Optical and Dynamic Properties of Water-Soluble Highly Luminescent

J. Phys. Chem. B 2007, 111, 12765-12771

12765

Optical and Dynamic Properties of Water-Soluble Highly Luminescent CdTe Quantum Dots Abhijit Mandal,† Junichi Nakayama,† Naoto Tamai,*,† Vasudevanpillai Biju,‡ and Mitsuru Isikawa‡ Department of Chemistry, School of Science and Technology, Kwansei Gakuin UniVersity, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan, and Nano-bioanalysis Team, Health Technology Research Center, National Institute of AdVanced Industrial Science and Technology (AIST), Hayashi-Cho, Takamatsu, Kagawa 761-0395, Japan ReceiVed: June 14, 2007; In Final Form: August 22, 2007

CdTe quantum dots (QDs) were synthesized in aqueous solution using thioglycolic acid (HS-CH2COOH, TGA) as a stabilizer. The phenomenon of “on” and “off” luminescence intermittency (blinking) of CdTe QDs in PVA and trehalose was investigated by single-molecule optical microscopy, and we identified that the intermittencies of single QDs were correlated with the interaction of water molecules absorbed on the QD surface. The “off” times, the interval between adjacent “on” states, remained essentially unaffected with an increase in excitation intensity. Every QD showed a similar power law behavior for the “off” time distribution regardless of the excitation intensity and aqueous environment of the QDs. In the case of “on” time distribution, power law behavior with an exponential cutoff tail is observed at longer time scales. The time traces indicated that the “on” time was inversely proportional to the excitation intensity; the duration of “on” time became shorter with increasing excitation intensity. An increase in the duration of “on” time was observed in trehalose with respect to that in PVA. We obtained a clear decrease in the power law exponent when PVA was replaced with trehalose. These observations indicate that the luminescence blinking statistics of water-soluble single CdTe QDs is significantly dependent on the aqueous environment, which is interpreted in terms of passivation of the surface trap states of QDs.

Introduction Colloidal semiconductor nanocrystals have received wide attention in the past few decades because of their unique optical properties. Nanometer-size semiconductor particles and their size-dependent physicochemical properties are current topics of active research in chemistry, physics, and biology.1,2 In particular, semiconductor quantum dots (QDs) have been adopted for in vitro bioimaging as an alternative to conventional organic fluorophores because of their small size, tunable optical emission, wide absorption and narrow emission bands, brightness, and superior photostabilily.3,4 The inherent advantages of QDs over organic dyes could allow the integration of nanotechnology and biotechnology, leading to major advances in medical diagnostics, molecular biology, and cell biology.5,6 However, QDs are embedded in different matrixes for different applications. In exploiting their potential applications, it is necessary to have a better understanding on how the optical properties of the QDs are influenced by different environments, more precisely on a single QD level.7 One of the major challenges in biological applications of QDs is compatibility in aqueous environments without affecting the brightness. Despite the unique properties and advantages of QDs, many studies suggested considerable heterogeneity in their emission properties, including “on” and “off” luminescence intermittency (blinking), long-lived dark states, spectral fluctuations, and variations of luminescence lifetime.8 These properties limit the applications of QDs especially in time-correlated imaging and investigation of slow dynamics of labeled mol* Corresponding author. E-mail: [email protected]. † Kwansei Gakuin University. ‡ AIST.

ecules. Therefore, the development of nonblinking QDs, suppression of blinking, and reduction of blinking time scale would be helpful for utilizing the superior luminescence properties of QDs more efficiently. Synthesis of biocompatible QDs with bright luminescence is still challenging for various applications. Aqueous synthetic routes have advantages over organometallic synthesis due to the low cost of production, safety, and possible biolabeling applications.9 Three different approaches have been used to obtain water-soluble semiconductor QDs: ligand exchange, encapsulation into a water-soluble shell (e.g., silica or phospholipids), and arrested precipitation in water. The ligandexchange methods have been applied to highly luminescent (quantum efficiency φf g 50%) CdSe(ZnS), CdSe(CdS), or CdTe QDs prepared in hot coordinating solvents, yielding watersoluble QDs with a lower values of φf (∼10-30%). In the case of CdSe QDs the luminescence was known to be considerably quenched after transfer into water, especially if a shell from a wider band gap material was not involved.10 The encapsulation of the QDs into a water-soluble shell typically yields lower luminescence quantum efficiencies of ∼20-30%. Both of the above-mentioned methods for transferring QDs into an aqueous phase involve several steps and thus have the additional disadvantage of being rather complicated and time-consuming. Arrested precipitation in water in the presence of stabilizers (e.g., thiols) is a faster and simpler method to obtain water-soluble QDs and has been applied to several semiconductors potentially relevant to biological applications (e.g., CdS, CdSe, CdTe). Single chromophore detection, that is, the study of single nano-objects (molecules, QDs, metal colloids, etc.), is a modern tool for investigating the physicochemical properties of a single object interacting with surrounding environments.11-14 One of

10.1021/jp074603+ CCC: $37.00 © 2007 American Chemical Society Published on Web 10/18/2007

12766 J. Phys. Chem. B, Vol. 111, No. 44, 2007 the most prominent features of single chromophore detection is the observation of luminescence intermittency or blinking, which is a random switching between an emitting and a nonemitting state, under steady laser illumination.15-27 This is a single particle effect, which is not observable in ensemble measurements. Luminescence blinking of single QDs was first observed and analyzed by Nirmal et al. using single-molecule spectroscopy and microscopy.28,29 In their investigations the luminescence intensity time trajectories of single QDs were observed for an extended period of time in contrast to single molecules, which are often suffer fast photobleaching. This high photostability of QDs offers better statistical accuracy with respect to the underlying kinetic phenomena even on a single particle. In contrast to exponential processes such as intersystem crossing related photon bunching in single molecules, a single CdSe QD follows an inverse power law behavior over many decades in probability density and in time. The probability distribution of “on” times P(ton) likewise follows a power law at short times but falls below the power law at longer “on” times (a bending tail observed) at room temperature.30 Such a broad distribution of kinetics with rates varying over 5 orders of magnitude is common to all studied QDs. Experiments carried out by Schuster et al.31 reported similar power law blinking behavior for single dye molecules as well. To date there are many kinetic models to explain this blinking dynamics. In this work, we prepared CdTe QDs in aqueous solution by a method reported previously. Buratto et al.32 found that in case of CdSe QDs luminescence can be strongly influenced by the interaction of water molecules adsorbed on the surface of QDs. We have examined this finding in our experiments on CdTe QD single particle experiments. We chose two different matrixes, poly(vinyl alcohol) (PVA) and trehalose, to disperse the QDs on a glass surface. The higher glass transition temperature of trehalose could explain its high water preservation efficiency.33 The work being presented in this paper is an investigation of the optical properties of CdTe QDs by steady state and time-resolved experiments at an ensemble level and how the different aqueous environments (PVA and trehalose) directly influence the intermittency statistics of single CdTe QDs. We found that the change in different aqueous environments from PVA to trehalose directly influences the intermittency statistics of CdTe QDs. The results of the current work added to the mechanism underlying the luminescence blinking of semiconductor QDs, which would be valuable during the preparation of nonblinking QDs, and QDs with reduced blinking. Experimental Section Materials and Samples. All chemicals were of the highest purity commercially available and were used without further purification. Cadmium perchlorate hexahydrate [Cd(ClO4)2‚ 6H2O] was purchased from Chameleon Reagent, Japan. Thioglycolic acid (TGA) was purchased from Aldrich Chemical Co. Aluminum telluride (Al2Te3) lumps was received from MP Biomedicals Inc. PVA was obtained from Wako Chemicals, Japan. Trehalose was purchased from Hyashibara Co., Japan. The water used throughout this research was Milli-Q water (Yamato, Millipore WQ 500). For the film measurements at first as-prepared CdTe QDs were diluted either by 5 wt % PVA or trehalose solution to approximately micromolar range, and then these solutions were spin cast onto the cover glass surfaces. For single-molecule experiments diluted CdTe QDs (∼10-10 M) were mixed in either poly(vinyl alcohol) (PVA) or trehalose (1 wt %) and spin coated with 3000 rpm onto a cover glass slide. At this low density particles were spatially well resolved.

Mandal et al. Synthesis of CdTe Quantum Dots. Water-soluble CdTe quantum dots capped with TGA were prepared according to the procedure reported in the literature.34-36 In a typical synthesis 2.35 mmol of Cd(ClO4)2‚6H2O was dissolved in 125 mL of water and 5.7 mM TGA was added followed by adjustment to the desired pH by adding 1 M NaOH solution under vigorous stirring. After this the solution was continuously stirred until the solution became optically clear. Separately, 0.2 g of Al2Te3 chunks was placed in a 50 mL three-necked flask. Dropwise addition of 20 mL of concentrated sulfuric acid was then introduced into the Al2Te3 chunks to produce H2Te gas and passed through the previous resulting mixture with a slow N2 flow for 20 min. The CdTe precursors were formed at this stage, accompanied by an orange color. The molar ratio of Cd2+:TGA: Te2- was fixed at 1:2.4:0.5. The size of the QDs was controlled by the duration of reflux time and was monitored by absorption and luminescence spectra. The evolution of the particle size during the growth at 100 °C was estimated from the absorption and luminescence spectra as described elsewhere.37,38 Three different CdTe QDs with diameters of 2.8, 3.2, and 4.6 nm were used in our experiments. Apparatus. The luminescence quantum yields of various (sized) CdTe QDs were determined at room temperature by comparing the integrated emission of the CdTe QDs in solution to the emission of rhodamine B of identical optical density at the excitation wavelength. UV-vis absorption and luminescence spectra were recorded using Hitachi U-3210 and FluoroMax-2 (Jobinyvon-spex) spectrophotometers, respectively. The bulk luminescence lifetimes of CdTe QDs were measured by using a frequency-doubled femtosecond cavity dumped Ti:sapphire laser (λex ) 410 nm, Kapteyn-Murnane Laboratories Inc.), operating at a repetition rate of 2 MHz with an overall temporal time resolution of 40 ps. The detector was a microchannel plate photomultiplier (Hamamatsu, MCP R2809U). The single photon counting technique comprised a constant-fraction discriminator (CFD, Tenelec TC 454), delay units (EG & G ORTEC, Model DB 463), and a time-to-amplitude converter (TAC, Tenelec TC 864) operated in reverse start-stop mode and a data card running on a PC. The quality of fit was judged in terms of a DW parameter, weighted residuals, and reduced χ2 values. The single QD luminescence was investigated through the cover glass slip on an Olympus IX-51 optical microscope with a 100× objective lens (NA ) 0.9). The sample was excited by a second harmonic of a Nd:YLF laser (at 524 nm, Microlase Inc.). The excitation laser intensity was varied using a variable ND filter. The laser beam was directed into the back aperture of the microscope objective lens through a dichroic mirror. Samples were globally illuminated by the laser beam through the microscope objective. Photons emitted from the QDs were collected by the same objective, separated from the excitation light by the dichroic mirror, a long pass filter, and then an interference filter (Asahi Spectra, PB0600-20, 600 ( 10 nm). Images and trajectories of individual QDs were recorded by a charge coupled device camera (iXon CCD, Andor Inc.). Results and Discussion To examine the effect of aqueous environment on luminescence properties of water-soluble CdTe QDs, we performed steady state and time-resolved studies on PVA and trehalose films prepared by spin casting. Ensemble luminescence spectra of CdTe QDs were measured in water as well. Figure 1 illustrates the luminescence spectra of CdTe QDs in PVA and trehalose media. We found that the luminescence spectra of CdTe QDs in films are red shifted compared to that in aqueous

Single CdTe Quantum Dots

Figure 1. Luminescence spectra (λexc ) 570 nm) of CdTe QDs in PVA (a) and trehalose films (b) on cover glass surfaces. Film samples for spectral measurements were prepared by spin casting.

solution. In PVA, the luminescence spectrum has a peak at 628 nm, which is ca. 4 nm red shifted compared with that in trehalose film. In addition, the spectral width of CdTe QDs in PVA is broader than that in trehalose film. In aqueous solution the 4.6 nm as-prepared CdTe QDs show a narrow luminescence peak centered at 615 nm without any trap state emission. However, in PVA film broad emission appears. Relatively high concentration QD solution (∼10-6 M) was used to prepare the film by spin casting. The spectral shift and the broad emission in PVA are probably due to the interaction between QDs. On the other hand, the small red shift of the luminescence peak and narrower bandwidth in trehalose film compared with PVA may originate from the high water preservation capability of trehalose as discussed below. We examined the luminescence decay behavior of CdTe QDs of three different sizes in PVA and trehalose media (Figure 2); the decays were monitored at the luminescence maxima. All decay behaviors are analyzed by a sum of three exponential decay functions.39 The average lifetime is increased with the increase in particle size in both cases. An interesting observation is that for the same size QDs the lifetime in trehalose media is longer compared to that in PVA matrix (Table 1). Besides the

J. Phys. Chem. B, Vol. 111, No. 44, 2007 12767 film decay measurements we also examined the luminescence decay in PVA and trehalose solution (5 wt %). We did not get any noticeable difference in lifetime in PVA and trehalose solution. This result can be interpreted by considering that the decay measurements in solution of either PVA or trehalose behave like an aqueous environment. It is best to mention here that the average decay time of CdTe QDs in aqueous solution (τav ) 28.8 for 4.6 nm CdTe QDs) is always longer compared to decay time on film. However, the average lifetime in aqueous solution is comparable to that in trehalose film. The plausible reason behind this is that we prepared the QDs in aqueous solution, hence the QDs preserve their luminescence properties better in aqueous environment. Trehalose is a common nonreducing disaccharide of glucose found in a variety of organisms such as fungi, plants, and animals, where it has a protective role in the case of water stress or dehydration and freezing.33,40 It is well-known that trehalose helps reduce the evaporation of water. These results suggest that trehalose, compared to PVA, could indeed reduce the evaporation of water from the QD environment. This interesting observation has been implemented in single quantum dot experiments. A time trace of luminescence intermittency (blinking) is shown in Figure 3 for CdTe QDs embedded in different matrixes. The trajectories in Figure 3 clearly reveal blinking dynamics, with “on” and “off” abruptly by continuous laser excitation. From these two trajectories one can preliminarily predict that in trehalose media a longer “on” time is observed compared to that in PVA matrix. In most of the trajectories we found this type of interesting behavior of single CdTe QDs, which we will explain briefly in the next section. We also found that the excitation intensity has a clear effect on the blinking behavior. Blinking dynamics and its dependence on excitation intensity have been examined by recording luminescence time trajectories and determining statistically the duration of the bright and dark periods for a large number of QDs at different excitation conditions. To define the threshold above which the QD is considered “on”, we measured the intensity of at least 10 dark regions of the sample (i.e., region with no QDs) and found the greatest intensity. This intensity corresponds to dark counts. The data were analyzed by a twothreshold system. In this technique, a histogram of the counts

Figure 2. Normalized luminescence decay curves for 2.8, 3.2, and 4.6 nm CdTe QDs (λex ) 410 nm, repetition rate of 2 MHz with an overall time resolution of 40 ps) in PVA (A) and trehalose (B). Luminescence wavelengths were monitored at 510, 530, and 620 nm, with QD diameters of 2.8, 3.2, and 4.6 nm, respectively.

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TABLE 1: Lifetime Data of CdTe QDs Embedded in Different Matrixes size of QDs (nm)

observed wavelength (nm)

decay components (ns) in PVA film

average decay time in PVA film (τ, ns)

decay components (ns) in trehalose film

average decay time in trehalose film (τ,ns)

2.8

510

5.2

530

4.6

615

τ1 ) 2.1 (0.52) τ2 ) 9.87 (0.42) τ3 ) 46.34 (0.06) τ1 ) 0.46 (0.73) τ2 ) 12.6 (0.18) τ3 ) 65.73 (0.09) τ1 ) 0.96 (0.29) τ2 ) 18.3 (0.41) τ3 ) 56.6 (0.30)

9.0

3.2

τ1 ) 0.78 (0.70) τ2 ) 9.66 (0.25) τ3 ) 58.2 (0.05) τ1 ) 0.37 (0.76) τ2 ) 12.1 (0.16) τ3 ) 64.2 (0.08) τ1 ) 1.4 (0.32) τ2 ) 11.3 (0.46) τ3 ) 38.9 (0.22)

per bin was generated for each trajectory, yielding bimodal distributions corresponding to regions of high and low luminescence intensity, which are defined as “on” and “off” states, respectively. In our single QD trajectory measurements the signal-to-noise ratio was over 3.0. By using different excitation intensities, we generated a histogram of luminescence of “on” and “off” times combining the results of over 40 different particles. For each time trajectory we measured a set of times ∆ton for the bright intervals and ∆toff for the dark intervals. Figure 4 shows the histograms of ∆ton distribution for a set of over 40 CdTe QDs in PVA matrix at room temperature. The intensity of the excitation beam was varied from 0.28 to 7.1 kW/cm2. At each excitation intensity, we generated a histogram of “on” and “off” times. Similar distributions of the “on” times have already been described in the literature.17,28 We found that with an increase in excitation intensity the duration of the bright interval values was reduced dramatically. This is qualitatively expected for a competing photophysical or photochemical branching process, which turns the particle luminescence off.16 The blinking behavior observed in our CdTe

7.5 14.1

24.6

QDs is strikingly similar to that observed at room-temperature luminescence from single CdSe QDs.19,30 This suggests that the mechanism used to explain the blinking observed in CdSe QDs could also describe the blinking observed in our CdTe QDs. We also measured the “on-off” distribution of single CdTe QDs in PVA and trehalose under identical experimental conditions (Figure 5) and compared the results. There is a clear effect on “on” time distributions by changing the surrounding matrix from PVA to trehalose, whereas the “off” time duration distributions are insensitive to the different environments of CdTe QDs. Buratto et al.32 observed that the luminescence intensity from CdSe QDs can be strongly influenced by the interaction of water molecules adsorbed on the surface of QDs. Recently, Gomez et al.41 demonstrated by using single QD experiments that the blinking statistics could be modified by adding octylamine, which can passivate unsaturated dangling bonds at the surface of the CdSe QDs. They found that the “on” time exponent (Ron) decreased significantly when CdSe QDs were capped with octylamine. Koberling et al.42 found that oxygen molecules adsorbed to the surface of semiconductor nanocrystals have a considerable effect on the luminescence properties of the particles. In presence of oxygen molecules the “on” time duration was decreased very much whereas “off” time remained same. Qualitatively it can be seen that the effect of trehalose on CdTe QDs is to increase the length and number of “on” time events. From a mathematical point of view, the blinking kinetics of these CdTe QDs can be quantified by analysis of the “on” and “off” time probability densities, P(ton) and P(toff), respectively. Figure 6 displays the luminescence intermittency statistics for CdTe QDs in PVA and trehalose environments. The distribution of “off” times involved in the blinking for all QDs studied here is almost linear on this scale, which indicates that the lengths of “off” times events are distributed according to an inverse power law of the type

P(toff) ) P0t-Roff

Figure 3. Typical luminescence intensity versus time trajectories of a single CdTe QD embedded in PVA (A) and trehalose (B) matrixes dispersed on a cover glass surface. The excitation intensity was 1.7 kW/cm2, and the integration time was 200 ms/bin. An expanded region of this trajectory is shown in the inset, which illustrates the similar nature of the blinking events at room temperature.

9.8

(1)

where P0 is the scaling coefficient and Roff is the power law exponent characterized by the statistics of each type of event. Fitting the data in Figure 6 to a power law distribution yields Roff ) 1.55 in PVA and Roff ) 1.55 in trehalose for all QDs. The universality of this statistical behavior indicates that the blinking statistics for the “off” times are insensitive to the different environments. These results are accordance with results obtained by other groups.11,42 The “on” times were fitted with a power law with an exponentially decaying tail with the relaxation time τon, thereby limiting very long “on” times.

P(ton) ) P0t-Ron exp(-t/τon)

(2)

Shimizu et al.30 observed a bending tail for “on” time distributions. They suggest a secondary, thermally activated and

Single CdTe Quantum Dots

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Figure 4. “On” time histogram of single CdTe QDs in PVA matrix by using different excitation intensities. All histograms were built from time traces that were recorded under identical experimental conditions, except for excitation power.

Figure 5. “On” and “off” time histograms for CdTe QDs in PVA and trehalose. The histograms are constructed from more than 40 individual emission time traces that were recorded under identical experimental conditions (excitation intensity ) 1.7 kW/cm2, time bin ) 200 ms, total acquisition time per QD ) 80 s). By comparing the time traces, it is seen that the length of the “on” times increases very much in the case of trehalose media.

photoinduced process causing the probability distribution of the “on” time statistics to be truncated at the tail of the power law distribution. At low temperature (10 K) and low excitation intensity, the “on” time power law was nearly same as that of “off” time distribution. Thus relatively long “on” times can be observed at low excitation intensity. This is also true for CdTe QDs as well in our case. The power law exponent for the “on” time is noticeably changed to smaller R when the embedding matrix was changed from PVA to trehalose. For the two different embedding matrixes, the blinking coefficients Ron and Roff are summarized in Table 2. The data to the power law distribution for all QDs studied here yield Ron ) 1.40 in PVA and Ron ) 1.24 in trehalose. Trehalose helps to reduce the evaporation of water and has a promising capability to preserve water. We

observed that trehalose, compared to PVA, could indeed reduce the evaporation of water from the QD environment. Trehalose binds to a large number of water molecules and forms an almost homogeneous mixture.33 The higher glass transition temperature (393 K) could explain its great preservation efficiency. Buratto et al.32 also suggested that the luminescence intensity from CdSe QDs can be strongly influenced by the interaction of water molecules absorbed on the surface. Their interpretation was that the water molecules are adsorbed to the surface of the QDs and passivate surface traps, which results in the increase of luminescence intensity. To make the QDs water-soluble, thioglycolic acid is used as a capping agent. This capping agent is bifunctional, with a polar group to make the nanocrystals water soluble and the mercapto group, which binds strongly to

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Figure 6. Log vs log plots of “on” and “off” probability distributions compiled from a set of CdTe QDs in PVA and trehalose matrixes at room temperature. The distributions show a power law behavior of the “off” times and a power law with exponential cutoff for “on” times.

TABLE 2: On-Off Power Law Exponents r for CdTe QDs Embedded in Different Matrixes PVA and Trehalosea matrix

Ron

τon (s)

Roff

PVA trehalose

1.40 ( 0.03 1.24 ( 0.03

1.80 ( 0.10 4.45 ( 0.17

1.55 ( 0.04 1.55 ( 0.05

a

The data are extracted from Figure 6.

the QD surface. Only in aqueous environment can TGA ionize and make it feasible to interact with QDs very well. We found that preparation of CdTe QDs is very sensitive to the pH of the medium. At relatively high pH (greater than 9.0), the capping agent works well and the luminescence intensity as well as the quantum yield is high. When we used PVA as an embedding matrix for QDs, the evaporation of water from the environment occurred with time. However, in trehalose matrix, water evaporation was not so fast compared to that in PVA. It is wellknown that trehalose has a good capability to preserve water. Once water is removed from the surface of QDs, there is a high chance to decrease the binding capability of the capping agents; hence QDs may be quenched quickly. We also found similar behavior in film decay measurements as mentioned previously. In the case of trehalose film we obtained a longer decay time compared to that in PVA film. These are the possible reasons why “on” times are increased when PVA is replaced by trehalose matrix. Issac et al.7 have observed that an increase in the dielectric constant of the matrix surrounding the QDs leads to a decrease in the value of Roff. The matrix in this case was thought to solvate the ejected charge carriers and stabilize the charge-separated state. Furthermore, they found that the value of the power law exponent of the “off” time (Roff) statistics scaled linearly with the reaction field factor [f()], which

correlates the stabilization energy with the dielectric properties of the matrix.

f() ) 1 - 1/

(3)

where  is the static dielectric constant of the matrix. This relationship predicts a minimum value for the Roff of about 1.41 for a matrix with a large dielectric constant ( ) ∞). Because of the water preservation capability of trehalose, the dielectric constant of trehalose and PVA ( ) 14) are high enough and hence the Roff values are very similar to each other. The data of Issac et al.7 are also at variance with the results obtained by Pelton et al.,11 who found that the blinking was independent of whether the QDs were in solution or immobilized on a glass coverslip. However, in our case, the values of Ron change significantly by changing the matrix from PVA to trehalose, whereas the values of Roff are insensitive to the difference in environmental materials. It seems that the properties of the “on” time distribution are more sensitive to the environmental conditions and to the QD preparation procedure. Though the origin of luminescence intermittency of QDs has been studied in detail previously,15,17,43-45 the blinking phenomenon of QDs and its unusual power law distribution in the histogram of on-off emission events are still not clearly understood. Verberk et al.46 suggested that an electron can tunnel from the excited QD to a trap state. After transfer, the charged QD still absorbs, but is dark because of fast Auger recombination, i.e., charge-induced nonradiative relaxation of the exciton energy. The dark period ends, and the QD becomes bright again when the trapped electron hops back. Marcus25,47 proposed a theoretical model for intermittency based on electron transfer between a QD and its localized surface states. Referring to the recently published nonadiabatic electron

Single CdTe Quantum Dots transfer theory with a diffusion-controlled electron transfer model for fluorescence intermittency of QDs, Tang and Marcus47 predict a power law distribution for the lifetime of the blinking statistics P(ton/off) followed by an exponential decay at longer times. They defined P(ton/off) as the rate of loss for the total population in either the neutral or the charge-separated state. The exponential part of the probability decay is associated with the parabolicity of the excited and dark state potential surfaces. The parabolic potential surfaces enhance the probability of populating the dark state (and vice versa) in such a way that the probability distribution of the bright state decays exponentially rather than by a power law at longer times. In conclusion, we have shown that the emission intermittency is dependent on the aqueous environment surrounding the CdTe QDs. Due to the good water preservation capability of trehalose, it could embed the QDs well and have a considerable influence on the luminescence properties of the QDs. The lifetimes of the bright states are influenced by the properties of the surrounding environment. The present paper has shown that the lifetime of the bright (“on”) state increases by changing the surrounding matrix from PVA to trehalose. Since the power law statistics for “off” times are excitation intensity independent, the process that couples a dark state to a bright state is a tunneling process and not the light-driven process. For the promising applications of nanocrystals in various fields, attention should be paid to the exact conditions in which the QDs will be used. The influence of the environment on the blinking behavior confirms that the charge rearrangements outside the QDs are relevant for the blinking statistics. The sensitivity of nanocrystals to their local environment complicates their use in many applications. It makes them very promising and sensitive nanoprobes. Careful tuning of the environment makes the nanocrystals even more suited for various applications or as model systems for different research. Acknowledgment. A.M. acknowledges the JSPS program of Postdoctoral Fellowship for Foreign Researcher. This work is partially supported by a Grant-in-Aid for Scientific Research on Priority Areas of Molecular Nano Dynamics, from MEXT, Japan. Supporting Information Available: Steady state and timeresolved spectra of different sized CdTe QDs and average “on” time probability distribution for single CdTe QDs at different excitation intensities. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Dabboussi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (3) Chan, C. W.; Nie, S. Science 1998, 281, 2016. (4) Bruchez, M.; Moronne, M.; Glin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (5) Niemeyer, C. M. Angew. Chem. 2001, 113, 4254. (6) Niemeyer, C. M. Angew. Chem., Int. Ed. 2001, 40, 4128. (7) Issac, A.; von Borczyskowski, C.; Cichos, F. Phys. ReV. B 2005, 71, 161302.

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