3998
J. Phys. Chem. C 2009, 113, 3998–4007
Kinetics of Photochromic Induced Energy Transfer between Manganese-Doped Zinc-Selenide Quantum Dots and Spiropyrans Saim M. Emin,† Norihito Sogoshi,† Seiichiro Nakabayashi,*,† Takashi Fujihara,† and Ceco D. Dushkin†,‡ Department of Chemistry, Faculty of Science, Saitama UniVersity, Saitama 338-8570, Japan, and Laboratory of Nanoparticle Science and Technology, Faculty of Chemistry, Sofia UniVersity, Sofia 1164, Bulgaria ReceiVed: NoVember 6, 2008; ReVised Manuscript ReceiVed: January 15, 2009
Fluorescence resonance energy transfer (FRET) is investigated with manganese-doped ZnSe nanoparticles (quantum dots) as long fluorescence lifetime donors and spiropyran molecules as acceptors. The kinetics of photochemical ring-opening reaction of spiropyran upon UV-illumination is exploited to achieve efficient quenching of the manganese fluorescence lifetimes at a low temperature of 78 K in methylcyclohexane/ isopentane mixture. Modulation of the acceptor structure from closed to open form allows us to investigate the temporal decrease in donor lifetime accompanied with an increase in the efficiency of FRET transfer. A theoretical model proposed by us describes very well the experimental data, thus providing the intimate parameters of FRET transfer in the system quantum dot-dye molecule. 1. Introduction The investigations on semiconductor nanocrystals (also known as quantum dots, QDs) in the last two decades have opened the perspectives for design and synthesis of new hybrid materials. The wide interest in quantum dots arises from their unique optical and electrical properties, including broad absorption and narrow emission spectra, large extinction coefficients, resistance to photobleaching, long fluorescence lifetime, and size-tunable emission.1 However, some of the semiconductor nanomaterials, such as cadmium-based nanocrystals (CdS, CdSe, CdTe), remain less favorable for bioapplications due to their toxicity.2 The transition-element-doped nanoparticles, including manganese-doped ZnS and ZnSe, can be an alternative for in vivo bioimaging,3 tunable lasers,4 and electroluminescent devices.5 The dopants in semiconductor nanocrystals lead to phenomena not found in the bulk state, because their electronic states are confined to a very small volume. For example, the emission peak from Cu2+ internal transitions in doped ZnSe can be tuned from 470 to 550 nm.6 Doping can introduce another big advantage such as long fluorescence lifetimes, compared to the pure host ZnSe nanocrystals, as in this study of Mn2+-doped ZnSe nanocrystals (ZnSe:Mn). The use of long-lived fluorophores, combined with time-resolved detection, minimizes prompt fluorescence interferences from the surrounding molecules. Fluorescence resonance energy transfer (FRET) is a powerful technique for probing very small (sub-nanometer scale) changes in the separation distance between donor and acceptor fluorophores, which is ideal for the sensitive detection of molecular binding events.7 FRET occurs when there is an appreciable overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. Recent reports have confirmed that luminescent quantum dots, such as CdS, CdSe, CdTe, Si, and InAs, can be used in FRET studies of biomolecular conformations, ligand-receptor binding, and so on.8,9 * To whom correspondence should be addressed. E-mail: sei@ chem.saitama-u.ac.jp. † Saitama University. ‡ Sofia University.
These QDs overcome some of the limitations associated with conventional organic dyes such as photostability and welldefined shapes. In contrast to cadmium-based QDs, which participate in FRET, we focus our attention on the long fluorescence lifetime from Mn2+ centers in doped zinc-based QDs, which serve as efficient energy donors. Considering the nature of dipole-dipole interaction between a donor (D) and acceptor (A), the elementary acts of FRET can be classified as the following types:10 singlet-singlet, triplet-singlet, singlet-higher triplet, or triplet-triplet. There are exceptional cases of FRET that follow different routes such as energy transfer from upper excited states, from metal-ligand complexes and so on.11 We like to enrich that variety of FRET systems by promoting the transition-element-doped semiconductor nanocrystals as the next generation of nanomaterials for FRET studies. Thakar et al. reported recently a similar energy migration mechanism from Mn2+ centers in doped semiconductor nanoparticles to covalently attach them dye molecules.12 They have synthesized multilayered nanoparticles consisting of two types of semiconductors: a core of ZnSe and a shell of ZnS deposited on it, which contains the Mn2+ dopant. The deposition of a second shell of ZnS completely passivates the Mn2+ centers. The complete structure of their multilayered nanomaterial can be denoted as ZnSe/ZnS:Mn/ZnS, where the Mn2+ ions are completely packed into a layer closed by two types of semiconductors. For the acceptor molecules, they have used BODIPY, a commercial dye molecule. The obtained FRET from the QD to the dye is of efficiency of 26%, which is rather low for a well-packed material. The energy propagation in their system is suppressed by the deposition of the second layer of ZnS onto the Mn2+ centers thus decreasing the FRET efficiency. From here, one can conclude that the FRET efficiency could be better for single cores of ZnSe doped with Mn2+ as the ones in our study. Moreover, we use the ability of acceptor molecules such as spiropyrans (SPO) to undergo structural transformation in order to investigate the temporal changes in the donor fluorescence lifetimes. The photochromic molecules (SPO) gain attention due to the ability of structure modulation, which can be used in drug delivery in cells,13 cell tagging,14 and protein
10.1021/jp809797x CCC: $40.75 2009 American Chemical Society Published on Web 02/18/2009
Kinetics of Photochromic Induced Energy Transfer labeling.15 On the basis of these advantages of SPO molecules, QD (cadmium-based) photoswitches were designed.16–18 In our study, ZnSe:Mn nanocrystals are used as FRET donors, while SPO molecules are the FRET acceptors in a glass-forming liquid at low temperature. By using glass-forming solvent, the effect of diffusion on the FRET efficiency is eliminated, while the distances between QDs and SPO molecules are fixed. Modulation of the acceptor configuration can activate FRET pathways, which are otherwise suppressed. Only the open form of SPO molecules can accept energy from the nearby QDs, whereas this process is forbidden for the closed form of SPO molecules due to the lack of spectral overlap with QDs. Hence, the overlap of QD emission at 575 nm with the absorption spectrum of the open-form SPO molecules leads to a resonance energy transfer among them. In our paper here, we exploit the Mn2+ long fluorescence lifetime, as well as the precise structure modulation of SPO molecules, to monitor the changes in the donor lifetimes. Toward this goal, we increase the effective concentration of acceptors in our system by constant illumination of the system with UV-light and measure the photoluminescence decay times and FRET efficiency. These data are interpreted straightforwardly using an original theoretical model developed by us here, which allows calculation of the microscopic parameters of FRET. The system ZnSe:Mn QDs (donors)-SPO molecules (acceptors), chosen by us in this study, is a compromise of many factors, technical and economical, which should obey a suitable candidate for FRET observation. The Zn-based QDs are a biologically friendly alternative; however, they require manganese doping in order to go to a visible light emission, which will overlap with the photochromic dye absorbance. Also, the doping makes the photoluminescence decay in the millisecond range, which is 103 times longer than that of the intrinsic ZnSe. SPO is an exotic dye with a suitable absorbance spectrum of its open-form molecule, which requires however a low temperature in order to be long living in the matrix. The latter necessitates kinetic measurements as the most appropriate way to change the active-dye concentration and, hence, to vary the distance between donors and acceptors. 2. Experimental Details 2.1. Sample Preparation. Mn2+-doped ZnSe nanocrystals were synthesized using the high temperature, organometallic synthesis of Norris et al.19 In a typical procedure, dimethylmanganese (MnMe2) was freshly prepared in a helium glovebox by reacting 0.25 mL of 0.2 M manganese chloride (MnCl2) slurry in anhydrous tetrahydrofuran (THF) with 0.1 mL of 3 M methylmagnesium chloride in THF. (All these chemicals were purchased from Aldrich.) The resulting clear golden solution was then diluted with 0.9 mL of anhydrous toluene. Subsequently, 0.25 mL of this solution was left for 5 min to form 0.04 M MnMe2. The MnMe2 solution (0.005 mmol) was later added in a glovebox to a syringe containing 2 mL of trioctylphosphine (TOP, Fluka), 0.5 mL of 1 M Se in TOP (Wako Chemicals), and 250 µL of 3 M diethylzinc (Kanto Chemicals, 0.75 mmol). The syringe was then removed from the glovebox and rapidly injected into a vigorously stirred reaction vessel with 15 mL of distilled hexadecylamine (HDA) at 310 °C under dry nitrogen. The nanocrystals then grew at 240-280 °C for about 3 h. The desired sizes of QDs were selected by monitoring their absorbance and emission spectra. The final stock of QDs was achieved by cooling down the HDA solvent to 60 °C and precipitating with 50 mL of methanol/chloroform mixture (8:1 v/v).
J. Phys. Chem. C, Vol. 113, No. 10, 2009 3999 The purification of as prepared nanocrystals from the unbound monomers and capping molecules was achieved by centrifugation of their solution at 7000 rpm for 3 min; it resulted in coagulation of the nanocrystals at the bottom of the centrifuge tube. The sediment was separated by decantation of the solution, which contained dissolved HDA. The solid was then dissolved in methanol/chloroform mixture and centrifuged again. This procedure was repeated several times until no segregation of white surfactant was observed at the top of the centrifuge tube. The resultant powder was measured on an analytical balance and placed in an inert gas atmosphere, and the particle concentration was calculated.20 Further, 65 mg of doped nanocrystals were redispersed in 5 mL of methylcyclohexane/isopentane, MCH:IP (1:4 v/v mixture), containing 3 µL of 1-undecanethiol. The quantity of 1-undecanthiol used was estimated from the polar surface area of molecules (2.53 nm2). In this regard, it was calculated that approximately 36 molecules of 1-undecanethiol adsorb on the surface of a 2.7 nm nanocrystal. The as obtained ZnSe:Mn nanocrystals are denoted as sample 1. The atomic percentage of Mn2+ in sample 1 is 0.6%. We prepared sample 2 with 0.4% Mn2+ in a similar procedure. The acceptor SPO, 1′,3′-dihydro-1′,3′,3′-trimethyl-6-nitrospiro-[2H-1-benzopyran-2,2′-(2H)-indole] (Aldrich), is known to have two forms: closed one and open one, which is formed under UV-illumination in a rather small quantity.21 While at a room temperature the transition closed-open form is reversible, at a low temperature the open form, once created, remains for a longer time. Only the open form has absorbance in visible light. Therefore, we designed two types of low temperature studies at 78 K: (i) The first type concerns the photocoloration of pure SPO in MCH:IP. Two types of cuvettes were used in this case: The first one with a thickness of 1 cm was to measure the absorption of open SPO in the dilute concentration range of 3 × 10-6 M. From these absorption data, the extinction coefficient of open SPO was calculated by the Beer-Lambert law. This value of extinction coefficient was used later to calculate the open-SPO concentrations in 1 × 10-3 M frozen dye solution. The second cuvette, a quartz cell of 1 mm thickness, was used in photoconversion studies of SPO in concentrated solutions of 1 × 10-3 M dye. (ii) The second type of study concerns FRET with the concentration of the compounds adjusted as 1 × 10-3 M for SPO and 4 × 10-5 M for ZnSe:Mn QDs. The acceptor concentrations in FRET investigations were kept higher in order to locate the SPO molecules closer to the donor nanocrystals. As mentioned above, all these studies were performed at a temperature of 78 K in MCH:IP glass-forming solvent. 2.2. Sample Characterization. First, we investigated the kinetics of photochromism of pure SPO in MCH:IP glassforming solvent at low temperatures using a UV/vis spectrophotometer (JASCO V-570) equipped with a cryostat at liquid nitrogen temperatures (Oxford Instruments). A temperature controller connected to the cryostat, which allows maintaining a constant temperature of 78 ( 0.1 K, was used to monitor the cooling of samples. Once placed inside the cryostat, the sample was illuminated with a portable UV-lamp (As One) generating 614 µW/cm2 of intensity at a wavelength of 365 nm. Upon illumination, the SPO molecules were converted from closed to open form. This process was recorded by the UV/vis spectrophotometer, and the concentration of open form SPO was determined for donor-free solutions.
4000 J. Phys. Chem. C, Vol. 113, No. 10, 2009 Second, the samples of QDs were analyzed by steady-state photoluminescence (PL) and photoluminescence excitation (PLE) techniques. The PL spectra were recoded at a fixed wavelength of excitation light λex ) 330 nm. The PLE spectrum was monitored at a fixed wavelength of emission λem ) 580 nm while scanning the wavelengths of excitation. The respective experiments were carried out at room temperatures.22 Third, we measured the fluorescence lifetimes of ZnSe:Mn nanoparticles which participate in FRET. The excitation light source for this study was an Nd:YAG laser (LUMONICS), generating laser pulses with full width at half-maximum (FWHM) < 15 ns at 355 nm. The photons were detected by using a photomultiplier tube (Hamamatsu H6780) with a risetime of 0.78 ns. A band pass filter with a FWHM equal to 16 at 580 nm was used to cut the band-edge emission at 450 nm. The laser beam intensity was
