOC Graphic

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FÃ¶rster Resonance Energy Transfer from Quantum Dots to Rhodamine

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