Systematic Studies on Emission Quenching of Cadmium Telluride

To whom correspondence should be addressed. Phone/Fax: +81-6-6879-7372. E-mail: [email protected]., †. Osaka University. , ‡. Nagoya...
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J. Phys. Chem. C 2009, 113, 21621–21628

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Systematic Studies on Emission Quenching of Cadmium Telluride Nanoparticles Taro Uematsu,†,§ Takahiro Waki,†,§ Tsukasa Torimoto,‡,§ and Susumu Kuwabata*,†,§ Department of Applied Chemistry, Graduate School of Engineering, Osaka UniVersity, Suita, Osaka 565-0871, Japan, Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan, and Japan Science and Technology Agency, CREST, Kawaguchi, Saitama 332-0012, Japan ReceiVed: August 27, 2009; ReVised Manuscript ReceiVed: NoVember 6, 2009

Quenching of emissions from cadmium telluride (CdTe) nanoparticles was distinctly observed by the addition of various electrochemically active organic molecules. The quenching ability of organic molecules is greatly influenced not only by their valency and redox potential but also by a type of capping ligand and the particle size of the CdTe nanoparticles. These effects are systematically studied through the fluorescence intensity and its lifetime changes based on photoinduced electron transfer reactions. The results suggest the existence of two different quenching mechanisms: diffusion-mediated electron transfer and electrostatic adsorption of the quenchers, which are both dependent on the valency of the quenchers. Besides conventional reaction schemes such as Rehm-Weller-type photoinduced electron transfer, a new reaction scheme has been successfully introduced by considering multiple adsorption of organic quenchers on a semiconductor nanoparticle. In this scheme, the kinetics of the electron transfer reactions between nanoparticles and quenchers became observable by emission quenching experiments, and they have been studied on the basis of Marcus theory. Introduction Semiconductor nanoparticles (NPs) have several desirable properties, such as durability to photoirradiation, wide absorption, and intense photoluminescence (PL), providing various potential applications to optoelectronics,1-5 photovoltaics,6-10 and biochemistry.11-13 In particular, their PL properties have been investigated for their use as fluorescence reagents, which are suitable for biolabeling and bioimaging. Furthermore, there is a usage as sensing materials by using PL quenching caused by some chemicals;14-16 for example, attaching Au NPs to semiconductor NPs almost completely quenches the PL via energy transfer. This phenomenon can be used for biosensing when both semiconductor and Au NPs are appropriately modified.17,18 Another mechanism that governs PL quenching is the electron transfer from/to redox-active chemicals. The electron transfer between organic fluorophores and electron scavengers has been extensively studied for scientific interest and for their application to photovoltaics and artificial photosynthesis. Such electron transfer reactions can easily be detected by observing the decrease in the PL, allowing elucidation of donor-acceptor interactions and several parameters related to the electron transfer reactions between them.19-27 The same phenomena should occur for semiconductor NPs that emit intense PL. In fact, some research groups have already reported that the PL from NPs is significantly quenched by the addition of appropriate redox quenchers.28-33 However, since these studies have only focused on a single combination of NPs and quenchers or mainly on the interaction between NPs and quenchers, their electron transfer mechanisms were not fully elucidated. * To whom correspondence should be addressed. Phone/Fax: +81-66879-7372. E-mail: [email protected]. † Osaka University. ‡ Nagoya University. § CREST.

Over the past two decades, many studies have examined the electron transfer reactions between NPs and other materials.10,34-38 In these studies, transient absorption spectroscopy proved to be a powerful technique for kinetic study of the reactions.35 Recently, Kamat et al. succeeded in observing the variation in electron transfer kinetics between TiO2 and differently sized CdSe NPs.10 The pump-probe measurements detected bleach of absorbance of CdSe NPs chemically bound to the surfaces of TiO2 and revealed that the electron transfer rate was dependent on the band gap energy of the CdSe NPs. On the other hand, Gaal et al. conducted systematic studies on electron transfer reactions between SnO2 NPs and a series of Ru- and Os-pyridine derivative complexes, which were electrostatically adsorbed on the surfaces of the SnO2 NPs.35 The studies focusing on electron transfer from the conduction band of SnO2 NPs to the oxidized complexes clearly showed that the reaction was in the inverted region of the Marcus theory when the corresponding potential gaps were large enough. Our research group has investigated fluorescent CdS,39 CdSe,40 CdTe,41 and (AgIn)xZn2(1-x)S242 NPs for improving the quantum efficiency of their PL and precise tuning of the PL wavelengths. Elucidation of parameters causing PL quenching became an essential subject for improving the quantum efficiency. Thus, we conducted systematic studies on PL quenching of NPs. In this paper, we report on PL quenching of CdTe NPs having different surface-capping reagents and particle sizes by the addition of various electrochemically active species. Systematic analysis of the PL quenching provided evidence of various quenching mechanisms. In particular, in cases of quenchers that were strongly adsorbed on the surfaces of NPs, the electron transfer rate from the conduction band edge of the NPs to the redox potential of the quenchers became observable with changes in the PL intensity. Then, we observed a phenomenon considered to be the inverted region of the Marcus theory when the potential difference between the conduction

10.1021/jp908279k  2009 American Chemical Society Published on Web 12/07/2009

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band edge of the CdTe NPs and the redox potential of the quencher was larger than 2 V. Experimental Section Materials and Methods. Cadmium chloride, sodium borohydride, thioglycolic acid (TGA), and sodium hydroxide were purchased from Wako Pure Chemicals Industries and used without further purification. N,N′-Dimethylaminoethanethiol (DMA) was purchased from TCI. Tellurium powder (99.8%, 200 mesh) was purchased from Aldrich. Water used in this study was purified by Milli-Q gradient A10 (Millipore, 18.2 MΩcm). PL and UV-vis spectra were recorded using a fluorospectrometer (Hitachi, F-4500) and a photodiode array spectrometer (Shimazu, MultiSpec-1500). Time-resolved fluorescence spectra were measured on a nitrogen laser (GL-3300)-pumped dye laser system (Photon Technology International, GL-302) equipped with a four-channel digital delay pulse generator (Stanford Research System Inc., DG535) and a motor driver (Photon Technology International MD-5020). The excitation wavelength was 460 nm with the use of coumarin 460 as a dye. The quantum yield was determined by a diode-array spectrometer equipped with an integrating sphere (HAMAMTSU, MPA-12). Observation of NPs was performed by transmission electron microscopy (TEM) (Hitachi, H-7650), and the amount of CdTe molecules was estimated by wavelength-dispersive X-ray fluorescence spectroscopy (XRF) (Rigaku, ZSX100e/MPS). Synthesis of CdTe Nanoparticles. An aqueous solution of TGA-stabilized CdTe NPs was synthesized by the previously reported method with some modification. Briefly, tellurium powder was reacted with sodium borohydride in aqueous solution at 0 °C. After 8 h, a pale-red-colored NaHTe solution was obtained. In another flask, cadmium chloride and thioglycolic acid were dissolved in 125 mL of water, the pH of which was adjusted to 9.0 by the addition of 1 M NaOH. Then, the solution was deaerated and NaHTe was added with stirring to obtain the precursor of the NPs. The molecular ratio of Cd: TGA:Te was 1:1.8:0.25. It was refluxed for several hours until the desired particle size was obtained. The quality of NPs was checked by UV-vis and fluorescence spectroscopy. A solution of DMA-stabilized CdTe NPs was synthesized by using methods similar to those for TGA-stabilized NPs. NaHTe solution was added to deaerated aqueous solution containing cadmium dichloride and DMA, whose pH was adjusted to 6.0, in the molecular ratio of Cd:DMA:Te ) 1:2.4: 0.25. The solution was refluxed for several hours until the NPs exhibited the desired optical properties. Photoluminescence Quenching by Organic Molecules. PL quenching of the CdTe NPs was measured in quartz cells. Typically, as-prepared CdTe NPs colloidal solution was diluted 50 times with water. Then, the quencher was added to the solution. The temperature was maintained at 0, 20, 30, or 60 °C during fluorescence and UV-vis spectra measurements. Results and Discussion Photoluminescence Quenching. Figure 1a shows changes in the PL intensity of TGA-CdTe NPs brought about by the addition of 9-aminoacridine of different concentrations. The increase in 9-aminoacridine concentration reduces the PL intensity. Since there are no spectral overlap between the PL of NPs and the absorption of the quencher, the occurrence of energy transfer is unlikely. On the other hand, absorption spectra measurements revealed that the addition of 9-aminoacridine did not cause any change in the spectra of the NPs. No change in NP quality was also confirmed by another experiment; PL

Figure 1. Changes in intensity of PL from TGA-CdTe (a, c) and DMACdTe (b, d) caused by addition of different concentrations of 9-aminoacridine (a, b) or anthraquinone-1-sulfonate (c, d).

intensity of the CdTe NPs was recovered when 9-aminoacridine was removed from the solution by ultrafiltration. The PL quenching was also checked by fluorescence lifetime measurements (Figure 2). Because the laser light source had a fwhm of 3 ns, as shown by a broken line (c) in Figure 2, the early processes of the fluorescence decay were distorted. By

Emission Quenching of CdTe Nanoparticles

Figure 2. Fluorescence lifetime measurements of TGA-CdTe (Ex ) 460 nm, Em ) 600 nm) in the absence (a) and presence (b) of 0.3 µM 9-aminoacridine. (c) Profile of the light source.

subtracting this influence, the experimental curves were well fitted to a single-exponential equation. The resulting fluorescence lifetime was decreased from τ0 ) 15.47 ns to τ ) 10.75 ns by adding 0.3 µM 9-aminocridine. The τ0/τ ratio of 1.439 was essentially the same as the I0/I ratio of 1.429 obtained by steadystate measurements in the same condition, clearly indicating that the PL decrease caused by adding 9-aminoacridine occurs via electron transfer processes. Rajh et al. investigated the electronic structure of thiol-capped CdTe NPs by pulse radiolysis and time-resolved absorption spectroscopy, estimating the potentials of the conduction band edge to be from -2.0 to -2.4 V (vs NHE) according to the size, leaving the valence band edge at 0.0 V independent of the size. Since the reductive potential of 9-aminoacridine is -0.916 V, the potential correlation strongly supports occurrence of electron transfer from CdTe NPs to 9-aminoacridine. Effects of Electrical Charges of CdTe Nanoparticles and Redox Species. Figure 1b shows the PL intensity of DMAcapped CdTe NPs having positive charges due to the terminal amino groups in the absence and presence of 9-aminoacridine. In this case, addition of an excess amount of 9-aminoacridine (100 µM) did not cause a significant decrease in the PL intensity. However, when negatively charged anthraquinone-1-sulfonate with a redox potential of -0.216 V was dissolved in place of 9-aminoacridine, distinct PL quench was observed, as shown in Figure 1d. As expected from these results, combination of anthraquinone-1-sulfonate and TGA-CdTe NPs, both of which have negative charges, resulted in no PL change (see Figure 1c). Thus, electrostatic attraction and repulsion between the NPs and redox species steadily control the quenching behavior. Several types of redox species were found to work as quenchers for TGA-CdTe, giving linear relationships in the Stern-Volmer plots, i.e., plots of (I0/I) - 1 vs quencher concentration, where I0 and I represent PL intensities in the absence and presence of the quencher. Figure 3 shows the Stern-Volmer plots obtained for the combination of DMACdTe and sulfonate-substituted anthraquinones. The SternVolmer coefficient (KSV) is influenced by several characteristics of the redox species, but the results shown in Figure 3 demonstrate that the number of substituted sulfonate groups dominantly influences the KSV value. In the case of molecular fluorophores and quenchers, quenching behavior is mostly classified into collisional and static

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Figure 3. Stern-Volmer plots of emission quenching of DMA-CdTe by various quenchers: anthraquinone-1,8-disulfonate (aq18ds, KSV ) 3.18 × 106 M-1), anthraquinone-2,7-disulfonate (aq27ds, KSV ) 3.31 × 106 M-1), anthraquinone-2,6-disulfonate (aq26ds, KSV ) 3.23 × 106 M-1), anthraquinone-1,5-disulfonate (aq15ds, KSV ) 1.38 × 106 M-1), anthraquinone-1-sulfonate (aq1s, KSV ) 4.663 × 104 M-1), and anthraquinone-2-sulfonate (aq2s, KSV ) 3.21 × 104 M-1).

SCHEME 1: Mechanisms of Dynamic and Static Quenching for NPsa

a

NP and Q represent a nanoparticle and quencher, respectively.

quenching.43 The collisional quenching is caused by photoinduced electron transfer that occurs when fluorophores collide with quenchers. The static quenching is caused by the formation of charge-transfer complexes that do not emit PL. Although these two are different processes, both cases satisfy the Stern-Volmer relationship of

I0 ) 1 + KSV[Q] I

(1)

where [Q] is the concentration of the quencher dissolved in solution. In particular, for the former case, the emission intensity in the presence of a certain concentration of quencher dissolved in solution ([Q]free) can be described as

Io ) 1 + KSV[Q]free ) 1 + kqτ0[Q]free I

(2)

where τ0 is the fluorescence lifetime without the quencher and kq is the quenching rate constant. Scheme 1 shows the quenching mechanisms for semiconductor NPs, which we propose based on the results obtained by the present study. Although a complete explanation will be presented later, the dynamic quenching mechanism is the same as that for the cases of molecular fluorophores. Using the variables given in the scheme, the quenching rate constant of the dynamic quenching is described as

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kq )

ket1kd ket1 + k-d

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(3)

where ket1 is the electron transfer rate and kd and k-d are the diffusion rates. When ket1 . k-d, kq is determined by diffusion, giving an approximation of kq = kd. This situation is anticipated in our case because of the large potential gap between NPs and quenchers (∆E > 1 V). Therefore, the diffusion-determined quenching rate constant (kqdiff) can be given by21

kdiff q = kd )

4π(DNP + DQ)NA -ZNPZQr0 1000 1 - exp(ZNPZQr0 /R)

(4) where DNP and DQ are the diffusion constants of the NPs and quenchers, ZNP and ZQ are the electrical charges of the fluorophores and quenchers, NA and r0 represent Avogadro’s number and the Onsager radii, and R is the reaction distance where electron transfer can occur. According to the literature, kq was estimated by substituting typical values for the variables given in eq 4: DNP + DQ (2.0 × 10-5 cm2 s-1), R (1.0 × 10-7 cm), and an effective charge of a CdTe NP (ZNP ) -3.0).44 Thus, kqdiff ) 3.6 × 1010 and 6.4 × 1010 M-1 s-1 are estimated for monosulfonate quenchers (ZNP × ZQ ) -3.0) and disulfonate quenchers (ZNP × ZQ ) -6.0), respectively, indicating that kqdiff for disulfonate quenchers is about two times as large as that of monosulfonate quenchers. This difference is inconsistent with the differences in KSV ()kqτ0) values obtained from the plots shown in Figure 3. For instance, KSV for anthraquinone-2,7disulfonate (3.31 × 106 M-1) is 100 times as large as that for anthraquinone-2-sulfonate (3.21 × 104 M-1). This large discrepancy prompted us to consider another quenching mechanism. It seems appropriate to introduce the concept of static quenching to explain the discrepancy. However, quenchers cannot make a complex with a CdTe NP because thiol molecules that are densely bound to the particle surface must prevent direct bonding of quenchers to CdTe. Instead, it would be better to consider electrostatic adsorption of quenchers to thiol molecules on an NP surface. Here, we assume that the association of quencher molecules to a NP and their dissociation are in equilibrium. Then, the quenching mechanisms of NPs are classified into three types: diffusion-controlled quenching (Rehm-Weller type), no diffusion quenching where one or more quenchers are electrostatically adsorbed on NPs (Marcus type), and both. For quenching by free and adsorbed quencher molecules, a new quenching rate constant, kqad, must be introduced. Thus, the PL quenching ratio is given by

I0 [ ]free + kad [ ]ad ) 1 + τ0(kdiff q Q q Q ) I

(5)

where [Q] ad is the concentration of adsorbed quencher molecules. This equation means that PL quenching fits to the Stern-Volmer equation if [Q]free . [Q]ad or [Q]free, [Q]ad. Determination of the Quenching Mechanisms. In order to know the contribution of free and adsorbed quenchers, we consider Stern-Volmer plots for different NPs concentrations. The results obtained for quenchers of anthraquinone-1,5disulfonate (Figure 4a), methyl viologen (Figure 4b), and 9-aminoacridine (Figure 4c) are shown in Figure 4. The concentration of as-prepared NPs was estimated to be 6.79 µM from the amount of CdTe measured by XRF and the mean diameter of the NPs measured by TEM. The plots shown in Figure 4 were obtained using an NP suspension diluted by 1/25 (0.27 µM), 1/50 (0.14 µM), 1/100 (0.07 µM), and 1/150 (0.05 µM). In all cases, KSV was dependent on the concentration of

Figure 4. Stern-Volmer plots of DMA-CdTe NPs quenched by anthraquionone-1,5-disulfonate (a) and TGA-CdTe NPs quenched by methyl viologen (b) and 9-amino-acridine (c) with NP dilution ratios of I (1/25), II (1/50), III (1/100), and IV (1/150).

NPs. Particularly in the cases of methyl viologen and anthrqauinone-1,5-disulfonate, KSV values are roughly proportional to the inverse of the NP concentration (1/[NP]). Then, the (I0/I) - 1 values were plotted again as a function of [Q]/[NP], as shown in Figure 5. The plots obtained for methyl viologen and anthrqauinone-1,5-disulfonate showed proportional relationships independent of [NP], implying that the (I0/I) - 1 value is determined exclusively by [Q]/[NP]. However, the same plots for 9-aminoacridine showed different behaviors; plots for dilution ratios of 1/25 and 1/50 showed a linear relationship, whereas those obtained at dilution ratios less than 1/100 deviated from the linear relation. To comprehend the obtained results, adsorption of the quenchers on NPs was considered based on the Langmuir adsorption isotherm. However, since the initially added quencher concentration ([Q]0, see Figure 4) is of the same order as the NP concentration (for example, 0.14 µM when diluted by 1/50), the dissolved quencher’s concentration in the presence of NPs ([Q]free) must be lower than the initial value due to the adsorption

Emission Quenching of CdTe Nanoparticles

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[Q]0 θ ) Yθ + [NP] Kad(1 - θ)[NP]

Figure 5. Plots of I0/I - 1 versus molar ratio of quencher to nanoparticle for DMA-CdTe NPs quenched by anthraquinone-1,5disulfonate (a) and TGA-CdTe NPs quenched by methylviologen (b) and 9-aminoacridine (c). The NP dilution ratios were I (1/25), II (1/50), III (1/100), and IV (1/150).

(9)

where Kad () kad/kde) is the adsorption equilibrium constant. The finding that [Q]0/[NP] is independent of [NP] (Figure 5) for methyl viologen and anthrqauinone-1,5-disulfonate means that quenching was caused under the condition where the second term of the right side of eq 9 is close to null. In this case, the resulting approximate equation of [Q]0/[NP] = Yθ is obtained, indicating that almost all the quencher molecules are adsorbed on NPs. In addition, [Q]ad ∝ Yθ, and eq 5 supports the validity of (I0/I) - 1 ∝ Yθ. Accordingly, it can be concluded that quenching of NP fluorescence by methyl viologen and anthrqauinone-1,5-disulfonate is caused by adsorbed species, and almost all the quenchers are adsorbed on NPs, leaving extremely low concentrations of dissolved quenchers ([Q]free = 0). In the case of 9-aminoacridine, plots for lower [NP] deviated from the linear relation observed for higher [NP]. This deviation also seems to be explained by the modified Langmuir equation (eq 9). If Kad is not as large as the term of θ /(Kad(1 - θ)[NP]) becomes negligible, Yθ must be lower than [Q]0/[NP]. Their difference widens as θ increases or [NP] decreases according to eq 9. Meanwhile, the contribution of dissolved species to quenching is quite small because of reasons mentioned below. Consequently, the plots are lowered from the linear relation of (I0/I) - 1 ∝ [Q]0/[NP]. The neutral solution used for quenching experiments cannot fully protonate both of the two nitrogen atoms in 9-aminoacridine molecules. It is then plausible that the species having less valence possesses less adsorbability to NPs. In the cases of monovalent species, such as anthraquinone1-sulfonate, much higher [Q] was required for sufficient quenching than the divalent species, as shown in Figure 3. For instance, I/I0 - 1 was less than 1 when 20 µM of anthraquinone monosulfonate was dissolved in 0.14 µM NP solution. Since [Q]0 was 140 times larger than [NP], the adsorbability of the quencher, if any, would be very small. In this case, it is natural to speculate that dissolved quencher molecules contribute to the quenching behavior, and the magnitude of the quenching by dissolved species was much smaller than that by the adsorbed species, as is generally comprehended. Two different quenching mechanisms were attempted to be distinguished by another way, i.e., by the temperature depen-

on NPs. If the maximum number of quenchers that can be adsorbed on one NP and the coverage factor of the adsorbed quenchers are denoted by Y and θ, respectively, [Q]free is given by

[Q]free ) [Q]0 - Yθ[NP]

(6)

Then, the adsorption rate (Vad) and desorption rate (Vde) of quencher are given by

Vad ) kadY(1 - θ)[NP][Q]free

(7)

Vde ) kdeYθ[NP]

(8)

where kad and kde are the rate constants of adsorption and desorption, respectively. In the equilibrium condition, the following equation is given based on Vad ) Vde with consideration of eq 6

Figure 6. Temperature dependence of the Stern-Volmer coefficients (KSV) of methyl viologen (a), anthraquinone-1,5-difulsonate (b), 9-aminoacridine (c), and anthraquinone-1-sulfonate (d).

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dence of KSV values (Figure 6). The KSV values obtained for four different quenchers, methyl viologen, anthraquinone-1,5disulfonate, 9-aminoacridine, and anthraquinone-1-sulfonate, were plotted as a function of temperature of the solution. It is well known that diffusion-mediated quenching and adsorptionbased quenching exhibit an increase and decrease in the KSV values, respectively, as the temperature increases. Decreases in the KSV values were obviously seen for quenching by methyl viologen, anthraquinone-1,5-disulfonate, and 9-aminoacridine, indicating that adsorbed species dominantly contribute to emission quenching. On the other hand, a slight increase in KSV for anthraquinone-1-sulfonate indicates an increase in the diffusion rate constant of the dissolved species. Effects of the Redox Potential of Quenchers. We defined the kinetics of the emission quenching of CdTe NPs as eq 5 together with the reaction shown in Scheme 1. kqdiff given in eq 5 has already been defined as the diffusion-determined rate constant, while kqad has not been defined yet. On the basis of the reaction scheme (Scheme 1), we attempted to define kqad. The adsorption-based quenching for semiconductor NPs can be expressed by the following equation.

Figure 7. Plots of the Stern-Volmer coefficients (KSV) as a function of the redox potential of quenchers.

d [NPm+ - (Qm-)(Qn-m)] ) nket2[NP* - Qn] dt m(k-et2 + kbt2)[NPm+ - (Qm-)(Qn-m)] ) 0 (10) The quenching rate is given by

kad q [NP* - Qn] ) nket2[NP* - Qn] mk-et2[NPm+ - (Qm-)(Qn-m)]

(11)

ad

These two equations define kq as

kad q ) nket2[1 - k-et2 / (k-et2 + kbt2)]

(12)

Since the difference in energy levels between the lower edge of the conduction band and redox potentials of the quenchers is very large, k-et2 , kbt2 can be assumed. Then, eq 13 is obtained as an approximate equation.

kad q ) nket2

(13)

The KSV value is also proportional to ket2 according to eq 5. As mentioned above, almost all the quenchers are adsorbed on NP surfaces under the condition where (I0/I) - 1 vs [Q]0/[NP] shows the linear relationship independent of [NP]. Then, the KSV values obtained in this situation are plotted as a function of the quencher’s redox potential, as shown in Figure 7. Apparently, there was a peak in the KSV-potential curve at around the redox potential of methyl viologen. The energy level of the upper valence band edge of the thioladsorbed CdTe NPs was experimentally estimated as 0 V vs NHE, which did not vary greatly by changing the particle size, whereas the energy level of its lower conduction band edge largely increased with decreasing particle size.45 Since the absorption spectrum of the CdTe NPs used for the experiments shown in Figure 7 exhibited the first exciton peak at 550 nm, the energy level of the lower conduction band edge was estimated to be -2.25 V vs NHE. Considering the large potential gap between the conduction band edge and LUMO of the quenchers, it is acceptable that the inverted region is observable; the electron transfer rate reached a maximum at methyl viologen and decreased with a positive shift in the quencher’s redox potential. Particle Size Effects. In addition to the above quenching experiments using the CdTe NPs exhibiting orange (O) fluorescence, the same experiments were conducted using CdTe NPs

Figure 8. log kqad versus potential gap (∆E) for four differently sized TGA-CdTe NPs quenched by various quenchers. The bandgaps of NPs were Eg ) 2.16 (a), 2.23 (b), 2.31 (c), and 2.44 V (d). Estimated reorganization energy λ ) 1.795 (a), 1.836 (b), 1.877 (c), and 1.945 V (d).

exhibiting different fluorescence colors of red (R), orange (O), yellow (Y), and green (G) whose energy levels of the lower conduction band edges were estimated to be 2.16, 2.23, 2.31, and 2.44 V, respectively. Their Stern-Volmer plots exhibited that KSV was apparently dependent on the energy level of the lower conduction band edge if the KSV values obtained for the same quencher were compared. The τ0 value of each NP was estimated by fluorescence lifetime measurement to evaluate the kqad values ()KSV/τ0) for the quenchers. Figure 8 shows plots of the logarithm of kqad ()log kqad) as a function of ∆E, the latter of which corresponds to the potential gap between the lower conduction band edge of NPs and redox potentials of the quenchers (E0 in Table 1). All the data in the figure are also summarized in Table 1. All plots strongly indicated the appearance of the inverted region, which gives maximum kqad at around 2 V. Such a situation allowed us to consider all the obtained data based on the Marcus theory; however, it is not possible in the present stage because of the following explanation.

Emission Quenching of CdTe Nanoparticles

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TABLE 1: Stern-Volmer Coefficient (KSV) and Quenching Rate Constants (kqad) of Several Types of Quenchers for Four Differently Sized NPs kqad/1014 NPa ∆E/Vb KSV/106 M-1 M-1 s-1

compound methylene blue E0 ) +0.01 V n-octylbipyridylbipyridinium dichloride E0 ) -0.26 V benzyl viologen E0 ) -0.37 V methyl viologen E0 ) -0.44 V proflavin E0 ) -0.781 V 9-aminoacridine E0 ) -0.916 V

R O Y G R

2.171 2.242 2.325 2.451 1.901

3.18 2.91 2.55 1.73 5.68

2.06 2.12 1.90 1.59 3.67

O Y G R O Y G R O Y G R O Y G R O Y G

1.972 2.055 2.181 1.791 1.862 1.945 2.071 1.721 1.792 1.875 2.001 1.380 1.451 1.534 1.660 1.245 1.316 1.399 1.525

5.07 4.20 2.50 4.84 4.42 3.37 2.01 4.50 3.82 3.38 2.09 2.01 2.35 2.79 1.88 1.51 1.79 2.19 2.12

3.71 3.14 2.56 3.13 3.23 2.52 2.55 2.91 2.80 2.52 2.96 1.30 1.72 2.08 2.49 0.973 1.31 1.64 2.34

a Band gap of nanoparticles: R (Eg ) 2.16 eV), O (2.23 eV), Y (2.31 eV), and G (2.44 eV). b Potential gap between the conduction band edge of NPs and the redox potential of quenchers.

The Marcus theory for nonadiabatic electron transfer is described as46,47

ket )

(

1 - (λ + ∆G°)2 2π 2 HAB exp p 4λkbT √4πλkbT

)

(14)

where λ is the reorganization energy, T and kb are the temperature and the Boltzmann constant, respectively, and ∆G° is the total Gibbs free energy change for the electron transfer reaction. By using eq 13, the equation can be rewritten as

kad q ) n × ket )

(

)

1 -(λ + ∆G°)2 2πn 2 HAB exp p 4λkbT √4πλkbT (15)

Here, fitting all the obtained data to eq 15 requires estimation of n values of the differently sized NPs. In our synthetic condition, the growth of CdTe NPs by reflux is reported to proceed with the diffusion-controlled Ostwald ripening process, which induces the growth of larger particles at the expense of smaller particles. Because the concentration of CdTe molecules is constant during the reflux, the concentration of NPs ([NP]) should decrease with an increase in particle size. It is more complicated, however, that the reaction mode was reported to broaden the size distribution of NPs by optical measurements, TEM observation, and Monte Carlo simulations.48-50 Considering that the particle volume is proportional to the cube of its diameter, the mean diameter of NPs does not simply give the mean n value. Therefore, we attempted to fit kqad obtained for several types of quenchers to eq 15, separately about each size of NPs. The results were displayed in the same figure, and the reorganization energy, λ, was numerically derived to be 1.795 (a), 1.836 (b), 1.877 (c), and 1.945 (d) V. Some deviation in

the values of λ may be attributed to the uncertainty of the absolute potential of NPs. We probably overestimated the shifts in the conduction band edge potentials that are partially due to the potential distribution caused by the size distribution of NPs. Conclusion We studied the systematic PL quenching of CdTe NPs caused by photoinduced electron transfer processes. The electrical charges of NPs and quenchers, particle size of NPs, and redox potential and concentration of quenchers are significant factors controlling the emission quenching. In a relatively low concentration range of multivalent quenchers, almost all quenchers are adsorbed on NPs. In these cases, the rate of the electron transfer step was observed as the intensity or lifetime of fluorescence. In particular, it should be significant that the existence of the inverted region is observable in the plots of quenching rate constant versus Gibbs free energy of reactions. This finding strongly suggests that the NPs emitting intense fluorescence are useful as probes for investigating electron transfer reactions. In the present study, it was recognized that the amount of quenchers adsorbed on a CdTe NP is required to completely understand the electron transfer based on the Marcus theory. However, full comprehension was prevented by some size distribution of CdTe NPs prepared by their growth in reflux medium. Recently, we developed a way to tune the CdTe NP size with a small size distribution, i.e., the size-selective photoetching technique. We believe that CdTe NPs whose sizes are tuned by this method may be more useful in clarifying the electron transfer behavior between NPs and redox species. Acknowledgment. This research was financially supported by a Grant-in-Aid for Scientific Research (18201022) from the Japan Society for the Promotion of Science (JSPS) and a Grantin-Aid for Scientific Research in Priority Areas “Science of Ionic Liquids” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). One of the authors (T.U.) expresses his gratitude to a Grant-in-Aid for JSPS Fellows and to the Global Center of Excellence (COE) Program “Global Education and Research Center for Bio-Environmental Chemistry” of Osaka University. The authors also thank Dr. T. Suenobu (Osaka University) for the fluorescence lifetime measurements. References and Notes (1) Gao, M.; Lesser, C.; Kirstein, S.; Mo¨hwald, E.; Rogach, A. L.; Weller, H. J. Appl. Phys. 2000, 87, 2297. (2) Schlamp, M. C.; Peng, X.; Alivisatos, A. P. J. Appl. Phys. 1997, 82, 5837. (3) Mattoussi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Thomas, E. L.; Bawendi, M. G.; Rubner, M. F. J. Appl. Phys. 1998, 83, 7965. (4) Kapitonov, A. M.; Stupak, A. P.; Gaponenko, S. V.; Petrov, E. P.; Rogach, A. L.; Eychmu¨ller, A. J. Phys. Chem. B 1999, 103, 10109. (5) Wang, C. J.; Shim, M.; Guyot-Sionnest, P. Science 2001, 291, 2390. (6) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385. (7) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2008, 130, 4007. (8) Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phys. ReV. B: Condens. Matter 1996, 54, 17628. (9) Barnham, K.; Marques, J. L.; Hassard, J.; O’Brien, P. Appl. Phys. Lett. 2000, 76, 1197. (10) Robel, I.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2007, 129, 4136. (11) Bruchez Jr, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (12) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016. (13) Ge, C.; Xu, M.; Liu, J.; Lei, J.; Ju, H. Chem. Commun. 2008, 8, 450.

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