Wavelength Dependence of Fluorescence Quenching of CdTe

Article pubs.acs.org/JPCC

Wavelength Dependence of Fluorescence Quenching of CdTe Quantum Dots by Gold Nanoclusters Huiping Wang,† Chengzhi Zheng,† Tianjiao Dong, Kanglei Liu, Heyou Han, and Jiangong Liang* College of Science, State Key Laboratory of Agricultural Microbiology, Institute of Chemical Biology, Huazhong Agricultural University, Wuhan 430070, PR China S Supporting Information *

ABSTRACT: Metal nanoclusters (NCs) are well-known for their distinct molecule-like luminescent behaviors. Currently, some research has been conducted concerning their quenching properties for various dyes, but little is known about the interaction between metal NCs and other fluorescent materials such as quantum dots (QDs). In this paper, we report efficient quenching of fluorescence emission of mercaptoacetic acid (TGA)-coated CdTe QDs having identical protective layers but differing core diameters (1.04, 1.61, and 2.11 nm) by the bovine serum albumin (BSA)-protected Au25 NCs (0.8 nm metal core diameter) that have negligible plasmon bands in PBS buffer solution at pH 7.4. With UV−vis absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy, we found that fluorescence emission of all QDs decreased significantly upon addition of Au NCs, in combination with no decrease in average fluorescence lifetime, which was attributed to static quenching of QDs by Au NCs. Interestingly, the 515 nm emitting QDs are at least 1 order of magnitude more efficiently quenched than the other two QDs in spite of the similar degree of spectral overlap of the emission spectrum with the excitation spectrum of Au NCs. This study not only has brought to light the quenching properties of metal NCs for QDs but also provided fundamental guidelines and new opportunities for further investigations into the interaction between metal NCs and other materials.

■

INTRODUCTION Over the past several decades, the study on the interactions between fluorophores and nanomaterials has aroused intensive interest among researchers.1−6 It is observed that fluorophores can exhibit significant changes in electronic and optical properties in close proximity to nanomaterials, which can be categorized into fluorescence (FL) quenching and emission enhancement.7−10 Distance-dependent effects were often observed in the FL quenching or emission enhancement of fluorophores affected by nanomaterials, such as nanoparticles, nanowires, and nanodisks, and the majority of attempts to date have been focused on explaining and understanding the behaviors.11−14 Metal nanoclusters (NCs) (such as Au and Ag) possess fascinating and superior properties due to their magic dimensions between metal atoms and nanoparticles.15,16 Sizedependent FL could often be observed when the sizes of metal clusters are smaller than 2 nm.17,18 Furthermore, quantum confinement effects result in discrete and size-tunable electronic transitions, as a single electron is excited between two molecule-like orbitals of NCs.19,20 Specifically, the spectral positions of these transitions are different from that of the plasmon band. With the advances in the successful synthesis of ultrasmall metal NCs and their application in biological sensing and imaging,21−23 the metal NC-fluorophore interaction can also be exploited for the development of new sensor applications. © 2013 American Chemical Society

Recently it is observed that metal NCs can also serve as efficient quenchers like those plasmonic nanoparticles, suggesting their great potential and promising future in biophysical measurements.24 The information available on the interaction between metal NCs and fluorophore is rather limited, and to the best of our knowledge, only a few recent studies have been published on explaining the interactions between Au NCs and nearby dye fluorophores. For example, Lu et al.25 reported Alexafluor 514 is minimally quenched by monolayer-protected Au NCs to which it is covalently bound. The small amount of quenching that was observed was ascribed to the combination of two effects: static quenching of some fraction of the Au NC-bound fluorophores and self-absorption. Using a collisional assay, Peteanu et al.24 measured the wavelength dependence of the quenching efficiency of Au25(SG)18 where SG is glutathione. These clusters efficiently quenched the emission of dyes in the wavelength range 500− 600 nm but not that of dyes with emission spectra which overlap the strongest absorption transitions of the clusters. The mechanism of this process is still under investigation. Liu et al.26 even investigated energy transfer between conjugatedoligomer-substituted polyhedral oligomeric silsesquioxane and BSA-Au NCs and then utilized the Förster resonance energy Received: September 4, 2012 Revised: January 22, 2013 Published: January 25, 2013 3011

dx.doi.org/10.1021/jp308782n | J. Phys. Chem. C 2013, 117, 3011−3018

The Journal of Physical Chemistry C

Article

N2 atmosphere for at least 20 min and then 30 μL of TGA was added. A white colloid was produced immediately in the solution, but this did not influence further synthesis. A 1.0 M NaOH solution was used to adjust the pH to 11.0. After that, 0.0132 g of Na2TeO3 and 0.0150 g of NaBH4 were transferred to the 3-neck round-bottom flask successively. Finally, different sized QDs were obtained under vigorous stirring at N2 atmosphere for a different period of time. After cooling to room temperature, the resulting products were precipitated by alcohol, and superfluous TGA, Na2TeO3, and Cd2+ were removed with centrifugation at 8000 rpm for 10 min. The resultant precipitate was redispersed in PBS buffer (0.01 M, pH = 7.4) and then stored in the dark at 4 °C for further use. Synthesis of BSA Protected Au NCs. The BSA coated Au NCs were synthesized in one pot as previously described by Ying et al. in 2009.33 All glassware used in the experiment was cleaned in a bath of freshly prepared 3:1 HCl/HNO3 and rinsed thoroughly in water prior to use. Typically, 5.00 mL of aqueous HAuCl4 solution (10 mM, 37 °C) was added to the BSA solution (5.00 mL, 50 mg/mL, 37 °C) under vigorous stirring. A volume of 0.50 mL of 1.0 M NaOH solution was introduced 2 min later, and the mixture was incubated at 37 °C for 24 h and the color of the solution changed from light yellow to deep brown. The as-synthesized Au NCs were dialyzed in membrane tubing with a molecular weight cutoff (MWCO) of 8−14 kDa (Biosharp.com) against 1.0 L of ultrapure water under continuous stirring at room temperature. After 24 h and three changes of water (at 8 h intervals) to remove unreacted HAuCl4 or NaOH, the tubing contents (Au NCs) were collected and then stored under 4 °C for further use. The concentration of the BSA in Au NCs was used to confirm the concentration of Au NCs in the following experiments. The concentration of Au NCs was obtained according to the method reported by ref 34. Spectroscopy Measurements. The QD stock solution was prepared at concentrations of 8.70 × 10−5, 2.40 × 10−5, and 6.00 × 10−6 M in PBS buffer solution (0.01 M, pH = 7.4), respectively. Typically, as concerned with each QD, varied volumes of Au NCs were added to a fixed volume of QD solution with a defined concentration. The PBS buffer solution was then added to make a fixed total volume for all mixtures. The same process is applied to steady-state and time-resolved spectroscopic measurements for the QDs and Au NCs mixture. The emission spectra were recorded in the wavelength range of 400−700 nm upon excitation at 380 nm. Isothermal Titration Calorimetry (ITC). A volume of 700 μL of QDs515 (2.67 × 10−6 M) was filled into a 1 mL stainless steel titration ampule, and the system equilibrated for 2 h. Then 250 μL of 3.23 × 10−4 M of Au NCs solution was filled into a Hamilton syringe. After that, 200 μL of Au NCs solution was titrated into the ampule for 20 injections after a heat flow with high stability. Background heats were obtained by titrating the BSA-Au NCs solution into PBS (pH 7.4). ΔH = Q/n was used to calculate the enthalpy change (ΔH) for the interactions between QDs and BSA-Au NCs, where Q (6.02 × 10−4 J) means the net heat and n (6.46 × 10−8 mol) stands for the mole number of Au NCs. The free energy change (ΔG) and the entropy change (ΔS) were calculated by the following equations: ΔG = ΔH − TΔS = −RT lnK, where R is the universal gas constant, T is the absolute temperature, and K is analogous to the Stern−Volmer quenching constant (Ksv). All experiments were conducted in the presence of 0.01 M PBS at pH 7.4 and 25 °C.

transfer (FRET) pair for mercury-ion sensing both in solution and in the cell. Thus, the mechanism of quenching for Au NCs is different from one another. In other words, the quenching properties of Au NCs are not completely revealed, while these are suggestive for further research. The study to date has mainly been focused on the interaction between metal NCs and dyes, but little research related to metal NCs and other materials, such as quantum dots (QDs), has been reported. As fluorescent probes, QDs have several advantages over conventional organic dyes: narrow, symmetrical, and tunable emission spectra according to their size and material composition, broad absorption spectra, and excellent photostability, which could make QDs a substitute for organic dyes in photophysical processes. So far, a few reports have been published on the interaction between QDs and metal nanoparticles and argued that in QDs-Au nanoparticle systems the quenching efficiency depends on several factors such as the particle size, shape, and separation distance between the donor and the gold quencher.27−29 Recently, a time-dependent density functional theory (TDDFT) study put foreword by Muñoz-Losa and co-workers30 suggested increasing the overlap of the donor dye emission with the specific absorption transitions of Au25 NCs would result in enhanced quenching. However, in the QDs-Au NCs system, the effect of emission wavelength of QDs on the quenching efficiency still remains unknown and exploring this effect experimentally is very essential for our understanding of the quenching properties of Au NCs for QDs. This is the focus of the current study. In this article, we describe the FL quenching phenomenon in the QDs-Au NCs system and report the information about degree of FL quenching as well as the influence of the Au NCs on the excited-state lifetime of three different sized QDs. The dependence of the quenching efficiency on the emission wavelength of QDs is also investigated.

■

EXPERIMENTAL SECTION Apparatus. The UV−vis absorption spectra were obtained in the range of 190−900 nm, with a 1.0 cm × 1.0 cm quartz cuvette on a Thermo Nicolet Corp. model evolution 300 UV− vis spectrometer (America). The FL intensity spectra were recorded by a Shimadzu RF-5301PC Spectrofluorometer (Japan) equipped with a 20 kW xenon discharge lamp as a light source. FL lifetime measurements of CdTe QDs with and without Au NCs were performed via a FLS920-st Steady State and TCSPC Fluorescence Lifetime Spectro-Fluorimeter from Edinburgh Instruments Ltd. An isothermal microcalorimeter TAM III (Thermometric AB, Sweden) was used to detect the enthalpy changes of the interaction between BSA-Au NCs and QDs, and a titration mode was taken in the microreaction system. Reagents. Bovine serum albumin (BSA, Biosharp.com), HAuCl4·4H2O, NaOH, CdCl2·2.5H2O, NaBH4, Na2TeO3, mercaptoacetic acid (TGA), and sodium citrate were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All reagents were of analytical reagent grade and used as received. Ultrapure water (18.2 MΩ) was used in all experiments. Synthesis of TGA Protected CdTe QDs. Thiol-capped CdTe QDs were synthesized via the hydrothermal route with some modifications.31,32 In a typical synthesis, 0.0685 g of CdCl2·2.5H2O was dissolved in 100 mL of ultrapure water in a 250 mL 3-neck round-bottom flask under vigorous stirring at 3012



■

Article

RESULTS AND DISCUSSION Au NCs with 25 atoms were synthesized according to the method developed by Ying et al.,33 possessing a common magic cluster size. The Au25 NCs are monodisperse and have an average diameter of ∼1 nm. Meanwhile, the NCs have been fully characterized by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and matrix-assisted laser desorption/ionization timeof-flight mass spectrometry (MALDI-TOF MS).33 As shown in Figure 1, a FL emission peak around 610 nm was observed

and 49.8%, respectively, estimated by Rhodamine 6G in ethanol as a reference sample (QY = 95%) as described in the literature.38 The identical surface characteristics and synthetic technique of the three CdTe QDs ensure the focus stays specifically on the dependence of the quenching properties on the size of the QDs. The three QDs were chosen to span the wavelength range where the excitation spectrum of BSA-Au NCs lies. Each of the three FL emission bands of the QDs was found to have a similar degree of spectral overlap with the excitation spectrum of Au NCs. Changes of the FL intensity of QDs with increasing the concentration of Au NCs in PBS buffer solution are shown in Figures 3 and 4. It can be seen that the strong influence of Au NCs on CdTe QDs occurred when they were mixed together. Initially, an increase in FL intensity of the three QDs was detected with addition of Au NCs, which can be attributed to the presence of surface protein BSA for stabilizing Au NCs. It has been demonstrated that BSA molecules can interact with TGA coated CdTe QDs, resulting in less aggregation, higher FL quantum yield, and enhanced stability, which are achieved by forming a new layer of quantum dot coating and passivating the defects on QDs surface.39 This effect can be explained by the presence of 35 thiol groups from the 35 cysteine residues in a BSA monomer attached to QDs by coordinating bonds. However, a further increase of the Au NCs concentration in the QDs solution instead induced a slight decrease in FL intensity. Finally, a gradual decrease in FL intensity of CdTe QDs was observed when increasing the concentration of Au NCs in the mixture. This is the tendency of fluorescent behaviors of the three sized QDs with addition of Au NCs. In particular, QDs515 were found to be much more efficiently quenched by Au NCs because a lower concentration of Au NCs can quench QDs515 and even completely quench the FL intensity. Furthermore, the gradual shift to shorter wavelengths of the maximum emission peaks of QDs with addition of Au NCs was also observed. In fact, there was also a blue shift of the FL emission peaks of QDs in the presence of only BSA (Figure S1 in the Supporting Information). Therefore, the blue shift was ascribed to the presence of BSA. The blue shift of the emission peaks of QDs indicated a reduction in the polarity of the local environment of CdTe QDs after interacting with BSA.40 Similar phenomena and explanations have been reported.41,42 On the basis of the ITC results and precise calculation, ΔH = −9.32 kJ mol−1, ΔS = 91.54 J mol−1 K−1, ΔG = −36.60 kJ mol−1. According to the paper,43 the electrostatic interactions often occur with positive ΔS and negative ΔH. Therefore, the electrostatic interaction is primarily responsible for the interaction of CdTe QDs with BSA-Au NCs to form a ground-state complex. Meanwhile, the electrostatic interaction has also been reported for playing a central role in the binding reaction between BSA and QDs.44−46 BSA consists of three homologous domains with each carrying different net charges at physiological pH: −11 (domain I), −7 (domain II), and +1 (domain III).47 QDs modified by TGA could bind to the surface of domain III.48 The magnitude of the FL quenching of each QDs as a function of the concentration of Au NCs was calculated using a Stern−Volmer (SV) plot. The ratio of the FL intensity without (F0) and with (F) the quencher is related to the concentration of the quencher ([Q]) by a coefficient KSV.

Figure 1. UV−vis absorption spectrum, excitation spectrum, and fluorescence emission spectrum (λex = 500 nm) of the as-prepared Au NCs.

upon excitation at 500 nm due to intraband transitions of free electrons of Au NCs.15 The optical excitation spectrum of BSAAu25 NCs exhibits a band centered at 504 nm, indicating discrete excitation−emission bands related to the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) electronic transitions.35,36 Figure 2 shows the spectral overlaps of the emission spectra of the three different sized CdTe QDs with the excitation

Figure 2. Extent of spectral overlaps of the excitation spectrum of BSA-Au NCs and the normalized emission spectra of the three sized QDs (λex = 380 nm).

spectrum of Au NCs. The overlap ratios between the three sized QDs emission spectra and the excitation spectrum of Au25 NCs are 83.49%, 83.36%, and 79.42%, respectively. The three TGA-capped QDs gradually increase in size and show FL emission peaks at 497, 515, and 527 nm (namely, QDs497, QDs515, and QDs527), respectively, with the absorption bands at 465, 478, and 492 nm. The three QDs synthesized via the hydrothermal method are identical in optical properties but different in size (1.04, 1.61, and 2.11 nm,37 respectively). Furthermore, they have similar quantum yield: 56.7%, 57.1%, 3013



Article

Figure 3. Fluorescence emission spectra of QDs497 (a), QDs515 (b), and QDs527 (c) (λex = 380 nm) in PBS showing fluorescence intensity as a function of the concentration of Au NCs: (1) 1.08 × 10−6 M, (2) 2.15 × 10−6 M, (3) 3.23 × 10−6 M, (4) 4.31 × 10−6 M, (5) 5.38 × 10−6 M, (6) 6.46 × 10−6 M, (7) 0, (8) 7.54 × 10−6 M, (9) 8.61 × 10−6 M, (10) 9.69 × 10−6 M, (11) 1.08 × 10−5 M in part a, (1) 3.59 × 10−7 M, (2) 0, (3) 7.18 × 10−7 M, (4) 1.08 × 10−6 M, (5) 1.44 × 10−6 M, (6) 1.79 × 10−6 M, (7) 2.15 × 10−6 M, (8) 2.51 × 10−6 M, (9) 2.87 × 10−6 M, (10) 3.23 × 10−6 M, (11) 3.59 × 10−6 M in part b, and (1) 1.08 × 10−6 M, (2) 2.15 × 10−6 M, (3) 3.23 × 10−6 M, (4) 4.31 × 10−6 M, (5) 0, (6) 5.38 × 10−6 M, (7) 8.61 × 10−6 M, (8) 7.54 × 10−6 M, (9) 6.46 × 10−6 M, (10) 1.08 × 10−5 M, (11) 9.69 × 10−6 M in part c. The concentrations of QDs are 2.32 × 10−6 M, 4.80 × 10−7 M, and 4.00 × 10−8 M, respectively.

Figure 5. Stern−Volmer plot showing dependence of fluorescence quenching on the concentration of Au NCs.

Figure 4. Plot of the normalized FL intensity versus the concentration of Au NCs.

F0 = 1 + KSV[Q] F

Table 1. Stern−Volmer Equation for the Interaction of the Three Sized CdTe QDs with Au NCs

(1)

Here, the intercept in this equation is actually not 1 due to the presence of BSA molecules on Au NCs, which causes a slight increase in the FL intensity of QDs initially, and therefore the equation needs some modifications. In so doing, the modified equation is shown as follows: F0 = a + KSV[Q] F

QDs

KSV (M−1)

QDs497 QDs515 QDs527

(5.1 ± 0.5) × 10 (2.6 ± 0.2) × 106 (6.3 ± 0.8) × 104 4

a

R2

0.64 ± 0.08 −0.26 ± 0.16 0.52 ± 0.04

0.99 0.96 0.97

QDs497/QDs515 mixture and the QDs515/QDs527 mixture with increasing the concentration of Au NCs (Figure S2 in the Supporting Information). As shown in Figure S2a in the Supporting Information, the maximum emission peaks of the QDs497/QDs515 mixture were initially shifted to shorter wavelengths quickly and the FL intensity of QDs decreased significantly upon addition of Au NCs. Further increase of Au NCs concentration in QDs solution instead induced a slight decrease in FL intensity and no blue shift, suggesting that QDs515 were quenched first, followed by QDs497. As noted, the significant blue shift (>10 nm) here was due to the decrease of the amount of QDs515 rather than the surface protein BSA. The quenching phenomena of both QDs were coincident with what has been shown in Figure 3. From Figure S2b in the Supporting Information, it can be seen that the maximum emission peaks of the QDs515/QDs527 mixture were initially shifted to longer wavelengths quickly and the FL intensity of QDs decreased significantly with an increase of the concentration of Au NCs, but then the FL intensity of QDs decreased slightly and no red shift occurred, indicating that QDs515 were quenched prior to

(2)

where “a” stands for a different constant from 1 and KSV, despite some modifications, still reflects the quenching efficiency under the same conditions. The modified Stern− Volmer plot (Figure 5) is linear and shows that the quenching efficiency varies with the emission wavelength of the three sized CdTe QDs. Furthermore, the quenching constants derived from the slope are presented in Table 1. A comparison of the measured KSV values shows that the quenching efficiency of QDs515 is 50-fold higher than that of the two other QDs, which is consistent with the FL quenching spectral results (Figure 3b). The striking dependence of the quenching efficiency on QDs emission wavelength for Au NCs is indeed evident, but in Figure 2, the emission band of the QDs with the maximum quenching efficiency shows a similar degree of spectral overlap with the excitation spectrum of Au NCs. To further demonstrate the wavelength dependence of the quenching phenomenon, experiments were designed to investigate the changes of FL emission spectra of the 3014



Article

Figure 6. Time-resolved fluorescence decay curves of QDs497 (a), QDs515 (b), and QDs527 (c) without and with Au NCs in PBS. The concentrations of QDs497, QDs515, and QDs527 are 6.09 × 10−5 M, 9.60 × 10−6 M, and 2.40 × 10−6 M, respectively. The concentration of Au NCs is 3.23× 10−5 M.

comparison with the free QDs samples prepared under the same solution conditions, the average FL lifetimes of all the three QDs exhibited a slight increase after addition of both the lower concentration (Figure S3 in the Supporting Information) and the higher concentration of BSA-Au NCs (Figure 6), which is probably due to the formation of effective surface passivation of BSA bound to the QDs.49 No decrease of QD lifetime upon addition of Au25 NCs indicates that the quenching of QDs by Au25 NCs belongs to static quenching, which is ascribed to the formation of a nonfluorescent complex between QDs and Au25 NCs.40 In some cases, static quenching can be recognized through subtle changes in absorption spectra caused by the formation of a quenched complex.40 The UV−vis absorption spectra of the QDs-Au NCs mixture are shown in Figure 7. The shoulder bands of the three QDs disappear gradually upon addition of Au NCs. Herein, the absorption spectra of the mixture do change in comparison with the mathematical superposition of absorption spectra of the free QDs and free Au NCs (Figure 8), which is assigned to the formation of a nonfluorescent complex between QDs and Au NCs, indicating the static quenching indeed occurred. In fact, Ganguly50 investigated the interactions of BSA-Au NPs and CdS QDs and found that FRET was primarily responsible for the quenching phenomenon. However, in the presence of only Au NPs, CdS QDs was statically quenched in nature. Herein, the different mechanisms from our experiment may result from the fundamentally different structural and optical properties between Au NCs and Au NPs. Despite the similar degree of spectral overlap with the excitation spectrum of Au NCs, QDs515 show the peak quenching efficiency by BSA-Au NCs. It can be concluded

QDs527. In conclusion, QDs515 were most efficiently quenched by BSA-Au NCs among the three sized QDs. To gain further insight into the FL quenching of the three sized QDs by BSA-Au NCs, lifetime measurements of QDs-Au NCs mixture and control samples were performed upon excitation at 380 nm. All data were well fitted by a doubleexponential decay as shown in Figure 6 and Figure S3 in the Supporting Information and are gathered in Table 2. In Table 2. Decay Parameters for CdTe QDs without and with Au NCs in PBS fitting parameters (two-component) sample 60.90 μM QDs497 60.90 μM QDs497/ 3.23 μM Au NCs 60.90 μM QDs497/ 32.30 μM Au NCs 9.60 μM QDs515 9.60 μM QDs515/ 3.23 μM Au NCs 9.60 μM QDs515/ 32.30 μM Au NCs 2.40 μM QDs527 2.40 μM QDs527/ 3.23 μM Au NCs 2.40 μM QDs527/ 32.30 μM Au NCs a

τ1 (ns)

α1a

τ2 (ns)

α2a

τ ̅ (ns)

χ2

5.42 5.46

0.2271 0.2151

27.68 28.00

0.7729 0.7849

26.47 26.86

1.041 1.110

5.51

0.1881

29.58

0.8119

28.58

1.084

4.12 4.04

0.3885 0.3240

16.54 17.69

0.6115 0.6760

14.84 16.34

1.216 1.098

3.40

0.3345

17.53

0.6655

16.27

1.099

6.50 6.75

0.1166 0.1164

29.86 30.57

0.8834 0.8836

29.21 29.90

1.054 0.995

6.64

0.0966

31.28

0.9034

30.73

1.067

αn (%) corresponds to the lifetime distribution for each component.

Figure 7. UV−vis spectra of QDs497 (a), QDs515 (b), and QDs527 (c) (λex = 380 nm) with the increasing concentration of Au NCs in PBS: (1) 0, (2) 1.08 × 10−6 M, (3) 2.15 × 10−6 M, (4) 3.23 × 10−6 M, (5) 4.31 × 10−6 M, (6) 5.38 × 10−6 M, (7) 6.46 × 10−6 M, (8) 7.54 × 10−6 M, (9) 8.61 × 10−6 M, (10) 9.69 × 10−6 M, (11) 1.08 × 10−5 M, respectively, in parts a and c and (1) 0, (2) 3.59 × 10−7 M, (3) 7.18 × 10−7 M, (4) 1.08 × 10−6 M, (5) 1.44 × 10−6 M, (6) 1.79 × 10−6 M, (7) 2.15 × 10−6 M, (8) 2.51 × 10−6 M, (9) 2.87 × 10−6 M, (10) 3.23 × 10−6 M, (11) 3.59 × 10−6 M in part b. The concentrations of QDs are 2.32 × 10−6 M, 4.80 × 10−7 M, and 4.00 × 10−8 M, respectively. 3015



■

Article

CONCLUSIONS The three TGA capped CdTe QDs with gradually increasing size can be effectively quenched by the BSA protected Au NCs, which is attributed to static quenching caused by the formation of a nonfluorescent complex between QDs and Au NCs. The quenching efficiency of both QDs497 and QDs527 are estimated to be at least 1 order of magnitude lower than that of QDs515 in an assay based on the concentration-dependent quenching of Au NCs. Interestingly, the maximum quenching efficiency occurs for QDs515 which have the similar degree of spectral overlap with the other two QDs, suggesting the wavelength dependence of the quenching efficiency of QDs by Au NCs. Although the quenching properties of Au NCs for QDs have been investigated here, further related work should be conducted to obtain more information.

Figure 8. The UV−vis absorption spectra of (a) QDs515, (b) Au NCs, (c) Au NCs and QDs515 mixture, and (d) the sum of Au NCs and QDs515. The concentration of Au NCs and QDs515 were 3.59 × 10−6 M and 4.80 × 10−7 M, respectively.

■

ASSOCIATED CONTENT

S Supporting Information *

that the magnitude of the FL quenching of each QDs is not related to the extent of the spectral overlap between the FL emission band of the QDs and the excitation spectrum of Au NCs, which is contrary to the previous report.30 Although few prior experimental studies are devoted to the understanding of the interactions between nonplasmonic metal NCs and nearby QDs, published reports concerning the quenching of dyes by metal NCs still provide clues. Muñoz-Losa and co-workers30 predicted that the FL of dyes having emission wavelengths coincident with specific electronic transitions of the metal NCs can be quenched by nonplasmonic small metal NCs such as Au25. Additionally, enhanced quenching will occur when the overlap of the donor dye emission with the specific absorption transitions of Au 25 NCs is increased. However, one experimental result25 confirmed that the presence of surface plasmons was not necessarily required in effective excitation energy transfer between fluorophores and Au NCs. Another study24 stated that no increase in the quenching efficiency was detected for dyes having emission wavelength coincident with any of the electronic transitions of Au NCs, and POPO-1 and TOTO-3 were poorly quenched by Au25 NCs despite the fact that both of them showed the strongest overlaps between the emission bands and the absorption transition in the absorption spectrum of the NCs, which were contrary to the prediction (ubi supra). Meanwhile, our experimental result demonstrated the quenching efficiency of QDs by Au NCs does not have direct relationship with the degree of spectral overlap between the emission band of QDs and the excitation spectrum of Au NCs. As described, QDs497 is poorly quenched by BSA-Au NCs in spite of the largest overlap of the emission band with the excitation spectrum of Au NCs, while QDs515 with a similar degree of spectral overlap were most efficiently quenched by BSA-Au NCs among the three sized QDs. This is the main point of our study. From the results of the ensemble spectroscopy experiments, it can be deduced that the FL quenching of QDs by Au NCs is a static process and a complex is formed between QDs and Au NCs via electrostatic interaction. However, it should be noted that there exists another interesting question unsolved: all the three sized QDs can be quenched by Au NCs with different quenching efficiencies, but QDs515 were far more efficiently quenched than the other two QDs and their emission wavelengths are just nearby. The reason resulting in the striking behavior is still unknown, and therefore further investigations are required to clarify the photophysical process.

Fluorescence emission spectra of three sized QDs with and without BSA, experiments using the QDs497/QDs515 mixture and the QDs515/QDs527 mixture to demonstrate the wavelength dependence of the quenching phenomenon, and the FL lifetime spectra of three sized QDs with and without Au NCs. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: +86-27-87282133. Phone: +86-27-87283712. E-mail: [email protected]. Author Contributions †

Equal contribution by the first two authors.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the support for this research by the National Nature Science Foundation of China (Grant 20905028), the Fundamental Research Funds for the Central Universities (Grant 2011PY009), and Huazhong Agricultural University Scientific & Technological Self-Innovation Foundation (Grant 2010SC05). The English teacher, Hanchang Zhu, is also acknowledged for the revision of the manuscript.

■

ABBREVIATIONS QDs, quantum dots; metal NCs, metal nanoclusters; FL, fluorescence; TGA, mercaptoacetic acid; BSA, bovine serum albumin; TDDFT, time-dependent density functional theory; HOMO−LUMO, highest occupied molecular orbital−lowest unoccupied molecular orbital

■

REFERENCES

(1) Thomas, K. G.; Kamat, P. V. Chromophore-Functionalized Gold Nanoparticles. Acc. Chem. Res. 2003, 36, 888−898. (2) Kamat, P. V. Photophysical, Photochemical and Photocatalytic Aspects of Metal Nanoparticles. J. Phys. Chem. B 2002, 106, 7729− 7744. (3) Mokashi, V. V.; Gore, A. H.; Sudarsan, V.; Rath, M. C.; Han, S. H.; Patil, S. R.; Kolekar, G. B. Evaluation of Interparticle Interaction between Colloidal Ag Nanoparticles Coated with Trisodium Citrate

3016



Article

and Safranine by Using FRET: Spectroscopic and Mechanistic Approach. J. Photochem. Photobiol. B 2012, 113, 63−69. (4) Guzatov, D. V.; Vaschenko, S. V.; Stankevich, V. V.; Lunevich, A. Y.; Glukhov, Y. F.; Gaponenko, S. V. Plasmonic Enhancement of Molecular Fluorescence near Silver Nanoparticles: Theory, Modeling, and Experiment. J. Phys. Chem. C 2012, 116, 10723−10733. (5) Sen, T.; Mandal, S.; Haldar, S.; Chattopadhyay, K.; Patra, A. Interaction of Gold Nanoparticle with Human Serum Albumin (HSA) Protein Using Surface Energy Transfer. J. Phys. Chem. C 2011, 115, 24037−24044. (6) Dulkeith, E.; Ringler, M.; Klar, T. A.; Feldmann, J.; Javier, A. M.; Parak, W. J. Gold Nanoparticles Quench Fluorescence by Phase Induced Radiative Rate Suppression. Nano Lett. 2005, 5, 585−589. (7) Yun, C. S.; Javier, A.; Jennings, T.; Fisher, M.; Hira, S.; Peterson, S.; Hopkins, B.; Reich, N. O.; Strouse, G. F. Nanometal Surface Energy Transfer in Optical Rulers, Breaking the FRET Barrier. J. Am. Chem. Soc. 2005, 127, 3115−3119. (8) Kasry, A.; Ardakani, A. A.; Tulevski, G. S.; Menges, B.; Copel, M.; Vyklicky, L. Highly Efficient Fluorescence Quenching with Graphene. J. Phys. Chem. C 2012, 116, 2858−2862. (9) Li, Y. Q.; Guan, L. Y.; Zhang, H. L.; Chen, J.; Lin, S.; Ma, Z. Y.; Zhao, Y. D. Distance-Dependent Metal-Enhanced Quantum Dots Fluorescence Analysis in Solution by Capillary Electrophoresis and Its Application to DNA Detection. Anal. Chem. 2011, 83, 4103−4109. (10) Bharill, S.; Chen, C. L.; Stevens, B.; Kaur, J.; Smilansky, Z.; Mandecki, W.; Gryczynski, I.; Gryczynski, Z.; Cooperman, B. S.; Goldman, Y. E. Enhancement of Single-Molecule Fluorescence Signals by Colloidal Silver Nanoparticles in Studies of Protein Translation. ACS Nano 2011, 5, 399−407. (11) Zhang, X.; Marocico, C. A.; Lunz, M.; Gerard, V. A.; Gun’ko, Y. K.; Lesnyak, V.; Gaponik, N.; Susha, A. S.; Rogach, A. L.; Bradley, A. L. Wavelength, Concentration, and Distance Dependence of Nonradiative Energy Transfer to A Plane of Gold Nanoparticles. ACS Nano 2012, 6, 9283−9290. (12) Acuna, G. P.; Bucher, M.; Stein, I. H.; Steinhauer, C.; Kuzyk, A.; Holzmeister, P.; Schreiber, R.; Moroz, A.; Stefani, F. D.; Liedl, T.; et al. Distance Dependence of Single-Fluorophore Quenching by Gold Nanoparticles Studied on DNA Origami. ACS Nano 2012, 6, 3189− 3195. (13) Zhu, J.; Li, J. J.; Zhao, J. W. Distance-Dependent Fluorescence Quenching Efficiency of Gold Nanodisk: Effect of Aspect RatioDependent Plasmonic Absorption. Plasmonics 2012, 7, 201−207. (14) Stoermer, R. L.; Keating, C. D. Distance-Dependent Emission from Dye-Labeled Oligonucleotides on Striped Au/Ag Nanowires: Effect of Secondary Structure and Hybridization Efficiency. J. Am. Chem. Soc. 2006, 128, 13243−13254. (15) Zheng, J.; Nicovich, P. R.; Dickson, R. M. Highly Fluorescent Noble-Metal Quantum Dots. Annu. Rev. Phys. Chem. 2007, 58, 409− 431. (16) Lu, Y. Z.; Chen, W. Sub-Nanometre Sized Metal Clusters: from Synthetic Challenges to the Unique Property Discoveries. Chem. Soc. Rev. 2012, 41, 3594−3623. (17) Peyser, L. A.; Vinson, A. E.; Bartko, A. P.; Dickson, R. M. Photoactivated Fluorescence from Individual Silver Nanoclusters. Science 2001, 291, 103−106. (18) Vosch, T.; Antoku, Y.; Hsiang, J. C.; Richards, C. I.; Gonzalez, J. I.; Dickson, R. M. Strongly Emissive Individual DNA-Encapsulated Ag Nanoclusters as Single-Molecule Fluorophores. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 12616−12621. (19) Zhu, M. Z.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. C. Correlating the Crystal Structure of a Thiol-Protected Au25 Cluster and Optical Properties. J. Am. Chem. Soc. 2008, 130, 5883−5885. (20) Zhu, M. Z.; Eckenhoff, W. T.; Pintauer, T.; Jin, R. C. Conversion of Anionic [Au25 (SCH2CH2Ph)18]− Cluster to Charge Neutral Cluster via Air Oxidation. J. Phys. Chem. C 2008, 112, 14221− 14224. (21) Hu, L. Z.; Han, S.; Parveen, S.; Yuan, Y. L.; Zhang, L.; Xu, G. B. Highly Sensitive Fuorescent Detection of Trypsin Based on BSAStabilized Gold Nanoclusters. Biosens. Bioelectron. 2012, 32, 297−299.

(22) Jin, L. H.; Shang, L.; Guo, S. J.; Fang, Y. X.; Wen, D.; Wang, L.; Yin, J. Y.; Dong, S. J. Biomolecule-Stabilized Au Nanoclusters as a Fluorescence Probe for Sensitive Detection of Glucose. Biosens. Bioelectron. 2011, 26, 1965−1969. (23) Shang, L.; Azadfar, N.; Stockmar, F.; Send, W.; Trouillet, V.; Bruns, M.; Gerthsen, D.; Nienhaus, G. U. One-Pot Synthesis of NearInfrared Fluorescent Gold Clusters for Cellular Fluorescence Lifetime Imaging. Small 2011, 7, 2614−2620. (24) Chowdhury, S.; Wu, Z. K.; Jaquins-Gerstl, A.; Liu, S. P.; Dembska, A.; Armitage, B. A.; Jin, R. C.; Peteanu, L. A. Wavelength Dependence of the Fluorescence Quenching Efficiency of Nearby Dyes by Gold Nanoclusters and Nanoparticles: The Roles of Spectral Overlap and Particle Size. J. Phys. Chem. C 2011, 115, 20105−20112. (25) Lu, Y.; Dasog, M.; Leontowich, A. F. G.; Scott, R. W. J.; Paige, M. F. Fluorescently Labeled Gold Nanoparticles with Minimal Fluorescence Quenching. J. Phys. Chem. C 2010, 114, 17446−17454. (26) Pu, K. Y.; Luo, Z. T.; Li, K.; Xie, J. P.; Liu, B. Energy Transfer between Conjugated-Oligoelectrolyte-Substituted POSS and Gold Nanocluster for Multicolor Intracellular Detection of Mercury Ion. J. Phys. Chem. C 2011, 115, 13069−13075. (27) Ray, P. C.; Fortner, A.; Darbha, G. K. Gold Nanoparticle Based FRET Asssay for the Detection of DNA Cleavage. J. Phys. Chem. B 2006, 110, 20745−20748. (28) Gersten, J. I.; Nitzan, A. Photophysics and Photochemistry Near Surfaces and Small Particles. Surf. Sci. 1985, 158, 165−189. (29) Griffin, J.; Singh, A. K.; Senapati, D.; Rhodes, P.; Mitchell, K.; Robinson, B.; Yu, E.; Ray, P. C. Size- and Distance-Dependent Nanoparticle Surface-Energy Transfer (NSET) Method for Selective Sensing of Hepatitis C Virus RNA. Chem.Eur. J. 2009, 15, 342−351. (30) Muñ oz-Losa, A.; Vukovic, S.; Corni, S.; Mennucci, B. Nonplasmonic Metal Particles as Excitation Energy Transfer Acceptors: an Unexpected Efficiency Revealed by Quantum Mechanics. J. Phys. Chem. C 2009, 113, 16364−16370. (31) Guo, J.; Yang, W. L.; Wang, C. C. Systematic Study of the Photoluminescence Dependence of Thiol-Capped CdTe Nanocrystals on the Reaction Conditions. J. Phys. Chem. B 2005, 109, 17467− 17473. (32) Bao, H. F.; Wang, E. K.; Dong, S. J. One-Pot Synthesis of CdTe Nanocrystals and Shape Control of Luminescent CdTe-Cystine Nanocomposites. Small 2006, 2, 476−480. (33) Xie, J. P.; Zheng, Y. G.; Ying, J. Y. Protein-Directed Synthesis of Highly Fluorescent Gold Nanoclusters. J. Am. Chem. Soc. 2009, 131, 888−889. (34) Shang, L.; Wang, Y. Z.; Jiang, J. G.; Dong, S. J. pH-Dependent Protein Conformational Changes in Albumin:Gold Nanoparticle Bioconjugates: a Spectroscopic Study. Langmuir 2007, 23, 2714−2721. (35) Lin, C. A. J.; Yang, T. Y.; Lee, C. H.; Huang, S. H.; Sperling, R. A.; Zanella, M.; Li, J. K.; Shen, J. L.; Wang, H. H.; Yeh, H. I.; et al. Synthesis, Characterization, and Bioconjugation of Fluorescent Gold Nanoclusters toward Biological Labeling Applications. ACS Nano 2009, 3, 395−401. (36) Muhammed, M. A. H.; Ramesh, S.; Sinha, S. S.; Pal, S. K.; Pradeep, T. Two Distinct Fluorescent Quantum Clusters of Gold Starting from Metallic Nanoparticles by pH-Dependent Ligand Etching. Nano Res. 2008, 1, 333−340. (37) Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G. Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15, 2854−2860. (38) Eaton, D. F. Reference Materials for Fluorescence Measurement. Pure Appl. Chem. 1988, 60, 1107−1114. (39) Poderys, V.; Matulionyte, M.; Selskis, A.; Rotomskis, R. Interaction of Water-Soluble CdTe Quantum Dots with Bovine Serum Albumin. Nanoscale Res. Lett. 2011, 6, 9−14. (40) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006. (41) Shang, L.; Brandholt, S.; Stockmar, F.; Trouillet, V.; Bruns, M.; Nienhaus, G. U. Effect of Protein Adsorption on the Fluorescence of Ultrasmall Gold Nanoclusters. Small 2012, 8, 661−665. 3017



Article

(42) Lacerda, S. H. D. P.; Park, J. J.; Meuse, C.; Pristinski, D.; Becker, M. L.; Karim, A.; Douglas, J. F. Interaction of Gold Nanoparticles with Common Human Blood Proteins. ACS Nano 2010, 4, 365−379. (43) Ross, P. D.; Subramanian, S. Thermodynamics of Protein Association Reactions: Forces Contributing to Stability. Biochemistry 1981, 20, 3096−3102. (44) Fan, J. Z.; Zhou, J.; Sun, T. Q.; Lü, S. Y.; Tang, J. Y.; Lü, J. Q. Multi-spectroscopic Study on the Interaction between CdTe Quantum Dots and Bovine Serum Albumin. Chin. J. Chem. 2010, 28, 2353− 2358. (45) Shao, L. W.; Dong, C. Q.; Sang, F. M.; Qian, H. F; Ren, J. C. Studies on Interaction of CdTe Quantum Dots with Bovine Serum Albumin Using Fluorescence Correlation Spectroscopy. J. Fluoresc. 2009, 19, 151−157. (46) Wang, Q. S.; Zhang, X. L.; Zhou, X. L.; Fang, T. T.; Liu, P. F.; Liu, P.; Min, X. M.; Li, X. Interaction of Different Thiol-Capped CdTe Quantum Dots with Bovine Serum Albumin. J. Lumin. 2012, 132, 1695−1700. (47) Peters, T., Jr. All About Albumin: Biochemistry, Genetics and Medical Applications; Academic Press: San Diego, CA, 1996. (48) Shcharbin, D.; Pedziwiatr, E.; Chonco, L.; Bermejo-Martín, J. F.; Ortega, P.; de la Mata, F. J.; Eritja, M. R.; Gómez, R.; Klajnert, B.; Bryszewska, M.; et al. Analysis of Interaction between Dendriplexes and Bovine Serum Albumin. Biomacromolecules 2007, 8, 2059−2062. (49) Gong, H. M.; Zhou, Z. K.; Song, H.; Hao, Z. H.; Han, J. B.; Zhai, Y. Y.; Xiao, S.; Wang, Q. Q. The Influence of Surface Trapping and Dark States on the Fluorescence Emission Efficiency and Lifetime of CdSe and CdSe/ZnS Quantum Dots. J. Fluoresc. 2007, 17, 715− 720. (50) Mandal, G.; Bardhan, M.; Ganguly, T. Occurrence of Förster Resonance Energy Transfer between Quantum Dots and Gold Nanoparticles in the Presence of a Biomolecule. J. Phys. Chem. C 2011, 115, 20840−20848.

3018


Wavelength Dependence of Fluorescence Quenching of CdTe

Recommend Documents