InP Quantum Dots: An Environmentally Friendly Material with

Article pubs.acs.org/JPCC

InP Quantum Dots: An Environmentally Friendly Material with Resonance Energy Transfer Requisites Anoop Thomas, Pratheesh V. Nair, and K. George Thomas* School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram (IISER-TVM), CET Campus, Thiruvananthapuram, 695 016, India S Supporting Information *

ABSTRACT: Growing demand for clean energy has intensified the interest in understanding the properties of environmentally friendly materials for future energy devices. Indium phosphide (InP) is relatively nontoxic as compared to cadmium chalcogenides, and herein we demonstrate the successful use of this material for resonance energy transfer applications. Three chromophoric dyes, namely, lissamine rhodamine B ethylene diamine (LiRh), Texas red cadavarine C5 (TxRed), and rhodamine 101 (Rh101), possessing free anchoring groups were used as acceptors in InP quantum dot (QD)-based donor−acceptor pairs. The energy transfer process was monitored by steady-state and time-resolved emission spectroscopic techniques. Large values of quenching constant (kq), in the range of 1013−1014 M−1 s−1, observed on addition of chromophoric dyes to InP overcoated with zinc sulphide (InP/ZnS), confirm that the interaction is predominantly static in nature. Selective excitation of the QD component at 405 nm showed a rapid decay of InP/ZnS emission and a concomitant growth of the acceptor emission (rise time of ∼200 ps), indicating that all these systems follow a nonradiative energy transfer mechanism. Time-resolved emission studies confirm that the photoexcited InP/ZnS QDs decays to the ground state by transferring the excitation energy to the chromophoric dyes leading to the formation of its excited state. The high efficiency of energy transfer observed in these systems further confirms that InP is an excellent energy harvester with potential use in biomedical and photovoltaic applications.

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toxicity is a major disadvantage.19 In contrast, III−V QDs such as InP are reported to be environmentally benign due to the structural robustness of the covalent bonds between component elements which blocks their disintegration and prevents the leakage of ions.20 More recently, it has been reported that InP overcoated with ZnS (InP/ZnS) is relatively nontoxic compared to II−VI QDs.20−22 To the best of our knowledge, there are no reports demonstrating the use of InPbased systems for FRET studies. Herein we demonstrate the energy transfer process from InP/ZnS QDs to three commonly used chromophoric acceptor systems, namely, lissamine rhodamine B ethylene diamine (LiRh), Texas red cadavarine C5 (TxRed), and rhodamine 101 (Rh101). Structure of acceptor molecules are presented as Scheme 1. Both LiRh and TxRed possess an amine moiety, and Rh101 has a carboxylate group that can effectively bind onto the QDs.

INTRODUCTION Dipole−dipole interactions between an electronically excited donor and a ground state acceptor, when brought into close proximity (10−100 Å) with suitable spectral overlap, can result in de-excitation of the former by resonant energy transfer to the latter.1 Over the years, Förster resonance energy transfer (FRET)2 has emerged as one of the most powerful tools for understanding various biomolecular processes.3,4 Conventionally, fluorescent systems such as organic chromophores as well as proteins are used as components in FRET-based systems;5 however, photobleaching and chemical degradation limit their application in biomolecular investigations.6 More recently, optically stable semiconductor quantum dots (QDs) have been accepted as a viable alternate to organic chromophoric systems due to their superior properties.7−12 For example, the broad absorption spectra of QDs allow selective excitation of the donor system far away from the absorption spectral region of the acceptor. Similarly, the size tunable narrow emission band of QDs provide good control over spectral overlap with the acceptor. High molar extinction coefficient (105−106 M−1 cm−1), longer luminescent lifetimes, and resistance to photobleaching are other attractive features of QDs.13,14 Among various QDs, II−VI systems such as CdS, CdSe, and CdTe are widely used in FRET-related applications;15−18 however their © 2014 American Chemical Society

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RESULTS AND DISCUSSION The core−shell InP/ZnS QDs were synthesized by following a reported procedure with minor modifications.23 Bare InP QDs Received: January 6, 2014 Revised: January 25, 2014 Published: January 27, 2014 3838

dx.doi.org/10.1021/jp500125v | J. Phys. Chem. C 2014, 118, 3838−3845

The Journal of Physical Chemistry C

Article

Scheme 1. Structure of Various Chromophoric Acceptors

The absorption spectrum of InP/ZnS showed sharp first excitonic peak at 457 nm, and the emission band possesses full width at half-maximum (fwhm) of 61 nm (Figure 1B). It is reported that overcoating with ZnS has a significant influence on the electronic spectrum of InP QDs which is attributed to the passivation of the surface trap states.34,35 The emission yield of InP/ZnS was estimated as 0.11 by following a relative method. An optically dilute solution of InP/ZnS (OD of 0.1 at the excitation wavelength) was used for determining the quantum yield with 8-hydroxy-1,3,6-pyrenetrisulfonic acid36 (ϕf = 0.7) as the standard. The emission lifetime of InP/ZnS core shell QDs was obtained in chloroform using time-correlated single photon counting (TCSPC) studies. The decay curve was fitted triexponentially, and the average lifetime (τavg) was found to be 37.6 ns (eq 9 and Table S2, Supporting Information for details). Chromophoric dye systems such as LiRh, TxRed, and Rh101 have been used extensively as energy acceptors in various molecular dyads37,38 and semiconductor based hybrid systems.15,39,40 These chromophoric systems possess a high molar extinction coefficient (ε) at their absorption maxima: 557 nm (ε = 1.22 × 105 M−1 cm−1) for LiRh,15 578 nm (ε = 8.5 × 104 M−1 cm−1) for TxRed,15 and 582 nm (ε = 9.6 × 104 M−1 cm−1) for Rh101.41 The main objective of the present investigation is to evaluate the efficiency of InP/ZnS QDs as an energy donor in transferring the energy to the chromophoric acceptors. The chromophores under investigation possess negligible absorption in the spectral region of 380−450 nm, which allowed the selective excitation of the InP/ZnS and prevented the acceptor molecule from direct excitation (Figure S4, Supporting Information). To achieve efficient energy transfer, it is necessary to have good spectral overlap between the emission of the donor and absorption of the acceptor. In the present case, the size of QDs was tuned during the synthesis to obtain green emitting QDs in such a way that precise spectral overlap between the emission of InP/ZnS and absorption of chromophoric acceptors is achieved (see shaded region in Figure 2A−C). Further, the energy transfer between InP/ZnS and the chromophoric dyes was monitored by following steadystate and time-resolved emission, and these results are summarized below. The absorption spectra of InP/ZnS in the absence and presence of chromophoric dyes are presented in Figure 2D−F. The steady-state emission spectrum was recorded by exciting the InP/ZnS donor at 400 nm wherein the absorption of chromophoric dyes is minimal (asterisk in Figure 2D−F). Successive addition of chromophoric dyes to a chloroform solution of InP/ZnS (0.66 μM) resulted in the quenching of InP/ZnS emission with a concomitant formation of a new band corresponding to the emission of chromophoric dyes (Figure

showed broad absorption spectral features with its first excitonic peak centered at 453 nm (Figure S1, Supporting Information). The emission spectrum of bare InP QDs was also found to be broad with low emission yield due to the presence of deep electronic trap states associated with dangling bonds on its surface.24 It is well established that the passivation of surface dangling bonds can be achieved by overcoating with a large bandgap semiconductor having minimum lattice mismatch.25−27 ZnS is widely used as an overcoating material since it possesses a bulk bandgap energy of 3.6 eV and minimum lattice mismatch of 7.6% with cubic zincblende phase of InP QDs.28,29 Compared to CdSe and CdTe systems, the optical properties of InP-based systems are not well established, mainly due to the challenges associated with the synthesis.30−33 In a typical synthesis of InP QDs, tris(trimethylsilyl)phosphine was injected to a solution of indium myristate in octadecene at ∼200 °C under inert atmosphere (details are provided in the Supporting Information). Growth of the QDs was followed by spectrophotometric methods and arrested upon attaining the desired size, by lowering the reaction temperature to 130 °C. A mixture of diethylzinc and hexamethyldisilathiane, required for overcoating three monolayers of ZnS over InP, was injected into the reaction mixture maintaining the temperature at 130 °C. The temperature was raised to 200 °C which allowed the growth of the shell. The InP/ZnS core−shell QDs were purified by repeated precipitation (three times) using a mixture (1:4) of methanol and acetone followed by centrifugation. The purified QDs were redispersed in chloroform and used for various photophysical studies. The TEM images indicate that both bare InP (Figure S2A, Supporting Information) as well as InP/ZnS core−shell QDs (Figure 1A) are highly crystalline and monodisperse. The average particle size is estimated as ∼3.0 nm for InP/ZnS QDs. The formation of the ZnS layer was confirmed by comparing the EDAX of bare InP and InP/ZnS QDs (Figure S3, Supporting Information).

Figure 1. Microscopic and spectroscopic characterization of InP/ZnS QDs. (A) TEM image of InP/ZnS and (B) absorption (a) and emission (b) spectrum of InP/ZnS QDs. 3839



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Figure 2. Spectral overlap (shaded portion) between the emission spectrum of InP/ZnS (trace a in A−C) and absorption spectrum of (A) LiRh (trace b), (B) TxRed (trace c), and (C) Rh101 (trace d). Steady-state absorption spectrum of InP/ZnS (0.66 μM) in the absence of chromophoric dyes (trace e in D−F) and in the presence of (D) LiRh (0.25 μM, trace f), (E) TxRed (0.40 μM, trace g), and (F) Rh101 (1.00 μM, trace h).

Figure 3. (A−C) Steady-state emission spectral changes on addition of chromophoric dyes excited at 400 nm: (A) LiRh, (B) TxRed, and (C) Rh101. (D−F) Excitation spectra of InP/ZnS in the absence of chromophoric dye (solid black trace a in D−F) collected at its emission maximum (515 nm). Excitation spectra of chromophoric dye in the absence (dotted red trace b) and in the presence (solid red trace c) of InP/ZnS collected at the corresponding emission maximum of the chromophoric dye: (D) LiRh (0.25 μM; 584 nm), (E) TxRed (0.40 μM; 614 nm), and (F) Rh101 (1.0 μM; 606 nm). (Note: the concentration of dye used in each case and the corresponding emission wavelength at which the excitation spectrum were collected are indicated in parentheses). (G−I) Relative change in the emission intensity of InP/ZnS as a function of the concentration of chromophoric dyes (G) LiRh, (H) TxRed, and (I) Rh101. In all the cases, the emission intensity was monitored at 515 nm.

3A−C; note: emission properties of dyes are given in Figure S5, Supporting Information). The quenching of emission of InP/ ZnS leveled off on increasing the concentration of acceptor molecule to around 0.25 μM for LiRh, 0.40 μM for TxRed, and 1.00 μM for Rh101. Formation of the new band, on successive addition of chromophoric dye, can be attributed to either (i) energy transfer from the excited donor or (ii) direct excitation of the dye. The latter possibility was ruled out by exciting the dye, in the absence of InP/ZnS, at 400 nm keeping the concentration of dye at its maximum (0.25 μM for LiRh, 0.40 μM for TxRed, and 1.00 μM for Rh101). In all the cases, the

emission from the dye was found to be minimal (Figure S6, Supporting Information). The energy transfer from InP/ZnS to chromophoric dyes was investigated by recording the excitation spectrum of InP/ ZnS in the absence and presence of chromophoric dyes (traces a and c in Figure 3D−F). The excitation spectrum of InP/ZnS in the presence of acceptor molecules in the spectral region of 380−450 nm (followed at the emission maximum of the chromophoric dyes) matches the absorption characteristics of InP/ZnS. These results confirm the energy transfer from QD to chromophoric dye. Further, the excitation spectrum of 3840



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Figure 4. (A−C) Luminescence decay profile of InP/ZnS in the absence (black trace a in A−C) and presence of chromophoric dye represented by red trace b: (A) LiRh (0.25 μM), (B) TxRed (0.40 μM), and (C) Rh101 (1.00 μM) monitored at 515 nm. (D−F) Time-resolved emission of chromophore bound QDs followed at the emission maximum of InP/ZnS (red trace a in D−F) and at the corresponding emission maximum of chromophoric dyes represented by blue trace b: (D) InP/ZnS-LiRh, (E) InP/ZnS-TxRed, and (F) InP/ZnS-Rh101. Black decay curve represents lamp profile (trace c in D−F) and fwhm of the pulse was found to be 96.4 ps. (G−I) TRES studies: emission spectrum of chromophore bound InP/ ZnS, recorded immediately after the laser pulse excitation (red trace a in G−I) and after a time delay as indicated in parentheses (blue trace b in G− I): (G) InP/ZnS-LiRh (250 ps), (H) InP/ZnS-TxRed (450 ps), and (I) InP/ZnS-Rh101 (450 ps). Fitted luminescent decay curves are shown as Figures S8 and S9 in Supporting Information.

that of bulk solvent) and results in the spectral shift of the chromophoric dye upon binding. The nature of binding between the QDs and chromophoric dyes was further confirmed through Stern−Volmer analysis. The Stern−Volmer quenching constant was determined by the following eq 1

chromophoric dyes in the absence and presence of InP/ZnS above 500 nm was analyzed (traces b and c in Figure 3D−F). The intense band observed in this spectral region corresponds to the long wavelength absorption band of the chromophoric dyes. Interestingly, this band underwent a large shift in the presence of QDs, indicating the strong interaction between InP/ZnS and chromophoric dyes in the ground state (vide infra). The nature of interaction between chromophoric dyes and InP/ZnS, whether static or dynamic, was investigated by analyzing the (i) spectral changes of chromophoric dyes in the presence and absence of QDs and (ii) Stern−Volmer plots. In the presence of InP/ZnS, the absorption and emission maximum of chromophoric dyes in chloroform showed a red shift of ∼15 nm (Figure S6; Supporting Information) compared to that of unbound chromophoric dyes. Similar shifts were also observed in the excitation spectra in all the cases (Figure 3D−F). These results clearly indicate that the interaction between chromophoric dyes and InP/ZnS QDs is predominantly ground state in nature. It is well established that the free amine as well as the carboxylate groups can effectively bind on to the surface of QDs.42,43 For example, it is reported that LiRh/TxRed, when bound onto the surface of CdSe QDs, undergo a spectral shift which is attributed to change in local polarity.15 In another report, it has been demonstrated that rhodamine 101, in its zwitterionic form, interacts strongly with single-walled carbon nanotubes resulting in a redshift of ∼15 nm in its absorption and emission maximum.44 In the present case, the long alkyl chain of the capping ligands near the surface of InP/ZnS creates a nonpolar local environment (compared to

I0 = 1 + KSV[Q ] I

(1)

KSV = kqτ0

(2)

where I0 is the emission intensity of the donor InP/ZnS QDs in the absence of the acceptor, I is the emission intensity in the presence of varying concentration of the acceptor and [Q] is the concentration of the quencher dyes. Plot of I0/I versus [Q] showed a linear behavior (Figure 3G−I), and the Stern− Volmer constants (Ksv) were estimated from their corresponding slopes (6.46 × 106 M−1 for LiRh, 8.57 × 106 M−1 for TxRed, and 2.56 × 106 M−1 for Rh101). Using this value and the lifetime of the donor (τ0), bimolecular quenching constant (kq) was further calculated (eq 2). Large values of quenching constant (kq) observed on addition of chromophoric dyes to InP/ZnS (1.71 × 1014 M−1 s−1 for LiRh, 2.28 × 1014 M−1 s−1 for TxRed, and 6.80 × 1013 M−1 s−1 Rh101) confirm that the interaction is predominantly static in nature.1,15 Control experiments were carried out using a chromophoric dye, namely, Nile Red, which do not possess any free anchoring groups and possess similar spectral features such as high spectral overlap (Supporting Information, Figure S7). Unlike in the previous cases, no spectral shifts were observed in the absorption and emission spectrum of Nile Red in the presence 3841



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Table 1. Energy Transfer Parameters for Chromophore Bound InP/ZnS Systemsa chromophore bound InP/ZnS

J(λ) (M−1 cm−1 nm4)

R0 (Å)

Eb (%)

Ec (%)

r (Å)

kT(r) (s−1)

InP/ZnS-LiRh InP/ZnS-TxRed InP/ZnS-Rh101

7.26 × 10 2.52 × 1015 2.17 × 1016

47.10 39.48 56.35

60 70 73

61 74 76

44.02 34.28 47.08

3.99 × 107 6.20 × 107 7.11 × 107

15

a R0 = Förster distance, J(λ) = overlap integral; E = efficiency calculated using blifetime (eq 3) and csteady state (eq 4) data; r = distance between the donor and acceptor calculated using eq 8; kT(r) = rate of energy transfer (eq 5).

Time-resolved emission spectroscopic (TRES) investigations of chromophore bound InP/ZnS systems were carried out by collecting the transient profiles in the spectral range of 450− 660 nm, with an interval of 2 nm. Time-resolved emission spectra were constructed by slicing the transient profiles obtained at each wavelength (Figure 4G−I). Spectrum recorded immediately after the laser pulse (trace a in Figure 4G−I) showed an emission band, with a maximum at ∼515 nm, originating from the direct excitation of InP/ZnS. Interestingly, the spectrum recorded after a time delay (250 ps for InP/ZnS-LiRh and 450 ps for both InP/ZnS-TxRed and InP/ZnS-Rh101; Table S3, Supporting Information for details) showed a decrease in emission intensity of InP/ZnS and an increase in the intensity of the emission band of the chromophoric dye (trace b in Figure 4G−I). The time-resolved emission features in all three cases correspond to the additive steady-state emission spectrum of InP/ZnS and the corresponding chromophoric dye. On the basis of TRES studies, it is concluded that the photoexcited InP/ZnS QDs decays to the ground state by transferring the excitation energy to the ground state of the chromophoric dyes leading to the formation of its excited state. The efficiency of energy transfer was calculated by following the lifetime (eq 3) as well as the fluorescence intensity of the donor (eq 4) in the presence and absence of the acceptor. The efficiency is then given as τ E = 1 − DA τD (3)

of InP/ZnS, ruling out the possibility of ground state interaction. Further, the emission band corresponding to Nile Red was not observed in the presence of InP/ZnS QDs (Supporting Information, Figure S7). These results clearly indicate that the free amine/carboxylic acid groups on LiRh, TxRed, and Rh101 strongly bind onto the surface of QDs resulting in the formation of strong donor−acceptor pairs (further defined as InP/ZnS-LiRh, InP/ZnS-TxRed, and InP/ ZnS-Rh101). Time-resolved emission spectroscopic investigations were carried out to understand the mechanism of energy transfer in chromophore labeled InP/ZnS QDs. The energy transfer process in a donor−acceptor system occurs either through nonradiative (resonance) or radiative energy transfer pathways which can be easily distinguished by following the emission lifetimes.45−47 The former process occurs by an exchange of energy through dipolar oscillation mechanism and a decrease in the emission lifetime of the donor molecule is observed. In contrast, the latter process involves the emission and reabsorption of photons wherein emission lifetime of the donor remains unaffected. The energy transfer process in chromophore-labeled InP/ZnS systems was analyzed using the TCSPC technique. Luminescence decay curves of InP/ZnS QDs, bare as well as chromophore labeled, are presented in Figure 4A−C. The complete decay of InP in the absence and presence of acceptor dyes (monitored in a time window of 1 μs) was best fitted using a triexponential fit. Interestingly, the average lifetime of InP/ZnS (τavg = 37.63 ns) decreased substantially when labeled with chromophoric dyes: LiRh (15.07 ns), TxRed (11.45 ns), and Rh101 (10.07 ns) (Table S2, Supporting Information for details). Reduction in the τavg of InP/ZnS, on labeling with chromophoric dyes, further indicates that the resonance energy transfer process is operating. Details are provided in the Supporting Information (Figure S8). A concomitant formation of the excited state of the chromophoric dyes can conclusively confirm the energy transfer process. However, the component corresponding to formation of the excited state of the acceptor was not observed in a time window of 1 μs, and hence we collected the emission in a shorter time scale (time window of 50 ns). Transient emission profiles of chromophore bound InP/ZnS were monitored at a sub-nanosecond time scale at two different wavelengths (at the emission maximum of InP/ZnS, and at the corresponding emission maximum of chromophoric dyes: details are given in the caption of Figure 4). Following the excitation at 405 nm, InP/ZnS labeled with chromophoric dyes showed a rapid decay at 515 nm (red trace a in Figure 4D−F) and concomitant growth of the acceptor emission (blue trace; b in Figure 4D−F). The fast component in the acceptor emission, having a negative pre-exponential factor, corresponds to the formation of the excited state of the chromophoric dye.1,48 The rise time was found to be 160 ps for InP/ZnSLiRh, 190 ps for InP/ZnS-TxRed, and 200 ps for InP/ZnSRh101 (Figure S9 and Table S3 in Supporting Information).

E=1−

FDA FD

(4)

where τDA and τD denote the lifetime of the donor (InP/ZnS) in the presence and absence of the acceptor, and FDA and FD denote the fluorescent intensities of the donor (InP/ZnS) in the presence and absence of the acceptor dyes. Energy transfer efficiency calculated from the lifetime was found to be 73% for InP/ZnS-Rh101, 70% for InP/ZnS-TxRed, and 60% for InP/ ZnS-LiRh. The energy transfer efficiencies obtained from the lifetime as well as steady-state emission intensity are in good agreement and summarized in Table 1. We have further analyzed the resonance energy transfer process from InP/ZnS to chromophoric dyes within the framework of Förster formalism.1 According to the mechanism, the rate of energy transfer depends on the Förster distance (R0), distance of separation between the donor−acceptor pair (r), and the decay time of donor (τD). For quantifying the energy transfer process in QD−dye conjugates by adopting Förster dipole−dipole interaction mechanism, two fundamental questions were debated: (i) can we take QDs as point dipoles and (ii) whether the sixth power dependence of FRET on the inverse of separation distance is valid. These issues were systematically analyzed by Mattoussi and co-workers by varying the distance between the QD−dye conjugates and concluded that Förster formalism holds well.49,50 The resonance energy 3842



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Scheme 2. Schematic Representation of Energy Transfer Process in InP/ZnS-LiRh and Time-Resolved Spectruma

a

Zero time represents immediately after laser pulse.

transfer rates, kT(r), from InP donor to LiRh, TxRed, and Rh101 acceptor systems were calculated based on eq 5.1 k T(r ) =

6 1 ⎛ R0 ⎞ ⎜ ⎟ τD ⎝ r ⎠

Rh101, respectively. Various energy transfer parameters for chromophore bound InP/ZnS systems are presented in Table 1, and schematic representation of energy transfer process in InP/ZnS-LiRh and corresponding time-resolved emission spectra are shown in Scheme 2. The rate of energy transfer can be conveniently tuned by varying the overlap integral (J(λ)).We have further tuned the overlap integral by varying the size of InP/ZnS QDs (Figures S10 and S11 in Supporting Information). It was found that InP/ZnS QDs emitting at 530 nm possess a higher J(λ), which results in an enhancement in the rate of energy transfer. In contrast, the J(λ) values of InP/ZnS QDs emitting at 500 nm was found to be lower, and a decrease in the rate of energy transfer was observed. These results are presented as Tables S4 and S5 in Supporting Information. We have further extended our studies to aqueous media since it is a prerequisite for biological investigations. Water-soluble InP/ZnS QDs, emitting at 515 nm, were synthesized by place exchanging with mercaptosuccinic acid.21 Water-soluble QDs were found to be stable under ambient conditions and were used as donor moieties in energy transfer investigations with TxRed and LiRh as acceptor molecules. These results are presented as Figure S12 in Supporting Information. Preliminary studies showed that water-soluble QDs can efficiently transfer energy to chromophoric dyes (E = 40% for InP/ZnS-LiRh and 50% for InP/ZnS-TxRed) expanding the scope of using InP/ZnS based systems for biological applications.

(5)

where R0 is given by ⎛ Q κ 2 9000(ln 10)J(λ) ⎞1/6 ⎟⎟ R 0 = ⎜⎜ D 128π 5Nn 4 ⎝ ⎠

(6)

and J (λ ) =

∫0

∞

FD(λ)εA (λ)λ 4 dλ

(7)

It is necessary to obtain two important parameters, namely, Förster distance (R0; eqs 6 and 7) and the donor−acceptor distance (r; eq 8), for evaluating the rate of energy transfer (kT(r)). The Förster distance term has contributions from (i) quantum yield of the donor (QD) (ii) the orientation of the donor and acceptor dipoles (κ2; taken as 2/3, taking into account that the dipoles are randomly oriented) (iii) overlap integral (J(λ)) representing the spectral overlap of the donor emission and acceptor absorption and (iv) the refractive index of the solvent (n) (note: N denotes Avogadro number). In the present case, the extent of interaction between the transition dipole moments of the InP/ZnS emission and the absorption of corresponding chromophoric dye (LiRh, TxRed, and Rh101) dictates the overlap integral, J(λ). This term is calculated from the normalized fluorescence intensity of the donor (FD) at a particular wavelength (λ) and the molar extinction coefficient of the acceptor at that wavelength as shown in eq 7. High J(λ) values were obtained for all the three systems and using these values, Förster distances (R0) were calculated (eq 6). The J(λ) and R0 values for InP/ZnS-LiRh, InP/ZnS-TxRed, and InP/ ZnS-Rh101 are presented in Table 1. Further, the distance between the InP/ZnS and chromophoric dye (r) was calculated using eq 8. E=

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CONCLUSIONS Potential use of InP/ZnS, an environmentally benign and relatively nontoxic QD, for energy transfer applications has been successfully demonstrated by labeling with three wellknown chromophoric dyes, namely, LiRh, TxRed, and Rh101, onto their surface. The large values of bimolecular quenching constants observed in the case of chromophore bound InP/ZnS systems confirm that the interaction is predominantly static in nature. The rate and efficiency of energy transfer in these systems were calculated within the framework of Förster formalism. Time-resolved emission spectroscopic investigations revealed that the resonance energy transfer involving rapid decay of photoexcited state of InP/ZnS and concomitant formation of the excited state of chromophoric acceptor occurs with high efficiency. The size-dependent emission properties of InP were further exploited for controlling the energy transfer parameters by tuning the spectral overlap between the donor− acceptor pairs. Results presented here clearly indicate that InP/ ZnS possesses the necessary requisites for energy transfer applications. Preliminary studies carried out using water-soluble

R 06 R 06 + r 6

(8)

where R0 is the Förster distance, and E is the efficiency of energy transfer (note: the value obtained through time-resolved studies was used to calculate the efficiency). Thus, the rate of energy transfer from InP/ZnS QDs to various chromophoric dyes were calculated as 3.99 × 107 s−1 for InP/ZnS-LiRh, 6.20 × 107 s−1 for InP/ZnS-TxRed, and 7.11 × 107 s−1 for InP/ZnS3843



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InP/ZnS expand the scope of these systems to biologically relevant conditions, and tagging these materials with biomolecules such as fluorescent proteins for in vivo and in vitro biological studies is under progress. In summary, InP/ZnS is a versatile and environmentally friendly material for energy transfer applications, meeting various photophysical requirements.

Microscopic Characterization. For HRTEM studies, samples were prepared by drop casting 100 μL of solution from the cuvette on a carbon-coated Cu grid, and the solvent was allowed to evaporate. The specimens were investigated with a FEI TECNAI 30 G2 transmission electron microscope operated at an acceleration voltage of 300 kV.

EXPERIMENTAL SECTION Materials. The following chemicals, indium acetate (99.99% trace metal basis), myristic acid (99%), octylamine (99%), octadecene (technical grade), hexamethyldisilathiane (synthesis grade) and diethylzinc solution (1 M in hexane), were purchased from Sigma Aldrich. Tris(trimethylsilyl)phosphine (98%) was purchased from Acros Organics. The chromophoric dyes lissamine rhodamine B ethylene diamine and Texas red cadavarine C5 were purchased from Invitrogen, and rhodamine 101 innersalt was purchased from Sigma Aldrich. All chemicals were used as received. Steady-State Spectroscopic Investigations. All photophysical experiments were carried out at room temperature in a quartz cuvette (make Starna, USA) having a path length of 1 cm. Absorption spectra were recorded on UV−vis−NIR spectrophotometer (model Shimadzu UV-3600) and emission as well as excitation spectra using spectrofluorimeter (model Horiba Jobin Yvon- Fluorolog 3). All steady-state emission studies were carried out by exciting the solution at 400 nm keeping the excitation as well as emission slit widths at 2 nm. Time-Resolved Emission Studies. Emission lifetimes were measured using a picosecond time-correlated single photon counting system (model Horiba Jobin Yvon-IBH). Solutions were excited at 405 nm using a pulsed diode laser (NanoLED-405L;

InP Quantum Dots: An Environmentally Friendly Material with

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